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levon altunyan P M M A - T H I N F I L M S A S D I E L E C T R I C L AY E R F O R P R I N TA B L E F I E L D E F F E C T T R A N S I S T O R S


PMMA - THIN FILMS AS DIELECTRIC L AY E R F O R P R I N TA B L E F I E L D E F F E C T TRANSISTORS levon altunyan

in partial fulfillment of the requirements for the degree of Bachelor of Science Institute for Nano Structures and Technology (NST) Faculty of Engineering University of Duisburg-Essen January 07, 2009 - April 07, 2009


Levon Altunyan: PMMA - thin films as dielectric layer for printable field effect transistors, in partial fulfillment of the requirements for the degree of Bachelor of Science, Š January 07, 2009 - April 07, 2009 supervisors: Prof. Dr. rer. nat. Roland Schmechel Prof. Dr.-Ing. Einar Kruis location: campus Duisburg time frame: January 07, 2009 - April 07, 2009


ABSTRACT

In this work, the potential of Poly(Methyl Methacrylate) (PMMA) as gate dielectric has been studied. Thin films of PMMA were prepared by spin coating on glass substrate. The spin process was optimized with respect to the material in use. The maximum process temperature was 160 (â—Ś C), which corresponds to the baking of the polymeric gate dielectric. A fabrication process for building up a MetalInsulator-Metal (MIM) structure was developed. Capacitance-voltage (C-V) and current-voltage (I-V) behaviour of the fabricated glass/silver/PMMA/silver, glass/Indium Tin Oxide (ITO)/PMMA/silver as well as glass/aluminium/PMMA/aluminium MIM structures were studied. The measurements were carried out at constant frequency of 100 kHz in the voltage range of -10 V to +10 V. Furthermore, the dielectric constant of the PMMA in use was verified. In addition, the field strength ranges at which breakdown occurs were examined. Frequency dependence of the electronic properties was also investigated. The realization of Metal Insulator Semiconductor Field-Effect Transistor (MISFET) glass/aluminium/PMMA/pentacene/aluminium and glass/aluminium/PMMA/C60 /aluminium structures as well as their characteristic curves are presented. Keywords: Poly(methyl methacrylate); Polymer gate dielectric; Organic thin film transistor;

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The smallest act of kindness is worth more than the grandest intention. — Oscar Wilde

ACKNOWLEDGMENTS

I would like to take the opportunity to say THANK YOU to Prof. Dr. rer. nat. Roland Schmechel and Dr. Ing. Dibakar Roy Chowdhury for their time and guidance during the development of this project. Without them this bachelor thesis would not have been possible. Furthermore, I would like to thank the whole team of the Institute for Nano Structures and Technology (NST) for their support concerning my work in the laboratory. Their advices contributed to the pleasant and fruitful experience that I obtained during this time. In addition, I would like to thank all those people that motivated me throughout the years to constantly try to make the best that I am capable of doing. The words would not fully express my gratitude to my family for their continuous support during my bachelor studies. Nevertheless, I would like to give my special thanks to my parents which have provided me with the opportunity to learn and face so many new things. Last but not least, I would like to thank one special member of my family, namely my dog - Archi, for the inspiration he has been giving me, during the times of hopeless laziness, independently from the distance which is dividing us.

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CONTENTS

i introduction 1 introduction ii 2 3 4 5

1 2

technology development 5 spin coating process 6 metal-insulator-metal (mim) structure organic field-effect transistor 36 conclusion and future work 44

bibliography

46

vi

17


LIST OF FIGURES

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 Figure 25

Poly(Methyl Methacrylate) (PMMA) 3 Dynamic Dispense Process - Schematic Representation 7 XP-200 High Resolution Stylus-Type Surface Profilometer, Ambios Technologies 8 Two Phase Spin Coating, ω2 ∈ [1000; 3500](rpm) 9 Two Phase Spin Coating, ω2 ∈ [1000; 7000](rpm) 10 Theoretical Model for Ultrathin PMMA Spin Coated Films [43] 10 3000 vs. 10000(rpm/s2 ) - Acceleration Comparison 12 PMMA Layer Thickness vs. Angular Velocity, tspin = 25(s) 13 PMMA Layer Thickness vs. Angular Velocity, tspin = 30(s) 13 Spin Coater, APT Spin150-v3-NPP 14 PMMA Layer Thickness vs. Spin Speed, One Spin Phase 15 High to Low Scattering PMMA Layer Heights Transition Region 16 MBraun 200B Glove Box System 17 Keithley 2612 18 High Leakage Currents 18 Top Contact, Mask Types - Part 1 20 3d Models of the MIM Structures - Part 1 23 3d Models of the MIM Structures - Part 2 25 Current-Voltage Characteristic of PMMA, High Currents 26 Current-Voltage Characteristic of PMMA, Low Currents 26 Crossed Contacts Structure, Side View 27 Top Contact, Mask Type 3, Design for CapacitanceVoltage Studies 28 Parallel Plate Capacitor Model; Semiconductor Characterization System 30 Capacitance Measurements 30 Dielectric Constant () vs. Contacts Area, dPMMA ≈ 375(nm) 31

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Figure 26 Figure 27 Figure 28 Figure 29 Figure 30 Figure 31 Figure 32 Figure 33 Figure 34

Figure 35 Figure 36 Figure 37 Figure 38 Figure 39 Figure 40 Figure 41

Dielectric Constant () vs. Contacts Area, dPMMA ≈ 475(nm) 32 Dielectric Constant () vs. Contacts Area, dPMMA ≈ 700(nm) 32 PMMA Layer Thickness vs. Dielectric Constant () 33 Breakdown Voltage, dPMMA ≈ 700(nm) 34 Breakdown Voltage, dPMMA ≈ 475(nm) 34 Breakdown Voltage, dPMMA ≈ 375(nm) 35 General Structure of the Realized Field-Effect Transistors (FETs) 36 Semiconducting Layer Materials 37 Characteristic Curve, Potential Curve and Cross Sectional View of the MISFET for Different Voltage Regions 38 Common FET Configurations 39 Realized MISFET Structures 40 Al/PMMA/C60/Al MISFET Structure, Characteristic Curve 1 40 Al/PMMA/C60/Al MISFET Structure, Characteristic Curve 2 41 Al/PMMA/Pentacene/Al MISFET Structure, Characteristic Curve 1 41 Al/PMMA/Pentacene/Al MISFET Structure, Characteristic Curve 2 42 MISFET - Non-Ideal Channel Interface 42

L I S T O F TA B L E S

Table 1 Table 2 Table 3 Table 4 Table 5

Two Phase Spin Coating Parameters, ω2 ∈ [1000; 3500](rpm) 8 Two Phase Spin Coating Parameters, ω2 ∈ [1000; 7000](rpm) 9 Material Data for Dielectric EG (PMMA) 11 Spin Parameters, 3000 vs. 10000(rpm/s2 ) - Acceleration Comparison 11 Spin Parameters, tspin = 25(s) vs. tspin = 30(s); Spin Time Comparison 12

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Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14

Spin Parameters, High to Low Scattering Transition Region 16 First MIM Structures, Types and Parameters 19 ITO Material Data 19 Realized MIM Structures, Samples: 5-8 21 Realized MIM Structures, Samples: 9-18 22 Realized MIM Structures, Samples: 19-27 24 Capacitance and Dielectric Constant () for Samples: 19-24 29 Dielectric Constant () Values for Different Contacts Geometric Areas and PMMA Thicknesses 33 Electric Field (E) at VBreakdownPMMA 35

ACRONYMS

PMMA

Poly(Methyl Methacrylate)

ITO

Indium Tin Oxide

MIM

Metal-Insulator-Metal

Ag

Silver

Al

Aluminium

FET

Field-Effect Transistor

OTFT

Organic Thin-Film Transistor

MIS

Metal Insulator Semiconductor

MISFET

Metal Insulator Semiconductor Field-Effect Transistor

OFET

Organic Field-Effect Transistor

MPS

Metal Polymer Semiconductor

SSE

Sum of Squared Error

RMSE

Root Mean Squared Error

ix


Part I INTRODUCTION


1

INTRODUCTION

The crucial process of charge accumulation and transport in fieldeffect transistors takes place at and very close to the interface between the gate dielectric and the semiconductor; hence, the properties of this interface and the dielectric have a huge influence on device characteristics as well as great impact on hole and electron transport in Field-Effect Transistors (FETs). Device parameters such as mobility, threshold voltage, subthreshold swing, etc. depend not only on the nature of the semiconductor but also on the chemical structure and dielectric properties of the insulator. The requirements for gate dielectrics in field-effect transistors are rigorous. They should show high dielectric breakdown strength, contain only minimal concentrations of impurities that could act as traps, and be environmentally stable, easily processable, and compatible with preceding and subsequent processing steps. Apart from their breakdown strength, gate dielectrics are mainly characterized by their dielectric constant  (also named Îş), which determines the capacitance C = d0 A of a dielectric layer of thickness d (0 is the permittivity in vacuum) and thus the amount of induced charges per applied gate electrode voltage (Vg ). Hence, in order to achieve a certain amount of charges in the transistor channel, one can either reduce the dielectric thickness or use a dielectric with a higher . On the other hand an important part in the modern field of printable electronics is the possibility to make low cost semiconducting devices from low-temperature-processable materials like polymers or nanoparticles. Furthermore, a material whose characteristics can be tuned over a wide range by changing its chemical structure should be preferred [44]. Polymer gate dielectrics have been used in top as well as bottom gate transistors, and their impact on morphology and mobility was investigated [10, 19, 25, 31, 32, 45]. They are easily applied in top gate transistors, where they are spun on top of the semiconductor from solvents orthogonal to the semiconductor and do not influence the interface morphology or damage the semiconductor [5, 35, 41]. Therefore, with respect to plastic as well as to transparent electronics, it is of significant importance to be investigated, how thin but still insulating polymer layers can be made as well as how the semiconducting active layers behave on the surface of such polymer layers. Poly(Methyl Methacrylate) (PMMA) is one of the promising polymeric materials and there are numerous papers for its application as a gate dielectric in Organic Thin-Film

2


introduction

(a) Structure of the PMMA Polymer

(b) Dielectric EG (PMMA)

Figure 1: Poly(Methyl Methacrylate) (PMMA)

Transistors (OTFTs) [33, 34, 40]. Therefore, the main goal of this work is to verify the feasibility of PMMA as a gate dielectric in the field of inexpensive electronics. PMMA is a polymeric resist commonly used in high resolution nanolithographic processes which use electron beam, deep UV (220-250 nm) or X-ray radiation. PMMA has also been used as a protective layer for wafer thinning. Its thermal and mechanical stability, together with a high resistivity (> 2 Ă&#x2014; 1015 â&#x201E;Ścm) [34] and suitable dielectric constant, similar to that of silicon dioxide ( = 3.9), make PMMA a good candidate as a dielectric layer in Metal Insulator Semiconductor (MIS) structures. Besides, PMMA can be easily deposited on large areas by spin-coating and baked at low temperatures (< 170(â&#x2014;Ś C)). Puigdollers et al. [33, 34] fabricated pentacene thin film transistor using PMMA and SiO2 as gate dielectrics. They stated that transistors using PMMA as a gate dielectric showed better electrical characteristics than SiO2 . They also observed that PMMA surface favors the formation of bigger crystalline grains than SiO2 surface, which consequently leads to improved field effect mobility. Uemura et al. [40] investigated the effect of surface modification of PMMA with clay mineral and showed that leakage current was smaller than unmodified PMMA diode structure. El-Shahawy [11] studied the dielectric constant () of solution cast PMMA film (thickness = 1 mm) and PMMA mixed with some organic laser dyes at different temperatures ranging from 30 â&#x2014;Ś C to 130 â&#x2014;Ś C and for various frequencies ranging from 0.6 to 10 kHz. They observed that the 

3


introduction

value increased from 3.6 to 5.1 with the increase of temperature for 10 kHz. Davis and Pathrick [9] reported the variation of dielectric constant and dielectric loss of PMMA (Mw = 136000) with frequency (1 Ă&#x2014; 102 to 1 Ă&#x2014; 105 Hz) for different annealing temperatures (30, 50 and 120 â&#x2014;Ś C) and for various annealing times (0 â&#x2C6;&#x2019; 64h). Na and Rhee [28] investigated characteristics such as Capacitance-Voltage (C-V) and Current-Voltage (I-V) behavior of aluminium/PMMA/p-Si MIS structure, aka Metal Polymer Semiconductor (MPS). They concluded, that the electronic properties of the annealed PMMA film at above glass transition temperature were degraded substantially with larger shift in flat band voltage, low dielectric constant and low breakdown voltage. In this work, the optimization of the spin-coating process [17, 30], for PMMA as well as the development of a Metal-Insulator-Metal (MIM) structure is demonstrated. Furthermore, studies of the CurrentVoltage as well as the Capacitance-Voltage characteristics of these structures are presented. In addition, the dielectric constant of the PMMA in use (PMMA@100kHz ) was verified. After optimization of the related parameters and electrically characterization of the developed MIMs was achieved, several Metal Insulator Semiconductor FieldEffect Transistor (MISFET) devices were built. Thus, the the realization of transistors using PMMA as a gate dielectric was achieved and is described.

4


Part II TECHNOLOGY DEVELOPMENT


2

S P I N C O AT I N G P R O C E S S

Fundamental progress has to do with the reinterpretation of basic ideas. â&#x20AC;&#x201D; Alfred North Whitehead In this section the optimization of the spin-coating process of PMMA will be described. Furthermore, the information and procedures needed to replicate the build up of the MIM structures will be explained. Moreover, the equipment and materials needed will be listed. Finally, the reasons, limitations and assumptions which influenced the choice of the methods in use will be stated. The MIM structures were fabricated on microscope cover slip transparent hydrolytic glasses (class 1). The substrates had a square shape, 15 Ă&#x2014; 15 (mm), and thickness of 0,5 - 0,6 (mm) [15]. As a first step each of the glasses was carefully cleaned so that oils and organic residues which appear on this type of surfaces are removed. The procedure that has been used involved the following ordered steps: 1. rinsing in acetone; 2. rinsing in ethanol; 3. rinsing in isopropanol; 4. rinsing in distilled water; 5. blow drying with compressed air; The next step included studies of the spin coating process for PMMA. One of the most important factors in spin coating is repeatability. Subtle variations in the parameters that define the spin process can result in drastic variations in the coated film. As a first approach a dynamic dispense process was preferred. The reason for this was the high viscosity PMMA in use. In this way dispensing is achieved while the substrate is turning at low speed (Ď&#x2030;1 ). A speed of about 1000 (rpm) was commonly used during this step of the process. This served to spread the fluid over the substrate as well as resulted in less waste of resin material since it was not necessary to deposit as much to wet the entire surface of the substrate. This is a particularly advantageous method when the fluid or substrate itself has poor wetting

6


t1

0

t2

t

ω

spin coating process

ω2 ω1 0

t1

t2

t

Figure 2: Dynamic Dispense Process - Schematic Representation

abilities and can eliminate voids that may otherwise form. After the dispense step the samples were accelerated to a relatively high speed (ω2 ) to thin the fluid to near its final desired thickness. Typical spin speeds for this step ranged from 1000 − 6000 (rpm). This step was studied in the range from 10 seconds to a minute. The combination of spin speed (ω2 ) and time (t2 ) selected for this step defined the final film thickness. In general, higher spin speeds and longer spin times created thinner films. The spin coating process involved a large number of variables that tended to cancel and average out during the spin process. That was the reason why sufficient time for this to occur was needed. The acceleration of the substrate towards the intermediate (ω1 ) and the final spin speed (ω2 ) also affected the coated film properties. Since the resin begins to dry during the first part of the spin cycle, it is important to accurately control acceleration. In many cases the substrate could retain topographical features from previous processes. It was therefore important to uniformly coat the resin over and through these features. While the spin process in general provides a radial (outward) force, it is the acceleration that provides a twisting force to the resin. This twisting aids in the dispersal of the resin around topography that might otherwise shadow portions of the substrate from the fluid. In operation the spin motor accelerates (a1 , a2 ) or decelerates (a3 ) in a linear ramp to the  final  spin speed. At first, relatively low accelerations such as 200 rpm , s2     500 rpm and 600 rpm were selected. A schematic representation s2 s2 of the two phases method can be seen in fig. 2. After changing the PMMA type from one with high viscosity to a one with considerably lower one - the number of steps had to be accordingly adjusted. Throughout the later experiments the PMMA

7


spin coating process

Table 1: Two Phase Spin Coating Parameters, ω2 ∈ [1000; 3500](rpm) Measurement Rate 1 sample/step size 8 points/sample step size = 500(rpm)

First Spin: ω1 = 1000(rpm) a1 = 500(rpm/s2 ) t1 = 20(s)

Second Spin: ω2 ∈ [1000; 3500](rpm) a2 = 500(rpm/s2 ) t2 = 40(s)

Baking Conditions: Tbaking = 150(◦ C) tbaking = 50(min)

material (Dielectric EG) had wt% = 4, 5 (%). This fact was initially not taken into consideration, which led to the observed in fig. 4 and fig. 5 results, based on the parameters listed in tables 1 and 2.

(a) Measurements of the Surface Mor-(b) Surface Profilometer and Benchtop Viphology bration Insulator Instrument Set

Figure 3: XP-200 High Resolution Stylus-Type Surface Profilometer, Ambios Technologies

The layer thickness has been measured with the XP-200 High Resolution Stylus-Type Surface Profilometer from Ambios Technologies (fig. 3a). To isolate vertical and horizontal vibration as well as vibration generated around the vertical axis of rotation as well as both horizontal axes of inclination, a Micro 40 benchtop unit from Halcyonics was utilized. The instruments’ arrangement needed for this step could be seen on fig. 3b. Diagrams 4 and 5 as well the final study of the PMMA layer thickness versus angular velocity have been fitted. The used fitting function (in red color) serves only as a good reference for the general behavior of the measured set of data points. This generalized correlation is a widely observed experimental result, and it is therefore accepted that the (empirically derived) mathematical relationship has the following form h = k1 ωα , where h is the film thickness, ω is the angular velocity, while k1 and α are empirically determined constants [24]. The α, has been observed to change only slightly for various polymer/solvent systems, and has, by most workers, been set in close vicinity of -0.5 [2, 3, 7, 8, 23, 26, 36] The goodness of the fit is reduced due to the fact that the concentration factor, cα was not taken into account. Using the suggested by

8


Film thickness, h [nm] →

spin coating process

750 700

PMMA Layer Thickness (nm) vs. Spin Speed (rpm) α

h = k1ω

650 600 550 500 1000

1500

2000 2500 Spin Speed, ω [rpm] →

3000

3500

Figure 4: Two Phase Spin Coating, ω2 ∈ [1000; 3500](rpm) Table 2: Two Phase Spin Coating Parameters, ω2 ∈ [1000; 7000](rpm) Measurement Rate 1 sample/step size 8 points/sample step size = 500(rpm)

First Spin: ω1 = 1000(rpm) a1 = 500(rpm/s2 ) t1 = 20(s)

Second Spin: ω2 ∈ [1000; 7000](rpm) a2 = 500(rpm/s2 ) t2 = 60(s)

Baking Conditions: Tbaking = 120(◦ C) tbaking = 50(min)

[43] values a sample plot of the layer thickness as a function of the material concentration and the angular velocity could be seen on fig. 6. After obtaining the kindly provided material data from Evonik (table 3), the benefits of the static dispense were found out and it became the preferred method. Thus, simply depositing a small puddle of the PMMA fluid on or near the center of the glass substrate was sufficient. The amount of material was selected in a way that the substrate is fully coated. As a general observation, once the substrate area was fully coated, the exact amount of material did not drastically influenced the final spin coated layer thickness. Therefore, a larger puddle, to ensure full coverage of the substrate during the high speed spin step, should be preferred. By choosing one, instead of several smaller interlocking circular drops, reduced the number of impurities as well as air bubbles inside the final PMMA layer. Consequently, the maximum possible acceleration of the spin coater in use was utilized. Thus, the time needed to reach the final angular velocity was reduced to minimum.  The maximum stated in the specifications acceleration

was 2000 rpm . Nevertheless, if the weight of the substrate is low s2 enough we could go for higher values. To verify if there is major difference in the acceleration parameter for values greater than the 0 stated maximum supported by the spin coater, tests at a1 = 3000

9


Film thickness, h [nm] →

spin coating process

PMMA Layer Thickness (nm) vs. Spin Speed (rpm)

650

h = k1ωα

600 550 500 450 400 1000

2000

3000 4000 5000 Spin Speed, ω [rpm] →

6000

7000

Figure 5: Two Phase Spin Coating, ω2 ∈ [1000; 7000](rpm) 1.56

d1 [µmeter] = 0.92*(c

−0.51

)*(ω

) 1

d1 [µmeter] →

1.5

0.8 1 0.6 0.5

0.4 0.2

0 20 10 0 8000 ← wt % [%]

2000 6000 4000 ← ω [rpm]

0

Figure 6: Theoretical Model for Ultrathin PMMA Spin Coated Films [43] 00

( rpm ) and a1 = 10000 ( rpm ) were carried out. All other parameters s2 s2 were kept constant (see table 4). The data plotted on fig. 7 , showed that actually for values higher than the amaxSpinCoater = 2000( rpm ) s2 there are no great differences in the obtained PMMA layer thicknesses. The majority of measured points tended to distribute themselves in the region between 370 (nm) and 420 (nm).Therefore, to reduce the ramp, as well as to assure the maximum possible acceleration in each case the value was set to a = 10000( rpm ). Furthermore, the time s2 dependency for the one spin phase was studied. Tests were made for 2 different time durations, with 2 different angular velocities each. 0 00 The selected times were t1 = 25(s) and t1 = 30(s). The respective 0 0 rotational velocities were ω1 = 3500( rpm ) and ω1 = 4000( rpm ). The s2 s2 reason to carry out this tests was also to compare this behavior with the reported in the supplied material data for PMMA. The rest of the

10


spin coating process

11

Table 3: Material Data for Dielectric EG (PMMA) Mw wt% Annealing step 3000 U SD %SD Roughness in nm(Ra 1) Roughness in nm(Rq 2) Waviness in nm(Wt ) Solvent Solvent data 1 2 3

1250000 1250000 4(%) 4, 5(%) ◦ ◦ 160( C) / 30min 160( C) / 30min 236 (nm) 385 (nm) +/- 10 +/- 15 4,15 % 3,79 % 2 2 2 3 10 17 E-Lact., BA a. Triethylin3 E-Lact., BA a. Triethylin Ethyllactat(61,5%) Butylacetat(38,5%)

1250000 5(%) ◦ 160( C) / 30min 625(nm) +/- 9 1,42 % 2 3 16 E-Lact., BA a. Triethylin Plasticizers(0,007%)

Arithmetic average of absolute values. Root mean squared. Ethyllactat, Butylacetat and Triethylin.

Table 4: Spin Parameters, 3000 vs. 10000(rpm/s2 ) - Acceleration Comparison

Measurement Rate 8 points/sample 3 samples 6 points/sample 7 samples

Spin Parameters: ω1 = 2000(rpm) t1 = 25(s) ω1 = 10000(rpm) t1 = 25(s)

Baking Conditions: Tbaking = 160(◦ C) tbaking = 30(min) Tbaking = 160(◦ C) tbaking = 35(min)

parameters was again kept constant for both cases (tspin =25 (s) and tspin =30 (s)) to the values given in table 5. The results, of this "play" with the time parameters can be seen on fig. 8 and fig. 9. For both cases the expected decrease in the PMMA layer heights with increase of the angular velocity was observed. Normally, with increase in spin coating time, the thickness should as well decrease. Nevertheless, from the measurements that were carried out comparing 25(s) and 30(s) this was not confirmed. This discrepancy between the theoretical model and the actual values should be accounted to the possible changes in the ambient conditions. For all future spins the tspin was kept constant to 25(s). In some cases the positions at which the substrate and the chuck of the spin coater were connected introduced local changes of the PMMA layer. It was following the shape of the circular chuck in use. One possible improvement for less surface inhomogeneities such as "waves" or others, would be the use of a special chuck, with a holder that would substitute the employment of vacuum. Additional parameters that are well known to affect the final spin coated layer thickness such as the drying rate of the resin fluid, factors like


spin coating process

Film thickness, h [nm] →

500 450 400 350 300 0

2000

4000 6000 8000 Acceleration, a [rpm/s2] →

10000

12000

Figure 7: 3000 vs. 10000(rpm/s2 ) - Acceleration Comparison Table 5: Spin Parameters, tspin = 25(s) vs. tspin = 30(s); Spin Time Comparison

Measurement Rate 6 points/sample 2 samples 6 points/sample 2 samples

Spin Parameters: ω1 = 3500(rpm) a1 = 10000(rpm/s2 ) ω2 = 4000(rpm) a1 = 10000(rpm/s2 )

Baking Conditions: Tbaking = 160(◦ C) tbaking = 45(min) Tbaking = 160(◦ C) tbaking = 45(min)

air temperature, humidity and other environmental effects were not in particular considered. Nevertheless, their influence was held approximately constant or was eliminated, during the spin process by the "closed bowl" design of the spin coater in use. All PMMA layer depositions were made on the Single Wafer Spin Processor for Manual Dispense (APT-SPIN150-v3-NPP), seen on fig. 10. Baking of the spin coated samples was an important step for the preparation of uniform thin PMMA films. Initially, the PMMA was baked at low temperatures (Tannealing = 120(◦ C) for tannealing ∈ [30; 60](min). The PMMA layer thickness, did not show any dependency on a longer heating time duration [17]. This as well as PMMA having a glass transition point Tg around 120(◦ C) [37], were the reasons why the temperature had to be increased a little bit more to Tannealing = 160(◦ C) and the duration was kept constant to tannealing = 30(min). The set of the initial spin coating procedure could be summarized as follows: • Use of 2 phases: – First Spin parameters:

12


spin coating process

Film thickness, h [nm] →

400

350

300

250 3000

3500 4000 Spin Speed, ω [rpm] →

4500

Figure 8: PMMA Layer Thickness vs. Angular Velocity, tspin = 25(s)

Film thickness, h [nm] →

500 450 400 350 300 250 3000

3500 4000 Spin Speed, ω [rpm] →

4500

Figure 9: PMMA Layer Thickness vs. Angular Velocity, tspin = 30(s)

* ω = 1000(rpm) 2 * a = 500(rpm/s ) * t = 20(s) – Second Spin parameters: * ω ∈ [1000(rpm); 7000(rpm)] * stepsize = 500(rpm) 2 * a = 500(rpm/s ) * t = 60(s)

– Baking Conditions: ◦ * Tannealing = 120( C) * tannealing = 50(min)

13


spin coating process

Figure 10: Spin Coater, APT Spin150-v3-NPP

– Preparation of 1 sample/step size – PMMA layer thickness measured at 8 points/sample Taking into account all this factors, the spin coating procedure for PMMA in use was optimized. The final outcome of all previously made considerations could be summarized in the following procedure: 1. Try to cover the whole substrate area; 2. After putting the PMMA drops on the substrate wait for some time before starting the spin coating. twait ≈ 10(s); 3. Do not use a "two step" (dynamic dispense) procedure → Use only one phase for approximately 25(s). tspin ≈ 25 (s); 4. Reduce the "ramp" to minimum →  Use as high as possible  accel eration. ("artificially" a = 10000

rpm s2

, aactual ≈ 2000

rpm s2

);

5. If possible do not use vacuum mode (special chuck is required); 6. Use temperature in the range of Tbaking = 160(◦ C) (Tbaking > Tg );

14


spin coating process

Film thickness, h [nm] →

900 800 700

PMMA Layer Thickness (nm) vs. Spin Speed (rpm) α

h = k1ω

600 500 400 300 1000

2000

3000 4000 Spin Speed, ω [rpm] →

5000

6000

Figure 11: PMMA Layer Thickness vs. Spin Speed, One Spin Phase

After the procedure has been optimized and the parameters have been fixed the final study of the PMMA layer thickness versus spin speed has been carried out. The study is based on the previously discussed procedure. PMMA was spun on glass substrate. The Spin Coater Parameters have been fixed to: • a = 10000( rpm ); s2 • t = 25(s); • Tannealing = 160(◦ C); • tannealing = 30(min); The considered spin speed was in the range ω ∈ [1000(rpm); 6000(rpm)]. For each step, stepsize = 250(rpm), 2 spin coated samples have been measured. For each of the samples 6 points were considered. The points were located on two different scratch lines, one through the center of the sample and one closer to the outer edge of the substrate. Three points were measured on each of the two lines. The first one close to the left, a center one and one close to the right edge of the scratch lines. In this way local deviations in the PMMA hight could be detected and a reasonable average hight for the spun layer assumed. Figure 11 is the final outcome of this work. The general trends between theoretical and measured data coincides - the final film thickness is inversely proportional to the spin speed and spin time. This results correspond also better to the thicknesses given by the material data sheet (see table 3) for the case of wt% = 4, 5(%). Less scattering in the obtained values is observed in the values of the PMMA layer thickness for ω > 1700(rpm). Therefore, the transition region between large inhomoginities and small scattering in the spin coated PMMA heights was studied better (see

15


spin coating process

fig. 12). For ω ∈ [1500; 2000](rpm), the parameters in table 6 were kept constant. Table 6: Spin Parameters, High to Low Scattering Transition Region

Measurement Rate 6 points/sample 2 samples

Film thickness, h [nm] →

650 600

Spin Parameters: a1 = 10000(rpm/s2 ) t1=25(s)

Baking Conditions: Tbaking = 160(◦ C) tbaking = 40(min)

PMMA Layer Thickness (nm) vs. Spin Speed (rpm) α

h=k ω 1

550 500 450 400 1500

1600

1700 1800 Spin Speed, ω [rpm] →

1900

2000

Figure 12: High to Low Scattering PMMA Layer Heights - Transition Region

Concerning the used fit, suggested in the literature the following observations can be made. After the maximum number of function evaluations was exceeded, the fit computation did not converge. Therefore, the current equation (d(ω) = k1 ∗ (ω(−α) ) may not be a good model for the data. The following coefficients (with 95% confidence bounds) have been computed: • k1 = 1.465e + 004(1.156e + 004, 1.774e + 004) • α = −0.4633(−0.4362, −0.4904) The goodness of the fit could be summarized within the following measures: • Sum of Squared Error (SSE): 5.195e+005 • R-square: 0.8105 • Adjusted R-square: 0.8097 • Root Mean Squared Error (RMSE): 45.58 From the values of α > −0.5 [24] suggests that the phenomenon is possibly caused by the effect of fluid inertia.

16


M E TA L - I N S U L AT O R - M E TA L ( M I M ) S T R U C T U R E

After optimizing the spin coating procedure for the PMMA layer, the bottom and top contacts of the MIM structure had to be designed. Different combinations of materials and structures have been implemented and their effectiveness and properties have been studied and verified. The material types that have been used were aluminium, silver and indium thin oxide. Initially, the bottom contacts have been made by using relatively thick (dAg = 220nm) layers of silver (ρ = 10, 49 g/cm3 ). They were fully metalized by thermal evaporation (see fig. 13a) under low pressure conditions of 2 × 10−6 (mbar). Typical deposition rates in the range

(a) MBraun, Evaporation Chamber

(b) MBraun 200B Glove Box System

Figure 13: MBraun 200B Glove Box System

of 3-5 (Å/s) were used. All metal evaporations were made inside the MB 200B glove box system’s chamber seen on fig. 13b. After the bottom metalization was ready, PMMA was spin coated with different thicknesses. At the end, the top contact (dAg ∈ [220; 250](nm)) has been evaporated using a mask of the type seen on figure 16a. After the first prototype MIM structures were prepared, I-V measurements have been carried out. The instrument used for this procedure was a 2612 Dual-Channel System SourceMeter Instrument from Keithley (fig. 14). The tests were driven in the ranges of -10 V to +10 V. The maximum current was capped to 1 mA. A critical issue occurred when using this arrangement. During the I/V measurements high leakage currents have been observed which can be seen on fig. 15. Several different scenarios for the possible reasons have been taken into consideration: • Possible diffusion of the silver contacts inside the PMMA layer;

17

3


metal-insulator-metal (mim) structure

Figure 14: Keithley 2612

• Probability that during measurements, the top contact’s electrode probe penetrates through, and touches the bottom contact and therefore causes high leakage currents; 6

Current, I [nA] →

1.5

x 10

1 0.5 0 −0.5 −1 −1.5 −1

−0.5

0 Voltage, V [V] →

0.5

1

Figure 15: High Leakage Currents

That was the reason why, as a first step a "dirty" method of placing thicker top contacts made of silver paste was considered as possible solution of the "too" thin top contact hypotheses. Nevertheless, the problems persisted. A summary of the parameters and types of structures used initially for the MIM structures, could be seen on table 7. To replicate a similar structure [39], Indium Tin Oxide (ITO) was used as the bottom contact. Prefabricated samples were used. The data for the bottom ITO contacts is given in table 8.

18


metal-insulator-metal (mim) structure

Table 7: First MIM Structures, Types and Parameters 1

2

3

1-st Spin:

2-nd Spin:

Structure Type:

Annealing:

ω1 = 1000(rpm) a1 = 200(rpm/s2 ) t1 = 20(s) ω1 = 1000(rpm) a1 = 500(rpm/s2 ) t1 = 20(s) ω1 = 1000(rpm) a1 = 500(rpm/s2 ) t1 = 20(s)

ω2 = 4000(rpm) a2 = 200(rpm/s2 ) t2 = 60(s) ω2 = 5000(rpm) a2 = 500(rpm/s2 ) t2 = 50(s) ω2 = 4000(rpm) a2 = 500(rpm/s2 ) t2 = 40(s)

PMMA≈ 1200(nm)

T = 110 (◦ C) theat = 40(min)

PMMA≈ 685(nm) Top Ag ≈ 235(nm)

T = 110 (◦ C) theat = 40(min)

TopAg≈ 245(nm)

T = 120 (◦ C) theat = 60(min)

Table 8: ITO Material Data ITO-Glass

6 Ω/sq. :

typ.6 Ω/sq. :

typ.ITO-film tk.:

Substrate thickness:

20

15

100(nm)

1,1(mm)

The surface of the ITO has been checked with the profilometer for probable spikes. The presence of the later could not been registered. Nevertheless, the ITO was rubbed so that the surface of the bottom contact is "polished". After this, the ITO-Glass structure was cleaned using the same procedure as for cleaning the glass substrates described previously. As a next step the PMMA layer was placed. Finally, by the use of shadow masks from the types presented on fig. 16a (sample 5, table 9) and fig. 16b (samples 6 and 7, table 9), the top contacts have been evaporated. Due to the small connection areas and the technological challenges related with the measurement of the MIM characteristics using the structure presented on figure 16a as a top contact, this type of mask was excluded from the further top metallisation procedures. That was the reason, why a mask from the type seen on fig. 16b was preferred. On top of the spun PMMA silver has been evaporated, as well as silver paste droplets have been placed as top contacts for several MIM structures of the type seen on fig. 17a (sample 8, table 9). Furthermore, a special design has been taken into consideration. The previously used mask, has been used for the bottom as well as for the top contacts but rotated by 90◦ (fig. 18a), using Silver (Ag) (fig. 17b). In this way regions with no overlapping metalization areas were achieved. Therefore, at this points the contacts with the probes were made. Thus, the probability that during measurements, the top contact’s probe penetrates through, and touches the bottom contact and therefore causes high leakage currents was considerably decreased. This idea was kept for several MIM samples with different PMMA layer thicknesses (samples 9-18, table 10). Unfortunately, the high leakage current problems persisted, which leaded to the conclusion that the silver material is dissolving

19


metal-insulator-metal (mim) structure

(a) Top Contact, Mask Type 1

(b) Top Contact, Mask Type 2

Figure 16: Top Contact, Mask Types - Part 1

inside the PMMA layer, which was the reason for the high leakage currents observed. Thus, the material for the bottom and top contacts has been changed to Aluminium (Al) (Ď = 2.70 g/cm3 ). By using the new bottom and top contacts arrangement design, as well as introducing Al as the contacts material, leakage current has reduced significantly. The highest currents flowing case is represented on fig. 19. In the rest of the cases a graph of the type of fig. 20 could be observed. Nevertheless, this was not a final proof that the measured low valued leakage currents are actually due to the isolating properties of the PMMA. After careful investigation of the contact lines, it turned out that the bottom pairs are difficult to connect with the tips of the measuring device. This was mainly due to the type of the needles in use, as well as the precautions not to scratch the bottom contacts while the PMMA layer is being slightly removed. Thus, the bottom contacts acted as if they were broken. In this case, the wrong impression of low leakage currents could be a consecuence of the absence of connection between the contacts and the tips of the measurement instrument. One possible solution for this was the introduction of a small paper during the spin coating phase. Thus, part of the bottom contacts could be prevented of being covered with PMMA. The problem with this method was the quality of prevention as well as the introduction of additional non-uniformities (e.g. glue impurities) over the contacts. That was the reason why, this method was eventually abandoned. Furthermore, chemically removement of the PMMA with acetone was also tried. The problem with this method was the fast distribution and difficulty of opening only a

20


metal-insulator-metal (mim) structure

Table 9: Realized MIM Structures, Samples: 5-8 Before Spin1: 5

True

6

True

7

2xClean-True

8

True

1

Spin Parameters: Ď&#x2030;1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) Ď&#x2030;1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) Ď&#x2030;1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) Ď&#x2030;1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Structure Type: Ag

PMMA ITO Glass Ag

PMMA ITO Glass Ag

PMMA ITO Glass Ag

PMMA ITO Glass

Annealing: T = 160(â&#x2014;Ś C) theat = 30(min) T = 160(â&#x2014;Ś C) theat = 120(min) T = 160(â&#x2014;Ś C) theat = 30(min) T = 160(â&#x2014;Ś C) theat = 35(min)

Substrate cleaning; t â&#x2030;&#x2C6;10(s) after placing PMMA, before spin coating;

certain bottom contact area with the needed preciseness. Therefore, to verify if the measured current voltage characteristics are correct the measurement of the MIM capacitance was carried out. By measuring CMIM â&#x2C6;&#x2C6; pF (based on the calculated contacts area, and expected dielectric constant ( â&#x2C6;&#x2C6; [3; 5]) was going to give us a sure indication, that the probes are connected properly at the penetration free areas. Therefore, it could be concluded that the measured I/V behavior is due to the used PMMA dielectric properties. Otherwise, due to the infinitely large area of air, the measured capacitance would be in the fF range or resulting in negative values indicating interconnection between the bottom and top conducting lines. One other issue that had to be addressed was the exactness of the PMMA layer thickness and its distribution over the bottom contacts. In our assumptions for the actual PMMA layer thickness, a study based on spin coating over flat glass surface has been used. Due to the non-flat surface of the bottom contacts, the spun PMMA layer thickness could be distributing in a different, difficult to model way as represented on fig. 21b. To assure the better PMMA distribution over the bottom contactâ&#x20AC;&#x2122;s plane the heights of the contacts had to be reduced. It was verified that conductance is still observable at Al thicknesses in the range of 25 nm. Therefore, the ranges of 50-75 (nm) for the bottom and 120-150 (nm) for the top contacts have been selected. Thus, the pressure caused by the top contact on the PMMA layer has decreased. Therefore, the MIM structures constructed thereafter, had even lower contactsâ&#x20AC;&#x2122; hight metalization in the ranges (dAl â&#x2C6;&#x2C6; [50; 75](nm). At this point in time, a MIM structure, for which CMIM â&#x2030;&#x2C6; 50(pF) was measured, was built

21


metal-insulator-metal (mim) structure

Table 10: Realized MIM Structures, Samples: 9-18 9

10

Before Spin1:

Spin Parameters:

True

ω1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 5000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = (rpm) a1 = (rpm/s2 ) t1 = 25(s) ω1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = (rpm) a1 = (rpm/s2 ) t1 = 25(s)

True

11

True

12

True

13

14

15

16

17

18

1

True

True

True

True

True

True

Substrate cleaning; t ≈10(s) after placing PMMA, before spin coating;

Structure Type: Ag

PMMA Ag

Annealing: T = 160(◦ C) theat = 35(min)

Glass Ag

PMMA Ag

T = 160(◦ C) theat = 35(min)

Glass

T = 160(◦ C) theat = 30(min) Ag

PMMA Ag

T = (◦ C) theat = (min)

Glass Ag

PMMA Ag

T = 160(◦ C) theat = 30(min)

Glass Ag

PMMA Ag

T = (◦ C) theat = 30(min)

Glass Ag

PMMA Ag

T =160(◦ C) theat = 30(min)

Glass Ag

PMMA Ag

T =160(◦ C) theat = 30(min)

Glass Ag

PMMA Ag

T =160(◦ C) theat = 30(min)

Glass Ag

PMMA Ag

Glass

T = (◦ C) theat = (min)

22


metal-insulator-metal (mim) structure

(a) ITO/PMMA/Ag Structure Model

(b) Ag/PMMA/Ag Structure Model

Figure 17: 3d Models of the MIM Structures - Part 1

(see sample 19 in table 12). Thus, the previously mentioned results concerning the current-voltage characteristics of the MIM structures, were proved to be correct. The next step, after the succesfull design of the MIM structure was achieved as well as the Current-Voltage studies have been made, was to verify the dielectric constant of the PMMA in use. In parallel to this, to study capacitance more carefully, a new set of mask was designed and ordered. The mask included open windows of different sizes and shapes. The general pattern was repeated in 4 × 4 blocks over the total mask’s area (15 × 15(mm)). A block consisted of 2 rows of rectangular shaped areas of 0, 1(mm) × 0, 2(mm), followed by 2 rows of rectangular shaped areas with increasing width and constant height. Finally, a set of circular shaped areas with increasing diameter, was also included. In the first two rows an increasing distance between the consecutive elements was designed. The step size for the first row was 0,05 mm, and therefore the distances here ranged from 0,1 mm to 0,3 mm for the final tuple of rectangles. This idea was kept also for the second row of rectangles. The difference for the set of rectangles included here, was in the step size, with the value of 0,02 mm. Thus, the distances ranged from 0,1 mm to 0,2 mm. The third row’s rectangular shaped areas were having hight of 0,1 mm and increasing width. The distances in between the shapes were kept constant at 0,2 mm. The widths for this set of rectangles started at 0,1 mm and ended at 0,4 mm, with the step size of 0,1 mm for the consecutive geometric figures. The widths of this row corresponded to the diameters of the last row’s circular areas. In between these two rows the fourth row of structures was present. Here the idea of keeping a constant hight (0,2 mm), as well as distances between

23


metal-insulator-metal (mim) structure

Table 11: Realized MIM Structures, Samples: 19-27 Before Spin1:

Spin Parameters:

19

True

20

True

21

True

22

True

23

True

24

True

25

True

26

True

27

True

ω1 = 1000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s) ω1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

1

Substrate cleaning; t ≈10(s) after placing PMMA, before spin coating;

Structure Type:

Annealing:

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

Al PMMA Al Glass

T = 160(◦ C) theat = 35(min)

24


metal-insulator-metal (mim) structure

(a) Bottom and Top Contacts, Top View (b) Al/PMMA/Al Crossed Contacts Structure Model

Figure 18: 3d Models of the MIM Structures - Part 2

the elements (0,067 mm) for increasing widths was preserved. The widths were in the ranges between 0,2 mm and 0,5 mm with the step size of 0,1 mm. The structure of this design can be seen on fig. 22a. A schematic representation of the MIM structure used to measure capacitance and thus determining PMMA can be seen on fig. 22b. By the time the special design for the top contact was fulfilled, initial capacitance measurements were made. For this purpose, the crossed contact structures (samples 19-24, table 11) from before have been used. The connections were made at positions of non-overlapping contact areas in vertical direction of the MIM as seen on fig. 24a. The previously mentioned technique of removing the PMMA layers over the bottom contacts during the I-V measurements can be observed at fig. 24b. A prerequisite for determining the dielectric constant, was the usage of the one phase PMMA layer thickness versus spin speed hight study (11). The Capacitance-Voltage measurements were carried out inside the MB 200B glove box system (fig. 13b). The ambient conditions inside were in the ranges O2 < 0, 1ppm, H2 O < 0, 1ppm. The instrument used for this purpose, was the 4200-SCS Semiconductor Characterization System, from Keithley. The capacitance measurements have been carried for at least 4 different points on each of the MIM structures with crossed Al contacts. They were executed predominately at f=100 (kHz). Several MIM structures were measured for comparison reasons as well as on f=1 (MHz). The difference of the measured capacitances in this case compared with the case of lower frequency was approximately

25


metal-insulator-metal (mim) structure

Current, I [nA] â&#x2020;&#x2019;

500

0

â&#x2C6;&#x2019;500 â&#x2C6;&#x2019;10

â&#x2C6;&#x2019;5

0 Voltage, V [V] â&#x2020;&#x2019;

5

10

Figure 19: Current-Voltage Characteristic of PMMA, High Currents â&#x2C6;&#x2019;3

Current, I [nA] â&#x2020;&#x2019;

5

x 10

0

â&#x2C6;&#x2019;5

â&#x2C6;&#x2019;10 â&#x2C6;&#x2019;10

â&#x2C6;&#x2019;5

0 Voltage, V [V] â&#x2020;&#x2019;

5

10

Figure 20: Current-Voltage Characteristic of PMMA, Low Currents

1%. After measuring a certain MIM structure at a given probe configuration the average value of the total number of 22 values in the -10 +10 V range, has been taken. Concerning the thickness of the dielectric material the scattered behavior of the obtained data was taken into account. Therefore, a range of around 50(nm) deviation of the PMMA layer thicknesses was assumed. After measuring several samples, with different isolator thickness the values for  were computed (see table 12). The area of the conductance plates was extracted from the "Draft Board" design of the mask (fig. 18a) and was also verified with the profilometer. The intersection regions for the bottom and top contacts were equal to the square of the stripe sides - 0, 4(mm) Ă&#x2014; 0, 4(mm) = 0, 16(mm2 ). The origin of the much higher than expected values can be explained by the following reasons. A simplified model, of parallel plate capacitor for the crossed

26


metal-insulator-metal (mim) structure

(a) Al/PMMA/Al Crossed Contacts Structure Model

27

(b) PMMA Layer Distribution

Figure 21: Crossed Contacts Structure, Side View

contacts structure has been used (fig. 23a). Taking into consideration the given structure, this model does not fully represent the correct relations concerning the stray field effects. Furthermore, it can be concluded that the PMMA would not distribute in the same way over the newly introduced bottom contact surface as on a flat glass substrate. In addition, the top contact will follow the morphology of the preceding PMMA layer, and thus it will also "bend" at the transition region between 2 layers (glass/PMMA) and 3 layers (glass/bottom contact/PMMA). Therefore, the main reason for the relatively high calculated results, is that the actual thickness of spin-coated PMMA at the metalizations edges, was actually much thinner compared to the assumed thicknesses. The different regions of contact possibilities could be summarized on fig. 21b. From the sketch the regions of much thinner PMMA layer can be also observed. These are the places defined by the transition step between the glass substrate and the bottom contact. At this positions the distance between the contacts is considerably lower and therefore, this causes additional effect on the observed high capacitance values. In summary, different materials and thicknesses for the design of the MIM structures have been used: • bottom and top contacts - Ag, Ag∈ [220; 250](nm); • bottom contact - ITO (ITO=100(nm)), top contact - Ag, Ag Paste; • bottom and top contacts - Al,Ag Contactbottom ∈ [27; 50](nm), Contacttop ∈ [50; 75](nm); Furthermore, different contact structures have been utilized:


metal-insulator-metal (mim) structure

(a) Mask Designed For Better Capaci-(b) Al/PMMA/Al "Capacitance" tance Measurements Structure Model

Type

Figure 22: Top Contact, Mask Type 3, Design for Capacitance-Voltage Studies

â&#x20AC;˘ bottom contacts: â&#x20AC;&#x201C; flat fully metallised; â&#x20AC;&#x201C; with rectangular "stripes"; â&#x20AC;˘ top contacts: â&#x20AC;&#x201C; with rectangular "stripes" over flat bottom contact; â&#x20AC;&#x201C; rectangular "stripes" perpendicular to bottomâ&#x20AC;&#x2122;s arrangement of the same type; The structure of the sucesfully eliminating contacts interconnections MIM structure can be summarized as follows: â&#x20AC;˘ Use of aluminium metalized contacts: â&#x20AC;&#x201C; 25(nm) 6 Albottom 6 50(nm) â&#x20AC;&#x201C; 50(nm) 6 Altop 6 75(nm) â&#x20AC;˘ Use of the "crossed contacts" design type (see fig. 18b) With the help of the crossed contacts aluminium type of structure (samples 19-24, table 11), high leakage currents problem was solved. Furthermore, the insulating properties of the PMMA were verified. In addition, capacitance measurements were made on the same structures. Due to the reasons described on page 27, the extracted dielectric constant () values were not feasible. Therefore, the previously mentioned designed mask (see fig. 22b) was used to verify the actual PMMA values over a fully metalized Al bottom contact. The relative static permittivity for different PMMA layer thicknesses was

28


metal-insulator-metal (mim) structure

Table 12: Capacitance and Dielectric Constant () for Samples: 19-24 Spin:

Thickness(nm):

C(pF):

19

Ï&#x2030;1 = 1000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop â&#x2030;&#x2C6; 150 PMMAâ&#x2C6;&#x2C6; [575; 625] Albottom â&#x2030;&#x2C6; 70

20

Ï&#x2030;1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop â&#x2030;&#x2C6; 150 PMMAâ&#x2C6;&#x2C6; [450; 500] Albottom â&#x2030;&#x2C6; 70

21

Ï&#x2030;1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop â&#x2030;&#x2C6; 150 PMMAâ&#x2C6;&#x2C6; [370; 420] Albottom â&#x2030;&#x2C6; 70

22

Ï&#x2030;1 = 1000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop â&#x2030;&#x2C6; 27 PMMAâ&#x2C6;&#x2C6; [575; 625] Albottom â&#x2030;&#x2C6; 47

23

Ï&#x2030;1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop â&#x2030;&#x2C6; 27 PMMAâ&#x2C6;&#x2C6; [450; 500] Albottom â&#x2030;&#x2C6; 47

24

Ï&#x2030;1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop â&#x2030;&#x2C6; 27 PMMAâ&#x2C6;&#x2C6; [370; 420] Albottom â&#x2030;&#x2C6; 47

C11 â&#x2030;&#x2C6; 47, 6743 C12 â&#x2030;&#x2C6; 48, 7387 C13 â&#x2030;&#x2C6; 51, 0907 C14 â&#x2030;&#x2C6; 48, 7434 C21 â&#x2030;&#x2C6; 70, 6584 C22 â&#x2030;&#x2C6; 67, 6498 C23 â&#x2030;&#x2C6; 63, 8898 C24 â&#x2030;&#x2C6; 72, 7390 C31 â&#x2030;&#x2C6; 92, 8357 C32 â&#x2030;&#x2C6; 94, 1920 C33 â&#x2030;&#x2C6; 92, 4802 C34 â&#x2030;&#x2C6; 97, 4294 C41 â&#x2030;&#x2C6; 46, 5045 C42 â&#x2030;&#x2C6; 44, 9528 C43 â&#x2030;&#x2C6; 46, 0825 C44 â&#x2030;&#x2C6; 45, 8466 C51 â&#x2030;&#x2C6; 60, 3045 C52 â&#x2030;&#x2C6; 61, 0021 C53 â&#x2030;&#x2C6; 98, 9066 C54 â&#x2030;&#x2C6; 60, 6441 C61 â&#x2030;&#x2C6; 139, 0350 C62 â&#x2030;&#x2C6; 109, 2150 C63 â&#x2030;&#x2C6; 174, 4985 C64 â&#x2030;&#x2C6; 169, 7310

r r11 r12 r13 r14 r21 r22 r23 r24 r31 r32 r33 r34 r41 r42 r43 r44 r51 r52 r53 r54 r61 r62 r63 r64

â&#x2C6;&#x2C6; [19, 35; 21, 03] â&#x2C6;&#x2C6; [19, 78; 21, 50] â&#x2C6;&#x2C6; [20, 74; 22, 54] â&#x2C6;&#x2C6; [19, 35; 21, 50] â&#x2C6;&#x2C6; [22, 44; 24, 94] â&#x2C6;&#x2C6; [21, 49; 23, 88] â&#x2C6;&#x2C6; [20, 29; 22, 55] â&#x2C6;&#x2C6; [23, 11; 25, 67] â&#x2C6;&#x2C6; [24, 25; 27, 52] â&#x2C6;&#x2C6; [24, 60; 27, 93] â&#x2C6;&#x2C6; [24, 15; 27, 42] â&#x2C6;&#x2C6; [25, 45; 28, 89] â&#x2C6;&#x2C6; [18, 88; 20, 52] â&#x2C6;&#x2C6; [18, 25; 19, 83] â&#x2C6;&#x2C6; [18, 70; 20, 33] â&#x2C6;&#x2C6; [18, 61; 20, 23] â&#x2C6;&#x2C6; [19, 16; 21, 28] â&#x2C6;&#x2C6; [19, 38; 21, 53] â&#x2C6;&#x2C6; [31, 42; 34, 91] â&#x2C6;&#x2C6; [19, 26; 21, 40] â&#x2C6;&#x2C6; [36, 31; 41, 22] â&#x2C6;&#x2C6; [28, 52; 32, 38] â&#x2C6;&#x2C6; [45, 58; 51, 73] â&#x2C6;&#x2C6; [44, 33; 50, 32]

29


metal-insulator-metal (mim) structure

Conductive Aluminium plates

A

d

PMMA Dielectric (a) Parallel Plate Capacitor Model

(b) 4200-SCS Semiconductor Characterization System, Probe Arrangement

Figure 23: Parallel Plate Capacitor Model; Semiconductor Characterization System

(a) Capacitance Measurements, samples(b) Capacitance Measurements, samples (19-21) (22-24)

Figure 24: Capacitance Measurements

computed using the simplified model for a parallel plate capacitor: CMIM =

0 PMMA A dPMMA

where CMIM the measured capacitance F 0 permittivity of free space, 0 â&#x2030;&#x2C6; 8, 8154187 Ă&#x2014; 10â&#x2C6;&#x2019;12 ( m ) A the are of the flat, parallel metallic (Al) plates dPMMA the thickness of the PMMA layer The areas of the flat, parallel metallic (Al) plates was as in the previous case extracted from the initial design "Draft Board" mask sketch. Here, the used voltage was as well in the -10 +10 V range. The frequency at which the measurements were made was kept constant to f=100 (kHz). After the measurements at the different existing geometric top contacts areas have been finished, an average capacitance

30


metal-insulator-metal (mim) structure

Dielectric Constant, ÎľPMMA [â&#x2C6;&#x2019;] â&#x2020;&#x2019;

value for each of them has been calculated. The MIM structures (samples 24-27, table 11) in use were having different isolator thicknesses (dPMMA ). The expected behavior of higher capacitance values for lower PMMA layer hight has been observed. As for the crossed contacts type of structure, certain tolerances in the spin coated dielectric thickness had to be assumed. The measurements of the dielectric constants (PMMA ) versus contacts area for different PMMA heights are showed on fig. 25, fig. 26 and fig. 27. A few values deviate highly from the majority of points. The main reason for their presence is accounted to two factors. First, dielectric constant values for which  < 1, were resulting as a consequence of the lower PMMA thicknesses at some areas of the examined MIM structures. On the other hand, the implementation by the responsible company of the shadow maskâ&#x20AC;&#x2122;s contacts areas were not so precise. The diviations from the actual designed "Draft Board" blue print, were the reason for the several higher dielectric constant measured values ( > 5, 5). When comparing same sized areas (data extracted from sketch) it was observed that, values obtained from the circular geometric objects are coinciding much better to the expected results than those from the rectangular shaped contacts. 5 4 3 2 1 0

0.02

0.04

0.06 0.08 0.1 Area, A [mm2] â&#x2020;&#x2019;

0.12

0.14

Figure 25: Dielectric Constant () vs. Contacts Area, dPMMA â&#x2030;&#x2C6; 375(nm)

In conclusion, the computed average value PMMAaverage â&#x2030;&#x2C6; 3, 72 based on the date in table 13 corresponds well to the values given in different sources [1, 14, 16, 22]. With this step, the dielectric constant for the PMMA in use has been verified. The probability of a failure at a given voltage was the next step in the studies of PMMA as gate dielectric. By definition, the breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. Due to the statistical nature of the breakdown voltage of a material, definite values

31


Dielectric Constant, ÎľPMMA [â&#x2C6;&#x2019;] â&#x2020;&#x2019;

metal-insulator-metal (mim) structure

6 5 4 3 2 0

0.02

0.04

0.06 0.08 0.1 Area, A [mm2] â&#x2020;&#x2019;

0.12

0.14

Dielectric Constant, ÎľPMMA [â&#x2C6;&#x2019;] â&#x2020;&#x2019;

Figure 26: Dielectric Constant () vs. Contacts Area, dPMMA â&#x2030;&#x2C6; 475(nm)

6 5.5 5 4.5 4 3.5 3 2.5 0

0.02

0.04

0.06 0.08 0.1 2 Area, A [mm ] â&#x2020;&#x2019;

0.12

0.14

Figure 27: Dielectric Constant () vs. Contacts Area, dPMMA â&#x2030;&#x2C6; 700(nm)

are not given in this work. The studies were carried on three samples (19-22, table 11) with different PMMA layer thickness, but same contact type (see 18b). The Breakdown Voltage measurements were carried out also inside the MB 200B glove box system (fig. 13b). The instrument used for this purpose, was the 4200-SCS Semiconductor Characterization System, from Keithley. For each of the measurements, several contact regions were selected. The valueâ&#x20AC;&#x2122;s ranges of Vbreakdown can be seen on fig. 29 for dPMMA â&#x2030;&#x2C6; 700(nm), fig. 30 for dPMMA â&#x2030;&#x2C6; 475(nm) and fig. 31 for dPMMA â&#x2030;&#x2C6; 375(nm) respectively. After reaching Vcrit , a sudden flow of current, within very short time is observed. Nevertheless, the expected completely destruction of the dielectric to a smoking hot mass of undefinable structure was not detected. The graphs show, an unexpected, fluctuating behaviour, rather than an irreversible and practically always destructive sud-

32


metal-insulator-metal (mim) structure

Table 13: Dielectric Constant () Values for Different Contacts Geometric Areas and PMMA Thicknesses Area Type Acircular (mm2 ) A1 = 0, 1256 A2 = 0, 0706 A3 = 0, 0314 Average Acircular Arectangular (mm2 ) A4 = 0, 04 A5 = 0, 03 A6 = 0, 02 Average Arectangular

dPMMA â&#x2030;&#x2C6; 700(nm)

dPMMA â&#x2030;&#x2C6; 475(nm)

dPMMA â&#x2030;&#x2C6; 375(nm)

 = 2, 96  = 2, 91  = 3, 23  = 3, 02

 = 2, 28  = 2, 64  = 3, 37  = 2, 64

 = 2, 92  = 2, 99  = 3, 30  = 3, 04

 = 4, 80 excluded = 5, 26 excluded = 5, 64  = 4, 80

 = 3, 51  = 4, 24  = 3, 23  = 3, 66

 = 4, 25  = 4, 57  = 3, 87  = 4, 23

ÎľPMMA vs PMMA Layer Thickness 5

ÎľPMMA â&#x2020;&#x2019;

4 3 2 1 0 350

400

450 500 550 600 650 PMMA Layer Thickness (nm) â&#x2020;&#x2019;

700

Figure 28: PMMA Layer Thickness vs. Dielectric Constant ()

den flow of current. Therefore it can be concluded, that the PMMA dielectric can recover its full dielectric strength once current flow has been externally interrupted. This "self-healing" property of PMMA thin films corresponds to the reported in literature behaviour [27]. Furthermore, the statistical nature of Vbreakdown could be observed from the different starting points of the breakdown regions. The change in voltage is defined as the work done per unit charge against the electric field. Assuming a positive charge moving along a curved path from the bottom to the top electrode plates requires work and raises voltage. Therefore, the general relation between voltage and electric field can be generalized to the line integral: R ~ Vf â&#x2C6;&#x2019; Vi = â&#x2C6;&#x2019; ~E ¡ ds.

33


metal-insulator-metal (mim) structure −6

5

x 10

Current, I [A] →

4 3 2 1 0 −1 0

10

20

30 40 Voltage, V [V] →

50

60

70

Figure 29: Breakdown Voltage, dPMMA ≈ 700(nm) −6

5

x 10

Current, I [A] →

4 3 2 1 0 −1 0

10

20

30 40 Voltage, V [V] →

50

60

70

Figure 30: Breakdown Voltage, dPMMA ≈ 475(nm)

For the case of charged parallel plate conductors (samples 19-21) a constant electric field could be assumed. Thus, the relationship between work and voltage, could be finally given as:

Rd

Vf − Vi = − ~E |es| ~ cos Θ 0 ds = −Ed The negative sign, shows the direction of the field. The results of the former equation solved for the magnitudes of the electric field (E) at the breakdown voltage points for different PMMA thicknesses is presented in table 14. Unfortunately, Ecrit is not a well defined material property, it depends on many parameters, the most notable (besides the basic material itself) being the production process, the thickness, the temperature, the internal structure (defects and the like), the age, the environment where it is used (especially humidity) and the time it experienced field stress. This behavior was observed

34


metal-insulator-metal (mim) structure −5

Current, I [A] →

3

x 10

2 1 0 −1 0

10

20

30 40 Voltage, V [V] →

50

60

70

Figure 31: Breakdown Voltage, dPMMA ≈ 375(nm) Table 14: Electric Field (E) at VBreakdownPMMA Spin:

Thickness(nm):

Breakdown Voltage:

Electric Field(MV/m):

19

ω1 = 1000(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop ≈ 150 PMMA∈ [575; 625] Albottom ≈ 70

V11 ≈ 13, 7(V) V12 ≈ 18, 0(V) V13 ≈ 16, 0(V)

E11 ∈ [21, 92; 23, 82] E12 ∈ [25, 60; 27, 82] E13 ∈ [20, 74; 22, 54]

20

ω1 = 1500(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop ≈ 150 PMMA∈ [450; 500] Albottom ≈ 70

21

ω1 = 2250(rpm) a1 = 10000(rpm/s2 ) t1 = 25(s)

Altop ≈ 150 PMMA∈ [370; 420] Albottom ≈ 70

V21 ≈ 12, 5(V) V22 ≈ 27, 0(V) V23 ≈ 27, 0(V) V24 ≈ 10, 5(V) V31 ≈ 13, 0(V) V32 ≈ 7, 0(V) V33 ≈ 11, 0(V)

E21 E22 E23 E24 E31 E32 E33

∈ [25, 00; 27, 77] ∈ [54, 00; 60, 00] ∈ [54, 00; 60, 00] ∈ [21, 00; 23, 33] ∈ [30, 95; 35, 13] ∈ [16, 66; 18, 91] ∈ [26, 19; 29, 72]

in the obtained slightly varying results. The majority of values were in the range between 25 to 30 (MV/m). These critical field strength ranges correspond to the typical for polymers [12]. More importantly, the average of the measured values Ecrit ≈ 34, 73 (MV/m) fitted exactly to those reported for PMMA [13, 18]. Therefore, the obtained results were considered as feasible and thus, the breakdown voltage and field studies were finalized.

35


4

ORGANIC FIELD-EFFECT TRANSISTOR

An idea that is developed and put into action is more important than an idea that exists only as an idea. — Siddhartha Buddha A field-effect transistor (organic or inorganic) requires the following components (shown in Figure 32): a thin semiconducting layer, which is separated from a gate electrode by the insulating gate dielectric; source and drain electrodes of width W (channel width) separated by a distance L (channel length) that are in contact with the semiconducting layer.

W

Source

L

Drain

UD Drain Contact Source Contact Semiconductor Insulator U Gate Contact Gate G Substrate Figure 32: General Structure of the Realized FETs

In this work, the used semiconducting layers were C60 (ρ = 1.32 g/cm3 ) and pentacene (ρ = 1.65 g/cm3 ). Pentacene (fig. 33b) is a promising candidate for the use in organic thin film transistors and OFETs. It is one of the most thoroughly investigated conjugated organic molecules with a high application potential due to a hole mobility in OFETs of up to 5,5 cm2 V −1 s−1 (almost comparable to amorphous silicon) [21]. Combined with buckminsterfullerene Pentacene is used in the development of organic photovoltaic devices [6, 29]. On the other hand, the fullerenes class of molecules and their derivatives part of which is C60 (fig. 33a) are characterized by exceptionally high electron affinity. They were shown to yield n-channel transistors with very high electron mobilities [4, 20, 42].

36


organic field-effect transistor

37

Ambipolar Transport in Organic FETs

(a) Buckminsterfullerene C60

(b) Pentacene

Figure 33: Semiconducting Layer Materials

The used gate electrode was made from Al metal placed on glass substrate. As gate dielectric, the previously studied PMMA polymeric insulator was used. The source and drain electrodes, which inject charges into the semiconductor, were also from Al with work function of Ď&#x2020; = 4, 0 eV. During the measurements the voltage was applied to the gate electrode (UG ) and the drain electrode (UD ). The source electrode was grounded (US = 0). Fig. 34 illustrates the basic operating regimes and associated currentvoltage characteristics of a field-effect transistor. The potential difference between the source and the gate is called the gate voltage (UGS or simply UG ), while the potential difference between the source and the drain is referred to as the source-drain voltage (UDS ). First, we can assume a simple MIS diode (that is, there is no potential difference between source and drain) with a voltage UG applied to the gate electrode. A positive gate voltage for example will induce negative charges (electrons) at the insulator/semiconductor interface that were injected from the grounded electrodes. For negative UG , positive charges (holes) will be accumulated. The number of accumulated charges is proportional to UG and the capacitance CPMMA of the insulator. However, not all induced charges are mobile and will thus contribute to the current in a field-effect transistor. Deep traps first have to be filled before the additionally induced charges can be mobile. That is, a gate voltage has to be applied that is higher than a threshold voltage UT , and thus, the effective gate voltage is UG â&#x2C6;&#x2019; UT . On the other hand, donor (for n-channel) or acceptor (for p-channel) states and interface dipoles can create an internal potential at the interface and thus cause accumulation of charges in the channel when UG =0 so that in some cases an opposite voltage has to be applied to turn the channel off. When no source-drain

co en me me of de po sim ele

2.5

ch the inj tra sem the ex wi fac the wo


organic field-effect transistor

Figure 34: Characteristic Curve, Potential Curve and Cross Sectional View of the MISFET for Different Voltage Regions

bias is applied, the charge carrier concentration in the transistor channel is uniform. A linear gradient of charge density from the carrier injecting source to the extracting drain forms when a small source-drain voltage is applied (UDS << UG , fig. 34, Part a.) ). This is the linear regime, in which the current flowing through the channel is directly proportional to UDS . The potential U(x) within the channel increases linearly from the source (x = 0, U(x) = 0) to UDS at the drain electrode (x = L, U(x) = UDS , fig. 34, Part b.) ). When the source-drain voltage is further increased, a point UDS = UG â&#x2C6;&#x2019; UT is reached, at which the channel is "pinched off". (Figure 34, Part c.) ). That means a depletion region forms next to the drain because the difference between the local potential U(x) and the gate voltage is now below the threshold voltage. A space-charge-limited saturation

38


2.3.

substrate interface, and by the bulk conductivity of the semiconductor, which can increase due to unintentional Device Structures 2.3. Device Structures doping, as for example often observed in P3HT transistors.53- 55

ductors is still 2.4. Charge Trans ductors is os

a clear distinctio a clear disti The exact nature o as po organic field-effect transistor 39amorphous as amorpho The physical nature of the semiconductor as well as the is still open The physical nature of the semiconductor as well as theductors crystals, at the crystals, 2.3. Device Structures employed gate dielectric may require or enable different clear distinctionat be employed gate dielectric may require or enable differenta transport in disor transport in as amorphous polym device structures that can show very different transistor current IDS,sat can flow across thissemiconductor narrowvery depletion zoneas astransistor carriers device structures thatof can show different The physical nature the as well the by thermally byatthermally the acti opp behavior. The most commonly found structures (in relation arebehavior. swept from the pinch-off point to the drain by the comparaemployed The gate most dielectric may require enable different commonly foundorstructures (in relationcrystals, tion of localized transport in disordere tion of loca highsubstrate) electric field inbottom the region. Further increasing to tively the substrate) are the contact/top gate (BC/TG, device structures that can depletion show very different transistor to the are the bottom contact/top gate (BC/TG,bydescribed this de thermally activate the behavior. source-drain voltage will not substantially increase the4b), current described th most commonly found structures (in relation Figure 4a), bottom contact/bottom gate (BC/BG, Figure Figure 4a),The bottom contact/bottom gate (BC/BG, Figure 4b),tion of localized stat order to model ch but leads to an expansion of the depletion region and thus a slight order to mod to the aregate the bottom contact/top gatestructures. (BC/TG, and top contact/bottom (TC/BG, Figure 4c) and topsubstrate) contact/bottom gate (TC/BG, Figure 4c) structures. this densit shortening of the channel. Since the potential at the Figure pinch-off point described The width of tho Figure 4a), bottom contact/bottom gate (BC/BG, 4b), The width Transistors with with the same components but different geomTransistors the same components but different geomorder to model charg remains U â&#x2C6;&#x2019; U and thus the potential drop between that point Gcontact/bottom T by the and and top gate (TC/BG, Figure 4c) structures. byspatial the spatia etries can show very dissimilar behavior. etries can show very dissimilar behavior. The width of the G andTransistors the source electrode stays approximately the same, the current with the same components but different geomand can be deter and can be Oneetries of at the major differences between these device the spatial and ene One of theIDS,sat major differences devicebymeasurements. saturates a level (Figure 34behavior. Part d.)between ). [38, 44] these 62 can show very dissimilar measuremen and can be determin geometries arises from the position of the injecting electrodes Transistors with the same components but different geometries can geometries arises from the position of the injecting electrodes One of the major differences between these device 62 a mobilities and mobilities A ab in show relation to the Inthe the bottom contact/bottom gate gatemeasurements. very dissimilar behavior. nature of the semiconin relation to gate. the gate. InThe thephysical bottom contact/bottom geometries arises from position of the injecting electrodes a stro A variable ran ductor ascharges well charges as gate dielectric may or enable structure, areemployed directly injected into therequire channel of ofmobilities Aand variabl in relation tothe the gate. the bottom contact/bottom gate structure, are In directly injected into the channel variable range distance different device structures show very different transistor structure, charges directly injected intodielectric the channel of inter- aA short accumulated charges atare the semiconductorintera short disth accumulated charges atthat thecan semiconductordielectric shortdistance distance wil behavior. found structures (in electrodes relation to the a distance with wit a accumulated charges at structures, the semiconductordielectric interface. In the other twocommonly structures, the source/drain face. InThe themost other two the source/drain electrodes distance with a low substrate) are the bottom contact/top gate (BC/TG, Figure 35a), face. In the other two structures, thesemiconducting source/drain electrodes and Matter and the channel are separated by the layer.layer. berg berg and 63 M and the channel aregate separated byFigure the semiconducting and Matters. bottom contact/bottom (BC/BG, 35b), and top contac- berg and the channel are separated by the semiconducting layer. distribution of lo Thus, charges first have to travel through several tens tens of ofdistribution distribution Thus, charges first have to travel through several of local t/bottom (TC/BG, Figure structures. Thus, gate charges first have to35c) travel through several tens of a Gaussian dens a Gaussian a Gaussian density characteristics a characteristi characteristics at lo berg-Matters Matters m bergMatte bergmode mobility with in mobility with increa mobility wi charge carriers charge carriers fill charge carr first semiconductor 4. Common field-effect transistor configurations: (a) semiconduc FigurFigur eFigur 4. eeCommon field-effect configurations: (a) (a)semiconductor (a) Bottom /(b) Bottom transistor Contact /(c) Topconfigurations: Contact / Bottom 4. Contact Common field-effect transistor layer bottom contact, top gate (BC/TG); (b) bottomcontact, contact, bottom gate accumulation la Top Gate Bottom Gate Gate bottom contact, top gate (BC/TG); (b) bottom bottom gate gateaccumulation accumulatio bottom contact, top gate (BC/TG); (b) bottom contact, bottom energies. Thus, add (BC/BG); (c) top contact, bottom gate (TC/BG). energies. Thus,T (BC/BG); (c) top contact, bottom gate (TC/BG). energies. (BC/BG); (c)Figure top contact, bottom (TC/BG). 35: Common FET gate Configurations One of the major differences between these device geometries arises from the position of the injecting electrodes in relation to the gate. In the bottom contact/bottom gate structure, charges are directly injected into the channel of accumulated charges at the semiconductor-dielectric interface. In the other two structures, the source/drain electrodes and the channel are separated by the semiconducting layer. Thus, charges first have to travel through several tens of nanometers of undoped semiconductor before they reach the channel. However, in the staggered BC/TG and TC/BG configurations, charges are injected not only from the edge of the electrode but also from those parts of the electrode that overlap with the gate electrode, contributing to the current depending on distance from the edge (current crowding). Other differences between transistor structures arise from the dielectric/semiconductor and electrode/semiconductor interfaces, such as different morphologies at the top and bottom surfaces of a semiconductor film (molecular orientation, roughness) or introduction of trap states during metal evaporation on organic semiconductors for top contact transistors [44].


organic field-effect transistor

Through this work, the top contact / bottom gate type of structure was implemented for the cases of C60 (fig. 36a) and pentacene (fig. 36b) isolation layers. The initially stated in the task description, transparent semiconducting nanoparticulate inorganic oxides (SnO2 , In2 O3 or ZnO) have been changed in favor of the previously mentioned organics. The reason for this decision was based on the relatively easier technological fabrication of the devices under consideration as well as on the broader knowledge based on previous work concerning their properties.

Al

C60 PMMA Al Glass

Al

Al

Pentacene PMMA Al Glass

(a) Al/PMMA/C60/Al MISFET Structure (b) Al/PMMA/Pentacene/Al Structure

Al

MISFET

Figure 36: Realized MISFET Structures

−6

15

x 10 V =0[V]

ID [A] →

g

10

Vg=10[V]

5

Vg=30[V]

V =20[V] g

V =40[V] g

0 −5 0

10

20 VDS [V] →

30

40

Figure 37: Al/PMMA/C60/Al MISFET Structure, Characteristic Curve 1

The procedure building the device was based on the MIM construction discussed in the previous chapter. After cleaning the glass substrates (see page 6), the bottom contacts have been evaporated. For both MISFET devices, full metalization with Albottom = 50(nm) has been used. In addition, the gate insulator (PMMA) has been spin coated. The layer thickness, was in the dPMMA ≈ 375(nm) region. At this point the two devices, differed from one another, namely in

40


organic field-effect transistor

−6

15

x 10 Vg=0[V]

ID [A] →

10

V =40[V] g

5 0 −5 0

10

20 VDS [V] →

30

40

Figure 38: Al/PMMA/C60/Al MISFET Structure, Characteristic Curve 2

2

ID [nA] →

0 V =0[V]

−2

g

Vg=−10[V]

−4

V =−20[V] g

Vg=−30[V]

−6

V =−40[V] −8 −40

g

−30

V

−20 [V] →

−10

0

DS

Figure 39: Al/PMMA/Pentacene/Al MISFET Structure, Characteristic Curve 1

the used semiconducting material. Edwards Auto 306 turbo evaporator was used for the thermal evaporation of C60 ≈ 50(nm) and pentacene ≈ 65(nm). Finally, the top contacts for the devices have been placed. For this purpose, the shadow mask from fig. 16b has been utilized. The bottom contact was with the hight of 100 (nm). After the structures were realized, their characteristic curve were measured with the help of the Keithley 4200-SCS Semiconductor Characterization System inside the glove box system (C60 ) and the 2612 Dual-Channel System SourceMeter Instrument from Keithley under room conditions (pentacene). The reason for the use of the two different instruments was caused due to the technological related issues of attaching the probes to visible contact areas. The outcome of this work is presented on figures 38 and 37, for C60 and on figures

41


organic field-effect transistor

100

Vg=0[V] V =â&#x2C6;&#x2019;10[V]

ID [nA] â&#x2020;&#x2019;

0

g

Vg=â&#x2C6;&#x2019;20[V]

â&#x2C6;&#x2019;100

Vg=â&#x2C6;&#x2019;30[V] Vg=â&#x2C6;&#x2019;40[V]

â&#x2C6;&#x2019;200

Vg=â&#x2C6;&#x2019;50[V] â&#x2C6;&#x2019;300

Vg=â&#x2C6;&#x2019;60[V]

â&#x2C6;&#x2019;400 â&#x2C6;&#x2019;60

â&#x2C6;&#x2019;50

â&#x2C6;&#x2019;40

â&#x2C6;&#x2019;30 â&#x2C6;&#x2019;20 VDS [V] â&#x2020;&#x2019;

â&#x2C6;&#x2019;10

0

Figure 40: Al/PMMA/Pentacene/Al MISFET Structure, Characteristic Curve 2

39 and 40 for the pentacene based MISFET structure. The C60 n-type as well as the pentaceneâ&#x20AC;&#x2122;s p-type behaviour can be observed from the given plots. Despite the fact that a sharp differentiation could not be observed, some field effect is noticed - a change in the gate voltage causes variation in the current from the source to the drain. Furthermore, in chapter 3, capacitance and a respective dielectric constant (PMMA ) (table 13), similar to silicon dioxideâ&#x20AC;&#x2122;s, has been observed. Therefore, it can be concluded that PMMA is a suitable gate dielectric for MISFET structures, but additional studies for the reasons causing the observed characteristic behaviour should be carried out. As discussed previously (page 39) molecular orientation,

W

Source

L

Drain

UD Drain Contact Source Contact Semiconductor Insulator U Gate Contact Gate G Substrate Figure 41: MISFET - Non-Ideal Channel Interface

roughness (see fig. 41) or introduction of trap states during metal

42


organic field-effect transistor

evaporation influence the properties of a transistor. In comparison to the ideal case of a smooth channel interface, the variations of the morphology of the PMMA layer would act as an additional barrier to the accumulated charges. Therefore, deep traps first have to be filled before the additionally induced charges can be mobile. This is the reason why it is important that the roughness of the channel interface is additionally examined. Moreover, implementation of the structures presented on fig. 35b (bottom contact/bottom gate) and fig. 35a (bottom contact/top gate) could be a more beneficial choice with respect to the materials in use. Their potential concerning lower channel roughness, and thus increased gate voltage effect could be further investigated. Therefore, it is important that their advantages are additionally examined.

43


5

CONCLUSION AND FUTURE WORK

Every end is a new beginning. â&#x20AC;&#x201D; Proverb In this thesis, the dielectric properties of Poly(Methyl Methacrylate) (PMMA) were studied. First, the material behaviour concerning spin coating has been examined. As a result, by optimizing the process, smooth and compact thin films of PMMA were obtained. Moreover, different Metal-Insulator-Metal (MIM) structures were implemented. Throughout this work, different technological problems were faced and were accordingly solved. Combinations of different contact materials were examined and their feasibility was studied. It was concluded that silver is dissolving into PMMA. Furthermore, it was verified that a thin aluminium layer of dAl = 25(nm) is still conductive. As an outcome of this work, a unique structure was developed. The "crossed" aluminium (Albottom â&#x2C6;&#x2C6; [25; 50](nm), Altop â&#x2C6;&#x2C6; [50; 75](nm)) contacts type of MIM has successfully prevented the shortenings caused by the measurement probes and thus has proved itself as an advantageous design arrangement. The studies of current-voltage (I-V) relationships of the MIM structures has given useful information about the properties of the gate insulator interface. Capacitance-Voltage (C-V) characteristics of the Glass/aluminium/PMMA/aluminium MIM showed low frequency dependency (<1%) comparing the f=100 kHz and f=1 MHz case. A major result of this work, measured by the capacitive method, was the verification of the dielectric constant () of the PMMA in use. The relative static permittivity for different contact areas and PMMA layer thicknesses were computed (PMMAaverage â&#x2030;&#x2C6; 3, 72). The obtained results correspond well to the data given in different literature. Furthermore, the breakdown voltage and the corresponding field strength were studied. As an outcome, "self healing" effect as well as an average of EcritPMMA â&#x2030;&#x2C6; 34, 72 (MV/m) of the specimen could be confirmed. A further study in this direction, could be the verification of the breakdown field versus different baking temperatures as well as ac voltages for various frequencies. Finally, an attempt to realize Organic Field-Effect Transistors (OFETs) using PMMA as gate insulator was made. The polymeric dielectric can be deposited easily by spin coating. The maximum temperature in the whole device manufacturing process is low 160 (â&#x2014;Ś C), corresponding to the PMMA baking. In addition, the verified dielectric constant is

44


conclusion and future work

similar to that of silicon dioxide. Based on the obtained results it can be concluded that PMMA can be used as a gate dielectric for pentacene or C60 MISFET structures. Possibility for future development is the implementation of the suggested methods for different insulator materials and layer thicknesses. In addition, capacitance measurements could be done on the MIS structures. Evaluation and testing could be carried out in the laboratory environment, focusing on design problems and interface issues. More precisely, the field effect dependence could be further investigated. A combination of these approaches would provide an additional set of observations which could be used for the further development of the MISFET structures.

45


BIBLIOGRAPHY

[1] Inc. Boedeker Plastics. Acrylic PMMA, Accessed 08.03.2009. URL http://www.boedeker.com/acryl_p.htm. (Cited on page 31.) [2] D.E. Bornside, C.W. Macosko, and L.E. Scriven. On the modeling of spin coating, . J. Imaging Technol., 13:122–130, 1987. doi: 10.1063/1.325357. URL http://link.aip.org/link/?JAPIAU/ 49/3993/1. (Cited on page 8.) [3] B. T. Chen. Investigation of the solvent-evaporation effect on spin coating of thin films. Polym. Eng. Sci., 23:399 – 403, 1983. (Cited on page 8.) [4] Masayuki Chikamatsu, Shuichi Nagamatsu, Yuji Yoshida, Kazuhiro Saito, Kiyoshi Yase, and Koichi Kikuchi. Solutionprocessed n-type organic thin-film transistors with high fieldeffect mobility. Appl. Phys. Lett., 87:203504, 2005. doi: 10. 1063/1.2130712. URL http://link.aip.org/link/?APPLAB/87/ 203504/1. (Cited on page 36.) [5] L.-L. Chua, P. K. H. Ho, H. Sirringhaus, and R. H. Friend. Highstability ultrathin spin-on benzocyclobutene gate dielectric for polymer field-effect transistors. Appl. Phys. Lett., 84:3400, 2004. doi: 10.1063/1.1710716. (Cited on page 2.) [6] Science Daily. Efficiently Organic: Researchers Use Pentacene To Develop Next-generation Solar Power, 2004, December 30, Retrieved March 19, 2009. URL http://www.sciencedaily.com/ releases/2004/12/041220005834.htm. (Cited on page 36.) [7] W.J. Daughton and F. L. Givens. An Investigation of the Thickness Variation of Spun-on Thin-films Commonly with the Semiconductor Industry. J. Electrochem. Soc., 129:173 – 179, 1982. (Cited on page 8.) [8] W.J. Daughton and F.L. Givens. On the uniformity of films:a new technique applied to polyimides. J. Electrochem. Soc., 126: 269 – 276, 1979. (Cited on page 8.) [9] W. J. Davis and R. A. Pethrick. Investigation of physical ageing in polymethylmethacrylate using positron annihilation, dielectric relaxation and dynamic mechanical thermal analysis. Pure Appl. Chem., 39(2):255 – 266, 1998. (Cited on page 4.)

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Pmma thin films as dielectric layer for printable field effect transistors