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International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol. 3, Issue 3, Aug 2013, 177-186 © TJPRC Pvt. Ltd.

DESIGN AND FABRICATION OF LOW PHASE ERROR SINGULAR PHASE BITS FOR MEMS PHASE SHIFTER ON HRS ANESH K SHARMA1, ASHU K GAUTAM2, ASUDEB DUTTA3 & S G SINGH4 1,2

Directorate of Radar Seekers, Research Centre Imarat, Hyderabad, Andhra Pradesh, India 1,3,4

Indian Institute of Technology Hyderabad, Andhra Pradesh, India

ABSTRACT The paper presents the design, fabrication and measurement of the MEMS based singular phase bits in Ku band for phase shifter development. The Phase Shifter bits have been designed in CPW configuration. The development architecture is based on the hybrid approach of the switched and loaded line topologies. All the switches have been monolithically manufactured on the 200 µm high resistivity silicon substrate using 4” diameter wafers. The three bits i.e. 180o, 90° and 45° are realized through switched microstrip lines using series ohmic MEMS switches whereas the two other bits i.e. 22.5° and 11.25° consist of microstrip line sections loaded by ohmic MEMS switch in shunt mode. Individual bits have been fabricated and evaluated for RF performance. The insertion loss from 0.5dB to 1.5dB across all five bits has been measured. A very low phase error with in ±2o has been achieved. These singular phase bits can be implemented to develop an integrated 5-bit phase shifter for radar applications.

KEYWORDS: Insertion Loss, Switched Line, Loaded Line, Phase Shifter, Phase Error INTRODUCTION Micro Electro Mechanical Systems (MEMS) are a highly innovative alternative to solid state technologies for the realization of RF MEMS since it offers low power consumption, improved RF performance, high isolation and linearity, high miniaturization and reduced costs [1]. The commonality with VLSI technology has been credited to a large extent for the rapid dissemination in the various potential applications. RF MEMS devices are finding direct applications in the circuit or in Sub systems. RF MEMS components like variable capacitors and low loss switches have been proven and the same can be extended to an important device like phase shifter for critical applications [2]. The study and development of phase shifters have gained lot of significance as these are the crucial components in the phased array antenna systems. MEMS phase shifters have been developed for its inherent advantages over other electronic phase shifters. The conventional phase shifters of pin diode, ferrite and MMICs suffer from high insertion loss and dc power consumption, weight, low bandwidth and non-linearity etc. In addition MEMS can be integrated straight forwardly into RF sub-modules to achieve a higher degree of functionality. RF Switch exhibits excellent RF properties such as low insertion loss, low power consumption, high isolation and linearity making this an integral part of the phase shifter circuit [3-4]. The electrostatic force is used for the device operation and the circuits require no quiescent current thus dissipate very negligible power. A 3-D full wave approach has been used for EM simulation. This paper is a continuation of the MEMS phase shifter development effort with a particular emphasis on low phase error.

DESIGN AND SIMULATION Different topologies such as switched [5-7] and loaded line, reflection-type and distributed MEMS transmission lines (DMTL) have been reported in literature. The most convenient depends on the specific requirements, such as frequency range, space occupation and loss, etc. Typically the loaded line topology is convenient in terms of insertion loss


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for high frequency applications (> 40GHz) and for small phase shift requirements. On the contrary the switched-line typology provides extremely linear and wideband performance (in terms of return loss) at lower frequencies (<20GHz) and when large phase shifts are required. The reflection-type phase shifters require the utilization of a hybrid device that typically leads to a rather high space occupation with respect to the switched or loaded line typologies. In order to optimize the low phase error, low insertion loss and small chip area, the hybrid architecture of switched and loaded line has been proposed in the study. The three bits namely 180 o, 90o and 45o have been designed using switched microstrip lines with series ohmic MEMS switches in view to achieve the large phase shift. The smaller phase bits namely 22.5o and 11.25o have been designed using microstrip line sections loaded by ohmic MEMS switches in shunt mode. This topology has been the best trade off among large phase shift, low loss and reduced space requirement in the defined frequency band. High resistivity (>5KΩ) Silicon substrate of thickness 200±10µm has been identified for fabrication as it provides higher Δ¢/mm and Δ¢ /dB with respect to low dielectric substrates like quartz leading to high compactness and low loss. Microstrip topology additionally provides lower loss and enhanced compactness with respect to CPW. The switch constituting the singular phase bits is presented in Figure 1.

Figure 1: Switch Layout of the MEMS SPST Cantilever Switch Constituting the Singular Bits The detailed design has been carried out using full wave electromagnetic simulators ADS Momentum and Ansoft HFSS. Simplified structures have been drawn in the simulation layout, in order to reduce the computation time and to increase the simulation accuracy. In particular an equivalent substrate dielectric constant has been used to account for the different thin dielectric layers covering the high resistivity Silicon substrate. The layouts of all the five bits i.e. 180o, 90o, 45o, 22.5o and 11.25o along with the simulated results are shown in the Figure 2 to Figure 6 in the 16-18 GHz frequency band.

Figure 2: Shows (a) Layout, (b) Simulated Return Loss, (c) Insertion Loss and (d) Phase Shift of Bit 1 (180 Degrees)


Design and Fabrication of Low Phase Error Singular Phase Bits for MEMS Phase Shifter on HRS

Figure 3: Shows (a) Layout, (b) Simulated Return Loss, (c) Insertion Loss and (d) Phase Shift of Bit 2 (90 Degrees)

Figure 4: Shows (a) Layout, (b) Simulated Return Loss, (c) Insertion Loss and (d) Phase Shift of Bit 3 (45 Degrees)

Figure 5: Shows (a) Layout, (b) Simulated Return Loss, (c) Insertion Loss and (d) Phase Shift of Bit 4 (22.5 Degrees)

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Anesh K Sharma, Ashu K Gautam, Asudeb Dutta & S G Singh

Figure 6: Shows (a) Layout, (b) Simulated Return Loss, (c) Insertion Loss and (d) Phase Shift of Bit 5 (11.25 Degrees) The simulations show return loss better than 18 dB and insertion loss better than 0.8dB for all individual bits in the 16-18 GHz frequency band. However slightly higher insertion loss is expected, since the simulation does not account for the switch contact resistance, which typically is about 0.9Ohm. The simulated phase shift error is <1degrees for all bits. The mask layout for all the five bits including the ohmic series switch is shown in Figure 7.

Figure 7: Shows the CPW Layout of Singular Bits of 180o, 90o, 45o, 22.5o and 11.25o

ANALYSIS OF MEMS STRUCTURES In order to achieve high isolation and return loss more emphasis has been given for the ohmic contact with very low contact resistance. Figure 8(a) shows the layout and (b) simplified equivalent circuit of the MEMS cantilever series ohmic switch. The estimated values of the resistance and capacitance are 0.9ohm and 10fF respectively. The layout shows the important sections like contact dimples and anchor springs which are significant for the contact force and the stress gradient.

Figure 8: (a) Configuration Details and (b) Simplified Equivalent Circuit of the Series Ohmic Cantilever MEMS Switch


Design and Fabrication of Low Phase Error Singular Phase Bits for MEMS Phase Shifter on HRS

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Electromechanical analysis of the MEMS parts of phase shifter has been carried out to optimize the critical parameters such as i) actuation voltage, ii) contact force due to series resistance and iii) deformation arising because of stress gradient, are studied in detail in order to ascertain the stable operation. Figure 9(a) shows the pull-in voltage and contact force while 9(b) shows the stiffness versus gradient for different spring lengths. The spring length of ls=10 µm has been found the optimized value for design.

Figure 9: (a) Shows the Pull in Voltage and Contact Force and (b) the Effect of Variation in Spring Length Stiffness versus Stress Gradient for Spring Lengths ls = 0, 10, 20 and 30 µm

FABRICATION FLOW Process Flow A batch of wafers has been successfully fabricated on 200±10µm thick high resistivity double polished p-type Silicon FZ 4” wafers using the eight mask RF MEMS process of FBK [6]. The flow of the line fabrication process details are summarized as given in figure 10(a-f).

Figure 10 (a-f): Schematic Process Flow of the 8 Mask Base Line Switch Process


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Anesh K Sharma, Ashu K Gautam, Asudeb Dutta & S G Singh

The Figure 10 shows the a) Growth of a 1000 nm thick oxide, deposition of 630 nm thick poly-silicon, Boron implantation, poly-silicon definition, B diffusion, 300 nm thick TEOS deposition and contact opening (resistors and actuation lines and electrodes), b) Deposition and definition of the underpass multilayer metal (TiN/Al/TiN), c) 100 nm thick LPCVD oxide deposition (LTO), via definition and etching. Floating metal deposition (05/150nm Cr/Au) and definition, d) Deposition and definition of the 3µm thick resist spacer, e) Deposition of the 2.5/25nm thick Cr/Au seedlayer, definition of the deposition area of the first Au electro-deposition (1.8µm thick bridges and pads), f) Second Au electro-deposition (3.5µm thick CPW lines), seed layer removal and final release. SEM Inspection Scanning electron microscope inspection has been performed to evaluate the surface topology of sections of the phase shifter. The SEM views of phase shifter parts are shown in Figure 11 (a-d) and inspection shows the complete removal of the sacrificial layer.

Figure 11: Shows (a) The Zoomed View of Cantilever Part, (b) the Zoomed View of Different Layers, (c) and (d) Shows the View of the 22.5o and 11.25o Bit Respectively

RF CHARACTERIZATION, RESULTS AND DISCUSSIONS On-wafer TRL Calibration Kit has been designed and fabricated on wafer in order to avoid measurement uncertainty and de-embedding the reference plane. The optical image and RF performance of the TRL calibration kit is shown in Figure 12 (a). This allows to move the measurement reference plane so as not to include in the measurements the loss contribution of the via-less coplanar-to-microstrip transition. The TRL calibration kit presents the Time delay: Topen=9.13ps, Tthru=18.17ps and Tline=33.76ps in 12-23 GHz frequency band which encompasses the frequency range of development. The return and insertion loss are better than 40dB and 0.2 dB respectively as shown in Figure 12 (b).

Figure 12: (a) Optical Image of TRL Calibration Kit with Reference Planes (Dot Line) and (b) LINE Measured Return Loss and Insertion Loss


Design and Fabrication of Low Phase Error Singular Phase Bits for MEMS Phase Shifter on HRS

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The 5-bit phase shifter RF measurement has been performed via 200Âľm pitch GSG microprobes using Agilent E8363C PNA vector analyzer and a voltage source. The actuation voltage of 55V has been applied for electrostatic actuation of the switch. The RF results are shown in Figure 13 to Figure 17

Figure 13: Shows (a) Photograph, (b) Measured Phase Shift, (c) Return Loss and (d) Insertion Loss of Bit 1 (180 Degrees)

Figure 14: Shows (a) Photograph, (b) Measured Phase Shift, (c) Return Loss and (d) Insertion Loss of Bit 2 (90 Degrees)

Figure 15: Shows (a) Photograph, (b) Measured Phase Shift, (c) Return Loss and (d) Insertion Loss of Bit 3 (45 Degrees)


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Anesh K Sharma, Ashu K Gautam, Asudeb Dutta & S G Singh

Figure 16: Shows (a) Photograph, (b) Measured Phase Shift, (c) Return Loss and (d) Insertion Loss of Bit 4 (22.5 Degrees)

Figure 17: Shows (a) Photograph, (b) Measured Phase Shift, (c) Return Loss and (d) Insertion Loss of Bit 5 (11.25 Degrees) The measurements show return loss better than 15dB and maximum insertion loss better than 1.5dB for all the singular bits in the 16-18 GHz frequency band. As described above, the insertion loss values are slightly higher than expected most likely due to the higher multi metal-gold contact resistance in the manufacturing process. The measured phase error is reported in Table 1. The very low phase errors for all the bits have been achieved with in 2°. Table 1: Summary of the Measured Phase Error for the 5 Single Bits Bit # 1 2 3 4 5

Type Switched Line Switched Line Switched Line Loaded Line Loaded Line

Theoretical Phase Shift[Deg] 180° 90° 45° 22.5° 11.25°

Phase Shift Error[Deg] -0.08° -1.37° -1.06° -1.94° -1.28°

The singular phase bits have been simulated and characterized as per the design and test plan. The design of CPW bits based on the switched line and loaded line topology has been established. The measured results show a good consonance with the simulated performance.


Design and Fabrication of Low Phase Error Singular Phase Bits for MEMS Phase Shifter on HRS

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SUMMARY The design, fabrication and RF characterization of the singular MEMS based phase bits in Ku band with CPW configuration has been successfully demonstrated. The presented phase bits based on the hybrid architecture of switched and loaded line were developed using cantilever ohmic series switch. The measured return loss better than 15dB and maximum insertion loss better than 1.5dB for all the bits in the 16-18 GHz frequency band has been achieved. The insertion loss values are slightly higher than expected most likely due to the higher multi metal-gold contact resistance in the manufacturing process. The above results make these phase shifter bits suitable for development of an integrated MEMS based phase shifter for implementation in transmit receive (T/R) module of active phased array application. The future scope of work is to develop an integrated version of the 5-bit phase shifter.

ACKNOWLEDGEMENTS This work has been coordinated by RF Microtech, Italy. The authors also would like to acknowledge FBK for the fabrication support.

REFERENCES 1.

Rebeiz G.M 2003 RF MEMS: Theory Design and Technology New York: J. Wiley & Sons

2.

Gabriel M. Rebeiz, Guan-Leng Tan, Joseph S. Hayden, “RF MEMS Phase Shifters: Design and Applications”, IEEE microwave magazine, June 2002.

3.

Juo-Jung Hung, Laurent Dussopt and Gabriel M. Rebeiz, “Distributed 2- and 3-Bit W-Band MEMS Phase Shifters on Glass Substrates”, IEEE Trans. Microwave Theory Tech., vol.52, no.2, Feb., 2004.

4.

Kai Tang,Yu-ming Wu, Qun Wu, Hai-long Wang, Huai-cheng Zhu, Le-Wei Li, “A Novel Dual –Frequency RF MEMS Phase Shifter”, 19th International Zurich Symposium on Electromagnetic Compatibility, 19-22 May 2008, Singapore.

5.

Zhu Jian, Yu-Yuan Wei, Chen Chen , Zhang Yong, Lu Le, “A Compact 5-bit Switched-line Digital MEMS Phase shifter’’ , Proceedings of the 1st IEEE International Conference on Nano/Micro Engineered and Molecular Systems , January 18 - 21, 2006, Zhuhai, China

6.

Zhu J, Zhou B Lin, el al, "A 4-bit digital MEMS phase shifter" [A], Proc. SPIE Smart Sensors, Actuators, and MEMS[C], 2003, 5116:571-576.

7.

Anesh K Sharma, Ashu K Gautam, DVK Sastry and S G Singh “Design & Simulation of low loss 5-bit Ku band Switched line MEMS Phase Shifter on GaAs” Advanced Materials Research Vols. 403-408 pp 5330 Trans Tech Publications, Switzerland, 2012.


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The paper presents the design, fabrication and measurement of the MEMS based singular phase bits in Ku band for phase shifter development. T...

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