Magazine fall01 investigation193nm

Lithography

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Investigation of 193 nm Resist Shrinkage During CD-SEM Measurements Thomas Hoffmann, Greet Storms, Monique Ercken, Mireille Maenhoudt, Ivan Pollentier, Kurt Ronse, IMEC, Belgium Franck Felten, Evelyn Wong, Jonathan England, KLA-Tencor, Europe

193 nm resists are known to shrink during CD-SEM measurements. The large size and non-linear behavior of this shrinkage must be characterized and understood if CD-SEM metrology is to be correctly applied in advanced lithography processing.

This paper describes a study in which recommendations for the best measurement conditions were developed and speculations on possible models for the observed shrinkage mechanisms could be made.

Introduction

It is well known that 193 nm resist features change size permanently during CD-SEM measurements.1-5 The size of the shrinkage, often up to 40 nm, should be compared to the CD metrology budget of 1 nm for features in the 100 nm design rule node, when 193 nm lithography is expected to enter production for critical layers. 1 The several classes of 193 nm resist chemistry (COMA, acrylate, cyclo-olefins, VEMA) and layer schemes (single, thin imaging layer and hybrid) all exhibit shrinkage to varying degrees depending on their formulations, process history and measurement conditions.

Shrinkage is observed to progress in a nonlinear way with applied e-beam dose and understanding the mechanisms that contribute to this shrinkage is complex. Several studies2, 3 have been reported the attempt to improve this understanding as a basis to improve the resist materials. As yet, complete elimination of e-beam-initiated shrinkage has yet to be achieved. This effect has largely been overcome in 248 nm resist metrology, but we may expect similar or worse effects in some 157 nm materials. It is, therefore, important to understand and minimize resist shrinkage in order to be able to meet the challenges for production worthy CD-SEM metrology of advanced materials. This 32

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paper discusses investigations of shrinkage effects carried out in joint work between IMEC and KLA-Tencor in a study to develop recommendations for CD-SEM conditions that can minimize shrinkage.

Experiments

This study investigates a 193 nm resist exhibiting above-average e-beam shrinkage. Wafers were uniformly exposed several days prior to CD-SEM measurements using an ASML 5500/900 argon fluoride scanner at IMEC. Trenches with a nominal CD of 150 nm were measured using five to ten fresh sites for each experiment. It should be noted that resist shrinkage cause the reported trench CD measurement values to increase. A first set of CD-SEM measurements were carried out at KLA-Tencor, San Jose on an 8200-R CD-SEM and these were repeated and extended in IMEC on 8100XP and 8100-ER systems.

It has already been widely reported that e-beam exposure of the measurement position must be minimized.

Therefore, a standard 193 nm resist measurement recipe was created with a low-magnification pattern recognition step (magnification of 6.25kX, 24 Âľm field of view (FOV)) to identify the region to be measured.

The e-beam spot size was then automatically focused in a region away from the measurement site. A highermagnification pattern recognition step (magnification 12.5kX, 12 Âľm FOV) was then used to identify the exact area of the trench to be measured.

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CD-SEMs have traditionally carried out measurements by analyzing high-magnification images of the feature to be measured. However, KLA-Tencor CD-SEMs directly collect the linescan (the intensity of the detected electrons signal as the electron beam is scanned across the feature) from the measurement location using an electron beam which is scanned at 120 Hz, four times the industry-standard TV rate. In this application, this technique has the advantages of being faster than when having to acquire complete images, and, more importantly, minimizes the total sample dose. For the measurements reported in this study, 768 linescans were collected at 128 locations equally spaced over 720 nm of the feature. The reported CD measurements were calculated from the average of these linescans using a 50-percent derivative algorithm. Experiments were carried out to investigate the effect of beam conditions and recipe parameters on shrinkage. Early measurements considered 10 static measurements (the sample is not moved between repeated measurement cycles), but the number of measurements was later extended, up to 1500 in some cases, to investigate more fully the various shrinkage mechanisms.

F i g u re 1a. The trench CD variation when using a 600 eV, 10pA beam.

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Observations

Figure 1 shows the increase in trench CD over 250 static measurements made using a 600 eV, 10pA beam.

Three regimes of shrinkage can be identified as reported elsewhere.3 Each regime can be fitted by an exponential term, each with a characteristic half dose analogous to a half-life in radioactive decay. In the data of Figure 1, there can be seen: i)

an initial fast-shrinkage, with a half-dose of nine measurements;

ii) an intermediate-term shrinkage with a half-dose of 55 measurements; iii) a long-term shrinkage with a half-dose of 540 measurements.

Variation with Landing Energy Shrinkage has previously been reported to change in an absolute way, rather than as a percentage of feature size.1 This implies that the shrinkage is a surface effect, which is easily understood due to the limited penetration depth of the electrons from the CD-SEM. In this study, decreasing the electron-beam energy reduced the size of all the shrinkage mechanisms. This is demonstrated in Figure 2, which shows comparative data to Figure 1 for measurements taken with a 400 eV beam. This dependency can be understood because the interaction volume is smaller and less energy is deposited in the resist as the energy decreases. Estimates of the range taken from published range tables 6 show that expected electronpenetration depths are consistent with energy dependence seen in the data. It should be noted that the lower energy data shows greater scattering because the smaller number of secondary electrons emitted from the sample has reduced the signal-to-noise ratio of the linescan signals.

The intermediate and slow contributions are shown below the data.

F i g u re 1b . The first 100 poi nts of t he trench CD cur ve. The fas t and

F i g u re 2. The equi va lent trench CD curve to Figur e 1a , b ut measure d

i n t e rmedia te cont ributions are shown b elow the data.

using a 400 eV, 10pA beam.

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F i g u re 3. Th e variation of th e trench shrinkage with beam current.

Dose Dependency Beam Current —First experiments on ten static mea-

surements indicated that beam current had little effect on the shrinkage. This surprising result has been reported in work elsewhere. 4 Figure 3 shows shrinkage measured for three beam currents over a larger range of measurements. It must be pointed out that the interpretation of the data in this study is complicated by the fact that we do not know the size of the undosed feature being measured. The first static measurement already includes some unknown amount of shrinkage.

Fresh samples have to be used for each experiment, and the CD control across the wafer (measured to be ±9 nm 3σ) does not allow data from each experiment to be compared without having to consider an offset between the collected data sets. The offsets between the sets of data in Figure 4 have been made so that the intermediate shrinkage region for all the beam currents overlap, in agreement with the early observations that this regime is independent of beam current. Under this interpretation, the fast shrinkage mechanism is observed to increase with beam current. The long-term shrinkage mechanism also changes with beam current. At 40pA, a higher than normal beam current, the trench can be seen to narrow once the other mechanisms have stabilized.

An alternative analysis of the beam current data with different applied offsets could lead to the conclusion that all the shrinkage regimes depended on beam current.

The precision of the data did not vary greatly until the beam current was reduced to 5pA. This reflects the reduced signal-to-noise at this low beam current, analogous to the trend with beam energy.

Effect of Scan Overlay after Each Measurement —An early experiment attempted to determine if a time delay placed between successive static measurements would

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change the rate of shrinkage by altering the induced temperature of the resist. A variable time delay between static measurements was introduced by using an option known as scan overlay. In this option, an image of the measured feature was acquired after each measurement at the magnification at which the measurement was defined (75kX, 2 µm FOV in this case). The measured linescan was then displayed over this image. Changing the time could be used to delay the period between successive static measurements. No difference in the intermediate shrinkage was observed when this delay was changed between one and five seconds, but it soon became apparent that the image acquisition itself was causing a difference. Figure 4 shows the overlap of shrinkage curves for the first 100 measurements with scan overlay, compared to the first 500 measurements without scan overlay. For clarity, only the fitted trend for the measurements without scan overlay is shown. In this figure it has been assumed that the scan-overlay step creates the same shrinkage as four measurement only sequences. Therefore, the horizontal scale for the data for measurement plus scan overlay has been multiplied by a factor of five. The dose applied to the wafer during imaging is different from the dose during a measurement by a factor of two. This implies that doses applied in different timescales have caused different amounts of shrinkage.

Proposed Model for 193 nm Shrinkage

Using the above interpretation of the data, it is possible to speculate on what processes might be occurring in the resist during electron bombardment. Confirmation of this model will require further experiments, including the use of complementary techniques to those used in this study, and it is hoped that the suggestions below

F i g u re 4. The shrinkage over the fi rst 100 measurements with sca n overl ay (red dot s) compared to th e trend (bl ack li ne) of the first 500 m e a s u rements without scan over lay. The beam conditions were 600eV, 10pA.

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might stimulate such further investigations and discussions.

The fast mechanism appears to change with energy and beam current, suggesting that it is related to the incident power. The mechanism may be a short-lived, thermally activated process such as the release of certain molecules, perhaps solvent, from near the surface of the resist before the surface has stabilized. This mechanism can be reduced after UV treatment of the surface.3 The intermediate mechanism is saturated with current, and has a lifetime of tens of microseconds. Perhaps this is cross-linking. When an electron impacts the resist, it will undergo many interactions with molecules as it slows down. Some of these interactions create radicals on the resist molecules. The process appears to be so efficient that, in the range of beam currents used in a CD-SEM, all possible radicals are created. The radicals may form cross links before they decay. After the first few dose events (approximately 20 in this study), the surface is cross-linked, and so the fast mechanism is suppressed. Once all the cross-links within the e-beam interaction zone have been made (after approximately 200 measurements in this study), the intermediate shrinkage mechanism stops. Electrons in the e-beam hit the sample on the tens of nanoseconds scale.

Therefore, altering the beam current changes events in this timescale. The proposal that beam current does not change the intermediate shrinkage mechanism suggests that the intermediate mechanism is saturated and longer-lived than tens of nanoseconds. During a measurement, the beam returns to the same spot on the sample approximately every ten microseconds. If the radicals have not decayed in this time, the returning beam cannot produce more radicals. During an image acquisition, the beam returns to the same spot on the sample at a slower rate, approximately every ten milliseconds. If the radicals have now decayed, the returning beam will now be able to re-create them. Therefore the increased shrinkage induced during imaging compared to measurement suggests the radicals have a lifetime longer than tens of microseconds, but shorter than ten milliseconds.

The slow shrinkage mechanism also proceeds at the same time the above two mechanisms are progressing.

This mechanism may be mass loss. There are suggestions that this could be molecular scission or solvent removal.2, 3 The mass loss gives slower shrinkage, which only becomes apparent after the medium mechanism has finished, but continues for a longer dose. When the

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resist has stabilized, carry-over can also become evident.

This is presumably due to the same mechanism (carbonization or “charging�) seen in 248 nm metrology. At the lower beam currents typically used for metrology, the trench continues to widen as mass loss dominates over carry-over. At extreme beam currents, such as 40pA investigated in this study, excessive carry-over can actually dominate over shrinkage. Under the normal beam current conditions in the KLA-Tencor CD-SEM, the amount of carry-over is low and hard to observe.

Best Measurement Conditions

Irrespective of the explanation for the different mechanisms occurring in the resist, the above work can be used to make recommendations for 193 nm resist measurements. Lithographic performance is best characterized by measuring feature dimensions before induced shrinkage. In production, after-develop inspection (ADI) is used to control and predict the after-etch inspection (ACI) feature size. The etch environment may quickly cause the resist to shrink in a similar way to which it shrinks in the CD-SEM. It is tempting to suggest that, under these conditions, measuring the fully shrunken dimension at ADI might give a reasonable prediction of the ACI dimension. In a related theme, suggestions have been made that resists could be stabilized, presumably both against e-beam-and etchinduced shrinkage, by introducing a pre-conditioning process such as UV irradiation, e-beam cure, or thermal processing.2, 4 However, measuring the un-dosed feature size does not require the assumption of systematic process offsets that are well controlled under all manufacturing conditions and does not incur an increased process cost.

In determining zero-dose dimensions, it is vital to consider sources of random and systematic error in the measurements. We can attribute random errors to vari ations in linescans caused by the usual effects that contribute to static and dynamic precision in a CD-SEM. Systematic errors may be attributed to uncertainties in the fits of successive measurements leading to the estimate of the CD of the undosed feature. In 248 nm resist metrology, systematic errors could largely be ignored, and the best conditions chosen to optimize dynamic precision. For 193 nm resist metrology, the systematic errors can no longer be ignored. To reduce systematic errors, multiple measurements should be taken in a dose regime where the medium term mechanism dominates. An e-beam current of 10pA will allow reduced contributions to the systematic errors from the Fall 2001

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fast shrinkage mechanism and still allow good signalto-noise to be obtained. A beam energy of around 500 eV should be the optimum balance point between “dynamic” and “systematic” errors. Choosing 400 eV would not give enough signal-to-noise ratio for good statistics on the linescans, and challenges the creation of truly robust production recipes. 600 eV may be tolerable, but higher energies would cause greater systematic uncertainties. Manual measurements cannot be used because the uncontrolled dosing of samples would lead to variations in shrinkage. Pattern recognition and focus steps can be set up in remote locations and at low magnifications to avoid shrinkage at the measurement site. Image refresh at the measurement magnification must be avoided at all costs. Once collected, the trend of the data has to be corrected for shrinkage. A linear fit (such as that used in 248 nm resist metrology) would no longer be sufficient, as the fast shrinkage has to be accounted for. Accurate correction of this fast shrinkage is likely to give the most problems in future metrology. The coefficients of the fit would depend on the resist and measurement conditions.

to speculate on possible mechanisms that could occur in the resist during e-beam exposure, but further work is required to refute or confirm this model.

Independent of the mechanisms, recommendations have been made for the best conditions to use for 193 nm resist metrology in which the balance between systematic and random error contributions has been considered. The above measurement conditions will be applied to automated focus exposure measurements of 193 nm resists and then in investigations of early 157 nm resists.

It is interesting to note that the early literature of 193 nm metrology includes several studies in which the intermediate regime has quickly been exceeded due to the high doses applied to samples. While this allows measurements to be taken in the region of slow shrinkage and would lead to measurements with low random errors, correction of systematic errors would be difficult. The use of non-image based metrology and four times TV rate scanning in KLA-Tencor CD-SEMs allows collection of many measurements before the intermediate regime is exceeded. Care must be taken when benchmarking the capabilities of different CDSEMs. By overdosing the sample and choosing a beam current at which carry over balances mass loss, it would be possible to show 193 nm measurements that appear to exhibit little initial shrinkage and then a low carryover regime over a long set of measurements. The precision would look very good, but there would be a penalty in accuracy.

References

Summary and Future Work

This work has shown that three regimes have to be accounted for in the shrinkage of a particular 193 nm resist. A fast regime is the most difficult to account for because it is so short-lived and uncertain in magnitude.

This creates difficulties in both interpreting the data of this study and for the corrections in metrology. Based on one interpretation of the data, it has been possible 36

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Acknowledgements

The authors would like to acknowledge the help of Diziana Vangoidsenhoven, Myriam Moelants, Nadia Vandenbroeck, and Christie Delvaux (IMEC) for wafer processing and exposure, and the many people at IMEC and KLA-Tencor for their useful discussions on this work, in particular Rob Watts, Amir Azordegan, Gian Lorusso, and Gianni Leonarduzzi.

1. I. Pollentier, M. Ercken, A. Eliat, C. Delvaux, P. Jaenen, K. Ronse, “Front-end of line development using 193 nm lithography”, Proceedings SPIE Micro e l e c t ronic and MEMS Technology Conference 2001 2. M. Neisser , T. Kocab, B. Beauchemin, T. Sarubbi, S. Wong, W. Ng, “Mechanism Studies of Scanning Electron M i c roscope Measurement Effects on 193 nm Photoresists and the Development of Improved Linewidth Meas u rement Methods”, Proceedings Interface2000, p. 4352 3. T. Kudo, J. Bae, R. Dammel, W. Kim, D. McKenzie, M. Rahman, M. Padmanaban, W. Ng, “CD Changes of 193 nm Resists During SEM Measurement”, Pro c e e d i n g s SPIE Microlithography Conference 2001 4. L. Pain, N. Monti, N. Martin, V. Ti r a rd, A. Gandolfi, M. Bollin, M. Vasconi, “Study of 193 nm Resist Behavior Under SEM Inspection : How to Reduce Line-width Shrinkage Effect ?”, Proceedings Interface2000, p. 233-248 5. B. Su, A. Romano, ‘Study on 193 nm Photoresist Shrinkage After Electron Beam Exposure”, Proceedings Interface2000, p. 249-264 6. L. Reimer, “Image Formation in Low-Voltage Scanning E l e c t ron Microscopy”, SPIE (1993) p52

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