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The impact of optical non-idealities in litho-litho-etch processing Stewart A. Robertsona, Michael T. Reillyb, Trey Gravesa, Mark D. Smitha, John J. Biaforea. a - KLA-Tencor Corp., FINLE Division, Austin, TX, USA. b - DOW Electronic Materials, Marlborough, MA, USA. ABSTRACT Experimental work reveals that a thermal cure freeze process can alter the refractive index of a 1st pass LLE resist designed for that purpose. Although negligible change in the real index (n) is observed at the actinic wavelength, a 20% increase in the imaginary index (k) occurred. It is also experimentally determined that a second pass resist coated over a frozen first layer, may have a planar or non-planar surface, depending upon its’ formulation.

Simulation studies show that a non-planarizing 2nd resist will exhibit lensing effects which result in the 2nd pass resist feature showing sensitivity to the CD and profile of the embedded resist features. Other simulations suggest that both non-planar 2nd resist surfaces and mismatching resist n & k values can have a negative impact on the alignment sensitivity of a LLE double patterning process.

Keywords: Thermal Cure Resist, Litho-litho-Etch (LLE), refractive index, non-planar resist surface, lensing effects 1. INTRODUCTION In previous work[1], it has been shown that non-planar interfaces induced by spin-coat processes over topography can result in significant variations in imaging that are not accurately predicted by considering the limiting cases of either a planarizing or a conformal coating. In the same work it was observed that when two coatings are applied by spin coat processes over topography the upper surface of the second material is near planar, assuming that the combined thickness exceeds the topography step height.

In most imaging cases, this leads to scenarios where the upper resist surface is close to planar and any topographical excursion from a flat interface are confined to the lower resist surface. The principal exception to this occurs in litho-litho-etch (LLE) double patterning processes. In such schemes, a resist film for the second pass is spun over a previously imaged resist structures. Typically, the thickness of the first and second resist layers will be approximately equal, thus the upper surface of the second resist can have significant topology.

Optical Microlithography XXIII, edited by Mircea V. Dusa, Will Conley, Proc. of SPIE Vol. 7640, 76400G · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.846553

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Moreover, when considering LLE techniques, it is usually assumed that the optical properties of the two layers are matched and negligible diffraction will occur at the internal interfaces.

In this work, we set out to explore the impact a non-planar upper resist surface has upon LLE imaging using lithographic simulation. Thus we determine whether it is desirable to design resist materials which exhibit good planarization or ones which follow the underlying topology.

Additionally, we experimentally determine the refractive index change a resist goes under when being frozen for a LLE process, using a Thermal Cure Resist (TCR) process as a typical example. The impact of this change (compared to a completely matched case) on second pass imaging is also explored via simulation.

2. EXPERIMENTAL WORK

A large variety of LLE process are under consideration, including resists that use incompatible solvent systems [2], thermal cure systems [3], chemical freeze techniques [4] and photostablization with short wavelength light [5]. In the experimental work here, only thermal cure resist of the kind described by Reilly et al. [6] is considered.

2.1 Second Resist Surface Shape Experiments were conducted to see how 2nd layer photoresist covers the cured first layer topography. A variety of 2nd layer resists were coated over an imaged 1st layer resist. Figure 1 shows the image 1st layer material after curing (48 nm Lines on a 120 nm Pitch). Also shown are identical structures after coating with 2 different 2nd layer resists; one which planarizes and one which only partially planarizes. It was seen that the degree of planarization could be controlled by the 2nd resist composition. It was clear that both the planarizing and non-planarizing 2nd resists could be manufactured. Simulation will be used to study the impact of different surface morphologies on lithographic responses and determine which response is more desirable.

2.2 Effect of Processing on Resist Index of Refraction Typically, the actinic optical properties of a photoresist are only important at the time of exposure and are thus measured for the film after spincoat and softbake. However, in a LLE process the frozen resist is

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Figure 1: Cross-section SEM images of (a) an imaged 1st pass resist after thermal curing (b) an imaged 1st pass resist covered by a planarizing 2nd pass resist (c) an imaged 1st pass resist covered by a non-planarizing 2nd pass resist. present during the second exposure and the actinic optical properties are important at this time too. Several processes can potentially cause the resist optical properties to change: Exposure (photolysis of the PAG), PEB (loss of leaving group and densification due to free volume loss) and the cure process. A simple experiment was conducted to explore the impact of each processing step on the actinic optical properties of the 1st photoresist. A standard resist film was applied to a 300 mm silicon wafer and softbaked as standard. It was then flood exposed at a relatively high dose (35 mJcm-2). It then received a standard PEB. Cure was administered without any development. After each point in the processing the resist film was measured using a Woollam VASE (Variable Angle Spectral Ellipsometer) to determine its’ thickness and index of refraction at 193 nm. In a real imaging situation a highly dosed area of resist would deprotect during PEB, rendering it soluble during development, however it should be remembered that the imaged resist features remaining after development are typically partially deprotected (up to approximately 40%). Studying the fully deprotected resist allows determination of a ‘worst case’ index change.

Table 1: Thickness and index of refraction data for a 1st pass TCR resist during different stages of processing.

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Table 1 documents the experimental results. Clearly exposure has negligible impact on thickness, but both the PEB and Cure bake processes result in film shrinkage. Although a very slight increase in real refractive index, n, is observed (0.4%) during processing, the change is comparable to the VASE’s resolution limit. The change in the complex index of refraction, k, is very pronounced (20%) and correlates closely to the thickness loss. This can be justified intuitively when it is considered that the primary source of absorption in the resist is the PAG chromaphore. During the bake processes, the mass loss is due to thermally assisted deprotection of the volatile leaving groups which ‘protect’ the polymer from dissolving in developer. Thus the film mass decreases, but all of the main absorbing species remains, resulting in an increase in the ‘per unit’ absorbance.

3. SIMULATION STUDY Simulation work was undertaken to investigate the impact of non-matched resist refractive indices and compare the behavior of planar versus non-planar 2nd resist coverage. A simple, hypothetical 32nm DHP (Double Half Pitch) process is considered, where the process conditions are as follows:

1st Pass Process

90 nm thick resist (typical ArF line resist model parameters)

Dual layer BARC

1.2NA, Immersion ArF, 0.96σo/0.76σi Quasar, XY polarization

37 nm Line, 128 nm pitch 6%AttPSM mask (wafer target 32 nm)

12 sec Development

Perfect freeze process (no profile change, but index of frozen image may be changed for 2nd pass imaging)

2nd Pass Process

75 nm thick resist (all parameters identical to 1st pass)

1.2NA, Immersion ArF, 0.96σo/0.76σi Quasar, XY polarization

37 nm Line, 128 nm Pitch 6%AttPSM Mask (wafer target 32 nm)

12 sec Development

The study looks at the impact of altering the 1st pass features (profile and CD) on the printed CD and placement error of the 2nd pass resist features for planar and non-planar resist coverage. Matched and mismatched refractive indices will be considered in both coating types. The mismatched index corresponds to our experimentally determined ‘worst case’ index mismatch. Obviously, this mismatch exceeds that which could happen in an experimental case with this material.

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3.1 First Pass Resist Through Focus Response Figure 2 shows the through focus response of the 1st pass resist when nominal sized (32 nm) at best focus. Considerable change in resist loss and sidewall shape is observed, in addition to the decrease in linewidth. If the 2nd pass resist is perfectly planarizing then this variation in the 1st pass features has no impact on the 2nd layer coverage, however in the case of a non-planarizing spincoat, the results are sensitive to theses changes. The maximum amount of surface topology will be seen at 1st pass ‘best focus’ where the features are tallest and widest. As 1st pass defocus increases the topography features get narrower and shorter resulting in less residual surface topography in the coated film. This is illustrated in Figure 3 which compares the 2nd resist surface topography for a perfectly planarizing resist and a non-planarizing resist as the 1st pass topography alters due to focus setting changes.

Figure 2: CD and profile simulation results for the 1st pass resist through focus at nominal sizing dose.

3.2 Second Pass Exposure Sensitivity to First Pass Process Deviations Both mismatched resist refractive indices and a non-planar 2nd pass resist surface will result in the scattering of incoming light during exposure. Such behavior is difficult to visualize in a patterned exposure, but can be understood easily when considering an un-patterned flood exposure. Figure 4 shows the light intensity in the 2nd pass resist, as the result of an open frame exposure with planar and non-planar resist surfaces. Two examples are given for each surface, one whether the refractive indices of both resists are

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matched and the other where the embedded pass resist indices are set to the experimentally determined ‘worst case’ values.

Figure 3: A 75nm 2nd resist coating over 1st resist images for various focus settings. Perfectly planarizing and a partially planarizing cases are considered.

Figure 4: Plots of relative light intensity in the resist films for the planar and non-planar cases.

For the case of the planar coating with matched indices, it can be seen that the light propogates uniformly through the resists without any scattering. Careful inspection of the planar case, with mis-matched indices shows that the uniform behavior is disrupted slightly in the region of the embedded structures, though the difference is subtle.

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The non-planar surface has a much more dramatic effect on the light distribution within the resist film. The convex regions in the resist surface above the embedded topography act as lenses altering the light distribution within the resist. Trivially, the intensity within the resist in the region of these ‘local lenses’ is higher than elsewhere. Comparing the matched and unmatched index cases, it is clear that the mismatched case exhibits a more severe redistribution of intensity than the matched one.

The effects that these scattering and lensing phenomena have on imaging will be studied by considering the CD of a feature printed in the 2nd resist at fixed dose and focus, as the 1st pass feature altered by varying the focus offset (as shown in Figure 2). The simulation results are plotted in Figure 5. For the case of a perfectly planar surface and perfectly matched indices, trivially, no change in CD is observed (blue line in Figure 5(a)).

In contrast the non-planar resist surface results in a 2nd pass resist CD which varies dramatically, even when the resist indices are matched (red line in Figure 5(a)). The introduction of the index mismatch results in even larger deviations (red line in Figure 5(b)).

A very small shift in CD can be seen for mismatched resist indices with a planar surface, but the magnitude of the change is less than 0.1 nm.

Figure 5: Second resist feature CD as a function of 1st pass focus offset for the planar and non-planar resist surface cases – Fixed 2nd pass exposure dose and focus offset. (a) Matched Index (b) Mismatched index.

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The impact of the lensing and scattering phenomena on mis-registration can be studied by taking the nominal 1st pass feature (at ‘best’ focus and nominal dose), applying the appropriate coating (planar or nonplanar) then looking at the placement error of the 2nd resist feature as a function of relative misalignment between the wafer and mask. Here the misalignment range considered was Âą10 nm. The simulation results are displayed in Figure 6.

For the planar, matched index example a perfect correlation between misalignment and feature placement error was observed, as expected (blue line in Figure 6(a)). The introduction of the non-planar 2nd resist surface increases the placement error above the actual misalignment by approximately 2% (red line in Figure 6(a)).

When resists with mismatched indices are considered the placement error is magnified above the actual misalignment value, even in the planar case (blue line in figure 6(b)). This increase of 1.9% increases to 3.9% when the non-planar surface is added.

Figure 6: Second resist feature placement error as a function of 2nd pass reticle misalignment for the planar and non-planar resist surface cases. (a) Matched Index (b) Mismatched index.

4. CONCLUSIONS Experimental work reveals that a thermal cure freeze process can substantially alter the absorbance (k) of a photoresist but has only a negligible impact on its real refractive index. It was also seen that it is possible to produce 2nd pass resists which planarize the pass one topography or exhibit residual surface topography where the 1st pass features are located.

The simulation studies suggest that when the 2nd resist does not planarize, features printed in that layer can show a strong sensitivity to the height and width of the features printed in the initial pass. With

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regard to 2nd resist feature CD a weak sensitivity to first pass optical properties was observed when a planar surface was used, however this sensitivity is much greater when the surface is non-planar.

Second layer feature placement errors were found to be sensitive to index mismatch and to non-planar surfaces, both of which had a detrimental effect in this study.

The overall conclusion of this study is that where possible a planarizing second resist is desirable and where possible the n&k values of the resist should be well matched.

It is relatively difficult to significantly alter the real refractive index of photoresist by more than about 0.05, however significant alterations of k values are easily obtainable. A more comprehensive study of the impact of index mismatch between the two resists should be undertaken, to determine whether such mismatches are always detrimental to imaging performance. Should it be discovered that beneficial behavior can be achieved, optimal optical parameters can be determined via simulation.

ACKNOWLEDGEMENTS The authors would like to thank and acknowledge the SEM metrology group at Dow Electronic materials and the PROLITH development team at KLA-Tencor, particularly David Blankenship, Greg Floyd and Chris Walker.

REFERENCES 1. Robertson, S. A., Reilly, M. T., et al., ‘Simulation of optical lithgraphy in the presence of topography and spin-coated films’, Proc. SPIE 7273, 727340 (2009) 2. Nakamura, T., Takeshita, M., Maemori, S., Uchida, R., Takasu, R., Ohmori, K., ‘Newly developed positive tone resists for posi/posi double patterning process’, Proc. SPIE 7273 (2009) 3. Bae, Y., Liu, Y., Cardolaccia, T., McDermott, J., Trefonas, P., Spizuoco, K., Reilly, M., Pikon, A., Joesten, L., Zhang, G., Barclay, G., Simon, J., Gaurigan, S., ‘Materials for single-etch double patterning process: surface curing agent and thermal cure resist’, Proc. SPIE 7273 (2009) 4. Wakamatsu, G., Anno, Y., Hori, M., Kakizawa, T., Mita, M., Hoshiko, K., Shioya, T., Fujiwara, K., Kusumoto, S., Yamaguchi, Y., Shimokawa, T., ‘Double patterning process with freezing technique’, Proc. SPIE 7273 (2009) 5. Wallow, T., Dai, J., Szmanda, C., Cervera, H., Truong, C., Kye, J., Kim, R., Levinson, H., Mori, G., ‘Photoresist stabilization for double patterning using 172 nm photoresist curing’ Proc. SPIE 7273 (2009) 6. Reilly, M.T., Bae, Y.C., Vohra, V., Koay, C., Colburn, M., ‘Evolution of Thermal Cure Photoresist for Double Exposure Process TCR/TCR Applications’, Proc. SPIE 7639 (2010).

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