Negative Tone Development: Gaining insight through physical simulation

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1 Negative Tone Development: Gaining insight through physical simulation Stewart A. Robertson a, Michael Reilly b, John J. Biafore a, Mark D. Smith a, and Young Bae b. a - KLA-Tencor Corp., PCI Division, Austin, TX, USA. b Dow Electronic Materials, Marlborough, MA, USA. ABSTRACT A simple analysis of aerial image quality reveals that negative tone imaging is superior to positive tone for small dimension contacts and trenches. Negative Tone Development (NTD) of positive chemically amplified (deprotecting) photoresist is currently the favored method for realizing such images on the wafer. When experimental process windows are determined for NTD systems, it is apparent that the results far exceed the upper limit predicted using current physical modeling. Since real data transcends the capabilities of the current model to predict, some important physical process is clearly missing. In this work, we explore whether resist shrinkage during PEB can account for the observed discrepancies. Two very simple shrinkage models are developed and tested. Results show that shrinkage in the vertical direction explains some profile artifacts observed in actual NTD processes but has negligible impact on conventional positive tone processes. The horizontal shrinkage model reveals that this type of phenomenon can significantly increase the exposure latitude of a negative tone process but has marginal impact on positive tone exposure latitude only introducing a small CD offset. While horizontal shrinkage does enhance exposure latitude appreciably, the effect does not seem large enough on its own to account for the entire increase observed in the experimental results. Further work is ongoing to investigate other potential mechanisms for observed behavior. Keywords: negative tone development, lithography simulation, shrinkage 1. INTRODUCTION It has long been known that certain features will print with superior image quality with a negative tone resist process. Mack 1 published on this as early as Despite this mass volume manufacturing processes at the 248nm and 193nm wavelengths has almost exclusively utilized positive tone chemically amplified photoresists developed in aqueous base solution. These resists operate on the principal of acid catalyzed de-protection to modulate the polymer solubility in the developer. Although negative tone chemically amplified resists have been developed, for one reason, or another, they have never been widely adopted in mainstream IC processing. In recent years, the ever diminishing process windows for back end features such as contacts and semi-dense trenches, has prompted renewed interest in negative tone imaging, since simple aerial image analysis shows substantial benefits in changing from the conventional dark field mask for positive tone imaging. Promising results have been shown for these layers utilizing a novel process for producing a negative tone resist image. Rather than using the standard cross-linking resist system 2,3 developed in an aqueous base, a standard de-protecting (positive type) resist 4,5 is used and dissolved in a solvent. It has been reported that such NTD processes do indeed image with superior quality to the standard positive tone approach 6,7. Physical simulation of lithography has a long history 8 of aiding researchers and engineers in understanding and optimizing real world processes. Ideally, this would be true for NTD as well. In this work, we investigate how well current physical models can represent the experimental results obtained from actual NTD processes and what may cause the observed deviations. Advances in Resist Materials and Processing Technology XXVIII, edited by Robert D. Allen, Mark H. Somervell, Proc. of SPIE Vol. 7972, 79720Y 2011 SPIE CCC code: X/11/$18 doi: / Proc. of SPIE Vol Y-1

2 2. APPROACH TO MODELING NEGATIVE TONE DEVELOPMENT In positive tone development of standard chemically amplified photoresist, the polymer dissolution properties in aqueous base of the polymer, increase as acid labile protecting groups are removed from the polymer. These leaving groups are de-protected in an acid catalyzed reaction during PEB (Post Exposure Bake) and volatilize out of the film. In the negative tone development process, a solvent developer is used, this exploits the fact that the polarity of the resist material increases with higher de-protection extent. As the polymer s polarity increases, its solubility in the solvent decreases. Since the entire chemistry of NTD and PTD seem near identical other than the direction of solubility on the development step, the obvious approach is to use the same chemically amplified resist model as currently employed for PTD but reverse the development rate equation. Several development rate equations 9-11 are routinely used in lithography simulation, however the original equation proposed by Mack 9 is still the most commonly used. This rate equation is given in Equation (1) R(M ) = R max ( a + 1)(1 M ) n a + (1 M ) n + R min (1) n + where a = 1 (1 M th n 1 ) n R(M) is the instantaneous bulk development rate, M is the local protection extent, R max is the maximum development rate of the resist (when M = 0), R min is the minimum development rate of the resist (when M = 1), n is the dissolution selectivity and M th is the threshold protection level when there is an inflection point in the R( M) curve. This equation can be reversed to describe negative tone development by substituting R(1-M) for R(M). The meanings of R max and R min and n are retained, but the threshold protection level where the inflection in the development rate occurs is now at 1-M th. Figure 1 shows plots of the Mack model in both PTD and NTD modes with typical input parameterr values. Figure 1: Plot of the Mack rate equation configured in both positive and negative tone dissolution modes, with typical measured maximumm and minimum development rates. Proc. of SPIE Vol Y-2

3 3. THE RESIST VECTOR CONCEPT In 2002, Hansen 12 introduced the concept of the resist vector, this work was followed up by Biafore 13 in The main principle of the resist vector concept is that basic lithographic performance is controlled by the projected image, but that the PEB reaction-diffusion processes and the development step cause divergence between the image behavior and the final resist response in a logical and consistent way. Both authors showed that the ultimate performance for a positive tone chemically amplified resist is the case where it is equivalent of a perfect threshold on the aerial image (the image-in-resist case). This corresponds to the case where there is no acid or quencher diffusion and the resist has infinite contrast. Although the resist model has flexibility to modify the processes isofocal CD and cross-pitch bias performance, this control always comes at the sacrifice of exposure latitude, i.e., the exposuree latitude of a resist model cannot exceed that of a perfect threshold of the aerial image. Figure 2 shows an examplee of this behavior, taken from the work by Biafore 13. This basic behavior has been central to the success of high accuracy model calibration for many years and can be considered to be true for virtually all PTD systems studied to date. In short, a study of many experimental PTD systems shows that the exposure latitude never exceeds that of the perfect image-in-resist. When NTD is studied it quickly becomes clear that it does not follow this same behavior. Figure 3 compares experimental maximumm exposure latitude versus isofocal CD for three NTD systems with the theoretical image-in- case the resist result for a 65nm dense trench exposed using 1.35NA, 0.9/0.6 annular, x/y polarization. In each experimental data has an exposure latitude well in excess of the calculated image-in-resist case. A few resist models are also shown in Figure 3, acid and quencher diffusion is varied from practically zero to an excessively large value and develop contrast is varied between 5 and 60. Unsurprisingly, none of these extreme parameter conditions can move the model behavior past the image-in-resist limit. It would appear that the NTD processes transcend the current model capabilities, strongly suggesting that some important physical process occurring in negative tone imaging is missing. Figure 2: Example of maximum exposure latitude versus isofocal CD for a 100nm dense line when exposure is by 0.75NA, dry ArF, 0.9 sigma. Taken from referencee 13. A number of physical phenomena could explain the divergence of actual NTD systems and the current chemically amplified resist model; these include, but are not limited to: 1. De-protection extent in the developed resist features. Physical shrinkage is typicallyy associated with acid catalyzed de-protection, corresponding to the mass loss of the leaving group. In positive tone development, the resist features remaining after development have a high protection level and have lost very little mass. In negative tone development, the protection level of the remaining resist is much lower and somewhat variable; therefore there is significantly more variation in mass loss. Proc. of SPIE Vol Y-3

4 2. There is a central assumption in the current resist model that the local dissolution rate is only dependent upon the local polymer protection level. Given that the dissolution mechanismm exploited in NTD is significantly different from thatt employed in PTD, this may no longer be true and the rate may be modulated by some other local resist property. 3. The observed behavior is consistent with a developer depletion effect, where a local increase in dissolved polymer concentration in the develop solution results in a drop in development rate. 4. SEM shrinkage. It is well knownn that chemically amplified resist shrinks during e-beam metrology. In PTD all resist features have a similar average protection level and can be expected to shrink comparably. In NTD the protection level of different features (even across an exposure latitude curve) varies considerably. Assuming that the resist can only shrink until all the leaving group is removed, this means that the available shrinkage for features varies on a case to case basis. Figure 3: Maximum exposure latitude versus isofocal CD for a 65nm dense trench when exposure is by 1.35NA, immersion ArF, 0.9/0.6 Annular, X/Y polarization. Comparison of three experimental NTD systems, the theoretical image-in-resist case and several model cases. 4. DEPROTECTION SHRINKAGE Although, many potential physical mechanisms can potentially describe the discrepanciess between the current physical model for NTD and reality, the remainder of this work will concentrate on de-protection shrinkage. Firstly, because this is a well known first order resist effect which has been omitted from modeling for a long period of time, and secondly because, the de-protection 4 shows a thickness versus dose curve for a resist film after PEB, as the exposuree dose rises, more acid characteristics of resist features formed by PTD and NTD processes are quite distinct. Figure is generated in the resist and the higher the de-protection extent in the film. On de-protection, the leaving group volatilizes from the film and it shrinks due to mass loss. The maximumm mass loss occurs when total de-protection occurs, for the material shown in Figure 4; this maximum would be about 15% of the original mass. Whilst many materials exhibit de-protection shrinkage at a lower level than this, this material is by no means unusual. Figure 5 compares the protection levels in the resist features on the wafer after a 65nm dense trench is imaged with a NTD and PTD process. Generic NTD and PTD processes were assumed. In the PTD case, the average protection level in the remaining structures is about 88% and less than 2% of the mass of the line would be volatized away thoughh leaving group loss. However, in the NTD case, the average protection level in the remaining structures is about 45% and over 8% of the mass would be lost. Figure 6 compares protection levels for the two processes for a 100nm trench on a 600nm pitch. In the PTD case, the average protection levell in the resist is about 91% and the lost mass is about 1.4% %, similar to that for the 65 nm dense feature of Figure 5. In the NTD case the average protection level is 3% (0% in most areas away from the feature edge) and the masss loss is around 14.6%, this is quite different from the 65nm dense trench case. Given that the protection levels in the resist are much lower in NTD than in PTD and that the protection level varies to a greater degree, it was decided to investigate the impact de-protection shrinkage would have on both PTD and NTD processes. Proc. of SPIE Vol Y-4

5 Figgure 4: Resist thickness t versuus dose after PEB for a de-prrotecting photooresist. Initial film fi thickness after spin-coat s and softbake s was Angstromss. Figure 5: Comparisoon of the protecction level in thhe resist featurees surroundingg a 65nm densee trench (1.35N NA, i immersion ArF F, 0.9/0.6 Annuular, X/Y polaarization) assum ming typical geeneric model parameters. Figure 6: Comparison C off the protectionn level in the reesist features suurrounding a 100nm isolated (600nm Pitch)) trench (1.35N NA, immersion n ArF, 0.9/0.6 Annular, X/Y Y polarization) assuming typiccal generic moodel parameterss. Proc. of SPIE Vol Y-5

6 4.1 Simple Vertical Shrinkage Model In a simple investigation of the impact of de-protection shrinkage it will, in the first instance, assume that all mass loss results in height loss (no lateral shrinkage). It will be assumed that 1. Shrinkage in z will be related to the average protection level between the top and bottom of the resist at each x position. 2. The shrinkage will be linearly dependent on the average protection level. 100% protection equals 0% film shrinkage; 0% protection equals 15% film shrinkage. 3. The same assumptions will be applied to both PTD and NTD modeling cases Vertical Shrinkage Model applied to PTD Figure 7 shows the de-protection profile of the resist surrounding a 100 nm isolated trench for a positive tone development case. It also shows the resist thickness versus x position for the resist after PEB using the simple shrinkage model described in 4.1. The expected dimple over the exposed region is clearly observed. Also shown is the resist profile after development when the vertical shrinkage model is applied Vertical Shrinkage Model applied to NTD Figure 8 shows the de-protection profile of the resist surrounding a 100nm isolated trench for a negative tone development case (the mask tone has been switched from the PTD case). It also shows the resist thickness versus x position for the resist after PEB using the simple shrinkage model described in 4.1. In this case we see that full resist thickness is retained in what will be the center of the space and the shrinkage occurs in the region which will remain after development. Also shown is the resist profile after development when the vertical shrinkage model is applied. This resulting profile exhibits rails on the edge of the trench resulting from a small region near the feature edge with increasing protection level. Figure 8 includes a SEM cross-section image for the described feature in an early NTD system, the predicted rails are evident in this micrograph. These profile artifacts are often, but not always, observed for NTD systems Comparison of PTD and NTD results with Vertical Shrinkage Model Figure 9 compares the resist profiles generated by PTD and NTD simulations that include the vertical shrinkage model with conventional simulations that do not. In the case, of PTD it is fairly evident that the impact of shrinkage is negligible; supporting the fact that not including this effect in normal chemically amplified resist simulations has little impact on their accuracy. However, for NTD we observe a more significant impact on resist profile prediction, including the presence of rail artifacts that are sometimes observed experimentally. This suggest that the shrinkage phenomena may be more important for NTD than PTD, although it must be remembered that in this model all shrinkage has been placed in the vertical direction, in reality some shrinkage will occur laterally and the image in z will be reduced. 4.2 Simple Horizontal Deformation Model In a simple investigation of the impact de-protection shrinkage might have on CD, a simple lateral deformation model is created. Here it is assumed that all mass loss results will results in a lateral CD deviation (no vertical shrinkage). It will be assumed that 1. Shrinkage will be applied to the solid resist part of the feature of interest. 2. The CD shrinkage will be linearly scaled based upon the average protection level in the solid resist feature. 100% protection equals 0% CD shrinkage; 0% protection equals 15% CD shrinkage. 3. For simplicity, it will be assumed that the resist CD can scale freely and no adhesion issues will occur at the resist substrate interface. 4. The same assumptions will be applied to both PTD and NTD modeling cases. Proc. of SPIE Vol Y-6

7 Figure 7: Pllots of (i) De-protection levell in the resist affter PEB for a 100nm isolatedd (600nm Pitchh) trench in usiing PTD (1.35NA, im mmersion ArF, 0.9/0.6 Annullar, X/Y polariization), (ii) Thhe correspondiing post-peb thhickness profille using the simple vertical v shrinkaage model, (iii) The final devveloped resist profile p after thee simple vertical shrinkage model m is applied. Figure 8: Plots of (i) De-prrotection level in the resist affter PEB for a 100nm isolatedd (600nm Pitchh) trench in usiing NTD mmersion ArF, 0.9/0.6 Annullar, X/Y polariization), (ii) Thhe correspondiing post-peb thhickness profille using (1.35NA, im the simple vertical v shrinkaage model, (iii) The final devveloped resist profile p after thee simple vertical shrinkage model m is applied, (iv) Cross-sectiion SEM of 100nm isolated trrench showingg the same raill features present on the moddified s simulation. Proc. of SPIE Vol Y-7

8 Figure 9: Comparison of simulated resist profiles with and without the shrinkage model Horizontal Deformation Model applied to PTD The primary reason for investigating horizontal deformation is to determinee whether this may be responsible for the enhanced exposure latitude exhibited by NTD systems. Therefore the impact of the simple model is explored for both tone systems. The exposure latitude of a positive tone develop system was studied for a 65nm dense trench system. Firstly, using the standard generic PTD system, the doses were calculated required to size the trench across the CD range 45nm to 80nm in 5nm increments. For each dose the protection level in the remaining resist was calculated, this value was then used to calculate the amount of resist shrinkage (CD increase for the trench) that would occur when the lateral deformation model was applied. The results are shown in Table 1. The first notable feature of the data is that through the exposure latitude curve the average protection in the remaining resist remains virtually unchanged. When the shrinkage is invoked, the main effect is to induce a slight CD offset. Although a small CD delta is evident between the highest and lowest dose, the effect is negligible. Figure 10 plots the 2 resulting exposure latitude curves; to the second decimal place the calculated exposure latitude of both curves is constant at 9.3% Table 1: Exposure latitude data for the PTD case with and without shrinkage. Proc. of SPIE Vol Y-8

9 Figure 10: Exposure latitude plot for the PTD case with and without shrinkage Horizontal Deformation Model applied to NTD The exposure latitude of a negative tone develop system was studied for a 65nm dense trench system. Firstly, using the standard generic NTD system, the doses weree calculated required to size the trench across the CD range 45nm to 80nm in 5nm increments. For each dose the protection level in the remaining resist was calculated, this value was then used to calculate the amount of resist shrinkage (CD increase for the trench) that would occur when the lateral deformation model was applied. The results are shown in Table 2. Unlike the PTD case, we see that the protection level in the resist alters considerably across the exposure range. In addition to introducing a CD offset, the lateral deformation appears to alter exposure latitude significantly. Figure 11 plots the 2 resulting exposure latitude curves. With the introduction of the lateral deformation the features exposure latitude increases from 12% to 16%. Whilst the lateral shrinkage/deformation appears to modulate the exposure latitude of a NTD process in the correct direction, it seems unlikely that it can entirely account for the observed discrepancies. In this example, we have taken one of the highest shrinkage systems, applied all the shrinkage in a laterall direction (obviously untrue) and allowed unrestricted movement of the feature edge (also obviously untrue), but the observed increase in exposure latitude was only 25% %. In the experimental results (Figure 3), increases of up to 80% above theoretical maximum are seen; de-protection shrinkage cannot account for all of this increase, but could well be partially responsible. Table 2: Exposure latitude data for the NTD case with and without shrinkage. Proc. of SPIE Vol Y-9

10 Figure 11: Exposure latitude plot for the PTD case with and without shrinkage. 5. CONCLUSIONS Experimental NTD process data shows that these processes transcend the prediction capabilities of current chemically amplified physical models. One obvious potential source of this errorr is de-protection shrinkage. Results from two simplified models show that shrinkage has negligible effect of the final resist profiles for PTD systems (hence it is omitted from current models) but it can have a significant impact on NTD systems. Firstly, shrinkage in the vertical direction modifies the resist profile adding observed profile artifacts. Secondly, lateral shrinkage can act to give a significant increase in exposure latitude; however the magnitude of this increase seems insufficient to entirely account for the observed experimental discrepancies. Further work is required to investigate other physical sources of increased exposure latitude and implement a more rigorously accurate 3D shrinkage model. REFERENCES 1. Mack, C. A., Connors, J. E., Fundamental differences between positive and negative tone imaging, Proc SPIE 1466, (1991) 2. Pugliano, N., Bolton, P., Barbieri, T., Reilly, M., King, M., Lawrence, W., Kang, D., Barclay, G., Negative tone 193 nm photoresists, Proc. SPIE 5039, (2003) 3. Ando, T., Abe, S.., Takasu, R., Iwashita, J., Matsumaru, S., Watababe, R.., Hirahara, K.., Suzuki, Y., Iwai, T., Topcoat-free ArF negative tone resist, Proc SPIE 7273, (2009) 4. Tarutani, S., Hideaki, H., Kamimura, S., Development of materials and processes for negative tone development towards 32-nm node 193-nm immersion double-patterning process, Proc. SPIE 7273 (2009) 5. Tarutani, S., Kamimura, S., Yokoyama, J., Process parameter influence to negative tone development process for double patterning, Proc. SPIE 7639, (2010) 6. Van Look, L., Bekaert, J., Truffert, V., Wiaux, V., Lazzarino, F., Maenhoudt, M., Vandenberghe, G., Reybrouck, M., Tarutani, S., Printing the metal and contact layers for the 32 and 222 nm node: Comparing positive and negative tone development process, Proc. SPIE 7640, (2010) 7. Bekaert, J., Van Look, L., Truffert, V., Lazzarino, F., Vandenberghe, G., Reybrouck, M., Tarutani, S., Comparing positive and negative tone development process for printing the metal and contact layers of the 32- and 22-nm nodes, J. Micro/Nanolith. MEMS MOEMS, Vol. 9(4), (2010) 8. Mack, C. A., Thirty years of lithography simulation, Proc. SPIE 5754 (2005) 9. Mack, C. A., Development of positive photoresist, J. Electrochemical Society, Vol. 134 No. 1, pp. 148, (Jan 1987) Proc. of SPIE Vol Y-10

11 10. Mack, C., A., New kinetic model for resist dissolution, J. Electrochemical Society, Vol. 139 No. 4, pp. L35, (Apr 1992) 11. Arthur, G., Mack, C., A., Martin, B., A new development model for lithography simulation, Proc Olin Microlithography Seminar Interface 97, pp 55, (1997) 12. Hansen, S. G., The resist vector: connecting the aerial image to reality, Proc. SPIE 4690, (2002) 13. Biafore, J. J., Kapasi, S., Robertson, S. A., smith, M. D., The divergence of image and resist process metrics, Proc. SPIE 7140, (2008) 14. Robertson, S. A., Biafore, J. J., Graves, T., smith, M. D., Rigorous physical modeling of a materials-based frequency doubling lithography process, Proc SPIE 6923, (2008) 15. Biafore, J. J., Robertson, S. A., Smith, M. D., Sallee, C., The accuracy of a calibrated PROLITH physical resist model across illumination conditions, Proc SPIE 6521 (2007) 16. Robertson, S. A., Kim, B. S., Choi W. H., Kim Y. H., Biafore, J. J., Smith, M. D., Evaluating the accuracy of a calibrated rigorous physical resist model under various process and illumination conditions, Proc. SPIE 6924 (2008) 17. Yong, L. H., Chun C. L., Chaing-Lin S., Biafore J. J., Roberson S. A., Advantages of a calibrated physical resist model for pattern prediction, Proc. SPIE 6923 (2008) 18. Vasek, J. E., Chin-Min, Y., Biafore J. J., Robertson, S. A., Site portability and extrapolative accuracy of a predictive resist model, Proc. SPIE 6925, (2008) Proc. of SPIE Vol Y-11