Observation of Swelling Behavior of ArF Resist during Development by using QCM Method (2)

Transcription

1 Journal of Photopolymer Science and Technology Volume 25, Number 4 (2012) CPST Observation of Swelling Behavior of ArF Resist during Development by using QCM Method (2) Many reports have discussed the swelling behavior of photoresists during development, as observed by the QCM method. Previously, we reported on the development of development analysis equipment based on the QCM method. In this paper, we report on a high-precision resist development analyzer also based on the QCM method. This equipment incorporates a high-precision developing solution temperature controller and features a high-precision air conditioning function for the measurement chamber. We also measured swelling behavior during development using a TBAH developer solution, which features larger molecules than TMAH, comparing these results with those obtained with TMAH. The results of this measurement indicate that the extent of resist swelling during development is less with TBAH developer solution than with TMAH developer solution. This result is consistent with results of a study by Itani et al. using high-speed AFM, suggesting the suitability of the measurement equipment used in our experiments. Keywords: QCM development analysis method, ArF resist, swelling, TMAH, TBAH 1. Many reports have discussed on the swelling behavior of photoresists during development, as observed by the QCM method.[1-7] Previously, we reported on the development of development analysis equipment based on the QCM method.[8] In this paper, we report on a high-precision resist development analyzer also based on the QCM method. This equipment incorporates a high-precision developing solution temperature controller and features a high-precision air conditioning function for the measurement chamber. We also measured swelling behavior during development using a TBAH developer solution, which features larger molecules than TMAH, comparing these results with those obtained with TMAH. The results of a study reported by Itani et al. [9] indicate less resist swelling during development with TBAH developer solution than with TMAH developer solution. Fig. 1 Structures of TMAH and TBAH developer solutions and an illustration illustrating the effects of photoresist swelling during development on the pattern Because TBAH is a bulkier quaternary ammonium salt than TMAH, only very small amounts penetrate the resist. This is expected to minimize the swelling of the resist during development. The finer the pattern, the higher the percentage of the swelled layer in the pattern. Extensive swelling during development is believed to lead to pattern collapse and the formation of pattern bridges that fuse the top parts of the pattern (see Fig. 1). Itani et al. studied swelling during development using high-speed AFM based on that Received March 27, 2012 Accepted May 10,

2 view. However, due to measurement resolution issues, high-speed AFM equipment proved unable to perform measurements with the 2.38% alkali concentration of standard developing solution (TMAH). In their measurements, Itani et al. diluted the developing solution by a factor of ten. Our equipment offers a measurement resolution of seconds. Hence, we used an undiluted developing solution at 2.38% TMAH concentrations and performed high-speed measurements by the QCM method, seeking to add to the data previously reported by Itani et al. 2. The equipment of experiment Fig. 2 shows the test equipment (RDA-Qz3) used in our study. The air conditioning system provides precise temperature control for the measurement chamber in which the developing solution and development tank are placed. By tweaking the software, we were able to achieve a measurement resolution of seconds.the photo in Fig. 3 shows the exterior of development tank. The development tank is made of Teflon to allow the use of organic solvents. A tube to circulate the developing solution and a solution dispense nozzle are connected to the development tank. The system maintains the developing solution precisely at C and the air inside the measurement chamber precisely within the range from 19 C to 25 C at an accuracy of 0.1 C. The QCM holder features a pointed helmet-like structure at the tip to minimize disturbances caused by air flows resulting from the pouring of the developing solution. The substrate insertion angle is fixed at 70 degrees to prevent any impact on the QCM substrate during insertion into the developing solution. Fig. 4 shows the QCM holder. Using a substrate capable of oscillating at 5 MHz, the equipment allows the observation of the dissolution and swelling behavior of the photoresist during development. Fig. 2 External view of the RDA-Qz3 The test equipment consists of a PC, frequency counter, voltmeter, QCM oscillator and holder, development tank, air conditioning system, and solution temperature controller with the development tank. Fig. 4 QCM holder Fig. 3 External view of development tank 3. Reducing heat shock If we insert a QCM substrate without resist into the developing solution and measure the frequency, the measurement results will show a change in the frequency at the moment when the substrate enters the developing solution, in addition to the change in frequency resulting from the phase change. This is attributable to the electromotive force generated between the electrodes due to the temperature difference between the substrate and the developing solution. This phenomenon is called as 468

3 heat shock. The change in frequency caused by heat shock is observed as melting or swelling. This poses as the problem of examining and determining a suitable combination of developing solution temperature and QCM substrate temperature (the temperature in the measurement chamber) to avoid heat shock. ArFES-3000mini (manufactured by Litho Tech Japan) ArF open-frame exposure system for the exposures. [10] Fig.6 Relationship between film thickness and exposure time (developing solution: TMAH) Fig. 5 Results of examination of conditions involving minimum heat shock Fig. 5 shows the extent of the change in the resonance frequency when the developing solution temperature is fixed at C and the measurement chamber air conditioning temperature is varied from 19 C to 25 C. No resist was applied to the QCM substrate. We mounted the substrate to the holder and let it stand for five minutes to allow the substrate temperature to stabilize. Our study showed that we could minimize heat shock at an air conditioning temperature of 22 C and a developing solution temperature of 23 C. We observed no swelling behavior at an exposure dose of 0 to 2.85 mj/cm 2. We observed swelling when we increased the exposure dose to 5.70 mj/cm 2 or higher. With exposure doses exceeding 8.55 mj/cm 2, we observed noticeable swelling during development, immediately followed by dissolution. Fig. 7 shows an enlarged view of the development starting area (development time: 5 sec) in Fig. 6. Fig. 7 shows a maximum swollen film thickness with a development time of around 0.5 seconds and exposure dose of mj/cm 2 and with a development time of around 0.2 seconds and exposure dose of mj/cm 2 or greater. 4. Experiments After determining the conditions that would result in minimum heat shock, we began measurements of ArF resist swelling behavior. We used an acrylic-based polymer as a resist and a TArF-P6111 (manufactured by TOK) dry-exposure ArF resist containing adamantyl as a protective group. We set resist film thickness to 260 nm, pre-bake conditions to 130 C for 90 seconds, and PEB to 130 C for 90 seconds. We used TMAH (2.38%) and TBAH (6.79%) as the developer solutions. Fig. 6 plots the relationship between film thickness and development time when varying the exposure dose from 0 to mj/cm 2, using TMAH developer solution. We used an Fig.7 Relationship between film thickness and development time during the first five seconds of development 469

4 Fig.8 Maximum swelling ratio and relationship of development time and exposure dose resulting in maximum swelling Fig. 8 shows the relationship between development time and exposure dose that results in the maximum swelling ratio (value obtained by normalizing maximum swelling by initial film thickness) and maximum swollen film thickness. The maximum swelling ratio increases when the exposure dose exceeds 5 mj/cm 2. It remains at roughly the same level when the exposure dose is 12 mj/cm 2 or above. The maximum swelling ratio is approximately 150%. The development time yielding the maximum swollen film thickness grew shorter with increasing exposure dose up to an exposure dose of 17 mj/cm 2, and leveled off past this point. 5. Accuracy of repeated measurements We confirmed the accuracy of repeated measurements by repeating our measurements with an exposure dose of mj/cm 2 using the TArF-P6111 resist five times under the same conditions and by examining deviations in maximum swelling ratio. Fig. 9 shows the development curves obtained from the five repeated measurements. Table 1 shows the results of measurements of development time that resulted in the maximum swelling ratio and maximum swollen film thickness. The deviations from the maximum swelling ratio were approximately 4%. The equipment provided accurate measurement results. Fig.9 Relationship between film thickness and development time with exposure dose of 17.1 mj/cm 2 Table 1 Results of five repeated measurements of maximum swelling ratio Measurement # Swelling ratio(%) AVERAGE MAX MIN R 9.6 STDEV 4.06 Fig.10 Relationship between sample number and development time yielding maximum Fig. 10 shows the swelling ratio and maximum swollen film thickness, based on five repeated measurements. 470

5 6. Observation of swelling behavior using TBAH developer solution Fig. 11 shows the relationship between film thickness and development time with TBAH developer solution in place of TMAH developer solution (all other variables unchanged) Fig.11 Relationship between film thickness and development time using TBAH developer solution obtained in measurements with TMAH. The development times giving the maximum swollen film thickness fell with increasing exposure, but leveled off at 17 mj/cm 2 and above. This was the same tendency observed with TBAH developer solution. Fig. 13 shows a comparison of the results of pattern transfers based on double patterning using the TMAH and TBAH developer solutions. The patterning conditions were as follows: 1.35 NA annular and Att-PSM with 56-nm mask and 128-nm pitch. We used TMAH developer solution for the first pattern development. We used TMAH and TBAH developer solutions for the second pattern development. The target pattern size was 32 nm for both the first and second development operations. We used the LWR smoothing ratio (%) to compare with LWR. The LWR smoothing ratio is obtained using formula (1) below. LWRTMAH LWReachdeveloper LWRSmootthingRatio (%) 100 LWRTMAH (1) No marked swelling behavior was observed with TBAH developer solution, in contrast to that observed with TMAH developer solution. Fig.12 Relationship between development time and exposure dose yielding the maximum swelling ratio and maximum swollen film thickness (TBAH developer solution) Fig. 12 shows the relationship between the development time and exposure dose that provided the maximum swelling ratio and maximum swollen film thickness with TBAH developer solution. The maximum swelling ratio increased once the exposure dose exceeded 5 mj/cm 2, then leveled off at an exposure dose of 12 mj/cm 2 or higher. We observed the same pattern with TMAH developer solution. However, the maximum swelling ratio was approximately 120%, less than the 150% Fig.13 Comparison of patterning results obtained with TMAH and TBAH developer solutions Based on the results, LWR was improved to 4.49 nm, compared to the 6.35 nm achieved with TMAH developer solution. The pattern collapse limit pattern also improved from 30.8 nm to 24.3 nm. The improvement of LWR smoothing ratio was -29.3%. We believe these improvements are attributable to the inhibition of the effects of swelling during development. 7. Summary The results of our experiments support the results reported in the study conducted by Itani and et al. [9] using high-speed AFM, which indicated less swelling with TBAH developer solution than 471

6 TMAH developer solution. The equipment we used in our study provided accurate measurements of photoresist swelling behavior during development. We now plan to evaluate resists characterized by minimal swelling such as molecular resists. 8. Acknowledgements We would like to express special thanks to Mr. Tomoya Kumagai of Tokyo Ohka Kogyo Co., Ltd. for supplying the TMAH and TBAH developer solutions used in this study and for providing patterning results. Referance [1] Gregory P. Prokopowizic, Jacque H. Georger Jr., Eyad Ayyash, James W. Thackeray, William R. Brunsvold, Laura L. Kosbar, Ali Afzali, Jeff D. Gelorme, Improved Resolution with Advanced Negative DUV Photo Resist with 0.26N Capabiloty, SPIE Proc. 3678, 1284 (1999). [2] William Hinsberg, Seok-won Lee, Hiroshi Ito, Donald Horne, Kay Kanazawa, Experimental approaches for assessing interfacial behavior of polymer films during dissolution in aqueous base, SPIE Proc. 4345, 1 (2001). [3] Thomas Wallow, Wendy Chan, William Hinsberg, Seok-Won Lee, Characterization of the Polymer-Developer Interface in 193nm Photoresist Polymers and Formulations During Dissolution, SPIE Proc. 4690, 299 (2002). [4] Minoru Toriumi, Toshio Itani, Dissolution characteristics of resist polymers studied by Quartz Crystal Microbalance transmission-line analysis and PKa acidity analysis, SPIE Proc. 4690, 904 (2002). [5] Hiroshi Ito, William D. Hinsberg, Larry F. Rhodes, Chun Chnag, Hydrogen bonding and aqueous base dissolution behavior of hexafluoroisopropanol-bearing polymers, SPIE Proc. 5039, 70 (2003). [6] Masamitu Shirai, Shinich Takashiba, Yusuke Horiguchi, Shigeo Irie, Toshiro Itani, Acid Components in outgassing from F2 resist: A Study Using In-Situ QCM Technique, J. Photopolym. Sci. Technology, 17, 645 (2004). [7] Minoru Toriumi, Theoretical analysis of development behavior of resist measured by QCM, SPIE Proc Y-1 (2009). [8] Atsushi Sekiguchi, Study of Swelling Action during Developing for ArF Resist by using QCM Method, J. Photopolym. Sci. Technol., 23, 421 (2010). [9] Toshiro Itani, Julius Joseph Santillan, In situ dissolution analysis of EUV resist SPIE Proc H-1 (2011). [10] Atsushi Sekiguchi, Kengo Ogawa, Kenji 472