The SMART Process for Directed Block Co-Polymer Self-Assembly

Journal of Photopolymer Science and Technology Volume 26, Number 5 (2013) 573 579 2013SPST The SMART Process for Directed Block Co-Polymer Self-Assembly Jihoon Kim*, Jingxiu Wan, Shinji Miyazaki, Jian Yin, Yi Cao, YoungJun Her, Hengpeng Wu, Jianhui Shan, Kazunori Kurosawa and Guanyang Lin* AZ Electronic Materials USA Corp., 70 Meister Avenue, Somerville, NJ 08876, USA AZ Electronic Materials (Korea) Ltd. #325-25, Boche-Ri, Miyang-Myeon, Anseong-Si, 456-883, KR AZ Electronic Materials (Japan) K.K., 3330 Chihama, Kakegawa, Shizuoka 437-1412, JP *jihoon.kim@azem.com and guanyang.lin@azem.com A new process for directed block co-polymer self-assembly (DSA), AZ SMART, for high resolution line and space patterning was introduced. The SMART process started with photoresist trench patterns generated through common photolithographic processes on top of a thin crosslinked neutral layer. A reactive ion etching (RIE) process removed the neutral layer at bottom of the resist trenches and followed by a resist stripping step which completely removed the resist material and uncovered the neutral surface protected by the resist film during etching step. DSA performances of the resultant SMART chemical pre-patterns without or with extra pinning material brushing step were compared. Results indicated that pinning material enhanced chemical pre-pattern directing power for DSA performance. The chemical prepattern without pinning material provided well aligned DSA performance for some specific pre-pattern structure and DSA multiplication factor, but it lacked general performance stability. On the other hand, process with added pinning material was demonstrated with stable performance for variable pre-pattern pitches with different DSA multiplication factors. SMART DSA pattern profile and its pattern etching transfer into hard masks were investigated. Keywords: block co-polymer, directed self-assembly, BCP, DSA, poly(styrene-bmethylmethacrylate), PS-b-PMMA, AZ SMART 1. Introduction Block co-polymer directed self-assembly extends existing photolithography patterning capability without requirement of significant new infrastructure development. It has quickly emerged as one of the leading candidates for advanced integrated circuit (IC) pattern printing.[1-4] Directed self-assembly (DSA) uses block co-polymer (BCP) with blocks having distinctly different chemical properties which results in segregation among the different blocks and formation of domains containing same polymer blocks. For a bis-block co-polymer, when volume fraction of the two blocks is properly constructed, a set of different morphologies can be assembled. Morphologies particularly interesting to the IC industry are the cylinder and the lamellae since they provide opportunities for printing high density contact and line/space patterns, respectively, which are commonly used for IC device fabrication. Cylinder and lamellae BCP assemblies can be directed via graphoepitaxy [5-14] or chemoepitaxy [15-19]. For line/space patterns, due to higher density and potentially better final DSA pattern LER/LWR, chemoepitaxy DSA has been extensively investigated. There are two DSA chemoepitaxy schemes reported in literature. One is the Lift-off scheme reported by Cheng et al. at IBM [20] and the other the UW flow published by Liu et al. from University Wisconsin [21]. Both flows start with Received May 24, 2013 Accepted June 10, 2013 573

isolated resist line patterns preferably generated by using positive tune development photolithography processes. In this publication, a novel new DSA flow, the AZ SMART (Surface Modification for Advanced Resolution Technology) flow, and its DSA performance were introduced. 2. Experimental 2.1. Materials Cross-linkable neutral material, AZEMBLY EXP NLD-175, brush pinning material AZEMBLY EXP NLD-226, poly(styrene-b-methylmethacrylate) (PS-b- PMMA) BCP and formulations of AZEMBLY EXP PME-053 with natural period of Lo 25.6nm, AZEMBLY EXP PME-190 (Lo 30nm), and resist stripper AZEMBLY EXP RSD-001 were all synthesized and formulated at AZ Electronic Materials. Propylene glycol monomethyl ether acetate (PGMEA) and n-butyl acetate (nba) were purchased from Sigma-Aldrich and used as received. 193nm immersion photoresist FAiRS M190 from FUJIFILM Electronic Materials was used for resist pattern formation. Spin-on Siantireflective coating (Si-ARC) SHB-9480 was a commercial product from Shin-Etsu Chemical Co., Ltd. and used as received. 2.2. Processes Resist nano-patterns were fabricated at facilities at IMEC, Leuven, Belgium. As shown in Figure 1, Si-ARC SHB-9480, 40nm, was spincoated on a 300 mm Si wafer on a Tokyo Electron Ltd. Clean Track ACT-12 and baked at 220 C for 90s. AZEMBLY EXP NLD-175 was coated with 7nm film thickness on top of the Si- ARC film and baked at 250 C in air for 2min, Figure 1-a. A 50nm photoresist film of FAiRS M190 was spin-coated on top of above substrate with a post-apply bake (PAB) of 100 C for 60s. Resist film pattern exposure was carried out on an ASML XT: 1950i scanner with NA of 1.35, dipole illumination σ ou t/σ in 0.76/0.66 and followed by a post exposure bake of 100 C for 60s and development by nba solvent for 30s to provide resist trench patterns with pitch of 90nm or 128nm depending on the mask pattern type using in the exposure step. The resist trench pattern critical dimensions (CD) rang from 20nm to 50nm depending on exposure dose applied (Fig. 1-b). The patterned wafer was then cut into small coupons for DSA studies. The coupon wafer was first subjected to an oxygen and argon (O 2 /Ar) plasma etching process (pressure: 50mtorr, 15sccm/O 2, 100sccm/Ar, power: 50W) to remove the NLD- 175 layer at bottom of resist trenches (Figure 1- c). A RIE-1701 Reactive Ion Etch Plasma system from Nordson MARCH was used. The etched wafer coupon was dipped into resist stripper RSD-001 at r.t for 15min to completely remove the resist material and uncover the NLD-175 neutral film surface which was protected by the resist film during the previous etching step. The wafer was then rinsed with nba or PGMEA and dried by blowing pressured air. A topographic chemical surface comprised of alternating NLD- 175 film stripes and shallow trenches, wherein Si-ARC film surface is at the bottom of the trenches, is formed (Figure 1-d). In some cases, this chemical surface could provide good DSA performance. However, for more stable DSA performance, AZEMBLY EXP NLD-226, a brush pinning material, was further spin-coated on the wafer and baked at 130 C for 120s (Figure 1-e). Fig. 1. Schematic of AZ SMART flow: a) substrate with cured AZEMBLY NLD-175 coating, b) resist trench pattern, c) after NLD-175 layer at bottom of trenches etching open, d) after resist stripping, e) after pinning material, AZEMBLY EXP NLD-226 coating and bake, f) after solvent rinsing to removal excess NLD-226, g) after coating PME and h) after PME film annealing. PGMEA or nba was used for rinsing off the excess NLD-226 coating and air blow dried. NLD-226 was designed with chemistry which has high reactivity towards Si-ARC surface but low reactivity with NLD-175 film surface. NLD- 226 is not self-crosslinkable. Therefore, after the NLD-226 film solvent rinse, NLD-226 material on top of the NLD-175 film surface was rinsed away. Only a monolayer of NLD-226 brush was 574

left into the trench patterns (Figure 1-f). AZEMBLY EXP PME formulation was spincoated (Figure 1-g) and annealed at 250 C for 5min in air (Figure 1-h). PMMA removal was done on a March RIE-1701 etcher with O 2 gas plasma. The resultant DSA pattern was inspected with a NanoSEM 3D from Applied Materials. DSA patterns were transferred into Si-ARC film through a plasma etching process conducted on a NE5000N etcher (ULVAC, Inc.) with CF 4 and He gas system. 3. Results and discussions 3.1. Negative tune development (NTD) process for small trench resist pattern Different from Lift-off and UW DSA schemes, AZ SMART DSA flow starts with resist trench patterns as shown in Figure 1. NTD process is widely used in IC device fabrication processes for high resolution contact and line/space pattern printing. NTD process provides advantages for small trench generation over positive tone development processes, in particular, for semi dense or isolated trench pattern printing. Figure 2 shows an example of very small trenchs, 29nm, generated by a 193nm immersion lithographic process using the NTD process. Trench CD: 29nm Fig. 2. High trench resolution via NTD processes (NA 1.35, dipole, σ out /σ in 0.76/0.66), resist pattern of 90nm pitch with 29 nm trench critical dimension. 3.2. AZ SMART pre-pattern structure optimization Given neutral and PME materials are well optimized, there are several key parameters affecting chemoepitaxy DSA performance: 1) commensurability of BCP Lo with chemical prepattern repeating pitch, 2) pinning line size, the trench width for AZ SMART, and 3) pinning line directing or pinning power. Chemical pre-pattern pattern repeating pitch was defined by photolithographic processes, which in general could be precisely controlled with very limited variation. BCP Lo can be finetuned through both BCP chemical synthesis and formulation optimizations. Since it is out of the scope of this publication, BCP Lo adjustment will not be discussed herein. Impacts of pinning feature size on DSA performance have been carefully studied. In general, in the lift-off process, its chemical prepattern has a topographical structure. The pinning stripe width, the trench CD, can vary between about half of BCP Lo to 1.5 times of BCP Lo without affecting BCP DSA performance [22]. On the other hand, in the UW flow, since its chemical pre-pattern has a flat surface, the pinning MAT width needs to be controlled to be less around 0.9x Lo and larger than 0.5x Lo in order to maintain a stable DSA performance. [23-24] In the AZ SMART flow, after resist stripping, a topographical trench structure as shown in Figure 1-d was fabricated. This structure is same type as the chemical prepattern from the Lift-off process except that the chemical natures of trench wall and bottom surfaces are different. In a lift-off process, the trench sidewalls are constructed by neutral material and the bottoms by substrate surfaces such as Si-ARC film. However, in a SMART process, although the trench sidewalls are also made of neutral material, in this case, the NLD- 175, the surfaces are interacted with plasma chemicals during the NLD opening etching process. Same interaction also incurs to the trench bottom surface. In fact, we tried coating BCP with proper Lo onto the SMART chemical structure right after resist stripping step. With proper control of the commensurability and the chemical pre-pattern trench size, well aligned DSA performance could be achieved as shown as an example in Figure 3-a. However, its repeatability was an issue with day to day and place to place variations, and very often, simple self-assembly fingerprint pattern was observed as illustrated by Figure 3-b SEM. We believe the instable performance mainly came from lack of control over the trench surfaces which provide pinning function and determine what surface energy should be on neutral material surface [25]. Etching plasma has complex chemical components. Slight difference in etching chamber pressure, gas mixing ratio or gas flow rate can affect plasma composition, 575

which in turn affects subtle surface properties of the etched substrates. To improve the performance stability, in particular, the pinning surface of the trench pattern in a SMART flow, a grafting pinning material, AZEMBLY EXP NLD-226 was developed. AZEMBLY EXP NLD-226 contains polymeric material with homopolystyrene block with a chain-end reactive a) Fig. 3. SMART DSA performance after resist stripping step. Good DSA performance was occationally achieved a), but simple fingerprint pattern was often produced as shown in b). functional group which reacts with inorganic substrate surface during a bake step to form stable chemical bond but has minimum interaction with the neutral surface of the NLD- 175 film. Figure 4-a summarized the water contact angles of the NLD-175 film and resulted surfaces after over coating NLD-226 onto NLD- 175 film followed by a bake and then solvent rinsing steps. Similar water contact angle data but for O 2 /Ar plasma treated Si-ARC films were presented in the Figure 4-b. Results revealed that NLD-226 could brush on etched Si-ARC surface with as low as 110ºC bake temperature. No further contact angle increase was observed when increasing the bake temperature. Bake time such as 2min and 5min made no difference on the resultant surface contact angle, indicating a fast grafting reaction between NLD-226 and treated Si-ARC surface. On the other hand, it required at least 160ºC bake temperature for NLD-226 to have partial reactivity with the NLD-175 film surface. In addition, even at high bake temperature such as 200 ºC, the resultant film contact angle (< 85 degree, Fig. 4-a) was still smaller than the contact angle on treated Si- ARC surface with bake temperature of 110 ºC, indicating there was only partial surface grafting for NLD-226 on NLD-175 surface. b) Fig. 4. CA measurement after NLD-226 coating bake, and solvent rinsing on a) NLD-175 film, and b) O 2 /Ar plasma treated Si-ARC film. NLD-226 brushing bake temperature influence on DSA performance was examined. As expected and summarized in Figure 5, higher bake temperatures, such as 170 ºC and 190 ºC, deteriorated the DSA performance and low NLD-226 brushing temperature at 150 ºC gave good DSA performance (see Figure 5-a). This observation was consistent with the contact angle study results that at high brushing temperatures such as 190 ºC, NLD-226 reacted with not only the treated Si-ARC surface, but also with NLD- 175 film surface. Grafting of NLD-226 on NLD- 175 film surface resulted in loss of neutrality, which in turn affected the pre-pattern DSA performance. At 150 ºC bake temperature, NLD- 226 reacted with the treated Si-ARC surface, but did not react with NLD-175 surface, therefore, a pre-pattern surface alternating with pinning and neutral stripes was formed, a foundation for good DSA performance. It is worthy of mention that although at temperature higher than 160 ºC, there were partial grafting of NLD-226 on NLD-175, some DSA performance could be obtained as shown in Figure 5-b, wherein the NLD-226 brushing temperature was 170 ºC. NLD-175 neutrality towards lamellar PS-b-PMMA BCP has wide window in term of its chemical composition. NLD-175 was an optimized 576

material for AZ SMART process. Uniform low density partial brushing of NLD-226 onto the film of NLD-175 does not completely damage NLD-175 neutrality in supporting a good DSA performance such in the case when the NLD-226 brushing temperature was at 170 ºC. Fig. 7. xsem of AZ SMART chemical pre-pattern structure a) before pinning material brushing and b) after pinning material brushing. c) Fig. 5. DSA performance of PME-053 (Lo 25.6 nm) vs. NLD-226 brushing temperature a) 150 C, b) 170 C, and c) 190 C for 2min. The selective reactivity of NLD-226 towards NLD-175 film and etched Si-ARC film was also examined by looking at cross section SEM at each step of the NLD-226 brushing processes. Figure 6-a is xsem of the structure after a 40nm NLD-226 coating and baking. There was a flat surface, indicating that the NLD-226 completely planarized the topographic surface. However, after solvent rinse, the topographic surface appeared again as shown in Figure 6-b. Carefully examining the Figure 6-b and comparing with the SEM picture after resist stripping step, the trench depth after NLD-226 brushing was apparently shallower than trench before the NLD-226 brushing step, indicating a NLD-226 layer was grafted inside the trench. Figure 7 presented a side by side comparison of those two xsems. Fig. 6. x-sem images after a) brushing material coating and baking and b) brushing material rinsing. Retention of the topographical structure after NLD-226 brushing process relaxes CD resolution requirement over the trench pinning pattern for well aligned DSA performance. Based on studies on impacts of pinning width on DSA performance, chemical pre-pattern with topographic trench pattern as a pinning spare can have good DSA performance with trench width CD up to 1.5x Lo. On the other hand, for a prepattern with flat surface, the pinning line CD need to be controlled to be less than 0.9x Lo. SMART DSA performance results were in agreement with those previous conclusions. As illustrated in Figure 8, good DSA performances were obtained based on a chemical pre-pattern with trench repeating pitch of 90nm and trench width of 45nm for AZEMBLY EXP PME-190 of Lo 30nm. Fig. 8. DSA performance of AZ SMART flow with chemical pre-pattern repeating pitch of 90nm and trench width of 45nm (3x pitch multiplication, BCP Lo 30nm. a) x-section and b) top-down SEM images 3.3 AZ SMART process stability We studied AZ SMART flow for different multiplication factors. Pre-pattern structures with pinning trench pitch of 90nm, 120nm and 128nm were prepared. AZEMBLY EXP PME-190 (Lo 30nm) were applied to the 90nm and 120nm prepattern wafer for 3x and 4x pitch multiplication DSA performance and the AZEMBLY EXP PME-053 (Lo 25.6nm) was applied to the 128nm pre-pattern wafer for 5x pitch multiplication DSA performance. As demonstrated in Figure 9, all practices gave well aligned DSA 577

performances, indicating SMART flow has stable DSA performance. We are now in process demonstrating its 12 wafer compatibility. c) Fig. 9. SEM images of DSA patterns a) 3x pitch multiplication with PME -190 (Lo 30nm), b) 4x pitch multiplication with PME-190 (Lo 30nm) and c) 5x pitch multiplication with PME-053 (Lo 25.6nm). To examine DSA pattern quality, DSA patterns of PME-190 (Lo 30nm) with 3x pitch multiplication was subjected to a RIE process to remove PMMA domains. Resultant pattern xsem was shown in Figure 10. It is seen that there was no 3-D network structure and all polystyrene (PS) lines were identical. No difference between PS line on top of neutral surface and pinning line surface. In fact, from the image, we cannot tell where trench and neutral surface were located. Fig. 10. SEM images after PMMA removal by dry etching. The image (b) is magnified image of (a). Fig. 11. SEM images of a) DSA PS line patterns after PMMA removal, and b) patterns of Si-ARC film after pattern transfer via RIE etching. We further pursued dry etching for pattern transfer into underlying Si-ARC layer with DSA polystyrene (PS) lines as masks. As shown in Figure 11, the pattern transfer preceded in parallel at all locations. The NLD-226 brushed trenches and NLD-175 film stripes below the DSA PS lines at different location did not affect etching pattern transfer uniformity. 4. Conclusions AZ SMART DSA flow starts with resist trench patterns, taking the advantages of high resolution patterning capability by NTD photolithographic processes. The flow for DSA line and space patterns were well demonstrated with good stability for multiple pitch multiplication factors. SMART flow utilizes lithographic processes commonly used in IC fabrications. The pinning material brushing process enables SMART to be substrate independent for stable DSA performance. AZ SMART flow should be easy to be implemented in IC industry. Acknowledgements We extend our gratitude to Tom Polson, Karen Dempkowski, Charles Hubka, Victor Monreal and Mike Healy for assisting SEM characterization and AZ Fab engineers for their support. Authors also would like to thank Durairaj Baskaran and Sung Eun Hong for their expert discussions. References 1. I.T.R.S., Lithography, http://www.itrs.net/ links/2010itrs/home2010.htm (2010). 2. C. T. Black, R. Ruiz, G. Breyta, J. Y. Cheng, M. E. Colburn, K. W. Guarini, H.-C. Kim, Y. Zhang, IBM J. Res. Dev., 51( 2007) 605. 3. J. Y. Cheng, C. A. Ross, H. I. Smith, E. L. Thomas, Adv. Mater., 18 (2006) 2505. 4. M. P. Stoykovich, P. F. Nealey, Mater.Today, 9 (2006) 20. 5. R. A. Segalman, H. Yokoyama, E. J. Kramer, Adv. Mater., 13 (2001) 1152. 6.R. Ruiz, N. Ruiz, Y. Zhang, R. L. Sandstrom, C. T. Black, Adv. Mater., 19 (2007) 2157. 7. J. Y. Cheng, A. M. Mayes, C. A. Ross, Nat. Mater., 3 (2004) 823. 8. J. Y. Cheng, C. A. Ross, E. L. Thomas, H. I. Smith, G. J. Vansco, Appl. Phys. Lett., 81 (2002) 3657. 578

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