AppLIcAtIon note




                                                        ImprovIng AdvAnced LIthogrAphy
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ImprovIng AdvAnced LIthogrAphy process defectIvIty




Improved Particle Retention                                well con...
ImprovIng AdvAnced LIthogrAphy process defectIvIty




Wafer Defect Reduction of                                        of...
ImprovIng AdvAnced LIthogrAphy process defectIvIty




   Case 2: Reducing Overall Coating Defects for                    ...
ImprovIng AdvAnced LIthogrAphy process defectIvIty




  Filter Priming                                                   ...
ImprovIng AdvAnced LIthogrAphy process defectIvIty




Conclusion                                                         ...
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Improving Advanced Lithography Process Defectivity with a Highly Retentive 5nm Asymmetric UPE Filter

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Semiconductors require ever increasing purity in fluids that come in contact with the devices to reduce the defectivity during the manufacturing process. Defect control is extremely critical and continues to be one of biggest challenges in lithography processes for integrated device manufacturers (IDMs) as the critical dimension (CD) size shrinks [1]. Any defect could result in an unusable device, resulting in a financial loss for the IDMs. Particularly in 193nm lithography processes, there have been widespread occurrences of various defects in the coated films, and many factors could play a role in defect formation. For example, photoresist and BARC coating defects could be affected by the dispense process, cleanliness of the materials, and filtration process.

Particle removal filters are used in almost every process step where a liquid comes in contact with a wafer. Implementation of polymer membrane-based microfiltration in the photochemical manufacturing process effectively improves the cleanliness of the materials. Furthermore, in today’s state-of-the art semiconductor fabs, an extra filtration step at the point of dispense on the coater module is adopted to further minimize coating defects by providing particle/bubble-free photochemical dispense on the wafer.

With the continuous demands for defect reduction and high productivity, Entegris has developed a new, highly retentive 5nm rated asymmetric ultrahigh molecular weight polyethylene (UPE) filter to address these demands particularly in advanced lithography processes. The purpose of this application note is to provide data to show the performance of 5nm asymmetric UPE filters and its effectiveness of reducing the wafer defects for various lithographic processes.

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Improving Advanced Lithography Process Defectivity with a Highly Retentive 5nm Asymmetric UPE Filter

  1. 1. AppLIcAtIon note ImprovIng AdvAnced LIthogrAphy process defectIvIty wIth A hIghLy retentIve 5 nm AsymmetrIc Upe fILter Authors: Aiwen Wu and Jennifer Braggin Introduction Design of 5 nm Asymmetric Semiconductors require ever increasing purity UPE Filter in fluids that come in contact with the devices to Various polymer materials could be used to manu­ reduce the defectivity during the manufacturing facture particle filters for photochemical filtration, process. Defect control is extremely critical and such as UPE, PTFE and polyamide. Traditionally, continues to be one of the biggest challenges in all these material­based filters have symmetric lithography processes for integrated device manu­ membranes, where pore size is constant across facturers (IDMs) as the critical dimension (CD) the membrane thickness. In contrast, Entegris size shrinks.1 Any defect could result in an unusable developed a new asymmetric UPE membrane using device, resulting in a financial loss for the IDMs. its proprietary co­extrusion technology, where Particularly in 193 nm lithography processes, pore size changes across the membrane thickness. there have been widespread occurrences of various The upstream surface of the asymmetric membrane defects in the coated films, and many factors has a larger pore size than on the downstream. could play a role in defect formation. For example, This type of pore structure provides the improved photoresist and BARC coating defects could be retention efficiency and capacity of the micro­ affected by the dispense process, cleanliness of porous membrane without sacrificing the flow. the materials, and filtration process. This optimized asymmetric structure is specifi­ Particle removal filters are used in almost every cally designed to meet the unique requirements process step where a liquid comes in contact with of advanced lithography processes below 65 nm a wafer. Implementation of polymer membrane­ technology node, where pressure drop might be based microfiltration in the photochemical a limiting factor in decreasing retention rating. manufacturing process effectively improves the It allows for a wider window of operation on cleanliness of the materials. Furthermore, in photochemical dispense pumps. Figure 1 shows the scanning electron microscope (SEM) image of today’s state­of­the­art semiconductor fabs, an the cross­section of asymmetric UPE membrane. extra filtration step at the point of dispense on the coater module is adopted to further minimize coating defects by providing particle/bubble­free photochemical dispense on the wafer. With the continuous demands for defect reduction and high productivity, Entegris has developed a new, highly retentive 5 nm rated asymmetric ultra­ high molecular weight polyethylene (UPE) filter to address these demands particularly in advanced lithography processes. The purpose of this applica­ tion note is to provide data to show the performance of 5 nm asymmetric UPE filters and its effectiveness of reducing the wafer defects for various litho­ graphic processes. Open layer Intermediate layer Highly retentive layer For increased flow High loading capacity Hard particle retention Increased gel retention 5 nm removal Increased impurity residence time Figure 1. Asymmetric membrane cross-section entegrIs, Inc. 1
  2. 2. ImprovIng AdvAnced LIthogrAphy process defectIvIty Improved Particle Retention well controlled. The experimental data shows that using smaller 26 nm fluorescent particles to Bubble point (BP) pressure is a common parameter challenge the filters results in more differentiation to characterize microporous membranes and is among the filters. Figure 2 provides a correlation defined as the minimum pressure required to begin between particle removal efficiency and bubble displacing wetting liquid from the pores of a wetted point for Entegris UPE membranes. As membrane membrane. This pressure is related to the largest pore size shrinks (BP increases), particle reten­ size pore in that membrane and is often determined tion increases. by visual detection of “bubble” as applied differen­ Retention vs. Bubble Point tial pressure is increased across the membrane. The equation for bubble point shows an inversely Retention BP Membrane Bubble Point proportional relationship between the pore Membrane Retention diameter and the bubble point value. 2 UPE UPE UPE UPE UPE UPE 5 nm 10 nm 10 nm 20 nm 30 nm 50 nm Asym- Asym- metric metric Figure 2. Correlation between 26 nm PSL bead retention for UPE membranes While 5 nm asymmetric UPE membrane greatly improves particle retention performance compared to 10 nm rating UPE, it doesn’t sacrifice the flow Filters with 20 nm and 10 nm retention ratings have performance. With pore size greatly shrinking and become the standard for 193 nm photoresist filtra­ thickness increasing, the pressure drop of 5 nm tion. These filters were tested for particle retention asymmetric UPE membrane does not significantly with monodispersed 33 nm polystyrene latex (PSL) increase. Figure 3 presents the flow comparison of beads and optical particle counters (OPC) using Entegris point­of­use (POU) photochemical filter a modified SEMATECH® test method.3 However, Impact® 2 V2. one of the issues with using an OPC to measure PSL 25 particles down to 33 nm in size is that it is difficult to differentiate very tight filters from one another. 20 Recently a new method has been developed by Pressure Drop (PSI) Entegris to measure particle removal efficiencies 15 Impact 2 V2 5 nm asymmetric for sub­30 nm pore size rated filters using fluo­ rescence spectroscopy. In this method, a filter is 10 challenged with fluorescent particles, which are Impact 2 V2 20 nm then counted by measuring fluorescent signals of 5 upstream and downstream solutions. Retention can Impact 2 V2 10 nm be calculated by comparing the fluorescent signals 0 of the two solutions. There are several advantages 0 5 10 15 20 to the fluorescence method compared to traditional Flow Rate for 1 cp Fluid (cc/sec) OPC techniques, including high sensitivity, easy Figure 3. Flow performance comparison of point-of-use filters spectral measurement and ability to detect fluores­ cent PSL particles whose size has been relatively entegrIs, Inc. 2
  3. 3. ImprovIng AdvAnced LIthogrAphy process defectIvIty Wafer Defect Reduction of of­use filtration methods on microbridging defectivity.6 This study focused on the ability of 5 nm Asymmetric UPE Filter point­of­use filtration on the lithography track to reduce the level of microbridging defectivity in a The decrease of critical line widths is requiring 45 nm line/space pattern created through immer­ the use of tighter filtration for critical photochem­ sion lithography. A design of experiment method­ icals. Filters rated at 10 nm and 20 nm filtration ology was used to determine the effect of filter have been implemented in both the photochemical retention rating, filtration rate and controlled manufacturing process and in the point­of­use filtration pressure on microbridging defectivity. spin­coating process in track systems, and have been shown to be effective in reducing defects.4, 5 In the experiment, wafers were coated and However, the implementation of finer filtration exposed on the Sokudo™ RF 3Si/ASML™ raises concerns as high differential pressure across XT:1900Gi cluster. Entegris’ IntelliGen® Mini the filter causes possible outgassing of the photo­ dispense system was used with Entegris’ Impact chemical and finer pore size approaches the size Mini point­of­use UPE filters with various pore of large molecular weight polymers in the photo­ sizes. IMEC’s Defect 45 mask was used for the expo­ chemical. These conditions can cause polymer sures. The Defect 45 mask is designed to print nine shearing, micro bubble formation and gel particle uniform sub­dies of 45 nm lines with 90 nm pitch. formation. Filter materials with finer pore size may The resist stack for this experiment consisted of 95 have affinity with certain species in the photo­ nm BARC and 105 nm TOK TARF Pi6 001ME resist chemical formulations, causing photochemical which intentionally contaminated high levels of changes to occur before it reaches the wafer. To microbridging defects. A post­soak step was added address all these concerns and demonstrate the after the exposure, and development was done benefits of highly retentive 5 nm asymmetric UPE with the slit­scan nozzle. In a particular test, an filter, a series of evaluations have been conducted intentionally contaminated resist with 10 times under actual production conditions at a number of the microbridging levels of the process of record different customer sites. (POR) resist chemistry was used to determine the effect of filtration parameters on defect density. Case 1: Reducing Microbridging Defects for Figure 4 shows the best results for combination of 193 nm Top-coatless Immersion Photoresist filtration rate and pressure applied to the filtrate Microbridging was recognized as one of the critical for 5 nm asymmetric UPE and 20 nm symmetric patterning defects that were frequently observed in UPE filters. Further analysis of the experimental 193 nm lithographic process in different formula­ data suggests that filter pore size is statistically tions from different manufacturers. The problem significant for reducing microbridging defects. The becomes remarkable particularly in dense line/ data also indicates that different filter designs will space patterns and seriously damages the pro­ require different, optimized filtration rates and duction yield. Entegris has engaged with the applied pressures to further reduce microbridging Interuniversity Microelectronics Center (IMEC) defect density. in Leuven, Belgium, to study the effects of point­ Normalized Defect Data Defective Chemistry Sample 120% No filter (Medium rate/ 20 nm (Low rate/ 5 nm (High rate/ Medium pressure) High pressure) High pressure) 100% 80% 60% 40% 20% 0 Total Irregular Embedded Burst Bubble Line Microbridging Bridging Contaminant Defect Nodule Figure 4. Results of 5 nm asymmetric UPE evaluation under best conditions at IMEC entegrIs, Inc. 3
  4. 4. ImprovIng AdvAnced LIthogrAphy process defectIvIty Case 2: Reducing Overall Coating Defects for Case 3: Reducing Residue-type Defects for 193 nm BARC Process 193 nm Top-coatless Immersion Photoresist The implementation of BARC processes in 193 nm A study had been started to reduce residue­type dry and immersion lithography has been accom­ defects for the latest 193 nm top­coatless immersion panied by defect reduction challenges on fine resist. To reduce this residue­type defect, 10 nm patterns. In a joint study with AZ Electronic asymmetric and 5 nm asymmetric UPE filters were Materials, the efficiency and performance of compared. The tested resist was well controlled for Entegris’ Impact 2 UPE filters with various pore elimination of microbridging­type defect. The focus sizes in reducing BARC process related defects was on eliminating residue­type defects. Table 1 was examined. AZ® ArF­1C5D BARC coating was is the brief summary of testing condition. The eval­ employed in this study. The testing was accom­ uation results in Figure 6 showed that the 5 nm plished using an Entegris IntelliGen Mini dispense asymmetric UPE filter reduced the residue­type pump integrated in the coating module of a defects by over 70% compared to the 10 nm TEL® CLEAN TRACK ACT 8. BARC was coated on asymmetric UPE filter. an unprimed silicon wafer. The dispense recipe and coat recipe were kept constant as the filter pore tAbLe 1. brIef sUmmAry of testIng condItIon size changed. Spin speed was adjusted such that Substrate 200 mm bare Si with BARC 37 nm BARC film thickness was achieved. The softbake condition was 200°C for 60 seconds. Top coat None Three ACI wafers were coated and the wafers were Mask 75 nm L/S measured for defects. A KLA­Tencor® 2360 High­ Exposure Dry ArF scanner Resolution Imaging Inspection System was used with a 0.20 µm pixel size under bright field imaging Dispense pump Single stage mode to count defects on the coated wafers. Then Inspection KLA 2360 the defect review was carried out using an Applied Materials® SEMVision™ cX defect review tool. 100 10 nm UPE Figure 5 shows the total defect results for the 5 nm UPE Normalized Defect Count (%) 80 tested filters. In the first run, 3 point­of­use UPE filters with various pore sizes were tested. 60 Compared to a 20 nm rating filter, smaller pore size filters significantly improved the BARC defectivity. 40 Furthermore, 5 nm asymmetric filter performed the best, with the lowest number of total defects. 20 Then a confirmation run was conducted using 20 nm and 5 nm asymmetric filters to confirm the findings in the first round of testing. The defect 0 Fall on Residue results were very similar to the first round of Figure 6. Effect of filter pore size on residue-type defect testing as shown in Figure 6. The 5 nm asymmetric reduction in ArF immersion resist filter showed much lower defect counts than the 20 nm filter. First run Confirmation run Total Defects Impact 2 V2 20 nm Impact 2 V2 10 nm Impact 2 V2 5 nm symmetric symmetric asymmetric Figure 5. Total defectivity comparison by filter entegrIs, Inc. 4
  5. 5. ImprovIng AdvAnced LIthogrAphy process defectIvIty Filter Priming 0.2 µm. This OPC was installed on the outlet line of the dispense system, monitoring the entire The most noticeable attribute of photochemical downstream of the testing filters. The effluent was POU filters is the ability to purge air and contami­ recycled to the reservoir. The filters were primed nants introduced during system maintenance or with the solvent ethyl lactate and the dispense filter changeout. Customers continue to have issues recipe was continually performed until particle with filter priming and associated tool downtime. counts leveled off. Since each new testing filter As the finer feature size of advanced lithography was installed after the particle counts reached very processes requires even finer filtration for photo­ low background with a filter in place, the particle chemicals, there is more concern on the finer levels shown by the counter indicated the level filter priming speed. The unique design of 5 nm of microbubbles in the dispense line during the asymmetric UPE membrane ensures a high level of testing. While optical particle counters are not retention to reduce advance process defects while designed to count bubbles, the results can be used also maintaining the ability of fast priming during in a semi­quantitative manner to see differences filter changeout. in filter performance. Laboratory experiments were conducted to examine The result of filter priming testing is shown in the priming performance of Impact 2 V2 5 nm Figure 7. The priming speed of Impact 2 V2 5 nm asymmetric UPE filter and compare to a standard asymmetric UPE filters is slightly better or equal Impact 2 V2 10 nm symmetric UPE filter on an to a standard Impact 2 V2 10 nm symmetric UPE Entegris two­stage­technology IntelliGen Mini filter. This result shows that when the membrane dispense pump. A recirculating chemical test structure is optimized correctly, the 5 nm asym­ stand was assembled using a chemical reservoir, metric filter can maintain the fast priming speed. a dispense pump, a filter manifold, a test filter As a result, the chemical waste and dispense­point and an OPC. The OPC is a PMS LiQuilaz® SO2, downtime are reduced. capable of detecting and sizing particles down to Bubble Cleanup Testing on IntelliGen Mini Pump with Ethyl Lactate (Dispense Volume = 5 mL; Dispense Rate = 1 mL/sec; Filtration Rate = 1 mL/sec 14 12 Impact 2 V2 10 nm symmetric UPE Impact 2 V2 5 nm asymmetric UPE #1 Impact 2 V2 5 nm aymmetric UPE #2 10 Particles/mL ≥0.2 µm 8 6 4 2 0 0 50 100 150 200 250 300 350 400 450 Dispense Cycle Figure 7. Bubble flushup of Impact 2 V2 5 nm asymmetric UPE filter entegrIs, Inc. 5
  6. 6. ImprovIng AdvAnced LIthogrAphy process defectIvIty Conclusion References Asymmetric UPE technology is a new applica­ 1. Tamada, M. Sanada, “Mechanism Study of tion­specific filter family designed to optimize Defect Improvement by Short Develop Time performance in critical photochemical filtration Process,” Proc. SPIE, Vol. 5753, 996 –1007, 2005. processes requiring a combination of high flow 2. Porter, Mark C., “Handbook of Industrial and extreme retention. The 5 nm asymmetric UPE Membrane Technology.” filter has been shown to provide superior retention 3. Lee, J.K. et al., “Latex Sphere Retention by that can lead to reduction on microbridging defects, Microporous Membranes in Liquid Filtration,” residue defects and overall defects in advanced Journal of the IES, January/February 1993, lithography processes. There is no evidence that 26 – 36. finer filtration down to a 5 nm rating changes the integrity of advance photolithography chemistries 4. Amari, M.; Wu, A.; Yang, H. J.; Chen, L.; resulting in increased defectivity. To enable Bowling T. and Watt, M., “Effect of Filter further reduction in chip line widths with mini­ Surface Chemistry and Morphology on mized defects, the finer filtration is required 193 nm Lithography Applications,” at both photochemical manufacturing and SEMICON Korea, 2005. point­of­use locations. 5. Wu, A. and Chow, W., “Defect Reduction in Advanced Lithography Processes Using a New Dual Functionality Filter,” International Symposium on Semiconductor Manufacturing, Tokyo, Japan, October 27– 29, 2008. 6. Braggin, J.; Gronheid, R.; Cheng S.; Van Den Heuvel, D.; Bernard, S.; Foubert, P. and Rosslee, C., “Analysis of the Effect of Point­of­ Use Filtration on Microbridging Defectivity,” SPIE 2009. Entegris®, Impact® and IntelliGen® are registered trademarks of Entegris, Inc. SEMATECH® is a registered trademark of Sematech, Inc. Sokudo™ is a trademark of Sokudo, USA, L.L.C. ASML™ is a trademark of ASML Holding N.V. AZ® is a registered trademark of AZ Electronic Materials. TEL® is a registered trademark of Tokyo Electron Kabushiki Kaisha. KLA­Tencor® is a registered trademark of KLA­Tencor Corporation. SEMVision™ is a trademark of Applied Materials, Inc. LiQuilaz® is a registered trademark of Particle Measuring Systems. entegrIs, Inc. Corporate Headquarters | 3500 Lyman Boulevard | Chaska, Minnesota 55318 USA Customer Service Tel. +1 952-556-4181 | Customer Service Fax +1 952-556-8022 www.entegris.com ©2009 Entegris, Inc. All rights reserved Printed in USA 4423-5836ENT-0909

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