IntroductionSemiconductors 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 the 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. Particu larly in 193 nm 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 membrane­based microfiltration in the photochemical manufacturing process effectively improves the cleanliness of the materials. Furthermore, in today’s state­of­the­art 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/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 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 5 nm asymmetric UPE filters and its effectiveness of reducing the wafer defects for various litho­graphic processes.

Design of 5 nm Asymmetric UPE FilterVarious polymer materials could be used to manu­facture particle filters for photochemical filtration, such as UPE, PTFE and polyamide. Traditionally, all these material­based filters have symmetric membranes, where pore size is constant across the membrane thickness. In contrast, Entegris developed a new asymmetric UPE membrane using its proprietary co­extrusion technology, where pore size changes across the membrane thickness. The upstream surface of the asymmetric membrane has a larger pore size than on the downstream. This type of pore structure provides the improved retention efficiency and capacity of the micro­ porous membrane without sacrificing the flow. This optimized asymmetric structure is specifi ­cally designed to meet the unique requirements of advanced lithography processes below 65 nm technology node, where pressure drop might be a limiting factor in decreasing retention rating. It allows for a wider window of operation on photochemical dispense pumps. Figure 1 shows the scanning electron microscope (SEM) image of the cross­section of asymmetric UPE membrane.

ImprovIng AdvAnced LIthogrAphy process defectIvIty wIth A hIghLy

retentIve 5 nm AsymmetrIc Upe fILter

Authors: Aiwen Wu and Jennifer Braggin

AppLIcAtIon note

entegrIs, Inc. 1

Figure 1. Asymmetric membrane cross-section

Open layer For increased flow

Intermediate layer High loading capacityIncreased gel retention Increased impurity residence time

Highly retentive layerHard particle retention5 nm removal

entegrIs, Inc. 2

ImprovIng AdvAnced LIthogrAphy process defectIvIty

Improved Particle RetentionBubble point (BP) pressure is a common parameter to characterize microporous membranes and is defined as the minimum pressure required to begin displacing wetting liquid from the pores of a wetted membrane. This pressure is related to the largest size pore in that membrane and is often determined by visual detection of “bubble” as applied differen­tial pressure is increased across the membrane. The equation for bubble point shows an inversely proportional relationship between the pore diameter and the bubble point value. 2

Filters with 20 nm and 10 nm retention ratings have become the standard for 193 nm photoresist filtra­tion. These filters were tested for particle retention with monodispersed 33 nm polystyrene latex (PSL) beads and optical particle counters (OPC) using a modified SEMATECH® test method. 3 However, one of the issues with using an OPC to measure PSL particles down to 33 nm in size is that it is difficult to differentiate very tight filters from one another. Recently a new method has been developed by Entegris to measure particle removal efficiencies for sub­30 nm pore size rated filters using fluo­rescence spectroscopy. In this method, a filter is challenged with fluorescent particles, which are then counted by measuring fluorescent signals of upstream and downstream solutions. Retention can be calculated by comparing the fluorescent signals of the two solutions. There are several advantages to the fluorescence method compared to traditional OPC techniques, including high sensitivity, easy spectral measurement and ability to detect fluores­cent PSL particles whose size has been relatively

well controlled. The experimental data shows that using smaller 26 nm fluorescent particles to challenge the filters results in more differentiation among the filters. Figure 2 provides a correlation between particle removal efficiency and bubble point for Entegris UPE membranes. As membrane pore size shrinks (BP increases), particle reten­tion increases.

While 5 nm asymmetric UPE membrane greatly improves particle retention performance compared to 10 nm rating UPE, it doesn’t sacrifice the flow performance. With pore size greatly shrinking and thickness increasing, the pressure drop of 5 nm asymmetric UPE membrane does not significantly increase. Figure 3 presents the flow comparison of Entegris point­of­use (POU) photochemical filter Impact® 2 V2.

Mem

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UPE 10 nm Asym-metric

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UPE 20 nm

UPE 30 nm

UPE 50 nm

Figure 2. Correlation between 26 nm PSL bead retention for UPE membranes

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Impact 2 V2 5 nm asymmetric

Figure 3. Flow performance comparison of point-of-use filters

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ImprovIng AdvAnced LIthogrAphy process defectIvIty

Wafer Defect Reduction of 5 nm Asymmetric UPE FilterThe decrease of critical line widths is requiring the use of tighter filtration for critical photochem­icals. Filters rated at 10 nm and 20 nm filtration have been implemented in both the photochemical manufacturing process and in the point­of­use spin­coating process in track systems, and have been shown to be effective in reducing defects.4, 5

However, the implementation of finer filtration raises concerns as high differential pressure across the filter causes possible outgassing of the photo­chemical and finer pore size approaches the size of large molecular weight polymers in the photo­chemical. These conditions can cause polymer shearing, micro bubble formation and gel particle formation. Filter materials with finer pore size may have affinity with certain species in the photo­chemical formulations, causing photochemical changes to occur before it reaches the wafer. To address all these concerns and demonstrate the benefits of highly retentive 5 nm asymmetric UPE filter, a series of evaluations have been conducted under actual production conditions at a number of different customer sites.

Case 1: Reducing Microbridging Defects for 193 nm Top-coatless Immersion Photoresist

Microbridging was recognized as one of the critical patterning defects that were frequently observed in 193 nm lithographic process in different formula­tions from different manufacturers. The problem becomes remarkable particularly in dense line/space patterns and seriously damages the pro­duction yield. Entegris has engaged with the Interuniversity Microelectronics Center (IMEC) in Leuven, Belgium, to study the effects of point­

of­use filtration methods on microbridging defectivity.6 This study focused on the ability of point­of­use filtration on the lithography track to reduce the level of microbridging defectivity in a 45 nm line/space pattern created through immer­sion lithography. A design of experiment method­ology was used to determine the effect of filter retention rating, filtration rate and controlled filtration pressure on microbridging defectivity.

In the experiment, wafers were coated and exposed on the Sokudo™ RF 3Si/ASML™ XT:1900Gi cluster. Entegris’ IntelliGen® Mini dispense system was used with Entegris’ Impact Mini point­of­use UPE filters with various pore sizes. IMEC’s Defect 45 mask was used for the expo­sures. The Defect 45 mask is designed to print nine uniform sub­dies of 45 nm lines with 90 nm pitch. The resist stack for this experiment consisted of 95 nm BARC and 105 nm TOK TARF Pi6 001ME resist which intentionally contaminated high levels of microbridging defects. A post­soak step was added after the exposure, and development was done with the slit­scan nozzle. In a particular test, an intentionally contaminated resist with 10 times the microbridging levels of the process of record (POR) resist chemistry was used to determine the effect of filtration parameters on defect density.

Figure 4 shows the best results for combination of filtration rate and pressure applied to the filtrate for 5 nm asymmetric UPE and 20 nm symmetric UPE filters. Further analysis of the experimental data suggests that filter pore size is statistically significant for reducing microbridging defects. The data also indicates that different filter designs will require different, optimized filtration rates and applied pressures to further reduce microbridging defect density.

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Normalized Defect Data Defective Chemistry Sample

Figure 4. Results of 5 nm asymmetric UPE evaluation under best conditions at IMEC

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ImprovIng AdvAnced LIthogrAphy process defectIvIty

Case 2: Reducing Overall Coating Defects for 193 nm BARC Process

The implementation of BARC processes in 193 nm dry and immersion lithography has been accom­panied by defect reduction challenges on fine patterns. In a joint study with AZ Electronic Materials, the efficiency and performance of Entegris’ Impact 2 UPE filters with various pore sizes in reducing BARC process related defects was examined. AZ® ArF­1C5D BARC coating was employed in this study. The testing was accom­plished using an Entegris IntelliGen Mini dispense pump integrated in the coating module of a TEL® CLEAN TRACK ACT 8. BARC was coated on an unprimed silicon wafer. The dispense recipe and coat recipe were kept constant as the filter pore size changed. Spin speed was adjusted such that 37 nm BARC film thickness was achieved. The softbake condition was 200°C for 60 seconds. Three ACI wafers were coated and the wafers were measured for defects. A KLA­Tencor® 2360 High­Resolution Imaging Inspection System was used with a 0.20 µm pixel size under bright field imaging mode to count defects on the coated wafers. Then the defect review was carried out using an Applied Materials® SEMVision™ cX defect review tool.

Figure 5 shows the total defect results for the tested filters. In the first run, 3 point­of­use UPE filters with various pore sizes were tested. Compared to a 20 nm rating filter, smaller pore size filters significantly improved the BARC defectivity. Furthermore, 5 nm asymmetric filter performed the best, with the lowest number of total defects. 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 results were very similar to the first round of testing as shown in Figure 6. The 5 nm asymmetric filter showed much lower defect counts than the 20 nm filter.

Case 3: Reducing Residue-type Defects for 193 nm Top-coatless Immersion Photoresist

A study had been started to reduce residue­type defects for the latest 193 nm top­coatless immersion resist. To reduce this residue­type defect, 10 nm asymmetric and 5 nm asymmetric UPE filters were compared. The tested resist was well controlled for elimination of microbridging­type defect. The focus was on eliminating residue­type defects. Table 1 is the brief summary of testing condition. The eval­uation results in Figure 6 showed that the 5 nm asymmetric UPE filter reduced the residue­type defects by over 70% compared to the 10 nm asymmetric UPE filter.

tAbLe 1. brIef sUmmAry of testIng condItIon

Substrate 200 mm bare Si with BARC

Top coat None

Mask 75 nm L/S

Exposure Dry ArF scanner

Dispense pump Single stage

Inspection KLA 2360

Tota

l Def

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Confirmation run

Impact 2 V2 20 nmsymmetric

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Impact 2 V2 5 nmasymmetric

Figure 5. Total defectivity comparison by filter

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0Fall on Residue

Figure 6. Effect of filter pore size on residue-type defect reduction in ArF immersion resist

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ImprovIng AdvAnced LIthogrAphy process defectIvIty

Filter PrimingThe most noticeable attribute of photochemical POU filters is the ability to purge air and contami­nants introduced during system maintenance or filter changeout. Customers continue to have issues with filter priming and associated tool downtime. As the finer feature size of advanced lithography processes requires even finer filtration for photo­chemicals, there is more concern on the finer filter priming speed. The unique design of 5 nm asymmetric UPE membrane ensures a high level of retention to reduce advance process defects while also maintaining the ability of fast priming during filter changeout.

Laboratory experiments were conducted to examine the priming performance of Impact 2 V2 5 nm asymmetric UPE filter and compare to a standard Impact 2 V2 10 nm symmetric UPE filter on an Entegris two­stage­technology IntelliGen Mini dispense pump. A recirculating chemical test stand was assembled using a chemical reservoir, a dispense pump, a filter manifold, a test filter and an OPC. The OPC is a PMS LiQuilaz® SO2, capable of detecting and sizing particles down to

0.2 µm. This OPC was installed on the outlet line of the dispense system, monitoring the entire downstream of the testing filters. The effluent was recycled to the reservoir. The filters were primed with the solvent ethyl lactate and the dispense recipe was continually performed until particle counts leveled off. Since each new testing filter was installed after the particle counts reached very low background with a filter in place, the particle levels shown by the counter indicated the level of microbubbles in the dispense line during the testing. While optical particle counters are not designed to count bubbles, the results can be used in a semi­quantitative manner to see differences in filter performance.

The result of filter priming testing is shown in Figure 7. The priming speed of Impact 2 V2 5 nm asymmetric UPE filters is slightly better or equal to a standard Impact 2 V2 10 nm symmetric UPE filter. This result shows that when the membrane structure is optimized correctly, the 5 nm asym­metric filter can maintain the fast priming speed. As a result, the chemical waste and dispense­point downtime are reduced.

Impact 2 V2 10 nm symmetric UPEImpact 2 V2 5 nm asymmetric UPE #1Impact 2 V2 5 nm aymmetric UPE #2

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Bubble Cleanup Testing on IntelliGen Mini Pump with Ethyl Lactate(Dispense Volume = 5 mL; Dispense Rate = 1 mL/sec; Filtration Rate = 1 mL/sec

Figure 7. Bubble flushup of Impact 2 V2 5 nm asymmetric UPE filter

entegrIs, Inc. Corporate Headquarters | 3500 Lyman Boulevard | Chaska, Minnesota 55318 USACustomer 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

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.

ConclusionAsymmetric UPE technology is a new applica ­ tion­specific filter family designed to optimize performance in critical photochemical filtration processes requiring a combination of high flow and extreme retention. The 5 nm asymmetric UPE filter has been shown to provide superior retention that can lead to reduction on microbridging defects, residue defects and overall defects in advanced litho graphy processes. There is no evidence that finer filtration down to a 5 nm rating changes the integrity of advance photolithography chemistries resulting in increased defectivity. To enable further reduction in chip line widths with mini­mized defects, the finer filtration is required at both photochemical manufacturing and point­of­use locations.

References 1. Tamada, M. Sanada, “Mechanism Study of

Defect Improvement by Short Develop Time Process,” Proc. SPIE, Vol. 5753, 996 –1007, 2005.

2. Porter, Mark C., “Handbook of Industrial Membrane Technology.”

3. Lee, J.K. et al., “Latex Sphere Retention by Microporous Membranes in Liquid Filtration,” Journal of the IES, January/February 1993, 26 – 36.

4. Amari, M.; Wu, A.; Yang, H.J.; Chen, L.; Bowling T. and Watt, M., “Effect of Filter Surface Chemistry and Morphology on 193 nm Lithography Applications,” SEMICON Korea, 2005.

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 Point­of­Use Filtration on Microbridging Defectivity,” SPIE 2009.

ImprovIng AdvAnced LIthogrAphy process defectIvIty