Improving global CD uniformity by optimizing Post ...

Improving global CD uniformity by optimizing Post Exposure Bake and Develop sequences S. Osborne, M. Mueller, H. Lem, D. Reyland and K. H. Baik Etec Systems, Inc., Applied Materials 26460 Corporate Avenue, Hayward, CA USA 94545 ABSTRACT Improvements in the final uniformity of masks can be shrouded by error contributions from many sources. The final Global CD Uniformity (GCDU) of a mask is degraded by individual contributions of the writing tool, the Post Applied Bake (PAB), the Post Exposure Bake (PEB), the Develop sequence and the Etch step. Final global uniformity will improve by isolating and minimizing the variability of the PEB and Develop. We achieved this decoupling of the PEB and Develop process from the whole process stream by using “dark loss” which is the loss of unexposed resist during the develop process. We confirmed a correspondence between Angstroms of dark loss and nanometer sized deviations in the chrome CD. A plate with a distinctive dark loss pattern was related to a nearly identical pattern in the chrome CD. This pattern was verified to have originated during the PEB process and displayed a ∆(Final CD)/∆(Dark Loss) ratio of 6 for TOK REAP200 resist. Previous papers have reported a sensitive linkage between Angstroms of dark loss and nanometers in the final uniformity of the written plate. These initial studies reported using this method to improve the PAB of resists for greater uniformity of sensitivity and contrast. Similarly, this paper demonstrates an outstanding optimization of PEB and Develop processes. Keywords: Dark loss, CAR develop, spray develop, puddle develop, CD uniformity, Remaining Film Thickness (RFT), Resist Thickness Loss (RTL), raster scan e-beam, lithography, mask.

1.

INTRODUCTION

The current need for reduced feature size and maximized product yield now force the specification for Global Critical Dimension Uniformity (GCDU) to below 5nm within a 132mm field. Attaining this requires superlative performance from both the write machine and mask processing. The Post Application Bake (PAB), Writing, Post Exposure Bake (PEB), Develop, Etch and Metrology all make a quadrille contribution to the final measure of global uniformity. Using Dark Loss allows us to de-couple the PEB and Develop steps from the other process errors in a very productive way. With the Writing and Etch steps eliminated, optimization can occur to an extent not possible with the inclusion of variability from these other steps. The time and expense of writing and etching are also eliminated. Dark loss is defined as the loss of unexposed resist material during the Develop. The procedure for performing a dark loss is described in Figure 1. The bubble plots that follow represent the distribution of change in resist thickness that occurs before and after the develop. Dark bubbles are related to areas that have greater loss than the mean. Clear bubbles indicate areas that have a smaller loss than the mean. Reports of the usefulness of dark loss appear several times since 1995, although the method has been referred to under varying terms1, 2, 3. Kobayashi et. al.1 reported the method of “DRAUGHT” (Dissolution Rate Analysis in Unexposed areas to Gain the Highest Tone of a resist) as a means of optimizing the uniformity of sensitivity and contrast of ZEP7000 over the coated plate. They did this by monitoring the uniformity of the “Remaining Film Thickness” (RFT) in relation to varied conditions for the PAB and subsequent cooling. The implications to the final CD uniformity were convincing. Five years later, Kashida2 quantified the magnitude of this linkage and demonstrated that the CD deviated linearly with deviations in “Resist Thickness Loss” (RTL) across the plate. More specifically, when the RTL, at specific sites across the plate, were observed to range over 2.7nm, the chrome 366

Proceedings of SPIE Vol. 5256 23rd Annual BACUS Symposium on Photomask Technology, edited by Kurt R. Kimmel, Wolfgang Staud (SPIE, Bellingham, WA, 2003) · 0277-786X/03/$15.00

CD at those same sites deviated over a range of 25nm. Their scatter plot revealed an approximate 1 order of magnitude proportionality between the chrome CD and the dark loss. Single Angstrom sized deviations in dark loss were translating into nanometer sized deviations in final global uniformity. The scatter of data around their trend axis likely represented the quadrille error contributions of all the processing steps mentioned above. The observation that the uniformity of resist loss across the plate had a meaningful proportionality to the final GCDU held potential for process optimization beyond just the PAB.

Coat Plate + PAB

Final Bake with PEB conditions but do not expose.

Measure on Nanospec or N&K. Accurately track orientation.

Determine (Initial - Final) thickness and bubble plot.

Remeasure the resist thickness using the exact same grid points.

Develop.

Figure 1: The flow chart describes how dark loss is measured. When the unexposed plate is developed, the relative resist losses will display a record of thermal inconsistencies experienced by the plate during bake—a thermal memory in the resist. The bake signature, so revealed, will also carry a fingerprint of the Develop steps.

This ∆(Final CD)/∆(Dark Loss) relation is useful to the extent that we can effectively measure Angstrom changes in film thickness. Angstrom sized measurements, though suspect in their absolute relevance, are highly reproducible and can be used to drive improvements in the final CD. Our present study is aided by the current capability of thickness measurement tools to support repeatable determinations down to 5 Angstroms. With the tools available, a two step iterative optimization became possible. First to improve the uniformity of the fundamentally critical PEB then to use dark loss again as an experimental indicator to improve the develop process—all without the cost, time and ambiguity associated the writing and etch tools.

Figure 2: This dark loss test shows what we believed to be a distinctive PEB signature. The bubble plots used to display the data use open bubbles to represent areas that have had less resist removed than the mean. Filled bubbles represent areas that have more resist removed than the mean.

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We were able to prove out Kashida’s and Kobayashi’s assertions in the following way. We recorded a very distinctive dark loss pattern for a plate before e-beam writing, as seen in the bubble plot of Figure 2. After writing the plate and completing metrology on the chrome image, we generated Figure 3. To verify that this pattern was indeed caused by the non-uniformity of the bake system, we rotated a second test plate 90 degrees with all other conditions being identical. Metrology was completed on this second plate in a standard orientation, so creating the effect of rotating the chrome bubble plot (Figure 4). Where Kashida’s group measured the proportionality of ∆(Final CD)/∆(Dark Loss) in ZEP 7000U to be 9, we measured a similar ratio for REAP 200 to be 6±1 (Figure 5).

Figure 3: Bubble plot of chrome 600nm Cross1 features of a plate believed to have the same bake signature revealed by dark loss in Figure 2.

Figure 4: These bubble plots of x and y Cross1 figures in chrome are from a second plate that was rotated to track and verify the signature seen in Figure 2 and 3. The 90 degree shift verifies the PEB process as the source of the signature.

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xCross1 Chrome CD Vs Dark Loss

Chrome CD Distribution (nm)

xCross1 (y/x=4.9) yCross1(y/x=7.3) Linear LSF AVGxyCross1y/x=6.1 20

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Dark Loss Distribution (nm)

Figure 5: Distributions from the chrome final and from dark loss for Figures 2 and 3 are plotted. The ratio of ∆(Final CD)/∆(Dark Loss) for REAP 200 is established to be about 6 using the 2.38 wt. % developer.

2.

EXPERIMENTAL

Tools and equipment used Tools and instrumentation used in this study include a SigmaMeltec CTS5000 Clean/Coat/Bake cluster tool for coating resist. A SigmaMeltec SFB2500 for baking. An N&K 1512 RT and Nanospec unit were used for thickness measurement. A SigmaMeltec SFD2500 was used for the develop. A SensArray Thermal Trak unit was used for bake plate initial setup. First order optimization of the SFB2500 bake plate was made using a SensArray system. The SensArray is designed to measure the environment of the resist by imbedding RTD sensors in the mask substrate. Seventeen of these RTDs are positioned uniformly over the test substrate. The process is ideal for making preliminary settings with two exceptions. The resist carries a solvent load that evaporates during the PAB and imparts a cooling effect that is not present for the SensArray test plate. Hence, the dark loss method presents a memory of the thermal environment experienced by the resist that is not simulated by the SensArray. Secondly, the point at which this thermal memory becomes frozen into the resist is unknown and it may not occur conveniently at the plateau of the equilibrium curves presented by the SensArray method. Final conditions determined for the SFB2500 Cool Plate Temperature Side Plate Temperature Dome Gap Nitrogen Curtain

23.5o C 112oC 1.2mm 350 cc/min

While our first internal verification of the linkage between dark loss and chrome CD uniformity was performed using TOK REAP200; our subsequent optimization of the SFB2500’s PEB plate used TOK REAP122. General process conditions and coating information can be found in the author’s prior publications4.

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The develop sequences Program 7

Program 9

Program 13

Streaming Aqueous Prewet 120s

Streaming Aqueous Prewet 120s

Streaming Aqueous Prewet 120s

Puddle Initial Contacting with 1.91wt% TMAH (