Kei Hayasaki

Defectivity reduction studies for ArF immersion lithography

Immersion lithography is widely expected to meet the manufacturing requirements of future device nodes. A critical development in immersion lithography has been the construction of a defect-free process. Two years ago, the authors evaluated the impact of water droplets made experimentally on exposed resist films and /or topcoat. (1) The results showed that the marks of drying water droplet called watermarks became pattern defects with T-top profile. In the case that water droplets were removed by drying them, formation of the defects was prevented. Post-exposure rinse process to remove water droplets also prevented formation of the defects. In the present work, the authors evaluated the effect of pre- and post-exposure rinse processes on hp 55nm line and space pattern with Spin Rinse Process Station (SRS) and Post Immersion Rinse Process Station (PIR) modules on an inline lithography cluster with the Tokyo Electron Ltd. CLEAN TRACKTM LITHIUS TM i+ and ASML TWINSCAN XT:1700Fi , 193nm immersion scanner. It was found that total defectivity is decreased by pre- and post-exposure rinse. In particular, bridge defects and large bridge defects were decreased by pre- and post-exposure rinse. Pre- and post-exposure rinse processes are very effective to reduce the bridge and large bridge defects of immersion lithography.

A novel post exposure bake technique to improve CD uniformity over product wafers

The impact of wafer warpage on critical dimension (CD) control is getting larger in ArF lithography. The product wafers with stacked films are warped due to the stress caused by the difference in the film stack structure between the top side and the back side of the wafers. A typical warpage of the product wafers is of convex shape, and the amount of the warpage is larger than 50 μm for 200mm wafer. On the other hand, proximity bake method is widely used in the Post Exposure Bake (PEB). When the warped wafer is placed on the hot plate, the gap between the wafer and the hot plate varies across the wafer. That is, the temperature of the wafer center is lower than that of wafer edge. Such a temperature variation affects CD uniformity within wafer. In particular the fact is obvious in ArF chemical amplified resist because PEB sensitivity of ArF resist is larger than 5nm/degree. In this study we optimize PEB zone temperature within wafer to suit the wafer warpage. This method is based on controlling zone temperature of the PEB hot plate with concentrically divided heaters. We carry out that the CD uniformity for the warped wafer is improved by 70% compared with the conventional process.

Dissolution performance of device pattern with low-impact development

For low-k1 lithography, high accurate control of the development process is required. For that purpose, low- impact dispensing is one of the most effective approaches. In that process, development time differs between start and end position of nozzle-scan. To reduce the time lag, the nozzle-scan-speed of 140mm/s was selected. But critical dimensions (CD) offset that depends on scan-direction was detected. From the results of the CD and dissolution performances for three resists, we found that the pull-back flow of the developer was the main cause of the CD offset. Thus, it is important that the developer does not flow by its pull-back-force. By observing and analyzing the flow of the dissolution product with a video camera, the best condition of the scan-speed (=60 mm/s) was selected. Under this nozzle-scan condition, the dissolution rates did not depend on the scan-direction of the dispenser-nozzle. As a result, the small CD offset could be observed for 200nm L&S patterns.

How to separate CD variation on local pattern density into error sources

We propose a methodology that separates critical dimension (CD) variation depending on local pattern density into error sources; CD error on exposure mask, mid-range flare of exposure tool, and acid evaporation during post exposure baking (PEB). This methodology consists of two particular processes. One is over-coating process onto resist film before exposure, and the other is double exposure process using test reticles. The test reticles have line-and-space (L/S) array region and peripheral region that contains opaque area to avoid overlapping with L/S array region in double exposure process. Over-coating process allows separation of acid evaporation from other error sources. On the other hand, double exposure process enables elimination of CD error on exposure mask depending on local pattern density since double exposure process can make various pattern densities around fixed L/S mask pattern. In fact, the experimental CD results could provide the good agreement with estimated CD results using mid-range flare model. It has become clear that the influence of local pattern density increases with design rule shrinkage. Furthermore, the methodology revealed that the influence of each error source is greatly dependent on local pattern density. Consequently, the methodology is effective to separate CD variation on local pattern density into error sources.

Dependence of ArF resist on exposed area ratio

We investigated dependence of ArF resist on Exposed Area Ratio (EAR). Because it can be one of the CD variation factor and it is difficult to correct by OPC. Acrylate polymer based resist showed dependence on EAR. At low EAR, resist showed T-top profile and its CD became large. It could be considered that the profile change was caused by acid evaporation and re-sticking. Resist profile simulation indicated that CD variation appeared at only low EAR. To decreasing the effect of acid evaporation and re-sticking, we tried to increase the amount of acid evaporation by increasing PAB temperature. CD variation by EAR was decreased with increasing PAB temperature.

The effect of gel layer formation during development on critical dimensions

Patterns unfaithful to the aerial images were obtained by the spin-off development method, in which a spin-off step is added to the static development method. The critical dimension (CD) of the isolated line decreased more than that of the dense line as wafer rotation speed increased. The CD difference between the isolated line and the dense line was not due to the alkaline concentration recovery by the stirring effect of the spin-off development method and the aerial image difference between pattern types. It was due to the difference in the ease of removing the semi-dissolving layer, which was observed to cover the sidewall of resist lines, at the boundary between developer and resist during spin-off. The developer flow along the wide space area and the wide area of the layer contacting developer during spin-off are thought to make the CD of the isolated line small. The mechanical removal of the semi-dissolving layer is one of the factors to determine CDs.

Novel development technique using ozonated water

Two kinds of development processes were investigated. One is two-step development in which surface treatment using ozonated water was employed between the two steps of the development. The other is development in which ozonated water and hydrogenated water were employed in the pre-treatment step and the post-treatment step. The above-mentioned processes were applied to KrF resist process of 130nm generation. By pre-treatment using ozonated water and two-step development using ozonated water in inter-treatment, the shot-to-shot CD variation of isolated line (line width = 180nm) and the intra-shot variation were improved from 6.6nm to 4.4nm and from 13.5nm to 8.6nm, respectively. And the total variation was greatly improved from 15.0nm to 8.6nm. Moreover, the number of defects was greatly decreased by post-treatment using ozonated water and hydrogenated water continuously.

The effect of gel layer formation during development on CDs

As fine patterns with small deviation are required, it is necessary to understand the resist development mechanism in order to control critical dimensions (CDs). When the sequence in which wafer rotation process was added to the static development was applied to line patterns, isolated lines and dense lines differed in their CD change behavior. This phenomenon was investigated from the viewpoint of the gel layer.

The effect of mixing development method on CD uniformity

It is required that the wafer size is bigger and the pattern size is finer, Bigger wafer size needs the CD uniformity in wider area and finer pattern size needs more uniform CDs. Mixing development method was found to improve CD uniformity. The effect will be reported.

High-accuracy development monitoring technology

A development monitor system capable of highly accurate control of pattern width has been established. This system is composed of a unique monitor pattern on the process wafer, the 0th order diffraction light measuring unit, and the image analysis and process control unit. In the conventional development process in which no monitor system is employed, the CD variation in 200nm line width was about 15nm when +/- 5 percent dose error exist. However, using the new system, 1nm of CD variation was obtained. In this article, a high-sensitivity monitor pattern is proposed and its performance in controlling 200nm line and space patterns in the development process is reported.