Yuichiro Yanagishita

Phase-shifting photolithography applicable to real IC patterns

A phase-shifting technique which simplifies mask fabrication and is applicable to actual IC patterns has been introduced into the i-line positive resist process. It combines edge-contrast enhancement and a chromeless mask. Although the effect of this technique on line and space patterns has turned out to be more restricted than that of the alternating mask technique, it can improve exposure and focus latitude in isolated hole patterning. The authors report on their estimation of the optimum shifter width which maximizes contrast enhancement on lines and spaces as well as on isolated hole patterns. Experimental data is presented to verify the improvements in photolithographic performance of isolated hole patterning due to this technique.

Mask pattern designing for phase-shift lithography

The authors have developed a mask pattern designing algorithm for a phase-shift lithography process. This algorithm produces pattern dimension linearity regardless of the pattern size and does not requires any special CAD (computer-aided design) technique. The authors also developed a mask fabrication technique with self-aligned shifters. One can avoid the alignment problem of chromium and shifter pattern by the self-aligning mask process.<<ETX>>

Method for accurately forecasting the effects of the "post exposure bake"

Post Exposure Bake (PEB) is one of the simple method for minimizing the influences of reflection from a substrate to a resist film. Although various references have been made to the heating process ,its mechanism has not been fully clarified yet and it has been difficult to forecast its effects accurately. To grasp PEB effects upon the resist performance exactely ,the next two kinds of change in the development rate by this process must be measured. (1) Change by PEB in the development rate curve for the remain of photoactive compounds. (2) Smoothing by PEB of the local development rate distribution. These data can be acquired by Development Rate Monitor. The first one is translated into the sets of the fitting parameters of the development rate curve in the lithography simulator. The second one will be represented by means of the thermal diffusion model. The change (2) has significant effects upon the lithography on highly reflective substrates (silicon, aluminum etc.), for these substrates generate large amount of standing wave in a resist film. Because the smoothing effect enhances the development contrast of photoresist, PEB process contributes to improvement of the lithography on these substrates. On the contrary, for low reflective substrates (silicon-oxide etc.) which give only small amount of standing wave the effects of (1) is relatively given more weight than the smoothing effect. Because the change (1) decreases the development contrast, PEB on low-reflective layers depresses lithographic performance. This report will simulate PEB effects, compare them with experimental data and clarify the relation between PEB performance and reflection of substrates.

Optimization of alternating phase shift mask structure for ArF laser lithography

An alternating phase shift mask (alt. PSM) must be fabricated in such a way that imbalances in optical intensities are minimized. The mask structure must be optimized to obtain a balanced distribution of optical intensities and this means that the shifter thickness/quartz depth that corresponds to a phase angle of 180 degrees and the correct amount of undercutting should be estimated. There are two key points in the optimization of an alt. PSM. One is to find the optimum structure in terms of reducing the amount of undercutting. Narrower chrome (Cr) line widths are required for ArF laser lithography than for KrF laser lithography, so the undercutting must be restricted to prevent peeling of the Cr patterns, degradation of cleaning durability, and so on. Another key point is to investigate the effect of Cr line widths and pattern pitches on imbalances in the optical intensities. A variety of pattern pitches and Cr line widths are available from actual devices. All patterns, however, have same shifter thickness and amount of undercutting on each mask produced by a given mask fabrication process. It is thus necessary to study the effect on optical intensities of changes in Cr line widths and pattern pitches so that it is possible to optimize mask structures for a variety of patterns. From our simulation and experimental results, we found that an alt. PSM with vertical sidewalls has advantages in terms of reducing the amount of undercutting and is effective in the fabrication of sub 100-nm devices. We also discovered that imbalances in optical intensities vary periodically with Cr line widths. It was found that a structure for an alt. PSM should be optimized for each Cr line widths on these bases.

Resist residue generated on TiN topographic substrates in positive chemically amplified resists

When positive chemically amplified resists are used on basic stepped substrates to produce patterns, a resist residue often forms at the bottom of the step. This paper discusses the results of investigating the cause of the chemically amplified resist residue formation as it is encountered during patterning on stepped substrates.