Yasuko Tabata

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>>

130-nm KrF lithography for DRAM production with 0.68-NA scanner

We have proposed a methodology for 130-nm DRAM patterning. We started by running a simulation to investigate the possibility of 130-nm DRAM production with KrF lithography. We optimized cell array features and isolate lines in the core circuits and peripheral circuits, corresponding to resist performance ((Delta) L). Using a half-tone phase-shift mask, off-axis illumination, and 0.68-NA KrF scanner, we found a high-performance resist of 40-nm (Delta) L that meets the requirement. Then, we screened resist samples using design of experiment. The result was a 40-nm (Delta) L positive resist that has small line edge roughness, a high- contrast resist profile, a small iso-dense bias and a low- blocking level to prevent defects. Finally, we applied this positive resist and OPC-mask to critical layers and achieved a sufficient production margin using a 0.68-NA KrF scanner.

Implementation of attenuated PSMs in DRAM production

We studied the use of attenuated phase shift mask (PSM) in DRAM production. There exists several problems with the use of an attenuated PSM compared to a conventional Cr mask. These include a need to form an opaque region, facilitate reticle alignment with a stepper, and optimize mask bias to prevent side peak printing. First, we investigated the characteristics of checkerboard patterns in achieving an opaque region. We confirmed the feasibility of making a mask to maintain opaqueness. Next we developed a mask fabrication process so to enable reticle alignment in some kinds of steppers by using an additional Cr layer under the attenuated layer. Finally, we tried to implement attenuated PSM in a previous generation stepper. We found that we must pay attention to lens aberration when optimizing mask bias.