Makoto Nakase

New phase shifting mask with self-aligned phase shifters for a quarter micron photolithography

In order to markedly improve the resolution of photolithography without improving the resolution of exposure systems, the authors propose a simple and effective phase shifting mask technology. The mask has self-aligned phase shifters which do not require assistant patterns and/or complicated design of the phase shifter patterns, which are essential to the conventional phase shifting mask. The mask with a phase shifter size of 0.5 mu m reduces the width of photointensity to 60% of that without phase shifters, while keeping high contrasts. The authors have fabricated the phase shifting mask and obtained 0.2- mu m line resist patterns with a high-contrast resist profile by a KrF excimer laser stepper with resolution capability of 0.4 mu m. The proposed phase shifting mask method is extremely attractive for a future ULSI lithography tool in 256-Mb DRAM (dynamic RAM) and beyond.<<ETX>>

Simulation Study on Phase-Shifting Masks for Isolated Patterns

Optimization for various phase-shifting masks to fabricate 0.25-µm isolated resist patterns was carried out, and their performances were compared among themselves by simulation. They were evaluated under the assumptions that isolated patterns were located on the same layer with 0.25-µm lines and spaces with the Levenson-type phase-shifters and that the exposure dose was adjusted to the dense patterns. It was found that isolated patterns need to be designed with regard to unavoidable errors in the feature size. None of the isolated line masks studied is effective in improving resolution, except in the case where the Levenson-type phase shifter (shifter edge) can be located. All isolated space masks with optimized dimensions, in contrast, are effective in improving depth of focus for the 0.25-µm isolated pattern.

Effect of basic additives on sensitivity and diffusion of acid in chemical amplification resists

The effect of amine additives in chemical amplification resists is discussed. Phenolic amines such as 4-aminophenol and 2-(4-aminophenyl)-2-(4-hydroxyphenyl) propane were investigated as model compounds from the viewpoint of sensitivity, diffusion and resolution. Equal molar amounts of acid and amine deactivated at the very beginning of post-exposure bake, and could not participate in decomposing the inhibitor as a catalyst. Only the acid which survived from the deactivation diffuses in the resist, decomposing the inhibitors from the middle to late stage of PEB. The basic additives reduce the diffusion range of the acid, especially for long-range diffusion, resulting in higher contrast at the interfaces between the exposed and unexposed areas. In addition, the amine concentration required is found to be less than the concentration which causes the resist sensitivity to start decreasing.

Mechanism of amine additive in chemically amplified resist visualized by using Monte Carlo simulation

A novel simulator using the Monte-Carlo method, which simulates molecular-like movements and reactions in chemically amplified resist systems, has been developed in order to clarify the mechanism of amine additive. The simulation program simulates diffusion phenomena with random-walked acids and amine molecules, and reactions of deactivation (neutralization) and inhibitor cleavage in every movement of the molecules. It was found that the amine in the exposed area was rapidly deactivated, and the concentration profile of the remaining amine was rapidly changed to an inversely shaped profile of that of the acid. This means that the remaining amine prevents diffusion of acids outside the exposed area by functioning as a barrier during almost the whole period of post-exposure bake (PEB). These concentration profiles lead to an enhancement of stability of pattern sizes and reduction of edge roughness of the pattern. Environmental stability was also calculated, and amine additive was also found to prevent contamination from the environment in the initial period of the PEB.

Highly transparent chemically amplified ArF excimer laser resists by absorption band shift for 193 nm wavelength

Naphthalene-containing chemically amplified resists for ArF excimer laser exposure are proposed, based on the concept of the absorption band shift by conjugation extension. Newly developed ArF excimer resists show a high transparency at 193 nm wavelength, a high sensitivity and a high contrast. The sensitivity of the resist is 150 mJ/cm2, which is 20 times greater than that of poly(methylmethacrylate) (PMMA). Furthermore, a 0.16 µ m pattern could be successfully fabricated by an ArF excimer laser stepper with 0.55 numerical aperture (NA) projection lens.

Optimization of partially coherent optical system for optical lithography

A new approach, based on the optimization algorithm of ‘‘simulated annealing,’’ is applied to maximize depth of focus for a partially coherent optical system. Optimization is carried out with practical constraints for optical lithography. A phase contrast lithography, consisting of an annular effective source and an annular phase filter on the pupil, is proposed as a definite result of the optimization procedure.

Submicron Optical Lithography Using A KrF Excimer Laser Projection Exposure System

The potential of excimer laser lithography was studied by using a newly developed KrF excimer laser exposure system which employed an achromatic lens of 0.37 NA. As a result, a resolution limit of 0.3 μm was achieved by the use of PMGI resist on a tri-level structure. However, for the case of resist exposure on a bare Si wafer, the resist film remailed locally in layers along the nodes of the standing waves, and fine pattern could not be obtained. This phenomenon is called spotted development in this paper. The spotted development, which is due to strong standing waves within the resist film, was successfully solved by the use of new resist process technologies such as a bias exposure method and excimer laser image reversal process.

Effect of remaining solvent on sensitivity, diffusion of acid, and resolution in chemical amplification resists

The effect of the remaining solvent on the diffusion of the catalytically active acid within the chemical amplification resists is discussed. A range of solvents was investigated from the viewpoint of sensitivity, diffusion, and resolution. The solvent remaining after the coating and baking processes is thought to one of the most probable factors among the several possible causes in the determination of the performance of the resists. It is also considered to result in the environmental instability which is currently one of the most serious problems in the use of the resists of this type. The remaining solvent drastically enhances the thermal diffusion of acid generated from photo‐acid generators upon photoanalysis. The log of the diffusion range of the acid is found to be proportional to the concentration of the remaining solvent. This phenomenon may be described by a theory which combines the hole theory of liquids and the free volume theory of polymers and was also found to be the correlation energy be...

KrF Excimer Laser Projection Lithography: 0.35 μm Minimum Space VLSI Pattern Fabrication by a Tri-Level Resist Process

An excimer laser exposure system has been newly developed, which adopts a 0.37 NA achromatic projection lens and a 248 nm wavelength KrF excimer laser as a light source. Irregular PMMA development was observed for exposures on a silicon substrate. Theoretical simulation clarified that an initial 0.02 µm thickness variation caused more than 0.16 µm thick resist residues due to intense standing waves. The standing waves could be suppressed by the use of a tri-level resist process so that VLSI patterns of 0.35 µm minimum space could be obtained.

Optimization of a high-performance chemically amplified positive resist for electron-beam lithography

We report on sub-0.1 μm electron-beam (EB) lithography using a new chemically amplified positive resist with a stabilizing additive. Diphenylamine (DPA) was incorporated into the resist formulation as a stabilizing additive. DPA improves the post-exposure delay (PED) stability. Even after a PED of 60 min in a clean-room atmosphere (NH 3 concentration ∼20ppb), no insoluble surface layer was observed for the resist with DPA. High sensitivity and resolution could be achieved by optimizing process conditions such as baking and developer. 0.1 μm lines-and-spaces (L/S) patterns and 0.08 μm hole patterns were obtained using a 50 kV variably shaped beam EB system. The practical sensitivity was 6 μC/cm 2 for L/S patterns. Our resist system also shows good performance as an etching mask. A 0.1 μm diameter hole which was 0.5 μm etched in silicon oxide using a 0.5 μm thick resist could be obtained in this experiment.