Akihiro Otaka

The Effect of an Organic Base in Chemically Amplified Resist on Patterning Characteristics Using KrF Lithography

A new resist system composed of an SEPR chemically amplified (CA) positive resist and an N-methyl pyrrolidone (NMP) organic base has been developed for KrF excimer laser lithography. Using 0.30-µ m l&s patterns formed with KrF stepper, we studied the effect of contamination from substrate films of plasma chemical vapor deposition silicon dioxide (P-CVD SiO2), low pressure CVD silicon nitride (LP-CVD Si3N4) and reactive sputtered titanium nitride (TiN), and of airborne contamination under the condition of an 8-ppb of ammonia. The results clarify the new resist system reduces the effect of substrate film as well as airborne contamination. The new resist system enables us to form fine patterns on any substrate and attains more than one hour post-exposure delay without overcoat and undercoat films.

Hole pattern fabrication using halftone phase-shifting masks in KrF lithography

A new concept for enhancing the depth of focus of fine hole patterns is proposed. The concept is based on modulating the complex amplitude of the hole pattern image. It is shown that shifting the origin of the complex amplitude plane enhances the depth of focus. Simulations show that the resolution limit improves to 0.5 λ/NA with the depth of focus of ±1.0 λ/NA2. It is also shown that halftone phase-shifting masks can be used to shift the origin. The optimum intensity transmittance of the halftone region for maximizing depth of focus is 17.3% for a 0.6 λ/NA hole pattern. Using the optimized halftone phase-shifting mask, a quarter-micron hole pattern can be fabricated with the large depth of focus of 1.4 µm in KrF lithography. Proximity effects for holes arrayed in a row are also discussed.

Sub-quarter micron logic-gate-pattern fabrication using halftone phase-shifting masks

A new technique for enhancing the resolution of isolated line patterns is proposed. It is based on the balance in the intensity and the phase between the 0th-order rays and the rays diffracted from a line pattern. We deduce this technique from a condition of a high-contrast image by using the complex amplitude. We show that this technique can be achieved with an optimized halftone phase-shifting mask and optimized off-axis illumination. The optimum transmittance of the mask is 30% for a 0.4-λ/ NA line pattern and the optimum illumination for isolated lines in the x and y direction is quadrupole-shaped illumination whose centers are (±0.4, ±0.4). We tested this method in KrF lithography with an 8% halftone phase-shifting mask and found that a complex 0.20-µ m gate pattern can be fabricated with a large depth of focus of 1.2 µ m.

New resist technologies for 0.25-μm wiring pattern fabrication with KrF lithography

A new resist system composed of a chemically amplified positive resist and an organic base designed to be compatible with an acid generator has been developed to improve resolution on TiN films. It was clarified that the non-uniformity of the organic base like the acid generator in the resist film significantly contributes to high resist performance. A new hardening method has also been developed to improve the thermal stability of resist patterns for metal etching. Using the new technologies, 0.25-µ m line and space (l&s) resolution on TiN films and highly accurate metal etching have been achieved.

Effect of lens aberration on hole pattern fabrication using halftone phase-shifting masks

In hole pattern fabrication using halftone phase-shifting masks (HT-PSM), we found that the spherical aberration has a large influence on the usable depth of focus for the full field (uDOF). Spherical aberration shifts the best-focus position, and the shift is 9 times larger for the HT-PSM than that for a conventional Cr mask. When the variation of the spherical aberration in the exposure field is more than 0.1 lambda, the uDOF of a 0.6-lambda/NA hole pattern using the HT- PSM becomes smaller than that using the conventional Cr mask. To eliminate the influence of the spherical aberration, we investigated the effect of shifting the phase of the HT-PSM from 180 degrees. Based on the results, we developed a new HT- PSM in which a phase distribution varies in accordance with the spherical aberration at each position of the field. We tested the new mask and showed that the new mask improves uDOF and that it is effective in the fabrication of fine hole patterns.

New spatial frequency doubling method for sub-0.15-um optical lithography

A new method for fabricating fine periodic patterns with a large depth of focus is proposed. It is based on flattening the 1st-order intensity distribution and extracting the 2nd-order intensity distribution which has a frequency twice that of the mask pattern. The resolution limit is improved up to the cutoff frequency of the projection system and the image can be created independently of the defocus and the pattern direction. This method is achieved by superposing the images at two focal planes under coherent illumination and it was demonstrated by KrF lithography with a conventional binary mask. A 0.14 micrometers lines-and- spaces pattern was fabricated with a large depth of focus of 2.0 micrometers . A contrast enhancement technique and pattern edge correction method needed for actual application of this method in device pattern fabrication are also discussed.

Spatial frequency doubling method by image superimposition for sub-0.15-μm optical lithography

A new spatial frequency doubling method by image superimposition (FREDIS) is designed to improve the resolution limit up to the cutoff frequency. With FREDIS, the periodic image with half the pitch of the mask pattern is fabricated independently of the defocus and the pattern direction. It is based on flattening the 1st-order intensity distribution and extracting the 2nd-order intensity distribution whose frequency is twice that of the mask pattern. This is achieved by superimposing the images at two focal planes under coherent illumination. Experiments using KrF lithography with a conventional binary mask show FREDIS can be used to fabricate 0.14-µ m lines-and-spaces pattern with a large depth of focus of 2.0 µ m. In FREDIS, imaging characteristic is affected by some parameters such as the distance between two focal planes and the coherence factor of the illumination and show the effects of these parameters. We also discuss two techniques that help make this method more suitable for replicating actual device patterns. One enhances the contrast and the other corrects the image of the periphery of a periodic pattern.

Spatial frequency doubling method for sub-0.15-μm optical lithography

A new method for fabricating fine periodic patterns with a large depth of focus is proposed. It is based on flattening the 1st-order intensity distribution at the image plane and extracting the 2nd-order intensity distribution which has double the spatial frequency of the mask pattern. The resolution is improved up to the incoherent cutoff frequency (2NA /λ) and the image can be created independently of the defocus and the pattern direction. This method is achieved by superimposing optical images with two focal planes under coherent illumination. Using this method, a 0.14-µ m lines and spaces pattern can be fabricated by KrF lithography with a large depth of focus of 2.0 µ m.