Minoru Yoshii

Multilevel imaging system realizing k1=0.3 lithography

The pursuit of ultimate resolution by optical lithography has given rise to many new technologies, such as PSM, oblique illumination etc. In order to realize the benefit of these new technologies in practice, a new exposure technology IDEAL is proposed. First exposure is for fine patterns, which are imaged with high contrast and large depth of focus, while second exposure is done with multileveled light distribution. These two exposures collaborate each other to form fine patterns with reasonable focus margin and good 2D profile. Experimental result of logic gate patterns are shown and demonstrate the possibility of k1 equals 0.3 lithography. Using IDEAL, KrF lithography can be extended to 100-110 nm and ArF to 80 nm resolution.

Assessment of trade-off between resist resolution and sensitivity for optimization of hyper-NA immersion lithography

The resist blur due to photoacid diffusion is a significant issue for 45-nm half-pitch node and beyond. Furthermore, it has been generally recognized that there is a trade-off between resist resolution and sensitivity. In this paper, we study the influence of the resist blur on resolution and sensitivity in hyper-numerical aperture ArF immersion lithography by utilizing a two-beam interferometric exposure tool. We evaluated the current photoresist performance for some of the latest commercial resists, and estimated their acid diffusion lengths as 8 to 9 nm in sigma assuming Gaussian blur kernel. In addition, we found that the acid diffusion length, that is, the resist resolution was controllable by PAG anion size, polymer resin size, and PEB temperature. We also found that there was the trade-off between resist resolution and sensitivity. Our results indicated that the resist blur is still a concern in order to extend ArF lithography for 45-nm half-pitch node and beyond, however, it will not likely be a showstopper. We consider that total optimization of resists and exposure tools is important in order to achieve ultimate resolution in hyper-NA immersion lithography.

ArF immersion lithography: critical optical issues

We present selected results of our feasibility study on ArF Immersion lithography from the viewpoint of the exposure-tool development. First, we show that utilizing finite bubble lifetime in degassed water can eliminate air bubbles that are generated by wafer scanning. Second, it is shown that thermal fluctuation of immersion liquid as well as vectorial diffraction effect from the mask is not significant in terms of imaging performance. Third, we demonstrate resist imaging of 60-nm and 45-nm line-and-space patterns in interferometric exposure experiments with an ArF laser at the power level of the actual exposure tools. Fourth, the increase of the depth of focus is confirmed using an alpha exposure tool of ArF immersion. All these results indicate that the ArF immersion lithography is promising for 65-nm half-pitch node and beyond.

What determines the ultimate resolution? The critical relationship between exposure tools and photoresists

As the resolution of optical lithography is being pushed for 45-nm half-pitch node, there is a growing concern about the printing capability of chemically amplified resists. The chemical amplification involves photoacid diffusion that causes contrast degradation of latent image or, in other words, resist blur. In this paper, we study the influence of the resist blur in high-NA ArF immersion lithography by using an interferometric exposure tool. Contrast ratio between the resist latent image and the original aerial image was measured for half pitch from 50 nm to 80 nm. Acid diffusion length for a high-resolution ArF resist was determined as 11 nm in sigma (26 nm in full width at half maximum) assuming Gaussian blur kernel. The results revealed that the influence of resist blur is in fact a significant issue for the 45-nm half pitch node. We consider that reduction of acid diffusion length is highly desirable. Given the tradeoff between the resist resolution and sensitivity, increasing illumination intensity in the exposure tools can be an effective means to overcome the challenge of the resist blur. We also demonstrate resist imaging of 30-nm line-and-space pattern with high-index immersion fluid. The reduction of acid diffusion will be even more important if the ArF immersion is to be extended beyond 45-nm half-pitch node with high-index fluids. While the focus of the paper is on high-NA ArF immersion lithography, our findings are also relevant to EUV lithography.

Influence of resist blur on ultimate resolution of ArF immersion lithography

The chemical amplification provides high sensitivity of resists for deep-uv and extreme-uv (EUV) lithography. On the other hand, the chemical amplification involves photoacid diffusion that causes contrast degradation of the latent image or, in other words, resist blur. We study the influence of the resist blur in high-numerical aperture ArF immersion lithography by using an interferometric exposure tool. The contrast ratio between the resist latent image and the original aerial image was measured for half pitches from 45 to 80 nm. Acid diffusion length for a high-resolution ArF resist was determined as 11 nm in sigma (26 nm in full width at half maximum) assuming a Gaussian blur kernel. The results revealed that the influence of the resist blur is a significant issue for the 45-nm half-pitch node. The reduction of acid diffusion length is highly desirable. Given the tradeoff between the resist resolution and sensitivity, increasing illumination intensity in exposure tools can be an effective means to overcome the challenge of the resist blur. We also demonstrate resist imaging of 30-nm line-and-space pattern with a high-index fluid. While our focus is on ArF immersion lithography, our findings are also relevant to EUV lithography.

New approach for realizing k1=0.3 optical lithography

A new exposure technology called IDEAL (Innovative Double Exposure by Advanced Lithography) which realizes k1 equals 0.3 optical lithography is introduced. In IDEAL exposure method, rough pattern mask and fine pattern mask are used. The rough pattern mask contributes to expand the degree of freedom in two-dimensional patterning and the fine pattern mask contributes to higher resolution and focus DOF enhancement. As an actual example, 120 nm gate array is formed using KrF 0.63 NA stepper. It has been confirmed that the double exposure method is effective in the pattern formation of irregularly arranged contact hole arrays of 150 nm. Furthermore, it is also shown that double exposure method is effective in the reduction of mask error enhancement factors (MEF).

New Ellipsometric Approach to Critical Dimension Metrology Utilizing Form Birefringence Inherent in a Submicron Line-and-Space Pattern

A new ellipsometric approach to critical dimension (CD) metrology is proposed, which utilizes the form birefringence of the submicron line-and-space (L&S) patterns of a photoresist. With the continued requirement of further narrowing of the period of the L&S to dimensions less than the probing wavelengths, higher-order diffraction will no longer occur and the 0th-order diffraction beam will be the only tool for CD metrology. Since ellipsometry uses this 0th-order diffraction beam, the method is promising for use in the evaluation of L&S patterns in the subwavelength region. We have been able to relate the large variation in the ellipsometric parameter to the form birefringence of the submicron pattern. Using the higher-order effective medium equations for the ordinary and extraordinary refractive indices, the ellipsometric parameter changes due to the linewidth changes are calculated. The calculated results agree well with the experimental results. Furthermore, the linewidths are determined from the best fit between the calculated and experimental data. The same is done for the latent images of the undeveloped resist patterns.

Practical applications of IDEAL exposure method

IDEAL (Innovative Double Exposure by Advanced Lithography) has been introduced as a new double exposure technique to realize k1 equals 0.3 optical lithography. IDEAL uses a rough pattern mask with patterns close to the actual device design and a simple fine pattern PSM to resolve very high contrast images on a wafer. IDEAL can be applied to complicated two dimensional patterns for actual device such as double, rectangular or T-shaped gate patterns. Results of IDEAL on different pattern types are shown. IDEAL significantly reduces MEF (Mask Error enhancement Factor). At various rough and fine dose ratios, IDEAL demonstrates the advantage especially at fine linewidths below 150 nm where the MEF of single conventional exposures increase sharply. Our extensive calculation of MEF with various patterns and experiments on complicated two dimensional patterns further confirm that IDEAL is a practical method in advanced device manufacturing.