Kenichi Murooka

IMPROVEMENT OF THE YOUNG'S MODULUS OF SIC FILM BY LOW-PRESSURE CHEMICAL VAPOR DEPOSITION WITH B2H6 GAS

SiC is one of the potential materials for use as an x‐ray mask substrate. It is preferable for an x‐ray mask substrate to have a large elastic modulus, so as to suppress any distortion of the extremely minute and precise patterns. The improvement of the Young’s modulus of polycrystalline SiC film using low‐pressure chemical vapor deposition with the introduction of B 2H6 in the source gas was investigated. The Young’s modulus increased with the addition of B2H6, and a maximum value of 600 GPa, which was 25% higher than in the case without B2H6, was reached at a source gas ratio B/Si=0.02. Two models which would possibly explain this phenomenon are discussed, and the theory which takes into account the interaction between carrier and ion core is found to provide a more plausible explanation of the results.

Solution for 100 nm: EBM-4000

Optical lithography will be extended down to 65nm to 50 nm. However, a mask with high accurate CD uniformity and resolution enhancement technology (RET) such as optical proximity effect correction (OPC) and phase shifting mask (PSM) are required to achieve resolution by exposure wave length. The mask technology is the key of the optical lithography extension. We developed the electron beam mask writer EBM-3000 for 180-150nm design rule 1), 2), and EBM-3500 for 150-130nm design rule 3), to achieve high accuracy CD uniformity mask and small OPC pattern writing. They were variable shaped electron beam mask writing system with continuous moving stage, at 50kV acceleration voltage, and had the functions of multi-pass field shift writing, real-time proximity effect correction, grid matching correction, and automatic adjustment for election optical column.The LSI road map calls for such small minimum feature size as that so close to optical resolution limitation where increasingly complex optical proximity corrections (OPC) as well as extremely good mask CD uniformity are required. What is making the challenge even more difficult is that writing time is exponentially increasing as the shot number is exploding to primarily cope with the complex and voluminous OPC and extremely good CD uniformity requirements. Thus the newly developed electron beam mask lithography system EBM-4000 is designed to overcome all these difficult problems associated with 100nm as well as 70nm node masks. In order to increase throughput, triangle/rectangle beam optical column, high current density/high resolution lens, and high speed DAC amplifiers have been developed. To achieve accurate CD uniformity, foggy electron correction/loading effect correction functions are developed.

Evaluation of Replicated Dynamic Random Access Memory Cell Patterns using X-Ray Lithography

We have evaluated the exposure latitude and the mask linearity of the lines and spaces (L/S), the isolated lines and the cell patterns used for DRAMs for various sizes down to 0.12 µ m. The exposure latitude was larger than 10% for all types of patterns with sizes larger than 0.12 µ m, using a mask of contrast of 3.7 in a mask-to-wafer gap of 20 µ m. We found that the mask linearity is insufficient and that mask pattern bias is required. The image shortening effect in the cell pattern was also evaluated. The results showed that the value of the shortening is higher when the design rule is smaller, the pattern density is sparser and the mask-to-wafer gap is larger. The value of the shortening in a 0.12-µ m- design- rule cell pattern was 13 nm, which is small enough to be corrected by simply giving a bias in the mask pattern.

The Effects of HCl Added to Chemical Vapor Deposition Source Gases for Producing a SiC X-Ray Mask Membrane

The effects of adding HCl to the chemical vapor deposition source gases on the stress, optical transparency, and surface roughness of a SiC X-ray mask membrane were examined. It was found that the stress dependence on the source gas carbon-to-silicon ratio was changed by adding HCl, and that a SiC membrane with low stress and high optical transparency is obtainable by adjusting these parameters. The surface roughness was about 15 nm from peak to valley under a good condition. The X-ray diffraction results of the SiC membranes showed that the peak sharpness, which indicates the crystal qualities such as densities of various intrinsic defects and the size of each crystal grain, is related to the optical transparency, and that the crystal orientation parameter is related to the surface roughness.

Initial results of a 50 kV electron beam writer EBM-4000 for a 90 nm node photomask

We have developed new 50 kV electron beam writer, EBM-4000, which is designed to fulfill the requirements for the 90 nm node mask. EBM-4000 makes use of the assets of our previous model, but major modifications are carried out on electron optics, writing circuits, and vacuum system. The written patterns are evaluated about CD uniformity, image placement accuracy, and throughput. The results indicate that CD uniformity and image placement accuracy meet the requirements for 90 nm node photomask, and EBM-4000 has a potential to extend its feasibility to 65 nm node photomask with acceptable throughput.

Growth rate and deposition process of silicon carbide film by low-pressure chemical vapor deposition

The growth rate and deposition process of polycrystalline silicon carbide by low-pressure chemical vapor deposition with the introduction of hydrogen chloride gas were investigated. Analogous to the case of silicon, the growth rate was well described by a model that included nucleation of particles in the gas phase, a change of the reaction species from hydrides to chlorides, and etching or inhibition of growth by hydrogen chloride. The film stress was considerably influenced by the introduction of hydrogen chloride, which is thought to change the reaction path.

Stress Control and Etching Study of W-Re as X-Ray Mask Absorber.

Stress control and etching of W-Re as X-ray mask absorber was studied. The W-Re film was deposited using an rf magnetron sputtering apparatus with two material targets. The substrates were revolved by themselves and over the W and Re targets to achieve uniform film thickness and stress. When the content ratio of W to Re was 1:1, non-columnar film structure was obtained. The stress repeatability, when the deposited film stress is near zero, was ±5 x 10 8 dyn/cm 2 . To further control the stress, implantation of Ar ions was carried out with an acceleration voltage of 180 kV. The stress change was 1 x 10 9 dyn/cm 2 compressive at the ion dose of about 6 x 10 15 ions/cm 2 . The etching of W-Re was carried out using SF 6 and CHF 3 gas mixture and a Cr etching mask by applying magnetron reactive ion etching. The etching selectivity between W-Re and Cr was over 250 in the CHF 3 content below 60%. We applied two-step etching to remove etch-obstructing films on the W-Re surface, and fabricated 0.1 μm lines and spaces of 0.5-μm-thick W-Re film.

Improvement in Optical Transparency of SiC Membrane by Modulating Source Gas Ratio in Chemical Vapor Deposition

A method to improve the optical transparency of a SiC membrane is studied by modulating the source gas carbon-to-silicon ratio (C/Si) during chemical vapor deposition. As a result of an investigation conducted under different C/Si conditions, the optical transparency was found to be highest at C/Si=0.9. Optical transparency decreased at lower C/Si probably due to the increase of surface roughness, and at higher C/Si due to the degradation of crystal quality. By alternate deposition under the condition of C/Si=0.8 and 0.95, the SiC membrane with an optical transparency of 68% was obtained without applying an antireflection coating.

System optimization of membrane mask distortion correction based on Fourier analysis

We have studied the distortion correction scheme of a membrane mask in the spatial frequency domain by using Fourier analysis. The relationship between stress and distortion tells us that distortion is inversely proportional to spatial frequency, which means the high spatial frequency component of the stress has little effect on the distortion. Also, the Fourier analysis of the thermal diffusion compensation tells us that the heat input is required to be large as the spatial frequency becomes large, i.e., the upper limit of the heat input power gives the upper limit of the correctable spatial frequency in the distortion correction. Assuming that the total heat input power is 100 W, and the membrane is 1 μm thick SiNx in air, it turned out that the acceptable stress in low spatial frequency region is about 200 MPa, and the distortion correction system is required to have only the spatial frequency corresponding to the wavelength of 0.26 mm or more, which can be easily implemented with a commercially availa...

X-ray phase-shift mask for proximity X-ray lithography with synchrotron radiation

In proximity X-ray lithography at the feasible gap size of approximately 10 micrometer, using attenuated phase-shift masks is the most effective method of achieving high resolution pattern transfer at the feature size of sub-100 nm. In this study, we have investigated the absorption and the phase-shift controllability of X-ray masks with various absorber materials by simulation and found that the phase- shift mask structure with Cu absorber is one of the best choices for proximity X-ray lithography using synchrotron radiation.