Kenji Marumoto

Validity of double and triple Gaussian functions for proximity effect correction in X-ray mask writing

An improved function is proposed to express the deposited energy intensity distribution for fine patterns or that for heavy metals such as X-ray absorbers. An experimental method is also proposed for obtaining the function parameters, and the optimal parameters are obtained for Si and W–Ti substrates at acceleration voltage of 25 kV. Using the improved function with the best-fit parameters, the validity of the double and triple Gaussian functions for the proximity effect correction is evaluated. The dose modulation ratios for ninety line-and-space patterns on the two substrates are calculated using the double Gaussian, triple Gaussian, and improved functions. From the comparison among the ratios, the following are found: for the double Gaussian function the ranges of correction errors on W–Ti and Si substrates are 8 and 30%, respectively; for the triple Gaussian function the ranges of errors on W–Ti and Si substrates are 6 and 12%, respectively.

Low stress and optically transparent chromium oxide layer for x‐ray mask making

This article describes the properties of dc‐sputtered chromium oxide layers such as internal stress, density, growth rate, and microstructure. Besides these mechanical properties, the composition of the layers and the optical properties such as transmissivity and refractive index were also investigated. Transparent, low‐stress, and smooth chromium oxide layers were obtained by optimizing the sputtering conditions: gas pressure, O2 content in the working gas, dc‐power density, and annealing temperature. The most critical parameter is the O2 content in the working gas. Using these optimized chromium oxide layers in the mask fabrication process as a masking, as well as an etch stop layer for the W etching, the etching behavior during absorber patterning in a subtractive x‐ray mask production process was improved.

Fabrication of Diamond Membranes for X-Ray Masks by Hot-Filament Method

Diamond thin films have been grown on a Si substrate by a hot-filament method with mixtures of CH4 and H2, and properties such as stress and optical transmittance were investigated for free-standing membrane films as a function of methane concentration (0.5-2.0%) and deposition pressure (8-80 Torr). Ellipsometric and Raman scattering measurements were also carried out to evaluate the film structure, and graphite and void components. It has been found that the loss in the optical transparency is mainly due to light scattering at the surface. It is also shown that boron doping is useful for decreasing the film resistivity, which is advantageous for electron beam (EB) writing and inspections.

Fabrication of x‐ray masks for giga‐bit DRAM by using a SiC membrane and W–Ti absorber

Advanced x‐ray masks for giga‐bit DRAM require an optimization of the mask fabrication process and a stress‐free absorber. In this article, we will describe the performance of the x‐ray masks fabricated by a membrane process where back‐etch is performed prior to electron beam writing. With other techniques such as stress control by step‐annealing and electron beam (EB) multiple writing, mask‐to‐mask overlay was reduced to less than 40 nm. In spite of multiple EB writing, the pattern size accuracies of 0.14 μm critical dimension were about 10% and 20% for the line and two‐dimensional patterns, respectively. For further improvement in the uniformity of absorber stress and resultant higher placement accuracy, a space variant annealing method is proposed, where the nonuniformity of the absorber stress is corrected by spatially variant temperature distribution during the annealing. Under this method, the uniformity of the amorphous W–Ti absorber improved to about 10 MPa in a 50 mm diam region.

Electron Beam Writing Techniques for Fabricating Highly Accurate X-Ray Masks

The problems in electron beam writing for the membrane-process were investigated; the resist thickness on the thinned membrane was adequately uniform (3σ=0.44%), and the resultant pattern size error was negligible. Deformation due to mask chucking onto the electron beam (EB) cassette was small enough to allow production of acceptable gigabit ULSI devices. We also confirmed that the SiC membrane is durable enough for the task of membrane-process, even with thermal impact of EB energy 40 times larger than that used in conventional writing. Moreover, the improvement of pattern size accuracy by using multiple writing was investigated in detail under various conditions of the beam step size, the writing time and the resist sensitivity. The pattern width deviation was improved from 20% to 13% for 0.15 µ m line-and-space patterns by multiple writing, and was little dependent on the beam step size and the resist sensitivity.

Replicating characteristics by SR lithography

Process optimization and pattern replication on various substrates by synchrotron radiation lithography was carried out to evaluate the problems for 0.15 micrometers level resists patters for 1-Gbit dynamic random access memory. It was found that the exposure latitude was rather restricted by the resist residue remaining between lines (scum) and the pattern collapse than the normally used +/- 10-percent critical dimensions. A simple Fresnel diffraction calculation including the phase-shifting effect and mask contrast showed that the occurrence of the scum was mainly determined by the optical images of x-rays, and could not be significantly improved by the resist process condition. We used the mask/wafer proximity gap of 20 micrometers to get a good optical image and 5000-angstrom resist thickness to suppress the pattern collapse. On the other hand, the replicating characteristics on the light element substrates were similar to that on the Si substrate, especially good on the SiN substrate, but the residues caused by secondary electrons ont he metal substrates and the catalytic reaction on the Pt substrate were observed. It was shown that protection layers could suppress those residues and serve a good pattern profile.

Sputtered W-Ti Film for X-Ray Mask Absorber

Properties of DC-sputtered W-Ti absorber films such as internal stress, density and microstructure were systematically investigated for X-ray mask application. Smooth, stress-free amorphous W-Ti films with a comparatively high density of 16.5-17.0 g/cm3 were obtained by optimizing several conditions: gas pressure, N2 content in the working gas, DC power density and annealing temperature. The reproducibility of the film stress was about ±5×107 Pa. Reproducibility was found to be mainly determined by the degree to which the sputtering tool used could be controlled. Furthermore, it was confirmed that the optimized absorber films offered good stability under stress within 1×107 Pa when stored in an air atmosphere for more than 250 days.

Fine Pattern Etching of W-Ti Absorber for X-Ray Mask with Electron Cyclotron Resonance Discharge Plasmas

The process of etching an amorphous W-Ti absorber using a Cr mask and electron cycrotron resonance discharge plasmas was investigated to fabricate an X-ray mask with a resolution below 0.1 μm. Highly selective and anisotropic etching has been achieved using a mixture of SF 6 , CHF 3 and He gases, by cooling the stage to about -50 o °C and with the control of the plasma conditions to increase ion assist reactions. It was also found that high selectivity (50-150) of W-Ti to Cr or ITO and a small microloading effect were advantageous to uniformly etch patterns of various sises. Moreover, the microfabrication of a smooth W-Ti absorber has been demonstrated for lines and spaces patterns below 0.1 μm and 1-Gbit-class dynamic random access memory patterns

A Principle of Deposition of Ultra Low and Uniform Stress Absorber for X-Ray Mask

We investigate the deposition of ultra low and uniform stress absorbers for highly accurate X-ray masks. We calculate the maximum overlay error between two masks having different absorber coverages as parameters of absorber stress peak-to-valley (pv) value and distribution shape in order to establish the criteria of absorber stress, which suggested that less than 10 and 6 MPa uniformity was required for 30 and 50 mm windows respectively to achieve the mask overlay of less than 5 nm. Then we perform several experiments to optimize the sputtering conditions from the viewpoint of the geometric relation between a sputtering target and mask substrates. We observe that the target-substrate (T-S) distance in the axial direction changes only the average stress, but does not affect the uniformity, while the T-S distance in the parallel direction affects both the average and distribution. A very uniform stress absorber of ?3 MPa for a 25 mm square area is deposited 60 mm away from the target center. From further experiments, we establish a simple but important principle that the linear stress distribution on the stage deposited without rotation cancels out by rotating the substrate, and as a result the absorber stress becomes sufficiently uniform.

Total evaluation of W-Ti absorber for x-ray mask

W-Ti film was comprehensively investigated for its application as an x-ray mask absorber. A stress-free and amorphous W-Ti film was successfully deposited by dc sputtering with a W-Ti (1 wt%) target using a gas mixture of Ar and N2. To obtain much lower stress, we firstly improve the accuracy of stress measurement up to +/- 1 MPa, and ultra-low stress film was obtained by step-annealing. The stress of the film has been found stable enough to apply to x- ray masks in an air atmosphere and for x-ray exposure. The density, composition, and microstructure of the film were also evaluated by SEM, XD, XPS, and TDS. The film surface was very smooth and the roughness measured by AFM was about 2 nm. The etching properties of the absorber using a Cr mask, ITO stopper and electron cyclotron resonance (ECR) discharge plasmas were also investigated in order to achieve a resolution of below 0.1 micrometers . Highly selective and anisotropic etching has been realized using a mixture of SF6 and CHF3, by cooling the stage to about -50 degree(s)C under controlled plasma conditions. Moreover, the microfabrication of a smooth W-Ti absorber has been demonstrated for lines and spaces patterns of 0.06 micrometers and for 1-Gbit-class dynamic random access memory patterns.