For high absolute pattern placement accuracy and high throughput in x- ray mask writing, it is very important to firmly hold the mask with little holding deformation and large thermal conduction. For these purposes we have developed a new 'triple-chuck' mask holding mechanism. This triple-chuck mechanism is a hybrid of three-point-contact and conventional electrostatic-chuck holding mechanism, and, as the name implies, it uses three small-area electrostatic chucks. To determine the suitable shape, area, and position of the electrostatic chucks, we performed deformation simulation using the finite element method, and also conducted thermal conduction simulations. The results suggested that the triple-chuck mechanism could attain targets set for an x-ray mask with a feature size of 0.2 micrometers . Accordingly, we installed the new holding mechanism in the EB-X1 writer and found that when holding 3-inch mask (2-mm thick, before bulk etching), there is no microslippage between the mask and holding mechanism when the XY-stage is moved with an acceleration of 0.3 G and the maximum holding deformation is 0.22 micrometers in a 25-mm-square patterning area. This corresponds to the absolute pattern placement accuracy degradation of less than 11 nm in the patterning area. About 30 minutes pass before the mask temperature is within 0.1 degree of the holding-mechanism temperature. This was determined by two different methods: a patterning method and marek detection. These experimental results confirmed the triple-chuck holding mechanism attained the targets set for an x-ray mask with a feature size of 0.2 micrometers .
The EB-X1 is an accurate X-ray mask writer with high-throughput that was developed by modifying one of our EB60 variable-shaped E-beam systems. For high resolution, we developed an electron optical system whose 50-nm beam edge sharpness, a 15 A/cm2 beam current density, and 1.0-micrometers X 0.5-micrometers maximum beam size with an acceleration voltage under 30 keV were determined by proximity-effect Monte Carlo simulation. We adopt a three-pronged approach for accurate pattern placement. First, we improve the beam positioning resolution from 20 nm to 5 nm. Because we suppress mechanical vibration, we can attain a 11-nm standard mark detection accuracy, resulting in a 20-nm compensation accuracy in the beam deflection distortion and a 25-nm field stitching accuracy. Second, our new column with its short beam-path and demagnification image of variable-shaped beam optics can attain a beam position stability within 30 nm over two hours. Finally, the use of an electrostatic chuck to firmly hold the mask-substrate with little holding- deformation and large heat transmission reduces mask-substrate deformation to 23 nm during pattern writing. Experiments confirm the EB-X1 can write a 0.2-micrometers minimum-feature sized pattern, has a pattern placement accuracy of 50 nm (3 (sigma) ) and a high throughput approximately ten times higher than that of a conventional point-beam exposure system. Using optimized correction coefficients for a specific layer, an average pattern placement accuracy of 33 nm (3 (sigma) ) can be achieved. The EB-X1 is now being used in the X-ray mask fabrication process line at NTT LSI Laboratories.
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