Stencil mask temperature measurement and control during ion irradiation

Abstract
In ion projection lithography a stencil mask consisting of a thin membrane of silicon is irradiated by a broad, uniform beam of ions. The resulting image is demagnified and projected on a resist-covered substrate. Energy deposited in the stencil mask by the ion beam can lead to temperature distributions resulting in nonuniform stress and unacceptably large feature displacement. Numerical simulations have shown that the presence of a cooled cylinder near the mask during irradiation is an effective means of temperature control. We have experimentally determined the temperature distribution in a thin glass disk which is chosen to be thermally equivalent to a Si stencil mask. We have found temperature distributions that approximately agree with the numerical computations, and have confirmed the effectiveness of a cooled cylinder in reducing the temperature variation over the disk. For example, a graphite coated (ε=0.85) glass disk, when irradiated by a 3.3 mW/cm2 ion beam, develops temperature differences of more than 5 °C between the center and edge of the disk. Introduction of a cylinder cooled 35 °C below room temperature reduces the temperature difference to 1.2°C, resulting in thermally induced feature displacement of less than 10 nm.

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