Electron beam array lithography

Abstract
Electron‐beam array lithography (EBAL) uses array optics to expose 108 to 1010 resolution elements without mechanical motion. The array optics are based on the use of a first stage of deflection (coarse deflection) which selects one of an array of lenslets. The lenslet array is followed by a second stage of deflection (fine deflection) which selects the final spot position. In order to maximize exposure rate and also minimize mechanical motion, it is proposed to use a 3×3 array of array optics channels to expose a 100 mm wafer. As a numerical example of the projected throughput of such a system consider a 20 MHz stepping rate per channel and 0.5 μm pixels. Then (3×1010 pixels)/(9×2×107 pixels per s) ≊170 s. Now if variable spot sizes and vector writing are used such that only 10% of the pattern is exposed using the smallest feature (pixel), the exposure time is reduced to ∠20 s. Consistent with these calculations, EBAL systems are presently under development with engineering goals of 0.5 μm smallest features, 100 mm wafers, and throughputs of 50 wafers/h. The electron optics of these systems are all electrostatic, and use either thermionic (e.g., LaB6) or field‐emission (e.g., W/Zr) cathodes. EBAL systems require that patterns be ’’stitched’’ across the boundaries of the lenslet fields in the array lens. This can be accomplished by the use of a standard calibration plate having fiducial marks at the corners and sides of lenslets. Measurement of the positions of these fiducials provides data from which to calculate a set of stitching constants for each lenslet of each electron‐beam channel. Overlay between different pattern levels on a wafer is accomplished by a similar process using data from fiducial marks at the corners of each chip which is being written on a wafer. By these means any pattern level on any wafer can be exposed in any EBAL exposure station.