Investigations into an electrostatic chuck design for 450mm Si wafer

Abstract

ABSTRACT We report on theoretical and experimental investigations into electrostatic chuck designs for use in future e-beam lithography on 450 mm Silicon wafers. Ultra-low thermal expansion glass (ULE) and Si infiltrated Silicon Carbide (SiSiC) designs were evaluated by finite element modeling, subject to a mass budget of 8 kg. In addition to massive chucks, light-weight designs were created by applying bore holes through the chuck body below its surface. Considerable chuck bending under gravity is observed with classical kinematic 3-point mounts. Out-of-plane distortions of about 1250 (650) nm and 400 (200) nm for the massive and light-weight designs of ULE (SiSiC), respectively, were calculated. The corresponding surface in-plane distortions for a chucked Si wafer of standard thickness 925 µm amount to about 3 (1.6) nm for the massive and 1 (0.5) nm for light-weight designs of ULE (SiSiC), respectively. By using the standard 6 th order polynomial correction upon e-beam wr iting, these values can be reduced to ” 0.7 nm for the massive designs with both materials. Various pin-pattern configurations for an ideally flat chuck surface were adopted to determine resulting wafer bending under the influence of electrostatic forces. At a typical electrostatic pressure of about 18 kPa, a square pin pattern of pin-pitch 3.5 mm and pin-diameter 0.5 mm results in wafer in-plane distortions <0.5 nm, which is considered tolerable for obtaining the desired tota l overlay accuracy of <4 nm. The pin structure manufacturing process for a corresponding ULE chuck surface was experimentally tested and verified. A nearly elliptic ULE plate, slightly larger than the wafer, was structured with a Chromium hard-mask and subjected to low pressure reactive ion etching to generate the pin-pattern. A homogeneity of about 7 % was obtained for the etching process, which is fully sufficient with respect to resulting variations in electrostatic attraction. Keywords: electrostatic chuck, 450 mm wafer chuck, pin structure, ultra-low expansion glass, Silicon Carbide