Cooled electrostatic chuck for high current serial ion implantation

A cooled electrostatic chuck for silicon wafers has been developed for use in serial process ion implantation, and incorporated into the SHC-80 high-current serial ion implanter. This platen uses the same six-phase power supply used on existing uncooled platens, incorporating positive wafer sensing. Monolithic construction allows the platen surface to be flat to 12 microns, and in fact the micro-roughness of the surface is exploited to accommodate cooling gas between the wafer and the platen. Electrostatic clamping provides uniform pressure of about 25 Torr between the wafer and the platen. Cooling gas is introduced through a ring of holes close to the wafer edge, designed to maintain a uniform gas pressure over the trapped region. There is no discrete seal, yet the rate of gas leakage is measured in 100ths of an sccm, which is too small to be resolved above processing pressures. Cooling rates have been measured by a variety of methods, both under laboratory conditions and under actual ion beam bombardment. A 50 keV 16 mA ion beam may be used for any high dose implant in the SHC-80 without causing the wafer temperature at any point to rise 80/spl deg/. Since the pressure falls to zero at the wafer edge, there will be a maximum in wafer temperature at the edge, which simplifies the cooling measurements to a single number-the edge cooling rate. The wafer center cooling rates achievable are many times greater than are needed to accomplish this real-world cooling rate.