Electrostatic chuck behaviour at ambient conditions

For advanced lithography applications, high-precision electrostatic chucks with diameters up to 12 inch are being developed at IOF. Although electrostatic chucking is mostly used in vacuum, the principle also works under ambient conditions. However, forces exerted on the wafer may not be the same in both cases. To quantify the influence of environmental conditions, electrostatic forces for various chuck dielectrics were measured under ambient conditions (humid air), dry nitrogen and in vacuum. The forces are found to be significantly reduced at ambient conditions as compared to the vacuum case. When flushing with dry nitrogen, the vacuum level is essentially retained. Clearly, water at the chuck-to-wafer interface reduces the chucking force. We suspect that dissociation of water occurs, entailing ionic charge transport and trapping in the dielectric. This eventually shields the wafer from the chuck electrode.