A conventional semiconductor process vacuum chamber typically includes a vacuum-tight chamber enclosure made of aluminum within which is mounted an aluminum platform or platen which supports a semiconductor wafer or other workpiece. For semiconductor fabrication processes such as etching or chemical vapor deposition, process gases are pumped into the chamber. Typically, the process gases are decomposed into reactive ions by a thermal or electrical energy source, most typically by employing a source of radio frequency (RF) power to excite some or all of the process gases into a plasma state.
Many semiconductor process chambers require the metal platen to be electrically and thermally insulated from the metal chamber wall. Electrical insulation typically is required because the platen is electrically powered by an RF power supply to create a desired voltage differential between the plasma body and the platen, which, in such cases, is also called the cathode. Thermal insulation typically is required because semiconductor processing generally requires maintaining the workpiece at a regulated high temperature, but it is desirable to minimize heating of the chamber wall and other chamber components.
Electrical and thermal insulation between the platen and the chamber wall conventionally is accomplished by either mounting the platen on a post which holds the platen a fixed distance from the chamber wall, or else by mounting a dielectric spacer between the platen and the chamber wall. Both approaches typically leave gaps between the platen and either the chamber wall or dielectric spacer. The process gases used for the semiconductor fabrication processes can enter these gaps and create undesirable deposits. The deposits must be removed periodically by cleaning or replacing the affected components. If the deposits are not removed, they can flake off in the form of particles which can settle on the wafer, thereby ruining at least a portion of the semiconductor circuitry being fabricated.
Some conventional semiconductor process chambers include a dielectric shield encircling the perimeter of the platen. However, such shields typically leave a gap between the shield and the platen through which process gases can enter and create deposits. Mounting the shield and platen tightly together typically is impractical because the platen expands and contracts in response to temperature changes. Specifically, the platen expands when it is raised to normal wafer processing temperatures, and it contracts when the chamber is turned off and opened for maintenance. In contrast, the dielectric shield generally is composed of alumina or other ceramic having a very low thermal expansion coefficient. For example, a 12.5-inch aluminum platen designed to support a 12-inch semiconductor wafer typically will expand 0.1 inch when its temperature is raised from 25.degree. C. to 400.degree. C., whereas any expansion of the ceramic shield typically will be negligible. If the shield and platen were rigidly attached to seal out process gases, mechanical stress due to differential thermal expansion could crack the dielectric shield. The thermal stress problem increases if larger platens are used to process larger diameter semiconductor wafers.
Therefore, there is a need for an improved system for mounting a cathode in a semiconductor process chamber which allows the platen to be electrically or thermally insulated from the chamber wall, and which minimizes mechanical stress due to differential thermal expansion, without requiring gaps behind the platen in which process gases can create undesirable deposits.