Development of a cooled microwave window. CRADA final report for CRADA Number Y-1293-0200

The objective of this Cooperative Research and Development project (CRADA) was to generate a new design for a microwave vacuum window to be used with ASTeX Corporation plasma processing equipment. This vacuum window allows transmission of microwave power from an input waveguide into a vacuum chamber for creation of plasma using the electron cyclotron resonance process. Requirements for the window design are: higher power capability, improved resistance to chemical attack, and physical compatibility with previous window models. In these applications, a significant portion of the input power is deposited in the window by plasma bombardment so the window must remove a great deal of heat to remain at a reliable operating temperature. A power level increase from 1.5 kW to 5 kW is desired by ASTeX for the new window which must have {approximately} 120 mm diameter and be compatible with existing hardware. New applications for these processing systems are being developed by ASTeX; these require the use of highly reactive fluorine plasmas which can rapidly etch some window materials. Therefore, the use of a fluorine compatible window ceramic is required. Two new window designs were investigated using advanced window-modeling techniques and low-power laboratory testing. It was determined that both concepts were capable of operating at significantly higher power levels than present commercial windows and would meet the CRADA design objectives. The compatibility of the window materials considered with fluorine plasmas are believed to be acceptable. ASTeX has a continuing interest in pursuing these window designs and will likely begin manufacturing design work of the improved design in the near future. There will also be a continuing effort to keep AlN ceramic manufacturers interested in improving the quality of large AlN disks. Additional window tests and development work could be performed by ORNL/MMES if a suitable funding source is available