RICHARD M ALEXY
Engineering in Newton, MA

2003021, Nov 20, 2003

May 16, 2002

10/150132

Weldon Williamson - Malibu CA, US
Robert Hallock - Newton NH, US
Charles Willingham - Framingham MA, US
Richard Alexy - Newton MA, US

H05B006/70

219/690000, 219/740000

A microwave window structure for a low-pressure chamber is provided. The window structure enables microwave energy to be introduced into the chamber from a source external to the chamber. The window structure includes a fixture having electrically conductive walls. Inner portions of the walls provide a peripheral region of an aperture within such fixture. The fixture is adapted for mounting to a sidewall portion of the chamber. A solid, microwave energy transparent dielectric window is included. The window includes: a periphery portion affixed to the fixture; and, an inner region disposed within the aperture of the fixture. A first surface of the inner region is disposed in the chamber and a second, opposite surface of the inner region widow being disposed external to the chamber. The window has a sidewall portion with a first end thereof terminating in the first surface and a second end thereof terminating at the periphery portion. The sidewall portion of the window is spaced from the walls of the fixture. The periphery portion of the window contacts the inner portions of the walls of the fixture adjacent a rounded region of such inner portions of the walls. The sidewall portion of the window is parallel to the inner walls of the fixture. The second surface of the window has affixed thereto a corrugated structure. The first surface of the window has peaks and walls therein, such peaks being separated one from the other by a length less than the nominal-operating wavelength of the microwave energy being introduced into the chamber through the window. With such an arrangement, the junction between the contact between conductive wall of the fixture, the dielectric window, and the vacuum within the chamber (a so-called “triple junction”) is displaced from the conductive walls of the fixture. More particularly, one key phenomenon in initiating microwave breakdown across a surface is injection of electrons onto the dielectric surface from the triple junction. Here, electrons are field-emitted from the conductive surface, especially from sharp edges or burrs in the walls. Each collision between the emitted electrons with the surface of the dielectric window multiplies the number of electrons because of secondary yield of most dielectrics is greater than unity (for example, up to a multiplier of 4 for silica). As the electrons across the surface of the dielectric window they produce avalanche ionization and gas breakdown on the window.