CHAPTER 4.4

Viewports Chuck Kraft

Larson Electronic Class

4.4.1

MATERIALS

The three most common viewport materials used in high- and ultra-high-vacuum systems are glass, quartz, and sapphire. The choice of which to use often depends on cost, transmission characteristics, bakeout temperature, style, and materials used for the viewport seal.

Glass viewports, the most widely used and least expensive of the three choices, are available in a wide variety of diameters. The transmission is approximately 90% between 0.32 and 2.6 microns. Glass viewports are bakeable to 400"" Celsius (centigrade). Zero-length viewports, where the surface of the glass is below the surface of the flange, are Kovar®' sealed. Also available are nonmagnetic view­ports with the glass sealed to the end of a stainless steel tube. The viewport is nor­mally mounted with the tube extending out from the vacuum chamber. In diame­ters up to 4 inches, the tube can be inverted to place the glass close to an object within the chamber. Tubular Kovar® viewports can be made the same way.

Quartz viewports have a transmission of approximately 90% in the range of 0.3 to 2.5 microns and 0.2 to 2.0 microns for UV-grade materials. They are available in both zero-length and standard-length. A standard-length quartz viewport has the quartz disk mounted on a tube. Quartz viewports are sealed to stainless steel

' Kovar® is a registered trademark of Carpenter Technology. Kovar is an iron, nickel, and cobalt al­loy used for glass sealing.

ISBN 0-12-325065-7 Copyright © 1998 by Academic Press $25.00 All rights of reproduction in any form reserved.

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4.4.1 Materials 445

frames with a low-temperature brazing alloy. The bakeout temperature is limited to 200° Celsius.

Sapphire viewports have the broadest transmission range. The transmission is above 80% at 0.3 to 4.0 microns and 0.25 to 4.5 microns for UV-grade sapphire. These viewports are sealed with a high-temperature braze material and can with­stand bakeout temperatures as high as 450° Celsius. The frame is Kovar®.

4.4.2

MOUNTING SYSTEMS AND PRECAUTIONS

Viewports of all three types are available with a variety of mounting flanges, and, with the exception of the glass and quartz zero-length types, without flanges. In use, it is important not to induce stress into the seal, which could cause cracking or breakage. The two types of stress are mechanical and thermal. Mechanical stress is created from uneven pressure applied to the viewport frame. Flange bolts should be tightened in a cross pattern, and foreign objects should not be allowed to make contact with the frame or the glass. Thermal stress is the result of uneven heat being applied to the frame and/or the glass during welding, bakeout, or in ac­tual use on the system. Viewports are designed for vacuum use. The zero-length glass viewports should be mounted in the normal configuration. Nonmagnetic glass viewports up to 4 inches in diameter, and quartz and sapphire viewports can be mounted with vacuum on either side. Precautions should be taken to avoid pres­sure differentials greater than 1 atmosphere. In some applications, safety pressure valves or shielding should be considered to protect operators and equipment.