TABLE OF CONTENTS Free-space packaging is sometimes referred to as an air, or air cavity, package. We will use the term cavity package that is less restrictive since the cavity may not contain air but will still have a cavity configuration. The original packages used for optoelectronic (OE) devices and early electronic systems were all free space and fully hermetic vacuum structures, still considered the best packages. The world's first packages were made of glass since it was well understood, widely available, and easy to work. Later, vacuum packages were made from metal, ceramic, and combinations, although glass was retained for display products. Early hermetic packages have been described in Chap. 1 in some detail.
Today's offerings for cavity-style, nonoptical packages include two mainstream materials metals and ceramics. Polymer-based near-hermetic packages that may be engineered to fully hermetic are just emerging. Metal cavity packages can be used for MEMS, MOEMS, and nondisplay OE devices as well as complete module assemblies. The metal package can be made in almost any size, but the shape is typically confined to a rectangular boxlike configuration since this is the easiest to manufacture. An exception to the rectangular shape is the cylindri-cally formed metal can style package such as the transistor outline (TO). This older hermetic design is still in general use today and is probably the lowest cost metal-hermetic package available. However, although the TO is a cavity fully hermetic package at reasonable cost,...
