ABSTRACT
Figure 19.1 Schematic diagram of a GaAs wafer bonded to a sapphire substrate with a polymer bonding material (Crystal Bond adhesive). Reprinted from Ref. [1] with permission. Copyright © 2010 CS MANTECH. 19.3 Wafer ThinningAfter the wafer is bonded to the sapphire, it is ready for thinning, which is generally done by grinding and in some cases followed by polishing. The ground wafer can be polished mechanically or by chemical mechanical polish (CMP) to give a finer surface that may make the subsequent processing easier. Typical grind thicknesses are 80 to 100 mm. Grinders can be fitted with real-time thickness monitoring to control thickness to +/–10 mm. Good thickness control is necessary for reproducible packaging process and control of TWV impedance. Thinned wafer thicknesses of below 60 mm have been reported, but in these cases a thick layer of gold is needed to provide mechanical support so that the wafer can be handled. Historically grinding used to be followed by lapping, but spray etching or polishing are better suited to high-volume production. A certain degree of roughness may be needed for good adhesion of metals, so smooth chemical polish is not necessary. Mechanical debris and polishing contaminants are removed at this step. Since fast grind speeds are utilized in production, there is a considerable amount of damage and stress in the surface layer. This is reduced by a chemical etch in an etch bath that removes damages layer and smoothens the surface (see Chapter 6). Few to 10 microns of material is removed to ensure a minimum required die strength of the finished die.
