The gallium arsenide ingots are wax-mounted to a graphite beam and sawed into individual wafers with the use an automatic inner diameter blade saw. Lubricants used in this operation generate a gallium arsenide slurry, which is collected, centrifuged, and recycled.
The wafers are then dismounted from the wax and graphite beam to prepare for cleaning. Cleaning consists of sequentially dipping the wafers in wet chemical baths containing methanol, de-ionized water, and an acid mixture of sulfuric acid, hydrogen peroxide, and water, in a 5:1:1 concentration. A visual inspection is also performed at this stage.
After cleaning, the wafers are wax-mounted to a lapping machine. The lapping machine operates at a set rotational speed and pressure and uses a lapping slurry containing aluminum oxide, glycerin, and water. After lapping, the wafers are dismounted on a hot plate, rinsed in a soap solution, and wiped dry.
Finally, the wafers are physically mounted to a mechanical polishing machine, and polished using a slurry consisting of sodium bicarbonate, 5 percent chlorine, water, and colloidal silica slurry.
The following are potential hazards of wafer preparation.
- Possible employee exposure to gallium arsenide (arsenic). The GaAs slurry presents a considerable dermal hazard, and there is potential for an airborne hazard if the slurry becomes dry.
- Possible employee exposure to chemicals used for cleaning. Common chemicals include acids (H2SO4), caustics (H2O2), and methanol.
- Occupational Health Guidelines for Chemical Hazards. US Department of Health and Human Services (DHHS), National Institute for Occupational Safety and Health (NIOSH) Publication No. 81-123, (1981, January). Provides a table of contents of guidelines for many hazardous chemicals. The files provide technical chemical information, including chemical and physical properties, health effects, exposure limits, and recommendations for medical monitoring, personal protective equipment (PPE), and control procedures.