Formation of silicon and gallium arsenide wafers  (Page 2/11)

Crystal shaping

Although an as-grown crystal ingot is of high purity (99.9999%) and crystallinity, it does not have the sufficiently precise shape required for ready wafer formation. Thus, prior to slicing an ingot into individual wafers, several steps are needed. These operations required to prepare the crystal for slicing are referred to as crystal shaping, and are shown in [link] .

Cropping

The as-grown ingots have conical shaped seed (top) and tang (bottom) ends that are removed using a circular diamond saw for ease of further manipulation of the ingot ( [link] a). The cuttings are sufficiently pure that they are cleaned and the recycled in the crystal growth operation. Portions of the ingot that fail to meet specifications of resistivity are also removed. In the case of silicon ingots these sections may be sold as metallurgical-grade silicon (MGS). Conversely, portions of the crystal that meet desired resistivity specifications may be preferentially selected. A sample slice is also cut to enable oxygen and carbon content to be determined; usually this is accomplished by Fourier transform infrared spectroscopic measurements (FT-IR). Finally, cropping is used to cut crystals to a suitable length to fit the saw capacity.

Grinding

The primary purpose of crystal grinding is to obtain wafers of precise diameter because the automatic diameter control systems on crystal growth equipment are not capable of meeting the tight wafer diameter specifications. In addition, crystals are seldom grown perfectly round in cross section. Thus, ingots are usually grown with a 1 - 2 mm allowance and reduced to the proper diameter by grinding [link] b.

Crystal grinding is a straightforward process using an abrasive grinding wheel, however, it must be well controlled in order to avoid problems in subsequent operations. Exit chipping in wafering and lattice slip in thermal processing are problems often resulting from improper crystal grinding. Two methods are used for crystal grinding: (a) grinding on center and (b) centerless grinding.

[link] shows a schematic of the general set-up for grinding a crystal ingot on center. The crystal is supported at each end in a lathe-like machine. The rotating cutting tool, employing a water-based coolant, makes multiple passes down the rotating ingot until the requisite diameter is obtained. The center grinder can also be used for grinding the identification flats as well as providing a uniform ingot diameter. However, grinding the crystal on centers requires that the operator locate the crystal axis in order to obtain the best yield.