Formation of silicon and gallium arsenide wafers  (Page 5/11)

Type of bow and warp	Surface appearance	Lattice curvature	Comments
	flat	flat	ideal
	curved	flat
	curved	curved
	flat	curved
	curved	flat	slips

Heat treatment

As-produced Czochralski grown crystals often have a level of oxygen impurity that may exceed the concentration of dopant in the semiconductor material (i.e., Si or GaAs). This oxygen impurity has a deleterious effect on the semiconductor properties, especially upon subsequent thermal processing, e.g., thermal oxide growth or epitaxial film growth by metal organic chemical vapor deposition (MOCVD). For example, when silicon crystals are heated to about 450 °C the oxygen undergoes a transformation that causes it to behave as an electron donor, much like an n-type dopant. These oxygen donors, or "thermal donors", mask the true resistivity of the semiconductor because they either add additional carrier electrons to a n-type crystal or compensate for the positive holes in a p-type crystal. Fortunately, these thermal donors can be "annihilated" by heat treating the materials briefly in the range of 500 - 800 °C and then cooling quickly through the 450 °C region before donors can reform. In principle thermal donor annihilation can be performed on wafers at any time during their fabrication; however, it is usually best to perform the heat treatment immediately after wafering since sub-standard wafers may be rejected before additional processing steps are undertaken and thus limiting additional cost. Donor annihilation is a bulk effect, and therefore the thermal treatment can be performed in air, since any surface oxide that may form will be removed in subsequent lapping and polishing steps.

Lapping or grinding

The as-cut wafers vary sufficiently in thickness to require an additional operation, the slicing operation does not consistently produce the required flatness and parallelism required for many wafer specifications, see [link] . Since conventional polishing does not correct variations in flatness or thickness, a mechanical two-sided lapping operation is performed. Lapping is capable of achieving very precise thickness uniformity, flatness and parallelism. Lapping also prepares the surface for polishing by removing the sub-surface sawing damage, replacing it with a more uniform and smaller lapping damage.

The process used for lapping semiconductor wafers evolved from the optical lens manufacturing industry using principles developed over several hundred years. However, as the lens has a curved surface and the wafers are flat, the equipment for lapping wafers is mechanically simpler than lens processing machines. The simplest double-side lapping machine consists of two very flat counter-rotating plates, carriers to hold and move the wafers between the plates, and a device to feed abrasive slurry steadily between the plates. The abrasive is typically a 9 μm Al ₂ O ₃ grit. Commercial abrasives are suspended in water or glycerin with proprietary additives to assist in suspension and dispersion of the particles, to improve the flow properties of the slurry, and to prevent corrosion of the lapping machine. Hydraulics or an air cylinder applies lapping pressure with low starting pressure for 2 to 5 minutes, which is then increased through most of the process. The completion of lapping may be determined by elapsed time or by an external thickness sensing device. The finished process gives a wafer with a surface uniform to within 2 μm. Approximately 20 μm per side is removed during the lapping process.