Processing Parameters
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Ramp Rate

Most diffusion furnaces are capable of heating at a rate of about 10°C/min. This is an acceptable rate for sources that are 2 or 3 inches in diameter. However, the slower heating rates given in Table I are recommended for the larger diameter sources to insure uniform heating across their diameters. If temperature gradients are too great, silicon damage and nonuniform doping can occur along the outside edge.

If controlled heating and cooling rates are not available, uncontrolled rates are usually acceptable. In this case, however, the two end zones must be slave-controlled to the center zone. If they are not slave-controlled, non-uniform heating of the boat may occur resulting in non-uniform doping across the boat.

Deposition Time

Sufficient time must be allowed at the deposition temperature for a solid source system to reach thermal equilibrium and for the sources to deposit enough B2O3 to uniformly dope the surfaces of the silicon wafers. The minimum time that is required to give good results increases with increasing source diameter. Experience has shown that times less than those given in Table I are generally too short to obtain acceptable uniformities.

Comments on Direct Insertion

Some users may choose to insert their diffusion carrier directly into the furnace at the deposition temperature. Direct insertion is not generally recommended for any size source. Process engineers who choose to use direct insertion may observe premature warpage of the sources and nonuniform doping of the silicon, especially across the boat.

If the direct insertion technique is selected, however, the diffusion engineer may find it necessary to tilt the furnace temperature profile to compensate for the longer time at temperature for sources and silicon at the gas inlet end (source end) of the boat (first-in last-out effect). An alternate technique to direct insertion into the hot zone that sometimes works for 2 and 3 inch diameter systems is to quickly push the diffusion carrier through the hot zone, allow the silicon and sources to stabilize near the gas inlet end of the tube, and then pull the boat slowly back into the hot zone. At the end of the deposition time, the boat is then slowly pulled back to the front of the tube from the hot zone. Since this technique makes the first-in end of the boat also the first-out end, variations across the boat are minimized.

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