Substrate heating
of wafer
Wafers can be heated by placing them on a hot substrate, or substrate heater, which is
typically heated using heating elements (e.g., electrical resistance
wire embedded in and electrically insulated from a heater block
or ceramic or metal material). Because the substrate heater stage
has a large thermal mass (compared to the wafer), its temperature
changes very slowly. When the wafer is placed on the substrate
heating stage, it is heated by radiation, conduction, and/or convection.
In order to cool the wafer at the end of the process, the wafer
is typically removed from the substrate heater and placed elsewhere
to expedite cooling. In this case the radiation heating is slower
than in rapid thermal processing, because the substrate heater
temperature is equal to the desired wafer temperature (cf. in
RTP, where the lamps are much higher in temperature than the desired
wafer temperature).
A
primary advantage of substrate heating is across-wafer temperature
uniformity. As with RTP, reactor walls may be kept at or near
room temperature. Substrate heating is predominantly used with
single-wafer processing, although multi-wafer batches may be used
as well.
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This exercise showed:
Substrate heating can raise the wafer temperature adequately, with the wafer temperature remaining close to the substrate temperature. With the wafer in close contact to the substrate heater, the temperatures of the two elements will be quite close at all times. This offers a good mechanism to control wafer temperature, since the substrate temperature is fairly easy to measure in real-time. However, this also means that wafer cooling may be quite slow, since if the wafer is on the substrate, it can only cool as fast as does the substrate heater, which has a considerably larger thermal mass (in this case about 10X that of the wafer).
Substrate heating raises the wafer temperature more slowly than does lamp heating. In this example the power input to the substrate heater (1250W in the default case) is similar to that for the lamp heating example. However, only a portion of the lamp power transferred to the wafer (25% here), yielding a similar power impingement to the wafer as for lamp heating. The additional thermal mass of the substrate heater is what slows down the wafer heating rate.