Page 339 - Sami Franssila Introduction to Microfabrication

318 Introduction to Microfabrication

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(a) (b)
Figure 31.3 Rapid-thermal oxidation uniformity: (a) vertical lamp bank geometry can be seen in oxide thickness chart
and (b) gas-flow patterns are seen in oxide thickness: incoming gas cools the wafer near the flat, and wafer edges are
cooler than the centre. Reproduced from Deaton, R. & Massoud, Z. (1992), by permission of IEEE

Metal chamber RTP tools are water-cooled to keep
them cold; quartz chambers are allowed to heat up;
that is, they are warm-wall systems. System walls do
not contribute to contamination because evaporation and 1000°C
desorption of material is minimized by keeping the
temperature low. Temp
A hybrid technology between resistively heated 800°C
furnaces and RTA is the fast ramp furnace. A heater,
typically made of silicon carbide, is kept at a very
high temperature, and the wafers are rapidly brought
to its vicinity. A massive radiation source emits at 30 s 60 s
much longer wavelengths than RTP lamps, and thermal Time
equilibrium is possible. This ramping arrangement can
significantly reduce wafer emissivity variation and Figure 31.4 Temperature profile in rapid-thermal anneal-
◦
temperature non-uniformities. Ramp rates for fast- ing: solid curve: 1000 C, 10 s anneal; dashed curve:
◦
ramping systems are 10 to 100 C/s, somewhat lower 1100 C spike anneal (zero-time anneal)
◦
than in RTP systems.
Rapid annealing times are typically tens of seconds
(Figure 31.4), very fast compared to 30 to 60 min furnace The main features of furnace and RTP systems are
anneals. In order to reduce unwanted diffusion during compared in Table 31.2.
annealing, high temperature/short time combination has When oxide thicknesses are scaled down, rapid-ther-
been refined to zero-time anneal (also known as spike mal oxidation becomes more competitive but furnaces
anneal): the anneal temperature refers to the highest are still the workhorses of oxidation. In implant
temperature reached by the system, but power is turned activation anneal, RTA is the only choice when shallow
off immediately after reaching that temperature. junctions are made, as discussed in Chapter 25.