Emissivity-Independent Rapid Thermal Processing using Radiation Shields
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and if a pyrometer is used to monitor the wafer's temperature, large errors can arise. As a result, the identification of methods of temperature control which are independent of the emissivity of the wafer has been an important aspect of the development of RTP technology [1]. In RTP systems which have two banks of lamps, one above and one below the wafer, it is possible to make the temperature control independent of the emissivity of the wafer backside by exploiting the intrinsic heat-transfer characteristics of this configuration. The method described in this paper relies on using an opaque plate beneath the wafer as a radiation shield, which masks the wafer's backside from the lower bank of lamps and provides a target whose temperature can be monitored easily by a pyrometer or a thermocouple [1]. Similar configurations, where the wafer is placed between two such plates, have been used extensively for processing GaAs wafers, partly because of the need to provide a semi-enclosed environment where a local arsenic overpressure can be generated, and partly because of the increase in lamp coupling and simplification in temperature measurement [2]. This paper explores the heat transfer phenomena which result in emissivityindependent temperature control when a radiation shield is used in RTP. A SIMPLE THERMAL MODEL FOR RTP WITH A RADIATION SHIELD Fig. 1 illustrates the main heat transfer mechanisms in an RTP system with a radiation shield. The system has lamps above and below the wafer, a quartz isolation tube to protect the wafer from
contamination, an opaque radiation shield beneath the wafer, and highly reflecting chamber walls. A simple zero-dimensional model for heat transfer was used to examine how the shield, combined 57 Mat. Res. Soc. Symp. Proc. Vol. 470 01997 Materials Research Society
Wall
Quartz, 15 f
LTS
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16
pWTL
P Wafer ,,",-,'","-",-- ', -13
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Shield 12
t•
18
19
4
Quartz, I11 INoJ
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Wall Fig. 1. A simple thermal model for a RTP system with a radiation shield. I1 to
1
0 are the radiant
energy fluxes in the system. There are 10 energy source / waveband combinations. The top and
bottom lamps emit Ilts and INbs respectively in the short waveband. The top surface of the wafer emits Pwtl and Pwts in the long and short wavebands, respectively. The wafer backside emits the corresponding terms Pwbl and Pwbs whereas the shield emits Psl and Pss from both its surfaces.
The top and bottom quartz plates emit Pqtl and Pqbl respectively in the long waveband. The model includes forced air cooling of the quartz and thermal conduction within the isolation tube. with double-sided lamp illumination, can provide excellent immunity to the influence of wafer backside variations. The details of the modelling approach were described previously, and only a
brief outline of the key features and assumptions will be given here [1]. The model assumes that the energy transfer
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