Silicon Rapid Thermal Processing with Ripple Pyrometry
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Mat. Res. Soc. Symp. Proc. Vol. 502 © 1998 Materials Research Society
ods compensate for the wafer emissivity and suppress interference from radiant sources other than the wafer itself. TEMPERATURE MEASUREMENT Accufiber introduced a "ripple technique" to measure the emissivity by exploiting the fluctuating component of reflected radiation [1-3]. This requires lamps driven by an AC power source. The ripple component in radiative output is typically several percent at 60 Hz. Figure 1 shows the arrangement of 2 light-pipe sensors in a rapid thermal processing chamber. The wafer sensor ("WAFER") receives radiation emitted by the wafer and reflected incident radiation. The lamp sensor ("LAMP") samples the incident radiation. The radiation received by the light pipes are guided to Si infrared photo diodes operating in the 950 nm wave length region where Si wafers are substantially opaque. Multiple reflections permit various portions of radiation from all sources within the chamber to enter both probes.
FIG. 1. Apparatus for Accufiber's ripple technique. The basic premise of the ripple technique is that the components of detector signals associated with AC lamp ripple originate largely from reflections. It assumes that the AC frequency is high enough so that wafer temperature fluctuations may be neglected. The ratio of the AC components in the two sensors is used to compute a reflection factor, R, given by, R = ACWAFER / ACLAMP .
(1)
Assuming an opaque body and the Kirchhoff relation with zero transmissivity and aborptivity equal to emissivity, one may compute an emittance factor, E, defined as, E=I-R .
(2)
Assuming that the quantity R also measures the fraction of reflected radiation, the signal representing the intensity of radiation thermally emitted by the wafer is found by subtracting the fraction R of the LAMP sensor quasi-DC signal, denoted as IL,from the WAFER sensor quasi-DC signal, denoted as Iw. The difference signal is expressed by the relation,
106
1E WAFER
= IW- R IL
(3)
•
Equation (3) provides the suppression of reflected background radiation. After dividing by the emissivity factor, one obtains the emissivity-corrected equivalent black-body radiative signal from the wafer, IE BLACKBODY
=
IE WAFER /
E
.
(4)
Temperature is computed from a calibration table or function for the black-body signal appropriate to the band pass of the optical system. Sensitivity factors for the WAFER and LAMP channels implicit in the above equations may be obtained with the aid of wafers with embedded thermocouple temperature sensors. REMOTE SENSOR PLACEMENT Figure 2 illustrates the sensor placement used in a ripple pyrometer developed at Lucent Technologies for a rapid thermal processor oven. The oven interior is gold plated for high reflectivity. The wafer is surrounded by a quartz isolation tube. Both sensors are located below a gap between a pair of lamps at the wall of the oven enclosure. The wafer sensor is a light pipe entrance behind a slotted opening that shields against direct lamp radiation. The lamp
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