Radiometric measurement of wafer temperatures during deposition

An infrared heat source is directed through a chopper or modulator and beam splitter to the surface of the water. A pair of radiometers are provided, one located behind the back surface of the wafer to measure transmittance, the other adjacent to the beam splitter to measure wafer reflectance. The wafer temperature may then be calculated using an experimentally determined relationship between wafer radiance W.sub.W and wafer temperature, with wafer radiance being provided by the relationship ##EQU1## where r.sub.BS is the reflectance of the beam splitter, W.sub.W is the blackbody radiance of the wafer, W.sub.a is the blackbody radiance equivalent to ambient temperature, and e.sub.W is the wafer emittance. Alternatively, rather than locate a radiometer behind the wafer to measure wafer transmittance, a mirror may be located behind the wafer to reflect the transmitted energy back through the wafer on a periodic basis for a short part of each duty cycle. A single radiometer can then measure both the reflected and transmitted energy.

FIELD OF THE INVENTION 
This invention is directed to the field of semiconductor wafer processing 
and more particularly to an improved method for temperature measurement of 
a wafer during processing of the wafer. 
BACKGROUND OF THE INVENTION 
During processing of a wafer to form semiconductor devices, accurate 
temperature control and monitoring of the wafer is critical to an 
effective process. Variations in the temperature can affect the dimensions 
of the devices, and therefore, the reproducibility of the process. 
To date, the common technique for measuring wafer temperatures is the use 
of a thermocouple. In using a thermocouple, it is put directly on the 
silicon wafer or on a monitor chip that sits near the silicon wafer. 
However, a thermocouple has a finite thermal mass. Therefore, it will 
modify the temperature of the water. Using the thermocouple on a monitor 
wafer is also relatively inaccurate, since it is at a distance from the 
wafer being processed, it cannot truly represent the temperature 
distribution on the wafer being processed. Further, the wafer being 
processed is not subject to the disturbing effects of the thermocouple and 
therefore, the temperature distribution on the wafer being processed and 
the monitor wafer are not truly identical. Further, use of the 
thermocouple has a high probability of introducing contamination on the 
surface of the wafer, because thermocouples are metallic elements. 
The more common technique today is used of a pyrometer, a non-contact 
temperature measurement technique. However, the classic use of pyrometry 
is extremely sensitive to surface optical properties, and therefore a 
wafer that has not been processed will have different emission 
characteristics than a wafer whose surface has been processed. Therefore, 
the difficulty with this process as presently practiced lies in the 
changing optical properties of the wafer as the layers are added to the 
surface of the silicon wafer. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of this invention to provide a non-invasive 
method for accurate temperature measurement of a wafer under process. 
It is a further objective of this invention to provide a more accurate 
method of constantly monitoring the temperature of a wafer being 
processed. 
More particularly, it is an objective herein to provide a measure of wafer 
temperature based on measurements of reflectance and transmittance of the 
wafer, with emittance of the wafer being calculated to provide a measure 
of wafer temperature in real time during the processing of the wafer. 
It is a further objective herein to provide a wafer temperature measurement 
system that may be directly incorporated into existing processing systems 
without major structural modifications of the system. 
In summary, this invention uses an infrared heat source directed through a 
chopper or modulator and beam splitter to the surface of the wafer. A pair 
of radiometers are provided, one located behind the back surface of the 
wafer to measure transmittance, the other adjacent to the beam splitter to 
measure wafer reflectance. The wafer temperature may then be calculated 
using an experimentally determined relationship between wafer radiance 
w.sub.w and wafer temperature, with wafer radiance being provided by the 
relationship 
##EQU2## 
where r.sub.BS is the reflectance of the beam splitter, W.sub.W is the 
blackbody radiance of the wafer, W.sub.a is the blackbody radiance 
equivalent to ambient temperature, and e.sub.W is the wafer emittance. 
In a preferred alternative embodiment, rather than locate a radiometer 
behind the wafer to measure wafer transmittance, a mirror may be located 
behind the wafer to reflect the transmitted energy back through the wafer 
on a periodic basis for a short part of each duty cycle. A single 
radiometer can then measure both the reflected and transmitted energy.

DESCRIPTION OF A PREFERRED EMBODIMENT 
When a heated wafer is measured using an infrared thermometer, the 
temperature accuracy depends primarily on the knowledge of wafer 
emittance. An infrared source 4 is located outside the wafer processing 
reactor (not shown), providing radiant heating to wafer 5 through a 
window. A chopper 6 interrupts the radiant emissions 7 at regular 
intervals, preferably operating at a 50% duty cycle. The emissions reach 
the wafer through a beam splitter 8. Reflected energy from the wafer 9 
reflects off the beam splitter to radiometer 1. Transmittance t.sub.w is 
measured by radiant energy 10 reaching radiometer 2. Although wafer 
emittance cannot be measured directly during the deposition process, it 
can be determined indirectly from the following relationships: 
EQU r.sub.w +a.sub.w +t.sub.w =1 (1-1) 
EQU a.sub.w =e.sub.w (1-2) 
so that 
EQU r.sub.w +e.sub.w +t.sub.w =1, (1-3) 
where r.sub.w, a.sub.w, e.sub.w, t.sub.w are the wafer reflectance, 
absorptance, emittance, and transmittance, respectively. 
The present method is based on measurements of reflectance and 
transmittance with emittance calculated from Eq. (1-3). The measurements 
are made during the deposition process and wafer temperature is determined 
"on-line" by a dedicated computer using the relationships derived in the 
following sections. 
FIG. 1 shows the configuration of the wafer, infrared source, and 
radiometers. Radiometer 1 is used to measure wafer reflectance; radiometer 
2 measures transmittance; and radiometer 3 is part of the source control 
loop. 
REFLECTANCE MEASUREMENT 
When the source is blocked, the signal from radiometer 1 is given by 
EQU S.sub.1 '=e.sub.w r.sub.BS (W.sub.w -W.sub.a)+W.sub.a, (1-4) 
where r.sub.BS is the reflectance of the beam splitter; W.sub.w is the 
blackbody radiance of the wafer; and W.sub.a is the blackbody radiance 
equivalent to ambient temperature. 
Solving Eq. (1-4) for wafer radiance, 
##EQU3## 
Using a look-up table for W.sub.w vs. temperature, the wafer temperature, 
T.sub.w, is determined from the measured quantities in Eq. (1-5). 
When the source is open, the signal from radiometer 1 is 
EQU S.sub.1 =t.sub.BS r.sub.w r.sub.BS (W.sub.s -W.sub.a)+e.sub.w r.sub.BS 
(W.sub.w -W.sub.a)-W.sub.a, (2-1) 
where t.sub.BS is the transmittance of the beam splitter; W.sub.s is the 
radiance of the source. 
With the source blocked, the signal is 
EQU S.sub.1 '=e.sub.w r.sub.BS (W.sub.w -W.sub.a)+W.sub.a, 
the signal difference is 
EQU S.sub.1 =S.sub.1 '=t.sub.BS r.sub.w r.sub.BS (W.sub.s -W.sub.a) (2-2) 
To calibrate the system for reflectance, the wafer is replaced by a highly 
reflective mirror and the signal is 
EQU S.sub.1c =t.sub.BS r.sub.m r.sub.BS (W.sub.s -W.sub.a)+w.sub.a (2-3) 
where r.sub.m is the mirror reflectance. 
With the source blocked, 
EQU S.sub.1c '=W.sub.a, 
so that 
EQU S.sub.1c -S.sub.1c '=t.sub.BS r.sub.m r.sub.BS (W.sub.s -W.sub.a) (2-4) 
The wafer reflectance is determined from 
##EQU4## 
TRANSMITTANCE MEASUREMENT 
When the source is open, the signal from radiometer 2 is 
EQU S.sub.2 =t.sub.BS t.sub.w (W.sub.s -W.sub.a)+e.sub.w (W.sub.w -W.sub.a) 
(3-1) 
and with the source blocked, 
EQU S.sub.2 '=e.sub.w (W.sub.w -W.sub.a) (3-2) 
The signal difference is 
EQU S.sub.2 -S.sub.2 '=t.sub.BS t.sub.w (W.sub.s -W.sub.a) (3-3) 
To calibrate the system for wafer transmittance, the wafer is removed and 
the signal from radiometer 2 is 
EQU S.sub.2c =t.sub.BS (W.sub.s -W.sub.a) (3-4) 
Transmittance is determined from 
##EQU5## 
Referring to Eq. (1-5), the blackbody radiance of the water is 
##EQU6## 
The emittance e.sub.w is determined from 
##EQU7## 
The reflectances of the beam splitter (r.sub.BS) and the mirror (r.sub.m) 
are measured independently. The ambient radiance (W.sub.a) is measured by 
intermittently blocking radiometer 1. 
The above values are calculated in an appropriately programmed computer 11; 
Wafer temperature is determined using a look-up table of calibrated values 
of W.sub.w as a function of blackbody temperature. 
ALTERNATIVE METHOD 
The alternative embodiment of FIG. 2 is substantially similar to the 
embodiment of FIG. 1, with the exception that no radiometer is provided 
behind the wafer. Rather, a mirror 20 is provided behond the wafer 
together with control means 22 for moving the mirror 20 from a position 
behind the wafer where the transmitted energy can be reflected back toward 
the beam splitter, and then out from behind the wafer. By providing this 
movement controller, the effects of using a mirror on the actual operating 
temperature of the wafer are minimized. Further, the elimination of the 
radiometer behind the wafer eliminates the need for a special window in 
the processing reactor behind the surface of the wafer. 
Rather, a quartz window which is cooled can now be used between the lamp 
source and the surface of the wafer. By providing a shutter 16 operating 
with a 1-second 50% duty cycle and interposing the mirror only a fraction 
of the duty cycle (perhaps 0.2 seconds), the effect of the presence of the 
mirror 20 on the wafer 5 is minimized, but the transmittance is still 
accurately measured. A theoretical derivation of the measurement of 
r.sub.w and t.sub.w (wafer reflectance and transmittance) follows below, 
with the symbols used in the equations being the same as those used in the 
set of equations used to describe the embodiment of FIG. 1, except in the 
situation where a specific alternative definition is given. 
Shutter open, no mirror: 
EQU S.sub.1 =e.sub.wf (W.sub.w -W.sub.a)r.sub.BS +e.sub.s (W.sub.s 
-W.sub.a)r.sub.wf t.sub.BS r.sub.BS +W.sub.a 
Shutter closed, no mirror: 
EQU S.sub.1 '=e.sub.wf (W.sub.w -W.sub.a)r.sub.BS +W.sub.a 
Shutter open, mirror: 
EQU S.sub.1 "=e.sub.wf (W.sub.w -W.sub.a)r.sub.BS +e.sub.s (w.sub.s 
-W.sub.a)r.sub.wf t.sub.BS r.sub.BS + 
EQU e.sub.wb (W.sub.w -W.sub.a)r.sub.m t.sub.w r.sub.BS +e.sub.s (W.sub.s 
-W.sub.a)t.sub.BS t.sub.w r.sub.m t.sub.w r.sub.BS +W.sub.a 
Shutter closed, mirror: 
EQU S.sub.1 '"=e.sub.wf (W.sub.w -W.sub.a)r.sub.BS +e.sub.wb (W.sub.w 
-W.sub.a)r.sub.m t.sub.w r.sub.BS +W.sub.a 
EQU S.sub.1 "-S.sub.1 '"=e.sub.s (W.sub.s -W.sub.a)r.sub.wf t.sub.BS r.sub.BS 
+e.sub.s (W.sub.s -W.sub.a)t.sub.BS t.sub.w.sup.2 r.sub.m r.sub.BS 
EQU S.sub.1 -S.sub.1 '=e.sub.s (W.sub.s -W.sub.a)r.sub.wf t.sub.BS r.sub.BS 
EQU .DELTA.=(S.sub.1 "-S.sub.1'")-(S.sub.1 -S.sub.1 ')=e.sub.s (W.sub.s 
-W.sub.a)t.sub.BS t.sub.s.sup.2 r.sub.m r.sub.BS 
EQU S.sub.1c =e.sub.w (W.sub.s -W.sub.a)r.sub.m t.sub.BS r.sub.BS +W.sub.a 
EQU .DELTA./[S.sub.1c -W.sub.a ]=t.sub.w.sup.2 
EQU t.sub.w =[.DELTA./(S.sub.1c -w.sub.a)].sup.1/2 
EQU r.sub.w =r.sub.m (s.sub.1 -s.sub.1 ')/s.sub.1c 
Alternative embodiments to this invention may become apparent to a person 
of skill in the art who studies this invention disclosure. Therefore, the 
scope of this invention is to be limited only by the following claims.