Temperature measurement and control for photohermal processes

The temperature of a surface undergoing a radiation assisted thermally driven process is sensed by observation of the thermal emission from that surface and used to control the process. In a preferred embodiment, the blue edge of the thermal emission spectral distribution is detected to determine the surface temperature of a workpiece during a process such as laser-assisted chemical vapor deposition, and used to control this temperature. The temperature measuring system has means for focusing workpiece thermal emission and defining the field of view, a spectrometer to separate shorter wavelength light from other spectral components of the thermal emission, and a photon-counting system to detect the shorter wavelength light and generate a surface temperature signal. Systems to determine surface temperature at a spot and along a line have an optical prism to disperse the thermal emission into component wavelengths, and a multichannel photon-counting detector comprised of an intensified photodetector array.

This invention relates to surface temperature determination during 
radiation assisted, thermally driven processes and use of this measurement 
to control the process, and more particularly to using short wavelength 
thermal emission to measure and control temperature during a chemical 
vapor deposition (CVD) process. 
In both pulsed and continuous wave laser-assisted chemical vapor deposition 
(LCVD), in applications where the deposition is assisted by laser heating 
of the surface, the amount of heating cannot be calculated because of 
changes in absorption and emissivity of the surface as it is modified by 
the deposition. Likewise because of the unknown variation in emissivity 
with wavelength, conventional infrared thermography cannot provide 
accurate temperature measurements. Many other temperature determination 
approaches, such as those for example using thermocouples, are rendered 
inaccurate by the strong variations in spatial and/or temporal properties 
of the laser beam. Temperature measurements are nevertheless desirable, 
because they appear to provide the most sensitive control parameter for 
many CVD processes of interest. 
The literature and known prior art of LCVD shows no attempts to control 
surface temperature of the part being processed. Typically, the gases are 
input at a known temperature and the reaction chamber is maintained at a 
constant temperature during LCVD. 
Conventional LCVD systems have a window in the sealed environment reaction 
chamber through which the laser beam is transmitted to a workpiece inside 
the chamber. Delivery of the laser beam via a fiber optic has many 
advantages. The gaseous reactant can be introduced into the vicinity of 
the workpiece by means of a gas nozzle on the optical fiber output 
coupler. 
SUMMARY OF THE INVENTION 
The short wavelength end of the Planck emission, toward the blue, is used 
to determine the surface temperature of a workpiece during a radiation 
assisted thermally driven process, one example of which is laser-assisted 
CVD. It is known that this component of the thermal emission spectral 
distribution varies much more strongly with temperature than longer 
wavelength emission further to the red. Consequently, unknown variations 
in emissivity will introduce a smaller error into resulting temperature. 
However, the received detector signal gets weaker rapidly as the selected 
wavelength is moved toward the blue, i.e. at shorter wavelengths, and less 
sensitive conventional solid state photodetectors cannot be employed. An 
element of the invention is the use of a photon-counting detector system 
to obtain the advantage of reduced emissivity sensitivity in temperature 
measurements based on observation of thermal emission at the shortest 
possible wavelengths. 
The invention is broadly characterized as a method of controlling a 
radiation assisted CVD process comprising the steps of delivering 
radiation to a portion of the surface of a workpiece to cause deposition 
of gaseous chemical reactants onto the workpiece by radiation heating of 
the surface, sensing the workpiece surface temperature resulting from this 
radiation, and utilizing the sensed temperature to control the radiation 
and the deposition process. 
An improved apparatus for a radiation assisted thermally driven process is 
comprised of a source of radiation and means for delivering radiation to 
the workpiece surface, a temperature measuring system for determining the 
temperature of a portion of the surface by detecting thermal emission from 
that portion and deriving a surface temperature signal, and means 
responsive to the temperature signal for controlling the radiation source 
and producing a preselected workpiece surface temperature. 
An illustrative embodiment of the invention is an improved apparatus for 
performing laser-assisted chemical vapor deposition of a gaseous chemical 
reactant. A laser for generating a laser beam is provided and means for 
delivering laser energy to heat the surface of the workpiece. A 
temperature measuring system is comprised of means for focusing the 
workpiece thermal emission, a spectrometer for separating shorter 
wavelength light toward the blue edge of the spectrum from other spectral 
components of the thermal emission, and a photon-counting detector for 
detecting the shorter wavelength emission and generating a surface 
temperature signal. Means are provided for utilizing the latter to control 
the laser and laser beam power and produce a predetermined workpiece 
surface temperature. 
Another aspect of the invention is a workpiece surface temperature 
measuring system comprising: means for focusing thermal radiation emitted 
during a chemical vapor deposition process, and defining the field of 
view; spectrometer means for separating shorter wavelength light from 
other components of the thermal emission spectrum; and a photon-counting 
system that detects the shorter wavelength thermal emission and generates 
a signal representative of the surface temperature. The photon-counting 
detector typically has multiple channels and is comprised of an image 
intensifier and a linear or two-dimensional photodetector array, the 
former when point measurements are made and the latter to make multiple 
point measurements along a line. Another single channel embodiment has a 
photomultiplier to detect, by a photon counting technique, short 
wavelength light passed by an interference filter. 
The preferred embodiments of the surface temperature determining system are 
comprised of: means for focusing thermal emission and defining source 
location for thermal emission or defining the field of view; means for 
collimating and dispersing the thermal emission into component wavelength 
and colors, as by using an optical prism or grating; a multichannel 
photon-detecting system comprised of an image intensifier and solid state 
photodetector array to generate in every channel a signal dependent on the 
intensity of the received component of the spectral distribution; and 
means for selecting the channel or channels receiving the shortest 
wavelength emission and outputting the respective detector signal which is 
indicative of workpiece surface temperature. The photodetector is a linear 
array or a two-dimensional array in instruments to respectively determine 
the temperature of a spot and a line on the workpiece surface.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates the point that a smaller error in temperature results, 
if there are unknown variations in emissivity, when the short wavelength 
region of thermal emission is observed to determine temperature. FIG. 1 is 
drawn from FIG. 4 of the technical paper by D. P. DeWitt, Optical 
Engineering, Vol. 25, No. 4, 596-601. The figure plots a factor here 
denoted G, which is the ratio of the fractional change in spectral 
radiance resulting from a fractional change in temperature, as a function 
of the wavelength (for a given temperature). The significance of this 
ratio can be derived as follows: the basic relationship between signal 
strength S in a particular wavelength channel from a surface at a 
temperature T is: 
EQU S.sub..lambda. =K.sub..lambda. e.sub..lambda. L.sub..lambda.,b (T) (1) 
or, suppressing these subscripts, 
EQU S=KeL(T) (1') 
where L(T) is the blackbody spectral radiance for wavelength .lambda. and 
temperature T, e is the emissivity, and K is a function of wavelength 
describing performance of the optical system which is known from 
calibration measurements. The thermal radiation is typically broken up 
into component colors and there are then a plurality of wavelength 
channels. 
A typical temperature measurement will produce a signal S.sub..lambda. for 
each wavelength channel but signals from channels toward the blue edge of 
the detected spectral distribution may be too small to determine 
accurately because of intrinsic or background noise. Let S stand for the 
signal from the first channel moving from blue to red along the thermal 
emission distribution which has a signal due to that emission which is 
significantly above the noise level. A value for temperature can be 
calculated from this signal if the emissivity is known, since e and L are 
the only unmeasured quantities in the equation, and L is known from theory 
as a function of .lambda. and T. In this example case, an accurate value 
for emissivity is assumed not available. Therefore, it is wished to make 
the temperature determination as insensitive as possible to emissivity. 
The determination is made by guessing a value for the emissivity, e.g., 
which is substituted into equation (1'). 
EQU S=Ke.sub.g L(T.sub.g) (2) 
This equation is solved to obtain the approximate value for estimated 
temperature T.sub.g. In order to obtain the fractional error in this 
result (T-T.sub.g /T) resulting from an error De in the value of e, 
EQU De=e-e.sub.g (3) 
the expression for S from equation (1') can be substituted into equation 
(2): 
EQU eL(T)=e.sub.g L(T.sub.g) (4) 
and a first order Taylor expansion may be used 
EQU L(T)=L(t.sub.g)+wDT (5) 
where w=.vertline.dL/dT.vertline..sub.T and DT=T-T.sub.g . Then from 
equations (3), (4) and (5), 
EQU (T-T.sub.g)/T.sub.g =(De/e) (1/G) (6) 
where small higher order terms have been neglected, as is usual in 
linearized analysis. 
Thus the fractional error in temperature is equal to the fractional error 
in emissivity divided by G, which was previously defined. It follows that 
a large value of G is desirable because that provides the smallest 
temperature measurement error. From FIG. 1, it is clear that the value of 
G increases steadily as one makes measurements further to the blue 
(decreasing values of .lambda.T in the range of 500 to 1000 .mu.m.K). 
However, the signal also gets weaker rapidly as the sample wavelength is 
moved toward the blue. Previous temperature measuring instruments based 
upon observation of thermal emission typically have used solid state 
detectors which are about one thousand times less sensitive than the 
photon-counting detectors proposed here. Use of less sensitive detectors 
prevents full realization of the reduced emissivity sensitivity advantage 
in temperature measurements. Accordingly, one element is the use of 
multichannel photon-counting detectors to provide additional accuracy and 
verification for the temperature measurement. Yet another element is the 
use of two-dimensional photon-counting detector arrays to provide this 
advantage along with multiple temperature measurements along a line. 
Configurations to accomplish these purposes are shown in FIGS. 2 and 5 for 
a point measurement and FIG. 4 for multiple point measurements along a 
line. The linear and 2-D photon-counting optical array detectors required 
for these measurements are now commercially available. 
An example of the advantage to be gained through use of photon-counting 
instruments can be obtained by consideration of a problem typical of a 
proposed application: temperature measurements are required on a surface 
of unknown emissivity with time resolution of one millisecond and spatial 
resolution of 20.times.20 microns. The approximate temperature of the 
surface is 1000.degree. C. Using the embodiment of FIG. 2, the number N of 
detected photons in an optical multichannel analyzer channel is given by: 
##EQU1## 
Here .eta. is the quantum efficiency of the detector (counts per photon 
incident upon the detector channel), .tau. is the exposure time, h is 
Planck's constant, c is the speed of light, D is the effective diameter of 
the spectrometer collimator, F is the focal length of the focusing 
collimator, A is the reciprocal dispersion of the prism (or grating in a 
grating spectrometer), x is the dimension of an individual detector in the 
direction of dispersion, y is the corresponding orthogonal dimension, T is 
the optical efficiency, and other quantities are as defined previously. 
Reasonable engineering assumptions for a spectral channel near 600 nm are: 
##EQU2## 
Although the signal level depends upon the spatial resolution, that 
dependence does not appear explicitly in the equation because spatial 
resolution is determined ultimately by detector pixel size and the 
magnification of the entrance optics. With the above numbers, a 0.2 
millimeter resolution is obtained at a working distance of approximately 
one meter. Likewise, signal level depends inversely on spectral resolution 
of each channel, but that quantity is not shown explicitly. Spectral 
resolution R is given by 
EQU R=A x/F (8) 
and for this case is 1 nm. Using the numbers above and the L- 10 spectral 
emittance for a 1000.degree. C. blackbody, it is found that N=10 at 
.lambda.=600 nm. Assuming these counts are detected photons following 
Poisson statistics, the standard deviation in this measurement is 
S.D.=.sqroot.10/10=31%, and it would be reasonable to expect a comparable 
or larger error due to the uncertainty in the emissivity. However, it is 
noted that at 1000.degree. C. and a wavelength of 0.6 microns, .lambda.T 
is 0.76 and from FIG. 1 factor G is approximately 20. Thus, a measurement 
done with photon-counting sensitivity on the blue edge of the Planck curve 
is characterized by a temperature measurement only 1/20 as large as the 
total uncertainty, or 1.5% in this case. In contrast, a measurement 
carried out with an unintensified solid state detector array, which is 
about 1000 times less sensitive than the photon-counting device, would be 
limited to wavelengths longer than 1.6 microns for these measurement 
conditions. There, according to FIG. 1, G=7 and the temperature error will 
be three times larger. 
Single or few channel photon-counting filter type instruments such as shown 
in FIG. 5 can be made much more sensitive than a spectrometer instrument 
and provide a comparable advantage over their less sensitive counterparts 
which are presently used extensively for temperature measurements. 
The multiple channels in a multiple channel detector can be used to further 
increase the accuracy of the temperature measurement. For example, the 
emissivity cannot be larger than 1, so any blue edge error band which 
includes temperature values for which the emissivity would have to be 
greater than 1 at a longer wavelength to account for the observed signal 
could be narrowed to eliminate those temperatures. However, another 
advantage of multiple channels, in number greater than the two or three 
channels presently used in some instruments, is to allow temperature 
measurements over a wide range. This advantage is realized because the 
most accurate temperature indication is obtained from the first few 
channels toward the blue side to see a signal, and this channel group 
shifts rapidly with temperature. For example, if the surface temperature 
is reduced to 900.degree. C., the first channels to provide a useful 
signal will be near 670 nm rather than 600 nm. It is noted that at the 
assumed resolution of 1 nm per channel, this shift is nearly 70 channels. 
Even another advantage of use of the multichannel detector is provided by 
its ability to confirm that the observed spectrum is characteristic of 
thermal emission. This capability allows avoidance of errors that would be 
introduced by using data containing, for example, absorption or emission 
lines. 
The temperature measurement system in FIG. 2 determines the temperature of 
a spot 10 on an observed surface 11 using an intensified linear array 
photodetector. The surface may be on a workpiece in a sealed environment 
reaction chamber in which is performed a CVD process or a laser-assisted 
CVD process. The emissivity of the hot surface is not known and changes as 
it is modified by the deposition. Thermal radiation emitted by the 
workpiece, particularly from the spot 10 on surface 11, is gathered and 
focused by a camera lens 12. To define the source location for thermal 
emission or limit the field of view of the system, a pinhole 13 in a plate 
14 is located at the focal point of the lens. Focused thermal emission 
passing through the pinhole is collimated by a collimating lens 15, and 
the emerging parallel beam of light falls on an optical prism 16 which 
disperses and breaks up the light into component wavelengths and colors. 
Since the index of refraction of optical materials varies with wavelength, 
the various wavelengths present in the light are deviated by different 
angles. Thermal emission toward the blue is deviated only a small amount 
as compared to thermal radiation toward the red edge of the thermal 
emission spectral distribution. 
The component wavelengths emerging from prism 16 are passed through a 
focusing lens 17 and focused onto different channels of a multichannel 
photon-counting system, in this case an intensified linear array detector 
18 comprised of an image intensifier 19 and solid state photodetector 
array 20. Focused light toward the blue edge of the spectrum is received 
by a known group of channels of the linear array detector 18, and light 
further to the red is detected by another group of channels as is shown in 
the drawing. The photodetector array 20 is scanned at given intervals and 
the read out voltage waveform 21 is sent to an electronics unit 22 to be 
analyzed. Every channel of the intensified linear array detector 18 
generates a detector signal whose magnitude depends on the intensity of 
the received component of the thermal emission spectral distribution. The 
electronics unit selects the channel or channels receiving shorter 
wavelength light toward the blue and outputs the respective detector 
signal which is indicative of the surface temperature of the workpiece. 
Other functions of the electronics subsystem have been described. 
The intensified linear array detector 18 and image intensifier 19 are shown 
in greater detail in FIG. 3. An image intensifier is also known as a light 
amplifier and is a device which, when actuated by a light image, 
reproduces a similar image of enhanced brightness, and is capable of 
operating at very low light levels without introducing spurious brightness 
variations into the reproduced image. Two types of image intensifiers are 
illustrated; there are other types that may be used in implementing the 
invention. The lower half shows a simple electrostatically focused image 
intensifier. Light strikes a semitransparent photocathode 23 which emits 
electrons 24 with a density distribution proportional to the distribution 
of light intensity incident on it. A positively charged phosphor screen 25 
at the other side of the intensifier converts the electron energy into 
visible light. The upper half illustrates a microchannel plate image 
intensifier which consists of a parallel bundle of small, hollow glass 
cylinders 26, where the inside walls of the cylinders are coated with a 
secondary emitting material. Electrons emitted from the photocathode 23 
strike the inside walls of the cylinders 26, causing secondary electron 
generation. The secondary electrons in turn continue to cascade down the 
inside walls of the cylinders to the phosphor screen 25, resulting in a 
high total current gain. Integrated circuit detector array 20 is comprised 
of a large number of parallel lines of individual photosensitive areas and 
the circuitry necessary to read the cells individually. Either 
charge-coupled device (CCD), charge-injection device (CID), or bipolar 
photodiode technologies can be employed. Roughly 1000 or so photons are 
needed to get a measurable signal. Thus an image intensifier providing a 
useful gain of 1000 will allow detection of single photons. 
FIG. 4 shows a temperature measuring system for making line temperature 
measurements using an intensified 2-D detector array. The surface 
temperature of multiple points along a line 27 on the observed surface 11 
are determined. Thermal emission from the plurality of points on the hot 
surface is focused by a camera lens 28 onto a slit 29 in a plate 30 which 
defines the field of view of the system. The focused thermal emission 
passing through the slit is presented to a collimating lens 31. The 
parallel beam of light is dispersed into component wavelengths and colors 
by an optical prism 32, and the different colors are focused by a lens 33 
onto an intensified 2-D array detector 34 which is comprised of an image 
intensifier 35 and a solid state 2-D photodetector array 36. 
The optical system is such that thermal radiation emitted from points along 
the line 27 in the x direction are detected at separate channels along the 
x dimension of the intensified 2-D array detector 34. The component 
wavelengths .lambda. of the thermal emission spread out along channels in 
the orthogonal direction as shown in the figure. The electronic system 37 
analyzes the read out detector signals, vertical channel by vertical 
channel, and selects the channel receiving shorter wavelength light toward 
the blue, and outputs that detector signal which is indicative of the 
surface temperature of the workpiece at a given point along line 27. This 
is done for every point along the line. 
In the embodiments of FIGS. 2 and 4, a prism spectrometer is used to break 
up the light beam into its component colors and discover what wavelengths 
are present in the light beam. A diffraction grating may be used as the 
spectrometer instead of a prism as a means of dispersing the light beam 
into spectra. If the grating spacing is known, then from a measurement of 
the angle of deviation of any wavelength, the value of this wavelength may 
be computed. The point temperature measurement system in FIG. 5 has a 
one-channel spectrometer in the form of an interference filter, and a 
single channel photon-counting system is comprised of a photomultiplier 
tube. Thermal emission from a spot 10 on the observed surface 11 is 
focused by a camera lens 38 and passes through a pinhole 39 in a stop 
plate 40 that serves to define and limit the field of view of the 
temperature measurement system. Parallel rays from a collimating lens 38' 
are incident on an interference filter 41 which selects a particular 
wavelength light to pass, toward the blue edge. A photomultiplier 42 
detects incident photons of light and has an output voltage dependent on 
the number of photons and the intensity of the received light. To provide 
a multichannel version of this system (not here illustrated), the 
interference filter can be tilted to isolate the color channels or there 
may be more than one filter. There are a plurality of photomultiplier 
detectors, one for every color channel. Refer to U.S. Pat. No. 4,081,215, 
Penney and Lapp, "Stable Two-Channel, Single-Filter Spectrometer", the 
disclosure of which is incorporated herein by reference. 
Temperature control for laser chemical vapor deposition using short 
wavelength thermal emission is illustrated in FIG. 6. This is a process in 
which a laser beam heats the surface and facilitates the deposition of 
gaseous chemical reactants onto the workpiece. A typical prior art LCVD 
system is shown here. A workpiece 43 is positioned within a sealed 
environment reaction chamber 44 having a reactant gas inlet port 45, a gas 
outlet port 46, and a pressure gage 47. A window 48 suitable for the 
transmission of a laser beam and an observation window 49 for viewing the 
workpiece are provided. A laser 50 generates a collimated laser beam 51 
that is reflected by a mirror 52 and focused by lens 53. The laser beam 
passes through window 48 and is focused to a spot on the surface of 
workpiece 43. Laser-assisted deposition of the gaseous chemical reactants 
onto the workpiece occurs either by a process of photolysis in which the 
laser beam causes the molecules of the gaseous reactant to dissociate and 
react with the substrate material or by a process of pyrolysis in which 
the laser beam heats the substrate and the gaseous reactant reacts 
directly with the substrate. 
Temperature control is achieved using a temperature measurement system 54 
of this invention to continuously determine workpiece surface temperature. 
The temperature signal generated by the system is supplied to the laser 
controls 55 which controls the laser 50 and the laser beam power to adjust 
surface temperature to a predetermined value. Alternatively, the laser 
beam may be delivered to the workpiece via an optical fiber. 
Some of the many applications of the invention are temperature 
determination and control during continuous and pulsed laser-assisted CVD 
of oxides, nitrides or carbides on steel surfaces. 
The invention is applicable to other radiation assisted, thermally driven 
processes, which are also known as photothermal processes. Examples are 
the heat treatment of a surface to cause surface components to diffuse 
into the material, and processes to recrystallize a surface. 
While the invention has been particularly shown and described with 
reference to several preferred embodiments, it will be understood by those 
skilled in the art that the foregoing and other changes in form and 
details may be made without departing from the spirit and scope of the 
invention as defined in the appended claims.