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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to multi-wavelength optical thermometry.
Specifically, a non-contact temperature measurement is performed where the
front and back surfaces of a workpiece, such as a semiconductor wafer, is
used in an interferometric arrangement in order to measure changes in
optical path lengths. Quite specifically, two laser beams, each having a
different wavelength of light, are used for providing optical beams useful
for measuring the temperature of a semiconductor wafer.
Measurement and control of the temperature of a substrate or wafer during
many semiconductor manufacturing processes can greatly enhance yield.
Preferably, non-contact methods are employed to avoid contamination during
measurement and the problems associated with thermal contact. Also, the
electronics used with the measurement instrument can be located remote
from the vicinity where the manufacturing process is being performed.
Non-contact optical thermometry permits absolute determination of
arbitrarily varying temperatures, opening up important applications, such
as temperature control, when the temperature can be expected to vary from
a set point.
Substrate temperature is widely recognized as an important processing
parameter in the fabrication of a wide variety of thin film materials and
devices, particularly in the microelectronics industry. Optical
thermometry utilizes laser interferometry to determine temperature changes
from the thermal expansion and refractive index changes of a transparent
substrate whose front and back faces are polished and approximately
parallel. Such techniques have been used to measure temperature of
optically absorbing semiconducting materials, such as silicon and gallium
arsenide, using IR lasers at wavelengths of 1.15 .mu.m, 1.5 .mu.m or 3.39
.mu.m.
The concept of optical temperature probes is well known in the art as
evidenced by the articles entitled "An Interference Thermometer and
Dilatometer Combined" by M. Luckiesh et al, J. Franklin Inst. 194, 251
(1922) and "An Interferometric-Dilatometer with Photographic Recording" by
F. C. Nix and D. MacNair, Rev. Sci. Instru., Feb. 12, 1941, pp. 66-70.
More recently, in articles entitled "Wavelength Modulated Interferometric
Thermometry for Measurement of Non-Monotonic Temperature Change", IBM
Technical Disclosure Bulletin, 34, Oct. 5, 1991, pp 350-353 and "Thickness
Measurements Using IR Tunable Laser Source", IBM Technical Disclosure
Bulletin, 35, 1B, June 1992, pp 465-468, there are disclosed a laser based
arrangement for temperature measurement in which infrared laser radiation
illuminates a silicon wafer and is reflected from both the front and back
surfaces of the silicon workpiece. The workpiece is transparent because of
the semiconductor band gap, has a dielectric constant of approximately 12
and is quite temperature dependent. As the workpiece temperature changes,
the path length through the workpiece changes primarily from a shift in
the dielectric constant. The resulting interference signal formed in the
cavity between the two silicon workpiece surfaces also changes. In the
arrangements described in the above articles, the laser frequency dithers
slightly to provide a derivative signal indicating whether the temperature
is increasing or decreasing. The temperature is then calculated by fringe
counting at a rate, for silicon, of 7.degree. C. per fringe.
The complexity of counting fringes over a process temperature range in the
order of hundreds of degrees will be evident to those skilled in the art.
Moreover, errors result if the rate of change of the temperature is
greater than the rate at which the measurement scheme can count the
fringes. If the temperature could be measured using a single fringe over
the entire temperature range, the ambiguity resulting from fringe counting
could be eliminated.
SUMMARY OF THE INVENTION
The present invention overcomes the limitations of the described
arrangements by simultaneously using two wavelengths of laser light to
make two independent measurements of the path length difference between
the front surface and the back surface of the workpiece. Using known
wavelengths, either of the two measurements provide a measure of the
temperature changes .DELTA.T corresponding to one fringe or less.
The present invention is most beneficial in semiconductor processing or
manufacturing, or in-situ monitoring of process temperatures and
temperature changes. The invention also has application for in-situ
temperature measurement during rework where reworking of a single chip
causes heating of an entire substrate containing many such chips.
In order to obtain a temperature change over a large temperature range, the
phase of the interference signals at each wavelength is measured and the
difference of the measured phases is calculated. The combined phase
corresponds to a beat wavelength
##EQU1##
where .lambda..sub.1 is the wavelength of the first laser beam and
.lambda..sub.2 is the wavelength of the second laser beam.
For a one percent difference of wavelengths centered at 1.5 .mu.m, the beat
interference repeats every 75 .mu.m, or approximately 700.degree. C.
Within the interval, the optical path length difference within the
workpiece, and hence the temperature, is uniquely determined.
In order to generate laser light at different wavelengths preferably two
solid state single mode infrared lasers operating at wavelengths of
approximately 1.3 .mu.m or at wavelengths of approximately 1.55 .mu.m are
used.
The use of two laser beams at different wavelengths for optical ranging
applications are known in the art and is described in the articles
entitled "Optical Ranging by Wavelength Multiplexed Interferometry" by C.
C. Williams et al, J. Appl. Phys. 60(b) Sep. 15, 1986, pp 1900-1903 and
"Absolute Optical Ranging with 200-nm Resolution" by C. C. Williams et al,
Optics Letters, 14(11), Jun. 1, 1989, pp. 542-544.
A primary advantage of the present invention is that minimal
post-processing of signals is required to obtain a measurement of the
temperature. After performing an initial calibration of the apparatus in
order to obtain an initial beat phase, subsequent temperature measurement
is not dependent on any subsequent fringe counting, providing the
temperature excursion allows the beat phase to remain within a single
fringe.
A principal object of the present invention is therefore, the provision of
an optical temperature sensor employing a two-color laser source.
Another object of the invention is the provision of a multiwavelength
optical thermometer for use in monitoring semiconductor processes.
Further and still other objects of the invention will become more clearly
apparent when the following description is read in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE is a schematic representation of a preferred embodiment of
the present invention.
DETAILED DESCRIPTION
Referring now to the FIGURE, there is shown schematically a preferred
embodiment of the present invention. A laser source 10 emits two laser
beams of different wavelengths .lambda..sub.1 and .lambda..sub.2. In a
preferred embodiment the laser source 10 comprises two solid state single
mode infrared lasers. The wavelengths are selected so that the sample 12
is transparent to the laser beam and characterized by a dielectric
constant. The difference in wavelengths (.lambda..sub.2 -.lambda..sub.1)
is selected to be an amount which results in a single fringe corresponding
to the range of temperatures to be measured. In the case of a silicon
wafer sample, the wavelengths are selected to be in the vicinity of 1.55
.mu.m, and preferably the difference in wavelengths .lambda..sub.2
-.lambda..sub.1 is approximately one percent or on the order of 0.015
.mu.m.
In order to better understand the invention, the operation will be
described in terms of a single channel interferometer, i.e., the signal
due to only one laser beam of wavelength .lambda..sub.1.
Light from the laser source 10 is transmitted into one end of an optical
fiber 14, travels through directional coupler 16 and is transmitted from
the end of fiber 18 directly at the entrant or front surface 20 of the
sample 12. The end of fiber 18 is polished at an angle of approximately
7.degree. or 8.degree. from a plane normal to the longitudinal axis of the
fiber so that back reflections from the end of the fiber 18 are
eliminated. A portion of the transmitted light beam is reflected from
surface 20 back into the fiber 18 and a portion of the light beam
continues through the sample 12 to the back surface 22 whereat the light
reflects back through the sample 12 and into the fiber 18. The two
reflected light beams, i.e. reflections from the surface 20 and surface 22
of the sample 12, travel back through the fiber 18, through directional
coupler 16 and thence to a photodetector, such as a photodiode detector.
The difference in path lengths of the light reflected from the two surfaces
of the sample, equal to twice the thickness of the sample, results in an
intensity which varies as the sin .phi., where .phi. is 2 knt, k is a
wavenumber equal to
##EQU2##
n is the index of refraction of the transparent sample at the wavelength
.lambda..sub.1, and t is the sample thickness. In the case of a silicon
wafer sample, the index of refraction is temperature dependent and the
change of intensity can be related to temperature. The relationship
applies to any sample where the index of refraction is temperature
dependent.
When only a single frequency laser beam is used as described above, various
methods have been employed to measure the sample temperature over a range
of several hundreds of degrees, a typical range for common wafer
processing steps. The fundamental problem with such methods resides in the
fact that a complete sinousoidal period at the photodetector corresponds
to only a limited portion of the desired measuring temperature range. For
an unambiguous measurement of the sample temperature it is most desirable
to increase the temperature range corresponding to a single fringe, that
is a single sinusoidal period. In accordance with the teachings of the
present invention, this is accomplished by using two laser beams of
closely spaced laser frequencies.
Referring again to the figure, laser source 10 transmits two laser beams at
different wavelengths .lambda..sub.1 and .lambda..sub.2, into fiber 14,
through directional coupler 16 into fiber 18 and at the sample 12. Light
is reflected by each beam from front surface 20 and back surface 22, as
described above, into fiber 18, through directional coupler 16 and into
fiber 24. The reflected light beams are passed through lens 26 onto an
optical grating 28 or similar device which separates the interfering beams
at wavelength .lambda..sub.1 to a photodetector 30 and the beam at
wavelength .lambda..sub.2 to a photodetector 32.
The output signal at detector 30 varies as the sine of the optical phase as
described above. The phase can be determined by monitoring the output
signal over a small temperature range or by dithering the wavelength by an
amount sufficient to provide a nearly full fringe change in the
interference pattern. After calibrating the device in the above described
manner, both optical phases may be determined by the equations
##EQU3##
The phase difference .DELTA..phi. is equal to
##EQU4##
Selecting the difference of the wavelengths (.lambda..sub.2
-.lambda..sub.1) to be, for example, one percent, i.e. .lambda..sub.2
=1.565 .mu.m and .lambda..sub.1 =1.55 .mu.m, results in a beat or
effective wavelength of 162 .mu.m. An effective wavelength of 162 .mu.m
permits continuous temperature measurement over a temperature range of 700
degrees without any ambiguity.
In an alternative embodiment the laser source 10 is selected so that the
wavelength .lambda..sub.2 is 1.313 and the wavelength .lambda..sub.1 is
1.30 when the laser beam wavelengths are approximately 1.3 .mu.m.
The two output signals from the photodetectors 30, 32 are processed by any
of a number of known methods to obtain the phase difference between the
two output signals. In order to extract phase information from a
measurement of intensity versus path length or temperature, a minimum of
three data points must be obtained. This can be done most simply by
extending the techniques described in the two IBM Technical Disclosure
Bulletins, supra. In these articles the intensity was tracked as a
function of temperature, and fringe counting was employed to extend the
temperature range. In accordance with the present invention, a preferred
method is to track the intensity over a single fringe or over a
sufficiently large amplitude to obtain a signal which can be readily
processed and to use the two color laser beam method to provide data over
a broad temperature range. Such a method requires only a limited knowledge
of the temperature history (i.e. the last few degrees of temperature
excursion) in order to unambiguously measure the temperature.
An alternative method is to measure the three data points of intensity as a
function of wavelength by varying the wavelength of both laser beams at a
frequency .omega., and then measuring the intensity by the variation in
intensity at the frequency .omega. and at the frequency 2.omega.. The
wavelength variation can be small, typically below 1 .ANG. change in
wavelength, and can be produced by a variation in laser bias current. The
three measured data points are the laser diode power output, and locking
amplifier measurements of the two frequency components. These three
measurements suffice to determine the phase of the interference as a
function of wavelength. This measurement must be done independently for
each of the two color (wavelengths) laser beams and then numerically the
phase difference can be determined to provide an extended temperature
measurement range as discussed above.
While there has been described and illustrated a preferred embodiment of
multi-wavelength optical thermometry, it will be apparent to those skilled
in the art that variations and modifications are possible without
deviating from the broad scope of the invention which shall be limited
solely by the scope of the claims appended hereto.
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