U.S. patent application number 10/961798 was filed with the patent office on 2005-05-19 for apparatus and method for real time measurement of substrate temperatures for use in semiconductor growth and wafer processing.
Invention is credited to Barlett, Darryl, Clarke, Roy, Perry, Douglas, Taylor, Charles A. II, Williams, Jason.
Application Number | 20050106876 10/961798 |
Document ID | / |
Family ID | 34576700 |
Filed Date | 2005-05-19 |
United States Patent
Application |
20050106876 |
Kind Code |
A1 |
Taylor, Charles A. II ; et
al. |
May 19, 2005 |
Apparatus and method for real time measurement of substrate
temperatures for use in semiconductor growth and wafer
processing
Abstract
The invention is an optical method and apparatus for measuring
the temperature of semiconductor substrates in real-time, during
thin film growth and wafer processing. Utilizing the nearly linear
dependence of the interband optical absorption edge on temperature,
the present method and apparatus result in highly accurate
measurement of the absorption edge in diffuse reflectance and
transmission geometry, in real time, with sufficient accuracy and
sensitivity to enable closed loop temperature control of wafers
during film growth and processing. The apparatus operates across a
wide range of temperatures covering all of the required range for
common semiconductor substrates.
Inventors: |
Taylor, Charles A. II; (Ann
Arbor, MI) ; Barlett, Darryl; (Dexter, MI) ;
Perry, Douglas; (Chelsea, MI) ; Clarke, Roy;
(Ann Arbor, MI) ; Williams, Jason; (Ann Arbor,
MI) |
Correspondence
Address: |
Marshall G. MacFarlane
YOUNG & BASILE, P.C.
Suite 624
3001 West Big Beaver Road
Troy
MI
48084-3107
US
|
Family ID: |
34576700 |
Appl. No.: |
10/961798 |
Filed: |
October 8, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60509762 |
Oct 9, 2003 |
|
|
|
Current U.S.
Class: |
438/689 |
Current CPC
Class: |
G01J 5/0007 20130101;
G01J 3/024 20130101; C23C 16/46 20130101; G01J 5/60 20130101; G01J
5/0821 20130101; G01J 5/0003 20130101; C23C 16/52 20130101; G01J
3/0291 20130101; G01J 3/0286 20130101; G01J 3/10 20130101 |
Class at
Publication: |
438/689 |
International
Class: |
H01L 021/302; H01L
021/461; H01L 021/31; H01L 021/469 |
Claims
What is claimed is:
1. In an apparatus for measuring the temperature of a substrate
material by inference from its bandgap measured by diffuse
reflectivity comprising lamp means for emitting broad spectrum
white light, focusing means for focusing the white light upon a
surface of a substrate material, detector means positioned at a
non-specular position on the front side of said substrate material
and computing means for determining the temperature dependent
bandgap absorption from onset wavelength of non-specular reflection
from a surface of the substrate material, the improvement
comprising: single optical fiber means for collecting
non-specularly reflected light detected by said detector means.
2. The apparatus of claim 1 which further comprises at least one
selectively positionable lens for collecting said non-specularly
reflected light from said surface of said substrate, whereby said
light is focused by said lens and directed to said optical fiber
means.
3. The apparatus of claim 1 which further comprises at least one
selectively positionable mirror for collecting said non-specularly
reflected light from said surface of said substrate, whereby said
light is focused by said mirror and directed to said optical fiber
means.
4. The apparatus of claim 1 which further comprises positioning
means on which said detector is mounted whereby said detector scans
in the aperture plane of said single optical fiber.
5. The apparatus of claim 1 which further comprises a tilt stage on
which said detector is mounted whereby said detector may be
aligned.
6. The apparatus of claim 1 which further comprises laser source
means for aligning said detector means.
7. The apparatus of claim 1 which further comprises intensity
control means for controlling said lamp means.
8. The apparatus of claim 1 which further comprises heating means
for heating said substrate, switch means for automatically
switching from the use of said lamp means to said heating means
when its temperature is sufficiently high to emit visible
radiation, whereby said heating means emits said light.
9. The apparatus of claim 1 which further comprises a quartz rod
positioned behind said substrate material for collecting broadband
light.
10. The apparatus of claim 1, wherein said lamp means further
comprises a lamp condensing mirror.
11. The apparatus of claim 1 which further comprises an array
spectrometer optimized in predetermined wavelength range coupled to
said optical fiber means.
12. The apparatus of claim 1 which further comprises condensing
optics for the purpose of collecting said reflected light.
13. The method of measuring the temperature of a substrate material
by inference from its bandgap measured by diffuse reflectivity
comprising: A. Generating a broad spectrum of light by
light-producing means; B. Directing said light upon a front surface
of a substrate material whereby a portion of said light is
non-specularly reflected from at least one surface of said
substrate; C. Collecting at least a portion of said non-specularly
reflected light at at least one focusing mirror; D. Selecting at
least a portion of said collected light to a single optical fiber
means; E. Transmitting said at least a portion of said
non-specularly reflected light through said optical fiber means to
a spectrometer; and F. Analyzing said non-specular reflected light
to improve band edge definition. G. Mapping the surface temperature
of the wafer by detector scanning stage means.
14. The method of measuring the temperature of a substrate material
by inference from its bandgap measured by diffuse reflectivity
comprising: A. Generating a broad spectrum of light by
light-producing means; B. Directing said light upon a front surface
of a substrate material whereby a portion of said light is
non-specularly reflected from at least one surface of said
substrate; C. Collecting at least a portion of said non-specularly
reflected light at at least one focusing lens; D. Selecting at
least a portion of said collected light to a single optical fiber
means; E. Transmitting said at least a portion of said
non-specularly reflected light through said optical fiber means to
a spectrometer; and F. Analyzing said non-specular reflected light
to improve band edge definition. G. Mapping the surface temperature
of the wafer by detector scanning stage means.
15. The invention of claim 13, wherein said light-producing means
comprises a heater placed in proximity to said substrate.
16. The invention of claim 14, wherein said light-producing means
comprises a heater placed in proximity to said substrate.
17. The invention of claims 1 through 12, which further comprises
feedback means for sensing and controlling the output of said lamp
means.
Description
PRIORITY CLAIM
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/509,762, filed Oct. 9, 2003.
FIELD OF THE INVENTION
[0002] The invention relates to methods and devices for making
precise non-contact measurements of the temperature of substrate
materials during the growth and processing of thin films,
particularly pertaining to semiconductor growth and wafer
processing.
BACKGROUND OF THE INVENTION
[0003] Precise temperature measurement during the growth of
deposited layers on a semi-conductor wafer is critical to the
ultimate quality of the finished, coated wafer and in turn to the
performance of the opto-electronic devices constructed on the
wafer. Variations in substrate temperature, including intra-wafer
variations in temperature ultimately affect quality and composition
of the layers of material deposited. During the deposition process,
the substrate wafer is normally heated from behind and rotated
about a center axis. Typically, a resistance heater positioned in
proximity to the wafer provides the heat source for elevating the
temperature of the wafer to a pre-determined value. Precise control
of the temperature associated with the process is most desirable,
and is best achieved through precise and real-time monitoring of
the substrate temperature.
[0004] An example application illustrating the necessity of precise
temperature control is the formation of semiconductor
nanostructures. Semiconductor nanostructures are becoming
increasingly important for applications such as "quantum dot"
detectors, which require the self-assembled growth of an array of
very uniform sizes of nano-crystallites. This can only be
accomplished in a very narrow window of temperature. Temperature
uncertainties can result in spreading of the size distribution of
the quantum dots, which is detrimental to the efficiency of the
detector.
[0005] The growth of uniform quantum dots is an example of a
thermally activated process in which the diffusion rates are
exponential in temperature. Therefore, it is important to be able
to measure, and have precise control over, the substrate
temperature when growth or processing is performed.
[0006] Numerous methods have been disclosed for monitoring these
temperatures. One simple, but largely ineffective approach has been
the use of conventional thermocouples placed in proximity to, or in
direct contact with the substrate during the thin film growth
operation. This methodology is deficient in many respects, most
notably, the slow response of typical thermocouples, the tendency
of thermocouples (as well as other objects within the deposition
chamber) to become coated with the same material being deposited on
the semi-conductor wafer, thereby effecting the accuracy of the
thermocouple, as well as the spot thermal distortion of the surface
of the semiconductor wafer resulting from physical contact between
the thermocouple and the substrate. In any event, the use of
thermocouples near or in contact with the substrate is largely
unacceptable during most processes because of the poor accuracy
achieved.
[0007] Optical pyrometry methods have been developed to overcome
these shortcomings. Optical pyrometry uses the emitted thermal
radiation, often referred to as "black body radiation", to measure
the sample temperature. The principal difficulties with this method
are that samples typically do not emit sufficient amounts of
thermal radiation until they are above approximately 450.degree.
C., and semiconductor wafers are not true black body radiators.
Furthermore, during deposition a semiconductor wafer has an
emissivity that varies significantly both in time and with
wavelength. Hence the use of pyrometric instruments is limited to
high temperatures and the technique is known to be prone to
measurement error.
[0008] In "A New Optical Temperature Measurement Technique for
Semiconductor Substrates in Molecular Beam Epitaxy", Weilmeier et
al. describe a technique for measuring the diffuse reflectivity of
a substrate having a textured back surface, and inferring the
temperature of the semiconductor from the band gap characteristics
of the reflected light. The technique is based on a simple
principle of solid state physics, namely the practically linear
dependence of the interband optical absorption (Urbach) edge on
temperature.
[0009] Briefly, a sudden onset of strong absorption occurs when the
photon energy, hv, exceeds the bandgap energy E.sub.g. This is
described by an absorption coefficient,
.alpha.(hv)=.alpha..sub.g exp[(hv-E.sub.g)/E.sub.0],
[0010] where .alpha..sub.g is the optical absorption coefficient at
the band gap energy. The absorption edge is characterized by
E.sub.g and another parameter, E.sub.0, which is the broadening of
the edge resulting from the Fermi-Dirac statistical distribution
(broadening .about.k.sub.BT at the moderate temperatures of
interest here). The key quantity of interest, E.sub.g, is given by
the Einstein model in which the phonons are approximated to have a
single characteristic energy, k.sub.B. The effect of phonon
excitations (thermal vibrations) is to reduce the band gap
according to:
E.sub.g(T)=E.sub.g(0)-S.sub.gk.sub.B.theta..sub.E[exp(.theta..sub.E/T)-1]
[0011] where S.sub.g is a temperature independent coupling constant
and .theta..sub.E is the Einstein temperature. In the case where
.theta..sub.E>>T, which is well-obeyed for high modulus
materials like Si and GaAs, one can approximate the temperature
dependence of the band gap by the equation:
E.sub.g(T)=E.sub.g(0)-S.sub.gk.sub.BT,
[0012] showing that E.sub.g is expected to decrease linearly with
temperature T with a slope determined by S.sub.gk.sub.B. This is
well obeyed in practice and is the basis for the band edge
thermometry.
[0013] Variations on this methodology are taught by Johnson et al.,
in U.S. Pat. No. 5,388,909, and U.S. Pat. No. 5,568,978. These
references teach the utilization of the filtered output of a wide
spectrum halogen lamp which is passed through a mechanical chopper,
then passed through a lens, then through the window of high vacuum
chamber in which the substrate is located, and in which the thin
film deposition process is ongoing. Located within the chamber is a
first mirror which directs the output of the source to the surface
of the substrate. The substrate is being heated by a filament or a
similar heater which raises the temperature of the substrate to the
optimum level required for effective operation of the deposition
process. A second mirror located within the chamber is positioned
to reflect the non-specular (i.e., diffuse) light reflected from
the back surface of the substrate, said reflection being directed
to another window in the chamber and thence through a lens to a
detection system comprising a spectrometer. The wavelengths of the
elements of the non-specular reflection are utilized to determine
the band gap corresponding to a particular temperature. Johnson et
al. teach that the temperature is determined from the "knee" in the
graph of the diffuse reflectance spectrum near the band gap.
[0014] While the prior art is in some ways effective, use of
optical fiber bundles, intra chamber optics, mechanical light
choppers and mechanically scanned spectrometers renders the
methodology deficient in many respects. The detected signal suffers
from temporal degradation of the optics within the deposition
chamber. The mechanical components are overly susceptible to
failure and the overall methodology of collecting the signal is
simply too slow for real-time measurement and control applications
in the industrial production environment. In addition, the
described means of the prior art is subject to variations in
accuracy dependent upon the fluctuation, over time, of the output
of the halogen light source.
[0015] Specifically, the prior art relies on one or more optical
elements within the deposition chamber to direct the incident light
to the wafer and to collect the diffusely reflected light. The
presence of optics within the deposition chamber is problematic,
since the material being deposited during the coating process tends
to coat all of the contents of the chamber, including the mirrors,
lenses, etc. Over time the coatings build up and significantly
reduce the collection efficiency of the optics and can lead to
erroneous temperature measurement.
[0016] More importantly, the prior art relies on a mechanical light
chopper and a mechanical scanning spectrometer for measurement of
the light signal. Not only do the mechanical components fail
frequently with extended use, but it is well known that gears in
scanning spectrometers wear, resulting in continual shifts in the
wavelength calibration. This leads to perpetually increasing errors
in temperature measurement unless the instrument is recalibrated
frequently, which is a very time consuming process. In addition, it
is well known that scanning spectrometers are quite slow, requiring
anywhere from 1-5 seconds to complete a single scan. In most
deposition systems the semiconductor wafers are rotating, typically
at 10-30 RPM. In this case, a temperature measurement that takes
1-5 seconds to complete is by default an average temperature and it
is impossible to make any type of spatially resolved measurement.
If the process chamber has many wafers rotating on a platter about
a common axis, as is typical in a production deposition system, the
slow response time of the prior art makes it impossible to monitor
multiple wafers.
[0017] Furthermore, the prior art utilizes a quartz halogen light
source with no consideration of any type of output stabilization or
intensity control. Quartz halogen lamps are known to degrade
rapidly over time leading to fluctuations in the lamp output that
result in measurement variations and further system downtime for
lamp replacement.
[0018] Basically, the many limitations of the prior art have
limited the applications of diffuse reflectance or "band edge"
thermometry in the commercial setting.
BRIEF SUMMARY OF THE INVENTION
[0019] The invention is an optical method and apparatus for
measuring the temperature of semiconductor substrates, in real-time
during thin film growth and wafer processing, utilizing the nearly
linear dependence of the interband optical absorption edge on
temperature.
[0020] The present invention utilizes simple, efficient collection
optics, external to the deposition system, connected via a single
small core optical fiber to a solid state array spectrometer. The
system requires no mechanical light chopper or other means to
modulate the light signal. The invention can operate in one of
three modes: 1.) the above described diffuse scattering reflectance
mode, by utilizing a unique feedback controlled, stabilized light
source that has all optics completely external to the deposition
system. 2.) transmission mode with external light source or 3.)
transmission mode utilizing the substrate heater as a light source
(requiring no external light source).
[0021] The invention utilizes sophisticated software algorithms to
analyze diffusely scattered light from the semiconductor substrate
to accurately and precisely determine the wavelength position of
the optical absorption edge. The measured position of the
absorption edge is compared to calibration data using a multi-order
polynomial equation that is specific to each semiconductor wafer
material. The data acquisition speed and software algorithms are
fast enough to provide typical temperature sampling rates of 20 Hz
or better. The invention operates across a wide range of
temperatures covering all of the required range for growth on
common substrates, including GaAs, Si, InP, ZnSe, and other
semiconductor wafers. In particular, the system design is optimized
for the temperature regime between ambient and .about.700.degree.
C. that is not currently served by existing non-contact sensors
(e.g., pyrometer-type sensors).
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic of one embodiment of the invention
depicting the light source and detector, and the other major
components of the system.
[0023] FIG. 2 is a top perspective view of the variable focus
quartz halogen light source with the respective placement of the
optics, components, and light intensity feedback sensor.
[0024] FIG. 3 is a top perspective view of one embodiment of the
detector assembly, that which utilizes lenses for collection and
imaging the diffusely scattered light, showing the respective
placement of the optics and collection fiber.
[0025] FIG. 4 is a top perspective view of a second embodiment of
the detector assembly, which utilizes a single focusing mirror for
collection and imaging of the diffusely scattered light, showing
the respective placement of the mirror and collection fiber.
[0026] FIG. 5A is a graph showing the raw, unprocessed, diffusely
scattered light spectra from a typical semiconductor wafer at
several predetermined wafer temperatures demonstrating the
wavelength dependence of the absorption edge on temperature.
[0027] FIG. 5B is a graph showing the spectra of FIG. 5A after the
spectra have been preprocessed to remove background light below the
absorption edge and normalize the maximum intensity.
[0028] FIG. 6 is a graph showing the diffusely scattered light
spectrum after it has been fully processed and a linear fit has
been performed in the region of the absorption edge to determine
the exact absorption edge wavelength.
[0029] FIG. 7 shows the spectra processing configuration dialog
from the software user interface.
[0030] FIG. 8 is graph showing the typical measured relationship
for the absorption edge wavelength position versus wafer
temperature measured by a thermocouple in direct contact with the
surface of the wafer.
[0031] FIG. 9 is a graph showing a multi-order polynomial fit to
the absorption edge wavelength position versus sample temperature
data.
[0032] FIG. 10 is a graph showing the long term stability of the
temperature measurement apparatus at a single predetermined wafer
temperature.
[0033] FIGS. 11A and 11B are simplified schematic drawings of the
apparatus in a multi-wafer deposition system demonstrating the
geometry for measurement of multiple wafers on a rotating wafer
platen.
[0034] FIG. 12 is a graph showing typical temperature data obtained
from a rotating platen of multiple semiconductor wafers in a
multi-wafer deposition system.
[0035] FIG. 13 is a detailed graph showing the variation in
temperature across the surface of multiple wafers in a multi-wafer
deposition system after a single rotation.
[0036] FIG. 14 is a graph showing the measured band-edge wafer
temperature as a function of time as the wafer set point
temperature is first set to 300 degrees Celsius, then 450 degrees
Celsius.
[0037] FIG. 15 is a schematic of a second embodiment of the
invention depicting the substrate heater as the light source and
the detector, in transmission geometry, in relation to a deposition
chamber.
[0038] FIG. 16 is a schematic of a third embodiment of the
invention depicting the detector, in transmission geometry, in
relation to an external light source.
DESCRIPTION OF THE EMBODIMENT
[0039] A schematic of one embodiment of the measurement apparatus
10, depicting the light source 12 and detector assembly 26 in
diffuse scattering reflectance geometry, in relation to a
deposition chamber 16, is shown in FIG. 1. The system comprises a
broad band light source 12 mounted in proximity to a transparent
view port 18 on the chamber. The light source 12 is typically a
quartz halogen lamp, mounted outside the deposition chamber 16
which illuminates a semiconductor wafer 20 (ghost lines in this
view) from its front (polished) surface 22. The apparatus also
comprises a detector assembly 26, also mounted outside the
deposition chamber 16 proximate to a transparent view port 18 at an
angle that is non-specular to the light source 12; an optical fiber
assembly 27, including a first optical fiber 28 coupled to an array
spectrometer 32, and a second optical fiber 30 running collinear to
first optical fiber 28 and coupled to a visible alignment laser 34
for aid in alignment of the detector assembly 26. The optical
components are optimized, using appropriate optical coatings, for
either infrared or visible operation depending on the
characteristics of the wafer 20 being measured. The light source 12
is connected to control assembly 35, containing light source power
and control unit 36 via light source power/data cable 38. Computer
control of the light source 12, alignment laser 34 and spectrometer
32 is maintained by computer 40 which is connected to light source
12, alignment laser 34 and spectrometer 32 by USB cable 42.
[0040] Typically the back surface 24 of a semiconductor wafer 20 is
optically rough and can act as a diffuse scattering surface for the
light source 12. If both sides of the wafer 20 are polished, which
is sometimes the case, a diffuser (e.g., pyrolytic boron nitride)
can be inserted between the back surface 24 of the wafer 20 and the
substrate heater 46 to enhance diffuse scattering, but this is not
a requirement. Light is diffusely scattered from the surfaces 22,
24 and from within the bulk of the wafer 20, a portion of which
light is scattered in the direction of the detector assembly 26,
and is imaged onto the entrance face of the optical fiber 28. The
light is analyzed by the solid state array spectrometer 32.
[0041] The first step in operation of the invention is to optimize
the optics configuration (light source, collection optics and
spectrometer) for the wavelength range required for the wafer
substrate material. FIG. 2 is a top perspective view of the
variable focus, 150W quartz halogen light source 12 with the
respective placement of the optics, components, and light intensity
feedback sensor 54. The light source 12 components are mounted to
enclosure 48. The light source 12 is optimized for either visible
or infrared output depending on the substrate material to be
measured. This involves selecting an appropriate bulb 50 for the
source with either an enhanced visible or infrared coating on the
lamp reflector 52. The bulbs 50 are readily available from several
vendors, as are suitable infrared collimating reflectors. In the
preferred embodiment, additional coatings, such as gold coatings
for infrared optimization, are added to the lamp reflector 52. The
light source 12 is driven by a computer-controlled 200W power
supply 36 with an integrated feedback control circuit that is
connected to a light feedback sensor 54 mounted in the vicinity of
the bulb 50. The purpose of the sensor 54 and feedback control
circuit is to maintain the bulb output at a constant value,
variable by computer 40 control, for the duration of the bulb
lifetime. Without the feedback control circuit the output of the
bulb 50 exhibits oscillations and the overall output slowly
decreases over the lifetime of the bulb 50. A variable aperture 56
within the light Source 12 controls the size of the spot of light
illuminating the semiconductor wafer 20. The light is focused onto
the wafer 20 using a pair of lenses, one fixed lens 58, the other
variable focusing lens 60 variable in position to obtain the best
focus at the wafer surface. The depth of field is sufficient for
use on most deposition systems. The lenses 58, 60 are coated with a
broadband antireflection coating to minimize back reflections,
hence maximizing the output of the light source. Because of the
high heat output of acceptable bulb/reflector combinations, a fan
62 is providing for cooling the light source 12.
[0042] Shown in FIG. 3 is a top perspective view of one embodiment
of the detector assembly 26. The assembly 26 mounts outside of the
deposition system to a transparent view port 18 on the chamber 16,
allowing the optics to remain clean and uncoated. The components of
the detector assembly 26 are mounted to frame 63. A first detector
lens 64 collects the diffusely scattered light and collimates it
into the second detector lens 66. The second lens 66 images the
light onto the optical fiber assembly 27 containing single-core
optical collection fiber 28. The lenses 64, 66 are coated with a
broadband anti-reflection coating to minimize signal loss at the
lens surfaces. The position of the second lens 66 can be adjusted
to obtain the best focus at the fiber face 68. The optical fiber 28
can also be positioned, utilizing an adjustor 72, in x,y and z
directions to assist in maximizing the amount of light collected
into the fiber. This particular embodiment of the detector assembly
26 also comprises a micrometer-actuated, single-axis tilt mechanism
70 built into the front of the assembly 26 to assist in pointing
the detector at the wafer 20 within the chamber 16.
[0043] A second embodiment of the detector assembly 26a, shown in
FIG. 4, uses a short focal length focusing mirror 74 mounted to
support 75 to collect and focus the diffusely scattered light onto
the first optical fiber assembly 27. This detector assembly 26a
design also mounts outside the deposition chamber 16 and the
coatings on the mirror 74 are optimized for the wavelength range
required for the particular substrate material. The advantage of
using a mirror 74 is that reflection losses from the surfaces of
lenses are eliminated completely and all wavelengths of light are
focused to same point, thus maximizing the collection efficiency.
The disadvantage is that the overall size of the detector assembly
is larger.
[0044] The single small core optical fiber 28 component within
fiber optic assembly 27 used to connect the detector assembly 26 or
26a to the spectrometer 32 eliminates many of the shortcomings of
the present fiber bundle methods and apparatus in use. It is well
known that fiber bundles have significant optical losses which are
associated with the empty spaces which exist between adjoining
fibers within the same bundle. Further, the existence of multiple
fibers increases the susceptibility of the bundle to interference
from stray light. It is equally well known that optical fibers have
a predetermined "acceptance angle" and that economically practical
optical fibers generally have a predetermined acceptance angle with
a tolerance of + or -2 degrees. While these tolerances are
satisfactory in the case of single fiber optics, optical fiber
bundles containing dozens of individual optical fibers and are much
more susceptible to stray light, with the susceptibility increasing
as the number in the bundle increases. The most important advantage
of a single fiber is the spatial selectivity afforded by their
small aperture (.about.400 .mu.m). This is important for stray
light rejection. Additionally, optical fiber bundles are relatively
expensive, typically in a range of $300 to $400 per foot. Single
optical fiber of approximately 400 micron cross-section, on the
other hand, costs less than $10 per foot.
[0045] With reference to FIG. 1, the fiber optic assembly 27 used
in the invention is a dual, bifurcated silica/silica fiber selected
for maximum transmission in the wavelength range required by the
particular semiconductor material. One optical fiber core of the
bifurcated fiber is used for collecting light from the lenses
within the detector. The other optical fiber core is used to
transmit laser light from a red visible semiconductor diode
alignment laser 34 to the semiconductor wafer 20, for use in
alignment of the detector assembly 26. When the detector assembly
26 is first attached to the deposition system, the alignment laser
34 can be activated to produce a visible red laser spot
illuminating the region where the detector assembly 26 is aimed.
The use a small single core fiber for light detection allows for
very precise selectivity of the region or spot on the wafer 20 for
the temperature measurement. The detector optics image an area of
the wafer surface. The magnification of the system is defined by
the focal length of the lenses and the position of the second
(variable position) lens. The image of the wafer 20 at the face of
the optical fiber is much larger than the diameter of the core.
This allows the system to spatially resolve temperature across the
wafer surface by either rotating the wafer or by moving the
position of the fiber using the x,y adjustment within the detector
assembly 26. Although it is not incorporated into the detector
assembly 26 shown, it is possible to use automated actuators to
scan the x,y-position of the fiber to create a 2-dimensional map of
the wafer 20 surface temperature.
[0046] A principal component to realizing this invention is the
very sensitive, fiber-coupled solid state array spectrometer 32.
Solid state array spectrometers (having no moving parts) are
becoming common in applications where speed and sensitivity are
essential. Their drawback is modest resolution (.about.few nm in
wavelength). This is not a limitation here, because the band-edge
features are relatively broad and can be determined by fitting
procedures to much greater precision than the spectrometer
resolution. The use of a fiber-coupled array spectrometer 32 for
this application has the following advantages:
[0047] a. Speed: array spectrometers measure typically 128-2048
wavelength channels simultaneously. Millisecond measurement times
are possible.
[0048] b. Sensitivity: array spectrometers are very compact,
promoting high light throughput (low numerical aperture: F1.8-F3.0
is typical). For InGaAs arrays, 1000 ADU/sec/picowatt at 1200 nm is
a typical sensitivity.
[0049] c. Wide spectral range: with careful selection of the
spectrometer grating one can cover the entire spectral range
required for this application (typically a wavelength range of
.about.300 nm would cover a temperature range from ambient to
.about.700.degree. C.).
[0050] d. Infrared sensitivity: the most challenging aspect of
band-edge thermometry concerns those semiconductors with small band
gaps, in the infrared region. Commercial array spectrometers with
InGaAs photo diode arrays have recently become available at
reasonable cost. Conventional InGaAs arrays extend the spectral
range beyond that offered by conventional Si CCD arrays
(.about.250-100 nm) up to 1700 nm. This opens up a wider range of
semiconductors to band-edge thermometry.
[0051] e. Spatial selectivity: when used with fiber-optic coupling,
array spectrometers have excellent rejection of stray light
signals. This is because the fiber core can range from 50 um to 800
um (matched to the spectrometer numerical aperture). Therefore, by
imaging the light scattered from the illuminated portion of the
wafer 20 onto the fiber entrance core, it is possible to eliminate
stray light that originates elsewhere in the vacuum chamber (hot
evaporation sources, gauge filaments, etc.).
[0052] The array spectrometer 32 used in this invention has
sufficient speed and sensitivity and to allow the collection of
complete spectra from the semiconductor wafer 20 at typical data
rates of 20 Hz and can exceed 50 Hz if required.
[0053] Shown in FIG. 5A is an example of diffuse reflectance
spectra collected from a semiconductor wafer 20 at four different
predetermined temperatures. The spectra as shown are unprocessed,
"raw" spectra. The band edge absorption is clearly visible at each
temperature. Shown in FIG. 5B are the same spectra after they have
been pre-processed by software routines to remove unwanted
background light below the absorption edge and normalize the
maximum intensity. An example of a fully processed spectrum showing
a linear fit to the absorption edge is shown in FIG. 6. The fit to
the linear portion of the absorption edge in the spectra is
extrapolated back to the x-axis to provide a highly accurate and
reproducible wavelength value for the band-edge. This wavelength
value is then correlated to the sample temperature.
[0054] The software algorithms used to process the spectra and
correlate the band-edge wavelength to a temperature can be
dependent on the type of semiconductor wafer material as well as
the specific geometry of the deposition chamber. Every deposition
chamber is slightly different and can produce different artifacts
into the raw spectra signal. The software processing algorithms
must be flexible to handle many applications. Shown in FIG. 7 is
the Spectra Processing Software Dialog from the system software.
The specific steps in the spectra preprocessing and final
absorption band-edge computation processing are described
below.
[0055] Preprocessing:
[0056] Noise Floor: allows the system to be configured to ignore a
specific level of light deemed noise based on experimental
conditions. If no portion of the current spectrum is above the
noise floor, the system ignores the spectrum and collects another
spectrum.
[0057] Clip spectra: removes expected anomalies in data beyond the
absorption band-edge and provides a consistent wavelength position
for normalizing the spectra.
[0058] Divide data point by reference: divides a reference lamp
spectrum from the collected spectrum to remove any unwanted
features introduced by the lamp.
[0059] Remove Background: using derivative calculations, the
parameters under this heading configure how the system will remove
black body radiation or other unwanted light from each collected
spectrum. The derivative of a spectrum is first smoothed to enhance
broad features and remove narrow features. The point of interest
within the derivative is then determined by one of two methods, 1)
a linear fit to the peak of the 1.sup.st derivative that satisfies
a specified height; or 2) an offset from the peak of the 2.sup.nd
derivative. The wavelength of this POI is used to find the
background level of light. This background level is then subtracted
from the spectrum.
[0060] Clip data point to min.: all wavelength data below the
wavelength with the minimum intensity is set to the minimum
intensity value. This creates a flat line up to the wavelength with
the minimum value.
[0061] Subtract data point offset: subtracts the minimum intensity
value determined in the previous step from the entire spectrum.
[0062] Compute Bandedge:
[0063] Preprocessed spectra are smoothed further to enhance broad
features and remove narrow features. The absorption edge is then
computed in one of two ways; 1) the x-intercept using a linear fit
at the wavelength position of the peak of the 1.sup.st derivative,
or 2) wavelength position of the peak of the 1.sup.st
derivative.
[0064] The preprocessing steps outlined above allow the system to
accurately and reproducibly determine an absorption band edge
wavelength from a given spectrum. This wavelength is then
correlated to a wafer temperature through the use of calibration
files. Calibration data is obtained by collecting spectra from
semiconductor wafers at well known temperatures. The temperature of
the wafer 20 is measured by a thermocouple in direct contact with
the wafer surface. A typical calibration data file, shown in FIG.
8, depicts the absorption band-edge wavelength versus thermocouple
(TC) temperature. The wavelength versus TC temperature plot is
slightly non-linear at low temperature but becomes very linear, as
predicted, at high temperature. Shown in FIG. 9 is the third order
polynomial fit to the data with the polynomial coefficients
computed and displayed at the top of the graph. The second and
third order polynomial coefficients are quite small. The software
uses the computed polynomial to relate the computed absorption
band-edge wavelength to a wafer 20 temperature.
[0065] The absorption wavelength versus temperature calibration
depends not only on the semiconductor material, e.g. Si, GaAs, InP,
but also very strongly on the wafer 20 thickness, dopant type, and
dopant density. This requires that calibration files must be
acquired for wafers 20 of different thickness, dopant type, and
dopant density. Once calibration files have been acquired for
several variations that establish a trend, for example the shift in
absorption edge due to wafer 20 thickness, the software can compute
calibration curves for modifications. When the proper calibration
file is selected, corresponding to the correct wafer material,
wafer thickness and dopant density, the system can precisely and
reproducibly measure the wafer temperature with high accuracy.
Shown in FIG. 10, is a long term stability plot for repeated
measurement of a semiconductor wafer 20 over a four hour period.
The wafer 20 was held at 200.0+/-0.1 degrees Celsius using a PID
temperature controller with a calibrated thermocouple mounted
directly to the wafer 20 surface. The plot shows that the
absorption band-edge measurement was repeatable with a maximum
error of 0.1 degrees Celsius and a standard deviation over a four
hour period of 0.04 degrees Celsius.
[0066] FIGS. 11A and 11B show a typical application of the
invention to a multi-wafer production deposition system. Multiple
wafers 20 are mounted on a platen 82 that rotates about a central
axis 80. The light source 12 is positioned on the outside of
chamber 16 so that as the platen 82 rotates, each wafer 20
individually passes beneath the light source 12. The diffusely
scattered light 100 is detected from a port of chamber 16. Platen
82 rotation speeds can be as high as 60 RPM resulting in each wafer
20 being illuminated by the light source 12 for as little as 50 ms.
The measurement speed of the invention is thus essential if every
wafer 20 is to be measured with each rotation. An example of actual
temperature data from a commercial production deposition system is
shown in FIG. 12. As shown in FIG. 11B, the wafer platen 82 holds
4,6-inch diameter wafers 20 and the invention is measuring each
wafer 20 on the platen 82 repeatedly as the platen 82 rotates. Each
wafer temperature is shown to be highly repeatable and if the data
is analyzed in detail, as shown in FIG. 13, it can be seen that the
invention can spatially resolve the temperature across each wafer
20. The measurement shows that some wafers have a much larger
temperature gradient than others. One wafer 20 is much hotter at
the center while another is much hotter at the edges. This type of
temperature non-uniformity can cause significant differences in
device performance depending on where the device originates from
the wafer 20.
[0067] The described invention has sufficient speed and accuracy
that the band-edge wafer 20 temperature signal can be used as an
input to a proportional-integral-differential (PID) control loop
for the purpose of controlling the output power of the substrate
heater. Shown in FIG. 14 is a graph of the wafer 20 temperature as
a function of time, measured using the band-edge absorption signal
in a direct feedback loop to a PID controller. The temperature
ramps and stabilizes very quickly to the set point values of 300
degrees Celsius and 450 degrees Celsius respectively.
[0068] In further embodiments of the invention, the system
utilization can be extended by operating the system in
transmittance rather than reflectance geometry. In a third
embodiment of the invention, shown in FIG. 16, an external light
source 12 can be mounted to illuminate either the front or back
side of the wafer 20 and the detector assembly 26 can be mounted on
the opposite side of the wafer 20 in a transmission geometry. In
some applications where there is limited space behind the substrate
heater 46, a quartz rod can be placed behind the wafer 20 to
collect and redirect the transmitted light 98 to a suitable port
where the detector can be mounted. Provided the quartz rod is
located behind the wafer 20, it will not be coated by the
deposition process. FIG. 16 is a schematic of a third embodiment of
the invention depicting the detector assembly 26, in transmission
geometry, utilizing the substrate heater 46, within the deposition
chamber 16, as the source of light. In this geometry no external
light source is required.
[0069] In conclusion, a new real-time, non-contact temperature
measurement system has been described-for use in semiconductor
growth and wafer processing applications. The invention is designed
to overcome the limitations of existing technology to provide a
versatile non-invasive temperature sensor for a much wider set of
applications in the thin film semiconductor arena. Taking advantage
of recent developments in fiber-coupled array spectrometers, the
new invention provides a powerful tool to characterize multi-wafer
temperature uniformity in production reactors, a measurement that
cannot be performed currently with other temperature measurement
techniques. Numerous obvious modifications may be made to the
invention without departing from the scope thereof.
* * * * *