U.S. patent application number 09/957478 was filed with the patent office on 2002-02-07 for apparatus for evaluating metalized layers on semiconductors.
Invention is credited to Chen, Li, Opsal, Jon.
Application Number | 20020015157 09/957478 |
Document ID | / |
Family ID | 25392023 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015157 |
Kind Code |
A1 |
Opsal, Jon ; et al. |
February 7, 2002 |
Apparatus for evaluating metalized layers on semiconductors
Abstract
An apparatus for characterizing multilayer samples is disclosed.
An intensity modulated pump beam is focused onto the sample surface
to periodically excite the sample. A probe beam is focused onto the
sample surface within the periodically excited area. The power of
the reflected probe beam is measured by a photodetector. The output
of the photodetector is filtered and processed to derive the
modulated optical reflectivity of the sample. Measurements are
taken at a plurality of pump beam modulation frequencies. In
addition, measurements are taken as the lateral separation between
the pump and probe beam spots on the sample surface is varied. The
measurements at multiple modulation frequencies and at different
lateral beam spot spacings are used to help characterize complex
multilayer samples. In the preferred embodiment, a spectrometer is
also included to provide additional data for characterizing the
sample.
Inventors: |
Opsal, Jon; (Livermore,
CA) ; Chen, Li; (Fremont, CA) |
Correspondence
Address: |
STALLMAN & POLLOCK LLP
Suite 290
121 Spear Street
San Francisco
CA
94105
US
|
Family ID: |
25392023 |
Appl. No.: |
09/957478 |
Filed: |
September 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09957478 |
Sep 20, 2001 |
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09688562 |
Oct 16, 2000 |
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6320666 |
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09688562 |
Oct 16, 2000 |
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09431654 |
Nov 1, 1999 |
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6191846 |
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09431654 |
Nov 1, 1999 |
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08887865 |
Jul 3, 1997 |
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5978074 |
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Current U.S.
Class: |
356/601 |
Current CPC
Class: |
G01N 21/211 20130101;
G01N 21/1717 20130101 |
Class at
Publication: |
356/601 |
International
Class: |
G01B 011/24 |
Claims
1. An apparatus for evaluating the characteristics of a sample,
comprising: an intensity modulated pump laser beam, said pump beam
being directed to a spot on the surface of the sample for
periodically exciting the sample; means for varying the modulation
frequency of said pump laser beam; a probe laser beam being
directed to a spot on the surface of the sample within a region
that has been periodically excited and is reflected therefrom; a
photodetector for measuring the power of the reflected probe beam
and generating an output signal in response thereto; steering means
for adjusting the lateral separation between the beam spots on the
surface of the sample; and processor for filtering the output
signal to provide a measure of the magnitude or phase of the
modulated optical reflectivity of the sample, said processor
further functioning to control the means for varying the modulation
frequency of the pump beam and the steering means in a manner so
that a plurality of measurements are taken and recorded at a
plurality of different modulation frequencies and at a plurality of
different separations between the beam spots, with the plurality of
measurements being used to evaluate the characteristics of the
sample.
2. An apparatus as recited in claim 1 wherein measurements are
taken as the separation between the beam spots is continuously
changed.
3. An apparatus as recited in claim 1 wherein measurements are
taken at discrete separation intervals between the beam spots.
4. An apparatus as recited in claim 1 wherein said steering means
functions to separate the beams over a range from an aligned
overlapping position to a spacing of at least 10 microns.
5. An apparatus as recited in claim 1 wherein the modulation
frequency of the pump laser beam can be at least 100 Mhz.
6. An apparatus as recited in claim 1 wherein the modulation
frequency of the pump laser beam can be varied from 100 KHz to 100
MHz.
7. An apparatus as recited in claim 1 wherein the photodetector
measures the periodic angular deflections of the probe laser beam
and wherein the processor uses the angular deflection measurements
to evaluate the characteristics of the sample.
8. An apparatus as recited in claim 1 further including a detector
means for measuring the intensity of rays within the probe beam as
a function of the angle of incidence with respect to the sample
surface and wherein the processor uses the angle of incidence
measurements to evaluate the characteristics of the sample.
9. An apparatus as recited in claim 1 further including a detector
means for measuring the change in polarization state of the
reflected probe beam and wherein the processor uses the
polarization state measurements to evaluate the characteristics of
the sample.
10. An apparatus as recited in claim 9 wherein the detector means
generates an output signal that represents an integration of rays
within the probe beam having multiple angles of incidence with
respect to the sample surface.
11. An apparatus as recited in claim 1 further including a means
for varying the size of either or both of the pump or probe beam
spots and wherein the reflected power of the probe beam is measured
as the size of the spot is varied and wherein the processor uses
the measurements at different spot sizes to evaluate the
characteristics of the sample.
12. An apparatus as recited in claim 1 further including a
broadband polychromatic light source for generating a polychromatic
probe beam, said polychromatic probe beam being directed to a spot
on the surface of the sample and is reflected therefrom, said
apparatus further including a detector means for measuring the
power of the reflected polychromatic light beam and generating a
plurality of second output signals corresponding to a plurality of
different wavelengths within the polychromatic beam and wherein the
processor uses the second output signals to evaluate the
characteristics of the sample.
13. An apparatus for evaluating the characteristics of a sample
comprising: an intensity modulated pump laser beam, said pump beam
being directed to a spot on the surface of the sample for
periodically exciting the sample; a probe laser beam being directed
to a spot on the surface of the sample within a region that has
been periodically excited and is reflected therefrom; a
photodetector for measuring the power of the reflected probe beam
and generating an output signal in response thereto; steering means
for adjusting the lateral separation between the beam spots on the
surface of the sample; and processor for filtering the output
signal to provide a measure of the magnitude or phase of the
modulated optical reflectivity of the sample, said processor
further functioning to control the steering means in a manner so
that a plurality of measurements are taken and recorded at a
plurality of different separations between the beam spots, with the
plurality of measurements being used to evaluate the
characteristics of the sample.
14. An apparatus as recited in claim 13 wherein measurements are
taken as the separation between the beam spots is continuously
changed.
15. An apparatus as recited in claim 13 wherein measurements are
taken at discrete separation intervals between the beam spots.
16. An apparatus as recited in claim 13 wherein said steering means
functions to separate the beams over a range from an aligned
overlapping position to a spacing of at least 10 microns.
17. An apparatus as recited in claim 13 wherein the modulation
frequency of the pump laser beam can be at least 100 MHz.
18. An apparatus as recited in claim 13 wherein the modulation
frequency of the pump laser beam can be varied from 100 KHz to 100
MHz.
19. An apparatus as recited in claim 13 wherein the photodetector
measures the periodic angular deflections of the probe laser beam
and wherein the processor uses the angular deflection measurements
to evaluate the characteristics of the sample.
20. An apparatus as recited in claim 13 further including a
detector means for measuring the intensity of rays within the probe
beam as a function of the angle of incidence with respect to the
sample surface and wherein the processor uses the angle of
incidence measurements to evaluate the characteristics of the
sample.
21. An apparatus as recited in claim 13 further including a
detector means for measuring the change in polarization state of
the reflected probe beam and wherein the processor uses the
polarization state measurements to evaluate the characteristics of
the sample.
22. An apparatus as recited in claim 21 wherein the detector means
generates an output signal that represents an integration of rays
within the probe beam having multiple angles of incidence with
respect to the sample surface.
23. An apparatus as recited in claim 13 further including a means
for varying the size of either or both of the pump or probe beam
spots and wherein the reflected power of the probe beam is measured
as the size of the spot is varied and wherein the processor uses
the measurements at different spot sizes to evaluate the
characteristics of the sample.
24. An apparatus as recited in claim 13 further including a means
for varying the modulation frequency of the pump laser beam and
wherein a plurality of measurements are taken at different
modulation frequencies and with different beam spot separations to
facilitate the evaluation of the sample.
25. An apparatus as recited in claim 13 further including a
broadband polychromatic light source for generating a polychromatic
probe beam, said polychromatic probe beam being directed to a spot
on the surface of the sample and is reflected therefrom, said
apparatus further including a detector means for measuring the
power of the reflected polychromatic light beam and generating a
plurality of second output signals corresponding to a plurality of
different wavelengths within the polychromatic beam and wherein the
processor uses the second output signals to evaluate the
characteristics of the sample.
26. An apparatus for evaluating the characteristics of a sample
comprising: an intensity modulated pump laser beam, said pump beam
being directed to a spot on the surface of the sample for
periodically exciting the sample; a probe laser beam being directed
to a spot on the surface of the sample within a region that has
been periodically excited and is reflected therefrom; a
photodetector for measuring the power of the reflected probe beam
and generating an output signal in response thereto; means for
adjusting the spot size of either or both of the pump or probe
beams on the surface of the sample; and processor for filtering the
output signal to provide a measure of the magnitude or phase of the
modulated optical reflectivity of the sample, said processor
further functioning to control the spot size adjustment means in a
manner so that a plurality of measurements are taken and recorded
at a plurality of different spot sizes with the plurality of
measurements being used to evaluate the characteristics of the
sample.
27. An apparatus as recited in claim 26 wherein the size of the
beam spots is varied from a few microns in diameter to ten microns
in diameter.
28. An apparatus as recited in claim 26 further including a
steering means for adjusting the lateral separation between the
pump and probe laser beam spots on the surface of the sample and
wherein a plurality of measurements are taken at different
separations between the pump and probe laser beam spots and wherein
the processor uses the measurements at spot separations to evaluate
the characteristics of the sample.
29. An apparatus as recited in claim 26 further including a means
for varying the modulation frequency of the pump laser beam and
wherein a plurality of measurements are taken at different
modulation frequencies and with different beam spot separations to
facilitate the evaluation of the sample.
30. An apparatus as recited in claim 27 wherein the modulation
frequency of the pump laser beam can be varied from 100 KHz to 100
MHz.
31. An apparatus as recited in claim 28 wherein the photodetector
measures the periodic angular deflections of the probe laser beam
and wherein the processor uses the angular deflection measurements
to evaluate the characteristics of the sample.
32. An apparatus as recited in claim 28 further including a
detector means for measuring the intensity of rays within the probe
beam as a function of the angle of incidence with respect to the
sample surface and wherein the processor uses the angle of
incidence measurements to evaluate the characteristics of the
sample.
33. An apparatus as recited in claim 28 further including a
detector means for measuring the change in polarization state of
the reflected probe beam and wherein the processor uses the
polarization state measurements to evaluate the characteristics of
the sample.
34. An apparatus as recited in claim 33 wherein the detector means
generates an output signal that represents an integration of rays
within the probe beam having multiple angles of incidence with
respect to the sample surface.
35. An apparatus as recited in claim 28 further including a
broadband polychromatic light source for generating a polychromatic
probe beam, said polychromatic probe beam being directed to a spot
on the surface of the sample and is reflected therefrom, said
apparatus further including a detector means for measuring the
power of the reflected polychromatic light beam and generating a
plurality of second output signals corresponding to a plurality of
different wavelengths within the polychromatic beam and wherein the
processor uses the second output signals to evaluate the
characteristics of the sample.
36. An apparatus for evaluating the characteristics of a sample,
comprising: an intensity modulated pump laser beam, said pump beam
being directed to a spot on the surface of the sample for
periodically exciting the sample; a probe laser beam being directed
to a spot on the surface of the sample within a region which has
been periodically excited and is reflected therefrom; a
photodetector for measuring the power of the reflected probe laser
beam and generating a first output signal in response thereto; a
broadband polychromatic light source for generating a polychromatic
probe beam, said polychromatic probe beam being directed to a spot
on the surface of the sample and is reflected therefrom; detector
means for measuring the power of the reflected polychromatic probe
beam and generating a plurality of second output signals
corresponding to a plurality of different wavelengths within the
polychromatic probe beam; and processor for filtering the first
output signal to provide a measure of the magnitude or phase of the
modulated optical reflectivity of the sample, said processor
further functioning to monitor the second output signals with the
first and second output signals being used to evaluate the
characteristics of the sample.
37. An apparatus as recited in claim 36 wherein the probe beam from
the polychromatic light source is directed to the same location as
the probe laser beam is directed.
38. An apparatus as recited in claim 36 further including a
steering means for adjusting the lateral separation between the
pump and probe laser beam spots on the surface of the sample and
wherein a plurality of measurements are taken at different
separations between the pump and probe laser beam spots.
39. An apparatus as recited in claim 36 wherein said steering means
functions to separate the pump and probe laser spots beams over a
range from an aligned overlapping position to a spacing of at least
10 microns.
40. An apparatus as recited in claim 36 further including a means
for varying the modulation frequency of the pump laser beam and
wherein a plurality of measurements are taken at different
modulation frequencies and with different beam spot separations to
facilitate the evaluation of the sample.
41. An apparatus as recited in claim 40 wherein the modulation
frequency of the pump laser beam can be varied from 100 KHz to 100
MHz.
42. An apparatus as recited in claim 38 further including a means
for varying the size of either or both of the pump and probe laser
beam spots and wherein the reflected power of the probe laser beam
is measured as the size of the spots is varied and wherein the
processor uses the measurements at different spot sizes to evaluate
the characteristics of the sample.
43. An apparatus as recited in claim 38 wherein the photodetector
measures the periodic angular deflections of the probe laser beam
and wherein the processor uses the angular deflection measurements
to evaluate the characteristics of the sample.
44. An apparatus as recited in claim 38 further including a second
detector means for measuring the intensity of rays within the probe
laser beam as a function of the angle of incidence with respect to
the sample surface and wherein the processor uses the angle of
incidence measurements to evaluate the characteristics of the
sample.
45. An apparatus as recited in claim 38 further including a second
detector means for measuring the change in polarization state of
the reflected probe beam and wherein the processor uses the
polarization state measurements to evaluate the characteristics of
the sample.
46. An apparatus as recited in claim 45 wherein the second detector
means generates an output signal that represents an integration of
rays within the probe beam having multiple angles of incidence with
respect to the sample surface.
47. A method for evaluating the characteristics of a sample
comprising the steps of: directing an intensity modulated pump
laser beam to a spot on the surface of the sample for periodically
exciting the sample; directing a probe laser beam to a spot on the
surface of the sample within a region that has been periodically
excited and is reflected therefrom; varying the modulation
frequency of the pump laser beam; varying the separation between
the pump and probe beam spots; measuring the power of the reflected
probe beam and generating an output signal in response thereto at
various modulation frequency and beam spot separations; and
filtering the output signals to provide a measure of the magnitude
or phase of the modulated optical reflectivity of the sample, at
various modulation frequencies and beam spot separations and using
said measurements to evaluate the characteristics of the
sample.
48. A method for evaluating the characteristics of a sample
comprising the steps of: directing an intensity modulated pump
laser beam to a spot on the surface of the sample for periodically
exciting the sample; directing a probe laser beam to a spot on the
surface of the sample within a region that has been periodically
excited and is reflected therefrom; varying the separation between
the pump and probe beam spots; measuring the power of the reflected
probe beam and generating an output signal in response thereto at
various beam spot separations; and filtering the output signals to
provide a measure of the magnitude or phase of the modulated
optical reflectivity of the sample, at various beam spot
separations and using said measurements to evaluate the
characteristics of the sample.
49. A method for evaluating the characteristics of a sample
comprising the steps of: directing an intensity modulated pump
laser beam to a spot on the surface of the sample for periodically
exciting the sample; directing a probe laser beam to a spot on the
surface of the sample within a region that has been periodically
excited and is reflected therefrom; varying the size of either of
both of the pump and probe beam spots on the sample; measuring the
power of the reflected probe beam and generating an output signal
in response thereto at beam spot sizes; and filtering the output
signals to provide a measure of the magnitude or phase of the
modulated optical reflectivity of the sample at various beam spot
sizes and using said measurements to evaluate the characteristics
of the sample.
50. A method as recited in claim 49 wherein the size of the spot on
the sample is varied between a few microns in diameter to ten
microns in diameter.
51. A method for evaluating the characteristics of a sample
comprising the steps of: directing an intensity modulated pump
laser beam to a spot on the surface of the sample for periodically
exciting the sample; directing a probe laser beam to a spot on the
surface of the sample within a region that has been periodically
excited and is reflected therefrom; measuring the power of the
reflected probe beam and generating an output signal in response
thereto; filtering the output signals to provide a measure of the
magnitude or phase of the modulated optical reflectivity of the
sample; directing a broadband, polychromatic light beam onto a spot
on the surface of the sample; measuring the intensity of the
reflected polychromatic light beam and generating a plurality of
second output signals corresponding to a plurality of different
wavelengths within the polychromatic beam; and evaluating the
characteristics of the sample using the modulated optical
reflectivity measurements and the measurements at different
wavelengths.
52. A method as recited in claim 51 further including the step of
varying the modulation frequency of the pump laser and measuring
the power of the reflected probe beam at a plurality of modulation
frequencies and using the measurements to characterize the
sample.
53. A method as recited in claim 52 further including the step of
varying the separation between pump and probe laser beam spots on
the sample surface and measuring the power of the reflected probe
beam at a plurality of separations and using the measurements to
characterize the sample.
54. A method as recited in claim 53 further including the step of
varying the size of either of both of the pump and probe laser beam
spots on the sample surface and measuring the power of the
reflected probe beam at a plurality of pump and probe beam spot
sizes and using the measurements to characterize the sample.
55. A method as recited in claim 52 wherein measurements are taken
as the separation between the pump and probe laser beam spots is
continuously changed.
56. A method as recited in claim 52 wherein measurements are taken
at discrete separation intervals between the pump and probe laser
beam spots.
57. A method as recited in claim 52 wherein the spacing between the
pump and probe beam spots is varied over a range from an aligned
overlapping position to a spacing of at least 10 microns.
58. A method as recited in claim 52 wherein the modulation
frequency of the pump laser beam can be varied from 100 KHz to 100
MHz.
59. A method as recited in claim 52 further including the step of
measuring the periodic angular deflections of the probe laser beam
and using those measurements to evaluate the characteristics of the
sample.
60. A method as recited in claim 52 further including the step of
measuring the intensity of rays within the probe laser beam as a
function of the angle of incidence with respect to the sample
surface and using the angle of incidence measurements to evaluate
the characteristics of the sample.
61. A method as recited in claim 52 further including the step of
measuring the change in polarization state of the reflected probe
beam and using the measurements to evaluate the characteristics of
the sample.
62. A method as recited in claim 61 wherein the step of measuring
the change of polarization state of the reflected probe beam
includes generating an output signal that represents an integration
of rays within the probe beam having multiple angles of incidence
with respect to the sample surface.
Description
TECHNICAL FIELD
[0001] The subject invention relates to a method and apparatus
particularly suited for the nondestructive characterization of
opaque layer structures on semiconductor samples.
BACKGROUND OF THE INVENTION
[0002] There is a great need in the semiconductor industry for
metrology equipment which can provide high resolution,
nondestructive evaluation of product wafers as they pass through
various fabrication stages. In recent years, a number of products
have been developed for the nondestructive evaluation of
semiconductor samples. One such product has been successfully
marketed by the assignee herein under the trademark Therma-Probe.
This device incorporates technology described in the following U.S.
Pat. Nos.: 4,634,290; 4,646,088; 5,854,710 and 5,074,669. The
latter patents are incorporated herein by reference,
[0003] In the basic device described in the patents, an intensity
modulated pump laser beam is focused on the sample surface for
periodically exciting the sample. In the case of a semiconductor,
thermal and plasma waves are generated in the sample which spread
out from the pump beam spot. These waves reflect and scatter off
various features and interact with various regions within the
sample in a way which alters the flow of heat and/or plasma from
the pump beam spot.
[0004] The presence of the thermal and plasma waves has a direct
effect on the reflectivity at the surface of the sample. Features
and regions below the sample surface which alter the passage of the
thermal and plasma waves will therefore alter the optical
reflective patterns at the surface of the sample. By monitoring the
changes in reflectivity of the sample at the surface, information
about characteristics below the surface can be investigated.
[0005] In the basic device, a second laser is provided for
generating a probe beam of radiation. This probe beam is focused
colinearly with the pump beam and reflects off the sample. A
photodetector is provided for monitoring the power of reflected
probe beam. The photodetector generates an output signal which is
proportional to the reflected power of the probe beam and is
therefore indicative of the varying optical reflectivity of the
sample surface.
[0006] The output signal from the photodetector is filtered to
isolate the changes which are synchronous with the pump beam
modulation frequency. In the preferred embodiment, a lock-in
detector is used to monitor the magnitude and phase of the periodic
reflectivity signal. This output signal is conventionally referred
to as the modulated optical reflectivity (MOR) of the sample.
[0007] This system has the advantage that it is a non-contact,
nondestructive technique which can be used on product wafers during
processing. Using lasers for the pump and probe beams allows for
very tight focusing, in the micron range, to permit measurements
with high spatial resolution, a critical requirement for
semiconductor inspection. The prior system has been used
extensively in the past to monitor levels of ion doping in samples
since the modulated optical reflectivity is dependent on ion dopant
levels in the sample. This dependence is relatively linear for the
low to mid-dose regimes (10.sup.11 to 10.sup.14 ions/cm.sup.2). At
higher dopant concentrations, the MOR signal tends to become
non-monotonic and further information is needed to fully analyze
the sample.
[0008] One approach for dealing with the problem of monitoring
samples with high dopant concentrations is to measure the DC
reflectivity of both the pump and probe beams in addition to the
modulated optical reflectivity signal carried on the probe beam.
Using the DC reflectivity data at two wavelengths, some ambiguities
in the measurement can often be resolved. The details of this
approach are described in U.S. Pat. No. 5,074,669, cited above.
[0009] Semiconductor fabrication technology is increasing in
complexity at a rapid pace. Various multilayer structures are being
developed which makes testing more difficult. In addition,
manufacturers are seeking to increase yields by fabricating chips
on larger diameter wafers. As the diameter of the semiconductor
wafers increases, the cost and value of each wafer increases. When
using large, valuable and expensive wafers, it is no longer
economically viable for manufacturers to rely on any forms of
destructive testing methodologies. Therefore, there is a great need
to provide equipment which can characterize complex structures with
many unknowns or variables in a nondestructive manner.
[0010] Inspection problems also arise where metalized layers are
deposited on semiconductors. If a typical metal layers is more than
100 angstroms thick, it will generally be opaque to more commonly
used optical wavelengths. Therefore, equipment designed to monitor
relatively transparent oxide layers cannot be effectively used to
inspect metalized layers. Therefore, some new methodologies are
required in order to inspect semiconductors with metalized layers.
These layers can be formed from materials, such as aluminum,
titanium, titanium nitride (TiN) and tungsten silicide (WSi).
SUMMARY OF THE INVENTION
[0011] In order to obtain sufficient information to characterize
more complex samples, a system has been developed which
substantially increases the amount of data that can be collected.
The system of the subject invention includes an intensity modulated
pump laser beam which is focused onto the sample in a manner to
periodically excite the sample. A probe laser beam is focused onto
the sample within the periodically heated area. A photodetector is
provided for monitoring the reflected power of the probe beam and
generating an output signal responsive thereto. The output signal
is filtered and processed to provide a measure of the modulated
optical reflectivity of the sample.
[0012] In accordance with the subject invention, the device further
includes a steering means for adjusting the relative position of
pump and probe beam spots on the sample surface. In the preferred
embodiment, the steering means is used to move the beam spots from
an overlapping, aligned position, to a separation of up to about 10
microns. Measurements can be taken as the separation of the beam
spots is gradually changed or at discrete separation intervals.
[0013] This approach is particularly useful for monitoring the
deposition of opaque, thin metal films. More specifically, the
measurements taken at different spatial distances between the pump
and probe beam spots can be used to help more accurately
characterize the thermal diffusivity of the layer. This information
can then be used by the processor to more accurately characterize
the sample.
[0014] It should be noted that the concept of taking a measurement
with a probe beam displaced from a pump beam can be found in the
prior art. For example, in U.S. Pat. Nos. 4,521,118 and 4,522,510,
both assigned to assignee herein and incorporated by reference,
deformations at the sample surface, induced by periodic heating,
are measured using a probe beam displaced from the pump beam. In
the latter patents, periodic angular deviations of the probe beam
are monitored. However, the latter patents do not teach or suggest
that it would be desirable to take multiple measurements as the
displacement between the two beams spots are varied.
[0015] Obtaining measurements from a probe beam displaced from a
pump beam is also disclosed in U.S. Pat. No. 5,228,776, assigned to
assignee herein and incorporated by reference. In this patent, an
effort is made to align the pump and probe beams at the opposed
ends of elongated conductive features within the sample. Further,
the focal planes of the two beams are displaced vertically. In the
principal embodiment of the U.S. Pat. No. 5,228,776, the lateral
spacing between the beams is selected and then fixed. There is no
teaching in the U.S. Pat. No. 5,228,776 that it would be desirable
to take multiple measurements as the displacement between the two
beams is varied.
[0016] In the preferred embodiment of the subject invention,
further information can be obtained by varying the modulation
frequency of the pump beam. While it has been known that obtaining
information as a function of modulation frequency is useful, the
subject invention expands upon the past teachings by increasing the
modulation range. In particular, in the prior art, the modulation
range was typically in the 100 KHz to 1 MHz range. Some experiments
utilized modulation frequency as high as 10 MHz. In the subject
device, it has been found useful to take measurements with
modulation frequencies up to 100 MHz range. At these high
frequencies, the thermal wavelengths are very short, enabling
information to be obtained for thin metal layers on a sample, on
the order of 100 angstroms.
[0017] In the preferred embodiment of the subject invention,
further information can be obtained by varying the spot sizes of
either the pump or probe beams. Varying the spot size of the pump
beam will vary the propagation characteristics of the thermal
waves. Varying the spot size of the probe beam will vary the
sensitivity of the system with respect to the depth of detection.
By taking measurements at different spot sizes, some depth
profiling information can be recorded and used to characterize the
sample.
[0018] In the preferred embodiment of the subject invention, still
further information can be derived by obtaining independent
reflectivity measurements at a plurality of wavelengths. More
specifically, the subject apparatus can further include a
polychromatic light source generating a second probe beam which is
directed to the sample surface. The reflected beam is captured by a
detector which is capable of measuring power as a function of
wavelength. These added measurements can also be used to help
better resolve ambiguities in the analysis and improve the
characterization of the sample.
[0019] It is also possible to add additional measurement modules
which measure either reflectivity or ellipsometric parameters as a
function of angle of incidence. Further, the system can also be
used to monitor the periodic angular deviations of the probe beam
to derive additional information.
[0020] Further objects of the subject invention will become
apparent from the following detailed description, taken in
conjunction with the drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic block diagram of the apparatus of the
subject invention.
[0022] FIG. 2 is a schematic diagram of a frequency synthesizer and
lock-in amplifier for electronically down mixing the high frequency
MOR signal to create a low frequency heterodyne signal output.
[0023] FIG. 3 is a schematic diagram of a quad cell
photodetector.
[0024] FIG. 4 is a schematic diagram of an array spectrometer.
[0025] FIG. 5 is a schematic diagram of the detection module for
simultaneously determining the reflectivity of rays within a beam
striking the sample at multiple angles of incidence.
[0026] FIG. 6 is a schematic diagram of a detection module for
determining ellipsometric parameters of a sample by analyzing a
reflected probe beam and which includes integrated data over
multiple angles of incidence with respect to the sample
surface.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0027] Turning to FIG. 1 there is shown a schematic diagram of the
apparatus 10 of the subject invention. The apparatus 10 is intended
to permit testing of a semiconductor sample 12 and in particular
can be used to derive information about layers on the sample.
Sample 12 is shown resting on a controllable stage 14. Stage 14 is
capable of precise movements and functions to scan the sample with
respect to the laser beams.
[0028] In the preferred embodiment, the apparatus 10 includes a
pump laser 20 for exciting the sample and a probe laser 22 for
monitoring the sample. Gas, solid state or semiconductor lasers can
be used. As described in the assignees earlier patents, other means
for exciting the sample can include different sources of
electromagnetic radiation or particle beams such as from an
electron gun.
[0029] In the preferred embodiment, semiconductor lasers are
selected for both the pump and probe lasers due to their
reliability and long life. In the illustrated embodiment, pump
laser 20 generates a near infrared output beam 24 at 790 nm while
probe laser 22 generates a visible output beam 26 at 670 nm.
Suitable semiconductor lasers for this application include the
Mitsubishi ML6414R (operated at 30 mW output) for the pump laser
and a Toshiba Model 9211 (5 mW output) for the probe laser. The
outputs of the two lasers are linearly polarized. The beams are
combined with a dichroic mirror 28. It is also possible to use two
lasers with similar wavelengths and rely on polarization
discrimination for beam combining and splitting.
[0030] Pump laser 20 is connected to a power supply 30 which is
under the control of a processor 32. The output beam of laser 20 is
intensity modulated through the output of power supply 30. The
modulation frequency has a range running from 100 KHz to 100 MHz.
In the preferred embodiment, the modulation frequency can be set up
to 125 MHz. As described in the above cited patents, if an ion
laser is used to generate the pump beam, the intensity modulation
can be achieved by a separate acousto-optic modulator.
[0031] Prior to reaching the beam combining mirror 26, the probe
beam 24 passes through a tracker 40 and a shutter 42. Tracker 40 is
used to control the lateral position of beam 24 with respect to the
probe beam as discussed more fully hereinbelow. The shutter 42 is
used to block the pump beam when other measurements which do not
require periodic excitation are being taken.
[0032] The beam 26 from probe laser 22 is turned by mirror 46 and
passed through a collimator 48 which is used to match the focal
plane of the probe laser spot with the pump laser spot. A shutter
50 is also located in the path of the probe beam 26 to block the
probe beam when measurements that do not use the probe are being
taken, so that no stray probe beam light hits the detector.
[0033] After the beams 24 and 26 are combined, they are redirected
through polarizing beam splitter 54 to another beam splitter 56. In
between, the beams pass through a quarter-wave plate 58 for
rotating the polarization of the beams by 45 degrees. The beams are
directed down to the sample 12 through a microscope objective 60.
Objective 60 has a high n.a., on the order of 0.9, and is capable
of focusing the beam to a spot size on the order of a few microns
and preferably close to one micron in diameter. The spacing between
the objective and the sample is controlled by an autofocus system
described in greater detail hereinbelow.
[0034] The returning reflected beams 24 and 26 pass through the
quarter-wave plate 58 a second time, resulting n another 45 degree
polarization rotation. This second rotation allows the beams to
pass through the beam splitter 54. A filter 64 is provided to
remove the pump beam light 24 allowing the probe beam light to fall
on the photodetector 70. Detector 70 provides an output signal
which is proportional to the power of the reflected probe beam 26.
Detector 70 is arranged to be underfilled so that its output can be
insensitive to any changes in beam diameter or position. In the
preferred embodiment, detector 70 is a quad cell generating four
separate outputs. When used to measure reflected beam power, the
output of all four quadrants are summed. When the subject apparatus
is operated to measure beam deflection, the output of one adjacent
pair of quadrants is summed and subtracted from the sum of the
remaining pair of quadrants. This later measurement will be
discussed below.
[0035] The output of the photodetector 70 is passed through a low
pass filter 71 before reaching processor 32. One function of filter
71 is to pass a signal to the processor 32 proportional to the DC
power of the reflected probe. A portion of filter 71 also functions
to isolate the changes in power of the reflected probe beam which
are synchronous with the pump beam modulation frequency. In the
preferred embodiment, the filter 71 includes a lock-in detector 72
for monitoring the magnitude and phase of the periodic reflectivity
signal. Because the modulation frequency of pump laser can be so
high, it is preferable to provide an initial down-mixing stage for
reducing the frequency of detection.
[0036] A schematic diagram of the frequency generation and
detection stage is illustrated in FIG. 2. As shown therein, a
frequency synthesizer 73 is provided for generating the various
pump beam modulation frequencies. Synthesizer 73 is under the
control of processor 32 and can generate an output from 100 KHz to
at least 125 KHz. This output is delivered as a signal to the power
supply 30 of laser 20.
[0037] Synthesizer 73 also generates an electronic heterodyne
signal for delivery to the lock-in amplifier 72. The heterodyne
signal will be close to, but different from the signal sent to the
pump laser. For example, the heterodyne signal can be 10 KHz higher
than the signal sent to the pump laser.
[0038] The heterodyne signal from the synthesizer is combined with
the output from the signal detector 70 in a mixer 74. The output of
the mixer will include signal components at both the sum and
difference of the two input signals. The difference signal will be
at the relatively low frequency of 10 KHz. All the signals are
passed through a low pass filter 75 to eliminate the high frequency
components from the synthesizer and the detector. The low frequency
signal is then demodulated by demodulator 76. The outputs of
demodulator 76 are the "in-phase" and "quadrature" signals typical
of a lock-in amplifier. The in-phase and quadrature signals can be
used by processor 32 to calculate the magnitude and the phase of
the modulated optical reflectivity signal.
[0039] In initial experiments, a model SR844 lock-in detector from
Stanford Research Systems was utilized. This device utilizes a
combination of analog and digital techniques to permit operation
over a wide frequency range. In this device, an internal frequency
synthesizer is used to modify the incoming reference signal from
the synthesizer to generate two reference signals which differ in
phase by 90 degrees. These reference signals are mixed with the
incoming signals from the detector 70. After filtering, the low
frequency signals are digitized by two 16-bit analog-to-digital
converters. The digital low frequency signals are supplied to a DSP
chip for analysis.
[0040] As an alternative to using an electronic heterodyne
down-mixing system, it is also possible to reduce the frequency of
detection using an optical heterodyne approach. Such an optical
approach is disclosed in U.S. Pat. No. 5,408,327, incorporated
herein by reference. In the latter system, both of the laser beams
are modulated but at slightly different frequencies. Both beams
generate thermal and plasma waves at their respective modulation
frequencies. The beam from one laser picks up an intensity
modulation upon reflection due to the modulated optical
reflectivity induced in the sample by the other beam. The MOR
signal picked up upon reflection "mixes" with the inherent
modulation of the beam, creating additional modulations in the beam
at both the sum and difference frequency. This process is analogous
to electrical heterodyning. The difference or "beat" frequency is
much lower than either of the initial beam modulation frequencies
and can therefore be detected by a low frequency lock-in
amplifier.
[0041] It should be noted that in the latter arrangement, both
beams carry the desired MOR signal and either one or both of the
reflected beams can be measured. Therefore, in the latter system,
the term probe beam could be applied to either laser beam. It
should also be noted that when some samples are exited with two
modulation frequencies, nonlinear effects in the sample may result
in periodic excitation at the sum and difference frequencies in the
sample. This effect is usually relatively small.
[0042] To insure proper repeatability of the measurements, the
signals must be normalized in the processor. As noted above, the DC
reflectivity of the probe beam is derived from detector 70. In
addition, the DC output powers of the pump and probe lasers are
monitored by incident power detectors 84 and 86 respectively. A
wedge 88 functions to pick off about one percent of the incident
beam power and redirects it to an edge filter 90 for separating the
two beams. The outputs of the detector 84 and 86 are passed through
the low pass portion of filter 71 and into the processor 32.
[0043] The signals are further normalized by taking a measurement
of the power of the pump beam 24 after it has been reflected. This
measurement is used to determine the amount of pump energy which
has been absorbed in the sample. The pump beam reflected power is
measured by detector 94. Wedge 88 functions to pick off about one
percent of the returning beams which are redirected to edge filter
96. The DC signal for both the incident pump and probe beam powers
as well as the reflected beam powers are used to correct for laser
intensity fluctuations and absorption and reflection variations in
the samples. In addition, the signals can be used to help calculate
sample parameters.
[0044] An autofocus mechanism is used to maintain the spacing
between the objective 60 and the sample 12 to be equal to the focal
length of the objective. This distance can be maintained to less
than one hundredth of a micron.
[0045] The autofocus mechanism includes a servo motor 108 for
varying the vertical position of the objective 60. The servo is
driven by an analog detection loop which determines if the
objective 60 is properly focusing the probe beam. As seen in FIG.
1, a small portion of the reflected probe beam light picked off by
the wedge 88 is redirected by filter 96 into the main elements of
the autofocus detection loop. The probe beam is focused by a lens
98 through a chopper wheel 104 located in the focal plane of the
lens 98. The light passing the chopper wheel 104 is imaged on a
split-cell photodetector 106.
[0046] If the objective 60 is out of focus, there will be a phase
difference in the light striking the two sides of the split cell
detector 106 which is measured by a phase detector 109. The phase
difference is used as an input to an amplifier 110 which in turn
drives the servo 108. This approach to autofocusing is known as the
automated Foucault knife edge focus test. This system can also be
used to intentionally defocus the beams, thereby varying the spot
sizes of the pump and probe beams. This adjustment can be used to
gain further information as discussed below.
[0047] In the preferred embodiment, a polychromatic, or white light
source 120 is provided to illuminate the sample to permit viewing
on a TV monitor (not shown). The beam 122 from the white light
source 120 is passed through a shutter 124. When measurements with
the lasers 20 and 22 are being taken, the shutter is closed so no
stray white light gets into the detection system.
[0048] A portion of the white light returning from the sample is
redirected by a beam splitter 130 to a CCD camera 132 for
generating an image for the TV monitor. The beam will pass through
a CCD lens 134 and a filter 136. Filter 136 functions to reduce the
amount of laser light reaching the camera. In the preferred
embodiment, a portion of the light is redirected by a beam splitter
140 into an array spectrograph 142 more fully described herein
below.
[0049] Having described the basic elements of the subject
invention, its operation will now be discussed.
[0050] In use, a sample 12 is placed on the stage 14. Measurements
at various points on the sample can be made by rastering the stage
to the desired location. During initial set up of a fabrication
procedure, a relatively high number of test points may be desired.
Once the fabrication procedure is up and running, only spot checks
of the wafer are typically made. This will often entail measuring
from five to twenty-five spots on a wafer.
[0051] At each measurement point, the pump and probe laser beams 24
and 26 are activated. In a typical measurement, the beams will be
initially colinearly aligned. Exact overlap will typically provide
the strongest modulated optical reflectivity signal. Measurements
can then be taken at a plurality of modulation frequencies of the
pump beam. The range of modulation frequencies and the number of
frequencies is selected by the operator. As noted above, in the
preferred embodiment, the range of pump beam modulation frequencies
covers from 100 KHz to 125 MHz. The output from the photodetector
70 is passed through the filter 71 (including lock-in 72) to the
processor which records the magnitude and/or phase of the modulated
optical reflectivity signal for each of the modulation frequencies
selected.
[0052] In accordance with the subject invention, once the desired
measurements are completed with the beam spots in an overlapping,
aligned position, the processor signals the tracker 40 to adjust
the position of the pump beam 24 so that pump and probe beam spots
on the sample surface are laterally displaced. In one possible
scenario, the beams are initially displaced a distance of one
micron. Once in this position, a series of measurements are taken
at different modulation frequencies in the manner described above.
Once again, the processor will record the modulated optical
reflectivity signal at each of these modulation frequencies.
[0053] In accordance with the subject invention, once the second
set of measurements are complete, the processor will again command
the tracker 40 to further separate the pump and probe beam spots to
a distance of two microns. Measurements will then be taken at this
two micron spacing and at subsequent spacings, each time increasing
the spacing by one micron. It is envisioned that for complex
samples, measurement might be taken at successive increments of 0.5
microns. Alternatively, measurements can be taken as the separation
of the two beam spots is continuously increased. The span of
spacings between beam spots can range from overlapping to about 10
microns.
[0054] It is envisioned that the user will be able to determine
what sort of scanning algorithm is best suited to the particular
test situation. Variables such as separation distance and number of
measurements at each separation can be entered by the user through
a software interface.
[0055] Once all of the measurements at various spacings and
modulation frequencies have been taken and stored, the processor
will attempt to characterize the sample. Various types of modeling
algorithms can be used depending on the complexity of the sample.
Optimization routines which use iterative processes such as least
square fitting routines are typically employed. One example of this
type of optimization routine used for thermal wave analysis is
described in "Thermal Wave Measurements and Monitoring of
TASi.sub.1 Film Properties," Smith, et. al., Journal of Vacuum
Science and Technology B, Vol. 2, No. 4, p. 710, 1984. An
optimization routine used for more general optical analysis can be
found in "Multiparameter Measurements of Thin Films Using
Beam-Profile Reflectivity," Fanton, et. al., Journal of Applied
Physics, Vol. 73, No. 11, p.7035, 1993.
[0056] It has been found that measurements taken at different beam
spot spacings are particularly useful in characterizing thermal
diffusivity of a sample. Identification of this parameter can be
used to determine the structure of the sample. In addition,
measurements at very high modulation frequencies are useful for
very thin films, since the wavelength of the thermal and plasma
waves is very short.
[0057] In the preferred embodiment of the subject invention,
further information can be obtained by varying the spot sizes of
either of both of the pump and probe beams. Varying the spot sizes
of the beams will vary the propagation and detection
characteristics of the thermal waves. In particular, when using a
highly focused pump beam spot size, the thermal waves will tend to
propagate in all directions (3 dimensionally) away from the area of
focus. In contrast, as the pump beam spot size increases, the
thermal waves will tend to propagate in a more 1-dimensional
manner, perpendicular to the area of focus.
[0058] With respect to the detection, a very large probe beam spot
tends to see mostly the 1-dimensional component of the thermal
wave. In contrast, a highly focused probe beam, being more
localized, is sensitive to the 3-dimensional character of the
thermal waves. This variation in the generation and detection
characteristics of thermal waves is characterized by the square
root of the sum of the squares of the pump and probe beam
diameters. When this value is small compared to the thermal
diffusion length, the measurement is 3-dimensional in character and
when the value is large compared to the thermal diffusion length,
the measurement is 1-dimensional in character. By taking
measurements at different pump and/or probe beam spot sizes, some
depth profiling information can be recorded and used to
characterize the sample.
[0059] As noted above, the spot size of both the beams can be
controlled by the autofocus system. In order to increase the size
of the beam spots, the processor can add an offset to the focusing
algorithm which would defocus the beams. The beam spots can be made
of different sizes by adjusting the collimator 48. In the preferred
embodiment, measurements are taken and recorded at various beam
spot sizes ranging from one micron to ten microns. This additional
information can be used to characterize the sample.
[0060] In the preferred embodiment of the subject invention, still
further measurements can be taken to reduce the ambiguities of
analysis. More specifically, in addition to the modulated optical
reflectivity measurements, it is also desirable to monitor the
periodic angular deflections of the probe beam due to deformations
in the surface of the sample induced by the periodic heating. This
type of measurement is described in detail in U.S. Pat. Nos.
4,521,118 and 4,522,510, cited above. As described in those
patents, because of the thermal expansion properties of samples,
the periodic heating by the pump beam will create a time varying
"bump" in the sample surface. If the pump and probe beam are spaced
apart, the probe beam will undergo periodic angular deviations at
the frequency of the modulated heating. These angular deviations
can be measured by a split cell detector. The output of the split
cell is sent to the filter and the processor. The processor
functions to calculate the magnitude and phase of the deflection
signal that is synchronous with the pump beam modulation
frequency.
[0061] As noted earlier, detector 70 is preferably a quad cell
detector, or a detector with four quadrants each generating
separate output signals. Deflections of the probe beam in both the
X and Y axis due to angular deviations of the probe beam can be
measured with such a detector. More specifically and as shown in
FIG. 3, to determine the deflection of the probe beam in the X
axis, the outputs of quadrants 200 and 202 are summed and
subtracted from the sum of quadrants 204 and 206. Alternatively, to
determine the deflection of the probe beam in the Y axis, the
outputs of quadrants 200 and 204 are summed and subtracted from the
sum of quadrants 202 and 206. It should be noted that these type of
measurements function to cancel out any changes in the reflected
power of the beam. As noted above, in operation, the output of all
four detector quadrants are fed to the filter 71 and processor 32.
If the output of the four quadrants is summed, the MOR signal is
obtained. If the quadrant halves are summed and subtracted, the
probe beam deflection signal can be obtained.
[0062] In use, the deflection signal can be recorded for each of
the beam spot positions and modulation frequencies. In practice,
there will usually be little or no deflection signal when the two
beam spots are colinearly aligned. However, as soon as the centers
of the beam spots are separated, a signal can be often be detected,
even if the beams are in a partially overlapping configuration.
[0063] In the preferred embodiment, the subject apparatus further
includes a spectrometer for providing additional data. As noted
above, a white light source 120 is necessary for illuminating the
sample for tracking on a TV monitor. This same light source can be
used to provide spectral reflectivity data.
[0064] As seen in FIG. 1, a beam splitter can be used to redirect a
portion of the reflected white light to a spectrometer 142. The
spectrometer can be of any type commonly known and used in the
prior art. FIG. 4 illustrates one form of a spectrometer. As seen
therein, the white light beam 122 strikes a curved grating 242
which functions to angularly spread the beam as a function of
wavelength. A photodetector 244 is provided for measuring the beam.
Detector 244 is typically a photodiode array with different
wavelengths or colors falling on each element 246 in the array. The
outputs of the diode array are sent to the processor for
determining the reflectivity of the sample as a function of
wavelength. This information can be used by the processor during
the modeling steps to help further characterize the sample.
[0065] It is also possible to provide a mechanism for measuring
sample reflectivity as a function of the angle of incidence of the
beam. To achieved this goal, a portion of the reflected probe beam
light 26 can be picked off by wedge 300. This light is sent to a
beam profile reflectometer module 302 as more clearly shown in FIG.
5. When taking these measurements, the pump beam is preferably
turned off so that the probe beam will not be modulated.
[0066] Module 302 is of the type described in U.S. Pat. No.
4,999,014 assigned to the assignee herein and incorporated by
reference. As described therein, if a probe beam is focused onto a
sample with a strong lens, various rays within the beam will strike
the sample surface at a range of angles of incidence. If the beam
is properly imaged with a relay lens 306, the various rays can be
mapped onto a linear photodiode array 308. The higher angles of
incidence rays will fall closer to the opposed ends of the array.
The output from each element 309 in the diode array will correspond
to different angles of incidence. Preferably, two orthogonally
disposed arrays 308a and 308b are provided to generate angle of
incidence information in two axes. A beam splitter 310 is used to
separate the probe beam into two parts so both axes can be detected
simultaneously. The output of the arrays will supplied to the
processor 32 for storage. The data can be used to further
characterize the sample.
[0067] In still a further preferred embodiment, ellipsometric
information can be derived from the sample using the reflected
probe beam. As in the previous example, the pump beam should be
turned off for these measurements. As seen in FIG. 1, a portion of
the reflected probe beam can be picked off by a beam splitter 400
and redirected to a beam profile ellipsometer 402 as shown in FIG.
6. A suitable beam profile ellipsometer is described in U.S. Pat.
No. 5,181,080, assigned to the assignee herein and incorporated by
reference.
[0068] As described above, the rays in the reflected probe beam
correspond to different angles of incidence. By monitoring the
change in polarization state of the beams (from the original linear
polarization, to elliptical polarization upon reflection),
ellipsometric information, such as .psi. and .DELTA., can be
determined.
[0069] To determine this information, the beam is first passed
through a quarter-wave plate 404 for retarding the phase of one of
the polarization states of the beam by 90 degrees. The beam is then
passed through a polarizer 408 which functions to cause the two
polarization states of the probe beam to interfere with each other.
The light is then passed through an imaging lens 410 an onto a
quadrant detector 412. Each quadrant, 414, 416, 418, and 420
generate separate output signals proportional to the power of the
probe beam striking that quadrant. These signals represents an
integration of the intensities of all the rays having different
angles of incidence with respect to the sample surface. As
described in U.S. Pat. No. 5,181,080, ellipsometric information can
be obtained if the signals from opposing quadrants 414 and 418 are
summed and subtracted from the sum of the signals from opposing
quadrants 412 and 420. The output of detector 412 is sent to the
processor 32 for storage and use in characterizing the sample.
[0070] As can be seen, the subject device can be used to provide a
large amount of measurement data in order to better resolve the
characteristics of the sample. Such complete measurements are often
necessary in order to determine the composition of a multilayer
structure.
[0071] While the subject invention has been described with
reference to a preferred embodiment, various changes and
modifications could be made therein, by one skilled in the art,
without varying from the scope and spirit of the subject invention
as defined by the appended claims.
* * * * *