Title: Reference signal for stitching of interferometric profiles
Abstract: Composite height profiles are produced by taking successive interferometric measurements of different sections of a sample surface by sequentially placing them within the field of view of the instrument. A reference signal is used to provide a full history of scanner motion during each measurement scan. The reference signal is independent of the fringes collected for profile-measurement purposes and is used to produce a z-position history of the scan that is independent of scanner nonlinearities and other error sources. As a result, errors caused by scanner nonlinearity and lack of repeatability are removed from the process and it is possible to combine profiles of sections that are spatially disconnected without loss of precision attributable to scanner imperfections.
Patent Number: 6,987,570 Issued on 01/17/2006 to Schmit, et al.
| Inventors:
|
Schmit; Joanna (Tucson, AZ);
Olszak; Artur (Tucson, AZ)
|
| Assignee:
|
Veeco Instruments Inc. (Tucson, AZ)
|
| Appl. No.:
|
109361 |
| Filed:
|
March 28, 2002 |
| Current U.S. Class: |
356/511 |
| Current Intern'l Class: |
G01B 9/02 (20060101) |
| Field of Search: |
356/511,512,513,514,515,516,489,495,479,497,2,601
|
References Cited [Referenced By]
U.S. Patent Documents
| 5061071 | Oct., 1991 | Fujita et al.
| |
| 5122648 | Jun., 1992 | Cohen et al.
| |
| 6069700 | May., 2000 | Rudnick et al.
| |
| 6473186 | Oct., 2002 | Kawasaki et al.
| |
| 2002/0180983 | Dec., 2002 | Ina et al.
| |
Primary Examiner: Lee; Hwa Andrew
Attorney, Agent or Firm: Durando; Antonio R., Quarles & Brady Streich Lang LLP
Parent Case Text
CROSS REFERENCES TO RELATED APPLICATIONS
This application is a continuing application of Ser. No. 09/888,826, filed on
Jun. 25, 2001 U.S. Pat. No. 6,624,894.
Claims
We claim:
1. A method of producing a topographic map of a test structure using a scanning
unit, the method comprising the following steps:
scanning a section of said test structure with said scanning unit to generate
first topographic data;
scanning a section of said test structure with said scanning unit to generate
second topographic data;
sensing the position of said scanning unit to generate positional reference data
for said scanning unit;
producing a first topographic map employing said first topographic data independently
of said positional reference data;
producing a second topographic map employing said second topographic data independently
of said positional reference data; and
adjusting for positional variation of said scanning unit between the scanning
steps employing said positional reference data independently of said topographic data.
2. The method of claim 1, further comprising the step of combining said first
topographic map with said second topographic map to form a composite map, the adjusting
step including establishing a relative position between said first and second topographic maps.
3. The method of claim 2, wherein one of the scanning steps is performed on a
first section of said test structure and the other of the scanning steps is performed
on a different second section of said test structure, the combining step including
forming a composite spatial map from said first and second topographic maps.
4. The method of claim 2, wherein the scanning steps are performed at different
times on the same section of said test structure, the combining step including
forming a composite temporal map from said first and second topographic maps.
5. The method of claim 1, wherein the adjusting step comprises establishing a
relative position between said first and second topographic maps.
6. The method of claim 1, wherein the sensing step comprises detecting said scanning
unit during each of the scanning steps.
7. The method of claim 1, wherein said scanning unit is designed to scan over
a range of scanning operation and the sensing step is performed using a reference
signal of coherence length sufficient to cover said range.
8. The method of claim 1, wherein said scanning unit is designed to scan over
a range of scanning operation and is calibrated to produce a sequence of nominal
phase steps within said range.
9. The method of claim 8, wherein each of said nominal phase steps is π/2.
10. The method of claim 8, wherein each of said nominal phase steps is 3π/2.
11. The method of claim 1, wherein the sensing step comprises generating interference
patterns for said scanning unit; and further comprising the step of performing
an irradiance analysis on said interference patterns to determine said positional
reference data.
12. The method of claim 11, wherein said interference patterns are generated
by directing a reference signal toward a reflective surface and a reference surface
aligned in an optical path.
13. The method of claim 12, wherein said reference signal is produced by a narrow-bandwidth source.
14. The method of claim 12, wherein at least one of the scanning steps comprises
directing a measurement signal toward said test structure, said reference signal
being produced by passing a portion of said measurement signal through a narrow-bandwidth filter.
15. The method of claim 12, wherein at least one of the scanning step comprises
directing a measurement signal toward said test structure, said measurement signal
and said reference signal being substantially coextensive.
16. The method of claim 12, wherein the sensing step comprises producing two
derivative signals in quadrature from said reference signal, and detecting each
of said derivative signals independently of the other.
17. The method of claim 12, wherein said reflective surface is mounted on said
scanning unit.
18. The method of claim 12, wherein said reflective surface is associated with
said test structure.
19. The method of claim 1, wherein the sensing step is performed using a linear encoder.
20. The method of claim 1, wherein the sensing step is performed using a capacitive sensor.
21. The method of claim 11, wherein the sensing step is performed using a linear
variable differential transformer.
22. The method of claim 1, wherein the sensing step is performed using an eddy-current sensor.
23. The method of claim 1, wherein the scanning steps are performed using an
interferometric profiler.
24. The method of claim 1, wherein the scanning steps are performed using a confocal microscope.
25. The method of claim 1, wherein the scanning steps are performed using a stylus profiler.
26. The method of claim 1, wherein the scanning steps are performed using an
atomic-force microscope.
27. The method of claim 1, wherein the scanning steps are performed using an
optical-coherence tomographic profiler.
28. Apparatus for producing a topographic map of a test structure comprising:
a scanning unit;
means for positioning the test structure and said scanning unit relative to one
another so as to permit said scanning unit to perform a scan on a selected section
of the test structure;
means for generating first topographic data from a first scan by said scanning
unit and second topographic data from a second scan by said scanning unit;
means for sensing the position of said scanning unit and for developing positional
reference data for said scanning unit;
means for producing a first topographic map employing the first topographic data
independently of the positional reference data and a second topographic map employing
the second topographic data independently of the positional reference data; and
means for adjusting for positional variation of said scanning unit between the
first scan and the second scan independently of the topographic data.
29. The apparatus of claim 28, further comprising means for combining the first
topographic map with the second topographic map to form a composite map, said adjusting
means including means for establishing a relative position between the first and
second topographic maps.
30. The apparatus of claim 29, wherein said combining means is designed to form
a composite spatial map from the first and second topographic maps when the first
scan is performed on a first section of the test structure and the second scan
is performed on a different second section of the test structure.
31. The apparatus of claim 29, wherein said combining means is designed to form
a composite temporal map from the first and second topographic maps when the first
and second scans are performed on the same section of the test structure at different times.
32. The apparatus of claim 28, wherein said adjusting means comprises means for
establishing a relative position between the first and second topographic maps.
33. The apparatus of claim 28, wherein said scanning unit is designed to scan
over a range of scanning operation, said sensing and developing means comprising
means for producing a reference signal of coherence length sufficient to cover
said range and for directing the reference signal toward said scanning unit.
34. The apparatus of claim 28, wherein said scanning unit is designed to scan
over a range of scanning operation and is calibrated to produce a sequence of nominal
phase steps within said range.
35. The apparatus of claim 34, wherein each of said nominal phase steps is π/2.
36. The apparatus of claim 34, wherein each of said nominal phase steps is 3π/2.
37. The apparatus of claim 28, wherein said sensing and developing means comprises
means for generating interference patterns for said scanning unit, and means for
performing an irradiance analysis on the interference patterns to develop the positional
reference data.
38. The apparatus of claim 37, wherein said interference pattern generating means
comprises means for producing a reference signal and for directing the reference
signal toward said scanning unit.
39. The apparatus of claim 38, wherein said reference signal producing means
comprises a narrow-bandwidth source for the reference signal.
40. The apparatus of claim 38, wherein said topographic data generating means
comprises means for producing a measurement signal and for directing the measurement
signal toward the test structure, said reference signal producing means including
a narrow-bandwidth filter and means for passing a portion of the measurement signal
through said filter.
41. The apparatus of claim 38, wherein said topographic data generating means
comprises means for producing a measurement signal and for directing the measurement
signal toward the test structure, said measurement signal producing means and said
reference signal producing means being designed to make the measurement signal
and the reference signal substantially coextensive.
42. The apparatus of claim 38, wherein said sensing and developing means comprises
means for producing two derivative signals in quadrature from the reference signal
and for detecting each of the derivative signals independently of the other.
43. The apparatus of claim 38, wherein said scanning unit is provided with a
reflective surface and said reference signal producing means is arranged to direct
the reference signal toward said reflective surface.
44. The apparatus of claim 28, wherein said sensing and developing means comprises
a linear encoder.
45. The apparatus of claim 28, wherein said sensing and developing means comprises
a capacitive sensor.
46. The apparatus of claim 28, wherein said sensing and developing means comprises
a linear variable differential transformer.
47. The apparatus of claim 28, wherein said sensing and developing means comprises
an eddy-current sensor.
48. The apparatus of claim 28, wherein said topographic data generating means
comprises an interferometric profiler.
49. The apparatus of claim 28, wherein said topographic data generating means
comprises a confocal microscope.
50. The apparatus of claim 28, wherein said topographic data generating means
comprises a stylus profiler.
51. The apparatus of claim 28, wherein said topographic data generating means
comprises an atomic-force microscope.
52. The apparatus of claim 28, wherein said topographic data generating means
comprises an optical-coherence tomographic profiler.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to the general field of topographic profilometry and,
in particular, to a technique for improving the process of stitching multiple maps
obtained by independent profilometric measurements of a sample to form a composite
map. More particularly, the invention relates to a stitching procedure for interferometric profilers.
2. Description of the Prior Art
It has become common practice in interferometric profilometry to combine profiles
of adjacent sections of a sample surface to form a composite profile. Each section
of the surface is profiled independently, thereby making it possible to retain
with each successive measurement the optical resolution provided by the field of
view corresponding to the size of the section being sampled. This process is commonly
referred to in the art as a "stitching" procedure.
The x,y translation of the sample stage with reference to the interferometric
objective may introduce tip, tilt and offset variations between measurements that
require adjustments to the profile data in order to avoid errors. Such corrections
are typically implemented by using information derived by measuring overlapping
regions of adjacent sections of the sample surface. Another source of errors lies
in the nonlinearity and lack of repeatability of the vertical motion of the scanner
employed to produce the interferometric fringes that allow profiling of the sample
surface. That is, the absolute position of the interferometric objective at the
origin and during the step sequence of the scan may vary between measurements because
of inertial and other effects, such that separate profiles cannot be reliably combined
without appropriate correction. This is again accomplished by taking measurements
of adjacent sections with overlapping regions, so that information relating to
both sections can be obtained and used to normalize the profiles of the sections
to a common reference surface.
For example, commonly owned U.S. Pat. No. 5,987,189, No. 5,991,461 and No. 6,185,315
describe techniques for correcting stitching errors introduced by the scanner and
the x,y translation of the sample (or of the interferometric objective) between
measurements of the various surface sections of interest. In essence, successive
measurements of adjacent sections of the sample surface are taken by sequentially
placing them within the field of view of the instrument and independently profiling
each section by conventional interferometric procedures, such as phase shifting
or vertical scanning interferometry. The x-y translation of the microscope between
successive measurements is carried out by maintaining a region of overlap between
sections, so that spatial continuity is retained between measurements. The height
data generated for each section are then combined to form a larger image corresponding
to the entire surface tested and discontinuities and/or errors introduced by the
x-y translation process are corrected by normalizing the overlapping portions to
a common reference plane. This is achieved by fitting a plane equation through
each set of measured heights in the overlapping regions, and the tip, tilt and
offset of one of the fitted planes are used to correct the height data of adjacent
sections and produce matching overlapping profiles. The measured height data for
the balance of the adjacent section are then also corrected by the same difference
in tip, tilt and offset to obtain a continuous normalized image.
Other stitching techniques in the art also rely on overlapping measurement
regions to provide the information necessary to normalize all data and provide
a consistent composite map of the sample surface. This limitation requires redundancy
of measurements and does not allow reliable stitching of data collected from sample
sections separated by intervening unprofiled regions. While x,y positioning errors
may be reduced to acceptable tolerances by using high-precision translation stages,
similar precision hardware is not yet available to reduce to acceptable levels
the z-axis (offset) errors produced by scanner-motion nonlinearity and non-repeatability.
Therefore, no stitching method has yet been disclosed in the art that avoids the
need for taking measurements of overlapping regions of profiled sections in order
to obtain correlating information that can be used to calculate appropriate correction
and produce reliable composite profiles. Thus, a stitching process that overcomes
these shortcomings would be very desirable.
Another limitation of current profiler technology resides in the measurement
of temporal changes occurring in sample surfaces or structures of interest. For
example, the objective of a measurement may be to identify changes in the topography
of a membrane as a result of its interaction with a chemical agent. That is obviously
accomplished by profiling the sample at different times and comparing the results
to calculate changes. Using prior-art profilometric techniques, these changes can
be measured accurately only when at least a portion of the sample within the field
of view remains unchanged. That is, the measurements related to the unchanged portion
are used to provide a common reference from which changes can be calculated. Accordingly,
it would also be very desirable to develop a process that allowed calculation of
temporal surface changes when the entire sample within the field of view varies
with time.
Commonly owned pending application Ser. No. 09/888,826 discloses the use
of an optical reference signal dedicated to providing a full history of the scanner
motion during interferometric measurements, thereby allowing a determination of
scanning errors at each step. The reference signal is independent of the fringe
information collected for measurement purposes during the scan. The concept may
be implemented by utilizing an additional light source with the same scanner used
for the measurement (with the same or a different detector), so that the optical
path difference (OPD) varies in synchronization with both the reference-signal
and the data-collection procedures. Alternatively, a high temporal-coherence filter
and/or a reduced numerical-aperture objective may be used with the same light source
and optical path used for the interferometric measurement.
The present invention is based on the realization that the reference signal described
in the referenced patent application provides a solution to the problem of stitching
interferometric profiles without requiring spatial continuity between them. Accordingly,
this disclosure also enables stitching without normalization of the profile data
to a common reference plane and without recourse to other equivalent solutions.
BRIEF SUMMARY OF THE INVENTION
The primary objective of this invention is a stitching procedure that permits
the accurate combination of separately acquired profiles of different sections
of a sample structure into a single composite map without taking measurements of
overlapping regions.
Another objective of the invention is the ability to combine the profiles
of adjacent sections of the sample structure without normalization to a common
reference plane or other equivalent approach.
Another object is the ability to combine the profiles of sections of the
sample surface that are not spatially connected to one another, thereby enabling
composite profiles of large surface areas where only partial measurements are taken.
Yet another object is the ability to measure temporal variations in the profile
of a sample structure even when the profile varies across the entire field of view
of the profilometer.
Still another object of the invention is a procedure that is computationally
efficient, stable, repeatable and consistent with measured data.
A further objective of the invention is a procedure capable of implementation
in
real time for on-line applications.
Another goal of the invention is its general application to conventional
interferometric profilers without limitation as to any specific field of testing.
Another objective is an approach that may be used with any topographic instrument,
whether to measure a surface, or a boundary or map of features within a structure,
and an approach that can be implemented in equivalent fashion using an interferometric
reference signal or other precision device for tracking the position of the scanner
during measurements.
Finally, another goal is the implementation of the above mentioned objectives
in a system that utilizes existing sensory, computing, and other hardware devices.
In accordance with these and other objectives, the preferred embodiment of this
invention consists of a reference signal dedicated to providing a full history
of scanner motion during each measurement scan. The reference signal is independent
of the fringes collected for profile-measurement purposes and is used only to produce
a correct z-position history of the scan. This is then advantageously utilized
as a reference for correlating all independent profile measurements. The profiles
obtained by the successive measurement of each sample-surface section are generated
with reference to the scanner positions associated with the reference-signal history
collected during the scan, and the various profiles are then combined using their
respective reference-signal histories as the correlating factor. As a result, errors
caused by scanner nonlinearity and lack of repeatability are removed from the process
and the measurement of overlapping regions is no longer necessary to avoid offset
errors. Accordingly, it is possible to combine profiles of sample-surface sections
that are spatially disconnected without loss of precision attributable to scanner
imperfections. Similarly, it is possible to measure temporal changes in the sample
profile without reference to an area of unchanged topography.
Various other purposes and advantages of the invention will become clear
from its description in the specification that follows, and from the novel features
particularly pointed out in the appended claims. Therefore, to the accomplishment
of the objectives described above, this invention consists of the features hereinafter
illustrated in the drawings, fully described in the detailed description of the
preferred embodiment and particularly pointed out in the claims. However, such
drawings and description disclose only some of the various ways in which the invention
may be practiced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a conventional interferometric profiler
including a reference-signal channel according to the present invention in the
form of a beamsplitter in the detection path to reflect a portion of the light
toward a narrow-bandwidth filter, a stop, and a reference-signal detector.
FIG. 2 shows a large interpixel step in a test surface and corresponding measurement
and reference modulation curves to illustrate the advantages of using a reference
signal concurrently with the acquisition of profiling data.
FIG. 3 shows a step between two separate sections of a test surface and corresponding
measurement and reference modulation curves to illustrate the use of a reference
signal to correlate and combine the height profiles of the sections into a composite
map according to the present invention.
FIG. 4 is a schematic representation of an embodiment of the invention with
an internal reference interferometer sharing part of the optical path with the profilometer.
FIG. 5 is a schematic representation of the preferred embodiment of the invention
having two reference detectors adapted to receive two reference signals in quadrature.
FIG. 6 is a schematic representation of an embodiment including a reference-signal
channel in the form of a separate laser source with a central wavelength outside
the bandwidth of the broadband source, so that it can be selectively directed by
a dichroic filter to a reference-signal module.
FIG. 7 is a schematic representation of an embodiment where the measurement
detector, instead of a separate reference detector, is also used for the reference signal.
FIG. 8 is a schematic representation of an embodiment of the invention wherein
the reference signal is produced by non-optical means.
FIG. 9 is a flow-chart of the steps of the procedure of the invention.
FIG. 10 is a schematic diagram illustrating the generic applicability of the
concept of the invention to any topographic profiler that uses a scanning mechanism
to effect profiling measurements in combination with any position gauge capable
of determining the position of the scanner during the measurement operation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
This invention lies in the recognition that the reference signal disclosed in
Ser. No. 09/888,826 for the purpose of correcting scanner nonlinearities could
also be used advantageously as a correlating reference to overcome the scanning
errors experienced during data acquisition of interferometric stitching procedures.
As a result of this independent reference source, the prior-art need for measuring
overlapping regions of adjacent sections is avoided and spatially separated profiles
of the sample surface can be independently measured and combined directly without
further processing. Furthermore, the concept of the invention is equally applicable
to other profiling apparatus and can be implemented with other, non-interferometric,
reference signals.
The invention is described with reference to x, y and z orthogonal coordinates
wherein x and y define the plane approximately parallel to the test surface and
z defines the vertical scanning direction, but it is obvious that the structure
and operation of the features detailed in this specification can be rotated in
any direction with equivalent results. Thus, "vertical scan" is intended to refer
to any motion of a component in profilometric apparatus required to produce a measurement
in a sample. The term "correlogram" is used to refer to the three-dimensional set
of interferograms produced by a multistep scan during a conventional interferometric
measurement. "Nominal" phase step and "nominal" scan step refer to the design phase
step and the corresponding design scan step, respectively, of the interferometric scanner.
The inventive concept of Ser. No. 09/888,826, hereby incorporated by reference,
resides in the idea of using a reference signal to track the actual behavior of
the scanner in an interferometer to produce scanner-position data that can be used
directly to calculate the height map of a sample surface, thereby avoiding dealing
with the errors introduced by scanner nonlinearities and other error sources. A
narrow-band light source is preferably utilized, so that the entire range of operation
of the scanner may be advantageously covered. The invention is suitable for implementation
with all types of conventional interferometric techniques.
For example, the reference signal may be acquired by adding a channel in the
detection part of the interferometer. Referring to the drawings, wherein like parts
are designated throughout with like numerals and symbols, FIG. 1 illustrates in
schematic view the basic configuration of a conventional interference microscope
modified to produce a reference signal according to this concept. The interferometer
10 comprises a light source
12 directing a beam of light through
an aperture
14 and field stop
16 toward a beam splitter
18,
which reflects the light in the direction of a test surface S. The light reflected
by the beam splitter
18 passes through a microscope objective
20
focused on the test surface S. The objective incorporates an interferometer
22,
such as Mirau, comprising a beam splitter
24 and a reference mirror
26
adapted for relative movement with respect to the test surface, so that two light
beams are generated for producing interference fringes as a result of the optical
path difference between the reference mirror and the test surface S. The beams
reflected from the reference mirror
26 and the test surface S pass back
up through the optics of the microscope objective
20 and through the beam
splitter
18 to a solid-state detector array
28 in a camera in coaxial
alignment with the objective
20. Typically, the detector array
28
consists of individual CCD cells or other sensing apparatus adapted to produce
a two-dimensional array of digitized intensity data corresponding to light signals
received at each sensor cell as a result of the interference of the coherent light
beams reflected from individual x-y coordinates or pixels in the surface S and
from corresponding coordinates in the reference mirror
26. Appropriate electronic
hardware (not shown) is provided to transmit the digitized intensity data generated
by the detector to a microprocessor for processing. The microscope objective
20,
as well as the interferometer typically incorporated within it, is adapted for
vertical movement to focus the image of the test surface on the detector array
28.
According to the invention described in Ser. No. 09/888,826, some of the
light directed to the measurement-signal detector
28 is diverted by a beamsplitter
30 to a reference channel, where the light is passed through a narrow bandwidth
filter
32 and collected by a reference-signal detector
34. The filter
32 is judiciously selected to provide a long coherence length, as required
to obtain usable irradiance data during the entire length of the scan. This approach
has the advantage that the reference signal travels much of the same path as the
measurement light, thus providing accurate information about the OPD change rate
during the scan. For low numerical-aperture (NA) lenses, this is a convenient way
to implement the reference-signal idea. For high NA lenses, though, interference
fringes become visible for smaller OPDs than would result simply from the coherence
of the light source (this is because of the interaction among rays hitting the
object's surface at high angles). Thus, the use of a narrow-bandwidth filter alone
in the reference signal channel does not appreciably extend the signal range with
respect to the measurement path. However, the influence of the lens's numerical
aperture can be minimized by inserting an aperture stop
36 in front of the
reference detector
34 to limit the effective NA of the lens and provide
the required large depth of fringe visibility.
As the scanner
38 of the interferometer
22 scans through its vertical
range of operation, the reference signal is recorded in conventional manner and
the irradiance data so collected are used to calculate the actual z position of
the scanner at each acquisition frame. Any of the many interferometric techniques
used in the art for calibration of profiler scanners and/or interferometric analysis
(including methods used in distance measurement interferometry, DMI) can be utilized
to calculate the distance between frames corresponding to each scanning step. For
example, an average step size between frames can be calculated from a few intensities
registered at each pixel using the well-known Schwider algorithm, which produces
the average phase step (in radians) of four steps between five consecutive frames
as follows:
ΔΦ
avg=arccos(I5-I1)/[2(I4-I2)] (1)
where I1-I5 are five consecutive frames (1-5) of intensity data measured for
a given pixel around the frame of interest (typically frame 3 in the sequence of
5, but the value could be used equivalently with respect to any intermediate step
between frames 1 and 5). Similar algorithms may be used, such as the arctangent
function disclosed as preferred in Ser. No. 09/888,826.
Since the reference-signal light source is preferably selected to have a coherence
length covering the entire scanning range of the interferometer, sufficient contrast
is available during the entire scan and no calculation of modulation threshold
is necessary. Thus, a good approximation of the local phase step can be calculated
throughout the scan based on the registered correlogram generated by the reference
signal. For processing convenience, the nominal phase step of current interferometric
apparatus is typically calibrated to produce a phase change of π/2 or 3π/2
at the mean wavelength λ being used, with a corresponding optical-axis scan
step of λ/8 and 3λ/8, respectively.
One advantage of the technique is that the phase step can be calculated during
the scan with each consecutively collected reference-signal irradiance data and
the resulting true step-size values can then be used with any interferometric technique
to correct scanner-motion deviations from the calibrated trajectory. That is, the
scanner positions obtained from the reference signal are used directly to produce
the profile of the sample surface. This correction can be carried out "on-the-fly"
to obtain an immediate precise measurement or after the scan is completed; in the
latter case, the correct step sizes are used in the form of a look-up table. Whether
phase shifting interferometry (PSI), vertical scanning interferometry (VSI, also
referred to as white-light interferometry), a combination of VSI and PSI, phase
shifting interferometry on-the-fly (PSIOTF), or lateral scanning interferometry
(LSI, disclosed in Ser. No. 09/569,131) is being implemented, the step sizes produced
by Equation 1 or any equivalent function with reference-signal irradiance data
can be used directly in the processing routines for the measurement-signal data
instead of the nominal scanner values. As those skilled in the art readily understand,
phase steps between frames (expressed in radians) are routinely converted to scanner
step sizes (expressed in nm or other length dimension) when the information about
the wavelength of the light used is taken into account.
The advantage of using the reference signal of the invention for measuring phase
steps and determining scan positions is illustrated by the examplary modulation
curves depicted in FIG. 2. In the typical situation where a test surface S features
a large interpixel step
40 (as shown), a VSI technique with a low-coherence
light source is generally used to profile the surface. In such cases, the modulation
information for the pixels separated by the step
40 (represented schematically
by the modulation curves
42 and
44) do not overlap and the relative
distance of the two modulation peaks has to be calculated using calibrated scanner
step information. That is, the distance between the top and the bottom of the step
40 is calculated using the nominal step size and the number of frames between
them. The use of a high-coherence reference signal, depicted by the modulation
curve
46, provides continuous information about the quality and history
of the scan even when no signal is produced by the measurement channel of the instrument.
Another advantageous consequence of the continuous scan information provided
by the reference signal, which is the basis for the present invention, is that
it can be used as a correlating reference between successive scans taken over spatially
discontinuous sections of the sample surface (as well as temporally discontinuous
measurements of the same section). As illustrated in FIG. 3 for two exemplary pixels
of separate sections A and B of the sample surface S, the z position of the pixel
in section A is estimated during a first scan by the peak of the modulation curve
48 with reference to a corresponding reference signal represented by the
modulation curve
50. Inasmuch as the reference signal is produced to overcome
scanning nonlinearities and other scanning errors occurring during such first scan,
as detailed above, the modulation curve
50 provides a correct representation
of scanner position at each scanning step. The z position of the pixel on surface
B is similarly estimated by the peak of the modulation curve
52 during a
subsequent scan with reference to a corresponding reference signal represented
by the modulation curve
54. The modulation curve
54 also provides
a corrected history of the subsequent scan. Therefore, it also provides a correct
representation of scanner position at each step with respect to a fixed reference,
such as the origin of the scan, which can be used advantageously to correlate the
measurements produced by the two independent scans.
So long as the scanner position is always tracked and based on the reference
signal,
each profile generated by separate scans is measured with reference to known scanner
positions. Therefore, a common reference is provided by the reference signal and
separate profiles can be combined into a composite map regardless of scanner nonlinearities
because of the corrective effect produced by the reference signal. For example,
if the z position of the measured pixel of surface A is calculated to be z1 with
reference to the origin of the scanner range using the reference signal, and if
the z position of the measured pixel of surface B is similarly calculated to be
z2, it is clear that the relative height distance between the two pixels (i.e.,
z1-z2) is independent of errors introduced by scanning nonlinearities. Therefore,
the two measurements can be combined to produce a correct single composite map
without further processing of the data. That is, stitching of separately acquired
profiles can be carried out without the need to maintain regions of overlap between
measurements and without further processing of the profile information. This property
of the invention is very advantageous when a sample surface contains only separate
regions of interest.
It is noted that this invention does not provide a method for correcting offset
errors (i.e, z position errors) generated by the relative translation of the sample
surface with respect to the interferometric objective in the x,y plane. It is only
directed at correcting offset errors produced by nonlinearities and other imperfections
of the scanning mechanism. With the current state of the art of precision motion,
though, these latter errors are significantly larger than x,y translation errors.
Therefore, the assumption of ideal x,y translation is acceptable if mechanisms
of appropriate mechanical quality are used.
The invention can also be used to measure surfaces that change with time as a
result of environmental effects. Because each independent measurement is tied to
the same reference system, different measurements can be compared meaningfully
without recourse to an area of unchanged topography to provide the reference necessary
to correlate the measurements. This property is useful, for example, for measuring
vertical shifts of MEMS (micro electro-mechanical systems) micro-mirror devices.
The common-path reference-signal implementation illustrated in FIG. 1 is an inexpensive
way of providing the necessary information about scanner motion, but it can be
limited by the amount of light available for detection after reflection by the
beamsplitter
30. Also, the reference-signal acquisition process is necessarily
limited to the same predetermined location on the sample surface corresponding
to the location of the reference signal in the image plane of the detector. This
limitation may affect the quality of the reference signal depending on the sample
being measured.
According to another embodiment
60 of the invention, illustrated
in FIG. 4, an additional reference-signal interferometer is built into the conventional
measurement interferometer of the instrument and is dedicated to monitor OPD changes
through a separate reference-signal channel. The two interferometers can share
a significant portion of the profiler's optics (such as the beam splitter
18
and various lenses), but provide independent signal channels that can be tuned
separately without affecting either the measurement or the reference path. In one
implementation of this embodiment, a laser
62 is used to provide enough
light for a good-quality reference signal and a beamsplitter
64 is added
in the optical measurement path to direct the laser beam toward the main splitter
18. A portion of the light is passed through to a reference mirror
66
for the reference signal, while the balance of the light is reflected toward a
mirror or other reflective element
68 (such as a corner cube or a reflecting
grating) placed on the moving part of the scanner
38 of the measurement interferometer.
In the preferred embodiment
70 of the invention, illustrated in FIG. 5,
the reference signal is split into two signals shifted by 90 degrees (in software
or by means of optical elements), which are then used as reference signals in a
synchronous detection technique. As one skilled in the art would readily understand,
the phase of one of the signals and correspondingly the position of the scanner
can be determined directly from the synchronous reference signals in quadrature
using any DMI technique. At each pixel, the phase of the measured signal with respect
to the reference signal is determined for every frame. Then, this phase is added
to a real shift (position) of the scanner determined from the reference signal.
According to another embodiment
80 of the invention illustrated
in FIG. 6, the separate laser source
62 is used for the purpose of producing
a reference signal that shares the optical path of the measurement light. Instead
of the narrow-bandwidth filter
32 of FIG. 1, the wavelength selection is
done by a dichroic filter
82 that deviates the laser light towards the reference-signal
detector
34 and blocks its passage towards the measurement-signal detector
28. As those skilled in the art would readily understand, the bandwidth
of the measurement light is either not affected, or only altered in a very narrow
bandwidth, which can be taken into account during analysis of the measurement data.
The advantage of this approach is that more light is available for the reference
detector
34. Accordingly, the narrow width of the laser beam and the narrow
bandwidth of its wavelength produce many good-contrast fringes over a long scanning
range. Moreover, the reference signal is available to monitor changes in OPD directly
and independently of the measurement signal.
The concept of the invention can also be implemented in a system where the reference
signal is recorded using the same in FIG. 7 with reference to the embodiment of
FIG. 1. In such a case, a portion of the measurement beam is passed through a narrow-band
filter
32 prior to detection by particular pixels in the measurement detector
28. In this configuration, the reference signal does not record actual changes
in the profiler/object distance, but, under the commonly accepted assumption that
most system errors are generated by the scanner, this simple approach can be used
advantageously to provide a reference signal that tracks scanner motion. At the
same time, the approach affords a great degree of freedom in the selection of the
light source for the reference signal and no other optical elements need to be
introduced in the measurement path. Also, the reference-signal acquisition location
on the object's surface is not limited to a preselected position.
Using any of the disclosed embodiments for data collection, Equation 1 or alternative
algorithms may be employed for calculating the phase step between frames directly
from a sequence of reference-signal intensity data collected during the scan. The
value of the effective wavelength used for the reference signal is selected for
good results and is kept as constant as possible. However, as is well understood
in the art, the effective wavelength detected by the camera may vary with different
bulb intensities because of the fairly large bandwidth used for the measurement
channel. For example, in VSI a large bandwidth is used to produce a contrast peak;
when the intensity of the light source is changed to adjust the measurement, changes
in the mean wavelength may also occur. In addition, the effective wavelength of
the measurement light is typically not as well known as for laser or narrow-bandwidth
filtered light. By having a constant wavelength in the reference signal, it is
also possible to measure changes due to scanner motion and vibrations and to determine
to what degree the effective wavelength of the measurement light detected by the
camera is affected by either the bulb's characteristics (i.e. changes in voltage
and time) or by spectral reflectance of the material.
The invention can be implemented with a single point detector for the reference
signal, or with a multiple point detector to reduce the influence of noise. Alternatively,
in order to reduce the noise in the average phase calculated from the reference
signal, a second camera can be used to detect reference-signal fringes of high
coherence from an entire surface, rather than from a few pixels.
It is also noted that the invention has been described in terms of an optical
reference signal, but one skilled in the art would readily appreciate that it must
not necessarily be so. The important feature of the invention is the idea of providing
a measure of actual scanner motion with respect to a known position independent
of the interferometric measurement data acquired during the scan. Therefore, any
position device
90 that provides a precise measure (through appropriate
hardware
92) of the scanner's relative displacement with respect to a known
position in the z direction at each data acquisition frame would be suitable for
practicing the invention in equivalent manner, as illustrated in FIG. 8. For example,
currently available displacement gauges, such as linear encoders, capacitive sensors,
linear variable differential transformers (LVDTs), and eddy-current sensors (all
represented schematically by reference numeral
90), provide measurements
with precision in the order of nanometers which could be used advantageously to
practice the invention.
Similarly, the concept of the invention is applicable to topographic apparatus
other than interferometric profilometers. In particular, confocal microscopes,
stylus profilometers, and atomic-force microscopes, all of which effect surface
measurements by recording the vertical position of a measuring unit during a vertical
scan of the surface, can be advantageously coupled to an independent reference
signal device to overcome the same problems discussed above with regard to interferometric
apparatus. The concept can also be used in confocal microscopy and optical coherence
tomography, where the map of interest is an internal feature of a sample structure,
such as an interface between separate solid phases. The general applicability of
the invention is illustrated schematically in FIG. 10, wherein component
94
refers to a generic topographic profiler (including any of the ones mentioned above)
that uses a scanning mechanism
96 to effect profiling measurements, and
component
98 similarly refers to a generic reference position gauge used
to determine the scanning position with respect to a fixed reference
100
during the measurement operation.
Thus, independently of the precise apparatus used to implement it, the invention
consists of taking successive measurements of different sections of interest in
a sample structure or surface. As each section is scanned, the position of the
scanner along the scanning range is tracked precisely at each data-acquisition
frame and the scanner's position history is correlated with the interferometric
data generated by the scan to provide a height profile that is unaffected by scanner
nonlinearities and inconsistent performance (lack of repeatability). A reference
signal operated in parallel with the measurement unit is most preferably used to
track the scanner position. As the relative x,y position of the sample surface
or structure and the measurement unit is shifted between scans, the scanner's vertical
position is kept unchanged or is tracked precisely with reference to the last reference-signal
position history, so as to provide a reference for the subsequent scan. Thus, the
profiles generated by the various scans over different sample sections can be combined
directly to form a composite profile. The steps of the invention are illustrated
in the diagram of FIG. 9.
Various changes in the details, steps and components that have been described
may be made by those skilled in the art within the principles and scope of the
invention herein illustrated and defined in the appended claims. For example, it
is understood that the reference signal in the interferometric implementation of
the invention may be sampled advantageously at much higher rates (e.g., at π/100)
than the measurement signal, thereby reducing the effect of noise. Since irradiance
signals are essentially sinusoidal, the reference value corresponding to each measurement
frame can then be fitted in conventional manner to provide better information than
could be derived from sampling at the measurement rate.
As one skilled in the art would readily understand, the invention can be readily
combined with variable-speed scanners used to measure surfaces separated by large
steps. Since the reference signal provides continuous information about the exact
position of the scanner, rapid vertical translations can be effected reliably without
concern for the loss of precision due to scanner's errant motion. Similarly, the
reference signal can be used advantageously to start the scan of a sample surface
from any arbitrary vertical position. The reference signal always provides a position
reference in relation to which the information collected from a scan can be processed.
Therefore, while the present invention has been shown and described herein
in what is believed to be the most practical and preferred embodiments, it is recognized
that departures can be made therefrom within the scope of the invention, which
is not to be limited to the details disclosed herein but is to be accorded the
full scope of the claims so as to embrace any and all equivalent processes and products.
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