U.S. patent application number 09/881026 was filed with the patent office on 2002-12-19 for automated overlay metrology system.
Invention is credited to Aiyer, Arun A., Fay, Bernard.
Application Number | 20020192577 09/881026 |
Document ID | / |
Family ID | 25377629 |
Filed Date | 2002-12-19 |
United States Patent
Application |
20020192577 |
Kind Code |
A1 |
Fay, Bernard ; et
al. |
December 19, 2002 |
Automated overlay metrology system
Abstract
Non-imaging measurement is made of misalignment of lithographic
exposures by illuminating periodic features of a mark formed by two
lithographic exposures with broadband light and detecting an
interference pattern at different wavelengths using a specular
spectroscopic scatterometer including a wavelength dispersive
detector. Misalignment can be discriminated by inspection of a
spectral response curve and by comparison with stored spectral
response curves that may be empirical data or derived by
simulation. Determination of best fit to a stored spectral curve,
preferably using an optimization technique can be used to quantify
the detected misalignment. Such a measurement may be made on-line
or in-line in a short time while avoiding tool induced shift,
contact with the mark or use of a tool requiring high vacuum.
Inventors: |
Fay, Bernard; (Los Gatos,
CA) ; Aiyer, Arun A.; (Fremont, CA) |
Correspondence
Address: |
Whitham, Curtis & Whitham
Reston International Center
Suite 340
11491 Sunrise Hill Rd.
Reston
VA
20190
US
|
Family ID: |
25377629 |
Appl. No.: |
09/881026 |
Filed: |
June 15, 2001 |
Current U.S.
Class: |
430/22 ;
430/30 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 2924/0002 20130101; G03F 7/70633 20130101; H01L 22/34
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
430/22 ;
430/30 |
International
Class: |
G03F 009/00; G03C
005/00 |
Claims
Having thus described my invention, what I claim as new and desire
to secure by Letters Patent is as follows:
1. A method of measuring overlay alignment of sequential
lithographic exposures, said method including steps of forming
first separated features on a surface, forming second separated
features on said surface interleaved between said first separated
features, and illuminating said first and second separated features
and detecting an interference pattern.
2. A method as recited in claim 1, including the further step of
calculating a spectrographic response corresponding to said
interference pattern.
3. A method as recited in claim 1, wherein said illuminating and
detecting step is performed with a specular spectroscopic
scatterometer.
4. A method as recited in claim 3 wherein said scatterometer is of
the reflectometer type.
5. A method as recited in claim 3 wherein said scatterometer is of
the ellipsometer type.
6. A method as recited in claim 5, wherein said ellipsometer
measures complex reflectivity spectral ratio for two orthogonal
polarizations with broadband illumination.
7. A method as recited in claim 1 wherein said illumination is
broadband light.
8. A method as recited in claim 1 wherein said detection measures
amplitude and phase.
9. A method as recited in claim 1, wherein said illumination and
detection step results in measured spectral curves and including
the further steps of modelling said first arid second features by
simulation to obtain simulated spectral curves, and comparing said
measured spectral curves with said simulated spectral curves.
10. A method as recited in claim 9, wherein said comparing step
includes use of an optimization technique to determine best fit and
to quantify a misalignment value.
11. A test mark including a plurality of marks formed by a
lithographic exposure, a mark formed between said plurality of
marks by another lithographic exposure, said mark and said
plurality of marks forming a periodic structure.
12. A non-imaging metrology apparatus comprising means for storing
spectral curves, a specular spectroscopic scatterometer for
measuring reflection from a plurality of marks formed by two
lithographic exposures and forming a periodic structure, and means
for comparing processed signals output from said specular
spectroscopic scatterometer with said spectral curves to evaluate
misalignment of said two lithographic exposures.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to multiple
sequential lithography processes and, more particularly, to
metrology techniques for measurement and characterization of
overlay and alignment accuracy for sequential lithographic
exposures and in-line and on-line lithographic exposure, scanner or
stepper tools.
[0003] 2. Description of the Prior Art
[0004] Lithography processes are currently used in many research
and manufacturing environments. Among these environments, one of
the more economically important is that of semiconductor integrated
circuit manufacture. In this field, increased functionality,
performance and potential economy of manufacture has driven the
development of numerous successive generations of devices having
minimum feature size regimes of increasingly small dimensions and
correspondingly increased device density. Currently, feature size
regimes of one-quarter micron are available in commercial devices
with significant further reductions of minimum feature size
foreseeable or in experimental stages of development.
[0005] While sophisticated processes have been developed allowing
production of structures much smaller than can be resolved by known
lithographic processes, it can be understood that at least one
lithographic process is required to establish the location and
basic dimensions of various electronic devices (e.g. transistors,
capacitors and the like) on the wafer and chip areas thereof.
However, as a practical matter, numerous lithographic exposures and
processes are generally required since formation of devices in
different areas and different layers are generally included in
current and foreseeable designs.
[0006] For example, at extremely small minimum feature size
regimes, if different structures of a particular device, such as
the gate and channel of a field-effect transistor must be formed
with different lithographic processes, overlay accuracy must be
maintained at a high level to avoid significantly altering
electrical characteristics of the device. In much the same manner,
interlayer connections (e.g. vias, studs and interconnect layer
patterns) must be overlaid on each other with extreme accuracy in
order to reliably and repeatably form low resistance connections
which will remain reliable when placed in service. The accuracy of
positioning in all of these circumstances will hereinafter be
referred to as overlay accuracy.
[0007] It has been observed that integration density in successive
generations of integrated circuits roughly doubles every two to
three years. This increase in integration density corresponds to a
reduction of minimum feature size of about 30% over similar
periods. The overlay accuracy requirement must be held to a
fraction (generally about one-third or less) of the minimum feature
size to maintain device geometry and connection reliability. To
maintain such accuracy, overlay and stitching measurement accuracy
(mean value plus or minus 3 sigma) must be maintained to a
metrology error budget of about 10% or less of the maximum
allowable overlay error or about 3% of the minimum feature size
(less than 6 nm for 0.18 micron minimum feature size
technology).
[0008] Commonly used metrology techniques are based upon optical
microscopy observation of overlay targets for quantifying the
overlay error. These techniques have relied upon a
feature-in-feature imaging process where a feature such as a square
or line is formed within another generally similar shape (e.g.,
box-in-box or line-in-line) to define the overlay target. The
smallest feature size of the overlay target is typically of the
order of 1 micron, so that it can be imaged with high contrast,
well within the resolution limit of a conventional optical
microscope.
[0009] Measurement of these overlaid features with, for example, a
scanning electron microscope (SEM) of atomic force microscope (AFM)
has allowed measurement data to be developed which can be processed
to provide measurement resolution somewhat greater than the
resolution of available lithography tools, even though the features
measured are typically larger than the features which can be
lithographically resolved. However, such measurement techniques
require complex and expensive SEM or AFM tools which are inherently
of low throughput and the measurement is necessarily destructive,
decreasing manufacturing yield.
[0010] Perhaps more importantly, decreases in minimum feature size
and increases in integration density have required increasingly
complex, expensive and difficult to use measurement tools while
measurements produced are of reduced repeatability,
reproducibility, tool induced shift (which are the principal
components of the metrology error budget) and quantitative
certainty (e.g. confidence factor) as limits of both lithographic
and microscopic resolutions are approached, particularly when the
imaged features measured are necessarily much larger than the
minimum feature size. Further, it is not only necessary to
quantitatively evaluate the positioning accuracy of overlaid or
stitched together features, but the profile of the exposed and
developed resist and/or lithographically produced structures must
also be evaluated in separate measurements in order to assure that
structures with the desired electrical properties are produced.
[0011] Thus, it is seen that, at the present state of the art,
known overlay measurement techniques can only be extended to
smaller regimes of feature size at relatively great tool expense
and process difficulty and complexity and increasing uncertainty
and decreasing repeatability of result. Further, it is not at all
clear that advances in microscopy processes or other inspection
devices which rely upon imaging of features will be able to support
manufacturing processes of foreseeable regimes of integrated
circuit feature size and integration density.
[0012] Spectroscopic reflectometry and spectroscopic ellipsometry
are known and well-understood techniques for making quantitative
observations of surfaces and structures but have only rarely been
applied to lithographic processes or characterization of the
performance of lithography tools. However, one such application is
the use of specular scatterometry to provide a non-destructive
measurement of profiles of resist grating patterns of high
resolution. This was presented by Spanos and his students (X. Niu,
N. Jakatdar) at the First Small Feature Reproducibility workshop
(UC SMART Program Review, of which the assignee of the present
invention is a funding participant), held at the University of
California on Nov. 18, 1998. Spanos outlined plans to use a
spectroscopic ellipsometry sensor to extract profiles of 180 nm and
150 nm linewidth resist features. FIG. 8, derived from that
presentation shows spectral response curves of Log(Tan .PSI.), the
amplitude ratio of complex 0th order TE and TM reflectivities, for
two sets of nominal linewidth features (250 nm and 100 nm) with
.+-.7% variation linewidths. The result for the 100 nm feature
shows a .+-.7% linewidth variation on a 100 nm nominal feature (or
7 nm) produces differing spectral curves that can be used to
measure linewidth. However, it should be recognized that the curves
are quite similar in shape since they are produced by regularly
spaced features, and that both the shape of the spectral curves and
the location of the various peaks and valleys can be used to
evaluate the linewidth corresponding to a measured spectral curve,
by best fit comparison to a library of spectral curves, previously
generated by simulation and verified by comparison to calibration
experimental data.
[0013] Specular spectroscopic ellipsometry measures the 0th order
diffraction responses of a grating at multiple wavelengths and
fixed incidence angle using a spectroscopic ellipsometer sensor. A
description of the measurement technique can be found in the
publication "Specular Spectroscopic Scatterometry in DUV
Lithography" by X. Niu et al. (SPIE, Vol. 3677, pp. 159-168, March,
1999. The ellipsometer sensor measures complex reflectivity for two
orthogonal light polarizations.
SUMMARY OF THE INVENTION
[0014] The meritorious effects of the invention include provision
of an optical metrology technique which does not rely upon imaging
of features for inspection, increased resolution and quantitative
accuracy and repeatability which can be performed with apparatus of
much reduced expense and complexity at greatly increased
throughput, and simultaneous and non destructive overlay position
and feature profile measurements.
[0015] In order to obtain these effects, a method and apparatus are
provided which perform non-imaging metrology apparatus comprising
storage of spectral curves, measurement with a specular
spectroscopic scatterometer of reflection from a plurality of marks
formed by two lithographic exposures and forming a periodic
structure, and providing comparison of processed signals output
from said specular spectroscopic scatterometer with said spectral
curves to evaluate misalignment of said two lithographic
exposures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of a
preferred embodiment of the invention with reference to the
drawings, in which:
[0017] FIGS. 1A, 1B and 1C are illustrations of a box-in-box image
used in known metrology techniques,
[0018] FIG. 2 shows respective feature levels in accordance with a
preferred form of the invention,
[0019] FIG. 2A shows the images of the features of FIG. 2 overlaid
in desired alignment,
[0020] FIG. 2B shows the images of the features of FIG. 2 overlaid
with misalignment,
[0021] FIG. 3 shows a simulated Fourier spectrum developed in
accordance with the invention and corresponding to FIG. 2A (correct
alignment)
[0022] FIG. 4 shows a simulated Fourier spectrum developed in
accordance with the invention and corresponding to FIG. 2b
(misalignment),
[0023] FIG. 5 shows a simulated Fourier phase spectrum developed in
accordance with the invention with aligned and misaligned responses
in accordance with the invention overlaid on each other,
[0024] FIG. 6A is a schematic diagram of the measurement apparatus
in accordance with the invention, including a spectroscopic
reflectometer sensor and detector and a wafer with composite
overlay targets with exact alignment,
[0025] FIG. 6B is a schematic diagram of the measurement apparatus
in accordance with the invention, including a spectroscopic
reflectometer sensor and detector and a wafer with composite
overlay targets with some misalignment,
[0026] FIG. 7 is a schematic illustration of the measurement
process in accordance with the invention, and
[0027] FIG. 8 is an illustrative chart from a presentation by
Spanos, Niu and Jakatdar at the First Small Feature Reproducability
workshop.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
[0028] Referring now to the drawings, and more particularly to
FIGS. 1A, 1B and 1C, there is shown a typical box-in-box metrology
feature exemplary of features for such purposes known and used in
the art. The box-in-box feature 16 shown in FIG. 1C is a composite
feature formed by two overlaid lithographic exposures corresponding
respectively to features 12 and 24 of FIGS. 1A and 1B,
respectively, which are generally sequentially performed and each
preceded by deposition of a layer of resist and followed by
development of resist layer and possibly including etching or
material deposition processes between the lithographic exposures.
One feature will be larger than the other and the smaller feature
should be of dimensions which, ideally, closely approach the
minimum feature size of interest.
[0029] At the present state of the art, however, the smaller and
larger features are generally produced with a transverse dimension
of the order of a few microns. Such dimensions are about an order
of magnitude or more larger than minimum feature size of the
current generation of commercially available integrated circuits.
Such a difference in minimum feature size puts stringent demand on
processing of measurement data to hold the overlay accuracy budget
within a small fraction of the minimum feature size and may provide
profile shapes which are not representative of the profiles of much
smaller features. The relative positions of these features and
their material profiles must then be observed by optical
microscopy, SEM or AFM in separate processes and the resulting data
processed. All of these processes are imaging techniques and all
have serious limitations. The optical microscopy method is limited
in image resolution. The AFM method is a quasi-contact technique
and is very slow. The SEM method requires that observation be
performed in a high vacuum and transfer of the wafer and pumping an
expensive vacuum chamber down to an appropriate pressure greatly
extends the amount of time required for measurement to be made;
which is, itself, of significant duration of about ten seconds or
more per measurement. It may be required for the wafer to be
sectioned and illuminated at different angles requiring different
set-up for different measurements. In any case, measurement is
destructive, indirect and of necessarily low throughput while
requiring apparatus and process methodology of high (and
increasing) complexity and cost as well as substantial processing
of the raw measurement data.
[0030] FIG. 2 shows two levels 20, 22 of exemplary features in
accordance with the present invention. In theory, any series of
repeated shapes with intervening repeated shapes in another level
could be used in accordance with the basic principles of the
invention. However, arrays of lines (which can be relatively short)
are preferred to minimize data processing complexity and time.
Further, while it is contemplated that the invention would
preferably be practiced using test patterns similar to those of
FIG. 2 and in which one level (sequentially second) was formed of
resist, longer lines may provide enhanced signal-to-noise ratio or
other improvement in raw data and it should be understood that the
invention can be practiced with completed structures such as
relatively long parallel connections (and which may include angled
portions). It is preferred that the two arrays of features be
similarly and regularly spaced (e.g. of constant pitch/spacing) and
of the same width but different widths and spacing can be
accommodated and, moreover, accurately evaluated by the practice of
the invention.
[0031] The exposures of the respective level patterns of FIG. 2 can
be made in a manner which is fortuitously aligned, as shown at 24
in FIG. 2A, but will generally be misaligned to some degree, as
shown at 26 in FIG. 2B. In accordance with the principles of the
invention, the composite pattern in accordance with, for example,
FIG. 2A or FIG. 2B will function as a diffraction grating and the
respective features need not be specifically imaged in the course
of the measurement process. By the same token and in accordance
with the invention, restrictions on the illumination source by
which the test pattern is observed are largely removed, greatly
simplifying the required measurement apparatus and the process of
its use while increasing throughput of the measurement process.
[0032] The theory and operation of a diffraction grating are
well-understood and software exists which allows analysis and
simulation of its effects which are extremely sensitive to geometry
and spacing of the elements which comprise the periodic structure
of a diffraction grating. Specifically, when a diffraction grating
is illuminated at a known angle with either monochromatic (coherent
or non-coherent) or broadband light, an interference pattern is
developed by differences in opacity or reflectivity of respective
areas therein. Depending on the geometry and spacing of the marks
forming the diffraction grating, the interference pattern will
exhibit specific and characteristic behaviors due to the
interaction of the wavelengths of the illuminating radiation (e.g.
light) and the spacing and other geometry of the marks which causes
reflected or transmitted light at certain angles and wavelengths to
be either reinforced or cancelled. Similar effects are observable
with respect to phase and polarization of the transmitted or
reflected radiation.
[0033] Measurement of amplitude and phase are generally referred to
as spectroscopic reflectometry. Measurement of amplitude, phase and
polarization are generally referred to as spectroscopic
ellipsometry. Either may be used in the practice of the invention;
the latter providing somewhat more detailed characterization of
spacing and material profile but reflects additional "degrees of
freedom" in the captured data. If spectroscopic reflectometry
provides sufficient accuracy in determining misalignment error,
then it is preferred for overlay metrology since less complex
processing and/or matching of data is required. This would be the
case when the overlay patterns have near ideal profiles of "square"
profiles. In this case, it is necessary to model a large number of
possible variations of the composite overlay structure and to build
up a library of response curves which can be compared and matched
to the measurement data.
[0034] However, if the profiles of the overlay patterns are rounded
or distorted, the ellipsometry with its additional degrees of
freedom can provide better accuracy in determining the misalignment
error. In this case, it is necessary to model an even larger number
of possible variations of the composite overlay structure and to
build up a library of response curves which can be compared and
matched to the measurement data. This library of response curves
needs to be generated for each new situation of level-to-level
overlay, for different degrees of misalignment error, profile
distortion and using the measured optical constants of the
substrate and layer materials. Fortunately, the wafer processes are
very well characterized so that once the library is generated for a
new situation, the misalignment error can be determined very
rapidly by comparing stored spectra with the measured one.
[0035] FIGS. 6A and 6B illustrate the measurement geometry and
apparatus in the case of reflecting aligned or misaligned overlay
targets. For simplicity, a simple reflectometry sensor is shown.
The measurement apparatus comprises a broadband light source
producing a collimated light beam incident on the overlay target
area at some fixed angle of incidence. The reflected light is
collected by a wavelength dispersive detector that measures
amplitude and phase of the reflected light across the desired
spectrum. In FIG. 6A, the alignment of the second mask level to the
first mask level is perfect and the composite overlay target
produces a spectral response as shown. In FIG. 6B, the alignment is
slightly off and a different spectral response is measured.
[0036] In both cases the spectral response is analyzed by a
dedicated computer, compared to stored signals and the misalignment
error, if any, is determined. Because the dimension of the
individual overlay target can be small (such as one micron or
less), the accuracy of the determination of the alignment error can
be very high. Other advantages are speed (because signal processing
is limited to comparison of the measured spectral curve to stored
data and determination of best fit or match), the avoidance of a
need for imaging, freedom from tool induced shift error and
non-contact operation.
[0037] The optical spectroscopic reflectometry or ellipsometry
sensor is very compact and can therefore be incorporated in a
process tool such as a resist track developer to provide on-line
metrology capability where it can provide direct feedback on the
alignment system performance of the stepper. The same sensor could
also be central to a stand-alone overlay metrology tool for in-line
metrology applications.
[0038] A simulation of the amplitude variation with wavelength from
the pattern of FIG. 2A is shown in FIG. 3 and a simulation of
amplitude variation with wavelength from the pattern of FIG. 2B is
shown in FIG. 4. It is assumed for purposes of this discussion that
the differently shaded portions 22, 24 of FIG. 2A (and 2B) have
different reflectivity and that the marks include at least one mark
which is of differing width. It is also assumed that illumination
is with broadband light and that the reflected light is analyzed
with a wavelength dispersive detector to provide a spectral curve
of reflectivity (amplitude and phase) as a function of wavelength.
The resulting curve will be similar in some respects to FIGS. 3 and
4 and can be processed to obtain the same overlay alignment
information, as will be readily understood by those skilled in the
art. The correctly aligned marks of FIG. 2A are assumed to be of
substantially constant pitch while two (or more) distinct pitches
or spacings are exhibited by the misaligned marks of FIG. 2B.
[0039] FIG. 3 shows a plurality of peaks of light amplitude at
different frequencies or wavelengths (calibrated as a function of
1/pixel which is basically equivalent to inverse wavelength but
specifically related by the calibration to the spatial period of
the composite overlay target and to the wavelength of reflected
light. Sharp peaks 32 and broad peaks 34 are evident and are an
incident of geometry, reflectivity and profile of individual lines.
In FIG. 3, both the sharp peaks and the broad peaks are
substantially symmetrical while in FIG. 4, substantial asymmetry is
evident, particularly in the broad peaks 42 and the sharp peaks 44
of longer wavelength. This asymmetry of peaks in FIG. 4 is due to
the different spacings caused by misalignment in the composite
pattern of FIG. 2B but is substantially absent from FIG. 3 since
the pitch of the marks is substantially constant. Thus, it is seen
that the shape of the spectral curve is extremely sensitive to the
existence of slight variation in spacing of a periodic structure
(which would include features at a plurality of pitches or periodic
spacings due to any misalignment) and even small degrees of
misalignment can be discriminated by inspection and quantified by
comparison with empirical or simulated data.
[0040] FIG. 5 shows a simulation of phase variation with wavelength
including two traces. The solid trace 52 corresponds to the aligned
overlay pattern of FIG. 2A while the dashed trace 54 corresponds to
slight misalignment. It should be noted that both the functional
variation (e.g. trace shape) with wavelength and the magnitude of
the variation varies with the degree of misalignment and is thus,
like amplitude variation with wavelength of FIGS. 3 and 4, a very
sensitive quantitative indicator of the degree of misalignment,
after processing.
[0041] It is desirable to develop a quantitative measurement of
overlay misalignment so that correction can be made or a decision
can be made as to whether or not overlay accuracy is sufficient for
processing to continue. (If the measurement is made including a
previously formed structure and a developed pattern of resist, if
the overlay misalignment is unacceptably great, the developed
resist can be stripped and another resist layer applied, exposed
and developed; thus saving the previous manufacturing expenditure
of processing the wafer to that point. This savings may be
substantial since many critical lithographic patterning processes
are performed in making connections to devices after processes for
forming those devices are substantially complete.) While the degree
of asymmetry or phase function with overlay misalignment distance
may be affected by other parameters such as mark size and profile,
particularly if spectroscopic ellipsometry techniques are employed,
the change in asymmetry or phase function with change in overlay
misalignment can be characterized and are incorporated in the
library data.
[0042] Therefore, for a given feature size regime and with at least
some similarity in feature geometry (e.g. pitch, width and profile)
a calibration or verification of the process in accordance with the
invention may be achieved by exposing overlaid patterns as
described above in connection with FIGS. 2, 2A and 2B with
differing misalignments and making spectroscopic observations such
as FIGS. 3 and 4, followed by processing of the spectral curves and
comparison to stored curves obtained from prior simulations to
determine the misalignment errors The same patterns can then be
observed or measured with SEM or AFM and the results compared to
the calculated misalignments. If the results do not agree, then it
may be necessary to perform additional simulations to better model
the composite overlay target physical properties using SEM
cross-sectional data.
[0043] Referring now to FIG. 7, the methodology of the invention
will now be summarized. Composite overlay targets 71 are obtained
by superimposition of two successive mask level patterns in a
reserved area of of the wafer 72, referred to as an overlay
measurement mark area The second mask level is defined as a resist
structure and the first mask level is defined as an etched
structure on the same area of the wafer substrate.
[0044] The wafer is brought under a specular spectroscopic
scatterometer 73 of either reflectometer or ellipsometer type which
is used to analyze the composite overlay target. Measurement data
(and amplitude and phase spectral curves) collected by a wavelength
dispersive detector 74 are processed by a data processor 75, such
as a workstation computer since processing power demands may be
high for running an optimization program for matching measured data
to stored data in a library 76 obtained by prior simulation with an
exact model representation of the composite target structure for a
full range of misalignment values. A global optimization technique
is used to determine the best fit to modeled data and to thus
quantify the misalignment value for the target.
[0045] In view of the foregoing, it is seen that the invention
provides a technique of measuring overlay misalignment which does
not require imaging of overlaid features and is thus applicable to
feature sizes well below one micron. The invention provides
improved resolution, repeatability, reproducibility and
quantitative accuracy using simplified apparatus and procedures of
reduced complexity and cost and which can be performed on-line or
in-line, possibly concurrently with other measurements of interest.
The invention utilizes an optical technique but, since it does not
require imaging and avoids the need for a microscope, it is free
from tool induced shift error (which is a measure of the impact of
tool asymmetry on measurement error in an imaging system). Further,
the invention provides for quantitative measurement of misalignment
determined from processing simple and direct observation of a
spectroscopic response corresponding to a detected interference
pattern using broadband illumination.
[0046] While the invention has been described in terms of a single
preferred embodiment, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the appended claims.
* * * * *