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Claims  |
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What is claimed is:
1. A method of inspecting an object and generating synthetic image data
comprising the steps of:
(a) using an interference optical system including an object channel and a
reference channel for simultaneously inspecting an object and a reflective
reference surface and developing a plurality of images formed by the
interference between object wave energy passing from said object and
through said object channel to an image plane and reference wave energy
passing from said reference surface and through said reference channel to
said image plane, each said image being formed in response to a change in
position of either said object or said reference surface;
(b) determining for each image the absolute value of the degree of
coherence between said object wave energy and said reference wave energy
by calculating the variance along each column of an array of mxn pixels in
said image plane, where m and n are integers, and generating absolute
value coherence data corresponding to each said column; and
(c) using said absolute value coherence data to generate synthetic image
data representative of a particular characteristic of said object, wherein
the brightness of each pixel element of a synthetic image developed using
said synthetic image data is proportional to said absolute value coherence
data.
2. A process for generating synthetic image data representative of a
cross-section of an at least partially reflective irregular surface of an
object formed by a portion of a semiconductor wafer having an elongated
strip of raised surface extending therethrough, comprising the steps of:
(a) illuminating the irregular object surface with light from a source of
illumination;
(b) illuminating a reflective reference surface with light from said source
of illumination, said reference surface being formed by an optically flat
mirror;
(c) collecting object light reflected from said object surface and
directing said object light along a first optical axis;
(d) collecting reference light reflected from said reference surface and
directing said reference light along a second optical axis at least a
portion of which is parallel to said first optical axis;
(e) focussing the light directed along said first and second optical axes
to form a fringed image pattern resulting from interference of said object
light and said reference light;
(f) orientating said wafer so that a selected scan line may be directed
substantially orthogonal relative to the length of said elongated strip;
(g) inspecting said image pattern to develop a series of coherence data
corresponding to the fringe amplitude at points along said scan line;
(h) incrementally changing the position of said object along said first
optical axis, each time repeating steps (a) through (e) and (g);
(i) processing the plurality of series of coherence data to develop
synthetic image data corresponding to a cross-sectional profile of said
object surface taken in a plane including said selected scan lines;
(j) displaying said synthetic image data to visually depict a
cross-sectional profile of said object surface taken in the plane
including said scan lines; and
(k) determining the position of said object along said first optical axis
at which the value of said coherence data corresponding to the crossing of
a first particular scan line over said raised surface is at a maximum
relative to the corresponding coherence data of the other scan lines and
identifying this position as corresponding to the top surface of said
strip.
3. A process as recited in claim 2, and further comprising the step of;
(l) detecting the width of the top of said raised surface by measuring the
length of the portion of said first particular scan line over which said
coherence data is at a maximum.
4. A process as recited in claim 3, and further comprising the steps of;
(m) determining the position of said object along said first optical axis
when the value of said coherence data corresponding to the crossing of
another particular scan line over portions of said surface other than said
raised surface are at a maximum relative to the corresponding data of the
other scanned lines; and
(n) determining the width of the base of said raised surface by measuring
the separation between the portions of said other particular scan lines
over which said data is at a maximum.
5. A process as recited in claim 4, and further comprising the step of:
(o) determining the height of said raised surface above the adjacent wafer
surface by measuring the distance between the position at which the object
is positioned along said first optical axis when the width of the top of
said raised surface is detected and the position along said first optical
axis when the width of the base of said raised surface is detected.
6. A process as recited in claim 5 and further comprising the step of;
(p) calculating the slopes of the side walls of said raised surface, in the
plane including said scan lines, as a function of the height of the top of
the raised surface above the base thereof and the difference in width of
the top and the base along the corresponding scan lines.
7. A process as recited in claim 2, wherein said synthetic image data is
developed by calculating the intensity of the interference fringes in said
fringed image pattern and by calculating the local variance of the image
intensity caused by the fringes.
8. A process as recited in claim 2, wherein said synthetic image data is
generated by calculating the variance of the intensity of each pixel in
the image plane as the path difference between the first and second
optical paths is made to change.
9. A method of measuring certain dimensions of an elongated strip of raised
surface formed on an object, such as a semiconductor wafer or photomask,
using an interference optical system to develop images formed by
interference between object wave energy passing from the object and
through an object channel to an image plane and reference wave energy
passing through a reference channel to the image plane, comprising the
steps of:
(a) illuminating the object surface with light from a source of
illumination;
(b) illuminating a reflective reference surface with light from said source
of illumination;
(c) collecting object light reflected from said object surface and directed
along a first optical axis through said object channel;
(d) collecting reference light reflected from said reference surface and
directed along a second optical axis through said reference channel, said
second optical axis having at least a portion thereof which is parallel to
said first optical axis;
(e) focusing the light directed along said parallel portions of said first
and second optical axes onto an image plane to form an interference image
pattern resulting from interference of said object light and said
reference light;
(f) inspecting the image pattern by detecting the light intensity at each
pixel in an array of mxn pixels extending across the image of said strip
to produce pixel data;
(g) scanning the pixel data and calculating therefrom coherence data
representing the absolute value or magnitude of the complex degree of
coherence of light incident upon said image plane and storing the
calculated coherence data for subsequent reference;
(h) incrementally changing the position of said object along said first
optical axis, each time repeating steps (a) through (g); and
(i) using the stored coherence data to generate data from which a synthetic
image corresponding to a transverse cross-section of the measured strip
may be developed.
10. A method as recited in claim 9 and further including the step of:
inspecting the stored coherence data to determine the height of the top
surface of said strip relative to the adjacent surface of said object,
such height being measured by determining a first position along said
first optical axis at which maximum coherence occurs at a point in an
array corresponding with said top surface, and by determining a second
position along said first optical axis at which maximum coherence occurs
at a point in an array corresponding with said adjacent surface, the
measured height of said top surface being equal to the distance between
said first position and said second position.
11. A method as recited in claim 10 and further comprising the step of
determining the width of the top surface of said strip by applying an edge
finding algorithm to coherence data taken from the image corresponding to
said top surface.
12. A method as recited in claim 10 and further comprising the step of
measuring the width of the bottom of the strip by applying an edge finding
algorithm to coherence data taken from the image corresponding to said
adjacent surface.
13. A method as recited in claim 9 wherein the step of calculating the
absolute value or magnitude of the complex degree of coherence is
accomplished by measuring the variance of the interference image along
pixel columns of an mxn array inspected at each position of said object
along said optical axis.
14. A method as recited in claim 9 wherein the step of calculating the
absolute value or magnitude of the complex degree of coherence is
accomplished by calculating the variance of the light intensity among
corresponding pixels of the several image patterns inspected as the
position of said object is changed along said first optical axis producing
a change in the path difference between said object channel and said
reference channel.
15. A method as recited in claim 9 wherein the step of calculating the
absolute value or magnitude of the complex degree of coherence is
accomplished by calculating the variance of the light intensity among
corresponding pixels of the several image patterns inspected as the
position of said reference mirror is changed along said second optical
axis producing a change in the path difference between said object channel
and said reference channel.
16. A method of inspecting an object and generating synthetic image data
comprising the steps of:
(a) using an interference optical system including an object channel and a
reference channel for simultaneously inspecting an object and a reflective
reference surface and developing a plurality of images formed by the
interference between object wave energy passing from said object and
through said object channel to an image plane and reference wave energy
passing from said reference surface and through said reference channel to
said image plane, each said image being formed in response to a change in
position of either said object or said reference surface
(b) determining for each image the absolute value of the degree of
coherence between said object wave energy and said reference wave energy
by calculating the variance in the intensity of each pixel over the said
plurality of images and generating corresponding absolute value coherence
data; and
(c) using said absolute value coherence data to generate synthetic image
data representative of a particular characteristic of said object, wherein
the brightness of each pixel element of a synthetic image developed using
said synthetic image data is proportional to said absolute value coherence
data. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to precision optical inspection
methods and apparatus, and more particularly to a method and apparatus for
performing microscopic inspection and measurement of integrated circuit
wafer geometry using interference microscopy in combination with
electronic image processing.
2. Discussion of the Prior Art
It has long been desired that means be provided to inspect and measure the
characteristics of microminiature surfaces such as those formed in
integrated circuit wafers. One such characteristic of interest is the line
widths of the various traces produced on a wafer surface during IC device
manufacture.
One prior art technique for integrated circuit metrology includes the use
of an ordinary microscope with some form of electronic detector positioned
at the image plane. For example, video cameras, scanning slits (see U.S.
Pat. No. 4,373,817), shearing systems and linear arrays, have all been
used as detectors with ordinary microscopes. However, the capability of
the ordinary microscope is limited in that it can only measure the
intensity of the optical wave amplitude and cannot measure the complex
phase of the amplitude. As a consequence, the three-dimensional nature of
integrated circuit surfaces makes use of the classical microscope
impractical for precision surface inspections and measurements of this
type.
Other prior art techniques have used confocal laser scanning microscopes to
obtain three dimensional data relating to integrated circuit surfaces. A
rather thorough treatment of the subject may be found in T. Wilson and C.
Shepard (1984), Theory and Practice of Scanning Optical Microscopy,
Academic Press.
Aside from the complexity and relatively high cost associated with the use
of confocal laser devices and techniques, the fact that such techniques
use monochromatic light makes them subject to inaccuracies caused by
destructive interference for certain thicknesses of transparent films
often found in semiconductor devices.
SUMMARY OF THE PRESENT INVENTION
It is therefore a principal object of the present invention to provide an
improved method and apparatus for accomplishing three dimensional
inspection of integrated circuits and the like.
Another object of the present invention is to provide an improved synthetic
imaging technique utilizing a two beam interference microscope.
Still another object of the present invention is to provide a method and
apparatus by which the top width, bottom width and height of an integrated
circuit line may be accurately measured.
Briefly, a preferred embodiment of the present invention includes a
specially adapted Linnik microscope in combination with a video camera, a
wafer transport stage and data processing electronics to form a novel
inspection apparatus based on the use of the two beam interference
microscope. The apparatus can utilize either broad band or narrow band
light to develop a plurality of interference images taken at different
axial positions relative to the surface under investigation. The
point-by-point brightness along scan lines across such images is then used
to develop data which is proportional to the degree of coherence (or to
the fringe amplitude, the variance of the fringes, or the amplitude of
oscillation of the fringes) as the optical path difference is varied in a
two beam optical or acoustic microscope.
Among the advantages of the present invention are that it provides a much
simpler and more economical technique than those using the confocal
microscope.
Another advantage is that it can use white light rather than monochromatic
light and as a result, can have a signal-to-noise ratio which is not
degraded by coherent speckle effects which affect any coherent optical
system. Moreover, by using white light the possibility of destructive
interference for certain thicknesses of transparent films is eliminated.
Furthermore, the theoretical resolution along the optical axis appears to
be better than that for a confocal microscope because the short coherence
length of white light effectively reduces the depth of focus of the
instrument. Empirically, it also appears that the present invention
substantially improves the lateral resolution of the microscope, at least
for the purpose of measuring linewidths of integrated circuits.
These and other objects and advantages of the present invention will no
doubt become apparent to those skilled in the art after having read the
following detailed description of the preferred embodiments which are
illustrated in the several figures of the drawing.
IN THE DRAWING
FIG. 1 is a schematic diagram depicting the basic functional components of
the present invention;
FIG. 2 is an isometric diagram illustrating an integrated circuit line and
five inspection levels;
FIGS. 3 through 7 are actual photographic depictions of interference images
taken at the levels 5 through 1 respectively, of FIG. 2;
FIG. 8 is a flow diagram functionally depicting operation of the electronic
processing electronics of the present invention;
FIG. 9 is an RMS profile of a central column developed in accordance with
the present invention; and
FIG. 10 is a depiction of a CRT display in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Interference microscopes can measure the topography of reflective surfaces
using standard techniques so long as the undulations in relief are within
the depth of field of the imaging system, and so long as the topography is
not so jagged as to confuse the fringe counting algorithm. The basic
formula is
##EQU1##
where .DELTA.h is the difference in height between two points in the
image, .DELTA..phi. is the phase difference, and .lambda. is the wave
length of light. The standard applications of the Linnik microscope in
this context are given in "Incident-Light Microscope Inteferometer for the
Orthoplan and Metalloplan", Instruction Manual for Use of the Linnik
Microscope Attachment by Ernst Leitz Gmbh, Wetzlar (1980); and in LEITZ,
"Incident-Light Interference Illuminator for the Orthoplan/Metalloplan, a
module which uses the wave length of light for measurement (19.sub.--)".
However, these standard techniques break down when any of the following
three conditions are present:
1. The topographic fluctuations on the object surface within the field of
view exceed the depth of focus of the microscope;
2. The object consists of transparent structures formed on an opaque
substrate; or
3. The object surface has steep cliffs or walls the vertical extent of
which exceeds a half wavelength of light.
When any of these cases occur, as they often do in integrated circuit
devices, the standard use of the Linnik or other two beam interference
microscopes simply does not give useful data because the fringe counting
algorithm provides hopelessly confused and incorrect results when combined
with equation 1.
The analysis process of the present invention described hereinafter
overcomes the difficulties encountered by the standard techniques of
Linnik interference microscopy, and when implemented in electronic
hardware offers new capabilities for automated inspection of semiconductor
devices.
The basic concept of the present invention is that broad band illumination
(white light) has a very short coherence length, and by measuring the
degree of coherence between an object and a reference beam at each point
in an image, a powerful light sectioning technique may be developed.
The principle can be illustrated with scalar diffraction theory. However,
the basic ideas apply in general even when scalar diffraction theory does
not provide a good approximation.
Consider the wave equation of light in a homogeneous medium:
##EQU2##
where c is the speed of light in the medium and u may be written as a
Fourier integral in the form
##EQU3##
The spectral density is
##EQU4##
where .alpha. is a normalization constant and .delta. is a delta function.
The degree of first order coherence is
##EQU5##
where the brackets <> denote ensemble average. If the wave u is a sum of
two constituent waves:
U=U.sub.1 +U.sub.2 (7)
then the degree of coherence between U.sub.1 and U.sub.2 may be defined
analogously as
##EQU6##
Referring now to FIG. 1 of the drawing, apparatus in accordance with the
present invention is schematically shown to include a spotting microscope
10, a LINNIK microscope 12, an X,Y stage 14 for carrying a wafer 16 and a
piezo-electric vertical motion system 18 between a set up position beneath
microscope 10 and an inspection position beneath microscope 12, a pair of
video cameras 20 and 22, data processing electronics 24, a CRT display 26
and an electronic controller and operator interface console 28.
In a two-beam interference microscope (such as the Linnik microscope), a
light wave from a source 31 reaching the image plane 36 is the sum of two
constituent waves; one reflecting off the surface of the object 16, and
the other reflecting off the surface of a reference mirror 34. Fringes are
seen in the image at 36, even when white light is used to illuminate the
object. If broad band illumination (white light) is used, strongest
fringing occurs when the path difference between the reference channel 32
and the object channel 30 is very small, on the order of a fraction of the
average wavelength, because the coherence length of white light is very
short. When the degree of coherence is high between the reference channel
and the object channel, the fringes are strong. Conversely, when the
degree of coherence is low, the fringes are weak. In the preferred
embodiment, white light Kohler illumination is provided by a Xenon arc
lamp 31, and a shutter 33 is included to flip the reference beam in and
out. The fringe rate and direction can be controlled on commercially
available Linnik microscopes by moving the microscope objective in the
reference channel off-axis. Accordingly, in the preferred embodiment the
lens 35 is positioned to make the fringes which appear at image plane 36
be parallel to camera 22's raster direction and the fringe spacing equal
to 32 horizontal raster rows of camera 22.
The connection between degree of coherence and fringe intensity may be
described as follows where U.sub.1 is the object wave and U.sub.2 is the
reference wave. At the image plane, the superposition of the object and
the reference wave results in the light intensity
<.vertline.U.sub.1 +U.sub.2 .vertline..sup.2 >=<.vertline.U.sub.1
.vertline..sup.2 >+<.vertline.U.sub.2 .vertline..sup.2 >+2R.sub.e <U.sub.1
*U.sub.2 > (9)
The overall path length difference between the reference channel and the
object channel can be varied so as to introduce a phase difference between
the object channel and the reference channel. In the narrow bandwidth
approximation, the phase shift will be the same for all frequencies of the
light. In this case, intensities at the image plane are of the form
<.vertline.U.sub.1 +e.sup.i.phi. U.sub.2 .vertline..sup.2
>=<.vertline.U.sub.1 .vertline..sup.2 >+<.vertline.U.sub.2
.vertline..sup.2 >+2R.sub.e [e.sup.i.phi. <U.sub.1 *U.sub.2 >](10)
The variance in equation 10, calculated by letting .phi. vary from -.pi. to
.pi. is easily found to be
Variance of <.vertline.U.sub.1 +U.sub.2 e.sup.i.phi. .vertline..sup.2
>=2.vertline.<U.sub.i *U.sub.2 >.vertline..sup.2 (11)
and therefore the degree of coherence may be expressed as
##EQU7##
In the present case, the illumination is actually broad band and the phase
shift is different for the different frequencies of the light. However, in
this case it is found that the following functional form for <U.sub.1
*U.sub.2 > is a good approximation for images taken in a two beam
interference microscope:
<U.sub.1 *U.sub.2 >=me.sup.R(l) e.sup.ilK(l), l=path difference, (13)
where R(l) and K(l) are slowly varying over the distance 2.pi./K(l) and
therefore equation (12) is still deriveable provided that is made to
vary through 2.pi. by letting l vary from -.pi./K(l) to .pi./K(l) in the
calculation of the variance. The parameter "m" in equation (13) is a
complex constant.
Therefore, one can define an easily measurable quantity C(x,t) which may be
taken as a practical measure of the degree of coherence as
##EQU8##
If U.sub.1 and U.sub.2 are not coherent, then C=0. In general, assuming
that R(l) and K(l) are slowly varying in equation (13), it can be shown
that
C=(<.vertline.U.sub.1 .vertline..sup.2 ><.vertline.U.sub.2 .vertline..sup.2
>).sup.1/2 G (15)
The technique of the present invention is to synthetically construct images
the brightness of which at each image point is proportional to C. This
amounts to imaging by means of a coherence probe.
The interference microscope 12 is set up in the following way prior to
calculation of C: With a first surface mirror (not shown) as the object
(at 16), the focus in the object channel 30 and the reference channel 32
are adjusted so that both the reference mirror 34 and the object mirror
are simultaneously in focus. Then the path difference is adjusted until
the maximum degree of coherence is obtained between the object wave and
the reference wave. This reference position is then the center point in
the variation of path difference used to measure the degree of coherence.
If fringe data is being used, as described below the setup is a little
different. In such case, a window in the center of the image plane 36 is
selected as the area of interest, and after focusing the reference and
object mirrors, the path is adjusted so that the fringe amplitude is the
greatest at center of the window. The object mirror is then replaced by an
object such as a silicon wafer 16 having an integrated circuit formed in
its upper surface.
All parts of the object surface which are at the same "level" as the
surface of the reference mirror will now produce a scattered wave which is
relatively coherent with respect to the reference wave, and those image
points end up being bright in the final image at 36 (FIG. 1). The very
brightest points are those where the object locally is a horizontally
reflective surface because at those points the object wave and reference
wave match is best. Parts of the object which are at a different level
than the reference mirror appear dark. Sectioning can then be accomplished
by moving the wafer 16 up or down to obtain successive images
corresponding to respective object planes passing through the wafer 16, as
illustrated in FIG. 2.
The degree of coherence C can be measured in a Linnik microscope in several
ways. One way is to vary the path length of the reference channel 32; for
example, through one or more wavelengths centered on the reference
position, and while doing this, calculating electronically the oscillation
in intensity at each point in the image plane of the microscope. The
amplitude of oscillation (variance) is proportional to C.
Alternatively, for object surface features which do not vary too quickly in
one direction (such as in the case of a semiconductor integrated circuit
line) the interference fringes may be adjusted so that they lie
perpendicular to a line to be inspected. The fringe spacing may also be
adjusted to any convenient value. In this case C is simply the amplitude
of the fringes within the window, i.e.,
C=Fringe Amplitude (16)
The advantage of this technique is that, as will be further explained
below, only one image is required to make a measurement of C at all points
across the line.
FIGS. 3-7 show photographs of actual fringe data taken at the different
object elevations generally depicted in FIG. 2. The object in this case
was a silicon wafer having a one micron high resist line 40 formed on its
upper surface. The vertical and horizontal white lines depicted in the
photographs are an electronic overlay produced on the display CRT 26 and
can be ignored for this discussion. The resist portion 40 is in the center
of each photo, and as depicted in FIG. 2, the five photos of FIGS. 3-7 are
taken at different positions of the object along the vertical (or Z) axis.
More specifically, the plane of focus of the image shown in FIG. 3 is
slightly above the top surface of the resist line 40, i.e., at level 5 in
FIG. 2. Within the central window 42, framed in black in the photos, the
fringe intensity is shown to be weak.
FIG. 4 shows the wafer raised a few thousand Angstroms to bring the top of
the resist into focus (at level 4 of FIG. 2). Fringes 43 in the window 42
are now strong in the central portion of the image (corresponding to the
top surface of the resist) but are weak on either side (where no resist is
present).
FIG. 5 shows the wafer again raised an additional few thousand Angstroms so
that the focal plane is between the top level of the resist and the
silicon substrate (level 3 in FIG. 2). Here again, the fringes are weak in
both the resist region and the silicon region since neither is in focus.
In FIG. 6 the wafer has again been raised another few thousand Angstroms to
level 2 to bring the silicon surface 44 (FIG. 2) into focus. Here the
fringes 45 are strong on the silicon and are fairly weak in the resist
portion of the image.
In FIG. 7, only the resist shows strong fringing due to the reflection of
the light off the bottom of the resist layer.
The process by which fringe amplitudes are used to measure line widths on
integrated circuits is shown in the flow chart of FIG. 8. The boxes
describe the algorithms used. The "column in window" reference in FIG. 8
refers to the columns of pixels, one of which is shown at 46 in FIG. 2,
scanned by the processor 24 to determine the variance values over the
length of the window 42, to thereby calculate the fringe coherence.
More particularly, the image 36 is scanned by video camera 22, which
develops an analog raster scan thereof for input to the processing
electronics 24. The first processing step is to convert the analog data
into 8 bit digital form and to store the data in a computer memory. A
"window" such as is illustrated at 42 in FIG. 2, is then scanned a pixel
column at a time, as illustrated at 46 and the data corresponding to each
column is moved by DMA transfer to a high speed arithmetic processor which
calculates the variance of each columnar array and stores its RMS value in
memory. After data is collected across the window 42 the stage is
incremented in the Z direction to another level, another scanning
operation is completed and the data stored. This operation is repeated at
levels separated by approximately 500 Angstroms until sufficient data is
obtained to evaluate all desired surfaces. The several sets of scan data
stored in memory is then itself scanned at the centermost point X.sub.R in
"X" of the resist line 40 (along the Z axis in FIG. 2) to determine the
level having the highest RMS value, and such level is determined to
coincide with and thus identify the top of the scanned line.
One such electronic scan is depicted in FIG. 9 wherein the ordinate
represents the RMS value of the central column data and the abscissa
represents the inspection level (or stage position along the Z axis). As
illustrated, the peak at level 4 corresponds to the top of the resist line
40 of FIG. 2 while the peak at level 2 corresponds to reflection off the
wafer substrate 44. The horizontal distance between the two peaks is thus
an indication of the vertical thickness of the resist line 40.
The memory is thereafter scanned at an X position X.sub.s over only the
substrate to find the scan line having the brightest level and this is
taken to be the bottom of the line. By subtracting the top level from this
bottom level information the height of the line can be determined.
The next step is to apply an edge finding algorithm to the memory location
corresponding to the top of the line in order to determine the raw top
width. An edge finding algorithm is then applied to the memory location
corresponding to the bottom of the line to determine the raw bottom width.
The final results for top and bottom width are calculated from the
formulas top width=A.sub.1 * raw top width+B.sub.1 * raw bottom
width+C.sub.1 bottom width=A.sub.2 * raw top width+B.sub.2 * raw bottom
width+C.sub.2.
The constants A, B, and C are determined by calibration procedure. Once
these widths are determined they can be reported to the CRT and displayed
to the user.
FIG. 10 shows an artist's rendition of the synthetic images produced on a
CRT screen by imaging C. The upper and lower righthand quadrants 50 and 52
show cross sections of the line, each row corresponding to a different
level (scan line). The elevated resist 54 appears as a cloud above the
silicon substrate. An electronic line 56 has been drawn in the right upper
quadrant through the algorithm's choice as the best row to call the top of
the line.
The upper left hand quadrant 58 shows this row expanded vertically to fill
the entire quadrant and to thereby look like a top down view. The lower
right hand quadrant 52 showns a line 60 drawn through the algorithm's
choice as the best candidate for the bottom substrate level.
The lower left quadrant 62 depicts an expanded top down view of the line
60. The edge finding algorithm then uses a threshold technique to find the
edges in both the upper and lower left hand quadrants.
In this way the height is known (by the difference in stage position
between the top and bottom rows), the top width is known by the distance
between the edges in the upper left quadrant, the bottom width is known by
the distance between the edges in the bottom left quadrant, and the wall
angles may be calculated. For best results, calibration to scanning
electron microscope results are required.
In operation the technique consists of first aligning a semiconductor IC
line (such as 40 in FIG. 2) in the field of view of the spotting
microscope 10. The wafer is then moved a fixed offset to the right, as
illustrated in FIG. 1, so that the same line is now viewed by the Linnik
microscope 12. The interference fringes are then pre-adjusted to lie
perpendicular to the line direction, and the fringe spacing is adjusted to
have two complete fringes within the window (of variance calculation) 42
(FIGS. 3-7) when a reflective plane in the object is in focus (see FIG. 4
for example.
The wafer is then dropped in the z direction so that the highest point in
the line is several thousand Angstroms below the focal plane, and the
resulting image at 36 is digitized by the electronics 24. The fringe
amplitude is then calculated for each scan column 46 (FIG. 2) in the
window 42, and the result is stored in memory.
One method of calculating the fringe coherence is to calculate the variance
of each column array 46 (FIG. 2) across the window 42. To accomplish this
for each array 46, the arithmetic processor in the electronics 24
calculates:
##EQU9##
The stage is then moved up a small distance (about 500 Angstroms) and
another image is digitized, fringe amplitudes calculated, and results
stored. The process of moving the stage up, digitizing the image, and
storing the fringe amplitudes is repeated until the stage has scanned a
sufficient distance to place the lowest point of the line above the optics
focal plane so that the entire depth of the line has been sectioned.
The top and bottom levels of the line are then determined by finding the
brightest scan rows at the appropriate positions in the image. Then the
edges of the line at the top and bottom levels are determined by an edge
finding algorithm. Finally, the output data is calculated by means of a
calibration formula which calculates the top and bottom width by the
formulas:
Top width=A.sub.1 *(Raw top width)+B.sub.1 *(Raw bottom width )+C.sub.1
Bottom width=A.sub.2 *(Raw top width)+B.sub.2 *(Raw bottom width)+C.sub.2
where A.sub.1, A.sub.2, B.sub.1, B.sub.2, C.sub.1 and C.sub.2 are
determined by a calibration procedure, and the results maybe reported to
the CRT 26. The results consist of the top width and bottom width, as well
as the positions of the four edges used in the calculation of the vertical
wall angles.
Although the present invention has been illustrated in a preferred
embodiment, it is anticipated that following a reading of this disclosure
numerous alterations and modifications thereof will become apparent to
those skilled in the art. It is therefore intended that the appended
claims be interpretted as covering all such embodiments as fall within the
true spirit and scope of the invention.
* * * * *
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