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Claims  |
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We claim:
1. An aperture-projecting measuring microscope comprising
means for supporting a workpiece to be observed and measured,
an illumination source producing a light beam directed along an optical
path toward said workpiece,
a stop having a transmissive aperture therein in said optical path, said
aperture having a size which is larger than the diffraction limit for said
aperture,
a microscope objective disposed in said optical path between said stop and
said workpiece so as to image said stop onto said workpiece, whereby a
portion of said workpiece is illuminated, the shape of said aperture being
apparent in the image of said stop on said workpiece, said microscope
objective also imaging said illuminated portion of said workpiece onto
said stop,
means for measuring at least one property of light passing through said
aperture from said workpiece, whereby an area of said workpiece determined
by said aperture can be characterized, said at least one property
including the intensity (I) of light, and
means for adjusting the relative distance (Z) between said microscope
objective and said workpiece so as to achieve and maintain a condition of
best focus, said condition being determined to occur when the rate of
change of said measured light intensity with respect to said relative
distance (dI/dZ) is zero.
2. The microscope of claim 1 wherein said measuring means comprises
means for spectrally resolving said light passing through said aperture
from said workpiece, and
means for measuring the intensity distribution with respect to wavelength
of light reflected from said workpiece.
3. The microscope of claim 1 wherein said illumination source lies on the
opposite side of said stop from said workpiece, light from said source
passing through said aperture to illuminate said workpiece.
4. The microscope of claim 1 wherein said illumination source lies on the
same side of said stop as said workpiece, said stop being light reflective
in the region surrounding said transmissive aperture, light from said
source being reflected by said stop to illuminate said workpiece.
5. The microscope of claim 1 further comprising means for indicating said
rate of change (dI/dZ) to a user, said adjusting means being manually
operable by said user.
6. The microscope of claim 1 wherein said adjusting means is operable
automatically, being servo-controlled in response to said rate of change
(dI/dZ).
7. A positive-confocal measuring microscope comprising
means for supporting a workpiece to be observed and measured,
an illumination source producing a light beam directed along an optical
path toward said workpiece,
a first field stop disposed in said optical path in front of said
illumination source, said first field stop having defined therein a
substantially transmissive central aperture and a partially transmissive
annular aperture surrounding said central aperture,
a microscope objective disposed in said optical path, said first field stop
being imaged by said microscope objective onto said workpiece, whereby a
small area of said workpiece to be measured is brightly illuminated by
light from said source passing through said central aperture and a larger
surrounding area of said workpiece to be observed visually is less
brightly illuminated by light from said source passing through said
annular aperture,
a second field stop having an aperture therein, said microscope objective
imaging said workpiece in an image plane, said image plane intersecting
said aperture of said second field stop when said workpiece is in best
focus,
means on the same side of said second field stop as said workpiece for
viewing said imaged workpiece, a dark spot corresponding to said aperture
of said second stop indicating the location of said small area to be
measured,
means lying behind said second field stop for measuring at least one
property of light passing through said aperture of said second stop from
said small brightly illuminated area of said workpiece, whereby said small
area can be characterized, said at least one property including the
intensity (I) of light,
means for adjusting the relative distance (Z) between said objective and
said workpiece so as to achieve and maintain a best focus condition, said
best focus condition being that relative distance when said measured
intensity (I) is a maximum.
8. The microscope of claim 7 wherein said measuring means comprises
means for spectrally resolving said light passing through said aperture of
said second stop reflected from said workpiece, and
means for measuring the intensity distribution with respect to wavelength
of said spectrally resolved light.
9. The microscope of claim 7 further comprising
means for scanning said workpiece transversely so as to measure light
passing through said aperture of said second stop corresponding to a
series of selected small areas on the surface of said workpiece, said
adjusting means continuously operable so as to maintain a condition of
best focus at each of said selected areas, and
means for forming a record of said relative distances (Z) of best focus for
each said selected area, said record representing a surface altitude
profile of said workpiece along the locus of said selected points.
10. The microscope of claim 7 further comprising means for indicating the
measured intensity (I) to a user, said adjusting means being manually
operable by said user.
11. An aperture-projecting measuring microscope with inverse-confocal
focusing comprising
means for supporting a workpiece to be observed and measured,
a first illumination source producing a light beam directed along an
optical path toward said workpiece,
a first field stop disposed in said optical path in front of said first
illumination source, said first field stop having a substantially
transmissive aperture therein,
a microscope objective disposed in said optical path, said first field stop
being imaged by said objective onto said workpiece, whereby a small area
of said workpiece is illuminated by light from said first source passing
through said aperture of said first field stop,
a second field stop having an aperture therein, said objective imaging said
workpiece in an image plane intersecting said aperture of said second
stop, said second stop being reflective,
a second illumination source producing a light beam directed along an
optical path toward said reflective second stop, said light beam from said
second source being reflected by said second stop toward said workpiece,
means for selecting which of said first and second illumination sources
illuminate said workpiece,
means for viewing said imaged workpiece, an area of said workpiece
illuminated by said second source being visible with a dark spot
corresponding to said aperture of said second stop indicating said small
area to be illuminated by said first source for measurement,
means behind said second stop for measuring at least one property of light
passing through said aperture of said second stop from said workpiece,
whereby an area of said workpiece determined by said aperture of said
second field stop can be characterized, said at least one property
including the intensity (I) of light, and
means for adjusting the relative distance (Z) between said objective and
said workpiece so as to achieve and maintain a best focus condition, said
best focus condition being met when the measured intensity (I) is a
minimum.
12. The microscope of claim 11 wherein said measuring means comprises
means for spectrally resolving said light passing through said aperture of
said second stop reflected from said small area of said workpiece
illuminated by said first source, and
means for measuring the intensity distribution with respect to wavelength
of said spectrally resolved light.
13. The microscope of claim 11 further comprising
means for scanning said workpiece transversely so as to measure light
passing through said aperture of said second stop corresponding to a
series of selected small areas of the surface of said workpiece, said
adjusting means being continuously operable so as to maintain a condition
of best focus at each of said selected areas, and
means for forming a record of said relative distances (Z) of best focus for
each of said selected areas, said record representing a surface altitude
profile of said workpiece along the locus of said selected points.
14. The microscope of claim 11 wherein said adjusting means is responsive
only when said selecting means selects said second light source to
illuminate said workpiece.
15. The microscope of claim 11 wherein said adjusting means is first
responsive to the measured intensity of light from said workpiece
illuminated by said first light source so as to provide gross focusing,
and is also responsive to the measured intensity of light from said
workpiece illuminated by said second light source so as to provide fine
focusing.
16. A microspectroreflectometer comprising,
at least one broad spectrum light source producing a light beam,
a microscope objective in the path of said light beam,
means for supporting a substantially flat object, said light beam being
focused to a spot by said microscope objective so as to illuminate at
least a portion of said object, said microscope objective forming an image
in an image plane of at least a part of said illuminated portion of aid
object,
means for viewing said image,
means defining a transmissive aperture intersecting said image plane for
passing light corresponding to a portion of said image,
means for diffracting said light passing through said aperture,
a linear detector array disposed in a position relative to said diffracting
means to receive first order diffracted light, and
means for adjusting the axial spacing between said microscope objective and
said object so as to thereby achieve and maintain an in-focus condition,
said object being considered in focus when the derivative with respect to
the object's axial position of the measured intensity of light passing
through said aperture is zero.
17. The microspectroreflectometer of claim 16 further comprising a focus
condition detector disposed in a position relative to said diffracting
means to receive zero order diffracted light, the measured intensity of
light received by said focus condition detector representing the intensity
of light passing through said aperture.
18. The microspectroreflectometer of claim 16 further comprising an
integrating circuit electrically connected to said linear detector array,
the sum of light intensities received by said linear detector array
representing the intensity of light passing through said aperture.
19. The microspectroreflectometer of claim 16 further comprising a light
source located in a position relative to said diffracting means
corresponding to zero order light diffraction, said light source directing
a light beam toward said aperture via said diffraction means.
20. The microspectroreflectometer of claim 16 wherein said at least one
light source comprises a first light source producing visible light and a
second light source producing ultraviolet light.
21. The microspectroreflectometer of claim 16 wherein said diffracting
means comprises a reflective holographic grating which is curved so as to
be concave with respect to impinging light.
22. The microspectroreflectometer of claim 16 wherein said viewing means
comprises a microscope eyepiece.
23. The microspectroreflectometer of claim 16 wherein said viewing means
comprises a camera.
24. A measuring microscope comprising
means for supporting a workpiece,
an illumination source producing a light beam directed along an optical
path toward said workpiece,
a stop having a transmissive aperture therein, said aperture having a first
diameter in a viewing mode, a second diameter smaller than said first
diameter in a spectroscopic measuring mode, and a third diameter smaller
than said second diameter in a focusing mode,
an objective lens imaging said stop onto a workpiece, where a variable size
portion of said workpiece is illuminated, said objective lens forming an
image of said illuminated portion of said workpiece in an image plane,
a spectrometer stop having an aperture therein, said aperture intersecting
said image plane,
means for viewing said image in said viewing mode,
means lying behind said spectrometer stop for spectrally resolving and
measuring the intensities of light passing through said aperture of said
spectrometer stop in said spectroscopic measuring mode, said resolving and
measuring means including means for determining the total intensity of
light passing through said aperture in said focusing mode, and
means responsive in said focusing mode to said intensity determining means
for adjusting the relative distance between said objective lens and said
workpiece so as to achieve a best focus condition, said condition being
determined to be met when the rate of change of measured light intensity
with respect to said relative distance is zero.
25. A measuring microscope comprising,
at least one broad spectrum light source producing a light beam,
a microscope objective in the path of said light beam,
means for supporting a substantially flat object, said light beam being
focused to a spot by said microscope objective so as to illuminate at
least a portion of said object, said microscope objective forming an image
in an image plane of at least a part of said illuminated portion of said
object,
a mirror with a transmissive aperture defined therein, said aperture
intersecting said image plane in a position for passing light
corresponding to a portion of said image,
means in light reflective relationship to said mirror for viewing said
image,
means for diffracting said light passing through said aperture,
a linear detector array disposed in a position relative to said diffracting
means to receive first order diffracted light,
a focus condition detector disposed in a position relative to said
diffracting means to receive zero order diffracted light, said object
being considered in focus when the light intensity measured by said focus
condition detector with respect to the axial position of said object is a
maximum, and
means for adjusting the axial spacing between said objective and said
object so as to thereby achieve and maintain an in-focus condition.
26. The microscope of claim 25 wherein said at least one light source
comprises a visible light source and an ultraviolet light source.
27. The microscope of claim 25 wherein said diffracting means comprises a
reflective holographic grating which is curved so as to be concave with
respect to impinging light.
28. A measuring microscope comprising,
at least one light source producing a light beam,
a microscope objective in the path of said light beam,
means for supporting a substantially flat object, said light beam being
focused to a spot by said microscope object so as to illuminate at least a
portion of said object, said microscope objective forming an image in an
image plane of at least a part of said illuminated portion of said object,
means for viewing said image,
means defining a transmissive aperture intersecting said image plane for
passing light corresponding to a portion of said image,
means for diffracting said light passing through said aperture,
a light source located in the zero order diffraction position relative to
said diffracting means and directing a light beam toward said aperture,
a linear detector array disposed in a position relative to said diffracting
means receive first order diffracted light,
an integrating circuit electrically connected to said linear detector
array, the sum of light intensities received by said linear detector array
representing the intensity of light passing through said aperture, and
means for adjusting the axial spacing between said objective and said
object so as to thereby achieve and maintain an in-focus condition, said
object being considered in focus when said sum of light intensities
received by said linear detector array is a maximum with respect to the
axial position of said object.
29. The microscope of claim 28 wherein said at least one light source
comprises a visible light source and an ultraviolet light source.
30. The microscope of claim 28 wherein said diffracting means comprises a
reflective holographic grating which is curved so as to be concave with
respect to impinging light. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention relates to a measuring microscope having an aperture which
determines the workpiece region to be measured, and incorporating a
focusing system based upon a generalization of the confocal microscope.
The invention relates especially to a microspectroreflectometer that is
employed to determine the local thickness of an object by light
interference, and which incorporates such a focusing system.
BACKGROUND ART
There is a broad class of measuring microscopes which project an image of a
workpiece upon an aperture, and which measure properties of the light
passing through this aperture, in order to characterize a particular small
area, or a sequence of such areas, on the workpiece. We may describe this
class as "aperture-projecting measuring microscopes."
One example of such an instrument is the microspectrophotometer, which
characterizes the light spectroscopically. A microspectrophotometer which
includes an illumination source for providing light to the workpiece, and
means for determining the ratio of reflected light intensity to incident
light intensity, as a function of wavelength, is called a
microspectroreflectometer.
All aperture-projecting measuring microscopes require means to establish
accurate focus of the microscope upon the selected area on the workpiece,
so that the light passing through the aperture will correctly represent
the properties of the selected area. In some cases it suffices for viewing
means and large-field illumination to be provided, so that the instrument
user can determine visually whether the instrument appears to be in focus.
To increase the speed and reproducibility of measurements, it is
preferable to provide automatic equipment to indicate when focus is
correct, and in some cases to provide means for automatic adjustment of
axial distance between the object and the microscope objective lens, so
that the instrument is automatically driven to best focus.
When the workpiece to be measured has topography whose depth is comparable
to or larger than the depth of field of the measuring microscope, it
becomes particularly important that the focusing mechanism be responsive
to the local surface altitude, in substantially the same region where the
measurement is to be made. One group of workpieces that often exemplify
this requirement are patterned semiconductor wafers used in the
fabrication of integrated circuits.
The focusing systems to be described in this specification are suitable for
use with aperture-projecting measuring microscopes. They are, in
particular, suitable for use with microspectroreflectometers.
It is known that the best focusing height of a microscope may be determined
by an apparatus in which the microscope objective projects upon the
workpiece the image of a pointlike light source, and reimages the
illuminated workpiece region on one or more pointlike apertures, behind
which lie photoelectric detectors. Such an apparatus is described by
Lacotte et al. in U.S. Pat. No. 3,912,922.
By "pointlike" is meant that the light source or aperture is smaller than
the diffraction limit, so that the size and shape of images of the light
source and aperture on the workpiece are determined primarily by the laws
of diffraction, most detailed geometric information about the original
shape being lost in the projected image. In the case of an essentially
perfect and unobstructed microscope objective, the image is the wellknown
Airy disc.
It has been shown possible to construct a profilometer (i.e. a measuring
microscope for measuring the altitude profile of a surface) by employing
such focussensing apparatus. Such a profilometer is described in D. K.
Hamilton et al., "Surface Profile Measurement Using the Confocal
Microscope", Journal of Applied Physics 53(7), 5320 (July, 1982), which is
incorporated herein by reference. In such a profilometer, the focus sensor
determines, at each of a series of points on the surface, the
objective-to-stage distance that best maintains the focus of the objective
on the surface; the record of the series of distance measurements
represents the profile of the surface.
To understand how the focusing systems of our invention differ from other
focusing systems, it is useful to review briefly some well-known
principles of the confocal microscope. Such microscopes are described more
fully in T. Wilson and C. Sheppard, Theory and Practice of Scanning
Optical Microscopy, Academic Press, 1984, which is incorporated herein by
reference.
FIG. 1 shows schematically a simple and common form of confocal microscope.
Laser 102 produces a beam of light, which is brought to a focus by lens
104 on pinhole 106. Pinhole 106 is small enough to be substantially
smaller than the diffraction limit for this optical system, so that laser
light coming through the pinhole is effectively a point source. Condenser
lens 108, which must be of high optical quality (often a microscope
objective is used for this function) forms an image of pinhole 106 on the
transparent object which is to be observed. The object (not shown) lies in
object plane 110, and may be moved transversely to the optical axis of the
instrument, so as to measure a profile of transmissivity vs. position.
This instrument is, in other words, a form of microdensitometer. Objective
lens 112, which typically has the same numerical aperture as condenser
108, forms an image of the illuminated spot on detector pinhole 114,
behind which lies detector 116. Detector pinhole 114 is smaller than the
diffraction limit.
Other known variations of the confocal microscope provide for building up a
map or an image of an object not by moving the object, but rather by
moving optical elements such as lenses or mirrors, so as to cause the
observed spot to move. For simplicity in presentation, our invention will
be described with respect to moving-object microscopes, but the
moving-optics variations are also contemplated.
The confocal microscope provides better spatial resolution than does a
conventional microscope. This point is illustrated by FIG. 2, which is the
graph of the radial distribution of intensity that would be observed by
the FIG. 1 microscope, as it scanned across a pointlike object.
Curve 201 is the intensity distribution that would be observed if either
pinhole 106 or pinhole 114 were absent. This curve is just the well-known
Airy intensity distribution that is observed with a conventional scanning
microscope.
Curve 202 is the distribution observed with both pinholes in place. The
observable enhancement in resolution is explained by the fact that the
resolution is a product of two Airy-disc images. As the point of
measurement moves away from the center of the actual pointlike object, the
intensity of illumination falls off according to curve 201, and the
sensitivity of the detector also falls off according to curve 201. The net
sensitivity curve 202 is the product of these two curves.
Curve 203, representing the sensitivity of a confocal microscope with
annular apodizing apertures (not shown) inserted in the pupils of each of
lenses 108 and 112, is shown as one example of the fact that more
elaborate versions of the confocal microscope can have even higher
resolution, typically at the expense of some residual sensitivity at large
distances from the center of the pattern. Thus the central portion 203a of
curve 203 is narrower than the central portions of curves 201 or 202, but
the sensitivity in rings 203b and 203c is higher than anything seen with
curves 201 and 202. The effect on imaging is that resolution improves at
the expense of introducing more artifacts in the image. For simplicity,
our invention is described without the presence of annular apodizing
apertures. We contemplate, however, the optional use of such apertures, or
of more complex apodizing apertures.
FIG. 3 illustrates a second known property of the confocal microscope which
is of importance in understanding our invention. This figure shows just
the detection half of a system like that of FIG. 1, in three different
conditions of focus. With reference to FIG. 3a, object 14 is imaged by
microscope objective 40 onto an image plane 90 coinciding with aperture 46
in field stop 44. This condition occurs when object 14 is "in focus" wit
respect to aperture 46. Note that the bundle of light rays 88 originating
from a pointlike region on object 14 lying on a focal plane 86 comes to a
focus at an image plane 90. All of these rays pass through aperture 46. In
FIG. 3b, the object 14 lies below focal plane 86, and the image plane 90
is located below stop 44. Only some of the light rays in bundle 88 pass
through aperture 46. Likewise, in FIG. 3c, object 14 lies above focal
plane 86, and the image plane 90 again does not coincide with stop 44.
Only some of the light rays in bundle 88 can pass through aperture 46.
This results in the condition shown in FIG. 4a, where the intensity I of
light passing through the aperture is at a maximum value I.sub.0 when the
axial position Z of the object coincides with the focal plane position
Z.sub.0.
This variation of the confocal microscope's response with focus condition
is the basis of profilometers such as that described in the Hamilton et
al. article cited above.
It is possible to construct a confocal microscope to work in reflective
mode. In such an instrument, one form of which is illustrated
schematically in FIG. 5, a single lens 510 acts both as condenser and
objective. Laser 502 emits a beam of light which is focused by lens 504 on
source aperture 506. Light which passes through aperture 506 then passes
through beamsplitter 508, and is focused by objective 510 on a workpiece
(not shown), which usually lies in object plane 512. Light reflected or
scattered from the workpiece is gathered by objective 510, and focused,
via beamsplitter 508, on detector aperture 514, behind which is located
detector 516. The operation of the reflection mode confocal microscope is
similar to that of the transmission confocal microscope previously
described.
It is also possible to construct a reflection confocal microscope in which
a single aperture is shared by the illumination and detection systems. One
example of such a configuration is shown in FIG. 6. An instrument
according to FIG. 6 is described in L. Reimer et al., "Lock-In Technique
For Depth-Profiling and Magnetooptical Kerr Effect Imaging in Scanning
Optical Microscopy", Scanning 9, 17-25 (1987). A particular advantage of
such an arrangement is that no precision pinhole alignment is required to
attain optimal performance. Whereas in two-pinhole instruments, the source
pinhole's image must be accurately positioned in relation to the second
pinhole, the single-pinhole system is automatically in alignment at all
times.
Although confocal microscope configurations are usually described to
include lasers, use of a laser is not in general strictly necessary. The
laser is employed because it is an unusually bright light source,
permitting high speed measurements with excellent signal to noise ratio.
In cases where achieving maximum brightness is not the dominant
consideration, it may be advantageous to construct a confocal microscope
with an incoherent light source such as a tungsten lamp or an arc lamp.
Most of the embodiments of our invention, to be described below, do in
fact use incoherent light sources.
We have recognized that, while confocal microscopes have previously been
described to use apertures whose size is less than the diffraction limited
optical spot at each aperture, it is possible and sometimes advantageous
to use larger apertures. In the aperture-projecting measuring microscopes
considered in our invention, the size of the aperture is determined by the
size of the workpiece area to be characterized, which is often larger than
the diffraction-limited spot. We will explain below that there is
considerable advantage in constructing a focus-sensing apparatus for use
in such instruments by using confocal microscope configurations or our own
inventive configuration, called the inverse confocal microscope.
Many aperture-projecting measuring microscopes suitable for modification to
include our inventive focusing means have been described in the
literature. For example, in U.S. Pat. No. 4,674,883, Baurschmidt discloses
a microspectroreflectometer for measuring the thickness and line width of
features upon an object, such as thin film structures on a semiconductor
wafer. The Baurschmidt microscope incorporates no provision for detecting
or automatically adjusting to a condition of best focus.
The accuracy with which the thickness of transparent films at specified
locations on wafers or other flat surfaces can be measured is limited by
the accuracy of focus of the microscope-spectrometer, which affects not
only the resolution of the location on the wafer being measured, but also
the amount of light reaching the spectrometer's detector elements. For
very thin films, the measurement accuracy may also be affected by both the
spectral resolution and the spectral range of the spectrometer. Measuring
microscopes currently available can accurately measure the film thickness
on unpatterned wafers down to about 10 nanometers. In the case of
patterned wafers and other somewhat flat objects having a rough or
profiled surface, either a large depth of focus is required or the
microscope must be able to bring areas of the surface into focus as the
object is scanned.
It is an object of the present invention to provide a measuring microscope
capable of automatically focusing on an object as that object is scanned.
It is another object of the present invention to provide a microscope
capable of accurately measuring characteristics of a workpiece area, such
as the thickness of thin films on patterned wafers for thicknesses in a
range from less than 2 nanometers to more than 5000 nanometers.
DISCLOSURE OF THE INVENTION
We have invented an aperture-projecting measuring microscope incorporating
one or more of a class of autofocusing systems that may be described
broadly as "generalized confocal microscopes." The class includes
variations on conventional confocal microscope systems, also referred to
herein as "positive confocal microscopes," and an entirely new optical
arrangement which may be called the "inverse confocal microscope." In a
case of particular interest, the measuring microscope is a
microspectroreflectometer. In another case of particular interest, the
measuring microscope is a profilometer
The inverse confocal microscope configuration has not previously been
employed, to our knowledge, for any purpose, and in particular has not
been employed in an autofocusing system for a microspectroreflectometer or
a profilometer. The positive confocal microscope configuration is well
known, but to our knowledge has not previously been used for focusing of a
microspectroreflectometer, or of any other aperture-projecting measuring
microscope whose field aperture exceeds diffraction-limited size.
According to one aspect of our invention, the measuring microscope may
include an autofocusing system based on a positive confocal microscope
configuration, incorporating an innovative field stop that provides
efficiently both for effective autofocusing and for large-field viewing of
the workpiece. An example of such a field stop is shown in FIG. 11. The
highly transmissive central portion 1103 of this stop provides a small
bright light source that may be used as the basis for focusing the
microscope while the larger, partially transmissive region 1102 provides
broad-area illumination for viewing.
According to another aspect of our invention, the measuring microscope
incorporates an inverse confocal microscope configuration. The innovative
nature of this configuration may be seen by noting that in all of the
confocal microscopes described in the Background section above, source and
detector apertures are always used in transmission. This is to say that in
the illumination path the light source is on one side of the illumination
aperture and the object plane is on the other side. Similarly, in the
detection path, the object plane is on one side of the detection aperture
and the detector is on the opposite side. This arrangement, that apertures
are used in transmission, is in fact common to all prior art confocal
microscopes.
The inverse confocal microscope differs from conventional confocal
microscopes in that at least one of the apertures, preferably the
illumination aperture, is used in reflection. One simple example of such
an arrangement is illustrated schematically in FIG. 7. A beam of light
emitted by source 702 is focused by lens 704 on aperture 716 in mirror
706. The size of the focused spot of light, in the vicinity of aperture
716, is large enough so that a significant portion of the light is
reflected from the mirror surface, rather than passing through the
aperture. The reflected light is focused by objective 708 on the object
709, in object plane 710. Light reflected or scattered by the object is
imaged by the objective in the vicinity of aperture 716, some of that
light passing through the aperture to strike detector 714. Light trap 712
traps rays coming through aperture 716 directly from source 702, so that
only rays coming from object 709 can reach detector 714.
Consider now the dependence of light intensity seen by detector 714 upon
the axial position of object 709. First imagine what the performance would
be, from the standpoint of geometrical optics, if objective lens 708 were
perfect and diffractive effects were of no importance. With the object in
perfect focus, an image of aperture 716 would be projected upon object 709
as a dark shape, surrounded by a bright spot. Objective lens 708 would
re-image this dark shape back upon aperture 716, and would reimage the
light coming from the object to strike the areas of mirror 706 that
surround aperture 716, so that no light from the object would go through
the aperture. With object 709 positioned above or below the optimal focal
plane, some of the light that originated at the mirror regions surrounding
aperture 716 would be so re-imaged as to pass through the aperture. Thus
there would be a null in detector output, corresponding to the location of
best focus.
In the presence of lens imperfections and of the real influence of
diffraction, imaging of the aperture shape will be imperfect, so that some
light will reach detector 714 even at best focus. Still, we find that a
well-defined minimum is observed, which can be used as the basis of an
autofocus system. A typical output pattern is shown in FIG. 8. The minimum
value of detected intensity, I.sub.min, occurs at the position of best
focus, z.sub.0.
An advantage of constructing a measuring microscope such as a
microspectroreflectometer with a focusing system configured as a
generalized confocal microscope is that many of the optical functions
required for focusing can be performed by optical elements that are
required by other functions of the instrument.
A second advantage of the generalized confocal microscope in this
application is that it avoids a major ambiguity associated with focusing
systems based on conventional image analysis methods. It is known that in
a conventional microscope, when the image is in best focus, the intensity
at each point in the image will be either a minimum or a maximum. Without
a priori knowledge of the object in view, it is impossible to predict
whether a minimum or a maximum will be seen (see G. Hausler et al. Applied
Optics 23 (15), 2468 (1 August 1984)). With a generalized confocal
microscope, it is predictable from the design of the microscope whether a
minimum or a maximum will be observed, which simplifies the operation and
the required control circuitry.
A third advantage of constructing a measuring microscope with a generalized
confocal microscope configuration to determine best focus is that such a
focusing system inherently establishes best focus in substantially the
same area of the viewed object where the measurement is to be made.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified side view of a transmission-type confocal microscope
of the prior art.
FIG. 2 is a graph of light intensity versus position for an image of a
single point object as seen through a conventional microscope, a confocal
microscope with two circular pupils, and a confocal microscope with two
annular pupils.
FIGS. 3a-3c are simplified side views of a confocal microscope portion for
an object respectively in focus, below the focal plane and above the focal
plane.
FIG. 4a is a graph of detected light intensity by confocal microscopes in
FIGS. 3a-3c. vs. the axial position of an object.
FIG. 4b is a graph of the rate of intensity change in FIG. 4a vs. axial
position.
FIGS. 5 and 6 are simplified side views of reflection-type confocal
microscopes of the prior art.
FIG. 7 is a simplified side view of an inverse confocal microscope of the
present invention.
FIG. 8 is a graph of light intensity measured by the microscope in FIG. 7
versus the relative distance between microscope objective and workpiece.
FIG. 9 is a simplified side view of a first embodiment of the present
invention.
FIG. 10 is a simplified side view of a second embodiment of the present
invention.
FIGS. 11 and 12 are plan views of two possible field stops for use in the
invention in FIGS. 9 and 10.
BEST MODE FOR CARRYING OUT THE INVENTION
With reference to FIGS. 9 and 10, each of measuring microscopes 10 and 20
has a platform or table 12 for supporting an object 14. One or more motors
16 move platform 12 in at least an axial direction Z. Typically, three
motors are used, each operating independently, to move platform 12 and
therefore object 14 in mutually orthogonal directions X, Y and Z. However,
for clarity, only one motor is seen in the drawings. Object 14 is
substantially flat. The term "substantially flat" does not mean that
object 14 must have a perfectly planar surface, nor even a surface which
is entirely within the depth of focus of the microscope optics to be
described below. Rather, the variations in height, if any, of the object
are substantially less than the length and width dimensions of the object,
such as in a semiconductor wafer with or without circuit patterns formed
thereon. The reflectivity of object 14 may be either specular or diffuse.
One use to which this invention may be put is to measure the thicknesses of
thin film coatings applied to a wafer surface so as to determine whether
any unacceptable thickness variations are present. Such an application
relies on variations in reflected light intensity for different
wavelengths of incident light due to the interference produced by partial
reflections from top and bottom surfaces of the thin film. Other
applications for measuring microscopes with spectrometers are also known.
While the invention described herein is a reflective system, a person
skilled in the art can easily see how to adapt the platform 12 and
microscope elements to illuminate a light transmissive object from below.
The measuring microscopes 10 and 20 have at least one light source 18 which
emits a broad spectrum of visible light 19. Typically, light source 18 is
a tungsten filament-type incandescent lamp, although other visible light
sources may also be used. A condenser lens 22 collects light 19 and
directs it at a field stop 24 with an aperture 25.
It should be understood that other and more complex illuminator
arrangements, which are well known, may be used with light source 18, and
with the other light sources which will be described below. It may be
useful, for example, to incorporate an additional lens and an aperture
stop in the illuminator, so as to reduce stray light in the system. In
describing this and the other illuminators in the instrument, we have
described very simple illumination optics, so as to concentrate on
essential components of the invention.
The light 19, now a generally columnar beam, travels along an optical path
26 to a beamsplitter 28, where it is deflected into the main axial beam
path 30.
A second light source 32 emitting ultraviolet light is preferred.
Typically, second light source 32 is a low-pressure mercury vapor
discharge lamp. However, any other lamp that emits ultraviolet light, such
as a deuterium continuum light source, may be used. Ultraviolet light 33
may be limited by a field stop 34 with aperture 35. Light 33 travels along
an optical path 36 to a beamsplitter 38, where it is deflected into main
axial beam path 30. Ultraviolet light extends the spectral range over
which thin film thickness may be measured, thereby enabling an accurate
thickness measurement to be made for thicknesses on the order of 2 nm or
less. The use of a second light source is optional.
A microscope objective 40 focuses the combined visible and ultraviolet
light beam 39 to a small spot 42 on the object 14. Microscope objective 40
further functions to form a magnified real image of at least a part of the
illuminated portion 42 of object 14. The image is formed in a plane
perpendicular to main optical axis 30. This plane intersects a
transmissive aperture 46 of a reflective stop 44 when the portion 42 is in
focus. Typically, the illuminated por | | |
