 |
Claims  |
|
|
What is claimed is:
1. A method of obtaining a representation of the surface profile of a
portion of a specimen such as a semiconductor wafer comprising the steps
of projecting a sharply defined beam through a confocal optical imaging
system having a very narrow depth of field to focus it into a small spot
upon the wafer surface and detecting a measurable characteristic of the
beam reflected from said spot, relatively moving the projected beam and
the wafer so that the spot scans a line on a portion of said wafer,
recording and storing a signal with respect to said measurable
characteristic at a plurality of closely spaced positions along said line,
successively changing the relative spacing between the imaging system and
the wafer by a small incremental distance after each scan of said line
such that the focus level of the system with respect to the wafer surface
is successively changed by said incremental distances in a given direction
perpendicular to said scan line, and separately determining the relative
spacing level for each of said points along said line at which a signal
most characteristic of a surface indication was obtained to provide a
cross-sectional representation of the surface of the wafer along said
line.
2. The method of claim 1 wherein the focus level is initially set above the
uppermost surface of the wafer and is successively lowered by said
incremental distances through the entire surface profile.
3. The method of claim 1 wherein the measurable characteristic of the beam
comprises its intensity and wherein the spacing levels for providing the
cross-sectional representation along said line are determined in
accordance with the maximum detected intensity of the reflected beam at
each of said closely spaced positions.
4. The method of claim 3 wherein said optical imaging system is fixed in
position except for changes in its focal depth and the wafer is moved in a
plane beneath the imaging system to obtain said line scan.
5. The method of claim 4 wherein the step of changing the relative spacing
between the imaging system and the wafer is accomplished by shifting the
vertical position of an objective lens in the imaging system.
6. The method of claim 1 including the step of displaying a graph of the
cross-sectional representation for permitting an observer to make
decisions with regard to further scanning of the wafer.
7. A method of scanning semiconductor wafers or the like for determining
surface pattern information in a given area on the wafer comprising the
steps of projecting a sharply defined beam through a confocal optical
imaging system having a very narrow depth of field to focus it onto a
small spot within said area upon the wafer surface and detecting a
measurable characteristic of the beam reflected from said spot, relatively
moving the projected beam and the wafer so that the spot scans a line
across said area of said wafer, recording and storing a signal with
respect to said measurable characteristic at a plurality of closely spaced
positions along said scan line, successively changing the relative spacing
between the imaging system and the wafer by a small incremental distance
after each scan of said scan line such that the focus level of the system
with respect to the wafer surface is successively changed by said
incremental distances in a given direction perpendicular to said scan
line, separately determining the relative spacing level for each of said
points along said scan line at which a signal most characteristic of a
surface indication was obtained to provide a cross-sectional
representation of the surface of the wafer along said scan line, selecting
from said cross-sectional representation those levels which provide the
maximum surface pattern information, and scanning the remainder of said
area by relatively moving the beam and the wafer over a plurality of lines
parallel to said scan line at each of the selected levels while recording
and storing said signals to thereby obtain all of the relevant surface
pattern information within said area of the wafer.
8. The method of claim 7 wherein the focus level is initially set above the
uppermost surface of the wafer and is successively lowered by said
incremental distances through the entire surface profile.
9. The method of claim 8 wherein said optical imaging system is fixed in
position except for changes in its focal depth and the wafer is moved in a
plane beneath the imaging system to obtain said line scan.
10. The method of claim 9 wherein the step of changing the relative spacing
between the imaging system and the wafer is accomplished by shifting the
vertical position of an objective ens in the imaging system.
11. The method of claim 7 wherein the measurable characteristic of the beam
comprises its intensity and wherein the spacing levels for providing the
cross-sectional representation along said scan line are determined in
accordance with the maximum detected intensity of the reflected beam at
each of said closely spaced positions.
12. The method of claim 7 including the step of displaying a graph of the
cross-sectional representation for permitting an operator to perform the
selecting step.
13. The method of claim 12 including the step of providing a two
dimensional image of said given area on the wafer by adding the signal
values obtained at each selected level for each of the closely spaced
positions of all of the parallel scan lines.
14. The method of claim 13 including the step of setting certain of the
signal values to zero prior to the adding step if such certain signal
values indicate that they represent reflections from surfaces
substantially out of focus of the imaging system.
15. The method of claim 12 including the step of providing a two
dimensional image of said given area on the wafer by determining the
maximum signal value of the selected scanning levels at each of the
closely spaced positions of all of the parallel scan lines.
16. A method of making linewidth measurements on the surface of a
semiconductor wafer or the line comprising the steps of projecting a
sharply defined beam through a confocal optical imaging system having a
very narrow depth of field to focus it onto a small spot upon the wafer
surface and detecting a measurable characteristic of the beam reflected
from said spot, relatively moving the projected beam and the wafer so that
the spot scans a line on a portion of said wafer, recording and storing a
signal with respect to said measurable characteristic at a plurality of
closely spaced positions along said line, successively changing the
relative spacing between the imaging system and the wafer by a small
incremental distance after each scan of said line such that the focus
level of the system with respect to the wafer surface is successively
changed by said incremental distances in a given direction perpendicular
to said scan line, separately determining the relative spacing level for
each of said points along said line at which a signal most characteristic
of a surface indication was obtained, providing a cross-sectional
representation of the surface of the wafer along said line by connecting
the points determined by said line, and measuring across the distance
between the generally vertical portions of said cross-sectional
representation to obtain the linewidths of a superimposed pattern of the
wafer surface.
17. The method of claim 16 wherein the focus level is initially set above
the uppermost surface of the wafer and is successively lowered by said
incremental distances through the entire surface profile.
18. The method of claim 16 wherein the measurable characteristic of the
beam comprises its intensity and wherein the spacing levels for providing
the cross-sectional representation along said line are determined in
accordance with the maximum detected intensity of the reflected beam at
each of said closely spaced positions.
19. The method of claim 16 wherein said optical imaging system is fixed in
position except for changes in its focal depth and the wafer is moved in a
plane beneath the imaging system to obtain said line scan.
20. The method of claim 16 wherein said cross-sectional representation is
provided in the form of a graph for permitting an operator to make said
measurements for obtaining linewidths.
21. A system for determining the cross-sectional profile of a specimen such
as a semiconductor wafer comprising a confocal optical imaging system
having a very narrow depth of field for focusing a beam on a small spot on
an underlying wafer and including a photodetector for receiving the
reflected spot from the wafer for detecting a measurable characteristic
thereof and providing an output signal, means for effecting relative
movement of a rapid oscillatory nature between the imaging system and the
wafer in a plane parallel to the surface of the wafer such that the spot
repeatedly scans along a line across a portion of the wafer,
means for obtaining and storing said photodetector signal at a plurality of
closely spaced points in a single one-directional scan along said line,
means for successively shifting the focus of said imaging system with
respect to the wafer by an incremental distance after each one-directional
scan along said line during the return relative movement of the system and
wafer,
means for determining the focus level for the imaging system at which the
maximum output signal is obtained for each of said spaced points along
said line,
and means for providing said focus levels as determined by said last named
means as a representation of the cross-sectional profile of said wafer
along said line.
22. A system according to claim 21 wherein the measurable characteristic of
the beam comprises its intensity, said focus level determining means
determining the maximum intensity as a representation of the maximum
reflectivity for the scanned levels at each point along said line.
23. A system according to claim 21 wherein said means for effecting
relative movement comprises means for oscillating said wafer in a linear
path.
24. A system according to claim 21 wherein said imaging system includes an
objective lens and said means for shifting the focus of the imaging system
includes means for altering the spacing between said objective lens and
the wafer along the path of the beam.
25. A system according to claim 21 wherein said means for providing the
determined focus levels comprises a graphical output display wherein the
determined focus levels are connected in a continuous line defining the
cross-sectional profile of the wafer.
26. A system according to claim 21 including means for determining separate
surface levels along the scanned wafer surface by providing from said
means for providing the determined focus levels a histogram of the various
focus levels versus the number of points along said line at which said
focus levels were found.
27. A system for determining the cross-sectional profile of a specimen such
as a semiconductor wafer comprising means for mounting said wafer for
oscillatory scanning movement along a line in a plane generally parallel
to the surface of the wafer; a confocal optical imaging system having a
very narrow depth of field overlying said wafer, said imaging system
including a light source, means for directing the beam from the light
source perpendicularly to the wafer surface and focusing it on a spot on
the wafer, and photodetector means for receiving the light reflected from
the spot and providing an output signal varying in accordance with the
intensity of the reflected spot; means for incrementally moving at least a
portion of said imaging system in a direction substantially perpendicular
to said plane to successively change the focal length of the imaging
system by a small amount relative to the surface profile of the wafer;
means for recording said output signals at times corresponding to a
plurality of closely spaced positions along said line during each scanning
movement of the wafer; means connected to said recording means for
determining the focal depth at which the maximum output signal was
received at each of said closely spaced positions; and means connected to
said last named means for providing the surface profile of said wafer
along said line as represented by the focal depth determined for each of
said positions.
28. A system according to claim 27 wherein said means for providing the
surface profile comprises a graphical output display wherein the focal
depth positions are connected in a continuous line defining the
cross-sectional profile of the wafer.
29. A system according to claim 27 including means for determining separate
surface levels along the scanned wafer surface by providing from said
means for providing the surface profile a histogram of the various focal
depths versus the number of positions along said line at which said focal
depths were found.
30. A system according to claim 27 wherein said imaging system includes an
objective lens and said means for moving at least a portion of said
imaging system comprises a piezoelectric crystal operatively connected to
said objective lens, and means for varying the voltage applied to said
crystal to cause it to expand or contract and thereby shift the spatial
position of said objective lens.
31. A system for scanning semiconductor wafers or the like for determining
surface pattern information in a given area on the wafer comprising a
confocal optical imaging system having a very narrow depth of field for
focusing a beam on a small spot on an underlying wafer and including a
photodetector for receiving the reflected spot from the wafer for
detecting a measurable characteristic thereof and providing an output
signal, means for effecting relative movement between the imaging system
and the wafer such that the spot scans along a line across a portion of
the wafer,
means for obtaining and storing said photodetector signal at a plurality of
closely spaced points in a single scan along said line,
means for successively shifting the focus of said imaging system with
respect to the wafer by an incremental distance after each scan along said
line,
means for determining the focus level for the imaging system at which the
maximum output signal is obtained for each of said spaced points along
said scan line,
means for providing said focus levels as determined by said last named
means as a representation of the cross-sectional profile of said wafer
along said scan line,
means for selecting from said representation of the cross-sectional profile
those levels which provide the maximum surface pattern information,
means for thereafter causing the focus shifting means to shift the focus of
the imaging system successively to each of the selected levels,
and means for causing the means for effecting relative movement to scan
over a plurality of lines parallel to said scan line while the focus level
is at each of the selected levels to thereby scan the remainder of said
given area on the wafer to obtain all of the relevant surface pattern
information within said area.
32. A system according to claim 31 wherein the measurable characteristic of
the beam comprises its intensity, said focus level determining means
determining the maximum intensity as a representation of the maximum
reflectivity for the scanned levels at each point along said line.
33. A system according to claim 31 wherein said means for effecting
relative movement comprises means for oscillating said wafer in a
generally linear path.
34. A system according to claim 33 wherein said means for causing the means
for effecting relative movement to scan over a plurality of lines
comprises a drive means operable to move the wafer at right angles to said
linear path at a slow linear speed relative to the oscillatory speed of
movement in said linear path.
35. A system according to claim 32 wherein said imaging system includes an
objective lens and said means for shifting the focus of the imaging system
includes means for altering the spacing between said objective lens and
the wafer along the path of the beam.
36. A system according to claim 31 wherein said means for providing the
determined focus levels acomprises a graphical output display wherein the
determined focus levels are connected in a continuous line defining the
cross-sectional profile of the wafer.
37. A system according to claim 31 wherein said means for selecting
includes means for providing a histogram of the various focus levels
versus the level of points along said line at which said focus levels were
found.
38. A system for making linewidth measurements on the surface of a
semiconductor wafer or the like comprising a confocal optical imaging
system having a very narrow depth of field for focusing a beam on a small
spot on an underlying wafer and including a photodetector for receiving
the reflected spot from the wafer for detecting a measurable
characteristic thereof and providing an output signal, means for effecting
relative movement of a rapid oscillatory nature between the imaging system
and the wafer in a plane parallel to the surface of the wafer such that
the spot repeatedly scans along a line across a portion of the wafer,
means for obtaining and storing said photodetector signal at a plurality of
closely spaced points in a single one-directional scan along said line,
means for successively shifting the focus of said imaging system with
respect to the wafer by an incremental distance after each one-directional
scan along said line during the return relative movement of the system and
wafer,
means for determining the focus level for the imaging system at which the
maximum output signal is obtained for each of said spaced points along
said line,
means for providing a cross-sectional representation of the surface of said
wafer by connecting the points determined by said last named means to form
a continuous cross-sectional line,
and means for measuring across the distance between the generally vertical
positions of said cross-sectional line to obtain the linewidth of a
superimposed pattern on the wafer surface.
39. A system according to claim 38 wherein the measurable characteristic of
the beam comprises its intensity, said focus level determining means
determining the maximum intensity as a representation of the maximum
reflectivity for the scanned levels at each point along said line.
40. A system according to claim 38 wherein said means for effecting
relative movement comprises means for oscillating said wafer in a linear
path.
41. A system according to claim 40 wherein said imaging system includes an
objective lens and said means for shifting the focus of the imaging system
includes means for altering the spacing between said objective lens and
the wafer along the path of the beam.
42. A system according to claim 41 wherein said means for providing a
cross-sectional representation comprises a graphical output display
wherein the determined focus levels are connected in a continuous line
defining the cross-sectional profile of the wafer. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to systems for scanning surface patterns on
specimens such as semiconductor wafers or the like, and more particularly,
it pertains to methods and apparatus for accurately obtaining a
cross-sectional surface profile of such specimens.
2. Description of the Prior Art
In the inspection of semiconductor wafers or the like to detect surface
pattern defects, a variety of techniques have been utilized that take
advantage of various forms of microscopes, optical, acoustical, and
scanning electron types. In optical imaging systems generally, devices
similar to T.V. cameras have been utilized wherein electromagnetic
radiation is reflected from a relatively large spot on the wafer and
processed through an optical system and imaging camera to provide a
multi-intensity image which, either digitally or by analog means, can be
recreated on an appropriate output device, such as a CRT.
The inspection of semiconductor wafers typically provides a means whereby
certain processing defects can be detected or whereby linewidth
measurements can be made so as to determine whether or not the
manufacturing process has been performed correctly. Since the tolerance
limits for the dimensions which must be detected and measured accurately
are in the micron or even submicron range, microscope imaging systems
generally require a high degree of imaging resolution.
In certain prior art wafer inspection systems, laser beams are focused
through optical systems having a very narrow depth of field. Then, by
scanning the laser beam along the top surface of the semiconductor wafer,
the patterned lines, or patterns on the wafer, can be measured by
utilizing special detector devices to denote the edges of such lines by
measuring the scattered light therefrom. It has been generally recognized
that with such wafer scanning systems of the aforedescribed type the beam
focus level can be adjusted as it is scanned across the wafer so as to
track the changing surface level thereof by noting when the reflected
image moves slightly out of focus and by adjusting the spacing between the
wafer and the optical system (by moving either one relative to the other)
so as to continually maintain the reflective surface of the wafer at the
proper focus. Prior art patents which describe such scanning systems
include U.S. Pat. No. 4,505,585 to Yoshikawa et al and United States
Defensive Publication T102,104 to Kirk et al.
SUMMARY OF THE INVENTION
With the present invention, methods and apparatus are provided for
systematically obtaining the cross-sectional profile within a given area
on the semiconductor wafer surface. The information provided by this
profile can thereafter be effectively utilized to make the conventional
pattern linewidth measurements with a generally greater degree of accuracy
than that provided by the systems of the prior art. Also, the profile
provides the necessary information to determine at what specific levels
the given wafer area should be scanned such that the entire area of the
wafer can be rapidly scanned only at such selected levels to provide all
of the relevant information necessary, reducing the processing times and
digital storage capacities required.
With the method and apparatus of the present invention. an optical imaging
system is provided to both project a sharply defined beam onto a small
spot upon the wafer surface and to detect the image of the reflected spot
with respect to a measurable characteristic of the reflected beam
indicative of the reflective surface at or near the focal plane. The
optical imaging system and the wafer are relatively moved in a plane
generally parallel to the surface of the wafer so that the projected spot
scans a line across a portion or given area of the wafer, and means are
provided for recording and storing a signal representative of the
measurable characteristic at a plurality of very closely spaced positions
along the scan line. The focus level of the imaging system is successively
changed by moving the wafer and imaging system closer together or further
apart after each pass along a scan line until a plurality of scans have
been made completely passing through the relevant surface detail of the
wafer. Then, for each single recording position along the scan line, that
focus level of the system is determined wherein a signal most
characteristic of a surface indication le.g.. a maximum relected intensity
signal) was obtained. The serial accumulation of the thus determined focus
levels for each of the closely spaced positions along the scan line
represents a cross-sectional profile of the surface of the wafer along the
scan line.
This surface profile information can then be utilized for directly making a
pattern linewidth measurement, or the information can be used for
selecting the particular surface levels at which the optical system needs
to be automatically focused during subsequent scans throughout the
particular portion or area on the wafer. This permits the selected wafer
area to be thoroughly scanned and three-dimensional images to be produced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of the semiconductor wafer scanning
and profiling system of the present invention.
FIG. 2 is a side elevation, partially in section, of the mechanical portion
of the apparatus of the present invention.
FIG. 3 is an exploded isometric view of the scanner and x-y planar drive
mechanism of the system of the present invention.
FIG. 4 is a front elevation, partially in section, of the focus control
device of the apparatus of FIG. 2.
FIG. 5 is a section taken along line 5--5 of FIG. 4.
FIG. 6 is a flow chart depicting the programming for the computer which
controls the various operative components of the system of the present
invention.
FIGS. 7A, 7B, and 7C collectively comprise a flow chart depicting a
subroutine of the program of FIG. 6 for respectively collecting,
processing and displaying the data for the cross-sectional profile.
FIG. 8 is a cross-sectional illustration of a portion of a scanned
semiconductor wafer surface and the corresponding profile and reflectivity
displays and focal level histogram obtained with the system of the present
invention.
FIG. 9 is a flow chart depicting the programming for the system of the
present invention wherein a "superfocus" image is obtained and displayed.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The profiling technique of the present invention is adapted to be carried
out by and to be useful with a wafer scanning system such as shown in FIG.
1 and more specifically described and claimed in our copending U.S. patent
application Ser. No. 725,082, filed Apr. 19, 1985, and entitled
"Semiconductor Wafer Scanning System". The disclosure of this prior
application is herein incorporated by reference into the present
application, and reference to such application may be had for a more
detailed explanation of the appratus of the present invention and the
method of operation thereof.
Referring now to FIG. 1, which very schematically illustrates the
mechanical apparatus of the present invention and, in block diagram form,
the circuitry of the present invention, it will be seen that an optics
module 30 is provided to focus a sharply defined beam from a laser source
40 on a small spot upon are underlying semiconductor wafer, w. The optics
module comprises a confocal optical imaging system which is controlled by
and provides data information signals to a computer system 22. The
computer outputs information to various display units including an image
display monitor 24a (where the "superfocus" image of the entire scanned
area is displayed) and a graphics video display unit 24b (where the
profiles, graphs and histograms are displayed). The surface of the
semiconductor wafer, w, to be inspected by the system underlies the
optical imaging system and extends in a plane generally perpendicular to
the projected beam. The wafer is arranged to be moved in this plane in x
and y orthogonal directions by x and y stages 34, 32, respectively and
also by a vibratory scanning mechanism 46 aligned for movement in the x
direction. Under the control of appropriate signals from the computer
system 22, the x and y stages are driven by conventional motor control
circuitry 36. Movement in the z direction, i.e., in a direction generally
parallel to the light beam projected from laser source 40, is accomplished
by a focus control mechanism 28 which shifts an objective lens 26 (the
last element of the optical system) over very small vertical distances in
order to change the focal plane of the optical system. The focus control
mechanism is operated from the computer system through a focus control
signal from conventional control circuitry 38 to shift the lens 26 up or
down. The beam from laser source 40 is sharply focused with a very narrow
depth of field, and it is adapted to be reflected from a surface on wafer
w (if one is present) at the focal plane back through the optical system
to a photodetector 42. The signal from the photodetector is sampled and
digitized by the control circuitry 4l and transmitted to the computer
system 22 and represents the intensity of the reflected light received
from the projected spot on the surface of wafer w. These digital signals
are provided as a function of the focus level, z, and also as a function
of separate, closely spaced positions in the x-y plane. Since the optical
system has a very narrow depth of field, reflected intensity peaks as the
focal plane coincides with an underlying reflective surface and drops off
rather sharply as the wafer surface is moved away from the focal plane.
Thus the height of the wafer at any particular planar (x,y) position
thereon can be readily detected by operating the focus control mechanism
28 to achieve a maximum output signal representing the intensity of the
reflected light. It is upon this fundamental principle that the present
invention is based. The computer system 22 tracks both the x, y positions
of the wafer with respect to the beam and the z level focal plane location
of the beam and coordinates this information with the intensity signals
from photodetector 42 in order to provide a three dimensional output
representation of the portion of the wafer that is scanned.
As pointed out previously, the wafer, w, is moved in the horizontal plane
by x and y stages 34 and 32, respectively, which are controlled by x, y
stage motor control circuitry 36 under the monitoring of the computer
system 22. The stages 32, 34 comprise conventional precision translation
tables provided with optical position encoders for submicron resolution
and accuracy. The motor control circuitry 36 is also conventional in
nature providing drive signals for moving the stages and including A/D
circuitry for receiving and processing the signals from the position
encoders so as to accurately monitor the position of the wafer at any
given instant. The z-axis focus control circuitry 38 provides an output
voltage for the focus control mechanism 28 which, in the present instance,
comprises a piezoelectric crystal that expands or contracts in the
vertical plane and responds to the applied voltage to shift the relative
vertical position of objective lens 26.
The control circuitry 44 for the entire system is adapted to receive a
continuous input light intensity signal from the photodetector 42 through
amplifier 45 and synchronize this data with the scanner 46 position
information. The control circuitry 44 also serves to output a scan drive
signal (a sinosoidal wave form) to the vibratory scanning mechanism 46
through an amplifier 47. The scanning mechanism 46 vibrates the wafer
rapidly in the x direction. The stage, or linear translator, 32, may be
adapted to simultaneously move the wafer w slowly in the y direction
during the vibratory scanning movement in the x direction when it is
desired to provide a two dimensional planar scan at a particular level on
the wafer. By scanning at a plurality of levels, a three dimensional scan
is obtained, such three dimensional scanning of an entire area (or site)
on a wafer being explained in detail in the aforementioned copending U.S.
patent application Ser. No. 725,082. As will be explained in greater
detail hereinafter, the basic profiling technique of the present invention
requires that the scanner 46 move only in the x direction making a
repeated number of scans over the same line on the wafer while
incrementally changing the level of lens 26 through focus control
mechanism 28 after each individual scan.
In the control circuitry 44 it will be seen that the scan drive voltage is
provided digitally out of the line scan wave form memory circuitry 48 and
that a D/A converter 49 converts the digital signals to an analog signal
for appropriate amplification by the amplifier 47. The memory 48 is
addressed by scan control and synchronization circuitry 50. The incoming
analog signal from the photodetector 42 is converted to a digital signal
by A/D converter 5l. Since the scanning mechanism 46 carrying the wafer,
w, will move at a varying linear velocity as the wafer, w, is scanned, the
timing of the digital photodetector signal sampling is such that the
recorded digital signal information will correspond to generally uniformly
spaced positions along the scan line on the wafer so that a distortion
free image of the wafer can be created in the ouput devices 24a and 24b.
In order to accomplish this objective, a line scan distortion memory 52 is
provided to control the timing between the samples. The timing information
from memory 52 is utilized by pixel timing and synchronizing circuitry 53
which controls a line scan pixel memory 54 that accepts and stores the
digital input signals at the appropriate times. Each sampled signal (from
the photodetector) corresponds to a pixel which is a representation of a
very small incremental area on the wafer with the sampled signal at the
time being a measurement of the reflected light from such incremental
area. For a further and more complete description of the control circuitry
44 reference is again made to our aforementioned copending U.S. patent
application Ser. No. 725,082.
The mechanical structure which comprises the semiconductor wafer scanning
system is shown in FIGS. 2 through 5. Referring first to FIG. 3, it will
be seen that the entire wafer drive apparatus and optical system is
arranged to be mounted upon a large surface plate 60 which is seated upon
a table 61 and isolated therefrom by four piston and cylinder type air
springs 62 located so as to support each corner of the surface plate. A
general frame structure 64 is elevated above the surface plate 60 to
provide support for the optics module 20 including the vertically
shiftable focus control mechanism 28.
The details of the focus control mechanism are best shown in FIGS. 2, 4,
and 5. The movable objective lens 26 will be seen to be mounted within a
cage 72 open at the top and the front and with a back face (FIG. 5)
adapted to slide within track 73 on the upright face of the frame
structure 64. A support bracket 70 is attached to one side of cage 72
projecting outwardly therefrom to support a DC servo motor 66 with a
projecting lead screw 67 thereof being adapted to engage the upper face of
a support bracket 68 secured to a main upright portion of frame 64. It
will be seen (from FIG. 2) that movement of the screw 67 within the motor
assembly 66 serves to raise or lower the objective lens 26 relative to the
underlying wafer support assembly. This lens movement is provided only for
gross alignment of the optical system relative to the wafer surface, i.e.,
to move the optical system so that the surface of wafer w lies in the
basic focal range of the optics. As will be explained presently, this
gross movement will initially place the focal plane of the optical system
close to but above the top surface of the wafer so that the lens 26 can
thereafter be successively moved closer to the wafer as the beam from
laser 40 is scanned across the wafer. Use of the motor 66 to elevate lens
26 well above the underlying wafer support structure also permits the
wafer w to be readily loaded and unloaded.
The fine focusing (i.e., fine vertical adjustment) of the objective lens 26
is accomplished by means of a piezoelectric crystal 76 of generally
cylindrical shape (FIGS. 4 and 5) which is attached between the base of
the cage 72 and an overhead annular support member 74 which has a central
hub 75 to which the upper end of the mount for lens 26 is threaded (FIG.
4). By varying the voltage to the electrical lead 77 (FIG. 5) the crystal
76 may be axially contracted or expanded in the direction of the arrows
(FIG. 4) so as to, in turn, lower or elevate the objective lens 26
relative to the underlying wafer. It will be appreciated that the movement
of lens 26 during the application of different electrical potentials to
crystal 76 will be in the submicron range (e.g , 0.01 microns per
increment) so that relatively small differences in surface levels on the
face of the wafer are capable of being distinguished.
The planar (i.e., x-y) drive arrangement is best shown in the exploded view
of FIG. 3. It will therein be seen that each of the x and y drive devices
or stages 34, 32 is comprised of a conventional precision translation
table which, in the presently described embodiment of the invention, is
designed to have about six to eight inches of linear travel. These tables
each include a drive motor 82 which serves to drive a slide block 80
within a channel shaped frame 83 by means of a lead screw (not shown)
which is threaded to a nut attached to the slide block 80. Although not
shown, it will be appreciated that each translation table includes an
optical position encoder therein with submicron resolution and accuracy
which serves to feed continuous position signals back to the computer 22
so that the precise position of the wafer in the x-y plane at any given
time can be controlled and correlated with the reflected intensity
measurements from the optical system during the operation of the
apparatus. A flat lower tilt plate 84 is attached to the upper face of the
slide block 80 of the upper, or y, stage translation table 32, and a
middle tilt plate 86 is secured thereto by means of a leaf spring 88 which
is rigidly bolted to the adjacent spaced ends of both of the tilt plates.
A tilt adjusting screw 87 is threaded through the end of tilt plate 86
opposite to the mounting of spring 88 so as to bear against the upper
surface of the lower tilt plate 84 so that the middle tilt plate (and the
structure supported thereabove) can be tilted about the x-axis by
adjustment of the screw 87. In a similar manner, an upper tilt plate 90 is
secured in spaced relationship to the middle tilt plate 86 by means of a
leaf spring 92 bolted to their rearward edges, and a tilt adjusting screw
91 is threaded through the forward edge of tilt plate 90 to bear against
the upper surface of | | |
