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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 measurements
of the surface profile of such specimens.
2. Description of the Prior Art
In the scanning 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, both optical and acoustical,
having high degrees of image resolution. 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 photodetector device 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 scanning 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 for
scanning specimens with a high degree of resolution are generally
required. Laser beams can be focused through such optical imaging systems
with a very narrow depth of field. Then, by scanning the laser beam along
the top surface of the semiconductor wafer, the conductive traces or
conductor pattern lines on the wafer can be measured by utilizing special
detector devices to denote the edges of such lines.
It has been generally recognized that with wafer imaging and 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 imaging 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 imaging systems include U.S. Pat. No. 4,505,585 to
Yoshikawa et al and U.S. Defensive Publication T102,104 to Kirk et al.
SUMMARY OF THE INVENTION
With the present invention, methods and apparatus are provided for
systematically measuring the cross-sectional profile, and particularly the
linewidths, within a given area on the semiconductor wafer surface with a
generally greater degree of accuracy than that provided by the systems of
the prior art.
With the method and apparatus of the present invention, an optical imaging
system is provided to project a sharply defined beam onto a small spot
upon the wafer surface and to detect the reflected spot with respect to a
measurable characteristic of the reflected beam indicative of a reflective
surface at 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 (e.g.. a
maximum reflected 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, or data, is then utilized for directly making a pattern
linewidth measurement.
Once a linewidth measurement is provided by making a measurement utilizing
the full profile data, such measurement can be compared with a
characteristic profile obtained using only that data provided by a single
scan at a single focal depth along the scan line. A depth offset distance
can then be computed where the linewidth on the single scan characteristic
profile equals the measured linewidth from the full profile data. This
depth offset distance lwhich, for example, might correspond to the
relative distance between measured maximum and minimum data levels on the
single scan characteristic profile) is then used as a measuring factor
during further scanning of an area of the wafer along scan lines generally
parallel to the initial scan line. Such area of the wafer can therefore be
scanned at a single focal depth to obtain reliable linewidth measurements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of the semiconductor wafer scanning
and linewidth measuring 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, 78, and 7C collectively comprise a flow chart depicting the
subroutine of the program of FIG. 6 for 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.
FIG. 10 is a flow chart depicting the subroutine for the programming step
of FIG. 6 of determining the threshold level (T.sub.L) for a linewidth
measurement.
FIG. 11 is a diagrammatic illustration of a semiconductor wafer surface
illustrating the types of linewidths which might be measured with the
system of the present invention.
FIG. 12 is a flow chart depicting the subroutine for the programming step
of FIG. 10 of determining the profile linewidth measurement (W.sub.p).
FIG. 13 is an illustration of a portion of the semiconductor wafer surface
profile and the linewidth measurement thereon as determined by the
subroutine of FIG. 12.
FIG. 14 is a flow chart depicting the subroutine for the programming step
of FIG. 10 of calculating a linewidth measurement (w).
FIG. 15 is an illustration of a portion of a single scan characteristic
profile at a single focal depth indicating the measurements calculated by
the subroutine of FIG. 14.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The linewidth measuring 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 applications Ser. No. 725,082, filed Apr. 19, 1985, and
entitled "Semiconductor Wafer Scanning System" and Ser. No. 752,160, filed
July 3, 1985, and entitled "Method and Apparatus for Determining Surface
Profiles". The disclosure of these prior applications are herein
incorporated by reference into the present application, and reference to
such applications may be had for a more detailed explanation of the
apparatus 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 an 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 44 and 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 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 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 51. 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.
ln 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 FIG. 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 a 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, thereof, and a tilt
adjusting screw 91 is threaded through the forward edge of tilt plate 90
to bear against the upper surface of tilt plate 86 so as to adjustably
rotate the tilt plate 90 about the y axis. It will be understood that in
setting up the apparatus initially and checking it thereafter, it is
essential that the tilt screws 87 and 91 are properly adjusted to insure
that the surface of upper tilt block 90 lies in a perfectly horizontal
plane precisely perpendicular to the path of the light beam from the
overhead optical system 20.
The vibratory scanner mechanism 46, by which the wafer w is rapidly
vibrated in the direction of the x-axis, is shown in detail in FIG. 3. It
will be seen that the scanner mechanism comprises a rectangular structure
including a pair of leaf springs 120a and 120b, for supporting, for
vibratory movement a drive bar 78, and a pair of tension adjusting leaf
springs 121a and 121b. The springs are arranged in a rectangular structure
by attachment to four corner blocks 122 with the ends of each of the
springs being tightly bolted to the corner blocks. The solid drive bar 78
is firmly attached to and extends between the midpoints of each of the
vibratory leaf spring 120a and 120b. Positioned atop the drive bar 78 (see
FIG. 2) is a vacuum chuck 89 which is supplied with a vacuum to hold the
wafer w securely upon its flat upper surface. The rearwardly projecting
end 78a of the drive bar 78 mounts a coil 79 to which a drive current is
applied from the control circuitry 44 through amplifier 47 (FIG. 1). A
plurality of fixed magnets 101 are mounted upon spaced upright mounting
blocks 100 between which the coil 79 is positioned so as to complete the
electromechancial drive arrangement for the scanner. The mounting blocks
100 are positioned upon and secured to an extension 90a of the upper tilt
plate 90, as shown in FIG. 3, and also serve to mount the terminals 1O1a
through which the coil 79 is connected to the drive circuitry. In order to
rigidly secure the scanner 46 to the upper tilt plate 90, U-shaped
mounting blocks 124 are bolted to the midpoint of each of the tensioning
springs 121a, 121b through attachment plates 128. Each of the attachment
plates has a threaded hole in the center thereof for receiving a set screw
127. Each screw extends freely through a passage 127a in the associated
U-shaped mounting block 124, as shown in FIG. 7 Abutment blocks 125 are
fixedly secured to the upper face of upper tilt plate 90 and provide
surfaces against which the set screws 127 abut. Each mounting block 124 is
also secured upon the upper face of upper tilt plate 90 by means of bolts
126 which are received in slots extending through the blocks so that
loosening of the bolts permits the blocks to be shifted laterally with
respect to the scanner. It will be appreciated that the mounting blocks
124 are thus free to slide upon the lateral faces of the abutment blocks
125 before the bolts 126 are fully tightened thereby permitting the
tension springs 121a, 122b to be bowed outwardly from their innermost
positions. This is done in order to apply the proper amount of tension in
the leaf springs 120a and 120b so as to adjust the mechanical resonant
frequency of the system to that desired. This mechanical resonant
frequency should be set just slightly higher than the operating or drive
frequency of the system so that the system will be energy efficient but so
that the oscillatory drive will never pass through the resonance point
wherein loss of control and damage to the structure could occur. It will
be seen that by rotating the set screws 127 to move the plates 128
outwardly of the abutment blocks 125, the tensioning springs 121a, 121b
bow outwardly to place an axial tensioning force on the springs 120a,
120b. Since each tensioning spring 121a, 121b can be adjusted separately
through its associated set screw 127, it will be recognized that the
separate adJustment of each side of the spring support system can be used
to compensate for any asymmetry in the spring system construction to
insure that a perfectly symmetrical drive arrangement is achieved.
It will be apparent that application of an alternating current to the coil
79 will shift the drive bar 78, and wafer w supported thereby, backwardly
and forwardly in the direction of the x axis, i.e., in the opposed
directions of arrow 110 (FIG. 3), at the frequency of the alternating
current applied thereby, bowing the support springs 120a, 120b
accordingly. This lateral vibratory movement, which comprises the scan
linewidth of the system along the x axis on the wafer, is set for typical
total excursion of about 2 millimeters.
The programming by which the computer system 22 controls the operation of
the aforedescribed mechanical and optical apparatus of the present
invention is shown in flow chart form in FIG. 6. Once the wafer w is
appropriately positioned upon the vacuum chuck 89, the basic x-y planar
drive mechanisms 34, 32 can be used to bring the wafer to a location
beneath the optical imaging system 20 wherein the beam from laser 40 will
overlie a particular site on the wafer. In a typical semiconductor wafer
inspection operation, it is conventional to look at only a plurality of
selected small areas or sites (e.g., four on the wafer rather than
scanning the entire wafer because of time limitations. Once the wafer has
been moved so as to locate under the beam from imaging system 20, a
starting x, y location within the first chosen wafer inspection site by
means of the x-y stage motor control circuitry 36 under command of signals
from the computer 22, the focus control mechanism 28 is operated to bring
the focus level to a focal plane z.sub.1 which is chosen so that it will
always be above the uppermost surface level of the wafer even if the wafer
may vary somewhat in thickness or not lie in a perfectly horizontal plane
(see the top figure in FIG. 8). The subroutine V(z) for obtaining and
displaying a z (vertical) profile along a line (in the x direction) on the
wafer is then carried out. This subroutine is shown specifically in FIGS.
7A, 7B, and 7C.
Referring first to FIG. 7A, the data collection phase, it will be seen that
the focus control mechanism 28 is initially operated (through control
circuitry 38) to bring the focal level of the optical system to its
uppermost scanning level z.sub.1 as explained previously. The vibratory
scanning mechanism 46 is now operated to scan the beam from laser 40 along
a line on the wafer while the control circuitry 44 (FIG. 1) samples the
reflectivity data from photodetector 42 along the line at n samples (e.g.,
512 samples) in a single (i.e., one-directional) scanning movement. These
samples represent generally uniformly spaced positions (x.sub.1
-x.sub.512) along the line from one lateral edge of the area or site to
the other. As the spring system drive of scanner 46 brings the wafer back
in the return direction along the scan line, no data is taken and the
focus control mechanism 28 is operated to lower the focal level by an
incremental distance (typically, a few hundredths of a micron). This
procedure is repeated as the focal level (the z level) is successively
lowered through m levels (e.g., 256 levels), as indicated in FIG. 7A, it
being understood that at each level, 512 samples along the x direction
will be obtained and all of this information will be stored within a
memory in the computer system 22.
At the conclusion of the data gathering phase, the data will be processed
in accordance with the programming shown in FIG. 7B. The data is saved
within the computer in an array x.sub.i by z.sub.j wherein i (the spaced
data taking positions along the x axis) will typically be about 512 while
j (the incremental focal levels of the optical system) will typically be
about 256. Thus, the data storage for the profiling operation must
accommodate 512 by 256 or approximately 131K bytes of information. As
shown in FIG. 7B, the system starts at z.sub.i and x.sub.1 and looks for
the maximum peak value (P.sub.m) and the reflectivity signal (R) at such
peak value and also looks for the first peak value P.sub.1. Thus, at data
position x.sub.1 along the x axis, the program steps through each z level
(1 through 256) successively testing the reflectivity values to first
locate a first peak value (i.e., where the reflected intensity first rises
to a peak value and then drops off) and then to locate a maximum peak
value (i.e.. the highest reflected intensity value). The maximum peak
value will occur at that z level where the basic reflecting surface on the
wafer lies precisely at the focal plane below the optical system, and the
first peak (if there is a peak prior to the maximum value) will occur at
that z level where a transparent or semi-transparent layer overlying the
underlying basic reflecting surface lies precisely at the focal plane.
The foregoing process can be appreciated by the graphic illustrations of
FIG. 8 which show a partial cross-sectional configuration for a typical
wafer (in the uppermost figure) and the corresponding output displays for
such profile provided in the graphics VDU 24b (FIG. 1). Thus, it will be
seen that with an underlying base layer of silicon at a base layer A, a
pair of spaced metallic lines of aluminum are provided at a level B and a
higher insulating line of silicon dioxide is provided at a level D.
Overlying the conductive material is some photoresist material of
semi-transparent nature left after the etching process. The photoresist on
the aluminum lines lies in a mound centered about level C while the
photoresist on the silicon dioxide is at a uniform level E as shown. Thus,
assuming that the data taking position x.sub.10 is being processed and
that this position lies within the silicon dioxide layer, as shown in FIG.
8, it will be appreciated that as the z levels are successively sequenced
by the programming, that z level representing a focal plane at level E
will provide a first peak reflectivity (R) value. As the z level
approaches E, there will be a rise in the level of the corresponding
signal R until it peaks at level E and then begins to drop off as the
focal plane drops below level E. As the focal plane (z level) approaches
level D however, another peak in the reflectivity signal is generated and
this peak will be higher than the peak at level E since the silicon
dioxide conductor at D is a non-transparent layer of photoresist at level
E. As the focal plane or z level drops below level D, several other
spurious peaks in the reflectivity signal may be generated of considerably
smaller value than the peak at level D for optical reasons unimportant to
an understanding of the present invention. Such peaks can be ignored. As
shown in FIG. 7B, the computer will store the z level value (z.sub.j) for
the x.sub.10 position and also the reflectivity value R at the maximum
peak P.sub.m. This process is repeated for each position along the x-axis,
i.e., x.sub.1 to x.sub.512, with all of the foregoing information being
recorded at each data taking position. For example, with reference to FIG.
8, it will be noted that at the x.sub.90 position there will be no first
peak separate from the maximum peak since only the silicon substrate level
(at A) will reflect any light. Hence. at x.sub.90 the system will not
store a separate P.sub.1 value.
The lower three graphs of FIG. 8 show the data display which is provided on
the graphics video display unit 24b in three separate arrays. The upper
graph represents z (depth at the wafer surface as referenced to the
optical system) vs. x (linear location across the scan area). Referring
now to the data display programming of FIG. 7C, it will be seen that the
stored data (FIG. 7B) is utilized so that for each x position, each z
level at which a maximum reflectivity signal was obtained is plotted and
connected by a solid line. It will be recognized that this solid line
comprises the z axis profile or cross-sectional profile of the wafer
surface as shown in the top illustration of FIG. 8. The reflectivity at
each maximum signal position is also plotted on a separate graph (R vs. x)
as shown in FIG. 8 and connected by a solid line. Referring to this graph,
it will be seen that the reflectivity for the highly reflective aluminum
lines is considerably greater than that of the silicon dioxide line--as
would be expected. Finally, referring again to the top graph (z vs. x),
the first peak (where one was found distinct from the maximum peak) is
plotted in dotted lines. As shown in FIG. 8. the dotted line plots are
only found overlying the conductive lines therein since these are the only
postions where multiple reflective layers of material are found.
It will be seen from the R vs. x graph of FIG. 8 that the underlying
silicon substrate level has a relatively low reflectivity; the silicon
dioxide layer has a higher reflectivity level; and the metallic aluminum
layers exhibit the highest reflectivity levels. The wavy surface of the
aluminum lines reflects the granular metallic nature of the relatively
flat metallic surface which inherently has variable reflectivity levels
therein.
Finally, a histogram is calculated and plotted and indicated in FIG. 7C and
as shown in the lowermost graphical display in FIG. 8. The histogram
utilizes the same data used in the uppermost graph (z vs. x), but it is
plotted in a different manner to indicate on a statistical basis the z
location of the maximum reflectivity signals. Thus, the number of pixels.
i.e., x positions x.sub.1 -x.sub.512, found at each z level where a
maximum reflect | | |
