Method and aparatus for determining surface profiles

A system for determining surface profiles of specimens such as semiconductor wafers includes a drive for mounting the wafer for oscillatory movement along a line and an optical imaging system overlying the wafer for focusing a beam on a small sport on the wafer and including a photodetector for detecting the reflected sport from the wafer. The spot is scanned along the line on the wafer while the focal depth of the imaging system is progressively changed while the photodetector and connected digital circuitry generate a plurality of spaced output signals for each scan along the line so that data comprised of a series of spaced signals are provided at a plurality of focus levels extending through the surface profile of the wafer. Computer means are provided for analyzing the data and providing a graphical output of the surface profile.

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.

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 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 plate 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 springs 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 101a 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 in 
the associated U-shaped mounting block 124. 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 a 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., 4) on the wafer rather than scanning 
the entire wafer because of the time limitations. Once the wafer has been 
moved so as to locate a starting x, y location within the first chosen 
wafer inspection site under the beam from the optical imaging system 20, 
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.1 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 and insulative material are 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 at D is a non-transparent layer of rather dull material 
but reflecting a much greater percentage of light then did the 
semi-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 and insulative superimposed 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 as 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. Thus, the number of pixels, i.e., x 
positions x.sub.1 -x.sub.512 found at each z level where a maximum 
reflectivity signal was observed are plotted. Comparing the upper and 
lower graphs of FIG. 8 it will be seen that the lowermost level A has the 
highest number of pixels while the level of the metallic lines B has the 
next highest and that all of these pixels are closely centered about the A 
and B elevations. It will be noted that a bell-shaped curve is formed 
about the C level: such curve representing the photoresist material 
capping the aluminum layers with the peak representing the average level 
of such material. Finally, the D and E levels are the smallest and are 
appropriately spaced at the relatively highest focal depth levels. The 
significance of preparing and displaying such a histogram is that either 
operator selection or conventional computer graphical analysis techniques 
can be utilized to locate each of the indicated peaks (A-E) which 
represent the specific levels of interest for scanning on the 
semiconductor wafer surface. Thus, as indicated in FIG. 7C, the automatic 
focusing function can be activated, wherein the z level at each clearly 
definable peak in the histogram will be selected for scanning, or the user 
can select only such levels as desired for further scanning. The system 
will thereafter be operated so that the focus control mechanism 28 will 
move the optical system to focus only on such selected levels rather than 
scanning the entire site area at each individual z level. In this way the 
z vs. x profile is utilized in an effective manner so that an entire site, 
i.e., the entire two-dimensional array of x and y positions may be scanned 
to obtain the three-dimensional representation but only at a few selected 
depth levels without losing any relevant information and while keeping the 
amount of stored data during one scan at a manageable level. 
As indicated in FIG. 7C, when the automatic focus selection function is 
activated, the automatically selected focus levels can be set and 
sequentially fed to the focus control circuitry 38 (FIG. 1) prior to each 
complete two dimensional scan at a given z level. When all of the selected 
z levels at a particular site have been scanned, the system is ready to 
rapidly shift the position of the wafer relative to the optical system (by 
translation stages 32, 34) so that a new site or area underlies the 
vibratory scanning area of the optical system, and the process can be 
repeated. 
Returning now to the flow chart of FIG. 6, which provides for the location 
of the z levels to be scanned where the user selects such levels, it will 
be seen that a nominal focus level (z.sub.nom) is selected as the 
uppermost detectable layer on the wafer; for example, in the wafer 
cross-section of FIG. 8, top level E would represent the nominal focus 
level as indicated. Then each level of interest for scanning at or below 
level E would be defined by a focus offset or .DELTA.z.sub.j value with 
.DELTA.z.sub.j being defined as z.sub.nom minus the z level of scanning 
interest (e.g., level A, B, C, etc.). In the example provided level E 
would be set at j=1 and the .DELTA.z.sub.1 value would be equal to 0 since 
level E is at the nominal focus level. Level D would be set at j=2 and the 
.DELTA.z value would equal the incremental z distance between D and E. In 
a similar manner each of the other j values (j3-j5) would be set for 
levels C, B and A with the offset (.DELTA.) values being set at the 
corresponding z level differential with level E. These focus offsets 
.DELTA.z.sub.j are thus determined and stored. Then, with j being 
initially set equal to one, the focus control mechanism 28 is operated to 
move the focal plane to the first focal level lor not moved at all if, as 
in the present case, the upper level of interest is at the nominal focus 
level (where the scanning operation will start), and the wafer area is 
scanned in the x, y plane in the manner previously described and as set 
forth in the aforementioned copending patent application Ser. No. 725,082. 
That is to say, the scanning mechanism 46 is operated in conjunction with 
a slow movement of y stage 32 so that reflectivity data is obtained for a 
matrix of closely spaced x positions and y positions throughout the 
scanned level. The reflectivity measurements R which are made at each of 
the x, y positions at the single z level can then be utilized for making 
linewidth measurements by noting the sharp changes in reflectivity levels 
and determining the distance therebetween in terms of the incremental x 
positions. Then, assuming that more z levels are to be scanned at the 
wafer site, the j value is increased by one, the control mechanism is 
operated to lower the focal plane, and the process is repeated until 
scanning has been accomplished at each of the selected focus offsets. 
After the final, and lowermost, z level is scanned, the program moves to a 
new site on the wafer. This is accomplished by first asking the question 
whether more sites are to be scanned on the wafer and, if so, utilizing 
gross movements of x, y stages 34, 32 to shift the wafer to the new 
scanning position. Upon reaching the new site, the system first moves 
through an autofocus step. In this step, the focus control mechanism 28 is 
operated to first bring the focal level of the optics to the z.sub.1 level 
known to be above the uppermost layer of the wafer. Then, the z level is 
successively lowered until a peak in the reflectivity signal R is 
recognized by the computer circuitry from the data supplied by line scan 
pixel memory 54 (FIG. 1). The successive lowering is accompanied by the 
sequential scanning of a line in the x direction and comparing the data 
with the previously received data ignoring any signals below a given 
threshold value (to eliminate the effects of noise). Once a peak is 
recognized (as distinguished from a random spike or other spurious 
signal), the optical system will be at the nominal focus level, i.e., 
z.sub.nom, with respect to the wafer surface (see FIG. 8). At this level, 
the aforedescribed scanning program will be repeated wherein the optical 
system will be vertically moved only to the selected levels of interest 
with a complete scan in the x, y plane at each such level being obtained. 
It will be recognized that autofocusing is needed as the wafter is shifted 
to permit a separate site thereon to be scanned since the mounting of the 
wafer on the scanner 46 might not set the wafer in a true horizontal 
plane, and, in addition, the etched top surface of the wafer might not be 
truly flat: hence, refocusing to find a known reference level at each new 
site is generally necessary. However, it will be noted that it is not 
necessary to collect new profiling V(z) data at each new site since the 
previously generated focus offsets (.DELTA.z.sub.j) will generally remain 
constant. 
While linewidth measurements can be made, as in the aforedescribed process, 
simply by noting the x spacing difference between distanct reflectivity 
signal changes on any given scan level, a better and more accurate way of 
measuring linewidths can be accomplished by utilizing the z vs. x profile 
display of FIG. 8. Since the data used to form this z vs. x graph is the 
maximum reflectivity signal at each individual x position along the 
scanning line, errors due to focusing problems, particularly at the edges 
of a line where the contour of the surface is changing, will be minimized. 
Conventional computer linewidth measurement techniques are utilized to 
actually make the measurements with the computer system 22 of the present 
invention. For example, with respect to the profile shown in FIG. 8, the 
first (silicon dioxide) line can be automatically recognized, the top and 
bottom levels thereof (D and A) defined, and a predetermined percentage 
(e.g., 50%) therebetween utilized as a defined measuring point. Then, it 
is a simple matter to determine the distance between the thus defined 
measuring points at the leading and trailing edges of the line in terms of 
the number of x positions therebetween. Standard interpolation methods can 
be utilized to improve the accuracy of the measurements by defining the 
measuring points in terms of fractions of an x position spacing (i.e., 
pixel width). As explained previously, the x positions are generally 
uniformly spaced across the wafer at known (e.g., submicron) spacings. 
The programming process to find the "superfocus" image is illustrated in 
the flow chart of FIG. 9. The "superfocus" image is the image of an entire 
scanned site or area on the wafer surface utilizing the information 
obtained by focusing the optical system at different levels. The image is 
displayed in the image display monitor 24a (FIG. 1) after the correct 
signals for the two dimensional video matrix are determined by the 
computer system 22 and stored in the video display memory 29. Once the 
wafer is moved so that the optics focus on the applicable site on the 
wafer, the V(z) data (for making the initial profile) is collected along a 
scan line at the center of the scanning field. This means that the x 
scanning line utilized is one halfway down the x-y maxtrix which is to be 
ultimately displayed in monitor 24a. The x-z profile is then obtained in 
the manner previously described, and a V(z) histogram (as in FIG. 8) is 
generated and displayed. The computer next asks the question whether or 
not the data provided by scanning along a single line is sufficient. For 
example, if the pattern within the entire x-y frame includes features 
which cannot be captured in a single x-line scan, then the y stage 32 must 
be driven to move the scanning mechanism 46 to a new location to provide a 
second x-line scan. This generates a new z profile which can be utilized 
for making linewidth measurements, and the z data obtained therefrom is 
simply added to the previously stored data in the histogram generating 
subroutine so that the data therein is accumulative. Thus, if there is a 
level found in the second x-line scan which was not present in the first 
x-line scan, a wholly new peak will be formed in the histogram. When it is 
determined that all of the necessary information has been obtained by a 
sufficient number of x-line scans, the program moves on to the process of 
obtaining the superfocus image. 
First, the main peaks (or z levels of interest) in the histogram are 
identified in the manner previously pointed out. This can be done manually 
by the operator or accomplished by conventional computer analysis 
techniques. The focus control meachanism 28 then moves the optical imaging 
system to focus on the first peak, i.e., the first level of interest, and 
the entire x-y plane is scanned and the data recorded. This data is stored 
in a buffer, and the focus control mechanism moves the imaging system to 
focus at the second peak, i.e., the second level of interest wherein the 
process is repeated with the new data being added to the previously stored 
data in the buffer. This process continues until each level of interest is 
scanned with all of the data for each x, y position being accumulated in a 
plurality of x, y matrices. The data can then be displayed as a single 
plane of data on the image display monitor 24a with the data for any given 
(x, y) point on the screen being derived from the accumulated information 
from all of the scanned levels. 
As one method of display of the superfocus image, it is possible to color 
code each individual level and display the maximum intensities at any 
given x, y position in accordance with the color coded level. This will 
produce a multi-color image with the different colors representing the 
different surface levels on the wafer. A second method of display involves 
simply adding all of the R signal values obtained at the different scan 
levels for each x, y position. An alternative to the foregoing method of 
display to improve the sharpness and quality of the image is to add the 
different R signal values for each x, y position but set a threshold level 
for each scan plane so that the grossly out-of-focus data would be set to 
zero. This threshold level could vary from plane to plane so that the 
sharpest possible image would be provided. Another alternative method of 
display would be to use only the maximum reflectivity signal at each x, y 
position so that the generated display image represents the x, z profile 
of FIG. 8 taken at each y level of the composite x, y matrix. The actual z 
vs. x profiles for each such y level could also be generated and stored. 
Such data could be displayed either graphically in a isometric (x, y, z) 
plot or used to modulate the intensity at each x, y position in a two 
dimensional display. 
From the foregoing it will be seen that the initial generation of a z-axis 
profile or cross-sectional image of the wafer surface permits all of the 
relevant wafer scanning data to be subsequently obtained in a rapid and 
highly efficient manner. First, direct linewidth measurements can be made 
directly from the z-axis profile more easily and more accurately than by 
utilizing conventional linewidth scanning techniques. Secondly, the 
initial generation of a z-axis profile by the computer permits a ready 
analysis (either by the operator or automatically by the apparatus) after 
scanning a single line (or a few lines) to develop the program for 
scanning a complete site or area on the wafer surface so that the computer 
time and storage capacity is used most effectively. This is accomplished 
by first identifying and then scanning only those particular levels of 
interest which provide all of the information necessary for providing a 
complete topographical image of the surface of the semiconductor wafer. 
Although the best modes contemplated for carrying out the present invention 
have been herein shown and described, it will be apparent that 
modification and variation may be made without departing from what is 
regarded to be the subject matter of the invention.