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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an X-Y stage for adjusting the position of a
patterned wafer for macro and micro optical inspection and, more
particularly, to a mechanism for accurately shifting the wafer from the
macro optical axis to the micro optical axis at which the respective
inspections are to occur.
2. Description of the Prior Art
It is well known to use a crossed roller type stage to move a patterned
wafer in two orthogonal axes during manual operator micro inspection of a
wafer. Where two optical systems are to be incorporated in one wafer
inspector, it is necessary to move the wafer from one station to the
other. In addition, a similar increment of travel is still required at
each station. While such movement could be accomodated by the crossed
roller technique alone, stage size would be increased by the need to move
the distance between two optical systems, in addition to the movement
required at each station separately. This invention discloses a technique
for using a small stage, capable of only moving the distance required at
one station, on which is mounted an arm and a chuck which can be moved a
precise fixed distance. The distance in this case being equal to the
distance between optical systems. This technique is less expensive,
smaller in size and faster in operation than a single stage capable of the
entire range of movement.
SUMMARY OF THE PRESENT INVENTION
It is an object of this invention to use a crossed roller type X-Y stage
for precise positioning of a wafer under two inspection stations known as
the macro and micro inspection stations.
It is another object of this invention to use a transport arm which pivots
about a vertical axis to move the wafer from the macro inspection station
to the micro inspection station on said X-Y stage.
It is yet another object of the invention to support said transport arm
above said X-Y stage by air bearings during transport.
It is a further object of this invention to use a spring-loaded mechanism
to drive said transport arm against hard stops in the macro and micro
station locations so as to correctly position the arm each time it is
moved from one station to the other.
Briefly, the invention comprises an X-Y stage with macro and micro
inspection stations adjacent opposed sides of said stage, transport means
for shifting a patterned wafer from one inspection station to the other
for inspection of said wafer, said transport means including means for
holding said wafer in position and means for rotating said wafer.
IN THE DRAWING
FIG. 1 is a block diagram illustrating the main elements of a wafer
inspection system in accordance with a preferred embodiment of this
invention;
FIG. 2 is a block diagram which generally illustrates the functions
performed by the wafer inspector system of this invention;
FIG. 3 is a block diagram which symbolically illustrates the automatic
handling of a wafer as it passes through micro and macro inspections in
accordance with a preferred embodiment of this invention;
FIG. 4 is a partially broken, front perspective view of wafer inspector
(10) in accordance with this invention illustrating the rigid stress frame
(104) and portions of the heavy aluminum castings (108) and (110);
FIG. 4A is a broken elevation view illustrating the air lock at the
junction between fixed and floating environmental covers.
FIG. 5 is a partially broken left front perspective view of wafer inspector
(10) which illustrates the macro inspection station (18), input wafer
cassette loaders (12) and (14), input wafer track (64), X-Y stage (28),
turntable (94), and macro optics;
FIG. 6 is a partially broken view of wafer inspector (10) illustrating the
input load pad (68) and input wafer arm (90) that are used in moving the
wafer from the wafer track (64) to the wafer vacuum chuck (92);
FIG. 7 is a partially broken view of the upper part of wafer inspector (10)
which illustrates in more detail the macro optics and the
interchangeability of the moveable macro mirror (114) and the pentaprism
(122);
FIG. 8 is a partially broken top view of wafer loading assembly (67), load
pad (68) and associated actuator assembly, and illustrates wafer tracks
for delivering a wafer to load pad (68) and the vacuum line (71) which
provides holding force for the wafer;
FIG. 8A is a top view of the load pad actuator assembly arm (70) and ball
slide assembly (124);
FIG. 8B is a view along line 8B--8B of FIG. 8A;
FIG. 8C is an elevation view of the garage (81) and associated components
used with alignment wafer (79);
FIG. 9 is an elevation view of the drive assembly for the wafer load
transfer arm (90);
FIG. 10 is a sectioned elevation view of the air piston (194) and drive
assembly for wafer transfer arm (90);
FIG. 11 is a partially broken top view of the distal end of wafer transfer
arm (90) illustrating the the recess (232) in wafer holding member (230)
and the vacuum holes (234) in the recess;
FIG. 11A is a partially broken top view of the arcuate part of wafer
transfer arm (98).
FIG. 12 is a top view of the X-Y stage (28) and shows the turntable (94),
vacuum chuck (92), and the flipper drive assembly (258);
FIG. 13 is an elevation view of the X-Y stage (28), turntable (94), vacuum
chuck (92) and the flipper drive assembly (258) shown in FIG. 12;
FIG. 14 is an isometric view of spring-loaded drive link (264);
FIG. 14A is a partially broken section view along the line 14A--14A of FIG.
14;
FIG. 15 is a section view of the vacuum chuck (92) and a broken view of
turntable (94) and illustrates how the vacuum chuck is mounted and how a
vacuum is applied to hold the wafer in place;
FIG. 16 is an elevation view illustrating the drive mechanism for the macro
lenses, moveable mirror and pentaprism;
FIG. 17 is an end view along the line 17--17 of FIG. 16 illustrating how
slider (372) is slideably attached to guide rail (368)
FIG. 18 is a schematic diagram which illustrates the basic optical elements
and their interrelationships;
FIG. 19 is an elevation view of some of the micro optical elements and
illustrates, in particular, the micro illuminator (408), imaging lens
housing (425) and microscope objective lens (642);
FIG. 19A illustrates darkfield control element (426);
FIG. 19B shows the partialy silvered mirror (428);
FIG. 20 illustrates how stepper motor (434) and pulley (438) drives
aperture and pupil stop (420) by belt (440);
FIG. 21 is an isometric view of a lens bracket used in the micro
illuminator (408);
FIG. 22 is a block diagram illustrating the physical arrangement of the
elements of the autofocus assembly;
FIG. 23 symbolically illustrates the location of the image of the autofocus
pupil stop within the back aperture of microscope objective;
FIG. 24 is a broken view which illustrates the masks and projected image
for autofocus control;
FIG. 25 shows graphs of the photodetector output currents which would
result from different occlusions of each of the masks shown in FIG. 24;
FIG. 26 is a graph of the control voltage obtained by subtracting the
output of photodetector (494) from the output of photodetector (488);
FIG. 27 is an isometric view of an adjustable optical module including a
reticle pattern;
FIG. 28 is an elevation view of a mount used to permit X, Y, Z and theta
adjustment of the lens optical module of FIG. 27;
FIG. 29 is a side view of the mount shown in FIG. 28;
FIG. 30 is an isometric view of a preferred embodiment of the turret
mounting assembly of the present invention;
FIG. 30A is a side view of the position control lever arm (560A) used in a
preferred embodiment of this invention;
FIG. 30B is a top view of that portion of lever arm (560A) which further
illustrates the crossed flexures employed;
FIG. 31 is a front view of the turret mounting assembly of FIG. 30;
FIG. 32 is a side view of the turret mounting assembly of FIG. 30 and
illustrates the well (668) and damper (670);
FIG. 32A shows a preferred embodiment of a damper assembly (670A) used in
the instant invention; and
FIG. 33 is a top view of the turret mounting assembly of FIG. 30;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
System Description
Referring now to FIGS. 1 and 2, it may be seen that the wafer inspection
system in accordance with this invention comprises three major
sub-systems; the wafer inspector 10, the control and data storage 46 and
the high speed image computer 56. Electrical interconnection paths are
shown and are designated as 40, 49, 51, 53, 55 and 57. A monitor 54 may
also be employed. The general functional inter-relationship among the
sub-systems comprising the wafer inspection system may be understood by
reference to FIG. 2; and the manner in which the wafer is transported from
the load pad 68 to vacuum wafer chuck 92 and thence to unload pad 76 is
generally illustrated in FIG. 3.
The system program is designed such that interaction with the system will
be through a touch screen on monitor 54, an associated joystick 52B and by
use of a concealable keyboard 52A. The monitor will display text and
graphics from system computer 50 via path 53 as well as wafer pattern
images from high-speed image computer 56. The use of joystick 52B is for
manual movement of the wafer and for graphics. The system computer program
is menu driven and, during routine wafer inspection, tests to be performed
are initiated by touching the appropriate "buttons" on the touch screen on
monitor 54. The keyboard 52A is also accessible but is for use by the test
designer. An entry code is required before parts of the program to be used
in an inspection, for a particular class of wafer, may be selected. The
procedure allows the designer to change the inspection and parameters of
the micro tests. Interactive "menus" will appear on the terminal
requesting the necessary input to either create an inspection, to inspect
wafer(s), or to obtain data output. The part of the program which the
operator routinely sees is designed such that the routine inspection of
production wafers can be done with just a few instructions given to the
system via the touch panel on the monitor.
Overall control of the system is provided by the system computer 50 which,
among other things, insures that the various step sequences and inspection
tests are performed in an orderly manner. The various sequences move a
wafer 16 from a cassette such as 12 to and through the macro and micro
inspection stations and finally outputs the wafer into a cassette such as
42. Each event in the sequence is accomplished in its sequence at the
programmed time. However, it should be understood that a number of
non-conflicting sequences are performed simultaneously to reduce
inspection time to a minimum. The wafers move to and away from the
inspection area simultaneously. During inspection, the system is
performing the following three major functions in parallel:
(1) moving the wafer into position and focusing on and grabbing the image;
(2) loading the test data from disk storage into RAM; and
(3) performing the computation required for the test.
Micro inspection includes a micro-measurement and a micro-comparison. The
test location is found by the use of a location image which was previously
chosen manually during training and is stored in the inspection as a
fundamental part of the micro-test. Micro-measurement is performed on
preselected geometries. Any field can be compared (micro-comparison) to a
reference image, or to the similar field in another die on the same wafer.
One source for the reference image is a previously inspected die on a good
wafer. The data base which was used to produce the image directly or
indirectly by optical or electron beam means can then be used as a
reference image. Disk memory 48 can also be programmed to provide a
standard reference image from the composite of several die for comparison
with the image obtained from the wafer during micro inspection, thus
removing random defects from the reference image by comparison.
There are four cassettes 12, 14, 42 and 44 that are shown. The cassettes
are standard types and are placed in indexers whch permit the transfer of
a wafer to the wafer track from the cassette and/or to the cassette from
the wafer track. Any of these four cassettes can be configured as an input
or output. For purposes of discussion, cassettes 12 and 14, hereinafter,
will be considered to be input cassettes and 42 and 44 will be considered
as output cassettes. During operation, however, a host input cassette
which is empty is used as an output cassette. This is done to accommodate
overflow as it is not expected that the number of wafers that pass and
fail will be equal in number. These cassettes can be configured to
accommodate industrial standard cassette sizes and stepping distances.
Wafer sizes of approximately 75 millimeters (3 inch) to 150 millimeters
(approximately 6 inch) can be accommodated.
Referring now to FIGS. 2 and 3, it may be seen that a wafer from one of the
input cassettes 12 or 14 is positioned in accordance with a control signal
along path 51 from movement controller 52. At the proper time, the control
signal will cause the wafer to be loaded onto wafer track 64. The wafer
track 64 moves the wafer 16 to the load pad 68 which is located between
the two input cassettes. As will be explained in detail subsequently, load
pad 68 is then caused to move vertically upward so as to position the
wafer above the track 64. Next wafer transfer arm 90 is moved across and
beneath load pad 68 so as to be positioned on the other side of the wafer
as shown. This places the recessed portion of the arcuate end of the arm
in position to accept the wafer. The load pad is next moved downward to an
intermediate position so that the wafer is slightly above the shelf which
is at the bottom of said recess in the transform arm 90. The wafer is now
in position to intersect the recessed side wall. The transfer arm is then
swung to a position adjacent the load pad. Because the wafer is not
precisely positioned on the load pad the side wall of said recess gently
and correctly moves the wafer so as to place the wafer in position above
the shelf. Next the load pad is lowered to its original position and is
now out of the path of the transfer arm. A vacuum is then drawn via the
vacuum holes in the shelf at the bottom of said recess to hold the wafer
thereto for transport. It is to be noted that no gripping of the wafer
edges is necessary. The wafer transfer arm 90 then moves the wafer 16
above the X-Y stage 28 and the wafer is placed on the top surface of a
vacuum chuck 92 that is attached to turntable 94. The turntable is in the
macro inspection station.
Prior to inspection the wafer edge and flat are found, the wafer having
been loaded onto the chuck in a random orientation and alignment. The
images picked up in macro view are first used to correct rotation and
positioning of the wafer pattern. The wafer surface is then tested for
pattern defects such as for bad spin, scratches, dust, etc., as well as
gross image defect variations due to areas being out of focus or not
receiving uniform exposure. Light for the macro inspection process is
supplied by ring lamps 20, 20A and 20B, and the fixed macro mirror 30
transmits the reflected image to the camera 38. Camera 38 converts the
image into electrical signals which are passed along path 40 to high speed
image computer 56 and the image appears on the screen of monitor 54.
Once the macro inspection is completed, movement controller 52 causes the
macro-micro transport arm (described below) to shift the assembly
containing the vacuum chuck to the micro inspection station on the X-Y
stage 28. The wafer 16 is held on the vacuum chuck 92 by pressure caused
by a vacuum pulled on the bottom surface of the wafer. As will be
explained in more detail later, this handoff between the macro and micro
stages is precisely performed so that the wafer is positioned for the
micro inspection test. Correction for mechanical positioning errors is
under the control of the system computer.
Tests are typically carried out at a number of sites, each of which is
positioned automatically by the machine. A stored series of images at each
site are used to correlate the image picked up from each wafer under test.
Thus, the mechanical stage is not required to position to high acuracy.
Correlation is used to select that portion of an approximately positioned
image which is to be tested. The machine may be programmed to go to a
particular series of test sites and the tests which may be different at
each site can also be specified. The system is designed to lead the user
through tests by a series of menus and "buttons" on a touch activated
screen. This makes the system self-teaching. The sites for tests are
selected by the user; subsequent wafers to be tested are simply loaded in
a cassette. The machine handles and aligns and tests the wafers and routes
each of them to pass or fail cassettes as a result of the tests which are
done entirely automatically. The sites selected for a test may or may not
be unique. If the machine is to find a particular site again, it must use
a series of unique references. These are automatically chosen by the
program and do not necessarily have to be within the field of view.
Because of the use of objective lenses and the requirement to focus at a
precise depth in the wafer pattern, each one of a plurality of objective
lenses must be brought into focus prior to the time that the pixel image
is recognized. As will be explained in more detail hereinafter, an
autofocus unit shown as 36 in FIG. 2 automatically adjusts the focus of
each of the objective lenses immediately after they are put into alignment
with the optical axis. During stage movement, shutter 37 is closed so that
no light reaches the tube face of camera 38 until the X-Y stage is steady
and the objective focused. Then, the shutter is opened so that the image
is presented to the input of the camera. The camera beam current is
controlled to allow integration of the light received during the entire
time the shutter is open. Once the shutter is closed, the signal is read
from the camera. The stage can move in parallel with this operation. Next,
the picture image is transmitted to the high speed image computer 56 where
it is compared against a standard reference, or against an image reference
which was obtained from the wafer itself and stored in temporary memory.
Once the micro inspection has been completed, a second wafer transfer arm
98 moves into a position adjacent vacuum chuck 92 and beneath wafer 16.
The vacuum is removed from chuck 92, arm 98 then lifts up to position the
wafer above chuck 92. A vacuum is pulled to hold the wafer on top of arm
98. Next, the arm 98 is caused to swing about a pivot point 99 so as to
position wafer 16 above unload pad 76. Once in position, controller 52
actuates the unload pad assembly causing it to move vertically upward and
lift the wafer 16 up above the arm 98. Then controller 52 causes the arm
98 to be swung so as to be free of the wafer downward path. Next, the
unload pad 76 moves vertically downward to position the wafer 16 for
transport on track 80. Because the computational analysis is carried on
during the inspection process, the determination as to whether the wafer
16 is a "good" wafer or a "bad" wafer has already been made. The wafer 16
may be classified as "pass" or "fail" and the wafer directed to cassette
42 or 44, depending upon its classification. Movement controller 52 then
activates that portion of track 80 that will move wafer 16 to the proper
cassette.
Wafer Inspector
Referring now to FIGS. 4-11, in conjunction with the following discussion,
the operational features of wafer inspector 10, in accordance with the
present invention, may be comprehended. A welded stress frame 104 provides
for rigid mounting of its base portion to a floor or other firm mounting
surface. The rigid frame provides support for two heavy aluminum castings
108 and 110 which are specially constructed to maximize their damping
constants. The base casting 108 sits on three air isolators 106 which rest
on the welded stress frame. The upper casting 110 is firmly attached to
the top surface of the base casting by means of bolts. (not shown). In
this way, critical elements of the inspection stations are isolated from
external movements.
Although not illustrated in detail, so as to not clutter the drawing, the
welded stress frame provides mounting facilities for the wafer handling
system. This isolates the moveable elements from the inspection. The
stress frame also provides for the mounting of environmental covers, see
FIG. 4A, which surround the area through which the wafer passes during
inspection so as to provide environmental control. Environmental covers
are also attached to the floating portion of the wafer inspector. In order
to maintain isolation between the stress frame and floating inspection
station, the environmental covers are not interconnected but lie in
different planes, which overlap, and are formed to provide a channel
adjacent the edge. One such arrangement is shown in FIG. 4A where a fixed
cover 107 is attached to fixed member 105 of stress frame 104, and a
floating cover 111 is attached to aluminum casting 110. This creates an
air lock 113 between the two overlapping covers because the interior of
the wafer inspector is maintained at a positive pressure. Air passing
through the air lock under pressure prevents outside contaminants from
entering the inspection area.
Wafer Transport
The wafers to be inspected are contained in input wafer cassette loaders 12
and 14 and these cassette loaders are positioned as shown adjacent the
input wafer track 64. If a wafer is to be loaded onto the wafer chuck 92,
the enabled loader outputs a wafer 16 onto an "O" ring belt track 64 where
it moves to a position midway between the cassette loaders 12 and 14.
While an "O" ring belt track has been shown, as a preferred way to move
the wafer, it could be understood that other techniques may be used to
provide the desired transport. For example, a walking beam may be used, or
direct moving arms may be adapted to accept the wafer and transport it to
the load pad. It is then raised up off the belt by a load pad 68, and the
wafer transfer arm 90 swings underneath the wafer 16.
The manner in which wafer transport is accomplished can be better
understood by referring to FIGS. 8-11. Referring now to FIGS. 8 and 8A, a
loading and unloading assembly 67 is shown which includes, for example,
cassettes 12 and 14, "O" ring tracks and the load pad 68 attached to a
vertical drive arm 70. A wafer to be loaded from cassette 12 is indexed
onto "O" ring track 58 which is driven by motor 60. An optical transmitter
62 positioned between and below the upper track 64 sends its optical
signal vertically upward to an optical receiver mounted in optical rail 63
(see FIG. 6). Interruption of the light signal by the wafer 16 stops motor
60 and activates motor 72 which drives track 64 so as to carry the wafer
toward load pad 68. An optical transmitter 66 is positioned below the load
pad aperture 68A and the light signal is transmitted through the aperture
68A to an optical receiver in the rail 63. Interruption of the light
signal from transmitter 66 notes the presence of the wafer on the load pad
68 and other programming information is used to determine what is to
happen next. For example, if only selected wafers are to be subjected to
the inspection, the wafer can be passed on along the track to be deposited
in one of the other cassettes. If the wafer is to be subject to
inspection, the wafer is stopped on the load pad 68, which is formed as a
part of drive arm 70.
The structure of the drive arm and the ball slide and air piston drive
arrangement are shown in more detail in FIGS. 8A and 8B. The drive arm 70
is L shaped with load pad 68 attached to the end of the transverse member
70A. A vacuum line 71 is attached to the edge of the longitudinal member
70B by mounting clamps 71A in a well known manner. The vacuum line 71
passes into the transverse member 70A adjacent the juncture of the
transverse and longitudinal members. The vacuum line 71 runs within the
transverse member to recess 69 in "T" shaped cut out 69A. Thus, a vacuum
can be pulled which is used to hold the wafer 16 on load pad 68 during
vertical movement thereof.
The drive arm 70 is held in a horizontally aligned position by an actuator
assembly comprising a ball slide assembly 124, which is driven vertically
up and down by a pair of air pistons 126 and 128. Air piston 126 has air
feed lines attached in a well known manner to air lines (not shown) via
couplings 138 and 139 and air piston 128 has air feed lines attached to
couplings 140 and 141. As air feed lines and air supply sources are well
known they are not shown to avoid drawing clutter. The manner in which the
air pistons operate is described in more detail hereinbelow. Once a wafer
has been selected for inspection its movement along the wafer track is
stopped when the wafer is positioned immediately above load pad 68 by the
interruption of the light signal through aperture 68A. The movement
controller 52 stops the wafer track with the wafer centered 10-30 miles
above load pad 68. A suction is applied to aperture 69 via vacuum line 71.
Then air pressure is applied to the air pistons to drive the ball slide
arrangement 124 upward. As it rises and the upper surface of the load pad
comes in contact with bottom of the wafer, the vacuum force becomes
effective to hold the wafer securely on the load pad during movement
thereof. The amount of upward displacement is controlled by a flag 130
and, in particular, cut outs 132 and 132A, which operate in conjunction
with optical detectors 134 and 136, to repeatably set the displacement. It
is to be noted that the drive arm 70 and, of course, its associated load
pad may rest in any one of three positions: (1) slightly below the wafer
track; (2) above the transfer arm (maximum upward displacement); and (b)
at an intermediate position. Initially the drive arm is raised to its top
position by the application of air pressure to air pistons 126 and 128.
Next, the wafer transfer arm 90 is swung under the drive arm 70 and is
positioned on the other side of the wafer. Then the drive arm 70 is
lowered to its intermediate position which positions the wafer slightly
above the recess 232 in the wafer transfer arm 90. As could be expected
the wafer setting on load pad 68 is not aligned so as to fit comfortably
in recess 232 without some adjustment. For this reason a sidewall 231 is
included on wafer transfer arm 90 to align the wafer as said arm is
maneuvered into a position adjacent load pad 68. Thus, it is seen that the
drive arm assembly provides a controlled vertical motion upward from a
rest position to a predetermined height above the "O" ring track and a
motion downward from said predetermined height to a position slightly
above the recess 232 in arm 90 and also to a position slightly below the
"O" ring track so as to be in position to readily accept a wafer from the
track.
The diameter of load pad 68 is smaller than the wafer so that when the
wafer transfer arm 90 swings adjacent the wafer, after the load pad is
lowered to its intermediate position, the wafer is aligned in the recess
232 without interference from load pad 68. Finally, the load pad can pass
through the aperture in the wafer transfer arm 90 as the load pad 68 moves
downward. Thus the wafer is transferred to the wafer transfer arm for
transport.
In order to ensure that the wafer inspector 10 is properly aligned and
ready to perform inspection test 5 tests, a calibration wafer 79 is first
run through the inspections. The calibration wafer is stored in garage 81
as shown in FIG. 8C. Garage 81 includes a top 81A, downwardly extending
sidewalls 81B and partial floor elements 81C extending inwardly from the
lower end of each of the sidewalls. When not in use, the calibration wafer
79 is stored in the garage which is then positioned above the wafer track
64A. When the calibration wafer is to be used, air piston 87 of actuator
assembly 85 is disabled and coil spring 89 acts to drive the garage 81
downward to its lowest position. The vertical movement is maintained by
guide assembly 83. As may be seen in FIG. 8C, the wafer 79 is deposited on
the wafer track 64A and the partial floor elements 81C are below the top
surface of the O-ring belts of track 64A. The track is enabled to move the
wafer toward load pad 68 from which it will be passed through the wafer
inspector as the calibration is checked and adjustment made as necessary.
Once back to the O-ring track the calibration wafer is directed to a
position within the garage. The air piston 87 is then enabled which raises
the garage and wafer above the track so that normal wafer inspections may
be effected.
As has been previously noted, it is possible to program the computer so
that any one of the cassettes may be used as the input or output, and, of
course, the output cassettes can be designed as pass and fail. Further,
the inspection can be limited to specifically designated wafers in each
cassette. If a wafer is not selected for test, it will be moved past load
pad 68 and along track 64A to track 80. If cassette 42 is designated to
accept uninspected wafers, optical signals from optical transmitter be
interrupted. This detects the presence of the wafer and causes the drive
motor 84 to stop. The wafer is then positioned adjacent "O" ring track 86
and motor 88 will then be activated to carry the wafer to cassette 42.
Conversely, when wafer 16 has been inspected, wafer transfer arm 98 will
move the wafer from the vacuum chuck 92 to a position above unload pad 76.
As described for load pad 68, unload pad 76 will be activated to rise
vertically and lift the wafer 16 from the transfer arm 98. The vacuum in
the arm having been removed so that the wafer readily moved upward with
the motion of the unload pad. A vacuum is applied to recess 75 of unload
pad 76 via line 77 to hold the wafer on the uload pad 76 during downward
motion.
The transfer arm 98 is then moved away and the unload pad actuator arm 74
moves vertically downward. The wafer can then be moved to the left or
right on track 80 or 80A, respectively, and the direction of motion will
depend upon the results of the inspection and the location of the pass and
fail cassettes. Only a part of the track 80A is shown. Further, not any
part of cassette 44 and its associated "O" ring track have been shown. It
should be understood, however, that operation of these elements will be
similar to that described for cassette 42 and its associated optical
detectors and "O" ring tracks. Consider the wafer to have passed
inspection and is to be lodged in cassette 42. The presence of the wafer
16 on unload pad 76 is recognized by the interuption of the light signal
from optical transmitter 78, which is centered below the aperture 76A. The
associated optical receiver being located in rail 63 which is located
above the O-ring track. Movement controller 52 will enable motor 84 so as
to cause track 80 to move the wafer from unload pad 76 past optical
transmitter 78 and over optical transmitter 82. This interupts the light
signal. The position of the wafer is recognized by movement controller 52
which stops motor 84. Next, motor 88 will activate belt 86 to move wafer
16 into cassette 42.
Wafer Transfer Arm
The manner in which the wafer transfer arms 90 or 98 are driven to move a
wafer between the load pad 68 or unload pad 76 and the vacuum chuck 92 may
be understood by reference to FIGS. 9-11. A stepper motor 170 has a shaft
172 connected to a bevel gear 174 which drives a bevel gear 176. A pinion
gear 178 is attached to a vertical drive shaft 180 that rotates in bearing
184 that is held in bearing support block 182. The drive shaft 180 is
attached to bevel gear 176, thus the motion of the stepper motor 170
causes pinion gear 178 to rotate. A spur gear 186 is driven by the pinion
gear 178. It should be noted that the pinion gear 178 is considerably
wider than the spur gear 186. Spur gear 186 is connected to an upper
bearing support block 188 by pins 208. These pin the gear to the hub. Pin
210 is used to pin the hub to the wafer arm drive shaft 196. A lower
bearing support block 202 has a threaded aperture 211 in which a set screw
212 is threaded to lock it to the shaft. Ball bearings in the upper and
lower bearing support blocks are designated 190 and 204, respectively, and
snap rings 192 and 200 hold the ball bearings in position in the upper and
lower support blocks.
Intermediate the upper and lower bearings is an air piston 194 which has an
air cylinder housing 214 that encloses an air cylinder piston 216 that
includes a piston drive connector 218. The snap rings 222 below and above
the piston drive connector 218 lock the air cylinder piston 216 to the air
cylinder shaft 228. O rings 224 and 226 minimize leakage around the
cylinder 216. Air may be applied via ports 195 or 197 depending upon
whether the wafer arm is to be driven down or up, respectively.
Thus, it is seen that the use of the wide pinion gear 178 allows a
controlled vertical movement of the wafer transfer arm 90, for example,
while still allowing for drive motion, when the arm is in either the upper
or lower vertical position. As will become clear in the subsequent
discussion, the transfer arm is driven at its lower vertical height to
pass under the load pad prior to accepting the wafer. In order to position
the wafer 16 on wafer vacuum check 92, the transfer arm 90 moves laterally
at its highest vertical position. Once the transfer arm is positioned
above the chuck, the transfer arm is dropped vertically downward to
transfer the wafer 16 to vacuum chuck 92. The transfer arm 90 is then
moved out of the macro inspection area.
To pick up a wafer from load pad 68, the load pad is raised to position the
wafer at its highest level and to place the load actuator arm 70 above the
path of transfer arm 90. The wafer arm 90 swings underneath the raised
load pad 68. The load pad is dropped down to the intermediate position
which positions the wafer 16 a few millimeter above shelf 233 in recess
232. Transfer arm 90 is then moved to a location adjacent the load pad and
the side wall 231 of recess 232 gently moves the wafer so that it is
correctly positioned on the shelf 233 of recess 232. The vacuum on load
pad 68 is released and the load pad is dropped down to its original
position. This motion causes the wafer to be deposited in the arcuate
recess of arm 90. The arcuate position is designed to accept the
particular size wafer being inspected and as noted hereinabove, the
arcuate position includes a recess 232 that provides a shelf 233 on which
the bottom edge of the wafer 16 may rest. Note that there is no gripping
action caused by the wafer holding member 230. The recess is sufficient to
insure that the edge of the wafer is not forced against the side wall of
the recess and the shelf extends radially inward far enough to provide
adequate support. In addition to the force of gravity, which tends to hold
the wafer in position, a series of vacuum holes 234 are provided in the
shelf and these interconnect with vacuum line 236 by which a vacuum
pressure is drawn to hold the wafer in position during transport.
Next, the wafer arm 90 is driven by stepper motor 170 until the wafer is
positioned over the wafer chuck 92 which at this time is centered in the
macro position. Air pressure is than applied via air inlet 195 of air
cylinder 214 causing the wafer arm to move vertically downward and deposit
wafer 16 on the wafer vacuum chuck 92. It being understood that the vacuum
pressure holding the wafer 16 in the recess 232 of wafer arm 90 is
released so as to avoid any undue pressure on the wafer 16 when the
underside of the wafer encounters th flat upper surface of the chuck 92.
The stepper motor 170 than drives the wafer arm into a neutral position
between the X-Y stage assembly 28 and the input load pad 68.
In removing a wafer 16 from the micro inspection station, the wafer
transport arm 98 is used. The drive motor moves the arm in its lowermost
position to a position adjacent the vacuum chuck 92 and beneath the wafer
16. The chuck holding vacuum is released. The arm moves up and a vacuum is
drawn through the vacuum holes 234 in the top of the arcuate position 235.
It is to be noted that a recessed position is not required for the arm 98,
and therefore the wafer rides on the top surface being held in place by
vacuum.
Vacuum Chuck
The wafer vacuum chuck 92 is a cylindrically shaped member having a flat
end wall 93 and is positioned on the X-Y stage 28 so as to centrally
locate the wafer when the stage is in either the macro or the micro
inspection station position. The diameter of the flat end wall 93 of chuck
92 is smaller than the wafer diameter and is small enough to allow the
passage of either the wafer holding members 230 or 235 when the wafer is
being deposited on or removed from the top surface of the vacuum chuck 92.
An example of a | | |
