U.S. patent application number 09/312583 was filed with the patent office on 2001-11-22 for edge gripping specimen prealigner.
This patent application is currently assigned to Kensington Laboratories, Inc.. Invention is credited to BACCHI, PAUL, FILIPSKI, PAUL S..
Application Number | 20010043858 09/312583 |
Document ID | / |
Family ID | 23212123 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010043858 |
Kind Code |
A1 |
BACCHI, PAUL ; et
al. |
November 22, 2001 |
EDGE GRIPPING SPECIMEN PREALIGNER
Abstract
Specimen edge-gripping prealigners (8, 80) grasp a wafer (10) by
at least three edge-gripping capstans (12) that are equally spaced
around a periphery (13) of the wafer. Each edge-gripping capstan is
coupled by a continuous synchronous belt (14) to a drive hub (15,
84) that is rotated by a drive motor (18, 88). The belts are
tensioned by idler pulleys (22, 92) that are rotated by a motive
force (25, 96, 102). The edge-gripping capstans and the drive drums
are mounted to hinged bearing housings (28, 112) that are spring
biased to urge the capstans away from the drive hub. Deactivating
the motive force rotates the idler plates into a belt tensioning
position that draws the capstans inward to grip the periphery of
the wafer. Once gripped, rotation of the drive hub is coupled
through the tensioned belts to the capstans. Driving all the
capstans provides positive grasping and rotation of the wafer
without surface contact with the wafer and thereby reduces wafer
damage and particle contamination.
Inventors: |
BACCHI, PAUL; (NOVATO,
CA) ; FILIPSKI, PAUL S.; (GREENBRAE, CA) |
Correspondence
Address: |
STOEL RIVES LLP
900 SW FIFTH AVENUE
SUITE 2600
PORTLAND
OR
97204
US
|
Assignee: |
Kensington Laboratories,
Inc.
|
Family ID: |
23212123 |
Appl. No.: |
09/312583 |
Filed: |
May 14, 1999 |
Current U.S.
Class: |
414/757 ;
414/816 |
Current CPC
Class: |
H01L 21/68707 20130101;
H01L 21/68728 20130101 |
Class at
Publication: |
414/757 ;
414/816 |
International
Class: |
B25J 001/00; B65H
001/00 |
Claims
We claim:
1. An apparatus for prealigning a substantially planar circular
specimen having an exclusion zone extending inwardly from a
periphery of the specimen, comprising: a frame; at least three
capstans movably mounted to the frame and positioned around the
periphery of the specimen so as to contact only the exclusion zone
of the specimen; a drive hub rotatable by a motor; a continuous
belt associated with each of the capstans, each belt coupling an
associated capstan to the drive hub; and an idler pulley associated
with each of the belts, each idler pulley movable to place an
associated belt in alternate tensioned and untensioned states, the
tensioned state drawing the capstans inwardly toward the specimen
to grip the specimen and the untensioned state releasing the
capstans outwardly to release the specimen.
2. The apparatus of claim 1 in which the specimen is a
semiconductor wafer.
3. The apparatus of claim 2 in which the semiconductor wafer has a
diameter ranging from about 200 millimeters to about 300
millimeters.
4. The apparatus of claim 1 in which the continuous belts are in
the tensioned state and in which a rotation of the drive hub
couples through the continuous belts to impart a rotation to the
capstans and, thereby, to the specimen.
5. The apparatus of claim 1 in which each capstan is coupled to an
associated drive drum that is movably mounted to the frame by a
housing.
6. The apparatus of claim 5 in which the drive drum is journaled
for rotation on bearings that are mounted to the housing.
7. The apparatus of claim 5 in which the housing is pivotably
mounted to the frame and the housing is urged away from the
specimen by a spring.
8. The apparatus of claim 1 in which each idler pulley is mounted
to an idler plate that moves each idler pulley to place the
associated belt in the alternate tensioned and untensioned
states.
9. The apparatus of claim 8 further including a motive force that
moves the idler plate in a rotary manner.
10. The apparatus of claim 1 in which the exclusion zone is an
annular band extending inwardly a distance ranging from about 2
millimeters to about 5 millimeters from the periphery of the
specimen.
11. The apparatus of claim 1 in which each capstan rotates about a
capstan axis and further includes a load/unload ramp portion that
extends radially away from the capstan axis and downward relative
to a plane of the specimen and a backstop portion that extends
radially away from the capstan axis and upward relative to the
plane of the specimen, the load/unload ramp portion and the
backstop portion together forming an intersecting pair of truncated
right conical sections having an included angle for gripping the
exclusion zone of the specimen.
12. The apparatus of claim 11 in which the rest pad ramp portion
extends downward by an angle ranging from about 0-degrees to about
5-degrees relative to the plane of the specimen.
13. The apparatus of claim 11 in which the backstop portion extends
upward by an angle of about 87-degrees relative to the plane of the
specimen.
14. A method for rotating a substantially planar circular specimen
having an exclusion zone extending inwardly from a periphery of the
specimen, comprising: providing a frame; movably mounting at least
three capstans to the frame; positioning the capstans around the
periphery of the specimen so as to contact only the exclusion zone
of the specimen; mounting a rotatable drive hub to the frame;
coupling the capstans to the rotatable drive hub by continuous
belts; placing a movable idler pulley in contact with each of the
belts; and moving the idler pulleys to place the belts in alternate
tensioned and untensioned states, the tensioned state drawing the
capstans inwardly toward the specimen to grip the specimen and the
untensioned state releasing the capstans outwardly to release the
specimen.
15. The method of claim 14 in which the specimen is a semiconductor
wafer.
16. The method of claim 15 in which the semiconductor wafer has a
diameter ranging from about 200 millimeters to about 300
millimeters.
17. The method of claim 14 further including rotating the specimen
by placing the continuous belts in the tensioned state and rotating
the drive hub to couple the rotation through the continuous belts
to the capstans and, thereby, to the specimen.
18. A method for prealigning a substantially planar circular
specimen having a periphery, comprising: providing a specimen
prealigner having at least three movable capstans distributed
around a circle sized to surround the specimen; coupling with
continuous belts the capstans to a rotatable drive hub; providing a
robot arm for placing the specimen in a position substantially
parallel to a plane passing through the capstans; tensioning the
belts to draw the movable capstans inwardly toward the specimen to
grip the specimen by the periphery; releasing the robot arm from
the specimen; rotating the drive hub; coupling the drive hub
rotation through the continuous belts to the capstans and, thereby,
to the specimen; prealigning the specimen; untensioning the belts
to release the movable capstans outwardly away from the specimen;
grasping the specimen with the robot arm; and moving the specimen
with the robot arm to another position.
19. The method of claim 18 in which the tensioning and untensioning
includes providing multiple movable idler pulleys, pressing the
idler pulleys against the belts for tensioning the belts, and
withdrawing the idler pulleys from the belts for untensioning the
belts.
20. The method of claim 18 in which the specimen is a semiconductor
wafer having a diameter less than about 300 millimeters.
Description
FIELD OF THE INVENTION
[0001] This invention is directed to a specimen prealigning
apparatus and method and, more particularly, to an edge gripping
semiconductor wafer prealigner that substantially reduces wafer
backside damage and particulate contamination.
BACKGROUND OF THE INVENTION
[0002] Integrated circuits are produced from wafers of
semiconductor material. The wafers are typically housed in a
cassette having a plurality of closely spaced slots, each of which
can contain a wafer. The cassette is typically moved to a
processing station where the wafers are removed from the cassette,
placed in a predetermined orientation (prealigned), and returned to
another location for further wafer processing.
[0003] Various types of wafer handling devices are known for
transporting the wafers to and from the cassette and among
processing stations. Many employ a robotic arm having a
spatula-shaped end that is inserted into the cassette to remove or
insert a wafer. The end of the robotic arm typically employs vacuum
pressure to releasably hold the wafer to the end of the arm. The
robotic arm enters the cassette through the narrow gap between an
adjacent pair of wafer slots and engages the backside of a wafer to
retrieve it from the cassette. After the wafer has been processed,
the robotic arm inserts the wafer back into the cassette.
[0004] U.S. Pat. No. 5,513,948 for UNIVERSAL SPECIMEN PREALIGNER,
which is assigned to the assignee of this application, and U.S.
Pat. No. 5,238,354 for SEMICONDUCTOR OBJECT PRE-ALIGNING APPARATUS
describe prior semiconductor wafer prealigners that include a
rotating vacuum chuck on which the wafer is placed by a robot arm
for prealigning.
[0005] Unfortunately, transferring the wafer among the cassette,
robot arm, and prealigner may cause backside damage thereto and
contamination of the other wafers housed in the cassette because
engagement with the wafer may dislodge particles that can fall and
settle onto the other wafers. Robotic arms and prealigners that
employ a vacuum pressure to grip the wafer can be designed to
minimize particle creation. Even the few particles created with
vacuum pressure gripping or any other non-edge gripping method are
sufficient to contaminate adjacent wafers housed in the cassette.
Reducing such contamination is particularly important to
maintaining wafer processing yields. Moreover, the wafer being
transferred may be scratched or abraded on its backside, resulting
in wafer processing damage.
[0006] What is needed, therefore, is a wafer gripping technique
that can securely, quickly, and accurately prealign wafers while
minimizing particle contamination and wafer scratching.
SUMMARY OF THE INVENTION
[0007] An object of this invention is, therefore, to provide an
apparatus and a method for prealigning semiconductor wafers.
[0008] Another object of this invention is to provide an apparatus
and a method for quickly and accurately prealigning specimens.
[0009] A further object of this invention is to provide an
apparatus and a method for prealigning wafers while minimizing
particle contamination and wafer scratching.
[0010] Specimen edge-gripping prealigners of this invention grasp a
wafer by at least three edge-gripping capstans that are preferably
equally spaced around the periphery of the wafer. Each of the
edge-gripping capstans is coupled by a continuous synchronous belt
to an axially centered, grooved drive hub that is rotated by a
drive motor. Each of the capstans is also coaxially connected to a
grooved drive drum that is coupled to the drive hub by one of the
continuous synchronous belts, and each belt is routed in a unique
location in a set of grooves in the drive drums and the drive hub.
The continuous synchronous belts are tensioned by idler pulleys
that are mounted to axially rotatable idler plates that are coupled
together for common rotation by a belt tensioning motor or some
other form of rotary biasing force, such as a spring, solenoid, or
vacuum pressure actuated piston.
[0011] The edge-gripping capstans and the grooved drive drums are
mounted to hinged bearing housings that are pivotally spring biased
to preload the grooved drive drums radially away from the axially
centered drive hub. The edge-gripping capstans can be driven
radially inward to grip the wafer by rotating the belt tensioning
motor to apply sufficient tension to overcome the spring preload
force on the idler plates. Once gripped, the wafer can be rotated
by energizing the drive motor to rotate the drive hub, which
rotation is coupled through the tensioned belts and drive drums to
the capstans.
[0012] The edge-gripping specimen prealigner of this invention is
suitable for prealigning semiconductor wafers. Simultaneously
rotating all the edge-gripping capstans provides positive rotation
of the wafer without wafer surface contact, which eliminates wafer
backside damage. Synchronously driving of all the capstans prevents
slippage between each capstan and the wafer and thereby results in
minimized edge contamination.
[0013] Additional objects and advantages of this invention will be
apparent from the following detailed description of preferred
embodiments thereof that proceed with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a sectional elevation view of a first embodiment
of an edge-gripping specimen prealigner of this invention showing
internal details of motors, belt drives, capstans, and a specimen
peripheral edge scanner.
[0015] FIG. 2 is a sectional top view taken along lines 2-2 of FIG.
1 showing belt driving and tensioning mechanisms coupling a drive
motor to three specimen edge gripping capstans.
[0016] FIG. 3 is a sectional elevation view taken along lines 3-3
of FIG. 2 showing internal details of a representative drive drum
and specimen edge gripping capstan of this invention.
[0017] FIG. 4 is an enlarged sectional view of an edge-gripping
capstan gripping a wafer periphery in a manner according to this
invention.
[0018] FIG. 5 is a sectional elevation view of a second embodiment
of an edge-gripping specimen prealigner of this invention showing
internal details of motors, belt drives, and capstans.
[0019] FIG. 6 is a bottom view of FIG. 5 showing belt driving and
tensioning mechanisms coupling a drive motor to six specimen edge
gripping capstans that are in a specimen edge-gripping
position.
[0020] FIG. 7 is a bottom view of FIG. 5 showing belt driving and
tensioning mechanisms coupling a drive motor to six specimen edge
gripping capstans that are in a specimen releasing position.
[0021] FIG. 8 is an enlarged sectional elevation view showing
internal details of a representative drive drum, specimen edge
gripping capstan, and specimen peripheral edge scanner of this
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0022] FIGS. 1 and 2 show sectional side and bottom views of a
first preferred embodiment of a specimen edge-gripping prealigner 8
(hereafter "prealigner 8".) Prealigner 8 is composed of a frame 9
to which three edge-gripping capstans 12 are movably mounted and
positioned to grasp a generally circular specimen, such as a wafer
10 (shown in phantom in FIG. 2). The capstans 12 are preferably
spaced equally apart and located along a circle generally defined
by a periphery 13 (shown in dashed lines in FIG. 2) of wafer 10.
Periphery 13 may include "flat" and "notch" features, which are
used for orientating wafer 10. Prealigner 8 may be adapted for use
with any generally circular specimens.
[0023] Edge-gripping capstans 12 are coupled by continuous
synchronous belts 14 to a grooved drive hub 15 that is journaled in
bearings 16 for rotation about a rotational axis 17 by a motor 18,
all of which are supported by frame 9. Edge-gripping capstans 12
are directly coupled to grooved drive drums 20. Each drive drum 20
is coupled to drive hub 15 by a different one of the three
continuous synchronous belts 14. Each of belts 14 is routed at a
different elevation around the same set of associated grooves in
its corresponding drive drum 20 and drive hub 15. The resulting
rotation of edge-gripping capstans 12 takes place about capstan
axes 21, which extend parallel to rotational axis 17.
[0024] Continuous synchronous belts 14 are tensioned by idler
pulleys 22 that are mounted to radially extending arms of an
axially rotatable idler plate 24, which is shown in FIG. 2 rotated
to a belt tensioning position 24A (solid lines) and an alternate
belt untensioned position 24B (phantom lines). Idler plate 24 is
rotated through a predetermined angular range about rotational axis
17 by a motor 25 or some other rotary biasing force, such as a
spring and a solenoid. Motor 25 and idler plate 24 are journaled
for rotation about bearings 26, all of which are supported by frame
9.
[0025] Referring to FIG. 3, each of grooved drive drums 20 is
journaled for rotation about bearings 27 that are mounted in
associated ones of hinged bearing housings 28. Bearing housing 28
are journaled for pivotal movement about bearings 29, which are
supported by frame 9. The pivoting of hinged bearing housings 28
allows radial displacement of capstan axis 21 relative to
rotational axis 17. The pivoting of hinged bearing housings 112
allows radial displacement of capstan axis 21 relative to
rotational axis 17. Each of hinged bearing housings 28 includes a
coil spring 30 that preloads drive drum 20 away from rotational
axis 17. To ensure proper movement of edge-gripping capstans 12,
each of hinged bearing housings 28 further includes a vane
120.sub.1 that protrudes from the end of hinged bearing housing 28
opposite pivot axis 116.sub.1. Depending on the rotational state of
hinged bearing housing 112, vane 120.sub.1 is positioned to
alternately interrupt (see FIG. 6 showing this position for an
alternative embodiment) or not interrupt (see FIG. 7 showing this
position for an alternative embodiment) a light beam within an
optical sensor 122.sub.1. All three of optical sensors 122.sub.1
acting together provide a positive electrical indication of whether
prealigner 8 is in a wafer gripping state or a wafer releasing
state.
[0026] FIG. 4 shows an enlarged view of a representative one of
edge-gripping capstans 12, which includes a wafer-contacting pulley
31 that may be formed from various materials, and preferably
polyetheretherketone "peek"), a semi-crystalline high temperature
thermoplastic manufactured by Victrex in the United Kingdom. The
material forming wafer-contacting pulley 31 may be changed to suit
the working environment, such as in high temperature applications.
Peek material provides a contamination resistant low scratching
wafer contacting surface.
[0027] Wafer-contacting pulley 31 includes a load/unload portion 32
ramped at a shallow angle for supporting wafer 10 when capstan 12
is in its specimen gripping and nongripping positions. Pulley 31
also includes an inwardly inclined ramp-backstop portion 34 that is
pressed against the periphery 13 of wafer 10 when capstan 12 is in
its specimen gripping position.
[0028] Load/unload ramp portion 32 has a radial width 36 that
allows adequate range for the wafer positioning variation of the
mechanism which loads the wafer onto the prealigner. Load/unload
ramp portion 32 is angled downwardly from the plane of wafer 10 by
an angle greater than 0 degrees, and preferably 1 to 5 degrees.
[0029] Inwardly inclined backstop portion 34 has a height 38 large
enough to capture wafer 10, preferably between about 1 mm and 2 mm
and is angled upwardly from the plane of wafer 10 to secure it by
about 3 degrees.
[0030] Load/unload ramp portion 32 and backstop portion 34 together
form an intersecting pair of truncated right conical sections
having an included angle for gripping periphery 13 of wafer 10.
[0031] When edge-gripping capstans 12 are actuated to press against
periphery 13 of wafer 10, the intersecting inclined conical
surfaces formed by load/unload ramp portion 32 and inwardly
inclined backstop portion 34 positively grip and maintain wafer 10
in a preferable horizontal attitude, although other attitudes are
possible. When edge-gripping capstans 12 are released from gripping
wafer 10, load/unload ramp portion 32 supports the periphery 13 of
wafer 10.
[0032] A typical operational sequence for prealigner 8 is described
below with reference to FIGS. 1 and 2.
[0033] Prealigner 8 is in an initial state in which no wafer 10 is
present and idler plate 24 is in belt untensioning position
24B.
[0034] A robot arm 50 (fragmentary view shown in FIG. 1) grips
wafer 10 by periphery 13 and positions wafer 10 at a wafer position
10A that is separated apart from but substantially parallel to a
plane passing through load/unload ramp portions 32 of edge-gripping
capstans 12. Robot arm 50 performs wafer 10 positioning movements
in one of the approximately 120-degree clearance spaces between
edge-gripping capstans 12. A specimen edge-gripping robot arm
suitable for use with this invention is described in copending U.S.
patent application Ser. No. 09/204,747, filed Dec. 2, 1998, for
ROBOT ARM WITH SPECIMEN EDGE GRIPPING END EFFECTOR, which is
assigned to the assignee of this application.
[0035] Robot arm 50 lowers wafer 10 to a wafer position 10B such
that wafer 10 is supported by the load/unload ramp portions 32 of
edge-gripping capstans 12.
[0036] Robot arm 50 disengages from wafer 10 and moves to a wafer
disengaged position (shown in dashed lines). Robot arm 50 may stay
at the wafer disengaged position during subsequent wafer
prealigning operations or it may be withdrawn from prealigner
8.
[0037] Motor 25 is actuated to rotate idler plate 24 from
untensioned position 24B to tensioned position 24A to provide
sufficient tension in belts 14 to overcome the preload force
applied to grooved drive drums 20 and to draw edge-gripping
capstans 12 radially inward to grip periphery 13 of wafer 10.
[0038] Once gripped, wafer 10 is rotated by energizing motor 18 to
rotate drive hub 15, which rotation is coupled through tensioned
belts 14 and drive drums 20 and, therefore, to edge-gripping
capstans 12. Preferably all of edge-gripping capstans 12 are driven
to prevent rotational slippage, even though wafer 10 is gripped
with minimal force.
[0039] During rotation of wafer 10, a linear charge-coupled device
("CCD") array 52 receives an image of a slice of periphery 13 of
wafer 10. Periphery 13 is illuminated through a collimating lens 53
by a light source 54 that casts a shadow of the periphery 13 on CCD
array 52. The "terminator" position of the shadow on individual
sensors in the CCD array 52 provides a signal from CCD array 52
that accurately represents a radial distance between rotational
axis 17 and periphery 13 for each of a set of rotational angles of
wafer 10. CCD array 52 may also sense when wafer 10 is gripped by
detecting a lateral movement of periphery 13.
[0040] An optical rotary encoder 56 provides feedback to control
the rotation of motor 25. A notch (not shown) in periphery 13
serves as an angular index mark for determining in cooperation with
optical rotary encoder 56 the actual rotational angles of wafer 10
since there is uncertainty of the actual effective radii of the
wafer 10 and the edge-gripping capstans 12.
[0041] Prealigning of wafer 10 may be carried out in the manner
described in the above-referenced U.S. Pat. No. 5,513,948 for
UNIVERSAL SPECIMEN PREALIGNER.
[0042] After wafer 10 is prealigned, motor 18 is deactivated, motor
25 rotates idler plate 24 to belt untensioning position 24B, and
robot arm 50 retrieves wafer 10 from prealigner 8.
[0043] FIGS. 5, 6, and 7 show respectively a sectional side view
and two bottom views of a second preferred embodiment of a specimen
edge-gripping prealigner 80 (hereafter "prealigner 80"). Prealigner
80 is composed of a frame 82 to which six edge-gripping capstans 12
are movably mounted and positioned to grasp a generally circular
specimen, such as wafer 10 (shown in phantom in FIGS. 6 and 7). The
capstans are spaced apart and located along a circular plane
generally defined by a periphery 13 (shown in dashed lines in FIGS.
6 and 7) of wafer 10. Periphery 13 typically includes a "notch"
feature for identifying a rotational index orientation for wafer
10. FIGS. 6 and 7 show periphery 13 of wafer 10 respectively
gripped and released by edge-gripping capstans 12.
[0044] Prealigner 80 may be adapted for use with generally circular
specimens, such as wafer 10 having a nominal diameter ranging from
about 200 mm to 300 mm, although other diameters would also be
applicable.
[0045] Edge-gripping capstans 12 are coupled by continuous
synchronous belts 14 to a drive hub 84 that is journaled in
bearings 86 for rotation about rotational axis 17 by a motor 88,
all of which are supported by frame 82. Edge-gripping capstans 12
are directly coupled to drive drums 90. Each drive drum 90 is
coupled to drive hub 84 by a different one of the six continuous
synchronous belts 14. Each of belts 14 is routed at different
elevations around the same set of associated grooves in its
corresponding drive drum 90 and drive hub 84. The resulting
rotation of edge-gripping capstans 12 takes place about capstan
axes 21, which extend parallel to rotational axis 17.
[0046] Continuous synchronous belts 14 are tensioned by idler
pulleys 92 that are mounted at the ends of arms that extend
radially from an axially rotatable idler plate 94, which is shown
in FIG. 6 rotated to a belt tensioning position and in FIG. 7
rotated to a belt untensioned position. Idler plate 94 is rotated
through an angular range about rotational axis 17 by a vacuum
pressure actuated piston 96 acting through a coupling link 98 that
is attached to the end of one of the arms of idler plate 94. Idler
plate 94 is journaled in bearings 100 for rotation about rotational
axis 17.
[0047] When vacuum pressure actuated piston 96 receives no vacuum
pressure and/or prealigner 80 is deenergized, a set of springs 102
extending between a rotationally adjustable hub 104 and the arms of
idler plate 94 provide a biasing force that rotates idler plate 94
to the belt tensioning position shown in FIG. 6. This is
advantageous because prealigner 80 will remain in a wafer gripping
state in the event of a power or vacuum pressure failure. The
amount of biasing force is adjustable by rotating adjustable hub
104. While a single spring 102 could provide the biasing force,
multiple springs are preferred because they provide a more uniform
and linear biasing force to idler plate 94. Of course, when moving
idler plate 94 to the belt relaxing position shown in FIG. 7,
vacuum pressure actuated piston 96 must provide sufficient force to
overcome the biasing force of springs 102.
[0048] Drive hub 84 and drive drums 90 have unequal diameters that
provide about a 3.6:1 drive ratio from drive hub 84 to drive drums
90 in a preferred embodiment. The rotational position of drive hub
84 is sensed by a conventional glass scale rotary encoder 106 and
an associated optical sensor 108.
[0049] Referring also to FIG. 8, each drive drum 90 is journaled on
bearings 110 that are mounted in associated ones of hinged bearing
housings 112. The hinged bearing housings 122 are journaled on
bearings 114 for pivoting about a pivot axis 116. The pivoting of
hinged bearing housings 112 allows radial displacement of capstan
axis 21 relative to rotational axis 17. Each of hinged bearing
housings 112 further includes a coil spring 118 that preloads drive
drum 90 radially away from rotational axis 17.
[0050] The preloading force provided by springs 118 is sufficient
to move edge-gripping capstans 12 radially away from rotational
axis 17 when belts 14 are in the untensioned state, but the
preloading force is insufficient when belts 14 are in the tensioned
state. Accordingly, edge-gripping capstans 12 alternate between
wafer gripping and wafer releasing positions in response to
actuation of vacuum pressure actuated piston 96. To ensure proper
movement of edge-gripping capstans 12, each of hinged bearing
housings 112 further includes a vane 120 that protrudes from the
end of hinged bearing housing 112 opposite pivot axis 116.
Depending on the rotational state of hinged bearing housing 112,
vane 120 is positioned to alternately interrupt (FIG. 6) or not
interrupt (FIG. 7) a light beam within an optical sensor 122. All
six of optical sensors 122 acting together provide a positive
electrical indication of whether prealigner 80 is in a wafer
gripping state or a wafer releasing state.
[0051] A typical operational sequence for prealigner 80 is
described below with reference to FIGS. 5, 6, 7, and 8.
[0052] Prealigner 80 is in an initial state in which no wafer 10 is
present and idler plate 94 is in the belt untensioning position
shown in FIG. 7.
[0053] A robot arm (not shown) grips wafer 10 by periphery 13 and
positions wafer 10 similar to the manner described-above for
prealigner 8.
[0054] The robot arm lowers wafer 10 such that wafer 10 rests on
load/unload ramp portions 32 of edge-gripping capstans 12.
[0055] The robot arm disengages from wafer 10 and moves to a wafer
disengaged position. The robot arm may stay at the wafer disengaged
position during subsequent wafer prealigning operations or it may
be withdrawn from prealigner 80.
[0056] Vacuum pressure actuated piston 96 is deactuated to rotate
idler plate 94 from the belt untensioned position shown in FIG. 7
to the belt tensioned position shown in FIG. 6, thereby drawing
edge-gripping capstans 12 radially inward to grip periphery 13 of
wafer 10.
[0057] Once gripped, wafer 10 is rotated by energizing motor 88 to
rotate drive hub 84, which rotation is coupled through tensioned
belts 14 and drive drum 90 and, therefore, to edge-gripping
capstans 12. Preferably all of edge-gripping capstans 12 are driven
to prevent rotational slippage, even though wafer 10 is gripped
with minimal force.
[0058] During rotation of wafer 10, a linear charge-coupled device
("CCD") array 124 receives an image of a slice of periphery 13 of
wafer 10. Periphery 13 is illuminated through a collimating lens
126 by a light source 128 that casts a shadow of the periphery 13
on CCD array 124. The "terminator" position of the shadow on
individual sensors in the CCD array 124 provides a signal from CCD
array 124 that accurately represents a radial distance between
rotational axis 17 and periphery 13 for each of a set of rotational
angles of wafer 10. CCD array 124 may also sense when wafer 10 is
gripped by detecting a lateral movement of periphery 13.
[0059] Rotational axis 17 is substantially coaxial with the
effective center of wafer 10 because of the angular spacing of
edge-gripping capstans 12 around periphery 13. Edge-gripping
capstans 12 are arranged in two groups of three, with the groups on
opposite sides of a first imaginary line 130 extending through
rotational axis 17 and CCD array 124. Adjacent capstans 12 in each
group are angularly spaced apart from each other, with the center
capstan in each group having its capstan axis 21 lying in a second
imaginary line 132 that extends perpendicular to the first
imaginary line 130 and through rotational axis 17.
[0060] The amount of angular rotation imparted by edge-gripping
capstans 12 to wafer 10 is sensed by rotary encoder 106 and optical
sensor 108 that is coupled to drive hub 84. A notch (not shown) in
periphery 13 serves as an angular index mark for determining in
cooperation with rotary encoder 106 and optical sensor 108 the
actual rotational angles of wafer 10. Because the diameter of wafer
10 is a variable and wafer periphery 13 may be square, chamfered,
or rounded, an angular encoding calibration is carried out as
follows. Wafer 10 is rotated until CCD array 124 senses the notch.
Wafer 10 is rotated one complete revolution until CCD array 124
again senses the notch. During one complete notch-to-notch
revolution of wafer 10, the distance travelled is measured by the
optical sensor 108. The total distance measured is divided by one
revolution in the appropriate unit system to derive the appropriate
relationship between the distance units of optical sensor 108 and
wafer rotational units. During a subsequent notch-to-notch rotation
of wafer 10, a set of radius measurements made at predetermined
angular intervals by CCD array 124 sensing periphery 13 of wafer 10
as described above.
[0061] Thereafter, rotational prealigning of wafer 10 may be
carried out in the manner described in the above-referenced U.S.
Pat. No. 5,513,948.
[0062] After wafer 10 is prealigned, vacuum pressure actuated
piston 96 is activated to rotate idler plate 94 to belt untensioned
position shown in FIG. 7, and the robot arm retrieves wafer 10 from
prealigner 80.
[0063] Skilled workers will recognize that portions of this
invention may be implemented differently from the implementations
described above for preferred embodiments. For example, different
drive hub to capstan ratios may be employed. Three and six capstan
embodiments are shown, but many embodiments with more than three
capstans are envisioned can be implemented. Also, the capstans
necessarily require neither equal angular spacing around the
specimen nor the spacings shown and described in the
above-described embodiments.
[0064] It will be obvious to those having skill in the art that
many changes may be made to the details of the above-described
embodiments of this invention without departing from the underlying
principles thereof. Accordingly, it will be appreciated that this
invention is also applicable to specimen handling applications
other than those found in semiconductor wafer processing. The scope
of the present invention should, therefore, be determined only by
the following claims.
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