U.S. patent application number 09/481055 was filed with the patent office on 2001-11-08 for apparatus and method for aligning a wafer.
Invention is credited to Chen, Ling, Lei, Lawrence C., Yu, Ying, Yudovsky, Joseph.
Application Number | 20010037771 09/481055 |
Document ID | / |
Family ID | 25401598 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010037771 |
Kind Code |
A1 |
Chen, Ling ; et al. |
November 8, 2001 |
Apparatus And Method For Aligning A Wafer
Abstract
A method for aligning a wafer on a support member within a
vacuum chamber includes increasing the pressure within the vacuum
chamber to at least about 1 Torr before aligning the wafer. The
wafer is introduced into the vacuum chamber on the support member,
the pressure is increased to at least about one Torr, and the
support member is lifted into a shadow ring that has a
frustoconical inner cavity constructed to funnel the wafer to a
centered, aligned position.
Inventors: |
Chen, Ling; (Sunnyvale,
CA) ; Yudovsky, Joseph; (Palo Alto, CA) ; Yu,
Ying; (Cupertino, CA) ; Lei, Lawrence C.;
(Milpitas, CA) |
Correspondence
Address: |
Applied Materials Inc
Patent Counsel
P O Box 450 A
Santa Clara
CA
95052
US
|
Family ID: |
25401598 |
Appl. No.: |
09/481055 |
Filed: |
January 11, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09481055 |
Jan 11, 2000 |
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08893461 |
Jul 11, 1997 |
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6063440 |
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Current U.S.
Class: |
118/728 |
Current CPC
Class: |
C23C 16/44 20130101;
H01L 21/68 20130101; C23C 16/4585 20130101 |
Class at
Publication: |
118/728 |
International
Class: |
C23C 016/00 |
Claims
1. A method for aligning a wafer within a vacuum chamber,
comprising the steps of: introducing the wafer into the vacuum
chamber; increasing the pressure within the vacuum chamber, and
subsequently moving the wafer into alignment.
2. The method of claim 1 wherein the pressure in the vacuum chamber
when the wafer is introduced therein is 1 milliTorr or less.
3. The method of claim 1 further comprising the step of increasing
the pressure in the vacuum chamber to at least about 1 Torr.
4. The method of claim 1 further comprising the step of increasing
the pressure in the vacuum chamber to a pressure that is at least
about 1 Torr and less than an operating pressure of the
chamber.
5. The method of claim 1 further comprising the step of increasing
the pressure in the vacuum chamber to a pressure between about 1
Torr and 100 Torr.
6. The method of claim 1 further comprising the step of increasing
the pressure in the vacuum chamber to a pressure between about 1
Torr and 10 Torr.
7. The method of claim 1 further comprising the step of waiting
until the pressure between the wafer and the support member is
equal to or greater than the pressure in the vacuum chamber before
aligning the wafer.
8. A method for aligning a wafer on a support member within a
vacuum chamber, comprising the steps of: providing a shadow ring
having a lower portion that is outwardly tapered for receipt of the
wafer and an upper aperture having a diameter that is slightly less
than the outer diameter of the wafer; introducing the wafer into
the vacuum chamber and onto the support member; increasing the
pressure within the chamber; and subsequently moving the support
member toward the shadow ring so that the shadow ring aligns the
wafer on the support member.
9. The method of claim 8 wherein the pressure in the vacuum chamber
when the wafer is introduced therein is 1 milliTorr or less.
10. The method of claim 8 further comprising the step of increasing
the pressure in the vacuum chamber to a pressure at least about 1
Torr.
11. The method of claim 8 further comprising the step of increasing
the pressure in the vacuum chamber to a pressure that is at least
about 1 Torr and less than an operating pressure of the
chamber.
12. The method of claim 8 further comprising the step of increasing
the pressure in the vacuum chamber to a pressure between about 1
Torr and 100 Torr.
13. The method of claim 8 further comprising the step of increasing
the pressure in the vacuum chamber to a pressure between about 1
Torr and 10 Torr.
14. The method of claim 8 further comprising the step of waiting
until the pressure beneath the wafer is equal to or greater than
the pressure in the vacuum chamber before aligning the wafer.
15. The method of claim 8 further comprising the step of raising
the wafer to a position below the shadow ring before increasing the
pressure within the chamber.
16. An apparatus for aligning a wafer on a support member in a
vacuum chamber, comprising: a support member is positioned within
the vacuum enclosure having a wafer receiving surface thereon; a
shadow ring located within the vacuum chamber, the shadow ring
comprising: an upper shield portion defining a circular upper
aperture therethrough, the upper aperture having a diameter that is
slightly less than the outer diameter of the wafer; a lower portion
extending from the upper shield portion having an annular cross
section defining a frustoconical inner cavity; the diameter of the
inner cavity decreases from a lower mouth aperture to an upper end;
and the diameter of the upper end of the inner cavity is slightly
greater than the outer diameter of the wafer; a gas supply in fluid
communication with the vacuum chamber; a gas flow controller that
controls the flow of gas from the gas supply to the vacuum chamber
and regulates the pressure within the vacuum chamber such that,
after the wafer is positioned on the support member and before the
wafer is raised into the shadow ring, the control member raises the
pressure within the chamber.
17. The apparatus of claim 16 wherein the control member raises the
pressure within the vacuum chamber to about 1 Torr.
18. The apparatus of claim 16 wherein the control member raises the
pressure within the vacuum chamber to a pressure between 1 Torr and
100 Torr.
19. The apparatus of claim 16 wherein the control member raises the
pressure within the vacuum chamber to a pressure between 1 Torr and
10 Torr.
20. The apparatus of claim 16 wherein the control member raises the
pressure within the vacuum chamber to a pressure that is at least
about 1 Torr and below an operating pressure.
21. The apparatus of claim 16 wherein the difference between the
diameter of the upper aperture in the upper shield portion and the
outer diameter of the wafer is no greater than 5 millimeters.
22. The apparatus of claim 16 wherein the difference between the
diameter of the upper aperture in the upper shield portion and the
outer diameter of the wafer is no greater than 3 millimeters.
23. The apparatus of claim 12 wherein the diameter of the upper end
of the inner cavity is substantially equal to the outer diameter of
the wafer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to the field of semiconductor
wafer processing equipment. More particularly, the present
invention relates to a method and apparatus for aligning a wafer on
a wafer support member.
[0003] 2. Background of the Related Art
[0004] In the fabrication of integrated circuits, the various
processes, such as physical vapor deposition (PVD), chemical vapor
deposition (CVD), and etch processes, are often carried out in a
vacuum environment to, among other things, reduce the particulate
level to which the wafers are exposed. Wafers are introduced into a
vacuum processing system through a loadlock where robots within the
vacuum processing system move the wafers from the loadlock into a
transfer chamber and then sequentially through the system
positioning the wafers in a series of processing chambers.
[0005] The processing steps carried out within the vacuum chambers
typically require the deposition, or etching of multiple metal,
dielectric and semiconductor film layers on the surface of a wafer.
During these processing steps, one must properly align and secure
the wafer in the processing chamber in which the desired deposition
or etch process is performed.
[0006] Typically, the wafer is supported in the chamber on a
support member, commonly called a susceptor or pedestal. The wafer
is placed on or secured to, the upper surface of the support member
prior to the deposition or etch process. To ensure proper
processing of the wafer, the wafer must be properly aligned
relative to the support member. The position of the support member
in the chamber is selected to provide a desired spacing and
relative geometry between the generally planar surface of the wafer
and other portions of the process chamber such as a gas plate in a
CVD process or a target in a PVD process.
[0007] Generally, a shadow or clamp ring is used to shield the edge
of a wafer and/or, in the case of a clamp ring, secure the wafer to
the support member. Although the present invention is equally
applicable to both shadow rings and clamp rings, the following
description will refer primarily to shadow rings such as those
typically used in CVD processes. In addition to acting as a shield,
shadow rings also function in wafer capturing or alignment on the
support member. Wing members extend downwardly and outwardly from
the shadow ring to form a funnel. As the support member moves the
wafer upward into the processing position, the support member moves
the wafer into the funnel which directs the wafer into alignment
with the shadow ring and the support member. Consequently, the
funnel applies vertical and lateral forces to the wafer when the
slanted wing members achieve lateral alignment of a misaligned
wafer with the shadow ring and support member as the support member
moves the wafer to the top end of the funnel and the shadow ring
settles on the support member.
[0008] A primary goal of wafer processing is to obtain as many
useful die as possible from each wafer. Many factors influence the
processing of wafers in the chamber and effect the ultimate yield
of die from each wafer processed therein including the existence of
contaminants within the chamber that can attach to the wafer and
contaminate one or more die therein. The processing chambers have
many sources of particle contaminants which, if received on the
wafer, reduce the die yield. One source of particulate
contamination occurs when a misaligned wafer is introduced into the
chamber. As the wing members of the shadow ring align with the
wafer, the wafer slides on the flat surface of the support member
and, due to the frictional forces between the wafer and the support
member, may create particulate contaminants. In some cases, the
frictional forces between the wafer and the support member cause
the misaligned wafer to actually move the shadow ring, thereby
preventing proper alignment of the wafer and reducing repeatability
of the zone of exclusion shielded by the shadow ring and the
process.
[0009] Prior efforts aimed at reducing the creation of particles
have reduced the alignment movement of the wafer on the support
member and simply increased the amount of overhang by the shadow
ring. In this way, the shadow ring is able to cover the wafer
without substantial movement of the wafer. One way that this is
accomplished is by increasing the diameter of the shadow ring
funnel upper end so that this diameter is larger relative to the
diameter of the wafer and the support member. Thus, rather than
substantially moving the wafers to align them, these systems simply
accept a greater misalignment and accept greater coverage of the
wafer upper surface area.
[0010] However, a second factor influencing the processing of
wafers in the chamber and affecting the ultimate yield of die from
each wafer processed therein is the repeatability of the
positioning of the wafer and the area covered by the shadow ring.
The wafer must be properly aligned relative to the support member
and the shadow ring to ensure that the film is properly deposited
on the wafer. Therefore, these prior efforts that avoid alignment
of the wafer and cover more surface area are not acceptable.
[0011] It would, therefore, be desirable to provide a relatively
simple system and method for reducing the coefficient of friction
between the support member and the wafer that would allow alignment
of the wafer without substantial particle generation.
SUMMARY OF THE INVENTION
[0012] In view of the foregoing, it is an object of the invention
to provide a relatively simple apparatus and method for reducing
the frictional forces between the support member and the wafer. It
is another object of the invention to enhance repeatability and to
provide a shadow ring that covers a minimal area of the upper
surface of the wafer. Yet another object of the invention is to
provide a system and method for aligning a wafer that is relatively
inexpensive, efficient, simple to implement, and reliable. Other
objects of the invention will become apparent from time to time
throughout the specification and claims as hereinafter related.
[0013] The present invention provides methods and apparatuses for
aligning a wafer on a support member in a vacuum chamber. In one
aspect of the invention, the method comprises the steps of
introducing the wafer into the vacuum chamber, increasing the
pressure within the vacuum chamber and moving the wafer into
alignment with a support member and/or shadow ring.
[0014] In another aspect, the method comprises providing a shadow
ring having a lower portion that is outwardly tapered for receipt
of a wafer and an upper aperture having a diameter that is slightly
less than the outer diameter of the wafer, introducing the wafer
into the vacuum chamber and onto the support member, increasing the
pressure within the chamber, and subsequently moving the support
member towards the shadow ring so that the shadow ring aligns the
wafer on the support member.
[0015] In accordance with the methods, the apparatus for aligning a
wafer on a support member in a vacuum chamber is an apparatus
comprising a support member positioned within the vacuum enclosure
and having a wafer receiving surface thereon, a shadow ring located
within the vacuum chamber, a gas supply in fluid communication with
the vacuum chamber, and a gas flow controller that controls the
flow of gas to the vacuum chamber and, thereby, regulates the
pressure within the vacuum chamber such that, after the wafer is
positioned on the support member and before the wafer is raised
into the shadow ring, the control member raises the pressure within
the chamber to about 1 Torr. The shadow ring used in this apparatus
comprises an upper shield portion defining a circular aperture
therethrough, the circular aperture having a diameter that is
slightly less than the outer diameter of the wafer, a lower portion
extending from the upper shield portion having an annular cross
section defining a frustoconical inner cavity, the diameter of the
inner cavity decreases from a lower mouth aperture to an upper end,
and the diameter of the upper end of the inner cavity is slightly
greater than the outer diameter of the wafer.
[0016] In each of these methods and apparatuses, the pressure is
preferably raised to a pressure greater than about 1 Torr and more
preferably to a pressure between about 1 Torr and 100 Torr and most
preferably between about 1 Torr and 10 Torr. Further, the pressure
is raised is to approximately equal to or less than the process
pressure. Also, the pressure between the wafer and the support
member is preferably equal to or greater than the pressure in the
chamber before the wafer is aligned.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] So that the manner in which the above recited features,
advantages and objects of the present invention are attained and
can be understood in detail, a more particular description of the
invention, briefly summarized above, may be had by reference to the
embodiments thereof which are illustrated in the appended
drawings.
[0018] It is to be noted, however, that the appended drawings
illustrate only typical embodiments of this invention and are
therefore not to be considered limiting of its scope, for the
invention may admit to other equally effective embodiments.
[0019] FIG. 1 is a partial cross sectional view of the vacuum
chamber.
[0020] FIG. 2 is a schematic drawing of the vacuum chamber and the
pressure control system.
[0021] FIG. 3 is a cross sectional view of a typical support member
having a wafer thereon that is partially covered by a shadow
ring.
[0022] FIG. 4 is a partial, cross sectional view of a shadow ring,
a wafer, and a support member showing the wafer misaligned on the
support member as they enter the inner cavity of the shadow
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] As shown in FIG. 1, the present invention relates to a
method and apparatus for aligning a wafer 20 on a support member 60
in a vacuum chamber 30. The alignment apparatus is depicted
generally as 10.
[0024] The preferred embodiment described below refers to an
alignment apparatus 10 that uses a shadow ring 40 to align the
wafer 20 on the support member 60. However, the invention is not
limited to this precise form of apparatus for it may apply to any
number of alignment mechanisms. As previously mentioned, the term
"shadow ring," as used herein, refers generally to both shadow
rings and clamp rings.
[0025] FIG. 1 shows a typical vacuum chamber 30 defined by an outer
body 32. The vacuum chamber 30 houses a support member 60 that may
take the form of a pedestal or susceptor mounted on a generally
vertically oriented shaft 62. The support member 60 serves to
support a wafer 20 on its flat upper supporting surface 64. The
support member 60 also includes a step formation 68 formed on its
outer perimeter to receive and support a shadow ring 40 and
includes four finger apertures 66.
[0026] In a typical vacuum chamber 30, the pressure within the
vacuum chamber 30 is controlled by a pressure control system such
as the one shown schematically in FIG. 2. In this system, a gas
supply 170 is provided in fluid communication with the vacuum
chamber 30. A gas flow controller 180 positioned intermediate the
gas supply 170 and the vacuum chamber 30 controls the flow from the
gas supply 170 to the vacuum chamber 30. Using a predetermined set
of instructions, the gas flow controller 180 selectively provides a
flow of gas to the vacuum chamber 30. As the gas flows into the
vacuum chamber 30, the pressure within the vacuum chamber 30
increases. In this way, the gas flow controller 180 controls the
pressure within the vacuum chamber 30. It is possible to provide
the gas to the chamber 30 through the support member 60 to the back
side of the wafer 20. When provided to the back side of the wafer
20, the gas creates a pressure between the wafer 20 and the support
member 60 that is initially greater than the pressure in the
chamber 30. This back side gas may be provided, for example, by a
bypass line 200 that provides communication from the gas flow
controller 180 to the upper surface 64 of the support member 60
between the support member 60 and the wafer 20.
[0027] FIG. 1 also illustrates a wafer lifting finger 90 received
in a finger aperture 66 passing through the body of the support
member 60. Typically, the processing chamber would include four
such lifting fingers 90. These lifting fingers 90 operate to lift
the wafer 20 clear of the upper supporting surface 64 of the
support member 60 after processing. This removal of the wafer 20 is
achieved by means of a conventional processing apparatus robot arm
(not shown) which enters the vacuum chamber 30 through the slit
valve opening 36. The same robot arm is also used to insert the
wafers 20 into the vacuum chamber 30. The lifting fingers 90 are
movable vertically under action of a lifting mechanism 92 of which
only the upper portion is shown.
[0028] A shadow ring 40 housed within the vacuum chamber 30
operates to provide an exclusionary zone where no deposition occurs
at the edge of the wafer 20. The shadow ring 40 also operates to
force a misaligned wafer 20 into alignment as the support member 30
moves from a lowered, or idle, position to a raised, or processing,
position. When the support member 30 is in the lowered position,
the shadow ring 40 is supported around its perimeter by an outer
support ring 38 that is, in turn, supported by a conventional
pumping plate 39 attached to the vacuum chamber 30. Together, the
two rings, 40 and 38, divide the vacuum chamber 30 into upper and
lower sections, 30a and 30b respectively.
[0029] During processing, the support member 60 moves upward into a
raised position lifting the shadow ring 40. The shadow ring 40 has
a lower portion 42 that rests on the upper surface 64 of the
support member 60 and supports the upper shield portion 50 of the
shadow ring 40 above the upper surface of the wafer 20. Preferably,
the shield portion 50 is held about 5 to 10 mils above the wafer
20. The upper shield portion 50 of the shadow ring 40 defines a
circular upper aperture 46 therethrough. The diameter of the upper
aperture 46 may be slightly less than the outer diameter of the
wafer 20 to form the exclusionary zone on the wafer 20. However,
new processes may require no overhang of the shadow ring 40 over
the wafer 20. In one typical processing operation, the step
formation 68, shown in FIG. 1, is in the range of 3.8 to 3.9 mm
high, the shadow ring 40 is in the range of 5 to 5.1 mm thick, and
the overhanging portion is in the range of 0.8 to 0.9 mm thick. The
overhanging portion defines an exclusionary zone of about 3 to 5 mm
about the edge of the wafer 20. However, in the preferred
embodiment, this exclusionary zone is no greater than 1.5 mm from
the edge of the wafer 20. To accommodate the current industry
standards, the exclusionary zone at any one edge is preferably
about 1.5 mm or less. This relatively small exclusionary zone is
necessary to allow deposition on the wafer 20 at a position 1.5 mm
from the wafer edge. Industry standards demand a film thickness at
1.5 mm from the wafer edge that is at least 90 percent of the film
thickness at the wafer center. No deposition is allowed on the
beveled edge of the wafer 20. Therefore, for a typical wafer 20
having a 0.5 mm chamfer about its edge, this allows a deviation of
only about 1 mm from center. As used herein, all dimensions account
for thermal expansion and are representative of measurements at
process temperatures.
[0030] Preferably, a purge gas is directed through the support
member 60 about the periphery of the wafer 20. The purge gas flows
between the shadow ring 40 and the wafer 20 to help shield the
exclusionary zone of the wafer 20.
[0031] A lower portion 42 of the shadow ring 40, as shown in FIG.
4, extends downwardly from the upper shield portion 50. The lower
portion 42 has an annular cross section throughout its length and
defines a frustoconical inner cavity 44 therein that is concentric
with the upper aperture 52. Because wafers 20 are circular in
shape, the support member 60 is circular as is the inner cavity
cross section. The diameter of the inner cavity 44 decreases from
the lower mouth portion 46 to the upper end 48 of the inner cavity
44 to form a funnel-like structure for aligning the wafer 20 on the
support member 60. Accordingly, the surface of the inner cavity 44
is relatively smooth to facilitate the sliding receipt and abutment
of the wafer 20 in the inner cavity 44. To allow receipt of the
wafer within inner cavity 44 and to properly align the wafer 20
with the shadow ring 40, the diameter of the upper end 46 of the
inner cavity 44 is slightly greater than and, preferably,
approximately equal to the outer diameter of the wafer 20. As
previously mentioned, current industry practice demands that the
thickness of the deposited film at a position 1.5 mm from the edge
of the wafer 20 to 90 percent of the thickness at the center of the
wafer 20. Accordingly, the wafer 20 must be aligned so that the
shadow ring overhangs the wafer 20 by no more than 1.5 mm about its
full periphery so that the film will be allowed to deposit on the
wafer 20 at 1.5 mm from the edge of the wafer 20. Therefore, the
diameter of the upper end 46 of the inner cavity 44 is preferably
at most only slightly more than 3 mm greater than the upper
aperture 52 and only slightly greater than the outer diameter of
the wafer 20 to ensure that the edge of the wafer 20 is within 1.5
mm of the periphery of the upper aperture 52. In this way, the
shadow ring 40 only overhangs the wafer 20 at most by about 1.5 mm
about the full periphery of the wafer 20. Because the wafer 20
rests on the upper surface 64 of the support member 60 and the
wafer 20 is relatively thin, the outer diameter of the support
member 60 must be sufficiently small that it can also be positioned
proximal the upper end 52 of the inner cavity 44. However, to
provide proper support for the wafer 20, the support member 60 must
cover substantially the full area of the wafer 20. Therefore, the
wafer must occupy most of the upper surface area of the support
member 60.
[0032] As shown in FIG. 1, once positioned in the vacuum chamber
30, a wafer 20 rests on the upper supporting surface 64 of the
support member 30. This placement is made with the support member
60 in its lowered position. Before processing may begin, the wafer
20 must first be raised by the support member 60 to the raised
position. It is during the movement from the lowered position to
the raised position that any misalignment of the wafer 20 is
corrected and the wafer 20 is aligned. As the support member 60
moves upward from the lowered position, a misaligned wafer 20
contacts the inner cavity 44 of the shadow ring 40 at a position
intermediate the upper end 48 and the lower mouth portion 46. FIG.
4 illustrates a misaligned wafer 20 on the support member 60. The
point of contact is dependent upon the magnitude of the
misalignment. Preferably, there is no misalignment. As the support
member 60 continues to move upward, the angled side of the
frustoconically-shaped inner cavity 44 exerts a lateral force on
the edge of the wafer 20 forcing the wafer 20 into alignment.
Consequently, when the support member 60 reaches its raised
position so that the wafer 20 is at the upper end 48 of the inner
cavity 44 of the shadow ring 40, the wafer 20 is aligned due to the
relative diameters of the wafer 20 and the shadow ring components.
When in this raised position, depending upon the type of process
involved, the outer portion of the wafer 20 may either bear against
the shadow ring 40 and slightly lift the shadow ring 40 under
action of the support member 60 or may rest on the shoulder 68 of
the support member 60 and, thereby, leave a small gap between the
shadow ring 40 and the wafer 20. For convenience, the application
refers primarily to those processes wherein the wafer 20 does not
contact the shadow ring 40 although the present invention is
applicable to all processes. With the support member 60 in the
raised position, the outer portion of the wafer 20 is covered by
the upper shield portion 50 of the shadow ring 40.
[0033] However, as mentioned previously, the sliding movement of
the wafer 20 on the support member 60 during alignment creates
particles within the vacuum chamber 30. These particles are
generated as a result of the friction between the wafer 20 and the
support member 60 which is generally characterized by the
coefficient of friction of the interface multiplied by the weight
of the wafer 20. Other forces acting upon the wafer 20 also affect
the magnitude of the frictional forces. For example, vacuum
chucking may affect the friction between the wafer 20 and the
support member 60. Likewise, the downward component of the force
exerted by the frustoconical inner cavity 44 increases the
frictional forces between the abutting surfaces. Nevertheless, the
friction force between the surfaces equals the coefficient of
friction between the surfaces multiplied by the downward, normal
forces exerted on the wafer 20 whatever their source. Generally,
the weights of the wafers 20 are relatively constant. Greater
frictional forces on the wafer 20 and the support surface 60 cause
greater particle generation and decrease the energy efficiency of
the system. In addition, high frictional forces may cause
misalignment and may cause the wafer 20 to move the shadow ring 40
out of alignment, rather than the shadow ring 40 moving the wafer
20 into alignment, if the lateral force applied on the wafer 20 by
the shadow ring is insufficient to overcome the frictional forces.
For the purposes of the present application, the relevant normal
and frictional forces are generally characterized by the following
formulas respectively wherein N represents the normal force applied
to the wafer 20, F is the frictional force applied to the wafer 20,
G is the weight of the wafer 20, A is the surface area of the wafer
20, P.sub.1 is the pressure in the chamber 30, P.sub.0 is the
pressure between the wafer 20 and the support member 60, and .mu.
is the coefficient of friction.
N=G-(P.sub.1-P.sub.0)A
F=.mu.N=.mu.(G-(P.sub.1-P.sub.0)A)
[0034] Thus, the normal force is equal to the weight of the wafer
20 less the force created by the pressure differential on the top
and bottom surfaces of the wafer 20. The force created by this
pressure differential equals the difference between the pressure
between the wafer 20 and the support member 60 and the pressure in
the chamber 30 multiplied by the surface area of the wafer 20. The
frictional forces equal the normal forces multiplied by the
coefficient of friction.
[0035] Reducing the frictional forces between the wafer 20 and the
support member 60 reduces the number of particles generated when
the wafer 20 is moved on the support member 60. Accordingly, in
order to reduce the number of particles generated, the coefficient
of friction or the normal force between the wafer 20 and the
support member 60 must be reduced. The present invention
accomplishes this by increasing the pressure within the vacuum
chamber 30 to at least about one Torr. Empirical studies, which are
more fully discussed below, have shown that increasing the pressure
within the vacuum chamber 30, so that the pressure between the
wafer 20 and the support member 60 is equal to or greater than the
pressure in the vacuum chamber 30, reduces the frictional forces
between the wafer 20 and the support member 60. In order for this
decrease in frictional force to occur, one of two things must
happen. One possibility is that the increased pressure somehow
lowers the coefficient of friction (e.g., by possibly creating a
cushion of gas between the wafer 20 and the support member 60).
Another possibility is that the increased pressure somehow lowers
the normal force on the wafer 20. Regardless of the manner in which
increasing the pressure affects the frictional forces, the result
is that the frictional forces are reduced and, thus, the wafer 20
may be moved on the support member 60 with less resistance and less
particle generation. The resulting decrease in frictional force
allows freer movement of the wafer 20 on the support member 60 and,
thereby, reduces the resulting scratches and generated particles.
Gas from the gas supply 170 is introduced into the vacuum chamber
30 to increase the pressure therein. The gas may be introduced
generally into the chamber 30 or through gas inlets positioned in
the upper surface 64 of the support member 60. It is in this latter
case that the pressure below the wafer 20 is greater than the
pressure above the wafer 20.
[0036] Therefore, the method of the present invention involves
increasing the pressure within the vacuum chamber 30 to at least
about one Torr before moving the wafer 20 on the support member 60
for alignment. Typically, the pressure within the vacuum chamber 30
when the wafer 20 is introduced therein is about one milliTorr or
less. The wafer is, thus, introduced into the vacuum chamber 30
onto the support member 60 which is in a lowered position. The
support member 60 is then raised to the lower mouth aperture 46 of
the shadow ring 40. However, before raising the support member 60
to the processing position the pressure within the vacuum chamber
30 is increased to at least about one Torr. Of course, this step of
increasing the pressure may take place at any time before the
support member 60 is raised into the inner cavity 44 of the shadow
ring 40. Preferably, the pressure is raised to between about 1 Torr
and 100 Torr or, more preferably, between about 1 Torr and 10 Torr
and approximately equal to or less than the operating pressure of
the process. The operating pressure of the process is the pressure
at which the process, such as a chemical vapor deposition process,
is carried out in the vacuum chamber 30. Also, before raising the
support member 60 to the raised position, the pressure between the
wafer 20 and the support member 60 is provided so that the pressure
between the wafer 20 and the support member 60 is approximately
equal to or greater than the pressure in the vacuum chamber 30.
Once the pressure in the vacuum chamber 30 is sufficiently raised
and the pressure beneath the wafer 20 is equalized, the support
member 60 is raised to the raised, or processing, position. As
previously discussed, when the support member 60 moves into the
shadow ring 40, any misaligned wafer 20 will contact the angled
sides of the inner cavity 44 which will force the wafer 20 into
alignment. After the support member 60 is in the raised position
and the wafer 20 is aligned, the pressure within the vacuum chamber
30 may be altered as needed.
[0037] As previously described, the pressure within the vacuum
chamber 30 is manipulated by a gas supply 170 and a gas flow
controller 180. In operation, the gas flow controller 180 uses
predetermined set of instructions to adjust the pressure within the
vacuum chamber 30 as needed. A vacuum pump 190, or series of vacuum
pumps 190, are used to evacuate the vacuum chamber 30.
[0038] Example
[0039] This system has been tested to determine its effectiveness
as follows. A misaligned wafer 20 was positioned upon a support
member 60 in a vacuum chamber 30 and was raised from a lowered
position to a raised position. The test was conducted under vacuum
conditions (i.e., moving the wafer 20 without first increasing the
pressure in the chamber) and under pressurized conditions (i.e.,
moving the wafer 20 only after increasing the pressure in the
chamber). When tested under pressurized conditions, the tests were
conducted with both the pressure beneath the wafer 20 equal to and
greater than the pressure in the chamber 30. In both of these
pressurized condition tests, the results were essentially the same.
The wafers 20 were then inspected using a SURISCAN 6200
manufactured by Tencor Instruments to determine the number of
particles generated as a result of the wafer 20 moving on the
support member 60. The results revealed that, without first
increasing the pressure in the chamber, alignment of the wafer
generated approximately 50 to 200 particles when the wafer 20
contacted the shadow ring and approximately 5000 backside
particles. In addition, without first increasing the pressure in
the chamber, the shadow ring 40 often moved with the wafer 20 as
the support member 60 lifted the shadow ring 40 due to the
frictional forces holding the wafer 20 to the support member 60.
This resulted in a misaligned wafer 20 and reduced repeatability of
the process. However, using the present invention, wherein the
pressure is raised to at least about one Torr before moving the
wafer 20, the movement of the wafer 20 on the support member 60
generated only approximately Twenty (20) particles when the wafer
20 contacted the shadow ring 40 and less than 2000 backside
particles. Further, the misaligned wafer 20 moved on the support
member 60 more readily and was, therefore, properly centered which
increased repeatability of the edge exclusion and the process.
[0040] While the foregoing is directed to the preferred embodiment
of the present invention, other and further embodiments of the
invention may be devised without departing from the basic scope
thereof, and the scope thereof is determined by the claims which
follow.
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