U.S. patent application number 10/803820 was filed with the patent office on 2005-09-22 for rails for semiconductor wafer carriers.
Invention is credited to Brown, Steven A., Gonzales, Manuel, Kopel, Claudia, Werninghaus, Thomas.
Application Number | 20050205502 10/803820 |
Document ID | / |
Family ID | 34964289 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050205502 |
Kind Code |
A1 |
Brown, Steven A. ; et
al. |
September 22, 2005 |
Rails for semiconductor wafer carriers
Abstract
A rail is provided for use as a support in an apparatus for
holding a plurality of semiconductor wafers. The rail has a
plurality of teeth arranged in a vertical column, such that the
space between a top surface of one tooth and a bottom surface of
the next higher adjacent tooth forms a slot for receiving a portion
of a semiconductor wafer. A support structure for supporting the
wafer is located on the top surface of substantially all teeth that
form the bottom of a slot, the support structure having sidewalls
and an upper surface spaced from the top surface. On each support
structure, a radius is formed at each intersection of the sidewalls
and the upper surface. The support structure extends for at least
approximately 50% of the length of each tooth.
Inventors: |
Brown, Steven A.;
(Bridgeport, TX) ; Gonzales, Manuel; (Fort Worth,
TX) ; Kopel, Claudia; (Denton, TX) ;
Werninghaus, Thomas; (Bockau, DE) |
Correspondence
Address: |
BRACEWELL & PATTERSON, L.L.P.
P.O. BOX 61389
HOUSTON
TX
77208-1389
US
|
Family ID: |
34964289 |
Appl. No.: |
10/803820 |
Filed: |
March 18, 2004 |
Current U.S.
Class: |
211/41.18 |
Current CPC
Class: |
H01L 21/67309
20130101 |
Class at
Publication: |
211/041.18 |
International
Class: |
A47G 019/08 |
Claims
I claim:
1. A rail for use as a support in an apparatus for holding a
plurality of semiconductor wafers, the rail comprising: a plurality
of teeth, each of the teeth having a top surface, a bottom surface,
and a length, the teeth being arranged such that a space between
the top surface of one tooth and the bottom surface of a next
higher adjacent tooth forms a slot for receiving a portion of a
semiconductor wafer; a raised support structure for contacting and
supporting said wafer and located on the top surface of
substantially all teeth that form a bottom of a slot, the raised
support structure having opposing sidewalls that intersect with and
define an upper surface therebetween, the upper surface being
spaced from the top surface, each raised support structure
extending for at least approximately 50% of the length of the
corresponding tooth; and wherein on each raised support structure,
a radius is formed at each intersection of at least selected
sidewalls and the upper surface.
2. The rail of claim 1, wherein on each raised support structure, a
radius is formed at each intersection of each of the sidewalls and
the upper surface.
3. The rail of claim 1, wherein the length of each tooth is greater
than 25 millimeters.
4. The rail of claim 1, wherein the length of each tooth is between
about 30 and about 100 millimeters.
5. The rail of claim 1, wherein the radius is at least 1
millimeter.
6. The rail of claim 1, wherein the radius is at least 1 millimeter
and not greater than 2.5 millimeter.
7. The rail of claim 1, wherein the raised support structure
extends at least approximately 70% of the length of each tooth.
8. The rail of claim 1, wherein each raised support structure is a
wedge-shaped protuberance running along one side of one of the
teeth.
9. The rail of claim 1, wherein each raised support structure runs
continuously from the front tip of one of the teeth to a point
located on the tooth at least 80% of the length of the tooth from
the tip of the tooth.
10. The rail of claim 1, wherein the rail is composed of silicon
carbide.
11. The rail of claim 1, wherein the rail is formed as a monolithic
structure.
12. A wafer carrier for supporting a plurality of semiconductor
wafers, the carrier comprising: at least one generally planar
plate; at least two support rails, each support rail having a
vertical axis, each support rail being mounted with its vertical
being generally normal to the plate; and wherein each support rail
has a plurality of teeth arranged in a vertical stack, the teeth
extending parallel to each other and generally parallel to a plane
of the plate, each tooth having raised support structure thereon,
the support structure comprising an upper surface spaced above a
top surface of the tooth and sidewalls connecting the upper surface
to the top surface, a radius being formed at an intersection of at
least selected sidewalls and the upper surface.
13. The wafer carrier of claim 12, wherein a radius is formed at
each intersection of each of the sidewalls and the upper
surface.
14. The wafer carrier of claim 12, wherein the length of each tooth
is greater than 25 millimeters.
15. The wafer carrier of claim 12, wherein the length of each tooth
is between about 30 and about 100 millimeters.
16. The wafer carrier of claim 12, wherein the radius is at least 1
millimeter.
17. The wafer carrier of claim 12, wherein the radius is at least 1
millimeter and not greater than 2.5 millimeter.
18. The wafer carrier of claim 12, wherein each support structure
extends at least approximately 70% of the length of each tooth.
19. The wafer carrier of claim 12, wherein each support structure
is a wedge-shaped protuberance running along one side of one of the
teeth.
20. The wafer carrier of claim 12, wherein each support structure
runs continuously from the front tip of one of the teeth to a point
located on the tooth at least 80% of the length of the tooth from
the tip of the tooth.
21. The wafer carrier of claim 12, wherein each support rail is
composed of silicon carbide.
22. The wafer carrier of claim 12, wherein each support rail is
formed as a monolithic structure.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] This invention generally relates to vertical carriers or
boats for holding semiconductor wafers during heat processing and
particularly relates to carriers designed to effectively support
large semiconductor wafers having nominal diameters equal to or
greater than about 200 millimeters, the invention includes features
for reducing crystal dislocations during processing.
[0003] 2. Description of the Related Art
[0004] Semiconductor wafers, especially those made of silicon, may
be conventionally processed by placing them horizontally into a
holding device or carrier at intervals in the vertical direction
and exposing the wafers' surfaces to high temperature gases in a
furnace to form an oxide film on these surfaces or to deposit films
such as nitride and polysilicon, or to anneal the wafers after ion
implantation. To maximize the amount of surface area exposed to the
heat treatment, the wafers are usually held in "boats," or
carriers, typically comprised of parallel vertical supports, or
rails, having relatively short slots evenly spaced along their
length. The slots in one support are normally aligned with slots in
the other supports so a corresponding slot in each support can
jointly receive a wafer. By placing wafers in appropriate slots on
the supports, the boat can carry a stack of wafers separated from
each other so that both sides of the wafer are exposed to the heat
treatment.
[0005] In the past, conventional vertical boats and carriers have
been designed to support wafers having nominal diameters of 200
millimeters (mm) or less. Wafers of this size are typically
supported by slots on the vertical rails that extend inward around
the edge of the wafer only a very short distance, usually less than
about 20 mm. Unfortunately, when such a design is utilized to
support larger wafers, i.e., wafers having a diameter greater than
about 200 millimeters, the wafers are deflected by their own weight
and tend to sag.
[0006] As the temperature in the furnace rises, this sagging or
deformation results in crystal dislocation, or "slip," and other
stresses on the wafer. Although "slip" typically begins to occur at
about 1200.degree. C. for wafers having nominal diameters of 200
mm, it may occur at a temperature of 1000.degree. C. or less for
wafers having diameters of 300 mm or larger. Crystal dislocations
caused by stresses on the wafers result in a decrease in the number
of chips that can be made on a wafer. This reduction in product
yield increases with wafer size, and therefore the processing of
larger wafers in conventional vertical boats has been generally
avoided.
[0007] Various techniques have been suggested in an attempt to
decrease the bending stress on wafers. One method suggested is to
locate the rails or vertical supports of the boat or carrier more
toward the front of the carrier where the wafers are loaded. This,
however, is difficult because of the need for an unobstructed wafer
loading path.
[0008] U.S. Pat. No. 5,931,666 discloses providing each tooth
forming the slots with a rounded, downwardly sloping tip at its
inner end. The rounded end reduces stress concentrations in the
wafers that normally develop along the sharp edge at the end of the
upper surface of each tooth in prior art designs.
[0009] Another technique for decreasing bending stress on large
wafers is disclosed in U.S. Pat. No. 5,492,229, which teaches the
use of relatively long support teeth, i.e., the support arms formed
by long slits or slots on the support rail, with small contact pads
located at or near the end of the teeth for supporting the wafers
toward their center and not at their edges. According to this
patent, the contact pads or support projections are located such
that the inner portion of each wafer is supported by the pad while
the peripheral portion, i.e., the portion of the wafer which
extends from the edge of the wafer inward a distance of up to 10%
of the wafer's radius, does not contact the pads or arms. By
supporting the wafers at their inner portion, this design not only
reduces the stress on the wafer caused by its own weight but also
decreases heat stress caused by direct heat transfer to the wafer
from the slits in the vertical supports.
[0010] Although the above-discussed patent proposes the use of long
support arms or teeth in order to decrease stress on the wafer, the
wafer support is far from uniform as it relies on small contact
pads located at or near the end of the support arms, the pads
occupying only a small portion of the length of the support arm and
contacting only a small area of each wafer. Moreover, the design
shown in the patent results in reduced tooth strength caused by the
removal of material from the top of the tooth to form the small
support pads or projections.
[0011] An alternate design for carrier rails is disclosed in U.S.
Pat. Nos. 6,171,400 and 6,357,604, which are both based on the same
application and are incorporated herein by reference. FIG. 1 of the
attached figures shows a prior-art semiconductor wafer carrier 11
that comprises a bottom portion or plate 10 on which are mounted
four vertical supports or rails 12, 14. Rails 12, 14 extend upward
between bottom plate 10 and a top portion or plate, which is not
shown in the figures. Two rails 12 located on the left side of
carrier 11 are identical and are mirror images of rails 14 located
on the right side of carrier 11. FIG. 2 of the attached figures
comprises perspective views illustrating the details of each side
of rails 14. Generally, the design of bottom plate 10 and top plate
is dependent on the type of apparatus used to move carrier 11 in
and out of the furnace where the wafers are to be processed and on
the design of the furnace itself.
[0012] A wafer 16 is shown in FIG. 1 as a dotted line in its
appropriate position after being inserted into wafer carrier 11.
Generally, the design of carrier 11 depends upon the size of wafers
16 to be held and supported. Typically, the nominal diameter of
wafers 16 held in carrier 11 ranges from about 200 mm to about 400
mm, although other diameter wafers 16 can be accommodated if
desired. Wafers 16 of this size usually have a thickness that
ranges from about 0.5 to about 1.5 mm.
[0013] Referring to both of FIGS. 1 and 2, each support rail 12, 14
is mounted on an upper surface 18 of plate 10, with two rails 12,
14 being located on both sides of the lateral centerline 20 and the
longitudinal centerline 22. Each rail 12, 14 contains a plurality
of support arms or teeth 24, 26, respectively, which in turn define
slots 28 into which wafers 16 are inserted. Slots 28 are aligned so
that a single wafer 16 can be jointly received by a corresponding
slot in each rail 12, 14, thereby allowing carrier 11 to hold
wafers 16 in a stack.
[0014] Although FIG. 1 shows four support rails 12, 14 on carrier
11, it will be understood that carrier 11 may have as few as two
rails 12, 14. Although three or four support rails 12, 14 are
normally preferred from a point of view of support and cost of
fabrication of carrier 11, more rails 12, 14 may be used if
desired.
[0015] Rails 12 are attached to and located on the left side of
bottom plate 10, whereas rails 14 are located on the right side of
plate 10. Ideally, for the most uniform support of wafers 16, rails
12, 14 should be equally spaced, i.e., 90.degree., from each other
in a circle on bottom plate 10. Unfortunately, such an arrangement
does not permit placement of wafers 16 into carrier 11. In order
for there to be sufficient clearance to load wafers 16 into the
front of carrier 11, each rail 12, 14 located on the front portion
of plate 10 normally must be spaced between about 150.degree. and
about 175.degree. from each other when measured the short way
around bottom plate 10. When it is desired to utilize only three
rails 12, 14 for support, two of rails 12, 14 are located toward
the front of carrier 11 as shown in FIG. 1 while one rail 12, 14 is
located at the back of carrier 11. Typically, rails 12, 14 in the
front support between about 55% and about 90% of the weight of each
wafer 16.
[0016] The '400 and '604 patents disclose a support structure or
ledge 30 that runs along a side of each tooth 24, 26 from its front
tip 32 toward its back edge 34, a distance equal to at least 70%,
usually at least 80%, of the length of the tooth. Ledge 30 is
designed to provide support for wafer 16 usually from the outer
edge of wafer 16 inward to a point located from the center of wafer
16, a distance equal to between about 25% and about 80%, preferably
between about 45% and 60%, of the radius of wafer 16. Although it
is preferred that ledge 30 support wafer 16 from its edge inward,
the actual support may begin as far as 9% of the radius of wafer 16
from the edge of wafer 16, more preferably less than 5% of the
radius of wafer 16 from the edge of wafer 16.
[0017] Although ledge 30 is normally designed to provide continuous
support to wafer 16 from its edge inward, it is preferable that the
contact area with the underside of wafer 16 be as small as possible
in order to expose the maximum amount of surface area of wafer 16
to the heat treating process and to reduce heat transfer by thermal
conductivity to the bottom of wafer 16, heat transfer causing
non-uniform expansion and stress on wafer 16. Of course, the actual
surface area of the top of ledge 30 will depend upon the size of
tooth 24, 26, which in turn depends upon the size of wafer 16 being
supported. Typically, for wafer 16 having a nominal diameter
between about 200 mm and 400 mm, the surface area of the top of
ledge 30 will range between about 20 and 200 mm.sup.2, preferably
between about 30 and 120 mm.sup.2.
[0018] The height of ledge 30 is normally sufficient to allow gases
in the furnace to access the area between the top surface of teeth
24, 26 and the underside of each wafer 16. Typically, the height
ranges between about 0.25 mm and about 2.5 mm, preferably between
0.5 mm and 1.25 mm. The distance between the top of ledge 30 and
the bottom surface of the next higher adjacent tooth 24, 26 usually
ranges between about 0.75 and about 4.0 mm, preferably between
about 1.5 and 3.0 mm.
[0019] It has been found that, when ledge 30 supplies support to
each wafer 16 beginning at a point near the edge of wafer 16 and
continuing inward, stress caused by the weight of wafer 16 is
substantially reduced as compared to when support is supplied only
at the inner portion of wafer 16. Moreover, utilizing ledge 30 that
is integral with and occupies at least 50% of the length of each
tooth 24, 26 increases the strength of each individual tooth 24,
26.
[0020] While the methods described above reduce slip during
processing of the wafers, there is a need for an improved rail
design for a wafer carrier. The improved design would provide a
ledge that supports the wafer above the top surface of each tooth,
with each ledge having rounded edges for further limiting slip by
reducing stress concentrations.
SUMMARY OF THE INVENTION
[0021] A rail is provided for use as a support in an apparatus for
holding a plurality of semiconductor wafers. The rail has a
plurality of teeth arranged in a vertical column, such that the
space between a top surface of one tooth and a bottom surface of
the next higher adjacent tooth forms a slot for receiving a portion
of a semiconductor wafer. A support structure for supporting the
wafer is located on the top surface of substantially all teeth that
form the bottom of a slot, the support structure having sidewalls
and an upper surface spaced from the top surface. On each support
structure, a radius is formed at each intersection of the sidewalls
and the upper surface. The support structure extends for at least
approximately 50% of the length of each tooth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The novel features believed to be characteristic of the
invention are set forth in the appended claims. The invention
itself however, as well as a preferred mode of use, further objects
and advantages thereof, will best be understood by reference to the
following detailed description of an illustrative embodiment when
read in conjunction with the accompanying drawings.
[0023] FIG. 1 is a perspective view of a lower portion of a
prior-art wafer carrier having four rails.
[0024] FIG. 2 is a perspective view of two prior-art rails of FIG.
1.
[0025] FIG. 3 is a perspective view of a lower portion of a wafer
carrier having four rails, the rails being constructed according to
the present invention.
[0026] FIG. 4 is a perspective view of two of the rails of FIG.
3.
DETAILED DESCRIPTION OF THE INVENTION
[0027] FIG. 3 is a perspective view of a wafer carrier of the
invention, and FIG. 4 is a perspective view of one embodiment of
the rails of FIG. 3. Rails 36 are direct replacements for rails 12
described above, and rails 38, which have a mirror image of rails
36, are direct replacements for rails 14.
[0028] FIG. 3 shows wafer carrier 40, which comprises bottom plate
10 and four vertical rails 36, 38. Rails 36, 38 extend upward
between bottom plate 10 and a top plate (not shown). Two rails 36
located on the left side of carrier 40 are identical and are mirror
images of rails 38 located on the right side of carrier 40. FIG. 4
shows the detail of rails 36.
[0029] Referring to FIGS. 3 and 4, each rail 36, 38 is formed as a
plate-like structure, having a narrow width and an extended height.
A plurality of support arms or teeth 42 define slots 44 into which
wafers 16 are inserted. When rails 36, 38 are installed on carrier
40, slots 44 are aligned so that a single wafer 16 can be jointly
received by a corresponding slot 44 in each rail 36, 38, thereby
allowing carrier 40 to hold wafers 16 in a stack. A vertical
support 48 depends from the lowermost tooth 42, the lower ends of
vertical support 48 and vertical section 46 being adjacent upper
surface 18 of bottom plate 10.
[0030] Each tooth 42 extends laterally outward from a vertical
section 46, teeth 42 preferably being parallel and equally spaced
from each other. Though shown as having a rectangular vertical
cross-section, each tooth 42 may have a vertical cross-section
having a different shape, for example, a wedge or a semi-circular
shape. Likewise, though rails 36, 38 are shown as plate-like
members, rails 36 can have other shapes. For example, they may have
U-shaped or C-shaped cross-sections. The vertical height of rails
36, 38 is dependent upon the height of the furnace in which wafers
16 are to be treated. Typically, rails 36, 38 will vary in length
between about 0.5 and 1.5 meters, but are usually somewhere between
about 0.6 and about 1.0 meter in length.
[0031] The number of slots 44 in each rail 36, 38 depends upon the
number of wafers 16 to be held by carrier 40. This, in turn,
depends upon the size of the furnace to be used for heat treatment
and the separation desired between wafers 16 to adequately expose
both the top and bottom surfaces of wafers 16 to the heat
treatment. Normally, each rail 36, 38 contains between about 50 and
about 240 slots. For a typical size furnace, the number of slots 44
normally ranges between about 80 and about 160.
[0032] A support structure 50 runs along a side of each tooth 42
from its front tip 52 toward its back edge 54, a distance
preferably equal to at least 70%, usually at least 80%, of the
length of each tooth 42. As can be seen in FIGS. 3 and 4, support
structure 50 has a narrow width near back edge 54 and a relatively
larger width near front lip 52.
[0033] Support structure 50 is designed to provide support for
wafer 16 usually from the outer edge of wafer 16 inward to a point
located from the center of wafer 16, a distance equal to between
about 25% and about 80%, preferably between about 45% and 60%, of
the radius of wafer 16. Although it is preferred that support
structure 50 support wafer 16 from its outer edge inward, the
actual support may begin inward of the outer edge of wafer 16.
[0034] For reasons described supra, it is preferable that the
contact area with the underside of wafer 16 be as small as
possible. Wafers 16 rest on upper surface 56, which is spaced from
top surface 58 of each tooth. Sidewalls 60, 62, 64 extend between
upper surface 56 and top surface 58, forming a substantially
triangular, or wedge-shaped, support structure 50. To reduce the
stress concentrations on wafer 16 during thermal processing,
sidewalls 60, 62, 64 are rounded, forming a radius at the
intersection of each sidewall 60, 62, 64 with upper surface 56. The
radius is preferably 1 mm to 2.5 mm, though other values may also
be used. The actual surface area of the top of support structure 50
will depend upon the size of each tooth 42, which in turn depends
upon the size of wafer 16 being supported. The combination of the
raised support structure 50 and the radius along the intersection
of upper surface 56 and sidewalls 60, 62, 64 effectively reduces
slip beyond the reductions achieved when using either of the
techniques alone.
[0035] The height of support structure 50 is normally sufficient to
allow gases in the furnace to access the area between top surface
58 of teeth 42 and the underside of each wafer 16. Typically, the
height ranges between about 0.25 mm and about 2.5 mm, preferably
between 0.5 mm and 1.25 mm. The distance between upper surface 56
of support structure 50 and the bottom surface of the next higher
adjacent tooth 42 usually ranges between about 0.75 mm and about
4.0 mm, preferably between about 1.5 mm and 3.0 mm.
[0036] Teeth 42 of rails 36, 38 respectively, are formed when slots
44 are cut into rails 36, 38, a process similar to that detailed in
the '400 and '604 references. The shape of the teeth typically
depends upon the shape of the plate-like member from which the
rails are fabricated. Although teeth 42, as shown in FIGS. 3 and 4,
are straight, teeth 42 may alternatively be wedge-shaped, tapering
outwardly from front tip 52 toward back edge 54. Teeth 42 can also
be curved, as is an arc of a circle. Generally, when the teeth are
straight, they range in length between about 20% to about 80% of
the radius of wafer 16, preferably between 40% and 60%. Normally,
the teeth are between about 20 and 150 millimeters long, preferably
50 to 100 millimeters.
[0037] Several advantages are realized with the present invention.
The rails of the invention provide raised support structures,
limiting the non-uniform expansion of semiconductor wafers during
heating that may increase stress and slip. In addition, the use of
long teeth provides support for a large portion of the radius of
the wafers, further limiting slip. Also, the use of rounded edges
at the upper surface of the support structure reduces stress
concentrations in this area, further reducing slip.
[0038] While the invention has been shown or described in only some
of its forms, it should be apparent to those skilled in the art
that it is not so limited, but is susceptible to various changes
without departing from the scope of the invention.
* * * * *