Wafer boat

Wafer supporting method and apparatus having at least two wafer supporting slots for each edge-wise oriented wafer in a plurality of coaxial edge-wise aligned wafers. The two wafer locations are essentially at the bottom and at an upper "side" i.e., slightly below the widest diameter dimension of and edge-wise wafer with a pair of spaced apart upper slots having the same loose fitting cross-sectional shape as the lower slot. The supported wafer at the upper location touches the slot support at one slot wall only. A cross sectional shape of the slots for each wafer includes: (a) a rounded bottom wafer-supporting wall having a diameter that is essentially the same as the wafer's thickness; (b) an angle-controlling side wall; and (c) a third gravity feed wall that joins the tight fitting bottom to a wide loose fitting wafer entry slot opening. Close spacing between adjacent slots is provided by a pair of slot walls that are parallel to each other and are at the wafer lean angle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to wafer handling apparatus and a method for 
manufacturing same. Thin wafers such as semiconductor masters are 
essentially circular and require coaxial alignment in wafer-carrying 
"boats". More particularly, this invention relates to a new and improved 
"boat" and a method of fabricating the boat so that a plurality of closely 
spaced wafer-supporting slots can be cut in a pre-assembled boat. 
2. Description of Related Art Including Information Disclosed Under 
.sctn..sctn.1.97-1.99 
The assignee of this invention has several patents relating to this general 
art area. The most relevant patent is U.S. Pat. No. 4,355,974 ('974). 
Additional patents and publications of relevance to this invention are 
cited in the '974 patent. In accordance with the cited '974 patent, slots 
are cut at an angle to the vertical into rods that are thereafter used to 
form a wafer boat that is used for handling and processing. 
Production yield is increased when angle-slotted boats are used and still 
further increases in production are achievable if the leaning angle of all 
the wafers is constant throughout the production life of a wafer boat. 
Such an art area has come to be known as angle-slotted boats to 
distinguish the Assignee's patented boats and the improved results 
achievable by the use thereof from the prior art's vertically-slotted 
boats. The invention of this application, however, is applicable to either 
type boat or related wafer handling device. 
A Japanese patent No. 53-153,861 cited in a Reexamination of the '974 
patent and many of the other references cited and discussed during the 
Reexamination proceedings discloses quite thoroughly the current state of 
the wafer boat art. In general the side (upper edge) and bottom (lower 
edge) slots in assignee's angle-slotted boats were formed in rods, or 
other wafer-supporting material, and the cuts which define the slots 
consisted of different configurations for the upper and lower slots. 
Several variations of different slot configurations for the bottom rods 
are shown in FIGS. 5 through 7 in the '974 patent. An additional prior art 
bottom slot configuration that has been in use by the assignee and which 
is not depicted in the '974 patent is in the nature of a small curved 
depression that is generally elliptically shaped. In accordance with the 
'974 patent, the side (upper) edges of the wafers are controlled in a 
desired leaning angle by sloped loosely cut slots and the wafer's weight 
was supported by a bottom rod which generally positioned the leaning 
wafers to provide an improved handling and processing operation. 
Applicant was the first to purposely make the upper slots wider than the 
thickness of the wafers to be supported and to divide the wafer-leaning 
function and the wafer-weight-supporting function into two separate 
locations, namely the loosely fitting upper and the wafer-positioning 
lower position. Additionally a pair of opposed upper slots for each wafer 
are purposely spaced wider apart than the widest expected width of the 
wafer so that the wafer has room for a side to side movement of the type 
that it will experience in expanding during the high temperature 
processing of a wafer batch. 
Cuts were first made in the rods and then the boats were assembled to 
fabricate the wafer boats of the '974 patent. Boat assembly involves high 
temperature glass fusion and the fusion creates slot alignment or slot 
deformation problems. It would be preferable if the slotting could be done 
in pre-assembled boats because the glass fusing of the slotted rods may 
create such alignment and deformation problems which are caused by high 
temperatures that are encountered during the assembly process. Slotting of 
pre-assembled boats, in contrast, avoids these problems. 
During use the slotted boats are subject to high semiconductor-processing 
temperatures and are repeatedly acid washed between furnace firings for 
cleanliness reasons that are well known to the art. An improved yield can 
be obtained when the slot angles are maintained at constant angles in 
spite of repeated acid washings. Most slot configurations do not assure 
that the coaxially-aligned wafers in the boat will constantly lean at a 
fixed angle with equal spacing between adjacent wafers after repeated acid 
washings. Moreover, the slot design problem is further complicated in that 
adjacent wafer slots must be very closely spaced to keep the production 
yield at a maximum with lowest possible production costs. 
Machine and computer control over X, Y and Z movements of the type 
necessary to slot angles in a slab or other type of wafer supporting 
material is well known. A plurality of essentially circular wafers which 
are coaxially aligned on a common center in a supporting surface, or boat, 
requires edge support at, or slightly below, the widest diameter position. 
This support allows use of wafer transfer combs of the type disclosed in 
the assignee's patent No. 4,568,234 issued Feb. 4, 1986, and now pending 
in a re-issue Application having Ser. No. 080,437 filed on July 31, 1987. 
As noted above, slotting of vertical and angled slots in pre-assembled 
boats is a known art. The different slot configurations for side (edge) 
and bottom (weight) wafer contact locations for angle-slotted boats, or 
tightly fitting slots of the same configuration, has heretofore prevented 
successful slotting in such boats. 
This invention for the first time discloses loose fitting angle slotting of 
side and bottom wafer locations having the same cross-sectional slot width 
of about twice the wafer thickness for adjacent wafers in a pre-assembled 
water boat wherein the leaning wafers touch one wall only at their upper 
edges. Suffice it to say that highly efficient production has not been 
achieved through the use of slotted pre-assembled boats prior to the 
advent of this invention. Moreover, the same unique shape for side and 
bottom slots which provides a loose fit for supported gravity-fed wafers 
while still allowing the lean angle of all wafers to be controlled 
throughout repeated acid washings are key features of this invention. 
SUMMARY OF THE INVENTION 
This invention discloses and claims a wafer supporting apparatus having at 
least two wafer supporting slots for each edge-wise oriented wafer in a 
plurality of coaxial edge-wise aligned wafers. The two wafer locations are 
essentially at the bottom and at an upper "side" i.e., slightly below the 
widest diameter dimension of an edge-wise wafer with a pair of spaced 
apart upper slots having the same loose fitting cross-sectional shape as 
the lower slot. The supported wafer at the bottom touches the slot support 
at two places and the supported wafer at the upper location touches the 
slot support at one slot wall only. A cross sectional shape of the slots 
for each wafer includes: (a) a rounded bottom wafer-supporting wall having 
a diameter that is essentially the same as the wafer's thickness; (b) an 
angle-controlling side wall; and (c) a third gravity feed wall that joins 
the tight fitting bottom to a wide loose fitting wafer entry slot opening. 
Close spacing between adjacent slots is provided by a pair of slot walls 
that are parallel to each other and are at the wafer lean angle. The 
unique slot shape of this invention allows the slots to be cut into a 
pre-assembled boat in which the edge-wise supported wafers have side to 
side translational movement (such as that a wafer experiences in 
high-temperature expansion) and also allows a gravity feed from a wide 
slot opening by a slanted non-supporting wall into an arcuate-shaped slot 
bottom. That bottom slot has an arcuate shape selected to be essentially 
as wide as the width of the wafer's thickness. The arcuate shape is a semi 
circle that is located between a slanted feed wall and a wafer-leaning 
wall of the wafer slot. 
One surprising result of the invention's slot configuration is that at 
least a bottom and one or more side slots for each wafer in an 
angle-slotted boat can be cut concurrently into a pre-assembled boat. The 
method of the invention uses a blade with a cutting diameter that is 
slightly over-sized relative to the widest support locations chosen in the 
boat for supporting a plurality of edge-wise wafers. The wafers are fed 
into slot openings which are initially loose fitting and are fed naturally 
by a narrowing gravity feed wall until the wafers reach a snug tight 
fitting rounded slot bottom where they are supported at a common and 
controlled angle that still provides edge to edge and front to back 
translational movement during processing and handling.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
FIG. 1 shows one of several typical "boats" that was pre-assembled in a 
slot-free configuration. Boat 100 has a pair of essentially U-shaped front 
and rear end braces 110 that are joined at the top by a pair of horizontal 
spaced parallel rods 120, 130. At the bottom of the boat 100 is located a 
bottom rod 140. The three rods 120, 130, and 140 are suitably spaced to 
hold a batch of wafers of a predetermined size when wafer-supporting slots 
have been cut into the boat 100. These rods have their ends flush with, or 
extending slightly beyond the outer edge of braces 110 so that the slots 
in the boat can be contiguous with the slots in another boat when the 
other boat is butted end-to-end with boat 100. The additional rods 111, 
112 of boat 100 are for support and form a stable base for the boat. 
Several other types of known prior art boats, not shown, are to be 
understood as being within those that can receive the method and apparatus 
of this invention. These other types include those that, rather than 
three, include four or more inner rods (see FIG. 5, for example ) or those 
having a mix of flat or round rods which form a wafer supporting 
semi-circular "saddle". Other known boats include those one-piece type 
which have flat or curved slabs for receiving the wafer slots. Any of 
these prior art boat types may be slotted by known techniques similar to 
that of FIGS. 1 or 5. 
Saw blade 150 (FIG. 1) is mounted for high speed cutting into boat 100 at 
the forward end of a rotatable shaft 160 that is driven by any suitable 
motor (not shown). There are several known ways to cut slots into a boat 
and FIG. 1 shows a way that is called a plunge cut. In this operational 
method, the saw blade 150 is selected to be slightly larger in diameter 
than the diameter of the wafers to be handled by the boat. Any suitable 
X-Y-Z plotting table in conjunction with a numerically or 
computer-controlled machine will center the blade 150 along the geometric 
center 151 (shown dashed) of the boat 100. Software control guides the 
movement of blade 150 into and out of the boat 100. Blade 150 cuts closely 
spaced slots (see FIG. 1A) at a predetermined depth into the three rods 
120, 130 and 140. (The depth of the cut, of course, must not unduly weaken 
the rods). 
Relative positioning of the boat 100 and the axis of rotation of the blade 
150 may be obtained in any of several known ways. The boat 100 may be held 
during cutting in a jig that is wedge shaped at the complementary angle to 
.phi. while the blade 150 is moved into and out of the boat 100 in a 
vertical direction. Alternatively the boat may be positioned in a 
horizontal position and the blade moved in and out of boat 100. 
The width of the cutting edge (FIG. 1A) is chosen to be only slightly wider 
than the thickness of the wafer to be supported. The prior art philosophy 
was to use these tight fitting slots so that random leaning at different 
angles was avoided. Unfortunately these tight prior art slots suffer width 
and other degradation with repeated acid washings and thus the hoped-for 
goals of the tight fitting prior art slots were not achieved. 
FIG. 2 shows an enlarged partial cross section of the cutting periphery of 
an improved blade 150 embedded in a cutting position in one of the rods 
such as bottom rod 140. The abrasive wheel periphery 150A has its 
geometric center line 155 at a slight angle to the vertical 161. In 
accordance with the teaching of the '974 patent .phi. may be between about 
two to ten degrees or so. The unique saw blade cross section 150A achieves 
an improved production yield over the prior art because it does not suffer 
from wafer pinching and distortion caused by tight ill fitting slots or, 
from wafer production degradation, resulting from non-uniform leaning of 
wafers that is caused by acid washing. FIG. 4 depicts, in a 
cross-sectional view of the top of the bottom rod 140, a series of 
finished slots 20 ) after they have been cut by blade 150A. 
As shown in FIG. 4, each slot 200 at the upper entrance level includes a 
pair of inwardly directed bevels 201 that may be at an angle of about 45 
to 60 degrees from the slot's geometrically-centered axis 155. These 
bevels 201 lead downwardly into a pair of parallel slot walls (202 and 
203) that are spaced apart by about twice the thickness --2t-- of the 
wafer 250. The rightmost wall 203 is tangent to an arcuate-shaped bottom 
wall 204. Bottom 204 is defined by the lower portion of a circle having a 
radius of --t/2--, where t is the thickness of a wafer to be held in slot 
200. Feed wall 205 joins the bottom 204 to the leftmost wall 202. The 
longer wall 203 of the parallel slot walls is the wall against which the 
wafer will lean and thus it is at the same chosen angle .phi. as that 
selected for type angle-slotted boat. 
Feed wall 205 is selected at a counter-angle of about 30 degrees relative 
to the wafer leaning angle. Feed wall 205 swerves the function of guiding 
a wager as it is received into the slot 200 to the slot's bottom 204. 
Gravity will cause the wafer to drop to the lowest point at the rounded 
bottom 204 of slot 200 and that same gravity feed will assure that the 
wafers all lean at a common angle in the boat 100 as determined by a 
series of parallel walls 203. All of the slots 200 are identical and all 
of them cooperate together to control the non-wedged support that is so 
important in the wafer processing art. 
By wedge free, I mean that the leaning wafers have a translational freedom 
from side to side, and back and forth, at the upper support location. 
Moreover, the slots in weight supporting rods at the bottom, for 
wedge-free processing, must not induce wafer bending or binding during the 
handling or processing. An abrasive blade, such as a diamond wheel, may be 
shaped as desired. One or more passes can be used to create the slot shape 
of the invention. Such blades wear slowly and many cuts can be made before 
blade wear creates any problem in the dimension of the slots that are made 
in boat 100. It is nevertheless a problem since, as noted above, the slot 
diameter must be slightly larger than the wafer diameter to provide 
wedge-free processing and the blade wear reduces the precise tolerances 
that are required in this art. As an example the saw blade diameter of 
FIG. 1, when employing the unique shape of this invention, may be about 
0.050 inches larger than the average wafer diameter to be handled, leaving 
about 0.020 to 0.025 inches between the slot bottom and the outer edge of 
each leaning wafer in the opposed slots in each side rod 120 and 130 for a 
given wafer. 
In one particular embodiment of this invention, the saw blade, whether a 
single blade of the plunge cut type (FIG. 1 or FIG. 3), or a scribed swath 
(as shown in FIG. 5) is chosen larger than the wafer to provide slots that 
are loose fitting. In FIGS. 5 and 6, the centers of a saw blade swath and 
a wafer 250 are relatively offset in a vertical direction from one 
another. The wafer's center 455 is coincident with the boats longitudinal 
axis, such as axis 151 of FIG. 1. By offsetting these two centers, and 
selecting the widest swath of the cut to be greater than the wafer's 
diameter the wafer can drop to the slot bottom and yet be loose at the 
upper edges. Note that a gradual increase is provided between the wafer's 
contact at rods 420, 430 and the upper rods 410, 440. The combination of 
the center offset and selecting a slot cut's diameter to be greater than 
the wafer's diameter, and the slots width to be about twice the wafer's 
thickness, provides a loose and wedge-free support of the wafers in 
accordance with the method and apparatus of this invention. In some wafer 
processing known as back-to-back processing, two wafers occupy each slot 
and chemical processing does not occur on the touching back surfaces of a 
wafer pair in a single slot. In such an instance, the slot width must be 
increased by an amount equal to the extra wafer thickness, and the 
principles of this inventions are still applicable. 
In the operational approach of FIG. 5, also suitable for practicing the 
method of making a wafer boat in accordance with this invention, a smaller 
abrasive wheel 450 (with the desired slot-making shape of FIG. 2 machined 
therein) is turning at very high speed on a movable shaft 460. The shaft's 
movement is controlled in essentially a semicircular path 475 as shown in 
FIG. 5 between point 476 and 477. The smaller blade 450 moves in a 
semicircular arc within the boat's four rods 410, 420, 430 and 440. This 
movement sequentially cuts, for example, first rod 410, next rod 420, then 
rod 430 and finally rod 440. The arc is precisely controlled so that the 
depth of the slots is within the strict tolerances that are required. In 
this alternate approach, the wear on a blade can be compensated for by 
slight variations in the software control over the depth of the shaft's 
movement in the arc. In either approach, however, the individual wafer 
slots for co-axial alignment of a batch of wafers is cut into the material 
of a non-slotted boat in order to loosely support them in a wedge-free 
manner. 
Since all of the slots are identical they all tend to suffer uniformly in 
the acid washing and improved results are obtained by the method and 
apparatus of this invention. 
The above description presents the best mode contemplated in carrying out 
my invention. My invention is, however, susceptible to modifications and 
alternate constructions from the embodiments shown in the drawing and 
described above. Consequently, it is not the intention to limit the 
invention to the particular embodiments disclosed. On the contrary, the 
invention is intended and shall cover all modifications, sizes and scope 
of the invention, as in the appended claims when read in light of the 
description and drawings.