Wafer transfer device

A wafer transfer device is intended to transfer wafers between a wafer boat in which a plurality of wafer support plates are vertically arranged at regular intervals and a cassette in which a plurality of wafer-mounted levels are vertically arranged at regular intervals. Each of the wafer support plates is ring-shaped having an opening in the center thereof and a passage is defined by openings of the wafer support plates, extending vertically in the boat. The wafer is horizontally transferred into and out of the wafer boat between the wafer support plates by a fork. The fork can be moved in vertical and horizontal directions and it can also be swung. A wafer push-up disk is arranged movable up and down through the passage in the boat. Three wafer supports are projected from the top of the push-up disk. These projections on the push-up disk are arranged contactable with the underside of the wafer without interfering with the fork. The fork and the push-up disk are associated with each other by a controller to achieve the transferring of wafers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a wafer transfer device and more 
particularly, a device for transferring wafers between wafer boat and 
cassette. 
2. Description of the Related Art 
A wafer housing case which is called carrier or cassette is usually used to 
transfer semiconductor wafers and others in the course of manufacturing 
semiconductor devices. 
The wafer cassette is made of resin, for example, light in weight and low 
in cost, and it is designed to house plural or 25 sheets of wafers 
therein. 
When a plurality of wafers are to be processed as a batch by the heat 
process system, they cannot be heat-processed under the condition that 
they are kept housed in the resin-made cassette. It is therefore usually 
needed that they are heat-processed after they are transferred into the 
heat processing wafer boat made of quartz, for example, chemically stable 
and excellent in heat resistance. 
As shown in FIGS. 1 and 2, a plurality wafer-mounted levels are vertically 
arranged in a wafer boat 10 so as to process a plurality of wafers as a 
batch. A ring-shaped support plate 11 having a recessed support face 11a 
thereon (see FIG. 2) has been used these days as the wafer-mounted level, 
because the circumferential portion of the wafer can be surrounded by the 
ring-shaped support plate 11. When the wafer is supported in this manner, 
its temperature can be uniformly lowered and raised so as to form a film 
of uniform thickness on the wafer. 
When the wafer is transferred between the ring-shaped support plate and a 
transfer fork, however, the up- and down-movement of the fork is disturbed 
by the ring-shaped support plate. This makes it necessary to move the 
wafer up and down independently of the fork. To meet this need, an 
inventor of the present invention has provided in U.S. patent application 
07/572,005 (now U.S. Pat. No. 5,162,047 issued Nov. 10, 1992) double fork 
structure in which two wafer transferring and pushing-up forks are used. 
When this double fork structure is employed, however, intervals between the 
wafer-mounted levels or ring-shaped support plates 11 must be made large 
to insert it. 
SUMMARY OF THE INVENTION 
The object of the present invention is therefore to provide a wafer 
transfer device capable of making smaller the intervals between 
ring-shaped wafer support plates in the heat processing wafer boat. 
According to the present invention, there can be provided a wafer transfer 
device for transferring wafers into and out of a wafer boat in which a 
plurality of wafer support plates are vertically arranged having intervals 
between them, the ring-shaped wafer support plates being ring-shaped and 
defining a vertically-extending passage by center openings thereof, said 
wafer transfer device comprising: carrying means for horizontally carrying 
the wafer into and out of the wafer boat between the wafer support plates; 
a wafer push-up disk arranged movable up and down through the passage; 
wafer support means projected from the top of the push-up disk and 
arranged contactable with the underside of the wafer without interfering 
with the carrying means; drive means for driving the push-up disk up and 
down; and means for controlling the carrying means and the push-up disk 
such that they can be associated with each other. 
According to the present invention, the underside of the wafer is released 
from the ring-shaped wafer support plate in the wafer boat by the push-up 
mechanism. This enables a gap, into which the carrying means is inserted, 
to be provided between the under-side of the wafer and the support plate. 
Further, the push-up mechanism is arranged movable up and down through the 
passage defined by center openings of the support plates. This makes it 
unnecessary to insert the wafer-lifting means into the wafer boat between 
the support plates. The intervals between the support plates in the boat 
can be thus made smaller. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims. dr 
BRIEF DESCRIPTION OF THE DRAWINGS 
The accompanying drawings, which are incorporated in and constitute a part 
of the specification, illustrate presently preferred embodiments of the 
invention and, together with the general description given above and the 
detailed description of the preferred embodiments given below, serve to 
explain the principles of the invention. 
FIG. 1 is a sectional view showing a wafer boat loaded in a 
pressure-reduced CVD system of the vertical type which is an example of a 
heat process system; 
FIG. 2 is a perspective view showing a part of the wafer boat enlarged; 
FIG. 3 is a plan view showing how each member of the heat process system is 
positioned with reference to a position just under the heating furnace and 
another position where the wafer transfer device of the present invention 
is applied; 
FIG. 4 is a perspective view showing the wafer transfer device according to 
a first embodiment of the present invention; 
FIG. 5 shows the wafer boat mounted on a stand when viewed from the top; 
FIG. 6 is a sectional view taken along a line VI--O--VI in FIG. 5; 
FIG. 7 is a perspective view showing in detail a drive section for a wafer 
push-up mechanism of the wafer transfer device according to the first 
embodiment; 
FIGS. 8A and 8B show how the wafer push-up mechanism of FIG. 7 is operated; 
FIG. 9 is a perspective view showing a wafer transfer fork; 
FIG. 10 is a perspective view showing an example of the manner of making 
the fork; 
FIG. 11 is a perspective view showing another example of the manner of 
making the fork; 
FIGS. 12 through 14 are sectional views taken along a line XII--XII in FIG. 
5 and showing how the wafer is transferred into the wafer boat; 
FIG. 15 is a sectional view showing how the fork is related to a push-up 
disk; 
FIG. 16 is a perspective view showing the conventional fork which is used 
as a comparison example; 
FIG. 17 is a perspective view showing the wafer transfer device according 
to a second embodiment of the present invention; 
FIG. 18 is a perspective view showing in detail a drive section for a wafer 
push-up mechanism of the wafer transfer device according to the second 
embodiment; and 
FIGS. 19A through 19C schematically showing how the wafer push-up mechanism 
of FIG. 18 is operated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a sectional view showing a wafer boat 10 loaded in the 
pressure-reduced CVD system of a vertical type which is an example of the 
heat process system. A heating furnace 1 includes an outer shell made of a 
metal plate 2a and a heat insulating layer 2b, and a heater 2c arranged 
along the inner wall of the outer shell. A cylindrical reactor tube 3 is 
erected in the heating furnace 2. A manifold 4 made of stainless steel, 
for example, is connected to the lower end of the reactor tube 3. Gas 
supply pipes 8a and 8b through which film forming gases are introduced 
into the reactor tube 3 are connected to the manifold 4. An exhaust pipe 9 
through which gases are exhausted outside after the reaction is also 
connected to the manifold 4. 
The wafer boat 10 is supported on a heat insulating sleeve 6 such that its 
lower end portion is inserted into the sleeve 6. The heat insulating 
sleeve 6 is on a turntable 6a. A rotating shaft 6b of the turntable 6a is 
connected to a drive mechanism in a support 7a of a lifter unit 7. A cap 5 
is attached to a base 6c of the rotating shaft 6b. When the lifter unit 7 
is moved up and down, the open bottom of the manifold 4 is closed and 
opened by the cap 5. 
FIG. 2 is a perspective view showing a part of the wafer boat 10 enlarged. 
The wafer boat 10 comprises four columns 12 and plural or 60 sheets of 
ring-shaped wafer support plates 11. Each of the support plates 11 has a 
wafer support face 11a which is so recessed as to keep the top surface of 
the wafer aligned with that of the wafer support plate 11 when the wafer 
is supported on the wafer support plate 11. 
The wafer boat 10 which is intended to hold the 6-inch wafers therein has 
the following dimensions: 
The outer diameter of each ring-shaped wafer support plate: .phi.156 mm, 
the inner diameter of each ring-shaped wafer support plate: .phi.123 mm, 
the outer diameter of the wafer support face: .phi.153 mm, 
the thickness of each ring-shaped wafer support plate: 3 mm (however, that 
of the wafer support face: 2 mm), and 
the intervals between the adjacent ring-shaped wafer support plates: 9.525 
mm. 
FIG. 3 is a plan view showing how each member of the heat process system is 
arranged with reference to a position P1 just under the heating furnace 
and another position P2 where the wafer transfer device of the present 
invention is applied. After the wafers which have been held in the wafer 
boat 10 as shown in FIG. 1 are heat-processed, the wafer boat 10 is 
lowered to the position P1 by the lifter unit 7. It is then released from 
the heat insulating sleeve 6 by a swing arm 1 and carried from the 
position P1 to the position P2 where it is mounted on the stand 15 (see 
FIG. 4). The wafers which have been processed are transferred from the 
wafer boat 10 to a cassette 50 by the wafer transfer device of the present 
invention which includes a transfer mechanism 30. 
On the other hand, the wafers which are not processed yet are transferred 
from a cassette 50 to the wafer boat 10 on the stand 15 at the position P2 
by the wafer transfer device of the present invention. The wafer boat 10 
is then released from the stand 5 by the swing arm 1 and carried from the 
position P2 to the position P1 where it is mounted on the heat insulating 
sleeve 6. It is loaded into the reactor tube 3, as shown in FIG. 1, by the 
lifter unit 7. 
FIG. 4 is a perspective view schematically showing the wafer transfer 
device according to a first embodiment of the present invention. This 
wafer transfer device of the present invention includes push-up and 
transfer mechanisms 20 and 30 which are positioned adjacent to the wafer 
cassette 50. The push-up and transfer mechanisms 20 and 30 are controlled 
and operated by a controller 35 and a computer system 36, associating with 
each other. 
The wafer cassette 50 is mounted on a cassette mount 55. It has plural or 
25 mount levels which are arranged at the same pitch in the vertical 
direction and the wafers are placed on these mount levels in it. 
The transfer mechanism 30 includes a transfer fork 31, a slide block 32 and 
a rotary base 33. The transfer fork 31 is a plate having a top surface 
contacted with the underside of the wafer, and it is arranged on the slide 
block 32 so as to slidably reciprocate in a direction X. The transfer fork 
31 can be swung in a direction .theta. and further reciprocated in a 
direction Z by the rotary base 33. 
FIG. 5 shows the wafer boat 10 mounted on the stand 15 when viewed from the 
top, and FIG. 6 is a sectional view taken along a line VI--O--VI in FIG. 
5. 
Grooves 13 are formed on each of the columns 12 of the wafer boat 10 at 
regular intervals and each of the ring-shaped wafer support plates 11 is 
supported by the columns 12 such that it is fitted into one of the grooves 
13 of the columns 12 which are at a same level. Each of the wafers is 
supported on the support plate 11 with its underside contacted with the 
wafer support face 11a. 
The wafer push-up mechanism 20 includes a drive section 23 housed in the 
stand 15, and a push-up disk 21 movable through a space or passage which 
is defined by openings of the ring-shaped wafer supports 11 of the boat 
10. The push-up disk 21 has three projections 22 which are contacted with 
the underside of the wafer to support the wafer on them. 
FIG. 7 is a perspective view showing a drive section 28 for the wafer push 
up mechanism 20 in detail. The drive section 28 comprises a motor 28m, a 
screw rod 28l rotated by the motor 28m. a guide 28g, and a first lifter 
plate 28a connected to the wafer push-up disk 21 and provided with a ball 
screw mechanism. The motor 28m and the guide 28g are housed in a 
cylindrical cover 29 (see FIG. 4). 
FIGS. 8A and 8B are intended to schematically show how the wafer push-up 
mechanism 20 of the first wafer transfer device is operated. As shown in 
FIGS. 8A and 8B, the wafer push-up disk 21 can be moved between its lower 
limit position (FIG. 8A) where it is waiting in the stand 15 and its upper 
limit position (FIG. 8B) where it is pushed up by the drive section 28. 
FIG. 9 is a perspective view showing the wafer transfer fork 31 in detail. 
This wafer transfer fork 31 is a plate, 210 mm long, 56 mm wide and 1.0 mm 
thick, made of alumina and intended to transfer 6-inch wafers. 
Wafer supports 61-64 having circular tops 61a-64a are projected from the 
top surface 60a of a body 60 of the fork 31. When the wafer is to be 
transferred, the tops 61a-64a of the supports 61-64 are contacted with the 
underside of the wafer to support the wafer on them. Each of the tops 
61a-64a of the supports 61-64 has a diameter of 10.5 mm and it also has a 
height h.sub.1 of 0.3 mm when measured from the top 60a of the fork body 
60. It is not necessarily shaped like a circle but it may be shaped like a 
rectangle. 
The wafer supports 61-64 are formed as follows: As shown in FIG. 10, 
recesses U.sub.1 -U.sub.4 are formed on the top 60a of a fork body 
material 60x. Column-shaped wafer support material 61x-64x are 
press-fitted into the recesses U.sub.1 -U.sub.4 and CVD coating process is 
then applied to these wafer support materials 61x-64x including their tops 
61ax-64ax. Or, as shown in FIG. 11, wafer support materials 61y-64y having 
those tops 61ay-64ay to which the CVD coating process has been applied may 
be bonded to the top of a fork body material 60x by adhesive. 
In the case of the fork 31, the CVD coating process of SiC is applied to 
the wafer supports 61-64 including their tops 61a-64a. A CVD coat layer 
which is composed of fine crystals of SiC is therefore formed on the whole 
surface of each of the wafer supports 61-64, thereby making their surfaces 
smooth like a mirror. The CVD coat layer of SiC has so high hardness that 
it cannot be broken by its contact with the wafer while transferring the 
wafer. In addition, it has extremely high purity and the amount of 
impurities such as heavy metal contained in it is smaller than that in 
quartz of which the wafer boat is made. Further, it is more advantageous 
particularly in that it contains extremely little of natrium which adds 
bad influence to the semiconductor wafer. The thickness of the CVD coat 
layer is about 50 .mu.m, for example. 
Reference numeral 65 in FIG. 9 denotes a cut-away portion provided at one 
end of the fork 31 and another reference numeral 66 represents members for 
preventing the wafer from being shifted from its position on the fork 31. 
These members are column-like projections located adjacent to their 
corresponding wafer supports 61-64 and having a diameter of 5 mm and a 
height h.sub.2 of 0.7 mm when measured from the top 60a of the fork body 
60. The wafer is supported at its rim portion by these stopper members 66 
not to shift from its position on the fork 31 while being transferred. 
According to the fork 31 of the first wafer transfer device, the wafer 
supports 61-64 including their tops 61a-64a which are contacted with the 
underside of the wafer are coated with SiC according to the CVD manner. 
Therefore, their surfaces can be made like a mirror, having high 
smoothness. In addition, their areas with which the underside of the wafer 
is contacted are quite small in total. This can reduce the creation of 
dust not to contaminate the surface of the wafer which is on the way of 
its being transferred. 
The fork 31 may be variously modified. For example, its whole surface may 
be coated like a mirror according to the CVD coating process. Further, it 
is preferable that the CVD coat layer contains Si from the viewpoint of 
preventing the wafers from being contaminated. Therefore, SiO.sub.2, SiN 
or others may be used as well. 
An example of the process of transferring the wafers from the wafer 
cassette 50 to the boat 10 will be described with reference to FIG. 4 and 
FIGS. 12 through 15. FIGS. 12 through 14 are sectional views taken along a 
line XII--XII in FIG. 5 and some of components are omitted for the sake of 
simplicity in them. 
1. The slide block 32 is set to oppose the fork 31 to the cassette 50. The 
fork 31 is then moved in the direction X or toward the cassette 50 and 
positioned under the wafer W. 
2. The slide block 32 is lifted to move the fork 31 in the direction Z. The 
tops 61a-64a of the wafer supports 61-64 on the fork 31 are thus contacted 
with the underside of the wafer W to support the wafer W on them or on the 
fork 31. 
3. The fork 31 is retreated in a direction reverse to the direction X to 
take the wafer W out of the cassette 50. 
4. The slide block 32 is swung in the direction .theta. to oppose the fork 
31 to the boat 10. 
5. The fork 31 on which the wafer W is supported is inserted between the 
support plates 11 (or 111) and 11 (or 112) of the boat 10, as shown in 
FIG. 12. 
6. The push-up disk 21 of the push-up mechanism 20 is lifted until the 
wafer W can be held on the projections 22 of the disk 21. The wafer W is 
thus supported on the tops of the projections 22 while the tops 61a-64a of 
the wafer supports 61-64 on the fork 31 are separated from the underside 
of the wafer W. FIG. 15 shows this state wherein the boat 10 is sectioned 
along a plane parallel to the face of the wafer W. As shown in FIG. 15, 
the projections 22 on the disk 21 are projected above the fork 31 through 
the cut-away portion 65 and on both sides of the fork 31 without 
contacting it. 
7. The fork 31 is retreated from the boat 10 in the direction reverse to 
the direction X. Only the push-up disk 31 is thus left in the opening of 
the ring-shaped support plate 11, as shown in FIG. 13. 
8. The push-up disk 21 is lowered. The wafer W is thus supported on the 
support plate 11 (or 112), as shown in FIG. 14. 
When the above procedure is repeated relative to every wafer on the support 
plate 11 of the boat, the wafers can be transferred from the cassette 50 
to the boat 10. 
When the above procedure is reversely repeated, the wafers which have been 
processed can be transferred from the boat 10 to the cassette 50. In 
short, the wafer on the support plate 11 is pushed up by the push-up 
mechanism 20 to release the underside of the wafer from the support plate 
11 and the fork 31 is inserted into a gap between the wafer and the 
support plate 11 to support the wafer on the tops 61a-64a of the wafer 
supports 61-64 thereof. The wafer thus supported on the fork 31 is then 
taken out of the boat 10. 
According to the wafer transfer device of the present invention, the 
underside of the wafer is separated from the support plate 11 of the boat 
10 by the push-up mechanism 20. The gap into which the fork 31 is inserted 
can be thus provided between the underside of the wafer and the support 
plate 11. 
Further, the push-up mechanism 20 is arranged to move up an down through 
the space or passage S which is defined by the openings of the plural 
ring-shaped wafer support plates 11. This makes it unnecessary to make 
large the intervals between the support plates 11 of the boat 10. In other 
words, the intervals can be kept smaller to thereby enable a larger number 
of the wafers to be processed every batch process without making the boat 
10 large-sized. 
The following test was conducted to study the effect of the CVD coat layer 
applied to the fork 31. 
The transferring of wafers was carried out by the wafer transfer device 
provided with the fork 31 and the number of particles stuck to the surface 
of each of the wafers thus transferred was measured. For comparison, same 
measurement was made about those wafers which had been transferred by the 
wafer transfer device provided with an alumina fork (material: Al.sub.3 
O.sub.3 (A479-SS) made by Kyosera Corp.). This alumina fork was shaped, as 
shown in FIG. 16, having wafer support faces 91. 
The transferring of wafers and the measuring of the number of particles 
stuck were carried out in a clean room (the class of cleanness: 10). The 
number of particles stuck was measured by Surfscan 5500 (made by Tencor 
Instruments). The target of this Surfscan 5500 was those particles which 
were larger than 0.2 .mu.m. 
The slide block 32 was reciprocated in directions X and Z without being 
swung in the direction .theta. to thereby transfer the wafers into and out 
of the cassette 50. The stroke and the speed of the slide block 32 driven 
in the direction X were 240 mm and 30 mm/sec.. Those in the direction Z 
were 4 mm and 4 mm/sec.. 
One cycle comprised exchanging the wafer with a new one, measuring the 
number of particles adhering to the new wafer, transferring the new wafer 
into and out of the cassette 50, and measuring the number of particles 
stuck to the wafer thus transferred. Average values were obtained after 
500 and 5000 cycles. Results thus obtained were as shown in Table 1. 
TABLE 1 
______________________________________ 
Number of Particles 
Area of wafer 
Stuck 
Contacted (mm.sup.2) 
500 Cycles 5000 Cycles 
______________________________________ 
Our Fork 78.5 15.4 16.8 
Fork of 602.5 28.4 19.9 
FIG. 16 
______________________________________ 
As shown in Table 1, the number of particles stuck because of dust created 
was smaller in the case of the fork of the present invention than in the 
case of the conventional fork. 
FIG. 17 is a perspective view schematically showing the wafer transfer 
device according to a second embodiment of the present invention. FIG. 18 
is a perspective view schematically showing a drive section for a push-up 
mechanism of this wafer transfer device. Same components as those of the 
first wafer transfer device shown in FIG. 4 will be denoted by same 
reference numerals and description on these components will be omitted. 
The second wafer transfer device is different from the first one in that 
the drive section for the push-up mechanism 20 is of 2-stage type 
comprising first and second lifter means 23 and 25. 
The first lifter means 23 comprises a motor 23m, screw rod 23l rotated by 
the motor 23m, a guide 23g, and a first lifter plate 23a connected to the 
push-up disk 21 and provided with the ball screw mechanism. The motor 23m 
and the guide 23g are housed in a cylindrical cover 24 and fixed to one 
end of a second lifter plate 25a which will be described later. When the 
screw rod 23l is rotated, the first lifter plate 23a is moved along the 
guide 23g to thereby move the wafer push-up disk 21 up and down in 
relation to the second lifter plate 25a. 
The second lifter means 25 comprises a motor 25m, screw rod 25l rotated by 
the motor 25m, a guide 25g, and the second lifter plate 25a having the 
ball screw mechanism, which are housed in a cylindrical cover 26. The 
cylindrical cover 26 and the stand 15 have slits through which the second 
lifter plate 25a is moved up and down. When the screw rod 25l is rotated, 
the second lifter plate 25a is moved along the guide 25g to move the first 
lifter means 23 up and down. 
FIGS. 19A through 19C are intended to show how the push-up mechanism 20 of 
the second wafer transfer device is operated. 
FIG. 19A shows the push-up disk 21 waiting in the boat mounted stand 15 or 
kept at its lower limit level. FIG. 19B shows the push-up disk 21 lifted 
to its upper limit level by the first and second lifter means 23 an 25. 
FIG. 19C shows the wafer W pushed up by the push-up mechanism 20. 
In FIG. 19B, Z.sub.1 denotes the stroke of the lifter plate 23a in the case 
of the first lifter means 23 and Z.sub.2 the stroke of the lifter plate 
25a in the case of the second lifter means 25. The sum (Z.sub.1 +Z.sub.2) 
is equal to the stroke of the lifter plate 28a driven by the drive section 
28 in the wafer transfer device according to the first embodiment and 
shown in FIGS. 4, 8A and 8B. In short, the push-up mechanism of the second 
wafer transfer device is driven by the drive section of the 2-stage type. 
Therefore, the space the second push-up mechanism occupies can be made 
smaller. This is a merit in addition to those achieved by the first wafer 
transfer device. 
When the stroke of the lifter plate driven by the drive section of the 
push-up mechanism is long, the wafer transfer device and the heat process 
system of the vertical type provided with this wafer transfer device are 
disadvantageous on the basis of a limit in height upon their being 
transported and carried into a clean room. This may make it impossible to 
provide the space the push-up mechanism occupies. Particularly when the 
wafers are to be transferred into and out of the high wafer boat of the 
vertical type. it is quite difficult to provide the vertically-extending 
space the push-up mechanism occupies. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and representative devices shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.