Apparatus for holding plates

The apparatus comprises a first holding member having a flat surface which is brought into contact with the nonprocess surface of the semiconductor wafer, a pair of second holding members having inclined surfaces which are spaced apart from said flat surface by a predetermined distance, and cooperate with the flat surface to receive the arcuated edge of the semiconductor wafer. The second holding members are spaced apart from each other in a direction perpendicular to a semiconductor wafer insertion direction. The inclined surfaces being inclined such that a distance between the inclined surfaces and the flat surface is decreased in the semiconductor wafer insertion direction, thereby clamping the arcuated edge portions of the semiconductor wafer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for holding plates such as 
semiconductor wafers having different sizes and thicknesses by a common 
holding portion. 
2. Description of the Related Art 
A conventional semiconductor wafer holding apparatus as a typical holding 
apparatus of this type is shown in FIGS. 1A and 1B. This holding apparatus 
vertically moves or conveys semiconductor wafers 1. A blade 2 constituting 
a holding portion has an arcuated upper edge 4 so as to match with a 
peripheral curve (almost an arc) of the wafer 1, as shown in FIG. 1A. A 
vertically tapered concave groove 3 is formed to receive and support the 
edge of the wafer 1 as shown in a sectional view of FIG. 1B along the line 
1B--1B of FIG. 1A. Therefore, the edge of the wafer 1 is held in the 
groove 3. The holding apparatus is used to take up the wafer and hold it 
in a vertical state or, to hold or move a plurality of wafers, for example 
twenty five wafers taken out from a wafer carrier or wafer boat, at the 
same time in a manner that the wafers are kept at regular intervals and in 
horizontal states. 
The groove 3 is set to have a width slightly larger than a maximum width of 
the wafer 1 in consideration of a tolerance. The holding apparatus 
described above presents the following various problems. 
A. Since the opening width of the groove 3 is considerably larger than a 
normal wafer thickness, the normal wafer cannot be stably supported at the 
upper end of the holding portion, resulting in poor positioning precision. 
B. In order to precisely match the width of the groove 3 with a wafer 
thickness, an allowance for a tolerance of groove formation becomes 
strict, and an exclusive jig for forming a groove is required, resulting 
in high cost. 
C. A process surface of a wafer is brought into contact with a side surface 
of the groove 3 although a contact area is small. 
D. Since there is no element for fixing a wafer, the wafer surface slightly 
vibrates in a direction parallel to the wafer surface and a direction 
perpendicular thereto. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a plate holding 
apparatus capable of improving holding precision of a plate, assuring 
versatility for appropriately holding plates regardless of their sizes and 
thicknesses, and reducing the manufacturing cost. 
The plate holding apparatus according to the present invention holds a 
plate by clamping it between a flat surface and an inclined surface 
opposite thereto. 
A plurality of inclined surfaces spaced apart from each other are 
preferably formed at positions opposite to the flat surface. In addition, 
the holding apparatus can also serve as a container. In this case, plate 
holding grooves of the container can be constituted by combinations of the 
flat surface and the inclined surfaces. 
The plate is held such that its one major surface is in contact with the 
flat surface, and edge portions of the other major surfaces are in contact 
with the inclined surfaces. At contact points (reaction points) of the 
plate on the inclined surfaces, a horizontal-component reaction force Rh 
of the weight of the plate acts as a force for urging the plate against 
the flat surface, and a rotational moment M for urging the plate against 
the flat surface is produced by an offset H between a gravitational 
direction of the plate and the reaction point, as shown in FIG. 2. 
Therefore, the plate is kept urged against the flat surface, and the 
center of the plate is not deviated from a reference position. The plate 
is not moved by external vibrations and is therefore appropriately held. 
The opposite surface portion of the holding portion is constituted by 
inclined surfaces. These inclined surfaces serve as guides for holding the 
plate. When the size, especially thickness, of the plate is changed, the 
positions of the reaction points are changed. The principle of holding the 
plates is not kept unchanged. Therefore, the plate holding apparatus 
according to the present invention has versatility which allows common use 
of the apparatus for various types of plates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A holding apparatus according to a preferred embodiment of the present 
invention applied to a semiconductor wafer push-up apparatus will be 
described in detail with reference to the accompanying drawings. 
The push-up apparatus vertically holds each, some, or all of a plurality of 
vertically held semiconductor wafers in a cassette, pushes the wafers 
upward (indicated by an arrow) by a driving mechanism 50 (FIG. 3B) and 
transfers them to a heating furnace loading susceptor. A wafer holding 
apparatus in the push-up apparatus will be described below. 
The holding apparatus comprises a vertical flat member 10 and two inclined 
surface members 20 which are spaced apart from each other in a direction 
perpendicular to a semiconductor wafer insertion direction and which are 
opposite to the vertical flat surface member 10, as shown in FIG. 3A. The 
two inclined surface members 20 are spaced apart from each other by a 
predetermined distance to be symmetrical about the vertical flat surface 
member 10. 
A large number of holding apparatuses are arranged in tandem with each 
other to constitute an assembly. A large number of wafers can be held by 
the assembly so as to be spaced apart from each other by predetermined 
intervals. When the assembly is moved, a large number of semiconductor 
wafers can be simultaneously moved. 
The semiconductor wafer 1 as an example of a plate has a diameter of 150 mm 
and is held such that an arcuated edge of the wafer 1 is clamped between 
the vertical flat surface member 10 and the opposite inclined surface 
members 20. The wafer 1 is set such that its process surface 1a is brought 
into contact with the inclined surface members 20. As shown in FIG. 3C 
along the line 3C--3C of FIG. 3B, a reaction point 5 as one point of the 
inclined surface member 20 is brought into contact with the wafer 1. 
Therefore, the inclined surface members 20 are not brought into contact 
with the process surface of the wafer 1, and the effective utilization 
area of the wafer 1 can be increased. 
The vertical flat surface member 10 comprises a stepped plate member, as 
shown in FIG. 3B. A rear surface (nonprocess surface) 1b of the wafer 1 
which is opposite to the process surface 1a is brought into contact with 
an upper vertical flat surface 11. The upper portion of the vertical flat 
surface 11 is constituted by a tapered surface 12, while the lower portion 
of the vertical flat surface 11 has two screw holes 13 for mounting the 
two inclined surface members 20. The stepped portion of the vertical flat 
member 10 has a horizontal flat surface 14 extending in a direction 
perpendicular to the vertical flat surface 11. 
The inclined surface members 20 are obtained by dividing a frustoconical 
member into substantially halves along two vertical surfaces perpendicular 
to each other. More specifically, each inclined surface member 20 
comprises a 1/4 sector plate having a flat surface portion whose upper 
surface is located at a corner and an inclined surface portion which is 
inclined downward along the edge from the flat surface portion. The 
inclined surface members 20 are mounted by Belleville screws 22 while the 
inclined surfaces 21 oppose the vertical flat surface 11 and the 
horizontal end faces are kept in contact with the horizontal flat surface 
14 of the vertical flat surface member 10. Accordingly the area below the 
reaction point 5, as seen in FIG. 3C, acts as an auxiliary inclined 
surface and is continuous with the inclined surface 21 above the reaction 
point 5. 
The above shape is determined due to manufacturing steps of the inclined 
surfaces 21. An inclined surface (this surface serves as inclined surfaces 
21) is formed on a disk-like member by a lathe and the member is divided 
into four members in consideration of the width of the holding apparatus. 
It should be noted that the inclined surface 21 may be an inclined surface 
of a frustoconical member as in the above embodiment or may be replaced 
with a flat surface. 
The vertical flat surface member 10 and the inclined surface member 20 may 
be made of various materials such as stainless steel, a material obtained 
by coating a ceramic material on stainless steel or a material obtained by 
coating Si0.sub.2 on stainless steel. In order to increase a 
pressure-receiving area and decrease a surface pressure (stress) when 
small chipping dust of the wafer 1 is produced at the reaction point 5 
between each inclined surface 21 and the wafer 1, a material having a low 
longitudinal modulus (Young's modulus) and a low hardness is preferably 
selected. 
A teflon-reinforced resin can be preferably used in favor of the above 
consideration. Ethylene trifluoride resin, hard vinyl chloride, a 
polyethylene-based high-density resin, or ABS resin can also be preferably 
used as a resin material having mechanical characteristics equivalent to 
or better than the teflon-reinforced resin. 
An angle of the inclined surface 21 of the inclined surface member 20 with 
respect to the vertical axis is minimized in order to increase a holding 
force for the wafer 1 (as will be described later). However, if the angle 
is extremely small, a very large force is required to remove the wafer 1, 
one end of which is held. 
Frictional coefficients of the flat vertical surface 11, the inclined 
surface 21, and the wafer 1 are considered as follows. When the frictional 
coefficient is expressed in the form of a tan (tangent) angle, a critical 
angle is given such that even if the wafer 1 is moved downward, it is not 
caught by the flat surface 11 and the inclined surfaces 21. A 
predetermined safety margin is generally added to the critical angle to 
determine the frictional coefficient. Assume that quartz glass is employed 
as the above material. Since a frictional coefficient of quartz glass is 
0.8, an angle formed by each inclined surface 21 and the horizontal 
direction is given as about 38.degree. .about. 39.degree. . Here, the 
angle between the inclined surface 21 and flat vertical surface 11 is 
about 51.degree. .about. 52.degree. . In consideration of the safety 
margin, horizontal component Ph an angle formed by the flat vertical 
surface 11 and each inclined surface 21 is determined to be 10.degree.. 
A general upper limit angle between the inclined surface 21 and the flat 
vertical surface 11 facing the surface 21 is preferably 20.degree. or less 
although the range slightly varies depending on materials. 
An operation will be described below. 
FIG. 2 dynamically shows the principle of operation of the holding portion 
constituted by the vertical flat surface 11 and the inclined surface 21. 
When the wafer 1 is inserted between the vertical flat surface 11 and the 
inclined surface 21, a reaction force R is produced by a weight W of the 
wafer 1 at the reaction point 5 as a contact position between the wafer 1 
and the inclined surface 21. The horizontal component Rh of the reaction 
force R is obtained to urge the wafer 1 against the vertical flat surface 
11. Therefore, it is possible to vertically hold the wafer 1 by the 
horizontal component Rh of the reaction force. 
The rotational moment M is produced by the offset H between the weight W of 
the wafer 1 and the reaction point 5. The rotational moment M acts to urge 
the wafer 1 against the vertical flat surface 11. 
A cooperative effect of the horizontal-component reaction force Rh and the 
rotational moment M allows vertical holding of the wafer 1 along the 
vertical flat surface 11. When the function of the inclined surface 21 and 
setting of the vertical flat surface are properly determined, a 
sufficiently large inclined surface reaction force R is obtained. 
Two holding portions constituted by the vertical flat surface 11 and the 
two opposite inclined surfaces 21 are formed to be aligned in the 
horizontal direction, as shown in FIG. 3B. Holding stability of the wafer 
in the horizontal direction can be achieved, and at the same time, the 
large vertical flat surface 11 which is in contact with the wafer 1 
surface can be assured. Thus, when the reaction point 5 serves as one 
fulcrum, vertical flat portions which are in contact with the wafer and 
which interpose the reaction point 5 therebetween serve as the other 
fulcrum against vibrations in a direction perpendicular to the surface of 
the wafer 1, thereby preventing vibrations of the wafer 1 and assuring a 
vertical holding function. 
The function of the inclined surfaces 21 does not only contribute to 
assurance of the holding force, but also to accurate reception and guide 
of the wafer 1 by the inclined surfaces 21 when the wafer 1 is inserted 
between the vertical flat surface 11 and the inclined surfaces 21 of the 
holding apparatus while being deviated from a proper position. 
According to a further effect of this embodiment, even if the size and 
thickness of the wafer 1 are changed, the wafer 1 can be vertically held 
although the positions of the reaction points 5 are changed. Therefore, 
one type of holding apparatus has versatility which allows holding of 
various types of wafers. 
Stability of the wafer 1 by a two-fulcrum support mechanism of the 
apparatus shown in FIGS. 3A through 3C against right-and-left movement 
will be described below. 
In a normal wafer holding state of this holding apparatus, the weight W of 
the wafer 1 is shared by the two reaction points 5 as W/2, as shown in 
FIG. 4A. 
As shown in FIG. 4B, in a balanced state immediately before the wafer 1 is 
about to move when an external force F such as a lateral load acts on the 
wafer 1, a balancing state of one reaction point 5 is given as follows: 
EQU Fr cos.theta.=WR sin.theta. 
where .theta. is the opening angle from the central line of the wafer to 
the reaction point 5. Condition F/W=tan.theta. is established from the 
above equation. 
The F/W value represents a ratio of an acceleration to G=980cm/sec.sup.2 as 
in the case wherein an acceleration is expressed with G (e.g., 0.5 of 
0.5G). A relationship between F/W and .theta. is shown in FIG. 4C. 
As shown in FIG. 4C, when the angle .theta. is increased, it is readily 
understood that the apparatus can withstand a large lateral load F. In 
this embodiment, the wafer 1 of a 150 mm diameter can be stably held even 
if the angle .theta. is set to be 21.degree. and a lateral load of about 
F=0.38 W or a lateral acceleration of about 0.38G is applied. However, if 
the angle .theta. is excessively increased, the wafer process surface may 
be adversely affected. Therefore, the angle .theta. falls preferably 
within the range of 20.degree. to 40.degree.. 
The present invention is not limited to the particular embodiment described 
above, and various changes and modifications may be made within the spirit 
and scope of the invention. 
The holding apparatus according to the present invention can be used to 
hold a plate in an inclined or horizontal state in addition to vertically 
holding the plate as in the above embodiment. In this case, if a 
horizontal flat surface 11 shown in FIG. 3A is held in an inclined or 
horizontal state, a sufficient holding force can be assured as compared 
with a conventional groove type apparatus although the holding force is 
smaller than the vertical holding force. 
The present invention is applicable to various devices for holding plates. 
In addition to a holding apparatus for holding semiconductor wafers, the 
present invention is applicable as a groove structure in a cassette for 
storing wafers and a convey boat. 
In this case, as shown in FIG. 5, when a plurality of grooves 25 are formed 
by a plurality of flat surfaces 25a and opposite inclined surfaces 25b in 
a cassette or boat, the same effect as in the above embodiment can be 
obtained. Equipment which employs the present invention can hold wafers 1 
without offset values. When the present invention is also applied to 
devices arranged at the inlet and outlet sides of the above equipment to 
transfer wafers 1, no transfer allowance is permitted and smooth transfer 
may not be performed. Therefore, it is preferable to hold the wafers in 
the inlet- and outlet-side devices with a given allowance. 
According to the present invention as has been described above, the plates 
can be positioned and held with high precision. In addition, the holding 
apparatus has versatility for allowing holding of plates even if their 
sizes and thicknesses are changed. 
Furthermore, the frictional force produced between the holding portion 
reaction point and the opposite surface serves as a restraint force for 
the plates. Therefore, a plate convey apparatus which can withstand 
vibrations can be obtained. 
The holding capacity of plates can cover the range from the vertical to the 
horizontal state with higher precision than that of the conventional 
apparatus. Therefore, the plate holding apparatus according to the present 
invention can be used in a variety of applications.