Holding apparatus for holding an article such as a semiconductor wafer

A holder which cleanly holds an article such as a semiconductor wafer and which is high in rigidity, light in weight, high in dimensional stability and excellent in dust resistance. The wafer holder includes a vacuum holding surface formed with a plurality of concentric or helical annular projections and annular vacuum holding grooves which are arranged at a given pitch. A plurality of vacuum holes for vacuum holding purposes are formed in the respective annular grooves so as to be arranged radially and each of the vacuum holes is subjected to pressure reduction by a vacuum source through the interior of the holder, thereby correcting the flatness of a wafer to conform with the upper surfaces of the annular projections. At least the portions of the holder which contact with the wafer (preferably the holder on the whole) are made of a sintered ceramic containing covalent bond-type conductive material such as a TiC-containing sintered Al.sub.2 O.sub.3 so that the contact portions exhibit conductivity and also less pores are present in the surface, thereby practically preventing the occurrence and deposition of fine particles.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a holding apparatus well suited for use in 
the transfer or vacuum holding of a thin sheet-type substrate such as a 
semiconductor substrate (wafer) or a glass plate. 
2. Description of the Prior Art 
In the manufacturing process of semiconductor integrated circuits, various 
holding apparatus are used for the purpose of transferring or firmly 
holding in position a wafer. For instance, in an exposure apparatus 
performing an important role during the lithographic operation, a 
substrate vacuum chuck (wafer holder) is used for the purpose of firmly 
holding a thin sheet-type wafer within a given plane. 
Generally, conventional holding apparatus of the above type includes 
transfer system components or a holder made of a metal or alumina ceramic 
(sintered Al.sub.2 O.sub.3). Also, Japanese Laid-Open Patent No. 2-186656 
proposes the use of a covalent bonding material such as diamond or 
sapphire for the portions which contact with a wafer so as to prevent the 
occurrence of fine particles due to abrasive wear. 
Then, the holding apparatus of the type used in the manufacturing process 
of semiconductor integrated circuits, for example, must satisfy the 
following characteristic properties (a) to (e). 
(a) Can prevent the deposition of fine particles (foreign particles) and 
the occurrence of chemical contamination of an article to be held: If 
foreign particles deposit on the wafer and the wafer supporting portions 
of the holding apparatus, this causes not only the breaking of connections 
and short-circuits in the transferred circuit pattern but also 
deterioration in the flatness of the wafer and hence deterioration in the 
resolution due to the deposited fine particles in cases where the holding 
apparatus is a wafer holder. 
(b) Is high in hardness: In order to prevent any abrasive wear at the 
portions which contact with an article to be held, the contacting portions 
should preferably be high in hardness. 
(c) Is high in rigidity/specific gravity value: In order to hold flat a 
thin-sheet wafer or to accurately transfer such wafer to a given position, 
the wafer supporting portions must be high in rigidity. In such case, the 
wafer supporting portions must be not only high in rigidity but also low 
in specific gravity in order to make the apparatus small in size and light 
in weight. 
(d) Is low in coefficient of thermal expansion: In the case of the wafer 
holder of an exposure apparatus, for example, the exposure results in the 
accummulation of heat. Thus, if the holder is high in coefficient of 
thermal expansion, this causes an increase in the dimensional variation by 
exposure of the wafer held by vacuum on the holder. As a result, in order 
to suppress any dimensional error threatening to occur during the transfer 
of circuitry, the coefficient of thermal expansion should preferably be as 
low as possible. 
(e) The working is easy: It is desirable that the grindability is so 
excellent as to produce a smooth surface and that the fine working 
required for vacuum holding is effected easily. 
However, these characteristic properties are not satisfied as yet by the 
previously mentioned conventional holding apparatus. 
To begin with, the holding apparatus of the type using the metal material 
for the wafer holding portions is not capable of avoiding the metal 
contamination of a wafer. Also, the metal material is not satisfactory in 
terms of wear resistance (hardness) and it is also disadvantageous from 
the standpoint of the rigidity/specific gravity value and the coefficient 
of thermal expansion. 
On the other hand, while the holding apparatus of the type using the 
alumina ceramic is advantageous in that the rigidity/specific gravity 
value is high as compared with the metal material thereby ensuring 
reduction in the size and weight of the apparatus and also the coefficient 
of thermal expansion is considerably low as compared with the metal 
material, there are disadvantages that fine particles tend to easily 
deposit in the large number of pores present in the surface of the 
sintered alumina ceramic and that the alumina ceramic is an insulator thus 
causing the deposition of dust due to the generation of static 
electricity. 
Also, with the apparatus proposed in the previously mentioned Japanese 
Laid-Open Patent No. 2-186656, while it is excellent from the standpoint 
of wear resistance, in the case of sapphire, for example, the deposition 
problem of fine particles due to static electricity remains unsolved since 
sapphire is an insulator. In addition, where the wafer holder, etc., are 
made of diamond or sapphire, three different methods including a method of 
using a single crystal material, a method of using a sintered powder and a 
method of using a surface coating are conceivable and they have the 
following deficiencies. Firstly, where the single crystal material is 
used, a large blank material of diamond is not available in the existing 
situation and a raw material of sapphire is not only expensive but also 
hard to work it. Further, where the sintered material is used, diamond and 
sapphire are materials which are hard to sinter. Even if the sintering is 
possible, it is considered that a great number of large pores are caused 
and moreover the use of cobalt (Co) or the like as a binder gives rise to 
a problem of metal contamination. Further, the method of using the coating 
has the danger of causing the separation of a coating film due to the 
difference in coefficient of thermal expansion between the coating film 
and its base material. 
As described hereinabove, there has been known in the prior art no holding 
apparatus which satisfies the required characteristic properties, i.e., 
the insurance of the desired mechanical strength, the reduction in weight 
and the dimensional stability and which prevents the contamination of an 
article to be held such as a wafer due to the deposition of fine particles 
or the like. 
SUMMARY OF THE INVENTION 
It is the primary object of the present invention to provide a holding 
apparatus which has excellent dust resisting properties while satisfying 
the characteristic properties required for holding apparatus of the type 
used in such applications requiring to maintain the cleanability of an 
article to be handled, i.e., the insurance of the required mechanical 
strength, the reduction in weight and the dimensional stability. 
In accordance with the present invention, the holding apparatus includes 
contact portions which directly contact with an article to be held such as 
a semiconductor substrate and at least the contact portions are made of a 
sintered ceramic including a covalent bond-type conductive material. 
In accordance with a preferred aspect of the present invention, the contact 
portions for the article to be held are made of a sintered aluminum oxide 
(Al.sub.2 O.sub.3) including titanium carbide (TIC). 
As mentioned previously, in the holding apparatus according to the present 
invention the contact portions for the article to be held are made of the 
sintered ceramic including the covalent bond-type conductive material with 
the result that the characteristic properties possessed by the covalent 
bond-type conductive material, i.e., high hardness and conductivity are 
added to the characteristic properties inherently possessed by the 
sintered ceramic such as high rigidity and light weight. As a result, not 
only the wear resistance of the contact portions is improved thus 
preventing the occurrence of fine particles but also the generation of 
static electricity at the contact portions is prevented by simply 
electrostatically grounding these portions by conventional procedures thus 
reducing the deposition of fine particles. Also, in accordance with the 
present invention, instead of using a single crystal or polycrystal of the 
covalent bond-type conductive material for the construction of the contact 
portions, the covalent bond-type conductive material is dispersed into the 
base material ceramic for the sintered material forming the contact 
portions and then the ceramic is sintered for use, thereby making the 
manufacture easy and inexpensive. 
Further, since the holding apparatus according to the present invention has 
excellent dust-resistant properties as well as such characteristic 
properties as high rigidity, light weight and high dimensional stability, 
it can be used advantageously in such applications where any dimensional 
variation due to the accumulation of heat presents a problem when the 
operation of high-speed transfer or the correction of the flateness of a 
thin sheet-type article to be held is effected. 
Still further, as regards the sintered material forming the contact 
portions of the holding apparatus according to the present invention, the 
conditions for its forming and/or sintering operation can be suitably 
selected to control the occurrence of pores in the surface of the sintered 
material. In other words, generally the addition of the covalent bond-type 
conductive material to the sintered ceramic deteriorates the sinterability 
as compared with the case of the base material ceramic alone, it is 
possible to produce a sintered material having reduced pores through the 
combined use of an HP (hot press) or HIP (hot isostatic press) process. 
For instance, while TiC or one of the covalent bond-type conductive 
materials is less sinterable when used singly, if it is added to Al.sub.2 
O.sub.3 and sintered under the proper process conditions, a sintered 
material with less pores than the heretofore used alumina ceramic can be 
produced. Thus, due to the fact that in the holding apparatus of the 
present invention the contact portions for an article to be held can be 
composed of a sintered material having less pores, the surface pores into 
which foreign particles tend to deposit are decreased and moreover any 
foreign particles deposited on the surface of the contact portions, if 
any, can be easily cleaned and removed. 
As the suitable covalent bond-type conductive materials for addition to the 
base material ceramic for the sintered material, specifically TiC, SiC, 
etc., may be cited. While its content in the sinter is suitably set, it 
must be added in an amount capable of ensuring the conductivity of the 
sinter (generally, materials having resistances of 10.sup.10 .OMEGA. 
.multidot.cm or less are called conductors), whereas the sinterability is 
deteriorated if the content of the covalent bond-type conductive material 
is excessively large. As a result, the content is selected between 5 and 
70% by weight, preferably on the order of 30% by weight. 
In accordance with the present invention, the ceramic used as a sinter base 
material may be any other material than Al.sub.2 O.sub.3 provided that it 
is excellent in sinterability and the desired mechanical strength is 
ensured. Where the requirement for the reduction in weight is not so 
great, ZrO.sub.2 or the like may for example be used. 
The following Table 1 illustrates the physical properties of various 
substances heretofore used as the materials for forming the contact 
portions of the holding apparatus and the sintered material used in the 
present invention (Table 1 shows by way of example the sintered Al.sub.2 
O.sub.3 containing TiC). 
TABLE 1 
__________________________________________________________________________ 
Young's Electric Coefficient of 
Electric 
modulus 
Specific 
Poros- 
resistance 
Hardness 
thermal expansion 
discharge 
Material 
kg/mm.sup.2 
gravity 
ity .OMEGA. cm 
Hv .times. 10.sup.-6 .degree.C. 
machinability 
__________________________________________________________________________ 
SUS 20000 
8.1 .smallcircle. 
7 .times. 10.sup.-5 
200 17.3 .smallcircle. 
Aluminum 
7000 
2.8 .smallcircle. 
200 23 .smallcircle. 
Al.sub.2 O.sub.3 
35000 
3.8 x .gtoreq.10.sup.14 
1800 7.1 x 
Zirconia 
26000 
6.0 .smallcircle. 
2 .times. 10.sup.12 
1700 8.9 x 
TiC + Al.sub.2 O.sub.3 
40000 
4.2 .smallcircle. 
3 .times. 10.sup.-3 
1900 7.8 .smallcircle. 
TiC x 180 .times. 10.sup.-6 
2900 .smallcircle. 
Sapphire 4 .smallcircle. x 
Diamond 3.4 .smallcircle. 
&gt;7000 
0.8 x 
(knoop 
hardness) 
__________________________________________________________________________ 
As will be seen from Table 1, the TiC-containing sintered Al.sub.2 O.sub.3 
used in the present invention is particularly apparent that its specific 
resistance is as low as about 10.sup.-3 .OMEGA. .multidot.cm and it 
ensures a sufficient conductivity for the prevention of static 
electricity. Also, its good conductivity makes possible the electric 
discharge machining and the machining of complicated shapes is made easy. 
Further, its characteristic properties, i.e., the porosity, Young's 
modulus and hardness are sperior to those of the conventionally used 
alumina ceramic (Al.sub.2 O.sub.3) and also the equivalent characteristic 
properties as the Al.sub.2 O.sub.3 are ensured with respect to the 
coefficient of thermal expansion and specific gravity. 
In Table 1, as will be seen from a comparison between the SUS (stainless 
steel) and aluminum and the TiC-containing sintered Al.sub.2 O.sub.3, the 
sintered material used in the present invention is higher in 
rigidity/specific gravity value than the metals and it is especially 
advantageous for use in holders holding articles for which flatness is 
required and in transfer holding apparatus which must be small in size and 
high in speed. Also, the sintered material used in the present invention 
is much smaller in coefficient of thermal expansion than the metals and 
the dimensional variation of the sintered material is not great even in 
case where the heat due to, for example, the exposure over a long period 
of time is accumulated in the sintered material. In other words, it is 
possible to reduce the dimensional error due to a thermal deformation of 
the holder which may possibly be caused during the transfer of circuitry 
to the wafer firmly held by vacuum on the holder. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description of its 
embodiments which are illustrative without any intention of limitation 
when taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 show schematically the construction of a wafer holder 
according to a first embodiment of the present invention. In the Figures, 
the wafer vacuum holding surface of a wafer holder WH is formed into a 
circular shape of a diameter which is slightly smaller than the diameter 
of a wafer W, and the wafer holding surface is formed with a plurality of 
annular projections 1 (wafer supporting portions) and annular grooves 
(vacuum holding grooves) which are concentrically (or helically) arranged 
radially from the center of the wafer holder WH at a constant pitch in the 
like manner as rims. 
In this embodiment, the wafer holder WH proper is wholly made of a 
TiC-containing sintered Al.sub.2 O.sub.3 and the pore distribution in the 
wafer holding surface is such that less than 300 pores of over 10 .mu.m in 
maximum size are present per mm.sup.2. Also, the annular projections 1 of 
the wafer holding surface are machined to have a ridge width of 0.02 to 
0.15 mm so as to reduce the probability of the deposition and riding of 
dirt. This machining can be easily effected by electric discharge 
machining or grinding operation. In addition, every side of the wafer 
holder WH is finished to produce a smooth surface having a surface 
roughness of 5 .mu.m Rmax or less and every boundary between the sides are 
smoothly chamferred. 
On the other hand, the annular grooves 2 are respectively formed with 
channels comprising vacuum holes or suction holes 3 which are arranged 
radially and the vacuum holes 3 are communicated with a manifold or 
sleeve-like hole 4 extended radially within the wafer holder WH. The hole 
4 is connected to a vacuum source for pressure reducing purposes so that a 
negative pressure is produced within the space enclosed by the back of the 
wafer W and the annular grooves 2 and the resulting holding action causes 
the back of the wafer W to be corrected in conformity with the tops of the 
plurality of annular projections 1, thereby making the wafer flat. 
At this time, due to the fact that the whole wafer holder WH is a good 
conductor, the charges by static electricity are grounded through the 
metal members supporting the holder and therofore there is caused no 
deposition of the fine particles around the holder due its charging. Also, 
since the wafer holder WH is high in hardness, practically its abrasive 
wear with the wafer and hence the resulting production of fine particles 
are eliminated and the amount of production of fine particles tending to 
deposite on the wafter is reduced extremely. 
Further, since there are less pores in the surface of the wafer holder WH, 
there is very rare possibility for the deposition of fine particles in the 
pores which may possibly be redeposited on the wafer W or alternatively 
the chances of fine particles existing between the wafer holder WH and the 
wafer W are so small that deterioration in the flatness of the wafer W 
hardly tends to occur. Also, even if the deposition of fine particles on 
the surface of the wafer holder takes place, there are less pores in the 
holder surface so that the particles can be removed easily and the 
cleaning of the wafer holder WH is simplified. 
Still further, the wafer holder WH of this embodiment is such that the 
coefficient of thermal expansion is small and the amount of dimensional 
variation due to the accumulation of heat during the exposure is reduced. 
Thus, even in cases where the exposure is effected with a low-intensity 
light over a long period of time, the dimensional error of the wafer can 
be reduced to less than a permissible limit. 
In addition, since the TiC-containing sintered Al.sub.2 O.sub.3 is low in 
specific gravity and thus the wafer holder of this embodiment is light in 
weight, where the step and repeat-type exposure is effected, the load on 
the driving system for moving the wafer holder WH is reduced and thus an 
excellent driving response characteristic is obtained. 
A second embodiment of the present invention will now be described. FIGS. 3 
and 4 show schematically the construction of a wafer holder (pin 
chuck-type holder) PWH according to the second embodiment. In the Figures, 
the wafer holder PWH is made of the same TiC-containing sintered Al.sub.2 
O.sub.3 as in the case of the first embodiment and its wafer holding 
surface is formed with a plurality of pin-type projections 25 (wafer 
supporting portions) by electric discharge machining, grinding operation 
or the like. The pin-type projections 25 are each formed into a 
cylindrical or square pillar shape having a diameter (or the length of 
each side) of about 0.02 to 0.1 mm and a pin height of 0.01 to 0.5 mm. 
Also, closed annular projections 27 and 28 of practically the same height 
as the pin-type projections are respectively formed on the innermost side 
and the outermost side of the wafer holding surface similarly by the 
electric discharge machining, grinding operation or the like, and each of 
these annular projections has an width of 0.02 to 0.15 mm. The surfaces of 
the various portions of the wafer holder PWH are subjected to smooth 
finish as in the case of the first embodiment. 
The underside wafer holding surface is formed with channels comprising 
vacuum holes or suction holes 29 which are arranged radially and the 
vacuum holes 29 are communicated with a sleeve-type hole or manifold 20 
extended radially inside the wafer holder PWH. By connecting the hole 20 
to a vacuum source for pressure reduction, a negative pressure is produced 
within the space enclosed by the wafer W and the annular projections 27 
and 28 so that the resulting holding action corrects the back side of the 
wafer W to conform with the tops of the plurality of pin-type projections 
25 and thus the wafer is held in its flattened condition. 
At this time, due to the fact that the wafer holder PWH of this embodiment 
is a good conductor as in the case of the first embodiment and that it 
possesses the various characteristic properties such as high hardness, 
high rigidity, light weight and high dimensional stability, an excellent 
dust resisting effect is obtained and it is also advantageous from the 
standpoint of operating characteristics and dimensional stability. 
It is to be noted that while, in the above-described first and second 
embodiments, the whole wafer holder is made of the TiC-containing sintered 
Al.sub.2 O.sub.3, depending on the circumstances, only the portions which 
directly contact with a wafer W, i.e., the annular projections 27 and 28 
and the pin-type projections 25 may be made of the TiC-containing sintered 
Al.sub.2 O.sub.3. However, there is also the danger of the fine particles 
deposited in the grooves of the holder being deposited on the wafer W and 
it is needless to say that the dust proof can be ensured more positively 
by composing the whole wafer holder with the TiC-containing Al.sub.2 
O.sub.3. Also, it is preferable that the whole wafer holder is made of the 
TiC-containing sintered Al.sub.2 O.sub.3 from the standpoint of mechanical 
strength, reduced weight and dimensional stability. 
Next, a third embodiment of the present invention will be described. FIGS. 
5 and 6 show schematically the construction of the principal parts of a 
wafer transfer apparatus according to the third embodiment. In the 
Figures, a plurality of projecting members 12 made of a TiC-containing 
sintered Al.sub.2 O.sub.3 are fixedly mounted on the upper surface (wafer 
W loading surface) of an arm 11 by such method as adhesion, bonding or 
clamping. In accordance with the present embodiment, there is a room in 
the thickness direction of the arm 11 and therefore the arm 11 is made of 
a conductive material such as aluminum, carbon fiber reinforced resin or 
conductive plastic material (any of these materials can ensure a usable 
mechanical strength if the thickness is large enough). Also, after the 
projecting members 12 have all been mounted on the arm 11, their upper 
surfaces are subjected to flat working by grinding or lapping operation 
and therefore they have a sufficient flatness. 
If the transfer apparatus shown in FIGS. 5 and 6 is used, during the 
transfer of a wafer there is practically the occurrence of no static 
electricity and the occurrence of no fine particles due to wear of the 
wafer and its supporting portions (the upper surfaces of the projecting 
members 12) and therefore there is practically no danger of any fine 
particles depositing on the wafer. 
A fourth embodiment of the present invention will now be described. FIGS. 7 
and 8 show schematically the principal parts of a wafer transfer apparatus 
according to the fourth embodiment. This wafer transfer apparatus is 
adapted for moving a wafer W to and from any selected one of the shelf 
positions within a wafer storage cassette 15 set on a pedestal 14. In the 
Figures, the cassette 15 is provided with an opening in its side on the 
right side of the paper plane and a plurality of wafer supporting shelves 
formed into substantially a C-shape are arranged at intervals of 2 to 3 mm 
in the height direction on both sides within the cassette 15. 
The transfer apparatus of this embodiment is provided with a transfer arm 
16 which is movable in the lateral and vertical directions within the 
plane of the paper and the transfer arm 16 is attached by a fixing member 
18 to an arm driver which is not shown. Formed on the upper surface of the 
arm 16 are a plurality of projecting members 17 each having a vacuum hole 
13. The width of the arm 16 is smaller than the width of the opening 
between the right and left shelves within the cassette 15 and the amount 
of arm movement is controlled by drive means (not shown) in accordance 
with the wafer storage pitch (the vertical shelf spacing) within the 
cassette 15. 
With the transfer apparatus according to this embodiment, when a wafer W is 
moved in or out, the arm 16 must be inserted into the gap of as small as 2 
to 3 mm and in this case the arm 16 must be prevented from contacting with 
the upper surface of the adjoining wafer W, thus making it necessary to 
ensure a high rigidity with a thin plate thickness for the arm 16. For 
this purpose, in the transfer apparatus of this embodiment both the arm 16 
and the projecting members 17 are wholly made of TiC-containing sintered 
Al.sub.2 O.sub.3. 
With the transfer apparatus of the present embodiment, in order to store a 
wafer W into the cassette 15, the wafer W is transferred onto the arm 16 
from above in the plane of the paper so that the back of the wafer W is 
held and supported by vacuum on the projecting members 17. Then, the 
position of the arm 16 is adjusted vertically in the plane of the paper by 
the driving means so that it is placed in a position corresponding to the 
selected shelf for storing the wafer W within the cassette 15. Thereafter, 
the arm 16 is moved toward the left in the plane of the paper and it is 
stopped upon reaching the given position within the cassette 15. Then, the 
arm 16 is slightly moved downward so that the wafer W is transferred onto 
selected one of the right and left shelves within the cassette 15. 
At this time, since the arm 16 and the projecting members 17, which are 
made of the TiC-containing sintered Al.sub.2 O.sub.3, are good conductors 
and are high in hardness, during the transfer of the wafer practically 
there are no generation of static electricity and no occurrence of fine 
particles due to abrasive wear of the wafer and its supporting portions 
(the upper surfaces of the projecting members 17) and therefore there is 
practically no danger of fine particles depositing on the wafer. Also, 
since the arm 16 of this embodiment is light in weight and high in 
rigidity, there is no danger of causing any damage to the wafers on the 
adjoining shelves due to any inclination or distortion of the arm 16 when 
the wafer is moved in and out and it is also advantageous in the case of 
high-speed operation. 
While, in the foregoing description, the holding apparatus for holding or 
transferring a wafer in the manufacture of semiconductor integrated 
circuits have been explained, it is needless to say that articles to be 
held according to the present invention are not limited to wafers and the 
present invention is also applicable to transfer apparatus for glass 
plates, etc., holders for various test apparatus and assembling apparatus 
for electronic components. In this case, the shape and construction of the 
holding portions are not limited in any way and therefore, in addition to 
carrying and holding an article to be held, the holding portions may for 
example be constructed so as to clamp an article to be held.