Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1998-04-21
2001-03-27
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S077000
Reexamination Certificate
active
06208408
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a projection exposure apparatus for exposure of a substrate such as a wafer, for example, for manufacture of semiconductor devices, for example.
FIG. 9
shows a projection exposure apparatus for use in the manufacture of semiconductor devices, for example, and it includes a wafer stage
101
for positioning a wafer W
0
, a projection lens
102
disposed above the wafer stage, a reticle stage
103
and a light source optical system
106
. The light source optical system
106
produces exposure light which passes a reticle R
0
placed on the reticle stage
103
. The light is then collected by the projection lens
102
onto the wafer W
0
, whereby a pattern of the reticle R
0
is transferred to the wafer W
0
.
The wafer stage
101
is supported at the central portion of a base
101
a
which is supported on the floor through an anti-vibration system
101
b
. At the outside peripheral portion of the base
101
a
, there is a cast frame
105
supported in an upstanding position. The top
105
a
of the frame
105
supports the projection lens
102
at the center, and also it supports at the outer portion a reticle stage supporting member
104
(for supporting the reticle stage
103
) as well as a light source optical system supporting member
106
a
, in an upstanding position.
The position of the wafer stage
101
is measured optically and without contact, by means of a laser interferometer
107
which is supported by the frame
105
. Similarly, the position of the reticle stage
103
is measured by another laser interferometer
108
.
Each of the frame
105
, the reticle stage supporting member
104
, the base
101
a
, and the light source optical system supporting member
106
a
is a structural supporting member, and they support the projection lens
102
, the reticle stage
103
, the light source optical system
106
, and the position measuring system. Thus, sufficient rigidity is required for them so as to prevent not only vibration but also deformation due to a movement load of the stage.
In semiconductor device manufacture, the size or diameter of a substrate has been enlarged more and more for further improvement of productivity. Currently, the device manufacture mainly uses 8-inch size silicon wafers, but 12-inch size wafers will be used soon. If the substrate size becomes 1.5 times larger, the stroke of a movement member such as a stage becomes longer correspondingly. The moving load of a movement member causes not a small deformation of a supporting structural member, and, with enlargement of the stroke of the movement member, the amount of such deformation increases. Particularly, if deformation occurs at the mount of a laser interferometer which measures a relative shift of the projection lens and the stage, an error is produced in measurement of the stage position which directly deteriorates the precision.
Moreover, enlargement of the stroke leads to an increase in size of the stage, and this necessitates enlargement of the size of a frame that supports the stage. In order to retain a similar frame rigidity, it is necessary to modify the shape, for example, such as making the thickness 1.5 times larger, for example. Consequently, there occurs a problem of enlargement of the system as a whole and increases in weight thereof.
Conventionally, cast iron is mainly used for the structural member as a supporting member. Since cast iron has several conveniences such as a large freedom in shape and low production cost, it is used widely as a material for a structural member. On the other hand, cast iron has a Young's modulus of 110 GPa which is relatively low as a metal material. Also, there are additional negative factors when used in an exposure apparatus, such as changeability of shape with time due to loosening of internal stress or thermal expansion caused by heat, for example.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide a solution for a problem to be caused when a supporting member for supporting a table for an original, a projection optical system or a substrate stage is made of cast iron.
In accordance with an aspect of the present invention to achieve the primary object above, a ceramics material which is light in weight and which has a high rigidity and a good shape stability is used for a supporting structural member. Examples usable as such ceramics material are alumina ceramics (Al
2
O
3
) which is cheap and has a good machining property, silicon carbide ceramics (SiC) having good heat conductivity, silicon nitride (Si
3
N
4
) having a good thermal expansion characteristic, and aluminum nitride (AlN).
The cast iron having been conventionally used as a supporting member has a Young's modulus of about 110-130 GPa and a specific gravity of 7.8. When alumina ceramics are used as the material therefor, a Young's modulus of 370 GPa and a specific gravity of 3.4 are provided. Namely, the Young's modulus increases about three times and the specific gravity becomes about a half. For the same shape, a three-times larger Young's modulus and a half weight can be established, which accomplishes light weight and high rigidity. Further, in regard to thermal expansion which may result in deterioration of precision of the exposure apparatus, it will become about {fraction (2/3+L )} as compared with the case of cast iron. This contributes to improvement of precision.
However, although use of ceramics material for a supporting structural member in a projection exposure apparatus may easily accomplish light weight and high rigidity, for the reason that the ceramics is a brittle material, the breaking strength is about {fraction (1/3+L )} that of cast iron. As is known in the art, generally ceramic has such characteristics that: it comprises ion bond and a covalent bond, and the inter-atomic distance is large and the surface energy is small, so that fractional brittleness breakage occurs.
Particularly, impact acceleration caused during transportation of the system applies an extraordinarily large load to a supporting member. For example, the weight to be supported by the supporting member may be 3t or more, and the largest impact acceleration to be produced during the transportation may be 30G. As a result, an enormous moment or instant load of 90t may be applied to the supporting member.
In regards to the supporting member
105
shown by hatching in
FIG. 9
, as illustrated in
FIG. 10
, an impact load FL from above the supporting member acts thereon as a tensile load, in the neighborhood of the bottom surface PC thereof. If the load is beyond the tolerance stress of the ceramics which are less resistive to a tension force, it may cause breakage of the apparatus which is very dangerous to operators.
It is a second object of the present invention to provide a ceramics supporting member having improved fraction toughness such that it is not easily broken.
In accordance with a second aspect of the present invention to achieve the second object, there is a reinforcing member for enhancing tensile strength, added to a supporting member of ceramics. More particularly, as first means, steel rods are inserted inside a ceramics material along longitudinal and lateral directions and, by sintering them together, these rods are embedded therein. Thus, by putting the tensile strength on the steel rods, the tensile stress to be applied to the ceramics is reduced. A tensile stress may be applied to the reinforcing member beforehand, and this enables that, upon deformation, a state lower than the breakage stress is maintained in response to application of an external force. As means for applying such pre-stress, in the present invention, a thermal expansion coefficient difference between the ceramics and the reinforcing member may be used.
For enhanced breakage strength, it is effective to reduce cracks (small defects or pores) in the neighborhood of the surface to which the largest tensile stress acts on. In one preferred form of the present invention, as second means, the s
Adams Russell
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Hung Henry
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