Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-03-13
2004-12-07
Kim, Peter B. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C349S120000
Reexamination Certificate
active
06829041
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a projection optical system and a projection exposure apparatus having the same, for the manufacture of devices such as semiconductor devices, CCDs, or liquid crystal devices, for example. In another aspect, the invention is concerned with a device manufacturing method using such a projection exposure apparatus. The present invention is particularly suitably usable in a projection exposure apparatus of a step-and-repeat type or step-and-scan type.
The density of semiconductor devices such as a DRAM or CPU, for example, has increased considerably. In the latest devices, a circuit pattern of a size not greater than 0.25 micron is required. Projection exposure apparatuses, called a stepper, are widely used because of their ability of forming such a fine pattern precisely. In such steppers, a pattern of a reticle is illuminated with light of a short wavelength in the ultraviolet region, and it is projected through a projection optical system onto a semiconductor (silicon) wafer in a reduced scale, whereby a fine circuit pattern is formed on the wafer.
For precision transfer of a reticle pattern, many strict conditions are applied to the projection optical system. Since the pattern size being resolvable with the projection optical system is in an inverse proportion to the numerical aperture (NA), the designing should be made to enable enlargement of the numerical aperture. Additionally, the aberration must be corrected precisely over the whole region corresponding to the semiconductor chip.
The designing can be done with the aid of high-speed computers and designing software. Naturally, for production of a projection optical system, it is necessary to make every lens of the projection optical system very precisely, exactly in accordance with the design. But, in addition to this, much attention has to be paid to the glass material or materials to be used. Since the refractive index of a glass material has a large influence to the imaging characteristic of a projection optical system, the uniformity thereof is very strictly controlled, generally to an order of 10
−6
or less. Further, the birefringence or double refraction property of a glass material is largely influential to the imaging characteristic and, therefore, the magnitude thereof should be suppressed to about 2 nm/cm, as is known in the art.
However, with a glass material for a projection optical system which may have a largest diameter of 200 mm, it is very difficult to control the double refraction property so precisely, uniformly over the whole surface. Usually, for reasons to be described below, birefringence would be produced to some degree.
A first reason is attributable to the manufacturing process of a glass material. For light in the ultraviolet region, currently, a quartz (silica) glass is widely used. Thus, the following explanation will be made with reference to quartz glass. As compared with optical crystals, quartz glasses to be used as a lens glass material has no directionality in its structure. Therefore, in an idealistic state, no birefringence is produced.
However, in quartz glasses, birefringence which might be considered as being attributable to remaining stresses such as thermal hysteresis or impurities may be observed experimentally. While the manufacture of quartz glass may be based on a direct method, a VAD (vapor axial deposition) method, a sol-gel method, or a plasma burner method, for example, in any of these methods, it is difficult for current technology to reduce a mixture of impurities to a level that can be disregarded.
Further, in cooling a quartz glass being formed in a high temperature state, it may be possible to reduce the stress, resulting from differences in the way of being cooled between the surface portion and the inside portion of the glass material (i.e., the stress due to thermal hysteresis), to some extent by a thermal treatment such as annealing, for example. But, in principle, it is difficult to completely remove it.
Referring now to
FIG. 24
, the process of manufacturing a lens element to be used in a lithographic projection optical system will be described. First, an ingot
100
of quartz glass is produced with a revolutionally symmetric shape. It is then sliced with a required thickness, by which a disk-like member
101
is provided. Since the ingot
100
is produced constantly symmetrically with respect to its central axis
100
a
, distribution of impurities remaining in the member
101
or distribution of stresses therein due to thermal hysteresis appears, as a matter of course, symmetrically with respect to the central axis
101
a
. At a final stage, cutting and polishing are made to the member
101
, whereby a lens element
102
is provided.
Now, distortion which may appear when impurities are mixed into the ingot
100
will be explained.
FIG. 25
is a sectional view of the ingot
100
. The peripheral hatching at
103
in this example shows a portion with a high impurity density. During an annealing process, the ingot
100
is heated. In the state with heat applied, the inside stress reduces to substantially zero. Through gradual cooling from that state, idealistically, a material without inside stress at room temperature can be provided. However, if impurities are mixed, the thermal expansion coefficient of the material changes. If the thermal expansion coefficient increases with the mixture of impurities, as a matter of course, it causes an increase of contraction during the cooling process.
As a result, although there is no stress in the heated state, the peripheral portion contracts largely with a temperature decrease. If particular attention is paid to the central portion of the glass material where a light flux is going to pass, it receives contraction from the peripheral portion as depicted by arrows in FIG.
25
. That is, inside stresses are produced. The inside stress is a cause for birefringence.
A second reason is attributable to a change, with time, of quartz glass when used in a stepper. As is known in the art, if light from a short-wavelength light source such as a KrF or ArF laser is projected to a quartz glass, a phenomenon called “compaction” may occur. Although details of how it occurs are not described here, what can be observed in that phenomenon is that the refractive index of the portion through which the light has passed increases but the volume of that portion decreases.
In
FIG. 26
, if laser light is projected to a hatched region
111
of the disk-like member
110
, the volume of that portion is likely to decrease. Since the peripheral portion not irradiated with laser light is not influenced by compaction, as the whole, the central portion is likely to contract whereas the peripheral portion is likely to act against the contraction.
In a balanced state, therefore, when particular attention is paid to the central portion of the glass material where light passes, it receives tension forces from the peripheral portion as depicted by arrows in FIG.
27
. Thus, inside stresses are produced. The inside stress is a cause for birefringence. The phenomenon described above may occur similarly in a projection optical system of a stepper. Since the phenomenon of compaction is particularly notable with use of ArF laser light, it may cause a large problem when a projection exposure apparatus with a light source of an ArF laser is practically developed.
As described, practically, it is very difficult to completely remove birefringence to be produced in a glass material. To the contrary, the requirement for birefringence in a stepper projection optical system is becoming strict, more and more. For providing a higher performance projection optical system, the number of lens elements constituting the projection optical system is increasing and, thus, the total glass material thickness is increasing. Therefore, even if the birefringence per unit length is kept to the above-described quantity (about 2 nm/cm), the total birefringence quantity of the system becomes large. Further, recent sh
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