Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-08-08
2002-04-16
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S067000, C355S053000
Reexamination Certificate
active
06373555
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light-exposure apparatus used in semiconductor fabrication, more particularly to the arrangement of the shutter and filters of the apparatus.
FIG. 1
schematically shows the structure of a conventional light-exposure apparatus
50
having a light source such as a mercury lamp
51
, and a shutter
52
mounted just below the light source. The apparatus
50
has three operating states: a shut state, an alignment state, and an exposure state.
FIG. 1
illustrates the light path in the alignment state.
The shutter
52
moves between a shut position (indicated by dotted lines) in which it blocks the light from the mercury lamp
51
, and an open position (indicated by solid lines) in which the light is not blocked and can be used for alignment and exposure purposes. The light is refracted by the transmitting part
53
a
of a convex secondary lens
53
, reflected by a primary mirror
54
, then reflected by an aluminum coating on the reflecting part
53
b
of the secondary lens
53
. After passing through a slit
55
, the light is reflected by the reflecting surface
56
a
of a toroidal mirror
56
, then passes through one of two filters, either an exposure filter
57
that transmits light of comparatively short wavelengths, or an alignment filter
58
that transmits light of longer wavelengths. In the alignment state, these filters are positioned so that the light passes through the alignment filter
58
, as shown.
The elements described so far constitute the illumination system
65
of the apparatus. The light emerging from the illumination system
65
is further reflected by three relay mirrors
59
,
60
,
61
, then illuminates a semiconductor wafer through a mask. The wafer and mask are not shown in FIG.
1
.
The wafer is coated with a photoresist material that is insensitive to the wavelengths transmitted by the alignment filter
58
. This light can therefore be used for alignment of the wafer and mask. After alignment is completed, the exposure filter
57
is moved into the light path, replacing the alignment filter
58
, and light of a shorter, more energetic wavelength is used to transfer the mask pattern to the photoresist.
In the shut state, the shutter
52
is moved to the position indicated by dotted lines to block the light from the mercury lamp
51
, so that the light does not reach the secondary lens
53
, primary mirror
54
, slit
55
, and toroidal mirror
56
in the illumination system
65
.
A problem in the conventional apparatus
50
is that the short-wavelength light that interacts with the photoresist also interacts to some extent with the optical elements in the illumination system
65
, gradually clouding or darkening the coatings on their surfaces, for example. Since these optical elements are exposed to all emitted wavelengths in both the exposure state and the alignment state, they receive continuous exposure to short-wavelength light during these two states. As a result, these optical elements, more specifically the secondary lens
53
, primary mirror
54
, and toroidal mirror
56
, tend to degrade comparatively quickly.
Continuous exposure to the light emitted by the mercury lamp
51
during the alignment and exposure states also raises the optical elements in the illumination system
65
to a comparatively high temperature. In the shut state, in which the shutter
52
is closed, these optical elements receive no illumination, and their temperature falls back toward room temperature. As the apparatus
50
cycles repeatedly among the shut, alignment, and exposure states, the optical elements in the illumination system
65
undergo repeated large temperature swings, which affect their optical properties and contribute to the degradation thereof.
SUMMARY OF THE INVENTION
An object of the present invention is to reduce the degradation of optical elements in an apparatus for projecting a mask pattern onto a wafer.
A further object is to reduce temperature variations in the illumination system of the apparatus.
The invented apparatus has an illumination system providing light for use in alignment and exposure, and a projection system using the provided light to project a mask pattern onto a wafer. The illumination system has a light source, two light selectors, and at least one optical element disposed between the two light selectors. The first light selector has an alignment filter, which it selectively places between the light source and the optical element, in the path of the light emitted by the light source. The second light selector has at least an exposure filter and a shutter, which it selectively places between the optical element and the projection system, in the path of the light received from the optical element.
The apparatus can be operated in a shut state, an alignment state, and an exposure state. The alignment filter is placed in the light path in the shut state and the alignment state, and is removed from the light path in the exposure state.
Degradation of the optical element is reduced because the optical element is exposed to unfiltered light only in the exposure state, and not in the alignment state.
Temperature variations are reduced because the optical element is illuminated in the shut state, as well as in the alignment state and exposure state.
REFERENCES:
patent: 6208707 (2001-03-01), Oshino
patent: 6271909 (2001-08-01), Suzuki et al.
patent: 57-164528 (1982-10-01), None
Adams Russell
Nguyen Hung Henry
Oki Electric Industry Co. Ltd.
Volentine & Francos, PLLC
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