Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-06-21
2001-12-25
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S035000, C355S053000, C355S055000, C355S067000, C430S269000, C349S004000, C349S074000, C349S082000
Reexamination Certificate
active
06333780
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to projection aligners in photolithography processes for use in the manufacture of semiconductor devices.
2. Description of the Background Art
Photolithography processes are adopted for various steps of the manufacture of semiconductor devices. In the photolithography processes, a projection aligner is employed for defining various patterns on a semiconductor wafer.
In the projection aligner, the intensity of the light applied to a photoresist disposed above the semiconductor wafer is controlled by a photomask (reticle).
FIG. 12
is a plan view of a conventional photomask
9
.
FIG. 13
is a cross section schematically illustrating the structure taken along the line B
1
—B
1
in FIG.
12
. The photomask
9
comprises a body
92
composed of quartz, and shields
93
selectively provided on the body
92
. The shields
93
are formed from chromium (Cr), for example, and they considerably reduce the light transmittance with respect to the position where only the body
92
exists.
In order to raise resolution limit due to miniaturization, there have been proposed phase shift masks such as Levenson type photomasks.
FIG. 14
is a plan view of a conventional Levenson type photomask
90
.
FIG. 15
is a cross section schematically illustrating the structure taken along the line B
2
—B
2
in FIG.
14
. The photomask
90
has three regions
90
a
,
90
b
and
90
c
. The phase of the light passing through the region
90
c
is shifted &pgr; with respect to the phase of the light passing through the region
90
b
. The region
90
a
reduces significantly the light transmittance with respect to the regions
90
b
and
90
c.
The photomask
90
comprises a body
92
, shields
93
selectively provided on the body
92
, and phase shifters
94
. The shields
93
open at the position corresponding to the region
90
a
or
90
b
, and the phase shifters
94
cover the opening of the shields
93
at the position corresponding to the region
90
c
. The photomask
90
can control both the intensity and phase of the light passing therethrough, thereby to optimize resolution and depth of focus.
Besides the stated technique, light source of shorter wavelength, modified illumination obtained by control of an aperture, and pupil filter method have been proposed, in order to raise resolution limit due to miniaturization. The pupil filter method comprises disposing a patterned spatial frequency filter at the pupil surface in a projection lens system. The spatial frequency filter can also control both the intensity and phase of the light passing therethrough, thereby to optimize resolution and depth of focus. It is however noted that the light passing through the projection lens system varies according to the pattern of the photomask
90
. It is therefore preferable to optimize the pattern of a spatial frequency filter for every photomasks.
The photomask
90
, however, calls for patterns which are independent one another in their respective steps of the manufacture of semiconductor devices, and there are also needed various mask making processes such as electron beam cutting, separating from wafer processes. Further, since the phase shift mask calls for the phase shifter
94
in addition to the body
92
and shields
93
, its design, making control and defect inspection are complicated. This causes an increase in the photomask revision. Consequently, the conventional photomasks have required a considerable amount of time for their making.
The projection lens system is the essential part in order that a photomask pattern is faithfully reproduced on a wafer, and it is necessary to maintain a high precision. The projection lens system is therefore placed in a closed case, in order to assure the rigid control of temperature, humidity and pressure. Since the spatial frequency filter is a component of the projection lens system, this cannot be replaced easily as is the case with aperture. It is therefore difficult to control the pattern of a spatial frequency filter for every photomasks.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, a projection aligner comprises: a spatial frequency filter having at least one liquid crystal element, provided in a projection lens system projecting light passed through a photomask onto an object to be processed; and a liquid crystal controller that controls at least one of transmittance and refractive index of the liquid crystal element.
In the first aspect, the transmittance and refractive index of a liquid crystal element are controlled by the liquid crystal controller. Thereby, the phase shift and transmittance required for the spatial frequency filter can be controlled without removing the filter from the projection lens system. This enables to obtain a spatial frequency filter in which a suitable function of pupil is employed for various photomask patterns.
Preferably, the projection aligner further comprises an aperture that introduces the light into the photomask and has at least one liquid crystal element.
According to a second aspect, the projection aligner of the first aspect is characterized in that the spatial frequency filter has a plurality of liquid crystal elements.
In the second aspect, even when it is difficult to control independently refractive index and transmittance by using a single liquid crystal element, a spatial frequency filter which has a pattern of desired phase shift and transmittance as a whole, can be obtained by stacking a plurality of liquid crystal elements.
Preferably, the projection aligner of the second aspect is characterized in that the liquid crystal element controller controls the transmittance and the refractive index of the liquid crystal elements independently.
Preferably, the projection aligner of first aspect further comprises a filter information storage storing the transmittance and phase shift of the spatial frequency filter.
More preferably, the projection aligner of the first aspect is characterized in that the phase shift of the spatial frequency filter is obtained by controlling an optical path difference according to the refractive index of the liquid crystal element.
According to a third aspect, a projection aligner comprises: a photomask having at least one liquid crystal element; and a liquid crystal element controller for controlling the transmittance of the liquid crystal element.
In the third aspect, it is possible to reduce significantly the number of mask making processes to be needed per photomask pattern. In addition, there is no need to replace the mask for each step in the photolithography process, thereby to make a pattern correction promptly.
Preferably, the projection aligner of the third aspect further comprises an aperture that introduces the light into the photomask and has at least one liquid crystal element.
According to a fourth aspect, the projection aligner of the third aspect is characterized in that the photomask has a plurality of liquid crystal elements.
In the fourth aspect, even when it is difficult to independently control refractive index and transmittance by using a single liquid crystal element, a photomask, e.g., a phase shift mask, which has a pattern of desired phase shift and transmittance as a whole, can be obtained by stacking a plurality of liquid crystal elements.
Preferably, the projection aligner of the fourth aspect is characterized in that the liquid crystal element controller controls the transmittance of the liquid crystal elements independently.
More preferably, the projection aligner of the fourth aspect is characterized in that the liquid crystal element controller controls the refractive index of the liquid crystal elements independently.
According to a fifth aspect, the projection aligner of the fourth aspect further comprises a projection lens system performing a reduction projection of the photomask onto the object to be processed.
In the fifth aspect, it is possible to relax the degree of miniaturization demanded in a liquid crystal when it is used as a photomask.
P
Brown Khaled
Mathews Alan A.
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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