Retainer

Details Retainer Retainer

2004-02-26

2004-11-30

Mack, Ricky (Department: 2873)

Optical: systems and elements

Lens

With support

C359S811000, C359S822000

Reexamination Certificate

active

06825998

ABSTRACT:

This application claims a benefit of priority based on Japanese Patent Application No. 2003-048365, filed on Feb. 26, 2003, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.
BACKGROUND OF THE INVENTION
The present invention relates generally to precision machines for mounting a lens, and more particularly to a projection optical system in an exposure apparatus, etc. More specifically, the present invention relates to a retainer that holds an optical element for a more precise imaging relationship in an exposure apparatus in projecting an image on an original sheet, such as a mask or reticle onto an object, such as a single crystal substrate for a semiconductor wafer, a glass plate for a liquid crystal display (“LCD”). The exposure apparatus is used to fabricate a semiconductor device, an image pick-up device (such as a CCD), and a thin film magnetic head.
The fabrication of a device using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a circuit pattern formed on a mask onto a wafer and the like, thereby transferring the circuit pattern. The projection optical system enables diffracted beams from the circuit pattern to interfere on the wafer and the like, so as to form an image.
The devices to be mounted on electronic apparatuses should be highly integrated to meet recent demands for miniaturization and low profile of electronic apparatuses, and finer circuit patterns to be transferred or higher resolution have been demanded increasingly. A short wavelength of a light source and an increased numerical aperture (“NA”) in a projection optical system are effective to the high resolution as well as a reduced aberration in the projection optical system.
An optical element, such as a lens and a mirror, when deforming in an projection optical system causes aberration because an optical path refracts before and after the deformation and light that is supposed to form an image at one point does not converge on one point. The aberration causes a positional offset and short-circuits a circuit pattern on a wafer. On the other hand, a wider pattern size to prevent short-circuiting is contradictory to a fine process. Therefore, a projection optical system with small aberration should hold its optical element(s) without changing a shape and a position relative to the optical axis of the optical element in the projection optical system so as to maximize the original optical performance of the optical element.
FIG. 12
is a schematic sectional view of a conventional retainer
1000
for holding an optical element
1100
. Referring to
FIG. 12
, the optical element
1100
is engaged with or slightly spaced from an inner circumference of a retaining member
1200
so that one surface of the optical element
1100
contacts a support part
1200
a
provided on the retaining member
1200
. Adhesive
1300
is inserted into an aperture between an outer circumference of the optical element
1100
and the inner circumference of the retaining member
1200
. After the adhesive
1300
cures, the optical element
1100
held integrally by the retaining member
1200
. The optical element
1100
thus held by the retainer
1000
can constitute an optical system held by a housing
2000
with other optical elements
1100
similarly held by the retainer
1000
.
FIG. 13
is a schematic sectional view of an optical system including the optical elements
1100
held by the conventional retainer
1000
.
In general, the support part
1200
a
in the retaining member
1200
, which contacts the optical element
1100
in a range of 360° around a rotational center axis of the optical element
1100
, as shown in FIG.
14
. However, the mechanical processing has a difficulty in maintaining the support part
1200
a
to be completely flat, and the optical element
1100
contacts the retaining member
1200
at plural points from the microscopic viewpoint irrespective of a design of contact in a range of 360°. Subject to the gravity influence in this state, the optical element
1100
undulates by its own weight around contact points as vertices. In particular, a projection lens tends to have a larger caliber and a larger lens capacity due to the recent high NA in the projection optical system, and easily deforms by its own weight. Here,
FIG. 14
is a schematic structure of the retaining member
1200
of the conventional retainer
1000
.
The projection optical system corrects aberrations that result from various errors in plural optical elements, such as a mirror and a lens, by adjusting a combination of these optical elements and a positional relationship among them, and should consider nanometer deformations about a surface shape of the optical element. However, use of the retainer
1000
would change contact points between the used optical element
1100
and the retaining member
1200
according to a combination between them, and thus vary deformed surface shapes. Therefore, aberrations scatter among retainers that hold different optical elements, and corrections of the aberrations become very arduous.
A retaining member
1500
can be used, as shown in
FIG. 15
, which arranges three support parts
1500
a
at 120° intervals around the rotational center of the optical element
1100
. The optical element
1100
always contacts and is supported by three support parts
1500
a
arranged at 120° intervals, is subject to the gravity force, and undulates around contact points (or these support parts
1500
a
) as vertices that are arranged at 120° intervals. Here,
FIG. 15
is a schematic structure of the remaining member
1500
in another conventional retainer.
Since a plane can be geometrically defined by three points, three support parts
1500
a
always define the same plane irrespective of the processing precision of the retaining member
1500
. Therefore, the optical element
1100
that contacts these support parts
1500
a
is supported under approximately the same condition even when the retaining member
1500
is replaced with another retaining member
1500
.
The optical element that undulates around three projections usually generates a trigonometric component of wave front aberration, but this aberration is correctable when the optical element is combined with another optical element in an optical system. This feature reduces scattering aberration among retainers that hold different optical elements, and facilitates an aberrational correction more easily than that for the retaining member
1200
that is configured to contact the optical element in a range of 360° around the rotational center axis of the optical element.
An optical element, such as a mirror and a lens, is often made of an optical glass material, such as quartz, due to excellent optical characteristics and manufacture convenience, whereas a retaining member for the optical element is made of a metallic material for strength and processability. In other words, the optical element and retaining member are made of different materials and have different coefficient of linear expansions. Thus, when the temperatures of the optical element and retaining member vary, for example, as the ambient temperature of the optical system varies and as the optical element heats up, the optical element and the retaining member have different expansion and contraction amounts due to different coefficients of linear expansion.
In the conventional retainers shown in
FIGS. 12
to
15
, an outer circumference of the optical element and an inner circumference of the retaining member connected to the optical element via the adhesive have different expansion and contraction amounts, and the optical element is subject to a tensile or compressive compulsory displacement in a radial direction and its top and bottom surfaces deform. The optical element consequently changes its optical performance, and the optical system that includes plural optical elements also changes the optical performance. In other words, an optical apparatus deteriorates the optical performance as the temp

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