Photocopying – Projection printing and copying cameras – Methods
Reexamination Certificate
2001-10-22
2003-12-16
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Methods
C355S046000, C355S053000, C355S072000
Reexamination Certificate
active
06665054
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to lithography instruments used for patterning and processing substrates such as semiconductor chips and wafers. More specifically, the invention is concerned with a method for positioning stages during the processing of the substrates affixed to those stages.
BACKGROUND OF THE INVENTION
In lithography processes the image from a reticle is transferred to a substrate, typically a wafer. System throughput is dependent upon the speeds of many separate steps that are performed in series. Throughput is therefore dependent on the duration of each step, which can include: loading, field image alignment, global alignment, and exposure.
The production of an acceptable final product requires the complex interaction of the systems necessary to implement each step. This complexity creates time requirements of its own. For example, when exposing patterns on wafers and other substrates, the reticle is moved at high speeds between discrete and precise positions to facilitate focusing the image on the substrate. This motion can generate dynamic reaction forces where the reticle is supported, leading to distortion of the reticle and, hence, distortion of the image focused on the substrate. Both reticle and wafer must be given time to settle to reduce the vibration that can cause distortion of the transferred pattern. Lithography processes typically occur in a clean room/vacuum environment; this is a source of further complexity and also an indication of the sensitivity of the processes.
A typical exposure apparatus employing a single wafer stage is shown in FIG.
1
and FIG.
2
. Exposure apparatus
10
transfers a pattern of an integrated circuit from reticle
12
onto semiconductor wafer
14
. Apparatus frame
16
preferably is rigid and supports the components of exposure apparatus
10
. These components include: reticle stage
18
, wafer stage
20
, lens assembly
22
, and illumination system
24
. Alternatively, separate, individual structures (not shown) can be used to support wafer stage
20
, reticle stage
18
, illumination system
24
, and lens assembly
22
.
Illumination system
24
includes an illumination source
26
and an illumination optical assembly
28
. Illumination source
26
emits an exposing beam of energy such as light or electron energy. Optical assembly
28
guides the beam from illumination source
26
to lens assembly
22
. The beam illuminates selectively different portions of reticle
12
and exposes wafer
14
. In
FIG. 1
, illumination source
26
is illustrated as being supported above reticle stage
18
. Typically, however, illumination source
26
is secured to one of the sides of apparatus frame
16
and the energy beam from illumination source
26
is directed to above reticle stage
18
with illumination optical assembly
28
. Where illumination source
26
is an electron beam, the optical path for the electron beam should be in a vacuum.
Lens assembly
22
projects and/or focuses the light passing through reticle
12
to wafer
14
. Depending upon the design of apparatus
10
, lens assembly
22
can magnify or reduce the image illuminated on reticle
12
.
Reticle stage
18
holds and precisely positions reticle
12
relative to lens assembly
22
and wafer
14
. Similarly, wafer stage
20
holds and positions wafer
14
with respect to the projected image of the illuminated portions of reticle
12
. In the embodiment illustrated in FIG.
1
and
FIG. 2
, wafer stage
20
and reticle stage
18
are positioned by shaft-type linear motors
30
. Depending upon the design, apparatus
10
may include additional servo drive units, linear motors and planar motors to move wafer stage
20
and reticle stage
18
, but other drive and control mechanisms may be employed.
The basic device as described may be used in different types of lithography processes. For example, exposure apparatus
10
can be used in a scanning type lithography system, which exposes the pattern from reticle
12
onto wafer
14
with reticle
12
and wafer
14
moving synchronously. In a scanning type lithography process, reticle
12
is moved perpendicular to an optical axis of lens assembly
22
by reticle stage
18
, and wafer
14
is moved perpendicular to an optical axis of lens assembly
22
by wafer stage
20
. Scanning of reticle
12
and wafer
14
occurs while reticle
12
and wafer
14
are moving synchronously.
Alternatively, exposure apparatus
10
may be employed in a step-and-repeat type lithography system that exposes reticle
12
while reticle
12
and wafer
14
are stationary. In the step-and-repeat process, wafer
14
is in a constant position relative to reticle
12
and lens assembly
22
during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer
14
is consecutively moved by wafer stage
20
perpendicular to the optical axis of lens assembly
22
so that the next field of semiconductor wafer
14
is brought into position relative to lens assembly
22
and reticle
12
for exposure. Following this process, the images on reticle
12
are sequentially exposed onto the fields of wafer
14
.
Processing a single wafer requires a significant time expenditure because of the complexity and sensitivity of the exposure apparatus and the steps involved. When a single wafer is undergoing one step, the apparatus for the others are normally idle. For example, when a single wafer is being exposed the apparatus for determining the alignment of the wafer relative to the wafer stage is typically idle. Consumer demand for the end product has created a need for increased throughput and, thus, the development of methods to decrease the idle time. A way to decrease idle time is to use two stages and position them so that each stage can undergo different steps of the process at the same time. The present invention is a method that uses two stages that run simultaneously, but with each stage at different steps in the process. This method relies upon a combination of encoders and interferometers to determine the position of each stage at any given point throughout processing. Encoders being rather less accurate than interferometers; the method preferably relies on them during the less position-sensitive steps of the process.
SUMMARY OF THE INVENTION
The present invention provides a two stage method where stage position may be determined using interferometers and one or more encoders. The stage assembly includes a plurality of interferometers mounted on a base for determining stage positions and encoders where interferometers are not feasible. The two stages move between multiple positions on the base and have mirrors affixed to them that cooperate with the other interferometer components to provide position data. At times, the two stages are positioned so that the first stage eclipses the second stage with respect to said at least one of the interferometers. Should such an eclipse occur, and another interferometer not be available for determining the eclipsed stage's position, an encoder is configured to supply position. The apparatus is designed so that encoders are required during the less position-sensitive steps of the process, such as when switching from one step to another.
A method incorporating the invention comprises: sizing the stages based on wafer and exposure apparatus parameters; dispersing interferometers and encoders about the base at appropriate positions based on the stage sizes; moving the stages as desired while using the exposure apparatus; and determining the positions of both stages at all times during the process.
REFERENCES:
patent: 4215938 (1980-08-01), Farrand et al.
patent: 5114234 (1992-05-01), Otsuka et al.
patent: 5151749 (1992-09-01), Tanimoto et al.
patent: 5187543 (1993-02-01), Ebert
patent: 5365342 (1994-11-01), Ayata et al.
patent: 5379115 (1995-01-01), Tsai
patent: 5392120 (1995-02-01), Kamiya
patent: 5504407 (1996-04-01), Wakui et al.
patent: 5523839 (1996-06-01), Robinson et al.
patent: 5528118 (1996-06-01), Lee
patent: 5537209 (1996-07-01), Lis
patent: 5677758 (1997
Fuller Rodney
Nikon Corporation
Pennie & Edmonds LLP
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