Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-09-21
2004-08-31
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06785005
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 an apparatus and method for using interferometers to determine the position of substrate stages during the simultaneous processing of the substrates affixed to these stages.
BACKGROUND OF THE INVENTION
Lithography processes require positioning a reticle between an electron beam and the substrate chip or wafer. System throughput is dependent upon the speeds of many separate processes that are performed in series. Throughput is therefore dependent on the duration of each process.
In a typical modern lithography process an individual wafer undergoes a number of sub-processes. These 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 sub-process. For example, in the sub-process for 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 held without slippage and in a way that does not cause distortion of the reticle pattern. The system is further complicated by the fact that lithography processes typically occur in a clean room/vacuum environment; this is also an indication of the sensitivity of the processes.
A typical exposure apparatus
10
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
.
This complexity and sensitivity of the exposure apparatus and the processes involved result in a significant time expenditure for each sub-process. When a single wafer is undergoing one of these sub-processes, the mechanisms for the others are normally 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. One current method 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, however, are less than ideal devices for this use for a number of reasons. The encoder must be placed in an area that does not interfere with the requirements of other sub-processes, such as substrate exposure. This leads to apparatus design problems in harmonizing the requirements of the encoder, interferometers, and the process. Also, encoders are less precise than interferometers. Precision in planar placement of the stage is necessary, since errors in reticle or wafer position result in similar errors in the final product and, therefore, reduced functionality of that final product.
SUMMARY OF THE INVENTION
The present invention provides a dual stage assembly and method where stage position may be determined using interferometers. The stage assembly includes a plurality of interferometers mounted on a base for determining stage positions. 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. Whenever such an eclipse occurs, the mirror on the second (eclipsed) stage is configured to cooperate with the non-eclipsed interferometers so that the position of said second stage is continuously determinable. This is achieved by appropriately dispersing the interferometers about one side of the base and by causing the mirror on the second stage to extend from behind the eclipsing shadow of the first stage. In a preferred embodiment, the second stage is the same size as the first and merely supports the larger mirror. In another preferred embodiment, the second stage is approximately the same size as the mirror in the relevant dimensions. In both the previously mentioned preferred embodiments the stages are the same size in the direction parallel to the axes of the interferometers. But the invention could also be practiced in two dimensions resulting in the need for interferometers on only two orthogonal sides
Lyons Michael A.
Morgan & Lewis & Bockius, LLP
Nikon Corporation
Turner Samuel A.
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