Photocopying – Projection printing and copying cameras – Detailed holder for photosensitive paper
Reexamination Certificate
2001-08-03
2004-01-13
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Detailed holder for photosensitive paper
C355S053000, C356S399000
Reexamination Certificate
active
06678038
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains, inter alia, to microlithography, which involves the transfer of a pattern, usually defined by a reticle or mask, onto a “sensitive” substrate using an energy beam. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, and micromachines. More specifically, the invention pertains to methods and devices, used in the context of a microlithography method and apparatus, respectively, for rotating a substrate as required for assessing undesired measurement error of the substrate position.
BACKGROUND OF THE INVENTION
As the density and miniaturization of microelectronic devices have continued to increase, the accuracy and resolution demands imposed on microlithographic methods also have increased. Currently, most microlithography is performed using, as an energy beam, a light beam (typically deep-UV light) produced by a high-pressure mercury lamp or excimer laser, for example. These microlithography apparatus are termed “optical” microlithography apparatus. Emerging microlithographic technologies include charged-particle-beam (“CPB”; e.g., electron-beam) microlithography and “soft-X-ray” (or “extreme UV”) microlithography. Because many contemporary microlithography machines operate according to the well-known “step-and-repeat” exposure scheme, they are often referred to generally as “steppers.”
All microlithographic technologies involve pattern transfer to a suitable substrate, which can be, for example, a semiconductor wafer (e.g., silicon wafer), glass plate, or the like. So as to be imprintable with the pattern, the substrate typically is coated with a “resist” that is sensitive to exposure, in an image-forming way, by the energy beam in a manner analogous to a photographic exposure. Hence, a substrate prepared for microlithographic exposure is termed a “sensitive” substrate.
For microlithographic exposure, the substrate (also termed herein a “wafer”) typically is mounted on a substrate stage (also called a “wafer stage”). The wafer stage is a complex and usually quite massive device that not only holds the wafer for exposure (with the resist facing in the upstream direction) but also provides for controlled movement of the wafer in the X- and Y-directions (and sometimes the Z-direction) as required for exposure and for alignment purposes. In most microlithography apparatus, a number of devices are mounted to and supported by the wafer stage. These devices include a “wafer table” and a “wafer chuck” attached to the wafer table. The wafer table can be used to perform fine positional adjustment of the wafer relative to the wafer stage, and often is configured to perform limited tilting of the wafer chuck (holding the wafer) relative to the Z-axis (e.g., optical axis).
The wafer chuck is configured to hold the wafer firmly for exposure and to facilitate presenting a planar sensitive surface of the wafer for exposure. The wafer usually is held to the surface of the wafer chuck by vacuum, although other techniques such as electrostatic attraction also are employed under certain conditions. The wafer chuck also facilitates the conduction of heat away from the wafer that otherwise may accumulate in the wafer during exposure.
Monitoring of the position of the wafer in the X-, Y-, and Z-directions must be performed with extremely high accuracy to ensure the attainment of the desired accuracy of exposure of the pattern from the reticle to the wafer. The key technology employed for such purposes is interferometry, due to the extremely high accuracy obtainable with this technology. Interferometry usually involves the reflection of light from mirrors, typically located on the wafer table, and the generation of interference fringes that are detected. Changes in the pattern of interference fringes are detected and interpreted as corresponding changes in position of the wafer table (and thus the wafer). To facilitate measurements in both the X- and Y-directions over respective ranges sufficiently broad to encompass the entire wafer, the wafer table typically has mounted thereto an X-direction movable mirror and a Y-direction movable mirror. The X-direction movable mirror usually extends in the Y-direction along a full respective side of the wafer table, and the Y-direction movable mirror usually extends in the X-direction along a full respective side of the wafer table.
Despite the extremely high accuracy with which modern microlithography apparatus are constructed and with which positional measurements can be performed in these apparatus, the measurements still are not perfect and hence are characterized by certain tolerances. With respect to these tolerances, a measurement error caused by the apparatus itself is termed a “tool-induced shift,” or “TIS,” an error attributed to variations in the wafers (or other substrates) is termed a “wafer-induced shift,” or “WIS.” The term “tool” is derived from the common reference to a microlithography apparatus as a “lithography tool.”
Whenever a wafer is mounted on the wafer chuck, the microlithography apparatus normally executes an alignment routine to determine the precise position and orientation of the wafer before initiating exposure of the wafer. To such end, the wafer chuck typically includes “fiducial” (reference) marks strategically placed around the wafer. Similarly, the wafer itself typically includes multiple alignment marks imprinted thereon.
Reference now is made to
FIG. 6
, depicting a schematic plan view of a conventional stepper machine S in the region of the wafer stage WS. The wafer stage WS includes a wafer table WT and a wafer chuck WC. The wafer table WT includes an X-direction movable mirror M
X
and a Y-direction movable mirror My. In the stepper S, the wafer stage WS is movable (to the left and right in the figure) to assume either of two positions, an alignment position P
A
and an exposure position P
E
. At the alignment position P
A
, the wafer table WT is positioned relative to an alignment axis A
A
extending in the Z-axis direction in the figure. At the exposure position P
E
, the wafer table WT is positioned relative to an exposure axis A
E
, also extending in the Z-axis direction parallel to the alignment axis A
A
. The alignment axis A
A
is coincident with the optical axis of an alignment microscope (not shown, but situated above the plane of the page of the figure). The exposure axis A
E
is coincident with the optical axis of a projection-optical system (not shown but situated above the plane of the page of the figure).
Whenever the wafer stage WS is in a loading position near the alignment position P
A
, a wafer W can be conveyed (usually robotically) into the stepper S and placed on the wafer chuck WC on the wafer table WT. Subsequently, the wafer stage WS moves to the alignment position P
A
, at which the alignment microscope is used to align the wafer W on the wafer chuck WC and perform other pre-exposure alignments of the wafer as required. (To such end, the wafer W can include alignment marks M, discussed below.) Upon completion of measurements and alignments performed at the alignment position P
A
, the wafer stage WS moves (note arrow AR) the wafer table WT (with wafer chuck WC and wafer W) to the exposure position P
E
. At the exposure position P
E
, further measurements and alignments of the wafer table WT usually are performed. Also, if conditions are appropriate, the wafer W is exposed with a pattern defined by a reticle (not shown but situated on the exposure axis A
E
above the plane of the figure).
As alignments of the wafer W are being performed with the wafer stage WS at the alignment position P
A
, the respective positions of the wafer table WT in the X-direction and the Y-direction are monitored and determined by respective interferometers IF
XL
, IF
YA
that direct respective laser light beams at the respective movable mirrors M
X
, M
Y
. Similarly, whenever the wafer stage WS is at the exposure position P
E
, the respective positions of the wafer
Adams Russell
Kim Peter B.
Klarquist & Sparkman, LLP
Nikon Corporation
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