X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
2002-08-02
2004-11-09
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S205000, C378S206000
Reexamination Certificate
active
06816568
ABSTRACT:
FIELD
This disclosure pertains, inter alia, to X-ray projection-exposure (“microlithography”) apparatus for transferring, for instance, circuit patterns defined on a photomask (either a mask or a reticle) onto a substrate such as a semiconductor wafer. The X-ray projection-exposure apparatus employ an X-ray beam having a wavelength in the range of 1 to 30 nm that transfers the pattern by passing through a reflection-type imaging-optical system. This disclosure also pertains to exposure methods performed using such an X-ray projection-exposure apparatus, and to semiconductor devices manufactured using the X-ray projection-exposure apparatus.
BACKGROUND
Exposure apparatus used for manufacturing semiconductor devices are typically configured to project, and thus “transfer,” a circuit pattern, defined on a mask or reticle (termed a “mask” herein), via a projection-optical system onto a suitable substrate such as a semiconductor wafer. During projection-exposure the mask is situated at an object plane and the substrate is situated at an image plane. In most projection-exposure apparatus currently in use (that utilize light in the deep-UV range), both the mask and the optical elements of the projection-optical system are transmissive to the light used for performing pattern transfer, and hence are termed “transmissive” optical systems. For example, a projection-exposure apparatus still currently in wide use utilizes, as an exposure-light source, “i-line” light produced by a high-pressure mercury lamp. Other deep-UV projection-exposure apparatus utilize, for example, a KrF excimer laser as a source of exposure light.
A conventional transmissive optical system
40
is depicted conceptually in FIG.
5
. The depicted apparatus includes a light source
49
, an illumination-optical system
50
, a projection-optical system
51
, a mask stage
53
for holding a mask
52
, a wafer stage
55
for holding a wafer
54
, a detection system
56
for detecting alignment marks on the wafer
54
, and a detection system
58
for detecting alignment marks on the mask
52
. A light beam, used for pattern transfer, downstream of the mask
52
is denoted as item
57
a
, and downstream of the projection-optical system
51
is denoted as item
57
b
. In
FIG. 5
the beam
57
a
,
57
b
is shown propagating along the optical axis of the projection-optical system
51
.
The apparatus
40
of
FIG. 5
also includes an optically based system
59
a
,
59
b
for detecting the surface position of the wafer
54
. The surface-position-detection system includes a light source
59
a
that obliquely illuminates a light beam onto the surface of the wafer
54
. Light of the beam reflected from the wafer
54
is detected by a photodetector
59
b
. Thus, the position of the wafer surface, in a direction along the optical axis, is detected. Examples of surface-position-detection systems of this type are described in Japan laid-open (Kôkai) Patent Application No. Hei 6-283403, Japan Kôkai Patent Application No. Hei 8-64506, and Japan Kôkai Patent Application No. Hei 10-214783. Such a detection system also may be used for detecting positions of a mark formed on the wafer and/or positions of a mark formed on the mask
52
.
The pattern defined on the mask
52
can be configured for projection, by the projection-optical system
51
, onto the wafer
54
at unity magnification or with demagnification. Demagnification is characterized by the image on the wafer being smaller, by a demagnification factor established by the projection-optical system
51
, than the corresponding pattern on the mask
52
.
The projection-optical system
51
normally comprises multiple lenses or the like that collectively function to form an image of the mask pattern on the surface of the wafer
54
. The entire mask pattern can be exposed in one exposure “shot,” or may require multiple exposure shots, depending upon the optical field (exposure-image field) of the projection-optical system
51
relative to the size of the pattern as projected onto the wafer. For example, if the projection-optical system
51
has an optical field of 20-mm square, then a die (or multiple dies) having a total area of no greater than 20-mm square can be exposed on one shot.
On the surface of the wafer
54
, patterns for microcircuits are projected and formed layer-by-layer in a superposed manner. Exposure and formation of the requisite number of layers results in formation of a micro-electronic device in which the layers are interconnected with each other in a three-dimensional manner. These layers must be formed in a manner requiring extremely accurate registration (“overlay”) of each new layer with existing layers formed in previous exposures. To achieve high overlay accuracy, the apparatus
40
of
FIG. 5
typically also includes respective devices for detecting the positions of the mask
52
and wafer
54
as exposures are being made. Each device normally includes a respective interferometer and the respective mark-position-detection system
58
,
56
. As the interferometers measure the respective positions of the mask stage
53
and wafer stage
55
in real time, the mark-position-detection systems
58
,
56
optically detect respective alignment marks defined on the wafer
54
and the mask
52
.
For example, the mark-position-detection system
56
can be configured as an optical microscope that produces a magnified image of the detected mark on the wafer
54
. The system
56
includes an image detector, such as a charge-coupled device (CCD), for detecting the magnified image. In many conventional apparatus, the mark-position-detection system
56
is mounted laterally adjacent the projection-optical system
51
due to space constraints. An example of such a mark-position-detection system is disclosed in Japan Kôkai Patent Application No. Hei 5-21314.
FIG. 6
depicts a representative relationship, at the image plane, between the optical field (exposure-image field) of the projection-optical system
51
and “detection centers” associated with the mark-position-detection system
56
. The hatched area in the center of the figure corresponds to the exposure-image field
62
, which has a center
61
. In this example, the exposure-image field
62
is rectangular. A straight line
64
denoted by a dot-dash line extends laterally from the center
61
in the X-direction. The wafer stage
55
is configured to move the wafer
54
in directions parallel to the line
64
. Another straight line
65
, denoted by a dot-dash line, extends vertically in the Y-direction (at a right angle to the line
64
) from the point
61
.
In a conventional projection-exposure apparatus, the central axis (i.e., the optical axis) of the projection-optical system
51
typically passes (in a Z-direction) through the center
61
of the exposure-image field
62
. The reason for this configuration is that the optical elements (lenses and/or mirrors) of the projection-optical system
51
typically are axially symmetrical in shape and situated along the optical axis, and the exposure light passing through the projection-optical system
51
is kept at or close to the optical axis to minimize optical aberrations. As a result, the exposure-image field on the image plane typically is located near the optical axis.
In instances in which the mark-position-detection system
56
(
FIG. 5
) is optical in configuration, the detection center
63
usually is situated at a position that is separated from the center point
61
of the exposure-image field
62
. The distance between these points
61
,
63
is a defined distance denoted by “BL” in the figure. Establishing the distance BL prevents the mark-position-detection system
56
from interfering with the projection-optical system
51
. In this regard, the detection center
63
denotes the intersection of the optical axis of the mark-position-detection system
56
with the image (wafer) plane. In this instance, the distance BL is essentially equal to a dimension that is the sum of the radius of the optical column of the projection-optical system
51
and the radius of the
Church Craig E.
Klarquist & Sparkman, LLP
Nikon Corporation
Thomas Courtney
LandOfFree
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