Overlay alignment system using polarization schemes

Details Overlay alignment system using polarization schemes Overlay alignment system using polarization schemes

2000-02-08

2003-02-25

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By alignment in lateral direction

With light detector

C356S369000

Reexamination Certificate

active

06525818

ABSTRACT:

FIELD OF THE INVENTION
The field of this invention relates to semiconductor manufacturing, and more particularly, to photolithography and to the use of alignment systems in photolithography, and to the use of polarized light in alignment systems.
BACKGROUND OF THE INVENTION
In fabricating microelectronic semiconductor devices and the like on a semiconductor wafer (body, substrate, or chip) to form an integrated circuit (IC) various metal layers and insulation layers are deposited in selective sequence, various openings are formed in these layers, various impurities may be deposited within these openings, and in some cases oxide layers are grown in situ on the wafer. The features formed on the various layers must be aligned with respect to, or placed in the proper spatial relation to, features which have been formed on the semiconductor wafer at an earlier step in the process sequence. To maximize integration of device components in the available wafer area to fit more components in the same area, increased IC miniaturization is utilized. Reduced dimensions of the features formed on the semiconductor wafer are needed for denser packing of components to meet the requirements of present day very large scale integration (VLSI). As the lateral size of the features is reduced, the thickness of the various layers is similarly decreased. The size of features formed on the semiconductor wafer are typically in the range of 100 nm or smaller. As the dimensions of these features are reduced, the features must be aligned with respect to one another to a greater and greater degree of precision.
The transfer of patterns from masks (reticles) to the semiconductor wafer is typically accomplished by projecting an image on the mask onto a layer of photoresist which has been deposited on the semiconductor wafer. The system used to accomplish this pattern transfer also typically includes apparatus to assure the correct alignment of the newly projected pattern with respect to the features previously formed on the semiconductor wafer.
One such system used to accomplish such pattern transfer and alignment is disclosed in U.S. Pat. No. 5,477,057 (David Angeley et al.), hereinafter “Angeley”, which is entitled “Off Axis Alignment System for Scanning Photolithography”, and is incorporated herein by reference.
FIGS. 1
,
2
,
3
,
4
,
5
, and
6
of this application are reproductions of
FIGS. 1
,
2
,
3
,
4
,
5
, and
7
, respectively, of Angeley et al. The system of
FIG. 1
contains an alignment system
12
that is mounted adjacent to a projection optical system used to project a mask pattern onto a photoresist layer
6
on a semiconductor wafer
18
. The alignment system
12
, which is shown in
FIG. 2
, uses a broadband light source
68
to illuminate two sets of alignment marks
34
and
34
′ on the semiconductor wafer
18
. The light from source
68
illuminates a diffusing glass
76
, which provides illumination of an alignment reticle
3
having a predetermined pattern
31
,
33
formed thereon which is shown in FIG.
3
. An optical system
10
images the alignment reticle pattern
31
,
33
into the plane of the semiconductor wafer
18
. The imaged light is reflected, scattered and diffracted by the alignment marks
34
,
34
′ on the semiconductor wafer
18
as the wafer is scanned past the stationery alignment reticle image
96
shown in FIG.
4
. The reflected, scattered and diffracted light is collected by optics
48
and
50
(see
FIG. 2
) and directed to the beam splitter
42
. Beam splitter
42
deflects the light to the optical detector sub-system
24
, where it is incident upon a detector mask
54
. The optical detector sub-system
24
consists of detectors
58
,
60
,
62
,
64
, and
66
which detect light passing through openings in the detector mask
54
and guided to the detectors by fiber optics
30
.
FIG. 5
shows a plan view of the detector mask
54
with openings (transmission regions)
58
′,
60
′,
62
′,
64
′, and
66
′, corresponding to the five detectors
58
,
60
,
62
,
64
, and
66
, respectively. Transmission region
58
′, which is a central region, collects light reflected from the semiconductor wafer
18
and the alignment marks
34
and
34
′. This is “bright-field” detection. The other regions
60
′,
62
′,
64
′, and
66
′ collect light scattered or diffracted from the alignment marks
34
and
34
′ (i.e., “dark-field” detection) and are located around the central region
58
′ in the orientation shown in FIG.
5
. These four other regions
60
′,
62
′,
64
′, and
66
′ further distinguish between the light scattered to the left and right of the central detector opening
58
′.
The alignment marks used in this system (See
FIG. 4
) are features formed on the semiconductor wafer which are typically rectangular in nature, one set of such marks being arranged in a linear array, with the major axis of the rectangular alignment mark at a 45 degree angle to the axis of the linear array, and a second set of such marks, whose major axis is perpendicular to that of the first set, is arranged in a similar linear array. The alignment reticle
32
typically has two orthogonal intersecting rectangular apertures
31
,
33
therein. The alignment reticle
32
is oriented such that light passing through one such rectangular aperture
31
illuminates the rectangular alignment marks
34
of one set of such marks, and light passing through a second rectangular aperture
33
illuminates the rectangular alignment marks
34
′ of the second set of such marks. The image
96
(see
FIG. 6
) of the alignment reticle
32
is scanned across the linear arrays of alignment marks
34
and
34
′ in a direction which is at an angle of 45 degrees with respect to the major axis of the arrays of alignment marks
34
and
34
′.
In this system (
FIG. 1
) a mask pattern is transferred through the projection optical system
14
to the photoresist layer
6
on the semiconductor wafer
18
using highly coherent deep ultra violet (DUV) light for which the projection optical system and photoresist properties have been optimized. The alignment portion of the system uses a broadband light source in a wavelength band where the photoresist is not sensitive, and uses an optical system which is optimized to the requirements of the alignment system. This alignment system uses non-polarized light to illuminate the patterns of alignment marks.
As the size of the features formed on the semiconductor wafer decreases, the dimensions of the alignment marks formed on the semiconductor wafer are decreased so as to allow an improvement in the ability to align the various features formed on the semiconductor wafer with one another. As the width of the rectangular alignment marks is decreased, and as the thickness of these features, and the thickness of the layers in which these features are formed, decrease, the magnitude of the light scattered and diffracted from the features is decreased also.
Another such system used to accomplish pattern transfer and alignment is disclosed in U.S. Pat. No. 5,285,258 (K. Kamon), hereinafter “Kamon”, which is entitled “Method of and an Apparatus for Detecting Alignment Marks”, and is incorporated herein by reference.
FIGS. 7
,
8
,
9
,
10
, and
11
are reproductions of
FIGS. 4
,
5
,
9
A,
7
A, and
7
B, respectively, of Kamon. This apparatus uses the same method as U.S. Pat. No. 5,477,057 of illuminating a pattern of alignment marks with a light beam while moving the semiconductor wafer relative to the light beam. This system differs from that of the system of U.S. Pat. No. 5,477,057 in that it makes use of a single detector to detect the light reflected from the alignment mark (i.e., “bright-field” detection), as opposed to the method of detecting the light scattered from the alignment marks which is known as “dark-field” detection. A general problem with this type of bright-field detection system is that the system readily detects not on

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