Geometrical instruments – Gauge – Collocating
Reexamination Certificate
1999-06-09
2001-10-16
Gibson, Randy W. (Department: 2859)
Geometrical instruments
Gauge
Collocating
C356S401000
Reexamination Certificate
active
06301798
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for measuring misalignment, and more particularly, to a method to measuring misalignment for exposure with an arc alignment mark.
2. Description of the Related Art
In the fabrication process of an integrated circuit, typically, a wafer is patterned by first transferring a pattern on a photomask to a photoresist layer via an exposure step. As the integration of the integrated circuit continuously increases, the alignment precision for the exposure step becomes more and more crucial.
In the conventional exposure step, an exposure alignment mark is formed on the wafer. While exposing the photoresist layer, a certain patterned is aligned with the exposure alignment mark, so as to precisely transfer the patterned from the photomask to a required position on the wafer. However, this kind of alignment method has error due to resolution limitation. When the fabrication process of integrated circuit approaches a submicron stage, the error is obvious to affect the quality of the wafer.
To avoid the alignment error to affect the product yield during the exposure step, currently, an exposure step is performed on a wafer among a batch of wafers (normally 25 wafers), followed by measuring misalignment on different positions of the wafer. A modification for exposure is then calculated on the other wafers in the same batch.
FIG. 1A
shows a top view of a conventional structure to measuring misalignment for exposure.
In
FIG. 1A
, a wafer
100
comprises exposure alignment marks
102
and an outer layer mark
104
. The outer layer mark
104
is arranged as a rectangle as shown in the figure. The outer layer mark
104
by formed by four trenches
104
a,
104
b,
104
c
and
104
d.
The wafer
100
further comprises an inner layer mark
106
made of photoresist. The inner layer mark
106
is formed by forming a photoresist layer on the wafer
100
first, followed by exposure and development steps using a certain pattern on the exposure alignment marks to confirm the position of the wafer
100
. As a result, the inner layer mark
106
has a rectangular shape and arranged within the rectangle of the outer layer mark
104
. Similarly, the inner mark
106
comprises four strip structures
106
a,
106
b,
106
c
and
106
d.
The inner layer mark
106
is predetermined to be formed in the center of the outer layer mark
104
. Each of the strip structures
106
a
to
106
d
is supposed to be equally distant to a corresponding trench of the trenches
104
a
to
104
b.
However, as misalignment occurs for exposure, the inner layer mark
106
deviates the central position of the outer layer mark
104
. The level of the deviation indicates the magnitude of misalignment for the wafer
100
beyond this region.
To introduce the conventional inspecting method of the misalignment in more details, in the following description, the horizontal direction is denoted as an X-axis, while the vertical direction is denoted as Y-direction as shown in
FIG. 1A. A
first measuring line
108
is selected parallel to the X-axis. The first measuring line intersects with the trenches
104
a
and
104
c
at A
1
and A
4
, and intersects with the strip structures
106
a
and
106
c
at A
2
and A
3
, respectively. The X-coordinates for A
1
, A
2
, A
3
and A
4
are X
1
, X
2
, X
3
and X
4
. A difference between a mean value of X
1
and X
4
and a mean value of X
2
and X
3
represents misalignment along the X-axis.
A second measuring line
110
is selected to be parallel to Y-axis. The same method is used to obtain a misalignment along the Y-axis. The misalignment as a form of displacement can thus be obtained and modified.
However, while a circular misalignment occurs as shown in
FIG. 1B
, each side of the inner layer mark
106
and a corresponding side of the outer layer mark
104
meets at an angle. The parallel relationship is not sustained any more. Therefore, using the above method, the misalignment is variable according to the location of the measuring lines. For example, in
FIG. 1B
, the misalignments obtained by measuring lines
112
and
114
are in different magnitude. This is the same for the measuring lines in vertical Y-direction. Thus, the calculated modification for exposure of the wafers is not correct. The incorrect modification thus causes the pattern to be transferred from the photomask to the wafers with a distortion.
On the other hand, before performing the exposure on the photoresist layer over the wafer, deposition steps are very often performed on the wafer. Some depositions are thus remained within the trenches of
104
. Though the trenches
104
are not filled thereby, in case that the deposition is not formed uniformly or is formed with defects, it would cause the outer layer mark
104
out of position to further cause the system fails to inspect the misalignment for exposure.
SUMMARY OF THE INVENTION
The invention provides a method for inspecting misalignment for exposure. An outer circular mark is formed on a wafer comprising an exposure alignment mark thereon. The outer circular mark has an central point. A photoresist layer is formed on the wafer. An exposure step is performed by aligning a certain pattern of the photomask with of the exposure alignment mark. A development step is performed to form an inner circular mark. The inner circular mark and the outer circular mark are concentric. However, since a misalignment might happen, in practical application, the central point of the outer circular mark and the inner circular point may not overlap. The central point of the inner circular mark is named the inner center, while the central point of the outer circular mark is named as the outer center.
By selecting some points on the outer circular mark, the outer center point can be obtained by a geometric method. Similarly, the inner center can be obtained. The difference between the inner center point and the outer center point indicates the magnitude of the exposure misalignment.
After obtaining the magnitude of exposure misalignment, a modification can be applied to the exposure step for wafers.
The invention further provides an aligning structure. The structure comprises a exposure alignment mark, an outer mark and an inner mark on a wafer. The inner and the outer mark have an inner circle and an outer circle, respectively. The inner circle has an inner center, and the outer circle has an outer center. The inner circle is within the outer circle.
With the circular geometry of both the inner and outer marks, measuring lines are not required. Apart from the misalignment for displacement, circular misalignment can also be measured. In addition, more points can be selected on the inner and the outer circles to obtain a more precise positions of the inner center and the outer center. The obtained magnitude of misalignment can be further more exact. Therefore, those problems caused by uniform depositions or depositions with defects within the outer mark can be overcome.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
REFERENCES:
patent: 4037969 (1977-07-01), Feldman et al.
patent: 4356223 (1982-10-01), Iida et al.
patent: 4600309 (1986-07-01), Fay
patent: 4643579 (1987-02-01), Toriumi et al.
patent: 4708484 (1987-11-01), Komeyama et al.
patent: 5906753 (1999-05-01), Aoyagi et al.
Kao Han-Pin
Liu En-Chuan
Gibson Randy W.
Martine & Penilla LLP
United Microelectronics Corp.
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