Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2002-05-21
2004-06-22
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S022000
Reexamination Certificate
active
06753120
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims benefit of Korea Patent Application No. 2001-74098, filed on Nov. 27, 2001, under 35 U.S.C. §119, the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND
1. Field of the Invention
The present invention relates to a method of measuring alignment of a photolithography process, and more particularly, to an alignment measuring method of a photolithography process by which a degree of misalignment for each shot region of a wafer is indexed, to more accurately determine whether an overlay fails.
2. Brief Description of Related Art
In general, a photolithography process is used to transcribe onto a wafer different pattern images formed on a plurality of reticles. The pattern images are sequentially transcribed and compounded onto the wafer, along with etching, layer deposition and other processing steps, to form a desired circuit pattern.
In such a photolithography process, it is important to design a precise circuit pattern and accurately align (or overlay) different pattern layers forming the circuit pattern.
For high-precision overlay of pattern layers, great efforts have been made to overcome a number of restrictions to form a circuit pattern with high-integration and high-precision by revising patterns of reticles and changing photoresist.
At this time, the dimension of patterns is usually determined by the specifications of the equipment and photoresist. However, the overlay of pattern images should be improved by periodical preventive maintenance and continuous development of measurement tools.
An overriding goal for the overlay step is to precisely overlay a pattern layer of transcribed pattern images with a pre-existing pattern layer as precisely as possible. In other words, the overlay of pattern layers is measured to make a standard database to determine whether another operational step, like a development step, should follow, or a re-alignment or maintenance should be performed to compensate for misalignment of the pre-existing pattern layer.
Therefore, there should be a database for a precise overlay of pattern layers. However, there is a problem in the overlay of pattern layers because respectively different formation relationships of align-marks have been used as a standard reference for transcription of pattern images in equipment. Besides, different overlay detection results may be obtained with the same align-marks in different equipment. Above all, the biggest problem has been that parameters mixed with linear and non-linear factors have been used for the overlay of pattern layers.
At this time, the aforementioned linear parameters include a level of misalignment along an X axis and Y axis of pattern images transcribed to the pre-existing pattern of a wafer, an expansion or contraction rate of an edge away from the center of a pattern, a degree of rotational angle and the like. The nonlinear parameters co-existing with the linear parameters as such are a level of misalignment of a pre-existing pattern, a level of precision in align-marks, an error in measurement equipment, and the like. It is required that the linear and non-linear parameters be separately analyzed on wafers or reticles, over again.
According to conventional overlay management with linear and non-linear parameters, a measurement has been taken regarding a level of the overlay of pattern images transcribed to a pre-existing pattern layer from respective shot regions of wafer, or a plurality of align-marks distributed instead of the shot regions. At this time, the amount of misalignment is calculated separately for a wafer field and a reticle field. While the calculation as such is performed with already measured data, the calculation is performed by correcting the misalignment with reference to the center of a wafer in the case of a wafer field, and with reference to the center of a reticle in the case of a reticle field.
According to the above-referenced method, there are various formulas for calculating the amount of misalignment in each shot region of a wafer field and the amount of misalignment in a reticle field. For the purpose of this disclosure, however, the amount of misalignment in the wafer field and the reticle field will be calculated only with formula 1 and formula 2, as examples:
Formula 1:
Xraw=a+bx−cy+&egr;;
Yraw=d+ey+fx+&egr;
; where
Xraw=the amount of misalignment that is measured relative to the x axis
Yraw=the amount of misalignment that is measured relative to the y axis
x=the distance along the x axis from the center of a wafer
y=the distance along the y axis from the center of a wafer
a=the distance of pattern images in a direction of x axis
b=the scale of enlargement of pattern images relative to x axis
c=the obliquity factor of the rotation angle of pattern images in a direction of x axis relative to y axis
d=the distance of pattern images in a direction of y axis
e=the scale of enlargement of pattern images relative to y axis
f=the obliquity factor of the rotation angle of pattern images in a direction of y axis relative to x axis
&egr;=error term (non-linearity element)
In the formula 1, if the terms a, b, c, d, e and f in each shot region are partial-differentiated so that the square value of the term &egr; may be minimized, the following 3 simultaneous equations for each of Xraw and Yraw are obtained as in formula 2.
Formula 2:
L=&Sgr;&egr;
2
=&Sgr;(
Xraw−a−bx+cy
)
2
∂
L
∂
a
=
-
2
∑
(
raw
-
a
-
bx
+
cy
)
=
0
∂
L
∂
b
=
-
2
∑
(
raw
-
a
-
bx
+
cy
)
x
=
0
∂
L
∂
c
=
-
2
∑
(
raw
-
a
-
bx
+
cy
)
y
=
0
L=&Sgr;&egr;
2
=&Sgr;(
Yraw−d−ey−fx
)
2
∂
L
∂
d
=
-
2
∑
(
Yraw
-
d
-
ey
-
fx
)
=
0
∂
L
∂
e
=
-
2
∑
(
Yraw
-
d
-
ey
-
fx
)
y
=
0
∂
L
∂
f
=
-
2
∑
(
Yraw
-
d
-
ey
-
fx
)
x
=
0
L=least-scale
If the above simultaneous equations are solved by using the Gauss-Jordan' elimination method, the overlay variables for each of the shot regions, i.e., the values of a(1~n), b(1~n), c(1~n), d(1~n), e(1~n) and f(1~n) in formulas 1 and 2 can be obtained.
At this time, another method can also be used to obtain the aforementioned simultaneous equations.
Accordingly, a relationship of overlay variables relative to the amount of misalignment will be described in further detail with reference to the accompanying drawings.
First of all, a and d indicated in formula 1 are, as shown in
FIG. 1
, respective distances apart from x and y axes (an intercept between x and y axes). If there is a misalignment relative to a simple position, the intercept will be kept constant as a total vector of the misalignment amount regardless of a position in a wafer.
Also, b and e indicated in formula 1 are, as shown in
FIG. 2
, an enlargement rate relative to x and y axes respectively (an enlargement/reduction ratio according to a regression formula). The misalignment amount increases or decreases in a constant proportional relationship depending upon a degree of distance apart from the center of a wafer to its edge.
Furthermore, c and f indicated in formula 1 are, as shown in
FIG. 3
, a rotational angle slope to an x axis direction relative to a y axis according to a regression formula, increasing or decreasing in a constant relationship depending upon the distance apart from the center of a wafer to its edge. At this time, if a value of the rotational angle slope is marked as (+), it means a counterclockwise direction.
Furthermore, all the aforementioned parameters shown in
FIGS. 1 through 3
are combined, resulting in a shape shown in FIG.
4
.
With such a result, a worker can determine whether the misalignment amount relative to respective shot regions falls within a preset range and, then, decide with refere
Samsung Electronics Co,. Ltd.
Volentine & Francos, PLLC
Young Christopher G.
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