Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
1999-08-12
2003-04-22
Mehta, Bhavesh (Department: 2625)
Image analysis
Applications
Manufacturing or product inspection
C382S151000, C355S055000
Reexamination Certificate
active
06553137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of increasing overlay accuracy and overlay displacement measuring devices, and more particularly to a method of increasing overlay accuracy and overlay displacement measuring device for determining a focus plane providing an optimum measuring accuracy.
2. Description of the Background Art
Recently, semiconductor devices such as ICs (Integrated Circuits) and LSIs (Large Scale Integrations) have been increasingly reduced in size. Especially, exposure devices for transferring a circuit pattern on a mask or reticle onto a circuit pattern formed on a semiconductor wafer are required to achieve increasingly high accuracy. As the degree of integration of the devices has been increased, accuracy from 0.20 &mgr;m to 0.10 &mgr;m is required, and most recently accuracy of 0.10 &mgr;m or smaller is required.
Referring to
FIGS. 10
to
12
B, a method of measuring overlay displacement in a conventional process of manufacturing a semiconductor device will be described.
Referring to
FIG. 10
, in the conventional method of measuring overlay displacement, positions of overlay inspection marks are measured at measuring points on selected ones of a plurality of chips
102
on a wafer
101
. At each of the measuring points, first and second overlay inspection marks
201
and
202
are formed during patterning of an interconnection or the like as shown in
FIGS. 11A and 11B
. First overlay inspection mark
201
is formed on a substrate, and second overlay inspection mark
202
is formed thereon. A set
204
of first and second overlay inspection marks
201
and
202
is referred to as a Box-in-Box mark
204
.
Generally, image recognition is used for measuring a magnitude of displacement of first and second overlay inspection marks
201
and
202
(a magnitude of displacement for Box-in-Box mark
204
). A broadband light such as a xenon lamp is used as a light source. By detecting an intensity of the light reflected from positions near edges of first and second overlay inspection marks
201
and
202
by a camera, edge positions of first and second overlay inspection marks
201
and
202
are recognized. Distances a and b shown in
FIGS. 11A
are obtained, and the magnitude of displacement of first and second overlay inspection marks
201
and
202
are calculated with the following expression (1).
Magnitude of displacement=(a−b)/2 (1)
Around 1991, a TIS (Tool Induced Shift) value of the magnitude of displacement would be used to increase overlay accuracy. The TIS value of displacement represents a difference between measured values of displacement for erecting and inverted images of a wafer, respectively shown in
FIGS. 12A and 12B
. The TIS value can be calculated with the following expression (2).
TIS value=(a1−b1)/2−(a2−b2)/2 (2)
a1, b1: distances a and b when erecting
a2, b2: distances a and b when inverted
However, when the TIS value is determined with the inspection mark which has been formed by a prescribed process, the resulting TIS value would be large. Such large TIS value has been resulted from a displaced measurement focus plane.
Then, a method of determining an optimum measurement focus plane using the TIS value had been developed by around 1993. Until now, this method has been used for determining the optimum measurement focus plane.
Now, the method of determining the measurement focus plane will be described.
A wafer
101
is loaded on a stage of an overlay displacement measuring device (not shown) for measuring a magnitude of overlay displacement. A rotational correction of wafer
101
is performed using an arrangement of a plurality of chips
102
formed on wafer
101
as a reference, and a reference point on wafer
101
is selected.
After an output level of a light source and a focus plane of a camera are set, an image of wafer
101
is incorporated.
Then, a magnitude of displacement of first and second overlay inspection marks
201
and
202
are measured in accordance with the above described method of measuring the magnitude of displacement for each of chips
102
, so that the TIS value of the magnitude of displacement is obtained.
The TIS value of displacement is repeatedly determined by changing the focus plane and, for each focus plane, a 3S (3 Sigma) of the magnitude of displacement is obtained. Thereafter, the focus plane providing the minimum 3S of the TIS value is selected as the focus plane providing the highest measurement accuracy for the magnitude of displacement.
However, our examination of the above described method of determining the focus plane by 3S of the TIS value has suggested that there is not any relation between 3S of the TIS value and 3S of the measured value of displacement, as shown in FIG.
13
. Therefore, even if the focus plane providing the smallest 3S of the TIS value is determined as the measurement focus plane, the magnitude of displacement is not always measured in the optimum manner.
SUMMARY OF THE INVENTION
The present invention is made to solve the aforementioned problem. An object of the present invention is to provide an overlay displacement measuring device capable of increasing accuracy for measuring a magnitude of displacement.
Another object of the present invention is to provide an overlay displacement measuring device for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement.
Still another object of the present invention is to provide an overlay displacement measuring device for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement in which an inspection mark having a large magnitude of displacement does not affect a variation in measured values.
Still another object of the present invention is to provide a method of increasing overlay accuracy capable of increasing accuracy for measuring a magnitude of displacement.
Still another object of the present invention is to provide a method of increasing overlay accuracy for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement.
Still another object of the present invention is to provide a method of increasing overlay accuracy for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement in which an inspection mark having a large magnitude of displacement does not affect a variation in measured values.
A method of increasing overlay accuracy according to one aspect of the present invention is performed in an exposure step in a process of manufacturing a semiconductor device. The above mentioned method of increasing overlay accuracy includes: a step of picking up an image of a semiconductor wafer including a plurality of chips each having first and second overlay inspection marks thereon by shifting a focus plane by a predetermined first distance in a predetermined first range with respect to a reference focus plane; a step of calculating a variation in measured values of displacement of the first and second overlay inspection marks for each focus plane; and a step of determining a measurement focus plane based on the variations in the magnitudes of displacement.
The measurement focus plane is determined by the variations in the measured values of displacement of the first and second overlay inspection marks. Unlike the conventional method using the TIS values, the focus plane is determined by using data which is directly related to the measurement of displacement. Therefore, the focus plane enabling accurate measurement of displacement of the first and second overlay inspection marks can be determined.
Preferably, the step of calculating the variation in the measured values of displacement includes: a step of calculating an average value of the measured values of displacement of the first and second overlay inspection marks for each pair of the first and second overlay inspection marks; a step of obtaining a value by su
Chawan Sheela
Mehta Bhavesh
Mitsubishi Denki & Kabushiki Kaisha
LandOfFree
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