Semiconductor device manufacturing: process – Including control responsive to sensed condition – Optical characteristic sensed
Reexamination Certificate
1999-10-21
2003-04-01
Fourson, George (Department: 2823)
Semiconductor device manufacturing: process
Including control responsive to sensed condition
Optical characteristic sensed
C430S311000
Reexamination Certificate
active
06541283
ABSTRACT:
TECHNICAL FIELD
The present claimed invention relates to the field of semiconductor wafer fabrication. More specifically, the present claimed invention relates to a method for determining the magnification portion of misalignment error in a stepper used to fabricate patterned layers on a wafer.
BACKGROUND ART
Integrated circuits (ICs) are fabricated en masse on silicon wafers using well-known photolithography, etching, deposition, and polishing techniques. These techniques are used to define the size and shape of components and interconnects within a given layer of material built on a wafer. The IC is essentially built-up using a multitude of interconnecting layers, one formed on top of another. Because the layers interconnect, a need arises for ensuring that the patterns on adjacent layers of the wafer are accurately formed.
Accurate formation of an image on a wafer using photolithography depends on several error-causing variables. These variables include, but are not limited to, rotational alignment error, translational alignment error, reticle writing error, and magnification error, between the reticle and the wafer. Magnification error is one of the more important variables for accurately forming an image on a wafer. Precise magnification of images formed on each layer is critical for several reasons. For example, proper magnification is necessary to accurately shape and size devices for proper performance, as well as to ensure proper location of insulators and interconnecting conductors. Hence, a need arises for ensuring accurate magnification of an image from a reticle formed on a layer of a wafer.
Each one of the error-causing variables can be corrected by a different part of the stepper. If errors are not segregated and measured independently, then the error measurements are confounded, and the resulting corrections for each variable may be contradictory and self-defeating. Thus, a need arises for a method to segregate other error-causing variables from the magnification error, so as to yield a true magnification error measurement.
Referring now to prior art
FIG. 1A
, a top view of a conventional alignment reticle is shown. Alignment reticle
126
includes multiple overlay patterns
110
a
-
110
e,
and a fine alignment target
132
located at an outer portion of the alignment reticle
100
b.
Each overlay pattern
110
a
-
110
e
includes a first overlay box
130
a
and a second overlay box
130
b,
though only shown in overlay pattern
110
a
for clarity. Hence, the fine alignment target
132
is located a significant distance,
136
and
138
, away from small overlay box
130
a
and large overlay box
130
b.
Large overlay box
130
b
is offset from small overlay box
130
a
by a distance
140
.
The conventional alignment reticle and conventional magnification error measurement process is corrupted by using an alignment target having magnification error, rotational error, and/or translational error. The conventional reticle includes an alignment target at an outer location of the reticle image,
132
of prior art
FIG. 1B and 126
b
of prior art
FIG. 1A
, that is projected through an outer portion
128
b
of the lens
128
of prior art FIG.
1
A. Consequently, the alignment target created on the wafer suffers from magnification error, rotational error and translational error as well as reticle writing error. Furthermore, the conventional magnification error measurement process compares a full-field shot on each of two layers. However, a full-field shot includes errors other than magnification. Hence, the magnification measurement is confounded with other these other errors. Consequently, the magnification measurement may not be accurate, and thus compromise yield of the wafer and performance of the IC formed on the wafer. Hence, a need arises for a more accurate reticle and for more accurate shots on a wafer, with which magnification error can be measured.
Additionally, the conventional fine alignment target includes duplicative magnification error. Magnification error, such as lens distortion, typically increases towards the outer regions of the lens, due to factors such as lens irregularities and to properties of light. Additionally, the alignment target created on the wafer suffers from reticle writing error because it is located a significant distance, e.g.
136
and
138
of prior art
FIG. 1B
, away from the overlay patterns, e.g.
110
a
and
110
e,
used to measure the magnification error of the stepper. That is, reticle writing error can have an error rate, linear or exponential, that accumulates over the distance between two images on the reticle. Hence, if an overlay pattern is located far away from an alignment target, then the prior art magnification error check will be measuring the translational misalignment of the alignment target along with the magnification error of the stepper.
Furthermore, a large distance between the overlay pattern and the alignment targets only serves to amplify any processing error for the steps used in the alignment process, e.g. magnification error. For example, if the wafer is realigned in the stepper using a charge coupled device (CCD) and digital signal processing for pattern matching, both having a given tolerance, then this tolerance may be amplified at a location far from the alignment target. In one instance, a given rotational error at the alignment will increase with the distance, or radius, from the alignment target. This scenario is shown in the following figure, prior art FIG.
1
B. Consequently, a need arises for creating an error-free alignment target. More specifically, a need arises for a method to measure magnification error using an alignment target that does not include reticle writing error, translational error, rotational error, or magnification error.
Referring now to prior art
FIG. 1B
, an example of a Preventative Maintenance (PM) wafer
150
with overlay boxes created therein is shown. Only one shot, shot
160
b,
is shown in this figure for clarity. Shot
150
has a small overlay box
160
a
and a large overlay box
160
b,
and a fine alignment target
162
formed therein. Alignment reticle
126
of prior art
FIG. 1A
is used to create the overlay boxes on wafer
150
. However, in this example, rotational error occurs when the stepper did not accurately align to fine alignment target
162
. This situation arises for the process that formed the second overlay box
160
b
on wafer
150
. Even though the rotational error during alignment was a small angle
164
, the large distance
166
between fine alignment target
162
and overlay box
160
a
magnifies the error to a substantial X error
162
and Y error
164
. Part of this rotational error in the fine alignment target, as well as other errors such as magnification error in the fine alignment target, may be interpreted as a magnification error between boxes
160
b
and
160
a.
Consequently, the prior art alignment reticle and misalignment measurement process may actually overcorrect the stepper and possibly cause more error than originally existed.
Confounding the magnification error also occurs by not separating out a translational portion of the misalignment error prior to forming images on a wafer for the magnification error process. The alignment of a wafer for a magnification error measurement process intrinsically includes a translational error. Conventionally, the translational error is not accounted for in a magnification error measurement. If this error is not compensated for, it will affect the results of the magnification error measurement. Thus, by using the magnification level to compensate for the translational portion of the alignment error, alignment accuracy can possibly be degraded, due to miscorrection. Consequently, a need arises for compensating for the transitional error in the magnification error measurement.
The confounding of errors in the conventional magnification measurement process becomes important when considering budget overlay requirements. Budget overlay is a value associated with the allowable tolerance for ma
Fourson George
Koninklijke Philips Electronics , N.V.
Simons Kevin
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