Abrading – Precision device or process - or with condition responsive... – Computer controlled
Reexamination Certificate
2001-04-18
2003-02-25
Hail, III, Joseph J. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
Computer controlled
C451S009000, C451S041000, C438S401000
Reexamination Certificate
active
06524163
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of semiconductor device manufacturing and, more particularly, to a method and apparatus for controlling a polishing process based on scatterometry derived film thickness variation.
2. Description of the Related Art
During the manufacture of semiconductor devices, semiconductor wafers, each including a plurality of individual die, are subjected to a number of processing steps. Typically, wafers are grouped into lots that are processed together. Each lot may contain, for example, 25 individual wafers. Certain of the processing steps are sensitive to the alignment of the wafer within the processing tool. For example, photolithography processing steps are highly sensitive to the alignment of the wafer. Other steps, including metrology steps, are also sensitive to wafer alignment, but to differing degrees.
FIG. 1
illustrates a typical semiconductor wafer
100
. The wafer
100
includes an orientation notch
110
useful as a reference point for a rough alignment of the wafer
100
. For identification purposes, a unique wafer identification code
120
is scribed on the wafer
100
beneath the notch
110
using a laser scribing process where small dots are burned into the surface of the wafer to construct the characters or symbols of the code. Exemplary wafer identification codes
120
may include alphanumeric identifiers or bar code identifiers (e.g., 1 or 2 dimensional codes). During the production process, process history and metrology information is stored in a database for each of the wafers
100
indexed by its respective wafer identification code
120
.
Typically, prior to performing an orientation-sensitive process, the wafer
100
is rotated until the notch
110
is located and placed in a predetermined position. Other techniques for performing rough alignments include using an edge alignment procedure where the wafer
100
is rotated and optically scanned to determine the profile of the edge at various positions about the rotation. Typically, a wafer
100
is not perfectly round. As such, the edge moves with respect to a fixed reference point as the wafer
100
is rotated. By determining the edge profile, the approximate center of the wafer
100
can be determined. The spatial relationship between the notch
110
and the approximate center point may be then used as a reference point for rough alignment of the wafer.
These rough alignment techniques are not suitable for highly sensitive processes such as photolithography. Accordingly, multiple sets of alignment marks
130
,
135
,
140
are etched into the wafer
100
near the periphery prior to the commencement of process steps for forming devices on the wafer
100
. A wafer
100
typically includes a plurality of individual semiconductor die
150
arranged in a grid
155
. Photolithography steps are typically performed by a stepper on approximately one to four die locations at a time, depending on the specific photomask employed. The alignment marks
130
,
135
,
140
provide an accurate reference point for aligning the stepper to the individual cells in the grid
155
that are to be exposed. The stepper includes sensitive optical scanning equipment to locate the alignment marks
130
,
135
,
140
and finely align the wafer
100
based on the alignment marks
130
,
135
,
140
such that the individual die
150
are accurately patterned.
Typically, the stepper selects one of the sets of alignment marks
130
,
135
,
140
for alignment. Although only three sets of alignment marks
130
,
135
,
140
are shown, more sets may be possible. The different sets of alignment marks
130
,
135
,
140
are disposed at different distances from the edge of the wafer
100
. For example, the alignment marks
130
,
135
may be disposed about 4 nm from the edge and the alignment marks
140
may be disposed 5 nm from the edge. Because of the different positions of the alignment marks
130
,
135
,
140
, they are not subject to the exact same processing environment. Accordingly, the ability of the stepper to align to one set of alignment marks
130
,
135
,
140
may differ from its ability to align to the other set of alignment marks
130
,
135
,
140
. If one particular set of alignment marks
130
,
135
,
140
is damaged by processing, resulting in a lower signal to noise ratio in the alignment process, the stepper may use an alternative set of alignment marks
130
,
135
,
140
having a higher signal to noise ratio.
Generally, the stepper selects one set of the alignment marks
130
,
135
,
140
as a default set, aligns the wafer
100
using the selected set of alignment marks
130
,
135
,
140
, and exposes a layer of photoresist material to form a desired pattern. If the set of alignment marks
130
,
135
,
140
chosen had a relatively low signal to noise ratio, the alignment may be incorrect. In some cases alignment errors may be detected, and the wafer
100
may be reworked. During the rework, a different set of alignment marks
130
,
135
,
140
is selected by the stepper to align the wafer
100
. In other cases, the alignment error is not detected until after a process that may not be reversed has been performed (e.g., etching). In such as case, the wafer
100
must be scrapped. Reworking or the scrapping the wafer is expensive and reduces the efficiency of the processing line.
As seen in
FIG. 2A
, an illustrative grating structure
200
used to define the alignment marks
130
,
135
,
140
is shown. The grating structure
200
includes trenches
210
formed in a silicon substrate
220
, shown in cross-section in
FIGS. 2B and 2C
. A variety of different constructs for the grating structure
200
may be used. For example, an alternate grating structure may comprise a single, rectangular group of trenches.
As shown in
FIG. 2B
, during the fabrication of shallow trench isolation (STI) structures on the wafer
100
, a layer of silicon nitride
230
is deposited on the wafer
100
for use as a stop layer for chemical mechanical polishing. A layer of silicon dioxide
240
formed using tetraethoxysilane (TEOS) is formed over the silicon nitride
230
(i.e., other layers, such as a silicon oxynitride antireflective coating layer (ARC) (not shown) and a liner oxide layer (not shown) may be disposed between the silicon nitride stop layer
230
and the silicon dioxide layer
240
). The silicon nitride stop layer
230
is deposited over the entire wafer
100
, including over the grating structure
200
. The silicon dioxide layer
240
is subsequently polished to remove excess material, and the silicon nitride stop layer
230
is stripped.
Typical CMP polishing processes do not tightly control the polish rates near the edges of the wafer
100
(i.e., where the alignment marks
130
,
135
,
140
are located), because no devices are present in that region and also because there are no available metrology techniques for monitoring the polishing rates near the edges. Accordingly, the edge regions may be overpolished or underpolished with respect to the other portions of the wafer
100
. If the edge region is over polished, all of the silicon nitride stop layer
230
is polished away and a portion of the silicon substrate
220
in which the trenches
210
are formed is also removed. If a portion of the edge region is underpolished, remnants of the silicon oxide layer
240
may remain over the trenches
220
and interfere with the subsequent stripping of the nitride stop layer
230
. As seen in
FIG. 2C
, in an overpolished region
250
, the depth of the trenches
210
is reduced. In an underpolished region
260
, remnants of the silicon nitride stop layer
230
remain in the bottom of the trenches
210
.
The remnants of the silicon nitride stop layer
230
result in a degradation of the signal to noise ratio when the alignment marks
130
,
135
,
140
are used for subsequent optical alignment. In some of the trenches
210
, no remnants may be present (i.e., no underpolishing). For the trenches
210
, with silicon nitride remnants
Advanced Micro Devices , Inc.
Hail III Joseph J.
Thomas David B.
Williams Morgan & Amerson P.C.
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