Optics: measuring and testing – By configuration comparison – With photosensitive film or plate
Reexamination Certificate
1998-12-11
2001-03-20
Pham, Hao Q. (Department: 2877)
Optics: measuring and testing
By configuration comparison
With photosensitive film or plate
C451S006000
Reexamination Certificate
active
06204922
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of film thickness measurement and film etching and deposition end point detection, and more specifically, to the field of film measurement and end point detection in an environment, such as semiconductor wafer fabrication and processing, in which the layer whose thickness is desired to be measured resides in a multi-layer or patterned sample.
Many industrial processes require precise control of film thickness. In semiconductor processing, for example, a semiconductor wafer is fabricated in which one or more layers of material from the group comprising metals, metal oxides, insulators, silicon dioxide (SiO
2
), silicon nitride (SiN), polysilicon or the like, are stacked on top of one another over a substrate, made of a material such as silicon. Often, these layers are added through a subtractive process in which a layer having a thickness greater than the desired thickness is added to the previous layer, and then polished or ground down to the desired thickness through a process known as chemical mechanical planarization (CMP). The level of precision which is required can range from 0.001 &mgr;m (a few atoms) to 100 &mgr;m (about the thickness of a human hair).
With reference to
FIG. 1
, a CMP station
1
is illustrated. The station comprises upper platen or pallet
2
and lower platen or pallet
6
configured to move relative to one another. In the specific example illustrated in the figure, pallet
2
is configured to axially move in a counter-clockwise direction relative to pallet
6
, and pallet
6
is configured to move horizontally or longitudinally relative to pallet
2
. The upper pallet
2
has a mount
3
for fixably mounting semiconductor wafer
4
. The lower pallet
6
is configured with a polishing pad
5
, which in operation comes into contact with a layer
4
a
on the underside of wafer
4
. A means
7
is provided for delivering a slurry to the upper surface
5
a
of the polishing pad
5
. As shown, the delivery means includes one or more channels, one of which is identified in the figure with identifying numeral
7
a,
for delivering the slurry to the upper face
5
a
of the polishing pad. The slurry facilitates polishing of the wafer face, and typically comprises an abrasive chemical composition such as an ultra-fine grit formed of solid alumina or silica particles in a solution.
In operation, the upper and lower platens are moved relative to one another such that the upper surface of the polishing pad
5
a
frictionally engages the lower surface of the layer
4
a
of the wafer
4
while slurry is delivered to the upper surface of the polishing pad
5
a
through means
7
. Through this procedure, polishing of the lower surface of layer
4
a
is accomplished. A mechanism (not shown) is provided to monitor the thickness of the layer
4
a.
This mechanism is used to control the CMP station so that the thickness of the layer
4
a
is not reduced below the desired thickness through excessive polishing. Exemplary embodiments of CMP stations configured to polish the surfaces of semiconductor wafers are described in U.S. Pat. Nos. 5,667,424; 5,663,797; 5,658,183; and 5,643,044, each of which is hereby incorporated by reference herein as though set forth in full.
In CMP processing, it is necessary to periodically if not continuously monitor film thickness or change in thickness in order to ensure that the thickness is not reduced below the desired amount though excessive polishing. Optical methods for monitoring film thickness in applications such as CMP have been proposed. According to such methods, an estimate of film thickness is derived through suitable analysis of light reflected from the surface of the film.
A problem with such methods as applied to CMP is that the monitoring of the film thickness or change in thickness cannot typically be done in situ, i.e., while chemical mechanical planarization of the wafer is underway. The reason is that the slurry introduces a distortion into the reflected light, making it difficult if not impossible to accurately measure film thickness.
The problem is illustrated in
FIG. 2
, in which, compared to
FIG. 1
, like elements are identified with like identifying numerals. As shown, a slurry
8
is interposed between the lower surface of film
4
a,
and the upper surface
5
a
of polishing pad
5
. Light
9
is directed to the film
4
a
for the purpose of measuring the thickness of the film. The reflected light
10
which ensues is captured and analyzed. As can be seen, the light
10
passes through the slurry
8
. This process introduces distortion or noise into the light
10
.
Consequently, the CMP procedure must typically be halted and the slurry removed from the film
4
a
in order to allow for accurate measurement of film thickness. Since monitoring of film thickness must be performed periodically if not continuously, the overhead involved in repeatedly stopping the CMP procedure, and removing the slurry, can be prohibitively expensive in terms of reduced throughput of the CMP procedure. In many cases, this renders the optical method of film measurement infeasible for application to CMP because of the overhead involved.
Another problem with such approaches is that distortion introduced by the optical components of the film measuring apparatus can adversely impact the accuracy of film thickness measurement. With reference to
FIG. 2
, each of the optical components involved in directing light
9
to the film
4
a,
and in capturing and analyzing the reflected beam
10
, can introduce such a distortion into the reflected light
10
.
A third problem with such methods is that, in the case in which the layer the thickness of which is desired to be measured resides in or on a multi-layer or patterned sample, interference from nearby layers can interfere with accurate measurement of the thickness of the desired layer. This problem can arise in contexts involving semiconductor fabrication other than CMP processing, such as the case in which a semiconductor layer is added to the top of a stack of thin films of semiconductor material through an additive process such as chemical vapor deposition (CVD). In this case, accurate measurement of the thickness of the top-most film using conventional approaches can be problematic due to the interference from nearby layers.
Optical methods for film thickness measurement have been proposed which employ spectral or Fourier analysis of the light reflected from the film. Representative examples of such methods are described in U.S. Pat. Nos. 5,646,734; 5,642,196; 5,587,792; 5,227,861; 4,984,894; 4,555,767; 3,985,447; and 3,880,524. However, such methods are computationally complex, require time-consuming comparisons with theoretical or expected waveforms, employ excessive numbers of analytical steps to determine film thickness, or require time-consuming angular and mechanical movements or sweeps of an optical device such as a mirror. Consequently, in many cases, the process of monitoring film thickness employing such methods cannot be performed in real time, i.e., concurrently with and without requiring stoppage of semiconductor processing. In such cases, the rate of semiconductor processing must be slowed down to permit film monitoring. The result is a reduced throughput of semiconductor processing.
Another problem with such methods in the context of individual film measurement in a multi-layer or patterned sample is that optical interference introduced from nearby layers can prevent accurate thickness measurement of the one layer.
Accordingly, it is an object of the present invention to provide a method and apparatus for achieving rapid and accurate thin film measurement and end point detection in a noisy environment.
Another object is a method and apparatus for thin film processing which permits in situ monitoring of film thickness and end point detection during CMP processing.
Another object is an optical method and apparatus for thin film measurement and end point detection during CMP processing which provides for ac
Filmetrics, Inc.
Howrey Simon Arnold & White , LLP
Pham Hao Q.
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