Method for optimizing a gap for plasma processing

An optimal gap is determined between a lower electrode and an upper electrode in a plasma processing device. A gap is set between the lower electrode and the upper electrode, and a substrate is processed in the plasma processing chamber. The processing results are obtained, and the processing rate and uniformity are determined from the processing results. The processing rate and uniformity are plotted with the gap setting. The steps of setting, processing, obtaining, determining, and plotting are repeated for additional substrates, the gap setting being different for each substrate. The optimal gap setting is selected as the gap setting corresponding to an optimal processing rate and an optimal uniformity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for optimizing plasma processing. 
More particularly, the present invention relates to a method for 
optimizing a gap for plasma processing to improve processing uniformity 
and throughput. 
2. State of the Art 
Plasma processing devices are widely used for chemical vapor deposition of 
materials on substrates and etching of substrates by supplying process gas 
to a plasma processing chamber and applying a radio frequency (RF) field 
to the gas. As shown in FIG. 1, a typical plasma processing device 
includes a chamber 100 enclosing a lower electrode 104, which supports a 
substrate 140, and an upper electrode 102. The lower electrode 104 is 
powered by an RF power source 114, and the upper electrode 102 is powered 
by an RF power source 112. Reactant gas supplied into the processing 
chamber is excited into a plasma by RF power applied through the 
electrodes 102 and 104, and the excited plasma causes deposition and/or 
etching on the substrate 140. Substrates typically processed in such a 
device include semiconductor wafers and flat panel displays. 
Examples of a plasma processing device include the LAM 4420 Polyetcher, 
which employs a mechanical clamp to hold the substrate on the lower 
electrode, and the LAM 4420XL Polyetcher, which employs an electrostatic 
chuck (ESC) to hold the substrate in place via an electrostatic field. 
It is important to maximize yield and throughput in a plasma processing 
device. Various processing parameters affect yield and throughput, 
including processing pressure. Plasma processing devices often include 
turbomolecular pumps used in combination with load locks to maintain a 
desired pressure for the plasma processing chamber. For example, FIG. 2 
illustrates a turbomolecular pumped plasma processing device. Referring to 
FIG. 2, the turbomolecular pumped plasma processing device includes a main 
plasma processing chamber 100, load locks 110 and 120, and a 
turbomolecular pump 130. Substrates are input into the load lock 110, 
processed through the main chamber 100, and output through the load lock 
120. The load locks 110 and 120 employ dedicated pumps to increase the 
substrate transfer time into and out of the main plasma processing chamber 
100, thus optimizing throughput of the plasma processing device. The 
turbomolecular pump 130 allows lower operating pressures at higher pumping 
rates which not only complements throughput but also allows a wider range 
of possible pressure settings for plasma processing. Thus, turbomolecular 
pumped plasma processing devices have proven effective for improving yield 
and enhancing throughput. 
Another processing parameter affecting throughput and yield in a plasma 
processing device is the gap between the upper electrode and the lower 
electrode. Different gap settings result in different concentrations of 
excited plasma above the substrate. Thus, the processing rate, and hence 
the throughput, vary depending on the gap setting. The concentration of 
excited plasma can also vary across the surface of the substrate, 
depending on the gap setting. Therefore, the gap setting also affects 
processing uniformity, therefore affecting the yield. 
Conventionally, gaps are set to optimize the processing rate, without 
concern for uniformity. Since the best gap setting for the processing rate 
can not be the best gap setting for uniformity, this conventional method 
of setting the gap does not produce an optimal yield. 
In addition to processing pressure and the gap between the upper electrode 
and the lower electrode, there are other factors that affect processing 
yield and throughput, including processing gas flow and RF power. If the 
plasma processing device is not producing sufficient yield or not 
operating at an acceptable throughput, the problem can be that one of the 
processing parameters is not properly set. Often, it is difficult to 
determine which parameter is not set properly. 
There is thus a need for a method to determine an optimal gap that provides 
both improved uniformity and throughput. There is also a need for a method 
for determining if a gap in a plasma processing device is properly set. 
SUMMARY OF THE INVENTION 
According to the present invention, a method for determining an optimal gap 
between a lower electrode and an upper electrode in a plasma processing 
device is provided. A gap is set between the lower electrode and the upper 
electrode, and a substrate is processed in the plasma processing chamber. 
The processing results are obtained, and a processing rate and uniformity 
are determined from the processing results. The processing rate and 
uniformity are plotted with the gap setting. The steps of setting, 
processing, obtaining, determining, and plotting are repeated for 
additional substrates, the gap setting being different for each substrate. 
The optimal gap setting is selected from the step of plotting as the gap 
setting corresponding to an optimal processing rate and an optimal 
uniformity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to the present invention, an optimal gap setting in a plasma 
processing device is determined based on uniformity of processing and 
processing rates. By improving the processing rate, the throughput of the 
plasma processing device is increased. By improving the uniformity, the 
yield is increased. 
FIG. 3A illustrates an exemplary method for optimizing a gap in a plasma 
processing device according to the present invention. Referring to FIG. 
3A, a gap is set at step 300. The gap can be set with any conventional gap 
setting tool, such as that shown in FIG. 4. Next, a substrate is processed 
at step 310, and the processing results are obtained for various locations 
on the substrate at step 320. The processing results can be obtained, for 
example, by comparing the thickness of a top layer of the substrate before 
processing and the thickness of the top layer of the substrate after 
processing. The measurement of thicknesses of the top layer of a substrate 
is described in more detail below with respect of FIGS. 5 and 6. 
At step 330, processing rates and uniformities for the various locations on 
the substrate are determined from the processing results. At step 340, the 
standard deviations of the uniformities and processing rates are obtained. 
The standard deviations are plotted on a graph with the gap setting at 
step 350. This is described in more detail below with reference to FIGS. 
7A and 7B. 
Next, at step 360, a determination is made whether the last substrate in 
the set of substrates to be used for setting an optimal gap has been 
processed. If not, the method returns to step 300. If the last substrate 
has been processed, the method proceeds to step 370 at which an optimal 
gap setting corresponding to an optimal processing rate and an optimal 
uniformity is selected from the graph. 
The method illustrated in FIG. 3A can be used, for example, in an initial 
set up process. Optimization of a gap can also be useful after the device 
has been set up, to determine, for example, whether the gap is still 
properly set. The gap setting can be off for a number of reasons, for 
example, because the upper electrode and the lower electrode have been 
replaced or cleaned. The method illustrated in FIG. 3A can be extended to 
determine whether a gap is properly set, keeping other processing 
parameters, such as the gas flow, RF power, and the processing pressure 
constant. 
FIG. 3B illustrates a method for determining if a gap is properly set. The 
method for determining if a gap is properly set begins after step 370 in 
FIG. 3A. As shown in FIG. 3B, the method proceeds from step 370 to step 
380 at which a worst gap setting corresponding to a worst uniformity, 
i.e., a gap setting having a highest standard deviation in uniformity, is 
selected from the graph. Next, at step 390, another substrate is processed 
at the worst gap setting. At step 400, the processing results are obtained 
for various locations on the substrate. At step 410, the uniformities at 
various locations on the other substrate are determined. At step 420, a 
standard deviation of the uniformities is calculated. The standard 
deviation is plotted at step 430. At step 440, the uniformities at the 
worst gap setting in the graphs are compared. If the uniformities are 
substantially the same, the gap setting is determined to be properly set. 
FIG. 4 illustrates a typical tool for setting a gap G between a bottom 
surface of the upper electrode 104 and a top surface of the lower 
electrode 102. The upper electrode 102 is lowered until it contacts the 
prongs 106 on the lower electrode 104. The prongs 106 have sensors which 
detect the contact with the upper electrode 102. The prongs have a 
definite fully extended length of, for example, up to 5 cm. The gap G is 
set by lowering the upper electrode 102, pushing the prongs 106 down 
toward the lower electrode 104 by a desired amount. Using this tool, the 
gap G can be set to any desired amount. The tool shown in FIG. 4 is shown 
as an example for setting a gap, and any tool can be used to set the gap G 
to a desired amount. 
The gap measurement tool can also be useful for measuring the uniformity of 
the gap G. The distance between the lower electrode 104 and the upper 
electrode 102 can vary across the surface of the lower electrode because, 
for example, the upper electrode can be tilted. By distributing multiple 
prongs 106 across the surface of the lower electrode 104, differences in 
the gap G across the surface of the lower electrode 104 can be measured. 
According to the present invention the method of optimizing a gap is 
applicable to deposition and etching plasma processing devices. In the 
interest of simplicity, however, the following description refers to an 
etching device. 
FIG. 5 illustrates an exemplary measurement layout on a substrate. As shown 
in FIG. 5, measurements of processing results can be made at nine 
locations on a substrate, numbered 1-9. In an exemplary embodiment, the 
measurement locations numbered 6-9 are positioned at approximately 3 mm 
from the edge of a 150 mm substrate. The measurement locations numbered 
2-5 are positioned at approximately 37.5 mm from the edge of the 
substrate. The measurement location numbered 1 is positioned approximately 
in the center of the substrate. As shown in FIG. 5, the flat 145 of the 
substrate 140 can be rotated 45 degrees to omit any aberrations in 
measurement. 
According to an exemplary embodiment, the processing results are obtained 
by measuring the thickness of a top layer of the substrate before the 
substrate is processed, measuring a thickness of the top layer of the 
substrate after the substrate is processed, and comparing the 
measurements. 
Measurements of substrates processed in an electrostatic chuck (ESC) plasma 
processing device are more accurate than measurements of substrates 
processed in a mechanical clamp plasma processing device. This is because 
the mechanical clamp plasma processing devices have poor backside helium 
cooling resulting in an overetching condition at the edge of the 
substrate. Thus, measurements can be made closer to the edge of the 
substrate if the substrate is processed in an ESC device. For example, 
measurements made from substrates processed in mechanical clamp devices 
require an extended edge exclusion of 10 mm or more compared to 
measurements made in ESC device, which can be made as close as 3 mm from 
the substrate edge. 
In an exemplary test, 150 mm substrates were processed that contained 1500 
.ANG.ng of doped, thermally grown PolySi on 400 .ANG.ng of thermal oxide 
on bare silicon. The thickness of the PolySi layer was measured before and 
after processing. These materials and thicknesses are mentioned only as an 
example, and the present invention is not limited to application to 
substrates containing these materials. 
The thickness of the top layer of the substrate can be measured by 
applying, for example, a spectral analysis. FIG. 6 illustrates an 
exemplary spectral analysis of a location on a substrate that has been 
processed. The measurement tool used to obtain the PolySi thickness 
measurements in this example utilized wavelengths ranging from 250 nm to 
750 nm. 
The reflectivity of the film is obtained throughout the wavelength range 
resulting in a graphical spectral analysis of the PolySi film over the 
thermal oxide of the substrate. The film is modeled for measurements made 
before and after processing to provide the most accurate goodness of fit 
(GOF) measurements. The GOF is measured by comparing the spectral analysis 
of each measurement location to the spectral analysis of the modeled film. 
FIG. 6 displays two curves in the 250 nm to 750 nm wavelength range. One 
curve 200 represents the film model, while the other curve 210 represents 
the spectral analysis of the measurement. As can be seen from FIG. 6, the 
spectral analysis of the measurement location overshoots the film model at 
the top of the curve, but at the bottom there is nearly a perfect overlay 
of the two curves. 
According to an exemplary embodiment of the present invention, a plurality 
of substrates are premeasured as described above. The substrates are 
processed in the plasma processing device, and the gap setting is modified 
for each substrate. The post PolySi thicknesses of the substrates are 
measured, and the mean and uniformity of the delta (loss) is obtained. The 
processing rate and %3 .sigma. uniformity are calculated from the mean and 
uniformity of the delta loss. The processing rate and %3 .sigma. 
uniformity are then plotted on a graph with the gap setting. 
FIGS. 7A and 7B illustrate exemplary plots of processing rates and 
uniformities for various gap settings using different plasma processing 
devices. FIG. 7A illustrates plots obtained for substrates processed in a 
LAM 4420XL Polyetcher, and FIG. 7B illustrates plots obtained for 
substrates processed in a LAM 4420 Polyetcher. 
Referring to FIGS. 7A and 7B, the uniformity and the etch rate are 
represented along the vertical axes and the gap setting is represented 
along the horizontal axis. The uniformity is represented by the line with 
superposed squares, and the etch rate is represented by the line with 
superposed diamonds. 
To establish the optimal gap setting, gap settings having an optimal 
uniformity are selected. Using 7%3 .sigma. as an optimal uniformity, three 
candidates for a gap setting can be selected from the graph shown in FIG. 
7A. 
Referring to FIG. 7A, the gap setting with the best uniformity is 1.4 cm. 
This gap setting corresponds to a uniformity deviation of 3%3 .sigma.. The 
etch rate at this gap setting is approximately 2550 .ANG.ng/Min. Thus, 
this gap setting offers low throughput. In addition, this gap setting 
suffers from a wide latitude in uniformity when compared to gap settings 
close to it. As shown in FIG. 7A, the 1.35 cm and 1.45 cm gap settings 
result in a uniformity deviation of approximately 8%3 .sigma., compared to 
the uniformity deviation of 3%3 .sigma. that is achieved by the 1.4 cm gap 
setting. This wide latitude in uniformity is undesirable because it means 
that if the gap setting is a small amount off from the desired setting, 
the processing uniformity will be affected adversely by a significant 
amount. If throughput and latitude were not an issue, the 1.4 cm gap 
setting would be the most preferable, in terms of uniformity. 
Another gap setting with optimal uniformity is 1.25 cm. The uniformity 
deviation at this setting is approximately 4%3 .sigma.. Increasing the gap 
setting to 1.3 cm or reducing the gap setting to 1.2 cm results in a 
slightly greater uniformity deviation. The increase in etch rate from the 
1.4 cm gap to the 1.25 cm gap is approximately 100 .ANG.ng/min. This 
higher etch rate provides a 7.7% throughput increase during the main etch 
step of the process when compared to the 1.4 cm gap setting. 
A gap setting of 0.9 cm offers a uniformity of approximately 5%3 .sigma. 
with an etch rate of about 3500 .ANG.ng/min. Thus, the a 0.9 cm gap 
setting offers an increase in throughput of 30% compared to the 1.4 cm 
setting and an increase in throughput of 24% compared to the 1.25 cm gap 
setting. Because the gap setting of 0.9 cm offers an optimal uniformity 
and the greatest throughput, this gap is selected as the optimal gap for 
uniformity and throughput. This gap setting can be used, for example, in 
the initial set up of the plasma processing device. 
Although, in the embodiment described above, gap setting candidates having 
an optimal uniformity are selected before selecting a gap setting having 
an optimal etch rate, the invention is not so limited. Alternately, gap 
setting candidates having an optimal etch rate can be selected first, and 
then the gap setting having the optimal uniformity can be selected. 
Another important aspect of the uniformity and processing rate plots shown 
in FIGS. 7A and 7B is the signature of the plots. The peaks and low points 
in the plots constitute the profile of the process and the plasma 
processing device. As shown in FIGS. 7A and 7B, the processing rate and 
uniformity plots obtained for substrates processed in two different 
devices have substantially the same peaks and low points. Thus, the 
determination of an optimal gap setting need only be made once for a 
number of different plasma processing devices. 
The method according to the present invention can be used to monitor, 
qualify, or improve process throughput, and this advantage is not only 
limited to Poly Etch processes. For example, although described with 
reference to the LAM 4420 and the LAM 4420XL Polyetchers, the present 
invention is applicable to any type of plasma processing device with a 
variable gap setting. Other examples of plasma processing devices to which 
the invention is applicable include the LAM 9000 Metal Etcher, the LAM 
4500 Oxide Etcher, the Applied 5000 Etcher, and etchers manufactured by 
Mattson Technologies, Tegal, and Hitachi. 
It will be appreciated by those skilled in the art that the present 
invention can be embodied in other specific forms without departing from 
the spirit or essential characteristics thereof. The presently disclosed 
embodiments are therefore considered in all respects to be illustrative 
and not restricted. The scope of the invention is indicated by the 
appended claims rather than the foregoing description and all changes that 
come within the meaning and range and equivalence thereof are intended to 
be embraced therein.