Electric heating – Metal heating – By arc
Reexamination Certificate
2000-05-09
2002-01-15
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121480, C219S121400, C156S345420, C118S7230IR
Reexamination Certificate
active
06339206
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to systems for adjusting spatial plasma densities/distributions and spatial distributions of chemicals within a plasma, and particularly to systems which use a plasma to process a substrate.
2. Discussion of the Background
In many electrical device and solid state manufacturing processes, a plasma reacts, or facilitates a reaction, with a substrate, such as a semiconductor wafer. In order to generate the plasma, power is supplied to a gas by an inductive or a capacitive plasma coupling element. Examples of inductive coupling elements include conductive and helical coils. Many conventional systems supply the RF power through an electrical matching network (MN). One known inductive plasma generating system is disclosed in U.S. pat. No. 5,234,529, issued to Wayne L. Johnson, the inventor of the present application. The contents of that patent are incorporated herein by reference.
One method of generating a plasma source
114
is described with reference to
FIG. 1. A
gas is supplied to a process chamber
102
through gas inlets
112
. An RF power source
110
having an output impedance R
s
supplies RF power to a helical coil
104
acting as an inductive coupling element. The coil
104
couples energy into the gas and excites it into a plasma within a plasma region
108
of the process chamber
102
. The plasma and energetic and/or reactive particles produced by the plasma (e.g., ions, atom, or molecules), can then be released through an output
120
of the plasma source
114
and used to process a substrate, e.g., a semiconductor wafer
106
or a flat panel display substrate.
During plasma processing, one factor controlling how processing occurs is “ambipolar diffusion.” The ambipolar diffusion process is illustrated in
FIG. 2
, which portrays a recombination surface
1306
to which electrons
1302
of the plasma are attracted. Upon reaching the recombination surface
1306
, the electrons
1302
adhere thereto, thereby producing a net negative charge which attracts ions
1304
from the plasma. The ions
1304
, upon reaching the recombination surface
1306
, recombine with electrons
1302
to produce neutral particles
1308
. This recombination lowers the ion density n
p
in the plasma since neutral particles
1308
do not contribute to the ion density n
p
. More importantly, the plasma density is reduced adjacent to the recombination surface
1306
as compared to further away from the surface
1306
. Consequently, the geometry of the walls acting as recombination surfaces affects the spatial distribution of a plasma within the source. In addition, since some ion species are more susceptible to this recombination process than other species, a recombination surface can cause one or more of the ion species to recombine disproportionately, thereby affecting the chemical composition of the plasma.
The ion density n
p
of the plasma in a particular region is also affected by the rates of several processes, including (1) the rate of production of ion-electron pairs, (2) the rate of recombination of ion-electron pairs, and (3) the rate of flow of electrons and ions into or out of the region (including pumping). The local plasma density n
p
in the region at a particular time is the value at which the aforementioned process rates are at an equilibrium. The value of n
p
also can be affected by the amount of power supplied to the region. More specifically, an increased amount of power supplied to the region tends to increase the local rate of production of ion-electron pairs, thereby increasing the value of n
p
in the region.
Non-uniform spatial distribution of the density of the plasma across the output
120
of the source
114
is disadvantageous. As shown in the graph of
FIG. 3A
, the local plasma density n
p
at a given location x across the output of a source can depend on the location, as well as the average plasma density <n
p
> of the source. The graph includes curves representing n
p
vs. x for two different plasmas, each having its own value of average density <n
p
>. For both plasmas of this example, n
p
is at a maximum in the center
320
of the source (and, therefore, in the center of the wafer
106
) and is smaller at the edges
322
. Further, this non-uniformity of n
p
is more pronounced when the average density is higher (high <n
0
>) than it is when the average density is lower (low <n
p
>).
As described above, the ion density n
p
also varies spatially based on the geometry of the source.
FIG. 3B
is a graph of local ion density n
p
as a function of location x for sources varying effective with and effective length L. As illustrated in the graph, the uniformity of plasma density can depend on the aspect ratio (L/W) of the plasma source. For examply, the ion density n
p
of each of the long and medium sources is greatest in the center
320
of the source and smallest at the edges of
322
whereas, for the short source, n
p
exhibits a relative dip near the center
320
. The relative peak in plasma density near the center (and the relatively low plasma density near the edges
322
) of a long source, can be caused by the proximity of a side wall
124
to the edge of the source. The side wall provides a recombination surface which increases the rate of recombination of ions and electrons. As a result, the plasma density can be reduced near the edges of a long source.
When processing a substrate, particularly a semiconductor water, non-uniformity of plasma density can cause non-uniformity of reaction characteristics (e.g., reaction rates) across the surface of the substrate. For example, as illustrated in
FIG. 3C
, if a plasma is used to etch a film on a substrate, and the plasma has a higher density near the center
320
of the wafer
106
, the etching rate can be higher in the center of the wafer
106
and lower at the edges
322
. Similarly to the example of
FIG. 3A
, the process of
FIG. 3C
can exhibit more pronounced non-uniformity in cases of high <n
p
> and less pronounced non-uniformity in cases of low <n
p
>.
The problems of non-uniformity of plasmo densities are discussed in severs U.S. patents which are incorporated herein by reference. Those patents are: U.S. Pat. No. 4,340,461 to Hendricks et al., entitled “Modified RIE Chamber for Uniform Silicon Etching”; U.S. Pat. No. 4,971,651 to Wantabe, entitled “Microwave Plasma Processing Method and Apparatus” in which local plasma density is absorbed, attenuated or diffused to produce a uniform plasma density, thereby uniformly processing a wafer; U.S. Pat. No. 5,444,207 to Sekine et al., entitled “Plasma Generating Device and Surface Processing Device and Method for Processing Wafers in a Uniform Magnetic Field”; U.S. Pat. No. 5,534,108 to Qian et al., entitled “Method and Apparatus for Altering Magnetic Coil Current to Produce Etch Uniformity in a Magnetic Field-Enhanced Plasma Reactor” in which a uniform plasma density is produced by a magnetic field rotating in a plane parallel to a horizontal plane of a processed substrate; U.S. Pat. No. 5,589,737 to Barnes et al., entitled “Plasma Processor for Large Workpieces” in which uneven processing is described as a result of non-uniform plasma density over large workpieces such as rectangular flat panel displays; and U.S. Pat. No. 5,593,539 to Kubota et al., entitled “Plasma Source for Etching” in which electrons are moved in a cycloid motion in order to produce a uniform plasma density.
Improvements in the performance of parallel plasma processors have been made by changing one or more electrodes in various ways. Gorin and Hoog (U.S. Pat. No. 4,209,357) describe increased uniformity of etching using different sized electrodes with adjustable spacing. Adjustable spacing has also been considered by Koch (U.S. Pat. No. 4,340,462). Hendricks et al. (U.S. Pat. No. 4,340,461) describe using a baffle plate to increase the size of the powered electrode. Non-planar electrodes of various shapes have been asserted to be beneficial. Some are simply curved (see Mu
Paschall Mark
Tokyo Electron Limited
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