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Description  |
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FIELD OF THE INVENTION
The invention is directed to a focus ring device for use in a method for
plasma processing of a semiconductor workpiece surface/substrate and, in
particular, for use in an apparatus and process for plasma etching.
DESCRIPTION OF THE BACKGROUND ART
Semiconductor processing involves a number of different chemical and
physical processes whereby minute integrated circuits are created on a
substrate. The integrated circuits are constructed using multilayers of
interrelated patterns of various materials, the layers being created by
such processes as chemical vapor deposition (CVD), physical vapor
deposition (PVD), and epitaxial growth. Some layers are patterned using
photoresist masks and wet and dry etching techniques. Patterns are created
within layers by the implantation of dopants at particular locations. The
substrate upon which the integrated circuit is created may be silicon,
gallium arsenide, glass, or other appropriate material.
In the production of semiconductor workpieces, plasma etching and reactive
ion etching (RIE), both of which employ reactive gas plasma, are presently
the most widely used processes to form fine wire patterns in the submicron
line width region. In general, plasmas provide higher etch rates, greater
anistropy (i.e. more vertical profiles), and lower foreign material
concentrations as compared to wet etchants. In plasma etching, a gas (or
combination of gases) is ionized to form a plasma. Depending on the
conditions of the system (e.g., pressure, power, bias to the electrodes,
etc.) as well as the nature of the ions, the exposed material can be
etched in a "physical" mode, a "chemical" mode, or a mode that is both
physical and chemical. In the physical etch mode, the ions are inert with
respect to the exposed materials, but have sufficient energy to physically
dislodge atoms from the exposed surface. In the chemical mode, the ions do
chemically react with the exposed surface to form gaseous reaction
products that are prepared from the chamber. In RIE both physical and
chemical etching take place.
In present plasma etching processes requiring submicron patterns, the
semiconductor workpiece substrate to be processed is placed on an
electrode pedestal and is surrounded by a focus ring which is a
cylindrical insulator. The focus ring functions to enhance the uniform
application of the etch reaction of the plasma on the surface of the
semiconductor workpiece/substrate being processed. Generally the progress
of the etching reaction is slower at the center portion of the workpiece
than at the peripheral portion thereof. This is due to an "internal
loading effect" which refers to the depletion of the etching reaction
seeds in the center portion of the workpiece/substrate due to the etching
reaction. The focus ring functions to decrease the progress and speed of
the etch reaction at the peripheral portion of the substrate, thereby
achieving a substantial etching uniformity over the entire
workpiece/substrate surface.
Many of the processes carried out within semiconductor processing systems
leave contaminant deposits on the elements of the process reactor and the
reactor chamber walls which accumulate and become the source of
particulate matter harmful to the creation of a semiconductor device. As
the geometries of semiconductor devices become ever so smaller, the
ability to maintain the uniformity and accuracy of critical dimensions
becomes strained. In this dimensional downsizing environment, the
avoidance of contaminant particulate matter upon the surface of the
semiconductor workpiece has become more critical.
Particulate contamination buildup on semiconductor process chambers and
other reactor elements such as focus rings is particularly significant in
the etch processing of semiconductor elements employing metal films. These
metal films are generally etched by employing a number of reactive gases,
including halocarbon gases, as plasma components. In the case of an
aluminum film, the etchant gases used are predominantly the chlorine
containing gases, chlorine (Cl.sub.2) and boron trichloride (BCl.sub.3),
which enables formation of volatile aluminum chloride compounds upon
etching, which volatile compounds can be removed from the etch processing
chamber by applied vacuum. However, simultaneously with the formation of
volatile aluminum chloride compounds, other active chlorine and boron
containing species are formed which can react with any oxygen and water
vapor present in the etch processing chamber or with organic species from
pattenting photoresist to form non volatile particulate compositions which
ultimately produce relatively large quantities of contaminant on the inner
walls of the process chamber. The non volatile particulate compositions
initially tend to remain inside the etch chamber in the form of loosely
attached particles to the chamber etch surfaces. These loosely attached
etch by-product compounds can easily break free of the surface to which
they are attached and fall upon a workpiece/substrate surface causing
contamination of the workpiece surface, thereby resulting in a defective
device.
This problem of contaminant generation and buildup becomes more acute in
metal etch processes employing a focus ring because the proximity of the
cylindrical walls shrouding the workpiece favor and enhance deposition and
coating of this wall surface with contaminants. Generally, the
non-volatile compositions generated in metal etch processes combine with
polymeric materials formed from photoresist and carbon containing etchant
gases (as by-products of the etch process) and accumulate to form a
contaminant coating on the inner wall of the cylindrical focus ring. As
the thickness of this contaminant coating increases, stability of the
deposited layer decreases due to its weight and/or the stress force
exerted on the coating from the curvature of the cylindrical wall,
eventually resulting in cracking and excessive particulate flaking or
powdering form the coating surface. The powder or flakes drop off the
cylindrical walls of the ring thereby causing contamination of the
semiconductor workpiece/substrate. As in the case of any semiconductor
process system, the apparatus employed in metal etch must be cleaned
periodically in order to avoid these problems and, of course, such
cleaning requires shutdown of the plasma operation with consequent loss of
production.
Known plasma chamber cleaning methods have involved opening the plasma etch
chamber, disassembling portions of the chamber, and removing the
contaminant deposits by physical of chemical methods. For example, the
chamber can be rinsed with a solution of hydrochloric acid, or hand wiped
with a solvent, to dissolve various contaminants. The etch chamber
alternatively may be washed with water and dried. The same cleaning
techniques are separately applied to the vacuum conductance channels and
pump system to avoid the inevitable diminished vacuum or suffocation
referred to above. All of these cleaning methods are complicated,
disruptive, time consuming and can be sources of additional contamination.
Plasma enhanced dry cleaning processes exist whereby contaminants attached
to the inside walls of a focus ring or a film deposition reaction chamber
are removed by plasma etching using carbon tetrachloride and oxygen.
However, presently known plasma enhanced dry cleaning systems require a
dry cleaning time period equal to about 5% to 10% of the time spent in the
aluminum etching process itself. Moreover, the dry cleaning plasma
processes are generally ineffective with respect to the vacuum exhaust
system which would have to be separately cleaned. It would clearly be
advantageous to delay cleaning of plasma etch process chambers and the
present invention effects such a result by providing a focus ring which
controls and stabilizes the buildup of contaminant coatings thereon.
Because a focus ring has proximity to the workpiece/surface substrate and,
consequently, is more susceptible to contaminant build-up in plasma etch
processing, it is desirable to provide a focus ring which accommodates and
stabilizes coatings of contaminant residues and requires less frequency of
cleaning.
SUMMARY OF THE INVENTION
The present invention is based on the discovery that texturizing the
functional surface of a generally cylindrical focus ring enables the
formation of substantially uniform and stable coatings on the ring's
textured surface in a plasma environment. The texturized surface of the
ring provides a roughened surface for a coating formation which reduces
stress forces on the coating otherwise presented with smooth surfaces.
Consequently, substantially uniform coatings of contaminants deposit on
the ring surface in plasma etch environments which coatings are not
subject to cracking or fissures because of the reduced stress forces
provided by texturizing. The present invention extends the time period
between semiconductor reactor process chamber cleaning otherwise required
in presently known semiconductor processing systems which use untexturized
or smooth surfaced focus rings.
The instant invention is primarily directed to a focus ring for surrounding
a workpiece/surface substrate during plasma processing comprising a hollow
annular assembly comprised of electrically insulating material and having
a texturized surface. The ring is preferably in the geometry of a
cylindrical structure and texturized over the entire surface of the ring.
The texturizing of the ring can be effected by any means of surface
abrasion such as etching or molding including bead blasting, chemical
etching, or use of a sculptured mold. The insulating materials of which
the instant focus ring is formed can be any engineering plastic or ceramic
including polycarbonate resins, polypropylene resins, ceramic compositions
(i.e. Al.sub.2 O.sub.3), and quartz.
The present invention is further directed to a semiconductor plasma
processing apparatus for processing workpieces comprising:
a) a semiconductor processing chamber having a reactive gas inlet and a
reactive gas outlet;
b) an electrode disposed in the processing chamber for supporting a
workpiece/surface substrate;
c) an power supply connected to the substrate supporting electrode; and
d) a focus ring for surrounding the substrate comprising a hollow annular
assembly comprised of insulating material and having a texturized surface.
And the instant invention is still further directed to a method for plasma
processing of a workpiece/surface substrate comprising:
a) placing a workpiece/surface substrate to be processed on an electrode
disposed in a plasma reactor chamber;
b) surrounding the workpiece/surface substrate with a focus ring comprised
of a hollow annular assembly further comprised of insulating material and
having an inner texturized surface;
c) evacuating said reactor chamber and introducing reactive gas into the
chamber; and
d) generating a plasma in the chamber by applying power to the substrate
supporting electrode whereby the workpiece/surface substrate is effected
by the plasma generated reactive gases.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical cross sectional view of a plasma etch apparatus
utilizing the texturized focus ring of the instant invention.
FIG. 2 is a perspective view of a cylindrical focus ring for surrounding a
workpiece/surface substrate in accordance with the present invention.
FIG. 3 is a sectional view of one texturized surface embodiment of the
instant focus ring.
FIG. 4 is a sectional view of another texturized surface embodiment of the
present focus ring invention.
FIG. 5 is a cross-sectional view of a stabilized formation of a contaminant
coating on the inner texturized surface of the instant focus ring
invention.
FIG. 6 is a cross-sectional view of a destabilized formation of a
contaminant coating on untexturized inner surfaces of prior art focus
rings.
FIG. 7 is a side view of a specific embodiment of the instant texturized
focus ring.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a focus ring useful in the control of
contaminants in the plasma processing of semiconductor workpieces. The
focus ring of the present invention is particularly useful in controlling
contaminant coating buildup on the ring during the plasma etch of metallic
workpieces. The instant focus ring is described in the following preferred
embodiments in terms of the various texturized surfaces of both the inner
and outer walls of the ring and the use of the ring as part of a plasma
etch apparatus and process to achieve control of contaminant coating
buildup on the surfaces of the ring. However, the concept of employing any
form of focus ring in which the entire surface is texturized or physically
altered to accommodate a stable coating of plasma etch by-product buildup
is applicable to semiconductor process chambers employing focus rings in
general. For example, contaminant control is important for focus rings
used for chemical vapor deposition, epitaxial growth, and dopant
implantation a well as focus rings used in metal etching.
FIG. 1 illustrates a plasma etching apparatus 10 employing the instant
focus ring invention. This is a reactive etching mode system available
commercially from Applied Materials, Inc. of Santa Clara, Calif. Plasma
etching apparatus 10 includes a cylindrical metal reaction chamber 12
comprising a top portion 14, sidewalls 16 and a baseplate 18, with a
connection 20 to a vacuum pump (not shown) for partially evacuating the
interior of the chamber. Gas supply 22 is connected to chamber 12 to
supply reactant gas to the chamber through valve and conduit arrangement
24 to gas distribution tubes 25 (only one shown) extending vertically from
the bottom of the chamber. A hexagonal cathode 28 is connected to an RF
power supply 26. Hexagonal cathode 28 is in the form of a surface
workpiece holder accessible for mounting workpieces 29 thereon for etch
processing. Each surface of hexode cathode 28 in FIG. 1 is designed to
have three 6-inch wafer workpieces mounted thereon. The walls of plasma
etch chamber 12, including sidewalls 16, top portion 14 and baseplate 18
form the grounded anode of the apparatus. In accordance with the present
invention, additionally mounted in each of the three holders on the six
faces of the hexagonal cathode 28 is a texturized cylindrical focus ring
30 in accordance with the present invention.
FIG. 2 shows the basic embodiment of the focus ring invention in the form
of a generally cylindrical hollow ring structure indicated by reference
numeral 40. The ring has an outer cylindrical wall 41 and an inner surface
42. The thickness of the ring 43 as well as the depth, or height, are
arbitrary and non-critical parameters dependant on the material used for
the ring and/or the particular etch process in which it is to be used. The
ring may also contain a fastening aid such as an internal or external
flange (not shown) but such a flange element is optional in that the focus
ring 40 is completely operable when seated on a pedestal and functionally
surrounding a workpiece in a plasma process without any flange or
fastening means. Focus ring 40 is comprised of electrically insulating
material in the form of engineering plastics or engineering ceramics such
as thermoplastic organic polymers, graphite, ceramics and quartz. Typical
organic polymers include polycarbonates and polypropylene resins. A
preferred resin is thermoplastic polycarbonate sold as Lexan by the
General Electric Company. Typical ceramic insulators include amorphous
sapphire (Al.sub.2 O.sub.3), pyrex glass, and other aluminum oxides.
It is preferable that the entire functional surface of the focus ring 40 of
FIG. 2 be texturized in that a pattern or finish is imparted to the
original insulating surface comprised of inner wall 42 and outer wall 41.
It is entirely within the spirit and scope of the invention to employ a
partially texturized focus ring the functional surfaces of which are
entirely texturized so as to be operable within the purview of the present
invention during plasma etching. The use of "texturize" within the purview
of the present invention means imparting to the entire ring surface (inner
and outer surface wall 42 and 41 of FIG. 2) any pattern, characteristic,
or profile which alters the inner surface topography of the insulator
focus ring so as to reduce stress forces on any coating formation thereon,
as will be demonstrated hereinafter. This texturizing enables the
formation of more stable coatings on inner and outer wall surfaces 42 and
41 of focus ring 40 than would be otherwise possible with an untreated or
untexturized surface. Textures such as "matte", "satin", or "silk screen"
can be imparted by well known texturizing methods to these focus ring
surfaces to achieve the favorable coating results obtained with the focus
ring 20 of the present invention. The surface texturizing can be also
characterized by pits, bumps or nodules which can be achieved by other
texturizing methods such as bead blasting or electrolytic deposition. The
entire surface of focus ring 40 is texturized as demonstrated because when
used in a plasma etching device such as that of FIG. 1, contaminant
deposition and buildup, generated in the etch process, occurs on the inner
wall 42 or the outer wall 41 or any edge surface, such as 43, of the focus
ring 40. All such contaminant coated surfaces are sources of contamination
of semiconductor workpieces processed in an apparatus shown in FIG. 1.
Consequently, the entire surface of the instant focus ring requires
texturization to stabilize such contaminant buildup anywhere on the
surface of the ring.
In FIGS. 3 and 4 there are demonstrated two different texturized inner
surface embodiments of the present focus ring invention. In FIG. 3 there
is shown a cut-away view of inner surface 42 of FIG. 2 demonstrating a
pitted pattern texturized surface 52 which was prepared by bead blasting
the entire original surface of focus ring 40 shown in FIG. 2. This is a
random pattern which creates a surface topography suitable for the
formation of more stable coatings than would be otherwise possible with
the untreated original surface. FIG. 4 additionally shows a cut-away view
of another texturized surface of inner wall 42 of focus ring 40 of FIG. 2.
In FIG. 4 there is demonstrated a micropatterned line configuration 62
which can be prepared by geometrically engraving, etching, or molding a
male or female micropattern surface having from about 50 to 350 repeating
geometric units or lines per inch, measured longitudinally across or
circumferentially about the inner and outer cylindrical wall surfaces.
As can be appreciated by those skilled in the art, the texturized surfaces
of the instant focus ring may be in the form of texturized coatings or
films applied to the entire surface of the ring. For example, focus ring
wall surfaces 41 and 42 of FIG. 2 may comprise a coating of a highly
crystalline polypropylene which has been texturized with a micropatterned
line configuration demonstrated as the focus ring surface 62 in FIG. 4.
Any functional coating known to those skilled in the art are operable in
the present focus rings useful in plasma processing.
In FIGS. 5 and 6 there is shown hemispherical sectional views of the
present focus ring and that of the prior art having coatings thereon. In
FIG. 5 there is shown the partial focus ring 70 having a curved partial
cylindrical structure 71 and a relatively uniform coating 73 on both the
inner and outer surfaces of the ring which surfaces have been texturized
prior to coating formation. The texturized surfaces are exaggeratedly
shown as interface layer 73 for illustration. In FIG. 6 prior art
untexturized focus ring 80 is demonstrated as having a cylindrical wall
and a contaminant coating 82 on both the inner and outer surfaces of the
ring which coating is replete with fissures and cracks because the
curvature of the ring causes stress forces to increase as coating
formation advances thereby resulting in cracking, fissuring and flaking of
the coating. This condition of the focus ring in a plasma etch environment
such as that of FIG. 1 would render it an unacceptable source of
contamination of the workpiece/substrate surface being worked. In the
absence of the texturized surface 73 demonstrated in FIG. 5, the
particulate contaminants flaking from the cracks 83 of FIG. 6 would likely
fall from the focus ring in relatively large quantities either directly
onto the surrounded workpiece or into the general environment of
neighboring workpieces where they would migrate to and contaminate such
workpieces. The only remedy for such a flaking condition of FIG. 6 is shut
down of the plasma apparatus and cleaning of the chamber and/or
replacement of the focus ring.
FIG. 7 demonstrates a side view of a particular focus ring 90 of a
generally cylindrical structure in which one planar end of the annular
ring is biased at an angle .theta. from the plane (shown as a vertical
plane) of the other end of the ring or tube. Pedestal rings on high
capacity etching machines such as that illustrated in FIG. 1 are mounted
at angles from the vertical so as to exploit gravity in the support of the
wafer workpieces on these rings and, accordingly, focus rings for use in
such plasma devices should be angularly constructed as in FIG. 7. This
angular focus ring embodiment 90 is constructed to offset and compliment
the angle of the pedestal ring mounting so that plasma will communicate
efficiently and effectively with the focus ring 90 and the shrouded wafer
(not shown); i.e. an offset focus ring from the plasma source will result
in unfocused etching. Other high capacity, automatic autoloading systems
are disclosed in U.S. Pat. No. 5,224,809 (issued Jul. 6, 1993 to Applied
Materials, Inc.) the disclosure of which is hereby incorporated by
reference. This patent demonstrates pedestal rings mounted to hexode
frames at approximately 3.degree. from the vertical and, consequently,
focus ring 90 of FIG. 7 would have an equal angle (.theta.=3.degree.) as
an embodiment of this focus ring invention to function in that patented
invention.
By way of illustration of the instant focus ring invention in a plasma
etching chamber similar to that described and shown in FIG. 1, texturized
focus rings 30 and wafers 29 are mounted on the hexagonal cathode 28. The
chamber may be maintained at a temperature of 60.degree. C., while a 90
volume percent of BCl.sub.3 gas and Cl.sub.3 gas is flowed into the
chamber at a rate of 120 sccm while the chamber is evacuated through a
vacuum pump to maintain the pressure in the chamber below 100 millitorr to
effect reactive ion etching. A plasma powered by an RF energy source can
be ignited in the chamber and maintained at a power level of 1400 watts
during the bombardment of the wafer. After 40 minutes the plasma may be
extinguished and the gas flow terminated.
After completion of the etch process, the entire surface (both inner and
outer walls of the ring) of the mounted focus rings will be found to have
deposited thereon a uniform adherent layer of contaminant material in a
thickness of about 1000 Angstroms without any noticeable cracking or
flaking. The condition of the focus rings enables their continued and
prolonged use in the chamber without presenting a source of contamination
for the enclosed wafer workpiece.
As indicated the instant texturized focus rings can be prepared by any
methods which treat and texturize the surface of the ring in the manner
described herein. Bead blasting a ring comprised of an engineering resin
is one such process. To illustrate, a polycarbonate ring of about 6 inches
in diameter, two inches high, and about 1/8th inch thickness is provided.
A blasting nozzle of 13/64 to 1/4 inch diameter and air pressure of 70 to
80 pounds per square inch (PSI) is aligned at an angle of from 60.degree.
to 90.degree. from the inside surface of the ring. Blasting commences at a
distance of form 21/2 to 31/2 inches from the surface of the ring using
1/8 inch diameter plastic beads. The blasting continues until a uniform
matte surface is produced throughout the entire 360.degree. of the inner
surface of the ring. The outside of the ring is treated in the same manner
until the entire exposed area of the cylindrical ring is texturized in a
light matte finish of 125 to 600 microinches root mean square (RMS). The
ring is then air blown and washed with isopropyl alcohol to remove
residual particles and other contaminant matter.
The above illustrations indicate that employing the texturized focus rings
of the present invention will result in an improved contaminant control
than otherwise obtainable with untexturized focus rings in that the
uniform coating on the inner ring surface will more effectively prevent
residue flaking onto a semiconductor workpiece during etch processing.
Moreover, these rings will work more efficiently in that uniform coating
of contaminant can be allowed to accumulate without contaminant risk to
the workpiece and the texturized ring will require cleaning or replacement
less often than ordinarily expected with an untexturized ring.
Having described the invention, it will be apparent to those skilled in the
art that various modifications can be made within the scope of the present
invention. For example, the process configuration of FIG. 1 is exemplary
as to the particular plasma processing systems demonstrated and described
herein and other semiconductor processing systems can employ the
texturized focus ring of the present invention.
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