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Description  |
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FIELD OF THE INVENTION
The present invention relates generally to cleaning devices utilized to
remove deposits from low pressure chemical vapor deposition chambers and,
more particularly, to such devices which utilize a plasma formed within
the deposition chamber to acheive such cleaning.
DESCRIPTION OF THE PRIOR ART
Low pressure chemical vapor deposition (LPCVD) and reduced pressure epitaxy
(RPE) by chemical vapor deposition are widely used methods in the
integrated circuit industry for the deposition of polysilicon, silicon
nitride, silicon dioxide, single crystal silicon, and other films. LPCVD
systems, as currently configured, employ a quartz tube situated in a
furnace as the reaction chamber, while RPE systems employ a quartz bell
jar. In LPCVE systems, wafers loaded into quartz boats are placed in the
quartz tube which is heated to 400.degree.-900.degree. C. by the furnace
(FIG. 1). The tube is evacuated and a mixture of gases is admitted. A
chemical reaction takes place, resulting in the deposition of a film on
all heated surfaces. Since the tube wall and the wafers are essentially at
the same temperature, film deposition occurs on both the wafers and the
tube wall. With each successive run, the wall deposit grows in thickness,
becomes less adherent and begins to flake off. Particles thus created can
lodge on the wafer surface and cause pinholes or non-uniform deposition.
Periodic cleaning or replacement of dirty tubes is thus required to
maintain system cleanliness and to avoid particle contamination. The
situation is much the same for RPE, with the exception that the wafers and
the susceptor on which they are placed are heated to
1100.degree.-1200.degree. C. by RF induction or radiant lamps. The quartz
bell jar reactor vessel is air-cooled, and is therefore at a much lower
temperature. Deposition nevertheless occurs on the inside surface of the
bell jar and thus periodic cleaning is required.
There is no established standard frequency for the cleaning of LPCVD tubes
or RPE bell jars in the industry, or even among manufacturers of these
systems. The cleaning frequency varies according to film type and the
maximum allowable particle level. For LPCVD polysilicon and silicon
nitride, cleaning is performed after 10 to 50 runs or a 5 to 25 micron
polysilicon or 1.5 to 7.5 micron silicon nitride deposit has accumulated.
The LPCVD silicon dioxide process is more prone to particle generation,
and cleaning is generally performed every 10 runs or 7-10 micron
cumulative deposit. RPE bell jars are typically cleaned every 10 to 40
runs.
Currently, the most common approach to maintain LPCVD tube or RPE bell jar
cleanliness is to remove the dirty tube or bell jar and replace it with a
clean one. The dirty tube or bell jar is subsequently cleaned with acids
or is discarded.
As illustrated in Table 1, the removal and replacement of dirty LPCVD tubes
or RPE bell jars involves several steps, is time consuming and is costly.
There are a number of problems inherent in this method which are
summarized in Table 2.
Table 1
Conventional Methods of LPCVD Tube Cleaning
A. REPLACE TUBE
1. Ramp down furnace temperature
*2. Remove cantilever
3. Disconnect vacuum and gas lines
*4. Remove LPCVD tube from furnace
5. Install clean LPCVD tube
6. Connect vacuum and gas lines
7. Leak check system
*8. Install cantilever
9. Ramp up furnace temperature
10. Profile furnace temperature
11. Perform test run
* if present
B. HANDLING OF DIRTY TUBE
1. Discard or
2. Cut out dirty center section and weld in new section or
3. Clean tube with acids, rinse and dry
TABLE 2
Disadvantages of Conventional Tube Cleaning Method
1. Procedure takes 4 to 16 hours
2. System downtime of 12 to 36%
3. Cost of $370 to $950 per tube clean
4. Removal of tube generates particles
5. Tubes often break upon removal
6. Tubes devitrify due to temperature cycling
7. Disconnection/reconnection of gas and vacuum lines causes leaks
8. Handling large tubes poses safety hazard
9. Use of acid for cleaning tubes poses safety and environmental hazards
10. Wet cleaning is ineffective for Si3N4
Most of the problems associated with the current method for cleaning LPCVD
tubes and RPE bell jars stem from the fact that the tube must be removed
from the system prior to cleaning. It is clear that an in-situ cleaning
technique would eliminate these problems.
Attempts have been made previously to use gaseous HCl to remove polysilicon
deposits from LPCV tubes and RPE bell jars by the reaction:
4HCl(g)+Si(s).fwdarw.SiCl4(g)+2H2(g). (1)
This reaction, however, does not proceed rapidly except at temperatures
above 900.degree. C., thus requiring the LPCVD furnace temperature to be
increased above its normal operating temperature of
600.degree.-650.degree. C. In RPE systems, where the bell jar temperature
does not exceed 200.degree. C., this method is ineffective. Furthermore,
the polysilicon deposit is actually composed of sequential layers of
polysilicon and silicon dioxide because each deposit of polysilicon
undergoes surface oxidation during wafer load/unload. HCl does not react
significantly with silicon dioxide and thus particles are created with
this process. Lastly, HCl is a corrosive and toxic gas (Tolerance Level
Value.+-.5 ppm).
Anhydrous HF can be used to effectively clean deposits of silicon nitride
(see "Method of cleaning a Reactor", UK Patent Application No. 803009, GB2
062 689 A, Date of filing: Sept. 17, 1980, Applicant: NV Philips'
Gloeilampenfrabriken, Eindhoven, The Netherlands, Inventor: E. P. G. T.
Van De Ven.) and silicon dioxide at elevated temperatures, but it is
ineffective for polysilicon. Anhydrous HF is toxic (TLV=3 ppm) and
extremely corrosive, requiring special plumbing materials and careful
handling.
Other gases such as ClF3, BrF3, BrF5, and IF5 liberate atomic fluorine when
heated (see D. E. Ibbotson, J. A. Mucha, D. L. Flamm and J. M. Cook, J.
Appl. Phys. 56, 2939 (1984)) and thus are effective in removing
polysilicon deposits via the fluorination reaction:
4F(g)+Si(s).fwdarw.SiF4(g). (2)
Silicon nitride and silicon oxide also react with atomic fluorine:
12F(g)+Si3N4(s).fwdarw.3SiF4(g)+2N2(g) (3)
4F(g)+SiO2(s).fwdarw.SiF4(g)+O2(g). (4)
However, the highest halogens present in ClF3, BrF3, BrF.sub.5, and IF5
preferentially adsorb on silicon nitride and silicon dioxide surfaces,
inhibiting the above fluorination reactions. Additionally, these gases are
extremely toxic (TLV=0.1 ppm).
The implementation of in-situ cleaning techniques has therefore been
hindered by the toxicity and corrosiveness of the gases involved and by
their limited ability to etch one or more of the films.
Plasmas are well known for their ability to produce reactive species such
as atomic fluorine from relatively nonreactive gases at temperatures far
below that required for thermal decomposition. The application of plasma
technology to in-situ chamber cleaning allows the use of less toxic and
easier-to-handle gases and, for the first time, makes effective in-situ
cleaning of LPCVD tubes and RPE bell jars a reality.
The proposed system would clean LPCVD tubes and RPE bell jars in situ,
thereby eliminating the time needed for furnace cool-down and heat-up and
significantly reduce the generation of particulates and the risk of tube
or bell jar breakage associated with their removal. Further, it offers
increased operator safety as it does not require the handling of large,
hot quartzware or the use of acids. Table 3 summarizes the many benefits
the proposed system offers over the current methods used to clean
deposition chambers.
TABLE 3
Advantages of Autoclean 10 Use for Tube Cleaning
1. Tube removal is not required
2. Procedure takes 40 to 90 minutes
3. System downtime of 1 to 4%
4. Cost of $75 to $235 per tube clean
5. No particle generation
6. Gas and vacuum lines left intact
7. Furnace and tube left at temperature
8. Safe and easy to use
9. Semi-automatic operation
In-situ cleaning of quartz tubes by means of a plasma has been demonstrated
heretofore. The Plasma-Enhanced Chemical Vapor Deposition (PECVD) systems
of Pacific Western Systems, Inc., Los. Gatos, CA and Advanced
Semiconductor Materials, Inc., Phoenix, AZ, both employ LPCVD-like quartz
tubes as reaction chambers. These tubes are situated horizontally in a
furnace. An electrode/susceptor assembly is inserted into the tube and
plasma deposition of silicon nitride, silicon oxide or polysilicon is
performed. Typical operating temperature of the furnace is
200.degree.-400.degree. C. Deposition occurs on the wafers as well as on
the electrode/susceptor assembly and the tubes walls. As in LPCVD, the
deposit on the electrode/susceptor assembly and the tube walls must be
periodically removed. Both manufactures indicate that this cleaning can be
performed by insertion of the electrode/susceptor assembly without wafers
into the tube and the creation of a plasma in a C2F6+O2 ambient. The
atomic fluorine generated within the plasma removes the film as in
reactions 2-4 above. The cleaning thus performed, however, is not complete
and periodic removal and wet cleaning with acids of the tube and
electrode/susceptor assembly must be performed. This is because the
electrode/susceptor assembly is not configured for cleaning, rather for
the deposition of films on wafers placed thereupon.
Several prior patents exist describing the in-situ cleaning of LPCVD tubes
and other deposition chambers. Many (such as "Washing of Reactor For
Plasma CVD Method", Japanese Pat. No. 57-27024(A), Mitsubishi Denki K.K.,
S. Tsuboi; "Plasma Chemical Vapor Deposition", Japanese Pat. No. 57-13737,
S. Yamazaki; and "Plasma Reactor", Japanese Pat. No. 57-69744(A),
Matsushita Denki Sangyo K.K., T. Kawaguchi.) describe the use of
electrodes placed on the outside of the LPCVD tube to generate a plasma
within the tube. The electrodes are either longitudinal along the entire
length of the tube, or are circumferential, with one placed at either end
of the tube. The electrodes thus placed can interfere with the uniform
heating of the chamber and thus are not suitable for permanent
installation on a production LPCVD system. Additionally, due to the
greater ion bombardment that exists at or near the electrodes, the film
removal is very nonuniform with these methods.
Several other patents (such as "Vacuum Vapor Deposition Apparatus",
Japanese Pat. No. 56-258143(A), Mitsubishi Denki K.K.; "Reduced Pressure
Chemical Phase Growth Device", Japanese Pat. No. 56-26539(A), Matsushita
Senki Sangyo K.K., O. Ishikawa; and "Apparatus for Chemical Vapor
Deposition", Japanese Pat. No. 56-166935(A), Mitsubishi Denki K.K.)
describe the use of a plasma formed external to the LPCVD chamber. The
reactant species formed in the plasma are transported by gas flow from the
plasma into the LPCVD tube where they react with the wall deposits. This
method suffers from extreme nonuniformity in film removal due to the
limited lifetime of the reactive species. The lifetime of the species is
much less than the time required to transport the specie from the plasma
to the deposits at the far end of the LPCVD tube and thus etching of
material there is much slower than near the front of the tube, closer to
the plasma.
One patent ("Apparatus For Semiconductor Device Fabrication", Japanese Pat.
No. 58-21826(A), Suwa Seikosha Co., Ltd.,) describes the use of an
inductive coil within a quartz tube that is axially placed within an LPCVD
tube. RF power applied to the coil causes a plasma to be formed within the
LPCVD tube, exterior to the coil. This results in a plasma that is uniform
both longitudinally and circumferentially. Inductive coils, however,
produce relatively less intense plasmas, especially at frequencies below 2
MHz. Higher frequencies cannot be effectively used due to the coupling of
the RF field into the furnace coils and, hence, into the facilties
electrical network, causing disruption of computers, microprocessors and
other electrical devices. Furthermore, the passage of current through the
coil induces a magnetic field longitudinally along its exterior. The
magnetic field "traps" electrons and ions near the surface of the coil,
resulting in a radially nonuniform plasma. The plasma is more intense near
the surface of the coil and thus the concentration of reactive species and
the ion bombardment at the inner surface of the LPCVD tube is reduced
resulting in a lower film removal rate.
IN THE DRAWINGS
FIG. 1 depicts a generalized low pressure chemical vapor deposition system;
FIG. 2 depicts a schematic drawing of the present invention;
FIG. 3 depicts an adaptor plate that can be used to facilitate the
interface of the invention with various sizes of tubes;
FIG. 4 depicts an electrode configuration of the present invention;
FIG. 4a depicts a sectional view of the electrode configuration of FIG. 4;
FIG. 4b depicts a cross-sectional view of the electrode configuration of
FIG. 4;
FIG. 5 depicts an alternative electrode embodiment of the present
invention;
FIG. 5a depicts a sectional view of the electrode configuration of FIG. 5;
FIG. 5b depicts a cross-sectional view of the electrode configuration of
FIG. 5;
FIG. 6 depicts another alternative electrode embodiment of the present
invention;
FIG. 6a depicts a sectional view of the electrode configuration of FIG. 6;
FIG. 7 depicts another alternative electrode embodiment of the present
invention;
FIG. 7a depicts a sectional view of the electrode configuration of FIG. 7;
FIG. 8 depicts another alternative electrode embodiment of the present
invention;
FIG. 8a depicts a sectional view of the electrode configuration of FIG. 8;
FIG. 9 depicts another alternative electrode embodiment of the present
invention;
FIG. 9a depicts a sectional view of the electrode configuration of FIG. 9;
FIG. 10 depicts another alternative electrode embodiment of the present
invention;
FIG. 10a depicts a sectional view of the electrode configuration of FIG. 7;
FIG. 10b depicts a cross-sectional view of the electrode configuration of
FIG. 10;
FIG. 11 depicts another alternative electrode embodiment of the present
invention;
FIG. 11a depicts a sectional view of the electrode configuration of FIG.
11;
FIG. 12 depicts another alternative electrode embodiment of the present
invention;
FIG. 12a depicts a sectional view of the electrode configuration of FIG. 12
.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 depicts a generalized Low Pressure Chemical Vapor Deposition (LPCVD)
system wherein wafers 2 are processed within a quartz LPCVD tube chamber 3
located within a furnace 4. The LPCVD tube 3 is evacuated to proper
processing pressure utilizing a vacuum pump 5 and valve system 6, and
processing gas is introduced through a suitable inlet 7.
The system 10 is shown schematically in FIG. 2. It consists mainly of an
electrode structure 12, an RF generator 14, matching network 16, an etch
gas source 18 and a process controller 20. All may be mounted on a mobile
cart to allow for servicing of several deposition chambers.
The electrode structure 12 is formed for insertion into an LPCVD tube 32 or
RPE bell jar 42 to effect the cleaning of deposited material from the
inner walls 33 or 43 thereof. The baseplate 22 of the electrode structure
12 has an o-ring seal 23 which matingly engages the base 34 of the tube 32
or the base 44 of the bell jar 42, such that a vacuum seal is created upon
the evacuation of the tube or bell jar utilizing the vacuum pump system
50. A pressure detector 24, which may take the form of a capacitance
manometer is utilized to measure the pressure within the tube or bell jar.
A gas flow control device 26, which may take the form of a mass flow
controller, is utilized to control the flow rate of etchant gas into the
tube or bell jar. Gas is uniformly distributed in the LPCVD tube 32 or RPE
bell jar 42 by a gas distribution tube 13. In order to make the system
adaptible to various size tubes or bell jars, an adaptor plate 28 may be
utilized (FIG. 3). The adaptor plate is doughnut-shaped to allow the
passage of the electrode structure 12 through its center hole 30 and has
an o-ring seal 29 which mating engages the base 34 of the tube 32 or the
base 44 of the bell jar 42. The other side of the adaptor plate 28
matingly engages with the o-ring seal 23 of the baseplate 24 of the
electrode structure 12.
To utilize the device, the electrode structure 12 is inserted into an LPCVD
tube 32 or RPE bell jar 42 with the baseplate 22 of the electrode
structure 12 making a vacuum tight seal with the loading end 34 of the
tube 32 of the loading end 44 of the bell jar 42. The tube 32 or bell jar
42 is evacuated via its own pumping system 50, and gas is introduced into
the tube 32 or bell jar 42 at a constant flow rate through gas
distribution tubes 13. Gas flow is controlled by a mass flow controller.
Several electrode structures and gas distribution tube configurations are
discussed hereinafter. The gas may be such known gases as CF4, CF4+O2,
C2F6+O2, SF6, or NF3. RF powr of 100 to 5000 watts at 50 to 2000 kilohertz
is applied from the RF generator 14 through the matching network 16 to the
electrode structure 12 creating a plasma within the tube 32 or bell jar
42. Etchant species created in the plasma react with the deposited
material to form a volatile product which is pumped out of the tube 32 or
bell jar 42 by the vacuum pumping system 50. Complete removal of the
deposited material is detected via a rise in the pressure in the tube 32
or bell jar 42 as measured by a capacitance manometer.
A first electrode structure 60 is shown in FIGS. 4, 4a and 4b. It consists
of four electrodes 62 fixtured on the inner circumference of a single
closed end quartz tube 64. The electrodes 62 are held against the inner
wall of the quartz tube 64 by a spring-loaded, differential length, quad
four-bar mechanism 66, which consists of four metal tension rods 68, each
indiviually and rigidly connected to one of four metal pivot mounts 70.
The pivot mounts 70 are each connected via pin joints to all four
electrodes 62 by four ceramic links 72, which are preferably at a 30 to 45
degree from a radial vector. Each electrode 62 is rigidly connected to a
metal electrode rod 63. The electrode rods 63 pass through clearance holes
76 in the ceramic load disk 74 and are buttressed against the ceramic load
disk 74 by retaining rings 65. The four tension rods 68 pass through four
clearance holes 76 in the disk 74. Four springs 78 are located coaxially,
one to each tension rod 68 and are held in compression against the ceramic
load disk 74 by retaining rings 79, thus placing each tension rod 68 in
adjustable tension. The tensile force placed on each tension rod 68 is
thus converted by this assembly into a radially outward force, pressing
the electrodes 62 against the inner wall of the quartz tube 64. The pivot
mounts 70 are located at different distances along the axis of the tube
64. The pivot mounts 70 have clearance holes 71 to allow tension rods 68
not connected to that particular pivot mount to pass through. The
baseplate 22 is a doughnut-shaped metal plate possessing an o-ring seal
23. A formed quartz flange 84 is rigidly affixed on the outer
circumference of the quartz tube 64 near its open end. The quartz tube 64
passes through the center hole 80 of the baseplate 22 which is of a
diameter slightly larger than the quartz tube 64 but less than the quartz
flange 84. The baseplate 22 is held to the quartz tube 64 and quartz
flange 84 by means of four metal brackets 86. An o-ring 88 between the
baseplate 22 and the quartz flange 84 provides for a vacuum tight seal,
while another o-ring 90 between the quartz flange 84 and the metal
brackets 86 is used to prevent breakage of the quartz. RF power is applied
to two diametrically opposing electrodes 62 while the other two are
grounded.
Etchant gas is introduced via two single closed end quartz gas distribution
tubes 92 which are located on 180 degrees from each other on the outside
of the quartz tube 64. The open end of each gas distribution tube 92 is
fixtured coaxially over one of two metal gas input tubes 94 located on the
baseplate 22. O-rings 96 are located between the outer circumference of
the gas input tubes 94 and the inner circumference of the gas distribution
tubes 92 and provide a leak-tight seal. The gas input tubes 94 pass
through the baseplate 22 and are connected to a common gas manifold 98.
Small holes 100 are fabricated in the walls of the gas distribution tubes
92. The holes 100 are distributed along the axial length of the gas
distribution tubes 92 on either side of the gas distribution tubes 92
along a vector tangential to the quartz tube 64. This electrode design
results in capacitive coupling to the plasma and yields a plasma that is
uniform both axially and circumferentially and that is confined near the
wall of the LPCVD tube or RPE bell jar. Etchant species created in the
plasma diffuse to the chamber walls and volatilize any material deposited
thereupon. Moreover, the electric field generated can couple to a degree
with the furnace coils 35 or radiant lamps 45, thus achieving a relative
degree of radial uniformity and ion bombardment of the LPCVD tube inner
wall, which results in higher and more complete material removal from the
walls.
Assuming that NF3 is used as the input gas, NF3 fragments in a plasma via
the following reactions:
NF3+e-.fwdarw.NF2+F+e- (5)
NF3+e-.fwdarw.NF2+F.sup.- (6)
NF3+e-.fwdarw.NF2+F+2e- (7)
NF2+e-.fwdarw.NF+F+e- (8)
NF+e-.fwdarw.N+F+e- (9)
Atomic fluorine thus created reacts with the deposited material to form
volatile products:
Si(S)+4F(g).fwdarw.SiF4(g) (10)
Si3N4(s)+12F(g).fwdarw.3SiF4(g)+2N2(g) (11)
SiO2(s)+4F(g).fwdarw.SiF4(g)+O2(g) (12)
W(s)+6F(g).fwdarw.WF6(g) (13)
B(s)+3F(g).fwdarw.BF3(g) (14)
P(s)+5F(g).fwdarw.PF5(g) (15)
The reactions rates of (10) and (12) were calculated by Flamm et al (D.
Flamm and V. Donnelly, The Design of Plasma Etchants, Plasm Chem. Plasma
Process. 1, p. 317 (1981).) to be:
R(Si)=2.9.times.10.sup.-12 T.sup.1/2 N.sub.f exp (-0.108/kT) (16)
R(SiO2)=6.14.times.10.sup.-13 T.sup.1/2 N.sub.F exp (-0.163/kT) (17)
where T is the temperature in degrees Kelvin, N.sub.F is the concentration
of F atoms, and k is Boltzmann's constant. For LPCVD applications, T is
700 to 1200 degrees K. Thus for a 50% fragmentation of NF3 into NF2+F
alone at 1 torr pressure, N.sub.F is approximately 4.times.10.sup.16 /cc
and the material removal rates for silicon and silicon dioxide are on the
order of 20 microns/minute and 2 micron/minute, respectively. Atomic
fluorine based etching of silicon nitride has shown its rate to be between
that of silicon and silicon dioxide. Based on typical deposition
thicknesses of 10 to 30 microns, one can see that these etch rates are
more than sufficient to clean the tube in a very short time, provided a
sufficient flux of reactant gas can be maintained. Table 4 calculates the
feed gas supply required to remove polysilicon at 0.5 micron/minute. The
calculated gas flow rate of 1.3 liter/minute can be easily handled by most
LPCVD systems while maintaining a 2 torr pressure. Silicon nitride and
silicon dioxide would require lower flows due to their lower density of
silicon.
TABLE 4
Feed Gas Requirements
Amount Of Si To Be Removed:
Tube Diameter: 15 cm
Deposition Zone Length: 120 cm
Deposition Thickness: 15 microns
Volume of Silicon to be Removed: 8.5 cm3 (0.67 mole)
Gas Flow Required:
Assume 30' clean time.
Assume 50% reaction efficiency; 2.7 moles of NF3 etch 1 mole of Si.
Molar etch rate=0.67 mole/30 min.=0.022 mole/min.
NF3 required
=0.06 mole/min.
=1.3 liters/min.
For RPE applications, the temperature of the walls and thus the deposited
silicon is typically 350 to 400 degrees K. The material removal rate
calculated from (16) using the above assumptions is 1 micron/minute,
demonstrating the feasibility of cleaning RPE bell jars in a short time.
A related configuration to that described in FIGS. 4, 4a and 4b is created
by decreasing the number of electrodes 62 to two or increasing the number
to six or any even number, where the electrodes are alternatively
connected to RF power and ground.
Another embodiment of the electrode structure depicted in FIGS. 4, 4a and
4b is shown in phantom in FIG. 4a. It is created by connecting all the
electrodes 62 to RF power by rotating switch 97 to pin a. In this
embodiment the furnace coils 35, which are electrically connected to
ground potential by the closing of switch 99, act as the sole ground
electrode.
The electrode structure depicted in FIGS. 4, 4a and 4b, and all other
electrode structures described herein can be adapted for use on either
LPCVD tubes or RPE bell jars. Electrode structures configured for use on
LPCVD tubes have baseplates and electrodes of smaller diameter and longer
electrodes than those designed for RPE bell jars.
The electrode structure 12 can take many forms, several of which are
described hereinafter. FIGS. 5, 5a and 5b depict and electrode structure
102 which is substantially similar to that depicted in FIGS. 4, 4a and 4b.
In this embodiment, the rectangular electrodes 62 of FIG. 4, 4a and 4b are
replaced by circumferential band electrodes 104, which are alternatively
connected to RF power and ground. Each set of band electrodes is affixed
to a pair of oppositely opposed conductive rods 106, which in turn are
connected to a differential length, quad four bar mechanism 66 as in FIGS.
4, 4a and 4b. The conductive rods 106 are formed such that there is a air
gap 108 between the rod 106 and the quartz tube 64 in those areas between
the band electrodes 104 to which the rod 106 is attached.
As will be obvious to one ordinarily skilled in the art, the electrode
configuration of FIGS. 5, 5a and 5b will function in much the same manner
as the electrode structure depicted in FIGS. 2 and 4.
FIGS. 6 and 6a depict an electrode structure 110 having a powered electrode
rod 112, a grounded electrode rod 114, and a gas distribution tube 116.
Both electrode rods are sheathed in closed-end quartz tubes 118. The gas
distribution tube 116 is formed with holes 120 to emit etchant gas into
the tube 32 or bell jar 42 when the electrode structure 110 is inserted
into the tube 32 or bell jar 42. Vacuum feedthroughs 122 for the electrode
rods are formed in the | | |
