Plasma enhanced chemical vapor processing system using hollow cathode effect

A high-efficiency, low-temperature, plasma-enhanced chemical vapor deposition (PECVD) system for growing or depositing various types of thin films on substrate surfaces, or etching such surfaces, using substrates of materials such as silicon, plastic, etc. The system uses a hollow-cathode-effect electron source with a surrounding confining electrode to create a plasma at the substrate surface to insure that the density of reactive species is both enhanced and localized at the substrate surface thus causing the rate of growth of the films, or the etch rate, to increase so that the process can take place at much lower temperatures and power levels. A particular embodiment involves the growing of hydrogenated amorphous silicon (a:Si:H), at room temperature, on silicon using a tubular reactor containing a cylindrical electrode lining the inside of the reactor walls acting as a counter electrode for an rf-powered, substrate-supporting electrode near the center of the reactor. A set of silicon wafers, on which the amorphous silicon is grown, is mounted on the latter electrode. The reaction gases (silane) flowing between the electrodes are decomposed in a plasma excited by an rf power source (13.56 MHz) connected to the substrate-supporting electrode. With the use of appropriate deposition parameters (silane flow rate, pressure, applied power and frequency, and substrate spacing) room temperature growth of a-Si:H is achieved at growth rates up to 15 .ANG./sec, while keeping a low hydrogen concentration (.about.10%) and the bonded hydrogen in the Si-H monohydride configuration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to plasma sputter/etching and more 
particularly to a high efficiency, low temperature system for growing or 
depositing various types of thin films on substrate surfaces, or etching 
such surfaces, using an improved hollow-cathode enhanced plasma at the 
substrate. 
2. Prior Art 
It is known that during plasma or RIE etching the use of a hollow-cathode 
electron source in the process significantly increases the presence of 
active etching species by increasing the electron density in the region of 
the etched surface so that the etching process takes place at a much 
faster rate or at a lower power level. See, for instance, U.S. Pat. No. 
4,637,853 to B. Bumble et al and IBM TDB Vol. 28, No. 10, pps. 4294-7, 
March, 1986. The phenomenon of hollow-cathode discharge is explained in 
detail by C. M. Horwitz in Appl. Surf. Science 22/23, 925 (1985). Also, 
see his U.S. Pat. No. 4,521,286. 
Enhanced growth of thin films is also possible with this phenomenon for the 
same reason and can be used to provide all the advantages to VLSI 
technology that low processing temperatures and high growth rate will 
offer. For example, reduced thermal exposure during oxidation can 
significantly reduce "bird's beak" which in turn will allow higher density 
circuits to be fabricated. In addition, less damage to the silicon/oxide 
interface will result due to the reduced power needed for this approach 
compared to other plasma systems. 
One important area of film growth in VLSI technology is the production of 
hydrogenated amorphous silicon. Hydrogenated amorphous silicon (a-Si:H) 
has been intensively studied since Chittick et al achieved a dramatic 
reduction of the gap defect state density by using a glow discharge growth 
technique. See R. C. Chittick, J. H. Alexander, and H. F. Sterling, J. 
Electrochem. Soc. 116, 77 (1969). Subsequently, Spear and LeComber were 
able to substitutionally dope these films. See W. E. Spear and P. G. 
LeComber, Solid State Commun. 17, 1193 (1975) in this regard. Amorphous 
Si:H is now widely used in devices that require a large-area, and 
low-cost, low-temperature processing, e.g., solar cells, thin-film 
transistors for displays, electrophotography sensors, photodetectors, and 
light-emitting diodes. For a comprehensive review of this technology, see, 
for example, A. Madan and M. P. Shaw, The Physics and Applications of 
Amorphous Semiconductors (Academic Press, Boston, 1988), or J. I. Pankove, 
Ed., Hydrogenated Amorphous Silicon, in Semiconductors and Semimetals, 
volume 21 (Academic Press, Orlando, 1984). 
As early as 1980, Knights concluded that optimal a-Si:H growth conditions 
led to a very low growth rate on the order of 0.5-1 .ANG./sec. See J. C. 
Knights, J. Non-Cryst. Solids 35/36, 159 (1980), and also A. Matsuda and 
K. Tanaka, J. Non-Cryst. Solids 97/98, 1367 (1987). A number of methods 
were suggested to increase the growth rate. For example, K. Ogawa, I. 
Shimizu and E. Inoue, in Japan. J. Appl. Phys. 20, L639 (1981), suggest 
the use of higher silanes as the silicon source gas. Very-high frequency 
rf glow discharge is discussed by H. Curtins, N. Wyrsch, M. Favre and A. 
V. Shah, in Plasma Chem. Plasma Process. 7, 267 (1987). The use of a 
grounded mesh peripherally surrounding parallel plate electrodes is 
described by T. Hamasaki, M. Ueda, A. Chayahara, M. Hirose, and Y. Osaka, 
in Appl. Phys. Lett. 44, 600 (1984). See also U.S. Pat. No. 4,633,809 to 
Hirose et al. Hydrogen-radical enhanced CVD, and hollow-cathode discharge 
are respectively suggested in Japan. J. Appl. Phys. 26, L10 (1987), by N. 
Shibata, K. Fukuda, H. Ohtoshi, J. Hanna, S. Oda, and I. Shimizu, and in 
the above-cited Appl. Surf. Science 22/23, 925 (1985) by C. M. Horwitz. 
Although optimal a-Si:H is usually deposited at temperatures between 
200.degree. and 300.degree. C., deposition at lower temperatures was also 
attempted as described by Y. Ziegler, H. Curtins, J. Baumann and A. Shah, 
in Mat. Res. Soc. Symp. Proc. 149, 81 (1989) and references therein, and 
by G. Lucovsky, B. N. Davidson, G. N. Parsons, and C. Wang, in J. of 
Non-Cryst. Solids 114, 154 (1989). The motivation for achieving lowering 
temperature deposition was the ability to use heat sensitive materials as 
substrates. Furthermore, the discovery, reported by Z. E. Smith and S. 
Wagner, in Phys. Rev. B 32, 5510 (1985), that defects in a-Si:H 
participate in chemical equilibrium reactions, stimulated the study of the 
properties of samples deposited at lower temperatures (from room 
temperature up to 100.degree. C.). reported by R. A. Street and K. Winer, 
Mat. Res. Soc. Symp. Proc. 149, 131 (1989). 
It is accordingly desirable in the art to have a high-efficiency system for 
depositing or growing films on, or etching, substrate surfaces, and 
particularly, to have, for example, an rf glow-discharge system for 
growing hydrogenated amorphous silicon (a-Si:H), at room temperature, on 
silicon. 
The present invention improves upon prior art hollow-cathode-effect devices 
for sputter/etching by a technique which enhances plasma-produced growth 
or deposition of films on substrate surfaces, or the etching of such 
surfaces, and an apparatus design which will do so efficiently on silicon 
wafers. In a particular embodiment the present invention provides a new 
high-growth-rate technique for a-Si:H deposition. 
SUMMARY OF THE INVENTION 
The present invention involves plasma-enhanced chemical vapor processing 
and particularly a high-efficiency, low-temperature system for growing or 
depositing various types of thin films on workpieces such as substrate 
surfaces, or etching such surfaces, using substrates of materials such as 
silicon, germanium, gallium arsenide, glass, plastic, and other suitable 
materials. The system uses a hollow-cathode-effect electron source with a 
surrounding confining electrode to create a plasma at the substrate 
surface, or preferably between spaced surfaces, which arrangement insures 
that the density of reactive species is both enhanced and localized at the 
substrate surfaces thus causing the rate of growth of the films, or the 
etch rate, to increase so that the process can take place at much lower 
temperatures and power levels. 
A preferred embodiment of the invention is its application in a 
plasma-enhanced chemical vapor deposition (PECVD) system and particularly 
an rf glow-discharge system for growing hydrogenated amorphous silicon 
(a-Si:H), at room temperature, on silicon. The embodiment comprises a 
tubular reactor, containing a generally concentric electrode arrangement 
including a set of spaced substrates supported axially therein, for 
accomodating hollow-cathode effect, plasma-enhanced, chemical vapor 
deposition (HC-PECVD) on, or processing of, the substrate surfaces. The 
cylindrical, concentric-electrode arrangement provides improved 
confinement of and enhanced hollow-cathode effects in the plasma produced 
at each substrate surface, permitting low temperature and pressure 
deposition of material from the chemical vapor onto the substrates. More 
particularly, the reactor consists of an outer quartz tube, defining the 
reaction chamber, with a cylindrical electrode adjacent the inside of the 
reactor wall acting as a counter electrode for an rf-powered electrode 
which is in electrical contact with and supports the substrates at the 
center of the reactor. The substrates are in the form of a set of spaced 
silicon wafers, on which the amorphous silicon is to be grown, and may be 
disposed in a wafer boat. The boat is mounted on the latter electrode 
which may be paddle-shaped to accomodate it. The plasma is produced from 
reaction gases, such as silane in the present embodiment, which are 
decomposed into a plasma excited by an rf power source (13.56 MHz) 
connected to the paddle-shaped electrode. The gas flows between the 
electrodes and may be admitted at one end of the reactor and pumped out at 
the opposite end. The electrode material is silicon carbide made 
conductive by the presence of crystalline silicon inclusions. Using 
appropriate deposition parameters (silane flow rate, pressure, electrical 
power and frequency, and substrate spacing) room temperature growth of 
a-Si:H is achieved at growth rates up to 15 .ANG./sec, while keeping a low 
hydrogen concentration (.about.10%) and the bonded hydrogen in the Si--H 
monohydride configuration. 
A particular example for growing device quality amorphous silicon at room 
temperature (23.degree. C.) in the disclosed reactor involves a set of 
parameters such as follows: 
Gas=100% SiH4 (silane) 
Pressure=25&lt;P&lt;100 millitorr 
Power=10&lt;Pw&lt;150 watts 
Gas Flow=10&lt;F&lt;50 sccm 
Temperature=23.degree. C. 
Growth Rate=100&lt;GR&lt;1000 A/min 
Using this process amorphous silicon films may be formed on substrates or 
devices that have serious temperature limitations, thus extending the 
range of materials that can be used in such device applications. 
The confining-electrode reactor of the invention may be used in a large 
number of applications with various gases and materials to produce 
improved treatment of substrate surfaces to deposit or grow films thereon 
or etch patterns therein.

DETAILED DESCRIPTION OF THE INVENTION 
A preferred plasma reactor in accordance with the present invention is 
shown in FIG. 1. The structure of the apparatus is made up essentially of 
three component sections conveniently identifiable by their preferred 
materials of construction, i.e., quartz, silicon carbide, and stainless 
steel. While these materials are preferred, those of skill in the art may 
wish to substitute other suitable materials as may be found to be 
appropriate. 
First, the quartz section is in the form of a cylindrical reaction chamber 
1 which is the heart of the reactor wherein the controlled plasma reaction 
takes place. The desirable properties of insulation, high temperature 
stability, and ultra-high purity make quartz the preferred material for 
this structure. 
Second, a silicon carbide cathode section is constructed from three 
components--an outer tubular electrode 3, an inner cantilevered electrode 
4, which may be in the form of a boat paddle for holding the third 
component, a wafer boat 5. All three of these parts are disposed within 
the quartz chamber 1 and are made of high purity silicon carbide because 
of its desirable combination of properties, i.e., ultra-high purity, very 
good thermal stability, strength at elevated temperatures (capable of 
supporting a loaded wafer boat), and conductivity, due to its being 
impregnated with high purity silicon powder during fabrication. 
Conductivity is important as the material is part of the cathode 
structure. FIGS. 1 and 2 illustrate one configuration for the wafer boat 
5, which when loaded with wafers 10, in a vertical orientation as shown, 
makes up part of the hollow-cathode arrangement. For descriptive purposes 
this vertical arrangement will be primarily discussed but it will be 
understood from the following description that the alternate wafer boat 
arrangement with horizontally oriented wafers, shown in FIG. 3, should 
give substantially equivalent results. While circular wafers are shown, it 
will also be understood that the workpieces disposed at the center of the 
reactor for operation of the plasma thereon may take many shapes and forms 
to suit the ends of any particular process being carried out in the 
reactor. Similarly, the reaction chamber 1 and surrounding electrode 3 may 
be other than circular in cross-section, but the cylindrical configuration 
is preferred, with the reactor having an elongated axis and the electrodes 
arranged parallel thereto. In some applications the surrounding electrode 
3 might act as the chamber wall. 
The third section shown in FIG. 1 is the stainless steel section which 
includes three members: dual-flanged end cap 6; end cap 7; and door 9. End 
caps 6 and 7 are vacuum sealed to the quartz center section 1 at its 
opposite flanged ends by O-rings 8, and door 9 is similarly joined to end 
cap 6. Provision is made between the flanges in end cap 6 for the 
attachment of gas input lines 15 and pressure sensors 16. The paddle 
electrode 4 is connected to the door 9 in a cantilevered arrangement which 
includes a paddle electrode holder 12 and a clamp 13 that joins the paddle 
4 and the holder 12. Electrical power to the paddle electrode 4, which 
acts as a cathode, is fed through a wire connection between holder 12 and 
a power electrode 14. The power electrode 14 and holder 12 are both 
electrically isolated from the door 9 by TEFLON and ceramic, vacuum-tight, 
feed-thrus 11. The surrounding tubular electrode 3 is supportingly 
connected to end cap 7, which also accomodates a connection 17 to a vacuum 
source or pump (not shown) for evacuating the chamber 1. 
The electrodes are powered from an rf generator 18 which may be connected 
in the manner of either of the two circuit arrangements shown in FIGS. 4 
and 5. As seen in FIG. 4, the tubular electrode 3 and end cap 7 are 
grounded and the rf voltage is applied to the paddle electrode 4 
containing the wafer boat 5 of FIGS. 1 and 2 with a set of vertical 
workpieces or wafers 10 mounted thereon, such that the electrode, boat, 
and wafers all act to form a cathode. In the arrangement of FIG. 5, end 
cap 7 is electrically isolated from tubular electrode 3 and is grounded 
along with door 9, while both electrode 3 and paddle electrode 4 are 
connected to the rf source 18. It is also contemplated that a source of dc 
power may be used as appropriate. In either event, the applied electrical 
power will create, either directly through a conductive substrate or 
through a "skin effect", an electric field at each substrate or wafer 
surface causing excitation of the gas in the spaces between the wafers to 
produce a plasma with enhanced hollow-cathode effect therein. 
A heating furnace may be provided, for producing any additional thermal 
energy necessary to the enhancement of the process, but is not shown for 
simplicity as the provision of a suitable device for this purpose will be 
within the purview of those of skill in the art. 
With the system shown in FIG. 1, high-efficiency deposition or growing of 
films on, or etching, the surfaces of the wafers may be achieved in the 
general manner as follows. A set of wafers 10, in the form, for example, 
of a number of silicon substrates, is disposed on electrode 4 in boat 5 in 
the reaction chamber 1. The wafers are spaced from each other to expose 
their surfaces for the desired reaction with the plasma to be formed. For 
instance, when the reactor is used to deposit a film on the wafer 
surfaces, the quality and uniformity of the deposited film will ultimately 
be a function of the spacing between the wafers and appropriate process 
parameters, such as the pressure in the reaction chamber 1, the species 
and flow rate of the reacting gas, and the power and frequency of the 
applied voltage, all of which may be adjusted to optimize the desired 
result. By way of a general example, the system of the preferred 
embodiment may be found to operate optimally with a judicious choice of a 
combination of parameters such as a pressure in the range from about 10 to 
500 millitorr with a constant flow of reactant gas through the chamber 1 
at a rate between 5 and 75 sccm. The reactant gas may be silane when 
depositing silicon film on a silicon wafer surface, and may be oxygen if 
growing an oxide on the silicon surface. The temperature may be at 
23.degree. C. (room temperature) or above and the applied electrical power 
may be in the range from 10 to 300 watts and at the conventional rf 
frequency of 13.56 MHz. The appropriate spacing of the wafers will depend 
upon the system sizing and typically may be of the order of about 0.1 to 
as much as 5 inches in a chamber of about 3 feet in length and 7 inch 
diameter. 
When rf power (e.g., 13.56 MHz) is applied to the electrode configuration 
of the present invention, a hollow-cathode-type glow discharge will result 
between each wafer. For a thorough understanding of the glow discharge 
phenomenon, a detailed explanation of the hollow cathode discharge may be 
found, as noted above, in Appl. Surf. Science 22/23, 925 (1985). With the 
present invention, the dense plasma produced by the hollow-cathode effect 
between the wafers will provide increased chemically-active species 
localized at the wafer surfaces thus enabling film growth, deposition, or 
etching, to take place at a significantly lower temperature than 
achievable with previous PECVD devices for this purpose. In particular, 
with regard to film deposition, the generally cylindrical, 
concentric-electrode arrangement provides improved confinement of and 
hollow-cathode effects in the plasma produced at the substrate surfaces, 
permitting low temperature and pressure deposition of material from the 
chemical vapor on the wafers. 
A particular example of an application of the present invention is the 
production of hydrogenated amorphous silicon. As noted above, hydrogenated 
amorphous silicon (a-Si:H) is widely used in devices that require a 
large-area and low-cost, low-temperature processing, e.g., solar cells, 
thin-film transistors for displays, electrophotography sensors, 
photodetectors, and light-emitting diodes. The present invention provides 
a new high-growth-rate technique for a-Si:H deposition. During the 
development of the technique, the deposition parameters were 
systematically varied in order to study their influence on the properties 
of the film deposited at room temperature. It is noted that an important 
characteristic of this deposition method is the degree of control of the 
hydrogen bonding configuration that can be achieved. 
An example of one embodiment of the present invention using the improved 
technique and involving the production of hydrogenated amorphous silicon 
will now be described in some detail with reference to the 
concentric-electrode PECVD reactor shown in FIG. 1. In this embodiment the 
reactor chamber 1 consists of a 3 feet long, 7-inch diameter quartz tube 
and contains a grounded, cylindrical electrode 3, lining the inside of the 
reactor walls, providing a counter electrode for the rf-powered, 
paddle-shaped electrode 4 near the center of the chamber. The electrode 
material is silicon carbide made conductive by the presence of crystalline 
silicon inclusions. The reaction gases are admitted to the chamber 1 
through an inlet 11 located at one end of the reactor and are decomposed 
in a plasma excited by an rf field at 13.56 MHz. The reactor is pumped at 
the opposite end by a roots blower connected in series to a roughing pump 
(not shown). 
With the use of such a reactor, a number of samples were deposited at room 
temperature from silane. Three series of experiments were performed. In 
the first series, the flow of silane was varied between 6 and 60 sccm 
(flow rate in cubic centimeters per minute at standard temperature and 
pressure), while the pressure and rf power were kept constant at 0.05 Torr 
and 75 W, respectively. In the second series, the pressure was 
systematically varied between 0.015 and 0.4 Torr, while keeping a silane 
flow of 15 sccm and a 75 W rf power. Finally, the third series covered a 
range of power levels between 25 and 275 W, with a constant silane flow 
rate (15 sccm) and pressure (0.05 Torr). Quartz substrates were used for 
the optical transmission measurements, while double-side-polished silicon 
wafers were used for the infrared absorption measurements. 
The thickness of the deposited films was measured using a TENCOR ALPHA-STEP 
200 surface profiler. The results of these measurements agreed with the 
thickness derived from the interference fringe spacing in the near-IR 
region measured using optical transmission. The optical transmission 
measurements were made using VARIAN CARY, 2400 ultraviolet-visible-near 
infrared spectrophotometer. The intercept from Tauc plots of 
(.alpha.h.omega.).sup.1/2 =B(h.omega.-E.sub.opt) was taken as E.sub.opt. 
The infrared transmittance spectra were measured in a range between 200 
and 4000 cm.sup.-1 using a PERKIN-ELMER 283 infrared spectrophotometer. 
The hydrogen concentration in the film was estimated from the integrated 
intensity of the bond stretching band in the 2000-2100 cm.sup.-1 region of 
the spectra using N.sub.H =1.4.times.10.sup.20 cm.sup.-2 
.intg..alpha.(.omega.)/.omega.d.omega., where N.sub.H is the concentration 
of hydrogen bonds, and .alpha. the absorption coefficient (See C. J. Fang, 
K. J. Gruntz, L. Ley and M. Cardona, J. Non-Cryst. Solids 35/36, 255 
(1980)). The Si--H/Si--H.sub.2 ratio was estimated (in accordance with P. 
J. Zanzucchi, in the above-noted Hydrogenated Amorphous Silicon, J. I. 
Pankove Ed., pg. 113, in Semiconductors and Semimetals, volume 21, Part B 
(Academic Press, Orlando, 1984) and references therein) as the ratio of 
the absorption coefficients at 2000 and 2100 cm.sup.-1, respectively. 
Raman spectra were measured in the backscattering geometry using 538.9 nm 
excitation and 2 cm.sup.-1 resolution. 
The improved results achieved with the reactor of the present invention are 
believed to arise from the combination of the plasma confinement and the 
hollow-cathode effects. It is theorized that the 
concentric-confining-electrode PECVD technique of the invention achieves a 
somewhat similar effect to that resulting from the use of a ground mesh 
surrounding the two parallel plate electrodes in conventional diode rf 
glow-discharge. As discussed by Hamasaki et al. in the reference cited 
above, the use of the ground mesh strongly increases the ratio of the area 
of the powered electrode (A.sub.P) to the area of all the other surfaces 
in contact with the plasma (A.sub.G). If the simplifying assumption is 
made that the distance (h) between the parallel electrodes in conventional 
diode glow-discharge is approximately equal to the electrode radius (r), 
i.e., r.about.h, this ratio becomes A.sub.P /A.sub.G .about.1/3 (since 
A.sub.P =.pi.r.sup.2 and A.sub.G =.pi.r.sup.2 +2.pi.rh). This enhancement 
of the A.sub.P /A.sub.G ratio causes an increase of the plasma potential 
(V.sub.P) and a decrease of the self bias potential (V.sub.S). The 
increase in V.sub.P is attributed by Hamasaki et al. and B. Chapman, in 
Glow Discharge Processes (John Wiley & Sons, New York, 1980), to the 
plasma confinement, and its net effect is equivalent to an increase in 
power. In the concentric-electrode configuration of the invention (FIG. 
1), far away from the electrode edges, A.sub.P /A.sub.G becomes 
.about.1/.pi. if a similar simplifying assumption is made, i.e., that the 
radius (r) of the outer electrode 3 -anode- is approximately equal to the 
width (l) of the inner electrode 4 -cathode- (since, in this case, A.sub.G 
=2.pi.rL and A.sub.P =2 lL, where L is the length of the reactor chamber). 
It is then speculated that a plasma with a high degree of dissociation and 
a very symmetric potential distribution results from the 
concentric-electrode configuration of this HC-PECVD system. In addition to 
the confinement effect, the more symmetric potential distribution induces 
a hollow-cathode effect. The hollow-cathode effect occurs when two 
similarly biased electrodes face each other, creating an "electron mirror" 
that enhances the electron confinement at low pressures. 
The glow discharge deposition of thin films is a complex multistep process. 
There is considerable discussion by Chittick et al. in J. Electrochem. 
Soc. 116, 77 (1969) and Shibata et al. in Japan. J. Appl. Phys. 26, L10 
(1987), both noted above, as to: which species, from those generated by 
the fragmentation of silane, is the main thin film precursor; which 
factors control the mobility of the adsorbed film precursor; and what is 
the dehydrogenation mechanism that allows the network propagation. These 
questions remain largely unresolved. It was therefore necessary, presented 
with a novel deposition technique, to perform an empirical study of the 
effect of the deposition conditions on the film quality. 
FIG. 6 illustrates the effect of varying the flow of silane on the film 
properties as obtained from such a study. The film properties are seen to 
be very sensitive to the silane flow. At low silane flows (6 sccm) a poor 
quality of a-Si:H film is obtained. This film shows high hydrogen 
concentration (x.sub.H .about.0.4), and predominance of the dihydride 
hydrogen bonding (the SiH/SiH.sub.2 ratio is less than 1) which suggests 
the presence of microstructure. The high hydrogen concentration raises the 
optical bandgap above 2 eV. Although these film characteristics are 
similar to those of low-temperature (&lt;150.degree. C.) a-Si:H grown using 
conventional diode rf glow-discharge (See Y. Ziegler, H. Curtins, J. 
Baumann and A. Shah, Mat. Res. Soc. Symp. Proc. 149, 81 (1989) and 
references therein), they also correspond to deposition conditions that 
cause silane depletion in the gas phase (i.e., when the dwell time of the 
species in the reactor is long compared with the reaction half time for 
the silane fragmentation). When the silane flow is increased to 15 sccm, 
the material quality improves dramatically: x.sub.H drops to .about.0.15; 
the dominant hydrogen bonding configuration is the Si--H monohydride; the 
optical gap is reduced to 1.7 eV; and the deposition rate increases 
7-fold. With the use of the set of diagnostics shown in FIG. 6, this 
room-temperature deposited material is indistinguishable from 
high-temperature (&gt;200.degree. C.) device-quality a-Si:H samples. The 
deposition rate, on the other hand, is much higher than for samples 
showing the same predominance of the monohydride configuration deposited 
using conventional diode rf glow discharge (See K. Ogawa, I. Shimizu and 
E. Inoue, Japan. J. Appl. Phys. 20, L639 (1981)). Further increase of the 
silane flow (60 sccm) induces a degradation in the film properties (as 
indicated by the optical gap increase and the low SiH/SiH.sub.2 ratio with 
respect to the sample grown at 15 sccm). It is interesting to note that 
although this pronounced shift from the monohydride to dihydride bonding 
configuration occurred when the silane flow was raised from 15 to 60 sccm, 
the total hydrogen content in the film did not appear to change 
appreciably. 
FIGS. 7 and 8 show the effects of the variation of deposition pressure and 
power, respectively. The dominant feature of FIG. 7 is the slow 
degradation of the film properties as the pressure is increased (as 
indicated by the increase of the hydrogen content and optical bandgap and 
the decrease of the monohydride to dihydride bonding ratio). During high 
pressure deposition (.about.0.4 Torr) extensive gas phase nucleation was 
observed. This phenomenon is apparently responsible for the decrease of 
the growth rate and the film quality degradation. At the other extreme 
pressure condition (.about.0.01 Torr) it appears that the mean free path 
of the species in the gas phase is high enough so that collisions with the 
reactor walls become the dominant deactivation mechanism and the 
confinement effect is lost. The effect of varying the power level on the 
properties of the a-Si:H films is shown in FIG. 8. As the power is 
increased, a steady degradation of the film properties is observed (as 
indicated by the increase of the hydrogen content and the optical bandgap, 
and by the decrease of the SiH/SiH.sub.2 ratio). The high power deposition 
results are comparable to that of low silane flow. At high power levels, 
silane depletion is also observed. 
Finally, FIG. 9 illustrates and emphasizes the dramatic effects that 
different deposition conditions have on the structural characteristics of 
the room-temperature deposited a-Si:H concentric-electrode PECVD films. 
The hydrogen bonding configuration is very sensitive to the deposition 
conditions. By increasing the flow of silane from 15 to 60 sccm, the 
dominant bonding configuration is changed from monohydride to dihydride. A 
two-fold increase in the deposition pressure (from 0.05 to 0.1 Torr) 
changes the bonding configuration to one in which both Si--H and 
Si--H.sub.2 are present in comparable amounts. On the other hand, Raman 
measurements yielded a full-width at half-maximum scatter between 68 and 
80 cm.sup.-1 for the TO-like band. 
It will be seen from the foregoing description that an improved 
concentric-confining-electrode configuration is presented for thin film 
deposition using an rf glow discharge plasma. The structure of a-Si:H 
films deposited at high growth rate and at room temperature can be tuned 
from one in which the dominant Si--H bonding configuration corresponds to 
the dihydride mode to one in which the monohydride form is exclusively 
present. The correlation between the film structure and the deposition 
conditions is established. The a-Si:H film quality is strongly dependent 
on the flow of silane during deposition but it is less sensitive to the 
reactor pressure and rf power. Finally, the two-dimensional 
characteristics of this tubular deposition system make it very attractive 
for large-area, high-throughput thin film production. 
An exemplary process for growing device quality amorphous silicon at room 
temperature (23.degree. C.) on one or more wafers in the described 
reactor, with the wafer spacing not being critical, has been found to be 
operable using the system within the following set of parameters: 
Gas=100% SiH4 (silane) 
Pressure=25&lt;P&lt;100 millitorr 
Power=10&lt;Pw&lt;150 watts 
Gas Flow=10&lt;F&lt;50 sccm 
Temperature=23.degree. C. 
Growth Rate=100&lt;GR&lt;1000 .ANG./min. 
With the use of such a process amorphous silicon films may be formed on 
substrates or devices that have serious temperature limitations, thus 
extending the range of materials that can be used in such device 
applications. Also the growth rate in the improved concentric-electrode, 
hollow-cathode configuration of the invention is faster than that 
achievable with other PECVD configurations by about 15 times. 
An example of the potential of this process is illustrated in the Table set 
forth in FIG. 10. The Table represents a comparison of a-Si:H film 
characteristics for a film resulting from conventional PECVD deposition 
processing (300.degree. C.) versus a film produced by the HC-PECVD process 
(room temperature) of the invention and the effects of annealing those 
films in an inert gas. The Table shows the effect of annealing on the 
electrical conductivity of HC-PECVD room temperature deposited a-Si:H. It 
will be seen that after being annealed for 20 hours, the dark conductivity 
of an a-Si:H sample remains unchanged but the photo conductivity increases 
3 fold. In addition, the activation energy also increases from 0.43 to 
0.72 eV. As a result of this annealing, both the activation energy and 
conductivity of HC-PECVD deposited a-Si:H approach the characteristics of 
conventional 300.degree. C. PECVD deposited a-Si:H but with a much higher 
deposition rate, i.e., 15 to 1. 
Because of the extremely high growth or deposition rate at low temperature 
achievable with this HC-PECVD system, a number of potential applications 
become feasible. These applications include the fabricating of wide 
bandgap materials such as: microcrystalline, poly- and epitaxy silicon at 
low temperature; silicon carbide; boron nitride; and diamond. 
A further application is the deposition of insulators, silicon dioxide or 
silicon nitride, at room temperature at a fast rate for use as an 
interlevel passivation layer. Additionally, alloying amorphous silicon 
with germanium to allow tuning of the bandgap at low temperature for 
optoelectric circuits is another potential process for this system. 
Furthermore, all of these films could be deposited through a photoresist 
mask for special applications because of the extremely low temperature of 
deposition. 
It will be appreciated by those of skill in the art in view of the 
foregoing description that the disclosed concentric-confining-electrode 
reactor may be used in a large number of applications with various gases, 
materials, and configurations to produce improved plasma treatment of 
workpieces in the depositing or growing of films thereon or the etching of 
patterns therein and the like.