Supported plasma sputtering apparatus for high deposition rate over large area

A supported plasma sputtering apparatus is described having shaped electrical fields in the electron discharge region between the cathode and anode and the sputter region between the target and substrate while such regions are free of any externally applied magnetic field to provide a high deposition rate which is substantially uniform over a wide area. Plasma shaping electrodes separate from the anode and target shape the electrical fields in the electron discharge region and the sputter region to provide a high density plasma. The anode surrounds the target to cause substantially uniform sputtering over a large target area. In one embodiment the anode is in the form of an annular ring surrounding a flat target surface, such anode being provided with a ribbed upper surface which shields portions of the anode from exposure to sputtered material to maintain the electron discharge for a long stable operation. Several other embodiments accomplish the same result by using different anodes which either shield the anode from sputtered material, remove the sputtered coating on the anode by heating, or simultaneously mix sputtered metal from the auxiliary target with sputtered insulator from the main target so the resultant coating is conductive. A radio frequency potential alone or together with a D.C. potential, may be applied to the target for a greater sputtering rate.

BACKGROUND OF THE INVENTION 
The subject matter of the present invention relates generally to a 
supported plasma sputtering apparatus which is capable of high deposition 
rates over a wide area, and in particular to triode sputtering apparatus 
having plasma shaping electrodes to provide a high density plasma without 
the use of any externally applied magnetic field to enable a more uniform 
deposition of sputtered material over a wide area. A "supported plasma" 
sputtering apparatus is one in which the plasma is maintained or supported 
by an electron discharge from a thermionic cathode to an anode and the 
sputtering target is immersed in such plasma. The plasma shaping 
electrodes shape the electrical field in the electron discharge region 
between the cathode and anode and in the sputter region between the target 
and substrate member to provide a high density plasma which increases the 
sputter deposition rate and enables a large area deposit of substantially 
uniform thickness. A radio frequency potential may be applied to the 
target to increase the sputtering rate and apparently increase the plasma 
density, which results in improved deposit thickness uniformity and 
greater deposit density, partially due to lower pressure. A long stable 
sputtering operation is achieved by maintaining the electron discharge 
using different types of anodes surrounding the target, which either 
shield anode surface portions from exposure to sputtering material or 
remove sputtered coatings on the anode by heating, or to simultaneously 
mix sputtered metal from the auxiliary target with sputtered insulator 
from the main target so that the resultant coating is conductive. 
The sputtering apparatus of the present invention is especially useful in 
sputtering metal, insulator or semiconductor materials over large areas, 
such as during the formation of solar panel photocells for the conversion 
of light to electrical power. 
Previously it has been proposed in the article, "High Rate RF Sputtering 
System", Journal of Vacuum Science and Technology, Volume 7, No. 2, by D. 
H. Grantham et al, pages 343-346, published 1969, and in U.S. Pat. No. 
3,901,784 of D. J. Quinn et al, granted Aug. 26, 1975, to deposit 
insulating material at a high deposition rate using radio frequency power. 
However, a magnetic field was provided in the sputter region to increase 
the deposition rate and no plasma shaping electrodes were employed so that 
a relatively small target area was sputtered and the sputtered deposit on 
the substrate would have a nonuniform thickness. Similar results would be 
achieved by triode sputtering apparatus shown in U.S. Pat. No. 3,514,391 
of Hablanian et al and U.S. Pat. No. 3,616,452 of Bessot et al, both of 
which apply radio frequency fields to insulating targets but employ 
magnetic fields in the sputter region which would result in a small area 
deposition and would produce nonuniform thickness deposits. An R.F. 
sputtering apparatus using a finned or ribbed anode to prevent sputtered 
material from completely coating such anode is shown in U.S. Pat. No. 
3,514,391 and in the article, "Initial Work on the Application of 
Protective Coatings to Marine Gas Turbine Components by High Rate 
Sputtering", by E. D. McClanahan et al in publication 74-GT-100 of the 
American Society of Mechanical Engineers, published March, 1974. None of 
these prior references disclose the use of plasma shaping electrodes 
separate from the anode which are connected to a more negative potential 
and shape the electrical fields in the electron discharge region and 
sputter region to increase the plasma density without using a magnetic 
field in the manner of the present invention. 
SUMMARY OF INVENTION 
It is therefore one object of the present invention to provide an improved 
sputtering apparatus for achieving high sputtering deposition rates over 
large areas. 
Another object of the invention is to provide such an improved sputtering 
apparatus using plasma shaping electrodes separate from the anode and 
target to shape the electrical field in the electron discharge region 
between the cathode and anode and in the sputter region between the target 
and substrate to provide a high density plasma without the use of a 
magnetic field. 
A further object of the present invention is to provide such a sputtering 
apparatus which provides a more uniform thickness deposition over a wider 
area. 
An additional object of the invention is to provide such a sputtering 
apparatus which is capable of a stable sputtering operation for a long 
period of time. 
Still another object of the invention is to provide a sputtering apparatus 
which accomplishes such stable operation by employing different types of 
anodes which either shield anode surface portions from exposure to 
sputtering material or remove sputtered material coated on the anode by 
heating such anode or coating the anode with conducting material sputtered 
from an auxiliary target. 
A still further object of the invention is to provide such a sputtering 
apparatus in which a radio frequency electrical potential is applied to 
the target to increase the sputtering rate.

DESCRIPTION OF PREFERRED EMBODIMENTS 
As shown in FIGS. 1 and 1A, one embodiment of the triode sputtering 
apparatus of the present invention includes an annular heated filament 
type cathode 10 serving as a source of electrons, a ring-shaped anode 12 
surrounding a flat annular target 14, and a flat annular substrate 16. The 
target 14 has its upper surface coated with a layer 18 of metal, insulator 
or semiconductor material to be sputtered, hereinafter called the 
"sputtering layer". A pair of annular first and second plasma shaping 
electrodes 20 and 22 are provided with electrode 22 being of a cup-shape 
surrounding the substrate 16 and the electrode 20 being in the form of a 
cylinder surrounding said coaxial with cathode 10, anode 12, substrate 16, 
and target 18. These plasma shaping electrodes are releasably attached by 
bolts or the like to a common support member 24 which may be made of metal 
and serves as the top portion of a sealed, evacuated envelope enclosing 
the sputtering apparatus and containing inert gas at a low pressure of 
about 1 to 5 millitorr. 
The envelope also includes a cylindrical envelope wall member 26 attached 
at its upper end to the support member 24 by an insulator ring 28. The 
envelope wall member 26 is attached at its lower end to an anode support 
ring 30. The anode 12 is releasably attached to support ring 30 and such 
ring is made of metal for applying an electrical potential to such anode, 
which is usually grounded. The anode support ring 30 is connected to a 
target support plate 32 through an insulator ring 34 so that the target 
may be maintained at a high negative D.C. potential with respect to the 
anode for sputtering purposes. 
Positive ions of inert gas are produced by the electron discharge from the 
filament cathode 10 to the anode 12 to produce a plasma in an annular 
shaped principal electron discharge region 36 surrounding the target 14, 
such region containing the major portion of the cathode to anode electron 
discharge for supporting or maintaining the plasma. These positive ions 
bombard the sputtering layer 18 on the surface of the target 14 to cause 
material to be sputtered from such target onto the substrate 16 through a 
centrally located sputter discharge region 38. Substrate 16 can be D.C. 
biased or R.F. self-induced biased to about -500 volts, but is preferably 
floating and assumes a bias of about -25 volts, as hereafter described. It 
should be noted that the plasma produced in the principal electron 
discharge region 36 surrounds and is distributed evenly over the sputter 
discharge region 38 and the target 14 for efficient uniform sputtering. 
The target 14 is preferably connected to a radio frequency (R.F.) signal 
A.C. power supply 39 through a coupling capacitor 40 of a few picofarads. 
In addition, for depositing conductor or semiconductor material, the 
target may be connected to a source of negative D.C. voltage of, for 
example, -2.0 kilovolts through a switch 41 and an isolating inductance 42 
so that such D.C. source and R.F. power supply are isolated from each 
other. One suitable R.F. power supply is a 10 kilovolts peak to peak 
amplitude R.F. power supply having a frequency of about 13.56 megahertz. 
This R.F. signal produced by the R.F. power supply induces an average D.C. 
voltage of about -2.5 to -5.0 kilovolts on the surface of the sputtering 
layer 18 in the manner described by H. S. Butler et al in Physics of 
Fluids, Volume 6, No. 9, Sept. 1963, pages 1346 to 1355. As stated 
earlier, the anode 12 is typically grounded. The filament cathode 10 is 
connected to a small negative D.C. potential of typically about -35 volts. 
The plasma shaping electrodes 20 and 22 are connected by a potentiometer 
23 to a D.C. potential of about -25 volts to +25 volts relative to the 
cathode 10 and in the example given, would be of a D.C. voltage between 
-10 volts and -60 volts with respect to ground. However, it is also 
possible to provide such plasma shaping electrodes so that they are 
floating in electrical potential and assume a negative D.C. potential of 
about -25 volts. The plasma shaping electrodes are negative with respect 
to the anode and accordingly repel electrons emitted by the cathode 10 
toward the anode 12. Because of the presence of positive ions of inert gas 
in the electron discharge region 36, the plasma shaping electrodes 20 and 
22 are not at a highly negative potential because this would result in the 
sputtering of metal from such electrodes due to positive ion bombardment. 
The annular ring-shaped filament cathode 10 is contained within a cavity 
formed by the plasma shaping electrodes 20, 22, and their support member 
24 so that substantially all of the electrons emitted by the cathode 10 
are repelled from the surface of such cavity and directed towards the 
anode. This increases the electron flow to such anode and also increases 
the density of the plasma formed by such electrons and the positive ions 
of inert gas in such electron discharge region because of the greater 
ionization caused by the increased electron flow through the discharge 
region. It should be noted that the cavity surface of the support member 
24 may be covered by shield plates 43 releasably attached thereto for 
replacement along with the electrodes 20, 22 and anode 12 when they become 
coated with sputtered material. The radio frequency signal applied to the 
target further increases ionization of the inert gas molecules by the 
electrons resulting in an even higher density plasma which increases the 
number of positive ions of inert gas which bombard the target. In the 
example given, incident target power densities of 50 to 100 watts/cm.sup.2 
were obtained. The result is that the sputtering apparatus of the present 
invention has an extremely high sputter deposition rate over a large area 
of the target which is maintained substantially uniformly across such 
target area due to the fact that no external magnetic field is applied to 
the sputtering apparatus so that the electron discharge region 36 and the 
sputter discharge region 38 are free of any such magnetic field. 
The substrate 16 is of a floating D.C. potential and is A.C. grounded 
through an R.F. signal bypass capacitor 44 of about 0.1 microfarads. The 
floating D.C. potential on the substrate 16 under typical operating 
conditions is about -25 volts. The target of the above example was 
approximately 12.7 centimeters in diameter while the bombarded area of the 
target was approximately 11.1 centimeters in diameter, giving a total 
effective target area of about 100 square centimeters. However, the 
effective sputtering target area can be increased considerably using 
higher R.F. power supplies. Target sizes of approximately 500 to 700 
square centimeters can be used with commercially available R.F. power 
supplies on the order of 30 to 35 kilowatts, which maintain a high 
sputtering rate over the larger area. 
An inert gas, such as krypton or argon, is supplied to the interior of the 
vacuum chamber through a coupling 46 in the side wall 26 and such chamber 
is normally maintained at a low pressure of between 1 to 5 millitorr, 4 
millitorr being typical for krypton, by means of a vacuum pump connected 
to such housing through coupling 46. The target and the substrate may have 
their temperatures regulated such as by water cooling in the conventional 
manner through hollow support stems 45 and 47. The target temperature is 
typically held at about 25.degree. Celsius while the substrate temperature 
varies over a wide range of between about -100.degree. to +900.degree. 
Celsius, depending on the materials being deposited. 
The anode to cathode spacing in the embodiment of FIG. 1 is about 7.62 
centimeters, while the target to substrate spacing may be adjusted from 
about 2.54 to 6.35 centimeters and is typically set at about 4.12 
centimeters. The substrate support stem 47 is sealed to the electrode 
support 24 by an insulative sleeve 49 which spaces the stem from such 
support by about 0.317 centimeters. The anode is spaced from the target 
approximately 0.317 to 0.635 centimeters for electrical insulation 
purposes and in the preferred embodiment may have its upper surface 
slightly raised above the upper surface of the target as shown. 
Using the above described apparatus, high deposition rates on the order of 
15,000 Angstroms per minute for metals and metal alloys were achieved, 
which can be increased to 25,000 Angstroms per minute with a greater R.F. 
signal power supply of, for example, 15 kilowatts power. This compares 
with a high of approximately 1000 Angstroms per minute deposition rate 
for a conventional R.F. diode sputtering system. Similarly, high 
deposition rates were obtained for insulating materials. Thus, a 
deposition rate of 2000 Angstroms per minute was obtained for aluminum 
oxide and 4000 Angstroms per minute was achieved for zirconium oxide. An 
even greater deposition rate could be achieved for insulators up to 
approximately 8000 Angstroms per minute using a greater R.F. power supply 
of 15 kilowatts. This increased deposition rate for insulators compares to 
a maximum deposition rate of about 500 Angstroms per minute for 
conventional R.F. diode sputtering systems. 
In addition to the increased sputtering rate, the sputtered deposit has a 
substantially uniform thickness of plus or minus 5% variation over much 
larger areas. To date these results have been achieved for targets of 110 
cm.sup.2 and we feel confident that similar results can be achieved on 
much larger targets of up to 500 to 700 cm.sup.2 using larger R.F. power 
supplies. It is also possible to deposit semiconductor materials at 
similar high deposition rates and large areas of substantially uniform 
thickness. 
As shown in FIG. 1, the anode 12 may be provided with a ribbed upper 
surface 48 in order to prevent the deposition of sputtered insulating 
material over the entire surface of the anode which would otherwise stop 
the electron discharge from the cathode to such anode. Thus, the tops of 
the ribs or ridges at the upper surface of the anode shield the valley 
portions of such ridges from the deposition of sputtered material. As a 
result, electron discharge between the cathode and anode is not prevented, 
which enables a long stable sputtering operation. In this regard, the 
ribbed anode 48 is similar to the anode used in the R.F. sputtering 
apparatus of U.S. Pat. No. 3,514,391 of Hablanian et al. 
FIG. 2 shows another embodiment of the sputtering apparatus similar to that 
of FIG. 1 which employs a modified anode 12' but is otherwise similar to 
the embodiment of FIG. 1. Anode 12' has an annular internal cavity 50 of 
rectangular cross section into which the electrons enter through a 
restricted opening 52 in the top of such cavity. As a result of the 
restricted opening 52, very little sputtered material enters the cavity 50 
and is deposited on the surface of the cavity. This enables the electron 
discharge from the filament cathode 10 to the anode 12' to be maintained 
for a longer stable operation. Of course, the sputtered material will 
eventually coat the entire cavity 50 which will stop the electron 
discharge if it is an insulator, but this will take a very long time. In 
the meantime the electrons emitted from the cathode 10 pass through the 
restricted opening 52 and strike the anode surface within the cavity 50 to 
maintain the sputtering action. It should be noted that in both the 
embodiments of FIGS. 1 and 2, the shape of the outer plasma shaping 
electrode 20 may be changed somewhat from that shown so that the bottom 
end of such electrode projects inward over the upper surface of the anode 
to further shield the anode from the deposition of sputtered material. 
A third embodiment of the sputtering apparatus of the present invention, 
shown in FIG. 3, is similar to that of FIG. 1, except for the use of a 
second modified anode 12" which is in the form of an annular metal wire of 
tungsten or other refractory metal. The wire anode 12" is mounted within 
an annular cavity 51 of rectangular cross section formed in the anode 
support member 30' by extending the inner edge of such support member 
upward to shield the anode wire from the target layer 18. The wire anode 
is heated either by electron bombardment with electrons emitted by the 
cathode 10 or by electrical current flowing through such anode wire from a 
conventional A.C. source of heating current (not shown). As a result of 
this heating of the wire anode, any sputtered material deposited on such 
anode is removed by evaporation. 
A fourth embodiment of the present invention is shown in FIG. 4, which is 
similar to that of FIG. 1 except that is employs a third modified anode 
12'". The anode 12'" is in the form of an annular ring releasably attached 
to the anode support 30' so that its outer surface 54 faces an auxiliary 
target 56. The auxiliary target 56 is in the form of an annular ring 
mounted within the cavity 51 in the anode support 30' and surrounding the 
anode. The auxiliary target 56 is made of a suitable conductor which, upon 
the application of a negative D.C. potential to such auxiliary target, is 
sputtered from such target onto the surface 54 of the anode. This results 
in maintaining a reasonably conductive surface on such anode by changing 
any sputtered insulator coating formed on such anode into a conductive 
coating. As a result, this insulating material does not prevent an 
electron discharge between the filament cathode 10 and the anode. An 
electrical lead 58 extends through an insulating seal 60 in the side of 
the envelope wall 26 in order to apply a negative D.C. potential of about 
- 150 volts in the range of -50 volts to -500 volts in the sputter 
position of switch 62 shown. In the other position of switch 62, the 
auxiliary target is deenergized. 
A fifth embodiment of the sputtering apparatus of the present invention is 
shown in FIG. 5. This embodiment is similar to that of FIG. 1 except that 
it employs a cylindrical target 14' which is mounted with its longitudinal 
axis coaxial with a surrounding cylindrical substrate member 16'. In this 
embodiment, it should be noted that the electron discharge region between 
filament 10 and anode 12 extends substantially perpendicular to the 
sputter discharge region extending between target 14' and substrate 16'. 
The upper end of the cylindrical target member 14' is provided with a 
hemispherical end portion 64 which is uniformly spaced from a 
corresponding hemispherical cavity 66 in the modified plasma shaping 
electrode 22' sufficiently to prevent electrical discharges between these 
members. The substrate cylinder 16' is mounted on the inner surface of the 
cylindrical metal support 68, whose upper end is joined to the envelope 
side wall 26 by an annular metal ring member 70 and whose lower end is 
joined to a second annular metal ring member 72. The substrate 16' and its 
support members 68, 70, and 72 are floating in electrical potential and 
normally assume a negative D.C. potential of about -30 volts so that they 
repel the electrons transmitted from cathode 10 across the surface of the 
substrate to anode 12. 
An annular insulator ring 74 connects the support ring 72 to the anode 
support 30 and electrically insulates these members from one another. The 
ribbed anode 12 is provided with an inner flange portion 76 which is 
supported on the upper edge of another insulator ring 78 whose lower edge 
is supported on an outer flange 80 projecting outward from the target 
support stem 45. A pair of rubber O rings 82 and 84 are provided between 
the insulator ring 78 and flanges 76 and 80 respectively, to form gas 
tight seals therewith. Similar O rings are also present between all 
adjacent separate members of the envelopes in FIGS. 1 to 5, but are not 
shown to simplify the drawings. A vacuum envelope is formed containing an 
inert gas, such as krypton or argon, at a low pressure of 1 to 5 millitorr 
provided by the vacuum pump and gas source connected to coupling 46. 
The embodiment of the sputtering apparatus shown in FIG. 5 has the 
advantage that it is capable of coating an even larger area of the 
substrate than the embodiments of FIGS. 1 to 4. In all embodiments, plasma 
shaping electrodes are employed to shape the electrical field in the 
electron discharge region between the cathode and anode and in the sputter 
region between the target and substrate to provide a high plasma density 
without using an externally applied magnetic field. The result is a high 
deposition rate giving high density sputter deposits of large area and 
substantially uniform thickness in all of these embodiments. 
It will be obvious to those having ordinary skill in the art that many 
changes may be made in the above-described preferred embodiments of the 
present invention without departing from the spirit of the invention. 
Therefore, the scope of the present invention should only be determined by 
the following claims.