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BACKGROUND OF THE INVENTION
Plasma sputter etching/deposition systems have long been used in the
electronics industry for fabricating various sophisticated items such as
LSI and VLSI circuits, memories, magnetic read/record heads, etc. The
systems may characteristically be used for the deposition of material onto
a target (sputter deposition) or for the selective removal (etching) of
material from such a target. The material removal processes remove
material by ion or electron bombardment or by reactive ion etching.
Typical plasma systems are RF sputtering, magnetron sputtering, diode (DC)
sputtering, ion beam sputtering, ion plating, etc. As with any industrial
process, any modifications which make the process more efficient either in
terms of time or expense greatly improves the value of the process.
The following description of the present invention is directed primarily
toward high plasma density magnetron sputter etching/deposition systems,
however it is to be understood that the concepts disclosed have broader
applicability.
A magnetron sputtering system is basically a diode plasma device with a
strong magnetic enhancement at the cathode. This magnetic enhancement
serves to form an electron trap, such that electrons undergo ExB drifting
paths which close upon themselves. The strong magnetic fields present also
increase the electron ionization probability and the plasma density,
leading to high ion bombardment rates of the cathode and high sputtering
rates. Two basic types of magnetrons have been developed which are: the
cylindrical post magnetron, and the planar magnetron.
A magnetron sputtering device may be characterized by two equations. The
first, by J. A. Thornton, J. Vac. Sci & Technology, Vol. 15 (1978) pp 171,
relates the lowest operating voltage, V, to the average energy required
for the production of an electron-ion pair, E:
V=E/.gamma.e.sub.1 e.sub.2 ( 1)
where .gamma. is the secondary electron coefficient for ion bombardment (of
the cathode), e.sub.1 is the probability that an ion will hit the cathode,
and e.sub.2 is the probability that a secondary electron will totally
utilize its energy in ionization. The second relevant equation is the
empirical relation
i=k V.sup.n ( 2)
where V is the operating voltage, i is the magnetron current and n is an
exponent in the range of 3-10. Higher values of n indicate more efficient
magnetron operation, a value of 5-7 being an average value.
The first equation predicts minimum or turn-on magnetron voltages of over
300-350 volts. Voltages in this range are indeed found experimentally,
although the magnetron currents at those energies are quite small and the
sputtering and deposition rates low. A typical magnetron sputtering system
operates in the 400-600 volt range at currents of a few amps, and up to 20
amps in very large systems (Op Cit., Thornton). The operating pressures of
a magnetron device are in the 3-10 millitorr range. At constant power, an
increase in the pressure will result in an increase in the magnetron
current and a decrease in the operating voltage. At high pressures,
though, scattering of the sputtered material becomes even more significant
and the actual deposition rate will decrease.
There are several drawbacks with magnetron devices. The first is the
operating pressure, which is by necessity in the 3-10 millitorr range. At
these pressures, the mean-free path of a sputtered atom is only a
centimeter or less. This short length means that sputtered material is
often scattered prior to deposition on a substrate. Typically only 35% of
the total material removed from the target is deposited on the substrate
area, the remaining 65% coat the various parts of the system, as well as
being redeposited on the target as shown in (W. H. Class, "Thin Solid
Films," Vol. 107, (1983), p 379). This scattering also limits the
effective target-to-substrate distance to a few centimeters, which can
result in significant substrate electron and ion fluxes. The scattering
also results in a loss of directionality of the depositing flux, which
makes such processes as "lift-off" more difficult. The second severe
operating problem with magnetrons is the high energies necessary for
operation. Typical energies of 400-600 eV are needed for useful sputtering
rates. These high energies can cause significant target damage, or
substrate damage in the case of samples being the sputtering target. To
increase the deposition or sputtering rates, it is necessary to increase
the magnetron voltage. As the electron energy increases, the ionization
probability near the target surface decreases, and the discharge becomes
more inefficient. The high energies can induce or inhibit various chemical
reactions at the target surface, which may not necessarily be desired. The
magnetron sputtering system is restricted by equation (1) to only
operating at high voltage, and only with the current/voltage
characteristic described by equation (2).
A hollow cathode is a plasma device which is capable of emitting a high
electron current. The actual operating procedure is well known and has
been described in detail in the (H. R. Kaufman, R. S. Robinson and D. C.
Trock, "J. Spacecrafts and Rockets," Vol. 20, (1983), p 77), and will not
be repeated here. By biasing the hollow cathode sufficiently negative of
some anode, a plasma can be produced due to electron ionization of the
background (working) gas. This plasma is characterized by a discharge
current, which is also equal to the emission current of the hollow
cathode. With even a small hollow cathode of diameter 1/8 inch, discharge
currents of up to 15 amps are possible at pressures in the 0.2-0.6
millitorr range in Argon.
The hollow cathode effect per se has been described in great detail in the
following three references, as well as quite a few others, and will not be
described in detail here:
1. H. R. Kaufman, R. S. Robinson and D. C. Trock, J. Spacecrafts and
Rockets, Vol. 20 (1983) p. 77.
2. H. R. Kaufman, in "Advances in Electronics and Electron Physics," Vol.
36, Academic Press, NY, (1974), p. 265.
3. J. L. Delcroix and A. R. Trindade, in "Advances in Electronics and
Electron Physics," Vol. 35, Academic Press, NY, (1974), p. 87.
In the past, hollow cathodes have only been reported which are based on a
cylindrical geometry, i.e., based on a tube, which is usually a refractory
material such as tantalum. The tube often has a constriction at its tip,
which serves to increase the internal pressure of the cathode. Usually, an
insert of foil or other material is added near the tip. Gas is incident on
the cathode from an external supply, which due to the smallness of the
aperture causes pressures of up to a Torr inside the tube. A plasma
discharge can be generated by biasing a keeper or anode, positive with
respect to the cathode. This plasma will exist in a region which is inside
the hollow cathode, which will be at much higher pressure and hence have a
much greater plasma density than those values outside of the hollow
cathode. The ion bombardment of the foil inside the tip, which is
insulated by the outer layers of foil, will cause the inner layers of the
foil to become quite hot, often 2000 K. At this high temperature, the foil
surface can thermionically emit electrons, which causes a greater
generation of plasma. Once this increase in plasma density occurs, the
relative potential of the keeper or anode with respect to the hollow
cathode can be reduced to voltages in the 30-50 volt range.
Multiple hollow cathodes of a sort have also been developed. The cathodes
consist of a number of tubes tightly bound together in an outer tube,
sharing a common gas and electrical power supply. The multiple tubes serve
to restrict the gas conduction through the tubes, allowing for higher
current operation at reduced gas flow. These multiple cathodes, however,
do not really depart from the above described mode of operation, and are
also restricted to the basic cylindrical geometry.
The ability to radically change the geometry of operation is necessary,
however, for a number of specialized applications, such as electron
injection into magnetron and other high energy plasmas or other large
chambers. In addition, the operation of multiple, separated but coupled
hollow cathodes is not possible with the classical design, due to gas flow
considerations and coupling problems with power supplies.
PRIOR ART
U.S. Pat. No. 4,431,473 discloses an RF magnetron device which can be used
for reactive ion etching. Special provision has been made to have several
chambers for the discharge, the magnet assembly, gas inlet, etc. The
present invention differs significantly from the patent in that a hollow
cathode electron source is used to inject energetic electrons into an
existing magnetron plasma. There is no source of electrons (other than the
normal cathode secondaries) in U.S. Pat. No. 4,431,473.
The present invention operates at low pressures, as low as 4.times.10(-5)
Torr, significantly below the range of a conventional magnetron (1
millitorr). The device in U.S. Pat. No. 4,431,473 operates in the 10
millitorr range. The low pressure of operation of the present invention
allows line-of-sight processes, as well as multiple plasma processes
within the same chamber. Also the present invention will operate in either
RF or DC modes. U.S. Pat. No. 4,431,473 is only an RF device.
The present invention also operates in either a sputtering mode, for
sputtering of the cathode or sputter deposition of cathode material onto a
sample, or in an Reactive Ion Etching (RIE) or Reactive Deposition mode,
where some chemical reaction is taking place at the cathode surface. U.S.
Pat. No. 4,431,473 is only a RIE device, for etching of a sample on the
cathode surface.
Finally, the present invention operates at voltages of 20 eV or lower,
because the plasma is sustained by the hollow cathode emission. U.S. Pat.
No. 4,431,473 has no provision for low energy operation, other than that
obtained from the addition of a magnetic field to an RF diode. This limits
low voltage operation still to 300-400 volts at the minimum.
An article by C. Horwitz, Applied Physics Letters, Vol. 44 (1984), pp 1041,
describes the reactive ion etching of materials in a modified RF device.
The device has been configured to form a partial electrostatic hollow
cathode glow, which changes the etching and polymerization rates in an
oxygen/freon plasma. The article makes no mention of magnetron sputtering
in any fashion and makes no mention of the injection of electrons into a
magnetron or RF plasma from an auxiliary electron source (hollow cathode
arc).
SUMMARY OF THE INVENTION
The present invention combines a hollow cathode electron emitting device in
a specific manner with an existing plasma sputter etching/deposition
device. In particular, the hollow cathode device has been combined with a
magnetron sputter etching/deposition system with surprising results. The
hollow cathode is utilized to provide additional ionization of the working
gas during normal magnetron operation, and provides all the gas ionization
at low magnetron energies. Low energy magnetron operation of this type was
not previously possible. At the high magnetron voltages (i.e., normal
operation) the hollow cathode has been found to increase the magnetron
current in the deposition mode by at least a factor of 10 times.
It has further been found that by successively operating the combined
system at low magnetron energy and high magnetron energy, it is possible
to switch from a sputtering to a sub-sputtering mode with attendant
isotropic and anisotropic etching, respectively. Utilizing this concept
unique results have been obtained with regard to controlling etch
profiles.
According to a further aspect of the invention, side discharge hollow
cathodes and multiple side discharge hollow cathodes have been developed
which depart from previous hollow cathode designs and which have
particular utility with the combined hollow cathode plasma sputter
etching/deposition systems of the present invention. These cathodes rather
than relying on specialized tips at the ends of the hollow cathode tube,
rely on apertures in the sides of a closed tube as the electron emission
point. It has further been possible to generalize the geometry of the
hollow cathode effect to non-cylindrical shapes which not only simplify
cathode construction, but also suits them better for use in specialized
plasma systems such as magnetrons.
OBJECTS OF THE INVENTION
It is a primary object of the present invention to provide an improved high
energy plasma sputter etching/deposition system.
It is another object of the invention to provide such a system which
combines a hollow cathode electron source with a high energy plasma
system.
It is still a further object to provide such a system wherein the
particular high energy plasma device is a magnetron sputter
etching/deposition system.
It is another object of the invention to provide such a combined system
capable of operation at lower pressures and lower energy levels with high
plasma density where desirable.
It is another object of the invention to provide such a combined system
capable of much higher sputtering rates than would normally be possible
without the hollow cathode enhancement.
It is another object of the invention to provide such a combined system
capable of performing different types of etching processes by merely
switching the current/voltage characteristics at a constant power level.
It is yet another object of the invention to provide such a switched
operating mode wherein the system will perform selectively isotropic or
anisotropic etching.
It is a further object of the invention to provide a novel hollow cathode
configuration for use in such combined systems providing improved electron
current distribution within a reaction chamber.
It is another object to provide such a hollow cathode configuration
utilizing emission from the side of the hollow cathode rather than axially
therefrom.
Other objects, features and advantages of the present invention will be
apparent from the subsequent description of the preferred embodiments
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 comprises a cross sectional view of a simplified hollow cathode
enhanced magnetron sputter etching/deposition system embodying the
principles of the present invention.
FIG. 2 comprises a graph illustrating the coupling efficiency of the hollow
cathode emission current into the magnetron device.
FIG. 3 comprises a graph illustrating the current relationships in a
constant voltage magnetron with a hollow cathode enhancement.
FIG. 4 comprises a graph showing the relationship between the deposition on
a target/hollow cathode emission current for a constant voltage magnetron
with a hollow cathode enhancement constructed in accordance with the
present invention.
FIG. 5 shows a series of curves showing the relationship between magnetron
current to magnetron voltage for a number of different emission
intensities for the hollow cathode.
FIG. 6 comprises a side view partially in section illustrating a side
discharge hollow cathode which may be utilized with the overall
combination of the present invention.
FIG. 7 comprises a side view partly in section of a further embodiment of a
side discharge hollow cathode having a pluarity of discharge points for
achieving even greater cathode current density.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The magnetic configuration of a planar magnetron is shown in cross section
in FIG. 1. In this figure, the magnetron target 10 is placed on a magnet
assembly 12, which consists of a central pole 13 of one magnetic polarity,
and a circular outer pole 14 of the opposite polarity. Magnetic field
lines 15 are shown connecting the two poles. The strength of the magnetic
field is related to the density of these virtual field lines 15. The
magnetron target 10 is biased by up to several hundred volts negative by
the magnetron power supply 16. In this embodiment the chamber 17 functions
as the anode, although often a separate anode closer to the magnetron
target 10 is present. The chamber 17 also functions as the vacuum
enclosure.
A critical aspect of the invention is the coupling between the hollow
cathode plasma and the magnetron plasma. This coupling of the two devices
depends critically on the positioning of the hollow cathode or multiple
cathodes. One example of this positioning is shown in FIG. 1 for a planar
magnetron. In this case, the hollow cathode 20 and keeper assembly 21 are
mounted above the magnetron target 10, close to the outer edge, and
projecting horizontally in towards the magnetron center. The radial
position of the hollow cathode 20 must be such that the magnetic field
lines that it intersects travel to the center pole 13, rather than the
bottom of the magnetic assembly, as field lines 18 do. The vertical
positioning of the cathode at this radial position (as shown in FIG. 1)
will determine the strength of the magnetic field at the cathode and the
coupling efficiency of the cathode to the magnetron plasma. This coupling
efficiency can be measured at constant magnetron voltage and pressure as a
percentage of the hollow cathode discharge current which is incident on
the magnetron target. A chart of this coupling efficiency is shown in FIG.
2.
The configuration shown in FIG. 1 is not the only position at which the
hollow cathode plasma will couple into the magnetron plasma. The hollow
cathode can be placed vertically at the same radial position, or else
closer to the target center. This can be extended to the extreme that the
hollow cathode can be placed vertically over the center of the target 10.
The primary constraint is that the field lines that the hollow cathode
intersect are ones which traverse the front of the target (shown in FIG. 1
as 15). There is, however, a point at which the hollow cathode can be
mounted too close to the magnetron target 10. This occurs when the hollow
cathode is mounted closer than 2-3 Larmor radii from the target surface in
the area between the center pole piece 13 and the outer pole piece 14. A
Larmor radius in this case is a few millimeters. When the hollow cathode
is in this position it physically impairs the magnetron ExB drift current.
This results in very poor magnetron operation, characterized by much
higher operating voltages and low deposition rates. The positioning
criteria for the planar circular magnetron and the rectangular planar
magnetron will be quite similar. For other magnetron geometries, such as
the cylindrical or cylindrical post magnetron, the physical positioning of
the hollow cathode in the magnetron will by necessity be different.
However, the two main criteria remain: the hollow cathode must be immersed
in the transverse magnetic field at the magnetron cathode surface, and the
hollow cathode must not be an electrical or physical impediment of the
magnetron ExB drift current.
The current-voltage characteristics of a magnetron at constant pressure
have been found to follow equation (2) above. Thus any increase in
magnetron current necessitates an increase in the magnetron voltage. With
hollow cathode enhancement of the magnetron, however, this is not the
case. For a configuration similar to FIG. 1, the magnetron voltage can be
held constant as the hollow cathode emission is increased. The increase in
the hollow cathode emission causes additional ionization of the gas in the
magnetron vicinity and increased magnetron current. A plot of this
behavior is shown in FIG. 3 for a constant magnetron voltage and gas
pressure. As can be seen from the figure, the magnetron current can be
increased almost a factor of 10 times by the addition of 3 amps of hollow
cathode discharge current. Measurements of the deposition rate on an
external crystal ratemonitor document a comparable increase in the
deposition rate of sputtered target material. (See FIG. 4.) The coupling
coefficient, as described above, is in this case approximately 46%, which
means 46% of the emitted hollow cathode discharge current is incident on
the magnetron target to produce enhanced sputtering.
A second feature of this enhanced operation is the low pressure of
operation. A prior art magnetron operates typically at gas pressures (in
Argon) of 3-10 millitorr. The enhanced operation described here operates
at comparable or higher magnetron currents but at pressures a factor of
ten lower, in the 0.3-0.5 millitorr range. This lower pressure is
accompanied by a much longer mean free path for the sputtered material,
which means that a higher percentage of target material is likely to land
on the substrate or conversely, that material sputtered from the target
(which may be the sample) is quite unlikely to redeposit on the target.
More importantly, operation at these lower pressures means that the
sputtered material travels in essentially a straight line path, as does
evaporated material.
The ability to operate at these low pressures allows the capability to
separate the sputtering target from the sample substrates by a significant
distance. Prior art magnetron sputtering systems typically have a target
to substrate distance of only a few centimeters, due to the high
scattering of the sputtered material. Additionally, the lower pressure of
operation allows other processes to occur simultaneously. For example,
evaporation techniques which will not operate in the pressure environment
of a prior art magnetron will operate well in the pressure range of the
hollow cathode enhanced magnetron. Additional processes, such as ion beam
bombardment or sputter deposition, ion plating or other ion, electron or
photon bombardments of the target or substrate surfaces are possible due
to the separation of the target and the substrate and the low pressure
operation capability.
Unlike prior art magnetrons which have a turn-on voltages of over -300
volts, the hollow cathode enhanced magnetron operates at any energy down
to the floating potential, which in this case is in the -15 to -20 volt
range or less. This feature is due to the presence of the hollow cathode
induced plasma, which is independent of the magnetron potential. With the
vacuum chamber at ground potential and functioning as the anode, a plasma
can be induced by biasing the hollow cathode at least 30 volts negative.
The plasma potential of the plasma in the magnetron vicinity will be close
to and slightly above ground. Alternatively, a separate anode can be
biased positively of ground by an equivalent voltage. This also gives
sufficient energy to the hollow cathode electrons to induce a plasma, but
in this case the plasma potential will be close to and slightly above the
anode potential. This latter case will increase the energy of the
bombarding ions somewhat to the magnetron target surface.
With the hollow cathode induced plasma established, biasing the magnetron
target negative (from anode potential) will induce bombardment of the
cathode surface at an energy equal to the sum of the magnetron potential
plus the plasma potential. A plot of the resultant magnetron current and
voltage as a function of hollow cathode discharge current (emission
current) is shown in FIG. 5. The most visible feature is the dramatic
shift to low ion energies at significant magnetron currents, due to the
hollow cathode emission current. Without any hollow cathode current, the
magnetron is incapable of operating at voltages (ion energies) below 300
volts (300 eV). The magnetron is then restricted to operating only at the
voltages and currents defined by the far right-hand curve in FIG. 5.
Stated differently, this would be its operating characteristic if there
were no hollow cathode present. With sufficient hollow cathode emission
current relatively high current operation (>1 amp) is possible at energies
below 100 eV. It should be noted that a magnetron current of 1 amp for the
present magnetron system corresponds to an ion current density of 7
mA/cm.sup.2.
At least three processes can be strongly enhanced by this high current
density low energy bombardment of the target. These processes are: (1)
"co-sputtering" or concurrent sample ion bombardment during a film
deposition, (2) reactive ion etching of the target surface, leading to
very high chemical etch rates, and (3) induced chemical reactions in a
depositing film. Each of these areas will be described below.
The ability to bombard films as they are growing with low energy ions has
been shown to strongly affect the film properties in (J. M. E. Harper, J.
J. Cuomo, R. J. Gambino and H. R. Kaufman, "10 V Bombardment Modification
of Surfaces," ed. by R. Kelly and O. Anciello (Elsevier, Amsterdam,
(1981). For example, the stress or adhesion of the film can be adjusted by
the ion bombardment to more favorable values. The energy of the ion
bombardment, however, must be sufficiently low as to not sputter off the
film at high current densities. The hollow cathode enhanced magnetron can
produce very high current densities on the target, or samples placed on
the target, at energies below the sputter threshold for the depositing
film. Thus, there is no possibility of sputtering off the depositing film.
Because the bombarding current densities can be quite high, this
co-sputtering process can occur at quite high rates of film deposition.
Reactive Ion Etching
As described above, the hollow cathode enhanced magnetron device is capable
of quite high bombarding ion current densities to the target, or samples
on the target, at energies below the sputter threshold of the film
species. This feature becomes important for the purposes of reactive ion
etching. As an example of a reactive ion etching process, an ion of a
reactive species (such as oxygen or freon) is directed to a sample
surface. At the surface a chemical reaction occurs with the surface
material, resulting in a volatile compound which leaves the surface and
can be pumped away. Reactive ion etching is an isotropic, dry etching
process. However, if the incident ion bombards the surface with energy
above the sputter threshold, physical sputtering is also likely to occur.
Therefore, for the purposes of reactive ion etching, the hollow cathode
enhanced magnetron is an appropriate device for high rate reactive ion
etching without sputtering as it can operate at high current densities at
energies below the sputter threshold.
Chemical Reactions In Depositing Films
The high current, low energy ion bombardment of the target surface in the
hollow cathode enhanced magnetron is also quite useful for inducing
chemical reactions in depositing films.
One example is nitride formation. A depositing film of aluminum in a
nitrogen background pressure will not form aluminum nitride as discussed
in J. M. E. Harper, J. J. Cuomo and H. T. G. Hentzell, Appl. Phys. Letter,
43, (1983) p 547. It is necessary to bombard the aluminum film with
energetic nitrogen to induce the reaction to occur. The enhanced magnetron
can be used to provide high current density nitrogen bombardment in this
case to a depositing film at ion energies below the sputter threshold, as
to not remove any of the film. Thus, very high rate compound film
formation can occur by inducing chemical reactions at the target surface
of the enhanced magnetron.
Switched Magnetron Operation
As described previously, magnetrons are magnetically enhanced diode
sputtering systems. They have been utilized either for the purposes of
depositing cathode material on other substrates by sputter deposition, or
for etching of samples on the cathode itself. It is the latter feature
which is of interest here. This etching process is an anisotropic etch,
i.e., the ions bombard the surface of the sample on the cathode (and the
cathode itself) at normal incidence. This causes sputtering, which will
produce vertical sidewalls when used in conjunction with a mask.
A second unique feature of such enhanced magnetron plasma devices is
reactive ion etching described above. In reactive ion etching, very low
energy ions of a reactive species (such as oxygen, freon or CCl.sub.4, for
example) are directed at a surface in a low energy gas discharge. These
reactive ions then react chemically with the atoms at the surface of the
cathode, forming a volatile species which leaves the surface and is pumped
away. This process is an isotropic etching process. This feature is due to
the low ion energy of the reactive ions (typically a few up to 10-20 eV)
and the chemical nature of the etching process. The isotropic process
results in substantial undercutting of the substrate when used with a
mask.
The hollow cathode enhanced magnetron system of the present invention emits
large numbers of electrons, which enhance the plasma in the magnetron
vicinity, and can lead to substantial increases in the magnetron current,
sputtering and deposition rates. A more subtle feature of the operation of
this invention will now be described. Referring again to FIG. 5, the
magnetron current and voltages are plotted as a function of hollow cathode
emission current in the hollow cathode enhanced magnetron system. As can
be seen in the figure, the curve at the far right is the operation of the
magnetron with no hollow cathode emission current. By increasing the
hollow cathode emission, the curve moves successively to lower energy
while keeping a relatively high magnetron current. For example, if a
horizontal line is drawn on the figure at 1.0 amps of magnetron current,
the energy of ion bombardment (magnetron voltage) can be varied from 600
eV with no hollow cathode current to 40 eV at 5.0 amps of hollow cathode
emission. This energy is below the sputter threshold for most materials.
However, the ion bombardment current, and hence the ion current density,
is not varied. Thus, by varying the amount of hollow cathode emission
current in the hollow cathode enhanced magnetron, the ion bombardment of
the cathode or samples on the cathode can be changed from a sputtering
energy (600 eV) to a sub-sputtering energy of approximately (40 eV).
The low energy energy bombardment is in the range of energies necessary for
reactive ion etching. By increasing the hollow cathode current higher,
this bombardment energy can be dropped further to less then 20 eV.
The invention utilizes the ability to dramatically change the energy of the
ion bombardment at the cathode surface to switch between a sputtering
bombardment, or anisotropic etch, to a non sputtering, reactive
bombardment, or isotropic etch. The gas, in this case, would be the same
for both processes. For example, oxygen bombardment at 600 eV will do
primarily sputtering, which is anisotropic, even though the oxygen is
somewhat reactive. At the sub-sputtering energy present due to high hollow
cathode electron emission, the effect of the oxygen will be only reactive,
which is isotropic. The ability to switch from one type of bombardment to
another will be limited in rate only by the power supplies which drive the
magnetron and the hollow cathode emission. These supplies can be run in a
DC mode, manually switching from one mode to another, or can be
electronically switched at frequencies up to ten of kilohertz or more.
The primary utility for the herein described switched magnetron operation
would be for the production of sloping sidewalls in samples etched on the
cathode surface. By switching between anisotropic and isotropic etching at
these high bombardment rates (tens of milliamps per square centimeter),
the slope of a sidewall cut below a mask can be controlled, or tapered
accordingly.
This switching process has the additional advantage of occurring at much
lower pressure than normal magnetron operation, or normal reactive ion
etching operation. A magnetron typically operates at 5-10 millitorr,
reactive ion etching occurs at 30-100 millitorr. The invention described
here, utilizing the hollow cathode enhanced magnetron system in a
switching mode, operates at high current densities of >10 milliamps per
square centimeter at pressures less than 1 millitorr range. This lower
pressure reduces the amount of reactive gas in the system and the loading
on the pumps, as well as reduces the problem of exhausting potentially
hazardous gases from the pumps. The lower pressures allow also
line-of-sight processes, as well as multiple processes, as described in
the earlier disclosure.
Special Hollow Cathode Configurations
As stated previously, the presently disclosed hollow cathode enhanced
magnetron sputter etching/deposition system makes a great many processes
possible which were not possible with the simple (non enhanced) magnetron
system. The simple cylindrical hollow cathode having an axial electron
emitting orifice at one end is often difficult to place in a reaction
chamber. And the single source of electrons is often inadequate to enhance
an otherwise feasible process. This problem gave birth to the side
(discharge) hollow cathode.
The side hollow cathode utilizes many of the features of the classical
cylindrical hollow cathode. A drawing of a side hollow cathode is shown in
FIG. 6.
The side hollow cathode is based on the same refractory metal tube 11 as
described previously above. Instead of a specialized tip assembly, a small
aperture 20 is drilled in the side of the tube 11. The end of the tube 21
may be sealed off, or it may be continued to other side cathodes, to be
described below. Gas 14 flows into the tube from the left end, although it
could also be incident from the right, sealed off side 21. Again, rolled
foil 13, which is also refractory is placed inside the cathode below the
hole.
Operation of the side hollow cathode is much the same as described above
for the classical cylindrical hollow cathode, and will not be repeated
here. Devices of this type operate at much the same levels as prior art
cathodes, both in gas flow, and discharge current and voltage.
A straightforward elaboration of this side hollow cathode is to simply
continue the tube and add a second or more apertures. This is shown in
FIG. 7. The additional apertures, labeled 22, 23, etc., can either share
the same foil insert 13 or have additional separate inserts. An equivalent
modification would be to have apertures on opposite sides of the tube,
either at the same or different locations. In each case, the hollow
cathode effect is unchanged. However, it should be understood that these
are only exemplary of the possibilities using this similar side cathode
geometry.
Operation of the multiple aperture side cathode is essentially unchanged
from the above described single aperture side hollow cathode. There is
some difficulty at low currents in attaining discharges at each aperture.
This effect is due simply to lack of sufficient bombarding ion current to
heat each of the foils below the apertures. This problem is eliminated
simply by increasing the discharge (or emission) current. In practice, it
is sufficient to have a keeper or anode located only over one of the
apertures in a multiple side cathode. The other apertures may then gain
enough heat by lateral conduction along the tube to initiate local
discharges. An alternative is to have one long keeper or anode which
encompasses all of the apertures. This may not be possible in all
environments. A second alternative would be to have a moving keeper or
anode, which moved along a row or array of apertures, igniting each
aperture in turn.
Single side hollow cathodes and multiple side hollow cathodes have a number
of desirable advantages over the conventional cylindrical hollow cathode.
These features are application oriented and include such applications as
electron bombardment ion sources, glow discharge plasma initiation and
plasma enhancement in devices such as magnetrons. In an ion source, the
ability to have multiple electron sources with a single power supply and
gas source greatly simplifies operation in large ion sources. Typically in
a large ion source, the electron current supplied from only a single
hollow cathode, and the non-uniformity of the gas distribution due to that
single cathode will be eliminated by using this multiple-aperture side
hollow cathode technique. In plasma cleaning, or etching as in the case of
RF sputtering or diode sputtering, it is desirable to inject electrons at
a number of positions. This invention allows that process with a single
device, rather than a number of devices which must be separately driven
and tediously balan | | |
