Note: Descriptions are shown in the official language in which they were submitted.
1~17~4
BACKG~OUN~ OF T~E I~VEN~ION
Field of the Invention:
This invention relates to magnetron sputtering devices.
BRIEF DESCRIPT10~ OF THE DRA~'Ir~G
Figures 1 and 2 are cross-sec~ional and perspective views
respectively of a ~rior art planar magnetron spu.terins aevice.
Figure 3 is a cross-sectional view of another prior art
device.
~igure 4 is a cross-sectional view of an illustrati~e mag-
netic structure in accordance with a presently, non-preferred
embodiment of the invention.
Figure 5 is a cross-sectional view of an illustrative mag-
netic structure in accordance with a preferred embodiment of
the invention.
Figure 6 is a perspective view of an illustrative stacked,
flexible magnetic tape for implementing the structure of Figure
5.
Figures 7 and 8 are plan views of stacked magnetic struc-
tures illustrating different corner arrangements thereof.
Figures 9 and 10 are cross-sectional views of illustrative
embodiments of further, preferred magnetic structures in accord-
ance with the invention.
Figure 11 is an illustrative embodiment of a masnetic struc-
ture applicable to small cathodes in accordance with an import-
ant aspect of the invention.
Figure 12 is a cross-sectional view of a further embodi~ent
of a magnetic structure applicable to small cathodes.
Figure 13 is a cross-sectional view of an illustrative er,-
bodimcnt of a further, preferred embodiment o, the ir.vention.
Figure 1~ illus,rates (a) a sraph showins the flux cistrl-
bution cstablishcc ~y t~.e structure of ~isure 13 and (b) a cross-
sec~ional view o~ an illustra~ive e~bociment of a fur~her, pre-
ferrcd c.,bodi.,crt of the i..vention.
~1~17~?4
Figure 15 is a cross-sectional view where the right side
portion thereof is an illustrative embodiment of a further, pre-
ferred magnetic structure in accordance with the invention and
the left side portion thereof is a moaified e~boai~lent of the
prior art structure illustrated in Figure 1.
Figures 16 and 17 are cross-sec~ional views of illustrati~e
e~bodiments of further, preferred masnetic s.ructures in
accordance with the invention where the left and right side por-
tions of each Figure illustrate a particular embodiment of the
invention.
Figure 18 illustrates a typical current-voltage character-
istic for a magnetically enhanced sputter cathode such as that
of the embodiment of Figure 1.
Figure 19 illustrates a family of current-voltage character-
istics correspon~ding to that of Figure 18 over a range of pres-
sures.
Figure 20 illusLrates current waveforms associated with
the characteristics of Figures 18 and 19.
Figure 21 illustrates a family of current-voltage character-
istics over a range of pressures typical of the magnetic struc-
tures of Figures 15-17.
Figure 22 illustrates various voltage ripple waveforms
which occur as the zero impedance portion of the characteristic
curves of Figure 21 is entered.
Figure 23 corresponds to the embodiment of Figure 13 where
the left side thereof illustrates ,ield shaping which occurs
with a shunt while the right side illustrates the field which
results without the shunt
Figure 24 is a cross-sectional view of illus.ra~ive e.-~odi-
ments of further, prefe.red magnetic s.ructures in accorcance
with the invention where the left and risht sides thereof illus-
trate particular e..~boà ..,er.ts o' the i,.~e..tion.
1~17~4
Figure 25 i5 a cross-sectional view of i].lustrative embod-
iments of further, preferred magnetic structures in accordance
with the invention where the le't and right sides thereof are
respective modifications of the le't and right sides of Fisure
24.
Figures 26, 27 and 28 are cross-sectional ~iews of illus-
trative embodiments of further magnetic structures in accorcar,ce
with the invention where the left and right sides of each Figure
illustrate a particular embodiment of the invention.
Figure 29 is a graph showing the flux distribution estab-
lishe~ by the embodiments of Figure `2~. .
Figure 30 is a cross-sectional view of an illustrative em-
bodiment of the invention suitable for sputter thic~er targets
used in industrial applications.
Figure 31 is a cross-sectional view of an illustrative em-
bodiment of the invent`ion for magnetically shifting plasma across
a sputtering surface.
2 Discussion of Prior Art:
Figures 1 and 2 are cross-sectional and perspective views
respectively of a representative prior art planar magnetron
sputtering device comprising inner magnet 10 and outer magnet
12 (both of which usually comprise a nu~ber of sections) where
the magnets are shunted by an iron pole plate 14. Disposed above
the magnetic structure is a cathode or target 16 (not shown in
Figure 2). The magnetic lines of force are as shown in Figure
1 where they exit from and return through cathode 16, a similar
technique being employed in U. S. Patent ~o. 3,878,085 where
the magnetic lines also enter and exit from the cathode surface.
~n electric field is established between (a) a ring~ e
anode 17, which may be disposed around and s?aced from cathode
17~4
16, (or the chamber wall may serve this function) and (b) the
target whereby electrons are removed from the cathode. Due to
the configuration of the lines of magnetic force (the illustra-
tion of which is approximate), the removed electrons tend to con-
centrate in regions A where the lines of force are substantiallyparallel to the upper surface of target 16. There the electrons
ionize gas particles which are then accelerated to the target to
dislodge atoms of the target material. The dislodged tarset
material then typically deposits as a coating film on an object
to be coated. Assuming the object to be coated is in strip form
or is mounted on a strip movins in the direction of the arrow
shown in Figure 2, the object will be uniformly coated, the
strip being narrower in width than the length of the sputterins
device.
~nce the ionizing electrons are removed from the target,
they travel long paths because they circulate in a closed
loop defined between inner magnet 10 and outer magnet 12, the
loop being above target 16. Hence, the electrons are effective
in ionizing the gas particles. However, since most of the ion-
izing electrons are concentrated in regions A, the ionized gas
particles will mainly erode cathode 16 in regions A'. Such un-
even disintegration of the target is undesirable in that the
target materials are most often extremely pure and accordingly,
very expensive.
Another prior art arrangement is shown in cross-section
in Figure 3 where parallel magnets 18 and 20 are employed with
pole pieces 22 and 24. However, this configuration is essenti-
1141~C)4
ally the same as that of Figures 1 and 2 in its function and is
subject to the same shortcomings.
SUMMARY OF THE INVENTION
It is an important object of this invention to provide an
improved planar magnetron sputtering device wherein the tarset
is more uniformly disintegrated.
It is a further object of this invention to provide an i~-
proved planar magnetro~ sputtering device of small dimensions
and high power output.
It is a further object of this invention to provide improved
planar sputtering devices which are stable at very low pressures
and which have an I-E characteristic ,hat exhibits a zero dynam-
ic impedance.
Other objects and advantages of this invention will be ap-
parent from a reading of the following specification and claimstaken with the drawing.
DETAILED_ DESCRIPTION OF THE DIFFERE~T EMB~DI~NTS
OF THE INVENTION
Reference should be made to the drawing where like numerals
refer to like parts.
In Figure 4 there is illustrated a magnetic structure com-
prising a flat coil solenoid 26 which was tested in an attempt
to provide a magnetic flux, which was more uniformly parallel
to the surface of the target 16 than that provided by the Figure
1 structure. As indicated hereinbefore wi.h respect to Figure
1, disintegration of the cathode predominantly occurs where the
lines of force are subs.antially parallel to the cathode sur ace
-- that is, at regions A'. However, the area over which ,he
7~4
lines of force are substantially parallel is rather minimal
and thus uneconomical utilization of the cathode results. The
Figure 4 embodiment did generate a desired type of parallel field
(the illustration of which is approximate), but the ampere-turns
S required to generate sufficient magnetic flux (typically over
100 Gauss at 1/2 inch above the coil) was very high. Accarding-
ly, the flat coil solenoid of Figure 4 is not considered to be
a preferred embodiment of the invention at this time.
Referring to Figures 5 and 6, there is shown an illustrative
1~ permanent magnet~structure which functionally approximates the
.~
6a
Figure 2 structure, where again the illustrative field is approx-
imate. The structure comprises a plurality of flexible magnetic
tapes 28 which are concentrically arranged or stacked to form a
flat coil as shown in Figure 6. Each ring of the coil comprises
a strip of the tape where the ends of each strip abut one another
as indicated at 30 for the outer ring. Together the strips are
substantially equivalent to a solid magnet where the directions
of the flux in each magnet are represented by arrows in Figure
5 and where the north and south poles of this "solid" magnet are
as shown, it being understood that the polarities shown are il-
lustrative and may be reversed, if desired. Rather than employ-
ing concentricorstacked strips as shown in Figures 5 and 6, a
single strip can be tightly wound to provide a spiral configura-
tion which is also very effective. Typically the strips of flex-
ible magnetic tape are oriented ferrite impregnated rubber strips1/16 inch or 1/8 inch thick. Further, rather than tapes, ferrite
block magnets (typically 3/16 inch thick) may also be employed
to construct a configuration corresponding to that of Figure 6.
Sputter cathodes magnetically enhanced by the magnetic
structures of the present invention possessed superior perform-
ance characteristics compared to those enhanced by the conven~
tional Figure 1 structure. They support extremely high density
plasmas, give better than usual target utilization and provide
higher power efflciency than conventionally achieved. They need
no pole pieces, can be built at lower cost and promise longer
maintenance free life time.
The rolled and stacked magnet assemblies differ from the
conventional magnetic arrays in that they represent a "solid"
magnet, as discussed above, rather than several individual mag-
nets pieced together magnetically via the pole pieces or poleplates. The performance of the resulting cathodes is closely
related to this "solid" form -- especially in the corners 34
shown in Figures 7 and 8. A degree of corner integrity can be
readily lost by any gaps between the layers of rubber magnet in
the corners. Trying to wind a magnet to best fit a rectangle on
the outside and gradually gapping the corners with magnetic
material 36 as in Figure 7 to generate a flattened ellipse in
the center will typically result in a much less powerful cathode
than will a tight wound flattened ellipse as shown in Figure 8.
When the rubber strip is used in either a stacked or rolled
construction several new factors are present. As indicated
above, there is a unique advantage in establishing the "corner
integrity" such that the plasma does not suffer corner losses as
typically occurs with stacked block angled corners or square
corners. Further, the electrical efficiency of the cathode in.
creases -- that is, more sputtering per watt-second of power
consumed. Improvement is typically sufficient to give 1.5 to
3 times the usual sputter efficiency. There is also a uniquely
greater stability to very high voltages and currents and to very
low inert gas pressures. Full power operation is also obtained
at pressure 10 times lower than usually required.
The power levels that can be supported by these cathodes
at 2 microns of argon -- or less, exceeds that previously observed
by the inventor. Due to limitations of power supply capability
and inability to adequately cool the target, the ultimate limita--
tions imposed by the magnetic structures have not been deter-
mined. However, it has been observed that two to four times the
usually employed power levels are readily attained without sug-
gestion of a break in the E-I curve.
The flexible magnet materials also make possible many struc-
tures that would be most difficult and expensive to achieve in
any other way. Even the Figure 1 type of magnet structure can
be given improved properties by interweaved stacking of the
~4~
corners to provide "corner integrity" using the rubber strips.
It may also be possible to achieve the corner integrity by use
of permeable metal sheets between layers of blocks or strips in
the corner regions.
In spite of the improvements effected by the Figure 5 em-
bodiments, they still suffer from uneven target utilization.
Where the lines of flux enter the center line of the target at
about 45 or more, there is no erosion of the target~ At the
outside edge prediction is less certain, for centrifical force
seems to overcome any simple angle value. Full 90 is a safe
value; however, this makes it possible to develop clamp rings and
guards that stop erosion at any desired point, as will be dis-
cussed further hereinafter. The lack of erosion in the target
center is of special concern due to the great cost of most of
1~ the targets. Increasing the area significantly eroded before
any point erodes all the way through the target is thus of great
importance. Accordingly, the magnetic structure illustrated in
Figure 9 may be employed whereby the magnets 28 are tipped away
from the perpendicular orientation shown in Figure 5. The angle
of the magnets with respect to the perpendicular can fall with-
in the 40-60 range shown in Figure 9 and preferably this angle
should be 50-55. Special orientations of the magnets to change
the pattern of erosion become quite easy when the flexible mag-
net system of the present invention is used. As the magnets are
tipped toward the center, it is observed that the plasma is af-
fected very little until approximately 40 is reached. At ap-
proximately this angle depending upon the geometrics, field
strengths, etc., a unique magnetic fused dome structure is formed
at the target center line, the illustration of which is approxi-
mate. There appears to become but a single line of perpendicular
flux where there had previously been about 1/2 - 1 inch or more
of this. This flux lines branch out of this center line at
~14~7~34
angles of 45 or less. The result is a unique plasma flow sit-
uation where the opposing streams of plasma overlap the center
line of the magnetics, providing erosion of the target across
its center whereby the uniformity of target erosion may be im
5 proved with respect to that of the Figure 5 embodiment. Al-
though the magnets 28 as shown in Figure 9 are polarized across
the thin dimension thereof, it is to be understood that they
may also be polarized along the width thereof - that is, from A
to B as shown in Figure 9.
A particularly preferred embodiment of the invention is il-
lustrated in Figure 10, this embodiment combining the effects
provided by the structures of Figures 5 and 9. Accordingly with
the embodiments of Figure 10, the strength of the magnetic field
above cathode 16 is enhanced by the perpendicular magnets 28l
15 while the erosion of the target center is enhanced by the tipped
magnets 28". Thus, for example, if the cathode has a width of
4 inches to 4-3/4 inches, the extent of magnets 28" on one side
of the cathode might be 1/2 inch to 3/4 inch, and the extent of
magnets 28' might be one inch. To provide a continuous solid
20 structure, a wedge-like insert 30 of magnetically permeable
material is preferably disposed between the perpendicular mag-
nets 28' and the tipped magnets 28". As indicated hereinbefore,
clamp rings may be provided to stop erosion at the outer edge
of the target. Such a ring is shown at 32 in Figure 10 where
25 the lines of force are perpendicular to the clamp rings. Fur-
ther, such clamp rings may be useful in positioning the cathode
structure of Figure 10 within the sputtering device.
An attempt was made to construct very small structures cor-
responding to that of Figure 1. It was found that at diameters
30 less than about 1-1/2 inches they would not work in that they
would not suppor~ a stable magnetically enhanced plasma. At
sufficiently high voltage they operated as sputter diodes, with
no change in behavior noted in the presence or absence of the
magnets. Such a structure is shown in Figure 11 where the struc-
ture corresponds to that shown in Figure 1 but where the dis-
tances between the magnets have been substantially decreased
to provide a small sputtering device where the cathode typical-
ly has a diameter of one inch or less. Such small devices are
useful in many applications.
It is thought the problem of center erosion and the prob-
lem of very small targets are one and the same. The radius of
curvature fro the plasma path may be a problem also. The elec-
trons traveling tight corners need very high magnetic fields to
keep them from centrifuging away from the cathode. A one inch
diameter cathode as shown in Figure 11 can be built using the
most powerful ferrite magnets, although it is to be understood
that the cathode of Figure 11 may be elongated rather than circ-
ular if so desired. Under most conditions this will not perform
in a magnetically enhanced mode. An iron filings picture which
gives an indication of the positions of the lines of force pro-
vides an explanation. The trapping dome is very short, pushed
down by strangely shaped lines of force from the outer half of
the ring magnet. Making the center magnet stronger would help
push up the dome, but the strongest commercially available ferrite
are now being used. The dome must clear the target surface by
at least 3/8 inch for the magnetic structure to be effective.
Because of the quadrupole-like form above the magnet, the dome
is very tightly defined. Changes in target height of a few
thousandths of an inch change this from an unenhanced cathode
to a violently effective enhanced one.
It can thus be seen that the small cathode structure of
Figure 11 has the same general quadrupole-like lines of force
as the center effective units of Figures 9 and 10 obtained by
tipping the magnets. As the small cathode field is reduced, or
~17~4
the cathode dimensions increased, the quadrupole effect becomes
undetectable. Other magnetic structures may be employed to ef-
fect the quadrupole-like lines of fsrce illustrated in Figures
9 and 11. Illustrative of such structures would be that of
Figure 12 which is also particularly applicable to small cath-
odes of either circular or elongated configuration. The magnet-
ic structure includes a pair of C-magnets 40 and 42 in opposing
polarity as shown in Figure 12 where the polarity may be re-
versed if so desired. A pole piece 44 connects the lower arms
10of the magnets 40 and 42. A center magnet 46 is disposed be-
tween the magnets 40 and 42 where the polarity of the upper pole
thereof is opposite that of the upper arms of the C~magnets and
where the magnet 46 may be an extension of pole piece 44, if
desired.
15It is a general teaching of the Figure 1 structures that
the outer pole area should be approximately the same as the inner
pole area. In the structures of Figures 9, 10 and 11, this teach-
ing has been totally violated. In tipping the magnets in the
Figures 9 and 10 structures, the lines of force are projected
upward from the outer edges, the return path being closed off
down the center. In the smaller structure of Figure 11, the pole
areas are loaded ~10:1 outside to inside. The results are unique.
Placing a steel pole piece in the center and/or around the
outside edges of the cathode has almost no effect on the perform-
ance of the cathodes of this invention. The unique effects arealmost totally caused by the form factor created by the stack-
ing. The form factor can be improved even further (with some
loss of flux) through the use of thin magnetic shunts 38, as
shown in Figure 13, which shows thin steel shunts 38 placed just
out of magnetic contact with the magnet edge surface. The Gauss
level parallel to the target surface (about 3/16 inch above the
surface seems to be the most meaningful indication) is shown
7~
with and without the shunt in Figure 14. The presence of the
correct thickness (typically 0.005-0.015 inch)and width of shunt
provides a significantly wider path of maximum erosion. A heavy
shunt destroys the pattern.
Modifying the cross section depth of the magnets can also
be used to help shape the parallel Gauss curve and thus the
erosion pattern indicated at "x" in Figure 14. Further, there
appears to be many ways the stacked and rolled parallel and
tipped magnetic structures of the present invention can be
varied to influence target utilization and other performance
criteria. Combinations of these effects can also be useful.
It should also be noted it has been a consistent teaching
of the prior art in the magnetically enhanced sputtering field
that it was necessary to provide a continuous line of force loop
system to provide significant plasma enhancement, the loop, as
stated hereinbefore, being defined in the Figure l embodiment
above cathode 16 between inner magnet lO and outer magnet 12.
With the flexible strips of the present invention it can be
shown that unique and productive configurations can be assembled
that are in opposition to this. In fact, unusually wide and
uniform sputtering patterns can be obtained in cases where there
is intentional disruption of the "race track" type of pattern.
By stacking an inch or more thick of long rubber magnet strips,
they can be folded, wound and twisted to explore configurations
where the ends do not meet end to end. Especially effective is
the configuration where an end butts 90 to a side. At such an
intersection (of the correct polarities) the plasma forms a 90
corner -- full into the corner -- and spreads to the full width
of the 45 limits of the line of force pattern. This wide
plasma seems to be compressed by negotiating corners -- as might
be predicted from centrifugal force and continuing acceleration
in the corners. Such configurations may lead to increased target
34
utilization and refinements far removed from the prior art.
Reference should now be made to Figures 15, 16 and 17 which
are cross-sectional views directed to further modifications of
the present invention which are stable at extremely low pres-
sures and which have an I-E characteristic that exhibits a zero
dynamic impedance in a predetermined band of low pressures.
Figure 15 is a modification of the Figure 1 embodiment of the
prior art. In particular, the portion on the right side of the
phantom center line of Figure 15 corresponds to the new zero
dynamic impedance embodiment of the present invention while the
left side portion corresponds to an embodiment which provides
the normally anticipated infinite dynamic impedance of satura-
tion -- that is, positive impedance, as will be discussed in
more detail hereinafter. Of course, in implementing the present
invention, the left side of Figure 1 would be the mirror image
of the right side thereof. In Figure 15 at the periphery of the
magnetic structure, a first plurality of horizontally disposed
magnetic strips 50 are disposed on top of a second plurality of
horizontally disposed strips 52 where the strips 52 are wider
than strips 50. At the center of the magnetic structure a third
plurality of horizontally disposed strips 54 are disposed on top
of a fourth plurality of strips 56 where the strips 56 are wider
than the strips 54. By widening the center magnet at the base
thereof, the magnetic field above cathode 16 is made more paral-
lel to and is bxought closer to the surface of the cathode thanis the case with the prior art cathode of Figure 1. The widened
base of the outer magnet provides a return for the lines of
force emanating from the base of the center magnet. According-
ly, the area of target erosion of the Figure 15 embodiment is
increased with respect to that of the Figure 1 embodiment.
Rather than use the stacked strips of tape as shown in Figure
15, a solid structure as employed in Figure 1 may also be used
~4~7~4
in the embodiment of Figure 15 or the remaining embodiments where,
of course, for the Figure 15 embodiment, the respective bases of
magnets 10 and 12 of Figure 1 would be widened.
The foregoing arrangement contrasts with the left portion
of Figure 15 where the magnets 10 and 12 are simply replaced
with respective stacks of horizontally disposed strips 58 and
60 as shown in the Figure. To obtain the advantages of the
present invention, it is necessary to widen the lower layers of
the magnetic strlps as shown in the right portion of Figure 15.
Referring to Figures 16 and 17, there are illustrated four
further embodiments of the invention, which are capable of achiev-
ing the zero dynamic impedance of the present invention. In
Figure 16, at the left side portion thereof, there is illus-
trated a first embodiment wherein centrally disposed tipped
strips 28" are surrounded by a plurality of horizontally dis-
posed strips 62. In implementing this embodiment, the right
side of Figure 1 would be the mirror image of the left side
thereof. Thus, this first embodiment corresponds to that of
Figure 10, where the slanted angle of strips 28" would prefer-
ably fall within the 40-60 range discussed hereinbefore with
respect to Figure 9. Further, insert 30 may also be employed
if desired although it is not needed if good contact is made
between an edge of magnets 28" and magnets 62. The directions
of the flux in magnets 28" and 62 are represented by arrows where
the north and south poles are as shown, it being understood that
the polarities shown for this embodiment and the other embodi-
ments of this invention are illustrative and may be reversed if
desired.
The right side portion of Figure 16 illustrates a second
embodiment capable of producing the zero dynamic impedance of
the present invention where the vertically disposed strips 28
correspond to those o~ Figure 5 and where they are surrounded
L7~4
by horizontally disposed strips 62. In implementing this ~m-
bodiment, the left side of Figure 16 would be the mirror image
of the right side thereof.
The left side portion of Figure 17 is a modification of the
left side portion of Figure 16 where, in implementing this embod-
iment, the right side of Figure 17 would be the mirror image of
the left side thereof. The slanted or tipped magnets 28''' have
a generally rectangular cross-section where the width of the
strips at A and B are more narrow than those at C as shown in
Figure 17. A further plurality of vertically disposed strips
64 are disposed between horizontally disposed strips 62 and
tipped strips 28"'.
The right side portion of Figure 17 illustrates a further
embodiment where tipped strips 28" are disposed between hori-
zontally disposed strips 62 and vertically disposed strips 66.Again in implementing this embodiment, the left side of Figure
17 would be the mirror image of the right side thereof.
As can be seen in Figures 16 and 17, a common characteris-
tic of all embodiments is the presence of the outer ring of
horizontally disposed strips 62. Preferably the outer edge of
strips 62 is disposed slightly inward of the outer edge of
cathode 16 although other relative placements are permissible.
The strips 62 cause the field on the other side of target 16 to
be substantially vertical at the approximate periphery thereof.
It is thought that this feature is instrumental in achieving
the many advantages associated with not only the embodiments
of Figures 16 and 17 but also those corresponding to Figure 15.
Increased field strength does not seem to be a contributing
factor. These advantages will be discussed in detail herein-
after with respect to Figures 21 and 22. However, before doingso, certain characteristics of conventional magnetrons will
be dlscussed with respect to Figures 18 through 20.
Magnetically enhanced sputter cathodes such as the embodi-
ment of Figure 1 typically display a current-voltage character-
istic such as that shown in Figure 18. The cathode reaction
impedance, Z, may be defined as delta E over delta I at any
point along the characteristic. This impedance usually assumes
a fixed value above a current of a few amperes. The initiation
voltage, Eo, may be defined as the zero current intercept of
the extrapolated linear portion of the current voltage charac-
teristic.
The family of characteristics over a range of pressures is
shown in Figure 19 for the conventional magnetron. Typically,
Zp, the reaction impedance that is constant at a given pressure
changes with pressure, becoming larger at lower pressures. The
characteristic curves start to break at high power as the pres-
sure is reduced. This is a strong function of magnetic qual-
ity, both field strength and corner integrity. ~ith a good
magnetic structure, some break will start to occur in the 2
micron argon pressure characteristic. These cathodes can some-
times be operated at pressures as low as one micron, but start
ing becomes difficult, and the discharge will sometimes pop out.
The lower pressure characteristics lean over or break to show
a saturation.
The current wave form assumes a most unexpected form, as
shown in Figure 20. The shape of the current peaks at low power
is a function of the three phase power. However, when the
voltage exceeds some critical value which is a function of
many factors, the current drops to a very low value, typically
a small fraction of an ampere. This is probably the raw elec-
tron emission from the target surface. The mechanism of this
current loss is not known for certain, but sputtering stops
during this period. As soon as the voltage drops below this
critical value the current starts again, and follows the wave
form as before.
Characteristics typical of the new cathode magnetic struc-
tures of Figures 15, 16 and 17 are shown in Figure 21. Several
very interesting differences become obvious. The characteristic
is stable to extremely low pressures. A factor of lOX lower
pressures becomes very practical. This has many meaningful
advantages. Sticking is better. Target to substrate distance
becomes a less critical factor. Good sticking has been obtained
at 18 inches. Also this expanded pressure range makes it pos-
sible to measure and control the pressure with an ionizationgauge, rather than the conventional thermocouple gauge. This
gives better sensitivity, better repeatability, and much faster
response. In addition, most pumping systems, such as that dia-
grammatically indicated at 68 in ~igure 16, can operate under
an argon load without interposing a baffle, or valve restriction
at this decreased value. This larger pumping speed gives better
impurity removal and more simple system design. Further, there
is reduced argon (or other inert gas) entrapment in the coating.
Because of the order-of-magnitude decrease in the operating
argon pressure over the conventional magnetron, there should
be an equivalent decrease in entrapment. The entrapment proves
to be even less than that, however, due to the increased coat-
ing particle energy resulting from fewer enroute collisions.
Thus, as stated above, the sticking is also improved du~ to this
same mechanism. Rate effects, as will be further discussed
hereinafter, are also improved where a factor of four times
the conventionally obtained rates seems feasible. In this new
mode of operation, it is also noted the erosion pattern spreads
across the center of the target, providing increased area of
target erosion. This rate in the center is less than in the
main ring, but does represent significant improvement.
It is noted Zp tends to be constant with the structures of
18
7~4
Figures 15, 16 and 17, independent of pressure over a wide range
of pressure. This suggests that the mechanism of the cathode
reaction is not changing over this range. The Eo change for
each doubling of pressure is constant at about 30 volts over a
wide range of pressures. Below about .5 microns Eo stops
changing, putting the .2 micron characteristic on top of the
.4 micron characteristic as indicated by curve ~ of Figure 21.
At a pressure of about 0.18 microns, a new effect occurs in
the characteristic. It starts to break upward -- just the op-
posite of saturation. Thus, a vertical characteristic, Zp =
0, occurs. This does not degenerate into an arc, in that the
critical voltage is needed to sustain the low impedance reaction.
It is thought this means that at the critical voltage a state
is achieved in which the reaction generates as much of the plasma
contents as is used. Thus, no voltage drop is involved~ and
current control while maintaining the critical voltage becomes
the new requirement on the power supply. Operation down to .07
microns has b een achieved, but appreciable currents at these
pressures are presently blocked by rather major oscillations
in the current and voltage. The voltage ripple i9 shown in
Figure 22 as the zero impedance portion of the characteristic
is entered.
A most important characteristic of the new cathode struc-
tures of Figures 15, 16 and 17 is a very high sputter rate.
Rates at all power levels appear to be enhanced by 50 to 100~.
In the zero impedance mode the rates are further enhanced to
give as much as 400% of the conventional rate at the same power
level. In the lower pressure range there appears to be a ~reater
diffusivity of the plasma, resulting in better target utiliza--
tion.
In Figures 13 and 14, iron shunt 38 is employed as a field
shaping expedient. This magnetic conductor provides a medium
1704
through which some of the lines of force from a coil 28 of mag--
netic source material are shunted, as illustrated in Figure 23
where the left side of the Figure approximately illustrates
the field shaping which occurs with shunt 38 and the right side
the field which results without the shunt. As can be seen, this
shunting action also bends or deflects some of the remaining
lines of force, bringing them into lower, more parallel (to the
target surface) position above the target, enhancing and render-
ing more uniform the plasma layer in the sputtering process.
However, since shunt 38 removes some of the lines of force from
above the target, it is preferable to employ means which pro
vide the bending action but which do not significantly decrease
the flux density.
There are many magnetic structural configurations that can
provide some degree of shaping such as those shown in Figure 24
at A and B. Magnets perpendicular to the fundamental coils or
stacks 28 are shown where in the embodiment illustrated at A a
ferrite block magnet 70 is employed and at B a coil or stack 72
of ferrite impregnated rubber strips is employed. The block
magnet 70 should be sufficiently strong so that it is not de-
magnetized by stack 28. Typical of suitable block magnets are
1/4" x 1" thick ferrite magnets made by Arnold Magnetics, Inc.
or Crucible Iron and Steel Co. The ferrite impregnated strips
of stack 72 may be of the same type employed in stack 28. Typi-
cal of suitable strips are 1/2" wide ferrite impregnated tapes(such as PL-1.4~ made by Minnesota Mining and Manufacturing Co.).
The magnet 70 and stack 72 pull down the lines of force issu-
ing from the center of fundamental stack 28 as indicated at B in
Figure 24. This action can establish the needed 45 relationship
between the lines of force and the target surface, as described
hereinbefore, very close to the center of the magnet assembly.
Magnets 70 and 72 are more effective than the edges of the
~1~17~3~
individual maynets in fundamental stack 28 due to their projec-
tion angle and greater single polarity width.
The similarity between the embodiments of Figures 5 and
24 should be noted, the basic difference, of course, being the
presence of magnets 70 and 72. The fundamental stack 28 of
Figure 5 establishes a flux therein which is substantially
parallel to the lower surface of the target. When a relatively
large cathode 16 is employed, the strength of the field and the
parallelism thereof with respect to the target are adequate to
achieve uniform target erosion and the other advantages described
hereinbefore. However, when the size of the cathode is reduced
to 4-1/2 inches or less (which may be the diameter of a circu-
lar target or the width from C to D of a generally rectangular
or oblong target as indicated in Figure 5), the field produced
by the magnetic structure of Figure 5 tends to be less parallel
to the target surface than is preferred whereby the erosion is
less uniform. Since practical target sizes tend to be 4-1/2
inches or less, it is thus preferable to modify the Figure 5
structure in such a way as to effect the desired field shaping
(target parallelism). Such modifications are effected by the
magnetic field deflectors 38, 70 and 72 of the embodiments of
Figures 23 and 24, as discussed above. The embodiment illus-
trated at B in Figure 24 can also play an important role in
shaping about the corners, etc.
When the magnetic field deflectors 70 and 72 of Figure
24 are employed, a portion of the fundamental field established
by stack 28 is displaced in that none is generated in the funda-
mental direction by the perpendicular deflectors 70 and 72.
In cases where the maximum fundamental field is needed, it is
expedient to continue the fundamental field source 28 beneath
the deflector magnets as shown in Figure 25 at 71 and 73. This
provides the full series magnet for the fundamental parallel
;
field.
The magnetic deflectors 70, 72, 70' and 72' are intended
for maximization of center target utilization. When the outer
stacks of magnets such as magnets 62 of Figures 16 and 17 are
5 employed to provide a sharply defined outer edge for the eros-
ion of the target, it is preferable to use outer deflectors 70"
and 72" also as shown in Figure 26. All of the principles re-
lated to the inner set 70, 72, 70' and 72' apply also to the
outer ones 70" and 72". This thereby extends erosion uniformity
10 both toward the center and toward the outside of the target 16
where further deflectors, in addition to deflectors 70, 72, 70
and 72', may be employed if so desired.
It is also possible, as shown in Figure 27 to use parallel
deflectors 74 and 76. Several aspects of the design criteria
15 for these are more critical than for the perpendicular types
of Figures 24-26. Basically these are like the shunt system
of Figure 13, but more powerful.
The embodiments of Figures 24-27 are particularly powerful
although they are somewhat costly for some routine sputter ap--
20 plications. Thus, where less power at the lowest pressures isrequired, the magnets 70, 72, 70', 72', 72", 74 and 76 may be
omitted from the embodiments of Figures 24-27 in accordance with
a further aspect of this invention. The foregoing is illus-
trated in Figure 28 where the left side thereof corresponds to
25 the embodiment shown at B in Figure 24 with an outer perpendic
ular ring 62 and the right side thereof has an open slot 78
corresponding to magnetic stack 72 where the slot may typical-
ly be about 1/2" thick. The latter embodiment is less pressure
critical for optimization of target utilization than the embod-
30 iments of Figures 24-27 although there is a slight decrease in
power/sputter efficiency.
The position of slot or gap 78 controls the shape of the
top of the parallel field above magnet 28 as does the position
of ring 72, this being illustrated in Figure 29 where the left
and right sides thereof respectively correspond to the left and
right side embodiments of Figure 28. However, in the embodi-
ment of Figure 28A, the optimum location of rin~ 72 is such
that the stacks or rings 28 on opposite sides thereof are ap-
pro~imately equal in length along the line from the center to
the periphery of the magnetics while in the embodiment of Figure
28B, the stack or ring 28 on the inside of slot 78 is optimally
about twice as long as the stack or ring 28 on the outside of
the slot.
It should also be noted slot 78 need not completely inter--
rupt stack 28 but may only partially interrupt it in a manner
analogous to that of the embodiment of Figure 25. Further,
there may be provided two or more complete or partial slots in
a manner analogous to Figure 26.
An iron ring or a non-magnetic ring (plastic or copper,
for example) (not shown) may also be employe~ in slot(s) 78
in lieu of ring 12 where typically the slot width would be
smaller if it contained a non-magnetic ring. An iron ring pro-
vides some slight advantage over an open slot 78, as indicated
in Figure 29B. Further, the iron ring gives slightly more power
to the outer rings of the magnetic structure than does open
slot 78.
By increasing the number of lines of force that have an
angle of 45 or less at the center of the target, the magnetic
structures of the present invention satisfy a first condition
for obtaining substantially uniform erosion of target 16. A
second condition for obtaining erosion is that the strength of
the magnetic field approximately parallel to and at least 3/8
inch above the target surface should preferably be at least
about 80 Gauss. In particular, the two conditions of ta) lines
~417~4
of force having a 45 angle or less with respect to the target
and (b) 80 Gauss parallel field strength at least 3/8 inch above
the target should both be satisfied over as much of the target
surface as possible to provide uniformity of target erosion.
The various embodiments of the present invention satisfy the
above requirements with varying degrees of success, the embodi-
ments of Figure 25 at A and B and of F'igure 26 at A appearing
at this time to be the most successful, although it is to be
understood that the embodiments of Figure 25 with an outside
vertical stack 62 should also be successful as would be the
slotted (whether open ornot) embodiments represented by Figure
28B for many routine sputter applications.
Reference should be made to Figure 30 which shows an illus-
trative embodiment for sputtering targets of about one inch.
With the materials employed in the present invention for magnet
construction and especially the ferrite impregnated plastic or
rubber materials (such as PL-1.4H made by Minnesota Mining and
Manufacturing Co.), many of the embodimen-ts discussed herein-
before are typically capable of operating with targets up to
1/2 inch in thickness. In most applications this is sufficient.
However, in heavier industrial coating operations such as those
in glass, autoparts, plastic film, and so forth, the cost of
the target change downtime is of sufficient concern that targets
of one inch thickness are desired.
To erode a thicker target, fields must be provided approach--
ing parallel above the target surface by at least 3/8 inch at
levels exceeding about 80 Gauss - just as for the thinner tar~
geted embodiments discussed above. Forcing fields of ~his mag-
nitude through the greater thickness of target involves more
substantial magnets, though the same principles apply. It has
been observed that the addition of a magnet 90 perpendicular
to target 16 at its center can significantly increase the field
24
17~
at this height parallel to the target surface. It should be
noted that this tends to widen the center area in which lines
of force pass into the target at greater than 45. This de-
creases the percentage of target than can be sputtered away and
can reduce the sputter effectiveness of the discharge power.
Thus, this approach to thick target sputtering is a compromise
in which one parameter is traded for another.
Reference should be made to Figure 31 which shows an il-
lustrative embodiment for increasing the width and uniformity
of the erosion pattern on magnetically enhanced sputter cath~
odes. Because of the high voltages on the cathode structure,
it is desirable that there be a minimum of complex operating
structure. The width and shape of the erosion patterns are a
function of magnetic field shapes and strengths as well as the
field position relative to the target surface. These parameters
may be simultaneously varied without mechanical intervention
by combining the embodiments described hereinbefore with a rela-
tively small electromagnet 94 which only provides a changing
aspect to the overall field. Electromagnet 94 should be ap~
plied with care on alnico magnet systems, in that their strength
can be degraded if the field modification is too severe. Figure
31 shows a cross-section of a rubber magnetics system 28 and 62
with auxiliary electromagnet 94. The magnetic field at the two
areas of contact between the fixed magnets 28 and the steel
frame 96, 98 are the same. Thus, the effect on the fixed field
should be very minimal. When current is passed through elec-
tromagnet coil 92, the two legs 96 and 98 of the frame are forced
to different magnetic polarities. This upsets the balance of
field and shifts the mechanical position of the center of the
magnetic field. The center line of the field on the target may
receive relatively little erosion. ~oving this line permits
enhanced erosion to occur across the center of the target,
7~4
significantly increasing the percentage of utilization of the
target. Use of AC on the electromagnet oscillates the center
automatically from side to side. Correct design of the coil
makes it possible to use a fixed voltage transformer to drive
the coil. Preferably the waveshape of the signal should be
rectangular or square and the frequency may be about 60 hz al-
though a rather broad range of frequencies may be employed.
As indicated hereinbefore, large voltages and currents
can be applied to the sputtering device of the present invention
at low pressures. Hence, contaminants and the like can be
readily pumped out and yet a high sputter rate is available.
A feature which has been observed with respect to the sputter~
ing device is the large ratio of circulatory Hall effect current
with respect to the discharge current passing between the anode
and cathode. The Hall effect circulatory current results from
the closed plasma loop, which may be generated by the magnetic
structure of Figure 6, for example, the plasma loop extending
over the sputtering surface in a well known manner as described,
for example7 in aforementioned U. S. Patent 3,878,085. The
circulatory current circulates around the plasma loop and is
primarily due to the more mobile ionizing electrons. ~hus,
the large circulatory current obtainable with the present in-
vention is indicative of the presence of a large number of ion-
izing electrons to thereby provide high sputtering rates at
low pressures.
The discharge current passing between the anode and cathode
is that which conventionally passes through the external circuit
connected across these two electrodes. Circulatory currents
five to one hundred times greater than the discharge current
have been observed. It appears the strength of the magnetic
field generated by the circulatory current approximates that of
the field generated by the magnetic structures of the present
26
~41~04
invention, this being a further indication of the magnitude of
this current. It further appears the magnetic field generated
by the circulatory current may tend to pinch the closed loop
plasma into a thin, intense, ribbon-like sheet although it is
not apparent to what extent this pinch effect is achieved. It
may be that this effect is instrumental in achieving the high
sputter rates at low pressures obtainable with the present in-
vention although there is no intent to be limited to a particu-
lar theory of operation.
Several factors or combinations thereof are thought to
provide the above improved performance features of the present
invention. One factor is that of enhancing the erosion of the
target across the width thereof -- that is, in the direction
of the lines of force over the target as generated by the vari-
ous embodiments of this invention. Ideally the width of uniform
target erosion corresponds to the width of the closed loop plas-
ma path discussed above. In addition to uniformity of target
erosion across the width of the closed loop path, uniformity
of the magnetic field around theclosed loop path (including, in
particular, the non-linear (curved or 90 turn, for example)
portions of the path) is also desirable. Yet another factor
is the maintenance of a sufficient magnetic field strength over
the target to provide electron entrapment. Another factor is
confinement of the plasma to the closed loop where the magnetic
lines of force may or may not pass through the target layer of
sputtering material during both their exit from and return to
the magnetic structure.
Various features of the magnetic structures described here-
inbefore are instrumental in implementing the various factors
described above or combinations thereof. Thus, with respect
to the provision of more parallel magnetic lines of force with
respect to the sputtering surface, the embodiment of Figure 5
27
70~
is available for larger targets although, of course, the other
embodiments of the invention may also be employed. For smaller,
more conventional targets, the field deflecting means described
hereinbefore with respect to Figures 13 and 23-28 are preferred.
Typically, these embodiments include at least one magnet (stack
28, for example) which establishes the generally parallel field
to the sputtering surface. This field may be rendered more
parallel with re~pect to the sputtering surface by additional
magnets in magnetic circuit with (a) the first magnet 28, such
as additional magnets 70, 72, 70', 72', etc. where magnets 70 and
72 completely interrupt the flux through stack 28 while magnets
70' and 72' only partially interrupt this flux and (b) the lines
of force over the target surface. Further, slot 78, which may
be open or which may contain an appropriate insert, may also act
as means for effecting the requisite field deflection to render
the lines of force more parallel to the target surface.
Vniformity of the magnetic field above the sputtering sur-
face around the closed loop at least in the central portion
of the field may be obtained by the use of the flexible magnetic
structures illustrated in Figures 5 and 8. Illustrative of
such flexible magnetic structures are the magnetic tape embod-
iments described hereinbefore.
The desired strength of the magnetic field (typically a
parallel field strength of at least 80 Gauss at least 3/8 inch
above the sputtering surface) is preferably obtained with the
ferrite magnets described hereinbefore where the rubber or plas-
tic tapes impregnated with ferrite particles are advantageous.
The presence of these particles, which are capable of produc-
ing a very strong magnetic field, in a low permeability binder
such as rubber or plastic, is apparently advantageous in gener-
ating fields having the requisite strength, although it is to
be understood that other ferrite magnets may be employed as may
28
~141~7~?4
be appropriate ferromagnetic magnets such as alnico magnets.
In order to confine the plasma to its closed loop path
and thus prevent erosion from extending beyond the target (or
cathode) edge, magnet 62 of Figures 15-17, 26-28, 30 and 31
may be employed for achieving this feature of the invention.
In the above embodiment the south pole, for example, of magnet
62 is adjacent that of magnet 28. Hence, the close juxtaposi-
tion of these like poles is a further factor in promoting
parallelism of the lines of force to the very edge of the
plasma path, at which point the lines of force are directed sub-
stantially perpendicularly through the target edge to thereby
limit further erosion thereof. ~ence, ideally the lines of
force should be rectangular in configuration (as opposed to
curved) over the sputtering surface and the field shaping tech-
niques of the present invention have been so utilized as to ap-
proach this ideal with varying degrees of success while at the
same time providing sufficient field strength over the target.
With further respect to magnets 28 and 62 (and the other
magnets described hereinbefore), it is emphasized both of these
members are magnets as opposed to members made of non-magnetized,
magnetically permeable material such as iron pole pieces fre-
quently used in magnetic structures to direct lines of force.
Such pole pieces and the like do not effect the requisite field
shaping and/or generate the requisite field strength as is ob
tained by the present invention. ~urther, the direction of the
flux in fundamental magnet 28 is preferably inclined at an angle
with respect to the flux in magnets 28", 62, 70, 71', etc.
Hence, the desired field shaping depends to a certain extent
on the respective inclinations of the fluxes in the different
~0 magnets as well as their respective strengths. Preferably, the
flux in magnet 28 is parallel to the target surface and those in
the field shaping magnets is at an angle of 90 with respect to
29
lt~4
that in magnet 28, although, as in Figure 27, the flux in the
field shaping magnet can be opposite to that in magnet 28. Fur--
ther, the strength of the field shaping magnets should be such
that they are not de-magnetized by fundamental magnet 28 or
vice versa. Also, with respect to the field shaping means,
various combinations of magnets, slots, iron rings, etc. may
be employed if so desired. Further, although permanent magnets
are preferred for establishing the fundamental field and for
shaping it, electromagnets may also be employed as indicated,
for example, in Figure 4.
As indicated hereinbefore, the magnetic structures of the
present invention may be emp]oyed with planar cathodes which
are circular or oblong. Oblong cathodes may be rectangular,
elliptical or oval. Also, the planar cathode may be annular.
Further, the planar cathode may include non-linear portions
such as the concave portions shown in the cathodes of Figures
5 and 7 of aforementioned U. S. Patent 3,878,085. In addition
to planar cathodes, cylindrical, conical, closed belt, etc.
cathodes may also be employed. Also, as the cathode is sput-
tered, there will be a tendency for it to become concave. Never
theless, the cathode may still be considered planar, cylindrical
or whatever its original shape was. Further, contoured surfaces
may be imparted to the cathode so that it is thicker in areas
of greatest expected erosion whereby the target will sputter
relatively uniformly. Again, such a cathode is to be considered
planar, cylindrical, etc. depending upon its general configura~
tion prior to sputtering thereof.
The target material to be sputtered may or may not be the
cathode of the device. If not, it may be clamped to the cathode
by a clamp similar to clamp 32 of Figure 10. Clamp 32 may also
be employed to secure the cathode to the magnetic structure
when the cathode and target are one and the same. ~lhen the
target is distinct from the cathode, it may or may not be co-
extensive therewith. If the target is smaller than the cathode,
the magnetic lines of force may return to the magnetic structure
through the cathode rather than the target. As long as the
lines of force are substantially perpendicular to the cathode
where they pass therethrough, sputtering of the cathode is mini--
mized.
As stated hereinbefore with respect to Figure 10, the lines
of force can be returned through clamp 32, the lines being sub-
stantially perpendicular to the surface thereof to thereby mini~mize erosion thereof. Typically, clamp 32 (whether it is used
to clamp the target to the cathode or to position the cathode
within the magnetic structure) is in electrical contact with
and connected to the cathode although it is not a part of the
cathode structure. In fact, member 32 may be slightly spaced
from the cathode so that the lines of force return perpendicu~
larly therethrough and yet little, if any, plasma escapes be--
tween member 32 and the cathode since the spacing is so small
that a plasma cannot be formed therebetween. The member 32 can
thus provide a path for return of the magnetic lines of force
and further prevent lines of force from returning to the anode
which would permit electrons to escape from the plasma. Fur-
ther, spaced member 32 may be biased somewhat negatively with
respect to the cathode to effect repulsion of electrons there--
from and maintain electron entrapment.
Regarding the anode referred to hereinbefore, it is usual-
ly so-called because sputtering systems are typically self-
rectifying when an AC potential is applied. ~ence, although
the term anode is employed in the following claims, it is to
be understood that it may be any other equivalent electrode
in the system. Further, the anode can be the container wall
of the sputtering device. DC, low frequency AC (60 Hz, for
example) or industrial radio frequencies, such as 13.56 MHz or
27.12 MHz, may be applied across the anode and cathode. To
effect RF isolation, the anode is almost always the container
wall when these high frequencies are employed although it is
quite often employed as the anode when DC is employed.
As to the gas employed in the system, it may be either
active or inert depending upon the type of sputtered layer de-
sired.
It should be further noted that the principles of the
present invention can be applied to sputter etching.
