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BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to radio frequency (RF) sputtering systems
and more particularly to a low contamination, high deposition rate
sputtering apparatus for the etching and deposition of insulator films, of
such materials as SiO.sub.2 on semiconductor device wafers.
DESCRIPTION OF RELATED ART
RF sputtering is a well known technique in the art of dry process etching
and deposition. In brief, it is a type of diode sputtering with a cathode
electrode, an effective anode, and a plasma within a vacuum chamber. The
electrodes are typically driven at high voltage (600-1500 volts) and high
frequency (13.56 megahertz). The plasma is formed by the high voltage
induced breakdown of the gas in the chamber, and typically assumes a small
positive potential relative to ground, which is the potential of the
chamber walls. All electrodes develop a negative dc bias with respect to
the plasma. However, the cathode is usually smaller than the anode so that
a high negative dc bias is developed on it with respect to the plasma. The
cathode electrode is bombarded at high energy by ions in the plasma during
the "negative" portion of the applied voltage and by electrons during the
"positive" part. Thus, no net charge flows through the cathode electrode.
The cathode electrode is sputtered by the high energy ion bombardment.
This sputtering is used either for erosion of samples on the cathode
surface, or net deposition on samples placed on an opposing surface with
suitable support. For a more detailed discussion of RF sputtering, one can
refer to B. Chapman, "Glow Discharge Process," J. Wiley and Sons, N.Y.
1980, pp. 135-173.
Current manufacturing capability for depositing sputtered SiO.sub.2 on
semiconductor device wafers has evolved from small batch tools to very
large batch tools capable of holding as many as 20-82 mm dia. wafers. In
these very large batch tools, the SiO.sub.2 target diameter may be as
large as 61 cm (24 in.) and deposition run times are approximately 5 to 6
hours at a deposition rate of 250 .ANG./min. for "standard" SiO.sub.2 and
125 .ANG./min. for "planar" SiO.sub.2. Long cycle times, plus the
necessity of batching wafers makes this operation expensive and
logistically difficult for manufacturing.
However, development of a single wafer tool, with comparable through-put to
the batch wafer tools, requires a tool with a deposition rate of about 20
times as the rate for current batch tools. This is a deposition rate of
approximately 5000 .ANG./min.
Such high rate diode sputtering of SiO.sub.2 has been reported by Grantham
et al, Journal of Vacuum Science Technology, Volume 7, pp. 343 (1970).
This unit used a strong axial magnetic field, differential pumping of the
cathode insulator and RF power density of about 50w/cm.sup.2 to achieve
deposition rate of about 5000 .ANG./min. However, at such high target
sheath voltages and power densities, sputtering efficiencies are known to
be low and losses very high. Furthermore, such use of magnetic fields
causes radial non-uniformity.
To obtain the sought after high deposition rates in an efficient process
for use in manufacturing, it is necessary to obtain significantly higher
plasma density than is conventionally achievable with known diode RF
discharges. In addition, with such high deposition rates, the apparatus
must be designed to reduce accumulation of material in the chamber which
can cause particulate contamination of the wafer.
In the art of radio frequency (RF) sputter deposition, one technique for
control of the degree of resputtering of the substrate electrode in a
diode (two-electrode) system has been by varying the ratio of source
electrode area to substrate electrode area. (Koenig and Maissel, IBM
Journal Res. & Dev., Vol. 14, No. 2, March 1970, pp. 168-171.) This, in
principle, will allow adjustment of the substrate material balance from
deposition to etching. However, unless one is using the entire substrate
area for useful material collection, part of the substrate area must be
periodically cleaned and will be potentially a source of particulates.
This has been also true, in general, for the 3-electrode systems
controlled by substrate-tuning (see U.S. Pat. No. 3,617,459), or by
control using a separate source of RF power. Here, material collects on
the third or "wall" electrode, as well as on unused portions of the
substrate electrode.
A different apparatus known in the prior art for high plasma density by
geometrical confinement has been the RF hollow-cathode, which has been
used for reactive ion etching.
In U.S. Pat. No. 4,521,286 issued to Horowitz on June 4, 1985 and assigned
to Unisearch Limited of Kensington, Australia, one type of RF hollow
cathode sputter etcher is taught for controlling the speed and
directionality of the plasma in a chemically reactive process. However,
this is a chemically reactive process, and the etched silicon (Si) is
pumped out of the chamber with the other etch products combined with the
reactive gas. Wherein, the removal of silicon by reactive gas is
inapplicable to physical sputtering and there is no teaching for
controlling material accumulation on the chamber walls.
It is therefore an object of the present invention to provide a physical
sputter etcher and deposition apparatus wherein very high deposition rates
can be achieved at low voltage or high plasma density.
It is a further object of the present invention that the design of the
chamber of the apparatus is such that material accumulation on the walls
of said chamber is greatly reduced or reduced to the point of being
removed entirely.
It is a still further object of the present invention that the plasma
potential within the chamber be controlled so that re-sputtering of the
substrate can be controlled.
SUMMARY OF THE INVENTION
A high rate, low contamination, non-reactive sputter etching or deposition
apparatus is comprised of a pair of parallel plate electrodes, cathode and
substrate and an additional or wall electrode means surrounding said other
electrode means. Said additional or wall electrode can be made to be
coplanar with said other electrodes. All the electrodes are housed in a
vacuum chamber with inlet means for introducing a non-reactive gas into
said chamber.
Means are also provided for supplying said RF voltage to said electrodes
and for varying the magnitude of the substrate electrode RF voltage with
respect to the magnitude of the cathode voltage.
Thick insulator rings are used to reduce stray capacitance between the wall
electrode and ground whereby the outer chamber (normally grounded) forms a
low inductance path between the cathode electrode and substrate electrode
and acts as a shield for the inner chamber RF potential.
In one embodiment, the in-phase mode, the area of said wall electrode is
made approximately equal to or slightly larger than the sum of said
cathode and substrate electrode areas and the substrate voltage is
adjusted with respect to the cathode voltage by a variable capacitor,
between the substrate electrode and the cathode.
In another embodiment, the tuned substrate mode, the area of said wall
electrode is made to be approximately equal to the substrate area, and an
inductive tuned network is used to vary the substrate RF bias voltage with
respect to the plasma in order to control resputtering of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic of a conventional tuned substrate sputtering
system as known in the prior art.
FIG. 2 shows a skeletal schematic cross section of the in-phase sputtering
apparatus according to the invention herein.
FIG. 3 shows a skeletal schematic cross section of the tuned substrate
sputtering apparatus according to the invention herein.
FIG. 4 shows, a further, more detailed, cross section of the in-phase
apparatus of FIG. 2.
FIG. 5 shows a flow calibration curve for the gas introduced into the
apparatus of the subject invention.
FIG. 6 is a graph of the measured deposition rates for low and high power
resputtering with the in-phase apparatus of FIG. 4.
FIG. 7 is a graph of the measured deposition rates for the tuned substrate
system of FIG. 3.
FIG. 8 is a graph of the target sheath voltage versus the system power
density for both tuned substrate (diode) and in-phase modes at separate
frequencies.
FIG. 9 is a graph of the results of uniformity of film deposit for an
in-phase system operating at 50 mTorr.
FIG. 10 is a graph of the results of uniformity of film deposit for the
apparatus of the tuned substrate of FIG. 3.
FIGS. 11.1 and 11.2 are graphs of the influence of changing the inner
diameter of the wall electrode on the thickness distribution of the
apparatus of FIG. 4.
FIG. 12 is a graph of SiO.sub.2 peak heights over metal linewidths for
various substrate voltages in the tuned substrate mode.
DETAILED DESCRIPTION OF THE INVENTION
A conventional tuned substrate sputtering system is shown schematically in
FIG. 1. The RF generator 1 is connected through an appropriate matching
network 3 to the cathode (top) electrode 5 to which is attached the target
material (SiO.sub.2). The substrate electrode 7 (bottom) holds the
substrate (silicon wafer) (not shown) and is insulated from the chamber 9
which is grounded. The RF current into the system flows through the
cathode 5, into the plasma, and then divides, some flowing into the
substrate electrode 7, and the remainder into the wall electrode 9 to
ground. An inductive tuned network 11 connects the substrate holder to
ground, and is used to vary the substrate RF bias voltage with respect to
the plasma in order to control resputtering of the substrate. In this
arrangement, the phase of the target and substrate sheath RF voltages is
at or near 180 degrees for any condition of resputtering.
Turning now to the subject invention, FIG. 2 shows a schematic drawing of
the in-phase RF sputtering apparatus. As can be seen in FIG. 2, RF power
13 is applied to the cylindrical inner metal annulus called the wall
electrode 15. We have chosen to ground the cathode electrode 17 to the
metal vacuum chamber 19, and to insulate the substrate electrode 21 and
couple it to ground by a variable capacitor 23. The RF current flow is
therefore from the wall electrode 15, into the plasma and then radially
inward to ground through both cathode 17 and substrate 21 electrodes.
Alternatively, one can ground the wall electrode, and drive both target
and substrate with the same connection, again using a variable capacitor
to adjust substrate coupling. In either case, the phase relationship
between cathode sheath voltage and substrate sheath voltage is near zero
for most conditions of resputtering.
As can be seen in the FIG. 2, the wall electrode of the in-phase sputtering
system is shaped as a hollow annulus. However, the shape of the wall
electrode can be any of a variety of shapes including, but not limited to
the annulus shown. The relative areas of the wall electrode (Aw), the
target electrode (At), and the substrate electrode (As) are chosen to
enhance the sputtering of the wall. For the in-phase mode, the areas
follow the approximate relationship Aw is approximately equal to or
slightly larger than As+At to cause equal sputtering on the wall and
target.
In practice, some additional modification is desirable to compensate for
the fact that the substrate sputtering must be less than the target
sputtering to obtain a net deposition.
As the substrate capacitance is decreased, less RF current will flow into
the substrate electrode, resulting in less substrate self-bias, and less
substrate sputter-etching. Therefore, the substrate can accumulate
material which has been sputter-etched from the cathode and the wall
electrodes. If the cathode and the wall electrode are covered with the
desired material, say SiO.sub.2, then SiO.sub.2 can be deposited on the
substrate electrode at a controlled level of re-sputtering. Furthermore,
the wall and the cathode electrodes will sputter-etch at approximately the
same rate, so that they will have no net accumulation of material.
FIG. 3 shows a skeletal schematic cross section of the tuned substrate
sputtering apparatus according to the invention herein. It should be
readily apparent that the tuned substrate apparatus of FIG. 3 is
mechanically similar in design to the in-phase apparatus of FIG. 2.
However, it should be noted that in FIG. 3 the area of the wall electrode
is made approximately equal to the substrate electrode and an inductor
network has replaced the capacitive network in FIG. 2 and the electrical
connections have been changed.
Referring now to FIG. 4, the silica target 31 was a 1.57 mm thick.times.150
mm dia. plate (Corning type 7940 optical grade) bonded to a water cooled
aluminum cathode electrode 33 with indium-tin solder. The thickness of the
target plate was chosen to limit the surface temperature rise at full
power (11.6 watts/cm.sup.2 ion heat load) to less than 120 deg. C. A hole
through the cathode structure to the target backside permitted a fiber
optic probe to be used for rate monitoring by reflectance of a HeNe laser
beam normally incident on the silicon substrate.
Argon gas is introduced from an inlet 35A on the base plate 37, using a gap
38 into the annular space between the water-cooled substrate electrode 39
and the glass-ceramic lower insulator ring 41. Argon was pumped out of the
inner chamber through a 0.5 mm throttling gap 43 between the wall
electrode 45 and the glass-ceramic upper insulator ring 47. Using this
arrangement, the flow-pressure calibration was measured as shown FIG. 5,
graphs 60 and 61; the inner chamber pressure 60 was sensed using the
cooling gas port 35b in the substrate holder, without a wafer in place, as
shown in FIG. 4.
A channel 35B through the substrate electrode 39 and the anodized plate
carried argon gas to the wafer backside. The substrate electrode 39 is
further surrounded by insulating ring 51.
The wall electrode 45 was a machined and brazed aluminum assembly supported
on four rods (not shown). Member 49 encloses the total apparatus. A large
pump port (not shown) is provided through member 49.
For the in-phase mode these rods were insulated from member 49. One of the
rods was used to supply the r.f. power to the system, and was water
cooled. Two of the rods were used to supply water to cool the wall
electrode and also to provide a convenient voltage measuring connection.
The cathode was grounded to member 49 with a metal ring 48A. Member 49
encloses the total apparatus.
For the tuned substrate mode, the four support rods were grounded to member
49 and the cathode 33 was insulated by a glass ring 48B which replaced
metal ring 48A.
The inside diameter of the wall electrode was initially 241 mm, and was
later reduced using an insert to 229 mm, giving area ratios (Aw/As+At) of
1.70 and 1.43 respectively in the in-phase mode. These ratios were larger
than intended and as a result, the resputtering of the wall electrode in
the in-phase mode was not optimized for the data reported herein. All data
was taken using a 10 Kw 40.68 MHz r.f. generator from RF plasma products.
This frequency was chosen based on a need to obtain the highest possible
plasma density at the lowest voltages. All data reported herein for the
in-phase mode was for the smallest diameter (229 mm) of the wall
electrode. However, in the tuned-substrate mode, additional insert rings
were made and tested down to 157 mm dia. giving tuned substrate area
ratios (Aw/As) down to 1.04, where substantial wall sputtering was
measured.
Deposition rates versus power are shown in FIG. 6 for low resputtering
level 62 and high resputtering level 63 of resputtering, for both modes. A
rate of 5000 A/min was exceeded at 7KW in the in-phase mode, at a low
resputtering level. In the tuned-substrate mode FIG. 7, resputtering level
64, the power was not raised as high because of the excessively high
system voltages which could exceed ratings of matching network components.
FIG. 8 shows the target sheath voltage vs the total system power density
(net rf power divided by target area) for 4 different cases; 13 MHz tuned
substrate mode 65 and in-phase mode 66, and 40.68 MHz tuned substrate mode
67 and in-phase mode 68. The use of the highest frequency and the in-phase
mode combined gave the lowest voltage/power density curve, which allowed
the use of a power density sufficient to reach the desired rate without
exceeding those target sheath voltages currently used by those in the art
(approximately 2,000).
FIGS. 9 and 10 show the thickness distribution for the in-phase (7.5 inch,
inner diameter) 69.lA and B and tuned substrate (9.0 inch, inner diameter)
modes 69.2A and B, respectively. Both apparatii, used symmetrical cathode
connections under "standard" resputtering conditions at 3 KW input power
and show a symmetrical thickness profile. The A and B curves represent
measurements across the orthogonal axii of the wafer.
Using the tuned substrate mode, the inner diameter of the wall electrode
was changed from 241 mm to 158 mm in four steps. The four steps used
resulted in FIG. 11 area ratios (Aw/As) of 5.45 Graphs (70, 74), 1.95
Graphs (71, 75), 1.26 Graphs (72, 76), and 1.04 Graphs (73, 77)
respectively. The influence of this change on thickness distribution both
on the wafer and on the wall electrode is shown in FIGS. 11.1 and 11.2 and
TABLE 1. As the diameter is reduced, the deposit on the wall is reduced
and the thickness on the wafer edge is increased. This is due to the
expected increase of wall electrode resputtering as the wall electrode
area is decreased. At a wall diameter of 158 mm, the wall itself is being
etched in some places. At a wall diameter of 163 mm, the wafer thickness
profile appears to be nearly ideal, while the wall deposit has been
reduced to about 30% at the inner edge, as compared to about 60% for the
largest inner diameter.
TABLE 1
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Tuned Substrate Mode:
Wall I.D. Area Ratio Max. wall deposit
(inches) Aw/As % of center
______________________________________
9.5 5.45 57%
7.0 1.95 40%
6.4 1.26 35%
6.2 1.04 0%
______________________________________
In-Phase Mode:
Wall I.D. Area Ratio Max. wall deposit
(inches) Aw/At + As % of Center
______________________________________
7.5 1.05 23%
______________________________________
Stress was measured on some runs by measuring wafer deflection using a
stylus profilometer. TABLE 2 contains the data for both rf hollow cathode
depositions and for tuned substrate depositions. All stress measurements
were compressive. The rf in-phase measurements at 3 different power levels
show that there is a trend of decreasing stress with increasing power
(deposition rate). During these runs, a backside gas pressure of 1 Torr
was used, so that the temperature of the wafer would be expected to
increase linearly with power. The measurements for the tuned substrate
mode were all at a single power using a 3 Torr cooling gas pressure, with
a variable substrate bias voltage (variable resputtering). These values
were all below 10.sup.9 dynes/cm.sup.2, and showed no apparent correlation
to resputtering level. This measurement accuracy is estimated to be
.+-.20%. For comparison, values for these films conventionally deposited
usually range between 1.5 and 2.2.times.10.sup.9 dynes/cm.sup.2. A low
compressive stress is usually desirable for mechanical strength.
TABLE 2
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FILM STRESS
IN-PHASE MODE - 9 IN. I.D. CHAMBER
STRESS
POWER DEPOSITION
BACKSIDE
(.times. 10*9
(WATTS)
RESPUTTERING
RATE(A/MIN.)
GAS(TORR)
DYNE/CM2)
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1000 PLANAR 491 1 3.9
3000 PLANAR 1560 1 1.48
5000 PLANAR 2250 1 1.09
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TUNED SUBSTRATE MODE - 9 IN. I.D. CHAMBER
SUBSTR. STRESS
POWER BIAS(P-P DEPOSITION
BACKSIDE
(.times. 10*9
(WATTS)
VOLTS) RATE(A/MIN.)
GAS(TORR)
DYNE/CM2)
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3000 350 2317 5 0.69
3000 400 2367 3 0.16
3000 500 2402 3 0.56
3000 600 2190 3 0.63
3000 700 1938 3 0.20
3000 800 1773 3 0.70
__________________________________________________________________________
Refractive index is an indicator of both density and stoichiometry.
Thermally grown oxide films on silicon are usually used as a reference.
TABLE 3 shows the measurements made at 6328 A and 5461 A using a Rudolph
automatic ellipsometer.
TABLE 3
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REFRACTIVE INDEX
TUNED SUBSTRATE MODE - 9 IN. I.D. CHAMBER
SUBSTR. REFR. REFER.
POWER BIAS DEPOSITION INDEX INDEX
(WATTS) (VOLTS) RATE(A/MIN.) (6328 A)
(5451 A)
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3000 400 2367 1.459 (9)
1.467 (16)
3000 500 2402 1.460 (3)
1.445 (8)
3000 600 2190 1.456 (14)
1.440 (13)
3000 700 1938 1.453 (9)
3000 800 1773 1.458 (7)
1.434 (9)
Thermal SiO.sub.2 1.457 1.460
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The only films characterized this way were films deposited in the tuned
substrate mode at different resputtering levels. A number of readings,
shown in parentheses, were averaged to get the values tabulated. The
values obtained are reasonably close to values for thermal oxide. Small
deviations from stoichiometry can increase the refractive index. Density
reduction can reduce the index proportionately. Except for the unlikely
event of compensating deviations in stoichiometry and density, the numbers
indicate near-bulk property films.
Measurements of SiO.sub.2 profile over aluminum metal patterns were made
for a series of substrate electrode voltages in the tuned substrate mode
only. FIG. 12 shows the peak heights of the SiO.sub.2 profile as measured
with a SLOAN DEKTAK II profilometer. "Standard" SiO.sub.2 conditions would
produce no planarization at all, so the 200 V or 300 V curves represent
that condition. "Planar" conditions are defined as a peak height of less
than 0.2 micron over a 5 micron wide line. Clearly, the 700 volt condition
or greater satisfies this condition. Therefore, the system can achieve
planarity in the tuned substrate mode at 3 KW.
Thus, while the invention has been described with reference to preferred
embodiments thereof, it will be understood by those skilled in the art
that various changes in form and details may be made without departing
from the scope of the invention.
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