(12) United States Patent
`Lantsman
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,190,512 B1
`Feb. 20, 2001
`
`US006190512B1
`
`(54)
`
`(75)
`
`SOFT PLASMA IGNITION IN PLASMA
`PROCESSING CHAMBERS
`
`Inventor: Alexander D. Lantsman, Middletown,
`NY (US)
`
`(73) Assignee: Tokyo Electron Arizona Inc., Tokyo
`(JP)
`
`Notice:
`
`Under 35 U.S.C. 154(b), the term of this
`patent shall be extended for 0 days.
`
`(21)
`
`(22)
`
`(51)
`(52)
`(58)
`
`(56)
`
`Appl. No.: 08/117,443
`
`Filed:
`
`Sep. 7, 1993
`
`Int. Cl.7 ................................................... .. C23C 14/34
`U.S. Cl.
`204/192.12; 204/192.13
`Field of Search ................................... .. 156/643, 646,
`156/626, 627, 345; 204/192.12, 192.13,
`298.08, 298.06, 298.34, 298.32, 192.33;
`219/121.57; 118/723 R, 723 E
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`................. .. 204/298
`4,557,819 * 12/1985 Meacham et al.
`. . . . . .. 156/643
`4,888,088 * 12/1989 Slomowitz . . . . . . . .
`
`5,288,971 *
`2/1994 Knipp ........................ .. 204/298.08X
`FOREIGN PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`Mashiro Kazuhiko, “Discharge Triggering Method of Sput-
`tering Device” Abstracts of Japanese Patent Application No.
`59—222580, Published December 14, 1984.
`Jeff Rowland, “Equipment Profile—Coherence One Series
`II Preamp and Model 7 Mono Amp” Audio Magazine, Apr.,
`1990.
`
`Audio Research Corporation, Product Brochure D70/D115/
`D250 and M100 Power Amplifiers.
`“Gaseous Conductors Theory and Engineering Applica-
`tions” James Dillon Cobine, Ph.D. (1941, 1958).
`“The Advanced Energy MDX Magnetron Drive” Sales
`Brochure of Advanced Energy Industries, Inc. (Jun., 1991).
`
`* cited by examiner
`
`Primary Examiner—Nam Nguyen
`(74) Attorney, Agent, or Firm—Wood, Herron & Evans,
`L.L.P.
`
`(57)
`
`ABSTRACT
`
`The specification discloses a power supply circuit which
`reduces oscillations generated upon ignition of a plasma
`within a processing chamber. A secondary power supply
`pre-ignites the plasma by driving the cathode to a process
`initiation voltage. Thereafter, a primary power supply elec-
`trically drives the cathode to generate plasma current and
`deposition on a wafer.
`
`59—222580
`
`12/1984 (JP) ............................. .. C23C/15/00
`
`7 Claims, 3 Drawing Sheets
`
`PROCESS GAS
`DC POWER
`DRIVE
`ENABLE‘?
`
`
`
`
`
`
`PLASMA DC
`POWER
`SUPPLY
`
`SECONDARY
`DC POWER
`SUPPLY
`
`TARGET
`(CATHODE)
`
`BIASING RF
` POWER
`
`SUPPLY
`
`GILLETTE 1025
`
`GILLETTE 1025
`
`

`
`U.S. Patent
`
`Feb. 20, 2001
`
`Sheet 1 013
`
`US 6,190,512 B1
`
`/fl
`
`PLASMA DC
`POWER
`SUPPLY
`
`+
`
`TAR ET
`
`
`
`(CATHODE)
`
`
`
`/Z
`
`/5
`
`BIASING RF
`POWER
`SUPPLY
`
`
`
`
`/4
`
`‘I
`
`SPUT CURRENT
`
`

`
`U.S. Patent
`
`Feb. 20, 2001
`
`Sheet 2 of 3
`
`US 6,190,512 B1
`
`c.0_“_
`
`
`
`uo<:o>._.
`
`3%
`
`

`
`U.S. Patent
`
`Feb. 20, 2001
`
`Sheet 3 013
`
`US 6,190,512 B1
`
`I -W
`PROCESS GAS
`"°""‘°E Cm
`ENABLE?
`'
`Dc POWER
`
`4%
`DRIVE
`
`SECONDARY
`PLASMA DC
`DC POWER
`POWER
`SUPPLY
`SUPPLY
`
`
`
`
`
`
`/5
`
`/4
`
`BACK PLANE
`
`22
`
`
`
`
`Zfl
`
`BIASING RF
`POWER
`SUPPLY
`
`
`
`
`
`
`
`I CATHODE Dc NEGATIVE voI..
`I
`I
`I
`:53
`'
`I/
`Dc POWER DRIVE
`N
`,
`-
`I
`I
`-L 54 i‘— Dc DRIVE
`H |<——GAs OFF
`'
`ON DELAY
`DELAY56
`
`I
`|
`I
`
`PROCESS GAS ENABLE COMMAMD
`
`GAS FLOW / PRESSURE
`
`I
`I
`
`4
`I
`
`I
`I
`
`FIG. 5
`
`I
`I
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`L
`
`545
`
`45
`
`6
`
`

`
`US 6,190,512 B1
`
`1
`SOFT PLASMA IGNITION IN PLASMA
`PROCESSING CHAMBERS
`
`BACKGROUND OF THE INVENTION
`
`This invention relates to reduction of device damage in
`plasma processes,
`including DC (magnetron or non-
`magnetron) sputtering, and RF sputtering.
`Atypical plasma processing apparatus is shown in FIG. 1.
`The apparatus includes a plasma power supply 10, which
`drives a cathode or target 12 to a large DC voltage (e.g.,
`-400 Volts) relative to the walls of vacuum chamber 14. The
`semiconductor substrate 16 (also known as the wafer) rests
`on a back plane 18 inside the chamber. The back plane may
`be driven by radio frequency (RF) AC voltage signals,
`produced by an RF power supply 20, which drives the back
`plane through a compensating network 22.
`The AC and/or DC power supplies generate a plasma in
`the area above the wafer and between the wafer and the
`
`target, and cause material from the target to deposit on the
`wafer surface.
`
`A typical DC power supply 10 includes a relatively
`sophisticated control system, designed to permit operation in
`constant power, constant voltage, or constant current modes.
`This control circuitry includes a damped control loop which,
`when the supply is engaged, produces a controlled ramping
`toward the desired output level. For example, as shown in
`FIG. 2, upon engagement of a typical DC power supply in
`an apparatus as shown in FIG. 1, the supply current (which
`represents the density of ionic transfer from the target due to
`sputter deposition on the wafer) ramps up to a constant value
`in a controlled manner with a small overshoot 24.
`
`Despite the otherwise carefully regulated output produced
`by typical power supplies, it is normal to observe a spike in
`the target voltage during process initiation. As shown in FIG.
`3, the magnitude of the spike 26 at process initiation may
`exceed the normal DC voltage level by a factor of 2 or more
`(e.g.,
`those shown in FIG. 3 reach -1100 Volts). This
`phenomenon, known as the “break down” spike, is typically
`viewed as a necessary, isolated event associated with the
`creation of a plasma in the chamber 14 (otherwise known as
`“plasma ignition”). Furthermore, a large magnitude break
`down spike has been seen as necessary to improve process
`quality.
`
`SUMMARY OF THE INVENTION
`
`Overvoltage in the processing chamber deteriorates the
`quality of sputtered films in several ways: High voltage
`events electrically damage layers and/or devices on the
`processing substrate (wafer). Furthermore, arcing which can
`be produced by overvoltages can cause local overheating of
`the target, leading to evaporation or flaking of target material
`into the processing chamber and causing substrate particle
`contamination and device damage. These sources of wafer
`damage become increasingly significant as integrated cir-
`cuits reach higher densities and become more complex.
`Thus,
`it is advantageous to avoid voltage spikes during
`processing wherever possible.
`With this in mind, careful analysis has revealed that the
`so-called break down spike is not, in fact, an isolated event
`necessarily associated with the creation of a plasma in the
`chamber. The spike is not caused by the creation of a plasma
`per se, but rather by harmonic oscillations within the cham-
`ber.
`
`As shown in FIG. 4, a gas-filled chamber generates
`sizable oscillations when driven by a DC voltage within a
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`55
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`60
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`65
`
`2
`in
`given voltage range. These oscillations are evident
`regions 26 and 28. Notably, however, the oscillations cease
`when the driving voltage exceeds a threshold voltage rep-
`resented by line 30. One explanation of this phenomenon is
`that complete plasma ignition occurs above threshold volt-
`age 30. When the power supply voltage is near to, but below
`this threshold voltage, unstable gas discharges, as well as
`related transitions between gas and plasma phases, occur in
`chamber 14. (Similar effects have been observed in gaseous-
`discharge tubes.) As a result, the gas-plasma system begins
`unstable oscillation, producing brief, but very large magni-
`tude voltage perturbations. This oscillation continues until
`the threshold voltage 30 is achieved, at which point the
`gas/plasma mixture fully transitions to a plasma, and oscil-
`lations cease.
`
`Voltage 30 will be referred to as the “oscillation threshold
`voltage”. The value of the oscillation threshold voltage will
`depend on the target (cathode) material, process gas and
`pressure, chamber geometry, electrical characteristics of the
`external power wiring, and possibly the volt-ampere curve
`of the sputtering chamber.
`Based on the preceding observations, the spike observed
`in region 26 of FIG. 3 is now understood to be an oscillation
`caused when the output voltage of primary supply 10 lingers
`at
`a voltage just below the oscillation threshold.
`Furthermore, careful inspection of region 28 of FIG. 3 also
`reveals oscillatory behaviors analogous to those which
`appear in region 28 of FIG. 4. (The oscillations in region 28
`have smaller magnitudes, in part because when the power
`supply is disabled, its output voltage drops relatively rapidly,
`whereas when the power supply is enabled its output voltage
`increases relatively slowly.)
`It has been found that the oscillation spike observed in
`FIG. 3 can be eliminated by elevating the target/cathode
`voltage above the oscillation threshold voltage before initi-
`ating gas flow into the chamber, and maintaining the cathode
`voltage above the oscillation threshold until processing is
`completed, gas flow is halted, and vacuum is restored. This
`technique prevents overvoltage during processing, and
`therefore can reduce device damage and particulate con-
`tamination.
`
`In brief summary, this technique is implemented by a
`power supply circuit comprising two power supply sections:
`an essentially conventional primary power supply, which
`provides the primary power to electrically drive the cathode
`during the plasma process, and a secondary power supply
`which supplies an initial plasma ignition voltage sufficiently
`in excess of the oscillation threshold voltage. This secondary
`power supply “pre-ignites” the plasma so that when the
`primary power supply is applied,
`the system smoothly
`transitions to final plasma development and deposition. This
`design thereby avoids oscillations when the primary power
`supply is engaged and disengaged, and any corresponding
`device damage.
`limiting resistor,
`In preferred embodiments, a current
`switch, and diode are connected in series between the
`secondary power supply and the cathode.
`The current limiting resistor limits the current flowing
`from the secondary power supply into the cathode. Only a
`minimal current is needed to elevate the cathode voltage
`above the oscillation threshold and pre-ignite the plasma; by
`interposing a current-limiting resistor, the secondary power
`supply current is held at this minimum level, thus avoiding
`the need for a high power secondary supply, and also
`limiting the plasma current and deposition while the sec-
`ondary power supply is enabled and the primary supply is
`disabled.
`
`

`
`US 6,190,512 B1
`
`3
`The diode automatically disconnects the secondary power
`supply from the cathode when the primary supply begins
`driving the plasma. To achieve this, the diode is connected
`so that it is “on”, i.e., current flows, when the magnitude of
`the secondary supply voltage exceeds the cathode voltage,
`and is “off” otherwise;
`thus, once the primary supply
`engages and begins driving the cathode, the diode turns “off”
`and the secondary supply is disconnected.
`The switch is used to turn the secondary power supply
`voltage on and off; at
`the beginning of processing,
`this
`switch is closed and gas is introduced into the chamber.
`When the plasma process is completed,
`the gas flow is
`stopped, and once vacuum is restored, the switch is opened.
`Because the switch is opened and closed while the chamber
`is at full vacuum (when there is very little gas in the
`chamber), gas/plasma transition oscillations are substan-
`tially reduced.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`The above and further aspects of the invention will be
`more fully understood with reference to the accompanying
`drawings, in which:
`FIG. 1 is a block diagram of a conventional sputter
`deposition apparatus;
`FIG. 2 is a trace of the sputter current measured at power
`supply 10 of FIG. 1;
`FIG. 3 is a trace of the sputter voltage measured at cathode
`12 of FIG. 1;
`FIG. 4 is a trace of the sputter voltage measured at cathode
`12 of FIG. 1, produced by manually adjusting the chamber
`voltage;
`FIG. 5 is a block diagram of a sputter deposition apparatus
`in accordance with the present invention;
`FIG. 6 is a timing diagram useful in understanding the
`operation of the apparatus of FIG. 5;
`FIG. 7 is a trace of the sputter current measured at the
`power supply 10 of FIG. 5; and
`FIG. 8 is a trace of the sputter voltage measured at the
`cathode 12 of FIG. 5.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`in one embodiment of a plasma
`Referring to FIG. 5,
`processing apparatus in accordance with the invention, a
`plasma ignition circuit 30 is added to the apparatus shown in
`FIG. 1. Other than circuit 30, the apparatus of FIG. 5 uses
`the same components as the apparatus of FIG. 1, including
`a primary DC power supply 10, cathode 12, substrate 16 and
`back plane 18 within chamber 14, RF power supply 20 and
`coupling network 22.
`Plasma ignition circuit 30 comprises a secondary DC
`power supply 32 which produces an output voltage greater
`than the oscillation threshold voltage of chamber 14. The
`output voltage of secondary supply 32 must be adjusted
`whenever the oscillation threshold changes, e.g. with every
`change in the cathode material, process gas and pressure,
`chamber geometry, electrical characteristics of the external
`power wiring, and (possibly) the volt-ampere curve of the
`sputtering chamber. The appropriate output voltage may be
`determined by monitoring the voltage of the cathode while
`manually adjusting the power supply voltage, in the manner
`discussed above with reference to FIG. 4. Through such an
`experiment,
`the oscillation threshold voltage can be
`measured, and a suitable output voltage above the oscillation
`threshold can be chosen.
`
`For example, FIG. 4 was generated from experiments
`using a deposition chamber 14 and a high purity aluminum
`
`5
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`10
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`15
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`20
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`25
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`30
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`35
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`65
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`4
`alloy target 12 sold by Materials Research Corporation of
`Orangeburg, N.Y. under the trademarks “ECLIPSE M3” and
`“RMX-10”, respectively, in combination with a DC power
`supply 10 sold by Advanced Energy Industries of Fort
`Collins, Colo. under the trademark “MDX-10kW”. Mea-
`surements were recorded by a thermal chart recorder, run-
`ning at 20 mm/sec. (FIGS. 2, 3, 7 and 8 were generated with
`similar apparatus.) FIG. 4 was produced under typical
`operating conditions, e.g.: The gas was sputtering purity
`argon, at a pressure of 10 mT (at 100 sccm) and a flow rate
`of 50-150 sccm. DC power supply 10 was set to output 6-10
`kW of power. The wafer temperature was approximately
`300° C. and the deposition rate was approximately 7000
`A/minute. These values are examplary only and are not
`critical to the oscillatory pheonomena shown in FIG. 4; for
`example, oscillations were also seen at ambient temperature.
`Based on FIG. 4, the oscillation threshold was estimated at
`just below -300 Volts, so the output voltage of secondary
`DC supply 32 was set at -300 Volts.
`Secondary DC power supply 32 is enabled and disabled
`by a voltage control signal on line 40, and is connected to
`cathode 12 through a resistor 34, relay switch 36, and diode
`38. The purpose of these elements is discussed below.
`Relay 36 connects and disconnects secondary power
`supply 32 to the cathode. The relay ensures that the voltage
`transition on the cathode is as rapid as possible. Relay 36 is
`opened and closed by the signal on line 42. In operation,
`relay 36 is left open while the chamber 14 is evacuated and
`secondary supply 32 is enabled (by an appropriate signal on
`line 40). Then, once the chamber is evacuated and secondary
`supply 32 has achieved the desired output voltage, relay 36
`is closed, causing a cathode 12 to transition te the voltage of
`secondary supply 32. After the cathode is at the secondary
`supply voltage, gas is permitted to flow into the chamber,
`producing a pre-ignited gas plasma. Thereafter, primary
`supply 10 is enabled to generate plasma current and depo-
`sition on wafer 16.
`
`To end processing, primary supply 10 is disabled, reduc-
`ing the plasma current and deposition on wafer 16. Then, gas
`flow is terminated and the chamber is fully evacuated. Once
`the chamber is fully evacuated, relay 36 is opened and the
`cathode voltage returns to ground. After relay 36 is opened,
`as desired, secondary supply 32 may be switched off by a
`signal on line 40.
`As noted above, resistor 34 serves as a current-limiter.
`The design shown in FIG. 5 attempts to separate ignition of
`the plasma from the initiation of deposition: the secondary
`supply 32 is used to pre-ignite the plasma, whereas the
`primary supply 10 is used to generate deposition. To ensure
`separation of these functions,
`it is desireable to limit the
`plasma current (and resulting deposition) generated by the
`secondary supply 32. The current produced by secondary
`supply 32 should be the minimum amount necessary to
`maintain plasma ignition. Resistor 34 provides the needed
`current-limiting function. During the pre-ignition period
`when gas is flowing into the chamber and the cathode is at
`the voltage of the secondary power supply 32, plasma
`current flows within the chamber 14. However, this current
`flow causes resistor 34 to develop a voltage drop, reducing
`the voltage between the cathode and the chamber 14, and
`thereby reducing the plasma current
`flow. As a result,
`although the voltage of secondary supply 32 is sufficient to
`pre-ignite a plasma in chamber 14, current limiting resistor
`34 limits the plasma current after the plasma is ignited. The
`value of the resistance should be chosen to limit the sec-
`ondary supply current to a few percent of the sputtering
`current produced by the primary supply (in the above
`example, about 200 mA).
`Diode 38 serves to isolate the secondary power supply
`after the primary power supply has initiated deposition.
`
`

`
`US 6,190,512 B1
`
`5
`Diode 38 will permit current flow from cathode 12 and into
`secondary supply 32, but will not permit reverse current flow
`from secondary supply 32 into the cathode. (Note that the
`cathode is driven to a negative voltage by supplies 10 and
`32.) When the primary supply 10 is enabled, in order to
`generate substantial sputtering current, supply 10 must pro-
`duce a voltage in excess of that produced by secondary
`supply 32. However, when magnitude of the primary supply
`voltage exceeds that of the secondary supply, diode 38 turns
`“off”, isolating secondary supply 32 from the cathode and
`primary supply.
`Diode 38 may also prevent undesired voltage drops
`during deposition. For example, most commercially avail-
`able plasma power supplies are designed to detect arcing in
`the chamber during processing, and to automatically sus-
`pend output when an arc is detected. Normally, output power
`is restarted after a brief delay (15-20 msec in the power
`supply described above). During this delay, the magnitude of
`the cathode voltage may decrease below the oscillation
`threshold, resulting in undesirable oscillation. Diode 38
`prevents such a result;
`if the magnitude of the cathode
`voltage drops below that of the secondary power supply,
`diode 38 will turn “on”, so that secondary power supply will
`hold the cathode voltage at a sufficient magnitude to main-
`tain plasma ignition, and prevent oscillation when the pri-
`mary supply power is restarted. Diode 38 similarly prevents
`the magnitude of the cathode voltage from dropping below
`the oscillation threshold when primary supply 10 is disabled
`at the end of sputtering. As a result, the plasma remains
`ignited until the chamber is evacuated after processing.
`The above timing is clarified in FIG. 6. As shown, when
`processing is initiated, the process gas enable signal on line
`42 (trace 46) is raised to a true value (the above-described
`deposition equipment sold by Materials Research Corpora-
`tion generates a Process Gas Enable signal which turns on
`before gas flow starts and remains on until after vacuum is
`re-established; in the depicted implementation, this signal is
`used to control relay 36.) At this time, relay 36 closes and
`secondary supply 32 is connected to the cathode, causing an
`essentially immediate, step change in the cathode voltage
`(trace 50) above the oscillation threshold voltage. Sometime
`thereafter, gas flow is initiated and the gas flow and pressure
`(trace 48) begin to ramp upwards toward normal processing
`levels. After a delay time (54), a normal pressure and flow
`rate are achieved, and primary supply 10 is enabled, causing
`a ramp increase in the power produced by the primary
`supply (trace 52). As the primary supply approaches full
`power,
`the magnitude of the cathode voltage (trace 50)
`increases slightly, causing plasma current flow and deposi-
`tion. This voltage increase also causes diode 38 to turn “off”,
`isolating secondary power supply 32 from the cathode.
`At the end of processing, primary supply 10 is disabled,
`causing a ramp decrease in the power produced by the
`primary supply (trace 52). As the primary supply power
`decreases, the magnitude of the cathode voltage (trace 50)
`decreases to the oscillation threshold, at which point diode
`38 turns “on”, secondary supply 32 is reconnected to the
`cathode, and secondary supply 32 holds the cathode above
`the oscillation threshold voltage. Thereafter, gas flow is
`turned off, causing the gas flow and pressure (trace 48) to
`ramp down toward zero. Once vacuum has been
`re-established, the process gas enable signal (trace 46) is set
`to false, opening relay 36 and causing a rapid decrease in the
`magnitude of the cathode voltage (trace 50).
`As shown in FIGS. 7 and 8,
`the FIG. 5 design can
`substantially reduce oscillations at the beginning and end of
`plasma processing. Although the current behaves in roughly
`the same manner (compare FIGS. 7 and 2, respectively), the
`cathode voltage generated by the FIG. 5 design shows no
`visible oscillations at the beginning and end of processing
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`55
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`60
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`65
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`6
`
`(regions 26 and 28 of FIG. 8). In the FIG. 1 design, these
`regions (see FIG. 3) evidenced substantial high voltage
`spikes.
`It should also be noted that the baseline voltage in FIG. 8,
`i.e., the voltage in regions 58 before and after processing, is
`approximately -300 Volts DC, rather than approximately 0
`Volts DC in the corresponding regions of FIG. 3. This
`confirms that secondary power supply 32 is holding cathode
`14 at a voltage of approximately -300 Volts DC, thereby
`maintaining ignition of plasma within chamber 14 before
`and after deposition.
`Although the invention has been described with reference
`to a specific embodiment, it will be understood that various
`modifications may now be made without departing from the
`inventive concepts described. For example, the inventive
`techniques described can be applied to any plasma process,
`including without
`limitation to DC (magnetron or non-
`magnetron) sputtering, RF sputtering, and sputter etching.
`The specific embodiment described above is to be taken as
`exemplary and not limiting, with the scope of the claimed
`invention being determined from the following claims.
`What is claimed is:
`
`1. A method of plasma processing a workpiece in a
`vacuum chamber having a cathode, comprising
`evacuating said chamber,
`elevating said cathode to a process initiation voltage
`relative to said chamber while said chamber is
`
`evacuated, said process initiation voltage being insuf-
`ficient to fully ignite or maintain a plasma within said
`chamber,
`flowing a gas into said chamber while maintaining said
`cathode at said process initiation voltage, and thereafter
`applying electrical power to said cathode to elevate said
`cathode to a processing voltage greater than said pro-
`cess initiation voltage to fully ignite a plasma from said
`gas within said chamber and cause electrical current to
`flow through said plasma,
`maintaining said cathode at said processing voltage to
`maintain ignition of a plasma in said chamber while
`processing said workpiece within said chamber.
`2. The plasma processing method of claim 1 wherein said
`cathode is elevated to said process initiation voltage by
`closing a switch and thereby connecting a power supply to
`said cathode.
`3. The plasma processing method of claim 1 wherein
`electrical power is applied to said cathode by enabling a
`primary power supply connected to said cathode.
`4. The plasma processing method of claim 3 wherein said
`cathode is elevated to said process initiation voltage by
`closing a switch and thereby connecting a secondary power
`supply to said cathode.
`5. The plasma processing method of claim 4 wherein said
`secondary power supply is connected to said cathode via a
`resistor which limits electrical current flow between said
`secondary power supply and said cathode.
`6. The plasma processing method of claim 5 wherein said
`secondary power supply is connected to said cathode via a
`diode which limits electrical current flow between said
`secondary power supply and said cathode when the magni-
`tude of the voltage of said cathode relative to said chamber
`exceeds the magnitude of said process initiation voltage.
`7. The plasma processing method of claim 4 wherein said
`secondary power supply is connected to said cathode via a
`diode which limits electrical current flow between said
`secondary power supply and said cathode when the magni-
`tude of the voltage of said cathode relative to said chamber
`exceeds the magnitude of said process initiation voltage.
`*
`*
`*
`*
`*