(12) United States Patent
`Lantsman
`
`III III IIIIIIIIII III 11IIIIIIIII
`US006190512B1
`US 6,190,512 Bl
`Feb. 20, 2001
`
`(io) Patent No.:
`(45) Date of Patent:
`
`(54) SOFT PLASMA IGNITION IN PLASMA
`PROCESSING CHAMBERS
`
`(75)
`
`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) Appl. No.: 08/117,443
`
`Sep. 7, 1993
`
`(22) Filed:
`Int. CI.7
`C23C 14/34
`(51)
`204/192.12; 204/192.13
`(52) U.S. CI
`(58) 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
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,557,819 * 12/1985 Meacham et al
`4,888,088 * 12/1989 Slomowitz
`5,288,971 * 2/1994 Knipp
`
`204/298
`156/643
`204/298.08 X
`
`FOREIGN PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`Mashiro Kazuhiko, "Discharge Triggering Method of Sput(cid:173)
`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(cid:173)
`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,
`L.L.P.
`
`(57)
`
`ABSTRACT
`
`Herron & Evans,
`
`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(cid:173)
`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
`
`VOLTAGE CTRL
`
`42. 'A JP
`
`ZL
`sz.
`SECONDARY ^ - F M-
`DC POWER
`SUPPLY
`
`PROCESS GAS
`ENABLE^
`
`o - "*
`
`3# £ j&
`
`H<H
`
`J£?
`
`V
`
`DC POWER
`DRIVE
`
`PLASMA DC
`POWER
`SUPPLY
`
`TARGET
`(CATHODE)
`
`I WAFER
`
`BACK PLANE
`
`V fit
`
`2?
`
`#7
`
`RF
`NETWORK
`
`BIASING RF
`POWER
`SUPPLY
`
`GILLETTE 1013
`
`

`

`U . S. P a t e nt
`
`Feb. 20,2001
`
`Sheet 1 of 3
`
`US 6,190,512 Bl
`
`Z?
`
`PLASMA DC
`POWER
`SUPPLY
`
`TARGET
`(CATHODE)
`
`fi?
`
`WAFER
`
`BACK PLANE
`
`/£
`
`RF
`NETWORK
`
`BIASING RF
`POWER
`SUPPLY
`
`£?
`
`<&?
`
`FIG. 1
`
`FIG. 2
`
`FIG. 3
`
`

`

`U.S. Patent
`US. Patent
`
`Feb.20,2001
`Feb. 20, 2001
`
`Sheet 2 of 3
`Sheet 2 0f 3
`
`US 6,190,512 Bl
`US 6,190,512 B1
`
`e.0.“—
`o
`
`mo<._.._o>
`
`.SQm
`
`6
`
`GO
`
`d w.0.“—
`
`uo<50>him
`
`\m/(rm.
`
`
`___________._______1____.._1\____4_____,_/::::_::_E_E_:Eigzgi::::_:___:::f
`.
`
`

`

`U.S. Patent
`
`Feb. 20,2001
`
`Sheet 3 of 3
`
`US 6,190,512 Bl
`
`VOLTAGE CTRL
`
`JS7
`L^l
`SECONDARY K+AAAr
`DC POWER
`SUPPLY
`
`PROCESS GAS
`ENABLE,,,
`
`&7
`
`H^
`
`DC POWER
`DRIVE
`
`PLASMA DC
`POWER
`SUPPLY
`
`TARGET
`(CATHODE)
`
`/4
`
`WAFER
`
`|
`
`BACK PLANE
`
`£?
`
`^7
`
`RF
`NETWORK
`
`BIASING RF
`POWER
`SUPPLY
`
`FIG. 5
`
`PROCESS GAS ENABLE COMMAMD
`
`4&
`
`J:
`
`GAS FLOW / PRESSURE
`
`\
`
`40
`
`JZ
`
`CATHODE DC NEGATIVE VOL.
`
`DC POWER DRIVE I
`
`•*! I-
`
`— &?\~- DC DRIVE
`ON DELAY
`
`&?
`-O
`
`5?
`-tZ
`
`GAS OFF
`
`DELAY.^ FIG. 6
`
`I
`
`

`

`US 6,190,512 Bl
`
`1
`SOFT P L A S MA IGNITION IN PLASMA
`PROCESSING CHAMBERS
`
`B A C K G R O U ND OF THE INVENTION
`
`5
`
`r
`
`a t i ng g as
`
`flow
`
`55
`
`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 invention relates to reduction of device damage in
`this threshold voltage, unstable gas discharges, as well as
`plasma processes,
`including DC
`(magnetron or non-
`related transitions between gas and plasma phases, occur in
`magnetron) sputtering, and RF sputtering.
`chamber 14. (Similar effects have been observed in gaseous-
`A typical plasma processing apparatus is shown in FIG. 1.
`The apparatus includes a plasma power supply 10, which 10 discharge tubes.) As a result, the gas-plasma system begins
`drives a cathode or target 12 to a large DC voltage (e.g.,
`unstable oscillation, producing brief, but very large magni-
`- 4 00 Volts) relative to the walls of vacuum chamber 14. The
`t u de v o l t age perturbations. This oscillation continues until
`semiconductor substrate 16 (also known as the wafer) rests
`t he
`threshold voltage 30 is achieved, at which point the
`on a back plane 18 inside the chamber. The back plane may
`gas/plasma mixture fully transitions to a plasma, and oscil-
`l at1 C ) ns cease.
`
`be driven by radio frequency (RF) AC voltage signals, 15
`produced by an RF power supply 20, which drives the back
`Voltage 30 will be referred to as the "oscillation threshold
`plane through a compensating network 22.
`voltage". The value of the oscillation threshold voltage will
`•
`ti
`TT- A^
`J/
`T->^
`i-
`depend on the target (cathode) material, process gas and
`Ine AC and/or DC power supplies generate a plasma in
`r
`,
`,
`,
`-
`,,
`.
`.
`r ,
`,
`.,
`c
`J u *
`tu
`c
`J
`ti.
`pressure, chamber geometry, electrical characteristics of the
`,,
`the area above the water and between the water and the
`,'
`.°
`;
`•, ,
`,
`,
`.
`,,
`,
`. ,
`,
`.,
`,,
`• , c
`external power wiring, and possibly the volt-ampere curve
`target, and cause material from the target to deposit on the 20
`.
`.
`.
`.
`..
`r
`=1
`^ jk sputtering chamber,
`%
`water surface.
`r
`&
`,
`.
`,
`,
`.
`. „
`,
`.
`,
`Based on the preceding observations, the spike observed
`, ^^
`A typical DC power supply 10 includes a relatively
`~,
`r ™^
`, .
`,
`.
`, ,
`,
`.., ..
`.
`, .
`.
`,
`,
`,
`.
`,
`.
`.
`/
`in region 26 of FIG. 3 is now understood to be an oscillation
`sophisticated control system, designed to permit operation in
`,
`,
`.,
`.
`•
`.
`,
`1 A,.
`c
`u
`r
`,
`,
`caused when the output voltage of primary supply 10 lingers
`constant power, constant voltage, or constant current modes.
`,,
`.
`,
`.,
`,
`.
`,,
`.,,
`..
`.
`u
`.
`r
`,
`, • ,
`„, at a v o l t a ge
`just b e l ow
`the oscillation
`t h r e s h o l d.
`.
`.
`.
`•
`,
`,
`,
`, ,
`rn
`1 his control circuitry includes a damped control loop which, 25
`c 1 •
`cnn 1 1
`.,
`r
`*•
`-to
`c
`
`,
`r
`„
`•
`Furthermore, careful inspection of region 28 01 FIG. 3 also
`,
`,
`,
`,
`.
`.
`when the supply is engaged, produces a controlled ramping
`, •
`.
`,
`,,
`, . ,
`.
`.,, .
`,
`.
`,
`1
`1 r.
`reveals oscillatory behaviors analogous
`to those which
`,
`,
`.
`1
`•
`toward the desired output level. For example, as shown in
`r ™^
`.
`- TO
`~„
`.,, .
`/rr,
`..
`-
`r
`•
`appear in region 28 of FIG. 4. (1 he oscillations in region 28
`, T^^
`,
`T n l^
`FIG. 2, upon engagement of a typical DC power supply in
`•» J
`•
`,
`n
`* 1,
`u
`*i.
`,r
`' ^
`&, &
`.Jr,
`. TnT^
`, . ,
`have smaller magnitudes, in part because when the power
`/
`an apparatus as shown m FIG. 1, the supply current (which
`,.
`f ..
`,
`,
`, ..
`,
`,
`.
`,,
`,
`.,
`.,,
`rr
`,
`on supply is disabled, its output voltage drops relatively rapidly,
`1
`i
`r •
`•
`r
`r
`represents the density of ionic transfer from the target due to
`JU
`u
`»u
`1 •
`ui J •*
`* * 1*
`r
`r
`.
`&
`. .
`J,
`„
`whereas when the power supply is enabled its output voltage
`,
`,
`sputter deposition on the water) ramps up to a constant value
`, ..
`.
`,
`,
`,
`\
`,
`• 1
`-*A
`increases relatively slowly.)
`.
`,,
`11
`1
`, .
`,
`.,
`.
`...
`in a controlled manner with a small overshoot 24.
`r
`,
`,
`.
`,
`T
`,
`It has been found that the oscillation spike observed in
`target/cathode
`F IG 3 c an be e l i m i n a t ed by
`eie v a ting
`t he
`t h r e s h oid v oit a ge before initi-
`v o l t a ge a b o ve
`t he o s c i l l a t i on
`i n to t he c h a m b e r; a nd maintaining the cathode
`t he o s cil l a ti0n threshold until processing is
`v o l t a ge a b o ve
`c o mp l e t e d, gas flow is halted, and vacuum is restored. TTiis
`technique prevents overvoltage during processing, and
`therefore can reduce device damage and particulate con-
`tamination
`tju
`1
`•
`•
`. - . u-
`i
`.
`,
`T
`. c
`In brief summary, this technique is implemented by a
`.
`..
`,
`.
`..
`,
`..
`power supply circuit comprising two power supply sections:
`?
`i
`V,-,
`.
`.
`1
`u- u
`an essentially conventional primary power supply, which
`45 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
`Overvoltage in the processing chamber deteriorates the
`power supply "pre-ignites" the plasma so that when the
`quality of sputtered films in several ways: High voltage
`is applied,
`the system smoothly
`events electrically damage layers and/or devices on the 50 primary power supply
`processing substrate (wafer). Furthermore, arcing which can
`transitions to final plasma development and deposition. This
`be produced by overvoltages can cause local overheating of
`design thereby avoids oscillations when the primary power
`the target, leading to evaporation or flaking of target material
`supply is engaged and disengaged, and any corresponding
`into the processing chamber and causing substrate particle
`device damage.
`contamination and device damage. These sources of wafer
`In pr efe r red embodiments, a current limiting resistor,
`damage become increasingly significant as integrated cir-
`switch, and diode are connected
`in series between
`the
`cuits reach higher densities and become more complex.
`secondary power supply and the cathode.
`Thus, it is advantageous to avoid voltage spikes during
`T he
`c u r r e nt
`l i m i t i ng
`r e s i s t or
`l i m i ts
`t he c u r r e nt
`flowing
`from t l le s e c 0n d a ry power supply into the cathode. Only a
`processing wherever possible.
`With this in mind, careful analysis has revealed that the go minimal current is needed to elevate the cathode voltage
`so-called break down spike is not, in fact, an isolated event
`above the oscillation threshold and pre-ignite the plasma; by
`necessarily associated with the creation of a plasma in the
`interposing a current-limiting resistor, the secondary power
`chamber. The spike is not caused by the creation of a plasma
`supply current is held at this minimum level, thus avoiding
`per se, but rather by harmonic oscillations within the cham-
`the need for a high power secondary supply, and also
`ber-
`65 limiting the plasma current and deposition while the see-
`in FIG. 4, a gas-filled chamber generates
`As shown
`ondary power supply is enabled and the primary supply is
`sizable oscillations when driven by a DC voltage within a
`disabled.
`
`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
`.
`v
`„
`,
`.
`.
`.
`„
`.
`„,
`,
`.
`,
`plasma ignition ). Furthermore, a large magnitude break
`,r
`.,
`,
`,
`•
`down spike has been seen as necessary to improve process
`^
`J
`f
`r
`^'
`
`^
`
`SUMMARY OF THE INVENTION
`
`

`

`US 6,190,512 Bl
`
`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, 5
`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 10
`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 15
`chamber), gas/plasma transition oscillations are substan(cid:173)
`tially reduced.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`25
`
`The above and further aspects of the invention will be 20
`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 35
`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.
`
`4Q
`
`DESCRIPTION OF THE PREFERRED
`E M B O D I M E N TS
`
`Referring to FIG. 5, in one embodiment of a plasma
`processing apparatus in accordance with the invention, a 45
`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.
`
`60
`
`65
`
`For example, FIG. 4 w as generated from experiments
`using a deposition chamber 14 and a high purity aluminum
`
`alloy target 12 sold by Materials Research Corporation of
`Orangeburg, N.Y. under the trademarks "ECLIPSE M 3" 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-lOkW". Mea(cid:173)
`surements were recorded by a thermal chart recorder, run(cid:173)
`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 w as sputtering purity
`argon, at a pressure of 10 mT (at 100 seem) and a flow rate
`of 50-150 seem. DC power supply 10 was set to output 6-10
`kW of power. The wafer temperature w as approximately
`300° C. and the deposition rate w as 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 - 3 00 Volts, so the output voltage of secondary
`DC supply 32 w as set at - 3 00 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(cid:173)
`sition on wafer 16.
`
`To end processing, primary supply 10 is disabled, reduc(cid:173)
`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 3 2, 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(cid:173)
`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 Bl
`
`15
`
`35
`
`10
`
`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 5
`generate substantial sputtering current, supply 10 must pro(cid:173)
`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(cid:173)
`able plasma power supplies are designed to detect arcing in
`the chamber during processing, and to automatically sus(cid:173)
`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 20
`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(cid:173)
`tain plasma ignition, and prevent oscillation when the pri(cid:173)
`mary supply power is restarted. Diode 38 similarly prevents 25
`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 3Q
`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(cid:173)
`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 40
`(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 45
`power, the magnitude of the cathode voltage (trace 50)
`increases slightly, causing plasma current flow and deposi(cid:173)
`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, 5Q
`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 60
`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 65
`cathode voltage generated by the FIG. 5 design shows no
`visible oscillations at the beginning and end of processing
`
`(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(cid:173)
`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(cid:173)
`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(cid:173)
`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(cid:173)
`tude of the voltage of said cathode relative to said chamber
`exceeds the magnitude of said process initiation voltage.
`
`