United States Patent (19)
`Sellers
`
`US005651865A
`Patent Number:
`Date of Patent:
`
`11
`45
`
`5,651865
`Jul. 29, 1997
`
`54 PREFERENTIAL SPUTTERING OF
`INSULATORS FROM CONDUCTIVE
`TARGETS
`75 Inventor: Jeff C. Sellers, Palmyra, N.Y.
`
`73) Assignee: ENI, Rochester, N.Y.
`
`21 Appl. No.: 261988
`22 Filled:
`Jun. 17, 1994
`
`56
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`5,300,205 4/1994 Fritsche .......................... 204/298.08 X
`5,303,139 4/1994 Mark .............................. 204/298.08 X
`5,427,669 6/1995 Drummond ........................ 204/298.08
`P
`FOREIGN PATENT DOCUMENTS
`0.591675 8/1993 European Pat. Off..
`0564789A1 10/1993 European Pat. Off..
`0 636 702 7/1994 European Pat. Off..
`WO87/05053 8/1987 WTPO
`Primary Examiner-Nam Nguyen
`Attorney, Agent, or Firm-Trapani & Moldrem
`[51] Int. Cl. ...................... C23C 14/34
`57
`ABSTRACT
`52 U.S. Cl. ............................... 204/192.13; 204/192.12;
`204/298.03; 204/298.08
`Pulses of positive voltage are applied to the target of a dc
`58 Field of Search ......................... 204/192.12, 192.13,
`Sputtering process to create a reverse bias. This charges
`204/298.03, 298.08
`insulating deposits on the target to the reverse bias level, so
`that when negative sputtering voltage is reapplied to the
`References Cited
`target, the deposits will be preferentially sputtered away. The
`reverse bias pulses are provided at a low duty cycle, i.e.,
`U.S. PATENT DOCUMENTS
`with a pulse width of 0.25-3 microseconds at a pulse rate of
`4,103,324 7/1978 vandervelden et al. ... 204298.08 x about 40-100 KHZ. This technique reduces sources for
`4,936,960
`6/1990 Siefkes et al. ................. 204/298.41 X arcing during a reactive sputtering process.
`5,241,152 8/1993 Anderson et al. ............. 204/298.08 X
`5286,360 2/1994 Szczyrbrowski et al. ......... 204/298.08
`
`5 Claims, 4 Drawing Sheets
`
`24
`12
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`3
`
`DC
`POWER
`SUPPLY
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`--
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`40- v99
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`50
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`PULSE CONTROL
`WIDTH & RATE
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`WACUUM
`PUMP
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`Page 1 of 12
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`APPLIED MATERIALS EXHIBIT 1007
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`U.S. Patent
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`Jul. 29, 1997
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`Sheet 1 of 4
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`5,651,865
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`14
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`DC
`SUPPLY
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`G.1
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`REACTIVE
`GAS
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`
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`VOLTAGE
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`- IGNITION
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`SUPER
`GLOW
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`- " - ARC
`DISCHARGE
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`CURRENT DENSTY
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`PASSIONE CURVE
`FIG.2
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`Page 2 of 12
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`U.S. Patent
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`Jul. 29, 1997
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`Sheet 2 of 4
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`5,651,865
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`ANODE
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`18
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`TARGET (AI)
`FIG.3
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`28 Y
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`N N
`2
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`Page 3 of 12
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`U.S. Patent
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`Jul. 29, 1997
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`Sheet 3 of 4
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`5,651,865
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`-WAPP
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`TIME -b-
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`IG.9
`Prior Art
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`Page 4 of 12
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`U.S. Patent
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`Jul. 29, 1997
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`Sheet 4 of 4
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`5,651,865
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`Page 5 of 12
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`5,651,865
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`1.
`PREFERENTAL SPUTTERNG OF
`NSULATORS FROM CONDUCTIVE
`TARGETS
`
`2
`sputtering occurs. Here, shutting off the dc power serves to
`extinguish early arcing. This means that unipolar pulses of
`power at a fixed duty cycle are fed to the target. This does
`have the beneficial effect of permitting charge to build up
`only partially across the dielectric deposition on the target,
`so that arcing is less likely to occur, and also can lead to a
`small amount of resputtering of the deposition. However,
`this system while slowing down the rate of insulating
`deposit on the target, does not reverse the deposition.
`Another system previously proposed is called a low
`energy small package arc repression circuit. In that system,
`an electronic switch cycles at a rate of about 2 KHz to cut
`out the current to the target. This in effect reverses the
`voltage on the target to several volts positive, and draws
`some electrons from the plasma to the front surface of the
`insulative deposition. This neutralizes the anions on the front
`surface of the deposition, to discharge the voltage buildup
`across the layer, thereby greatly reducing the occurrence of
`dielectric breakdown and arcing. Also, discharge of the front
`surface of the insulative layer lowers the surface potential to
`approximately that of the target. Discharging of the dielec
`tric deposition also permits the argon ions in the plasma to
`collide with the insulating dielectric material. This does
`result in some resputtering of the molecules of the deposited
`material, thus slowing the rate of deposit on the target.
`However, this approach does not resputter the molecules
`of the deposited compound as effectively as the atoms of the
`target material, and so this approach has not been totally
`effective in removing deposits from the target during reac
`tive sputtering processes.
`Different materials require different voltages to be applied
`to the targets to effect sputtering. For example, because a
`gold atom is a much heavier atom than an aluminum atom,
`it requires a much more energetic ion to free it from the
`target. Typically, in a process that employs an aluminum
`target, the applied voltage needed is about-450 volts. while
`in a similar process that employs a gold target the applied
`voltage has to be about -700 volts.
`Considering that an aluminum oxide (Al2O3) molecule is
`significantly heavier than an aluminum atom, one can under
`stand that a higher potential would be required to energize
`the argon ion enough to resputter the coating. This is, of
`course, true for other materials as well.
`Another approach to solve this problem involves a pair of
`sputtering targets, with one serving as cathode and the other
`as the anode. The applied electrical voltage is periodically
`reversed so that the sputtering occurs first from the one
`target and then from the other. This process also reverses the
`charge on the deposited insulating material as well, which
`reduces the possibility of arcing and also resputters some of
`the insulating material on the targets. However, this
`arrangement, requiring plural targets, can be cumbersome
`and expensive to employ.
`These previous solutions, which employ unipolar pulsing
`or alternately cycled targets, have been effective in reducing
`voltage stress on insulating films redeposited on the targets,
`but have not been effective in removing the redeposit or in
`preventing it. None of these approaches sputters off the
`insulator from the beginning before it has a chance to
`accumulate, and none of these techniques has been effective
`in eliminating or halting the redeposition of insulating film
`on the target.
`OBJECTS AND SUMMARY OF THE
`INVENTION
`It is an object of this invention to enhance reactive
`sputtering in a fashion which avoids accumulations of
`insulating deposition on the conductive target.
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`BACKGROUND OF THE INVENTION
`This invention relates to the deposition of thin films and
`is more concerned with a reactive sputtering process
`wherein atoms of a target material are freed from a conduc
`tive target and are reacted with a reactive gas to form a
`coating which is deposited on a substrate. This process can
`be employed, for example, in creating dielectric insulating
`layers on electrical parts, or wear-resistant layers on
`mechanical parts.
`The invention is more specifically directed to a dc sput
`tering process in which the dielectric coating material that
`becomes lodged on the conductive target is removed, thus
`avoiding a major cause of arcing.
`Sputtering is a vacuum deposition process in which a
`sputtering target is bombarded with ions, typically an ion
`ized noble gas, and the atoms of the target material are
`mechanically freed by momentum transfer. The target mate
`rial then coats a nearby substrate.
`In a reactive sputtering process a reactive gas is intro
`duced into the deposition chamber, and the freed target
`material reacts with the reactive gas to form a coating
`material. For example, the target material can be aluminum,
`and oxygen can be introduced as the reactive gas to produce
`a coating of aluminum oxide. A carbonaceous gas, e.g.
`acetylene can be used as the reactive gas to produce carbide
`coatings such as SiC., WC, etc., and ammonia can be
`introduced to produce a nitride coating such as TiN. In any
`event, the conductive target atoms and the reactive gas react
`in plasma in the chamber to produce the compound that
`serves as a coating. In a typical example, aluminum atoms
`freed from an aluminum target enter plasma of argon and
`oxygen to produce a deposition of aluminum oxide.
`DC sputtering is a random process and the insulating
`coating material is deposited on all available surfaces. This
`means that not only does the insulating material coat the
`article in question, but it also coats other surfaces in the
`chamber as well, including the target. Thus, in a reactive
`sputtering process for depositing aluminum oxide, mol
`ecules of Al2O land on the surface of the aluminum target.
`45
`This deposition of an insulator on the target causes severe
`problems, including a reduction of sputtering rate and a
`propensity to induce arcing.
`Contamination of the target can also result, even in
`conventional dc sputtering, due to atmospheric gases, water
`droplets, inclusions, and other contaminants. Each of these
`can be a source for arcing, and their presence will also
`reduce the deposition rate over time because of a reduced
`active sputtering area on the target. Accordingly, these
`problems necessitate a frequent cleaning of the target sur
`face.
`This problem has been known to exist for some time, but
`its causes have not been completely appreciated. Procedures
`to deal with these problems, such as arc control in reactive
`sputtering have not been completely satisfactory.
`A standard approach involves sensing the presence of
`arcing, and then interrupting current flow. This will control
`the arcing, but does nothing about the insulating coating that
`continues to cover the target.
`One early attempt to deal with arcing in a blind fashion
`involves periodically interrupting the current flow between
`the dc power supply and the plasma generator in which the
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`3
`It is another object to carry out reactive sputtering under
`conditions that eliminate sources of arcing and which maxi
`mizes the rate at which sputtering can take place.
`According to an aspect of the present invention, a reactive
`dc sputtering process is carried out in a plasma chamber by
`applying an electric potential to a conductive target so that
`target material is sputtered from the target and is reacted
`with a reactive gas that is introduced into the chamber. Adc
`voltage of a suitable level e.g. -500 volts, is applied from a
`power supply to the sputtering target, which serves as
`cathode, with a conductive surface in the plasma chamber
`being held at ground to serve as anode. A noble gas, e.g.,
`argon, present in the chamber is ionized and creates a
`plasma, e.g., argon anions and free electrons. The electrons
`are drawn off in the anode, and the positive argon ions are
`accelerated towards the cathode, that is, towards the con
`ductive target. The argonions knockatoms of target material
`free from the target by momentum transfer. The argon ions
`pick up electrons from the negatively charged target, and
`migrate back to the plasma. The freed target atoms enter the
`plasma and react with a reactive gas that has been introduced
`into the chamber. The reactive gas can be, for example,
`oxygen, borane, acetylene, ammonia, silane, arsene, or vari
`ous other gases. The reaction product is deposited on a
`substrate positioned adjacent the plasma. The substrate can
`be a masked semiconductor wafer on which a compound
`such as a Al2O3, SiO2, or another insulator or dielectric is to
`be deposited. In some processes, the substrate can be a drill
`bit, wear plate, valve spindle, or other mechanical part on
`which a wear resistant coating, such as WC or TiN, is
`deposited.
`As mentioned before, the reaction product of the reactive
`sputtering process is deposited randomly, and not only coats
`the workpiece substrate, but also coats other surfaces includ
`ing walls of the chamber and the sputtering target. Accu
`35
`mulation of the insulating coating can induce arcing, and
`also reduces the available area of the sputtering target,
`thereby reducing the sputtering rate over time.
`In this invention the negative dc voltage that is applied to
`the target is interrupted periodically by reverse-bias pulses
`of a positive dc voltage relative to the anode. Preferably, the
`reverse bias pulses are at a level of 50 to 300 volts above
`ground potential, and these are applied at a pulse frequency
`of 40KHz to 100 KHz with a pulse width of 1 usec to 3 usec.
`This results in low duty cycle pulses (about 10% or less).
`The reverse bias creates a reversal of the charge across the
`insulating material. These accumulations behave as a
`capacitor, with the conductive target being one plate and the
`conductive plasma being the other plate. The reverse voltage
`is applied long enough (e.g. 2 usec) for the polarity of the
`capacitive charge to be reversed, up to -300 volts, on the
`plasma side of the deposition.
`Then when the normal or negative sputtering voltage is
`again applied, the argon ions in the plasma are accelerated
`preferably towards the reverse-charged dielectric material.
`These ions are accelerated to an increased energy due to the
`additional potential difference. As a result, the molecules of
`the deposition are re-sputtered off the target. This process
`keeps the target clear of depositions, and thereby eliminates
`the main sources of arcing. This process also permits the
`active sputtering surface of the target to remain as large as
`possible.
`This process also cleans out otherinsulative contaminants
`from the surface of the target whether used for reactive
`sputtering or conventional sputtering.
`The above and many other objects, features, and advan
`tages of this invention will become more apparent from the
`
`4
`ensuing description of a preferred embodiment, which
`should be readin conjunction with the accompanying Draw
`ing.
`
`BRIEF DESCRIPTION OF THE DRAWING
`FIG. 1 is a schematic view of a dc sputtering arrangement
`for explaining the principles of reactive dc sputtering.
`FIG. 2 is a Passione curve of voltage versus current
`density, showing the various regions of plasma behavior in
`a sputtering operation.
`FIGS. 3 to 5 are schematic views for explaining the
`adverse effects of film accumulations on the sputtering
`target.
`FIGS. 6 and 7 are schematic representations of an ideal
`sputtering process and one in which an arc discharge con
`dition occurs.
`FIGS. 8A and 8B are schematic views illustrating equiva
`lency of an insulative film accumulation on the sputtering
`target with a capacitor.
`FIG. 9 is a chart showing a pulse waveform for unipolar
`dic sputtering according to the prior art.
`FIG. 10 is a chart showing a pulse waveform for low duty
`cycle reverse bias dc sputtering, according to this invention.
`FIG. 11 is a waveform chart showing the behavior of
`electric potential at the plasma-facing surface of an insulator
`film accumulation on the conductive target, according to an
`embodiment of this invention.
`FIG. 12 is a schematic view of an arrangement for
`practicing reactive dc sputtering according to the principles
`of this invention.
`
`DETALED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`With reference to the Drawing and initially to FIG. 1, a dc
`sputtering arrangement 10 employs a plasma chamber 12
`and a regulated dc power supply 14. The plasma chamber is
`evacuated by a vacuum pump (not shown) and is supplied
`with a controlled amount of a noble gas, typically argon. A
`conductive target 16 is located within the chamber 12 and
`serves as a cathode. The target 16 is connected to a negative
`terminal of the power supply 14. The target 16 can be a flat
`plate of aluminum, tungsten, silicon or another conductive
`material. An anode 18 disposed within the chamber is
`coupled to the positive terminal of the power supply 14. The
`anode can have a relatively large Surface area with respect
`to the target 16. A gas conduit 20 brings a reactive gas at a
`controlled rate into the chamber to react with atoms of the
`target material that become freed from the target. This
`reaction produces a reaction product that is deposited as a
`film on surfaces within the chamber. A plasma 22 is formed
`in the noble gas in a zone between the anode 18 and the
`target 16, and a substrate 24 to be coated is positioned in the
`chamber adjacent the plasma 22. The substrate is passive in
`this process, and can be any part or element on which a
`coating is desired. For example, the substrate can be a steel
`mechanical part, such as a bearing, on which the reactive
`sputtering process deposits a layer of titanium nitride as a
`hard wear coating. As another example, the substrate can be
`a semiconductor wafer on which a dielectric layer such as
`aluminum oxide or nitride is deposited.
`The sputtering arrangement can also incorporate suitable
`means for removing heat from the target.
`Actual deposition of a film requiresignition of the plasma
`22. The plasma is created by applying a voltage to the space
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`between the anode 18 and the cathode or target 16 high
`enough to cause ionization of at least a small portion of the
`Sputtering gas, i.e., the argon.
`The intense electric field strips outer electrons from the
`gas atoms, creating anions, e.g., Art and electrons. This
`leads to a steady state which is fed by a steady state applied
`current from the dc power supply 14.
`The positive ions Art are accelerated by the steady state
`field towards the target 16 or cathode, where they recombine
`with electrons in the target. To achieve a sputtering effect,
`the ions are made energetic enough so that their kinetic
`energy will knock atoms of the target material off the target
`16 when the ions collide with it. The freed target atoms then
`enter the plasma and react with the reactive gas that is
`introduced into the chamber. The product of this reaction
`deposits itself on any available surface, and preferably on
`the substrate 24. The reaction product also redeposits on the
`target 16, as well as other surfaces in the plasma chamber.
`Accumulations of insulating deposit on the target can lead to
`arcing and other problems.
`Control of the sputtering process requires control of
`voltage, current flow, and current density.
`Current flow is required for sputtering, and all else being
`equal, higher current produces proportionately more colli
`sions of the Art ions onto the target. Therefore a high
`deposition rate requires higher current flow.
`Different target materials require different levels of
`applied voltage. For example, a gold atom is significantly
`heavier than an aluminum atom, and therefore requires a
`much more energetic ion to knockit free from the target. In
`a typical dc sputtering process that can use either a gold
`target or an aluminum target, a voltage of about 450 volts is
`required for aluminum, whereas a much higher 700 volts is
`required for gold. There is thus allower limitat which plasma
`exists, but no sputtering takes place because the ions are not
`Sufficiently energetic. Each process will have a lower limit
`On Sputtering Voltage depending on the target material.
`Current density is an important factor and has to be
`controlled. In the sputtering process it is desirable to maxi
`mize the deposition rate but it is also imperative to avoid arc
`discharges.
`The upper limit of deposition rate occurs when the current
`density of the plasma is in a so-called superglow region. This
`can be explained with reference to FIG. 2, which shows the
`relation of current density in plasma to the voltage drop
`across the plasma. This curve is the so-called Passione
`CWC.
`In an initial region shown at the extreme left side of FIG.
`2, before the plasma is ignited there, is a zero current density
`and the entire voltage necessary for ignition appears across
`the gaseous medium. For low current densities, there is a
`negative-resistance region indicated as the ignition region,
`where the voltage across the plasma drops with increased
`current density, until a generally flat region, i.e. a glow
`region, is reached. As current density increases, a superglow
`region is reached. Here the plasma becomes more energetic
`and thus more resistive to the movement of argonions across
`it; hence the Voltage increases across the plasma. Beyond
`this region the current density becomes so high that the
`plasma self-heats the ions to the point that the plasma
`generates thermal electrons and photons which then cause
`runaway ionization. This drastically increases the number of
`ions, i.e., the number of charge carriers in the plasma. The
`plasma impedance drops, and current flow increases. This
`produces a sharp negative resistance region leading to an
`arcing region where the impedance is very small and current
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`6
`is limited only by the output impedance of the power source.
`The arcing that is produced here creates deleterious effects
`on the target, such as pitting, flaking and cracking, as well
`as localities of extreme heat. Moreover, as soon as arcing
`occurs, sputtering stops.
`Therefore, in a sputtering process, a number of factors
`come into play: deposition rates are directly related to
`current flow; applied voltages are determined by the char
`acteristics of target materials; and arc discharging directly
`follows from excess current density conditions.
`The problems that arise from redeposition of insulator
`reaction products onto the target 16 can be explained with
`reference to FIGS. 3, 4, and 5. As shown in FIG. 3, the
`potential difference between the positive anode 18 and the
`negative target 16 produces electric field lines from the
`anode to the target. An applied voltage War, typically on
`the order of about 300 to 500 volts dc, is applied from the
`dc power supply 14 to the anode 18 and target 16. In this
`example, the target is made of aluminum, the plasma 22 is
`formed of argon ions Ar", and oxygen O is introduced as a
`reactive gas. Initially, the electric field lines are distributed
`more or less uniformly over the surface of the target, so that
`the argon ions bombard the target 16 over its entire surface.
`Wherever the argon ion impacts, aluminum atoms are
`knocked free by momentum transfer. The argonions pickup
`electrons from the negative target 16, and electrons in the
`plasma are absorbed into the anode 18.
`The freed aluminum atoms enter the plasma 22 and react
`with oxygen in the plasma, forming alumina Al-O. The
`alumina molecule is neutral and non-conducting, and depos
`its itself on any convenient surface which it contacts. Some
`of the alumina lands on the target 16 and produces as a
`deposition an insulating layer 26, as shown in FIG. 4.
`Because the insulating deposition 26 is a dielectric, i.e., an
`insulator, the Surface that faces the plasma, that is the upper
`surface in FIG. 4, accumulates a positive change. This
`deflects the electric field lines around the deposition 26. The
`ions tend to follow the field lines and thus are accelerated
`towards the uncoated aluminum areas of the target. The
`argon ions collide with and sputter aluminum, but do not
`resputter the Al-O material. Consequently, the alumina
`continues to coat the exposed surface of the target, while the
`relative area of aluminum metal that is still available for
`sputtering shrinks. The result is to increase the current
`density while reducing the overall current flow, thus reduc
`ing the deposition rate. Eventually nearly the entire surface
`of the target 16 becomes coated, and only a small area of the
`target is uncovered and available for sputtering. This causes
`the electric field lines to converge on localized spots as
`shown in FIG. 5. Ions impact these spots at a high current
`density, leading to arc discharge.
`The adverse effects of insulative redeposit on the target
`can be explained with reference to FIGS. 7 and 8. Under
`glow or superglow conditions, the plasma 22 can be con
`sidered to function as thousands of high-value resistances
`connected in parallel, with leads positioned uniformly over
`the face of the target 16. In other words, the plasma can be
`considered as several thousand-megohm resistors as shown
`in FIG. 6. As the coating of insulating material develops on
`the target, the available area on the target is diminished,
`ultimately to Small uncoated gaps 28. The current density at
`the gaps 28 enters the negative resistance zone of the
`Passione curve (FIG. 2) and the current path becomes
`restricted. The effective resistance drops to a very small
`value, e.g., 0.01 ohm, as illustrated in FIG. 7. Under these
`conditions, arcing occurs.
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`7
`The above-described problem comes about because the
`insulating film 26 forms a dielectric, sandwiched between
`the electrodes formed by the conductive plasma 22 and the
`conductive target 16, respectively. The top surface, i.e., the
`plasma side of the insulating film 26, charges up to the
`applied voltage +V because of charge received from the
`positively charged argon ions.
`Because of electrostatic repulsion between the positively
`charged ions, the argon ions Ar" are steered away from the
`film-coated areas of the target toward the remaining
`uncoated metal areas. This raises the current density on
`those areas. Therefore the total current flow has to be
`reduced to keep the current density within the superglow
`range and this of course reduces the deposition rate. Also,
`because the argon ions are steered away from the coated
`areas of the target 16, there is little resputtering of the
`insulating film 26. Consequently, the redeposition of the
`insulating film continues unabated, until the sputtering pro
`cess has to be terminated.
`The Zone of insulating film can be considered a capacitor
`to which a dc voltage is applied. The insulating film 26 on
`the target 16 charges up to the applied voltage V. when
`the capacitor is charged, no further current can flow, i.e., the
`area covered by the insulating film is unavailable for sput
`tering. Plasma current is directed to the remaining areas and
`the current density increases to the remaining uncovered
`gaps 28 on the target. Eventually, the current density will
`creep up to the arc discharge region of the Passione curve
`(FIG. 2) which establishes a limit for reactive dc sputtering.
`Another problem that can arise from the accumulation of
`the dielectric layer 26 is that the dielectric layer may not be
`strong enough to tolerate the entire applied voltage V. In
`that case, the dielectric layer will punch through and fail.
`When this happens the failure region will be flooded with
`charge causing a radical increase in local conductance, and
`a consequent increase in local current density. This can
`produce local arcing, and can bring the entire plasma 22 to
`an arc discharge condition.
`Returning to the Passione curve of FIG. 2, in the glow and
`superglow regions the plasma 22 has a positive resistance
`and exhibits stable behavior. The arc discharge region is
`entered whenever the current density, even for a local area,
`exceeds the Superglow limit. The arc discharge behaves like
`a short from anode to target, and exhibits a negative resis
`tance so the more that current flows, the lower the resistance
`becomes. To stop the arc discharge, the current density in the
`plasma has to drop to the point where it crosses back to the
`glow or Superglow regions.
`The most reliable way to do this is to remove all current
`flow as quickly as possible. However, when the current flow
`resumes, there is nothing to prevent the arc conditions from
`recurring.
`Modern power supplies for dc sputtering often incorpo
`rate arc detecting circuitry which shuts the current off when
`an arc is detected. The current must remain off long enough
`for the arc current to decay. This can reduce effective net
`power to the plasma chamber 12.
`One attempt to deal with this problem of arc prevention
`has been the introduction of unipolar pulsing. This technique
`involves shutting off the power supply, creating pulses at a
`fixed rate and duty cycle. This produces alternate application
`of negative and ground voltage to the target, which allows
`the positively charged dielectric material of the layer 26 to
`be partly charged, and then partly discharged around a bias
`point. This reduces the voltage stress on the insulating layer,
`and does permit some re-sputtering of the insulating layer.
`
`45
`
`50
`
`55
`
`65
`
`5,651,865
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`8
`This system can be understood with reference to FIGS.
`8A, 8B and 9. As shown in FIGS. 8A and 8B, the plasma
`chamber can be considered as a capacitor circuit in which
`the target 16 serves as one electrode or plate, the deposition
`of insulating material 26 serves as the capacitor's dielectric
`layer, and the plasma 22 serves as a series resistance 22a and
`also as the other (positive) electrode or plate 22b of the
`capacitor. The voltage applied to the target V can be a
`pulsed dc voltage alternating between the applied potential
`V and ground potential, i.e., 0 volts. The anode 18 is
`considered to be at ground potential. In this prior-art uni
`polar pulsed power system, the upper plate 22b of the
`capacitor, i.e., the dielectric layer 26 is permitted to decay
`back towards target potential. In FIG. 8B, the voltage at the
`upper capacitor plate is represented as Vcap. Typically, the
`applied voltage is interrupted for periods of 10 microseconds
`or more with a pulse rate of about 2 kilohertz, i.e., a pulse
`period of 500 microseconds. This technique is intended to
`prevent high-voltage build-up on the dielectric or insulating
`layer 26, but does not actually remove the positive charges.
`Consequently, even though some resputtering of the layer
`may occur from ion bombardment, the net effect is simply
`to slow the rate at which the insulting layer accumulates on
`the target.
`The technique of this invention can be explained with
`reference to FIGS. 10 and 11. In this technique, pulses 30 of
`reverse bias voltage --V
`are inserted onto the negative dc
`sputtering voltage -V. The effect of this is to bring the
`target voltage V above ground potential to the reverse bias
`voltage +V. This lifts the capacitor voltage Voa, i.e. the
`potential of the upper surface of the dielectric layer 26 to a
`high potential so that the layer 26 absorbs free electrons
`from the plasma 22 until it achieves ground potential, i.e.,
`corresponding to the potential of the anode 18. This decay is
`shown at curves 32 in FIG. 11.
`As shown in FIG. 10, the reverse pulses create asymmet
`ric bipolar pulsed dic. The positive reverse voltage has
`excursions of smaller magnitude than the negative sputtering
`voltage, e.g., +V-150 volts vs. -VA-300 to -700
`volts. As also shown, the pulse duty cycle is quite Small, that
`is, the pulse width of the positive reverse voltage +V is
`Small relative to its period, e.g., about 2 usec.
`Then the applied negative voltage is resumed at the end of
`the pulse. This brings the lower electrode of the capacitor,
`i.e., the target 16 to the negative voltage -V.
`The upper, or plasma-facing surface of the dielectric
`deposition layer 26 is then brought to a very negative
`potential, more negative than the target potential by the
`amount -V.
`For a practical example of a reactive sputtering system,
`e.g., Al to produce Al2O3, the applied voltage V can be
`-400 vac, and the reverse bias voltage can be +150 volts. In
`this case, the voltage V, which appears on the upper
`surface of the layer 26 follows the voltage wave form of
`FIG. 11. At the onset of applications of -400 volts to the
`target to the target 16, the dielectric voltage drops to -550
`volts. This is more negative than the target potential, and
`tends to attract the positively charged argonions towards the
`accumulations of Al2O on the target. Because of the greater
`potential, i.e. 550 volts, the ions that bombard the Al-O are
`more energetic than those that strike the uncoated target.
`These more energetic ions are able to free the relatively
`heavy oxide molecules from the insulating layer 26, and
`these molecules deposit themselves elsewhere. Thus, the
`reverse bias pulses serve to clean the target of the insulating
`deposition, so that sputtering can continue unabated until the
`target is consumed.
`
`Page 9 of 12
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`O
`
`15
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`25
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`30
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`35
`
`20
`
`The reverse bias pulses are also effective in re