`
`(cid:44)(cid:49)(cid:55)(cid:40)(cid:47)(cid:3)EXHIBIT 100(cid:26)
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`US. 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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`12
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`14
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`'
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`DC
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`SUPPLY +
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`I
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`REACTIVE
`GAS
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`IG.1
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`IGNITION
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`SUPER
`GLOW
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`ARC
`DISCHARGE
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`CURRENT DENSITY
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`PASSIONE CURVE
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`FIG.m
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`VOLTAGE
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`Page 2 of 12
`Page 2 of 12
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`US. Patent
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`Jul. 29, 1997
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`Sheet 2 0f 4
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`5,651,865
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`Page 3 of 12
`Page 3 of12
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`US. Patent
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`Jul. 29, 1997
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`Sheet 3 0f 4
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`5,651,865
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`"VAPP
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`TIME —~+
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`|G.9
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`Prior Art
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`Page 4 of 12
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`5,651,865
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`1
`PREFERENTIAL SPUTTERING OF
`INSULATORS FROM CONDUCTIVE
`TARGETS
`
`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
`fieed 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 A1203 land on the surface of the aluminum target.
`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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`sputtering occurs. Here. shutting 01f 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 are 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.
`Ditferent materials require different voltages to be applied
`to the targets to efiect 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 (A1203) 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 etfective in removing the redeposit or in
`preventing it. None of these approaches sputters oil 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.
`OBIECI‘S AND SUMMARY OF THE
`lNVENTION
`
`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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`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. A dc
`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 011‘ in the anode. and the positive argon ions are
`accelerated towards the cathode, that is, towards the con-
`ductive target. The argon ions knock atoms 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 areactive 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 A1203. Si02, 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—
`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 40 KHz to 100 KHz with a pulse width of 1 usec to 3 psec.
`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 of 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 other insulative 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
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`ensuing description of a preferred embodiment, which
`should be read in 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
`dc 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.
`
`DETAILED 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 anoflrer 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 filmrequires ignition of the plasma
`22. The plasma is created by applying a voltage to the space
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`5,651,865
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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.. Ar+ 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 Ar+ 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 eifect.
`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 Ar+ ions onto the target. Therefore a high
`deposition rate requires higher current flow.
`levels of
`Different
`target materials require different
`applied voltage. For example, a gold atom is significantly
`heavier than an aluminum atom. and therefore requires a
`much more energetic ion to knock it 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 a lower limit at which plasma
`exists. but no sputtering takes place because the ions are not
`sufliciently energetic. Each process will have a lower limit
`on sputtering voltage depending on the target material.
`Orrrent 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
`curve.
`
`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. Le. 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 argon ions 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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`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 VA". 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 02 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 argon ions pick up
`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 A1203. 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 A1203 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 are 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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`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 +VAPP 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 VAPP. 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 VAPP. 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 crn've of FIG. 2, in the glow and
`superglow regions the plasma 22 has a positive resistance
`and exhibits stable behavior. The are discharge region is
`entered whenever the current density, even for a local area,
`exceeds the superglow limit. The are 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 are conditions from
`recurring.
`'
`Modern power supplies for dc sputtering often incorpo—
`rate arc detecting circuitry which shuts the current oif when
`an arc is detected The crnrent must remain oif 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 OR 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.
`
`8
`This system can be understood with reference to FIGS.
`8A. 8B and 9. As shown in FIGS. 8A and SB, 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 221) of the
`capacitor. The voltage applied to the target VT can be a
`pulsed dc voltage alternating between the applied potential
`VAPP 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 k‘ilohertz, 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 eifect 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 +VREV are inserted onto the negative dc
`sputtering voltage —VAPP. The effect of this is to bring the
`target voltage VT above ground potential to the reverse bias
`Voltage +VREV. This lifts the capacitor voltage VCAP, 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 dc. The positive reverse voltage has
`excursions of smaller magnitude than the negative sputtering
`voltage, e.g., +VREV=+150 volts vs. —VAPP=—300 to —700
`volts. As also shown, the pulse duty cycle is quite small, that
`is, the pulse width of the positive reverse voltage +VREV 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 —VAPP
`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 —VREV.
`For a practical example of a reactive sputtering system,
`e.g., A1 to produce A1203, the applied voltage VAPP can be
`—-400 vdc, and the reverse bias voltage can be +150 volts. In
`this case, the voltage VCAP 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 argon ions towards the
`accumulations of A1203 on the target. Because of the greater
`potential. i.e. 550 volts, the ions that bombard the A1203 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.
`
`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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`45
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`50
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`55
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`5.651.865
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`9
`The reverse bias pulses are also effective in removing
`impurities and inclusions from the target 16. This technique
`can also be employed to prepare or “condition” an aluminum
`target which has been exposed to the atmosphere. In a
`standard aluminum reactive sputtering technique. a separate
`conditioning phase is required. and this can damage the
`aluminum target due to pitting and stressing. This precon-
`ditioning stress damage can actually produce arcing during
`a subsequent sputtering process. On the other hand. the
`reverse bias pulsing technique of this invention sputters oflc
`oxides and impurities with little or no damage to the target
`and without arcing.
`A principal benefit of this technique is that it removes
`sources for arcing. rather than simply interrupting or limit—
`ing arcing. This technique permits a reactive sputtering
`operation to run as close as possible to the high end of the
`superglow region. which maximizes deposition rates with-
`out risk of encountering arc discharge conditions.
`In a preferred mode. the reverse bias pulse can be pro-
`vided at 150 volts. at period of 2 user: and a pulse repetition
`rate of 50 KHz. The duty cycle should be kept small so that
`sputtering is not unduly interrupted The pulse period should
`be of sufficient length to allow the dielectric layer the reverse
`biased. and this period can be between 0.25 usec and 3 usec.
`The pulse repetition rate can be between 40 KHz and 200
`KHz. The positive reverse bias voltage is tailored to the
`particular sputtering operation. as is the negative applied
`voltage. That is the applied voltage VA” can be between
`—300 and -—700 volts, and the reverse bias voltage can be
`between +50 and +300 volts.
`
`A practical embodiment of a reactive dc sputtering
`arrangement, with a reverse bias pulsing feature of this
`invention. is shown in FIG. 12. Here the chamber 12 has the
`target 16 coupled to a negative terminal of the power supply
`14 and its anode 18 coupled to the positive terminal of the
`power supply 14. The substrate 24 is situated within the
`plasma chamber; argon or another suitable sputtering gas in
`introduced through a gas c