Europaisches Patentamt
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`European Patent Office
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`Office europeen des brevets
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`I lllllll llllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`0 421 430 A2
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`@ Publication number:
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`®
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`EUROPEAN PATENT APPLICATION
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`@ Application number: 90119065.2
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`@ Int. Cl.5; H01J 37/32, H05H 1/46
`
`@ Date of filing: 04.10.90
`
`@) Priority: 03.10.89 US 416750
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`@ Date of publication of application:
`10.04.91 Bulletin 91/15
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`@ Designated Contracting States:
`BE DE ES FR GB IT NL
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`@ Applicant: APPLIED MATERIALS, INC.
`P.O. Box 58039 3050 Bowers Avenue
`Santa Clara California 95052(US)
`
`@ Inventor: Mintz, Donald M.
`1633 S. Mary Ave.
`Sunnyvale, CA 94087(US)
`
`@ A plasma process, method and apparatus.
`
`@) A plasma process method and apparatus ca(cid:173)
`pable of operation significantly above 13.56 MHz can
`produce reduced self-bias voltage of the powered
`electrode to enable softer processes that do not
`damage thin layers that are increasingly becoming
`common in high speed and high density integrated
`circuits. A nonconventional match network is used to
`enable elimination of reflections at these higher fre(cid:173)
`quencies. Automatic control of match network com(cid:173)
`ponents enables the rf frequency to be adjusted to
`ignite the plasma and then to operate at a variable
`frequency selected to minimize process time without
`significant damage to the integrated circuit.
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`....
`N
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`0.. w
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`Inventor: Hanawa, Hiroji
`3427 Flora Vista
`Santa Clara, CA 95051 (US)
`Inventor: Somekh, Sasson
`25625 Moody Road
`Los Altos Hills, CA 94022(US)
`Inventor: Maydan, Dan
`12000 Murietta Lane
`Los Altos Hills, CA 94022(US)
`
`@ Representative: Diehl, Hermann Dr.
`Diehl & Glaeser, Hiltl & Partner
`FIUggenstrasse 13
`W-8000 MUnchen 19(DE)
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`a -
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`·
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`Igniting Plasma
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`b
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`Maintaining Pressure
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`a2
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`Adjusting Match Network
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`c
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`Providing Power
`At Optimized Frequency
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`Figure 3
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`Xerox Copy Centre
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`Samsung Exhibit 1012
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`EP 0 421 430 A2
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`A PLASMA PROCESS, METHOD AND APPARATUS
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`This invention relates in general to semicon(cid:173)
`ductor wafer processing method and apparatus.
`Plasma etching of wafers is attractive because
`it can be anisotropic, can be chemically selective,
`can produce etch under conditions far from ther(cid:173)
`modynamic equilibrium, utilizes a reduced amount
`of etchant chemicals compared to wet etch pro(cid:173)
`cesses and produces a significantly
`reduced
`amount of waste products. The reduction of etchant
`chemicals and waste products produces a cost
`savings. The anisotropic etch enables the produc(cid:173)
`tion of substantially vertical sidewalls which is im(cid:173)
`portant in present day processes in which the
`depth of etch and feature size and spacing are all
`comparable. The ability to choose etch chemicals
`and process parameters to produce chemical se(cid:173)
`lectivity of the etch enables these choices to be
`made to etch the desired material without substan(cid:173)
`tially etching other features of the integrated cir(cid:173)
`cuits being produced. Choices of process param(cid:173)
`eters that produce process conditions far from ther(cid:173)
`modynamic equilibrium can be used to lower pro(cid:173)
`cess temperature, thereby avoiding high tempera(cid:173)
`tures that can deleteriously affect the integrated
`circuits under fabrication.
`In Figure 1 is shown a plasma reactor 10. This
`reactor includes an aluminum wall 11 that encloses
`a plasma reactor chamber 12. Wall 11 is grounded
`and functions as one of the plasma electrodes.
`Gases are supplied to chamber 12 from a gas
`source 13 and are exhausted by an exhaust sys(cid:173)
`tem 14 that actively pumps gases out of the reac(cid:173)
`tor to maintain a low pressure suitable for a plasma
`process. An rf power supply 15 provides power to
`a second (powered) electrode 16 to generate a
`plasma within chamber 12. Wafers 17 are trans(cid:173)
`ferred into and out of reactor chamber 12 through a
`port such as slit valve 18.
`A plasma consists of two qualitatively different
`regions: the substantially neutral, conductive plas(cid:173)
`ma body 19 and a boundary layer 110 called the
`plasma sheath. The plasma body consists of sub(cid:173)
`stantially equal densities of negative and positive
`charged particles as well as radicals and stable
`neutral particles. The plasma sheath is an electron
`deficient, poorly conductive region in which the
`electric field strength is large. The plasma sheath
`forms between the plasma body and any interface
`such as the walls and electrodes of the plasma
`reactor chamber and the rf electrodes.
`Semiconductor process plasmas are produced
`by a radio frequency (rf) field at 13.56 MHz that
`couples energy into free electrons within the cham(cid:173)
`ber, imparting sufficient energy to many of these
`electrons that ions can be produced through colli-
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`sions of these electrons with gas molecules. Typi(cid:173)
`cally, the walls of the reactor chamber are metal
`(though often coated with thin insulating layers) so
`that they can function as one of the rf electrodes.
`s When the walls do not function as one of the
`electrodes, they still affect the process by confining
`the plasma and by contributing capacitive coupling
`to the plasma reactor.
`The 13.56 MHz frequency is substantially uni-
`versally utilized in plasma reactors because this
`frequency is an ISM (Industry, Scientific, Medical)
`standard frequency for which the government man(cid:173)
`dated radiation limits are less stringent than at non(cid:173)
`ISM frequencies, particularly those within the com·
`15 munication bands. This substantial universal use of
`13.56 MHz
`is further encouraged by the
`large
`amount of equipment available at that frequency
`because of this ISM standard. Other ISM standard
`frequencies are at 27.12 and 40.68 MHz, which are
`first and second order harmonics of the 13.56 MHz
`ISM standard frequency. A further advantage of the
`13.56 MHz frequency is that, since the lowest two
`order harmonies of this frequency are also ISM
`standard
`frequencies, equipment utilizing 13.56
`25 MHz is less likely to exceed allowable limits at
`harmonies of the fundamental frequency of such
`equipment.
`When the powered rf electrode is capacitively
`coupled to the rf power source, a de self bias Vdc
`of this electrode results. The magnitude of this self
`bias V de is a function of the ion density and elec(cid:173)
`tron temperature within the plasma. A negative self·
`bias de voltage Vdc of the powered electrode on
`the order of several hundreds of volts is commonly
`produced (see, for example, J. Coburn and E. Kay,
`Positive-ion bombardment of substrates in rt
`diode glow discharge sputtering, J. Appl. Phys.,
`43 p. 4965 (1972). This self bias de voltage V de is
`useful in producing a high energy flux of positive
`ions against the powered electrode. Therefore, in a
`plasma etching process, a wafer 17 to be etched is
`positioned on or slightly above the powered elec(cid:173)
`trode 16 so that this flux of positive ions is incident
`substantially perpendicular to the top surface of the
`wafer, thereby producing substantially vertical etch(cid:173)
`ing of unprotected regions of the wafer.
`These high voltages enable etch rates that are
`required for the etch process to be commercially
`attractive. Because of the susceptibility of the small
`(submicron) geometry devices available today to
`damage by a small amount of particulates, in(cid:173)
`tegrated circuit (IC) process systems are available
`that enable several IC process steps to be ex(cid:173)
`ecuted before reexposing the wafer to ambient
`atmosphere (see, for example, the multichamber
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`EP 0 421 430 A2
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`system illustrated in U.S. patent 4,785,962 entitled
`Vacuum Chamber Slit Valve, issued to Masato
`Tashima on November 22, 1988). This small geom(cid:173)
`etry has also produced a trend toward single wafer
`process steps (as opposed to multiwafer process(cid:173)
`ing steps that are common in
`larger geometry
`devices) so that processing can be sufficiently uni(cid:173)
`form over the entire wafer that these small geome(cid:173)
`try features can be produced throughout the wafer.
`Because the wafer throughput of the system is
`limited to the throughput of the slowest of the
`series of process steps within such a system, it is
`important that none of these sequential steps take
`significantly longer than the other steps in the
`process or else such slow step will serve as a
`bottleneck to system throughput. Presently, typical
`system throughput is on the order of 60 wafers per
`hour. For example, the· fundamental etch prior to
`metal 2 deposition is performed at a rate equivalent
`to a 250 Angstroms • per minute silicon dioxide
`etch rate. This permits the removal of approxi(cid:173)
`mately 70 Angstroms of aluminum oxide in con(cid:173)
`tacts to aluminum metal 1 in approximately 40
`seconds using a nonselective argon-only process.
`These etch conditions are used routinely in wafer
`fabrication and produce a 1500-1600 volt self bias
`at the powered electrode.
`Transistor speed specifications and high device
`densities in the most modern MOS integrated cir(cid:173)
`cuits have required the use of shallow junctions
`and thin gate oxides. Unfortunately, such IC struc(cid:173)
`tures are sensitive to ion bombardment by high
`energy ions such as those utilized with conven(cid:173)
`tional 13.56 MHz plasma etch apparatus. There(cid:173)
`fore, it is advantageous in such IC processing to
`reduce the self-bias voltage of the powered elec(cid:173)
`trode to less than 500 volts negative self-bias using
`a nonselective argon-only process. Because wafer
`damage decreases with decreasing self-bias volt(cid:173)
`age, it would be even more advantageous to op(cid:173)
`erate at self-bias voltages closer to 300 V. Unfortu(cid:173)
`nately, at 13.56 MHz, this reduction of self-bias
`results in a much slower etch rate, which thereby
`significantly degrades process throughput.
`One solution has been to enhance the etch rate
`by use of magnets that produce containment fields
`that trap ions within the vicinity of the wafer, there(cid:173)
`by increasing the ion density at the wafer. The
`magnetic field confines energetic ions and elec(cid:173)
`trons by forcing them to spiral along helical orbits
`about the magnetic field lines. Such increased ion
`density at the wafer produces a concomitant in(cid:173)
`crease in etch rate without increasing the self bias
`potential, thereby enabling throughputs on the or(cid:173)
`der of 60 wafers per hour without damaging the
`wafers.
`In effect, the etch rate is preserved by
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`* 1 Angstrom = .1 NM.
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`increasing the current level to counter the de(cid:173)
`creased voltage drop across the plasma sheath at
`the wafer. Unfortunately, nonuniformities of the
`magnetic field of such "magnetically enhanced"
`plasma etching systems exhibit a decreased uni(cid:173)
`formity of etch rate over the surface of the wafer.
`To improve uniformity over the surface of the
`wafer, in one such system, the wafer is rotated
`about an axis that is perpendicular to and centered
`over the surface of the powered electrode. This
`produces at the wafer surface a time-averaged field
`improved
`that has cylindrical symmetry and an
`uniformity over the wafer and therefore produces
`increased etch uniformity over the surface of the
`wafer. However, this rotation produces within the
`plasma chamber undesirable mechanical motion
`that can produce particulates and increase con(cid:173)
`tamination. Alternatively, a rotating magnetic field
`can be produced by use of two magnetic coils
`driven by currents that are ninety degrees out of
`phase. Unfortunately, the controls and power sup(cid:173)
`plies for this scheme are relatively expensive and
`the etch uniformity is still not as good as in a
`plasma etch apparatus that does not include such
`25 magnets.
`Another solution to enhance the rate of plasma
`processing of wafers is the recently developed
`technique of electron cyclotron resonance. This
`technique has application to wafer cleaning, etching
`and deposition processes. In this technique, a plas(cid:173)
`ma is produced by use of a microwave source and
`a magnetic containment structure. Unfortunately,
`this technique, when applied to etching or chemical
`vapor deposition, exhibits poor radial uniformity
`and low throughput. In addition, it requires expen(cid:173)
`sive hardware that includes; (1) a complex vacuum
`pumping system; (2) a microwave power supply
`that must produce microwave power at an ex(cid:173)
`tremely accurate frequency and power level; (3) a
`large magnetic containment system that may in·
`clude large electromagnets; and (4) an rf or de
`power supply connected to the wafer electrode.
`The invention provides a high frequency semi(cid:173)
`conductor wafer processing method and apparatus
`and relates more particularly to the use of a plasma
`apparatus for wafer cleaning, wafer deposition and
`wafer etching.
`The invention provides a plasma process meth·
`od according to independent claim 1 and a plasma
`process apparatus according to independent claim
`14. Further advantageous features of the method
`and apparatus are evident from
`the dependent
`claims.
`the description and
`the drawings. The
`claims are intended to be a first non-limiting ap-
`proach of defining the invention in general terms.
`In a conventional plasma reactor, an
`igniter
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`EP 0 421 430 A2
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`electrode produces, within a low pressure gas, high
`energy electrons that have enough energy to ionize
`within the reactor chamber atoms and molecules
`struck by these high energy electrons. This results
`in a cascade of electrons that produce a plasma
`consisting of electrons, ions, radicals and stable
`neutral particles. The plasma is then maintained by
`a powered electrode of voltage lower than that of
`the igniter electrode. Sufficient rf power is coupled
`into the plasma to maintain a desired ion con(cid:173)
`centration, typically on the order of 108 -1011 cm-3.
`Typical the frequency of the rf power is in the
`range from 10 kHz to 30 MHz but the most com(cid:173)
`mon frequency is 13.56 MHz because this is high
`enough to produce reasonable ion concentrations
`and is an ISM (industry, scientific, medical) stan(cid:173)
`dard frequency that does not interfere with tele(cid:173)
`communications.
`Because the electrons are on the order of
`thousands to hundreds of thousands of times light(cid:173)
`er than the plasma ions, the electrons experience a
`proportionately greater acceleration than the ions
`and therefore acquire kinetic energies that are sig(cid:173)
`nificantly greater than those acquired by the ions.
`The effect of this is that the energy from the rf field
`is strongly coupled into electron kinetic energy and
`is only very weakly coupled into ion kinetic energy.
`These high energy electrons are also referred to as
`high temperature electrons. As a further result of
`this large mass difference between the electrons
`and the ions, collisions between the high energy
`electrons and the ions does not transfer much of
`the electron energy to the ions. The effect of this is
`that the electrons acquire a temperature that is
`typically on the order of 1-5 ev even though the
`other particles in the plasma remain substantially at
`the temperature of the walls of the plasma reactor
`chamber (on the order of 0.03 ev) or up· to a few
`hundred degrees Centigrade hotter.
`Because the electrons are much more mobile
`than the ions, they initially strike the walls of the
`reactor chamber at a greater rate than do the ions.
`The effect of this is that the plasma body becomes
`slightly electron deficient while the boundary layer
`sheath becomes substantially electron deficient.
`This also produces a positively charged layer at
`the interface between the plasma body and the
`plasma sheath. This net positive charge of the
`plasma body and boundary layer results in a plas(cid:173)
`ma body electrical potential Vp (usually called the
`"plasma potential") on the order of several times
`the electron mean kinetic energy divided by the
`electron charge. The potential in the bulk of the
`plasma is nearly constant while the largest part of
`the potential variation is across the sheaf. In an rf
`plasma, this sheath potential variation is also de(cid:173)
`pendent on various parameters including the area
`of the reactor chamber wall, the area of the power-
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`ed electrode, the pressure in the reactor chamber
`and the rf power input.
`In accordance with the illustrated preferred em(cid:173)
`bodiment, a plasma apparatus is presented that
`operates at frequencies higher than the 13.56 MHz
`frequency utilized in conventional plasma appara(cid:173)
`tus. Useful frequencies for plasma processing have
`been found to range from 30 to 200 MHz. These
`processes include wafer cleaning, chemical vapor
`deposition and plasma etching. The frequency that
`is selected is dependent on which of these pro(cid:173)
`cesses is being utilized which in turn determines
`the required choice of plasma density and
`ion
`bombardment energy.
`In the case of a nonreactive etch process, the
`lower frequency limit is controlled by the maximum
`self bias voltage that can be used without damag(cid:173)
`ing the integrated circuit under fabrication The up(cid:173)
`per frequency limit is controlled by the minimum
`self bias voltage (about 150 volts for a nonreactive
`etch process and 50 volts for a reactive ion etch
`process) that produces sufficient energy to etch the
`wafer. As a practical matter, when this etch process
`is used in a serial wafer fabrication system, this
`upper limit is further reduced by the requirement
`that such etching be achievable within a period that
`is short enough that this etching step does not
`create a bottleneck to fabrication throughput. This
`range of frequencies has required modification of
`the match network that prevents reflections of rf
`power at the transition from the 50 ohm char(cid:173)
`acteristic impedance of the rf transmission line to
`the much lower impedance of the plasma reactor
`chamber. The plasma is generated by detuning the
`35 match network to make the powered electrode
`function as an igniter and then is tuned to reduce
`the self-bias voltage to a level appropriate for etch(cid:173)
`ing wafers without damage to the wafers.
`In the case of plasma wafer cleaning, the fre-
`quency is chosen to produce a high current density
`at voltages that do not etch the wafer or implant
`ions into the wafer. In the case of plasma enhanced
`chemical vapor deposition, the bombardment volt(cid:173)
`age and current should be compatible with good
`deposition uniformity, high film purity and the ap-
`propriate level of film stress.
`Figure 1 illustrates the structure of a typical
`plasma reactor.
`Figure 2 illustrates a match network suitable for
`coupling rf power at frequencies significantly
`above 13.56 MHz to a plasma reactor.
`Figure 3 is a flow diagram of a process utilizing
`the plasma reactor and match network of figures
`1 and 2, respectively.
`In the figures, the first digit of a reference
`numeral will indicate the first figure which is pre(cid:173)
`sented the element indicated by that reference
`numeral.
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`EP 0 421 430 A2
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`pressure should be on the order of 4 mTorr. A
`reactive ion etch normally employs fluorine-contain(cid:173)
`ing or chlorine-containing gases. For example, the
`reactive ion etch containing 10 mole percent Nf3
`or 5 mole percent BCb, the power should be on
`the order of 10-50 Watts and the pressure should
`be on the order of 10-40 mTorr. •
`For a cleaning process, the pressure is typi(cid:173)
`cally selected in the range from 1 to 40 milliTorr
`and the self-bias voltage is selected in the range
`from 5 to 300 volts. Preferably, the pressure is on
`the order of 5 milliTorr and the self-bias voltage is
`on the order of 15 volts. To achieve these param(cid:173)
`eter values, the frequency should be selected in
`the range from 100 to 200 MHz. Particularly useful
`gases for wafer cleaning are pure argon, hydrogen
`and gas mixtures that include a fluorine-containing
`gas.
`The frequency can also be selected to op-
`timize various plasma enhanced chemical vapor
`deposition processes. For example, for that of sili(cid:173)
`con dioxide, the total process pressure can range
`from 0.5 to 20 milliTorr. The optimum pressure is
`on the order of 5 milliTorr. The self-bias voltage is
`typically in the range from 1 O to 400 volts and is
`preferrably selected to be on the order of 150 volts.
`To achieve these parameter values, the frequency
`should be selected in the range from 100 to 200
`MHz. Particularly useful gases
`for plasma en-
`hanced chemical vapor deposition are argon, silane
`and TEOS.
`A pair of electromagnetic coils 114 and 115
`and associated power supply 116 are included to
`produce a weak magnetic field that deflects plasma
`ions away from the walls of the plasma reactor.
`This is important to avoid contamination of the
`wafer during processing. Unlike in the prior art, it is
`not necessary that these fields be uniform at the
`surface of the wafer because they are too weak (on
`the order of 1-20 Gauss at the surface of the wafer)
`to significantly affect process uniformity. However,
`this range of magnetic field is sufficient to prevent
`plasma ions from impacting the walls with sufficient
`energy to desorb contaminants from those walls.
`A match network is used to couple power from
`the 50 ohm impedance rt power line to the much
`lower impedance plasma without producing a sig(cid:173)
`nificant amount of reflected power at the match
`network. For the frequencies significantly above
`13.56 MHz (i.e., on the order of or greater than 40
`MHz}, the conventional match network design can·
`not be used.
`
`The fundamental nonreactive etch prior to met(cid:173)
`al 2 deposition typically is executed at an etch rate
`the order of 250 Angstroms per minute
`on
`(equivalent silicon dioxide etch rate) in order to
`complete this etch step within about 40 seconds.
`At this rate, this etch step duration plus system
`overhead to transfer wafers into and out of the
`reactor meets a system throughput requirement of
`60 wafers per hour. Many IC circuit designs today
`contain layers that can be damaged by bombard(cid:173)
`ment by high energy ions. To avoid such damage
`while retaining an etch time on the order of 40
`seconds, the plasma is generated by an rt field of
`frequency higher than the 13.56 ISM (industry,
`scientific, medical} frequency that is conventionally
`utilized.
`Because it is important to maintain system
`throughput, as the frequency was varied in these
`tests, the power was adjusted to achieve a 250
`Angstroms per minute equivalent silicon dioxide
`etch rate For such etch rate, the negative self bias
`voltage was measured to be just over 500 volts for
`a 40 MHz frequency and was measured to be
`approximately 310 volts for a 60 MHz frequency.
`This confirms that soft etches (i.e., etches with self
`bias on the order of or less than 500 volts) with
`acceptable throughput can be produced at these
`frequencies. The self-bias voltage for comparable
`powers at 137 MHz is -125 volts. Therefore, it
`appears that a useful range of frequencies for
`achieving a soft etch that etches the wafer without
`damaging thin layers extends from 30 MHz to 200
`MHz. Frequencies above 137 MHz are particularly
`useful for wafer cleaning and plasma enhanced
`chemical vapor deposition. The preferred choice of
`frequency is in the range 50-70 MHz because this
`produces the required etch rate under very soft
`conditions (self-bias near -300 V). For a frequency f
`in the range 13.56 MHz < f < 200 MHz and an rf
`power that produces a 250 Angstroms per minute
`equivalent silicon dioxide etch rate, the etch uni(cid:173)
`formity is comparable to the uniformity exhibited
`by conventional 13.56 MHz nonmagnetized pro(cid:173)
`cesses. For a nonreactive etch, the gas pressure is
`selected in the range from 1 to 20 milliTorr.
`A soft etch process that uses high frequency rf
`power is useful in both nonreactive, nonselective
`etch processes and in reactive ion etch processes.
`The power is generally higher and the pressure is
`generally lower for the nonreactive ion processes.
`For example, for an etch with argon ions, the power
`should be on the order of 300 Watts and the
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`•1 Torr = t.333 mbar.
`== 0.00133 mbar.
`1 mTorr
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`• 1 ft = .305 meters.
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`At rf frequencies, the wavelength of the signals
`becomes small enough that phase variation of sig·
`nals over the lengths of cables can produce signifi·
`cant interference effects. In this frequency range,
`cables should be substantially equal to an integral
`number of quarter wavelengths. In particular, cable
`111 from rf power supply 15 to match network 112
`and cable 113 from match network 112 to powered
`electrode 16 should each be substantially equal to
`an
`integral number of quarter wavelengths. By
`"substantially equal to" is meant that this length is
`equal to an integral number of quarter wavelengths
`plus or minus 0.05 quarter wavelength. This re(cid:173)
`quirement is easily met at 13.56 MHz where a
`quarter wavelength is on the order of 15 feet, * so a
`small cutting error of the length of such cable will
`not be significant. However, at 60 MHz a quarter
`wavelength is about 3 feet so that cable length
`errors are proportionately 5 times as significant.
`The addition of a single extra connector or circuit
`element can violate this cable length criterion
`In addition, at these frequencies, the discrete
`components, such as interdigitated blade capaci(cid:173)
`tors and multiple coil
`inductors, conventionally
`used in the match network for a 13.56 MHz plasma
`apparatus are unsuitable for use at the higher fre·
`quencies. The inductances of such discrete com(cid:173)
`ponents are too large for frequencies on the order
`of or greater than 40 MHz. In the chosen range of
`frequencies, an inductor can be a single strip of
`conductor. Likewise, the multiblade interdigitated
`blade capacitors of 13.56 MHz systems are re(cid:173)
`placed by a simple pair of parallel conductive
`plates spaced by a nonconductor such as a teflon
`sheet.
`Figure 2 illustrates a match network design that
`can be used at frequencies above 40 MHz. A
`ground 20 is connected to the outer conductor of a
`located in a wall 22 of match
`first rf connector 21
`network 112. A conductor 23 electrically connects
`the inner conductor of connector 21 to a first plate
`24 of an input capacitor 27. This capacitor also
`contains a second plate 26 and a dielectric spacer
`25. Plate 26 is electrically connected to a first plate
`28 of a shunt capacitor 211 that also contains a
`dielectric spacer 29 and a second plate 210. Plate
`28 is also connected to a first plate 212 of a third
`capacitor 215, that also includes a dielectric spacer
`213 and a second plate 214. Plate 214 connects
`through an inductor 216 to rf electrode 16.
`To permit tuning of this match network at a
`given frequency in the range 40-100 MHz, capaci(cid:173)
`tors 27 and 211 are variable capacitors. In this
`embodiment, these capacitances are varied by
`variation of the. spacing between plates 24 and 26
`and between the spacing between plates 28 and
`21 O. Variation of these spacings is achieved by
`means of a motor 221 connected by a rotary-to-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`linear-displacement coupling 222 and a motor 223
`connected by a rotary-to-linear-displacement cou(cid:173)
`pling 224. An automated control circuit 225 auto·
`matically adjusts these two capacitances to mini-
`5 mize the amount of power reflected at rf coupler
`21. To enable such adjustment, a detector 226,
`connected between rf power supply 15 and rf con(cid:173)
`nector 21 provides to control circuit 225 information
`about the relative phase between the current and
`voltage components of the rf power input; and the
`ratio of the magnitudes of the current and voltage
`components of the rf power input signal. Control
`circuit 225 is a conventional feedback control cir(cid:173)
`cuit that adjusts the plate spacings of capacitors 27
`and 211 until the relative phase and ratio of mag(cid:173)
`nitudes of
`the
`rf current and voltage signals
`reaches preset values that are selected to produce
`substantially zero reflection of power back toward
`the rf power source. Typically, for a tuned match
`network, this system will produce less than 10
`Watts of reflected power from a 300 Watt input
`signal.
`For operation over the range of rf frequencies
`from 40-100 MHz, components 27, 211, 215 and
`216 should have the values 10-100 pf, 50-400 pf,
`100 pf and 0.5 /.LH, respectively.
`The variable control of the capacitors enables
`electrode 16 to function as an igniter electrode to
`generate a plasma as well as the powered elec-
`trode to maintain the plasma. When it is utilized as
`the igniter electrode, the feedback control of ca(cid:173)
`pacitors 27 and 211
`is
`inactivated and
`these
`capacitances are set respectively to 100 pf and
`400 pf. This produces at electrode 16 an electric
`field strength which is sufficiently large to produce
`a cascade of electrons that ignites the plasma.
`After ignition and tuning, the self-bias voltage on
`the powered electrode is on the order of -300 volts
`for 300 Watts of power at 60 MHz.
`Because this plasma apparatus can be op·
`erated over a range of frequencies, the chosen
`frequency will not in general be one of the ISM
`(industrial, scientific, medical) frequencies. There·
`fore, rf gasketing is used in all vacuum flanges of
`reactor 10, no windows into chamber 12 are al(cid:173)
`lowed, and all elongated openings in the chamber
`are eliminated or shielded so that rf radiation from
`reactor 10 is eliminated to an extent that reduces rf
`emissions below the United States governmentally
`allowed level of 15 /.L Vim at a distance of 300 m
`from the apparatus. This avoids interference with
`TV and other rf communication near such reactors.
`Although tests have indicated that a faster etch
`without significant damage to the wafer is achieved
`at 60 MHz than at 40 MHz, the frequency of 40.68
`MHz is an attractive choice because it is an ISM
`standard frequency with reduced radiation limits.
`Harmonics of this frequency are still a problem, but
`
`35
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`40
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`45
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`50
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`55
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`6
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`Page 6 of 11
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`11
`
`EP 0 421 430 A2
`
`12
`
`the power in these harmonics is generally substan(cid:173)
`tially less than at the fundamental.
`
`Claims
`
`1. A plasma process method comprising the steps
`of:
`
`(a) igniting a plasma within a reactor chamber in
`a plasma reactor.
`(b) maintaining within said reactor chamber a
`gas pressure in the range 0.5 • 30 milliTorr • ;
`and
`(c) providing rf power to a powered electrode at
`which a wafer is placed for processing within the
`reactor chamber, said power being supplied at a
`power level P sufficient to maintain a plasma
`within the reactor chamber and at a frequency f
`> 13.56 MHz that is selected to optimize pro·
`cessing of the wafer.
`2. A method as in claim 1 wherein the frequency f
`is in the range 30·200 MHz.
`3. A method as in claim 1 or 2 wherein the fre(cid:173)
`quency f is in the range 50-70 MHz, whereby this
`method is particularly suitable for a soft plasma
`etch process.
`4. A method as in claim 1 or 2 wherein the fre(cid:173)
`quency f is greater than 137 MHz, whereby this
`method is particularly suitable for plasma cleaning
`and plasma enhanced chemical vapor deposition.
`5. A method as in one of claims 1 to 3 wherein, in
`step (b), the power P and frequency f are selected
`to produce a powered electrode negative self-bias
`that is less than 350 volts, whereby this method is
`particularly suitable for a soft plasma etch process.
`6. A method as in claim 5 wherein the power and
`frequency are selected to produce at the powered
`electrode a negative self-bias voltage greater than
`150 volts, whereby this method is particularly suit·
`able for a nonreactive soft etch process.
`7. A method as in claim 5 wherein the power and
`frequency are selected to produce at the powered
`electrode a negative self-bias voltage greater than
`50 volts, whereby this method is particularly suit·
`able for a soft reactive ion etch process.
`8. A method as in one of claims 1 to 7, wherein in
`step (a}, rf power is supplied to said power elec(cid:173)
`trode through a match network that is detuned to
`produce a sufficiently