Plasma Sources Sci. Technol. 1 (1992) 109-116. Printed in the UK
`
`Review of inductively coupled
`
`J Hopwood
`IBM Research Division, T J Watson Research Center, Yorktown Heights.
`New York 10598, USA
`
`Received 7 March 1992, in final form 1 April 1992
`
`Abstract. The need for large-area, high-density plasma sources for plasma-aided
`manufacturing of integrated circuits has created a renewed interest in inductively
`coupled plasmas (ICPS). In this paper several ICP reactor geometries are briefly
`reviewed. Typically, inductive coupling of RF power (0.5-28 MHz) can produce
`ion densities in excess of 10l2 cm-’ even at sub-millitorr pressures. Existing
`electromagnetic field models of ICPS are examined and found to be in reasonable
`agreement with experimental results. Sputter deposition, anodic silicon oxidation
`and polymer etching using ICPS are also described. It is concluded that ICPS are
`promising candidates for meeting the future requirements of plasma processing,
`although considerable process development. plasma characterization and
`modelling are still needed
`
`1. I n t r o d u c t i o n
`
`For over I 0 0 years inductively coupled plasmas (ICPS)
`have been generated and studied. Recently the trend
`toward high-rate, single-wafer processing in integrated
`circuit (IC) fabrication has motivated the development of
`low-pressure (c: 1 Torr) ICPS for plasma-aided materials
`processing applications. The requirements for modern
`processing plasmas
`include high densities of ions,
`electrons and radicals, excellent uniformity over dia-
`meters of at least 20 cm, low and controllable ion energies
`and negligible contamination from reactor sputtering or
`particulate generation. A review of inductively coupled
`plasmas reveals that most of these requirements can
`already be met using a readily available RF (13.56 MHz)
`heating frequency and relatively simple source designs.
`It is naturally desirable to understand the physics of
`the ICP as an aid to controlling the plasma process. Since
`low-pressure ICP implementations for IC processing plas-
`mas are relatively new, these sources have not been
`investigated as extensively as other high-density plasmas
`such as electron cyclotron resonance (ECR) plasmas [I]
`and helicon wave plasmas [2,3]. For an overview of
`high-density plasma sources the reader is referred to [4].
`This paper contains a brief review of the current under-
`standing of the properties of ICPS with an emphasis on
`low-pressure, plasma-processing applications.
`
`2. Review of inductively.coupled p l a s m a
`t e c h n o l o g y
`
`As the name implies, the inductively coupled plasma uses
`an inductive circuit element adjacent to (or immersed
`
`0963-0252/92/020109+08 $04.50 @ 1992 IOP Publishing Ltd
`
`inside) a discharge region in order to couple energy from
`an RF power source to an ionized gas. The inductive
`circuit element is typically a helical or spiral-like con-
`ductor. An additional electrical reactance is used to tune
`the inductor such that an electrical resonance at the RF
`driving frequency is obtained. Properly implemented, the
`resonant circuit causes large RF currents to flow in the
`inductive element. The RF magnetic flux generated by
`these currents then penetrates into the adjacent discharge
`region. Using Faraday’s law, V x E = -dB/at, one can see
`that the time-varying RF magnetic flux density (B) induces
`a solenoidal RF electric field (E). It is this ‘inductive’
`electric field which then accelerates free electrons in the
`discharge and sustains the plasma.
`Since the inductive coupling element is driven in an
`electrical resonance condition, one can expect high pot-
`entials t o exist on the structure. Such RF potentials will
`lead to capacitive coupling to the discharge as one would
`observe in RF planar parallel-plate plasma reactors,
`Capacitively coupled plasmas are characterized by high
`voltages and observable sheaths. Ions are accelerated
`across the sheaths t o the walls at high energy and can
`cause sputtering and heating of the walls. It is not
`uncommon to observe weak, capacitively coupled E-
`discharges in inductively coupled plasma sources at low
`absorbed powers [5-71. As RF power is increased, a
`sudden increase in luminosity and density is observed at
`[SI, signaling the onset of
`pressures above -3OmTorr
`inductive coupling or the H-discharge. The mode of
`coupling.(capacitive or inductive, E or H) has frequently
`,been a matter of debate since the original inductive
`plasmas [7]. Some implementations of ICP reactors
`attempt to minimize the degree of capacitive coupling
`(and its negative side-effects) by placing split Faraday
`
`109
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`
`J Hopwood
`
`shields between the inductive coupler and the discharge
`wall. Since high potentials exist on the inductive coup-
`lers, it is arguable that any ICP is entirely inductively
`coupled and, for the purpose of this review, high-density
`plasma sources which may be only partly inductively
`coupled will be considered as ICPS.
`In what follows an attempt has been made to classify
`the various forms of inductively coupled plasma sources.
`Conspicuously absent from the listing below is the
`helicon wave source [2,3] which is sometimes referred to
`as a resonant inductive plasma etcher [S, 91 (RIPE). In this
`source, however, the plasma is magnetized longitudinally
`by solenoidal electromagnets, and coupling is achieved
`hy an R F transverse electroma_enetic helicon wave. The
`divisions among non-magnetized Icps reviewed here are
`made primarily along geometrical features and are:
`(i) helical inductive couplers-cylindrical
`plasmas;
`(ii) helical resonators-cylindrical
`plasmas;
`(iii) spiral inductive couplers-planar
`plasmas;
`(iv) immersed inductive couplers;
`(v) transformer-coupled plasmas.
`
`2.1. Helical inductive couplers
`Historically, Hittorf is credited by Eckert [IO] with
`producing the first plasma by induction using a coil
`surrounding a tube in 1884. These early inductive plas-
`mas were often referred to as ‘ring’ discharges since the
`limited skin depth of the exciting field in the discharge
`caused the periphery to glow more brightly. More
`recently, high-pressure (- 1 atm) induction plasmas or
`‘induction arcs’ have been extensively studied and used.
`Eckert [ll] gives a comprehensive review of high-
`pressure ICPS. Applications include spectral chemical
`analysis, plasma-assisted chemical synthesis, crystal
`growth and thermalization of gases to produce thrust
`from a plasma jet. At high pressures, however, the plasma
`is dominated by volume recombination rather than
`diffusion which results in a non-uniform, small-volume
`plasma. In addition, high-pressure arcs are characterized
`by high neutral gas temperatures ( l ~ - l O O O O K ) . These
`properties make the induction arc unsuitable for process-
`ing damage-sensitive, large-area wafers and dictate low-
`pressure operation.
`The geometry of the helical-class Icp is shown in
`figure I@). The induced electric field within the plasma is
`azimuthal, forming closed loops about the axis, and the
`RF magnetic field is directed along the central axis of the
`discharge [S, 10, 121. Several models [S, 11-13] have
`been published which describe the induction fields while
`taking into account such factors as finix conductivity
`and radial variation of plasma density. In general, the
`induction field is maximum at the plasma tube circum-
`ference and decreases monotonically toward the centre.
`The result at higher pressures can be a ring-shaped
`discharge as viewed along the axis. At low pressures,
`however, diflusion processes increase the plasma density
`near the centre, where the induction field is low, thus
`providing a more radially uniform discharge.
`
`110
`
`It is possible in the helical-class Icp for the high RF
`potential which forms end-to-end across the inductive
`coupler to produce an axial electric field through the
`discharge. The axial field will produce a weak capacitive
`discharge, particularly during low-power operation
`[S, 61. A cylindrical conducting shield (Faraday shield)
`placed around the discharge chamber will short-out the
`axial electric field. Longitudinal slots must be cut from
`the shielding, however, so that the shield does not
`function as a short-circuited secondary transformer
`winding with the inductive coupler acting as the primary,
`thus inhibiting induction fields from the discharge region.
`When operated in the inductive mode, argon ion dens-
`ities 161 in excess of 10”
`have been reported at
`64 mTorr.
`The barrel etcher [I41 is a traditional plasma process-
`ing tool which is very similar in appearance to figure l(a).
`Plasma ion density in barrel reactors is typically only of
`the order of 1010cm-3 or less, suggesting that the
`coupling mechanism is capacitive. In addition, wafers are
`loaded into the centre of the cylindrical discharge vessel
`for processing where they would significantly disrupt the
`inductive fields. For this reason, plasma processing in
`helical inductive plasmas and helical resonators (discus-
`sed below) frequently occurs in a remote, downstream
`chamber which.is separate from the plasma generation
`region.
`
`2.2. Helical resonator
`The helical resonator plasma source [IS-lS] also con-
`sists of a cylindrical discharge tube within a helical coil.
`The coil, however, is designed with an electrical length of
`(A/4+ni./2) or (A/2+n,l/2), where n=O, 1,2,. . . and i, is
`the wavelength of the excitation frequency. The former
`design is a quarter-wave resonator and the latter is a half-
`wave resonator. As shown in figure I(b) the coil is within
`a conducting enclosure which provides a parasitic
`capacitance from the coil to ground. In addition, a
`trimming capacitor is usually connected between the coil
`and ground to adjust the capacitance to ground such that
`the structure resonates. Typically one end of the coil is
`grounded and, in the quarter-wave resonator, the other
`end is floating. In the half-wave design, both ends of the
`coil are grounded. RF power is applied at a centre tap of
`the coil. The presence of the discharge chamber and
`plasma within the helical resonator will perturb the
`resonant frequency of the reactor. A treatment of this
`perturbation is given in [19].
`Although there is currently some debate as to
`whether the helical resonator is truly an inductively
`coupled plasma, high-density plasmas can be generated
`using a split Faraday shield (described above) between
`the coil and the plasma. This shield will short-out
`capacitive fields and allow primarily inductive excitation
`of the discharge. In addition, ion densities of 10“-
`IO” C I I - ~ have been reported in commercially available
`helical resonators [ 181 thus exceeding densities produced
`
`by capacitively coupled plasmas (typically - 10” ~ m - ~ ) .
`
`LAM Exh 1006-pg 2
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`

`
`inductively coupled plasmas for plasma processing
`
`(6)
`
`(4
`
`RF Power In,
`
`Gas
`Inlet
`
`Figure 1. Cross-sectional schematic diagrams of various inductively coupled
`plasma reactors: ( a ) helical coupler. ( b ) helical resonator, ( c ) spiral coupler, (d)
`immersed coupler and (e) transformer-coupled plasmas. See text for details and
`references.
`
`111
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`
`J Hopwood
`
`2.3. Spiral inductive couplers
`In processing applications where it is desirable to gen-
`erate uniform plasmas over large, planar areas or in ion
`sources where one wishes uniform ion generation over
`planar extraction grids, spiral-like inductive couplers
`which lie in (or nearly in) a plane have been developed
`120-231. A typical planar design is shown in figure 2(c)
`where a * spiral-like coil (shown in cross section) is
`separated from the low-pressure discharge chamber by a
`(typically quartz) vacuum window. A
`dielectric
`capacitance in series with the spiral inductor is tuned
`such that the driving circuit resonates a t the RF fre-
`quency. These reactors sometimes use permanent-
`magnet multipolar buckets in the discharge chamber to
`confine the plasma, improve uniformity and increase the
`discharge density. The effects of capacitive coupling
`between the coil and the plasma are reduced by using a
`thick-cross-section dielectric window.
`The geometry of the spiral ICP holds particular
`advantages in processing of planar surfaces such as
`wafers. Since the skin depth of a 13.56 MHz RF induction
`in plasmas with electron
`field is approximately I-2cm
`densities of the order of 1011cm-3, the substrate may
`be placed in close proximity to the inductive coil.
`Typically, the coil is separated from the plasma by a
`1-3cm quartz window and the substrate is positioned
`5-1Ocm below the window (2.5-10 skin depths). The
`induction electric field decays exponentially within the
`plasma such that the field strength is attenuated by a
`factor of at least 12 (i.e. exp2.5) at the substrate. By
`processing in close proximity to the region of plasma
`generation, plasma losses such as electron-ion and
`neutral-neutral recombination are reduced as compared
`to remote, downstream processing. This results in impro-
`ved ion generation efficiencies as measured at the sub-
`strate. For the source in figure I(c), argon-ion generation
`efficiency measured at the substrate varies from 150-
`300eV/ion for argon densities of (1-4)x lO"cm-'
`at
`5 mTorr and 100-1000 W RF power. Although it can be
`argued that RF power is inexpensive relative to the other
`costs of IC fabrication, reduced power absorption may
`provide other benefits. In downstream source configura-
`tions, for example, the plasma in the generation region
`may typically be an order of magnitude more dense than
`at the substrate [24]. Such an excessively dense plasma in
`the source region may increase sputter contamination,
`uv damage and neutral gas heating at the substrate.
`Although the principle is not proved, it is at least
`intuitively appealing to produce a plasma of exactly the
`proper density, and no more, at the point of use.
`Examples of the performance and modelling of planar,
`spiral ICPS which approach this ideal are given later in the
`paper
`
`2.4. Immersed inductive couplers
`In the ICPS discussed so far, the inductive coupling
`element has been physically outside the discharge region.
`In the immersed-coil class, the inductor is positioned
`
`112
`
`.,.U6,.-L'~
`m.,nnn+:"
`
`.U..Y"II*
`
`.+O,,L,
`
`I - - - C -
`,"'C
`
`1 1 A 1
`
`within the plasma vessel (figure I(d)). This design has a
`distinct advantage in metal sputtering 1251 applications.
`Metal deposition on the dielectric plasma-vessel walls of
`non-immersed ICPS eventually suppresses plasma gen-
`eration by acting as a single-turn, short-circuited second-
`ary winding which is closely coupled to the inductive
`driver. Although Yamashita [25] reports no measurable
`impurities in the metal films deposited by immersed ICP
`sputter deposition, contamination sputtered from an
`immersed inductive element is a concern in other pro-
`cessing applications such as etching and deposition.
`Immersed coils have successfully been used in ion beam
`sources [26-281 where a loop antenna is located within a
`bucket .."-.,.._ .i----l
`r,gu,r; *,U,,. To
`prevent sputter erosion of the antenna, glass cloth is
`fused to the metal inductor. Helium-ion densities of
`2 x 101'-5 x 10" cm-3 at 0.5-5 mTorr have been re-
`ported in pulsed RF (3-100 kW, 1 MHz) immersed ICP ion
`sources [26].
`
`2.5. Transformer-coupled plasmas
`ICPS have also been used in laser design 129-311 where a
`ferrite core transformer couples low- or high-frequency
`(2.5 kHz-I MHz) energy to a ring-shaped plasma cham-
`ber (see figure I@)). Here the reader may observe that the
`plasma functions as a single-turn secondary winding
`around the closed path of the vacuum vessel. The
`induction electric field resides along the axis of the tube,
`rather than in the azimuthal loops of the helical and
`spiral ICPS. Reference [30] describes an ICP with a similar
`closed-loop plasma tube, but a single-turn, air-core
`primary winding is used. This implementation has been
`operated from 3.5-28MHz. The plasma
`in
`these
`configurations is most intense in the narrow capillary
`region which is typically 1-3"
`in diameter. For laser
`operation an optical cavity is positioned to include the
`capillary volume. While the narrow cylindrical geometry
`described here is not appropriate for large planar areas
`encountered in semiconductor fabrication, it may prove
`useful for continuous-strand processing systems and
`other applications.
`
`3. Modelling of ICPS
`
`The simplest model of the ICP is a discrete circuit element
`model known as the transformer model [5] shown in
`figure 2. In this model the inductive coupling structure is
`described as an N-turn primary transformer winding
`with self-inductance, L,. The high-density plasma is
`modelled as a single-turn secondary winding and a series-
`connected plasma impedance, Z,,,,,..
`The coupling coef-
`ficient, K , between the primary and secondary windings
`may approach unity
`for helical,
`immersed and
`transformer-coupled reactors. For spiral-like inductive
`.
`couolers. K mav be somewhat less than one DO]. This
`-~,
`. .
`,
`reduction in coupling coefficient is due. to the leakage
`magnetic flux from the primary which does not intersect
`the secondary defined by the plasma. The transformer
`
`~~
`
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`
`~
`
`~~~
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`LAM Exh 1006-pg 4
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`

`
`Impedance
`
`Source
`
`RF p r q (-Q
`N: 1 1
`2
`
`IcP applicator
`
`Plasma
`
`Figure 2. Schematic diagram of the transformer model of
`inductively coupled plasmas [5]
`
`model is sufficient for relating external circuit parameters
`such as coil current and voltage to the 'plasma im-
`pedance', but does not directly describe the fields within
`the plasma or any plasma physics.
`More sophisticated electromagnetic models have
`been developed to describe the induction fields within the
`discharge. Thomson [13], in one of the earliest theoret-
`ical treatments of the ICP, produced analytical expres-
`sions for the field required to sustain a plasma by
`induction. In his work, Tbomson concludes that 'it does
`not require a very intense magnetic field to produce he
`discharge'. In the same work, Thomson also identified
`the tendency of currents induced near the periphery of
`the discharge to shield the central regions from induction
`currents. Recent measurements [32] in spiral-like ICPS
`(see figure l(c)) are shown in figure 3. Notice that the
`13.56 MHz magnetic flux density is quite low at only a
`few gauss. The corresponding induced electric field is of
`the order of only 4-8 V cm-'. Figure 3 also demonstrates
`the shielding of the inductive fields by plasma currents.
`Induction electric fields which are parallel to a planar
`plasma surface are expected to decay spatially with an
`exponential dependence and a decay constant given by
`
`6 1
`
`I
`
`- 4
`cn
`v1
`3
`s3
`U
`
`1
`
`I
`I
`Key ond exponential least squares fits:
`(0) 300 W, B,(r) = 2.76 e-o2h G
`G
`(U) 500 W, B,(z) = 3.78 e-'&
`G
`(A) 700 W, B,(r) = 4.24 e-'.*
`G
`(0) 900 W, E,(?) = 5.12
`50 S C C ~ 0,. 5.0 mlorr
`
`CC ' . ' ~
`
`0
`
`6
`2
`4
`Distance Below Window, z (cm)
`Figure 3. The radial component of the induction
`magnetic flux density decreases exponentially away from
`the spiral-shaped coupler.
`
`a
`
`Inductively coupled plasmas for plasma processing
`
`6 i- c/o,,, where c is the speed of light in a vacuum and
`wps is the electron plasma frequency [33]. The full curves
`in figure 3 are exponential least-squares fits to the
`experimental data which confirm the theoretical shield-
`ing depth (or skin depth) of the induction field.
`The Thomson 1131 model assumes uniform and real
`conductivity across the helical-type ICP discharge dia-
`meter. Eckert [IO]
`refined this model by combining
`positive column diffusion theory with the electromagne-
`tic model and the effects of finite collision frequency.
`Henriksen er al [I21 offer a similar solution with the
`assumption of a parabolic radial electron density profile.
`The preceding models of helical ICPS all make an a
`priori assumption that the magnetic field is axial and the
`electric field and current are entirely azimuthal. For the
`spiral-like, planar ICP (figure l(c)) these simplifying as-
`sumptions are not easily justified. A schematic represen-
`tation of the induction electric and magnetic fields for the
`spiral coil ICP is shown in figure 4. The reader may
`observe that the B-field is not directed entirely along the
`axis as in helical coil ICPS. The RF fields, however, can be
`numerically modelled by solving Maxwell's equations
`using finite element analysis (EA). Solutions for the
`electric field are shown in figure 5 for such a planar ICP
`[32]. The electric field in a plane 2.5cm below the
`vacuum window is shown by arrows where the relative
`size of the arrow is proportional to the strength of the
`field. The square-spiral coil is shown in outline to assist
`the reader. This model is a three-dimensional numerical
`solution to Maxwell's equations assuming a cold, colli-
`sionless and uniform plasma with a relative permittivity
`given by E,=
`1 -(w,,/w)~
`where wp. is the electron
`plasma frequency and w is the RF frequency. Direct
`measurement of the magnetic flux by a small loop
`antenna, movable within the plasma, has experimentally
`confirmed the accuracy of this model. It is interesting to
`note that the electric field is in fact dominantly azimuthal
`within the bulk of this high-electron-density plasma. One
`would expect that axial electric fields due to capacitive
`coupling from the coil to the plasma would quickly decay
`over the distance of several Debye lengths [33] (which is
`of the order of 100pm). Nonetheless, such capacitive
`fields may play an important role in the plasma inter-
`actions at the window such as sputtering.
`
`I
`
`I
`Plasmo'
`Figure 4. Schematic representation of the induction fields
`for a planar, spiral coupler above a dense plasma.
`
`I
`
`113
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`

`
`J Hopwood
`
`Figure 5. Induction electric field vectors from a three-
`dimensional numerical solution to Maxwell’s equations are
`shown in a plane parallel to the inductive element 2.54cm
`within the plasma. The outline of the square, spiral
`coupler is superimposed over the field plot.
`
`The results of modelling of spiral-coupled ICPS in-
`dicate that scaling the diameter of the plasma may he
`accomplished by simply increasing the diameter of the
`coil. The larger plasma volume will necessarily require an
`increase in RF power to maintain the power density
`within the discharge. The subtle but practical problem of
`parasitic capacitance arises during scale-up, however, if
`the self-inductance of the coil increases as the diameter is
`enlarged (e.g. by adding more turns to the coil). In order
`to produce the large RF currents required for inductive
`coupling, the coil is tuned by an external capacitance to
`resonate a t the RF frequency. For a typical 25 cm coil
`the inductance is - 2 pH. The corresponding series
`capacitance required to achieve resonance a t 13.56 MHz
`is -70 p F according to w2 = I/LC. As the inductance of
`the coupler increases, the series capacitance must de-
`crease to maintain the resonance condition. Eventu-
`ally the scaled-up inductance will reduce the series
`capacitance to the order of IO pF. Under these conditions
`parasitic capacitances between ground and the coupling
`circuit, which are also of the order of IOpF, will inhibit
`tuning of the coil. Care must be taken in the engineering
`of the large ICPS to minimize parasitic capacitances and
`the self-inductance of the coupler. Somewhat lower
`frequency operation will also alleviate the potential
`problem of parasitic capacitance in large ICPS. Practically
`speaking, however, manufacturing environments are
`constrained to use only Fcc-allowed RF heating fre-
`quencies which are quite broadly spaced. Currently,
`13.56 MHz operation appears suitable for I C P sources
`capable of 200 mm diameter wafer processing (see
`section 5).
`
`114
`
`4. Plasma characteristics
`
`Ion and electron density, electron temperature and
`plasma potential with respect to ground can all be readily
`measured with a Langmuir probe, although at RF fre-
`quencies care must be taken to allow the probe to follow
`the RF fluctuation of the plasma potential [34]. The
`literature gives several examples of such measurements in
`ICPS which show ion densities in excess of IO”
`Immersed-coupler, pulsed RF ion sources [26] achieve up
`to 6 x IOL2 cm-3 in He at 1-5 mTorr with up to 100 kW
`pulses at 1% duty cycle and 10Hz repetition rate. The
`discharge is surrounded by a magnetic bucket which
`givcs 10% iinifoimitj: ovci a 30 ciii diaiiii:ci ion cx:rzc-
`tion surface. Electron temperatures in such a plasma are
`in the range of 2-7 eV. Ion temperatures at 0.4 mTorr are
`reported to be as high as 0.8 eV as measured by Doppler
`line broadening of argon under high-power (55 kW)
`conditions. This phenomenon is attributed to RF power
`coupling to the ions. Ion heating is not surprising since
`the excitation frequency (1 MHz) is much less than the
`ion plasma frequency (wDi r 100 MHz).
`Amorim et RI [6] report electron temperature in a
`3.8 cm diameter helical ICP of lOeV and an ion density in
`the m i d - l O ” ~ m - ~ range at 64-130mTorr with only
`370 W of absorbed RF (1 1.4 MHz) power (see figure 6).
`The higher electron temperature is likely to have been
`caused by increased diffusion losses due to the small
`diameter of the discharge vessel and absence of magnetic
`confinement. The plasma density versus power in figure 6
`shows discontinuous transition from a low-density mode
`where ni < 1 0 ’ l ~ m - ~
`to a high-density mode where
`n, z 10Lzcm-3. The authors attribute this phenomenon,
`which they observe above 30mTorr, to a transition from
`a low density E-discharge to a high-density ICP (H-
`discharge). The discharge is easily excited in this plasma
`
`c
`
`._ c
`10”
`
`I
`I
`I
`
`I , . -
`
`100
`
`10’0
`
`0
`
`E-discharge.
`capoeitively coupled
`
`300
`200
`RF Power (W)
`
`400
`
`500
`
`Figure 6. Ion density versus power in a helical ICP
`increases abruptly as the coupling mode changes from
`capacitive to inductive. At pressures below 30 mTorr the
`transition is continuous (after [6]).
`
`LAM Exh 1006-pg 6
`
`

`
`.------I
`LG,,,&JGL'JL"LG
`
`^I
`L q J G L L , GilGic.LLVI,
`
`source since no split Faraday shield (see figure I(a)) was
`used to eliminate capacitive coupling.
`In the ICP shown in figure l(c) with a chamber width
`of 27 cm, the electron temperatures vary between 3 and
`8eV for pressures of 30-0.3mTorr respectively [35]. As
`... :+I-
`.-. -:-L.
`,,rrg,lrr
`",IC
`Illblr'vnc1 W l l l l
`decreasing pressure. Argon ion densities are typically
`1011-1012cm-3 for RF powers of 200-12OOW. Average
`or DC plasma potentials in argon and oxygen also
`increase with decreasing pressure from 1OV at 30mTorr
`to approximately 25 V at 1 mTorr. Low DC plasma
`potentials are due in part to the multipolar magnetic
`confinement used in this system which slows electron
`diffusion losses. The RF variation of the plasma potential
`has also been measured using a calibrated capacitive
`probe. For RF powers above 500W the RF potential
`variation in the plasma is typically less than IOV,,, and
`is dominated by the fundamental frequency with little
`harmonic content. Low plasma potentials are important
`to low-contamination plasma processing since ions ac-
`celerated from the plasma to the chamber walls by high
`plasma potentials will cause sputtering of wall materials.
`
`5. A p p l i c a t i o n s of ICPS
`
`Yamashita [25] describes an immersed ICP sputtering
`apparatus which obtains densities of IO" cm-' in argon.
`The metal target is Dc-biased at one end of the helical coil
`and the substrate is placed facing the target at the
`opposite end. In this deposition configuration the sput-
`tered material can be ionized as it passes through the
`inductively coupled discharge. It is estimated that the
`ionization fraction of sputtered material at the substrate
`is as high as 65%. Metal deposition rates are also
`reported for AI (42508, min-I), Ti (2200A min-I), Fe
`(44008, min-I), and Cu (112008, min-') in 10 mTorr
`argon with 700 W RF power and 800 W DC sputter target
`power.
`Ho and Sugano [36] have used an ICP for selective
`anodic oxidation of silicon. Substrates were positively
`biased (typically +3OV) and heated to 600°C in a 0.2
`Torr oxygen plasma generated by 1 kW RF power at
`420 kHz. Oxygen ion density was reported at 1 x IOL2
`cm-3 and oxide growth rates of > 0 . 8 p n h-' were
`achieved. The measured
`interface-state density after
`annealing was comparable to thermal oxides, which
`attests to low contaminant levels.
`Large-area plasma processing has been demonstrated
`in a planar, spiral inductor ICP [37]. Polyimide coated on
`glass substrates has been etched at 38008, min-l
`in
`5mTorr oxygen with 500" RF power. No external bias
`was applied to the substrate, but ion impact energies are
`estimated to be 2 10-15 eV from the difference of the
`plasma potential and the floating potential V,-V,. The
`substrates were not cooled, but the process time was only
`1 min and the relatively massive glass substrates were not
`hot upon removal from the plasma system. In this
`reactor, uniformity of better than 3.5% (standard de-
`viation/average) can be achieved on substrates with
`
`Inductively coupled plasmas for plasma processing
`
`0.50 I
`
`n 0.30 1
`I
`
`1: 2
`0.20 I
`0
`
`120
`160
`80
`200
`40
`Diagonal Substrate Position (mm)
`Figure 7 . Polyimide ash rate uniformity of 3.5% across
`diagonals of greater than 20cm is shown for the planar,
`spiral coupled reactor.
`
`I
`240
`
`diagonals exceeding 200 mm as shown in figure 7. The
`substrate-to-window separation is typically 5-10cm, but
`in the high-density inductively coupled mode the skin
`depth of inductive fields, 6, is only of the order of 1-3 cm
`(figure 3). One can observe that the substrate is shielded
`from RF induction fields by the plasma.
`Radio-frequency ion thrusters [38] are an application
`of ICPS which date back to the 1960s. Ion beams extracted
`at 1-4 kV from helical ICP plasmas of Hg and Xe at (1-
`Torr are designed to provide station-keeping
`5) x
`thrust to satellites. Ion densities of the order of IO" cm-3
`and electron temperatures of z 1 eV are reported. Source
`diameters range from 15 to 45 cm. Of particular interest
`to ICP manufacturing concerns are proven operational
`lifetimes of about 10000 hours for these gridded ion
`sources.
`
`6. S u m m a r y
`
`A brief review of inductively coupled plasma technology
`has identified several reactor variations. Large-area
`plasma can be generated using helical- and spiral-shaped
`inductive coupling circuit elements which are either
`external to or immersed in the discharge. Small-volume
`ICPS have also been produced by transformer coupling to
`a ring-shaped discharge tube with applications to lasers.
`The issue of whether a particular source geometry is
`inductively coupled or capacitively coupled has histori-
`cally been (and continues to be) a point of debate. In
`general, if high-density plasma generation is achieved by
`an inductive structure in a Faraday shielded chamber,
`coupling which is primarily capacitive may be ruled out.
`The degree of capacitive coupling impacts the con-
`tamination in the plasma from sputtering by large RF
`potentials between the plasma and the inductive coupling
`structure.
`Existing models for inductively coupled processing
`plasmas are relatively simple from a plasma physics
`perspective. The primarily 'field models' treat the plasma
`as a conductive or dielectric medium and predict only
`induction field geometry and power deposition. There is
`
`115
`
`LAM Exh 1006-pg 7
`
`

`
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