Low Pressure RF Plasma Sources For Industrial Applications .
Low Pressure RF Plasma Sources forIndustrial Applications (ICP versus CCP)Valery GodyakRF Plasma ConsultingBrookline, MA, USAegodyak@comcast.netWorkshop on Radio Frequency DischargesDublin City University, August 26-27, 2011, Dublin, Ireland1
Main Plasma Sources in Processing of MaterialsM. Lieberman’slecture, 20072
CCP Processing ChamberGas inletMain RFPowered electrodeCapacitive Coupled Plasmas (CCP) at 13.56 MHzwere the first in plasma processing applicationsWaferSheathPlasmaChuckSheathGrounded electrodeHe coolantBias RFGas outletF. F. Chen, AVS, 2003In CCP the discharge current and plasma density are controlledby the electrode rf sheaths at the plasma boundary Good plasma uniformity due to rf sheath ballasting effect* Simple and relatively inexpensive constructionb u t: No independent control of ion flux and ion energy Low plasma density at low gas pressure At large rf power and low gas pressure, most of it goesfor ion acceleration rather than for plasma production*Sheath stabilizing effect is due to different sheath and plasma impedancedependence on plasma density, Zp Np-1: Zsh Np-1/2 (a good subject for study)
CCP equivalent circuit and rf power distributionbetween electron and ion heatingGeneralω 2 ω02νen2 ω02Prf Ppl PiSince in plasmaEp const, Np Id and Vdc VrfPpl Id and Pi Id2V. Godyak et al, IEEE Trans. PS, 19, 660, 19914
According to CCP analytical modelId Np (Aω3νeffm2L2/8πe3Vp){1 (νeff/ω)[(Vrf/Vp)2 – 1]1/2}where Vp Re(EpLp) is the minimal discharge sustaining voltage, andA(pL,M) is a geometric factor accounting for ion space non-uniformityAt large voltage(Vrf Vp), Id Np Vrfω2 ; Ssh ω-1V. Godyak, Sov. J. Plasma Physics 2, 78, 1976CCP in Hg vapors at 40.8 MHz shows a linear V/A characteristic at Vrf VpDue to large M andrelatively small rfvoltage, Pi isnegligibly smallV. Godyak et al, inproceedings of XIIICPIG, p. 347, Berlin,Germany (1977).5
Electrical characteristics of a symmetrical CCP, Argon, 13.56 MHzL 6.7 cm, D 16 cmFor moderate rf voltage (Vp V 1 kV), V/A discharge characteristics are nearly linear,that does not obey to Childs-Langmuir law followed from Lieberman rf sheath model.V. Godyak et al, IEEE Trans. PS, 19, 660, 19916
CCP modes and transitions between them1. Volume/boundary heating mode transition,argon 13.56 MHz, L 2 cm, D 16 cmJ. K. Lee et al, (2003)I.V. Schweigert et al,V. Vahedi et al, (1994)Experiment, (1990)Godyak & PiejakV. Godyak & R. Piejak PRL 65, 996, 19907
2. α to γ-mode transition(S. Levitsky, half century ago)(13.56 MHz, He, 0.3 Torr)V. Godyak et al, PRL 61, 40,19928
3. CCP resonant mode , (Hg 1.2 mTorr,L 7.8 cm, D 7 cm)Np ω3{1 (νeff/ω)[(Vrf/Vp)2 – 1]1/2}(1976)77.6 MHz Series (geometric) resonance of inductive plasmaand capacitive sheath Double valued rf current and plasma density withcapacitive and inductive discharge impedance Discharge parameters are not sensitive to dischargevoltage when ω νeffPeak discharge voltage In the resonance, the rf current does not depends ongas pressure, while Np ω355 MHzThe analytic expression above and experimentssuggest to utilize a higher frequency to achievehigher plasma density at fixed discharge voltage100 MHzVHFCCP operates close the resonance conditionV. Godyak & O. Popov, Sov. J. Plasma Physics 3, 238 (1977)9
Very High Frequency CCP (VHFCCP)(Dual and triple frequency CCP)M. Lieberman’slecture, 2007Main concept expectations:Typical for CCP uniformity, large plasma density (Np fh2) and independent control ofion flux and ion energy. High frequency fh to control plasma density (ion flux), while lowfrequency fl to control ion energy and its specter (IED)fh 27-162 MHz , fl 2-13.56, sometime both 2 and 13.56 MHz to tailor IEDToday, Dual (Triple) Frequency CCPs are the mainstream technology10
Ion flux vs RF power:are 27 MHz and 2 MHz decoupled?2 MHz power (Fixed 27 MHz power)27 MHz power (Fixed 2 MHz power)1.5250 W1.00W0.527.12 MHz600 W500 W400 W-2750 W500 WIon Current (mA.cm )2 MHz-2Ion Current (mA.cm )1.51.0250 W0.50.00.0010020030040050027.12 MHz Power (Watts)60070002505007502 MHz Power (Watts)J-P Booth et al, Dry Process Symposium, Jeju, Korea, 2005Ion flux and ion energy in VHFCCP are not independable!11
Plasma density profile in CCP for different source frequencyIncrease in f1 leads to increase in plasma (and process) non-uniformity!
Plasma non-uniformity in front of 300 mm wafer in CCPdriven at 100 MHz in Argon and processing gas mixtures(mode jump)Ar, 10 mTorr10 mTorr750 WAr, 80 mTorr80 mTorr750 WV. Volynets et al, J. Vac. Sci. Technol. A26, 406, 200813
VHF CCP problemsI.Low frequency bias significantly affects plasma parametersII.Standing surface waves (λr [1 d/s]-1/2 λo/3, radial non-uniformity)III.Edge effect (enhances edge plasma density)IV.Skin effect (radial non-uniformity when δ 0.45d/R)V.E to H transition (rf power is magnetically coupled to plasma)VI.Plasma-Sheath local resonances on F, 2F , 3F (destroy plasma uniformity)VII.Resonance effects and mode jumps prevent smooth plasma control*All these problems became more severe at larger:rf frequency, wafer size and plasma density(Gas flow distribution, segmenting and profiling of rf electrode have limited successes)The interplay of many fundamental electro-magnetic effects withresonant conditions makes VHF CCPs too complicated for reliabletheir control in a wide range of their parameters14
Inductively Coupled Plasma (ICP) SourcesMain ICP topologies in applicationsToroidal lampFerrite coreInduction coil(N turns)SecondaryCurrent pathWafer processingRemote plasma sourcesrf powersupplyLightingLighting15
Skin Effect, δ E(dE/dx)-1In ICP, the rf current forms closed loop within plasma without rf sheathRf power absorption is localized within a skin layer at the plasma boundary1. Geometric skin depth (the most important and neglected)is due to multi-dimensionality of the real ICP structure2. High frequency SE, δ0 c/ω0 @ ω νen and ω vTe/δ *3. Low frequency, normal SE, δn δ0(2ν/ω)1/2@ ω νen *Erf Ewrw/rδ rVp Vt4. Anomalous SE @ ω and νen vTe/δ **5. Non-linear SE @ ω vrf/δ **(eBrfvrf /c eE rf)* Formulae 2 and 3 are valid only for an uniform plasma with planar boundary!** Cases 4 and 5 process non exponential spatial variation of E & B16
Transformer formalismR. Piejak et al, PSST 1, 179, 1992Physicists, do not be arrogant, these eqs. are integrals of Maxwell Equations, but resultin some universal relationships between ICP integral parameter independently of ICPgeometry and specific mechanism of electron heating in RF field.17
ICP plasma and electrical parameters Electron temperature is defined by the ionization balance, Te Te (pΛ) Plasma density is defined by power absorbed by electrons, Np Pd By measuring of transmitted power, Ptr, and the coil current Ic, with and w/oplasma, one can infer the power absorbed by plasma, Pp, and power loss in antennaand surrounding hardware, Po (in antenna, matcher and chamber)Ptr Pi - Pr I2(R0 Rp); P0 I02R0R0RpPd I2Rp Ptr – P0I2/I02Surprisingly, this simple RF diagnostics is neglected in characterization ofcommercial plasma reactors and of many laboratory rf plasmas, where plasma ischaracterized by the power consumed from the power source, Ptr. This power isalways smaller (sometimes significantly) than Pd, and is not proportional to PdR. Piejak et al, PSST 1, 179, 199218
Argon ICP electrical characteristics(experiment, Godyak et al PSST, 11, 525, 2002)19
More electrical parametersFrequency dependenceTransformed plasma resistancePower Transfer efficiency20
Argon pressure dependence of EEDF and plasmaparameters in ICP driven at 6.78 MHz, 50 W6.78 MHz, 50 W1050Inelasticcollisions andescapeto the wall50W1110)10Collision dominatedand Near-collisionless121010Plasma potential910810300 mT100710 * i101130.36.78 MHz, 50 W50W8.3mT50W-310plasma density (cm )eepf (eV-3/2cm-350101210111010642610010203040electron energy (eV)5010-1012101010gas pressure (mT)effective electron temperature (eV)10Transition from normal toanomalous skin effect13031021
EEDF power and frequency dependenceIn a high density plasmas, EEPF at low energy must be Maxwellian22
Power Transfer Efficiency and Frequency EffectPc I2rc E2 Edc2 (1 ω2/υ2eff); excessive coil loss at ω υeff23
Frequency effect exists at low density plasma in anomalous skin effect regime, butdisappears at larger plasma density due to e-e collisions.Operation at lower frequency is desirable because: Lower cost and more efficient rf equipment Capacitive coupling and transmission line effect can be eliminated Easier management of rf power and simpler and more reliable electrical diagnosticse-e interactions diminish frequency dependence ofEEDF, approaching it to a Maxwellian distribution24
Pros and Cons of conventional ICP sourcesPositive (expected) Independent control of ionflux and energy to the wafer Operates at a wide range ofgas pressure. Relative simple construction Operates in widerange offrequencies. Possibility to operate at lowfrequency Possibility of plasma profilecontrol with multiple coils?Negative (experienced) Inability to operate at lowplasma density (Np 1011 cm-3)in inductive mode Can not operate with small gap(large residual time) Stray capacitive coupling(plasma non-uniformity andwindow erosion) Transmission line effect(plasma non-uniformity) Bead process uniformitycontrol25
Mentioned above negative ICP features have promoted VHFCCP.Many of negative opinions on ICP limitations are based onexperience with poorly designed commercial ICP reactors.Contrary to prevailed lore: ICP can operate in inductive mode at small wattage and low plasma density ICP can operate with small gap (small residual time) Many commercial ICPs with two antennas do not produce peripheral maxima. ICP can provide process uniformity control over the large area Capacitive coupling and transmission line effect can be eliminated ICP source can operate at much lower frequency, more efficient and is lessexpensive than VHFCCP and ICP used today in the plasma processing ofsemiconductor materialsAll above can be achieved by properly designed ICP source26
ICP does operate in inductive mode at low plasma densitySmall power ICP in RF lamps and in lab experiments, Pd 2.5 W!ICP pancake configuration2R 20 cm, L 10 cmr 0, z 5 cmSpherical ICP withand without ferritecore Ar-Hg at 0.5 TorrPa/Pdrelative power lossno core1rod2.5 MHzU-core0.10.01110100discharge power (W)Inefficient coupling and huge antenna loss prevent low plasma density operation incommercial ICPs.Pa Pd n is the condition for stable ICP operation27
320028002400200016001200800400012008004000-150Ar Sputter RateTh-Ox E/R-5050X-axis [ mm , e.e.3mm ]150Sputtering and etching rate profilesAr sputter rate uniformity : /- 2.5%Th-Ox etch rate uniformity : /- 2.6%300 mm , Th-Ox wafer (blanket)In 4 cm gap theuniformity isbetter than inany VHFCCPG. Vinogradov, FOI, 200628Th-Ox E/R [ A/min ]Sputter Rate [ A/min ]Groovy ICP, Process : Ar/C4F6/O2. FOI (Japan)
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Toroidal ICP plasmas with ferromagnetic corein industrial applicationsToroidal lampFerrite coreSecondaryCurrent pathToroidal plasmas for fusion, 1960Kogan & Ulanov, 1993100 kW, 10 kHz, 1 atm.Induction coil(N turns)rf powersupplyInduction lighting, Andersen, 1970Smith et al, 1998(5-10) kW, 400 kHz, 1-10 Torr30
ICP enhanced with ferromagnetic core2.5 MHz, spherical ICP with internal inductor, D 7 cm, Ar-Hg 0.5 Torr250no core2.5 MHz2.5 MHzcoil voltage (V)coil current (A)200no core10rod core1110100discharge power (W)power factorrod core2.5 MHz0.011relative power lossno core0.11rod2.5 MHzU-core0.1no core10100discharge power (W)U-core10000.1U-core15050U-core1rod core0.01110100discharge power (W)110100discharge power (W)Introduction of aferromagnetic corereduces antenna coilcurrent and voltage,together with increasingICP power factor (Cosφ)and power transferefficiency31V. Godyak, Proc.XVthIntern. Conf. on Gas Discharge and their Appl. V. 2, p. 621, Toulouse, France, 2004
ICP with ferromagnetic toroilal coresMagnetic flux is closed in ferromagnetic coreIE & B field distributionBEE &IBInductor currentInductorcurrentBE32
Distributed ICP with 18 core ferrite toroidal cores2R 10 cm, h 4.7 cm, 400 kHz, 400 W, Xe 0.3-100 mTV. Godyak, PEUG, Santa Clara, 200333
18 core distributed ferrite ICPXenon, 400 kHz, 10 mTorr, 400 W500 mTorr5 mTorr5 mTorrV. Godyak, PEUG, Santa Clara, 200334
Plasma uniformity control and coupler losses@ 400 W, p 0.3-100 mT, Cosφ 0.95-0.97 and Po/Pd (16 - 1)%Ii @ 2 mm from the chamber bottom0.5Xe, 400 W18 cores0.4ion current (mA)0.410 mTh 40 mm0.3180.2400 WXe, 400 W18 cores1000.3 mT, 16%h 47mm0.310coupler loss (W)0.5ion current (mA)Coupler loss versus coupler voltage h50.22 mT12 cores0.161mT, 9.1%1010 mT, 2.5%0.1100 mT, 1%00020406080radial position (mm)100020406080radial position (mm)100100200300primary voltage (V)400Power factor and efficiency exceed those in a conventional transformer at 60 Hz!V. Godyak, PEUG, Santa Clara, 200335
6 core large diameter ICP operating at 400kHC-W Chang. 2005RFI36
Plasma and Electrical parametersC-W Chang. 200512ne [cm ]2.8x10112.4x10112.0x10111.6x10111.2x1011-3-3Ne [cm 02mTorr, 1.5kW 28.07%2mTorr, 2.5kW r20mtorr50mtorr0.80.710-50Position [mm]100Power(W)1000Power factorPower transfer efficiencyPower 01001000Power [W]37
Low Frequency ICP Reactor Enhanced with Ferrite Core0.4-2 MHz, argon, d 1.5 - 8 cmferrite coresIncreased coupling provides better plasma spatial controlsupport platering windowdisk windowring angeflat probeswindingwindingNo Faradayscreen!chamberchambermovable pedestal2rf plasma potential (V pk-pk)Pd 100 W1.510.50110100gas pressure (mTorr)V. Godyak, PSST 20, 025004, 2011100038
Power transfer efficiency and plasma density control1power transfer efficiency0.8 Coil voltage and current arefalling with power!? Theinductor coil voltage followsthe plasma EMF (negativeplasma V/A characteristic)10000.61000.41 mTorr High power transferefficiency is due to strongcoupling (thin window and1000ferrite core)100.2010100discharge power (W)10130.610002ion flux (mA/cm ))-3plasma density (cm Wide range of plasmadensity, including low plasmadensity in inductive mode2mT0.5100101210101 mTorr110.45500.3100.2 Ability to create peripheralplasma maximum (spatialpower transfer selectivity)20Pt 200 W; d 25 mm0.1Vb -15V1010100100discharge power (W)1000-8-6-4-202468distance (cm)V. Godyak et al, PSST 20, 025004, 201139
Etch profile control in two inductor ICPDom window with two inductorsPlasma density profiles for different etch ratenon-uniformities. Strong local coupling provideseffective control of the plasma profilePlasma density profiles for different gases andfixed power in the coils, P1 400 W, P2 300 WV. Nagorny & D. Lee, 20th ISPC, Philadelphia, PA, 2011 Etch profile uniformity takes placeat non-uniform plasma profile40
Twelve reasons why the future belongs tothe distributed ICPs with ferromagnetic core No external E-M radiation (all E-M flux is directed to plasma) No capacitive coupling (no plasma rf potential, no window erosion) No transmission line effect (no azimuthal non-uniformity) Ability to operate at low power and low plasma density Low driving frequency (0.4 - 2 MHz) reduces cost of rf equipment Simple, compact and ridged construction Good plasma spatial control due to strong local coupling High power transfer efficiency reduces power dissipation in hardware High power factor allows for considerable simplification of matching network Effective thermal management due to large contact area of the rf inductor Wide processing window (mixture, pressure, plasma density) Possibility of scaling up for uniform processing of very large substrates41
Summary VHFCCPs have fundamental limitations preventing them toscale up for next generation wafers processing (450-670 mm) The main problems in commercial ICPs are weak inductiveand large capacitive coupling They are far from the optimaldesign and thus have room for improvement Properly designed ICP can operate without capacitivecoupling and transmission line effect at low plasma density,small gap and can provide good process uniformity Low frequency distributed ICPs with ferromagnetic cores cando everything that other plasma source do, but more efficientlyand cost effective42
Plasma-Sheath local resonances on F, 2F , 3F (destroy plasma uniformity) VII. Resonance effects and mode jumps prevent smooth plasma control* All these problems became more severe at larger: rf frequency, wafer size and plasma density (Gas flow distribution, segmenting
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