1Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Etching• Etching Terminology• Etching Considerations for ICs•Wet Etching• Reactive Ion Etching (plasma etching)2Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Etch Process - Figures of Merit• Etch rate• Etch rate uniformity• Selectivity• Anisotropy3Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05(1) Biassubstratedfhf filmdmetching maskdfsubstratedmBias B d dB can be orfm≡−><00.4Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Complete Isotropic EtchingBhf=×2Vertical Etching = Lateral Etching RateLateral Etching rate = 0B = 0Complete Anisotropic Etching5Professor N Cheung, U.C. BerkeleyLecture 15EE143 F051021≤≤−≡fffAhBA02==∴BfhBanisotropicisotropic(2) Degree of Anisotropy6Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Etching of Steps with a SlopesubstratestartfinalfilmlargeθmakexminimizeTorateetchlateralvrateetchverticalvttimeetchingLetlv⇒∴===()xvtxvtxxxvvtvlvl1212=⋅=⋅=+=+⋅cotcotθθθvtv⋅x1x2vtv⋅θcot θ = 1/ tan θ = b / a7Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05oo090==θθvvtvltstartfinalfilmsubsubfilmExample8Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Worst-Case Design Considerations for EtchingstepSubstratestep heightvariationvariationof filmthicknessacross waferetching maskcan be erodedduringfilm etchingMaskfilmtarget thickness value(a) Film thickness variation:()()δ+⋅=1max ffhhvariationfactor9Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05(b) Film etching rate variation(c) Overetching around step()()()()()()vvWorst case etching time required to etch thefilmfilmhvhvffffffffminmaxmin=−∴−==⋅+−111φδφsteph2h2h1Overetch time (fraction)=+hhh122variation10Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05()()()∆+⋅−+=∴111fffTvhtφδ(d) With Mask ErosionLet be the verticaland lateral etching rates of the mask.Let be the vertical etching rate of the film.(ignoring lateral film rate forsimplicity)vvmm⊥,//vftotal∆=hh12beforefilmW/2afterfilmθhf11Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05()()()()WvvtvvhvvmmTmfffmm2111=+⋅=⋅⋅++−+⊥⊥⊥cotcot////θδφθ∆Goal : Minimize WSmallWvνhsmallofmf→>>⊥θ9012Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Etching Selectivity Sfm between film material and mask material()onlycomponentsverticalVVSmffm⊥⊥≡Wet EtchingSfmis controlled by: chemicals, concentration, temp. RIESfmis controlled by:plasma parameters, plasma chemistry,gas pressure, flow rate & temperature.13Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05SiO2SimaskSSiO2, Si HF solutionSelectivity is very large ( ~ infinity)SiO2/Si etched by HFsolutionSiO2/Si etched by RIE (e.g. CF4)SSiO2, Si Selectivity is finite ( ~ 10 )Examples14Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05For a given allowable W/2 , what is the minimumselectivity required?()()()()()+−∆++=⊥mmfffmvvWhS//mincot1112θφδ[Note] If varies from run-to-run,vm⊥()()()()( )+−+∆++=∴+=⊥=⊥⊥mmfactoruniformityfmUfmffmmmmvvWhSvv//(min)maxcot111121θφφδφ15Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Wet EtchingReactant transport to surfaceSelective and controlled reaction of etchantwith the film to be etchedTransport of by-products away from surface12312316Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Wet Etching (cont.)• Wet etch processes are generally isotropic• Wet etch processes can be highly selective• Acids are commonly used for etching:HNO3<=> H++ NO3-HF <=> H++ F-H+is a strong oxidizing agent=> high reactivity of acids17Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Si wet etching mechanisms18Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Wet Etch Processes(1) Silicon DioxideTo etch SiO2film on Si, useHF + H2OSiO2+ 6HF → H2+ SiF6+ 2H2ONote: HF is usually buffered with NH4F to maintain [H+] at a constant level (for constant etch rate)NH4F → NH3+ HFEtch rate (A/min)T(oC)6:1 BOE18 26650120019Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Wet Etch Processes (cont.)(2) Silicon NitrideTo etch Si3N4film on SiO2, useH3PO4(phosphoric acid)(180oC: ~100 A/min etch rate)Typical selectivities:– 10:1 for nitride over oxide– 30:1 for nitride over Si20Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Wet Etch Processes (cont.)(3) AluminumTo etch Al film on Si or SiO2, useH3PO4+ CH3COOH + HNO3+ H2O(phosphoric acid) (acetic acid) (nitric acid) (~30oC)6H++ 2Al → 3H2+ 2Al3+(Al3+ is water-soluble)21Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Wet Etch Processes (cont.)(4) Silicon(i) Isotropic etchingUse HF + HNO3+ H2O3Si + 4HNO3→ 3SiO2+ 4NO + 2H2O3SiO2+ 18HF → 3H2SiF6+ 6H2O(ii) Anisotropic etching (e.g. KOH, EDP)22Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Effect of Slow {111} Etching with KOH or EDPMask opening aligned in <110> direction => {111} sidewalls23Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05[110]-Oriented Silicon{111} planes oriented perpendicular to the (110) surface=> possible to etch pits with vertical sidewalls!Bottom of pits are• flat ({110} plane) if KOH is used{100} etches slower than {110}• V-shaped ({100} planes) if EDP is used{110} etches slower than {100}24Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05(3) Field-Emission Tips25Professor N Cheung, U.C. BerkeleyLecture 15EE143 F05Drawbacks of Wet Etching• Lack of anisotropy• Poor process control• Excessive particulate contamination=> Wet etching used for noncritical feature
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