PLASMA RIE BirckETCHING Nanotechnology Center FUNDAMENTALS AND APPLICATIONS 1 Ou tline 1. Introductory Concepts 2. Plasma Fundamentals 3. The Physics and Chemistry of Plasmas 4. AiAniso tropy Mec han isms 5. The Etchinggp of Si and its Compounds 6. The Etching of Other Materials 2 DEFINITIONS ¾ Electron (e-) + + + + ¾ Positive ion (Ar , Cl , SiF4 , CF3 ) Positive ion mass in RIEs >>mass of electron ¾ Radical (F, Cl, O, CF3) Uncharged atoms with unsatisfied chemical bonding 3 DEFINITIONS ( con tinued) ¾ Mean free path averaggpe distance a particle travels before collisions 5 λ(cm) ≈ ((pDependent on the s pecies ) P(mT ) ¾ Pressure 1atmosphere= 760 Torr = 1*105 Pascals ¾ Pumping speed (S) [liters/sec] ¾ Gas flow rate (Q) [Torr-liters/sec] or [sccm] 4 Plasma Vacuum Sys tem Gas lines Mixed gas [Q] valve MFC box APC & gate Chamber valve Vacuum pump [S] VRF Matching network He backside cooling dP(()t) (Q -S⋅P(())t)) = dt V V= chamber volume In Steady state : Q = S⋅P 5 MhilPMechanical Pumps Wet and Dry Pumps Pumping speed: 20-500 m3/h Ultimate pressure: 1-10 mTorr [BOC Edwards Dry Pump] [Kurt J. Lesker] 6 TbPTurbo Pumps Rotation speed= 20000-90000 rpm Pumping speed: 50-3000 l/s Ultimate pressure: 10-5-10-8 Torr [Wiki] [BOC Edwards Turbo Pump] [TP controller] 7 MFlCtll(MFC)Mass Flow Controller (MFC) Thermal-based flow meter Heater Q [sccm] T1 T2 T2 – T1= Cp ×Q Cp is specific heat. [HORIBASTEC] 8 MFC or Gas Box Mixed gas line Gas lines Panasonic MFC Box 9 AtAutoma tiPtic Pressure C ont roll ll(APC)er (APC) & Gate Valve Pendulum valve Butterfly valve Q = S × P 10 [VAT] Clamp or El ect rost ati c Ch uck Clamp Electrostatic Chuck (ESC) Si wafer Dielectric ------- +++++ +++++ ------- Base Plate +V + - -V He 11 RF Genera tor & Ma tc hing N e twork Matching Chamber Z P P L RF Generator F Network L ZIn PR ZS= 50Ω PL=PF-PR In general: ZL≠ ZS Purpose of Matching Network: Zin= ZS to maximize power delivery from source. ZS L C1 Manual or Automatic ZL Matching Network C 2 [Gambetti] ZIn 12 Ou tline 1. Introductory Concepts 2. Plasma Fundamentals 3. The Physics and Chemistry of Plasmas 4. Anisotropy Mechanisms 5. The Etching of Si and its Compounds 6. The Etching of Other Materials 13 [Plasmas.org] Wha t is Pl asma? •Plasmais the fourth state of matter. It is an ionized gggas, a gas into which sufficient energy is provided to free electrons from atoms or molecules and to allow both species, ions and electrons, to coexist . [Plasmas. org] 14 HtMkPl?How to Make Plasma? ¾ Capacitive RIE Plasma - Low density plasma 9 3 ne ≈ 10 [electron/cm ] Ionization efficiency ≈ 10-7 ¾ Inductive RIE Magnetic field - High density plasma 13 3 Wafer ne ≈ 10 [electron/cm ] Ionization efficiency ≈ 10-3 [Oxford Instruments] 15 DC Glow Discharge ¾Only used for sputtering system not for etching . Vc=0 Vc= -100 n Red: ni X Black: ne V(x ) V(x) Vp Vp X X -100v Vp= a few volts 16 RF Glow Discharge ¾Used for any materials (insulating and conductive) . VRF(t) V RF ZG VpRF (t)= V (t) ZZGE+ Vp(t) AAEGGE⇒ ZZ VpRF (t) V (t) VRF(t) ZE ZG V(x) VRF X 17 RF Glow Discharge Z Actual RIE G V(t)pRF= V (t) ZZGE+ AAEGGE⇒ ZZ AE<<AG ZE>>ZG VRF(t) V(t)pRFVp(t) ≈ V 0 (t) ¾Ion transit time (Tion) is the time it takes the ion to traverse the sheath. ¾1/Freq << Tion ! Freq= 13.56 MHz 18 Ph’LPaschen’s Law Describes how the breakdown voltage depends on electrode separation and the pressure based on ideal gas law. a ×(p.d) V V = ln(p.d)+ b 800 600 V: Voltage p: Pressure d: gap distance 400 a & b: constants 10-1 100 101 Pd [Torr cm] 19 IdInduc tive C oupl ldPled Plasma RIE STS ASE and AOE systems 20 Why High Density Plasmas? ¾ Lower ion bombardment energies improve selectivity and reduce ion- bombardment-induced physical damage of the wafer surface. ¾ Lower ion energg,ies, however, result in the lower etch rates and reduced anisotropy! ¾ However, the etch rate can be increased by using much higher ion fluxes due to high density plasmas. ¾ The anisotropy can also be restored by operating at low pressure . 21 Ou tline 1. Introductory Concepts 2. Plasma Fundamentals 3. The Physics and Chemistry of Plasmas 4. Anisotropy Mechanisms 5. The Etching of Si and its Compounds 6. The Etching of Other Materials 22 Electron-Molecule Collisions ¾ An energetic electron colliding with a neutral etch gas molecule can create any of the following processes: - - - - 9 Dissociation AB + e A+B + e CF4 + e CF3+F+e 9 Ionization AB + e - AB+ +2e+ 2e- Ar +e+ e- Ar+ +2e- Often dissociation and ionization Occur in one collision: - + - CF4 + e CF3 + F+ 2e 23 RdilRadicals and dI Ions i n Pl asmas ¾ Positive ions are veryyp important for etching gp processes. ¾ Radicals are more numerous than ions in gas glow discharges because: 1. The electron energy required in order to break chemical bonds in the molecules is usually less than the energy required to ionize these molecules. 2. Radicals have a longer lifetime in the plasma compared to ions because an ion is almost always neutralized during a collision with a surface while radicals often do not react with a surface and are reflected back into the plasma. 24 Wha t is Plasma Et chi ng? CF4 - + - CF4 + e CF3 + F+ 2e valve Chamber Si + 4F SiF (gas) 4 Gate valve Vacuum pump + CF3 , F SiF Si Wafers 4 1- Need an etching gas 2- Establish a glow discharge 3- Choose chemistry so that the reactive species react with the substrate to form a volatile by-product 4- Pump away the volatile by-product 25 Why Plasma Etchi ng? ¾ Clean process ¾ Compatible with automation ¾ Anisotropic etching ¾ PiPrecise pa tttttern transf er especi illfNally for Nano-scalfle fea tures Mask Substrate Isotropic etch Directional etch Vertical etch 26 Gas-Solid Systems Solid Etch Gas Etch Product Silicon CF4,Cl2, SF6 SiF4, SiCl4, SiCl2 SiO2, SiNx CF4, C4F8, SiF4, CO, O2, N2, CHF3, SF6 FCN Al BCl3/Cl2 Al2Cl6, AlCl3 Ti, TiN Cl2, CF4 TiCl4, TiF4 Organic Solids O2, O2/CF4 CO, CO2, GaAs & III-V Cl2/Ar, BCl3 Ga2Cl6, AsCl3 Cr Cl2/O2 CrO2Cl2 Difficult materials to etch: Fe, Ni, Co, Au, Ag, Pt halides not volatile C Cu Cu3Cl3 is volatile above 200 27 Halogen Siz e Effect [Handbook of Advanced Plasma Processing Techniques by Pearton] 28 Ou tline 1. Introductory Concepts 2. Plasma Fundamentals 3. The Physics and Chemistry of Plasmas 4. Anisotropy Mechanisms 5. The Etching of Si and its Compounds 6. The Etching of Other Materials 29 De fin ition • Etch rate Mask GldGold •Mask (Photoresist, Metal, SiO2, …) Substrate • Selectivity L • Anisotropy degree L H A1≡ − f H 30 Reactive etching ¾RtithiiReactive etching is an i itisotropic process! ¾Has very high selectivity! Si + 4F SiF4 (gas) Mask SiF4 F Substrate Isotropic etch 31 Ion etching ¾Ion etching or mechanical etching is an anisotropic process! ¾Has lower selectivity and etch rate! Si + Ar+ Si + Ar+ + Si Mask Ar Substrate Directional etch Vertical etch 32 Reactive ion etching ¾Reactive ion etching is an anisotropic process! ¾Has better selectivity and much higher etch rate! Effect of Ions: Breaks bonds, raises temperature locally on the surface and provides activation energy [J. Appl. Phys. 50, 3189 (1979)] 33 Sidewall Passi vation ¾ Deposition of carbon polymer material on the sidewalls where: (a) Either the carbon is provided by the feed gas through the chamber such as CHF 3, C4F8. (b) Or the carbon is provided by the erosion of the photoresist etch mask. 34 Sidewall Passi vation ¾Oxidation of the sidewall by adding O2 gas. 35 BhPBosch Process 500um Silicon Etched by T. Switching SF6 and C4F8 Maleki using STS ASE (8um/min etch rate) The sid ewa ll film t hic kness depen ds to t he depos it ion or pass ivat ion t ime. 36 TtfftilthiTemperature effects in plasma etching ¾Wafer surface temp. depends on: - Chuck temperature - I’Ion’s energy an dditd density ¾Reaction probability of radicals depends on substrate temperature. ¾Helium backside cooling helps anisotropic etch by preventing or reducing reaction of F and Cl species with sidewalls. 37 Notch Effect ¾ Notching effect due to charging oxide by ions ¾ Can be reduced by using low frequency (LF) 380kHz bias generator in pulsed mode e- + +++++++ RF bias RF bias on off [J. Vac. Sci. Technol. B 19.5., Sep/Oct 2001] STS ASE has LF pulsed generator!! 38 Other Effects Grass or micromaskinggy issue mainly because of metal mask sputtered on the wafer Aspec t Ra tio Depen den t Effec t (ARDE) Typically large open areas etch faster than smaller features! 39 Other Effects Microloading effect: Feature of the same size etch more slowly in dense patterns Than in wide open areas! Etchi ng Species Density x 40 Ou tline 1. Introductory Concepts 2. Plasma Fundamentals 3. The Physics and Chemistry of Plasmas 4. Anisotropy Mechanisms 5. The Etching of Si and its Compounds 6. The Etching of Other Materials 41 STS ASE DRIE It’s a 6” ICP Bosch process dedicated for silicon etching! Gases: SF6 C4F8 O2 Ar 42 STS ASE DRIE High etch-rate recipe: Switching Pressure RF coil power RF bias Gas flow time power [sccm] Etch 858.5 sec 40m Torr 2200W 40W 450 SF6 Passivation 3 sec 14mTorr 1500W 20W 200 C4F8 Etch rate ≈ 8µm/min for 500 µm feature size with ~ 20% exposed area High selectivity to PR ≈ 75-100 43 STS ASE DRIE Low etch-rate recipe: Switching Pressure RF coil power RF bias Gas flow time power [sccm] Etch 13 sec 5T5mTorr 800W 25W 160 SF6 Passivation 7 sec 1mTorr 600W 20W 85 C4F8 Etch rate ≈ 2µm/min for 500 µm feature size with ~ 20% exposed area High selectivity to PR ≈ 50 Smooth side wall 44 STS AOE DRIE It’s a 4” ICP tool dedicated for etching of oxide, nitrite, and polymers! Gases: SF6 C4F8 CF4 O2 Ar He Pressure RF coil power RF bias Gas flow power [sccm]