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EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie Dry Etching Lecture 2 Dry Etching I Agenda: Ê DC Plasma – Plasma discharge zones – Paschen’s Law Ê Ê Ê RF Plasma High-density Plasmas DRIE – Microloading – Silicon grass Reading: M. Madou, Chapter 2, pp. 77-107 Most figures in this presentation are adapted from M. Madou, Chapter 2 EEL6935 Advanced MEMS 2005 H. Xie 1/7/2005 1 EEL6935 Advanced MEMS 2005 H. Xie Plasmas Glow Discharge Plasma Glow occurs when a DC voltage is applied between two electrodes in a gas Ê Ê Ê Ê Ê Ê EEL6935 Advanced MEMS 2005 H. Xie 2 3 Low pressure (0.001~10 Torr) High voltage (~1kV) Electrons from cathode accelerated in the electric field ionize gas molecules and provide the plasma-sustaining current Energetic collisions create avalanche of ions and electrons Electrons move much faster than ions Neutral species greatly outnumber electrons and ions by 4 to 6 orders of magnitude EEL6935 Advanced MEMS 2005 H. Xie Ions Neutrals Electrons 4 Glow Discharge Plasma Ê Ê Average particle energy is given by <Ee>=kBTe for electrons <Ei>=kBTi for ions Glow Discharge Plasma Ions Neutrals Electrons Ê Dissociation Typical values Highly reactive radicals Ê Ê e- + Ar → Ar* + eAr* → Ar + hv Photons 5 EEL6935 Advanced MEMS 2005 H. Xie Reactive Plasma Etching e- + CF4 → CF3+ + F + 2e- Excitation e - + F → F* + e F* → F + hv Je = neq<ve>/4 Ji = niq<vi>/4 → <ve> is much greater than <vi>, so Je >> Ji ⇒ Permanent positive charge ⇒ electrons lost to the walls 2005 H. Xie Ionization e- + Cl2 → 2Cl+ + 2e- Effective current density EEL6935 Advanced MEMS e- + CF4 → CF3 + F + e- e- + Cl2 → 2Cl + e- <Ee>: 1~10eV (hot) <Ei>: 0.02~0.1eV (cold) Thus, Te>>Ti e.g., Ee~2eV; Ei~0.4eV Then, Te = 23,000 K! But Ti = 490 K Ê Electron-Molecule Collisions 6 Glow Discharge Plasma Chemical etching Isotropic Ê EEL6935 Advanced MEMS 2005 H. Xie 7 Color of light emission depends on gas, ionization energy, pressure and electric field EEL6935 Advanced MEMS 2005 H. Xie 8 Glow Discharge Plasma Ê Ê Ê Ê Ê Ê Special zones Aston dark space: low energy electrons Cathode glow: electrons gain sufficient energy to excite gas atoms Crookes dark space: electrons gain too much energy and luminescence is weak due to inefficient excitation Negative glow (brightest region): low electric field Faraday dark space: electrons slows down due to collisions and low electric field Positive column: quasineutral, low electric field, uniform; not important for etching or deposition Breakdown voltage (V) Paschen’s Law EEL6935 Advanced MEMS 2005 H. Xie 9 The breakdown voltage is a function of the product of the gas pressure and the gap distance, i.e., V = f(Pxd) The curves have minima. For large pxd, increasing pxd results in larger breakdown voltages. For small pxd, breakdown voltages increase with pxd decreasing. This is because when the pressure is too low or the distance is too small, most electrons reach the anode without any collisions. In air, the minimum breakdown voltage is 327 V. EEL6935 Advanced MEMS 2005 H. Xie RF Plasmas 10 RF Plasmas Capacitive coupling RF voltage source 2VP ≈ (VRF ) p − p 2 − VDC Vp: plasma potential VDC: self-bias VRF: applied RF signal Electrons oscillates between the electrodes with the AC voltage. No need for electron emission from cathode. Can sustain RF plasma at lower pressures than DC plasma. RF plasma allows etching of dielectrics as well as metals. EEL6935 Advanced MEMS 2005 H. Xie Self-bias VDC: electrons move faster than ions and charge up the cathode (electrons cannot cross over the capacitor) to build up a negative potential. ( •The maximum energy of positive ions striking the cathode is e VDC + VP •The maximum energy of positive ions striking the anode is 11 EEL6935 Advanced MEMS 2005 H. Xie eVP ) ~300eV ~20eV 12 RF Plasmas RF Plasmas Equivalent electrical circuit of RF plasma Child-Langmuir equation for the ion-current density V 3/ 2 Ji ∝ 2 d VT = VDC + VP where A is the area of each electrode; d is the thickness of the dark space. 2005 H. Xie The RF voltage is split between the two capacitors in series, i.e., The ion-current densities on both the anode Ji(P) and cathode Ji(T) must be equal, i.e., VT CP = VP CT VP 3 / 2 VT 3 / 2 = dP2 dT 2 EEL6935 Advanced MEMS Combining the above three equations yields VT A d A V = P T = P T VP AT d P AT VP 13 EEL6935 Advanced MEMS 4 where AP is the area of anode; AT is VT = VDC + VP EEL6935 Advanced MEMS 2005 H. Xie VT AP = VP AT 4 14 High-Density Plasmas High etching rate requires high plasma densities (> 1011/cm3) Higher pressures (more gas atoms) ⇒ higher plasma densities But smaller mean free path and thus less directionality Better solution: Increase the number of collisions of each electron. But how to realize this? the area of cathode. 3/ 4 2005 H. Xie RF Plasmas A VT = P V P AT A d C∝ where V is the voltage drop across a dark space; d is the thickness of the dark space. Each of the cathode and anode dark spaces behaves like a diode and can be modeled as a capacitor. The above equation shows that the smaller electrode has greater voltage drop. Thus, for plasma etching where the substrate is placed on the cathode, the anode area must be larger than that of the cathode. This can be done by connecting the anode to the walls of the chamber. In practice, the exponent (i.e., 4) in the above equation is not a constant. Instead, it varies with the area ratio. Reducing VP by increasing the anode area will also help reduce the damage of the plasma to the chamber. New plasma sources Electron Cyclotron Resonance (ECR) Inductively Coupled Plasma (ICP) 15 EEL6935 Advanced MEMS 2005 H. Xie 16 High-Density Plasmas High-Density Plasmas Electron Cyclotron Resonance (ECR) K K K F = qv × B • Lorentz force Inductively Coupled Plasma (ICP) Ê • An electron in a static and uniform magnetic field will move in a circle. Ê Ê • Applying an alternating electrical field will result in a cycloid. The frequency of this cyclotron motion is given by eB ω0 = Ê A 13.56-MHz RF signal applied to a coil (helical or planar) induces an alternating magnetic field Electron density can reach > 1012/cm3 An outer shield isolates RF field from surrounding equipment A slotted inner shield may be used. Planar Coil ICP m • This is called electron cyclotron resonance frequency. • When the frequency of the electric field is set to ωo, electron resonance occurs. • For the commonly used microwave frequency 2.45 GHz, the resonance condition is met when B = 875 G = 0.0875 T. Cross-section view Top view • Electron density up to 1011 /cm3 EEL6935 Advanced MEMS 2005 H. Xie 17 EEL6935 Advanced MEMS 2005 H. Xie Physical/Chemical Etching Physical/Chemical Etching Two etching mechanisms < 100 mTorr Ê Chemical etching Ê Physical etching (sputtering, ion milling) SiF Ar+ Higher pressure Reactive Ion Etching (RIE) Reactive Plasma Etching • Chemical • Fast • Isotropic • Highly selective • Less prone to radiation damage Si 2005 H. Xie Higher excitation energy • Physical and chemical • Directional • Selective Si Ar+ EEL6935 Advanced MEMS Physical Sputtering • Physical momentum transfer • Directional • Poor selectivity • Radiation damage possible 100 mTorr range F 18 19 EEL6935 Advanced MEMS 2005 H. Xie 20 Frequently Used Gases Frequently Used Gases , SF6 , SF6 EEL6935 Advanced MEMS 2005 H. Xie 21 EEL6935 Advanced MEMS 2005 H. Xie Etching Profiles 22 Anisotropy • Energy-driven anisotropy • Etch rate increases with increasing bias voltage • Undercut x is determined by the etch rate at zero bias Vx • The etch depth z ~ Vz, etch rate at a bias ⇒ x/z = Vx/Vz → Zero undercut if no etch at zero bias → Small undercut if very high bias EEL6935 Advanced MEMS 2005 H. Xie 23 EEL6935 Advanced MEMS 2005 H. Xie 24 Anisotropy Some Simple Rules • Inhibitor-driven anisotropy 1. Fluorine-to-carbon ratio (F/C) – Fluorine → etching – Hydrocarbons → polymerization – Adding oxygen reduces polymer due to CO and CO2 formation but increases resist attack. NF3 or ClF3 may be used instead. – Adding hydrogen increases polymer due to HF formation 2. Selective versus unselective dry etching – Higher polymerization rates typically lead to higher selectivity – Small additions of halogens significantly increase the selectivity of fluorine-based recipes 3. Substrate bias – negative bias reduces the polymerization tendency • Etch rate decreases with increasing hydrogen concentration • But undercut rate decreases even faster • This is because the formation of HF reduces F to C ratio and thus more polymer is formed. • But too much hydrogen will make the etching very slow. EEL6935 Advanced MEMS 2005 H. Xie 25 EEL6935 Advanced MEMS 2005 H. Xie 26 Bosch Process Some Simple Rules Deep Reactive Ion Etch 4. and 5. Dry etching of III-V compounds – Group III halides (fluorides in particular) tend to be nonvolatile – Chlorine-based etchants are often used – And elevated substrate temperatures – Crystallographic etch patterns • • • Advanced Silicon Etch (ASE ) Inductively Coupled Plasma (ICP) Invented by Robert Bosch Corp. ¾ Simple, but very clever idea ¾ Huge impact to MEMS 6. Metal etching – Chlorocarbons and fluorocarbons – Chlorines are preferred for Al etching (AlF3 is not volatile) Passivation Si ICP etch Scallops EEL6935 Advanced MEMS 2005 H. Xie 27 EEL6935 Advanced MEMS 2005 H. Xie Si ICP etch Alternative etching and passivation Ê Sucessive SF6 silicon etch/CHF3 (or similar fluorinecarbon compound) deposition) Ê Sidewall passivation via ‘teflon-like’ compound Separate control of plasma generation and directionality Ê High density plasma Ê Tunable bias voltage 28 Bosch Process Bosch Process STS ICP Etcher Alcatel 601E ICP Etcher Other Deep Silicon ICP etcher providers: Alcatel, Plasma Therm EEL6935 Advanced MEMS 2005 H. Xie Other Deep Silicon ICP etcher providers: STS, Plasma Therm 29 EEL6935 Advanced MEMS 2005 H. Xie Bosch Process 30 Homework 1 Common Issues • Silicon Grass or Black Silicon • micromasking • Al2O3 contamination from mask and/or chamber walls • Native oxide or dusts • Redeposition of mask material 1.1 1.2 • Solutions: • Cleaning samples • Cleaning chambers • Good thermal contact • Ion energy (RF power, bias) 1.3 FEM simulation and 3D model (a) Design a cantilever beam with a resonator frequency of 1 MHz. (b) Build its 3D model using Coventorware and verify the resonant frequency. • Microloading • RIE lag • Diffusion limited etching • For deep trench etches, increase SF6 flow rate. EEL6935 Advanced MEMS 2005 H. Xie 31 EEL6935 Advanced MEMS 2005 H. Xie 32