Lecture 2 Dry Etching I

Transcription

1 EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie Dry Etching Agenda: DC lasma lasma discharge zones aschen slaw RF lasma High-density lasmas DRIE Microloading Silicon grass Lecture 2 Dry Etching I Reading: M. Madou, Chapter 2, pp Most figures in this presentation are adapted from M. Madou, Chapter 2 EEL6935 Advanced MEMS 2005 H. Xie 1/7/ EEL6935 Advanced MEMS 2005 H. Xie 2 lasmas Glow Discharge lasma Glow occurs when a DC voltage is applied between two electrodes in a gas Low pressure (0.001~10 Torr) High voltage (~1k) 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 Ions Neutrals Electrons EEL6935 Advanced MEMS 2005 H. Xie 3 EEL6935 Advanced MEMS 2005 H. Xie 4

2 Glow Discharge lasma Glow Discharge lasma Average particle energy is given by <E e >=k B T e for electrons <E i >=k B T i for ions Ions Neutrals Electrons Electron-Molecule Collisions Dissociation e - + Cl 2 2Cl + e - e - + CF 4 CF 3 + F + e - Typical values <E e >: 1~10e (hot) <E i >: 0.02~0.1e (cold) Thus, T e >>T i e.g., Ee~2e; Ei~0.4e Then, T e = 23,000 K! But T i = 490 K Effective current density J e = n e q<v e >/4 J i = n i q<v i >/4 <v e > is much greater than <v i >, so J e >> J i ermanent positive charge electrons lost to the walls Ionization Highly reactive radicals e - + Cl 2 2Cl + + 2e - e - + CF 4 CF 3+ + F + 2e - Excitation e - + F F * + e - F * F + hv hotons e - + Ar Ar * + e - Ar * Ar + hv EEL6935 Advanced MEMS 2005 H. Xie 5 EEL6935 Advanced MEMS 2005 H. Xie 6 Reactive lasma Etching Glow Discharge lasma Chemical etching Isotropic Color of light emission depends on gas, ionization energy, pressure and electric field EEL6935 Advanced MEMS 2005 H. Xie 7 EEL6935 Advanced MEMS 2005 H. Xie 8

3 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 ositive column: quasineutral, low electric field, uniform; not important for etching or deposition Glow Discharge lasma Breakdown voltage () aschen s Law The breakdown voltage is a function of the product of the gas pressure and the gap distance, i.e., = f(xd) 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. EEL6935 Advanced MEMS 2005 H. Xie 9 EEL6935 Advanced MEMS 2005 H. Xie 10 RF lasmas RF lasmas Capacitive coupling RF voltage source 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. ( ) RF p p 2 DC 2 p : plasma potential DC : self-bias RF : applied RF signal Self-bias DC : 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 + ~300e The maximum energy of positive ions striking the anode is ( ) DC e ~20e EEL6935 Advanced MEMS 2005 H. Xie 11 EEL6935 Advanced MEMS 2005 H. Xie 12

4 RF lasmas RF lasmas Child-Langmuir equation for the ion-current density Ji d 3/2 2 where is the voltage drop across a dark space; d is the thickness of the dark space. Equivalent electrical circuit of RF plasma Each of the cathode and anode dark spaces behaves like a diode and can be modeled as a capacitor. A C d where A is the area of each electrode; d is the thickness of the dark space. T = DC + The ion-current densities on both the anode J i () and cathode J i (T) must be equal, i.e., 3/2 3/2 T = 2 2 d dt The RF voltage is split between the two capacitors in series, i.e., T C = C T Combining the above three equations yields T A dt A T = = AT d AT 3/4 4 A T = AT EEL6935 Advanced MEMS 2005 H. Xie 13 EEL6935 Advanced MEMS 2005 H. Xie 14 RF lasmas High-Density lasmas T A = AT where A is the area of anode; A T is the area of cathode. 4 High etching rate requires high plasma densities (> /cm 3 ) Higher pressures (more gas atoms) higher plasma densities But smaller mean free path and thus less directionality T = DC + 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 by increasing the anode area will also help reduce the damage of the plasma to the chamber. Better solution: Increase the number of collisions of each electron. But how to realize this? New plasma sources Electron Cyclotron Resonance (ECR) Inductively Coupled lasma (IC) EEL6935 Advanced MEMS 2005 H. Xie 15 EEL6935 Advanced MEMS 2005 H. Xie 16

5 High-Density lasmas Electron Cyclotron Resonance (ECR) Lorentz force F = qv B An electron in a static and uniform magnetic field will move in a circle. Applying an alternating electrical field will result in a cycloid. The frequency of this cyclotron motion is given by eb ω 0 = m This is called electron cyclotron resonance frequency. Inductively Coupled lasma (IC) High-Density lasmas A MHz RF signal applied to a coil (helical or planar) induces an alternating magnetic field Electron density can reach > /cm 3 An outer shield isolates RF field from surrounding equipment A slotted inner shield may be used. lanar Coil IC When the frequency of the electric field is set to ω o, electron resonance occurs. For the commonly used microwave frequency 2.45 GHz, the resonance condition is met when B = 875 G = T. Electron density up to /cm 3 EEL6935 Advanced MEMS 2005 H. Xie 17 Cross-section view Top view EEL6935 Advanced MEMS 2005 H. Xie 18 hysical/chemical Etching hysical/chemical Etching Two etching mechanisms Chemical etching hysical etching (sputtering, ion milling) < 100 mtorr hysical Sputtering hysical momentum transfer Directional oor selectivity Radiation damage possible Higher excitation energy F S i F Ar + S i Ar mtorr range Reactive Ion Etching (RIE) hysical and chemical Directional Selective S i Higher pressure Reactive lasma Etching Chemical Fast Isotropic Highly selective Less prone to radiation damage EEL6935 Advanced MEMS 2005 H. Xie 19 EEL6935 Advanced MEMS 2005 H. Xie 20

6 Frequently Used Gases Frequently Used Gases, SF 6, SF 6 EEL6935 Advanced MEMS 2005 H. Xie 21 EEL6935 Advanced MEMS 2005 H. Xie 22 Etching rofiles Anisotropy Energy-driven anisotropy Etch rate increases with increasing bias voltage Undercut x is determined by the etch rate at zero bias x The etch depth z ~ z, etch rate at a bias x/z = x / z 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

7 Anisotropy Some Simple Rules Inhibitor-driven anisotropy 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. 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 EEL6935 Advanced MEMS 2005 H. Xie 25 EEL6935 Advanced MEMS 2005 H. Xie 26 Some Simple Rules 4. and 5. Dry etching of III- compounds Group III halides (fluorides in particular) tend to be nonvolatile Chlorine-based etchants are often used And elevated substrate temperatures Crystallographic etch patterns 6. Metal etching Chlorocarbons and fluorocarbons Chlorines are preferred for Al etching (AlF3 is not volatile) Deep Reactive Ion Etch Advanced Silicon Etch (ASE ) Inductively Coupled lasma (IC) Invented by Robert Bosch Corp. Simple, but very clever idea Huge impact to MEMS Bosch rocess Si IC etch assivation Si IC etch Scallops Alternative etching and passivation Sucessive SF 6 silicon etch/chf 3 (or similar fluorinecarbon compound) deposition) Sidewall passivation via teflon-like compound Separate control of plasma generation and directionality High density plasma Tunable bias voltage EEL6935 Advanced MEMS 2005 H. Xie 27 EEL6935 Advanced MEMS 2005 H. Xie 28

8 Bosch rocess Bosch rocess STS IC Etcher Alcatel 601E IC Etcher Other Deep Silicon IC etcher providers: Alcatel, lasma Therm EEL6935 Advanced MEMS 2005 H. Xie Other Deep Silicon IC etcher providers: STS, lasma Therm 29 EEL6935 Advanced MEMS 2005 H. Xie Bosch rocess 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 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