Plasma etching Bibliography 1. B. Chapman, Glow discharge processes, (Wiley, New York, 1980). - Classical plasma processing of etching and sputtering 2. D. M. Manos and D. L. Flamm, Plasma etching; An introduction, (Academic, Boston, 1989). - Most helpful textbook for the researcher majoring the dry etching. 3. M. Sugawara, Plasma etching; Fundamentals and applications, (Oxford Univ. Press, New York, 1998). - Mostly dedicated to the high density plasma sources such as ICP and ECR 4. W. N. G. Hitchon, Plasma processes for semiconductor fabrication, (Cambridge Univ. Press, Cambridge, 1999) - Theoretical approach to the plasma etching and plasma deposition process 5. R. J. Shul and S. J. Pearton, Handbook of advanced plasma processing techniques, (Springer, Heidelberg, 2000). - Helpful textbook for the researcher in the field of compound semiconductor process 6. http://newton.hanyang.ac.kr/plasma/ - Dedicated to the plasma physics for graduate student in physics 1
Contents 1. Introduction 2. What is plasma?? 3. Reaction processes in plasma 4. Mechanism of plasma etching 5. Dry etch reactor 6. Process requirement of dry etching 7. In-situ diagnostic method of plasma etch 8. Device damage from plasma 9. Case study 9.1 Silicon etch 9.2 Metal etch 9.3 GaAs and InP etch 9.4 GaN and related material etch 2
7.1. Introduction Etch removal of unwanted area during the fabrication of semiconductor Etching is the most important step in the fabrication of semiconductor devices along with a lithography technique. InP via-hole etched by RIE GaAs laser-facet etched by ICP InGaN mesa for LED etched by ICP 3
Dry etching by using plasma. Anisotropic feature profile Fig. 2 (c) and (d) High aspect ratio etching Fig. 1 Wet etching by using wet chemical solution. Isotropic feature profile Fig. 2 (a) Low aspect ratio etching Fig. 2 (b) 4 Figure 1 Figure 2 4
Advantage of plasma etching Etching can be anisotropic Less consumption of chemicals; cost, environment impact Clean process (vacuum) Compatible with automation Precise pattern transfer Deep silicon etching for sensor application 5
Formation of sheath region The fast-moving electrons hit the wall before the ions do and some stick to the wall. The wall charges up negatively and this negative charge pushes other electrons away at the same time as attracting positive ions. The field near the wall holds the electrons away from the wall and accelerates the positive ions toward the wall. High energy ion bombardment cf) Generally, the voltage drop couldn t be measured. In practice process engineers usually monitor the dc potential (relative to ground) of the electrode instead, which is called dc-bias. 6
Processes in the sheath region 7
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7. 4. Mechanism of plasma etching 7. 4. 1. Etch mechanism (a) (b) Physical sputtering - purely physical process by energetic ion bombardment Chemical etching - purely chemical process by forming volatile byproduct through chemical reaction between substrate and active radicals in plasma (c) Accelerated ion-enhanced etching - chemical etching + physical etching: removal of volatile product is accelerated by energetic ion bombardment (d) Sidewall-protected (inhibitor driven) ion-enhanced etching deposition of etch-resistant layer with ion bombardment anisotropic etching 9
7. 4. 2. Sequential steps in etching cf) If any of these steps fails to occur, the overall etch cycle ceases and the step failed is a rate-limiting step 1 Formation of active etchant by electron collisions 2 Transport of active etchant to the wafer surface 3 Adsorption of etchant to wafer surface 4 Reaction of etchant and wafer to form etch-product 5 Desorption of etch-product from the wafer surface 6 Acceleration of desorption of etchproduct by ion bombardment 7 Transport of etch-product to the bulk plasma 8 Redissociation of etch-product in the plasma or pumped out 9 Redeposited on the reactor wall or pumped out 10
7. 4. 3. Radicals in plasma Radicals are generated through dissociation and ionization ex) e + O 2 O + + O* + 2e, e + CF 4 CF 3+ + F* + 2e Radicals are much more abundant than ions in plasma because; (1) They are generated at a higher rate due to; - lower threshold energy and ionization is often dissociative (2) Radicals survive longer than ions Although the concentration of radicals is much larger than that of positive ions, the reactive fluxes incident on the surfaces can be comparable, since ions are moving faster because they have large energy obtained from the electric field in the sheath. 11
7. 4. 4. Volatility and evaporation Volatility of etch-products is a key distinction between plasma etching and sputtering. In general, desorption is a rate-limiting steps in the plasma etching Highly volatile by-product formation is important. Evaporation rate of material (a) with molecular weight M a is proportional to its vapor pressure, p a, (refer to Chap. II) M a a 2 RT a C a 1 2 M a 2 RT p The evaporation rate is increased with increasing temperature. However, plasma etching generally done at room temperature. H formation of volatile product at RT is most important. a, 1 2 e p a C RT a e H RT 12
Boiling point of etch product (Si and metal) Si (SiO 2, Si 3 N 4 ) Metal (Ag, Al, Ti, Au, Co, Cr, Cu, Ni, Pb, Pt, Ta, W, Zn) Etch product Boiling point ( ) Comment SiH 4-111.6 Gas at RT SiF 4-95.7 Gas at RT Si 2 H 6-15 Gas at RT SiHCl 3 31.7 SiCl 4 56.7 Si 2 OCl 6 135.5 Si 2 Cl 6 147 AgCl 1550 AlCl 3 182.7 Sublimation TiCl 4 136.45 TiF 4 284 Sublimation Au 2 Cl 3 - Non volatile Au 2 Br 3 - Non volatile CoCl 2 1050 CrO 2 Cl 2 117 Cr(CO) 6 151 CuCl 2 655 Non volatile CuBr 2 900 Non volatile Ni(CO) 4-25 PbCl 2 954 PtF 6 69.1 TaF 5 229.5 WF 6 17 Sublimation (CH 3 ) 2 Zn 46 ZnCl 2 756 Non volatile 13
Boiling point of etch product (III-V semiconductor) III-V semiconductor (GaAs, InP, GaN) Etch product Boiling point ( ) Comment Ga 2 H 6-63 Gas at RT GaCl 3 201.3 GaCl 2 535 GaF 3 ~ 1000 GaBr 3 279 GaI 3 < 345 (CH 3 ) 3 Ga 55.7 (C 2 H 5 ) 3 In -32 Gas at RT (CH 3 ) 3 In 88 InCl 3 418 Sublimation InBr 3 371 Sublimation AsH 3-54.8 Gas at RT AsF 5-52.9 Gas at RT AsF 3 63 AsCl 3 130.4 AsBr 3 221 PF 3-101 Gas at RT PH 3-88 Gas at RT PF 5-75 Gas at RT PCl 5 62 NCl 3 < 71 NF 3-129 Gas at RT NI 3 - Explode NH 3-33 Gas at RT N 2-196 Gas at RT (CH 3 ) 3 N -33 Gas at RT 14
Examples that show extremely low etch rate. Etch Al in fluorine-based gas : AlF 3 is not volatile Etch Ni in chlorine-based gas : NiCl 2 is not volatile Etch Al 2 O 3 in Cl 2 plasma: Al 2 O 3 + Cl 2 AlCl 3 + O 2 (Uphill thermodynamically, but etched with UV laser irradiation) Etch SiO 2 in Cl 2 plasma: Uphill thermodynamically, but etched with energetic ion bombardment. 15
Typical gases used for plasma etching n type Si Si Feed gas Mechanism Selective to Cl 2 Chemical Cl 2 /C 2 F 6 SiO 2 Ion-inhibitor SiCl 4 CCl 4 /O 2 Ion-energetic SiO 2 Cl 2 SiCl 4 /O 2 Sl 2 /SiCl 4 Al Cl 2 /CCl 4 Ion inhibitor /energetic SiO 2, some resist, Si 3 N 4 Cl 2 /CHCl 3 Cl 2 /BCl 3 Cl 2 Chemical Cl 2 /BCl 3 III-V semiconductor Cl 2 /CH 4 Cl 2 /CCl 4 Ion-inhibitor SiO 2, resist CCl 4 / O 2 SiCl 4 /O 2 III-V semiconductor Without Al Cl 2 /O 2 CF 2 Cl 2 Chemical Ion-inhibitor Al-containing alloy, SiO 2 16
7. 5. Dry etch method and reactor type 7.5.1. Dry etch method Plasma method (a) Plasma etching (PE) (b) Reactivel ion etching (RIE) (c) High density plasma etching: Electron cyclotron resonance etching (ECR) and inductively coupled plasma etching (ICP) Ion beam method (a) Ion beam etching (IBE) (b) Reactive ion beam etching (RIBE) (c) Chemically-assisted ion beam etching (CAIBE) 17
Detailed characteristics of dry etching technique Parameter PE RIE MERIE ICP ECR frequency 13.56MHz 13.56MHz 13.56MHz 13.56MHz 2.45GHz - IBE (sputter) Pressure 0.001 0.001 0.1 ~ 10 0.01 ~ 0.1 0.01 (torr) ~ 0.01 ~ 0.01 0.001 ~ 0.1 T e (ev) ~ 8 ~ 8 ~ 5 ~ 4 ~ 4 Plasma density ~ 3e8 cm -3 ~ 1e10cm -3 ~5e10cm -3 ~5e11cm -3 ~5e11cm -3 - Wafer location Grounded electrode Powered electrode Powered electrode Powered electrode Powered electrode Powered electrode Ion voltage 25 ~ 100 V 250 ~ 500 V 400 ~ 1000 V 0 ~ 0 ~ 500 ~ 1000 V 1000 V 2000 V Ion energy Not Not Not Controllable Controllable controllable Controllable Controllable Controllable Chemical reaction Yes Yes Yes Yes Yes No Physical reaction No Yes Yes Yes Yes Yes Selectivity Excellent Good Good Good Good Poor Anisotropy poor Good Good Good Good Excellent 18
Comparison of dry etching technique RIE CAIBE ECR ICP advantage economical Relatively fast etch rate fast etch rate fast etch rate, low plasma damage disadvantage slow etch rate, plasma damage low versatility high price, low scalability 19
7. 5. 2. Reactor types of dry etch (a) Plasma etching (PE) and Reactive ion etch (RIE) 20
Plasma etching (PE) Same reactor geometry as PECVD system Low ion bombardment energy due to the low sheath voltage drop sample was loaded on the grounded electrode (anode) Mainly chemical reactions and negligible physical etching Isotropic etch profile At relatively high pressure: 0.1 ~ 10 Torr Reactive ion etch (RIE) Combination of chemical activity of reactive radicals with physical effects due to high sheath drop sample was loaded on the powered electrode (cathode) Ion bombardment strongly enhances the chemical process Anisotropic etch profile due to ion bombardment Lower operation pressure of 0.01 ~ 0.1 Torr 21
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(b) Magnetically enhanced reactive ion etching (MERIE) Reduce the plasma loss on the chamber wall using magnetic field by electromagnet bucket Electron collisional efficiency increase by interaction of E and B field Substrate rotation for the increase of uniformity 23
(c) Electron Cyclotron Resonance etching z Quartz window Magnet k B Plasma generation y x E R Circularly polarized wave - v F e( E V B) Electron cyclotron motion 24
ECR: One of the high density plasma source (5 x 10 11 cm -3 ) ECR: Plasma generation by combining microwave(2.45 GHz) and the magnetic field by additional magnet. Plasma generation mechanism Microwave (2.45 GHz) is introduced into reaction chamber through quartz window Magnetic field is generated in the reaction chamber by magnet (permanent or electro magnetic) Electrons rotate around the magnetic line of force with the electron cyclotron angular frequency of ω c : eb c m When the electric field E of microwave is perpendicular to the magnetic field and the circular wave of magnetic field satisfies ω = ω c, electrons are continuously accelerated by the electric field of the microwave, obtaining high energy, and then ionizing the gas molecules by collisions. If microwave of 2.45 GHz are used, the ECR takes place at the magnetic field flux density of 875 G. e 25
(d) Inductively coupled plasma(icp) 13.56 MHz currents pass through ICP coil Z RF magnetic field formation along z axis Induction of vortex electric field Electrons oscillation Increase of electron collision efficiency More effective plasma generation than conventional RIE high radical density Electrostatic shield configuration eliminates capacitive coupling Independent ion energy control by table power 26
Types of Inductively coupled plasma(icp) Cylindrical type ICP Planar type ICP Contamination-free geometry Contamination of wafer by sputtering of window material. 27
Inductively Coupled Plasma(ICP) in NSL Laser interferometer on chamber-top Optical emission spectroscopy through sidewall window Electrostatic shield btw quartz and coil ICP/PECVD cluster tool at GIST 28
(e) Ion beam-based reactor IBE inert gas ion (Ar + ) formation in external RF ion source and extracted to the reaction chamber by acceleration electrode (grid). RIBE reactive gas besides inert gas ions are extracted from the external source to the reaction chamber. Etch rate is increased by the additional chemical reaction CAIBE inert gas ion (Ar + ) are extracted from the external source and the reactive gas are independently supplied to the wafer surface through shower-ring just above the wafer. 29