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

Contents Introduction What is plasma?? Reaction processes in plasma Mechanism of plasma etching Dry etch reactor Process requirement of dry etching In-situ diagnostic method of plasma etch Device damage from plasma Case study 9.1 Silicon etch 9.2 Metal etch 9.3 GaAs and InP etch 9.4 GaN and related material etch

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. InGaN mesa for LED etched by ICP InP via-hole etched by RIE GaAs laser-facet etched by ICP

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 2 Figure 1

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

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”.

Processes in the sheath region

7. 4. Mechanism of plasma etching 7. 4. 1. Etch mechanism Physical sputtering - purely physical process by energetic ion bombardment Chemical etching - purely chemical process by forming volatile by-product through chemical reaction between substrate and active radicals in plasma Accelerated ion-enhanced etching - chemical etching + physical etching: removal of volatile product is accelerated by energetic ion bombardment Sidewall-protected (inhibitor driven) ion-enhanced etching – deposition of etch-resistant layer with ion bombardment anisotropic etching

7. 4. 2. Sequential steps in etching 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 etch-product 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 cf) If any of these steps fails to occur, the overall etch cycle ceases and the step failed is a rate-limiting step

7. 4. 3. Radicals in plasma Radicals are generated through dissociation and ionization ex) e + O2  O+ + O* + 2e, e + CF4  CF3+ + 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.

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 Ma is proportional to its vapor pressure, pa, (refer to Chap. II) The evaporation rate is increased with increasing temperature. However, plasma etching generally done at room temperature.  formation of volatile product at RT is most important.

Boiling point of etch product (Si and metal)   Etch product Boiling point (℃) Comment Si (SiO2, Si3N4) SiH4 -111.6 Gas at RT SiF4 -95.7 Si2H6 -15 SiHCl3 31.7 SiCl4 56.7 Si2OCl6 135.5 Si2Cl6 147 Metal (Ag, Al, Ti, Au, Co, Cr, Cu, Ni, Pb, Pt, Ta, W, Zn) AgCl 1550 AlCl3 182.7 Sublimation TiCl4 136.45 TiF4 284 Au2Cl3 - Non volatile Au2Br3 CoCl2 1050 CrO2Cl2 117 Cr(CO)6 151 CuCl2 655 CuBr2 900 Ni(CO)4 -25 PbCl2 954 PtF6 69.1 TaF5 229.5 WF6 17 (CH3)2Zn 46 ZnCl2 756

Boiling point of etch product (III-V semiconductor)   Etch product Boiling point (℃) Comment III-V semiconductor (GaAs, InP, GaN) Ga2H6 -63 Gas at RT GaCl3 201.3 GaCl2 535 GaF3 ~ 1000 GaBr3 279 GaI3 < 345 (CH3)3Ga 55.7 (C2H5)3In -32 (CH3)3In 88 InCl3 418 Sublimation InBr3 371 AsH3 -54.8 AsF5 -52.9 AsF3 63 AsCl3 130.4 AsBr3 221 PF3 -101 PH3 -88 PF5 -75 PCl5 62 NCl3 < 71 NF3 -129 NI3 - Explode NH3 -33 N2 -196 (CH3)3N

Examples that show extremely low etch rate. Etch Al in fluorine-based gas : AlF3 is not volatile Etch Ni in chlorine-based gas : NiCl2 is not volatile Etch Al2O3 in Cl2 plasma: Al2O3 + Cl2  AlCl3 + O2 (Uphill thermodynamically, but etched with UV laser irradiation) Etch SiO2 in Cl2 plasma: Uphill thermodynamically, but etched with energetic ion bombardment.

Typical gases used for plasma etching   Feed gas Mechanism Selective to n type Si Cl2 Chemical SiO2 Cl2/C2F6 Ion-inhibitor SiCl4 Si Ion-energetic CCl4/O2 SiCl4/O2 Al Sl2/SiCl4 Ion inhibitor /energetic SiO2, some resist, Si3N4 Cl2/CCl4 Cl2/CHCl3 Cl2/BCl3 III-V semiconductor SiO2, resist Cl2/CH4 CCl4/ O2 Without Al Cl2/O2 Al-containing alloy, SiO2 CF2Cl2

7. 5. Dry etch method and reactor type Plasma method Plasma etching (PE) Reactivel ion etching (RIE) High density plasma etching: Electron cyclotron resonance etching (ECR) and inductively coupled plasma etching (ICP) Ion beam method Ion beam etching (IBE) Reactive ion beam etching (RIBE) Chemically-assisted ion beam etching (CAIBE)

Detailed characteristics of dry etching technique Parameter PE RIE MERIE ICP ECR IBE (sputter) frequency 13.56MHz 2.45GHz - Pressure (torr) 0.1 ~ 10 0.01 ~ 0.1 0.01 0.001 ~ 0.01 0.001 ~ 0.1 Te (eV) ~ 8 ~ 5 ~ 4   Plasma density ~ 3e8 ㎝-3 ~ 1e10㎝-3 ~5e10㎝-3 ~5e11㎝-3 Wafer location Grounded electrode Powered electrode Ion voltage 25 ~ 100 V 250 ~ 500 V 400 ~ 1000 V 0 ~ 500 ~ 2000 V Ion energy Not Controllable Not controllable Chemical reaction Yes No Physical reaction Selectivity Excellent Good Poor Anisotropy poor

Comparison of dry etching technique

7. 5. 2. Reactor types of dry etch (a) Plasma etching (PE) and Reactive ion etch (RIE)

Reactive ion etch (RIE) 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

(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

(c) Electron Cyclotron Resonance etching z Quartz window Magnet k B Plasma generation y - x ER v Circularly polarized wave Electron cyclotron motion

ECR: One of the high density plasma source (5 x 1011 ㎝-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 ωc: When the electric field E of microwave is perpendicular to the magnetic field and the circular wave of magnetic field satisfies ω = ωc, 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.

(d) Inductively coupled plasma(ICP) 13.56 MHz currents pass through ICP coil 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 Z

Types of Inductively coupled plasma(ICP) Planar type ICP Contamination of wafer by sputtering of window material. Cylindrical type ICP Contamination-free geometry

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

(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.