JePPIX Course Processing Wet and dry etching processes Huub Ambrosius
Material removal: etching processes Etching is done either in dry or wet methods: Wet etching uses liquid etchants with wafers immersed in etchant solution. Wet etch is cheap and simple, but hard to control (not reproducible), not popular for nanofabrication for pattern transfer purpose. Dry etch uses gas phase etchants in plasma, both chemical and physical (sputtering process). Dry plasma etch works for many dielectric materials and some metals (Al, Ti, Cr, Ta, W ). For other metals, ion milling (Ar + ) can be used, but with low etching selectivity. (as a result, for metals that cannot be dry-etched, it is better to pattern them using liftoff) Etching is consisted of 3 processes: Mass transport of reactants (through a boundary layer) to the surface to be etched. Reaction between reactants and the film to be etched at the surface. Mass transport of reaction products from the surface through the surface boundary layer. Figures of merit: etch rate, etch rate uniformity, selectivity, and anisotropy. 2
Figures of merit: selectivity Etching selectivity: The ratio of etching rate between different materials, usually the higher the better. Generally, chemical etching has high selectivity, physical etching (sputtering, milling) has low selectivity. For fabrication, the selectivity is usually between film material and mask material, and is defined by S fm. (f: film; m: mask) Temperature affects selectivity 3 Etching with mask erosion
Selective over-etch of different materials The film is etched through to the bottom, plus over-etch to etch laterally for under-cut profile. 4
Figures of merit: anisotropy Isotropic: etch rate is the same along all directions. Anisotropic: etch rate depends on direction, usually vertical vs. horizontal. For isotropic, R I =1. For complete anisotropic, R I =0. CD: 5 critical dimension
Figures of merit: anisotropy Generally speaking, chemical process (wet etch, plasma etch) leads to isotropic etch; whereas physical process (directional energetic bombardment) leads to anisotropic etch. Isotropic: Best to use with large features when sidewall slope does not matter, and to undercut the mask (for easy liftoff). Large critical dimension (CD, i.e. feature size) loss, generally not for nano-fabrication. Quick, easy, and cheap. Anisotropic: Best for making small features with vertical sidewalls, preferred pattern transfer method for nano-fabrication and some micro-fabrication. Typically more costly. 6
Etching The reverse of Epitaxy Etching = Removal of Material SiCl 4 + H 2 <--> SiHCl 3 + HCl; SiHCl 3 + H 2 <--> SiH 2 Cl 2 + HCl; SiH 2 Cl 2 <--> SiCl 2 + H 2 ; SiHCl 3 <--> SiCl 2 + HCl; SiCl 2 + H 2 <--> Si + 2HCl Etching reaction: SiCl 4 (g) + Si(s) <-- > 2 SiCl 2 (g) Effect of SiCl 4 concentration on silicon Epitaxial Growth PAGE 7
Wet chemical etching Extensively used for: Clean surface before processes like epitaxy Controlled removal of material through lithographically defines masks PAGE 8
Etch Rate Surface reaction limited etch process: Etch rate, R, depends on temperature: R = Kexp(-E a /kt) Material transport (through diffusion) limited etch process: Etch rate depends on agitation or stirring Sometimes both reaction rate and diffusion contribute Reaction limited etching is preferred PAGE 9
Silicon etching as example Step one: Oxidation - Si + 4HNO 3 SiO 2 + 2 H 2 O + 4NO 2 Step two: Oxide dissolution - SiO 2 + 6HF H 2 SiF 6 + 2H 2 O Use a mixture of nitric acid and hydrofluoric acid, dilute it with water or acetic acid PAGE 10
Orientation dependent Si Etching More bonds to break gives a slower etch rate in some etchants (or etch solutions) - (111) slowest = most dense, most bonds - (110) - (100) fastest = most sparse, fewer bonds Orientation dependent etchant for Si: KOH: water: Isopropyl alcohol Strong orientation dependence can be used to fabricate submicron device structures Orientation dependent etching: a) Through window patterns on <100> oriented Silicon. b) Through window patterns on <110> oriented Silicon PAGE 11
Wet Chemical Etching In III-V: Polarity along [111] (111)B (111)A Si vs III-V PAGE 12
III-V materials etching Oxidation ( or reduction ) of semiconductor surface - H 2 PO 4, HNO 3, H 2 SO 4, HCl Removal of soluble reaction product - H 2 O 2 Dilute the etch solution with - H 2 O Extensively used etching systems - GaAs: H 2 SO 4 : H 2 O 2 and H 3 PO 4 : H 2 O 2 : H 2 O - InP: HCl : H 2 O Selective etching: - H 2 SO 4 : H 2 O 2 : H 2 O etches InGaAs and InGaAsP but not InP - HCl : H 2 O etches InP but not InGaAs and InGaAsP PAGE 13
Limitations of wet etching Wet etching tends to be isotropic (etches all directions) this causes undercuts (b) Not very suitable for small pattern transfer (roughly minimum pattern around 2 µm) Use dry etching (plasma etching) for small pattern transfer (c) PAGE 14
Wet Chemical Etching diffusion-limited or mass-transportlimited etching Stirring reaction-rate-limited, surface-limited, or kinetically-limited etching Stirring 1. Diffusion of etching species to the surface 2. Chemical reaction 3. Diffusion of etching products out of the surface PAGE 15
Wet Chemical Etching Principle of etching 1. oxidising the semiconductor 2. Forming a complex (with acid or alkali) 3. dissolving the complex in the solvent Etching GaAs: H 2 O 2 /H 2 SO 4 /H 2 O H 2 O 2 /H 3 PO 4 /CH 3 OH H 2 O 2 can not oxidise InP (too low potential) Etching InP: Etching InGaAsP: H 3 PO 4 / HCl H 2 O 2 /H 2 SO 4 /H 2 O PAGE 16
Wet Chemical Etching (110) Anisotropic Etching / Crystal Planes Resist Mask A (Ga) B (As) A (Ga) B (As) (110) Etch-Rate {111}B > Etch-Rate {100} > Etch-Rate {111}A PAGE 17
Wet Chemical Etching In III-V: Polarity along [111] (111)B (111)A Si vs III-V PAGE 18
Wet Chemical Etching (111)B or (111)As (111)A or (111)Ga Element III (Ga or In) has no available electrons; hence difficult to remove As has available electrons; hence chemically active In III-V chemical etching along plane (111)A is slower than etching along plane (111)B. Asymmetric charge distribution along [111] in III-V (polarisation). PAGE 19
Wet Chemical Etching Stripe // [011] Stripe // [011] PAGE 20
Common Planes and Axes C B A H Angle OCH = Arc tg (1 2 a)/a = 35 16 PAGE 21
Wet Chemical Etching 54 44 Stripe // [011] Stripe // [011] Etching InP with Br 2 :CH 3 COOH, 1 min at 25 C 4% Br 2 2%Br 2 Ref: Adachi et al., J. Electrochem. Soc. June 1981, P 1342-1349 PAGE 22
Wet Chemical Etching 54 44 35 16 Stripe // [011] Plane (112) Stripe // [011] Etching InP with H 3 PO 4 :HCl (4:1) Selective etching of InP towards Q PAGE 23
Wet Chemical Etching Etching InP with HCl:H 2 O (4:1) at RT Along [011] self-limiting at (112) plane Along [011], etch rate ~5 µm/min PAGE 24
Wet Chemical Etching Suspended membrane: Etching InP with HCl:H 2 O (4:1) at 2 C Along [011] Along [011] PAGE 25
InP Wet Etching InP can be etched using HCl:H 3 PO 4 (1:4) Etch rate of InP: ~ 1 µm/min at RT This etch is selective towards Quaternary. Masking: photoresist, SiO x or SiN x HCl:H 3 PO 4 PAGE 26
InGaAs and InGaAsP Wet Etching InGaAs, and InGaAsP can be etched using a solution of H 2 O 2 :H 2 SO 4 :H 2 O (1:1:10) Preparation of the solution Add H 2 SO 4 to H 2 O, wait till the solution cools down, add H 2 O 2 Ech rate of Q1.3» 150 nm/min at RT Etch rate of Q 1.1» 900 nm/min at RT Etch rate of InGaAs» 900 nm/min. There is very little etching of InP Masking: photoresist, SiO x or SiN x PAGE 27
Reactive Ion Etching Capacitive RF discharge in a parallel plate reactor DC bias (ion energy) is a result of RF power, pressure and gas feedstock PAGE 28
Reactive Ion Etcher at PAGE 29
Reactive Ion Etching Isotropic Etch Ion bombardment leads to anisotropic etch Role of sidewall passivating films in anisotropic etch Less anisotropy due to mask etching PAGE 30
Reactive Ion Etching Chemical etching Ion-radical synergism Physical etching Si etch rate (nm/min) Time (s) Role of ions in reactive ion etching: Tenfold increase of etching rate with XeF 2 +Ar as compared to XeF 2 or Ar alone From: Coburn and Winters, J. Appl. Phys. 50, 3189 (1979); Surf. Sci. Rep. 14, 162 (1992). PAGE 31
Reactive ion etching PAGE 32
CH 4 -H 2 Reactive Ion Etching CH 4 -H 2 RIE a combination of processes Chemical: InP + 3 CH 4 **= In(CH 3 ) 3 + PH 3 CH 4 **= CH 3, CH 2, CH, H radicals and ions Reaction needs activation-energy E act ~(½ mv 2 ) Physical: Ion bombardment by accelerated Ions Not selective (½ mv 2 ) ~ bias voltage Introduce (surface) roughness Substrate temperature increases > 100 o C Polymer deposition: x CH 4 = CH 3 - (CH 2 ) x-2 - CH 3 + x H 2 ++ high selectively for any masking material Anisotropy by sidewall protection by polymer Sidewall roughness due to polymer deposition Trade off between anisotropy and optical loss. PAGE 33
Influence of Polymer Influence of descum CH 4 -H 2 etching for 30 min CH 4 -H 2 - etching + O 2 -descum PAGE 34
CH 4 -H 2 Reactive Ion Etching Polymer deposition on electrodes stops the process Bottom electrode: Quartz plate with 2 inch InP-substrate and deposited polymer Top electrode with deposited polymer Process to clean the electrodes (O 2 + CHF 3 ) PAGE 35
Size Evolution Sidewall angle and descumming Etching only Increased descumming time PAGE 36
Shallow and deep waveguides Shallow etching PAGE 37 Deep etching High-contrast
Shallow and deep waveguides Double-Etch step RIE CH 4 /H 2 : 20/80 sccm, 220W, 60mTorr, 2 min O 2 : 100 sccm, 220W, 100mTorr, 4 sec Typical losses of 3µm RWG on undoped structures: 1-1.5 db/cm (shallow) and 2-3 db/cm (deep) PAGE 38
Definition of all waveguides 50nm SiN x masking layer PAGE 39
Patterning area for deep etching (HPR) + 1 st RIE HPR photo-resist PAGE 40
Remove resist + 2 nd RIE PAGE 41
Litho + 3 rd RIE for isolation etch (if required) PAGE 42
High Density Plasma High density plasma sources with plasma production spatially separated from substrate zone Inductively coupled plasma (ICP) Microwave (MW) plasma Expanding thermal plasma (ETP) PAGE 43
ICP etch tool in Clean room PAGE 44
High Density Plasma Rf diode Operating conditions High density Pressure p Gas flow Φ Power P Frequency f 5-1000 Pa 5-200 sccm 10-2000 W 13.56 MHz (50 khz-50 MHz) Capacitive coupling Plasma parameters 1-10 Pa 5-200 sccm 100-5000 W 13.56-2450 MHz Inductively coupling/ wave-heated Electron density n e Electron temperature T e Ion acceleration energy E i Ionization degree Rf diode 10 15-10 17 m -3 3 ev (1-5 ev) 200-1000 ev 10-6 -10-3 PAGE 45 High density 10 16-10 18 m -3 2-7 ev 20-500 ev 10-4 -10-1
ICP for Deep Etching DBR gratings Chemistry: Cl 2 :Ar:H 2 ICP plasma PAGE 46
2a- Integrated laser + DBR Gratings Double etching process: shallow for laser and deep for DBR grating Shallow etched SOA area Deeply etched DBR area PAGE 47
2a- Integrated laser + DBR Gratings 3 rd order DBR grating PAGE 48
2a- Integrated laser + DBR Gratings Double etch after planarization with BCB Deep etch: > 5 µm deep Shallow etch: > 1.8 µm deep PAGE 49
ICP for Deep Etching Photonic Crystals Pillars Chemistry: Cl 2 :Ar:H 2 ICP plasma PAGE 50
2b- PhC Pillars Cl 2 :Ar:H 2 (7:4:12sccm), ICP=1000W, RF140W, 4 mtorr, 200 C, 2min using Cr-lift-off with PMMA on 430nm SiO x PAGE 51
2b- PhC Pillars Cl 2 :Ar:H 2 (7:4:12sccm), ICP=1000W, RF140W, 4 mtorr, 200 C, 2min using Cr-lift-off with PMMA on 430nm SiO x PAGE 52
ICP for Deep Etching ZEP/Cr/SiO x (320/50/500nm) Etching: 1min 40sec ICP Cl 2 :O 2 (14:2 sccm) 1000W ICP 160W RF 1.2 mtorr, 200ºC PhC Holes Hole 160nm Aspect ratio: >18 PAGE 53
ICP for Deep Etching Well defined Patterns and very smooth morphology PAGE 54
Criteria for Deep Etching in 1D and 2D PhC q Anisotropy of the etching process q Suitability of used chemistry q Suitability of used mask v Etching selectivity between InP/Q vs mask v Mask thickness limitation Capability of opening the mask using optical of E-beam lithography PAGE 55
RIE Lag Effect RIE lag effect: the narrower the trench the lower the etch depth PAGE 56
RIE Lag Effect Stripe Width (nm) 1110 950 790 710 570 450 350 235 145 Depth (µm) 2.87 2.68 2.56 2.44 2.33 2.17 1.97 1.64 1.02 ICP-RIE Lag Effect Etch Depth (µm) 3.5 3 2.5 2 1.5 1 0.5 0 0 200 400 600 800 1000 1200 Stripe Width (nm) PAGE 57