Chapter 5-1. Chemcal Etching

Outline Terminology Wet Etching Dry Etching DRIE Si / SiO2/ SiN/ Metal Dry etching

Etching terminology

Etching terminology

Etching terminology

Etching terminology

Etching terminology

Mechanism of wet etching

Mechanism of wet etching

Mechanism of wet etching

Mechanism of wet etching

Mechanism of wet etching

Experimental conditions for Si wafer etching Pattern size 100㎛ 100-1.414a=0 a= 100/1.414=70.72 Etch time 70/Etch rate of KOH per a hour =70/31=2.26hour Pattern size 400㎛ 400-1.414a=0 a= 400/1.414=282.89 282.89/Etch rate of KOH per a hour =282.89/31=9.13hour <100> Wafer

100um patterns by Si wafer etching

Wet etching VS Dry etching - 식각용액을 사용, 화학적인 반응만으로 박막 식각 - Selectivity 가 좋음 - 고가 장비 필요 없음, low cost 한꺼번에 많은 기판 처리, productivity ↑ undercut 발생, 용액의 측면 침식으로 미세 pattern 구현이 어려움 (3um 이하 어려움) - 화학약품의 과다 사용으로 환경문제 대두 - 비등방성 식각 가능 - 측면 침식이 거의 없음 - 식각 가공 resolution이 좋음 (1um 이하 가능) - Gas 사용으로 습식식각에 비해 상대적으로 깨끗하고 안전 - 물리적 충돌에 의한 식각도 일어나므로 완벽하게 특정 물질의 선택적 식각이 어려움

Dry etching # Advantage # Disadvantage - Uses small amounts of chemical - Isotropic or Anisotropic etch profile - Directional etching without using the crystal orientation of Si - High resolution and cleanliness - Less Undercutting # Disadvantage - Some gases are quite toxic - Need for specialized (expensive) equipment - Re-deposition of non-volatile compounds

Atom elastic collision Excitation & Relaxation Mechanism of dry etching Vacuum Chamber Electrode : 전자의 충분한 가속 Mean free path ↑ Electrode Plasma DC or RF - 반응가스 도입 후, 용기 내 압력을 10-3~1 torr 유지한 상태에서 plasma 이용 식각 - 글로우 방전(Glow discharge)에 의한 plasma 형성 압력이 매우 낮은 용기의 양쪽 전극에 전력을 인가, 한쪽 전극에서 전자 방출 - 방출된 전자는 가스 입자와 충돌하여 plasma 형성 - plasma 중에 형성된 이온과 라디칼이 식각하고자 하는 박막과 반응하여 식각 Atom elastic collision Excitation & Relaxation Ionization

Mechanism of dry etching What is Plasma? The 4th state of material (e.g., Solid, Liquid, Gas, Plasma) Quasi Neutral State with Collectively behavior (‘Plasma’; 1927’ Nomenclature by Langmuir) Hot Plasma (e.g., ICP-AES for analysis), Cold Plasma (e.g., ICP, etc for etching) The Plasma Universe (99.9%) Ar O2 Ar+ O2+ e- Breakdown Gas state Plasma state

Plasma Surface Interactions Equivalent flux density (/cm2sec) 1022 10W/cm2 1020 Ion beam modification (implantation, surface treatment) 1018 Film deposition 1016 1014 Etching 1012 accelerator Plasma chemistry 1010 10-2 100 102 104 106 108 Kinetic energy (eV) Thermal activation of atom migration Sputtering Electronic excitation Displacement of lattice atoms Increased sticking Implanatation Desorption

Plasma Surface Interactions General Plasma Generation Nonconductive material → Dielectric Breakdown → Conductive material High frequency electric field generate following reactions Precursors Evacuation Surface reactions Electrode - Ionization Dissociation R + Recombination Drift/Accelerating Diffusion Ion-molecule reactions Radical-molecule reactions Optical emission Main Plasma Reactions Excitation Dissociation Ionization Recombination Absorption Sputtering Polymerization

Plasma enhanced chemical etching Bulk Plasma + reactive ion reactive species electron PR particle + e- e- + + + e- mask (mask erosion) + volatile product sidewall passivation + + Sheath λD ~ <1 mm Substrate e- e- e- e- e- e- e- e- x Electrode plasma presheath sheath s Sheath edge

Various Plasma Etching Reactors Schematic configuration of several dry etching reactors; (a) chemically assisted ion beam etching (CAIBE) reactor, (b) reactive ion etching (RIE) chamber, (c) inductively coupled plasma (ICP) reactor, (d) electron cyclotron resonance (ECR) reactor.

ICP (Inductively Coupled Plasma) # ICP Equipment Gas Inlet Ceramic Process Chamber Process Height Wafer/Sample Helium Cooling Pumping Port RF Matching Unit ~ Plasma - 반응용기 외부에 coil을 감아 RF 전원 을 인가하면 패러데이의 전자유도법칙에 의해 coil에 유도자장이 발생하게 되고, 이에 따른 유도전자기장이 반응용기 내부에 형성 고밀도 plasma 가 생성 - Platen 의 RF 전원 은 plasma 를 substrate 로 당기는 역할

ICP (Inductively Coupled Plasma) # ICP Equipment – remote plasma Processing Height ~ Matching Unit Gas Inlet Coil Helium Cooling Gas Inlet

Silicon plasma etching Conventional Plasma Etching System High Density Plasma Source : ICP(DRIE), ECR, etc Relatively Low Density Plasma Source : RIE, Ion Milling, Sputter Advantages : HAR st., Relatively contamination free process, etc For MEMS HAR devices and structures, Optical applications, etc ICP(DRIE) - STS RIE ECR-asher RIE-PECVD

Silicon etching – RIE system High rate Isotropic Oxide/Nitride/Si etch with good uniformity:   Thermal oxide etch rate ~ 818Å/min ± 5% across 6” wafer, sel to AZ6615 PR ~ 0.5:1 Single-crystal Si etch rate ~ 870 Å/min, sel to AZ6615 ~ 0.53:1 Adjust pressure or power for uniformity. CF4 35sccm (or CHF3) O2 3sccm 150mT 300W (on 240mm diameter electrode) -579V 20C, graphite cover plate Silicon etch using an SiO2 mask (150mT, 300W) Silicon wet etching result Si (100) for convex cornor Henning Schröder, Ernst Obermeier, Anton Horn, and Gerhard K. M. Wachutka, J. Micros. Sys., 2001, 10, 88

DRIE etch Principle # Bosch process - 1st step : Etch (SF6 gas) - 2nd step : Deposit passivation layer (C4F8 → CFx 계열로 분해) - 3rd step : Wallside polymer etch << Bottom side polymer etch - 1st step again : polymer and Silicon etch (SF6 gas)

< Deposition step> DRIE etch Principle # Passivation step and Etch step < Deposition step> < Etch step > - Coil power ↑ ▶ passivation rate ↑ - Coil power ↑ , Wafer Temperature ↑ ▶ passivation rate ↓ ∴ He back side cooling - Coil power, Gas flow ↑ ▶ SF6 로부터 F• etchant species 많이 발생 ▶ etch rate ↑ - High pressure ▶ 높은 F• etchant species로 etch rate ↑

DRIE etch Principle # repeat the etch and passivation steps

High Aspect Ratio Etch # High Aspect Ratio Etch

< Fluidic Chennels > High Aspect Ratio Etch < Gas Turbine > < Gyroscope > < Fluidic Chennels > < Optic switch >

< scallop effect > # Bosch process and scallop effect < sidewall > < scallop effect >

Scallop effect # Bosch process and scallop effect < Top > Mask undercut : 0.11um Top Scallop : 47.6nm < Top > Bottom Scallop : 47.6nm < bottom >

Scallop effect # removal of the scallop effect < before thermal oxidation > < thermal oxidation after DRIE process > < after oxidation removal >

Etch rate & Selectivity & Uniformity In same Plasma Condition EA = Etch Rate of Layer A EB = Etch Rate of Layer B SA/B : Selectivity of A to B # Uniformity Ei : Etch Rate at Several Points Emax : Maximum Etch Rate Emin : Minimum Etch Rate

Profile after DRIE 90-α 90+α t = (A-B)/2 θ = tan-1 (t/d) A B t d (depth) θ t = (A-B)/2 θ = tan-1 (t/d)

Range of Profile ← negative anisotropic positive →

Profile faults # Bowing < Bowing > * Reason - too high a platen power and too high pressure - poor ion directionality - secondary ion etching with ions that have bounced off the bottom of trench (increasing deposition time, increasing the polymer gas flow, reducing the etch time) ※ usual solution - to drop the platen power - pressure reduction

< Undercut by chemical isotropic etching > Profile faults # Undercut Undercut < Undercut by chemical isotropic etching > ※ solution - reducing the total cycle time to as low as possible (maintaining the same etch/deposition time ratio) - etch rate가 높으면 undercut의 주된 원인이 됨. but, undercut을 줄이려다 보면 etch rate가 낮아짐.

Microloading effect * reason - Pattern의 open된 area 차이에 따라 etch rate이 달라짐. - 압력이 높고, Open Size가 아주 적을 때는 상대적으로 반응 부산물이 Wafer의 표면에서 머물게 될 확률 ↑ ▶ 식각 공정의 활성도 저해 * solution - 반응 부산물 생성 억제 or 반응 부산물을 Wafer의 표면에서 제거 - 반응 부산물의 생성을 억제하는 방법은 selectivity 저하 등 또 다른 문제점 초래 - 후자의 방법 선택, 압력을 낮게 유지하는 방법을 사용

Notch (footing effect) OXIDE NOTCH MASK SILICON 1. Build-up of negative charge on side walls and top of trench due to isotropic electron flux 2. Reduced electron flux to base of trench due to repulsion at top of trench 3. Accumulation of positive ions on insulator surface 4. Further positive ions deflected to sidewalls =>Sidewall Notch. ※ solution - Increasing deposition characteristic : polymer gas flow ↑   - Increasing dep time, decreasing etch time, reducing platen power

Notch (footing effect) * Footing 현상의 mechanism - 식각 gas의 양이온이 바닥에 충돌 후, 방전되지 못하고 남아있음 - 뒤따르는 양이온들과 척력을 발생, 원하지 않는 방향으로 발생 식각됨.

Notch (footing effect) * 기판의 관통 공정시 최종적으로 드러나는 바닥면이 절연막이거나 유리기판인 경우 바닥 면에 전하의 charging이 일어나 식각 pattern 안쪽으로 식각이 확장 * 정확한 공정시간의 조절로 최소화해야 함. * Microloading 현상으로 큰 pattern의 경우 필연적으로 나타남.

Back scattering effect * DRIE etch through 공정에서의 footing 현상

Etching gas

Etching parameters

Etching parameters

Etching parameters

Etching parameters

Dielectric plasma etching Low Density Plasma : very well established process, C/O control High Density Plasma : HAR st., High etch rate, surface morphology Advantages : Anisotropic etching, HAR st., Simple process, etc than Dielectric wet etching process For MEMS and Micro Optical parts devices and structures, etc

Requirements

Silicon Oxide etching mechanism (ICP)

- Energy transfer to surface Silicon Oxide etching mechanism (ICP) Chemical dominant reaction Ion Bombardment - Energy transfer to surface SiF4 CO CFx adsorption layer SiO2 ( O ) Etching reaction products Ion driven etching reaction Blocking film (eg. C deposition) Low density plasma 3/4 SiO2 + CF3 = 3/4 SiF4 + CO + 2 O -- Excess Oxygen atoms -> low selectivity to PR Medium density plasma 1/2 SiO2 + CF2 = 1/2 SiF4 + CO -- Optimum etching mode High density plasma 1/4 SiO2 + CF = 1/4 SiF4 + 1/2 CO + 1/2 C -- Carbon deposition -> etch stop

Courtesy of Wavesplitter Technologies Inc SiNx etch, (PR mask) Courtesy of Cambridge Univ. 20 um Core etch, (Silicon mask) Courtesy of Wavesplitter Technologies Inc 6 um Core etch, (PR mask) 25 um Quartz Lens etch

Dielectric etching – RIE Etchrate : 345.13A/min Selectivity : 7.2 : 1 Uniformity : 1.12% Profile : 84.7deg CHF3 35sccm Ar 15sccm 30mT 200W (on 240mm diameter electrode)

Metal plasma etching Chlorine and Inert gas based plasma used Advantages : Anisotropic etching, Simple process, etc than Metal wet etching process For well defined metal pattern and reducing surface contamination

Al - ICP etching 2.2um deep Al etch, Vertical Profile, No corrosion Etch rate : 1500A/min Selectivity : 2.5 : 1 Uniformity : <<9.66%

Cr - RIE etching Cl2 : 60sccm Etch rate:393A/min O2 : 3sccm Pressure : 200mtorr RF : 75Watts Etch rate:393A/min Selectivity:6.6 : 1 Uniformity:3.48%

Conclusion 1. Plasma etching is quite useful tool for fabricating ultra high precision machining with few micron scale or below. 2. Various materials (e.g., Si, SiO2, Si3N4 and some metals) are etched with vertical profile and quick and simple procedure. 3. For nano-scale fabrication one’s needs will require an unique solution as an advanced plasma etching method.