Microsoft PowerPoint Etching

Transcription

1 1 Wet etching VS Dry etching # Wet Etching - 식각용액을 사용, 화학적인 반응만으로 박막 식각 - Selectivity 가 좋음 -고가장비필요없음, low cost - 한꺼번에 많은 기판 처리, productivity - undercut 발생, 용액의 측면 침식으로 미세 pattern 구현이 어려움 (3um 이하 어려움) - 화학약품의 과다 사용으로 환경문제 대두 # Dry Etching - 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

2 2 Dry etching # Dry etching Advantage - 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 # Dry etching Disadvantage - Some gases are quite toxic - Need for specialized (expensive) equipment - Re-deposition of non-volatile compounds

3 3 Dry Etching # Dry Etching Equipment Electrode Vacuum Chamber : 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 Atom elastic collision Excitation & Relaxation - plasma 중에 형성된 이온과 라디칼이 식각하고자 하는 박막과 반응하여 식각 Ionization

4 4 Introduction to Plasma and Plasma etching What is Plasma? The 4 th 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%) Gas state Ar O2 Breakdown Plasma state e - Ar+ O2+ e -

5 Plasma Surface Interactions 5 sec) 2 Equivalent flux density (/cm W/cm 2 Film deposition Plasma chemistry Etching Ion beam modification (implantation, surface treatment) accelerator Kinetic energy (ev) Thermal activation of atom migration Sputtering Displacement of lattice atoms Electronic excitation Desorption Increased sticking Implanatation

6 6 General Plasma Generation Nonconductive material Dielectric Breakdown Conductive material High frequency electric field generate following reactions Electrode Recombination - + Ionization - Dissociation + R R R Surface reactions Electrode Drift/Accelerating Diffusion Precursors Ion-molecule reactions Optical emission Radical-molecule reactions Evacuation Main Plasma Reactions Excitation Dissociation Ionization Recombination Absorption Sputtering Polymerization

7 7 General Plasma etching mechanism (plasma enhanced chemical etching) + e - Bulk Plasma e e - e - + reactive ion reactive species electron PR particle + mask (mask erosion) Sheath λ D ~ volatile product + + sidewall passivation Φ <1 mm Substrate Φ p Φ p e - e - e - e - e - e - e - e - Electrode Φ(0) = 0 Φ (0) = 0 plasma presheath sheath Sheath 0 s edge Φ w x

8 8 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.

9 9 ICP (Inductively Coupled Plasma) equipment # ICP Equipment Gas Inlet Ceramic Process Chamber ~ RF Matching Unit Plasma Wafer/Sample Process Height Pumping Port ~ RF Matching Unit Helium Cooling Gas Inlet - 반응용기 외부에 coil을감아rf 전원을 인가하면 패러데이의 전자유도법칙에 의해 coil에 유도자장이 발생하게 되고, 이에 따른 유도전자기장이 반응용기 내부에 형성 고밀도plasma 가생성 - Platen 의 RF 전원 은plasma 를substrate 로당기는역할

10 10 ICP (Inductively Coupled Plasma) equipment # ICP Equipment remote plasma ~ Matching Unit Gas Inlet Processing Height Coil ~ Matching Unit Helium Cooling Gas Inlet

11 11 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

12 12 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. Silicon etch using an SiO 2 mask (150mT, 300W) CF4 35sccm (or CHF3) O2 3sccm 150mT 300W (on 240mm diameter electrode) -579V 20C, graphite cover plate 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

13 13 DRIE etch Principle # Bosch process -1 st step : Etch (SF 6 gas) -2 nd step : Deposit passivation layer (C 4 F 8 CF x 계열로 분해) -3 rd step : Wallside polymer etch << Bottom side polymer etch - 1st step again : polymer and Silicon etch (SF 6 gas)

14 14 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 높은F etchant species로 etch rate

15 DRIE etch Principle 15 # repeat the etch and passivation steps

16 16 High Aspect Ratio Etch # High Aspect Ratio Etch

17 17 < Gas Turbine > < Gyroscope > < Fluidic Chennels > < Optic switch >

18 18 Scallop effect # Bosch process and scallop effect < sidewall > < scallop effect >

19 19 Scallop effect # Bosch process and scallop effect Mask undercut : 0.11um Top Scallop : 47.6nm < Top > Bottom Scallop : 47.6nm < bottom >

20 20 # removal of the scallop effect < before thermal oxidation > < thermal oxidation after DRIE process > < after oxidation removal >

21 21 Etch rate & Selectivity & Uniformity # Etch rate # Selectivity In same Plasma Condition E A = Etch Rate of Layer A E B = Etch Rate of Layer B S A/B : Selectivity of A to B # Uniformity E i : Etch Rate at Several Points E max : Maximum Etch Rate E min : Minimum Etch Rate

22 22 Profile after DRIE A B 90-α 90+α t θ d (depth) t = (A-B)/2 θ = tan -1 (t/d)

23 23 Range of Profile negative anisotropic positive

24 24 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

25 25 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 높으면undercut의 주된 원인이 됨. but, undercut을 줄이려다 보면 etch rate가 낮아짐.

26 26 Microloading effect # 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 저하등또다른문제점초래 - 후자의 방법 선택, 압력을 낮게 유지하는 방법을 사용

27 27 Notch (footing effet) # Notch (footing effect) SILICON OXIDE MASK NOTCH 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 t rench 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

28 28 Notch (footing effet) * Footing 현상의 mechanism - 식각 gas의 양이온이 바닥에 충돌 후, 방전되지 못하고 남아있음 - 뒤따르는 양이온들과 척력을 발생, 원하지 않는 방향으로 발생 식각됨.

29 29 # 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.

30 30 # Back scattering effect * DRIE etch through 공정에서의 footing 현상

31 31 Etch rate의 Major controlling parameters # Coil power : W 일때 1. switching times -etch step etch rate -cycle time etch rate 2. platen power -etch time의 증가만큼 etch rate를 크게 증가시키지는 않지만, etch rate 증가의 원인이 된다.

32 32 Uniformity의 Major controlling parameters # Pressure -pressure uniformity 향상 * etch와 deposition에서 압력은 다르다. etch gas flow는 depo gas flow보다 높기 때문에 etching step 압력은 deposition 압력보다 높다. etch step 초기 압력은 overshoot 되어 매우 높고, 이후 설정 압력으로 조정(drop back)된다. 반면, depo step 초기 압력은 보통 매우 낮고, 이후 설정압력으로 조정 (back up)된다. etch step에서 압력은 uniformity에 큰 영향을 준다. APC (Auto Pressure Control) 는 overshoot를 줄여주어 uniformity를 향상 -etch step etch rate - cycle time etch rate

33 33 Main effects of the controlling parameters A. Platen power -Platen power는 profile, etch rate, selectivity에 영향을 끼침 - 높으면 profile은 보다오목(re-entrant), negative 따라서, passivation 을 증가시켜야 함. but passivation 증가 etch rate - 하지만, platen power 증가에 의해 늘어난 etch rate 증가량에 비해서는 작다. 따라서, platen power를 올려주면 Si etch rate는 증가한다. B. Coil power -Coil power를 800W에서 600W로 줄이면 etch rate는 약간감소 - 800W 이상으로 coil power Si etch rate는 20%에서 50%까지 증가 - 보통 coil power 증가에 의한 etch rate 증가는 etch gas flow rate에 의존

34 34 Main effects of the controlling parameters C. Z-distance - Z-distance : wafer와 coil bottom 까지의 거리 -Z가 짧으면(즉, platen과 high intensity plasma region 거리가 가까우면) etch rate, selectivity (반응 gas가 반응물질에 바로 닿음) uniformity 안 좋아짐 (반응 gas가 충분히 퍼지지 못함) Z가 길어지면 etch rate는 떨어지는 경향을 보이지만, profile control은 좋아지며 좋은 CD (Critical Dimension) control 가짐 D. Gas flow -etch gas : etch rate - deposition gas : positive한 profile gas flow의 변화는 pressure에 영향, etch step 초기에 overshoot으로 압력변화 E. Exposed Si Area - Exposed Si area : etch rate, more positive profile, Uniformity

35 35 Main effects of the controlling parameters F. Pressure -Uniformity를 control 하는 가장 주된 factor는 pressure이다. - 낮은 압력은 uniformity -etch step에서 높은 pressure은 etch rate, selectivity uniformity 와 selectivity 는 trade-off 관계 G. Switching Times - Switching time은 profile control에 주된factor -etch/deporatio, more negative profile -etch/deporatio, more positive profile 좋은anisotropiy profile을 얻기 위해서 switching time 조정이 중요 H. Mask Type - 보통 oxide와 Photoresist 가 mask(식각 방지막)으로 주요 사용 - oxide의 selectivity는 PR보다 높다. - 너무 식각되지 않는 hard mask는 redeposit 되어 문제를 야기시킴

36 Silicon Oxide and Nitride plasma etching 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

37 37 Different requirements for silicon and silicon dioxide etching: Silicon etching Silicon dioxide etching Required etch depth um 1-50 um Aspect ratio ~ 30 ~ 10 Mask material Photoresist, silicon dioxide Photoresist, silicon, metal Etch-stop layer Buried oxide None Etchant species F* atoms CF x + ions Required ion energy "Low" "High"

38 38 Silicon Oxide etching mechanism (ICP) RFEnergy + + Ion Deposition Layer (CxF y) Rea ction Layer Wall Reaction SiO 2 Rea ction Product

39 39 Silicon Oxide etching mechanism (ICP) Chemical dominant reaction Ion Bombardment - Energy transfer to surface SiF 4 ( O ) CO Etching reaction products CF x adsorption layer Ion driven etching reaction SiO 2 Blocking film (eg. C deposition) Low density plasma 3/4 SiO 2 + CF 3 = 3/4 SiF 4 + CO + 2 O -- Excess Oxygen atoms -> low selectivity to PR Medium density plasma 1/2 SiO 2 + CF 2 = 1/2 SiF 4 + CO -- Optimum etching mode High density plasma 1/4 SiO 2 + CF = 1/4 SiF 4 + 1/2 CO + 1/2 C -- Carbon deposition -> etch stop

40 40 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

41 41 Dielectric etching RIE system CHF3 35sccm Ar 15sccm 30mT 200W (on 240mm diameter electrode) Etchrate : A/min Selectivity : 7.2 : 1 Uniformity : 1.12% Profile : 84.7deg

42 Metal plasma etching 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

43 43 Al - ICP etching result Al - RIE etching result Etch rate : 1500A/min Selectivity : 2.5 : 1 Uniformity : <<9.66% 2.2um deep Al etch, Vertical Profile, No corrosion

44 44 Cr - RIE etching result Cl2 : 60sccm O2 : 3sccm Pressure : 200mtorr RF : 75Watts Etch rate:393a/min Selectivity:6.6 : 1 Uniformity:3.48%

45 45 PG65211 Etching

46 46 PG65211 Etching

47 47 PG65211 Etching

48 48 PG65211 Etching

49 49 PG65211 Etching

50 50 PG65211 Etching

51 51 PG65211 Etching

52 52 PG65211 Etching

53 53 Results and Discussions 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.