Reactive Ion Etching (RIE)

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Spencer Watson
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1 Reactive Ion Etching (RIE) RF ~ MHz plasma Parallel-Plate Reactor wafers Sputtering Plasma generates (1) Ions (2) Activated neutrals Enhance chemical reaction 1

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3 Remote Plasma Reactors Plasma Sources (1) Transformer Coupled Plasma (TCP) (2) Electron Cyclotron Resonance (ECR) e.g. quartz plasma coils wafers -bias Pressure pump1mtorr 10mTorr bias~ 1kV 3

4 Processes Occurring in Plasma Etching

5 Synergism of ion bombardment AND chemical reaction give the high RIE rates. 5

6 REMOVAL of surface film and DEPOSITION of plasma reaction products can occur simultaneously 6

7 RIE Etching Sequence gas flow diffusion of reactant absorption diffusion of by product desorption 4 3 X chemical reaction gaseous by products Substrate 7

8 Volatility of Etching Product * Higher vapor pressure * e. g. Si 4 F SiF 4 higher volatility (high vapor pressure) e. g. Cu Cl CuCl (low vapor pressure ) Example Difficult to RIE Al-Cu alloy with high Cu content mask Al-Cu Metal Do not want CuCl residues 8

9 Vapor pressure of by-product has to be high P P0 e P H v kt Example Difficult to RIE Al-Cu alloy with high Cu content 1500oC CuCl AlCl3 1~2% typical 200oC 1/T [Al-Cu alloy] Cl2 as etching gas. 9

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11 Examples Use CF4 gas For etching Si * CF4 F CF3 3 * CF4 e CF F 2e * Si 4F SiF4 F* are Fluorine atoms with electrons 11

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13 Aluminum + * CCl4 e CCl3 Cl 2e * Al 3Cl AlCl3 Photoresist C x H y Oz O2 COx HOx 13

14 How to Control Anisotropy? 1) ionic bombardment to damage expose surface. 2) sidewall coating by inhibitor prevents sidewall etching. 14

15 This exaggerated picture shows a passivation layer so thick that it can peel off from the sidewall Photoresist on top of Si Si Sidewall passivation films HCl/O2/BCl3 chemistry 15

16 How to Control Selectivity? E.g. SiO2 etching in CF4+H2 plasma Rate SiO2 S Rate Si S Rates P.R. SiO2 Si SiO2 Si Reason: F * %H2 in (CF4+H2) H HF F SiF H 2% 4 * content 16

17 Example: Si etching in CF4+O2 mixture Rates 1 Reason: Si (1)O CF x COF x F * 2 F * increases Si etching rate ( 2 )Si O 2 SiO 2 rate SiO2 %O2 in CF4 Poly-Si Oxide 17

18 For reference only 18

19 Effect of RIE process variables on etching characteristics Control variable effect 19

20 Temperature Dependence of Selectivity R1 A1e Q1 R2 A2 e kt Q2 R= etching rates A = proportional constants Q = activation energies kt R1 A1 Q1 Q2 kt S e R2 A2 S if Q1<Q2 77oK 1/T 20

21 Example: RIE of Aluminum Lines * It is a three-step sequence : 1) Remove native oxide with BCl3 2) Etch Al with Cl-based plasma 3) Protect fresh Al surface with thin oxidation 2 1 BCl3 Cl2-based RIE P.R. native Al2O3 Al 3 Al Form oxide again (gently) Al 21

22 Example: Etching of Deep Trenches ~1 m mask erosion mask mask ballooning Si trenching by-product residue ideal problems 22

23 Approaches to minimize deep trench etching problems Ballooning: Use chemistry with a good sidewall inhibitor. Trenching: -Use high pressure to increase ion-neutral scattering (ion trajectory less directional) Bottom Roughness: Increase vapor pressure of etching byproduct. 23

24 Hard Mask for Etching RIE 1 RIE 2 Photoresist oxide poly To minimize CD distortion, sometimes a two-step RIE process is used. Example: Process 1 to transfer pattern from resist; followed by Process 2 to transfer pattern from oxide to poly. EE243S2010 Lec22 24

25 A better Solution: Multiple step RIE sequence 25

26 EE243S2010 Lec22 26

27 * Can etch through whole Si wafer thickness 27

28 Local Loading Effect Less etchant consumption More etchant consumption Wsmall Wlarge 28

29 RIE Lag * smaller trenches etch at a slower rate than larger trenches. CCl2F2/O2 RIE 29

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31 Etching Profile Simulation ( For reference only Material and Isotropic and Directional Etch rates in nm/s Resist Oxide Silicon Substrate Many features such as mask erosion, sidewall angles, undercut, etc. can be predicted from the simple etching models. Q = sin-1 (5.0/( )) = 17.4o Angle appears larger due to unequal scales in x and y.

32 SUMMARY OF ETCH MODULE Etch Bias, Degree of Anisotropy, Etch Selectivity Worst-case considerations for etching Wet etch qualitative KOH/EDP etch of Si (anisotropic) Reactive Ion Etch equipment- qualitative Synergism of ion bombardment and chemical etching Selectivity Control - Gas mixture, Temperature Anisotropy Control Inhibitor deposition, Substrate bombardment RIE examples: Aluminum, deep trench etching. Pattern and Aspect ratio Dependence - qualitative

Section 3: Etching. Jaeger Chapter 2 Reader

Section 3: Etching. Jaeger Chapter 2 Reader Section 3: Etching Jaeger Chapter 2 Reader Etch rate Etch Process - Figures of Merit Etch rate uniformity Selectivity Anisotropy d m Bias and anisotropy etching mask h f substrate d f d m substrate d f