Reactive Ion Etching (RIE) RF 13.56 ~ MHz plasma Parallel-Plate Reactor wafers Sputtering Plasma generates (1) Ions (2) Activated neutrals Enhance chemical reaction 1
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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
Processes Occurring in Plasma Etching
Synergism of ion bombardment AND chemical reaction give the high RIE rates. 5
REMOVAL of surface film and DEPOSITION of plasma reaction products can occur simultaneously 6
RIE Etching Sequence gas flow 5 1 2 diffusion of reactant absorption diffusion of by product desorption 4 3 X chemical reaction gaseous by products Substrate 7
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
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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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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Aluminum + * CCl4 e CCl3 Cl 2e * Al 3Cl AlCl3 Photoresist C x H y Oz O2 COx HOx 13
How to Control Anisotropy? 1) ionic bombardment to damage expose surface. 2) sidewall coating by inhibitor prevents sidewall etching. 14
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
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
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
For reference only 18
Effect of RIE process variables on etching characteristics Control variable effect 19
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
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
Example: Etching of Deep Trenches ~1 m mask erosion mask mask ballooning Si trenching by-product residue ideal problems 22
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
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
A better Solution: Multiple step RIE sequence 25
EE243S2010 Lec22 26
* Can etch through whole Si wafer thickness 27
Local Loading Effect Less etchant consumption More etchant consumption Wsmall Wlarge 28
RIE Lag * smaller trenches etch at a slower rate than larger trenches. CCl2F2/O2 RIE 29
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Etching Profile Simulation (http://cuervo.eecs.berkeley.edu/volcano/) For reference only Material and Isotropic and Directional Etch rates in nm/s Resist 0.83 0.0 Oxide 0.0 8.0 Silicon 11.7 5.0 Substrate 0.0 0.0 Many features such as mask erosion, sidewall angles, undercut, etc. can be predicted from the simple etching models. Q = sin-1 (5.0/(11.7 + 5.)) = 17.4o Angle appears larger due to unequal scales in x and y.
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