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Section 3: Etching

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

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Section 3: Etching

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  1. Section 3: Etching Jaeger Chapter 2 Reader

  2. Etch rate Etch rate uniformity Selectivity Anisotropy Etch Process - Figures of Merit EE143 – Ali Javey

  3. dm etching mask hf film º - Bias B d d substrate f m df dm substrate df Bias and anisotropy Complete Isotropic Etching Vertical Etching = Lateral Etching Rate Complete Anisotropic Etching Lateral Etching rate = 0 B = 0 EE143 – Ali Javey

  4. Degree of Anisotropy rlat: lateral etch rate rver: vertical etch rate Af: degree of isotropy isotropic anisotropic EE143 – Ali Javey

  5. Etching Selectivity S Wet Etching S is controlled by: chemicals, concentration, temperature RIE S is controlled by: plasma parameters, plasma chemistry, gas pressure, flow rate & temperature. EE143 – Ali Javey

  6. SiO2 Si Selectivity Example SiO2/Si etched by HF solution SSiO2, Si Selectivity is very large ( ~ infinity) SiO2/Si etched by RIE (e.g. CF4 plasma) SSiO2, Si Selectivity is finite ( ~ 10 ) EE143 – Ali Javey

  7. Uniformity (a) Film thickness variation across wafer • The variation factor d is dictated by the deposition method, • deposition equipment, and manufacturing practice. (b) Film etching rate variation variation factor EE143 – Ali Javey

  8. Reactant transport to surface Selective and controlled reaction of etchant with the film to be etched Transport of by-products away from surface Wet Etching 1 3 2 1 2 3 EE143 – Ali Javey

  9. Wet etch processes are generally isotropic Etch rate is governed by temperature, concentration, chemicals, etc. Wet etch processes can be highly selective Acids are commonly used for etching: HNO3 <=> H+ + NO3- HF <=> H+ + F- H+ is a strong oxidizing agent => high reactivity of acids Wet Etching (cont.) EE143 – Ali Javey

  10. (1) Silicon Dioxide To etch SiO2 filmon Si, use HF + H2O SiO2 + 6HF® H2 + SiF6 + 2H2O Note: HF is usually buffered with NH4F to maintain [H+] at a constant level (for constant etch rate). This HF buffer is called Buffered Oxide Etch (BOE) NH4F ® NH3 + HF Wet Etch Processes Etch rate(A/min) 6:1 BOE 1200 650 18 26 T(oC) EE143 – Ali Javey

  11. (2) Silicon Nitride To etch Si3N4 film on SiO2, use H3PO4 (phosphoric acid) (180oC: ~100 A/min etch rate) Typical selectivities: 10:1 for nitride over oxide 30:1 for nitride over Si Wet Etch Processes (cont.) EE143 – Ali Javey

  12. (3) Aluminum To etch Al film on Si or SiO2, use H3PO4 + CH3COOH + HNO3 + H2O (phosphoric acid) (acetic acid) (nitric acid) (~30oC) 6H+ + 2Al ® 3H2 + 2Al3+ (Al3+ is water-soluble) Wet Etch Processes (cont.) EE143 – Ali Javey

  13. (4) Silicon (i) Isotropic etching Use HF + HNO3 + H2O 3Si + 4HNO3® 3SiO2 + 4NO + 2H2O 3SiO2 + 18HF ® 3H2SiF6 + 6H2O (ii) Anisotropic etching (e.g. KOH, EDP) for single crystalline Si Wet Etch Processes (cont.) EE143 – Ali Javey

  14. Lack of anisotropy Poor process control Excessive particulate contamination => Wet etching used for noncritical feature sizes Drawbacks of Wet Etching EE143 – Ali Javey

  15. Reactive Ion Etching (RIE) Parallel-Plate Reactor RF 13.56 MHz plasma ~ wafers Plasma generates (1) Ions (2) Activated neutrals Enhance chemical reaction EE143 – Ali Javey

  16. Remote Plasma Reactors e.g. quartz Plasma Sources (1) Transformer Coupled Plasma (TCP) (2) Electron Cyclotron Resonance (ECR) coils plasma wafers -bias Pressure 1mTorr 10mTorr bias~ 1kV pump EE143 – Ali Javey

  17. RIE Etching Sequence gas flow 5 1 diffusion of by product desorption diffusion of reactant 2 3 4 X chemical reaction gaseous by products absorption Substrate EE143 – Ali Javey

  18. Volatility of Etching Product * Higher vapor pressure higher volatility (high vapor pressure) Example Difficult to RIE Al-Cu alloy with high Cu content EE143 – Ali Javey

  19. Examples Use CF4 gas For etching Si F* are Fluorine radicals (highly reactive, but neutral) Aluminum Photoresist EE143 – Ali Javey

  20. How to Control Anisotropy ? • 1) ionic bombardment to damage expose surface. • 2) sidewall coating by inhibitor prevents sidewall etching. EE143 – Ali Javey

  21. How to Control Selectivity ? E.g.SiO2 etching in CF4+H2 plasma S Rates P.R. SiO2 Si SiO2 H2% Si %H2 in (CF4+H2) Reason: EE143 – Ali Javey

  22. Example: Si etching in CF4+O2 mixture Reason: Rates Si 1 2 %O2 in CF4 Poly-Si Oxide EE143 – Ali Javey

  23. Cl2-based RIE 2 BCl3 P.R. 1 native Al2O3 Al Form oxide again (gently) 3 Al Al 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 EE143 – Ali Javey

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