Section 3: Etching Jaeger Chapter 2 Reader

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

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

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

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

Selectivity Example SiO2 Si 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

Uniformity variation factor (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

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

Wet Etching (cont.) 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 EE143 – Ali Javey

Wet Etch Processes (1) Silicon Dioxide To etch SiO2 film on Si, use Etch rate (A/min) (1) Silicon Dioxide To etch SiO2 film on 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 6:1 BOE 1200 650 18 26 T (oC) EE143 – Ali Javey

Wet Etch Processes (cont.) (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 EE143 – Ali Javey

Wet Etch Processes (cont.) (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) EE143 – Ali Javey

Wet Etch Processes (cont.) (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 EE143 – Ali Javey

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

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

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

RIE Etching Sequence Substrate 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

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

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

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

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

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

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