Etching and Cleaning Cleaning remove contaminated layers Etching remove defect layers, form pattern by selective material removal Wet etching: using reactive chemical solution Dry etching: using reactive/energetic gas-phase species, including ions, atoms, radicals, plasma, laser… Basic steps: reaction + dissolution (or desorption)

Patterning by lithography and wet etching transparent glass Cr patterned film Mask Si photoresist SiO 2 film Al film Si UV exposure Si Develop solution Si Pattern transfer to photoresist Si Etching of Al film

Etchants for Si

Isoetch Curves for Si and GaAs H 2 SO 4 :H 2 O 2 :H 2 O For GaAs HF:HNO 3 :diluent For Si

Anisotropic etching Isotropic etching undercut mask

Anisotropic etching of Si Si(100) Si(110)

Wet etching of noncrystalline films SiO 2 : HF + NH 4 F + H 2 O (buffered HF, BHF) PSG: BHF or HF + HNO 3 + H 2 O (3:2:150, fast for PSG) Si 3 N 4 : buffered HF, but very slow with H 3 PO 4, rate for Si 3 N 4 : SiO 2  10 : 1 Poly-Si: HF + HNO 3 + H 2 O (3 : 50 : 20) Al: H 3 PO 4 + HNO 3 Au and Pt: HCl + HNO 3 (3 : 1, aqua regia) W: KH 2 PO 4 + KOH + K 3 Fe(CN) 6 + H 2 O

Isotropic etching undercut Film etching undercut Undercut at interface due to stress Layer 2 Layer 1 Undercut in wet etching

Wet etching : advantages and disadvantages nHigh selectivity, simple facility Severe undercut, rough edges, large amount of chemical waste Dry etching : physical and chemical n Physical: ion bombardment, directional flux n Chemical: reactive atoms, molecules, radicals n Chemical + Physical: enhanced reaction/desorption by electrons, ions, photons, plasma…

Etching by Ion Sputtering Neutralized ion beam: good for conductor & insulator

Etching by Ion Sputtering Plasma Sputtering Weak selectivity in ion beam etching

Voltage ratio in RF sputtering: V C /V A = (A A /A C ) n RF sputtering: can be applied to insulator DC & RF Sputter Etching

Chemical Etching Plasma Reactors Downstream In situ Shielded tunnel

Physical + Chemical Effects Reactive Ion Etching (RIE) Deep via holes for inter-level connections

Vapor pressure of metal halides

Etch chemistry for different materials n Halogen etching: for Si, W, Mo, Ti e.g. W + 6F  WF 6 n Fluorocarbon (CF 4, C 2 F 6, CHF 3 ): for SiO 2, PSG, Si 3 N 4 e.g. e - + CF 4  CF 3 + F + e - (plasma dissociation) 12F + Si 3 N 4  3SiF 4 + 2N 2 n CF 4 -H 2 mixture: increasing HF, reducing F concentration Increasing SiO 2 etching, decreasing Si and photoresist etching n Cl-based RIE: low volatility of SiCl 4, desorption stimulated by ion and electron flux  increased anisotropy n Etching of Aluminum: by BCl 3, CCl 4

Etching rate in CF 4 -H 2 plasma Increasing selectiveity