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2 Major Fabrication Steps in CMOS Process Flow UV light oxygen Silicon dioxide Silicon substrate Oxidation (Field oxide) photoresist Photoresist Coating Mask exposed photoresist Mask-Wafer Exposed Alignment and ExposurePhotoresist oxide Photoresist Develop RF RF Power Power RF RF Power Power RF RF Power Power Ionized CF 4 gas photoresist oxide RF RF Power Power Ionized oxygen gas oxide oxygen gate oxide Dopant gas Silane gas polysilicon Ionized CCl 4 gas oxide poly poly gate gate Oxide Etch CF 4 or C 3 F 8 or CHF 3 O 3 CF 4 +O 2 or CL 2 Photoresist Strip Oxidation (Gate oxide) Polysilicon Deposition Polysilicon Mask and Etch Scanning ion beam ox S G D resist resist resist S G D S silicon nitride top nitride G Ion Active Nitride Contact Metal Implantation Regions Deposition Etch Deposition and Etch D Contact holes S G D Metal contacts drain G S D

3 CMOS manufacturing 1. Twin-well Implants 2. Shallow Trench Isolation 3. Gate Structure 4. Lightly Doped Drain Implants 5. Sidewall Spacer 6. Source/Drain Implants 7. Contact Formation 8. Local Interconnect 9. Interlayer Dielectric to Via First Metal Layer 11. Second ILD to Via Second Metal Layer to Via Metal-3 to Pad Etch 14. Parametric Testing 8 Via LI metal n M-4 Passivation layer Bonding pad metal ILD-6 M-3 M-2 M-1 9 Poly gate 3 ILD-5 ILD-4 ILD-3 ILD-2 ILD-1 LI oxide p + p + STI n + n n-well p - Epitaxial layer p + Silicon substrate 6 p-well 14 p

4 Etching? Etching : Chemical Gas Photo Pattern Layer Wet Etching Dry Etching P/R Strip : H 2 SO 4 + H 2 O

5 Dry and Wet Etching

6 Wet etching wafer wafer spray 1. wafer (Byproduct) 4.,,

7 Etching Etch bias = Wb(photo dimension ) Wa(etch dimension) Overetch & Undercut Etch bias : Photo. Overetch : Undercut : Wet etching open ( lateral etch / side etch )

8 Etch profile Isotropic profile - etched equally in all directions - wet etchs - quick, easy, cheap - best to use with large geometries when sidewall slople does not matter, undercut/release. - rounding of sharp anisotropic conrners to avoid stress concentration Anisotropic profile - etched in the preferred direction only - only dry etch - best for making small gaps and vertical sidewalls - lithographic patterns for smaller features - typically more costly

9 Etching (etch rate) : - - ( ) = ( ) / (selectivity) : - masking S( ) = target / mask

10 Selectivity

11 Uniformity

12 Wet Etching process Undercut

13 Wet etching parameters Wet Etching Parameter - Etchant - Etchant - Etching. Dipping. Spray. Dip & Spray - : Thin Film, PR

14 Wet etching mechanism 1 Wet etching - Oxidation - Dissolution of the oxide Wet etchant - Oxidation chemical - Dissolution chemical - Diluent

15 Wet etching mechanism 2 Wet chemical etching usually proceeds by oxidation, followed by the dissolution of the oxide by a chemical reaction. Si etchant : HNO 3 + HF + H 2 O or CH 3 COOH The overall reaction is where H 2 SiF 6 is soluble in water. (oxidation) (dissociation of the oxide)

16 Wet etching - oxide Constant temperature HF + NH 4 F (BOE) 1. HF H + + F -, F - H + ph 2. NH 4 F NH F -, F - ph NH 4 F : HF 6 : 1 7:1 PSG Thin Oxide 20:1 50:

17 Hydrofluoric acid SiO

18 Buffered HF SiO

19 Wet etching Si & SiO

20 Phosphoric acid Si x N y

21 Wet etching nitride Phosphorour acid at C ( ) 30~60 etch rate 200mL / 2L

22 Wet etching Al H 3 PO 4 + CH 3 COOH + HNO 3 + H 2 O Al etching heated etchant (35 ~ 45 C) Etching Al etching snow ball residue - Al 4 Al + 2 HNO 3 2 Al 2 O 3 + N 2 + H 2 - Al 2 O 3 2 H 3 PO 4 + Al 2 O 3 2 Al(PO 4 ) + 3 H 2 O - -H 2 O

23 Wet PR stripper - organic Stripper - Carbitol (Diethylene Glycol Monoethyl Ether) CH 3 CH 2 O(CH 2 ) 2 O(CH 2 ) 2 OH PR resin - Amine (MEA) PAC resin cross-link - NMP (Normal Methyl 2-Pyrrolidone) Solubility acid - Surfactant chemical PR

24 Etchants Material Etchant composition Etch rate SiO2 28mL HF 170mL H 2 O 113g NH 4 F 1000A/min 15mL HF 10mL HNO 3 300mL H 2 O 120A/min Si3N4 Buffered HF 5A/min H 3 PO 4 100A/min Al 1mL HNO 3 4mL CH 3 COOH 4mL H 3 PO 4 1mL H 2 O 350A/min Au 4g KI 1g I 2 40mL H 2 O 10000A/min Mo 5mL H 3 PO 4 2mL HNO 3 4mL CH 3 COOH 150mL H 2 O 5000A/min Pt 1mL HNO 3 7mL HCl 8mL H 2 O 500A/min W 34g KH 2 PO g KOH 33g K 3 Fe(CN) 6 H 2 O to make 1 liter 1600A/min Material Si 3 N 4 Al Ti, TiN TiSi 2 PR Etchant HF H 3 PO 4 :H 2 O:HNO 3 :CH 3 COOH (16:2:1:1) NH 4 OH:H 2 O 2 :H 2 O (1:1:5) NH 4 F:HF (6:1) Piranha H 3 PO 4 :H 2 O Organic strippers

25 Wet etch reaction rates

26 Dry etching Undercut Dry etching P/R film Dry etching gas film plasma radical ion pattern (SiO 2 ), (Si 3 N 4 ), (Al), (Poly-Si)

27 Dry etcher - plasma Dry etcher (a) Plasma etcher chamber (b) Plasma Etching : Plasma atom, radical, ion gas species film film vacuum pump. SiO 2 etching CF 4, Ar, CHF 3 3 gas CF 4, Ar, CHF 3 CF 3-, CF 2, CF*, F, F*, F + e (dissociation) SiO 2 + CF 3- SiF 4 (volatile) + CO 2 (gas phase )

28 Plasma etching

29 Etching free radicals

30 Etching plasma

31 Etching ion bombardment

32 Etching gas products

33 Plasma gas chemistries

34 Physical vs. Chemical etching

35 Etching parameters Etching rate - gas flow / gas composition - pressure - source power - bias voltage - wafer temperature Profile : isotropic, sloped, anisotropic - neutral / ion flux ratio - directionality of ions - deposition or passivation chemistries - temperature - reactivity of neutrals Selectivity to mask or underlayer Uniformity (etch rate, profile, selectivity) - pressure - spatial plasma uniformity (ion flux, energy) - spatial neutral flux uniformity - wafer temperature uniformity

36 Etching profile

37 Etching profile

38 Etching failure