EEL5225: Principles of MEMS Transducers (Fall 2003) Fabrication Technology, Part I Agenda: Oxidation, layer deposition (last lecture) Lithography Pattern Transfer (etching) Impurity Doping Reading: Senturia, Ch. 3 pp. 50-56. 1 Lecture 4 by H.K. Xie
Lithography Overview Components Process Lithography 2
Lithography--Overview Cleanroom Class 100: the maximum number of particles that are larger than 0.5um is 100 per cubic foot of air Class 10 or 1 for lithography area Lithography Technique used to transfer pattern from master copy to surface of solid material Photolithography--Lithography via Photons (Light) Light Source Idealized picture: Contact and proximity lithography Optics hν Photomask Photoresist Deposited/grown layer Semiconductor substrate 3
Lithography Light Sources Ultraviolet light from Hg discharge g-line (λ= 438 nm) h-line (λ= 405 nm) i-line (λ= 365 nm) 0.3µm (using 5:1 stepper) Excimer lasers KrF (λ= 248 nm) 0.18 µm ArF (λ= 193 nm) 0.10 µm 4 May & Sze, Fundamentals of Semiconductor Fabrication, pp.66
Lithography Exposure Methods Shadow Printing Contact lithography ~1um resolution Particle damage Proximity lithography Degraded resolution: 2 to 5um Critical dimension (CD) or minimum linewidth: CD λg where λ: wavelength; g: gap between mask and wafer including photoresist thickness Projection Printing nx reduction (Ex. 2x, 5x, 10x) Projection (photomask for full wafer) Step & repeat (reticle for individual die) Contact/Proximity Projection/Stepper 5
Lithography--Photomask Photomask (1x) Clear-field Dark-field Pattern (hν opaque) Ex. Chrome, emulsion Plate (hν transparent) Ex. Quartz, borosilicate, soda-lime Clear Field Dark Field Reticle (5x, 10x) Flat Low coefficient of thermal expansion High transmission 6
Lithography--Photoresist Positive Negative Polymethylmethacrylate (PMMA) Exposure speed: slow Developer (aqueous base) Poor adhesion No swelling in developer bis(aryl)axide rubber Exposure speed: fast Developer (organic solvent) Good adhesion Swelling in developer Positive: Exposed resist becomes soluble in developer Negative: Exposed resist becomes insoluble. Ref. R. C. Jaeger, Intro. To Microelectronic Fabrication, p. 19. 7
Lithography--Developer Developer Stripper Positive resist Aqueous basic hydroxides NaOH, NH 4 OH, TMAH Negative resist Organic solvent Xylene Positive resist Over oxide steps: Acid (3 H 2 SO 4 :1 H 2 O 2 ) piranha Over metal: Simple solvent (Nanostrip) Negative resist Over oxide steps: Acid (piranha) Over metal: Chlorinated solvent 8
Lithography--Process Photomask Design Clean as needed Apply photoresist using spinner Prebake (soft-bake) Align and expose NOT OK Develop Metrology-Check alignment OK Post-bake (hard-bake) To Etch or Lift-off Strip resist 9
Next Generation Lithography Electron Beam (e-beam) Lithography Raster scan Vector scan Electron resist Extreme Ultraviolet Lithography (EUV) Wavelength: 10~14 nm 50-nm features EUV beam scanning Expensive X-Ray Lithography Wavelength: ~ 1nm Shadow printing 100-nm features Ion Beam Lithography High resolution 10
Pattern Transfer Overview Wet Etching Dry Etching Liftoff Plating Pattern Transfer (etching) 11
Pattern Transfer--Overview Lithography Etching Plating Liftoff Wet (liquid) Dry (gas) ADDDITIVE SUBTRACTIVE 12
Pattern Transfer--Wet Etching Mechanism 1) Reactant transport 2) Surface chemical reaction 3) Product removal Advantages High selectivity Batch process Disadvantages Isotropic Fluid issues (mixing, bubbles, etc.) Waste 2 1 3 Silicon substrate Resist Layer 13
Pattern Transfer--Wet etching Examples Oxide etch Buffered Hydrofluoric acid (BHF) 5:1 NH 4 :HF Thermal SiO 2 100 nm/min etch rate at R.T. Silicon ~ 0 nm/min Aluminum etch H 3 PO 4 :CH 3 COOH:HNO 3 Baker Aluminum etch Aluminum 100 nm/min at R.T. Silicon ~0 nm/min Silicon etch Isotropic etch: HNO 3 :HF:H 2 O Anisotropic etch: KOH 14
Pattern Transfer--Dry etching Plasma Reactive ++, Applied Field: low Removal via chemical reaction by ionized species Selectivity: very good Directionality: poor (isotropic) Silicon substrate Reactive ion etch (RIE) Reactive ++, Applied Field: moderate Removal via chemical reaction by ionized species Removal via momentum transfer Selectivity: good Directionality: good (fairly anisotropic) Silicon substrate Sputtering (ion milling) Reactive 0, Applied Field: high Removal via momentum transfer Selectivity: poor Directionality: very good (very anisotropic) Silicon substrate 15
Pattern Transfer--Dry etching Etching Gases Plasma Cl and F containing compounds CF 4, BCl 3, etc Reactive ion etch (RIE) Cl and F containing compounds CF 4, BCl 3, etc. Sputtering/Ion milling Inert heavy ions Ar Masking layers Depends on application Photoresist mask for SiO 2, Si SiO 2 mask for Si etch 16
Pattern Transfer--Dry etching Etching Profiles 17 from Madou, p.91
Pattern Transfer--Liftoff Process Lithography (positive resist) Film deposited on top of resist pattern Resist (mask layer) removed Advantages No need for etching film Disadvantages Film thickness less than 1/5-1/3 resist thickness Need for temperature control Contamination of film/substrate interface Silicon substrate Silicon substrate Silicon substrate Resist Film (metal) Resist 18
Impurity Doping Overview Diffusion Ion Implantation Impurity Doping 19
Impurity Doping--Overview Process Incorporation of specific impurities Diffusion from external source Ion implantation Redistribution Diffusion Vacancy diffusion Interstitial diffusion Homogenous distribution of impurity atoms Equilibrium Limited impurity source (internal) diffusion Non-equilibrium Constant impurity source (external) diffusion 20
Impurity Doping--Diffusion 21 Physics Flux proportional to concentration gradient Flux gradient proportional to time rate of change of concentration (Continuity equation for particle flux) Model N J = D x N J = t x D: Diffusion coefficient Fick s Diffusion Equation (or Fick s Law): Combining yields 1-dimensional diffusion equation where J is the particle flux, D is the diffusion coefficient, and N is the impurity concentration (#/volume). N t 2 N = D 2 x
Impurity Doping--Diffusion Case I: Constant impurity source (external at surface) N( x, t) = N0erfc 2 Dt Q= dose= 2N = π x Dt # cm 0 2 erfc: complementary error function Case II: Limited impurity source (internal) 2 Q x N( x, t) = exp π Dt 4Dt Q = constant Diffusion coefficients: D=D 0 exp(-e A /kt) (1st order) Boron: D 0 =10.5 cm2/s, E A =3.69eV Phosphorus: D 0 =10.5 cm2/s, E A =3.69eV E A : activation energy Arsenic: D 0 =0.32 cm2/s, E A =3.56eV 22
Impurity Doping--Diffusion Diffusion profiles Constant Source Limited Source Dt Q = dose = 2N Q = constant 0 π Ref. R. C. Jaeger, Intro. To Microelectronic Fabrication, p. 52-53. 23