Fabrication Technology, Part I

Transcription

1 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 Lecture 4 by H.K. Xie

2 Lithography Overview Components Process Lithography 2

3 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

4 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

5 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

6 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

7 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

8 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

9 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

10 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

11 Pattern Transfer Overview Wet Etching Dry Etching Liftoff Plating Pattern Transfer (etching) 11

12 Pattern Transfer--Overview Lithography Etching Plating Liftoff Wet (liquid) Dry (gas) ADDDITIVE SUBTRACTIVE 12

13 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 Silicon substrate Resist Layer 13

14 Pattern Transfer--Wet etching Examples Oxide etch Buffered Hydrofluoric acid (BHF) 5:1 NH 4 :HF Thermal SiO 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

15 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

16 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

17 Pattern Transfer--Dry etching Etching Profiles 17 from Madou, p.91

18 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

19 Impurity Doping Overview Diffusion Ion Implantation Impurity Doping 19

20 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

21 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

22 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

23 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