Introduction to microfabrication Based on chapter 1 sami

Introduction to microfabrication (Based on chapter 1) sami. franssila@aalto. fi Figures from: Franssila: Introduction to Microfabrication unless indicated otherwise.

Videos • • • 3 min basics https: //www. youtube. com/watch? v=0_vtd. BXy. GPw 3 min <Si> crystal growth & wafering: https: //www. youtube. com/watch? v=AMg. Q 1 -Hd. El. M Microscope images: https: //www. youtube. com/watch? v=Fxv 3 Jo. S 1 u. Y 8 • Show a photo of all circulated items before sending them round the classroom. Then return to that photo after circulation.

Dimension in microworld Fig. 1. 12

Microfabrication vs. Nanofabrication ? Fig. 1. 3: Electron beam lithography defined gold-palladium nanobridge Fig. 24. 4: Focussed ion beam patterned Aalto vase

Materials thin film 2 thin film 1 substrate 1 -1000 nm Substrate: thick piece of material (0. 5 mm = 500 µm) silicon most often

Common materials Substrates: Thin films: Others: Silicon Si. O 2 Si. Nx Polysilicon Al Cu W Pt Photoresist

Sheet resistance Rs /T Rs is in units of Ohm, but it is usually denoted by Ohm/square to emphasize the concept of sheet resistance. Resistance of a conductor line can now be easily calculated by breaking down the conductor into n squares: R = n. Rs Aluminum film 1 µm thick, sheet resistance ? Tungsten film, 1Ω resistance, thickness ?

Sheet resistance example (1) Aluminum film 1 µm thick, sheet resistance ? = 3 µΩ-cm, Rs /T Rs = 3 µΩ-cm /10 -4 cm = 30 mΩ = 0. 03 Ω

Sheet resistance example (2) Tungsten film, 1 Ω sheet resistance; how thick ? = 10 µΩ-cm, Rs /T T = /Rs T = 10 µΩ-cm/1 Ω = 1*10 -5 cm = 10 -7 m = 100 nm Why is tungsten resistivity 10 µΩ-cm and not 5. 6 µΩ-cm as in handbook ?

Resistor sheet resistance Figure 2. 8: Conceptualizing metal line resistance: four squares with sheet resistance Rs in series gives resistance as R = 4 Rs. Tungsten resistor of previous page: 4 Ω resistance.

Resistor design, homework L W How to change resistor resistance ? 1. Change L: vary its length 2. Change W: vary its width 3. Change T: vary its thickness 4. Change ρ: choose a different material T

The most important film: Si. O 2 Thermal oxidation O 2 O 2 O 2 Si Si. O 2 1000 o. C Si + O 2 Si. O 2 Si 1 µm 500 µm

Si. O 2 thin films: two cases O 2 O 2 O 2 W Si O 2 O 2 Si 1000 o. C ? 400 o. C ? Al

Solution: CVD Si. H 4 O 2 Si. H 4 + O 2 Si. O 2 + 2 H 2 O 2 Al Si 400 o. C CVD = Chemical Vapor Deposition Si. O 2 Si Al

Metallic films Evaporation conductors (Al, Au, Cu): Low resistivity wafer resistors (Ta, W, Pt, Si) High and stable resistivity crucible electron beam gun capacitor electrodes (poly-Si, Al, Mo) Good interface against the dielectric.

Patterning: lithography and etching Fig. 9. 1

Photoresist exposure UV light photomask photoresist silicon wafer Positive resist: exposed parts become soluble Negative resist: exposed parts crosslinked and insoluble Fig. 9. 10

After lithography a b c f e d Fig. 9. 14 a) b) c) d) e) f) ion implantation (Ch 15) wet etching (Ch 11) moulding (Ch 18) plasma etching (Ch 11) electroplating (Ch 29) lift-off (Ch 23)

6 lithography step MEMS process Liqun et al: Sensors and Actuators A 270 (2018) 214– 222

38 g air-bag inertial switch

CMP Grenoble

Silicon wafers scribe lines for chip dicing edge exclusion alignment marks for lithography wafer flat for orientation checking Fig. 1. 4: 100 mm diameter silicon wafer Fig. 1. 20 Real estate allocation on a wafer

Silicon strengths • • • silicon is a good mechanical material silicon is good thermal conductor silicon is transparent in infrared silicon is a semiconductor silicon is optically smooth and flat silicon is known inside out consider silicon first, alternatives then

Single crystalline silicon (a. k. a. monocrystalline) <100> silicon Fig. 4. 6

Real silicon wafers Not ideal, but ALWAYS contain: -dopants (B, P, e. g. 1 dopant atom per billion silicon atoms ≈ 1013 cm-3/5*1022 cm-3) -oxygen from crucible (15 pmma) -impurities (Fe, Zn, … e. g. 1010 cm-3) -defects (voids, dislocations, precipitates, …) In order to control conductivity, we will intentionally add dopants.

Videos • Doping: if you need to refresh your memory about semiconductors and doping, this video is useful: • https: //www. youtube. com/watch? v=k 12 GM jt. N 8 a. A TKK MICRONOVA, 2010 Microfabrication 26

Doping top metallization antireflection coating (ARC) n -diffusion p-substrate n-diffusion (e. g. 1016 cm-3 phosphorous) p-substrate (e. g. 1015 cm-3 boron) p+ diffusion backside metallization Fig. 25. 2 p+ diffusion (e. g. 1018 cm-3 boron)

Silicon substrate (p-type with boron majority) boron phosporous Impurities (e. g. Fe, Cu)

Phosphorous diffusion top layer turned into n-type Bulk of the wafer remains p-type, because diffusion does not extent to depth of wafer.

Junction depth xj Cdopant Xj is the depth where diffused dopant concentration equals wafer dopant concentration (of opposite type). xj ≈ 1. 2 µm 1020 cm-3 1018 cm-3 1016 cm-3 0. 5 1. 0 1. 5 depth/µm

Polycrystalline and amorphous materials Fig. 1. 6

Fig. 1. 1: Microtechnology subfield evolution from 1960’s onwards.

Silicon microelectronics 0. 5 µm CMOS in SEM micrograph Fig. 1. 15 65 nm CMOS in TEM micrograph Fig. 1. 11

Optoelectronics Fig. 1. 14: Silicon solar cell Fig. 6. 2: Ga. As multiple quantum well solar cell

MEMS: Micro Electro Mechanical Systems Fig. 29. 21: Microgears, courtesy Sandia National Labs. Fig. 21. 3: comb-drive actuator

Power MEMS Fabricated by bonding together 5 silicon wafers. Fig. 1. 17: Microturbine

MOEMS (Micro Opto Electro Mechanical Systems) Fig. 21. 4: variable optical attenator Fig. 1. 2: Micromirror made of silicon, 1 mm diameter, is supported by 1. 2 µm wide, 4 µm thick torsion bars (detail figure), from ref. Greywall.

Micro-optics Fig. 1. 7: Aluminum oxide and titanium oxide thin films deposited over silicon waveguide ridges, courtesy Tapani Alasaarela. Fig. 7. 13: Refractive index Si. O 2/Si. Ox. Ny/Si. O 2 waveguide: nf 1. 46/1. 52/1. 46. From ref. Hilleringmann.

Microfluidics and Bio. MEMS Fig. 1. 13: silicon microneedle Fig. 1. 11: Oxy-hydrogen burner flame ionization detector

Cleanrooms Fig. 1. 19

Yield of a total process is a product of yield of individual process steps 50 step MEMS process Y 0 = 0. 999 95% 500 step DRAM process, Y 0 = 0. 999 61%

Yield (2) Yield depends on chip area (A) and defect density (D) D = 0. 01 mm-2 (= 1/cm 2) A= 10 mm 2 Y = 90% A= 100 mm 2 Y = 37%

Microindustries are big Integrated circuits Other semiconductors Flat panels displays Solar cells Hard disks MEMS $330 B $130 B $100 B $30 B $10 B Equipment Materials $60 B $10 B