Thin Film Materials (I)

1. Thin Film Materials (I) Thin films are used extensively in sensor applications. The typical thin-film deposition is less than 10 m thick. Many films are less than 1 m thick thus implying nanoscale dimensions. There are several methods of applying thin-films.

2. Physical vapor deposition • Chemical vapor deposition • Electrodeposition • Langmuir-Blodgett

3. Physical Vapor Deposition • Evaporative Vapor Deposition • Electron Beam Deposition • Sputter Deposition • Cathodic Arc Deposition • Pulsed Laser Deposition

4. Physical Vapor Deposition • Always performed in vacuum. • Vacuum increases mean free path of ions or atoms. • Chemical reactions do not occur in the process. • Vacuum is typically less than 10–4 Torr. • Substantial investment required for equipment. • Radial deposition – outward from source.

5. Evaporative Deposition • Resistive process heats metal above BP. • Uses low vacuum, typically 10-4 Torr. • Low voltage, high current (Joule heating). • Metal is placed in tungsten boat and heated. • Most economical of all vacuum processes.

6. Electron Beam Deposition • Electron beam heats metal above BP. • Uses high vacuum, typically 10-6 Torr. • Requires electron source. • Metal is placed in graphite crucible and bombarded. • Somewhat economical of all vacuum processes.

7. Sputter Deposition • Plasma discharge creates metal ions. • DC magnetron generates plasma using argon gas. • Requires high vacuum, 10-6 Torr. • Requires small argon pressure, 10-3 Torr.

8. Cathodic Arc Deposition • Electric arc vaporizes metals from cathode. • High current, generates very hot arc. • Requires low vacuum, 10-4 Torr. • Simple process, can be done with metal rods.

9. Pulsed Laser Deposition • High-power laser ablates target. • Vapor deposition occurs in plasma plume. • Requires high vacuum, 10-5 Torr. • Expensive vacuum system.

10. Chemical Vapor Deposition • Chemical reaction occurs at substrate surface. • Chemical reaction typically forms bonds at substrate. • Cost effective, excellent for large-scale production. • Also useful for experimental research. • Typically used for semiconductor wafer processing. • Typically used in silicon photovoltaic processing.

11. Chemical Vapor Deposition • Several CVD methods exist. • Classified by pressure / vacuum level. • Classified by technology.

12. Chemical Vapor Deposition • Deposition of polycrystalline Si on oxide substrates. • Low pressure CVD (LPCVD). • Typically 600 °C. SiH4 → Si + 2 H2(silane)

13. Chemical Vapor Deposition • Deposition of silicon dioxide on substrates. • Low pressure CVD (LPCVD). • Typically 900 °C. SiH4 + O2 → SiO2 + 2 H2 (silane) SiCl2H2 + 2 N2O → SiO2 + 2 N2 + 2 HCl (dichlorosilane) Si(OC2H5)4 → SiO2 + byproducts (tetraorthosilicate)

14. Chemical Vapor Deposition • Deposition of silicon dioxide on substrates. • Low pressure CVD (LPCVD). • Typically 900 °C. SiH4 + O2 → SiO2 + 2 H2 (silane) SiCl2H2 + 2 N2O → SiO2 + 2 N2 + 2 HCl (dichlorosilane) Si(OC2H5)4 → SiO2 + byproducts (tetraorthosilicate)

15. Chemical Vapor Deposition • Deposition of metals on substrates. • Copper or aluminum for bus structures. • Metal halides are typical precursors. 2MCl5 + 5H2 → 2M +10HCl WF6 → W + 3F2 WF6 + 3H2 → W + 6HF

16. Chemical Vapor Deposition • Plasma enhanced CVD (PECVD). • Plasma / ionization catalyzes reactions. • Lower temperatures can be used.

17. Chemical Vapor Deposition • Hot-wall thermal CVD (HWCVD). • Large barrel holds wafers / substrates. • High temperatures (1000 °C).

18. Chemical Vapor Deposition • Metalorganic CVD (MOCVD). • Metal ion bound to organic ligand. • Ligands are typically low MW; methyl, ethyl

19. Electrodeposition • Chemical reaction using electrolytes, electroplating. • Ions in solution. • Reactions occur at electrodes. • Nernst behavior. • Two coupled half-cell reactions. • Voltage source needed to drive reaction. Ecell = E0cell - (RT/nF)lnQ

20. Electrodeposition • Basic chemistry concepts, but complicated kinetics. • Typical two-electrode system is simple.

21. Electrodeposition • Three-electrode system uses potentiostat. • Precise control of process.

22. Langmuir-Blodgett Deposition • Deposition of organic layers. • Successive formation of monolayers by dipping, removing and drying and repeating. • Initially studied by Benjamin Franklin. • Used for applying biomolecules on sensor substrates.