THIN FILM MATERIALS 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.

DIFFERENT METHODS OF DEPOSITION Physical vapor deposition Chemical vapor deposition Electro deposition Langmuir-Blodgett

DIFFERENT TYPES OF DEPOSITION Evaporative Vapor Deposition Electron Beam Deposition Sputter Deposition Cathodic Arc Deposition Pulsed Laser Deposition

PHYSICAL VAPOUR 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.

EVAPORATIVE DEPOSITION Resistive

EVAPORATIVE DEPOSITION EXPLANATION This process heats metal above BP. Uses low vacuum, typically Torr. Low voltage, high current (Joule heating). Metal is placed in tungsten boat and heated. Most economical of all vacuum processes.

ELECTRON BEAM DEPOSITION processes.

ELECTRON BEAM DEPOSITION EXPLANATION Electron beam heats metal above BP. Uses high vacuum, typically Torr. Requires electron source. Metal is placed in graphite crucible and bombarded. Somewhat economical of all vacuum

SPUTTER DEPOSITION pressure, Torr.

SPUTTER DEPOSITION EXPLANATION Plasma discharge creates metal ions. DC magnetron generates plasma using argon gas. Requires high vacuum, Torr. Requires small argon

CATHODIC ARC DEPOSITION

CATHODIC ARC DEPOSITION EXPLANATION Electric arc vaporizes metals from cathode. High current, generates very hot arc. Requires low vacuum, Torr. Simple process, can be done with metal rods.

PULSED LASER DEPOSITION.

PULSED LASER DEPOSITION EXPLANATION High-power laser ablates target. Vapor deposition occurs in plasma plume. Requires high vacuum, Torr. Expensive vacuum system..

CHEMICAL VAPOUR 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.

DIFFERENT CVD METHODS Several CVD methods exist. Classified by pressure / vacuum level. Classified by technology.

CVD AT 600 °C. Deposition of polycrystalline Si on oxide substrates. Low pressure CVD (LPCVD). Typically 600 °C. SiH 4 Si + 2 H 2 (silane)

CVD AT 900 °C Deposition of silicon dioxide on substrates. Low pressure CVD (LPCVD). Typically 900 °C. SiH 4 + O 2 SiO H 2 (silane) SiCl 2 H N 2 O SiO N HCl (dichlorosilane) Si(OC 2 H 5 ) 4 SiO 2 + byproducts (tetraorthosilicate)

CVD Deposition of metals on substrates. Copper or aluminum for bus structures. Metal halides are typical precursors. 2MCl 5 + 5H 2 2M +10HCl WF 6 W + 3F 2 WF 6 + 3H 2 W + 6HF

PLASMA ENHANCED CVD

PLASMA ENHANCED CVD EXPLANATION Plasma enhanced CVD (PECVD). Plasma / ionization catalyzes reactions. Lower temperatures can be used.

CVD AT 1000 °C

CVD AT 1000 °C EXPLANATION Hot-wall thermal CVD (HWCVD). Large barrel holds wafers / substrates. High temperatures (1000 °C).

METAL ORGANIC CVD Metal organic CVD (MOCVD). Metal ion bound to organic ligand. Ligands are typically low MW; methyl, ethyl

ELECTRO DEPOSITION 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. E cell = E 0cell - (RT/nF)lnQ

ELECTRO DEPOSITION

Three-electrode system uses potentiostat. Precise control of process. ELECTRODEPOSITION

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.