Thin Film Materials (I)

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

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Physical Vapor Deposition

• Evaporative Vapor Deposition
• Electron Beam Deposition
• Sputter Deposition
• Cathodic Arc Deposition
• Pulsed Laser Deposition

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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 104 Torr.
• Substantial investment required for equipment.
• Radial deposition outward from source.

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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.

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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.

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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.

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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.

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Pulsed Laser Deposition

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

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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.

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Chemical Vapor Deposition

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

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Chemical Vapor Deposition

• Deposition of polycrystalline Si on oxide
  substrates.
• Low pressure CVD (LPCVD).
• Typically 600 C.

SiH4 ? Si 2 H2 (silane)
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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)
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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)
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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
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Chemical Vapor Deposition

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

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Chemical Vapor Deposition

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

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Chemical Vapor Deposition

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

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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
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Electrodeposition

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

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Electrodeposition

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

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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.