Diamond like carbon (DLC)

Diamond vs DLC

Hybridisation of Carbon

DLC• Carbon exist in SP3 , SP2 , SP hybridised states• SP3 – Diamond, SP2 - Graphite • DLC is a metastable form of amorphous carbon, with or

without hydrogen, which contains a significant fraction of sp3 bonded carbon atoms.

• DLC has network of graphitic clusters linked into islands by SP3

bonds.• DLC has similar properties of diamond, but these are achieved

by the isotropic thin films with no grain boundaries.• structurally they are amorphous in nature with sp2 and

sp3 bonded carbon atoms

Carbon-Hydrogen alloys

Ternary phase diagram of bonding in amorphous carbon-hydrogen alloys

ta-C – Tetrahedral amorphous carbonta-C:H – Hydrogenated ta -C a-C:H – amorphous hydrogenated -C

DLC

Mixture of SP3 and SP2 sites. DLC has always SP2 sitesta-C with > 70% of SP3

Properties depend on the composition of films

Schematic of SP2 clusters in a-C:H

DLC - properties• The structure and properties of DLC films are largely dependent on

the hydrogen content and the ratio of sp2 to sp3 bonded carbon atoms• Hydrogen content reduces hardness and density• Hydrogen content increases the band gap and the electrical resistivity• Hydrogen content also affect the optical properties and decreases the

refractive index• Dopants ( B, N, O, F, Si, Ti, W, Nb) manipulate the properties of DLC

films • DLC has high mechanical hardness, very low surface roughness,

chemical, optical transparency, electrochemical inertness and a wide band gap semiconductor.

DLC-Properties• Unlike Diamond, DLC can be p and n doped• At high N content, band gap reduces and new SP2 sites

increases.• DLC has low surface energy • Density dependent on number of SP3 sites• Mechanical properties depend on the local C-C

coordination. SP3 increases youngs modulus increases.• Electronic properties depend on number of SP2 sites, this

controls the band gap. All DLC have bonds.• Optical band gap decreases with increase in SP2 fraction

Comparison of different C forms

DLC

Preparation of DLC

• DLC films may contain significant amounts of hydrogen depending on the source of carbon and deposition process.

• Hydrogen-free DLC coatings are prepared by solid carbon or graphite targets with arc physical vapor deposition, pulsed laser deposition, and magnetron sputtering techniques

Preparation of DLC

• DLC are easy to prepare compared to diamond• DLC contains amorphous (a-C), hydrogenated

alloys (a-C:H)• Deposition methods like Plasma enhanced

chemical vapour deposition (PECVD), sputtering are used to prepare a-C with with higher SP3 content

• More SP3 with less hydrogented films were deposited by PECVD.

General deposition methods

• Chemical vapour deposition (CVD) – High temperature, specific choice of substrates, polycrystalline film, more hydrogen content and more grain boundaries.

• Physical vapour deposition (PVD) – Sputtering, ion beam, mass selected ion beam (MSIB) plasma, Pulse laser, cathodic vacuum arc

DLC deposition methods

• Ion beam• PECVD• Sputtering• Cathodic vaccum arc• Pulse laser deposition

Deposition techniques

Deposition mechanism• Diamond like carbon deposition takes place with optimum ion

( carbon or hydrocarbon ions) energy bombardment ~ 100 eV • SP3 fraction and H contents of DLC films depends mainly on

ion energy

Ion beam depositionCarbon or hydrocarbon ions are generated by plasma sputtering of graphite cathode or gas (methane) ionization in a plasma.

Ions are extracted and accelerated using power grids

Ion beam were directed into deposition vacuum chamber

Deposition on the substrate

Mass selected Ion beam (MSIB)

• Controlled deposition from single ion species with well-defined ion energy

• Accelerated ions passed are through the magnetic filters

• It filter out neutral species and selects the C+ • Ions can be decelerated to desired ion energy

using electrostatic lens and deposited on the substrate

SputteringMost common method for depostion of DLC

Ar Plasma generation

Magnetron sputtering is used to increase the yield (magnets are placed behind the target causes the Electron to achieve higher path length and thereby It increases the plasma ionisation).

DC bias is applied to the substrate to vary the ion energy

a-C:H produced by Reactive sputtering ( Ar, and H or CH4)

a-CNx can be produced by using ( Ar + N plasma)

Disadvantage – Less ratio of Ions to neutral species (less hard)

Ion assisted sputteringA beam of Ar ion is used to sputter the graphite traget

Additional Ar beam can be used to bombard the growing film to densify the film or encourage SP3 bonding

Cathodic vacuum arcTouching graphite cathode with carbon strikerelectrode and withdrawing initiate the arc. (In vacuum)

High ion density (1013 cm3 )plasma is generated by above process

Low voltage, high current density power supply(cathode spot is very small [1-10 m])

Particulate and plasma can be filtered by using magnetic filter ducts. (Filtered Cathodic Vacuum Arc[FCVA]) (Shown in the next slide)

FCVA are used to prepare highly ionised plasma with an energetic species low ion energy distribution and high growth rate (1 nm/s)

Unlike ion beam deposition, the depositing beam in FCVA is neutral plasma beam,which can be deposited on the insulating substrates.

Filtered Cathodic vacuum arc (FCVA)• Particulates cannot follow the field. they hit the walls of the filters. ( S

bends gives improved filtration)• Neutral species also hit the walls, so the filters raises the ionisation of

plasma from 30% to 100 %.• Finally plasma beam are condensed on the substrate to produce ta-C

Single bend S bend

Plasma depositionPlasma decomposition of hydrocarbons ( acetylene)

Two electrodes with different area

Higher mobility of electrons than ions create a sheath next to electrode with excess of ions

For DLC deposition the plasma has to operated at lowest possible pressure

This will increase the fraction of ions to radical of the plasma

In pressure plasma, use of magnetic field, increase the path length of electron and the ionisation efficiency

Pulsed laser depositionA very short pulse of intense laser vaporise materials as intense plasma

The expanding ions in plasma strikes the substrate and deposit as film

This is used to coat many different materials

Characterisation methods

• Raman spectroscopy• IR Spectroscopy• Nuclear magnetic resonance ( C13 NMR)• Electron energy loss spectroscopy• Electron spectroscopy for chemical analysis (ESCA)• UV spectroscopy• Ellipsometry• X Ray reflectivity, Neutron diffraction

Applications• Optical windows - a-C:H forms transparent thin films. (UV, Visible)

• Magnetic storage disks – higher capacity and less wear of disk materials. No pinholes even with 1.2 nm thick film

• Antifuses - as the high current passes it affords less resistance. DLC acts as semiconductor, hence increase in temperature increases the conductivity. Nitrogen doped DLC are better antifuses

• Low dielectrics films – Device dimension decreases with DLC films. Lower dielectric constant than SiO2

Applications• Field emission – Emission of electrons under ambient

temperature. FE Devices made of DLC shows emission at low applied field. Thin film carbon emitters are better than the tips of Si, Mo in chemical, physical stability and their cost.

• Field effect transistors – ta-C can be used in thin film transitors. Now Carbon nanotubes are found to be better than DLC.

• Nitrogen doped ta-C retain its electrochemical stability as boron doped diamond electrodes

Applications

• DLC have low friction coefficient- unlubricated DLC on steel has same friction as lubricated steel on steel

• High wear resistance – ta-C has low rate of wear.

• tribological properties depends on the chemical composition of the surface film, method of preparation.

• Useful in precision machining and manufacturing

• Keeps razor blade tips very sharp

Applications

• Microelectromechanical devices (MEMs)

• Biomedical coatings – biocompatible coatings – replacement hip joints, heart valves and stents. ( hydrogenated DLC films)

• Protective coatings - Automobile coatings, corrosion resistant coatings, abrasion resistant coatings, ultra smooth surfaces

• Ultra-hydrophobic surfaces – fluorinated DLC films

Applications

References

• Diamond-like amorphous carbon. J.Robertson, Material science and Engineering, R(37), 2002, 129-281