Diamond like carbon (DLC)

Diamond like carbon (DLC) Diamond vs DLC Hybridisation of Carbon DLC • • • Carbon exist in SP3 , SP2 , SP hybridised states • • DLC has network of graphitic clusters linked into islands by SP3 bonds • structurally they are amorphous in nature with sp2 and sp3 bonded carbon atoms 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 similar properties of diamond, but these are achieved by the isotropic thin films with no grain boundaries Carbon-Hydrogen alloys ta-C – Tetrahedral amorphous carbon ta-C:H – Hydrogenated ta -C a-C:H – amorphous hydrogenated -C Ternary phase diagram of bonding in amorphous carbon-hydrogen alloys DLC Schematic of SP clusters in a-C:H 2 Mixture of SP and SP sites DLC has always SP sites ta-C with > 70% of SP Properties depend on the composition of films 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 • Electronic properties depend on number of SP2 sites, this controls the band gap All DLC have π bonds • Optical band gap decreases with increase in SP2 fraction 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 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 Ion beam deposition Carbon 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 Sputtering Most 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 sputtering A 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 SP bonding Cathodic vacuum arc Touching graphite cathode with carbon striker electrode and withdrawing initiate the arc (In vacuum) High ion density (10 13 cm )plasma is generated by above process Low voltage, high current density power supply (cathode spot is very small [1-10 µm]) Particulate and plasma can be filtered by using magnetic filter ducts (Filtered Cathodic Vacuum Arc) (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 deposition Plasma 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 deposition A 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 [...]... 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 deposition Carbon or hydrocarbon ions are generated by plasma sputtering of graphite cathode or gas (methane) ionization...Preparation of DLC • • • DLC are easy to prepare compared to diamond • More SP3 with less hydrogented films were deposited by PECVD Hydrogenated tetrahedral carbon – (ta-C:H) 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... 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... 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 ... 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 deposition Plasma 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... Ar ion is used to sputter the graphite traget Additional Ar beam can be used to bombard the growing film 3 to densify the film or encourage SP bonding Cathodic vacuum arc Touching graphite cathode with carbon striker electrode and withdrawing initiate the arc (In vacuum) High ion density (10 13 3 cm )plasma is generated by above process Low voltage, high current density power supply (cathode spot is... filter ducts (Filtered Cathodic Vacuum Arc) (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 ... 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... and sp3 bonded carbon atoms 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... has similar properties of diamond, but these are achieved by the isotropic thin films with no grain boundaries Carbon- Hydrogen alloys ta-C – Tetrahedral amorphous carbon ta-C:H – Hydrogenated