TMT4320 Nanomaterials November 10 th, Thin films by physical/chemical methods (From chapter 24 and 25)

Transcription

1 1 TMT4320 Nanomaterials November 10 th, 2015 Thin films by physical/chemical methods (From chapter 24 and 25)

2 2 Thin films by physical/chemical methods Vapor-phase growth (compared to liquid-phase growth) Chemical vapor deposition (CVD) Molecules as vapor, chemical reaction Plasma-enhanced CVD (PECVD) Physical vapor deposition (PVD) Atoms as vapor Evaporation, Molecular beam epitaxy (MBE) Sputtering (cathodic, magnetron, ion beam) Cathodic arc evaporation Pulsed laser deposition (PLD) Low-energy cluster beam deposition

3 3 Chemical vapor deposition (CVD) Chemical reaction of volatile precursors with other gases to produce a non-volatile solid that deposits atomistically on a substrate Chemical reactions can be activated by Heat energy (in hot-walled reactors or in cold-walled reactors where only the substrate is heated) With the help of a plasma Reaction types include: Pyrolysis or thermal decomposition Reduction/oxidation Compound formation Advantages: Moderately high growth rates (1 5 μm/hr) Uniform growth on substrates with complex geometries Disadvantages: Toxic or other hazardous gases Very high temperature in thermally activated CVD process (T > 1200 K)

4 4 Hot-wall: Tubular reactor with walls surrounded and heated by resistor elements Cold-wall: Substrate is directly heated inductively by graphite susceptors

5 5 CVD contd. Can synthesize monocrystalline, polycrystalline, amorphous and epitaxial films Can tailor composition to produce a wide variety of films Thickness is controlled by amount of precursor and reaction times Other types of CVD methods: Plasma enhanced (PECVD) Metalorganic (MOCVD) Low pressure (LPCVD) Pressure enhances mass flux of gaseous reactants and products through the boundary layer between the laminar gas stream and substrates Reduces unwanted gas-phase reactions Laser enhanced (LECVD) Aerosol-assisted (AACVD) Ultrahigh vacuum (UHVCVD) Hot wire (HWCVD)

6 6 Plasma-enhanced CVD (PECVD) Deposition of thin films from a gas state (vapor) to a solid state on a substrate Chemical reactions occur after creation of a plasma of the reacting gases The plasma is generally created by RF (AC) frequency or DC discharge between two electrodes, the space between which is filled with the reacting gases Reduced substrate temperature compared to CVD while maintaining high deposition rates due to the enhanced reactivity of the precursors and the possibility of accelerating active species towards the substrate in ionic form NTNU NanoLab PECVD Based on Si materials Si, P, B, N, O, C, H

7 7 Physical vapor deposition (PVD) Transferring growth species from a source or target and deposit them on a substrate to form a film The processes proceed atomistically and mostly involves no chemical reactions Molecular dynamics computer simulation of the basic physical process underlying physical vapor deposition: a single Cu atom deposited on a Cu surface. Wikipedia

8 8 PVD contd. Variations of PVD include: Evaporative deposition: the material to be deposited is heated to a high vapor pressure by electrically resistive heating in "low" vacuum Electron beam physical vapor deposition: the material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum Sputter deposition: atoms or molecules are dislodged from a solid target through impact of gaseous ions (plasma) Cathodic arc deposition: A high power arc directed at the target material blasts away some into a vapor Pulsed laser deposition: A high power laser ablates material from the target into a vapor Ion implantation: Differs from deposition, ions are implanted into the substrate material to form inclusions or metastable materials

9 9 Evaporation The growth species are removed from the source by thermal means Pressure 10-3 to torr Thermodynamic equilibrium Typically large grain films Multi-component materials are difficult to deposit (fractionation)

10 10 Molecular beam epitaxy (MBE) Special case of evaporation for single crystal film growth Ultrahigh vacuum: ~10-10 torr The molecular beam(s) is generated by heating the precursor using resistive heating Effusion cell (Knudsen cell) The evaporated atoms or molecules do not interact with each other in the vapor phase due to the low pressure Main attributes: Extremely clean environment Low growth temperature Slow growth rate (up to 1 m/h) Simple growth mechanism better understanding of process due to ability to individually control evaporation of sources Variety of in situ analysis capabilities better understanding and ability to refine the process

11 11 NTNU MBE system (Varian Gen II) III: Ga, In, Al V: As, Sb Dopants: Si, Be, Te RHEED-gun High vacuum high purity Heterostructures with abrupt interfaces Growth rate can be accurately controlled In situ characterization 8 Effusion cells In situ RHEED (during growth!)

12 12 Electron beam physical vapor deposition A target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum The electron beam causes atoms from the target to transform into the gaseous phase Pressure: ~10-4 Torr Electron gun(s) Power from few tens to hundreds of kw The electron beam is accelerated to a high kinetic energy and focused towards the target Advantages A high deposition rate: μm / min Relatively low substrate temperatures High material utilization efficiency

13 13 Schematic of electron beam physical vapor deposition. Wikipedia

14 14 Sputtering Atoms are ejected from a solid target material due to bombardment of the target by high energy ions (> 30 ev) This is a surface erosion phenomenon which results from elastic collisions with energy and momentum transfer between the incident ions and the target atoms Wikipedia

15 15 Cathodic sputtering The simplest sputtering process A plasma is produced in a chamber by applying a potential difference of kv order between the substrate holder (anode) and the target (cathode) An inert gas is introduced (usually argon) at low pressure ( Pa) Schematic of a DC cathodic sputtering system. The dotted line indicates the potential between anode and cathode. R. Waser (ed.), Nanoelectronics and Information Technology

16 16 Magnetron sputtering Magnetron sputtering constitutes an important step forward in the development of PVD processes A special magnetic device is associated with the cathode to confine electrons close to the target surface and increase the plasma density, and hence the sputtering rate The high ion density means that the discharge can be maintained at lower pressure (up to 10 2 Pa) Another important parameter is the ion energy, and hence the energy of the atoms deposited on the substrate, which is higher in this process This plays a key role in the growth of the films, which are denser and adhere better

17 17 Plane magnetron target showing the magnetic field lines parallel to the target surface. E is the electric field which produces the discharge and the Ar+ plasma, while B is the magnetic field produced by the magnets of the magnetron. The grey square represents the sputtered atom.

18 18 Ion beam sputtering A target is sputtered using a beam of ions with controlled flux and energy Other sputtering techniques: the target is sputtered by plasma ions whose energy is not accurately determined Monokinetic ion sources are used (usually producing Ar + ), with which the energy can be varied over the range kev Current densities are high (~ 1mA/cm 2 ) and the beam can have a broad cross-sectional area (typical diameter ~ 10 cm) Deposits can be made in higher vacuum (~ Pa) than with plasma sputtering

19 19 Advantages Can produce very dense layers with 2D rather than columnar growth This happens due to the high energy of the sputtered atoms Disadvantages Low deposition rate, which is only of the order of μm/hr Large amount of maintenance required to keep the ion source operating Setup for ion beam sputtering with non-reactive assistance (Ar + ) or reactive assistance (N 2+ /O 2+ )

20 20 Cathodic arc evaporation An arc is produced at low pressure between the metal to be evaporated (cathode) and an anode The cathode material evaporates due to a very sharp increase in its temperature, accompanied by local melting The evaporated atoms are ionized by collisions with plasma electrons and accelerated out of the cathode towards the substrate Advantages Highly robust Widely used in industry due to the fact that very high deposition rates can be achieved Cathodic arc evaporation is used industrially to produce hard zirconium, molybdenum or titanium carbide or nitride coatings Disadvantage Presence of microparticles (1 100 μm) in the form of liquid droplets in the beam, which leads to films of lower quality

21 21 Cathodic arc evaporation, in which the cathode comprises the material to be evaporated.

22 22 Pulsed laser deposition (PLD) A high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material to be deposited This material is vaporized from the target (in a plasma plume) and deposits as a thin film on a substrate This process can occur in ultrahigh vacuum or in the presence of a background gas (i.e. oxygen)

23 23 PLD contd. To synthesize multilayers, the targets are placed successively in the beam and rotated to avoid local damage Controlling parameters: Substrate temperature Substrate target distance Residual pressure in the chamber Intrinsic characteristics of the laser beam Advantages: The material is transferred stoichiometrically from the target to the substrate This facilitates deposition of multi-element materials such as oxides (Al 2 O 3, SiO 2, YBCO, etc.). There is a wide choice of materials that can be deposited, limited only by absorption at the laser wavelength

24 24

25 25 Questionable!!! Other sources give values of max 3000 nm per hour R. Waser (ed.), Nanoelectronics and Information Technology

26 26 Summary and learning objectives thin films by physical/chemical methods Be able to describe the different methods below, what kind of materials/films can be made and some advantages/limitations: Chemical vapor deposition (CVD) molecules as vapor How does a plasma (PECVD) change/enhance the process? Physical vapor deposition (PVD) atoms as vapor Evaporation Electron beam PVD Sputter deposition (magnetron, ion beam) Cathodic arc deposition Pulsed laser deposition (PLD) Molecular beam evaporation (MBE) be able to compare this method in particular to both CVD and other PVD methods You should know which of these methods would be suitable for epitaxial growth (and why) and what types of structures you can grow in addition to thin films. And make sure to not mix method (i.e. CVD, MBE) and growth mechanism (i.e. VLS growth, Volmer-Weber growth)