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Sputtering

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sputtering

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Sputtering

  1. 1. PRESENTED BY: SANDEEP NAIR CB.EN.P2MFG15018 M.TECH- MFG SPUTTERING AMRITA SCHOOL OF ENGINEERING, COIMBATORE
  2. 2. SPUTTERING  SPUTTERING is a widely used and highly versatile vacuum coating system used for the deposition of a variety of coating materials.  A plasma at higher pressure is used to “knock "metal atoms out of a “target”. These energetic atoms deposit on a wafer located near the target. The higher pressure produces better step coverage due to more random angled delivery. The excess energy of the ions also aids in increasing the surface mobility (movement of atoms on the surface)
  3. 3. SPUTTERING PROCESS Wafer surface Al target _ Dark space or sheath Al Ar+ Al Al Aro Al Al O- Negative glow Ar+ e- Aro Ar+ e- e- O On the left side, sputter off an Al atom. On the right side, generate secondary electrons, which are accelerated across the sheath region and 1) ionize/excite an Ar; or 2) ionize an impurity atom, here O, to generate O- (for Ar, always positive ion Ar+). This O- is accelerated toward substrate and may go into the film (bad). After collision ionization, there are now TWO free electrons. This doubles the available electrons for ionization. This ongoing doubling process is called "impact ionization”, which sustains a plasma.
  4. 4. SPUTTERING PROCESS  Energy of each incoming ion is 500-1000eV. Energy of sputtered atoms is 3-10eV.  Thus, the sputtering process is very inefficient from the energy point of view, 95% of incoming energy goes to target heating & secondary electron.  High rate sputter processes need efficient cooling techniques to avoid target damage from overheating (serious problem).  The sputtered species, in general, are predominantly neutral.  The energy of the ejected atoms shows a Max-wellian distribution with a long tail toward higher energies.  The energies of the atoms or molecules sputtered at a given rate are about one order of magnitude higher than those thermally evaporated at the same rate, which often lead to better film quality.  However, since sputtering yields are low and the ion currents are limited, sputter-deposition rates are invariably one to two orders of magnitude lower compared to thermal evaporation rates under normal conditions.
  5. 5. SPUTTERING MECHANISM  The fundamental mechanism of sputtering is the collision interaction between the impinging ions and the lattice atoms of the target. The ion dissipates it's energy in collision cascades. The ejection depth of the target atoms is approx. 1 nm.  For quantitative statements one has to apply the formalism of transport theory to the mechanism of a collision cascade:  At ion energies Ei < 1keV the Born-Mayer potential may be used.  At Ei > 1 keV the Thomas-Fermi potential is valid.
  6. 6. CHARACTERISTICS OF SPUTTERING  Emissions from cathode include neutral atoms, ions (both positive and negative), electrons, neutral clusters of atoms and charged clusters of atoms. Of these, the vast majority are neutral atoms. These atoms have kinetic energies approximately 50 to 100 times that of neutral atoms generated from thermal evaporation sources.  This additional energy is thought to be the reason for the greater adhesion often observed for sputter deposited films over thermally evaporated films.  Due to the relatively high pressure in an operating sputter deposition chamber, the mean-free path of sputtered species is short. The numerous gas-phase collisions which the sputtered material suffers between the target and substrate tend to reduce the amount of kinetic energy the depositing species have upon arrival. the density and crystal structure of the thin film are affected.
  7. 7. SPUTTERING YIELD Elastic energy transfer E2 is greatest for M1=M2. There is also inelastic energy transfer, which leads to secondary electrons emission…   angle/incidentstrikingondepends: metaltargetofenergyBonding:U ionbombingofenergykinetic:E ionbombingofmass:m atomtargetofmass:M U E mM Mm ionsbombing atomssputtered Y M m M m 2     • Sputter yield Y: the number of sputtered atoms per impinging ion. • Obviously, the higher yield, the higher sputter deposition rate. • Sputter yield is 1-3: not too much difference for different materials. • The sputter yield depends on: (a) the energy of the incident ions; (b) the masses of the ions and target atoms; (c) the binding energy of atoms in the solid; and (d) the incident angle of ions. • The yield is rather insensitive to the target temperature except at very high temperatures where it show an apparent rapid increase due to the accompanying thermal evaporation.
  8. 8. SPUTTER DEPOSITION TECHNOLOGIES 1. DC Diode Sputtering 2. Magnetron Sputtering  Unbalanced Magnetron  Balanced Magnetron 3. RF Sputtering 4. Reactive Gas Sputtering 5. Ion Beam Sputtering 6. Pulse DC/AC Sputtering
  9. 9. MAGNETRON SPUTTERING Magnetron sputtering is the most commonly used method for a sputter deposition . It usually utilizes a strong electric and magnetic fields to trap electrons close to the surface of the magnetron, which is known as the target. The electrons follow helical paths around the magnetic field lines undergoing more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur The extra argon ions created as a result of these collisions leads to a higher deposition rate. It also means that the plasma can be sustained at a lower pressure. The sputtered atoms are neutrally charged and so are unaffected by the magnetic trap.
  10. 10. ION BEAM SPUTTERING Ion beam sputtering utilizes an ion source to generate a relatively focused ion beam directed at the target to be sputtered. The ion source consists of a cathode and anode with a common central axis.  Applying a high voltage field of 2-10 kV to the anode creates an electrostatic field inside the ion source, confining electrons around a saddle point in the center of the source. When argon gas is injected into the ion source, the high electric field causes the gas to ionize, creating a plasma inside the source region. The ions are then accelerated from the anode region to the exit aperture (cathode) creating a “collimated” ion beam. The resulting ion beam impinges upon a target material and, via momentum transfer between the ion and the target, sputters this material onto the sample.
  11. 11. ADVANTAGES Able to deposit a wide variety of metals, insulators, alloys and composites. Replication of target composition in the deposited films. Capable of in-situ cleaning prior to film deposition by reversing the potential on the electrodes . Better film quality and step coverage than evaporation. Can use large area targets for uniform thickness over large substrates. Sufficient target material for many depositions. No x-ray damage.
  12. 12. DISADVANTAGES Substrate damage due to ion bombardment or UV generated by plasma. Higher pressures 1 –100 m torr ( < 10-5 torr in evaporation), more contaminations unless using ultra clean gasses and ultra clean targets. Deposition rate of some materials quite low. Most of the energy incident on the target becomes heat, which must be removed. Difficult to deposit uniformly on complex shapes such as turbine blades High performance thick coatings are hard to produce due to high internal residual stress levels
  13. 13. COMPARISON OF TYPICAL THIN FILM DEPOSITION TECHNOLOGY
  14. 14. APPLICATION Aerospace & Defense:  Heads-up cockpit displays  Jet turbine engines  Mirrors for optical and x-ray telescopes  Night vision equipment. Wear Coatings:  Anti-corrosion coatings  Anti-seize coatings  Dies and molds  Sewing needles  Tool and drill bit hardening.
  15. 15. APPLICATION Automotive:  Auto headlights and tail-lights  Auto trim components  Drive train bearings and components  Wheels and rims Optics:  Anti-reflective/Anti-glare coatings  Cable communications  Laser lenses  Optical filters for achromatic lenses  Spectroscopy
  16. 16. THANK YOU
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sputtering

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