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Cvd and pvd

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  • 1. ECE614: Device Modelling and Circuit Simulation Unit 2 PVD & CVD By Dr. Ghanshyam Singh
  • 2. Content Physical vapor deposition (PVD) – Thermal evaporation – Sputtering – Evaporation and sputtering compared – MBE – Laser sputtering – Ion Plating – Cluster-Beam Chemical vapor deposition (CVD) – Reaction mechanisms – Step coverage – CVD overview Epitaxy
  • 3. Physical vapor deposition (PVD) The physical vapor deposition technique is based on the formation of vapor of the material to be deposited as a thin film. The material in solid form is either heated until evaporation (thermal evaporation) or sputtered by ions (sputtering). In the last case, ions are generated by a plasma discharge usually within an inert gas (argon). It is also possible to bombard the sample with an ion beam from an external ion source. This allows to vary the energy and intensity of ions reaching the target surface.
  • 4. Physical vapor deposition (PVD): thermal evaporation Heat Sources Advantages Disadvantages Resistance No radiation Contamination e-beam Low contamination Radiation RF No radiation Contamination Laser No radiation, low contamination Expensive N = No exp- Φe kT 6 The number of molecules leaving a unit area of evaporant per second No = slowly varying function of T ϕe = activation energy required to evaporate one molecule of material k = Boltzman’s constant T = temperature
  • 5. Physical vapor deposition (PVD): thermal evaporation Si Resist d ββββ θθθθ Evaporant container with orifice diameter DD Arbitrary surface element Kn = λλλλ/D > 1 A ~ cosββββ cos θθθθ/d2 N (molecules/unit area/unit time) = 3. 513. 1022 Pv(T)/ (MT)1/2 The cosine law This is the relation between vapor pressure of the evaporant and the evaporation rate. If a high vacuum is established, most molecules/atoms will reach the substrate without intervening collisions. Atoms and molecules flow through the orifice in a single straight track,or we have free molecular flow : The fraction of particles scattered by collisions with atoms of residual gas is proportional to: The source-to-wafer distance must be smaller than the mean free path (e.g, 25 to 70 cm)
  • 6. Physical vapor deposition (PVD): thermal evaporation ββββ2222 = 70= 70= 70= 700000 ββββ1111 = 0= 0= 0= 00000 t2 t1 Substrate t1 t2 = cos ββββ1 cos ββββ2 ≈≈≈≈ 3 Surface feature Source Source Shadow t1/t2=cosββββ1111/cosββββ2222 λλλλ = (ππππRT/2M)1/2 ηηηη/PT From kinetic theory the mean free path relates to the total pressure as: Since the thickness of the deposited film, t, is proportional to the cos β, the ratio of the film thickness shown in the figure on the right with θ = 0° is given as:
  • 7. Physical vapor deposition (PVD): sputtering W= kV i PTd -V working voltage - i discharge current - d, anode-cathode distance - PT, gas pressure - k proportionality constant Momentum transfer
  • 8. Evaporation and sputtering: comparison Evaporation Sputtering Rate Thousandatomic layers per second (e.g. 0.5 µm/min for Al) One atomic layer per second Choice of materials Limited Almost unlimited Purity Better (no gas inclusions, very high vacuum) Possibility of incorporating impurities (low-medium vacuum range) Substrate heating Very low Unless magnetron is usedsubstrate heating can be substantial Surface damage Very low, with e-beam x-ray damage is possible Ionic bombardment damage In-situ cleaning Not an option Easily done with a sputter etch Alloy compositions, stochiometry Little or no control Alloy composition can be tightly controlled X-ray damage Only with e-beam evaporation Radiation andparticle damage is possible Changes in source material Easy Expensive Decomposition of material High Low Scaling-up Difficult Good Uniformity Difficult Easy over large areas Capital Equipment Low cost More expensive Number of depositions Only one deposition per charge Many depositions can be carried out per target Thickness control Not easy to control Several controls possible Adhesion Often poor Excellent Shadowing effect Large Small Film properties (e.g. grain size and step coverage) Difficult to control Control by bias, pressure, substrate heat
  • 9. Physical vapor deposition (PVD): MBE, Laser Ablation - MBE – Epitaxy: homo-epitaxy hetero-epitaxy – Very slow: 1µm/hr – Very low pressure: 10-11 Torr Laser sputter deposition – Complex compounds (e.g. HTSC, biocompatible ceramics)
  • 10. Chemical vapor deposition (CVD): reaction mechanisms Mass transport of the reactant in the bulk Gas-phase reactions (homogeneous) Mass transport to the surface Adsorption on the surface Surface reactions (heterogeneous) Surface migration Incorporation of film constituents, island formation Desorption of by-products Mass transport of by-produccts in bulk CVD: Diffusive-convective transport of depositing species to a substrate with many intermolecular collisions-driven by a concentration gradient SiH4SiH4 Si
  • 11. Chemical vapor deposition (CVD): reaction mechanisms Fl = D ∆c ∆x δ(x) = ηx ρU       1 2 δ = 1 L δ(x)dX = 2 30 L ∫ L η ρUL       1 2 ReL = ρUL η δ = 2L 3 ReL Energy sources for deposition: – Thermal – Plasma – Laser – Photons Deposition rate or film growth rate (Fick’s first law) (gas viscosity η, gas density ρ, gas stream velocity U) (Dimensionless Reynolds number) Laminar flow L δ(x) dx (U) (Boundary layer thickness) Fl = D ∆c 2L 3 ReL (by substitution in Fick’s first law and ∆x=δ)
  • 12. Mass flow controlled regime (square root of gas velocity)(e.g. AP CVD~ 100-10 kPa) : FASTER Thermally activated regime: rate limiting step is surface reaction (e.g. LP CVD ~ 100 Pa----D is very large) : SLOWER Chemical vapor deposition (CVD) : reaction mechanisms Fl = D ∆c 2L 3 ReL R = Ro e - Ea kT
  • 13. Chemical vapor deposition (CVD): step coverage Fl = D ∆c 2L 3 ReL R = Ro e - Ea kT Step coverage, two factors are important – Mean free path and surface migration i.e. P and T – Mean free path: λ = αααα w z θ=180θ=180θ=180θ=1800000 θ=270θ=270θ=270θ=2700000 θ=90θ=90θ=90θ=900000 θ is angle of arrival kT 2 1 2 PTπa2 > α Fldθ∫ θ = arctan w z
  • 14. Chemical vapor deposition (CVD) : overview CVD (thermal) – APCVD (atmospheric) – LPCVD (<10 Pa) – VLPCVD (<1.3 Pa) PE CVD (plasma enhanced) Photon-assisted CVD Laser-assisted CVD MOCVD
  • 15. The L-CVD method is able to fabricate continuous thin rods by pulling the substrate away from the stationary laser focus at the linear growth speed of the material while keeping the laser focus on the rod tip, as shown in the Figure . LCVD was first demonstrated for carbon and silicon rods. However, fibers were grown from other substrates including silicon, carbon, boron, oxides, nitrides, carbides, borides, and metals such as aluminium. The L-CVD process can operate at low and high chamber pressures. The growth rate is normally less than 100 µm/s at low chamber pressure (<<1 bar). At high chamber pressure (>1 bar), high growth rate (>1.1 mm/s) has been achieved for small-diameter (< 20 µm) amorphous boron. Chemical vapor deposition (CVD) : L-CVD
  • 16. Epitaxy VPE: – MBE (PVD) (see above) – MOCVD (CVD) i.e.organo-metallic CVD(e.g. trimethyl aluminum to deposit Al) (see above) Liquid phase epitaxy Solid epitaxy: recrystallization of amorphous material (e.g. poly-Si) Liquid phase epitaxy
  • 17. Epitaxy Selective epitaxy Epi-layer thickness: – IR – Capacitance,Voltage – Profilometry – Tapered groove – Angle-lap and stain – Weighing Selective epitaxy
  • 18. Homework Homework: demonstrate equality of λ = (πRT/2M)1/2 η/PT and λ = kT/2 1/2 a 2 π PT (where a is the molecular diameter) What is the mean free path (MFP)? How can you increase the MFP in a vacuum chamber? For metal deposition in an evaporation system, compare the distance between target and evaporation source with working MFP. Which one has the smaller dimension? 1 atmosphere pressure = ____ mm Hg =___ torr. What are the physical dimensions of impingement rate? Why is sputter deposition so much slower than evaporation deposition? Make a detailed comparison of the two deposition methods. Develop the principal equation for the material flux to a substrate in a CVD process, and indicate how one moves from a mass transport limited to reaction-rate limited regime. Explain why in one case wafers can be stacked close and vertically while in the other a horizontal stacking is preferred. Describe step coverage with CVD processes. Explain how gas pressure and surface temperature may influence these different profiles.

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