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  1. 1. THIN FILMS (Evaporation and Sputtering) Dr. U P Singh School of Electronics Engineering
  2. 2. Course Coverage • Thin films : • Physical deposition, – Evaporation and sputtering ; – Chemical Vapor Deposition techniques; – Epitaxial growth
  3. 3. • Two main types of deposition methods have been developed and are used in CMOS technology: • Physical Vapor Deposition (PVD) - evaporation, sputter deposition • Chemical Vapor Deposition (CVD) - APCVD, LPCVD, PECVD, HDPCVD
  4. 4. • Many films, made of many different materials are deposited during a standard CMOS process. • These layers include silicon dioxide, silicon nitride, poly silicon and metal. • In this set of notes we describe the requirements, methods and equipment used to deposit these thin films.
  5. 5. Film parameters • Thickness/uniformity • Surface flatness/roughness • Composition/grain size • Stress free • Purity • Integrity
  6. 6. Thin Film Characteristics  Good step coverage  Ability to fill high aspect ratio gaps (conformality)  Good thickness uniformity  High purity and density  Controlled stoichiometries  High degree of structural perfection with low film stress  Good electrical properties  Excellent adhesion to the substrate material and subsequent films
  7. 7. Solid Thin Film Silicon substrate Oxide Width Thickness Thin films are very thin in comparison to the substrate.
  8. 8. Film Coverage over Steps Conformal step coverage Nonconformal step coverage Uniform thickness
  9. 9. Aspect Ratio for Film Deposition Aspect Ratio = Depth Width = 2 1 Aspect Ratio = 500 Å 250 Å 500 Å D 250 Å W
  10. 10. High Aspect Ratio Gap Photograph courtesy of Integrated Circuit Engineering
  11. 11. Stages of Film Growth Continuous film Gas molecules Nucleation Coalescenc e Substrate
  12. 12. Simple Evaporator Roughing pump Hi-Vac valve Hi-Vac pump Process chamber (bell jar) Crucible Evaporating metal Wafer carrier Base pressure is as low as possible • Heat the crucible to produce a vapor of the charge material • Vapor travels in a straight line to the wafers producing a thin film • Wafers positioned on a planetary for uniformity
  13. 13. •In evaporation, source material is heated in high vacuum chamber. (P <10-5 torr) • Mostly line-of-sight deposition since pressure is low. • Deposition rate is determined by emitted flux and by geometry of the target. The evaporation source can be considered either a point source or as a small area surface source (latter is more applicable to most evaporation systems).
  14. 14. Simple Thermal Evaporator Resistive evaporator sources (A) Simple sources including heating the charge itself and using a coil of refractory metal heater coil and a charge rod. (B) More standard thermal sources including a dimpled boat in resistive media.
  15. 15.  High deposition rate afforded by modern cathode and target design  Capability to deposit and maintain complex alloy compositions  Ability to deposit high-temperature and refractory metals  Capability to maintain well-controlled, uniform deposition on large (200 mm and larger) wafers  Ability, in multi-chamber systems, to clean the contact before depositing metal
  16. 16. • Sputtering is a term used to describe the mechanism in which atoms are ejected from the surface of a material when that surface is stuck by sufficiency energetic particles. • Alternative to evaporation. • First discovered in 1852, and developed as a thin film deposition technique by Langmuir in 1920. • Metallic films: Al-alloys, Ti, TiW, TiN, Tantalum, Nickel, Cobalt, Gold, etc.
  17. 17. • Sputtering is usually carried out in an argon plasma and by biasing the target (source of metal) negatively, argon ions are attracted to the target • The momentum of the Ar ions is transferred to the target resulting in the ejection of one or more atoms from the surface of the target • The sputtered atoms, mostly neutral, fly into the plasma and land on the wafer
  18. 18. • Like evaporation, sputter deposition occurs essentially along a line-of-sight path with a cosine distribution • Poor step coverage can result if the surface topography of the wafer is abrupt • The uniformity of the deposited film can be improved by raising the substrate temperature (enhancing surface migration), using a larger target, or inserting a collimator between the sputtering cathode and wafer • Better at depositing alloys and compounds than evaporation.
  19. 19. Reasons for sputtering • Use large-area-targets which gives uniform thickness over the wafer. • Control the thickness by Dep. time and other parameters. • Control film properties such as step coverage (negative bias), grain structure (wafer temp), etc. • Sputter-cleaned the surface in vacuum prior to deposition.
  20. 20. Simple Parallel Plate DC Diode Sputtering System Exhaust e- e- e- DC diode sputterer Substrate 1) Electric fields create Ar+ ions. 2) High-energy Ar+ ions collide with metal target. 3) Metallic atoms are dislodged from target. Anode (+) Cathode (-) Argon atoms Electric field Metal target Plasma 5) Metal deposits on substrate 6) Excess matter is removed from chamber by a vacuum pump. 4) Metal atoms migrate toward substrate. Gas delivery + + + + +
  21. 21. Dislodging Metal Atoms from Surface of Sputtering Target + 0 High-energy Ar+ ion Sputtered metal atom Metal atoms Cathode (-) Rebounding argon ion recombines with free electron to form a neutral atom.
  22. 22. • Rate of sputtering depends on the sputtering yield, S, defined as the number of atoms or molecules ejected from the target per incident ion. • S is a function of the energy and mass of ions, and the target material. It is also a function of incident angle. • S does not vary between target materials as much as the vapor pressure does. • Yield vary from ~0.5-30, depending also on the momentum of ion. • Controlling composition of alloys is easier with sputtering than with evaporation.
  23. 23. Step coverage tends to be very poor due to line-of- sight deposition process •Heating allows surface diffusion and so improves step coverage •Generally only acceptable for aspect ratios of < 1:1
  24. 24. • Calculate the mean free path of a particle in the gas phase of a deposition system and estimate the number of collisions it experiences in traveling from the source to the substrate in each of the cases below. Assume that in each case the molecular collisional diameter is 0.4 nm, the source-to-substrate distance is 5 cm, and that the number of collisions is approximately equal to the source-to-substrate distance divided by the mean free path. a. An evaporation system in which the pressure is 10-5 torr and the temperature is 25°C. b. A sputter deposition system in which the pressure is 3 mtorr and the temperature is 25°C The mean free path of a gas particle λ = kT/2 πd2P where k =1.36x10-22 cm3 atm K-1, T is the temperature in K, d is the collision diameter of the molecule in cm (approximately 4x10-8 cm for most molecules of interest), and P is the pressure in atm. The # collisions is approximately equal to the source-to substrate distance divided by the mean free path in each case. Putting the values gives: λ(in cm) = λ = kT/2 πd2P = 1.36x10−22 cm3 ⋅ atm ⋅K−1 ∗ T(K) /  2π(4x10−8 cm)2 P(torr) /760torr / atm = 1.45x10−5 T(K) /P(torr) a. 433 cm, 1.2x10-2 collisions; b. 1.44 cm, 3.5 collisions;
  25. 25. Chemical Vapor Deposition •Excellent step coverage •Large throughput •Low-temperature processing •A number of metals and metal compounds, such as Al, Cu, WSi2, TiN, and W, can be deposited by chemical reaction or thermal decomposition of precursors •Usually the wafer needs to be heated to 100oC to 800oC to provide the initial thermal energy to overcome the reaction barrier
  26. 26. Chemical Vapor Deposition The Essential Aspects of CVD 1. Chemical action is involved, either through chemical reaction or by thermal decomposition (referred to as pyrolysis). 2. All material for the thin film is supplied by an external source. 3. The reactants in a CVD process must start out in the vapor phase (as a gas). Chemical Vapor Deposition (CVD) - APCVD (Atm pressure CVD), - LPCVD, (Low Pressure CVD) - PECVD, (Plasma Enhanced CVD) - HDPCVD, (High Density Plasma CVD)
  27. 27. CVD Chemical Processes 1. Pyrolosis: a compound dissociates (breaks bonds, or decomposes) with the application of heat, usually without oxygen. 2. Photolysis: a compound dissociates with the application of radiant energy that breaks bonds. 3. Reduction: a chemical reaction occurs by reacting a molecule with hydrogen. 4. Oxidation: a chemical reaction of an atom or molecule with oxygen. 5. Reduction-oxidation (redox): a combination of reactions 3 and 4 with the formation of two new compounds.
  28. 28. A simple CVD reactor
  29. 29. Steps involved in a CVD process: 1. Transport of reactants to the deposition region. 2. Transport of reactants from the main gas stream through the boundary layer to the wafer surface. 3. Adsorption of reactants on the wafer surface. 4. Surface reactions, including: chemical decomposition or reaction, surface migration to attachment sites (kinks and ledges); site incorporation; and other surface reactions (emission and re-deposition for example). 5. Desorption or reemission of by-products. 6. Transport of by-products through the boundary layer. 7. Transport of by-products away from the deposition region.
  30. 30. Epitaxy • Epitaxy Growth Model • Epitaxy Growth Methods – Vapor-Phase Epitaxy (VPE) – Metalorganic CVD (MOCVD) – Molecular-Beam Epitaxy (MBE) Epitaxy is the process of the controlled growth of a crystalline doped layer of silicon on a single crystal substrate.
  31. 31. Silicon Epitaxial Growth on a Silicon Wafer Si Si Cl Cl H H Si Si Si Si Si Si Si Si Si Si Si Cl H Cl H Chemical reaction By-products Deposited silicon Epitaxial layer Single silicon substrate
  32. 32. Illustration of Vapor Phase Epitaxy Dopant (AsH3 or B2 H3) H2 SiH2 Cl2 RF induction-heating coils Susceptor Wafers Vacuum puimp
  33. 33. Silicon Vapor Phase Epitaxy Reactors Exhaust Exhaust Exhaust RF heating RF heating Gas inlet Gas inlet Horizontal reactor Barrel reactor Vertical reactor
  34. 34. Effects of Keyholes in ILD on Metal Step Coverage b) SiO2 is planarized c) Next layer of aluminum is deposited Metal void caused by keyhole defect in SiO2 a) SiO2 deposited by PECVD SiO2 Keyhole defect in interlayer dielectric Aluminum ILD: Interlayer Dielectric