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EPITAXY

EPITAXY. Topics Covered. Epitaxy Basics Advantages Types VPE and Epitaxy Growth model Liquid Phase Molecular Beam Defects. EPITAXY. It is a process used to grow a crystalline layer on a crystalline substrate. Used for high purity layer growth (denuded zone) Provides Isolation

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EPITAXY

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  1. EPITAXY

  2. Topics Covered • Epitaxy Basics • Advantages • Types • VPE and Epitaxy Growth model • Liquid Phase • Molecular Beam • Defects

  3. EPITAXY • It is a process used to grow a crystalline layer on a crystalline substrate. • Used for high purity layer growth (denuded zone) • Provides Isolation • The lattice constant of the epitaxially grown layer needs to be close to the lattice constant of the substrate wafer. Otherwise the bonds cannot stretch far enough and dislocations will result.

  4. Epitaxial growth processes can be divided into a) Commensurate, b) Strain relaxed incommensurate and c) Incommensurate but pseudomorphic Problems due to lattice mismatch

  5. A Micrograph showing the (111) planes of Al epitaxially grown on (111) Silicon

  6. Advantages of Epitaxy • Better doping profiles than Diffusion and Ion Implantation • Can form very thick doped structures devices(30-100 um) (required in power devices) • Less Oxygen and Carbon content in Epilayer

  7. Types of Epitaxy Processes • Vapour phase epitaxy • Liquid phase epitaxy • Molecular Beam Epitaxy

  8. Gas inlet Gas inlet RF heating RF heating Exhaust Exhaust Exhaust Horizontal reactor Vertical reactor Barrel reactor Vapor Phase Epitaxy Equipment

  9. Vapor Phase Epitaxy Sources • Silane (SiH4) is preferred at low temperatures since at higher temperatures it results in gas phase decomposition of Silane resulting in particles. These particles tend to fall onto the wafers leading to contamination. • Silicon tetrachloride is preferred at low temperature • Higher quality (fewer defects) material occurs at higher temperatures than 800 C. At Higher temperature Dichlorosilane (SiH2Cl2) is most commonly used and allows growth at ~800 to 1050 °C • For growing P type epi layer, Gaseous Diborane(B2H6) is used and for N type epi layer, Phosphine(PH3) and Arsine(As3) are used

  10. Behaviour of variety of Silicon containing growth species

  11. VPE Reactions Overall Reaction Detailed Reactions

  12. Semiconductor Surface Gas Cg Si Bulk Cs F1 F2 Epitaxy Growth Model Cg = Concentration of reactant species in gas Cs = Concentration of reactant species at semiconductor surface Ks = surface reaction rate constant Hg=gas phase mass transfer coefficient F1 = flux of reactant species from bulk of gas to Si interface = hg (Cg-Cs) ------(1) Under Steady state, F1= F2 F2 = flux of reactant species consumed in epitaxial reactions=Ks Cs ------(2)

  13. Epitaxy Growth Model Let Ca=Number of semiconductor atoms incorporated per unit volume Growth rate of Epitaxial layer = Let Ct=Total number of molecules per cubic cm in gas = Cg/y Where y = mole fraction of reactant species Case 1: Ks <<hg Case 2 : hg<<Ks Surface Reaction Controlled Mass transfer controlled

  14. Liquid Phase Epitaxy • Advantages: • Higher dopant concentrations • Low defect occurrence • Uses: • growing GaAs and other III-V compounds • Used for growing thin layers since growth rate is small • Types of processes • Equilibrium cooling • Two Step Cooling

  15. LPE EQUIPMENT

  16. Molecular Beam Epitaxy (MBE) -Process • In an UHV chamber, ultra high purity materials (Si or other dopants) are evaporated. • The evaporated species is transported at relatively high velocity in vacuum to the substrate • Because of the very low pressure, the mean free path is very long.Thus, the evaporated material travels in a straight line (a molecular beam) toward a hot substrate. • Once on the substrate, the atom or molecule moves around until it finds an atomic site to chemically bond to. • Shutters can be used to turn the beam flux on and off • The flux of atoms/molecules is controlled by the temperature of the “effusion cell”

  17. Molecular Beam Epitaxy (MBE)

  18. Molecular Beam Epitaxy (MBE) • Advantages: • Offers the highest purity material (due to UHV conditions) and the best layer control (almost any fraction of an atomic layer can be deposited and layers can be sequenced one layer at a time (for example Ga then As then Ga etc…). • Low temperature Process – out diffusion and auto doping is minimized • Precise control of doping possible • Disadvantages: • Low throughput • Equipment is Expensive

  19. Epitaxial Defects • Line dislocations in substrate can extend to epi layer • Tripyramids, Hillock formation. • Misfit Dislocations

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