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Charged Defects Gettering and Crystal Growing Processes Presentation Transcript
1.Charged Defects, Gettering and Crystal Growing Processes
2.Charged Defects Vacancies ? may be charged also Charge interaction in a solid = chemical reactions in solutions, Concentration of vacancies ? determined by Mass-action relationships, Impurity conc. Gradient ? electric field in semiconductors Conc. Of neutral vacancies ? unaffected by this electric field. Hence, this conc. is same in extrinsic and intrinsic semiconductors .
3.Conc. Of neutral vacancies ? fn of Temperature Conc. Of charged vacancies ? fn of Temp. and Impurity Conc. (due to E field) Hence Total Vacancy conc. ? fn of Impurity conc. Reactions involved: Vo + h+ ? V+ Vo + e- ? V- Vo + 2e- ? V2- Charged vacancies have self compensating effect on the semiconductor P-type material will have V+ which is donor- like N-type material will have V- which is acceptor-like .
4.Solid Solubility Solid solubility is the conc. Of the impurity that the host lattice can accept in a solid solution of itself and impurity, Retrograde solubility: Solubility decreases with decreasing temp. Effect: Precipitation of impurity at lower temp. ? generation of volume defects.
5.How? Maximum conc. At high temp Cool down the crystal, lower solid solubility at lower temp. So the solution is supersaturated To achieve equilibrium impurity atoms in excess of solid solubility (at lower temp.) are precipitated as a second phase. (silicides) Precipitates ? undesirable ? sites of dislocation generation.
6.Gettering What is Gettering? A process that removes harmful impurities or defects from the regions in a wafer where devices are fabricated, Advantages: Improvements in junction leakage current How it is done? Trapping sites are intentionally created to trap the harmful impurities so that they become immobile Bulk of the wafer does not participate in device functioning. Therefore traps are introduced on the back surface of the wafer,
7.Gettering Techniques Pre-Gettering: Gettering before device fabrication ? provides sinks that absorb impurities during device fab. Back surface damaging ? done intentionally Mechanically ? lapping, sand blasting By LASER beam Thermal Treatments at different temps.
8.Gettering Techniques Intrinsic Gettering: Oxygen present in the wafer is precipitated to provide trapping sites, High Temperature Cycle (>1050 o C in N2) to evaporate Oxygen from near the wafer surface, Thus supersaturation condition near the surface is avoided. A defect free zone is obtained for device fab. This zone is called Denuded Zone Low temperature treatment to precipitate oxygen in the bulk,
9.Gettering Techniques Metallic Gettering; A metal e.g. Nickel ? evaporated on the ack surface of wafer. Heat treatment for adhesion, Impurities e.g. gold having higher solubility in Nickel get trapped in it. Polysilicon Gettering: Deposit 1 micron thick polysilicon on the back surface Grain boundaries in poly act as traps for impurities.
10.Mono-crystal Silicon Growth Crystal Pulling CZ Method Floating Zone Method .
11.Czochralski (CZ) crystal growth It involves the crystalline solidification of atoms from a liquid phase at an interface. The process consists of the following steps: A fused silica crucible is loaded with a charge of undoped EGS together with a precise amount of diluted silicon alloy. Evacuate the growth chamber. Back-fill with inert gas ? prevent atmospheric gases from entering the chamber.
12.Czochralski (CZ) crystal growth Heat Crucible to 1421o C to melt Silicon. Introduce seed crystal in molten silicon. Seed crystal is slim ( 5 mm dia) and 100 – 300 mm long with precise orientation The seed crystal is then withdrawn at a very controlled rate. The seed crystal and the crucible are rotated in opposite directions while this withdrawal process occurs.
13.Surface Tension Weight of the liquid silicon pulled up is supported by the surface tension of the melt. For 300 mm diameter wafer and 1 mm pulled melt, weight = 165 gm Weight of 1 meter long crystal = 165 Kg. It is important that the pulled liquid solidifies quickly and does not melt again. The temp. gradient across the pulled crystal should be very large.
14.Crystal Growing Theory Every impurity has a solid solubility in Silicon and a different equilibrium solubility in the melt. Segregation Coefficient k0 = Cs / Cl Cs , Cl = equilibrium concentrations of the impurity in the solid and melt near the interface .
15.Solidification Process Temperature gradient, solidification and transport phenomena involved in CZ growth is represented in the figure Positions 1 and 2 represent the locations of isotherms associated with the equation and the crystal solidification at the interface. Impurity atoms and silicon atoms are transported across the boundary layer and incorporated in the growing crystal.
16.Distribution of impurity in the grown crystal
17.Pull Rate and Growth Rate .
18.The FZ Process consists of the following steps: A polysilicon rod is mounted vertically inside a chamber, which may be under vacuum or filled with an inert gas. A needle-eye coil that can run through the rod is activated to provide RF power to the rod, melting a 2 cm long zone in the rod. This molten zone can be maintained in stable liquid form by the coil. The coil is then moved through the rod, and the molten zone moves along with it. The movement of the molten zone through the entire length of the rod melts the polysilicon and solidifies it into a single crystal.
19.Alternative Alternatively the coil can be kept stationary and the rod can be moved up or down through the coil.
20.Review Questions Explain with the help of diagrams various crystal defects. Describe crystal growing theory using CZ growth. Explain, in sequence, various process steps for obtaining finished wafer from single crystal ingot. Compare CZ and FZ processes for growing single crystal silicon. Numerical questions based on Cs = ko Co (1 – X)ko - 1
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