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Oxidation Techniques and Systems Presentation Transcript
1.UNIT PROCESS OXIDATION
2.Oxidation Techniques and Systems
3.Growth of Silicon di Oxide
4.Dry Oxidation
5.Typical dry oxidation process sequence Stabilize furnace temperature with purge nitrogen flow Idle with process nitrogen flow Load furnace (push in wafer boat) Oxidation with dry oxygen, HCl; stop nitrogen flow (for calculated time) Start process N2 flow; stop O2 & HCl Unload furnace (pull out wafer boat) Stop process N2; idle with purge N2 flow
6.Wet Oxidation
7.Dependence of oxidation rate on orientation
8.Comparison of Dry and Wet Oxide growth rates
.Typical pyrogenic wet oxidation process sequence Stabilize furnace temperature with purge nitrogen flow Idle with process nitrogen & O2 flow Load furnace (push in wafer boat) Turn off N2; stabilize in O2 flow Turn on H2 flow, ignition & H2 flow stabilization Oxidation for calculated time Turn off H2, keep O2 flowing Turn on N2 flow, stop O2 flow Unload furnace (pull out wafer boat) 10.High Pressure Oxidation
11.High Pressure Oxidation For thick Oxides: The oxidation rate depends upon the parabolic rate coefficient (B). B depends upon Cg , the equilibrium concentration of oxygen in the gas phase. Increasing the oxygen pressure in the furnace will therefore increase B and can therefore decrease the time or the temperature for growing the same thickness of the oxide.
12.Atomic Structure of the Oxide Film Q. Explain the kinetics of thin oxide growth. Raj. Univ. 2006 Q. Explain the Grove–Deal model of oxidation and also derive linear and parabolic rate coefficients of oxidation. Raj. Univ. 2008
13.Grove-Deal model describes the kinetics of silicon oxidation. It derives a relationship between ‘oxidation time’ and ‘thickness of the grown oxide’. Effect of temperature and pressure on oxide thickness can also be estimated by this model. The model is valid for: oxidation temperatures 700 – 10000 C partial pressures o.2 – 1.0 atmosphere oxide thickness 300 – 20,000 ? oxygen and water ambient
14.Grove – Deal Model According to this model the oxidizing species (1) are transported from the bulk of the gas phase to the gas-oxide interface with flux F1 (the flux is the number of atoms or molecules crossing a unit area in a unit of time) (2) are transported across the existing oxide toward the silicon with a flux F2 , and (3) react at the Si-SiO2 interface with the silicon with flux F3 .
15.Growth Mechanism and Kinetics
16.Basic model for thermal oxidation of silicon
17.For steady state F1 = F2 = F3 , The flux of oxidant from the bulk of the gas phase to the gas – oxide interface is proportional to the difference between the oxidant concentration in the bulk of the gas CG and the oxidant concentration adjacent to the oxide interface CS . F1 = hG(CG – CS) (1) Where hG is the gas-phase mass transfer coefficient
18.To relate the equilibrium oxidizing species concentration in the oxide to that in the gas phase, we invoke Henry’s Law: C0 is the equilibrium concentration in the oxide at the outer surface is given by C0 = HpS (2) and C* is the equilibrium bulk concentration in the oxide is given by C* = HpG (3) Where pS is the partial pressure in the gas adjacent to the oxide, pG is the partial pressure in the bulk of the gas and, H is Henry’s Law Constant Using Henry’s Law along with the ideal gas law F1 = h(C* - C0) (4) where h is the gas-phase mass-transfer coefficient in terms of concentration in the solid,
19.H is given by h = hG /HkT ? Oxidation is a non-equilibrium process The flux of this oxidizing species across the oxide is taken to follow Fick’s Law at any point d in the oxide layer. Following the steady state assumption, F2 must be the same at any point within the oxide, resulting in F2 = D(C0 – Ci) / d0 (5) where D is the diffusion coefficient, Ci is the oxidizing species concentration in the oxide adjacent to the oxide-silicon interface, d0 is the oxide thickness.
20.Assuming that the flux corresponding to the silicon-silicon di oxide interface reaction is proportional to Ci F3 = kSCi (6) where kS is the rate constant of the chemical surface reaction for silicon oxidation. For steady state condition F1 = F2 = F3 By solving the simultaneous equations, expressions for Ci and C0 can be obtained.
21.2Limiting cases When the diffusivity is very small, Ci ? 0 and C0 ? C* This is called the diffusion-controlled case. It results from the flux of oxidant through the oxide being small (due to D being small) compared to the flux corresponding to the Si-SiO2 interface reaction. Hence the oxidation rate depends on the supply of oxidant to the interface, as opposed to the reaction at the interface.
22.2Limiting cases (contd.) When the diffusivity is large, Ci = C0 ; This is called the reaction-controlled case. An abundant supply of oxidant is provided at the Si-SiO2 interface. The oxidation rate is controlled by the reaction rate constant kS and by Ci (which equals C0).
23.Rate of oxide growth Define N1 as the number of oxidant molecules incorporated into a unit volume of the oxide layer. Since the oxide has 2.2X1022 SiO2 molecules/cm3 and one O2 molecule is incorporated into each SiO2 molecule, N1 equals 2.2X1022 cm-3 for dry oxygen. The number for water vapor oxidation is twice as big because two H2O molecules are incorporated into each SiO2 molecule.
24.Rate of oxide growth Combining various equations and assuming that an oxide may be present initially from a previous step or may grow before the assumptions in the model are valid, i.e. d0 = di at t=0, The following equation can be generated: d20 + Ad0 = B(t + ?) (7) A = 2D[1/kS + 1/h] (7a) B = 2DC*/N1 (7b) ? = [d2i + Adi]/B (7c)
25.Rate of oxide growth The quantity ? represents a shift in the time coordinate to account for the presence of the initial oxide layer di. Equation (7) is the well known mixed linear-parabolic relationship. Solving eq. (7) for d0 as a function of time gives D0/(A/2) = [1 + (t+?)/{A2/4B}]½ - 1, (8)
26.Two limiting cases: One limiting case occurs for t >> ? and t >> A2/4B, d20 = Bt (9) Equation (9) is the parabolic law, where B is the parabolic rate constant. The other limiting case occurs for short oxidation times when (t+?) << A2/4B, d0 = {B/A} (t+?) , (10) Equation (10) is the linear law, where B/A is the linear rate constant. Equations (9) and (10) are the diffusion-controlled and reaction-controlled cases respectively.
27.Linear and Parabolic rate constants Dopant Redistribution at the Si-SiO2 interface:
28.Pre-oxidation Clean
29.Critical aspects of the oxidation process
30.Plasma Oxidation
31.Anodic Oxidation
32.Oxide Induced Stacking Faults
33.Dependence of oxidation rate on orientation
34.Effect of impurities and damage on the oxidation rate
35.Effect of impurities and damage on the oxidation rate
36.Selective Oxidation LOCOS Process
37.Difference between Dry & Wet Oxidation
38.Oxide Thickness Measurement .
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