Section 4: Thermal Oxidation

1. Section 4: Thermal Oxidation Jaeger Chapter 3 EE143 - Ali Javey

2. Properties of SiO2 SiO2 Thermal SiO2 is amorphous. Weight Density = 2.20 gm/cm3 Molecular Density = 2.3E22 molecules/cm3 Crystalline SiO2 [Quartz] = 2.65 gm/cm3 <Si> (1) Excellent Electrical Insulator Resistivity > 1E20 ohm-cm Energy Gap ~ 9 eV (2) High Breakdown Electric Field > 10MV/cm (3) Stable and Reproducible Si/SiO2 Interface EE143 - Ali Javey

3. Properties of SiO2 (cont’d) (4) Conformal oxide growth on exposed Si surface (5) SiO2 is a good diffusion mask for common dopants e.g. B, P, As, Sb. *exceptions are Ga (a p-type dopant) and some metals, e.g. Cu, Au SiO2 Si EE143 - Ali Javey

4. Properties of SiO2 (cont’d) (6) Very good etching selectivity between Si and SiO2. HF dip SiO2 Si Si EE143 - Ali Javey

5. Thermal Oxidation of Silicon EE143 - Ali Javey

6. Thermal Oxidation Equipment Horizontal Furnace Vertical Furnace EE143 - Ali Javey

7. Kinetics of SiO2 growth Oxidant Flow (O2 or H2O) Gas Flow Stagnant Layer Gas Diffusion Solid-state Diffusion SiO2 Si-Substrate SiO2 Formation EE143 - Ali Javey

8. SiO2 Si Si Silicon consumption during oxidation 2.17 mm 1mm 1mm Si oxidized 2.17 mm SiO2 molecular density of SiO2 atomic density of Si EE143 - Ali Javey

9. The Deal-Grove Model of Oxidation stagnant layer CG Cs SiO2 Si Note Cs Co Co Ci X0x F3 F2 F1 gas transport flux reaction flux at interface diffusion flux through SiO2 • F: oxygen flux – the number of oxygen molecules that crosses a plane of a certain area in a certain time EE143 - Ali Javey

10. The Deal-Grove Model of Oxidation (cont’d) Mass transfer coefficient [cm/sec]. “Fick’s Law of Solid-state Diffusion” Diffusivity [cm2/sec] Oxidation reaction rate constant EE143 - Ali Javey

11. Diffusivity: the diffusion coefficient EE143 - Ali Javey

12. The Deal-Grove Model of Oxidation (cont’d) • CS and Co are related by Henry’s Law • CG is a controlled process variable (proportional to the input oxidant gas pressure) Only Co and Ci are the 2 unknown variables which can be solved from the steady-state condition: F1 = F2 =F3 ( 2 equations) EE143 - Ali Javey

13. The Deal-Grove Model of Oxidation (cont’d) Henry’s Law Henry’s constant partial pressure of oxidant at surface [in gaseous form]. from ideal gas law PV= NkT  EE143 - Ali Javey

14. The Deal-Grove Model of Oxidation (cont’d) Define Similarly, we can set up equations for F2 and F3 Using the steady-state condition: We therefore can solve for Co and Ci 1 2 EE143 – Vivek Subramanian

15. The Deal-Grove Model of Oxidation (cont’d) We have: At equilibrium: F1=F2=F3 Solving, we get: Where h=hg/HkT EE143 - Ali Javey

16. The Deal-Grove Model of Oxidation (cont’d) We can convert flux into growth thickness from: Oxidant molecules/unit volume required to form a unit volume of SiO2. SiO2 Si F EE143 - Ali Javey

17. The Deal-Grove Model of Oxidation (cont’d) Initial Condition: At t = 0 , Xox = Xi xox SiO2 SiO2 Si Si Solution Note: hg >>ks for typical oxidation condition EE143 - Ali Javey

18. for O2 as oxidant for H2O as oxidant Dry / Wet Oxidation Note : “dry” and “wet” oxidation have different N1 factors EE143 - Ali Javey

19. Summary: Deal-Grove Model Xox t Oxide Growth Rate slows down with increase of oxide thickness EE143 - Ali Javey

20. Solution: Oxide Thickness Regimes (Case 1) Large t [ large Xox ] (Case 2) Small t [ Small Xox ] EE143 - Ali Javey

21. Thermal Oxidation on <100> Silicon EE143 - Ali Javey

22. Thermal Oxidation on <111> Silicon EE143 - Ali Javey

23. Thermal Oxidation Example A <100> silicon wafer has a 2000-Å oxide on its surface (a) How long did it take to grow this oxide at 1100o C in dry oxygen? (b)The wafer is put back in the furnace in wet oxygen at 1000o C. How long will it take to grow an additional 3000 Å of oxide? EE143 - Ali Javey

24. Thermal Oxidation ExampleGraphical Solution (a) According to Fig. 3.6, it would take 2.8 hr to grow 0.2 mm oxide in dry oxygen at 1100o C. EE143 - Ali Javey

25. Thermal Oxidation ExampleGraphical Solution (b) The total oxide thickness at the end of the oxidation would be 0.5 mm which would require 1.5 hr to grow if there was no oxide on the surface to begin with. However, the wafer “thinks” it has already been in the furnace 0.4 hr. Thus the additional time needed to grow the 0.3 mm oxide is 1.5-0.4 = 1.1 hr. EE143 - Ali Javey

26. Thermal Oxidation ExampleMathematical Solution EE143 - Ali Javey

27. Thermal Oxidation ExampleMathematical Solution EE143 - Ali Javey

28. 1 2 3 Effect of Xi on Wafer Topography SiO2 SiO2 Xi Si EE143 - Ali Javey

29. 1 2 3 1 3 2 Effect of Xi on Wafer Topography SiO2 SiO2 more oxide grown more Si consumed Xi Si less oxide grown less Si consumed EE143 - Ali Javey

30. Temperature Ambient Type (Dry O2, Steam, HCl) Ambient Pressure Substrate Crystallographic Orientation Substrate Doping Factors Influencing Thermal Oxidation EE143 - Ali Javey

31. Dry oxidation, 900oC High Doping Concentration Effect Coefficients for dry oxidation at 900oC as function of surface Phosphorus concentration SiO2 n+ n+ n n EE143 - Ali Javey

32. Transmission Electron Micrograph of Si/SiO2 Interface Amorphous SiO2 Crystalline Si EE143 - Ali Javey

33. Thermal Oxide Charges potassium sodium EE143 - Ali Javey

34. Oxide Quality Improvement • To minimize Interface Charges Qf and Qit • Use inert gas ambient (Ar or N2) when cooling down at end of oxidation step • A final annealing step at 400-450oC is performed with 10%H2+90%N2 ambient (“forming gas”) after the IC metallization step. EE143 - Ali Javey

35. Oxidation with Chlorine-containing Gas • Introduction of halogen species during oxidation • e.g. add ~1- 5% HCl or TCE (trichloroethylene) to O2 • reduction in metallic contamination • improved SiO2/Si interface properties SiO2 Na+ K+ Si Cl2 Na+ or K+ in SiO2 are mobile! EE143 - Ali Javey

36. Effect of HCl on Oxidation Rate EE143 - Ali Javey

37. Local Oxidation of Si [LOCOS] ~100A SiO2 (thermal) - pad oxide to release mechanical stress between nitride and Si. Oxidation Nitride Etch EE143 - Ali Javey

38. Local Oxidation of Silicon (LOCOS) Standard process suffers for significant bird’s beak Fully recessed process attempts to minimize bird’s beak EE143 - Ali Javey

39. Si SiO2 C1 CB C2 Dopant Redistribution during Thermal Oxidation conc. e. g. B, P, As, Sb. Si CB (uniform) x Segregation Coefficient equilibrium dopant conc. in Si equilibrium dopant conc. in SiO2 Fixed ratio (can be>1 or <1) EE143 - Ali Javey

40. Four Cases of Interest (A) m < 1 and dopant diffuses slowly in SiO2 SiO2 Si C2 CB e. g. B (m = 0.3) D C1 flux loss through SiO2 surface not considered here. B will be depleted near Si interface. EE143 - Ali Javey

41. Four Cases of Interest (B) m > 1, slow diffusion in SiO2. SiO2 Si C1 e.g. P, As, Sb CB C2 dopant piling up near Si interface for P, As & Sb EE143 - Ali Javey

42. Four Cases of Interest (C) m < 1, fast diffusion in SiO2 e. g. B, oxidize with presence of H2 SiO2 Si C2 CB C1 EE143 - Ali Javey

43. Four Cases of Interest (D) m > 1, fast diffusion in SiO2 SiO2 Si CB e. g. Ga (m=20) C1 C2 EE143 - Ali Javey

44. Polycrystalline Si Oxidation poly-Si SiO2 grain boundaries (have lots of defects). fast slower SiO2 a roughness with Xox b Overall growth rate is higher than single-crystal Si EE143 - Ali Javey

45. 2-Dimensional oxidation effects (100) Thinner oxide (110) Thicker oxide (100) (100) (110) Si cylinder Top view Mechanical stress created by SiO2 volume expansion also affects oxide growth rate EE143 - Ali Javey