Silicon Oxidation

1. Silicon Oxidation ECE/ChE 4752: Microelectronics Processing Laboratory Gary S. May January 15, 2004

2. Outline • Introduction • Deal/Grove (Kinetic) Model • Impurity Redistribution • Masking Properties of SiO2 • Oxide Quality • Oxide Thickness Measurement

3. Definition • Process by which a layer of silicon dioxide (SiO2) is grown on a silicon substrate • Applied exclusively to Si, since GaAs, Ge, and other semiconductors don’t form native oxides • Uses: 1) implant/diffusion mask 2) surface passivation 3) isolation 4) key component of MOS structures 5) dielectric for multilevel interconnect

4. Reactions • Dry oxidation: Si + O2→ SiO2 (better quality) • Wet oxidation: Si + 2H2O → SiO2 + 2H2 (faster growth rate)

5. Silicon Consumption • During growth, 1 mole of SiO2 takes up more volume than 1 mole of Si • To grow an oxide layer of thickness d, a layer of Si of thickness 0.44d is consumed

6. Outline • Introduction • Deal/Grove (Kinetic) Model • Impurity Redistribution • Masking Properties of SiO2 • Oxide Quality • Oxide Thickness Measurement

7. Model Assumptions • Temperature: 700 - 1300 oC • Pressure: 0.2 - 1.0 atm • SiO2 thickness: 0.03 - 2 mm

8. Basic Diagram Co = concentration of oxidizing species at oxide surface (cm-3) Cs = concentration of oxidizing species at Si surface (cm-3) d = oxide thickness F’s = fluxes (cm-2s-1)

9. Flux D = diffusion coefficient of oxidizing species x = thickness of existing oxide layer k = surface reaction rate constant At steady-state, F1 = F2 = F, so:

10. Growth Rate where: C1 = # molecules of oxidizing species/unit volume = 2.2 × 1022 cm-3 for O2 = 4.4 × 1022 cm-3 for H2O

11. Solution • Initial condition: x(0) = d0 where:

12. Compact Form x2 + Ax = B(t + t) where: A = 2D/k B = 2DCo/C1

13. Limiting Cases • Short times (reaction rate-limited): “Linear Regime” • Longer times (diffusion-limited): x2 = B(t + t) “Parabolic Regime”

14. Thin, Dry Oxides • For wet oxidation, initial oxide thickness d0 is very small (or t≈ 0). • For dry oxidation, extrapolated value of d0 at t = 0 is about 25 nm. • Thus, dry oxidation on bare silicon requires a value for t that can be generated using this initial thickness.

15. Example A silicon sample is oxidized in dry O2 at 1200 oC for one hour. (a) What is the thickness of the oxide grown? SOLUTION:  From Table 3-2, for dry O2 @ 1200 oC A = 0.04 mm, B = 0.045 mm2/h, t = 0.027 h Using these parameters, we obtain an oxide thickness of x = 0.196 mm

16. Example (cont.) (b) How much additional time is required to grow 0.1 mm more oxide in wet O2 at 1200 oC? SOLUTION:  From Table 3-1, for wet O2 at 1200 oC are A = 0.05 mm, B = 0.72 mm2/H Since d0 = 0.196 mm from the first step, = 0.067 h The final desired thickness is x = d0 + 0.1 mm = 0.296 mm. Using these parameters, we obtain an additional time of t = 0.76 h = 4.53 min

17. Temperature Variation

18. Outline • Introduction • Deal/Grove (Kinetic) Model • Impurity Redistribution • Masking Properties of SiO2 • Oxide Quality • Oxide Thickness Measurement

19. Segregation Coefficient • When two solids come together, an impurity in one will redistribute until it reaches equilibrium. • The ratio of equilibrium concentration of the impurity in Si to that in SiO2 is:

20. 4 Cases of Redistribution

21. Outline • Introduction • Deal/Grove (Kinetic) Model • Impurity Redistribution • Masking Properties of SiO2 • Oxide Quality • Oxide Thickness Measurement

22. Oxides as Dopant Masks • SiO2 can provide a selective mask against diffusion at high temperatures. • Oxides used for masking are ~ 0.5-1 mm thick.

23. SiO2 Masks for B and P

24. Outline • Introduction • Deal/Grove (Kinetic) Model • Impurity Redistribution • Masking Properties of SiO2 • Oxide Quality • Oxide Thickness Measurement

25. Dry vs. Wet Oxides • Wet oxides are usually used for masking • SiO2 growth rate is much higher when water is the oxidant. • Dry oxidation results in a higher quality oxide that is denser and has a higher breakdown voltage (5 – 10 MV/cm). • Thin gate oxides in MOS devices are usually formed using dry oxidation.

26. Oxide Charge Definitions • Interface trapped charge (Qit): located at Si/SiO2 interface • Fixed oxide charge (Qf): positive charge located within 3nm of Si/SiO2 interface • Oxide trapped charges (Qot): associated with defects in the SiO2 • Mobile ionic charges (Qm): result from contamination from Na or other alkali ions

27. Oxide Charge Locations

28. Outline • Introduction • Deal/Grove (Kinetic) Model • Impurity Redistribution • Masking Properties of SiO2 • Oxide Quality • Oxide Thickness Measurement

29. Color Chart • Not very accurate • Colors repeat periodically at higher thicknesses

30. Profilometry • Requires a step feature • Accurate for thicknesses in 100 nm – 0.5 mm range

31. Ellipsometry • Polarization changes are a function of optical properties, thickness, and wavelength and angle of incidence of the light beam. • Differences in polarization measured by an ellipsometer, and oxide thickness can be calculated. • Polarization changes occur when light is reflected from or transmitted through a medium.