1 / 31

Silicon Oxidation

Silicon Oxidation. ECE/ChE 4752: Microelectronics Processing Laboratory. Gary S. May January 15, 2004. Outline. Introduction Deal/Grove (Kinetic) Model Impurity Redistribution Masking Properties of SiO 2 Oxide Quality Oxide Thickness Measurement. Definition.

carter
Download Presentation

Silicon Oxidation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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.

More Related