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

Outline  Introduction Deal/Grove (Kinetic) Model Deal/Grove (Kinetic) Model Impurity Redistribution Impurity Redistribution Masking Properties of SiO 2 Masking Properties of SiO 2 Oxide Quality Oxide Quality Oxide Thickness Measurement Oxide Thickness Measurement

Definition Process by which a layer of silicon dioxide (SiO 2 ) 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

Reactions Dry oxidation: Si + O 2 → SiO 2 (better quality) Wet oxidation: Si + 2H 2 O → SiO 2 + 2H 2 (faster growth rate)

Silicon Consumption During growth, 1 mole of SiO 2 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

Outline Introduction Introduction  Deal/Grove (Kinetic) Model Impurity Redistribution Impurity Redistribution Masking Properties of SiO 2 Masking Properties of SiO 2 Oxide Quality Oxide Quality Oxide Thickness Measurement Oxide Thickness Measurement

Model Assumptions Temperature: o C Pressure: atm SiO 2 thickness:  m

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

Flux D = diffusion coefficient of oxidizing species x = thickness of existing oxide layer  = surface reaction rate constant At steady-state, F 1 = F 2 = F, so:

Growth Rate where: C 1 = # molecules of oxidizing species/unit volume = 2.2 × cm -3 for O 2 = 2.2 × cm -3 for O 2 = 4.4 × cm -3 for H 2 O = 4.4 × cm -3 for H 2 O

Solution Initial condition: x(0) = d 0 Initial condition: x(0) = d 0 where:

Compact Form x 2 + Ax = B(t +  ) where: A = 2D/  B = 2DC o /C 1

Limiting Cases Short times (reaction rate-limited): Short times (reaction rate-limited): “Linear Regime” Longer times (diffusion-limited): Longer times (diffusion-limited): x 2 = B(t +  ) “Parabolic Regime”

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

Example A silicon sample is oxidized in dry O 2 at 1200 o C for one hour. (a) What is the thickness of the oxide grown? SOLUTION: From Table 3-2, for dry O 1200 o C A = 0.04  m, B =  m 2 /h,  = h Using these parameters, we obtain an oxide thickness of x =  m

Example (cont.) (b) How much additional time is required to grow 0.1  m more oxide in wet O 2 at 1200 o C? SOLUTION: From Table 3-1, for wet O 2 at 1200 o C are A = 0.05  m, B = 0.72  m 2 /H Since d 0 =  m from the first step, = h = h The final desired thickness is x = d  m =  m. Using these parameters, we obtain an additional time of t = 0.76 h = 4.53 min

Temperature Variation

Outline Introduction Introduction Deal/Grove (Kinetic) Model Deal/Grove (Kinetic) Model  Impurity Redistribution Masking Properties of SiO 2 Masking Properties of SiO 2 Oxide Quality Oxide Quality Oxide Thickness Measurement Oxide Thickness Measurement

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 SiO 2 is: The ratio of equilibrium concentration of the impurity in Si to that in SiO 2 is:

4 Cases of Redistribution

Outline Introduction Introduction Deal/Grove (Kinetic) Model Deal/Grove (Kinetic) Model Impurity Redistribution Impurity Redistribution  Masking Properties of SiO 2 Oxide Quality Oxide Quality Oxide Thickness Measurement Oxide Thickness Measurement

Oxides as Dopant Masks SiO 2 can provide a selective mask against diffusion at high temperatures. Oxides used for masking are ~  m thick. Dopants Diffusion Constants at 1100 o C (cm 2 /s) B 3.4 × – 2.0 × Ga 5.3 × P 2.9 × – 2.0 × As 1.2 × – 3.5 × Sb 9.9 ×

SiO 2 Masks for B and P

Outline Introduction Introduction Deal/Grove (Kinetic) Model Deal/Grove (Kinetic) Model Impurity Redistribution Impurity Redistribution Masking Properties of SiO 2 Masking Properties of SiO 2  Oxide Quality Oxide Thickness Measurement Oxide Thickness Measurement

Dry vs. Wet Oxides Wet oxides are usually used for masking SiO 2 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.

Oxide Charge Definitions 1.Interface trapped charge (Q it ): located at Si/SiO 2 interface 2.Fixed oxide charge (Q f ): positive charge located within 3nm of Si/SiO 2 interface 3.Oxide trapped charges ( Q ot ): associated with defects in the SiO 2 4.Mobile ionic charges (Q m ): result from contamination from Na or other alkali ions

Oxide Charge Locations

Outline Introduction Introduction Deal/Grove (Kinetic) Model Deal/Grove (Kinetic) Model Impurity Redistribution Impurity Redistribution Masking Properties of SiO 2 Masking Properties of SiO 2 Oxide Quality Oxide Quality  Oxide Thickness Measurement

Color Chart Thickness (  m) Color 0.07Brown 0.31Blue 0.39Yellow 0.41 Light orange 0.47Violet Not very accurate Colors repeat periodically at higher thicknesses

Profilometry Requires a step feature Accurate for thicknesses in 100 nm – 0.5  m range

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.