1OxidationD. W. Parent2Silicon dioxide•What is SiO2?•What is SiO2used for?• Advantages and Disadvantages of SiO2• How is it grown?–Dry–Wet• Numerical Examples3What is SiO2?• Two forms– Single crystal (quartz)– Amorphous• We are interested in Amorphous SiO2– Random three dimensional network of SiO2 constructed from polyhedra of oxygen ions.– This material is more porous than Quartz (density of 2.15-2.25g/cm3compared to 2.65 25g/cm3)4What is SiO2?Nonbridging oxygenNetwork modifierBridging oxygenSiliconNetwork formerThe O-Si-O Bondangle is 109oTetrahedral distancebewtween Si and O ions is1.6Å5What is SiO2used for?• MOS Metal Oxide Semiconductor• Device passivation– Combines with dangling bonds to reduce surface statesN+N+Metal or Poly SiSiO2SiP6What is SiO2used for?• Diffusion Masks– Block the diffusion of B and P for example• Antireflective coating for Photodevices23124thickness,nnnnλ==Ar Coating (n2)Device (n3)Air (n1)LightDevice (n3)B or PSiO2SiO27Advantages and Disadvantages of SiO2• CMOS digital logic gates use little power when not switching logic state, thus high levels of integration are possible because the standby power consumption is low.•SiO2 is a native film that is quite easy to grow. All that is required is heat and oxygen or steam.8Advantages and Disadvantages of SiO2•SiO2 consumes Si while growing. 44% of the SiO2 layer comes from the original Si.– This leads to a non-planer structure after each oxidation step.• Due to the large increase in volume there is 2-4×109 dyn cm-1 of compressive strain.– This causes dislocations.• Oxidation-Induced Stacking Faults (these can be removed by a high temp treatment.9Advantages and Disadvantages of SiO2• The large dielectric constant leads to larger capacitance values for a given thickness (compared to silicon nitride).10How is it grown?• The oxidizing species must diffuse through the SiO2 layer that has already grown. This leads to a linear regime of growth and a parabolic regime of growth. Given by the equation:)()/()(22hrthrmBXmAXμμ=+11How is it grown?• Dry oxidation: Flow dry O2 over sample at elevated temperatures.22SiOOSi ⎯→⎯+• Wet oxidation: Bubble N2through a water bubbler @95Co over sample at elevated temperatures.22222 HSiOOHSi +⎯→⎯+12Linear Rate Constant versus Tempurature-3.50-3.00-2.50-2.00-1.50-1.00-0.500.000.501.000.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1Tempurature 1000/T(K-1)Log(B/A(microns/hour))Dry <100>Dry <111>13Wet Oxidation-1.50-1.25-1.00-0.75-0.50-0.250.000.250.500.751.001.250.70 0.73 0.75 0.78 0.80 0.83 0.85 0.88 0.90Tempurature 1000/K-1Log(B/A (microns/hr))Wet<100>Wet<111>14Parabolic Rate Constant versus Tempurature-3.00-2.75-2.50-2.25-2.00-1.75-1.50-1.25-1.00-0.75-0.50-0.250.000.65 0.70 0.75 0.80 0.85 0.90 0.95Tempurature 1000/T (k-1)Log (B(microns2/hr))Wet<111> or <100>Dry<111>, or <100>15Numerical Examples• How long do we need to grow SiO2at 1155Cousing a wet process<111> to protect against a 30 minute1100CoP diffusion?• How long do we need to grow SiO2at 1265Cousing a dry process<100> to create a MOS insulator capacitance (Ci=εi/d) of 69nF? – Note: For SiO2