OXIDATION- Overview

1. OXIDATION- Overview • Process Types • Details of Thermal Oxidation • Models • Relevant Issues

2. Uses • As a part of a structure • e.g. Gate Oxide • For hard masks • e.g. In Nitride Etch, implant mask ... • Protecting the silicon surface (Passivation ) • Insulator (ILD/IMD) • As part of ‘mild etch’ (oxidation / removal cycles) • Whether useful or not, automatically forms in ambient • Native Oxide ( ~ 20 A thick) • except H-terminated Si (111)

3. Processes • Thermal Oxidation (Heating) • Dry vs Wet • Electrochemical Oxidation (Anodization) • Oxide (and nitride) • adhere well to the silicon • good insulator • Breakdown voltage 10 MV/cm • ==> Can make a very thin gate

4. Structure • Tetrahedral Structure • each Si to four O • each O to two Si • Single crystal quartz (density 2.6 g/cm3) • Fused silica (density 2.2 g/cm3) ©Time Domain CVD • Reaction with water • Si-OH termination is stable • structure is more porous than Si-O-Si

5. Thermal Oxidation Wet oxidation Dry oxidation • Dense oxide formed (good quality, low diffusion) • slow growth rate • NEED TO KEEP WATER OUT OF THE SYSTEM • Overall reaction • Relatively porous oxide formed (lower quality, species diffuse faster) • Still good quality compared to electrochem oxidation, for example • faster growth rate Dry oxide for gate ox Wet oxide for masking

6. Wet Oxidation • Proposed Mechanism • Hydration near Silicon/ Silicon oxide interface • Oxidation of silicon • Hydrogen rapidly diffuses out • Some hydrogen may form hydroxyl group

7. Diffusivities in Oxide • Oxygen diffuses faster (compared to water) • Sodium and Hydrogen diffuse very fast Hydrogen Oxygen Sodium Diffusivity (log scale) Water 1/T

8. Oxide Growth (Thermal) Original Si surface Oxide Si • To obtain 1 unit of oxide, almost half unit of silicon is consumed (0.44) • Oxidation occurs at the Si/SiO2 interface • i.e. Oxidizing species has to diffuse through ‘already existing’ silicon oxide

9. Oxide Growth (Thermal) • At any point of time, amount of oxide is variable ‘x’ • Usually, concentration of oxidizing species (H2O or O2) is sufficiently high in gas phase • ==> Saturated in the oxide interface Air (BL) Oxide Silicon o Concentration i x Distance

10. Oxidation Kinetics • At steady state • diffusion through oxide = reaction rate at the Si/SiO2 interface • Oxygen diffuses faster than Water • However, water solubility is very high (1000 times) • ==> Effectively water concentration at the interface is higher • ==> wet oxidation faster Diffusion At steady state Reaction

11. Oxidation Kinetics • 6.023x1023 molecules • =1 mol of oxide = x g of oxide • = y cm3 of oxide (from density) • 2.2 x 1022 molecules/cm3 • One O2 per SiO2 • Two H20 per SiO2 Flux at steady state • Oxide Growth Rate • = Flux/ # oxidizing species per unit volume (of SiO2) • n = 2.2 × 1022 cm-3 for O2 • = 4.4 × 1022 cm-3 for H2O Initial Condition Eqn

12. Bruce Deal & Andy Grove where Deal-Grove Model Solution OR t is the time needed to grow the ‘initial’ oxide • A and B depend on diffusivity “D”, solubility and # oxidizing species per unit volume “n” • A and B will be different for Dry and Wet oxidation

13. Linear & Parabolic Regimes If one starts with thin oxide (or bare silicon) • Very short Time • Longer Time • Linear vs Parabolic Regimes • Kinetic Controlled vs Mass Transfer Controlled • Initially faster growth rate, then slower growth rate

14. Exponential Regime If one starts with bare oxide • For dry oxidation, one finds that t is not zero in the model fit • A t corresponding to an initial thickness of 25 nm provides good fit • Initial growth at very high rate • Approximated by exponential curve • Hypothesis 1 • Charged species forms • holes diffuse faster / set up electrical field • diffusion + drift ==> effective diffusivity high • space charge regime controls • length = 15 nm for oxygen, 0.5 nm for water • ==> wet oxidation not affected

15. Exponential Regime • Hypothesis 2 • In dry oxidation, many ‘open’ areas exist • oxygen diffuses fast in silicon • hence more initial growth rate • once covered by silicon di oxide, slow diffusion • Hypothesis 3 • Even before reaction (at high temp), oxygen dissolved in silicon (reasonable diffusion) • once temp is increased, 5 nm quick oxide formation

16. Temp Variation of Linear/Parabolic Coeff Solubility and Diffusion function of temp Parabolic [B] Linear [B/A] © May & Sze

17. Effect of Doping • Doping increases oxidation rate • Segregation • ratio of dopant in silicon / dopant in oxide • e.g. Boron incorporated in oxide; more porous oxide • more diffusion • parabolic rate constant is higher • P not incorporated in oxide • no significant change in parabolic rate constant © May & Sze

18. Issues • Na diffuses fast in oxide • Use Cl during oxidation • helps trap Na • helps create volatile compounds of heavy metals (contaminant from furnace etc) • use 3% HCl or Tri chloro ethylene (TCE) Ref: VLSI Fabrication Principles by S.K. Ghandhi

19. Electrochemical • Use neutral solution and apply potential • Pt as counter electrode (Hydrogen evolution) • Use Ammonium hydrogen Phosphate or Phosphoric acid or ammonia solution • Silicon diffuses out and forms oxide • Increase in oxide thickness ==> increase in potential needed • self limiting • Oxide quality poor • Used to oxidize controlled amount and strip • for diagnosis