Chapter5_Oxidation.ppt
- 1. Chapter 6: Thermal Oxidation and the Si/SiO2 Interface
- 2. 2 Contents 1. Introduction 2. Structure 3. Manufacturing Methods and Equipment 4. Measurement Methods 5. Growth kinetics – Deal Grove model – Influence ambient 6. Trends 7. Summarisation
- 3. 3 Thermally grown oxides: • Si is unique, its oxide (SiO2) and the Si/SiO2 interface are very stable. Nothing in this world is perfect: • When SiO2 is formed, misfit at the interface causes interface states and some Si atoms in the SiO2 near the interface are positively ionised (fixed oxide charge)=> shift in MOST Vt (threshold voltage). • Sodium (Na) and Potasium (K) are incorporated and form mobile ions => instabilities of MOST Vt 1. Introduction
- 4. 4 1. Introduction Thermal oxides are used for: • Gate oxide 2-10 nm Dry oxidation (slow) Si + O2 => SiO2 • Pad oxide (20 nm, dry) between Si3N4 and Si for LOCOS (LOCal Oxidation of Silicon) • Electrical isolation between devices (lateral); Field Oxide (0.5-1 mm) Wet oxidation (fast) Si+2H2O => SiO2+2H2 • Masking layers (0.5-1.0 mm wet) for implantation and diffusion of doping atoms • The dielectric between conductors and the protection on top are made by CVD (Chemical Vapor Deposition)
- 5. 5 3. Structure • tetrahedron • amorphous • no periodic structure of lattice, density low diffusion rel. easy • quartzite: crystalline structure
- 6. 6 • diffusion of oxidant through already grown SiO2-layer • at Si-SiO2 interface: reaction of oxidant with Si thickness SiO2 = x consumed Si-thickness = 0.44x 2 2 2 2 2 2 2 H SiO O H Si SiO O Si
- 7. 7 mole cm cm g mole g / 18 . 27 / 21 . 2 / 08 . 60 3 2 volume of 1 mole of Si = • volume of 1 mole of SiO2 = • 1 mole SiO2 uses 1 mole of Si • To grow 100 Å thick SiO2, 44 Å Si is consumed density weight molecular mole of volume 1 mole cm cm g mole g / 06 . 12 / 33 . 2 / 09 . 28 3 3 area thickness volume 44 . 0 18 . 27 06 . 12 2 SiO Si h h
- 8. 8 Field oxide - LOCOS • Lateral isolation between devices • Thick : Wet oxidation • LOCOS: Local oxidation of Silicon • bird's beak! Fully recessed LOCOS: prior to oxidation etch trench in Silicon Pad oxide
- 9. 9 Field Oxide - Shallow Trench Isolation + Less area loss ( higher packing density) + planar surface + easy downscaling Trenches, reoxidation + B-implant, CVD oxide + CMP, spacers, etching pad oxide, gate -oxidation + -deposition
- 10. 10 3. Manufacturing Methods and Equipment Oxidation Techniques • Thermal Oxidation • Rapid Thermal Oxidation Thermal Oxidation Techniques • Wet Oxidation Si (solid) + H20 => SiO2 (solid) + 2H2 • Dry Oxidation Si (solid) + O2 (gas) => SiO2(solid)
- 11. 11 3. Manufacturing Methods and Equipment Furnace: Temperature: 700C - 1200C Ambient: oxygen ("dry oxidation") water vapour or burning hydrogen flame ("wet oxidation")
- 12. 12 Wafers are placed in wafer load station • Dry nitrogen is introduced into chamber - Nitrogen prevents oxidation from occurring • Nitrogen gas flow shut off and oxygen added to chamber - Occurs when furnace has reached maximum temperature - Oxygen can be in a dry gas or in a water vapor state • Nitrogen gas reintroduced into chamber - Stops oxidation process • Wafers are removed from furnace and inspected Dry Thermal Oxidation Characteristics • Oxidant is dry oxygen • Used to grow oxides less than 1000Å thick • Slow process - 140 - 250Å / hour
- 13. 13 Thin Oxide Growth • Thin oxides grown (<150Å) for features smaller than 1mm - MOS transistors, MOS gates, and dielectric components • Additional of chemical species to oxygen decreases oxide growth rate (only in special cases) - Hydrochloric acid (HCI) - Trichloroethylene (TCE) - Trichloroethane (TCA) • Decreasing pressure slows down oxide growth rate
- 14. 14 Wet Thermal Oxidation Characteristics • Oxidant is water vapor • Fast oxidation rate - Oxide growth rate is 1000-1200Å / hour • Preferred oxidation process for growth of thick oxides
- 15. 15 Oxidation occurs in tube furnace - Vertical Tube Furnace - Horizontal Tube Furnace Modern Equipments
- 16. 16 4.Measurement Methods Physical Measurements Step measurements: --step -SEM -AFM -TEM Si SiO2 TEM image Si SiO2 SEM image
- 17. 17 Optical Characterisation n1/n0 = sinf/sinb > lmin,max = 2 n1x0 cosb/m m= 1,2,3… for maximum, 0.5, 1.5, 2.5… for minimum 4.Measurement Methods
- 18. 18 Schematic drawing of an ellipsometer • The instrument relies on the fact that the reflection at a dielectric interface depends on the polarization of the light while the transmission of light through a transparent layer changes the phase of the incoming wave depending on the refractive index of the material. Ellipsometer Method • An ellipsometer can be used to measure layers as thin as 1 nm up to layers which are several microns thick. 4.Measurement Methods
- 19. 19 Electrical characterisation Charges in oxide • Mobile charges Qm :1010 -1012 cm-2 clean furnace tube, clean chemicals , do not ever touch wafers with bare hands Trapped oxide charge Qot : 109 -1013 cm-2 low-T anneal • Fixed oxide charge (+) Of : 1010 -1012 cm-2 rapid cooling or N2-anneal Qf<100> < Qf<111> • Interface states (E in gap) Qit 1010 cm-2 low T (450ºC) hydrogen anneal neutralisation potassium sodium incomplete Si-Si / Si-O bonds; 3 nm from interface 4.Measurement Methods
- 20. 20 C-V Measurements • There are a number of measurement techniques used to characterize SiO2 and the Si/SiO2 interface. • The most powerful of these is the C-V method which is described in the text in detail. 4.Measurement Methods
- 21. 21 • Electric field lines pass through the “perfect” insulator and Si/SiO2 interface, into the substrate where they control charge carriers. • Accumulation, depletion and inversion result. • HF curve - inversion layer carriers cannot be generated fast enough to follow the AC signal so Cinv is Cox + CD • LF curve - inversion layer carriers follow the AC signal so Cinv is just Cox. • Deep depletion - “DC” voltage is applied fast enough that inversion layer carriers cannot follow it, so CD must expand to balance the charge on the gate. • C-V measurements can be used to extract quantitative values for: • tox - oxide thickness • NA - the substrate doping profile • Qf, Qit, Qm, and Qot - oxide & interface charges. • See text for more details on these measurements. 4.Measurement Methods
- 22. 22 5. Growth kinetics Deal-Grove model • The basic model for oxidation was developed in 1965 by Deal and Grove. Si+O2 => SiO2 Si+2H2O => SiO2+2H2 ) • Three first order flux equations describe the three series parts of the process.
- 23. 23 5. Growth kinetics • Three first order flux equations describe the three series parts of the process. • Under steady state conditions, F1 = F2 = F3, so • Note that the simplifications are made by neglecting F1 which is a very good approximation. • Combining (3) and (4), we have (4) (5) (1) (2) (3) (6)
- 24. 24 • Integrating this equation (6) (see text), results in the linear parabolic model. Where ( parabolic rate constant) (linear rate constant) (7) • (7) can also be written with oxide thickness as a function of time. where t>>A2/4B (long time) xo 2=B(t+τ) Parabolic rate t+ τ <<A2/4B (short time) xo=(B/A)(t+τ) Linear rate
- 25. 25 • The rate constants B and B/A have physical meaning (oxidant diffusion and interface reaction rate respectively).
- 26. 26
- 27. 27 • Calculated dry O2 oxidation rates using Deal Grove.
- 28. 28 • Calculated H2O oxidation rates using Deal Grove.
- 29. 29 Thin Oxide Growth Kinetics • A major problem with the Deal Grove model was recognized when it was first proposed - it does not correctly model thin O2 growth kinetics. • Experimentally O2 oxides grow much faster for ~ 200 Å than Deal Grove predicts. • MANY suggestions have been made in the literature about why. None have been widely accepted. 1. Reisman e t. a l. Model • Simple power law “fits the data” over the whole range of oxide thicknesses. • a and b are experimentally extracted parameters. • Physically - interface reaction controlled, volume expansion and viscous flow of SiO2 control growth.
- 30. 30 2. Han and Helms Model • Second parallel reaction added - “fits the data” ” over the whole range of oxide thicknesses. • Three parameters (one of the A values is 0). • Physically - second process may be outdiffusion of OV and reaction at the gas/SiO2 interface. 3. M assoud e t. a l. Model • Second term added to Deal Grove model which gives a higher dx/dt during initial growth. • L ~ 70 Å so the second term disappears for thicker oxides. • Because it is simply implemented along with the Deal Grove model, this model has been used in process simulators. • Experimental data agrees with the Reisman, Han and Massoud models. (800°C dry O2 model comparison below.)
- 31. 31
- 32. 32 Pressure Dependence B and B/A C0 and thus also P : P growth rate enables oxide growth at lower temperatures O H for P B B P B A B A i i 2 ) ( O for P B B P B A B A i n i 2 ) (
- 33. 33 Dependence on Si Substrate Orientation Wafer Orientation • Oxide grows faster on <111> wafers - more silicon atoms available to react with oxidant • Affects oxide growth rate during Linear Stage
- 34. 34 Influence of impurities • H2O impurity in O2-gas: hydroxyl groups are formed more open oxide structure R • HCl gas: is able to react with impurities and form volatile chlorides quality of oxide and Si-SiO2 interface improves • high concentrations of P and B change fermi level and vacancy concentration: – B segregates into oxide weakens the Si-O2 bond enlarges the diffusion – P segregates into Si surface concentration of P B/A
- 35. 35 Influence Halogens (Chlorine) increase of growth rate 0-5% HCl Is often added to improve gate oxide quality
- 36. 36 Oxidation of doped Si Boron doped Si Phosphorus doped Si
- 37. 37 2D SiO2 Growth Kinetics • 950 °C oxidation (left), 1100 °C oxidation right
- 38. 38 • These effects were investigated in detail experimentally by Kao et. al. about 10 years ago. • Typical experimental results (from Kao et.al.)
- 39. 39 Trends • Improve Si/SiO2 interface properties – non-stoichometric monolayer due to incomplete oxidation - dangling bonds – strained region (1-4 nm) due to lattice mismatch Solve by – Fluorine incorporation (Si-F) – Nitridation of oxides N2O, NH3 (Si-N) Further: Si-N: improved barrier to B penetration Optimum: N2O; 0.5 - 1 at% N at interface • Ultra thin dielectrics down to 2 nm (<0.1 mm MOS) • Deposited gate dielectrics (less strain, less defects) • Ultra clean technology (reduce defect density) • Cluster tools (more clean process)
- 40. 40 Summarisation • Furnace oxidation is very well controlled process and is widely used in IC fabrication • Model of Deal-Grove widely accepted, except for thin oxide growth in dry O2 • Oxide growth rate depends on crystal orientation, mechanical stress, ambient (dry, wet, pressure, impurities, …) • Si(100) most used material • LOCOS - STI • Electrical characterisation of quality of oxide • Trends to improve quality