1. How to turn an accident into a great experiment
Chapter 6 OXIDATION
How to turn an accident
into a great experiment

2. 1955 - Development of Oxide Masking
“… Bell Labs researchers encountered a major problem with pitting on the surface of silicon wafers during high-temperature diffusion. This problem was overcome by chemist Carl Frosch during a serendipitous accident in which the hydrogen gas carrying impurities through the diffusion furnace briefly caught fire, introducing water vapor into the chamber. The resulting “wet-ambient” diffusion method had covered the silicon surface with a layer of glassy silicon-dioxide (SiO2).”
From

3. Introduction
Si is unique in that its surface can be passivated with an native oxide
Layers are easily grown thermally
It has few defects
It is stable over time
Its mechanical and electrical properties are almost ideal
Good adhesion
Prevents the penetration (in-diffusion) of dopants
Resistant to most chemicals used in fabrication
Easily patterned and etched with specific chemicals or dry etched with plasmas

4. Thermal oxide properties
Amorphous
lattice constant of quartz ~0.5nm
Density: 2.27g/cm3
Dielectric constant: 3.9
DC 25C: -cm
Energy gap: 9eV
Thermal conductivity: W/m-C
Refractive index: 1.46
Melting point: 1700C
Molecular weight: g/cm3
Molecules: 2.3x1022/cm3
Specific heat: 1 J/g-C
Film stress: Compressive GPa
Etch rate: 6:1 BOE 100nm/min

5. http://www. enigmatic-consulting

6. Introduction Oxide is used for several applications Gate dielectric
Mask against ion implantation or diffusion
For isolation of various lateral regions
As insulator between various metal layers

7. Introduction

8. NTRS Roadmap

9. Introduction Si will oxidize at room temperature
nm (5- 10 Å) thick layer forms in about 5 minutes
The reactions slows and stops after a few hours when the layer is 1 – 2 nm thick
Properties of the oxide depends on the surface treatment and crystalline structure
Which plane of the unit cell is exposed

10. Introduction The most critical application is the gate oxide
2.5 – 5 nm thick
Projected to be < 1 nm in a few years
Will sustain about 50% of the theoretical breakdown voltage of the bulk material of 10 – 15 MV/cm
Thicknesses must be controlled to about 1 atomic layer

11. Introduction
Amorphous arrangement of atoms means that there is a possibility that multiple Si atoms will be connected
Thin ‘wires’ of Si within SiO2 layer enables leakage currents to flow
When films get this thin, quantum mechanical effects including tunneling become important
Finite probability that an electron can penetrate through an energy barrier
Tunneling is usually undesirable, but some devices are now built using this phenomenon (nonvolatile memory)

12. Basic Concepts
Oxide grows by diffusion of oxygen/H2O through the oxide to the Si/SiO2 interface
Thus, a new interface is continuously growing and moving into the Si wafer
The process is known as:
Dry oxidation when oxygen only is used.
Wet oxidation when water vapor (with or without oxygen) is used.

13. Basic Concepts

14. Basic Concepts The process involves an expansion
the density of an equal volume of Si occupies less space than a volume of oxide containing the same number of Si atoms
Nominally, the oxide would like to expand by 30% in all directions; but it cannot expand sideways because it is constrained by the Si atoms
Thus, there is a 2.2  expansion in the vertical direction
In figure 6-4, note the growth of the LOCOS (Local Oxidation of Silicon) oxide above the surface
Also note the “bird’s beak” of oxide under the nitride layer – a stress-induced rapid growth of oxide

15. Basic Concepts

16. Basic Concepts

17. Basic Concepts
If there are shaped surfaces where oxide must grow, this expansion may not be so easily accommodated
The oxide layers are amorphous (i.e., there is only short range order among the atoms)
There are no crystallographic forms of SiO2 that match the Si lattice
The time required for transformation to a crystalline form at device temperatures is very very long

18. Basic Concepts The oxide that grows is in compressive stress
This stress can be relieved at temperatures above oC by viscous flow
There is a large difference in the TCE (thermal coefficient of expansion) between Si and SiO2
This increases the compressive stresses in the oxide and results in tensile stresses in the Si near its surface
Si is very thick while the oxide is very thin
Si can usually sustain the stress
Since the wafer oxidizes on both sides, the wafer remains flat; if you remove the oxide from the back side, you will see a warping of the wafer
The stress can be measured by measuring the warp of the wafer

19. Basic Concepts
The electrical properties of the Si/SiO2 interface have been extensively studied
To first order, the interface is perfect
The densities of defects are 109 – 1011 /cm2 as compared to Si atom density of 1015 /cm2
Most defects are associated with incompletely oxidized Si
Deal (1980) suggested a nomenclature that is now used to describe the various defects

20. Defect Nomenclature

21. Defect Nomenclature There are four type of defects
Qf is the fixed oxide charge.
It is very close (< 2 nm) to the Si/SiO2 interface
Surface concentration of 109 –1011/cm2
Related to the transition from Si to SiO2
Incompletely oxidized Si atoms
Positively charged and does not change under normal conditions

22. Defect Nomenclature Qit is the interface trapped charge
Appears to incompletely oxidized Si with dangling bonds
Located very close to the interface
Charge may be positive, neutral, or negative
Charge state may change during device operation due to the trapping of electrons or holes
Energy levels associated with these traps are distributed throughout the forbidden band, but there seem to be more near the valence and conductions bands
Density of traps is 109—1011 cm-2 eV-1

23. Defect Nomenclature Qm is the mobile oxide charge
It is not so important today but was very serious in the 1960’s
It results from mobile Na+ and K+ in the oxide
Shift in VTH is inversely proportional to COX and thus, as oxides become thinner, we can tolerate more impurity

24. Defect Nomenclature Qot is charge trapped anywhere in the oxide
Broken Si-O bonds in the bulk oxide well away from the interface
by ionizing radiation or by some processing steps such as plasma etching or ion implantation
Metal ions from surface of Si or introduced during growth
Fe, Mn, Cr, Cu
Normally repaired by a high-temperature anneal
They can trap electrons or holes
This is becoming more important as the electric field in the gate oxide is increased
They result in shifts in VTH

25. Defect Nomenclature
All four types of defects have deleterious effects on the operation of devices
High temperature anneals in Ar or N2 near the end of process flow plus an anneal in H2 or forming gas at the end of process flow are used to reduce their effect

26. Manufacturing Methods
Furnace capable of 600 – 1200 oC with a uniform zone large enough to hold several wafers
Gas distribution system to provide O2 and H2O
Generally, H2 is burnt with O2 at the entrance of the furnace to create water vapor
TCA or HCl may be used to remove metal ions
Control system that holds the temperatures and gas flows to tight tolerances (0.5 C)

27. PRODUCTION FURNACES
Commercial furnace showing the furnace with wafers (left) and gas control system (right).

28. PRODUCTION FURNACES
Close-up of furnace with wafers.

29. PRODUCTION FURNACES

30. Models The first major model is that of Deal and Grove (1965)
This lead to the linear/parabolic model
Note that this model cannot explain
the effect of oxidation of the diffusion rate
the oxidation of shaped surfaces
the oxidation of very thin oxides in mixed ambients
The model is an excellent starting place for the other more complicated models

31. CHEMICAL REACTIONS Process for dry oxygen Si + O2  SiO2
Process for water vapor
Si + 2H2O  SiO2 + 2H2

32. OXIDE GROWTH
Si is consumed as oxide grows and oxide expands. The Si surface moves into the wafer.
Original
surface
54%
SiO2
46%
Silicon
wafer

33. MODEL OF OXIDATION Oxygen must reach silicon interface
Simple model assumes O2 diffuses through SiO2
Assumes no O2 accumulation in SiO2
Assumes the rate of arrival of H2O or O2 at the oxide surface is so fast that it can be ignored
Reaction rate limited, not diffusion rate limited

34. Deal-Grove Model of Oxidation
Fick’s First Law of diffusion states that the particle flow per unit area, J (particle flux), is directly proportional to the concentration gradient of the particle.
We assume that oxygen flux passing through the oxide is constant everywhere.
F1 is the flux, CG is the concentration in the gas flow, CS is the concentration at the surface of the wafer, and hG is the mass transfer coefficient

35. J
Distance from surface, x N No Ni
Silicon
dioxide
SiO2 Si Xo

36. Deal-Grove Model of Oxidation
Assume the oxidation rate at Si-SiO2 interface is proportional to the O2 concentration:
Growth rate is given by the oxidizing flux divided by the number of molecules, M, of the oxidizing species that are incorporated into a unit volume of the resulting oxide:

37. Deal-Grove Model of Oxidation
The boundary condition is
The solution of differential equation is

38. Deal-Grove Model of Oxidation
xox : final oxide thickness
xi : initial oxide thickness
B/A : linear rate constant
B : parabolic rate constant

39. There are two limiting cases:
Very long oxidation times, t >> 
xox2 = B t
Oxide growth in this parabolic regime is diffusion controlled.
Very short oxidation times, (t + ) << A2/4B
xox = B/A ( t +  )
Oxide growth in this linear regime is reaction-rate limited.

40. Deal-Grove Model of Oxidation