Amorphous arrangement of atoms means that there is a possibility that multiple Si atoms will be connected - PowerPoint PPT Presentation

1 / 30
About This Presentation
Title:

Amorphous arrangement of atoms means that there is a possibility that multiple Si atoms will be connected

Description:

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 ... – PowerPoint PPT presentation

Number of Views:175
Avg rating:3.0/5.0
Slides: 31
Provided by: BallF4
Category:

less

Transcript and Presenter's Notes

Title: Amorphous arrangement of atoms means that there is a possibility that multiple Si atoms will be connected


1
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)

2
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.

3
Basic Concepts
4
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 birds beak of oxide under the
    nitride layer a stress-induced rapid growth of
    oxide

5
Basic Concepts
6
Basic Concepts
7
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

8
Basic Concepts
  • The oxide that grows is in compressive stress
  • This stress can be relieved at temperatures above
    1000oC 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

9
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

10
Defect Nomenclature
11
Defect Nomenclature
  • There are four type of defects
  • Qf is the fixed oxide charge.
  • It is very close (lt 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

12
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 1091011 cm-2 eV-1

13
Defect Nomenclature
  • Qm is the mobile oxide charge
  • It is not so important today but was very serious
    in the 1960s
  • 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

14
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

15
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

16
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)

17
PRODUCTION FURNACES
  • Commercial furnace showing the furnace with
    wafers (left) and gas control system (right).

18
PRODUCTION FURNACES
  • Close-up of furnace with wafers.

19
PRODUCTION FURNACES
20
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

21
CHEMICAL REACTIONS
  • Process for dry oxygen
  • Si O2 ? SiO2
  • Process for water vapor
  • Si 2H2O ? SiO2 2H2

22
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
23
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

24
Deal-Grove Model of Oxidation
  • Ficks 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

25
(No Transcript)
26
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

27
Deal-Grove Model of Oxidation
  • The boundary condition is
  • The solution of differential equation is

28
Deal-Grove Model of Oxidation
xox final oxide thickness xi initial oxide
thickness B/A linear rate constant B
parabolic rate constant
29
  • There are two limiting cases
  • Very long oxidation times, t gtgt ?
  • xox2 B t
  • Oxide growth in this parabolic regime is
    diffusion controlled.
  • Very short oxidation times, (t ?) ltlt A2/4B
  • xox B/A ( t ? )
  • Oxide growth in this linear regime is
    reaction-rate limited.

30
Deal-Grove Model of Oxidation
Write a Comment
User Comments (0)
About PowerShow.com