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Background material science ideas that may be of use. This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide. SiO. - PowerPoint PPT Presentation
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Background material science ideas that may be of use.
This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide.
II Background Engineering Science Page
A) Starting Material 2- 2B) Unit Process Model for Process Control 2- 3
Crystal Gro wth Model 2- 3Growth A pproach Adjustments to Mo del Constraints 2- 3
Single Crystal Silicon Growth 2- 3Czochralski Metho d (C Z Gro wth) 2- 4Impurity Sources 2- 5
Single Crystal GaAs Growth 2- 5C) Wafer Material Performance Adjustments 2- 5
A bridged Perio dic Table of Elements 2- 6Impurities in the Crystal Structure 2- 6
Two Views of a Cubic Crystal Structure 2- 7Doping During Crystal Gro wth 2- 7 Equilibrium Segregation Coefficient 2- 8
Effective Segregation Coefficient 2- 8Two Views of a Doped Cu bic Crystal Structure 2- 9Local an d Universal Charge Characteristics 2-10 Lo w Temperature 2-10
At High Temperature 2-10After Ion A ddition 2-10
D) Bare Wafer Issues 2-11Orientation 2-12
Surface Planes an d Directions ( Miller In dices) 2-12Concentration of Constituents 2-13
Ty pical Intrinsic Densities 2-13Slab Density for Si 2-13Face Densitites for Si 2-13
Highly Doped Si Wafer Cross Section 2-13Important Parameters 2-14
Resistivity 2-14Epitaxial Film 2-15
E) Electronic Influence of Contaminates 2-15Energy Level A pproximate Values for Isolated Atom in Space 2-15Related Position of Silicon Energy Levels 2-15Compensate d Devices 2-16
Charge Neutrality 2-16
SiO2
(Quartzite)
Coke Furnace
gas
SiHCl3
Finish with EGS
Poly-crystalline
A) Starting Materials
SiCSolid
Siliquid
Heat
Exch
an
ger
SiSolid
Pulverizer
MGS
Micron sized particles
Flu
idiz
ed
BedHCl
Metallugical grade silicon
Start with Sand
Distillation Column
pureTrichorosilane
Hydrogen gas
Fuel gas
hot
Chemical Vapor Deposition Reactor
Electronic Grade solid silicon
B) Unit Process Models for Proper Control (Process control is best achieved when the equipment follows the model)
Crystal growth is an excellent example of how equipment and model cooperate to accomplish the task
Crystal Growth Model
Growth Approach Adjustments to Model Constraints (Three adjustments to equipment to make growth process match simplify model)
1)Uniformly Heat and insulate the Melt. ( This makes the liquid (dT/dx)Liquid
= 0
2)Slow Pull Rates. (This makes (dx) small value and allows use of calculus)
3)Crucible Surface Area Approximates As and Crystal Rotates Slowly. (This makes heat of fusion the only new heat source and (dT/dx)
S predictable)
This Gradient has Known Shape and Values
Thermal Conductivity for Crystal Near Liquid Interface
The small change in solid crystal mass because of a small change in time
Temperature gradient in solid crystal near the solid crystal interface
Temperature gradient in liquid at a location near the liquid interface
(dm/dt)solid
(dT/dx)Solid
- (dT/dx)liquid
s(dm dt ) s(k s Hs ) (A ) ( dT dx )s
Pulled single crystal rod is checkedby X-ray camera for crystal alignment
Aligned rod carefully polished andthen sliced to near wafer dimensions.
Wafer Finish
Linear Pull Rate
Density of Crystal Near Melt Interface
(d v dt) (ks Hs ) (1 s ) (dT dx)s
Single Crystal of Wafer Material with
know Amount of Dopent Added.Single Crystal crystal attached to chuck to obtain a specific crystalorientation with plane of melt
Seed single crystal is slowly rotatedwhile being pulled out of the melt
treated in an arc furnacefollowed by HCl rinses and
Melt comes from Sand ( Quartzite ) that has been
factional distillation to become EGS
Chuck that Holds Single Crystal
Fused Silica Crucible Adds oxygen to Melt
Crucible HolderUniformly Heated
Degenerately DopedElectronic Grade Si
Czechralski Method (CZ Growth)
=
Impurity Sources ( PPM)
MGS EGS Crucible
Metal Grade Electronic Grade Silica
B 50 0.001 0.23
Fe 2,100 5. 6
P 30 0.002 -
As - 0.01 0.005
C 80 0.6 -
0 - 0.02 0.05
“VLSI Technology”, 2nd Edition, S.M. Sze, McGraw Hill, 1988
Note:
CZ Growth used over 99% of time. Other option is Float Zone Crystal Growth Process.
Good reference for CZ growth,
“Characterization & Engineering of the Antimony Hero-Antisite Defect in LEC Gallium Arsenide, Ph.D. Dissertation, Marshall Wilson, U. South Fl. 1997
BrSeGallium Arsenic
Lewis diagrams show the atom as its symbol plus its electrons in the outer orbitEvery atom in a Group has the same number electrons in outer orbit.
Ge As
Impurities the Crystal Structure
Although unwanted impurities exist within a crystal structure, in most
micro and nano applications, a “special” impurity, the dopant, is added
to the crystal structure.
C)Wafer Material Performance Adjustments
ArAl P S ClSi
Ga
B N O FC
He
III V VI VII VIII
(with Some Lewis Electron Structures)
Group Number I
H
IV
Si
Two views of a cubic crystal structure 3 D Perspective
Ab
ou
t 6 A
Lattice Point
About 6 AO
Interstitial Location
-10o1 A = 10 Meters
About 6 A
Si
Si
Si
Si
Si Si
Si
Si
Si
Si
Si
SiSi Si SiSi
SiSiSi SiSi
Si
Si
SiSiSi
Si
Si
Si
Doping During Crystal Growth
Equilibrium Segregation Coefficient
A dopant will be driven by equilibrium considerations
to a specific concentration ratio between two possible phases
if given enough time and stable conditions to do so.Equilibrium Segregation Coefficient
With C s s = Concentration of Dopant in the Crystal Being Pulled
C ll = Concentration of Dopant in the Melt below the Crystal
kequilibrium
Cs /
C
Effective Segregation Coefficient
Often the driving equilibrium considerations are to complicatedto understand because of the arrangement of the equipment andany additional components within the melting system. In this situation an effective segregation coefficient, kseq, is used.
Segregation Coefficient
With:v = Pull Rate;
B = Boundary Layer;
D = Diffusion CoefficientNote:
kseqseq takes on values from keqeq to 1.0
kseqseq k eq ( 1 / [ keqeq +( 1 - keqeq ) ( e - (B/D) (vv ) ]=
Summary Two Views of a Doped Cubic Crystal StructurePhosphorus atom on a substitutional lattice location
n-type Doped Arrangementp-type Doped Arrangement
Boron atom on a substitutional lattice location
(Phosphorus's non-bonded outer electroncan move about the slab of material andgenerate local areas of negative charge)
(Boron’s unoccupied outer orbit hole can move about the slab of material andgenerate local areas of positive charge)
P
B P
BSi
SiSi
Si
Si
Si
Si Si
Si Si
Si
Si
Si
Si
Si
SiSi Si SiSi
SiSi Si SiSi
B Si
SiSi
Si
Si
Si Si
Si Si
Si
Si
Si
Si
Si
SiSi Si SiSi
SiSi Si SiSi
Si P
Local and Universal Charge Characteristics
The local charge around the boron adds up to zero.
Substrate withAcceptor atom
the phosphorus in the n doped slabalso adds up to zero.
The local charge around the
No localized charge inequality in eitherof these slabs of doped silicon.
1) At Low Temperatures
p-type Doped Arrangement
Both of these Slabs (n-type and p-type) Remain Overall Neutral
n-type Donor Arrangement
N =A
Substrate withDonor atom
Si Si
B
Si
Si
Si
Si
Si Si
Si Si
P
Si
Si
Si
Si
Si Si
Number Density of Acceptors N =D
Number Density of Donors
Local and Universal Charge Characteristics
3) Add ion to slab so it finally exchanges with a lattice location
An Ion
Si Si
B
Si
Si
Si
Si
Si Si
electron left this location soregion is now more positive than it was before it left.
electron has entered this location soregion is now more negative than itwas before it got there.
N = Density of Charged AcceptorsA
p = Number Density of Holes
Density of Charged Donors
n = Number Density "Free" Electrons
“Ionized” acceptor
Concentration of ions = Concentration of carrier
2) Raise the temperature of the lattice
Si Si
B
Si
SiSi
Si Si
“ionized” donor atom
Si Si
Si Si
P
Si
SiSi
Si Si
DN =
Lattice with new ion becomes charged
Orientation
(001)
(110)
(111)
a
a
Surface planes and directions based on Miller Indices
-10o1 A = 10 Meters
About 6 AO
D) Wafer Issues
(100)(010)
(001)
(100)
(100)
(The 111 perspective)
Concentration of Constituents
Typical Intrinsic Densities
Slab Density N total = 5 x 10 22 atoms/cm 3
Face Density N (100) = 6.8 x 10 14 atoms/cm 2
Face Density N (110) = 9.6 x 10 14 atoms/cm2
Highly Doped Wafer Cross-Section ( p +)
Number Density of Constituents
N Boron = 1 x 10 1818 atoms/ cm33
N Silicon = 5 x 10 2222 atoms/ cm33
Resistivity
(Sometimes before the first process step the wafer may have an excess amount of dopant that defines the wafer’s resistivity.)
10 -4
10 +2
10 +3
10 0
10+1
10 -1
10 -2
10 -3
1014
1015
10 1017
1018
1019
1020
Dopant Density #/cm3
Conductivity = (Charge/Carrier)(Mobility of Carrier)(Density of Carrier)
Conductivity = (1/ Resistivity)
When Dopant is an n-type Material
When Dopant is a p-type Material
Graph for Educational Value Only.Do not use Values for Accurate Work.
16
Important Parameters
Res
isti
vely
Oh
m-C
m
5 x 10 atoms/cm15 -310 ohm-cm
5 x 10 atoms/ cm16 -31 ohm-cm;
How many Boron (dopant) atoms should be put into an epi layer with a resistivity of;
B) 10 ohm-cm
Primary reason to build an epi layer coated substrate is to adjustthe resistance between the circuit to be built on the top of the epi layer
and the back side of the wafer below the epi layer.
Resistance of a Material
Resistance = (resistivity) ((length material)/ (cross-section area))
R = ( )
A) 1 ohm-cm;from Resistivity plot, 5 x 10 atoms/ cm are needed.16 -3 (i.e. about 1 PPM)
Epitaxial Film
665 micrometers
Less than 20 micrometersepi layer
Common p-epi layer resistivities values are from 1 0hm-cm through 10 ohm-cm.
Example- Epitaxial Film Concentration
5 x 10 atoms/cm are needed.15 -3 (i.e. about 0.1 PPM)from Resistivity plot,
(( L ) / ( A ))
Energy Levels(levels further away from the nucleus)
Energy of Orbit closest to nucleus
E5
E4
( E - E ) =5 4
Conductance Band
TheBandGap
(Alone and Lonely)Single Atom
Many Atoms Close Together
(In a Solid Crystal Lattice)E
E9
6E5
En
erg
y V
alu
es
Hig
her
Neg
ati
ve V
alu
es
With n being integer energy levelsand 13.6 electron volts being Bohr’senergy value for the first orbit of a hydrogen atom
Energy Levels from Bohr’s Model
En (1/n) (13.6 ev)2
=Not
to
Scale
1 2 3 4 5 6 7 8 9 10
-13.6 ev
-0.136 ev
-3.40 ev
-1.51 ev
-0.85 ev
-0.21 ev-0.55 ev
Related Position of Silicon Energy Levels
More
Posit
ive E
nerg
y V
alu
es
E4
E
E3
2E1
More
Posit
ive E
nerg
y V
alu
es
Valance Band
E1
E2
E3
E5
E9
Energy Level Approximate Values for Isolated Atom in Space
E) Electronic Influence of Contaminates
( -0.55 - [ -0.85] ) = + 0.30 ev
Phosphorous in Interstital Spaces
Phosphorous in Substitutional Spaces
p-type Dopent at Substitutional SitesMetallurgical Junction
(Equal Density of Positive and Negative Entities)
(the letter Sigma indicates that all of the items in the region of interested are added together.)
Note:Charge neutrality occurs when;
( NA
n )
(If both n-type and p-type materials are present, the device is said to be compensated)
Compensated DeviceMask Protecting a Piece of Boron Doped Silicon
( D
p )
N
Number of ionized acceptors
Number of holes
Image Triggering Vocabulary
Metallurgical Grade Silicon (MSG)
Electronic Grade Silicon (ESG)
CZ Growth
Lattice
Interstitial Locations
Substitutional Locations
Lewis Diagrams
Miller Indicies
Resistivity
Epitaxial Film
Compensated Device
Donors
Acceptors
Equal Charge Density
Metallurigical Junction
Image Triggering Vocabulary
Metallurgical Grade Silicon (MSG)
Electronic Grade Silicon (ESG)
CZ Growth
Lattice
Interstitial Locations
Substitutional Locations
Lewis Diagrams
Miller Indicies
Resistivity
Epitaxial Film
Compensated Device
Donors
Acceptors
Equal Charge Density
Metallurigical Junction