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Transcript of In Do German
Uma Parthavi M
Dept. of Electrical Engineering,
Indian Institute of Technology Delhi.
Tutor: Prof. N Dasgupta
Doping by Diffusion and Implantation
Contents
Doping by Diffusion and Implantation2
Doping Two step doping process
Diffusion equipment & sources
Diffusion-Microscopic & Macroscopic point of view
Fick‟s Laws – solutions
Diffusivity
Influence of Electric Field, Defects
Oxidation Enhanced Diffusion
Ion Implantation Implantation Basics
Ion implanter
Implantation profiles
Channeling
Damage annealing
Comparison between diffusion and ion implantation
References
Contents
Doping Silicon
Doping by Diffusion and Implantation3
Diffusion :
The spread of particles through random motion from regions of
higher concentration to regions of lower concentration
Ion implantation
Bombarding the substrate with ions accelerated to high
velocities
Introduction
Creating Doped regions
Doping by Diffusion and Implantation4
Step1 : Pre-deposition
Controllably introduce desired dopant atoms
Methods: Solid phase diffusion from glass
layers
Gas phase diffusions
Ion Implantation
Step2 : Drive-in
The introduced dopants are driven deeper into the wafer without further introduction of dopant atoms
Two step process for producing a junction
Diffusion
Diffusion sources
Doping by Diffusion and Implantation5 Diffusion
Diffusion- Equipment
Doping by Diffusion and Implantation6
Diffusion Equipment(showing predep. Of BSG)[2]
Diffusion
Diffusion
Doping by Diffusion and Implantation7
Microscopic Point of View :
Considers the motion of dopant at atomic scale
Computationally expensive and used in simulation tools
More accurate
Macroscopic Point of View :
Considers overall motion of dopant profile
Fick‟s Laws
Considering the macroscopic point of view is important
because it gives a sufficiently accurate first hand picture
Diffusion
Microscopic Point of View
Vacancy Assisted Diffusion Interstitial Assisted diffusion
Doping by Diffusion and Implantation8
Impurity atom
Diffusion
Fick’s First Law
Doping by Diffusion and Implantation9
Diffusive flux has a magnitude proportional to spatial
concentration gradient
F is flux(atoms/cm2sec); D is the diffusivity(cm2sec-1);
is the concentration gradient.
Flow is opposite to the direction of concentration gradientx
C
CDF
x
CDF
Diffusion
Fick’s Second Law
Doping by Diffusion and Implantation10
Increase in the concentration in a cross section of unit area with time is simply the difference between the flux into the volume and the flux out of the volume.
“what goes in and doesn’t go out stays there”
If D is a constant,
x
FF
x
F
t
C outin
Flux in and out of a volume element
).(. CDFt
C
Diffusion
Solutions to Fick’s Equations
Doping by Diffusion and Implantation11
Steady state – linear
Limited source in infinite medium - Gaussian
Limited source at surface - Gaussian
Infinite source – Error function
Diffusion
Steady state
Doping by Diffusion and Implantation12
Steady state – dopant concentration in constant with time
Solving for the above gives, C=a+bx
02
2
x
CD
Diffusion
Limited source in infinite medium
Doping by Diffusion and Implantation13
Boundary conditions:
0C
C
0t
0t
as
as
for
for
0x
0x
QdxtxC ),(
A constant dose of dopants introduced in an infinite
mediumDopants
+ve
-ve
Si Wafer
Diffusion
Consequences
Doping by Diffusion and Implantation14
Solution has an evolving Gaussian form
Symmetric about the origin
Peak concentration decreases by and is given by C(0,t)
Diffusion length =
It is an approximate measure of how much the dopant has diffused
t/1
Dt2
Time evolution of Gaussian profile[1]
Diffusion
Limited source at the surface
Doping by Diffusion and Implantation15
Dopant dose Q introduced at the surface
Can be treated as an effective dose of 2Q being
introduced in a virtual infinite medium
Dopants introduced at the surface
Si Wafer
Real Dopants
Virtual mediumVirtual Dopants
Diffusion
Infinite source
Doping by Diffusion and Implantation16
Consider series of slices,
each with thickness ,
having a dose of C .
The solution for this case is
simply the linear
superposition of Gaussian
solutions for thin slices
Boundary conditions:
C=0 at t=0 for x>0
C=C at t=0 for x<0
x
x
dDt
x
Dt
CtxC
0 2
4)(exp
2),(
Diffusion from an infinite source
Diffusion
Doping by Diffusion and Implantation17
Cs is the concentration at the surface and Cs=C/2
Surface conc. is constant
Total Dose
DtC
dxDt
xerfCQ s
s
2)]
2(1[
0
Time evolution of erfc profile[1]
Diffusion
Diffusivity
Doping by Diffusion and Implantation18
For common impurities in
silicon,
k is the Boltzmann
constant, EA is the
activation energy in eV and
T is the temperature in
degrees Kelvin.
)exp(0
kT
EDD A
Diffusivity for common dopants [3]
Diffusion
Solid solubility
Doping by Diffusion and Implantation19
Maximum Thermodynamic concentration of dopant that can be
dissolved in silicon without forming a separate phase
In reality, electrical solubility is less than the solid solubility
because of formation of neutral clusters with vacancies
Solid solubility plots for common dopants[1]
Diffusion
Influence of Electric field
Doping by Diffusion and Implantation20
Dominant when doping concentrations exceed intrinsic
carrier concentrations.
F is the flux as discussed earlier, C is
the net doping concentration at x
h is upper bounded by 2
x
ChDF
22 41
inC
Ch
Diffusion
Effect of electric field on low
concentration regions[1]
Doping by Diffusion and Implantation21 Diffusion
Influence of defects
Doping by Diffusion and Implantation22
DA is the effective diffusivity ,DA* is the normal equilibrium diffusivity under inert
conditions, fI is the fraction of dopants diffusing with interstitial mechanism, fv is the
fraction of dopants diffusing with vacancy-type mechanism, CI is the interstitial
concentration, CV is the vacancy concentration, CI* is the interstitial concentration at
equilibrium, CV* is the vacancy concentration at equilibrium
Diffusion
Oxidation enhanced diffusion
Doping by Diffusion and Implantation23
P,B diffusion – enhanced ; Sb– retarded
Oxidation of Si to SiO2 causes volume to increase –induces stress which is relieved by the Si atoms moving to interstitial spaces
Oxidation injects interstitials ; P,B prefer interstitial type diffusion
Interstitials combine with vacancies – decrease in vacancies ; Sb prefers vacancy type diffusion
Plot showing effect of oxidation in diffusion of As and Sb
implants[1]
Diffusion
Ion Implantation Basics
Doping by Diffusion and Implantation26
Energetic and violent technique – Dominant doping
technique for past 20 yrs
Direct bombardment of accelerated dopant ions onto the
substrate
Cascade of damages created in the perfect Si lattice –
removed by annealing
Precise control on the amount and distribution of the dose
Energy of ions control the distribution
Ion beam current controls the dose
Implantation
Ion Implanter
Doping by Diffusion and Implantation27
Ion Sources: Gas : Arsine, Phosphine, Boron difluoride in a zeolite matrix ; allow rapid beam
tuning
Solid : elemental sources of As, P ; vaporized
Ion Implantation System[5]
Implantation
Ion Implanter
Doping by Diffusion and Implantation28
Gas from the source is ionized by electrons from a filament/plasma discharge
Ions are extracted by voltage and mass analyzed to select only one ion species
B is the magnetic field , proportional to the current I, V is the external voltage applied, m is the mass of a ion, v is the velocity of an ion, q is the charge of an ion
Different ions can be chosen by varying the external voltage and the current to the coils
Iq
mVr
qvBr
mv
mvqVEK
IB
12
21..
2
2
Beam of B11(top) and B10 separated
Courtesy: Albion Systems
Implantation
Ion Implanter
Doping by Diffusion and Implantation29
The radius of curvature is proportional to square root of the mass
Ions are further accelerated depending on the requirements and incident on the target
The implant dose is measured by locating the sample at the end of a „Faraday cup‟
I is the collected beam current, A is the implant area, t is the integration time and q is the charge on the ion
dtq
I
AQ
1
Range of Energy and Dose needed for different applications [6]Implantation
Implantation profiles
Doping by Diffusion and Implantation30
Range of an ion is the actual distance travelled by it before stopping
Projected Range Rp is the average distance travelled normal to the surface
ΔRp is the standard deviation of the projected range also called straggle
Heavy ions – Smaller Rp and ΔRp
Lighter ions – Greater Rp & ΔRp
Implantation profiles of commonly used dopant
atoms[6]
Implantation
Implantation profiles[7]
Doping by Diffusion and Implantation31 Implantation
Implantation profiles
Doping by Diffusion and
Implantation
32
Can be approximated to a Gaussian
C(x) is the concentration distribution, Rp is the range,ΔRp is the straggle, Cp is the peak concentration
The 2D distribution is usually assumed to be a product of vertical and lateral distribution
)2
)(exp()(2
Rp
RpxCpxC
RpCpQ 20
0,1
0,2
0,3
0,4
0,5
0,6
0 50 100 150 200 250
Ran
ge
(um
)
Energy(KeV)
As
P
B
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
0 50 100 150 200 250
Stra
gg
le(u
m)
Energy(KeV)
As
P
B
Range and Straggle for As,P,B
Data from BYU’s Range and Straggle calculatorImplantation
Pearson Model
Doping by Diffusion and Implantation33 Implantation
Channeling
Doping by Diffusion and Implantation34
Crystalline Si – planar and axial
channels
Once an ion enters a channel, it can be
steered along the channel until it comes
to rest either by drag or sharp collision
High doses – less channeling
Implantation
Impact of channeling on profiles
Doping by Diffusion and Implantation35
Impact of channeling on B profile[8]
Implantation
Avoiding channeling
Doping by Diffusion and Implantation36
Channeling can be reduced by –
Oxide screening
Tilting the wafer (ideally 7degrees)
Screening by amorphous Si
Implantation
Avoiding channeling
ImplantationDoping by Diffusion and Implantation37
From: http://www.silvaco.com/tech_lib_TCAD/simulationstandard/1996/dec/a1/a1.html
Ion stopping mechanism
Doping by Diffusion and Implantation38
Nuclear Stopping: Collision of ions with lattice atoms
Depends on Ion energy
Tends to dominate at the end of the stopping process when ions have lost much of their
energy
Produces damage
Electronic Stopping: Nonlocal electronic Stopping
Drag experienced by the ion in a dielectric medium; dissipative, does not alter the trajectory
Directly proportional to the ion velocity
Depends on ionization state of the ion
Local electronic Stopping
If the ion comes close enough to a lattice atom, momentum transfer due to e-transfer possible
Subtly alters the trajectory – minor compared to nuclear stopping
Depends on the ion velocity
Implantation
Stopping power for common ions
Doping by Diffusion and Implantation39
Total stopping power =
electron stopping power+
nuclear stopping power
Nuclear stopping
dominates at low energies
Electron stopping
dominates at higher
energies, for lighter atoms
Stopping powers of dopants[1]
Implantation
Stopping mechanisms
Doping by Diffusion and Implantation40 Implantation
Damage During implantation
Doping by Diffusion and Implantation41
Nuclear stopping – ions transfer energy to lattice atoms; crystalline structure damaged
Energy required to displace a Si atom to create a Frenkel pair (I +V) is 15eV
Damage to the crystal is in the following ways:
Creation of interstitials and vacancies
Creation of local zones of amorphous material
High dose implants might turn crystal to amorphous state
The above two types of damage are called Primary crystalline damage ; Repaired by thermal process known as annealing
But subjecting wafer to thermal process for a long time might cause diffusion of dopants - undesirable
Implantation
Annealing
Doping by Diffusion and Implantation43
Primary damage anneals at 400oC
Firstly I and V combine in the bulk ; this leaves only I‟s originating from introduction of extra atom
Later vacancies and interstitials recombine at the surface
Above 400oC extra I‟s condense into rod shaped defects – {311} planes
Upon annealing after 900oC, they start disappearing
Damage less than a critical value can be repaired.
For damage above critical value, {311} defects form stable dislocation loops –secondary damage
Steps in Annealing with time [1]
{311} Ribbon Defects[1]
Implantation
Annealing
Doping by Diffusion and Implantation44
Largest concentration @ interface between crystalline and
amorphous Si – EOR(End of Range) Defects
These EOR loops are known to disappear in some instances
after 60 sec anneal at 1100oC
EOR loops detrimental if present at junctions
Annealing cycles are chosen to cause enough dopant diffusion
so that the loops are contained in highly doped regions and
are shielded from any depletion regions
Implantation
Dopant Activation
Doping by Diffusion and Implantation45
Activation Dopants should occupy
substitutional sites Broken bonds should be repaired
to improve mobility
Low primary Damage: all damage anneals out
High primary Damage : Amorphization Solid Phase Epitaxy provides
nearly ideal soln
Partial Damage: Formation of secondary damage 950 -1050oC required
Fraction of atoms activated for boron implant [9]F
ract
ion
of
ato
ms
act
ive
Implantation
Annealing
Doping by Diffusion and Implantation46
Annealing can be done in two ways:
Furnace Annealing
Rapid Thermal Annealing
Implantation
Furnace Annealing
Doping by Diffusion and Implantation47
Inert ambient – Nitrogen or Argon
Oxide capping layer recommended to avoid evaporation of dopants
Temperature range – 750-1100oC
Time >30 mins
Problem of Diffusion of implanted dopants
Transient enhanced Diffusion – not suited for shallow junctions Typical Furnace used for annealing
Implantation
Rapid Thermal Annealing
Doping by Diffusion and Implantation48
Bank of lamps that rapidly
heat a wafer
Optical energy transfer
Ramp rate of 100oC/s
Wafer attains uniform
temperature in few ms
Annealing time: 1-100 s
No diffusion during anneal
RTA furnace Schematic
Implantation
Comparison of Diffusion and Ion
implantation
Diffusion Ion Implantation
Doping by Diffusion and Implantation50
Advantages: No damage created
Batch fabrication possible
Disadvantages: Limited to solid solubility
Low dose predeps difficult
High temperature process
Shallow junctions difficult
Advantages: Low temperature process
Precise dose and junction depth control
Implantations through thin layers of oxide/nitride possible
Short process times
Disadvantages: Implant damage enhances
diffusion
Additional cost of annealing
Dislocations may cause junction leakage
Channeling
Comparision
References
Doping by Diffusion and Implantation51
[1] J D Plummer, M D Deal and P B Griffin, “Silicon VLSI Technology: fundamentals, practice and modelling”,Pearson Edu. Inc.,2001
[2] John (2010, June 1), “Diffusion of impurities for IC fabrication” [online].Available:http://www.circuitstoday.com/diffusion-of-impurities-for-ic-fabrication
[3] H.Puchner , “Advanced Process Modelling for VLSI Technology,” Ph.D. dissertation, Dept. Elect. Eng., Technical Univ. of Vienna,Vienna,Austria, 1996
[4] National Technology Roadmap for Semiconductors (NTRS); SIA: San Jose, 1997.
[5] John (2010, June 2), “Ion Implantation” [online].Available:http://www.circuitstoday.com/ion-implantation
[6] L Rubin and J Poate(2010, Dec 2), “Ion Implantation in silicon technology” [online].Available: http://www.aip.org/tip/INPHFA/vol-9/iss-3/p12.html
[7] (2010, Dec 2),”Ion Implantation Processes in Semiconductor Manufacturing” [online].Available: http://www.leb.e-technik.uni erlangen.de/lehre/mm/html/implant.htm
References
References
Doping by Diffusion and Implantation52
[8] C Tian,S Gara,G Hobler and G Stingeder , “Boron Implantation in Si: Channeling
Effects Studied by SIMS and Simulation ,” Mikrochim. Acta, ser. D, vol. 107, pp.
161-169, 1992
[9] B. L. Crowder and F. F. Morehead, Jr, “Annealing characteristics of n-type dopants
in ion-implanted silicon,” Applied physics letters, ser. D, vol. 14, pp. 313-315,
May 1969
References
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