Doping by Diffusion and Implantation
Uma Parthavi M
Dept. of Electrical Engineering,
Indian Institute of Technology Delhi.
Tutor: Prof. N Dasgupta
Contents
 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
2
Doping by Diffusion and Implantation
Contents
Doping Silicon
 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
3
Doping by Diffusion and Implantation
Introduction
Creating Doped regions
 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
4
Doping by Diffusion and Implantation
Two step process for producing a junction
Diffusion
Diffusion sources
5
Doping by Diffusion and Implantation
Diffusion
Diffusion- Equipment
Diffusion Equipment(showing predep. Of BSG)[2]
6
Doping by Diffusion and Implantation
Diffusion
Diffusion
 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
7
Doping by Diffusion and Implantation
Diffusion
Microscopic Point of View
Vacancy Assisted Diffusion
Interstitial Assisted diffusion
Impurity atom
8
Doping by Diffusion and Implantation
Diffusion
Fick’s First Law
 Diffusive flux has a magnitude proportional to spatial
concentration gradient
C
F  D
x
F is flux(atoms/cm2sec); D is the diffusivity(cm2sec-1);
C
is the concentration gradient.
x
 Flow is opposite to the direction of concentration gradient
F  DC
9
Doping by Diffusion and Implantation
Diffusion
Fick’s Second Law
 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”
C F Fin  Fout


t
x
x
Flux in and out of a volume element
 If D is a constant,
10
Doping by Diffusion and Implantation
C
 .F  .( DC )
t
Diffusion
Solutions to Fick’s Equations
 Steady state – linear
 Limited source in infinite medium - Gaussian
 Limited source at surface - Gaussian
 Infinite source – Error function
11
Doping by Diffusion and Implantation
Diffusion
Steady state
 Steady state – dopant concentration in constant with time
 2C
D 2 0
x
 Solving for the above gives, C=a+bx
12
Doping by Diffusion and Implantation
Diffusion
Limited source in infinite medium
Boundary conditions:
C 0
C 



A constant dose of dopants introduced in an infinite
medium
as t  0 for x  0
as t  0 for x  0
C ( x, t )dx  Q
-ve
Dopants
Si Wafer
+ve
13
Doping by Diffusion and Implantation
Diffusion
Consequences
 Solution has an evolving
Time evolution of Gaussian profile[1]
14
Doping by Diffusion and Implantation
Gaussian form
 Symmetric about the
origin
 Peak concentration
decreases by 1 / t and is
given by C(0,t)
 Diffusion length = 2 Dt
It is an approximate
measure of how much the
dopant has diffused
Diffusion
Limited source at the surface Virtual medium
Virtual Dopants
Real Dopants
Si Wafer
Dopants introduced at the surface
 Dopant dose Q introduced at the surface
 Can be treated as an effective dose of 2Q being
introduced in a virtual infinite medium
15
Doping by Diffusion and Implantation
Diffusion
Infinite source
 Consider series of slices,
each with thickness , x
having a dose of C x.
 The solution for this case is
simply the linear
superposition of Gaussian
Diffusion from an infinite source
solutions for thin slices
 Boundary conditions:
0
( x   )2
C
C=0 at t=0 for x>0
exp 
C ( x, t ) 
d


4 Dt
2 Dt
C=C at t=0 for x<0
16
Doping by Diffusion and Implantation
Diffusion
 Cs is the concentration at the surface and Cs=C/2
 Surface conc. is constant
 Total Dose
Q

0
2Cs
x
Cs [1  erf (
)]dx 
2 Dt

Dt
Time evolution of erfc profile[1]
17
Doping by Diffusion and Implantation
Diffusion
Diffusivity
 For common impurities in
silicon,
 EA
D  D exp(
)
kT
0
k is the Boltzmann
constant, EA is the
activation energy in eV and
T is the temperature in
degrees Kelvin.
Diffusivity for common dopants [3]
18
Doping by Diffusion and Implantation
Diffusion
Solid solubility
 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]
19
Doping by Diffusion and Implantation
Diffusion
Influence of Electric field
 Dominant when doping concentrations exceed intrinsic
carrier concentrations.
F  hD
h  1
C
x
C
C 2  4ni
2
 F is the flux as discussed earlier, C is
the net doping concentration at x
 h is upper bounded by 2
20
Doping by Diffusion and Implantation
Diffusion
Effect of electric field on low
concentration regions[1]
21
Doping by Diffusion and Implantation
Diffusion
Influence of defects
 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
22
Doping by Diffusion and Implantation
Diffusion
Oxidation enhanced diffusion
 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
23
Doping by Diffusion and Implantation
Plot showing effect of oxidation in diffusion of As and Sb
implants[1]
Diffusion
Ion Implantation Basics
 Energetic and violent technique – Dominant doping





26
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
Doping by Diffusion and Implantation
Implantation
Ion Implanter
Ion Implantation System[5]
 Ion Sources:
 Gas : Arsine, Phosphine, Boron difluoride in a zeolite matrix ; allow rapid beam
tuning
 Solid : elemental sources of As, P ; vaporized
27
Doping by Diffusion and Implantation
Implantation
Ion Implanter
 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  I
K .E.  qV 
1 2
mv
2
mv 2
 qvB
r
2mV 1
r
q I
Beam of B11(top) and B10 separated
Courtesy: Albion Systems
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
28
Doping by Diffusion and Implantation
Implantation
Ion Implanter
 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‟
1 I
Q   dt
A q
 I is the collected beam current, A is
the implant area, t is the integration
time and q is the charge on the ion
29
Doping by Diffusion and Implantation
Range of Energy and Dose needed for different applications [6]
Implantation
Implantation profiles
 Range of an ion is the actual
Implantation profiles of commonly used dopant
atoms[6]
30
Doping by Diffusion and Implantation
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
Implantation profiles[7]
31
Doping by Diffusion and Implantation
Implantation
Implantation profiles
0,6
 Can be approximated to a
( x  Rp ) 2
C ( x)  Cp exp( 
)
2Rp
Range(um)
Gaussian
0,5
0,4
0,3
As
P
0,2
B
Q  2 RpCp
0,1
 C(x) is the concentration
0
Straggle(um)
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
0
50
100
150
200
250
Energy(KeV)
0,1
0,09
0,08
0,07
0,06
0,05
0,04
0,03
0,02
0,01
0
As
P
B
0
50
100
150
200
250
Energy(KeV)
32
Range and Straggle for As,P,B
Data from BYU’s Range and Straggle calculator
Doping by Diffusion and
Implantation
Implantation
Pearson Model
33
Doping by Diffusion and Implantation
Implantation
Channeling
 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
34
Doping by Diffusion and Implantation
Implantation
Impact of channeling on profiles
Impact of channeling on B profile[8]
35
Doping by Diffusion and Implantation
Implantation
Avoiding channeling
 Channeling can be reduced by –
 Oxide screening
 Tilting the wafer (ideally 7degrees)
 Screening by amorphous Si
36
Doping by Diffusion and Implantation
Implantation
Avoiding channeling
37
From: http://www.silvaco.com/tech_lib_TCAD/simulationstandard/1996/dec/a1/a1.html
Doping by Diffusion and Implantation
Implantation
Ion stopping mechanism
 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
38
Doping by Diffusion and Implantation
Implantation
Stopping power for common ions
 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]
39
Doping by Diffusion and Implantation
Implantation
Stopping mechanisms
40
Doping by Diffusion and Implantation
Implantation
Damage During implantation
 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
41
Doping by Diffusion and Implantation
Implantation
Annealing
 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]
43
Doping by Diffusion and Implantation
Implantation
Annealing
 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
44
Doping by Diffusion and Implantation
Implantation
Dopant Activation
 Activation
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
Fraction of atoms active
 Dopants should occupy
nearly ideal soln
 Partial Damage:
 Formation of secondary damage
 950 -1050oC required
45
Doping by Diffusion and Implantation
Fraction of atoms activated for boron implant [9]
Implantation
Annealing
Annealing can be done in two ways:
 Furnace Annealing
 Rapid Thermal Annealing
46
Doping by Diffusion and Implantation
Implantation
Furnace Annealing
 Inert ambient – Nitrogen or





47
Argon
Oxide capping layer
recommended to avoid
evaporation of dopants
Temperature range – 7501100oC
Time >30 mins
Problem of Diffusion of
implanted dopants
Transient enhanced Diffusion
– not suited for shallow
junctions
Doping by Diffusion and Implantation
Typical Furnace used for annealing
Implantation
Rapid Thermal Annealing
 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
48
Doping by Diffusion and Implantation
Implantation
Comparison of Diffusion and Ion
implantation
50
Diffusion
Ion Implantation
 Advantages:
 No damage created
 Batch fabrication possible
 Advantages:
 Low temperature process
 Precise dose and junction depth
 Disadvantages:
 Limited to solid solubility
 Low dose predeps difficult
 High temperature process
 Shallow junctions difficult
 Disadvantages:
 Implant damage enhances
Doping by Diffusion and Implantation
control
 Implantations through thin layers
of oxide/nitride possible
 Short process times
diffusion
 Additional cost of annealing
 Dislocations may cause junction
leakage
 Channeling
Comparision
References
[1] J D Plummer, M D Deal and P B Griffin, “SiliconVLSI 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 forVLSI 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
51
Doping by Diffusion and Implantation
References
References
[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
52
Doping by Diffusion and Implantation
References
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