1. Doping: Depositing impurities into Si in a controlled manner
Diffusion
Doping: Depositing impurities into Si in a controlled manner

2. Overview
Diffusion vs Implantation
Mechanism,Models
Steps
Equipment

3. Goal: Controlled Junction Depth
Controlled dopant concentration and profile
Preferred location of maximum concentration need not be the surface P+ P+
Drain
Source
N “well”
Wafer (Substrate): P Type

4. Diffusion & Ion Implanatation
Ion Implantation
Bombardment of ions
SOURCE
Ions
Electric
Field
Junction is where N = P
Can also be used when doping N in N
OXIDE
BLOCK
Wafer (Substrate)`

5. Diffusion & Ion Implantation
Solid-in-solid
high temperatures (1000 C)
Distances covered are in um or nm
Diffusion
OXIDE
BLOCK
Wafer (Substrate)`

6. Mechanism , Models Substitutional (10-12 cm2/s)
Interstitial replacement (10-6 cm2/s)
Interstitial movement
Substitutional preferred (better control)
Au, Cu diffuse by interstitial mechanism
B, P etc by substitutional mechanism
Two ideal cases
Constant source, limited source
Using Fick’s First & second law
J = Flux
D - Diffusivity of A in B
N- Concentration
x - distance

7. Models Limited source Constant Source Dose Q = constant
Approx by Delta Fn
Constant Source
Concentration at x=0 is No
Complementary Error Function
Total Dose Q

8. Models Constant Source Concentration at x=0 is No
Important Parameter : Dt
species, temp and time N0
Concentration
Impurity 3 2 1
Distance from Surface

9. Models Limited Source Dose Q Important Parameter : Dt N0
Area under the curve is constant
If you normalize, erfc drops faster than Gaussian
Concentration
Impurity 3 1 2
Distance from Surface

10. Diffusivity Diffusivity Follows Arrhenius behavior
Wafer goes through heating cycles many times in the process
Effective Diffusivity * time = sum (Diffusivity * time)
Concept of thermal budget

11. Diffusion Max absorption (at a given temp) Usually quite high
Good for emitter and collector, but not for base
Not all dopant can contribute to electron/hole near solubility limit
Solubility limit in the range of 10 20/cm3 at 1000o C
Diffusion into silicon
Faster on grain boundaries
10 times in poly silicon
Diffusivity in SiO2 usually very low (Segregation occurs)

12. Junction Formation N Carrier Conc Impurity Conc P Jn Distance
from surface

13. Diffusion: Drive In: Dopant re distribution
Deposited dopant must be pushed into Si
Re-distribution of dopant
Oxidation of exposed Si to protect
OXIDATION
Dopant Diffusion
*Dopant profile changes due to diffusion
* Also due to preference for Oxide/Silicon: N-type piles up in Si, P-type depletes in Si

14. Diffusion: Steps Dep Diffusion OXIDE BLOCK 1.Pre Clean
To remove particles
Thin oxide grows
OXIDE
BLOCK
2.HF Etch
To remove oxide
Not too much!
3.Deposit (pre dep)
Deposit enough to be higher than the solubility limit
4.Drive In
High temp to enable diffusion inside Si
Also forms SiO2 (with high dopant concentration)
2-STEP diffusion (usual)
5.Deglaze (HF Etch)
Oxide may act as dopant source in future steps
Removing highly doped oxide may be problem (for dry etch)

15. Diffusion: Dep: schematic
Wafers are Horizontal
Gas
Flow
Better Uniformity
Less wafers per batch
Vertical
Poor Uniformity
More wafers per batch (or can have smaller chamber)
Gas
Flow
Dummy wafers placed in the beginning & end

16. Doping: Gas phase
Dopant can be in Gas/Liquid/Solid state, but is typically carried using N2 in gaseous form
*Carrier gas may be bubbled through liquid source
*Carrier gas may pass over heated solid source
* inert gas can provide volume to maintain laminar flow
Chamber
Carrier Gas (N2) + Source
Reaction gas

17. Doping: Gas phase Reaction/Diffusion Limited Phosphorus oxy chloride
Phosphine
Arsenic Oxide
Diborane
Boron Tribromide

18. Solid phase Solid Source Slugs between wafers
Lower through put
Cleaning is issue (slugs can break)
Safer to handle(no toxic vapor at room temp)
Spin coating (with solvents)
Similar to photo resist coating
Cost of extra spin/bake steps
thickness variations

19. Doping: Solid phase Phosphorous pentoxide Arsenic Oxide
Antimony Tri Oxide
Boron Trioxide
Tri Methyl Borate (TMB)

20. Issues Side diffusion Maximum dopant concentration is near surface
Increases with temperature/time
Limits the space between devices
Maximum dopant concentration is near surface
==> majority of current near surface
(Surface tends to have max defects)
==> less control
Dislocation generation (thermal drive in)
Surface contamination (dep)
Low dopant concentration and thin junction (small junction depth) are difficult
At 0.18 um , junction depth is ~ 40 nm
At 0.09 um, junction depth may be 20 nm

21. Issues: Side diffusion
Side diffusion (Lateral Diffusion)
BLOCK
Wafer (Substrate)`
Diffusion
OXIDE
BLOCK

22. Example of Real systems :
Hitachi systems
*Hitachi-Zestone VII
*2m x 3m x 3m
*300 mm wafer
*one wafer at a time
* lower thermal budget,
* better control, uniformity
* low throughput
*Hitachi-Vertron V
*1m x 3.5m x 3.3m
*200 mm wafer
*150 wafers at a time
* higher thermal budget,
* good control, uniformity
* high throughput

23. Example of Real systems : Protemp
Protemp system picture

24. Gettering To remove unwanted impurities
Try to get them to the back of wafer
Defects
Ar implant
Dep SiN/SiO2 (stress)
Oxygen during crystal growth (intrinsic)
High Conc P on back of wafer

25. Measurement Sheet Resistance (average)
Four point probe, VDP (Van der Pauw)
Bevel
Interference
Dye
SIMS

26. Diffusion: Summary Diffusion Temp, Time, Thermal budget
Doping (more important for older nodes)
Relevant for all nodes
2 step (constant source, limited source)
Solid/Liq/Gas

27. Ion Implantation “Somewhat similar” to Sputtering
Dopant goes inside the silicon
sputtering deposits on the surface
Used for controlled doping
concentration
profile (depth)
Equipment
Mechanism
Issues
Summary

28. Equipment
Schematic from
© Peter van Zant

29. 1. Ion Source Gas or solid source (no liquid source)
Solid heated to obtain vapor (P2O5)
effectively gas source
Mass flow meters (to control the flow better)
Gas usually Fluorine based
Ionization chamber
low pressure (milli/ micro torr) to ionize and minimize contamination
heated filament (thermionic emission)
positively charged ions created

30. 2. Analyzing Selection, analyzing, mass analyzing, ion separation
Similar to Mass Spectroscope
Usually the second stage (before acceleration)
Magnetic field to control the path
Charge to Mass Ratio
Some of the species from BF3 source
Selection of B+

31. 3. Acceleration Acceleration needed for implantation
Positive ions accelerated with ring anodes
Energy range: 5 keV for low, 2 MeV for high
Medium current : 1 mA
High current: 10 mA
Current ~ Dose
Beam Focus (magnetic/electric)
SOI
100 mA
High
Current
Oxygen
High energy ==> high throughput
few seconds per wafer
10 mA
High
Current
Beam Current
Low Energy
High Energy
Low
Current
1 mA
keV
MeV
Accln Energy

32. 4. Scanning Beam size ~ 1 sqr cm Wafer size 200 mm or 300 mm Issues:
neutral atoms need to be removed because...
dose calculated by current integrator
Electrical (beam) scanning & Mechanical (wafer) scanning
Beam Scan:(medium current)
beam moves outside the wafer for turn
controlling XY plates may be destroyed by discharge
Rotate wafer for uniformity
Wafer scan: (high current)
Beam shuttering: (electrical/mechanical) turn beam off when not on wafer

33. 5. Target chamber End chamber low particle, high vacuum Wafer held on
clamp (more particles) OR ESC (less particles)
Anti-static devices on the chamber
Integrate the current to measure dose
For 2+ ions, divide by 2 and so on...
Wafer charging:
minimize by connecting wafer to ground (with a charge counter)
dielectrics may get damaged
use flood gun to provide electron (and count it in measurement)

34. Stopping cross section
Electrons attract the +vely charged ions
Nuclei repel the +vely charged ions
Mechanism
Inelastic collision:
Electron (ionization)
Nuclear (nuclear reactions)
Elastic collision
Electron
Nuclear (atom substitution)
Stopping cross section vs
energy (plots
from web)
At low energy Nuclear collisions predominant
At high energy electronic collisions predominant
Variation in ‘stopping cross section’
Gaussian profile expected (projected range Rp)

35. Implantation Mask with Photoresist or oxide
resist for medium and low energy, moderate dose
high energy/high dose: increase in temp
Resist re-flow
Cross link (for organics)
less soluble (stripping an issue)
Faraday Cage
Retain secondary electron from wafer
Otherwise, wafer under dosed
-Ve Bias e-

36. Issue: Transverse Straggle
implant
Gaussian
OXIDE
BLOCK
Transverse Straggle
(Diffraction)
Even in implantation, dopants present in lateral direction

37. Channeling Some ions will move through “channels” without experiencing
nuclear
or electron
collision for
a “long” time
==> No Gaussian Profile

38. Channeling 1. Hold the wafer at an angle (~ 8 degree)
Also causes “shadow”
==> increase transverse straggle(called undercut)
BLOCK
==> Too much angle is
also a problem
Shadow
Undercut

39. Channeling 2. Dep amorphous material on the top It has to be very thin
and not stop ions
implant
OXIDE
BLOCK
3. Damage top
of wafer and make it
amorphous (eg high
energy silicon implant)

40. Channeling 4. Increase temperature ==> reduce channel cross section
Channeling critical angle ~ (Z/E) 1/2
==> Low energy implants more likely to channel

41. TED Transient Enhanced Diffusion ©Solid State Technology
Damage during implantation
==> point defects (vacancies)
interstitial silicon atoms
reduced during anneal
Channel dopant diffuse to surface
==> VT modification
©Solid State Technology

42. RTA Anneal to heal the damage Diffusion during anneal an issue
High temp repair is faster than anneal
Repair energy barrier 5 eV, diffusion barrier 3 or 4 eV
1. Adiabatic (laser, heats surface , < micro sec)
profile control difficult (not used)
2. Thermal flux ( micro to 1 sec)
laser, ebeam, flash lamp
surface+bulk heating
rapid cooling ==> point defects
3. Iso thermal (W-Halogen lamp)
30 sec (1100 C)

43. Diffusion vs Ion Implantation
Dep+Diffusion: depends on chemical nature and solubility
Implantation: on energy of ion beam
Expensive
Better Control of junction depth, dose, profile
Less ‘transverse straggle’