1. Chapter 8 Ion Implantation

2. ION IMPLANTATION SYSTEM
Ion implanter is a high-voltage accelerator of high-energy impurity ions
Major components are:
Ion source (gases such as AsH3 , PH3 , B2H6)
Mass Spectrometer (selects the ion of interest)
HV Accelerator (voltage > 1 MeV)
Scanning System (x-y deflection plates for electronic control)
Target Chamber (vacuum)

3. ION IMPLANTATION SYSTEM
Cross-section of an ion implanter 2 1 3
90o
analyzing
magnet
25 kV
Ion
source
Resolving aperture
R R R C
0 to 175 kV
Acceleration
tube
Focus
Neutral beam trap
and beam gate
Neutral beam
Beam trap
y-axis
scanner
x-axis 4 5
Wafer in process chamber
Integrator Q
m/q=(B2R2)/(2V)
Or Faraday cup
Acceleration energy = voltage x charge on ion

5. ION IMPLANTATION
High energy ion enters crystal lattice and collides with atoms and interacts with electrons
Types of collisions: Nuclear and electron
Each collision or interaction reduces energy of ion until it comes to rest
Amount of energy loss is dependent on ion, the energy it has at the time of the scattering event, and the type of scattering.

6. From Handbook of Semiconductor Manufacturing Technology
by Yoshio Nishi and Robert Doering

7. From Handbook of Semiconductor Manufacturing Technology
by Yoshio Nishi and Robert Doering

8. Channeling
Deep penetration by the ion because it traveled along a path where no semiconductor atoms are situated
Process is used for materials characterization: Rutherford backscattering
To prevent channeling
Implantation is performed at an angle of about 8° off the normal to the wafer surface.
The wafer surface is amorphorized by a high dose, low energy implantation of a nonelectrically active ion.
Hydrogen, helium, and silicon are common ions used

9. Determining the Dose
The implanted dose can be accurately measured by monitoring the ion beam current using a Faraday cup
The integrated current during the implant divided by the charge on the ion is the dose.

10. Post Implantation Anneals
An annealing step is required to repair crystal damage (recrystallization) and to electrically activated the dopants.
Dislocations will form during the anneal so times and temperatures must be chosen to force dislocations disappear.
If the anneal time is long and the temperature is high, a drive of the implanted ions may occur.

11. ION IMPLANTATION
Projected range (RP): the average distance an ion travels before it stops.
Projected straggle (RP): deviation from the projected range due to multiple collisions.

12. MODEL FOR ION IMPLANTATION
Distribution is Gaussian Cp = peak concentration
Rp = range
Rp = straggle

13. MODEL FOR ION IMPLANTATION
For an implant contained within silicon, the dose is

14. ION IMPLANTATION MODEL
Model developed by Lindhard, Scharff and Schiott (LSS)
Range and straggle roughly proportional to energy over wide range
Ranges in Si and SiO2 roughly the same
Computer models now available

15. Range of impurities in Si10
100
1000
Acceleration energy (keV) Rp
0.01
0.1
1.0 B P As Sb
Projected range (mm)

16. Straggle of impurities in SiB Sb As P
DRp DR
0.10
0.01
0.002 10
100
1000
Acceleration energy (keV)
Normal and transverse straggle
(mm)

17. Si
SiO2
AZ-7500 resist
Si3N4

20. Xox = RP + RP (2 ln(10CP/CBulk))0.5
SiO2 AS A BARRIER
The minimum oxide thickness for selective implantation:
Xox = RP + RP (2 ln(10CP/CBulk))0.5
An oxide thickness equal to the projected range plus six times the straggle should mask most ion implants.

21. Other Materials
A silicon nitride barrier layer needs only be 85% of the thickness of an oxide barrier layer.
A photoresist barrier must be 1.8 times the thickness of an oxide layer under the same implantation conditions.
Metals are of such a high density that even a very thin layer will mask most implantations.
Nickel is one of the most commonly used metal masks

22. ADVANTAGES Low temperature process Wider range of barrier materials
The wafer is cooled from the backside during high energy, high current diffusions are performed
Less change of stress-induced dislocations due to thermal expansion issues
Wider range of barrier materials
Photoresist
Wider range of impurities
No concern about solid solubility limitations
Implantation of ions such as oxygen, hydrogen, helium, and other ions with low solid solubility is possible.

23. Advantages over Diffusion
Better control and wider range of dose compared to predep diffusions
Impurity concentration profile controlled by accelerating voltage
Very shallow layers
Lateral scattering effects are smaller than lateral diffusion.

24. Complex-doping profiles can be produced by superimposing multiple implants having various ion energies and doses.
200 KILOELECTRON
VOLTS
FINAL PROFILE
100 50 20 10 15 5
150
200
250
300
350
DEPTH (NANOMETERS)
NITROGEN CONCENTRATION (ATOMIC PERCENT)

25. RADIATION DAMAGE
Impact of incident ions knocks atoms off lattice sites
With sufficient dose, can make amorphous Si layer

26. RADIATION DAMAGE
Critical dose to make layer amorphous varies with temperature and impurity
1018
1017
1016
1015
1014
1013
Temperature, 1000/T (K-1) B P Sb
Critical dose (atom/cm2)

27. Recrystallization
Radiation damage can be removed by annealing at oC for 30 min. After annealing, a significant percentage of the impurities become electronically active.
Point defects coalesce into line dislocations
Line dislocations merge into loop dislocations
Loop dislocations slowly disintegrate as interstitial Si atoms move on to lattice sites

28. Ion Implantation Implanting through a sacrificial oxide layer:
Large ions (arsenic) can be slowed down a little before penetrating into the silicon.
The crystal lattice damage is suppressed (at the expense of the depth achieved).
Collisions with the thin masking layer tends to cause the dopant ions to change direction randomly, thereby suppressing channeling effect.
The concentration peak can be brought closer to the silicon surface.

29. Ion Implantation
For deep diffusion (>1µm), implantation is used to introduce a certain dose, and thermal diffusion is used to drive in the dopants.
The resulting profile after diffusion can be determined by: