1. Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah

2. ION IMPLANTATION
Two problems associated with diffusion especially for IC fabrication:
High-temperature process
Unable to provide shallow junction depths
Ion implantation is a relatively simple means to place a known number of atoms in a wafer.
Ion implantation process:
Ionization of the dopant source to form positive ions
Acceleration of ions through a high voltage field to reach the required energy
Projection of high-energy ions towards the wafer surface (target)
Collision of ions with silicon atoms resulting in energy loss
End of penetration of ions in the substrate (coming to rest)

3. SYSTEM REQUIREMENTS
May achieve better control of distribution of dopants versus depth with ion implantation
Process can be faster
Process does not require as much thermal processing

4. 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. Gives excellent purity control)
HV Accelerator (voltage up to 1 MeV)
Scanning System (x-y deflection plates for electronic control)
Target Chamber (vacuum)

5. ION IMPLANTATION SYSTEM
Cross-section of an ion implanter 2 1 3
90o
analyzing
magnet
25 kV
Ion
souce
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

6. ION IMPLANTATION SYSTEM
Cross-section of an
ion implanter

7. ION IMPLANTATION
High energy ion enters crystal lattice and collides with atoms and interacts with electrons
Each collision or interaction reduces energy of ion until it comes to rest
Interactions are a complex distribution. Models have been built and tested against observation

8. ION IMPLANTATION
To prevent channeling, implantation is normally performed at an angle of about 8° off the normal to the wafer surface.
An annealing step is required to repair crystal damage and to electrically activated the dopants.
The implanted dose can be accurately measured by monitoring the ion beam current.
Complex-doping profiles can be produced by superimposing multiple implants having various ion energies and doses.
Lateral scattering effects are smaller than lateral diffusion.
Expensive $$$$$

9. 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.

10. MODEL FOR ION IMPLANTATION
Distribution is Gaussian
Np = peak concentration
Rp = range
Rp = straggle

11. MODEL FOR ION IMPLANTATION
The implanted impurity profile can be approximated by the Gaussian distribution function.
For an implant contained within silicon: Q = (2π)0.5 NP RP

12. MODEL FOR ION IMPLANTATION
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 at low cost for PCs

13. MODEL FOR ION IMPLANTATION
Range of impurities in Si 10
100
1000
Acceleration energy (keV) Rp
0.01
0.1
1.0 B P As Sb
Projected range (mm)

14. MODEL FOR ION IMPLANTATION
Straggle of impurities in Si B Sb As P
DRp DR
0.10
0.01
0.002 10
100
1000
Acceleration energy (keV)
Normal and transverse straggle
(mm)

15. SiO2 AS A BARRIER
SiO2 serves as an excellent barrier against ion-implantation
SiO2
Silicon Np NB
N(X0) Rp X0
Depth, x

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

17. ADVANTAGES Advantages over diffusion: low temperature process
allows wider range of barrier materials
permits wider range of impurities
better control of dose
wider range of dose
can control impurity concentration profile
can introduce very shallow layers

18. PROFILE CONTROL
Various shapes of profiles can be created by varying the energy of the incident beam
200 KILOELECTRON
VOLTS
FINAL PROFILE
100 50 20 10 15 5
150
200
250
300
350
DEPTH (NANOMETERS)
NITROGEN CONCENTRATION (ATOMIC PERCENT)

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

20. 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)
Radiation damage can be removed by annealing at oC for 30 min. After annealing, almost all impurities become electronically active.

21. 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.

22. 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:

23. Ion Implantation
A boron implantation is to be performed through a 50nm oxide so that the peak concentration is at the Si-SiO2 interface. The implant dose in silicon is to be 1013/cm2. What are the energy of the implant and the peak concentration at the interface?
Peak at Si-oxide interface  RP = 0.05µm  Energy = 15keV (RP=0.023µm)
Implanted dose in silicon = 1013  Q=2x1013 NP = Q/2.5RP = 3.5x1018/cm3
How thick should the oxide layer be to mask the implant if the background concentration is 1016/cm3?
Xox = (2 ln(10 x 3.5 x 1018/1016))0.5 = 0.14µm
If the oxide layer is 50nm, how much photoresist is required on top of the oxide to completely mask the implant?
PR thickness = 1.8 x (oxide thickness) = 1.8 x (0.14 – 0.05) = 0.16µm