Ion implantation is a technique for doping semiconductors by accelerating ions to high energies and bombarding a wafer with them. During implantation, the wafer is kept at ambient temperature to prevent diffusion. However, a post-implant annealing step above 900°C is required to repair damage to the wafer's crystal structure caused by nuclear collisions with ions. Ion implantation offers more control over dopant dose and depth profile than diffusion and allows for precise doping of semiconductors.

2. Ion Implantation-Overview
is a low-temperature technique for the introduction of
impurities (dopants) into semiconductors and offers
more flexibility than diffusion.
Wafer is Target in High Energy Accelerator
Impurities “Shot” into Wafer

5. Ion Implantation
x
Blocking mask
Si
+ C(x) as-implant
depth profile
Depth xEqual-Concentration
contours
Reminder: During implantation, temperature is ambient. However,
post-implant annealing step (>900oC) is required to anneal out defects.
y

6. Ion Stopping
Ions gradually lose their energy as they travel through
the solid
The loss of ion energy in the target is called stopping.

7. Ion Implantation Energy Loss
Mechanisms
Si+
+
Si
Si
e
e
+
+
Electronic
stopping
Nuclear
stopping
Crystalline Si substrate damaged by collision
Electronic excitation creates heat

8. Ion Energy Loss Characteristics
EE143 – Ali Javey
Light ions/at higher energy more electronic stopping
Heavier ions/at lower energy more nuclear stopping
EXAMPLES
Implanting into Si:
H+
B+
As+
Electronic stopping
dominates
Electronic stopping
dominates
Nuclear stopping
dominates

9. Nuclear collisions: ions collide with atoms. The
positively charged ions are coulombically repealed by
the positive cores of the wafers lattice atoms. This
coulombic repulsion is “screened” by the cloud of
electrons surrounding each atom.
Electronic stopping: If ions graze the lattice atoms,
they do not interact with the lattice atom’s electrons
and not the positive core. This interaction slows the
ions by “viscous friction” similar to a rock thrown into
water.

10. Advantages of Ion Implantation:
Precise control of dose and depth profile
Very fast (1 6" wafer can take as little as 6 seconds for a
moderate dose)
Wide selection of masking materials
e.g. photoresist, oxide, poly-Si, metal
Less sensitive to surface cleaning procedures
Excellent lateral uniformity (< 1%
variation across 12” wafer)

11. Disadvantages of Ion
Implantation:
Not all the damage can be corrected by annealing.
Typically has higher impurity content than does
diffusion.
Often uses extremely toxic gas sources such as arsine
(AsH3), and phosphine (PH3).
Expensive

12. Application
Doping
Nitrogen or other ions can be implanted into a tool
steel target (drill bits, for example).
prosthetic devices such as artificial joints, it is desired
to have surfaces very resistant to both chemical
corrosion and wear due to friction.
ion beam mixing, i.e. mixing up atoms of different
elements at an interface.

13. Range Distributions
The range of a distribution with a discrete random
variable is the difference between the maximum
value and the minimum value. For a distribution
with a continuous random variable, the range is the
difference between the two extreme points on the
distribution curve, where the value of the function
falls to zero. For any value outside the range of a
distribution, the value of the function is equal to 0.
Range-is the difference between the highest and the
lowest values in a frequency distribution.
-It also the average total of a path length .

14. Where:
n(x= Rp) = is the peak concentration
Rp = is the projected range
∆Rp(Ợp) = is the standard deviation
Gaussian distribution

17. Implant Uniformity
 Non uniformity of implanted dopants across a wafer
can be readily untraveled from sheet resistance
measurements.
 Sheet resistance maps of 3 samples implanted with
50keV phosphorous as a function of dose.
(a) 5x 1013 cm-2, (b) 2x 1014 cm-2, (c) 2 x1015 cm-2.

18. Annealing
 Annealing- It is a process that produces conditions by
heating to above the critical temperature, maintaining a
suitable temperature, and then cooling.
 Annealing is used to induce ductility, soften material,
relieve internal stresses, refine the structure by making
it homogeneous, and improve cold working properties.

19. Diffusion During Subsequent Anneals
 For implantations far away from the surface and for
reasonable short characteristic diffusion lengths , the
new profile can be approximated by: