Ion implantation

Question 1

What is the expression for electronic stopping power S_e?

Answer 1

S_e = k_e*sqrt(E)

k_e = const. of proportionality depending on ion and target

Question 2

What happens with the electronic stopping power S_e when the ion energy is increased?

Answer 2

S_e peaks, and then decreases

Question 3

Describe the stopping power (nuclear and electronic) as a function of energy.

Answer 3

S_e is proportional to sqrt(E) for low E, and decreases for high E. S_n increases with E to a point where it falls off and decreases, and peaks at a lower E than S_e.

Question 4

Why does the nuclear stopping power decrease for high energies?

Answer 4

At high energies the collision time is very short, and hence the momentum transfer, delta_p = integral(F)dt, will be small.

Question 5

What is the expression for projected range, R_p?

Answer 5

R_p = integral dx from 0 to R_p = integral (dE/(dE/dx)) from E_0 to 0 = integral (dE/(S_n + S_e) from E_0 to 0

Question 6

How can range and standard deviation be determined?

Answer 6

By LSS tables or Monte Carlo simulations

Question 7

Which will have the longest projected range - high or low energy ions?

Answer 7

High energy ions

Question 8

Which will have the longest projected range - heavy or light ions?

Answer 8

Light ions

Question 9

Why will boron implanted in Si have higher concentration on the surface than what is predicted by a Gaussian distribution?

Answer 9

Boron is lighter than Si, and will hence suffer from significant backscattering. (The effect is most pronounced at high energies.)

Question 10

When does channeling occur in ion implantation, and why?

Answer 10

When the ion velocity is parallel to a major crystal orientation; because the ion can travel long distances with low energy loss due to the nuclear and electronic stopping power being low in that specific direction.

Question 11

For what atoms is channeling most pronounced, and why?

Answer 11

Light atoms implanted on axis, since the ion radius is much less than the atomic lattice spacing

Question 12

What is impurity activation?

Answer 12

The process of moving implanted ions into lattice sites

Question 13

What processes must be done after ion implantation?

Answer 13

Annealing - to repair damage, and impurity activation - to move implanted ions into lattice sites. (Both methods usually done by annealing)

Question 14

What is the threshold dose Φ_th?

Answer 14

Above this threshold, the damage is complete - meaning that an amorphous layer is rendered. (No long-range order is left)

Question 15

Why is the threshold dose Φ_th high at high temperatures?

Answer 15

Because the lattice self-anneals

Question 16

Why is the threshold dose Φ_th high for light atoms?

Answer 16

Because electronic stopping dominates, which results in less damage

Question 17

What are primary and secondary defects?

Answer 17

Primary defects: Point defects such as interstitials and vacancies.
Secondary defects: Defects occurring from annealing, by recombination or agglomeration of point defects. (Recombinations: interstitials being captured by a vacancy)

Question 18

What is a typical annealing time?

Answer 18

30-60 min

Question 19

At what temperatures does primary and secondary defects anneal out?

Answer 19

Primary defects (interstitials and vacancies): 500-600ᵒC
Secondary defects (agglomerations): 850-1100ᵒC

Question 20

What is SPE?

Answer 20

Solid Phase Epitaxy: Reforming an amorphous crystal by using the underlying undamaged substrate as a template

Question 21

What is the SPE regrowth velocity at 600ᵒC for Si in the 100 and 111 directions?

Answer 21

100: 300Å/min for
111: 30Å/min for

Question 22

How do 2D and 3D defects such as twins and stacking faults occur?

Answer 22

From microislands of single-crystal material slighlty shifted from the lattice