Ion Implantation

Question 1

What are the advantages of ion implantation?

Answer 1

1. Fine dose control
2. Shallow, step-like junctions (no diffusion profile)
3. Tailored dopant profiles

Question 2

What are the major parameters to be controlled in ion implantation?

Answer 2

Ion species, ion energy, implant dose (concentration)

Question 3

What is the basic process of ion implantaion?

Answer 3

First, the desired species is ionized. Ions are used since charged particles can be directed and accelerated.

Then, the ions are directed into a beam and the wafer is scanned under the beam. The process is monitored and stopped when the desired dose is reached.

Lastly, rapid annealing known as rapid thermal processing is used to re-crystallized ion-implantation amorphized silicon.

Question 4

Why is a mass spectrometer used in ion implantation?

Answer 4

In ion implantation, ions are created by applying current to a gas or sputtered solid. Often, this source is a polyatomic molecule, so more than one ion can be created.

In order to select only the specific desired ion out of the many possible ions and neutral molecules, the ionized material is accelerated through a mass spectrometer.

Inside the mass spectrometer, the ions travel through a magnetic field that is perpendicular to the direction of ion travel. This causes them to curve. The turn radius depends on the mass - higher mass = greater turn radius.

The ions exiting the mass spectrometer are directed through an opening that it placed according to the turn radius of the desired species.

Question 5

How is the ion implantation dose measured?

Answer 5

A Faraday cup/cage is connected to the wafer. The wafer is grounded, and the number of electrons entering the Faraday cup is measured.

Question 6

Why is the wafer cooled in ion implantation?

Answer 6

To prevent diffusion of the dopants.

Question 7

How are neutral atoms eliminated in ion implantation?

Answer 7

Before the beam reaches the wafer, a bend is used to trap neutral atoms/molecules.

Question 8

What ion-Si interactions take place in ion implantation?

Answer 8

1. Repulsive interactions with the nucleus - short range interactions that lead to Rutherford scattering. This is mostly elastic, eg. directional change without energy loss

Most Rutherford scattering is forward scattering, aka deflection to an angle, but in the specific case of a head-on approach backscattering can take place.

2. Attractive interactions between the positively-charged ion and the electron cloud. This leads to ‘drag’, slowing the ion velocity/causing energy loss.

Question 9

What is the energy loss per unit length caused by drag?

Formula

Answer 9

Energy loss per unit length = dE/ds = ke*sqrt(energy)

The factor ke depends on the masses and atomic numbers of the ion and target atoms.

Question 10

What is the use of a Monte Carlo simulation in ion implantation?

Answer 10

Statistical simulations that trace the path of an ion in the target. These are repeated millions of times to devise a Gaussian profile of ion implantation.

Question 11

What is the parameter Rp?

Monte Carlo simulation

Answer 11

In Monte Carlo simulations of ion implantation, the parameter Rp is the projected range of ion implantation. Rp is the peak of the Gaussian that describes the distance an ion travels in the substrate. At this depth, the concentration of implanted ions is highest.

Question 12

What is the parameter Np?

Monte Carlo simulation

Answer 12

In Monte Carlo simulations of ion implantation, the parameter Np is the maximum dose (the concentration at depth Rp).

Question 13

What is straggle?

Answer 13

∆Rp - straggle - the distribution of ions at greater or shallower depths compared to the maximum.

⊥∆Rp - lateral/transverse straggle - distribution of ions widthwise relative to the desired area of implantation. Lateral straggle is greater in lighter ions due to increased backscattering.

At edges, the lateral straggle profile resembles the error function.

Question 14

What is channeling in ion implantation?

Answer 14

If the ion beam is aligned with a crystallographic axis, channeling can occur. This means that the beam travels between crystal planes to a greater depth than desired.

To reduce channeling, ion implantation can be carried out at a small angle (usually 7˚, for everything except low energy B).

Question 15

What issue is presented by shadowing, in ion implantation? How can this issue be prevented?

Answer 15

Ion implantation is carried out at a set angle to prevent channeling. In this case, shadowing (due to topography) can take place. This is particularly significant in the ion implantation of the source/drain since the gate acts as a shield (the process is “self aligned”), leading to a lack of symmetry between the source and drain.

This can be solved by rotating the wafer, or by changing the implantation angle from -7˚ to 7˚ during the implantation.

Question 16

What type of masks are used in ion implanation?

Answer 16

Masks are used to prevent ion implantation from occurring where it is not desired. The mask must be thick enough to prevent 99.99% of ions from entering.

In the source/drain implantation, the gate serves as a mask (“self aligned”).

In other processes, poly-Si, field oxide, or photoresists can be used. Since the photoresist is made from polymer, it must be relatively thick to be effective; the field oxide can be thinner and the poly-Si can be thinnest.

Question 17

What types of damage does ion implantation cause? How is this damaged repaired?

Answer 17

Above a certain dose, ion implantation can cause the Si to become amorphous. This is more likely with heavier dopants, and can be predicted by statistical modeling. Amorphous Si is problematic since it does not have the required electrical properties.

To recrystallize the Si, a short, high-temperature annealing known as rapid thermal processing is used. This is high temperature (~1000˚C) to ensure the recrystallization but short enough that the dopants do not diffuse.

Question 18

What is the relationship between ion implantation and the solubility limit?

Answer 18

Since ion implantation is not an equilibrium process, the solubility limit (for ion concentration) can be exceeded without risking the formation of precipitates.