Ion Implantation

Implantation Basics

Ion implant is a critical technology in the fabrication of semiconductor devices. The ion implanter is a large machine that processes semiconductor wafers. The ion implanter is used to provide doping for the semiconductor device, which is the introduction of impurity atoms to change the electrical properties of the silicon crystal and thereby form a transistor structure.

Implant Process

The process of ion implantation includes the steps of :

1) Creating ions of the desired impurity atom

2) Using electric fields to accelerate the ions to the required energy

3) Transporting the ions down a beamline to the silicon wafer

4) Scanning the beam, or moving the silicon wafer, or both, such that uniform dosing of the silicon wafer is accomplished.

All semiconductor fabrication processes use many (15-25) steps of ion implantation to create a completed semiconductor device. The primary parameters of ion implantation are species, energy and dose.  The species is what atom is being implanted, and there are two main categories: N-type and P-type, which are denoted by the electrical activity of the impurity in the silicon. N-type dopants are usually arsenic or phosphorus and P-type doping is usually boron. The energy determines how deep into the silicon the atoms will go: higher energy implants go deep, low energy implants are shallow. The dose determines how conductive the layer will be when complete. All of these parameters are chosen by the transistor designer for each implant step to optimize the device characteristics.

Implanter Types and Vendors

A significant business exists in providing the ion implant systems for semiconductor manufacturers.  Currently, three vendors dominate the US ion implant market: Varian Semiconductor Equipment (VSEA), Axcelis Technologies (ACLS) and Applied Materials (AMAT). There are three main types of ion implanters:  medium current, high current and high energy, and each have special uses in semiconductor processing. Medium current (MC) implanters are used for those steps where the dose is moderate or low and good control is essential. Examples of MC processes are threshold adjust and channel stop implants. High current (HC) implanters are used where the dose is very high and control is not so important.  HC processes include source/drain formation and polysilicon gate doping. High energy (HE) implanters are used where the dopants need to go deep into the silicon. The need for HE implant is reduced as technology scales, with the well formation, especially retrograde wells, being the primary requirement.