Ion Implantation

In ion implantation, charged dopants (ions) are accelerated in an electric field and directed onto the wafer. The penetration depth can be set very precisely by reducing or increasing the voltage used to accelerate the ions. Since the process takes place at room temperature, dopants introduced earlier cannot diffuse out. As with diffusion, regions that should not be doped are covered with a mask, though for implantation a photoresist mask is sufficient.

An implanter consists of the following components:

• Ion source: the dopant gas (e.g. boron trifluoride BF₃) is ionized (electrons emitted from a hot cathode collide with the gas particles; impact ionization continuously generates positive ions and free electrons)
• Pre-accelerator: the ions are drawn out of the ion source at roughly 30 kiloelectronvolts
• Mass separator: the charged particles are deflected by 90° in a magnetic field. Particles that are too light or too heavy are deflected more or less than the desired ions and are caught by screens behind the separator
• Acceleration lane: several hundred keV accelerate the particles to their final energy (200 keV accelerate boron ions to roughly 2,000,000 m/s)
• Lenses: lenses are distributed throughout the system to focus the ion beam
• Beam deflection: electric fields deflect the ions to irradiate the desired location
• Wafer station: the wafers are brought into the ion beam either individually or mounted on large rotating wheels

Illustration of an ion implanter

Penetration depth of ions in the wafer

Unlike diffusion, the particles do not penetrate the crystal through their own thermal motion but are shot into the crystal lattice at high velocity. Collisions with silicon atoms slow them down along the way. The impacts knock silicon atoms out of their lattice sites, while the dopant ions themselves mostly come to rest on interstitial sites. There, they are not electrically active, since they form no bonds with neighboring atoms that could give rise to free charge carriers. The displaced silicon atoms must be reincorporated into the crystal lattice, and the electrically inactive dopants must be activated.

Annealing the crystal lattice and activating the dopants

A temperature step at around 1000 °C moves the dopants onto lattice sites (beforehand, only about 5 % of the dopant atoms sit on lattice sites). Lattice damage from the collisions is already cured at around 500 °C. Because the dopant atoms keep moving through the substrate at these high temperatures, these steps are carried out for only a very short time.

This is exactly where the trade-off of the process lies: the temperature must be high enough to move the dopants onto lattice sites, and the time short enough that they do not migrate away while doing so. Both can only be achieved together by continually shortening the heating duration. What began as an hours-long furnace process first became rapid thermal processing over seconds, then spike annealing, where the target temperature is only passed through rather than held, and finally flash-lamp or laser annealing in the millisecond range. Here, only the topmost layer of the wafer is heated while the substrate underneath stays cold.

For very shallow doping profiles, the ion mass is additionally increased: instead of implanting individual boron atoms, molecules containing several boron atoms are implanted. At the same acceleration voltage they carry less energy per boron atom and therefore penetrate less deeply – a way to create very shallow junctions without having to operate the implanter at impractically low voltages.