THE MODERN METHOD

Fire the dopant in

ions, not heat, do the work

Ion implantation strips electrons off dopant atoms, accelerates the resulting ions through a strong electric field, and steers them into the wafer as a beam. Unlike diffusion, implantation places dopant mostly straight down, which is what a small transistor needs.

A beam of ions goes where it's aimed, instead of spreading in every direction.

INSIDE THE IMPLANTER

Source, accelerator, magnet, target

An ion source makes dopant ions, an accelerator gives them energy, and a mass-analyzing magnet bends the beam so only ions of the right mass reach the wafer, filtering out contamination. The wafer itself is tilted a few degrees so the beam doesn't run straight down open channels in the crystal lattice.

The magnet is a filter: only the dopant you asked for reaches the wafer.

DOSE AND ENERGY

Two independent knobs

Dose Ions per square centimeter: sets how much dopant lands, roughly 10¹¹ to 10¹⁶ per cm²

Energy Kilo-electron-volts: sets how deep the ions stop, from nanometers to a micron

Independent control Change one without disturbing the other, unlike a diffusion profile

Dose and energy are set separately, which is the real advantage over diffusion.

MASKING THE BEAM

Dopant only where you want it

🧴

Photoresist

A thick enough layer of patterned resist stops the ions completely

🧱

Oxide or nitride

A hard mask survives higher-energy implants that would punch through resist

🎭

The pattern

Whatever lithography defined in Part 3 decides where the dopant lands

Implantation inherits its precision from the mask lithography already printed.

CHANNELING

Why the wafer sits tilted

tilt of about 7 degrees

Silicon atoms line up in a crystal lattice, and an ion aimed straight down a row can skip along open channels far deeper than intended. Tilting the wafer a few degrees, commonly about 7 degrees, points the beam off-axis from those channels, so ions stop at a predictable, repeatable depth.

A few degrees of tilt keeps the implant depth from becoming a matter of luck.

UNIT 21 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered the modern way to dope silicon: firing dopant atoms in as a beam of ions.