Ion Implantation I have a very basic question regarding ion plantation: when Silicon is doped with Boron, for example, it is doped by Boron atoms (3 valent electrons). When it is doped with phosphorus, for example, it is also doped by phosphorus atoms (5 valent electrons), so how come the implantation is of ions?
Thank you very mutch
 A: The reasons to use ion implantation to introduce dopants into semiconductors (or specific impurities into, say, optically active thin films) are at least threefold. All were envisioned quite early in transistor technology by William Shockley (co-inventor of the transistor) in a 1954 patent application, granted in 1957. The big three reasons are: 


*

*Get one specific impurity, and only one: Once you have made ions, you can accelerate them and then use a magnetic field to select a specific charge:mass ratio to send down the beamline to your devices. From a practical perspective, this allows you to pick one element (and one isotope) to put into your material. So, using a phosphine gas as the feed for an ion source, you are accelerating phosphorous and hydrogen ions out of the ion source. You use a magnet to pick only the phosphorous. Phosphorous only has one isotope, so no further selection is necessary. Boron, however, has two isotopes, and you'd get one (usually the isotope with the highest natural occurrence, but there may be other reasons to select a specific isotope).

*Place the impurity at just the right depth: semiconductor devices are in three dimensions, and often you want one doping region underneath another. A given ion at a given implant energy will have a resulting impurity profile in the substrate. Higher incident energy means it goes further in, lighter ions will go further than heavier ions at the same energy. This profile can be thought of to first order as a gaussian centered on a specific depth with a width. There are also higher fidelity models with, e.g., skew and kurtosis. Now, this as-implanted distribution will change with the anneal sequence needed to get the implanted dopants on to lattice sites, but still you get to control the depth. This is most obvious in technologies that had a so-called deep-well implant, often several microns deep, well below the transistors. In a classic surface in-diffusion process from the 1960's this would be essentially impossible since the surface concentration of the well doping would be insanely high.

*You also want a specific amount of the dopant. Note that as device dimensions have shrunk, the amount (and allowed variation of the amount) have shrunk as well, so a high degree of control of the amount of dopant implanted is required. With ions this is (pretty) easy - you scan the ion beam across an aperture, with the wafer behind it. Overscan to make the beam go fully off the aperture. Then, the number of ions per square area that go into the device is just the charge collected on the wafer divided by the area of the implant on the wafer. 
So, you use ion implantation to get the right amount of the right impurity in the right place.
