A proton in a vacuum channel can be controlled similar to electrons in a metal gate field effect transistor. A low voltage (<10v) controls each channel via a metal electrode embedded in the dialectric above the channel. Protons are gated into the first cell of each accelerator channel. The first several accelerator stages bunch the protons. The pixel data is applied to the gates selecting which of the 250,000 channels write protons. These bunches of protons are accelerated as a linear array and writes on the wafer much like an inkjet printer.
Controlling the Dose
175 protons are needed to expose a 20nm pixel. Additional gate electrodes can control how full the first stage is loaded.
It is not known how important dose control will be. Since the mass of a proton is large relative to its charge, the focus of the proton bunches should stay constant regardless of the number of protons in the bunch. The length of the bunches is long relative to the diameter.