A coating process to infill high aspect-ratio vias and trenches in semiconductor substrates with dense boron for the production of neutron detectors and other devices uses a vacuum cathodic arc or other source of fully ionized boron plasma. Biasing of the substrate is used to impart energies to the plasma ions directing them toward the substrate, while repulsing the electrons. The full ionization produced by the source allows control of the energies of the boron ions by means of the bias voltage. The bias is alternated between coating deposition at low ion energies and sputtering of already coated material by energetic ions. Most of the sputtered material comes off the substrate top surface and between the trenches or vias and much of it is redeposited, thereby contributing to the infill. The process is suitable for carbon, boron or similar light elements, and is of particular interest for 10B, an element having exceptionally high thermal neutron cross-section.