Control of thin-film silicon microstructure with low-energy particle bombardment in reactive sputtering

Reactive magnetron sputtering is a physical vapor deposition process that can produce amorphous, nanocrystalline, or polycrystalline silicon thin films by sputtering a Silicon target in argon, with or without hydrogen. This work demonstrates a new growth parameter than can modify most microstructural phases of thin-film silicon: controlled, low-energy ion bombardment, characterized by the Ar+ ion to neutral silicon flux ratio. Additionally, we investigate the kinetic role of H in the nanocrystalline silicon phase transition, and compare these two very different bombardment effects.; An external magnetic field is applied to our growth chamber in order to shift and collimate the plasma; this increases the flux of Ar+ to the growth surface, while the ion energy is limited to 20eV via substrate biasing. The energy of Ar+ as well as the neutral silicon growth specie, is preserved due to very low pressure (1.8mTorr) operation. This high flux of low energy ions increases the energy provided to the surface of the growing film without introducing damage; this has several beneficial effects on microstructure, which we hypothesize to be the result of enhanced local mobility of adatoms. The grain size of polycrystalline silicon grown on glass is increased (from 40 to 100 nm) while remaining remarkably smooth (1.9 nm), and the non-crystalline transition layer is completely eliminated. In the amorphous silicon growth regime, we find that the ion flux controllably increases structural order on the 1–2 nm length scale in the films. This medium range order is a recently discovered characteristic of vapor deposited amorphous silicon.; Hydrogen can be added to the growth chamber to promote a phase change to nanocrystalline silicon at moderate growth temperatures. The growth flux with hydrogen includes energetic (∼100eV) neutral H, produced by the reflection of H₂+ions from the target. We determine the kinetic role of these fast H neutrals in the nanocrystalline phase formation by substituting D for H during growth. Crystallization is enhanced with D; we suggest that this higher mass specie enhances a sub-surface restructuring process by enhanced momentum transfer to silicon atoms.