Comparative study of thermal and radical-enhanced methods for growing boron nitride films from diborane and ammonia

Abstract

This work studies the deposition of boron/boron nitride (B/BN) composite films at low substrate temperature (275-375 °C) by alternating pulses of diborane (B 2 H 6 ) and ammonia (NH 3 ) with argon purging in between to avoid gas-phase reactions of the precursors. This process is similar to atomic layer deposition in which the dominance of surface reactions simplifies the growth mechanism. However, non-self-limiting decomposition of B 2 H 6 and incomplete nitridation lead to the incorporation of pure boron (pure-B), causing deviation from the desired 1:1 B:N stoichiometry. Using the pure-B fraction as a measure of incomplete nitridation, this article describes consecutive experiments to control this effect and ultimately understand it in the context of a surface reaction model. First, it is demonstrated that, in a purely thermal mode, the growth of the layers and their composition strongly depend on the total gas pressure. The pure-B content (not to be confused with the total boron content) could thus be varied in the range of ∼6-70 vol. %. Next, enhancement of nitridation by the dissociation of NH 3 into reactive radicals using a hot-wire was found to be insufficient to produce stoichiometric BN. Finally, plasma-assisted deposition at 310 °C resulted in nearly stoichiometric polycrystalline BN with an interplane distance matching that of hexagonal BN; the material was stable in air for at least six months. The pressure dependence in the purely thermal mode is consistent with a growth model of BN from B 2 H 6 and NH 3 via the so-called surface-adduct mechanism. The effects of the radical-enhanced methods on nitridation are explained using this model.

Low temperature pure boron layer deposition for Si diode and micromachining applications

“…The major difference with respect to the ULPCVD deposition was that the precursor in LPCVD furnace was 0.2% B2H6 in H2 gas, and the processing pressure was around 1 mbar. For B-deposition in both furnaces on patterned wafers, the thickness of the B-layer would presumably be more uniform because the deposition on thermal oxide has a much longer incubation time [132], probably because there are too few nucleation centers for B-adsorption. Therefore, gas depletion, which is responsible for thickness non-uniformity, will be reduced.…”

Study of the phase nature of boron- and nitrogen-containing films by optical and photoelectron spectroscopy

“…The pulsed approach simplifies the growth mechanism by suppressing gas phase reactions between precursors, as discussed in our recent report. 24 The films were obtained in the pressure range of 10 −3 -0.05 mbar, at substrate temperatures between 270 and 375 °C. The typical pulse-and purge-durations were 0.5-2.5 s of the B 2 H 6 pulse, 4-6 s of post-B 2 H 6 purge, 2.5-3 s of NH 3 pulse, and 4-6 s of post-NH 3 purge.…”

“…The films, with various compositions, were obtained either in purely thermal deposition mode or with assistance by a hot wire. 24 A reference layer of nearly stoichiometric boron nitride (ns-BN, x = 0.53 by XPS) was deposited in a Picosun R-200 reactor from the same precursors and in the same manner but by adding a plasma assistance. Reference layers of pure-B were deposited on Si by CVD from B 2 H 6 (5% in Ar) by either using (1) a constant flow of B 2 H 6 (200 SCCM) at 1 mbar and 220 °C or using (2) pulses of B 2 H 6 (a 2-s pulse every 17 s) at 10 −3 mbar and 300 °C.…”

“…The present work aims to provide and analyze experimental data toward addressing the phase question for layers of B 1−x N x (0.21 ≤ x ≤ 0.47), grown by pulsed chemical vapor deposition (CVD) of diborane (B 2 H 6 ) and ammonia (NH 3 ) as recently reported. 24 This method is of particular interest as it exposes the growing layer to pulses of one precursor at a time, which in certain conditions may be expected to produce layered (lamellar) composites. Specifically, samples grown with incomplete nitridation during the NH 3 pulse showed optical absorption resembling that of pure-B (see Sec.…”

“…For more details on the deposition process and conditions, the reader is referred to our recent report. 24 The films were studied with x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). XPS is used because it conveys information on the chemical environment of atoms, which is expected to vary between the phases of a composite.…”