Abstract
We report on low-temperature and low-pressure deposition conditions of 140 °C and 1.5 mTorr, respectively, to achieve high-optical quality silicon nitride thin films. We deposit the silicon nitride films using an electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) chamber with Ar-diluted SiH4, and N2 gas. Variable-angle spectroscopic ellipsometry was used to determine the thickness and refractive index of the silicon nitride films, which ranged from 300 to 650 nm and 1.8 to 2.1 at 638 nm, respectively. We used Rutherford backscattering spectrometry to determine the chemical composition of the films, including oxygen contamination, and elastic recoil detection to characterize the removal of hydrogen after annealing. The as-deposited films are found to have variable relative silicon and nitrogen compositions with significant oxygen content and hydrogen incorporation of 10–20 and 17–21%, respectively. Atomic force microscopy measurements show a decrease in root mean square roughness after annealing for a variety of films. Prism coupling measurements show losses as low as 1.3, 0.3 and 1.5 ± 0.1 dB/cm at 638, 980 and 1550 nm, respectively, without the need for post-process annealing. Based on this study, we find that the as-deposited ECR-PECVD SiOxNy:Hz films have a suitable thickness, refractive index and optical loss for their use in visible and near-infrared integrated photonic devices.
Highlights
As thin-film silicon nitride (Si3N4) becomes a standard material in integrated photonics, the need for a broad range of deposition recipes for different applications becomes apparent
Low pressures of 1–2 mTorr are realized in ECR-Plasma-enhanced CVD (PECVD) by using a strong magnet to confine and stabilize the plasma during deposition, whereas inductively coupled plasma (ICP) techniques typically operate at pressures >10 mTorr [13]
In the case of ICP-PECVD and ECR-PECVD, the reaction chamber is separate from the plasma chamber, allowing for high-quality films to be produced at low temperatures and pressures without the risk of surface damage from high energy bombardment
Summary
As thin-film silicon nitride (Si3N4) becomes a standard material in integrated photonics, the need for a broad range of deposition recipes for different applications becomes apparent. We demonstrate that the associated optical losses of hydrogenated non-stoichiometric PECVD silicon nitride (SiOxNy:Hz), as reported in the literature [13], are reasonable for specific applications requiring low-temperature deposition conditions and access to thicker films up to 650 nm. The system’s low deposition pressure with operating conditions shown here of 1–2 mTorr ensures a low concentration of Si-H bonds to form during deposition while using SiH4 precursors, limiting interstitial hydrogen incorporation during deposition, as is a common drawback in PECVD systems Another expected source of hydrogen is invasive H2O in the form of water vapour likely entering the chamber through the ~5 m-long gas transit lines and potentially residual from the sample transfer process.
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