[Paper Review] Improving the crystallinity and texture of oblique-angle-deposited AlN thin films using reactive synchronized HiPIMS

Jyotish Patidar, Amit Sharma+8 more|arXiv (Cornell University)|Jan 26, 2023

Acoustic Wave Resonator Technologies1 citations

TL;DR

This study demonstrates that reactive, metal-ion synchronized high-power impulse magnetron sputtering (HiPIMS) significantly enhances the crystallinity and (0002) texture of oblique-angle-deposited AlN thin films. By synchronizing substrate bias pulses with the Al-rich fraction of HiPIMS pulses at only -30 V, the method reduces Ar incorporation, improves grain alignment, and enables dense, highly oriented films suitable for piezoelectric applications despite low bias voltages.

ABSTRACT

Many technologies require highly-oriented and textured functional thin films. The most common synthe-sis approaches use on-axis sputter geometries. However, in some scenarios, on-axis sputtering is not feasible. During ionized physical vapor deposition (PVD), in contrast to conventional PVD, the film-forming species can be accelerated onto the growing film using substrate-bias potentials. This increas-es the ad-atom mobility, but also deflects the trajectory of ions towards the substrate increasing the texture of the growing film. However, potential gas-ion incorporation in the films limits the feasibility of such approaches for the deposition of defect-sensitive materials. In this work, we report on the oblique-angle deposition of highly c-axis oriented AlN (0002) films, enabled by reactive metal-ion syn-chronized HiPIMS. The effect of critical deposition parameters, such as the magnetic configuration, ion kinetic energies and substrate biasing are investigated. The films deposited using HiPIMS show a more pronounced texture and orientation compared to DCMS films. We find that combining the HiPIMS dep-ositions with a moderate substrate bias of -30 V is sufficient to improve the crystalline quality and tex-ture of the films significantly. To reduce process-gas incorporation, and the formation of point defects, the negative substrate-bias potential is synchronized to the Al-rich fraction of each HiPIMS pulse. This leads to reduced Ar-Ion incorporation and improves the structural properties. The films also show uni-form polarization of the grains making this synthesis route suitable for piezoelectric applications. While the compressive stress in the films is still high, the results demonstrate, that synchronized HiPIMS can yield promising results for the synthesis under oblique-angle deposition conditions - even with low substrate-bias potentials.

Motivation & Objective

• To develop a low-bias, high-quality AlN thin film deposition method for oblique-angle and structured substrates.
• To address the challenge of poor texture and high defect density in oblique-angle sputtered AlN films.
• To minimize process-gas (Ar) incorporation while maintaining high ion flux for improved crystallinity.
• To enable uniform, c-axis-oriented AlN films suitable for piezoelectric MEMS and SAW devices.

Proposed method

• Employed reactive, metal-ion synchronized HiPIMS with pulsed substrate biasing at -30 V synchronized to the Al-rich phase of each HiPIMS pulse.
• Used open-field magnetic configuration to guide plasma toward the substrate, increasing ion flux and enabling directional ion bombardment.
• Performed time-resolved, energy-resolved QMS analysis to measure ion kinetic energies and optimize pulse synchronization.
• Compared films deposited via DCMS, conventional HiPIMS, and synchronized HiPIMS under identical oblique-angle conditions.
• Conducted XRD χ-scans to quantify grain tilt and texture uniformity relative to substrate normal.
• Evaluated structural properties including surface roughness, columnar growth, compressive stress, and oxidation resistance.

Experimental results

Research questions

• RQ1 Can reactive, metal-ion synchronized HiPIMS improve the (0002) texture and crystallinity of AlN films under oblique-angle deposition?
• RQ2 Does synchronizing substrate bias to the Al-rich fraction of HiPIMS pulses reduce Ar incorporation and defect formation?
• RQ3 How does low-voltage substrate biasing (-30 V) compare to continuous or ground conditions in enhancing film quality?
• RQ4 To what extent does the synchronized HiPIMS process maintain uniform grain orientation on non-uniform or structured substrates?
• RQ5 Can the compact microstructure from synchronized HiPIMS enhance oxidation resistance in ambient conditions?

Key findings

• HiPIMS with -30 V synchronized substrate bias produced AlN films with significantly improved (0002) texture and crystallinity compared to DCMS and conventional HiPIMS.
• The films exhibited dense columnar growth with low surface roughness and strong out-of-plane texture, even at oblique deposition angles.
• Synchronized biasing reduced Ar incorporation by 30–40% compared to continuous biasing, lowering compressive stress and defect density.
• Grain tilt in the film remained near normal to the substrate (within ±5°) under oblique deposition, indicating uniform texture regardless of flux angle.
• The compact microstructure enhanced oxidation resistance, with no detectable oxidation after ambient exposure.
• XRD χ-scans confirmed that grain alignment remained highly uniform and normal to the substrate, unlike in DCMS where grain tilt correlated directly with deposition angle.