Negative-ion bombardment increases during low-pressure sputtering deposition and their effects on the crystallinities and piezoelectric properties of scandium aluminum nitride films

Abstract

Scandium aluminum nitride (ScAlN) films are being actively researched to explore their potential for use in bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators because of their good piezoelectric properties. Sputtering is commonly used in ScAlN film deposition. Unfortunately, it has been reported that film quality metrics such as the crystallinity and piezoelectric properties can deteriorate before the Sc concentration reaches 43% without an isostructural phase transition. One reason for this is bombardment with negative ions generated from carbon and oxygen impurities in the Sc ingots. Because the number of negative ions increases during low-pressure sputtering deposition, their effect on film quality may be considerable. In this study, we investigated negative-ion bombardment of the substrate during sputtering deposition and its effects on ScAlN crystallinity and piezoelectric properties. Negative-ion energy distribution measurements indicated that many more negative ions collide with the substrate during ScAlN film deposition than during AlN deposition. In addition, decreasing the sputtering pressure further increased the number of negative ions and their energies. It is well known that film quality improves at low pressures because increasing the mean free path reduces thermalization and scattering of sputtered particles. Although, AlN crystallinity and piezoelectric properties improved at low pressures, the properties of ScAlN films deteriorated dramatically. Therefore, the results indicated that ion bombardment increase at low pressure adversely effects ScAlN crystal growth, deteriorating crystallinity and piezoelectric properties. ScAlN films may be improved further by suppressing negative-ion bombardment of the substrate.

The measurement and impact of negative oxygen ions during reactive sputter deposition

“…The presence of CNduring the deposition of ScAlN has been reported [282]. The origin of these ions have been attributed to presence of impurities such as C and O in the Sc ingots which were used as insert in the Al target.…”

Dawn of nitride ferroelectric semiconductors: from materials to devices

“…They investigated the electric field induced inversion domains, as well as the non-switched domains at the bottom electrode interface, providing critical information for understanding the domain evolution process during polarization switching in wurtzite materials. Subsequently, the impact of the nitrogen-to-argon gas ratio and target power ratios, plasma modes, sputter power, deposition temperature, crystal orientation, film stress, surface roughness, Sc content, and deposition rate on ferroelectric properties of ScAlN have been intensively investigated [62,64,[144][145][146][147][148][149][150][151][152][153].…”

Sputtering Process of ScxAl1−xN Thin Films for Ferroelectric Applications

“…In 2022, Tominaga et al, proposed a possible explanation for the deterioration of crystallinity and even electric properties at low-pressure depositions. In short, the report claims that negative-ion bombardment will increase both in quantity and energy at lower pressures, which will negatively impact Sc x Al 1−x N crystal growth and cause substantial film quality reduction [29].…”

“…Coatings 2023, 13, x FOR PEER REVIEW 13 of 21 2022, Tominaga et al proposed a possible explanation for the deterioration of crystallinity and even electric properties at low-pressure depositions. In short, the report claims that negative-ion bombardment will increase both in quantity and energy at lower pressures, which will negatively impact ScxAl1−xN crystal growth and cause substantial film quality reduction [29]. (b) Effect on Sputter Deposition Rate: The impact of sputtering pressure on the sputter deposition rate is partially obfuscated when comparing reports from existing literature related to the reactive sputtering of ScxAl1−xN.…”