F. Ali, N. Rady, N. Jackson
University of New Mexico,
United States
Keywords: aluminum nitride, piezoelectric, sputter, polymer, stainless steel
Summary:
This paper reports on the development of sputter deposited single orientation textured piezoelectric Aluminum Nitride (AlN) thin films on various wafer substrates. Deposition of piezoelectric AlN has been extensively investigated on single crystal substrates such as silicon and sapphire. However, there is a high demand for thin film piezoelectrics on various substrates including polymers and metals due to their mechanical properties. This study investigated optimizing key deposition parameters and their impact on depositing high quality AlN on flexible polymer substrates as well as high tensile strength metals such as stainless steel (SS). In this study we investigated deposition of AlN on three different types of SS (304, 420, and 17-4) as well as polymers such as low CTE polyimide and semi-crystalline parylene. The films consisted of a multi-layer stack of AlN (seed layer ~100 nm), Mo (200 nm), and AlN (piezo ~700 nm) (Figure 1). Sputtering parameters for all the layers are shown in Table 1. The deposition process was first carried out on Si (111) substrates and then later compared to films deposited on the various wafer substrates. Surface roughness and crystallinity of the substrates was critical for ensuring high quality piezoelectric films. The quality of the AlN was highly dependent on surface roughness and crystallinity of the substrate. Surface roughness of the samples was varied by polishing and roughening the surfaces. Parylene substrates were altered through a thermal annealing process to increase the crystallinity of the film prior to stacked layer deposition on all of the substrates. X-ray diffraction results indicate only a single peak for both AlN (002) and Mo (110), with an additional peak for the stainless steel and polymer substrates. Additional peaks of AlN have been known to decrease the piezoelectric properties. AlN on rough SS and rough parylene had significantly lower crystallinity compared to films deposited on Si. The piezoelectric d33,f properties for the various films/substrates are illustrated in in the figure below. The results show that AlN on Si had the highest piezoelectric properties at 5.81 pC/N. AlN deposited on polished surface of 304 and 420 had piezoelectric values of 3.79 and 3.52 pC/N, whereas polished 17-4 only had a value of 1.89 pC/N. This is believed to be due to the increased surface roughness of 17-4. The unpolished SS substrates resulted in a much lower piezoelectric values <1 pC/N. Polyimide substrates resulted in a relatively low piezoelectric value of 1.12 pC/N due to the poor crystallinity of the film. However, parylene-N substrates with high crystallinity resulted in piezoelectric properties of 4.98 pC/N whereas AlN on a rougher surface had a value of 2.88 pC/N. Crystallinity of the parylene also had a significant impact as lower crystallinity parylene with FWHM (0.81°) compared to annealed parylene-N (0.48°) resulted in piezoelectric properties of 3.8 vs 4.98 pC/N. The results illustrate that high quality piezoelectric AlN can be deposited on various substrates, however the performance will be lower when compared to Si due to reduced surface roughness and high crystallinity.