c-axis orientation and piezoelectric coefficients of AlN thin films sputter-deposited on titanium bottom electrodes

• Published in: Applied surface science 2012-10, Vol.259, p.59-65
• Main Authors: Ababneh, A., Alsumady, M., Seidel, H., Manzaneque, T., Hernando-García, J., Sánchez-Rojas, J.L., Bittner, A., Schmid, U.
• Format: Article
• Language: English
• Subjects: Aluminum nitride; c-axis orientation; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Deposition; Exact sciences and technology; Laser vibrometry; Metal electrode; Microstructure; Orientation; Physics; Piezoelectric coefficients; Piezoelectricity; Roughness; Sputter deposition technique; Surface roughness; Thin film; Thin films; Titanium; XRD analyses
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2012.06.086

► The surface roughness of the titanium bottom electrode (Ti) is a dominating factor on the piezoelectric coefficients and structure of AlN thin films. ► Increasing the surface roughness of Ti bottom electrode results in open-porous surface characteristics and low piezoelectric coefficients of the AlN top layer. ► Decreasing the surface roughness of Ti bottom electrode results in a smooth and dense surface which is a necessary requirement for high piezoelectric coefficients. ► Both piezoelectric coefficients d33 and d31 were determined by comparing the experimental and simulated displacement profile of the piezoelectric film. Aluminum nitride (AlN) reactively sputter deposited from an aluminum target is an interesting compound material due to its CMOS compatible fabrication process and its piezoelectric properties. To obtain high piezoelectric coefficients it is a necessary pre-request to synthesize films with c-axis orientation. Besides the influence of sputter conditions on the microstructure of AlN thin films the condition of the substrate surface is another important factor of utmost importance. In this study, the influence of 350nm thick titanium metallization DC sputter-deposited on SiO2/Si substrates at varying back pressure levels bp,Ti in the range of 2×10−3 to 14×10−3mbar on the c-axis orientation and the piezoelectric coefficients of 600nm thick AlN thin films is investigated. Besides the plasma power for Ti deposition (Pp,Ti=100W) the parameters for AlN synthetization are fixed to Pp=1000W and bp,AlN=4×10−3mbar in 100% N2 atmosphere. Basically, the surface roughness of the Ti bottom layer is the dominating factor resulting either in a high degree of c-axis orientation (i.e. at low bp,Ti values) or in an amorphous AlN microstructure (i.e. at high bp,Ti values). Under low pressure conditions, a smooth and dense surface characteristics is achieved due to a higher kinetic energy associated with the adatoms what is especially important at nominally unheated substrate conditions. The piezoelectric coefficient d33 decreases from 2.55 to 1.7pm−1 when increasing the titanium sputter pressure from 2×10−3 to 14×10−3mbar. When decreasing the Ti film thickness to 60nm and hence, reducing the root mean square roughness by a factor of about 2, the intensity associated with the AlN (002) peak is increased by a factor of about 1.7 demonstrating the direct impact. Furthermore, the highest values for d33 and d31 (i.e. 3.15pmV−1 and −1.28pmV−1) are determined.