352 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 51, no. 3, march 2004 Effect of Particle Bombardment on the Orientation and the Residual Stress of Sputtered AlN Films for SAW Devices Enrique Iborra, Marta Clement, Jes´ us Sangrador, Alfredo Sanz-Herv´ as, Luc´ ıa Vergara, and Miguel Aguilar Abstract—We present a study of the effect of particle bombardment on the preferred orientation and the resid- ual stress of polycrystalline aluminum nitride (AlN) thin films for surface acoustic wave (SAW) applications. Films were deposited on silicon (100) substrates by radio fre- quency (RF) sputtering of an aluminum target in an ar- gon and nitrogen gas mixture. The main deposition pa- rameters were changed as follows: the total pressure from 4 mTorr to 11 mTorr, the N2 content in the gas mixture from 20% to 80%, and the substrate self-bias voltage from 10 V to 30 V. If a sufficiently high negative substrate self-bias voltage is induced, (00.2)-oriented films are ob- tained over the full ranges of pressure and N2 content. Such films have values of residual stress ranging from 3 GPa to +1 GPa, depending on the deposition conditions. Our re- sults suggest that the energy of the Ar ions colliding with the substrate controls the preferred orientation of the films, whereas the directionality of the ions (for the same en- ergy) is the main factor determining the residual stress. To demonstrate the suitability of our material for the intended application, SAW filters with good electroacoustic response have been fabricated using AlN thin films with optimized (00.2) orientation and controlled residual stress. I. Introduction A luminum nitride (AlN) is a well-established mate- rial for thin film surface acoustic wave (SAW) appli- cations due to its good properties, such as an outstanding ultrasonic velocity and good thermal and chemical stabil- ities. Among the different techniques investigated to de- posit high-quality thin films for SAW transducers, reactive sputtering is the most popular method because it is an in- expensive procedure that yields films of sufficient crystal quality at low temperature on a great variety of substrates. For the fabrication of SAW devices, it is essential to de- termine the relation between the deposition parameters and the preferred orientation and the crystal properties of the AlN films. In order to propagate Rayleigh waves, the AlN films must exhibit c-axis preferred orientation as their main characteristic. Other film properties—such as the crystal quality, the surface roughness, and the residual stress—should be optimized to improve the performance of SAW devices. Manuscript received February 7, 2003; accepted October 20, 2003. This work has been supported by the Ministerio de Ciencia y Tec- nolog´ ıa of Spain through project no. MAT2001-350. The authors are with the Departamento de Tecnolog´ ıa Electr´ onica, E.T.S.I. Telecomunicaci´on, Universidad Polit´ ecnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain (e-mail: eiborra@etsit.upm.es). The c-axis oriented AlN thin films can be achieved by the sputtering technique as long as sufficient kinetic en- ergy is supplied to the adatoms on the surface of the film during its growth. This energy can be provided by heating the substrate [1] and by bombarding the surface of the film with energetic particles from the plasma [2]. However, the energetic bombardment of the substrate may have negative consequences, such as the formation of crystal defects that would induce high residual stress in the film [3], [4]. Sput- tering at low pressure is the most common way to enhance the ionic bombardment of the growing films and, there- fore, to induce the growth of c-axis oriented films. As the pressure is decreased, the energetic particles traveling to- ward the substrate suffer fewer collisions with the plasma and reach the substrate with higher kinetic energy. In a previous work [5], in which we used a mixture of Ar and N 2 as the discharge gas, we showed that the Ar ions, ac- celerated toward the substrate by the difference between the plasma and substrate potentials, were the bombard- ing species that most effectively contributed to the supply of energy to the substrate. This result suggests that it is possible to control the crystal properties of the films by adjusting the substrate bias voltage. In this work we have analyzed the effects of particle bombardment on the orientation and the residual stress of AlN films as the sputter parameters (pressure, N 2 content, and substrate bias voltage) were modified. First, we have investigated the crystal properties of a series of AlN films grown with the substrate holder electrically isolated from the ground; in this case the substrate self-bias voltage is de- termined by the pressure and the gas composition. Second, we have analyzed another series of films grown with an in- tentional control of the substrate bias potential. Third, the effect of the pressure and the nitrogen content on the orientation and the residual stress of films grown at a high- negative substrate voltage is investigated. To demonstrate the suitability of our material for SAW applications, we present the piezoelectric response of SAW filters fabricated with representative good quality AlN films. II. Experimental Aluminum nitride thin films were deposited by RF diode sputtering of a 150 mm Al target in a high purity (99.9995%) N 2 and Ar mixture. The vacuum chamber was pumped down to a base pressure lower than 5 · 10 −7 Torr. Films were grown without intentional heating on silicon 0885–3010/$20.00 c  2004 IEEE