Shear elastic waves are excited in ZnO thin films. The ZnO films are prepared by co-sputtering of zinc with small amounts of aluminum in an oxidizing atmosphere. The films exhibit shear mode coupling factors of 0.05 to 0.08 in the 0.3 to 1.3 GHz fre- quency range. Electmacoustic constants of sputtered ZnO films of polycrystalline form have been measured by several groups of workers in recent years in relation to the excitation and detection of bulk e1ast)ic waves (l), or surface elastic waves (Z). The majority of the electroacoustic measurements has dealt with ZnO films with c-axis normal to the film plane (normal orientation). One of the reasons is that t'he sputtered ZnO films on metallized crystalline substrat>es or nonideal lattice configurations usually show the normal orientation. Tho ZnO films with other oriented axes arc difficult to prepare with high reproducibility bp using conventional sput,tering techniques. This paper describes a new reproducible sputtering technique for controlling the directions of the oriented axis (S) and shows the electroacoustic properties of ZnO films with the c-axis in the film plane (parallel oricntation) in relation to the rxcitation of bulk shear mode elast'ic waves. A planar electrode dc-sputtering system with an ultra- high-vacuum oil diffusion pump was used for preparing the ZnO films. Thc elect,rode construction is shown in Fig. 1. A 25 mm diameter disk of zinc metal (purity 99.99yG7,) was used as cathode and a 60 mm diameter disk of alumi- num (purity 99.9oj,) was used as anode. An auxiliar- electrode of aluminum wire (purity 99.99%) was posi- tioned noar the cathode and substrates (70.59 glass, fused quartz) were placed behind the anode. The distancrs from cathode to anode and cathode to substrate wcrc 20 mm and 27 mm, respectively. Sputtering was carried out at 6 X loF2 Torr of Ar and 0, with an oxygen concentration of 50%. The background pressure before sputtering was 2XlO-'j Torr. By co-sputtering the auxiliary alunlinum electrode and the zinc cathode, aluminum was added to sputtered films. The contents of the aluminum were con- trolled by adjusting the rat'io Ig/Ic, where I, is the current flow in the auxiliary aluminum elcctrode, and I, is the zinc cathode current, which could be regulated by the circuit resistance R, in Fig. 1.