Deposition of thick zinc oxide films with a high resistivity

ZnO is a well known piezoelectric material. Unfortunately it is not easy to deposit thin films onto silicon with a high resistivity by using common deposition technologies. The use of such films is therefore strictly limited to high frequency applications. The goal of our work was to find out a new deposition technology that allows the deposition of ZnO-films with a high resistivity. Furthermore we were looking for the deposition of film thicknesses in a range up to 20 μm for SAW-sensor and microactuator applications. The deposition of the ZnO-films was carried out in a programmable RF-magnetron-sputtering-system. We sputtered from a pure zinc target with a variable gas composition that consists of argon and oxygen. We worked in an alternating mode to achieve a high resistivity of the films. After a deposition cycle at a sample temperature of about 30°C with a ramp shaped power the silicon-samples were cooled during the following cycle in the gas atmosphere. The deposition rate we measured was dependent from the gas composition and the applied power in a range between 1,5 μm/h and 2,2μm/h. We deposited films of a thickness of 20 μm. Between two sputtered aluminium electrodes the films had a resistivity in a range between 2*10¹⁰ ωcm and 2*10¹¹ ωcm. The stress of the films could be influenced by the composition of the gases. The measured minimum stresses of the films were in a range of about 180 MPa. The films were also characterised by means of XRD-measurements. We found a week 〈101〉-orientation of the layers perpendicular to the surface.