Effect of substrate material and electrode surface preparation on stress and piezoelectric properties of aluminum nitride

Intrinsic stress and electromechanical coupling coefficient are the most important parameters of piezoelectric thin films used in membrane as well as cantilever type MEMS such as energy harvesters, transducers and others. In this paper, practical aspects controlling stress and crystallinity in aluminum nitride (AlN) film used in manufacturing of Bulk Acoustic Wave (BAW)/ Film Bulk Acoustic Resonator (FBAR) filters have been presented. Controlling stress by modifying deposition parameters was compared to controlling stress by surface modification. Stress was found to be an important indicator in predicting crystallinity of AlN films. Impact of the substrate material and electrode deposition on stress and crystal orientation of piezoelectric AlN was studied. In process applications where control of the surface layer thickness is critical and surface topography is variable, use of ion mill to reduce surface roughness is preferable to CMP. It was generally found that for most amorphous materials such as silicon dioxide or silicon nitride that are placed under the electrode or piezoelectric AlN, it is critical to get the best possible surface smoothness. On the other hand for the electrode material such as molybdenum, it is more important to get material with the best crystal orientation. Polishing of the well oriented electrode material did not provide further improvement. It was also observed that if AlN is deposited on a smooth or well oriented layer, orientation of the AlN is improved when the stress in the film is more tensile. On the other hand if the tensile stress is caused by residuals on the surface or surface roughness, tensile stress usually leads to poorly oriented films.