Thomas Modes: "Piezoelectric force microscopy on magnetron-sputtered (Al,Sc)N layers"
T. Modes, O. Zywitzki, S. Barth, H. Bartzsch
Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, Dresden, Germany
Aluminum scandium nitride layers with hexagonal wurtzite structure and a polar c axis were deposited by reactive pulse magnetron sputtering on silicon wafers. By incorporation of scandium into wurtzite structure the piezoelectric charge constant and electromechanical coupling are improved, which are important for application in radio frequency filters.The piezoelectric properties were determined by a Berlincourt piezometer (PM300, Piezotest) and by piezoelectric force microscopy PFM in off-resonance mode (AFM NX20, Park System). Piezometer measurements revealed for pure AlN layers a piezoelectric charge coefficient d33 of 8.4 pC/N. With increasing scandium content, the piezoelectric charge coefficient increased up to 27 pC/N for a scandium concentration of 43 %. The results of PFM measurements show that mean PFM amplitude is directly proportional to the piezoelectric charge coefficient determined by the piezometer.
In contrast to the piezometer, the PFM measurement allows a high lateral resolution of the piezoelectric properties. For non-optimized samples the PFM amplitude shows areas with lower piezoelectric response than the surrounding matrix. This can be attributed to grains with abnormal grain growth and different crystallographic orientation (Fig. 1). After optimizing the pulse parameters of magnetron sputtering, the growth of these abnormal oriented grains could be suppressed and the PFM amplitude exhibits a homogenous signal.
Furthermore, the PFM measurement allows to assess local changes of the polarity. It can be shown that in dependence of the process parameters the polarity of the (Al,Sc)N layer is changed from N polar to bipolar and further to Al polar.
The results show that the high-resolution PFM measurement enables a further optimization of piezoelectric properties of (Al,Sc)N layer by detection of local changes in amplitude and phase of piezoelectric signal.
Figure 1: (Al,Sc)N layer with some crystallites of abnormal grain growth which show a lower piezoelectric amplitude (left AFM topography, right PFM amplitude)