Herstellung von Aluminiumnitrid Dünnschichten mittels Magnetron Sputtern auf Diamant für potentielle Pseudo-Surface-Acoustic-Wave-Anwendungen

In this dissertation high quality, piezoelectrical AlN coatings were synthesized and their suitability as SAW sensor material have been tested. For thin film synthesis reactive radiofrequency magnetron sputtering of high purity aluminum in a N2 atmosphere was utilized.
For the optimization of the growth mechanisms and to elucidate the coatings’ structure the process parameters in correlation to comprehensive materials characterization studies were carried out. In detail, the investigated process parameters were rf-power, substrate temperature, composition of process gases and the process pressure. High quality (002)-fibretextured AlN coatings were synthesized at room temperature in pure N2 atmosphere. These films were fully
c-axis oriented with a (002)-AlN-rocking-curve exhibiting 0.1° of full width at half maximum.
The thickness and surface roughness of the synthesized coatings were 6.6 µm and less than 2 nm, respectively. Measurements by means of nanoindentation displayed hardness values of 21.8 GPa and Young’s modulus of 338 GPa. According to scanning acoustic microscopy the SAW-, SLAW-, and transversal-velocities were 5.7 km / s, 10.9 km / s and 6.7 km / s, respectively. To test the feasibility of PSAW devices based on AlN coatings, nanocrystalline,
polished diamond substrates, synthesized by microwave-plasma-enhanced CVD were utilized as substrate material. The AlN coatings’ thickness was simulated and tested successfully to match the needs for a successful PSAW stimulation. To generate surface waves IDT structures were produced in a FIB-SEM by Platinum deposition and electron beam lithography. A symmetrical comb structure with finger thickness of 2 µm each have been chosen corresponding to a wavelength of 8 µm. As per simulation the AlN film thickness should be 720 nm for excitation of the first PSAW mode. The IDTs were arranged to work as a SAW filter showing a resonance frequency at 1 GHz in the case of AlN-Si-systems and 2 GHz in the case of AlN-NCD. Measurements were conducted by employing a frequency generator and an oscilloscope. The corresponding phase velocity of measured PSAWs were 8.2 km / s and 16.1 km / s in the case of AlN-Si and AlN-NCD, respectively. Simulation matched the measurements with a deviation of 1 % only, thus confirming the successful stimulation of the PSAW modes. The results found in this study present the advantage of AlN-NCD compared to other piezoelectric materials. Moreover, the possibility of combining the piezoelectric thin-film
material (AlN) with a variety of substrate materials tremendously extends its applications.