Improving Piezoelectric Properties of Polycrystalline AlN Thin Films by Doping

Piezoelectric thin films are used in a wide range of MEMS/NEMS based sensor and actuator applications such as vibrational energy harvesters, actuators in e.g. resonantly excited gyroscopes, surface accustic wave (SAW) devices, all electrical atomic force microscope (AFM) cantilevers for nano-scaled biological and material analyses and many more. To pave the way for novel MEMS/NEMS-based sensor and actuator concepts it is the aim of the proposed project to improve by doping with Scandium (Sc), Chromium (Cr) or Copper (Cu) key film properties especially the piezoelectric response of sputter deposited aluminium nitride (AlN) thin films as they suffer in the pure configuration from moderate piezoelectric coefficients compared to alternatives such as lead zirconate titanate (PZT). Alloying with the two latter elements is preferred as materials are used which are well- established in the MEMS and the microelectronics world. Basically, these impurities increase the piezoelectric properties due to a lattice distortion and a modified distribution in electronegativity inducing local polarities. The influence of different doping concentrations especially when using chromium and copper, and of the sputter conditions on the microstructure, the coexisting phases and the distortion of the wurtzite lattice as well as on the degree of c-axis orientation is measured enabling in a fundamental approach to complete and carefully extend crystallographic models of this material class. One focus is on the local chemical analysis of the doped AlN in the polycrystalline film using HR-TEM analyses to determine the implementation probability in the grain and at grain boundaries, respectively. Next, electrical, mechanical and electromechanical key properties are determined varying in addition geometrical parameters such as the film thickness. In detail, systematic measurement series to determine the longitudinal and the transversal piezoelectric coefficients (d 33, d31) of the transition-metal- doped AlN films are performed. Furthermore, mechanical (i.e. Young`s Modulus, biaxial film stress) and electrical (i.e. permittivity, leakage current mechanism) properties are investigated being of utmost importance for the design of advanced MEMS/NEMS based devices. Additionally, modified AlN thin films can act as diluted magnetic semiconductor (DMS) material. This effect will be investigated by means of magnetic force microscopy (MFM) and a new arranged measurement set-up. Ferromagnetic behaviour of AlN thin films enables the development of novel sensor concepts since ferromagnetic elements as iron and nickel are typically avoided in CMOS-related MEMS technology.

New disruptive technologies such as the Internet of Things, Industry 4.0 or the smartphone trend require reliable, low power sensors and novel energy sources to provide useful solutions that enable economic growth. There, improved piezoelectric thin films are the very basic technology that is needed to build new systems that fulfil these requirements. Within the scope of this research project a significant efficiency improvement of a common piezoelectric thin film material: Aluminum nitride (AlN), was demonstrated by adding other elements such as Scandium, Yttrium and Chromium. Preparation of piezoelectric films via this technique, scientifically called doping, is applied in two specific applications: The first one is a sensor system, intended for online monitoring liquid properties of fluids, eg. oil quality in large industrial engines. Related to Industry 4.0 an industrial company can save money and avoid equipment failure by changing oil only when the oil quality deteriorates instead of at a fixed maintenance interval. The improved, doped piezoelectric material was successfully applied for such a sensor system and two direct advantages were confirmed: An increase of the sensor level leads to a higher measurement range in terms of oil viscosity and on the other hand, the higher signal level enables operation at lower voltages, which equals to lower power consumption. The second area is Energy harvesting, where minute amounts of energy are generated from equipment vibrations, which is usually energy that is not exploited and therefore wasted. The energy harvesting system in turn can power wireless sensor notes and serves in that context as a sustainable alternative to batteries. With the doped piezoelectric thin films we were able to increase the power output hence, the fraction of vibrational energy that can be converted to useful electricity. In addition, doped AlN films are at this stage able to compete with established piezoelectric materials, for instance PZT a compound consisting of lead, zirconium, titanium and oxygen. The main advantages of AlN is that it is lead free, biologically not harmful (in contrast to lead) and compatible with established production facilities of semiconductor companies. At the start of the project only Scandium was known as a potential dopant material which is a rare earth metal and therefore expensive and scarcely available. While we have taken Scandium doped AlN films to a level where it can be applied in actual sensor systems we have taken the doping approach one step further and also introduced Yttrium and Chromium to enhance AlN. Both of these materials are found at higher volume and more are available more economical on the market compared to Scandium.