A great many applications and products today are based on thin-film technology. Innovative layer deposition and coating technologies, materials, and material combinations are often what make new applications and products feasible in the fields of optics, electronics, sensors, energy, and medical technology. The requirements placed on these layers and layer systems are wide-ranging and demanding.
For example, electrically insulating layers facilitate the functionality of ever smaller sensors and components. The smaller the physical features, the higher the demands on the coatings and the corresponding deposition technologies. Excellent insulation layer materials include aluminum oxide (Al2O3), silicon dioxide (SiO2) and silicon nitride (Si3N4), characterized by dielectric strengths of several MV/cm (megavolts per centimeter). They can be deposited by reactive magnetron sputtering at rates of 2-4 nm/s. This facilitates economical deposition of substantial layers with stand-off voltage levels of over 2000 V. Table 1 summarizes the typical insulation properties of these layers.
Thanks to their protective effects (such as scratch resistance and their ability to act as barrier and passivation layers), these coatings can be used in demanding environments, such as in chemically aggressive media, at high temperatures, under mechanical loads, and in contact with electrolytes. Pores, cracks, and defects in the layers, substrate roughness, and structured substrates present particular challenges in humid or aqueous environments, as moisture can penetrate and create conductive paths that destroy the insulating property. Suitable process control at the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP has made it possible to deposit insulation layers using a smoothing step that fills trenches and structured areas needing to be backfilled – up to an aspect ratio of approximately 1:1 (Fig. 1).
Examples of applications for these kinds of electrically insulating layers include:
- electrodes for pacemakers,
- electrodes for measuring blood glucose,
- pressure sensors in metal technology, and
- electrically insulating yet thermally conductive coatings for diode lasers and power electronics.
Piezoelectric thin films, such as zinc oxide (ZnO) and aluminum nitride (AlN), are used for a number of applications including high-frequency filters, ultrasonic transducers, pressure sensors, microfluidic actuators (Zhou, et al., 2014), biosensors (Fu, et al., 2017), and for micro energy harvesting.
Despite low piezoelectric coefficients (d33 piezoelectric charge coefficient for AlN is 6 pC/N, for example) compared to other piezoelectric materials like lead zirconate titanate, they nevertheless have various advantages. Thanks to their wurtzite crystal structure, no polarization of the layers is necessary. Accordingly, there is no depolarization over time that reduces the piezoelectric activity. In addition, they are also characterized by high acoustic wave velocity (> 6,300 m/s for ZnO and > 10,000 m/s for AlN longitudinally), low dielectric constant (ZnO 8.6 and AlN 8.5-10), and by good mechanical strength (ZnO modulus of elasticity 110-130GPa and AlN 330 GPa) (Fu, et al., 2017). Moreover, they are lead-free and can also be employed at high temperatures (AlN >1,000 °C).
Depositing Thin Films
Piezoelectric thin films are usually deposited by magnetron sputtering, though other thin-film deposition methods can also be employed. Depending on the application, layer thicknesses in the range of a few nanometers to a few micrometers can be achieved. A limiting factor here is generally the mechanical stress in the layers. Excessive or inhomogeneous film stress can lead to deformations, functional impairments, and in extreme cases to adhesion problems, cracks, or flaking. In multilayer systems (this includes the electrode layers), the non-piezoelectric layers are also usually strained, so that film stress adaption or compensation is necessary. The Fraunhofer FEP has developed a process to deposit piezoelectric AlN at up to 200 nm/min on 200 mm diameter while adjusting the film stress over a range of compressive to tensile stress (Barth S. , 2015). For example, layers of up to 50 µm were deposited on 100 mm silicon wafers. Depending on the parameter set, moderate layer stress between -230 MPa and +130 MPa could be achieved by suitable process control (Barth, et al., 2014).
An improvement in piezoelectric properties can be achieved by doping the AlN films. A large number of studies have been carried out in recent years on different materials, with scandium (Sc) doping of AlN providing the best results by far. At an Al:Sc atomic ratio of 57:43, the d33 piezoelectric coefficient was able to be increased by up to 400% to 27.6pC/N. With this, in ultrasonic applications, for example, the signal levels can be increased many times (Figs. 2 and 3), and in energy harvesting applications, the energy generated can be considerably increased for the same mechanical stress. Depending on the structure and excitation, generating several hundred µJ per impulse and several hundred µW with continuous excitation is possible (Fig. 4). This allows potentially sufficient energy to be generated for supplying low-power electronics (such as position sensors and pacemakers).
Comprehensive know-how has been built up at Fraunhofer FEP for development of coatings and processes, sputter sources, and control technology tailored to customer-specific requirements. The scientists are ready to work with industry in translating these coatings into innovative applications.
The results were partly the result of publicly funded projects:
The KMU-innovativ Nanotechnologie (NanoChance) program funded by German Federal Ministry of Education and Research (BMBF) supporting SBEs researching innovations in nanotechnology through the group project on nanofunctionalised coating systems for sensors in hydrogen technology („Nanofunktionalisierte Schichtsysteme für Sensoren in der Wasserstofftechnik“ NaFuSS)
FEP Subproject: “Nanofunktionale Isolations- und Barriereschichten für die Wasserstofftechnik“ (nanofunctional insulation and barrier layers for hydrogen technology)
BMBF promotional reference: 13N13171
Project period: Aug. 1, 2014 - July 31, 2017
Cooperative project: “Neue Wege für die Qualitätssicherung mikrotechnischer Erzeugnisse“ (GigaSonic, new paths for quality assurance of microtechnical products)
FEP Subproject: “Grundlagen für piezoaktive Dünnschichten“ (fundamentals of piezoactive thin films)
Project number: 13555/2317
Project period: June 1, 2009 - Aug. 31, 2010
Cooperative project: “Erforschung von Dünnschicht- und Abgleichtechnologien für die nanoskalige Akustoelektronik“ (DANAE, research into thin-film and alignment technologies for nanoscale acoustoelectronics)
Subtopic: “Beschichtungstechnologien für piezoelektrisch und akustisch wirksame Schichten in der Akustoelektronik“ (coating technologies for piezoelectric and acoustically effective layers in acoustoelectronics)
Saxon State Ministry of Economy, Labour and Transport, Grant number: 100206218, Period: Feb. 1, 2015 - Jan. 31, 2018
Dr. Stephan Barth1, Jan Hildisch1, Dr. Hagen Bartzsch1
1 Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, Dresden, Germany
Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP
Barth, S. (2015). Hochrate-Abscheidung von piezoelektrischen Aluminiumnitrid-Dünnschichten mittels reaktiven Magnetron Sputterns. Dresden: TUDpress.
Barth, S., Bartzsch, H., Gloess, D., Frach, P., Herzog, T., Walter, S., & Heuer, H. (2014). Sputter deposition of stress controlled piezoelectric AlN and AlScN films for ultrasonic and energy harvesting applications. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 61, no. 8, S. 1329-1334. doi:http://dx.doi.org/10.1109/TUFFC.2014.3040
Fu, Y. Q., Luo, J. K., Nguyen, N. T., Walton, A. J., Flewitt, A. J., Zu, X. T., Milne, W. (2017). Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications. Progress in Materials Science 89, S. 31–91. doi:http://dx.doi.org/10.1016/j.pmatsci.2017.04.006
Zhou, J., DeMiguel-Ramos, M., Garcia-Gancedo, L., Iborra, E., Olivares, J., Jin, H.,. Fu, Y. Q. (2014). Characterisation of aluminium nitride films and surface acoustic wave devices for microfluidic applications. Sensors and Actuators B: Chemical 202, S. 984-992. doi:http://dx.doi.org/10.1016/j.snb.2014.05.066