Manufacturing processes Deposition of piezoelectric films for energy harvesting

Autor / Redakteur: Stephan Barth and Hagen Bartzsch* / Dipl.-Ing. (FH) Hendrik Härter

Autonomous sensors and smaller mobile electronics systems will become more important in the future of the Internet of Things. We introduce more efficient materials and production processes.

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Future of Internet of Things: Autonomous sensors and smaller mobile electronics systems get the power by energy harvesting.
Future of Internet of Things: Autonomous sensors and smaller mobile electronics systems get the power by energy harvesting.
(Bild: © iconimage - Fotolia)

The trend towards energy autonomous sensors and ever smaller mobile electronics systems is continuing and will further gain importance in the future (“Internet of Things”). Autonomous sensors can be used, for example, to monitor the status of the components under high stresses, are used as pressure sensors in tires, or for medical implants. The energy supply of such systems by means of batteries or cables is often too complicated or complex. One solution provides the on-site energy generation from the environment, e.g. utilizing light (solar cells), temperature differences (thermoelectric harvesters) or mechanical vibrations (piezoelectric harvesters).

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This allows utilization in areas that were previously off-limits due to various constraints. Such constraints may be the weight/volume of the sensor system. Other can be the position of the sensor at difficult to access location, e.g. near hazardous materials or inside a structure or machine, where energy supply by cable or battery – that needs to be changed at regular intervals - is not feasible. Piezoelectric materials can convert mechanical vibrations into electric energy because the effect of mechanical force results in a charge separation.

Up to now, the piezoelectric material of choice has mainly been lead zirconate titanate (PZT). Aluminum nitride (AlN) is another option. While AlN has a significantly lower piezoelectric coefficient d33 compared to PZT, this is (at least partially) compensated by a considerably lower dielectric constant and more advantageous mechanical characteristics.

Other advantages compared to PZT are: lead-free (according to EC regulations), higher stability, is biocompatibility and the deposition of AIN is compatible with common microelectronic processes. In recent years Japanese researchers found the doping of AlN with Scandium results in a significant increase of the piezoelectric coefficient. This is also relevant for application in energy harvesting.

New process for manufacturing piezoelectric coatings

At Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology (FEP), AlN layers were deposited by reactive magnetron sputtering of aluminum targets in an argon/nitrogen atmosphere. This process enables the deposition of AlN layers homogeneously onto a large coating surface of up to 200 mm diameter with deposition rates of up to 200 nm/min. The process is applicable for economical manufacturing of piezoelectric AlN films. An upscaling to even larger substrates sizes, e.g. 500 mm x 500 mm is feasible. The piezoelectric coefficient d33, depicting the charge generation per applied strain in direction of polar axis, has reached values of up to 7 pC/N.

The typical values described in available research literature for d33 of AlN range between 5 to 7 pC/N. At the same time, the mechanical stress of the layers can be adjusted in a wide range between compressive and tensile according to the needs of the application. The stress in the films influences for example the adhesion strength of the coating, the electromechanical coupling and the values of the energy produced. Sc-doped AlN films were deposited by co-sputtering of Al and Sc targets in an argon/nitrogen atmosphere.

By changing the ratio of the applied power between the sputter targets, the film composition can be adjusted. Depending on the composition and process parameters, an increase of d33 by up to 300 percent compared to AlN could be achieved by FEP process (Image 1). This corresponds well to the maximum published values by other groups, although at significantly higher deposition rates and larger deposition area.

Coatings were done on silicon cantilever

Thus, the process of FEP has the potential for highly productive manufacturing of Sc-doped AlN films. Working in collaboration with the Technical University of Dresden and Oulu University in Finland, tests on energy harvesting with AlN coatings of up to 50 µm were performed. The coatings were done on silicon cantilever of the size approx. 70 mm x 5 mm. These served as standard measurement samples, to characterize the film material. The measurement setup consists of an electromagnetic shaker system for the generation of defined mechanical vibrations regarding frequency and displacement amplitude and an electrical measurement circuit (Image 2).

The shaker system simulates the vibration environment of the harvester. The coated silicon cantilever is stimulated to oscillate and is electrically connected with the measurement circuit. In the most basic form, an AC measurement circuit with purely resistive load is used. With that, the measured powers at resonance of the cantilever for pure AlN with thickness 10 µm reached 70 µW at vibration amplitudes at the basis of ±2.5 µm. The usage of Sc-doped AlN resulted in a significant increase of generated power (5 times higher for Al60Sc40N compared to AlN), confirming the expectations from piezoelectric film characterization (Table).

Supply autonomous sensors with energy

However, to supply autonomous sensors with energy, the harvesters need to be able to charge a battery or capacitor: a DC output is required. To realize this, an AC/DC- or a DC/DC-converter has to be included. In the most basic form, a diode rectifier bridge can be used. However, due to the inevitable electric losses, the power output at the end is lower than for the pure AC circuit. An improved circuit based on non-linear processing of the voltage according to the SSHI-principle (Synchronised Switch Harvesting on Inductor) can remedy this (Image 3). In the SSHI-circuit, an inductor and an electronic switch are connected in series with the piezoelectric element and the rectifier input (Series-SSHI).

The switch is triggered at the maximum value of displacement of the Si cantilever. In that moment, the switch closes and a part of the energy stored in the capacitor of the piezoelement is transferred to the filter capacitor. A voltage inversion is occurring. This setup significantly improves the energy conversion efficiency, thereby increasing the harvested power.

Support several industrial partners

A DC power of 380 µW was measured for a 60 mm x 10 mm silicon cantilever coated with 10 µm Al60Sc40N at resonance (1.17 kHz, base vibration amplitude ±2.5 µm). This is an almost two-times increase compared to the basic diode rectifier, which resulted at same conditions in a generated power slightly above 200 µW (Image 4). The scientists now are prepared to support industrial partners across the entire scale of product development, starting from feasibility studies under laboratory conditions up to integrated packages for the deposition of AlN and AlScN films, consisting of key components, fully automatic process and control systems as well as technology.

* Dr. Stephan Barth and Dr. Hagen Bartzsch are collaborate at Fraunhofer FEP in Dresden, Germany.