In an additive manufacturing process, miniature loudspeakers can be produced efficiently and cost-effectively as part of piezoelectric microelectromechanical systems - so-called piezo-MEMS - using a combination of inkjet printing and laser technology. This has been demonstrated by scientists at the Fraunhofer Institute for Laser Technology ILT, the Institute for Materials in Electrical Engineering 2 (IWE2) at RWTH Aachen University and the Fraunhofer Institute for Silicon Technology ISIT. The partners manufactured a corresponding demo component as part of the recently completed BMBF joint project “Generative Manufacturing of Efficient Piezo-MEMS for Microactuators (GENERATOR) ”.
Piezo-MEMS are true technical all-rounders because the ultra-thin piezoelectric layers fulfill actuator or sensor functions: Either they expand when an electric field is applied or they convert mechanical motion into electrical voltage. Accordingly, they are in demand in communications or medical technology, for example, like sensors or actuators in pumps, valves or loudspeakers – in each case in miniature format. The thin films are usually made of lead zirconate titanate (PZT), currently the most powerful and functional piezoelectric ceramic. Preferably, piezoelectric layers with a thickness of a few μm are used, which can be structured very precisely by etching or direct printing, for example.
Until now, conventional vacuum and mask-based manufacturing methods have been used to produce piezo-MEMS, but these methods are very time-consuming and cost-intensive, especially for the manufacture of small batches. As part of the GENERATOR project Fraunhofer ILT, together with partners developed a process combination of digital inkjet printing and laser crystallization as a favorable alternative: After PZT special ink is applied to 8” silicon wafers, crystallization follows through laser radiation at local temperatures of over 700 °C. Quality is ensured by a temperature-controlled process that limits temperature fluctuations to ± 5 °C.
A multilayer actuator with a total layer thickness of 2 to 3 µm was built out of several 20 to 30 nm thin PZT layers. Alternately, a total of up to 30 layers of functional ceramics and electrodes can be built on top of each other to form a micro loudspeaker. Thanks to this design, the actuator is said to offer better performance and higher reproduction quality than conventional actuators. The PZT layers and electrode layers interlock like two very fine combs. The fast laser processing of the layers reduces the processing time per layer, which would otherwise take minutes, down to just a few seconds. Instead of the common and very expensive platinum, the scientists use the electrically conductive ceramic lanthanum nickel oxide (LNO) as the electrode material. By dispensing with metallic components, they can significantly increase the durability of these purely ceramic multi-material stacks and reduce material costs at the same time.
Conventional systems for manufacturing thin-film electronics cost several million euros and only make financial sense for large-scale production. For smaller batch sizes, the additive hybrid process becomes attractive, especially if the component consists of several layers like the micro loudspeaker. The process is, therefore, particularly suitable for small and medium-sized enterprises (SMEs), because their investment in the system technology turns out to be significantly less than that of conventional technology.
Until now, the process has been used to coat silicon substrates. These substrates have had to undergo relatively complex post-processing after the multi-stack system has been built to produce components that are ready for use. However, the properties of the laser-based manufacturing process mean that other substrates such as ultra-thin glass could also conceivably be used, an advantage that would simplify production even further and open up a wide range of possible applications.
Contact: Fraunhofer ILT, info@ilt.fraunhofer.de
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