It has six corners and is about the size of a 1-cent piece: In an additive manufacturing process, miniature loudspeakers as part of piezoelectric, microelectromechanical systems - so-called piezo-MEMS - can be efficiently and cost-effectively manufactured using a combination of inkjet printing and laser technology. This is proven by scientists from 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. A corresponding demo component was created as part of the recently completed BMBF joint project »Generative production of efficient piezo-MEMS for micro-actuators (GENERATOR).
Piezo-MEMS are real technical all-rounders, because the wafer-thin piezoelectric layers fulfill either actuator or sensor functions: either they expand when an electrical field is applied or they convert mechanical movement into electrical voltage. They are accordingly in demand in communication or medical technology, for example as sensors or actuators in pumps, valves or loudspeakers - each in miniature format. The thin layers usually consist of lead zirconate titanate (PZT), currently the most powerful piezoelectric functional ceramic. Piezoelectric layers a few μm thick are preferably used, which can be structured very precisely, for example by etching or direct printing.
Laser-assisted printing processes as an alternative to conventional high-vacuum coating
So far, conventional vacuum- and mask-based manufacturing methods have been used in the production of piezo-MEMS, which, however, are very time-consuming and cost-intensive, especially in the production of small series. As part of the GENERATOR project funded by the Federal Ministry of Education and Research (BMBF), Fraunhofer ILT, together with Fraunhofer ISIT and IWE2 at RWTH Aachen University, developed a process combination of digital inkjet printing and laser crystallization as an inexpensive alternative: after applying PZT special ink 8 “silicon wafers are crystallized by means of laser radiation at local temperatures of over 700 ° C. Quality is ensured by a temperature-controlled process that limits temperature fluctuations to ± 5 ° C.
Trend zum Multi-Material-Stack
A multi-layer actuator with a total layer thickness of 2 to 3 µm is built up from several 20 to 30 nm thin PZT layers. "In the beginning we only applied a single layer, now a multi-material stack is created layer by layer," explains ILT scientist Samuel Fink. Alternately, up to 30 layers of functional ceramic and electrodes build up on top of each other to form a micro loudspeaker. Thanks to this construction, the actuator should have a better performance and higher playback quality than conventional actuators. PZT layers and electrode layers interlock like two very fine combs. Thanks to the fast laser processing of the layers, the processing time, which would otherwise be minutes, is reduced to 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 doing without metallic components, the durability of these purely ceramic multi-material stacks can be significantly increased and material costs can be reduced at the same time.
If you now apply an alternating voltage to this multi-material stack, the PZT layers deform in fractions of a second and thus stimulate the entire stack to vibrate. Since the entire system is only a few µm thick and therefore has a very low mass, acoustic signals can be transmitted excellently, especially in the high frequency range. "The nice thing about this production method is the digitally controllable printing and laser processes, which enable an instant design change of the layers produced without additional costs for masks or tools and thus also the production of smaller lot sizes," says Dr. Christian Vedder, head of the Thin Film Processing group at Fraunhofer ILT.
Opportunity for small and medium-sized businesses
Conventional systems for the manufacture of thin-film electronics cost several million euros and are therefore only worthwhile for large-scale production. The additive hybrid process becomes interesting for smaller batches, especially when the component, like the micro loudspeaker, consists of several layers. The process is therefore particularly suitable for small and medium-sized enterprises (SMEs), because the investment in the system technology is significantly cheaper compared to conventional technology. Fink: »For this, the user needs the appropriate printing and laser system technology as well as specially adapted PZT and LNO inks. Even very small job shops could therefore set up small series production for microactuators in the future. "
There is an easier way: glass instead of silicon
So far, the method has been used for the coating of silicon substrates. After the multi-stack system has been set up, these have to be reworked in a relatively complex manner in order to produce usable components. Due to the properties of the laser-based manufacturing process, however, other substrates such as, for example, ultra-thin glass are also conceivable, which would considerably simplify manufacture and open up a wide range of possible applications. “In the course of the project, in addition to process development, we were able to develop very exciting results on the fundamental mechanisms of laser crystallization of ceramics, which take place in the millisecond range. New possibilities are opening up here, which are of great personal interest to me and which will hopefully soon be transferred to other materials and thus areas of application.
The project on which this report is based was funded by the Federal Ministry of Education and Research under grant number 03VP02223.