Piezoelectric material lead-zirconate-titanate (PZT) is the heart of the smart-MEMPHIS project. During the first year-and-a-half, MEMS foundry Silex has reached world-class figure of merit results with a repeatable and improved PZT process.
During the first year and a half, one task in the focus of the project coordinator Silex has been the development of beyond state of the art piezoelectric materials. Piezoelectric materials respond to mechanical stress by producing an electric field - or vice versa, contracting when a voltage is applied. Traditionally piezoelectric materials have been used for instance in ultrasound imaging, where they and detect the reflected ultrasonic waves.
In MEMS applications in particular, thin-film (< 0.5-10 µm) piezomaterials in particular have recently attracted more attention. This is because they offer more sensitivity and in actuating applications require lower power levels. They also hold promise for more cost-effective solutions. PZT and aluminium nitride are the two most common thin-film piezo materials. The challenge that also smart-MEMPHIS answers has been to mass-produce high quality piezoelectric thin films repeatably and reliably.
In smart-MEMPHIS, PZT was chosen for its high electromechanical coupling factor e31, which describes how effectively the piezoelectric material converts mechanical energy to electrical energy (and vice versa). In energy harvesting, the higher the electromechanical coupling, the better.
High-efficiency, thick and crack-free PZT
Silex has been able to improve the electromechanical coupling factor e31 to world-class figures better than -19 C/m2. Crystallographic properties of the PZT layer - the crystal orientation - affect the coupling factor dramatically. PZT with a <100> crystal orientation (think looking at a cube of atoms straight ahead) has a 20 % higher coupling factor than <111>-oriented PZT (looking at a cube from one of the corners). The objective of Silex has then been to grow <100>-oriented PZT on top of silicon, on which PZT typically grows in the <111> direction - it has previously been a challenge to create arbitrarily oriented PZT films that do not crack under mechanical stress. Additional goal in a foundry environment is to do this repeatably and quickly.
This has been achieved through using TiO2/Pt/Metal oxide stack as the bottom electrode of the PZT structure. The key is to deposit optimal quality <111>-oriented Pt with the best crystalline structure possible, on which a high-quality <100> metal oxide layer can be deposited. How defect free the Pt-layer forms depends in turn on the quality of the TiO2 layer, which is needed both for adhesion and to prevent Pb from diffusing from the PZT to the silicon substrate. After 1,5 years of process development to improve the quality of both the TiO2, the Pt and the metal oxide, Silex has succeeded in the optimization of this entire stack magnificently.
Another factor in the improvements of the e31 coefficient has been doping of PZT. Enhancing the piezoelectric properties of PZT with various dopants has led to another 20 % increase in the e31 coefficient at Silex. With rigorous process development in smart-MEMPHIS, Silex can now grow thick layers of PZT that have excellent qualities for energy harvesting and that do not crack under physical stress. Right now extensive HAST and long time reliability and fatigue testing are on-going at aixACCT with initially very promising results.
Smart-MEMPHIS benefits all piezo MEMS
Silex contributions to PZT development benefit not only energy harvesting, but other future MEMS as well. Piezoelectric materials hold a lot of promise for producing smaller MEMS actuators with better performance and lower power consuption. For instance, EPSON released PZT-based MEMS printer heads already in 2013, and the start-up USound is producing piezo-MEMS loudspeakers. In sensors, such as microphones, accelerometers and gyroscopes, piezomaterials offer better sensitivity, lower voltages and much lower power consumption while being smaller. In actuators, the benefit of piezoelectric materials is their almost immediate response as well as lack of moving parts, which makes piezoelectric actuators very durable. By developing better processes and materials, smart-MEMPHIS contributes to the development of the neverending variety of emerging MEMS products.
Thorbjörn Ebefors, Silex
Samira Nik, Silex
Anna Mannila, Spinverse