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Introduction to piezo MEMS printing This article covers recent progress on the fabrication of epitaxial piezoelectric thin films. The crystalline quality of these epitaxial thin films is such that optimal piezoelectric properties with long-term stability can be obtained. This is important for many piezoelectric applications, such as piezoelectric inkjet printing. Inkjet print heads represented 15% of a US$10 billion microelectromechanical systems (MEMS) market in 2011.1 While thermal print heads, mostly for small office/home office (SOHO), have currently the major share of the MEMS inkjet market because they enable complementary metal oxide semiconductor compatible manufacturing, piezoelectric print heads are an emerging product in the professional printing arena. When comparing the two printing (droplet generation, see Figure 1) principles—thermal versus piezoelectric—the major advantage of the former, which is often decisive for the final product, is an extremely low price-per-nozzle combined with very high integration density (high dot-per-inch resolution). This makes thermal inkjet (TIJ) devices with throwaway print heads very attractive for SOHO and the low-end professional printing markets. However, thermal inkjets impose very stringent demands on the ink due to the fundamental limitation of the droplet formation process, which requires explosive liquid evaporation to create pressure in the print head for ink ejection.

Therefore, TIJ print heads can only operate with water-based inks with a rather limited viscosity and temperature range. In contrast, the piezoelectric inkjet (PIJ) (see Figure 1b), which utilizes pure mechanical pressure created by the piezoelectric element, though more expensive, is able to jet a wide range of fluids over an extended temperature region. This universality makes PIJ an attractive tool not only for the high-end professional market, but also paves the way for industrial printing (e.g., electronics, 3D prototyping, bio- and medical printing) (see Figure 2). Additionally, PIJ print heads are more reliable and robust due to the possibility of early nozzle failure detection by using the piezoelectric actuator as a sensor while idle.2,3 As mentioned previously, the disadvantage of PIJ is its high cost price. Until recently, PIJ print heads were manufactured using bulk Pb(Zr,Ti)O3 (PZT) ceramics as the piezoelectric material in conjunction with micromachining. Scaling up of the thin PZT film deposition process to a 150–200 mm wafer diameter in combination with MEMS processing is a technology enabler for piezo-MEMS print heads with a drastically reduced price. As of today, two companies have introduced printing solutions containing piezo MEMS print heads. Additionally, several companies are considering the application of piezoMEMS technology for manufacturing piezo-MEMS based print heads for the future.

Hiroshi Funakubo, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology; [email protected] Matthijn Dekkers, SolMateS, The Netherlands Alessia S