New corn-based sacrificial layer for MEMS based on screen-printed PZT ceramics
Introduction
Printed MEMS ceramics attract more and more attentions, combining the ease of fabrication with the unique performances of ceramics in dielectric, piezoelectric and thermally-stable parts. A large proportion of these MEMS devices involves the use of sacrificial layers in order to achieve free-standing parts [5], [11], [15], [20], [27] or to generate pores [1], [18], [26]. Depending on the application and especially on the materials involved, several constraints, such as the maximum process temperature allowed or the use of specific solvents, condition the use of a certain sacrificial layer. Table 1 compares the deposition and the removal processes of different sacrificial layers with those of the literature, with an emphasis on processes involving screen-printed MEMS. For these ceramic-based MEMS, often requiring high temperature or annealing steps [5], [17], [26], [28], two types of removal processes emerge: chemical-based and thermal.
We propose here a simple formulation for a thermally-decomposed sacrificial layer made of a mixture of industrial corn starch and commercial screen-printing epoxy paste. The paste is used to make PbZr0.52Ti0.48O3 (PZT) discs and microcantilevers transducers, demonstrating high densification and compatibility with the other commercial screen-printing pastes used.
Section snippets
Materials and methods
The paste is a composite made of industrial corn starch (36 wt%) and an epoxy-based organic vehicle (64 wt%) (ElectroScience Laboratory, ESLCV59). The corn starch is first mixed with a mortar and a pestle and then homogenised in a 3-roll mill (EXAKT 80, EXAKT Technologies). The fabrication of this paste is similar to a previous work where a composite paste made of SrCO3 and epoxy was used [15].
The resulting paste is successfully screen-printed on an alumina substrate (96 wt% Al2O3) using a DEK
Sacrificial layer characteristics
The thickness of the deposits for the polyester and the corn starch-based pastes are respectively 40 μm and 90 μm after curing. This is due to the different solvent concentrations and nature of the charges contained in both inks. While the same screen-printer settings were kept, the polyester-based ink contains more solvents, resulting in a thinner deposit after drying. Roughness profiles are measured using an ALTISURF 500 profilometer. The corn starch and polyester sacrificial layers show an
Conclusion
This work demonstrated the use of an affordable, thermally decomposable and screen-printable sacrificial layer paste based on corn starch. PZT ceramics were successfully released and a very low porosity was obtained (2.2%) using this paste in fully screen-printed multilayers. We believe that improvements could be made by milling the corn grains and an optimising the organic vehicle proportion for the inking, making such an ink valuable for ceramic-based printed MEMS.
Authors’ contributions
Simon Grall: Investigation, Data curation, Visualization, Formal analysis, Writing – Original draft, Writing – Review & editing, Conceptualization.
Onuma Santawitee: Investigation, Formal analysis.
Isabelle Dufour: Writing – Review & editing, Supervision, Validation, Methodology, Conceptualization.
Vincent Aubry: Supervision.
Hélène Debéda: Writing – Review & editing, Supervision, Validation, Methodology, Conceptualization.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Simon Grall majored in Chemistry and Physical Engineering in ENSCBP (Bordeaux, France) with a specialization in Nanomaterials. He obtained his engineering degree in 2015 and started an industrial thesis in partnership with the automotive manufacturer PSA in 2016 focusing on screen-printed sensors for environment and piezoelectric ceramics.
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Simon Grall majored in Chemistry and Physical Engineering in ENSCBP (Bordeaux, France) with a specialization in Nanomaterials. He obtained his engineering degree in 2015 and started an industrial thesis in partnership with the automotive manufacturer PSA in 2016 focusing on screen-printed sensors for environment and piezoelectric ceramics.
Onuma Santawitee graduated from Silpakorn University with a bachelor degree of Petrochemicals and Polymeric Materials in 2002 and from the Graduate School, Silpakorn University with a master degree of Polymer Science and Engineering in 2006. She worked as a technician of Physical Characterization Laboratory at National Metal and Materials Technology Centre (MTEC), Thailand from 2005 to 2016. Currently, she has been granted a full scholarship from The Royal Thai Government to undertake a PhD study at the Doctoral School, University of Bordeaux. Her thesis concerns developments of screen-printed piezo-ceramic transducers with sensitive coating for volatile organic compounds (VOCs) detection.
Isabelle Dufour graduated from the École Normale Supérieure de Cachan, Cachan, France, in 1990, and earned the Ph.D. and H.D.R. degrees in engineering science from the University of Paris-Sud, Orsay, France, in 1993 and 2000, respectively. She was a CNRS Research Fellow from 1994 to 2007, first in Cachan, working on the modelling of electrostatic actuators (micromotors and micropumps) and then, after 2000, in Bordeaux, working on microcantilever-based chemical sensors. She is currently a Professor of Electrical Engineering with the University of Bordeaux, Bordeaux, France. Her research interests are in the areas of microcantilever-based sensors for chemical detection, rheological measurements, material characterization and energy harvesting.
Vincent Aubry is R&D engineer in Electronics and Microelectronics devices. He is currently in charge of air quality sensors at the Scientific and Future Technologies Department of PSA Groupe.
Dr Hélène Debéda is engineer in Physics of materials and received the PhD degree from Bordeaux University (UB) in 1996 for her work on thick-film gas sensors. From 1996-1998 she obtained a European post-doctorate grant at the IMT Laboratory of Karlsruhe, Germany, to work on LIGA piezoactuators. In 2013, she received the HDR degree in electronics from the UB. She is currently associate professor at University of Bordeaux-IMS Laboratory. Up to now, she was more involved on alternative technologies to silicon ones applied to sensors or MEMS. Today, she principally studies piezoelectric devices for gas or dust detection and energy harvesting using screen-printed thick films of inorganic and composite layers.