In the world of applications based on MEMS (Micro-ElectroMechanical Systems), the use of a piezoelectric material with direct and/or inverse effects is of real interest for sensing, actuating, energy harvesting, or Structural Health Monitoring applications [1
]. MEMS classification depends on the nature of the substrate, so ceramic, silicon, and organic MEMS are all discussed. According to the type of MEMS, specific fabrication techniques are required. In silicon MEMS, for example, either bulk or surface micromachining is used to achieve free-standing layers [2
]. These processes require numerous and quite complicated fabrication steps. For each instance of micromachining, humid, or dry etching is the final process stage leading to movable microstructures. Considering sacrificial surface silicon micromachining, the microstructure is fabricated on top of both a silicon substrate and a sacrificial thin film whose thickness determines the final gap between the substrate and the mechanical structure. In the case of bulk micromachining, the microsystem is fabricated from the silicon wafer thanks to an etching process of the exposed silicon regions. It is important to point out here that in silicon piezoelectric MEMS, the integrated piezoelectric layers are mainly thin films of AlN, ZnO, or PZT [3
]. Nevertheless, for specific applications that require a higher volume of active material, thick films are of real interest because they bridge the gap between thin films and bulk ceramics. Today, different processing routes such as 3D printing, ink-jet, sol-gel, and aerosol are considered to shape thick films with thicknesses in the range 1–100 µm [4
]. Among them, the screen-printing technology, widely used in microelectronics and more recently in flexible electronics, appears to be an attractive technique due to its simplicity, low cost, and wide range of targeted materials. For instance, silicon bulk micromachining technology associated with screen-printing Pb(ZrTi)O3
(PZT) was successfully developed for preparing micropumps, actuators, accelerometers, and energy harvesters [6
]. However, MEMS processing had to be improved and adapted to overcome issues such as the Pb diffusion towards silicon with the formation of lead silicate compounds, the incompatibilities with standard silicon wet etching, and the fragility of the mechanical silicon structure due to the pressure applied by the squeegee during printing. The use of a sacrificial layer is easy to implement in MEMS processing and has allowed one to optimize the process with a reduction in the number of fabrication steps. Its role depends on the targeted device and is thus adapted to the process. In the case of ceramic MEMS, the sacrificial layer allows one to partially remove a ceramic layer from its substrate leading to free standing structures. This method based on a carbon type sacrificial layer was introduced in the early 1980s to develop the first ceramic MEMS for pressure sensors based on screen-printed thick films [7
]. Carbon or mineral type sacrificial layers were also considered for releasing in the late 2000s for screen-printed PZT MEMS with different geometries [9
]. The use of the sacrificial layer approach was also considered at that time for manufacturing LTCC (low-temperature co-fired ceramic) ceramic MEMS [11
] to overcome the collapse of the structures or cavities during hot rolling and/or final firing. The nature of the sacrificial layer is obviously related to the type of MEMS (Si, organic, or ceramics) and to the active layer that is subsequently released [12
In this paper, LTTC ceramics MEMS that use green tape to structure the active ceramic layer and thick-film ceramic MEMS based on screen-printed layers are considered.
The first part is devoted to a survey of literature highlighting three main types of sacrificial layers whose removal during or at the end of the process will depend on their composition. Particular emphasis will then be paid to the processing by conventional sintering of PZT-based thick-film MEMS and their characteristics. It will be shown that, according to the active piezoelectric material and the required conventional thermal treatment, both the nature of the sacrificial layer and the releasing approach will have an impact on the chemistry, the microstructure, and strain/stress issues. Controlling the whole process and selecting the relevant materials are crucial for the properties of the printed piezoelectric thick-film MEMS.
In the second part of this paper, original approaches targeting bulk ceramic and thick-film ceramic MEMS produced using advanced sintering techniques associated with protective layers are introduced. While sacrificial materials have already been shown to act as a pore forming agent or as a thermal or diffusion barrier for bulk ceramics [15
], carbonate based protective materials have recently appeared to efficiently prevent PZT from chemical reduction during spark plasma sintering (SPS) [17
]. Chemistry, microstructure, and stress/strain effects are again issues strongly linked to sintering and whose control has an impact on the performance of the electroactive multilayer. An approach combining SPS and a protective layer is illustrated based on our recent investigations on PZT thick-film ceramic MEMS with an emphasis on the pros and cons of such a strategy targeting a complex MEMS design.
Finally, we conclude with the challenges and opportunities of eco-friendly approaches aiming a global improvement in processing in the field of electroactive ceramic MEMS.