Abstract
Piezoelectric thick films are of real interest for devices such as ceramic Micro-ElectroMechanical Systems (MEMS) because they bridge the gap between thin films and bulk ceramics. The basic design of MEMS includes electrodes, a functional material, and a substrate, and efforts are currently focused on simplified processes. In this respect, screen-printing combined with a sacrificial layer approach is attractive due to its low cost and the wide range of targeted materials. Both the role and the nature of the sacrificial layer, usually a carbon or mineral type, depend on the process and the final device. First, a sacrificial layer method dedicated to screen-printed thick-film ceramic and LTCC MEMS is presented. Second, the recent processing of piezoelectric thick-film ceramic MEMS using spark plasma sintering combined with a protective layer approach is introduced. Whatever the approach, the focus is on the interdependent effects of the microstructure, chemistry, and strain/stress, which need to be controlled to ensure reliable and performant properties of the multilayer electroceramics. Here the goal is to highlight the benefits and the large perspectives of using sacrificial/protective layers, with an emphasis on the pros and cons of such a strategy when targeting a complex piezoelectric MEMS design.
Highlights
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]
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 [7,8]
Three main types of sacrificial layer compositions can be identified from a survey of literature focused on thick films and LTTC ceramic MEMS: A glass- or metal-based composition that will be chemically dissolved in a solution of strong acid or base at the end of the process, as in silicon technology; a composite or all-organic composition based on carbon, corn starch, or polyester that is consumed during sintering and bypasses the problem of removing the sacrificial layer at the end of the process; and a mineral composition based on carbonate, presenting good compatibility with most of the structural layers and removable at the end of the process with a weak acid
Summary
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]. 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. 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 [7,8]. 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.
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