The general objective of the present thesis is to develop and optimize the fabrica- tion techniques for the three dimensional processing of harsh compatible materials in order to produce MEMS demonstrators. These demonstrators will be studied in order to assess the material properties and create behavioral models of the devices. The
main material systems chosen for these studies are polar (grown along the c-axis) group III nitrides and nanocrystalline diamond (NCD). This selection is justified by the excellent properties of these materials, as above listed, although there is a lack of a mature MEMS technology as in opposition to other harsh environment compatible materials such as SiC.
The proposed development implies the optimization of a compatible processes for metalization and etching in order to produce the demonstrator structures. Both wet and dry etching techniques have to be investigated and optimized for the different processing steps. Using these processes, test structures will be produced for the study of the properties of the functional materials. Hence, the investigated data will allow building theoretical models which will allow the development of further functional devices using the optimized processing.
Therefore, the partial objectives of the thesis can be itemized in the following subpoints related with
1. Fabrication processes
2. Investigation of the material properties as inputs for the modeling of the demon- strators
3. The production and characterization of the demonstrators.
1.3.1
Optimization of etching processes for the three dimensional
fabrication of nitride and diamond MEMS structures
The first step addressed in this thesis is the development of etching processes compatible with the proposed materials and usual substrates. On the one hand, dry and wet techniques need to be explored in order to find a pattern transfer process to the functional material. This process has to be selective, anisotropic and induce low damage to the patterned material.
On the other hand, an etching process for the mechanical release of the functional structures has to be developed. This process can be used for the etching of the sub- strate or of a sacrificial layer. Dry and wet techniques have to be investigated in order to find the optimal process for the material combinations of substrate, functional lay- ers and metalization. This process needs to be highly selective and isotropic.
Additionally, those etching techniques can be used for the modification of the ma- terial. The good stability, durability and biocompatibility of the nitrides have also led to the development of GaN based chemical and biological gas sensors. These devices
have been developed using Schottky diodes with platinum or palladium contacts on GaN and AlGaN/GaN structures [69,70] and showed high sensitivity. However, the response may be further improved if the effective surface area is increased, allowing a more efficient accumulation of the gas induced dipole layer [71]. Thus, the pro- duction of nanoporous GaN layers will be discussed and the relation between pore morphology, etching conditions and material properties, such as defect density or doping level, will be addressed.
1.3.2
Study of the mechanical and electrical properties of mi-
cromechanised structures
The main properties that have to be assessed in the functional materials are the residual stress and Young’s modulus. In order to measure these properties test struc- tures have to be fabricated and tested using different techniques for a consistent and complete characterization. Static and dynamic loading techniques need to be used in order to compare the obtained numbers and assure that the data are consistent with the operation mode of the devices. For the residual stress characterization structural and optical measurements have to be performed, in order to correlate the data and its influence on the other material properties.
Additionally, nitrides are piezoelectric materials and their properties may be used for mechanical sensing and actuation. Polycristalline AlN can be specially used since it can be easily deposited on a wide range of substrates by the reactive sputtering technique. The layers deposited by this technique have to be investigated to optimize the deposition conditions and improve the material properties. The most relevant for this thesis are conductivity, density, residual stress, Young’s modulus and piezoelectric parameters.
1.3.3
Development and characterization of NCD and nitride res-
onators and sensors
The third and final work package that has to be developed in this thesis is the fab- rication and characterization of demonstrators. Simple micromechanised structures based in single and double clamped beams, as well as membranes, will be produced using the different material systems under study in this thesis.
The resonators will be studied measuring the resonant frequency and quality fac- tor of the various harmonics. These measurements will be performed varying the functional material properties (Young’s modulus, residual stress state) and ambient
conditions, mainly the ambient pressure. Therefore, a behavioral characterization under different conditions will be performed for both, resonance frequency and qual- ity factor.
Additionally, static actuation of piezoelectric structures will also be studied. Sin- gle and double clamped micromechanised structures based on group III-nitride and NCD materials will be analyzed for determining the optimal design parameters. Sim- ple structures based on beams and membranes will be modeled for the optimization analysis.
Finally, the acquired data will be used for assembling complete models for the design of complex MEMS structures. Various actuation principles will be taken into account and the device paramenter influence on performance will be investigated.