This volume has been structured following three different parts. The first block is an introduction to the research topic, the pursued objectives, the methodology and the employed resources and techniques are also explained. As mentioned before the experimental techniques (for fabrication, characterization and modeling) used in this thesis are described in chapter 2.
The second part of the dissertation is devoted to the presentation of the results re- lated to the fabrication technology that has been specifically optimized or developed for the fabrication of the devices presented in this dissertation. It is composed of:
• Chapter 3. Dry etching of functional materials. The dry etching optimiza-
tion for several functional materials is described in this chapter. The etching of III-N materials that can serve as functional material or sacrificial layer is ad- dressed here. A section devoted to the NCD dry etching optimization is also included. In this section etch rate, etched surface morphology and selectivity among materials of the optimized dry etching processing is analyzed.
• Chapter 4. Patterning MEMS structures by sacrificial layer etching. In
this chapter several approaches for producing free-standing III-N and NCD microstructures are proposed and deeply studied. The processing techniques based on sacrificial etching are presented and explained. The processes pre- sented in this chapter were used to produce the microstructures used for this thesis.
• Chapter 5. AlN deposition by pulsed DC reactive sputtering. The results of
the deposition of thin layers of AlN by RF reactive sputtering on different sub- strates are here described. Additionaly, the conditions leading to the deposition of thin AlN layers with piezoelectric response are discussed.
• Chapter 6. Nanoporous Gallium Nitride. The use of a specially developed
etching technique of GaN for the production of nanoporous (nP ) layers is pre- sented in this chapter. Preliminary results on material properties suggesting that nP − GaN could be a promising material for enhanced chemical sensors are also discussed.
The third part is devoted to present how the technology optimized with the afore- mentioned researched has been applied to the design and characterization of several demonstrators.
• Chapter 7. III-N MEMS structures. This chapter is devoted to discussing the
results obtained from the mechanical characterization of various III-N MEMS structures. The elastic modulus and strain relaxation effects of free-standing devices is here determined for various III-N materials.
• Chapter 8. Nanocrystalline diamond resonators. In this chapter the results
for statical and dynamical characterization of NCD MEMS, mainly focused on resonators, are presented. The research of ambient and geometric effects on the response of the MEMS is thoroughly discussed.
• Chapter 9. MEMS design and simulation. Analysis, design and theoreti-
cal results on various structures with diverse actuation principles (piezoelectric and electrostatic) are presented in this chapter. Optimal design parameters are determined and used for the modeling of beam and membrane based devices. Finally, The dissertation concludes with two additional chapters devoted to the general conclusions and to the various researched topics which can be followed after the work presented in this volume.
1.5.1
Notes about conventions
Several conventions have been assumed in this text and are here stated.
When naming heterostructures the substrate will be omitted unless otherwise stated. The substrate will of course be mentioned when describing the wafers used at the experiments but when presenting the results it will be omitted to ease the reading.
Additionally, when naming heterostructures two conventions exist, the material stack may be named starting by the material at the top of the wafer or at the inter- face with the substrate. Device engineers prefer the former whereas growers tend to use the later convention. Since this dissertation is about fabrication technology and devices the reader may eventually become confused. In order to avoid misinterpreta- tion, throughout the entire text the top-bottom convention will be used. For instance, if the reader finds the heterostructure GaN/AlN in this text it has to be interpreted as the GaN being on top of the wafer and the AlN between the GaN and the growth substrate.
The punctuation at the formulas is omitted and considered to be implicit. Al- though many texts and schools follow the convention of punctuating the equations, the commas, stops, colons and semicolons at the end of the equations will be consid- ered implicit in this dissertation in the sake of clarity.
The list of the acronims used in this document and the meaning of the most common symbols may be found at appendix C.
Design, fabrication and experimental
techniques
In this chapter the fabrication, characterization and analysis techniques used in this thesis are described. The objective of this chapter is to provide the necessary background about the principles of the techniques so it can be used as a reference for a better understanding of the discussed results in the following chapters. The common techniques are just mentioned and the used equipment is listed. On the other hand, the special techniques used in this thesis are thoroughly described and the principles for understanding the technique, either fabrication or measurement acquisition, are presented.
2.1
Simulation tools
In this section the scientific software used for simulating the devices and analyzing the measured data are listed and explained. The software explanation is intended to be illustrative rather than exhaustive. For deeper details of the software packages refer to the vendor or project sites, where in depth documentation is available.
2.1.1
ANSYS finite element method SW
Complex structures with 3D geometry and various materials have been simulated and analyzed by means of a commercial software package, ANSYS Multiphysics. This package provides a user friendly interface for defining and running 3D finite element analysis for coupled mechanical, thermal, electrical and fluid problems. The soft- ware suite provides an analyzer for extracting data for the simulated geometries and
has a scripting language which has been used for running batch simulations with parametrization.
ANSYS software allows performing static, dynamic, modal and transient analy-
sis. It incorporates a wide library of elements that can be used for defining coupled analysis (combining several domains such as thermal, electrical and mechanical) or even non linear simulations. The preprocessor allows the use of materials with user defined properties, where piezoelectricity or anisotropic elasticity can be used for properly modeling the materials used for this dissertation. The solver is optimized and although it is difficult to adjust an learn how to use correctly it allows a better control and suppression of simulation artifacts than scripts written from scratch.
In this thesis ANSYS Classic suite was used, although some simulations have been easily ported to ANSYS Workbench providing the same results. For more information the reader is invited to visit the ANSYS website where documentation on the up to date features of the ANSYS suite.
Finite element method
Finite element method (FEM) is a numerical technique for finding approximate solutions to boundary value problems for partial differential equations. It uses subdi- vision of a whole domain into smaller parts, called elements, and a special formula- tion of the equation, the weak formulation, for solve the problem over the subdomain using error minimization techniques. If the discretization function is also used as the weighting function for the error equation the method is said to use the Galerkin method, which is the most common approach in FEM.
The advantages of subdivision into several subdomains are: • Accurate representation of complex 2D or 3D geometry • Inclusion of dissimilar material properties
• Easy assembly of complete solution
• Capture of local effects by using finer meshes in the areas of interest
The application of FEM to the analysis of engineering problems and for simulat- ing complex structures as an aid for data interpretation is usually referred as Finite Element Analysis (FEA).
The description of the method is a very wide topic and providing a detailed ex- planation is completely out of the scope of this text. A huge amount of literature
about the method is available. The interested reader may refer to the excellent text by Claes Johnson [72] for introduction. A detailed derivation of the method is avail- able at the classical texts by Bathe [73] or Zienkiewicz and Taylor [74]. A more modern approach is provided by Ern and Guermond [75] or Brenner and Scott [76]. MEMS specific FEM analysis review is included in the book by Beeby, Ensell, Kraft and White [77]. For a general theory description of resonant MEMS with a chapter devoted to numerical analysis is given by the recent book by Brand, Dufour, Henry and Josse [78].
2.1.2
Other software
Other software was used for fitting and analyzing data. Mainly Octave was used as a versatile scripting language for performing mathematical calculations, perform- ing simple simulations, plotting and analyzing data. Other scripts for data analysis automation were written using Python with the numpy, scipy and matplotlib pack- ages. For analytical analysis wxMaxima graphical interface to the Maxima Computer
Algebra System was used. Yet another tool for data plotting and curve fitting was
used, Origin Pro. Lastly, the HRXRD diffractograms were analyzed using an ad hoc simulator, as later described at 2.3.1.
Several masks were produced for fabricating the devices used in this thesis. The masks were designed using the AutoCAD software suite.