AlGaN/GaN based high electron mobility transistors (HEMTs), Schottky diodes and/or resistors have been presented as sensing devices for mechanical or chemical sensors operating in extreme conditions. In addition we investigate ferroelectric thin films for integration into micro-electro-mechanical-systems (MEMS). Creation of appropriate diaphragms and/or cantilevers out of SiC is necessary for further improvement of sensing properties of such MEMS sensors. For example sensitivity of the AlGaN/GaN based MEMS pressure sensor can be modified by membrane thickness. We demonstrated that a 4H-SiC 80μm thick diaphragms can be fabricated much faster with laser ablation than by electrochemical, photochemical or reactive ion etching (RIE). We were able to verify the feasibility of this process by fabrication of micromechanical membrane structures also in bulk 3C-SiC, borosilicate glass, sapphire and Al<sub>2</sub>O<sub>3</sub> ceramic substrates by femtosecond laser (520nm) ablation. On a 350μm thick 4H-SiC substrate we produced an array of 275μm deep and 1000μm to 3000μm of diameter blind holes without damaging the 2μm AlN layer at the back side. In addition we investigated ferroelectric thin films as they can be deposited and micro-patterned by a direct UV-lithography method after the ablation process for a specific membrane design. The risk to harm or damage the function of thin films was eliminated by that means. Some defects in the ablated membranes are also affected by the polarisation of the laser light. Ripple structures oriented perpendicular to the laser polarisation promote creation of pin holes which would perforate a thin membrane. We developed an ablation technique strongly inhibiting formation of ripples and pin poles.
Optical fibres are widely used in various applications as a medium for optical signals or optical transfer. This transport can be realized on long distance, compared to free space optics, which significantly extends reach of applications. Free space optics and fibre optics are combined in practice to yield the maximum performance of individual components forming a particular system. In such cases, light coupling from free space into fibres is required and it is frequently implemented with the use of lenses. An optical signal coupled into a fibre may also need certain modifications of spectral and spatial properties to allow its propagation down the fibre or reduce the amount of power carried in. The above requirement has been fulfilled by modifying surface of facets of photonic crystal fibres. By extrusion of a certain amount of host material from the surface, it is possible to obtain a structure resembling a thin film or an opaque layer for certain wavelengths. Several different structures of photonic crystal fibres and materials are considered to show influence of such thin-film on signal properties. This investigation is carried out in context of abilities of ablation of material from surfaces of photonic crystal fibres. Only certain shapes and geometrical arrangements can be considered. One of the goals is to specify, which of them are key for potential modification of spectral characteristics of photonic crystal fibres. The printed structures could potentially work like a thin-film ablation. Rigorous and versatile finite difference method has been employed to model propagation of light, its incidence onto a surface of the photonic crystal fibre, and subsequent propagation down the fibre. The simulations are carried on small pieces of photonic crystal fibres, with the length of tens of micrometres, due to well-known demands of the simulation technique on computational resources. Nevertheless, such a simplification is valid, since the structure is longitudinally uniform beyond the thin-film layer. However, this is aspect is not covered in the presented paper and it is our ongoing effort. Finally, the goal is to verify if the investigated structures can work as a slot waveguide.
We present the design and implementation of a MEMS pressure sensor with an operation potential under harsh
conditions at high temperatures (T = 300 – 800°C). The sensor consists of a circular HEMT (C-HEMT) integrated on a
circular AlGaN/GaN membrane. In order to realize MEMS for extreme conditions using AlGaN/GaN material system,
two key issues should be solved: (a) realization of MEMS structures by etching of the substrate material and (b)
formation of metallic contacts (both ohmic and Schottky) to be able to withstand high thermal loads. In this design
concept the piezoresistive and piezoelectric effect of AlGaN/GaN heterostructure is used to sense the pressure under
static and/or dynamic conditions. The backside bulk micromachining of our SiC wafer in the first experiment started
with FS-laser ablation down to ~200 -270μm deep holes of 500μm in diameter. Because no additional intermediate layer
can stop the ablation process, the number of laser pulses has to be optimized in order to reach the required ablation
depth. 2D structural-mechanical and piezoelectric analyses were performed to verify the mechanical and piezoelectric
response of the circular membrane pressure sensor to static pressure load (in the range between 20 and 100kPa). We
suggested that suppressing the residual stress in the membrane can improve the sensor response. The parameters of the
same devices previously fabricated on bulk substrates and/or membranes were compared. The maxima of drain currents
of our C-HEMT devices on SiC exhibit more than four times higher values compared to those measured on silicon