Johann Zehetner

AlGaN/GaN diaphragm-based pressure sensor with direct high performance piezoelectric transduction mechanism

The piezoelectric response of AlGaN/GaN circular HEMT pressure sensing device integrated on AlGaN/GaN diaphragm was experimentally investigated and supported by the finite element method modeling. The 4.2 μm thick diaphragm with 1500 μm diameter was loaded by the dynamic peak-to-peak pressure up to 36 kPa at various frequencies. The piezoelectric charge induced on two Schottky gate electrodes of different areas was measured. The frequency independent maximal sensitivity 4.4 pC/kPa of the piezoelectric pressure sensor proposed in a concept of micro-electro-mechanical system was obtained on the gate electrode with larger area. The measurement revealed a linear high performance piezoelectric response in the examined dynamic pressure range.

MEMS pressure sensor fabricated by advanced bulk micromachining techniques

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 substrates.

Laser ablation: A supporting technique to micromachining of SiC

We present an effective fabrication method of AlGaN/GaN membrane on SiC for MEMS sensors applications. It employs laser ablation as a supporting technique to the plasma enhanced etching methods. Circular patterns transferred deeply into bulk SiC substrates fabricated by ablation using (1) excimer laser and (2) femtosecond (fs) laser tools were compared. We found that the fs laser tool is more suitable for bulk micromachining of SiC because of the clearness of the process. The additional higher thermal load can be also suppressed. A simple laser cleaning procedure was found allowing us to fabricate deep structures without the ablation process retardation by debris formation.

Nanostructuring by femtosecond laser ablation and RIE for MEMS and microfluidic systems fabrication

AlGaN/GaN based sensors should be integrated into micro-electro-mechanical-systems (MEMS) and microfluidic devices used in biotechnology. The creation of appropriate diaphragms, surface structures or a combination of them is important for the fabrication of devices required in biotechnology and interdisciplinary research. Laser ablation as a direct mask writing procedure and deep reactive ion etching were used for nanostructuring of silicon nanopillars from the opposite site of the sensor device.

Investigation of optical thin films printed on the surface of facets of photonic crystal fibers

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.

Using of laser ablation technique in the processing technology of GaN/SiC based MEMS for extreme conditions

In order to improve the stability, sensitivity or efficiency of AlGaN/GaN based sensors employing high electron mobility transistors (HEMTs), Schottky diodes and/or resistors they should be integrated into micro-electro-mechanical-systems (MEMS). The creation of appropriate diaphragms and/or cantilevers is necessary for the verification of sensing properties of such MEMS sensors. In this paper, we present possible approaches to improve the fabrication of micromechanic structures in bulk SiC substrates with epitaxial AlGaN/GaN heterostructures using femtosecond laser ablation to fabricate SiC diaphragms. The objective of this work is also to point at the backside damaging effects and to find an optimal method for its elimination or suppression.

Investigation of optical stability issues with embedded semiconductor quantum dots used as colour conversion material in LED lighting applications

In order to further improve the efficiency and the spectral quality of white-light LEDs new colour conversion materials which allow a selective and narrow banded conversion of the blue excitation light are needed. Quantum dots could be promising in this regard as they exhibit exactly these properties. For commercially used light sources a stable colour distribution has to be ensured over the entire lifetime. Our experiments revealed a very different degradation characteristic of phosphor and quantum dots. In this paper we present the results of different experiments which aim on the understanding of the mechanism for optical stability. We focused mainly on two degradation effects, a decrease of the photoluminescence intensity and a shift of the emission peak wavelength towards the blue.

Systematic study of the roughness of biodegradable polymer surfaces processed by ultrashort laser pulses

Polymer-based materials are increasingly common in medical fields, ranging from simple disposable products to complex implants. Polymers have a large variety of physical and chemical properties, fit for use in different medical applications; in particular, biodegradable polymers are interesting for the use as temporary implants - for example in the case of stents, where, after the support phase of a hollow organ the stent can be decomposed by the human body. Roughness of the polymer surfaces is an important parameter for applications. For example, it influences strongly the clinical outcome of stents treatments [1]; as another example, an appropriate surface quality provides a good breeding ground for cell growth [2]. Therefore, precise control over this parameter is desirable. Surface roughness achieved using ultrashort-pulse laser machining depends on processing conditions. The goal of this paper is a systematic investigation of the surface roughness of laser-ablated surfaces as a function of laser processing parameters. To this purpose we machined an array of squares on a sheet of poly(Lactic acid) (PLA) using laser pulses of 350 fs duration at the wavelength of 518 nm. We varied systematically across the array the pulse-to-pulse translation distance, the repetition frequency and the fluence of the laser pulses. We measured the topography of the machined surfaces with a scanning confocal microscope; from the measured surface topography we calculated the standard roughness parameters (arithmetic average) and (root mean square average) [3], obtaining in this way a map of roughness as a function of processing parameters. This map can be used to select appropriate processing parameters for machining surfaces with desired roughness characteristics.Polymer-based materials are increasingly common in medical fields, ranging from simple disposable products to complex implants. Polymers have a large variety of physical and chemical properties, fit for use in different medical applications; in particular, biodegradable polymers are interesting for the use as temporary implants - for example in the case of stents, where, after the support phase of a hollow organ the stent can be decomposed by the human body. Roughness of the polymer surfaces is an important parameter for applications. For example, it influences strongly the clinical outcome of stents treatments [1]; as another example, an appropriate surface quality provides a good breeding ground for cell growth [2]. Therefore, precise control over this parameter is desirable. Surface roughness achieved using ultrashort-pulse laser machining depends on processing conditions. The goal of this paper is a systematic investigation of the surface roughness of laser-ablated surfaces as a function of laser proc...