Johannes G.E. Gardeniers

Characteristics of high quality ZnO thin films deposited by pulsed laser deposition

This paper show that under optimized deposition condition, films can be grown having a full width at half maximum (FWHM) value of the (002) x-ray diffraction (XRD) line a factor of 4 smaller than the previously published results using PLD and among the best reported so far by any technique. Under optimized conditions, c-axis oriented ZnO films having a FWHM value of the (002) XRD reflection line less than 15°, electrical resistivities around 5 × 10-2 Ω cm and optical transmittance higher than 85% in the visible region of the spectrum were obtained. Refractive index was around 1.98 and the Eg = 3.26 eV, values characteristic of very high quality ZnO thin films.

Micromachining of buried micro channels in silicon

A new method for the fabrication of micro structures for fluidic applications, such as channels, cavities, and connector holes in the bulk of silicon wafers, called buried channel technology (BCT), is presented in this paper. The micro structures are constructed by trench etching, coating of the sidewalls of the trench, removal of the coating at the bottom of the trench, and etching into the bulk of the silicon substrate. The structures can be sealed by deposition of a suitable layer that closes the trench. BCT is a process that can be used to fabricate complete micro channels in a single wafer with only one lithographic mask and processing on one side of the wafer, without the need for assembly and bonding. The process leaves a substrate surface with little topography, which easily allows further processing, such as the integration of electronic circuits or solid-state sensors. The essential features of the technology, as well as design rules and feasible process schemes, will be demonstrated on examples from the field of /spl mu/-fluidics.

Preferred orientation and piezoelectricity in sputtered ZnO films

Thin ZnO films were deposited on various types of substrates with rf magnetron reactive sputtering of a Zn target in pure O2 atmosphere. The layers were characterized by x-ray diffraction and electrical measurements. The piezoelectric strain constant d31 of the films was determined via optical interferometric measurements on the piezoelectrically forced vibration of silicon cantilevers. Because of the high resistivity of the ZnO samples (higher than 109 Ω cm), piezoelectric excitation down to frequencies of 100 Hz was found to be feasible. The relation between the piezoelectric strain constant and the c-axis orientation distribution of the film [obtained from the full width at half maximum of the x-ray rocking curves at the ZnO (002) diffraction] was determined both experimentally and theoretically. It was found that the experimental effective piezoelectric strain constants are, at most, 60% of the value predicted by the theoretical calculations. It is thought that this discrepancy is due to the cancellat...

The influence of nanoscale grooved substrates on osteoblast behavior and extracellular matrix deposition

To fight bone diseases characterized by poor bone quality like osteoporosis and osteoarthritis, as well as in reconstructive surgery, there is a need for a new generation of implantable biomaterials. It is envisioned that implant surfaces can be improved by mimicking the natural extracellular matrix of bone tissue, which is highly a organized nano-composite. In this study we aimed to get a better understanding of osteoblast response to nanometric grooved substrates varying in height, width and spacing. A throughput screening biochip was created using electron beam lithography. Subsequently, uniform large-scale nanogrooved substrates were created using laser interference lithography and reactive ion etching. Results showed that osteoblasts were responsive to nanopatterns down to 75 nm in width and 33nm in depth. SEM and TEM studies showed that an osteoblast-driven calcium phosphate (CaP) mineralization was observed to follow the surface pattern dimensions. Strikingly, aligned mineralization was found on even smaller nanopatterns of 50 nm in width and 17 nm in depth. A single cell based approach for real time PCR demonstrated that osteoblast-specific gene expression was increased on nanopatterns relative to a smooth control. The results indicate that nanogrooves can be a very promising tool to direct the bone response at the interface between an implant and the bone tissue.

The effect of surface roughness on direct wafer bonding

A theory is presented which describes the initial direct wafer bonding process. The effect of surface microroughness on the bondability is studied on the basis of the theory of contact and adhesion of elastic solids. An effective bonding energy, the maximum of which is the specific surface energy of adhesion, is proposed to describe the real binding energy of the bonding interface, including the influence of the wafer surface microroughness. Both the effective bonding energy and the real area of contact between rough surfaces depend on a dimensionless surface adhesion parameter, theta. Using the adhesion parameter as a measure, three kinds of wafer contact interfaces can be identified with respect to their bondability; viz. the nonbonding regime (theta > 12), the bonding regime (theta < 1), and the adherence regime (1 < theta < 12). Experimental data are in reasonable agreement with this theory

LPCVD silicon-rich silicon nitride films for applications in micromechanics, studied with statistical experimental design*

A systematic investigation of the influence of the process parameters temperature, pressure, total gas flow, and SiH2Cl2:NH3 gas flow ratio on the residual stress, the refractive index, and its nonuniformity across a wafer, the growth rate, the film thickness nonuniformity across a wafer, and the Si/N incorporation ratio of low pressure chemical vapor deposition SixNy films has been performed. As a tool for complete characterization of the property-deposition parameter relations, a full factorial experimental design was used to determine the dominant process parameters and their interactions. From this study it could be concluded that, in decreasing order of importance, the gas flow ratio of Si and N containing precursors, temperature, and pressure are the most relevant parameters determining the mechanical and optical properties of the films and the deposition rate and nonuniformity in film properties across a wafer. The established relations between properties and deposition parameters were fitted with physical–chemical models, including a film growth model based on a Freundlich adsorption isotherm. The optimal deposition conditions for films to be used in micromechanical devices will be discussed.

High-resolution liquid- and solid-state nuclear magnetic resonance of nanoliter sample volumes using microcoil detectors

The predominant means to detect nuclear magnetic resonance (NMR) is to monitor the voltage induced in a radiofrequency coil by the precessing magnetization. To address the sensitivity of NMR for mass-limited samples it is worthwhile to miniaturize this detector coil. Although making smaller coils seems a trivial step, the challenges in the design of microcoil probeheads are to get the highest possible sensitivity while maintaining high resolution and keeping the versatility to apply all known NMR experiments. This means that the coils have to be optimized for a given sample geometry, circuit losses should be avoided, susceptibility broadening due to probe materials has to be minimized, and finally the B(1)-fields generated by the rf coils should be homogeneous over the sample volume. This contribution compares three designs that have been miniaturized for NMR detection: solenoid coils, flat helical coils, and the novel stripline and microslot designs. So far most emphasis in microcoil research was in liquid-state NMR. This contribution gives an overview of the state of the art of microcoil solid-state NMR by reviewing literature data and showing the latest results in the development of static and micro magic angle spinning (microMAS) solenoid-based probeheads. Besides their mass sensitivity, microcoils can also generate tremendously high rf fields which are very useful in various solid-state NMR experiments. The benefits of the stripline geometry for studying thin films are shown. This geometry also proves to be a superior solution for microfluidic NMR implementations in terms of sensitivity and resolution.

An electrochemical microactuator: principle and first results

A novel electrochemical microactuator made with the use of silicon micromachining techniques, and its feasibility, are presented. Gas pressure is generated by electrolysis of an aqueous electrolyte solution. The pressure built up is used to change the deflection of a membrane. The actuator has three states: the electrolysis state, in which the pressure is built up; the passive state, in which the circuit is open and the pressure is maintained; and the pressure reduction state, in which the electrodes are short-circuited in order to reverse the electrolysis reaction. The advantage of this type of actuation is a relatively large pressure generation with low energy consumption. Power is required only for pressure build-up and for changing the states. Therefore, this type of actuation has promising applications in pumps or active valves.

Pulsed-laser deposited ZnO for device applications

The study investigates the growth by pulsed-laser deposition (PLD) of ZnO thin films for the eventual incorporation into piezo-electric actuators and other sensors being developed at the University of Twente. All films are purely c-axis oriented, and results are presented which suggest the production of some of the highest quality ZnO thin films yet reported. These include films with rocking curve full-width half-maxima (FWHM) down to 1.2° and (002) 2 θ peak FWHM (corrected) of 0.085°. Principally, X-ray diffraction analysis is detailed, and the shift in (002) peak position with changing deposition conditions is explored.

Growth of ZnO thin films on GaAs by pulsed laser deposition

ZnO thin films have been grown on GaAs substrates using the pulsed laser deposition technique with or without a photodeposited SiO2 buffer layer. The presence of the SiO2 layer has a beneficial effect on the crystalline quality of the grown ZnO films. Highly c-axis oriented ZnO films having a full width at half maximum value of the (002) X-ray diffraction line of less than 0.13 ° have been grown on such buffer layers at a substrate temperature of only 350 °C.