J. Elders

Stiction of surface micromachined structures after rinsing and drying : model and investigation of adhesion mechanisms

The mechanisms causing stiction of polysilicon structures fabricated by surface micromachining techniques have been investigated. It is found that during drying from rinse liquids attractive dynamic capillary forces are responsible for bringing micromechanical structures into contact with the underlying substrate. Measured adhesion energies of sticking microbridges indicate that van der Waals forces are responsible for the stiction of hydrophobic surfaces and that hydrogen bridging is an additional adhesion mechanism for hydrophilic surfaces. Methods to reduce the stiction problem are indicated.

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.

Low temperature growth of highly transparent c-axis oriented ZnO thin films by pulsed laser deposition

The effects of the oxygen partial pressure, substrate temperature and laser wavelength on the structural and optical properties of thin films of ZnO grown on silicon and glass substrates by pulsed laser deposition have been studied. Regardless of thickness, all the grown layers are c-axis oriented and optically transparent. At substrate temperatures as low as 300°C, featureless layers with a FWHM value for the (002) XRD reflection less than 0.18° and exhibiting an optical transmission higher than 80% in the visible region were produced. For otherwise identical deposition conditions, the KrF excimer laser (at 248 nm) was always found to produce better quality thin films than the frequency-doubled Nd: YAG laser (532 nm). This is explained by the large difference between the optical absorption coefficients of ZnO at the two wavelengths employed, which play a keu role in the laser-target interaction. SEM investigation of the target surface after deposition revealed very different surface morphologies for the two wavelengths employed supporting this assumption.

Applications of fluorocarbon polymers in micromechanics and micromachining

Several thin-film deposition and etching techniques of the polymer fluorocarbon are investigated and the resulting thin-film properties will be compared with those of commercially available bulk polytetrafluoroethylene. The most promising deposition technique is performed in a conventional reactive ion etcher using a carbonhydrotrifluoride (CHF3) plasma. By changing the deposition parameters, control of the properties and step coverage of the deposited thin films within a certain range is possible, eg., uni-directional and conformal step coverage of deposited thin films can be obtained. Etching is performed with the help of an evaporated aluminium oxide mask using an oxygen, nitrogen, or sulfurhexafluoride plasma for isotropic etching, or a CHF3 plasma giving a directional etch profile. The combination of the unique properties, deposition and etching techniques make fluorocarbon thin films a promising tool for micromachining; a number of applications will be discussed and demonstrated.

EFFECTS OF LASER WAVELENGTH AND FLUENCE ON THE GROWTH OF ZNO THIN-FILMS BY PULSED-LASER DEPOSITION

Transparent, electrically conductive and c-axis oriented ZnO thin films have been grown by the pulsed laser deposition (PLD) technique on silicon and Corning glass substrates employing either a KrF excimer laser (? = 248 nm) or a frequency-doubled Nd:YAG laser (? = 532 nm). The crystalline structure, surface morphology, optical and electrical properties of the deposited films were found to depend not only on the substrate temperature and oxygen partial pressure, but also on the irradiation conditions. The quality of the ZnO layers grown by the shorter wavelength laser was always better than that of the layers grown by the longer wavelength, under otherwise identical deposition conditions. This behaviour was qualitatively accounted for by the results of the numerical solution of a one-dimensional heat diffusion equation which indicated a strong superheating effect of the melted target material for the case of frequency-doubled Nd:YAG laser irradiations. By optimizing the deposition conditions we have grown, employing the KrF laser, very smooth c-axis oriented ZnO films having a full-width at half-maximum value of the (002) X-ray diffraction value less than 0.16° and optical transmittance around 85% in the visible region of the spectrum at a substrate temperature of only 300°C.

DEEMO: a new technology for the fabrication of microstructures

The recent innovations in dry etching make it a promising technology for the fabrications of micromoulds. The high aspect ratios, directional freedom, low roughness, high etch rates and high selectivity with respect to the mask material allow a versatile fabrication process of micromoulds for subsequent electroplating and embossing, as is demonstrated with the DEEMO process. DEEMO is an English acronym and stands for Dry Etching, Electroplating and Moulding.

Materials analysis of fluorocarbon films for MEMS applications

In this paper the results of the materials analysis of fluorocarbon (FC) films are presented. The properties of the fluorocarbon films are comparable to those of polytetrafluoroethylene (PTFE), better known under the trademarks such as teflon and fluon. The properties of PTFE are desirable for MEMS applications and enable new designs, new applications and new technological processing routes for microsystems. Therefore, FC films have a tremendous potential for MEMS applications. Furthermore, FC films can easily be deposited via spin coating, e-beam evaporation, in conventional reactive ion etchers and in plasma-enhanced deposition chambers using a carbonhydrotrifluoride plasma facilitating the use of the films for micro electro-mechanical structures. The films deposited in a reactive ion etcher are extremely chemical resistant. The X-ray photoelectron spectroscopy (XPS) analyses results are presented.

Mechanical parameter extraction using resonant structures

In order to model and design micromechanical sensors and actuators, it is of crucial importance that thin film mechanical parameters, such as the Youngs modulus and the residual stress, are known. In this paper, the parameter extraction method using resonant structures have been used to obtain these data and has been compared with the bulge test. Circular aluminum diaphragms have been used as samples