Bistable SiC MEMS membranes: the potential for medical applications

Abstract

The aim of this thesis is to fabricate and characterize silicon carbide coated and hence, biocompatible, bistable membranes, which are controlled by integrated piezoelectric thin film actuators. Due to different membrane configurations and electrical excitation signals it is possible to switch the buckled membrane from one stable state to the other and vice versa. The first part of this work concentrates on the stress behaviour of a-SiC:H thin layers. By coating the top surface of the membranes, biocompatibility to any human tissue is ensured with an electrically insulating material. Silicon carbide as thin layer can be deposited by PECVD in different compositions of silicon and carbon, which has a huge impact on the thickness and layer stress of the resulting thin layer. However, not only the stress behaviour of a-SiC:H layers will be characterised, but also its biocompatibility and the likeliness of CaCo-2 cells attaching to the substrates surface. Epithelial cells of the small intestine were used to characterise the adhesiveness and proliferation on a-SiC:H layers, deposited with different gas flow ratios of silane and methane. Silicon is known as a hydrophile material, while SiC with an increasing amount of carbon gets more hydrophobic, which tends to directly influence the attachment behaviour of cells. The more hydrophile a surface is, the more likely is the attachment of a cell. Living mammalian cells adhere especially well to surfaces, whose electrochemical potential is similar to their own. Since carbon fits this aspect very well, carbon-rich surfaces are preferred with respect to the attachment behaviour of such cells, which stands in contradiction with the hydrophobicity of SiC substrates with high contents of carbon. This work found a balanced composition of silicon and carbon in an a-SiC:H thin layer, where cells adhere best, and compares the stress behaviour of different SiC composed layers with the proliferation and adhesiveness of cells. For this purpose, 50.000 CaCo-2 cells were placed on 13 different processed 6 x 6 mm2 a-SiC:H coated silicon samples. To get a broader result, each of the 13 samples was produced six times, whereby every sample was either pre-treated in an O2-plasma or additionally coated with collagen or Poly-D-Lysine in order to create different growing surroundings, while the same procedures for cell planting, feeding, growing and measuring were applied. After finding the right deposition conditions for a preferably low stress state, the SiC layer got deposited on several different diaphragms, which were produced with diameters from 600 800 m. The fabrication process starts with an SOI-wafer as base material, where different thin film layers were subsequently deposited. These layers were treated with several etching steps, until the final structure of the membrane was created. Through a careful adjustment of the layer stress of all other thin films involved, the diaphragm buckled randomly in one of its two stable states. As piezoelectric layer aluminium nitride (AlN) was used as the material of choice. Secondly, the vibrational behaviour of bistable membranes under the load of an electrical signal was measured, as well as the directly related characteristic resonance frequencies, to predict the switching behaviour of such biocompatible diaphragms in air and different fluids. These measurements were conducted with a Laser-Doppler-Vibrometer, a White-Light- Interferometer, wafer-bow-measurements, contact angle measurements and an oscilloscope. In this thesis, the fabrication process, the measurements approach as well as the results of the switching behaviour of bistable membranes in both air and fluids are described and discussed. Finally, the correlation of living cells growing on substrates of different Si-C compositions and its correlation to the mechanical stress state will be presented.