Very deep trenches (up to 200 pm) with high aspect ratios (up to 10) in silicon and polymers are etched using a fluorine-based plasma (SFd02/CHF3). Isotropic, positively and negatively (i.e. reverse) tapered as well as.fully vertical walls with smooth suriaces are achieved by controlling the plasma chemistry. A convenient way to find the processing conditions needed for a vertical wall is described the black silicon method. This new procedure is checked for three different reactive ion etchers (RIE), two parallel-plate reactors and a hexode. The influence of the RF power, pressure and gas mixture on the profile will be shown. Scanning electron microscope (SEM) photos are included to demonstrate the black silicon method, the influence of the gases on the profile, and the use of this method in fabricating microelectromechanical systems (MEMS).

Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces. N Tas et al

This article is a brief review of dry etching as applied to pattern transfer, primarily in silicon technology. It focuses on concepts and topics for etching materials of interest in micromechanics. The basis of plasma-assisted etching, the main dry etching technique, is explained and plasma system configurations are described such as reactive ion etching (RIE). An important feature of RIE is its ability to achieve etch directionality. The mechanism behind this directionality and various plasma chemistries to fulfil this task will be explained. Multi-step plasma chemistries are found to be useful to etch, release and passivate micromechanical structures in one run successfully. Plasma etching is extremely sensitive to many variables, making etch results inconsistent and irreproducible. Therefore, important plasma parameters, mask materials and their influences will be treated. Moreover, RIE has its own specific problems, and solutions will be formulated. The result of an RIE process depends in a non-linear way on a great number of parameters. Therefore, a careful data acquisition is necessary. Also, plasma monitoring is needed for the determination of the etch end point for a given process. This review is ended with some promising current trends in plasma etching.

The capillary filling speed of water in nanochannels with a rectangular cross section and a height on the order of 100nm has been measured over a length of 1cm. The measured position of the meniscus as a function of time qualitatively follows the Washburn model. Quantitatively, however, there is a lower than expected filling speed, which we attribute to the electro-viscous effect. For demineralized water in equilibrium with air the elevation of the apparent viscosity amounts up to 24±11% in the smallest channels (53nm height). When using a 0.1M NaCl (aq) solution the elevation of the apparent viscosity is significantly reduced.

An intensive study has been performed to understand and tune deep reactive ion etch (DRIE) processes for optimum results with respect to the silicon etch rate, etch profile and mask etch selectivity (in order of priority) using state-of-the-art dual power source DRIE equipment. The research compares pulsed-mode DRIE processes (e.g. Bosch technique) and mixed-mode DRIE processes (e.g. cryostat technique). In both techniques, an inhibitor is added to fluorine-based plasma to achieve directional etching, which is formed out of an oxide-forming (O 2) or a fluorocarbon (FC) gas (C 4 F 8 or CHF 3). The inhibitor can be introduced together with the etch gas, which is named a mixed-mode DRIE process, or the inhibitor can be added in a time-multiplexed manner, which will be termed a pulsed-mode DRIE process. Next, the most convenient mode of operation found in this study is highlighted including some remarks to ensure proper etching (i.e. step synchronization in pulsed-mode operation and heat control of the wafer). First of all, for the fabrication of directional profiles, pulsed-mode DRIE is far easier to handle, is more robust with respect to the pattern layout and has the potential of achieving much higher mask etch selectivity, whereas in a mixed-mode the etch rate is higher and sidewall scalloping is prohibited. It is found that both pulsed-mode CHF 3 and C 4 F 8 are perfectly suited to perform high speed directional etching, although they have the drawback of leaving the FC residue at the sidewalls of etched structures. They show an identical result when the flow of CHF 3 is roughly 30 times the flow of C 4 F 8 , and the amount of gas needed for a comparable result decreases rapidly while lowering the temperature from room down to cryogenic (and increasing the etch rate). Moreover, lowering the temperature lowers the mask erosion rate substantially (and so the mask selectivity improves). The pulsed-mode O 2 is FC-free but shows only tolerable anisotropic results at −120 • C. The downside of needing liquid nitrogen to perform cryogenic etching can be improved by using a new approach in which both the pulsed and mixed modes are combined into the so-called puffed mode. Alternatively, the use of tetra-ethyl-ortho-silicate (TEOS) as a silicon oxide precursor is