Henri V. Jansen

The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control

Very deep treches (up to 200 um) with high aspect ratios (up to 10) in silicon are etched using a fluorine-based plasma (SF6/O2/CHF3). Isotropic, positively and megatively (i.e. reverse) tapered as well as fully vertical walls with smooth surfaces 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 r.f. 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 Micro Electro Mechanical Systems (MEMS).

Stiction in surface micromachining

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.

A survey on the reactive ion etching of silicon in microtechnology

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 μ-flown: a novel device for measuring acoustic flows

An acoustic wave consists of two elements, the acoustic pressure and the acoustic flow. Up to now one has to measure the pressure and calculate the flow to determine the acoustic flow, so it would be convenient to have a sensor that is able to measure acoustic flows. At the University of Twente a novel device has been developed which fulfils this need. In this paper a short introduction to the governing principles of this dynamic flow sensor, the fabrication process, the electronics and some of its interesting applications will be presented. This micromachined device measuring acoustic flows is called the microflown or μ-flown.

RIE lag in high aspect ratio trench etching of silicon

While etching high 3 aspect ratio trenches into silicon with reactive ion etching (RIE) using an SF6/O2 chemistry it is observed that the etch rate is depending on the mask opening. This effect is known as RIE lag and is caused by the depletion of etching ions and radicals or inhibiting neutrals during their trench passage. In order to decide which source is the main cause, we constructed special horizontal trenches where only radicals are controlling the etching. The experiment showed that radicals are not responsible for RIE lag. Inhibitor depletion will result in inverse RIE lag. This effects is not found during our experimentation which leaves us with ion depletion to explain RIE lag. Depletion of ions is caused by ions captured by the sidewalls due to the angular distribution of incoming ions into the trench opening and the deflection of ions in the trench due to electrostatic fields. The analysis given in this paper indicates that the influencing field causes ion deflection, ion depletion, and therefore RIE lag in micron-sized Si trenches for low-energetic ions. In all cases, thus independent of the feature size, the angular distribution of incoming ions is thought to have a major contribution to RIE lag at higher pressures. These phenomena will be treated theoretically and simulated using a program, written in c++ under windows, in order to give a quantitative analysis of RIE lag.

Wet anisotropic etching for fluidic 1d nanochannels

In this paper a method is proposed to fabricate channels for fluidic applications with a depth in the nanometer range. Channels with smooth and straight sidewalls are constructed with the help of micromachining technology by etching shallow trenches into langle110rangle silicon using native oxide as a mask material and OPD resist developer as the etchant. Sub-50 nm deep fluidic channels are formed after bonding the nanopatterned wafers with silicon or borofloat-glass wafers. The nanofabrication process is significantly simplified by using native oxide as the main mask material. The etch depth of the nanochannels is limited by the thickness of the native oxide layer, and by the selectivity of the oxide/silicon etch rate (estimated to be at least 250 for langle110rangle silicon at room temperature).

The electrolysis of water: an actuation principle for MEMS with a big opportunity

In this paper the theory of water electrolysis in a closed electrochemical cell, that contains two electrodes, an electrolyte and a pressure sensor is described. From the leakage and electrochemical experiments done with this macrocell it is possible to obtain information about the applicability of the electrochemical principle in a closed cavity, the choice of the electrodes and electrolyte, and different types of leakage. To control the pressure of the electrochemical actuator automatically, an electronic feedback system was connected to the cell. A value of the pressure is set and the regulator will actuate the electrochemical cell in such a way to get the desired pressure.

The black silicon method. IV. The fabrication of three-dimensional structures in silicon with high apect ratios for scanning probe microscopy and other applications

The recently developed black silicon method (BSM) is presented as a powerful tool in finding recipes for the fabrication of MEMS building blocks such as Ay-stages. scanning probe tips, inkjet filters, multi-electrodes for neuro-electronic interfaces, and mouldings Lor direct patterning into polymers. The fabrication of these blocks in silicon with high aspect ratios and smooth surface textures will be described and discussed by using the BSM and standard reactive ion etching (ME).

Periodic arrays of deep nanopores made in silicon with reactive ion etching and deep UV lithography

We report on the fabrication of periodic arrays of deep nanopores with high aspect ratios in crystalline silicon. The radii and pitches of the pores were defined in a chromium mask by means of deep UV scan and step technology. The pores were etched with a reactive ion etching process with SF(6), optimized for the formation of deep nanopores. We have realized structures with pitches between 440 and 750xa0nm, pore diameters between 310 and 515xa0nm, and depth to diameter aspect ratios up to 16. To the best of our knowledge, this is the highest aspect ratio ever reported for arrays of nanopores in silicon made with a reactive ion etching process. Our experimental results show that the etching rate of the nanopores is aspect-ratio-dependent, and is mostly influenced by the angular distribution of the etching ions. Furthermore we show both experimentally and theoretically that, for sub-micrometer structures, reducing the sidewall erosion is the best way to maximize the aspect ratio of the pores. Our structures have potential applications in chemical sensors, in the control of liquid wetting of surfaces, and as capacitors in high-frequency electronics. We demonstrate by means of optical reflectivity that our high-quality structures are very well suited as photonic crystals. Since the process studied is compatible with existing CMOS semiconductor fabrication, it allows for the incorporation of the etched arrays in silicon chips.

Direct integration of micromachined pipettes in a flow channel for single DNA molecule study by optical tweezers

We have developed a micromachined flow cell consisting of a flow channel integrated with micropipettes. The flow cell is used in combination with an optical trap setup (optical tweezers) to study mechanical and structural properties of /spl lambda/-DNA molecules. The flow cell was realized using silicon micromachining including the so-called buried channel technology to fabricate the micropipettes, the wet etching of glass to create the flow channel, and the powder blasting of glass to make the fluid connections. The volume of the flow cell is 2 /spl mu/l. The pipettes have a length of 130 /spl mu/m, a width of 5-10 /spl mu/m, a round opening of 1 /spl mu/m and can be processed with different shapes. Using this flow cell we stretched single molecules (/spl lambda/-DNA) showing typical force-extension curves also found with conventional techniques. These pipettes can be also used for drug delivery, for injection of small gas bubbles into a liquid flow to monitor the streamlines, and for the mixing of liquids to study diffusion effects. The paper describes the design, the fabrication and testing of the flow cell.