Jan H.J Fluitman

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.

Electrostatic curved electrode actuators

This paper presents the design and performance of an electrostatic actuator consisting of a laterally compliant cantilever beam and a fixed curved electrode, both suspended above a ground plane. A theoretical description of the static behavior of the cantilever as it is pulled into contact with the rigid fixed-electrode structure is given. Two models are presented: a simplified semi-analytical model based on energy methods, and fully three-dimensional (3-D) coupled electromechanical numerical simulations using CoSolve-EM. The two models are in qualitative agreement with each other, and predict stable actuator behavior when the beam deflection becomes constrained by the curved electrode geometry before electrostatic pull-in can occur. The pull-in behavior depends on the shape of the curved electrode. Test devices have been fabricated by polysilicon surface micromachining techniques. Experimental results confirm the basic theoretical results. Stable behavior with relatively large displacements and forces can be generated by these curved electrode actuators. Depending on the design, or as a result of geometrical imperfections, regions of unstable (pull-in) deflection behavior are also observed.

Micro resonant force gauges

A review of micro resonant force gauges is presented. A theoretical description is given of gauges operating in a flexural mode of vibration, including a discussion of non-linear effects. Gauge factor and quality factor are defined and their relevance is discussed. Performance issues such as sensitivity, stability and resolution are addressed. Design aspects, including the means for excitation and detection of the vibration, and examples of silicon microfabrication technologies are described.

Towards integrated microliquid handling systems

In this paper we describe components for integrated microliquid handling systems such as fluid injection analysis, and first results of planar integration of components. The components discussed are channels, passive and active valves, actuators for micropumps, micromixers, microflow sensors, optical detectors, pumps and dosage systems. The dosage system described comprises a flow sensor and a pump micromachined on a single silicon wafer sandwiched between Pyrex wafers. The liquid pump is of the reciprocating type with a thermo-pneumatic actuator. The microliquid flow sensor is based on the thermal anemometer type. Both pump and flow sensor are realized in a 3 inch (100)- Si wafer using a KOH bulk etch from both sides of the wafer. The dosing system allows accurate dosing of liquid in the mu l regime and can easily be integrated with components as mixers and detectors to microliquid handling systems. A new concept for micromixing of liquids is introduced and its feasibility is demonstrated. The mixer allows fast mixing of small amounts of two liquids and it is applicable to microliquid handling systems. The mixer has a channel for the liquid, an inlet port for the reagent, and a mixing area, the bottom of which has 400 micronozzles (15 mu m*15 mu m). Through these nozzles, a reagent is injected into the sample liquid, making many microplumes. These plumes speed up mixing by diffusion over a short distance.

Micro-liquid flow sensor

A simple to realise micro-liquid flow sensor with high sensitivity is presented. The sensor is based on well known thermal anemometer principles. An analytical model for the sensor behaviour applicable for gas/liquid fluids is presented. The realisation process of the sensor is described. Model and experimental results agree well. The sensor is simple to integrate with other micro-liquid handling components such as pumps, mixers, etc.

Micro mixer with fast diffusion

A concept for micromixing of liquid is introduced, and its feasibility is demonstrated. The mixer allows fast mixing of small amounts of two liquids and is applicable to microliquid handling systems. The mixer has a channel for the liquid, an inlet port for the reagent, a 2.2-mm*2-mm*330- mu m mixing area, and 400 micronozzles (15 mu m*15 mu m) through with a reagent is injected into the sample liquid. The resulting microplumes greatly increase the contact surface between the two liquids and hasten the speed of the mixing by diffusion. The fabrication process is extremely simple. The mixing is complete within a few seconds; a homogeneous state of mixing is reached in 1.2 s when the total volume injected is 0.5 mu l and the injection flow rate is 0.75 mu l/s.<<ETX>>

Nonlinearity and hysteresis of resonant strain gauges

The nonlinearity and hysteresis effects of the electrostatically activated voltage-driven resonant microbridges have been studied theoretically and experimentally. It is shown that in order to avoid vibration instability and hysteresis to occur, the choices of the ac and dc driving voltages and of the quality factor of a resonator, with a given geometry and choice of materials, are limited by a hysteresis criterion. The limiting conditions are also formulated as the hysteresis-free design rules. Expressions for the maximum allowable quality factor and maximum attainable figure of merit are given. Experimental results, as obtained from electrostatically driven vacuum-encapsulated low-pressure chemical-vapor deposition (LPCVD) polysilicon microbridges, are presented and show good agreement with the theory.

Integrated micro-liquid dosing system

An integrated micro-liquid dosing system consisting of a micropump and a microliquid flow sensor is demonstrated. The dosing system allows accurate dosing of liquid in the microliter regime and can easily be integrated with components such as mixers and detectors in micro-liquid handling systems. The liquid pump is of the reciprocating type with a thermopneumatic actuator. The microliquid flow sensor is based on the thermal anemometer type. Both pump and flow sensor are realized in a 3-in

Single element excitation and detection of (micro-)mechanical resonators

The authors describe a single-element approach for the excitation and detection of the vibrational motion of (micro-)mechanical resonators. An equivalent electrical one-port network is derived for an electrostatically and a piezoelectrically driven resonator. In this way, the effect of the mechanical resonator is transformed into the electrical domain and can easily be accounted for in a circuit simulation. A detection circuit, based on an (on-chip) bridged design, is proposed as a way to compensate for the parasitic parallel load of the one-port. A criterion is given for the accepted level of unbalance of the bridge if a minimal phase shift of 90 degrees in the transfer characteristic around resonance is required.<<ETX>>

Q-factor dependence of one-port encapsulated polysilicon resonator on reactive sealing pressure

Micromachined encapsulated polysilicon resonators have been fabricated in different reactive sealing pressure, 200, 50 and 20 mTorr, in order to investigate the dependence of the Q-factors on the sealing pressure. Q-factors as high as 2700 have been measured. The experimental results show that the q-factors of one-port encapsulated resonators are proportional to 1/p and the resonant frequency is independent of the sealing pressure. However, the measured Q-factors are more than two orders of magnitude lower than theoretical prediction.