Table of contents

Micromachined device technology has emerged during the last three decades. At first it was mainly a technological spin-off from microelectronics/integrated circuit technology. Sensor applications gave the main market pull, batch processing the key to high quality at low cost and silicon micromachining established itself as a unique process technology with distinctive features. Today, these devices have matured into a separate industry sector with their own market and manufacturing infrastructure, also with micromachining of other materials than silicon. They are used in microelectronic systems with widespread applications, ranging from low-cost, high-volume automotive applications to high-cost, low-volume instrumentation applications. The micromachined devices have during these years shown a much slower learning curve than microelectronics in general, making them bottlenecks for performance and cost improvements in their systems. The herald of the rapid development of integrated circuit technology-batch processing-is one of the important keys to ease these bottlenecks. The most important batch processes for micromachined devices are highlighted, and recommendations for future batch processing developments for micromachined devices are given.

Micromechanical structures must be assembled where monolithic integration is not feasible because of incompatible processes, complex geometries or different materials involved. In this way, a system can be built up of products from various suppliers, which may promote the development of a market for microsystems. In micromechanics, assembly must establish mechanical and fluidic contacts in addition to electrical connections. Assembling single components is relatively expensive. This problem may be overcome if batch processes are conducted. A batch process for LIGA structures was developed which has been used to make microfluidic components by transferring a diaphragm to microstructures.

Although silicon planar technology has generated the field of micromechanics, it has been realized that this technology has intrinsic limitations for the fabrication of truly so micromechanisms. A number of alternative technologies based either on silicon or on different materials are presently investigated in order to overcome the limitations of planar silicon technology. This paper outlines the motivations for developing new microfabrication technologies, especially those that are considered as 'non-traditional' in the microelectronics domain, and the perspectives offered by this approach for fabricating miniature, micro and nanodevices. Four representative 'non-traditional' technologies are considered. LIGA process, micro electro-discharge machining (EDM), micro stereo lithography, and the combination of biological and artificial microfabricated structures ('hybrid' technologies).

Practical applications of microfabricated sensors and actuators are, with a few noticeable exceptions, only slow to evolve. This paper will try to shed some light on the differences between the micro and macro world and discuss a number of examples of successful employment of microsystem technology in macro instrumentation.

In this paper I would like to present the recent progress in the field of micro-optics including miniaturized light sources, bulk and planar optical components and the key role played by miniaturized optomechanical devices in the general development of future optical microsystems.

The manipulation and characterization of single microparticles and living cells is possible by the application of high-frequency electric fields using ultra-microelectrode arrays fabricated on planar silicon or glass wafers using semiconductor technology. Devices can be developed for cell trapping, manipulation and cultivation. Electrode miniaturization and partial insulation allows the use of highly conductive media, such as cell culture or physiological solutions. Normal growth of animal cell occurs in the high field (>50 kV min^-1) between continuously energized multielectrodes. This opens up new biomedical applications. These microtools may be combined to develop cell separators, microsensors and controlled-biocompatibility surfaces.

This paper describes a single-chip microphone structure and its fabrication using polysilicon for both condenser electrodes. The fabrication technology is IC-compatible with KOH-etching on one wafer surface only to form the membrane in a last step. The membrane dimensions range from 100 mu m by 100 mu m to 500 mu m by 500 mu m. The membrane thickness and the air gap is 1.5 mu m while the thickness of the backplate is 3.5 mu m. For increased mechanical sensitivity slotted membranes were also fabricated. The static deflection behaviour due to electrostatic forces was simulated and measured.

To preserve the shape of convex corners when etching in aqueous KOH, corner compensation structures have to be used. The etching of convex corners is due to the fact that some planes etch faster than others, resulting in a loss of the desired structure. By adding extra structures at these convex corners, these structures will be removed during etching resulting in the desired convex corners. The basic corner compensation structures reported in the literature are oriented along the (100) direction or the (110) direction. This paper shows a step-by-step analysis of the etching of these structures. The results show that the (411) planes are responsible for the undercutting, which implies that a perfect compensation of a convex corner using structures oriented along the (100) direction is not possible. Moreover, it is shown that each compensation structure leaves an imprint on the membrane, which cannot be removed by further etching, as the convex corner would otherwise be etched.

Silicon surfaces become very rough and extremely dull when etched in a KOH solution that is contaminated with stainless steel. Membranes that are produced using the ECE stop also exhibit an increased surface roughness, despite the general assumption that very smooth surfaces are always obtained when using this etch-stop technique. This contamination originates from the holder in which the wafer is mounted during the etching. In order to achieve mirror-quality membranes, it must be ensured that, besides using an appropriate solution concentration and temperature, no steel comes into contact with the etchant.

The electrostatic bonding of silicon and Pyrex glass was studied in order to find out the influence of process parameters on the final result and to be able to optimize the process with regard to the fabrication of silicon sensors. The most important parameters are the temperature and the voltage. The main criterion used for the optimization of their values was the induced stress in the silicon part of the bonded ensemble. It was found that a temperature of 360 degrees C and voltages in the range 750-1000 V are suitable.

A vital requirement for a resonator-based sensor is a high degree of balance in the resonator's chosen mode of vibration. A systematic study of the balance and stress sensitivity of a number of important resonator geometries has been performed. New methods have been developed both to evaluate and optimize the balance of a resonator structure. This paper presents the results of the study, details the methods used and provides some simple design rules for resonators. All the geometries evaluated are realizable as microsensors using silicon micromachining techniques.

In this paper a non-traditional, but IC-compatible fabrication technology for micromachined acoustical silicon sensors for airborne sound is described. The microphones are based on the piezoelectric effect of P(VDF/TrFE)-layers, which are placed on top of a very thin silicon nitride membrane (area: 1 mm²). The P(VDF/TrFE)-layers are spin-coated, annealed and poled by corona discharge, in order to get piezoelectric behaviour. As a result of the design and the technology process, a maximum sensitivity of 150 mu V Pa^-1, a bandwidth of about 16 kHz and an equivalent noise level of less than 60 dB(A) were measured. These values are much better in comparison to those measured at former silicon microphones with piezoelectric polymer layers.

This paper deals with the compensation of convex corner undercutting during anisotropic etching of (100)-oriented silicon wafers in aqueous KOH with respect to micromachined devices. Several compensation structures are examined focusing on the etching of two intersecting (110)-oriented V-grooves formed by (111)-planes. A novel structure with reduced spatial requirements is shown. Using this structure, crossing V-grooves for chip separation are realized.

Long U-shaped grooves that have been anisotropically etched into (110)-oriented silicon with KOH are required for many microsensors and actuators like for microcoolers, force sensors, separating trenches, printheads, etc. Specifications for the etched structures (such as minimum roughness, steep walls or high etch uniformity) require different etch parameters. The etch rates, etch-rate ratios and roughness of the (110) planes are investigated as a function of the KOH temperature and concentration. An important influence on the etch process, especially during the etch of narrow grooves, is also exerted by the (311) planes observed at KOH concentrations above 30%.

Very deep trenches (up to 200 mu m) with high aspect ratios (up to 10) in silicon and polymers are etched using a fluorine-based plasma (SF₆/O₂/CHF₃). Isotropic, positively and negatively (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 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).

The effect of high-temperature annealing on Young's modulus E and the intrinsic stress sigma of thin films made of LPCVD-polysilicon was investigated. The films were annealed for 2 hours in a nitrogen atmosphere at temperatures between 600 degrees C and 1100 degrees C. Then Young's modulus and the intrinsic stress were determined by the membrane deflection method. An extended analytical theory for the membrane deflection was developed and the results correspond well with FEM analysis of Pan J.Y. et al. (1990 Technical Digest, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, USA p 70). LPCVD-polysilicon was produced with a SiH₄ flow rate of 70 sccm and a total pressure of 100 mTorr at 620 degrees C. The film thickness was 460 nm. For the as deposited films the method of membrane deflection yields a Young's modulus of 151+or-6 GPa and an intrinsic stress of -350+or-12 MPa. After annealing at temperatures higher than the deposition temperature the compressive stress started to decrease with increasing annealing temperature. It relaxed nearly completely after annealing at 1100 degrees C. Young's modulus seems to increase a little with increasing annealing temperature up to 162+or-8 GPa at 1100 degrees C. The values for E and sigma obtained with the membrane deflection method were compared with the values obtained by the method of ultrasonic surface waves. The method of ultrasonic surface waves yields systematically higher values for E. The discrepancy can be explained by the uncertainty of Poisson's ratio of polysilicon.

Borophosphosilicate glass (BPSG) demonstrates superior reflow properties at low reflow temperature compared with phosphosilicate glass (PSG). However, the latter is conventionally used in surface micromachining, because of the high underetch rate in HF. This paper reports on the BPSG composition required to achieve both satisfactorily reflow at 850 degrees C and adequate etch rate in BHF to enable the application of BPSG as reflow and sacrificial material. The effect of the boron content in the BPSG and the application of a HMDS film before low-temperature reflow on the reflow performance have been investigated using LPCVD deposition and reflow in a wet oxidation furnace.

Up to now, mainly uniaxial accelerometers are described in most publications concerning this subject. However, triaxial accelerometers are needed in the biomedical field. Commercially available triaxial accelerometers consisting of three orthogonally positioned uniaxial devices do not meet all specifications of the biomedical application. Therefore, a new highly symmetrical inherently triaxial accelerometer is being developed, the advantages of which are higher sensitivity and reduction of off-axis sensitivity.

In this work, microRaman spectroscopy is applied for the stress analysis of LPCVD polysilicon films deposited on SiO₂ sacrificial layers. The features of the first-order Si Raman signal (shape, width and position of maximum) are analyzed taking into account the presence of structural defects and stress distribution in the layers. These measurements are correlated with the results obtained by using micromachined test structures.

A process is described for fabrication of planar coils with ferromagnetic yoke as parts of a micromachined torque sensor. These parts are located on the rear side of a silicon wafer where CMOS sensor devices and circuitry had been already formed on the front side. Only 'cold' process steps are necessary, such as metal evaporation, electroplating, wet etching, sputtering and low-temperature CVD. The wires of the coils are realized by electroplated gold. To avoid tearing off of further layers over the gold edges, a special etch process was developed. This process provides the possibility of adjusting the gradient of the gold wire slopes. The yoke legs are formed by KOH etched pits and electroplated NiFe.

This paper reports on the technology and performance of piezoresistive pressure sensors that utilize GaAs/AlGaAs membranes for pressure transduction into stress to induce resistance changes of p doped GaAs resistors. We have tested the sensor to differential pressure up to 8*10⁴ Pa and in a temperature range of room temperature up to 433 K.

This paper presents a resonant force sensor comprising piezoelectric ZnO thin-film transducers for excitation and detection of resonant beam vibrations. A short description of the processing technique is given, i.e. deposition and passivation of the ZnO layer and separation of beam structures. The electrical behaviour of the sensor was optimized by patterning ZnO areas to minimize electrical crosstalk effects.

A new water permeability sensor based on the sensitivity of polyimide to humidity has been fabricated for the measurement of the water penetration of thin metal films. To evaluate the sensitivity of the polyimide layer a special humidity sensor was also fabricated. This humidity sensor demonstrates good linearity, a relatively fast response time (1.5-2 min), and good sensitivity. To evaluate the permeability sensor a 5000 AA gold film with a pinhole density of approximately 100 per cm² was measured. The results show a capacity change from 100 to 109 pF and saturation after 300 h.

This paper reports on the development of shape memory alloy based microactuators that can be integrated in teleoperated microsystems for medical applications. The use of shape memory alloys as actuator will increase the performance of micromechanical devices by several orders of magnitude. The paper presents the results of a material study and an analysis of the current technological limitations of shape memory alloys for application in micro actuators. The possibilities for using thin rolled wires, ground wires, and melt-spun ribbons in micro-applications are studied in particular. These could offer an attractive alternative to sputtered shape memory thin films.

A conventional sol-gel process was used to spin-cast PZT films on oxidized Si wafers coated with sputtered Pt layers. After annealing at 550 degrees C-800 degrees C, the resulting perovskite-type PZT films showed different textures and surface morphologies, depending on whether or not a Ti adhesion layer was used. If a Ti layer was present, Ti diffusion into and through the Pt film leads to a compound Pt₃Ti, which facilitates crystallization of the perovskite PZT phase; without Ti, crystallization is more difficult and occurs via the growth of dendritic crystallites. Several optical and electrical properties of the PZT films have been measured; the first results indicate high dielectric constants ( epsilon approximately=480) and acceptable ferroelectric behaviour.

A low-power IC-compatible electrostatically driven microrelay is presented. The mechanical part of the microrelay consists of a polysilicon/silicon nitride/polysilicon microbridge realized by sacrificial layer technology. Its fabrication, operation up to 100 kHz and net switching voltages in the 1 to 10 V range are reported.

A differential capacitive atomic force microscope (AFM) transducer with integrated tip for use in ultra-high vacuum is presented. It is fabricated by the dissolved wafer technique with multiple etch stop using highly boron-doped epitaxial layers. The tip is fabricated using anisotropic etching and radii of curvature of the order of 60 nm were measured. Measured sensitivities agree well with the presented model.

This paper describes the fabrication of the tubular guidance channel and of a type of microfabricated through-pass hole dice, which together form a prototype neural connector. The final goal is to develop a hybrid implantable microsystem capable of bidirectionally interfacing the peripheral nervous system to an external device for actuation (artificial limb) or for functional neuromuscular stimulation.

A process for the fabrication of microvalve systems by thermoplastic molding and membrane techniques has been developed. The valve system consists of three individual valves formed by two parts molded from polymethylmethacrylate PMMA and a polyimide membrane. The mold inserts were manufactured by milling of a brass substrate using a 300 mu m diameter head. The three-dimensional microstructure of the inserts consists of four different levels for valve seats, orifices, alignment pins and cavities. The overall diameter and height of the whole valve system is 7 mm and 1.9 mm, respectively. The valves are designed to be normally open. To close the valves, the pressure in an actuator chamber above the membrane is raised by a heater coil and the membrane is pressed onto the valve seat. First measurements at a difference pressure of 1000 hPa showed a rate of water flow through a single valve of 171 mu l s^-1. An actuator pressure of 180 hPa was reached by heating air with a resistive heater and continuous electrical power of 158 mW. A valve supplied with nitrogen at 130 hPa was closed by an electrical power of 116 mW.

Biocompatible three-dimensional structures have been fabricated using micromachined silicon as a mold. Techniques have been developed for patterning conductive silicone rubber on insulating silicone rubber substrates. Furthermore, polyurethane replica of micromachined silicon wafers were formed. Applications of biocompatible microstructures comprise cell culture substrates, neural implants and prostheses, and microsubstrates for bioartificial organs.

We present a new technology that uses the selective generation and etching of porous silicon. This technology combines the advantages of surface micromachining (small area consumption, CMOS compatibility, front side lithography) with the advantage of bulk micromachining (large distance, no sticking). The practical application is shown in two examples: a free-standing polysilicon bridge for flow measurement and a thin-film sensor for a thermal transducer.

The design and fabrication of hydraulic microcomponents obtained by stereolithography are proposed in this paper. A piezoelectric micropump and microchannels have been fabricated and tested extensively. The pump body and the microchannels are made out of ultraviolet-photocurable polymer material and manufactured by a single stereolithographic process. Stereolithography has been selected since it allows the microfabrication of three-dimensional structures of any complex shape, even incorporating 'integral' movable parts that may require no assembly. The design of the pump body and the flow channels has been based on optimum hydraulic criteria, and aimed to obtain long life and quick, cheap and easy manufacturing. Design rules have been studied in order to obtain hydraulic components with specific behaviours (flow rate, head, loss and charge). The paper describes the finite-element analysis of the thin plate pumping element and of the actuator, as well as the fabrication and experimental performance of the pump. Experimental results show good agreement with theoretical prediction obtained by simulation, and values of flow rate and discharge head that are among the highest reported in the literature for pumps for similar size and working principles.

In this paper we describe the design, fabrication and testing of an active endoscope for minimal invasive surgery (MIS). The device has a silicone rubber body with 8 mm outer diameter and 40 mm length. This configuration has two bending degrees of freedom. The device is actuated by three miniature SMA springs, which are heated by Joule effect and cooled by a forced air flow. The actuators are located in three lumina (1.5 mm diameter) of the endoscope. The active endoscope incorporates a fibre optic bundle for visual inspection which has been used in the current experiments to measure the performance of the endoscope tip in terms of bending angles and directivity.

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.

A novel device for controlling the radiative transfer of heat is proposed. An Active Radiator tile (ART) is composed of two parallel plates separated by small thermally insulating posts. In the rest state, the ART presents a large thermal resistance. Application of a suitable potential across the plates causes one plate to deflect into direct contact with other plate, allowing a large conductive heat flow. Thus the ART device acts as a thermal valve. We report the modelling of the mechanical and thermal properties of the device, and progress towards the construction of a laboratory prototype.

A technique is presented that provides planarization after a very deep etching step in silicon. This offers the possibility for not only resist spinning and layer patterning but also for realization of bridges and cantilevers across deep grooves or holes. The technique contains a standard dry film lamination step to cover a wafer with a 38 mu m thick foil. Next the foil is etched back to the desired thickness of a few micrometres. This thin film facilitates resist spinning and high-resolution patterning. The planarization method is demonstrated by the fabrication of aluminium bridges across a deep groove in silicon.

We present a micromachined infrared IR detector based on the principle of the Golay cell. The detector basically consists of a sealed cavity, in which the heat generated by absorbed in light results in an increased gas pressure. This pressure rise is detected capacitively. The theoretical responsivity and noise equivalent of a micromachined device are calculated, and it is shown that a detectivity of 3.6*10⁹ cm Hz^1/2 W^-1 can be expected for a 1 mm² micromachined version. Further, we propose the use of a micromachined pneumatic gas leak in order to avoid thermal drift. A prototype was fabricated using Si and Pyrex micromachining techniques, and confirmed the principle of operation. The preliminary experimental results are compared to theory.

The creation of new products in LIGA technology starts with the layout of X-ray masks. In this state the designer has to take into account not only the shapes and the sizes of the intended structures, but also a set of layout and design rules imposed by the various LIGA process steps and the materials used. Like in microelectronics automatic rule checking can eliminate the need for frequent redesign cycles and can hence result in a considerable reduction of design cost. The precondition for an automatic rule check of the layout data is a suitable rule formalization that leads to a technology database which CAD tools can access to process the rules. This paper introduces a method to formalize LIGA design rules using the object-oriented process description language DINGO-ML.

The working principle of a dynamic micropump is presented and its basic parameters are discussed. The dynamic micropump has a very simple and cheap structure consisting of a pump chamber, an oscillating membrane and two truncated pyramid shaped microchannels produced by anisotropic etching of silicon. The latter show a direction-dependent behaviour of their flow resistances, for which reason they provide as so-called dynamic passive valves, a partial rectifying of an alternating flux. The simple mechanical system of the micropump enables high working frequencies between 100 Hz and 10 kHz; the zero-load pump rate is equal to or higher than 250 mu l min^-1, being up to one-order of magnitude more than the micropumps so far developed can provide. The pump parameters can be chosen in a wide range by simply changing the size of the dynamic valve channels.