Josef Binder

Additive electroplating technology as a post-CMOS process for the production of MEMS acceleration-threshold switches for transportation applications

This paper presents an acceleration-threshold sensor fabricated with an electroplating technology which can be integrated on top of a pre-processed CMOS signal processing circuit. The device can be manufactured using a standard low-cost CMOS production line and then adding the mechanical sensor elements via a specialized back-end process. This makes the system especially interesting for automotive applications, such as airbag safety systems or transportation shock monitoring systems, where smaller size, improved functionality, high reliability and low costs are important.

Simulation, design and fabrication of electroplated acceleration switches

This paper describes the simulation, design and fabrication of acceleration switches for automotive applications. A simulation model is introduced which describes the dynamical behaviour of the sensor. The main focus of the development is on the specifications of the accelerometer, such as the switching time, the overload protection, the self-test feature and the damping behaviour. The simulated acceleration switches are fabricated with the use of a sacrificial layer technique and electroplated moulding of microforms. The 3D-UV Microform technology was preferred because it enables high aspect ratio patterning of commercially available thick photoresists. First devices of the microswitch sensors were characterized by means of optical measurement facilities for the determination of resonance frequencies and of the dynamical behaviour during test pulses. Measurements on a shaker were carried out to confirm the validity of the model.

Comparison of stress measurement techniques in surface micromachining

Stress measurement techniques using micromachined structures are compared with the wafer curvature technique. Buckling bridges and rotating indicator structures are simulated, designed and fabricated using thin polysilicon films. Stress ranging from the high compressive to the high tensile state can be detected using appropriate indicator designs.

A low-voltage torsional actuator for application in RF-microswitches

This paper reports a new surface micromachined torsional actuator for integrated microswitches. The device requires an actuation voltage below 10 V and is fabricated in a BiCMOS compatible process, allowing for monolithic integration with an actuation circuitry. Insensitivity against mechanical vibrations is achieved by a bistable operation principle. Measured pull-in voltages of the structures are in good agreement with the electromechanical simulations. The dynamic behavior of the actuator is investigated by interferometric determination of the resonance modes. Measurements of the switching times of completed devices result in values below 10 μs.

Laterally driven accelerometer fabricated in single crystalline silicon

Abstract The request for wireless sensor operations grows in medical and automotive applications. These sensors receive their energy and send their data by a telemetric unit. The wireless transferred energy restricts the power consumption of the sensor and signal processing to less than 3 mW. Therefore, the sensor has to be operated in open-loop. Furthermore, a main focus is directed to increase the sensitivity of the mechanical–electrical transducer. Considering both open-loop and sensitivity, the sensor has to be optimized by referring to the structure height. The way for realizing high structures, as described in this paper, is the micromachining of silicon wafers with a specified thickness. The superior mechanical properties of single crystalline silicon compared to electroplated metals or surface-micromachined devices confirm the use of silicon as sensor material. A laterally driven accelerometer is simulated, designed and fabricated comprising the technologies of deep reactive ion etching (DRIE) of silicon, silicon direct bonding (SDB) and chemical mechanical polishing (CMP). Characterization results confirm the performance of this new technology. The open-loop sensor, which was characterized, had a height of 50 μm with damping constant greater than 0.1.

Acceleration threshold switches from an additive electroplating MEMS process

An acceleration threshold sensor fabricated with an electroplating technology on top of a CMOS signal processing circuit is presented in this paper. Manufacturing of the device is achieved using a standard low-cost CMOS production line and employing a specialized back-end process for adding the mechanical sensor-elements. This production technology makes the system especially interesting for automotive applications in airbag systems or transportation shock monitoring systems. These applications demand smaller size, improved functionality, high reliability and low costs, factors that can be satisfied with this new technology. The initial additive electroplating technology (AET) has been improved by coating the basic structures with an electroplated alloy. Additionally, a specialized packaging procedure has been developed to hermetically seal the sensor-structures following their release after the sacrificial layer removal. The devices are finally SMD-packaged to allow for easy handling. Samples of the device have been tested extensively and have successfully proven its functionality.

Fluid characterization using sensor elements based on interdigitated electrodes

Examinations of uncoated interdigitated structures (IDSs) for fluid characterization are presented in this paper. The measuring principle is based on the determination of the capacitance and resistance of the electrode arrangement in fluids. A binary fluid mixture can be characterized from the measured variables if the components are known. Common IDS transducers are coated with a sensitive layer that changes its capacitive or resistive characteristics when exposed to a substance, e.g., a gas. In the presented application the fluid itself serves as dielectric and resistive medium. The sensor element is analysed by measurements in pure fluids and in binary fluid mixtures. Additionally, theoretical methods for the calculation of resistance and capacitance of an IDS in fluid are presented. These methods are based on FEM simulation and conformal mapping.

Quasianalog accelerometer using microswitch array

Abstract This paper describes the development of a quasianalog accelerometer which uses an array of acceleration switches. The investigations are based on results of fabricated and evaluated single-microswitch sensors. The new acceleration sensor concept combines the advantage of high EMI immunity and simple signal pre-conditioning circuitry. Important characteristics of the laterally moving cantilevers, such as resonance frequency and switching time, can be determined by a single mask. First test devices have been fabricated by the use of a special sacrificial-layer additive technology. This Microform technology combines UV patterning of very thick photoresist layers and moulding of the resulting patterns by electroplated metals. This technology is used for the fabrication of laterally driven microswitches, each having a different acceleration threshold. Under an applied acceleration, the cantilever beam deflects laterally and closes a contact with an electrode. With an electrode on each side of the cantilever, two directions of acceleration can be measured. A simulation model has been developed which enables the design of quasianalog sensors for different applications. Different layouts are created to investigate the basic characteristics. The simulation results are verified on a shaker using the fabricated test structures of a single-microswitch sensor.

Fabrication of gap-optimized CMUT

A recently introduced set up of capacitive micromachined ultrasonic transducers (cMUT) combines a conductive membrane above a structured sacrificial layer. All previous approaches either require an additional metallic electrode or do not possess a structured sacrificial layer and, consequently, may make exact adjustment of the membrane dimensions difficult. The present set ups are especially suited for the fabrication of cMUT with gap heights ranging between 50 nm and 2 /spl mu/m between the electrodes. Large gaps are a prerequisite to enabling sufficient deflections of the membrane and, therewith, to generating high pressure gradients. On the other hand, small gap sizes are desirable for detecting weak ultrasonic sources. This paper focuses on the fabrication process of cMUT to realize electrode separation above 500 nm and, in addition, on the manufacturing of cMUT with gaps below 500 nm. The successful realization has been proven by some basic experimental investigations. Finally, the fundamental equations of a frequently chosen simulation model are documented, as a number of ambiguities exist in the common literature.

Multi-electrode substrate for selectivity enhancement in air monitoring

A new microstructured silicon substrate for thin gas sensitive layers has been developed composing a thermoresistor at the chip surface and a buried polysilicon heater. The process flow of the largely MOS-compatible fabrication is described. The two pairs of planarized interdigitated electrodes with both a width and spacing of 2 microns were manufactured by a special lift-off process. A second interdigitated electrode is placed above the sensing tin oxide layer and can also serve as a measuring electrode. As a further possibility, we suggest the application of a transverse electric field to the sensing layer by using the upper interdigitated electrode as a gate electrode and the heater as a counter electrode. This additional electric field can influence the effective signal values caused by desorption and adsorption of gas specimen thus enhancing the selectivity of resistive gas sensors. In order to detect oxidizing gases like nitrogen oxides with a metal oxide layer we have developed a reactive sputter process for tin oxide with a homogenous unstoichiometry across the complete grains. Measuring the d.c. resistance of these strongly reduced layers at relatively low temperatures of about 175°C we have achieved a sensitivity in the sub-ppm range with regard to nitrogen dioxide in dry air.