Section 5

MEMS/Microsystems Device and Process Technologies

MEMS devices or microsystems which have successfully established high volume commercial markets include accelerometers and pressure sensors for automotive applications, inkjet print heads, and digital micro-mirrors for image projection. The automotive MEMS supplier group has strong European representation by companies such as Bosch, TEMIC, SensoNor and VTI-Hamlin, which are major players in this market. The other two device classes are mostly supplied commercially by U.S and Japanese companies. The MCC tour did not visit the established European MEMS suppliers, but rather focused on companies, government laboratories and universities pursuing novel technologies and emerging applications in order to find out about the future of MEMS and microsystems in Europe.

Among the institutions and companies the MCC/WTEC team visited throughout central Western Europe (Germany, Switzerland, France, Belgium, Netherlands), the device classes being pursued mostly fell into the following categories:

The process and device technologies used in these efforts include all approaches and a variety of materials and processes. The European microsystems community appears to be very pragmatic in the ways it approaches miniaturization; the Europeans are not partisan to a particular process or technology but usually have a variety of process capabilities at their disposal either internally or through collaborations within the numerous European and national programs. They focus on the goals (products), not the means (processes) to achieve them. This being said, there appears to be a lot of work on high aspect ratio devices through processes such as deep reactive ion etching (DRIE) and LIGA-like processes.

Silicon micromachining is only one of the tools employed in the quest for miniaturization. We heard of work on polymers, glass, and even metals. CMOS-based research centers (such as IMEC, Delft, or Siemens/Infineon) are naturally approaching the microsystems field from the point of view of enhancing the integrated circuit capabilities through micromachining and forming an integrated, low cost device. However, the majority of the institutions visited do not have the silicon CMOS focus and place less emphasis on silicon integration as they are targeting small to medium volume applications, such as medical applications, with a less restrictive cost environment.

In summary, the focus in the European microsystems community is on device and system miniaturization using the processes and materials fulfilling the application requirements. The push seems to be towards simple miniaturized devices that provide incremental performance advantages over their traditional counterparts, not revolutionary new concepts and break-through applications. There is a large amount of work going on in the micro-fluidics area with medical, biological and chemical applications targeted as the next area where microsystems will move into the commercial markets.

This section summarizes some of the most interesting device and process developments that the MCC STT group encountered during its visit to Europe. The chosen technologies are meant to be examples of the type of activities underway in Europe and to illustrate the diversity of materials, devices and applications that are being envisioned. Many of the presented examples do not fall under the strict U.S. definition of MEMS, but are qualified as microsystems in Europe. The selected devices and technologies are also in this sense representative of the efforts presented by our host companies.

Fluidic Microsystems

Piezoelectric Micro-Pump (FhG-IFT)

After some experimentation with electrostatically activated micro-pumps, the Insititut fur Festkoerpertechnik (IFT) in Munich has now developed piezoelectrical micro-membrane pumps. A bulk piece of piezoelectric material is attached to a thin silicon membrane and forms the pumping mechanism. The flow rate of the micro-pump can be adjusted by the frequency and the amplitude of the control signal. It is self-priming and bubble-tolerant. The IFT has also developed the control electronics, a package which integrates pump and electronics, and has demonstrated various prototypes.

The micro-pump works for various liquids (water-based, organic) and gases. IFT targets medical applications, especially precise dosing of drugs. A working prototype was passed around during our visit and samples are available for evaluation. The prototype size is 7 mm x 7 mm x 1 mm and can pump up to 1 ml/min of liquid or 3 ml/min of gas. IFT representatives did not want to discuss the external connection and packaging know-how.


Fig. 5.1. (a) Photograph illustrating size of FhG-IFT piezoelectric micropump (Source: FhG-IFT).


(b) Working principle: A bulk piece of piezoelectric material actuates the silicon pump diaphragm. The inlet and outlet microvalves open in an alternating fashion as the pressure in the pump chamber oscillates.

Micro-Channel Dosing (FhG-IFT)

The IFT has developed silicon micro-capillaries for precise dosing of medications. The micro-channels provide excellent reproducibility, miniaturization, batch fabrication and significantly higher dosage accuracy. This appears to be a very simple technology with a large potential market. IFT is currently working on a portable drug delivery system concept (MEDOS) which combines the microflow restrictor with a pressurized drug reservoir, a pinch valve, a temperature sensor, and a micro-controller.


Fig. 5.2. Micro-capillary devices for precise drug dosing (Source: FhG-IFT).

Impedimetric Bio-Sensors (IMEC)

IMEC is developing, in collaboration with Innogenetics, impedimetric biosensors that allow for the detection of affinity binding of biomolecular structures (e.g., antigens, DNA) by impedimetric measurements. The presence of these DNA sequences or antigens/antibodies can be detected by looking for the binding of these molecules to selective probes.

Nanoscaled interdigitated electrode arrays are made with deep U.V. lithography. Electrode widths and spacings from 500 nm down to 250 nm are produced on large active areas (0.5 mm x 1 mm). An impedimetric DNA- or immunosensor measures the binding of a target molecule to selective probes by measuring changes in the electric properties in the vicinity of an electrode. For instance, when antibodies bind to antigens, the change in electric properties results in a change of impedance, enabling the measurement of a direct electrical signal. Claimed advantages over optical detection methods are that no labeling or enzymatic reactions are needed. The direct electrical read-out also eliminates the need for optical components.


Fig. 5.3. IMEC impedimetric biosensor for DNA-and immunosensing (Source: IMEC).

ISFET (TIMA, IFT, MESA)

The structure of the ion-selective field-effect transistor (ISFET) is essentially a MOS transistor which can be rendered ion-sensitive by eliminating its gate electrode in order to expose the gate insulator to a solution. The gate insulator or ion-sensitive membrane senses the specific ion concentration (i.e., measures the pH), generating an interface potential on the gate; a corresponding drain-source current change in the semiconductor channel is observed. The only difference between the electrical circuits is the replacement of the gate of the MOS transistor by the series combination of the reference electrode, electrolyte and chemically sensitive insulator or membrane.

TIMA has developed a novel architecture of the ISFET sensor interface circuit for a biomedical microsystem using standard CMOS technologies. It is a differential configuration with two ISFET devices (one with Si3N4 ion sensitive layer, the other with SiO2 sens. layer) and realized in a 2.5 Ám CMOS compatible technology. The sensor interface has a current output signal and low silicon area requirements. The circuit architecture provides digital facilities enabling system calibration.

Several other European research laboratories (IMEC, IFT, MESA) are also working on ISFET microsystems for chemical and biological applications. For instance, IMEC is developing ISFET devices as blood gas sensors in collaboration with Siemens. MESA (University of Twente) is focused on improving the drift and stability properties of ISFETs and integrating ESD protection.


Fig. 5.4. Schematic ion-sensitive field effect transistor (ISFET) diagram (Source: TIMA).

Micro-Pneumatic Valve (FhG-IFT)

The IFT is also pushing the development of miniaturized pneumatic valves. This work was funded by Hoerbiger Origa GmbH. IFT developed and produced a small volume of normally-open (NO) silicon microvalves with electrostatic actuation for dry air applications.

The microvalves have a size of 6 mm x 6 mm x 1 mm, an operating pressure of up to 16 Bars and a switching time of 1 ms.


Fig. 5.5. Pneumatic microvalve (Source: FhG-IFT).b

Silicon Micro-Reactors (FhG-IFT)

Miniaturized chemical reactors at IFT target analytical applications. The micro-reactors contain the solid-state reagents which immobilize biochemical reactive components (e.g., enzymes, antigens and antibodies), i.e., the reactor walls are coated with chemically reactive substances. The results of the reactions are tested either inside the reaction chamber itself (on-column) or in a post-column. The advantages of miniaturization include short diffusion pathways, high surface-to-volume ratios which increase reaction speeds, low internal volume, reduced amount of reagents, and well-defined flow behavior.

The silicon micro-reactors developed at IFT are meander-shaped or straight channels surface-etched into silicon. The inlet and outlet connections are to the back-side of the chip. The micro-channels are sealed by anodic bonding to pyrex glass. The channel walls are then prepared with the reagents which can also be regenerated after use. Reaction detection can be done by on-column photometry and chemiluminescence, for instance.

Practical testing of prototypes was done in collaboration with the University of Regensburg. First tests included enzyme-linked-immunosorbent assay (ELISA) for the detection of toxins and pesticides.

The same technology is being extended to fabricate micro-mixers and micro-reaction-based micro-total-analysis systems (ÁTAS). For the micro-mixers the micro-channels with several inlets and one outlet are etched into a thinned silicon membrane backed by a bulk piezo-disk. Given the small dimensions of the channels, the fluid flow is laminar. The vibrations of the piezo-actuator are needed to mix the reagents fed in through the various inlets.


Fig. 5.6. 1st and 2nd generation microreactors. (Source: FhG-IFT)
(a) 1st generation: 220 mm2 area with post-column detection
(b) 2nd generation: 24.5 mm2 area with optical on-column detection
The 3rd generation will be reduced to an area of 8 mm2.

ÁTAS (MESA, EPFL, IFT)

Many research laboratories in Europe are working on micro total analysis systems (ÁTAS).

The MESA Research Institute at the University of Twente is pursuing a ÁTAS effort for bioprocess monitoring in collaboration with Delft University. The objective is to monitor continuously nutrients and agents in the production of therapeutic agents by living cells in a bio-reactor. The technique to separate and analyze components in a solution used by MESA is capillary electrophoresis. A high electric field is applied along a small capillary or microchannel; this results in the separation of the solution components in the microchannel. Heat dissipation is a major concern in order to maintain a controlled temperature environment.


Fig. 5.7. Schematic diagram of micro-Total Analysis System (ÁTAS) (Source: FhG-IFT).

MESA uses a miniaturized transparent insulating channel (ÁTIC) process technology to construct micro-channels on a glass substrate (see Fig. 6.8). These devices were fabricated using deep reactive ion etching (DRIE), insulating film deposition, anodic bonding and back etching of the silicon. This application takes advantage of the processability of silicon and thermal conduction of silicon and combines these properties with the use of glass to withstand the high applied voltages to yield a compromise between electrical insulation and thermal dissipation requirements.

EPFL is similarly developing a DNA-microanalyzer for biomedical applications and IFT is working on various aspects of micro total analysis systems.


Fig. 5.8. MESA ÁTAS fabrication approach combining glass and silicon (Source: MESA).

Mechanical MEMS

Low-power Accelerometers (CSEM)

CSEM has been manufacturing a family of capacitive accelerometers for several years and the most recent addition to the 61xx family was discussed during our visit. The targeted applications are battery-powered monitoring and alarm systems for automatic shock detection-recording, patient monitoring, seismic activity, GPS systems, and tilt control of drilling equipment.

The accelerometer is built using a 3-wafer capacitive sensor technology. This uses a 3-wafer silicon micromachining and fusion bonding process to produce a proof mass, spring and fixed capacitor plate. The interface circuits are designed to function with different sensors. The specific sensor configuration and calibration data are written into an EEPROM. It is a 2 g device with a sensitivity of 2 mg.

CSEM has also collaborated with CERME, a French aeronautical company, to develop accelerometers with high stability. The sensing cell supplied by CSEM is a capacitive sensor of the CSEM family. These accelerometers are used for aerospace navigation, navigation in small commercial airplanes, and in helicopters.


Fig. 5.9 CSEM 6100 accelerometer with interface circuitry in a T08 package (Source:CSEM).

Pressure sensors (LETI)

The Laboratoire d’Electronique et de Technologie d’Instrumentation (LETI) of the Commission d’Energie Atomique (CEA) in France has developed a variety of pressure sensor approaches.

The first approach uses bulk micromachining of silicon-on-insulator (SOI) wafers and multi-wafer bonding with piezoresistive silicon gauges insulated from the substrate to detect pressure changes. The device measures approx. 6 mm x 6 mm and achieves good linearity (0.01%) without temperature compensation from 0 to 2 Bars and over an extended temperature range of –20 to 160 degrees centigrade. The application targets are geophysics, aeronautic, industrial process control, and instrumentation.

A second type of pressure sensor is surface micromachined into SOI wafers. It uses epitaxial silicon on insulator substrates as starting material. It uses deep reactive ion etching (DRIE) and the buried oxide layer serves as the sacrificial release layer. In this case pressure changes are measured capacitively. The device measures 1 mm x 1 mm and operates from 0 to 1 Bar with a sensitivity of 3 pF/Bar over a temperature range of –40 to 125 degrees centigrade. The application targets are medical.

A third approach to pressure sensors which was mentioned briefly during our visit at LETI is the use of bulk micromachined quartz with wet etching.


Fig. 5.10. SOI pressure sensors (Source: LETI)
(a) Bulk micromachined


(b) Surface micromachined

Side-Impact Pressure Sensors and Accelerometers (Siemens/Infineon)

At Siemens the MEMS device development takes place on its standard 6-inch BiCMOS line at the Corporate Research Center. In order to achieve the goal of low cost devices fully integrated with the signal processor, Siemens engineers have to date operated under the constraint of only using existing processes and equipment. This approach guarantees a low cost manufacturable product, but limits the type of devices and markets they can get involved with.

Siemens has developed side-impact crash detectors based on a pressure sensor. They are claimed to be faster and cheaper than accelerometers for this application. Silicon oxide is used as the sacrificial layer underneath a poly-silicon membrane. The integrated circuitry is fabricated first, then the MEMS devices are added and finally the wafer is metallized. The devices are calibrated and trimmed electrically at the wafer level. The MEMS device processing is said to add less than 10% to the BiCMOS wafer processing cost.

Development of accelerometers is also underway. In this case the devices are capped on wafer and then cavity micro-machined. Again the interface electronics is integrated on-chip.

Development of CMOS gyroscopes will require addition of equipment (DRIE) to the existing line.

Accelerometers (LETI)

LETI is also exploring several versions of accelerometers. All are based on one of LETI’s core competencies, the fabrication and processing of SOI substrates.

The first one is a surface micromachined device which uses deep reactive ion etching on SOI substrates with the buried oxide as a sacrificial layer, similarly to LETI’s micromachined pressure sensor. The structure is a standard inter-digitated comb with capacitive read-out. It is a low-g design (Full scale: 2 g) targeting automotive and medical applications.

The second effort uses the same surface micromachining technology in combination with a different design for a 50 g crash detector to be used in transportation and automotive applications.


Fig. 5.11. Low-g accelerometer on SOI (Source:LETI)

These devices are very interesting, not so much due to the device technology, but because of the associated packaging development, which was the strongest that the MCC team observed on its tour. LETI has invested considerable effort into two packaging approaches: flip-chip and “plip-chip.”

The flip-chip approach shown in Fig. 5.12 utilizes microbumps to assemble the MEMS device to the interface ASIC. The solder balls end up approximately with a 60 Ám diameter and a 40 Ám height. A solder preform is also deposited around the MEMS chip in order to form a hermetic seal during reflow. LETI claimed this to be a flux-less flip-chip process.


Fig. 5.12. LETI flip-chip packaging of MEMS accelerometers (Source:LETI).

The “plip-chip” approach is an even more aggressive wafer-scale packaging approach that results in SMT-ready chip-scale packages. Vias are etched anisotropically into a thin silicon lid to reroute the electrical connection pads in 3D to the external surface of the package.


Fig. 5.13. LETI "plip-chip" packaging for MEMS accelerometers (Source:LETI).
a) completed chip-scale SMT package


(b) Via etched into silicon lid to bring out electrical connections to package surface

These technologies have been transferred by LETI to its spin-off company, Tronics.

Telemetry System For Animal Monitoring (KU Leuven)

The MICAS (Medical & Integrated Cicruits and Sensors) group at the Department of Electrical Engineering (ESAT) at the University of Leuven is focusing on biomedical implantable systems with remotely programmable data acquisition systems. Wireless data transmission is performed either by a continuous (radio frequency) link, or by a close proximity inductive field (transponder). In order to avoid the use of batteries in implants, in some cases an inductive powering link is used by closely coupled coils. Typical power levels are in the order of 20 mW, but recently a functional system capable of transmitting a power level of 20 W for an implantable system has been developed (to drive an implanted liver pump).

A recently deployed device is an intelligent miniaturized injectable biotelemetry transponder for identification and measurement of temperature and activity (i.e., acceleration) for use in large scale animal husbandry (e.g., pigs). The transponder is capable of transmitting on a close range (50 cm) the physiological data that are captured over a given period. By means of a bi-directional telemetry link between the device and an external transceiver all the operation modes (temperature and activity range/sensitivity, sampling periods, oscillator frequencies, etc.) are programmable, even when the device is already injected in the animal. The lifetime of the device is claimed to be 6 to 12 months, depending on the sampling frequencies. The sensor and electronics are packaged hermetically in a bio-glass capsule with an external diameter of 6 mm and a length of 40 mm which is injected into the pigs. The monitoring takes place while the pigs are at their feeding stations.


Fig. 5.14. Injectable sensor capsules for temperature and activity monitoring in animals. The capsule diameter is 6 mm and its length is 40 mm. (Source: KU Leuven).

GyrAcc (MESA)

The bio-sensor group in the MESA Research Institute at the University of Twente is applying MEMS accelerometers and gyroscopes to assist paraplegic persons. The Functional Electrical Stimulation (FES) project is directed towards restoration of mobility in paralyzed individuals. The research encompasses sensing of human movement and interface forces using natural and artificially engineered sensors in combination with electrical stimulation of the neuromuscular system. The ultimate goal is the restoration of mobility. This project is done in collaboration with BMTI and Roessingh Research and Development.

Within this FES project there is a need for monitoring rotations as well as accelerations in three dimensions in order to monitor the movements of human limbs. Until now experimental clinical measurements were done by means of 6 separate sensors which produced a cumbersome system (4 cm x 4 cm x 4 cm). The goal of the GyrAcc project is to produce an integrated micromachined system (3 mm x 3 mm x 3 mm) which ultimately can be implanted.


Fig. 5.15. "3-D-Gyracc" under development. Target size is 3 mm x 3 mm x 3 mm. (Source: MESA).

Optical Microsystems

Optical micro-switch (LETI)

LETI developed an electro-statically controlled 1 x 2 micro-optical switch for fiber connection applications. A comb drive moves the incoming wave guide between two outgoing waveguides (see Fig. 5.16).


Fig. 5.16. Micro-optical switch (Source: LETI).
(a) view of waveguides and electrostatic comb drives


(b) magnified view of movable waveguide

This device offers switching speeds of milliseconds and low driving power. The insertion loss is less than 4 dB in either position. 1 x 4 and 1 x 8 switches are under development.

Micro-optical vibration sensor (LETI)

This device was developed by LETI for deployment on electrical power generators where the high electro-magnetic field environment makes electrical read-out devices impractical. It is a modified version of LETI’s micro-optical fiber switch. In this design the incoming optical waveguide is part of a vibrating seismic mass, which, depending on its position, feeds the light into one of two outgoing fibers. Vibrations result in intensity variations for both outputs, which can be analyzed to provide information on the vibrational amplitude.


Fig. 5.17 Micro-optical vibration sensor. (Source: LETI)

Micro-Optical Scanner (EPFL)

EPFL is developing an integrated optical micro-scanner head. The application target is a low-cost handheld barcode reader. This work is supported by the Swiss MINAST priority program.

The scanning function is produced by a micromachined bimorph actuator based on silicon and thin film technology. The device consists of a metal mirror located on the tip of a thermal bimorph actuator beam (Fig. 5.18). Mirrors of 500 x 300 to 800 x 800 Ám2 with resonant frequencies varying from 600 to 100 Hz have been fabricated (Fig. 5.18). Mechanical scan angles of above 90░ were achieved and devices have run through fatigue tests of billions of cycles at 300 Hz and 90░ deflection. The power consumption of the device is typically 1 mW for 30░ mirror deflection.


Fig. 5.18. Scanning mirror prototype. The mirror diameter is 600 Ám (Source: EPFL).

Laser Beam Scanner (LETI)

LETI is developing this device technology with multiple applications in mind. Examples include bar code readers, holographic storage, automotive collision avoidance, etc. It is a one-dimensional scanner with two lenses etched into a silicon oxide layer. One of the lenses is fixed, the other lens can be moved using an electrostatic comb drive. A 150 V drive results in approximately 5 degrees of beam deflection.


Fig. 5.19. One-dimensional laser beam scanner (Source: LETI).
(a) complete device (5 mm x 1.4 mm)


(b) magnified view of 2 lenses and electrostatic drive

Uncooled Micro-Bolometer (LETI, IMEC)

There are several efforts we observed at various European institutions regarding the development of uncooled infra-red detectors.

LETI is targeting low cost surveillance applications, preventive maintenance, driving aids, etc. LETI’s approach uses amorphous silicon pixels with a 50 Ám pitch and integrated CMOS read-out. The silicon pixels are suspended on vias above the substrate in order to provide thermal isolation. A demonstrator prototype with 256 x 64 bit resolution was developed by LETI. It exhibits a 90% fill factor, sensitivity of 50 mK at room temperature (using an f/1 stop and a 30 Hz scan rate). This technology is being transferred to SOFRADIR.


Fig. 5.20. Room temperature micro-bolometer (Source: LETI).

IMEC is also applying its state-of-the-art integrated circuit and materials capabilities to the development of room temperature bolometers. Its approach uses CVD-deposited poly-SiGe, which has a thermal conductivity four times lower than poly-Si. This helps with the thermal isolation of the suspended structures. Also, the built-in stress in poly-SiGe can be minimized at low deposition temperature, a property which enables easy integration with CMOS circuitry.


Fig. 5.21. Magnified view of poly-SiGe pixel in IMEC’s room temperature bolometer effort (Source: IMEC).

Spatial Light modulator (FhG-IFT)

The Fraunhofer Institute in Dresden has developed a spatial light modulator using a viscoelastic control layer. Applications range from display to optical image processing.

This device uses electrostatic forces to change the distance between two electrodes separated by a low modulus elastic dielectric. An array of pixels is covered with silicone gel on top of which a metallic mirror film is deposited. A high voltage (approx. 250 volts) is applied across the silicone dielectric on all pixels. This puts the silicone into uniform compression. An alternating voltage of +/-15 volts is then applied between neighboring pixels and causes the silicone to deform. The deformation profile acts as a phase grating creating a phase modulation.

Pixel sizes range from 16 x 16 Ám2 to 24 x 24 Ám2 and array sizes are as large as 256 x 256 pixels.


Fig. 5.22. Principle of operation of Fraunhofer spatial light modulator.

Electrical MEMS switches

Electro-Static Micro-Relay (FhG-IFT)

The goal is miniaturization of relays, thin-film fabrication and integration with integrated circuits. Applications range from low-power battery-powered devices to high frequency signal switching.

The design of the IFT microrelay is a T-type configuration in order to be able to isolate the signal path from the switch circuitry. To date no separate control electrode has been used. Rather the conductors were built up on an insulating polyimide layer and the substrate biased to close the relay. Performance to date was reported to us as a device through-resistance of 3 W, actuation voltages of 20-70 volts, and a switching time of less than 5 Ás.


Fig. 5.23. FhG-IFT micro-relay with electrostatic actuation (Source: FhG-ITF).
(a) SEM photograph of 1st generation device


(b) next generation design

Magnetic Micro-Relay (IMEC, CP Clare)

This device development was driven by CP Clare and undertaken in collaboration with IMEC, CSEM, ARITECH and SPEA.

The device is a miniaturized electromagnetic relay assembled from two parts. One part contains an electroplated copper coil, plated FeNi poles and lower gold contacts deposited on an FeSi substrate. The other part contains an NiFe armature with the upper gold contacts fabricated on a silicon chip. The two parts are assembled using a flip-chip process in which a solder seal ring is also reflown around the periphery to form a hermetic cavity with a protective atmosphere for the contacts. The whole device is then packaged into a standrad plastic SMT package.

When a current is applied to the copper coils the FeNi is being magnetized and closes the gold contacts. The magnetic actuators provide higher contact forces than their electrostatic counterparts and yield a relay on-resistance of only 0.4 W. The switching speed is slower however at 1-2 ms. Life times in excess of 106 cycles have been demonstrated.


Fig. 5.24. Electro-magnetic micro-relay (Source: H. Tilmans, IMEC).

(a) Schematic of device
(b) Schematic of packaged device
(c) Electromagnetic chip
(d) Armature chip
(e) Assembled device
(f) Packaged device

Siemens Electrostatic Microrelay

Siemens Electromechanical Components (EC) has developed and is bringing to market an electrostatically actuated microrelay. The application markets are automated test and measurement systems, telecommunications, battery-operated consumer electronics, etc.

The device is fabricated out of two parts. The actuation electrodes and one electrical contact are thin-film deposited and patterned on a glass substrate. The actuation counter electrode and the other signal contact are thin-film deposited and patterned on silicon. The silicon is back etched to release the thin-films which curl up due to built-in stress. When bonded together the thin films therefore form a wedge-like contact with the two actuation electrodes only separated by a thin oxide layer at the device edge. This yields a low actuation voltage (< 15 V) and high contact force (contact resistance < 1 W), as the contact gradually rolls down upon application of an actuation voltage (see Fig. 5.25). The electrostatic actuation provides low power operation and fast switching times (approx. 0.2 ms).

The microrelay was developed at Siemens’ R&D facilities in Berlin. It has been transferred to a manufacturing unit in the Far East and samples are available at this time. This appears to be the first new device to be commercialized in the class of electrical applications.


Fig. 5.25. Siemens electrostatic silicon microrelay (Source: Siemens).
(a) Packaged microrelay in SO-8 package


(b) Schematic cross-section of microrelay


(c) Principle of operation

Passive Microstructures

Micro-Integrated Hall Sensor (EPFL)

The Institute for Microsystems at the Swiss Federal Institute of Technology in Lausanne (EPFL) undertook this project as a co-development with the industrial partner Sentron AG of Zug (Switzerland), illustrating the tight coupling between university research and industrial applications that the MCC/WTEC group noticed throughout Europe. Sentron AG is a small company specialized in sensor microsystems and the initiator of the highly sensitive Hall sensor patent. This device work appears to be quite unique to EPFL.

The driving factor for this project was the quest for very high sensitivity integrated sensors, which are both compact and economic to use, for contactless switches, position indicators, and meters as well as in systems for the measurement of speed and rotation. In the automobile Hall sensors can be found in systems such as ABS, speed meters, electric windows, control systems for doors, headlights, the engine and the gear box. Many Hall probes can be found in home appliances, domestic electronics and computers.

Traditionally, bulky flux concentrators are used to increase the sensitivity of the device. The goal of this project is to use low cost integrated flux concentrators with microelectronic integration processes. The key development is a process to add soft magnetic material to an IC. The ferromagnetic material is simply attached by an adhesive process to the wafer, and subsequently patterned through a lithography and wet etch step. The concentrators are integrated directly onto the silicon wafer already containing thousands of Hall sensors and the wafer is then cut into single Hall probes (Fig. 5.26) which are packaged like standard integrated circuits (Fig. 5.27).

Fig. 5.26. (a) Macro-Flux-Concentrators used in industry
(b) Hall device with integrated flux
concentrators (Note the scale difference)


Fig. 5.27. The cylindrical Hall device measures the circular field under the air gap between the two flux concentrators (Source: EPFL)

The unit cost for the integration of the concentrators is claimed to add only 25 percent to the price of the sensor, whereas the equivalent detectivity of the sensor is multiplied by five.

The active area of their Hall sensor has a cylindrical shape. The makes it very suitable for measuring the circular field under the air gap between two magnetic flux concentrators (Fig. 6.27).

This device was demonstrated to the MCC/WTEC team and the sensitivity compared to commercially available Hall sensors. The integration technology for ferromagnetic layers opens new possibilites in the area of magnetic sensors: EPFL is currently working on different projects using two and three dimensional Hall sensors, magnetic “choppers” and “fluxgates.”

Angular Position Sensor (EPFL)

The objective of this project is to develop a contactless angular position sensor with an accuracy equal or greater than the commercial sensors, but in a much smaller volume. One of the applications shown during the laboratory tour was as a sensor used in the feedback control of a brushless micromotor. This approach is going to be used to improve control and to achieve higher reliability of brushless micromotors. This work was done with RMB SA as an industrial partner and co-funded by the Swiss Federal Microsystem Program, MINAST.

As measurement principle, a small magnet rotating above a multidimensional Hall sensor is used to indicate the angular position of an axis. This sensor microsystem is produced in standard CMOS technology, in order to enable the integration of the first amplification levels.

The sensor is based on the vertical Hall principle. Two vertical Hall devices are disposed in a cross shape (Fig. 5.28) and each element gives a component (sin, cos of the angle) of the magnetic field in the plane of the sensor.


Fig. 5.28 Schematic of the 2-D sensor with the cross-shape of two vertical Hall sensors on the same substrate (Source: EPFL).


Fig. 5.28. The sensor measures the magnet’s angular position with a precision of about 1░ (Source: EPFL).

In order to determine the angular position of a rotary axis, a permanent magnet is fixed on the engine axis just above the sensor. To extract the angle between the magnetic field and the sensor plane, one processes both signals obtained from the 2-D sensor.

A prototype was developed using the above principle (Fig. 5.29); the diameter of the magnet was 1.5 mm and the dimensions of the 2-D sensor was 3 mm x 1 mm.


Fig 5.29. LETI magnetic devices on silicon (Source: LETI).
(a) vertical SiO2 gap and electroplated polar pieces for hard disk drive heads


(b) plated copper coil for integrated inductors


(c) plated magnetic NiFe microstructures

An accuracy of less than 1░ is obtained in the temperature range of –10░C to 60░C, with the offset and temperature dependence electronically compensated. Under stabilized temperature this accuracy can be better than 0.2░. This approach is going to be used for improved control and to achieve higher reliability of brushless micromotors.

Magnetic Devices On Silicon At LETI

LETI has developed various technologies necessary to implement magnetic microtechnology on silicon. Among these are electroplating of giant magneto-resistive (GMR) films, patterned ferromagnetic layers and copper windings. The targeted applications include magnetic recording planar silicon heads for hard disk drives, and helical scan thin-film recording magnetic heads.

Inductive Microsensors (CSEM)

Researchers at the Centre Suisse d’Electronique et de Microtechnique SA (CSEM) have developed the processing technology know-how to produce copper coils with electro-plated thin layers of ferromagnetic material. The process technology was first developed for a magnetographic printing head. They are now applying this process to fabricate planar spiral coils with a ferromagnetic core on chip as key elements of integrated inductive microsensors with interface circuits.

The inductive microsensor detects the position of a structured metallic target such as a linear or angular gear. The inductive micro-sensor is composed of two silicon chips, one for the integrated micro-coils and the other for the integrated interface circuit. This solid-state sensor has no mobile parts and therefore promises to be robust and reliable. The sensing magnetic AC-field is generated by an oscillator circuit and one of the micro-coils. The amplitude of the magnetic field is modulated by a structured metallic target such as a gear tooth or slot. The position of the target is detected by sensing micro-coils and an amplifier-demodulator arrangement.

The applications for this type of device include angular encoders in electrical motors, linear encoders for valves, machine tools and robotics, and automotive ABS gearwheel sensors for position and speed.


Fig. 5.31. Inductive position sensor (Source: CSEM).
(a) Finished product in application


(b) Sensor before packaging

UV Flame Detector (EPFL)

Even though not a MEMS device under the more restrictive U.S. definition of the term, this effort was presented to us as a highlight of integrated microsystem technology at EPFL. EPFL developed a fully integrated sensor microsystem for blue/ultraviolet (UV) radiation detection using low cost CMOS technology. It is intended for a high-volume application, namely flame detection in gas burners. This sensor is meant to be the key element of the burner security system. This project was funded by the Swiss MINAST program with Landis & Staefa (Germany) as the industrial partner.

The sensor, covering a silicon area of 2.2 mm2, was fabricated using a low-cost CMOS 0.5 Ám technology. It is composed of a 1 mm2 UV-selective stripe-shaped photodiode, combined with a smaller infrared (IR) photodiode to achieve a high UV selectivity. This photodiode system has maximal responsivity at a wavelength of 420 nm with 43 percent quantum efficiency. A ratio of the responsivities at the wavelengths 420 nm and 1 Ám of 560 is achieved without the use of optical filters. The realization of the photodetection part in a CMOS process allowed on-chip co-integration of additional electrical functions.

This optoelectronic microsystem for UV detection was demonstrated to MCC/WTEC group.


Fig. 5.32. UV-sensor microsystem (Source: EPFL).
(a) Photograph: the UVS-photodiode is in blue, the IR-photodiode is the upper rectangle, the interface electronics are at the chip center


(b) Measured responsivity vs wavelength of EPFL photodiode. For comparison, the spectral response of a commercially-available UV-enhanced photodiode is shown.

References

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Published: January 2000; WTEC Hyper-Librarian