Bob Puers

Piezoelectric actuation for application in RF-MEMS switches

RF-MEMS switches are commonly electrostatically actuated. This way of actuation has the advantage of technological simplicity. However the actuation voltage is relatively high. Piezoelectric actuation can have significantly lower actuation voltages, depending on the used materials and geometries. Analysis shows that clamped-free beams and clamped-clamped beams can have a reasonable deflection when aluminum nitride is used as the piezoelectric material. A five mask monolithic process has been developed for the realization of piezoelectrically actuated cantilevers and RF-MEMS switches. The complexity of this process is comparable with the complexity of the process for electrostatically actuated switches. Deflections of piezoelectrically actuated cantilever beams have been measured. Due to a high stress gradient in the beams, the assumptions that have been made in the analysis are not valid anymore. Finit element simulations were needed to verify the measurement data. The simulations fit with the measurements when the following values are taken for the properties of the aluminum nitrde film: Youngs modulus Ep = 320 GPa and piezoelectric coefficient d31 = -3.2 pC/N.

Auto-adhesion model for MEMS surfaces taking into account the effect of surface roughness

Although a lot of work has been done on understanding auto-adhesion (stiction) in MEMS, the effect of surface roughness has never been extensively addressed. In this paper, a model is presented, which describes the auto-adhesion interaction energy of micromachined surfaces in contact. Included in the model is the capillary force, in such a way that it can be readily extended to accommodate electrostatic and van de Waals forces as well, in combination with the roughness of the contacting surfaces. By investigating the effect of the height distribution of the surfaces, a term is derived for the surface interaction energy in different environment conditions as a function of the mean separation between the rough surfaces. To obtain an equilibrium distance between the surfaces, the repulsive part of the interaction is also modeled. The combination of these terms gives us the equilibrium distance between the surfaces and the corresponding surface interaction energy, thereby quantifying the effect environmental conditions have on auto-adhesion. The results of the model agree well with surface interaction energies in MEMS known from literature for different environmental conditions.

A Biomedical Microphysiometer

We report on a micromachined silicon chip that is capable of providing a high-throughput functional assay based on calorimetry. A prototype twin microcalorimeter based on the Seebeck effect has been fabricated by using IC technology process steps in combination with micromachined postprocessing techniques. A biocompatible liquid rubber membrane supports two identical 0.5×2 cm2 measurement chambers, situated at the cold and hot junction sites of a thermopile. The thermopile consists of 666 aluminum/p+-polysilicon thermocouples. The chambers can house up to 106 eukaryotic cells cultured to confluence. The advantage of the device over microcalorimeters on the market, is the integration of the measurement channels on chip, rendering microvolume reaction vessels, ranging from 10 to 600 µl, in the closest possible contact with the thermopile sensor (no springs are needed). Power and temperature sensitivity of the sensor are 23 V/W and 130 mV/K, respectively. The small thermal inertia of the microchannels results in the short response time of 70 s, when filled with 50% of water.Biological experiments were done with cultured kidney cells of Xenopus laevis (A6). The thermal equilibration time of the device is 45 minutes. Stimulation of transport mechanisms by reducing bath osmolality by 50% increased metabolism by 20%. Our results show that it is feasible to apply this large-area, small-volume whole-cell biosensor for drug discovery, where the binding assays that are commonly used to provide high-throughput need to be complemented with a functional assay.Solutions are brought onto the sensor by a simple pipette, making the use of an industrial microtiterplate dispenser feasible on a nx96-array of the microcalorimeter biosensor. Such an array of biosensors has been designed based on a new set of requirements as set forth by people in the field.

Silicon microphysiometer for high-throughput drug screening

We report on a micromachined silicon chip that is capable of providing a high-throughput functional assay based on calorimetry. A prototype twin microcalorimeter based on the Seebeck effect has been fabricated by IC technology and micromachined postprocessing techniques. A biocompatible liquid rubber membrane supports two identical 0.5 X 2 cm2 measurement chambers, situated at the cold and hot junction of a 666-junction aluminum/p+-polysilicon thermopile. The chambers can house up to 106 eukaryotic cells cultured to confluence. The advantage of the device over microcalorimeters on the market, is the integration of the measurement channels on chip, rendering microvolume reaction vessels, ranging from 10 to 600 (mu) l, in the closest possible contact with the thermopile sensor (no springs are needed). Power and temperature sensitivity of the sensor are 23 V/W and 130 mV/K, respectively. The small thermal inertia of the microchannels results in the short response time of 70 s, when filled with 50 (mu) l of water. Biological experiments were done with cultured kidney cells of Xenopus laevis (A6). The thermal equilibration time of the device is 45 min. Stimulation of transport mechanisms by reducing bath osmolality by 50% increased metabolism by 20%. Our results show that it is feasible to apply this large-area, small- volume whole-cell biosensor for drug discovery, where the binding assays that are commonly used to provide high- throughput need to be complemented with a functional assay. Solutions are brought onto the sensor by a simple pipette, making the use of an industrial microtiterplate dispenser feasible on a nx96-array of the microcalorimeter biosensor. Such an array of biosensors has been designed based on a new set of requirements as set forth by people in the field as this project moved on. The results obtained from the prototype large-area sensor were used to obtain an accurate model of the calorimeter, checked for by the simulation software ANSYS. At present, the sensor chip has been designed. Future publication(s) will deal with this part of the work.