Lou Hermans

Characterization and optimization of infrared poly SiGe bolometers

In this paper, we present a complete characterization of poly SiGe bolometers. Devices having different dimensions and different geometry have been fabricated. The dependence of the low-frequency noise and of the temperature coefficient of resistance (TCR) on resistivity in poly SiGe has been measured and modeled. The impact of resistivity, bias voltage, thermal conductance, thickness, and dimensions of the active element on the device performance has been investigated. It has been demonstrated that, by using the appropriate absorber and by optimizing the device parameters, poly SiGe bolometers are suitable for realizing high-performance focal plane arrays (FPAs).

Nanoscaled interdigitated titanium electrodes for impedimetric biosensing

Nanoscaled interdigitated electrodes (IDEs) are developed for the purpose of being used as miniature and sensitive affinity biosensors. Because of the ease to derivatise its surface, oxidized Ti is chosen as an electrode material on a SiO2 substrate. For proof of principle, oxidized and non-oxidized Ti IDEs are characterised in salt solutions and the immobilisation of glucose oxidase is monitored using impedance spectroscopy. Beside transducer development and demonstration, tailored bio interfaces are a prerequisite for the development of sensitive affinity biosensors. Therefore, a characterisation of immobilisations (based on silanisations) on TiO2 and SiO2 is conducted. For this study, different analytical techniques are identified and evaluated. The use of these techniques will enable a thorough characterisation of immobilisation processes, ultimately leading to miniature and sensitive affinity biosensors.

Structural and mechanical properties of polycrystalline silicon germanium for micromachining applications

In this paper, we propose polycrystalline silicon germanium (poly SiGe) as a material suitable for MEMS applications. Films are prepared by chemical vapor deposition (CVD) at atmospheric pressure (AP) or reduced pressure (RP). The structure of the films is investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) for different deposition and annealing conditions. The stress in the as-grown and annealed layers is measured, and the correlation between stress and structural properties is discussed. It is demonstrated that by adjusting the deposition conditions, the stress of the as-grown material can be varied from -145 to 60 MPa. Examples of poly SiGe micromachined devices, prepared at 650/spl deg/C, are presented. It is shown that by using as-grown poly SiGe, it is possible to realize surface-micromachined suspended membranes having 0.6-/spl mu/m-wide and 50-/spl mu/m-long supports. The effect of the average stress and stress gradient on the mechanical stability of surface-micromachined structures is illustrated. Finally, the strain in poly SiGe is measured, and it is found to vary, according to the deposition conditions from -6.88/spl times/10/sup -4/ to 3.6/spl times/10/sup -1/ These values are compared to those measured for APCVD poly Si.

Characterization of bolometers based on polycrystalline silicon germanium alloys

In this paper, we report on the first realization and characterization of uncooled Infra Red (IR) bolometers, based on polycrystalline alloys of silicon and germanium (poly SiGe). Responsivity, thermal conductance, thermal time constant and noise will be analyzed. It will be shown that poly SiGe provides high thermal insulation. An average detectivity of 10/sup 8/ cm/spl radic/(Hz)/W has been measured. We expect that modifications in the device structure could allow to achieve detectivities of 10/sup 9/ cm /spl radic/(Hz)/W.

Project SVAVISCA: a space-variant color CMOS sensor

The paper describes the result of the first phase of the ESPRIT LTR project SVAVISCA. The aim of the project was to add color capabilities to a previously developed monochromatic version of a retina-like CMOS sensor. In such sensor, the photosites are arranged in concentric rings and with a size varying linearly with the distance from the geometric center. Two different technologies were investigated: 1) the use of Ferroelectric Liquid Crystal filters in front of the lens, 2) the deposition of color microfilters on the surface of the chip itself. The main conclusion is that the solution based on microdeposited filters is preferable in terms of both color quality and frame rate. The paper will describe in more detail the design procedures and the test results obtained.

32-pixel FIRGA demonstrator: testing of a gallium arsenide photoconductor array for far-infrared astronomy

A gallium arsenide photoconductive detector, which is sensitive in the far-infrared wavelength range from approximately 60 micrometers to 300 micrometers , offers the advantage of extending considerably the long wavelength cut-off of presently available photodetectors. FIRGA is an ESA sponsored GaAs detector development program which is approaching completion. The FIRGA study is intended to prepare the technology for large 2D GaAs detector arrays for far-infrared astronomy. The primary goal of the development is the preparation of a monolithic 32 element demonstrator array module with associated cryogenic read-out electronics. Continuous progress in material research has led to the production of pure and doped n-type GaAs layers using liquid phase epitaxy. We prepared sample detectors from those materials and investigated their electrical and infrared characteristics. Finally, a multi-layer structured detector device was manufactured. The 4 X 8 element array configuration is defined by sawing a split pattern into the layers with pixel size 1 mm X 1 mm. The device is back illuminated. The 32 pixels are connected to two cryogenic read-out electronics chips mounted close-by. Results of the initial detector performance tests are reported. We determined dark current, responsivity and response transients. Ongoing development activities will concentrate on material research, i.e. the production of n-GaAs layers of ultra-high purity and those with improved FIR characteristics using new centrifugal techniques for material growth.

4x32 FIRGA array: a pacesetter for a 52x32-element gallium arsenide focal plane array

FIRST and SOFIA are both future IR observatories with 3m class main mirrors having sophisticated instrumentation aboard. The present design of the FIRST imaging spectrometer PACS requires two large far-IR photoconductor arrays of 25 X 16 pixels each, the baseline material is stressed and unstressed Ge:Ga. A gallium arsenide photoconductive detector which is sensitive in the far IR (FIR) wavelength range from about 60 micrometers to 300 micrometers might offer the advantage of extending considerably the long wavelength cut- off of presently available photodetectors. FIRGA is an ESA sponsored detector development program on this matter involving international partners. The aim is a monolithic 4 X 32 demonstrator array module with associated cryogenic read-out electronics. Recent progress in material research has led to the production of Te-doped n-type GaAs layers using liquid phase epitaxy. We prepared sample detectors from those material and investigated their electrical and IR characteristics. First measurements indicate that GaAs has in principle considerable potential as a FIR photon detector. Theoretical modeling of GaAs detectors can help with the detector design and allows the prediction of response transients as a function of detector parameters. Present development activities are mainly concentration on material research, i.e. the production of GaAs:Te with improved FIR characteristics. Results of the current test and measurements are reported. The FIRGA study is intended to prepare the technology for large 2D GaAs detector arrays for far IR astronomy.

Modular 25x16 pixel stressed array for PACS aboard FIRST

For the Photoconductive Array Camera and Spectrometer (PACS) 2 sensor arrays consisting of each 16 X 25 pixels are foreseen. The sensors arranged in linear arrays with 16 detectors are tuned to the wavelength ranges 60 micrometer to 130 micrometer and 130 micrometer to 210 micrometer by applying different levels of stress to the Ge:Ga crystals utilizing a special leaf spring which is part of each of the 25 modules. The electronics of the sensors are mounted on the same module but thermally isolated from the sensor level which is linked to a 1.7 K stage. The sensors are read out by a new generation of the integrating and multiplexing cryogenic readout electronics (CRE). With the optical design a 100% filling factor is achieved and with a fore optics made of light cones in front of the detector cavities a high detection efficiency close to 1 is expected. In order to achieve extreme high stress uniformity among all detectors and therefore equal cutoff wavelengths, a high degree of the quality of the Ge:Ga detectors and of the assembling components used for this dedicated stress mechanism is required. The first two engineering modules have been successfully manufactured and tested afterwards. The relative responsivity of a set of pixels has been determined and a good performance demonstrated for the sensors which are very close to fulfill the requirements for PACS aboard the infrared telescope FIRST.

Nanoscaled impedimetric sensors for multiparameter testing of biochemical samples

In this contribution, the development of a nanoscaled interdigitated electrode structure for impedimetric measurements, made by deep U.V. lithography is reported. This biochemical impedimetric detection system has several potential advantages over other detection systems and is easy to integrate in a multiparameter testing system.

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.