Michael J. Schoening

Semiconductor-based field-effect structures for chemical sensing

Silicon sensors can be fabricated as small, rugged and reliable chip devices with a broad field of applications in medicine, biotechnology, food analysis and environmental monitoring. Thus, there is an increasing demand in realizing such sensors for the determination of, e.g. chemical and biological quantities in aqueous solutions. By developing semiconductor-based field-effect structures, moreover, their main advantage is due to the combination of both the physical effect as the transducer principle and the deposition of the sensitive layers directly onto the silicon chip. In this work, different sensor types that are originated from the field effect are presented: The capacitive ElS (electrolyte-insulator-semiconductor) sensor is suitable for the pH detection using the capacitance/voltage technique. By immobilizing an additional enzyme layer, e.g. of penicillinase, a biosensor has been realized. Both sensors can be integrated as an EIS sensor array. The utilization of the porous silicon technology offers the possibility of a further miniaturization. The LAPS (light-addressable potentiometric sensor) is based on the identical ElS structure. Here, each measuring point on the surface can be arbitrarily addressed by a probing light. The resulting photocurrent is generated as the sensor signal. This arrangement also allows a two-dimensional mapping of the spatial distribution of ions or molecules.

Pulsed-laser deposition as a novel preparation technique for chemical microsensors

The application of sensitive layers for chemical microsensors consisting of multicomponent compositions and dielectric materials requires specific deposition techniques, since the different chemical and physical properties of the respective components can be significantly disturbed during the deposition process. To avoid this drawback, the pulsed laser deposition technique is suggested as a novel thin film preparation method for such sensor devices.

A Novel Gas-phase Hydrogen Peroxide Sensor Basing on a Combined Physical/chemical Transduction Mechanism

In this work, different set-ups as well as different transducer materials have been investigated in order to develop a hydrogen peroxide (H 2 O 2 ) sensor for the gas phase. The sensor is based on a combined physical/chemical transduction mechanism and should be able to detect high H 2 O 2 concentrations up to 10 Vol.%. Different sensor arrangements are presented that are based on a “three sensor” cell and a diffusion cell. As transducer materials manganese oxide and copper alloys have been investigated. For the reference part of the sensor set-up, Teflon and enamel have been tested as passivating material.

Thin-film electrodes for trace metal analysis by dc resistance changes

The lateral d.c. resistivity of thin metal films with layer thicknesses of less than 30 nm is increased due to the adsorption of certain particles and is decreased by their desorption. The contribution of the adatoms to the film resistivity can be understood similarly to the effect of foreign atoms in a bulk metal. The magnitude of the resistivity increase is related to the surface coverage of the thin metal film. Using thin metal films of gold as working electrodes in a conventional three-electrode arrangement, a novel electrochemical microsensor, based on the described mechanism of the surface resistivity changes has been developed. The thin film sensor has been prepared by means of process steps of silicon planar technology. With this sensor the trace analysis of heavy metals, such as cadmium, lead, nickel, thallium, and zinc ions as well as cadmium-EDTA complexes in aqueous solutions is possible. The different species could be distinguished from each other due to their characteristic stripping potentials. For the investigated species a linear signal relation has been obtained over a wide range of concentrations from several ppb to some ppm.

Application of Thin-Film Amorphous Silicon to Chemical Imaging

A thin-film amorphous silicon (a-Si) deposited on a glass substrate was employed as a semiconductor material for the chemical imaging sensor, which can visualize the distribution of ion concentration in a solution. The sensing properties and the spatial resolution of the a-Si sensors were investigated. Nearly-Nernstian pH sensitivities and submicron resolution were demonstrated, which suggests the superior performance of the chemical imaging sensor based on thin-film a-Si.

Insect chemoreceptors coupled to silicon transistors as innovative biosensors

Applications like the warning about agricultural pest infestations, the detection of spoilt food during storage and transport as well as the monitoring of smoldering fires require highly selective odor sensing techniques. Insect antennae that have been optimized by evolution over million of years are most suitable for such a sensitive and selective detection of certain organic substances in air. The utilization of this highly specialized sense of smell from insects needs in terms of analytical tools, however, an adaptation of the antenna to the microelectronic technique. Therefore, a beetle/FET (field-effect transistor) interface as an innovative biosensor has been developed. This BioFET (biologically sensitive FET) is based on the direct combination of the intact chemoreceptor of an insect with the gate of a FET by means of an electrolyte solution. Depending on the experimental set-up, two different biosensor configurations, namely a whole-beetle BioFET and an isolated-antenna BioFET have been designed. In both configurations, the organic compound that is detected by the beetle initiates a recognition process at its nerve cell membranes, which results in a net potential over the whole insect antenna. Then, this potential drop modified the gate conductivity and consequently, the drain current of the FET. By applying various kinds of insect antennae (e.g. ofthe Colorado potato beetle and the steelblue jewel beetle) different odor concentrations, such as cis- 3-hexen-1-ol, guaiacol and 1-octen can be detected down to the low ppb range.

Capacitive field-effect (bio-)chemical sensors based on nanocrystalline diamond films

Capacitive field-effect electrolyte-diamond-insulator-semiconductor (EDIS) structures with Oterminated nanocrystalline diamond (NCD) as sensitive gate material have been realized and investigated for the detection of pH, penicillin concentration, and layer-by-layer adsorption of polyelectrolytes. The surface oxidizing procedure of NCD thin films as well as the seeding and NCD growth process on a Si-SiO 2 substrate have been improved to provide high pH-sensitive, non-porous thin films without damage of the underlying SiO 2 layer and with a high coverage of O-terminated sites. The NCD surface topography, roughness, and coverage of the surface groups have been characterized by SEM, AFM and XPS methods. The EDIS sensors with O-terminated NCD film treated in oxidizing boiling mixture for 45 min show a pH sensitivity of about 50 mV/pH. The pH-sensitive properties of the NCD have been used to develop an EDIS-based penicillin biosensor with high sensitivity (65-70 mV/decade in the concentration range of 0.252.5 mM penicillin G) and low detection limit (5 µM). The results of label-free electrical detection of layer-by-layer adsorption of charged polyelectrolytes are presented, too.