Magnetoresistive biosensors use a new detection method for molecular recognition reactions based on two recently developed techniques and devices: Magnetic markers and XMR sensors, where XMR means either giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR). The markers are specifically attached to the target molecules, and their magnetic stray field is picked up by an embedded magnetoresistive sensor as a change of the electrical resistance. Compared to established, e.g., fluorescent, detection methods, magnetic biosensors have a number of advantages, including low molecular detection limits, flexibility, and the direct availability of an electronic signal suitable for further automated analysis. This makes them a promising choice for the detection units of future widespread and easy-to-use lab-on-a-chip systems or biochips. In this article, we discuss recent advances in this field and compare possible approaches toward single molecule detection.
Large opportunities in magnetolectronic devices are opened by the spin dependent tunnelling resistance, where a strong dependence of the tunnelling current on an external magnetic field can be found. Within a short time, the quality of the junctions increased dramatically. We will briefly address some important basics depending on the material stacking sequence of the underlying thin film system with special regard to the ferromagnetic electrodes. Scaling issues, i.e. the influence of the geometry of small tunnelling junctions especially on the magnetic switching behaviour are considered down to junction sizes below 0.01 µm 2 . The last part will give a short overview on applications beyond the use of the tunnelling elements as storage cells in MRAMs. This concerns mainly field programmable logic circuits, where we demonstrate the clocked operation of a programmed AND gate. The second 'unconventional' feature is the use as sensing elements in DNA or protein biochips, where molecules marked magnetically with commercial beads can be detected via the dipole stray field in a highly sensitive and relatively simple way.
Magnetoresistive Biosensors use a new detection method for molecular recognition reactions based on two recently developed techniques and devices: Magnetic markers and XMR -sensors, where XMR means either GiantMagneto-(GMR) or Tunneling-MagnetoResistance (TMR). The markers are specifically attached to the target molecules, and their magnetic stray field is picked up by the embedded magnetoresistive sensor as a change of the electrical resistance. Compared to established, e.g. fluorescent, detection methods, magnetic biosensors have a number of advantages, including low molecular detection limits, flexibility and the direct availability of an electronic signal suitable for further automated analysis. This makes them a promising choice for the detection units of future widespread and easy to use lab-on-a-chip systems or biochips.Both the measurement technique using XMR-sensors as well as new developments in the preparation of magnetic carriers are discussed here. Different configurations are discussed and the results for Giant Magnetoresistance sensors are compared to an analysis of the same biological systems marked with fluorescence dyes. Down to a concentration of about 10 pg/µl of, e.g., DNA molecules, the magnetoresistive technique is competitive with nowadays standard analysis methods. The capability of the TMR sensors to detect even single markers is additionally demonstrated by a model experiment using the tip of a magnetic force microscope to meamic the presence of a magnetic particle on top of the sensor surface.The magnetic carriers (beads) usually detected by the sensors consist of paramagnetic magnetite particles embedded in a polymer matrix with sizes from some µm down to about 100nm. They are linked to, e.g., DNA or proteins (often by a avidin-biotin bond) and thereby enable highly specific detection of complementary molecules. These magnetic particles often suffer from their broad size distribution and the relatively small magnetic moment. With the new colloidal synthesis of superpara-or ferromagnetic Co, CoFe and FePt nanocrystals by, e.g., pyrolythic decomposition of CVD precursor molecules, magnetic markers with superior magnetic moments, smaller size and size distribution can be produced. Here, the question about their potential to replace magnetite is addressed. Starting from a magnetic analysis of the corresponding magnetophoretic mobility of Co and FeCo based alloys their synthesis and resulting microstructural and magnetic properties as function of the underlying particle size distribution and the stability of the oleic acid ligand are discussed.Moreover, the magnetic particles offer an additional feature: They can be manipulated on chip via currents running through specially designed line patterns. We show, that this manipulation can be performed in a precise and reproducible manner, enabling locally enhanced concentration or even the measurement of binding forces with very low loading rates.
We report on the development and first experimental results of a “at wavelength” full-field imaging technique for defect inspection of multilayer mask blanks for extreme ultraviolet (EUV) lithography. According to the International Semiconductor Roadmap by Sematech, less than 5×10−3 defects per cm2 should be present on such multilayer mask blank to enable mass production of microelectronics using EUV lithography, thus fast high-resolution methods for mask defect inspection and localization are needed. Our approach uses a photoemission electron microscope in a normal incidence illumination mode at 13 nm to image the photoelectron emission induced by the EUV wave field on the multilayer mask blank surface. We show that by these means, buried defects in the multilayer stack can be probed down to a lateral size of 50 nm.
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