An eddy-current microsensor for proximity measurement was developed. To achieve an optimal design, the sensor was modeled using the finite-element method. By studying the sensor response, but staying within the technological boundaries, an optimal design was reached. The sensor had a soft-magnetic core and separate excitation and pickup coils. The fabrication technologies included electroplating for core and coil and sputter deposition for insulation and seed layers. Photoresists were used for creating micromolds for electroplating both core and coil, as well as for sensor embedding. Completed sensor samples had a frequency range of 100 kHz to 2 MHz and proved their capability of performing proximity measurement.
Biomolecules such as the enzyme horseradish peroxidase and the protein streptavidin immobilized on superparamagnetic magnetic labels (2pm encapsulated microconed iron oxide (15%) poly(styrene-co-maleic acid) microspheres (Micromer@-M, Micromod), 20-40 biomolecules per label) were detected using spin valve sensors (2 x 6pm2, MR-5%). Using an on-chip field of 15 Oe (extemal field) MR signals of -1OOpV were obtained when single 2um particles were placed over the spin valve sensor. Sensor saturation occurs near -1mV ( Fig.1) with a noise level of -1OpV. The moment of the 2um diameter microspheres at 15 Oe was estimated at 5.10-'2emu and the calculated maximum MR signal for one microsphere directly on top of the sensor was -4OOpV (this assumes zero space between the microsphere and sensor surface). The microspheres were focussed at the sensor site and moved to and from the sensor by integrating the sensors within Al current line structures. The lines were tapered in diameter from 150ym to 5pm at the sensor area to enable (10-20mA current) the positioning of small numbers of beads at sites (5x15pm2) adjacent to the sensor. Single microspheres could then be moved over the sensor by switching the current line off (Fig.2) and back to the current line by switching the current line on.Present work is concentrating on the detection of biomolecular recognition via the selective immobilization of biotin to a TiW(N2) layer deposited on the sensor surface and subsequent binding of streptavidin functionalized beads. Biotin binds to streptavidin with high biological affinity (&-10'3M-', binding strength -200pN). We foresee a range of biotechnological applications for this system including high sensitivity detection assays and protein and DNA biochips. 1203- Sensor Clear (fluid ml$ IntroductionEddy current measurement for non-destructive testing excels due to its simplicity and reliability [ 11. While originally developed as an inspection technique for detecting large scale defects, there is a growing need to also identify micron scale cracks. This application necessitates smaller sensors with improved resolution and sensitivity [2]. In this paper, we are presenting an eddy current microsensor for the detection of surface microcracks in both ferrous and non-ferrous metals.
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