On-chip eddy current sensor for proximity sensing and crack detection

In this paper, we present a new integrated eddy current sensor for proximit y sensing and for the detection of microcracks on the surface of metals. Th e device consists of two slacked planar coils fabricated onto a glass subst rate and encapsulated on one side by a Ni/Fe permalloy magnetic core. Fabri cation of the device is achieved by a UV-LIGA thick photoresist lithography process, which involves the lithographic patterning of 15-25 mum thick mol ds using AZ-4000 series photoresist. The introduction of the permalloy core coupled with the thick conductor lines produces a high inductance, low res istance device capable of generating large magnetic fields at low driving c urrents. The device has been tested in the frequency range of 10-500 kHz an d has been shown to work as both a proximity sensor and crack detector at i nput powers of 30 mW or less. When used as a proximity sensor; the unamplif ied output voltage on the sensing coil changes by as much as 75 mV with an aluminum target placed at a distance of 400 mum from the coil. The device h as also shown the capability of clearly detecting cracks with depths of as little as 8 mil (200 mum) in both aluminum and titanium. Results show an ex tremely linear relation between crack depth and output signal voltage with an unamplified signal strength of several millivolts. (C) 2001 Elsevier Sci ence B.V. All rights reserved.