Magnetic Pattern Recognition Sensor Arrays using CCD Readout

Magnetic encoding is currently widely employed in cheques, transaction cards, access cards and bank notes because of its robustness, economy, security, and ease of updating coded information. Coded magnetic information is currently read using either inductive metal-in-gap (MIG) or magnetoresistive (MR) heads.¹⁾ Due to various loss mechanisms, the signal-to-noise ratio of MIG heads peaks at around 100 kHz, decreasing rapidly at higher frequencies. The fabrication of both the MIG head²⁾ as well as the accompanying signal processing circuitry³⁾ is also non-trivial. MR heads provide higher SNR and signals that are independent of spatial frequency. They are however fragile, non-linear, and have a high temperature coefficient In cheques and bank notes, human-readable magnetic ink character recognition (MICR) characters are employed. Each MICR character has been designed to produce a distinct inductive head signal pattern. Unlike magnetic stripes, MICR characters signals are not binary when read using conventional read heads, resulting in increased read error rates. To avoid costly misreads, a closely spaced array of magnetic sensors can be utilized. Fabrication of read head arrays is, however, difficult in both technologies. A silicon magnetic sensor array fabricated using the charge-coupled device (CCD) technology has been designed to overcome these limitations. The magnetic sensor pixels are buried-channel MOSFET´s with geometries designed to optimize magnetic sensitivity. The use of buried-channel, as opposed to surface-channel, MOSFET´s results in enhanced sensitivity, lower noise, and higher signal resolution.