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Sensor UnderlayersGMR and SDT materials are usually deposited on SiNinsulating coatings over Si wafers. GMR materials havealso been successfully integrated with both BiCMOS andbipolar semiconductor underlayers. This integration hasbeen demonstrated in both integrated sensors and inmagnetic couplers or isolators. A major obstacle thatneeded to be overcome was the planarization of theelectronics underlayer to obtain the necessary “smoothness”for successful sensor depositions. Figure 7, an AtomicForce Microscopy (AFM) image of the electronicsunderlayer, shows the roughness of the substrate before theplanarization required for integrated sensor arraydeposition.roughness has been reduced from 2.9 nm to 0.2 nm. Thelarge individual peaks are probably debris.Another method of planarizing sensor underlayers,especially between layers of metalization such asinterconnects and on-chip coils have been deposited is touse spin-on organic insulators such as BCB. Figure 9shows the cross section of an SDT sensor with severallayers of metal for interconnects and field coils. Note theBCB insulation layers.shield / flux concentratorNiFe 14 µm1500 Å Si Nitride(Au 2 µm)1000 Å seed layer (NiFeCo) --also use as hard mask for BCB etchBCB ~2 µm500 Å silicon nitride surface sealing layerM3 1.8 µmCrSi 150 Å1500 Å Si NitrideBCB ~2 µmFigure 7. Atomic force Microscope (AFM) image showingthe surface roughness of a substrate beforeplanarization. RMS roughness 2.9 nm.M31.8 µmM2 1.8 µmM1 0.5µm500 Å Si nitride surface sealing layer1500 Å Si NitrideBCB~2 µm500 Å silicon nitride surface sealing layerwindow nitride 2500ÅFigure 9. Cross section of SDT sensor showing BCBinsulation used to planarize layers between metaldepositions.Signal ProcessingCrSi150 Å2k Å silicon nitride LPCVDsilicon waferM2 1.8 µmM1CrSi 150 ÅAl 200 ÅCrPtMnFeCo/Ru/FeCoNiFe 120 Å Al2O3 barrierA block diagram of the data acquisition system formonitoring a 16-element array such shown earlier is shownin Figure 10. Two 8-channel multiplexers (AD 7501 & AD7503) are combined for 16-channel selection. This circuitis made possible by using multiplexers with the samespecifications and pin configurations where one of themhas an enable input that is active ‘true’ and the other isactive ‘false’. Because the output impedance is high in thedisabled state, the outputs from the multiplexers can bedirectly connected. A host computer provides control usinga nonlatched (no handshaking) digital signal using 4 bits ofa single port on the AT MIO16X, series E, data acquisitionboard.Figure 8. AFM image showing the surface roughness of asubstrate after CMP Planarization. RMS roughness 0.2nm.A similar substrate after chemical mechanicalpolishing for 10 minutes is shown in Figure 8. The rms4
Enable8 elementarraySensing currentHostAD7501Device selectV refV outAT MIO16Xseries E I/OAD7503O/PBand-passfilterMultiplexed sense current and output3 bit data busLock-inamplifierV +X and Y signalsFigure 10. Block diagram of the multiplexed dataacquisition system for multiplexing a 16-element GMRarray.Device selectV refV outData acquisition systems can also include digitizersand control. The system shown in Figure 11 takes thesignals from 64 sensors in an array for biosensing,multiplexes them to an amplifier, and digitizes them. Thecomputer controls the current to an electromagnet whichapplies an ac magnetic bias to the sensing elements.Multiplexed output, Parallel sense currentFigure 12. Schematics of multiplexed data acquisition byon-chip FETs. Either the sense current or the outputcan be multiplexed.ApplicationsEdge Connector64 x 1MultiplexerGMR Array ChipDiff.Amp.Digitizer BoardConnector Board12 BitA to DConverterCoil BoardCurrentAmpLaptop / PCFigure 11. Data acquisition system for a 64-elementbiosensor array. Computer control of applied magneticfield is also shownOn-chip multiplexing of sensors reduces the numberof connections to the chip. The multiplexing can beaccomplished by using FETs to select the device to beconnected to an external digitizer as is shown in Figure 12.Multiplexing can be accomplished by selecting the sensorto which the sensing current is directed or by using parallelsense current and multiplexing the output.Imaging of magnetic media – There are a variety ofapplications that utilize arrays of magnetic sensors to formimages of various types of magnetic media. Informationrecorded on audiotapes by simple tape recorders and eventelephone answering machines has played important rolesin legal cases. It is important to ascertain whether theserecordings are genuine or have been edited. Direct imagingof the magnetic patterns on the tape surface can revealstarts and stops and erasures. In some cases, magneticimaging can recover information that was erased and is nolonger available in audio playback. The originalinformation may remain as patterns bordering the erasehead or even may be recovered as a faint image in theerased section. Research has been done at the NationalInstitute of Standards and Technology (NIST) in Boulder,Colorado on methods to recover this seemingly lostinformation. 7 An image is slowly acquired one scan at atime using a commercial magnetoresistive read head from acomputer hard drive. Magnetic sensor arrays cantremendously reduce the time to acquire such informationand bring this technology out of the research laboratory andinto the forensic laboratory. 8Magnetic imaging using high-resolution arrays ofmagnetic sensors has many potential uses in commerce.5
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