Sensors - March 2003 - High-Resolution, Chip-Size Magnetic Sensor Arrays

You may have heard of the technique before, but the materials and applications are new. Now you can manufacture arrays that measure small magnetic fields or changes in magnetic fields. And you can use the signal conditioning and logic of integrated circuits to optimize system performance.

Arrays of micron-size sensors on a single chip are not new. Arrays of visible-light-sensing diodes (photodiodes) were available in the mid-to-late 1970s. This development was followed by IR sensor arrays targeted at the same consumer market. On the other hand, single-chip arrays of magnetic sensors are new.

An atomic force microscopy photo of an electronics underlayer shows the roughness of the substrate before the planarization required for integrated sensor array deposition.

The advent of more sensitive giant magnetoresistive (GMR) and spin-dependent tunneling (SDT) materials has made it possible to manufacture devices that can measure small magnetic fields, or changes in magnetic fields, associated with magnetic biosensors, nondestructive test/inspection/evaluation, precision position sensing, document and item validation (including currency and credit cards), and magnetic imaging. By using a silicon substrate, you can use the signal conditioning and logic of ICs to optimize system performance. This integration reduces the effect of noise and simplifies the sensor/signal-processing interface.

Sensor Elements
The design of sensor arrays depends largely on the application. Arrays that cover a significant width require small sensing elements that operate at low power levels. For different applications, GMR elements can be patterned to form simple resistors, half bridges, Wheatstone bridges, and X,Y sensors. Single-resistor elements are the smallest devices and require the fewest components, but they have poor temperature compensation and usually require the formation of some type of bridge by using external components. Alternatively, they can be connected in series with one differential amplifier per sensor resistor. Half bridges take up more area on a chip but offer temperature compensation because both resistors are at the same temperature. You can use half bridges as field gradient sensors if one of the resistors is a distance from the other. They can also function as field sensors if one of the resistors is shielded from the applied field.

Figure 1. The diagram shows part of a 16-element array of half-bridge GMR sensors with 5 µm spacing. The first two elements have portions removed to show the three layers-GMR, metals 1, and metals 2-along with their interconnections.

Figure 1 shows a portion of an array of half-bridge GMR elements with 5 µm spacing. The elements are 1.5 µm wide and 6 µm high, with a similar size element above the center tap. The bottoms of the stripes are connected to a common ground connection, and the tops of the half bridges are connected to a current supply. The center taps are connected to 16 separate pads on the die. A bias strap passes over the lower elements to provide a magnetic field to bias the elements. Wider spacing between the elements can be achieved while retaining the directional sensitivity of narrow stripes by using serpentine resistors as the lower elements.

For greater sensitivity, you can use arrays of full bridges, which require four resistors per sensor. Small magnetic shields plated over two of the four equal resistors in a Wheatstone bridge protect the resistors from the applied field and let them act as reference resistors with matching temperature coefficients. The two remaining resistors are exposed to the external field. The bridge output is twice the output of a bridge with only one active resistor. The output for a 16% change in the resistors is ~8% of the voltage applied to the bridge. Arrays of full bridges require more chip area than simple resistors or half bridges and more interconnects and connections to the outside world. An array of n full bridges requires 2N+2 connections.

If you have sufficient room on the chip, you can make magnetoresistive materials more sensitive by adding permalloy structures plated to the substrate. These act as flux concentrators. Place the active resistors in the gap between two flux concentrators. These

Figure 2. A 2D array of X,Y GMR sensors can scan an extended area without moving the probe.

resistors experience a field larger than the applied field by approximately the ratio of the gap width between the flux concentrators to the length of one of the flux concentrators (gap/flux concentrator length).

Two-directional arrays of sensors can be used to scan an area nonmechanically if the area covered by the array encompasses the region of interest (see Figure 2). One possible use for an array of 2-directional GMR sensors is eddy current probes for crack detection in conducting materials.

An additional use of 2-directional arrays is to increase the spatial resolution of a mechanically scanned array. If each row is offset from the adjacent row, it will cover a slightly different track. For example, an array with three rows of sensors would achieve a resolution three times smaller than a single row as it is mechanically scanned. The scanning direction is perpendicular and in plane with the offset row.

Materials
The GMR materials used in these sensors are a variety of antiferromagnetically coupled multilayers. The material used in commercial GMR sensors has a saturation field of 300 Oe and GMR of 15%. Newer FeCo/Cu multilayer materials with saturation fields below 100 Oe and GMR >10% can also be used. Figure 3 shows the MR response of various multilayer materials.

The slopes of the GMR curves in Figure 3 are 0.04%/G for a conventional multilayer, 0.07%/G for a low-hysteresis multilayer, and 0.2%/G for a high-sensitivity multilayer material.

Figure 3. Varying sensitivities and field ranges are available in multilayer GMR materials. The traces of GMR vs. field for conventional multilayer (ML), low hysteresis multilayer (LH-ML), and high sensitivity multilayer (OD-ML) materials demonstrate the trade off between sensitivity, hysteresis, and field range.

In a half-bridge configuration, these values correspond to outputs of 0.2, 0.35, and 1.0 mV/V/G (20, 25, and 100 nV/nT @ 10 V).

Electronics
GMR materials have been successfully integrated with both BiCMOS and bipolar semiconductor underlayers. This integration has been demonstrated in both integrated sensors and in magnetic couplers or isolators. A major obstacle that had to be overcome was vhe planarization of the elec.tronics underlayer to obtain the necessary smoothness for successful sensor deposition.

Onchip multiplexing of sensors reduces the number of connections to the chip. This can be accomplished by using FETs to select the sensor to which the sensing current is directed or by using parallel sense current and multiplexing the output (see Figure 4).

Figure 4. These schematics show two methods of data acquisition using onchip FETs. In (A), the sense current is multiplexed; in (B), the output is multiplexed. Both are executed by sequentially addressing FETs.

Applications
Imaging of Magnetic Media. A variety of applications use magnetic sensor arrays to form magnetic-media images. It’s important to ascertain whether the media are genuine or if they have been altered or edited. Direct imaging of the magnetic patterns on the tape surface can reveal alterations and erasures, and in some cases, it can recover information that was erased and is no longer available through ordinary means. The original information may remain as patterns bordering the erase-head track or may even be recovered as a ghost image in the erased section. Magnetic imaging using high-resolution arrays of magnetic sensors has many potential uses in commerce, including magnetic validation of documents, such as currency.

Magnetic Bioassay. For many years, researchers have used magnetic particles in biological assays. These particles range in size from few nanometers to a few microns, and they vary in composition from pure ferrite to small percentages of ferrite encapsulated in plastic or ceramic. The beads are coated with a chemical or biological species (e.g., DNA or antibodies) that selectively bind to the target analyte.

By integrating an online magnetic detector, the selectivity of sample and target can be used as a rapid, sensitive detection strategy. The development of solid-state GMR sensors as the magnetic detectors facilitates this integration. These sensors have the unique advantage of being compatible with silicon IC fabrication, which makes it possible to build multiple detectors on a single chip, along with any required electronic circuitry. The miniature nature of GMR sensor elements lets an array simultaneously test for multiple biological molecules of interest. Results from theoretical modeling, as well as laboratory results, show that GMR detector arrays can detect single, micron-size magnetic beads.

Nondestructive Testing. Eddy current methods are used in nondestructive evaluation, inspection, and testing of conducting metals. Recent developments have included the use of GMR and SDT sensors and sensor arrays to detect eddy currents.

Conclusions
The development of onchip magnetic arrays is less than 10 years old, and there are still many technology issues to be resolved. Yet the list of potential applications is growing rapidly, and exciting new opportunities are constantly emerging. Rooted in tÖe same magnetic technology that is driving MRAM, arrays of onchip magnetic sensors are expected to continue to expand rapidly in areas of application, and the number of companies supplying them will grow as well.

Future developments will include arrays using spin-dependent tunneling sensors. Sensors with a 1.5 nm Al₂O₃ barrier thickness have demonstrated up to 40% GMR and sensitivities of 30 mV/V/G (3 µV/nT @ 10 V).

Acknowledgments
The authors acknowledge support for research sponsored by the AFRL, NSF, NASA, and NIST.