Thin Film Vector Magnetometer

1. Field of the Invention

The present invention relates generally to magnetoresistive sensors, but more particularly it relates to an improved thin film vector magnetometer that produces an output voltage which varies linearly with the component of magnetic field in the plane of the thin film, of which it is fabricated, along a predetermined sensitivity axis.

2. Description of the prior art

The detection of magnetic fields is a common function required of many systems. Magnetometers of various types are found in torpedoes, mines, buoys, intrusion detectors, vehicle detectors, and magnetic anomaly detectors to name a few systems. Accordingly, there are wide ranges in the levels, dynamic ranges, and frequency of fields which must be detected. In addition, system level requirements impose constraint such as the maximum power consumption, physical size and operational conditions, e.g., temperature and ambient noise.

Several thin film magnetometers are disclosed in the prior art. In the publication, R. S. Hebbert and L. J. Schwee, "Thin Film Magnetoresistance Magnetometer", The Review Of Scientific Instruments, Vol. 37, No. 10, pp. 1331-1323, October 1966, there is disclosed a magnetometer that uses an etched thin film layer arranged in a Wheatstone bridge configuration. The magnetoresistive properties of the thin film layer are used to provide a resistance change in the arms of the Wheatstone bridge when subjected to a dynamic magnetic field. The dynamic magnetic field produces an imbalance in the arms of the bridge. This imbalance is a function of the intensity of the dynamic magnetic field thereby providing a corresponding output signal amplitude. The thin film layer is arranged apparently with its easy axis aligned orthogonal to a constant hard axis of a bias field. The bias field is applied parallel to the plane of the thin film layer and is of an intensity much greater than the anisotropy field H.sub.k of the thin film layer. This arrangement, due to the fast domain switching characteristics of the thin film layer, has an inherently fast response time with a stated flat response to dynamic magnetic fields of frequencies in the order of 65 MHz.

Paul et al, in U.S. Pat. No. 3,546,579, patented on Dec. 8, 1970, disclose a magnetometer that uses four anisotropic thin films arranged in a Wheatstone bridge configuration. The magnetoresistive properties of the thin films are used to change the resistance in the arms of the bridge when subjected to a dynamic magnetic field. The four thin films are arranged with their easy axes aligned orthogonally to a constant hard axis bias field. The pair of thin films in the balancing legs of the bridge are subjected to bias fields of opposite polarities such that the bridge network is rendered substantially insensitive to ambient or other hard axis field components. Paul et al consider their bias scheme to be an improvement over the bias scheme used by Hebbert and Schwee, aforementioned.

In practice the foregoing magnetometers suffer from several problems. In both the Hebbert et al and Paul et al magnetometers, size and impedance are critical factors in that the devices are larger and have lower impedances than desired. Also, in the Paul et al magnetometer, there is a question of whether there is a practical technique for fabricating their Wheatstone bridge using multiple bias fields. Thus the problem presented is to maintain their self-cancelling feature while producing a magnetometer that is small in physical size.

Consequently, there is a need in the prior art to configure a thin film vector magnetometer that can produce an output voltage which varies linearly with the component of a magnetic field in the plane of the thin film along a predetermined sensitivity axis while maintaining low power consumption and miniaturization of the finished product.

The prior art, as indicated hereinabove, include some advancements in thin film vector magnetometers. However, insofar as can be determined, no prior art thin film vector magnetometer incorporates all of the features and advancements of the present invention.

Accordingly, an important object of the present invention is to configure a thin film vector magnetometer that will produce an output voltage which varies linearly with the component of magnetic field in the plane of the thin film along a predefined sensitivity axis while maintaining low power consumption and miniaturization.

A further object of the present invention is to accomplish the foregoing object while optimizing the sensitivity and linear dynamic range of the thin film vector magnetometer.

Another object of the present invention is to configure the thin film vector magnetometer to be economic in design and amenable to the mass production techniques used in thin film processing.

Still another object of the present invention is to optimize the magnetic material geometry of the chip portion of the thin film vector magnetometer so as to increase the sensitivity and linearity thereof.

In accordance with the above stated objects, other objects, features and advantages the present invention has as a primary purpose the fabrication of a thin film vector magnetometer that will produce an output voltage which varies linearly as a vector component of the external magnetic field in the plane of the thin film along a predetermined sensitivity axis.

The essence of the present invention is in designing the thin film vector magnetometer so that its properties such as sensitivity, linear dynamic range, power consumption and physical size can be tailored to specific applications.

The purpose of the present invention is carried out by configuring the thin film vector magnetometer to include a magnetoresistive bridge, a thin film chip carrier and a bias magnet. In fabricating the unique geometry of the magnetoresistive bridge, permalloy (nickel-iron) is used. To fabricate the magnetoresistive bridge, a set of two photolithographic masks are necessary. One mask is used in etching the permalloy level. The second mask is used to define vias through a passivation level through a photolithographic process and to etch electrical bonding pads in an aluminum-copper level deposited after the passivation step.

The thin film vector magnetometer is completed by connecting trim resistors to the magnetoresistive bridge grounding one end thereof and attaching a power source to the other end thereof so as to form a Wheatstone bridge configuration. The magnetoresistive bridge is affixed to the thin film chip carrier and the bias magnet is affixed to the top of the thin film chip carrier so as to apply a constant bias field which is oriented orthogonal to the easy axis of the magnetoresistive bridge. The thin film vector magnetometer, as configured, is most sensitive to signal fields having the same relative alignment as the easy axis of the magnetoresistive bridge. Its output varies with the vector component of the signal field as referenced to the easy axis.

The foregoing and other objects, novel features and advantages of the present invention will be more apparent from the following more particular description of the preferred embodiment as illustrated in the accompanying drawings, in which:

FIG. 1 is a partially cutaway schematic representation of a thin film vector magnetometer, according to the present invention, illustrating, inter alia, a magnetoresistive bridge including a thin film chip having a plurality of magnetoresistors, a thin film chip carrier and a bias magnet;

FIG. 2 is a sectional view illustrating the physical relationship of the thin film chip, a thin film chip carrier and the bias magnet of FIG. 1; taken along lines 2--2 thereof; and

FIG. 3 is a sectional view illustrating the levels of fabrication of the thin film chip of FIG. 1; taken along lines 3--3 thereof.

The following is a description of a thin film vector magnetometer employing the present invention to produce an output voltage that varies linearly with the component of a magnetic field in the plane thereof along a predefined sensitivity axis. A description of a process of fabricating a thin film chip portion thereof is also discussed in some detail.

Referring then to FIGS. 1 and 2 as viewed concurrently, a thin film vector magnetometer 10 comprises a magnetoresistive bridge 12 which is configured in a traditional Wheatstone bridge topography, a thin film chip carrier 14 of a ceramic material in which the magnetoresistive bridge 12 is affixed, and a bias magnet 16 configured to be affixed to the thin film chip carrier 14.

The magnetoresistive bridge 12 of FIG. 1 comprises a thin film chip 18, a first trim resistor 20, a second trim resistor 22, a power source 24 and an output connector 26.

Referring now to FIGS. 1 and 3 as view concurrently, the thin film chip 18 comprises a left arm magnetoresistor 28, a right arm magnetoresistor 30, a left leg magnetoresistor 32 and a right leg magnetoresistor 34. The thin film chip 18 further comprises a substrate 36 of silicon (Si), a first level material 38 of silicon dioxide (SiO.sub.2), a second level magnetic material 40 of permalloy or nickel-iron alloy a third level material 42 of silicon nitride (Si.sub.3 N.sub.4), and a fourth level material 44 of aluminum copper (AlCu). The fourth level material 44 includes a plurality of electrical bonding pads 44a through 44f, as shown in FIG. 1. For purposes of the present invention, the substrate 36 may be glass, and, accordingly, the first level material 38 would not be necessary. However, the use because of silicon is preferred of its superior heat sinking capability and its compatibility with integrated circuit technology.

Still referring to FIGS. 1 and 3 as viewed concurrently, a well defined easy axis in the direction shown for magnetic field H.sub.S is produced in the magnetoresistors 28 through 34 during the deposition process. A bias field H.sub.B is produced by application of the bias magnet 16, which for the purposes of the present invention is fabricated out of a ferromagnetic material such as barium ferrite. The bias field H.sub.B also defines the sensitivity axis of the thin film magnetometer 10 in the direction shown. The sensitivity axis or the direction for maximum response to an applied signal of magnetic field H.sub.S is bidirectional and orthogonal to the unidirectional hard axis of magnetic field Hs and bias field H.sub.B as illustrated.

The geometry of the thin film chip 18 including the orientation of the left arm magnetoresistor 28, the right arm magnetoresistor 30, the left leg magnetoresistor 32 and the right leg magnetoresistor 34 with respect to the easy axis, the bias field H.sub.B and to each other, produces a true vector response. The foregoing is made possible by etching the right arm magnetoresistor 30 and the left leg magnetoresistor 32 such that their resistances will increase for the bias field H.sub.B direction shown and for a right directed signal field H.sub.S. On the other hand, the left arm magnetoresistor 28 and the right leg magnetoresistor 34 are etched such that their resistances will decrease for the same field directions. The output of the magnetoresistive bridge 12, and, accordingly, the thin film vector magnetometer 10 is produced at the output connector 26 for a power source 24 producing a constant current that flows into the electrical bonding pad 42a. The output of the magnetoresistive bridge 12 at the center conductor, as referenced to the outer conductor of the output connector 26, will be positive for a signal field H.sub.S in the direction to the right. On the other hand, for a signal field H.sub.S in the direction to the left, the output at the center conductor of the output connector 26 will be negative. It has been shown, that a power source 24 comprising a constant current source allows the thin film vector magnetometer 10 to perform better with changes in temperatures than with a constant voltage source. This occurs because a constant current source will compensate for the increase in the non-magnetic part of the resistance of the thin film chip 18 which increases with temperature.

Still referring to FIG. 1, some of the quality operational characteristics of the thin film vector magnetometer 10 can be attributed to the geometry of the magnetoresistors 28 through 34 in respect to the easy axes. As shown, the meandering resistor lines of the left arm magnetoresistor 28 and the right leg magnetoresistor 34 are oriented at minus 45.degree. with respect to the easy axis. Contrariwise, the meandering resistor lines of the right arm magnetoresistor 30 and the left leg magnetoresistor 32 are oriented at plus 45.degree. with respect to the easy axis. In order to be able to balance the thin film vector magnetometer bridge 10 in actual operation, the trim resistors 20 and 22 are connected as shown and used to balance out any imbalances due to resistances in the electrical bonding pads 44a through 44f or mismatching in the magnetoresistors 28 through 34.

Still referring to FIGS. 1 and 2 but particularly to FIG. 3, a section view of thin film chip 18 suitable for use in a Wheatstone bridge configuration along with a sectional view thereof is shown. In FIG. 3, the vertical scale is approximately 5000 times the horizontal scale in order to better show the levels of fabrication of the thin film chip 18. A first level of thin film chip 18 is deposited on a suitable substrate 36 of silicon. This first level comprises a silicon dioxide layer 38. A second level of the thin film chip 18 is fabricated by first depositing permalloy of 81-19 Ni--Fe using a physical vapor deposition process (either resistance heating or electron beam heating) or a sputtering process. This level which comprises a permalloy layer 40 is patterned with the meandering resistor lines and geometry depicted in the magnetoresistors 28 through 34 by using conventional photolithographic techniques. Then the permalloy layer 40 is etched chemically or with an ion beam miller. A third level of thin film chip 18 comprises a silicon nitride layer 42 or silicon monoxide having vias (not shown) for making connections to the magnetoresistors 28 through 34. The vias in this level are created using a lift off technique whereby photoresist plugs are removed in a heated acetone bath agitated with an ultrasonic agitator. This technique is believed to give superior stripping of the photoresist. A fourth level comprises a plurality of electrical bonding pads 44a through 44f of 95-5 Al--Cu. The electrical bonding pads 44a through 44f are fabricated to corresponding arms and legs of the thin film chip 18 to form external connection points to associated circuit elements. This level is fabricated using an etching process. The Al--Cu is deposited in an ion beam coater. Then the metalization pattern is created by patterning a layer of photoresist on the areas intended to be electrical bonding pads 44a through 44f. The AlCu layer 44 is then etched to define the electrical bonding pads 44a through 44f after which the photoresist is stripped. It should be mentioned that during deposition of the permalloy layer 40 a large d.c. magnetic field is applied which produces a well defined easy axis in the permalloy layer 40, and, accordingly, the thin film chip 18. Without this d.c. magnetic field, the easy axis would be unpredictable and possibly have a high degree of dispersion. Thus, using this technique, a reproducible easy axis can be achieved.

For purposes of the present invention, the first level of silicon dioxide 38 is about 1500 .ANG., the second level of magnetoresistors 28 through 34 having meandering resistor lines of permalloy layer 40 is about 400 .ANG., the third level 44 of silicon nitride 44 is about 2000 .ANG. and the fourth level of electrical bonding pads 44a through 44f is about 4000 .ANG.. Also, for purposes of the present invention, the width of the thin film chip 18 is about 3.90 millimeters (mm) and the length is about 7.80 mm. The thickness of the thin film chip is about 0.40 mm. The width of each of the meandering resistor lines of the magnetoresistors 28 through 34 is about 100 microns with the separation between the lines, in a particular magnetoresistor, being about 5 microns. Each of the electrical bonding pads 44a through 44f are about 0.40 mm in diameter.

The desired properties of magnetic material suitable for use with the present invention are zero magnetostriction, H.sub.k .about.3.5 Oe, H.sub.c .about.1 to 1.5 Oe, and the magnetoresistance ratio, R/R.sub.o, greater than 2.3 percent. Isotropic films may also be used in the fabrication of the present invention.

To those skilled in the art, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the present invention can be practiced than otherwise than as specifically described herein and still be within the spirit and scope of the appended claims.