MAGNETIC FIELD SENSOR COMPRISING A WHEATSTONE BRIDGE

Abstract: A magnetic field sensor comprising: a plurality of resistive elements (11a, 11a'; 11b, 11b') in a Wheatstone bridge configuration (1), whereby at least one element demonstrates a magneto-resistive effect; means for causing a measurement current to pass from a first point (5) through the bridge (1) to a second point (7); a conductive track (19) which runs in proximity to the resistive elements (11a, 11a'; 11b, 11b') but is electrically insulated therefrom, for the purpose of magnetically biasing the resistive elements with a biasing current, whereby the second point (7) is electrically connected to the conductive track (19), so that the measurement current is also employed as the biasing current.

Description: "Magnetic field sensor comprising a Wheatstone bridge" The invention relates to a magnetic field sensor comprising: a plurality of resistive elements in a Wheatstone bridge configuration, whereby at least one element demonstrates a magneto-resistive effect; means for causing a measurement current to pass from a first point through the bridge to a second point; a conductive track which runs in proximity to the resistive elements but is electrically insulated therefrom, for the purpose of magnetically biasing the resistive elements with a biasing current. Magnetic field sensors of this type may be employed inter alia: - as magnetic heads, which can be used to decrypt the magnetic flux emanating from a recording medium in the form of a magnetic tape, disc or card; in compasses, for detecting the terrestrial magnetic field, e.g. in automotive, aviation, maritime or personal navigation systems; as field sensors in medical scanners, and as replacements for Hall probes in various other applications; as memory cells in Magnetic Random- Access Memories (MRAMs); as current sensors, whereby the magnetic field produced by such a current is detected. Magneto-resistance is a phenomenon whereby the electrical resistance of a body can be influenced by magnetic flux. In particular, the electrical resistance of the body changes in a predictable manner in response to a varying magnetic flux, making such a body suitable for use as a magnetic-electric transducer in a magnetic field sensor. However, as with any resistive body, the electrical resistance of such a body can also be influenced by other environmental factors, particularly temperature. A problem in (sensitive) practical applications is thus to devise some means of differentiating between transducer signals resulting from (varying) magnetic flux and (unwanted) transducer signals emanating from other environmental sources. One approach is to connect a number of magneto-resistive elements in a Wheatstone bridge arrangement. If a pair of resistive elements can be magnetically biased in such a manner as to have opposite responses (in the sense of opposite polarity) to a given magnetic flux but not to other environmental factors, then subtractive comparison of the electrical resistances of the two resistive elements will cause cancellation of any unwanted response to spurious environmental factors, while exposing any response to magnetic flux. Magnetic field sensors employing a Wheatstone bridge in this manner are known from the prior art. However, among the sensors thus known, there are various different approaches when it comes to magnetically biasing the magneto-resistive elements. For example: (a) in Japanese Patent Application (Kokai) No. 61-711 (A), each of the resistive elements in the Wheatstone bridge is magnetically biased in a given direction using an appropriately poled permanent magnet positioned in the vicinity of that element; (b) On the other hand, in an article in Philips Electronic Components and Materials Technical Publication 268 (1988) entitled "The magnetoresistive sensor", the individual resistive elements are biased using a so-called "barber pole" (a term generally known and understood in the art, and thus receiving no further elucidation here). The use of biasing on the basis of permanent magnets as in case (a) above is highly unsatisfactory: not only is very careful tuning of the strength and position of the permanent magnets required, but the permanent magnets are themselves unacceptably sensitive to temperature variations. In addition, the use of permanent magnets necessarily makes any such biased magnetic field sensor bulky, and sets a limit on the attainable degree of miniaturization. On the other hand, while the biasing method in case (b) may be suitable for resistive elements demonstrating the so-called Anisotropic Magneto-Resistive (AMR) effect, it cannot be employed in conjunction with resistive elements demonstrating the considerably larger (and thus more interesting) Giant Magneto-Resistive (GMR) effect; this is because the GMR effect does not depend on the direction of current flow through a GMR resistive element, so that a barber pole cannot be used. An alternative means of biasing the resistive elements involves the use of a current-carrying conductor running in proximity to the elements so as to produce a biasing magnetic field within them (so-called current biasing). The form of the path followed by this conductor determines the direction of the biasing current for each resistive element, so that the biasing direction of each element can be determined in advance. Although the approach referred to in the previous paragraph alleviates the problems associated with option (a) above, a marked disadvantage is that it consumes extra electrical power, due to the fact that it requires a biasing current through the separate current-carrying conductor. Such extra power consumption (with the accompanying increase in Ohmic dissipation) hampers attempts at device miniaturization. It is an object of the invention to provide an improved magnetic field sensor. In particular, it is an object of the invention that such a sensor should be compatible with the use of GMR resistive elements in the Wheatstone bridge configuration. Moreover, it is an object of the invention that the said sensor should employ novel biasing means which produce a satisfactory result and are compatible with trends toward miniaturization. These and other objects are achieved in a sensor as specified in the opening paragraph, characterized in that the second point is electrically connected to the conductive track, so that the measurement current is also employed as the biasing current. The sensor according to the invention provides an elegant, compact realization of the objects put forth hereabove. Because the measurement current is also employed as a biasing current, the power consumption of the sensor is advantageously kept low. In addition the number of external electrical connections is reduced, and the quantity of electronics necessary to power the sensor is kept to a minimum. It should be explicitly noted that the term "Wheatstone bridge" as here employed is intended to refer to either a full or a half Wheatstone bridge. In the former, two branches are connected in parallel between the first and second point, each branch containing two series-connected resistive elements; on the other hand, in a half Wheatstone bridge, only one such branch is connected between the first and the second point. It should also be noted that the term "magneto-resistive element" or "resistive element" (shorthand) is intended to refer to any type os sensor element which changes its effective resistance in response to an applied magnetic field; in particular, a spin-tunnel junction should be considered as falling within the scope of this term. In a preferential embodiment of the sensor according to the invention, each of the resistive elements in the Wheatstone bridge demonstrates a magneto-resistive effect. Such an embodiment has an increased sensitivity compared to a bridge which contains both ordinary resistive elements and magneto-resistive elements. Magneto-resistance effects can be realized in various material configurations. In particular, a GMR effect can be achieved in structures such as antiferromagnetically coupled magnetic multilayers (e.g. Co/Cu and Fe/Cr), exchange-biased spin-valve multilayers (e.g. FeMn/NiFe/Cu/NiFe), and discontinuous NiFe/Ag multilayers: see, for example, the treatise "Magnetic thin films and multilayer systems: analysis and industrial applications", Springer Series in Materials Science, U. Hartmann (ed.), Springer Verlag (1997). In principle, all such material configurations and structures can be successfully employed in the magneto-resistive elements of the sensor according to the invention. If so desired, soft-magnetic material(s) can be deposited in the vicinity of (some or all of) the resistive elements in the Wheatstone bridge, so as to locally reinforce the magnetic biasing fields produced at the location of those elements. A further refinement of the sensor according to the invention is characterized in that, in the vicinity of some or all of the resistive elements, the conductive track contains a narrowed portion. In general, this will serve to increase the strength of the biasing field at the location of the adjacent resistive elements. Yet another refinement of the sensor according to the invention is characterized in that, in the vicinity of some or all of the resistive elements, the conductive track is wound into a coil form (e.g. a planar coil). This serves to increase the biasing magnetic field strength at the location of those elements. A particular embodiment of the sensor according to the invention is characterized in that: the resistive elements are arranged in a first plane; the conductive track runs within a second plane substantially parallel to the first plane and separated therefrom by an intervening electrically insulating layer. An advantage of such a planar configuration is that it requires a minimum of layers (viz. substrate, structured bridge layer, blanketing insulating layer and structured conductive track layer). A refinement of the inventive sensor described in the preceding paragraph is characterized in that it comprises a third plane substantially parallel to the first plane, separated therefrom by an intervening electrically insulating layer, and at the side thereof remote from the second plane, the third plane containing a second conductive track which serves to carry a second biasing current for the purpose of magnetically biasing the resistive elements in the Wheatstone bridge, whereby the second conductive track is electrically connected to the conductive track in the second plane in such a manner that, after passing through the conductive track in the second plane, the biasing current in the second plane passes into the second conductive track, so that the measurement current is also employed as the second biasing current. Such an embodiment employs double (i.e. two-sided) biasing with one and the same electrical current, and thus provides a stronger biasing effect than the single (i.e. one-sided) biasing described in the previous paragraph. In an advantageous embodiment of the sensor as described in the preceding two paragraphs, the first and second planes are electrically connected by means of a via connection (in the case of the structure claimed in Claim 6, the second and third planes may also be electrically connected by means of a via connection). Such an embodiment is particularly suitable when the inventive sensor is manufactured using thin-film technology. Methods for the manufacture of suitable via connections between two points are well known in the art, and generally comprise the etching of a narrow tunnel between the two points (through an intervening insulating body) and the subsequent filling of this tunnel with conductive paste. An alternative embodiment of the sensor according to the invention is characterized in that the resistive elements are arranged above one another in a stack configuration, and that the conductor track follows a path which runs between neighbouring resistive elements in a sandwich arrangement. Such a stack configuration has the advantage that it is more compact than the planar configuration described above, since the thickness of the resistive elements is generally at least an order of magnitude smaller than their lateral dimensions. Both the planar and the stacked embodiments hereabove described can be manufactured using standard thin-film deposition and structuring techniques well known in the art, and thus receiving no further attention here. The invention and its attendant advantages will be further elucidated with the aid of exemplary embodiments and the accompanying schematic drawings, whereby: Figure 1 illustrates a plan view of a magnetic field sensor according to the invention; Figure 2 renders a perspective view of a magnetic read head (magnetic field sensor) according to the invention, having flux guides and electrical connections. Embodiment 1 Figure 1 illustrates a plan view of a particular embodiment of a magnetic field sensor according to the invention (planar embodiment). The sensor comprises a (full) Wheatstone bridge 1 containing two branches 3a; 3b connected in parallel between a first point 5 and a second point 7 and located within a first plane 9. The plane 9 is depicted schematically here, but it may be regarded as being the plane of an electrically insulating substrate on which the bridge 1 is provided. The bridge 1 is constructed in such a manner that an electrical current I delivered to the first point 5 by an electrical conductor 13 is split into a current I, through the branch 3a and a current I b through the branch 3b. After passing through the respective branches 3a;3b, these currents I.,I b recombine at the second point 7, and are carried off by an electrical conductor 15. Each of the branches 3a;3b contains two series-connected magneto-resistive elements 11a, 11a'; l ib, lib', thus arranged that the current I, passes through the elements 11a, 11a' and the current I b passes through the elements llb.llb'. In this particular embodiment, the elements 11a, 11a'; lib, lib' demonstrate the GMR effect. To this end, these elements may, for example, be comprised of 4 nm Co 2o Ni 65 Fe 15 / 1.5 nm Co 95 Fes / 1.5 nm Cu / 1.5 nm Co 95 Fe 5 / 4 nm Cθ 2 oNi 65 Fe, 5 (alternatively, the Cu may be replaced by a CuAg alloy, for example). The substrate 9 is here comprised of Si(100). The elements 11a, 11a'; lib, lib' are mutually interconnected by Cu tracks so as to form the branches 3a;3b. Each element 11a, 11a'; lib, lib' has lateral dimensions of approximately 8 μ , and a thickness of approximately 50 nm, whereas the Cu tracks are about 0.5 μm thick and 10 μ wide. The whole bridge 1 has approximate lateral dimensions of 1 X 1 mm 2 . According to the invention, the electrical conductor 15 runs from the second point 7 to a second plane 17 extending parallel to the first plane 9 and electrically insulated therefrom. Such a construction may, for example, be achieved by providing a covering layer of insulating material (such as Si 3 N 4 , SiO 2 , Al 2 O 3 , A1N, etc.) on top of the bridge 1, and providing a via connection 15 through this insulating layer from the point 7; the second plane 17 may then be regarded as the major surface of the insulating layer remote from the substrate 9. Once it has emerged onto the second plane 17, the electrical conductor 15 contacts a conductive track 19 located within the second plane 17 and successively passing directly above each of the four magneto-resistive elements l la, lla';l lb,llb'. As here depicted, the track 19 passes above each of the elements 11a, 11a'; lib, lib' so as to run parallel to their long axes. As it passes through the track 19 from the second point 7, the measurement current I (being the sum of I, and I b ) generates a biasing magnetic field in each of the elements 11a, 11a'; lib, lib'. The arrows 25a,25a';25b,25b' demonstrate the direction of these respective biasing fields for each of the elements 11a, 11a'; lib, lib' (as can be verified using the well-known "right hand rule" in electromagnetism). The track 19 is thus shaped that each of the arrow-pairs (25a,25a'), (25b,25b'), (25a,25b) and (25a' ,25b') contains two oppositely oriented arrows. In this particular case, the insulating layer between the first plane 9 and the second plane 17 has a thickness of about 0.5 μm. The track 19 takes the form of a Cu strip with a thickness of 1 μm and a width of 10 μm (approximately). Assuming I to have an approximate value of 5 mA, the magnetic biasing field H generated at each element 11a, 11a'; lib, lib' will have a value of approximately 0.25 kA/m. It should be noted that: - the depiction of the resistive elements 11a, 11a'; lib, lib' in the Figure is schematic. In practice, it is advantageous to embody each such element as a thin meandering track, so as to achieve a relatively large resistance in a relatively compact area; the "planes" 9,17 will generally not be exact planes in the mathematical sense, but will typically demonstrate relatively small surface irregularities (arising from the underlying relief of conductor tracks, etc.). The term "plane" should therefore be interpreted as referring substantially to a plane in the physical sense. Embodiment 2 In an Embodiment otherwise identical to Embodiment 1 (see Figure 1), a third plane is located beneath the plane 9, i.e. at the side of the plane 9 remote from the plane 17. A via connection then runs from the terminal point 27 on the track 19 through the intervening space and into the third plane, where it is connected to the starting point of a second conductive track. This second conductive track follows a path which successively runs beneath each of the resistive elements l la, lla';l lb, l lb' (just as the track 19 runs above them). As a result of the via connection into the third plane from the point 27, the biasing current through the second conductive track will be equal to that through the track 19. The shape of the second conductive track is chosen in such a way that the local biasing fields which it generates are also in the directions 25a,25a';25b,25b' (when viewed on location at each of the respective resistive elements 11a, 11a'; l ib, lib'). In this manner, the biasing produced by the track 19 is reinforced and doubled. Embodiment 3 Figure 2 renders a schematic perspective view of part of a magneto- resistive magnetic read head (magnetic field sensor) according to the invention. The head comprises a transducer S (e.g. a planar Wheatstone bridge as described in Embodiment 1, or a stacked sensor as described in Claim 9) with electrical connections 65. The head additionally comprises flux guides 59,59', which are positioned relative to the transducer S so as to form a magnetic circuit. The end faces 61,61' form part of the pole face of the head, the magnetic gap 63 being located between said faces 61,61'. If a magnetic medium, such as a magnetic tape, disc or card, passes before the faces 61,61' in close proximity thereto, the magnetically-stored information on that medium will generate a varying magnetic flux in the above-mentioned magnetic circuit, which magnetic flux is also fed through the transducer S. The transducer S transcribes this varying magnetic flux into electrical resistance variations, which can be measured via the electrical connections 65. The head may also contain an electrical coil, which can be employed in the recording of magnetic information on magnetic media.

Claims: CLAIMS 1. A magnetic field sensor comprising: a plurality of resistive elements in a Wheatstone bridge configuration, whereby at least one element demonstrates a magneto-resistive effect; means for causing a measurement current to pass from a first point through the bridge to a second point; a conductive track which runs in proximity to the resistive elements but is electrically insulated therefrom, for the purpose of magnetically biasing the resistive elements with a biasing current, characterized in that the second point is electrically connected to the conductive track, so that the measurement current is also employed as the biasing current. 2. A sensor according to Claim 1, characterized in that each of the resistive elements in the bridge demonstrates a magneto-resistive effect. 3. A sensor according to Claim 1 or 2, characterized in that soft- magnetic material is deposited in the vicinity of at least one of the resistive elements, so as to locally reinforce the magnetic biasing field produced at the location of that element. 4. A sensor according to any of the Claims 1-3, characterized in that, in the vicinity of at least one of the resistive elements, the conductive track contains a narrowed portion. 5. A sensor according to any of the Claims 1-4, characterized in that, in the vicinity of at least one of the resistive elements, the conductive track is wound into a coil form. 6. A sensor according to any of the Claims 1-5, characterized in that: the resistive elements are arranged in a first plane; the conductive track runs within a second plane substantially parallel to the first plane and separated therefrom by an intervening electrically insulating layer. 7. A sensor according to Claim 6, characterized in that it comprises a third plane substantially parallel to the first plane, separated therefrom by an intervening electrically insulating layer, and at the side thereof remote from the second plane, the third plane containing a second conductive track which serves to carry a second biasing current for the purpose of magnetically biasing the resistive elements in the Wheatstone bridge, whereby the second conductive track is electrically connected to the conductive track in the second plane in such a manner that, after passing through the conductive track in the second plane, the biasing current in the second plane passes into the second conductive track, so that the measurement current is also employed as the second biasing current. 8. A sensor according to Claim 6 or 7, characterized in that, where two planes are electrically connected, such connection is achieved by means of a via connection. 9. A sensor according to any of the Claims 1-5, characterized in that the resistive elements are arranged above one another in a stack configuration, and that the conductor track follows a path which runs between neighbouring resistive elements in a sandwich arrangement.

Inventors: LENSSEN KARS-MICHIEL HUBERT, LENSSEN, KARS-MICHIEL, HUBERT
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: G01R33
IPC codes: G01R33