Current detecting device using ferromagnetic magnetoresistance element

Abstract: A current detecting device comprises a substrate, a magnetism sensing member made of ferromagnetic magnetoresistance elememt, an insulating member, and a conducting member, and these members are formed by a photolithography technique such that the magnetism sensing member is electrically isolated from the conducting member by the insulating member. When a current flows into the conducting member, the magnetism sensing member responds to a magnetic field generated by the current. At this time, the resistance of the magnetism sensing member changes due to a magnetoresistance effect in accordance with the intensity of the current. Therefore, the current can be measured by detecting the resistance change. In such a current detecting device, there is no reactance component thereby achieving high sensitivity when detecting the current.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to apparatus for measuring the amount of current flowing through a circuit, and particularly to current detecting device used for detecting current flowing through a circuit formed on a substrate. 2. Prior Art As is well known in the art, in a current measuring apparatus, a coil is used for generating a magnetic field, and a current is detected by means of a detecting device such as a Hall generator and a magnetoresistor. One example of the current measuring apparatus is disclosed in a Japanese Laid-Open Patent Application No. 53-118175, and Fig. 8 shows an example of such a conventional current detecting device. In this example, a wire line 311, in which a current I′ flows, is inserted into a ring-shaped magnetic core 312 around which a coil 313 is wound, and a magnetic field H′ made by the current I′ is detected by measuring a potential difference between two terminals 314a and 314b of the coil 313. However, the reactance component of the coil 313 exists in the circuit through which a current, which is to be measured, is flowed thereby undesirably influencing input-output characteristics of the current measuring apparatus. Accordingly, it is difficult to produce a highly accurate current measuring apparatus. In addition, since the coil 313 and the detecting device are difficult to unify as one body, the current measuring apparatus cannot be satisfactorily miniaturized. Besides, in the case of assembly, a further manufacturing step is required for adjusting a gap width between the coil 313 and the detecting device because the gap width is technically difficult to control in an assembly line. IBM Technical Disclosure Bulletin 18 , 11, April 1976 discloses a current sensor incorporating a magnetoresistance element. The current sensor requires the application of a bias magnetic field to sense a current. IBM Technical Disclosure Bulletin Vol. 18, No. 3, January 1976 discloses a magnetoresistive sensor. The sensor comprises a four element bridge on a silicon chip with four permalloy magnetoresistive sensors. The sensors are in the shape of slim rectangular elements joined in two parallel pairs joined end to end in a bridge by connections which link them to four gold pads. A thin film conductor overlies and completely overlaps the sensors and has two connection pads. SUMMARY OF THE INVENTION The present invention has been developed in order to remove the above-described drawbacks inherent to the conventional current detecting device. It is, therefore, an object of the present invention to provide a new and useful current detecting device which can be produced by a photolithography technique using a ferromagnetic thin film. It is another object of the invention to provide a satisfactorily miniaturized current detecting device. It is a further object of the invention to provide a highly accurate current detecting device. It is a still further object of the invention to provide a current detecting device which is able to be made as one body with semiconductor integrated circuit. In accordance with the present invention there is provided a current detecting device incorporating a magnetoresistance element, the device comprising: (a) an insulating substrate; (b) an elongate conducting member, both ends of the conducting member being respectively connected to terminals of a first two-terminal means for the application of a current to be measured; (c) an elongate magnetism sensing member, both ends of the magnetism sensing member being respectively connected to terminals of a second two-terminal means for the measurement of the resistance of the magnetism sensing member; and (d) an insulating member sandwiched between the conducting member and the magnetism sensing member to electrically isolate the conducting member and the magnetism sensing member whereby the laminate formed of the conducting member, the magnetism sensing member, and the insulating member forms a detecting member, the detecting member being formed on the substrate characterised in that the conducting member is located below the magnetism sensing member and next to the substrate, and in that the conducting member has two edges each of which forms a step between the top surface of said conducting member and the top surface of said insulating substrate, said insulating member covers said two edges of said conducting member, and the pattern width of said magnetism sensing member defined as the distance between two edges is larger than the pattern width of said conducting member such that said magnetism sensing member overlies said edges and covers said steps. Preferably a ratio WM/WA is determined as at least 1.3 where WM is a pattern width of said magnetism sensing member and WA is a pattern width of said conducting member so that magnetic flux due to said current to be measured is effectively received by said magnetism sensing member. BRIEF DESCRIPTION OF THE DRAWINGS The object and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Fig. 1 is a top view of a current detecting device according to a first embodiment of the present invention; Fig. 2 is a sectional view taken along line D-D of Fig. 1; Fig. 3 is a graph showing input-output characteristics of three elements each exhibiting galvanomagnetic effect; Fig. 4 is a graph showing a relation between the changing rate of resistance and the ratio of the pattern width of a magnetism sensing layer and the pattern width of a conducting layer; Fig. 5 is a circuit diagram of a first embodiment of a detecting circuit which can be used for the current detecting device according to the present invention; Fig. 6 is a detecting circuit of a second embodiment of a detecting circuit which can be used for the current detecting device according to the present invention; Fig. 7 is a detecting circuit of a third embodiment of a detecting circuit which can be used for the current detecting device according to the present invention; and Fig. 8 is a perspective view of a conventional current detecting device. The same or corresponding elements and parts are designated at like reference numerals throughout the drawings. Referring now to Fig. 1, a top plan view of a current detecting device is shown as a first embodiment of the present invention and a sectional view taken along line D-D of Fig. 1 is shown in Fig. 2. In this embodiment, the current detecting device comprises an insulating substrate 34, a magnetism sensing layer made of a magnetoresistor or a magnetoresistance element, i.e. a ferromagnetic magnetoresistance thin film 31, a thin insulating film 32 made by sputtering SiO₂ or Si₃N₄, and a thin conducting film 33. The conducting thin film 33 is formed by etching a conducting layer made by vapor-depositing aluminum on the insulating substrate 34 wherein the insulating substrate is partially covered. The insulating thin film 32 is provided on the conducting thin film 33. The ferromagnetic magnetoresistance thin film 31 is formed by etching a magnetism sensing layer made by vapor-depositing a nickel alloy such as Ni-Co and Ni-Fe on the insulating thin film 32. The conducting layer 33 is located substantially below the magnetism sensing layer 31 when viewed from the magnetism sensing layer 31. Both ends of the conducting layer 33 are respectively connected to terminals 35a and 36a for the application of a current Id which is to be measured, and terminals 37a and 38a are respectively connected to both ends of the magnetism sensing layer 31. Thus, in the present current detecting device, the magnetism sensing layer 31 and the conducting layer 33 are electrically isolated from each other. When forming the layer of the ferromagnetic magnetoresistance element 31 and such a conducting layer 33, a photolithographic technique can be used, and when producing the magnetism sensing layer 31 and the conducting layer 33, the method of sputtering, ion-beam deposition, or CVD is also applicable. In addition, the ferromagnetic magnetoresistance thin film 31 can be made of an alloy mainly including Fe, Co or Ni. The operation of the current detecting device will be described hereinbelow. The above-mentioned magnetoresistance element 31 has a property that its own resistance value varies when receiving a magnetic field in the direction making the right angle with a current flowing direction. When the current Id flows from the terminal 35a toward the terminal 36a, the magnetism sensing layer 31 receives the magnetic field of Hd in the direction of an arrow. Therefore, the resistance between the terminal 37a and the terminal 38a varies in accordance with the intensity of the magnetic field Hd. As a result, the current Id can be measured by detecting the resistance therebetween. Fig. 3 is a graph showing input-output characteristics of three elements of a ferromagnetic magnetoresistance element, a Hall generator, i.e. Hall element (InSb), and a semiconductor magnetoresistance element. As will be understood from the graph of Fig. 3, the sensitivity of ferromagnetic magnetoresistance element is higher than those of others. Besides, as to the temperature characteristics of those elements, the temperature coefficient of the ferromagnetic magnetoresistance element where the current is constant is -0.05%/°C; that of the Hall element is -0.13%/°C; and that of the semiconductor magnetoresistance element is -2%/°C. Therefore, the present current detecting device using a ferromagnetic magnetoresistance element exhibits satisfactory input-output characteristics. In such a structure, there are some advantageous effects as follows. (1) since the current which is to be measured is not required to flow through any coil, there is no undesirable influence due to the reactance component of the coil. Therefore, the response of the current detecting device is improved in comparison with the conventional device. (2) Since each of the magnetism sensing layer 31, the insulating layer 32, and the conducting layer 33 is a thin film and the forming of these films is performed by a photolithographic technique, the current detecting device is miniaturized with high precision. (3) Since the ferromagnetic magnetoresistance element is used in the present embodiment, the present current detecting device has high sensitivity thereby exhibiting satisfactory input-output characteristics. Here, assuming that the pattern width of the conducting layer is WA, and that the pattern width of the magnetism sensing layer is WM, a preferable relation is obtained between WA and WM as follows. As will be seen from Fig. 4, a changing rate of the resistance becomes large when a ratio WM/WA ≧ 1.3, because the magnetic flux effectively passes through the magnetism sensing layer 31. In other words, the magnetic flux due to the current Id is effectively received by the magnetism sensing layer 31. In this case, the sensitivity of the current detecting device becomes high. Here, assuming that a resistance value is R(0) when the current Id is 0mV, and that a resistance value is R(50) when Id is 50mV, the changing rate of the resistance is given by: (R(0) - R(50)) / R(0) In addition, Ni-Fe alloy is used as a magnetism sensing layer 31 for measuring the changing rate of the resistance. The change of the resistance between the both end portions 31a and 31b of the magnetism sensing layer 31 is very small because the intensity of the magnetic field Hd falls off at the inverse square of the distance between the conducting layer 33 and the magnetism sensing layer 31. Therefore, in the case of WM/WA > 1.3, the changing rate of the resistance is saturated. When using the above-mentioned current detecting device, generally, an operational amplifier is used in order to process a small signal indicative of a resistance value. Since the offset voltage of the operational amplifier is approximately 3mV or below in general, when the changing rate of the resistance is 0.3% or more, the current is detectable without any problem even if there is such an offset. Figs. 5 through 7 show detecting circuits which can be used for the present current detecting device. In Figs. 5 - 7, references 31, 33, and 35-38 are corresponding to the same parts as those used in Fig. 1. In Fig. 5, the detecting circuit comprises a bridge circuit formed of the magnetism sensing layer 31 and resistors R1, R2, and R3, and a comparator 40. The terminal 38a is connected to one input terminal of the comparator 40, and a terminal electrode A′ is connected to the other input terminal of the comparator 40. In this structure, for example, if the voltage of the terminal electrode A′ is desirably predetermined by adjusting the resistors R1 and R2, an outputted signal can be obtained when the voltage of the terminal 38a reaches the predetermined value. In other words, the current I reaches a certain value, a signal is outputted from the comparator 40. In Fig. 6, the detecting circuit comprises a bridge circuit formed of the magnetism sensing layer 31 and three resistors R4, R5, and R6, and two operational amplifiers 41 and 42. A terminal electrode B′ is connected to a noninverting input terminal of the operational amplifier 41 acting as a voltage follower, and the terminal 38a is connected to a noninverting input terminal of an operational amplifier 42 whose inverting input terminal is connected via a resistor R8 to the output terminal of the operational amplifier 41. The output terminal of the operational amplifier 42 is connected via a resistor R7 to the inverting input terminal of the operational amplifier 42. In this structure, a signal is outputted from the output terminal of the noninverting amplifying circuit in accordance with the current I which is to be measured. Besides, as shown in Fig. 7, the bridge circuit can be formed by using two magnetism sensing layers 31 and resistors R9 and R10. In this case, there is an advantageous effect that the sensitivity of this circuit is doubled. From the embodiment , it will become apparent that a high sensitivity current detecting device having no reactance component can be produced. The above-described embodiment is just an example of the present invention, and therefore, it will be apparent for those skilled in the art that many modifications and variations may be made without departing from the scope of the present invention.

Claims: A current detecting device incorporating a magnetoresistance element, the device comprising: (a) an insulating substrate (34); (b) an elongate conducting member (33), both ends of the conducting member being respectively connected to terminals (35a,36a) of a first two-terminal means for the application of a current to be measured; (c) an elongate magnetism sensing member (31), both ends of the magnetism sensing member being respectively connected to terminals (37a,38a) of a second two-terminal means for the measurement of the resistance of the magnetism sensing member (31); and (d) an insulating member sandwiched between the conducting member (33) and the magnetism sensing member (31) to electrically isolate the conducting member (33) and the magnetism sensing member (31) whereby the laminate formed of the conducting member (33), the magnetism sensing member (31), and the insulating member forms a detecting member, the detecting member being formed on the substrate (34) characterised in that the conducting member (33) is located below the magnetism sensing member (31) and next to the substrate (34), and in that the conducting member (33) has two edges each of which forms a step between the top surface of said conducting member (33) and the top surface of said insulating substrate (34), said insulating member covers said two edges of said conducting member, and the pattern width of said magnetism sensing member 31 defined as the distance between two edges is larger than the pattern width of said conducting member (33) such that said magnetism sensing member overlies said edges and covers said steps. A current detecting device as claimed in claim 2, characterised in that a ratio WM/WA is determined as at least 1.3 where WM is the pattern width of said magnetism sensing member (31) and WA is the pattern width of said conducting member (33) so that the magnetic flux due to said current to be measured is effectively received by said magnetism sensing member (31). A current detecting device as claimed in any preceding claim, characterised in that said magnetism sensing member (31) is formed of a material with a characteristic that changes based on a magnetic field. A current detecting device as claimed in claim 3, wherein said magnetism sensing member (31) is made of a ferromagnetic magnetoresistance element. A current detecting device as claimed in claim 3 or 4, characterised in that said characteristic is resistance. A current detecting device as claimed in claim 4 or claim 5, wherein said magnetism sensing member is made of a Ni-Fe alloy. A current detecting device as claimed in any preceding claim, wherein said edges of said conducting member (33) define a width in a direction making a right angle with a direction of connection to said magnetism sensing member (31). A current device as claimed in any preceding claim wherein the said members (31,33) are deposited as layers.

Inventors: AO KENICHI, ARASUNA TOSHIKAZU, IZAWA ICHOROH, KAKEHI TATUYA, SAKURAI HIROSHI, YOSHINO YOSHIMI, AO, KENICHI, ARASUNA, TOSHIKAZU, IZAWA, ICHIROH, KAKEHI, TATUYA, SAKURAI, HIROSHI, YOSHINO, YOSHIMI
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1988
Main classification: G01R15
IPC codes: G01R15, G01R17