Magnetic field sensor

The invention relates to a magnetic field sensor according to the preamble of claim 1.

Magnetoresistive sensors of this type, as they are known, for example, from US Pat. No. 5,485,334 A or JP-8-203032 A, are used to measure weak to medium magnetic fields. Depending on the direction of magnetization with respect to the direction of current flow in a thin film, its ohmic resistance is influenced by an external magnetic field, which is used to determine this external field.

A large number of disturbance variables can be eliminated by suitable switching of the resistors formed from thin magnetic films. This is done, for example, by means of a Wheatstone bridge circuit of four groups of resistors, whereby the spontaneous magnetization of the films is periodically reversed (flipped) (this causes a current-carrying conductor loop, which is formed from a thin flip conductor) and the change in resistance caused by the magnetic field to be measured is displayed as an alternating signal on the Bridge output tapped.

It is also known to compensate for the magnetic field to be measured at the location of the sensor by means of compensation conductors, whereby the sensor works as a zero indicator and the result is independent of the sensor characteristic in a first approximation. Flip and compensation conductors can be integrated on a sensor chip and due to the proximity of the current-carrying conductor layers to the resistance films, even weak currents are sufficient for these auxiliary fields.

A decisive disadvantage of these structures, however, is the fact that the field acting inside the magnetoresistive film is generally less than the external magnetic field to be measured. This is to be clarified in the following. The spatial position of the spontaneous magnetization depends essentially on the geometric shape of the layer. Free magnetic poles can occur at the geometrical boundary of the film, which cause a stray field with a non-negligible energy. Stray fields could be completely avoided by closed flux rings. However, in the case of alloy compositions that are not free from magnetostriction, for example, these flux rings lead to considerable magnetostrictive stresses in the crystal, so that, in general, complete freedom from stray fields is not optimal in terms of energy. In addition, a domain structure in magnetoresistive layers is undesirable anyway because of the poorly reproducible magnetization process.

The effective field Hi in the layer results from the external field H._a with the demagnetizing field H_e according to the demagnetization tensor N_e and magnetization

Λ / to

H_i = HO + He = H _a -N_e · M.

N_e can be calculated as the sum of the geometric demagnetization tensor N_eG and a tensor N_e, which results from inhomogeneities in the layer and is called the internal demagnetization tensor. N_elcan only be taken into account statistically. The following results for the energy in the stray field:

<img file="AT407803B_D0001.tif" />

Strictly speaking, these equations only apply provided that a homogeneous external field results in a homogeneous magnetization in the layer, and this magnetization in turn results in a homogeneous demagnetizing field. Exactly this only applies to homogeneous, ellipsoidal samples. For a general ellipsoid with the main axes Α, B and C in the main position, the demagnetization tensor becomes a diagonal matrix with the elements N._a, N_b and N_c. These geometric demagnetization factors can be calculated for the three axis directions by means of elliptic integrals. It applies

N_a+ N "+ N_c=1.

For the energy E_F. in the stray field, depending on the direction cosine "xyz" of the magnetization with respect to the ellipsoid axes results:

2Ao

<img file="AT407803B_D0002.tif" />

[A_Oa_x² + N_ba² +

<img file="AT407803B_D0003.tif" />

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To this so-called shape anisotropy energy E_F. To overcome this, work has to be done by the external field, which ultimately rotates the magnetization in the direction of the field. In addition, in the edge regions of a, for example, rectangular layer, the internal field will not be homogeneous and the coherent rotation of the magnetization will be disturbed.

According to the invention, this problem is solved in a magnetic field sensor of the type mentioned at the beginning by the features cited in the characterizing part of patent claim 1. This results in improvements in sensitivity and the signal-to-noise ratio, the demagnetizing field is minimized and the homogeneous field conditions mean that the spontaneous magnetization is rotated coherently up to the edge areas. These improvements are supported when proceeding according to the features of claim 2. The effect of the overall arrangement of the resistors is increased with the features of claim 5.

The layers or films forming the resistors can be applied in several steps in order to achieve approximately the spatial shape of an ellipsoid or semi-ellipsoid for the sensor field, whereby the field conditions in the interior are homogeneous and the magnetization rotation is also coherent in the edge areas. For example, an approximately ellipsoidal or semi-ellipsoidal structure of the sensor field can be produced in a simple manner by means of vapor deposition technology and various elliptical masks.

Sensor noise can occur particularly in the high-resolution measurement of very weak fields, and it is therefore proposed according to the invention to proceed in accordance with the features of patent claims 3 and 4, respectively. The use of the features according to claim 6 is advantageous for improving the resolution and sensitivity.

It is not only particularly advantageous to design the sensor field in at least one plane parallel to the base or to the carrier to be elliptical or to approximate an ellipse and, if necessary, the spatial shape of the sensor field as a whole in the form of an ellipsoid or of a semi-ellipsoid, but it can also be provided according to the invention to form the individual resistors forming the sensor field, which are spaced apart on the base, in at least one plane parallel to the base or an ellipse. Advantageously, one of claims 7 to 11 is followed.

It is particularly easy to build the ellipsoidal or semi-ellipsoidal structure of a sensor field if the individual resistors forming the sensor field are built up from layers, with the resistors on the outside in the sensor field being made up of fewer layers or thinner layers than the closer ones layers located at the center of the sensor field. Using appropriate vapor deposition techniques, the layer thickness or the number of layers of which a resistor is built up can be varied as desired.

Further advantageous embodiments of the invention can be found in the following description, drawings and claims.

In the following the invention is explained in more detail with reference to the drawings, for example.

1 and 1a schematically show a magnetic field sensor according to the invention in plan view and in longitudinal section. Fig. 2 shows the interaction of individual magnetic quantities in a magnetized ellipsoid. Fig. 3 shows the amount of the demagnetizing magnetic field. Fig. 4 shows schematically the structure of an ellipsoidal sensor field. 5 shows schematically the layer structure of a magnetic field sensor according to the invention. Fig. 6th shows schematically the structure of a single, ellipsoidal resistor. Figures 7a and 7b schematically show a longitudinal and a cross section through a semi-ellipsoidal resistor. 8 shows a schematic section through an embodiment of a magnetoresistive resistor. Fig. 9 shows a section through a number of resistors.

1 shows a schematic plan view of a magnetic field sensor with a structure that is known in principle. The series-connected, magnetic resistance films or resistors 10 are provided with highly conductive barber pole structures 11 with strips 11a in order to achieve a current flow of approximately 45 ° for spontaneous magnetization in order to optimize the operating point. The barber pole structures 11 can be applied to the bottom and / or top of the resistors 10; At least the application of a barber pole structure on the side of the resistor 10 facing away from the carrier or the base 30 is sufficient.

A short current pulse in the flip conductor or in the remagnetization line 12 defines the

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Direction of the magnetization M in the resistors 10. The field of the compensation line 13, which can be seen from the section according to FIG. 1a, is used to determine the field H to be measured_ato compensate at the location of the resistors 10. According to the schematic section in FIG. 1a, at least one shielding layer 15 or 18 is provided or formed between the remagnetization line 12 and the compensation line 13 and / or between the compensation line 13 and the resistor 10. This shielding layer 15 or 18 can be formed by one or more electrically conductive film layers, optionally isolated from one another. The shielding layers 15 or 18th are advantageously earthed in terms of alternating current. An insulation layer J is expediently formed at least on one side of the shielding layer 15 or 18, so that the compensation line 13 and the remagnetization line 12 are separated by the shielding layers 15 or 18 and the insulation layers J or are electrically shielded from the resistors or resistance layers 10. If necessary, only one of the two shielding layers 15 or 18 can be provided, in particular if the mutual influence of the remagnetization line 12 and the compensation line 13 is irrelevant for the measurement result.

The resistors 10 provided can be connected in series or as a potentiometer or as a bridge; the circuit depends on the intended use of the magnetic field sensor.

It should be noted that the illustrations are not true to scale.

The procedure according to the invention can advantageously also be used for simple magnetic field sensors which only consist of a magnetically active layer or a magnetoresistive resistor (for example cores of flux gates, simple magnetoresistive resistors, flux guide layers for Hall probes ...).

Fig. 2 shows the interaction of the fields H_a, Η ,, H_e and M in a magnetized ellipsoid. In the long main axis, the demagnetizing field is minimal and thus the sensitivity of the sensor is increased.

3 shows the magnitude of the demagnetizing field H._e a rod-shaped (long ellipsoid) and a disk-shaped sample (flat ellipsoid).

Both along the rod axis and in the plane of the disk, H becomes_e minimal.

4 shows schematically the structure of an ellipsoidal sensor field 40. The sensor field 40 has an elliptical outline 41 in a plane parallel to the carrier 30. This ellipse has a major axis A and a minor axis B. A multiplicity of resistors 10 are arranged within this elliptical outline 41 and, overall, approximately fill the area of ​​an ellipse. The elliptical resistors 10 lie with their longer axis A perpendicular to the long axis A of the ellipse 41 or the long axis A of the spaced apart resistors 10 runs parallel to the short axis B of the sensor field 40 Section parallel to axis A and parallel to axis B, advantageously likewise elliptical cross-sectional shape or a cross-sectional shape as close as possible to an ellipse. In this context it is noted that the sensor field 40 can also have the shape of a semi-ellipsoid or that in this case the cutting planes along the axes A and B have the outline shape of a semi-ellipse or are as close as possible to this outline shape.

The desired cross-sectional shape of the sensor field 40 in planes parallel to the axes A and B is achieved by correspondingly varying the thicknesses or heights h of the individual resistors 10 on the carrier 30 or by building ellipsoids in a carrier layer.

In the production of such a sensor field 40, a number of preferably elliptical or approximately elliptical circumference having magnetoresistive resistors 10 is initially applied to a base 30 in such a way that these resistors are parallel or perpendicular to each other in the directions of the axes A and B of the sensor field 40 are aligned.

As shown in FIG. 9, the lowermost layer 21 of a resistor 10 is first applied to a carrier 30, further layers 23, 25, etc. being applied to this layer 21 in each case. These further layers 23, 25, 27 have smaller dimensions, in particular with regard to the semi-axes. The thickness and the length of the semi-axes of the layers 21, 23, 25, 27 are matched to one another so that in the respective side or Sectional views for the resistors 10 result in elliptical cross-sectional shapes, as shown in particular in FIG

AT 407 803 B is shown in more detail in FIGS. 7a and 7b. It is advantageous if not only the sensor field 40 has an ellipsoidal or semi-ellipsoidal shape, but also the individual resistors 10 forming the sensor field have an ellipsoidal or semi-ellipsoidal shape. FIG. 9 shows a section along the axis B of a sensor field 40 according to FIG. 4, the external resistors 10 being made up of two layers 21, 23; the row of resistors further inside consists of three layers 21, 23 and 25; the inner layer consists of four layers 21, 23, 25 and 27; from this middle layer the number of layers of the individual resistors 10 decreases again towards the outside. A similar structure of the resistors can also be carried out along the individual rows of resistors parallel to axis A. Since the thickness of the individual layers can also be varied, with the large number of resistors 10 present along the axis A, the layers of the resistors located in the outer regions of the sensor field 40 can be thinner or have a lower height h or the thickness can increase in the direction of axis B may be different from the direction of axis A. Ultimately, however, the resistors located on the outer circumference of the elliptical sensor field should have approximately the same height as one another, or the resistors 10 located in the central region of the ellipse should have an essentially comparable height to one another.

The approximation to the shape of a semi-ellipsoid is easier to implement for the sensor field, because when producing the lower half of an ellipsoid, support layers adjoining the magnetoresistive layer are necessary in order to be able to apply planar elliptical layers with rising semi-axes. In any case, it is essential that the resistors 10 in the middle of the sensor field are thicker than those in the edge zones, whereby the approximately ellipsoidal structure of the sensor field is ensured and at the same time an approximately homogeneous field distribution within the sensor is achieved.

The number of layers forming the resistors 10 depends on the required accuracy and on the manufacturing complexity. The axis ratio or the axes A, B and C of the sensor field 40 are not shown to scale in FIG. 4, but are exaggerated. In order to achieve a low demagnetization factor in the plane of the sensor field 40, the ellipsoid is advantageously designed to be very flat. For this reason, three layers for the resistors 10 will suffice in many cases.

5 shows schematically a section through a magnetic field sensor according to the invention similar to FIG of the shielding layers 15, 18 are arranged. Depending on the desired shielding effect, one or two shielding layers 15 or 18 are provided. Depending on the materials used and the type of production, these layers are connected to one another by appropriate adhesion promoter layers H and / or passivation layers P in order to achieve a compact structure that electrically isolates the layers present in each case from one another.

Fig. 6 shows schematically the structure of an ellipsoidal resistor 10. A lowermost magnetoresistive layer 21, preferably with an elliptical outline, is applied to the provided base 30 or the carrier, with a non-magnetic layer 22 of the same thickness around this layer 21 by making appropriate masking and coatings is arranged around. In the next coating step, a further layer 23, preferably an elliptical layer with larger semiaxes, is applied to these applied layers 21, 22 directly above the first layer 21, at the edge of which a further, non-magnetic layer 24 is attached, etc. In this way it is possible to produce an ellipsoidal resistor 10 that is at least perpendicular and transverse to the base 30. Advantageously, the thickness of the individual layers and the length of the semi-axes of the elliptical layers 21, 23,

Side views results in an approximately elliptical cross section and thus the spatial shape of an ellipsoid is approximated as closely as possible.

A semi-ellipsoid is built up in the same way, except that the base layer is started with the layer having the largest elliptical outline and smaller layers are applied to it with respect to the semi-axes a and b, respectively.

The number of layers is determined by the required accuracy and the manufacturing effort. The axis ratio of the ellipses is shown exaggerated in FIGS. 2 and 4a and 4b.

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In order to achieve a low demagnetization factor in the plane of the film, the ellipsoid is advantageously designed to be very flat. In most cases, sufficiently homogeneous fields in the resistance layer 10 will be achieved with just three layers.

The construction of the sensor field 40 takes place in such a way that at the same time a plurality of resistors 10 are formed on a carrier in a corresponding manner ellipsoidal or semi-ellipsoidal.

FIGS. 7a and 7b show a longitudinal section and a cross section through a resistor 10 made up of three layers and approximated to a semi-ellisoid. It can be seen that a flat semi-ellipsoid can be produced or approximated well with just three layers, which at most can also be varied or graded with regard to their height or thickness h.

FIG. 8 shows schematically in a side view the structure of a resistance layer or a resistor 10 which is built up on a base 30. Power supply lines 31, 32 are connected to the lowermost layer 21 of the resistor 10 in both end regions, and these power lines extend over the resistance layer 21 in the end region at the bottom and at the top. Intermediate layers such as passivation and / or adhesion promoter layers have not been shown in order to improve clarity. The layer thicknesses are also not shown true to scale. The lower power supply lines 31 can be applied to the electrically insulating substrate or the base 30, for example by means of sputtering, the lower barber pole structures being dispensed with in the present case. The barber pole strips 11a are applied to the upper surface of the magnetoresistive film of the lowermost layer 21 and the upper power supply 32 is applied in the end regions.

The next following magnetoresistive layer 23 can be partially introduced into the recess 35 formed by the elevation or step 34 for the connection of the current leads 31, 32 in the lowermost layer 21 to homogenize the internal field, whereby the desired shape of the Ellipsoids or semi-ellipsoids is approximated. Since this layer 23 reproduces the structure of the strips 11a on its outside, the approximation to an elliptical contour course is improved. The gradation in the layer 21 by the lower power supply 31 also contributes to this. These two layers 21, 23 already result in a relatively good structure approximating an ellipsoid, over which the insulation layer J is applied, on which insulation layer J the further layers, as already shown in connection with FIGS. 1 and 5, follow.

If the layer 23 is omitted or no further layers are applied, then the magnetic field compensating power supply 31, 32 also results in advantages for non-ellipsoidal resistors 10.

In Fig. 1, the resistors 10 are shown rectangular in the longitudinal direction; this is done for illustrative reasons. The resistors 10 advantageously have an elliptical cross section in a plane parallel to the base 30. It can be seen that the power supply lines 31, 32 are arranged laterally offset with respect to the longitudinal axis LA through the individual resistors 10. This laterally offset arrangement takes place with opposite end regions of the resistors 10 in each case on the same side of these resistors 10 or on the same side of the longitudinal axis LA. The power supply lines 31 and 32 are thus approximated to the middle area of ​​the respective first barber pole strip 11a. The current through the resp. Each resistor 10 should, if possible, enclose an angle of 40-50 °, in particular 45 °, with the magnetization (which is parallel to the longitudinal axis). This is achieved by the connection geometry of the current supply and discharge lines 31, 32 shown in FIG.

These inventive measures make it possible to optimally compensate for the magnetic fields generated by the current flow and overall improvements in the interference signal distance of magnetoresistive magnetic field sensors are achieved. The ohmic and capacitive repercussions of the compensation line 13 and the magnetic reversal line 12 on one another and on the resistance films 10 are avoided by the shielding layers 15, 18 which are arranged between these planes. The sensor noise is reduced in particular by the ellipsoidal or semi-ellipsoidal shape of the resistors 10, since in this case the rotation of the spontaneous magnetization takes place coherently as far as the edge regions. The interfering magnetic fields generated by the power supply lines 31 and 32 and the barber pole structures 11 in the area of ​​the resistors 10 are transferred to the barber pole 6 by the double-sided connection and by one

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Structures 11 parallel coupling of the current flow avoided.

It is stated that the ellipsoidal or semi-ellipsoidal design of the resistors supports the effect which is achieved by the arrangement of the individual resistors in the form of an ellipsoidal or semi-ellipsoidal sensor field 40.

About 100 to 1000 resistors 10 are arranged in a sensor field 40 designed according to the invention. The resistors 10 are arranged in the sensor field 40 at the same or regular, mutual intervals, in particular in the form of a regular network.