DE10257253A1 - Giant magnetoresistive sensor for motor vehicle cam and crank shafts has eight magnetoresistive elements in two Wheatstone bridges - Google Patents

Abstract

A giant magnetoresistive (GMR) sensor comprises a fully rotational symmetric arrangement of eight GMR resistive elements (I1-4,II1-4) connected together so as to form two Wheatstone full bridges.

Description

The invention relates to a GMR sensor element according to the main claim and its use.

The G iant M agneto- R esistive effect (GMR effect) can be spin valve structures use (spin-valves or "spin valves") for the Winkelsensierung in the form of so-called. This is for example in WO 00/79298 or in EP 0 905 523 A2 described.

GMR spin valves essentially consist of two ferromagnetic thin layers with a resulting magnetization m ₁ or m ₂ , which are separated from one another by an intermediate, non-magnetic thin layer. The electrical resistance R (α) of such a layer system then shows a cosine dependence on the angle α between the direction of the magnetization m ₁ and the direction of the magnetization m _{2 of} the type:

The maximum relative change in resistance ΔR _GMR / R denotes the GMR effect and is typically 5% to 15%.

GMR spin valve coating systems Furthermore usually by cathode sputtering of the respective materials, and then using conventional Photolithography and etching techniques structured.

Essential for the intended spin valve function is a rigid direction of the magnetization m _{1 of} the first ferromagnetic layer, which is at least approximately unchangeable, due to a magnetic field acting on the layer system from the outside, which is to be detected in particular with regard to its direction and / or strength. of the so-called reference layer (RL) or reference layer, and a direction of the magnetization m _{2 of} the second ferromagnetic layer, the so-called free layer (FL) or detection layer, which is oriented slightly at least approximately parallel to the external magnetic field. To achieve both, the two ferromagnetic layers are magnetically decoupled by a sufficient thickness of the non-magnetic intermediate layer, the so-called non-magnetic layer (NML), typically of a few nanometers, and the magnetization of the reference layer (RL), for example by an additional, directly adjacent antiferromagnetic layer, a so-called natural antiferromagnet (AF), and their mutual magnetic coupling fixed ("pinned") by exchange interaction.

This is shown schematically in 1a shown where the GMR layer system or GMR sensor element is under the influence of a magnetic field of a transmitter magnet.

A further improved stabilization of the reference magnetization is achieved by adding an additional so-called synthetic or "artificial" antiferromagnet (SAF). This SAF exists accordingly 1b from two ferromagnetic layers strongly coupled antiferromagnetically via a non-magnetic intermediate layer. The one of these two ferromagnetic layers which lies directly next to or on the natural antiferromagnet AF is referred to as the pinned layer (PL) since its magnetization M _{P is} fixed ("pinned") as a result of the coupling to the natural antiferromagnet (AF). The second ferromagnetic layer of the SAF, whose magnetization M _R is oriented opposite to that of the pinned layer (PL) due to the antiferromagnetic coupling, serves as a reference layer (RL) for the GMR spin valve layer system already described above.

In order to extract the angle-dependent useful signal, four spin valve resistor elements are connected together in a GMR sensor element according to the prior art, for example by means of aluminum thin-film conductor tracks, to form a Wheatstone bridge circuit (Wheatstone full bridge). The maximum signal amplitude is obtained accordingly 2 oppositely oriented reference magnetizations M _{R of} the bridge resistances within the half-bridges and identically oriented reference magnetizations M _{R of} the resistors lying diagonally in the full bridge.

A GMR angle sensor usually has a second full bridge made of GMR resistors, the reference directions of which, as in 2 shown, are rotated by 90 ° relative to those of the first full bridge. The signal U _sin provided by the second full bridge is thereby phase-shifted by 90 ° relative to the signal of the first full bridge U _cos .

The formation of an arc tangent or corresponding algorithms (eg CORDIC algorithm) is then used to determine the unique angle α to the direction of an external magnetic field B from the two cosine or sinusoidal bridge signals U _sin , U _cos over a full 360 ° revolution.

The different reference magnetization directions according to 2 are realized, for example, in that the individual GMR bridge resistances correspond locally to a temperature T above the blocking temperature (Néel temperature) of the antiferromagnetic layer (AF) but below the Curie temperature of the ferromagnetic layers (PL, RL) 1a respectively. 1b be heated so that the antiferromag netic spin order is canceled in the antiferromagnetic layer, and then cooled in an external magnetic field suitable field direction. When the antiferromagnetic order is formed again, the spin configuration resulting from the exchange interaction at the interface of the antiferromagnetic layer (AF) and the adjacent ferromagnetic layer (PL) is frozen. Consequently, the direction of magnetization of the adjacent ferromagnetic layer (pinned layer PL) is fixed. Local heating of the GMR bridge resistors can take place, for example, using a short laser or current pulse. The current pulse can be driven directly through the GMR conductor structure and / or an additional heating conductor.

In known GMR angle sensors, the reference magnetization M _{R of} the individual bridge resistors is selected either parallel or perpendicular to the direction of the strip-shaped structured GMR resistance elements. This serves to keep the influence of the shape anisotropy low. Furthermore, the strip-shaped structured GMR resistance elements within a full bridge are in accordance with 2 preferably aligned in parallel. This serves to suppress a signal contribution due to a superimposed anisotropic magnetoresistive effect (AMR effect). The AMR signal contribution is based on a dependence of the electrical resistance on the angle α between the current and the magnetization direction of the shape:

If, on the other hand, the GMR resistors are implemented within a half-bridge with their GMR strips aligned orthogonally, as is shown, for example, in 10 is the case in WO 00/79298, the AMR signal contribution is even favored to the maximum. This affects the angular accuracy of the GMR angle sensor.

Advantages of invention

Point for the reasons mentioned therefore known GMR angle sensors do not have a rotationally symmetrical arrangement of the bridge resistances. Both full bridges are rather common arranged laterally next to each other. This results in a consequence the lack of rotational symmetry an increased sensitivity known Sensors regarding the directional inhomogeneity the encoder field, i.e. the external magnetic field, as well as regarding Temperature gradient.

The fact that with known GMR angle sensors the pinning or reference direction within a bridge resistor always have a fixed angle to the direction of the strip these sensors do not offer the possibility of shape anisotropy influences on the pinning behavior and related disadvantages on the angle sensing accuracy compensate.

For a 360 ° one Angle sensor, on the other hand, is a rotational symmetry in the sensor design advantageous to not already due to an asymmetry in the arrangement to obtain additional direction-dependent angular error contributions of the individual GMR resistance elements.

By the rotationally symmetrical according to the invention Arrangement of the GMR resistance elements in the two Wheatstone bridges is therefore both a reduced sensitivity to field direction and Temperature inhomogeneities achieved as well as an undesirable AMR signal contribution suppressed and also the shape anisotropy influence on the pinning behavior and reduces the angle sensing accuracy of the GMR sensor element.

It is also particularly advantageous if in addition to the rotationally symmetrical arrangement of the GMR resistance elements in the two Wheatstone bridges an interleaved arrangement of these resistors is chosen. this leads to to a further reduced sensitivity to field direction and temperature inhomogeneities.

The suppression of the interfering AMR signal contribution is achieved by additionally dividing each individual GMR bridge resistance element into two equal halves or partial bridge resistances with orthogonally oriented GMR strip directions. In particular, this also leads to an increase in the angle measurement accuracy. In this context, it is also advantageous that the direction of the strip-shaped structured GMR resistance elements (“GMR strip direction”) is chosen to be parallel for one of the two partial bridge resistances and perpendicular to the pinning or reference direction for the other partial bridge resistance. sets an average of the influence of pinning directions parallel and perpendicular to the stripe direction within each of the GMR bridge resistance elements. The pinning behavior is then identical for all two-part GMR bridge resistance elements (averaging over both parts). In this case, the two bridge output signals U ₁ , U ₂ also advantageously have a 45 ° phase shift with respect to one another.

If the GMR resistance elements have a pinning or reference direction that is selected at least approximately at 45 ° to the direction of the strip-shaped structured GMR resistance elements, this advantageously leads to identical pinning behavior of the individual GMR resistance elements te, ie in particular to an improved signal stability and long-term stability of the GMR sensor element. In this case, the two bridge output signals U ₁ , U ₂ also have a 45 ° phase shift with respect to one another.

Bridge output signals U ₁ , U ₂ which are phase-shifted with respect to one another by any angle φ, where φ is preferably 45 ° or is 45 °, can finally be advantageously mapped by a coordinate transformation to orthogonal signals with a 90 ° phase shift. The angle α to the direction of the external magnetic field B can then be determined from the latter by arctangent formation or a corresponding algorithm, for example the CORDIC algorithm.

The coordinate transformation also offers the advantage that fluctuations in the phase difference of the two bridge external signals U ₁ , U ₂ caused by production can be compensated for when mapping to the orthogonal signals.

drawings

It shows 1a a simplified GMR spinvalve layer structure with two ferromagnetic layers RL and FL with the magnetizations m ₁ and m ₂ , a non-magnetic intermediate layer NML and an antiferromagnetic layer AF. The latter is used for fixing (pinning) the reference magnetization m ₁ . In addition, a transmitter magnet for generating an external magnetic field B is provided. The angle α denotes the angle between the field or magnetization direction of the free ferromagnetic layer (FL) and thus also the direction of the external magnetic field B in the plane of the GMR sensor element and the reference magnetization direction.

The 1b shows a GMR spin valve layer system with a natural antiferromagnet AF and an additional synthetic antiferromagnet SAF as well as a further non-magnetic intermediate layer NML and a ferromagnetic free layer FL.

The 2 shows an equivalent circuit diagram for an angle sensor element based on the GMR effect with two full bridges (Wheatstone bridge circuits), the reference magnetizations M _{R are} oriented in opposite directions within the two bridges and are rotated from bridge to bridge by 90 ° to each other. The direction of the reference magnetization M _R is further parallel or perpendicular to the direction of the individual, strip-shaped structured GMR resistance elements, which for example according to 1a or 1b are set up. This "stripe direction" is represented by the indicated streak family within the individual GMR resistance elements. Next to it is in 2 indicated the direction of an external magnetic field B, which includes the angle α to be measured with the GMR sensor element with a reference direction. The reference or zero direction is defined by the choice of the reference magnetization directions in the two full bridges, one of which is designed as a sin full bridge and one as a cos full bridge.

The 3 shows a rotationally symmetrical arrangement of meandering, nested GMR bridge resistance elements I / 1 to I / 4 (bridge I) and II / 1 to II / 4 (bridge II). The directions of the reference magnetization (see arrows in 3 ) in bridge I each oriented at 45 ° to the direction of the individual, strip-shaped structured GMR resistance elements, and the reference magnetization directions in bridge II in each case rotated by 45 ° with respect to those in bridge I. Next to it is in 3 indicated the direction of an external magnetic field B, which includes the angle α to be measured with the GMR sensor element with a reference direction. The reference or zero direction is defined by the choice of the reference magnetization directions in bridge I and bridge II, bridge I being intended to deliver a cosine-shaped signal curve over the angle α.

The 4 shows an equivalent circuit diagram according to the layout of the GMR sensor element 3 , The pinning or reference magnetization direction M _A is in each case at 45 ° to the GMR strip direction, which is again analog 2 is indicated by the family of strips drawn within the individual GMR resistance elements, oriented, and additionally rotated in bridge II by 45 ° with respect to that in bridge I. There is an amplification of the AMR signal contribution as a result of mutually orthogonal stripe directions of the resistances of each half-bridge.

The 5a shows GMR sensor output signals U ₁ and U ₂ with 45 ° phase difference in accordance with a pinning or reference magnetization direction M _R at 45 ° to the strip direction accordingly 3 or 4. The 5b shows correspondingly transformed, mutually orthogonal GMR sensor signals U _cos and U _sin with 90 ° phase difference. The AMR signal contribution is in 5a and 5b not shown. On the x axis is in 5a respectively. 5b each the direction of the external magnetic field B in degrees, ie the angle α, plotted while on the y-axis at 5a the GMR sensor output signal in mVolt / Volt and at 5b the transformed GMR sensor signal is plotted in mVolt / volt.

The 6 shows a rotationally symmetrical, at least approximately circular or octagonal, nested arrangement of meandering GMR bridge resistance elements, wherein the AMR signal contribution was suppressed by dividing each of the individual bridge resistance elements into two equal halves with mutually orthogonal stripe directions.

The 7 shows an equivalent circuit diagram according to the layout of the GMR resistance elements 6 , Suppression of the AMR signal contribution is achieved here by dividing each bridge resistance element I / 1, I / 2 to II / 4 into two halves a and b with mutually orthogonal GMR strip directions. The respective pinning or reference magnetization M _R is oriented at 45 ° to the respective GMR strip direction. The latter is indicated by the family of strips drawn within the individual GMR resistance elements.

The 8th shows an equivalent circuit diagram according to the layout of the GMR resistance elements 6 with to 7 alternative pinning or reference magnetization directions M _R at 0 ° and 90 ° to the GMR strip direction for each of the individual bridge resistances I / 1, I / 2 to I / 4. The influence of the pinning direction is averaged here by means of the pinning or reference magnetization direction, which is both parallel and perpendicular to the GMR strip direction, within each two-part bridge resistance I / 1, I / 2 to I / 4.

embodiments

a.) rotationally symmetrical arrangement

The 3 shows a possible rotationally symmetrical arrangement of a total of eight bridge resistance elements of two full bridges (Wheatstone bridges). In contrast to AMR sensors, in which the reference direction is given by the current direction, which is defined by the strip direction, the GMR angle sensor defines the reference direction by the direction of the magnetization of the reference layer (RL). In principle, the pinning or reference direction can be chosen arbitrarily, but in order to obtain the same pinning behavior for all bridge resistance elements, an orientation of the pinning or reference direction at 45 ° to the strip direction is selected here. This is further illustrated in 4 , where the direction of the reference magnetization M _{R is also} indicated in addition to the direction of the stripes (family of stripes within the resistance symbols).

b.) Mapping to orthogonal signals

In the case of a pinning direction or direction of the reference magnetization at 45 ° to the GMR strip direction, the two bridge output signals U ₁ and U _{2 have} according to 5a not the usual phase shift of 90 °, but only a 45 ° phase shift. However, these signals U ₁ , U ₂ can be easily related to the orthogonal, cosine and sinusoidal signals 5b be transformed. For this purpose, the following transformation is carried out in a sensor evaluation electronics:

Here φ denotes the phase shift of the second bridge signal relative to the first bridge signal. In principle, this phase shift can be chosen arbitrarily, however, a phase shift of 45 ° is preferably set.

According to the cosine and sinusoidal signals obtained by means of this transformation 5b can be determined by arc tangent formation or by using an appropriate algorithm such as the CORDIC algorithm in the sensor evaluation electronics:

The implementation of this coordinate transformation also has the important advantage that manufacturing-related fluctuations in the phase shift of the two bridge signals U ₁ , U ₂ can be detected and compensated for in a sensor-specific manner when mapping to orthogonal signals (90 ° phase shift). For this purpose, for example, in an offset and amplitude balance of the signals U _1, U ₂ is at the end of a production line and this phase shift φ example by means of Fourier analysis of the two bridge signals U _1, U ₂ is determined, and stored transmitter sensor in.

c.) rotationally symmetrical Arrangement with suppression of the AMR signal contribution

In the 3 The resistor arrangement shown favors the AMR signal contribution, since the GMR strip directions of the two bridge resistors of each half-bridge are orthogonal to one another. This disadvantage can be avoided by according to the preferred, likewise rotationally symmetrical arrangement 6 Each bridge resistor is composed of two equal halves with GMR strip directions perpendicular to each other. The AMR component is then filtered out by the series connection of the two partial resistors, each with an identical reference magnetization M _R , while the GMR signal portion remains unchanged due to the identical direction of the reference magnetization M _R for both partial resistors. This fact is illustrated by the following relationship for a two-part GMR bridge resistance element:

Here α denotes the angle between the field or magnetization direction of the free ferromagnetic Layer (FL) and the reference magnetization direction; referred to the angle between the field or magnetization direction of the free Layer (FL) and the GMR strip direction of the first partial resistor. The strip direction of the second partial resistor is -90 ° to that of the first partial resistance rotated.

d.) Pinning behavior ^`

The 7 illustrates the division of the bridge resistances into two halves with mutually orthogonal stripe directions but identical reference magnetization direction M _R. In principle, the pinning direction or the direction of the reference magnetization M _R can be chosen arbitrarily. However, an angle of 45 ° to the respective strip direction is preferred, since this achieves identical pinning behavior for all partial resistors.

Alternatively, a pinning direction or a direction of the reference magnetization M _R can also be set, which is oriented parallel to the stripe direction for one of the two partial resistors and perpendicular to the stripe direction for the other partial resistance. This results in a different pinning behavior for the individual partial resistors, but for each of the bridge resistance elements in the form of a series connection of the two partial resistors, an identical pinning behavior.

This choice of the pinning or reference magnetization direction offers opposite Known sensors have the advantage that within each bridge resistance element over the different pinning behavior from parallel and perpendicular Alignment of the pinning or reference magnetization direction to the GMR strip direction is averaged.

The described 360 ° GMR angle sensor is particularly suitable for the detection of the absolute position of the camshaft or the crankshaft in a motor vehicle, especially in one camshaft-free engine with electric or electro-hydraulic Valve control, an engine position of an electrically commutated motor or a detection of a wiper position, or in the Steering angle sensors in motor vehicles.

Claims (7)

GMR sensor element with a rotationally symmetrical arrangement of in particular eight GMR resistance elements, which form two Wheatston full bridges with one another are connected. GMR sensor element according to claim 1, characterized in that the GMR resistance elements into each other are nested. GMR sensor element according to claim 1 or 2, characterized in that that the GMR resistance elements in strips are structured. GMR sensor element according to one of the preceding claims, characterized in that each GMR resistance element of the Wheatston full bridges is divided into two identically constructed halves with directions of the strip-shaped structured GMR resistance elements oriented orthogonally to one another. GMR sensor element according to one of the preceding claims, characterized characterized that it is a clear measurement of an angle (α) an external magnetic field (B) opposite a direction of magnetization of a reference layer (RL) can be carried out over 360 °. GMR sensor element according to one of the preceding claims, characterized characterized in that the GMR resistance elements at least approximately circular or octagonal are arranged. Use of a GMR sensor element according to one of the preceding Expectations in an angle sensor for detecting the absolute position of a camshaft or a crankshaft in a motor vehicle, in particular with a camshaft-free engine with electrical or electro-hydraulic Valve control, an engine position of an electrically commutated motor or a detection of a wiper position, or in the Steering angle sensors in motor vehicles.