WO2002082111A1

WO2002082111A1 - Method for adjusting magnetization in a layered arrangement and use thereof

Info

Publication number: WO2002082111A1
Application number: PCT/DE2002/000900
Authority: WO
Inventors: Gunther Haas; Andrew Johnson; Gilbert Moersch
Original assignee: Robert Bosch Gmbh
Priority date: 2001-04-07
Filing date: 2002-03-14
Publication date: 2002-10-17
Also published as: DE10117355A1

Abstract

Disclosed is a method for adjusting magnetization, especially local modifications, in the resulting magnification (m2) in a layered arrangement (2) with a ferromagnetic layer (2a) and an adjacent antiferromagnetic layer (2b). The antiferromagnetic layer (2b) is initially heated, at least partially, to above a threshold temperature (Tb), especially with the aid of a laser. Above said temperature, the influence of said area on the resulting direction of magnetization (m2) of the adjacent area (5, 6, 8, 9) of the ferromagnetic layer (2a) disappears substantially. At least the area (5,6,8,9) of the ferromagnetic layer (2a) adjacent to the heated area of the antimagnetic layer (2b) is exposed to an external magnetic field (H) of a given direction and the antiferromagnetic layer (2b) is finally cooled to below the threshold temperature (Tb). The inventive method is particularly suitable for production of a magnetoresistive layered system working according to the spin-valve principle with partially different directions of magnetization (m2) which are connected in the form of Wheatstone bridges.

Description

Method for setting a magnetization in a layer arrangement and its use

The invention relates to a method for setting, in particular for local change, a resulting direction of magnetization in a layer arrangement according to the type of the main claim, and the use of this method for producing a magnetoresistive layer system working according to the spin valve principle.

State of the art

A magnetoresistive layer system which operates according to the spin valve principle has a soft magnetic or ferromagnetic detection layer, an adjacent, non-magnetic, electrically conductive intermediate layer and a reference layer which is as hard as possible and is adjacent to the intermediate layer, with a predetermined spatial orientation of the direction of the resulting magnetization on. With a suitable design of the thicknesses of the individual layers, such a layer system then shows a change in the electrical resistance of the intermediate layer in accordance with:

R = R ₀ + C cos θ

Here, θ denotes the angle between the magnetization πti belonging to the detection layer or its direction and the magnetization m ₂ belonging to the reference layer or its direction. Since the magnetization rri _ι in the soft direction of the magnetic detection layer can be changed by an externally applied magnetic field, aligning itself as far as possible parallel to this, a corresponding change in resistance occurs in the intermediate layer, which is typically in the range of 5% and 15% ("Giant Magneto Resistance "(GMR)).

Magnetoresistive coating systems are widely used in magnetic disks and reading heads, but they are also suitable for measuring magnetic field strengths and directions of magnetic fields and in particular for contactless detection of speeds and angles as well as quantities derived therefrom, for example in motor vehicles.

In rαagnetoresistive layer systems based on the spin valve principle, it is also known to design the hard magnetic reference layer from two adjacent partial layers arranged one above the other, a relatively soft magnetic, ferro-magnetic layer with the magnetization m ₂ directly adjacent to the intermediate layer, and one the underlying antiferromagnetic layer, which defines the spatial orientation of the magnetization m ₂ in the soft magnetic, ferromagnetic layer via the so-called "Exchange Bias Effect". Since the antiferromagnetic layer has no or hardly any magnetic properties after being generated by an external magnetic field Alignment or unidirectional anisotropy in the reference layer must be induced during deposition of the antiferromagnetic layer by applying an external magnetic field.

Such a construction of the reference layer from ferromagnetic and antiferromagnetic layers has the advantage that even relatively strong external magnetic fields do not become one Change the direction of the magnetization m ₂ in the reference layer.

For practical applications of magnetoresistive layer systems, it is often essential to operate them in a Wheatstone bridge circuit in order to eliminate the relatively large temperature dependence of the GMR effect. For this purpose it is known to deposit such a layer system on a substrate, in four individual resistors which

L0 are designed, for example, as meandering conductor tracks, rectangles or circles, to be structured, and then to be interconnected by means of conductor tracks to form a Wheatstone bridge. For example, for an angle measurement, a direction-dependent one from an externally applied magnetic field

Obtaining L5 bridge _{output signal} is further known to distinguish the magnetization directions of the regions of the reference _layer which form the four individual resistors R _1A R ₂ , R ₃ and R. The magnetization directions of the regions of the reference layer that are caused by the resistances

20 the Ri and R _{3 are} taken, in relation to the areas which are occupied by the resistors R ₂ and R ₄ , rotated by 180 °. This results in a voltage Uß as the bridge output voltage according to:

25 U _B = 2ü ₀ C cosθ

However, an angle measurement over an angular range of 360 ° is only possible with such a bridge circuit if two interconnected Wheatstone bridges are used at the same time, which are rotated by 90 ° with respect to the directions of the magnetizations.

Bridge circuits are problematic in a magnetoresistive layer system with two Wheatstone rotated against one another, in order to produce a sensor that detects locally different and simultaneously defined directions of the resulting magnetization m ₂ in particular on a chip. For this purpose, it has already been proposed to replace the antiferromagnetic partial layer by a so-called “artificial” antiferromagnet, which has a resulting magnetic moment. In this way, the resulting magnetization m ₂ in the reference layer can be made retrospectively, ie even after the deposition of the layer system of artificial antiferromagnet and reference layer, locally change again by means of an external magnetic field or set L0. However, one has to accept that the direction of the magnetization m ₂ in the reference layer can inevitably also be changed by external interference fields. Such a GMR sensor element is offered by Infineon AG, Munich, under the designation GMR-B6.

L5

In addition, the application DE 199 49 714.1 describes a magnetically sensitive component that works on this principle. The 360 ° angle measurement using two Wheatstone bridges is also explained there.

20

The object of the invention was to provide a method with which locally different directions of a resulting magnetization in a layer arrangement, in particular on a chip, or these directions as well

25 can be changed again after the layer arrangement has been deposited. In particular, it was an object to provide a method by means of which magnetoresistive layer systems working according to the spin valve principle can be produced, for 360 ° angle measurement and in particular in motor vehicles

30 can be used in ABS wheel sensors, steering angle sensors or as a potentiometer replacement, and which deliver an offset-free output voltage over the widest possible temperature range.

35 advantages of the invention The method according to the invention for setting, in particular for locally changing, a resulting direction of magnetization in a layer arrangement has the advantage over the prior art that a magnetoresistive layer system which works according to the spin valve principle can be produced in a particularly simple manner, in which Areas are present within the reference layer, each with a different, in particular pairwise perpendicular magnetization direction.

LO

It is particularly advantageous here that a sensor element based on the GMR effect can be produced, in which locally different directions of the resulting magnetization are present on a chip or a substrate, so L5 that these areas can be interconnected to form a Wheatstone bridge in order to to achieve a large degree of temperature independence of the specific electrical resistance in the current-carrying intermediate layer.

It is also advantageous that two Wheatstone bridge circuits can now also be implemented on the substrate at the same time, the directions of the resulting magnetization of the individual regions in the first Wheatstone bridge relative to the resulting directions of the magnet

_5 in the individual areas of the second Wheatstone bridge are rotated 90 ° against each other. In this way, in addition to a temperature-independent output signal of the GMR sensor element, an offset-free bridge output voltage U _{B can also be} achieved. In addition, an angle is

30 solution over 360 ° possible.

Another advantage of the method according to the invention is that it is possible to dispense with the use of an “artificial” antiferromagnet, so that external interference fields do not impair the layer arrangement or the locally Different directions of the resulting magnetization remain unchanged due to such external interference fields.

In addition, the method according to the invention can easily be integrated into the mass production of sensor elements and the usual processes. In addition, one has the possibility of simply determining the shape of the regions of different magnetization directions via the local heating on the substrate, i.e. For example, to generate locally meandering, circular or rectangular areas which are then interconnected.

Advantageous developments of the invention result from the measures mentioned in the subclaims.

L5

It is particularly advantageous if the antiferromagnetic layer, in particular locally, is heated above the threshold temperature T _b by irradiation with a laser. A laser can be particularly simple, defined and localized.

10 cal limits heat in the layer arrangement and in particular the antiferromagnetic layer to be heated. It has also proven to be particularly advantageous with regard to a locally defined energy input if the irradiation with the laser in the form of short-term

15 ger pulses with a pulse duration of 10 ns to 100 μs.

It is also advantageous if the irradiation with the laser is carried out by scanning strips to be irradiated, so that stripes with on the substrate in the ferromagnetic layer adjacent to the antiferromagnetic layer

30 different resulting magnetization directions can be induced. Local or punctual laser pulses or the scanning of the antiferromagnetic layer with the laser, in particular, isolate surfaces with a size of typically 5 μm ² to 500 μm ² or strips of a typical

35 see width from 5 μm to 100 μm and a length of 1 mm up to 120 mm, depending on the size of the substrate or wafer used.

Because successively in different, locally limited areas or strips of the antiferromagnetic

Layer is heated above the threshold temperature T, an adjustment, in particular a change, of the locally resulting magnetization directions m ₂ can advantageously be made in the assigned areas of the ferromagnetic layer by an external magnetic field applied during the heating. The locally different magnetization directions m ₂ are preferably aligned perpendicular to one another.

It has also proven to be particularly simple and advantageous if the external magnetic field used to change or set the local direction of magnetization during heating up already during the heating of the antiferromagnetic layer above the threshold temperature T _b and in particular during the entire time, within which the respective range the antiferromagnetic layer is above this threshold temperature is maintained. In principle, however, it is also sufficient if the external magnetic field is only applied above the threshold temperature after it has been heated up and is maintained at least until the temperature drops below the threshold temperature. In any case, the result is that the resulting direction of magnetization of the ferromagnetic layer in the region adjacent to the heated region of the antiferromagnetic layer after cooling at least approximately to that during the

Time of heating above the threshold temperature applied direction of the external magnetic field is aligned in parallel.

Thus, the heated area of the antiferromagnetic layer, even after cooling below the threshold temperature T _b again a stabilization of the area of the ferromagnetic layer adjacent to this area with respect to the direction of the resulting magnetization m _{2 there} . Overall, a local stabilization direction of the resulting magnetization in the ferromagnetic layer is defined by the local heating.

With regard to the materials for the ferromagnetic layer and the antiferromagnetic layer, customary materials can advantageously be used. A ferromagnetic layer is particularly suitable for a soft magnetic layer, for example a nickel layer, an iron layer, a cobalt layer or a layer with an alloy of two or three of the elements mentioned. A nickel oxide layer or an iridium-manganese layer, for example, is suitable as the antiferromagnetic layer.

drawings

The invention is explained in more detail with reference to the drawings and in the description below. 1 shows a schematic diagram of a magnetoresistive layer system based on the spin valve principle, FIG. 2a shows a section through FIG. 1 below the threshold temperature, FIG. 2b shows a section through FIG. 1 after heating above the threshold temperature and cooling with an external magnetic field H applied 2c shows the magnetoresistive layer system according to FIG. 1 and FIG. 2b on a substrate. FIG. 3 shows the local setting of the magnetization direction m ₂ in the form of strips, while FIG. 4 explains an interconnection of local areas with different magnetization directions to form two Wheatstone bridge circuits. embodiment

The invention is based on a magnetoresistive layer system according to the spin valve principle shown in FIG. 1, which has a GMR effect. It is provided that an electrically conductive intermediate layer 3, which is current-carrying during operation, is arranged on a reference layer 2, which at least locally has a resulting magnetization m ₂ with a predetermined, fixed or “pinned” magnetization direction, and a detection layer 1 is arranged thereon detection layer 1 is, for example, a soft magnetic layer whose magnetization m _**. always at least approximately aligns parallel to an externally applied magnetic field. Since, in such an external magnetic field, the direction of magnetization m _2, as already explained, at least remains largely unaffected, resulting an angle-dependent electrical resistance of the intermediate layer (GMR effect).

In detail, FIG. 2c shows that an optional buffer layer 11, which consists of tantalum and is a few nanometers thick, has first been applied to a substrate 10 made of, for example, thermally oxidized silicon, for example using sputtering technology. The detection layer 1 was then deposited on this buffer layer 11, which consists for example of a nickel-iron layer a few nanometers thick or a cobalt layer. The ferromagnetic detection layer is preferably a soft magnetic, ferroelectric layer. The intermediate layer 3 was then in a known manner on the detection layer 1 in the form of a few

Nanometer thick layer, for example made of copper, deposited.

Finally, a ferromagnetic layer 2a made of a preferably relatively soft magnetic material such as a nickel Iron alloy or cobalt deposited with a thickness of a few nanometers before an antiferromagnetic layer 2b was deposited thereon, which consists, for example, of a few nanometer thick nickel oxide layer or an iridium-manganese layer.

The ferromagnetic layer 2a and the adjacent antiferromagnetic layer 2b form the reference layer 2 according to FIG. 1. At this point it should also be emphasized that the layer sequence according to FIG. 2c can also be reversed, i.e. the reference layer 2 is deposited on the buffer layer 11, then the intermediate layer 3 and then the detection layer 1.

In FIG. 2c it is further provided that at least when the reference layer 2 is generated from the two sub-layers 2a, 2b by applying an external magnetic field during the deposition or deposition, a homogeneous alignment of the resulting magnetic moment or the magnetization m ₂ in the first ferromagnetic layer 2a is set. In particular, this application of the external magnetic field during the deposition or deposition favors unidirectional anisotropy in the reference layer 2, which is also referred to as the “pinning” direction.

For local adjustment or change of the “pinning” direction in the reference layer 2 or in particular in the ferromagnetic layer 2a, ie specifically the direction of the magnetization m ₂ resulting locally there, it is now further provided that at least the antiferromagnetic layer 2b, but preferably the antiferromagnetic layer 2b and the ferromagnetic layer 2a are heated above a threshold temperature T _b by local irradiation with the aid of a laser. This threshold temperature is also referred to as the "blocking temperature" of the antiferromagnetic layer 2b. The heating is based on the knowledge that when an antiferromagnetic layer is heated above this threshold temperature T _b , the so-called “Exchange 5 bias effect” disappears, ie the antiferromagnetic

Layer 2b no longer induces a preferred direction of magnetization m ₂ in the adjacent ferromagnetic layer 2a above this threshold temperature T _b . In this respect, the stabilization of the direction of the magnetization m ₂ , L0, which was caused by the antiferromagnetic layer 2b, is also lost above this threshold temperature T _b .

In detail, according to FIG. 2a it is provided that, in a plan view of the antiferromagnetic layer 2b, locally

L5 th areas of the antiferromagnetic layer 2b, for example insulated areas with a size of 5 μm ² to 500 μm ² , or alternatively also strips with a width of 5 μm to 100 μm and a length of 1 mm to 120 mm, successively with a laser be heated. The laser offers

20 the possibility to heat even surfaces with a size of μm ² very precisely. In principle, however, other heating methods are also possible with which such local or strip-shaped heating of the antiferromagnetic layer 2b is possible. The threshold temperature T _b depends on

25 otherwise depend on the material of the antiferromagnetic layer 2b. In the case of the materials mentioned above, it is approx. 200 ° C.

FIG. 2a first shows the state below the threshold temperature T _b at which the antiferromagnetic

Layer 2b in the ferromagnetic layer 2a, which is adjacent to it, induces a unidirectional anisotropy of the magnetization m ₂ via the "exchange bias effect". FIG. 2b then shows how radiation with a Laser first heated the antiferromagnetic layer 2b above the threshold temperature T _b , whereby at least after the threshold temperature T _{b has been} exceeded and during the subsequent cooling, an external magnetic field H of the indicated direction has been applied. If now gnetfeld with applied external ma- H by turning off the laser irradiation, the antiferromagnetic layer 2b again below the threshold temperature T _b cools, sets of _b by the heating above the threshold temperature T off "exchange bias effect" again, ie the antiferromagnetic layer 2b "pins" or fixes again a resulting magnetization m ₂ in the layer 2a via the boundary layer between the antiferromagnetic layer 2b and the ferromagnetic layer 2a with the direction shown in FIG. 2b corresponding to the direction of the temporarily applied external magnetic field H. ,

Overall, it is achieved by the method explained that in the area of the ferromagnetic layer 2a, which is adjacent to the heated area of the antiferromagnetic layer 2b, the direction of the magnetization m ₂ is oriented in accordance with the direction of the external magnetic field H applied during the heating above the threshold temperature. This alignment only occurs in the areas that are adjacent to the heated areas. Other areas are not affected by the change in the direction of magnetization.

In summary, according to FIG. 2b it was achieved that the resulting magnetization m ₂ in the ferromagnetic layer 2a above the threshold temperature T _{b followed} the direction of the external magnetic field H, and that during the cooling below the threshold temperature T _b the antiferromagnetic layer 2b over the coupling "exchange bias effect", so that the direction of the magnetization m ₂ below the threshold temperature T _b of the antiferromagnetic layer 2b is again induced or stabilized.

Since the explained heating with a laser is a local effect, it is now possible in a simple manner to generate areas with different directions of the resulting magnetization m ₂ locally and defined in the ferromagnetic layer 2a on the surface of the substrate 10. This is explained with the aid of FIG. 3, which is a top view of the ferromagnetic layer 2a according to FIG

2c shows. The antiferromagnetic layer 2b covering the ferromagnetic layer 2a was not shown in FIG. 3.

3 shows a first strip 5, a second strip 6, a third strip 8 and a fourth strip 9, each of which has a different direction of magnetization m ₂ in the rectangular areas shown. In particular, the rectangular regions of the ferromagnetic layer 2a adjacent to the heated regions of the antiferromagnetic layer 2b are designed in the form of insulated areas with a size of 5 μm ² to 500 μm ² . In particular, it is provided in FIG. 3 that the individual areas or strips 5, 6, 8, 9 with different directions of the magnetization m _{2 are at} a minimum distance of 20 μm to 100 μm from one another.

In order to achieve the indicated directions of magnetization m ₂ in the strips 5, 6, 8, 9, they were each heated one after the other by laser irradiation above the threshold temperature T _b , whereby, as explained, a respective one of the indicated directions of magnetization m ₂ in the individual strips 5, 6, 8, 9 parallel external magnetic field H has been applied. This is exemplarily illustrated in FIG. 3 using the example of the second strip 6. tert. For the other strips 5, 8, 9, the external magnetic field H was rotated by 90 °.

A corresponding mask is preferably used to generate the rectangular or alternatively also meandering areas with locally different magnetization directions according to FIG. 3 by heating corresponding assigned areas of the antiferromagnetic layer 2b. However, it is also possible to work without a mask, in that locally limited areas are heated, for example with a fine, circular laser pulse with a pulse duration of 10 ns to 100 μs.

In addition, it should be emphasized that the heating can take place on a wafer use or alternatively also on a sensor element that has already been processed with or without an additional passivation layer applied on the reference layer 2. In particular, the laser treatment and thus the local change in the resulting magnetization direction according to FIG. 3 can also be carried out in a final backend test.

The method steps following FIG. 3, such as structuring the generated reference layer 2, applying line layers for the electrical interconnection of the individual regions produced, and suitable insulation layers or protective layers correspond to the prior art.

FIG. 4 explains the connection of two Wheatstone bridge circuits generated on the substrate 10 in the ferromagnetic layer 2a according to FIG. 2c or FIG. 1. For this purpose, the areas with different directions of the resulting magnetization m ₂ according to FIG. 3 were routed to a first Wheatstone bridge 40 and a second Wheatstone bridge 41 via conventional conductor layers or conductor tracks. interconnected. In this example, the first Wheatstone bridge 40 is formed by the areas within the first strip 5 and the second strip 6. To form the second Wheatstone bridge 41, areas lying within 5 of the third strip 8 were connected to areas lying within the fourth strip 9 as shown. In this way, the first Wheatstone bridge 40 is rotated by 90 ° with respect to the second Wheatstone bridge 41.

L0 Within the individual Wheatstone bridges 40, 41, two areas with a parallel magnetization direction and two areas with an anti-parallel magnetization direction are further interconnected in a manner known per se. Each of the two Wheatstone bridges 40, 41 delivers

L5 thus a bridge output voltage U _B as a function of an input voltage U ₀ according to:

U _B = 2U ₀ C cos θ

By rotating the Wheatstone bridge 40 relative to the second Wheatstone bridge 41 by 90 °, the first Wheatstone bridge 40 delivers a cos signal while the second delivers a sin signal. With the help of the well-known Arctan evaluation method, the absolute

Determine 25 angles of the direction of an external magnetic field over the entire angular range of 360 °.

Claims

Expectations

1. Procedures for recruitment, especially local changes

L0 change of a resulting magnetization direction in a layer arrangement (2) with a ferromagnetic layer (2a) and an adjacent antiferromagnetic layer (2b), the ferromagnetic layer (2a) resulting magnetization with an associated, by the anti-

L5 has a magnetization direction (m ₂ ) induced or influenceable, in particular stabilizable, resulting in a ferromagnetic layer, characterized in that at least the antiferromagnetic layer (2b) is at least partially heated above a threshold temperature (T _b )

20, above which the influence of this area of the antiferromagnetic layer (2b) on the resulting direction of magnetization (m ₂ ) of the adjacent area (5, 6, 8, 9) of the ferromagnetic layer (2a) at least largely disappears, so that at least that heated loading

25 region (5, 6, 8, 9) of the ferromagnetic layer (2a) which is adjacent to the antiferromagnetic layer (2b) is exposed to an external magnetic field (H) in a predetermined direction, and that the antiferromagnetic layer (2b) is then again below the threshold temperature ( T _b ) is cooled.

30

2. The method according to claim 1, characterized in that the external magnetic field (H) is already applied when heating up and / or after reaching the threshold temperature (T _b ).

35

3. The method according to claim 1 or 2, characterized in that the external magnetic field (H) is maintained after application until cooling below the threshold temperature (T _b ).

4. The method according to any one of the preceding claims, characterized in that the resulting direction of magnetization (m ₂ ) of the ferromagnetic layer (2a) in the area adjacent to the heated area (5, 6, 8, 9) after cooling at least approximately parallel to the Direction of the external magnetic field (H) is aligned.

5. The method according to claim 4, characterized in that by the direction of cooling (m ₂ ) of the ferromagnetic layer (2a) resulting in the cooling in the area adjacent to the heated area (5, 6, 8, 9) by the adjacent antiferromagnetic Layer (2b) in this area (5, 6, 8, 9) is stabilized.

6. The method according to any one of the preceding claims, characterized in that a soft magnetic layer, in particular a NiFe layer, is used as the ferromagnetic layer (2a).

7. The method according to any one of the preceding claims, characterized in that a NiO layer or an IrMn layer is used as the antiferromagnetic layer (2b).

8. The method according to any one of the preceding claims, characterized in that the resulting magnetization direction (m ₂ ) before heating in the ferromagnetic layer (2a) of the layer arrangement (2) by a when the antiferromagnetic layer (2b) or in one Deposit the antiferromagnetic layer (2b) and the ferromagnetic layer (2b) applied external magnetic field.

9. The method according to any one of the preceding claims, 5 characterized in that the heating is carried out by irradiation with a laser.

10. The method according to claim 9, characterized in that the irradiation with the laser in the form of short-term pulses

L0 with a pulse duration of 10 ns to 100 μs.

11. The method according to claim 9 or 10, characterized in that the irradiation with the laser by scanning strips to be irradiated (5, 6, 8, 9).

L5

12. The method according to any one of the preceding claims, characterized in that locally limited areas of the antiferromagnetic layer (2b), in particular insulated areas with a size of 5 μm ² to 500 μm ² or strips (5,

20 6, 8, 9) with a width of 5 μm to 100 μm and a length of 1 mm to 120 mm.

13. The method according to any one of the preceding claims, characterized in that successively in different,

25 locally delimited areas (5, 6, 8, 9) of the ferromagnetic layer (2a) an adjustment, in particular a change, of the magnetization directions (m ₂ ) resulting locally there.

14. The method according to any one of the preceding claims, characterized in that in the ferromagnetic layer (2a) a plurality of regions (5, 6, 8, 9) with locally different resulting magnetization direction (m ₂ ), in particular

5 those with mutually perpendicular resulting magnetization direction (m ₂ ) are generated.

15. The method according to any one of the preceding claims, characterized in that the layer arrangement (2) with the

L0 ferromagnetic layer (2a) and the antiferromagnetic layer (2b) located above or below it is partially heated above the threshold temperature (T _b ).

16. Use of the method according to one of the preceding L5 claims for producing a magnetoresistive, according to

Layer-valve working layer system, in particular a magnetoresistive layer system working according to the spin-valve principle, in which areas with a different, in particular mutually perpendicular resulting magnetization direction (m ₂ ) are present within the reference layer (2) which are interconnected in the form of a Wheatstone bridge (40, 41).