Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
  • G01R33/06—Measuring direction or magnitude of magnetic fields or magnetic flux using galvano-magnetic devices
  • G01R33/09—Magnetoresistive devices

Definitions

• the invention relates to a thin-film magnetic field sensor.
• magnetotransport phenomena have the advantage of being able to be easily integrated with reading electronics.
• two types of effects are used: on the one hand the magnetoresistance of magnetic materials, and on the other hand the Hall effect.
• French patent application No. 93 15551 describes a weak magnetic field sensor based on a photosensitive element with planar Hall effect. Such a sensor is based on the transverse measurement of the anisotropic magnetoresistance effect in a thin ferromagnetic film.
• the voltage measured along the axis YY ' varies as a function of the angle ⁇ between the magnetization of the film and the measurement current, by following the relationship below:
• V l— SIN5 (2 ⁇ ) where ⁇ R is the resistivity anisotropy which essentially depends on the material and the thickness of the active part of the sensor.
• planar Hall effect sensors compared to magnetoresistive sensors with longitudinal measurement are on the one hand a great simplification of the associated technology, on the other hand a reduction of approximately four orders of magnitude of the thermal drift, the main component of noise at low frequency (around 1 Hz).
• this sensor can be made sensitive only to the component of the magnetic field perpendicular to its direction of supply. Its size can be reduced to dimensions smaller than those of the magnetic domains, which makes it possible to eliminate the source of noise associated with wall movements. Measurements on prototypes of such sensors have shown a linear response of these sensors on four orders of magnitude (see document A. Schuhl, F. Nguyen-Van-Dau and JR Childress, Applied Physics Letters, 66, 15 May 1995 ).
• the invention therefore relates to a magnetic field sensor comprising a first planar element in a thin layer of crystalline magnetoresistive material with anisotropy of resistivity in the plane, having a first and a second easy magnetization axes, this element having means of electrical connections.
• the two easy axes of magnetizations are substantially equivalent, and in that it comprises a first electrical conductor arranged parallel to an easy axis of magnetization, isolated from the magnetoresistive material and allowing the circulation of an electric current of the control inducing in the element a field magnetic which imposes, when the sensor is at rest s, an orientation of the magnetization direction of the element parallel to the second easy magnetization axis.
• planar Hall effect sensors experimentally exhibit a non-zero resistance at zero field.
• This offset can have two origins: - faults in the definition of the geometry of the sensor making it asymmetrical; - poor alignment of the pattern with respect to the crystal axes of the active layer.
• the invention also relates to a sensor characterized in that it comprises:
• a second element made of magnetoresistive material similar to the first element, the two easy magnetization axes of which are parallel to those of the first element;
• each of the two detectors is then sensitive to a component of the magnetic field. It can be seen that the precision of such a biaxial sensor will certainly be limited by the precision of the assembly of the two detectors.
• the two components of the magnetic field are measured by two detectors which have been assembled. The measurement accuracy is then limited by the difference in sensitivity between the two sensors. In case of mass production, large diameter substrates will be used. We can then expect significant differences between the sensitivities of the different sensors. To solve this problem, you must either locate the sensors to assemble two neighboring sensors, or measure the sensitivity of each sensor. In all cases, this requires significant operations and therefore the cost of the sensor will be affected. According to the invention, it is therefore provided that the two sensors are produced in the same magnetoresistive layer and are at a short distance from each other.
• a substrate 5 carries an element 1, preferably square but possibly rectangular, in a thin layer of a magnetoresistive material making it possible to obtain a planar Hall effect.
• This material has an anisotropy oriented in the plane of the layer such that there are two easy axes of magnetization. These two magnetization axes are identical and are oriented along the perpendicular axes XX "and YY '.
• the thickness of the layer is between 0.01 and 1 ⁇ m (0.02 ⁇ m by example) and the width and length of element 1 are between 10 and 50 ⁇ m (20 ⁇ m for example).
• the element 1 has at its two ends connection pads 2, 2 ′ allowing the connection of a current supply device and allowing the flow of a measurement current i in the element 1 This current i is preferably continuous and constant.
• connection elements 3 and 3 ′ are connected on either side of the element 1 along the YY axis.
• connection elements allow the connection of a voltage or resistivity measurement device.
• the connection areas 2, 2 'and 3, 3' are at least the width of one side of the element 1.
• These connections can be made in a material other than that of element 1.
• this conductor is parallel to the axis XX '. It is separated from the element 1 by an insulating layer 4.
• Element 1 is produced in the form of a thin layer of ferromagnetic material having a cubic crystal structure and causing a quadratic magneto-crystalline anisotropy in the plane of the layers.
• a thin film produced in such a material thus has in its plane two easy axes of magnetizations equivalent and perpendicular to each other. Current techniques allow such a film to be produced.
• the active area is such that it has the shape of a square whose characteristic size (of the order of ten microns) is sufficiently small so that the element or practically a magnetic monodomain. So in the absence of magnetic field applied, the magnetization is oriented in the square along one of the equivalent directions of easy magnetization.
• conductor 6 is provided and provision is made for the circulation of an electric current (direct or alternating) in this conductor. This current generates a transverse magnetic field in element 1.
• the conductor is arranged above the sensor for example parallel to the direction XX '.
• the dimensions of the wire and its distance from the sensor must be such as to enable a magnetic field on the sensor to be sufficient to saturate the magnetization in the direction parallel to the current leads.
• the direction of the magnetization is controlled by the direction of the current flowing in the wire. After circulating an electric control current + 1 in the conductor 6, the direction and the direction of the magnetization are controlled and the sensor delivered a voltage V +:
• V + V 0 + S * i * H (1)
• Vo the offset voltage
• S the sensitivity of the sensor
• V / T.A i is the measuring electrical current of the sensor
• H is the component of the external field parallel to the voltage taps.
• Field measurement can be done either after pulse I which places the magnetization of the sensors in the right direction, or if necessary during the pulse. In the latter case, the sensitivity of the sensor will depend on the intensity of the current flowing in the conductor 6.
• V- V 0 - S * i * H (2)
• FIG. 3 represents an application of the sensor of FIG. 2 to the production of a biaxial compass.
• At least two sensors are produced on the same substrate making it possible to measure different components of the magnetic field.
• these are two orthogonal components. Control of the direction of the magnetization and therefore of the direction sensitive to the field in each sensor is ensured by a control conductor such as the conductor 6 of FIG. 2.
• FIG. 3 represents on the same substrate two magnetoresistive elements 1 and 10.
• the electrodes 2 and 2 'of the element 1 are aligned in a direction XX' of an easy magnetization axis XX 'of the element 1.
• the electrodes 3 and 3' are aligned along the other axis YY 'd easy magnetization.
• the electrodes 12, 12 ' are aligned along the axis YY and the electrodes 13, 13' are aligned along the axis XX '.
• Voltage measuring devices are connected to the electrodes 3, 3 'on the one hand and 13, 13' on the other hand.
• the two sensors 1 and 10 To operate as a magnetic field direction detector (compass for example), it measures two components of a magnetic field.
• the two sensors 1 and 10 must be controlled by currents traversing orthogonal conductors 6 and 16 so that under the effect of the control current I the magnetizations of the two sensors are orthogonal.
• the two sensors are therefore sensitive to two orthogonal components of the magnetic field.
• Elements 1 and 10 are produced on a substrate S by photolithography and / or ion etching. An insulating layer 4 is then deposited on the two sensors.
• connection wire F1 made of a good electrical conductor material, preferably not magnetic (gold, silver, copper, aluminum, etc.) produced during a lithography step.
• FIG. 4 shows another embodiment of the sensor of the invention.
• the two magnetoresistive elements 1, 10 are supplied with current in the same direction YY '. According to FIG. 4, they are connected in series by the connection F1 and are traversed by the same current i of the same direction.
• the voltage measurements at connections 3, 3 'and 13, 13' are made in directions parallel to XX '.
• the conductors 6 and 16, above the elements 1 and 10 are parallel l 'one at XX' and the other at YY '(that is to say perpendicular to each other). Recall that it was mentioned at the start of the description that these directions XX ′ and YY are parallel to the easy axes of magnetization of the elements 1 to 10.
• the substrate is semiconductor (for example Si), it is possible to monolithically integrate all or part of the compass reading electronics on the same substrate.
• the positioning of the conductors 6 and 16 relative to the sensors makes it possible to use the offset suppression method described in relation to FIG. 2.
• the production method described above where the two sensors are produced during the same masking step makes it possible to substantially improve the precision as regards their relative positions. This improves the angular resolution of the biaxial compass compared to a hybrid integration.
• any misalignment of the patterns with respect to the crystallographic axes of the magnetoresistive material can be compensated for by the offset removal procedure by reducing the extent of this misalignment by at least an order of magnitude.
• the invention therefore makes it possible to produce a set of planar Hall effect sensors in order to obtain a biaxial compass making it possible to determine the direction of the magnetic field without a priori knowing the sensitivity of the individual elements 1 and 10.
• the two elements make it possible to measure two orthogonal components of the magnetic field. They are produced at the same time on the same substrate. As they are close to each other the manufacturing parameters are identical. Both elements have the same sensitivity. To determine the direction of the magnetic field, it suffices to make the relationship between the two measurements. The result is then independent of the individual sensitivity of the elements.
• the invention is perfectly suited to the manufacturing processes of low cost and large series of compasses, on large surface substrates for which very high uniformity of deposition is difficult to obtain.
• the invention requires the introduction of an additional current line to place the magnetization of the material. sensitive of the two sensors in two directions perpendicular to each other. This same current line is used in the invention to achieve elimination of the offset and therefore obtain better angular resolution.
• the invention is also applicable to a three-axis magnetic field direction sensor.
• FIG. 5 schematically represents such a sensor. It comprises in a XY plane a first bi-axis sensor as described above and having two detectors (1, 10) each measuring a component of the magnetic field in a direction (X, Y) in the YX plane. Each detector is represented in this figure by its magnetoresistive sensitive element 1, 10. This first sensor therefore measures the direction of the component of the magnetic field in the XY plane.
• a second bi-axis sensor also having two detectors (1 ′, 10 ′) is arranged in an XZ plane perpendicular to the XY plane.
• This sensor measures the direction of the magnetic field in the XZ plane. It follows that the assembly of the two sensors makes it possible to obtain the direction of the magnetic field in space.
• Such a sensor is easier to produce because it only requires the assembly of two sensors only (preferably in perpendicular planes) instead of three sensors in traditional techniques.
• Figures 6a and 6b show an alternative embodiment of a tri-axis sensor comprising a bi-axis sensor such as that of Figure 3 and a conventional Hall effect sensor both integrated on the same substrate.
• the Hall effect sensor comprises a layer 20 of material with high electronic mobility located on the substrate 7.
• This layer 20 is for example a layer of doped semiconductor material. It is covered with an insulating layer 21 on which the magnetoresistive elements 1 and 10 and their connection means are produced which are not shown in FIGS. 6a, 6b.
• An insulating layer 4 covers at least elements 1 and 10. We also find the conductor (6, 16) located above elements 1 and 10. Finally connections 22, 22 ', 23, 23' allow taking the contacts on the Hall 20 effect element. To simplify the figures, the contact points to the connections to elements 1 and 10 have not been shown.
• the sensor constituted by elements 1 and 10 operate as described above and make it possible to measure the component of the magnetic field in the XY plane.
• the conventional Hall effect sensor comprising the layer 20 makes it possible to measure the component of the magnetic field along the Z axis. It follows from this that the assembly of the two sensors makes it possible to determine the direction of a magnetic field in space.
• switching means can be associated with each conductor 6, 16 to allow the direction of flow of the control currents in these conductors to be reversed.
• Voltage measurement means are associated with each magnetoresistive element and a difference circuit makes it possible to make the difference in the voltage measurements made for the two directions of flow of the control currents.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Hall/Mr Elements (AREA)
• Measuring Magnetic Variables (AREA)

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• 1996
  • 1996-07-05 FR FR9608395A patent/FR2750769B1/fr not_active Expired - Fee Related
• 1997
  • 1997-07-04 JP JP10504863A patent/JP2000514920A/ja active Pending
  • 1997-07-04 EP EP97931872A patent/EP0909391A1/de not_active Withdrawn
  • 1997-07-04 US US09/147,473 patent/US6191581B1/en not_active Expired - Fee Related
  • 1997-07-04 WO PCT/FR1997/001205 patent/WO1998001764A1/fr not_active Application Discontinuation

Non-Patent Citations (1)

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