Use of multi-layer thin films as stress sensor

Details Use of multi-layer thin films as stress sensor Use of multi-layer thin films as stress sensor

2000-02-10

2004-02-24

Lefkowitz, Edward (Department: 2855)

Measuring and testing

Specimen stress or strain, or testing by stress or strain...

Specified electrical sensor or system

C073S728000

Reexamination Certificate

active

06694822

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to Giant MagnetoResistive (GMR) devices in conjunction with micromachined beams to measure stresses with high sensitivity, and methods of making and using the same.
BACKGROUND OF THE INVENTION
Stress and Pressure Sensors
The GMR effect has been widely reported in multi-layer thin film sensors, where there are alternating ferromagnetic layers
12
, made from materials such as Cobalt, Iron or Nickel, separated by nonmagnetic conductor layers
14
, such as chromium or copper to form a sensor
10
such as illustrated in FIG.
1
. When a current is passed along the length direction of the sensor, the electrical resistance of the multi-layer stack of films varies as the relative angle between the magnetizations of the individual ferromagnetic layers, as shown in FIG.
1
. The resistance is minimum when the magnetization vectors between the neighboring ferromagnetic layers are parallel to each other, and is maximum when the two vectors are antiparallel to each other (at 180°), as shown in FIG.
2
. It is to be noted that this is in contrast to the conventional AMR effect, where the resistance is maximum when the magnetization vector within a single magnetoresistive film is parallel to the direction of the current, and minimum when it makes an angle of 90° to the direction of the current. The AMR effect is shown in
FIGS. 3 and 4
.
Typically, the change in electrical resistance of a GMR multi-layer stack for a full. rotation of the magnetization vector from a parallel to an antiparallel state can be anywhere from 2% to greater than 50%, and for the AMR effect, the change in resistance for a 90° rotation is 1.5-3.5%.Therefore, if the magnetizations of some layers in a GMR multi-layer stack can be made to rotate under the application of a magnetic field, the GMR stack theoretically will provide a greater sensitivity magnetic field sensor than a conventional AMR film. However, one challenge in doing this is that the exchange coupling magnetic field between the alternating ferromagnetic layers in a GMR stack is very large, on the order of 2000 Oersted. As a result, to make an individual ferromagnetic layer rotate in relation to a neighboring ferromagnetic layer requires enormous magnetic fields. If the interlayer distance between the neighboring ferromagnetic layers is increased to reduce the exchange coupling field between the layers, then the GMR ratio (percentage resistance change) decreases correspondingly. As a result, it has not been possible to exploit the full extent of the classical GMR effect.
One approach to overcome the problem described above is the spin valve magnetic field sensor, a device that utilizes a version of the GMR effect. The spin valve essentially consists of two ferromagnetic layers
52
and
54
separated by a nonmagnetic conducting spacer layer
56
as shown in FIG.
5
. Of these two layers
52
and
54
, layer
54
is a “pinned layer”, in which the magnetization vector is pinned in one direction. The other layer
52
is a soft ferromagnetic layer, called the “free layer”, whose magnetization vector is free to rotate in the plane of the film. The separation between the pinned layer
54
and the free layer
52
is chosen such that the coupling field between the two layers is not too large. In the quiescent state, the magnetization of the free layer
52
is oriented at 90° to that of the pinned layer
54
, as shown by the bold arrow in FIG.
5
. Under the application of a relatively weak magnetic field, the rotation of the magnetic moment of the free layer
52
(as shown by the dashed arrows in
FIG. 5
) leads to a change in the relative angle of the magnetization between the pinned layer
54
and free layer
52
, and results in a change in resistance of the device.
FIG. 6
depicts a typical resistance change of the device as a function of the applied magnetic field to the device. In the quiescent state, the resistance of the device is represented by the point X on the graph, and the change in resistance is linear for changes in magnetic field almost up to the point of saturation of the device as shown in FIG.
6
.
The magnetization vector of the pinned layer in a spin valve device is usually held in place through antiferromagnetic exchange coupling between the pinned layer and a hard magnetic material (the “pinning layer”
58
shown in
FIG. 5
for example) such as CrMnPd, PtMn, FeMn, NiMn, etc. Other methods to fix the magnetization of the pinned layer include permanent magnet biasing, current induced biasing, etc.
The classical Giant MagnetoResistive (GMR) effect, as it is described above, has also been contemplated as being used to measure mechanical strain induced by stress. This principle involves generating a rotation of the magnetization vector of the ferromagnetic film under the application of mechanical stress even in the absence of a magnetic field, which results in a resistance change of the film, which in turn can be used to infer the degree of stress. However, one still needs to overcome the large exchange coupling field between the alternating ferromagnetic layers, and in order to do this, it has been suggested to use an externally applied magnetic field to aid the rotation of one of the layers under an applied stress, and to measure the resulting change in resistance. However, in practice, this is very difficult to implement, since it is not possible to apply such large magnetic fields in sensors that are widely deployed in the field. Additionally, this method causes serious accuracy problems, since the effects of the externally applied magnetic field and the stress on the sensor need to be decoupled.
It has also been suggested, such as in U.S. Pat. No. 5,856,617, to use a in valve device of the type described above to measure strains in the cantilever tip of an atomic force microscope. In such a suggested strain gauge device, an example of which is illustrated in
FIG. 5
, the free layer
52
is made to be of non-zero magnetostriction, so that under zero magnetic field conditions, the free layer magnetization vector rotates under the application of stress to the cantilever beam, and the resulting change in relative magnetization vector angle between the free layer
52
and the pinned layer
54
leads to a resistance change in the device. The strain gage device is thus a conventional top spin valve, with the free layer
52
comprising an alloy of NiFe, Ni and Co and being deposited directly onto the substrate, a non-magnetic conducting layer
56
disposed between the free layer
52
and the pinned layer
54
, and with the antiferromagnetic (AFM) layer
58
that is used for pinning the pinned layer
54
being on top of the stack. Although this device may find some use in measuring strains on atomic force microscope cantilever tips, there are several disadvantages to the use of this device as a general purpose strain gauge. The device's drawbacks relate mostly to the performance, reliability and processing limitations that are inherent with this type of design, and are listed below.
First, since an antiferromagnetic (AFM) layer
58
is used to pin the pinned layer
54
through exchange coupling, the device is subject to reliability concerns, since extended exposure of the AFM material to elevated temperatures around 150-200 C can cause “depinning” of the pinned layer, which destroys the effectiveness of the sensor. This is especially true if the antiferromagnetic material that is chosen has a low “Blocking temperature” (the temperature at which the antiferroniagnet starts to lose its exchange anisotropy). Furthermore, if the AFM material chosen is one that needs high temperature annealing, this introduces other processing problems such as the compatibility of the film with the substrate on which the multi-layer stack is being deposited, due to thermal mismatch concerns and delamination of the stack. Moreover, most manganese based AFMs have poor corrosion resistance.
Second, it is very difficult to maintain the magnetization of the free layer
52
to be pointing in a direction that is at 90°

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