Tunneling effect element and physical quantity to electrical quantity transducer

A tunneling effect element, including an insulating layer that forms a tunneling barrier, a lower electrode that is conductive and non-magnetic, and is formed on a bottom surface of said insulating layer, an upper electrode that is conductive and non-magnetic, and is formed on a top surface of said insulating layer, and a transmission member. The transmission member is made of insulating material that is formed surrounding the insulating layer and the lower and upper electrodes. The transmission member is also formed on a surface of an object to be detected, and transmits deformation of the object to be detected to the insulating layer. The tunneling effect element detects a change in stress of the object to be detected as a change in electric resistance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a tunneling effect element that receives a physical-quantity such as acceleration, pressure, load, displacement or the like and produces a strain, and shows a change in resistance that corresponds to that strain, and relates to a physical-quantity to electrical-quantity transducer such as an acceleration transducer, pressure transducer, load transducer, displacement transducer or the like that converts the aforementioned physical-quantity to an electrical-quantity based on the resistance change of the tunneling effect element.

2. Description of the Related Art

In conventional micro-detector elements, there are elements in which the sensor unit for detecting a pressure change in the detected object comprises a tunneling-magnetoresistive-effect element having a magnetostriction section in part of the magnetic member (for example, see Patent Document 1). Hereafter this technology will be called the first prior art.

Also, in conventional stress sensors comprising a tunneling-magnetoresistive-effect element, there are stress sensors in which the tunneling-magnetoresistive-effect element is surrounded by magnetic shielding (for example, see Patent Document 2). Hereafter, this technology will be called the second prior art.

According to the above mentioned first and second prior art, when compared with a strain gage having a small amount of resistance change on the scale of several hundred mΩ, the first and second prior art have an advantage in that (1) the circuit pattern is simple since there is no need for a Wheatstone bridge circuit or the like, and (2) localized displacement in minute locations can be detected since the area contributing to the pressure sensitivity is small.

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

However, according to the first prior art described above, as can be seen from the fact that, (1) in the detection operation a uniform external magnetic field must be applied to the tunneling-magnetoresistive-effect element, and (2) besides a tunneling-magnetoresistive-effect element, it is possible to use a magnetoresistive-effect element, giant-magnetoresistive-effect element, magneto-impedance element or electromagnetic induction coil as the sensor unit, the characteristic unique to a tunneling effect element that the tunneling-transition probability changes is not used. In other words, above described the operating principle of the micro-detection element is that the moment rotates by applying stress to the magnetostriction member, and the resistance value of the tunneling-magnetoresistive-effect element changes due to that. Therefore, when a change in resistance due to a disturbance magnetic field is added as noise to the change in resistance that corresponds to the pressure that is originally being detected, there is a problem in that stable output cannot be obtained. Therefore, there is a problem in that it is difficult to apply the micro-detection element to locations where a fairly large disturbance magnetic field occurs, for example, in a magnetic disk drive as an acceleration sensor, in an automobile as an impact sensor.

On the other hand, according to the second prior art, magnetic shielding surrounds the magnetoresistive-effect element, so stable output can be obtained even in conditions where a large disturbance magnetic field occurs such as inside a magnetic disk drive or inside an automobile. However, since an element having an anisotropic-magnetoresistive effect, or an element having a giant-magnetoresistive effect (GMR effect) can also be used instead of a tunneling-magnetoresistive-effect element as the magnetoresistive-effect element that is used in this second prior art, the characteristic unique to a tunneling effect element that the tunneling-transition probability changes is not used. Therefore, naturally, when the object to be detected is a magnetic material, the element cannot be employed, so there is a problem in that the element lacks versatility.

Moreover, in the first prior art described above, the tunneling-magnetoresistive-effect element is constructed by layering a thin magnetic conductive film having a high spin polarization such as Fe20—Ni80 film, an aluminum-oxide (alumina) (Al2O3) film, and magnetic thin film having a large magnetostriction constant such as Fe—Co50 film in order onto an insulating substrate. In other words, since the material of the bottom thin film and that of the top thin film, between which a tunneling barrier is located, differ, there probably exists a difference between the thermal-expansion coefficient of each. Therefore, when there are fluctuations in the temperature of the environment in which the object to be detected is placed, thermal-expansion stress occurs near the tunneling barrier, which could cause thermal drift. In the case of the second prior art described above, detailed construction of, and the materials used in the tunneling-magnetoresistive-effect element are neither disclosed nor suggested, so the aforementioned problem cannot be solved.

Taking the circumstances described above into consideration, the object of the present invention is to provide a tunneling effect element and physical-quantity to electrical-quantity transducer capable of solving problems as described above.

Means for Solving the Problem

In order to solve the aforementioned problems, a tunneling effect element according to an aspect of the invention comprises: an insulating layer that forms a tunneling barrier; a lower electrode that is conductive and is formed on a bottom surface of said insulating layer; an upper electrode that is conductive and is formed on a top surface of said insulating layer; and a transmission member that is formed around said insulating layer and said lower and upper electrodes, and transmits the behavior of an object to be detected to said insulating layer.

Also, in one aspect, the tunneling effect element detects a change in stress of said object to be detected as a change in electric resistance.

Furthermore, in another aspect, a resistance area product of the tunneling effect element, which is the product of the resistance value and the surface area, is 100 k Ω·μm2 or less.

Also, inyet another aspect, lower and upper electrodes are non-magnetic.

Moreover, in some aspects the lower and upper electrodes are made of the same material.

Furthermore, in some aspects, the tunneling effect element has an internal inherent strain due to stress which is less than ±3%.

Also, in other aspects, a physical-quantity to electrical-quantity transducer is provided which includes a tunneling effect element according to one of the above-noted aspects, which includes a first wiring section that is electrically connected to the lower electrode and a second wiring section that is electrically connected to the upper electrode.

Advantage of the Invention

According to this invention, it is possible to provide a tunneling effect element that has versatility and that does not receive the effects of drift due to differences in the thermal-expansion coefficient of the lower and upper electrodes, and is not easily affected by external magnetic fields.

EXPLANATION OF REFERENCE NUMBERS

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the Preferred Invention

FIG. 1is a external view showing the overall structure of the tunneling effect element of a first embodiment of the invention;FIG. 2is a perspective view showing the external construction of the sensor unit2forming the tunneling effect element1shown inFIG. 1; andFIG. 3is a partially transparent and perspective view showing the state of the tunneling effect element1shown inFIG. 1mounted on the object21to be detected. The tunneling effect element1of this example comprises a sensor unit2, wiring sections3and4, and a transmission member (filling material)5that is filled in around the element1. As shown inFIG. 2, the sensor unit2comprises: an insulating layer11forming a tunneling barrier, a lower electrode12and an upper electrode13. The insulating layer11is made, for example, from aluminum oxide (Al2O3), magnesium oxide (MgO) or the like, and has an approximately circular column shape. The insulating layer11, for example, has a diameter of approximately 100 nm, and thickness of approximately 1 nm. Both the lower electrode12and upper electrode13are, for example, made from a non-magnetic, good conductor such as Tantalum (Ta) and have an approximately circular column shape. Both the lower electrode12and upper electrode13have a diameter of approximately 100 nm and a thickness of approximately 30 nm, for example.

Also, inFIG. 1, the wiring section3is made from copper (Cu), for example. The wiring section3comprises: an approximately T-shaped plate section3a(as seen from the top), an approximately circular column-shaped section3b, and an approximately disk-shaped pad section3c. Integrating a wide section3aaand a narrow section3abunitarily forms the plate section3a. The thickness of the plate section3ais 200 nm, for example. The top surface in the approximately central position of the wide section3aais electrically connected to the bottom surface of the lower electrode12. On the other hand, the top surface of the end of the narrow section3abthat is on the opposite side from the wide section3aais electrically connected to the bottom surface of the circular column section3b. Also, the top surface of the circular column section3bis electrically connected to the bottom surface of the pad section3c. A probe6made of gold (Au) and the like, comes in contact with the top surface of the pad section3c, and the other end of the probe6(not shown in the figure) is electrically connected to a detection circuit (not shown in the figure).

Moreover, the wiring section4is made from copper (Cu), for example. The wiring section4comprises: an approximately T-shaped plate section4a(as seen from the top), and an approximately disk-shaped pad section4b. Integrating a wide section4aaand a narrow section4abunitarily forms the plate section4a. The thickness of the plate section4ais 200 nm, for example. It is not shown inFIG. 1, however, the bottom surface in the approximately central position of the wide section4aais electrically connected to the top surface of the upper electrode13. On the other hand, the top surface of the end of the narrow section4abthat is on the opposite side from the wide section4aais electrically connected to the bottom surface of the pad section4b. A probe7made of gold (Au) and the like, comes in contact with the top surface of the pad section4b, and the other end of the probe7(not shown in the figure) is electrically connected to a detection circuit (not shown in the figure).

The filling material5is made from an insulating material such as aluminum oxide (Al2O3), magnesium oxide (MgO) or the like. As shown inFIG. 3, for example, the filling material5is firmly filled on the top surface of the object21to be detected, and surrounds the tunneling effect element1so that the behavior of the object21to be detected is efficiently transmitted to the tunneling effect element1via the filling material5. As was described above, concerning the tunneling effect element1and filling material5, everything except for the insulating layer11forming the tunneling effect element1, is formed on the top surface of the object to be detected using a thin-film formation technique such as chemical vapor deposition (CVD), vacuum evaporation or sputtering, a lithography technique, etching technique or plating technique.

On the other hand, when the insulating layer11is made of aluminum oxide (Al2O3), it is formed by any one of the following methods: (1) natural oxidation in air after forming metallic aluminum on the lower electrode12; (2) oxidation by the plasma oxidation method in air or in a vacuum after metallic aluminum is formed on the lower electrode12; (3) forming an aluminum oxide (Al2O3) film using a thin-film-formation technique such as CVD, vacuum evaporation or sputtering.

Next, the reasons for setting the size, shape and material of the sensor unit2as described above will be described. For the sensor unit2, smaller is better performance to detect localized displacement of a minute location, however, in the case of making the sensor unit2small, it increases the resistance value of the sensor unit2and is generally inconvenient for process. In the case that the resistance value of the sensor unit2is high (for example 100 MΩ or greater), the voltage applied between the lower electrode12and upper electrode13becomes high, so non-typical circuit elements must be used in the direct-current power supply. On the other hand, in the case that the resistance value of the sensor unit2is low (for example 10Ω or less), there is a possibility of current leakage occurring in the insulating layer11. Therefore, it is preferred that the resistance value of the sensor unit2be 100Ω to 1 MΩ.

In order to maintain the resistance value of the sensor unit2at 100Ω to 1 MΩ even if the sensor unit2is made small, it is necessary to keep the resistance area product RA, which is the product of the resistance value and surface area of the sensor unit2, low, and as the result of devoted study by the inventors, it was found that a resistance area product (RA) of 100 kΩ·μm2or less is preferred.FIG. 4shows the approximation results for the relationship between the diameter and resistance area product RA of the sensor unit2having an approximately circular column shape. InFIG. 4, the shaded area is the practical range. Therefore, in this first embodiment, the diameter of the sensor unit2is 100 nm. Also, in the case of designing very small sensors in which the diameter of the sensor unit2is 10 nm or less, it is preferred that the resistance area product RA be 1 kΩ·μm2or less.

Next, the relationship between the thickness and resistance area product RA of the insulating layer11forming the sensor unit2will be described. First, the inventors consideres a sensor unit2that operates based on the following operating principles:

(a) When a physical-quantity such as acceleration, pressure, load, displacement or the like is applied to the sensor unit2, the physical-quantity is transmitted to the insulating layer11forming the tunneling barrier, and the insulating layer11deforms. The most notable deformation of the insulating layer11is one concerning the thickness of the insulating layer11.

(b) When the thickness of the insulating layer11changes, the resistance value of the sensor unit2changes logarithmically.

Therefore, in order for the sensor unit2to obtain large changes in resistance value with respect to the applied physical-quantity, the thickness of the insulating layer11is an important element.FIG. 5is an example that shows the relationship between the thickness and resistance area product RA of aluminum that is formed on a top surface of the lower electrode forming the sensor unit (refer to Kazuo Kobayashi et al., ‘Tunneling type Giant Magnetoresistive, a Spin Valve Type Tunnel Junction’, Materia Japan, Vol. 37, No. 9 (1998), p. 736-740). Here, the thickness of the aluminum refers to the thickness of the metallic aluminum that is formed on a lower electrode before natural oxidation occurs in the case of constructing the insulating layer forming the sensor unit using aluminum oxide (Al2O3). As can be seen fromFIG. 5, in areas where the thickness of the aluminum is thinner than 1.3 nm, the resistance area product RA greatly relies on the thickness of the aluminum and large changes in resistance can be obtained as a sensor unit.

The inventors experimentally found the relationship between the thickness and resistance area product RA of aluminum for areas where the thickness of aluminum is even thinner than that shown in aboveFIG. 5.FIG. 6shows an example of the relationship between the thickness and resistance area product RA of aluminum that was found experimentally. The experiment results that are shown inFIG. 6use Tantalum (Ta) film having a thickness of 30 nm as the lower and upper electrodes. InFIG. 6, it can be seen that when the thickness of the aluminum changes by 10%, the resistance area product RA change linearly only 70%. In other words, the resistance area product RA greatly relies on the thickness of the aluminum and large changes in resistance can be obtained as the sensor unit2in areas where the thickness of the aluminum is 1.3 nm or less. Aluminum having a thickness of 1.3 nm becomes an aluminum oxide film having a thickness of 2.0 nm to 2.5 nm by natural oxidation, so in this first embodiment, the thickness of the insulating layer11is made to be approximately 1 nm. The maximum thickness for which the resistance area RA product changes linearly does not change much even when the material of the insulating layer11changes. The minimum thickness of the insulating layer11for which a uniform film can be obtained is about 0.4 nm.

Also, the reason that the insulating layer11, as well as both the lower electrode12and upper electrode13, have an approximately circular column shape, and that the tunnel junction section has a circular shape is because importance was placed on ease of processing, however the shape of the insulating layer11, lower electrode12and upper electrode13, and the shape of the tunnel junction can be any shape such as an approximately square column shape or rectangular shape as long as there is no problem in processing them. Next, both the lower electrode12and upper electrode13forming the sensor unit2are made from a non-magnetic, good conductor such as Tantalum (Ta). Therefore, since the thermal expansion coefficient of both the lower electrode12and upper electrode13is the same, they are not affected by drift due to differences in the thermal expansion coefficient. Also, since both the lower electrode12and upper electrode13are made from a non-magnetic such as Tantalum (Ta), it is difficult for them to be affected by an external magnetic field.

Next, the Resistance Change Ratio obtained in the case that the tunneling effect element1constructed as described above is mounted on the object to be detected will be described. As shown inFIG. 7, a plurality of tunneling effect elements1are formed at desired intervals on the front surface of an approximately square column-shaped object22to be detected which is made of, for example, AlTiC and the like, and filled with a filling material5(not shown in the drawing). The width of the object22to be detected is approximately 0.25 mm. InFIG. 7, only one tunneling effect element1is shown.FIG. 8shows an enlarged view of the area A inFIG. 7. The tunneling effect element1is mounted so that the sensor unit2forming the tunneling effect element1is located at an offset of just 0.1 mm from the center O of the object22to be detected. Also, the object22to be detected is supported by support members23and24that are located on the top surface of the object22to be detected separated by a distance of 40 mm (distance between each fulcrum of supports), and the bottom surface of the object22to be detected is pressed upward from the bottom so that the object22to be detected has a downward bend of about 1 mm. A pair of probes is brought in contact simultaneously with the two pads3cand4bof each of the tunneling effect elements1, and the resistance value is measured. Therefore, as shown inFIG. 9, a 0.2% change in resistance (no-load resistance: approximately 300Ω) was measured. Inside the tunneling effect element1there exists a maximum ±3% inherent strain due to stress that occurs during the manufacturing process, however, this inherent strain is within the elastic deformation range of the components of the tunneling effect element1. Therefore, detection of the change in stress using the tunneling effect element1is performed assuming this inherent strain.

The strain occurring in the sensor unit2due to bending of the object22to be detected is calculated, and the Resistance Change Ratio of the sensor unit2is estimated based on this.FIG. 10is a schematic view for approximating the Resistance Change Ratio of the sensor unit2. InFIG. 10, h is half the distance between each fulcrum of supports, and in this case it is 20 mm. Also, a is the amount that the object22to be detected is pressed in order to cause it to bend (pressing amount), and in this case it is 1 mm. Furthermore, r is the radius of curvature around the centerline O of the object22to be detected. Therefore, h, a and r are related by the following equation (1).
r2=h2+(r−a)2(1)

By substituting h=20 mm and a=1 mm into Equation (1), the radius of curvature r becomes 200.5 mm.

Here, as shown inFIG. 8, since the sensor unit2is offset just 0.1 mm from the center O of the object22to be detected, the radius of curvature of the sensor unit2becomes 200.4 mm. Therefore, the horizontal compressibility of the sensor unit2becomes 0.049%, as given by Equation (2).
{(200.5−200.4)/200.5}×100=0.049(%)  (2)

The rate of change in the film thickness of the insulating layer11when the sensor unit2is pressed from the horizontal direction in the case that the insulating layer is made using aluminum oxide is expressed by Equation (3), where the Poisson's ratio for aluminum oxide is 0.24.
0.24×0.049=0.1176≈0.012(%)  (3)

From the relationship between the thickness and resistance area product RA of aluminum shown inFIG. 6, it can be experimentally seen that when the thickness of the aluminum changes by 10%, the resistance area product RA change linearly only 70%. In this case, the thickness of the insulating layer11changes just 0.012%, so, as shown by Equation (4), the Resistance Change Ratio can be approximated as 0.084%.
0.012×70/10=0.084%  (4)

When comparing this approximated value ‘0.084%’ with the actual measured value ‘0.2%’, the approximated value is a little less. However, this can be considered to be due to measurement error or factors that were not considered when performing the approximation such as effects due to the solid structure around the sensor unit2, so the aforementioned approximated value is considered to be reasonable.

Next, as shown inFIG. 11, a plurality of tunneling effect elements1are formed at specified intervals on the front surface of an approximately triangular column-shaped object31to be detected, and filled with a filling material5(not shown in the figure). Similar to the detected object22shown inFIG. 7, the width of the object31to be detected inFIG. 11is approximately 0.25 mm, also similarly, the tunneling effect elements which are not shown in the figure are mounted so that there is an offset of just 0.1 mm from the center O of the object31to be detected. Also, after an initial state (SS) of no external force is applied to the object31to be detected, a cycle consisting of an upward bend (UB), released state (RE), downward bend (DB) and released state (RE) was executed for just two cycles, and a pair of probes25were brought into contact simultaneously with the two pads3cand4bof each tunneling effect element1, and the resistance values were measured.

As shown inFIG. 12A, the upward bend (UB) referred to here is performed by supporting the object31to be detected by support members32and33located on the bottom surface of the object31and separated by a distance of 40 mm (distance between each fulcrum of supports), and then pressing the top surface of the object31to be detected downward from the top approximately 2 mm to bend the object31to be detected upward. On the other hand, as shown inFIG. 12B, the upward bend (UB) referred to here is performed by supporting the object31to be detected by support members32and33located on the top surface of the object31and separated by a distance of 40 mm (distance between each fulcrum of supports), and then pressing the bottom surface of the object31to be detected upward from the bottom approximately 2 mm to bend the object31to be detected downward.

FIG. 13shows an example of measurement results. InFIG. 13, line a shows the characteristics of the Resistance Change Ratio in area a shown inFIG. 11, line b shows the characteristics of the Resistance Change Ratio in area b shown inFIG. 11where no external force is applied, and line c shows the characteristics of the Resistance Change Ratio in the area c shown inFIG. 11where no external force is applied. FromFIG. 13, it can be seen that for an upward bend (UB) and downward bend (DB) nearly opposite characteristics occur. In other words, this tunneling effect element1can also detect the direction of bend of the object31to be detected.

In this way, with this first embodiment of the invention, in comparison with a strain gage having a small amount of resistance change on the order of several 100 mΩ, it is of course possible to obtain general effects for a tunneling effect element, which are: (1) simple circuit construction in which a Wheatstone bridge circuit or the like is not necessary, and (2) it is possible to detect localized displacement since the surface area contributing to pressure sensitivity is small. Furthermore, with this first embodiment of the invention, both the lower electrode12and upper electrode13are made of a non-magnetic material having good conductivity such as Tantalum (Ta). Also, the thermal expansion coefficient of both the lower electrode12and upper electrode13are the same, so they are not affected by drift due to differences in the thermal expansion coefficient, and it is difficult for them to be affected by external magnetic fields. Therefore, stable output can be obtained even in conditions where large disturbance magnetic fields occur such as inside a magnetic disk drive or inside an automobile. Furthermore, since this embodiment of the invention can be employed even when the detected object is a magnetic material, it has sufficient versatility.

FIG. 14is a plan view showing the external construction of the sensor unit41forming the tunneling effect element of a second embodiment of the invention. The sensor unit41of this example comprises insulating layers42xand42ythat form a tunneling barrier, a lower electrode43and upper electrodes44xand44y. The insulating layers42xand42yare made from aluminum oxide (Al2O3), magnesium oxide (MgO) or the like, and are approximately rectangular. The thickness of the insulating layers42xand42yis approximately 1 nm, for example. The insulating layer42xis formed on one rectangular section of the object to be detected (not shown in the figure) by way of a transmission member46that transmits the behavior of the object to be detected to the insulating layer, and it is formed such that it is parallel with the x direction in the figure. On the other hand, the insulating layer42yis formed on one rectangular section of the object to be detected (not shown in the figure) by way of the transmission member46, and it is formed such that it is parallel with the y direction in the figure.

The lower electrode43, and upper electrodes44xand44yare all made from a non-magnetic, for example, being good conductive such as Tantalum (Ta) and have an approximately rectangular shape. The thickness of the lower electrode43and upper electrodes44xand44yis approximately 30 nm, for example. The lower electrode43is formed so that it nearly covers the entire surface of one rectangular section of the object to be detected (not shown in the figure). The upper electrode44xis formed on one rectangular section of the object to be detected (not shown in the figure) such that it is parallel with the x direction in the figure. On the other hand, the upper electrode44yis formed on one rectangular section of the object to be detected (not shown in the figure) such that it is parallel with the y direction in the figure. An insulating layer45is formed on the top surface of the lower electrode43except in the area of the insulating layers42x,42yand a wiring area43a. The method for forming the insulating layers42xand42y, lower electrode43and upper electrodes44xand44yforming the sensor unit41described above is the same as in the first embodiment described above. Also, detectors, having this kind of construction are located on the object to be detected at specified intervals. The insulating layer45is similarly made from an insulating material such as an oxide or the like, as in the case of the insulating layers42xand42y, however it does not need to form a tunneling barrier. In this way, with this second embodiment of the invention, it is possible to easily obtain the two-dimensional distribution of change in a physical-quantity in the X-axis direction and Y-axis direction of the object to be detected.

The preferred embodiments of the invention were described above with reference to the drawings, however, detailed construction is not limited by these embodiments, and changes to design within a range that does not depart from the scope of this invention are also included in this invention.

For example, in the first and second embodiment described above, no particular mention was made regarding the material of the object to be detected on which the sensor unit is installed. The material of the object to be detected can be any material, such as an ultra-thin glass substrate, plastic film, or the like that can be bent.

Also, in the first embodiment described above, an example was described in which the tunneling effect element1was installed on the front surface of the object22to be detected, and bending in the upward and downward direction of the object22to be detected was detected, however, the invention is not limited to this. For example, in the case where the sensor unit2is formed on the top surface of the object22to be detected, and bending in the planar direction is being detected, the sensor unit2can be formed such that it is rotated 90 degrees with respect to the direction shown inFIG. 1so that its axis is the lengthwise direction of the object22to be detected, and the direction of the wiring section can be changed.

Moreover, in the second embodiment described above, an example was given in which the object to be detected is just on the side of the bottom surface of the sensor unit41, however, the invention is not limited to this. For example, in the case where the object to be detected is on the top surface of the sensor unit41, a separate transmission member can be formed on the top surface of the insulating layers42x,42yand insulating layer45and the like.

Also, in each of the embodiments described above, examples were given in which the tunneling effect element was formed directly on the object to be detected, however, the invention is not limited to this, and similar to the conventional strain gage described above, it is possible, for example, to attach the tunneling effect element to the object to be detected after forming it first on a strain-causing body such as a diaphragm and the like.

Moreover, each of the embodiments described above can use the same technology as long as there are no particular discrepancies or problems with the purpose or construction.