Pressure sensor

A pressure sensor according to an embodiment includes: a support member; a membrane supported by the support and having flexibility; and a strain detection element formed on the membrane. The strain detection element includes a first magnetic layer formed on the membrane and having a magnetization, a second magnetic layer having a magnetization, and an intermediate layer formed between the first magnetic layer and the second magnetic layer. A direction of at least one of the magnetization of the first magnetic layer and the magnetization of the second magnetic layer changes relatively to that of the other depending on a strain of the membrane. Moreover, the membrane includes an oxide layer that includes aluminum.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority from prior Japanese Patent Application No. 2014-136503, filed on Jul. 2, 2014, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described in the present specification relate to a pressure sensor.

BACKGROUND

In recent years, a pressure sensor employing spin technology has been proposed. The pressure sensor employing spin technology is a device that measures a pressure using a principle that magnetization directions in a plurality of magnetic layers change relatively due to a pressure changing, whereby a value of electrical resistance of an element changes. Such a pressure sensor has a strain detection element employing spin technology disposed on a membrane formed on a support member; and converts a strain of the membrane caused by the pressure into a resistance change of the strain detection element, thereby detecting the pressure applied to the membrane.

A structure in which such a pressure sensor has a plurality of strain detection elements employing spin technology disposed on one membrane that bends by the pressure, has been proposed. The pressure sensor including these plurality of strain detection elements desirably shows a response in which there is no occurrence of unintended variation in characteristics among the strain detection elements.

In order to achieve an improvement in performance of such a pressure sensor, characteristics of the membrane including a film thickness of the membrane are preferably uniform. However, in a pressure sensor employing a conventional material of the membrane, it has been difficult to render characteristics of the membrane uniform, hence it has not been easy to improve performance of the pressure sensor.

DETAILED DESCRIPTION

A pressure sensor according to an embodiment described below includes: a support member; a membrane supported by the support and having flexibility; and a strain detection element formed on the membrane. The strain detection element includes a first magnetic layer formed on the membrane and having a magnetization, a second magnetic layer having a magnetization, and an intermediate layer formed between the first magnetic layer and the second magnetic layer. A direction of at least one of the magnetization of the first magnetic layer and the magnetization of the second magnetic layer changes relatively to that of the other depending on a strain of the membrane. Moreover, the membrane includes an oxide layer that includes aluminum.

Pressure sensors according to embodiments will be described below with reference to the drawings. Note that the drawings are schematic or conceptual, and that a relationship of film thickness and width of each of the portions, a ratio of size between portions, and so on, are not necessarily identical to those in reality. Moreover, even when expressing the same portions, those portions are also sometimes expressed with mutually different dimensions or ratios depending on the drawing. Note that in the specification and each of the drawings of the present application, an element similar to that previously mentioned regarding an already-described drawing will be assigned with a reference symbol identical to that previously assigned, and a detailed description of that element will be appropriately omitted.

First Embodiment

First, a pressure sensor according to a first embodiment will be described with reference toFIG. 1, and so on.

FIG. 1is a schematic perspective view exemplifying a pressure sensor110A and a strain detection element200according to the first embodiment. Note that in order to makeFIG. 1more easily seen,FIG. 1displays only part of the strain detection element200, moreover omits illustration of an insulating portion, and mainly depicts a conductive portion.

Moreover,FIG. 2is a schematic cross-sectional view looking from A-A′ ofFIG. 1. In addition,FIG. 3includes schematic plan views showing configurations of the pressure sensor110A. Furthermore,FIG. 4is a schematic perspective view showing a configuration of the strain detection element200, andFIG. 5is a schematic perspective view for explaining operation of the pressure sensor110A.

As shown inFIG. 2, the pressure sensor110A includes: a membrane120; and the strain detection element200formed on the membrane120. The membrane120has flexibility whereby it bends in response to a pressure from external. The strain detection element200strains in response to bending of the membrane120, and changes its electrical resistance value according to this strain. Therefore, by detecting a change in the electrical resistance value of the strain detection element200, the pressure from external is detected. Note that the pressure sensor110A may detect a sound wave or an ultrasonic wave. In this case, the pressure sensor110A functions as a microphone or an ultrasonic sensor.

As shown inFIG. 1, the pressure sensor110A includes: a substrate110; the membrane120provided to one surface of the substrate110; and the strain detection element200provided on the membrane120. Moreover, provided on the membrane120are a wiring line C1, a pad P1, a wiring line C2, and a pad P2that are connected to the strain detection element200. Hereafter, a direction perpendicular to the substrate110is assumed to be a Z direction. Moreover, a certain direction perpendicular to this Z direction is assumed to be an X direction, and a direction perpendicular to the Z direction and the X direction is assumed to be a Y direction.

As shown inFIG. 2, the substrate110is a plate-like substrate including a hollow portion111, and functions as a support member; supporting the membrane120such that the membrane120bends in response to an external pressure. In the present embodiment, the hollow portion111is a hole having, for example, a cylindrical shape (it may have another shape, as will be mentioned later) that penetrates the substrate110. The substrate110is configured from, for example, a semiconductor material such as silicon, a conductive material such as a metal, or an insulating material. Moreover, the substrate110may include the likes of silicon oxide (SiOx) or silicon nitride (SiNx), for example. On the other hand, the membrane120is formed by an oxide that includes aluminum, for example, aluminum oxide.

The hollow portion111is formed by carrying out etching on the substrate110to process the substrate110until the membrane120is exposed.

The inside of the hollow portion111is designed to allow the membrane120to be bent in, for example, a direction (Z axis direction) perpendicular to a principal plane of the substrate110. For example, the inside of the hollow portion111may be in a decompressed state or a vacuum state. Moreover, the inside of the hollow portion111may be filled with a gas such as air or an inert gas, or a liquid. Furthermore, the hollow portion111may be in communication with external.

As shown inFIG. 2, the membrane120is formed thinly compared to the substrate110. Moreover, the membrane120includes: a vibrating portion121that is positioned directly above the hollow portion111and that bends in response to an external pressure; and a supported portion122that is formed integrally with the vibrating portion121and that is supported by the substrate110. As shown in, for example,FIG. 3A, the supported portion122surrounds the vibrating portion121. Hereafter, a region positioned directly above the hollow portion111of the membrane120will, be called a first region R1. The vibrating portion121and the supported portion122are both formed by an oxide that includes aluminum (Al) (as an example, aluminum oxide). Note that an overall thickness t1of the membrane120can be set to, for example, not less than 50 nanometers (nm) and not more than 3 micrometers (μm). In this case, it can preferably be set to not less than 100 nm and not more than 2 μm.

The first region R1may be formed in a variety of forms. For example, the first region R may be formed in a substantially true circular shape as shown inFIG. 3A, may be formed in an elliptical shape (for example, a flattened circular shape) as shown inFIG. 3B, may be formed in a substantially square shape as shown inFIG. 3C, or may be formed in a rectangular shape as shown inFIG. 3D. Moreover, the first region R1may also be formed as a polygon or regular polygon. In addition, the first region R1may be a combination of the above-described shapes. Note that in the case where the first region R1is the likes of a rectangle, a square, and a polygon, its corner portions may be formed sharply, or its corner portions may be provided with a roundness.

As shown also in the embodiments below, in the case of a strain detection element employing spin technology, the shape of the membrane120is more preferably a shape where X-Y anisotropy of strain generated in the membrane increases when a pressure is applied to the membrane. For example, it is preferably a shape close to a rectangular shape. This makes it possible to dispose a large number of strain detection elements employing spin technology. It also improves signal-to-noise ratio (SNR) according to the number of elements N. When individual strain detection elements are assumed to show identical outputs, an improvement effect of SNF when a plurality of N elements are employed is 20 log√N. That is, SNR improves as the number N increases. This is due to the fact that when the strain detection elements are each connected in series, whereas a signal increases N times proportionally to the number of elements N, while noise increases proportionally to √N according to the number of elements N, hence SNF effectively improves by 20 log(N/√N)=20 log√N.

Note that when a planar shape of the first region R1is a perfectly circular shape, a diameter of the first region R1can be set to, for example, not less than 1 μm and not more than 1000 μm. In this case, it can preferably be set to not less than 60 μm and not more than 600 μm.

When the planar shape of the first region R1is a square, a length of one side of the first region R1can be set to, for example, not less than 1 μm and not more than 650 μm. In this case, it can preferably be set to not less than 50 μm and not more than 550 μm. When the planar shape of the first region R1is a rectangle, a length of a short side of the first region R1can be set to, for example, not less than 1 μm and not more than 500 μm. In this case, it can preferably be set to not less than 50 μm and not more than 400 μm.

FIG. 3Eis a schematic plan view showing a placement position on the membrane120of the strain detection element200. As an example, a rectangle whose corner portions are provided with a roundness is adopted as the shape of the first region R1. In this way, the corner portions of the shape of the first region R1are provided with a roundness. The rounding of the corner portions is provided for the following reason. That is, when the membrane undergoes an etching process by RIE (Reactive Ion Etching) or the like, the central portion and the corner portions have different etching rates. Rounding the corner portions may suppress a negative influence due to a film thickness distribution of the membrane120.

A roundness of R of the corner portions in this case depends also on area of the membrane120, but a desirable design is to set R=30 to 100 μm. It is suitable to set R=about 70 μm in order to perform etching processing uniformly, while maintaining X-Y anisotropic strain large.

It is suitable for the strain detection element200to be disposed at an edge of the first region R1. As shown inFIG. 3E, “an edge of the first region R1” herein indicates a position between a point120P6on a boundary of the supported portion122and the vibrating portion121, and a central point120P7of a line segment joining a centroid120P1of the first region R1and the point120P6. This is because at an edge of the first region R1, a strain of the vibrating portion121is easily generated, and detection sensitivity of the strain rises. In addition, this is because the strain detection element200detects strain by rotation of magnetization in a magnetic layer, hence it is easier for directionality of that strain to be discriminated when the strain detection element200is positioned at an edge of the first region R1. However, in the case where dimensions are different for an up-down direction and a left-right direction as inFIG. 3B(ellipse) andFIG. 3D(rectangle), the strain detection element200may be disposed close to the center of the first region R1, instead of being disposed at an edge of the first region R1.

Moreover, as shown inFIG. 3E, when the first region R1of the membrane120is projected on a plane (for example, the X-Y plane) parallel to the first region R1, a minimum circumscribed rectangle120S of the first region R1can be formed in a region surrounded by points120P2,120P3,120P4, and120P5. The minimum circumscribed rectangle120S includes: a region120S1formed by joining the point120P2, the point12023, and the centroid120P1by line segments; a region120S2formed by joining the point120P4, the point120P5, and the centroid120P1by line segments; a region120S3formed by joining the point120P3, the point120P4, and the centroid120P1by line segments; and a region120S4formed by joining the point120P2, the point120P5, and the centroid120P1by line segments.

Moreover, as shown inFIG. 3E, a region where the first region R1and the region120S1overlap, of the membrane120has a plurality of the strain detection elements200disposed thereon. In addition, at least two of the plurality of strain detection elements200disposed on the region where the first region P1and the region120S1overlap are different from each other in a direction parallel to a line segment120S11joining the point120P2and the point120P3.

Next, a schematic configuration of the strain detection element200according to the present embodiment will be described with reference toFIG. 4.FIG. 4is a schematic perspective view showing the configuration of the strain detection element200according to the first embodiment. As shown inFIG. 4, the strain detection element200according to the present embodiment includes a first magnetic layer201, a second magnetic layer202, and an intermediate layer203provided between the first magnetic layer201and the second magnetic layer202. The first magnetic layer201and the second magnetic layer202both have a magnetization, and are disposed separated from each other via the intermediate layer203.

When a strain occurs in the strain detection element200, a direction of magnetization of at least one of the magnetic layers201and202changes relatively to the magnetization of the other. Accompanying this, an electrical resistance value between the magnetic layers201and202changes. Therefore, by detecting this change in the electrical resistance value, the strain that has occurred in the strain detection element200can be detected.

In the present embodiment, the first magnetic layer201is configured from a ferromagnetic body and functions as, for example, a magnetization free layer. Moreover, the second magnetic layer202is also configured from a ferromagnetic body and functions as, for example, a reference layer. The second magnetic layer202may be a magnetization fixed layer or may be a magnetization free layer. That is, it is only required that a change in magnetization of the first magnetic layer201be made more easily than a change in magnetization of the second magnetic layer202.

Note that it is also possible for the first magnetic layer201to be formed larger in the XY plane than the second magnetic layer202, for example. Moreover, it is also possible for one of the first magnetic layer201and the second magnetic layer202to be divided.

Next, operation of the strain detection element200according to the present embodiment will be described.

FIGS. 5A to 5Care schematic perspective views exemplifying operation of the strain detection element200according to the first embodiment.

FIG. 5Acorresponds to a state (tensile state) when a tensile force ts has been applied to the strain detection element200and a strain has occurred.FIG. 5Bcorresponds to a state (unstrained state) when the strain detection element200does not have a strain.FIG. 5Ccorresponds to a state (compressive state) when a compressive force cs has been applied to the strain detection element200and a strain has occurred.

In order to make the drawings more easily seen,FIGS. 5A to 5Cdepict the first magnetic layer201, the second magnetic layer202, and the intermediate layer203. In this example, the first magnetic layer201is a magnetization free layer, and the second magnetic layer202is a magnetization fixed layer.

Operation where the strain detection element200functions as a strain sensor is based on application of an “inverse magnetostriction effect” and a “magnetoresistance effect”. The “inverse magnetostriction effect” is obtained in a ferromagnetic layer employed in a magnetization free layer. The “magnetoresistance effect” is expressed in a stacked film of a magnetization free layer, an intermediate layer, and a reference layer (for example, a magnetization fixed layer).

The “inverse magnetostriction effect” is a phenomenon where magnetization of a ferromagnetic body changes due to a strain occurring in the ferromagnetic body. That is, when an external strain is applied to a stacked body of a strain detection element, a magnetization direction of the magnetization free layer changes. As a result, a relative angle between the magnetization of the magnetization free layer and the magnetization of the reference layer (for example, the magnetization fixed layer) changes. A change in electrical resistance is caused by the “magnetoresistance effect (MR effect” at this time. The MR effect includes, for example a GMR (Giant magnetoresistance) effect or a TMR (Tunneling magnetoresistance) effect, and so on. The MR effect is expressed by passing a current through the stacked body and reading a change in relative angle of inclination of magnetizations as an electrical resistance change. For example, a strain occurs in the stacked body (strain detection element), magnetization direction of the magnetization free layer changes due to the strain, and the relative angle between the magnetization direction of the magnetization free layer and the magnetization direction of the reference layer (for example, the magnetization fixed layer) changes. That is, the MR effect is expressed due to the inverse magnetostriction effect.

When a ferromagnetic material employed in the magnetization free layer has a positive magnetostriction coefficient, the direction of magnetization changes such that an angle between the direction of magnetization and a direction of a tensile strain becomes smaller, and an angle between the direction of magnetization and a direction of a compressive strain becomes larger. When a ferromagnetic material employed in the magnetization free layer has a negative magnetostriction coefficient, the direction of magnetization changes such that an angle between the direction of magnetization and a direction of a tensile strain becomes larger, and an angle between the direction of magnetization and a direction of a compressive strain becomes smaller.

In the case where a combination of materials of the stacked body of the magnetization free layer, the intermediate layer, and the reference layer (for example, the magnetization fixed layer) has a positive magnetoresistance effect, electrical resistance decreases when the relative angle between the magnetization free layer and the magnetization fixed layer is small. In the case where a combination of materials of the stacked body of the magnetization free layer, the intermediate layer, and the reference layer (for example, the magnetization fixed layer) has a negative magnetoresistance effect, electrical resistance increases when the relative angle between the magnetization free layer and the magnetization fixed layer is small.

Described below is an example of change in magnetization for an example of the case where the ferromagnetic materials employed in the magnetization free layer and the reference layer (for example, the magnetization fixed layer) each have a positive magnetostriction constant and the stacked body including the magnetization free layer, the intermediate layer, and the reference layer (for example, the magnetization fixed layer) has a positive magnetoresistance effect.

As expressed inFIG. 5B, in an unstrained state STo (for example, an initial state) where there is no strain, the relative angle between the magnetization of the first magnetic layer (the magnetization free layer)201and the magnetization of the second magnetic layer (for example, the magnetization fixed layer)202is set to a certain value. A direction of magnetization of a magnetic layer in an initial state of the first magnetic layer201is set by, for example, hard bias or shape anisotropy of the magnetic layer, and so on. At this time, a preferable example of an initial magnetization direction setting due to hard bias is a setting of a direction inclined at substantially 45 degrees to a direction of application of a stress. With a view to broadened range, an angle of inclination is preferably 30 to 60 degrees. Doing so makes it possible to obtain an output signal that changes linearly whichever of the cases of a tensile force ts or a compressive force cs has occurred. In this example, the magnetization of the first magnetic layer201and the magnetization of the second magnetic layer202intersect each other in the initial state.

As shown in FIG. SA, when the tensile force ts is applied in a tensile state STt, a strain corresponding to the tensile force ts occurs in the strain detection element200. At this time, the magnetization of the first magnetic layer201in the tensile state STt changes from the unstrained state STo such that a relative angle between the magnetization of magnetization free layer210and a direction of the tensile force ts becomes smaller. In the example shown in FIG. SA, the relative angle between the magnetization of the first magnetic layer201and the magnetization of the second magnetic layer202becomes smaller in the case that the tensile force ts is applied, compared to in the unstrained state STo. As a result, the electrical resistance in the strain detection element200decreases compared to the electrical resistance during the unstrained state STo.

On the other hand, as shown inFIG. 5C, when the compressive force cs is applied in a compressive state STc, the magnetization of the first magnetic layer201in the compressive state STc changes from the unstrained state STo such that an angle between the magnetization of the first magnetic layer201and a direction of the compressive force cs becomes larger.

In the example shown inFIG. 5C, the relative angle between the magnetization of the first magnetic layer201and the magnetization of the second magnetic layer202becomes larger in the case that the compressive force cs is applied, compared to in the unstrained state STo. As a result, the electrical resistance in the strain detection element200increases.

Thus, in the strain detection element200, a change in strain occurring in the strain detection element200is converted into a change in electrical resistance of the strain detection element200. In the above-described operation, an amount of change in electrical resistance (dR/R) per unit strain (dε) is called a gauge factor (GF). Employing a strain detection element having a high gauge factor makes it possible to obtain a strain sensor of high sensitivity.

Next, configuration examples of the strain detection element200according to the present embodiment will be described with reference toFIGS. 6 to 12. Note that below, a description of “material A/material B” indicates a state where a layer of material B is provided on a layer of material A.

FIG. 6is a schematic perspective view showing one configuration example200A of the strain detection element200. As shown inFIG. 6, the strain detection element200A is configured having stacked therein, sequentially from below: a lower electrode204; a base layer205; a pinning layer206; a second magnetization fixed layer207; a magnetic coupling layer208; a first magnetization fixed layer209(the second magnetic layer202); the intermediate layer203; a magnetization free layer210(the first magnetic layer201); a cap layer211; and an upper electrode212. The first magnetization fixed layer209corresponds to the second magnetic layer202. The magnetization free layer210corresponds to the first magnetic layer201. Moreover, the lower electrode204is connected to, for example, the wiring line C1(FIG. 1), and the upper electrode212is connected to, for example, the wiring line C2(FIG. 1). However, when, for example, the first magnetic layer201is divided, the upper electrode connected to one of the first magnetic layers201may be connected to the wiring line C1(FIG. 1) and the upper electrode connected to the other of the first magnetic layers201may be connected to the wiring line C2(FIG. 1). Similarly, when, for example, the second magnetic layer202is divided, the lower electrode connected to one of the second magnetic layers202may be connected to the wiring line C1(FIG. 1) and the lower electrode connected to the other of the second magnetic layers202may be connected to the wiring line C2(FIG. 1).

Employed in the base layer205is, for example, a stacked film of tantalum and ruthenium (Ta/Ru). A thickness (length in a Z axis direction) of a Ta layer thereof is, for example, 3 nanometers (nm). A thickness of a Ru layer thereof is, for example, 2 nm. Employed in the pinning layer206is, for example, an IrMn layer having a thickness of 7 nm. Employed in the second magnetization fixed layer207is, for example, a Co75Fe25layer having a thickness of 2.5 nm. Employed in the magnetic coupling layer208is, for example, a Ru layer having a thickness of 0.9 nm. Employed in the first magnetization fixed layer209is, for example, a Co40Fe40B20layer having a thickness of 3 nm. Employed in the intermediate layer203is, for example, a MgO layer having a thickness of 1.6 nm. Employed in the magnetization free layer210is, for example, Co40Fe40B20having a thickness of 4 nm. Employed in the cap layer211is, for example, Ta/Ru. A thickness of a Ta layer thereof is, for example, 1 nm. A thickness of a Ru layer thereof is, for example, 5 nm.

Employed in the lower electrode204and the upper electrode212is, for example, at least one of aluminum (Al), an aluminum copper alloy (Al—Cu), copper (Cu), silver (Ag), and gold (Au). Employing such materials having a comparatively small electrical resistance as the lower electrode204and the upper electrode212makes it possible to pass a current efficiently through the strain detection element200A. A nonmagnetic material may be employed in the lower electrode204and the upper electrode212.

The lower electrode204and the upper electrode212may, for example, include: a base layer dedicated for the lower electrode204and the upper electrode212(not illustrated); a cap layer dedicated for the lower electrode204and the upper electrode212(not illustrated); and a layer of at least one of Al, Al—Cu, Cu, Ag, and Au, provided between the base layer and cap layer. For example, employed in the lower electrode204and the upper electrode212is the likes of tantalum (Ta)/copper (Cu)/tantalum (Ta). Employing Ta as the base layer dedicated for the lower electrode204and the upper electrode212results in adhesion between the substrate110and the lower electrode204and upper electrode212being improved, for example. Titanium (Ti) or titanium nitride (TiN), and so on, may be employed as the base layer dedicated for the lower electrode204and the upper electrode.

Employing Ta as the cap layer dedicated for the lower electrode204and the upper electrode212makes it possible to avoid oxidation of the likes of copper (Cu) below the cap layer. Titanium (Ti) or titanium nitride (TiN), and so on, may be employed as the cap layer dedicated for the lower electrode204and the upper electrode212.

Employable in the base layer205is, for example, a stacked structure including a buffer layer (not illustrated) and a seed layer (not illustrated). This buffer layer eases surface roughness of the lower electrode204or the membrane120, and so on, and improves crystallinity of a layer stacked on this buffer layer, for example. Employed as the buffer layer is, for example, at least one selected from the group of tantalum (Ta), titanium (Ti), vanadium (V), tungsten (W), zirconium (Zr), hafnium (Hf), and chromium (Cr). An alloy including at least one material selected from these materials may be employed as the buffer layer.

A thickness of the buffer layer in the base layer205is preferably not less than 1 nm and not more than 10 nm. The thickness of the buffer layer is more preferably not less than 1 nm and not more than 5 nm. If the buffer layer is too thin, a buffer effect is lost. If the buffer layer is too thick, the strain detection element200A becomes excessively thick. The seed layer is formed on the buffer layer, and that seed layer may have a buffer effect. In this case, the buffer layer may be omitted. Employed in the buffer layer is, for example, a Ta layer having a thickness of 3 nm.

The seed layer in the base layer205controls crystalline orientation of a layer stacked on the seed layer. The seed layer controls the crystalline particle diameter of the layer stacked on the seed layer. Employed as the seed layer are the likes of a metal of fcc structure (face-centered cubic structure), hcp structure (hexagonal close-packed structure), or bcc structure (body-centered cubic structure).

Employing ruthenium (Ru) of hcp structure, or NiFe of fcc structure, or Cu of fcc structure as the seed layer in the base layer205makes it possible to set a crystalline orientation of a spin valve film on the seed layer to an fcc (111) orientation. Employed in the seed layer is, for example, a Cu layer having a thickness of 2 nm, or a Ru layer having a thickness of 2 nm. When raising crystalline orientation of the layer formed on the seed layer, a thickness of the seed layer is preferably not less than 1 nm and not more than 5 nm. The thickness of the seed layer is more preferably not less than 1 nm and not more than 3 nm. As a result, a function as a seed layer of improving crystalline orientation is sufficiently displayed.

On the other hand, when, for example, there is no need to cause crystalline orientation of the layer formed on the seed layer (when, for example, forming an amorphous magnetization free layer, and so on), the seed layer may be omitted. Employed as the seed layer is, for example, a Cu layer having a thickness of 2 nm.

The pinning layer206gives unidirectional anisotropy to the second magnetization fixed layer207(ferromagnetic layer) formed on the pinning layer206, and thereby fixes magnetization of the second magnetization fixed layer207. Employed in the pinning layer206is, for example, an antiferromagnetic layer. Employed in the pinning layer206is, for example, at least one selected from the group of Ir—Mn, Pt—Mn, Pd—Pt—Mn, Ru—Mn, Rh—Mn, Ru—Rh—Mn, Fe—Mn, Ni—Mn, Cr—Mn—Pt, and Ni—O. It is also possible to employ an alloy having an additional element further added to the Ir—Mn, Pt—Mn, Pd—Pt—Mn, Ru—Mn, Rh—Mn, Ru—Rh—Mn, Fe—Mn, Ni—Mn, Cr—Mn—Pt, and Ni—C. A thickness of the pinning layer206is appropriately set to give sufficiently strong unidirectional anisotropy.

In order to perform fixing of magnetization of the ferromagnetic layer contacting the pinning layer206, heat treatment during magnetic field application is performed. Magnetization of the ferromagnetic layer contacting the pinning layer206is fixed in a direction of the magnetic field applied during the heat treatment. Annealing temperature is set to, for example, a temperature greater than or equal to a magnetization fixing temperature of an antiferromagnetic material employed in the pinning layer206. Moreover, when an antiferromagnetic layer including Mn is employed, Mn sometimes diffuses to a layer other than the pinning layer206to lower an MR change rate. Hence, the annealing temperature is desirably set to a temperature less than or equal to a temperature at which diffusion of Mn occurs. The annealing temperature may be set to, for example, not less than 200° C. and not more than 500° C. Preferably, it may be set to, for example, not less than 250′C and not more than 400° C.

When PtMn or PdPtMn are employed as the pinning layer206, the thickness of the pinning layer206is preferably not less than 8 nm and not more than 20 nm. The thickness of the pinning layer206is more preferably not less than 10 nm and not more than 15 nm. When IrMn is employed as the pinning layer206, unidirectional anisotropy may be given by a pinning layer206which is thinner than when PtMn is employed as the pinning layer206. In this case, the thickness of the pinning layer206is preferably not less than 4 nm and not more than 18 nm. The thickness of the pinning layer105is more preferably not less than 5 nm and not more than 15 nm. Employed in the pinning layer206is, for example, an Ir22Mn78layer having a thickness of 7 nm.

A hard magnetic layer may be employed as the pinning layer206. Employed as the hard magnetic layer is, for example, a hard magnetic material of comparatively high magnetic anisotropy and coercivity such as Co—Pt, Fe—Pt, Co—Pd, Fe—Pd, and so on. Moreover, an alloy having an additional element further added to Co—Pt, Fe—Pt, Co—Pd, and Fe—Pd, may be employed. Employable as the hard magnetic layer is, for example, CoPt (where a percentage of Co is not less than 50 at. % and not more than 85 at. %), (CoxPt100-x)100-yCry(where x is not less than 50 at. % and not more than 85 at. %, and y is not less than 0 at. % and not more than 40 at. %), or FePt (where a percentage of Pt is not less than 40 at. % and not more than 60 at. %), and so on.

Employed in the second magnetization fixed layer207is, for example, a CoxFe100-xalloy (where x is not less than 0 at. % and not more than 1.00 at. %), a NixFe100-xalloy (where z is not less than 0 at. % and not more than 100 at. %), or a material having a nonmagnetic element added to these alloys. Employed as the second magnetization fixed layer207is, for example, at least one selected from the group of Co, Fe, and Ni. It is also possible to employ as the second magnetization fixed layer207an alloy including at least one material selected from these materials. Also employable as the second magnetization fixed layer207is a (CoxFe100-x)100-yByalloy (where x is not less than 0 at. % and not more than 100 at. %, and y is not less than 0 at. % and not more than 30 at. %). Employing an amorphous alloy of (CoxFe100-x)100-yByas the second magnetization fixed layer207makes it possible to suppress variation of characteristics of the strain detection element200A even when size of the strain detection element is small.

A thickness of the second magnetization fixed layer207is preferably not less than 1.5 nm and not more than 5 nm, for example. As a result, for example, intensity of a unidirectional anisotropic magnetic field due to the pinning layer206can be more greatly strengthened. For example, intensity of an antiferromagnetic coupling magnetic field between the second magnetization fixed layer207and the first magnetization fixed layer209can be more greatly strengthened, via the magnetic coupling layer formed on the second magnetization fixed layer207. For example, magnetic film thickness (product (BE·t) of saturation magnetization Bs and thickness t) of the second magnetization fixed layer207is preferably substantively equal to magnetic film thickness of the first magnetization fixed layer209.

Saturation magnetization of Co40Fe40B20with a thin film is approximately 1.9 T (tesla). For example, when a CoxFe40B20layer having a thickness of 3 nm is employed as the first magnetization fixed layer209, the magnetic film thickness of the first magnetization fixed layer209is 1.9 T×3 nm, that is, 5.7 Tnm. On the other hand, saturation magnetization of Co75Fe25is approximately 2.1 T. The thickness of the second magnetization fixed layer207at which a magnetic film thickness equal to that described above can be obtained is 5.7 Tnm/2.1 T, that is, 2.7 nm. In this case, a Co75Fe25layer having a thickness of approximately 2.7 nm is preferably employed in the second magnetization fixed layer207. Employed as the second magnetization fixed layer207is, for example, a Co75Fe2layer having a thickness of 2.5 nm.

In the strain detection element200A, a synthetic pin structure of the second magnetization fixed layer207, the magnetic coupling layer208, and the first magnetization fixed layer209is employed. Instead, a single pin structure configured from a single magnetization fixed layer may be employed. When the single pin structure is employed, a Co40Fe40B20layer having a thickness of 3 nm, for example, is employed as the magnetization fixed layer. The same material as the above-mentioned material of the second magnetization fixed layer207may be employed as the ferromagnetic layer employed in the single pin structure magnetization fixed layer.

The magnetic coupling layer208generates antiferromagnetic coupling between the second magnetization fixed layer207and the first magnetization fixed layer209. The magnetic coupling layer208forms a synthetic pin structure. Employed as a material, of the magnetic coupling layer208is, for example, Ru. A thickness of the magnetic coupling layer208is preferably not less than 0.8 nm and not more than 1 nm, for example. A material other than Ru may be employed as the magnetic coupling layer208, provided it is a material generating sufficient antiferromagnetic coupling between the second magnetization fixed layer207and the first magnetization fixed layer209. The thickness of the magnetic coupling layer208may be set to a thickness of not less than 0.8 nm and not more than 1 nm corresponding to a second peak of RKKY (Ruderman-Kittel-Kasuya-Yosida) coupling. Furthermore, the thickness of the magnetic coupling layer208may be set to a thickness of not less than 0.3 nm and not more than 0.6 nm corresponding to a first peak of RKKY coupling. Employed as the material of the magnetic coupling layer208is, for example, Ru having a thickness of 0.9 nm. As a result, highly reliable coupling can be more stably obtained.

A magnetic layer employed in the first magnetization fixed layer209(second magnetic layer202) contributes directly to the MR effect. Employed as the first magnetization fixed layer209is, for example, a Co—Fe—B alloy. Specifically, a (CoxFe100-x)100-yByalloy (where x is not less than 0 at. % and not more than 100 at. %, and y is not less than 0 at. % and not more than 30 at. %) may also be employed as the first magnetization fixed layer209. When an amorphous alloy of (CoxFe100-x)100-yByis employed as the first magnetization fixed layer209, variation between elements due to crystalline particles can be suppressed even when, for example, size of the strain detection element200A is small.

A layer formed on the first magnetization fixed layer209(for example, a tunnel insulating layer (not illustrated) can be planarized. Planarization of the tunnel insulating layer makes it possible to reduce defect density of the tunnel insulating layer. As a result, a larger MR change rate can be obtained by a lower sheet resistivity. For example, when Mg—O is employed as a material of the tunnel insulating layer, employing an amorphous alloy of (CoxFe100-x)100-yByas the first magnetization fixed layer209makes it possible to strengthen (100) orientation of an Mg—O layer formed on the tunnel insulating layer. More greatly raising the (100) orientation of the Mg—O layer enables an even larger MR change rate to be obtained. The (CoxFe100-x)100-yByalloy crystallizes adopting a (100) surface of the Mg—O layer as a template during annealing. Therefore, good crystal conformity can be obtained between the Mg—O and the (CoxFe100-x)100-yByalloy. Obtaining good crystal conformity enables an even larger MR change rate to be obtained.

An Fe—Co alloy, for example, may be employed as the first magnetization fixed layer209, besides the Co—Fe—B alloy.

If the first magnetization fixed layer209is thicker, a larger MR change rate is obtained. In order to obtain a larger fixed magnetic field, it is more preferable for the first magnetization fixed layer209to be thin. There is a tradeoff relationship in the thickness of the first magnetization fixed layer209between the MR change rate and the fixed magnetic field. When a Co—Fe—B alloy is employed as the first magnetization fixed layer209, the thickness of the first magnetization fixed layer209is preferably not less than 1.5 nm and not more than 5 nm. The thickness of the first magnetization fixed layer209is more preferably not less than 2.0 nm and not more than 4 nm.

Employed in the first magnetization fixed layer209, besides the above-mentioned materials, is a Co90Fe10alloy of fcc structure, or Co of hcp structure, or a Co alloy of hcp structure. Employed as the first magnetization fixed layer209is at least one selected from the group of Co, Fe, and Ni. Employed as the first magnetization fixed layer209is an alloy including at least one material selected from these materials. Employing a bcc structure FeCo alloy material, a Co alloy including a cobalt composition of 50% or more, or a material (Ni alloy) having a Ni composition of 50% or more, as the first magnetization fixed layer209results in, for example, a larger MR change rate being obtained.

It is also possible to employ as the first magnetization fixed layer209a Heusler magnetic alloy layer of the likes of, for example, Co2MnGe, Co2FeGe, Co2MnSi, Co2FeSi, Co2MnAl, Co2FeAl, Co2MnGa0.5Geo0.5, and Co2FeGa0.5Ge0.5. For example, employed as the first magnetization fixed layer209is a Co40Fe40B20layer having a thickness of 3 nm.

The intermediate layer203decouples magnetic coupling between the first magnetic layer201and the second magnetic layer202, for example. Employed in a material of the intermediate layer203is, for example, a metal or an insulator or a semiconductor. Employed as the metal is, for example, Cu, Au, or Ag, and so on. When a metal is employed as the intermediate layer203, a thickness of the intermediate layer is, for example, about not less than 1 nm and not more than 7 nm. Employed as the insulator or semiconductor are, for example, the likes of a magnesium oxide (MgO, and so on), an aluminum oxide (Al2O3, and so on), a titanium oxide (TiO, and so on), a zinc oxide (Zn—O, and so on), or gallium oxide (Ga—O). When an insulator or semiconductor is employed as the intermediate layer203, the thickness of the intermediate layer203is, for example, about not less than 0.6 nm and not more than 2.5 nm. Also employable as the intermediate layer203is, for example, a CCP (Current-Confined-Path) spacer layer. When a CCP spacer layer is employed as the spacer layer, a structure in which, for example, a copper (Cu) metal path is formed in the insulating layer of aluminum oxide (Al2O3), is employed. For example, employed as the intermediate layer is a MgO layer having a thickness of 1.6 nm.

A ferromagnetic body material is employed in the magnetization free layer210(first magnetic layer201). Employable in the magnetization free layer210is, for example, a ferromagnetic body material including Fe, Co, and Ni. Employed as a material of the magnetization free layer210are, for example, an FeCo alloy, an NiFe alloy, and so on. Furthermore, employed in the magnetization free layer210are the likes of a Co—Fe—B alloy, an Fe—Co—Si—B alloy, an Fe—Ga alloy of large λs (magnetostriction constant), an Fe—Co—Ga alloy, a Tb-M-Fe alloy, a Tb-M1-Fe-M2 alloy, an Fe-M3-M4-B alloy, Ni, Fe—Al, or ferrite. In the previously mentioned Tb-M-Fe alloy, M is at least one selected from the group of Sm, Eu, Gd, Dy, Ho, and Er. In the previously mentioned Tb-M1-Fe-M2 alloy, M1is at least one selected from the group of Sm, Eu, Gd, Dy, Ho, and Er. M2 is at least one selected from the group of Ti, Cr, Mn, Co, Cu, Nb, Mo, W, and Ta. In the previously mentioned Fe-M3-M4-B alloy, M3 is at least one selected from the group of Ti, Cr, Mn, Co, Cu, Nb, Mo, W, and Ta. M4 is at least one selected from the group of Ce, Pr, Nd, Sm, Tb, Dy, and Er. Examples of the previously mentioned ferrite include Fe3O4, (FeCo)3O4and so on. A thickness of the magnetization free layer210is, for example, 2 nm or more.

Employable in the magnetization free layer210is a magnetic material containing boron. Employable in the magnetization free layer210is, for example, an alloy including at least one element selected from the group of Fe, Co, and Ni, and boron (B). For example, the likes of a Co—Fe—B alloy or an Fe—B alloy can be employed. For example, a Co40Fe40Bo alloy can be employed. When an alloy including at least one element selected from the group of Fe, Co, and Ni, and boron (B) is employed in the magnetization free layer210, the likes of Ga, Al, Si, or W may be added as an element promoting high magnetostriction. For example, an Fe—Ga—B alloy, an Fe—Co—Ga—B alloy, or an Fe—Co—Si—B alloy may be employed. Employing such a magnetic material containing boron results in coercivity (Hc) of the strain detection element200lowering and facilitates change in magnetization direction with respect to strain. This enables a high strain sensitivity to be obtained.

Boron concentration (for example, composition ratio of boron) in the magnetization free layer210is preferably not less than 5 at. % (atomic percent). This makes it easier for an amorphous structure to be obtained. Boron concentration in the magnetization free layer is preferably not more than 35 at. %. If boron concentration is too high, the magnetostriction constant decreases, for example. Boron concentration in the magnetization free layer is preferably not less than 5 at.3and not more than 35 at. %, and is more preferably not less than 10 at. % and not more than 30 at. %, for example.

Employing Fe1-yBy(where 0<y≤0.3) or (FeaX1-a)1-yBy(where X═Co or Ni, 0.8≤a<1, and 0<y≤0.3) in part of the magnetic layer of the magnetization free layer210makes it easy to obtain both a large magnetostriction constant λ and a low coercivity, hence is particularly preferable from a viewpoint of obtaining a high gauge factor. For example, Fe80B20(4 nm) may be employed as the magnetization free layer210. Co40Fe40B20(0.5 nm)/Fe80B20(4 nm) may be employed as the magnetization free layer.

The magnetization free layer210may have a multi-layer structure. When a tunnel insulating layer of MgO is employed as the intermediate layer203, a portion of the magnetization free layer210that contacts the intermediate layer203is preferably provided with a layer of a Co—Fe—B alloy. As a result, a high magnetoresistance effect is obtained. In this case, the Co—Fe—B alloy layer is provided on the intermediate layer203, and another magnetic material having a large magnetostriction constant is provided on the Co—Fe—B alloy layer. When the magnetization free layer210has a multi-layer structure, the likes of Co—Fe—B (2 nm)/Fe—Co—Si—B (4 nm), for example, is employed in the magnetization free layer210.

The cap layer211protects a layer provided below the cap layer211. Employed in the cap layer211are, for example, a plurality of metal layers. Employed in the cap layer211is, for example, a two-layer structure (Ta/Ru, of a Ta layer and a Ru layer. A thickness of this Ta layer is, for example, 1 nm, and a thickness of this Ru layer is, for example, 5 nm. Another metal layer may be provided instead of the Ta layer or Ru layer, as the cap layer211. There may be any configuration of the cap layer211. For example, a nonmagnetic material may be employed as the cap layer211. Another material may be employed as the cap layer211, provided said material is capable of protecting the layer provided below the cap layer211.

When a magnetic material containing boron is employed in the magnetization free layer210, a diffusion prevention layer not illustrated, of an oxide material or a nitride material, may be provided between the magnetization free layer210and the cap layer211, in order to prevent diffusion of boron. Employing a diffusion prevention layer configured from an oxide layer or a nitride layer makes it possible to suppress diffusion of boron included in the magnetization free layer210and maintain an amorphous structure of the magnetization free layer210. Employable as the oxide material or nitride material employed in the diffusion prevention layer is, specifically, an oxide material or nitride material including an element such as Mg, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Pu, Rh, Pd, Ag, Hf, Ta, W, Sn, Cd, Ga, and so on.

Now, since the diffusion prevention layer does not contribute to the magnetoresistance effect, its sheet resistivity is preferably low. For example, sheet resistivity of the diffusion prevention layer is preferably set lower than sheet resistivity of the intermediate layer that contributes to the magnetoresistance effect. From a viewpoint of lowering sheet resistivity of the diffusion prevention layer, an oxide or a nitride of Mg, Ti, V, Zn, Sn, Cd, and Ga whose barrier heights are low, is preferable. An oxide having stronger chemical bonding as a function for suppressing diffusion of boron, is preferable. For example, MgO of 1.5 nm can be employed. Moreover, an oxynitride may be regarded as either an oxide or a nitride.

When an oxide material or nitride material is employed in the diffusion prevention layer, a film thickness of the diffusion prevention layer is preferably not less than 0.5 nm from a viewpoint of sufficiently displaying a function of preventing boron diffusion, and is preferably not more than 5 nm from a viewpoint of lowering sheet resistivity. In other words, the film thickness of the diffusion prevention layer is preferably not less than 0.5 nm and not more than 5 nm, and more preferably not less than 1 nm and not more than 3 nm.

Employable as the diffusion prevention layer is at least one selected from the group of magnesium (Mg), silicon (Si), and aluminum (Al). Employable as the diffusion prevention layer is a material including these light elements. These light elements bond with boron to generate a compound. Formed in a portion including an interface between the diffusion prevention layer and the magnetization free layer210is at least one of a Mg—B compound, an Al—B compound, and a Si—B compound, for example. These compounds suppress diffusion of boron.

Another metal layer, and so on, may be inserted between the diffusion prevention layer and the magnetization free layer210. However, if a distance between the diffusion prevention layer and the magnetization free layer210becomes too large, boron diffuses between said layers whereby boron concentration in the magnetization free layer210ends up lowering, hence the distance between the diffusion prevention layer and the magnetization free layer210is preferably not more than 10 nm, and more preferably not more than 3 nm.

FIG. 7is a schematic perspective view showing a configuration example of the strain detection element200A. As shown inFIG. 7, the strain detection element200A may include an insulating layer (insulating portion)213filled between the lower electrode204and the upper electrode212.

Employable in the insulating layer213is, for example, an aluminum oxide (for example, Al2O3) or a silicon oxide (for example, SiO2), and so on. A leak current of the strain detection element200A can be suppressed by the insulating layer213.

FIG. 8is a schematic perspective view showing another configuration example of the strain detection element200A. As shown inFIG. 8, the strain detection element200A may include: two hard bias layers (hard bias portions)214provided separated from each other between the lower electrode204and the upper electrode212; and the insulating layer213filled between the lower electrode204and the hard bias layer214.

The hard bias layer214sets the magnetization direction of the magnetization free layer210(first magnetic layer201) to a desired direction by a magnetization of the hard bias layer214. The hard bias layer214makes it possible to set the magnetization direction of the magnetization free layer210(first magnetic layer201) to a desired direction in a state where a pressure from external is not applied to the membrane.

Employed in the hard bias layer214is, for example, a hard magnetic material of comparatively high magnetic anisotropy and coercivity such as Co—Pt, Fe—Pt, Co—Pd, Fe—Pd, and so on. Moreover, an alloy having an additional element further added to Co—Pt, Fe—Pt, Co—Pd, and Fe—Pd may be employed. Employable in the hard bias layer214is, for example, CoPt (where a percentage of Co is not less than 50 at. % and not more than 85 at. %), (CoxPt100-x)100-yCry(where x is not less than 50 at. % and not more than 85 at. %, and y is not less than 0 at. % and not more than 40 at. %), or FePt (where a percentage of Pt is not less than 40 at. % and not more than 60 at. %), and so on. When such materials are employed, applying the hard bias layer214with an external magnetic field larger than the coercivity of the hard bias layer214makes it possible for a direction of magnetization of the hard bias layer214to be set (fixed) in a direction of application of the external magnetic field. A thickness (for example, a length along a direction from the lower electrode204toward the upper electrode212) of the hard bias layer214is, for example, not less than 5 nm and not more than 50 nm.

When the insulating layer213is disposed between the lower electrode204and the upper electrode212, SiOx or AlOxmay be employed as a material of the insulating layer213. Furthermore, a base layer not illustrated may be provided between the insulating layer213and the hard bias layer214. When a hard magnetic material of comparatively high magnetic anisotropy and coercivity such as Co—Pt, Fe—Pt, Co—Pd, Fe—Pd, and so on, is employed in the hard bias layer214, the likes of Cr or Fe—Co may be employed as a material of the base layer for the hard bias layer214. The above-described hard bias layer214may also be applied to any of the later-mentioned strain detection elements.

The hard bias layer214may have a structure of being stacked on a hard bias layer-dedicated pinning layer not illustrated. In this case, the direction of magnetization of the hard bias layer214can be set (fixed) by exchange coupling between the hard bias layer214and the hard bias layer-dedicated pinning layer. In this case, employable in the hard bias layer214is a ferromagnetic material configured from at least one of Fe, Co, and Ni, or from an alloy including at least one kind of these metals. In this case, employable in the hard bias layer214is, for example, a CoxFe100-xalloy (where x is not less than 0 at. % and not more than 100 at. %), a NixFe100-xalloy (where x is not less than 0 at. % and not more than 100 at. %), or a material having a nonmagnetic element added to these alloys. Employable as the hard bias layer214is a material similar to that of the previously mentioned first magnetization fixed layer209. Moreover, employable in the hard bias layer-dedicated pinning layer is a material similar to that of the previously mentioned pinning layer206in the strain detection element200A. Moreover, when the hard bias layer-dedicated pinning layer is provided, a base layer of a similar material to that employed in the base layer205may be provided below the hard bias layer-dedicated pinning layer. Moreover, the hard bias layer-dedicated pinning layer may be provided to a lower portion of the hard bias layer, or may be provided to an upper portion of the hard bias layer. The magnetization direction of the hard bias layer214in this case can be determined by magnetic field-accompanied heat treatment, similarly to in the case of the pinning layer206.

The above-described hard bias layer214and insulating layer213may also be applied to any of the strain detection elements200described in the present embodiment. Moreover, when the above-mentioned stacked structure of the hard bias layer214and the hard bias layer-dedicated pinning layer is employed, an orientation of magnetization of the hard bias layer214can be easily maintained even when a large external magnetic field is instantaneously applied to the hard bias layer214.

FIG. 9is a schematic perspective view showing another configuration example200B of the strain detection element200. The strain detection element200B differs from the strain detection element200A in having a top spin valve type structure. That is, as shown inFIG. 9, the strain detection element200B is configured having stacked therein, sequentially from below: the lower electrode204; the base layer205; the magnetization free layer210(first magnetic layer201); the intermediate layer203; the first magnetization fixed layer209(second magnetic layer202); the magnetic coupling layer208; the second magnetization fixed layer207; the pinning layer206; the cap layer211; and the upper electrode212. The first magnetization fixed layer209corresponds to the second magnetic layer202. The magnetization free layer210corresponds to the first magnetic layer201. Moreover, the lower electrode204is connected to, for example, the wiring line C1(FIG. 1), and the upper electrode212is connected to, for example, the wiring line C2(FIG. 1).

Employed in the base layer205is, for example, a stacked film of tantalum and copper (Ta/Cu). A thickness (length in a Z axis direction) of a Ta layer thereof is, for example, 3 nm. A thickness of a Cu layer thereof is, for example, 5 nm. Employed in the magnetization free layer210is, for example, Co40Fe40B20having a thickness of 4 nm. Employed in the intermediate layer203is, for example, a MgO layer having a thickness of 1.6 nm. Employed in the first magnetization fixed layer209is, for example, Co40Fe40B20/Fe50Co50. A thickness of a Co40Fe40B20layer thereof is, for example, 2 nm. A thickness of an Fe50Co50layer thereof is, for example, 1 nm. Employed in the magnetic coupling layer208is, for example, a Ru layer having a thickness of 0.9 nm. Employed in the second magnetization fixed layer207is, for example, a Co75Fe25; layer having a thickness of 2.5 nm. Employed in the pinning layer206is, for example, an IrMn layer having a thickness of 7 nm. Employed in the cap layer211is, for example, Ta/Ru. A thickness of a Ta layer thereof is, for example, 1 nm. A thickness of a Ru layer thereof is, for example, 5 nm.

In the previously mentioned bottom spin valve type strain detection element200A, the first magnetization fixed layer209(second magnetic layer202) is formed more downwardly than (−Z axis direction) the magnetization free layer210(first magnetic layer201). In contrast, in the top spin valve type strain detection element200B, the first magnetization fixed layer209(second magnetic layer202) is formed more upwardly than (+Z axis direction) the magnetization free layer210(first magnetic layer201). Therefore, the materials of each of the layers included in the strain detection element200A may be used as materials of each of the layers included in the strain detection element200B, by inverting them in an upside-down manner. Moreover, the above-mentioned diffusion prevention layer may be provided between the base layer205and the magnetization free layer210of the strain detection element200B.

FIG. 10is a schematic perspective view showing another configuration example200C of the strain detection element200. The strain detection element200C is applied with a single pin structure employing a single magnetization fixed layer. That is, as shown inFIG. 10, the strain detection element200C is configured having stacked therein, sequentially from below: the lower electrode204; the base layer205; the pinning layer206; the first magnetization fixed layer209(second magnetic layer202); the intermediate layer203; the magnetization free layer210(first magnetic layer201); the cap layer211; and the upper electrode212. The first magnetization fixed layer209corresponds to the second magnetic layer202. The magnetization free layer210corresponds to the first magnetic layer201. Moreover, the lower electrode204is connected to, for example, the wiring line C1(FIG. 1), and the upper electrode212is connected to, for example, the wiring line C2(FIG. 1).

Employed in the base layer205is, for example, Ta/Ru. A thickness (length in a Z axis direction) of a Ta layer thereof is, for example, 3 nm. A thickness of a Ru layer thereof is, for example, 2 nm. Employed in the pinning layer206is, for example, an IrMn layer having a thickness of 7 nm. Employed in the first magnetization fixed layer209is, for example, a Co40Fe40B20layer having a thickness of 3 nm. Employed in the intermediate layer203is, for example, a MgO layer having a thickness of 1.6 nm. Employed in the magnetization free layer210is, for example, Co40Fe40B20having a thickness of 4 nm. Employed in the cap layer211is, for example, Ta/Ru. A thickness of a Ta layer thereof is, for example, 1 nm. A thickness of a Ru layer thereof is, for example, 5 nm.

Materials similar to those of each of the layers of the strain detection element200A may be employed as materials of each of the layers of the strain detection element200C.

FIG. 11is a schematic perspective view showing another configuration example200D of the strain detection element200. As shown inFIG. 11, the strain detection element2000is configured having stacked therein, sequentially from below: the lower electrode204; the base layer205; a lower pinning layer221; a lower second magnetization fixed layer222; a lower magnetic coupling layer223; a lower first magnetization fixed layer224; a lower intermediate layer225; a magnetization free layer226; an upper intermediate layer227; an upper first magnetization fixed layer228; an upper magnetic coupling layer229; an upper second magnetization fixed layer230; an upper pinning layer231; the cap layer211; and the upper electrode212. The lower first magnetization fixed layer224and the upper first magnetization fixed layer228correspond to the second magnetic layer202. The magnetization free layer226corresponds to the first magnetic layer201. Moreover, the lower electrode204is connected to, for example, the wiring line C1(FIG. 1), and the upper electrode212is connected to, for example, the wiring line C2(FIG. 1).

Employed in the base layer205is, for example, Ta/Ru. A thickness (length in a Z axis direction) of a Ta layer thereof is, for example, 3 nanometers (nm). A thickness of a Pu layer thereof is, for example, 2 nm. Employed in the lower pinning layer221is, for example, an IrMn layer having a thickness of 7 nm. Employed in the lower second magnetization fixed layer222is, for example, a Co75Fe25layer having a thickness of 2.5 nm. Employed in the lower magnetic coupling layer223is, for example, a Ru layer having a thickness of 0.9 nm. Employed in the lower first magnetization fixed layer224is, for example, a Co40Fe40B20layer having a thickness of 3 nm. Employed in the lower intermediate layer225is, for example, a MgO layer having a thickness of 1.6 nm. Employed in the magnetization free layer226is, for example, Co40Fe40B20having a thickness of 4 nm. Employed in the upper intermediate layer227is, for example, a MgO layer having a thickness of 1.6 nm. Employed in the upper first magnetization fixed layer228is, for example, Co40Fe40B20/Fe50Co50. A thickness of a Co40Fe40B20layer thereof is, for example, 2 nm. A thickness of an Fe50Co50layer thereof is, for example, 1 nm. Employed in the upper magnetic coupling layer229is, for example, a Ru layer having a thickness of 0.9 nm. Employed in the upper second magnetization fixed layer230is, for example, a Co75Fe25layer having a thickness of 2.5 nm. Employed in the upper pinning layer231is, for example, an IrMn layer having a thickness of 7 nm. Employed in the cap layer211is, for example, Ta/Ru. A thickness of a Ta layer thereof is, for example, 1 nm. A thickness of a Ru layer thereof is, for example, 5 nm.

Materials similar to those of each of the layers of the strain detection element200A may be employed as materials of each of the layers of the strain detection element200).

FIG. 12is a schematic perspective view showing one configuration example200E of the strain detection element200. As shown inFIG. 12, the strain detection element200E is configured having stacked therein, sequentially from below: the lower electrode204; the base layer205; a first magnetization free layer241(the first magnetic layer201); the intermediate layer203; a second magnetization free layer242(the second magnetic layer202); the cap layer211; and the upper electrode212. The first magnetization free layer241corresponds to the first magnetic layer201. The second magnetization free layer242corresponds to the second magnetic layer202. Moreover, the lower electrode204is connected to, for example, the wiring line C1(FIG. 1), and the upper electrode212is connected to, for example, the wiring line C2(FIG. 1).

Employed in the base layer205is, for example, Ta/Cu. A thickness (length in a Z axis direction) of a Ta layer thereof is, for example, 3 nm. A thickness of a Cu layer thereof is, for example, 5 nm. Employed in the first magnetization free layer241is, for example, Co40Fe40B20having a thickness of 4 nm. Employed in the intermediate layer203is, for example, Co40Fe40B20having a thickness of 4 nm. Employed in the cap layer211is, for example, Cu/Ta/Ru. A thickness of a Cu layer thereof is, for example, 5 nm. A thickness of a Ta layer thereof is, for example, 1 nm. A thickness of a Ru layer thereof is, for example, 5 nm.

Materials similar to those of each of the layers of the strain detection element200A may be employed as materials of each of the layers of the strain detection element200E. Moreover, a material similar to that of, for example, the magnetization free layer210of the strain detection element200A (FIG. 6) may be employed as materials of the first magnetization free layer241and the second magnetization free layer242.

Advantages of First Embodiment

The membrane120(vibrating portion121and supported portion122) of the first embodiment are each formed by an oxide that includes aluminum (Al) (as an example, aluminum oxide). As previously mentioned, the hollow portion111is formed by carrying out etching on the substrate110to process the substrate110until the membrane120is exposed. However, there is a problem that if at that time, the membrane120gets etched, then a film thickness of the exposed membrane120ends up differing by place depending on a degree of the etching, whereby desired characteristics for the membrane120cannot be obtained, leading to lowering of precision of the pressure sensor110A. This problem will be explained with reference toFIGS. 13 to 17.

FIG. 13Ais a schematic view showing a problem in a processing step of the hollow portion111of the pressure sensor110A of the first embodiment. For simplification of illustration, only the detection element200is displayed on the membrane120, and wiring lines, and so on, are not displayed.

The hollow portion111is formed by etching the substrate110by a RIE method. During processing, etching proceeds by an etching gas72and the substrate110coming into contact and causing a chemical reaction.

Processing of the hollow portion111proceeds, and as a depth of the hollow portion111increases, a difference occurs in ease-of-reach of the etching gas72at the bottom of the hollow portion111. Generally, it becomes more difficult for the etching gas72to reach an edge, compared to a central portion, of the hollow portion111.

Because a difference occurs in ease-of-reach of the etching gas72at the bottom of the hollow portion111during processing of the hollow portion111as described above, a difference also occurs in speed of etching depending on a position in the bottom of the hollow portion111. As a result, if, for example, it is more difficult for the etching gas72to reach the edge compared to the central portion at the bottom of the hollow portion ill, then, as shown inFIG. 13B, a film thickness Tc of the central portion of the membrane120after hollow portion111processing becomes thin compared to a film thickness Te of the edge thereof.

The supported portion222at the edge of the membrane120is fixed at an upper surface of the substrate110, hence, as shown inFIG. 14, when an applied pressure80is applied from a hollow portion111side and the central portion of the membrane120deforms in a convex shape, that edge deforms in a concave shape. Therefore, an inclination of a force applied to the strain detection element200by a change in shape of the membrane120inverts bounded by a point120c. A force Ps shows a large value in a narrow range from a boundary point120dof the membrane120and the substrate110to the point120c. Furthermore, there is a distribution in magnitude of the force applied to the strain detection element200by deformation of the membrane120even between the point120dand the point120c, and there exists an extremely narrow region120ewhere the force becomes greatest.

The strain detection element200of the present embodiment has a smaller volume compared to an ordinary strain detection element using a piezo element, hence has excellent spatial resolution. Therefore, as shown inFIG. 14, the strain detection element200can be disposed pinpointedly on the region120ebetween the point120cand the point120dwhere a value of the force applied to the strain detection element200becomes large on the membrane120, whereby performance of the strain detection element200can be used to a maximum and sensitivity of the pressure sensor can be raised.

As mentioned above, superior performance can be shown by using a strain detection element employing spin technology than in the case of using a conventional piezoelectric element as a strain detection element. However, the technology of the present invention that employs aluminum oxide in a membrane bending by a pressure displays an improving effect even when a piezoelectric element is used. Specifically, it is possible for an element in which a voltage is generated by a polarization effect of electrons of an insulating material when a strain is applied in the manner of PZT, AlN, and so on, to be employed on a membrane120of embodiments of the kinds ofFIGS. 18, 22, and23, as a piezoelectric element. In this case also, the membrane120of aluminum oxide of the present invention shows an improving effect.

It is possible to know which portion on the membrane120corresponds to the region120e, by theoretical calculation. When performing the theoretical calculation, a structure in which film thickness is uniform is employed in a model of the membrane120. However, in reality, a distribution exists in film thickness of the membrane120as inFIG. 13B. If the actual shape of the membrane120at this time differs greatly from the model used in calculation, then a position of the region120eon the membrane120gets misaligned from a position derived by the theoretical calculation. As a result, performance of the strain detection element200cannot be used to a maximum, and it ends up being impossible for the pressure sensor of the embodiment to sufficiently extract the performance. It is therefore necessary to bring the shape of the membrane120closer to a state where film thickness is uniform, that is, close to the model of the theoretical calculation. As an example, a ratio (Tc/Te) of minimum film thickness Tc and maximum film thickness Te of the membrane120can be set to, for example, 0.9 or more, and preferably 0.95 or more.

In order to render the shape of the membrane120into a shape where film thickness is uniform, tolerance of the membrane120to PIE during formation of the hollow portion111must be raised.FIGS. 15A and 15Bshow a manufacturing step when etching the substrate110to form the hollow portion111.

As shown inFIG. 15A, when processing the substrate110by RIE and forming the hollow portion111, the tolerance to RIE of the membrane120is low, hence a depth of the hollow portion111due to a difference in ease-of-reach of the etching gas ends up differing greatly by position.FIG. 15Ashows as an example the case where it is easier for the etching gas72to reach the central portion of the hollow portion111than the edge of the hollow portion111, as a result of which etching is faster and the depth of the hollow portion111increases more in the central portion than at the edge of the hollow portion111. At this time, a difference between a depth of a shallowest portion and a depth of a deepest portion of the hollow portion111is assumed to be hc1.

For example, even if etching has reached a lower surface of the membrane120at the central portion of the hollow portion111, a residual portion111R must be removed by etching for the vibrating portion121to achieve its function. However, as shown inFIG. 15B, when it is attempted to remove this residual portion111F by etching, the membrane120close to the central portion of the hollow portion111also ends up being partially etched, besides the residual portion111R. That is, the film thickness of the membrane120is not uniform, and a film thickness difference hc3occurs according to position. As previously mentioned, this is undesirable from a viewpoint of sensitivity of the pressure sensor.

Accordingly, in the present embodiment, the membrane120(vibrating portion121and supported portion122) are each configured as a single film formed by an oxide that includes aluminum (Al) (as an example, aluminum oxide (AlOx). The oxide that includes aluminum has a high etch selectivity with respect to silicon. When the membrane120is formed by an oxide that includes single aluminum, the thickness of the membrane can be set to not less than 100 nm and not more than 2 μm.

FIG. 16Ais a table showing etch selectivity with respect to silicon. In the case where etching employing RIE is performed under the same conditions on silicon and a sample A, when an etching amount of the sample A is 1/X times that of silicon, the etch selectivity with respect to silicon of the sample A is assumed to be X. When the etch selectivity with respect to silicon is defined in the above manner, the etch selectivity with respect to silicon of a silicon oxide film (SiOx) and aluminum oxide (AlOx) are as shown inFIG. 16A. As shown inFIG. 16A, aluminum oxide shows a high etch selectivity of 1050 with respect to silicon.

As a result, in the case that the membrane120is configured by aluminum oxide, the film thickness of the membrane120is maintained substantially uniformly upward of the hollow portion111, even when the residual portion111R is removed by etching and etching for forming the hollow portion111is performed in the region R1until the membrane120is exposed. As a result, the film thickness of the membrane120can be set to a value as designed, and sensitivity of the pressure sensor110A can be improved. Moreover, the membrane120formed by aluminum oxide has a high tolerance also in etching for forming the strain detection element200formed on the membrane120, hence planarization of an upper surface is secured, whereby uniformity of film thickness of the membrane120is maintained. Therefore, performance of the strain detection element200can be used to a maximum and sensitivity of the pressure sensor110A can be raised.

As shown inFIGS. 3A to 3D, sometimes, a plurality of the strain detection elements200are disposed on the membrane120of the pressure sensor110A. As a result, improvement of SNR can be achieved as previously mentioned. Electrically connecting a plurality of N of the strain detection elements200in series or in parallel in this way enables an improving effect of 20 log√N to be obtained for SNR. Sensitivity of the pressure sensor310A can be further raised compared to when a single strain detection element200is disposed. When it is attempted to raise sensitivity of the pressure sensor by this method, there is a need to align outputs from the disposed individual strain detection elements200, that is, uniformly align performance of the disposed individual strain detection elements200. In this respect also, the membrane120formed by aluminum oxide and capable of having the film thickness of the entire membrane120aligned uniformly is well matched to the pressure sensor110A.

FIG. 16Bis a schematic view for explaining a device for evaluating sensitivity to applied pressure of the vibrating portion121of the membrane220, and an evaluation method thereby.FIG. 16Bshows a schematic configuration of the device for evaluating sensitivity of the vibrating portion121. The pressure sensor110A is fixed on a plate M2. The plate M2has a hole M21of about the same size as the vibrating portion121opened therein, and the pressure sensor110A is fixed such that the hollow portion111of the pressure sensor110A comes above the hole M21. The plate M2to which the pressure sensor110A is fixed is attached to a measurement jig M1. The plate M2configures a lid of the measurement jig M1, and an airtight hollow portion M11can be made by attaching the plate M2. At this time, the plate M2is attached to the measurement jig M1such that the pressure sensor110A attached to the plate M2is present on an opposite surface to the hollow portion M11.

A pressure generator (not illustrated) is attached to the hollow portion M11, and an applied pressure80of a set magnitude can be generated within the hollow portion M11. The applied pressure80is applied also to the vibrating portion121of the pressure sensor110A linked to the hollow portion M11via the hole M21. The shape of the membrane120changes due to the applied pressure80being applied to the vibrating portion121. This change in shape of the membrane120is measured using a laser microscope M3provided directly above the pressure sensor110A.FIG. 16Cis a schematic view of the change in shape of the membrane120when the applied pressure80is applied. The vibrating portion121bends due to the applied pressure80being applied to the membrane120via the hollow portion M11. At this time, a displacement amount D in a direction perpendicular to the membrane120(Z axis direction) from an initial state when the applied pressure30is not applied, of the centroid120P1of the vibrating portion121, is measured by the laser microscope M3. When sensitivity to applied pressure of the vibrating portion121is good, a value of the displacement amount D is large even when magnitude of the applied pressure80is small. Moreover, when a value of the applied pressure80is changed in a small range, the change in value of the displacement amount D is also large.

It will be described byFIGS. 17A to 17Dhow using an oxide that includes aluminum as the membrane120is effective for the pressure sensor110A.

FIG. 17Ais actual image data showing a measurement result by the laser microscope M3in the initial state where the applied pressure80from external is not applied, in the case that sputtering-deposited aluminum oxide (AlOx) is employed as the material of the membrane120. Residual stress of the membrane120before processing of the hollow portion111is adjusted to an appropriate value and a circle is adopted as the shape of the vibrating portion121. Moreover, the diameter of the vibrating portion121is set to 530 μm, and the thickness of the membrane120is set to 500 nm. Note that for simplicity,FIG. 17Ashows a membrane120not having the likes of the strain detection element200or electrodes connected to the strain detection element200disposed thereon. InFIG. 17A, the inside of the circular portion corresponds to the vibrating portion121, and the outside of the circular portion corresponds to the supported portion122.

FIG. 17Bis a view showing by color contrast a height distribution in a vertical direction (Z axis direction) of the membrane120shown in the image data ofFIG. 17A. It is found from the fact that color ofFIG. 17Bis uniform, that the membrane120is flat in the initial state. As will be mentioned later, sometimes, when a large bending occurs in the membrane120in the initial state, the strain detection element200cannot sufficiently display its performance.

FIG. 17Cshows a result of measuring a change in shape in the B-B′ cross-section ofFIG. 17Aby the laser microscope M3, in the case that values of pressure80applied to the membrane120are adjusted to −10 kPa, −5 kPa, −1 kPa, −0.5 kPa, 0 kPa, 0.5 kPa, 1 kPa, 5 kPa, and 10 kPa. It is found that shapes of the film to left and right bounded by the centroid120P1of the membrane120are equal, and forces applied to the strain detection element200disposed at the edge of the vibrating portion121when the vibrating portion121is deformed are equal.

FIG. 17Dis a graph assuming the horizontal axis to be the applied pressure80and the vertical axis to be the displacement amount D of the centroid120P1of the membrane120, in the case ofFIG. 17C. It is found from this graph that the displacement amount D of the centroid120P1of the vibrating portion121shows a steep change in a small range of the applied pressure80from external. In other words, the membrane120responds to a change in applied pressure with good sensitivity. Displacement inclination (m/kPa) as a change in the displacement amount. D per unit applied pressure is defined as an index of steepness of change of the displacement amount D.

The membrane120shown inFIG. 17Ahas a displacement inclination of 3.0 μm/kPa in a range of applied pressure of −0.5 kPa to 0.5 kPa. When the device of the present invention is employed as an acoustic sensor and microphone, the pressure range used is a smaller range, hence it becomes possible to have an even larger displacement inclination in such a pressure range and detect a faint sound with high sensitivity.

It is found from the measurement results shown inFIGS. 17C and 17Dthat when an oxide that includes aluminum is employed as the membrane120, it becomes possible to produce a pressure sensor110A having a membrane in which bending in the initial state is small, moreover in which the shape of the film when bending has occurred is symmetrical, and which responds with good sensitivity to an applied pressure.

Second Embodiment

Next, a pressure sensor according to a second embodiment will be described with reference toFIG. 18. The pressure sensor of this second embodiment has a configuration of the membrane120which differs from that of the first embodiment. Other configurations are similar to those of the first embodiment. InFIG. 18, configurations identical to those of the first embodiment are assigned with reference symbols identical to those assigned in the first embodiment, and detailed descriptions thereof will be omitted below.

FIG. 18is a schematic cross-sectional view of the A-A′ cross-section ofFIG. 1. As shown inFIG. 18, the membrane120is formed by a three-layer structure of a first film131positioned on a strain detection element200side, a second film133positioned on a substrate120side, and an intermediate film132between the first film131and the second film133. As will be mentioned later, adopting such a three-layer structure makes it possible to provide a flat membrane120in which bending does not occur in the initial state when an applied pressure from external is not applied. In a more preferable embodiment, a difference between a film thickness of the first film131and a film thickness of the second film133is set to a certain value or less, from a viewpoint of suppression of residual stress.

The first film131and the second film133are both formed by an oxide that includes aluminum (Al). In the first embodiment, the entire membrane120is formed by an oxide that includes aluminum, but in this second embodiment, only an upper surface and a lower surface of the membrane120are formed by an oxide that includes aluminum. Since the first film131(upper surface of the membrane120) and the second film133(lower surface of the membrane120) are configured from an oxide that includes aluminum, the pressure sensor110A of the second embodiment can secure uniformity of film thickness of the membrane120and improve precision of the pressure sensor110A, similarly to in the above-mentioned advantages of the first embodiment. Moreover, in the case of the second embodiment, physical properties such as Young's modulus or Poisson coefficient of the membrane120can be controlled to preferable values for the pressure sensor110A by choosing a material of the intermediate film132. Note that film thicknesses of the first film131and the second film133may be set to not less than 10 nm and not more than 300 nm. In this case, the film thicknesses may preferably be set to not less than 30 nm and not more than 150 nm.

The intermediate film132can be formed from at least one material selected from the group of an oxide that includes silicon and a nitride that includes silicon, in addition to the oxide that includes aluminum. Besides these, an organic material such as a polymer material may also be used as the material of the intermediate film132. Examples of the polymer material include the following. For example, the following can be employed, namely acrylonitrile butadiene styrene, a cyclo olefin polymer, elastic ethylene propylene, a polyamide, a polyamide imide, polybenzimidazole, polybutylene terephthalate, a polycarbonate, polyethylene, polyethylene ether ketone, a polyetherimide, polyethylene imine, polyethylene naphthalene, polyester, polysulfone, polyethylene terephthalate, phenol formaldehyde, a polyimide, polymethyl methacrylate, polymethyl pentene, polyoxymethylene, polypropylene, m-phenyl ether, poly p-phenyl sulfide, a p-amide, polystyrene, polysulfone, polyvinyl chloride, polytetrafluoroethylene, perfluoroalkoxy, ethylene propylene fluoride, polytetrafluoroethylene, poly ethylene tetrafluoroethylene, polyethylene chlorotrifluoroethylene, polyvinylidene fluoride, melamine formaldehyde, a liquid crystal polymer, or urea formaldehyde. A film thickness of the intermediate film132may be set to not less than 100 nm and not more than 1 μm. In this case, the film thickness may preferably be set to not less than 150 nm and not more than 800 nm.

Note that a buffer film, or the like, illustration of which is omitted, may be interposed between the intermediate film132and the first film131or second film133. Moreover, the intermediate film132is sometimes a single-layer film and is sometimes a film having a stacked structure.

The overall thickness t1of the membrane120may be set to, for example, not less than 50 nanometers (nm) and not more than 3 micrometers (μm). In this case, the overall thickness t1may preferably be set to not less than 300 nm and not more than 1.5 μm.

FIG. 19is a schematic view showing film thicknesses h1, h2, and h3of the first film131, the intermediate film132, and the second film133configuring the membrane120, and residual stresses σ1, σ2, and σ3of the first film131, the intermediate film132, and the second film133. For simplification of description,FIG. 19shows a state after the hollow portion111has been formed, but the residual stresses σ1, σ2, and σ3are residual stresses respectively occurring in the first film131, the intermediate film132, and the second film133, before formation of the hollow portion111. In order to apply a large strain to the strain detection element200with respect to a pressure from external and raise sensitivity of the pressure sensor110A, it is desirable for a value of the residual stress σ of the membrane120to be close to 0 MPa. An average residual stress σave of the membrane120configured from the stacked structure is calculated by the formula below using the film thicknesses h1to h3and the residual stresses σ1to σ3of the first film131, the intermediate film132, and the second film133.
σave=(h1*σ1+h2*σ2+h3*σ3)/(h1+h2+h3)  Mathematical Expression 11

When the oxide that includes aluminum is deposited by sputtering to form the first film131and the second film133, the residual stresses σ1and σ3of the first film131and the second film133can be controlled by adjusting a pressure of a sputter gas. At this time, the first film131and the second film133are deposited as amorphous aluminum oxide.

Note that the first film131undergoes etching due to milling for processing of the strain detection element200positioned above the first film131, while the second film133undergoes etching due to a RIE method at a time of processing the hollow portion111. If the film thickness h1of the first film131and the film thickness h3of the second film133have ended up changing due to the etching, then a value of the average residual stress σave of the membrane120as understood from the formula [Mathematical Expression 1] ends up changing.

However, the first film131and the second film133formed by the oxide that includes aluminum (Al) have a strong tolerance to milling and RIE, hence thickness of the films does not change before and after a manufacturing process. As a result, adopting a structure in which the intermediate film132is sandwiched by the first film131and the second film133as shown inFIG. 18makes it possible for the value of the average residual stress σave of the membrane120to be easily controlled.

A reason why bending of the membrane120in the initial state can be suppressed by the above-described three-layer structure will be described below with reference toFIG. 20. In the description below referring toFIG. 20, values of the residual stress a are expressed as follows. That is, a residual stress a when a tensile residual stress occurs in the membrane120is expressed as a positive value, and conversely, a residual stress a when a compressive residual stress occurs in the membrane120is expressed as a negative value, with 0 MPa therebetween. Note thatFIG. 20shows the shape in the initial state of the membrane120when the pressure from external is not applied to the membrane120.

In the case that there is a distribution of residual stress along the Z axis direction (a direction normal to the membrane120) in the membrane120before processing/formation of the hollow portion111, a moment acting in a direction causing the residual stress σ of the membrane120to increase is generated after processing/formation of the hollow portion111.

First, as shown inFIG. 20A, consideration is given to the case where a difference in thickness of the film thickness h1of the first film131and the film thickness h3of the second film133is large, and a distribution causing the residual stress of the membrane120to increase occurs along the Z axis direction (a direction from the hollow portion111side toward the strain detection element200side). In this case, as shown inFIG. 20A, a moment M1which is upwardly inclined in the Z direction is generated in the membrane120.

FIG. 20Ashows as an example the case of h1>>h3and σ1>σ2. Moreover, it is also assumed that since h1>>h3, there is no contribution from the residual stress σ3.

Since the membrane120has the upwardly inclined moment M1, the membrane120has a convex shape with a large bending65ain the initial state. As a result, a large compressive force Ps is applied to the strain detection element200.

Next, as shown inFIG. 20B, consideration is given to the case where the difference in thickness of the film thickness h1of the first film131and the film thickness h3of the second film133is large, and a distribution causing the residual stress of the membrane120to decrease occurs along the Z axis direction (direction from the hollow portion111side toward the strain detection element200side). In this case, as shown inFIG. 208B, a moment M2which is downwardly inclined in the Z direction is generated in the membrane120.FIG. 20Bshows as an example the case of h1>>h3and σ1<σ2. Moreover, it is also assumed that since h1>>h3, there is no contribution from the residual stress σ3. Since the membrane120has the downwardly inclined moment M2, the membrane120has a concave shape with a large bending65bin the initial state. As a result, a large tensile force P1is applied to the strain detection element200.

When large forces Ps and P1are applied to the strain detection element200from the initial state, a change in magnetization of the magnetic layer due to the magnetostriction effect is not sufficiently caused and sensitivity of the pressure sensor110A does not rise, even when the value of the residual stress σ of the membrane120is small and sensitivity of the membrane120to a pressure from external is good.

Next, referring toFIG. 20C, consideration is given to the case where the difference in thickness of the film thickness h1of the first film131and the film thickness h3of the second film133is small, the residual stress a increases as the hollow portion111side is approached from the intermediate film132of the membrane120, and the residual stress c increases as the strain detection element200side is approached from the intermediate film132of the membrane120. In this case, a moment M3and a moment M4respectively caused by the residual stress σ1of the first film131and the residual stress σ2of the intermediate film132, and the residual stress σ3of the second film133and the residual stress σ2of the intermediate film132, are generated in the membrane120.FIG. 20Cshows as an example the case of σ1>σ2and σ3>σ2. When σ1<σ2and σ3<σ2, respective inclinations of the moment M3and the moment M4are inversed. Since the moments M3and M4are generated in directions that cancel each other out, bending in the initial state of the membrane120is suppressed. Therefore, a force applied to the strain detection element200in the initial state is configured to be minute.

Note that some of the strain detection elements200may obtain highest sensitivity when a tensile or compressive force is not applied in the initial state where a pressure from external is not applied, and some other strain detection elements200may obtain highest sensitivity when a minute tensile or compressive force is applied. This depends on the thickness or material of the film configuring the strain detection element200.

One method of applying a minute force to the strain detection element200in the initial state is to provide minute bending to the membrane120in the initial state. When the membrane120has a substantially symmetrical three-layer structure in the Z axis direction as inFIG. 18, magnitudes of each of the moments M3and M4can be finely adjusted by adjusting magnitudes of the film thicknesses h1, h2, and h3, whereby magnitude of bending in the initial state of the membrane120can be controlled with good precision.

The pressure sensor of the embodiment undergoes annealing for fixing of magnetization of the magnetic layer in a manufacturing process. In the case of different thermal expansion coefficients, thermal stresses are generated at an interface between the first film131and the intermediate film132or at an interface between the intermediate film132and the second film133. Influence on the initial state of the membrane120due to moments generated from these thermal stresses can also be relieved by providing the membrane120with symmetry in the Z axis direction as inFIG. 13.

Moreover, as shown inFIG. 21A, at the interface between the first film131and the intermediate film132and at the interface between the intermediate film132and the second film133, a third film134or a fourth film135may be newly formed at portions where composition has been modified by migration of an element configuring the membrane120. Values of residual stress occurring in the third film134or the fourth film135may be different to those of the first film131or the second film133. Influence on the initial state of the membrane120due to moments generated as a result of residual stresses of the third film134or the fourth film135can also be relieved by providing the membrane120with symmetry in the Z axis direction as inFIG. 18.

Advantages of Second Embodiment

As described above, in the pressure sensor110A of the second embodiment, the upper surface and the lower surface of the membrane120are configured by an oxide that includes aluminum. Therefore, uniformity of film thickness of the membrane120can be secured and sensitivity of the pressure sensor110A can be improved, similarly to the above-mentioned advantages of the first embodiment. That is, the second film133functions as a stopper film in etching for formation of the hollow portion111, and the first film131functions as a stopper film in etching for sputtering of the strain detection element200.

In addition, by adopting the above-mentioned three-layer structure in the membrane120, the pressure sensor110A of the second embodiment enables physical properties such as residual stress of the membrane120to be controlled, and enables bending of the membrane120in the initial state to be suppressed or adjusted, whereby sensitivity of the pressure sensor can be improved.

Moreover, it will be described with reference toFIGS. 21B to 21Ehow the membrane120configured from the first film131, the intermediate film132, and the second film233of the the second embodiment is effective for the pressure sensor110A. The evaluation device and evaluation method shown inFIG. 16Bare utilized in evaluation of the membrane220.

FIG. 21Bis actual image data showing a measurement result by the laser microscope M3in the initial state where the applied pressure from external is not applied, in the case that sputtering-deposited AlOx is employed as the material of the first film131and the second film133and a CVD (Chemical Vapor Deposition)-deposited SiNx film is employed as the material of the intermediate film132. Residual stress of the film before processing of the hollow portion111is adjusted to an appropriate value and a circle is adopted as the shape of the vibrating portion121. Moreover, the diameter of the vibrating portion121is set to 530 μm, and the film thickness of the first film131is set to 100 nm, the film thickness of the second film133is set to 50 nm, and the film thickness of the intermediate film132is set to 550 nm. Moreover,FIG. 21Bshows a membrane120not having the likes of the strain detection element200or electrodes connected to the strain detection element200disposed thereon. InFIG. 21B, the inside of the circular portion corresponds to the vibrating portion121, and the outside of the circular portion corresponds to the supported portion122.

FIG. 21Cis a view showing by color contrast a height distribution in a vertical direction (Z axis direction) of the membrane120shown in the image data ofFIG. 21E. It is found from the fact that color ofFIG. 21Cis uniform, that the membrane120is flat in the initial state. As previously mentioned, sometimes, when a large bending occurs in the membrane120in the initial state, the strain detection element200cannot sufficiently display its performance.

FIG. 21Dshows a result of measuring a change in shape in the B-B′ cross-section ofFIG. 21Bby the laser microscope M3, in the case that applied voltages of −10 kPa, −5 kPa, −1 kPa, −0.5 kPa, −0.2 kPa, 0 kPa, 0.2 kPa, 0.5 kPa, 1 kPa, 5 kPa, and 10 kPa are applied to the membrane120. It is found that shapes of the film to left and right bounded by the centroid120P1of the membrane120are equal, and forces applied to the strain detection element200disposed at the edge of the vibrating portion121when the vibrating portion121is deformed are equal.

FIG. 21Eis a graph assuming the horizontal axis to be the applied pressure80and the vertical axis to be the displacement amount D of the centroid120P1of the membrane120, in the case ofFIG. 210. It is found from this graph that the displacement amount D of the centroid120P1of the vibrating portion121shows a steep change in a small range of the applied pressure80from external. In other words, the membrane120responds to a change in applied pressure with good sensitivity. The membrane120shown inFIG. 21Bhas a displacement inclination of 3.6 μm/kPa in a range of applied pressure of −0.2 kPa to 0.2 kPa.

It is found from the measurement results shown inFIGS. 21D and 21Ethat in the case of employing a membrane120in which sputtering-deposited AlOx is utilized as the material of the first film131and the second film133and a CVD-deposited SiNx film is employed as the material of the intermediate film132, it becomes possible to produce a pressure sensor110A having a membrane in which bending in the initial state is small, moreover in which the shape of the film when bending has occurred is symmetrical, and which responds with good sensitivity to an applied pressure.

Third Embodiment

Next, a pressure sensor according to a third embodiment will be described with reference toFIG. 22. The pressure sensor of this third embodiment has a configuration of the membrane120which differs from that of the first embodiment. Other configurations are similar to those of the first embodiment. InFIG. 22, configurations identical to those of the first embodiment are assigned with reference symbols identical to those assigned in the first embodiment, and detailed descriptions thereof will be omitted below.

FIG. 22is a schematic cross-sectional view of the A-A′ cross-section ofFIG. 1. As shown inFIG. 22, the membrane120is formed by a two-layer structure of a film133disposed on the substrate110side, and a film132disposed upwardly of the film133. The film133is configured by an oxide that includes aluminum similarly to the film133of the second embodiment, and the film132is configured from a material identical to that: of the intermediate film132of the second embodiment. That is, the membrane120of this third embodiment adopts a configuration in which the first film131is removed from the membrane120of the second embodiment. In other words, in the membrane120of the third embodiment, only a first surface on a side of the substrate110acting as a support member; is configured from the oxide that includes aluminum. Expressing this in yet another way, the membrane120of the third embodiment includes: the first film including the oxide that includes aluminum; and the third film, and the third film is positioned between the first film and the strain detection element. Note that a film thickness of the film133may be set to not less than 10 μm and not more than 300 μm, and more preferably to not less than 20 nm and not more than 200 nm.

Advantages of Third Embodiment

As described above, in the pressure sensor110A of the third embodiment, a lower surface (the film133) of the membrane120is configured by an oxide that includes aluminum. There is no film of an oxide that includes aluminum on an upper surface of the membrane120, hence flatness at the upper surface of the membrane120is somewhat lost, but at the lower surface of the membrane120, the film133can be caused to function as a stopper film in etching for formation of the hollow portion111. Therefore, uniformity of film thickness of the membrane120can be secured and advantages similar to those of the first embodiment can be obtained.

Fourth Embodiment

Next, a pressure sensor according to a fourth embodiment will be described with reference toFIG. 23. The pressure sensor of this fourth embodiment has a configuration of the membrane120which differs from that of the previously mentioned embodiments. Other configurations are similar to those of the previously mentioned embodiments. InFIG. 23, configurations identical to those of the previously mentioned embodiments are assigned with reference symbols identical to those assigned in the previously mentioned embodiments, and detailed descriptions thereof will be omitted below.

FIG. 23is a schematic cross-sectional view of the A-A′ cross-section ofFIG. 1. As shown inFIG. 23, the membrane220is formed by a two-layer structure of a film131on which the strain detection element200is disposed, and a film132disposed downwardly of the film131. The film131is configured by an oxide that includes aluminum similarly to the film131of the second embodiment, and the film132is configured from a material identical to that of the intermediate film132of the second embodiment. That is, the membrane120of this fourth embodiment adopts a configuration in which the second film133is removed from the membrane120of the second embodiment. In other words, in the membrane120of the fourth embodiment, only a second surface on a side of the strain detection element200is configured from the oxide that includes aluminum. Expressing this in yet another way, the membrane120of the fourth embodiment includes: the second film including the oxide that includes aluminum; and the third film, and the second film is positioned between the third film and the strain detection element. Note that a film thickness of the film131may be set to not less than 10 μm and not more than 300 μm, and more preferably to not less than 20 nm and not more than 200 nm.

Advantages of Fourth Embodiment

As described above, in the pressure sensor110A of the fourth embodiment, an upper surface (the film131) of the membrane120is configured by an oxide that includes aluminum. There is no film of an oxide that includes aluminum on a lower surface of the membrane120, hence flatness at the lower surface of the membrane120is somewhat lost, but at the upper surface of the membrane120, the film131can be caused to function as a stopper film in etching for formation of the strain detection element200. Therefore, uniformity of film thickness of the membrane120can be secured and advantages similar to those of the first embodiment can be obtained.

FIG. 24Ashows an example of design of the pressure sensor10A according to the first through fourth embodiments.

FIG. 24Ais an example of the case where a circle is adopted as the shape of the vibrating portion121, and the diameter of the vibrating portion121is designed to be 530 μm. A length of one side of the strain detection element200is 10 μm, and a total of 20 or more, in the illustrated example a total of 30, of the strain detection elements200are disposed, divided into two places, on one vibrating portion121. An electrode124connected to the strain detection element200is routed so as to pass as much as possible over the supported portion122in order not to hinder movement of the vibrating portion121. A shape of a beam123on the vibrating portion121can be changed to match a method of placement of the strain detection elements200, and the beam123is sometimes also removed.

FIG. 24Bis an example of the case where a rectangle is adopted as the shape of the vibrating portion121, and a length of a long side of the vibrating portion121is designed to be 578 μm and a length of a short side of the vibrating portion121is designed to be 376 μm. A length of one side of the strain detection element200is 10 μm, and a total of 30 of the strain detection elements200are disposed in parallel close to the two long sides on the vibrating portion121. An electrode124connected to the strain detection element200is routed so as to pass as much as possible over the supported portion122in order not to hinder movement of the vibrating portion121. A shape of a beam123on the vibrating portion121can be changed to match a method of placement of the strain detection elements200, and the beam123is sometimes also removed.

Note that inFIGS. 24A and 24B, the beam323is provided on the membrane120that bends due to pressure, but these beams123need not be present. The beam120is formed on the membrane120by a material different from that of the membrane120.

FIG. 24Cis a schematic view of a cross-sectional structure of the pressure sensor110A in the case where a single film formed by an oxide that includes aluminum of the kind shown in the first embodiment is used in the membrane120. The lower electrode204and the upper electrode212are disposed such that a current in the Z axis direction (direction perpendicular to the membrane120) flows in the strain detection element200. Parts of the lower electrode204and the upper electrode212are present on the vibrating portion121. Therefore, a material capable of lowering of residual stress is employed in the lower electrode204and the upper electrode212, so as not to hinder movement of the vibrating portion121.

On the other hand, on the supported portion122, a gold pad300is attached to the lower electrode204and the upper electrode212. In order to prevent leak of current, a periphery of the lower electrode204, the upper electrode212, and the strain detection element200is protected by a lower electrode-embedding insulating film303, a strain detection element-embedding insulating film302, an insulating film301surrounding other than a portion contacting the strain detection element200of the upper electrode212, and an insulating film304. In the case that the oxide that includes aluminum forming the membrane120shows insulating properties, a material similar to that of the membrane120can be used in the insulating films301,302,303, and304. In other words, the insulating films301,302,303, and304are also capable of lowering of residual stress. Moreover, it also becomes possible to avoid a problem such as film peeling generated by a difference in materials at an interface of the membrane120and the lower electrode-embedding insulating film303.

Moreover, in order to increase adhesion of the substrate110and the membrane120, an adhesion film305is sometimes provided between the membrane120and the substrate110. The adhesion film305is a thin film, hence is shaved off during hollow portion111processing in a region of the vibrating portion121. Therefore, the adhesion film305never exerts an influence on mechanical characteristics of the vibrating portion121. A magnetic body306is sometimes disposed in a periphery of the strain detection element200. A hard magnetic body of the likes of CoPt, CoCrPt, and FePt is employed as the magnetic body306, as a bias layer for applying to the strain detection element. As a result, stable characteristics as a strain detection element are shown, and it also becomes possible to reduce noise. A preferable embodiment is to set the initial magnetization direction of the strain detection element to be at substantially 45 degrees to a direction of application of stress. Considering also the likes of angular misalignment, and so on, setting to be 30 to 60 degrees represents a realistic example of design.

FIG. 24Dis a schematic view of a cross-sectional structure of the pressure sensor110A in the case of adopting a configuration of the membrane120of the kind shown in the second embodiment. The lower electrode204and the upper electrode212are disposed such that a current in the Z axis direction (direction perpendicular to the membrane120) flows in the strain detection element200. Parts of the lower electrode204and the upper electrode212are present on the vibrating portion121. Therefore, a material capable of lowering of residual stress is employed in the lower electrode204and the upper electrode212, so as not to hinder movement of the vibrating portion121. In order to prevent leak of current, a periphery of the lower electrode204, the upper electrode212, and the strain detection element200is protected by a lower electrode-embedding insulating film303, a strain detection element-embedding insulating film302, an insulating film301surrounding other than a portion contacting the strain detection element200of the upper electrode212, and an insulating film304.

In the case that the oxide that includes aluminum forming the first film131shows insulating properties, a material similar to that of the first film131can be used in the insulating films301,302,303, and304. As a result, it also becomes possible to avoid a problem such as film peeling generated by a difference in materials at an interface of the first film131and the lower electrode-embedding insulating film303. In order to increase adhesion of the substrate110and the membrane120, an adhesion film305is sometimes provided between the membrane120and the substrate110. The adhesion film305is a thin film, hence is shaved off during hollow portion111processing in a region of the vibrating portion121. Therefore, the adhesion film305never exerts an influence on mechanical characteristics of the vibrating portion121. A magnetic body306is sometimes disposed in a periphery of the strain detection element200. A hard magnetic body of the likes of CoPt, CoCrPt, and FePt is employed as the magnetic body306, as a bias layer for applying to the strain detection element. As a result, stable characteristics as a strain detection element are shown, and it also becomes possible to reduce noise. A preferable embodiment is to set the initial magnetization direction of the strain detection element to be at substantially 45 degrees to a direction of application of stress. Considering also the likes of angular misalignment, and so on, setting to be 30 to 60 degrees represents a realistic example of design. An additional element may be added to the above-described hard magnetic body.

Fifth Embodiment

Next, a fifth embodiment will be described with reference toFIG. 25.FIG. 25is a schematic cross-sectional view showing a configuration of a microphone150according to the present embodiment. The pressure sensor110A installed with the strain detection element200according to the first through fourth embodiments can be installed in a microphone, for example.

The microphone150according to the present embodiment includes: a printed board151installed with the pressure sensor110A; an electronic circuit152installed on the printed board151; and a cover153covering the pressure sensor110A and the electronic circuit152along with the printed board151. The pressure sensor110A is a pressure sensor installed with the strain detection element200according to the first through fourth embodiments.

The cover153is provided with an acoustic hole154from which a sound wave155enters. When the sound wave155enters inside the cover153, the sound wave155is detected by the pressure sensor110A. The electronic circuit152passes a current through the strain detection element installed in the pressure sensor110A and detects a change in resistance value of the pressure sensor110A, for example. Moreover, the electronic circuit152may amplify this current value by an amplifier circuit, and so on.

The pressure sensor installed with the strain detection element200according to the first through fourth embodiments has high sensitivity, hence the microphone150installed therewith can perform detection of the sound wave155with good sensitivity.

Sixth Embodiment

Next, a sixth embodiment will be described with reference toFIGS. 26 and 27.FIG. 26is a schematic view showing a configuration of a blood pressure sensor160according to the sixth embodiment.FIG. 27is a schematic cross-sectional view of the blood pressure sensor160as seen from H1-H2. The pressure sensor110A installed with the strain detection element200according to the first through fourth embodiments can be installed in the blood pressure sensor160, for example.

As shown inFIG. 26, the blood pressure sensor160is affixed over an artery166of an arm165of a human, for example. Moreover, as shown inFIG. 27, the blood pressure sensor160is installed with the pressure sensor110A installed with the strain detection element200according to the first through fourth embodiments, whereby blood pressure can be measured.

The pressure sensor110A installed with the strain detection element200according to the first through fourth embodiments has high sensitivity, hence the blood pressure sensor160installed therewith can perform detection of blood pressure continuously with good sensitivity.

Seventh Embodiment

Next, a seventh embodiment will be described with reference toFIG. 28.FIG. 28is a schematic circuit diagram showing a configuration of a touch panel170according to the seventh embodiment. The touch panel170is installed in at least one of an inside or an outside of a display not illustrated.

The touch panel170includes: a plurality of pressure sensors110A disposed in a matrix; a plurality of first wiring lines171disposed in plurality in a Y direction and respectively connected to one ends of a plurality of the pressure sensors110A disposed in an X direction; a plurality of second wiring lines172disposed in plurality in the X direction and respectively connected to the other ends of a plurality of the pressure sensors110A disposed in the Y direction; and a control unit173that controls the plurality of first wiring lines171and the plurality of second wiring lines172. The pressure sensor110A is the pressure sensor according to the first through fourth embodiments.

Moreover, the control unit173includes: a first control circuit174that controls the first wiring line171; a second control circuit175that controls the second wiring line172; and a third control circuit176that controls the first control circuit174and the second control circuit175.

For example, the control unit173passes a current through the pressure sensor110A via the plurality of first wiring lines171and the plurality of second wiring lines172. Now, when a touch surface not illustrated is pressed, the pressure sensor110A has a resistance value of its strain detection element changed according to that pressure. The control unit173specifies a position of the pressure sensor110A where a pressure due to pressing was detected, by detecting this change in resistance value.

The pressure sensor110A installed with the strain detection element200according to the first through fourth embodiments has high sensitivity, hence the touch panel170installed therewith can perform detection of pressure due to pressing with good sensitivity. Moreover, the pressure sensor110A is miniature, and a high resolution touch panel170can be manufactured.

Note that the touch panel170may include a detection element for detecting touch, other than the pressure sensor110A.

Other Application Examples

Application examples of the pressure sensor110A installed with the strain detection element200according to the first through fourth embodiments were described above with reference to specific examples. However, the pressure sensor110A can be applied to a variety of pressure sensor devices, such as an atmospheric pressure sensor or tire air pressure sensor, and so on, in addition to the fifth through seventh embodiments shown.

Moreover, regarding specific configurations of each element such as the membrane, the strain detection element, the first magnetic layer, the second magnetic layer, and the intermediate layer included in the strain detection element200, the pressure sensor110A, the microphone150, the blood pressure sensor160, and the touch panel170, such specific configurations are included in the scope of the present invention provided they can be similarly implemented by a person skilled in the art by appropriately selecting from a publicly-known scope and provided that they allow similar advantages to be obtained.

Moreover, combinations in a technically possible range of two or more elements of each of the specific examples are also included in the scope of the present invention provided that they fall within the spirit of the present invention.

In addition, all strain detection elements, pressure sensors110A, microphones150, blood pressure sensors160, and touch panels170capable of being implemented by appropriate design change by a person skilled in the art based on the strain detection element, pressure sensor110A, microphone150, blood pressure sensor160, and touch panel170mentioned above as embodiments of the present invention also belong to the scope of the present invention provided that they fall within the spirit of the present invention.