WO2012165110A1 - Ferroelectric film and piezoelectric element provided therewith - Google Patents

Abstract

A ferroelectric film is produced from perovskite crystals. The crystalline orientation is a <100> main orientation, and rhombohedral crystals and tetragonal crystals are mixed. In the tetragonal crystals, an a-axis orientation and a c-axis orientation are mixed. When a function (F) indicating a <100> peak by the 2θ/θ measurement of X-ray diffraction is divided into functions (F1, F2, F3) indicating the diffraction intensities of X rays incident on the tetragonal crystals in the a-axis orientation, the tetragonal crystals in the c-axis orientation, and the rhombohedral crystals, respectively, the ratio of the integral value of the function (F3) corresponding to the rhombohedral crystals to the total of the integral values of the three functions (F1, F2, F3) is 0.2-0.8 inclusive.

Description

Ferroelectric film and piezoelectric element including the same

The present invention relates to a ferroelectric film made of a perovskite crystal and a piezoelectric element provided with the ferroelectric film.

In recent years, piezoelectric materials such as Pb (Zr, Ti) O ₃ have been used as electromechanical transducers for application to drive elements and sensors. Such a piezoelectric body is expected to be applied to a MEMS (Micro Electro Mechanical Systems) element by being formed as a thin film on a substrate such as Si.

In the manufacture of MEMS elements, high-precision processing using semiconductor process technology such as photolithography can be applied, so that the elements can be miniaturized and densified. In particular, the cost can be greatly reduced by manufacturing the elements at a high density on a relatively large Si wafer having a diameter of 6 inches or 8 inches, compared to single wafer manufacturing in which the elements are individually manufactured. it can.

In addition, by making the piezoelectric thin film and making the device MEMS, the mechanical and electrical conversion efficiency is improved, and new added value such as improved sensitivity and characteristics of the device is also created. For example, in a thermal sensor, it is possible to increase measurement sensitivity by reducing thermal conductance due to MEMS, and in an inkjet head for a printer, high-definition patterning can be achieved by increasing the density of nozzles.

As a material for the piezoelectric thin film, a crystal made of Pb, Zr, Ti, and O called PZT is often used. Since PZT exhibits a good piezoelectric effect when it has an ABO ₃ type perovskite structure shown in FIG. 8, it needs to be a perovskite single phase. The shape of the unit cell of the PZT crystal having the perovskite structure varies depending on the ratio of Ti and Zr, which are atoms entering the B site. That is, when Ti is large, the crystal lattice of PZT is tetragonal, and when Zr is large, the crystal lattice of PZT is rhombohedral. When the molar ratio of Zr to Ti is around 52:48, both of these crystal structures exist, and the phase boundary that takes such a composition ratio is called MPB (Morphotropic Phase Boundary). In this MPB composition, since the maximum of piezoelectric characteristics such as piezoelectric constant, polarization value, and dielectric constant can be obtained, a piezoelectric thin film having an MPB composition is actively used.

When a piezoelectric thin film is used as a MEMS drive element, the piezoelectric thin film must be formed with a thickness of 3 to 5 μm in order to satisfy the required displacement generation force. In order to form a piezoelectric thin film on a substrate such as Si, a chemical film-forming method such as a CVD method, a physical method such as a sputtering method or an ion plating method, and a liquid phase growth method such as a sol-gel method are known. It is important to find the conditions for obtaining a perovskite single phase film according to the film forming method.

By the way, in recent years, a MEMS driving element with higher density and higher output has been demanded. In order to realize such a drive element, a ferroelectric film is required that can obtain a value (absolute value) of 150 [pm / V] or more as the value (absolute value) of the piezoelectric constant d ₃₁ that is an index of piezoelectric characteristics. It is done. Note that the piezoelectric constant d ₃₁ can be calculated by the following formula 1, which is disclosed in Non-Patent Document 1, for example.

Therefore, in order to realize a ferroelectric film having high piezoelectric characteristics, studies are being made to develop piezoelectric characteristic improvement measures used in bulk into a thin film. One way to improve this piezoelectric property is to control the polarization domain and use non-180 ° domain rotation. This will be described in detail below.

Perovskite crystal materials such as PZT are ferroelectrics having spontaneous polarization Ps when no voltage is applied. The polarization domain refers to a region where the directions of spontaneous polarization Ps are aligned. The direction that the spontaneous polarization Ps can take varies depending on the shape of the unit cell of the crystal. If it is tetragonal, it is the <100> axial direction, and if it is rhombohedral, it is the <111> axial direction. Note that the <100> axial direction collectively refers to [100], [010], [001] and a total of six equivalent directions opposite to these directions. The <111> axis direction collectively refers to [111], [−111], [1-11], [−1-11], and a total of eight equivalent directions opposite to these directions. . 9A shows the direction of tetragonal spontaneous polarization Ps, and FIG. 9B shows the direction of rhombohedral spontaneous polarization Ps. Among them, particularly in the tetragonal crystal, a domain composed of the spontaneous polarization Ps in the [100] direction and the [010] direction and the opposite direction thereof is called an a-axis orientation domain, and the spontaneous polarization in the [001] direction and the opposite direction thereof. A domain composed of Ps is called a c-axis orientation domain.

Also, the boundary between domains is called a domain wall. There are two types of domain walls: 180 ° domain walls and non-180 ° domain walls. The former 180 ° domain wall is the boundary between a domain in one direction and a domain in its inverted direction. On the other hand, the latter non-180 ° domain wall is a boundary between domains in directions other than 180 °. As an example of the non-180 ° domain wall, a domain wall in which an a-axis orientation domain and a c-axis orientation domain are adjacent to each other in a tetragonal crystal can be given. The domain wall at this time becomes a boundary between domains whose orientation directions are different from each other by 90 °.

10 shows a piezoelectric strain ΔX1 when an electric field is applied in the c-axis direction to a tetragonal c-axis orientation domain, and the same electric field in the c-axis direction with respect to a tetragonal a-axis orientation domain. The piezoelectric strain ΔX2 when applied is shown. In the figure, a thick black arrow indicates the polarization direction (the same applies to other drawings). When an electric field is applied to the a-axis orientation domain in the c-axis direction, the polarization direction of the a-axis orientation domain changes to the c-axis direction, causing a phenomenon that the domain direction rotates by 90 °. At this time, the 90 ° domain rotation from the a-axis direction to the c-axis direction is larger than the piezoelectric strain ΔX1 (normal piezoelectric strain) when an electric field is applied to the c-axis orientation domain in the c-axis direction. Since ΔX2 is generated, if such non-180 ° domain rotation can be efficiently generated, the piezoelectric characteristics can be improved.

However, the energy required to generate distortion due to non-180 ° domain rotation is higher than that in the case of generating normal piezoelectric distortion, and even if a single domain a-axis oriented crystal is formed, With the device drive voltage, non-180 ° domain rotation cannot be caused. The reason is as follows.

Non-180 ° domain rotation is considered to occur by the following mechanism. As shown in FIG. 11, it is assumed that a domain 101 whose polarization direction is in the direction of electric field application and a non-180 ° domain 102 in a different direction are in contact with each other by a non-180 ° domain wall W. . When an electric field is applied, first, the domain 101 generates a normal piezoelectric strain, and is dragged by this strain, so that the polarization direction of the adjacent non-180 ° domain 102 is rotated in the electric field application direction via the non-180 ° domain wall W. At this time, starting from the non-180 ° domain wall W, the polarization direction of the crystal in the non-180 ° domain 102 adjacent to the non-180 ° domain wall W rotates one after another, and the non-180 ° domain wall W moves. Domain rotation occurs.

Therefore, in bulk, by introducing a large number of domains having spontaneous polarization Ps in different directions and reducing the size of each domain, the density of non-180 ° domain walls that are the starting point of non-180 ° domain rotation is increased, Non-180 ° domain rotation is caused to improve the piezoelectric characteristics.

Actually, in PZT having a bulk MPB composition, tetragonal crystals and rhombohedral crystals are mixed in a relatively large crystal grain having a particle size of several μm, so that non-180 in a region where the same crystal structure is gathered. In addition to the domain, the domain in the <100> direction and the domain in the <111> direction are mixed in a complex manner between the regions, the domain size is reduced, and the density of the non-180 ° domain walls is increased. As a result, it is considered that non-180 ° domain rotation occurs efficiently and good piezoelectric characteristics can be obtained.

As shown in FIG. 12, the piezoelectric distortion is reversibly generated by applying / not applying an electric field, and non-180 ° domain rotation applies an electric field over a certain level (for example, 400 V / mm or more in FIG. 12). Occurs. At this time, the sum of the normal piezoelectric strain (true piezoelectric strain) due to the c-axis orientation domain and the piezoelectric strain due to non-180 ° domain rotation is observed as the total piezoelectric strain (electric field induced strain amount).

However, in general, a thin film has a crystal grain size smaller than that of a bulk except in special cases such as an epitaxial film. For example, in a ferroelectric film formed on Si via a lower electrode such as Pt or Ir, the crystal has a columnar structure and the average crystal grain size is about 100 to 500 nm. If the crystal grain size is small, as shown in Non-Patent Document 2, the number of domains entering the crystal is reduced, and the number of domain walls is also reduced accordingly, which causes non-180 ° domain rotation. It becomes difficult. As a result, it becomes difficult to improve the piezoelectric characteristics.

The relationship between the crystal grain size and the number of domain walls will become clearer with reference to the following drawings. FIG. 13A is a plan view showing a set of a plurality of crystal grains having different grain sizes, and FIGS. 13B and 13C are small grain sizes included in the plurality of crystal grains. FIGS. 13D and 13E are a plan view and a cross-sectional view of a crystal grain 201 (for example, a grain size of 0.3 μm or less). FIGS. 13D and 13E show a crystal grain 202 having a large grain size included in the plurality of crystal grains. It is the top view and sectional drawing of (for example, a particle size of 1 micrometer or less). From these, it can be seen that the small crystal grains 201 constituting the thin film have fewer domains and domain walls than the large crystal grains 202. In particular, since the grain size of the crystal grain 201 is small, the number of non-180 ° domain walls W1 that are the starting point of non-180 ° domain rotation is extremely lower than that of the crystal grain 202, and the domain wall is 180 ° domain. In some cases, the wall W2 alone is used. For this reason, in a thin film having a small crystal grain size, it is difficult to improve the piezoelectric characteristics by causing non-180 ° domain rotation.

On the other hand, unlike the bulk, the thin film can be epitaxially grown using the crystal orientation of the substrate, or the crystal can be oriented due to the relationship between the stress of the substrate and the film. Conventionally, a device for causing non-180 ° domain rotation in a thin film has been made by using these.

For example, in Patent Document 1, a <100> -oriented tetragonal epitaxial film is grown on a substrate, and an a-axis orientation domain and a c-axis orientation domain are mixed in this epitaxial film, so that non-180 ° Domain rotation is caused to enhance the piezoelectric characteristics. Note that since the epitaxial film is strongly bound by the substrate, Non-Patent Document 3 discloses that the epitaxial film becomes a tetragonal single crystal even with the MPB composition. In Patent Document 2, a non-180 ° domain wall is obtained by obtaining an a-axis single alignment film in which the alignment axis is inclined from the vertical direction of the substrate using a special Si substrate in which the crystal plane of the substrate surface is inclined. However, domain rotation is generated at a relatively low voltage.

JP 2008-218675 A (refer to claims 1 and 2, paragraphs [0019], [0020], [0026], etc.) JP 2008-277672 A (refer to claims 1, 5, 6 and paragraphs [0039], [0062], [0063], [0068] to [0072], etc.)

However, as described above, since the epitaxial film becomes a tetragonal single crystal even with the MPB composition, in the configuration of Patent Document 1, different crystal structures of tetragonal crystals and rhombohedral crystals are mixed. The effect cannot be obtained. In other words, the density of non-180 ° domain walls cannot be increased by mixing different crystal structures, and non-180 ° domain rotation cannot be efficiently generated.

Moreover, in the structure of patent document 2, since it is necessary to use a special board | substrate, when performing a MEMS process by a post process, since a uniform process is difficult and a process becomes complicated, the application to MEMS is difficult. It becomes.

The present invention has been made to solve the above-described problems, and its object is to efficiently generate non-180 ° domain rotation and obtain high piezoelectric characteristics without using a special substrate. An object of the present invention is to provide a ferroelectric film that can be easily applied to MEMS and a piezoelectric element including the ferroelectric film.

A ferroelectric film according to one aspect of the present invention is a ferroelectric film made of a perovskite crystal, the crystal orientation is <100> main orientation, rhombohedral and tetragonal crystals are mixed, and the tetragonal crystal In FIG. 5, a-axis orientation and c-axis orientation are mixed, and function F indicating <100> peak by 2θ / θ measurement of X-ray diffraction is expressed as tetragonal crystal with a-axis orientation, tetragonal crystal with c-axis orientation, rhomboid A function corresponding to the rhombohedral crystal with respect to the sum of integral values of the three functions F1, F2 and F3 when divided into functions F1, F2 and F3 indicating the diffraction intensities of X-rays incident on the respective rhombohedral crystals. The ratio of the integral values of F3 is 0.2 or more and 0.8 or less.

According to the above configuration, even a thin film such as a ferroelectric film can efficiently cause non-180 ° domain rotation and obtain high piezoelectric characteristics.

(A)-(c) is sectional drawing which shows the film-forming process of the PZT film | membrane as a ferroelectric film based on one Example of this invention. It is a graph which shows the result of 2theta / theta measurement of XRD with respect to the said PZT film | membrane. It is a graph which shows the result of having divided the function which shows <100> peak of the PZT film into a plurality of functions. It is explanatory drawing which shows the integrated value of each of the said several function, and the piezoelectric constant of a PZT film | membrane about each Example and each comparative example of this invention. It is a perspective view which shows the schematic structure of a piezoelectric displacement measuring meter. It is a top view which shows the structure of the piezoelectric element provided with the PZT film | membrane of each Example. FIG. 7 is a cross-sectional view taken along line A-A ′ of FIG. 6. It is explanatory drawing which shows typically the crystal structure of PZT. (A) is explanatory drawing which shows the polarization direction of a tetragonal crystal, (b) is explanatory drawing which shows the polarization direction of a rhombohedral crystal. It is explanatory drawing which shows each piezoelectric distortion at the time of applying an electric field to a c-axis direction with respect to the tetragonal c-axis orientation domain and a-axis orientation domain. It is explanatory drawing for demonstrating the mechanism of a non-180 degree domain rotation. It is a graph which shows the relationship between the electric field applied to PZT, and the electric field induced distortion amount observed. (A) is a plan view showing a set of a plurality of crystal grains having different grain sizes, and (b) and (c) are plan views of crystal grains having a small grain size included in the plurality of crystal grains. It is sectional drawing, (d) and (e) are the top view and sectional drawing of a crystal grain with a big particle size contained in the said several crystal grain.

An embodiment of the present invention will be described below with reference to the drawings. First, the PZT film forming method of this embodiment will be described as Examples 1 and 2. For comparison with Examples 1 and 2, Comparative Examples 1 to 3 will also be described.

Example 1
FIG. 1A to FIG. 1C are cross-sectional views showing a process of forming a PZT film as a ferroelectric film of this example. First, as shown in FIG. 1A, a thermal oxide film 2 made of, for example, SiO _{2 having a} thickness of about 100 nm is formed on a substrate 1 made of a single crystal Si wafer having a thickness of about 400 μm. The substrate 1 may be a standard substrate having a thickness of 300 μm to 725 μm and a diameter of 3 inches to 8 inches. The thermal oxide film 2 can be formed by exposing the substrate 1 to a high temperature of about 1200 ° C. in an oxygen atmosphere using a wet oxidation furnace.

Next, as shown in FIG. 1B, an adhesion layer made of Ti having a thickness of about 20 nm and a Pt electrode layer having a thickness of about 100 nm are sequentially formed on the thermal oxide film 2. The adhesion layer and the Pt electrode layer are collectively referred to as a lower electrode 3. Ti and Pt are formed by sputtering, for example. The sputtering conditions for Ti at this time were Ar flow rate: 20 sccm, pressure: 0.4 Pa, RF power applied to the target: 200 W, and the sputtering conditions for Pt were Ar flow rate: 20 sccm, pressure: 0.4 Pa, on the target Applied RF power: 150 W, substrate temperature: 530 ° C. Although Pt becomes a film having <111> orientation due to its self-orientation, it has a high crystallinity because it affects the quality of the PZT film formed on Pt.

When Ti is exposed to a high temperature in a subsequent process (for example, formation of a PZT film), it diffuses into the Pt film, forming hillocks on the surface of the Pt layer, and leakage of the drive current of the PZT film, There is a risk of problems such as orientation degradation. Therefore, in order to prevent these problems, the adhesion layer may be made of TiOx instead of Ti. TiOx can also be formed by introducing reactive oxygen during Ti sputtering and forming a film by reactive sputtering, or heating at about 700 ° C. in an oxygen atmosphere in an RTA (Rapid Thermal Annealing) furnace after Ti film formation. It can also be formed by performing.

Next, as shown in FIG. 1C, a PZT film 4 that is a ferroelectric film is formed on the substrate 1 with Pt by sputtering. The formation method of the ferroelectric film is not limited to the sputtering method, but other physical film formation methods such as a pulse laser deposition (PLD) method and an ion plating method, and chemical film formation such as an MOCVD method and a sol-gel method. The law may be used.

Here, a sputtering target having a Zr / Ti molar ratio of 52/48 having an MPB (Morphotropic Phase Boundary) composition was used. Further, Pb contained in the target is likely to be re-evaporated during high-temperature film formation, and the formed thin film tends to be Pb deficient. Therefore, it is desirable to add the Pb to the target in a larger amount than the stoichiometric ratio of the perovskite crystal. For example, the amount of Pb added is preferably 10 to 30% higher than the stoichiometric ratio, although it depends on the film formation temperature.

The PZT sputtering conditions were Ar flow rate: 25 sccm, O ₂ flow rate: 0.4 sccm, pressure: 0.4 Pa, substrate temperature: 500 ° C., RF power: 500 W, and a PZT film 4 having a thickness of 5 μm was formed. By forming the film under such conditions, a PZT film 4 having an average particle diameter of 1 μm could be formed.

FIG. 2 shows the result of 2RD / θ measurement of XRD (X-ray diffraction) for the PZT film 4 of Example 1. In addition, the intensity | strength (diffraction intensity, reflection intensity) of the vertical axis | shaft of FIG. 2 is shown by the count rate (cps; count per second) of the X-ray per second. Details of the 2θ / θ measurement will be described later. In FIG. 2, since the diffraction intensity peak for the PZT film 4 is found only in the primary and high-order reflection peaks due to the <100> orientation, the crystal orientation of the PZT film 4 is the <100> main orientation. I can say that.

Here, the main orientation means that the orientation rate F measured by the Lotgering method is 80% or more and has crystal orientation. The orientation rate F at this time is represented by the following formula.
F (%) = (P−P ₀ ) / (1−P ₀ ) × 100
However, P refers to the ratio of the total reflection intensity from the orientation plane to the total reflection intensity. For example, in the case of <100> orientation,
P = I <100> / [I <100> + I <110> + I <111>]
It is. P ₀ is P of the non-oriented sample. In the non-oriented sample, F = 0%, and in the fully oriented sample, F = 100%. Incidentally, in this example, the orientation rate F was 100%.

If the crystal orientation direction of the PZT film 4 is random, piezoelectric distortion may be canceled between adjacent crystals when a voltage is applied. However, as in this example, the crystal orientation of the PZT film 4 is the <100> main orientation, and the crystal orientation directions are aligned in almost one direction, so that the piezoelectric strain is canceled between adjacent crystals when a voltage is applied. Can be suppressed, and the piezoelectric characteristics can be improved.

Further, FIG. 3 shows that for the <100> diffraction intensity peak (hereinafter referred to as <100> peak) of the PZT film 4 by 2θ / θ measurement of X-ray diffraction, fitting with a forked function is performed using dedicated software. The results are shown in which the function F indicating the <100> peak is divided into a plurality of functions F1, F2 and F3. The functions F1, F2 and F3 are functions indicating the diffraction intensity of X-rays incident on the a-axis oriented tetragonal crystal, the c-axis oriented tetragonal crystal and the rhombohedral crystal, respectively. In FIG. 3, the vertical axis of the graph showing the functions F1, F2 and F3 indicates the relative intensity of the function F with respect to the maximum intensity.

Here, 2θ / θ measurement of X-ray diffraction means that X-rays are incident on the sample at an angle θ from the horizontal direction (at an angle θ relative to the crystal plane) and reflected from the sample. This is a technique for investigating an intensity change with respect to θ by detecting X-rays having an angle of 2θ with respect to incident X-rays. In X-ray diffraction, the diffraction intensity increases when the Bragg condition (2 d sin θ = nλ (λ: wavelength of X-ray, d: atomic plane spacing of crystal, n: integer)) is satisfied. There is a correspondence between the surface spacing (lattice constant) and the above 2θ. Therefore, based on the value of 2θ at which the diffraction intensity increases, the crystal structure of the sample on which the X-rays are incident can be grasped.

For example, in a PZT tetragonal crystal, the lattice constant in the a-axis direction is 4.04 angstroms, the lattice constant in the c-axis direction is 4.14 angstroms, and the lattice constant of rhombohedral crystals is 4.07 angstroms. Therefore, the values of 2θ corresponding to the crystal structures are different from each other. Therefore, based on the 2θ value at which the diffraction intensity increases, it is determined whether the crystal structure of the sample on which the X-rays are incident is an a-axis oriented tetragonal crystal, a c-axis oriented tetragonal crystal, or a rhombohedral crystal. be able to.

FIG. 4 shows the integrated values (areas) of the functions F1, F2 and F3, respectively. Here, the sum of the integrated values of the three functions F1, F2 and F3 corresponding to the crystal structures of the a-axis oriented tetragonal crystal, the c-axis oriented tetragonal crystal and the rhombohedral crystal is 1. As a result, in this example, it was found that the PZT film 4 includes both tetragonal crystals and rhombohedral crystals, and in the tetragonal crystals, a-axis orientation and c-axis orientation are mixed.

Thus, in the present embodiment, since the a-axis orientation and the c-axis orientation are mixed in the tetragonal crystal of the PZT film 4, a 90 ° domain rotation from the a-axis direction to the c-axis direction, ie, non- A 180 [deg.] Domain rotation can be generated to produce a strain greater than the normal piezoelectric strain due to the c-axis oriented domain.

In addition, since rhombohedral crystals and tetragonal crystals are mixed in the PZT film 4, the domains in the <111> direction and the domains in the <100> direction are mixed in a complicated manner, and the domain size is reduced. As a result, the density of non-180 ° domain walls increases, and non-180 ° domain rotation can be efficiently generated.

In this example, the lattice constant of Si used as the material of the substrate 1 is about 5.4 angstroms, which is much larger than the lattice constant of PZT described above, so it is clear that the PZT film 4 is not an epitaxial film. It is. Therefore, rhombohedral crystals and tetragonal crystals can be mixed in the PZT film 4, and the above-described effects can be obtained.

Next, on the PZT film 4, for example, Pt is sputtered to form the upper electrode 5 (see FIGS. 6 and 7), and the piezoelectric element 10 (see FIG. 5) is completed. The piezoelectric element 10 was separated and taken out, and the piezoelectric displacement of the piezoelectric element 10 was measured with a piezoelectric displacement meter. FIG. 5 shows a schematic configuration of the piezoelectric displacement meter.

In this piezoelectric displacement measuring instrument, the end of the piezoelectric element 10 is clamped with a fixed portion 11 so that the movable length of the cantilever is 10 mm, and a cantilever structure is formed. A voltage of 0 V and a minimum voltage of −20 V was applied to the lower electrode 3 at a frequency of 500 Hz, and the displacement of the end of the piezoelectric element 10 was observed with a laser Doppler vibrometer 13. As a result, the piezoelectric displacement of the piezoelectric element 10 was 2.2 μm. From this piezoelectric displacement, the piezoelectric constant d ₃₁ can be calculated using the following calculation formula disclosed in Non-Patent Document 1.

Here, h ^s is the substrate thickness, the s ^p elastic compliance of the thin film PZT, s ^s is the elastic compliance of the substrate, L is the length of the cantilever, V is the applied voltage, [delta] is the displacement of the cantilever. Therefore, when h ^s = 400 μm, s ^p = 90 GPa, s ^s = 180 GPa, and L = 10 mm, the piezoelectric constant d ₃₁ = −175 pm / V. Therefore, in Example 1, it turns out that the high piezoelectric characteristic is acquired. In Comparative piezoelectric constant d _31, since sufficient in comparison of the absolute value, in the following, demonstrate the value of piezoelectric constant d ₃₁ in absolute value. FIG. 4 also shows the value (absolute value) of the piezoelectric constant d ₃₁ in this example.

(Example 2)
In this example, the PZT film 4 was formed on the wafer manufactured up to the lower electrode 3 as in Example 1 using the same target as in Example 1. However, among the sputtering conditions for the PZT film 4, only the O ₂ flow rate was changed to 0.8 sccm, and the other conditions were the same as in Example 1 to form the PZT film 4. Thereafter, as in Example 1, the upper electrode 5 was formed on the PZT film 4 to complete the piezoelectric element 10.

FIG. 4 shows the results of crystal structure evaluation and piezoelectric displacement measurement by XRD for the formed PZT film 4 and piezoelectric element 10 as in Example 1. From the figure, it was found that in the PZT film 4 of this example, as in Example 1, a-axis oriented tetragonal crystals, c-axis oriented tetragonal crystals, and rhombohedral crystals were mixed. The piezoelectric constant d ₃₁ was slightly lower than that in Example 1, but a relatively good value of 150 [pm / V].

(Example 3)
In this example, the PZT film 4 was formed by sputtering using the same target as in Example 1 on the wafer manufactured up to the lower electrode 3 as in Example 1. The sputtering conditions at this time are Ar flow rate: 30 sccm, O ₂ flow rate: 0.5 sccm, pressure: 0.9 Pa, substrate temperature: 550 ° C., RF power: 500 W. Thereafter, as in Example 1, the upper electrode 5 was formed on the PZT film 4 to complete the piezoelectric element 10.

FIG. 4 shows the results of crystal structure evaluation and piezoelectric displacement measurement by XRD for the formed PZT film 4 and piezoelectric element 10 as in Example 1. The piezoelectric constant d ₃₁ was slightly lower than that in Example 1, but was 153 [pm / V], which was a relatively good value similar to that in Example 2.

(Comparative Example 1)
In this comparative example, a PZT film 4 is formed under the same sputtering conditions as in Example 1, using a target having a Ti ratio that is several percent higher than that in Example 1 on the wafer manufactured up to the lower electrode 3 as in Example 1. Formed. By using such a target, it is possible to form the PZT film 4 in which the proportion of rhombohedral crystals is reduced. Thereafter, as in Example 1, the upper electrode 5 was formed on the PZT film 4 to complete the piezoelectric element 10.

FIG. 4 shows the results of crystal structure evaluation and piezoelectric displacement measurement by XRD for the formed PZT film 4 and piezoelectric element 10 as in Example 1. The piezoelectric constant d ₃₁ in this comparative example is 115 [pm / V], which is considerably lower than that in the first embodiment. This is because, compared with Example 1, the proportion of rhombohedral crystals contained in the PZT film 4 decreased, and the proportion of tetragonal crystals with c-axis orientation increased, so that the composition ratio deviated from the MPB composition. This is thought to be due to the fact that the domain rotation of ° does not occur efficiently.

(Comparative Example 2)
In this comparative example, the PZT film 4 was formed on the wafer manufactured up to the lower electrode 3 in the same manner as in Example 1, using the same target as in Comparative Example 1 and under the same sputtering conditions as in Example 2. Thereafter, as in Example 1, the upper electrode 5 was formed on the PZT film 4 to complete the piezoelectric element 10.

FIG. 4 shows the results of crystal structure evaluation and piezoelectric displacement measurement by XRD for the formed PZT film 4 and piezoelectric element 10 as in Example 1. The piezoelectric constant d ₃₁ in this comparative example was 100 [pm / V], which was even lower than that in Comparative example 1. Compared to Comparative Example 1, the ratio of a-axis-oriented tetragonal crystals contained in the PZT film 4 is decreased, and the ratio of c-axis-oriented tetragonal crystals is increased. This is thought to be due to an increase in the rate of piezoelectric strain and a decrease in the rate of piezoelectric strain using non-180 ° domain rotation from the a-axis direction to the c-axis direction.

(Comparative Example 3)
In this comparative example, a PZT film 4 is formed under the same sputtering conditions as in Example 1 using a target having a Zr ratio that is several percent higher than that in Example 1 on the wafer manufactured up to the lower electrode 3 as in Example 1. Formed. By using such a target, the PZT film 4 having an increased rhombohedral ratio can be formed. Thereafter, as in Example 1, the upper electrode 5 was formed on the PZT film 4 to complete the piezoelectric element 10.

FIG. 4 shows the results of crystal structure evaluation and piezoelectric displacement measurement by XRD for the formed PZT film 4 and piezoelectric element 10 as in Example 1. The piezoelectric constant d ₃₁ in this comparative example is 130 [pm / V], which is lower than that in Example 2. This is because when the proportion of rhombohedral crystals contained in the PZT film 4 increases too much, the a-axis orientation and the c-axis orientation are not mixed in the tetragonal crystal, and the effect of non-180 ° domain rotation cannot be obtained. It is considered that the piezoelectric characteristics are deteriorated.

From the results of Examples 1 to 3 and Comparative Examples 1 to 3, the integral values of the three functions F1, F2, and F3 corresponding to the a-axis oriented tetragonal crystal, the c-axis oriented tetragonal crystal, and the rhombohedral crystal, respectively. If the ratio of the integral value of the function F3 corresponding to the rhombohedral crystal to the total is 0.2 or more, high piezoelectric characteristics can be obtained, and if it is 0.4 or more, the effect is surely obtained. I can say that. The upper limit of the ratio is preferably smaller than 0.94 in Comparative Example 3 and close to 0.73 in Example 3 from the viewpoint of obtaining high piezoelectric characteristics. Is desirably 0.8 or less, and more desirably 0.5 or less.

Furthermore, from the results of Examples 1 and 2, the ratio of the integral value of the function F1 corresponding to the tetragonal crystal with the a-axis orientation to the sum of the integral values of the three functions F1, F2, and F3 is 0.4 or more and 0.00. It can be said that it is desirable to be 5 or less.

(Application examples of PZT films)
Since the PZT film 4 shown in Examples 1 and 2 can be formed using a normal Si wafer as described above without using a special substrate having a tilted surface crystal plane, the MEMS Application is also easy. Hereinafter, an example in which the PZT film 4 is applied to a piezoelectric element manufactured by the MEMS technology will be described.

FIG. 6 is a plan view showing a configuration when the PZT film 4 of Examples 1 and 2 is applied to a diaphragm (diaphragm) as a piezoelectric element 10 ′ manufactured using the MEMS technology, and FIG. FIG. 7 is a cross-sectional view taken along line AA ′ in FIG. The piezoelectric element 10 ′ is configured by laminating a thermal oxide film 2, a lower electrode 3, a PZT film 4 as a ferroelectric film, and an upper electrode 5 in this order on a substrate 1. The PZT film 4 is disposed in a necessary area of the substrate 1 in a two-dimensional staggered pattern.

In addition, a region corresponding to the formation region of the PZT film 4 in the substrate 1 is a recess 1a in which a part in the thickness direction is removed with a circular cross section, and the upper portion of the recess 1a in the substrate 1 (the bottom side of the recess 1a). ) Remains a thin plate-like region 1b. The lower electrode 3 and the upper electrode 5 are connected to an external control circuit by wiring not shown.

By applying an electrical signal from the control circuit to the lower electrode 3 and the upper electrode 5 sandwiching the predetermined PZT film 4, only the predetermined PZT film 4 can be driven. That is, when a predetermined electric field is applied to the upper and lower electrodes of the PZT film 4, the PZT film 4 expands and contracts in the left-right direction, and the PZT film 4 and the region 1b of the substrate 1 are bent up and down by the bimetal effect. Therefore, when the recess 1a of the substrate 1 is filled with gas or liquid, the piezoelectric element 1 can be used as a pump, which is suitable for an inkjet head, for example.

Further, the deformation amount of the PZT film 4 can be detected by detecting the charge amount of the predetermined PZT film 4 through the lower electrode 3 and the upper electrode 5. That is, when the PZT film 4 is vibrated by sound waves or ultrasonic waves, an electric field is generated between the upper and lower electrodes due to an effect opposite to that described above. By detecting the magnitude of the electric field and the frequency of the detection signal at this time, The element 1 can also be used as a sensor (ultrasonic sensor). Furthermore, since the PZT film 4 exhibits a pyroelectric effect, the piezoelectric element 1 can be used as a pyroelectric sensor (infrared sensor).

In addition, the piezoelectric element 1 can also be used as a frequency filter (surface acoustic wave filter) by using the piezoelectric effect of the PZT film 4, and the piezoelectric element 1 can be made non-volatile by using the PZT film 4 as a ferroelectric substance. It can also be used as a memory.

(Constituent material of ferroelectric film)
In the above description, PZT has been described as an example of the constituent material of the ferroelectric film. As can be seen from the fact that PZT is also expressed as Pb (Zr, Ti) O ₃ , when the perovskite crystal is represented by the general formula ABO ₃ , Pb ions are included at the A site, and Zr ions and Ti ions are present at the B site. A lead-based metal oxide (see FIG. 8). Such a lead-based metal oxide exhibits a good piezoelectric characteristic by adopting a perovskite structure, and is therefore suitable for the ferroelectric film of the above-described embodiment.

At this time, an additive may be contained in at least one of the A site and the B site of PZT. As an additive for the A site, for example, a lanthanoid metal containing Nd or La, or a metal ion of at least one of Sr and Bi can be considered. Moreover, as an additive of B site, the metal ion of at least any one of Nb, Ta, W, and Sb can be considered, for example. As a ferroelectric film in which such an additive is added to PZT, for example, PLZT ((Pb, La) (Zr, Ti) O ₃ ), PSZT ((Pb, Sr) (Zr, Ti) O ₃ ) , PNZT (Pb (Zr, Ti, Nb) O ₃ ).

Even a metal oxide obtained by adding an additive to PZT exhibits a good piezoelectric characteristic by adopting a perovskite structure, and thus is suitable for the ferroelectric film of the above-described embodiment.

The ferroelectric film may be made of a lead-free metal oxide as long as it has a perovskite structure. As such a lead-free metal oxide, when the perovskite crystal is represented by the general formula ABO ₃ , the A site contains at least one of Sr, Ba, and Bi metal ions, and the B site contains Ti ions or The thing containing Ta ion can be considered. Specifically, BST ((Ba, Sr) TiO ₃ ) or SBT (SrBi ₂ Ta ₂ O ₉ ) can be considered.

Since lead-free metal oxides such as BST and SBT exhibit a good piezoelectric characteristic by adopting a perovskite structure, they are suitable for a ferroelectric film of a piezoelectric element.

That is, even when a ferroelectric film is formed using a material other than the above-described PZT (including an additive added to PZT), non-180 ° domain rotation is efficiently generated, High piezoelectric characteristics can be obtained, and the ferroelectric film can be easily applied to MEMS.

The ferroelectric film described above is a ferroelectric film made of a perovskite crystal, in which the crystal orientation is <100> main orientation and rhombohedral and tetragonal crystals are mixed. Axial orientation and c-axis orientation coexist, and the function F indicating the <100> peak by 2θ / θ measurement of X-ray diffraction is expressed as a tetragonal crystal with an a-axis orientation, a tetragonal crystal with a c-axis orientation, and a rhombohedral crystal. Integration of the function F3 corresponding to the rhombohedral crystal with respect to the sum of the integration values of the three functions F1, F2 and F3 when divided into functions F1, F2 and F3 indicating the diffraction intensity of the incident X-rays. The value ratio is 0.2 or more and 0.8 or less.

According to this configuration, since the crystal orientation is the <100> main orientation and the crystal orientation directions are substantially aligned in one direction, piezoelectric distortion (piezoelectric displacement) is canceled between adjacent crystals when a voltage is applied. Can be suppressed. In addition, since the a-axis orientation and the c-axis orientation are mixed in the tetragonal crystal, a 90 ° domain rotation from the a-axis direction to the c-axis direction, that is, a non-180 ° domain rotation is caused. A strain larger than the normal piezoelectric strain due to the axially oriented domain can be generated.

In addition, since rhombohedral crystals and tetragonal crystals are mixed, the <111> direction domain and the <100> direction domain are mixed in a complicated manner, the domain size is reduced, and the non-180 ° domain wall density is reduced. Also gets higher. Thereby, non-180 ° domain rotation can be efficiently generated.

In particular, the function F indicating the <100> peak by 2θ / θ measurement of X-ray diffraction indicates the diffraction intensity of X-rays incident on each of an a-axis oriented tetragonal crystal, a c-axis oriented tetragonal crystal, and a rhombohedral crystal. When divided into the three functions F1 to F3, the ratio of the integral value of the function F3 corresponding to the rhombohedral crystal to the sum of the integral values of the three functions F1 to F3 is 0.2 or more and 0.8 or less. As a result, non-180 ° domain rotation can be reliably generated and high piezoelectric characteristics can be obtained.

Thus, even a thin film such as a ferroelectric film can efficiently generate non-180 ° domain rotation and obtain high piezoelectric characteristics. In addition, it is not necessary to use a special substrate (a substrate whose surface crystal plane is inclined) when rotating the domain, and the application of the ferroelectric film to MEMS is facilitated.

In the ferroelectric film, if the ratio is 0.4 or more and 0.5 or less, higher piezoelectric characteristics can be obtained with certainty.

The ferroelectric film described above is a lead-based metal oxide containing Pb ions at the A site and Zr ions and Ti ions at the B site when the perovskite crystal is represented by the general formula ABO ₃ . Can be configured.

Since lead-based metal oxides such as PZT exhibit a good piezoelectric characteristic by adopting a perovskite structure, they are suitable for the ferroelectric film.

In the above ferroelectric film, the metal oxide includes an additive at at least one of the A site and the B site, and the additive at the A site is at least one of a lanthanoid metal, Sr, and Bi. It is a metal ion, and the additive at the B site may be a metal ion of at least one of Nb, Ta, W, and Sb.

Even a metal oxide obtained by adding an additive to PZT is suitable for the ferroelectric film because it exhibits good piezoelectric characteristics by adopting a perovskite structure.

Further, when the perovskite crystal is represented by the general formula ABO ₃ , the ferroelectric film contains at least one of Sr, Ba, and Bi metal ions at the A site, and Ti ions or Ta ions at the B site. It may be composed of a lead-free metal oxide.

Lead-free metal oxides such as BST (barium strontium titanate) and SBT (strontium bismuth tantalate) exhibit good piezoelectric properties by adopting a perovskite structure, and are therefore suitable for the ferroelectric film. .

The piezoelectric element described above is a piezoelectric element in which a lower electrode, a ferroelectric film, and an upper electrode are laminated in this order on a substrate, and the ferroelectric film is composed of the ferroelectric film described above. Has been.

The above-described ferroelectric film does not require a special substrate to cause non-180 ° domain rotation, and can be easily applied to MEMS. Therefore, the ferroelectric film is high in a piezoelectric element manufactured using MEMS technology. Piezoelectric characteristics can be easily realized.

As described above, even a thin film such as a ferroelectric film can efficiently cause non-180 ° domain rotation and obtain high piezoelectric characteristics. In addition, since it is not necessary to use a special substrate, the ferroelectric film can be easily applied to MEMS.

The present invention can be used in, for example, MEMS actuators (inkjet printers and projector actuators), MEMS sensors (pyroelectric sensors, ultrasonic sensors), frequency filters, and nonvolatile memories.

1 Substrate 3 Lower electrode 4 PZT film (ferroelectric film)
5 Upper electrode 10 Piezoelectric element 10 'Piezoelectric element

Claims (6)

1. A ferroelectric film made of a perovskite crystal,
   The crystal orientation is <100> main orientation, and rhombohedral and tetragonal crystals are mixed,
   In the tetragonal crystal, a-axis orientation and c-axis orientation are mixed,
   A function F1 indicating the diffraction intensity of X-rays incident on each of an a-axis oriented tetragonal crystal, a c-axis oriented tetragonal crystal, and a rhombohedral crystal is represented by a function F indicating a <100> peak by 2θ / θ measurement of X-ray diffraction. , F2 and F3, the ratio of the integral value of the function F3 corresponding to the rhombohedral crystal to the sum of the integral values of the three functions F1, F2 and F3 is 0.2 or more and 0.8 or less A ferroelectric film characterized by the following.
2. 2. The ferroelectric film according to claim 1, wherein the ratio is 0.4 or more and 0.5 or less.
3. When the perovskite crystal is represented by a general formula ABO ₃ , the perovskite crystal is composed of a lead-based metal oxide containing Pb ions at the A site and Zr ions and Ti ions at the B site. Item 4. The ferroelectric film according to Item 1.
4. The metal oxide includes an additive in at least one of the A site and the B site,
   The additive at the A site is a metal ion of at least one of a lanthanoid metal, Sr, and Bi,
   4. The ferroelectric film according to claim 3, wherein the additive at the B site is a metal ion of at least one of Nb, Ta, W, and Sb.
5. When the perovskite crystal is represented by the general formula ABO ₃ , a lead-free metal oxide containing at least one metal ion of Sr, Ba, Bi at the A site and Ti ion or Ta ion at the B site The ferroelectric film according to claim 1, comprising:
6. A piezoelectric element in which a lower electrode, a ferroelectric film, and an upper electrode are laminated in this order on a substrate,
   2. The piezoelectric element according to claim 1, wherein the ferroelectric film is composed of the ferroelectric film according to claim 1.

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