JP2020170781A - Film structure - Google Patents

Abstract

To provide a film structure capable of improving the piezoelectric characteristics of a piezoelectric film containing lead zirconate titanate.SOLUTION: A film structure includes a base 11, a lower piezoelectric film 21 formed on the base 11 and containing lead zirconate titanate, and an upper piezoelectric film 22 formed on the lower piezoelectric film 21 and containing lead zirconate titanate. The base 11 has compressive stress, the lower piezoelectric film 21 and the upper piezoelectric film 22 both have tensile stress, and the elastic constant of the lower piezoelectric film 21 is larger than the elastic constant of the upper piezoelectric film 22.SELECTED DRAWING: Figure 2

Description

The present invention relates to a membrane structure.

As a film structure having a substrate, a conductive film formed on the substrate, and a piezoelectric film formed on the conductive film, a substrate, a conductive film containing platinum formed on the substrate, and a conductive film on the conductive film. A film structure having a piezoelectric film containing lead zirconate titanate (Pb (Zr, Ti) O ₃ : PZT) formed in the above is known.

Further, as the piezoelectric film containing PZT, for example, on the first film containing PZT having a Zr-rich composition in which the composition ratio of zirconium (Zr) to titanium (Ti) is relatively large, zirconium (Zr) to titanium (Ti) A piezoelectric film in which a second film containing PZT having a Ti-rich composition having a relatively small composition ratio of Zirconium is formed is known.

WO 2017/221649 (Patent Document 1), in a membrane structure, a substrate, formed on a substrate, and, first represented by a composition formula _{Pb (Zr 1-x Ti x} ) O 3 a first layer containing a composite oxide, is formed on the first layer and a second layer including a second composite oxide represented by a composition formula _{Pb (Zr 1-y Ti y} ) O 3, the X satisfies 0.10 <x≤0.20, y satisfies 0.35≤y≤0.55, the first film has tensile stress, and the second film compresses. Techniques with stress are disclosed.

International Publication No. 2017/221649

When a piezoelectric film containing PZT formed on such a substrate is used as a pressure sensor, for example, when a stress in a direction perpendicular to the upper surface of the substrate or a stress in a direction parallel to the upper surface of the substrate is applied, It is desirable to increase the charge generated on the upper and lower surfaces of the piezoelectric film. That is, it is desirable to improve the piezoelectric characteristics (positive piezoelectric characteristics) of the piezoelectric film. However, in the piezoelectric film containing PZT formed on the above-mentioned substrate, it is difficult to improve the piezoelectric characteristics of the piezoelectric film because, for example, the piezoelectric film has tensile stress.

The present invention has been made to solve the above-mentioned problems of the prior art, and can improve the piezoelectric characteristics of the piezoelectric film in a membrane structure having a piezoelectric film containing lead zirconate titanate. It is an object of the present invention to provide a membrane structure that can be formed.

A brief outline of the typical inventions disclosed in the present application is as follows.

The film structure as one aspect of the present invention is formed on a substrate, a first piezoelectric film formed on the substrate and containing lead zirconate titanate, and a first piezoelectric film, and lead zirconate titanate. It has a second piezoelectric film containing lead acid acid. The substrate has compressive stress, and both the first piezoelectric film and the second piezoelectric film have tensile stress. The first elastic constant of the first piezoelectric film is larger than the second elastic constant of the second piezoelectric film.

Further, as another aspect, the ratio of the first thickness to the sum of the first thickness of the first piezoelectric film and the second thickness of the second piezoelectric film may be 0.25 to 0.75.

Further, as another aspect, the first piezoelectric constant of the first piezoelectric film may be smaller than the second piezoelectric constant of the second piezoelectric film.

As another aspect, the first piezoelectric film contains lead zirconate titanate represented by the following general formula (Chemical formula 1), and the second piezoelectric membrane is represented by the following general formula (Chemical formula 2). It may contain lead zirconate titanate.
Pb (Zr _1-a Ti _a ) O ₃ ... (Chemical formula 1)
Pb (Zr _1-b Ti _b ) O ₃ ... (Chemical formula 2)
a may satisfy 0.48 <a ≦ 0.78, and b may satisfy 0.28 ≦ b ≦ 0.48.

The film structure as one aspect of the present invention is formed on a substrate, a first piezoelectric film formed on the substrate and containing lead zirconate titanate, and a first piezoelectric film, and lead zirconate titanate. It has a second piezoelectric film containing lead acid acid. The substrate has compressive stress, and both the first piezoelectric film and the second piezoelectric film have tensile stress. The first elastic constant of the first piezoelectric film is smaller than the second elastic constant of the second piezoelectric film. The ratio of the first thickness to the sum of the first thickness of the first piezoelectric film and the second thickness of the second piezoelectric film is 0.25 to 0.75.

Further, as another aspect, the first piezoelectric constant of the first piezoelectric film may be larger than the second piezoelectric constant of the second piezoelectric film.

As another aspect, the first piezoelectric film contains lead zirconate titanate represented by the following general formula (Chemical Formula 3), and the second piezoelectric membrane is represented by the following general formula (Chemical Formula 4). It may contain lead zirconate titanate.
Pb (Zr _1-c Ti _c ) O ₃ ... (Chemical formula 3)
Pb (Zr _1-d Ti _d ) O ₃ ... (Chemical formula 4)
c may satisfy 0.28 ≦ c ≦ 0.48, and d may satisfy 0.48 <d ≦ 0.78.

As another aspect, the sum of the first product of the first elastic constant and the first thickness and the second product of the second elastic constant and the second thickness is the third elastic constant of the substrate and the substrate. It may be smaller than the third product with the third thickness of.

Further, as another aspect, the lower layer portion of the first piezoelectric film may have tensile stress.

As another aspect, the first elastic constant may be the elastic stiffness C ₁₃ of the first piezoelectric film, and the second elastic constant may be the elastic stiffness C ₁₃ of the second piezoelectric film.

In another aspect, the first piezoelectric constant may be the piezoelectric constant d ₃₁ of the first piezoelectric film, and the second piezoelectric constant may be the piezoelectric constant d ₃₁ of the second piezoelectric film.

Further, as another aspect, the substrate may be made of silicon.

Further, as another aspect, the substrate may be made of (100) oriented silicon. The film structure is further formed on a first film having a cubic crystal structure and containing (100) oriented zirconium oxide, and a cubic crystal structure formed on the first film. It may have a conductive film having a crystal structure and containing (100) oriented platinum. The first piezoelectric film is formed on the conductive film, has a tetragonal crystal structure, and contains (001) oriented lead zirconate titanate, and the second piezoelectric film has a tetragonal crystal structure. It may also contain (001) oriented lead zirconate titanate.

By applying one aspect of the present invention, it is possible to improve the piezoelectric characteristics of the piezoelectric film in a membrane structure having a piezoelectric film containing lead zirconate titanate.

It is sectional drawing of the membrane structure of Embodiment 1. FIG. It is sectional drawing of the membrane structure used for the calculation of Embodiment 1. FIG. 6 is a graph showing the effect of the ratio of the thickness of the lower piezoelectric film to the thickness of the entire piezoelectric film portion in the film structure of the first embodiment on the generated charge. It is sectional drawing of the membrane structure of Embodiment 2. It is sectional drawing of the membrane structure used in the calculation of Embodiment 2. 3 is a graph showing the effect of the ratio of the thickness of the lower piezoelectric film to the thickness of the entire piezoelectric film portion in the film structure of the second embodiment on the generated charge. It is a graph which shows the voltage dependence of the polarization of the membrane structure of the comparative example 1. It is a graph which shows the voltage dependence of the displacement of the film structure of the comparative example 1. It is a graph which shows the voltage dependence of the polarization of the membrane structure of the comparative example 2. It is a graph which shows the voltage dependence of the displacement of the film structure of the comparative example 2. It is a graph which shows the result of having measured the amount of electric charge generated when compressive stress was applied from the top and bottom to the piezoelectric film part in the film structure of Comparative Example 1. It is a graph which shows the result of having measured the amount of electric charge generated when compressive stress was applied from the top and bottom to the piezoelectric film part in the film structure of Comparative Example 2. It is a graph which shows the result of having measured the amount of electric charge generated when compressive stress was applied from the top and bottom to the piezoelectric film part in the film structure of Example 2.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

It should be noted that the disclosure is merely an example, and those skilled in the art can easily conceive of appropriate changes while maintaining the gist of the invention are naturally included in the scope of the present invention. Further, in order to clarify the description, the drawings may schematically represent the width, thickness, shape, etc. of each part as compared with the embodiment, but this is just an example, and the interpretation of the present invention is used. It is not limited.

Further, in the present specification and each figure, the same elements as those described above with respect to the above-mentioned figures may be designated by the same reference numerals, and detailed description thereof may be omitted as appropriate.

Further, in the drawings used in the embodiments, hatching (shading) added to distinguish the structures may be omitted depending on the drawings.

In the following embodiments, when the range is indicated as A to B, A or more and B or less are indicated unless otherwise specified.

(Embodiment 1)
First, the membrane structure of the first embodiment, which is one embodiment of the present invention, will be described. FIG. 1 is a cross-sectional view of the membrane structure of the first embodiment.

As shown in FIG. 1, the film structure 10 of the first embodiment has a substrate 11 and a piezoelectric film portion 12 formed on an upper surface 11a as a main surface of the substrate 11, and has a piezoelectric film portion. 12 has a lower piezoelectric film 21 and an upper piezoelectric film 22 formed on the lower piezoelectric film 21. The lower piezoelectric film 21 contains lead zirconate titanate. The upper piezoelectric film 22 contains lead zirconate titanate. The substrate 11 has a compressive stress (compressive stress CS1 shown in FIG. 2 described later) applied in a direction parallel to the upper surface 11a of the substrate 11, that is, applied along the upper surface 11a. Both the lower piezoelectric film 21 and the upper piezoelectric film 22 are applied in a direction parallel to the upper surface 11a of the substrate 11, that is, a tensile stress applied along the upper surface 11a (tensile stress TS1 shown in FIG. 2 to be described later). And TS2).

The elastic constant of the lower piezoelectric film 21 is larger than the elastic constant of the upper piezoelectric film 22. That is, the elastic constant of the upper piezoelectric film 22 is smaller than the elastic constant of the lower piezoelectric film 21. In other words, the lead zirconate titanate contained in the lower piezoelectric film 21 is a so-called hard PZT, and the lead zirconate titanate contained in the upper piezoelectric film 22 is a so-called soft PZT. That is, the amount of strain when a constant voltage is applied to the lower piezoelectric film 21 is smaller than the amount of strain when the same voltage is applied to the upper piezoelectric film 22.

As will be described with reference to FIG. 2 described later, the film structure 10 of the first embodiment has such a structure, so that, for example, the piezoelectric film portion 12 has an upper surface 11a of the substrate 11 (see FIG. 1). When a stress in a direction perpendicular to the above or a stress in a direction parallel to the upper surface 11a of the substrate 11 is applied, the electric charge generated on the upper surface and the lower surface of the piezoelectric film portion 12 can be increased. That is, the piezoelectric characteristics (positive piezoelectric characteristics) of the piezoelectric film portion 12 can be improved.

As shown in FIG. 1, the film structure 10 of the first embodiment preferably has an alignment film 13, a conductive film 14, and a conductive film 15. The alignment film 13 is formed on the substrate 11. The conductive film 14 is formed on the alignment film 13. The conductive film 14 is a lower electrode. The piezoelectric film portion 12, that is, the lower piezoelectric film 21, is formed on the conductive film 14. The conductive film 15 is formed on the piezoelectric film portion 12, that is, on the upper piezoelectric film 22. The conductive film 15 is an upper electrode.

As shown in FIG. 1, consider a case where the film structure 10 is formed on the upper surface side of the SOI (Silicon On Insulator) substrate 16. The SOI substrate 16 includes a silicon substrate 17, a BOX (Buried Oxide) layer 18 which is an embedded oxide film formed on the silicon substrate 17, and an SOI (Silicon On Insulator) layer 19 formed on the BOX layer 18. including. In such a case, the SOI layer 19 made of silicon can be used as the substrate 11.

Further, as shown in FIG. 1, the silicon substrate 17 is penetrated from the lower surface of the silicon substrate 17 by etching a part of the silicon substrate 17 using, for example, a photolithography technique and an etching technique using an alkaline etching solution. The opening 17a that reaches the BOX layer 18 can be formed. Further, for example, by using the silicon substrate 17 on which the opening 17a is formed as a mask and using an etching technique using an etching solution such as hydrofluoric acid, the portion of the BOX layer 18 exposed to the opening 17a is etched. As shown in FIG. 1, it is possible to form an opening 18a that penetrates the BOX layer 18 from the lower surface of the BOX layer 18 to reach the SOI layer 19 and communicates with the opening 17a. Further, as shown in FIG. 1, a part of the conductive film 15 as the upper electrode can be etched and patterned by, for example, a photolithography technique and an etching technique.

As a result, as shown in FIG. 1, in the plan view, the substrate 11 made of the SOI layer 19, the alignment film 13, the conductive film 14, and the piezoelectric film portion 12 are formed in the opening 17a and the opening 18a. A piezoelectric element made of a film structure 10 having a conductive film 15 and a conductive film 15 can be formed. Then, a piezoelectric actuator made of a plurality of piezoelectric elements (Micro Electro Mechanical Systems: MEMS) formed on the upper surface side of the SOI substrate 16 with high shape accuracy can be easily formed.

When the SOI layer 19 made of silicon having a high impurity concentration and high conductivity is used as the substrate 11, the alignment film 13 and the conductive film 14 can be omitted. Further, when the substrate 11 is used, for example, when a probe having conductivity is brought into contact with the surface of the piezoelectric film portion 12, the conductive film 15 can be omitted. Further, as the substrate 11, various films such as an oxide film such as silicon oxide can be used in addition to silicon.

Further, various substrates can be used instead of the substrate 11. That is, the piezoelectric film portion 12 does not have to be formed on the substrate 11 having a film shape, and can be formed on various substrates such as silicon and glass.

Next, the technical idea of the film structure of the first embodiment will be described using the result of calculating the influence of the ratio of the thickness of the lower piezoelectric film 21 to the thickness of the entire piezoelectric film portion 12 on the generated charge. .. FIG. 2 is a cross-sectional view of the membrane structure used in the calculation of the first embodiment. In FIG. 2, in order to simplify the understanding of the tensile stress of the piezoelectric membrane portion 12 of the membrane structure 10, the radius of curvature of the membrane structure 10 when the membrane structure 10 is curved, that is, warped, is shown. It is exaggerated to be smaller than the actual radius of curvature. Further, in FIG. 2, the compressive stress of the substrate 11 is indicated as the compressive stress CS1, the tensile stress of the lower piezoelectric film 21 is indicated as the tensile stress TS1, and the tensile stress of the upper piezoelectric film 22 is indicated as the tensile stress TS2. are doing. As shown in FIG. 2, when the substrate 11 has a compressive stress CS1, the lower piezoelectric film 21 has a tensile stress TS1, and the upper piezoelectric film 22 has a tensile stress TS2, the film structure 10 is convex downward. It will be curved or warped so that it has the shape of.

Regarding the film structure of the first embodiment, the present inventors consider the sum of the thickness TH1 of the lower piezoelectric film 21 and the thickness TH2 of the upper piezoelectric film 22, that is, the lower portion with respect to the thickness of the entire piezoelectric film portion 12. The effect of the ratio RT1 on the generated charge was calculated when the ratio RT1 of the thickness TH1 of the piezoelectric film 21 was changed. As a result, the present inventors have found that the lead zirconate titanate contained in the lower piezoelectric film 21 is composed of a hard PZT, and the lead zirconate titanate contained in the upper piezoelectric film 22 is composed of a soft PZT, that is, a hard system. When the soft PZT is laminated on the PZT, the piezoelectric characteristics are higher than those in the case where the entire piezoelectric film portion 12 is composed of only the soft PZT and the case where the entire piezoelectric film portion 12 is composed of only the hard PZT. Was found to improve.

As a model for calculating the effect of the ratio RT1 of the thickness of the lower piezoelectric film 21 to the thickness of the entire piezoelectric film portion 12 on the generated charge, as shown in FIG. 2, the alignment film 13 (see FIG. 1) and A simplified model of the film structure was used by omitting the conductive film 14 (see FIG. 1) and the conductive film 15 (see FIG. 1). Further, the substrate 11 was made of silicon, the thickness THB of the substrate 11 was 1 μm, and the total thickness of the piezoelectric film portion 12 was 1 μm. The relative permittivity, piezoelectric constant d ₃₁ and Young's modulus of each of the lead zirconate titanate (PZT) contained in the lower piezoelectric film 21 and the lead zirconate titanate (PZT) contained in the upper piezoelectric film 22. The numerical values used in the calculation are shown in Table 1.

Under such conditions, the ratio RT1 of the thickness of the lower piezoelectric film 21 to the thickness of the entire piezoelectric film portion 12 is changed to 0, 0.25, 0.5, 0.75, and the ratio RT1 is generated. The effect on charge was calculated. The result is shown in FIG. FIG. 3 is a graph showing the effect of the ratio of the thickness of the lower piezoelectric film to the thickness of the entire piezoelectric film portion in the film structure of the first embodiment on the generated charge. The horizontal axis of FIG. 3 shows the ratio RT1 of the thickness of the lower piezoelectric film 21 to the thickness of the entire piezoelectric film portion 12, and the vertical axis of FIG. 3 is the amount of electric charge generated on the upper surface and the lower surface of the piezoelectric film portion 12. It shows the generated charge index, which is an index showing. Further, in FIG. 3, the ratio RT1 of the thickness of the lower piezoelectric film 21 to the thickness of the entire piezoelectric film portion 12 is referred to as “lower layer ratio”, and is shown using a value obtained by multiplying the ratio RT1 by 100. ing.

As shown in FIG. 3, when the ratio RT1 is 0.25 to 0.75, the generated charge index is increased as compared with the case where the ratio RT1 is 0 and the case where the ratio RT1 is 1. .. That is, when the soft PZT is laminated on the hard PZT and the ratio RT1 is 0.25 to 0.75, the entire piezoelectric film portion 12 is composed of only the soft PZT (ratio RT1). It has been clarified that the piezoelectric characteristics are improved as compared with the case where (0) and the case where the entire piezoelectric film portion 12 is composed of only the hard PZT (when the ratio RT1 is 1).

Preferably, the ratio RT1 of the thickness TH1 to the sum of the thickness TH1 of the lower piezoelectric film 21 and the thickness TH2 of the upper piezoelectric film 22 is 0.25 to 0.5. As shown in FIG. 3, when the ratio RT1 is 0.25 or more, the upper layer of the piezoelectric film portion 12 is compared with the case where the ratio RT1 is less than 0.25 when stress is applied to the piezoelectric film portion 12. The tensile stress of the portion is reduced, and charges are likely to be generated on the upper surface and the lower surface of the piezoelectric film portion 12. Further, as shown in FIG. 3, when the ratio RT1 is 0.5 or less, the piezoelectric constant of the upper layer portion of the piezoelectric film portion 12 becomes larger than that when the ratio RT1 exceeds 0.5, and the piezoelectric film When stress is applied to the portion 12, electric charges are likely to be generated on the upper surface and the lower surface of the piezoelectric film portion 12. Further, when the ratio RT1 is close to 0.5, the thickness of the lower piezoelectric film 21 and the thickness of the upper piezoelectric film 22 can be made substantially equal to those when the ratio RT1 is far from 0.5. The time required to form the piezoelectric film portion 12 per unit film thickness can be shortened most, the productivity of the film structure 10 can be improved, and the manufacturing cost of the film structure 10 can be reduced.

Preferably, the piezoelectric constant of the lower piezoelectric film 21 is smaller than the piezoelectric constant of the upper piezoelectric film 22. That is, the piezoelectric constant of the upper piezoelectric film 22 is larger than the piezoelectric constant of the lower piezoelectric film 21. In such a case, as described above, when the thickness TH1 of the lower piezoelectric film 21 is equal to or less than the thickness TH2 of the upper piezoelectric film 22, for example, the piezoelectric film portion 12 is perpendicular to the upper surface of the substrate 11. It is possible to surely increase the charge generated on the upper surface and the lower surface of the piezoelectric film portion 12 when the stress in the above direction or the stress in the direction parallel to the upper surface of the substrate 11 is applied. That is, the piezoelectric characteristics (positive piezoelectric characteristics) of the piezoelectric film portion 12 can be reliably improved.

Preferably, the lower piezoelectric film 21 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical Formula 5).
Pb (Zr _1-a Ti _a ) O ₃ ... (Chemical formula 5)
Here, a satisfies 0.48 <a ≦ 0.78. The general formula (Chemical formula 5) represents the same composite oxide as the general formula (Chemical formula 1).

In such a case, the lower piezoelectric film 21 has a Ti composition ratio larger than that of the Morphotropic Phase Boundary (MPB), that is, has a Ti-rich composition, and tends to be a hard PZT.

In the composite oxide composed of lead zirconate titanate represented by the general formula (Chemical formula 5), a part of lead (Pb) is used to improve the insulating property or piezoelectric characteristics of lead zirconate titanate. It may be substituted with element A. The element A comprises one or more selected from the group consisting of Li, Na, K, Rb, Ca, Sr, Ba, Bi and La. In such a case, the lower piezoelectric film 21 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical formula 6) instead of the above general formula (Chemical formula 5).
(Pb _1-x A _x ) (Zr _1-a Ti _a ) O ₃ ... (Chemical formula 6)
Here, x satisfies 0 <x ≦ 0.04, and a satisfies 0.48 <a ≦ 0.78.

Further, preferably, the upper piezoelectric film 22 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical Formula 7).
Pb (Zr _1-b Ti _b ) O ₃ ... (Chemical formula 7)
Here, b satisfies 0.28 ≦ b ≦ 0.48. The general formula (Chemical formula 7) represents the same composite oxide as the general formula (Chemical formula 2).

In such a case, the upper piezoelectric film 22 has a Zr composition ratio larger than that of MPB, that is, has a Zr-rich composition, and tends to be a soft PZT.

In the composite oxide composed of lead zirconate titanate represented by the above general formula (Chemical Formula 7), a part of Pb is element B in order to improve the insulating property or piezoelectric characteristics of lead zirconate titanate. It may be replaced. The element B comprises one or more selected from the group consisting of Li, Na, K, Rb, Ca, Sr, Ba, Bi and La. In such a case, the upper piezoelectric film 22 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical formula 8) instead of the above general formula (Chemical formula 7).
_{(Pb 1-y B y)} (Zr 1-b Ti b) O 3 ··· ( of 8)
Here, y satisfies 0 <y ≦ 0.04, and b satisfies 0.28 ≦ b ≦ 0.48.

Further, preferably, the sum of the product PR1 of the elastic constant EL1 of the lower piezoelectric film 21 and the thickness TH1 and the product PR2 of the elastic constant EL2 of the upper piezoelectric film 22 and the thickness TH2 is the elasticity of the base 11. It is smaller than the product PRB of the constant ELB and the thickness THB of the substrate 11.

In such a case, for example, when the coefficient of thermal expansion of the substrate 11 is smaller than the coefficient of thermal expansion of the piezoelectric film portion 12, the substrate 11 has compressive stress, and the piezoelectric film portion 12 is pulled. It will have stress. Further, when the sum of the product PR1 and the product PR2 is smaller than the product PRB, compression is performed from the lower surface of the substrate 11 toward the upper surface of the piezoelectric film portion 12 in the thickness direction of the substrate 11 and the piezoelectric film portion 12. The position where the stress is switched to the tensile stress, that is, the stress center position SCP is located on the base 11 side of the interface between the base 11 and the piezoelectric film portion 12, that is, inside the base 11. As a result, the piezoelectric film portion 12 has tensile stress at any position in the thickness direction from the lower surface to the upper surface. Therefore, when the film structure of the first embodiment is used as the piezoelectric element, for example, the stress of the piezoelectric film portion 12 is less likely to fluctuate depending on the mounting state of the piezoelectric element, so that the piezoelectric element can be easily designed. Can be done.

Further, preferably, the lower layer portion of the lower piezoelectric film 21 has a tensile stress. Due to the arrangement relationship with the substrate 11, the lower layer portion of the lower piezoelectric film 21 is most likely to have compressive stress. However, even the lower layer portion of the lower piezoelectric film 21 which is most likely to have a compressive stress has a tensile stress, so that the lower piezoelectric film 21 has a tensile stress at any position in the thickness direction from the lower surface to the upper surface. become. Therefore, the stress center position SCP is surely located on the side of the base 11 side of the boundary surface between the base 11 and the piezoelectric film portion 12, that is, inside the base 11. Therefore, the piezoelectric element can be designed more easily.

Further, preferably, the elastic constant of the lower piezoelectric film 21 is the elastic stiffness C ₁₃ of the lower piezoelectric film 21, and the elastic constant of the upper piezoelectric film 22 is the elastic stiffness C ₁₃ of the upper piezoelectric film 22. In such a case, as the elastic constants of the lower piezoelectric film 21 and the upper piezoelectric film 22, the stress applied to each layer in the thickness direction of the substrate 11 and the upper surface 11a of the substrate 11 in each layer (see FIG. 1). Elastic stiffness C ₁₃ is used, which shows the relationship with the strain in the direction parallel to. Therefore, when a stress in a direction perpendicular to the upper surface 11a of the substrate 11 is applied to the piezoelectric film portion 12, the electric charges generated on the upper surface and the lower surface of the piezoelectric film portion 12 can be evaluated more accurately.

Further, preferably, the piezoelectric constant of the lower piezoelectric film 21 is the piezoelectric constant d ₃₁ of the lower piezoelectric film 21, and the piezoelectric constant of the upper piezoelectric film 22 is the piezoelectric constant d ₃₁ of the upper piezoelectric film 22. As a result, as the piezoelectric constants of the lower piezoelectric film 21 and the upper piezoelectric film 22, the electric field applied in the direction perpendicular to the upper surface 11a (see FIG. 1) of the substrate 11 with respect to the piezoelectric film portion 12, that is, the piezoelectric film. Piezoelectricity showing the relationship between the electric field applied in the thickness direction of the portion 12 and the strain of the piezoelectric film portion 12 in the direction parallel to the upper surface 11a of the substrate 11, that is, the strain in the direction parallel to the upper surface of the piezoelectric film portion 12. The constant d ₃₁ is used. Therefore, it is possible to evaluate the voltage in the thickness direction of the piezoelectric film portion 12 generated when a stress in the direction parallel to the upper surface 11a of the substrate 11 is applied to the piezoelectric film portion 12. Therefore, when a stress is applied to the piezoelectric film portion 12 in a direction parallel to the upper surface 11a of the substrate 11, the electric charges generated on the upper surface and the lower surface of the piezoelectric film portion 12 can be evaluated more accurately.

Preferably, the substrate 11 is made of (100) oriented silicon. The alignment film 13 is formed on the substrate 11 and has a cubic crystal structure, and contains (100) oriented zirconium oxide. The conductive film 14 is formed on the alignment film 13 and has a cubic crystal structure. It has a structure and has a conductive film containing (100) oriented platinum. The lower piezoelectric film 21 is formed on the conductive film 14, has a tetragonal crystal structure, and contains (001) oriented lead zirconate titanate. The upper piezoelectric film 22 has a tetragonal crystal structure and contains (001) oriented lead zirconate titanate. When the PZT having a tetragonal crystal structure is (001) oriented, the polarization direction parallel to the [001] direction and the electric field direction parallel to the thickness direction of the piezoelectric film portion 12 are parallel to each other. Piezoelectric properties are improved.

Here, the fact that the alignment film 13 is (100) oriented means that the (100) plane of the alignment film 13 having a cubic crystal structure is the main surface of the substrate 11 made of silicon and is made of the (100) plane. It means that it is along the upper surface 11a made of silicon, and preferably is parallel to the upper surface 11a made of the (100) plane of the substrate 11 made of silicon. Further, the (100) plane of the alignment film 13 is parallel to the upper surface 11a made of the (100) plane of the substrate 11 only when the (100) plane of the alignment film 13 is completely parallel to the upper surface 11a of the substrate 11. This includes the case where the angle formed by the plane completely parallel to the upper surface 11a of the substrate 11 and the (100) plane of the alignment film 13 is 20 ° or less. The same applies not only to the alignment film 13 but also to the orientation of the films of other layers.

Preferably, the alignment film 13 is epitaxially grown on the substrate 11, and the conductive film 14 is epitaxially grown on the alignment film 13. As a result, when the lower piezoelectric film 21 has a tetragonal crystal structure, the lower piezoelectric film 21 can be epitaxially grown on the conductive film 14, and the lower piezoelectric film 21 is likely to be (001) oriented. Further, when the upper piezoelectric film 22 has a tetragonal crystal structure, the upper piezoelectric film 22 can be epitaxially grown on the lower piezoelectric film 21, and the upper piezoelectric film 22 is likely to be (001) oriented. When the PZT having a square crystal structure is (001) oriented, the polarization direction parallel to the [001] direction and the electric field direction parallel to the thickness direction of the piezoelectric film portion 12 are surely parallel to each other. Therefore, the piezoelectric characteristics are further improved.

In FIG. 1, two directions orthogonal to each other in the upper surface 11a as the main surface of the substrate 11 are the X-axis direction and the Y-axis direction, and the direction perpendicular to the upper surface 11a is the Z-axis direction. In such a case, the epitaxial growth of a certain film means that the film is oriented in any of the X-axis direction, the Y-axis direction, and the Z-axis direction.

Further, although not shown in FIG. 1, preferably, the film structure 10 is formed between the conductive film 14 and the piezoelectric film portion 12, has a perovskite structure, and has the following general formula (Chemical Formula 9). It has an oxide film containing a composite oxide represented by).
_{Sr (Ti z Ru 1-z} ) O 3 ··· ( of 9)

Here, z preferably satisfies 0 ≦ z ≦ 0.4, and more preferably 0.05 ≦ z ≦ 0.2. When z is less than 0.01, the resistance of the composite oxide represented by the above general formula (Chemical Formula 9) becomes high, and there is a possibility that an electric field cannot be sufficiently applied to the lower piezoelectric film 21 and the upper piezoelectric film 22. On the other hand, when z exceeds 0.4, the composite oxide represented by the above general formula (Chemical Formula 9) becomes powder and may not be sufficiently hardened.

Since the film structure 10 has an oxide film having the above-mentioned perovskite structure between the conductive film 14 and the piezoelectric film portion 12, it is included in the piezoelectric film portion 12, particularly the lower piezoelectric film 21 of the piezoelectric film portion 12. Lead zirconate titanate tends to be oriented (001).

(Embodiment 2)
Next, the membrane structure of the second embodiment will be described. The film structure of the second embodiment is different from the film structure of the first embodiment in that the hard PZT is laminated on the soft PZT, and the soft PZT is laminated on the hard PZT.

FIG. 4 is a cross-sectional view of the membrane structure of the second embodiment. As shown in FIG. 4, the film structure 10 of the second embodiment has a substrate 11 and a piezoelectric film portion 12 formed on an upper surface 11a as a main surface of the substrate 11, and has a piezoelectric film portion. Reference numeral 12 denotes a lower piezoelectric film 23 and an upper piezoelectric film 24 formed on the lower piezoelectric film 23. The lower piezoelectric film 23 contains lead zirconate titanate. The upper piezoelectric film 24 contains lead zirconate titanate. The substrate 11 has a compressive stress (compressive stress CS2 shown in FIG. 5 described later) applied in a direction parallel to the upper surface 11a of the substrate 11, that is, applied along the upper surface 11a. Both the lower piezoelectric film 23 and the upper piezoelectric film 24 are applied in a direction parallel to the upper surface 11a of the substrate 11, that is, a tensile stress applied along the upper surface 11a (tensile stress TS3 shown in FIG. 5 described later). And TS4).

In the second embodiment, unlike the first embodiment in which the elastic constant of the lower piezoelectric film 21 is larger than the elastic constant of the upper piezoelectric film 22, the elastic constant of the lower piezoelectric film 23 is larger than the elastic constant of the upper piezoelectric film 24. small. That is, the elastic constant of the upper piezoelectric film 24 is larger than the elastic constant of the lower piezoelectric film 23. In other words, in the second embodiment, unlike the first embodiment, the lead zirconate titanate contained in the lower piezoelectric film 23 is a so-called soft PZT, and the lead zirconate titanate contained in the upper piezoelectric film 24 is It is a so-called hard PZT, and is laminated upside down from the first embodiment. Further, the amount of strain when a constant voltage is applied to the lead zirconate titanate contained in the lower piezoelectric film 23 is larger than the amount of strain when the same voltage is applied to the lead zirconate titanate contained in the upper piezoelectric film 24. large.

As will be described with reference to FIG. 5 described later, the film structure 10 of the second embodiment has such a structure, so that, for example, the piezoelectric film portion 12 has an upper surface 11a of the substrate 11 (see FIG. 4). When a stress in a direction perpendicular to the above or a stress in a direction parallel to the upper surface 11a of the substrate 11 is applied, the electric charge generated on the upper surface and the lower surface of the piezoelectric film portion 12 can be increased. That is, the piezoelectric characteristics (positive piezoelectric characteristics) of the piezoelectric film portion 12 can be improved.

As shown in FIG. 4, the film structure 10 of the second embodiment also preferably has the alignment film 13, the conductive film 14, and the conductive film 15 in the same manner as the film structure 10 of the first embodiment. , Have. The alignment film 13 is formed on the substrate 11. The conductive film 14 is formed on the alignment film 13. The conductive film 14 is a lower electrode. The piezoelectric film portion 12, that is, the lower piezoelectric film 23 is formed on the conductive film 14. The conductive film 15 is formed on the piezoelectric film portion 12, that is, on the upper piezoelectric film 24. The conductive film 15 is an upper electrode.

As shown in FIG. 4, in the second embodiment as well, the case where the film structure 10 is formed on the upper surface side of the SOI substrate 16 is considered as in the first embodiment. The SOI substrate 16 includes a silicon substrate 17, a BOX layer 18 formed on the silicon substrate 17, and an SOI layer 19 formed on the BOX layer 18. In such a case, the SOI layer 19 made of silicon can be used as the substrate 11.

Further, also in the second embodiment, as in the first embodiment, as shown in FIG. 2, an opening 17a is formed from the lower surface of the silicon substrate 17 through the silicon substrate 17 to reach the BOX layer 18, and the BOX is formed. An opening 18a that penetrates the BOX layer 18 from the lower surface of the layer 18 to reach the SOI layer 19 and communicates with the opening 17a is formed, and a part of the conductive film 15 as an upper electrode is etched and patterned. Can be done. As a result, in a plan view, the substrate 11 made of the SOI layer 19, the alignment film 13, the conductive film 14, the piezoelectric film portion 12, and the conductive film 15 are provided in the opening 17a and the opening 18a. A piezoelectric element made of the membrane structure 10 can be formed.

In the second embodiment as well, as in the first embodiment, when the SOI layer 19 made of silicon having a high impurity concentration and high conductivity is used as the substrate 11, the alignment film 13 is used. , The conductive film 14 can be omitted. Further, when the substrate 11 is used, for example, when a probe having conductivity is brought into contact with the surface of the piezoelectric film portion 12, the conductive film 15 can be omitted. Further, as the substrate 11, various films such as an oxide film such as silicon oxide can be used in addition to silicon.

Further, various substrates can be used instead of the substrate 11. That is, the piezoelectric film portion 12 does not have to be formed on the substrate 11 having a film shape, and can be formed on various substrates such as silicon and glass.

Next, the technical idea of the film structure of the second embodiment will be described using the result of calculating the influence of the ratio of the thickness of the lower piezoelectric film 23 to the thickness of the entire piezoelectric film portion 12 on the generated charge. .. FIG. 5 is a cross-sectional view of the membrane structure used in the calculation of the second embodiment. In FIG. 5, in order to simplify the understanding of the tensile stress of the piezoelectric membrane portion 12 of the membrane structure 10, the radius of curvature of the membrane structure 10 when the membrane structure 10 is curved, that is, warped, is shown. It is exaggerated to be smaller than the actual radius of curvature. Further, in FIG. 5, the compressive stress of the substrate 11 is indicated as the compressive stress CS2, the tensile stress of the lower piezoelectric film 23 is indicated as the tensile stress TS3, and the tensile stress of the upper piezoelectric film 24 is indicated as the tensile stress TS4. are doing. As shown in FIG. 5, when the substrate 11 has a compressive stress CS2, the lower piezoelectric film 23 has a tensile stress TS3, and the upper piezoelectric film 24 has a tensile stress TS4, the film structure 10 is convex downward. It will be curved or warped so that it has the shape of.

Regarding the membrane structure of the second embodiment, similarly to the membrane structure of the first embodiment, the present inventors add the thickness TH3 of the lower piezoelectric film 23 and the thickness TH4 of the upper piezoelectric film 24. That is, the effect of the ratio RT2 on the generated charge when the ratio RT2 of the thickness TH3 of the lower piezoelectric film 23 to the thickness of the entire piezoelectric film portion 12 was changed was calculated. As a result, the present inventors have found that the lead zirconate titanate contained in the lower piezoelectric film 23 is composed of soft PZT, and the lead zirconate titanate contained in the upper piezoelectric film 24 is composed of hard PZT, that is, soft system. When the hard PZT is laminated on the PZT, the piezoelectric characteristics are higher than those in the case where the entire piezoelectric film portion 12 is composed of only the hard PZT and the case where the entire piezoelectric film portion 12 is composed of only the soft PZT. Was found to improve.

As a model for calculating the effect of the ratio RT2 of the thickness of the lower piezoelectric film 23 to the thickness of the entire piezoelectric film portion 12 on the generated charge, as shown in FIG. 5, the alignment film 13 (see FIG. 4) and A simplified model of the film structure was used by omitting the conductive film 14 (see FIG. 4) and the conductive film 15 (see FIG. 4). Further, the substrate 11 is made of silicon, and the thickness of the substrate 11 is THB.
Was set to 1 μm, and the overall thickness of the piezoelectric film portion 12 was set to 1 μm. Further, as the relative permittivity of lead zirconate titanate contained in the lower piezoelectric film 23, the piezoelectric constant d ₃₁ , and the young rate, the relative permittivity and piezoelectric constant of lead zirconate titanate contained in the upper piezoelectric film 22 shown in Table 1 are shown. d ₃₁ , Young rate was used. Further, as the relative permittivity of lead zirconate titanate contained in the upper piezoelectric film 24, the piezoelectric constant d ₃₁ , and the young rate, the relative permittivity and piezoelectric constant of lead zirconate titanate contained in the lower piezoelectric film 21 shown in Table 1 are shown. d ₃₁ , Young rate was used.

Under such conditions, the ratio RT2 of the thickness of the lower piezoelectric film 23 to the thickness of the entire piezoelectric film portion 12 is changed to 0, 0.25, 0.5, 0.75, 1 to generate the ratio RT2. The effect on charge was calculated. The result is shown in FIG. FIG. 6 is a graph showing the effect of the ratio of the thickness of the lower piezoelectric film to the thickness of the entire piezoelectric film portion in the film structure of the second embodiment on the generated charge. The horizontal axis of FIG. 6 shows the ratio RT2 of the thickness of the lower piezoelectric film 23 to the thickness of the entire piezoelectric film portion 12, and the vertical axis of FIG. 6 is the amount of electric charge generated on the upper surface and the lower surface of the piezoelectric film portion 12. It shows the generated charge index, which is an index showing. Further, in FIG. 6, the ratio RT2 of the thickness of the lower piezoelectric film 23 to the thickness of the entire piezoelectric film portion 12 is referred to as “lower layer ratio” and is shown using a value obtained by multiplying the ratio RT2 by 100. ing.

As shown in FIG. 6, when the ratio RT2 is 0.25 to 0.75, the generated charge index is increased as compared with the case where the ratio RT2 is 0 and the case where the ratio RT2 is 1. .. That is, when the hard PZT is laminated on the soft PZT and the ratio RT2 is 0.25 to 0.75, the entire piezoelectric film portion 12 is composed of only the hard PZT (ratio RT2). It has been clarified that the piezoelectric characteristics are improved as compared with the case where (0) and the case where the entire piezoelectric film portion 12 is composed of only the soft PZT (when the ratio RT2 is 1).

Preferably, the ratio RT2 of the thickness TH3 to the sum of the thickness TH3 of the lower piezoelectric film 23 and the thickness TH4 of the upper piezoelectric film 24 is 0.5 to 0.75. As shown in FIG. 6, when the ratio RT2 is 0.5 or more, the piezoelectric constant of the lower layer portion of the piezoelectric film portion 12 is larger than that when the ratio RT2 is less than 0.5, and the piezoelectric film portion 12 has a larger piezoelectric constant. When stress is applied to the piezoelectric film portion 12, electric charges are likely to be generated on the upper surface and the lower surface of the piezoelectric film portion 12. Further, as shown in FIG. 6, when the ratio RT2 is 0.75 or less, the piezoelectric film portion 12 is compared with the case where the ratio RT2 exceeds 0.75 when stress is applied to the piezoelectric film portion 12. The tensile stress of the lower layer portion is reduced, and charges are likely to be generated on the upper surface and the lower surface of the piezoelectric film portion 12. Further, when the ratio RT2 is close to 0.5, the thickness of the lower piezoelectric film 23 and the thickness of the upper piezoelectric film 24 can be made substantially equal to those when the ratio RT2 is far from 0.5. The time required to form the piezoelectric film portion 12 per unit film thickness can be shortened most, the productivity of the film structure 10 can be improved, and the manufacturing cost of the film structure 10 can be reduced.

Preferably, the piezoelectric constant of the lower piezoelectric film 23 is larger than the piezoelectric constant of the upper piezoelectric film 24. That is, the piezoelectric constant of the upper piezoelectric film 24 is smaller than the piezoelectric constant of the lower piezoelectric film 23. In such a case, as described above, when the thickness TH3 of the lower piezoelectric film 23 is equal to or greater than the thickness TH4 of the upper piezoelectric film 24, for example, the piezoelectric film portion 12 is perpendicular to the upper surface of the substrate 11. It is possible to surely increase the charge generated on the upper surface and the lower surface of the piezoelectric film portion 12 when the stress in the above direction or the stress in the direction parallel to the upper surface of the substrate 11 is applied. That is, the piezoelectric characteristics (positive piezoelectric characteristics) of the piezoelectric film portion 12 can be reliably improved.

Preferably, the lower piezoelectric film 23 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical Formula 10).
Pb (Zr _1-c Ti _c ) O ₃ ... (Chemical formula 10)
Here, c satisfies 0.28 ≦ c ≦ 0.48. The general formula (Chemical formula 10) represents the same composite oxide as the general formula (Chemical formula 3).

In such a case, the lower piezoelectric film 23 has a Zr composition ratio larger than that of MPB, that is, has a Zr-rich composition, and tends to be a soft PZT.

In the composite oxide composed of lead zirconate titanate represented by the above general formula (Chemical Formula 10), a part of Pb is element C in order to improve the insulating property or piezoelectric characteristics of lead zirconate titanate. It may be replaced. The element C comprises one or more selected from the group consisting of Li, Na, K, Rb, Ca, Sr, Ba, Bi and La. In such a case, the lower piezoelectric film 23 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical Formula 11) instead of the above general formula (Chemical Formula 10).
(Pb _{1-u Cu} ) (Zr _1-c Ti _c ) O ₃ ... (Chemical 11)
Here, u satisfies 0 <u ≦ 0.04, and c satisfies 0.28 ≦ c ≦ 0.48.

Preferably, the upper piezoelectric film 24 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical Formula 12).
Pb (Zr _1-d Ti _d ) O ₃ ... (Chemical formula 12)
Here, d satisfies 0.48 <d ≦ 0.78. The general formula (Chemical formula 12) represents the same composite oxide as the general formula (Chemical formula 4).

In such a case, the upper piezoelectric film 24 has a Ti composition ratio larger than that of MPB, that is, has a Ti-rich composition, and tends to be a hard PZT.

In the composite oxide composed of lead zirconate titanate represented by the above general formula (Chemical Formula 12), a part of Pb is element D in order to improve the insulating property or piezoelectric characteristics of lead zirconate titanate. It may be replaced. D comprises one or more selected from the group consisting of Li, Na, K, Rb, Ca, Sr, Ba, Bi and La. In such a case, the upper piezoelectric film 24 contains a composite oxide composed of lead zirconate titanate represented by the following general formula (Chemical formula 13) instead of the above general formula (Chemical formula 12).
(Pb _1-v D _v ) (Zr _1-d Ti _d ) O ₃ ... (Chemical formula 8)
Here, v satisfies 0 <y ≦ 0.04, and d satisfies 0.48 <d ≦ 0.78.

Further, preferably, the sum of the product PR3 of the elastic constant EL3 of the lower piezoelectric film 23 and the thickness TH3 and the product PR4 of the elastic constant EL4 of the upper piezoelectric film 24 and the thickness TH4 is the elasticity of the substrate 11. It is smaller than the product PRB of the constant ELB and the thickness THB of the substrate 11.

In such a case, for example, when the coefficient of thermal expansion of the substrate 11 is smaller than the coefficient of thermal expansion of the piezoelectric film portion 12, the substrate 11 has compressive stress, and the piezoelectric film portion 12 is pulled. It will have stress. Further, when the sum of the product PR3 and the product PR4 is smaller than the product PRB, the stress center position SCP is set in the thickness direction of the substrate 11 and the piezoelectric film portion 12 as described above in the first embodiment. , The substrate 11 is located closer to the substrate 11 than the interface between the substrate 11 and the piezoelectric film portion 12. As a result, the piezoelectric film portion 12 has tensile stress at any position in the thickness direction from the lower surface to the upper surface. Therefore, when the film structure of the second embodiment is used as the piezoelectric element, for example, the stress of the piezoelectric film portion 12 is less likely to fluctuate depending on the mounting state of the piezoelectric element, so that the piezoelectric element can be easily designed. Can be done.

Further, preferably, the lower layer portion of the lower piezoelectric film 23 has a tensile stress. As a result, as described above in the first embodiment, the lower piezoelectric film 23 has tensile stress at any position in the thickness direction from the lower surface to the upper surface. Therefore, the stress center position SCP is surely located inside the substrate 11. Therefore, the piezoelectric element can be designed more easily.

Further, preferably, the elastic constant of the lower piezoelectric film 23 is the elastic stiffness C ₁₃ of the lower piezoelectric film 23, and the elastic constant of the upper piezoelectric film 24 is the elastic stiffness C ₁₃ of the upper piezoelectric film 24. In such a case, as the elastic constants of the lower piezoelectric film 23 and the upper piezoelectric film 24, the stress applied to each layer in the thickness direction of the substrate 11 and the upper surface 11a of the substrate 11 in each layer (see FIG. 4). Elastic stiffness C ₁₃ is used, which shows the relationship with the strain in the direction parallel to. Therefore, when a stress in a direction perpendicular to the upper surface 11a of the substrate 11 is applied to the piezoelectric film portion 12, the electric charges generated on the upper surface and the lower surface of the piezoelectric film portion 12 can be evaluated more accurately.

Further, preferably, the piezoelectric constant of the lower piezoelectric film 23 is the piezoelectric constant d ₃₁ of the lower piezoelectric film 23, and the piezoelectric constant of the upper piezoelectric film 24 is the piezoelectric constant d ₃₁ of the upper piezoelectric film 24. As a result, as the piezoelectric constants of the lower piezoelectric film 23 and the upper piezoelectric film 24, an electric field applied in a direction perpendicular to the upper surface 11a (see FIG. 4) of the substrate 11 with respect to the piezoelectric film portion 12, that is, the piezoelectric film. Piezoelectricity showing the relationship between the electric field applied in the thickness direction of the portion 12 and the strain of the piezoelectric film portion 12 in the direction parallel to the upper surface 11a of the substrate 11, that is, the strain in the direction parallel to the upper surface of the piezoelectric film portion 12. The constant d ₃₁ is used. Therefore, it is possible to evaluate the voltage in the thickness direction of the piezoelectric film portion 12 generated when a stress in the direction parallel to the upper surface 11a of the substrate 11 is applied to the piezoelectric film portion 12. Therefore, when a stress is applied to the piezoelectric film portion 12 in a direction parallel to the upper surface 11a of the substrate 11, the electric charges generated on the upper surface and the lower surface of the piezoelectric film portion 12 can be evaluated more accurately.

Also in the second embodiment, similarly to the first embodiment, the substrate 11 is preferably made of (100) oriented silicon. The alignment film 13 is formed on the substrate 11 and has a cubic crystal structure, and contains (100) oriented zirconium oxide. The conductive film 14 is formed on the alignment film 13 and has a cubic crystal structure. It has a structure and has a conductive film containing (100) oriented platinum. The lower piezoelectric film 23 is formed on the conductive film 14, has a tetragonal crystal structure, and contains (001) oriented lead zirconate titanate. The upper piezoelectric film 24 has a tetragonal crystal structure and contains (001) oriented lead zirconate titanate. As a result, as in the first embodiment, the polarization direction parallel to the [001] direction and the electric field direction parallel to the thickness direction of the piezoelectric film portion 12 are parallel to each other, so that the piezoelectric characteristics are improved.

Also in the second embodiment, similarly to the first embodiment, preferably, the alignment film 13 is epitaxially grown on the substrate 11, and the conductive film 14 is epitaxially grown on the alignment film 13. As a result, when the lower piezoelectric film 23 has a tetragonal crystal structure, the lower piezoelectric film 23 can be epitaxially grown on the conductive film 14, and the lower piezoelectric film 23 is likely to be (001) oriented. Further, when the upper piezoelectric film 24 has a tetragonal crystal structure, the upper piezoelectric film 24 can be epitaxially grown on the lower piezoelectric film 23, and the upper piezoelectric film 24 is likely to be (001) oriented. When the PZT having a square crystal structure is (001) oriented, the polarization direction parallel to the [001] direction and the electric field direction parallel to the thickness direction of the piezoelectric film portion 12 are surely parallel to each other. Therefore, the piezoelectric characteristics are further improved.

Further, although not shown in FIG. 4, in the second embodiment as well, the film structure 10 is preferably formed between the conductive film 14 and the piezoelectric film portion 12, as in the first embodiment. , Has a perovskite structure, and has an oxide film containing a composite oxide represented by the above general formula (Chemical Formula 9).

Hereinafter, the present embodiment will be described in more detail based on the examples. The present invention is not limited to the following examples.

[Measurement pattern and cantilever production]
(Example 1)
In the film structure of the first embodiment, the thickness of the lower piezoelectric film 21 is 250 nm, the thickness of the upper piezoelectric film 22 is 750 nm, and it corresponds to the case where the ratio RT1 is 0.25 in FIG. A film structure, that is, a film structure in which the entire piezoelectric film portion 12 is composed of a laminate of a hard PZT and a soft PZT on the hard PZT, was produced as the film structure of Example 1.

Hereinafter, the method for forming the film structure of Example 1 will be described. First, the SOI substrate 16 (see FIG. 1) was prepared. The SOI substrate 16 includes a silicon substrate 17 (see FIG. 1), a BOX layer 18 formed on the silicon substrate 17, and a substrate 11 composed of an SOI layer 19 formed on the BOX layer 18. Had an upper surface as a main surface composed of a (100) surface.

Next, a zirconium oxide (ZrO ₂ ) film was formed on the substrate 11 as an alignment film 13 (see FIG. 1) by an electron beam vapor deposition method. The conditions at this time are shown below.
Equipment: Electron beam vapor deposition equipment Pressure: 7.00 x 10 ^-3 Pa
Thin film deposition source: Zr + O ₂
Acceleration voltage / emission current: 7.5kV / 1.80mA
Thickness: 24 nm
Substrate temperature: 500 ° C

Next, a platinum (Pt) film was formed on the alignment film 13 as a conductive film 14 (see FIG. 1) by a sputtering method. The conditions at this time are shown below.
Equipment: DC sputtering equipment Pressure: 1.20 x 10 ^-1 Pa
Deposition source: Pt
Electric power: 100W
Thickness: 150 nm
Substrate temperature: 450-600 ° C

Next, on the conductive film 14, as the lower piezoelectric film 21 (see FIG. 1), as a hard material _Pb a _{(Zr 0.30 Ti} 0.70) _{O 3} film (PZT film) was formed by sputtering .. The thickness of the lower piezoelectric film 21 was 250 nm. The conditions at this time are shown below.
Equipment: RF magnetron sputtering equipment Power: 1750W
Gas: Ar / O ₂
Pressure: 1 Pa
Substrate temperature: 380 ° C

Next, on the lower piezoelectric film 21, as the upper piezoelectric film 22 (see FIG. _{1), (Pb 0.98 La 0.02} ) as a soft material _{(Zr 0.58 Ti} 0.42) _{O 3} film ( The PLZT film) was formed by a sputtering method. The thickness of the upper piezoelectric film 22 was 750 nm. The conditions at this time are shown below.
Equipment: RF magnetron sputtering equipment Power: 1750W
Gas: Ar / O ₂
Pressure: 1 Pa
Substrate temperature: 380 ° C

Next, a platinum (Pt) film was formed on the upper piezoelectric film 22 as the conductive film 15 (see FIG. 1) by a sputtering method.

Next, a portion of the conductive film 15 arranged directly above the silicon substrate 17 via the piezoelectric film portion 12 was etched and patterned. As a result, the measurement pattern of Example 1 for measuring the piezoelectric constant d ₃₃ was produced.

Although detailed description will be omitted, when evaluated using the X-Ray Diffraction (XRD) method, the zirconium oxide (ZrO ₂ ) film has a cubic crystal structure and ( It was found that the platinum (Pt) film had a cubic crystal structure and was (100) oriented. It was found that both the lower piezoelectric film 21 and the upper piezoelectric film 22 had a tetragonal crystal structure and were (001) oriented.

Further, an opening 17a is formed from the lower surface of the silicon substrate 17 through the silicon substrate 17 to reach the BOX layer 18, and from the lower surface of the BOX layer 18 through the BOX layer 18 to reach the SOI layer 19 and the opening. An opening 18a communicating with the 17a was formed, and portions of the conductive film 15 arranged in the opening 17a and the opening 18a in a plan view were etched and patterned. As a result, a film having a substrate 11 made of an SOI layer 19, an alignment film 13, a conductive film 14, a piezoelectric film portion 12, and a conductive film 15 in the opening 17a and the opening 18a in a plan view. A cantilever of Example 1 was made of the structure 10 for measuring the piezoelectric constant d ₃₁ .

(Example 2)
The thickness of the lower piezoelectric film 21 is 300 nm and the thickness of the upper piezoelectric film 22 is determined by the same procedure as in Example 1 except that the film formation time for forming the lower piezoelectric film 21 and the upper piezoelectric film 22 is changed. A film structure corresponding to the case where the thickness is 700 nm and the ratio RT1 is 0.3 in FIG. 3 was produced as the film structure of Example 2.

(Example 3)
The thickness of the lower piezoelectric film 21 is 500 nm and the thickness of the upper piezoelectric film 22 is determined by the same procedure as in Example 1 except that the film formation time for forming the lower piezoelectric film 21 and the upper piezoelectric film 22 is changed. A film structure corresponding to the case where the thickness is 1000 nm and the ratio RT1 is 0.33 in FIG. 3 was prepared as the film structure of Example 3.

(Example 4)
The thickness of the lower piezoelectric film 21 is 500 nm and the thickness of the upper piezoelectric film 22 is determined by the same procedure as in Example 1 except that the film formation time for forming the lower piezoelectric film 21 and the upper piezoelectric film 22 is changed. A film structure corresponding to the case where the thickness is 1500 nm and the ratio RT1 is 0.25 in FIG. 3 was prepared as the film structure of Example 4.

(Comparative Example 1)
The thickness of the lower piezoelectric film 21 is 1000 nm according to the same procedure as in Example 1 except that the upper piezoelectric film 22 is not formed and only the lower piezoelectric film 21 is formed, and the ratio is shown in FIG. A film structure corresponding to the case where RT1 is 1, that is, a film structure in which the entire piezoelectric film portion 12 is composed of only a hard PZT was produced as the film structure of Comparative Example 1.

(Comparative Example 2)
The thickness of the upper piezoelectric film 22 is 1000 nm according to the same procedure as in Example 1 except that the lower piezoelectric film 21 is not formed and only the upper piezoelectric film 22 is formed, and the ratio is shown in FIG. A film structure corresponding to the case where RT1 is 0, that is, a film structure in which the entire piezoelectric film portion 12 is composed of only a soft PZT was produced as the film structure of Comparative Example 2.

[Voltage dependence of polarization and voltage dependence of displacement]
(Comparative Example 1)
With respect to the film structure of Comparative Example 1, the voltage dependence of polarization was measured by applying a voltage between the conductive film 14 and the conductive film 15 using the produced measurement pattern. FIG. 7 is a graph showing the voltage dependence of the polarization of the film structure of Comparative Example 1. Further, with respect to the membrane structure of Comparative Example 1, the voltage dependence of the displacement of the membrane structure was measured using the produced cantilever. FIG. 8 is a graph showing the voltage dependence of the displacement of the film structure of Comparative Example 1.

As shown in FIG. 7, the residual polarization _{P r} is positive in a 50.5μC / cm ^2, a -54.3μC / cm ² in the negative side, the anti-voltage _{V c} is 34.1V with positive On the negative side, it was -13.2 V, and the relative permittivity ε _r was 126. Further, as shown in FIG. 8, the piezoelectric constant d ₃₁ is −100 pm / V (pC / N), and the piezoelectric constant g ₃₁ (piezoelectric constant d ₃₁ / relative permittivity ε _r ) is −79 × 10 ^{−. It was 3} Vm / N (m ² / C).

(Comparative Example 2)
For the film structure of Comparative Example 2, the voltage dependence of polarization was measured by applying a voltage between the conductive film 14 and the conductive film 15 using the produced measurement pattern. FIG. 9 is a graph showing the voltage dependence of the polarization of the film structure of Comparative Example 2. Further, with respect to the film structure of Comparative Example 2, the voltage dependence of the displacement of the film structure was measured using the produced cantilever. FIG. 10 is a graph showing the voltage dependence of the displacement of the film structure of Comparative Example 2.

As shown in FIG. 9, the residual polarization _{P r} is positive in a 19.1μC / cm ^2, a -36.3μC / cm ² in the negative side, the anti-voltage _{V c} is 11.9 V in the positive side On the negative side, it was -4.0 V, and the relative permittivity ε _r was 268. Further, as shown in FIG. 10, the piezoelectric constant d ₃₁ is −208 pm / V (pC / N), and the piezoelectric constant g ₃₁ (piezoelectric constant d ₃₁ / relative permittivity ε _r ) is −78 × 10 ^{−. It was 3} Vm / N (m ² / C).

Comparing FIGS. 7 and 9, in Comparative Example 1, the area of the portion surrounded by the hysteresis curve showing the voltage dependence of polarization is larger than that in Comparative Example 2, and the hysteresis curve is larger near the coercive voltage. It was excellent in so-called squareness, which changed parallel to the vertical axis. This is consistent with the fact that in the film structure of Comparative Example 1, the entire piezoelectric film portion 12 is composed of only the hard PZT, and in the film structure of Comparative Example 2, the entire piezoelectric film portion 12 is composed of only the soft PZT. It was.

[Piezoelectric constant]
The piezoelectric constant d ₃₃ , the piezoelectric constant d ₃₁ and the piezoelectric constant g ₃₁ were evaluated using the measurement patterns and cantilever for the six types of film structures of Examples 1 to 4 and Comparative Examples 1 and 2. Table 2 shows the evaluation results of the piezoelectric constant d ₃₃ , the piezoelectric constant d ₃₁ and the piezoelectric constant g ₃₁ . In Table 2, the piezoelectric constant d ₃₁ and the piezoelectric constant g ₃₁ are shown as absolute values, omitting the notation of negative symbols. In Table 2, the amount of warpage of the SOI substrate 16 before the cantilever is manufactured is also shown. When the sign of the amount of warpage is negative, it means that the SOI substrate 16 is curved or warped so as to have a downwardly convex shape.

As shown in Table 2, when the piezoelectric film portion 12 is a hard PZT on which a soft PZT is laminated (Examples 1 to 4), the piezoelectric constant d ₃₃ is 255 to 298 pC / N. When the piezoelectric constant d ₃₃ is 199 pC / N and the piezoelectric film portion 12 is composed of only the hard PZT (Comparative Example 1), and when the piezoelectric constant d ₃₃ is 250 pC / N and the piezoelectric film portion 12 is composed of only the soft PZT. (Comparative Example 2), the piezoelectric constant d ₃₃ was larger than in any of the cases.

As shown in Table 2, when the piezoelectric film portion 12 is a hard PZT on which a soft PZT is laminated (Examples 1 to 4), the absolute value of the piezoelectric constant d ₃₁ is 196 to 245 pC /. N, the absolute value of the piezoelectric constant d ₃₁ is 145 pC / N, the piezoelectric film portion 12 is composed of only the hard PZT (Comparative Example 1), and the absolute value of the piezoelectric constant d ₃₁ is 173 pC / N. The absolute value of the piezoelectric constant d ₃₁ was larger than any of the cases where the film portion 12 was composed of only the soft PZT (Comparative Example 2).

As described above, in the measurement results of the piezoelectric constant d ₃₃ and the piezoelectric constant d ₃₁ , when the piezoelectric film portion 12 is a hard PZT on which a soft PZT is laminated (Examples 1 to 4). The piezoelectric characteristics (positive piezoelectric characteristics) are improved as compared with the case where the piezoelectric film portion 12 is composed of only the hard PZT (Comparative Example 1) and the case where the piezoelectric film portion 12 is composed of only the soft PZT (Comparative Example 2). What was done was consistent with the calculation result of FIG. Therefore, when the soft PZT is laminated on the hard PZT, it is compared with either the case where the entire piezoelectric film portion 12 is composed of only the hard PZT or the case where the entire piezoelectric film portion 12 is composed of only the soft PZT. However, it was clarified that the piezoelectric characteristics were improved.

It was also measured amount of charges generated upon application of a compressive stress from the upper and lower piezoelectric film 12 by using the measurement pattern and d ₃₃ meter for Comparative Example 1, each of the film structure of Comparative Example 2 and Example 2 The results are shown in the graphs of FIGS. 11 to 13. FIG. 11 corresponds to Comparative Example 1, FIG. 12 corresponds to Comparative Example 2, and FIG. 13 corresponds to Example 2. The horizontal axis of the graphs of FIGS. 11 to 13 displays the number of measurements performed a plurality of times as steps, and the vertical axis of the graphs of FIGS. 11 to 13 shows the measured charges in a standardized manner. .. The charges shown on the vertical axis of the graphs of FIGS. 11 to 13 correspond to the generated charge index of FIG. 3 and correspond to the piezoelectric constant d ₃₃ shown in Table 2.

The charges generated in the first step in each of the film structures of Comparative Example 1, Comparative Example 2 and Example 2 are referred to as charge CR1, charge CR2 and charge CR3. At this time, as shown in FIGS. 11 to 13, the charge CR3 was larger than either the charge CR1 and the charge CR2. Therefore, according to FIGS. 11 to 13, when the piezoelectric film portion 12 is a hard PZT on which a soft PZT is laminated (Example 2), the piezoelectric film portion 12 is composed of only the hard PZT. The improvement in the piezoelectric characteristics (positive piezoelectric characteristics) as compared with both the case of (Comparative Example 1) and the case where the piezoelectric film portion 12 is composed of only the soft PZT (Comparative Example 2) is consistent with the calculation result of FIG. It was. Therefore, when the hard PZT is laminated on the soft PZT, it is compared with either the case where the entire piezoelectric film portion 12 is composed of only the hard PZT or the case where the entire piezoelectric film portion 12 is composed of only the soft PZT. However, it was clarified that the piezoelectric characteristics were improved.

Although the invention made by the present inventor has been specifically described above based on the embodiment thereof, the present invention is not limited to the embodiment and can be variously modified without departing from the gist thereof. Needless to say.

Within the scope of the idea of the present invention, those skilled in the art can come up with various modified examples and modified examples, and it is understood that these modified examples and modified examples also belong to the scope of the present invention.

For example, a person skilled in the art appropriately adds, deletes, or changes the design of each of the above-described embodiments, or adds, omits, or changes the conditions of the process of the present invention. As long as it has a gist, it is included in the scope of the present invention.

10 Membrane structure 11 Base 11a Upper surface 12 Piezoelectric film 13 Alignment film 14, 15 Conductive 16 SOI substrate 17 Silicon substrate 17a, 18a Opening 18 BOX layer 19 SOI layer 21, 24 Lower piezoelectric film 22, 23 Upper piezoelectric film CR1 ~ CR3 Charges CS1, CS2 Compressive stress SCP Stress center position TH1 to TH4, THB Thickness TS1 to TS4 Tension stress

Claims (13)

With the base
A first piezoelectric film formed on the substrate and containing lead zirconate titanate,
A second piezoelectric film formed on the first piezoelectric film and containing lead zirconate titanate,
Have,
The substrate has compressive stress and
Both the first piezoelectric film and the second piezoelectric film have tensile stress and have tensile stress.
A film structure in which the first elastic constant of the first piezoelectric film is larger than the second elastic constant of the second piezoelectric film. In the membrane structure according to claim 1,
A film structure in which the ratio of the first thickness to the sum of the first thickness of the first piezoelectric film and the second thickness of the second piezoelectric film is 0.25 to 0.75. In the membrane structure according to claim 2,
A film structure in which the first piezoelectric constant of the first piezoelectric film is smaller than the second piezoelectric constant of the second piezoelectric film. In the membrane structure according to any one of claims 1 to 3,
The first piezoelectric film contains lead zirconate titanate represented by the following general formula (Chemical Formula 1).
Pb (Zr _1-a Ti _a ) O ₃ ... (Chemical formula 1)
The second piezoelectric film contains lead zirconate titanate represented by the following general formula (Chemical Formula 2).
Pb (Zr _1-b Ti _b ) O ₃ ... (Chemical formula 2)
The a satisfies 0.48 <a ≦ 0.78, and
The b is a film structure satisfying 0.28 ≦ b ≦ 0.48. With the base
A first piezoelectric film formed on the substrate and containing lead zirconate titanate,
A second piezoelectric film formed on the first piezoelectric film and containing lead zirconate titanate,
Have,
The substrate has compressive stress and
Both the first piezoelectric film and the second piezoelectric film have tensile stress and have tensile stress.
The first elastic constant of the first piezoelectric film is smaller than the second elastic constant of the second piezoelectric film.
A film structure in which the ratio of the first thickness to the sum of the first thickness of the first piezoelectric film and the second thickness of the second piezoelectric film is 0.25 to 0.75. In the membrane structure according to claim 5,
A film structure in which the first piezoelectric constant of the first piezoelectric film is larger than the second piezoelectric constant of the second piezoelectric film. In the membrane structure according to claim 5 or 6,
The first piezoelectric film contains lead zirconate titanate represented by the following general formula (Chemical Formula 3).
Pb (Zr _1-c Ti _c ) O ₃ ... (Chemical formula 3)
The second piezoelectric film contains lead zirconate titanate represented by the following general formula (Chemical Formula 4).
Pb (Zr _1-d Ti _d ) O ₃ ... (Chemical formula 4)
The c satisfies 0.28 ≦ c ≦ 0.48, and
The d is a membrane structure satisfying 0.48 <d ≦ 0.78. In the membrane structure according to claim 2 or 5,
The sum of the first product of the first elastic constant and the first thickness and the second product of the second elastic constant and the second thickness is the third elastic constant of the substrate and the first product of the substrate. A membrane structure that is smaller than the third product with three thicknesses. In the membrane structure according to any one of claims 1 to 8.
The lower layer portion of the first piezoelectric film is a film structure having tensile stress. In the membrane structure according to any one of claims 1 to 9,
The first elastic constant is the elastic stiffness C ₁₃ of the first piezoelectric film.
The second elastic constant is the elastic stiffness C ₁₃ of the second piezoelectric film, film structure. In the membrane structure according to claim 3 or 6,
The first piezoelectric constant is the piezoelectric constant d ₃₁ of the first piezoelectric film.
The second piezoelectric constant is a film structure having a piezoelectric constant d ₃₁ of the second piezoelectric film. In the membrane structure according to any one of claims 1 to 11.
The substrate is a film structure made of silicon. In the membrane structure according to claim 12,
The substrate was made of (100) oriented silicon.
The membrane structure further
A first film formed on the substrate, having a cubic crystal structure, and containing (100) oriented zirconium oxide.
A conductive film formed on the first film, having a cubic crystal structure, and containing (100) oriented platinum,
Have,
The first piezoelectric film is formed on the conductive film, has a tetragonal crystal structure, and contains (001) oriented lead zirconate titanate.
The second piezoelectric film is a film structure having a tetragonal crystal structure and containing lead zirconate titanate oriented (001).

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