Patent Description:
A piezoelectric material is utilized widely for a functional electronic component such as a sensor and an actuator. For example, potassium sodium niobate (KNN) may be used as the piezoelectric material (see patent documents <NUM> and <NUM>, for example). In recent years, there is a strong demand for piezoelectric materials comprising KNN, which has a higher versatility.

<CIT> discloses a piezoelectric contact sensor. <CIT> describes piezoelectric film structures and their use in sensors and display assemblies.

Said publications deal with transparency of the piezoelectric material and devices. Furthermore, a piezoelectric thin film of KNN containing Mn or Cu at a concentration of <NUM> at% or more and <NUM> at% or less, formed by sputtering, is known from <CIT>, for example.

An object of the present invention is to provide a piezoelectric film having an enhanced versatility, and a related manufacturing technique thereof.

According to an aspect of the present invention, there is provided a piezoelectric stack and a related manufacturing technique thereof, including:.

According to the present invention, there is provided a piezoelectric film having an enhanced versatility, and a related manufacturing technique thereof.

An embodiment of the present invention will be described hereafter, with reference to <FIG>.

As illustrated in <FIG>, a stack (stacked substrate) <NUM> (also referred to as a piezoelectric stack <NUM> hereafter) having a piezoelectric film according to the present embodiment, includes a substrate <NUM>, a bottom electrode film <NUM> formed (deposited) on the substrate <NUM>, a piezoelectric film (piezoelectric thin film) <NUM> deposited on the bottom electrode film <NUM>, and a top electrode film <NUM> deposited on the piezoelectric film <NUM>.

A substrate (transparent substrate) with a light transmittance of, for example, <NUM> % or more at least in a wavelength region of visible light (about <NUM> to <NUM>), preferably in a wavelength region of visible light and near-infrared rays (<NUM> to <NUM>) can be preferably used as a substrate <NUM>. For example, a strontium titanate (SrTiO<NUM>, abbreviated as STO) substrate, a quartz glass (SiO<NUM>) substrate, a sapphire (Al<NUM>O<NUM>) substrate, a gallium nitride (GaN) substrate, or a gallium oxide (Ga<NUM>O<NUM>) substrate can be used as the substrate <NUM>. A thickness of the substrate <NUM> is, for example, <NUM> to <NUM>.

The bottom electrode film <NUM> is preferably comprised of an electrode (transparent electrode) with a light transmittance of, for example, <NUM> % or more at least in the wavelength region of visible light, preferably in the wavelength region of visible light and near-infrared rays. The bottom electrode film <NUM> may be deposited using various metallic oxides such as strontium ruthenium oxide (SrRuO<NUM>, abbreviated as SRO), lanthanum nickel oxide (LaNiO<NUM>, abbreviated as LNO). The bottom electrode film <NUM> is a single-crystal film or a poly-crystal film. When the bottom electrode film <NUM> is deposited using SRO, crystals comprised in the bottom electrode film <NUM> (SRO-film) are preferably oriented preferentially in (<NUM>) direction with respect to a surface of the substrate <NUM>. That is, a surface of the SRO-film (a surface to be a base of the piezoelectric film <NUM>) is preferably mainly comprised of SRO-(<NUM>). The same applies to LNO. That is, when the bottom electrode film <NUM> is deposited using LNO, a surface of the LNO-film is preferably mainly comprised of LNO-(<NUM>). The bottom electrode film <NUM> can be deposited by a method such as a sputtering method, or an evaporation method. Instead of SRO, LNO, the bottom electrode film <NUM> can also be deposited using indium oxide such as indium tin oxide (ITO), indium zinc oxide (IZO), indium gallium zinc oxide (IGZO). A thickness of the bottom electrode film <NUM> is, for example, <NUM> to <NUM>.

The bottom electrode film <NUM> can also be a thin film (thin metal film) deposited using various metals such as platinum (Pt), gold (Au), or an alloy containing the above various metals as its main components. When the bottom electrode film <NUM> is deposited using Pt, the bottom electrode film <NUM> (Pt-film) is preferably oriented preferentially in (<NUM>) direction with respect to the surface of the substrate <NUM>. That is, a surface of the Pt-film is preferably comprised of Pt-(<NUM>). The bottom electrode film <NUM> (Pt-film) can be deposited by a method such as a sputtering method, or an evaporation method. The thickness of the bottom electrode film <NUM> (Pt-film) is, for example, <NUM> to <NUM>, preferably <NUM> to <NUM>.

The piezoelectric film <NUM> uses an alkali niobium oxide which contains potassium (K), sodium (Na), and niobium (Nb), and is represented by a composition formula of (K<NUM>-xNax)NbyO<NUM>, namely, potassium sodium niobate (KNN). A coefficient x [= Na / (K + Na)] in the above-mentioned composition formula is a value in a range of <NUM> < x < <NUM>, preferably <NUM> ≤ x ≤ <NUM>. A coefficient y [= (K + Na) / Nb] in the above-mentioned composition formula is a value in a range of <NUM> ≤ y ≤ <NUM>. The piezoelectric film <NUM> is a KNN polycrystalline film (also referred to as a KNN-film <NUM> hereafter). A crystal structure of KNN is a perovskite structure.

Preferably, crystals comprised in the KNN-film <NUM> are oriented preferentially in (<NUM>) direction with respect to the surface of the substrate <NUM>. That is, a surface of the KNN-film <NUM> (a surface to be a base of the top electrode film <NUM>) is preferably mainly comprised of KNN-(<NUM>). By depositing the KNN-film <NUM> directly on the bottom electrode film <NUM> (the SRO-film oriented preferentially in (<NUM>) direction or the Pt-film oriented preferentially in (<NUM>) direction, with respect to the surface of the substrate <NUM>), the crystals comprised in the KNN-film <NUM> can be easily oriented preferentially in (<NUM>) direction with respect to the surface of the substrate <NUM>. For example, <NUM> % or more crystals in a crystal grain group comprised in the KNN-film <NUM> can be oriented in (<NUM>) direction with respect to the surface of the substrate <NUM>, and <NUM> % or more regions of the surface of the KNN-film <NUM> can be KNN-(<NUM>). A thickness of the KNN-film <NUM> is, for example, <NUM> to <NUM>.

The KNN-film <NUM> can be deposited by a method such as a sputtering method, a PLD (Pulsed Laser Deposition) method, or a sol-gel method. According to the present invention, the KNN-film <NUM> is deposited by sputtering. A composition ratio of the KNN-film <NUM> can be adjusted by controlling a composition of a target material used during sputtering deposition, for example. The target material can be produced by mixing and burning K<NUM>CO<NUM>-powder, Na<NUM>CO<NUM>-powder, and Nb<NUM>O<NUM>-powder, for example. In this case, the composition of the target material can be controlled by adjusting a mixed ratio of K<NUM>CO<NUM>-powder, Na<NUM>CO<NUM>-powder, Nb<NUM>O<NUM>-powder, and the like.

The KNN-film <NUM> has optical transmission (transparency). An average light transmittance through the KNN-film <NUM> in the wavelength region of visible light and near-infrared ray (<NUM> to <NUM>) is <NUM> % or more. An average light transmittance through the KNN-film <NUM> in the wavelength region of visible light (<NUM> to <NUM>) is, for example, <NUM> % or more. There is no particular limitation on an upper limit of the average light transmittance, and the upper limit is preferably <NUM> %. However, according to a current technique, the upper limit of the average light transmittance through the KNN-film <NUM> in the wavelength region of visible light and near-infrared rays is about <NUM> %.

The term "average light transmittance" used herein means an average value of the light transmittance through the KNN-film <NUM> in a prescribed wavelength region (range). The light transmittance through the KNN-film <NUM> can be measured by a known light transmittance measuring device. In the present embodiment, the light transmittance through the KNN-film is measured by a spectroscopic ellipsometer (M-<NUM>, manufactured by J. Woollam Co.

An average light transmittance through the KNN-film <NUM> in a wavelength region of purple visible light (<NUM> or more and less than <NUM>) is preferably <NUM> % or more and <NUM> % or less. An average light transmittance through the KNN-film <NUM> in a wavelength region of blue visible light (<NUM> or more and less than <NUM>) is preferably <NUM> % or more and <NUM> % or less. An average light transmittance through the KNN-film <NUM> in a wavelength region of green visible light (<NUM> or more and less than <NUM>) is preferably <NUM> % or more and <NUM> % or less. An average light transmittance through the KNN-film <NUM> in a wavelength region of yellow visible light (<NUM> or more and less than <NUM>) is preferably <NUM> % or more and <NUM> % or less. An average light transmittance through the KNN-film <NUM> in a wavelength region of orange visible light (<NUM> or more and less than <NUM>) is preferably <NUM> % or more and <NUM> % or less. An average light transmittance through the KNN-film <NUM> in a wavelength region of red visible light (<NUM> or more and less than <NUM>) is preferably <NUM> % or more and <NUM> % or less. An average light transmittance through the KNN-film <NUM> in a wavelength region of reddish purple visible light (<NUM> or more and <NUM> or less) is preferably <NUM> % or more and <NUM> % or less.

For example, a mixed gas (Ar/O<NUM>-mixed gas) of argon (Ar) gas and oxygen (O<NUM>) gas is used as an atmosphere gas during sputtering deposition of the KNN-film <NUM>. In order to increase the light transmittance through the KNN-film <NUM>, it is effective to increase a partial pressure (H<NUM>O-partial pressure) of water contained in Ar/O<NUM>-mixed gas during sputtering deposition, or to perform a heat treatment on the KNN-film <NUM> in an ambient air or in an oxygen-containing atmosphere after deposition of the KNN-film <NUM> and before deposition of the top electrode film <NUM> described later. Thereby, the KNN-film <NUM> can be sufficiently oxidized and oxygen deficiencies in the KNN-film <NUM> can be reduced. As a result, the light transmittance through the KNN-film <NUM> can be increased.

For example, the light transmittance through the KNN-film <NUM> can fall within the above-mentioned range by performing the heat treatment on the KNN-film <NUM> for <NUM> hours or more, preferably <NUM> hours or more, and more preferably <NUM> hours or more, per <NUM> thickness of the KNN-film <NUM>, under conditions of <NUM> to <NUM>, preferably <NUM> to <NUM> in an ambient air or in an oxygen-containing atmosphere. The heat treatment is preferably performed at a temperature equal to or higher than a deposition temperature of the KNN-film <NUM>. The light transmittance through the KNN-film <NUM> can fall within the above-mentioned range, for example, by setting the H<NUM>O-partial pressure during the deposition of the KNN-film <NUM> to <NUM> Pa or more, instead of performing the heat treatment.

The KNN-film <NUM> contains a metallic element selected from a group consisting of copper (Cu) and manganese (Mn) at a concentration within a range of <NUM> to <NUM> at%.

By adding at least one of Cu or Mn within the above-mentioned concentration range into the KNN-film <NUM>, a film property of the KNN-film <NUM> can be enhanced. For example, an insulation property (a leak resistance) of the KNN-film <NUM> can be enhanced, and a dielectric constant of the KNN-film <NUM> can be a value suitable for applications of the piezoelectric stack <NUM>. Further, by adding Cu within the above-mentioned concentration range into the KNN-film <NUM>, a resistance to a fluorinated etching liquid (e.g., a buffered hydrofluoric acid (BHF) solution containing hydrogen fluoride (HF) and ammonium fluoride (NH<NUM>F) at respective prescribed concentrations), that is, an etching resistance can be enhanced, in addition to the above-mentioned insulation property. Thereby, a formation of a protect film for protecting an exposed surface of the KNN-film <NUM> is not required. That is, the BHF solution can be used as an etching liquid with no need to form the protect film. As a result, processes after forming the piezoelectric stack can be simplified.

When the KNN-film <NUM> contains a metal element such as Cu or Mn, the light transmittance through the KNN-film <NUM> tends to decrease. However, even when the KNN-film <NUM> contains Cu, Mn, or the like, the light transmittance through the KNN-film <NUM> can fall within the above-mentioned range by increasing the H<NUM>O-partial pressure in the deposition atmosphere of the KNN-film <NUM> or by performing the heat treatment after the deposition of the KNN-film <NUM>, as long as a total concentration of Cu and Mn contained in the KNN-film <NUM> is within the above-mentioned range.

Further, with the total concentration of Cu and Mn in the KNN-film <NUM> falling within the above-mentioned range, the dielectric constant of the KNN-film <NUM> does not become excessively high, and a sensitivity tends to be less likely to decrease when the piezoelectric stack <NUM> is utilized, for example, as a sensor. One reason can be considered as follows: an addition amount of Cu or Mn is appropriate, and it is less likely to be difficult to preferentially orient the crystals comprised in the KNN-film <NUM> in (<NUM>) direction with respect to the surface of the substrate <NUM>. Even when the H<NUM>O-partial pressure in the deposition atmosphere of the KNN-film <NUM> is increased or the heat treatment is performed after the deposition of the KNN-film <NUM>, the light transmittance through the KNN-film <NUM> tends to fall within the above-mentioned range.

The KNN-film <NUM> may contain an element other than K, Na, Nb, Cu, and Mn such as lithium (Li), Ta, antimony (Sb) at a concentration where the light transmittance through the KNN-film <NUM> can be maintained within the above-mentioned range, for example, at the concentration of <NUM> at% or less.

The top electrode film <NUM> is preferably comprised of an electrode (transparent electrode) with a light transmittance of, for example, <NUM> % or more at least in the wavelength region of visible light, preferably in the wavelength region of visible light and near-infrared rays The top electrode film <NUM> can be deposited using a metal oxide such as SRO or LNO, or an indium oxide such as ITO, IZO, or IGZO. The top electrode film <NUM> does not greatly affect the crystal structure of the KNN-film <NUM>, unlike the bottom electrode film <NUM>. Therefore, a crystal structure of the top electrode film <NUM>, and a deposition method of the top electrode film <NUM> are not particularly limited. The top electrode film <NUM> can be deposited by a method such as a sputtering method, an evaporation method, a plating method, or a metal paste method. A thickness of the top electrode film <NUM> is, for example, <NUM> to <NUM>. The top electrode film <NUM> can also be a thin film (thin metal film) deposited using various metals such as Pt, Au, or an alloy containing the above various metals as its main components. When the top electrode film <NUM> is deposited using Pt, Au, or the like, the thickness of the top electrode film <NUM> is, for example, <NUM> to <NUM>, preferably <NUM> to <NUM>. The top electrode film <NUM> can also be a fine wire (metal fine wire) formed using the various metals such as Pt, Au, or an alloy containing the above various metals as its main components.

Since the piezoelectric stack <NUM> includes the above-mentioned substrate <NUM> (transparent substrate), the bottom electrode film <NUM> (transparent electrode), the KNN-film <NUM> having transparency, and the top electrode film <NUM> (transparent electrode), the light transmittance (average light transmittance) through the entire piezoelectric stack <NUM> at least in the wavelength region of visible light, preferably in the wavelength region of visible light and near-infrared rays can be, for example, <NUM> % or more, preferably <NUM> % or more.

<FIG> is a schematic configuration view of a piezoelectric device <NUM> according to the present embodiment. The piezoelectric device <NUM> includes at least a piezoelectric element <NUM> obtained by shaping the above-mentioned piezoelectric stack <NUM> into a prescribed form, and a voltage detection means 11a or a voltage application means 11b connected to the piezoelectric element <NUM>.

By connecting the voltage detection means 11a between the bottom electrode film <NUM> and the top electrode film <NUM> of the piezoelectric element <NUM>, the piezoelectric device <NUM> can function as a sensor. When the KNN-film <NUM> is deformed according to a variation of some physical quantity, a voltage is generated between the bottom electrode film <NUM> and the top electrode film <NUM> due to the deformation. By detecting this voltage using the voltage detection means 11a, the physical quantity applied to the KNN-film <NUM> can be measured. As mentioned above, since the KNN-film <NUM> has optical transmission, the piezoelectric device <NUM> can be suitably used for applications requiring optical transmission (transparency). For example, the piezoelectric device <NUM> can be suitably used as a sensor for a touch panel or a mobile phone display.

By connecting the voltage application means 11b between the bottom electrode film <NUM> and the top electrode film <NUM> of the piezoelectric element <NUM>, the piezoelectric device <NUM> can function as an actuator. By applying a voltage between the bottom electrode film <NUM> and the top electrode film <NUM> using the voltage application means 11b, the KNN-film <NUM> can be deformed. Various structures connected to the piezoelectric device <NUM> can be actuated due to such a deformation motion.

Next, a method of manufacturing the above-mentioned piezoelectric stack <NUM> will be described. First, the bottom electrode film <NUM> is deposited on any one of main surfaces of the substrate <NUM>. It is also acceptable to prepare the substrate <NUM> with the bottom electrode film <NUM> deposited in advance on any one of its main surfaces. Subsequently, the KNN-film <NUM> is deposited on the bottom electrode film <NUM> using, for example, the RF sputtering method. After that, the heat treatment is performed on the KNN-film <NUM>. Then, the top electrode film <NUM> is deposited on the KNN-film <NUM> after the heat treatment using, for example, the RF sputtering method. Thereby, the piezoelectric stack <NUM> can be obtained. The piezoelectric element <NUM> is obtained by shaping this piezoelectric stack <NUM> into a prescribed form using an etching, etc., and the piezoelectric device <NUM> is obtained by connecting the voltage detection means 11a or the voltage application means 11b to the piezoelectric element <NUM>.

The following conditions are given as conditions for the deposition of the bottom electrode film <NUM>, the KNN-film <NUM>, and the top electrode film <NUM>, and for the heat treatment of the KNN-film <NUM>.

When forming the piezoelectric stack <NUM>, the H<NUM>O-partial pressure in the deposition atmosphere of the KNN-film <NUM> may be increased, instead of performing the heat treatment on the KNN-film <NUM>. Thereby, the light transmittance through the KNN-film <NUM> can be increased similarly to the above-mentioned case where the heat treatment is performed. In addition to increasing the H<NUM>O-partial pressure in the deposition atmosphere of the KNN-film <NUM>, the heat treatment may be performed on the KNN-film <NUM> after the deposition of the KNN-film <NUM> and before the deposition of the top electrode film <NUM>. Thereby, the light transmittance through the KNN-film <NUM> can be further increased.

When the piezoelectric stack <NUM> is shaped into a prescribed form by etching or the like, for example, a dry etching method such as reactive ion etching or a wet etching method using a prescribed etching liquid can be used as the etching method.

When the piezoelectric stack <NUM> is shaped by the dry etching, a photoresist pattern as an etching mask for the dry etching is formed on the piezoelectric stack <NUM> (the top electrode film <NUM>, or the KNN-film <NUM> in a case where the top electrode film <NUM> is not provided) by a photolithography process or the like. As the etching mask, a noble metal film (metal mask) such as a chromium (Cr) film, a nickel (Ni) film, a platinum (Pt) film, or a Ti-film may be formed by a sputtering method. Then, the dry etching is performed on the piezoelectric stack <NUM> (the top electrode film <NUM>, the KNN-film <NUM>, etc.) using a halogen element-containing gas as an etching gas. Examples of the halogen element include chlorine (Cl), fluorine (F) and the like. As the halogen element-containing gas, BCl<NUM>-gas, SiCl<NUM>-gas, chlorine (Cl<NUM>) gas, CF<NUM>-gas, C<NUM>F<NUM>-gas, or the like can be used.

When the piezoelectric stack <NUM> is shaped by the wet etching, a silicon oxide (SiOx) film or the like as an etching mask for the wet etching is formed on the piezoelectric stack <NUM> (the top electrode film <NUM>, or the KNN-film <NUM> in a case where the top electrode film <NUM> is not provided). Then, for example, the piezoelectric stack <NUM> is immersed in an etching liquid containing an alkaline aqueous solution of a chelating agent and not containing hydrofluoric acid, and the wet etching is performed on the piezoelectric stack <NUM> (the top electrode film <NUM>, the KNN-film <NUM>, etc.). As the etching liquid containing the alkaline aqueous solution of the chelating agent and not containing hydrofluoric acid, an etching liquid obtained by mixing ethylenediaminetetraacetic acid as the chelating agent, aqueous ammonia, and aqueous hydrogen peroxide can be used.

According to the present embodiment, one or more of the following effects can be obtained.

With the average light transmittance through the KNN-film <NUM> in the wavelength region of visible light and near-infrared rays less than <NUM> %, the image displayed on the display by the piezoelectric device <NUM> (the KNN-film <NUM>) may be blurred or fuzzy when the above-mentioned piezoelectric device <NUM> is applied, for example, to a touch panel. Therefore, the above-mentioned piezoelectric device <NUM> cannot be applied to a touch panel in some cases.

(b) Since the KNN-film <NUM> contains metal elements selected from a group consisting of Cu and Mn at a concentration within a range of <NUM> at% to <NUM> at%, the insulation property and the etching resistance of the KNN-film <NUM> can be enhanced, and the dielectric constant of the KNN-film <NUM> can be a value suitable for applications of the piezoelectric stack <NUM>.

(c) By increasing the H<NUM>O-partial pressure in the deposition atmosphere of the KNN-film <NUM> or by performing the prescribed heat treatment after the deposition of the KNN-film <NUM>, the KNN-film <NUM> can be sufficiently oxidized to increase the light transmittance through the KNN-film <NUM>. Therefore, even when the KNN-film <NUM> contains Cu, Mn, and the like within the above-mentioned range, the average light transmittance through the KNN-film <NUM> can fall within the above-mentioned range. As mentioned above, the present embodiment can make the average light transmittance through the KNN-film <NUM> fall within the above-mentioned range, while enhancing the insulation property of the KNN-film <NUM>, or adjusting the dielectric constant, the etching resistance, or the like.

The present embodiment is not limited to the above-mentioned aspects, and can be modified as follows.

The piezoelectric stack <NUM> does not have to include the bottom electrode film <NUM>. That is, the piezoelectric stack <NUM> may include the substrate <NUM>, the KNN-film (piezoelectric film) <NUM> deposited on the substrate <NUM>, and the top electrode film <NUM> (electrode film <NUM>) deposited on the KNN-film <NUM>.

<FIG> is a schematic configuration view of the piezoelectric device <NUM> produced using the piezoelectric stack <NUM> according to this modified example. The piezoelectric device <NUM> includes at least the piezoelectric element <NUM> obtained by shaping the piezoelectric stack <NUM> into a prescribed form, and the voltage detection means 11a and the voltage application means 11b connected to the piezoelectric element <NUM>. In this modified example, the piezoelectric element <NUM> includes a pattern electrode formed by shaping the electrode film <NUM> into a prescribed pattern. For example, the piezoelectric element <NUM> includes a pair of positive and negative pattern electrodes 4p<NUM> on an input side, and a pair of positive and negative pattern electrodes 4p<NUM> on an output side. Examples of the pattern electrodes 4p<NUM>, 4p<NUM> include comb-shaped electrodes (Inter Digital Transducer, abbreviated as IDT).

By connecting the voltage application means 11b between the pattern electrodes 4p<NUM> and connecting the voltage detection means 11a between the pattern electrodes 4p<NUM>, the piezoelectric device <NUM> can function as a filter device such as a surface acoustic wave (abbreviated as SAW) filter. By applying a voltage between the pattern electrodes 4p<NUM> using the voltage application means 11b, SAW can be excited on the surface of the KNN-film <NUM>. A frequency of SAW to be excited can be adjusted, for example, by adjusting a pitch between the pattern electrodes 4p<NUM>. For example, the shorter the pitch of IDT as the pattern electrodes 4p<NUM>, the higher the frequency of SAW, and the longer the above-mentioned pitch, the lower the frequency of SAW. The voltage is generated between the pattern electrodes 4p<NUM>, due to SAW having a prescribed frequency (frequency component) determined according to the pitch of IDT or the like as the pattern electrodes 4p<NUM>, among SAWs which are excited by the voltage application means 11b, propagate in the KNN-film <NUM>, and reach the pattern electrodes 4p<NUM>. By detecting this voltage using the voltage detection means 11a, SAW having a prescribed frequency among the excited SAWs can be extracted. The "prescribed frequency" as used herein can include not only a prescribed frequency but also a prescribed frequency band whose center frequency is the prescribed frequency.

The above-mentioned embodiment has been described for a case where the piezoelectric stack <NUM> is produced by depositing the bottom electrode film <NUM>, the KNN-film <NUM>, and the top electrode film <NUM>, in this order, on the substrate <NUM>, but is not limited thereto. The piezoelectric stack <NUM> may be produced as follows. In this modified example, the same components as those in the above-mentioned embodiment are marked with the same numerals, and the explanation therefor will be omitted.

First, as illustrated in <FIG>, a sacrificial layer <NUM> is formed on a first substrate <NUM>, a first electrode film <NUM> is deposited on the sacrificial layer <NUM>, the piezoelectric film (KNN-film <NUM>) is deposited on the first electrode film <NUM>, a second electrode film <NUM> is deposited on the KNN-film <NUM>, and a second substrate <NUM> is bonded to the second electrode film <NUM> to form a stack <NUM>. Also in this modified example, by increasing the H<NUM>O-partial pressure in the deposition atmosphere of the KNN-film <NUM> or by performing the prescribed heat treatment after the deposition of the KNN-film <NUM> and before the deposition of the second electrode film <NUM>, the KNN-film <NUM> can be sufficiently oxidized to increase the light transmittance through the KNN-film <NUM>. Then, as illustrated in <FIG>, the sacrificial layer <NUM> of the stack <NUM> is etched, and the stack <NUM> is separated into the first substrate <NUM> and the piezoelectric stack <NUM> including the first electrode film <NUM>, the KNN-film <NUM>, the second electrode film <NUM>, and the second substrate <NUM>.

The first substrate <NUM> is a substrate that will be separated from the stack <NUM>, as described later. For this reason, the first substrate <NUM> does not have to be a transparent substrate like the above-mentioned substrate <NUM>. As illustrated in <FIG>, a single-crystal silicon (Si) substrate 41a on which a surface oxide film (SiO<NUM>-film) 41b such as a thermal oxide film or a CVD (Chemical Vapor Deposition) oxide film is formed, that is, a Si-substrate having the surface oxide film, can be preferably used as the first substrate <NUM>. Further, as illustrated in <FIG>, a Si-substrate 41a having an insulating film 41d formed on its surface may also be used as the first substrate <NUM>, the insulating film 41d being comprised of an insulating material other than SiO<NUM>. Further, a Si-substrate 41a in which Si-(<NUM>) or Si-(<NUM>), etc., is exposed on a surface thereof, namely, a Si-substrate not having the surface oxide film 41b or the insulating film 41d may also be used as the first substrate <NUM>. Further, an SOI (Silicon On Insulator) substrate, a gallium arsenide (GaAs) substrate, a metal substrate comprised of a metal material such as stainless steel can also be used as the first substrate <NUM>. A thickness of the single-crystal Si-substrate 41a is, for example, <NUM> to <NUM>, and a thickness of the surface oxide film 41b is, for example, <NUM> to <NUM>.

The sacrificial layer <NUM> is formed using a material that disappears upon an etching described later. The sacrificial layer <NUM> can be formed using, for example, Ti, SRO, zinc oxide (ZnO). The sacrificial layer <NUM> can be formed by a method such as a sputtering method, or an evaporation method. A thickness of the sacrificial layer <NUM> can be, for example, <NUM> to <NUM>.

The first electrode film <NUM> is preferably comprised of an electrode (transparent electrode) with a light transmittance of, for example, <NUM> % or more at least in the wavelength region of visible light, preferably in the wavelength region of visible light and near-infrared rays The first electrode film <NUM> is a film to be a base for the KNN-film <NUM>. Therefore, the first electrode film <NUM> preferably has a crystal structure similar to that of the bottom electrode film <NUM> in the above-mentioned embodiment. The first electrode film <NUM> can be deposited using the materials, deposition method, conditions, etc., similar to those for the above-mentioned bottom electrode film <NUM>. The first electrode film <NUM> is a film that is to be the top electrode film in the piezoelectric element <NUM> (piezoelectric device <NUM>) produced using the piezoelectric stack <NUM>.

The second electrode film <NUM> is preferably comprised of an electrode (transparent electrode) with a light transmittance of, for example, <NUM> % or more at least in the wavelength region of visible light, preferably in the wavelength region of visible light and near-infrared rays. The second electrode film <NUM> does not greatly affect the crystal structure of the KNN-film <NUM>, unlike the first electrode film <NUM>. Therefore, a crystal structure of the second electrode film <NUM>, and a deposition method of the second electrode film <NUM> are not particularly limited, like the top electrode film <NUM> in the above-mentioned embodiment. The second electrode film <NUM> can be deposited using the materials, deposition method, conditions, etc., similar to those for the above-mentioned top electrode film <NUM>. The second electrode film <NUM> is a film that is to be the bottom electrode film in the piezoelectric element <NUM> (piezoelectric device <NUM>) produced using the piezoelectric stack <NUM>.

A substrate (transparent substrate) with a light transmittance of, for example, <NUM> % or more at least in the wavelength region of visible light, preferably in the wavelength region of visible light and near-infrared rays can be preferably used as the second substrate <NUM>. The second substrate <NUM> does not greatly affect the crystal structure of the KNN-film <NUM>, like the second electrode film <NUM>. Therefore, a material, a crystal structure, surface (main surface) conditions such as surface roughness, a formation method, a thickness, and the like, of the second substrate <NUM> are not particularly limited. The second substrate <NUM> preferably has flexibility in addition to optical transmission. For example, a resin substrate (resin film) such as a polyimide substrate (polyimide film) can be preferably used as the second substrate <NUM>.

The second substrate <NUM> can be bonded onto the second electrode film <NUM> by adhesion, fusion, or the like. When the bonding is performed by adhesion, an adhesive containing an epoxy resin, a silicone resin, or the like as a main component can be used. For example, the above-mentioned adhesive is applied onto the second electrode film <NUM> by a spin coating method or the like to form an adhesive layer, and the second substrate <NUM> is placed on the adhesive layer. Thus, the second substrate <NUM> can be bonded on the second electrode film <NUM>. When the bonding is performed by fusion, a heat-fusible material, for example, a metal such as gold (Au) or a heat-fusible film is used instead of the adhesive, and the second substrate <NUM> is placed on the above-mentioned material in a molten state. After that, the above-mentioned material is solidified to accomplish the bonding.

The etching of the sacrificial layer <NUM> can be performed by the wet etching using, for example, a solution containing any one of hydrogen chloride (HCl), Diammonium Cerium (IV) Nitrate ((NH<NUM>)<NUM>[Ce(NO<NUM>)<NUM>]), or acetic acid (CH<NUM>COOH) as the etching liquid. Etching conditions such as a concentration of the etching liquid, an etching time, and an etching temperature are adjusted according to the forming material, thickness, plane area, etc., of the sacrificial layer <NUM>. For example, when the sacrificial layer <NUM> is formed using Ti, the stack <NUM> is immersed in a solution containing HCl at a concentration of, for example, <NUM> % to etch the sacrificial layer <NUM>. For example, when the sacrificial layer <NUM> is formed using SRO, the stack <NUM> is immersed in a solution containing Diammonium Cerium (IV) Nitrate at a concentration of, for example, <NUM> mol% to etch the sacrificial layer <NUM>. For example, when the sacrificial layer <NUM> is formed using ZnO, the stack <NUM> is immersed in a solution containing acetic acid at a concentration of, for example, <NUM> %, and heated to <NUM> to <NUM> to etch the sacrificial layer <NUM>. The present inventors have confirmed that when the etching liquid is supplied to the stack <NUM> under the above-mentioned conditions, only the etching of the sacrificial layer <NUM> of the stack <NUM> can be allowed to proceed. The sacrificial layer <NUM> disappears upon the etching.

In a case of the piezoelectric stack <NUM> which does not require the bottom electrode film as mentioned in the above-mentioned modified example <NUM>, it is not necessary to deposit the second electrode film <NUM>. That is, the second substrate <NUM> may be directly bonded onto the KNN-film <NUM>. In this case as well, the bonding method and bonding conditions similar to those mentioned above can be used.

Also in this modified example, the piezoelectric stack <NUM> including the KNN-film <NUM> having optical transmission similar to that of the above-mentioned embodiment can be obtained, and the effect similar to that of the above-mentioned embodiment can be obtained.

For example, an adhesion layer mainly comprised of titanium (Ti), tantalum (Ta), titanium oxide (TiO<NUM>), nickel (Ni), ruthenium oxide (RuOx), iridium oxide (IrOx), etc., may also be provided between the substrate <NUM> and the bottom electrode film <NUM> or between the KNN-film <NUM> and the top electrode film <NUM> in order to enhance an adhesion between them as long as the above-mentioned light transmittance through the entire piezoelectric stack <NUM> is maintained, for example, at <NUM> % or more. Thicknesses of these adhesion layers can be, for example, <NUM> to <NUM>.

The substrate <NUM> may be removed from the piezoelectric stack <NUM> when shaping the above-mentioned piezoelectric stack <NUM> into the piezoelectric element <NUM>, as long as the piezoelectric device <NUM> produced using the piezoelectric stack <NUM> (piezoelectric element <NUM>) can be applied to desired applications such as a sensor or an actuator.

As mentioned above, explanation has been given specifically for the embodiments of the present invention. However, the present invention is not limited thereto, and can be variously modified within the scope of the appended claims.

The above-mentioned embodiment has been described for a case where the piezoelectric device <NUM> produced by processing the piezoelectric stack <NUM> is used for the application requiring optical transmission as mentioned above, but is not limited thereto. For example, the piezoelectric device <NUM> can also be used for applications where it is not required to have optical transmission, including sensors such as angular velocity sensors, ultrasonic sensors, pressure sensors, and acceleration sensors, and actuators such as heads for inkjet printers, MEMS mirrors for scanners, vibrators for ultrasonic generators.

When the piezoelectric device <NUM> produced by processing the piezoelectric stack <NUM> is not required to have optical transmission, the substrate <NUM> does not have to be comprised of a transparent substrate. For example, a substrate similar to the first substrate <NUM> in the above-mentioned modified example may be used as the substrate <NUM>. Similarly, the bottom electrode film <NUM> (second electrode film <NUM>) does not have to be comprised of a transparent electrode. The bottom electrode film <NUM> (second electrode film <NUM>) can be deposited using various metals such as Pt, Au, ruthenium (Ru), or iridium (Ir), or an alloy containing the above various metals as its main components. In this case, a thickness of the bottom electrode film <NUM> (second electrode film <NUM>) can be, for example, <NUM> to <NUM>. Similarly, the top electrode film <NUM> (first electrode film <NUM>) does not have to be comprised of a transparent electrode. The top electrode film <NUM> (first electrode film <NUM>) can be deposited using various metals such as Pt, Au, aluminum (Al), or Cu, or an alloy of these various metals, for example. In this case, a thickness of the top electrode film <NUM> (first electrode film <NUM>) can be, for example, <NUM> to <NUM>. When the above-mentioned adhesion layer is provided between the substrate <NUM> and the bottom electrode film <NUM> or between the KNN-film <NUM> and the top electrode film <NUM>, a thickness of the adhesion layer can be, for example, <NUM> to <NUM>.

When the piezoelectric device <NUM> produced by processing the piezoelectric stack <NUM> is not required to have optical transmission, a substrate formed of a metal material such as stainless steel or a substrate formed of a plastic material may be used as the second substrate <NUM> of the above-mentioned modified example. Various substrates such as a Si-substrate having a silicon nitride (SiN) film formed on a surface thereof, and a poly-Si-substrate can also be used as the second substrate <NUM>. An SOI-substrate or a SiO<NUM>-substrate, having a quality lower than that required for the substrate <NUM> or the first substrate <NUM> can also be used as the second substrate <NUM>. An example of such a low quality substrate is a substrate having a rougher surface than the substrate <NUM> or the first substrate <NUM> (having a larger surface roughness than that of the substrate <NUM>).

Explanation will be given for an experimental result supporting an effect of the above-mentioned embodiment hereafter.

A Si-substrate (surface is oriented preferentially in (<NUM>) direction, thickness: <NUM>, diameter: <NUM> (<NUM> inches)) was prepared as a substrate. A thermal oxide film (thickness: <NUM>) was formed on the surface of the Si-substrate. Then, a piezoelectric stack was produced by depositing a Pt-film (oriented preferentially in (<NUM>) direction with respect to the surface of the substrate and having a thickness of <NUM>) as a bottom electrode film, and a KNN-film (oriented preferentially in (<NUM>) direction with respect to the surface of the substrate and having a thickness of <NUM>) as a piezoelectric film, in this order, on the Si-substrate (thermal oxide film). Cu was added into the KNN film so that a Cu-concentration (CuO-concentration) in the KNN-film was <NUM> at%. Then, the heat treatment was performed on the KNN-film under prescribed conditions. In this example, from a viewpoint of measuring the light transmittance through the KNN-film only, the piezoelectric stack including an opaque (non-light transmitting) Si-substrate, an opaque Pt-film, and the KNN-film having optical transmission was produced.

The Pt-film was deposited by the RF magnetron sputtering method. Conditions for depositing the Pt-film were as follows.

The KNN-film was deposited by the RF magnetron sputtering method. Conditions for depositing the KNN-film were as follows.

(K<NUM>-xNax)NbO<NUM> sintered ceramics having a composition of (K + Na) / Nb = <NUM> to <NUM> and Na / (K + Na) = <NUM> to <NUM>, and containing Cu at a concentration of <NUM> at%, was used as a sputtering target material for depositing the KNN-film into which Cu was added. The target material was produced as follows: K<NUM>CO<NUM>-powder, Na<NUM>CO<NUM>-powder, Nb<NUM>O<NUM>-powder, and CuO-powder were mixed for <NUM> hours using a ball mill, the mixture was provisionally burned at <NUM> for <NUM> hours, then pulverized using again the ball mill, and molded under a pressure of <NUM> MPa, and thereafter burned at <NUM>. The composition of the target material was controlled by adjusting a mixed ratio of K<NUM>CO<NUM>-powder, Na<NUM>CO<NUM>-powder, Nb<NUM>O<NUM>-powder, and CuO-powder, and measured by EDX (energy dispersive X-ray spectrometry) before performing the deposition.

Conditions for the heat treatment performed on the KNN-film were as follows.

The evaluation of the average light transmittance through the KNN-film was performed according to the following procedure. The light transmittance through the piezoelectric film (KNN-film) included in the piezoelectric stack in the wavelength region of <NUM> to <NUM> was measured using a spectroscopic ellipsometer before and after performing the heat treatment. The light transmittance was measured every <NUM> in the wavelength region of <NUM> to <NUM>. Then, an average value (arithmetic mean value) of the measured values of the light transmittance within a prescribed wavelength range was calculated, and the calculated value was defined as the average light transmittance. The calculation results of the average light transmittance are as illustrated in the following Table <NUM>.

Claim 1:
A piezoelectric stack (<NUM>), comprising:
a substrate (<NUM>);
an electrode film (<NUM>, <NUM>); and
a piezoelectric film (<NUM>) comprising an alkali niobium oxide of a perovskite structure represented by a composition formula of (K<NUM>-xNax)NbO<NUM> with <NUM> < x < <NUM>,
wherein the piezoelectric film (<NUM>) contains a metallic element selected from the group consisting of copper and manganese at a concentration of <NUM> at% or more and <NUM> at% or less, and
an average light transmittance through the piezoelectric film (<NUM>) in a wavelength region of visible light and near-infrared rays is <NUM> % or more.