JP7490796B2 - Piezoelectric element - Google Patents

Description

The present disclosure relates to piezoelectric laminates and piezoelectric elements.

Lead zirconate titanate (Pb(Zr,Ti) _O3 , hereinafter referred to as PZT) is known as a material having excellent piezoelectricity and ferroelectricity. Taking advantage of its ferroelectricity, PZT is used in FeRAM (Ferroelectric Random Access Memory), which is a non-volatile memory. Furthermore, in recent years, MEMS piezoelectric elements having a PZT film are being put into practical use through the fusion with MEMS (Micro Electro-Mechanical Systems) technology. The PZT film is used as a piezoelectric film in a piezoelectric element having a lower electrode, a piezoelectric film, and an upper electrode on a substrate. This piezoelectric element is deployed in various devices such as inkjet heads (actuators), micromirror devices, angular velocity sensors, gyro sensors, ultrasonic elements (PMUT: Piezoelectric Micromachined Ultrasonic Transducer), and vibration power generation devices.

Piezoelectric elements with a piezoelectric film have a problem in that the piezoelectric properties of the piezoelectric film are deteriorated by the electric field applied for driving. JP 2006-86223 A describes providing a metal oxide layer between the electrode and the piezoelectric film to suppress the deterioration of the piezoelectric properties caused by the electric field. JP 2006-86223 A describes that the deterioration of the piezoelectric properties occurs when an electric field is applied to a piezoelectric element with water droplets attached thereto, and the water is electrolyzed and enters the piezoelectric film in the form of ions, reducing the material that constitutes the piezoelectric layer and generating oxygen vacancies. The deterioration of the piezoelectric properties is suppressed by compensating for the oxygen vacancies in the piezoelectric film with oxygen in a metal oxide layer provided adjacent to the piezoelectric film.

In a piezoelectric film made of a perovskite oxide containing Pb such as PZT, Pb is easily removed during film formation, so it is common to form the film with a Pb composition ratio greater than the stoichiometric ratio. In the formed piezoelectric film, PbO or _PbO2 exists between the grain boundaries as an excess Pb component. Since Pb has a high ionization tendency, it is easily ionized by the intrusion of moisture from the outside world, and ion migration is likely to occur due to the application of an electric field when the piezoelectric device is driven. The diffusion of Pb into the electrode layer due to the migration of Pb causes deterioration of the piezoelectric characteristics and also causes insulation breakdown. This diffusion of Pb into the electrode layer can occur in either the upper electrode layer or the lower electrode layer, but it is a problem that occurs more prominently on the upper electrode layer side, which is closer to the outside world and more susceptible to moisture intrusion.

JP 2006-86223 A aims to suppress deterioration of piezoelectric characteristics by providing a metal oxide layer, but makes no mention of Pb migration in the PZT film or the suppression of migration.

The technology disclosed herein has been developed in consideration of the above circumstances, and aims to provide a piezoelectric laminate and piezoelectric element capable of suppressing the diffusion of Pb from the piezoelectric film to the upper electrode layer.

Specific means for solving the above problems include the following aspects.
＜1＞
A piezoelectric laminate comprising, on a substrate, a lower electrode layer, a piezoelectric film including a perovskite oxide layer containing Pb, and an amorphous oxide layer containing In, in this order.
＜2＞
The piezoelectric laminate according to <1>, wherein the carrier density of the amorphous oxide layer is 1×10 ¹⁵ to 1×10 ²² /cm ³ .
＜3＞
The piezoelectric laminate according to <1> or <2>, wherein the amorphous oxide layer further contains at least one of Ga, Zn, Sn, Cu, Al, Sr, Zr, Ni, and Ru.
＜4＞
<1> to <3>, and
an upper electrode layer provided on the amorphous oxide layer of the piezoelectric laminate.

The piezoelectric laminate and piezoelectric element disclosed herein can suppress the diffusion of Pb from the piezoelectric film to the upper electrode layer.

FIG. 2 is a cross-sectional view showing a layer structure of a piezoelectric element according to an embodiment. FIG. 11 is a diagram showing a Pb distribution in the film thickness direction of a Ti layer formed on a piezoelectric film in Comparative Example 1. FIG. 4 is a diagram showing the Pb distribution in the film thickness direction of an amorphous oxide layer formed on a piezoelectric film in Example 1.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the thicknesses and ratios of each layer are appropriately altered for ease of viewing, and do not necessarily reflect the actual layer thicknesses and ratios.

"Piezoelectric element of one embodiment"
Fig. 1 is a cross-sectional schematic diagram showing the layer structure of a piezoelectric element 1 including a piezoelectric laminate 5 according to one embodiment. As shown in Fig. 1, the piezoelectric element 1 includes a piezoelectric laminate 5 and an upper electrode layer 18. The piezoelectric laminate 5 includes a substrate 11, and a lower electrode layer 12, a piezoelectric film 15, and an amorphous oxide layer 16 that are laminated on the substrate 11. In other words, the piezoelectric element 1 has a configuration in which the upper electrode layer 18 is formed on the amorphous oxide layer 16 of the piezoelectric laminate 5.

The piezoelectric film 15 includes a perovskite oxide containing Pb. The piezoelectric film 15 is basically made of a Pb-containing perovskite oxide. However, the piezoelectric film 15 may contain inevitable impurities in addition to the Pb-containing perovskite oxide. Perovskite oxides are generally represented by _ABO3 . Pb is an A-site element, and is preferably contained as the main component of the A-site. In this specification, the term "main component" means a component that occupies 50 mol% or more. In other words, "containing Pb as the main component of the A-site" means that 50 mol% or more of the A-site elements are Pb. There are no particular limitations on the other elements in the A-site other than Pb in the Pb-containing perovskite oxide and the elements in the B-site.

As the Pb-containing perovskite oxide constituting the piezoelectric film 15, for example, a perovskite oxide represented by the following general formula (1) is preferable.
(Pb _a1 α _a2 ) (Zr _b1 Ti _b2 β _b3 ) O _c (1)
In the formula, Pb and α are A-site elements, and α is at least one element other than Pb. Zr, Ti, and β are B-site elements. a1≧0.5, b1>0, b2>0, b3≧0, and (a1+a2):(b1+b2+b3):c=1:1:3 is standard, but may deviate from the standard value within the range in which a perovskite structure can be obtained.

It is preferable that a2≦0.55.
Also, it is preferable that 1.3≧a1≧0.5, 1.0≧b1>0, 1.0≧b2>0, and 0.2≧b3≧0, and it is more preferable that 1.2≧a1≧0.9, 0.6≧b1>0.3, 0.6≧b2>0.3, and 0.15≧b3≧0.05.

In perovskite oxides containing Pb, A-site elements other than Pb include lithium (Li), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), lanthanum (La), cadmium (Cd), and bismuth (Bi). α is one or a combination of two or more of these.

Other B-site elements besides Ti and Zr include scandium (Sc), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), ruthenium (Ru), cobalt (Co), iridium (Ir), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), and antimony (Sb). β is one or a combination of two or more of these.

The thickness of the piezoelectric film 15 is not particularly limited and is usually 200 nm or more, for example, 0.2 μm to 5 μm. It is preferable that the thickness of the piezoelectric film 15 is 1 μm or more.

The substrate 11 is not particularly limited, and examples thereof include substrates of silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, etc. The substrate 11 may be a laminated substrate in which a _SiO2 oxide film is formed on the surface of a silicon substrate.

The lower electrode layer 12 is an electrode for applying a voltage to the piezoelectric film 15. The main component of the lower electrode layer 12 is not particularly limited, and may be a metal or metal oxide such as gold (Au), platinum (Pt), iridium (Ir), ruthenium (Ru), titanium (Ti), molybdenum (Mo), tantalum (Ta), aluminum (Al), copper (Cu), and silver (Ag), or a combination thereof. Indium tin oxide (ITO), LaNiO ₃ , SRO (SrRuO ₃ ), and the like may also be used. Various adhesion layers and seed layers may be included between the lower electrode layer 12 and the substrate 11.

The upper electrode layer 18 is an electrode that is paired with the lower electrode layer 12 and applies a voltage to the piezoelectric film 15. The main component of the upper electrode layer 18 is not particularly limited, and in addition to the metal materials exemplified for the lower electrode layer 12, chromium (Cr), zinc (Zr), tungsten (W), and combinations thereof can be mentioned. The upper electrode layer 18 is preferably a metal layer that exhibits N-type semiconductor properties when oxidized, and preferably contains one of Ti, Zn, Cu, and W. Since an amorphous oxide layer containing In has an N-type carrier, if the upper electrode layer is a metal layer that exhibits N-type semiconductor properties when oxidized, it is preferable because a PN junction is not generated between the amorphous oxide layer 16 and the upper electrode layer 18.

Here, "lower" and "upper" do not mean upper and lower in the vertical direction, but simply refer to the electrode arranged on the substrate 11 side across the piezoelectric film 15 as the lower electrode layer 12, and the electrode arranged on the opposite side of the piezoelectric film 15 from the substrate 11 as the upper electrode layer 18. The lower electrode layer 12 and the upper electrode layer 18 are not limited to single layers, and may be laminated structures consisting of multiple layers.

There are no particular limitations on the thickness of the lower electrode layer 12 and the upper electrode layer 18, but it is preferable that the thickness be approximately 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

The amorphous oxide layer 16 contains In. Here, containing In means that the amorphous oxide layer contains 3 at% or more of In among the metal elements contained in the layer. There is no limit to the In content as long as it is 3 at% or more, but the In content of the metal elements contained in the amorphous oxide layer is preferably 5 at% or more and 90 at% or less, and more preferably 30 at% or more. Amorphous oxide means an oxide having an amorphous structure. Amorphous means a state that does not have a long-period structure like a crystal, and does not show a peak in the X-ray diffraction pattern, but shows a broad halo pattern.

In the perovskite oxide layer containing Pb, lead oxide (PbO or PbO ₂ ) is present at the grain boundaries between the particles of the perovskite structure. Lead oxide is easily ionized when moisture penetrates from the outside air, and migration of Pb ions is likely to occur. The diffusion of Pb into the electrode layer due to the migration of Pb ions may cause a decrease in piezoelectricity and cause insulation breakdown. This problem is particularly noticeable on the upper electrode layer side, which is greatly affected by moisture from the outside air. In the piezoelectric element 1 of this embodiment, since the amorphous oxide layer 16 containing In is provided between the piezoelectric film 15 and the upper electrode layer 18, it is possible to suppress the diffusion of Pb to the upper electrode layer 18 side associated with the migration of Pb ions in the piezoelectric film 15. In other words, the amorphous oxide layer 16 functions as a Pb diffusion suppression layer. In the prior art, it is described that a metal oxide layer is provided, but it is not described that it is amorphous. In the case of a polycrystalline metal oxide layer, the crystal grain boundaries become paths for Pb ions, and migration of Pb ions into the metal oxide layer cannot be suppressed, and Pb diffusion into the upper electrode layer 18 cannot be sufficiently suppressed. On the other hand, in the present embodiment, since the amorphous oxide layer 16 does not have grain boundaries, the formation of paths through which Pb ions pass is suppressed. Therefore, migration of Pb ions can be suppressed. Even if Pb ions penetrate into the amorphous oxide layer 16, since the amorphous oxide layer 16 is an oxide layer, the Pb ions are oxidized to lead oxide, which has the effect of suppressing ion migration. Note that since an oxide of In is highly conductive and inexpensive, the amorphous oxide layer 16 can be obtained inexpensively.

The carrier density of the amorphous oxide layer 16 is preferably 1×10 ¹⁵ to 1×10 ²² /cm ^3. If the carrier density is in this range, the conductivity is between the conductivity of the piezoelectric film 15 and the conductivity of the upper electrode layer 18, and the decrease in piezoelectricity that occurs when an insulating oxide layer is provided can be suppressed. The carrier density of the amorphous oxide layer 16 is more preferably 1×10 ¹⁸ /cm ³ or more, and even more preferably 1×10 ²¹ /cm ³ or more. In addition, when the carrier density of the amorphous oxide layer 16 is sufficiently high, for example, 1×10 ²¹ /cm ³ or more, the amorphous oxide layer 16 can function as an electrode layer together with the upper electrode layer 18. In addition, when the carrier density of the amorphous oxide layer 16 is sufficiently high, it is also possible to have the amorphous oxide layer 16 also function as the upper electrode layer without providing the upper electrode layer 18. In other words, the piezoelectric laminate 5 shown in FIG. 1 itself can function as a piezoelectric element.

The amorphous oxide layer 16 may further contain at least one of Ga, Zn, Sn, Cu, Al, Sr, Zr, Ni, and Ru in addition to In. By adding a metal other than In, it is possible to adjust the carrier density of the amorphous oxide layer. As the amorphous oxide layer, ITO and IGZO (InGaZnO ₄ ) are preferable, and ITO is particularly preferable. ITO is a material used as a transparent electrode layer in the field of liquid crystal displays. Target and film formation technology are established, film formation is easy, and the target is relatively inexpensive, so that a piezoelectric laminate capable of suppressing Pb migration can be produced inexpensively.

The piezoelectric element 1 or piezoelectric laminate 5 of the above embodiment can be applied to ultrasonic devices, mirror devices, sensors, memories, etc.

Below, we will explain the examples and comparative examples of the present disclosure. First, we will explain the methods for producing the piezoelectric elements of the comparative examples and examples.

(Production method)
A 6-inch Si wafer with 100 nm-thick thermal oxide films on both sides was used as the substrate. A 20 nm-thick TiW layer and a 100 nm-thick Ir layer were sequentially laminated on the substrate by sputtering as the lower electrode. Furthermore, a Nb-doped PZT film was deposited as a piezoelectric film to a thickness of 2 μm by RF (Radio Frequency) sputtering on the lower electrode formed by laminating the TiW layer and the Ir layer formed as described above. The substrate temperature was 550° C., and the sputtering gas was an _O2 to Ar flow rate ratio of 3%. The RF power was 1 kW.

For the laminate substrate in which a lower electrode and a piezoelectric film were formed on a Si wafer as described above, a diffraction pattern was obtained by X-ray diffraction using an X-ray diffraction device manufactured by Malvern Panalytical, and it was confirmed that the substrate was made into a perovskite crystal.

The wafer was then diced into 1-inch (25 mm) square pieces. The following comparative examples and examples each used a 1-inch square laminate substrate obtained by dicing.

A 100 nm thick Pb diffusion suppression layer was formed by sputtering on the 1 inch square laminated substrate obtained above, and a 100 nm thick Au layer was further formed by sputtering as an upper electrode layer to obtain a piezoelectric element. The Pb diffusion suppression layers of Comparative Examples 1 and 2 and Examples 1 to 9 were formed with the materials and film formation temperatures shown in Table 1. In Comparative Example 1, the Pb diffusion suppression layer was made of metal, in Comparative Example 2, the Pb diffusion suppression layer was made of a polycrystalline metal oxide containing In, and in Examples 1 to 9, the Pb diffusion suppression layer was made of a metal oxide containing In. In Comparative Examples 1 and 2 and Examples 1 and 4 to 9, the composition ratio of In and Sn in ITO was In:Sn ≒ 9:1. In Example 2, the composition ratio of In and Zn in IZO (a substance composed of In, Zn, and O) was In:Zn ≒ 9:1. The composition ratio of In, Ga, and Zn in IGZO (a material composed of In, Ga, Zn, and O) in Example 3 was In:Ga:Zn ≒ 1:1:1. The composition ratio of In, Ga, and Zn in IGZO in Example 4 was In:Ga:Zn = 1:2:1. In Examples 5 to 9, the Pb diffusion suppression layer was formed by co-sputtering. In Examples 5 to 9, the film formation rate was controlled to 9:1 so that the ratio of Cu, Al, SrRuO ₃ , Ni, or ZrO ₂ to ITO was 9:1. When forming the Pb diffusion suppression layer and the upper electrode layer, a metal mask having a circular opening with a diameter of 400 μm was used, and the Pb diffusion suppression layer and the upper electrode layer were formed in a circular shape with a diameter of 400 μm.

In addition to the circular opening with a diameter of 400 μm, a 7 mm square opening was provided in the metal mask, and a laminated portion of the Pb diffusion suppression layer and the upper electrode layer was formed on the laminated substrate for XRD measurement of a 7 mm square. The crystallinity of the Pb diffusion suppression layer was evaluated by XRD measurement, and it was confirmed that Comparative Examples 1 and 2 were polycrystalline and Examples 1 to 9 were amorphous.

(Sample for calculating carrier density)
A sample for calculating the carrier density of the Pb diffusion control layer was prepared as follows.

The Pb diffusion suppression layer of each example was formed under the same conditions as in each example on a synthetic quartz substrate measuring 1 cm square and 1 mm thick, and a 20 nm Ti layer and a 100 nm Au layer were laminated in sequence as electrodes for AC (Alternating Current) Hall measurement. The AC Hall measurement electrodes were formed using a metal mask to a size of 1 mm square at the four corners of the substrate. The carrier density of the Pb diffusion suppression layer in each example was calculated using the carrier density calculation sample thus formed. The carrier density of the Pb diffusion suppression layer in each example was as shown in Table 1.

The piezoelectric elements of the examples and comparative examples prepared as described above were evaluated as follows.

<Changes in piezoelectricity over time>
The initial dielectric constant was measured for Comparative Examples 1 and 2 and Examples 1 to 9. The dielectric constant was measured using an impedance analyzer manufactured by Agilent.
After measuring the initial dielectric constant, an AC voltage of 30 Vp-p amplitude and 1 kHz frequency was applied, and after driving for a certain driving time, the dielectric constant (dielectric constant after driving) was measured again. The driving time was 72 hours, and the driving atmosphere was 25.2°C and humidity 55%.

Table 1 shows the initial dielectric constant (dielectric constant before driving), the dielectric constant after driving, and the ratio of the dielectric constant after driving to the initial dielectric constant.

As shown in Table 1, in Comparative Examples 1 and 2, the dielectric constant after driving decreased to about 70% of the dielectric constant before driving, whereas in Examples 1 to 9, the dielectric constant hardly changed between before and after driving. In other words, the piezoelectric elements of Examples 1 to 9 showed results in which the decrease in piezoelectricity was suppressed. In the case of Comparative Examples 1 and 2, this is thought to be because changes occurred in the film composition, etc. caused by migration of Pb ions within the piezoelectric stack before and after driving.

<Interface Pb Diffusion Distance>
Secondary ion mass spectrometry (SIMS) analysis was performed on Comparative Examples 1 and 2 and Examples 1 to 9. The analysis was performed while irradiating Ar ⁺ ions and cutting from the Au layer side. As an example, Figures 2 and 3 show the Pb distribution in the film thickness direction of the Ti layer or amorphous oxide layer obtained from SIMS data for the piezoelectric elements of Comparative Example 1 and Example 1. The Ti layer in Comparative Example 1 and the amorphous oxide layer in Example 1 are layers formed on the upper layer of the piezoelectric film as Pb diffusion suppression layers.

In Figure 2, the horizontal axis represents the position in the thickness direction of the piezoelectric laminate, with 0 being the interface between the piezoelectric film and the Ti layer provided as a comparative example of a Pb diffusion prevention layer. As shown in Figure 2, Comparative Example 1 shows high Pb intensity at the interface between the piezoelectric film and the Ti layer, and the Pb intensity becomes almost constant from 70 nm onwards. The Pb intensity of about 20 at which this Pb intensity becomes constant is the background, indicating a state in which there is almost no Pb (the same applies to Figure 3). From Figure 2, the Pb diffusion distance from the piezoelectric film side into the Ti layer is determined to be 70 nm.

In Figure 3, as in Figure 2, the horizontal axis represents the position in the thickness direction of the piezoelectric laminate, with 0 being the interface between the piezoelectric film and the Pb diffusion prevention layer (here, the amorphous oxide layer). As shown in Figure 3, in Example 1, even at the interface between the piezoelectric film and the Ti layer, the Pb intensity is sufficiently small compared to Comparative Example 1, and is almost constant at the 1.4 nm position. In other words, from Figure 3, the Pb diffusion distance from the piezoelectric film side to the inside of the Pb diffusion prevention layer can be determined to be 1.4 nm.

For Comparative Example 2 and Examples 2 to 9, the Pb diffusion distance was calculated by obtaining graphs similar to those in Figures 2 and 3 from the SIMS data. The Pb diffusion distance from the interface between the piezoelectric film and the Pb diffusion prevention layer to the Pb diffusion prevention layer side for each example is shown in Table 1.

As shown in Table 1, for Comparative Example 1, diffusion of Pb elements was confirmed over a distance of 70 nm from the interface with the piezoelectric film into the Ti layer (see Figure 2), and for Comparative Example 2, diffusion of Pb elements was confirmed over a distance of 60 nm from the interface with the piezoelectric film into the polycrystalline ITO layer. For Examples 1 to 9, the diffusion distance was approximately 1 to 2.5 nm, which is considered to be within the analytical precision error, and it can be considered that almost no Pb diffusion occurred.

The Ti layer and the ITO layer formed as the Pb diffusion suppression layer in Comparative Examples 1 and 2 are both polycrystalline, so it is believed that grain boundaries exist, and Pb ⁺ ions move through these grain boundaries to cause the diffusion of Pb. In addition, for Comparative Examples 1 and 2, if the driving time is further extended, the Pb diffusion distance may become even longer and reach the upper electrode. Since the Pb diffusion suppression layers in Examples 1 to 9 are all amorphous, it is believed that the diffusion of Pb is suppressed because there are no grain boundaries and there is no migration path. In addition, when the Pb diffusion suppression layer is an oxide, the oxygen supplied from the Pb diffusion suppression layer causes the Pb ⁺ ions to become PbO, and the effect of the electric field is no longer applied, so it is believed that ion migration is suppressed and the diffusion of Pb can be suppressed.

The disclosure of Japanese Patent Application No. 2020-166407, filed on September 30, 2020, is incorporated herein by reference in its entirety.
All publications, patent applications, and standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or standard was specifically and individually indicated to be incorporated by reference.

Claims (2)

a piezoelectric laminate comprising, on a substrate, a lower electrode layer, a piezoelectric film including a perovskite oxide layer containing Pb, and an amorphous oxide layer containing In , in that order, and the amorphous oxide layer has a carrier density of 1×10 ¹⁵ to 1×10 ²² /cm ³ ;
an upper electrode layer provided on the amorphous oxide layer of the piezoelectric laminate . 2. The piezoelectric element of claim 1, wherein the amorphous oxide layer further comprises at least one of Ga, Zn, Sn, Cu, Al, Sr, Zr, Ni, and Ru.