WO2018180276A1 - 圧電体膜、圧電素子、及び、圧電素子の製造方法 - Google Patents
圧電体膜、圧電素子、及び、圧電素子の製造方法 Download PDFInfo
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- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/0021—Reactive sputtering or evaporation
- C23C14/0036—Reactive sputtering
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- H10N30/80—Constructional details
- H10N30/85—Piezoelectric or electrostrictive active materials
- H10N30/853—Ceramic compositions
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- H10N30/8554—Lead-zirconium titanate [PZT] based
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- C01G33/00—Compounds of niobium
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- H10N30/079—Forming of piezoelectric or electrostrictive parts or bodies on an electrical element or another base by depositing piezoelectric or electrostrictive layers, e.g. aerosol or screen printing using intermediate layers, e.g. for growth control
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Definitions
- the present invention relates to a piezoelectric film, a piezoelectric element, and a method for manufacturing the piezoelectric element.
- a piezoelectric film using a perovskite oxide (ABO 3 ) such as lead zirconate titanate is known.
- a piezoelectric element having the piezoelectric film and an electrode provided so that an electric field can be applied to the piezoelectric film is used in various devices as an actuator, for example.
- Patent Document 1 discloses that “A 1 -b B 1 -a X a O 3 is represented by a general formula, A includes Pb, and B includes at least one of Zr and Ti.
- X is composed of at least one of V, Nb, Ta, Cr, Mo, and W, a is in the range of 0.05 ⁇ a ⁇ 0.3, and b is 0 ..
- the inventors of the present invention have studied the piezoelectric film described in Patent Document 1, and have found that there is a problem that the leakage current increases in a high temperature environment.
- an object of the present invention is to provide a piezoelectric film in which an increase in leakage current is suppressed even under a high temperature environment (hereinafter also referred to as “having the effect of the present invention”).
- Another object of the present invention is to provide a piezoelectric element and a method for manufacturing the piezoelectric element.
- the unit of q may be a in formula (2).
- the content of Pb in the perovskite oxide is 20.7 atm% or more and less than 22 atm% with respect to the total number of atoms in the perovskite oxide, according to [1] or [2] Piezoelectric film.
- a piezoelectric element comprising the piezoelectric film according to any one of [1] to [7] and an electrode.
- a method of manufacturing a piezoelectric element comprising a substrate, a lower electrode, the piezoelectric film according to any one of [1] to [7], and an upper electrode in this order.
- obtaining a substrate with electrode to form by maintaining the temperature of the electrode-attached substrate in T 1, by vapor deposition, depositing a perovskite oxide with electrodes on a substrate, perovskite oxide The temperature of the substrate with the electrode on which is deposited is maintained at T 2 , a step of obtaining a piezoelectric film by depositing a perovskite oxide by vapor phase growth, and an upper electrode is formed on the piezoelectric film
- T 1 and T 2 satisfy the formula (3) 1.04 ⁇ T 1 / T 2 ⁇ 1.12.
- the present invention it is possible to provide a piezoelectric film in which an increase in leakage current is suppressed even under a high temperature environment. Moreover, according to this invention, the manufacturing method of a piezoelectric element and a piezoelectric element can also be provided.
- a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
- FIG. 1 is a schematic cross-sectional view of a piezoelectric element according to an embodiment of the present invention.
- the piezoelectric element 10 includes a substrate 11, an adhesion layer 12, a lower electrode 13, a piezoelectric film 14, and an upper electrode 15.
- the piezoelectric film 14 is sandwiched between a pair of electrodes (lower electrode 13 and upper electrode 15).
- an electric field can be applied to the piezoelectric film 14 through a pair of electrodes.
- the piezoelectric element 10 according to the present embodiment includes the adhesion layer 12, the piezoelectric element may not include the adhesion layer when there is no problem in the adhesion between the substrate and the lower electrode. Moreover, it may replace with the contact
- the piezoelectric film according to the above embodiment contains a perovskite oxide represented by the following formula (1), and the Nb content q (atm%) relative to the total number of atoms in the perovskite oxide, A piezoelectric in which the ratio r of the diffraction peak intensity from the (200) plane to the diffraction peak intensity from the (100) plane of the perovskite oxide measured by the X-ray diffraction method satisfies the formula (2) described later. It is a body membrane.
- the reason why the piezoelectric film according to the present embodiment having such a configuration can solve the above-mentioned problem is not necessarily clear, but the inventors presume as follows.
- the mechanism by which an effect is acquired is not restrict
- the leakage current means a current measured by the method described in the examples.
- the piezoelectric film described in Patent Document 1 contains V, Nb, Ta, Cr, Mo, and W that can take a larger valence in addition to atoms that can take a tetravalence of Zr and Ti. According to the study by the present inventors, it is considered that when an element capable of having a large valence is added to the B site of the perovskite oxide (ABO 3 ), it becomes difficult to balance the charge in the perovskite oxide crystal. It is done.
- the piezoelectric film according to the above embodiment is characterized in that the relationship between the diffraction intensity ratio (200) / (100) by X-ray diffraction and the lead content is controlled within a predetermined range.
- (200) / (100) is a parameter estimated to reflect the amount of defects existing on the (200) plane. That is, it is estimated that (200) / (100) increases as the number of defects on the (200) plane decreases.
- the (200) plane defect is presumed to be filled with Pb contained in the perovskite oxide. On the other hand, if there is too much Pb contained in the perovskite oxide, it is presumed that excess Pb is not taken into the crystal lattice and exists at the grain boundaries.
- the piezoelectric film according to the above embodiment has an r (r is (r is the content of Nb (atm%) when the total number of atoms contained in the perovskite oxide is 100 atm%)). 200) / (100).) Ratio is 0.35 or more and less than 0.58. When r / q is 0.35 or more, there are few defects generated by containing Nb, and the above defects are filled with Pb. When r / q is less than 0.58, excessive Pb Leakage current hardly occurs. Therefore, according to the piezoelectric film according to the above embodiment, desired characteristics can be obtained even in a high temperature environment.
- the piezoelectric film according to this embodiment preferably has a perovskite oxide preferentially oriented in the (100) plane and the (200) plane.
- the preferential orientation refers to a state where the orientation direction of the crystal is not disordered and a specific crystal plane is oriented in a substantially constant direction.
- “preferentially oriented in the (100) plane” means from the (100) plane, the (110) plane, and the (111) plane of the perovskite oxide generated when the piezoelectric film is measured by the X-ray diffraction method.
- the ratio (100) / ((100) plane + (110) plane + (111) plane) of the diffraction peak intensity is greater than 0.5, and the same applies to the (200) plane.
- the C axis of the columnar crystal is oriented in the film thickness direction (T direction in FIG. 1) of the piezoelectric film. It is preferable.
- the piezoelectric film according to the embodiment contains a perovskite oxide represented by the following formula (1).
- the content of the perovskite oxide in the piezoelectric film is not particularly limited, but 90% by mass or more based on the total mass of the piezoelectric film is obtained in that a piezoelectric film having a better effect of the present invention can be obtained.
- 99 mass% or more is more preferable, and it is preferable that the piezoelectric film is substantially made of a perovskite oxide.
- the phrase “substantially composed of a perovskite oxide” means that the piezoelectric film does not contain components other than the perovskite oxide (however, impurities that are not intentionally mixed are excluded).
- A represents an A element containing Pb, and x and y each independently represent a number exceeding 0 and less than 1.
- the perovskite oxide may contain an element other than the above as an impurity as long as the perovskite structure can be maintained.
- A is referred to as an A site element, and Zr, Ti, and Nb are also referred to as a B site element.
- A represents an A element containing Pb, and typically represents Pb. A may be partially substituted with a metal element other than Pb. Examples of metal elements other than Pb include Ba and Bi.
- the perovskite oxide represented by the formula (1) is represented by the following formula (1-1).
- the value of y is not particularly limited. However, a morphotropic phase boundary (MPB) is obtained in that a piezoelectric film having more excellent effects of the present invention can be obtained. Boundary) near 0.52 is preferable.
- MPB morphotropic phase boundary
- the value of r / q is determined based on the Nb content in the perovskite oxide (unit: atm%, two significant digits) and (100) and (100) of the perovskite oxide measured by the X-ray diffraction method.
- 200 represents the ratio of two significant digits obtained from the measurement result of diffraction peak intensity from the plane (three significant digits).
- (200) / (100) is 0.830
- the Nb content is 2.4 atm%
- the third digit is rounded off to calculate r / q of 0.35.
- the Nb content (atm%) in the perovskite oxide means a value measured by XRF (X-ray Fluorescence) or ICP (Inductively Coupled Plasma).
- the lower limit of r / q is 0.35 or more, but it is preferably more than 0.36, more preferably 0.37 or more, in that the obtained piezoelectric film has more excellent effects of the present invention. 0.39 or more is more preferable. Although an upper limit is less than 0.58, 0.57 or less is preferable and 0.55 or less is more preferable at the point which the piezoelectric material film obtained has the effect of this invention more excellent.
- the content of Nb at the B site (representing the content of Nb when the total number of atoms at the B site is 100 atm%, the unit is atm%) is not particularly limited. From the standpoint of obtaining a piezoelectric film having a more excellent effect of the present invention, when the total number of atoms at the B site is 100 atm%, 10 to 15 atm% is preferable, and 10 to 12 atm% is more preferable.
- x is preferably 0.10 to 0.15, more preferably 0.10 to 0.12.
- the Nb content (atm%) in the perovskite oxide is not particularly limited, but when the total number of atoms contained in the perovskite oxide is 100 atm%, the lower limit is 2.0 atm% or more.
- the upper limit is preferably 3.0 atm% or less, more preferably less than 3.0 atm%, further preferably 2.9 atm% or less, and particularly preferably 2.4 atm% or less.
- the Pb content (atm%) in the perovskite oxide is not particularly limited, but when the total number of atoms contained in the perovskite oxide is 100 atm%, the lower limit is 20.0 atm% or more.
- the upper limit is preferably 22.0 atm% or less, more preferably less than 22.0 atm%, further preferably 21.5 atm% or less.
- the ratio of the number of atoms contained in the Pb content (atm%) to the Nb content (atm%) in the perovskite oxide is not particularly limited, a piezoelectric film having a more excellent effect of the present invention can be obtained. Therefore, the Pb content (atm%) / Nb content (atm%) is preferably 6.9 or more, more preferably more than 7.0, still more preferably more than 8.6, 8 .7 or higher is particularly preferable, and 8.8 or higher is most preferable. As an upper limit, 12 or less is preferable and 11 or less is more preferable.
- the unit of Nb content and Pb content is atm% when the total number of atoms contained in the perovskite oxide is 100 atm%.
- the piezoelectric film has a more excellent effect of the present invention.
- the Pb content (atm%) / Nb content (atm%) is a coefficient calculated with two significant digits.
- the Nb content in Example 2 described later is 2.0 atm%
- the Pb content (atm%) / Nb content (atm%) is more than 7.0, and all atoms contained in the perovskite oxide are included.
- the number is 100 atm%
- the Nb content (atm%) is 2.3 atm% or less
- the Pb content (atm%) is 20.8 atm% or more.
- the piezoelectric film is preferably a columnar crystal film made of columnar crystals in which the crystal structure of the perovskite oxide extends in the film thickness direction of the piezoelectric film (direction T in FIG. 1).
- the columnar crystal film is preferably composed of a large number of the columnar crystals.
- the average column diameter of the columnar crystals is not particularly limited, but is preferably 30 nm to 1.0 ⁇ m. When the average column diameter of the columnar crystals is 30 nm or more, the influence of the domain boundary portion is further reduced, so that a piezoelectric film in which leakage current is further suppressed can be obtained.
- the average column diameter of the columnar crystals is 1.0 ⁇ m or less, a piezoelectric film having better shape accuracy can be obtained when the piezoelectric film is patterned.
- the film thickness of the piezoelectric film is not particularly limited, but is preferably 500 nm or more, more preferably 1.0 ⁇ m or more, and even more preferably 2.0 ⁇ m or more, from the viewpoint that the durability of the piezoelectric film is more excellent.
- the upper limit of the thickness of the piezoelectric film is not particularly limited, but is preferably 20 ⁇ m or less, and more preferably 10 ⁇ m or less.
- the method for producing the piezoelectric film (hereinafter also referred to as “film formation”) is not particularly limited, and a known method can be used.
- the method for forming the piezoelectric film include vapor deposition methods such as sputtering, plasma CVD (Chemical Vapor Deposition), MOCVD (Metal Organic Chemical Vapor Deposition), and PLD (Pulse Laser Deposition).
- vapor deposition methods such as sputtering, plasma CVD (Chemical Vapor Deposition), MOCVD (Metal Organic Chemical Vapor Deposition), and PLD (Pulse Laser Deposition).
- liquid phase methods such as sol-gel method and organometallic decomposition method; aerosol deposition method.
- the vapor phase growth method is preferable as the method for manufacturing the piezoelectric film because the film forming conditions are easily controlled. Further, according to the vapor phase growth method, it is possible to suppress the generation of lateral streaks in the piezoelectric film during film formation, and a piezoelectric film having higher durability can be obtained.
- the method for producing the piezoelectric film by the vapor phase growth method is not particularly limited, but typically, the substrate and the target are opposed to each other, and a film containing the constituent elements of the target is formed on the substrate using plasma. A method is mentioned.
- substrate the base material mentioned later, a board
- Examples of the vapor phase growth method include a bipolar sputtering method, a tripolar sputtering method, a direct current sputtering method, a radio frequency sputtering method (RF: Radio Frequency Sputtering method), an ECR (Electron Cyclotron Resonance) sputtering method, a magnetron sputtering method, and a counter target. Examples thereof include a sputtering method, a pulse sputtering method, and an ion beam sputtering method.
- RF Radio Frequency Sputtering method
- ECR Electro Cyclotron Resonance
- a sputtering method (especially a high frequency sputtering method is preferable), an ion plating method, or a plasma CVD method is preferable in that a piezoelectric film having the better effect of the present invention can be obtained.
- Sputtering is preferable.
- the manufacturing method of the piezoelectric film is a sputtering method, the perovskite oxide of the obtained piezoelectric film tends to be columnar crystals that are C-axis oriented in the thickness direction of the piezoelectric film (T direction in FIG. 1).
- the substrate 11 in the piezoelectric element 10 according to the present embodiment is not particularly limited, and a known substrate can be used.
- the substrate include silicon, glass, stainless steel, yttrium-stabilized zirconia (YSZ), SrTiO 3 , alumina, sapphire, and silicon carbide.
- YSZ yttrium-stabilized zirconia
- SrTiO 3 alumina, sapphire, and silicon carbide.
- a laminated substrate such as an SOI (Silicon on Insulator) substrate in which a SiO 2 film and a Si active layer are sequentially laminated on silicon may be used.
- the piezoelectric element 10 includes an adhesion layer 12 between the substrate 11 and the lower electrode 13 for improving the adhesion between them.
- the adhesion layer 12 is not particularly limited, and a known material can be used. Examples of the material for the adhesion layer include Ti and TiW.
- the lower electrode 13 is an electrode for applying an electric field to the piezoelectric film 14 and makes a pair with the upper electrode 15.
- the material for the lower electrode 13 is not particularly limited, and a known material can be used.
- the material of the lower electrode 13 include metals such as Au, Pt, Ir, IrO 2 , RuO 2 , LaNiO 3 , SrRuO 3 , ITO (Indium Tin Oxide), and TiN (titanium nitride), metal oxides, And transparent conductive materials, and combinations thereof.
- the lower electrode contains Ir.
- the thickness of the lower electrode is not particularly limited, but is preferably 50 to 500 nm.
- the material of the upper electrode 15 is not particularly limited, and a known material can be used.
- Examples of the material of the upper electrode 15 include the materials described as the material of the lower electrode 13, electrode materials generally used in semiconductor processes such as Al, Ta, Cr, and Cu, and combinations thereof. Can be mentioned.
- the thickness of the upper electrode is not particularly limited, but is preferably 50 to 500 nm.
- the piezoelectric element according to this embodiment can be used for various applications.
- the piezoelectric element according to the present embodiment can be used as an actuator, and specifically, can be applied to a wearable device, a touch pad, a display, a controller, and the like.
- the piezoelectric element according to the present embodiment can also be used as a sensor or the like.
- the piezoelectric element according to the present embodiment can exhibit predetermined characteristics even in a high temperature environment, and thus is suitable for a machine part or the like that requires high reliability.
- the method for manufacturing the piezoelectric element according to the embodiment of the present invention is not particularly limited, and a known method can be used. For example, a method of laminating a lower electrode, an adhesion layer, a piezoelectric film, and an upper electrode in this order on a substrate by using the film forming method described in the method for manufacturing a piezoelectric film. Especially, it is preferable to have the following processes as a manufacturing method of a piezoelectric element at the point from which the piezoelectric element which has the more excellent effect of this invention is obtained.
- Step of forming a lower electrode on a substrate to obtain a substrate with an electrode (2) Step of depositing perovskite oxide on substrate with electrode by vapor phase growth method while maintaining temperature of substrate with electrode at T 1 (initial step) (3) A step of obtaining a piezoelectric film by depositing a perovskite oxide by vapor phase growth method while maintaining the temperature of the electrode-attached substrate on which the perovskite oxide is deposited at T 2 (later step) (4) Step of forming an upper electrode on a piezoelectric film to obtain a piezoelectric element (upper electrode forming step)
- each process is explained in full detail. In the following description, the members and materials used are as already described, and the description is omitted.
- the lower electrode forming step is a step of forming the lower electrode on the substrate.
- a method for forming the lower electrode is not particularly limited, and a known method can be used.
- As a method for forming the lower electrode for example, the method already described as the method for forming the piezoelectric film can be cited.
- the initial step is a step of depositing a perovskite oxide on the substrate with electrodes obtained in the lower electrode formation step by vapor deposition.
- the temperature T 1 of the substrate with electrodes in the initial step is not particularly limited, but the temperature at which the perovskite oxide can be deposited on the substrate, in other words, the oxide represented by the formula (1) can be grown as the perovskite. It is preferable to set the temperature. Under general film forming conditions, the temperature T 1 is preferably 450 ° C. to 700 ° C., more preferably 500 ° C. to 650 ° C., and further preferably 550 ° C. to 600 ° C.
- T 1 / T 2 is not particularly limited as long as it is larger than 1.04, preferably 1.05 or more, and more preferably 1.06 or more.
- the upper limit is not particularly limited as long as it is less than 1.12 and is preferably 1.10 or less.
- the perovskite oxide deposited in the initial step has a thickness of 100 nm or more.
- the perovskite oxide film functions as a seed layer when further depositing the perovskite oxide in a later step to be described later.
- the upper limit of the thickness of the perovskite oxide deposited in the initial step is not particularly limited, but is generally preferably not more than half of the total film thickness of the piezoelectric film.
- the vapor phase growth method is not particularly limited, and may be a known method (the method already described). Among these, the sputtering method is preferable and the high-frequency sputtering method is more preferable in terms of easier control of the film thickness.
- Late step is to hold the temperature of the electrode substrate with perovskite oxide is deposited T 2, by vapor deposition, and further, a step of obtaining a piezoelectric film by depositing perovskite oxide.
- the temperature T 2 of the electrode-bearing substrate in later steps includes the temperature T 2 of the electrode-bearing substrate in later steps, perovskite oxide on the substrate deposition temperature capable, in other words, as the oxide perovskites represented by the formula (1), capable of growth It is preferable to set the temperature.
- the temperature T 2 is preferably 400 ° C. to 650 ° C., more preferably 450 ° C. to 600 ° C., and further preferably 500 ° C. to 550 ° C.
- the temperature T 2 of the electrode-bearing substrate as compared to the initial step is controlled to be low, it is easy to control the content of Pb perovskite oxide (so-called lead loss hardly occurs).
- the relationship between the temperature T 1 , the temperature T 2 , and x in the formula (1) is not particularly limited, but a piezoelectric film having a more excellent effect of the present invention can be obtained. It is preferable to satisfy
- the vapor phase growth method is not particularly limited, and may be a known method (the method already described). Among these, the sputtering method is preferable and the high-frequency sputtering method is more preferable in terms of easier control of the film thickness.
- the upper electrode formation step is a step of obtaining a piezoelectric element by forming an upper electrode on the piezoelectric film obtained in the later step.
- the method for forming the upper electrode is not particularly limited, and a known method can be used.
- As a method for forming the upper electrode for example, the method already described as the method for forming the piezoelectric film can be cited.
- the piezoelectric element manufacturing method according to the embodiment of the present invention may have other steps as long as the effects of the present invention are exhibited. Examples of other steps include an adhesion layer forming step.
- the adhesion layer forming step typically includes a step of forming an adhesion layer between the substrate and the lower electrode, and a known method can be used.
- a 10-nm-thick Ti adhesion layer (corresponding to an adhesion layer) and a film on a 25 mm square SOI (corresponding to a silicon on insulator, substrate).
- a substrate with an electrode was prepared in which an Ir lower electrode (corresponding to a lower electrode) having a thickness of 300 nm was sequentially laminated.
- the substrate with the electrode is placed in an RF (Radio Frequency) sputtering apparatus, and the target is used under the conditions of a vacuum degree of 0.3 Pa and an Ar / O 2 mixed atmosphere (O 2 volume fraction of 2.0%).
- RF Radio Frequency
- Tb whose Zr / (Zr + Ti) ratio is 0.52 and whose Nb content is adjusted to 10%, 12%, and 15% at the B site of the formed piezoelectric film in advance.
- a piezoelectric film having a thickness of 2 ⁇ m was formed using a lead zirconate acid target.
- the diffraction peak intensity r ((200) / (100) ratio) of the perovskite oxide was changed by controlling the substrate temperature during film formation. Specifically, in the manufacturing process of the piezoelectric film, an initial process in which the film thickness of the piezoelectric film reaches 100 nm or more, and a later process in which the film thickness to the target piezoelectric film thickness is formed following the initial process.
- the film shown in Table 1 was produced by changing the film formation temperature stepwise.
- the film-forming conditions regarding each example and each comparative example are as shown in Table 1.
- the piezoelectric films of the examples and comparative examples formed by the method described in Table 1 were analyzed for crystal structures by X-ray diffraction, and both were preferentially oriented in the (100) plane and (200) plane.
- the piezoelectric film was a single-phase film of a perovskite oxide represented by the formula (1).
- a Pt upper electrode having a thickness of 100 nm was laminated on the piezoelectric film to obtain a piezoelectric element.
- A The increase in leakage current density was less than 1.0 ⁇ 10 ⁇ 8 A / cm 2 .
- B The increase in leakage current density was 1.0 ⁇ 10 ⁇ 8 A / cm 2 or more and less than 1.0 ⁇ 10 ⁇ 7 A / cm 2 .
- C The increase in leakage current density was 1.0 ⁇ 10 ⁇ 7 A / cm 2 or more and less than 5.0 ⁇ 10 ⁇ 6 A / cm 2 .
- D The increase in leakage current density was 5.0 ⁇ 10 ⁇ 6 A / cm 2 or more and less than 1.0 ⁇ 10 ⁇ 5 A / cm 2 .
- E Increase in leakage current density was 1.0 ⁇ 10 ⁇ 5 A / cm 2 or more.
- the film formation conditions, structures, and evaluations relating to the piezoelectric films of the respective examples and comparative examples are described for each row over Table 1 part 1 and Table 1 part 2.
- the Nb doping amount to the B site is adjusted to be 10 atm% when the total of B site atoms is 100 atm%, and the film formation temperature in the initial step is set to 570.
- the film formation temperature in the latter step was 540 ° C. (thus T 1 / T 2 was 1.06), and the film formation conditions were such that the thickness of the piezoelectric film was 2.0 ⁇ m.
- the ratio r of the diffraction peak intensity from the (200) plane to the diffraction peak intensity from the (100) plane is 0.82
- Nb in the perovskite oxide is Nb.
- the content of Pb was 2.0 atm% and the content of Pb was 20.7 atm% (therefore, the Pb content / Nb content was 10).
- the increase in leakage current density of the piezoelectric film of Example 1 was 6.1 ⁇ 10 ⁇ 9 A / cm 2 , and the evaluation was “A”.
- it measured by the method similar to the above also about the other Example and the comparative example, and was described in Table 1.
- the piezoelectric films of Examples 1 to 7 had the effects of the present invention.
- the piezoelectric films of Comparative Examples 1 and 2 did not have the effect of the present invention.
- the piezoelectric film of Example 1 in which the atomic ratio of the content of Pb to the content of Nb in the perovskite oxide exceeds 7.0 is higher than that of the piezoelectric film of Example 7 in terms of leakage current density. The increase was smaller. Further, when the total number of atoms contained in the perovskite oxide is 100 atm%, the Nb content is 2.3 atm% or less, or the Pb content is 20.8 atm% or more. Compared with the piezoelectric film of Example 4, the piezoelectric film of Example 1 had a smaller increase in leakage current density.
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Abstract
Description
上記のような技術として、特許文献1には、「A1-bB1-aXaO3の一般式で示され、Aは、Pbを含み、Bは、Zr及びTiのうちの少なくとも一つからなり、Xは、V、Nb、Ta、Cr、Mo、及びWのうちの少なくとも一つからなり、aは、0.05≦a≦0.3の範囲であり、bは、0.025≦b≦0.15の範囲である、圧電体膜。」が記載されている。
[2] 式(1)におけるxが0.1~0.15である、[1]に記載の圧電体膜。
[3] ペロブスカイト型酸化物中におけるPbの含有量が、ペロブスカイト型酸化物中における全原子数に対して、20.7atm%以上、22atm%未満である、[1]又は[2]に記載の圧電体膜。
[4] ペロブスカイト型酸化物中におけるNbの含有量に対するPbの含有量の含有原子数比が7.0を超える、[1]~[3]のいずれかに記載の圧電体膜。
[5] ペロブスカイト型酸化物に含有される全原子数を100atm%としたとき、Nbの含有量が2.3atm%以下であるか、又は、Pbの含有量が20.8atm%以上である、[4]に記載の圧電体膜。
[6] ペロブスカイト型酸化物の柱状結晶からなる柱状結晶膜である、[1]~[5]のいずれかに記載の圧電体膜。
[7] 膜厚が1μm以上である、[1]~[6]のいずれかに記載の圧電体膜。
[8] [1]~[7]のいずれかに記載の圧電体膜と、電極と、を有する圧電素子。
[9] 基板と、下部電極と、[1]~[7]のいずれかに記載の圧電体膜と、上部電極と、をこの順に備える圧電素子の製造方法であって、基板上に下部電極を形成して電極付き基板を得る工程と、電極付き基板の温度をT1に保持して、気相成長法によって、電極付き基板上にペロブスカイト型酸化物を堆積させる工程と、ペロブスカイト型酸化物が堆積された電極付き基板の温度をT2に保持して、気相成長法によって、更に、ペロブスカイト型酸化物を堆積して圧電体膜を得る工程と、圧電体膜上に上部電極を形成して圧電素子を得る工程と、を有し、T1及びT2が式(3)1.04<T1/T2<1.12を満たす、圧電素子の製造方法。
[10] 気相成長法がスパッタリングである、[9]に記載の圧電素子の製造方法。
以下に記載する構成要件の説明は、本発明の代表的な実施態様に基づいてなされることがあるが、本発明はそのような実施態様に限定されるものではない。
なお、本明細書において、「~」を用いて表される数値範囲は、「~」の前後に記載される数値を下限値及び上限値として含む範囲を意味する。
図1は、本発明の実施形態に係る圧電素子の断面模式図である。圧電素子10は、基板11、密着層12、下部電極13、圧電体膜14、及び、上部電極15を備える。圧電体膜14は、一対の電極(下部電極13及び上部電極15)により挟まれる。圧電素子10においては、一対の電極を介して圧電体膜14に電界を印加できる。
本実施形態に係る圧電素子10は、密着層12を備えるが、基板と下部電極との間の密着性に問題が無い場合には圧電素子は密着層を備えなくてもよい。また、密着層12に代えて、又は、密着層12とともに、バッファ層を備える形態であってもよい。以下では、上記実施形態に係る圧電素子10の各部材について、その形態を説明する。
上記実施形態に係る圧電体膜は、後述する式(1)で表されるペロブスカイト型酸化物を含有し、ペロブスカイト型酸化物中における、全原子数に対するNbの含有量q(atm%)と、X線回折法によって測定される、ペロブスカイト型酸化物の(100)面からの回折ピーク強度に対する、(200)面からの回折ピーク強度の比rとが、後述する式(2)を満たす、圧電体膜である。
このような構成を有する本実施形態に係る圧電体膜が上記課題を解決できる理由としては、必ずしも明らかではないが、本発明者らは以下のとおり推測する。なお、下記推測により、効果が得られる機序が制限されるものではない。言い換えれば、下記の機序以外の機序により効果が得られる場合でも、本発明の範囲に含まれる。
特許文献1に記載された圧電体膜は、Zr及びTiという4価をとり得る原子に加えて、より大きな価数をとり得るV、Nb、Ta、Cr、Mo、及び、Wを含有する。本発明者らの検討によれば、ペロブスカイト型酸化物(ABO3)のBサイトに大きな価数をとり得る元素が加わると、ペロブスカイト型酸化物結晶内において電荷のバランスがとりにくくなるものと考えられる。そのようなペロブスカイト型酸化物を用いた圧電体膜を高温環境下において使用すると、ペロブスカイト型酸化物の結晶内に欠陥が生成されやすくなるものと推測している。この欠陥は、高温環境下においてリーク電流を生じさせ、また、発生したリーク電流を増加させる原因となるものと推測される。すなわち、ペロブスカイト中のNbの含有量が多すぎると、リーク電流も大きくなってしまう可能性がある。
ここで、(200)/(100)は、(200)面に存在する欠陥の量を反映すると推測されるパラメータである。すなわち、(200)面における欠陥が少なくなるほど(200)/(100)が大きくなるものと推測される。(200)面の欠陥は、上記ペロブスカイト型酸化物に含有されるPbによって埋められるものと推測される。一方で、ペロブスカイト型酸化物に含有されるPbが多すぎると、過剰のPbは結晶格子に取り込まれず、その粒界に存在すると推測される。この場合、Pbがリークパスとして働き、結果として高温環境下でのリーク電流は増加してしまう。
上記実施形態に係る圧電体膜は、q(qはペロブスカイト型酸化物が含有する全原子数を100atm%としたときの、Nbの含有量(atm%)を表す。)に対するr(rは(200)/(100)を表す。)の比を、0.35以上、0.58未満とするものである。r/qが0.35以上であると、Nbを含有することによって発生する欠陥が少なく、かつ、上記の欠陥がPbによって埋められ、r/qが0.58未満だと、過剰のPbによるリーク電流が発生しにくい。従って、上記実施形態に係る圧電体膜によれば、高温環境下においても所望の特性が得られる。
特に、本発明の実施形態における圧電体膜がペロブスカイト型酸化物の柱状結晶膜である場合には、柱状結晶のC軸が、圧電体膜の膜厚方向(図1のT方向)に配向していることが好ましい。
式(1) A1+δ[(ZryTi1-y)1-xNbx]Oz
式(1)中、AはPbを含有するA元素を表し、x及びyはそれぞれ独立に0を超え、1未満の数を表す。δ=0かつz=3が標準値だが、これらの値はペロブスカイト型酸化物がペロブスカイト構造を取り得る範囲で標準値からずれてもよい。
また、上記ペロブスカイト型酸化物は、ペロブスカイト構造が保持できる範囲内であれば、上記以外の元素を不純物として含有してもよい。
なお、本明細書において、AをAサイト元素といい、Zr、Ti、及び、NbをBサイト元素ともいう。
AがPbからなる場合、式(1)で表されるペロブスカイト型酸化物は以下の式(1-1)で表される。
式(1-1) Pb1+δ[(ZryTi1-y)1-xNbx]Oz
xは0超1未満、yは0超1未満であり、δ=0及びz=3が標準であるが、これらの値はペロブスカイト構造を取り得る範囲内で標準値からずれてもよい。
式(1)及び式(1-1)においてyの値としては、特に制限されないが、より優れた本発明の効果を有する圧電体膜が得られる点で、モルフォトロピック相境界(MPB:Morphotropic phase boundary)の0.52近傍が好ましい。
式(2)0.35≦r/q<0.58
なお、本明細書において、ペロブスカイト型酸化物におけるNbの含有量(atm%)は、XRF(X-ray Fluorescence)又はICP(Inductively Coupled Plasma)で測定される値を意味する。
Pbの含有量(atm%)/Nbの含有量(atm%)が8.8以上、11以下であると、圧電体膜はより優れた本発明の効果を有する。
なお、本明細書において、Pbの含有量(atm%)/Nbの含有量(atm%)は、有効数字2桁で計算される係数である。例えば後述する実施例2におけるNb含有量は、2.0atm%であり、Pb含有量は21.0atm%である。従ってPbの含有量(atm%)/Nbの含有量(atm%)は、21.0/2.0=10.5となり、3桁目を四捨五入して11となる。
柱状結晶の平均柱径としては特に制限されないが、30nm~1.0μmが好ましい。柱状結晶の平均柱径が30nm以上であると、ドメイン境界部分の影響がより小さくなるため、リーク電流がより抑制された圧電体膜が得られる。一方、柱状結晶の平均柱径が1.0μm以下であると、圧電体膜をパターニングした場合に、より優れた形状精度を有する圧電体膜が得られる。
上記圧電体膜を製造する(以下、「成膜する」ともいう。)方法としては特に制限されず、公知の方法を用いることができる。
圧電体膜の成膜方法としては、例えば、スパッタ法、プラズマCVD(Chemical Vapor Deposition)法、MOCVD(Metal Organic Chemical Vapor Deposition)法、及び、PLD(Pulse Laser Deposition)法等の気相成長法;ゾルゲル法及び有機金属分解法等の液相法;エアロゾルデポジション法;等が挙げられる。
なかでも、より優れた本発明の効果を有する圧電体膜が得られる点で、気相成長法としては、スパッタリング法(特に高周波スパッタリング法が好ましい)、イオンプレーティング法、又は、プラズマCVD法が好ましく、スパッタリング法がより好ましい。
圧電体膜の製造方法がスパッタリング法であると、得られる圧電体膜のペロブスカイト型酸化物が圧電体膜の厚さ方向(図1のT方向)にC軸配向した柱状結晶となりやすい。
本実施形態に係る圧電素子10における基板11としては特に制限されず、公知の基板を用いることができる。基板としては、例えば、シリコン、ガラス、ステンレス鋼、イットリウム安定化ジルコニア(YSZ:Yttria-stabilized zirconia)、SrTiO3、アルミナ、サファイヤ、及び、シリコンカーバイド等の基板が挙げられる。
また、基板としては、シリコン上にSiO2膜とSi活性層とが順次積層されたSOI(Silicon on Insulator)基板等の積層基板を用いてもよい。
本実施形態に係る圧電素子10は、基板11と下部電極13との間に、両者の密着性を良好にするための密着層12を備えている。密着層12としては、特に制限されず、公知の材料を用いることができる。密着層の材料としては、例えば、Ti、及び、TiW等が挙げられる。
下部電極13は、圧電体膜14に電界を印加するための電極であり、上部電極15と一対をなす。下部電極13の材料としては特に制限されず、公知の材料を用いることができる。下部電極13の材料としては、例えば、Au、Pt、Ir、IrO2、RuO2、LaNiO3、SrRuO3、ITO(Indium Tin oxide)、及び、TiN(窒化チタン)等の金属、金属酸化物、及び、透明導電性材料、並びに、これらの組合せが挙げられる。なかでも、下部電極は、Irを含有することが特に好ましい。
下部電極の膜厚は特に制限されないが、50~500nmが好ましい。
上部電極15の材料としては特に制限されず、公知の材料を用いることができる。上部電極15の材料としては、例えば、下部電極13の材料として説明した材料、Al、Ta、Cr、及び、Cu等の一般的に半導体プロセスで用いられている電極材料、並びに、これらの組合せが挙げられる。
上部電極の膜厚は特に制限されないが、50~500nmが好ましい。
本発明の実施形態に係る圧電素子の製造方法としては、特に制限されず、公知の方法を用いることができる。例えば、圧電体膜の製造方法の部分で説明した成膜方法を用いて、基板上に、下部電極、密着層、圧電体膜、及び、上部電極をこの順に積層する方法が挙げられる。なかでも、より優れた本発明の効果を有する圧電素子が得られる点で、圧電素子の製造方法としては、以下の工程を有することが好ましい。
(1)基板上に下部電極を形成して電極付き基板を得る工程(下部電極形成工程)
(2)電極付き基板の温度をT1に保持して、気相成長法によって、電極付き基板上にペロブスカイト型酸化物を堆積させる工程(初期工程)
(3)ペロブスカイト型酸化物が堆積された電極付き基板の温度をT2に保持して、気相成長法によって、更に、ペロブスカイト型酸化物を堆積して圧電体膜を得る工程(後期工程)
(4)圧電体膜上に上部電極を形成して圧電素子を得る工程(上部電極形成工程)
以下では、各工程について詳述する。なお、以下の説明において、用いられる部材、及び、材料については、既に説明したとおりであり、説明を省略する。
下部電極形成工程は、基板上に下部電極を形成する工程である。下部電極を形成する方法としては特に制限されず、公知の方法を用いることができる。下部電極を形成する方法としては、例えば、圧電体膜の成膜方法として既に説明した方法等が挙げられる。
初期工程は、下部電極形成工程で得られた電極付き基板上に、気相成長法によってペロブスカイト型酸化物を堆積させる工程である。
式(3)1.04<T1/T2<1.12
後期工程は、ペロブスカイト型酸化物が堆積された電極付き基板の温度をT2に保持して、気相成長法によって、更に、ペロブスカイト型酸化物を堆積して圧電体膜を得る工程である。
後期工程では、初期工程と比較して電極付き基板の温度T2が低く制御されるため、ペロブスカイト型酸化物中のPbの含有量を制御しやすい(いわゆる、鉛抜けが起こりにくい)。
式(4)T1/T2>0.8x+0.96
上部電極形成工程は、後期工程で得られた圧電体膜上に上部電極を形成して圧電素子を得る工程である。上部電極を形成する方法としては特に制限されず、公知の方法を用いることができる。上部電極を形成する方法としては、例えば、圧電体膜の成膜方法として既に説明した方法等が挙げられる。
本発明の実施形態に係る圧電素子の製造方法は、本発明の効果を奏する限りにおいて、他の工程を有していてもよい。他の工程としては例えば、密着層形成工程が挙げられる。 密着層形成工程としては、典型的には、基板と下部電極との間に密着層を形成する工程が挙げられ、公知の方法を用いることができる。
次に、RF(Radio Frequency)スパッタリング装置内に上記電極付き基板を載置し、真空度0.3Pa、Ar/O2混合雰囲気(O2体積分率2.0%)の条件下で、ターゲット中のZr/(Zr+Ti)比を0.52とし、形成される圧電体膜のBサイトへのNbドープ量が10%、12%、15%となるように予めNbの含有量を調整したチタン酸ジルコン酸鉛ターゲットを用いて、膜厚2μmの圧電体膜を成膜した。
なお、成膜時の基板温度を制御することでペロブスカイト型酸化物の回折ピーク強度r((200)/(100)比)を変化させた。具体的には、圧電体膜の製造工程において圧電体膜の膜厚が100nm以上になるまで成膜する初期工程と、その初期工程に引き続き目標の圧電体膜の膜厚まで成膜する後期工程とで、成膜温度を段階的に変化させることで表1の膜を作製した。なお、各実施例、及び、各比較例に関する成膜条件は、表1に示したとおりである。
次に、上記圧電体膜上に、膜厚100nmのPt上部電極を積層して、圧電素子を得た。
25℃の環境下において、各実施例及び比較例の圧電体膜の電極間に、-2V~-50Vまで-2V間隔で電圧を変えるスイ―プを5回繰り返し実施し、5回目と1回目の測定における-50V印加時の電流差をリーク電流として測定した。このリーク電流の大きさを、電極面積で除して、リーク電流密度(A/cm2)を算出した。次に、200℃の環境で、上記と同様の試験を実施し、リーク電流密度(A/cm2)を算出した。その結果から、以下の式を用いてリーク電流密度の増加(A/cm2)を計算し、以下の基準で評価した。計算されたリーク電流密度の増加、及び、その結果を表1に示した。
(式)リーク電流密度の増加(A/cm2)=200℃環境下でのリーク電流密度(A/cm2)-25℃環境下でのリーク電流密度(A/cm2)
A:リーク電流密度の増加が、1.0×10-8A/cm2未満だった。
B:リーク電流密度の増加が、1.0×10-8A/cm2以上、1.0×10-7A/cm2未満だった。
C:リーク電流密度の増加が、1.0×10-7A/cm2以上、5.0×10-6A/cm2未満だった。
D:リーク電流密度の増加が、5.0×10-6A/cm2以上、1.0×10-5A/cm2未満だった。
E:リーク電流密度の増加が、1.0×10-5A/cm2以上だった。
ペロブスカイト型酸化物中におけるNbの含有量に対するPbの含有量の含有原子数比が7.0を超える実施例1の圧電体膜は、実施例7の圧電体膜と比較してリーク電流密度の増加がより小さかった。
また、ペロブスカイト型酸化物に含有される全原子数を100atm%としたとき、Nbの含有量が2.3atm%以下であるか、又は、Pbの含有量が20.8atm%以上である、実施例1の圧電体膜は、実施例4の圧電体膜と比較して、リーク電流密度の増加がより小さかった。
11 基板
12 密着層
13 下部電極
14 圧電体膜
15 上部電極
Claims (10)
- 式(1)で表されるペロブスカイト型酸化物を含有し、
前記ペロブスカイト型酸化物中における、全原子数に対するNbの含有量qと、
X線回折法によって測定される、前記ペロブスカイト型酸化物の(100)面からの回折ピーク強度に対する、(200)面からの回折ピーク強度の比rとが、式(2)を満たす、圧電体膜であり、
式(1) A1+δ[(ZryTi1-y)1-xNbx]Oz
式(2)0.35≦r/q<0.58
この場合、式(1)中、AはPbを含有するAサイト元素を表し、x及びyはそれぞれ独立に0を超え、1未満の数を表し、δ=0かつz=3が標準値だが、これらの値は前記ペロブスカイト型酸化物がペロブスカイト構造を取り得る範囲で標準値からずれてもよく、かつ、式(2)中、qの単位はatm%である、圧電体膜。 - 前記式(1)におけるxが0.1~0.15である、請求項1に記載の圧電体膜。
- 前記ペロブスカイト型酸化物中におけるPbの含有量が、前記ペロブスカイト型酸化物中における全原子数に対して、20.7atm%以上、22atm%未満である、請求項1又は2に記載の圧電体膜。
- 前記ペロブスカイト型酸化物中におけるNbの含有量に対するPbの含有量の含有原子数比が7.0を超える、請求項1~3のいずれか一項に記載の圧電体膜。
- 前記ペロブスカイト型酸化物に含有される全原子数を100atm%としたとき、Nbの含有量が2.3atm%以下であるか、又は、Pbの含有量が20.8atm%以上である、請求項4に記載の圧電体膜。
- 前記ペロブスカイト型酸化物の柱状結晶からなる柱状結晶膜である、請求項1~5のいずれか一項に記載の圧電体膜。
- 膜厚が1μm以上である、請求項1~6のいずれか一項に記載の圧電体膜。
- 請求項1~7のいずれか一項に記載の圧電体膜と、電極と、を有する圧電素子。
- 基板と、下部電極と、請求項1~7のいずれか一項に記載の圧電体膜と、上部電極と、をこの順に備える圧電素子の製造方法であって、
基板上に下部電極を形成して電極付き基板を得る工程と、
前記電極付き基板の温度をT1に保持して、気相成長法によって、前記電極付き基板上にペロブスカイト型酸化物を堆積させる工程と、
前記ペロブスカイト型酸化物が堆積された電極付き基板の温度をT2に保持して、気相成長法によって、更に、ペロブスカイト型酸化物を堆積して圧電体膜を得る工程と、
前記圧電体膜上に上部電極を形成して圧電素子を得る工程と、を有し、
前記T1及びT2が以下の式(3)
式(3)1.04<T1/T2<1.12
を満たす、圧電素子の製造方法。 - 前記気相成長法がスパッタリングである、請求項9に記載の圧電素子の製造方法。
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