JP5848549B2 - Method for manufacturing acoustic wave device - Google Patents

Description

  The present invention relates to a method for manufacturing an acoustic wave device.

  In recent years, communication devices such as mobile phones have become widespread. An elastic wave device using an elastic wave may be used as a filter, a duplexer, or the like of a communication device. Examples of the elastic wave device include a device using a surface acoustic wave (SAW) and a device using a thickness acoustic wave (BAW). Piezoelectric thin film resonators are devices utilizing BAW, and include types such as FBAR (Film Bulk Acoustic Resonator) and SMR (Solidly Mounted Resonator). Some devices use Lamb Wave. By increasing the electromechanical coupling constant of the piezoelectric film, the frequency characteristics of the acoustic wave device can be improved and a wider band can be realized. Non-Patent Document 1 discloses a technique for improving the electromechanical coupling constant of a piezoelectric thin film resonator by controlling the orientation of a piezoelectric thin film made of aluminum nitride (AlN).

IE Transactions on Ultrasonics Ferroelectronics and Frequencies Control, Volume 47, page 292, 2000 (IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS AND FREQUENCY CONTROL, vol.47, p.292, 2000)

  However, even when the orientation of the piezoelectric thin film is increased, it may be difficult to manufacture an acoustic wave device having a large electromechanical coupling constant. In view of the above problems, an object of the present invention is to provide a method for manufacturing an acoustic wave device having a large electromechanical coupling constant.

The present invention includes a step of forming a piezoelectric film having a wurtzite crystal structure, a step of measuring a lattice constant in the c-axis direction and a lattice constant in the a-axis direction of the piezoelectric film, and a lattice constant in the c-axis direction. and when the ratio of the a-axis direction of the lattice constant is a predetermined value or more, so that the ratio of the lattice constant is smaller than the predetermined value, the piezoelectric film in the step of forming the piezoelectric film And a step of changing the film-forming conditions of the elastic wave device. ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the elastic wave device which has a big electromechanical coupling constant can be provided.

The present invention is a method for manufacturing an acoustic wave device for manufacturing a piezoelectric thin film resonator comprising a piezoelectric film and a lower electrode and an upper electrode sandwiching the piezoelectric film, wherein the lower electrode has a wurtzite crystal structure. a step of forming the piezoelectric film, and a step of measuring the lattice constant of the c-axis direction, when the c-axis direction of the lattice constant is a predetermined value or more, so that the lattice constant is smaller than the predetermined value to a method for manufacturing the acoustic wave device characterized by having the steps of changing the film forming conditions of the piezoelectric film in the step of forming the piezoelectric film. According to this configuration, it is possible to provide a method for manufacturing an acoustic wave device having a large electromechanical coupling constant.

The present invention provides the piezoelectric film in a step of forming a piezoelectric film having a wurtzite crystal structure, a step of measuring a lattice constant of the piezoelectric film, and a step of forming the piezoelectric film based on the lattice constant. A step of changing a film forming condition; a step of measuring a distribution of a lattice constant of the piezoelectric film in the plane of the piezoelectric film; and a distribution of the lattice constant in the plane of the piezoelectric film is a predetermined size or more. If it is, a method for manufacturing the acoustic wave device characterized by having the steps of reducing the distribution of the lattice constant in the plane of the piezoelectric film. According to this configuration, the characteristics of the acoustic wave device can be stabilized.

  In the above configuration, the step of reducing the distribution of the lattice constant may be performed after the step of changing the film formation conditions. According to this configuration, the characteristics of the acoustic wave device can be further stabilized.

  In the above configuration, the step of measuring the lattice constant and the step of measuring the distribution of the lattice constant can be performed simultaneously. According to this configuration, the process can be simplified.

  In the above configuration, the step of forming the piezoelectric film is a step of forming a piezoelectric film on a substrate or a lower electrode of the acoustic wave device, and the step of measuring the lattice constant is performed on the substrate or the substrate. It can be a step of measuring the lattice constant of the piezoelectric film formed on the lower electrode. According to this configuration, the lattice constant can be measured with high accuracy.

  In the above configuration, the step of forming the piezoelectric film is a step of forming a piezoelectric film in which a temperature compensation film is inserted, and the step of measuring the lattice constant is a piezoelectric film in which the compensation film is inserted. It is possible to measure the lattice constant. According to this configuration, the lattice constant can be measured with high accuracy. In addition, the temperature characteristics of the acoustic wave device are stabilized.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the elastic wave device which has a big electromechanical coupling constant can be provided.

FIG. 1A is a plan view illustrating an FBAR, and FIG. 1B is a cross-sectional view illustrating the FBAR. FIG. 2A is a schematic view illustrating an experiment of the θ-2θ method. FIG. 2B is a graph showing the relationship between the lattice constant and the electromechanical coupling constant. FIG. 3A is a graph showing the relationship between residual stress and lattice constant. FIG. 3B is a graph showing the relationship between the residual stress and the change rate of the lattice constant. FIG. 4A is a graph showing the relationship between the residual stress and the change rate of the ratio of the lattice constant. FIG. 4B is a graph showing the relationship between the residual stress and the rate of change of the electromechanical coupling constant. FIGS. 5A to 5D are cross-sectional views illustrating the configuration of the wafer used in the lattice constant management process. FIG. 6 is a flowchart illustrating the lattice constant management process. FIG. 7A to FIG. 7C are cross-sectional views illustrating a method for manufacturing the FBAR according to the first embodiment. FIG. 8 is a flowchart illustrating the method for manufacturing the FBAR according to the second embodiment. FIG. 9A and FIG. 9B are cross-sectional views schematically illustrating the reduction in lattice constant distribution due to the residual stress of the upper electrode. FIG. 10 is a graph showing the distribution of lattice constants. FIG. 11 is a cross-sectional view illustrating an FBAR according to a modification of the second embodiment. 12A and 12B are cross-sectional views illustrating another example of the acoustic wave device. FIGS. 13A and 13B are cross-sectional views illustrating another example of the acoustic wave device.

  Below, FBAR is demonstrated as an example of an elastic wave device. First, the configuration of the FBAR will be described. FIG. 1A is a plan view illustrating the FBAR, FIG. 1B is a cross-sectional view illustrating the FBAR, and illustrates a cross-section along AA in FIG.

  As shown in FIGS. 1A and 1B, the FBAR 100 includes a substrate 10, a lower electrode 12, a piezoelectric thin film 14, and an upper electrode 16. The lower electrode 12 is provided on the substrate 10. A dome-shaped air gap 18 is formed between the substrate 10 and the lower electrode 12. In other words, the distance between the substrate 10 and the lower electrode 12 is large in the central part of the gap 18, and the distance between the substrate 10 and the lower electrode 12 is small in the peripheral part of the gap 18. For example, the lower electrode 12 is exposed in the gap 18. The piezoelectric thin film 14 is provided on the lower electrode 12. The upper electrode 16 is provided on the piezoelectric thin film 14. In other words, the lower electrode 12 and the upper electrode 16 sandwich the piezoelectric thin film 14. The lower electrode 12, the piezoelectric thin film 14, and the upper electrode 16 are overlapped to form the resonance region 11. The elastic wave excited in the resonance region 11 vibrates in the thickness direction (vertical direction in FIG. 1B) and travels in the plane direction (lateral direction in FIG. 1B). A part of the lower electrode 12 exposed from the opening of the piezoelectric thin film 14 functions as a terminal part for taking out an electric signal. The lower electrode 12 is provided with an introduction path 13 connected to the gap 18. A hole 15 is formed at the tip of the introduction path 13. The introduction path 13 and the hole 15 are used in the process of forming the gap 18.

  The substrate 10 is made of an insulator such as silicon (Si), glass, gallium arsenide (GaAs). The lower electrode 12 has a two-layer structure of ruthenium / chromium (Ru / Cr), and the upper electrode 16 has a two-layer structure of Cr / Ru. In other words, the lower electrode 12 and the upper electrode 16 are each formed by laminating a Cr layer and a Ru layer in order from the side closer to the piezoelectric thin film 14. The Cr layer of the lower electrode 12 has a thickness of 100 nm, for example, and the Ru layer has a thickness of 250 nm, for example. The Cr layer of the upper electrode 16 is 20 nm, for example, and the Ru layer is 250 nm, for example. The piezoelectric thin film 14 is made of AlN or zinc oxide (ZnO) whose main axis is the (002) direction. Thus, the piezoelectric thin film 14 has an orientation such that the c-axis faces the thickness direction and the a-axis faces the surface direction.

Next, the relationship between the crystal structure of the piezoelectric thin film and the electromechanical coupling constant will be described. First it will be described the relationship between the orientation and the electromechanical coupling constant k ₃₃ ^2. Electromechanical coupling constant k ₃₃ ² higher c-axis orientation of the piezoelectric thin film 14 is high. The higher the electromechanical coupling constant k ₃₃ ^{2 of the} piezoelectric thin film 14, the better the characteristics of the FBAR.

表１の左端の列はサンプルを示す。表１の左から２番目のＦＷＨＭ１は、基板１０上における圧電薄膜１４のＦＷＨＭを示す。表２の左から３番目のＦＷＨＭ２は、下部電極１２の上における圧電薄膜１４のＦＷＨＭを示す。左から４番目は電気機械結合定数ｋ_３３ ^２を示す。 Using a sample in which a piezoelectric thin film 14 made of AlN was formed on a substrate 10 made of Si, the relationship between the orientation and the electromechanical coupling constant k ₃₃ ² was verified. The orientation of the piezoelectric thin film 14 was evaluated by the X-ray rocking curve method in the (002) plane direction of AlN. The FWHM (Full Width Half Maximum) of the rocking curve represents the orientation. The smaller the FWHM, the higher the orientation. The electromechanical coupling constant k ₃₃ ² was evaluated by forming an FBAR using each sample. Table 1 is a table showing the relationship between the orientation and the electromechanical coupling constant k ₃₃ ² in samples B1, B2, and B3 having different film forming conditions for the piezoelectric thin film 14. The substrate 10 is made of Si, and the piezoelectric thin film 14 is made of AlN having a thickness of 1200 nm.

The leftmost column of Table 1 shows a sample. The second FWHM 1 from the left in Table 1 indicates the FWHM of the piezoelectric thin film 14 on the substrate 10. The third FWHM 2 from the left in Table 2 indicates the FWHM of the piezoelectric thin film 14 on the lower electrode 12. Fourth from the left shows the electromechanical coupling constant _{k 33} ^2.

As shown in Table 1, FWHM1 is 1.3 deg and FWHM2 is 2.7 deg in any of samples B1 to B3. In other words, the orientation in the samples B1 to B3 is the same. On the other hand, the electromechanical coupling constant k ₃₃ ² of sample B is 6.07%, k ₃₃ ^{2 of} sample B2 is 6.11%, and k ₃₃ ^{2 of} sample B3 is 6.31%. Thus, even with the same orientation of the piezoelectric thin film 14, a difference occurs in the electromechanical coupling constant k ₃₃ ^2, may not be obtained the desired electromechanical coupling constant k ₃₃ ^2. As a result, it is difficult to obtain a piezoelectric thin film resonator having desired characteristics, and the yield may be reduced.

It will now be described experiments for verifying the relationship between the lattice constant and the electromechanical coupling constant k ₃₃ ^2. A plurality of AlN films having different lattice constants were formed on the substrate 10 by changing the film formation conditions. The lattice constant of each AlN film was measured by θ-2θ measurement (θ-2θ method) by X-ray diffraction. Further, each of the AlN layer having a different lattice constant and identical thickness to create a FBAR as a piezoelectric thin film 14 was determined electromechanical coupling constant k ₃₃ ^2. Thus it was verified the relationship between the lattice constant and the electromechanical coupling constant k ₃₃ ^2. Manufacturing conditions other than the film forming conditions of the piezoelectric thin film 14 are the same between the piezoelectric thin film resonators. The thickness of the piezoelectric thin film 14 is 1200 nm. Let c be the lattice constant of the c-axis of AlN and a be the lattice constant of the a-axis. The θ-2θ method will be described. FIG. 2A is a schematic view illustrating the measurement of the θ-2θ method.

  As shown in FIG. 2A, an X-ray generation source 20, an X-ray detector 22, and a sample 24 are used for the θ-2θ method. Sample 24 is an AlN film formed on a substrate made of Si. The X-ray generation source 20 irradiates the sample 24 with X-rays. X-rays are scattered by the sample 24. The X-ray detector 22 detects scattered X-rays.

  The surface of the sample 24 facing the X-ray generation source 20 and the X-ray detector 22 is a (002) surface. The angle between the surface of the sample 24 facing the X-ray generation source 20 and the X-ray detector 22 and the scattered X-ray is θ. The external angle formed between the irradiated X-rays and the scattered X-rays is 2θ. By rotating the X-ray generation source 20 and the sample 24 in the direction of the arrow in the figure, θ and 2θ are changed. For example, when the sample 24 is rotated by 1 °, the X-ray generation source 20 is rotated by 2 ° in the same direction as the sample 24. This keeps the angle ratio constant. While changing the angle as described above, an angle θ at which the intensity of the X-ray detected by the X-ray detector 22 exhibits a peak is obtained.

上記の数１から結晶面間隔ｄを求めることができる。なお、（００２）面におけるＸ線強度がピークを示すのは２θが約３６ｄｅｇ（θが約１８ｄｅｇ）のときである。また、ＡｌＮのようなウルツ鉱型結晶構造においては、結晶の面方位（ｈｋｌ）、結晶面間隔ｄ、格子定数ａ及びｃの関係は次の式で表される。

ＡｌＮの（００２）面にＸ線を照射するので、ｈ＝ｋ＝０、かつｌ＝２である。数１から算出したｄを数２に代入することで、ｃ軸方向の格子定数ｃを求めることができる。なお、Ｘ線強度が３６ｄｅｇ付近でピークを示すのは、Ｘ線の管球がＣｕ管球の場合である。他の管球を用いる場合、異なる角度にピークが発生する。管球の種類によってλが異なるためである。 When the X-ray intensity shows a peak, the relationship between the crystal plane distance d, the angle θ, and the X-ray wavelength λ of the sample 24 is expressed by the following equation.

The crystal plane distance d can be obtained from the above equation (1). The X-ray intensity on the (002) plane shows a peak when 2θ is about 36 deg (θ is about 18 deg). In a wurtzite crystal structure such as AlN, the relationship between the crystal plane orientation (hkl), the crystal plane spacing d, and the lattice constants a and c is expressed by the following equation.

Since X-rays are irradiated to the (002) plane of AlN, h = k = 0 and l = 2. By substituting d calculated from Equation 1 into Equation 2, the lattice constant c in the c-axis direction can be obtained. The X-ray intensity shows a peak around 36 deg when the X-ray tube is a Cu tube. When other tubes are used, peaks occur at different angles. This is because λ varies depending on the type of tube.

The experimental results will be described. FIG. 2B is a graph showing the relationship between the lattice constant and the electromechanical coupling constant. The horizontal axis represents each lattice constant c, the vertical axis represents the electromechanical coupling constant k ₃₃ ^2.

As shown in FIG. 2 (b), the electromechanical coupling constant k ₃₃ ² as the lattice constant c is small, increases. For example, when the lattice constant c is about 4.988 電 気, the electromechanical coupling constant k ₃₃ ² is about 6.10%. On the other hand, when the lattice constant c is about 4.980 Å, the electromechanical coupling constant k ₃₃ ² is about 6.35%. From the results in FIG. 2 (b), in order to increase the electromechanical coupling constant k ₃₃ ² it can be seen that it is sufficient small lattice constant c.

  Next, conditions for controlling the lattice constant c will be described. Experiments and simulations were conducted to verify the relationship between the lattice constant c and the residual stress remaining in the piezoelectric thin film 14. First, the experiment will be described. The experiment verified the relationship between the residual stress and the lattice constant c in the piezoelectric thin film 14 having different film forming conditions.

  FIG. 3A is a graph showing the relationship between residual stress and lattice constant. The horizontal axis represents the residual stress, and the vertical axis represents the lattice constant c. When the residual stress is a tensile stress, the residual stress takes a positive value. When the residual stress is a compressive stress, the residual stress takes a negative value. The compressive stress is a stress in the direction of narrowing the upper surface of the piezoelectric thin film 14 in FIG. The tensile stress is a stress in a direction in which the upper surface of the piezoelectric thin film 14 is expanded. The piezoelectric thin film 14 is made of AlN. As shown in FIG. 3A, the lattice constant c is smaller when the residual stress is positive than when the residual stress is negative. In other words, when the residual stress is a tensile stress, the lattice constant c is small.

  Next, simulation will be described. In the simulation, first-principles calculation of the electronic state of AlN including structural optimization was performed using the pseudopotential method as a calculation method and ABINIT as a calculation program. Lattice constants a and c at which AlN has a stable structure when stress was input were determined. According to the JCPDS (Joint Committee on Powder Diffraction Standards) card chart, the lattice constant c when the stress is zero is 0.498 nm (4.98 mm), and the ratio between the lattice constants a and c (ratio of lattice constants). c / a is 1.6.

  FIG. 3B is a graph showing the relationship between the residual stress and the change rate of the lattice constant. The vertical axis represents the rate of change of the lattice constant c. The rate of change of the lattice constant c is the rate of change of the lattice constant c based on the lattice constant c when the stress is zero. As shown in FIG. 3B, the rate of change of the lattice constant c decreases as the residual stress value increases. Specifically, when the residual stress is a compressive stress (negative value), the rate of change of the lattice constant c is a positive value. In other words, the lattice constant c is larger than when the stress is zero. When the residual stress is a tensile stress (positive value), the rate of change of the lattice constant c is a negative value. In other words, the lattice constant c is smaller than when the stress is zero.

  Further, the rate of change of the lattice constant ratio c / a was verified. FIG. 4A is a graph showing the relationship between the residual stress and the change rate of the ratio of the lattice constant. The vertical axis represents the lattice constant ratio c / a. As shown in FIG. 4A, when the residual stress is a compressive stress, the rate of change of c / a is a positive value. In other words, c / a increases. When the residual stress is a tensile stress, the change rate of c / a is a negative value. In other words, c / a becomes small.

数３に基づき電気機械結合定数ｋ_３３ ^２を算出することができる。応力がゼロの場合における安定構造のＡｌＮの結晶格子に歪みを加えることで、応力がゼロの場合の電気機械結合定数ｋ_３３ ^２を求めることができる。応力が入力された場合における安定構造のＡｌＮの結晶格子に歪みを加えることで、応力が入力された場合の電気機械結合定数ｋ_３３ ^２を求めることができる。 It will now be described a simulation of the electromechanical coupling constant _{k 33} ^2. Similar to the above, the simulation is a first principle calculation of the electronic state of AlN. By applying strain to the crystal lattice of AlN having a stable structure, the piezoelectric constant e ₃₃ , elastic constant C ₃₃ and dielectric constant ε ₃₃ in the c-axis direction of AlN were obtained. The following relationship is established between the electromechanical coupling constant k ₃₃ ² , the piezoelectric constant e ₃₃ , the elastic constant C _33, and the dielectric constant ε ₃₃ .

Based on Equation 3, the electromechanical coupling constant k ₃₃ ² can be calculated. Stress By adding distortion in the crystal lattice of AlN in a stable structure in the case of zero, it is possible to stress seek electromechanical coupling constant k ₃₃ ² in the case of zero. Stresses that applies a distortion in the crystal lattice of the AlN stable structure when entered, it is possible to obtain an electromechanical coupling constant k ₃₃ ² when the stress is inputted.

FIG. 4B is a graph showing the relationship between the residual stress and the electromechanical coupling constant. The horizontal axis residual stress and the vertical axis represents the rate of change of the electromechanical coupling constant k ₃₃ ^2, respectively. The rate of change of the electromechanical coupling constant is to stress relative to the electromechanical coupling constant k ₃₃ ² in the case of zero. As shown in FIG. 4 (b), when the residual stress is compressive stress, the change rate of the electromechanical coupling constant k ₃₃ ² has a negative value. In other words, if the residual stress is compressive stress, the electromechanical coupling constant k ₃₃ ² decreases. If the residual stress is tensile stress, rate of change of the electromechanical coupling constant k ₃₃ ² has a positive value. If the residual stress as described above is a tensile stress, the ratio c / a of the lattice constants c and lattice constant becomes smaller, the electromechanical coupling constant k ₃₃ ² increases.

  Example 1 based on the above knowledge will be described. The method for manufacturing an FBAR according to the first embodiment includes a lattice constant management process for managing the lattice constant of the piezoelectric thin film 14. FIGS. 5A to 5D are cross-sectional views illustrating the configuration of the wafer used in the lattice constant management process.

The wafer shown in FIG. 5A is obtained by forming a piezoelectric thin film 14 on a substrate 10. The wafer shown in FIG. 5B has a piezoelectric thin film 14 formed on the lower electrode 12. The wafer shown in FIG. 5C has a piezoelectric thin film 14 formed on the lower electrode 12 and an upper electrode 16 formed on the piezoelectric thin film 14. The wafer shown in FIG. 5D is obtained by forming a piezoelectric thin film 14 on the lower electrode 12 and inserting a temperature compensation film 19 into the piezoelectric thin film 14. Temperature compensation film 19 is, for example, silicon oxide _(SiO 2), or made of a SiO ₂ such as fluorine (F) _doped, SiO ₂ doped with like. The temperature compensation film 19 is inserted in the plane direction of the piezoelectric thin film 14 between the lower electrode 12 and the upper electrode 16.

  The wafer used for the lattice constant management process may be a dummy wafer or a product wafer. The dummy wafer is a wafer used for managing a lattice constant, not for forming a product. A product wafer is a wafer for forming a product (FBAR). Both a dummy wafer and a product wafer may be used.

  FIG. 6 is a flowchart illustrating the FBAR lattice constant management process according to the first embodiment. In the following description, a case where a dummy wafer having the configuration shown in FIG.

As shown in FIG. 6, a piezoelectric thin film 14 (piezoelectric film) is formed on the substrate 10 by using, for example, a sputtering method (step S10). The lattice constant of the formed piezoelectric thin film 14 is measured (step S11). What is measured is the lattice constant c in the c-axis direction. As a measuring method, for example, the θ-2θ method can be used. Lattice constant c determines whether less than a predetermined value _{c 0} (step S12). The predetermined value c ₀ is a value of a lattice constant corresponding to a desired electromechanical coupling constant k ₃₃ ² , for example. For example, based on FIG. 2B, when the electromechanical coupling constant k ₃₃ ² is desired to be 6.3% or more, c ₀ may be set to about 4.982 Å.

If No, i.e. the lattice constant c may be _{c 0} or more, changing the conditions for forming the piezoelectric thin film 14 (step S13). Deposition conditions are changed to the lattice constant c is less than c _0. The film forming conditions will be described later. After step S13, the process returns to step S10. The piezoelectric thin film 14 is formed on another dummy wafer using the changed film formation conditions. In the case of Yes in step S12, the lattice constant management process ends.

  The FBAR is manufactured using the film formation conditions changed by the lattice constant management process. FIG. 7A to FIG. 7C are cross-sectional views illustrating a method for manufacturing the FBAR according to the first embodiment.

  As shown in FIG. 7A, a sacrificial layer 17 is formed on the substrate 10 by using, for example, a sputtering method or a vapor deposition method. The sacrificial layer 17 is made of, for example, magnesium oxide (MgO) and is provided in a region where the void 18 is formed. The thickness of the sacrificial layer 17 is, for example, 20 nm.

  As shown in FIG. 7B, the lower electrode 12 is formed by using, for example, a sputtering method. The sputtering method is performed in an argon (Ar) gas atmosphere under a pressure of 0.6 to 1.2 Pa, for example. The pressure and atmosphere are the pressure and atmosphere in the sputtering apparatus. After the film formation, the lower electrode 12 is formed into a predetermined shape using, for example, an exposure technique and an etching technique. As shown on the right side of FIG. 7B, one end of the lower electrode 12 and one end of the sacrificial layer 17 overlap. On the other hand, the lower electrode 12 extends to the upper surface of the substrate 10 as shown on the left side of FIG.

As shown in FIG. 7C, the piezoelectric thin film 14 is formed by using, for example, a sputtering method. The piezoelectric thin film 14 is made of AlN having a thickness of, for example, 400 nm and having a c-axis as a main axis. The sputtering method is performed, for example, in an argon / nitrogen (Ar / N ₂ ) mixed gas atmosphere under a pressure of about 0.3 Pa. As the film forming conditions, the conditions changed in FIG. 6 are used.

For example, the upper electrode 16 is formed by sputtering. The sputtering method is performed in an Ar gas atmosphere under a pressure of 0.6 to 1.2 Pa, for example. For example, the upper electrode 16 and the piezoelectric thin film 14 are formed into a predetermined shape using an exposure technique, an etching technique, or the like. A region where the lower electrode 12, the piezoelectric thin film 14, the upper electrode 16 and the sacrificial layer 17 overlap is formed. The lower electrode 12 is exposed from the opening formed in the piezoelectric thin film 14. An etching solution is introduced from the hole 15 and the introduction path 13 shown in FIG. 1A to remove the sacrificial layer 17. The residual stress of the composite film composed of the lower electrode 12, the piezoelectric thin film 14, and the upper electrode 16 is a compressive stress. Therefore, when the etching of the sacrificial layer 17 is completed, the composite film swells, and a void 18 having a dome shape is formed between the lower electrode 12 and the substrate 10 on the composite film side. Through the above steps, an FBAR having a desired electromechanical coupling constant k ₃₃ ² is formed.

According to Example 1, a step of forming the piezoelectric thin film 14 having a wurtzite crystal structure, a step of measuring the lattice constant c of the piezoelectric thin film 14, if the lattice constant c is a predetermined value c ₀ or more And changing the film forming conditions. By changing the film forming conditions, the lattice constant c is smaller than a predetermined value c _0. As a result, an FBAR having a large electromechanical coupling constant k ₃₃ ² can be obtained.

The film forming conditions changed in step S13 in FIG. 6 are, for example, the flow rate of Ar gas in the sputtering method for forming the piezoelectric thin film 14. By increasing the Ar flow ratio in the Ar / N ₂ mixed gas, the stress of the piezoelectric thin film 14 becomes tensile stress, and by decreasing the Ar flow ratio, the stress of the piezoelectric thin film 14 becomes compressive stress. In step S13, the stress of the piezoelectric thin film 14 is set as the tensile stress by increasing the Ar flow rate ratio to, for example, Ar flow rate / (Ar flow rate + N ₂ flow rate) = 0.25. The Ar flow rate ratio may be changed according to the apparatus. As shown in FIG. 3A to FIG. 4A, the lattice constant c and the ratio c / a of the lattice constant are reduced by the residual stress becoming a tensile stress. As a result, the electromechanical coupling constant _{k 33} ² increases.

In step S12 of FIG. 6, a value obtained by one measurement may be used as the lattice constant c, or an average value of a plurality of measurement results may be used. Further, for example, the lattice constant may be managed by managing the lattice constant ratio c / a. In this case, the lattice constants a and c are measured, and c / a is calculated (step S11). It is determined whether the ratio c / a is less than a predetermined value c ₀ / a ₀ (step S12). In the case of No, that is, when c / a is equal to or greater than c ₀ / a ₀ , the film forming conditions are changed so that c / a is less than a predetermined value c ₀ / a ₀ (step S13). One or both of the lattice constants c and c / a may be managed.

  The lattice constant management process shown in FIG. 6 may be performed every manufacturing process. However, performing the lattice constant management process in each manufacturing process leads to an increase in the number of processes, a complicated manufacturing process, and the like, and increases the cost of the FBAR. Therefore, it is preferable to periodically perform the lattice constant management process. “Periodically” means, for example, when the target used in the sputtering method is changed every day or every week, when the production lot is changed, or when the production process is repeated a predetermined number of times.

  When the wafer having the configuration shown in FIG. 5B is used in the lattice constant management step, the lower electrode 12 and the piezoelectric thin film 14 are formed in step S10 of FIG. In step S11, the lattice constant of the piezoelectric thin film 14 on the lower electrode 12 is measured. When the wafer having the configuration shown in FIG. 5C is used, the lower electrode 12, the piezoelectric thin film 14, and the upper electrode 16 are formed in step S10. In step S11, the lattice constant of the piezoelectric thin film 14 provided between the lower electrode 12 and the upper electrode 16 is measured. When the wafer having the configuration shown in FIG. 5D is used, the lower electrode 12, the piezoelectric thin film 14, the upper electrode 16, and the temperature compensation film 19 are formed in step S10. In other words, the piezoelectric thin film 14 with the temperature compensation film 19 inserted is formed. In step S11, the lattice constant of the piezoelectric thin film 14 in which the temperature compensation film 19 is inserted is measured. It is preferable to place the piezoelectric thin film 14 of the wafer used in the lattice constant management process in the same situation as the piezoelectric thin film 14 in the FBAR. As a result, the lattice constant can be measured with high accuracy.

  As described above, a product wafer may be used in the lattice constant management process, or both a dummy wafer and a product wafer may be used. The FBAR can be formed by the product wafer that has undergone the lattice constant management process. When it is determined No in step S12 of FIG. 6, the product wafer may be discarded or the FBAR may be formed using the product wafer.

Although the lattice constant c in Example 1 shows an example of adjusting to a predetermined value c ₀ or less, for example, a lattice constant c may be adjusted to a predetermined value c ₀ or more. That is, when the lattice constant c is a predetermined value _{c 0} smaller (Yes in step S12 in FIG. 6) to adjust the deposition conditions in step S13. Thereby, the lattice constant c is set to a predetermined value c ₀ or more, and a desired electromechanical coupling constant k ₃₃ ² can be obtained.

  The piezoelectric thin film 14 may be made of a piezoelectric body having a wurtzite crystal structure other than AlN and ZnO. The lower electrode 12 and the upper electrode 16 have a two-layer structure, but may have a single-layer structure or a structure of three or more layers. As materials for the lower electrode 12 and the upper electrode 16, aluminum (Al), copper (Cu), chromium (Cr), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru) Metals such as rhodium (Rh) and iridium (Ir) can be used. The lower electrode 12 may be exposed to the gap 18 or may not be exposed.

表２に示すように、位置Ｃ１〜Ｃ３のいずれにおいてもＦＷＨＭ１は１．３ｄｅｇであり、ＦＷＨＭ２は２．７ｄｅｇである。言い換えれば、位置Ｃ１〜Ｃ３における配向性は同じである。これに対し、位置Ｃ１におけるｋ_３３ ^２は６．０７％、位置Ｃ２におけるｋ_３３ ^２は６．１９％、位置Ｃ３におけるｋ_３３ ^２は６．２５％である。このように、圧電薄膜１４の面内において電気機械結合定数ｋ_３３ ^２に差異が生じることがある。このため、圧電薄膜共振子の周波数特性が不安定になることがある。 The second embodiment is an example in which the distribution of the electromechanical coupling constant k ₃₃ ² is reduced. Table 2 is a table showing the measurement results of the orientation and the electromechanical coupling constant k ₃₃ ² at different positions C1~C3 within the plane of the piezoelectric thin film 14.

As shown in Table 2, FWHM1 is 1.3 deg and FWHM2 is 2.7 deg at any of positions C1 to C3. In other words, the orientations at the positions C1 to C3 are the same. On the other hand, k ₃₃ ^{2 at the} position C1 is 6.07%, k ₃₃ ^{2 at the} position C2 is 6.19%, and k ₃₃ ^{2 at the} position C3 is 6.25%. Thus, a difference may occur in the electromechanical coupling constant k ₃₃ ² in the plane of the piezoelectric thin film 14. For this reason, the frequency characteristics of the piezoelectric thin film resonator may become unstable.

It will be described the cause of the distribution of the electromechanical coupling constant k ₃₃ ^2. The piezoelectric thin film 14 is formed by sputtering, for example. When the sputtering method is performed, the surface temperature of the piezoelectric thin film 14 may be about 100 ° C. When the piezoelectric thin film 14 is taken out of the sputtering apparatus, thermal stress may remain in the piezoelectric thin film 14 due to temperature changes. Further, residual stress may occur due to lattice defects that may occur in the piezoelectric thin film 14 during film formation. The distribution of the residual stress causes the distribution of the lattice constant c or the distribution of the ratio c / a. Distribution of the lattice constant c, or distribution of the ratio c / a is sometimes cause distribution of the electromechanical coupling constant k ₃₃ ² as shown in Table 2.

In order to reduce the distribution of the electromechanical coupling constant k ₃₃ ^2, the distribution of the lattice constant may be reduced. The method for manufacturing an FBAR according to the second embodiment includes a distribution reduction process for reducing the distribution of the lattice constant. FIG. 8 is a flowchart illustrating the method for manufacturing the piezoelectric thin film resonator according to the second embodiment. A product wafer is used for the distribution reduction process.

Step S10 shown in FIG. 8 is the same as that shown in FIG. The lattice constant c in the plane of the piezoelectric thin film 14 is measured (step S11). In Example 2, the lattice constant c is measured at a plurality of locations in the plane of the piezoelectric thin film 14. In other words, the distribution of the lattice constant c in the plane of the piezoelectric thin film 14 is measured. Steps S12 and S13 are the same as those shown in FIG. If Yes in step S12, it is determined whether the distribution of the lattice constant c in the plane of the piezoelectric thin film 14 is large (step S14). For example, it is determined whether the difference Δc between the maximum value and the minimum value of the lattice constant c measured at a plurality of locations is equal to or greater than a predetermined value Δc ₀ . In the case of Yes, the post-process for forming FBAR is performed (step S15). For example, when the product wafer has the configuration shown in FIG. 5B, a step of providing the upper electrode 16 and a step of forming the air gap 18 are performed as a post process. In the case of No in step S14, a post process is performed (step S16). After step S16, the distribution of the lattice constant c in the plane is reduced (step S17). After step S14 or after step S17, the process ends.

  A specific method for reducing the distribution of the lattice constant will be described. In order to reduce the lattice constant distribution, the residual stress distribution may be reduced. In order to reduce the distribution of the residual stress, for example, the residual stress of the upper electrode 16 can be used. The distribution of the residual stress in the upper electrode 16 is generated by changing the film formation conditions such as the film formation pressure of the upper electrode 16, for example. FIG. 9A and FIG. 9B are cross-sectional views schematically illustrating the reduction in lattice constant distribution due to the residual stress of the upper electrode. FIGS. 9A and 9B show the FBAR with the piezoelectric thin film 14 and the upper electrode 16 extracted and with hatching omitted. For simplification of the drawing, the piezoelectric thin film 14 and the upper electrode 16 have the same thickness. Block arrows represent strong residual stresses, and solid arrows represent weak residual stresses. A strong residual stress means a residual stress having a large absolute value, and a weak residual stress means a residual stress having a small absolute value.

  In the example shown in FIG. 9A, the residual stress of the piezoelectric thin film 14 is a tensile stress. A weak residual stress exists at the center of the piezoelectric thin film 14. A strong residual stress exists in the periphery of the piezoelectric thin film 14. Thus, the residual stress of the piezoelectric thin film 14 may have different magnitudes in the plane. On the other hand, a strong residual stress exists at the center of the upper electrode 16. A weak residual stress exists in the periphery of the upper electrode 16. The residual stress of the upper electrode 146 is a tensile stress. The strong residual stress of the central portion of the upper electrode 16 is applied to the central portion of the piezoelectric thin film 14. The weak residual stress of the peripheral portion of the upper electrode 16 is applied to the peripheral portion of the piezoelectric thin film 14. As a result, the residual stress of the piezoelectric thin film 14 is approximately the same in the central portion and the peripheral portion. In other words, the residual stress distribution of the piezoelectric thin film 14 is reduced. As a result, the distribution of the lattice constant c of the piezoelectric thin film 14 is also reduced.

  In the example shown in FIG. 9B, a weak compressive stress exists in the central portion of the piezoelectric thin film 14, and a strong compressive stress exists in the peripheral portion. A weak tensile stress exists at the center of the upper electrode 16 and a strong tensile stress exists at the periphery. The piezoelectric thin film 14 and the upper electrode 16 have opposite directions of residual stress. Therefore, the residual stress of the piezoelectric thin film 14 and the residual stress of the upper electrode 16 cancel each other. As a result, the residual stress distribution of the piezoelectric thin film 14 is reduced. In order to more effectively reduce the distribution of residual stress, the piezoelectric thin film 14 and the upper electrode 16 are preferably in contact with each other.

  An experiment verifying the reduction of the lattice constant distribution will be described. In the experiment, as shown in FIG. 9A and FIG. 9B, the distribution of the lattice constant was reduced by reducing the distribution of residual stress of the piezoelectric thin film 14 by the upper electrode 16. The piezoelectric thin film 14 is made of AlN. The upper electrode 16 is formed by laminating a Cr layer and a Ru layer from the side closer to the piezoelectric thin film 14. The piezoelectric thin film 14 has a thickness of 1200 nm, and the upper electrode 16 has a thickness of 250 nm.

  FIG. 10 is a graph showing the distribution of lattice constants. The horizontal axis represents positions (in-plane positions) 1 to 10 in the plane of the piezoelectric thin film 14. The vertical axis represents the lattice constant c. A black circle represents a lattice constant before the distribution is reduced, and a white circle represents a lattice constant after the distribution is reduced. Before the distribution reduction means before the upper electrode formation, and after the distribution reduction means after the upper electrode formation. As shown by black circles in FIG. 10, before the distribution is reduced, the minimum lattice constant is about 4.985 約 and the maximum lattice constant is about 4.99Å. On the other hand, as shown by white circles in FIG. 19, after the distribution is reduced, the minimum lattice constant is about 4.983 and the maximum lattice constant is about 4.984. Thus, the distribution of the lattice constant c is greatly reduced.

According to the second embodiment, the step of measuring the lattice constant distribution of the piezoelectric thin film 14 and the step of reducing the lattice constant distribution when the lattice constant distribution is equal to or larger than a predetermined size are performed. By distribution of the lattice constant of the piezoelectric thin film 14 is reduced, the distribution of the electromechanical coupling constant k ₃₃ ² is also reduced. As a result, the characteristics of the FBAR are stabilized.

As shown in FIG. 8, the step of reducing the distribution of lattice constant (step S17) is performed after the step of changing the film formation conditions (step S13). Accordingly, in the piezoelectric thin film 14 having a desired electromechanical coupling constant _{k 33} ^2, the distribution of the electromechanical coupling constant _{k 33} ² is reduced. As a result, the characteristics of the FBAR are more stable.

  As described in step S11 of FIG. 8, the step of measuring the lattice constant and the step of measuring the distribution of the lattice constant are performed simultaneously. In other words, both step S12 and step S14 are performed using the value of the lattice constant c measured in step S11. For this reason, a process can be simplified. However, the step of measuring the lattice constant and the step of measuring the distribution of the lattice constant may be performed separately. For example, a step of measuring the lattice constant distribution may be performed after step S12 of FIG. 8 and before step S14. The distribution of the lattice constant ratio c / a may be reduced. The distribution of one or both of the lattice constant c and the ratio c / a of the lattice constant may be reduced.

  9A and 9B show an example in which the residual stress distribution of the piezoelectric thin film 14 is reduced by the residual stress of the upper electrode 16. In other words, the step of reducing the lattice constant distribution shown in step S17 of FIG. 8 is a step of providing the upper electrode 16. Other methods may be used to reduce the distribution of residual stress. An example using an additional film is a modification of the second embodiment.

FIG. 11 is a cross-sectional view illustrating an FBAR according to a modification of the second embodiment. The description of the configuration already described in FIGS. 1A and 1B is omitted. As shown in FIG. 11, the FBAR 110 includes an additional film 30. The additional film 30 is provided on the upper electrode 16. The additional film 30 may be formed of an insulator such as SiO ₂ or silicon nitride (SiN), or may be formed of a metal. The residual stress of the piezoelectric thin film 14 can be relaxed by the residual stress of the upper electrode 16 and the residual stress of the additional film 30.

  Step S17 may be a step of heating the piezoelectric thin film 14. The residual stress distribution can be reduced by heating the piezoelectric thin film 14 to, for example, 600 ° C. It is preferable to heat the piezoelectric thin film 14 to a temperature higher than the maximum temperature in the FBAR manufacturing process. Both the reduction of the distribution due to the residual stress of the upper electrode 16 and the reduction of the distribution due to heating may be performed.

  The piezoelectric thin film 14 may be heated before or after the upper electrode 16 is formed. In other words, step S16 may be performed after step S17. As shown in FIG. 9A and FIG. 9B, the distribution can be reduced by providing the upper electrode 16. In other words, step S16 may include step S17.

A large lattice constant distribution means that, for example, the difference Δc between the maximum value and the minimum value of the lattice constant c measured at a plurality of locations is large. .DELTA.c in step S14 of FIG. 8, it is determined whether the predetermined value .DELTA.c ₀ or more. If Δc is equal to or greater than Δc _0, it is determined as Yes. Further, the size of the distribution may be determined using the standard deviation of the lattice constant c.

  The manufacturing method according to each of Example 1 and Example 2 can be applied to other acoustic wave devices. 12A to 13B are cross-sectional views illustrating another example of the acoustic wave device. The description of the configuration already described in FIGS. 1A and 1B is omitted.

  As shown in FIG. 12A, a gap 32 is formed in the substrate 10 of the FBAR 120. The gap 32 overlaps the lower electrode 12, the piezoelectric thin film 14, and the upper electrode 16. The lower electrode 12 is exposed in the gap 32. The gap 32 is formed by removing a part of the substrate 10 by an etching method or the like, for example. The gap 32 may penetrate the substrate 10 in the thickness direction.

  As shown in FIG. 12B, the SMR 130 includes an acoustic reflection film 34. The acoustic reflection film 34 is provided between the substrate 10 and the lower electrode 12. The acoustic reflection film 34 is a laminated film of a film having a high acoustic impedance and a film having a low acoustic impedance.

  As shown in FIG. 13A, the acoustic wave resonator 140 includes a piezoelectric film 31, a first support substrate 38, a second support substrate 40, and an electrode 42. The lower surface of the first support substrate 38 is bonded to the second support substrate 40 by, for example, surface activation bonding, resin bonding, or the like. A piezoelectric film 31 is provided on the upper surface of the first support substrate 38. The first support substrate 38 is formed with a hole that penetrates the first support substrate 38 in the thickness direction. The hole functions as a gap 44 between the piezoelectric film 31 and the second support substrate 40. An electrode 42 is provided in a region overlapping the gap 44 on the upper surface of the piezoelectric film 31. The piezoelectric film 31 is made of a piezoelectric material having a wurtzite crystal structure, such as AlN. The elastic wave resonator 140 is a resonator using Lamb waves.

  As shown in FIG. 13B, the FBAR 150 includes a temperature compensation film 19. The temperature compensation film 19 is inserted into the piezoelectric thin film 14 and is in contact with the piezoelectric thin film 14. The sign of the temperature coefficient of the elastic constant of the temperature compensation film 19 is opposite to the sign of the temperature coefficient of the elastic constant of the piezoelectric thin film 14. Therefore, the temperature characteristics of the FBAR 150 are stabilized. Note that the acoustic wave device shown in FIGS. 12A to 13A may include the temperature compensation film 19. With the method for manufacturing an acoustic wave device according to Example 1 or Example 2, an acoustic wave device as shown in FIGS. 12A to 13B can be manufactured. The method for manufacturing an acoustic wave device according to Example 1 or Example 2 can also be applied to a method for manufacturing an acoustic wave device such as a filter and a duplexer including a resonator, and a module including the filter and the duplexer.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

DESCRIPTION OF SYMBOLS 10 Substrate 12 Lower electrode 14 Piezoelectric thin film 16 Upper electrode 17 Resonance region 18, 32, 44 Air gap 19 Temperature compensation film 31 Piezoelectric film 100, 110, 120, 150 FBAR
130 SMR
140 Elastic wave resonator

Claims (7)

Forming a piezoelectric film having a wurtzite crystal structure;
Measuring a lattice constant in the c-axis direction and a lattice constant in the a-axis direction of the piezoelectric film;
When the ratio between the c-axis direction of the lattice constant between the a-axis direction of the lattice constant is a predetermined value or more, so that the ratio of the lattice constant is smaller than the predetermined value, forming the piezoelectric film And a step of changing a film forming condition of the piezoelectric film in the step of forming a film. An acoustic wave device manufacturing method for manufacturing a piezoelectric thin film resonator comprising a piezoelectric film and a lower electrode and an upper electrode sandwiching the piezoelectric film,
Depositing the piezoelectric film having a wurtzite crystal structure on the lower electrode;
measuring the lattice constant in the c-axis direction;
When the lattice constant in the c-axis direction is equal to or greater than a predetermined value, the film formation conditions of the piezoelectric film in the step of forming the piezoelectric film are changed so that the lattice constant is smaller than the predetermined value. And a method of manufacturing an acoustic wave device. Forming a piezoelectric film having a wurtzite crystal structure;
Measuring the lattice constant of the piezoelectric film;
Changing the film forming conditions of the piezoelectric film in the step of forming the piezoelectric film based on the lattice constant;
Measuring the distribution of the lattice constant of the piezoelectric film in the plane of the piezoelectric film;
And a step of reducing the distribution of the lattice constant in the plane of the piezoelectric film when the distribution of the lattice constant in the plane of the piezoelectric film is equal to or greater than a predetermined size. Manufacturing method.   The method for manufacturing an acoustic wave device according to claim 3, wherein the step of reducing the distribution of the lattice constant is performed after the step of changing the film formation conditions.   5. The method for manufacturing an acoustic wave device according to claim 3, wherein the step of measuring the lattice constant and the step of measuring the distribution of the lattice constant are performed simultaneously. The step of forming the piezoelectric film is a step of forming a piezoelectric film on the substrate or the lower electrode of the acoustic wave device,
6. The step of measuring the lattice constant is a step of measuring a lattice constant of a piezoelectric film formed on the substrate or the lower electrode. Manufacturing method of elastic wave device. The step of forming the piezoelectric film is a step of forming a piezoelectric film in which a temperature compensation film is inserted,
7. The method of manufacturing an acoustic wave device according to claim 1, wherein the step of measuring the lattice constant is a step of measuring a lattice constant of the piezoelectric film in which the temperature compensation film is inserted. .

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