JP2008244653A - Manufacturing method for thin-film bulk wave resonator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method for a thin-film bulk wave resonator suitable to adjust a resonance frequency and an anti-resonant frequency. <P>SOLUTION: A protecting film 60 is formed on an upper electrode 50B of a parallel arm resonator 10B, and a plurality of apertures are formed in a mesh-like fashion on the protecting film 60 by dry etching to expose a part of the upper electrode 50B. By adjusting the aperture ratio of the protecting film 60, the anti-resonant frequency of the parallel arm resonator 10B can be matched with the resonance frequency of a serial resonator 10A. Since the protecting film 60 with the plurality of apertures formed in the mesh-like fashion has a facility for adjusting the resonance frequency and the anti-resonant frequency of the parallel arm resonator 10B by its weight, the protecting film and a frequency regulation film are formed by one manufacturing process. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a thin film bulk acoustic wave resonator.

  Numerous resonances using piezoelectric materials such as thin film bulk acoustic resonators and surface acoustic wave resonators as resonators that make up bandpass filters for wireless LAN and mobile communication devices The vessel has been put into practical use. In particular, thin film bulk acoustic wave resonators are commercialized as RF filters for PCS (Personal Communication Service) systems, which have an excellent Q value due to the structure in which elastic waves propagate in the film thickness direction, and require high frequency selectivity. ing. The thin film bulk acoustic wave resonator has a substrate and a laminated resonator formed on the substrate. The laminated resonator has a piezoelectric film and a pair of upper and lower electrodes sandwiching the piezoelectric film from above and below, and when a high frequency signal is applied between the upper electrode and the lower electrode, the laminated resonance The body is excited in the longitudinal direction at a resonance frequency at which the film thickness is equal to ½ wavelength.

  As a ladder type filter, the basic configuration consists of a one-port resonator connected in series and parallel in a ladder configuration, and the resonance frequency of the series arm resonator and the anti-resonance frequency of the parallel arm resonator are substantially matched. In a filter that obtains bandpass characteristics, the resonance frequency or anti-resonance frequency of each resonator must match the target frequency. Thus, a technique for matching the resonance frequency or anti-resonance frequency of the resonator with the target frequency has been studied.

For example, Non-Patent Document 1 discloses a method of adjusting the resonance frequency by patterning the upper electrode of a thin film bulk acoustic wave resonator in a mesh shape and substantially reducing the density of the upper electrode. In Patent Document 1, an additional film made of a metal or a dielectric is provided below a lower electrode or an upper electrode of a resonator that sets a resonance frequency relatively lower than other resonators among a plurality of resonators. A method of forming is disclosed. Patent Document 2 discloses a technique for adjusting a resonance frequency by removing a sacrificial mass load layer disposed on a piezoelectric resonator.
JP 2001-326553 A JP-T-2001-514456 Proc. Symp. Ultrason. Electron., Vol 27, (2006) pp13-14

  However, in any of the methods disclosed in any of the above-mentioned documents, problems such as a complicated manufacturing process or a decrease in the resonance characteristics of the resonator have been pointed out. For example, in the method disclosed in Non-Patent Document 1, since a large number of minute holes are opened in the upper electrode, there is a possibility that the resistance component increases and the resonance characteristics deteriorate. Furthermore, when a hole is formed in the upper electrode by using ion milling that physically etches the material to be processed, there is a problem that burrs are generated in the opening. If a hole is formed in the upper electrode using wet etching that chemically etches the material to be processed, not only precise processing cannot be performed, but also the crystal structure of the piezoelectric film may be damaged. In order to form the hole of the upper electrode by using the reactive dry etching for physicochemically etching the material to be processed, the electrode material that can be processed is limited.

  In the method disclosed in Patent Document 1, it is necessary to separately form an additional film below the lower electrode or below the upper electrode, which complicates the manufacturing process. In the technique disclosed in Patent Document 2, the sacrificial mass load layer is formed on the electrode of the piezoelectric resonator and physically removed by ion sputtering or the like, so that the manufacturing process becomes complicated.

  Therefore, an object of the present invention is to solve such problems and to provide a method for manufacturing a thin film bulk wave resonator suitable for adjusting the resonance frequency and the antiresonance frequency.

  In order to solve the above problems, a method of manufacturing a thin film bulk acoustic wave resonator according to the present invention includes forming a protective film on the upper electrode, forming a plurality of openings in the protective film in a mesh shape by dry etching, A part of the electrode is exposed. Here, the thin film bulk acoustic wave resonator includes a piezoelectric film, a lower electrode formed on one surface of the piezoelectric film, and an upper electrode formed on the other surface of the piezoelectric film.

  The protective film in which a plurality of openings are formed in a mesh shape has a function of adjusting the resonance frequency and anti-resonance frequency of the thin film bulk acoustic wave resonator by its weight. Can be formed. Further, by patterning the protective film by dry etching, high-precision patterning is possible, which is suitable for adjusting the frequency of the thin film bulk acoustic wave resonator.

  Here, the opening area of the openings or the arrangement pitch of the openings is set so that the resonance frequency or antiresonance frequency of the thin film bulk acoustic wave resonator matches the target frequency. For example, in order to manufacture a bandpass filter in which a series arm resonator and a parallel arm resonator are connected in a ladder shape, a protective film is provided on both the series arm resonator and the parallel arm resonator or only on the parallel arm resonator. The protective film may be patterned by dry etching so that the anti-resonance frequency of the parallel arm resonator matches the resonance frequency of the series resonator. Here, the patterning includes forming a plurality of openings in a mesh shape.

  A dielectric film is suitable as the protective film, and reactive ion etching is suitable as the dry etching. Thereby, highly accurate anisotropic etching excellent in selectivity can be realized.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of a thin film bulk wave resonator suitable for adjustment of a resonance frequency and an antiresonance frequency can be provided.

  Embodiments according to the present invention will be described below with reference to the drawings. Devices with the same reference numerals indicate the same devices, and redundant description is omitted. The drawings are schematic, and the relationship between the thickness and the planar dimensions and the ratio of the thickness of each layer are different from the actual ones. Moreover, the part from which the relationship and ratio of a mutual dimension differ between drawings is contained.

  FIG. 1 shows a cross-sectional structure of a filter 10 according to this embodiment. The filter 10 includes a series arm resonator 10 </ b> A and a parallel arm resonator 10 </ b> B formed on the substrate 20. The material of the substrate 20 is not particularly limited as long as it has an appropriate mechanical strength and is suitable for fine processing such as etching. For example, a silicon single crystal substrate or a sapphire single crystal substrate is used. Ceramic substrates, quartz, glass substrates and the like are suitable.

  The series arm resonator 10A includes a lower electrode 30A formed on the insulating film 21 on the substrate surface, a piezoelectric film 40 formed on the lower electrode 30A, and an upper electrode 50A formed on the piezoelectric film 40. A cavity 22A is opened on the back surface of the substrate so that the first multilayer resonator can be freely vibrated in the thickness direction without being constrained by the substrate 20.

  The parallel arm resonator 10B includes a lower electrode 30B formed on the insulating film 21 on the substrate surface, a piezoelectric film 40 formed on the lower electrode 30B, an upper electrode 50B formed on the piezoelectric film 40, and an upper part. The second laminated resonator including the protective film 60 partially formed on the electrode 50B is provided, and the second laminated resonator is free to vibrate in the thickness direction without being constrained by the substrate 20 below the electrode 50B. A cavity 22B is opened on the back surface of the substrate so that it can be formed.

The material of the piezoelectric film 40 is preferably a piezoelectric material having a large electromechanical coupling coefficient, a small propagation loss and a power flow angle, a small delay time temperature coefficient, and a small frequency dispersion of propagation speed. (ZnO), aluminum nitride (AlN), potassium niobate (KNbO ₃ ), lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), lead zirconate titanate (PZT), barium titanate (BaTiO ₃ ) Etc. are suitable. As a material of the lower electrodes 30A and 30B and the upper electrodes 50A and 50B, the piezoelectric film 40 is preferably a conductive material capable of forming an appropriate orientation, for example, ruthenium (Ru), aluminum (Al), Molybdenum (Mo), titanium (Ti), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), or an alloy containing any two or more thereof is suitable. In particular, the upper electrodes 50A and 50B are preferably made of a conductive material having low selectivity to the etching gas when the protective film 60 is dry-etched by reactive ion etching or the like.

  The protective film 60 is a film for protecting the filter 10 from the outside air, and is formed so as to substantially cover the entire surface of the element formed on the substrate 20. However, the protective film 60 is patterned so that the entire surface of the upper electrode 50A of the series arm resonator 10A is exposed and a part of the upper electrode 50B of the parallel arm resonator 10B is exposed in a mesh shape. Since the protective film 60 partially formed on the upper electrode 50B has a function of adjusting the resonance frequency and antiresonance frequency of the parallel arm resonator 10B, it functions as a frequency adjustment film.

The material of the protective film 60 is preferably a material that has appropriate durability, heat resistance, and water resistance and that can be etched by reactive dry etching such as reactive ion etching (RIE). For example, a silicon dioxide film Dielectric films such as (SiO ₂ ), aluminum oxide film (Al ₂ O ₃ ), silicon nitride film (Si ₃ N ₄ ), and tantalum oxide film (Ta ₂ O ₅ ) are suitable.

  In the above device configuration, when a high frequency signal is applied between the upper electrode 50A and the lower electrode 30A of the series arm resonator 10A, the piezoelectric film 40 vibrates in the thickness direction due to the reverse piezoelectric effect of the piezoelectric film 40, Electrical resonance characteristics are shown. Further, the elastic wave or vibration generated in the piezoelectric film 40 is converted into an electric signal by the piezoelectric effect of the piezoelectric film 40. This elastic wave is a thickness longitudinal vibration wave having a main displacement in the thickness direction of the piezoelectric film 40, and is excited at a resonance frequency at which the film thickness of the first laminated resonator becomes equal to ½ wavelength. Even when a high-frequency signal is applied between the upper electrode 50B and the lower electrode 30B of the parallel arm resonator 10B, the second stacked resonator vibrates in the thickness direction according to the same principle as described above. Since the protective film 60 is formed in a mesh shape on 50B, the resonance frequency and anti-resonance frequency of the parallel arm resonator 10B are shifted to lower frequencies due to the weight of the protective film 60.

2 shows an equivalent circuit of the filter 10, FIG. 3A shows the reactance characteristics of the series arm resonator 10A and the parallel arm resonator 10B, and FIG. 3B shows the band pass characteristics of the filter 10. FIG. .
As shown in FIG. 2, the filter 10 is a ladder type filter in which a series arm resonator 10A and a parallel arm resonator 10B are connected in a ladder shape. In such a ladder-type filter, as shown in FIG. 3A, when the antiresonance frequency fa1 of the parallel arm resonator 10B matches the resonance frequency fr2 of the series resonator 10A, as shown in FIG. In addition, a bandpass characteristic is obtained in which the resonance frequency fr1 of the parallel arm resonator 10B and the antiresonance frequency fa2 of the series arm resonator 10A are attenuation poles, and the frequency band therebetween is the passband. Here, the solid line in FIG. 3A shows the reactance characteristic of the parallel arm resonator 10B, and the broken line shows the reactance characteristic of the series arm resonator 10A.

  FIG. 4 shows a planar pattern of the protective film 60 partially formed on the upper electrode 50B of the parallel arm resonator 10B. The protective film 60 has a plurality of openings 61 that are opened in a mesh shape so that a part of the upper electrode 50B is exposed. The length of one side of the opening 61 is sufficiently shorter than the wavelength of the elastic wave at the time of resonance, and the arrangement pitch of the openings 61 (the distance between the centers of the adjacent openings 61) is sufficiently larger than the wavelength of the elastic wave at the time of resonance. It is preferable to set it short. As a result, the elastic wave cannot be recognized by separating the openings 61 one by one, and an elastic characteristic change equivalent to the fact that the density of the protective film 60 is substantially reduced occurs.

  The shape of the opening 61 is not particularly limited, and may be, for example, a square shown in FIG. 5 in addition to the circle shown in FIG. 4, or an arbitrary plane pattern (eg, an ellipse, a polygon, a triangle). But you can. 4 to 5, the hatched portion may be removed from the protective film 60, and a planar pattern such as a circle or a square may be left as the protective film 60 on the upper electrode 50B.

  FIG. 6 is a graph showing the correspondence between the aperture ratio of the protective film 60 partially formed on the upper electrode 50B of the parallel arm resonator 10B and the resonance frequency fr and antiresonance frequency fa of the parallel arm resonator 10B. . It is shown that as the aperture ratio of the protective film 60 is increased, the mass of the protective film 60 formed on the upper electrode 50B is decreased, so that both the resonance frequency fr and the anti-resonance frequency fa are increased. Here, the aperture ratio indicates the ratio of the total value of the areas of all the openings 61 to the area of the upper electrode 50B. By adjusting the aperture ratio of the protective film 60, the anti-resonance frequency fa1 of the parallel arm resonator 10B can be matched with the resonance frequency fr2 of the series resonator 10A. The aperture ratio is appropriately adjusted according to the frequency difference between the anti-resonance frequency fa1 of the parallel arm resonator 10B and the resonance frequency fr2 of the series resonator 10A, the film thickness of the protective film 60, the density of the protective film 60, and the like. As a range of the aperture ratio, for example, a range of about 20% to 80% is preferable. The aperture ratio can be adjusted by increasing or decreasing the opening area of the openings 61 or the arrangement pitch of the openings 61.

Next, the manufacturing process of the filter 10 will be described with reference to FIG.
First, as illustrated in FIG. 7A, for example, a (100) silicon substrate is prepared as the substrate 20, and the insulating film 21 is formed on the surface of the substrate 20. For example, in order to form a silicon oxide film as the insulating film 21, a thermal oxidation method or the like may be applied. Further, a metal film having a thickness of about 150 to 600 nm is deposited on the insulating film 21 using an RF magnetron sputtering method, an etching mask is formed by photolithography, and then the metal film is patterned by reactive ion etching to form an island shape. Separated lower electrodes 30A and 30B are formed. The material of the lower electrodes 30A and 30B is preferably a conductive material with low selectivity to dry etching such as reactive ion etching, and for example, platinum (Pt), ruthenium (Ru), and the like are suitable.

  Then, a piezoelectric material is deposited on the lower electrodes 20A and 20B by RF magnetron sputtering. The film thickness of the piezoelectric material may be adjusted according to the resonance frequency. For example, if the resonance frequency is set to about 2.0 GHz using AlN, the film thickness is about 1.2 μm to 2 μm, depending on the electrode material used. What is necessary is just to set to a film thickness. Subsequently, a metal film having a thickness of about 150 to 600 nm is deposited by RF magnetron sputtering so as to cover the entire surface of the piezoelectric film 40, an etching mask is formed by photolithography, and then the metal film is selectively etched by wet etching. Electrodes 50A and 50B are formed.

Next, as shown in FIG. 7B, a protective film 60 is formed to a thickness of about 1000 to 3000 so as to cover the upper electrodes 50A and 50B and the piezoelectric film 40 exposed on the surface, and the protective film is formed by photolithography. An etching mask 70 is formed on 60. In the etching mask 70, the entire surface of the upper electrode 50A of the series arm resonator 10A is exposed, a part of the upper electrode 50B of the parallel arm resonator 10B is exposed in a mesh shape, and the other part is covered with the protective film 60. Patterning. As the material of the protective film 60, the selectivity to the reactive dry etching in which the material to be processed is physicochemically etched by the sputter etching of the material to be processed by ion irradiation and the generation of the volatile substance by radicals is higher than the upper electrodes 50A and 50B. The film is preferably a film having durability, heat resistance, and water resistance that is extremely high and can function as a protective film. For example, a dielectric film is preferable. Among the dielectric films, a silicon dioxide film (SiO ₂ ), an aluminum oxide film (Al ₂ O ₃ ), a silicon nitride film (Si ₃ N ₄ ), and a tantalum oxide film (Ta ₂ O ₅ ) are particularly preferable.

  Then, as shown in FIG. 7C, under the predetermined etching conditions (substrate temperature, pressure, etching gas flow rate, high frequency output, etc.), the upper electrode 50A, The protective film 60 is patterned until 50B is exposed. According to reactive ion etching, anisotropic etching with less undercut can be realized, so that the mask pattern of the etching mask 70 can be faithfully transferred to the protective film 60 and the selectivity to the material to be processed is high. 50A and 50B function as etching stoppers.

  Finally, as shown in FIG. 7D, after an etching mask is formed on the back surface of the substrate 20 by photolithography, the back surface of the substrate is opened by reactive ion etching using a fluoride-based gas, and the cavities 22A, 22B. Form. Through the above steps, the series arm resonator 10A and the parallel arm resonator 10B are formed.

  The protective film 60 formed on the upper electrode 50B of the parallel arm resonator 10B is patterned in a mesh shape, and the aperture ratio is adjusted to thereby set the antiresonance frequency fa1 of the parallel arm resonator 10B to the series resonator 10A. However, the protective film 60 is formed in a mesh shape on each of the upper electrode 50B of the parallel arm resonator 10B and the upper electrode 50A of the series arm resonator 10A. A configuration in which the anti-resonance frequency fa1 of the parallel arm resonator 10B is matched with the resonance frequency fr2 of the series resonator 10A by adjusting may be applied.

  In the above description, the case where the present invention is applied to an FBAR type piezoelectric resonator has been described. However, an SIR type piezoelectric resonance using an AirGap type piezoelectric resonator or an acoustic reflection film in which a recess is formed on the surface of a substrate. The present invention can be applied to all laminated piezoelectric resonators using piezoelectric films, such as ceramics.

FIG. 8 shows a circuit configuration of the duplexer 80 according to the present embodiment.
The duplexer 80 includes an antenna terminal ANT1 connected to an antenna (not shown), a transmission signal terminal SX1 connected to a transmission circuit (not shown) that outputs a transmission signal to the antenna, and reception received via the antenna. A reception signal terminal RX1 connected to a reception circuit (not shown) for inputting a signal, a transmission filter 81 that transmits a transmission signal and blocks the reception signal, and reception that transmits a reception signal and blocks the transmission signal Filter 82. Either one or both of the transmission filter 81 and the reception filter 82 has a circuit configuration in which the filters 10 according to this embodiment are cascade-connected in a plurality of stages.

  According to the present embodiment, the protective film 60 in which the plurality of openings 61 are formed in a mesh shape has a function of adjusting the resonance frequency fr1 and the anti-resonance frequency fa1 of the parallel arm resonator 10B by the weight, and therefore once The protective film and the frequency adjusting film can be formed by the manufacturing process. Further, by patterning the protective film 60 by dry etching, high-precision patterning becomes possible, which is suitable for frequency adjustment of the thin film bulk acoustic wave resonator.

It is a sectional structure figure of a filter concerning this embodiment. It is an equivalent circuit diagram of the filter concerning this embodiment. FIG. 3A is a graph showing the reactance characteristics of the series arm resonator and the parallel arm resonator, and FIG. 3B is a graph showing the bandpass characteristics of the filter. It is a schematic diagram which shows the plane pattern of a protective film. It is a schematic diagram which shows the other plane pattern of a protective film. It is a graph which shows the correspondence of the aperture ratio of a protective film, and the resonant frequency and antiresonance frequency of a parallel arm resonator. It is sectional drawing which shows the manufacturing process of the filter concerning this embodiment. It is a circuit block diagram of the duplexer concerning this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Filter 10A ... Series arm resonator 10B ... Parallel arm resonator 20 ... Substrate 22A, 22B ... Cavity 21 ... Insulating film 30A, 30B ... Lower electrode 40 ... Piezoelectric film 50A, 50B ... Upper electrode 60 ... Protective film 70 ... Etching mask

Claims (3)

A method for manufacturing a thin film bulk acoustic wave resonator comprising: a piezoelectric film; a lower electrode formed on one surface of the piezoelectric film; and an upper electrode formed on the other surface of the piezoelectric film. There,
Forming a protective film on the upper electrode;
Forming a plurality of openings in the protective film in a mesh shape by dry etching, exposing a part of the upper electrode;
A method of manufacturing a thin film bulk acoustic wave resonator. A method of manufacturing a thin film bulk acoustic wave resonator according to claim 1,
The method of manufacturing a thin film bulk acoustic wave resonator, wherein the aperture area of the aperture or the arrangement pitch of the apertures is set so that the resonance frequency or antiresonance frequency of the thin film bulk acoustic wave resonator matches a target frequency. A method of manufacturing a thin film bulk acoustic wave resonator according to claim 1 or 2,
The method for manufacturing a thin film bulk acoustic wave resonator, wherein the protective film is a dielectric film, and the dry etching is reactive ion etching.

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