JP5096695B2 - Thin film acoustic resonator - Google Patents

Description

  The present invention relates to a thin film acoustic resonator provided with an acoustic reflecting portion.

  In recent years, with the increase in the number of mobile phone terminals, duplexers, interstage filters, and the like are required to have a lower insertion loss and a steep attenuation characteristic than before. A thin film acoustic resonator (FBAR) is an acoustic resonator that uses resonance in the thickness direction of an elastic wave propagating in a piezoelectric thin film. A ladder-connected FBAR filter has low loss and sharp attenuation. It has attracted attention as a filter that can realize the characteristics.

  The thin-film acoustic resonator includes a piezoelectric film and a resonance part including an upper electrode and a lower electrode disposed on the upper and lower surfaces of the piezoelectric film. Furthermore, in order to confine the acoustic wave generated by exciting the piezoelectric film in the resonance part, it is necessary to provide a cavity part or an acoustic reflection part above and below the resonance part. The acoustic reflection unit includes a low acoustic impedance layer that is alternately stacked on the substrate, and a high acoustic impedance layer that has a larger acoustic impedance than the low acoustic impedance layer.

  The resonance characteristics of the thin-film acoustic resonator are affected by the crystallinity of the piezoelectric body, and a piezoelectric film with good crystallinity is indispensable for realizing excellent filter characteristics. Furthermore, it is generally known that in order to obtain a piezoelectric film having good crystallinity, it is necessary to smooth the upper surface of the lower electrode that is the base and the acoustic reflection portion that is the base of the lower electrode. ing.

When the acoustic reflection part is formed by alternately depositing the low acoustic impedance layer and the high acoustic impedance layer on the substrate by using a normal chemical vapor deposition (CVD) method or a sputtering method, the surface of each layer The unevenness is increased by lamination, and it is very difficult to smooth the surface of the acoustic reflection portion. For this reason, after forming the acoustic reflection portion, the surface of the acoustic reflection portion is smoothed using a chemical mechanical polishing (CMP) method, or the acoustic reflection portion is deposited using a bias sputtering method. A method of smoothing the surface has been attempted (for example, see Patent Document 1).
Japanese Patent Laid-Open No. 2005-136761

  However, in the conventional method of forming the acoustic reflection portion, the problem that the smoothness of the upper surface of the obtained acoustic reflection portion is not sufficient, or when the upper surface of the acoustic resonator is smoothed using the CMP method, There is a problem that the process becomes complicated.

  An object of the present invention is to solve the above-described conventional problems and to realize a thin film acoustic resonator including a piezoelectric film having a good crystallinity formed on a smooth surface.

  In order to achieve the above object, the present invention has a configuration in which a thin film acoustic resonator is provided with a smoothing layer formed between an acoustic reflector and an acoustic resonator.

  Specifically, the thin film acoustic resonator according to the present invention includes a substrate, a low acoustic impedance layer on the substrate, and a high acoustic impedance layer having a higher acoustic impedance than the low acoustic impedance layer. A reflection unit, and an acoustic resonance unit in which a lower electrode, a piezoelectric film, and an upper electrode are sequentially stacked on the acoustic reflection unit, and the acoustic reflection unit includes a smoothing layer above the acoustic reflection unit. It is characterized by.

  In the thin film acoustic resonator of the present invention, since the acoustic reflection part includes a smoothing layer on the upper part, the smoothness of the upper surface of the acoustic reflection part is improved. Thereby, the crystallinity of the piezoelectric film formed on the acoustic reflector is improved, and a thin film acoustic resonator having excellent resonance characteristics can be realized.

  In the thin film acoustic resonator, the smoothing layer is preferably formed on the uppermost portion of the acoustic reflection portion. In this case, the smoothing layer preferably functions as a low acoustic impedance layer.

  In the thin film acoustic resonator, the smoothing layer may be formed between the high acoustic impedance layer and the low acoustic impedance layer that are stacked on the uppermost side of the acoustic reflection portion.

  In the thin film acoustic resonator of the present invention, the smoothing layer is preferably a film formed of a spin coatable material. In this case, the film formed of a spin coatable material is preferably a spin-on-glass film, a fluorine resin film, a silicon resin film, or an epoxy resin film. With such a configuration, the upper surface of the smoothing layer can be surely smoothed.

  In the thin film acoustic resonator of the present invention, the smoothing layer preferably has a root mean square roughness of 1 nm or less on the upper surface. With such a configuration, it is possible to reliably form a piezoelectric film with good crystallinity.

  In the thin film acoustic resonator of the present invention, the high acoustic impedance layer is preferably made of an insulating material. By setting it as such a structure, when a some acoustic resonance part is formed on an acoustic reflection part, it can prevent that each acoustic resonance part mutually couple | bonds.

  In the thin film acoustic resonator of the present invention, a plurality of acoustic resonance parts are formed on the acoustic reflection part at intervals, and a high acoustic impedance is provided in a region between the acoustic resonance parts in the acoustic reflection part. It is preferred that the layers are separated. By adopting such a configuration, even when a metal film is used for the high acoustic impedance layer, it is possible to prevent the acoustic resonance portions from being electrically coupled to each other.

  According to the thin film acoustic resonator of the present invention, the upper surface of the acoustic reflecting portion can be smoothed, and the crystallinity of the piezoelectric film formed on the acoustic reflecting portion can be improved, so that excellent resonance characteristics can be obtained. .

(First embodiment)
A first embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a cross-sectional configuration of a thin film acoustic resonator according to a first embodiment of the present invention. As shown in FIG. 1, the acoustic resonator according to the present embodiment has a high acoustic impedance layer 22 made of tungsten and a low acoustic impedance layer 23 made of silicon oxide alternately stacked on a substrate 21 made of silicon for four periods. The formed acoustic reflection unit 24 and the acoustic resonance unit 28 formed by sequentially laminating the lower electrode 25, the piezoelectric film 26 and the upper electrode 27 on the acoustic reflection unit 24 are provided.

  The acoustic reflector 24 has a smoothing layer 29 that is a spin-on-glass (SOG) film formed on the top. For this reason, the upper surface of the acoustic reflection part 24 is a very smooth surface. Thereby, the smoothness of the lower electrode 25 can be improved, and the crystallinity of the piezoelectric film formed on the lower electrode 25 can be improved.

  In this embodiment, in order to set the resonance frequency of the thin film acoustic resonator to 2 GHz, a high acoustic impedance layer 22 made of tungsten having a thickness of 600 nm corresponding to a quarter of the acoustic wavelength on the substrate 21 made of silicon and 680 nm. When the acoustic reflection portion 24 is formed by stacking four periods of the low acoustic impedance layer 23 made of silicon oxide having a thickness of 5 mm, the root mean square on the upper surface of the low acoustic impedance layer 23 formed on the uppermost portion of the acoustic reflection portion 24. The roughness (Rms) was 2 nm. By forming the smoothing layer 29, which is an SOG film having a thickness of 100 nm, on the acoustic reflector 24 having an Rms of 2 nm, the Rms on the upper surface of the smoothing layer 29 was 0.8 nm. For this reason, by providing the smoothing layer 29 on the acoustic reflector 24, the piezoelectric film 26 having excellent crystallinity can be obtained.

  The SOG film is spin-coated with an SOG solution and subjected to heat treatment, thereby embedding surface irregularities and obtaining high flatness and uniform film thickness. Usually, it is used for embedding and flattening the unevenness of the wiring, so it is generally used with a film thickness of several μm. However, the present applicants have found that by adjusting the solid concentration in the solution of the SOG solution, the flatness and the uniformity of the film thickness can be maintained even when the film thickness after the heat treatment is 100 nm or less.

  FIG. 2 shows the relationship between the solid concentration in the SOG solution and the film thickness of the SOG film after the heat treatment in the case where the SOG film was actually formed at a spin coating speed of 3000 rpm and a heat treatment temperature of 450 ° C. Yes. As shown in FIG. 2, it is apparent that the thickness of the SOG film can be controlled well by changing the solid concentration in the SOG solution. Also, under the present experimental conditions, the thickness of the SOG film could be reduced to 100 nm or less when the solid concentration in the SOG solution was less than 5 percent. As described above, the smoothing layer 29 having a thin and very smooth surface can be formed on the acoustic reflection portion 24 based on the knowledge of the applicants of the present application.

  In addition to the SOG film, the smoothing layer 29 may be a spin-coating fluorine-based resin material, a silicon-based resin material, an epoxy-based resin material, or the like. In this embodiment, silicon oxide is used as the low acoustic impedance layer 23, but an insulator such as silicon nitride or a metal material such as aluminum or titanium may be used. The high acoustic impedance layer 22 may be a high melting point metal such as tungsten, platinum, iridium, or molybdenum, or an insulator such as hafnium oxide or aluminum nitride.

  In the present embodiment, the smoothing layer 29 is formed on the uppermost part of the acoustic reflection unit 24. However, since the upper surface of the layer formed on the surface smoothed by the smoothing layer 29 is also smoothed, the smoothing layer 29 does not necessarily need to be formed on the uppermost part of the acoustic reflector 24. For example, when the high acoustic impedance layer 22 and the low acoustic impedance layer 23 are laminated in a plurality of periods, the smoothing layer 29 is provided between the high acoustic impedance layer 22 and the low acoustic impedance layer 23 formed on the uppermost side. It may be formed.

  In the present embodiment, the acoustic reflector 24 is formed by laminating four periods of the high acoustic impedance layer 22 and the low acoustic impedance layer 23, but the lamination period of the high acoustic impedance layer 22 and the low acoustic impedance layer 23 is appropriately changed. can do. In addition, the stacking order of the high acoustic impedance layer 22 and the low acoustic impedance layer 23 may be changed.

(Second Embodiment)
The second embodiment of the present invention will be described below with reference to the drawings. FIG. 3 shows a cross-sectional configuration of the thin film acoustic resonator according to the second embodiment. In FIG. 3, the same components as those in FIG.

  As shown in FIG. 3, in the thin film acoustic resonator of the present embodiment, the smoothing layer 29 is used as the low acoustic impedance layer 23 in the uppermost layer of the acoustic reflector 24. Thus, by using the smoothing layer 29 as the low acoustic impedance layer 23, the smoothing layer 29 functions as a part of the acoustic reflector 24, and a thin film acoustic resonator having good characteristics is obtained.

  As in the first embodiment, the smoothing layer 29 can be made of an SOG material, a fluorine resin material, a silicon resin material, an epoxy resin material, or the like. These materials have an acoustic impedance lower than that of a silicon oxide film widely used as a low acoustic impedance layer. For this reason, since the acoustic reflection characteristics are improved as compared with the acoustic reflection section 24 using a silicon oxide film, and Rms directly under the lower electrode can be reduced, a thin film acoustic resonator having good characteristics can be realized.

(Third embodiment)
The third embodiment of the present invention will be described below with reference to the drawings. FIG. 4 shows a cross-sectional configuration of the thin film acoustic resonator according to the third embodiment. In FIG. 4, the same components as those of FIG.

  As shown in FIG. 4, in the thin film acoustic resonator of this embodiment, the low acoustic impedance layer 23 is formed of an SOG film, and the lamination period of the low acoustic impedance layer 23 and the high acoustic impedance layer 22 is 1.5 periods. It has become.

  The SOG film has an acoustic impedance that is an order of magnitude lower than a silicon oxide film that is generally used as a low acoustic impedance layer, and has good acoustic reflection characteristics. For this reason, it becomes possible to implement | achieve by accumulating the acoustic reflection part which should usually laminate | stack 4-6 periods of high acoustic impedance layers and low acoustic impedance layers about 1.5-2 periods. Thereby, the formation process of the acoustic reflection part 24 can be reduced significantly.

  Further, since the acoustic impedance of the low acoustic impedance layer 23 is lowered, silicon oxide, silicon nitride, or the like can be used as the high acoustic impedance layer in place of conventional tungsten or the like. Thereby, formation of the acoustic reflection part 24 becomes easy and reduction of manufacturing cost is also attained.

  For the low acoustic impedance layer 23, a fluorine resin film, a silicon resin material, an epoxy resin material, or the like may be used instead of the SOG film.

(Fourth embodiment)
The fourth embodiment of the present invention will be described below with reference to the drawings. FIG. 5 shows a cross-sectional configuration of the thin film acoustic resonator according to the fourth embodiment. In FIG. 5, the same components as those of FIG.

  As shown in FIG. 5, in the thin film acoustic resonator of the present embodiment, a first acoustic resonance unit 28 </ b> A and a second acoustic resonance unit 28 </ b> B are formed on the acoustic reflection unit 24. In addition, the low acoustic impedance layer 23 in the acoustic reflector 24 is integrally formed in a region where the first acoustic resonator 28A is formed and a region where the second acoustic resonator 28B is formed. The acoustic impedance layer is formed independently of each other in a region where the first acoustic resonance unit 28A is formed and a region where the second acoustic resonance unit 28B is formed. That is, in the acoustic reflection unit 24, the high acoustic impedance layer 22 is separated between the first acoustic resonance unit 28A and the second acoustic resonance unit 28B. Thereby, even when a metal material such as tungsten is used for the high acoustic impedance layer 22, the capacitance value between the lower electrode 25 of the first acoustic resonance unit 28A and the high acoustic impedance layer 22 and the second acoustic resonance are obtained. It is possible to suppress the first acoustic resonance unit 28A and the second acoustic resonance unit 28B from being electrically coupled via the capacitance value between the lower electrode 25 of the unit 28B and the high acoustic impedance layer 22.

  Although FIG. 5 shows an example in which two acoustic resonance parts are formed, three or more acoustic resonance parts may be formed. Also in the configuration of the first embodiment or the second embodiment, a plurality of acoustic resonance portions are formed on the acoustic reflection portion, and the high acoustic impedance layer 22 is not formed in a portion between the acoustic resonance portions. By doing so, the same effect can be obtained.

  The thin film acoustic resonator according to the present invention can be realized as a thin film acoustic resonator including a piezoelectric film having a good crystallinity formed on a smooth surface, and useful as a thin film acoustic resonator including an acoustic reflector. It is.

It is sectional drawing which shows the thin film acoustic resonator which concerns on the 1st Embodiment of this invention. It is a graph for demonstrating the formation method of the planarization layer in the thin film acoustic resonator which concerns on the 1st Embodiment of this invention. It is sectional drawing which shows the thin film acoustic resonator which concerns on the 2nd Embodiment of this invention. It is sectional drawing which shows the thin film acoustic resonator which concerns on the 3rd Embodiment of this invention. It is sectional drawing which shows the thin film acoustic resonator which concerns on the 4th Embodiment of this invention.

Explanation of symbols

21 Substrate 22 High acoustic impedance layer 23 Low acoustic impedance layer 24 Acoustic reflection portion 25 Lower electrode 26 Piezoelectric film 27 Upper electrode 28 Acoustic resonance portion 28A First acoustic resonance portion 28B Second acoustic resonance portion 29 Smoothing layer

Claims (4)

A substrate,
An acoustic reflector formed by sequentially laminating a low acoustic impedance layer and a high acoustic impedance layer having a higher acoustic impedance than the low acoustic impedance layer on the substrate;
An acoustic resonance unit in which a lower electrode, a piezoelectric film, and an upper electrode are sequentially stacked on the acoustic reflection unit;
The acoustic reflection unit includes a smoothing layer formed between the high acoustic impedance layer and the low acoustic impedance layer stacked on the uppermost side of the acoustic reflection unit,
On the acoustic reflection portion, a plurality of the acoustic resonance portions are formed at intervals from each other,
In the region between the acoustic resonance parts in the acoustic reflection part, the high acoustic impedance layer is separated,
The smoothing layer is a thin film acoustic resonator, wherein the thickness is Ri der less 100 nm, the root-mean-square roughness of the upper surface is 1nm or less.   The thin film acoustic resonator according to claim 1, wherein the smoothing layer is a film formed of a spin coatable material.   The thin film acoustic resonator according to claim 2, wherein the film formed of the spin coatable material is a spin-on-glass film, a fluorine resin film, a silicon resin film, or an epoxy resin film. The high acoustic impedance layer is a thin film acoustic resonator according to any one of claims 1 to 3, characterized in that it consists of insulating material.

Images