JP4678261B2 - Piezoelectric thin film vibrator - Google Patents

Description

  The present invention relates to a piezoelectric thin film vibrator, and more particularly to a vibrator structure suitably used as a high frequency filter.

  In general, communication devices such as cellular phones are becoming higher in frequency and smaller in size, and for this reason, higher performance and smaller size of the RF circuit section have been demanded. Among them, as a high-frequency element such as a high-frequency filter used in a transmission / reception unit of a communication device, it is possible to realize performance equivalent to that of a conventional SAW (surface acoustic wave) element. A BAW (Bulk Acoustic Wave) element that is easy to realize has attracted attention. This BAW element has a laminated structure in which a piezoelectric layer is sandwiched between electrodes, and generates an acoustic wave in the thickness direction in the piezoelectric layer. It is easy to increase the frequency, have good withstand voltage, and easily downsize. (See, for example, Patent Documents 1 and 2 below).

As the above BAW element, the above laminated structure is formed on a substrate made of a silicon substrate or the like, and then the substrate portion behind the laminated structure is removed by etching or the like, thereby enabling longitudinal vibration of the laminated structure portion. , An FBAR (Film Bulk Acoustic Resonator) type device structure (FIGS. 1 and 3 of Patent Document 1) and an acoustic reflection multilayer film in which layers having different acoustic impedances are alternately and repeatedly stacked as described above 2. Description of the Related Art An SMR (Solid Mounted Resonator) type element structure (FIG. 2 of Patent Document 1) disposed between a structure and a substrate is known.
JP 2001-156582 A JP 2002-344279 A

  As described above, the above-mentioned BAW element has an advantage over the SAW element in that it is easy to increase the frequency, but the acoustic impedance of a general piezoelectric body is usually about 30 [Pa · s / m]. Therefore, there is a problem that it is difficult to further increase the frequency. In addition, since the resonance frequency is affected not only by the acoustic characteristics of the piezoelectric body but also by the mass of the laminated structure, if the electrode must be formed thick due to restrictions such as the electrical resistance of the electrode, the mass of the electrode There is also a problem that a desired high frequency cannot be realized due to the footsteps.

  Therefore, the present invention solves the above-mentioned problems, and its object is to provide a structure of a piezoelectric thin film vibrator capable of further increasing the frequency without being restricted by the acoustic impedance value of the piezoelectric body. is there.

  In view of such circumstances, the piezoelectric thin film vibrator of the present invention includes a piezoelectric layer and a pair of electrodes that are disposed on both the front and back sides of the piezoelectric layer and apply an electric field in the stacking direction to the piezoelectric layer. In the piezoelectric thin film vibrator having a structure, a diamond layer is disposed on one side in the stacking direction of the piezoelectric layer, and the stacked structure has an acoustic impedance of each layer from the piezoelectric layer to the diamond layer. It is characterized by increasing sequentially from the layer toward the diamond layer.

  According to the present invention, since the acoustic impedance of each layer sequentially increases from the piezoelectric layer toward the diamond layer, the acoustic vibration generated by the piezoelectric layer is likely to occur inside the diamond layer. The higher longitudinal wave velocity of the layer allows higher frequencies.

  That is, when the acoustic impedance on the incident side is Z1 and the acoustic impedance on the output side is Z2 with respect to the interface between adjacent layers, the acoustic reflection ratio (ratio of the sound pressure of the incident wave and the reflected wave) at the interface is R. = (Z2−Z1) / (Z1 + Z2). If Z1 <Z2, R is positive and the phase of the reflected wave is normal, and if Z1> Z2, R is negative and the phase of the reflected wave is inverted. Therefore, the acoustic impedance of each layer from the piezoelectric layer to the diamond layer increases sequentially, and the smaller the difference in acoustic impedance at the interface, the easier the acoustic vibrations penetrate into the diamond layer. The effect of acoustic characteristics is increased.

  In the present invention, an acoustic relaxation layer is preferably formed between the piezoelectric layer and the diamond layer. According to this, since the acoustic relaxation layer is formed between the piezoelectric layer and the diamond layer, the difference in acoustic impedance between the adjacent layers can be reduced, so that it is possible to further increase the frequency. Since the reflection of acoustic vibration at each layer interface can be suppressed, the insertion loss can be reduced.

  The acoustic relaxation layer is not limited to one layer, and a plurality of layers may be provided. By providing a plurality of acoustic relaxation layers, the difference in acoustic impedance between adjacent layers can be further reduced.

  In the present invention, it is preferable that the adjacent piezoelectric layer and the acoustic relaxation layer are lattice-matched. According to this, when the acoustic relaxation layer is lattice-matched with the piezoelectric layer, the crystallinity of the piezoelectric body formed on the acoustic relaxation layer can be improved. Examples of such a combination of an acoustic relaxation layer and a piezoelectric layer include AlN and ZnO.

  In this invention, it is preferable that the said electrode arrange | positioned at the said lamination direction one side of the said piezoelectric material layer is arrange | positioned on the opposite side of the said piezoelectric material layer with respect to the said diamond layer. If the electrode arranged on one side in the stacking direction of the piezoelectric layer has an acoustic impedance lower than that of the diamond layer, the acoustic impedance condition described above is not limited even if the electrode is arranged on the piezoelectric layer side of the diamond layer. However, by arranging the electrode on the opposite side of the piezoelectric layer with respect to the diamond layer, the above acoustic impedance condition can be satisfied regardless of the acoustic impedance value of the electrode. An electrode can be configured using the electrode material.

  In this invention, it is preferable that the said electrode arrange | positioned at the said lamination direction one side of the said piezoelectric material layer is comprised with the said diamond layer to which electroconductivity was provided. By using a diamond layer with conductivity as an electrode, there is no need to provide a separate electrode, so the layer structure can be simplified and materials with acoustic impedance that satisfy the above conditions can be selected easily. Become.

[First Embodiment]
Next, a first embodiment of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic longitudinal sectional view schematically showing the structure of the piezoelectric thin film vibrator of the present embodiment.

  The piezoelectric thin film vibrator 10 is formed on the substrate 1. As the substrate 1, various substrates such as a semiconductor substrate such as a silicon substrate, a glass substrate, a sapphire substrate, and a ceramic substrate can be used. In particular, since various semiconductor circuits can be formed in the substrate 1 by using a semiconductor substrate such as a silicon substrate, the piezoelectric thin film vibrator and the circuit can be integrated. Among these, using a silicon substrate is advantageous in that a general semiconductor manufacturing technique can be used.

A base layer 2 is formed on the substrate 1. The underlayer 2 is an insulating film such as a silicon oxide layer (SiO ₂ ) or a silicon nitride layer (Si ₃ N ₄ ), and may be composed of two or more composite layers. The underlayer 2 can be formed by a thermal oxidation method, a CVD method, a sputtering method, or the like.

  The substrate 1 is provided with an opening 1a formed by etching (wet etching or dry etching) from the back surface. The opening 1a can be formed using the underlayer 2 as an etching stop layer. By providing the opening 1a, a mechanical restraining force with respect to a laminated structure described later is reduced, and the laminated structure can be vibrated relatively freely. The illustrated example shows an FBAR type element structure.

  A piezoelectric thin film vibrator 10 having a structure in which a lower electrode layer 11, a diamond layer 12, an acoustic relaxation layer 13, a piezoelectric layer 14, and an upper electrode layer 15 are sequentially laminated is formed on the base layer 2. ing.

  The lower electrode layer 11 can use any electrode material. However, although not particularly limited, an electrode material having a high acoustic impedance, for example, Pt (Z = 69.7 [Pa · s / m]), W (Z = 98.2 [Pa · s / m]) ), Mo (Z = 64.4 [Pa · s / m]) or the like is preferably used. Further, on the electrode material having a low acoustic impedance, for example, a layer of Al (Z = 17.0 [Pa · s / m]), Si (Z = 19.7 [Pa · s / m]) or the like, It is good also as a structure which laminated | stacked the electrode material with high acoustic impedance. Since the thickness of the lower electrode layer 11 is preferably light in order to increase the frequency, the lower electrode layer 11 is preferably thin if sufficient conductivity is exhibited.

  The diamond layer 12 has an acoustic velocity (longitudinal wave velocity) VL that is far higher than other materials, and has a high acoustic impedance Z = ρ · VL = 61.6 [Pa · s] that is at least twice that of a general piezoelectric material. / M] (ρ is density). When the resonance wavelength is λ, the film thickness h of the diamond layer 12 is preferably 0.9 × λ / 4 or more and 1.1 × λ / 4 or less, and particularly preferably h to λ / 4. . This is for obtaining the confinement effect of the sound wave excited in the laminated structure.

The acoustic relaxation layer 13 is formed between the diamond layer 12 and the piezoelectric layer 14 and is provided to reduce the difference in acoustic impedance between the layers in the laminated structure. The acoustic relaxation layer 13 is made of a material that is larger than the acoustic impedance of the piezoelectric body constituting the piezoelectric layer 14 and smaller than the acoustic impedance of the diamond constituting the diamond layer 12. For example, when the acoustic relaxation layer 13 is made of a dielectric material (including a piezoelectric material), if the piezoelectric material layer 14 is ZnO (Z = 34.6 [Pa · s / m]), AlN (Z = 35). 0.0 [Pa · s / m]), Si ₃ N ₄ (Z = 52.0 [Pa · s / m]), Al ₂ O ₃ (Z = 43.3 [Pa · s / m]), SrTiO ₃ (Z = 40.3 [Pa · s / m]).

  Further, although the lower electrode layer 11 is provided in the present embodiment, the acoustic relaxation layer 13 may be configured as an electrode without forming the lower electrode layer 11. Also in this case, similarly to the above, the acoustic impedance is made of a material larger than the acoustic impedance of the piezoelectric body constituting the piezoelectric layer 14 and smaller than the acoustic impedance of the diamond constituting the diamond layer 12. Examples of such a material include Fe (Z = 43.2 [Pa · s / m]), Ag (Z = 40.0 [Pa · s / m]), and Cr (Z = 450.2 [Pa · s / m]). S / m]), Cu (Z = 43.4 [Pa · s / m]), Ni (Z = 52.5 [Pa · s / m]), and the like. In this case, it is possible to use a diamond substrate as the diamond layer 12 and omit the substrate 1 and the underlayer 2.

  The film thickness h of the acoustic relaxation layer 13 may be smaller than λ / 4, as long as it functions as a buffer for acoustic characteristics. Specifically, the film thickness h is preferably about λ / 100 to λ / 10. In this way, by setting the film thickness h of the acoustic relaxation layer 13 to a range that is significantly smaller than λ / 4, the function as the acoustic relaxation layer is ensured and the influence on the resonance frequency is minimized. Can do.

  The acoustic relaxation layer 13 adjacent to the piezoelectric layer 14 is preferably composed of a thin film lattice-matched with the piezoelectric layer 14. Thereby, the cohesion of the piezoelectric layer 14 formed thereon can be improved. For example, since both AlN and ZnO are materials that can be formed as a thin film having c-axis orientation, the crystallinity of the piezoelectric layer 14 can be increased by using one as the acoustic relaxation layer 13 and the other as the piezoelectric layer 14. It is possible to improve the performance of the piezoelectric thin film vibrator. Other thin film materials having c-axis orientation that can be used include CdS, CdSe, and ZnS.

The piezoelectric layer 14 may be any material that exhibits piezoelectricity. For example, AlN, ZnO, PZT (Z = 31.8 [Pa · s / m]), KNbO ₃ (Z = 29.3 [Pa · s / m]). The film thickness h of the piezoelectric layer 14 is preferably 0.9 × λ / 2 or more and 1.1 × λ / 2 or less, particularly h to λ / 2, where λ is the resonance wavelength. Is desirable. This is to confine the acoustic wave in the laminated structure.

  An arbitrary electrode material can be used for the upper electrode layer 15. However, although not particularly limited, it is preferable to use the materials listed as the material of the lower electrode layer 11. In addition, it is preferable to use an electrode material having as small a mass as possible while ensuring sufficient conductivity.

As a specific configuration of the present embodiment, the acoustic relaxation layer 13 is formed of Si ₃ N ₄ and the piezoelectric layer 14 is formed of ZnO. Other combinations of the acoustic relaxation layer 13 and the piezoelectric layer 14 include SrTiO ₃ and ZnO, Al ₂ O ₃ and ZnO, SrTiO ₃ and AlN, Al ₂ O ₃ and AlN, Si ₃ N ₄ and AlN, and SrTiO. ₃ and PZT, Al ₂ O ₃ and PZT, Si ₃ N ₄ and PZT, SrTiO ₃ and KNbO ₃ , Al ₂ O ₃ and KNbO ₃ , Si ₃ N ₄ and KNbO _{3, and the} like.

  In the present embodiment, since the acoustic impedance increases sequentially from the piezoelectric layer 14 to the diamond layer 12 in the range from the piezoelectric layer 14 to the diamond layer 12 through the acoustic relaxation layer 13, the lower electrode layer Since the acoustic vibration generated by the fluctuating electric field generated between the upper electrode layer 15 and the upper electrode layer 15 is substantially present inside the diamond layer 12, it is possible to increase the frequency by the high sound speed in the diamond layer 12. Become.

  In the present embodiment, instead of the substrate 1, a substrate 1 ′ according to a second embodiment described later or a substrate 1 ″ according to a third embodiment described later may be used.

[Second Embodiment]
Next, a second embodiment according to the present invention will be described with reference to FIG. The piezoelectric thin film vibrator 10 ′ of the second embodiment has the same basic configuration as that of the first embodiment, but a plurality of acoustic relaxation layers 13 </ b> A and 13 </ b> B between the diamond film 12 and the piezoelectric layer 14. Is different from the first embodiment in that is arranged.

  The acoustic relaxation layer 13A is disposed on the diamond layer 12 side, and the acoustic relaxation layer 13B is disposed on the piezoelectric layer 14 side. The acoustic impedance of the acoustic relaxation layer 13A is larger than that of the acoustic relaxation layer 13B.

  Also in the configuration of this embodiment, the acoustic impedance sequentially increases in the order of the piezoelectric layer 14, the acoustic relaxation layer 13B, the acoustic relaxation layer 13A, and the diamond layer 12. By doing so, as in the first embodiment, the acoustic vibration caused by the fluctuating electric field applied to the upper electrode 15 and the lower electrode 11 is also present inside the diamond layer 12, thereby causing a high frequency of the resonance frequency. Can be achieved.

Specific examples of the plurality of acoustic relaxation layers 13A and 13B include a case where Si ₃ N ₄ , Al ₂ O ₃ , and SrTiO ₃ are stacked in this order.

  In particular, in the present embodiment, since two (plurality) of acoustic relaxation layers 13A and 13B are formed between the piezoelectric layer 14 and the diamond layer 12, in the stacking range from the piezoelectric layer 13 to the diamond layer 12. The difference in acoustic impedance between the layers can be reduced. Therefore, since reflection of acoustic vibrations at the interface between the layers can be suppressed, it is easy to further increase the frequency, and loss of acoustic vibrations can be reduced.

  In the present embodiment, unlike the first embodiment, a space portion 1b interposed between the substrate 1 'and the piezoelectric thin film vibrator 10 is formed in the surface layer portion of the substrate 1'. The space portion 1b forms a sacrificial layer (a material that can be selectively removed with respect to the substrate material, for example, a silicon oxide film, a PSG film, etc.) on the substrate 1 ′, and then, on the sacrificial layer. After the piezoelectric thin film vibrator 10 is formed, finally, the piezoelectric thin film vibrator 10 can be left as it is, and only the sacrificial layer can be removed.

  In this embodiment, either of the acoustic relaxation layers 13A and 13B may be configured as a lower electrode layer. However, as described above, the acoustic relaxation layer configured as the lower electrode layer needs to be formed of an electrode material that satisfies the above-described gradual increase requirement of acoustic impedance.

  In this embodiment, instead of the substrate 1 ′, the substrate 1 having the opening 1a similar to that of the first embodiment or the substrate 1 ″ of the third embodiment described later may be used.

[Third Embodiment]
Next, a third embodiment according to the present invention will be described with reference to FIG. In the piezoelectric thin film vibrator 10 ″ of this embodiment, instead of providing the lower electrode, conductivity is imparted to the diamond layer 12 ′, and this diamond layer 12 ′ is used as an electrode (lower electrode). Different from the previous embodiments.

  The diamond layer 12 'is imparted with conductivity by doping B (boron) into diamond. The diamond layer 12 'can be formed by, for example, a CVD method (plasma CVD method) using an organic solvent such as methane as a carbon source and boron oxide or the like as a B source. However, conductive diamond can be produced by N (nitrogen) doping or surface hydrogenation treatment in addition to B doping.

  In the present embodiment, since the diamond layer 12 'is used as an electrode, it is not necessary to separately provide a lower electrode, so that the laminated structure can be easily constructed and the mass of the laminated structure constituting the vibrator can be reduced. Moreover, it becomes easy to select the material structure of the laminated structure so as to satisfy the gradually increasing condition of the acoustic impedance.

  In the present embodiment, unlike the above-described embodiments, an acoustic reflection multilayer film 1c formed by alternately and repeatedly laminating layers having different acoustic impedances is formed on the surface portion of the substrate 1 ″. The sound wave generated by the piezoelectric thin film vibrator 10 ″ is reflected so that the energy of the acoustic vibration is not leaked to the substrate 1 ″, and the acoustic vibration is confined in the laminated structure. That is, it has a so-called SMR type element structure. ing.

  In this embodiment, the substrate 1 or 1 ′ of the previous embodiment may be used instead of the substrate 1 ″.

  FIG. 4 shows another configuration of a diamond layer that can be used in this embodiment. This diamond layer 12 ″ is provided with conductivity by applying B doping or the like only to the lower surface layer portion 12x, and the thickness portion other than the surface layer portion 12x is an insulating portion 12y having no conductivity. In the diamond layer 12 ″, the surface layer portion 12x functions as an electrode, and the surface layer portion 12x is increased in hardness by B doping and has a higher acoustic impedance than the insulating portion 12y. Therefore, it is possible to obtain a higher resonance frequency than the above.

[Application examples and frequency characteristics]
FIG. 5 is a schematic configuration diagram illustrating a configuration of the transmission / reception unit 20 of a communication device such as a mobile phone. The transmission / reception unit 20 is connected to a transmission / reception shared antenna 21, a signal separator 22 connected to the antenna 21, and a low noise amplifier 23, an RF filter 24, and an oscillator 26 connected to the signal separator 22. A high-frequency receiving unit having the mixer 25, and a high-frequency transmitting unit having a mixer 27, an RF filter 28, and a power amplifier 29 connected to the oscillator 26. Both the high frequency receiving unit and the high frequency transmitting unit are connected to a signal processing unit 30 including a baseband processing circuit and the like.

  A high frequency filter can be configured by connecting a plurality of piezoelectric thin film vibrators of the above embodiment in a ladder type, a lattice type, a lattice-ladder type, or the like. Such a high frequency filter can be used when a duplexer as the signal separator 22 of FIG. 5 is configured, and can also be used as the RF filters 24 and 28. Therefore, by constructing the high-frequency circuit shown in FIG. 5 on the substrate 1 together with the piezoelectric thin film vibrators constituting these high-frequency filters, the transmission / reception unit 20 can be integrated (one-chip).

  FIG. 6 is a graph showing frequency characteristics of insertion loss of the signal separator 22 (duplexer) shown in FIG. 5 and transmission / reception signals passing through the signal separator 22. As can be seen from the frequency characteristics of the insertion loss of the high-frequency filter, in this embodiment, both the transmission-side filter characteristic Ft and the reception-side filter characteristic Fr have steep cut-off characteristics. The transmission signal St and the reception signal Sr can be easily separated.

  The piezoelectric thin film vibrator of the present embodiment has the effect that it is easier to increase the frequency than conventional elements without hindering downsizing, without deteriorating power durability, and suppressing an increase in manufacturing cost. This makes it possible to further increase the frequency of circuit elements such as filters.

  In addition, the piezoelectric thin film vibrator of the present invention is not limited to the illustrated examples described above, and it is needless to say that various modifications can be made without departing from the scope of the present invention.

FIG. 3 is a schematic cross-sectional view schematically showing the structure of the piezoelectric thin film vibrator of the first embodiment. FIG. 5 is a schematic cross-sectional view schematically showing the structure of a piezoelectric thin film vibrator according to a second embodiment. FIG. 10 is a schematic cross-sectional view schematically showing the structure of a piezoelectric thin film vibrator according to a third embodiment. The schematic sectional drawing which shows another example of a diamond layer. The schematic block diagram which shows a mode that a piezoelectric thin film vibrator is utilized as a high frequency filter of the transmission / reception part of a communication circuit. The graph which shows the frequency characteristic of the duplexer using a high frequency filter, and the signal isolate | separated by this.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 1a ... Opening part, 1b ... Space, 1c ... Acoustic reflection multilayer film, 2 ... Underlayer, 10 ... Piezoelectric thin film vibrator (laminated structure), 11 ... Lower electrode layer, 12 ... Diamond layer, 13 ... Sound Relaxation layer, 14 ... piezoelectric layer, 15 ... upper electrode layer

Claims (6)

A first electrode;
A piezoelectric layer having a first acoustic impedance on the first electrode;
A diamond layer having a second acoustic impedance greater than the first acoustic impedance;
An acoustic relaxation layer disposed between the piezoelectric layer and the diamond layer and having an acoustic impedance that is greater than the first acoustic impedance and less than the second acoustic impedance;
A piezoelectric thin film vibrator, wherein a second electrode is laminated on the diamond layer .   The piezoelectric thin film vibrator according to claim 1, wherein a plurality of the acoustic relaxation layers are laminated. 3. The piezoelectric thin film vibrator according to claim 1, wherein the acoustic relaxation layer is adjacent to the piezoelectric layer, and the piezoelectric layer and the acoustic relaxation layer are lattice-matched. 4. The piezoelectric thin film vibrator according to claim 1, wherein the second electrode includes the diamond layer to which conductivity is imparted. 5.   A filter comprising the piezoelectric thin film vibrator according to any one of claims 1 to 4.   A transmission / reception apparatus comprising the piezoelectric thin film vibrator according to claim 1.

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