Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-11-12
2001-08-07
Enad, Elvin (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06271619
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a piezoelectric thin film device with good production yield and reliability, in particular, to a piezoelectric thin film device which has a floating structure having openings in a surface of a substrate and is suitably used at a high frequency.
BACKGROUND ART
A piezoelectric thin film device is a device, which uses a bulk resonance of a piezoelectric film, for use as a filter or a resonator capable of activation in a certain frequency ranging from several hundreds Mega-Hertz to Giga-Hertz. Likewise, the resonator is used in a crystal oscillator; however, it is activated at lower frequency. When the piezoelectric thin film device is used in communication instruments for a mobile communication, for example, a frequency to be used is considerably high and ranges from several hundreds Mega-Hertz to Giga-Hertz.
For a device which uses a bulk resonance of the piezoelectric film, a resonance frequency of the film is mainly determined by a thickness of the piezoelectric film, so that the thickness is needed to be reduced to several micro-meters or less in order to resonate the film with frequency of Giga-Hertz. The crystal oscillator mentioned above is made by polishing a single crystal, thereby it is difficult to thin the oscillator up to about several micro-meters.
Recently, devices using the bulk resonance has been studied actively, in which piezoelectric thin films thinned up to about 1 to 2 &mgr;m by various thin film fabrication method are typically used. Also, thin film electrodes formed on both sides of the piezoelectric thin film is applied with AC voltage for resonating the film. Also, in order to decrease an energy loss of a resonation, employed is a floating structure in which a portion positioned under resonant section of the film is partly etched.
One well-known method for fabricating the floating structure has fabricating an upper structure on a gallium arsenide (GaAs) substrate, etching the substrate from its bottom using an etchant such as sulfuric acid. For example, JP(A) 6-350154 discloses one fabrication method in which the floating structure is fabricated by forming an insulating film, a lower thin film electrode, a piezoelectric thin film, and an upper thin film electrode successively on surface of a substrate, and etching a part of the substrate from its opposite surface to the insulating film to form a floated resonant section on the etched part. The floating structure mentioned above can be fabricated using relatively simple processes.
However, the etched part or recess formed in the bottom surface of the substrate is closed at its top by the insulating film. Therefore, when the structure is mounted on a substrate with its bottom surface fixed to the substrate using an adhesive, the recess is disadvantageously closed. As a result, a pressure in the closed recess varies considerably at the vibration of the resonant section, which results in damages of the. device and a decrease in its reliability.
Also, a wire connected to the upper electrode is provided on the piezoelectric thin film and, in this instance, it should be extended over a step caused by the existence of the piezoelectric thin film, which may cause a disconnection of the wire and an increase of parasitic capacitance.
Also, the recess of the structure decrease the strength of the resonant section in particular as well as the yield of the device.
SUMMARY OF THE INVENTION
The first object of the present invention is to provide a piezoelectric thin film device having a floating structure with a high durability and reliability.
The second object of the present invention is to reinforce a structure of a resonant section and thereby to prevent the device from being damaged, increasing a production yield and reliability of the device.
The piezoelectric thin film device of the present invention mainly comprises a substrate, an insulating film formed on the substrate, a piezoelectric thin film formed on the insulating film, and thin film electrodes formed on both upper and lower surfaces of the piezoelectric thin film, wherein the piezoelectric thin film resonated by an application of alternating voltage between the thin film electrodes, characterized in that the substrate below the piezoelectric thin film is partially or entirely removed to form a removed section, so that the removed section is opened air at top and bottom surfaces of the device.
That is, one of the feature of the present invention is that the removed section below the piezoelectric thin film is opened to air at top and bottom surface of the device through corresponding opening. Therefore, the removed section does not define a closed space even when it is mounted and fixed on a substrate, so that the piezoelectric thin film is prevent from being damaged by the pressure variation which would otherwise be caused if the removed section is not opened to air at the top surface of the device.
A substrate may be made of any material to which etching can be applicable, such as a semiconductor (e.g., Si, GaAs) and an oxide (e.g., MgO). The insulating film, which is not needed when the substrate has a significant high insulation resistance, may be selected from materials having a chemical stability against the etchant used for an etching of the substrate, for example, silicon oxide (SiO, SiO
2
), silicon nitride (SiNx), tantalum oxide (TaOx), strontium titanate (SrTiO
3
), and magnesium oxide (MgO) etc.
Preferably, the thin film electrode below the piezoelectric thin film may be made of materials having a good adhesion property to the substrate and the insulating film and being stable to the piezoelectric thin film, such as noble metal (e.g., Pt, Au), multi-layer material consist of noble metal and titanium (Ti), ruthenium oxide (RuO
2
), or iridium oxide (IrO
2
) etc. Alternatively, when the piezoelectric thin film is formed at relatively lower temperature, for example, 300° C. or less, aluminum may be used.
The piezoelectric thin film may be made of any piezoelectric thin film, for example, zinc oxide (ZnO), aluminum nitride (AlN), lead titanate (PbTiO
3
), lead zirconate titanate (Pb(Zr,Ti) O
3
),and lithium niobate (LiNbO
3
).
The thin film electrode on the piezoelectric thin film is not subjected to high temperature and therefore may be made of any above mentioned materials.
Note that, the above mentioned structure having an opening on its top surface can prevent the device from being damaged by the pressure variation. However, the mechanical strength of the resonant section is insufficient. The mechanical strength, on the other hand, can be increased by increasing the thickness of the insulating film formed on the substrate, which in turn results in that the piezoelectric thin film is resonated with the thick and heavy insulating film, decreasing an efficiency of the resonance. It is therefore needed for the resonate structure to be light in order to keep the resonance of the structure and to be held positively.
Inventors of the present invention found that it is effective making a bridge between the resonant section above the removed section and the substrate in order to reinforce the resonant section. And the present invention is completed.
That is, the piezoelectric thin film device of the present invention includes at least one bridge supported at one end thereof on at least the piezoelectric thin film, the thin film electrode, or an insulating film formed between the substrate and the piezoelectric thin film and at the other end thereof on at least a portion of the substrate outside the recess, wherein the portion to be resonated is supported by the bridge.
The reinforcing structure is useful for the device having a recess opened to air at one surface of the substrate. This in turn means the structure is effective to the device which has a recess opened at the top surface of the substrate but closed at its bottom surface.
That is, a piezoelectric thin film device, comprising: a substrate; a piezoelectric thin film formed on the substrate; and thin film electrodes formed on the upper
Ishikawa Takahide
Maeda Chisako
Misu Koichiro
Uchikawa Fusaoki
Umemura Toshio
Enad Elvin
Medley Peter
Mitsubishi Denki & Kabushiki Kaisha
Sughrue Mion Zinn Macpeak & Seas, PLLC
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