KR100790749B1 - Saw device with composite electrode structures - Google Patents

Abstract

A SAW(Surface Acoustic Wave) device with composite electrode structures is provided to reduce acoustic loss by separating a metal electrode from a piezoelectric substrate by a bottom layer. A SAW(Surface Acoustic Wave) device(100) includes a substrate(101) and a plurality of composite electrode structures. The substrate has a polished surface for SAW propagation, and a piezoelectric characteristic. The plurality of composite electrode structures are arranged on the substrate, and are composed of at least two layers. Each composite electrode structure has a metal electrode(102) arranged on the substrate, and a bottom layer(103) arranged between the substrate and the metal electrode. The bottom layer separates the metal electrode from the substrate, and has lower acoustic loss than the metal electrode.

Description

Surface acoustic wave device with composite electrode structure {SAW Device With Composite Electrode Structures}

1 is a schematic cross-sectional view of a conventional surface acoustic wave device.

2 is a diagram showing a stress distribution in an electrode cross section of a conventional surface acoustic wave element.

3 is a schematic cross-sectional view of a surface acoustic wave device according to one embodiment of the present invention.

4 is a schematic cross-sectional view of a surface acoustic wave device according to another embodiment of the present invention.

5 is a schematic cross-sectional view of a surface acoustic wave device according to still another embodiment of the present invention.

6 is a cross-sectional view schematically illustrating a modification of the surface acoustic wave device of FIG. 5.

<Description of the symbols for the main parts of the drawings>

100, 200, 300, 300 ': SAW element 101: piezoelectric substrate

102, 112 and 122: metal electrodes 103, 113 and 123: base layer

104: dielectric plate (cover) 105: spacer

106: cavity

FIELD OF THE INVENTION The present invention relates to surface acoustic wave (hereinafter also referred to as SAW) devices, and more particularly to low loss surface acoustic wave devices with improved passband performance in the GHz frequency range.

SAW devices are not only used in various communication applications, but also as important components for mobile phones and base stations. The most commonly used SAW device types are passband filters and resonators. In addition to low cost, small size and excellent technical parameters (low loss, selectivity, etc.) make SAW devices substantially more competitive than devices based on other physical principles.

1 is a schematic cross-sectional view of a typical SAW device. Referring to FIG. 1, the SAW element 10 includes a single crystal piezoelectric substrate 11 and an interdigital transducer (IDT) electrode 12 disposed thereon. The electrodes 12 of different polarities are alternately arranged to form the IDT transducer 15. The role of this transducer electrode 12 is the same as that of an antenna for SAW. The transducer electrode 12 exchanges the electric field in time and space, which passes through the piezoelectric substrate 11 and generates mechanical stress due to the piezoelectric effect. In this manner, the transducer electrode 12 can generate surface acoustic waves that propagate on the substrate surface. Such propagation of surface acoustic waves can be effected in other ways (eg surface acoustic waves can be reflected, deflected, absorbed, waveguided), and generated by one IDT. Surface acoustic waves may be received by other IDTs. The generation of surface acoustic waves changes the impedance of the transducer. These phenomena associated with SAW are now well understood and commonly used in the design and manufacture of various types of SAW filters.

In particular, recently required SAW applications are mobile phones, which require high filtering performance with low insertion loss. Typical relative frequency passbands for many cell phone standards are around 1-5%, which is a filter substrate material with strong couplings such as Lithium Niobate or Lithium Tantalate. strong coupling) requires the use of a material. To satisfy this criterion, so-called "leaky wave" cuts of LiNbO3 (64LN, 41LN) and LiTaO3 (36LT, 42LT) are often used. These leaky wave cuts provide high coupling and relatively high SAW rates. The so-called leaky longitudinal wave has a dominant longitudinal component and requires a rather thick (h _electrode / lambda = 8%, h _electrode electrode thickness, lambda wavelength) and precisely controlled aluminum electrodes.

However, the current SAW technology appears to be facing some limitations, and further developments are needed to overcome these limitations and achieve qualitatively improved performance. In particular, in the 2 GHz range, the minimum loss in the SAW device is typically still around 2 dB, which is an obstacle for some applications. Power handling of SAW devices is reduced with increasing frequency, which remains a problem in applications such as duplexers in mobile phones. In addition, sealing packaging substantially increases the manufacturing cost of SAW devices.

Due to the rapid development of communication systems, the operating frequency is required to be further increased. In new applications such as LAN, "Bluetooth", the 2.5 GHz frequency range is used. In new communication systems (e.g., HLAN), the frequencies of 3 GHz to 6 GHz will be used. In some applications a relative band of 5% to 6% is required, which is also a problem for devices based on commonly used substrates.

In all these systems, filtering of frequencies will be the most important function for reliable operation and efficient use of the available resources of the allocated frequencies. Insertion loss is one of the most important parameters for mobile communication applications.

Within the SAW device there are a number of important loss mechanisms as follows.

1. The interaction between an acoustic wave and a thermal phonon is an inherent loss mechanism present in all solids and depends not only on temperature but also on the crystal properties and crystal quality of the material (LiNb is one of the best piezoelectric materials in this sense). ).

2. The acoustic loss at the metal electrode accounts for a large part of the total loss. Metals are always more lossy materials than dielectrics. Aluminum is known as a material that exhibits relatively low acoustic attenuation compared to other metals such as gold or copper. However, even in Al, the elastic loss is much higher than that of LiNb.

3. Ohmic losses in conductors are becoming more important at higher frequencies because of the smaller electrode thickness.

4. Acoustic-electric interaction due to RF current generated in the metal electrode by the electric field near the piezoelectric substrate as the SAW propagates along the piezoelectric substrate surface.

5. Interaction with the gas (air) on the working surface results in a loss of elastic energy.

6. Defects, contamination, residues on the surface cause scattering and absorption of SAW energy, especially after technical treatment.

7. In classical leaky SAW and longitudinal leaky SAW, losses due to elastic energy deviating from the filter structure, such as losses due to "leaky" bulk wave components. .

Among the loss mechanisms described above, those related to the presence of metal electrodes (numbered "2" and "3" in the above list) are responsible for a very large portion of the total loss (evaluated as about 2/3 for 1 GHz devices). Attenuation of the seismic wave is higher in the metal than in the dielectric. Although other properties, such as conductivity or oxidation, work more advantageously on gold electrodes, aluminum is most commonly used as electrode material because aluminum (Al) exhibits relatively low attenuation. The specific resistance of Al is significantly higher than that of gold or silver, but the elastic losses in gold are unacceptably high. The thickness of the electrodes used in SAW devices is limited to about 10% of the characteristic wavelength. This is because thicker electrodes reduce the coupling while loading the substrate surface more strongly and increasing losses more. High reflectivity of thick electrodes can be problematic in some design methods, but can be used efficiently in resonator elements to limit outward SAW energy loss.

All of the above mechanisms can lead to filter losses, which typically can range from 1.0 dB to 2 dB in the device passband for mobile phone applications for the 900 MHz range frequency, about 3 dB for the 2 GHz frequency. It can even reach. In order for mobile communications to evolve further, these losses must be significantly reduced. The energy lost in the filter is eventually converted to heat and increases the temperature of the working filter. For example, in applications such as duplex, front-end TX filters, the applied power can reach 2-3 W and the temperature increase can reach 40 ° C. or higher. The increased temperature results in reduced filter performance, aging promotion, reduced power handling capabilities, and the like.

Al electrodes commonly used in SAW devices cause two other known drawbacks of the device-limited power handling and aging. The high density of acoustic energy concentrated in the electrode results in an acoustically induced migration of aluminum. The electrodes are deformed and eventually short-circuited, resulting in unacceptable performance degradation and even device failure.

The oxidation of aluminum and the corrosion of aluminum in the presence of moisture are important aging mechanisms and sealing packaging is required to reduce these results. The dependence of SAW properties on mass-loading by the electrodes also results in low yields due to technical variations in electrode dimensions.

International Application Publication No. WO 03/010888 discloses the use of hard dielectric layers with high elastic impedance. In this document the dielectric layer separates the IDT electrode from the piezoelectric leakage wave substrate. However, the dielectric layer is formed uniformly and covers the entire surface of the substrate. As described in the published international application, this dielectric layer can reduce "leaky" radiation in the substrate, but does not reduce the absorption of elastic energy at the metal electrode. The dielectric layer also reduces the reflectivity of the electrode.

In another International Application Publication No. WO 03/075458, an electrode structure is disclosed comprising a dielectric layer separating a metal electrode from a piezoelectric substrate, but the choice of dielectric layer material is determined by the problem addressed in this patent document, i.e. The dielectric material is selected to reduce the capacitance between the IDT fingers. Therefore, a dielectric material having a low dielectric permittivity is used and the thickness of the dielectric layer is thin compared to the metal electrode. This configuration does not have a significant effect on the energy absorbed by the metal electrode, and deteriorates the piezoelectric coupling with a decrease in capacitance.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object thereof is to significantly reduce attenuation loss due to high elastic damping in a metal electrode, to reduce resistance loss, to increase power handling capability, and to improve aging characteristics. It is to provide an element.

In order to achieve the above technical problem, the surface acoustic wave (SAW) device according to the present invention,

A substrate having a polished surface for SAW propagation and having piezoelectric properties;

A plurality of composite electrode structures arranged on the substrate, each comprising at least two layers,

Each composite electrode structure,

A metal electrode disposed on the substrate;

A base layer disposed between the substrate and the metal electrode to separate the metal electrode from the substrate, the base layer having a lower elastic loss than the elastic loss at the metal electrode, wherein the thickness h of the base layer is Satisfy the relation.

(h / λ) · ε <1

Is the SAW wavelength of the piezoelectric substrate at the center frequency of the device, and ε is the ratio of the dielectric constant of the substrate to the dielectric constant of the base layer (ε = ε _substrate / ε _{base layer} ). According to the composite electrode structure, the substrate region between the metal electrodes is exposed by the base layer.

According to an embodiment of the present invention, the base layer may be made of the same piezoelectric material as the substrate. In particular, the base layer may be a ridge portion formed by etching the substrate material to form a groove in a region between the metal electrodes.

According to another embodiment of the present invention, the base layer may be made of a mechanically hard material having a higher elastic impedance than the elastic impedance of the substrate, and may have a dielectric constant higher than that of the substrate. .

According to another embodiment of the present invention, the base layer may be made of a mechanically soft material having a lower elastic impedance than the elastic impedance of the substrate. In particular, the base layer may be made of a powder of small particles having a high dielectric constant. In addition, the base layer may be a gas or a vacuum. The base layer may be made of a mechanically soft solid material having an elastic impedance lower than that of the substrate.

When the base layer is made of powder, gas or vacuum, a dielectric plate used to support the metal electrode may be additionally installed. The dielectric plate is spaced apart from the substrate surface up a distance equal to the total thickness of the electrode and base layer, and the metal electrode is mechanically attached to the dielectric plate.

The distance between the substrate and the dielectric plate may be controlled by a spacer formed outside the composite electrode structures. In particular, the dielectric plate has a cavity of controlled depth, and protrusions can be formed around the cavity to serve as the spacer.

According to an embodiment of the present invention, the base layer may be made of a semiconductor or other metal having a lower elastic absorption than the elastic absorption at the metal electrode.

The metal electrode may have a thickness in the range of 0.01 to 0.25 times the wavelength of the SAW excited in the device on the substrate at the center frequency of the device.

Various known means for controlling SAW propagation may be installed on the substrate, such as reflectors, transducers, screens, absorbers, and the like.

A detailed analysis of the loss mechanism of a conventional SAW device shows that the energy absorption at the metal electrode is not uniform across the electrode cross-section and is concentrated at a higher mechanical stress, ie at the bottom of the electrode attached to the substrate surface. Shows. 2 shows this stress distribution in the electrode cross section. In terms of shear wave wavelength at Al, an electrode thickness of 8 to 10% of the SAW wavelength λ is close to one quarter of the wavelength at _Al (λ _Al / 4), so that the stress distribution is It is essentially nonuniform across the thickness. For leaky SAWs, shear ploarization prevails, and the stresses in the electrodes go to zero on the unfixed electrode top, roughly following the cosine law, and the interface between the electrodes and the substrate. Maximum in the vicinity. Reducing stress in the metal reduces the absorption of elastic energy, which in turn reduces the attenuation of the SAW and consequently the loss of the SAW device.

The above object of the invention is achieved by partial acoustic isolation of the metal electrode from the substrate in which the SAW in the IDT and reflector is excited and propagated on the surface. Partial elastic separation is realized by separating the upper metal electrode from the substrate by a bottom layer with reduced elastic absorption. The base layer having a specific thickness h,

1. made of the same crystalline material as the substrate, or

2. made of a dielectric material with low elastic impedance, such as soft solid, small solid powder, gas or vacuum;

3. made of a dielectric with high elastic impedance, of a hard solid material, or

4. It can be made of a semiconductor or other metal with a significantly lower elastic absorption than the elastic absorption in the upper metal layer (metal electrode).

Substantially, this requires a composite electrode structure comprising at least two materials, the upper metal layer (metal electrode) having high conductivity and the base layer separating the metal layer from the substrate and having low elastic absorption, wherein the 2 Corresponds to the branch material.

To avoid a reduction in electro-mechanical coupling, it is necessary to concentrate the electric field generated by the charge on the metal electrode inside the substrate. For this reason, the thickness h of the base layer separating the electrode and the piezoelectric substrate satisfies the following expression.

(h / λ) · ε <1

Is the SAW wavelength of the piezoelectric substrate at the center frequency of the device, and ε is the ratio of the dielectric constant of the substrate to the dielectric permittivity of the _{base layer} (ε = ε _substrate / ε _{base layer} ). The substrate surface and the base layer surface are parallel to each other.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art. Accordingly, the shape and size of elements in the drawings may be exaggerated for clarity, and the elements denoted by the same reference numerals in the drawings are the same elements.

3 is a schematic cross-sectional view of a SAW device according to an embodiment of the invention. Referring to FIG. 3, the SAW element 100 includes a piezoelectric substrate 101 and a plurality of metal electrodes 102 disposed thereon. The piezoelectric substrate 101 has a polished top surface for SAW propagation, and the metal electrode provides various known means for controlling SAW propagation, such as IDTs, resonators, and reflectors. The base layer 103 is disposed between the metal electrode 102 and the piezoelectric substrate 101. Base layer 103 separates metal electrode 102 from the substrate and has a lower elastic loss than the elastic absorption or elastic loss at the metal electrode. By the base layer 103, the metal electrode 102 is partially elastically separated from the piezoelectric substrate 102. This metal electrode 102 and underlying base layer 103 form a composite electrode structure that provides reduced elastic energy loss.

In order to concentrate the electric field generated by the electrode into the piezoelectric substrate, the thickness h of the base layer 103 particularly satisfies the following expression.

(h / λ) · ε <1

Is the SAW wavelength of the piezoelectric substrate at the center frequency of the device, and ε is the ratio of the dielectric constant of the substrate to the dielectric constant of the base layer (ε = ε _substrate / ε _{base layer} ). According to the composite electrode structure, the substrate region between the metal electrodes is exposed by the base layer.

To provide reduced elastic absorption, the base layer 103 may be formed of a hard dielectric material having an impedance that is significantly greater than the elastic impedance of the piezoelectric substrate 101. Since deformation is reduced in hard material, the dielectric base layer with high elastic impedance also partially separates the metal electrode from the substrate. However, in order not to provide a significant loss of piezoelectric coupling on a LiNb or LiTa substrate, it is preferred that the dielectric base layer material has a high _dielectric constant ( ε _dielectric ≫50) greater than 50. Oxides such as TiO ₂ or Nb ₂ O ₅ can be selected as such dielectric material. Using a hard base layer can be advantageous in terms of minimizing "leakage" losses for LiNb and LiTa substrates.

As the base layer 103 material, a semiconductor such as Si or another metal such as Mo (a metal different from the metal electrode material) may be used. In this case, the condition that the electric field should be concentrated inside the piezoelectric substrate is automatically satisfied. If such base layer 103 of semiconductor or other metal has a lower elastic absorption or elastic loss than the upper metal electrode 102, a reduction in loss is provided. In this case, the conductivity of the base layer 103 may not be good, and the low resistance of the composite electrode structure is provided by the upper metal electrode 102.

Alternatively, the base layer 103 may be made of a soft solid dielectric having an elastic impedance considerably lower than that of the piezoelectric substrate. If base layer 103 has a low elastic impedance, this base layer can provide elastic separation of metal electrode 102 from substrate 101.

In order to manufacture the SAW device 100 according to the embodiment of FIG. 3, a dielectric layer (or a semiconductor layer or another metal layer) is first deposited on the piezoelectric substrate 101. Thereafter, metal electrodes or electrode patterns 102 are formed on the dielectric layer (or semiconductor layer or other metal layer), and the dielectric layer (or semiconductor layer or other metal layer) is etched between the metal electrodes 102. Accordingly, the base layer 103 made of a dielectric (or semiconductor or other metal) is formed only under the metal electrodes 102, and the piezoelectric substrate 101 is exposed in the region between the metal electrodes 102.

4 is a cross-sectional view of a SAW device 200 according to another embodiment of the present invention. In the present embodiment, the base layer is made of the same material as the piezoelectric substrate, and is formed in particular by groove etching of the piezoelectric substrate material.

According to the present embodiment, the dielectric portion (ie, base layer 113) of the composite electrode structure is made of the same piezoelectric material as the substrate 101. The electric field is automatically concentrated on the piezoelectric material and there is no reduction in coupling. The dielectric constants of the base layer 113 and the piezoelectric substrate 101 are the same, so that ε = 1, and the above equation, i.e., (h / λ) · ε <1 requires that the thickness of the base layer 113 is smaller than the SAW wavelength. .

Moreover, the device structure having the ridge portion (ie, the base layer 113) of the piezoelectric substrate 101 and the metal electrode 112 formed on the ridge portion upper surface leads to an increase in the electro-mechanical coupling. This embodiment is also advantageous in that it provides high reflectivity of the composite electrode structure.

In the device 200 structure, if the thickness of the base layer 113 corresponds to the thickness of the metal electrode 112, an essential part of the mechanical stress is concentrated in the base layer 113. On the other hand, the metal electrode 112 moves somewhat as an entire block (an integer number of metal electrodes), and the internal deformation is significantly reduced. As a result, the elastic energy loss is significantly reduced.

For leaky waves on some substrates (eg, 41-LiNb substrates), the increase in Al electrode thickness in conventional SAW devices results in stronger attenuation due to the leakage characteristics of the leaky waves. The presence of a thick and "slow" Al layer obstructs the boundary conditions in such a way that the radiation of the slow shear bulk wave increases. This effect does not appear in the present embodiment, because the base layer 113 made of the same piezoelectric material (eg, LiNb) as the piezoelectric substrate material does not have a mass loading effect.

Preferably, the piezoelectric substrate 101 is a piezoelectric material, such as 36-LiTaO ₃ , 64-LiNbO ₃ , 41-LiNbO ₃ (or YZ-LiNb for leakage longitudinal wave-based SAW devices) having a high piezoelectric coupling. It is formed of a substrate material having properties. The metal electrode 112 is made of Al, Au, Mo, Cu or other suitable high conductivity metal and may be connected to a bus bar or the like.

The polarity of the metal electrode 112, the number of electrodes per wavelength [lambda], the metallization coefficient, the thickness, etc., can be used to determine the type of SAW used for a particular device, desired piezoelectric coupling value, capacitance, electrode reflectivity and other designs. Determined by the parameter. The electrode 112 is separated from the piezoelectric substrate by the dielectric base layer 113. In this embodiment, the piezoelectric base layer 113 is made of the same piezoelectric material having the same directionality as the substrate 101, and may be formed by etching with a groove A of a predetermined depth between the metal electrodes 112. .

Simulation with FEM / BEM simulation software shows that, for leakage longitudinals, this device 200 configuration can increase electromechanical coupling while maintaining very high reflectivity of the composite electrode structure.

The overall height of the electrode should be optimized according to the characteristics of the SAW used. For leakage SAWs on 64LN or 41LN, an increase in Al electrode thickness results in an unacceptable increase in SAW attenuation due to radiation of the volume leakage component. Since the reflectivity of these electrodes is not sufficient for wide-band resonator filters (CRF / DMS, etc.), this places a strong limitation on device design. Limited Al thicknesses (<3% to 4%) and bottom ridges made of materials such as piezoelectric substrates solve this problem, resulting in a strong increase in reflectivity of the composite electrode structure while keeping volume component radiation at a low level. Can be.

For the leakage longitudinal, the overall thickness of the composite electrode structure having a top Al layer (metal electrode) and a base layer of YZ-LiNb such as a piezoelectric substrate has an optimal value at about 6% to 7% in h _total / λ. The simulation shows the fact.

In this embodiment, the metal electrode 112 is firmly mounted on the piezoelectric substrate 113, and no top cover or the like for mechanical support of the metal electrode 112 is necessary.

Substantially, the above-described composite electrode structure (metal electrode 112 and piezoelectric base layer 113) is easily manufactured by etching to form grooves A between the metal electrodes using a conventional lithography process. Can be. A pedestal or ridge formed between the grooves A (and of the same crystalline material as the substrate) forms the base layer 113. The ridge portion (that is, the base layer 113) and the metal electrode 112 formed on the upper surface form a composite finger (composite electrode structure).

5 is a cross-sectional view of a SAW device 300 in accordance with another embodiment of the present invention. In the present embodiment, the base layer 123 has a significantly lower impedance than the piezoelectric substrate 101, and is made especially of gas, vacuum or powder.

In the extreme case where the base layer 123 is a gas or vacuum layer, the elastic separation is almost complete. In this case no mechanical movement is transmitted from the piezoelectric substrate 101 to the metal electrode 122. In addition, the loss in the base layer 123 itself is only a very small amount. Because of the difference in elastic impedance, only a small fraction of the SAW elastic energy passes through the gas or vacuum layer (ie, base layer 123) described above. This is ideal for the purpose of reducing SAW energy absorption. In order to realize such a gas / vacuum layer, two main things must be satisfied. That is, the metal electrode 122 must be mechanically supported, and a very thin gap must be provided between the metal electrode 122 and the piezoelectric substrate 101. The base layer 123 of gas or vacuum cannot mechanically support the metal electrode 122, and the metal electrode 122 must be attached to the same piezoelectric substrate at some point or supported from the other side by an additional dielectric plate. (See Figure 5).

A method of implementing such a gas / vacuum base layer 113 is to form a metal electrode 122 on a dielectric plate (lid) 104 that serves to support the metal electrode. The dielectric plate 104 is disposed close to the surface of the piezoelectric substrate 101 at a distance D equal to the total thickness of the metal electrode 122 and the gas / vacuum base layer 123 of the composite electrode structure. do. The distance between the piezoelectric substrate 101 and the plate (or cover) 104 may be controlled by the spacer 105. The spacer 105 is disposed outside the composite electrode structures 122 and 123 to separate the dielectric plate 104 from the surface of the substrate 101 by a distance D.

The dielectric plate 104 may be made of a non-piezoelectric material such as glass, fused quartz, or a weak piezoelectric material such as quartz. The electrode system supported by the dielectric plate 104 may form interdigital transducers, reflectors, busbars, and other electrode elements commonly used in SAW devices.

Various materials can be used for the spacer 105. Spacer 105 may be made of a metal such as a contact bar and may partially coincide with the contact pad. Alternatively, it may be made of a material used to join the two plates (piezoelectric substrate 101 and dielectric plate 104). The spacer 105 may also be formed of a dielectric film such as AlN. By being placed on both sides from the elastic channel, the spacer 105 not only defines the distance between the two plates 101 and 104, but also serves to improve the waveguiding of the SAW inside the elastic channel. . AlN spacer films on LiTa or LiNb piezoelectric substrates are well suited for this role.

The cavity between the piezoelectric substrate 101 and the plate 104 may be vacuumed or filled with air. Alternatively, nitrogen or an inert gas may be filled. In order to prevent the piezoelectric substrate 101 and the plate 104 from contacting each other, the gas pressure in the cavity may be increased.

In FIG. 5, the spacer 105 is coupled to the plate 104 as a separate component distinct from the plate 104, but the present invention is not limited thereto. In addition, the spacer 105 may be formed of the same material as the plate 104.

6 is a cross-sectional view illustrating a modification of the SAW device of FIG. 5. Referring to the SAW element 300 'of FIG. 6, the dielectric plate 104' has a cavity 106 of controlled depth D formed by etching or the like, and acts as a spacer around the cavity 106. Protruding portion 105 'is formed. The metal electrode 122 is disposed in the cavity 106.

After forming the cavity 106 in the dielectric plate 104 'and placing the electrode at the bottom of the cavity, the piezoelectric substrate 101 is attached to the protrusion 105' around the cavity 106. Therefore, according to this embodiment, the piezoelectric substrate 101 is directly attached to the dielectric plate 104 without having to make a separate spacer. In this case, the protrusion 105 'becomes a kind of spacer.

Alternatively, a material such as nano-particle powder having a high dielectric constant may be used. Nano-sized particles are automatically concentrated below the electrode (at the point of high electric field) and the concentration of the electric field within the piezoelectric body is greatly increased.

In fact, the main role of the IDT electrode is to generate an electric field in the piezoelectric substrate. Whether the transducer electrode is disposed directly on the piezoelectric surface or separated from the piezoelectric surface by a thin dielectric base layer for this purpose is not critical as long as the distance to the piezoelectric surface is small enough. More specifically, the electric field generated by the metal electrode should be mainly concentrated inside the piezoelectric body, which mathematically results in the relation: (h / λ). When the dielectric base layer 123 is in the form of a gas or vacuum gap, precise control of the gap is not required as long as the gap is sufficiently narrow and the condition is satisfied. The dielectric plates 104, 104 'itself may be made of a non-piezoelectric substrate such as glass. However, in the case of using a LiNb or LiTa piezoelectric substrate, a very thin gap filled with gas or vacuum with a dielectric constant ( ε _substrate ) of about 50 is required (the dielectric constant ε _gap of the _gap is about 1).

In order to fabricate the SAW elements 300, 300 ′ with the base layer 123 of gas / vacuum, means for supporting the metal electrode 122 must be provided. Clearly, the most important step in fabricating a device with such a gas / vacuum base layer 123 is the gap between the operating surface of the piezoelectric substrate 101 and the electrode 122-this gap should be minimized as described above. -Is a limitation. For example, for a 1 GHz device with a wavelength of about 4 μm, the gap should be smaller than 0.1 μm (1000 μs) to avoid a reduction in coupling. This gap can be defined by forming a support layer (ie, spacer layer 105) having an appropriate thickness located outside the elastic channel. For example, the contact pad may be used to act as a spacer by increasing the thickness of the contact pad according to the electrode thickness.

The amplitude of the elastic displacement within the SAW is about 1 Hz and is still incomparably smaller than all other distances associated with the device. Even surfaces polished to “optical” quality still have roughness on the order of about 0.1 μm. On the other hand, mechanical contact between two surfaces does not necessarily form an important "acoustic contact". That is, no matter what point the two surfaces are in contact with, this mechanical contact is not sufficient for sufficient transfer of elastic energy from one surface to the other. In order to provide elastic contact, close physicochemical bonding such as gluing, molecular bonding, liquid inter-layer, etc. is required. Thus, substantially, the metal electrode 122 separated from the piezoelectric substrate 101 by the base layer 123 of gas or vacuum may be in contact with the piezoelectric substrate 101 at some points spaced from each other.

In addition, powder-like materials of nano-sized particles having a higher dielectric constant than those of piezoelectric substrates can solve the above problem by concentrating an electric field under the electrode while providing significantly reduced elastic contact.

According to the embodiment of FIGS. 5 and 6, the power handling capability is increased because the electrode is only partially exposed to elastic energy and subjected to reduced mechanical stress. By selecting a dielectric plate having a low coefficient of thermal expansion, the TCD of the SAW device can also be improved.

5 and 6, the thickness h of the gap between the electrode 122 surface and the piezoelectric substrate 101 should be minimized. This gap can be treated as a capacitor connected in series with the standard capacitance between the electrodes for the case where the electrodes are placed directly on the surface. The voltage drop across this gap capacitor decreases with piezoelectric coupling. The capacitance (C _gap ) of this capacitor can be easily calculated as follows.

C _gap = ε _o ㆍ ε · (a · L) / h = 8.86 · 10 ^-6 ㆍ a / h pF / 1 μm

Where a is the width of the electrode, h is the thickness of the gap, L is the length of the electrode, ε _o is the vacuum permittivity, and ε is the base layer (gap in this embodiment) that separates the metal electrode from the piezoelectric substrate. Relative dielectric permittivity.

The value is compared with the capacitance (C _o) of the IDT electrode in the following.

C _O = 23 · 10 ^-5 pF / 1 μm

If the _gap C »C _O, that is, if h / a« 0.04, the influence of the gap it can be seen that good negligible. For 900 MHz filters (a to 1 μm), this means that the gap should be thin enough that the electrode is substantially in contact with the piezoelectric substrate.

Solid dielectrics with relatively low elastic admittance (SiO ₂ ) and high permittivity are also suitable as dielectric base layers. The problem with solid layers is that typically soft materials have rather high damping (eg Al, etc.). However, the presence of the soft layer usually reduces the reflectivity of the electrode, which can be a problem in some applications.

Elastic separation of the upper metal electrode from the acoustic wave provides a number of advantages:

Elastic damping at the metal electrode, one of the main mechanisms of attenuation, is partially removed. In particular, the accumulation of elastic energy at the Al electrode is reduced.

In embodiments using base layers of gas, vacuum, powder, etc. having low impedance:

Since there is no mechanical loading, the "leak wave" cut of LiTa and LiNb is optimal with minimal loss due to "leak" of the volume wave.

The electrode can be made of gold (Au), which has a low resistance (and therefore a low resistance loss) and blocks aging due to oxidation.

The electrode can be made somewhat thick, up to electrode thickness / λ = 25%, further reducing the resistance without the introduction of mass loading. The electrode thickness can range from 0.01 to 0.25 times λ (typical SAW wavelength).

Device performance is virtually unaffected by electrode thickness and metallization rate, simplifying device design and increasing production yield.

For Au electrodes, the device does not require packaging, or a simple, non-hermetic packaging technique may be sufficient.

In the case of an additional lid (dielectric plate) supporting the electrode, photolithography can be performed on cheap materials such as glass, having a 6 inch wafer, and at the same time can eliminate the problem of ptoelectricity.

The reduced intensity (or absence) of the acoustic waves in the electrodes thoroughly reduces the mechanical stresses induced in the electrodes, thus eliminating the effects of elastic induction migration. In addition, power handling capability can be improved without the use of complex “sandwich” type electrodes of several metals and alloys. Pure aluminum or pure gold can be used.

It is intended that the invention not be limited by the foregoing embodiments and the accompanying drawings, but rather by the claims appended hereto. In addition, it will be apparent to those skilled in the art that the present invention may be substituted, modified, and changed in various forms without departing from the technical spirit of the present invention described in the claims.

In the above-mentioned preferred embodiment, when the electrode is firmly attached to the solid base layer on the substrate, no additional cover (dielectric plate) for mechanically supporting the electrode is used. However, even in such a case, the use of the cover can be justified if the base layer has an elastic impedance significantly different from that of the metal electrode. If a metal electrode is attached to both the solid base layer and the lid (dielectric plate), the speed of the SAW in the lid should be higher than the speed of the substrate to prevent radiation of elastic energy into the lid.

The piezoelectric substrate may include a multilayer structure that contributes to increasing thermal stability or reducing "leakage" losses in the "leakage SAW" substrate. All known piezoelectric materials used in SAW devices can be used as the piezoelectric substrate.

As described above, according to the present invention, the elastic loss is significantly reduced by separating the metal electrode from the piezoelectric substrate by the base layer. In addition, resistance loss can be reduced, power handling capability can be increased, and aging characteristics can be improved.

Claims (13)

A substrate having a polished surface for SAW propagation and having piezoelectric properties; And A plurality of composite electrode structures arranged on the substrate, each comprising at least two layers, Each composite electrode structure, A metal electrode disposed on the substrate; A base layer disposed between the substrate and the metal electrode to separate the metal electrode from the substrate, the base layer having a lower elastic loss than the elastic loss at the metal electrode, wherein the thickness h of the base layer is A surface acoustic wave device characterized by satisfying a relational expression. (h / λ) · ε <1 Is the SAW wavelength of the piezoelectric substrate at the center frequency of the device, and ε is the ratio of the dielectric constant of the substrate to the dielectric constant of the base layer (ε = ε _substrate / ε _{base layer} ). The method of claim 1, And said base layer is made of the same piezoelectric material as said substrate. The method of claim 2, And the base layer is a ridge portion formed by etching the substrate material to form a groove in a region between the metal electrodes. The method of claim 1, And the base layer is made of a mechanical hard material having a higher elastic impedance than the elastic impedance of the substrate, and has a dielectric constant higher than that of the substrate. The method of claim 1, And the base layer is made of a mechanical soft material having a lower elastic impedance than the elastic impedance of the substrate. The method of claim 5, The base layer is a surface acoustic wave device, characterized in that made of a powder of small particles having a high dielectric constant. The method of claim 5, The base layer is a surface acoustic wave device, characterized in that the gas or vacuum. The method according to claim 6 or 7, Further comprising a dielectric plate for supporting the metal electrode, And the dielectric plate is spaced apart from the substrate surface by a distance equal to the total thickness of the electrode and base layer, and the metal electrode is mechanically attached to the dielectric plate. The method of claim 8, And the distance between the substrate and the dielectric plate is controlled by a spacer formed outside the composite electrode structures. The method of claim 9, The dielectric plate has a cavity having a controlled depth, and a protrusion is formed around the cavity to serve as the spacer. The method of claim 5, And the base layer is made of a mechanical soft solid material having an elastic impedance lower than that of the substrate. The method of claim 1, The base layer is a surface acoustic wave device, characterized in that made of a semiconductor or other metal having a lower elastic absorption than the elastic absorption in the metal electrode. The method of claim 1, And said metal electrode has a thickness in the range of 0.01 to 0.25 times the wavelength of SAW excited in said device on said substrate at the center frequency of said device.

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