Description

BACKGROXJND OF THE INVENTION

Field of the Invention

The present invention relates to surface acoustic wave (SAW) devices, and more specifically to low loss SAW devices, with improved passband performance particularly in the GHz frequency range. SAW devices have become key components for mobile communication handsets and base stations, for TV sets, as well for other numerous communication applications. Passband filters and resonators are the most frequently used types of SAW devices. Small size, excellent technical parameters (low loss, selectivity, etc.), as well as low cost put them practically out of competition with devices based on other physical principles.

Description of the Related Art

Typical SAW device (see Fig.l) includes single crystal piezoelectric substrate 1 with an interdigital transducer (BDT) structure 2 placed on the surface of the substrate. The role of transducer is that of antenna for SAW - it creates alternating in time and space electric fields, which penetrate into piezoelectric substrate and which generate mechanical stresses due to the piezoeffect. In this way the transducer can generate surface acoustic waves, which propagate on the surface. The waves propagation can be influenced in different ways (waves can be reflected, deflected, absorbed, waveguided, etc.), waves launched by one transducer can be received by another IDT. The generation of the waves changes the impedance of the transducer. AU these phenomena are well understood now and are routinely used for the design and fabrication of SAW filters of different types. Particularly demanding applications are for mobile phones, where high filtering performance is demanded together with a low insertion loss. Typical relative frequency passband for many mobile phone standards is about 3% to 4%, which demands the use of high coupling materials such as Lithium Niobate or Lithium Tantalate for the filter substrates. To satisfy these criteria so-called "leaky wave" cuts of LiNbO3 (64LN, 41LN) and LiTaO3 (36LT, 42LT) are often used, which provide high coupling and relatively high SAW velocity. However, the SAW technology seems to have reached certain limits where further development demands a new breakthrough to qualitatively improve performance. In particular at 2 GHz range the minimal losses in SAW devices become typically higher than 2 dB which is an obstacle for some applications. The power handling of SAW devices decreases with increasing the frequency and remains a problem for applications such as duplexers in handsets of mobile phones. In addition, heπnetic packaging increases substantially the fabrication cost of the device - the package now costs more than the device encapsulated in it. The rapid development of communication systems demands further increase of the operating frequencies. New applications, such as LAN, "Bluetooth", etc. use 2.5 GHz frequency range. The new communication systems (HLAN, for example) will use 3 GHz - 6 GHz frequencies. In all these systems the filtering of frequencies will be the most crucial function for reliable work and efficient use of available recourses of frequencies. That guarantees brilliant future for SAW devices. The insertion loss is one of the most important parameters for mobile applications.

There are a number of significant loss mechanisms in SAW devices as follows: 1. Interaction of the acoustic wave with thermal phonons is an intrinsic loss present in dielectric material and dependent on the material crystalline nature and quality as well as temperature (LiNb is one of the best piezoelectric materials in this sense ). 2. Acoustic losses in the metal electrodes represent a significant part of the total losses. The metals are usually much more "lossy" materials than dielectrics. Aluminum is known as a material giving relatively low attenuation compared to other metals, such as gold or copper. 3. Resistive (Ohmic) losses in conductors become more important at high frequencies, because the thickness of electrodes is small. 4. Acousto-electric interactions due to RF currents created in metals by the electric fields near the piezoelectric substrate with SAW propagating along its surface. 5. Interaction with gas (air) loading of the working surface 6. Imperfections, contamination, residues on the surface , in particular after technological treatment 7. Losses due to acoustic energy escaping from the filter structure, for example by "leaky" bulk wave component of "leaky waves"

Of all these loss mechanisms the ones related to the presence of metal electrodes (numbered "2" and "3" in the above list) are responsible for a very significant part of the total loss (estimated as 2/3 for IGHz devices ) . The attenuation of acoustic waves is much higher in the metals, than in dielectrics. Early experiments with gold gave unsatisfactory results. That is why Aluminum is the most common electrode material since it gives relatively low attenuation, despite of other properties, such as conductivity or oxidation are much more favorable for gold electrodes. Although the resistivity of Al is significantly higher than that of gold or silver, the acoustic losses in gold are unacceptably high. The thickness of the electrodes used in SAW devices is limited to about 10% of the characteristic wavelength, because the electrodes with even higher thickness could load too strongly the surface further increasing loss and decreasing coupling.

All these mechanisms result in filter losses, which may achieve typically from 1.0 dB to 2dB in the device passband for mobile phone applications for the 900 MHz range or even about 4 dB for 2 GHz frequencies. Further development of mobile communications demands a significant decrease of these losses. The energy dissipated in a filter is transformed into heat and increases the operating filter temperature. For some important applications (duplexers, front-end TX filters) the applied power may reach 2W to 3 Watts, and the increase of temperature is up to 4O0C or more. The increased temperature results in a decrease of filter performance, accelerated aging, decrease of power handling capability, etc. (That is one more reason why the decrease of loss is desirable. - unnecessary)

The Al electrodes typically used in SAW devices are responsible for two other known drawbacks of the devices - limited power handling and aging. A high density of acoustic energy concentrated in the electrodes eventually results in acoustic induced migration of Aluminum. The electrodes change their shape and can short-circuit the device, resulting in performance deterioration and even failure.

The oxidation of Aluminum and its corrosion in the presence of water is an important aging mechanism and demands hermetic packaging to decrease its consequences.

The dependence of the SAW characteristics on the mass-loading by electrodes results in poor yield because of technological variations of the electrode dimensions.

SUMMARY OF THE INVENTION

It is the goal of the present invention to propose a SAW device wherein the attenuation losses due to high acoustic attenuation in the metal are eliminated, the resistive losses are decreased, while the power handling capabilities are increased and aging characteristics are improved.

The above mentioned goals of the present invention are achieved by isolating acoustically the electrodes in both the D)T and the reflectors from the piezoelectric substrate on the surface of which the SAW are excited and propagated. Acoustic isolation is achieved by impedance mismatch, Le. by separating the electrodes from the acoustic path by a thin medium of very low acoustic impedance. This medium can be vacuum, gas or indeed any other low impedance material. In practice, this requires that the electrodes be physically suspended right above the acoustic path. One way of achieving this is to form the electrodes onto a dielectric plate (lid) which serves to support the former in close proximity to the surface of the piezoelectric substrate without making physical contact with the latter. The separation distance h between the electrodes and the piezoelectric h surface satisfies the following relation: — • £ « 1 , where λ is the SAW wavelength in Λ said piezoelectric substrate at center frequency of the device, and ε is the dielectric permittivity of said substrate and both surfaces being parallel to each other(Fig.2). In fact, the main role of the IDT is to create an electric field in the piezoelectric substrate. For that purpose it is of no importance whether the transducer is placed directly onto the piezoelectric surface or is suspended just above the surface of the latter, provided that the distance to the piezoelectric surface is sufficiently small. More precisely, the electric fields created by the electrodes must predominantly concentrate inside said piezoelectric, which h mathematically results in the formula given above: — • ε « 1. From the technological point λ of view no exact control of the gap is required, provided that the latter condition is satisfied. The lid itself can be made of a non-piezoelectric substrate, such as glass. The acoustic isolation of the electrodes from the acoustic wave has numerous advantages as follows: • one of the main mechanisms of attenuation - acoustic attenuation in the metal electrodes is completely eliminated - there is no significant acoustic energy in the Al electrodes in the proposed device • The piezoelectric surface is not involved in any processing step, apart from the last bonding step, and thus remains virgin, i.e. free from contamination or any modification. • The 36-degree "leaky wave" cut of LiTa remains optimal with minimal losses by "leakage" of bulk waves, since there is no mass-loading now. • The electrodes can be made of gold(Au) which decreases their resistivity and excludes aging due to oxidation • the electrodes can be made rather thick, up to h/λ = 25 % , which allows to further decrease their resistivity, without introducing any mass-loading • the device performance becomes almost insensitive to the electrodes thickness and metallization ratio (almost as in semiconductors!), which simplifies the design of the devices and increases the yield during mass fabrication. • The absence of acoustic waves in the electrodes decreases drastically the mechanical stress induced in the electrodes and hence eliminates the effect of acoustically induced migration. The power handling capability can now be improved without using complicates "sandwich" type electrodes with several metals and alloys. Pure Aluminum or pure gold can be used. • In the case of Au electrodes the device requires no packaging, or very simplified non- hermetic packaging technology may suffice. • The photolithography can be made on a cheap material, like glass, with 6 inch wafers, eliminating at the same time the problems of pyroelectricity. • The piezoelectric material is needed only in the area of the acoustic channels, resulting in efficient use of expensive materials.

Clearly, the most critical step during the fabrication of the proposed devices is the definition of the gap between the working surface of the first piezoelectric substrate and the electrodes and which gap must, as discussed above, be minimized. For example for a 1 GHz device with a wavelength about 4 microns, the gap must be less then 0.1 micron (lOOOA) to avoid a decrease in coupling. Such a gap can be defined by creating a supporting layer of appropriate thickness (spacer layer) located outside the acoustic channel. For example, the contact pads may be used to play the role of spacers by increasing their thickness accordingly with respect to the electrode thickness. (The amplitude of acoustic displacement in SAW is about 1 A and is still incomparably smaller than all other distances involved.) It is noted that surfaces polished to the "optical" quality still have a roughness of the order of 0.1 micron or so. On the other hand a mechanical contact between two surfaces does not necessarily create a significant "acoustic contact", that is although the surfaces are in contact at certain points this mechanical contact is not sufficient for efficient transmission of acoustic energy from one surface to the another. To provide an acoustic contact an intimate physico-chemical bonding is necessary, say gluing, molecular bonding, etc. Thus in practice the two substrate may even touch each other. The device thus proposed has an increased power handling capability because the electrodes are not exposed to the acoustic energy and experience no mechanical stresses. By choosing the dielectric plate with a low temperature expansion coefficient the TCD of device can also be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l illustrates the prior art - a conventional SAW device Fig. 2 schematically illustrates the SAW device proposed in the present invention - substrates separated by spacers of controlled height D. Fig. 3 is a schematic illustration of another realization of the proposed SAW device, wherein the distance between the wafers is controlled by etching a cavity in the dielectric plate Fig. 4 illustrates schematically an alternative realization of the proposed SAW device wherein the electrodes are suspended above the surface of said piezoelectric substrate, while the bus bars and contact pads rest on the latter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Example 1

Fig. 2 schematically illustrates the first preferred embodiment of a filter. The filter includes a first substrate 1 with piezoelectric properties, preferably a single crystal material, such as 36- LiTaOs , 64-LiNbOs, or 41-LiNbOs with high piezoelectric coupling and the dielectric plate 2, which can be made from a non-piezoelectric material, such as glass, fused quartz , or from a weak piezoelectric material such as quartz.

The electrode system 3 is placed onto the dielectric plate 2 and the two substrates are brought in close proximity with their surfaces parallel to one another, the distance between them being controlled by spacers 4 placed outside the acoustic channel area. The electrode system serves to create the interdigital transducers, reflectors, busbars, and other elements normally used in SAW devices.

Various materials can be used for the spacers 4 - they may be made from the same metal as the contact bars and may partly coincide with the contact pads. Alternatively they can be made from a material which is simultaneously used for bonding the two substrates. The spacers may also be done from a dielectric film, such as AlN. Placed on both sides from the acoustic channel the spacers not only define the distance between the substrates, but also serve to improve waveguiding of the SAW inside the acoustic channel. AlN film on LiTa or LiNb will be perfectly suitable for such a role. The cavity between said substrate and said plate may be evacuated or be filled with air; alternatively nitrogen or inert gases may be used. Increased gas pressure may be applied to prevent said surfaces from touching each other.

Example 2

Fig.3 shows the second preferred embodiment of the filter , wherein the cavity 5 of controlled depth D is created in the dielectric plate and the electrodes are placed in said cavity. After that the first piezoelectric substrate is attached directly to the second one.

The gap d between the surface of the electrodes and the first substrate must be minimized. This gap can be treated as a capacitor connected in series with the standard capacitance between electrodes for the case when the electrodes are placed directly onto the surface. The voltage lost over this gap capacitor decreases accordingly the piezoelectric coupling. One can easily estimate the capacitance of this capacitor

Q^ = *o — = 8.86.10-6 ^.i^- d d Iμm where a is the electrode width. This value must be compared with the capacitance of one electrode in the DDT, which is about

One can see that the influence of the gap will be negligible if CQAP » CO, that is if

- « 0.04 a For a 900 MHz filter (a ~ lμtri) it means that the electrodes must practically touch the piezoelectric substrate.

While particular embodiments of the present invention have been shown and described, it will be obvious to those of ordinary skill in the art that changes and modifications may be made without departing from this invention in its broader aspects. Thus for instance, although the preferred embodiments of the present invention make use of a lid to support the electrodes and at the same time define the separation gap, the same results can be achieved by suspending the electrodes over the acoustic path without the use of a lid as illustrated in Fig. 4. Thus, an alternative approach would be to eliminate a thin sacrificial layer over the acoustic path of the device and onto which sacrificial layer the electrodes are defined. Subsequently, this layer may be removed to fill the gap with gas or evacuate it, or alternatively use the sacrificial layer itself as an acoustic isolation between the electrodes and the acoustic path. Further, to provide rigidity of the electrodes, the latter may be anchored to said piezoelectric substrate at a number of points. Another evident alternative would be to replace vacuum or air gap with powder- like material with extremely small particles, having big dielectric permittivity. Said piezoelectric substrate can include multilayered structure which serves to increase thermal stability, or to decrease "leaky" losses, in " leaky SAW " substrate, etc. - all known in state of the art piezoelectric materials used for SAW devices.