Bonded wafer SAW filters and methods

Improved coupling coefficients and desirable filter characteristics are exhibited in a SAW filter including an electrode pattern deposited on a piezoelectric substrate bonded directly to an anti-reflective layer, wherein the anti-reflective layer is bonded to a carrier through an adhesive layer such that a preselected thickness of the anti-reflective layer is sufficient for enhancing an acoustic match between the piezoelectric substrate and the adhesive layer.

FIELD OF INVENTION

The present invention relates to surface acoustic wave devices, and in particular to surface acoustic wave filters having improved coupling coefficients while maintaining desirable filter characteristics.

BACKGROUND OF THE INVENTION

Surface acoustic wave (SAW) filters typically comprise interdigital transducer elements deposited on a piezoelectric substrate. SAW filters are widely used in various telecommunication systems due to small size, low loss characteristics of the implemented SAW resonators. SAW filter performance is generally dependent upon the features of the piezoelectric substrate. Low loss, smooth broadband, good rejection and sharp shape factor filter characteristics may be achieved by providing a piezoelectric substrate which exhibits high coupling coefficient, temperature compensation, high resonant Q factor and strong suppression of plate modes. Lithium Tantalate (LT) and Lithium Niobate (LN) substrates exhibit a high coupling coefficient. Rotated Y-cuts of LT exhibit coupling coefficient in the range of 8% to 10% and have been described by Ueda et al. in U.S. Pat. No. 6,037,847 and by Naumenko et al. in U.S. Pat. No. 6,556,104, the disclosures of which are herein incorporated by reference in their entirety. The coupling coefficient of LN has been shown to exhibit a coupling coefficient as high as 23%. However, these high coupling piezoelectric substrates typically exhibit an undesirably significant temperature drift.

Two approaches have been investigated to reduce the temperature drift of the high coupling substrates by bonding the high temperature coefficient expansion piezoelectric substrate to a low temperature coefficient expansion substrate. Taguchi et al. in U.S. Pat. No. 5,998,907, the disclosure of which is herein incorporated by reference in its entirety disclose a number of bonded SAW device structures for purpose of temperature compensation. Taguchi discloses combining two piezoelectric layers with one layer bonded directly to a second layer having a lower temperature coefficient of expansion (TCE). In another embodiment disclosed by Taguchi comprises of a thin film insulation layer of silicon dioxide of 1000 Angstrom joined directly with the piezoelectric substrate and a TCE carrier of Si for purpose of temperature compensation. While attempts to control temperature characteristics are sought, structures as proposed above lead to spurious and undesirable responses due to a direct capacitance coupling between the electrode pattern and carrier, typically a Silicon material. This results in a poor filter performance characteristics.

Abbott et al in U.S. Pat. No. 7,105,980 discloses a SAW composite device comprising a piezoelectric substrate, a surrogate carrier and a bonding film securing the piezoelectric substrate to the carrier wherein the bonding film is a silicon oxide layer with at least one micron thickness to reduce capacitance coupling.

Other known embodiments include bonded wafer SAW filters in which the piezoelectric substrate is bonded with an adhesive layer to a lower temperature coefficient expansion carrier wafer. The TCE material used may be Si or alumina. Unfortunately, and as will be detailed late in this specification, such device structures produce plate mode resonances that distort the response of the resonators. There is a need to overcome this disadvantage.

SUMMARY OF THE INVENTION

In view of the foregoing background and in keeping with the teachings of the present invention, a SAW filter may comprises an electrode pattern deposited on a piezoelectric substrate bonded directly to an anti-reflective layer, wherein the anti-reflective layer is bonded to a carrier through an adhesive layer.

One SAW filter may be described as comprising a piezoelectric substrate having first and second opposing surfaces, an electrode pattern deposited on the first surface of the piezoelectric substrate, an anti-reflective layer directly bonded to the second surface of the piezoelectric substrate, a carrier substrate, and an adhesive material securing the anti-reflective layer to the carrier substrate, wherein the anti-reflective layer is used for enhancing an acoustic match from the piezoelectric substrate to the adhesive layer.

The anti-reflective layer may comprise a layer of Silicon, Silicon Oxide, Aluminium Oxide, or a combination thereof. Yet further, the anti-reflective layer may comprise a Silicon Oxide layer having a thickness greater than 1.5 microns. The Silicon Oxide layer may comprise a thickness substantially equally to 1.8 microns+n*(5.2), where n takes on zero and positive integer values. Additionally, the anti-reflective layer having the characteristics exhibiting an acoustic impedance and wave velocity that lie between that of the piezoelectric substrate and the adhesive layer. The piezoelectric substrate may be Lithium Tantalate or Lithium Niobate. The adhesive layer may be glue, resinous material or an epoxy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference initially toFIG. 1, One embodiment of the present invention may be described as a SAW filter10comprising a piezoelectric substrate12having first and second opposing surfaces14,16, respectively, with an electrode pattern18deposited on the first surface14of the piezoelectric substrate12. An anti-reflective layer20is bonded directly to the second surface16of the piezoelectric substrate12. An adhesive material22secures the anti-reflective layer20to a carrier substrate24. The anti-reflective layer20is such that an acoustic match from the piezoelectric substrate12to the adhesive layer22is enhanced.

Before continuing with details regarding features of the embodiment herein presented by way of example, and for an better appreciation of problems in the art solved by the present invention, consider one known embodiment of a bonded wafer SAW filter26in which the piezoelectric substrate12is bonded with an adhesive layer22to a lower temperature coefficient expansion carrier wafer24, as illustrated with reference toFIG. 2. The TCE material used may be Si or alumina. Such device structures produce plate mode resonances that distort the response of the resonators. By way of example,FIG. 3illustrates results of an analysis exhibiting spurious responses due to the plate modes for the filter24ofFIG. 2. By way of example, any plate mode resonances in the frequency range between the dashed (---) vertical lines will produce distortion in the passband frequency range of the filter.

The degree to which the plate modes distort the response of the resonators is dependent upon multiple physical factors. Two prominent factors are metal thickness28of the electrodes30and the angle Y-rotation of the substrate12. Known devices show much evidence as to what the thicknesses of the electrodes must be in order to obtain high performance SAW resonators on Y-rotations of LT and LN. For example, for 36-48° Y rotated LT the relative thickness of Al electrodes30is generally in the range of 0.16<h/p<0.22 where h is the thickness28and p is the periodicity32. By way of example,FIGS. 4a,4band4cillustrate a variation of the plate mode suppression for different Y-rotations of LT (36°, 42°, and 46° respectively). As the Y-rotation is varied from an angle of 36° to 46°, the effect of the plate mode is modulated. For a metal thickness of h/p=0.18 the coupling to the plate mode's effect reaches a minimum at about 46° Y-rotated X propagation Lithium Tantalate. By way of further example,FIGS. 5aand5billustrate variations of a leaky SAW (LSAW) coupling and slow shear BAW coupling, respectively, for various electrode thicknesses and cut angle of LT. The modulation of the plate mode is a result of the variation of its piezoelectric coupling coefficient. Both the piezoelectric coupling of the dominant leaky surface acoustic wave modes and the plate modes are affected by the Y-rotation of the piezoelectric substrate. Practical devices in the prior art are generally found in the regions encompassed by the dashed (---) boxes providing a strong LSAW coupling coefficient and a weak slow shear BAW coupling. For practical devices to be constructed using the bonded wafer as disclosed, the Y-rotation of the LT is desirably restricted so as to provide sufficient suppression of the plate modes. With continued reference toFIG. 5a, this region lies in the vicinity of the dot-dash line. The overlap of regions of practical electrode thickness and lower coupling to the plate modes yield a piezoelectric coupling of about 8.9%. As compared to the common 9.5% piezoelectric coupling for h/p=0.18 for 42° LT, the 8.9% coupling coefficient can be an appreciable disadvantage. The lower coupling would result in higher loss and narrower bandwidth. There is clearly a need to overcome this disadvantage.

Referring again toFIG. 2, the filter26as described includes the electrode pattern18deposited on the piezoelectric substrate12bonded through an adhesive material/layer22to the carrier substrate24. In contrast, the embodiment as illustrated with reference toFIG. 6includes an array of electrodes30deposited on a high coupling piezoelectric substrate12such as lithium tantalate. The electrodes30shown have a periodicity (p)32with a thickness (h)28. The angle (α) at which a BAW propagates in the piezoelectric substrate12is dependent upon the electrode period32, the velocity of the BAW, the frequency of excitation, and the type of BAW. The three types of BAW modes generated are vertical shear, horizontal shear and longitudinal shear. The velocities and angles of these three BAW modes are illustrated in the plots ofFIGS. 7aand7b. As illustrated, the vertical shear BAW mode propagates in the LT at an angle of 23° to 30° falling in the filter passband region can significantly degrade the performance of the filter. These shear waves will be incident at the interface of LT and adhesive (glue) bonding to the carrier substrate along with the resulted reflected shear and transmitted shear waves as illustrated with reference toFIG. 8.

For LSAW devices on YX orientations of LT, the shear wave transmission into the adhesive material22or glue is relatively weak while the reflected shear wave is relatively strong. As adhesive materials22, such as glues and/or epoxies, have low acoustic impedances and low acoustic velocities, an interface between the piezoelectric substrate12or plate and the adhesive material22will produce large acoustic reflections which will result in the guiding of plate modes in the piezoelectric plate. These plate modes will produce spurious resonance which destructively interference with the desired SAW mode. Suppressing these reflections will result in suppressing the spurious plate modes. Embodiments of the invention provide methods and structures to reduce/suppress spurious acoustic modes which accompany the desired acoustic response of an IDT constructed on a thin piezoelectric plate which is adhesively bonded to a carrier substrate.

With reference again toFIG. 1, to reduce or suppress the plate modes, it is desirable to form the anti-reflective layer20between the piezoelectric substrate (plate)12and the adhesive material22. An additional layer of material is placed between the piezoelectric substrate (plate)12and the adhesive material22. In order to result in a reduction or suppression of the acoustic reflections, this additional material preferably has an acoustic impedance and velocity that is generally half way between that of the piezoelectric plate and the adhesive. By way of example for the case of LiTaO3 or LiNbO3 substrates12and an epoxy adhesive22, silicon oxide is an excellent candidate for this anti-reflective “intermediate” layer20. The thickness34of this intermediate layer20is preferably selected so as to suppress the composite acoustic reflections.FIGS. 9aand9billustrate a wave pattern at an interface of LT (substrate12) to silicon oxide layer (anti-reflective layer20). At an incident angle of 62° off normal, the transmitted shear propagates downward from the interface at a slight angle. While at an incident angle of 64°, the transmitted shear is evanescent with a slow decay into the oxide layer. From these two illustrations ofFIGS. 9aand9b, it is evident that the critical angle for the shear wave reflection is approximately 63° at the interface between the LT and silicon oxide layer. As illustrated with reference toFIGS. 10aand10b, the critical angles are clearly identified for shear wave transmission from LT to SiO2and shear wave reflection at the interface. By way of example,FIG. 11illustrates a wave pattern at the interface of SiO2(anti-reflective layer20) to the glue (adhesive material22) and clearly illustrates that an intermediate layer of SiO2provides efficiently transmitting acoustic energy at small glancing angles into an acoustic field that steeply propagates downward into the glue.

As a result, one embodiment of the acoustic wave filter10having desirable characteristics as above described comprises the electrode pattern18deposited on a high coupling piezoelectric substrate12as described earlier with reference toFIG. 1. The piezoelectric substrate12may be lithium tantalate or lithium niobate. The piezoelectric substrate12is directly bonded to an anti-reflective coating layer20. The anti-reflective layer20desirably results in an evanescent field that decays slowly with depth. The anti-reflective layer may be comprised of silicon oxide. The silicon oxide layer20is attached to a layer of glue22which is then attached to a carrier substrate24, such as a ceramic including aluminum oxide as herein illustrated by way of example with reference again toFIG. 1. At the interface between the silicon oxide layer20and the glue22, the evanescent field should efficiently convert into a downward propagating field. This results in a substantially reduced reflection of the acoustic energy incident from the LT and upon the lower layers. The cumulative effect is a substantial reduction in the distortions brought about by the spurious plate modes. Additionally, the anti-reflective layer20preferably has an acoustic impedance and/or wave velocity that is between that of the piezoelectric substrate12and that of the adhesive layer22thereby reducing the shear wave reflection. The thickness of the anti-reflective material is sufficient for enhancing an acoustic match between the piezoelectric substrate and adhesive layer.

By way of further example, the carrier substrate24may be the Al2O3as above illustrated, Si, SiO2, or any other material which meets the desirable requirements of a low TCE as compared to the piezoelectric substrate material, sufficient thickness and Young's Modulus so at to produce a compressional strain in the piezoelectric plate as the temperature is increased, and a tensile stress as the temperature is decreased.

By way of example,FIGS. 12aand12bprovide simulation results of a vertical shear BAW coupling and Q factor for various cut angles of LT over different thicknesses of the silicon dioxide layer. The minimums in the coupling and Q factors are representative of the silicon thicknesses where the combinations of reflections at each interface of the silicon dioxide are destructive. When the reflections combined destructively both the effective coupling factor and the Q factor is reduced. The result for a 900 MHz SAW filter resonator shows a substantial reduction in spurious response of the plate modes at periodic intervals of silicon dioxide thicknesses which is given as h(SiO2)≈1.8μ+n*(5.2)μ where n takes on zero and positive integer.FIGS. 13aand13billustrate a resonator response of one preferred embodiment for the 42° and 46° cut angle of LT. As illustrated, the resonator responses exhibit minimum BAW modes.

Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.