Abstract:
In accordance with the invention, magnetostrictive saw devices are provided with improved transducer structures for enhanced performance. In one improved device, the transducers are in the form of gratings with interconnected ends for reduced resistance and inductance. In another embodiment, the transducers are shaped to provide apodization. In yet a third embodiment, transducer performance is enhanced by patterning composite structures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/137,431, now U.S. Pat. No. 6,046,657 filed by Glenn B. Alers et al. on Aug. 21, 1998 and entitled “Magnetostrictive Acoustic Wave Device and Microelectronic Circuit Including Same”, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to magnetostrictive surface acoustic wave (SAW) devices and, in particular, to magnetostrictive SAW devices provided with improved transducers for enhanced performance. 
     BACKGROUND OF THE INVENTION 
     SAW devices are important components in RF circuits, especially in wireless communication devices. SAW devices are particularly important as delay elements. They provide low-velocity, non-dispersive propagation with low attenuation up to microwave frequencies and a propagation path which is accessible at a substrate surface. 
     While conventional SAW devices are not readily integrated into silicon microelectronic circuits, applicants&#39; above-referenced U.S. application Ser. No. 09/137,431 describes magnetostrictive SAW devices which can be fabricated on silicon substrates. These devices can be integrated with microelectronic circuits useful in wireless communications. 
     In essence, a magnetostrictive SAW device comprises a substrate, a film of an appropriate magnetostrictive material disposed on the substrate, an input transducer for generating horizontally polarized shear waves along the film and an output transducer for receiving the shear waves. The substrate can comprise silicon and include one or more microelectronic circuit elements interconnected with the magnetostrictive SAW device. The transducers are typically serpentine (meander-type) electrodes. 
     The present invention provides devices of this type with improved transducer structures for enhanced performance. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, magnetostrictive saw devices are provided with improved transducer structures for enhanced performance. In one improved device, the transducers are in the form of gratings with interconnected ends for reduced resistance and inductance. In another embodiment, the transducers are shaped to provide apodization. In yet a third embodiment, transducer performance is enhanced by patterning composite structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments described in connection with the accompanying drawings. In the drawings: 
     FIG. 1 is a perspective view of a SAW device using conventional transducers. 
     FIGS.  2 ( a ) and  2 ( b ) are schematic views of improved transducers having comb or interconnected grid configurations; 
     FIGS.  3 ( a ) and  3 ( b ) are schematic views of improved transducers for an apodized SAW device; and 
     FIGS.  4 ( a )  4 ( b ) and  4 ( c ) are schematic cross sections of improved patterned layer transducers. 
    
    
     It should be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
     DETAILED DESCRIPTION 
     Referring to the drawings, FIG. 1 is a perspective view of a magnetostrictive SAW device  10  using conventional transducers  18  and  22 . The device  10  typically comprises a substrate  12  coated with a thin film  14  of magnetostrictive material followed by a film  16  of insulating material. An input magnetoelastic transducer  18 , typically in the form of a serpentine conductor, is disposed on a first region of insulating film  16 , and a similar output transducer  22  is disposed on a second region of film  16  for receiving acoustic waves propagated from input transducer  18 . The transducers each comprise a conductor with periodic discrete excursions perpendicular to the direction of surface wave propagation. 
     Application of an oscillatory electrical signal to the ends of input transducer  18  generates, through the magnetostrictive response of film  14 , shear acoustic waves in both film  14  and substrate  12 . These waves propagate within both film  14  and substrate  12  from the first region to the second region where they produce a corresponding oscillatory electrical signal in output transducer  22 . 
     In typical devices, the thin film  14  of magnetostrictive material has a thickness in the range 0.1-1.0 μm and the insulating film  16  has a thickness of about 0.1-1.0 μm. Suitable magnetostrictive materials include polycrystalline ferromagnetic alloys such as Ni x Fe 1-x , preferably with 0.4&lt;×&lt;0.6 or 0.15&lt;×&lt;0.25 and Co x Fe 1-x , preferably with 0.3&lt;×&lt;0.7; amorphous ferromagnetic alloys of Fe and/or Co with early transition metals such as Co 1-x-y Ta x Zr y , preferably with 0.01&lt;×&lt;0.15 and 0.01&lt;y&lt;0.15, and Co 1-x-y Nb x Zr y , preferably with 0.01&lt;×&lt;0.15 and 0.01&lt;y&lt;0.15; amorphous ferromagnetic alloys of Fe and/or Co with metalloids such as Fe 1-x-y Co x P y , preferably with 0.01&lt;×&lt;0.7 and 0.05&lt;y&lt;0.2; and rare-earth transition-metal ferromagnetic alloys such as Tb x Dy 1-x Fe y , preferably with 0.2&lt;×&lt;0.3 and 1.9&lt;y&lt;2.1. Ferromagnetic oxides, such as NiFe 2 O 4 , (NiZn)Fe 2 O 4 , and Fe 3 O 4 , can also be used for the thin film  14 . Since these oxides are insulating, they permit omission of the insulating film  16 . 
     In the SAW device described in the parent application Ser. No. 09/137,431, the substrate  12  comprises a silicon substrate including at least one microelectronic circuit  23 , and the in put transducer, the output transducer or both are coupled to the microeletronic circuit. In such arrangements, the SAW device  10  provides non-dispersive, low-loss delay particularly useful in wireless communication circuits. Further details concerning the structure and operation are set forth in the above-identified application. 
     The present invention is directed to SAW devices provided with improved transducers for enhanced performance. FIGS.  2 ( a ) and  2 ( b ) are schematic views of two different improved transducers having a comb configuration. As distinguished from a conventional serpentine transducer, a comb structure transducer, comprises a grid (array) of parallel electrodes, and the respective ends of the electrodes are interconnected. Thus, for example, the improved transducer  20  of FIG.  2 ( a ) comprises a plurality of parallel conductive grid conductors  25 A and  25 B, . . . ,  25 E extending transversely between longitudinal conductors  24 A and  24 B. Thus the top ends of the grid electrodes are connected by  24 A and the bottom ends by  24 B. The grid conductors are transverse to the direction of acoustic wave propagation. In the unchirpped transducer embodiment of FIG.  2 ( a ), the grid lines are equally spaced by a distance d which is advantageously given by: 
     
       
         d=ν/f  (Eq. 1) 
       
     
     where ν is the acoustic velocity of the substrate and f is the desired resonant frequency of the device. 
     FIG.  2 ( b ) illustrates a second comb structure transducer  21  adapted to produce a magnetostrictive SAW device apodized by chirping. Here the spacing x n  between consecutive transverse conductive grid lines is a function of the distance x along the propagation direction. Advantageously x n  varies linearly with x. The advantage of the comb structure transducer, as compared with the conventional serpentine transducer, is substantially reduced resistance and inductance. 
     The device can be apodized by varying the transducer grid conductor spacing x n  with x (as shown in FIG.  2 B), by varying the element length A n , or by varying both x n , and A n . The frequency response of the device, denoted by R c (f), is given by:                  R   c          (   f   )       =       ∑     n   =       -     (     N   -   1     )       /   2           +     (     N   -   1     )       /   2                         A   n          exp        (       -   2        π                 j          fx   n     v       )                   (Eq.  2)                                
     Where ƒ is frequency, and N is the number of elements in the transducer, and R c  is given in arbitrary units. This expression is approximate. It ignores second-order effects, such those caused by reflection of the acoustic wave, interactions between elements, and the finite size of the elements. 
     FIGS.  3 ( a ) and  3 ( b ) are schematic views of alternative transducers for apodized SAW devices. FIG.  3 ( a ) shows an apodized transducer  30  comprising a serpentine conductor  31  with periodic excursions transverse to the direction x of wave propagation. In contrast with the conventional serpentine transducers, the amplitudes A n  of the periodic excursions vary as a function of x. The excursion amplitudes start off large, diminish with increasing x and then again become large. 
     FIG.  3 ( b ) shows a comb structure transducer  32  apodized by variation of element length. Here the longitudinal conductors  34 A and  34 B are no longer parallel but rather extend generally in the direction x of wave propagation. The transverse conductive grid lines  35 A,  35 B, . . . ,  35 D are preferably equally spaced, but their lengths A n  vary with x in a manner similar to that shown in FIG.  3 ( a ). 
     FIGS.  4 ( a ),  4 ( b ) and  4 ( c ) are schematic cross sections of improved transducer structures utilizing multiple configured layers. In the FIG.  4 ( a ) transducer  40  the insulating and magnetostrictive films  16 ,  14 , rather than continuously covering the substrate surface, are patterned similar to the configuration of the overlying conductor  41 . Patterning the magnetostrictive film is advantageous in that it reduces direct coupling between transducers. Such direct coupling, either capacitive or through a direct ohmic contact, is undesirable in that it reduces the device&#39;s ability to filter out unwanted frequency components. Furthermore, such patterning, in combination with an insulating substrate, eliminates the need for the insulating layer  16 . 
     FIG.  4 ( b ) shows an alternative transducer  42  wherein not only are the insulating and magnetostrictive films patterned, but also a film of magnetic material  43  overlies the transducer conductors and is patterned in a configuration similar to the conductors. The magnetic film and the transducer conductors can be patterned together using conventional photolithographic techniques. 
     FIG.  4 ( c ) is an alternative transducer  44  where the magnetic material  43  and the magnetostrictive material  14  essentially surround the transducer conductor  41 . Here the conductor is patterned first, then the overlying magnetic film is deposited and patterned. 
     The overlying magnetic layer  43  is advantageously a soft magnetic material having a relatively small uniaxial anisotropy. The anisotropy should be as low as possible consistent with a ferromagnetic resonance frequency above the desired frequency of operation. The overlying magnetic layer  43  can be a magnetostrictive material to increase the magnetostrictive response. 
     In the embodiments of FIGS.  4 ( b ) and  4 ( c ), the overlying magnetic film  43  reduces the magnetic reluctance of the magnetic circuit surrounding the conductor, thereby increasing the flux level in the magnetostrictive material and thus the magnetostrictive response. 
     The invention can now be better understood by consideration of the following specific examples: 
     EXAMPLE 1 
     An exemplary device of the type shown in FIG.  4 ( a ) can comprise an insulating a substrate  12  of silicon, a magnetostrictive layer  14  of sputtered CoFeTaZr (46%, 46%, 3%, 5%) alloy (CFTZ) (typically about 250 nm thick) an insulating layer  16  of sputtered SiO2 (typically about 500 nm thick), and a conductive layer of evaporated Al (typically 1 micrometer). 
     EXAMPLE 2 
     An exemplary device of the type shown FIG.  4 ( b ) can comprise substrate  12  of borosilicate glass (typically 0.4 mm), magnetostrictive layers  14  and  43  of sputtered CFTZ (each typically 250 nm) and a conductive layer  41  of sputtered copper (typically 500 nm). No insulating layer  16  need be used. 
     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention.