Recent Advances in Shallow Bulk Acoustic Wave Devices

Abstract

I n t h i s paper we discuss the recent advances i n a r e l a t i v e l y new c l a s s o f planar a c o u s t i c dev i c e s employing an a c o u s t i c wave which t r a v e l s j u s t below the surface. These waves a r e r e f e r r e d t o as shallow b u l k a c o u s t i c waves (SBAW's) o r as surface-skimming b u l k waves (SSBW's).I n the l a s t few years t h e r e has been a g r e a t deal o f a c t i v i t y i n t h i s area because o f the inherent advantages which SBAW devices possess over conventional b u l k and s u r f a c e a c o u s t i c wave devices. These devices can be employed as bandpass f i l t e r s and delay l i n e o s c i l l a t o r s , and thus can be incorporated i n t o a v a r i e t y o f s i g n a l processing systems. The mater i a l aspects, e q u i v a l e n t c i r c u i t , energy t r a p p i n g and devices development w i l l be presented i n t h i s overview. . I n t r o d u c t i o nShallow b u l k a c o u s t i c wave o r surface skimning b u l k wave devices u t i l i z e shear h o r i z o n t a l waves which propagate j u s t below the s u r f a c e o f t h e subs t r a t e . They were f i r s t r e p o r t e d by Yen, e t a l . and Lewis, e t a l . i n 1977.1*2 Since then t h e r e has been a g r e a t deal o f a c t i v i t y i n t h i s area because o f the inherent advantages t h a t they possess over conventional b u l k and surface a c o u s t i c wave devices.3-IB bandpass f i l t e r s and o s c i l l a t o r s , and thus can be incorporated i n t o a v a r i e t y o f s i g n a l processing systems. I n t h i s paper we w i l l discuss the recent advances of t h i s r e l a t i v e l y new c l a s s of planar b u l k a c o u s t i c wave devices. a r e the m a t e r i a l aspects, e q u i v a l e n t c i r c u i t , energy t r a p p i n g and devices development. These devices can be employed as Topics t o be discussed 2. SBAW M a t e r i a l s The important c r i t e r i a uses i n s e l e c t i n g SBAW/SSBW m a t e r i a l s a r e : 1. Large p i e z o e l e c t r i c c o u p l i n g t o shear h o r i z o n t a l wave. 2. Zero o r small p i e z o e l e c t r i c c o u p l i n g t o SAW and o t h e r b u l k waves. 3. Zero o r small beam s t e e r i n g b o t h on the surface and i n t o t h e b u l k . 4. Zero o r small temperature c o e f f i c i e n t o f d e l ay. Using these c r i t e r i a , we have concentrated on w i d e l y used SAW m a t e r i a l s : quartz, b e r l i n i t e , LiNbO and LiTaO 3 ' 3 For s i n g l y r o t a t e d Y-cut q u a r t z and b e r l i n i t e , the above c r i t e r i a a r e s a t i s f i e d w i t h wave propag a t i o n normal t o the X-axis. Here, the SAW and o t h e r b u l k waves c o u p l i n g c o e f f i c i e n t a r e i d e n t ic a l l y zero, so t h a t very spurious f r e e responses a r e obtained. I n a d d i t i o n , the shear h o r i z o n t a l wave s u f f e r s no beam s t e e r i n g on the surface, and f o r two small regions near AT-and BT-crystal, b o t h the temperature c o e f f i c i e n ...

Applications of surface acoustic and shallow bulk acoustic wave devices

“…Also, other review papers deal with SAW filters [24], components for electronic warfare receivers [25], SAW-device applications [26]- [29], SAW resonators [30]- [32], SAW materials [33], SAW waveguides [34], SAW waves and acoustoelectric interactions [35], [36], SAW reflective array structures [37], acoustooptic Bragg modulators for integrated optic communications [38], SAW-based fixed and adaptive filtering on GaAs [39], hybrid programmable transversal filters [40], and SAW-based acoustic charge transport (ACT) processes and structures on GaAs [41]. Principles of SBAW propagation and devices are contained in [42] and [43]. A sensor classification scheme is presented in [44].…”

Temperature Compensation of Surface Transverse Waves for Stable Oscillator Applications

“…namely to find temperature compensated gratings for LiN'bO 3 and LiTa'O 3 . These results show that pure STW propagating normal to the X-axis on rotated Y-cut lithium niobate and lithium tantalate can be temperature-compensated with grooves alone.…”

“…By incorporating a shallow grating between transducers it was found that the losses could be reduced to, typically, 10 dB. 3 The grating trapped and guided the wave along the surface and reduced diffraction, which was the cause of the high insertion loss. This effect, which can be explained by a straightforward space harmonic analysis of gratings on isotropic substrates, was exploited in fabricating delay line oscillators operating at frequencies as high as 2 and 3.…”

“…This effect, which can be explained by a straightforward space harmonic analysis of gratings on isotropic substrates, was exploited in fabricating delay line oscillators operating at frequencies as high as 2 and 3. 3 and LiTaO. Since multi-compensation effects are known to occur in SCcut crystals, where stress-compensation also occurs, another interesting direction for this project will be to look for STW cuts and grating geometries having properties similar to those of the SC-cut.…”

Surface Transverse Wave Mode Analysis and Coupling to Interdigital Transducers

“…The trapping, or waveguiding effect, is physically generated by the slowing of the acoustic wave a t the substrate surface. The slowing is generated by three phenomena : (1) surface mass loading, (2) In order to establish notation as well as make the discussion relatively self-contained, the calculation of acoustic modes for a substrate with a free surface will first be described. These solutions will encompass the previously mentioned SSBW modes, which will later be trapped as STW's when the surface boundary conditions are modified to include the effects of a deposited metal grating.…”