Strip Type Resonator of Lithium Tetraborate

FUJITSU LIMITED 1015 Kamikodanaka Nakahara-ku Kawasaki 211 , JAPAN The t h i c k n e s s s h e a r s l o w mode of 51° ro ta ted Y-cut l i t h i u m t e t r a b o r a t e h a s z e r o t e m p e r a t u r e c o e f f i c i e n t o n f i r s t o r d e r , a h igh coup l ing c o e f f i c i e n t o f 26% and v e l o c i t y o f 3240 m/sec. The c o u p l i n g c o e f f i c i e n t o f t h i c k n e s s s h e a r first mode i s z e r o , t h a t o f t h e t h i c k n e s s e x t e n s i o n mode i s 2 0 . 5 % a n d t h e v e l o c i t y i n t h i c k n e s s e x t e n s i o n mode is 6880 m/sec. We h a v e s t u d i e d t h e s t r i p t y p e r e s o n a t o r o f 51° r o t a t e d Y-cut p l a t e . W i t h t h e l o n g i t u d i n a l d i r e c t i o n s e t up p a r a l l e l t o t h e d i s p l a c e m e n t d i r e c t i o n ( p a r a l l e l t o Z ' a x i s ) , t h e s t r i p t y p e r e s o n a t o r o p e r a t e s i n s i n g l e mode. The w i d t h o f t h e s t r i p p l a t e is d e s i g n e d t o s e p a r a t e t h e main mode f rom spur ious modes which a r e g e n e r a t e d by t h e s h e a r mode t r a v e l i n g l a t e r a l l y and r e f l e c t e d a t t h e e d g e o f s t r i p . The opt imum r a t i o o f e l e m e n t w i d t h t o t h i c k n e s s is 4.0. The l e n g t h o f t h e s t r ip p l a t e is d e s i g n e d t o r e a l i z e small s i z e b u t t o k e e p a h igh Q and t o b e f r e e f rom edge r e f l ec t ion . The optimum r a t i o of e l e m e n t l e n g t h t o t h i c k n e s s is about 30. The mechanical Q f a c t o r is more than 10,000 and c a p a c i t a n c e r a t i o is 20. The resonance mode i s f r e e f r o m s p u r i o u s r e s p o n s e i n t h e p r a c t i c a l f requency range . The t e m p e r a t u r e c h a r a c t e r i s t i c curve i s a pa rabo la wi th tu rna round po in t a t room tempera ture and the second order tempera ture c o e f f i c i e n t i s -0.26 ppm/OC2. 1 . I n t r o d u c t i o n E l e c t r o n i c d e v i c e s a r e b e c o m i n g more and more d i g i t i z e d and m i n i a t u r i z e d , g r e a t l y i n c r e a s i n g t h e demand f o r s t a b l e c h i p t y p e r e s o n a t o r s . T h e r e f o r e , m e c h a n i c a l r e s o n a t o r s u s i n g p i e z o e l e c t r i c m a t e r i a l are a t t r a c t i n g a t t e n t i o n d u e t o t h e i r h i g h Q , h igh s t a b i l i t y , and small s i z e . Many p i e z o e l e c t r i c materials, q u a r t z , LiTaO3, LiNb03, Li2B407, and p i e z o e l e c t r i c ce ramics , have been s tud ied fo r u se i n bu lk a c o u s t i c wave dev ices , u sed in the f r equency r ange f r o m s e v e r a l MHz t o s e v e r a l t e n s o f MHz. because of its low t empera tu re coe f f i c i en t and h i g h s t a b i l i t y , b u t it h a s a low electro-mechanical c o u p l i n g f a c t o r , so t h a t i t is d i f f i c u l t t o make a m i n i a t u r e r e s o n a t o r o f q u a r t z . P i e z o e l e c t r i c ceramic i s s u i t a b l e material f o r m i n i a t u r i z e d r e s o n a t o r s b e c a u s e of i ts h i g h c o u p l i n g f a c t o r s Q u a r t z is t h e m o s t u s e f u l m a t e r i a l and low m a t e r i a l c o s t b u t i t s tempera ture s t a b i l i t y is n o t so good . L iTa03 c rys t a l has a p a r a b o l i c t e m p e r a t u r e c h a r a c t e r i s t l c c u r v e w i t h h i g h e r c o u p l i n g coe f f i c i en t t han AT-qua r t z . Tab le tu rna round po in t a t room tempera ture and 20 times 1 l ists t h e r e l e v a n t c h a r a c t e r i s t i c s o f t h i c k n e s s mode r e s o n a t o r s u s i n g v a r i o u s m a t e r i a l s . The newly deve loped l i t h ium t e t r abora t e is an i n t e r e s t i n g m a t e r i a l . L i 2 B 4 0 7 h a s t h e f i r s t o r d e r zero t e m p e r a t u r e c o e f f i c i e n t i n r o t a t e d Y-cut p l a t e [11 [21[31 . On t h e o t h e r h a n d , t h e m i n i a t u r i z a t i o n o f r e s o n a t o r s u s i n g t h i c k n e s s mode has progressed u s i n g t h e p i e z o e l e c t r i c s t r i p t e c h n i q u e [ 4 1 . 'When t h e d i s p l a c e m e n t d i r e c t i o n o f t h e v i b r a t i o n mode i s p a r a l l e l t o t h e l o n g i t u d i n a l d i r e c t i o n o f t h e s t r i p and t h e d r i v i n g e l e c t r o d e s r e a c h t h r o u g h o u t t h e la teral d i r e c t i o n , t h e s p u r i o u s r e s p o n s e caused bv t h e twist ove r tone canno t be g e n e r a t e d . Many p a p e r s r e p o r t e d o n s t r i p t y p e r e s o n a t o r s [51[61[71[81. T h i s p a p e r r e p o r t s t h e t e m p e r a t u r e c h a r a c t e r i s t i c s o f t h i c k n e s s s h e a r s l o w mode O f L i B 0 r e s o n a t o r u s i n g d o u b l e r o t a t i o n Li2B407 resonator and s t u d i e d t h e d i m e n s i o n s of t h e s t r i p t o separate t h e s p u r i o u s r e s p o n s e s f o r enough from the main mode and t o m i n i a t u r i z e t h e s t r i p . We a p p l i e d t h e p i e z o e l e c t r i c s t r ip t o Table 1 R e l e v a n t c h a r a c t e r i s t i c s o f t h i c k n e s s mode r e s o n a t o r u s i n g v a r i o u s m a t e r i a l s . MATERIALS 1 Coupling Temperature factor ( I C ) stabilitv (-20-70°C) . . . . AT-Quartz 1 5 0 pprn 0.61 163"-Y LiNb03 + 30 ppm 0.086 X LiTaOB f 3000 ppm 0.47 2. Tempera ture Coef f ic ien t To a n a l y z e t h e e l e c t r o m e c h a n i c a l c o u p l i n g c o e f f i c i e n t a n d t e m p e r a t u r e c o e f f i c i e n t , we used t h e c l a s s i c a l i n f i n i t e , o n e d i m e n s i o n a l r e s o n a t o r m o d e l a d d i n g t h e e l e c t r i c f i e l d a l o n g t h e n o r m a l d i r e c t i o n o f t h e p i e z o e l e c t r i c p l a t e [ 9 1 . The p l a t e o r i e n t a t i o n o f d o u b l e r o t a t i o n p l a t e is defined by two a n g l e s c$ and e a s shown i n F i g . 1 . 35 1 CH2186-0/85/0000-035l$l.0001985lEEE The resu l t s a re presented i n a l t i t u d e c h a r t s in polar diagrams showing only a quar te r p a r t because Liz8407 is c l a s s 4mm c r y s t a l so t h a t X and Y axes are symmetrical. Electro-mechanical coupling Factor i s shown in F ig . 2 , First order resonance Frequency temperature coefficient in Fig. 3 , and f i rs t order ant i resonance frequency temperature coeff ic ient i n Fig. 4 . The Frequency response of thickness shear slow mode on 51° ro ta ted Y-cut Li2B407 resonator was s ingle mode with high Q , a s shown i n Fig. 5. The resonator was a d isk 8 mm in diameter and 0.25 mm thick with gold electrodes 3 mm i n diameter. This resonator did not generate spurious responses without thickness overtone, as shown in Fig. 6. We confirmed, experimentally, the second order temperature coefficient of rotated Y-cut plate using the above d isk . The resonator had ze ro f i r s t o rde r t empera tu re coe f f i c i en t a t 51' ro ta ted Y-cut p l a t e , and the resonance frequency temperature character is t ic indicated the parabol ic curve as shown i n Fig. 7 . The second order temperature coeff ic ient was -0.26 ppm/OC2 which i s the same order value as For LiTa03.