Planar-Structure Focusing Lens for Operation at 200 MHz and Its Application to the Reflection-Mode Acoustic Microscope

Abstract

A b s t r a c t I n t h e p r e v i o u s r e p o r t , t h e a u t h o r s proposed a new p l a n a r -s t r u c t u r e a c o u s t i c -m i c r o s c o p e l e n s d e s igned a c c o r d i n g t o t h e t h e o r y of F r e s n e l zone p l a t e , and d e m o n s t r a t e d i t s good f o c u s i n g c a p a b i l i t y . S i n c e t h e l e n s i s composed o f a number o f concent r i c a n n u l a r grooves of uniform d e p t h on i t s r a d i at i o n p l a n e , i t has an advantage o v e r t h e c o n v e n t i on a l a c o u s t i c l e n s w i t h a s p h e r i c a l concave i n b e i n g f a b r i c a t e d e a s i l y u s i n g a p h o t o l i t h o g r a p h i c t e chnique. I n o u r f i r s t model o f t h e p l a n a r l e n s , which had an a p e r t u r e a n g l e of 47.3" and o p e r a t e d a t 100 MHz, t h e grooves c o u l d be formed s u c c e s s f u ll y u s i n g an o r d i n a r y chemical e t c h i n g t e c h n i q u e . However, t h e formation o f f i n e r grooves i s i n e v i t ab l y l i m i t e d by t h e u n d e r c u t t i n g e f f e c t s o f a r as a wet-process i s u s e d . T h e r e f o r e , some advanced p h o t o l i t h o g r a p h i c t e c h n i q u e s h o u l d be employed f o r f a b r i c a t i n g t h e l e n s w i t h a w i d e r a p e r t u r e a n g l e and a h i g h e r o p e r a t i o n f r e q u e n c y . R e c e n t l y , t h e a u t h o r s have i n t r o d u c e d t h e ECR ( e l e c t r o n c y c l o t r o n r e s o n a n c e ) r e a c t i v e i o n e t c h i n g t e c h n i q u e f o r forming f i n e grooves. Using t h i s d r y -p r o c e s s , a p l a n a r l e n s d e s i g n e d f o r an o p e r a t i o n frequency o f 200 MHz and an a p e r t u r e a n g l e o f 90' h a s been o b t a i n e d . I n t h e p r e s e n t r e p o r t , w e d e s c r i b e t h e f a b r i c a t i o n p r o c e s s and some r e s u l t s o f imaging experiments by an a c o u s t i c microscope equipped w i t h t h e p l a n a r lens. 1. I n t r o d u c t i o n I n t h e p r e v i o u s r e p o r t , t h e a u t h o r s demons t r a t e d t h e s a t i s f y i n g f o c u s i n g performance o f t h e new p l a n a r -s t r u c t u r e a c o u s t i c microscope l e n s d e s igned and f a b r i c a t e d f o r o p e r a t i o n a t 100 MHz [I]. The l e n s i s composed of a number o f c o n c e n t r i c a n n u l a r grooves of uniform d e p t h a r r a n g e d on t h e r a d i a t i o n p l a n e a c c o r d i n g t o t h e t h e o r y o f F r e s n e l zone p l a t e [2].

Fabrication and Optimization of High Frequency ZnO Transducers for Both Longitudinal and Shear Emission: Application of Viscosity Measurement using Ultrasound

“…The choice of ZnO as a piezoelectric material can be related to its ease of preparation and its increased scope in high-frequency applications. One can site, for example, the acoustic microscopy applications in 100s MHz to 1s GHz frequency range [14,15]. The ZnO piezoelectric transducer's fabrication starts with the deposition of 10 nm of a titanium layer by cathode sputtering on the surface of a 400 μm thick silicon wafer, followed by deposition of 80 nm of platinum.…”

High frequency acoustic microscopy with Fresnel zoom lens

“…Practically the phased transducer array only works below several ten MHz [3] . Acoustic Fresnel zoom lens [4][5][6][7] seems to be the trade-off of the two ways mentioned above. It not only can work at very high frequency, but also can scan its focus along the depth (i.e.…”