Transduction and propagation of Surface Acoustic Waves in three-layered media with an electrically conductive substrate

Transduction and propagation

of Surface Acoustic Waves

in three-layered media

with an electrically

conductive substrate

Ui I — Np O - J UJ CD - J

Transduction and propagation of Surface Acoustic Waves

in three-layered media with an electrically conductive substrate BIBLIOTHEEK TU Delft P 1637 5278 474858

of Surface Acoustic Waves

in three-layered media

with an electrically

conductive substrate

PROEFSCHRIFT

Ter verkrijging van de graad van doctor in de technische wetenschappen aan de Technische Hogeschool te Delft, op gezag

van de rector magnificus, voor een commissie aangewezen door het college van dekanen, te verdedigen op donderdag 13 november 1980 te 14.00 uur.

door

Adrian Venema

elektrotechnisch ingenieur geboren te Cimahi (Indonesië)

f V

81

0 0 0 1 6 8

poa.ENsnuQi

Dit proefschrift is goedgekeurd door de promotor prof.dr.ir. S. Middelhoek

Aan mijn Moeder Aan Jaap en Apollien

Aan Toon Aan Vim en Mare

6

CONTENTS

1. INTRODUCTION 8

1.1. Statement of the problem 8

1.2. Outline of a SAW delay l i n e 9

1.2.1. Homogeneous piezoelectric media 11

1.2.2. Two-layered media 12 1.2.3. Three-layered media 14

References 17

2. SAW-DEVICE TECHNOLOGY OF A CDS-SI02-SI LAYERED MEDIUM 19

2.1. Introduction 19 2.2. S i l i c o n oxidation and lumped network approximation

of s i l i c o n d i o x i d e - s i l i c o n interface properties 22

2.3. Interdigital metal-pattern fabrication 44

2.4. Substrate and wire bonding 46

2.5. Piezoelectric cadmium-sulfide overlay technology 48 2.5.1. Deposition and properties of piezoelectric

cadmium s u l f i d e 48

2.5.2. Etching of piezoelectric overlay 56

2.6. Packaging 57

References 58

3. PROPAGATION OF SAW OVER A THREE-LAYERED MEDIUM 60

3.1. Introduction 60

3.2. Basic equations 61

3.3. Boundary conditions 66

hi)

3.4. The influence of crystal symmetry on VuJ 70

3.5. Numerical results 75

3.5.1. Phase velocity, displacement and e l e c t r i c

-f i e l d amplitudes 75 3.5.2. Piezoelectric coupling coefficient 93

4 . TRANSDUCTION OF SAW IN A THREE-LAYERED MEDIUM 110

4 . 1 . Introduction n o 4 . 2 . Equivalent c i r c u i t for SAW transduction i l l

4 . 3 . Interdigital transducer-admittance calculations 114

4 . 3 . 1 . Methods of calculating the interdigital

transducer admittance 115

4 . 3 . 2 . The influence of a nonconducting depletion

layer on the admittance Y. 141

° %m 4 . 3 . 3 . The influence of surface states on the

admittance Y. 145

vn

4 . 4 . Interdigital-transducer admittance partitioning 154

4 . 4 . 1 . S'ummary of parameters for 158

4 . 5 . Equivalent-circuit applications 160

4 . 5 . 1 . The influence of Q. R and C on the

J J ' c e transducer performance 174 4 . 5 . 2 . Non-inductive matching 181 References 182 5. EXPERIMENTAL RESULTS 183 5 . 1 . Introduction 183 5 . 2 . Presentation of experimental results 183

5 . 3 . Discussion and conclusions 192

Reference 194 Appendices 195

A. Surface charge Qg($s) and space-charge

capacitance C ) 195 B. Single-level surface-state admittance Y$ and

continuum of surface-state admittance Ys s 200

C. Material constants 207 L i s t of symbols 21 o

Summary 216 Samenvatting 218 Acknowledgement 220

8

1. INTRODUCTION

1.1. Statement of the problem

I n t h i s t h e s i s t h e t r a n s d u c t i o n and p r o p a g a t i o n o f s u r f a c e a c o u s t i c waves o v e r t h e s u r f a c e o f t h r e e - l a y e r e d media w h i c h have an e l e c t r i c a l l y c o n d u c t i v e s i l i c o n s u b s t r a t e a r e e x p e r i m e n t a l l y and t h e o r e t i c a l l y i n v e s t i g a t e d . The e x p e r i m e n t a l d e v i c e s were f a b r i c a t e d w i t h the use o f t h e p l a n a r s i l i c o n t e c h n o l o g y . D e s i g n c r i t e r i a a r e e v a l u a t e d f o r t h e s e s u r f a c e a c o u s t i c wave d e v i c e s o f w h i c h o n l y t h e p a s s i v e t y p e s a r e c o n s i d e r e d .

At p r e s e n t , and f o r the coming d e c a d e s , e l e c t r o n i c d e v i c e s r e a l i s e d as i n t e g r a t e d c i r c u i t s a r e the most i m p o r t a n t components f o r a system d e s i g n e r . T h i s s i t u a t i o n has been made p o s s i b l e by t h e p l a n a r s i l i c o n t e c h n o l o g y . The need t o o b t a i n the h i g h e s t p o s s i b l e number o f a n a l o g o r d i g i t a l n e t w o r k f u n c t i o n s p e r u n i t s u r f a c e a r e a w i t h i n a d i s t a n c e o f t h e o r d e r o f 10 pm from t h e s i l i c o n s u r f a c e w i t h maximum r e p r o d u c i b i l i t y has pushed p l a n a r s i l i c o n t e c h n o l o g y t o a v e r y s o p h i s t i c a t e d l e v e l . M o r e o v e r , an i n c r e a s i n g i n t e r e s t i s found i n t h e i m p l e m e n t a t i o n o f t r a n s d u c e r f u n c t i o n s i n a s i l i c o n e n v i r o n m e n t , p r o v i d e d t h a t t h e f u n c t i o n s a r e r e a l i s a b l e w i t h the p h y s i c a l p r o p e r t i e s o f t h e s i l i c o n . The t r a n s d u c e r as a s i l i c o n d e v i c e can be j o i n e d on a c e r a m i c s u b s t r a t e w i t h o t h e r i n t e g r a t e d c i r c u i t s u s i n g h y b r i d b o n d i n g t e c h n i q u e s o r can be f u l l y i n t e g r a t e d w i t h t h e a u x i l i a r y e l e c t r o n i c b i p o l a r o r MOS c i r c u i t s on one s i l i c o n c h i p . A t r a n s d u c e r i s used t o c o n v e r t n o n e l e c t r i c a l energy i n t o e l e c t r i c a l energy o r v i c e v e r s a . S i g n a l p r o c e s s i n g i s then u s u a l l y p e r f o r m e d by e l e c t r o n i c i n s t r u m e n t a t i o n t e c h n i q u e s . I n s t e a d o f b e i n g c o n s i d e r e d s o l e l y as a means o f i d e n t i f y i n g a n o n e l e c t r i c a l s i g n a l , a t r a n s d u c e r can have o t h e r i n t e r e s t i n g f e a t u r e s as soon as t r a n s d u c t i o n o f a s i g n a l and i t s s i m u l t a n e o u s m o d u l a t i o n w i t h i n the b o u n d a r i e s o f t h e t r a n s d u c e r d i m e n s i o n s a r e p o s s i b l e . The t r a n s d u c t i o n o f an e l e c t r i c a l i n p u t s i g n a l i n t o an a c o u s t i c wave ( m e c h a n i c a l s i g n a l ) , w h i c h i n c l u d e s s i g n a l p r o c e s s i n g o f t h e i n p u t s i g n a l and where e v e n t u a l a d d i t i o n a l s i g n a l h a n d l i n g o c c u r s d u r i n g t h e p r o p a g a t i o n o f t h e wave t h r o u g h a f r e q u e n c y - d i s p e r s i v e l a y e r e d medium, i s r e a l i s a b l e by s u r f a c e a c o u s t i c wave f i l t e r s .

^ u r f a c e a c o u s t i c waves (SAW) are m e c h a n i c a l d i s t u r b a n c e s t h a t p r o p a g a t e o v e r t h e s t r e s s - f r e e s u r f a c e o f an e l a s t i c h a l f s p a c e . I t t h e r e f o r e comes as no s u r p r i s e t o f i n d t h a t t h e s e waves have t h e i r r o o t s i n s e i s m o l o g y . P h y s i c a l l y ,

s u r f a c e a c o u s t i c waves were f i r s t r e c o g n i s e d and c a t a l o g u e d by L o r d R a y l e i g h [ 1 . 1 ] a t the t u r n of the l a s t c e n t u r y . Subsequent g e o p h y s i c i s t s i n v e s t i g a t e d t h e s e wave types i n d e t a i l . As a consequence, a c o n s i d e r a b l e body of

l i t e r a t u r e on t h e n a t u r e of s u r f a c e a c o u s t i c waves [ 1 . 2 , 1.3, 1.4] e x i s t s . The knowledge of s u r f a c e a c o u s t i c waves s t r e t c h e s back i n t o h i s t o r y . What i s new i s t h e i r usage i n m i c r o e l e c t r o n i c s .

A c o u s t i c waves ( o r e l a s t i c waves) are t y p i c a l l y f i v e o r d e r s of m a g n i t u d e s l o w e r than t h e i r e l e c t r o m a g n e t i c c o u n t e r p a r t s , a p r o p e r t y t h a t has been e x p l o i t e d i n a c o u s t i c d e l a y l i n e s f o r a l o n g t i m e . However, s u r f a c e a c o u s t i c waves and, i n p a r t i c u l a r , R a y l e i g h waves r a t h e r than o t h e r wave t y p e s , a r e the o b v i o u s c h o i c e f o r m i c r o e l e c t r o n i c a p p l i c a t i o n s i n v o l v i n g the p r o c e s s i n g o f s i g n a l s . The p r i n c i p a l r e a s o n f o r t h i s c h o i c e i s p r e c i s e l y t h a t w h i c h has l e d to the p l a n a r s i l i c o n t e c h n o l o g y , t h a t one can g e t t o the s u r f a c e of the s o l i d .

In a r e m a r k a b l y s h o r t p e r i o d of t i m e , components u t i l i s i n g SAW have made a major impact on the e l e c t r o n i c s market w o r l d w i d e . T h i s i s because SAW d e v i c e s r e a l i s e a n a l o g and d i g i t a l s i g n a l p r o c e s s i n g f u n c t i o n s t h a t a r e d i f f i c u l t to a c h i e v e by o t h e r means. One of the p r i m e movers i n t h i s e x p l o s i o n of i n t e r e s t i n SAW d e v i c e s was the d i s c o v e r y of the i n t e r d i g i t a l t r a n s d u c e r

(IDT) i n 1965 [ 1 . 5 ] .

In the subsequent s e c t i o n s of t h i s c h a p t e r a summary i s g i v e n of the a p p l i c a b i l i t y of an IDT i n a SAW d e l a y l i n e ( s e c t i o n 1.2), and of SAW p r o p a g a t i o n o v e r homogeneous p i e z o e l e c t r i c media ( s e c t i o n 1.2.1) and o v e r two-and t h r e e - l a y e r e d media ( s e c t i o n s 1.2.2 two-and 1.2.3).

1.2. Outline of a SAW delay line

In t h i s s e c t i o n a s h o r t d e s c r i p t i o n i s g i v e n of a SAW d e l a y l i n e f a b r i c a t e d on the h i g h l y p o l i s h e d p l a n e boundary of a homogeneous p i e z o -e l -e c t r i c m-edium. Th-e SAW d -e l a y l i n -e c o n s i s t s of two u n i f o r m IDTs s -e p a r a t -e d by a p r o p a g a t i o n p a t h i n w h i c h the g e n e r a t e d s u r f a c e a c o u s t i c wave a c h i e v e s i t s d e l a y t i m e .

The IDT i s a p l a n a r i n t e r l e a v e d m e t a l e l e c t r o d e s t r u c t u r e of w h i c h a d j a c e n t e l e c t r o d e s a r e g i v e n e q u a l b u t o p p o s i t e p o t e n t i a l s and w h i c h r e l i e s on the p i e z o e l e c t r i c e f f e c t t o c o u p l e d i r e c t l y e l e c t r i c a l and m e c h a n i c a l energy. The s p a t i a l l y p e r i o d i c e l e c t r i c IDT f i e l d p r o d u c e s u l t i m a t e l y a c o r r e s p o n d i n g p e r i o d i c m e c h a n i c a l s t r a i n p a t t e r n , w h i c h g i v e s r i s e t o a s u r f a c e a c o u s t i c wave, p r o v i d e d the s u r f a c e i s s t r e s s f r e e . Because the

1 0

t r a n s d u c t i o n p r o c e s s i s r e c i p r o c a l the IDT i s used f o r b o t h g e n e r a t i o n and d e c t e c t i o n . The IDT can be c o n s i d e r e d as a t h r e e p o r t w i t h two a c o u s t i c a l p o r t s and one e l e c t r i c a l p o r t ; the i n p u t power i s d i v i d e d i n t o two e q u a l p a r t s .

A SAW d e l a y l i n e w i t h u n i f o r m IDTs, as shown i n F i g . 1.1.a i s c o n s i d e r e d to be the b a s i c element of most SAW d e v i c e s .

Fig. 1.1. a. SAW delay line with uniform three fingerpair-IDTs on a homogeneous piezoelectric substrate. b. Interdigital metal pattern of a uniform IDT with period p and aperture A.

A u n i f o r m IDT has e q u a l e l e c t r o d e w i d t h and space between two a d j a c e n t

e l e c t r o d e s and a c o n s t a n t e l e c t r o d e o v e r l a p ( a c o u s t i c a p e r t u r e A) ( F i g . l . l . b ) . F o r a p a r t i c u l a r s i g n a l f r e q u e n c y ƒ where the w a v e l e n g t h A of the SAW .equals the p e r i o d p of the IDT, the r e i n f o r c e m e n t of the i n d i v i d u a l s t r a i n

c o n t r i b u t i o n s of each f i n g e r p a i r i s m a x i m a l . The s p l i t t i n g of power i n t o e q u a l p a r t s r e s u l t s i n an i n h e r e n t minimum 3 dB t r a n s d u c e r c o n v e r s i o n l o s s a t ƒ . A m i n i m a l i n s e r t i o n l o s s of 6 dB i s found f o r the d e l a y l i n e of F i g . 1.1.a. For f r e q u e n c i e s d i f f e r i n g from f the i n t e r f e r e n c e i s l e s s c o n s t r u c t i v e , r e s u l t i n g i n an i n c r e a s i n g c o n v e r s i o n l o s s . A bandpass t r a n s f e r f u n c t i o n f o r an IDT and SAW d e l a y l i n e i s a c h i e v e d . The a c o u s t i c a p e r t u r e A d e f i n e s the e f f e c t i v e r e g i o n of t r a n s d u c t i o n between two a d j a c e n t e l e c t r o d e s . M a t c h i n g n e t w o r k s between t h e t r a n s d u c e r and the s o u r c e and the t r a n s d u c e r and the l o a d

a l l o w f o r t h e o p t i m a l t r a n s f e r o f s i g n a l energy.

1.2.1. Homogeneous piezoelectric media

I n h e r e n t i n i t s p i e z o e l e c t r i c p r o p e r t i e s , a s i n g l e c r y s t a l medium has an a n i s o t r o p i c n a t u r e . A s u r f a c e wave mode known as t h e g e n e r a l i s e d R a y l e i g h wave [ 1 . 6 , 1.7] c a n p r o p a g a t e o v e r t h e s t r e s s - f r e e s u r f a c e o f a p i e z o e l e c t r i c e l a s t i c h a l f s p a c e .

G e n e r a l l y , t h e mode has t h r e e p a r t i c l e ( m e c h a n i c a l ) d i s p l a c e m e n t s , w h i c h a r e c o n f i n e d t o a l a y e r a p p r o x i m a t e l y one w a v e l e n g t h t h i c k from t h e s u r f a c e of the e l a s t i c s o l i d , and w h i c h have a damped o s c i l l a t o r y a m p l i t u d e d e c a y . The d i r e c t i o n o f wave power f l o w i s n o t n e c e s s a r i l y c o l l i n e a r w i t h t h e wave v e c t o r .

I f i n t h e c r y s t a l t h e s a g i t t a l p l a n e i s one of m i r r o r symmetry o r i s p e r p e n d i c u l a r t o an a x i s o f t w o f o l d r o t a t i o n , pure-mode axes a r e p r o d u c e d , and the power f l o w w i l l be c o l l i n e a r w i t h t h e wave v e c t o r . F o r m i r r o r symmetry t h e p a r t i c l e d i s p l a c e m e n t s a r e c o n f i n e d t o t h e s a g i t t a l p l a n e and c o u p l e d t o t h e e l e c t r i c f i e l d o f t h e wave w h i l s t t h e h o r i z o n t a l p a r t i c l e d i s p l a c e m e n t i s u n c o u p l e d from t h e e l e c t r i c f i e l d and s a g i t t a l p l a n e d i s p l a c e m e n t s . A s t i f f e n e d R a y l e i g h wave t h e n p r o p a g a t e s . I n t h e c a s e o f t h e a x i s o f t w o f o l d r o t a t i o n t h e s a g i t t a l p a r t i c l e d i s p l a c e m e n t i s u n c o u p l e d from t h e h o r i z o n t a l d i s p l a c e m e n t w h i c h i s c o u p l e d t o t h e e l e c t r i c f i e l d . T h i s i s a B l e u s t e i n - G u l y a e v wave, and shows a l a r g e r p e n e t r a t i o n i n t o , t h e s o l i d . W h i l e t h e g e n e r a l i s e d R a y l e i g h wave has i t s c o u n t e r p a r t i n n o n p i e z o e l e c t r i c m e d i a , t h e B l e u s t e i n - G u l y a e v wave does n o t .

C e r t a i n c o m b i n a t i o n s o f p l a n e s o f o r i e n t a t i o n and d i r e c t i o n s o f p r o p a g a t i o n i n t h e p l a n e g i v e r i s e t o p s e u d o - s u r f a c e waves. I n t h a t c a s e s u r f a c e wave energy i s r a d i a t e d i n t o t h e b u l k by means o f s h e a r waves.

I f m e c h a n i c a l d i s s i p a t i o n c a n be i g n o r e d [ 1 . 7 ] t h e n t h e phase v e l o c i t y o f SAW i s a c o n s t a n t f o r one d i r e c t i o n o f p r o p a g a t i o n i n a c r y s t a l c u t .

SAW, w h i l e p r o p a g a t i n g over an e l a s t i c s o l i d , a r e prone t o a t t e n u a t i o n

[ 1 . 8 ] . T h i s i s caused b y :

( i ) s c a t t e r i n g from s u r f a c e i m p e r f e c t i o n s ( s c r a t c h e s , p o l i s h i n g d e f e c t s ) ( i i ) s c a t t e r i n g from c r y s t a l d e f e c t s ( d i s l o c a t i o n s and g r a i n b o u n d a r i e s

i n p o l y c r y s t a l 1 i n e m a t e r i a l s )

( i i i ) i n t e r a c t i o n s w i t h t h e r m a l phonons ( s u r f a c e and b u l k phonons) ( i v ) i n t e r a c t i o n s w i t h c h a r g e c a r r i e r s i n m e t a l s and s e m i c o n d u c t o r s . An e x p r e s s i o n f o r t h e a t t e n u a t i o n o f a g e n e r a l i s e d R a y l e i g h wave p r o p a g a t i n g over an a n i s o t r o p i c medium has been d e v e l o p e d by K i n g and Sheard

12

[1.9] i n terms of a v i s c o s i t y t e n s o r .

F l e x i b i l i t y i n SAW component d e s i g n i s o b t a i n e d by the i n t r o d u c t i o n of an e q u i v a l e n t c i r c u i t model. A w i d e l y a c c e p t e d e q u i v a l e n t c i r c u i t model was p r e s e n t e d by S m i t h e t a l . i n 1969 [ 1 . 1 0 ] . I t r e q u i r e s a knowledge of the p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t of t h e m a t e r i a l and of the IDT s t a t i c c a p a c i t a n c e . The p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t i s a measure f o r the e f f i c i e n c y of the c o n v e r s i o n of e l e c t r i c a l energy i n t o a c o u s t i c a l energy and v i c e v e r s a [ 1 . 1 1 , 1.12], Under the same c o n d i t i o n s as t a k e n f o r the phase v e l o c i t y , the p i e z o e l e c t r i c c o u p l i n g i s a c o n s t a n t . The IDT s t a t i c c a p a c i t a n c e has been c a l c u l a t e d by b o t h Engan [ 1 . 1 3 ] and F a r n e l l e t a l . [ 1 . 1 4 ] . F o r most p i e z o e l e c t r i c media c u r r e n t l y i n use t h e p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t and o t h e r r e l e v a n t d e s i g n p a r a m e t e r s a r e g i v e n by S l o b o d n i k [ 1 . 1 5 ] .

A v a s t body of l i t e r a t u r e e x i s t s [ 1 . 1 6 ] c o n c e r n i n g the a d a p t i o n of the IDT geometry f o r the r e a l i s a t i o n of s p e c i f i c t r a n s f e r f u n c t i o n s by a p e r t u r e a p o d i s a t i o n and/or v a r i a t i o n of the IDT p e r i o d i c i t y . A p r o b l e m o f t e n found i n SAW d e v i c e d e s i g n i s r e l a t e d t o the d i f f r a c t i o n of the a c o u s t i c beam (beam s p r e a d i n g ) caused by the f i n i t e a c o u s t i c a p e r t u r e of the IDT. A n i s o t r o p y c o n s i d e r a b l y c o m p l i c a t e s t h i s d i f f r a c t i o n p r o c e s s and i n a d d i t i o n , causes beam s t e e r i n g [ 1 . 1 7 ] . N o n l i n e a r p r o c e s s i n g i s r e a l i s e d by the n o n l i n e a r i n t e r a c t i o n of two c o u n t e r p r o p a g a t i n g SAW of s u f f i c i e n t l y h i g h power l e v e l by the anharmonic m o t i o n of t h e l a t t i c e of the p i e z o e l e c t r i c s u b s t r a t e . A s t r o n g e r n o n l i n e a r i t y i s o b t a i n e d when SAW can be c o u p l e d to c h a r g e c a r r i e r s i n a nearby s e m i c o n d u c t o r [ 1 . 1 8 ] . 1.2.2. Two-layered media The b a s i c c o m p o s i t i o n of a t w o - l a y e r e d medium i s t h a t of a s u b s t r a t e ( h a l f s p a c e ) c o v e r e d w i t h an o v e r l a y m a t e r i a l . S u b s t r a t e and o v e r l a y can b o t h be p i e z o e l e c t r i c or n o n p i e z o e l e c t r i c or the s u b s t r a t e i s n o n p i e z o e l e c t r i c w i t h a p i e z o e l e c t r i c o v e r l a y or v i c e v e r s a . S u r f a c e a c o u s t i c wave p r o p a g a t i o n i n p i e z o -e l -e c t r i c t w o - l a y -e r -e d m-edia i s d -e s c r i b -e d i n d -e t a i l by F a r n -e l l and A d l -e r [ 1 . 1 9 ] , f o r the c a s e of n o n c o n d u c t i v e l i n e a r e l a s t i c homogeneous m a t e r i a l s . R a y l e i g h , S t o n e l e y and Lamb waves a r e d i s c u s s e d . A l l t h e s e wave t y p e s and t h e i r h i g h e r modes have a common p r o p e r t y , the SAW o r p l a t e wave v e l o c i t y i s f r e q u e n c y d i s p e r s i v e as i s the p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t . Freedom o f m a t e r i a l c h o i c e f o r s u b s t r a t e and o v e r l a y was the main s t i m u l u s f o r i n v e s t i g a t i o n s i n SAW t r a n s d u c t i o n and p r o p a g a t i o n i n t w o l a y e r e d media. The use of p o l y -c r y s t a l l i n e l a y e r s and amorphous s u b s t r a t e s w i l l g e n e r a l l y l e a d t o a h i g h e r

SAW a t t e n u a t i o n [ 1 . 8 ] . C r y s t a l l i n e s u b s t r a t e s such as s a p p h i r e ( A ^ O ^ ) a r e t h e r e f o r e recommended, a l t h o u g h beam s t e e r i n g e f f e c t s can be i n t r o d u c e d .

A n o t h e r c o n t r i b u t i o n to l o s s of s i g n a l power i s due to the f r e q u e n c y d i s p e r s i o n i n wave v e l o c i t y w h i c h may cause a c o r r e s p o n d i n g d i s t r i b u t i o n i n s i g n a l power such t h a t an u n d e s i r a b l e shape of the a c o u s t i c r e s p o n s e o c c u r s .

An i m p o r t a n t c l a s s of t w o - l a y e r e d media f o r p r a c t i c a l p u r p o s e s i s the c o m b i n a t i o n of a n o n p i e z o e l e c t r i c s u b s t r a t e w i t h a p i e z o e l e c t r i c o v e r l a y [ 1 . 2 0 , 1.21]. The p i e z o e l e c t r i c o v e r l a y m a t e r i a l i s u s u a l l y found amongst the I I - V I (ZnO, ZnS, CdS) or I I I - V (A1N, GaN) compounds. The s u b s t r a t e m a t e r i a l can be c r y s t a l l i n e A^O^ ( s a p p h i r e ) or amorphous ( A ^ O ^ , g l a s s , MgA^O^

( s p i n e l ) ) .

I n t w o - l a y e r e d media SAW can be g e n e r a t e d and d e t e c t e d by means of an IDT, under the c o n d i t i o n of a s t r e s s - f r e e s u r f a c e , i n f o u r p o s s i b l e t r a n s d u c e r c o n f i g u r a t i o n s [ 1 . 2 2 , 1.23]. F o r each t r a n s d u c e r c o n f i g u r a t i o n the p i e z o -e l -e c t r i c c o u p l i n g c o -e f f i c i -e n t has a c h a r a c t -e r i s t i c f u n c t i o n a l r -e l a t i o n s h i p t o o v e r l a y t h i c k n e s s and SAW w a v e l e n g t h .

A d e t a i l e d t h e o r y of SAW g e n e r a t i o n i n a t w o - l a y e r e d medium by an IDT i n a l l f o u r t r a n s d u c e r c o n f i g u r a t i o n s was p r e s e n t e d by K i n o and Wagers [ 1 . 2 4 ] , The t h e o r e t i c a l f o u n d a t i o n f o r t h i s work was g i v e n by b o t h A u l d and K i n o [ 1 . 2 5 ] and K i n o and Reeder [ 1 . 2 6 ] . The a c o u s t i c impedance f o r m u l a e f o r a r a d i a t i n g , I D T - p i e z o e l e c t r i c medium c o m b i n a t i o n d e r i v e d by S m i t h e t a l . [ 1 . 1 0 ] , c o u l d a l s o be found by K i n o and Wagers [ 1 . 2 4 ] . I t has been shown t h a t the S m i t h e q u i v a l e n t c i r c u i t i s a p p l i c a b l e t o t w o - l a y e r e d media p r o v i d e d t h a t the IDT s t a t i c c a p a c i t a n c e and the p i e z o e l e c t r i c c o u p l i n g a r e known..Comparison between t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s u s i n g the S m i t h e q u i v a l e n t c i r c u i t i s g i v e n by Evans, L e w i s and P a t t e r s o n [ 1 . 2 7 ] , H i c k e r n e l l [ 1 . 2 8 ] and S a s a k i , C h u b a c h i and K i k u c h i [ 1 . 2 9 ] .

Measurement of c o n v e r s i o n e f f i c i e n c y f o r two of the f o u r p o s s i b l e

t r a n s d u c e r c o n f i g u r a t i o n s , from e l e c t r i c t o s u r f a c e - a c o u s t i c - w a v e power, as a f u n c t i o n of the t h i c k n e s s of an e v a p o r a t e d ZnS f i l m on a b o r o s i l i c a t e g l a s s s u b s t r a t e was g i v e n by I n a b a , K a j i m u r a and M i k o s h i b a [ 1 . 3 0 ] . H u r l b u r t [ 1 . 3 1 ] has c a l c u l a t e d the t r a n s d u c e r a d m i t t a n c e c h a r a c t e r i s t i c s as a f u n c t i o n of f r e q u e n c y f o r low p i e z o e l e c t r i c c o u p l i n g m a t e r i a l s (ZnO or CdS) on f u s e d q u a r t z . The c a l c u l a t e d r e s u l t s a r e found to be i n e x c e l l e n t agreement w i t h the e x p e r i m e n t a l ones.

Schmidt and V o l t m e r [ 1 . 3 2 ] have c a l c u l a t e d the SAW v e l o c i t y , the m e c h a n i c a l d i s p l a c e m e n t s and the e l e c t r i c f i e l d as a f u n c t i o n of the o v e r l a y

14

e x p e r i m e n t a l r e s u l t s were i n good agreement w i t h t h e o r y . L o f t u s [ 1 . 3 3 ] has c a l c u l a t e d t h e phase v e l o c i t y , t h e m e c h a n i c a l d i s p l a c e m e n t s and t h e l o c i o f d i s p l a c e m e n t f o r CdS on s a p p h i r e . D a n i e l and de K l e r k [ 1 . 3 4 ] measured t h e d i s p e r s i o n o f t h e phase v e l o c i t y o f A1S on s a p p h i r e . O ' c l o c k and D u f f y [ 1 . 3 5 ] have g i v e n e x p e r i m e n t a l r e s u l t s o f t h e p i e z o e l e c t r i c c o u p l i n g v e r s u s o v e r l a y

t h i c k n e s s and w a v e l e n g t h r a t i o f o r b o t h A1N and GaN on s a p p h i r e . L i m , K r a u t and T i t t m a n [ 1 . 3 6 ] c a l c u l a t e d t h e d i s p e r s i v e p r o p e r t i e s o f t h e f u n d a m e n t a l R a y l e i g h wave mode f o r ZnO on s a p p h i r e , i n o r d e r t o d e t e r m i n e t i m e - b a n d w i d t h p r o d u c t s . P i z a r e l l o [ 1 . 3 7 ] has t h e o r e t i c a l l y and e x p e r i m e n t a l l y i n v e s t i g a t e d

the phase v e l o c i t y , t h e group v e l o c i t y and t h e p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t as a f u n c t i o n o f o v e r l a y t h i c k n e s s and w a v e l e n g t h r a t i o f o r t h e f i r s t t h r e e R a y l e i g h modes o f ZnO on s a p p h i r e . M i h a l y and Rosner [ 1 . 3 8 ] c a l c u l a t e d t h e d i s p e r s i o n c u r v e s o f phase v e l o c i t y and p i e z o e l e c t r i c c o u p l i n g f o r ZnO- o r C d S - s a p p h i r e c o m b i n a t i o n s . A1N on s a p p h i r e has been i n v e s t i g a t e d by Hagon, D y a l and L a k i n [ 1 . 3 9 ] f o r t h e r e a l i s a t i o n o f a p u l s e c o m p r e s s i o n f i l t e r . Improved t e m p e r a t u r e s t a b i l i t y has been shown by Sandbank and B u t l e r [ 1 . 4 0 ] by u s i n g ZnO on i s o p a u s t i c g l a s s .

1.2.3. Three-layered media

The g e n e r a t i o n and d e t e c t i o n o f SAW i n t h r e e - l a y e r e d media a r e r e a l i s a b l e by t h e t r a n s d u c e r c o n f i g u r a t i o n s s i m i l a r t o t h o s e f o r t w o - l a y e r e d media.

W i t h m o n o l i t h i c i n t e g r a t i o n as t h e u l t i m a t e o b j e c t i v e , t h e s u b s t r a t e m a t e r i a l i s c o n d u c t i v e s i l i c o n . F o r t h e two t r a n s d u c e r c o n f i g u r a t i o n s t h a t g i v e t h e h i g h e s t p i e z o e l e c t r i c c o u p l i n g t h e IDT i s embedded between t h e p i e z o -e l -e c t r i c l a y -e r and t h -e s u b s t r a t -e , and thus t h -e i n c l u s i o n o f a n o n z -e r o

s u b s t r a t e c o n d u c t i v i t y adds t o t h e c o m p l e x i t y o f SAW d e v i c e d e s i g n [ 1 . 4 1 , 1.42, 1.43]. I t i s n o t e d t h a t t h e s i g n a l l o s s c a n be reduced by t h e

a p p l i c a t i o n o f an e x t e r n a l b i a s v o l t a g e between t h e combs o f t h e IDT and t h e s i l i c o n back c o n t a c t [ 1 . 4 1 , 1.44] a l o n g l i n e s s i m i l a r t o those d e s c r i b e d f o r GaAs by Takada e t a l . [ 1 . 4 5 ] .

The i n t r o d u c t i o n of an i n s u l a t i n g l a y e r between t h e IDT and s i l i c o n a l s o r e d u c e s t h e p r o b l e m o f s i g n a l l o s s and a t t h e same t i m e p r o v i d e s c o m p a t i b i l i t y w i t h p l a n a r s i l i c o n t e c h n o l o g y where a t h e r m a l l y grown s i l i c o n d i o x i d e l a y e r i s u s e d . D e f i n i t e l i m i t s a r e imposed on t h e p e r m i s s a b l e o x i d e t h i c k n e s s and s i l i c o n r e s i s t i v i t y o f s i l i c o n i n t e g r a t e d c i r c u i t s i n t e n d e d f o r volume p r o d u c t i o n . A t y p i c a l v a l u e o f o x i d e t h i c k n e s s i s 0.5 ym w i t h an upper l i m i t

of 1.5 ]im. S i l i c o n r e s i s t i v i t i e s n o r m a l l y r a n g e from 10_ 1* t o 1 fi.m. C u r r e n t l y ,

however, t h e r e i s c o n s i d e r a b l e i n t e r e s t i n s i l i c o n microwave i n t e g r a t e d c i r c u i t s (MIC) [ 1 . 4 6 ] w h i c h has pushed t h e r e s i s t i v i t y r e q u i r e m e n t s up t o

10 ti.m.

I n t h r e e - l a y e r e d media f l e x i b i l i t y i n SAW d e v i c e d e s i g n p r o c e d u r e i s o b t a i n e d , t h r o u g h t h e u s e o f an e q u i v a l e n t c i r c u i t model. The b a s i c S m i t h model a p p l i c a b l e t o t h r e e - l a y e r e d media w i t h n o n c o n d u c t i n g l a y e r s and

s u b s t r a t e i s extended t o a l l o w f o r t r a n s d u c t i o n w i t h a c o n d u c t i v e s u b s t r a t e . An e v a l u a t i o n o f t h e p i e z o e l e c t r i c c o u p l i n g and t h e s t a t i c IDT

c a p a c i t a n c e f o r a s p e c i f i c s t r u c t u r e w i t h t h e s i l i c o n c o n d u c t i v i t y and s i g n a l f r e q u e n c y as p a r a m e t e r s i s r e q u i r e d [ 1. 4 7 , 1 . 4 8 ] .

I t i s shown by Venema and Humphryes [ 1 . 4 2 ] t h a t f o r most p r a c t i c a l c a s e s S m i t h ' s model c a n be a p p l i e d w i t h t h e p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t c a l c u l a t e d f o r a e i t h e r a z e r o o r an i n f i n i t e s u b s t r a t e c o n d u c t i v i t y .

I n the l i t e r a t u r e a t t e n t i o n has been f i r s t g i v e n t o n o n c o n d u c t i v e l a y e r s and s u b s t r a t e s . A r m s t r o n g and Crampin [ 1 . 4 9 ] p r e s e n t e d t h e o r e t i c a l r e s u l t s f o r the phase v e l o c i t y and p i e z o e l e c t r i c c o u p l i n g i n a Z n 0 - S i 0 2_S i medium and

D a v i e s [ 1 . 5 0 ] found a d i s p e r s i o n - f r e e r e g i o n f o r t h e SAW phase v e l o c i t y f o r a p a r t i c u l a r c o m b i n a t i o n o f t h e t h i c k n e s s o f t h e i n t e r f a c i a l and t h e p i e z o -e l -e c t r i c l a y -e r i n a ZnO-Al^O^ c o m b i n a t i o n .

Venema and D e k k e r s [ 1 . 5 1 ] have c a l c u l a t e d t h e phase v e l o c i t y and t h e p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t f o r a C d S - S i O ^ - S i l a y e r e d medium as a f u n c t i o n o f t h e p i e z o e l e c t r i c o v e r l a y t h i c k n e s s and the w a v e l e n g t h r a t i o w i t h the i n t e r f a c i a l ( S i 0 2 ) l a y e r t h i c k n e s s w a v e l e n g t h r a t i o as a p a r a m e t e r . The r e s u l t s were r e l a t e d t o f o u r p o s s i b l e t r a n s d u c e r c o n f i g u r a t i o n s . I n a d d i t i o n an enhancement i n t h e p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t was f o u n d f o r each t r a n s d u c e r c o n f i g u r a t i o n .

Good e x p e r i m e n t a l r e s u l t s were o b t a i n e d by H i c k e r n e l l and Brewer [ 1 . 5 2 ] and by H i c k e r n e l l [ 1 . 2 8 ] w i t h Z n 0 - S i 02- S i c o m b i n a t i o n s . The r a t i o o f t h e SAW

w a v e l e n g t h and t h e t h e r m a l l y grown s i l i c o n d i o x i d e t h i c k n e s s was such t h a t t h e c o n d u c t i v i t y e f f e c t s o f t h e s i l i c o n s u b s t r a t e were m i n i m i z e d . D i r e c t

c o m p a r i s o n o f t h e measured phase v e l o c i t y and p i e z o e l e c t r i c c o u p l i n g w i t h t h e t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s f o r ZnO f i l m s on f u s e d q u a r t z i s t h e n p e r m i t t e d .

S h i o s a k i , Yamamoto and Kawabata [ 1 . 5 3 ] have c a l c u l a t e d t h e phase v e l o c i t y and t h e p i e z o e l e c t r i c c o u p l i n g c o e f f i c i e n t f o r t h e f i r s t t h r e e R a y l e i g h modes i n b o t h two- and t h r e e - l a y e r e d media as a f u n c t i o n o f SAW w a v e l e n g t h and l a y e r t h i c k n e s s . One o f t h e i r c o m b i n a t i o n s o f m a t e r i a l was Zn0-Si0„-Si. L a y e r s and

1 6

s u b s t r a t e s were t a k e n to be n o n c o n d u c t i v e . They found d e v i a t i o n s between t h e t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s f o r the p i e z o e l e c t r i c c o u p l i n g , due t o the n o n z e r o s i l i c o n c o n d u c t i v i t y .

The phase and group v e l o c i t y , the t e m p e r a t u r e c o e f f i c i e n t s and the p i e z o -e l -e c t r i c c o u p l i n g c o -e f f i c i -e n t of a Z n O - S i C ^ - S i m-edium hav-e b-e-en s t u d i -e d by Ono, Wasa and Hayakawa [ 1 . 5 4 ] . The t h i c k n e s s e s of the ZnO and S i C ^ l a y e r s and t h e i r c r y s t a l l o g r a p h i c o r i e n t a t i o n s , f o r t h o s e Z n O - S i C ^ - S i c o m b i n a t i o n s t h a t y i e l d a h i g h 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 and t e m p e r a t u r e s t a b i l i t y , a r e d i s c u s s e d .

I n the r e m a i n i n g c h a p t e r s of t h i s t h e s i s the f o l l o w i n g s u b j e c t s a r e p r e s e n t e d . C h a p t e r 2 d e a l s w i t h SAW d e v i c e t e c h n o l o g y of a C d S - S i C ^ - S i medium. C h a p t e r 3 i s d e v o t e d t o the t h e o r e t i c a l e v a l u a t i o n of p r o p a g a t i o n p r o p e r t i e s of SAW, i n p a r t i c u l a r R a y l e i g h waves, o v e r t h r e e - l a y e r e d media. I n C h a p t e r 4 the SAW t r a n s d u c t i o n p r o c e s s i s d e s c r i b e d . A lumped e l e c t r i c a l network model t o d e a l w i t h the s i l i c o n c o n d u c t i v i t y i s p r e s e n t e d . I n C h a p t e r 5 the

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2. SAW-DEVICE TECHNOLOGY OF A CDS-SI02-SI LAYERED MEDIUM

2.1. Introduction

SAW d e v i c e t e c h n o l o g y of l a y e r e d media d i f f e r s s u b s t a n t i a l l y f r o m t h a t of homogeneous p i e z o e l e c t r i c media. The h i g h l y complex t e c h n o l o g y and the

s p e c i f i c p r o p e r t i e s of l a y e r e d media were r e a s o n s why the development o f SAW d e v i c e s on homogeneous p i e z o e l e c t r i c media was g i v e n p r i o r i t y . I t was s t a t e d i n s e c t i o n 1.2.2 t h a t freedom of m a t e r i a l c h o i c e f o r s u b s t r a t e and o v e r l a y was the m a i n s t i m u l u s f o r i n v e s t i g a t i o n s i n t o SAW t r a n s d u c t i o n and p r o p a g a t i o n i n t w o - l a y e r e d m e d i a . G e n e r a l l y s p e a k i n g t h i s i s c o r r e c t . However, when i t comes to r e a l i s a t i o n , some r e s t r i c t i o n s have t o be made due to m a t e r i a l c o m b i n a t i o n s or p r o c e s s s t e p s w h i c h a r e not c o m p a t i b l e . I n t h e c a s e o f t w o - l a y e r e d media w i t h a c o n d u c t i v e s i l i c o n s u b s t r a t e an amorphous s i l i c o n - d i o x i d e l a y e r has to be i n s e r t e d f o r c o m p a t i b i l i t y w i t h p l a n a r s i l i c o n t e c h n o l o g y . T h i s t h e r m a l l y grown l a y e r , w i t h an e s t i m a t e d c o n d u c t i v i t y v a l u e of l O- 1" * S/m s i m u l t a n e o u s l y

r e d u c e s the s i g n a l l o s s of e l e c t r i c a l o r i g i n . The r e a l i s a b i l i t y of the f o u r t r a n s d u c e r c o n f i g u r a t i o n s , shown i n F i g . 2.1, i s of c e n t r a l i m p o r t a n c e .

C o n f i g u r a t i o n s A and E have the IDT-metal p a t t e r n a t the i n t e r f a c e of the p i e z o e l e c t r i c and s i l i c o n - d i o x i d e l a y e r , w h i l e c o n f i g u r a t i o n B has an

a d d i t i o n a l m e t a l p l a t e on top of the p i e z o e l e c t r i c l a y e r . C o n f i g u r a t i o n s C and

D have the IDT-metal p a t t e r n on top of the p i e z o e l e c t r i c l a y e r and

c o n f i g u r a t i o n D has an a d d i t i o n a l m e t a l p l a t e a t the i n t e r f a c e of the p i e z o -e l -e c t r i c and s i l i c o n - d i o x i d -e l a y -e r .

The p i e z o e l e c t r i c o v e r l a y c r y s t a l m a t e r i a l has t o be c h o s e n such t h a t low c o n d u c t i v i t y i s e n s u r e d and t h a t i t s a n i s o t r o p y d i s t u r b s the SAW p r o p a g a t i o n o v e r the t h r e e - l a y e r e d medium m i n i m a l l y . A h e x a g o n a l c r y s t a l o f f e r s the s o l u t i o n f o r the a n i s o t r o p y p r o b l e m , p r o v i d e d t h a t the c r y s t a l c - a x i s i s normal to the o x i d i s e d s i l i c o n s u b s t r a t e . For t h i s o r i e n t a t i o n a h e x a g o n a l c r y s t a l behaves e l a s t i c a l l y i s o t r o p i c f o r s u r f a c e waves p r o p a g a t i n g o v e r the b a s a l p l a n e . The t e c h n o l o g i c a l p r o b l e m o f s u p p l y i n g s u b s t r a t e s w i t h p i e z o -e l -e c t r i c c r y s t a l p l a t -e l -e t s i n h i b i t s th-e r -e a l i s a t i o n of SAW-d-evic-e mass p r o d u c t i o n . The d e p o s i t i o n of p i e z o e l e c t r i c f i l m s by means of s p u t t e r i n g or e v a p o r a t i o n s o l v e s t h i s p r o b l e m , and a l s o e n a b l e s the f o r m a t i o n of p a t t e r n s i n the p i e z o e l e c t r i c f i l m s by c h e m i c a l e t c h i n g [ 2 . 1 ] . ZnO and CdS f i l m s are commonly used as a p o l y c r y s t a l l i n e p i e z o e l e c t r i c o v e r l a y , due to t h e i r ease of d e p o s i t i o n on an amorphous s u b s t r a t e . H i g h q u a l i t y p i e z o e l e c t r i c f i l m s r e q u i r e

20

c) d)

Fig. 2.1. Structural identification of four transducer configurations for SAW transduction in silicon.

s t o i c h i o m e t r i c c o m p o s i t i o n , low c o n d u c t i v i t y (max. 1 0- 6 S/m) and a s t r o n g c r y s t a l l i t e e-axes o r i e n t a t i o n n o r m a l t o t h e s u b s t r a t e . The i n d i r e c t

e v a p o r a t i o n method d e s c r i b e d i n s e c t i o n 2.5.1 i s v e r y u s e f u l f o r p r o d u c i n g a p i e z o e l e c t r i c CdS o v e r l a y w i t h t h e s e s p e c i f i c a t i o n s . The t r a n s d u c e r

c o n f i g u r a t i o n s A and B a r e most adequate f o r volume p r o d u c t i o n of m o n o l i t h i c a l l y i n t e g r a t e d SAW d e v i c e s . A s u c c e s s f u l r e a l i s a t i o n o f a

m o n o l i t h i c s u r f a c e wave t r a n s v e r s a l f i l t e r u s i n g c o n f i g u r a t i o n A was r e p o r t e d by H i c k e r n e l l , O l s o n , Adamo and Bush [ 2 . 2 ] . The m e t a l l i s a t i o n a t the top o f c o n f i g u r a t i o n B o f f e r s a d v a n t a g e s a t low kh v a l u e s (fe-wavenumber, ^Q p i e z o

-e l -e c t r i c l a y -e r t h i c k n -e s s ) . At h i g h -e r kh0 v a l u e s the p e r f o r m a n c e s of the two

c o n f i g u r a t i o n s a r e p r a c t i c a l l y i d e n t i c a l [ 1 . 5 1 ] . As the optimum kh^ v a l u e f o r maximum 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 e x i s t s w i t h i n t h i s r e g i o n , the e x t r a e l e c t r o d e i s r e d u n d a n t . C o n f i g u r a t i o n s C and D r e a c h a l o w e r p i e z o e l e c t r i c c o u p l i n g o v e r a w i d e range of kh^, whereas the p i e z o e l e c t r i c c o u p l i n g of c o n f i g u r a t i o n D shows a r e l a t i v e l y h i g h peak s i m i l a r to the one f o r c o n f i g u r a t i o n B a t low khg v a l u e s [ 1 . 5 1 ] . I n c o n c l u s i o n , i t s ease of f a b r i c a t i o n , i t s p o t e n t i a l l y h i g h p i e z o e l e c t r i c c o u p l i n g and the s e n s i t i v i t y to an e x t e r n a l l y a p p l i e d b i a s v o l t a g e makes c o n f i g u r a t i o n A most f a v o u r a b l e f o r SAW-device d e s i g n i n m o n o l i t h i c i n t e g r a t i o n . U n l e s s o t h e r w i s e s p e c i f i e d t r a n s d u c e r c o n f i g u r a t i o n A w i t h an u n i f o r m IDT and a CdS p i e z o e l e c t r i c o v e r l a y i s c o n s i d e r e d i n t h i s t h e s i s .

W i t h the a d o p t i o n of an e q u i v a l e n t c i r c u i t f o r an IDT embedded i n a l a y e r e d medium, f l e x i b i l i t y i s i n t r o d u c e d i n t o the d e s i g n p r o c e d u r e . From the l i t e r a t u r e i t i s c l e a r t h a t two f i e l d s of e x t e n s i v e r e s e a r c h can be embodied i n such an a p p r o a c h , i . e . MOS-device d e s i g n and i t s a s s o c i a t e d t e c h n o l o g y and SAW-device d e s i g n on n o n c o n d u c t i n g homogeneous p i e z o e l e c t r i c s u b s t r a t e s . Most of t h e p r o p e r t i e s of MOS s t r u c t u r e s a r e w e l l - k n o w n and can be a d e q u a t e l y a c c o u n t e d f o r [2.3 ( C h a p t e r 9 ) , 2 . 4 ] , p r o v i d e d t h a t a lumped network a p p r o x i m a t i o n i s a v a i l a b l e f o r t h e i n t e r d i g i t a t e d M O S - s t r u c t u r e a d m i t t a n c e .

The t r a n s d u c e r c o n f i g u r a t i o n can be c o n s i d e r e d as an i n t e r d i g i t a l m e t a l e l e c t r o d e p a t t e r n - S i C ^ - S i system (IDT-MOS) w i t h a p i e z o e l e c t r i c o v e r l a y , where MOS s t a n d s f o r m e t a l - o x i d e - s e m i c o n d u c t o r . The p e r f o r m a n c e of the t r a n s d u c t i o n

and p r o p a g a t i o n p r o c e s s e s of SAW i n t h i s c o m b i n a t i o n of m a t e r i a l s (assuming a h i g h q u a l i t y p i e z o e l e c t r i c • f i l m ) i s c l o s e l y r e l a t e d to the e l e c t r i c a l c h a r a c t e r i s t i c s of the IDT-MOS. I n C h a p t e r 4 a lumped network w i l l be p r e s e n t e d i n w h i c h the a d m i t t a n c e r e p r e s e n t s the IDT-MOS. The a d m i t t a n c e of one f i n g e r p a i r i s supposed t o be composed of a s e r i e s arrangement of two lumped n e t w o r k s each r e p r e s e n t i n g an i d e a l MOS c a p a c i t o r . E x p r e s s i o n s a r e d e r i v e d f o r the c o n d u c t i v e and the c a p a c i t i v e p a r t of t h i s a d m i t t a n c e as a f u n c t i o n of f r e q u e n c y , o x i d e l a y e r t h i c k n e s s , e f f e c t i v e e l e c t r i c f i e l d p e n e t r a t i o n i n t o the s i l i c o n s u b s t r a t e and s i l i c o n c o n d u c t i v i t y . MOS c a p a c i t o r s h a v i n g a r e a l S i 0 2 ~ S i i n t e r f a c e have been i n v e s t i g a t e d e.g. by N i c o l l i a n and G o e t z b e r g e r [ 2 . 5 ] . The i n t e r f a c e p r o p e r t i e s a r e r e p r e s e n t e d by lumped n e t w o r k s of w h i c h the c o n d u c t i v e p a r t of the a d m i t t a n c e i s a measure of the i n t e r f a c e s t a t e l o s s . By t h i s a p p r o a c h , the i n f l u e n c e of the i n t e r f a c e s t a t e s on the IDT-MOS can be e v a l u a t e d by a m o d i f i c a t i o n of the i d e a l MOS c o n d u c t a n c e - c a p a c i t a n c e network. A s h o r t d e s c r i p t i o n of the u n d e r l y i n g

2 2

p r i n c i p l e s of b o t h t h e MOS t h e o r y and t h e N i c o l l i a n - G o e t z b e r g e r approach i s p r e s e n t e d i n s e c t i o n 2.2 and A p p e n d i c e s A and B.

S e c t i o n 2.5 d e s c r i b e s t h e p i e z o e l e c t r i c c a d m i u m - s u l f i d e o v e r l a y

t e c h n o l o g y u s e d . The c a d m i u m - s u l f i d e f i l m s a r e o b t a i n e d by a d e p o s i t i o n method u s i n g i n d i r e c t e v a p o r a t i o n r a t h e r than d e p o s i t i o n from t h e m o l e c u l a r beam. F o r some SAW d e v i c e s o n l y a bounded p i e z o e l e c t r i c r e g i o n m i g h t be p r e f e r r e d . A method of e t c h i n g t h e CdS f i l m s i s p r e s e n t e d [ 2 . 1 ] .

The s h o r t s e c t i o n 2.6 on p a c k a g i n g c o n c l u d e s t h i s c h a p t e r .

2.2. S i l i c o n oxidation and lumped network approximation of s i l i c o n dioxide-s i l i c o n interface propertiedioxide-s

B o t h p - and «-type s i l i c o n w a f e r s w i t h i n t h e c o n d u c t i v i t y range 1 t o 100 S/m and of (111) and (100) c r y s t a l - s u r f a c e o r i e n t a t i o n were used i n t h e i n v e s t i g a t i o n s . The f o l l o w i n g c l e a n i n g p r o c e d u r e f o r t h e w a f e r s was u t i l i s e d : - p o l i s h i n g w i t h m e t h a n o l - w e t t e d c o t t o n w o o l , - u l t r a s o n i c a l l y c l e a n i n g i n f u m i n g n i t r i c a c i d , - b o i l i n g i n h o t n i t r i c a c i d ( 6 5 % ) , - r i n s i n g w i t h d e m i n e r a l i z e d w a t e r (0.2 t o 0.1 y S ) , - d r y i n g by w h i r l i n g .

The w a f e r s were s u b j e c t e d t o t h e r m a l o x i d a t i o n . Wet o x i d a t i o n was f o l l o w e d by a d r y o x i d a t i o n and a s h o r t d r y n i t r o g e n a n n e a l i n g s t e p , a l l a t a f u r n a c e t e m p e r a t u r e of 1100°C. The p i e z o e l e c t r i c c o u p l i n g of the t r a n s d u c e r c o n f i g u r a t i o n i s s t r o n g l y i n f l u e n c e d by t h e p a r a m e t e r kh , where h i s t h e o x i d e l a y e r t h i c k n e s s . ox ox Sometimes t h i c k o x i d e l a y e r s o f about 1 um a r e n e c e s s a r y t o p r e v e n t e x c e s s i v e s i g n a l l o s s . G e n e r a l l y a s i l i c o n d i o x i d e - s i l i c o n ( S i O ^ - S i ) system i s prone t o d e t r i m e n t a l e f f e c t s , some o f them showing t h e i r p r e s e n c e t o a g r e a t e r e x t e n t w i t h i n c r e a s i n g o x i d e t h i c k n e s s . These e f f e c t s a r e w e l l - k n o w n b u t n o t a l l of them a r e s a t i s f a c t o r i l y u n d e r s t o o d .

Four b a s i c t y p e s o f c h a r g e s have been shown t o be a s s o c i a t e d w i t h t h e r m a l l y o x i d i s e d s i l i c o n of w h i c h t h e f i r s t t h r e e a r e l o c a t e d i n t h e o x i d e l a y e r and t h e l a s t a t t h e S i 0 2 ~ S i i n t e r f a c e [ 2 . 3 , 2.4, 2 . 5 ] . A s c h e m a t i c p r e s e n t a t i o n i s g i v e n i n F i g . 2.2, and a b r i e f d e s c r i p t i o n o f t h e t y p e s of c h a r g e f o l l o w s : 1. S u r f a c e - s t a t e charge o r f i x e d o x i d e c h a r g e . O r i g i n o f t h i s p o s i t i v e c h a r g e , w h i c h i s l o c a t e d i n t h e o x i d e w i t h i n

metal electrode

Si02

+

+

I I

Na^J — m o b i l e impurity ions X — fos* surfoce states - ) - — t r a p s ionised by radiation

— surface - state charge

Fig. 2.2. Basic classification of states in a nonideal MIS diode (after Sze 12.21).

20 nm (200 1) o f t h e S i 0 2_S i i n t e r f a c e , i s i n t h e d e v i a t i o n from o x i d e s t o i c h i o m e t r y . I t s d e n s i t y ranges from 1Q1" t o 1 01 6 m- 2 and depends on t h e s i l i c o n s u r f a c e o r i e n t a t i o n .

2. M o b i l e i o n i c c h a r g e .

A l k a l i i o n s N a+, K+ and L i+ a r e t h e main s o u r c e s o f t h i s c h a r g e . They can m i g r a t e i n t h e r m a l l y grown o x i d e a t low t e m p e r a t u r e s o f 100°C o r l e s s and c a n o r i g i n a t e from any p r o c e s s i n g s t e p .

3. R a d i a t i o n i n d u c e d c h a r g e .

G e n e r a t i o n o f e l e c t r o n - h o l e p a i r s i n t h e o x i d e l a y e r by i o n i s i n g r a d i a t i o n r e s u l t s i n a p o s i t i v e space c h a r g e due t o t r a p p e d h o l e s . 4. F a s t s u r f a c e - s t a t e s .

Tamm-Shockley s t a t e s a r e caused by l a t t i c e i n t e r r u p t i o n and g i v e a s u r f a c e - s t a t e d e n s i t y o f t h e o r d e r o f 1 01 9 m- 2 f o r an a t o m i c c l e a n s u r f a c e . The f o r m a t i o n o f t h e o x i d e l a y e r , however, uses up most o f the a v a i l a b l e c h e m i c a l bonds s u c h t h a t t h e a c t u a l s u r f a c e - s t a t e d e n s i t y a t a c l e a n S i O ^ - S i i n t e r f a c e i s s u b s t a n t i a l l y s m a l l e r . These i n t e r f a c e s t a t e s a r e d i s t r i b u t e d i n t h e s i l i c o n band gap.

A l l t h e s e types o f c h a r g e s c a n be m o n i t o r e d u s i n g t h e MOS C -V method o f a n a l y s i s . A r e v i e w o f t h e s e methods i s g i v e n by D e a l [ 2 . 6 ] , G o e t z b e r g e r and

2 4

Sze [ 2 . 7 ] and D a s c a l u [ 2 . 8 ] , Whelan [ 2 . 9 ] has t h e o r e t i c a l l y and e x p e r i m e n t a l l y i n v e s t i g a t e d a C^-V method to study the energy d i s t r i b u t i o n and d e n s i t y of o x i d e c h a r g e and s u r f a c e s t a t e s .

The c a p a c i t a n c e C of a MOS s t r u c t u r e , as shown i n F i g . 2.3 i s measured w i t h a c a p a c i t a n c e b r i d g e w i t h a v o l t a g e V a c r o s s the c a p a c i t o r . The v o l t a g e

i s a s u p e r p o s i t i o n of a s m a l l aa v o l t a g e w i t h r a d i a n f r e q u e n c y co and a da v o l t a g e V , The C (u), V) c u r v e s a r e o b t a i n e d w i t h u> as p a r a m e t e r . (The s i l i c o n back c o n t a c t i s assumed to be ohmic and p e r f e c t l y c o n d u c t i n g . )

Fig. 2.3. Metal-oxide-semiconductor (MOS) structure. The silicon dioxide and silicon wafer thicknesses are denoted by h and h , respectively.

The measured C^-V p l o t a t a s p e c i f i c r a d i a n f r e q u e n c y i s t h e n compared to a c a l c u l a t e d i d e a l C^-V c u r v e of the MOS c a p a c i t o r , h a v i n g no energy d i f f e r e n c e between m e t a l and s e m i c o n d u c t o r work f u n c t i o n s as w e l l as no c h a r g e w i t h i n the o x i d e and s u r f a c e - s t a t e c h a r g e . An i m p o r t a n t c o n d i t i o n f o r the c a l c u l a t i o n of the c u r v e i s the a s s u m p t i o n of t h e r m a l e q u i l i b r i u m i n the MOS s t r u c t u r e . T h i s a s s u m p t i o n i s g e n e r a l l y c o r r e c t w i t h o x i d e l a y e r s t h i c k enough t o p r e v e n t s i z a b l e dc c u r r e n t s . I d e a l c u r v e s f o r h i g h and low f r e q u e n c y M O S - c a p a c i t a n c e b e h a v i o u r a r e computed by G o e t z b e r g e r [ 2 . 1 0 ] f o r s i l i c o n , w i t h o x i d e t h i c k n e s s and d o p i n g as p a r a m e t e r s .

B a l k [ 2 . 1 1 ] has d i s c u s s e d i n d e t a i l b o t h the m e r i t s and the problems of l a y e r e d d i e l e c t r i c s as a l t e r n a t i v e i n s u l a t o r s f o r S i 0 2 - The u n i q u e and

f a v o u r a b l e p r o p e r t i e s of Si02> however, r e s u l t i n c o m b i n a t i o n s of S i 0 2 w i t h a second d i e l e c t r i c , where S i O ^ i s a l w a y s i n c o n t a c t w i t h the s i l i c o n . Commonly used second d i e l e c t r i c s a r e p h o s p h o s i l i c a t e g l a s s (PSG), s i l i c o n n i t r i d e

( S i ^ N ^ ) and a l u m i n i u m o x i d e ( A ^ O ^ ) . To i n d i c a t e t h e i r u t i l i t y , PSG, f o r i n s t a n c e , a c t s as a g e t t e r m a t e r i a l f o r Na and t h e r e f o r e b l o c k s N a+ d r i f t i n the S i 0 2 ~ l a y e r . The a p p l i c a t i o n o f l a y e r e d d i e l e c t r i c s as d e s c r i b e d above has n o t been c o n s i d e r e d i n t h i s t h e s i s .

Energy-band b e n d i n g a t t h e s e m i c o n d u c t o r s u r f a c e i s caused by a c h a r g e (C/m2) on t h e t o p m e t a l e l e c t r o d e ( g a t e ) o f t h e MOS c a p a c i t o r . A space c h a r g e

(C/m2) and a c o u p l e d s u r f a c e p o t e n t i a l ip (V) o c c u r s . F o r r e a l MOS s t r u c t u r e s t h e m e t a l - s e m i c o n d u c t o r work f u n c t i o n d i f f e r e n c e and t h e

c o m b i n a t i o n o f the o x i d e and t h e s u r f a c e - s t a t e charge d e t e r m i n e QgWg)' From Sg( l |J s) t h e d i f f e r e n t i a l c a p a c i t a n c e (F/m ) o f t h e s e m i c o n d u c t o r space c h a r g e as a f u n c t i o n o f ij; i s d e r i v e d ( A p p e n d i x A) : DQ c C?J) ) = -rr^ _{s s dtfj} ( p - t y p e s i l i c o n ) £ £ „ . O Si 1 - e x p ( - & j y +-2^ ( e x p ( B ^ ) - O po -) po (2.1)

W

0 ) £ £„ . o S^ p po In . t n i + -£° P '- po W i t h £ o £S i po po f r e e space p e r m i t t i v i t y ; e = 8.8542 x 10 F/m, r e l a t i v e s i l i c o n p e r m i t t i v i t y ; £^^ = 11.8 [ 2 . 3 ] , t h e r m a l e q u i l i b r i u m b u l k m a j o r i t y c a r r i e r c o n c e n t r a t i o n ( m- 3) i n p - t y p e s i l i c o n , t h e r m a l e q u i l i b r i u m b u l k m i n o r i t y c a r r i e r c o n c e n t r a t i o n ( m- 3) i n p - t y p e s i l i c o n , - i n t r i n s i c c a r r i e r c o n c e n t r a t i o n ( m- 3) ; n • = 1.48 x 1 01 6 m- 3 a t T = 300 K, -§= , q - e l e c t r o n c h a r g e ; q = 1.602 x 10 1 9 C, K - B o l t z m a n n c o n s t a n t ; K = 1.38 X 1 0 ~2 3 J/K, T - t e m p e r a t u r e i n K; 38.69 V-1 f o r T = 300 K, - c y \i I- ( i n t r i n s i c Debye l e n g t h ) ; L„ = 23.86 ym f o r T = 300 K. D . \ 2\fiqn . D • rp o

{exp(-34i ) + W ~ 1} + — iexp(6i|j ) - gip - 1 } s s p s s

epo

> 0 .

26 and s u r f a c e - s t a t e c h a r g e , c h a r g e n e u t r a l i t y of t h e MOS s t r u c t u r e r e q u i r e s QG- Q8 = « • <2-2) In a d d i t i o n p o t e n t i a l b a l a n c i n g r e q u i r e s : V - V - IT = 0 , (2.3) OX S where V i s t h e v o l t a g e d r o p a c r o s s t h e o x i d e l a y e r .

From e q s . (2.2) and (2.3) t h e MOS c a p a c i t a n c e c a n be f o u n d . By t a k i n g

DV DIP ,

1 _ DV OX s _ _ J _1_ ,„ 41

C ~ DQ„ ~ A QN DQ ~ C C '

M G G S OX S

i t i s shown t h a t t h e o x i d e c a p a c i t a n c e C a p p e a r s i n s e r i e s w i t h t h e space charge c a p a c i t a n c e t o form t h e MOS c a p a c i t a n c e C^. The o x i d e c a p a c i t a n c e C c a n be d e f i n e d as OX C = (F/m2) . (2.5) OX H OX The f u l l ijj^-range e x p r e s s i o n f o r ( e q . ( 2 . 1 ) ) c a n be a p p r o x i m a t e d i n t h e c h a r a c t e r i s t i c modes f o r QS^8) (Appendix A ) . A l s o a p p r o x i m a t i o n s f o r a r e

o b t a i n e d u s i n g e q s . (2.1) and (2.4) and assuming a p - t y p e s e m i c o n d u c t o r w i t h p » n . RP O PO ( i ) a c c u m u l a t i o n o f h o l e s o c c u r s f o r V < 0; ill < 0. O ' 8 C = C , because M OX 2 \ \ n i ) v

\

2

I

C now r e a c h e s i t s maximal v a l u e . C w i l l d e c r e a s e when V tends

M M O to z e r o . ( i i ) f l a t - b a n d c o n d i t i o n i s a t t a i n e d f o r lp = 0. C C e £ _ . FP

Y /

N n OX S . „H „ O SZ RP O , po C = t; =- , w i t h C = —= 1 + -t— M C + C S L„ \ IN. I \ P OX 8 0. \ 4. / V ^PO ( i i i ) d e p l e t i o n o f h o l e s o c c u r s f o r V > 0; > Ui > 0.

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C Cj e e„. /p

C = where c c , oSi (1E£ i i w 1 x

A d e p l e t i o n l a y e r near the s e m i c o n d u c t o r s u r f a c e i s formed, a c t i n g as a d i e l e c t r i c l a y e r .

The c a p a c i t a n c e C r e a c h e s a minimum (C . ) and t h e n i n c r e a s e s m rmn w i t h i n c r e a s i n g due t o the f o r m a t i o n of an i n v e r s i o n l a y e r of e l e c t r o n s a t the S i O ^ - S i i n t e r f a c e . ( i v ) i n v e r s i o n f o r i n c r e a s i n g V > 0: ill > -4i„ > 0. o s B C i n c r e a s e s w i t h i n c r e a s i n g V, because m T h i s a p p r o x i m a t i o n i s f o l l o w e d by f o r f r e q u e n c i e s up t o about 100 Hz [ 2.4 ] , as the i n c r e a s e of C depends on t h e a b i l i t y of the

m

m i n o r i t y c a r r i e r ( e l e c t r o n ) c o n c e n t r a t i o n i n the i n v e r s i o n l a y e r to f o l l o w the a p p l i e d aa s i g n a l . At h i g h e r f r e q u e n c i e s does not show an i n c r e a s e w i t h i n c r e a s i n g V b u t r a t h e r a lower minimum i s a t t a i n e d w h i c h i s c o n s t a n t f o r an i n c r e a s i n g Vj. The c o n d i t i o n of t h e r m a l e q u i l i b r i u m i s no l o n g e r f u l f i l l e d . The h i g h f r e q u e n c y v a l u e of the d e p l e t i o n l a y e r c a p a c i t a n c e C, , where the d e p l e t i o n l a y e r t h i c k n e s s hj r e a c h e s a maximum ?2, ( A p p e n d i x A) i s d e f i n e d as cctn -

"-irr

(F/m2) (2

-

6) dm Hence f o r h i g h f r e q u e n c i e s £ £ £„ . m e „ .h + £ h, ST, OX OX dm

The eqs. (2. I , 2.4, 2.5, 2.6, 2.7) f i n d t h e i r a p p l i c a t i o n i n the t r a n s d u c e r p e r f o r m a n c e c a l c u l a t i o n s of C h a p t e r 4. F i g u r e 2.4 [ 2 . 1 0 ] shows the r e l a t i o n -s h i p between C and V , (C -V p l o t ) f o r the c a -s e of a low and a h i g h

r m o m a

m e a s u r i n g f r e q u e n c y f o r an i d e a l MOS c a p a c i t o r w i t h p - t y p e s i l i c o n . The o x i d e l a y e r t h i c k n e s s h i s chosen as p a r a m e t e r .

28 0.55 pm 0,45 jjm 0.35 j i m 0.25 jjm -02 2.0 2.5 3.0 •Vo(V)

Fig. 2.4. MOS Cm~V plot for p-type silicon (N = 1020 m~3) (after Goetzberger

12.101) a. low frequency, b. high frequency; ^-ox varies from 0.25 to

0. 55 urn.

(2.8)

I n a r e a l MOS s t r u c t u r e one has t o a c c o u n t f o r t h e m e t a l - s e m i c o n d u c t o r work f u n c t i o n d i f f e r e n c e tp , t h e t o t a l o x i d e c h a r g e d e n s i t y Q (C/m2) and

ms ox the s u r f a c e s t a t e c h a r g e d e n s i t y Q ( C / m2) . The c h a r g e s Q and Q a r e

ss ozc ss assumed t o be l o c a t e d a t t h e SiO„-Si i n t e r f a c e (x = 0 ) . Charge n e u t r a l i t y i n

z o the MOS s t r u c t u r e r e q u i r e s % ~ %x -%s-% - ° • P o t e n t i a l b a l a n c i n g r e q u i r e s V - cp - V - i> = 0 (2.9) ms ox s where i p ^ = ^ - {X + ( ^ - cp^) + ( ^ - cptf) 12} u> - m e t a l work f u n c t i o n (V) m X - s e m i c o n d u c t o r e l e c t r o n a f f i n i t y ( V ) . P o t e n t i a l s cp. ( j = a, V, i, F) , w h i c h a r e a s s o c i a t e d w i t h t h e l o w e s t energy

l e v e l E^ i n t h e c o n d u c t i o n band, t h e h i g h e s t energy l e v e l i n t h e v a l e n c e band, t h e i n t r i n s i c F e r m i l e v e l E. and t h e F e r m i l e v e l E , r e s p e c t i v e l y , c a n be d e f i n e d : E . tp. = - — + a c o n s t a n t ( J = e, V, i, F) . i] q A p i c t o r i a l p r e s e n t a t i o n of eq. (2.9) i s g i v e n i n F i g . 2.5 i n w h i c h ip i s t h e F p o t e n t i a l a s s o c i a t e d w i t h t h e F e r m i l e v e l i n t h e m e t a l .

merol oxide silicon

Fig. 2.5. Model for the potential distribution in a MOS capacitor (p-type silicon) in thermal equilibrium. The mode of depletion is shown.

B o t h (p and Q cause t h e C -V p l o t t o s h i f t c o n g r u e n t l y w i t h r e s p e c t t o t h e ms ox m r v i d e a l MOS C-V p l o t . The p r e s e n c e o f u n i f o r m c o n t a m i n a t i o n by m o b i l e i o n i c c h a r g e c a n n o t be s e p a r a t e l y d e t e c t e d by t h i s s h i f t u n l e s s an a d d i t i o n a l s t r e s s v o l t a g e i s a p p l i e d , w i t h t h e MOS s y s t e m a t an e l e v a t e d t e m p e r a t u r e . I t i s t h e n p o s s i b l e t o c a l c u l a t e t h e ( u n i f o r m ) m o b i l e i o n i c c h a r g e and f i x e d o x i d e c h a r g e [ 2 . 6 ] . The f l a t b a n d F__ o c c u r s a t a n o n z e r o v a l u e of t h e b i a s v o l t a g e V . The FB o o c c u r r e n c e of ( f a s t ) s u r f a c e s t a t e s i s found i n t h e s l o p e o f t h e t r a n s i t i o n from t h e a c c u m u l a t i o n t o t h e d e p l e t i o n mode. I n a d d i t i o n t o t h e e s t a b l i s h e d C -V methods t h e c o n d u c t a n c e G of a MOS m m c a p a c i t o r as a f u n c t i o n of V w i t h t h e r a d i a n f r e q u e n c y CO as parameter c a n be