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Microwave acoustic delay lines
Microwave acoustic delay lines R. W. H a r c o u r t
The e s t a b l i s h e d t e c h n o l o g y for m a k i n g e v a p o r a t e d thin f i l m p i e z o e l e c t r i c t r a n s d u c e r s h a s m a d e the c o n s t r u c t i o n of d e lay l i n e s at m i c r o w a v e f r e q u e n c i e s p o s s i b l e . T h i n f i l m s of CdS on s a p p h i r e d e l a y m e d i a have b e e n u s e d to m a k e c.w. and p u l s e d d e l a y l i n e s f r o m 1-11GHz, with d e l a y s f r o m
1 -14~ts o p e r a t i n g at 77°K and 300°K. The p e r f o r m a n c e of s o m e of t h e s e l i n e s i s d e s c r i b e d . T h e p a r a m e t e r s g o v e r n i n g the choice of d e l a y m e d i a , t r a n s d u c e r m a t e r i a l and e l e c t r o d e s t r u c t u r e a r e d i s c u s s e d and the i m p o r t a n c e of c a r e f u l e l e c t r i c a l m a t c h i n g of t r a n s d u c e r to g e n e r a t o r i s e m p h a s i s e d .
The d e v e l o p m e n t of a t e c h n i q u e for m a k i n g thin f i l m s of p i e z o e l e c t r i c m a t e r i a l s t h a t can have f u n d a m e n t a l r e s o n a n c e s a s high a s 12GHz h a s m a d e it p o s s i b l e to m a k e delay l i n e s up to that f r e q u e n c y . So it is now f e a s i b l e to i n s e r t d e l a y s at r . f . r a t h e r than at i. f. in a r a d a r s y s t e m . T h i s p a p e r d e s c r i b e s the c o n s t r u c t i o n and p e r f o r m a n c e of s o m e typical l i n e s m a d e by the A u t h o r while w o r k i n g at S t a n d a r d T e l e c o m m u n i c a t i o n s L a b o r a t o r i e s and s o m e of the p r o b l e m s a s s o c i a t e d with t h e m .
L Band. T h e m a i n p r o b l e m with the t r a n s d u c e r s o f o r m e d i s the d o m i n a n t effect the c o n d u c t i n g f i l m will h a v e on the b a n d p a s s a s it h a s to be m o r e than an a c o u s t i c w a v e l e n g t h thick.
TRANSDUCERS The m e t h o d s of d e p o s i t i o n o f p i e z o e l e c t r i c thin f i l m s h a v e b e e n d e s c r i b e d in m u c h detail e l s e w h e r e 1,2, 3 and n e e d no f u r t h e r e n l a r g e m e n t h e r e . All the d e l a y l i n e s d e s c r i b e d had t r a n s d u c e r s p r e p a r e d by the c o e v a p o r a t i o n of c a d m i u m and s u l p h u r . T h i s m e t h o d i s to be p r e f e r r e d f o r m i c r o w a v e u s e a s it p r o d u c e s f i l m s of u n i f o r m high r e s i s t i v i t y . The f i l m s a r e p o l y c r y s t a l l i n e with a p r e f e r r e d o r i e n t a t i o n of the c - a x i s n o r m a l to the s u b s t r a t e and a s s u c h could g e n e r a t e l o n g i tudinal m o d e s o u n d w a v e s with v e r y s m a l l s h e a r m o d e c o m p o n e n t s . The a b s e n c e of a n o t h e r m o d e i s i m p o r t a n t if long d e l a y s a r e r e q u i r e d f o r which the p u r e s t e x p o n e n t i a l d e l a y is essential. S h e a r w a v e s can be g e n e r a t e d by thin f i l m t r a n s d u c e r s p r o duced both by d i r e c t v a p o u r s t r e a m f r o m a CdS s o u r c e and by c o e v a p o r a t i o n of c a d m i u m and s u l p h u r . T h e u s u a l m e t h o d of a c h i e v i n g t h i s is to have the s u b s t r a t e tilted r e l a t i v e to the i n c i d e n t a t o m i c s t r e a m s u c h that the c - a x i s is m a d e to lie at s o m e angle to the n o r m a l . The t r a n s d u c e r t h e n g e n e r a t e s both l o n g i t u d i n a l and s h e a r m o d e c o m p o n e n t s but the f o r m e r m a y be m i n i m i s e d by s e t t i n g the c - a x i s at s o m e c r i t i c a l a n g l e 4. If t h i s t i l t i n g w e r e a c h i e v a b l e e i t h e r i m m e d i ately a f i l m s t a r t e d to f o r m o r a f t e r only a few h u n d r e d A n g s t r o m s t h e n s u c h t r a n s d u c e r s could be u s e d to g e n e r a t e s h e a r w a v e s at m i c r o w a v e f r e q u e n c i e s . In p r a c t i c e , h o w e v e r , the c - a x i s b e n d s o v e r slowly and it r e q u i r e s s e v e r a l t h o u s a n d s of A n g s t r o m s t h i c k n e s s b e f o r e the c r i t i c a l angle i s r e a c h e d , t h e r e f o r e only low f r e q u e n c y f u n d a m e n t a l t r a n s d u c e r s can n o r m a l l y be p r o d u c e d . If d u r i n g the initial g r o w t h the t r a n s d u c e r w e r e doped so a s to be c o n d u c t i n g until the c r i t i c a l angle a l i g n m e n t had b e e n r e a c h e d and then a high r e s i s t i v i t y f i l m d e p o s i t e d a s a c o n t i n u a t i o n of the g r o w t h p r o c e s s , that u p p e r f i l m could be m a d e r e s o n a n t at m i c r o w a v e f r e q u e n c i e s with the l o w e r f i l m a c t ing a s b a s e e l e c t r o d e . D r D u n c a n 5 et al at G l a s g o w U n i v e r s i t y have t h u s m a d e s h e a r wave t r a n s d u c e r s o p e r a t i n g at R. W. H a r c o u r t , BSc I T T C o m p o n e n t s Group E u r o p e , S t a n d a r d T e l e p h o n e s and C a b l e s L i m i t e d , Q u a r t z C r y s t a l D i v i s i o n , E d i n b u r g h Way, Harlow, E s s e x .
230
ULTRASONICS O c t o b e r 1970
A n o t h e r m e t h o d of m a k i n g s h e a r wave e v a p o r a t e d t r a n s d u c e r s i s to g r o w t h e m e p i t a x i a l y on a c r y s t a l l i n e s u b s t r a t e , t h u s c r e a t i n g o r d e r e d o r i e n t a t i o n in the a, b and c - a x e s d i r e c t i o n . de K l e r k 1 h a s u s e d s u c h a m e t h o d to g e n e r a t e longitudinal, f a s t and slow s h e a r w a v e s f r o m a s i n g l e t r a n s d u c e r . P u t t i n g a m e t a l f i l m b e t w e e n s u b s t r a t e and t r a n s d u c e r m a k e s e p i t a x ial g r o w t h h a r d to r e a l i s e and o m i t t i n g it l e a d s to high insertion loss transducers. The b a n d p a s s of a t r a n s d u c e r i s of p r i m a r y i m p o r t a n c e to the o v e r a l l p e r f o r m a n c e of a d e l a y line, in s o m e i n s t a n c e s it i s e v e n m o r e i m p o r t a n t than the i n s e r t i o n l o s s . In o r d e r to p r e d i c t what the s h a p e of the f r e q u e n c y r e s p o n s e will be t h e e q u i v a l e n t c i r c u i t of the t r a n s d u c e r d e l a y line s y s t e m h a s to be a n a l y s e d . It c a n be shown t h a t the e q u i v a l e n t c i r c u i t of a t r a n s d u c e r in d i r e c t c o n t a c t with the delay line, and with a v e r y thin m e t a l b a s e e l e c t r o d e , i s that given in F i g 1 w h e r e C O i s the c l a m p e d c a p a c i t a n c e of the t r a n s d u c e r , r s i s the s e r i e s l o s s due to the e l e c t r o d e s , B T is the s u s c e p t a n c e and G T the c o n d u c t a n c e of the t r a n s d u c e r . In g e n e r a l , for low coupling c o n s t a n t t r a n s d u c e r s , B T << wC o and m a y be i g n o r e d . Away f r o m the half w a v e l e n g t h r e s o n a n c e f r e q u e n c y , fo, of the t r a n s d u c e r G w i s given by rKw2 Gw -
X0 ~
(1 -- Cos0) 2 (Sin20+r2Cos2~)
where X0 = (2~f C6) - I r - Acoustic impedance of delay medium/Acoustic impedance of transducer KT is the piezoelectric coupling constant and 0 --
~f fo
T h i s c o n d u c t a n c e t e r m c h a r a c t e r i s e s the b a n d - w i d t h of the t r a n s d u c e r and, when r e l a t e d to the p o w e r d e l i v e r e d to it and the s e r i e s l o s s , g i v e s the i n s e r t i o n l o s s a s well. When the m e t a l b a s e e l e c t r o d e i s m a d e of the s a m e o r d e r of t h i c k n e s s a s the t r a n s d u c e r or s e v e r a l r e s o n a n t f i l m s a r e i n t r o d u c e d b e t w e e n the t r a n s d u c e r and delay m e d i u m then the f r e q u e n c y d e p e n d a n t p a r t of G w i s m o d i f i e d . T h i s c a n be s e e n in Fig 2 w h e r e the t h e o r e t i c a l b a n d - w i d t h i s shown of a 0 . 2 5 ~ m thick CdS f i l m on a gold e l e c t r o d e , one q u a r t e r w a v e l e n g t h t h i c k n e s s at fo on a s a p p h i r e delay m e d i u m . The q u a r t e r w a v e length d i p s in the c u r v e a r e d e e p e n e d both by i n c r e a s i n g the gold f i l m t h i c k n e s s and by u s i n g an e l e c t r o d e with an a c o u s t i c i m p e d a n c e not well m a t c h e d to b o t h line and t r a n s d u c e r . T h u s i n s e r t i o n l o s s m a y be i m p r o v e d but at the e x p e n s e of bandwidth. It i s t h u s p o s s i b l e to d e s i g n the p a s s b a n d s h a p e
rs
o~
/k/~tVM ~
I
p1
CO ~ / / / / / / / /
I
0
F i g 1 Equivalent c i r c u i t of a t r a n s d u c e r r i g i d l y connected to a delay medium. 60
50 $ 4O _9o
I II de[a)'
~-,
~'~:Matching fiim--~smedium N : '=f° . . . . . . . . .
..'/]
;==45,0
/.~y
.........
._~ 30
g -o
20
~-
10 0
Tungsten ;zl= 103 !
0"2
I
Gotd ~t =61"5 I
I
No matching film fl =oo I
I
I
I
I
0-4 0"6 0"8 1.0 1.2 1.4 1.6 1.8 2.0 NormaLized frequency fifo [GHz]
F i g ~. T h e o r e t i c a l transduction l o s s v e r s u s n o r m a l i z e d f r e q u e n c y f o r t r a n s d u c e r s having: no e l e c t r o d e ; h a l f w a v e l e n g t h thick gold e l e c t r o d e at fo; h a l f - w a v e l e n g t h t h i c k t u n g s t e n e l e c t r o d e a t fo-
to suit the ultimate r e q u i r e m e n t of the line. This can be r e a l i s e d in p r a c t i c e as shown in Fig 3 w h e r e e x p e r i m e n t a l r e s u l t s for a p a r t i c u l a r t r a n s d u c e r a r e c o m p a r e d with theory. The double m i n i m a c u r v e s shown in Fig 2 a r e c h a r a c t e r i s t i c of a t r a n s d u c e r of h a l f - w a v e l e n g t h t h i c k n e s s at fo which has b e e n d e p o s i t e d on a delay m e d i u m where the a c o u s t i c i m p e dance r a t i o r is a s high as 2. The depth of t h e s e m i n i m a may be i n c r e a s e d by u s i n g m o r e than one matching film and, if a single m i n i m a is r e q u i r e d at fo, e i t h e r r m u s t be n e a r e r unity o r the e l e c t r o d e f i l m s m u s t be made one wavelength thick at fo" T h e o r e t i c a l c u r v e s for the l a t t e r a r e shown in Fig 4. The a v e r a g e i n s e r t i o n l o s s of the t r a n s d u c e r is g o v e r n e d by K 2 . It has not been found to a g r e e with the value given for the bulk, and t h e r e has been s o m e d i s c r e p a n c y between its value obtained by R e e d e r 6 and that given by Dahr and Court 7. If the quality of f i l m s u s e d in both c a s e s a r e a s s u m e d to be about equal, the d i s c r e p a n c y may well be due to doubt in the value of X o. The optimum value of K w f o r CdS is that of the bulk m a t e r i a l and is s e t at 0.22, if ZnO w e r e u s e d i n s t e a d it could be i n c r e a s e d to 0.34 and the a v e r a g e i n s e r t i o n l o s s l o w e r e d 8. If the s e r i e s l o s s t e r m r s w e r e absent, then by the use of a t r a n s f o r m e r to m a t c h G T to the line and by tuning out the c a p a c i t a n c e of the t r a n s d u c e r (which is c o n t r o l l e d only by its active a r e a and t h i c k n e s s ) a p e r f e c t m a t c h would be p o s sible with z e r o i n s e r t i o n l o s s . However the s e r i e s l o s s t e r m m a k e s this i m p o s s i b l e as it d o m i n a t e s the s y s t e m . It i s f a r m o r e p r o d u c t i v e to t r a n s f o r m the line i m p e d a n c e down to m a t c h r s and again tune out C O to the new situation. A1though l o s s i s given d i r e c t l y by the r e l a t i o n b e t w e e n r s and GT, the o p t i m u m bandwidth of the t r a n s d u c e r is achieved without i t s being l i m i t e d by the tuning c i r c u i t r y . The use of such a t r a n s f o r m e r is d e s c r i b e d l a t e r .
s e v e r a l well defined p r o p e r t i e s that the m a t e r i a l should have for it to give low attenuation to acoustic waves above 1GHz. It m u s t be a single c r y s t a l with c o m p l e x s t r u c t u r e and low m a s s a t o m s . It m u s t have a low Debye T e m p e r a t u r e and low t h e r m a l conductivity. T h e s e a r e n e c e s s a r y but not sufficient conditions. C r y s t a l s that s a t i s f y them for which m e a s u r e m e n t s a r e available a r e l i s t e d in Table 1. All t h e s e values a r e for 20°C at which t e m p e r a t u r e the attenuation c h a n g e s a s the s q u a r e of the f r e q u e n c y . The table shows that s a p p h i r e is as good a m a t e r i a l as any for longitudinal waves except Lithium Tantalate and Lithium Niobate. N e i t h e r of the l a t t e r have been m e a s u r e d at 1GHz or above so that t h e i r potential low l o s s has not been p r o v e d . F u r t h e r , they a r e difficult m a t e r i a l s to make in s t o i c h i o m e t r i c , high quality s i n g l e c r y s t a l s and so a r e v e r y e x p e n s i v e . By using s h e a r waves which have l o w e r v e l o c i t i e s , s o m e advantage may be gained using YIG o r Spinel but the technical difficulties in g r o w i n g s h e a r wave t r a n s d u c e r s offset t h e s e advantages somewhat. Both YIG and Spinel can be grown to a high o r d e r of c r y s t a l l i n e p e r f e c t i o n and to l a r g e enough s i z e s for delays g r e a t e r than 10gs, t h e r e f o r e f o r high f r e q u e n c i e s or long d e l a y s they would be v e r y useful once the s h e a r wave t r a n s d u c e r technology has been i m p r o v e d . All the delay lines made by the Author had s a p p h i r e a s the medium, o t h e r m a t e r i a l s w e r e m e a s u r e d for l o s s but not u s e d in any d e v i c e s . Sapphire can be grown in two ways, e i t h e r by f l a m e fusion o r by pulling the s e e d f r o m the m e l t (Czochralski). The f o r m e r i s the c h e a p e r method but does not p r o d u c e such a good c r y s t a l a s does the l a t t e r . The pulling p r o c e s s can be c o n t r o l l e d to give a p a r t i c u l a r o r i e n tation depending on the s e e d and that o r i e n t a t i o n can be m a i n t a i n e d throughout the growth f o r lengths of c r y s t a l g r e a t e r than 120mm. F l a m e fusion usually p r o d u c e s c r y s t a l with the c - a x i s n o r m a l to the rod and the a - a x e s at s o m e
50
~'1At2o~
&0
CdS//7L-
/
.u
oj/
o
30
.~_
20 ,-
10
i,=.
0 0-8
I 12
Theory oooo Experiment t I A I t I 20 24 28 32 36 40 Frequency [GH ~]
t 16
F i g 3 Comparison of theoretical tranoduetion loss v f r e quency with experiment, for Cds (half-wave) f i l m at 2 . 1 G H z on gold half-wave f i l m at 2 . 5 G H z .
60 50 ~, .-o. 40
Cds ~ Transduce'l]
;~
Z= 20.7
A[203
I a~,~.
~'[~]~' 7
Matching
-
-
l:
~.=45 . ~ . ........_ ~ . / :
/~-" L:
o ~, 30 ~ 20
E=10
SiO 2
~
~=15
C~Is :~=20.7
10
0.2
0.4
0.6
0:8
NormaLized
MATERIALS
Fig In m o s t m i c r o w a v e delay line applications, delays in e x c e s s of 1/~s a r e r e q u i r e d and t h e r e f o r e the l o s s in the m a t e r i a l has to be low. Oliver and Slack 9 have shown that t h e r e a r e
fl= 2.5GHz-
fo = 2.1GHa
1.0
1.2
frequency f i f o
1.4. 1.6
1.8 2.0
[-GH~.]
4 T h e o r e t i c a l transduction l o s s v e r s u s n o r m a l i z e d
f r e q u e n c y for t r a n s d u c e r s having e l e c t r o d e s of; m a g n e s i u m , s i l i c o n dioxide and c~dmiuln sulphide all of one w a v e l e n g t h t h i c k n e s s at foULTRASONICS October 1970
231
Table 1 C o m p a r i s o n of m a t e r i a l s which might be u s e d a s d e l a y line m e d i a at m i c r o w a v e f r e q u e n c i e s
Mode
Direction [dB m m -1]
Attenuation at 1GHz [dB m m -1]
4.4
L
a or c
0. 025
0.34
1.9
S
c
0. 044
1.0 1.0
0.06 0.04
0.6 0.22
L S
c c
0.06 0.04
C
1.0 1.0
0.3 0.21
1.7 1.3
L L
C
1.0
0.044
0.4
S
a
0.044
C
1.0
0.11
0.55
S
a
0.11
F r e q u e n c y Attenuation [GHz] [dB m m -1]
[dS ~s -1]
4.0
0.40
A
2.8
B Quartz SiO 2 Periclase MgO
Material
Reference
Sapphire A120 3
This author
Rutile TiO 2
YAG Y3A15012
Xcut Z cut
0.3 0.21
YIG Y3Fe5012
D
1.0
0.034
0.13
S
Y3Ga5012
D
1.0 1.0
0. 025 0.1
0.1 0.7
S L
a or c aorc
0. 025 0.10
Li I ~ O 3
E
0.5 0.5
0. 0045 0.0090
0.03 0. 065
L L
a or c b
0. 018 0. 036
Li TaO 3
E
0.5
0.0017
0.01
L
c
0. 0068
Spinel MgA1204
F
0.034
9.0
36
L
<100)
0. 045
9.0
10
6.5
s
<100>
o. 012
9.0 9.0
20 25 20 9 42
21 12.5 20 5.9 18
L S
(111)
0. 025 0.032 o. 025 0.011 0. 052
9.0 9.0 9.0
32
a
L
F.S S. S
<110> (110>
References A Shaw, H. J. et al, Applied Physics Letters, Vol 4 (1964), No 2, p 28
D Holland, J o u r n a l of Applied P h y s i c s , Vol 38 (1967), No 10
B Midford, T.A.M. & Warnuga, S., Journal of Applied Physics, Vol 36
E S p e n c e r & Lenzo, J o u r n a l of Applied P h y s i c s , Vol 38 (1967), No 1, p 1.
(1965), p 3362 C Oliver & Slack, J o u r n a l of Applied P h y s i c s , V o l 37 (1966) No 4 , p 1542 angle to the r o d a x i s - - s e e d i n g to c o n t r o l t h e s e i s p o s s i b l e but m o r e expensive. The r e s u l t i n g c r y s t a l i s not p e r f e c t and can have a change in o r i e n t a t i o n of the c r y s t a l l i n e axis, a c r o s s a 20ram d i a m e t e r , of a s much as 1°. The question of o r i e n t a t i o n is a v e r y i m p o r t a n t one f o r double ended o r long delay l i n e s . If the rod axis of a line i s not a l i g n e d to a c r y s t a l axis then the e n e r g y contained in the a c o u s t i c b e a m will move off at s o m e i n t e r m e d i a t e angle b e tween rod and c r y s t a l axes, depending on the e l a s t i c t e n s o r c o m p o n e n t s . F o r a single ended line this is not i m p o r t a n t for, as long a s the ends a r e p a r a l l e l the e n e r g y will r e t u r n along the s a m e path. F o r double ended l i n e s this 'walk off' may be sufficient to c o m p l e t e l y m i s s the output t r a n s d u c e r s i n c e the a c o u s t i c b e a m is n o r m a l l y both n a r r o w and well c o l l i m a t e d . If the c r y s t a l axis w a n d e r s along the a c o u s t i c path then the b e a m may not r e t u r n to the s a m e point again. The total effect of the v a r i a t i o n s in c r y s t a l l i n e axis o r i e n t a tion o b s e r v e d in flame fusion grown s a p p h i r e may be such that an a c o u s t i c b e a m moving in one d i r e c t i o n will be deviated d i f f e r e n t l y f r o m one moving in the opposite d i r e c t i o n . It follows that for long lines, both double and single ended, pulled s a p p h i r e is p r e f e r a b l e , while for s h o r t lines, providing the rod axis i s made a s n e a r p a r a l l e l to the a v e r a g e c r y s t a l line axis, f l a m e fusion grown s a p p h i r e may be u s e d . The l o s s in s a p p h i r e may be r e d u c e d by cooling. F r o m 293°K to 100°K the i m p r o v e m e n t is not great, f r o m 100°K to 60°K a v e r y s t e e p change in attenuation o c c u r s and f r o m 60°K down, the change is again s m a l l . T h e r e f o r e liquid n i t r o g e n (BPt 77°K) may be u s e d to v e r y good effect to cool 232
ULTRASONICS October 1970
L e w i s & P a t t e r s o n , J o u r n a l of Applied P h y s i c s , Vol 39 (1968), p 3420 the delay line when long d e l a y s at high f r e q u e n c i e s a r e r e q u i r e d . The i m p r o v e m e n t is v e r y m a r k e d around X band w h e r e a r o o m t e m p e r a t u r e l o s s of 2 d B / m may be r e d u c e d to 0 . 2 d B / m m , but at 77°K the r a t e of change of attenuation with f r e q u e n c y is not a s f2 but much s l o w e r ie around fo.5.
DELAY
LINE P E R F O R M A N C E
The Author has made l i n e s f r o m 1 to 12GHz with delays f r o m 1 to 14gs. T h r e e will be d e s c r i b e d in o r d e r to outline the p r o b l e m s involved. The m o s t s t r a i g h t - f o r w a r d l i n e s a r e t h o s e that can be u s e d in a p u l s e d r.f. s y s t e m w h e r e the pulse is s h o r t c o m p a r e d with the delay. T h e s e can be single ended with the s a m e t r a n s d u c e r as t r a n s m i t t e r and r e c e i v e r and a r e usually fitted on one a r m of a c i r c u l a t o r . The r e f l e c t e d e n e r g y f r o m the t r a n s d u c e r i s g e n e r a l l y l a r g e r than f r o m all delayed e c h o e s but for p u l s e d operation this may be u s e d a s a r e f e r e n c e signal. If n e c e s s a r y it may be s u p p r e s s e d by gating the i.f. a m p l i f i e r . In g e n e r a l it is not p o s s i b l e to run a single ended line c.w. If the line can a l s o be n a r r o w band then a s i m p l e double stub coaxial t u n e r may be used to e q u a l i s e C O and m a t c h the line i m p e d a n c e to the s e r i e s l o s s r e s i s t a n c e . If t h i s is to be done at X-band it is b e t t e r to use an E - H t u n e r in the waveguide s i n c e a coaxial t u n e r would have too g r e a t a l o s s . An e x a m p l e of such a device was a 14.25ps line working at 3GHz which had a total i n s e r t i o n l o s s of 74dB and a b a n d - w i d t h of 2flMHz. It c o n s i s ted of a s a p p h i r e rod 10mm d i a m e t e r and 80mm long with p o l i s h e d ends flat to 1/10 of a sodium light band and p a r a l l e l
to 3s of arc'. T h e m e t a l b a s e e l e c t r o d e w a s 800~, of gold and the t r a n s d u c e r w a s a q u a r t e r of a w a v e l e n g t h thick at 3GHz. ' f h e l o s s w~s m a d e up of a t r a n s m i s s i o n l o s s of 48dB and a I o s s p e r t r a n s d u c e r of 13dB. W h e n the line h a s to be u s e d c.w. it h a s to be double ended with one t r a n s d u c e r a s t r a n s m i t t e r and t h e o t h e r a s r e c e i v e r . U n l e s s the r o d u s e d i s high quality pulled s a p p h i r e s o m e a l l o w a n c e h a s to be m a d e to a d j u s t the pick up point of the output t r a n s d u c e r to o p t i m i s e the i n s e r t i o n l o s s . Such l i n e s h a v e b e e n m a d e for u s e at 4 . 3 G H z f o r r a d i o a l t i m e t e r c a l i b r a t i o n . T h e y w e r e 150, 390 and 1520m (500, 1300 and 5000ft) e q u i v a l e n t - h e i g h t long ie l g s , 2 . 6 g s a n d 10gS r e s p e c t i v e l y . T h e l o s s r e q u i r e m e n t s w e r e s u c h t h a t the s h o r t e s t could be u n t u n e d a n d h e n c e give a b a n d width >100MHz, the s e c o n d had output tuned only a n d g a v e a bandwidth of 30MHz while the l o n g e s t had to h a v e both e n d s t u n e d a n d had a band width of 20MHz. T h e s e w i d t h s s a t i s f i e d the r e q u i r e m e n t s of the a l t i m e t e r a n d the m a t c h i n g w a s a c h i e v e d with double s t u b t u n e r s . T h e l o n g e s t line had a total l o s s of 80dB and w a s m a d e f r o m c - a x i s - o r i e n t a t e d pulled s a p p h i r e with v e r y well p o l i s h e d e n d s a n d r o d a n d c r y s t a l a x e s p a r a l l e l to within 1° of a r c . T h e m a i n p r o b l e m with double ended l i n e s i s that t h e r e is a l e a k a g e of s i g n a l f r o m input to output which can be s o l a r g e that the d e l a y e d s i g n a l r e m a i n s u n d e t e c t e d . A l t i m e t e r s have a f r e q u e n c y s e l e c t i v i t y s u c h that n e a r r e f l e c t i o n s a r e a t t e n u a t e d m o r e t h a n f a r , t h u s f o r a 1520m (5000ft) line the p i c k up s i g n a l m u s t be l e s s t h a n 15dB above t h e d e l a y e d s i g n a l if the i n s t r u m e n t i s not to lock on z e r o . T h i s l e a k a g e can be r e d u c e d by v e r y c a r e f u l e a r t h i n g of the t r a n s d u c e r b a s e e l e c t r o d e but it is m o s t e a s i l y e l m i n a t e d by m o u n t i n g the d e l a y line s u c h t h a t the s a p p h i r e i s the d i e l e c t r i c of a c i r c u l a r w a v e g u i d e below cut off. T h i s w o r k s well f o r d e l a y s l o n g e r t h a n I ~ S at 4 . 3 G H z when the rod d i a m e t e r i s 10ram. F o r s h o r t e r d e l a y s e v e n t h e s e m e t h o d s will not work a s the b a s e e l e c t r o d e i s t h i n n e r than the s k i n depth and e n e r g y will r a d i ate t h r o u g h it. Both the above l i n e s w e r e n a r r o w band b e c a u s e the m a t c h i n g w a s not o p t i m u m . A s h a s b e e n m e n t i o n e d in the b e g i n n i n g of t h i s p a p e r the w i d e s t b a n d - w i d t h with low i n s e r t i o n l o s s i s o b t a i n e d when the line i m p e d a n c e i s m a d e n e a r l y equal to the s e r i e s l o s s r e s i s t a n c e and C O i s tuned out. One m e t h o d of a c h i e v i n g t h i s i s by the u s e of a c o a x i a l t a p e r , the d i m e n s i o n s of which a r e s u c h a s to r e d u c e f r o m a Z o of 50f~ to one of 512 in a l e n g t h e q u a l to half a w a v e l e n g t h or i t s odd m u l t i p l e s . Such a t a p e r h a s b e e n m a d e to be half a w a v e l e n g t h l o n g a t 4GHz. The t r a n s d u c e r r e a c t a n c e w a s s e t at 5 ~ by e v a p o r a t ing an a r r a y of s p o t s s e v e n t h o u s a n d t h s of an inch in d i a m e t e r on i t s f r e e face. By m o v i n g the delay line a r o u n d r e lative to the end of the t a p e r , c o n t a c t could be m a d e to any of t h e s e s p o t s . No d i r e c t a t t e m p t w a s m a d e to tune out C o but the c a p a c i t a n c e due to c h a n g e in d i m e n s i o n s at the 5012 end of the t a p e r , half a w a v e l e n g t h away, gave s o m e f o r t u i t o u s t u n i n g a n d c o n t r i b u t e d to the low i n s e r t i o n l o s s o b s e r v e d . T h e b a n d width a n d i n s e r t i o n l o s s of a t r a n s d u c e r u s i n g s u c h a m a t c h i n g s y s t e m i s shown in F i g 5, t h e bandwidth w a s v e r y wide and w a s e n t i r e l y l i m i t e d by the t r a n s d u c e r . T h e b a n d width of the t r a n s f o r m e r , m e a s u r e d f r o m i t s v . s . w . r , with t r a n s d u c e r a s load, w a s f r o m 3 to 6GHz. T h e s a m e t a p e r w a s a l s o u s e d at X - b a n d . F i g 6 s h o w s the B a n d - p a s s of a 1 . 6 5 p s l i n e w o r k i n g a t r o o m t e m p e r a t u r e . In t h i s c a s e the bandwidth i s l i m i t e d by the c h a n g e in a t t e n u a t i o n in the s a p p h i r e a s the s q u a r e of the f r e q u e n c y , coupled with the c h a n g e in C O with f r e q u e n c y . T h e t r a n s d u c e r i t s e l f had a v e r y b r o a d bandwidth at t h i s f r e q u e n c y a s it w a s half a w a v e l e n g t h thick at 9GHz on a thin gold film, the b a n d - p a s s of which i s the c o n t i n u o u s line shown in F i g 2. A n o t h e r t r a n s f o r m i n g m e t h o d i s to m a k e a w a v e g u i d e to c o a x i a l t r a n s f o r m e r to m a t c h d i r e c t to 5 ~. T h i s h a s b e e n t r i e d with l i m i t e d s u c c e s s . T h e t a p e r gave b e t t e r r e s u l t s but w a s m o r e bulky. By c a r e f u l m a t c h i n g , b a n d w i d t h s have b e e n obtained which a r e l i m i t e d only by the t r a n s d u c e r i t s e l f . By e x a c t d e s i g n of m a t c h i n g c i r c u i t s and t r a n s d u c e r s t r u c t u r e s m a n y r e q u i r e d b a n d p a s s s h a p e s m a y be s y n t h e s i s e d up to a m a x i m u m width of a r o u n d 50% at low f r e q u e n c i e s . T h e u p p e r f r e q u e n c y l i m i t i s s e t m o r e by the c h a n g e of a t t e n u a t i o n with f r e q u e n c y than by the t r a n s d u c e r .
30 I ¢--1
en
o 20
1'Ya dB
"o
,-10
3
3.1
Frequency [GH,]
4
4.25
F i g 5 T r a n s d u c e r b a n d - w i d t h of a 0 . 2 5 ran t h i c k CdS f i l m
on a thin gold f i l m with a counter d i a m e t e r e l e c t r o d e , e l e c t r i c a l l y matched with a 5 0 - 5 f l c o a x i a l taper h a l f a wavelength long at 4GHz.
[]
Tuned at each frequency Instantaneous bendpass
70'~ .~.
°
c 65.o
0
c
D 601
o
0 55
I 8,O
8.5
I
I
9.O 9.5 ~uency ~H~
I
10.0
F i g 6 B a n d - p a s s of 1 . 6 5 / ~ s delay Une at Xband. Matched with a 5 0 - 5 ~ c o a x i a l taper one and a half w a v e l e n g t h s long at 10GHz. A l s o shown i s the i n s e r t i o n l o s s of the delay line tuned at e a c h point. Upward trend e n t i r e l y due to the change in t r a n s m i s s i o n l o s s with frequency.
REFERENCES 1
de K l e r k , J. and Kelley, E. F. R e v i e w of Scientific I n s t r u m e n t s , Vol 36 (1965) No 4, p 506. A l s o ' T h i n f i l m zinc oxide t r a n s d u c e r s f o r u s e in m i c r o w a v e d e v i c e s ' , U l t r a s o n i c s , Vol 8, No 3.
2
F o s t e r , N . F . IEEE T r a n s a c t i o n s on Sonics and U l t r a s o n i c s SU-11 (1964) No 2, p 63.
3
B e e c h a m , D. U l t r a s o n i c s Vol 5, (1967), No 1, p 19.
4
Mitchell, R . F .
5
Duncan, W., H u t c h i n s , R. H. and Stewart, P . E . M . V a c u u m Science and T e c h n o l o g y , Vol 6, No 4, p 555.
U l t r a s o n i c s Vol 6, No 2 , ( 1 9 6 8 ) , p 112.
6
R e e d e r , T. M., P r o c IEEE, Vol 55 (1967), p 1099.
7
B a h r , A. J. and Court, I. N., IEEE Sonics and U l t r a s o n i c s S y m p o s i u m ( V a n c o u v e r 1967).
8
Malbon, R. M., W a l s h , D. J . and Winslow, A. K., Applied P h y s i c s L e t t e r s , Vol 10, No 1, (1967), p 9.
9
O l i v e r , D. W. and Slack, G. A., J o u r n a l of Applied P h y s i c s , (1966),Vol 37, No 4, p 1542.
ULTRASONICS O c t o b e r 1970
233
The e s t a b l i s h e d t e c h n o l o g y for m a k i n g e v a p o r a t e d thin f i l m p i e z o e l e c t r i c t r a n s d u c e r s h a s m a d e the c o n s t r u c t i o n of d e lay l i n e s at m i c r o w a v e f r e q u e n c i e s p o s s i b l e . T h i n f i l m s of CdS on s a p p h i r e d e l a y m e d i a have b e e n u s e d to m a k e c.w. and p u l s e d d e l a y l i n e s f r o m 1-11GHz, with d e l a y s f r o m
1 -14~ts o p e r a t i n g at 77°K and 300°K. The p e r f o r m a n c e of s o m e of t h e s e l i n e s i s d e s c r i b e d . T h e p a r a m e t e r s g o v e r n i n g the choice of d e l a y m e d i a , t r a n s d u c e r m a t e r i a l and e l e c t r o d e s t r u c t u r e a r e d i s c u s s e d and the i m p o r t a n c e of c a r e f u l e l e c t r i c a l m a t c h i n g of t r a n s d u c e r to g e n e r a t o r i s e m p h a s i s e d .
The d e v e l o p m e n t of a t e c h n i q u e for m a k i n g thin f i l m s of p i e z o e l e c t r i c m a t e r i a l s t h a t can have f u n d a m e n t a l r e s o n a n c e s a s high a s 12GHz h a s m a d e it p o s s i b l e to m a k e delay l i n e s up to that f r e q u e n c y . So it is now f e a s i b l e to i n s e r t d e l a y s at r . f . r a t h e r than at i. f. in a r a d a r s y s t e m . T h i s p a p e r d e s c r i b e s the c o n s t r u c t i o n and p e r f o r m a n c e of s o m e typical l i n e s m a d e by the A u t h o r while w o r k i n g at S t a n d a r d T e l e c o m m u n i c a t i o n s L a b o r a t o r i e s and s o m e of the p r o b l e m s a s s o c i a t e d with t h e m .
L Band. T h e m a i n p r o b l e m with the t r a n s d u c e r s o f o r m e d i s the d o m i n a n t effect the c o n d u c t i n g f i l m will h a v e on the b a n d p a s s a s it h a s to be m o r e than an a c o u s t i c w a v e l e n g t h thick.
TRANSDUCERS The m e t h o d s of d e p o s i t i o n o f p i e z o e l e c t r i c thin f i l m s h a v e b e e n d e s c r i b e d in m u c h detail e l s e w h e r e 1,2, 3 and n e e d no f u r t h e r e n l a r g e m e n t h e r e . All the d e l a y l i n e s d e s c r i b e d had t r a n s d u c e r s p r e p a r e d by the c o e v a p o r a t i o n of c a d m i u m and s u l p h u r . T h i s m e t h o d i s to be p r e f e r r e d f o r m i c r o w a v e u s e a s it p r o d u c e s f i l m s of u n i f o r m high r e s i s t i v i t y . The f i l m s a r e p o l y c r y s t a l l i n e with a p r e f e r r e d o r i e n t a t i o n of the c - a x i s n o r m a l to the s u b s t r a t e and a s s u c h could g e n e r a t e l o n g i tudinal m o d e s o u n d w a v e s with v e r y s m a l l s h e a r m o d e c o m p o n e n t s . The a b s e n c e of a n o t h e r m o d e i s i m p o r t a n t if long d e l a y s a r e r e q u i r e d f o r which the p u r e s t e x p o n e n t i a l d e l a y is essential. S h e a r w a v e s can be g e n e r a t e d by thin f i l m t r a n s d u c e r s p r o duced both by d i r e c t v a p o u r s t r e a m f r o m a CdS s o u r c e and by c o e v a p o r a t i o n of c a d m i u m and s u l p h u r . T h e u s u a l m e t h o d of a c h i e v i n g t h i s is to have the s u b s t r a t e tilted r e l a t i v e to the i n c i d e n t a t o m i c s t r e a m s u c h that the c - a x i s is m a d e to lie at s o m e angle to the n o r m a l . The t r a n s d u c e r t h e n g e n e r a t e s both l o n g i t u d i n a l and s h e a r m o d e c o m p o n e n t s but the f o r m e r m a y be m i n i m i s e d by s e t t i n g the c - a x i s at s o m e c r i t i c a l a n g l e 4. If t h i s t i l t i n g w e r e a c h i e v a b l e e i t h e r i m m e d i ately a f i l m s t a r t e d to f o r m o r a f t e r only a few h u n d r e d A n g s t r o m s t h e n s u c h t r a n s d u c e r s could be u s e d to g e n e r a t e s h e a r w a v e s at m i c r o w a v e f r e q u e n c i e s . In p r a c t i c e , h o w e v e r , the c - a x i s b e n d s o v e r slowly and it r e q u i r e s s e v e r a l t h o u s a n d s of A n g s t r o m s t h i c k n e s s b e f o r e the c r i t i c a l angle i s r e a c h e d , t h e r e f o r e only low f r e q u e n c y f u n d a m e n t a l t r a n s d u c e r s can n o r m a l l y be p r o d u c e d . If d u r i n g the initial g r o w t h the t r a n s d u c e r w e r e doped so a s to be c o n d u c t i n g until the c r i t i c a l angle a l i g n m e n t had b e e n r e a c h e d and then a high r e s i s t i v i t y f i l m d e p o s i t e d a s a c o n t i n u a t i o n of the g r o w t h p r o c e s s , that u p p e r f i l m could be m a d e r e s o n a n t at m i c r o w a v e f r e q u e n c i e s with the l o w e r f i l m a c t ing a s b a s e e l e c t r o d e . D r D u n c a n 5 et al at G l a s g o w U n i v e r s i t y have t h u s m a d e s h e a r wave t r a n s d u c e r s o p e r a t i n g at R. W. H a r c o u r t , BSc I T T C o m p o n e n t s Group E u r o p e , S t a n d a r d T e l e p h o n e s and C a b l e s L i m i t e d , Q u a r t z C r y s t a l D i v i s i o n , E d i n b u r g h Way, Harlow, E s s e x .
230
ULTRASONICS O c t o b e r 1970
A n o t h e r m e t h o d of m a k i n g s h e a r wave e v a p o r a t e d t r a n s d u c e r s i s to g r o w t h e m e p i t a x i a l y on a c r y s t a l l i n e s u b s t r a t e , t h u s c r e a t i n g o r d e r e d o r i e n t a t i o n in the a, b and c - a x e s d i r e c t i o n . de K l e r k 1 h a s u s e d s u c h a m e t h o d to g e n e r a t e longitudinal, f a s t and slow s h e a r w a v e s f r o m a s i n g l e t r a n s d u c e r . P u t t i n g a m e t a l f i l m b e t w e e n s u b s t r a t e and t r a n s d u c e r m a k e s e p i t a x ial g r o w t h h a r d to r e a l i s e and o m i t t i n g it l e a d s to high insertion loss transducers. The b a n d p a s s of a t r a n s d u c e r i s of p r i m a r y i m p o r t a n c e to the o v e r a l l p e r f o r m a n c e of a d e l a y line, in s o m e i n s t a n c e s it i s e v e n m o r e i m p o r t a n t than the i n s e r t i o n l o s s . In o r d e r to p r e d i c t what the s h a p e of the f r e q u e n c y r e s p o n s e will be t h e e q u i v a l e n t c i r c u i t of the t r a n s d u c e r d e l a y line s y s t e m h a s to be a n a l y s e d . It c a n be shown t h a t the e q u i v a l e n t c i r c u i t of a t r a n s d u c e r in d i r e c t c o n t a c t with the delay line, and with a v e r y thin m e t a l b a s e e l e c t r o d e , i s that given in F i g 1 w h e r e C O i s the c l a m p e d c a p a c i t a n c e of the t r a n s d u c e r , r s i s the s e r i e s l o s s due to the e l e c t r o d e s , B T is the s u s c e p t a n c e and G T the c o n d u c t a n c e of the t r a n s d u c e r . In g e n e r a l , for low coupling c o n s t a n t t r a n s d u c e r s , B T << wC o and m a y be i g n o r e d . Away f r o m the half w a v e l e n g t h r e s o n a n c e f r e q u e n c y , fo, of the t r a n s d u c e r G w i s given by rKw2 Gw -
X0 ~
(1 -- Cos0) 2 (Sin20+r2Cos2~)
where X0 = (2~f C6) - I r - Acoustic impedance of delay medium/Acoustic impedance of transducer KT is the piezoelectric coupling constant and 0 --
~f fo
T h i s c o n d u c t a n c e t e r m c h a r a c t e r i s e s the b a n d - w i d t h of the t r a n s d u c e r and, when r e l a t e d to the p o w e r d e l i v e r e d to it and the s e r i e s l o s s , g i v e s the i n s e r t i o n l o s s a s well. When the m e t a l b a s e e l e c t r o d e i s m a d e of the s a m e o r d e r of t h i c k n e s s a s the t r a n s d u c e r or s e v e r a l r e s o n a n t f i l m s a r e i n t r o d u c e d b e t w e e n the t r a n s d u c e r and delay m e d i u m then the f r e q u e n c y d e p e n d a n t p a r t of G w i s m o d i f i e d . T h i s c a n be s e e n in Fig 2 w h e r e the t h e o r e t i c a l b a n d - w i d t h i s shown of a 0 . 2 5 ~ m thick CdS f i l m on a gold e l e c t r o d e , one q u a r t e r w a v e l e n g t h t h i c k n e s s at fo on a s a p p h i r e delay m e d i u m . The q u a r t e r w a v e length d i p s in the c u r v e a r e d e e p e n e d both by i n c r e a s i n g the gold f i l m t h i c k n e s s and by u s i n g an e l e c t r o d e with an a c o u s t i c i m p e d a n c e not well m a t c h e d to b o t h line and t r a n s d u c e r . T h u s i n s e r t i o n l o s s m a y be i m p r o v e d but at the e x p e n s e of bandwidth. It i s t h u s p o s s i b l e to d e s i g n the p a s s b a n d s h a p e
rs
o~
/k/~tVM ~
I
p1
CO ~ / / / / / / / /
I
0
F i g 1 Equivalent c i r c u i t of a t r a n s d u c e r r i g i d l y connected to a delay medium. 60
50 $ 4O _9o
I II de[a)'
~-,
~'~:Matching fiim--~smedium N : '=f° . . . . . . . . .
..'/]
;==45,0
/.~y
.........
._~ 30
g -o
20
~-
10 0
Tungsten ;zl= 103 !
0"2
I
Gotd ~t =61"5 I
I
No matching film fl =oo I
I
I
I
I
0-4 0"6 0"8 1.0 1.2 1.4 1.6 1.8 2.0 NormaLized frequency fifo [GHz]
F i g ~. T h e o r e t i c a l transduction l o s s v e r s u s n o r m a l i z e d f r e q u e n c y f o r t r a n s d u c e r s having: no e l e c t r o d e ; h a l f w a v e l e n g t h thick gold e l e c t r o d e at fo; h a l f - w a v e l e n g t h t h i c k t u n g s t e n e l e c t r o d e a t fo-
to suit the ultimate r e q u i r e m e n t of the line. This can be r e a l i s e d in p r a c t i c e as shown in Fig 3 w h e r e e x p e r i m e n t a l r e s u l t s for a p a r t i c u l a r t r a n s d u c e r a r e c o m p a r e d with theory. The double m i n i m a c u r v e s shown in Fig 2 a r e c h a r a c t e r i s t i c of a t r a n s d u c e r of h a l f - w a v e l e n g t h t h i c k n e s s at fo which has b e e n d e p o s i t e d on a delay m e d i u m where the a c o u s t i c i m p e dance r a t i o r is a s high as 2. The depth of t h e s e m i n i m a may be i n c r e a s e d by u s i n g m o r e than one matching film and, if a single m i n i m a is r e q u i r e d at fo, e i t h e r r m u s t be n e a r e r unity o r the e l e c t r o d e f i l m s m u s t be made one wavelength thick at fo" T h e o r e t i c a l c u r v e s for the l a t t e r a r e shown in Fig 4. The a v e r a g e i n s e r t i o n l o s s of the t r a n s d u c e r is g o v e r n e d by K 2 . It has not been found to a g r e e with the value given for the bulk, and t h e r e has been s o m e d i s c r e p a n c y between its value obtained by R e e d e r 6 and that given by Dahr and Court 7. If the quality of f i l m s u s e d in both c a s e s a r e a s s u m e d to be about equal, the d i s c r e p a n c y may well be due to doubt in the value of X o. The optimum value of K w f o r CdS is that of the bulk m a t e r i a l and is s e t at 0.22, if ZnO w e r e u s e d i n s t e a d it could be i n c r e a s e d to 0.34 and the a v e r a g e i n s e r t i o n l o s s l o w e r e d 8. If the s e r i e s l o s s t e r m r s w e r e absent, then by the use of a t r a n s f o r m e r to m a t c h G T to the line and by tuning out the c a p a c i t a n c e of the t r a n s d u c e r (which is c o n t r o l l e d only by its active a r e a and t h i c k n e s s ) a p e r f e c t m a t c h would be p o s sible with z e r o i n s e r t i o n l o s s . However the s e r i e s l o s s t e r m m a k e s this i m p o s s i b l e as it d o m i n a t e s the s y s t e m . It i s f a r m o r e p r o d u c t i v e to t r a n s f o r m the line i m p e d a n c e down to m a t c h r s and again tune out C O to the new situation. A1though l o s s i s given d i r e c t l y by the r e l a t i o n b e t w e e n r s and GT, the o p t i m u m bandwidth of the t r a n s d u c e r is achieved without i t s being l i m i t e d by the tuning c i r c u i t r y . The use of such a t r a n s f o r m e r is d e s c r i b e d l a t e r .
s e v e r a l well defined p r o p e r t i e s that the m a t e r i a l should have for it to give low attenuation to acoustic waves above 1GHz. It m u s t be a single c r y s t a l with c o m p l e x s t r u c t u r e and low m a s s a t o m s . It m u s t have a low Debye T e m p e r a t u r e and low t h e r m a l conductivity. T h e s e a r e n e c e s s a r y but not sufficient conditions. C r y s t a l s that s a t i s f y them for which m e a s u r e m e n t s a r e available a r e l i s t e d in Table 1. All t h e s e values a r e for 20°C at which t e m p e r a t u r e the attenuation c h a n g e s a s the s q u a r e of the f r e q u e n c y . The table shows that s a p p h i r e is as good a m a t e r i a l as any for longitudinal waves except Lithium Tantalate and Lithium Niobate. N e i t h e r of the l a t t e r have been m e a s u r e d at 1GHz or above so that t h e i r potential low l o s s has not been p r o v e d . F u r t h e r , they a r e difficult m a t e r i a l s to make in s t o i c h i o m e t r i c , high quality s i n g l e c r y s t a l s and so a r e v e r y e x p e n s i v e . By using s h e a r waves which have l o w e r v e l o c i t i e s , s o m e advantage may be gained using YIG o r Spinel but the technical difficulties in g r o w i n g s h e a r wave t r a n s d u c e r s offset t h e s e advantages somewhat. Both YIG and Spinel can be grown to a high o r d e r of c r y s t a l l i n e p e r f e c t i o n and to l a r g e enough s i z e s for delays g r e a t e r than 10gs, t h e r e f o r e f o r high f r e q u e n c i e s or long d e l a y s they would be v e r y useful once the s h e a r wave t r a n s d u c e r technology has been i m p r o v e d . All the delay lines made by the Author had s a p p h i r e a s the medium, o t h e r m a t e r i a l s w e r e m e a s u r e d for l o s s but not u s e d in any d e v i c e s . Sapphire can be grown in two ways, e i t h e r by f l a m e fusion o r by pulling the s e e d f r o m the m e l t (Czochralski). The f o r m e r i s the c h e a p e r method but does not p r o d u c e such a good c r y s t a l a s does the l a t t e r . The pulling p r o c e s s can be c o n t r o l l e d to give a p a r t i c u l a r o r i e n tation depending on the s e e d and that o r i e n t a t i o n can be m a i n t a i n e d throughout the growth f o r lengths of c r y s t a l g r e a t e r than 120mm. F l a m e fusion usually p r o d u c e s c r y s t a l with the c - a x i s n o r m a l to the rod and the a - a x e s at s o m e
50
~'1At2o~
&0
CdS//7L-
/
.u
oj/
o
30
.~_
20 ,-
10
i,=.
0 0-8
I 12
Theory oooo Experiment t I A I t I 20 24 28 32 36 40 Frequency [GH ~]
t 16
F i g 3 Comparison of theoretical tranoduetion loss v f r e quency with experiment, for Cds (half-wave) f i l m at 2 . 1 G H z on gold half-wave f i l m at 2 . 5 G H z .
60 50 ~, .-o. 40
Cds ~ Transduce'l]
;~
Z= 20.7
A[203
I a~,~.
~'[~]~' 7
Matching
-
-
l:
~.=45 . ~ . ........_ ~ . / :
/~-" L:
o ~, 30 ~ 20
E=10
SiO 2
~
~=15
C~Is :~=20.7
10
0.2
0.4
0.6
0:8
NormaLized
MATERIALS
Fig In m o s t m i c r o w a v e delay line applications, delays in e x c e s s of 1/~s a r e r e q u i r e d and t h e r e f o r e the l o s s in the m a t e r i a l has to be low. Oliver and Slack 9 have shown that t h e r e a r e
fl= 2.5GHz-
fo = 2.1GHa
1.0
1.2
frequency f i f o
1.4. 1.6
1.8 2.0
[-GH~.]
4 T h e o r e t i c a l transduction l o s s v e r s u s n o r m a l i z e d
f r e q u e n c y for t r a n s d u c e r s having e l e c t r o d e s of; m a g n e s i u m , s i l i c o n dioxide and c~dmiuln sulphide all of one w a v e l e n g t h t h i c k n e s s at foULTRASONICS October 1970
231
Table 1 C o m p a r i s o n of m a t e r i a l s which might be u s e d a s d e l a y line m e d i a at m i c r o w a v e f r e q u e n c i e s
Mode
Direction [dB m m -1]
Attenuation at 1GHz [dB m m -1]
4.4
L
a or c
0. 025
0.34
1.9
S
c
0. 044
1.0 1.0
0.06 0.04
0.6 0.22
L S
c c
0.06 0.04
C
1.0 1.0
0.3 0.21
1.7 1.3
L L
C
1.0
0.044
0.4
S
a
0.044
C
1.0
0.11
0.55
S
a
0.11
F r e q u e n c y Attenuation [GHz] [dB m m -1]
[dS ~s -1]
4.0
0.40
A
2.8
B Quartz SiO 2 Periclase MgO
Material
Reference
Sapphire A120 3
This author
Rutile TiO 2
YAG Y3A15012
Xcut Z cut
0.3 0.21
YIG Y3Fe5012
D
1.0
0.034
0.13
S
Y3Ga5012
D
1.0 1.0
0. 025 0.1
0.1 0.7
S L
a or c aorc
0. 025 0.10
Li I ~ O 3
E
0.5 0.5
0. 0045 0.0090
0.03 0. 065
L L
a or c b
0. 018 0. 036
Li TaO 3
E
0.5
0.0017
0.01
L
c
0. 0068
Spinel MgA1204
F
0.034
9.0
36
L
<100)
0. 045
9.0
10
6.5
s
<100>
o. 012
9.0 9.0
20 25 20 9 42
21 12.5 20 5.9 18
L S
0. 025 0.032 o. 025 0.011 0. 052
9.0 9.0 9.0
32
a
L
F.S S. S
<110> (110>
References A Shaw, H. J. et al, Applied Physics Letters, Vol 4 (1964), No 2, p 28
D Holland, J o u r n a l of Applied P h y s i c s , Vol 38 (1967), No 10
B Midford, T.A.M. & Warnuga, S., Journal of Applied Physics, Vol 36
E S p e n c e r & Lenzo, J o u r n a l of Applied P h y s i c s , Vol 38 (1967), No 1, p 1.
(1965), p 3362 C Oliver & Slack, J o u r n a l of Applied P h y s i c s , V o l 37 (1966) No 4 , p 1542 angle to the r o d a x i s - - s e e d i n g to c o n t r o l t h e s e i s p o s s i b l e but m o r e expensive. The r e s u l t i n g c r y s t a l i s not p e r f e c t and can have a change in o r i e n t a t i o n of the c r y s t a l l i n e axis, a c r o s s a 20ram d i a m e t e r , of a s much as 1°. The question of o r i e n t a t i o n is a v e r y i m p o r t a n t one f o r double ended o r long delay l i n e s . If the rod axis of a line i s not a l i g n e d to a c r y s t a l axis then the e n e r g y contained in the a c o u s t i c b e a m will move off at s o m e i n t e r m e d i a t e angle b e tween rod and c r y s t a l axes, depending on the e l a s t i c t e n s o r c o m p o n e n t s . F o r a single ended line this is not i m p o r t a n t for, as long a s the ends a r e p a r a l l e l the e n e r g y will r e t u r n along the s a m e path. F o r double ended l i n e s this 'walk off' may be sufficient to c o m p l e t e l y m i s s the output t r a n s d u c e r s i n c e the a c o u s t i c b e a m is n o r m a l l y both n a r r o w and well c o l l i m a t e d . If the c r y s t a l axis w a n d e r s along the a c o u s t i c path then the b e a m may not r e t u r n to the s a m e point again. The total effect of the v a r i a t i o n s in c r y s t a l l i n e axis o r i e n t a tion o b s e r v e d in flame fusion grown s a p p h i r e may be such that an a c o u s t i c b e a m moving in one d i r e c t i o n will be deviated d i f f e r e n t l y f r o m one moving in the opposite d i r e c t i o n . It follows that for long lines, both double and single ended, pulled s a p p h i r e is p r e f e r a b l e , while for s h o r t lines, providing the rod axis i s made a s n e a r p a r a l l e l to the a v e r a g e c r y s t a l line axis, f l a m e fusion grown s a p p h i r e may be u s e d . The l o s s in s a p p h i r e may be r e d u c e d by cooling. F r o m 293°K to 100°K the i m p r o v e m e n t is not great, f r o m 100°K to 60°K a v e r y s t e e p change in attenuation o c c u r s and f r o m 60°K down, the change is again s m a l l . T h e r e f o r e liquid n i t r o g e n (BPt 77°K) may be u s e d to v e r y good effect to cool 232
ULTRASONICS October 1970
L e w i s & P a t t e r s o n , J o u r n a l of Applied P h y s i c s , Vol 39 (1968), p 3420 the delay line when long d e l a y s at high f r e q u e n c i e s a r e r e q u i r e d . The i m p r o v e m e n t is v e r y m a r k e d around X band w h e r e a r o o m t e m p e r a t u r e l o s s of 2 d B / m may be r e d u c e d to 0 . 2 d B / m m , but at 77°K the r a t e of change of attenuation with f r e q u e n c y is not a s f2 but much s l o w e r ie around fo.5.
DELAY
LINE P E R F O R M A N C E
The Author has made l i n e s f r o m 1 to 12GHz with delays f r o m 1 to 14gs. T h r e e will be d e s c r i b e d in o r d e r to outline the p r o b l e m s involved. The m o s t s t r a i g h t - f o r w a r d l i n e s a r e t h o s e that can be u s e d in a p u l s e d r.f. s y s t e m w h e r e the pulse is s h o r t c o m p a r e d with the delay. T h e s e can be single ended with the s a m e t r a n s d u c e r as t r a n s m i t t e r and r e c e i v e r and a r e usually fitted on one a r m of a c i r c u l a t o r . The r e f l e c t e d e n e r g y f r o m the t r a n s d u c e r i s g e n e r a l l y l a r g e r than f r o m all delayed e c h o e s but for p u l s e d operation this may be u s e d a s a r e f e r e n c e signal. If n e c e s s a r y it may be s u p p r e s s e d by gating the i.f. a m p l i f i e r . In g e n e r a l it is not p o s s i b l e to run a single ended line c.w. If the line can a l s o be n a r r o w band then a s i m p l e double stub coaxial t u n e r may be used to e q u a l i s e C O and m a t c h the line i m p e d a n c e to the s e r i e s l o s s r e s i s t a n c e . If t h i s is to be done at X-band it is b e t t e r to use an E - H t u n e r in the waveguide s i n c e a coaxial t u n e r would have too g r e a t a l o s s . An e x a m p l e of such a device was a 14.25ps line working at 3GHz which had a total i n s e r t i o n l o s s of 74dB and a b a n d - w i d t h of 2flMHz. It c o n s i s ted of a s a p p h i r e rod 10mm d i a m e t e r and 80mm long with p o l i s h e d ends flat to 1/10 of a sodium light band and p a r a l l e l
to 3s of arc'. T h e m e t a l b a s e e l e c t r o d e w a s 800~, of gold and the t r a n s d u c e r w a s a q u a r t e r of a w a v e l e n g t h thick at 3GHz. ' f h e l o s s w~s m a d e up of a t r a n s m i s s i o n l o s s of 48dB and a I o s s p e r t r a n s d u c e r of 13dB. W h e n the line h a s to be u s e d c.w. it h a s to be double ended with one t r a n s d u c e r a s t r a n s m i t t e r and t h e o t h e r a s r e c e i v e r . U n l e s s the r o d u s e d i s high quality pulled s a p p h i r e s o m e a l l o w a n c e h a s to be m a d e to a d j u s t the pick up point of the output t r a n s d u c e r to o p t i m i s e the i n s e r t i o n l o s s . Such l i n e s h a v e b e e n m a d e for u s e at 4 . 3 G H z f o r r a d i o a l t i m e t e r c a l i b r a t i o n . T h e y w e r e 150, 390 and 1520m (500, 1300 and 5000ft) e q u i v a l e n t - h e i g h t long ie l g s , 2 . 6 g s a n d 10gS r e s p e c t i v e l y . T h e l o s s r e q u i r e m e n t s w e r e s u c h t h a t the s h o r t e s t could be u n t u n e d a n d h e n c e give a b a n d width >100MHz, the s e c o n d had output tuned only a n d g a v e a bandwidth of 30MHz while the l o n g e s t had to h a v e both e n d s t u n e d a n d had a band width of 20MHz. T h e s e w i d t h s s a t i s f i e d the r e q u i r e m e n t s of the a l t i m e t e r a n d the m a t c h i n g w a s a c h i e v e d with double s t u b t u n e r s . T h e l o n g e s t line had a total l o s s of 80dB and w a s m a d e f r o m c - a x i s - o r i e n t a t e d pulled s a p p h i r e with v e r y well p o l i s h e d e n d s a n d r o d a n d c r y s t a l a x e s p a r a l l e l to within 1° of a r c . T h e m a i n p r o b l e m with double ended l i n e s i s that t h e r e is a l e a k a g e of s i g n a l f r o m input to output which can be s o l a r g e that the d e l a y e d s i g n a l r e m a i n s u n d e t e c t e d . A l t i m e t e r s have a f r e q u e n c y s e l e c t i v i t y s u c h that n e a r r e f l e c t i o n s a r e a t t e n u a t e d m o r e t h a n f a r , t h u s f o r a 1520m (5000ft) line the p i c k up s i g n a l m u s t be l e s s t h a n 15dB above t h e d e l a y e d s i g n a l if the i n s t r u m e n t i s not to lock on z e r o . T h i s l e a k a g e can be r e d u c e d by v e r y c a r e f u l e a r t h i n g of the t r a n s d u c e r b a s e e l e c t r o d e but it is m o s t e a s i l y e l m i n a t e d by m o u n t i n g the d e l a y line s u c h t h a t the s a p p h i r e i s the d i e l e c t r i c of a c i r c u l a r w a v e g u i d e below cut off. T h i s w o r k s well f o r d e l a y s l o n g e r t h a n I ~ S at 4 . 3 G H z when the rod d i a m e t e r i s 10ram. F o r s h o r t e r d e l a y s e v e n t h e s e m e t h o d s will not work a s the b a s e e l e c t r o d e i s t h i n n e r than the s k i n depth and e n e r g y will r a d i ate t h r o u g h it. Both the above l i n e s w e r e n a r r o w band b e c a u s e the m a t c h i n g w a s not o p t i m u m . A s h a s b e e n m e n t i o n e d in the b e g i n n i n g of t h i s p a p e r the w i d e s t b a n d - w i d t h with low i n s e r t i o n l o s s i s o b t a i n e d when the line i m p e d a n c e i s m a d e n e a r l y equal to the s e r i e s l o s s r e s i s t a n c e and C O i s tuned out. One m e t h o d of a c h i e v i n g t h i s i s by the u s e of a c o a x i a l t a p e r , the d i m e n s i o n s of which a r e s u c h a s to r e d u c e f r o m a Z o of 50f~ to one of 512 in a l e n g t h e q u a l to half a w a v e l e n g t h or i t s odd m u l t i p l e s . Such a t a p e r h a s b e e n m a d e to be half a w a v e l e n g t h l o n g a t 4GHz. The t r a n s d u c e r r e a c t a n c e w a s s e t at 5 ~ by e v a p o r a t ing an a r r a y of s p o t s s e v e n t h o u s a n d t h s of an inch in d i a m e t e r on i t s f r e e face. By m o v i n g the delay line a r o u n d r e lative to the end of the t a p e r , c o n t a c t could be m a d e to any of t h e s e s p o t s . No d i r e c t a t t e m p t w a s m a d e to tune out C o but the c a p a c i t a n c e due to c h a n g e in d i m e n s i o n s at the 5012 end of the t a p e r , half a w a v e l e n g t h away, gave s o m e f o r t u i t o u s t u n i n g a n d c o n t r i b u t e d to the low i n s e r t i o n l o s s o b s e r v e d . T h e b a n d width a n d i n s e r t i o n l o s s of a t r a n s d u c e r u s i n g s u c h a m a t c h i n g s y s t e m i s shown in F i g 5, t h e bandwidth w a s v e r y wide and w a s e n t i r e l y l i m i t e d by the t r a n s d u c e r . T h e b a n d width of the t r a n s f o r m e r , m e a s u r e d f r o m i t s v . s . w . r , with t r a n s d u c e r a s load, w a s f r o m 3 to 6GHz. T h e s a m e t a p e r w a s a l s o u s e d at X - b a n d . F i g 6 s h o w s the B a n d - p a s s of a 1 . 6 5 p s l i n e w o r k i n g a t r o o m t e m p e r a t u r e . In t h i s c a s e the bandwidth i s l i m i t e d by the c h a n g e in a t t e n u a t i o n in the s a p p h i r e a s the s q u a r e of the f r e q u e n c y , coupled with the c h a n g e in C O with f r e q u e n c y . T h e t r a n s d u c e r i t s e l f had a v e r y b r o a d bandwidth at t h i s f r e q u e n c y a s it w a s half a w a v e l e n g t h thick at 9GHz on a thin gold film, the b a n d - p a s s of which i s the c o n t i n u o u s line shown in F i g 2. A n o t h e r t r a n s f o r m i n g m e t h o d i s to m a k e a w a v e g u i d e to c o a x i a l t r a n s f o r m e r to m a t c h d i r e c t to 5 ~. T h i s h a s b e e n t r i e d with l i m i t e d s u c c e s s . T h e t a p e r gave b e t t e r r e s u l t s but w a s m o r e bulky. By c a r e f u l m a t c h i n g , b a n d w i d t h s have b e e n obtained which a r e l i m i t e d only by the t r a n s d u c e r i t s e l f . By e x a c t d e s i g n of m a t c h i n g c i r c u i t s and t r a n s d u c e r s t r u c t u r e s m a n y r e q u i r e d b a n d p a s s s h a p e s m a y be s y n t h e s i s e d up to a m a x i m u m width of a r o u n d 50% at low f r e q u e n c i e s . T h e u p p e r f r e q u e n c y l i m i t i s s e t m o r e by the c h a n g e of a t t e n u a t i o n with f r e q u e n c y than by the t r a n s d u c e r .
30 I ¢--1
en
o 20
1'Ya dB
"o
,-10
3
3.1
Frequency [GH,]
4
4.25
F i g 5 T r a n s d u c e r b a n d - w i d t h of a 0 . 2 5 ran t h i c k CdS f i l m
on a thin gold f i l m with a counter d i a m e t e r e l e c t r o d e , e l e c t r i c a l l y matched with a 5 0 - 5 f l c o a x i a l taper h a l f a wavelength long at 4GHz.
[]
Tuned at each frequency Instantaneous bendpass
70'~ .~.
°
c 65.o
0
c
D 601
o
0 55
I 8,O
8.5
I
I
9.O 9.5 ~uency ~H~
I
10.0
F i g 6 B a n d - p a s s of 1 . 6 5 / ~ s delay Une at Xband. Matched with a 5 0 - 5 ~ c o a x i a l taper one and a half w a v e l e n g t h s long at 10GHz. A l s o shown i s the i n s e r t i o n l o s s of the delay line tuned at e a c h point. Upward trend e n t i r e l y due to the change in t r a n s m i s s i o n l o s s with frequency.
REFERENCES 1
de K l e r k , J. and Kelley, E. F. R e v i e w of Scientific I n s t r u m e n t s , Vol 36 (1965) No 4, p 506. A l s o ' T h i n f i l m zinc oxide t r a n s d u c e r s f o r u s e in m i c r o w a v e d e v i c e s ' , U l t r a s o n i c s , Vol 8, No 3.
2
F o s t e r , N . F . IEEE T r a n s a c t i o n s on Sonics and U l t r a s o n i c s SU-11 (1964) No 2, p 63.
3
B e e c h a m , D. U l t r a s o n i c s Vol 5, (1967), No 1, p 19.
4
Mitchell, R . F .
5
Duncan, W., H u t c h i n s , R. H. and Stewart, P . E . M . V a c u u m Science and T e c h n o l o g y , Vol 6, No 4, p 555.
U l t r a s o n i c s Vol 6, No 2 , ( 1 9 6 8 ) , p 112.
6
R e e d e r , T. M., P r o c IEEE, Vol 55 (1967), p 1099.
7
B a h r , A. J. and Court, I. N., IEEE Sonics and U l t r a s o n i c s S y m p o s i u m ( V a n c o u v e r 1967).
8
Malbon, R. M., W a l s h , D. J . and Winslow, A. K., Applied P h y s i c s L e t t e r s , Vol 10, No 1, (1967), p 9.
9
O l i v e r , D. W. and Slack, G. A., J o u r n a l of Applied P h y s i c s , (1966),Vol 37, No 4, p 1542.
ULTRASONICS O c t o b e r 1970
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