Monitoring Thin-Film Properties with Surface Acoustic Wave Devices

Chapter 14

Monitoring Thin-Film Properties with Surface Acoustic Wave Devices

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Diffusion, Surface Area, and Pore Size Distribution

G. C. Frye, S.J.Martin, A. J. Ricco, and C. J. Brinker Sandia National Laboratories, Albuquerque, NM 87185 The ability of surface acoustic wave (SAW) devices to monitor adsorption of N onto the surfaces of porous films and diffusion of species into polymer films has been demonstrated. Calculations based on the N2 adsorption isotherms illustrate how sol-gel solution chemistry can be used to tailor the surface area and pore size distribution of thin films. BET surface areas from unity to over 30 cm/cm of film have been obtained on various samples with median pore diameters from less than 0.4 nm to greater than 6 nm. SAW frequency transients occurring during the diffusion of small molecular species into polymer films have been used to determine diffusion coefficients from 10 to 10 cm /sec. 2

2

2

-9

-15

2

When a surface acoustic wave, also called a Rayleigh wave, travels along a substrate surface, the wave velocity and amplitude are affected by changes occurring at the surface. In general, since the acoustic energy is concentrated within one wavelength of the surface, SAW devices are inherently more sensitive to these surface changes than bulk crystal oscillators which have mechanical energy distributed throughout the substrate. As an example of the sensitivity of SAW devices, surface mass changes as small as 100 pg/cm (1) can be detected. This extreme sensitivity has been applied to the construction of several SAW-based chemical sensors (2-7). In addition, since the SAW is sensitive to minute perturbations occurring in thin films which are in intimate contact with the surface, SAW devices can be used to monitor physical and chemical processes occurring in these overlayers. Based on this effect, SAW devices have recently found applications in the characterization of the properties of thin films (8-10). In this paper, we report on the utility of SAW devices to characterize: (1) the surface area and pore size distribution of porous thin films based on N adsorption isotherms and (2) diffusion coefficients (D) for thin polymer films based on absorption transients (i.e., mass absorbed as a function of time) as indicated by SAW velocity transients (i.e., SAW velocity 2

2

© 1989 American Chemical Society

Murray et al.; Chemical Sensors and Microinstrumentation ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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s h i f t as a f u n c t i o n o f t i m e ) . F o r t h e f i r s t a p p l i c a t i o n , t h e SAW d e v i c e enhances t h e d e t e c t i o n l i m i t f o r t h e amount o f adsorbed N by more t h a n f o u r o r d e r s o f magnitude over c o n v e n t i o n a l t e c h n i q u e s c u r r e n t l y used f o r t h i s type o f c h a r a c t e r i z a t i o n . F o r t h e second a p p l i c a t i o n , t h e use o f t h i n f i l m s d e c r e a s e s t h e d i f f u s i o n a l l e n g t h s c a l e from t h a t o b t a i n e d w i t h b u l k samples used i n some c o n v e n t i o n a l gravimetric techniques. T h i s r e s u l t s i n a d r a m a t i c decrease i n t h e time r e q u i r e d t o o b t a i n D v a l u e s . 2

Background S u r f a c e A c o u s t i c Wave D e v i c e s . A SAW d e v i c e t y p i c a l l y c o n s i s t s o f i n p u t and o u t p u t i n t e r d i g i t a l t r a n s d u c e r s on a p i e z o e l e c t r i c sub s t r a t e such as q u a r t z o r l i t h i u m n i o b a t e ( s e e F i g u r e 1 ) . When an a l t e r n a t i n g voltage i s applied to the input transducer, the a l t e r n a t i n g s t r a i n generated i n the p i e z o e l e c t r i c s u b s t r a t e launches the SAW. The SAW probes the m e c h a n i c a l p r o p e r t i e s o f an o v e r l a y i n g t h i n f i l m as i t t r a v e l s a c r o s s t h e s u b s t r a t e b e f o r e b e i n g c o n v e r t e d back i n t o an e l e c t r i c a l s i g n a l a t t h e o u t p u t t r a n s d u c e r . Changes i n mechanical p r o p e r t i e s (e.g., d e n s i t y , s t i f f n e s s ) o f the f i l m r e s u l t i n changes i n wave p r o p a g a t i o n v e l o c i t y and a m p l i t u d e . As i l l u s t r a t e d i n F i g u r e 1, a s i m p l e and h i g h l y s e n s i t i v e method f o r m o n i t o r i n g these changes i s t o use t h e SAW d e v i c e as the feedback element o f an o s c i l l a t o r c i r c u i t . I n t h i s way, changes i n t h e f r e q u e n c y o f o s c i l l a t i o n c a n be d i r e c t l y r e l a t e d t o changes i n t h e v e l o c i t y o f p r o p a g a t i o n o f t h e wave. F o r s i t u a t i o n s where o n l y mass changes i n an o v e r l a y i n g f i l m a r e i m p o r t a n t , f r e q u e n c y s h i f t c a n be r e l a t e d t o t h e mass l o a d i n g / a r e a o f f i l m (m) u s i n g (1): Af f«

Δν v

-

-KC f m

(1)

m n

rt

i n w h i c h κ i s t h e f r a c t i o n o f t h e a c o u s t i c wave p a t h l e n g t h between t r a n s d u c e r s c o v e r e d by t h e f i l m , c i s t h e mass s e n s i t i v i t y o f t h e d e v i c e (1.3 χ 10" cm -s/g f o r q u a r t z (11)) and v and f a r e unper t u r b e d wave v e l o c i t y and o s c i l l a t o r f r e q u e n c y , r e s p e c t i v e l y . To o p t i m i z e t h e s e n s i t i v i t y o f t h e d e v i c e , a l l t h e f i l m s used i n t h i s s t u d y c o v e r e d t h e e n t i r e a c t i v e a r e a o f t h e SAW d e v i c e (κ - 1). R e l a t i o n s h i p s t o account f o r f r e q u e n c y s h i f t s due t o changes i n other p r o p e r t i e s , such as f i l m c o n d u c t i v i t y (12) o r s t i f f n e s s (13)» a r e g i v e n elsewhere. S i n c e f r e q u e n c y c a n e a s i l y be measured t o w i t h i n 1 Hz, 10 ppb changes i n f r e q u e n c y c a n be m o n i t o r e d f o r t h e 97 MHz d e v i c e s used i n t h i s study. T h i s g i v e s a l i m i t o f mass r e s o l u t i o n o f a p p r o x i m a t e l y 80 pg/cm . Even though t h e f r e q u e n c y s t a b i l i t y i s l e s s t h a n 10 Hz (11) under i d e a l s i t u a t i o n s ( i . e . , m i n i m a l e n v i r o n m e n t a l p e r t u r b a t i o n s ) , t h e n o i s e l e v e l observed d u r i n g an experiment i s sometimes on t h e o r d e r o f 1 ppm (100 Hz). T h i s n o i s e l e v e l p r o b a b l y r e s u l t s from f l u c t u a t i o n s i n t h e t o t a l p r e s s u r e o r c o n c e n t r a t i o n o f t h e s o r b i n g s p e c i e s i n t h e gas phase over t h e d e v i c e . F o r a p p l i c a t i o n s where t h e t o t a l f r e q u e n c y response i s s m a l l , improvements i n t h e e x p e r i m e n t a l gas f l o w system may l e a d t o a s i g n i f i c a n t r e d u c t i o n i n t h i s noise l e v e l . m

6

2

Q

Q

2


210

CHEMICAL SENSORS AND MICROINSTRUMENTATION

C h a r a c t e r i z a t i o n o f Porous Samples. A d s o r p t i o n isotherms a r e a u s e f u l a n a l y t i c a l method f o r c h a r a c t e r i z i n g porous samples. This measurement c o n s i s t s o f m o n i t o r i n g the amount o f a m o l e c u l a r s p e c i e s t a k e n up b y t h e sample as a f u n c t i o n o f the r e l a t i v e s a t u r a t i o n o f the s p e c i e s i n the gas phase over the sample. T h i s r e l a t i v e s a t u r a t i o n i s d e f i n e d as p/p , where ρ i s the p a r t i a l p r e s s u r e o f the t e s t s p e c i e s and p i s i t s s a t u r a t i o n vapor p r e s s u r e a t t h e temperature of the run. I n a t y p i c a l measurement, p / p i s i n c r e a s e d from z e r o t o a v a l u e near 1 ( o n s e t o f b u l k c o n d e n s a t i o n ) and t h e n d e c r e a s e d back down t o z e r o . For many porous samples, h y s t e r e s i s i s o b s e r v e d between t h e a d s o r p t i o n and d e s o r p t i o n branches o f t h e i s o t h e r m o b t a i n e d I n t h i s manner ( 1 4 ) . The s u r f a c e a r e a o f a sample can be d e t e r m i n e d from an adsorp t i o n i s o t h e r m u s i n g a w e l l known model d e v e l o p e d by Brunauer, Emmett and T e l l e r ( 1 5 ) . T h i s BET a n a l y s i s i s based on u s i n g one b i n d i n g energy f o r the a d s o r p t i o n o f the f i r s t monolayer on the s u r f a c e s and a second b i n d i n g energy f o r a d s o r p t i o n o f subsequent l a y e r s . A l i n e a r form o f the r e s u l t i n g r e l a t i o n s h i p (15.16) i s g i v e n b y : Q

Q

D

β

=

P/P« n ( l - p/p ) 0

1 n^c

,

(c - D r^c

£_ p

(2)

0

i n which η i s the number o f adsorbed m o l e c u l e s , n^ i s the number o f adsorbed m o l e c u l e s c o r r e s p o n d i n g t o one monolayer on the a v a i l a b l e s u r f a c e s , and c i s a c o n s t a n t which depends on t h e two b i n d i n g energies. Linear behavior i n the p l o t o f β vs. p/p i s t y p i c a l l y observed f o r p / p v a l u e s between 0.05 and 0.3 (14.16). The slope (S) and i n t e r c e p t ( I ) v a l u e s a r e used t o c a l c u l a t e n^ - 1/(S + 1 ) and c - 1 + S/I. The s u r f a c e a r e a (A) o f the sample i s then c a l c u l a t e d u s i n g A - «ηΐ&η,ι where i s t h e adsorbed a r e a p e r m o l e c u l e . The s t a n d a r d a d s o r b a t e used f o r t h i s a n a l y s i s i s N a t i t s b o i l i n g p o i n t (77 K) due i n p a r t t o the r e l a t i v e c o n s t a n c y o f a^ a t 0.162 nm /mol e c u l e on a wide v a r i e t y o f sample m a t e r i a l s ( 1 4 ) . A pore s i z e d i s t r i b u t i o n (PSD) o f a sample i s a measure o f the c u m u l a t i v e o r d i f f e r e n t i a l pore volume as a f u n c t i o n o f pore d i a meter. PSDs can be c a l c u l a t e d from a d s o r p t i o n i s o t h e r m s b a s e d on an a n a l y s i s which accounts f o r c a p i l l a r y c o n d e n s a t i o n i n t o p o r e s . This a n a l y s i s (14.16) uses a model o f t h e pore s t r u c t u r e combined w i t h the K e l v i n e q u a t i o n (17) t o r e l a t e t h e pore s i z e t o t h e v a l u e o f p/p a t which pore " f i l l i n g " o c c u r s . Due t o l i m i t a t i o n s i n t h i s t e c h n i q u e , o n l y pores w i t h d i a m e t e r s from about 3 t o 50 nm, c a l l e d mesopores ( 1 4 ) , can be c h a r a c t e r i z e d . T h i s pore s i z e range, however, i s t y p i c a l o f many porous samples o f i n t e r e s t . F o r samples w i t h pores s m a l l e r o r l a r g e r than t h i s range, a l t e r n a t i v e t e c h n i q u e s , such as mercury i n t r u s i o n f o r l a r g e pores (14.16), a r e t y p i c a l l y more s u i t a b l e . Q

G

2

2

0

Determination o f D i f f u s i o n C o e f f i c i e n t s . Even though d i f f u s i o n a l c h a r a c t e r i s t i c s o f polymers can be complex, t h e r e a r e many s i t u a t i o n s where s i m p l e c o n c e n t r a t i o n - i n d e p e n d e n t F i c k i a n d i f f u s i o n o c c u r s f o r s m a l l m o l e c u l a r s p e c i e s (18.19). F o r a polymer f i l m o f c o n s t a n t t h i c k n e s s L, t h e c o n c e n t r a t i o n C ( x , t ) o f s p e c i e s a t p o s i t i o n χ (measured p e r p e n d i c u l a r t o the s u r f a c e ) and time t i s governed by the one-dimensional F i c k ' s law (20):


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Monitoring Thin-Film Properties ac at

(3)

where D i s the d i f f u s i v i t y . W i t h the f i l m on an impermeable sub s t r a t e , the boundary c o n d i t i o n s a r e : (1) dC/dx - 0 a t χ - 0 ( i . e . , the s u b s t r a t e a c t s as a d i f f u s i o n b a r r i e r ) and (2) C(L,t) - C f o r t > 0 ( i . e . , the c o n c e n t r a t i o n a t the g a s / f i l m i n t e r f a c e i s h e l d c o n s t a n t ) . A u n i f o r m c o n c e n t r a t i o n i n the f i l m p r i o r t o the s t a r t o f d i f f u s i o n i s t y p i c a l l y assumed: C ( x , t ) - C ' f o r t < 0. I f the d i f f u s i o n c o e f f i c i e n t i s c o n s t a n t , E q u a t i o n 3 can be s o l v e d u s i n g these boundary c o n d i t i o n s t o y i e l d the f o l l o w i n g a n a l y t i c a l e x p r e s s i o n (20) f o r the amount o f absorbed s p e c i e s as a f u n c t i o n o f time (M(t)): Q

0

n-1 where φ - n(n-h)/L and i s the i n c r e m e n t a l amount a b s o r b e d a t film saturation (t «>). T h i s e x p r e s s i o n p r e d i c t s t h a t M(t) i s p r o p o r t i o n a l t o Jt u n t i l M ( t ) / M > 0.6. Thereafter, " r e f l e c t i o n " o f s p e c i e s a t the impermeable SAW s u b s t r a t e d e c r e a s e s the n e t f l u x i n t o the f i l m . I f the SAW f r e q u e n c y s h i f t i s l i n e a r w i t h amount a b s o r b e d ( A f ( t ) » k M ( t ) , where k i s a c o n s t a n t ) , E q u a t i o n 4 can be u s e d t o o b t a i n an e x p r e s s i o n f o r the t r a n s i e n t f r e q u e n c y response A f ( t ) o f the SAW o s c i l l a t o r ( h e r e a f t e r r e f e r r e d t o as a f r e q u e n c y transient). m a x

Experimental SAW D e v i c e s . SAW d e v i c e s were d e s i g n e d a t S a n d i a N a t i o n a l Labs and m a n u f a c t u r e d on c r y s t a l l i n e ST-cut q u a r t z s u b s t r a t e s by C r y s t a l Tech n o l o g i e s ( P a l o A l t o , CA). T r a n s d u c e r s , c o n s i s t i n g o f 50 f i n g e r - p a i r s w i t h a p e r i o d i c i t y (Λ) o f 32 μπι, were formed p h o t o l i t h o g r a p h i c a l l y from 200 n m - t h i c k Au-on-Cr m e t a l l i z a t i o n . F i n g e r s a r e 8 μτα wide and 1.7 mm l o n g . C e n t e r - t o - c e n t e r s e p a r a t i o n o f t r a n s d u c e r s i s 7.36 mm. Each SAW d e v i c e was mounted i n a s t a n d a r d 25 χ 13 mm f l a t p a c k u s i n g two beads o f RTV s i l i c o n e r u b b e r t o damp r e f l e c t i o n s from the ends o f the c r y s t a l . An u l t r a s o n i c bonder was u s e d t o a t t a c h 25 μτα. g o l d w i r e s between the d e v i c e and the f l a t p a c k l e a d s . The f l a t p a c k was mounted i n a b r a s s t e s t case. Thin F i l m Formation. The porous f i l m s u s e d i n t h i s s t u d y were p r e p a r e d from s o l - g e l s o l u t i o n s by d i p - c o a t i n g SAW d e v i c e s a t 20 cm/min, f o l l o w e d by d r y i n g and h e a t i n g i n a i r a t 400 °C f o r 5 min. The s o l - g e l system used, denoted Four-Component, c o n t a i n s S i 0 , B2O3, A1 0 and BaO i n r a t i o s o f 71:18:7:4 (by w e i g h t ) . The r e a c t i o n c o n d i t i o n s were s i m i l a r t o those d e s c r i b e d e l s e w h e r e (21) except t h a t t h i s s o l u t i o n d i d not c o n t a i n Na 0. T h i n f i l m s were formed using the a s - p r e p a r e d s o l u t i o n as w e l l as a s o l u t i o n w h i c h had been "aged" a t pH 3 and 50 °C u n t i l g e l a t i o n o c c u r r e d (about t h r e e weeks). The g e l was u l t r a s o n i c a l l y d i s r u p t e d t o form a f l u i d s o l s u i t a b l e f o r d i p coating. T h i s a g i n g p r o c e s s i s known t o r e s u l t i n l a r g e r p o l y m e r i c s p e c i e s i n the s o l - g e l s o l u t i o n (22) which should r e s u l t i n a l a r g e r pore s t r u c t u r e i n the d e p o s i t e d f i l m . F i l m t h i c k n e s s e s and r e f r a c 2

2

3

2


212


t i v e i n d i c e s were o b t a i n e d u s i n g G a e r t n e r Model L119 o r Rudolph Auto ELIV e l l i p s o m e t e r s . P e r c e n t p o r o s i t y was e v a l u a t e d from the r e f r a c t i v e i n d i c e s u s i n g t h e L o r e n t z - L o r e n z (23) r e l a t i o n s h i p assuming a s k e l e t a l r e f r a c t i v e i n d e x o f 1.45. The polymer f i l m used t o s t u d y d i f f u s i o n a l p r o p e r t i e s was formed by s p i n c o a t i n g a t 4000 rpm u s i n g Dupont PI-2545. T h i s f o r m u l a t i o n i s one o f a c l a s s o f p o l y a m i c a c i d s o l u t i o n s w h i c h c o n v e r t s t o a f u l l y a r o m a t i c p o l y i m i d e c o a t i n g when h e a t e d . After being p a r t i a l l y i m i d i z e d a t 115 C f o r 30 rain, a f i n a l f i l m t h i c k n e s s o f 1.8 Mm (measured u s i n g a s u r f a c e p r o f i l i m e t e r ) was o b t a i n e d . e

O s c i l l a t o r C i r c u i t r y . As shown i n F i g u r e 1, the o s c i l l a t o r c i r c u i t r y c o n s i s t s o f a wide band a m p l i f i e r (HP 8447D) and v a r i a b l e attenuators (HP 335C and 355D) t o a d j u s t the n e t g a i n , a t u n a b l e bandpass f i l t e r (K&L 5BT-95/190-5N) t o p r e v e n t s p u r i o u s o s c i l l a t i o n a t o t h e r frequen c i e s , and a phase s h i f t e r (Merrimac PSL-4-100B) t o tune t h e o s c i l l a t i o n frequency. A 20 dfi d i r e c t i o n a l c o u p l e r s p l i t s o f f a f r a c t i o n o f the power t o a f r e q u e n c y c o u n t e r (HP 5385A). The synchronous f r e q u e n c y , a t w h i c h the t r a n s d u c e r s most e f f i c i e n t l y e x c i t e a SAW, i s g i v e n b y f - ν/Λ. F o r o u r d e v i c e s , ν 3100 m/s and Λ - 32 |*m r e s u l t i n g i n f - 97 MHz. The optimum operat i n g f r e q u e n c y i s determined b y network a n a l y s i s u s i n g an HP 8656A s i g n a l g e n e r a t o r and an HP 8405A v e c t o r v o l t m e t e r . f

Gas Flow System. The e x p e r i m e n t a l gas f l o w system u s e d i s shown i n F i g u r e 2. A s t a i n l e s s s t e e l l i d c o n t a i n i n g gas i n l e t and o u t l e t was s e a l e d t o the f l a t p a c k u s i n g a t e f l o n gasket. ForN adsorption r u n s , t h e d e v i c e t e s t case and a s t a i n l e s s s t e e l c o i l o n t h e gas i n l e t s i d e were p l a c e d i n a Dewar f l a s k and submerged i n l i q u i d N . F o r d i f f u s i o n r u n s , the case and c o i l were p l a c e d i n a w e l l - i n s u l a t e d chamber equipped w i t h a h e a t exchanger f e d by a temperature bath (Haake A81). SAW d e v i c e response was m o n i t o r e d u s i n g the computer (HP 9816) c o n t r o l l e d t e s t apparatus shown i n F i g u r e 2. A Data A c q u i s i t i o n / C o n t r o l U n i t (HP 3497A) was used t o c o n t r o l flow r a t e s o f v a r i o u s N and He streams u s i n g T y l a n Model FC-260 mass f l o w con t r o l l e r s and F l u o r o c a r b o n Corp. s e r i e s DV-224 s o l e n o i d - o p e r a t e d t e f l o n v a l v e s . The N was d r i e d by passage through a bed c o n t a i n i n g D r i e r i t e and m o l e c u l a r s i e v e s . To p r e v e n t c o n t a m i n a t i o n , a l l f i t t i n g s and t u b i n g were t e f l o n o r s t a i n l e s s s t e e l . Before a r u n was s t a r t e d , t h e t e s t f i l m s were purged b y f l o w i n g N o v e r t h e d e v i c e a t 165 C (aged Four-Component f i l m ) o r 25 °C ( a l l o t h e r runs). To measure a n a d s o r p t i o n i s o t h e r m , the o s c i l l a t i o n frequency was m o n i t o r e d as t h e computer a d j u s t e d the p a r t i a l p r e s s u r e ρ o f N i n the gas phase o v e r the d e v i c e a t 77 K. T h i s was done b y c o n t r o l l i n g the r e l a t i v e f l o w r a t e s o f a He mix-down stream and a N c a r r i e r stream. S i n c e He i s n o n a d s o r b i n g a t l i q u i d N temperature ( 1 6 ) , ρ 0 f o r t h e mix-down stream, w h i l e f o r t h e c a r r i e r stream, ρ - p s i n c e t h e d e v i c e temperature i s m a i n t a i n e d a t the b o i l i n g p o i n t o f N. The v a l u e o f p / p was incremented e v e r y 3 sec from a v a l u e o f 0 a t t h e s t a r t o f t h e r u n t o a v a l u e around 0.95 and t h e n b a c k t o 0. I t was found t h a t two h o u r s f o r a f u l l a d s o r p t i o n i s o t h e r m was s u f f i c i e n t l y slow t o m a i n t a i n a d s o r p t i o n e q u i l i b r i u m ( 1 0 ) . F o r d i f f u s i o n r u n s , the mix-down stream was N a t 25 *C and the c a r r i e r stream was N s a t u r a t e d w i t h a n o r g a n i c s p e c i e s by passage 2

2

2

2

2

e

2

2

2

c

2

Q

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14 FRYE ET AL.


Device

M

213

M

Variable Attenuator Coupler

Phase

Bandpass

Shifter

Filter

Frequency

Computer

Counter

F i g u r e 1. Schematic o f a SAW d e v i c e as t h e feedback element o f an o s c i l l a t o r c i r c u i t . I n p u t and o u t p u t i n t e r d i g i t a l t r a n s d u c e r s a r e used t o e x c i t e and d e t e c t a SAW i n the p i e z o e l e c t r i c ST-cut quartz substrate.

DEWAR OR ENVIRONMENTAL FLOW

CHAMBER

. —

Ί

DEVICE T E S T C H A M B E R

1

CONTR

SAW

DEVICE

CARRIER

FLOW CONTR

COIL

Ν

MIX-DOWN O S C I L L A T O R CIRCUITRY

COMPUTER

FREQUENCY

COUNTER

F i g u r e 2. Schematic o f t h e e x p e r i m e n t a l s e t - u p f o r c o n t r o l o f v a p o r f l o w and measurement o f SAW o s c i l l a t o r f r e q u e n c y .


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through a bubbler containing the test l i q u i d . To maintain the c a r r i e r stream at ρ - p , the same temperature for the bubbler and the device i s required. Therefore, the bubbler was immersed i n the temperature bath. After a stable o s c i l l a t i o n frequency was obtained, the p/p value was changed stepwise and the frequency transient that occurred was monitored u n t i l a new stable frequency was obtained. 0

0

Results and Discussion Characterization of Sol-Gel Films. Adsorption isotherms for the two Four-Component films are shown i n Figure 3. Since the only e f f e c t giving SAW device response should be the mass loading that occurs due to adsorption onto the surfaces of the f i l m , the experimental f r e quency s h i f t s (right axis) have been converted into an adsorbed mass/area of f i l m ( l e f t axis) using Equation 1. For comparison, the adsorption of one monolayer of N onto the f l a t SAW substrate gives a mass loading of 28.7 ng/cm (1/a^ - 6.2 χ 10 molecules/cm ) which should r e s u l t i n a 3.6 ppm frequency s h i f t . With this i n mind, i t i s clear that the f i l m from the unaged solution (Figure 3a) exhibits a small amount of adsorption. The adsorption isotherm shape resembles what has been c a l l e d a Type II isotherm system (14), t y p i c a l of nonporous samples. The apparent hysteresis i n this isotherm i s probably the result o f long-term d r i f t over the course of the two hour experiment. This r e l a t i v e l y small s h i f t (1-2 ppm) may be the result of temperature changes due to increases i n the concentration of 0 (condensed from the atmosphere) i n the l i q u i d N surrounding the test case. A l i n e a r BET plot i s obtained from these data (0.05 < p/p < 0.3) and the r e s u l t i n g surface area i s calculated to be 1.3 cm /cm of f i l m . This rather unconventional unit of f i l m surface area/nominal f i l m area i s appro priate here since i t i s the sample area, and not i t s mass, which i s the well-known parameter. The experimental surface area value i s very close to the value of 1.0 cm /cm expected for a f l a t , nonporous f i l m on the SAW substrate. The s l i g h t l y higher value may be due to surface roughness rather than f i l m porosity. Furthermore, the lack of measurable porosity with N (no pores > 0.4 nm) i s consistent with the r e f r a c t i v e index value (1.454) being indistinguishable from the value expected for a dense glass of t h i s composition (1.45). The f i l m formed from the aged solution exhibits over 30 times as much adsorption as the unaged f i l m (see Figure 3b). The shape of the isotherm, e s p e c i a l l y the hysteresis loop at higher p/p values, resembles a Type IV isotherm system (14), t y p i c a l of samples contain ing mesopores. The BET surface area of 33 cm /cm of f i l m indicates a large amount of porosity for a f i l m which has a thickness of only 148 nm. As discussed previously, pore size d i s t r i b u t i o n s can be obtained from adsorption isotherms due to the dependence of c a p i l l a r y conden sation on pore s i z e . A PSD obtained from the desorption branch of the data i n Figure 3b using standard c a l c u l a t i o n procedures (16) i s shown i n Figure 4. A r e l a t i v e l y unimodal d i s t r i b u t i o n i s obtained with a median diameter of around 6 nm. As shown i n Table I, r e s u l t s with films having shorter aging times indicate that the median pore diameter increases with aging time. This i s expected based on the e f f e c t s of aging on the size of the polymeric species i n the precur sor s o l - g e l solution (22). 2

2

14

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FRYE ET AL


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F i g u r e 3. A d s o r p t i o n i s o t h e r m s o b t a i n e d u s i n g c o a t e d SAW d e v i c e s f o r (a) unaged and (b) aged Four-Component s o l - g e l f i l m s .


216


"I"

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0

I

I

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«

4

I

1

8

i

1

I

I

I

!

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12

16

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PORE DIAMETER (nm) F i g u r e 4. A pore s i z e d i s t r i b u t i o n o b t a i n e d from t h e d e s o r p t i o n b r a n c h f o r the aged Four-Component f i l m ( F i g u r e 3 b ) .


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Monitoring Tkm-FUm Properties

Table I :

E f f e c t o f A g i n g T i n e on F i l a P o r o s i t y

A g i n g Time (Days)

Adsorption Isotherm

Median Pore Diameter

0 7 14 21

F i g u r e 3a Not shown Not shown F i g u r e 3b

Monitoring Thin-Film Properties with Surface Acoustic Wave Devices

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