13
Improved Room-Temperature Vulcanized ( R T V ) Silicone Elastomers as Integrated Circuit (IC) Encapsulants CHING-PING WONG Western Electric Co., Inc., Engineering Research Center, Princeton, NJ 08540 Of all the commercially available organic and inorganic polymeric materials, RTV silicone elastomer has proved to be one of the most effective encapsulants used for mechanical and moisture protection of the Integrated Circuitry (IC) devices. A general overview of the RTV silicone elastomer and its commercial preparation and cure mechanism are described. Improved electrical performance of the RTV silicone encapsulant, by immobilizing the contaminant ions, such as Na+, K+, Cl-, with the addition of the heterocyclic poly-ethers as the contaminant ion scavengers seems to have a potential application as the contaminant ionic migration preventor in the electronic applications. Since World War II silicone (organosiloxane) polymers have been used in a variety of applications where properties of high thermal stability, hydrophobicity and low dielectric constant are necessary (see Figure 1). One particular application of interest to us is the use of silicones as encapsulants or conformal coatings for integrated circuits. Work in 1969 at Bell Laboratories demonstrated that silicone RTVs exhibited excellent performance as moisture protection barriers for Integrated Circuitry (IC) devices (1). Since that time, a number of different silicone RTVs have been adapted for use on ICs within the Bell System. However, as the design of ICs has steadily moved to smaller and smaller dimensions, the requirements placed on the encapsulation have risen. For example, early ICs were made with 50 to 75 micron design rules, present devices are at the 3-5 micron level and the trend is now to submicron geometries. At this level, the silicone cure mechanism, surface chemistry and level of contaminants become critical, and it is these areas that we are investigating.
0097-615 6/ 82/0184-0171 $05.00/0 © 1982 American Chemical Society
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H
H C-CH -CF 2
I
2
—Si
HC
I
O
CH
3
CH
CH
I CH
CH
1
H-C-H Si
OH
O-
-Si-
To Other Units
H—C—H I H
CH H Figure 1. Silicone structure consists of Si—O—Si backbone that provides thermal stability of the material. Hydrocarbon radicals that attach to silicon atoms provide water-repelling properties.
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The primary commercial source of s i l i c o n e polymers i s the Rochow process wherein a stream of a l k y l or a r y l monohalide ( t y p i c a l l y the c h l o r i d e ) i s passed through a heated bed of pure s i l i c o n a l l o y e d with copper metal. The exact mechanism of t h i s process i s not well c h a r a c t e r i z e d but i s presumed to proceed through an organocopper intermediate (2)• The major product of the r e a c t i o n i s the d i o r g a n o d i h a l o s i l a n e , however, some production of monoorganotrihalo- triorganohalo-tetraorgano-and t e t r a h a l o s i l a n e s i s observed. The major product i s p u r i f i e d by d i s t i l l a t i o n and i s then c a t a l y t i c a l l y hydrolyzed to d i s i l a n o l s which are unstable and combine to form a mixture of c y c l i c siloxane oligomers ( p r i m a r i l y trimer and tetramer) and l i n e a r hydroxy end-blocked (HEB) siloxane polymers. The c y c l i c oligomers can be ring-opened and condensed into l i n e a r polymers. The molecular weight of the HEB siloxanes i s c o n t r o l l e d by the r e a c t i o n c o n d i t i o n s . End-blocking can be changed by a v a r i e t y of r e a c t i o n s . The HEB siloxanes are t y p i c a l l y f l u i d s of v i s c o s i t i e s v a r y i n g from a few c e n t i s t o k e s to r e s i n s and gums (with s i l i c o n e gum the molecule weight could go up to m i l l i o n s ) . In these cases, they are not s u i t a b l e as coatings and must be c r o s s l i n k e d or v u l c a n i z e d . Two major c r o s s l i n k i n g r e a c t i o n types are a v a i l a b l e , free r a d i c a l i n i t i a t e d and condensation c u r i n g . For the purposes of t h i s d i s c u s s i o n , we w i l l l i m i t the d i s c u s s i o n to the condensation cures• The condensation cures can be further d i v i d e d i n t o four sub-classes; a) the carboxylate cure, b) alkoxide cure, c) oxime cure and d) amine cure. For e l e c t r o n i c a p p l i c a t i o n s , alkoxide cure i s p r e f e r r e d . However, the alkoxide cure system has not been well defined. Figure 2 i n d i c a t e s one of the reasonable alkoxide cure system mechanisms. The methoxy end-blocked s i l i c o n e , with the c a t a l y t i c e f f e c t of some organotitanates or some other types of organotin compounds, w i l l provide one of the methoxy end-group from the s i l i c o n e polymer to react with a hydroxy group from the alumina s u b s t r a t e . The e l i m i n a t i o n of a methanol molecule and the formation of a silicone-oxygen-substrate bond w i l l r e s u l t . At the other end of a s i l i c o n e polymer u n i t , another methoxy end group w i l l react with moisture i n a i r . T h i s r e s u l t s i n the e l i m i n a t i o n of an a d d i t i o n a l methanol molecule and the formation of the s i l i c o n e hydroxy end-group (see Figure 2a). This s i l i c o n e hydroxy end-group i s r e a c t i v e and f u r t h e r reacts with another s i l i c o n e polymer's methoxy end-group r e s u l t s i n chain propagation (see Figure 2b). The e l e c t r i c a l performance of the encapsulant i s g r e a t l y dependent on i t s p u r i t y . Ionic i m p u r i t i e s , such as sodium, potassium and c h l o r i d e s , are harmful contaminants i n the encapsulant. I t has long been shown that i o n i c m a t e r i a l s ,
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(a)
CH
CH,
3
O-Si
CH -0-Si 3
.
C
CH
3
-O-SI-
"3. CH
3
J77f7TrmTm7TnTr7f7T7T77777T Substrate
t=£>
(b)
CH CHg-O-Si-
3
CH
3
O-Si i CH 3
CH,
-O-Sl-O^T 0
CH.
CH~ »3 i 0 S i-0-CH C H 3 - q ) - S i O-Si O i O L i CH n CHCH
C
5
3
fl
>///////////////////////////////// Substrate Figure 2.
Proposed alkoxide cure mechanism:silicon polymer cross-linking and bonding to surface.
(a) One of the methoxy end-groups reacts with a substrate hydroxy I group to form a Si—O substrate bond. Another methoxy end-group reacts with moisture in air to produce an active hydroxy I end-group, (b) Active hydroxy I end-group reacts intermolecularly with other methoxy group to cross-link polymers.
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whether from t h e d e v i c e s u r f a c e , e n c a p s u l a t i o n m a t e r i a l s o r t h e environment, a f f e c t the e l e c t r i c a l r e l i a b i l i t y o f encapsulated IC d e v i c e s . I t h a s been shown (3>) t h a t e x p o s u r e t o h y d r o g e n c h l o r i d e a c c e l e r a t e s the d e t e r i o r a t i o n o f e l e c t r i c a l p r o p e r t i e s of s i l i c o n e encapsulated t r i p l e - t r a c k c o n d u c t o r s and resistors. M i c h a e l and A n t o n e n r e p o r t t h a t s a l t a t m o s p h e r e t e s t i n g (M.I. S t d 883A Method 1009.1) d r a m a t i c a l l y i n c r e a s e s the F a i l u r e I n Time ( F I T ) r a t e f o r s i l i c o n e encapsulated d e v i c e s over those d e v i c e s which have n o t been exposed t o s a l t (4). E x p e r i m e n t s i n o u r l a b o r a t o r y w i t h s i l i c o n e RTVs d e l i b e r a t e l y doped w i t h HC1 showed t h a t F I T r a t e s a l s o increased. From t h e s e r e s u l t s , i t i s l o g i c a l t o c o n c l u d e t h a t i o n i c c o n t a m i n a n t s do i n d e e d c a u s e an i n c r e a s e i n F I T r a t e , e s p e c i a l l v u n d e r h o t , humid c o n d i t i o n s . Sodium, p o t a s s i u m , and c h l o r i d e a r e t h e most l i k e l y i o n s p r e s e n t i n m a t e r i a l s and e n v i r o n m e n t . T h i s i s m a i n l y due t o t h e i r abundance i n n a t u r e . Certainly, material specifications c a n be made t o l i m i t t h e l e v e l s o f t h e s e i o n s h u t t h i s makes no p r o v i s i o n f o r the r e i n t r o d u c t i o n o f these ions from the environment. S i n c e t h e s o u r c e o f i o n s c a n n o t be e l i m i n a t e d , i t was f e l t t h a t i f we c o u l d i n c o r p o r a t e a m e c h a n i s m f o r t r a p p i n g o r i m m o b i l i z i n g t h e s e i o n s , t h e s i l i c o n e RTVs would demonstrate b e t t e r r e l i a b i l i t y . Our i n v e s t i g a t i o n o f a number o f d i f f e r e n t t y p e s o f i o n t r a p p i n g compounds showed t h a t t h i s was the case. I n 1967, C. J . P e d e r s o n o f DuPont deNemours C o . s y n t h e s i z e d t h e c y c l i c p o l y e t h e r s (j>)• These c y c l i c polyethers a r e commonly r e f e r r e d t o as "crown e t h e r s " ( s e e F i g u r e 3 ) . I n s o l u t i o n , crown e t h e r s a r e e x t r e m e l y e f f e c t i v e l i g a n d s f o r a wide r a n g e o f m e t a l i o n s . The s i z e o f t h e r i n g c a v i t y and t h e i o n i c r a d i u s o f the metal a f f e c t the s t a b i l i t y o f the complex. T a b l e s I and I I l i s t t h e c a v i t y d i a m e t e r s f o r t h e crown e t h e r s and t h e i o n i c r a d i i o f a number o f m e t a l i o n s (6-11). The crown e t h e r s and t h e r e l a t e d c r y p t a t e s ( c r y p t a t e s were f i r s t r e p o r t e d b y J-M L e h n ( 1 2 ) o f F r a n c e i n 1969 ( F i g u r e 3 ) ) , h a v e b e e n used i n a v a r i e t y o f s y n t h e t i c p r o c e d u r e s p r i m a r i l y because o f t h e i r a b i l i t y t o s o l v a t e i o n i c m a t e r i a l s i n organic s o l v e n t s (_13, 1 4 ) . R e c e n t l y , i t h a s b e e n shown t h a t crown e t h e r s i n t h e s o l i d s t a t e form " s a n d w i c h " l i k e c o m p l e x e s w i t h most m e t a l s and t h a t t h e c o u n t e r i o n i s a l s o t i g h t l y bound (1_5). I t i s t h i s e v i d e n c e t h a t s u g g e s t e d t h e i r u s e as i o n t r a p s i n s i l i c o n e RTV f o r m u l a t i o n s .
+
(b) Metal chelated cryptate
Figure 3. Structure of a typical crown ether and cryptate: (a) -q — 1, 18-crown-6-ether and (b) rj = 1, Kr opto fix 22. Contaminant ions (such as Na*, K ) are immobilized (coordinated) within the cavity of the heterocycle compound.
(a) Metal chelated Crown Ether
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Table I Atomic and i o n i c radius of some important a l k a l i and t r a n s i t i o n metals• Atomic Radius (A) Na K Cu Ag Au
Ionic
Radius
+
Na K Cu** Ag** Au+1
1.95 2.35 1.28 1.44 1.46
0.95 1.35 0.96, 1.26 1.37
+
2
Cu+ 0.69
Table II Structure and diameter of some crown ethers
Crown ethers dibenzo-12-crown-4 dibenzo-14-crown-4 dibenzo-15-crown-5 dibenzo-18-crown-6
Cavity Diameter 1.8-1.9 1.8 - 1.9 2.7 4.0
(A)
Comments
No coplanar Coplanar & symmetrical Coplanar & symmetrical Coplanar & symmetrical
Experimental Discussion A t y p i c a l encapsulation formula c o n s i s t s p r i m a r i l y o f a s i l i c o n e prepolymer and a c r o s s l i n k i n g agent. Other c o n s t i t u e n t s such as s o l v e n t s , c a t a l y s t s , f i l l e r s , and various a d d i t i v e s may be included to enhance the a p p l i c a t i o n or f i n a l p r o p e r t i e s of the encapsulant. The d i f f i c u l t y i n compounding these encapsulant m a t e r i a l s l i e s i n understanding the i n t e r p l a y of each o f the components and i n determining the amount and type o f each one which w i l l provide the best degree of r e p r o d u c i b i l i t y and r e l i a b i l i t y . Our i n i t i a l experiments consisted o f blending s e l e c t e d c r o s s l i n k e r s and HEB s i l o x a n e s with a commercially a v a i l a b l e s i l i c o n e RTV encapsulant and monitoring the e f f e c t o f the new ingredient on the e l e c t r i c a l performance o f the encapsulant. This procedure proved u s e f u l i n i d e n t i f y i n g a large number of s u i t a b l e components for an encapsulant formulation. This method was also used to determine e f f e c t i v e c a t a l y s t s f o r the alkoxide system. A f t e r the i n i t i a l screening process, i t was necessary to develop model systems f o r both the carboxylate and alkoxide cured systems on which f u r t h e r experimentation could be performed. For the purposes o f our t e s t i n g ,
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t r i a c e t o x y m e t h y l s i l a n e was chosen as a c r o s s l i n k e r and 8000 c e n t i s t o k e HEB s i l i c o n e as the prepolvmer. Cure studies and e l e c t r i c a l t e s t i n g demonstrated that, although there i s a minimum l e v e l of c r o s s l i n k e r to insure f u l l cure, i n excess i t does not have an adverse e f f e c t . The acetoxy system i s a r e l a t i v e l y simple system to compound and, provided that the a c e t i c a c i d by-product i s not harmful to the m e t a l l i z a t i o n , i t i s an e x c e l l e n t e l e c t r o n i c encapsulant. On the other hand, the alkoxide system presented several problems i n formulation. The system f i r s t chosen as a model c o n s i s t e d of a trimethoxymethyl s i l a n e c r o s s l i n k e r , 8000 c e n t i s t o k e HEB s i l o x a n e , and a c a t a l y s t . A number of c a t a l y s t s were used and each e x h i b i t e d d i f f e r e n t cure rates and e l e c t r i c a l p r o p e r t i e s . DuPont t e t r a a l k o x y t i t a n t e - T y z o r appears to be one o f the b e t t e r c a t a l y s t s used i n t h i s type of c u r i n g system. F i l l e r s are u s u a l l y incorporated i n t o the s i l i c o n e formulation to improve mechanical p r o p e r t i e s , promote adhesion, and to serve as l i g h t screening and pigment agents. C a b - o - s i l ^ , a form of fumed s i l i c a , carbon-black, titanium dioxide and calcium carbonate are then used as RTV f i l l e r s . RTV f i l l e r s have g r e a t l y improved the RTV elastomer and have proven to be good ingredients f o r the IC encapsulants. For i n c o r p o r a t i o n of crown ethers and cryptates i n t o the RTV encapsulant system as sodium and potassium ion scavengers, the t o t a l i o n i c contaminants must f i r s t p r e c i s e l y be determined. Atomic absorption i s used to measure these ions i n commercial s i l i c o n e RTVs and s i l i c o n e f l u i d s . Values of ^10 ppm f o r sodium and potassium were obtained i n the best samples. C h l o r i d e l e v e l was determined by p o t e n t i o m e t r i c t i t r a t i o n of the s i l i c o n e with AgN03 A quantity of ion trap ( e i t h e r crown ethers or c r y p t a t e s ) was then added to the RTV s i l i c o n e encapsulant, and i t s molar c o n c e n t r a t i o n was equal to the combined sodium and potassium contaminant l e v e l s . The formulated RTV s i l i c o n e i s u s u a l l y cured at room temperature f o r 16 hours and then at 120°C f o r 4 hours to ensure the complete removal of organic solvent. A rubbery and non-tacky elastomer i s u s u a l l y obtained a f t e r the c u r i n g c y c l e . The e l e c t r i c a l r e l i a b i l i t y of s i l i c o n e RTV formulations was determined by u s i n g a Biased Humidity Temperature (BHT) t e s t i n g procedure employing alumina ceramic IC devices with e i t h e r t r i p l e track meandering r e s i s t o r or conductor m e t a l l i z a t i o n (see Figure 4) (2, JJ>)- The m e t a l l i z a t i o n f o r a r e s i s t o r IC i s tantalum n i t r i d e , and f o r a conductor, titanium-palladium-goid. These IC devices have 75 micron design parameters and, a f t e r c o a t i n g with an RTV, are subjected to a c c e l e r a t e d t e s t i n g c o n d i t i o n s . In normal t e s t i n g procedures, we employ 96% r e l a t i v e humidity, 100°C and 180 v o l t dc bias to t e s t the encapsulant. The e v a l u t i o n of the R
#
Triple-track testing device (3). 2
3
IEEE Proceedings
2
s
4
Triple-track resistor and conductor coupons are made by deposition of Ta N and Ti-Pd-Au metallization, respectively, on the Al O substrate. This test pattern consists of three parallel meandering lines with 3-mil spaces between lines. Each line is approximately 3-mil wide and has 2.86 X JO' squares, with an overall length of 8.5 in. The number of squares of insulator between adjacent lines is approximately 3.5 X 10 .
Figure 4.
o
3
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encapsulant i s based on t h e i r BTH t e s t i n g e l e c t r i c a l performance. In general, the lower the r e s i s t a n c e and c u r r e n t , the b e t t e r the encapsulant.
leakage
Results and D i s c u s s i o n To formulate a s u i t a b l e RTV s i l i c o n e as an IC encapsulant based on the acetoxy cure system i s r e l a t i v e l y simple. The acetoxy cure system t y p i c a l l y employs an a c e t o x y s i l a n e as a c r o s s l i n k i n g agent. T h i s r e a c t i o n i s r a p i d , even i n the absence of c a t a l y s t s and produces tough, durable rubbers. The by-product, a c e t i c a c i d , can cause c o r r o s i o n of aluminum m e t a l l i z a t i o n on IC d e v i c e s , p a r t i c u l a r l y with thick coatings where entrapment o f the acid can occur. P r e s e n t l y a v a i l a b l e high performance RTVs employ an alkoxide cure- the by-product being a non-corrosive a l c o h o l . However, t h i s type of alkoxide cure r e a c t i o n i s r e l a t i v e l y slow even when catalyzed and because of the need for a c a t a l y s t , the cure mechanism i s more complex than that for the acetoxy system. Nevertheless, they a l l seem f e a s i b l e . The r e s u l t s of the BHT t e s t i n g on T r i p l e Track R e s i s t o r (TTR) t e s t i n g showed that the i n c l u s i o n of crown ethers and cryptates i n t o a s i l i c o n e RTV formulation d r a m a t i c a l l y enhances the e l e c t r i c a l r e l i a b i l i t y of our t e s t v e h i c l e s (Figure 5). In the t r i p l e track r e s i s t o r experiment, we grounded the two outer tracks and biased the center track. Then we measured the r e s i s t a n c e change between the centers of the conductor l i n e s . This process measures the degree of " e l e c t r o - o x i d a t i o n " . Leakage currents due to i m p u r i t i e s can cause the r e s i s t o r to anodize. The change of the r e s i s t a n c e with respect to the o r i g i n a l r e s i s t a n c e w i l l increase with time. This i s mainly due to the o x i d a t i o n process. The l e s s the r e s i s t a n t changes with t e s t i n g time, the b e t t e r the encapsulant m a t e r i a l w i l l be. This data a d d s f u r t h e r evidence that sodium and potassium ions c o n t r i b u t e to the f a i l u r e of devices (17) i n as much as the crown ethers with the smaller c a v i t i e s outperform those with l a r g e r c a v i t i e s (18). To our s u r p r i s e , the 12-crown-4, with a c a v i t y diameter of 1.8 X may be more s u i t a b l e f o r complexine Na"*" with an i o n i c diameter of 1.8 A . The 15-crown-5 at 2.7 A may be e f f e c t i v e for K at 2.6 X (_18). In our experiment, the sodium and potassium ions both seem to have been trapped w i t h i n the crown ether quite securely even under the most severe t e s t i n g c o n d i t i o n s . Formation of an ' i o n - p a i r ' between the metal cation-crown ether and halogens counterion has been observed (15). The p a i r i n g of metal crown ether with halogen ions ( i . e . c l ~ ) would be b e n e f i c i a l i n the trapping of c h l o r i d e contaminant m a t e r i a l s . Since most halogens are p o t e n t i a l harmful contaminants i n an encapsulant m a t e r i a l , the formation of 'dendrites' - which k
+
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0.01 10
Elastomers
100 TIME (hrs.)
181
1000
Figure 5. Triple-track resistor electrical testing performance of crown ethers in commercial RTV silicon encapsulants. Conditions: bias, 180 V; relative humidity, 96%; temperature, 100°C.
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causes the leakage current between conductor path is greatly enhanced by the presence of halogens especially under high electrical potential bias, temperature and humidity. Such an argument has been confirmed, and has been well documented (3, 19). Cryptates have also been known to coordinate hydrochloric acid. Hydrochloric acid and chloride ion have been associated with the metal migration in the IC devices. The addition of these cryptate compounds in RTV silicone encapsulant may thus also have potential as HC1 and Cl~ scavengers (18). This finding may also be used to prevent silver, gold and copper ions migrations in electronic industry applications. The thermal and hydrolytic stability of crown ethers and cryptates must be taken into consideration, however, when chosen as additives. Also, since these compounds may be potentially hazardous to our health (20), caution must be taken in using these types of compounds. Ways to eliminate the leaching of cryptates from the encapsulant were proposed by incorporating the cryptate into the backbone or grafting into the substituent side chain of the siloxane polymer. This has been shown to be feasible (2_l). The use of crown ethers and cryptates to eliminate contaminant ions may have some potential application in electronic applications. Literature Cited 1. 2. 3. 4.
5. 6. 7. 8. 9. 10.
White, M. L., Proc. IEEE, 57, 1610 (1969). Rochow, E. G., "An Introduction to the Chemistry of the Silicones", 2nd Ed. New York, John Wiley and Sons, Inc., 1951. Sbar, N., IEEE Proc, 26th Elec. Comp. Conf., 277 (1976). Michael, K. W., Antonen, R. G., "The Properties of Silicone/Epoxy Electronic Grade Molding Compound", Proceedings of the Soc. of Hybrid and Microelectronics Conf., P. 253, Anaheim, Calif., 1978. Pederson, C. J., Journal of Amer. Chem. Soc., 89, 7017 (1967). Bush, M. A., Truter, M. R., J. Chem. Soc., Chem. Comm., 1439 (1970). Dalley, N. K., Smith, J. S., Larson, S. B., Christenson, J.J., Izatt, R. M., Journal of Chem. Soc., Chem. Comm., 43 (1975). Mallison, P. R., Journal of Chem. Soc., Perkin, 261 and 266 (1975). Neman, M. A., Steiner, E. C., Van Remoirtere, F. P., Boer, F. D., Inorg. Chem., 14, 734 (1975). Hughes, D. L., Journal of Chem. Soc., Dalton, 2374 (1975).
13. WONG 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Silicone Elastomers
Harman, M. E., Hart, F. A., Hursthouse, M. B., Moss, G. P., Raithby, P. R., Journal of Chem. Soc., Chem. Comm., 396 (1976). Lehn, J. M., Dietrich, B., Savvage, J. P., Tetrahedron Letter, 2885 (1969). Liotte, C. L., Harris, H. P., Journal of Amer. Chem. Soc., 96, 2250 (1974). Sam, D. J., Simmons, H. E., Journal of Amer. Chem. Soc., 94, 4024 (1972). Poonia, N. S., Ajaj, A. V., Chem. Rev., 29(S), 389 (1979) Mancke, R. G., The Proceedings of 31st Electronic Components Conference, p. 119, at Atlanta, Georgia, May (1978). Kaneda, A., Watanabe, Y., Japanese Patent (76-11377). Wong, C. P., "Encapsulated Electronic Devices Having Improved Silicone Encapsulant", U. S. Patent 4,271,425, June 2, 1981. DerMarderosian, A., The Proceedings of International Soc. for Hybrid and Microelectronics Symposium, P. 134. Minneapolis, Minnesota, Sept. 1978 and reference therein. Crown Ethers - PCR, Product Technical Report. Wong, C. P., "Encapsulated Electronic Devices and Encapsulating Compositions", U.S. Patent (allowed, in press).
RECEIVED
October 23, 1981.
