8 Polyimide for Multilevel Very Large-Scale Integration (VLSI) GAY
SAMUELSON
Process Technology Laboratory, SRDL, Motorola, SG, Phoenix, A Z 85008
Multilevel structures consisting of alternating metal and di electric layers are necessary to achieve interconnection in high density or VLSI circuits using either MOS or bipolar technology. The function of the interlevel d i e l e c t r i c of the multilevel struc ture is three-fold: (1) i t must provide planarization of under lying topography while allowing high resolution patterning of v i a holes necessary for contact between metal layers, (2) i t must pro vide insulation integrity, and (3) i t must contribute minimally to device capacitance. A l i k e l y candidate for the role of interlevel dielectric is polyimide on the basis of i t s relative purity and planarizing spin -on application. In fact, planarizing metal with polymer or so -called PMP technology was pioneered by Hitachi to develop two metal level transistors (1,2,3). More recently, several other companies, TI (4), and IBM (5) have reported use of polyimide for multilevel interconnect systems. T. Herndon and R. Burke have reported a process for constructing polyimide-aluminum multilevel 64K MNOS memories (6). The present work is a report of the properties of polyimide which define functionality as an interlevel dielectric/passivant. Thus, the planarizing and patterning characteristics and e l e c t r i cal characteristics of current vs voltage, dissipation, break down f i e l d strength, d i e l e c t r i c constant, charge and crossover isolation are discussed i n addition to the r e l i a b i l i t y - r e l a t e d passivation properties. Experimental Baseline Process. DuPont PI2545, PI2555 and Hitachi PIQ as received from the manufacturer, were spun in a class 100 clean room environment at appropriate spin speeds to achieve 0.5 - 6 μ film thickness. The s i l i c o n wafer substrates were pre-spun (5K rpm, 30") with 0.05% DuPont VM651 (γ-amino propyltriethoxy silane) adhesion promoter i n 95/5 (v/v) methanol/H2O. The p o l y i mide film cast on the silane-coated s i l i c o n wafer was pre-baked 0097-6156/82/0184-0093$05.00/0 ©
1982 American Chemical Society
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30 min at 130°C a f t e r which p o s i t i v e r e s i s t such as KTI-809 was spun, soft-baked and exposed. R e s i s t development and concomitant polyimide etch occurred on spray a p p l i c a t i o n of a p o s i t i v e r e s i s t developer l i k e DE-3. Acetone was used as a r e s i s t s t r i p and methanol was used as a f i n a l r i n s e . The patterned polyimide f i l m was cured as f o l l o w s : 1 hr 200°C 1 hr 300°C 15 min 400-450°C Dry Etch C o n d i t i o n s . F u l l y cured polyimide f i l m s were used f o r a l l dry e t c h i n g . The degree of d e s i r e d r e s o l u t i o n determined the mask. For l a r g e r geometries (e.g., 4 - 5 y) , hard-baked KTII I r e s i s t was chosen. Since the etch r a t e of r e s i s t was twice that of f u l l y cured polyimide, the r e s o l u t i o n achieved with t h i s system was l i m i t e d by the t h i c k (2.7 u) r e s i s t necessary to maint a i n mask i n t e g r i t y f o r a 1.2 u t h i c k polyimide f i l m . For smaller geometries (e.g., 0.5 - 3 y) , a m o d i f i c a t i o n of the B e l l Labs technique was used (7). Plasma enhanced (PE) Si02 or SiN was deposited at a thickness of 1200 S on 1.2 y of f u l l y cured p o l y imide. The i n o r g a n i c f i l m was plasma etched and subsequently used as a mask f o r r e a c t i v e i o n etching (R.I.E.) or r e a c t i v e i o n m i l l i n g (R.I.M.) of polyimide. Three d i f f e r e n t dry etch techniques were i n v e s t i g a t e d : i s o t r o p i c O2 plasma etching i n a Tegal 200 r e a c t o r , R.I.E. i n a para l l e l - p l a t e in-house modified Tegal 400 r e a c t o r and R.I.M. i n a Veeco, Model RG-830. The c o n d i t i o n s of o p e r a t i o n f o r each system were as f o l l o w s where time i s the time to etch 1.2 y of f u l l y cured polyimide. i ) Tegal 200 - 1.2 t o r r 0 p r e s s . , 300 watts, 5 min i i ) M o d i f i e d Tegal 400 - 100 u 0 press., 350 watts, 9 min i i i ) Veeco, RG-830 - 9 x 10""* t o r r 0 p r e s s . , 15° angle of incidence of the incoming beam, 500 V a c c e l e r a t i n g v o l t a g e , 0.55 ma/cm current d e n s i t y , 20 min 2
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E l e c t r i c a l Measurements. D i e l e c t r i c and I-V c h a r a c t e r i s t i c s were determined on simple guard r i n g - d o t MIS s t r u c t u r e s c o n s i s t i n g of A l - polyimide - degenerate s i l i c o n of r e s i s t i v i t y 0 - .02 ten. The t e s t s t r u c t u r e f o r C-V c h a r a c t e r i s t i c determination was s i m i l i a r except that r e s i s t i v i t y of the s i l i c o n wafer substrate was 6 - 1 2 ficm. I-V c h a r a c t e r i s t i c s were determined using a K e i t h l e y 616 electrometer, a Kepco model BPO 500M b i p o l a r h i g h v o l t a g e power supply and a Fluke 8502A h i g h r e s o l u t i o n DVM. C-V c h a r a c t e r i s t i c s and d i e l e c t r i c p r o p e r t i e s were determined using an HP 4275A LCR meter. The p i n h o l e d e n s i t y of polyimide was assessed by a s t a t i s t i c a l e v a l u a t i o n of shorts u s i n g an TiWAu - polyimide - TiWAu m u l t i l e v e l s t r u c t u r e where each d i e contained 3275 crossovers of f i r s t and second metal. The p r o b a b i l i t y of good crossovers was taken as
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( 1 - P ) where P i s the p r o b a b i l i t y of one bad crossover and N i s the t o t a l number of c r o s s o v e r s . The p r o b a b i l i t y of good c r o s s overs was determined experimentally from the t o t a l number of open d i e T the t o t a l number of d i e probed. Results and
Discussion
The p r o p e r t i e s of polyimide which p e r t a i n to the f u l f i l l m e n t of each f u n c t i o n a l requirement of an i n t e r l e v e l d i e l e c t r i c / p a s s i vant w i l l be discussed i n t u r n . P l a n a r i z a t i o n and P a t t e r n a b i l i t y . Polyimide, because of i t s spin-on a p p l i c a t i o n , i s an i d e a l choice f o r p l a n a r i z i n g u n d e r l y i n g topography. An example i s provided i n F i g u r e 1 which shows a scanning e l e c t r o n micrograph (SEM) of a c r o s s s e c t i o n through the b i r d ' s beak created at the periphery of an oxide i s o l a t i o n area of a b i p o l a r d e v i c e . The p l a n a r i z i n g e f f e c t of 1.6 y PI2545 on the 6000 § step created by the beak, i s evident. According to Rothman (8), because of geometry e f f e c t s , i t i s probably impossib l e to t o t a l l y p l a n a r i z e but step coverage i s v a s t l y improved with an u n d e r l y i n g coat of polyimide. P a t t e r n i n g the planar polyimide i s h i g h l y process dependent i n terms of the r e s o l u t i o n and w a l l slope achieved. Wet chemical etch and b a r r e l O2 plasma etch are i s o t r o p i c processes producing s l o p i n g v i a w a l l s and a r e s o l u t i o n l i m i t of 3 - 5 u. An example of i s o t r o p i c O2 plasma e t c h i n g i s provided i n F i g u r e 2 where a 5 x 5 y v i a i s shown etched i n 1.5 u PI2545 o v e r l y i n g l a r g e g r a i n s i z e aluminum. Using a SiN or Si02 mask, d i r e c t i o n a l etch t e c h niques such as R.I.E. or R.I.M. can provide r e s o l u t i o n to _< 1 y. V i a w a l l s , i n t h i s case, are e s s e n t i a l l y v e r t i c a l which may r e q u i r e innovations i n subsequent m e t a l l i z a t i o n to avoid step coverage d i f f i c u l t y . T h i s p a r t i c u l a r problem i s unique to VLSI where high r e s o l u t i o n p a t t e r n i n g requirements i n d i e l e c t r i c s , organic or i n o r g a n i c , d i c t a t e use of d i r e c t i o n a l etch techniques. Insulation Integrity. I n s u l a t i o n i n t e g r i t y i s a f u n c t i o n of an i n t e r l a y e r d i e l e c t r i c / p a s s i v a n t d e f i n e d by s p e c i f i c e l e c t r i c a l , mechanical and p a s s i v a t i o n p r o p e r t i e s . The D.C. e l e c t r i c a l prope r t y of i n t e r e s t i s the I-V c h a r a c t e r i s t i c which i s used to deduce c o n d u c t i v i t y and breakdown f i e l d s t r e n g t h . The corresponding A.C. e l e c t r i c a l property i s d i s s i p a t i o n f a c t o r . The p e r t i n e n t mechani c a l and p a s s i v a t i o n p r o p e r t i e s are, r e s p e c t i v e l y , p i n h o l e d e n s i t y and performance r a t i n g as a d i f f u s i o n b a r r i e r to N a and H2O. Both bulk and s u r f a c e I-V c h a r a c t e r i s t i c s were determined f o r PI2545, PI2555 and PIQ. A r e p r e s e n t a t i v e bulk c u r r e n t d e n s i t y J , vs e l e c t r i c f i e l d E or Je i s shown i n F i g u r e 3 f o r 0.5 y t h i c k PIQ. Log J appears to be a n o n - l i n e a r f u n c t i o n of e i t h e r E or i n the broad f i e l d range i n v e s t i g a t e d although i n the narrow range of 5 x 10**- 5 x 10 V/cm, l o g J v s . i s apparently l i n e a r as has been reported (9). The overlapping m u l t i p l e t r a c e s r e p r e s e n t i n g +
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Figure 1.
SEM of the cross section through a 6000 A bird's beak showing the planarizing effect of 1.6 ym PI2545.
Figure 2.
SEM of a 5 X 5 fxm via in 1.5 ^ PI2545. Isotropic 0 ditions were used.
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d i f f e r e n t wafer areas i n d i c a t e good e l e c t r i c a l u n i f o r m i t y f o r t h i s t h i n f i l m . However, below 1 y, e l e c t r i c a l u n i f o r m i t y was shown to be polyimide chemistry and/or s o l v e n t dependent. At a t y p i c a l use f i e l d c o n d i t i o n of 5 x 1 0 V/cm, c o n d u c t i v i t y i n polyimide i s c l o s e l y s i m i l a r to that f o r thermal S i 0 ( i . e . , ^ 1 0 ~ ft^cm"" ) but s h i f t s s e v e r a l orders of magnitude l a r g e r than thermal S i 0 (to ^3 x 1 0 - S r ^ c n r ) at higher f i e l d s such as E = 2 x 1 0 V/cm. Surface I-V c h a r a c t e r i s t i c s a r e i n d i c a t e d i n F i g u r e 4 f o r a p o l y imide s u r f a c e before and a f t e r A r backsputter used as a pre-metal c l e a n . A r backsputter has been shown to remove 70 - 140 & organic r e s i d u e i n v i a s r e s u l t i n g from polyimide p r o c e s s i n g (6). From F i g u r e 4 i t i s evident that A r backsputter a c t i v a t e s the polyimide s u r f a c e such that i t becomes ohmically conductive with a measured sheet r e s i s t i v i t y of approximately 1.5 x 1 0 Q/Q. Furthermore, the e l e c t r i c a l l y a c t i v e s u r f a c e does not r e a d i l y decay to i t s o r i g i n a l background I-V c h a r a c t e r i s t i c as i n d i c a t e d i n the I-V scan F i g u r e 4 ( c ) , made 72 hours f o l l o w i n g A r backs p u t t e r . A s i m i l a r s u r f a c e I-V c h a r a c t e r i s t i c and c a l c u l a t e d sheet r e s i s t i v i t y were observed f o r a polyimide f i l m f o l l o w i n g wet chemical etch i f there was inadequate removal of r e s i s t and/or developer. The problem was solved by i n c r e a s i n g the r e s i s t s t r i p time i n acetone and subsequently, r i n s i n g i n methanol. 5
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Another property p e r t i n e n t t o t h e f i l f i l l m e n t of the r e q u i r e ment f o r i n s u l a t i o n i n t e g r i t y i s d i s s i p a t i o n f a c t o r (D). D i s a s e n s i t i v e i n d i c a t o r of cure c o n d i t i o n s (10) and i t s value i s , consequently, dependent on the cure regime as i l l u s t r a t e d i n F i g u r e 5 where D (1 MHz) i s p l o t t e d vs time f o r temperatures i n the range 200 - 400°C. Such a s e r i e s of curves i s unique f o r a p a r t i c u l a r polyimide chemistry and f i l m t h i c k n e s s range. D decreases with time a t T
