- Email: [email protected]
Macrodefects in BaTiO3 capacitors
Mat. R e s . B u l l . , Vol. 21, p p . 1073-1082, 1986. P r i n t e d i n t h e USA. 0025-5408/86 $3.00 + .00 C o p y r i g h t (c) 1986 P e r g a m o n J o u r n a l s L t d .
MACRODEFECTS IN BaTiO3 CAPACITORS W.A. Bahn* and R.E. Newnham Materials Research Laboratory The Pennsylvania State U n i v e r s i t y U n i v e r s i t y Park, PA 16802 ( R e c e i v e d J u n e 9, 1986; R e f e r e e d )
ABSTRACT Controlled macrodefects (pores and delaminations) were introduced into single layer capacitors. The e f f e c t s of these defects on the d i e l e c t r i c properties and d.c. e l e c t r i c breakdown strength were studied. Macrodefects did not cause a reduction in the o v e r a l l density of the d i e l e c t r i c , but a reduction in the capacitance was observed in the samples containing defects. Large voids reduced the capacitance by increasing the a c t i v e d i e l e c t r i c thickness by I0 percent. Delaminations also increased the a c t i v e d i e l e c t r i c thickness by I0 percent, but a f u r t h e r reduction of the capacitance was observed and was modelled using s e r i e s - p a r a l l e l mixing. The capacitors were subjected to a d.c. voltage ramp u n t i l they f a i l e d . There were no s i g n i f i c a n t differences in the breakdown strengths of the various sample types with the average strength being 300 kV/cm. The f a i l u r e mechanisms were explained by the microstructures of the breakdown paths. Breakdown was thermal in nature with the formation of a chemically reduced d i e l e c t r i c tunnel between the electrodes. The f a i l u r e path wa~ occasionally traced to a defect, but more often occurred in the dense ceramic The defects did not s i g n i f i c a n t l y change the e l e c t r i c f i e l d d i s t r i bution in the d i e l e c t r i c between the electrodes, and hence, the breakdown strength of the d i e l e c t r i c layer in the region of the defect was not g r e a t l y reduced. MATERIALSINDEX: barium t i t a n a t e Introduction M u l t i l a y e r ceramic capacitors u t i l i z e high p e r m i t t i v i t y rr,a t e r i a ! s to achieve volumetric e f f i c i e n c y . Unfortunately, ceramics are i n h e r e n t l y *Now at Coming Glass Works, Corning, NY 14830. 1073
1074
W.A.
BAHN, et al.
Vol. 21, No. 9
defective and r e q u i r e careful processing to achieve high densities and maintain high p e r m i t t i v i t i e s . Despite t i g h t control of the processing of ceramic capacitors, gross defects often appear in the f i n a l product. Voids and delaminations usually occupy an i n s i g n i f i c a n t f r a c t i o n of the total volume, but are often linked to premature f a i l u r e s . For this reason, the influence of these defects on the d i e l e c t r i c properties and on the e l e c t r i c a l breakdown behavior of ceramic capacitors is of concern to many manufacturers. Mathematical models have been developed to explain the reduction in d i e l e c t r i c p e r m i t t i v i t y by the presence of a defect or second phase, but pred i c t i o n of the e l e c t r i c breakdown strengths has proven a more d i f f i c u l t problem and is p a r t i c u l a r l y important given the trend towards increased volumetric e f f i c i e n c y . Knowledge of the e l e c t r i c a l f a i l u r e mechanism in ceramic materials is l i m i t e d . E l e c t r i c breakdown has been a t t r i b u t e d to sample o r o s i t y (1,2), grain size (3), phase t r a n s i t i o n s (4,5), grain boundary phases 6), d i e l e c t r i c p e r m i t t i v i t y (7), and the thermal environment (8-10). Breakdown strength is dependent on the specimen, environmental conditions, and the measurement methods.
I
A technique f o r introducing controlled macrodefects into ceramic d i e l e c t r i c s is described in this paper. The e f f e c t of voids and delaminations on the d i e l e c t r i c properties and breakdown strengths were investigated. Experimental Procedure Capacitor Fabrication The materials used in this i n v e s t i g a t i o n are l i s t e d in Table I. Ceramic capacitors with one e f f e c t i v e d i e l e c t r i c layer were fabricated using a conventional m u l t i l a y e r tape casting process. The stacking sequence of the tapes was as follows (Fig. I ) : 1 2 1 2 1 3
screened marker layer blank layers electrode layer ink side down blank layers electrode layer ink side up blank layers
The defects were formed by f u g i t i v e organics that were placed in the capacitor during stacking. The defects were inserted by hand both adjacent to the electrode ( i n t e r l a y e r ) and between the tapes forming the active d i e l e c t r i c (intralayer). The tapes were laminated in a vacuum die under 5000 pounds per square inch at 75°C f o r 2 minutes. The parts were diced and f i r e d at 1290°C f o r 45 minutes using a globar furnace. A f t e r f i r i n g , several samples were ground and polished to determine the average d i e l e c t r i c thickness and overlap area. E l e c t r i c a l Measurements The capacitance and loss were monitored as a function of temperature f o r ten samples of each defect type. The capacitors were loaded into a Delta Design Model 2300 temperature control box and cooled from 150°C to -50°C at a rate of 3°C/min. The capacitances and losses at 0 . I , 1.0 and I0 kHz were measured semicontinuously using a Hewlett Packard HP 4274A LCR bridge i n t e r faced with a Hewlett Packard 9816 computer. Both a short-time and a step-by-step t e s t were u t i l i z e d f o r determination of the e l e c t r i c breakdown strength of the capacitors. The short time t e s t was accomplished by sending a t r i a n g u l a r wave signal from an I n t e r s t a t e sweep generator F74 to a Trek Cor-a-trol 610A a m p l i f i e r . The signal was amplified
Vol. 21, No. 9
BaTiO 3
1075
TABLE 1 M a t e r i a l s Used in t h i s I n v e s t i g a t i o n D i e l e c t r i c Powder - TAMTRON X7PI82H (BX BaTi03) D i e l e c t r i c Constant (5.5 g/cm 3)
2100
D i s s i p a t i o n Factor
(%)
1.0
Breakdown Strength
(V/mil) (kV/cm)
768 300
Binder System - Cladan CB73115 ( a c r y l i c based) Electrode System - Captrode I00 Pt Defects Polymethyl methacrylate (pores) A c r y l o i d B7 (delaminations)
by I000 then applied to the capacitance sample. The voltage through the voltage d i v i d e r was monitored on a N i c o l e t Model #204A o s c i l l o scope. A c u r r e n t - m o n i t o r i n g c i r c u i t was i n s e r t e d a f t e r the sample to observe the c u r r e n t through the c a p a c i t o r , and was also tested with an o s c i l l o scope. The c i r c u i t used f o r the s h o r t - t i m e t e s t shown in Fig. 2 used a voltage ramp of 0 to 6300 v o l t s in 12.19 seconds. The breakdown voltage was determined by conlparing the voltage read on the o s c i l l o s c o p e to a c a l i b r a t i o n curve f o r the voltage d i v i d e r , and was checked by c a l c u l a t i n g the maximum voltage from the time to f a i l u r e . Both techniques gave the same breakdown voltage. The average breakdown strength was c a l c u l a t e d from a l o t of t h i r t y samples of each defect type.
I I I
FIG. 1 Schematic of stacking sequence of tapes f o r lamination. For c l a r i t y , the f i r s t electroded l a y e r has been shown face up.
The s t e p - b y - s t e p t e s t was c a r r i e d out manually by changing the voltage in increments of 250 v o l t s a f t e r an i n i t i a l voltage of 1250 v o l t s was applied to the c a p a c i t o r . The voltage was held at each l e v e l f o r ten seconds and when breakdown occurred between l e v e l s , the breakdown voltage was taken to be the previous l e v e l . The i n i t i a l voltage of 1250 v o l t s was determined by the r e s u l t s of the short time t e s t and was s l i g h t l y less than 50% of the expected breakdown voltage. The average breakdown strength f o r each sample type was determined by a l o t of ten c a p a c i t o r s .
Vol. 21, No. 9
W. A. BAHN, et al.
1076 INTERSTATE
V~.TAS[ D,V,O[M
|wlrl~ p IrUNCTION
p,~o~c'r,oN ¢mcurr
II[H!
FIG. 2 d.c. e l e c t r i c breakdown system. The sweep function generator controlled the ramping of the signal that was fed into a high voltage a m p l i f i e r . The breakdown was monitored on the oscilloscope. Results and Discussion Monolith Fabrication and D i e l e c t r i c Properties High density single layer capacitors were fabricated with controlled defects. Macropores of approximately 50 microns in diameter and delaminations 2mm x 3mm x 15 microns were reproducibly implanted in 5.7mm x 5.2mm x 100 micron p a r a l l e l plate capacitors. Consistency in sample densities was desirable so comparisons of the e l e c t r i c a l properties could be a t t r i b u t e d to the defects and not to density v a r i a t i o n s . The defects occupied a small f r a c t i o n of the t o t a l volume and had no apparent e f f e c t on the sample density. The defects, however, altered the e f f e c t i v e d i e l e c t r i c thickness. Microstructures of several samples are presented in Figures 3a, b, and c. Average capacitances and losses for several sample types are presented in Table 2. The samples with no defects had the highest capacitances, while those with macropores were somewhat smaller, and specimens with delaminations were the smallest. Variations in capacitance were a r e s u l t of increases in d i e l e c t r i c thickness of the samples containing macrodefects. The d i e l e c t r i c thicknesses of the capacitors containing no defects, macropores, and delaminations were 90, I00 and llO microns r e s p e c t i v e l y . Further reduction in capacitance of the delaminated samples was a t t r i b u t e d to the area f r a c t i o n of the delamination with respect to the total active electrode area. Seriesp a r a l l e l modelling of the phases resulted in reasonable agreement with measured values. TABLE 2 Average Capacitances and Losses f o r the Various Defect Types, with Standard Deviations. Sample Type ilo Defects Interlayer Intralayer Interlayer Intralayer
Porosity Porosity Delamination Delamination
Capacitance (nF) Avg. S.D.
Loss Avg.
6.22 5.31 5.56 4.44 4.56
0.008 0.007 0.010 0.008 0.011
0.75 0.93 0.74 0.68 0.65
S.D. 0.0005 0.0008 0.0008 0.0005 0.001
Vol.
21, No. 9
BaTiO 3
(a)
1077
(b)
FIG. 3 Micrographs showing typical capacitor cross-sections: a) optical micrograph of a capacitor without defects, b) optical micrograph of a capacitor with i n t r a l a y e r porosity, c) SEM micrograph of a capacitor with an i n t e r l a y e r de I ami na t i on.
(c) d.c. E l e c t r i c Breakdown The e l e c t r i c strengths of the various capacitor types are summarized in Table 3. The breakdown strengths observed in this i n v e s t i g a t i o n were comparable to the single crystal barium t i t a n a t e strengths observed by Inuishi and Uematsu ( I I ) , and two to three times the values observed by Fang and Brower (4), Bogdanov (5), and Schomann (6) f o r p o l y c r y s t a l l i n e BaTiO3. The increase in strength was probably a r e s u l t of dopants, as well as the thinner d i e l e c t r i c , and improved electrode geometry. The previous studies were performed on high p u r i t y materials using recessed disc specimens with thicknesses of 0.5 to 5.0 millimeters. During the voltage a p p l i c a t i o n , discharging by the sample occasionally occurred. Sometimes the discharges were audible and appeared on the o s c i l l o scope screen as an instantaneous drop in the voltage. The discharges may be caused by localized gaseous discharges and seemed to have no e f f e c t on the breakdown strength. Ouring the voltage ramp, the magnitude of the current through the sample was i n s i g n i f i c a n t u n t i l breakdown occurred, and then i t increased almost instantaneously. There were no current spikes observed at the discharges.
1078
W.A.
B A H N , et al.
Vol. 21, No. 9
TABLE 3 E l e c t r i c Breakdown Strengths of Capacitors ~Jith and Without Hacrodefects. Sample Type No Defects Interlayer Intralayer Interlayer Intralayer
Pores Pores Delamination Delamination
Short Time Test x(kV/cm) S.D. 280 95 280 92 300 70 310 I00 320 80
Step by Step x(kV/cm) 330 310 290 290 310
In order to determine the cause of the e l e c t r i c breakdown, microstructures of the broken-down samples were examined. Several samples fractured during testing making the location of the f a i l u r e very easy. A number of unfractured samples were mounted in epoxy and slow ground away to locate the f a i l u r e patl,. Two types of breakdown paths were observed. Typical f a i l u r e paths are shown in Figures 4 to 7. As shown in the micrographs, a tunnel was formed between the electrodes. The tunnels displayed a smooth i n t e r i o r , often with evidence of microcracking, i n d i c a t i v e of melting of the d i e l e c t r i c . When the surface fractured, the tunnel was located t y p i c a l l y at the center of a H e r t z i a n - l i k e f r a c t u r e (except when the f a i l u r e s i t e was at the end of the electrode, then i t was not c e n t r a l l y located). The second type of breakdown path is shown in Fig. 8. The conduction path was formed by a narrow channel between the electrodes and was detected only in one sample that f a i l e d at a s l i g h t l y lower f i e l d level (200 kV/cm). Resistance measurements of the capacitors a f t e r breakdown showed a two orders of magnitude decrease in resistance between 25 and 135°C. The magnitude and temperature dependence of the resistance suggested that the conduction
(a)
(b) FIG. 4
Microstructures of capacitor with i n t e r l a y e r delamination. Sample fractured during breakdown with f a i l u r e s i t e not located at the defect (EB = 300 kV/cm); a) 150x and b) 500x.
Vol. 21, No. 9
BaTiO3
(a)
1079
(b) FIG. 5
M i c r o s t r u c t u r e of capacitor with i n t e r l a y e r delamination showing H e r t z i a n - l i k e f r a c t u r i n g of the surface and l o c a t i o n of conduction tunnel. F a i l u r e s i t e not located at the defect (E B = 480 kV/cm); a) 150x and b) 500x.
(a)
(b) FIG. 6
M i c r o s t r u c t u r e s of a capacitor with i n t e r l a y e r pore t h a t did not f r a c t u r e during breakdown. Breakdown s i t e is located at a macropore (E B = 300 kV/cm); a) 250x and b) 500. path a f t e r breakdown may have been composed of a reduced BaTiO3-SrTiO 3 phase. This p r e d i c t i o n was upheld by the black c o l o r of the tunnel observed using l i g h t microscopy. For most of the samples, thermal runaway was the apparent cause of breakdown. Localized reduction of the ceramic d i e l e c t r i c probably occurred r e s u l t i n g an increase in c o n d u c t i v i t y . R e s i s t i v e heating caused a rapid r i s e
1080
W . A . BAHN, et al.
FIG. 7 Microstructure of a capacitor containing an i n t r a l a y e r delamination. Breakdown s i t e is located at delamination (EB > 450 kV/cm).
Vol. 21, No. 9
FIG. 8 Microstructure of a capacitor with i n t e r l a y e r porosity. Sample f a i l e d at a low breakdown strength and shows a d i f f e r e n t type of f a i l u r e path.
in temperature, and melting of the ceramic formed a conduction path between the electrodes. Figure 9 shows a conchoidal piece, that fractured from the surface of a capacitor containing an i n t r a l a y e r delamination. The portion of the platinum electrode missing from the capacitor surface was attached to t h i s ceramic piece. Unlike the ceramics in Figures 4 to 7, the electrode showed no i n d i c a t i o n of melting. This indicated that at the electrode-ceramic i n t e r face, the temperatures reached during breakdown were between the melting temperature of the d i e l e c t r i c (approximately 1500°C) and the melting temperature of platinum (1772°C). The temperature w i t h i n the tunnel could not be determined but was probably in the same range. The f i n i t e width of the tunnel wall and the melting around the tunnel at the ceramic-electrode i n t e r f a c e suggested that the heat was generated at a rapid rate. Once the tunnel was formed, the heat was dissipated by the electrode at the ceramic-electrode i n t e r f a c e . When the f i e l d dropped instantaneously to zero and i2R heating ceased, the Pt electrode continued d i s s i p a t i n g heat. Eventually a large temperature gradient formed at the ceramic electrode i n t e r f a c e because of the large differences in the thermal c o n d u c t i v i t i e s and heat capacities of the m a t e r i a l s . These differences resulted in a thermal expansion mismatch and hence f r a c t u r i n g of the capacitor. When f r a c t u r i n g did not occur, gaseous expansion from the high temperatures was a l l e v i a t e d by forming large holes in the melted d i e l e c t r i c tunnel. The presence of defects did not reduce the breakdown strengths, contrary to what was predicted by Gerson and Marshall ( I ) , and Rupaal, et al. (2). There were three major differences between t h e i r studies and this one: the sample configuration, the presence of randomly dispersed macroporosity and the d i e l e c t r i c thickness. In t h e i r i n v e s t i g a t i o n s , tlle breakdown path was located in the region of the defects because of a s i g n i f i c a n t reduction in the d i e l e c t r i c thickness. The d i e l e c t r i c thickness was not s i g n i f i c a n t l y reduced in t h i s i n v e s t i g a t i o n and the apparent increase in strength of the d i e l e c t r i c material from reducing the d i e l e c t r i c thickness was w i t h i n the s c a t t e r of the data.
Vol.
21, N o .
9
BaTiO 3
1081
Only in a few samples were the breakdown s i t e s traced to a d e f e c t despite what was p r e d i c t e d by f i e l d enhancement models. In t h i s i n v e s t i g a t i o n , i t was not e v i d e n t t h a t f i e l d enhancement around the defects was s i g n i f i c a n t enough to cause f a i l u r e . Capacitors w i t h f a i l u r e s i t e s located at defects are shown i n Figures 7 and 8. The m i c r o s t r u c t u r e s i n Figure 5 were of a d e f e c t i v e c a p a c i t o r but the breakdown tunnel was not associated with a defect. The m i c r o s t r u c t u r e s do not provide i n f o r m a t i o n as to the o r i g i n of the f a i l u r e s . FIG. 9 Summary and Conclusions
M i c r o s t r u c t u r e of piece the f r a c t u r e from the surface of a c a p a c i t o r cont a i n i n g an i n t r a l a y e r d e l a m i n a t i o n during breakdown.
C o n t r o l l e d delaminations and voids were placed i n t o s i n g l e layer capacitors fabricated with m u l t i l a y e r technology (tape c a s t i n g screen p r i n t i n g , l a m i n a t i o n , e t c . ) . The s i n t e r e d ceramic d e n s i t y was approximately 94% of t h e o r e t i c a l , and the defects did not r e s u l t i n a r e d u c t i o n of the d i e l e c t r i c d e n s i t y . The pores averaged between 25 and 50 microns i n diameter and the delaminations were approximately 2 mm x 3 mm x 15 microns. The thickness of the a c t i v e d i e l e c t r i c was increased approximately 10% i n the samples w i t h d e f e c t s . The t o t a l thickness of the ceramic d i e l e c t r i c in the region of a d e f e c t was reduced between I0 and 25%. The defects caused a decrease i n the capacitance of the samples as a r e s u l t of the increase in the d i e l e c t r i c t h i c k n e s s . For the samples w i t h pores, t h i s was the only c o n t r i b u t i o n to the capacitance change. For the samples w i t h d e l a m i n a t i o n s , the d e f e c t area was s i g n i f i c a n t as compared to the e l e c t r o d e area and a s e r i e s - p a r a l l e l mixing p r e d i c t e d s i m i l a r capacitances as the measured values. The d.c. breakdown studies showed t h a t the samples w i t h defects had s t r e n g t h s s i m i l a r to those w i t h o u t defects probably because the defects did not extend between the e l e c t r o d e s and did not s i g n i f i c a n t l y reduce the t o t a l ceramic d i e l e c t r i c thickness i n the region of the d e f e c t . In most of the samples, e l e c t r i c a l breakdown was thermal in nature as was i n d i c a t e d by the m i c r o s t r u c t u r e s . The breakdown paths i n the c a p a c i t o r s w~re tunnels between the e l e c t r o d e s w i t h a smooth i n t e r i o r and evidence of microcracking, in some o f the samples, s u f f i c i e n t pressure was present w i t h i n the region of breakdown to cause the sample to f r a c t u r e w i t h a conchoidal piece missing from the surface. In samples t h a t did not f r a c t u r e , large holes i n the i n t e r i o r of the conduction tunnel were created as a means f o r the gaseous pressure to escape. The e l e c t r i c a l breakdown probably f o l l o w e d a sequence s i m i l a r to the f o l l o w i n g :
1082
W0 A. BAHN, et al.
Vol. 21, No. 9
l)
The large e l e c t r i c f i e l d that was applied to capacitor resulted in a chemical reduction of the d i e l e c t r i c , or perhaps in e l e c t r o s t r i c t i v e stresses that caused propagation of a microcrack between electrodes.
2)
In the case of a microcrack, surface conduction along the crack resulted in near instantaneous discharges, and localized heating of the surface and reduction of the ceramic.
3) The reduction of the ceramic led to increased conduction and localized heating. The temperature rises very rapidly to the melting temperature forming a tunnel between the electrodes.
4)
Once the tunnel was formed, conduction of heat along the platinum electrode resulted in a temperature d i f f e r e n t i a l between the ceramic and the electrode, and the thermal expansion mismatch caused the ceramic to thermal shock. Acknowledgements
We would l i k e to thank the Center f o r D i e l e c t r i c Studies f o r sponsoring this i n v e s t i g a t i o n . We are also grateful to Paul Moses for his help in setting up the e l e c t r i c a l measurements and to Jack tlecholsky and William Carlson f o r t h e i r assistance in the i n t e r p r e t a t i o n of the microstructures. References I.
R. Gerson and T.C. Marshall, J. Appl. Phys. 30 ( I I ) ,
1650 (1959).
2.
A.S. Rupaal, J.E. Garnier, and J.L. Bates, Comm. Amer. Cer. Soc., ClO0 (1981).
3.
E.K. Beauchamp, J. Amer. Cer. Soc. 54 ( I 0 ) , 484 (1971).
4.
P.H. Fang and W.S. Brower, Phys. Rev. ]13 (2), 456 (1959).
5.
S.V. Bogdanov, Sov. Phys. - Solid State 4 (8), 1596 (1963).
6.
K.D. Schomann, Appl. Phys. 89 (6), 89 (1975).
7.
H. Domingos, D.P. Quattro, and J. Scattero, IEEE Trans. CHMT - 1 (4), 423 (1978).
8.
H. Mizaura and J. Okada, J. Phys. Soc. Jpn. 61 ( I ) ,
9.
E.J. B r i t t and M.V. Davis, IEEE Thermionic Conversion S p e c i a l i s t Conf. held in San Diego, CA, October 4-7, 137 (1971).
55 (1951).
lO.
M. Yashimura and H.K. Bowen, J. Amer. Cer. Soc. 64 (7), 404 (1981).
II.
Y. Inuishi and S. Uematsu, J. Phys. Soc. Jpn. 13, 761 (1958).
MACRODEFECTS IN BaTiO3 CAPACITORS W.A. Bahn* and R.E. Newnham Materials Research Laboratory The Pennsylvania State U n i v e r s i t y U n i v e r s i t y Park, PA 16802 ( R e c e i v e d J u n e 9, 1986; R e f e r e e d )
ABSTRACT Controlled macrodefects (pores and delaminations) were introduced into single layer capacitors. The e f f e c t s of these defects on the d i e l e c t r i c properties and d.c. e l e c t r i c breakdown strength were studied. Macrodefects did not cause a reduction in the o v e r a l l density of the d i e l e c t r i c , but a reduction in the capacitance was observed in the samples containing defects. Large voids reduced the capacitance by increasing the a c t i v e d i e l e c t r i c thickness by I0 percent. Delaminations also increased the a c t i v e d i e l e c t r i c thickness by I0 percent, but a f u r t h e r reduction of the capacitance was observed and was modelled using s e r i e s - p a r a l l e l mixing. The capacitors were subjected to a d.c. voltage ramp u n t i l they f a i l e d . There were no s i g n i f i c a n t differences in the breakdown strengths of the various sample types with the average strength being 300 kV/cm. The f a i l u r e mechanisms were explained by the microstructures of the breakdown paths. Breakdown was thermal in nature with the formation of a chemically reduced d i e l e c t r i c tunnel between the electrodes. The f a i l u r e path wa~ occasionally traced to a defect, but more often occurred in the dense ceramic The defects did not s i g n i f i c a n t l y change the e l e c t r i c f i e l d d i s t r i bution in the d i e l e c t r i c between the electrodes, and hence, the breakdown strength of the d i e l e c t r i c layer in the region of the defect was not g r e a t l y reduced. MATERIALSINDEX: barium t i t a n a t e Introduction M u l t i l a y e r ceramic capacitors u t i l i z e high p e r m i t t i v i t y rr,a t e r i a ! s to achieve volumetric e f f i c i e n c y . Unfortunately, ceramics are i n h e r e n t l y *Now at Coming Glass Works, Corning, NY 14830. 1073
1074
W.A.
BAHN, et al.
Vol. 21, No. 9
defective and r e q u i r e careful processing to achieve high densities and maintain high p e r m i t t i v i t i e s . Despite t i g h t control of the processing of ceramic capacitors, gross defects often appear in the f i n a l product. Voids and delaminations usually occupy an i n s i g n i f i c a n t f r a c t i o n of the total volume, but are often linked to premature f a i l u r e s . For this reason, the influence of these defects on the d i e l e c t r i c properties and on the e l e c t r i c a l breakdown behavior of ceramic capacitors is of concern to many manufacturers. Mathematical models have been developed to explain the reduction in d i e l e c t r i c p e r m i t t i v i t y by the presence of a defect or second phase, but pred i c t i o n of the e l e c t r i c breakdown strengths has proven a more d i f f i c u l t problem and is p a r t i c u l a r l y important given the trend towards increased volumetric e f f i c i e n c y . Knowledge of the e l e c t r i c a l f a i l u r e mechanism in ceramic materials is l i m i t e d . E l e c t r i c breakdown has been a t t r i b u t e d to sample o r o s i t y (1,2), grain size (3), phase t r a n s i t i o n s (4,5), grain boundary phases 6), d i e l e c t r i c p e r m i t t i v i t y (7), and the thermal environment (8-10). Breakdown strength is dependent on the specimen, environmental conditions, and the measurement methods.
I
A technique f o r introducing controlled macrodefects into ceramic d i e l e c t r i c s is described in this paper. The e f f e c t of voids and delaminations on the d i e l e c t r i c properties and breakdown strengths were investigated. Experimental Procedure Capacitor Fabrication The materials used in this i n v e s t i g a t i o n are l i s t e d in Table I. Ceramic capacitors with one e f f e c t i v e d i e l e c t r i c layer were fabricated using a conventional m u l t i l a y e r tape casting process. The stacking sequence of the tapes was as follows (Fig. I ) : 1 2 1 2 1 3
screened marker layer blank layers electrode layer ink side down blank layers electrode layer ink side up blank layers
The defects were formed by f u g i t i v e organics that were placed in the capacitor during stacking. The defects were inserted by hand both adjacent to the electrode ( i n t e r l a y e r ) and between the tapes forming the active d i e l e c t r i c (intralayer). The tapes were laminated in a vacuum die under 5000 pounds per square inch at 75°C f o r 2 minutes. The parts were diced and f i r e d at 1290°C f o r 45 minutes using a globar furnace. A f t e r f i r i n g , several samples were ground and polished to determine the average d i e l e c t r i c thickness and overlap area. E l e c t r i c a l Measurements The capacitance and loss were monitored as a function of temperature f o r ten samples of each defect type. The capacitors were loaded into a Delta Design Model 2300 temperature control box and cooled from 150°C to -50°C at a rate of 3°C/min. The capacitances and losses at 0 . I , 1.0 and I0 kHz were measured semicontinuously using a Hewlett Packard HP 4274A LCR bridge i n t e r faced with a Hewlett Packard 9816 computer. Both a short-time and a step-by-step t e s t were u t i l i z e d f o r determination of the e l e c t r i c breakdown strength of the capacitors. The short time t e s t was accomplished by sending a t r i a n g u l a r wave signal from an I n t e r s t a t e sweep generator F74 to a Trek Cor-a-trol 610A a m p l i f i e r . The signal was amplified
Vol. 21, No. 9
BaTiO 3
1075
TABLE 1 M a t e r i a l s Used in t h i s I n v e s t i g a t i o n D i e l e c t r i c Powder - TAMTRON X7PI82H (BX BaTi03) D i e l e c t r i c Constant (5.5 g/cm 3)
2100
D i s s i p a t i o n Factor
(%)
1.0
Breakdown Strength
(V/mil) (kV/cm)
768 300
Binder System - Cladan CB73115 ( a c r y l i c based) Electrode System - Captrode I00 Pt Defects Polymethyl methacrylate (pores) A c r y l o i d B7 (delaminations)
by I000 then applied to the capacitance sample. The voltage through the voltage d i v i d e r was monitored on a N i c o l e t Model #204A o s c i l l o scope. A c u r r e n t - m o n i t o r i n g c i r c u i t was i n s e r t e d a f t e r the sample to observe the c u r r e n t through the c a p a c i t o r , and was also tested with an o s c i l l o scope. The c i r c u i t used f o r the s h o r t - t i m e t e s t shown in Fig. 2 used a voltage ramp of 0 to 6300 v o l t s in 12.19 seconds. The breakdown voltage was determined by conlparing the voltage read on the o s c i l l o s c o p e to a c a l i b r a t i o n curve f o r the voltage d i v i d e r , and was checked by c a l c u l a t i n g the maximum voltage from the time to f a i l u r e . Both techniques gave the same breakdown voltage. The average breakdown strength was c a l c u l a t e d from a l o t of t h i r t y samples of each defect type.
I I I
FIG. 1 Schematic of stacking sequence of tapes f o r lamination. For c l a r i t y , the f i r s t electroded l a y e r has been shown face up.
The s t e p - b y - s t e p t e s t was c a r r i e d out manually by changing the voltage in increments of 250 v o l t s a f t e r an i n i t i a l voltage of 1250 v o l t s was applied to the c a p a c i t o r . The voltage was held at each l e v e l f o r ten seconds and when breakdown occurred between l e v e l s , the breakdown voltage was taken to be the previous l e v e l . The i n i t i a l voltage of 1250 v o l t s was determined by the r e s u l t s of the short time t e s t and was s l i g h t l y less than 50% of the expected breakdown voltage. The average breakdown strength f o r each sample type was determined by a l o t of ten c a p a c i t o r s .
Vol. 21, No. 9
W. A. BAHN, et al.
1076 INTERSTATE
V~.TAS[ D,V,O[M
|wlrl~ p IrUNCTION
p,~o~c'r,oN ¢mcurr
II[H!
FIG. 2 d.c. e l e c t r i c breakdown system. The sweep function generator controlled the ramping of the signal that was fed into a high voltage a m p l i f i e r . The breakdown was monitored on the oscilloscope. Results and Discussion Monolith Fabrication and D i e l e c t r i c Properties High density single layer capacitors were fabricated with controlled defects. Macropores of approximately 50 microns in diameter and delaminations 2mm x 3mm x 15 microns were reproducibly implanted in 5.7mm x 5.2mm x 100 micron p a r a l l e l plate capacitors. Consistency in sample densities was desirable so comparisons of the e l e c t r i c a l properties could be a t t r i b u t e d to the defects and not to density v a r i a t i o n s . The defects occupied a small f r a c t i o n of the t o t a l volume and had no apparent e f f e c t on the sample density. The defects, however, altered the e f f e c t i v e d i e l e c t r i c thickness. Microstructures of several samples are presented in Figures 3a, b, and c. Average capacitances and losses for several sample types are presented in Table 2. The samples with no defects had the highest capacitances, while those with macropores were somewhat smaller, and specimens with delaminations were the smallest. Variations in capacitance were a r e s u l t of increases in d i e l e c t r i c thickness of the samples containing macrodefects. The d i e l e c t r i c thicknesses of the capacitors containing no defects, macropores, and delaminations were 90, I00 and llO microns r e s p e c t i v e l y . Further reduction in capacitance of the delaminated samples was a t t r i b u t e d to the area f r a c t i o n of the delamination with respect to the total active electrode area. Seriesp a r a l l e l modelling of the phases resulted in reasonable agreement with measured values. TABLE 2 Average Capacitances and Losses f o r the Various Defect Types, with Standard Deviations. Sample Type ilo Defects Interlayer Intralayer Interlayer Intralayer
Porosity Porosity Delamination Delamination
Capacitance (nF) Avg. S.D.
Loss Avg.
6.22 5.31 5.56 4.44 4.56
0.008 0.007 0.010 0.008 0.011
0.75 0.93 0.74 0.68 0.65
S.D. 0.0005 0.0008 0.0008 0.0005 0.001
Vol.
21, No. 9
BaTiO 3
(a)
1077
(b)
FIG. 3 Micrographs showing typical capacitor cross-sections: a) optical micrograph of a capacitor without defects, b) optical micrograph of a capacitor with i n t r a l a y e r porosity, c) SEM micrograph of a capacitor with an i n t e r l a y e r de I ami na t i on.
(c) d.c. E l e c t r i c Breakdown The e l e c t r i c strengths of the various capacitor types are summarized in Table 3. The breakdown strengths observed in this i n v e s t i g a t i o n were comparable to the single crystal barium t i t a n a t e strengths observed by Inuishi and Uematsu ( I I ) , and two to three times the values observed by Fang and Brower (4), Bogdanov (5), and Schomann (6) f o r p o l y c r y s t a l l i n e BaTiO3. The increase in strength was probably a r e s u l t of dopants, as well as the thinner d i e l e c t r i c , and improved electrode geometry. The previous studies were performed on high p u r i t y materials using recessed disc specimens with thicknesses of 0.5 to 5.0 millimeters. During the voltage a p p l i c a t i o n , discharging by the sample occasionally occurred. Sometimes the discharges were audible and appeared on the o s c i l l o scope screen as an instantaneous drop in the voltage. The discharges may be caused by localized gaseous discharges and seemed to have no e f f e c t on the breakdown strength. Ouring the voltage ramp, the magnitude of the current through the sample was i n s i g n i f i c a n t u n t i l breakdown occurred, and then i t increased almost instantaneously. There were no current spikes observed at the discharges.
1078
W.A.
B A H N , et al.
Vol. 21, No. 9
TABLE 3 E l e c t r i c Breakdown Strengths of Capacitors ~Jith and Without Hacrodefects. Sample Type No Defects Interlayer Intralayer Interlayer Intralayer
Pores Pores Delamination Delamination
Short Time Test x(kV/cm) S.D. 280 95 280 92 300 70 310 I00 320 80
Step by Step x(kV/cm) 330 310 290 290 310
In order to determine the cause of the e l e c t r i c breakdown, microstructures of the broken-down samples were examined. Several samples fractured during testing making the location of the f a i l u r e very easy. A number of unfractured samples were mounted in epoxy and slow ground away to locate the f a i l u r e patl,. Two types of breakdown paths were observed. Typical f a i l u r e paths are shown in Figures 4 to 7. As shown in the micrographs, a tunnel was formed between the electrodes. The tunnels displayed a smooth i n t e r i o r , often with evidence of microcracking, i n d i c a t i v e of melting of the d i e l e c t r i c . When the surface fractured, the tunnel was located t y p i c a l l y at the center of a H e r t z i a n - l i k e f r a c t u r e (except when the f a i l u r e s i t e was at the end of the electrode, then i t was not c e n t r a l l y located). The second type of breakdown path is shown in Fig. 8. The conduction path was formed by a narrow channel between the electrodes and was detected only in one sample that f a i l e d at a s l i g h t l y lower f i e l d level (200 kV/cm). Resistance measurements of the capacitors a f t e r breakdown showed a two orders of magnitude decrease in resistance between 25 and 135°C. The magnitude and temperature dependence of the resistance suggested that the conduction
(a)
(b) FIG. 4
Microstructures of capacitor with i n t e r l a y e r delamination. Sample fractured during breakdown with f a i l u r e s i t e not located at the defect (EB = 300 kV/cm); a) 150x and b) 500x.
Vol. 21, No. 9
BaTiO3
(a)
1079
(b) FIG. 5
M i c r o s t r u c t u r e of capacitor with i n t e r l a y e r delamination showing H e r t z i a n - l i k e f r a c t u r i n g of the surface and l o c a t i o n of conduction tunnel. F a i l u r e s i t e not located at the defect (E B = 480 kV/cm); a) 150x and b) 500x.
(a)
(b) FIG. 6
M i c r o s t r u c t u r e s of a capacitor with i n t e r l a y e r pore t h a t did not f r a c t u r e during breakdown. Breakdown s i t e is located at a macropore (E B = 300 kV/cm); a) 250x and b) 500. path a f t e r breakdown may have been composed of a reduced BaTiO3-SrTiO 3 phase. This p r e d i c t i o n was upheld by the black c o l o r of the tunnel observed using l i g h t microscopy. For most of the samples, thermal runaway was the apparent cause of breakdown. Localized reduction of the ceramic d i e l e c t r i c probably occurred r e s u l t i n g an increase in c o n d u c t i v i t y . R e s i s t i v e heating caused a rapid r i s e
1080
W . A . BAHN, et al.
FIG. 7 Microstructure of a capacitor containing an i n t r a l a y e r delamination. Breakdown s i t e is located at delamination (EB > 450 kV/cm).
Vol. 21, No. 9
FIG. 8 Microstructure of a capacitor with i n t e r l a y e r porosity. Sample f a i l e d at a low breakdown strength and shows a d i f f e r e n t type of f a i l u r e path.
in temperature, and melting of the ceramic formed a conduction path between the electrodes. Figure 9 shows a conchoidal piece, that fractured from the surface of a capacitor containing an i n t r a l a y e r delamination. The portion of the platinum electrode missing from the capacitor surface was attached to t h i s ceramic piece. Unlike the ceramics in Figures 4 to 7, the electrode showed no i n d i c a t i o n of melting. This indicated that at the electrode-ceramic i n t e r face, the temperatures reached during breakdown were between the melting temperature of the d i e l e c t r i c (approximately 1500°C) and the melting temperature of platinum (1772°C). The temperature w i t h i n the tunnel could not be determined but was probably in the same range. The f i n i t e width of the tunnel wall and the melting around the tunnel at the ceramic-electrode i n t e r f a c e suggested that the heat was generated at a rapid rate. Once the tunnel was formed, the heat was dissipated by the electrode at the ceramic-electrode i n t e r f a c e . When the f i e l d dropped instantaneously to zero and i2R heating ceased, the Pt electrode continued d i s s i p a t i n g heat. Eventually a large temperature gradient formed at the ceramic electrode i n t e r f a c e because of the large differences in the thermal c o n d u c t i v i t i e s and heat capacities of the m a t e r i a l s . These differences resulted in a thermal expansion mismatch and hence f r a c t u r i n g of the capacitor. When f r a c t u r i n g did not occur, gaseous expansion from the high temperatures was a l l e v i a t e d by forming large holes in the melted d i e l e c t r i c tunnel. The presence of defects did not reduce the breakdown strengths, contrary to what was predicted by Gerson and Marshall ( I ) , and Rupaal, et al. (2). There were three major differences between t h e i r studies and this one: the sample configuration, the presence of randomly dispersed macroporosity and the d i e l e c t r i c thickness. In t h e i r i n v e s t i g a t i o n s , tlle breakdown path was located in the region of the defects because of a s i g n i f i c a n t reduction in the d i e l e c t r i c thickness. The d i e l e c t r i c thickness was not s i g n i f i c a n t l y reduced in t h i s i n v e s t i g a t i o n and the apparent increase in strength of the d i e l e c t r i c material from reducing the d i e l e c t r i c thickness was w i t h i n the s c a t t e r of the data.
Vol.
21, N o .
9
BaTiO 3
1081
Only in a few samples were the breakdown s i t e s traced to a d e f e c t despite what was p r e d i c t e d by f i e l d enhancement models. In t h i s i n v e s t i g a t i o n , i t was not e v i d e n t t h a t f i e l d enhancement around the defects was s i g n i f i c a n t enough to cause f a i l u r e . Capacitors w i t h f a i l u r e s i t e s located at defects are shown i n Figures 7 and 8. The m i c r o s t r u c t u r e s i n Figure 5 were of a d e f e c t i v e c a p a c i t o r but the breakdown tunnel was not associated with a defect. The m i c r o s t r u c t u r e s do not provide i n f o r m a t i o n as to the o r i g i n of the f a i l u r e s . FIG. 9 Summary and Conclusions
M i c r o s t r u c t u r e of piece the f r a c t u r e from the surface of a c a p a c i t o r cont a i n i n g an i n t r a l a y e r d e l a m i n a t i o n during breakdown.
C o n t r o l l e d delaminations and voids were placed i n t o s i n g l e layer capacitors fabricated with m u l t i l a y e r technology (tape c a s t i n g screen p r i n t i n g , l a m i n a t i o n , e t c . ) . The s i n t e r e d ceramic d e n s i t y was approximately 94% of t h e o r e t i c a l , and the defects did not r e s u l t i n a r e d u c t i o n of the d i e l e c t r i c d e n s i t y . The pores averaged between 25 and 50 microns i n diameter and the delaminations were approximately 2 mm x 3 mm x 15 microns. The thickness of the a c t i v e d i e l e c t r i c was increased approximately 10% i n the samples w i t h d e f e c t s . The t o t a l thickness of the ceramic d i e l e c t r i c in the region of a d e f e c t was reduced between I0 and 25%. The defects caused a decrease i n the capacitance of the samples as a r e s u l t of the increase in the d i e l e c t r i c t h i c k n e s s . For the samples w i t h pores, t h i s was the only c o n t r i b u t i o n to the capacitance change. For the samples w i t h d e l a m i n a t i o n s , the d e f e c t area was s i g n i f i c a n t as compared to the e l e c t r o d e area and a s e r i e s - p a r a l l e l mixing p r e d i c t e d s i m i l a r capacitances as the measured values. The d.c. breakdown studies showed t h a t the samples w i t h defects had s t r e n g t h s s i m i l a r to those w i t h o u t defects probably because the defects did not extend between the e l e c t r o d e s and did not s i g n i f i c a n t l y reduce the t o t a l ceramic d i e l e c t r i c thickness i n the region of the d e f e c t . In most of the samples, e l e c t r i c a l breakdown was thermal in nature as was i n d i c a t e d by the m i c r o s t r u c t u r e s . The breakdown paths i n the c a p a c i t o r s w~re tunnels between the e l e c t r o d e s w i t h a smooth i n t e r i o r and evidence of microcracking, in some o f the samples, s u f f i c i e n t pressure was present w i t h i n the region of breakdown to cause the sample to f r a c t u r e w i t h a conchoidal piece missing from the surface. In samples t h a t did not f r a c t u r e , large holes i n the i n t e r i o r of the conduction tunnel were created as a means f o r the gaseous pressure to escape. The e l e c t r i c a l breakdown probably f o l l o w e d a sequence s i m i l a r to the f o l l o w i n g :
1082
W0 A. BAHN, et al.
Vol. 21, No. 9
l)
The large e l e c t r i c f i e l d that was applied to capacitor resulted in a chemical reduction of the d i e l e c t r i c , or perhaps in e l e c t r o s t r i c t i v e stresses that caused propagation of a microcrack between electrodes.
2)
In the case of a microcrack, surface conduction along the crack resulted in near instantaneous discharges, and localized heating of the surface and reduction of the ceramic.
3) The reduction of the ceramic led to increased conduction and localized heating. The temperature rises very rapidly to the melting temperature forming a tunnel between the electrodes.
4)
Once the tunnel was formed, conduction of heat along the platinum electrode resulted in a temperature d i f f e r e n t i a l between the ceramic and the electrode, and the thermal expansion mismatch caused the ceramic to thermal shock. Acknowledgements
We would l i k e to thank the Center f o r D i e l e c t r i c Studies f o r sponsoring this i n v e s t i g a t i o n . We are also grateful to Paul Moses for his help in setting up the e l e c t r i c a l measurements and to Jack tlecholsky and William Carlson f o r t h e i r assistance in the i n t e r p r e t a t i o n of the microstructures. References I.
R. Gerson and T.C. Marshall, J. Appl. Phys. 30 ( I I ) ,
1650 (1959).
2.
A.S. Rupaal, J.E. Garnier, and J.L. Bates, Comm. Amer. Cer. Soc., ClO0 (1981).
3.
E.K. Beauchamp, J. Amer. Cer. Soc. 54 ( I 0 ) , 484 (1971).
4.
P.H. Fang and W.S. Brower, Phys. Rev. ]13 (2), 456 (1959).
5.
S.V. Bogdanov, Sov. Phys. - Solid State 4 (8), 1596 (1963).
6.
K.D. Schomann, Appl. Phys. 89 (6), 89 (1975).
7.
H. Domingos, D.P. Quattro, and J. Scattero, IEEE Trans. CHMT - 1 (4), 423 (1978).
8.
H. Mizaura and J. Okada, J. Phys. Soc. Jpn. 61 ( I ) ,
9.
E.J. B r i t t and M.V. Davis, IEEE Thermionic Conversion S p e c i a l i s t Conf. held in San Diego, CA, October 4-7, 137 (1971).
55 (1951).
lO.
M. Yashimura and H.K. Bowen, J. Amer. Cer. Soc. 64 (7), 404 (1981).
II.
Y. Inuishi and S. Uematsu, J. Phys. Soc. Jpn. 13, 761 (1958).