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Electrical and stability properties and ultrasonic microscope characterisation of low temperature co-fired ceramics resistors
Microelectronics Reliability 41 (2001) 669±676
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Electrical and stability properties and ultrasonic microscope characterisation of low temperature co-®red ceramics resistors Andrzej Dziedzic a,*, Leszek J. Golonka a, Jaroslaw Kita a, Heiko Thust b, Karl-Heinz Drue b, Reinhard Bauer c, Lars Rebenklau c, Klaus-Jurgen Wolter c
a
Institute of Microsystem Technology, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, PL-50-370 Wroclaw, Poland b Ilmenau University of Technology, P.O. Box 100565, D-98684 Ilmenau, Germany c Dresden University of Technology, HelmholtzStrasse 18, D-01062 Dresden, Germany Received 16 October 2000; received in revised form 14 December 2000
Abstract This paper presents systematic investigations of electrical and stability properties of various low temperature co-®red ceramics (LTCC) resistors. One of the goals of this work was to check the compatibility of LTCC materials (tapes, resistive and conductive inks) from various manufacturers. Three commercially available green tapes and three LTCC resistor/conductor systems were examined. The resistive inks with 1 kX=sq. nominal sheet resistance were used. Buried (inside) and surface resistors were laminated and ®red according to the tape manufacturersÕ recommendations. The in¯uence of dimensional eect on sheet resistance and hot temperature coecient of resistance, the temperature dependence of resistance in a wide temperature range (from 180°C to 130°C), long-term stability of thermally aged as®red resistors (150°C, 500 h) and durability to high-voltage micro- or nanosecond pulses (50 ns pulses with 4000 V/mm maximum electric ®eld or 10 ls ones with 700±1000 V/mm electrical ®eld) were carried out for electrical and stability characterisation of LTCC resistors. Non-destructive scanning acoustic microscope diagnostics was applied for structure investigation and estimation of lamination and co®ring process quality of buried LTCC resistors. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Most of commercially available high-temperature thick-®lm inks is compatible with alumina (96% Al2 O3 ) substrate. However, composition of glass±ceramic tapes, which are a key component in the low temperature co®red ceramics (LTCC) materials system, diers considerably not only from alumina but also from each other. The fabrication process of LTCC components and circuits is somewhat dierent from standard cermet thick®lm technology, too. Moreover, each manufacturer of thick-®lm electronic materials oers its own LTCC materials, consisting of a tape based on a dierent devitrifying glass±ceramic system [1,2] and a set of resistor and conductor pastes highly recommended for this tape. *
Corresponding author. Tel./fax: +48-71-355-48-22. E-mail address: [email protected] (A. Dziedzic).
There is an interesting question from userÕs point of view if the LTCC materials, in particular tapes and resistor inks from dierent manufacturers, could be applied in one process, or dierences between various green tapes make such compatibility impossible. For example, with positive answer the user will not become fully subordinated to one producer of LTCC materials. The above problem is similar to behaviour of thick®lm resistors fabricated on dielectrics, intensively investigated dozen or so years ago [3,4]. These works shown, that the electrical properties were associated with interactions between substrates and resistor glassy phase constituents. But among others, stable and repeatable temperature sensor with good linearity was fabricated by selecting the correct resistor/dielectric paste combination and resistor aspect ratio [5]. This paper presents basic electrical and stability properties of buried and surface resistors made from three dierent resistor systems (from manufacturers A, B
0026-2714/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 0 0 4 - X
670
A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
and C) on/in three dierent LTCC tapes from the same manufacturers A, B and C. The sheet resistance, hot temperature coecient of resistance (HTCR), temperature dependence of resistance in a wide temperature range, long-term stability and high-voltage pulse durability were analysed and discussed for various resistor/ tape con®gurations, resistor placement as well as its aspect ratio. The classical physicochemical analysis of interactions that occur between LTCC components and tapes is presented very rarely [6] because of unknown initial composition of such complicated, non-equilibrium systems as commercially available thick-®lm materials. Moreover, most of such test techniques demand a long preparation of the samples and are almost always destructive. Therefore, non-destructive scanning acoustic microscope (SAM) [7,8] was applied in investigation of possible delamination and/or existence of the air bubbles inside the LTCC test structure.
in this paper could not be transferred in a simple way for other inks of tested resistor systems. The resistive ®lms and appropriate conductors were screen-printed through 325 mesh screens and then dried at 70°C for 10 min. The test pattern consisting of six resistors with 1 mm width and 0.5, 1, 3, 5, 7 and 9 sq. length was made on/in 15 15 mm2 4-layer LTCC structure (Fig. 1). The lamination process was carried out in an isostatic press at 210 bar for 10 min held at 70°C. Next, independently on the resistor position (buried or surface), the laminates were ®red under timetemperature pro®le recommended by adequate tape manufacturers (Fig. 2). In general, 18 technological combinations with different LTCC tapes, types of resistive inks as well as resistor positions were prepared and subjected to standard and non-standard electrical characterisation. Two-part marks were used for description of the above variants. For example, the C-B3s code means surface resistors made using 103 X/sq. resistor ink from manufacturer B on the LTCC tape supplied by manufacturer C.
2. Test sample fabrication The three various resistor systems, oered by manufactures A, B and C as well as the LTCC tapes from the same vendors were applied in our experiment. In order to limit the number of experimental variants only 1 kX/sq. inks were used in fabrication of buried (b) or surface (s) resistors. But the readers have to be conscious that names of commercially available resistor series are related in larger part to their electrical properties than to their constituents. Within the same resistor series its particular members (with various nominal sheet resistance) could have not only dierent conductive phase/ glass ratio but also glass content or kind and amount of modi®ers. Therefore the electrical properties presented
3. Dimensional eects Systematic investigations of electrical properties of thick-®lm resistors at dierent LTCC systems are presented in this paper. Analysis of dimensional eects is one of the most common electrical characterisation of such components. As one can see from Table 1 the 1000 X/sq. members of A, B and C series show very dierent behaviour. Only in a very few cases these are apparent as insigni®cant reduction in sheet resistance (maximum by about 40% for C-C3s and B-A3b resistors). The reverse eect, this is the increase of actual sheet resistance by factor from 5 to
Fig. 1. Test pattern geometry of LTCC resistors.
A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
Fig. 2. Firing pro®les of various LTCC systems.
671
Fig. 3. Normalised sheet resistance versus length of A3 resistors (values of resistance for l w (1 sq.) are given in Table 1).
Table 1 Sheet resistance of 1 sq. length resistors Technological variant
R (kX)
A-A3b A-A3s B-A3b B-A3s C-A3b C-A3s
1.38 0.66 0.57 0.78 10.6 6.70
A-B3b A-B3s B-B3b B-B3s C-B3b C-B3s
10.9 9.10 13.7 1.42 7.10 3.00
A-C3b A-C3s B-C3b B-C3s C-C3b C-C3s
4.20 1.11 5.30 2.53 1.67 0.57
14 in comparison with nominal one, occurs in much more number of cases (C-A3b, C-A3s, A-B3b, A-B3s, B-B3b, C-B3b, A-C3b and B-C3b). The plots of normalised sheet resistance as a function of resistor length for A3, B3 and C3 resistor inks are shown in Figs. 3±5, respectively. Various contact eects could be noted. Most desired behaviour, i.e. almost constant R independently on resistor length is visible for B-A3b, B-A3s, C-C3s or A-C3s structures. So-called normal contact eect, manifested itself as increase of sheet resistance with resistor length, is characteristic for A-A3s, C-A3b or B-B3s samples. Strong or even very strong anomalous contact eect, where sheet resistance is decreased with increase of resistor aspect ratio, is visible in C-B3b, B-C3s or A-C3b variants. The resistor/con-
Fig. 4. Normalised sheet resistance versus length of B3 resistors (values of resistance for l w (1 sq.) are given in Table 1).
Fig. 5. Normalised sheet resistance versus length of C3 resistors (values of resistance for l w (1 sq.) are given in Table 1).
ductor sets, proposed by ink producers, were used. Therefore the reported dimensional eects are associated
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A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
Fig. 6. Dependence of HTCR on resistor length for B3 test resistors.
Fig. 7. Normalised temperature dependence of resistance of B-B3s resistors with various length.
with interactions between tapes and resistive ®lms. It is worth to notice, that geometrical area of such interactions is increased approximately twofold for buried resistors. Dependence of HTCR, where HTCR R2
R1 =R1 T2
T1
1
and R2 ± resistance at T2 125°C, R1 ± resistance at T1 25°C, versus resistor length is correlated strongly with R f l=w characteristics (l, w ± resistor length and width, respectively). The 500 ppm/K changes of HTCR (between 150 and 350 ppm/K for A3, from 350 to 100 ppm/K for B3 and between 350 and 150 ppm/K for C3 ®lms) were found in dependence of applied tape material and resistor con®guration. Moreover, as is shown in Fig. 6, HTCR does not depend on resistor length when R is independent on aspect ratio. But it is increased slightly in the case of anomalous contact eect and is decreased when R is increased with resistor aspect ratio. 4. Temperature properties of LTCC resistors Resistance measurement of polymer, cermet and/or LTCC thick-®lm resistors and capacitors in a wide temperature range [9,10] is very important during analysis of conduction processes in such systems. The resistance versus temperature was measured automatically in the range from 180°C to 130°C. The Keithley 2000 multimeter interfaced to an IBM PC for data acquisition and presentation was used for this purpose. Majority of investigated resistors exhibits low or even very low temperature sensitivity of resistance within the whole operating temperature range. Typical temperature characteristics have a minimum (Fig. 7). This means that the dierential TCR of these compositions is increased, starting from the negative values goes to the positive ones
Fig. 8. Normalised temperature dependence of resistance of B-B3b resistors with various length.
through zero. The test samples with negligible dimensional eect have almost identical normalised R versus T curves independently on resistor geometry (Fig. 8). Different values of HTCR for samples with noticeable normal or abnormal dimensional eect are connected with signi®cant movement of characteristic minimum of resistance. In the case of anomalous dimensional eect this minimum shifts towards lower temperatures when aspect ratio is increased. And it shifts in opposite direction when increase of sheet resistance versus resistor length is noted (Figs. 4 and 8). 5. Long-term stability Long-term stability was analysed based on resistance drift during 500 h exposures at 150°C (almost standard long-term stability test conditions). Fractional resistance changes (DR=R0 ) were measured for all technological variants and aspect ratios.
A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
Fig. 9. Long-term stability of various B3 resistors with 1 sq. length.
About a half of combinations exhibited small resistance decrease (not more than 0:1%) after initial several ageing hours. Further ageing process at 150°C caused in principle gradual but very small resistance increase. The fractional changes in resistance could be roughly approximated as straight lines in DR=R0 log t coordinates (Fig. 9). Very small resistance changes were measured after 500 h ageing at 150°C. All of them were contained in the range from 0:1% to 0.5% (Table 2). Moreover, there was no distinct relationship between long-term stability
673
Fig. 10. In¯uence of resistor length on fractional changes in resistance of various A3 structures after 500 h ageing at 150°C.
and kind of tape or resistor ink as well as resistor placement. But stability was dependent on aspect ratio. The income of resistor/conductor interfacial resistance to the total one is dependent on resistor aspect ratio. Stability behaviour of contact area and resistor volume are somewhat dierent. Some examples are presented, e.g., in Ref. [11]. This is why the relative change in resistance of ``short'' components were somewhat dierent from ``long'' ones (Fig. 10).
6. Durability to high-voltage pulses Table 2 Comparison of long-term stability of ``short'' (0.5 sq.) and ``long'' (9 sq.) LTCC resistors (DR=R0 (%) after 500 h ageing at 150°C) Test structure
0.5 sq.
A-A3b A-A3s B-A3b B-A3s C-A3b C-A3s
0.08 0.09 0.03 0.06 0.09 0.14
9 sq. 0.01 0.04 0.01 0.02 0.03 0.06
A-B3b A-B3s B-B3b B-B3s C-B3b C-B3s
0.14 0.31 0.12 0.30 0.07 0.51
0.09 0.10 0.10 0.14 0.05 0.37
A-C3b A-C3s B-C3b B-C3s C-C3b C-C3s
0.20 0.17 0.15 ± 0.12 0.04
0.04 0.01 0.05
±
0.05 0.25
There is no generally accepted test method of electrostatic discharge investigation [12±14]. Therefore highvoltage susceptibility to two kinds of high-voltage pulses has been tested. The ®rst one consisted of series of 50 pulses with the waveform shown in Fig. 11a and 4000 V/mm electric ®eld at the peak. Fifty rectangle pulses with 10 ls duration, 400 ms pulse o-time and electrical ®eld from the range between 700 and 1000 V/mm have been applied in the second case (Fig. 11b). These investigations were performed on 0.5 sq. samples.
Fig. 11. Shape of (a) nanosecond and (b) microsecond highvoltage pulses.
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Table 3 Pulse durability of 0.5 sq. LTCC resistors (DR=R0 (%)) Test structure
Pulses from Fig. 11a
Pulses from Fig. 11b
A-A3b A-A3s B-A3b B-A3s C-A3b C-A3s
0.36 1.40 1.61 0.58 0.01 0.33
5000 180 1000 500 30:0 36:0
A-B3b A-B3s B-B3b B-B3s C-B3b C-B3s
0.41 0.65 0.41 0.99 3.04 1.84
0:31 0:32 0:03 116 1:02 3:23
28.0
1:26 2:12 1:52
A-C3b A-C3s B-C3b B-C3s C-C3b C-C3s
± ±
36.0 9.0
±
0:16 0:62
The opposite behaviour was noted for A3 and C3 resistors (Table 3). The 10 ls, 700 V/mm pulses create almost complete destruction of A3 ®lms independently on technological and constructional variants. But even 50% higher electric ®eld does not create catastrophic damages in B3 and C3 samples. On the other hand C3 resistors exhibit much larger changes than A3 ones under nanosecond pulses. Buried components behave a little better than surface resistors. The high-voltage susceptibility is dependent signi®cantly on resistor manufacturer. This is in agreement with Ref. [14] where it was also shown that such exposition to high-voltage pulses prior to long-term stability test allowed much more selective analysis of various resistorsÕ quality. 7. Ultrasonic microscope Most of physicochemical test techniques necessitate a long preparation of the samples and are almost always destructive. The SAM, using high-frequency ultrasonic waves as an investigation tool, is very attractive because of three particular features: · it is non-destructive technique, · it allows subsurface imaging, · it provides information about dierent sample properties (adhesion, density, topography, hardness, viscosity) under the surface [7]. Planar orientation of layers is predominant in LTCC structures. Therefore the ultrasonic microscopy gives
interesting information about quality of buried LTCC components [8,15]. Of course, knowledge about design and fabrication process of LTCC multilayer (e.g. kind, number and thickness of laminated tapes, geometrical layout of printed ®lms, number and position of tapes with screen-printed ®lms) makes easier successful interpretation of ultrasonic images. In order to satisfy the varied needs inherent in hybrid technology it is necessary to have high-frequency transducer (from 400 to 2000 MHz) for surface or near subsurface analysis and lower ultrasonic frequency transducer (from 30 to 100 MHz) for deeper subsurface analysis [7]. The SONOSCAN 6000D microscope with 100 MHz transducer was used for such subsurface analysis. Most of tested structures showed good lamination without any delamination or blistering traces. Such situation is shown in Fig. 12a (structure B-B3b). But local delamination areas were detected in certain mixed tape/resistor combinations, for example in B-A3b sample (Fig. 12b ± black area on the resistive ®lm near the common termination). Small number of test structures, e.g. C-C3b (Fig. 12c), exhibits blistering defects (white lines along the resistor tracks). Mentioned air bubbles and delamination in the region between buried resistors and LTCC tapes could be caused by improper burnout of the organic vehicles from green tapes and resistive ®lms. But parameters of lamination process could aect delamination, too. Small multilayer curvature in the area of resistor patterns (no larger than 50 lm) re¯ects dierences in LTCC tape and resistive ®lm shrinkage during typical co®ring process. The resistors subjected to high-voltage pulses were also investigated in ultrasonic microscope. But even for signi®cant resistance changes there was no dierences in ultrasonic images of as-®red and voltage stressed LTCC resistors. This means, that structure modi®cation affected by voltage pulses was less than horizontal resolution of applied transducer (according to Ref. [7] the planar resolution of 100 MHz transducer is equal to about 10 lm).
8. Conclusions One of the goals of this work was to check the compatibility of LTCC materials from various sources. Three commercial green tapes and three proper resistor/ conductor systems were examined. The sheet resistance, HTCR, temperature dependence of resistance in a wide temperature range, long-term stability and high-voltage pulse durability were analysed for various LTCC resistor/tape con®gurations, nominal R of resistive ®lms as well as resistor con®guration and topology. Nondestructive ultrasonic observations completed these investigations.
A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
6.
7. 8. 9.
675
of aspect ratio for combinations with abnormal dimensional eect and in opposite manner for those ones with normal dimensional eect. Practically all of as-®red tape/resistor combinations possess very good long-term stability DR=R0 6 0:2% but this test seems to be unselective for modern thick-®lm resistors. The dimensional eect, contrary to R and HTCR dependence on resistor aspect ratio, does not aect the long-term stability level. The high-voltage susceptibility is dependent signi®cantly on resistor ink producer. Most of test structures is well-laminated (without delamination or blistering traces) independently on small curvature of the outside LTCC surface observed sometimes in the area of paste pattern and arising from dierence in tape and resistive layer shrinkage.
References
Fig. 12. Ultrasonic microscope view of LTCC test structure without delamination (a ± B-B3b resistors), with delamination traces (b ± B-A3b resistors) and with blistering (c ± C-C3b structure).
During the experiment the following was found: 1. The properties of materials from the same source exceed mixed combinations. Of course particular LTCC systems could dier each other strongly. 2. Some mixed combinations guarantee very similar electrical properties as technological variants recommended by manufacturers. 3. Sometimes the signi®cant dierences between actual and nominal R could be noted. 4. Negligible, normal or abnormal termination eect, time to time enhanced by additional interactions with tapes, aects very strongly the dimensional behaviour of sheet resistance and HTCR. 5. Most of investigated resistors exhibits weak temperature dependence of resistance typical for such class of components. The characteristic resistance minimum is moved towards lower temperature with increase
[1] Kinzy Jones W, Liu Y, Larsen B, Weng P, Zampino M. Chemical, structural and mechanical properties of LTCC tapes. Proc. IMAPS-US, Boston, September 2000. p. 669± 74. [2] Dziedzic A, Golonka LJ, Kita J, Kozøowski JM. Macroand microstructure of LTCC tapes and components. Proc 24th IMAPS-Poland Conf, Rytro, Poland, September 2000. p. 163±8. [3] Joshi B, Pitt KEG, Winster AE. The properties of thick ®lm resistors on dielectrics. Microelectronics J 1982;13(5): 4±11. [4] Golonka LJ, Kozøowski JM. Properties of thick ®lm resistors inside multilayer structure. Proc Eighth European Hybrid Microelectronics Conf, Rotterdam, 1991. p. 377± 84. [5] Janoska I, Haskard MR. Thick ®lm standard paste temperature sensor applications. Sensors and Actuators 1985;8:3±9. [6] Sutterlin RG, Dayton GO, Biggers JV. Thick-®lm resistor/ dielectric interactions in a low temperature co-®red ceramic package. IEEE Trans CHMT ± Part B 1995;18:346±51. [7] Krause J, Ousten Y, Fremont H, Danto Y, Schwierzi B. Scanning acoustic microscope: an attractive tool for the thick ®lm technology characterisation. Proc Eighth Int Electronic Manuf Technol Symp, Stresa, Italy, 1990. p. 313±21. [8] Harsanyi G, Semmens JE, Martell SR. A new application of acoustic microimaging: screening MCM-C multilayer defects. Microelectron Reliab 2000;40:477±84. [9] Golonka LJ, Dziedzic A, Henke M. Temperature properties of thick-®lm resistors for LTCC applications. Proc 43rd Int Sci Coll, vol. 2. Ilmenau, Germany, 1998. p. 203± 7. [10] Dziedzic A, Golonka LJ, Mielcarek W. New con®guration of LTCC passive components. Proc 12th European Microelectronics Conf, Harrogate, UK, 1999. p. 3±9.
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[11] Dziedzic A, Golonka LJ, Licznerski BW, Hielscher G. Heaters for gas sensors from thick conductive or resistive ®lms. Sensors and Actuators B 1994;18±19:535±9. [12] Tobita T, Takasago H. New trimming technology for a thick ®lm resistor by the pulse voltage method. IEEE Trans CHMT 1991;CHMT-14:613±7. [13] Muller J, Thust H, Sjoling B, Turvey S. Trimming of buried resistors in LTCC-circuits. Proc 33rd ISHM-Nordic Conf., Helsingor, Denmark, 1996. p. 166±73.
_ [14] Kita J, Dziedzic A, Golonka LJ, Zuk G. Pulse durability of polymer, cermet and LTCC thick-®lm resistors. Proc 12th European Microelectronics Conf, Harrogate, 1999. p. 313±9. [15] Kita J, Wolter K-J, Bauer R, Luniak M, Teterycz H, Golonka LJ, Nitsch K. Investigations on a gas sensor in low temperature co®ring ceramics technology. Proc. 20th Int. Spring Seminar on Electronics Technology, Szklarska Poreba, Poland, 1997. p. 39±44.
www.elsevier.com/locate/microrel
Electrical and stability properties and ultrasonic microscope characterisation of low temperature co-®red ceramics resistors Andrzej Dziedzic a,*, Leszek J. Golonka a, Jaroslaw Kita a, Heiko Thust b, Karl-Heinz Drue b, Reinhard Bauer c, Lars Rebenklau c, Klaus-Jurgen Wolter c
a
Institute of Microsystem Technology, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, PL-50-370 Wroclaw, Poland b Ilmenau University of Technology, P.O. Box 100565, D-98684 Ilmenau, Germany c Dresden University of Technology, HelmholtzStrasse 18, D-01062 Dresden, Germany Received 16 October 2000; received in revised form 14 December 2000
Abstract This paper presents systematic investigations of electrical and stability properties of various low temperature co-®red ceramics (LTCC) resistors. One of the goals of this work was to check the compatibility of LTCC materials (tapes, resistive and conductive inks) from various manufacturers. Three commercially available green tapes and three LTCC resistor/conductor systems were examined. The resistive inks with 1 kX=sq. nominal sheet resistance were used. Buried (inside) and surface resistors were laminated and ®red according to the tape manufacturersÕ recommendations. The in¯uence of dimensional eect on sheet resistance and hot temperature coecient of resistance, the temperature dependence of resistance in a wide temperature range (from 180°C to 130°C), long-term stability of thermally aged as®red resistors (150°C, 500 h) and durability to high-voltage micro- or nanosecond pulses (50 ns pulses with 4000 V/mm maximum electric ®eld or 10 ls ones with 700±1000 V/mm electrical ®eld) were carried out for electrical and stability characterisation of LTCC resistors. Non-destructive scanning acoustic microscope diagnostics was applied for structure investigation and estimation of lamination and co®ring process quality of buried LTCC resistors. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Most of commercially available high-temperature thick-®lm inks is compatible with alumina (96% Al2 O3 ) substrate. However, composition of glass±ceramic tapes, which are a key component in the low temperature co®red ceramics (LTCC) materials system, diers considerably not only from alumina but also from each other. The fabrication process of LTCC components and circuits is somewhat dierent from standard cermet thick®lm technology, too. Moreover, each manufacturer of thick-®lm electronic materials oers its own LTCC materials, consisting of a tape based on a dierent devitrifying glass±ceramic system [1,2] and a set of resistor and conductor pastes highly recommended for this tape. *
Corresponding author. Tel./fax: +48-71-355-48-22. E-mail address: [email protected] (A. Dziedzic).
There is an interesting question from userÕs point of view if the LTCC materials, in particular tapes and resistor inks from dierent manufacturers, could be applied in one process, or dierences between various green tapes make such compatibility impossible. For example, with positive answer the user will not become fully subordinated to one producer of LTCC materials. The above problem is similar to behaviour of thick®lm resistors fabricated on dielectrics, intensively investigated dozen or so years ago [3,4]. These works shown, that the electrical properties were associated with interactions between substrates and resistor glassy phase constituents. But among others, stable and repeatable temperature sensor with good linearity was fabricated by selecting the correct resistor/dielectric paste combination and resistor aspect ratio [5]. This paper presents basic electrical and stability properties of buried and surface resistors made from three dierent resistor systems (from manufacturers A, B
0026-2714/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 0 0 4 - X
670
A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
and C) on/in three dierent LTCC tapes from the same manufacturers A, B and C. The sheet resistance, hot temperature coecient of resistance (HTCR), temperature dependence of resistance in a wide temperature range, long-term stability and high-voltage pulse durability were analysed and discussed for various resistor/ tape con®gurations, resistor placement as well as its aspect ratio. The classical physicochemical analysis of interactions that occur between LTCC components and tapes is presented very rarely [6] because of unknown initial composition of such complicated, non-equilibrium systems as commercially available thick-®lm materials. Moreover, most of such test techniques demand a long preparation of the samples and are almost always destructive. Therefore, non-destructive scanning acoustic microscope (SAM) [7,8] was applied in investigation of possible delamination and/or existence of the air bubbles inside the LTCC test structure.
in this paper could not be transferred in a simple way for other inks of tested resistor systems. The resistive ®lms and appropriate conductors were screen-printed through 325 mesh screens and then dried at 70°C for 10 min. The test pattern consisting of six resistors with 1 mm width and 0.5, 1, 3, 5, 7 and 9 sq. length was made on/in 15 15 mm2 4-layer LTCC structure (Fig. 1). The lamination process was carried out in an isostatic press at 210 bar for 10 min held at 70°C. Next, independently on the resistor position (buried or surface), the laminates were ®red under timetemperature pro®le recommended by adequate tape manufacturers (Fig. 2). In general, 18 technological combinations with different LTCC tapes, types of resistive inks as well as resistor positions were prepared and subjected to standard and non-standard electrical characterisation. Two-part marks were used for description of the above variants. For example, the C-B3s code means surface resistors made using 103 X/sq. resistor ink from manufacturer B on the LTCC tape supplied by manufacturer C.
2. Test sample fabrication The three various resistor systems, oered by manufactures A, B and C as well as the LTCC tapes from the same vendors were applied in our experiment. In order to limit the number of experimental variants only 1 kX/sq. inks were used in fabrication of buried (b) or surface (s) resistors. But the readers have to be conscious that names of commercially available resistor series are related in larger part to their electrical properties than to their constituents. Within the same resistor series its particular members (with various nominal sheet resistance) could have not only dierent conductive phase/ glass ratio but also glass content or kind and amount of modi®ers. Therefore the electrical properties presented
3. Dimensional eects Systematic investigations of electrical properties of thick-®lm resistors at dierent LTCC systems are presented in this paper. Analysis of dimensional eects is one of the most common electrical characterisation of such components. As one can see from Table 1 the 1000 X/sq. members of A, B and C series show very dierent behaviour. Only in a very few cases these are apparent as insigni®cant reduction in sheet resistance (maximum by about 40% for C-C3s and B-A3b resistors). The reverse eect, this is the increase of actual sheet resistance by factor from 5 to
Fig. 1. Test pattern geometry of LTCC resistors.
A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
Fig. 2. Firing pro®les of various LTCC systems.
671
Fig. 3. Normalised sheet resistance versus length of A3 resistors (values of resistance for l w (1 sq.) are given in Table 1).
Table 1 Sheet resistance of 1 sq. length resistors Technological variant
R (kX)
A-A3b A-A3s B-A3b B-A3s C-A3b C-A3s
1.38 0.66 0.57 0.78 10.6 6.70
A-B3b A-B3s B-B3b B-B3s C-B3b C-B3s
10.9 9.10 13.7 1.42 7.10 3.00
A-C3b A-C3s B-C3b B-C3s C-C3b C-C3s
4.20 1.11 5.30 2.53 1.67 0.57
14 in comparison with nominal one, occurs in much more number of cases (C-A3b, C-A3s, A-B3b, A-B3s, B-B3b, C-B3b, A-C3b and B-C3b). The plots of normalised sheet resistance as a function of resistor length for A3, B3 and C3 resistor inks are shown in Figs. 3±5, respectively. Various contact eects could be noted. Most desired behaviour, i.e. almost constant R independently on resistor length is visible for B-A3b, B-A3s, C-C3s or A-C3s structures. So-called normal contact eect, manifested itself as increase of sheet resistance with resistor length, is characteristic for A-A3s, C-A3b or B-B3s samples. Strong or even very strong anomalous contact eect, where sheet resistance is decreased with increase of resistor aspect ratio, is visible in C-B3b, B-C3s or A-C3b variants. The resistor/con-
Fig. 4. Normalised sheet resistance versus length of B3 resistors (values of resistance for l w (1 sq.) are given in Table 1).
Fig. 5. Normalised sheet resistance versus length of C3 resistors (values of resistance for l w (1 sq.) are given in Table 1).
ductor sets, proposed by ink producers, were used. Therefore the reported dimensional eects are associated
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Fig. 6. Dependence of HTCR on resistor length for B3 test resistors.
Fig. 7. Normalised temperature dependence of resistance of B-B3s resistors with various length.
with interactions between tapes and resistive ®lms. It is worth to notice, that geometrical area of such interactions is increased approximately twofold for buried resistors. Dependence of HTCR, where HTCR R2
R1 =R1 T2
T1
1
and R2 ± resistance at T2 125°C, R1 ± resistance at T1 25°C, versus resistor length is correlated strongly with R f l=w characteristics (l, w ± resistor length and width, respectively). The 500 ppm/K changes of HTCR (between 150 and 350 ppm/K for A3, from 350 to 100 ppm/K for B3 and between 350 and 150 ppm/K for C3 ®lms) were found in dependence of applied tape material and resistor con®guration. Moreover, as is shown in Fig. 6, HTCR does not depend on resistor length when R is independent on aspect ratio. But it is increased slightly in the case of anomalous contact eect and is decreased when R is increased with resistor aspect ratio. 4. Temperature properties of LTCC resistors Resistance measurement of polymer, cermet and/or LTCC thick-®lm resistors and capacitors in a wide temperature range [9,10] is very important during analysis of conduction processes in such systems. The resistance versus temperature was measured automatically in the range from 180°C to 130°C. The Keithley 2000 multimeter interfaced to an IBM PC for data acquisition and presentation was used for this purpose. Majority of investigated resistors exhibits low or even very low temperature sensitivity of resistance within the whole operating temperature range. Typical temperature characteristics have a minimum (Fig. 7). This means that the dierential TCR of these compositions is increased, starting from the negative values goes to the positive ones
Fig. 8. Normalised temperature dependence of resistance of B-B3b resistors with various length.
through zero. The test samples with negligible dimensional eect have almost identical normalised R versus T curves independently on resistor geometry (Fig. 8). Different values of HTCR for samples with noticeable normal or abnormal dimensional eect are connected with signi®cant movement of characteristic minimum of resistance. In the case of anomalous dimensional eect this minimum shifts towards lower temperatures when aspect ratio is increased. And it shifts in opposite direction when increase of sheet resistance versus resistor length is noted (Figs. 4 and 8). 5. Long-term stability Long-term stability was analysed based on resistance drift during 500 h exposures at 150°C (almost standard long-term stability test conditions). Fractional resistance changes (DR=R0 ) were measured for all technological variants and aspect ratios.
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Fig. 9. Long-term stability of various B3 resistors with 1 sq. length.
About a half of combinations exhibited small resistance decrease (not more than 0:1%) after initial several ageing hours. Further ageing process at 150°C caused in principle gradual but very small resistance increase. The fractional changes in resistance could be roughly approximated as straight lines in DR=R0 log t coordinates (Fig. 9). Very small resistance changes were measured after 500 h ageing at 150°C. All of them were contained in the range from 0:1% to 0.5% (Table 2). Moreover, there was no distinct relationship between long-term stability
673
Fig. 10. In¯uence of resistor length on fractional changes in resistance of various A3 structures after 500 h ageing at 150°C.
and kind of tape or resistor ink as well as resistor placement. But stability was dependent on aspect ratio. The income of resistor/conductor interfacial resistance to the total one is dependent on resistor aspect ratio. Stability behaviour of contact area and resistor volume are somewhat dierent. Some examples are presented, e.g., in Ref. [11]. This is why the relative change in resistance of ``short'' components were somewhat dierent from ``long'' ones (Fig. 10).
6. Durability to high-voltage pulses Table 2 Comparison of long-term stability of ``short'' (0.5 sq.) and ``long'' (9 sq.) LTCC resistors (DR=R0 (%) after 500 h ageing at 150°C) Test structure
0.5 sq.
A-A3b A-A3s B-A3b B-A3s C-A3b C-A3s
0.08 0.09 0.03 0.06 0.09 0.14
9 sq. 0.01 0.04 0.01 0.02 0.03 0.06
A-B3b A-B3s B-B3b B-B3s C-B3b C-B3s
0.14 0.31 0.12 0.30 0.07 0.51
0.09 0.10 0.10 0.14 0.05 0.37
A-C3b A-C3s B-C3b B-C3s C-C3b C-C3s
0.20 0.17 0.15 ± 0.12 0.04
0.04 0.01 0.05
±
0.05 0.25
There is no generally accepted test method of electrostatic discharge investigation [12±14]. Therefore highvoltage susceptibility to two kinds of high-voltage pulses has been tested. The ®rst one consisted of series of 50 pulses with the waveform shown in Fig. 11a and 4000 V/mm electric ®eld at the peak. Fifty rectangle pulses with 10 ls duration, 400 ms pulse o-time and electrical ®eld from the range between 700 and 1000 V/mm have been applied in the second case (Fig. 11b). These investigations were performed on 0.5 sq. samples.
Fig. 11. Shape of (a) nanosecond and (b) microsecond highvoltage pulses.
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Table 3 Pulse durability of 0.5 sq. LTCC resistors (DR=R0 (%)) Test structure
Pulses from Fig. 11a
Pulses from Fig. 11b
A-A3b A-A3s B-A3b B-A3s C-A3b C-A3s
0.36 1.40 1.61 0.58 0.01 0.33
5000 180 1000 500 30:0 36:0
A-B3b A-B3s B-B3b B-B3s C-B3b C-B3s
0.41 0.65 0.41 0.99 3.04 1.84
0:31 0:32 0:03 116 1:02 3:23
28.0
1:26 2:12 1:52
A-C3b A-C3s B-C3b B-C3s C-C3b C-C3s
± ±
36.0 9.0
±
0:16 0:62
The opposite behaviour was noted for A3 and C3 resistors (Table 3). The 10 ls, 700 V/mm pulses create almost complete destruction of A3 ®lms independently on technological and constructional variants. But even 50% higher electric ®eld does not create catastrophic damages in B3 and C3 samples. On the other hand C3 resistors exhibit much larger changes than A3 ones under nanosecond pulses. Buried components behave a little better than surface resistors. The high-voltage susceptibility is dependent signi®cantly on resistor manufacturer. This is in agreement with Ref. [14] where it was also shown that such exposition to high-voltage pulses prior to long-term stability test allowed much more selective analysis of various resistorsÕ quality. 7. Ultrasonic microscope Most of physicochemical test techniques necessitate a long preparation of the samples and are almost always destructive. The SAM, using high-frequency ultrasonic waves as an investigation tool, is very attractive because of three particular features: · it is non-destructive technique, · it allows subsurface imaging, · it provides information about dierent sample properties (adhesion, density, topography, hardness, viscosity) under the surface [7]. Planar orientation of layers is predominant in LTCC structures. Therefore the ultrasonic microscopy gives
interesting information about quality of buried LTCC components [8,15]. Of course, knowledge about design and fabrication process of LTCC multilayer (e.g. kind, number and thickness of laminated tapes, geometrical layout of printed ®lms, number and position of tapes with screen-printed ®lms) makes easier successful interpretation of ultrasonic images. In order to satisfy the varied needs inherent in hybrid technology it is necessary to have high-frequency transducer (from 400 to 2000 MHz) for surface or near subsurface analysis and lower ultrasonic frequency transducer (from 30 to 100 MHz) for deeper subsurface analysis [7]. The SONOSCAN 6000D microscope with 100 MHz transducer was used for such subsurface analysis. Most of tested structures showed good lamination without any delamination or blistering traces. Such situation is shown in Fig. 12a (structure B-B3b). But local delamination areas were detected in certain mixed tape/resistor combinations, for example in B-A3b sample (Fig. 12b ± black area on the resistive ®lm near the common termination). Small number of test structures, e.g. C-C3b (Fig. 12c), exhibits blistering defects (white lines along the resistor tracks). Mentioned air bubbles and delamination in the region between buried resistors and LTCC tapes could be caused by improper burnout of the organic vehicles from green tapes and resistive ®lms. But parameters of lamination process could aect delamination, too. Small multilayer curvature in the area of resistor patterns (no larger than 50 lm) re¯ects dierences in LTCC tape and resistive ®lm shrinkage during typical co®ring process. The resistors subjected to high-voltage pulses were also investigated in ultrasonic microscope. But even for signi®cant resistance changes there was no dierences in ultrasonic images of as-®red and voltage stressed LTCC resistors. This means, that structure modi®cation affected by voltage pulses was less than horizontal resolution of applied transducer (according to Ref. [7] the planar resolution of 100 MHz transducer is equal to about 10 lm).
8. Conclusions One of the goals of this work was to check the compatibility of LTCC materials from various sources. Three commercial green tapes and three proper resistor/ conductor systems were examined. The sheet resistance, HTCR, temperature dependence of resistance in a wide temperature range, long-term stability and high-voltage pulse durability were analysed for various LTCC resistor/tape con®gurations, nominal R of resistive ®lms as well as resistor con®guration and topology. Nondestructive ultrasonic observations completed these investigations.
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6.
7. 8. 9.
675
of aspect ratio for combinations with abnormal dimensional eect and in opposite manner for those ones with normal dimensional eect. Practically all of as-®red tape/resistor combinations possess very good long-term stability DR=R0 6 0:2% but this test seems to be unselective for modern thick-®lm resistors. The dimensional eect, contrary to R and HTCR dependence on resistor aspect ratio, does not aect the long-term stability level. The high-voltage susceptibility is dependent signi®cantly on resistor ink producer. Most of test structures is well-laminated (without delamination or blistering traces) independently on small curvature of the outside LTCC surface observed sometimes in the area of paste pattern and arising from dierence in tape and resistive layer shrinkage.
References
Fig. 12. Ultrasonic microscope view of LTCC test structure without delamination (a ± B-B3b resistors), with delamination traces (b ± B-A3b resistors) and with blistering (c ± C-C3b structure).
During the experiment the following was found: 1. The properties of materials from the same source exceed mixed combinations. Of course particular LTCC systems could dier each other strongly. 2. Some mixed combinations guarantee very similar electrical properties as technological variants recommended by manufacturers. 3. Sometimes the signi®cant dierences between actual and nominal R could be noted. 4. Negligible, normal or abnormal termination eect, time to time enhanced by additional interactions with tapes, aects very strongly the dimensional behaviour of sheet resistance and HTCR. 5. Most of investigated resistors exhibits weak temperature dependence of resistance typical for such class of components. The characteristic resistance minimum is moved towards lower temperature with increase
[1] Kinzy Jones W, Liu Y, Larsen B, Weng P, Zampino M. Chemical, structural and mechanical properties of LTCC tapes. Proc. IMAPS-US, Boston, September 2000. p. 669± 74. [2] Dziedzic A, Golonka LJ, Kita J, Kozøowski JM. Macroand microstructure of LTCC tapes and components. Proc 24th IMAPS-Poland Conf, Rytro, Poland, September 2000. p. 163±8. [3] Joshi B, Pitt KEG, Winster AE. The properties of thick ®lm resistors on dielectrics. Microelectronics J 1982;13(5): 4±11. [4] Golonka LJ, Kozøowski JM. Properties of thick ®lm resistors inside multilayer structure. Proc Eighth European Hybrid Microelectronics Conf, Rotterdam, 1991. p. 377± 84. [5] Janoska I, Haskard MR. Thick ®lm standard paste temperature sensor applications. Sensors and Actuators 1985;8:3±9. [6] Sutterlin RG, Dayton GO, Biggers JV. Thick-®lm resistor/ dielectric interactions in a low temperature co-®red ceramic package. IEEE Trans CHMT ± Part B 1995;18:346±51. [7] Krause J, Ousten Y, Fremont H, Danto Y, Schwierzi B. Scanning acoustic microscope: an attractive tool for the thick ®lm technology characterisation. Proc Eighth Int Electronic Manuf Technol Symp, Stresa, Italy, 1990. p. 313±21. [8] Harsanyi G, Semmens JE, Martell SR. A new application of acoustic microimaging: screening MCM-C multilayer defects. Microelectron Reliab 2000;40:477±84. [9] Golonka LJ, Dziedzic A, Henke M. Temperature properties of thick-®lm resistors for LTCC applications. Proc 43rd Int Sci Coll, vol. 2. Ilmenau, Germany, 1998. p. 203± 7. [10] Dziedzic A, Golonka LJ, Mielcarek W. New con®guration of LTCC passive components. Proc 12th European Microelectronics Conf, Harrogate, UK, 1999. p. 3±9.
676
A. Dziedzic et al. / Microelectronics Reliability 41 (2001) 669±676
[11] Dziedzic A, Golonka LJ, Licznerski BW, Hielscher G. Heaters for gas sensors from thick conductive or resistive ®lms. Sensors and Actuators B 1994;18±19:535±9. [12] Tobita T, Takasago H. New trimming technology for a thick ®lm resistor by the pulse voltage method. IEEE Trans CHMT 1991;CHMT-14:613±7. [13] Muller J, Thust H, Sjoling B, Turvey S. Trimming of buried resistors in LTCC-circuits. Proc 33rd ISHM-Nordic Conf., Helsingor, Denmark, 1996. p. 166±73.
_ [14] Kita J, Dziedzic A, Golonka LJ, Zuk G. Pulse durability of polymer, cermet and LTCC thick-®lm resistors. Proc 12th European Microelectronics Conf, Harrogate, 1999. p. 313±9. [15] Kita J, Wolter K-J, Bauer R, Luniak M, Teterycz H, Golonka LJ, Nitsch K. Investigations on a gas sensor in low temperature co®ring ceramics technology. Proc. 20th Int. Spring Seminar on Electronics Technology, Szklarska Poreba, Poland, 1997. p. 39±44.