THE INTERACIONS OF CONDUCTIVE AND GLASS PHASE IN THICK-FILM RESISTORS DURING FIRING
"'Marko Hrovat, ^Darko Belavič, ''Janez Hole, ^Janez Bernard, "'Andreja Benčan,
^Jena Cilenšek
"'Jožef Stefan Institute, Ljubljana, Slovenia ^HIPOT-R&D d.o.o., Šentjernej, Slovenia
Key words: thick-film resistors, characterisation, ruthenium oxide, ruthenates, phase equilibria
Abstract: Some thick-film resistors based on RuO?, ruthenates or a mixture of RUO2 and ruthenates, were evaluated. The resistors were fired at different temperatures to determine the influence of firing temperature on the electrical and microstructural characteristics. The microstructures of the thick-film resistors were analysed with scanning electron microscopy and energy-dispersive X-ray analysis. The temperature coefficients of resistivity, noise indices and gauge factors were measured as a function of firing temperature. After a long term high temperature firing ruthenate based conductive phase transform into RUO2 coinciding with a significant increase of the temperature coefficients of resistivity and decrease of the resistance. Glass phase in thick-film resistors was analysed by EDS. All glass compositions are rich in Si02 with the molar ratio Si02 / PbO between 2 and 2.5. Subsolidus equilibria in the RUO2 - PbO - Si02 diagram were determined with the aim to verify the interactions between conductive phase (either ruthenium oxide or ruthenate) and silica-rich glasses. The tie line between RUO2 and PbSiOa indicates that the lead ruthenats are not stable in the presence of the silica-rich glass phase.
Interakcije med prevodno in stekleno fazo v debeloplastnih
uporih med procesom žganja
Kjučne besede: debeloplastni upori, karakterizacija, rutenijev oksid, rutenati, fazni diagrami
Izvleček: Karakterizirali smo nekatere debeloplastne uporovne materiale na osnovi RUO2, rutenatov ali zmesi RUO2 in rutenatov. Uporte smo žgali pri različnih temperaturah, da bi ugotovili vpliv temperature žganja na električne in mikrostrukturne karakteristike. Mikrostrukture debeloplastnih uporov so bile preiskane z elektronskim vrstičnim mikroskopom in rentgensko analizo. Izmerili smo temperaturne koeficiente upornosti, indekse tokovnega šuma in faktorje gauge v odvisnosti od temperature žganja. Ugotovili smo, da v debeloplastnih uporih, žganih dolgo časa pri povišanih temperaturah, rutenat preide v rutenijev oksid. Pri tem se zelo zniža plastna upornost in poviša temperaturni koeficient upornosti. Stekleno fazo v debeloplastnih uporih smo analizirali z EDS (Energy Disspersive X-ray Analysis). Ugotovili smo, da so vsa stekla bogata na Si02 z razmerjem Si02 / PbO med 2 in 2,5. Preiskali smo fazna ravnotežja v sistemu RUO2 - Si02 - PbO, Rezultati so potrdili, da rutenat ni stabilen v prisotnosti stekel bogatih na 8102.
Introduction
Thick-film resistors consist basically of a conducting phase, a lead-borosiiicate-based glass phase and an organic vehicle. The organic material is burned out during the high-temperature processing. The ratio between the conductive and the glass phases roughly determines the specific resistivity of the resistor. In most modern resistor compositions the conductive phase is either RUO2 or ruthenates; mainly, as reported in the literature, lead or bismuth ruthenates. The main change during firing is the transition from a mixture of glass grains and, usually, much finer grains of the conductive phase in a thick-film paste, into conductive chains through the sintered glass in the fired resistor. During the firing cycle all the constituents of the resistor paste react with each other and the melted glass also interacts with the substrate The resistors are only a relatively short time (typically 10 min) at the highest temperature (typically 850°C). Because of this the reactions between the constituents of the resistor material do not reach equilibrium so that the required characteristics of fired materials (e.g long-term stability, low noise indices and a low tempera-
ture coefficient of resistivity) are, in a way, a compromise as a consequence of this frozen non-equilibrium /1-5/. The aim of this paper is to present the results on some thick film resistor material, fired either at the required 850°G for 10 min or at higher firing temperatures for significantly longer times. The aim was to gain some insight into the changes in the electrical and microstructural characteristics, and gauge factors if the resistors are fired long enough at the high temperature to allow the reactions within the resistorto reach the equilibrium. Thick-film resistors with a nominal resistivity of 10 kohm/sq. (Du Pont 8039 and 2041, and Heraeus 8241) were evaluated. The conductive phase in 8039, 2041 and 8241 resistors is based on (Bi2-xPbx)Ru207-x/4, a mixture of RUO2 and Pb2Ru206.5, and RUO2, respectively /6,7/. Data on the conductive phase and the qualitative results of an energy-dispersive X-ray analysis (EDS) of the glass composition of the thick-film resistors are summarized in Table 1. All glasses contain, as main elements, lead, silicon and aluminum oxides. Boron oxide, which is also present in the glass phase, cannot be detected in the EDS spectra because of the low relative boron weight fraction in the glass and the strong
absorption of the boron Ka line during EDS analysis in the glass matrix.
Table 1. Conductive phase and qualitative results of EDS microanalysis of elements detected in glass phase of thick-film resistors /17/.
Resistor	Conductive phase	Ma in elements	Other elements detected
8039	ruthenate	Si, Pb, Al	Zr
2041	RuO 2+ ruthenate	Si, Pb, Al	Mg, Zn, Ca, Ba
8241	RU0 2	Si, Pb,Al	Zn, Cu
The X-ray analysis of conductive phase in investigated thick film resistors will be given. The change of conductive phase (from ruthenate to the ruthenium oxide) at high firing temperatures, depending on the composition of glass phase will be discussed.
Experimental
Thick-film resistors with dimensions 1.6x1.6 mm^ were printed on 96% alumina substrates and fired forlO min at 850°C and for 6 hours at 950°C. The resistors were terminated with a Pd/Ag conductor that was prefired at 850°C. ColdTCRs (from -25°C to 25°C) and hotTCRs (from 25°C to 125°C) were calculated from resistivity measurements at - 25°C, 25°C, and 125°C. Current noise was measured in dB on 100 mW loaded resistors by the Quan Tech method (Quan Tech Model 315-C). Gauge factors (GFs) were measured. The resistors were examined by X-ray powder-diffraction (XRD)analysisAJEOLJSM 5800scanning electron microscope (SEM) equipped with an energy-dispersive X-ray analyser (EDS) was used for the micro-structural analysis.
Results and discussion
Sheet resistivities, cold (-25°C to 25°C) and hot (25°C to 125°C) TCRs, noise indices and gauge factors of the in-
vestigated thick-film resistors that were 10 min at 850°C and 6 hours at 950°C are shown in Table 2.
After firing at 950°C for 6 hours, the resistivities of all the resistors significantly decreased to around 5% of the resistivities after firing at 850°C for the 2041 resistors, and to 1 % or less for the 8039 and 8241 resistors. The GFs of all the resistors, as well as the sheet resistivities, decreased with increasing firing temperature. The TCR values of the resistors after firing at the "normal" temperature of 850°C are below 100x10'®/K. After firing for 6 hours at 950°C the absolute values of the TCRs of the 8039 and 8241 resistors increased significantly. The noise indices decrease with increased firing temperature. The 2041 resistor material has the lowest noise, around or under -20 dB, regardless of the firing temperature.
X-ray diffraction (XRD) spectra of ruthenate-based "equilibrated" resistors showed that at higher firing temperatures the ruthenate decomposes forming Ru02, while the conductive phase in Ru02-based resistors stays unchanged. This is shown in Figs. I.a, l.b and l.c for 10 kohm/sq. Du Pont 8039 and 2041 thick film resistors, and Heraeus 8241 thick-film resistors, respectively/6/. As mentioned before, the 8241 resistor is based on Ru02 and the 2041 material is based on a mixture of (mainly) ruthenate and RUO2. The resistors were fired for 10 min at 850°C and for 6 hours at 950°C. After 6 hours of firing at 950°C the ruthenate peaks of the 8039 resistors disappearwhile the spectrum of RUO2 based 8241 resistors remains unchanged. Presumably because of the interaction with the molten glass the ruthenate decomposes.
The decomposition of the ruthenate phase in the ruthen-ate-based 8039 resistor after high-temperature firing and the formation of RUO2 was confirmed with SEM. Micro-structures of the 8039 resistors that were fired for 10 min at 850°C and for 6 hours at 950°C are as an example in Figs. 2.a and 2.b. The microstructure of the 8039 resistor, fired at 850°C (Fig. 3.a) consists of light sub micrometer-sized particles of a conductive phase in a grey glass matrix. The dark particles are SiZr04. After 6 hours firing
Table 1: Sheet resistivities, cold and hotTCRs, noise indices and gauge factors of the thick-film resistors, fired 10 min at 850°C and 6 hours at 950°C
Resistor	T firing	Resistivity	Cold TCR	Hot TCR	Noise	GF
	(°C)	(ohm/sq.)	(10-6/K)	(10-6/K)	(dB)	
8039	850	7,3 k	50	90	-14.3	11.0
	950, 6 h	37	1845	1810	-29.9	1.5
2041	850	6.6 k	-35	20	-23.3	11.0
	950, 6 h	280	-90	-85	-32.0	7.0
8241	850	5.4 k	20	60	-4.5	15.5
	950, 6 h	36	1950	1990	-25.5	2.0
400 350 300 250 200 150 100 50
8039
950, 6h

Ru02 R"02 / /

40 45 50 2theta (deg.)
Fig. 1a: XRD spectra of 2039 thick-film resistor, fired for 10 min at 850°C and for 6 hours at 950°C. Spectra of ruthenate (RU) and of RuOg (Ru02) are also Included.
2041

40 45 50 2 theta (deg.)
Fig. lb: XRD spectra of 2041 thick-film resistor, fired for 10 min at 850°C and for 6 hours at 950°C. Spectra of ruthenate (RU) and of Ru02 (RUO2) are also Included.
300
250
100
Ru02			8241 A i	A ^ A A A
950, 6 h	, 1			......iv A ft . A
850			i A. ....... ............ii.. . A ^.....A A A	
RU ^ j		1 . ., i i .		
20 25 30 35 40 45 50 55 60 65 70
2 theta (deg.)
Fig. 1c: XRD spectra of 8541 thick-film resistor, fired for 10 min at 850°C and for 6 hours at 950°C. Spectra of ruthenate (RU) and of RUO2 (Ru02) are also included.
at 950°C the ruthenate particles in the 8039 resistor have nearly all disappeared.
Adachi and Kuno /8,9/ studied high-temperature interactions between Pb0-B203-Si02 glasses and Pb2Ru206.5
Fig. 2a: MIcrostructure of a ^cross-section of the thick-film resistor 8039, fired for 10 m In at 850°'^. Alumina substrate is on the right. Light particles are conductive phase - (Bl2-xPbx)Ru207-x/4.
5 |.im
Fig. 2b: MIcrostructure of a cross-section of the thick-film resistor 8039, fired for 6 hours at 950°C. Alumina substrate is on the right. After firing at 950°C the ruthenate particles in the 8039 resistor have nearly ail disappeared.
or RUO2. They showed that in glasses poor in PbO the Pb2Ru206.5 disappears and the RUO2 is formed while for PbO-rich glasses the RUO2 reacts with the PbO from the glass and forms Pb2Ru206.5. Their results are summarised in Fig. 3. Three regions are marked in the Pb0-B203-Si02 phase diagram. In the first region in the silica rich part of diagram ruthenates decomposes into RUO2. In third region (PbO rich) ruthenates are stable while RUO2 reacts with glass forming Pb2Ru206.5. In glasses with roughly 1/1 Si02 / PbO ratio (second region) the RUO2 and the ruthenate coexist.
To confirm these findings, the subsolidus ternary phase diagram of the RUO2 - PbO - Si02.system was investigated. The glass phase in different commercial thick-film resistors was analysed by SEM and the Pb0/Si02 ratio was
determined. All analysed glass compositions are rich in SiOa with the molar ratio Si02 / PbO between 2 and 2.5. The molar ratio SiOa / PbO in glass phases of thick-film resistors is also graphically shown as a short bold bar near Si02 in the PbO-poor part of the Ru02 - PbO - Si02 system in Fig. 4. The PbO-rich part of phase diagram, which was not investigated, is shown with dotted lines. No ternary compound was found in the system. There is no binary compound between Ru02 and Si02. The tie lines are between Pb2Ru206.5 and PbSiOs, and between Ru02 and PbSiOs. The results therefore indicate that the lead-ruth-enate-based conductive phase in thick-film resistors is indeed unstable when in contact with the silica-rich glass phase, as shown by dashed lines in Fig. 4.
1 f 8 fe (5J3.!i
SiOv


^ i V ' i—. i—^ \ ! f
ir.'V'Wv-H.-x
/■ ____-^fM^X
V	N/ ^ .

I ,U )
SO! ?š!C£-in
Fig. 3: The Pb0-B203-Si02 system (after Adachi and Kuno/8/). Leadruthenate is stabie in tine region HI and unstable in the region I.
Acknowledgement
The financial support of the Ministry of Education, Science and Sport of the Republic of Slovenia is gratefully acknowledged.
References
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PbO
Pb^uQ
PbßiO , •. PbßiO , PbgiO, PbSIOj
k Resistor glass
Marko Hrovat, Janez Hole, Janez Bernard, Andreja Benčan, Jena Cilenšek Jožef Stefan Institute, Ljubljana, Slovenia
Darko Belavič HIPOT-R&D d.o.o., Šentjernej, Slovenia
RuO,
SiO,
Fig. 4: The proposed subsolidus ternary phase
diagram of the PbO-poor part of the RuOz - PbO - S/O2. The molar ratio SiOg / PbO in glass phases of some thick-film resistors is shown as a short bold bar near S/O2 in the Pb0-Si02 system.