Effect of conduction particle size on LaB6 thick film resistor

Thin Solid Films, 198 (1991) 17-27 ELECTRONICS AND OPTICS

17

E F F E C T O F C O N D U C T I O N P A R T I C L E SIZE ON LaB 6 T H I C K FILM RESISTOR OSAMU ITO, TADAMICHI ASAI, TOSHIO OGAWA, MITSURU HASEGAWA AND AKIRA IKEGAMI

Hitachi Research Laboratory, Hitachi Ltd., 1-1, Saiwaicho 3-chome, Hitachi-shi, lbarak i-ken 317 (Japan) YOSHISHIGE ENDOH AND TAKESHI ARAYA Mechanical Engineering Research Laboratory, Hitachi Ltd., 502, Kandatsu-cho, Tsuchiura-shi, lbarakiken 300 (Japan) KAZUHIRO ATOH AND TAKAO KOBAYASHI Tokai Works, Hitachi Ltd.. 1410 lnada, Katsuta-shi. Ibaraki-ken 312 (Japan) (Received April 24, 1990; accepted August 7, 1990)

In order to obtain guidelines to develop a thick film resistor (TFR) fireable in an N2 atmosphere and covering a wide practical range of sheet resistivities, i.e. 10-10 6 ~/I-q, the microstructures of TFRs were observed, the sintering behaviour of the conduction particles and glass was analysed and the particle size effect on the formation of the conduction path was simulated. Through these investigations, conductive material properties which influence the characteristics of the T F R have been found. (1) As conduction particles fireable in N 2, LaB 6 particles have suitable properties such as poor sinterability and good wettability with glass. (2) Particle size also has a large effect on the characteristics of the TFR, and it is necessary to use LaB 6 ultrafine particles (UFPs) as conduction particles to cover a wide range of sheet resistivities. Using UFPs with specific surface areas of 8.9 and 83.0 m z g - ~, we found the possibility of developing TFRs fireable in N2 with resistivities of 10-106 ~/I---I in the LaB6-glass system.

1. INTRODUCTION

The copper hybrid integrated circuit (IC) system has recently been substituted for the A g - P d system especially in high frequency circuits, although the Ag-Pd conductor and RuO2-glass system have been commonly used in thick film technology. The reasons are the better performance, excellent reliability and lower impedance of the copper conductor. Since non-noble metals such as copper are easily oxidized when fired in air, firing in N2 is necessary to avoid oxidation. However, there is no thick film resistor (TFR) for the copper hybrid IC system which meets the usual requirements such as stability in the firing atmosphere (N2) and a wide range of resistivities to fabricate thick film circuits like the RuO2-glass system. In this study, TFRs composed of different conduction particles, i.e. RuO 2, platinum, LaB 6 and TiN, were examined. Characterization of the TFRs and computer simulation of the particle size effect on the formation of the conduction path in the TFRs were carried out in order to develop new TFRs fireable in N2. 0040-6090/91/$3.50

© ElsevierSequoia/Printedin The Netherlands

o. IT() et al.

18 2. EXPERIMENTAL METHOD

Table I lists the physical properties and particle sizes of the conductive materials used in this study. All of them have almost the same order of resistivity and a metal-like, positive temperature coefficient of resistivity (TCR). TABLE 1 PHYSICAL PROPERTIES AND SIZ['S OI- ( ' O N D t J ( ' T t ( ) N PARTICLES

Particles

R(laf~cm~

T ( p p m C t)

pitaml

RuO2 Pt LaB¢, TiN

40 11 15 25

4950 3900 2680 2480

0. I 0.1 5 6 5 6

R, resistivity. T, TCR. P, particle size.

The resistor pastes are composed of conduction particles (RuO2, platinum, LaB6 and TIN), glass particles and organic vehicles. We used lead borosilicate glasses for air firing and borosilicate for N 2 firing. These were produced from oxide powders. They were mixed and heated at 1500 ~C for 1 h and cooled rapidly to room temperature. Then they were mechanically crushed. The average particle size of glass after crushing was about 5 ~tm. The resistor pastes were screen printed on prefired terminations made of copper which were formed on 96~, AI20 3 substrate. Then the LaB6-glass and TiN-glass systems were fired in N 2 by conveyor furnace at 900 °C for 5 rain, while the RuOz-glass and platinum-glass systems were fired in air at 750 ~C for 10 rain. 3.

RESULTS AND DISCUSSION

3.1. Effect of volume fraction of glass on TFR properties Figures 1 and 2 show the effect of the volume fraction of glass on the sheet resistivity and TCR in each TFR. Brief descriptions are given below of the electrical properties of each TFR when the volume fraction of glass varies. (1) In the RuO2-glass system the sheet resistivity increases gradually with the volume fraction of glass and the system covers a wide range of sheet resistivities. The TCR shows a favourable value (within ± 500 ppm) for practical use. (2) In the platinum-glass system the change in sheet resistivity is small (1-10 f~/[S]). When the volume fraction of glass increases to more than 95 vol.'~o, a transition of the conductive property is observed: the T F R suddenly becomes insulating. (3) In the LaB6-glass system the sheet resistivity ranges from 10 to 100 f~/[]. However, the electrical properties (sheet resistivity and TCR) vary drastically at around a glass composition of 70 vol.'Uo, which means that the resistivity is not controllable from the practical viewpoint.

La&

1

18

THICK FILM RESISTOR

”

:RuCI,-Glass -Glass B:La&-Glass A:TiN -G.lass 8

.‘R

-

Fig. 1. Relationship between sheet resistivity arid votume fk%kx~ of g&s.

Vofuee Fraction

of Glasdvolif

Fig. 2. Relationship between TCR and volume fraction of glass.

20

o. fro et al.

(4) In the TiN-glass system the sheet resistivity is in the high range. Moreover, the TCR shows a large negative value even when the TFR includes a high content of TiN conduction particles. 3.2. Effect Of characteristics q[conductit, e materials on TFR properties 3.2.1. Contact state qf conduetion particles in TFR As mentioned previously, the oxide and non-oxide conductive materials used in this study have metallic properties and a large positive TCR. According to X-ray analysis of these TFRs, no crystallographic change was observed before and after firing. These results and the T C R property in Fig. 2 suggest that the sheet resistivities in these systems are determined by the dispersion state of the conduction particles in the glass matrix. Since an electric current flows through the chain of conduction particles, it is considered that the resistivity depends mainly on the contact state between the particles 1. In Fig. 3 the microstructure of the platinum glass system after etching out the glass is shown. Necking is observed between platinum particles. This fact shows that platinum particles are sintered together easily during firing.

Fig. 3. SEM imageof the Pt-glass system(afteretching of the glass). To investigate the contact state of the other conduction particles of R u O 2 , LaB 6 and TiN during firing, these powders were pressed and fired in the same process as that of TFR firing. Figure 4 shows the microstructures of these compacted bodies. There is no neck formation between neighbouring particles. Even for these powders pressed more densely than those printed o n A 1 2 0 3 substrate, sintering does not occur in any of these cases. Therefore it is concluded that no neck is formed in printed powders with vehicles in the R u O 2 , L a B 6 - and TiN-glass systems. Following this result, a current is expected to flow through the thin glass layers which exist between the conduction particles in these systems. Therefore it may be concluded that the glass between the particles plays an important role in connecting conduction particles with one another 2. 3.2.2. Wettability between conduction particles and glass To investigate the role of the glass in the formation of the conduction path, we carried out a special experiment. Compacted bodies of conduction particle and glass

LaB 6

THICK

21

FILM RESISTOR

Fig. 4. SEM images of RuO2,

LaB 6 and TiN compact bodiesafter firing.

powders were made separately. They were put together and fired in the same firing process as for the TFR fabrication. Then line analyses of the main elements of the glass perpendicular to the boundary were carried out to examine the degree of glass permeation into the compacted body of the conduction particles. The results are shown in Fig. 5. The glass permeates into the compacted bodies of RuO2 and LaB 6 but not into the TiN compact. When the glass flows into the compacted body of conduction particles by softening, the degree of permeation is considered to correspond to the wettability between conduction particles and glass. Therefore the wettability with glass is good for RuO z and LaB6 but not for TiN. Interface betueen Glass and Coapsct Body of Conduction Particle before P i r i n l

=;~ RuO,IRuO,

=4~.

~ Glass

_1_

_J_

L,B.iLaB.' /

', Glass

',

!Glass I

-,,-

GIsss

ZiN

i I

Glass,

,

5 0 # m

Fig. 5. Lineanalysesof the compact bodiesafterfiring,showingthe degreeof glass permeation.

3.2.3. Effect of properties of conductive materials on TFR properties The conductive properties of the conduction particles do not always show a direct relation with those of the TFR. In this study some properties of the conductive materials, i.e. wettability and sinterability, have been investigated to determine how these properties influence the conduction mechanism. These results are briefly summarized in Table II taking the conduction mechanism into account. In the platinum-glass system, since platinum powder sinters easily, a neck forms the conduction path by platinum. In this case the resistance between particles

22

o. ITO et al.

TABLE II EFFECTOF PROPERTIESOF CONDUCTIONMATERIALSON ELECTRICALPROPERTIESOF TFRs Property

Pt-glass

RuOe~lass

LaBo glass

TiN glass

Wettabilitywith glass Self-sinterability Particle size (lam) TCR of TFR Possible conduction mechanism Schematic i~loUS:arca:i~nt°f

Poor Good 0.1 Positive Metallic

Good Poor 0.1 Almost zero Tunnelling

Good Poor 5 6 Zero or negative Tunnelling or hopping

Poor Poor 5 6 Negative Hopping

~

~

5

a ~

C, conduction particle. G, glass.

is negligible. The electrical properties of the conduction path are mainly determined by the properties of platinum metal itself. Therefore this system shows metallic properties and has a positive TCR. The RuO2-glass system has been investigated from various aspects by many authors, and several conduction mechanisms have been proposed in this system. However, the main mechanism is believed to be tunnelling conduction, which does not depend on the temperature very much 3. Tunnelling conduction occurs when the insulating layer between metallic materials is thin 4'5. Owing to their good wettability with glass, the conduction particles attract each other like a liquid phase sintering process. This behaviour during firing might provide a suitable structure for tunnelling conduction (a thin glass layer between conduction particles). Therefore both good wettability with glass and unsinterability are indispensable to realize tunnelling conduction in the TFR. Like RuO2, LaB6 shows both good wettability with glass and unsinterability. Thus it may be expected that tunnelling conduction can occur in the LaB6-glass system as well. Actually, in the low range of sheet resistivities the TCR is small as in the RuOz-glass system. However, in the high range of sheet resistivities the LaB 6glass system shows a large negative TCR. Hopping conduction is possible in this range. These results suggested comparing other differences between RuO2 and LaB 6. Further discussion will be given in the next subsection. As mentioned above, the condition of particle contact depends mainly on the wettability with glass. In the TiN-glass system the TiN particles do not contact each other well during the firing process owing to their poor wettability with glass. Therefore the glass layer between particles becomes thick. In this case, when the glass layer increases its thickness, a possible conduction mechanism is thermally assisted conduction or hopping conduction, which means a large negative TCR.

LaB 6 THICK FILM RESISTOR

23

3.3. Effect of particle size on TFR properties 3.3.1. Particle size effect on conduction path in computer simulations Figure 6 shows micrographs of the RuO2-glass and LaB6-glass systems. The dispersion state of the conduction particles is different, although the number of conduction particles is the same. According to Tables I and II, one difference between RuO 2 and LaB 6 is their particle sizes. Therefore the effect of particle size on the TFR properties was focused on next.

JO~

L'~ I~[~?~3

I~'~ i

I

50pm

(a) RuO2 (10vo1%) (106•/C])

(b) LaB0 (10vo1%) (>1014Q/I-1)

Fig. 6. Micrographs using transmission light (dark region: conductive material; bright region: glass).

A lot of research to explain the conduction mechanism of TFRs by mathematical models has been carried out, especially on the percolative behaviour 6-9. However, no previous study has directly elucidated the particle size effect on the conduction path in TFRs by computer simulation. In order to confirm the effect of particle size on the formation of the conduction path, computer simulation based on the Monte Carlo method was employed in this study. Simulation was carried out on the two-dimensional unit square shown in Fig. 7. First, insulating circles representing the glass phase and conducting circles were generated randomly in the unit square; then conducting circles representing conduction particles were generated until there was no room to generate new circles. We assumed that the overlapping distance between these circles was to be less than 0.01 of the unit length. If this distance was more than 0.01, the circle was omitted and we continued to generate another circle. Through this procedure we obtained a pseudostructure of the TFR. Secondly, all the conducting circles linked to the left electrode were investigated to estimate the length of the conduction path. Linking means that a circle has some overlapping part with neighbouring circles. The purpose here is to find the rightmost conducting circle linked to the left electrode. We assumed the x coordinate of the rightmost conducting circle to be the maximum length of the conduction path, which is Xm,x in Fig. 7. [fit linked to the right electrode, the conduction path in this unit square was regarded as completed. Xmax (the maximum length of the conduction path) in this case is 1.0.

24

o.

In~ulatlng Circle

Conducting Circle

--Insulating Circles and Conducting Circles Dispersed Randomly

V0=0.54

Xmax

-

-

Conducting Circles Linked to the Left Electrode

d~:0.10

d c:0.04

Vc:Ratio of Conducting Phase d, :Diameter of Insulating Circle

Xma

x=0.64

Xmax : X Coordinate of the Cemter of the Rightmost conducting Circle Linked to the Left Electrode

d c : o i a m e t e r of Conducting Circle

Fig. 7.

IT() et al.

Example of the c o n d u c t i o n path by computer simulation.

In this way the particle size effect on the formation of the conduction path was investigated by varying the diameter of the conducting circle while the diameter of the insulating circle was fixed. It should be noted that the diameters of the conducting and insulating circles are fixed relative to one another, and making the conducting circles small corresponds to making the insulating circles large. Therefore we can consider the effects of both diameters in this calculation ~°. Figure 8 shows the effect of the diameter of the circles on the formation of the conduction path on varying the diameter of the conduction particles. In this figure Vc is defined as Vc

- -

Yale2 ]Edc2 + Y,di 2

*'°ki J I i ~ I i I P~ -

LC~

0

3

--

0.5

d o =0.06 d ~ =0.i0

k 1.0

0

*-o~q-i~[

-i

~ o

oo

Vo

Fig. 8.

o

°°i

o 0

~-E--i@~!~

,r-

~1

~

*'°~-I l i -L

2

'--

o

o

o°°° 0.5

Vc d o =0.04 d ~ --0.I0

~_

' 1.0

o

' 0.5

Vo d c =0.02 d ~ =0.i0

Influence of diameter of the conducting circle on the formation of the conduction path.

1 .0

LaB 6 THICK FILM'RESISTOR

25

where V~is the fraction of the conducting phase, d c is the diameter of a conducting circle and d i is the diameter of an insulating circle. In other words, V~ roughly corresponds to the volume fraction of conduction particles. Obviously, it is possible to vary V~ by changing the number of conducting circles and insulating circles. In Fig. 8 the left plot shows that the conduction path is not completed until V~reaches more than 0.9, while the right plot shows that the conduction path is completed at Vc = 0.45. It is clear that the relative difference of diameter causes a remarkable change in the formation of the conduction path. The important result obtained here is that the conduction path is completed at a smaller volume of conduction particles when the conduction particle size becomes smaller than that of the insulating particles, while it is not possible to form the conduction path in the case of larger conduction particles. In addition, we can expect a higher sheet resistivity at this volume because the conduction path has to become narrow in order to form a path by a small number of conduction particles. 3.3.2. Effect of particle size of LaB 6 on LaB6-glass system According to these results, if the particle size of LaB6 is small enough, it is possible to form the conduction path even when the amount of LaB 6 is low. Moreover, the sheet resistivity is expected to be relatively high in this case. To confirm the particle size effect on the TFR properties, LaB 6 powders with four different particle sizes were prepared. Figure 9 shows scanning electron microscope images of the LaB6 particles used here. The LaB6 of Fig. 9(a) was used in the LaB6-glass system in Fig. 1. The particles of Figs. 9(a) and 9(b) were made by a mechanical crushing process. The other LaB6 ultrafine particles (UFPs) were made by a process at a temperature higher than the melting point of LaB 6. Using the two mechanically crushed powders and a combination of LaB 6 UFPs, three kinds of LaB6-glass system were made. The electrical properties of these TFRs are shown in Figs. 10 and 11. The following is a brief description of the electrical properties of these TFRs. (1) In the system with 5-6 lam LaB 6 particles the properties of the TFR change drastically at a volume fraction of glass of about 70~. (2) In the system with 1 ~tm LaB 6 particles the TFR does not show the insulating property in the area of glass composition 70-90 vol.Yo, while the TFR with 5-6 ~tm LaB 6 shows the insulating property with the same composition. This is

(a) 5 - 6 ~ m

(b) 1 ~tm

Fig. 9. SEM images of LaB 6 particles.

(c) UFP1 8.9 m2/g

(d) UFP2 83.0 m2/g

I

! 5 Mm

o. ITO et al.

26

,o,

,0,

lOS

j

10'

10'

.

.

.

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8

~ 10'

° '°'10~

n- 10~

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10:

~/~

lo: o o

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.

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.

10L

to

OO

100

Volume Fraction of Glass(

80

70

00

%)

10~0

80

70

80

90

Volnme Fraction of Glass(

'°~o

loo

7o

oo

(C) LaBs Ultra

( b ) L a B 8 (1pro)

(a) LaBs (5-6vm)

eo

~oo

oo

Volume Fraction of Glass(

%)

Fine

%)

Particle

Fig. 10. Sheet resistivity in the LaB 6 glass system.

4OOOF

4oool

4000

~oo0[

2OO[

UFP

1 i-

~o~

E

o

o

~_ o.o,"

%

'\o

O

1-

-~oooF

I - -20(

-4OOO . . . . . . . .

-4000]-

+4OOO- -

i° -ooooso

0o

70

oo

oo

Volume Fraction of Glass(

loo

-oo°~5o ' so ' 7'0 ' e'o ' ~

%)

(a) LaB~ (5-6prn)

Volume Fraction of Glass( ( b ) L a B 0 (1pro)

' too %)

000050

60

70

80

90

100

Volume Fraction of Glass(%) (c) LaBe Ultra

Fine

Particle

Fig. l l. T C R in the LaB6 glass system.

in agreement with the computer simulation result. Although a wide range of sheet resistivities is covered and the TCR is small in the 1 jam LaB 6 system, the variation of sheet resistivities is large when the volume fraction of glass is larger than 85°~,. (3) In the systems with U F P I and UFP2 LaB 6 particles, different sizes of UFPs were used as shown in Figs. 9(c) and 9(d). The TFR with the larger UFPs (UFPI) has a lower range of sheet resistivities (10-104 Q/[]) while the TFR with the smaller U F P s (UFP2) has a higher range of sheet resistivities (104-107 I~/D), thus covering the practical range completely. The variation of sheet resistivity is considerably smaller than with the mechanically crushed particles and this system has a rather small TCR, which indicates its possible application in copper hybrid IC systems. 4. CONCLUSIONS

In order to develop a thick film resistor fireable in N 2 which covers a wide range of sheet resistivities, the effects of the characteristics of the conductive materials on

LaB 6 THICK FILM RESISTOR

27

the properties of the TFR were studied. We found a number of important properties of the conduction particles for the fabrication of TFRs. (1) The conduction particles should not be self-sintered in the firing process. (2) Wettability is necessary between the conduction particles and glass to get good contact and allow tunnelling conduction between particles. (3) The size of the conduction particles should be considerably smaller than that of the glass particles for forming the conduction path in the glass-rich TFR, which realizes high sheet resistivity stably. Following these guidelines, the LaB6-glass system with different particle sizes was investigated. It was found that the TFR with a combination of LaB 6 UFPs covers a wide range of sheet resistivities ( 1 0 - 1 0 6 ~/[-']), which is necessary for thick film circuits. ACKNOWLEDGMENTS

The authors would like to thank Dr. Tadahiko Miyoshi and Mr. Satoru Ogihara of Hitachi Ltd. and Dr. Susumu Hioki, Mr. Tateo Tamamura, Mr. Katsuo Ebisawa, Mr. Sadao Deyama and Mr. Michio Ootani of Hitachi Powdered Metals Co. Ltd. for their encouragement and helpful discussions in this study. REFERENCES 1 2 3 4 5 6 7 8 9 10

A. Kusy, Thin Solid Films, 43 (1977) 243. P. Palanisamy et al., J. Am. Ceram. Soc., 68 (1985) c-215. G.E. Pike and C. H. Seager, J. Appl. Phys., 48 (1977) 5152. J.C. Fisher and 1. Giaever, J. Appl. Phys., 32 (1961 ) 172. R. Stratton, J. Phys. Chem. Solids, 23 (I 962) 1177. M. Prudenziati, Electrocomp. Sci. Technol., 10 (1983) 285. P.F. Carcia et al., J. Appl. Phys., 53 (1982) 5258. G.E. Pike and C. H. Seager, Phys. Rev. B, 10 (1974) 1421. L. Orger et al., Powder TechnoL, 46 (1986) 133. T. lnokuma et al., IEEE Trans., CHMT-7 (1984) 166.