Development of Ultra-Low Fire COG and X7R Dielectric ... · The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number 1, First Quarter 1999 (ISSN 1063-1674)

The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number 1, First Quarter 1999 (ISSN 1063-1674)

8 International Microelectronics And Packaging Society

Development of Ultra-Low Fire COG and X7R Dielectric Compositions for Multilayer Ceramic Chip Capacitorand Integrated Passive Component Applications

Development of Ultra-Low Fire COG and X7RDielectric Compositions for Multilayer CeramicChip Capacitor and Integrated PassiveComponent Applications

Brian C. Foster and Walter J. SymesFerro CorporationTranselco Division1789 Transelco DrivePenn Yan, New York 14527Phone: 315-536-3357Fax: 315-536-0376e-mail: [email protected]

Abstract

A series of COG and X7R dielectric compositions have been developed which densify at temperatures below 1000 oC. These compositionshave been demonstrated to be compatible with 90:10 and 95:5 Ag:Pd internal electrode metal systems in multilayer ceramic capacitordevices. In addition, these devices have been subjected to mechanical, electrical, and environmental reliability testing and have exhibitedfailure rates that meet accepted industry standard pass/fail criteria. A feasibility study in which multiple dielectric constant materials werecofired in a monolithic structure indicates that these materials are good candidates for integrated passive component applications. A multi-layer ceramic capacitor cost model shows that when these materials are used with 95:5 Ag:Pd, the total material cost independent ofprocessing cost is lower than the material cost for Ni electrode material systems.

Key words:

Ultra-Low Fire, Dielectric, Capacitor, Multilayer, Integration, andElectronic Packaging.

1. Introduction

Continuing price erosion in the surface mount multilayer ceramicchip capacitor market coupled with a significant increase in the marketprice of palladium has led to the development of dielectric composi-tions compatible with nickel and copper internal electrode materi-als. This technology has become dominant in Japan and is in thedevelopment or introductory phase in the U.S. and Europe. Barriersto the implementation of this technology outside of Japan have beenthe lack of merchant raw materials and/or formulated dielectric com-

positions that are reduction resistant and the lack of required pro-cess technology. In addition, processing costs, including the initialcapital investment in equipment designed for sintering in a low oxy-gen partial pressure atmosphere, are substantially higher than noblemetal processing costs.

An alternative approach to the base metal system is the develop-ment of dielectric compositions that sinter at temperatures < 1000oC. As sintering temperature is reduced 90% or greater, silver palla-dium internal electrode compositions can be employed. A furtherreduction in sintering temperature to < 900 oC allows the fabricationof multi-element integrated passive component structures with 100%silver metallization.

Liquid phase sintering of perovskite type dielectric compositionsto promote densification below 1100 oC has been an area of interestinvestigated by numerous authors in the past1-13. Although the mecha-nisms of flux assisted sintering through the formation of low melt-ing glasses or eutectic liquids are well understood, commercializa-tion of dielectric compositions with densification temperatures be-low 1100 oC utilizing this approach have not been widely devel-oped. Difficulties in controlling microstructure evolution during sin-

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Intl. Journal of Microcircuits and Electronic Packaging

tering and adverse electrode-ceramic interactions in cofired struc-tures have been the primary limiting factors. In this paper, a series ofCOG and X7R dielectric compositions which exhibit fully densemicrostructures at sintering temperatures from 850-960 oC will bepresented. In addition, cofired multilayer capacitor configurationsusing 90:10 and 95:5 Ag:Pd internal electrode metal systems willbe characterized for both physical and electrical property perfor-mance. Reliability data including HALT, standard life and 85:85temperature and humidity testing with and without bias voltage willbe also detailed. A multilayer ceramic capacitor cost model compar-ing Ag:Pd and Ni internal electrode material construction costs in-dependent of processing costs will be presented in the analysis pro-cedure.

2. Results and Discussion

2.1. COG Dielectric Development

Two temperature-compensating dielectric compositions that meetEIA Class I requirements and exhibit dielectric constant values of32 and 95, respectively, have been developed which densify below1000 oC5. The first composition is a substituted BaTi

4O

9 modified

with ZrO2 and SiO

2. A lead borosilicate glass frit is added as a sin-

tering aid. The second composition is a BaO-TiO2-Nd

2O

3 material

with rare earth oxide modifiers and a high bismuth glass frit addedas a sintering aid. Table 1 shows the physical characteristics for eachof these compositions.

Table 1. Physical properties data for COG dielectriccompositions.

pDielectric constant

Average particle

size

Surface Area

Specific Gravity

Tap Density

pH @ 10%

solids

microns m2/g g/cm3 g/cm3

32 1.05 3.60 4.60 1.04 8.7

95 0.80 7.80 5.70 1.04 9.6

Figure 1 shows the sintering behavior of the two compositionsusing Thermomechanical Analysis (TMA). The time/temperatureprofile used was a linear ramp of 3 oC/min to a peak temperature of1100 oC. Densification begins to occur at approximately 500 oC forthe K32 material coincident with liquid phase formation. The changein shrinkage rate at approximately 570 oC indicates the formation ofa second liquid phase. The K95 composition has a continuous slopeshrinkage behavior characteristic of a single step sintering mecha-nism. The onset of densification for the K95 material occurs at ap-proximately 660 oC. Figure 2 shows a thermally etched cross sec-tion of the K95 composition.

Figure 1. TMA curves for the K32 and K95 compositions.

Figure 2. Thermal etch of the K95 composition.

2.2. X7R Dielectric Development

A series of EIA Class II compositions exhibiting X7R charac-teristic behavior have been developed using the same liquid phasesintering approach proven successful in the COG dielectric devel-opment initiative. These compositions densify at temperatures be-tween 850 and 950 oC. The chemistry of these compositions is amodified BaTiO

3 where Nb and Nd are used to promote a core/shell

structure. A complex glass frit is added as a sintering aid to promotevery low temperature densification. Table 2 shows the typical physi-cal characteristics for the X7R compositions.

Table 2. Physical properties data for X7R dielectriccompositions.

Average particle

size

Surface Area

Specific Gravity

Tap Density

pH @ 10%

solids

microns mg/cm2 g/cm3 g/cm3

0.75 3.10 6.10 1.42 9.2

14

m2/g g/cm3 g/cm3

m2/g g/cm3 g/cm3




Figure 3 shows the sintering behavior of a K2400 X7R materialusing an overlay of Differential Thermal Analysis (DTA) and TMAplots. The endothermic peaks in the DTA curve represent liquid phaseformation. These peaks correspond to the onset of densification atapproximately 660 oC in the TMA curve with slope changes at 700oC and 800 oC where the most rapid densification occurs. Completedensity is achieved at 940 oC which is considerably lower in tem-perature than either of the COG dielectric compositions.

DTA

TMA

K2400 X7R

Figure 3. DTA and TMA plots for the K2400 X7Rcomposition.

Figure 4 shows the thermally etched microstructure of the sameK2400 X7R material. The average fired grain size is approximately0.7 micron.

Figure 4. Thermal etch of the K2400 X7R microstructure.

2.3. Cofired Multilayer Structures

Cofired multilayer ceramic capacitors were fabricated from allthree dielectric compositions using 90:10 and 95:5 Ag:Pd internalelectrodes. A tape transfer prototype manufacturing process was used

to produce the test components. Dielectric tape was cast on a PETcarrier film using a floating doctor blade and internal electrodes weredeposited using a vision aligned screen printer. This process wassufficient to produce 5 micron dielectric thickness and 1 micron elec-trode thickness cofired composites.

Tables 3 and 4 show a comparison of electrical data at three fir-ing temperatures for each of the COG compositions. The internalelectrode metal composition was 90:10 Ag:Pd. A soak time of 1.5hours was used for each peak firing temperature. The test chip con-figuration for the K32 composition was an EIA type 0805, 100 pFdesign. The test chip configuration for the K95 composition was anEIA type 1206, 400 pF design.

Table 3. Electrical data for the K32 composition.

Firing temp

MHz % df

Diel. Thk. µm K

TCC -55°C

TCC 125°C

RC 25°C

RC 125°C

960 0.001 20.61 34 N024 N009 >2391 113

980 0.002 21.04 32 N021 N004 >2156 86

1000 0.021 20.37 26 N014 P002 >1840 49

1020 0.081 20.80 24 N003 N001 >1736 48

Table 4. Electrical data for the K95 composition.

Firing temp

KHz % df

MHz % df

Diel. Thk. µm K

TCC -55°C

TCC 125°C

RxC 25°C

RxC 125°C

980 0.029 0.042 17.7 95 P023 N003 >7828 992

1000 0.025 0.040 17.6 95 P020 N005 >7732 891

1020 0.030 0.050 17.5 92 P023 N004 >7800 1427

A review of the electrical data for the K32 composition shows areduction in the dielectric constant, an increase in the dissipationfactor, and a reduction in the insulation resistance as firing tempera-ture exceeds 980 oC. This effect is a function of increasing porosityin the fired microstructure. This increase in porosity is associatedwith grain growth and flux component volatility.

The K95 composition is very stable with respect to peak firingtemperature. The slight decrease in the dielectric constant and theincrease in the dissipation factor at 1020 oC is the result of a reduc-tion in internal electrode continuity at that firing temperature for the90:10 Ag:Pd metal system.

Multilayer ceramic capacitors were made using two different X7Rcompositions with dielectric constants of 2400 and 3000, respec-tively. A range of fired dielectric thickness from 5 to 14 micronswas evaluated. Both 90:10 and 95:5 Ag:Pd metal systems were usedin the construction of the prototype test devices. Sintering tempera-tures from 860-950 oC were employed with soak times of 1.5 to 3hours. No significant difference in electrical performance was ob-served between the two internal electrode systems.

Figure 5 shows the temperature coefficient of capacitance (TCC)behavior for each of the X7R compositions. Both values are wellwithin the +/- 15% EIA specification limit. Figure 6 plots the TCCas a function of dielectric thickness for the K3000 material. The

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reduction in dielectric thickness from 13.5 microns to 5.4 micronsrotates the TCC curve clockwise slightly but stays within the X7Rspecification.

-10

-8

-6

-4

-2

0

2

4

-55 -45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155

Te m pe ra ture °C

% C

apac

itanc

e Ch

ange

K2400

K3000

Figure 5. TCC behavior for the K2400 and K3000 X7Rcompositions.

U ltra-low Fire X7R TC C Data K 3000

-10

-8

-6

-4

-2

0

2

4

-55 -45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155

Te m pe ra ture °C

% C

apac

itanc

e Ch

ange

5.4 m ic ron

13.5 m ic ro n

Figure 6. TCC vs. dielectric thickness for the K3000 X7Rcomposition.

Figure 7 shows how the dissipation factor changes as a functionof the dielectric thickness for each of the X7R compositions. TheK3000 material has a dissipation factor that is approximately 0.5%higher than the K2400 composition at an equivalent dielectric thick-ness.

Figure 8 is a thermally etched cross section of a 1206 chip ca-pacitor made using the K2400 X7R dielectric with a fired dielectricthickness of 5.7 microns. The peak firing temperature was 900 oCand the internal electrode was 95:5 Ag:Pd.

The ultra-low firing temperature of the COG and X7R dielectriccompositions described in the preceding pages combined with theircompatibility with low resistivity, high Ag content metal systemsmake them very attractive candidates for integration in integratedcircuit packaging or discrete, multifunction passive component ap-plications. The ability to incorporate multiple dielectric constant

materials within the same functional block allows for increased min-iaturization, lower assembly cost, reduced parasitic coupling, im-proved impedance matching, and reduced losses at higher operatingfrequencies. Previous authors have described various dielectric ma-terial systems designed for these applications14-23. In general, chemi-cal and physical property incompatibility problems have preventedthe successful incorporation of multiple dielectric constant materi-als within the same cofired structure. The ultra-low firing composi-tions presented in this work are not ceramic filled glass or recrystal-lizing glass materials but rather polycrystalline ceramics with < 15wt% glass forming constituents to promote densification by liquidphase sintering.

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

0 5 10 15 20

Die le ctric th ickne ss in m icrons

Dis

sipa

tion

fact

or in

per

cent

K3000

K2400

Figure 7. Dissipation factor vs. dielectric thickness for theK2400 and K3000 X7R compositions.

Figure 8. Thermally etched cross section of a 12-6 chipcapacitor made with the K2400 X7R dielectric.

To evaluate the feasibility of using the ultra-low firing COG andX7R dielectric materials in a cofired integrated passive componentapplication, a series of multilayer composites were produced. Struc-tures using dielectric constants of 10, 32, 95 and 2400 were fabri-

16




cated in two and three component composite variations with a 90:10Ag:Pd internal electrode. The tape/transfer process was used in theconstruction of these prototype devices. With the exception of theK10, K32 composite, there was acceptable microstructure develop-ment with no adverse interaction between the different dielectricmaterials. Figures 9-11 show several cross sections of these con-figurations.

Figure 9a. Composite of K10 and K2400 materials.

Figure 9b. Composite of K10 and K2400 materials.

Figure 10. Composite of K32 and K2400 materials.

Figure 11. Composite of K10 and K32 materials.

2.4. Reliability

A full battery of reliability tests have been performed on the ul-tra-low fire COG and X7R dielectric compositions in a multilayerceramic capacitor format with both 70:30 and 90:10 Ag:Pd elec-trodes. Temperature/humidity with and without bias, Life, and HALTtesting, along with mechanical tests for bend strength, thermal shock,and thermal cycle were also performed. Failure rates have all metcustomer and industry standard pass/fail criteria. In addition, an EDSX-ray elemental mapping evaluation was also performed on themultilayer capacitor devices to discern Ag mobility between dielec-tric layers. No Ag presence between dielectric layers could be de-tected.

3. Cost modeling

A multilayer ceramic capacitor material cost model was devel-oped to quantify the impact of high silver internal electrodes on con-struction cost. The costs of dielectric and internal electrode materi-als were compared for Ni and AgPd systems. This model does notconsider differences in processing costs, such as the cost of sinter-ing in a controlled PO

2 atmosphere for the base metal system. In

addition, termination material cost is excluded from the model. Alist of the key assumptions used to develop the model are providedin this section.

3.1. Cost Model Assumptions:

· Capacitor design is an EIA type 1206 (3.2mm x 1.6mm), 1microfarad value.

· Internal active layer dielectric thickness is 6 microns.· Dielectric constant for Ni system is 2300.· Dielectric constant for AgPd system is 3000.· Nickel metal cost is $4.62/TO.· Weight deposit of Ni electrode paste is 12 mg/in2.· Effective fired Ni electrode thickness is 2 microns.

17




· Ag metal cost is $5.50/TO.· Weight deposit of Ag-Pd electrode paste is 7 mg/in2.· Effective fired Ag-Pd electrode thickness is 0.9 micron.· Ni system and Ag-Pd system dielectric material costs are equiva-

lent at $25/lb.Figure 12 shows the system material cost comparison as a func-

tion of four different Pd costs. The Ni system baseline cost is 0.386.As Pd cost increases, the cross over point where AgPd systems be-come equivalent to Ni systems moves toward the 100% Ag end ofthe scale. At a market price of $300/TO for Pd, the 95:5 Ag:Pd sys-tem is approximately 8% lower in material cost than the Ni system.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 5 10 15 20 25 30

Percent Palladium in Ag/Pd Electrode System

Mat

eria

ls C

ost p

er M

LCC

(ce

nts

Pd @ $400/TO

Nickel

Pd @ $100/TO

Pd @ $200/TO

Pd @ $300/TO

Figure 12. Multilayer ceramic capacitor material costmodel.

4. Conclusions

A series of COG and X7R ultra-low fire dielectric compositionshave been developed for multilayer ceramic capacitor applications.These materials have been demonstrated to be compatible with 90:10and 95:5 Ag:Pd internal electrode metal systems. The optimum sin-tering temperatures for these materials vary from 860-960 oC. Chipcapacitors fabricated using these compositions and 90:10 Ag:Pd elec-trodes have passed a full screen of reliability testing including THB,Life, and HALT measurements.

Cost modeling of multilayer ceramic chip capacitors fabricatedwith the ultra-low firing dielectrics using 95:5 Ag:Pd internal elec-trodes shows that material costs independent of processing costs arelower than those for Ni internal electrode systems.

A feasibility study has shown that multiple dielectric con-stant ultra-low fire compositions can be integrated into a singlemonolithic composite structure. This study indicates that integra-tion of complex multifunction passive elements may be possiblewith this material technology.

Acknowledgments

The authors would like to acknowledge Julie Burton, SkipQuackenbush, Everette Davis, Tim Jaenecke, and the entire TranselcoECM R&D team for their contributions to this paper and the devel-opment of the ultra-low fire dielectric technology.

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Documents

Development of Ultra-Low Fire COG and X7R Dielectric ... · The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number 1, First Quarter 1999 (ISSN 1063-1674)