tantalum thin film applications anew approachfor

Electrocomponent Science and Technology1977, Vol. 4, pp. 69-73

(C)Gordon and Breach Science Publishers Ltd., 1977Printed in Great Britain

TANTALUM THIN FILM APPLICATIONS A NEWAPPROACH FOR CAPACITORS

A. PERINATI

Telettra S.p.A. 20059 Vimercate, Milano, ltaly

(Received May 1, 1977)

Tantalum-based materials and process developments have increased the use of tantalum films, combined with siliconintegrated circuits, to form hybrid integrated circuits which can meet the demands of telecommunication systems.This paper, after a historical review, describes the status of tantalum thin film technology used at Telettra in theimplementation of circuits for telecommunication equipment. In particular, this paper focusses on the effortsdevoted to the improvement of the -Ta capacitor process in terms of yield and reliability for RC active filterrealization. The results of a new Ta O -# Ta deposition process, where a TaO layer is first formed by d.c.reactive sputtering in oxygen in the same vacuum batch where the -Ta film is sequentially formed, are given.

1. INTRODUCTION

Twenty years after the first investigations of thin filmmaterials used in microelectronics, thin film technol-ogy has reached a remarkable stage of developmentand application especially in telecommunication.

The basic reason is to be found in the comple-mentary characteristic of thin film technology withrespect to semiconductor integrated circuits. Thinfilm technology has limited applications in the fieldof active components, typical of semiconductortechnology, but it is suitable for implementingresistors and capacitors with peculiar characteristic,range of values, temperature coefficients, stabilityetc., which would not be easy to achieve with semi-conductor integrated circuit technology. The first tenyears of thin film technology have been characterizedby the study of film properties and by the implemen-tation of discrete components; in the following tenyear period the development of applications has takenover the outcome of which being circuits havingseveral R and C components on one substrate and,in general, hybrid circuits of remarkable complexity.

In the last few years efforts have been orientatedtowards the improvement of industrial processes aswell as towards cost reduction as a consequence ofthe trend in international economy; for example, thedegree of miniaturization in hybrid integratedcircuits has increased and the use of highly pricedmetals has been limited.

If the NiCr and Ta technologies are considered,and thee represent the trend followed by the

majority of manufacturers in the field of thin filmhybrids, the following can be observed.

From early studies on Cr, as thin film materialfor resistors because of its high vapour tension andresistivity, and on NiCr because of the property ofresistivity and temperature coefficient of such thinfilm alloy,2 ,3 new deposition techniques havedeveloped together with improved processes whichhave led to new NiCr thin film resistors withrepeatable electrical characteristics and low tempera-ture coefficient as required in professional circuits.Ta technology has developed essentially from the"Bell Labs" where Basseches4 obtained in 1957 thefirst resistive film of Ta by means of cathode sput-tering. In the same year Berry ,6 began research onthe technology which eventually led to the inventionof Ta thin film capacitor. From then on a vastresearch and development program was carried onin the Bell Labs and in other research institutions inthe USA, Japan, Europe, etc.

As a result of studies on the effect of suchelements as N, C, O on the structure and electricalproperties of films as resistors, the development ofTantalum Nitride,7 as the most suitable film forresistors, took place due to its characteristics ofresistivity, temperature coefficient, stability etc.A deeper knowledge of sputtering techniques led tothe development of "Low Density Ta" for highresistivity films.8 Films deposited with N2 and 02reactive sputtering led to simultaneous implementa-tion of RC groups temperature compensated byvirtue of variation of the film TCR in such a way as

69

70 A. PERINATI

to balance out the capacitor coefficient (TCC).9

The research on RC integrated networks led to thedevelopment of Ta-A1 alloy films which provides highvalue resistors with high stability and low TCR. o ,11

As for Ta thin film capacitors their TM structure isat the basis of a high degree of accuracy, stability,of repetitive TCC and dissipation factor which makesthem suitable for active RC hybrid integrated circuits.Moreover, the need of miniaturizing, hence ofincreasing the sheet capacitance, has led to the develop-ment of the TMM z structure where, thanks to self-healing properties of materials such as manganesedioxide, and lead dioxide, the influence of the draw-backs of tantalum pentoxide have been balanced out,especially for small dielectric thickness.

The study on the N, C, O effect on Ta, with respectto stability has brought to capacitors high quality interms of leakage current, yield, breakdown voltage,realized with/3-Ta N2 doped films, 3 Also, thebehaviour at high temperature of capacitor TCC,dissipation factor, capacitance variation etc. hasbeen optimized. 4 Besides a presentation of thestate of Ta thin film technology in Telettra, thepresent work reports some results obtained in thefirst production exercise of/-Ta capacitors used inthe implementation of RC active filters; it alsodescribes the efforts produced towards theimprovement of the fabrication process in terms ofyield and reliability.

In particular, the deposition process of Ta20s-Ta films is described where a tantalum pentoxide

layer is first formed by d.c. reactive sputtering inoxygen in the same vacuum batch where the -Ta filmis sequentially formed.

2. STATUS OF RESISTOR ANDCAPACITOR PROCESSES IN TELETTRA

Ta thin film technology has been used at Telettra inthe implementation of circuits for telecommunicationequipment for about 10 years. Resistors are realizedboth with tantalum nitride films and tantalumoxynitride films depending on the required charac-teristics. Capacitors mostly make use of structuresmade of/Ta-Ta20s-NiCr + Au; the interdigitatedstructure is also used in some applications.

2.1 Resistors

Tantalum nitride resistors are adopted in digital,analog, and microwave circuits. Film deposition isby means of reactive sputtering on both glazed andunglazed alumina substrates, with 50 f/sq, sheetresistance.

Tantalum oxynitride resistors are used fortemperature compensated RC active filters. They aredeposited by means of reactive sputtering with100 2/sq. sheet resistance and feature about-200 ppm/C TCR which balances out the TCC ofthe capacitor. Sequence NiCr, Au, as well as Ti, Pd,Au is used for conductors; the latter is used in hybrid

5 15 25TIME (WEEKS)

FIGURE Yield of Ta thin film capacitors in pilot production. 50 V, 125C, 30 minutes test.

TANTALUM THIN FILM APPLICATIONS 71

circuits with beam leaded active components where,as a general rule, the working or process temperatureis high.

As for microwave circuits the microstrip thicknessis increased with an electrochemical process up to afew micron. For the adjustment of resistors to theirfinal value, the anodizing process is used. Equipmenthas been produced in which all resistors containedin one 2" x 2" substrate may be anodized in a fewseconds. Circuit scribing is obtained with a CO2 lasersystem featuring 100 Wcw output power.

2.2 Capacitors

Ta thin film capacitors are produced using standardoverlap, interdigitated and bipolar interdigitateddesigns to meet the required circuit characteristic

ni

.i,

<I’I

|,,

10-11 10-10 10-9 10-8 10-750 VOLT LEAKAGE (AMPS)

FIGURE 2 Leakage current distribution of 0.36 cmTa-film capacitors formed at 230 V.

TABLEComparison of original and new processes for deposition of

Ta Os-/Ta layers

Original process steps New process steps

Substrate cleaning Substrate cleaningSputter #-Ta film Sputter Ta Os-#Ta filmsThermally oxidize/3-Ta filmSubstrate cleaningSputter #-Ta film

such as capacitance values, dissipation factor etc.The standard TM structure is primarily used for theproduction of RC active filters temperature com-pensated with tantalum oxynitride resistors.

/3-Ta films are sputtered on glazed aluominasubstrates to a thickness of about 4000 A.A tantalum pentoxide layer is used to protect theglazed substrate from subsequent hydrofluoric acidetchants. The tantalum oxide dielectric is producedby a conventional process using anodic oxidation atroom temperature in 0.01% aqueous citric acidelectrolyte, at a current density of 0.3 x 10-3 A/cm2

Formation voltage of 230 V and soak time of hare used. Nichrome-gold counterelectrodes, about2000 A thick are evaporated using filamentevaporation sources. Equipment has been developedto control capacitance value and dissipation factorand to make a screening test at 50 V for minute ofcapacitors arrays by on-line computer, for highproduction rates.

3. A NEW APPROACH FORCAPACITOR PRODUCTION

Production of Ta thin film capacitors is likely to beaffected by yield problems, especially for highcapacitor values.

Figure shows the results of a pilot productionline of a batch of 3500 Ta film capacitor networkswith 56 nF/cm2 sheet capacitance, made up of fourequal capacitors for a total of 80 nF. Data arereferred to a condition whereby arrays can stand ascreening test of 50 volts at 125C for 30 minuteswithout catastrophic breakdown. The same figureshows yield results of the single 20 nF capacitor asan array component. Data show in terms of yieldthe price necessary to realize high value capacitors.

The average yield value drops from about 85% toabout 60% when the capacitor value rises from20 nF to 80 nF, in the condition of 50 V, 125C for30 minutes. Process control has indicated the factors

72 A. PERINATI

TABLE IIEconomic advantages of TaOs-Ta sequential deposition

using a bell jar batch system

Cycle-time reduction 40%Yield increase 10%Thin film capacitor cost reduction 15%

responsible for the large yield variation amongbatches of capacitors; these factors are often super-ficial irregularities of the glazed substrates or causedby handling, and impurities left over after cleaningprocesses, especially at interfaces between Ta-Ta20s,Ta2 Os NiCr + Au. In order to improve yield andreliability efforts have been concentrated towards thereduction of the above mentioned factors by meansof modifications in the fabrication process.

In particular, the process of Ta20s-3Ta filmdeposition has been simplified by means of sequentialdeposition within the same vacuum cycle. Also athick layer of photoresist material has been used,deposited immediately after the definition of counter-electrode geometry, in order to protect thecapacitors during the operation of testing, scribing,snapping, soldering etc. The layer is removed justbefore the final encapsulation.

In the new process a tantalum pentoxide layer,used to protect the glazed substrate from subsequenthydrofluoric acid etchants is formed by d.c. reactivesputtering in oxygen in the same vacuum batchwhere/3-Ta film, about 4000 A thick, is sequentiallyformed by d.c. sputtering.A comparison of the original and new proceszes

is shown in Table I. In regard to time and costreductions and to yield increase, some process-stepsand non productive time of the sputtering cycle havebeen eliminated. The results, using a 71 cm

commercially available diffusion pumped vacuumsystem, are given in Table II. Sheet resistance,resistivity and TCR values of -Ta films, formed byd.c. sputtering on glazed substrates using the newTa20s-/3Ta deposition process, are reported inTable III.

An oxygen doped Ta layer, about 500 k thick isformed at the Ta: Os-/3Ta interface as shown by TCRbehaviour with film thickness. The capacitors proper-ties are similar to those obtained from a standardprocess. The 50 V, R.T., leakage current distributionof 20 nF capacitors, evaluated on about 5,000samples, is shown in Figure 2. Data of aging test atdifferent voltage-temperature values show a goodbehaviour of capacitors formed in this manner, withrespect to the original process and related reliability.

4. SUMMARY AND CONCLUSIONS

Tantalum-based materials and process developmentshave increased the use of tantalum films, combinedwith silicon integrated circuits, to form hybridintegrated circuits which can meet the demands oftelecommunication systems. The RC active networkstemperature compensated using resistor-capacitortechnology on a single substrate, is one majorapplication of tantalum thin film technology. In thepast few years the search for greater economies hasled to circuit miniaturization and to replacement ofprecious metals for less costly circuit metalizations.In order to improve the capacitor yield and reliabilityfor RC active filters, temperature compensated inthe production of Telettra telecommunicationsystems, a new process has been developed fordepositing Ta: Os-flTa films by sputtering. In thisprocess a Ta20s layer, used to protect the glazed

TABLE IIISheet resistance, resistivity and TCR of -Ta films sequentially deposited

on to reactive sputtered Ta O

TCR (ppm/C)Runs R/sq. (/sq.) p(/d cm) (-196C to 25C)

4.5 209 -1462 4.8 213 -1343 4.7 209 -1324 4.7 208 -1285 4.4 200 -1056 4.6 204 1147 4.7 207 -1328 4.7 212 -1409 4.7 210 -13010 4.5 201 -100

TANTALUM THIN FILM APPLICATIONS 73

substrate from subsequent hydrofluoric acid etchants,is first formed by d.c. reactive sputtering in oxygenin the same vacuum batch where/-Ta film issequentially formed. The processing innovation hasproven to be valuable for capacitor production,resulting in economy of labor in handling,processing and in increased yield. The initial proper-ties as well as the long term behaviour of capacitorsformed in this manner are typical of a standardcapacitor process.

REFERENCES

1. C. T. Graham, Proc. of the 1952 ElectronicComponents Conference of the IRE.AIEE, pp. 61-63.

2. R. H. Alderson and F. Ashworth, Brit. J. App. Phys.,8, (1957) 205.

3. J. G. Swanson and D. S. Campbell, Thin Solid Films, 1,(1967) 183.

4. Bell Lab. Rec., 36, (1958) 426.5. R.W. Berry and D. Sloan, Proc. 1RE, 47, (1959) 1070.6. R.W. Berry, U.S.P., 2, 993,266, dated July 25, 1961.7. D. Gerstenberg and E. H. Mayer, Proc. Electronic

Components Conference, Washington, D.C., 1962,p. 57.

8. H. J. Schuetze, H. W. Ehlbeck and C. G. Doerbeck,A. V. S. 1963 Trans. l Oth Natl. Vac. Symp., MacmillanCo., New York, 1963, p. 434.

9. G. I. Parisi, Proc. Electronic Components Conference,Washington, D.C., 1969, p. 367.

10. C. A. Steidel, J. Vac. Sci. Technol., 6, (1969) 694.11. A. Schauer and M. Roschy, Proc. Electronic Components

Conference, Washington, D.C., 1973, p. 245.12. D. A. McLean and F. E. Rosztoczy, Electrochem.

Technol., 4, (1966) 523.13. H. Y. Kumagai, I. M. Fletcher, K. R. Bauerle,

J. R. Hightower and C. A. Steidel, Proc. ElectronicComponents Conference, Washington, D.C., 1973 p. 257.

14. W. Anders, Thin Solid Films, 27, (1975) 135.

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