FILM BELL TELECOMMUNICATION CIRCUITS

Electrocomponent Science and Technology, 1981, Vol. 8, pp, 3-80305-3091/81/0802-0003 $06.50/0

(C) 1981 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

THIN FILM TECHNOLOGY USED IN BELL SYSTEMTELECOMMUNICATION CIRCUITS

WALTER WOROBEY

Bell Telephone Laboratories, 555 Union Boulevard, Allentown, Pennsylvania 18103

This paper describes the thin film technology used for telecommunication circuits in the Bell System. Vacuumdepositio,n and photolithography form the basis for the film processing steps. Nitrogen doped tantalum films are usedto form the very stable thin film resistors and capacitors. Laser trimming is used to obtain precision RC active filters.Line widths down to 75 pm for conductors and 50 pm for resistors are readily achieved in production. The propertiesof the various components used in thin film integrated circuits are described and examples of completed circuits usedin transmission, station, and switching systems are shown. Some key advantages of this technology are low cost due tobatch processing techniques, high packing density and high precision of passive components.

1. INTRODUCTION

Thin film technology used within the Bell System meetsthe needs of a wide range of applications in trans-mission, switching, station, and microwave areas.These needs place increasing demands on thetechnology and require continuing developments inprocessing, materials and design. The recent trends inthis technology are toward smaller sizes, more complexcircuits and improved stability or precision.

Thin film integrated circuits can be divided into threebasic categories: conductor circuits, resistor circuits andresistor-capacitor (RC) circuits. Each type can includecrossovers to facilitate the interconnection betweencomponents and terminals of the hybrid. Also eachtype, except the RC circuits can be designed as a singlelevel or bi-level circuit. A bilevel design has conductorspatterned on both sides of the ceramic which are inter-connected by means of metallized via holes.2 The viaholes are formed in the substrate by laser drilling.

This paper describes the status of thin film tech-nology as it is presently used in production. First it willreview the thin film components that are used in hybridcircuits. These components, which include resistors andcapacitors, are based on sputtered tantalum and theTiPdAu metallization systems. Performancecharacteristics and design considerations for currentapplications will be discussed. Next the. processing stepsrequired to make thin film circuits will be described.For most applications beam leaded silicon integratedcircuits are thermocompression bonded to the

metallization on the ceramic surface. Finally examplesof specific thin film circuits will be shown.

2. THIN FILM COMPONENTS

2.1 Conductors

There are two basic types of metallization systems usedfor conductors in thin fim circuits. 3,4 The first type hasan end-of-life sheet resistance of 0.05 ohms per squareand consists of Ti-Pd-Au. The Ti and Pd are evaporatedor sputtered on to-the substrate and ten gold plated toa thickness of approximately 18 KA. The second typemetallization system is used for circuits which requirelower conductor resistance. They have an end-of-lifesheet resistance of less than 0.005 ohms per square andcan consist of either Ti-Pd-Au with 50 KA of Au orTi-Pd-Cu-Ni-Au with 40 KA of Cu and 20 KA of Au.The latter system is less expensive since it requires lessgold. Selective plating is used where possible in order toreduce the amount of gold that must be removed fromthe substrate surface to define conductor patterns.Typical conductor line widths are 125 pm (5 mils),however, line widths down to 75 pm (3 mils) are usedwhere necessary.

2.2 Crossovers

There are three types of crossovers than can be used forthin film circuits. They are the gold beam crossover,

4 W. WOROBEY

the batch bonded crossover6 and the thick film cross-under.7 The first two are formed by photolithographictechniques and the third type (crossunder) by standardthick film screening and firing techniques. The advan-tages of the first two are higher crossover density due tosmaller line widths (down to 125/m) and lower capaci-tance per crossing point. The advantage of the cross-under type is lower cost due to simpler processingand higher yields. In the fabrication of the thick filmcrossunder the bottom conductor and the glazeinsulator are screened and fired using conventionaltechniques. The top conductor consists of thin filmmetallization which is photolithographically patternedto form precision lines down to 100/m wide. Hundredsof crossovers can be formed on a thin film circuit withoverall yields well above 95%.

2.3 Resistors

Thin film resistors consist of reactively sputtered Ta2Nhaving the required sheet resistance. The film is photo-etched to form the desired pattern and then pre-aged at

300C for 4 hours or 350 for 1 hour to form the highlystable component that is required for many precisionapplications. Typical properties for the thin film Ta2Nresistors are shown in Table I. Sheet resistances as highas 300 f2/E] are standard and used for most

applications. Typical TCR for TaeN is -90 ppm/C,however, this can be adjusted by varying the sputteringparameters to a value around 140 ppm/C. This more

negative value is desired to match the TCC for -Tathin film capacitors and thus give a less temperaturesensitive RC product. Present technology can provideoverall control of the temperature coefficient of the RCproduct to within __-50 ppm/C over the temperature

TABLEThin film resistor properties.

Material TaaNTypical sheet resistances 100 /E3 and 300 f2/U]TCR -90 ppm/C, 140 ppm/CEnd of life aging +0.1% (20 years at 65C)Trimming accuracy AR/R ---0.05%Line width 50/m (2 mils)

range from -40C to +85C which includes theoperating range of most circuits.The initial tolerance of resistors is determined by the

capability of the laser-trimming process and the controlof the short-term stability of the laser trimmed sectionof the resistor. A typical pattern which allows resistorsto be adjusted over a wide range of values is shown in

Figure 1. The loops and ladder sections provide theadjustment range. Continuous laser adjustment of thetop hats and supplementary tuning sections allowsresistors over a few thousand ohms to be trimmed towithin 0.05 percent of a specified value.Long-term resistor aging is determined by the aging

of both the untrimmed and laser trimmed parts of aresistor. Laser trimming within the current path of aresistor tends to degrade the stability of the resistor filmand, therefore, should be kept to a minimum. The loopand ladder sections of the resistor are unaffected bylaser trimming because the laser-trimmed edge is not inthe vicinity of the current path. Accelerated agingstudies of typical laser trimmed resistors indicate a20-year resistance aging of less than 0.1 percent at

65C.Line widths for resistor patterns can be as low as 50

/m (2 mils) and still result in acceptable yields usingstandard photolithographic techniques. This line width

FIGURE Thin film resistor pattern for laser trimming.

THIN FILM TECHNOLOGY IN BELL SYSTEM CIRCUITS

in combination with 300 f/E3 film results in a resistancedensity of approximately 6 Mf/cmz.

over a 20-year period at 85C. The allowable ac work-ing voltage at 85C is about 7 Vrms.

2.4 Capacitors

Thin film capacitors are made by anodizing a patternedtantalum film to 190 volts in a 0.01 percent aqueoussolution of citric acid and forming an adherent coun-terelectrode of NiCr-Pd-Au or Ta2N-Ti-Pd-Au on topof the dielectric. Of great importance in fabricatinggood quality thin film capacitors is the base materialfrom which the dielectric is formed. Sputtered tantalumwith approximately 15 atomic percent of nitrogen is thepreferred material.9,1 This film has a body centeredcubic structure and an electrical resistivity of about 100/f-cm. It is often referred to as a-Ta because of itsstructure. Capacitors formed in this manner will typi-cally have the properties shown in Table ii after theyhave been stabilized at 300C for 4 hours. The capaci-tance density is 64 nF/cm2, the dissipation factor (tan 6)at 1 kHz is 0.0015 and the temperature coefficient ofcapacitance (TCC) is 145 ppm/C. Accelerated agingstudies of pre-stabilized thin film capacitors indicate anestimated 20-year capacitor aging of about -0.15 per-cent at 65C.The catastrophic failure characteristics of tantalum

thin film capacitors depend on the direction of the fieldin the dielectric. These capacitors can withstand higherdc voltages in the forward direction (i.e. tantalum posi-tive) than in the reverse direction. Acceleration.factorsare usually calculated from static stress tests over alimited range of stress levels and extrapolations madeto determine the working stress conditions for longperiods of time. Capacitors with dc working voltages ofup to 10 volt forward bias or 0.5 volt reverse bias willhave a maximum instantaneous failure rate of below100 FIT’s (1 FIT 1 failure in 109 component hours)

TABLE IIThin film capacitor properties.

Property

N concentrationTa crystal structureTa resistivityCapacitance density (190 volt anodization)Dissipation factor at kHzTCCCapacitor aging (20 years, at 65C)Initial capacitance toleranceDC working voltage at 85Cforward bias (Ta positive)reverse bias (Ta negative)

AC working voltage at 85C

---15 at %bcc100/f cm64 nF/cm20.0015145 ppm/C-0.15%+- 5 percent

10V0.5 V7 V rms

3. CIRCUIT FABRICATION

Circuit fabrication can be broken into two main parts;film integrated circuit (FIC) processing and hybridintegrated circuit (HIC) assembly. A typical RC circuitfabrication procedure is shown in Table iii. The details

TABLE IIIRC circuit fabrication procedure.

FIC process steps

Selectively glaze ceramic substrateSputter Ta and oxidize for underlaySputter -Ta capacitor filmPhotoetch capacitor base electrode patternAnodize capacitor areas to 190 voltsSputter Ta2N resistor filmDeposit TiPdAu conductor layerPhotoetch conductor patternPhotoetch Ta2N resistor patternThermally stabilize R’s and C’s at 300C for 4 hoursTest capacitorsLaser trim resistors

Hybrid assembly steps

Bond SIC’s and attach discrete componentsLaser scribe and separate circuitsAttach lead framesTest circuitsEncapsulate with RTVFinal test

of this fabrication procedure have been described inprevious papers.TM Most of the FIC process stepsinclude vacuum depositions and pattern delineationsusing standard photolithographic techniques. All ofthese are batch processing steps in which handling isreduced to a minimum and many circuits are pro-cessed simultaneously.The FIC processing steps are carried out using .large

ceramic substrates which measure 9.5 11.4 cm.These large substrates contain many individual circuitswhich are later separated for assembly and testingsteps. For example the substrate shown in Figure 2 con-tains 60 individual RC filter circuits. This allows morecircuits to be processed per hour and thus reduces thecost per circuit to a minimum.The HIC assembly steps include: attaching compo-

nents, circuit separation, attaching external leads,encapsulating and testing. The encapsulating material is

6 W. WOROBEY

FIGURE 3 Thin film RC circuit used as an active filterbuilding block (STAR).

FIGURE 2 Large substrate (9.4 x 11 cm) with 60 thin filmcircuits.

an RTV silicone rubber13 which forms a conformalcoating over the film integrated circuits and theattached devices.

4. EXAMPLES OF THIN FILM CIRCUITS

Examples of completed hybrid integrated circuits priorto encapsulation are shown in Figures 3, 4, 5 and 6.These circuits show the wide range of application ofthin film technology within the Bell System. The circuitshown in Figure 3 is an active filter building blockcalled the STAR (Standard Tantalum ActiveResonator) circuit. It is comprised of ninelaser-adjustable resistors, two 5100 pF thin-filmcapacitors and an operational amplifier which isthermal-compression bonded in the centre. It isfabricated on a 1.93 0.66 cm ceramic substrate andassembled to form a machine-insertable 16-pin dualin-line package (DIP). By laser trimming the resistorsto predetermined values and interconnecting thecomponents on the hybrid via the printed wiring board,the STAR circuit can be used to realize a wide varietyof low-pass notch, high-pass notch, bandpass, or otherfilter functions. 14,15

An example of a custom design RC circuit used in atransmission system is shown in Figure 4. This is a highstability RC active filter providing a fourth order bandelimination filter and a second order bandpass filter. Itcontains 26 thin film resistors, 6 thin film capacitorsand 4 beam leaded SIC’s. All of these components withtheir interconnections are contained on a ceramic sub-strate which is 3.3 1.4 cm.An example of a thin film circuit used in station

apparatus is shown in Figure 5. This is a line logic hy-brid for a PBX system. The substrate which measures4.1 1.4 cm contains 4 MSI beam leaded silicon chipsas well as thin film conductors and crossovers forinterconnections. The external leads are thermo-compression bonded on 0.254 cm centres and arebent to fit on a printed wiring board on rows 1.51 cmapart.

Finally, Figure 6 shows an example of a thin filmcircuit used in a switching system. 16 This 3-port lineunit contains 4 large gate array chips having a total of500 gates. The conductors forming the high densityinterconnection pattern on the ceramic consist of75/m (3 mil) lines and spaces. This hybrid circuitreplaced 31 16-pin DIPs which were interconnected ona 6-layer multi-layer printed wiring board. The hybridmeasures 4.6 x 2.0 cm and contains numerouscrossovers to facilitate routing.

FIGURE 4 Precision RC circuit used in a transmission system.

FIGURE 5 Line logic hybrid for PBX system.

8 W. WOROBEY

FIGURE 6 3 Port line circuit used as a switching peripheral

5. CONCLUSION

This paper described the thin film technology whichwas developed to meet the needs of today’s communi-cation systems within the Bell System. The require-ments for high precision, highly conductive paths andhigh packing density for both analog and digital circuitsare met with this technology, in addition, the combina-tion of thin film components, high density interconnec-tions and sophisticated silicon integrated circuits offeradvantages in miniaturization, low cost and design flex-ibility for today’s system applications. As photolitho-graphic and laser trimming techniques are refined,further reductions in circuit size can be realized forfuture applications.

REFERENCES

1. W. H. Orr and T. R. Robillard, "Thin Film Hybrid Inte-grated Circuits for Communication Systems"--IEEETrans. Parts, Hybrids & Packaging Vol. PHP-8, No. 2p. 51 (June, 1972).

2. T. E. Brady and D. K. Hinderman, "A Bi-Level ThinFilm Hybrid Integrated Circuit Containing Crossovers,Resistors and Capacitors," Proc. Elect. Comp. Conf., p.220 (1973).

3. J. M. Morabito, J. H. Thomas, III, and N. G. Lesh, "Mat-erial Characterization of Ti-Cu-Ni-Au (TCNA)--A NewLow Cost Thin Film Conductor System," IEEE Trans.Parts, Hybrids & Packaging Vol. PHP-11, No. 4, p. 253(1975).

4. N. L. Sbar and L. G. Feinstein, "Performance of NewCopper Based Metallization Systems in an 85C, 80 per-cent RH CI Contaminated Environment," IEEE Trans.Parts, Hybrids & Packaging Vol. PHP-13, No. 3, p. 208(1977).

5. A. Pfahnl and H. Basseches, "Crossovers for Interconnec-tions on Substrates," Proc. Elect. Comp. Conf., p. 78(May, 1969).

6. J.A. Burns and D. A. DiLeo, "Batch Bonded Crossoversfor Thin Film Circuits"---Solid State Technology (July,1976).

7. D.H. Klockow, "The Design of Economical Hybrid Inte-grated Circuits Utilizing Combined Thin and Thick FilmTechnologies," Proc. Intern. Micro. Symp., p. 139 (1975).

8. G. A. Bulger, "Stability Analysis of Laser Trimmed ThinFilm Resistors," IEEE Trans. Parts, Hybrids & Packag-ing, Vol. PHP 11, pp. 172-177 (Sept., 1975).

9. M. H. Rottersman, M. J. Bill and D. Gerstenberg, "TaFilm Capacitors with Improved AC Properties," Proc.27th Elect. Comp. Conf., p. 462 (1977).

10. W. Anders, "Nitrogen-Doped Tantalum Thin FilmCapacitors with Improved Temperature Stability"mThinSolid Film, Vol. 27, p. 135 (1975).

11. O.J. Duff, G. J. Koerckel, E. H. Mayer and W. Worobey,"A High Stability RC Circuit Using High Nitrogen DopedTantalum," IEEE Trans. Comp. Hybrids & ManufTechn., Vol. CHMT-2, No. 2, p. 221 (June, 1979).

12. O. J. Duff, F. Arcidiacono and G. J. Koerckel, "AMethod of Reducing Cost and the Production of HighStability Thin Film RC Networks," 1980 Elect. Comp.Conf., April 28, 1980, San Francisco.

13. D. Jaffe, "Encapsulation of Integrated Circuits Contain-ing Beam Leaded Devices with a Silicone RTV Disper-sion," Proc. Elect. Comp. Conf., pp. 376-381 (May,1976).

14. W. Worobey and J. Rutkiewicz, "Tantalum Thin Film RCCircuit Technology for a Universal Active Filter," IEEETrans. Parts, Hybrids & Packaging, Vol. PHP-12, No. 4p. 276 (1976).

15. J.J. Friend, C. A. Harris and D. Hilberman, "STAR: AnActive Biquandratic Filter Section"---IEEE Trans. Cir-cuits and Systems, Vol. CAS-22 (Feb., 1975).

16. C. T. Goddard, "The Role of Hybrids in LSI Systems,"IEE Trans. Comp. Hybrids & Manuf. Techn., Vol.CHMT-2, No. 4, pp. 367-371 (Dec., 1979).

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