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  • CMOS ICs are being producedusing a variety ofprocesses,and considerable data is nowavailable on their reliabilityand failure mechanisms.

    Reliability of CMOSIntegrated Circuits

    There are basic differences between MOS andbipolar digital integrated circuits, and betweenCMOS and other digital MOS technologies. Some ofthose differences have an impact on the reliability ofthe various types of digital integrated circuits.Accelerated-life tests and field use, along with otheravailable data on CMOS reliability, indicate thatCMOS devices, properly made, are equal in reliabili-ty to bipolar digital circuits of equal complexity. Inthis article, we look at CMOS packaging, circuitcomplexity, and electrostatic gate protection andcompare CMOS to other types of digital ICtechnology.

    CMOS IC Technology

    The basic building block for CMOS integratedcircuitry1-4 is shown in Figure 1. In contrast to othertypes of MOS integrated circuits, the CMOS cir-cuits contain no load resistors; this results in verylow quiescent dissipation. The voltage transfercharacteristics of a basic CMOS inverter are shownin Figure 2, and typical curves of dynamic dissipa-tion versus frequency in Figure 3.CMOS ICs were originally produced in volume in

    1968. The technology has evolved from that used forthe original RCA CD4000 series' devices (6-15 volts)to the CD4000A series (3-15 volt) devices2 in 1971,and to the 4000B series (3-20 volt) devices 3,6 in1974. The introduction, in 1970, of plastic-encapsulated devices5 was instrumental in achiev-ing even wider acceptance of the popular 4000series.CMOS ICs are being produced by a number of

    manufacturers using a variety of different pro-cesses.2'2w5-'9 Ion implantation is being used to formp-wells, 3,20 to adjust thresholds,6 and to avoid fieldinversion. Metallization materials have includedaluminum, polycrystalline silicon, and titanium-palladium-gold or titanium-platinum-gold.52'2 Tech-

    niques for attaching the die to the substrate have in-cluded gold-silicon eutectic bonding and epoxy at-tachment.' Although most CMOS ICs beingmanufactured today are bulk silicon types, devicesfab'ricated on thin-film silicon-on-sapphiresubstrates are being produced.3'7"1'9-1419At present, 4000A series CMOS ICs are commer-

    cially available from more than 10 suppliers, andstandardized 4000B series devices22 are or will beavailable from at least nine. Devices qualified toMIL M-38510, Class A are also commerciallyavailable.4The packages used for CMOS devices are similar

    to those used for other types of MOS devices and forbipolar devices. The same potential failuremechanisms apply to chip-to-substrate bonds, wirebonds, and packages.2 All MOS devices are,however, more surface-sensitive than digital bipolardevices, and higher voltages are applied to MOSdevices than to digital bipolar devices.

    MOS vs. bipolar ICs

    MOS ICs have had a major impact on the digitalelectronics industry. Not only have they displacedbipolar ICs for many applications, but they havealso made possible a large number of totally new ap-plications. As a result, they are now being producedin volumes roughly comparable to those of bipolardevices. Because bipolar devices antedate MOSdevices, more information has been published on thereliability of bipolar circuits than on the reliabilityof MOS circuits. Also, bipolar circuits were initiallyused to a large extent in high-reliability military andaerospace applications, whereas'MOS circuits havebeen used principally in consumer and commercialapplications. Also, while most early MOS deviceswere hermetically packaged, a large portion of allMOS ICs produced today are encapsulated inplastic.1,13

    6 0018-9162/78/1000-0006$00.75 O 1978 IEEE COMPUTER

  • INPUT Q-

    + VDD

    SUBSTRATE

    GATES G 0 OUTPUT

    l_n

    p - WELL

    -vss

    Figure 1. The basic building block for CMOS integratedcircuits. Figure 2. VoltageCMOS inverter.

    transfer characteristics of a basic

    The first commercially available MOS deviceswere based on p-channel enhancement-mode MOStransistors with aluminum gates. Accordingly, con-siderable information is available on the reliabilityof this type of device, and on possible failuremechanisms.ss-27 Devices that have since becomecommercially available include CMOS, silicon-gate,n-channel, depletion-mode, floating-gate, CCD, andCMOS/SOS devices. Considerable data is nowavailable on the reliability and possible failuremechanisms of CMOS devices, on ICs con-taining silicon-gate transistors,51-" and on devicesbased on n-channel transistors.55-"A number of fundamental differences between

    MOS and bipolar devices have an impact onreliability. The principal differences are that MOSICs have a higher substrate resistivity and usehigher applied voltages, and the properties of thegate oxide of MOS devices are more important.The process of MOS fabrication is simpler than

    bipolar fabrication.5,23 Accordingly, it is easier to at-tain higher chip complexity with MOS, and thushigher gate-to-pin ratios. Since wire-bond failuresare a significant factor in limiting the reliability ofsmall-scale ICs, MOS can significantly improvereliability by reducing the number of wire bonds andexternal interconnections. Moreover, with MOStechnology, there is lower power dissipation perfunction, which improves reliability by loweringchip temperatures. In typical bipolar ICs (TTL),device dissipation is significant.MOS, particularly CMOS, also has an advantage

    over bipolar devices in that the high impedance ofMOS devices does not result in high current den-sities in the metal interconnections, and thus elec-tromigration (current-induced mass transport) isnot a common problem in MOS devices. Problems ofhigh current density at metal-silicon contacts arealso less frequent. The high impedance of MOSdevices also makes multilevel interconnectionsfeasible in complex arrays without significantly

    October 1978

    compromising circuit properties. Diffusedcrossunders in the single-crystal silicon are effec-tive, and if another level of interconnections is re-quired in addition to that provided by the metalliza-tion layer, polycrystalline silicon, deposited as partof the silicon-gate process, is quite effective as an in-terconnection level. By contrast, an additional levelof interconnections. in bipolar arrays means use ofmetal-over-metal 'crossovers, which requires addi-tional technology and introduces possible newfailure mechanisms.Since localized defects in silicon are a factor in IC

    reliability, one advantage of MOS compared tobipolar circuits is that no epitaxial layer is requiredfor conventional monolithic MOS devices. MOSdevices are thus fabricated in silicon of better

    AMBIENT TEMPERATURE (TA) - 250CPOWER DISSIPATION P = CVDD2f + PQUIESCENT

    10 103 104 105 106 107INPUT FREQUENCY (fl) - Hz

    Figure 3. Typical relationship between dynamic dissipation of CMOSintegrated circuits and frequency.

    7

    VDD

    VOUT

    VIN

  • crystallographic perfection, with no possibility ofepitaxial stacking faults or of epitaxial spikes thatcause device problems and damage the masks usedfor photolithography. Finally, since MOS process-ing is simpler than bipolar processing and requiresfewer steps, fewer manufacturing errors are possi-ble.

    Failure rates for devices of various complexitiesare often lumped together and reported as a failurerate for a particular device family. Since MOSdevices tend to be more complex than bipolardevices, equal reported failure rates per packagedpart actually represent lower failure rates per gate.MOS ICs use many of the same materials and pro-

    cesses that bipolar ICs and small-signal transistorsdo. Accordingly, improvements in siliconmaterials,'9 oxidation,65-69 photolithography,50'70-7'diffusion,20'72 metallization,7' passivation,78'" andplastic encapsulation,"23"3'97-79 and also in devicephysics, design, process control,'8 automation,80 andelectrical characterization have resulted in substan-tial improvements in the reliability of both types ofdevices.

    Reliability advantages of CMOS

    CMOS technology provides a number of relia-bility advantages over other MOS tech-nologies, 34,7-9,14,21,28 For example, since both p-typeand n-type diffusions are part of the normal process,both are available to use as channel stoppers or aspart of a more effective input protection circuit. Thelow dissipation of CMOS ICs results in lower chiptemperatures, which substantially improve reliabili-ty. The wide range of CMOS operating voltages per-mits greater reliability of operation, including allow-ing functional testing at voltages substantially_above and below the ultimate operating voltage.Moreover, because of low dissipation per gate,CMOS can be used to fabricate very complex chipswithout introducing reliability problems resultingfrom excessively high chip temperatures."'0"4 Bycontrast, power dissipation in large chips is a pro-blem with TTL integrated circuits and, to some ex-tent, PMOS and NMOS circuits. Finally, CMOStechnology represents a mature, high-volumetechnology with an extensive history of reliability.Potential failure mechanisms are well understood,and production processes and process controls havebeen specifically selected to ensure against thepossibility of manufacturing errors that mightadversely affect reliability.Other factors of importance to the reliability of

    CMOS ICs include device and design features>(dielectric thickness and quality, design rules,device complexity, in-process controls), specifica-tions (maximum and minimum operating voltages,operating temperature range), electrical testing(tests performed on the wafer and on packageddevices), and screening (amount of burn-in or otherscreens applied). Operating conditions of impor-,tance include chip temperature, applied voltage,

    voltage transients, moisture con

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