Regular Articles Historical Overview of Semiconductor Device Reliability for Telecommunication Networks–– Field Data, Prediction Model of Device Failure Rate, and Wear-out Failure Analyses at NTT Noboru Shiono, Eisuke Arai, and Shin’ichiro Mutoh Abstract The 960s brought about the advent of the intense development of semiconductor devices for industrial and commercial applications. Since then, NTT has taken the initiative in introducing semiconductor devices into telecommunication networks to realize highly reliable and miniaturized equipment with high performance. This article reviews the failure physics approach we have been using to develop highly reliable semiconductor devices. It also summarizes our findings regarding device failures in field use and discusses a failure rate prediction model based on the findings. 1. Introduction (0-9/h)), depending on the type of device (from diodes to LSIs (large-scale integrated circuits)) and From the 960s to the 980s, NTT’s use of semi- the kind of equipment [4]. For semiconductor devices conductor devices was the primary factor in the suc- that had been developed at that time, accelerated life cessful development of highly reliable and miniatur- tests (longer than 0,000 hours) and long-term opera- ized high-performance telecommunications equip- tion tests at room temperature using a large number ment. In the 960s, the semiconductor industry was of devices were conducted to confirm that those tar- in its infancy. Therefore, it was not really known how gets had been achieved. In addition, field failure tests reliable semiconductor devices would be, and experi- were carried out with actual working equipment to ence in using them in telecommunications equipment confirm the high reliability of semiconductor devices was lacking. At that time, NTT was the first company and of passive and mechanical components as well. in the world to introduce transistors into telecommu- On the basis of the results of those field failure nications equipment []–[7]. tests, we developed a failure rate prediction model for In the early 960s, NTT introduced a failure phys- electronic components including semiconductor ics approach (also known as the physics of failure), devices to use in the reliability design of the next described in the next section, in order to develop stage of our telecommunications equipment [8], [9]. highly reliable semiconductor devices for telecom- The objectives of this failure rate prediction model munications equipment. The target values for device were the same as those of the well-known MIL- reliability required in network system design were a HDBK-27 (Military Handbook) model for reliabil- lifetime longer than 25 years and a failure rate lower ity prediction of electronic equipment [0], and the than 0.2–50 FIT (failure in time, which is defined as procedures were similar. the number of failures per billion device hours The development of reliability assurance standards NTT Technical Review Regular Articles New telecommunications system planning (Target value of system life and failure rate is determined.) Mass production Setting of lifetime and failure rates for No good semiconductor devices Qualification testing of C mass-produced devices Selection of appropriate design and fabrication Go technology (reliability design) Screening Design and fabrication of reliability test No good structures D Lot-acceptance testing No good Accelerated life testing A Go of test structures Shipment Go Trial fabrication of real devices No good Reliability testing B of real devices Go Criteria for each testing point: A B C D - Failure mode extraction - Whether degradation modes - Whether lifetime and - Whether the produced lot using test structures are a matter or not failure rate are lower than is normal or not - Whether the estimated - Whether lifetime and the target values for mass- failure rate is lower than failure rate of wear-out produced devices or not the target value or not failures are lower than the target values or not Fig. 1. Procedure for developing highly reliable semiconductor devices for telecommunications equipment. such as MIL-STD (Military Standard) -9500 [11] associated failure mechanisms are clarified, and and MIL-STD-3850 [2] started in the late 950s, device reliability is improved by optimizing device and NTT applied them to semiconductor devices structures and fabrication processes. Finally, a reli- installed in NTT equipment to ensure their high reli- ability assurance procedure is carried out; this proce- ability [8]. Over the years, NTT also developed and dure includes conducting wafer acceptance tests, periodically revised its own assurance standards to screening tests of encapsulated devices for eliminat- reflect advances in device technology. These stan- ing early failures, and lot-acceptance tests. dards were finally withdrawn in 987 because the The procedure we followed for developing highly reliability of semiconductor devices had been reliable semiconductor devices is shown in Fig. 1, improved to the level where such standards were no and examples of devices used in NTT equipment are longer necessary. summarized in Table 1 [3], [4]. The relationship between failure rate and working time is known as the 2. Development of highly reliable devices bathtub curve, which depicts the early failure period, based on the failure physics approach random failure period, and wear-out failure period. Early failures are caused by latent defects induced The failure physics approach for developing highly during fabrication, and the failure rate decreases with reliable devices is as follows; first, various failure time (β < , where β is the shape parameter in a modes are investigated in accelerated life tests. Next, Weibull distribution). Random failures are caused by Vol. 11 No. 5 May 2013 2 Regular Articles Table 1. Main devices used in various telecommunication systems, failure modes, and countermeasures for higher reliability. System (System name/ Main devices Main failure modes Countermeasures for higher year installed) (type or process used) reliability Coaxial cable transmission Bipolar transistors - Wire bond breakage due to - Control thermal process system (direct wire bonding on Au-Al compound formation during wire bonding and (CP-12MTr/1967) metallization of E-B junction) between Au wire and Al control bond strength metallization pad on junction - Change Au wire to Al wire - E-B junction short due to Au penetration of Au wire into junction Electronic exchange Logic ICs (bipolar CSL type) - Open failure of Al - Improve passivation film system (D10/1972) metallization due to corrosion Electronic exchange 4K DRAM - Increase in leakage current - Control contamination system (D10-HCP/1977) (n-channel MOS process) - Decrease in holding time - Optimize screening conditions Submarine coaxial cable Bipolar transistors (Au/Pt/Ti - Metal penetration into - Improve electrode metal system electrode, BeO substrate for junction structure from Al to Ti/Pt/Au (CS-36M/1977) high thermal conductivity) - hFE degradation electrode - Improve passivation film from single layer of SiO2 to SiO2 /SiN - Use individual device assurance method Digital electronic exchange High-voltage bipolar LSIs - Open failure of Al - Adopt SiN passivation film system (dielectric isolation, pn metallization due to - Adopt EPIC dielectric (D70/1982) junction isolation, plastic corrosion isolation structure combined encapsulation) - Degradation of breakdown with field plate voltage for isolation 64K DRAM - Soft error - Use chip coating with resin - Adopt ECC technique Optical fiber transmission InGaAsP/InP BH-LDs - Increase in thermal - Improve metallized layer and system (Fabry-Perot type, buried- resistance in die bonding to adopt silicon heat sink (F-400M/1983) heterostructure) substrate - Use Au-rich AuSn die - Short failure due to Sn bonding whisker growth around die - Use individual device bonding assurance method - Degradation of optical power output ISDN network Submicrometer custom LSIs - Open failure due to - Use multilayer metallization (ISDN/1988) (CMOS, BiCMOS) electromigration and stress structure (Al-Si-Cu/Ti/TiN/Ti, migration of Al metallization etc.) - Vth degradation due to hot- - Control process of H2O carrier effect content in interlayer dielectric - TDDB of gate oxide film - Use high-voltage and low- temperature screening BeO: beryllium oxide InGaAsP: indium gallium arsenide phosphide BH: buried heterostructure ISDN: integrated services digital network CSL: controlled saturation logic Pt: platinum Cu: copper Si02: silicon dioxide E-B: emitter-base SiN: silicon nitride ECC: error-correcting code Sn: tin EPIC: epitaxial passivated integrated circuit TDDB: time-dependent dielectric breakdown (gate oxide breakdown) H2O: hydrogen dioxide (water) small latent defects that occur after relatively long- out failures occur after long-term operation due to term operation at a constant failure rate (β = ). Wear- wear-out and fatigue, and the failure rate tends to 3 NTT Technical Review Regular Articles Diodes 100 Transistors Bipolar ICs 10 1 Failure rate in field use (FIT) 0.1 1965 1970 1975 1980 Year installed Fig. 2. Failure rate in field use vs. year installed in NTT equipment for semiconductor devices. increase rapidly during this period (β > ). typically operated in an air-conditioned room with a Countermeasures consisting of fabrication process maximum junction temperature of 55ºC (ambient control and proper screening were effective in reduc- temperature 40ºC) and relative humidity of 20–70%