JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-2, NO. 6, DECEMBER 1984 939

Ultra-High Reliability Ultra-High Speed Silicon Integrated Circuits for Undersea

Optical Communications Systems

Abstruct -A line of ultra-high-speed ultra-high reliability integrated circuits have been designed for use in undersea optical communication systems. These are bipolar circuits using junction isolation and are realized in both standard buried collector, microwave junction isolated monolithic (MJIM) and microwave complementary bipolar integrated circuit (MCBIC) technology. The design of these circuits depends heavily on the technology used to realize the discrete transitors for undersea systems which are now deployed in a number of transoceanic locations. The reliability of the circuits is assured by application of the methodology distilled and refined from that used in previous undersea system develop ments. This paper summarizes how these previous developments have influenced the design of these circuits and presents the early reliability results within the context of the data accumulated in u) years of monitor- ing undersea semiconductor device reliabilty.

A I. INTRODUCTION

line of silicon integrated circuits have been designed for use in the regenerator that will be a crucial part

of the new optical undersea system known as SL [l]. These circuits, microwave junction isolated monolithic (MJIM) and microwave complementary bipolar integrated circuit (MCBIC) have been described in previous publications [2],

They are of a medium scale of integration (MSI) com- plexity which are realized with microwave transistors as the active components. MJIM circuits employ only n-p-n tran- sistors, whereas the MCBIC circuits have complementary n-p-n and p-n-p transistors with extrapolated unity gain frequency ( f , ) in the range of 4 and 2.5 GHz, respectively.

These circuits are fabricated using an all ion-implanted technology, are silicon nitiide passivated, and use a single- level metal formed from a composite layer of titanium, titanium nitride, platinum, and gold. This technology is identical to that used in a broad class of discrete and integrated devices designed and manufactured by AT&T

These UHF IC's for SL actually represent the sixth generation of semiconductor devices developed for under- sea applications, the SF [5] and SG [6] systems as well as AT&T Bell Laboratories designed military systems.

Over the two decades that these developments spanned, a methodology for assuring reliability of components for

131.

t41.

The author is with ATBET Bell Laboratories, Reading, PA 19604. Manuscript received August 1, 1984; revised August 24, 1984.

undersea applications has been devised. This methodology had its roots in the procedures adopted for the electron tubes used in the earliest undersea cables realized with electron tube repeaters [7].

However, the early electron tube methodology has been altered and refined significantly over this period. The methodology as applied to discrete semiconductors has been described elsewhere [8] and has evolved because of the following factors:

First, ~ semiconductors are becoming more designable. This is derived from their well-behaved adherence to thor- oughly understood and documented physical principles. This permits a greater reliance on analysis and less need for elaborate empirical procedures adopted in the early vacuum tube repeater developments.

Also, experience with development and manufacture of the semiconductors for the five previous generations of undersea devices has evolved a battery of proven qualifica- tion and screening procedures which can identify poten- tially short-lived devices on lots of devices and censoring such potential short-lived devices from a population being considered for repeater manufacture.

Finally, these UHF IC's share a common technology with the discrete silicon transistor that has a proven record of reliability in the SG Undersea systems as well as the discrete microwave transistor that has also established a record of high reliability in the T4M terrestrial high-speed (274-Mbit) digital coaxial system [9].

Thus the IC's for SL represent an extrapolation of a proven technology arid methodology to a structure provid- ing an improvement in performance for use in lightwave systems.

11. COMPARISON OF MJIM AND MCBIC TO PROTOTYPE DISCRETES

As noted in the previous section, the MJIM and MCBIC circuits that will be used in SL are derivatives from two older proven product lines and share a common technol- ogy. These are the SG transistor and the microwave tran- sistor, which is the active element in these UHF IC's.

These components not only share a common technology, their development overlapped in time and, because of this, a synergy developed which benefited each, in tu%.

0733-8724/84/1200-0939$01.00 01984 IEEE

940

' For example, the discrete microwave transistor was in- troduced into manufacture in 1973 and was used in ter- restrial high bit rate coaxial installations. These devices were not hermetically packaged but in beam-lead form, were applied to thin-film microwave circuits, and coated with room-temperature vulcanizing (RTV) silicone rubber

There was no screening procedure or burn-in used on these devices or circuits. In spite of the presence of a small number of early failures embedded in the statistics, they have compiled a record of 20 FIT'S in a total of 4 000 000 000 device hours of service by mid-1979.

During the planning period in the early 1970's prior to availability of SG transistors, life acceleration tests per- formed on the discrete microwave transistor were used to evaluate whether its technology was capable of meeting SG reliability goals as shown in Fig. 1.

This life-test acceleration curve evaluated the microwave transistor against the usual transistor life-test end points of stability of breakdown voltages, forward resistances, and leakage currents as well as the more stringent requirements of stability of dc current gain. The devices exhibited no changes in breakdown voltages and forward resistances and a decrease in leakage currents accompanied by an increase in current gain. The failure mode was a general drift upwards in current gain beyond the 2.5-percent limit, a change characteristic of that resulting from annealing of surface states. The activation energy of approximately 1.5 eV displayed by Fig. 1 is also typical of that observed when surface states are annealed.

Subsequently, when prototype MJIM circuits were fabricated, their long-term reliability capability was com- pared not only to the discrete microwave transistor but the low-frequency discrete transistor (f, - 300 MHz) which is the active element in all of the general purpose digital and analog bipolar integrated circuits manufactured by AT&T. These VHF IC's are currently being manufactured by AT&T by the hundreds of millions per year and are ubiquitous in virtually all of AT&T electronic apparatus. Life performance of these circuits have been carefully monitored and reproducibly demonstrate FIT rates of bet- ter than 10 without the need for bum-in [ll].

The technology for these circuits has been refined aver the 20-year period of their development and manufacture and represents the core technology from which the technol- ogy for the discrete microwave transistor, the SG tran- sistor, and the IC's was derived.

A comparison of the results of. life-test acceleration curves as shown in Figs. 2 and 3 will be discussed in some detail because they provide a self-consistent summary of the long-term life capability of the MJIM/MCBIC tech- nology.

These figures follow the usual custom of plotting junc- tion temperature in degrees centigrade (in a scale which is linear in 1/K) versus log time for several relevant devices [12]. The shaded area in Fig. 2 represents the range within which the median life of the VHF prototype transistor falls. It is plotted for reference since, as was noted, it is the

P O I .

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-2, NO. 6, DECEMBER 1984

3001 I I 1 1 1 1 ~ 1 1 1 1 1 1 1 ~ 1 1 1 1 1 ~ 1 1 1 ~ 1 1 I 1 ~ ~ 1 ~ ~ ~ ~

250

200 95% CONFIDENCE

9 a 3 t- 2 150 n

; 100

3 t

50

25 100 101 102 103 104 105 106

HOURS OF LIFE Fig. 1. Capability of microwave transistor technology. SG criteria.

600 I 1 1 t t 1 1 1 1 I ~ I I I I I I ~ I 1 1 1 1 1 1 1 1 I I I I I ~

[MEDIAN LIFE OF VhF 500 SEALED JUNCTION

L w -cC MEDIAN

-0-+ - 1 r

I -

- -

L 100 10' lo2 l o 3 lo4

TIME (HOURS)

Fig. 2 . Reliability of discrete silicon transistors.

basic active element used in all the bipolar logic and linear silicon integrated circuits manufactured by AT&T and because the technology used in its manufacture has been refined for these as well as the discrete microwave tran- sistor depicted in Fig. 2 by the experimental points high- lighted by the solid and dashed curves. These curves show that i i ~ spite-of the fact that narrower lines and spaces and thinner metallization and dielectrics are used in the micro- wave transistor, the same long-term life will be realized.

If one extrapolates to use condition, one predicts ex- tremely low failure rate. And, in fact, the failure rate in the

MILLER: INTEGILATED CIRCUITS FOR UNDERSEA OPTICAL COMMUNICATIONS SYSTEMS 941

high-speed AT&T digital coaxial installation, as has been noted, has by now verified this prediction.

The data of Fig. 2 is repeated on Fig. 3. Superimposed on Fig. 3 are some results with prototype MJIM circuits. The points arranged vertically were obtained by using short time (10-h) intervals and increasing stress in incre- ments until the failure distribution represented by the sigma and the median plotted on the figure were obtained. The solid square is derived from a constant stress test where a small number of. failures were used to extrapolate to the 16th percentile. If one were to extrapolate further, one would find that the median of the MJIM circuits would also fal l within the shaded area represented by the VHF discrete (as well as the microwave discrete).

The dashed curve representing the sigma of the MJIM failure distribution is parallel to the median line of the VHF and microwave discretes. It also is very close to the sigma of the microwave discrete failure distribution.

This self-consistency of behavior of the prototype MJIM circuits with the discrete microwave prototype as well as the VHF discrete from which they were all derived is very persuasive evidence that the basic technology used in these devices provides devices of similar long-term life capability and that it is extremely repeatable. (The data was gener- ated over a long period of time by different individuals and in different structures made in several manufacturing loca- tions.)

Moreover, as is shown in Fig. 1 and the discussion associated with it, the SG transistor is a part of this body of data on the reliability capability of the basic technology. Data accumulated on the SG transistor during the produc- tion lot qualification procedures to be discussed in the next sections adds significantly to the confidence of this data Lase.

111. QUALIFICATION PROCEDURES

As was noted in an earlier section, accelerated life tests conducted on the discrete microwave prototype transistors were used as a basis for deciding that SG reliability goals could be met.

When production of the SG transistor was initiated, stress acceleration tests similar to those in Figs. 1, 2, and 3 were used to verify that each lot of devices measured up to expectations. For this type of evaluation, a lot is generally defined by the wafers in a diffusion run, the common denominator which establishes the quality of passivation. This might range from five wafers in development to 50 wafers in high-level production. Typically, in production of devices for undersea applications, wafer lot sizes range from 5 to 20 wafers.

Fig. 4 represents a compilation of that lot-by-lot qualifi- cation procedure which was used on the production runs for the SG transistors used in TAT6 and TAT7.

Three samples from each lot were stress-tested by dis- sipating 1.5 W in each device while maintaining constant heatsink temperatures for 20,150, and 2000 4 respectively.

300

250

200

150 v ' G- -

I oo

75

LIFE (HOURS)

Fig. 4. Qualification tests on the SG transistor. Pt = 1.5-W failure crite- rion AhFE d 2.5 percent.

INFANT MORTALITY

t, U.

~

0 TIME - Fig. 5. Bathtub reliability model.

~

The heatsink temperature was raised an increment after completion of each time interval and the failure distribu- tion at each time of stress interval using' an end point of A h , Q 2.5 percent was determined.

The shaded area in Fig. 4 represented a growing data base within which 95 percent of all the SG transistor production sample medians fell. The criterion for accep- tance of the lot for continued processing was that the sample median fall within the shaded area and that the failure distribution also fall within normal limits as typified by the solid curves plotted on Fig. 4. The significance of the dashed - 40 line indicates that one should expect less than 1 device failure in a 330 repeater system in 20 years if no early failures are contained in the population.

As in the case of Fig. 1, the failure mode displayed by these devices is a drift upwards in current gain beyond the 2.5-percent limit. There were no failures due to breakdown voltages, forward resistances, or leakage currents. Since these devices received a gain stabilization bake, the activa- tion energy of the acceleration curve increased from 1.5 to 2 eV because the more shallow surface states were annealed by the gain stabilization treatment.

IV. PLAN FOR RELIABILITY ASSURANCE OF TAT8 SL SIC'S

The methodology which has evolved for assuring reliabil- ity of semiconductor devices for undersea applications, which has been described in detail in [SI, involves three important procedures.

942 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-2, NO. 6 , DECEMBER 1984

C E R T I F Y ( 1 0 0 % ) - QUALIFY (SAMPLE) SCREEN (700%)

I . ELECTRICALLY TEST CHIP

2 . V ISUALLY INSPECT CHIP

3. VISUALLY INSPECT DEVICE ASSEMBLY

4 . SERIALIZE

5 . FIRST DC TEST PACKAGED DEVICES

t6. TEMPERATURE CYCLE DEVICES

t 7. MECHANICALLY STRESS DEVICES

8. LEAK TEST FOR PACKAGE INTEGRITY

9 . SECOND DC TEST AND COMPARE

1 0 . HTRB DEVICES

1 I . THIRD DC TEST AND COMPARE

1 2 . F IRST DYNAMIC TEST

t 13. POWER STRESS DEVICES

1 4 . FOURTH DC TEST AND COMPARE

21. DEW POINT TEST

22. VISUALLY INSPECT

23. SELECT AND ASSIGN

24. CODE AND RELEASE

t OVERSTRESS BL'T WITHIN DESIGN CAPABILITY

Fig. 6. Procedure for reliability assurance of SL circuits.

The first is to use a technology that offers an intrinsic long life so that the failures due to wear-out mechanisms (the rising portion of the famihar bathtub curve) shown in Fig. 5 are not present during normal life; and to use lot-by-lot qualification techniques as demonstrated in Fig. 4 to assure that wear-out mechanisms extend beyond the planned useful life of the system.

The second, which is increasingly more important with the realization of the former, is to devise techniques which censor atypical devices: those whch fall in the decreasing region of the curve of Fig. 5. This process has been characterized as censoring, screening, or, more recently, purging devices exhibiting infant mortality.

Finally, a burn-in procedure which is intended to verify (certify) that the previously described two procedures have been effective is used. This certification procedure is usu- ally an extended (typically 1000 h) operational life test at or modestly above the maximum use condition of the device.

Fig. 6 contains a concise summary of the steps included in the 3 phases of reliability assurance which is being used for SL devices.

The methodology summarized in the preceding para- graphs and in Fig. 6 was used on the prototype MJIM circuits which were prepared for the SL system field trial thereby demonstrating for a short period the efficacy of the methodology when applied to UHF IC's [13].

V. RESULTS OBTAINED USING UNDERSEA COMPONENT RELIABILITY ASSURANCE

METHODOLOGY

Certification procedures for the discrete transistors used in SF and SG systems were dominated by the need to forecast long-term stability of current gain ( h F E ) to assure that these analog systems did not drift out of alignment [14]. Therefore, precision long-term aging equipment was established in the manufacturing line to measure [15], and hopefully, to permit extrapolation of current gain [16].

In spite of the fact that this life-test measurement system was designed and functioned at a precision unprecedented in device life-test art, the stability of the devices was such that one could not distinguish parameter aging trends as shown by Figs. 7 and 8.

Data from certification aging such as described above, as well as records of in-service performance, have been care- fully recorded and periodically updated [17]. These data are summarized in Fig. 9 which depicts 20 years of experi- ence with undersea semiconductor device reliability. It includes the cumulative number of device life hours in certification aging, as well as hours in service.

Fig. 9 plots failure rate in FIT'S versus cumulative device hours of operation. The solid line represents a theoretical curve that represents the best FIT rate that one can dem- onstrate with a 90-percent confidence level as a function of device life-test hours when one has no failures.

I- 4 16.090 w (3

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a. 15.860

f

11

- Q - I

15.630 I Y 4 UI I 5 YEARS 7 YEARS a m U

0 600 1200 1800 2400 3000 AGING (DAYS)

Fig. 7 . Certification aging of an SG surge protection diode. 100 devices aged at 13-V reverse bias.

+ .50 + .40 + .30

E + .20 ," f.10

. -. - -. -. -. -. -. -. -. -. -. - - - - ACCEPTANCE LIMIT

w

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-.20 0 . 20 40 60 80 100 120 140 160 180

TIME (DAYS) Fig. 8. SG transistor certification. 100 devices aged at 1.5 W.

tn

W

a ... G TRANSISTOR 1

LL

DEVICE HOURS OF OPERATION

Fig. 9. Experience with undersea devices.

Also shown on Fig. 9 are the SL IC allocation of 0.5 FIT and data representing five different kinds of components; silicon diodes, discrete germanium UHF power transistors, discrete silicon UHF power transistors, silicon VHF IC's, and silicon UHF IC's.

The components which have accumulated the most life hours fall in the lower right portion of Fig. 9, while those with the least are in the upper left.

The diodes ' which are used in the SF, SG, and SDC systems are all of the same design and have accumulated the most life hours with no failures. These components have demonstrated a FIT rate of approximately 0.3, thus surpassing the target allocation for SL.

The SF germanium transistor, which was first placed on certification aging in 1965 and entered service in 1968, has accumulated a total of 2 X l o9 device hours and no failures have been experienced. These results establish a FIT rate of 1.2 at a 90-percent confidence level. The SF system FIT rate goal of 5.0 was demonstrated to a 90-percent confi- dence level in 1980, 12 years after the SF system entered service.

Similarly, the first SG transistor was placed on the LTA racks in 1973 and entered service in 1976. With the one failure that occurred during system laying, 7.2 FIT's have been demonstrated in 5.4X 10' device hours of aging.

The results discussed above emphasize the tyranny of numbers associated with attempting to demonstrate, via laboratory or factory tests, the very low FIT rates consid- ered necessary for undersea system reliability.

While the data just summarized demonstrates the ef- fectiveness of the qualification, screening, and certification procedures for discrete semiconductors, similar procedures have been applied to integrated circuits used in a new undersea system designed and manufactured by AT&T for the U.S. Government.

The results of screening these devices, as well as certifi- cation age, as of mid-August 1984, are also plotted on Fig. 9. These IC's, which represent the first significant use of ICs in undersea repeaters, have demonstrated a FIT rate of approximately 864, a figure which is approximately equal to that demonstrated by the SF and SG transistors (indi- vidually) at the time when the systems were placed in service.

Finally, the data point in the upper left portion of Fig. 9 represents the accumulated experience of 1,500 FIT's with prototype SL UHF IC's in August 1984.

VI. CONCLUSIONS

This paper has reviewed the methodology which has been developed and refined for providing assurance of reliable operation of semiconductor components in under- sea applications as well as the application of this methodol- ogy to increasingly more sophisticated components. The empirical data supports the thesis that the methodology is universally applicable. Its most recent application has been to the prototype silicon UHF IC's used in the AT&T SL sea trial, and it is now being applied to the IC's which will be deployed in the SL undersea lightwave system.

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[ l ] P. K. Runge and P. R. Trischitta, "The SL undersea lightwave system," pp. 744-153, this issue.

[2] D. G. Ross, "Integrated circuits for high speed digital transmission on optical fiber," presented at High Speed Digital Technol. Conf.,

[3] D. G . Ross, R. M. Paski, D. G. Ehrenberg, W. H. Eckton, Jr., and San Diego, CA, Jan. 1981.

S. F. Moyer, "A regenerator chip set for high speed digital transmis- sion,'' presented at ISSCC, San Francisco, CA, Feb. 1984.

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A. J. Wahl, W. McMahon, N. G. Lesh, and,W. J. Thompson, “SF 1983, MD2 paper 8.

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I. G. Abrahamson et ai. “Statistical methods for studying aging and for selecting semiconductor devices,” in Trans. 23rd A n n . Tech. Conf. Amer. SOC. for Quality Control (Los Angeles, CA), May 5-7,

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[17] A. J. Wahl, “Ten years of power aging of the same group of submarine cable semiconductor devices,” Bell Syst. Tech. J . , vol. 56, pp. 987-1005, July-Aug. 1977.