Evaluation of the thermal resistance of AlCu electromigration test structures

MICROELECTRONICS RELIABILITY

PERGAMON

Microelectronics Reliability 40 (2000) 1317-1322 www.elsevier.com/locate/microrel

Evaluation of the thermal resistance of A1-Cu electromigration test structures A. Braghieri a

a, I. De

Munari

a, M.

Impronta b, A. Scorzoni c

Centro MTI, Universit?t di Parma, parco Area delle Scienze 181/A, 43100 Parma, Italy b CNR - Istituto LAMEL, via P. Gobetti 101, 40129 Bologna, Italy c Dipartimento di Ingegneria Elettronica e dell'Informazione (DIEI), Universit?t di Perugia, via G, Duranti 93, 06125 Perugia, Italy

Abstract The knowledge of the actual temperature of a metal line is fundamental in electromigration tests. It depends on the feasibility of thermal resistance measurements on the test structures. This work deals with the typical problems associated with the evaluation of the thermal resistance of AI-Cu test structures. As a consequence of significant copper-precipitation-induced resistance drops during high temperature electrical and thermal characterisation of the lines, it is shown that the derived thermal resistance value is a decreasing function of the measurement elapsed time. Thermal characterisations performed at temperatures lower than those typically used allowed us to overcome this kind of problem. © 2000 Elsevier Science Ltd. All rights reserved.

1.

Introduction

In an accelerated electromigration (EM) life test, a high constant temperature T and a high constant current density j are applied to the test line until a given failure criterion is reached. The activation energy Ea of the relevant EM process can be calculated from Black's equation stating that the Median Time to Failure (MTF) is given by [1] MTF =

Aj -n exp(Ea/kBT )

(1)

where A is a phenomenological parameter, n is the so called "current acceleration factor" and kB is Boltzmann' s constant.

Since the MTF depends exponentially on the reciprocal of the test temperature T, it is evident the importance of knowing the actual temperature the metallization undergoes during the test, which is given by the sum of the oven temperature and the temperature increase AT due to Joule self-heating induced by the high stress current. The evaluation of AT implies a complete electrical and thermal characterisation of the test structures to be tested, from which the Temperature Coefficient of Resistance (TCR) of the test line should be derived and used in the subsequent AT assessment. The dependence of the electrical resistance of aluminum-based metallizations on temperature is very well approximated by a linear function of the temperature [2]. Starting from this fact, both the JEDEC Standard 33-A [3] and the proposal for a

0026-2714/00/$ - see front matter. © 2000 Elsevier Science Ltd. All rights reserved. PII: S0026-2714(00)00155-4

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A. Braghieri et aL/ Microelectronics Reliability 40 (2000) 1317-1322

standard presented within the PROPHECY project [4] suggest, for TCR evaluation, to measure the resistance of the metal line at four or more different temperatures, uniformly distributed over a range that should include the test temperature. The TCR is then calculated from the ratio between the slope S and the intercept R(0) at T = 0 ° C of the straight-line determined after an unweighted least-square fitting procedure of the measured resistance values. The goodness of this approximation can be characterised by the correlation coefficient r obtained from the above mentioned fitting procedure. The correlation coefficient can therefore serve as a convenient indicator of a possible measurement problem. Correlation coefficients greater than 0.999 should, in fact, indicate that the resistance vs. temperature measurements have been adequately performed. According to the proposed procedure [4], an estimation for the temperature increase due to Joule self-heating can be obtained by applying the stress current IH to the test structure at the stress temperature TH. After a time long enough to account for thermal response of the specimen (in the case of packaged test chips, the thermal response time might be many minutes long), the electrical resistance Rn of the test line is measured. The temperature increase due to Joule self-heating induced by the stress current In can be then estimated by A T = [R H - R( T H ) ]/ S

(2)

where R(TH) is the electrical resistance of the test line measured at TH applying a measurement current sufficiently low to avoid Joule self-heating. The temperature rise due to Joule heating can also be written as A T = RthP = RthRH 12

(3)

where P is the electrical power dissipated in the line while Rth is the thermal resistance between the line and the heated sample holder. The combination of Eqs. (2) and (3) yield gth = [R H - R ( T H )]/SRH [21_ I

(4)

which is used to determine the thermal resistance by means of electrical measurements only. It should be noted that the thermal resistance is a physical property of the specimen/heated-sampleholder system and therefore, despite the form of

Eq. (4), it does not depend on the current at which its evaluation is carried out and it is only weakly dependent on the temperature. Instead, the thermal resistance is strongly influenced by the materials surrounding the metallization, by the package and by the way the samples are mounted. An accurate estimation of the thermal resistance of each specimen to be tested can therefore serve as a useful tool to detect possible differences in the test conditions of samples belonging to the same measurement batch.

2.

Experimental results and discussion

A set of EM life tests has been scheduled on 30001xm long, 0.45~tm wide and 0.80~tm thick A1-Cu lines with a TiN/Ti barrier, a TiN ARC and ended with two longitudinal in-line vias. Each test structure was packaged in 40-DIL ceramic packages. The structures were mounted on a temperature-controlled hot plate enclosed in a thermally isolated chamber. Before every EM run, electrical and thermal characterisations of the samples to be tested have been performed and the thermal resistance between each sample and the heated surface has been evaluated, according to the standard procedures, at the current density scheduled for the EM test and at a sample holder temperature close to the desired stress temperature. From a characterisation of 14 samples to be tested at T=220°C and j = 1MA/cm 2, negative thermal resistance values have been derived notwithstanding the quite good correlation coefficients (greater than 0.9996) obtained in the TCR determination. Since no measurement problems have been detected, we interpreted these senseless results as a consequence of phenomena such as copper precipitation causing resistance variation on the samples characterised at high temperature. In previous works, significant resistance decays have been observed during the early stages of EM in A1-Cu lines [5,6,7]. They have been justified as a consequence of CuA12 particles precipitation in the host A1 matrix. Copper precipitation is a typical thermally activated process therefore it makes sense to account for the role it could play during electrical and thermal characterisation of A1-Cu test structures since the temperature the metallizations undergo in these cases might be high enough to give rise to the phenomenon. In other words during electrical and

A. Braghieri et aL/ Microelectronics Reliability 40 (2000) 1317-1322

thermal characterisation steps, the electrical resistance of the metallizations could change just because of the characterisation procedure itself. Regarding the way the thermal resistance measurements could be influenced by electrical resistance instability, it should not be difficult to realise that in case of time-dependent electrical resistance, the quantity R H - R ( T H), as well as any other

derived

quantity

such

as

the

estimated

value /~th for the thermal resistance

Rth ,

becomes a

function of the elapsed time between R(TH) and R~ acquisition. Thus, if copper-precipitation-induced electrical resistance decay is fast enough and/or the measurement elapsed time is too long, thermal resistance assessment would easily yield negative values. In order to verify this hypothesis, a set of experiments has been performed on two nominally identical samples. One sample underwent a preparation step, i.e. it was kept at a constant temperature of 220°C for about 60 hours with the specific aim to pre-stabilise its electrical properties by inducing as much copper precipitation as possible. After this step, electrical and thermal characterisations of the as-received sample and of the pre-stabilised sample were accomplished in two different temperature ranges. A "Low Range" (LR), spanning from 60°C up to 120°C, was first considered in order to avoid (or at least minimise) the effects of copper precipitation process, which is supposed to be very slow at these temperatures. Then a "High Range" (HR), spanning from 160°C up to 220°C, was examined. In both cases, the TCR was estimated measuring the test line electrical resistance at four different temperatures uniformly distributed in the corresponding range. Furthermore, in order to achieve a good thermal stability of our system, the electrical resistance measurements have been taken one hour after the desired T value was set up on the temperature controller. In the LR, the TCR evaluation fitting procedure yielded very high correlation coefficients (greater than 0.999995) for both samples. Such a good correlation coefficient have been obtained also in the HR but for the pre-stabilised sample only while, for the as-received one, the correlation coefficient was not better than 0.9998. The observed correlation coefficient degradation is consistent with the hypothesis of significant electrical resistance drops occurring in the as-

1319

received sample during its characterisation in the HR. In fact, the higher is the temperature the metallization undergoes, the faster the copper precipitation process and the heavier the corresponding electrical resistance decay induced in a given time. Since the TCR is derived from electrical resistance measurements performed at different temperature values and equally delayed in time, the copper-precipitation phenomenon will yield to a distortion (and not to a simple rotation) of the R vs. T plot, which lowers the degree of linearity. The fractional changes A/~th(t)/A/~th(0)of the estimated thermal resistance value as a function of time t are reported in Figs. 1 and 2, which are referred to as the LR and the HR measurements respectively. In these figures the starting time is defined as the time at which the first RH measurement was acquired, i.e. two minutes after the acquisition of R(TH) and one minute after the application of the stress current IH. AS one can see from data reported in Fig. 1, the thermal resistance evaluation achieved in the LR does not seem to depend both on the annealing procedure and on time within an acceptable 10% variation due to long term fluctuation of the heated surface temperature. This behaviour still stands in the HR only if the pre-stabilised sample is considered (curve (a) of Fig. 2).

Appreciable/~th changes

are

in

fact

detectable in the case of the as-received sample (curve (b) of Fig. 2) for which the assessed thermal resistance value decreases with time making easy to argue that should the measurements have been carried on for a long enough time, ]~thwould have gone negative. On the other hand, basing on the fact that during the first two minutes the samples seem to behave more or less in the same manner, one could object that there is no reason to wait such a long time between R(TH) and RH acquisition. It is nevertheless worth noting that for a different measurement system or for a different type of package (or both) the time necessary to the samples to reach a steady state condition after the application of the stress current IH could be longer than just two minutes. Moreover, what we set to verify here is simply the hypothesis that copper-precipitation-induced errors in the thermal resistance evaluation are really possible. Under this point of view, the fact that only in the HR the as-received sample exhibits a different behaviour with respect to the pre-stabilised one can be

A. Braghieri et al./ Microelectronics Reliability 40 (2000) 1317-1322

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Fig. 1. Fractional changes of the estimated thermal resistance vs. time for the pre-stabilised sample and for the as-received one, Data were acquired at T= 120°C starting from one minute after the application of the stress current density jH = 1MMcm 2.

Fig. 2. Fractional changes of the estimated thermal resistance vs. time for the pre-stabilised sample and for the as-received one. Data were acquired at T=220°C starting from one minute after the application of the stress current density jH = 1MMcm 2.

considered as a first rough confirmation of our explanation for physically flimsy results occurred in the previously reported thermal resistance assessments. The next step was to thermally stabilise the asreceived sample also and then to repeat the electrical and thermal characterisation. The as-received sample was indeed kept at a temperature of 220°C for three days and during this time its electrical resistance R was continuously monitored and compared to the resistance of the prestabilised sample kept at the same thermal and electrical conditions. It should be noted that a very small current density (j = 0.07 MA/cm 2) has been used for the continuous resistance measurements in order to avoid any possible EM-induced resistance change on the samples. The observed fractional electrical resistance changes AR(t)/R(O) as a function of time t are plotted in Fig. 3. The electrical resistance of the pre-stabilised sample appears to be substantially constant confirming also the absence of any EM effect. On the contrary, the electrical resistance of the as-received sample monotonously decreases with time following a curve, which was found to be fairly well fitted by a linear combination of two exponential decay functions with two different time constant values.

From the data in Fig. 3, a rate of resistance decay of the as-received sample can be estimated and used to calculate the corresponding rate of /~thchanges reported in Fig. 2. The achieved decrease of about 3.5%/minute agrees well with the curve (b) of Fig. 2 confirming that responsible for the observed progressive drop of /~th vs. t is the decay of the test line electrical resistance occurring during the thermal resistance characterisation. During the TCR evaluation accomplished on both the samples after the thermal stabilisation of the as-received one, correlation coefficients greater than 0.999996 have been obtained both in the LR and in the HR. Fig. 4 shows the fractional J~thchanges vs. time obtained in both LR and HR thermal resistance characterisation of the as-received sample after it was thermally stabilised. Now the thermal resistance estimation is no longer dependent on time regardless the particular range of temperatures considered. Comparing Figs. 2 and 4, we can state that, dealing with as-received test structures, timedependent thermal resistance assessments are to be expected when executed at temperatures sufficiently high to give rise to a significant copperprecipitation-induced electrical resistance drops. Nevertheless, once the samples are thermally stabilised at a given temperature (220°C in our case), time-independent thermal resistance evaluations

A. Braghieri et al./ Microelectronics Reliability 40 (2000) 1317-1322

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Fig. 4. Fractional changes of the estimated thermal resistance vs. time for the as-received sample after it was thermally stabilised. Data have been taken at T = 120°C (LS) and at T = 220°C (HS) starting one minute after a stress current density j = 1MA/cm 2 was applied.

become feasible at any temperature lower than the stabilisation one. On the other hand, it should be considered that it could not be acceptable to thermally stabilise a sample to be tested in an EM lifetime run because in this way a Time To Failure (TTF) underestimation is to be expected. From this point of view, it should be possible to obtain time-independent thermal resistance estimation without resort to any thermal stabilisation procedure simply characterising the sample in the LR, as a comparison of Figs. 1 and 4 would clearly suggest.

refine a new procedure.

thermal

resistance

evaluation

Acknowledgments The authors would like to thank the CNRMADESS II project for financial support, Mr. F. Dieci for his contribution in performing part of the presented measurements and Dr. C. Caprile (ST Microelectronics) for helpful discussions.

References 3.

Conclusions

In conclusion we have shown that the standard procedures [3,4] may be inadequate if used to evaluate the thermal resistance of A1-Cu lines using the same thermal and electrical conditions to be used during the EM lifetime test, i.e. at the stress temperature. The experimental data presented would suggest a safe thermal resistance characterisation using a temperature range, which spans between 60°C and 120 °C. Once these observations will be confirmed on a large number of samples, it could be necessary to

[1] F. Fantini, J.R. Lloyd, I. De Munari, and A. Scorzoni, <>, Microelectronic Engineering, Vol. 40, pp. 207221, 1998. [2] H.A. Shafft, J.S. Suehle, <>, Solid-State Electronics, Vol. 35, No. 3, pp. 403410, 1992. [3] Standard Method for Measuring and Using the TCR to Determine the Temperature of a Metallization Line, EIA/JEDEC33-A, (Revision of JEDEC Standard No. 33), October 1995.

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A. Braghieri et aL/ Microelectronics Reliability 40 (2000) 1317-1322

[4] A. Scorzoni, M. Impronta, I. De Munari, and F. Fantini, ~A Proposal for a Standard Procedure for Moderately Accelerated Electromigration Tests on Metal Lines~, Microelectronics and Reliability, Vol. 39, pp. 615-626, 1999. [5] J.E. Sanchez, Jr., V. Pham, ~dnterpretation of Resistance Changes During Interconnect Reliability Testing~,, Mat. Res. Soc. Symp. Proc., Vol. 338, pp. 459-464, 1994. [61 A. Scorzoni, I. De Munari, R. Balboni, F. Tamarri, A. Garulli, and F. Fantini, ~Resistance Changes due to Cu Transport and Precipitation During Electromigration in Submicrometric A1-0.5%Cu Lines~, Microelectronics and Reliability, Vol. 36, No. 11/12, pp. 1691-1694, 1996. [7] A.G. Domenicucci, R.G. Filippi, and K.W. Choi, ~Effect of Copper on Microstructure and Electromigration Lifetime of Ti-A1Cu-Ti Fine Lines in the Presence of Tungsten Diffusion Barriers~, J. Appl. Phys., Vol. 80, No. 9, pp. 4952-4959, 1996.