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Ohmic contact electromigration
Microelectron. Reliab., Vol. 32, No. 1/2, pp. 167-174, 1992. printed in Great Britain.
0026-2714/92/$5.00 + ,00 © 1991 Pergamon Press plc
OHMIC CONTACT ELECTROMIGRATION ANDREA SCORZONI and GIAN LUIGI BALDINI CNR - Istituto LAMEL, via Castagnoli
I, 40126, Bologna
(ITALY)
and CANDIDA CAPRILE SGS - Thomson Microelectronics,
via Olivetti 2, 20041, Agrate Brianza
(Received for publication
25 September
(ITALY)
1990)
Abstract In thispaper ohnficcontact electromigrationis brieflyreviewed. After a short introductionon the main contact failuremechanisms and technologicalsolutions,the effectof contact scalingon electromigrationis considered. Then, guidelineson teststructuresand lifetestset-up for contact electromigration studies are given, with particular reference to the thermal characterization needed to extract rehable data from lifetestexperiments. Finally,the most recent experimental results obtained with both aluminum based metallizationsand contacts making use of silicides or barrier layers are reviewed and compared.
1
I n t r o d u c t i o n and failure m e c h a n i s m s
One of the main reliability problems to be solved, at the present stage, for VLSI interconnections, is the electromigration (EM) of the contacts on shallow junctions [1-14]. Almninunl based metallizations are usually employed in devices and IC's. The damage induced by EM in A1/Si contacts depends on the direction of the electron current. If current is introduced into a contact (i.e. electrons flow out the contact) Si depletion in the region surrounding the contact and A1 spiking across the junction take place [2]. Moreover, aluminum EM at the oxide steps causes thinning of the conductor and open circuit failure. This is enhanced by gradients in A1 diffusivity caused by temperature gradients located close to the contacts [9]. Finally, when using alunfinum metalhzations containing silicon, if the current flows out through a contact (i.e. electrons are introduced into the contact), E M brings to Si accumulation at the contact surface. As a consequence, two reliability indicators can be defined: a contact resistance increase or a sharp rise in the junction reverse current.
2
Technological solutions
The nmst viable technological solution to prevent, or retard, contact EM lies, at present, in the interposition of a diffusion barrier between Si and the A1 alloy used for the interconneetions. The typical materials used as diffusion barriers are partially oxidized refractory metal films, e.g. W, or their alloys like Ti-W, refractory metal compounds such as nitrides (TIN) or oxides, or a metallic amorphous phase with a high crystallization temperature. These barriers slow down interdiffusion kinetically. They are kinetic barriers, meaning that the barrier and its neighboring fihns are not in thermodynamic equilibriunl.
3
Device
scaling and effect of low a contact resistivity
Several papers in the past have underlined the need for high dopant concentrations at the metal/silicon interface [15],in order to decrease the corresponding values of contact resistivityPc. Typical contact resistivitieslie in the range 10 -7 to I0 -s f/cm 2. The effect of a low contact resistivityon contact reliability has been taken into account in [3-14], where it was shown that the resulting current crowding underneath the contact can give rise to concerns because of the particularly elevated current density at the contact leading edge. In fact, it is conmlonly accepted that the Median Time to Failure ( M T F ) of a contact is related to the current density across the contact through the relationship I:
(1) teq. (l) is usually written substituting d with the total current I. This procedure is obviously incorrect because it doesn't take into account current crowding effects. 167
168
A. SCORZON1 et al. where E~ is the activation energy of the s t a n t , "/peak the peak current density at 1-11, with n increasing with the average By m e a n s of a 1-D model it is simple contact is given by:
prevailing t r a n s p o r t m e c h a n i s m , k s the B o l t z m a n n conthe contact and the value of ,~ is typically in the range current density [5, 11]. to d e m o n s t r a t e [3, 16] t h a t the peak current density in a
I Jpeak ~ m i n ( f t , g ) x w
(2)
where I is the total current, f and w are the length and the width of the contact, respectively, and it = x / p ¢ / R , k is the transfer length of the contact, being R,k the sheet resistance under the contact. If ~ is the scaling factor, considering a constant voltage scaling (which implies I c ( ~ ) and s u b s t i t u t i n g (2) in (1), the M T F for large contacts (g > ft) is cx ~ - 2 n while for small contacts (g < f t ) is ~x ~ a,~.
Using a 2-D simulation p r o g r a m [151 it is possible to calculate the current density distribution in a contact of 1 × l # m 2 , with a tolerance d -- 0.5#m, as a function of the contact resistivity. The average current density in the length Aa, = 0.1pro (starting from the leading edge of the contact) has been calculated for different values of the contact resistivity and s u b s t i t u t e d into eq. (1) to derive the corresponding values of M T F . In Fig. 1 are reported the resulting values of contact resistance and M T F as a function of the contact resistivity. The deternfination of M T F was performed by a s s u m i n g n = l and n = 2 . This represents an optimistic prediction since m o s t of the calculated current densities would imply a higher current exponent. F r o m Fig. 1 it can be seen t h a t a decrease in contact resistivity from 10 -~ to 10-Tftcm ~ is a c c o m p a n i e d by a s u b s t a n t i a l decrease in contact resistance, whereas the corresponding M T F decreases. On the other h a n d , if values of pc in the range 10 - s - 10-Tf~cm ~ are obtained, the corresponding contact resistance does not change appreciably, while the corresponding M T F is lowered by almost an order of m a g n i t u d e . Anyway, it. m u s t be remarked that the model is quite far from being rigorous, since t e m p e r a t u r e effects related to different Joule contributions, as a function of p¢. , are not taken into account. As a conclusion, it's to be pointed out t h a t , based on previous considerations, a potential problem exists as far as EM is concerned, and the effect of an excessively low pc on the e n h a n c e m e n t of the contact failure rate should be better kept in mind.
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Figure 1: Calculated M T F and contact resistance as a function of the contact resistivity for a 1 × l p m ~ t e r m i n a l contact of a device, with alignment tolerance ~ = 0.5/ml and a sheet resistance of lOOfl/[::],
O h m i c contact electromigration
4
169
Test s t r u c t u r e s for e l e c t r o m i g r a t i o n e x p e r i m e n t s
Electromigration experiments on Test Structures (TS) generally constitute the final validation step for a new technology. Due to the interaction of the different layers, the main difficulty to be overcome, for a correct interpretation of contact EM data, is the normalization of the MTF's to the actual temperature of the TS's during the lifetest. In fact the determination of the actual device temperature during the stress is important for two reasons: first to make a correct association between MTF and test conditions, which differ from the nominal ones because of Joule heating; second to be sure that the dispersion of the Times To Failure is related only to the statistical fluctuations intrinsic to the manufacturing process and it is independent of the dispersion of stress conditions. The normalization is especially important when comparing TS's that come from different technologies, where the layers (diffusions, contacts and metal) may have different thermal properties. Moreover it has to be verified that all the layers involved behave as ohnlic resistors. Recently, a method has been presented [14] to separate the contribution of the contacts and of the diffusions to the overall temperature increase, making use of two TS's placed on the same wafer. In this way it is possible both to verify the thermal and electrical behaviour of the layers and to extract the individual temperature coefficients and thermal resistances, for the structures that behave ohmically. For contact EM lifetests, single contact chains should be used (see Fig. 2). In these structures both failure mechanisms are likely to take place. The distance between the two contacts must be kept small to mininfize the series resistance of the structure. This also helps in avoiding the onset of a critical temperature in the diffused region, above which the intrinsic carriers become dominant for conduction [12, 13]. The metM taps should be overdimensioned to avoid EM problems in the metal itself. Moreover the structure should be provided with Kelvin voltage taps for resistance monitoring. A contact to the substrate is also necessary to detect junction leakage current. The second TS that should be employed is a Cross Kelvin Resistor (CKR) [17]. This structure is used for contact resistance measurements as a function of temperature. The CKR consists of two "L" shaped resistors, one diffused and the other metallic, that cross over the contact to be measured (Fig. 3). Contact resistance measurements are performed injecting the current through the diffusion and extracting it at the opposite metal terminal; the voltage drop is detected across the two remaining terminals. Thermal offsets can be reduced by reversing the polarity of the current and taking the average voltage.
5
Thermal characterization
A crucial point of every lifetest is the determination of the actual test conditions for each TS during the stress, both in terms of temperature and of current density. Different temperature increases from item to item arise both due to oven temperature disunifornfities and to the geometrical and thermal resistance differences always present in an homogenous population. These differences reflect in the Joule heating of the samples and therefore affect the observed time to failure (TTF) distributions. In order to be sure that the resulting standard deviation is not influenced by the
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Figure 2: Single chain test structure.
170
A. SCORZON! et al.
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Figure 3: Cross Kelvin resistor test structure. temperature dispersion, each observed TT.F (ti) should be normalized to the temperature T,,, the mean value of the individual actual temperatures Ti, by means of the formula:
1)]
w h e r e t~i is the i-th normalized T T F a n d E a is the activation energy. A n a priori k n o w n value of E ~ can be used, or, better, an iterative procedure should be employed. In order to obtain the actual temperature of each device during the life test, a standard thermal characterization can be defined. It is important to underline that this procedure is appropriate
only in the case of structures with reliable thermal behaviour, i.e. structures in which both the total resistance increases linearly (or at least monotonically) with temperature and the temperature increase is proportional to the dissipated power. In all the other cases the procedure cannot be applied and only approximate methods can be employed. For single contact chains the temperature increase due to Joule heating has contributions both front the diffused region and from the contacts (the contribution of the metal lines can be neglected due to their low resistance). Since these layers have different thermal properties it is hard to estmlate the individual contributions, making use of a structure in which only the series effect of the layers may be observed (as in the TS of Fig. ]). The CKR's therefore have to be employed to analyze the contact resistance behaviour with temperature. 2 A non-linear behaviour with temperature, at least in the high temperature range (T _> 150°C), is expected in the case of Al-l%Si/n+Si single contact chains [14], while the interposition of, e.g., a TiN/Ti barrier layer between A]-I%Si and the semiconductor yields a good linear relationship. Since the resistance of the diffused region between the two contacts increases linearly in the temperature range considered [R.T.-250°C], the reason for the non linearity is to be ascribed to the contacts. To experinlentally verify this assertion, the resistance of CKR structures was measured as a function of temperature and current. At high current levels, a contact resistance decreasing with temperature was measured. A possible explanation of this behaviour can be given keeping in mind that aluminum-doped silicon nodules usually precipitate at the Al-l%Si/n+Si interface after conventional annealing. It is well known that this effect increases as the contact area decreases. As a consequence, spurious interfaces can be formed in parallel to the olmfic contact and complex conduction phenomena may occur as a function of current and temperature.
2To be rigorous, also the C K R suffersfrom parasiticeffectsrelated to the sheet resistancesunderneath and outside the contact [15]. These act as additional resistances,which should be linear with temperature. The parasiticresistance c~n overcome the resistance due to the contact inlerface when very low contact resistivity(_< 10-rncm 2) interfaces are used. In this case, however, the good thermM behaviour of the contact is indeed assured by the negligibilityof the contact interfacecontribution.
Ohmic contact electromigration
5.1
171
Thermal characterization of structures with reliable thermal behaviour
In principle, for structures with reliable thermal behaviour, the temperature of the i-th device T~ during the stress is simply determined~ by measuring the resistance Ri and using the following equation:
( Ri - Roi)
Ti = T0 + -
-
c~i Roi
(4)
where ~i, the temperature coefficient, and R0i, the resistance at a reference temperature To, are obtained in a previous experiment by resistance measurements at different temperatures and low injected current. During this previous experiment the measuring current should be kept low to mininfize Joule heating. Moreover current should be reversed and the measured voltages should be averaged to minimize thermoelectric effects. Unfortunately the effectiveness of this procedure is limited by the Joule heating effect taking place (luring the high current tests. A more efficient approach is to estimate cti and R01 at a.higher current level (at which the thermoelectric effects are negligible); following this approach the thermal resistance Rthl of each device can also be determined. Ia practice, the resistances of the various devices must be measured as functions of both hot plate temperature T,i and current I (the typical temperature range to be explored is between B.T. and 250°C, with current levels depending on the contact window size, e.g. ht the range 3--13 In& tbr 1.5 × 1.51LIn2 windows), keeping in mind that the high current test must be short enough to avoid EM. By means of the relation: Ti = T~i + R t h i R i l "2
(5)
where Ri is the resistance at the stress conditions, and using eq. (4), it is possible to implement a fitting procedure to calculate at the same time cti, R0i and RthI Finally the stress temperature of the i-th device is easily calculated by means of eq. (5). For the single chains the determination of the ~r, 's during the test can be accomplished using the temperature dependence of the intrinsic diodes. The direct voltage, at a fixed current level of the V-I curve, can be expressed as a function of the temperature as: V ( T ) = V'o - f i T (6) where V0 is the direct voltage extrapolated at T = 0°C and f~ is the slope of the straight line. The value of these parameters can be calculated for each device, with a linear regression, by means of a temperature ramp performed right before the lifetest. Typical values for/3 are within 1.82.8 nW/°C.
5.2
Case of non-linear thermal behaviour
We have seen that an effective temperature correction during the lifetest for the AI-I%Si contacts or, in general, for non-ideal structures is not feasible. On the other hand, temperature differences in the order of 10-20°C are expected between the contact chain and the substrate even in the case of very low resistance ohmic contacts [14]. Anyway, apart from sporadic cases, most of the works published in the literature on contact EM neglect in toto this problem. This results in very dispersed values of the exponent n of eq. (1). In fact, as current increases, the temperature difference between the device under test and the substrate increases as well. If this incremental temperature is not taken into account, fitting the experimental MTF's implies the choice of high n values. A possible partial correction, suggested in [14], can be applied when different contact schemes, realized with structures having same geometries and diffused regions, but with ideal and non-ideal behaviours, respectively, are available. In this case the TTF values obtained for the non-ideal contacts can be normalized using, as actual temperatures, the sum of the T,i values and the mean value of the Joule heating obtained for the single chain of ideal structures. The rationale for this procedure is that only a small amount of the temperature increase comes from the contacts, while the dominant Joule heating effect is due to the Si diffusion.
6
Lifetest set-up
A computerized set up must be developed, which affords a separate monitoring of the whole structure resistance and of the junction leakage current (at a reverse bias at which the leakage current of the junction is well characterized, e.g. VR = -10 V) during the lifetest In this way it is possible to check independently on the evolution of the two failure mechanisms for the structures under test. Hot plates with high thermal capacity and accurate temperature control must be used to maintain the TS's at the desired temperature. Special test fixture and thermal insulation must also be used to guarantee a constant surface temperature (within +1.5°C up to 260°C). Each hot plate should have a capacity of about 20 devices. A scanner can be used to perform practically continuous resistance measurements; the leakage current and the substrate temperature should be measured every 15' and the software must automatically bypass each device when the leakage current exceeds a previously defined "failure reverse current" (usually in the range 50-200 pA).
A. SCORZONI et al.
172
All d a t a are stored in a m a s s m e m o r y ; an a u t o m a t i c software n m s t be developed for the extraction of the t e m p e r a t u r e coefficient (a,:) and the t h e r m a l resistance (/~thi) of each device from t h e r m a l characterization data. During the lifetest, the t e m p e r a t u r e and the current should be m e a s u r e d and recorded at the beginning of each m e a s u r i n g cycle, so t h a t current and t e m p e r a t u r e variations can be taken into account during d a t a analysis.
7
R e s u l t s w i t h a l u m i n u m metallizations containing silicon
Electromigration in A1-Si to Si contacts has been studied extensively since late sixties, when Black [1, 2] reported for the first time the formation of etch pits in the contact window. This was due to: 1. Solid s t a t e dissolution of silicon into a l u m i n u m to satisfy the solid solubility limit at the device t e m p e r a t u r e (in [1, 2] the a l u m i n u m was deposited as an alloy with 3 percent silicon on p+ diffused silicon; then very little silicon was initially dissolved from the substrate). 2. T r a n s p o r t of the silicon t h r o u g h the a l u m i n u m and away from the aluminum-silicon interface by n l o m e n t m n exchange between t h e r m a l l y activated silicon solute ions and the conducting electrons. 3. Further dissolution of the silicon in the a l m n i n u m which is continuously being depleted of silicon by electron wind forces. At the s a m e time whiskers a n d hillocks grew from the negative resistor contact region, while voids, some of which grew to form open electrical circuits, developed at the positive resistor t e r m i n a l j u s t d o w n s t r e a m (in t e r m s of electron flow) from the contact. F o r m a t i o n of a l u m i n u m etch pits in n+Si was also reported by Gargini et al. {5], together with step coverage failure around the thinning edge of the positive contacts of single chains. In their study, l t t m thick, passivated Al-1%Si was used. Failures were defined as lea~affe failures when total current to the s u b s t r a t e exceeded the arbitrary value of 100#A under 10 V reverse bias at r o o m teml)erature. W h e n e v e r resistance doubled, the contacts wore said to fail by opening. Leakage failures h a d an activation energy of 0.9 :k 0.1 eV, typical of diffusion of Si in A1. In this case the M T F was found to be proportional to the j u n c t i o n depth squared (X 2) and a current density acceleration factor n in the range 1-10 was found. T h e open failures had an E~ = 0,5 eV, with r~ = 2.3, attd were detected only when a CVD t u n g s t e n barrier was interposed between Al-1%Si and Si. It was found t h a t at operating conditions, for 3 × 3pro 2 contacts, failure at the contact sidewall d o m i n a t e s leakage current failure. This was no longer true for 1.5 × 1 . 5 # m = contacts. In b o t h cases, "classical" conductor opening by EM was negligible. Similar results were obtained by Ondrusek et al. [11] with A l - l % S i / n + S i contacts. T h e failure m e c h a n i s m was silicon depletion at the positive contacts, with an activation energy of 0.84 eV a n d n =5-11. C h e r n et al. {8], using Al-l.5%Si on n + and p+ diffused silicon, detected an increasing leakage current only in contacts to n+Si, t h o u g h etch pits formed also in A1-Si/p+Si contacts. To justify this behaviour, they hypothesized that Al contacting n-type Si s u b s t r a t e forms a Schottky harrier, the area of which is negligible with respect to the total j u n c t i o n area. Very low current levels and a wafer level set-up were used in [8]. As a consequence, a current acceleration factor n = 1.2 was evaluated, together with an activation energy of 0.83 eV, in very good agreement with other results. T h e same a u t h o r s studied also the open circuit failure m e c h a n i s m in multiple contact chains [9]. Al-1.5%Si/n+Si contacts were evaluated. A l m n i n u m depletion at the contact close to the positively biased b o n d i n g pad was detected. It was concluded t h a t , while m o m e n t u m transfer between conducting electrons and conductor ions provides the driving force for a l u m i n u m EM, the supply of vacancies plays a key role in determining the location of failure. In this case E~ = 0.5 ± 0.1 eV (typical of A1 grain b o u n d a r y migration) and n = 2.5 ! 0.5 were extracted. Because of this low activation energy, open failure is likely to occur at lower t e m p e r a t u r e s . This conclusion is in agreement with the results of Gargini et al.. On the other h a n d , because of the smaller current acceleration factor for leakage failure with respect to open failure (1 vs. 2.5), leakage failure is likely to occur at a lower current level. Caprile et al. [14] studied the A l - l % S i / n + S i metallization scheme with single contact chains having 1.5 × 1.5#m 2 contact windows. Two failure criteria were adopted: A R / R o >_ 20% and a leakage current I/ > 50tLA. W i t h the failure criterion of A R / R o >_ 20% they found the usual value of E~ - 0.9 eV. Failure analysis showed contact windows completely filled with silicon. T h e y pointed out t h a t the entity of this effect m i g h t have been influenced by the overdimensioned m e t a l stripes t h a t constitute an a h n o s t infinite silicon reservoir. For the leakage current increase the calculated activation energy was Ea = 0.53 eV. From failure analysis two morphologies of degradation were identified. One is the typical pitting of AI in the silicon substrate; the other is related to electromigration induced Si depletion in the A1 just above the contact area, so t h a t A1-Si interdiffusion takes place a n d brings to the j u n c t i o n short.
Ohmic contact elcctromigration
8
R e s u l t s w i t h silicides or barrier layers i n t e r p o s e d b e t w e e n alum i n u m and silicon
Sacrificial n+poly-Si barrier layers were employed by Vaidya et al. [6] in the undoped A1/n+poly Si/n + Si metallization scheme. They considered contact windows of 2 and 4 ILnl in size and observed again a high density of electromigrated silicon, together with etch pits, at the positive contacts. As a conclusion the polysilicon was not effective in reducing EM failures. E~ =0.85-1.0 eV and n -- 10 ± t were extracted from life test data. Also in this case the typical relation MTFo¢ X] was found (here Xj nmst take into account the polysilicon thickness). Regarding the extrapolation to operating conditions, opposite results with respect to Gargini et al. were found. In fact the open runner failure mechanism should always dominate at operating conditions in this metallization scheme. The A1-0.5%Cu/(TiN)/CoSi2/n+Si metallization scheme was successively studied by the same group [7]. Contact sizes between 2.5 and 4 #m were employed. Time to failure was defined as the time taken to produce a ~ 1001LA leakage at 10 V. While contacts without the TiN barrier underwent a rapid increase in leakage with time high current stress, contacts with an intermediate TiN layer retained their integrity almost up to the catastrophic failure point. It was concluded that, using an arbitrary activation energy of 0.9 eV, a TiN layer as thin as 350 -~ increases the lifetime of contacts by several orders of magnitude over that of A1/CoSi2 or A1/n+poly-Si. Ondrusek et al. [11] studied the Al-l%Si/TiSi~/(poly-Si} contact scheme to n ÷ and p+Si, with and without a poly-Si layer. They found that the silicide delays junction spiking, probably because of a TiN layer formed during TiSi2 growth in N2. Moreover the silicide improves of two orders of magnitude the Si accumulation at the negative contact, probably due to a lower incidence of initial silicon formation in the silieided contact window during standard 450°C anneal. For Si accumulation the usual value of E~ =0.87-0.88 eV was found. The A1-0.75%Si-0.5%Cu/TaSi2 polycide was studied by Steenwyk et al. [10]. Open circuit failures (R doubled) were detected with high temperature (> 215°C) experiments conducted with eight-window chains. An E~ > 1.1 eV was extracted. Two possible reasons were proposed for this unusual value: 1. The rate of vacancy transport is limited by large grain regions in the head of the contact and by the "barrier" in the floor. 2. Short, finer grained segments are interspersed between large, boundary blocking grains, giving rise to a pressure gradient which retards matter flow along grain boundaries, then causing high Ea. An high value of n (_~ 6) was also reported, probably because the temperature increase due to Joule heating and temperature gradients were unaccounted for. While few Si precipitates were detected on negative contacts at these relatively high temperatures, at lower temperatures (~ 200°C) the 1.25 tLnl windows exhibited a new failure mode, i.e. a non-olmfic I-V characteristic due to Si precipitation at the negative contact window. This is likely to happen because Si transport has a lower activation energy (0.8-0.9 eV) and should be dominant at the lower temperatures. Caprile et al. [14] compared the AI-I%Si/TiN/Ti/n+Si metallization scheme with the A1l%Si/n+Si one (see previous section). The only failure mode identified for the contacts with the diffusion barrier was the leakage current increase due to the spiking of A1 through the junction. For this degradation mechanism an activation energy Ea = 1.42 eV was calculated. A possible failure mechanism is the Si diffusion through the defects in the barrier layer, which open the path to the A1 spiking in the silicon substrate. Due to the presence of the barrier practically no resistance increase was observed with stress time; the failure analysis confirmed that only a very small amount of Si randomly deposited on the barrier.
9
Conclusions
In this paper ohmic contact electromigration has been reviewed. The main contact failure mechanisms and reliability indicators have been described, together with the most important technological solutions to slow-down contact electromigration. The effect of contact scaling and low contact resistivities has been considered. Then, guidelines on test structures and lifetest set-up for contact eleetronfigration studies have been given, with particular reference to the thermal characterization needed to extract reliable data from lifetest experiments. Finally, the most recent experimental resultsobtained both with aluminum based metallizationsand with contacts containing silicidesor barrier layershave been reviewed. Activation energiesof the various failuremechanisms have been reported, and comparisons between resultsobtained by differentauthors have been attempted.
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References [1} J. R. Black, Proc. 6th IEEE IRPS, (1967). [2] J. R. Black, in Ohmic Contacts to Semiconductors, B. Schwartz ed., 1969, p. 311. [3] G. S. Prokop and R. R. Joseph, J. Appl. Phys., Vol. 43, p. 2595 (1972). [4] J. R. Black, Proc. 16th IEEE IRPS, p. 233 {1978). [5] P. A. Gargini, C. Tseng and M. H. Woods, Proc. 20th IEEE IRPS, p. 66 (1982). [6} S. Vaidya and A. K. Sinha, Proc. 20th IEEE IRPS, p. 50 (1982). [7] S. Vaidya, R. H. Schutz and A. K. Sinha, J. Appl. Phys., Vol. 55, p. 3514 (1984). 181 J. G. J. Chern, W. G. Oldham and N. Cheung, IEEE Trans. Electron Devices, Vol. ED-32, p. 1341 (1985). [9] J. G. J. Chern, W. G. Oldham and N. Cheung, IEEE Trans. Electron Devices, Vol. ED-33, p. 1256 (1986). [10] S. D. Steenwyk and E. F. Kankowsky, Proc. 24th IEEE IRPS, p. 30 (1986). {1l] J. C. Ondrusek, C. F. Dunn and J. W. McPherson, Proc. 25th IEEE IRPS, p. 154 (1987). [121 K-Y. Fu and B. E. Pyle, IEEE Trans. Electron Devices, Vol. ED-35, p. 2151 (1988). [13] S. W. Sun and K-Y. Fu, IEEE Electron Dev. Lett., Vol. EDL-10, p. 126 (1989). [141 C. Caprite and G. Specchiulli, Proc. IEEE ICMTS 1989, p. 115. [15] A. Scorzoni and M. Finetti, Material Science Rep., Vol. 3, p. 79 (1988). [16] P. J. Wright, W. M. Loh and K. C. Saraswat, IEEE Trans. Electron Devices, Vol. ED-35, p. 1328 (1988) [17] S. J. Proctor and L. W. Linholm, IEEE Electron Device Letters, Vol. EDL-3, p. 294 (1982). [18] R. W. Thomas et al., ISTFA Proe. 1985, p. 98.
0026-2714/92/$5.00 + ,00 © 1991 Pergamon Press plc
OHMIC CONTACT ELECTROMIGRATION ANDREA SCORZONI and GIAN LUIGI BALDINI CNR - Istituto LAMEL, via Castagnoli
I, 40126, Bologna
(ITALY)
and CANDIDA CAPRILE SGS - Thomson Microelectronics,
via Olivetti 2, 20041, Agrate Brianza
(Received for publication
25 September
(ITALY)
1990)
Abstract In thispaper ohnficcontact electromigrationis brieflyreviewed. After a short introductionon the main contact failuremechanisms and technologicalsolutions,the effectof contact scalingon electromigrationis considered. Then, guidelineson teststructuresand lifetestset-up for contact electromigration studies are given, with particular reference to the thermal characterization needed to extract rehable data from lifetestexperiments. Finally,the most recent experimental results obtained with both aluminum based metallizationsand contacts making use of silicides or barrier layers are reviewed and compared.
1
I n t r o d u c t i o n and failure m e c h a n i s m s
One of the main reliability problems to be solved, at the present stage, for VLSI interconnections, is the electromigration (EM) of the contacts on shallow junctions [1-14]. Almninunl based metallizations are usually employed in devices and IC's. The damage induced by EM in A1/Si contacts depends on the direction of the electron current. If current is introduced into a contact (i.e. electrons flow out the contact) Si depletion in the region surrounding the contact and A1 spiking across the junction take place [2]. Moreover, aluminum EM at the oxide steps causes thinning of the conductor and open circuit failure. This is enhanced by gradients in A1 diffusivity caused by temperature gradients located close to the contacts [9]. Finally, when using alunfinum metalhzations containing silicon, if the current flows out through a contact (i.e. electrons are introduced into the contact), E M brings to Si accumulation at the contact surface. As a consequence, two reliability indicators can be defined: a contact resistance increase or a sharp rise in the junction reverse current.
2
Technological solutions
The nmst viable technological solution to prevent, or retard, contact EM lies, at present, in the interposition of a diffusion barrier between Si and the A1 alloy used for the interconneetions. The typical materials used as diffusion barriers are partially oxidized refractory metal films, e.g. W, or their alloys like Ti-W, refractory metal compounds such as nitrides (TIN) or oxides, or a metallic amorphous phase with a high crystallization temperature. These barriers slow down interdiffusion kinetically. They are kinetic barriers, meaning that the barrier and its neighboring fihns are not in thermodynamic equilibriunl.
3
Device
scaling and effect of low a contact resistivity
Several papers in the past have underlined the need for high dopant concentrations at the metal/silicon interface [15],in order to decrease the corresponding values of contact resistivityPc. Typical contact resistivitieslie in the range 10 -7 to I0 -s f/cm 2. The effect of a low contact resistivityon contact reliability has been taken into account in [3-14], where it was shown that the resulting current crowding underneath the contact can give rise to concerns because of the particularly elevated current density at the contact leading edge. In fact, it is conmlonly accepted that the Median Time to Failure ( M T F ) of a contact is related to the current density across the contact through the relationship I:
(1) teq. (l) is usually written substituting d with the total current I. This procedure is obviously incorrect because it doesn't take into account current crowding effects. 167
168
A. SCORZON1 et al. where E~ is the activation energy of the s t a n t , "/peak the peak current density at 1-11, with n increasing with the average By m e a n s of a 1-D model it is simple contact is given by:
prevailing t r a n s p o r t m e c h a n i s m , k s the B o l t z m a n n conthe contact and the value of ,~ is typically in the range current density [5, 11]. to d e m o n s t r a t e [3, 16] t h a t the peak current density in a
I Jpeak ~ m i n ( f t , g ) x w
(2)
where I is the total current, f and w are the length and the width of the contact, respectively, and it = x / p ¢ / R , k is the transfer length of the contact, being R,k the sheet resistance under the contact. If ~ is the scaling factor, considering a constant voltage scaling (which implies I c ( ~ ) and s u b s t i t u t i n g (2) in (1), the M T F for large contacts (g > ft) is cx ~ - 2 n while for small contacts (g < f t ) is ~x ~ a,~.
Using a 2-D simulation p r o g r a m [151 it is possible to calculate the current density distribution in a contact of 1 × l # m 2 , with a tolerance d -- 0.5#m, as a function of the contact resistivity. The average current density in the length Aa, = 0.1pro (starting from the leading edge of the contact) has been calculated for different values of the contact resistivity and s u b s t i t u t e d into eq. (1) to derive the corresponding values of M T F . In Fig. 1 are reported the resulting values of contact resistance and M T F as a function of the contact resistivity. The deternfination of M T F was performed by a s s u m i n g n = l and n = 2 . This represents an optimistic prediction since m o s t of the calculated current densities would imply a higher current exponent. F r o m Fig. 1 it can be seen t h a t a decrease in contact resistivity from 10 -~ to 10-Tftcm ~ is a c c o m p a n i e d by a s u b s t a n t i a l decrease in contact resistance, whereas the corresponding M T F decreases. On the other h a n d , if values of pc in the range 10 - s - 10-Tf~cm ~ are obtained, the corresponding contact resistance does not change appreciably, while the corresponding M T F is lowered by almost an order of m a g n i t u d e . Anyway, it. m u s t be remarked that the model is quite far from being rigorous, since t e m p e r a t u r e effects related to different Joule contributions, as a function of p¢. , are not taken into account. As a conclusion, it's to be pointed out t h a t , based on previous considerations, a potential problem exists as far as EM is concerned, and the effect of an excessively low pc on the e n h a n c e m e n t of the contact failure rate should be better kept in mind.
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'
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' I
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~
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,
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,
,
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Figure 1: Calculated M T F and contact resistance as a function of the contact resistivity for a 1 × l p m ~ t e r m i n a l contact of a device, with alignment tolerance ~ = 0.5/ml and a sheet resistance of lOOfl/[::],
O h m i c contact electromigration
4
169
Test s t r u c t u r e s for e l e c t r o m i g r a t i o n e x p e r i m e n t s
Electromigration experiments on Test Structures (TS) generally constitute the final validation step for a new technology. Due to the interaction of the different layers, the main difficulty to be overcome, for a correct interpretation of contact EM data, is the normalization of the MTF's to the actual temperature of the TS's during the lifetest. In fact the determination of the actual device temperature during the stress is important for two reasons: first to make a correct association between MTF and test conditions, which differ from the nominal ones because of Joule heating; second to be sure that the dispersion of the Times To Failure is related only to the statistical fluctuations intrinsic to the manufacturing process and it is independent of the dispersion of stress conditions. The normalization is especially important when comparing TS's that come from different technologies, where the layers (diffusions, contacts and metal) may have different thermal properties. Moreover it has to be verified that all the layers involved behave as ohnlic resistors. Recently, a method has been presented [14] to separate the contribution of the contacts and of the diffusions to the overall temperature increase, making use of two TS's placed on the same wafer. In this way it is possible both to verify the thermal and electrical behaviour of the layers and to extract the individual temperature coefficients and thermal resistances, for the structures that behave ohmically. For contact EM lifetests, single contact chains should be used (see Fig. 2). In these structures both failure mechanisms are likely to take place. The distance between the two contacts must be kept small to mininfize the series resistance of the structure. This also helps in avoiding the onset of a critical temperature in the diffused region, above which the intrinsic carriers become dominant for conduction [12, 13]. The metM taps should be overdimensioned to avoid EM problems in the metal itself. Moreover the structure should be provided with Kelvin voltage taps for resistance monitoring. A contact to the substrate is also necessary to detect junction leakage current. The second TS that should be employed is a Cross Kelvin Resistor (CKR) [17]. This structure is used for contact resistance measurements as a function of temperature. The CKR consists of two "L" shaped resistors, one diffused and the other metallic, that cross over the contact to be measured (Fig. 3). Contact resistance measurements are performed injecting the current through the diffusion and extracting it at the opposite metal terminal; the voltage drop is detected across the two remaining terminals. Thermal offsets can be reduced by reversing the polarity of the current and taking the average voltage.
5
Thermal characterization
A crucial point of every lifetest is the determination of the actual test conditions for each TS during the stress, both in terms of temperature and of current density. Different temperature increases from item to item arise both due to oven temperature disunifornfities and to the geometrical and thermal resistance differences always present in an homogenous population. These differences reflect in the Joule heating of the samples and therefore affect the observed time to failure (TTF) distributions. In order to be sure that the resulting standard deviation is not influenced by the
/ Sub.
v+
Figure 2: Single chain test structure.
170
A. SCORZON! et al.
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Figure 3: Cross Kelvin resistor test structure. temperature dispersion, each observed TT.F (ti) should be normalized to the temperature T,,, the mean value of the individual actual temperatures Ti, by means of the formula:
1)]
w h e r e t~i is the i-th normalized T T F a n d E a is the activation energy. A n a priori k n o w n value of E ~ can be used, or, better, an iterative procedure should be employed. In order to obtain the actual temperature of each device during the life test, a standard thermal characterization can be defined. It is important to underline that this procedure is appropriate
only in the case of structures with reliable thermal behaviour, i.e. structures in which both the total resistance increases linearly (or at least monotonically) with temperature and the temperature increase is proportional to the dissipated power. In all the other cases the procedure cannot be applied and only approximate methods can be employed. For single contact chains the temperature increase due to Joule heating has contributions both front the diffused region and from the contacts (the contribution of the metal lines can be neglected due to their low resistance). Since these layers have different thermal properties it is hard to estmlate the individual contributions, making use of a structure in which only the series effect of the layers may be observed (as in the TS of Fig. ]). The CKR's therefore have to be employed to analyze the contact resistance behaviour with temperature. 2 A non-linear behaviour with temperature, at least in the high temperature range (T _> 150°C), is expected in the case of Al-l%Si/n+Si single contact chains [14], while the interposition of, e.g., a TiN/Ti barrier layer between A]-I%Si and the semiconductor yields a good linear relationship. Since the resistance of the diffused region between the two contacts increases linearly in the temperature range considered [R.T.-250°C], the reason for the non linearity is to be ascribed to the contacts. To experinlentally verify this assertion, the resistance of CKR structures was measured as a function of temperature and current. At high current levels, a contact resistance decreasing with temperature was measured. A possible explanation of this behaviour can be given keeping in mind that aluminum-doped silicon nodules usually precipitate at the Al-l%Si/n+Si interface after conventional annealing. It is well known that this effect increases as the contact area decreases. As a consequence, spurious interfaces can be formed in parallel to the olmfic contact and complex conduction phenomena may occur as a function of current and temperature.
2To be rigorous, also the C K R suffersfrom parasiticeffectsrelated to the sheet resistancesunderneath and outside the contact [15]. These act as additional resistances,which should be linear with temperature. The parasiticresistance c~n overcome the resistance due to the contact inlerface when very low contact resistivity(_< 10-rncm 2) interfaces are used. In this case, however, the good thermM behaviour of the contact is indeed assured by the negligibilityof the contact interfacecontribution.
Ohmic contact electromigration
5.1
171
Thermal characterization of structures with reliable thermal behaviour
In principle, for structures with reliable thermal behaviour, the temperature of the i-th device T~ during the stress is simply determined~ by measuring the resistance Ri and using the following equation:
( Ri - Roi)
Ti = T0 + -
-
c~i Roi
(4)
where ~i, the temperature coefficient, and R0i, the resistance at a reference temperature To, are obtained in a previous experiment by resistance measurements at different temperatures and low injected current. During this previous experiment the measuring current should be kept low to mininfize Joule heating. Moreover current should be reversed and the measured voltages should be averaged to minimize thermoelectric effects. Unfortunately the effectiveness of this procedure is limited by the Joule heating effect taking place (luring the high current tests. A more efficient approach is to estimate cti and R01 at a.higher current level (at which the thermoelectric effects are negligible); following this approach the thermal resistance Rthl of each device can also be determined. Ia practice, the resistances of the various devices must be measured as functions of both hot plate temperature T,i and current I (the typical temperature range to be explored is between B.T. and 250°C, with current levels depending on the contact window size, e.g. ht the range 3--13 In& tbr 1.5 × 1.51LIn2 windows), keeping in mind that the high current test must be short enough to avoid EM. By means of the relation: Ti = T~i + R t h i R i l "2
(5)
where Ri is the resistance at the stress conditions, and using eq. (4), it is possible to implement a fitting procedure to calculate at the same time cti, R0i and RthI Finally the stress temperature of the i-th device is easily calculated by means of eq. (5). For the single chains the determination of the ~r, 's during the test can be accomplished using the temperature dependence of the intrinsic diodes. The direct voltage, at a fixed current level of the V-I curve, can be expressed as a function of the temperature as: V ( T ) = V'o - f i T (6) where V0 is the direct voltage extrapolated at T = 0°C and f~ is the slope of the straight line. The value of these parameters can be calculated for each device, with a linear regression, by means of a temperature ramp performed right before the lifetest. Typical values for/3 are within 1.82.8 nW/°C.
5.2
Case of non-linear thermal behaviour
We have seen that an effective temperature correction during the lifetest for the AI-I%Si contacts or, in general, for non-ideal structures is not feasible. On the other hand, temperature differences in the order of 10-20°C are expected between the contact chain and the substrate even in the case of very low resistance ohmic contacts [14]. Anyway, apart from sporadic cases, most of the works published in the literature on contact EM neglect in toto this problem. This results in very dispersed values of the exponent n of eq. (1). In fact, as current increases, the temperature difference between the device under test and the substrate increases as well. If this incremental temperature is not taken into account, fitting the experimental MTF's implies the choice of high n values. A possible partial correction, suggested in [14], can be applied when different contact schemes, realized with structures having same geometries and diffused regions, but with ideal and non-ideal behaviours, respectively, are available. In this case the TTF values obtained for the non-ideal contacts can be normalized using, as actual temperatures, the sum of the T,i values and the mean value of the Joule heating obtained for the single chain of ideal structures. The rationale for this procedure is that only a small amount of the temperature increase comes from the contacts, while the dominant Joule heating effect is due to the Si diffusion.
6
Lifetest set-up
A computerized set up must be developed, which affords a separate monitoring of the whole structure resistance and of the junction leakage current (at a reverse bias at which the leakage current of the junction is well characterized, e.g. VR = -10 V) during the lifetest In this way it is possible to check independently on the evolution of the two failure mechanisms for the structures under test. Hot plates with high thermal capacity and accurate temperature control must be used to maintain the TS's at the desired temperature. Special test fixture and thermal insulation must also be used to guarantee a constant surface temperature (within +1.5°C up to 260°C). Each hot plate should have a capacity of about 20 devices. A scanner can be used to perform practically continuous resistance measurements; the leakage current and the substrate temperature should be measured every 15' and the software must automatically bypass each device when the leakage current exceeds a previously defined "failure reverse current" (usually in the range 50-200 pA).
A. SCORZONI et al.
172
All d a t a are stored in a m a s s m e m o r y ; an a u t o m a t i c software n m s t be developed for the extraction of the t e m p e r a t u r e coefficient (a,:) and the t h e r m a l resistance (/~thi) of each device from t h e r m a l characterization data. During the lifetest, the t e m p e r a t u r e and the current should be m e a s u r e d and recorded at the beginning of each m e a s u r i n g cycle, so t h a t current and t e m p e r a t u r e variations can be taken into account during d a t a analysis.
7
R e s u l t s w i t h a l u m i n u m metallizations containing silicon
Electromigration in A1-Si to Si contacts has been studied extensively since late sixties, when Black [1, 2] reported for the first time the formation of etch pits in the contact window. This was due to: 1. Solid s t a t e dissolution of silicon into a l u m i n u m to satisfy the solid solubility limit at the device t e m p e r a t u r e (in [1, 2] the a l u m i n u m was deposited as an alloy with 3 percent silicon on p+ diffused silicon; then very little silicon was initially dissolved from the substrate). 2. T r a n s p o r t of the silicon t h r o u g h the a l u m i n u m and away from the aluminum-silicon interface by n l o m e n t m n exchange between t h e r m a l l y activated silicon solute ions and the conducting electrons. 3. Further dissolution of the silicon in the a l m n i n u m which is continuously being depleted of silicon by electron wind forces. At the s a m e time whiskers a n d hillocks grew from the negative resistor contact region, while voids, some of which grew to form open electrical circuits, developed at the positive resistor t e r m i n a l j u s t d o w n s t r e a m (in t e r m s of electron flow) from the contact. F o r m a t i o n of a l u m i n u m etch pits in n+Si was also reported by Gargini et al. {5], together with step coverage failure around the thinning edge of the positive contacts of single chains. In their study, l t t m thick, passivated Al-1%Si was used. Failures were defined as lea~affe failures when total current to the s u b s t r a t e exceeded the arbitrary value of 100#A under 10 V reverse bias at r o o m teml)erature. W h e n e v e r resistance doubled, the contacts wore said to fail by opening. Leakage failures h a d an activation energy of 0.9 :k 0.1 eV, typical of diffusion of Si in A1. In this case the M T F was found to be proportional to the j u n c t i o n depth squared (X 2) and a current density acceleration factor n in the range 1-10 was found. T h e open failures had an E~ = 0,5 eV, with r~ = 2.3, attd were detected only when a CVD t u n g s t e n barrier was interposed between Al-1%Si and Si. It was found t h a t at operating conditions, for 3 × 3pro 2 contacts, failure at the contact sidewall d o m i n a t e s leakage current failure. This was no longer true for 1.5 × 1 . 5 # m = contacts. In b o t h cases, "classical" conductor opening by EM was negligible. Similar results were obtained by Ondrusek et al. [11] with A l - l % S i / n + S i contacts. T h e failure m e c h a n i s m was silicon depletion at the positive contacts, with an activation energy of 0.84 eV a n d n =5-11. C h e r n et al. {8], using Al-l.5%Si on n + and p+ diffused silicon, detected an increasing leakage current only in contacts to n+Si, t h o u g h etch pits formed also in A1-Si/p+Si contacts. To justify this behaviour, they hypothesized that Al contacting n-type Si s u b s t r a t e forms a Schottky harrier, the area of which is negligible with respect to the total j u n c t i o n area. Very low current levels and a wafer level set-up were used in [8]. As a consequence, a current acceleration factor n = 1.2 was evaluated, together with an activation energy of 0.83 eV, in very good agreement with other results. T h e same a u t h o r s studied also the open circuit failure m e c h a n i s m in multiple contact chains [9]. Al-1.5%Si/n+Si contacts were evaluated. A l m n i n u m depletion at the contact close to the positively biased b o n d i n g pad was detected. It was concluded t h a t , while m o m e n t u m transfer between conducting electrons and conductor ions provides the driving force for a l u m i n u m EM, the supply of vacancies plays a key role in determining the location of failure. In this case E~ = 0.5 ± 0.1 eV (typical of A1 grain b o u n d a r y migration) and n = 2.5 ! 0.5 were extracted. Because of this low activation energy, open failure is likely to occur at lower t e m p e r a t u r e s . This conclusion is in agreement with the results of Gargini et al.. On the other h a n d , because of the smaller current acceleration factor for leakage failure with respect to open failure (1 vs. 2.5), leakage failure is likely to occur at a lower current level. Caprile et al. [14] studied the A l - l % S i / n + S i metallization scheme with single contact chains having 1.5 × 1.5#m 2 contact windows. Two failure criteria were adopted: A R / R o >_ 20% and a leakage current I/ > 50tLA. W i t h the failure criterion of A R / R o >_ 20% they found the usual value of E~ - 0.9 eV. Failure analysis showed contact windows completely filled with silicon. T h e y pointed out t h a t the entity of this effect m i g h t have been influenced by the overdimensioned m e t a l stripes t h a t constitute an a h n o s t infinite silicon reservoir. For the leakage current increase the calculated activation energy was Ea = 0.53 eV. From failure analysis two morphologies of degradation were identified. One is the typical pitting of AI in the silicon substrate; the other is related to electromigration induced Si depletion in the A1 just above the contact area, so t h a t A1-Si interdiffusion takes place a n d brings to the j u n c t i o n short.
Ohmic contact elcctromigration
8
R e s u l t s w i t h silicides or barrier layers i n t e r p o s e d b e t w e e n alum i n u m and silicon
Sacrificial n+poly-Si barrier layers were employed by Vaidya et al. [6] in the undoped A1/n+poly Si/n + Si metallization scheme. They considered contact windows of 2 and 4 ILnl in size and observed again a high density of electromigrated silicon, together with etch pits, at the positive contacts. As a conclusion the polysilicon was not effective in reducing EM failures. E~ =0.85-1.0 eV and n -- 10 ± t were extracted from life test data. Also in this case the typical relation MTFo¢ X] was found (here Xj nmst take into account the polysilicon thickness). Regarding the extrapolation to operating conditions, opposite results with respect to Gargini et al. were found. In fact the open runner failure mechanism should always dominate at operating conditions in this metallization scheme. The A1-0.5%Cu/(TiN)/CoSi2/n+Si metallization scheme was successively studied by the same group [7]. Contact sizes between 2.5 and 4 #m were employed. Time to failure was defined as the time taken to produce a ~ 1001LA leakage at 10 V. While contacts without the TiN barrier underwent a rapid increase in leakage with time high current stress, contacts with an intermediate TiN layer retained their integrity almost up to the catastrophic failure point. It was concluded that, using an arbitrary activation energy of 0.9 eV, a TiN layer as thin as 350 -~ increases the lifetime of contacts by several orders of magnitude over that of A1/CoSi2 or A1/n+poly-Si. Ondrusek et al. [11] studied the Al-l%Si/TiSi~/(poly-Si} contact scheme to n ÷ and p+Si, with and without a poly-Si layer. They found that the silicide delays junction spiking, probably because of a TiN layer formed during TiSi2 growth in N2. Moreover the silicide improves of two orders of magnitude the Si accumulation at the negative contact, probably due to a lower incidence of initial silicon formation in the silieided contact window during standard 450°C anneal. For Si accumulation the usual value of E~ =0.87-0.88 eV was found. The A1-0.75%Si-0.5%Cu/TaSi2 polycide was studied by Steenwyk et al. [10]. Open circuit failures (R doubled) were detected with high temperature (> 215°C) experiments conducted with eight-window chains. An E~ > 1.1 eV was extracted. Two possible reasons were proposed for this unusual value: 1. The rate of vacancy transport is limited by large grain regions in the head of the contact and by the "barrier" in the floor. 2. Short, finer grained segments are interspersed between large, boundary blocking grains, giving rise to a pressure gradient which retards matter flow along grain boundaries, then causing high Ea. An high value of n (_~ 6) was also reported, probably because the temperature increase due to Joule heating and temperature gradients were unaccounted for. While few Si precipitates were detected on negative contacts at these relatively high temperatures, at lower temperatures (~ 200°C) the 1.25 tLnl windows exhibited a new failure mode, i.e. a non-olmfic I-V characteristic due to Si precipitation at the negative contact window. This is likely to happen because Si transport has a lower activation energy (0.8-0.9 eV) and should be dominant at the lower temperatures. Caprile et al. [14] compared the AI-I%Si/TiN/Ti/n+Si metallization scheme with the A1l%Si/n+Si one (see previous section). The only failure mode identified for the contacts with the diffusion barrier was the leakage current increase due to the spiking of A1 through the junction. For this degradation mechanism an activation energy Ea = 1.42 eV was calculated. A possible failure mechanism is the Si diffusion through the defects in the barrier layer, which open the path to the A1 spiking in the silicon substrate. Due to the presence of the barrier practically no resistance increase was observed with stress time; the failure analysis confirmed that only a very small amount of Si randomly deposited on the barrier.
9
Conclusions
In this paper ohmic contact electromigration has been reviewed. The main contact failure mechanisms and reliability indicators have been described, together with the most important technological solutions to slow-down contact electromigration. The effect of contact scaling and low contact resistivities has been considered. Then, guidelines on test structures and lifetest set-up for contact eleetronfigration studies have been given, with particular reference to the thermal characterization needed to extract reliable data from lifetest experiments. Finally, the most recent experimental resultsobtained both with aluminum based metallizationsand with contacts containing silicidesor barrier layershave been reviewed. Activation energiesof the various failuremechanisms have been reported, and comparisons between resultsobtained by differentauthors have been attempted.
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