‘High-field-Induced Degradation in Ultrathin Sio2 Films

. IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 35. NO. 12, DECEMBER 1988 2259

High-Field-Induced Degradation in Ultra-Thin SO2 Films

PIER0 OLIVO, THAO N. NGUYEN, MEMBER, IEEE, AND BRUNO RICCO, MEMBER, IEEE

Abstract-Very thin thermal oxides are shown to exhibit a failure mode that is undetected by conventional breakdown tests. This failure mode appears in the form of excessive leakage current at low field and is induced by high-field stresses. The stress-induced oxide leakage is permanent and stable with time and thermal annealing. It becomes the dominant failure mode of thin oxides because it always precedes destructive breakdown. Experimental results and theoretical calculations show that the leakage current is not caused by positive charge generation and accumulation in the oxide. It is proposed that the oxide leakage originates from localized defect-related weak spots where the insulator has experienced significant deterioration from electrical stress. The leakage conduction mechanism appears to he thermally assisted tunneling through the locally reduced injection harrier, and the model seems to be consistent with both I-V measurements at temperatures from 77 K to 250C and theoretical calculations.

I. INTRODUCTION HE integrity and reliability of the insulators ( < 10 T nm ) are of great importance to metal-oxide semicon-

ductor (MOS) ultra-large-scale integration (ULSI) tech- nology and have consequently received increasing atten- tion in the last decade [ 11-[3]. Early publications [ 13, [2] reported that the integrity of thermal oxide decreases with reducing film thickness while their reliability increases. More recently, further work has shown that the integrity of thin oxide can be maintained by careful processing [4] and exploiting the fundamental aspects of oxide growth kinetics [5], while there seems to be a general consensus that reliability (for instance, as measured by the time to failure rsD) improves significantly with decreasing oxide thickness [3], [6 ] , [7]. With regards to reliability, however, it must be stressed that all the data presented so far in the literature have been obtained with the same techniques used for thick insulators, which, as shown in this work, can lead to very serious errors and overestimations.

The conventional breakdown tests assume that oxide breakdown causes a large and sudden increase in electrical conductivity (Le., shorting of the electrodes in MOS structures). Under such an assumption, the breakdown gate voltage V,, is determined from a large jump in the

Manuscript received April 19, 1988; revised July 25, 1988. P. OIivo was with the IBM Thomas J. Watson Research Center, York-

town Heights, NY 10598. He is now with the DEIS, Universith di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy.

T. N . Nguyen is with the IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598.

B. Ricco is with the DEIS, Universia di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy.

IEEE Log Number 8823672.

measured current of the voltage-ramped time-zero-breakdown (TZB) test. From VBD, the breakdown field FBD can then be calculated if the insulator thickness, flat-band voltage, and semiconductor band bending are known (these parameters must be determined with great care in the case of thin insulators [8]). In addition to FED, the oxide quality is often characterized by the total injected charge prior to breakdown QBD and the time to breakdown tBD. These quantities are measured from time-dependent breakdown (TDB) experiments where a constant voltage/ current is applied to the device until a sudden current/ voltage change is detected.

In this paper we show that catastrophic breakdown related to the large current jump is not the dominant failure mechanism of very thin oxides. A new failure mode in the form of excessive leakage current at low fields is observed after stressing thin oxides at high fields and occurs long before destructive breakdown [9].

The characteristics of the oxide leakage are studied in detail using a variety of measurement techniques. The implications of the oxide leakage to the results of conventional TZB and TDB tests are discussed. The origin and conduction mechanism of the leakage current is then investigated. Finally, a model .for the oxide degradation during high-field stress is proposed.

11. EXPERIMENTS AND RESULTS

A . Experimental Conditions

The following experimental results have been obtained from n+ polycrystalline Si gate-thermal oxide-n+ Si substrate MOS capacitors whose area was cm2, but devices with an A1 gate and different substrate types and resistivities have also been considered and have been found to produce qualitatively similar results. The gate oxide was grown in O2 + 4.5-percent HC1 at 800C up to a final thickness to, varying from 5.1 to 9.7 nm. Sub- sequently, the oxide was annealed in Ar ambient.

I-I/ curves have been obtained using an HP4 145B semiconductor parameter analyzer and an HP4 140B picoam- meter. Constant-voltage TDB measurements have been performed by means of an HP4140B. High-frequency C-Vplots were carried out by means of an HP4275A in- ductance-capacitance-resistance (LCR) meter. Measure- ments have been performed at different temperatures from 77 K up to 250C.

0018-9383/88/1200-2259$01 .OO 0 1988 IEEE

2260 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 35. NO. 12, DECEMBER 1988

1 2 3 4 5 6 7 IGate Voltage1 [V I

Fig. 1 . Ramp I-Vcharacteristics of two similar 5.1-nm oxide n+ poly gate capacitors (area of cm2) up to an oxide field of = 12 MV/cm. The initial I -V ramps (solid lines) show no signs of breakdown, while the after-stress curves (dashed-dotted lines) exhibit large low-field leakage currents. Curves a and b have been obtained with a positive gate voltage, c and d with a negative one.

B. Voltage Ramp Stress The sequence of I-V curves shown in Fig. 1 illustrates

the presence of a low-field leakage current after a high field stress. Curve a is the I-V characteristic of a virgin device obtained with a positive voltage ramp ( 5 mV/s) increasing until the tunneling current density I J , I reaches a predefined value (0.1 A/cm2, corresponding to an average oxide field of = 12 MV/cm.) The curve indicates normal behavior (including well known oscillations [lo]) and without visible signs of breakdown. However, when the measurement is repeated (curve b ) , a large current increase at low voltages is evident, indicating that a change has occurred during the previous ramp. At high voltages, the current is still essentially identical to that from the first ramp, and the entire I-Vcurve does not show any features resembling the breakdown occurring in much thicker oxides. Similar behavior is found by applying negative voltage ramps to the device (curves c and d ).

The same result can also be obtained by ramping up and down the gate voltage without disconnecting the device from the measurement setup. Therefore, it is different from the low field variations that were observed only after a disconnection of the device and attributed to a charge redistribution within the oxide [ 111.

I) Stress Polarity: The behavior of the leakage current induced by opposite stress polarities can be observed from the curves of Fig. 2. The dot-dashed curves ( c and d ) represent, respectively, the after-stress characteristics measured with positive gate voltage after positive (curve a ) and negative (curve 6 ) gate voltage-ramp stress on similar devices. For each polarity, the voltage ramp stress was stopped when the tunneling current density lJ,l reached 0.1 A/cm2 to insure an identical electric field being applied to the oxide. The similarity of the after- stress I-V curves, which were reproducible in many pairs of devices, suggests that the leakage current is independent of the interface from which electrons are injected into the oxide.

2) Repeated Ramps: The leakage current can be in-

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Fig. 2. Dependence of the leakage current on the stress polarities. Curves c and d represent after-stress I - V characteristics with positive gate voltage on two similar devices. Curve c was obtained after a positive gate voltage stress (curve a ) , curve d after a negative voltage stress (curve b ) . The maximum oxide field ( = 12 MV/cm) was the same for both stresses.

creased progressively by repeating the voltage ramp. When the stop voltage of the ramps is kept constant, the low-field leakage current is enhanced, in the form of a vertical I-V shift, after each ramp, but the additional enhancement gradually diminishes (Fig. 3(a)). The enhancement depends on the stop voltage and tends to di- minish quickly after the first few ramps with high stop voltage. When the stop voltage is successively increased after each ramp, the amount of leakage current enhancement becomes more or less constant and exhibits a strong dependence on the size of the voltage increase. Fig. 3(b) shows the I-V curves of the same capacitor after repeated ramps with increasing stop voltages. The leakage current begins to increase after the ramp with 5.2-V peak voltage and the oxide broke destructively at 5.9 V during the last ramp (curve i ).

3) Temperature Behavior: The temperature dependence of the leakage current has been studied by repeating the I-V measurements on one capacitor at 23, 100, 150, 200, and 250C and on a similar device (but bonded and packaged for cooling purposes) at 23C and at liquid-nitrogen temperature. The voltage ramp of these measurements was limited to 4.5 V, which had been determined from previous experiments to have no measurable field- stress-induced oxide leakage during the sequence of measurements. The temperature cycling of the test devices could potentially introduce thermal stress and affect the oxide tunneling current. The thermal stress effect was therefore monitored by performing I- V measurements (at room temperature) before and after each temperature cycle. For all cases, the I-V characteristics were identical, suggesting that the thermal stress effect was insignificant. The result of measurements at 77 K , 23"C, lOO"C, and 250C are shown in Fig. 4 by solid curves a , b, c , and d, respectively. The test devices were field stressed at room temperature by ramping voltage until the tunneling current density IJ, I reached 0.1 A/cm2. The stress- induced leakage was measured at the same set of temperatures using the same procedure as carried out previously. The thermal stress was found to have no effects on the

OLIVO et ai.: HIGH-FIELD-INDUCED DEGRADATION IN SiO, FILMS 2261

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(b) Fig. 3 . Leakage current increase induced by repeated voltage ramps. (a)

Sequence of curves (from a , virgin device, t o j ) with the same maximum voltage (5.5 V). The leakage current increase depends on the maximum applied voltage. With 6 V applied (Fig. 1, curves a and b ) , the after- stress characteristic is reproduced by successive ramps. (b) Dependence of the measured current on the stop voltages of I-V ramps. The voltage ramp is repeated on the same capacitor with increasing stop values: 5 V (curvea, virgin sample), 5.2 V (curveb), 5.4 V (curvec), 5.5 V (curve d),5.6V(curvee),5.7V(curvef),5.8V(curveg),and5.9V(curve h ) . The leakage current begins to increase after the ramp with 5.2-V peak voltage and the capacitor broke destructively at 5.9-V during (curve i ) ramp.

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Fig. 4. Before (solid lines) and after-stress (dashed-dotted lines) characteristics at different temperatures (77 K (curve a ) , 23OC (curve b ) , 100C (curve c), and 250C (curve d ) ) . Curves b , c, and d were obtained from the same device, curve a from a similar device presenting the same virgin I-Vcharacteristic at room temperature. The experimental conditions are described in the text.

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Fig. 5. Dependence of the leakage current on the oxide thickness. Solid and dashed-dotted lines represent I-V curves on virgin and stressed sam- ples, respectively. The oxide thickness was: 5.1 nm (curve a ) , 6.0 nm (curve b ) , 7.5 nm (curve c), and 9.7 nm (cu rved) .

oxide leakage and the oxide leakage is stable at high temperatures. The data for 77 K, 23"C, lOO"C, 250C measurements are shown as dashed-dotted curves a, 6 , c, and d , respectively, in Fig. 4.

As temperature rises, both the stress-induced oxide leakage and the tunneling current before stress increase, but the temperature behavior is very different. For the oxide leakage, the temperature effect is larger at low fields, and the field dependence is weaker at high temperatures. The tunneling current from unstressed devices shows a constant increase with temperature at all fields as one would expect.

4) Oxide Thickness Dependence: The oxide-thickness dependence of the leakage current has been investigated using capacitors with different oxide thickness but pre- pared by the same fabrication process. Fig. 5 shows the I- V characteristics before (solid) and after stress (dot- dashed) for 5.1 nm (curve a ) , 6.0 nm (curve b ) , 7.5 nm (curve c ) , and 9.7 nm (curve d ) thermal oxides subjected to the same stress conditions. The low-field leakage phenomenon becomes increasingly significant in very thin oxides while the predominant effect of electrical stress in thicker films is electron trapping, as indicated by the positive voltage shift in case d .

C. Constant Voltage Stress The oxide leakage current can also be induced by a con-

stant voltage stress commonly used in wear-out or TDB tests. Fig. 6 illustrates the progressive development of the leakage current (calculated as the difference between the measured and the initial tunneling current) as a function of the injected charge density, obtained by periodically interrupting the stress (5.5 V) and monitoring the current at low gate voltages. The stress-induced leakage begins to occur at low levels of electron fluence and long before catastrophic breakdown. The leakage increases with time and electron fluence at a decreasing rate and becomes essentially saturated beyond an electron fluence of lo2' cm-2. The destructive breakdown occurred at 95 h of stress at a fluence of 3.5 x cmP2 or 5600 C/cm-2.

IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 35. NO. I ? . DECEMBER 1988

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Fig. 6 . Increase of the low-field leakage current (calculated as the difference between the measured and the initial tunneling current) as a function of electron fluence for a capacitor stressed at a constant voltage of 5.5 V . The stress was periodically interrupted to monitor the current increase at 2 .5 V (curve a), 3 V (curve b ) , 3.5 V (curve c ) , 4 V (curve d 1 , and 4.5 V (curve e ) until catastrophic breakdown (tbd = 95 h, Qbd = 5600 C/cm*).

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stresses similar to those of Fig. 1. The dashed line represents the C-V plot after a positive gate voltage stress and is practically indistinguish- able from the virgin curve. The dashed-dotted line is the C-V plot after a negative voltage stress.

D. C- V Meusurements C-V plots have been performed on virgin and stressed

devices as shown in Fig. 7. A noticeable C-V shift corresponding to positive charge within the insulator is observed after a negative voltage stress. This result is similar to that for thick oxides [12]. The positive voltage stress produces insignificant shift in the C-V curve.

111. DISCUSSION A . Implications of Stress-Induced Low-Field Leakage

Most conventional TZB and TDB tests cannot detect the occurrence of stress-induced oxide leakage at low voltages because the measurements and breakdown determination concentrate in the high-current and high-field re- gion. During a high-field stress such as the voltage ramp stress illustrated in Fig. 1, the oxide was degraded and became leaky, but this degradation was not at all obvious from the original I- V curve. Only after repeated I - V measurements at low voltages did the oxide leakage reveal

itself. Although the oxide leakage is not a catastrophic breakdown, it presents a serious problem and should be recognized as an oxide failure. Many devices such as dynamic random access memory (DRAM) and electrically erasable-programmable read-only memory (E2PROM) cannot tolerate much leakage currents, especially at low fields comparable to the devices' operating conditions. These devices will fail when the oxide becomes suffi- ciently leaky, not when the oxide breaks down catastro- phically.

Because oxide leakage always precedes destructive breakdown, it becomes the dominant failure mode of thin oxides. It is more relevant, from the device application viewpoint, to determine the onset of a critical oxide leakage rather than catastrophic breakdown. The criterion for oxide failure should be derived from the leakage require- ment of specific device application. For ramp tests, the voltage inducing the critical current is defined as the failure voltage to differentiate from conventional destructive breakdown voltage. The failure voltage is difficult to determine exactly but can be estimated by repeating I-V ramps and increasing the stop voltage by a small increment, as shown in Fig. 3(b), until the critical leakage current is observed. The accuracy of the failure voltage improves with a decreasing voltage increment. However, small voltage steps result in a large number of I-V ramps before the critical leakage is detected, and many repeated ramps can potentially weaken or wear out the thin oxide and lower the failure voltage. A similar technique [13], in which the I-V ramps are replaced by a series of voltage pulses with increasing amplitude (the device is stressed during the high-voltage level of the pulse and the leakage current is monitored during the low-voltage level), can minimize the oxide wear out. Based on an arbitrary current criterion of 10-pA leakage current at 3 V, the failure voltage of the device of Fig. 3(b) is between 5.7 and 5 . 8 V. The conventional TZB test applied to similar devices on the same wafer produces a very tight distribution of destructive breakdown voltages, centered around 7.5 V ( F E D ,= 14.6 MV/cm). It should be noticed that the destructive breakdown voltage is 30 percent higher than the failure voltage, and the conventional TZB test overesti- mates the field strength of thin oxides.

For constrant stress tests, the total injected charge to failure and the time to failure can be defined as the cu- mulative charge and time at which the critical leakage current is produced. These parameters should be differ- entiated from Q B D and f B 0 at the destructive breakdown event. The total charge and time to failure can be determined by periodically interrupting the stress and monitoring the oxide leakage, as illustrated in Fig. 6, until the critical leakage current is reached. Using the failure criterion of 10 pA at 3 V as before, the device of Fig. 6 fails at an electron fluence of lOI9 cm-? (1 .6 C/cm2) after about 100 s of stress. This device exhibits catastrophic breakdown at QBD = 5600 C/cm2 and = 95 h, which represents a more than three-orders-of-magnitude increase from the values of the charge and time to failure.

OLlVO el al . : HIGH-FIELD-INDUCED DEGRADATION IN SiO, FILMS 2263

Part of this increase might be due to the relaxation induced by the interruption of high-field stress [14], nec- essary to monitor the leakage current. However, it is rea- sonable to conclude that the conventional TDB test that determines destructive breakdown in the high-current/ voltage regime also leads to overestimation of the actual reliability of thin oxides.

B. Origin and Mechanism of Oxide Leakage

Low-field current enhancement similar to Fig. 1 has been observed in ultra-thin-oxide metal-gate capacitors after negatively biased stress [lo]. The effect was attributed to the generation and accumulation of positive charge centers located near the Si-SiO, (anode) interface. These charge centers create potential wells that reduce the tunneling barrier and enhance the current injection. It was also reported that the positive charge could be annealed out at relatively low (100C) temperatures, and its density could be determined from the C-V shift. However, stud- ies on the effects of stress polarity and C-Vmeasurements suggest that the current increase observed here is not due to this kind of positive charge.

First of all, the after-stress Z-V characteristics (Fig. 2 ) are essentially independent of the polarity used during the stress. This implies that if accumulated charges are responsible for the oxide leakage they must be symmetri- cally distributed within the oxide. Any positive charge near the anode interface, with reversed bias, would end up at the cathode interface, and its influence on the tunneling current would be completely different. The inde- pendence of stress polarity can be explained by a similar distribution of positive charge being generated by each stress and located at the midpoint of the oxide film.

This positive charge should then produce an identical negative voltage shift in the C-V curve after each stress. The C-V data of Fig. 7 reveal that there was indeed positive charge in the oxide, but the negative voltage shift is considerably smaller for positive than for negative voltage stress. In either case, the magnitude of the shift is too small to account for the current enhancement. One can still argue that the positive charge is very localized or un- evenly distributed in the lateral dimension and results in a small average charge density from C- V measurements. Yet it is difficult to reconcile the different effects of the positive charge generated by opposite stresses on the I-V and C- V curves.

In a recently reported model [15], high-field catastrophic breakdown in oxides is attributed to localized accumulation of trapped holes at weak spots in the oxide. The trapped holes enhance current injection by increasing the cathode field and lead to the formation of local high- field/current density regions. The total current is then composed of two parallel components: one coming from the weak spots and the other from the remaining good oxide area. This model proposes that positive charge accumulation occurs near the cathode interface rather than the anode interface as in an earlier model [ 101.

The enhancement of tunneling current by positive charge is a positive-feedback process. The enhanced current generates more positive charge, which then increases the current further. The relationship between positive charge and tunneling current is exponential because the positive charge affects the current through reducing the injection barrier or increasing the cathode field. The process can lead to self-destruction of the insulator in a very short time [ 151. Because of the positive feedback and exponential relationship, the current enhancement under constant voltage stress grows at an increasing rate with time and produces a rapid current increase [ 161. The behavior of the leakage current in Fig. 6 is not consistent with the characteristic of positive charge. The current increases at a decreasing rate with time and eventually stops growing. One might argue that the periodic interruptions of the stress could potentially cause the observed behavior through charge relaxation, but the oxide leakage does not decrease with time after the electrical stress is discontin- ued. The recombination of positive charge by tunneling electrons may appear as a plausible explanation for the saturation of the leakage current (Fig. 6), but it is hard to explain an increasing recombination rate when the tunneling electron current decreases because of electron trapping.

Based on these considerations and the inconsistency of I-V and C-V data after opposing stresses, one can rule out the possibility of localized positive charge being responsible for the low-field leakage current. This conclu- sion is also supported on a theoretical basis. Numerical calculations have been performed to evaluate the feasibil- ity of the model of localized positive charge near the cathode interface. The calculations employed the Wentzel- Kramers-Brillouin (WKB) expression to compute the transmission coefficients through a potential barrier. The shape of the tunneling barrier was determined from the one-dimensional Poissons solution.

In general, the injected current can be expressed as I , = AJo T, where A is the device area, Jo the current density hitting the injecting interface, and T the transmission coefficient. The calculation of Jo requires a knowledge of the distribution of injecting levels, which is difficult to determine because of two-dimensional quantum effects at the semiconductor interface. For the sake of simplicity, a sin- gle injecting level at the silicon conduction band edge can be assumed. As a consequence, Jo is assumed to be independent of the oxide charge so that only the transmission coefficient needs to be computed. The obtained results will not be exact but should reveal the qualitative effects of localized positive charge.

In the presence of a localized charge within the oxide, the injected current can be expressed as

where A, and T,, A , and T,, are the area and the transmission coefficients of the good and weak part of the oxide, respectively. From this equation, a global transmis-

2264 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 35. NO. 12, DECEMBER 1988

2 3 4 5 6

Gate V o l t a g e [ V I Fig. 8. Calculated global transmission coefficients in presence of localized

positive charge. The oxide thickness was 5 nm. Traps, whose concen- tration was 8 .5 x 10'' cm-3, were uniformly distributed between 0 and 1 nm (dashed-dotted lines) and between 4 and 5 nm (dotted lines) from the Si-SiO, injecting interface. The ratio between the weak and good area of the device was: IO-'(curve a ) , (curve b ) , and (curve c). The solid line represents the completely good device.

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Gate V o l t a g e [ V I Fig. 9. Calculated global transmission coefficients in presence of localized

positive charge. The oxide thickness was 5 nm. Traps were uniformly distributed between 0 and 1 nm (dashed-dotted lines) and between 4 and 5 nm (dotted lines) from the Si-SiO, injecting interface. The ratio between the weak and good area of the device was lo-'. The trap concen- tration was: 7 . 5 x IOzo cm-' (curve a ) , 8.5 x IO2" cm-' (curve b ) , and 10" cm-j (curve c ) . The solid line represents the completely good device.

sion coefficient T can be defined as

T = - A A , 2; + - T,. A A

Two series of T calculations have been performed. In the first series (Fig. 8), the oxide charge density was fixed at 8.5 X lo2' cmP3 and was uniformly distributed within 1 nm from either interface of a 5-nm SiOz film while the ratio of weak to good oxide area was vaned. In the second series (Fig. 9), the area ratio was kept constant at while the charge density was varied.

From Figs. 8 and 9 it can be concluded that: 1) the location of the localized charge has a very strong influence on the transmission coefficient; 2) positive charge near the cathode interface is not likely to be responsible for the oxide leakage because the resulting I-Vcurve does not agree qualitatively with experiments; 3) positive

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ature. Curvesf, g, h , i are from Fig. 3(b).

charge near the anode interface can, in principle, be responsible for the leakage current. However, the charge density required to account for the leakage is unrealisti- cally high.

At this point we have to rule out the positive charge model on both experimental and theoretical grounds. We believe that the stress-induced oxide leakage originates from localized defect-related weak spots where the insulator has experienced significant deterioration from electrical stress. The deterioration is not of electrical nature but in the form of physical and/or chemical changes because the oxide leakage is permanent and stable with time and thermal annealing.

The conduction mechanism of the leakage current I , was investigated by fitting experimental data obtained at different temperatures with many possible I- V expressions. The Frenkel-Pool and Schottky emission agreed reason- ably well with the field dependence of ZI but only at room temperature and when field-induced barrier lowering and a large series resistance were included. The field emission (Fowler-Nordheim) [17] offers the best fit to both field and temperature dependence and appears to be the most likely mechanism for conduction at weak spots. Fig. 10 plots the fitting curves based on the tunneling equation

ZI = GV2 exp ( - P / V ) (3) and the room-temperature I-V characteristics of the leakage current taken from Fig. 3(b). In (3), G represents a pre-exponential factor containing the area of the weak spot, V the applied gate voltage, and 0 a constant factor depending on the barrier height. If the tunneling barrier can be considered to be triangular, for the sake of simplicity, then its height is 0.91 eV. The curvature of the measurable after-stress I-V characteristics is consistent with a conduction mechanism due to Fowler-Nordheim tunneling through a low barrier. In fact, a flex in the I-V characteristics, denoting a transition of the conduction mechanism from direct to Fowler-Nordheim tunneling [18] is present only in the virgin devices. Very good fitting to all measured I-V curves is achieved by changing G and keeping the barrier height constant.

OLIVO el a / . : HIGH-FIELD-INDUCED DEGRADATION IN SiO, FILMS

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ferent temperatures. Curves a (77 K), b (23"C), c (IOO"C), and d (250C) are from Fig. 4. The resulting triangular equivalent barrier heights are: 1 . 1 eV (curve a ) , 0.91 eV (curve b ) , 0.85 eV (curve c ) , and 0.66 eV (cu rved) .

Experiments on the temperature dependence of the tunneling current show that, as temperature increases, the virgin current rises but maintains the same field dependence. On the contrary, the leakage current displays a weaker field dependence at high temperatures, and the temperature effects tend to vanish at high fields.

The fitting of experimental curves at temperatures varying from liquid nitrogen to 250C (Fig. 11) to the tunneling expression (see (3)) reveals that the equivalent triangular barrier height reduces from 1.1 eV at 77 K to 0.66 eV at 250C. The experimental temperature dependence and the reduction of the equivalent triangular barrier height are consistent with the thermally assisted tunneling process with low injection barrier.

The decrease in effective barrier height can be attributed to an increase of the thermal energy of electrons as temperature rises. The temperature behavior of the leakage can be explained with the aid of Fig. 12, which plots the distributions of electrons injected through a barrier height of 1.1 eV at different temperatures for two values of gate voltage. The distributions are computed from the product of the energy-dependent transmission coefficients and the distribution of electrons in the conduction band, based on the method in [19]. At low voltage (Fig. 12(a)), the tunneling transmission coefficients are small and the distributions are strongly dependent on the Fermi-Dirac prob- aDi!ity of ~ccupancy , which varies exponentially with temperature. The maximum value of the distribution is rather the same for temperatures over 0C but, at the highest temperatures, it is far from the bottom of the conduction band, assumed as a reference, and the distribution itself broadens over a large energy interval. Hence, the oxide leakage should be sensitive to temperature at low fields. At high voltage (Fig. 12(b)), the tunneling transmission coefficients are very large and the distributions become less dependent on the energy of electrons. The distributions of injected electrons peak near the bottom of the conduction band and the temperature effects are practically negligible.

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of 1 . 1 eV at different temperatures for two values of gate voltage. The oxide thickness was 5 nm. The temperatures were: 77 K (curve a ) , 23C (curve b ) , 100C (curve c), and 250C (curve d ) . (a) gate voltage of 2 V; (b) gate voltage of 4 V.

The following model for the evolution of thin-oxide degradation is proposed. Very thin thermal oxides have localized weak spots that probably come from imperfec- tions such as submicrometer particles and small-scale surface roughness on the silicon surface before oxidation. High-field stress destroys the integrity of thin oxides at these weak spots by changing the oxide physically andlor chemically. This change leads to a reduced tunneling barrier and causes a local enhancement of the tunneling current that appears as the stress-induced oxide leakage at low fields. The onset of this enhancement depends on the magnitude of the electrical stress and the leakage current progressively increases with the continuation of the stress. The parallelism of the after-stress I-V characteristics (see Fig. 10) indicates that the leakage conduction maintains the same field dependence as the oxide proceeds toward catastrophic failure. -This property, along with the simulation results showing that only the area-dependent parameter G need be changed in order to fit different characteristics, suggests that: 1) the weak spots propagate laterally (i.e., enlarging the defect area instead of pene- trating through the oxide film) and 2) the barrier height remains constant during the evolution of the weak spots. The total defect area widens gradually with time but tends to saturate after some time (Figs. 3(a), 6). The constant triangular equivalent barrier height also suggests that the reduced tunneling barrier is not due to accumulation of defects but to the formation of a different and well defined phase of the involved material.

2266 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 35. NO. 12. DECEMBER 1988

The described phenomenon is present (or, at least, is measurable) predominantly in thin insulators. This result may be due: 1) to different growing conditions (as many weak spots will disappear or become less critical in thicker oxides [ 5 ] , [20]) or 2) to the difficulty, in thicker oxides, of creating a low-barrier high-conductivity path connect- ing the two interfaces.

Finally, it should be mentioned that the familiar sudden and catastrophic breakdown occurs some time after the leakage saturation, and it is not yet known whether this event takes place at a new location or at the same weak spots responsible for the increased leakage.

IV. CONCLUSIONS A new failure mode of very thin Si02 films has been

investigated. High field stress was found to produce a leakage current (observable at low field) which cannot be tolerated in many circuit applications and should be re- garded as the first and true oxide failure. conventional time-zero and time-dependent breakdown tests fail to detect this stress-induced leakage and therefore result in significant overestimation of the integrity and reliability data. The origin of the increased leakage is studied and shown to be unrelated to the generation and accumulation of positive charge. It is proposed that the leakage originates from localized defect-related weak spots which experienced significant deterioration during electrical stress and become more conductive due to a low interface barrier. The I- I/ measurements at different temperatures and theoretical calculations suggest that the conduction mechanism of the leakage current is thermally-assisted tunneling through a barrier of about 1 eV. The stress-induced degradation was shown to evolve gradually with time by enlarging defect area.

This work demonstrates that very thin oxides behave differently from thicker oxides and as a result, they require extra care in the measurements as well as data inter- pretation.

ACKNOWLEDGMENT The authors are very grateful to S. A. Cohen for his

help during the experiments, to Dr. A. Modelli (SGS- Thomson) for helpful discussion, to Dr. G. W. Rubloff for comments on the manuscript and encouragement, and to Dr. N. 0. Lipad for support in this work.

REFERENCES [l] S. P. Li and J . Maserjian, Effective defect density for MOS break-

down: Dependence on oxide thickness, IEEE Trans. Electron De- vices, vol. ED-23, p. 52.5, 1976.

[2] Y.-P. Han, J . Mize, T . Mozdzen, T. OKeepe, J. Pinto, and R. Wor- fey. Ultra-thin gate-oxide characteristics and MOS/VLSI scaling implications, in IEDM Tech. Dig. , p. 98, 1982.

[3] I. C. Chen, S. Holland, K. K. Young, C. Chang, andC. Hu, Sub- strate hole current and oxide breakdown, Appl. Phys. Lett., vol. 49, p. 669, 1986.

141 D. A. Baglee, Characteristics and reliability of 100 A oxides, in Proc. 21st Int. Rel. Phys. Symp. (Las Vegas, NV), p. 1.52, 1984.

151 T. N. Nguyen and D. L. Quinlan, High quality 100 A thermal oxide, in Proc. MRS Symp. Materials Issues in Silicon Integrared Cir- cuit Processing. M. Wittmer, J. Stimmell, and M. Strathman, Eds., vol. 71, p. S05, 1986.

161 I . C . Chen, S. Holland. and C. Hu, Hole trapping and breakdown in thin SiO,, IEEEElecrron Device Lett., vol. EDL-7, p. 164. 1986.

M.-S. Liang and J. Y. Choi, Thickness dependence of oxide breakdown under high field and current stress, Appl. Phys. Lett., vol. 50, p. 104, 1987. B. Riccb, P. Olivo, T . N. Nguyen, T. S. Kuan, and G. Ferriani, Oxide thickness determination in thin insulator MOS structures, IEEE Trans. Electron Devices, vol. 35, p. 432, 1988. T. N. Nguyea, P. Olivo, and B. Ricc6, A new failure mode of very thin ( < 5 0 A ) thermal SiO, films, in Proc. IEEE Int. Rel. Phys. Symp. (IRPS), p. 66, 1987. J . Maserjian and N. Zamani, Behavior of the Si-SiO, interface observed by Fowler-Nordheim tunneling, J. Appl. Phys., vol. 53, p. 559, 1982. P. Olivo, B. Riccb, and E. Sangiorgi, High-field-induced voltage- dependent oxide charge, Appl. Phys. Lett., vol. 48, p. 1135, 1986. P. Olivo, B. Riccb, T. N. Nguyen, T. S. Kuan, and S. J. Jeng, Evi- dence of the role of defects near the injecting interface in determining SiO, breakdown, Appl. Phys. Lett., vol. 51, p. 2245, 1987. P. A. Heimann, An operational definition for breakdown of thin thermal oxides in silicon, IEEE Trans. Electron Devices, vol. ED- 30, p. 1366, 1983. Y. Fong, I. C. Chen, S. Holland, J . Lee, and C. Hu, Dynamic stress of thin oxide, in IEDM Tech. Dig. , p. 664, 1986. I. C. Chen, S. E. Holland, and C. Hu, Electrical breakdown in thin gate and tunneling oxides, IEEE Trans. Electron Devices, vol. ED- 32, p. 413, 1985. Z. A. Weinberg and T. N. Nguyen, The relation between positive charge and breakdown in metal-oxide-silicon structures, J . Appl. Phys., vol. 61, pp. 1947, 1987. M. Lenzlinger and E. H. Snow, Fowler-Nordheim tunneling into thermally grown SiOz, J. Appl. Phys., vol. 40, p. 278, 1969. L. A. Kasprzak, R. B. Laibowitz, and M. Ohring, Dependence of the Si-Si0 barrier height on SiO, thickness in MOS tunnel structures, J. Appl. Phys., vol. 48, p. 4281, 1977. J . L. Moll, Physics of Semiconductor. New York: McGraw-Hill.

T. N. Nguyen, P. Olivo, D. L. Quinlan, and G. W. Rubloff, presented at the Amer. Vacuum Soc. Symp., Baltimore MD, 1986.

1964, pp. 84-85.

* Piero Olivo was born in Bologna, Italy, in 1956. He received the B.S. degree in 1980 and the Ph.D. degree in 1987, both from the University of Bo- logna.

Since 1983, he has been an Assistant Professor of Applied Electronics at the University of Bo- logna. In 1986-1987, he was a Visiting Scientist at the IBM Thomas J. Watson Research Center, Yorktown Heights, NY. His scientific interests are in the area of solid-state devices. His research ac- tivities include SiO, physics, electron transport - . .

and trapping through thin SiOl structures, hot-carrier effects in MOS- FETs, oxide breakdown and reliability, MOS measurement techniques, and thin oxide properties. He is currently interested in IC testing, with emphasis on design for testability and fault simulation.

* Thao N. Nguyen (S75-M83) received the B.E degree (honors class I and university medal) from the University of New South Wales, Australia, in 1978 and M.S. and Ph.D. degrees in electrical engineenng from Stanford University in 1980 and 1984, respectively. His undergraduate and grad- uate research work involved device physics, mod- eling, simulation, fabrication, and characterization of power DMOSFETs, CCDs, and small- geometry surface-channel and buried-channel MOSFETs.

He joined the IBM Thomas J. Watson Research Center in 1983 as a research staff member working on exploratory silicon matenals and processes. He became Manager of Growth, Interfaces and Analysis in 1987 His research interests include advanced low-temperature processes (UHV/ CVD epitaxy, PECVD insulators, and CVD tungsten) and their applications to ULSI devices, thin oxide growth and characterization, oxide breakdown and reliability, MOS measurement techniques, and process-induced defects. He has authored or coauthored over 30 technical papers related to these areas and has received an Outstanding Technical Achievement Award from IBM.

Dr. Nguyen is a member of Eta Kappa Nu, Tau Beta Pi, the MRS, the APS, and the Electrochemical Society

OLIVO et ai.: HIGH-FIELD-INDUCED DEGRADATION IN SiO, FILMS 2267

Bruno Ricc6 (M85) was born in Parma, Italy, in 1947. He graduated in electrical engineenng from the University of Bologna, Bologna, Italy, in 1971 and received the Ph.D. degree from the Univer- sity of Cambridge, Cambndge, United Kingdom, in 1975.

While at the University of Cambridge, he worked at the Cavendish Laboratory. In 1980, he became a full Professor of Applied Electronics at the University of Padova, Italy. In 1983, he joined the Department of Electronics of the University of

Bologna. In 1981, he was a Visiting Scholar at the University of Stanford, Stanford, CA, and, later, at the IBM Thomas J. Watson Research Center, Yorktown Heights, NY. His scientific interests concern solid-state devices and integrated circuits. In particular, he has worked on electron transport in polycrystalline silicon, tunneling in heterostructures, silicon dioxide physics, hot-electron effects in MOSFETs, latch up in C-MOS structures, and Monte Carlo simulation. He is also interested in circuit design and testing.

Dr. Ricc6 serves as the Associate Editor for Europe of the IEEE TRANS- ACTIONS ON ELECTRON DEVICES.

‘High-field-Induced Degradation in Ultrathin Sio2 Films

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