Polarity dependence of cumulative properties of charge-to-breakdown in very thin gate oxides

Pergamon PII:

Solid-State Electronics Vol. 41,No. 7,pp. 995-999.1991 0 1997Elsevier Science Ltd. All rights reserved Printed in Great Britain S003&1101(97)00012-9 0038-1101/97$17.00+ 0.00

POLARITY DEPENDENCE OF CUMULATIVE OF CHARGE-TO-BREAKDOWN IN VERY OXIDES T. BROiEK*, Electrical

Engineering

Department,

(Recehed

E. C. SZYPER University

and

PROPERTIES THIN GATE

C. R. VISWANATHAN

of California U.S.A.

at Los Angeles,

Los Angeles,

CA 90095,

15 April 1996; in revised form 24 July 1996)

Abstract-The cumulative character of wear-out of thin silicon oxide layers is investigated under unipolar and DC bipolar stress. It is found that for bipolar degradation occurring under constant-current stress, the total QM significantly exceeds values expected from a combination of polarity-dependent progressive wear-out. The results are explained by a model in which regions where degradation takes place under stress of either polarity are separated physically within the oxide film. 0 1997 Elsevier Science Ltd

I. INTRODUCTION Long-term oxide reliability and degradation hardness are crucial characteristics of MOS devices; they are also important parameters for oxide quality evaluation and process characterization. It is well known that wear-out of thin silicon oxide layers has a cumulative, additive character[l]. The time-to-breakdown under high-field Fowler-Nordheim (F-N) injection conditions remains the same, independent of stress continuity: it does not change when the prolonged stress, which eventually leads to breakdown, is broken into a series of stress periods, the last of which causes failure. The total time remains the same. It was also shown recently[2], that the cumulative nature of oxide degradation is also valid for the total charge-to-breakdown (Qw), the amount of charge which should pass through the oxide until breakdown occurs. The above characteristic is almost obvious if one considers the main mechanism of high-field oxide wear-out, namely generation of electron traps in the oxide bulk. Electron traps are constantly generated and filled by F-N injected electrons until the density of traps exceeds some critical value[3,4] which triggers breakdown. Traps, once generated in the oxide layer by stress, cannot be annealed below 8OO”C[2] and remain there, thus lowering the Qbd measured later. Recently, the idea of critical density of trapped charges leading to breakdown was supported by theoretical considerations on polarization effects in dielectric layers[5]. The idea of damage accumulation and the additive character of Qbd is schematically shown in Fig. 1, where straight lines illustrate progressive oxide *On leave from IMiO, Warsaw University of Technology, Warsaw, Poland. 995

wear-out under either stress polarity. Two lines of different slopes represent different degradation rates resulting from different injection conditions of both interfaces and yielding in different Qbd values under a unipolar stress (the polarity dependence of QM has been consistently observed in MOS structures[ I ,6,7]). If wear-out takes place under subsequent stresses of the same polarity, degradation proceeds along a single line, until 100% of degradation (breakdown) is achieved. In contrast, when stress polarity during wear-out is switched to the opposite one, the amount of damage introduced under the first stress (prestress) serves as a starting point for further degradation, which now occurs at a different rate. In Fig. 1 this concept is illustrated as a transition between two degradation lines. If the injection of Q;, under positive gate bias introduces fractional damage corresponding to F(Q&), switching the polarity will translate the state a into the corresponding state a’, after which degradation will proceed beyond a’ until degradation reaches 100%. Similarly, for the opposite case, the transition from b to b’ may be considered. The cumulative feature of QM is often used to determine the amount of charging damage introduced in the oxide during processing steps such as plasma etching and deposition or ion implantation. The measured Qbd of fully-processed devices. when compared to a reference oxide, shows the amount of degradation (fraction of QM) caused by process-related charging. Since the polarity of stress during processing may vary depending on the tool used during a particular step (etching, ashing, implantation), and may also vary across the wafer for a given tool[8], the oxide may undergo a series of high-field injections under different stress polarities and conditions. It is important then, for damage

T.

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24 structures in each) and each group was subjected to a different amount of charge injection (pre-stress) under 0.05 A cme2 F-N injection of either polarity. For both polarities, Qinjwas chosen to be a significant part of QM for each polarity (QM under positive stress-injection from silicon-is usually significantly larger than that under negative stress).

Injected

Charge

3. RESULTS

Fig. 1. The concept of progressive and additive linear degradation of oxide under unipolar and bipolar stress

conditions. The meaning of symbols is explained in the text. evaluation from Qbd analysis, to determine wear-out dependence and cumulative character of oxide degradation under such switchiqg conditions. Since existing experimental data on that issue are limited only to single polarity cases or frequency dependence of QM under unipolar and bipolar AC voltage stress[9], the aim of this work is to conduct a wider study of the additive nature of oxide wear-out.

2. EXPERIMENTAL Experiments were performed on 6.5 nm oxide, n+ poly-silicon gate MOS capacitors on p-type epi-silicon with a gate area of 10-l cm2. Special test structures with contact to the surrounding n+ region were used, allowing us to perform stress of either polarity. Qbd was measured under both positive (electron injection from the substrate) and negative (injection from the gate) F-N stress in a constant-current mode (current density 0.05 A cm-?). Prior to QM tests, capacitors were split into groups (with at least

Qbd values corresponding to 50% of breakdown distribution, measured on pre-stressed and virgin (pre-stress of Q,“, = 0) samples, are summarized in Fig. 2. The results obtained for subsequent stresses of the same polarity may be analyzed directly by simply adding the amount of injected charge. As can be seen from Fig. 2(a), the additive character of degradation holds for Qbd measured under positive stress on devices pre-stressed with positive gate voltage injection. The total QM (Q,“, + measured QM) is independent of pre-injected charge. The data obtained for devices pre-stressed under negative stress and tested under negative gate bias conditions show a similar behavior (Fig. 2(b)) though the total Qbd decreases slightly here as the amount of pre-injected charge increases. The two remaining cases, when the test polarity was opposite to that of pre-stress, show a strong dependence of total Qbd on Q,, during pre-stress. Their analysis requires additional precautions, since in these cases one cannot use total charge-to-breakdown, obtained by simply adding QM + Q,, as a measure of wear-out. To reflect the different in degradation rate (as discussed in Fig. I), we used the ratio of QM under both polarities as a scaling factor. We define a fraction of

After the change of Injection polarity

Electron injection under positive gate bias

-4-t p-sub&.

AND DISCUSSION

/ tl

,

region of trapped electron charge and newly generated traps

Fig. 2. Values of measured QM and total QM (obtained as lQi,,jl+ measured ~QMI)for capacitors pre-stressed with various amount of charge and at different stress polarity. QM was measured with constant-current electron injection under: (a) positive gate polarity; (b) negative gate polarity. Negative values of charge injected during the pre-stress correspond to the negative stress polarity.

997

Polarity dependence of cumulative properties

8) ti i nogatlve pra-stress

1 i

I -10

-5

0

i positive pre-atrraa 5

10

15

20

Charge injected during pre-stress [Clcm~?]

b)

negative pm-stress positive pre-stress

-5

0

5

10

Charge

injected

during

pre-strew

15

20

[C/CnPZ]

Fig. 3. The total Qw, calculated from measured Qw and pre-injected charge, increases as the amount of charge injected during stress of opposite polarity increases. The total QM was calculated using eqn (2) and then compared with QM value of virgin (previously unstressed) devices.

wear-out, which occurs because of pre-stress of a given polarity, as F(QM), a fraction of QM, injected during the stress (Fig. 1). Hence, we can write:

both cases (positive pre-stress and negative Qw test and vice-versa) the total QM increases as Qn, increases. The increase in QM is as high as 25%, when compared to unipolar values. The above results indicate that, under bipolar switching stress conditions, the oxide may withstand higher degradation than lOO%, defined for unipolar case. This suggests that the bipolar stress either annihilates a part of previously introduced damage, or that the amount of traps, generated and filled in the oxide may be larger than the critical concentration. Electron traps, however, are not annealable at room temperature. The hypothesis of larger density of traps (and hence the total trapped charge) is confirmed by measurements of the final voltage, recorded in the last moment of stress preceding breakdown. As can be seen in Fig. 4, the voltage required to inject the same level of current just before breakdown is, in both cases of bipolar stress (positive/negative or negative/positive), appreciably higher than for unipolar cases; for bipolar stress it also depends on the amount of charge injected during the first stress, increasing as Q,, increases. According to the concept of the critical density of electron traps, breakdown is triggered during unipolar stress when the density of generated traps reaches the critical value. The charge stored at these traps also reaches its maximum value, depending on trap occupation level, which is determined by the electric field in the a) Qbd teat stress at waitive oate voltage

zi 0

charge mjectad during p&-stress

n measured charge-lo-breakdown

for positive stress

F(Q&) = Q&/Q&;

for negative stress

F(Q&) = Q,/QG

1

-

(1)

where Qlnj is the amount of charge injected during the pre-stress and QM is the charge-to-breakdown measured at a given polarity on virgin, undamaged

devices. Hence, for devices pre-stressed under positive bias and tested for Qbd under negative gate bias, the normalized total QM will be given by; QM(total) = F(Q&)*Qbd + Qbd(test),

(2a)

-6

-4.5 -3 negative pre-stress Charge

InjeCted

ICI 20 posltlve pm-stress

0

during

pre-stress

[C/CW2]

b,

and for the case of opposite polarities; Q&otal)

= F(QG)*Q;~ + Q&(test).

(2b)

In the above formulas, QM (test) corresponds to the charge-to-breakdown value, determined during the final breakdown test (regardless of any previous stresses). We used eqns (2a) and (2b) to calculate effective values of QM and to compare them with QM for unipolar cases. The results are shown in Fig. 3. As can be seen, even after the scaling procedure, the total effective QM indicates a significant dependence on the amount of pre-injected charge, which contradicts the concept of cumulative degradation. In

-4.5 -3 -6 negalive prs-stress Charge

inlected

0 during

15 20 10 poslllve prestress pra-slrass

(ClCtW21

Fig. 4. Final voltage during constant-current QM tests performed under: (a) positive; (b) negative stress polarity on samples degraded under various pre-stress conditions. When polarities of pre-stress and Qw tests are opposite, the voltage is higher; it also increase as the amount of charge injected during the pre-stress increases.

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as that used in this study, the regions which suffer

injected Charge Fig. 5. Band diagrams of MOS structure degraded under unipolar (left) and bipolar stress (right). For the bipolar case, the stress-induced degradation may take place in two physically separated regions, because of the small oxide thickness.

charge build-up in the oxide affects the magnitude of gate voltage necessary to sustain constant-current injection. The voltage increases continuously during the stress and reaches its maximum value (determined by the amount of electron trapped charge) just before the moment of breakdown. The difference between final voltages recorded in devices tested for breakdown (shown in Fig. 4) infers that the negative charge, built in the oxide after bipolar stress, is higher than the critical value corresponding to breakdown threshold after unipolar stress. The results obtained suggest that the observed dependence of degradation may be specific for very thin oxides. The tunneling distance for the oxide under investigation is almost half of the oxide thickness. Although the whole volume of the oxide gets damaged during stress. it seems that electrons, after being injected by tunneling through a half-oxide thickness barrier, damage more than the remaining half of the oxide, close to the anode (Fig. 5). After switching the stress polarity, the charge trapped in this region affects the effective barrier height and increases the voltage required to sustain constantcurrent injection. The damage occurs now in another portion of the oxide, which served as a tunneling barrier during pre-stress. Thus, for the oxide as thin oxide. Negative

5c-10

la-lo

strong degradation under stress of each polarity are physically separated and the overlap region, in which damage accumulates under bipolar stress is rather limited. The above model is further confirmed by the QM data presented in Fig. 2 for pre-stressed oxides. As can be seen, the QM value, measured under a polarity opposite to that of pre-stress, is almost independent of the amount of charge injected during pre-stress. The pre-stress introduces some amount of damage (as can be concluded from the comparison of Qbd for pre-stressed and virgin oxides), but does not influence the development of breakdown conditions. As shown in the schematic in Fig. 5, the breakdown occurs when the damage level in the region which currently suffers degradation (near the anode) reaches a critical level, and this effect is almost independent of the level of damage in the other regions of the oxide (near the cathode). To further confirm that the electron charge density in the oxide after bipolar wear-out is higher than that obtained after unipolar injection, devices were stressed under various conditions (both polarities, with and without pre-stress of opposite polarity) almost to breakdown. Capacitance-voltage characteristics measured on these devices are shown in Fig. 6. Independent of the stress polarity used for the final stress (which brought the oxide to the degradation level corresponding to the critical density of electron traps), the curves, measured on devices previously pre-stressed with the opposite polarity, are clearly shifted to more positive voltage values. This indicates that the effective density of trapped electrons (and hence that of electron traps) is larger than the critical value for these samples under unipolar stress. Again, the effect can be explained if one assumes a physical separation of regions in which electronic charge builds up under stresses of different polarity. In that case, the validity of critical defect density may still be applied to any localized region of the oxide.

4.

,

,

-2

-1

I

Gab “o&a

1

I

1

2

M

Fig. 6. Capacitance-voltage characteristics of MOS devices stressed almost to breakdown with unipolar and bipolar electron injection under constant-current conditions.

CONCLUSION

Experimental study of oxide wear-out under constant-current unipolar and DC bipolar stress indicates that the concept of cumulative character of charge-to-breakdown does not apply to very thin films. It is found that the charge-to-breakdown under bipolar stress exceeds the values significantly expected from combination of polarity-dependent progressive wear-out. The results show also that the concept of cumulative oxide wear-out cannot be used directly for determination of the level of degradation of very thin oxides (e.g. because of process-induced charging), unless the polarity of previous stress is unambiguously known.

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4. Dumin, D. J, J. Electrochem. Sot., 1995, 142, 1272. Blake, G., Microelectronics Eng., 1995, 28, 55. DiMaria, D. J., App. Phys. L&r., 1996, 68, 3004. Han, L. K., et al., Microelectronics Eng., 1995, 28, 89. Shin, H., et al., IEEE Electron Dev. Lett., 1993, 14, 88. Rosenbaum, E., Liu, Z. and Hu, C., IEEE Trans.

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