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Microelectron, Reliab., Vol. 37, No. 7, pp. 1147-1150, 1997 © 1997 Elsevier Science Ltd

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RESEARCH NOTE

A METHOD FOR SEPARATING THE EFFECTS OF INTERFACE FROM BORDER AND OXIDE TRAPPED

CHARGE DENSITIES IN MOS TRANSISTORS

ZORAN SAVIC Military Technical Institute, Katani6eva 15, 11000 Belgrade, Yugoslavia

and

BRANISLAV RADJENOVI(~-

Institute of Nuclear Sciences "Vin6a", Atomic Physics Laboratory, PO Box 522, 11001 Belgrade, Yugoslavia

(Received 18 January 1996; in revisedjbrm 26 June 1996)

Abstract--This paper presents a procedure for a more accurate separation of interface trap effects in the presence of large border trap densities after irradiation of MOS devices. It is based on the standard subthreshold technique, hut a special measurement procedure is applied which eliminates the drifts produced by border traps via the tunneling effect. The procedure is demonstrated on pMOS dosimetric transistors, and it is shown that it gives different and, we claim, better estimates of interface trap density than standard techniques. © 1997 Elsevier Science Ltd.

1. INTRODUCTION

It has become evident that the terminology for the oxide charges developed in 1979 by the Deal committee [1] concerning oxide traps (that lie within the oxide and do not communicate with the Si) and interface traps (that are located at the Si/SiO2 interface and communicate directly with the under- lying Si) is not appropriate for explaining recent experimental work in the field of Si/SiO2 systems. It turns out that near interfacial oxide traps that communicate with the underlying Si over a wide range of time scales (border traps [2]) can play a significant role in determining MOS device radiation response and long term reliability [3]. The micro- scopic defects responsible for these traps seem to be quite distinct. As resolved by ESR studies, interface traps are concerned primarily with Pb [4] and border [5] (and also oxide [6]) traps are primarily concerned with E' centers.

The line between interface and border traps is hard to draw based solely on electrical measurements. This is especially true for those methods which are based on recording voltage-current characteristics which are usually done quasi-statically or at low frequency in order to achieve reasonable accuracy. These include the well-known subthreshold method of

tAuthor to whom correspondence should be addressed.

McWhorter and Winokur [7], the mobility method of Galloway et al. [8], or the similar method of Dimitriev and Stojadinovi6 [9]. With these methods some border traps could be mistaken for interface traps. This can lead to errors in microstructural models of interface and border traps.

Recently Paulsen and co-workers [10, 11], using a variable frequency charge-pumping technique, were able to distinguish slow and fast states, and attributed them to border and interface trap densities. Similar methods were also used by Fleetwood and co- workers [12, 13], and Autran et al. [14] in estimating oxide-trap, interface-trap and border-trap charge densities. In this paper we suggest an alternative method based on the measurements of the threshold voltage drift.

Starting from our earlier investigation of border trap contributions to the threshold voltage shift in dosimetric pMOS transistors [15] and its connection with the voltage drift, we have analyzed the possibility of improving the standard subthreshold technique so that exact interface trap densities can be found in the presence of large border trap densities.

2. EXPERIMENTAL TECHNIQUE AND RESULTS

Ionizing radiation, hot carriers, high-field stress and many other conditions can lead to threshold voltage shifts in MOS transistors. The subthreshold

1147

1148 Z. Savi6 and B. Radjenovi6

technique for separating the contribution of interface traps and trapped oxide charges to the threshold voltage shift is based on the recording of the ID =f(V~s) characteristic of MOS transistors in saturation. Besides a few physical constants and the doping density of the bulk silicon, this method requires knowledge of the current-voltage data from the subthreshold part of the ID =f(V~s) curve, and also from the above threshold part of that curve. Using these data "midgap" current can be calculated from the theoretical relation and "midgap" voltage Vm~ can be found by extrapolating the subthreshold region of the ID =f(Vcs) curve [7]. The threshold voltage V,h is also found by extrapolation from the plot of the square root of the drain current vs the gate voltage in the above threshold part of the ID = f(VGs) characteristic. The shift of the midgap voltage A Vm~ is then attributed to the influence of trapped oxide charges and the shift of the stretchout voltage A Vso = A V,, - A Vmg is attributed to the influence of interface traps. Once the threshold voltage shift due to interface traps is known, the increase of the number of interface traps, produced by the ionizing radiation, hot carriers, etc. can be determined by

AN. = Co,AV~o/q, (1)

where AN~, [cm-:] represents the increase in total number of interface traps between the midgap and threshold; Co, is the oxide capacitance per unit area given by Co~ = eo~/tox, with to~ being the thickness of the gate oxide and Cox the oxide dielectric constant.

The implementation of the above method is carried out on a specially produced pMOS dosimetric transistor whose ID=f(VGs) characteristics are presented in Fig. 1. Details of its production are given elsewhere [16] and here it is only pointed out that the starting material was (100) n-type Si of 4-7 f~cm resistivity, and that the thickness of radiation soft gate oxide was 0.99 #m. This oxide was formed by dry thermal oxidation (at 1150°C in 02 ambient for 22 rain) and a layer of CVD oxide. The transistor shows a considerable radiation sensitivity of 3.04mV/cGy(Si). The Io=f(Vcs) curves were

10 -2

10 "3

10"4

~ 1 0 - 5

~ 10 -6

10-7

l0 "8

10 -9 3

Fig.

No 13.97 (tox = 0.99 ~m)

/ / / i tr J O D = 0 Gy(Si)

I • D = 20 Gy(Si)

. / . 2 L I 6 9 12

Voltage [V] 15

Gate voltage--drain current characteristics for transistor before and after irradiation.

-0.025

-0.05c

-0.075 I

"~ _O.lOO I

I~ -0.125

-0.151]

-0.175 10 .2

No 13.97 (tox= 0.99 p,m) I = 10 p,A

\ ITIMI v ITIMI " I III

"-. II II IIII

I I JiiJiil O D = 0 Gy(Si) ~ . . • D = 20 Gy(Si)

Jltlt lll J lJ I Ill," 2 3 10-1 2 3 100 2 3 101 2 3

Time Is]

Fig. 2. Threshold voltage drifts for the transistor before and after irradiation.

recorded using the Keithley current source 263 and Hewlett Packard multimeter 3457A. They were generated quasi-statically by forcing 42 values of the current through the transistor and measuring the corresponding voltage values. During the measure- ment the temperature was recorded and, when necessary, the temperature compensation was calcu- lated in data processing. The standard instrumental error was estimated to be smaller than 0.01%. This means that the voltage shift of approximately 0.5 mV should be regarded as real, and not the consequence of noise or instrumental drift. The irradiation was performed at a Co-60 facility at a dose rate of approximately 40 Gy(air)/h. During irradiation the transistor was in short-circuit.

Using the above mentioned data, and the values of voltages and currents from the pre- and post- irradiation ID =f (Vrs ) curves (Fig. 1), it was found that after a dose of 20 cGy(Si) the threshold voltage shift was AVth = 2.253 V. For the sake of convenience the current and voltage are shown with positive signs in Fig. 1, although they are usually shown as negative values. Consequently, threshold voltage shift also has a positive sign. Some 30% of this value, exactly 0.669 V, is attributed to the influence of the charges in interface traps in the conventional subthreshold method [7]. The increase in interface trap density produced by irradiation obtained from relation (1) is AN~, = 1.49 x 10 '° cm -2. This value of the estimated interface trap density is not correct due to the method of recording the ID =f(VGs) curves as we will now explain.

It is well documented [15, 17] that, at least for pMOS transistors with radiation soft oxides, the conventionally defined threshold voltage of irradiated MOS transistors considerably drifts with time after step current excitation. This is shown for the above mentioned transistor in Fig. 2. This voltage is defined as the voltage at 10/~A drain current while drain and gate are in short-circuit. But the drift exists for any applied current and is present all the time during the recording of the ID =f(Vos) curve. This is usually

Interface effects in MOS transistors 1149

done by linear ramp voltage at low frequency or by staircase voltage quasi-statically in order to achieve a reasonable accuracy. For pMOS transistors the drift leads to additional stretch out of the ID =f(V~s) curve and this effect directly leads to an error in estimates of interface trap density using the subthreshold technique.

In our recent paper [15] we have shown that the drift of the threshold voltage can be described by the tunnel effect theory and is concerned with near interfacial oxide traps, or border traps. It depends on a number of factors. It is a function of time duration, received dose, oxide thickness and technology, temperature, and the pre-measurement history. In that paper we have emphasized that the drift is virtually independent of the applied voltage in the range of the recorded ID =f(Vcs) curve and we speculated that this is a consequence of the domination of the local field produced by trapped charges in the vicinity of the Si/SiO2 interface. This means that the drift for equal times after the step excitation is approximately equal no matter whether a 10nA or 10#A current is forced through the transistor. In the case of the N-channel transistors the same does not hold, since they can show different drifts when different currents are forced through them.

For an improved calculation of interface trap density by the subthreshold method, the effect of border traps on the Io =f(Vcs) curves must be eliminated. In other words, the stretchout of the It, =f(V~s) curves produced by the drift during its recording must be corrected. But, as the drift starts some 10-J3 s after the excitation the exact value of the drift voltage is experimentally hard to find. Fortunately, the subthreshold technique is actually mathematically based on voltage differences. This is the case in definitions of A Vmg, A V,h and A V,o, and it can also be shown for extrapolating procedures. So, for the correction procedure the difference of the drift voltage is needed and it is obtainable from experimental measurements like those presented in Fig. 2. Moreover, if the drift of the voltage can be made exactly equal during the measurements of the points of interest on the Io =f(V~s) curves by a special measurement procedure, then the correction of the subthreshold method is automatically done using original measurement results.

It is important to notice that the drift depends on the pre-measurement history. I.e. in succeeding measurements on the same transistor the drift is equal only if there is enough time for border trap charges to relax to their equilibrium value after excitation. If the elapsed time between the succeeding measure- ments is not long enough, some hysteresis voltage will be found and the slope of the drift will be smaller. Such a procedure, of course, does not lead to the same drift for the points on the Io =f(V~s) curves. This is presented in Fig. 3 where the drift voltage for exact timing used for recording the curves on Fig. 1

No 13 (tox = 0.99 I~m) 0.150 0.123

0.096 ~ 0.069

0.042 0.015

,.W. -0.012

~ -0.039

-0.066

-0.093

-0.120 10 -2

I = 10 I~A

o Continuous .e xi.tation

2 10-1 2 10 0 2 101 2 Time [s]

Fig. 3. Threshold voltage drifts for the irradiated transistor obtained by continuous and impulse excitations.

(0.88 s excitation followed by 0.69 s pause for 42 measurements) is shown together with the drift for continuous excitation. It can be seen from Fig. 3 that, although the drift is much smaller than for the continuous case, which refers to the linear ramp or staircase excitation, the drift between the recording points on the ID =f(V~s) curve from Fig. 1 is noticeable and is not equal.

Generally, the drift and the subsequent hysteresis is smaller if the excitations are shorter and pauses between the succeeding measurements are longer. So. in order to eliminate the hysteresis effect one has to perform measurements which are as short as possible and with reasonably long pauses between them. In that case the drifts are repeatable, and if the time between step current excitations and measurements is fixed, then the drifts for different measurements are the same. Following these guidelines we have programmed the measurements with current exci- tations of 55 ms and pauses of 5min for four points on the ID =f(V,~s) curves needed for the extrapolating procedure of the subthreshold method. The measurements of the voltage at each point were carried out 7 ms after the step current excitation.

Using the values of voltages and currents obtained by the above mentioned method, before and after irradiation for the transistor from Fig. 1, it was found that after a received dose of 20 cGy(Si) the threshold voltage shift was AV, h=2.194V. Of this value, 0.608 V can be attributed to the influence of the charges in the interface traps and the increase of their density produced by irradiation can be estimated to be AN, = 1.36 × 10 ~° cm -2. This value is 8.7% smaller than the estimated interface traps obtained previously without drift correction. A larger error than this could be expected with higher border trap charge densities [13] or with a less favorable measurement procedure. We have estimated from Fig. 2 that in the case of linear ramp or staircase excitation the error could be as high as 25%.

1150 Z. Savi~ and B. Radjenovi~

3. CONCLUSION

In radiation soft oxides large densities of border traps can be present after irradiation, or after hot carrier excitation or high field stress. These traps are responsible, via a tunneling process, for the voltage drift after a current excitation of the MOS transistor. This effect distorts voltage-current characteristics of these devices, and this induces errors in standard techniques for separating the contributions of interface and oxide trap densities in the threshold voltage shift produced by ionizing radiation, or some other stress of gate oxides.

In the case of the subthreshold technique we have found that the drift effects can be reduced by a special measurement procedure in which the drifts for needed points of voltage-current characteristics are equal. By investigating and analyzing the drift effect, we have found that this procedure requires measure- ments with as short as possible step current excitation (of the order of ms) and reasonably long pauses (of the order of minutes) between the succeeding measurements.

We have demonstrated by experimental measure- ments on dosimetric pMOS transistors that the above measurement procedure can give different and, we claim, better estimates of interface trap contributions in the threshold voltage shift after irradiation of these devices than the standard subthreshold technique based on recording the ID =f(V~s) curves. In this way, we have clarified the conditions which are to be satisfied if the standard subthreshold method is to be correctly used in the case of a large density of border traps.

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5. Warren, W. L., Shaneyfelt, M. R., Fleetwood, D. M., Schwank, J. R., Winokur, P. S. and Devine, R. A. B., Microscopic nature of border traps in the MOS oxides. IEEE Trans. Nucl. Sci., 1994, NS-41(6), 1817-1827.

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7. McWhorter, P. J. and Winokur, P. S., Simple technique for separating the effects of interface traps and trapped oxide charge in metal-oxide-semiconductor transistors. Appl. Phys. Lett., 1986, 48(2), 133-135.

8. Galloway, K. F., Gaitan, M. and Russell, T. J., A simple model for separating interface and oxide charge effects in MOS device characteristics. 1EEE Trans. Nucl. Sci., 1984, NS-31(6), 1497-1501.

9. Dimitriev, S. and Stojadinovi6, N., Analyzing of CMOS transistor instabilities. Solid State Electron., 1987, 30, 991-1003.

10. Paulsen, R. E., Sergiej, R. R., French, M. L. and White, M. H., Observation of near-interface oxide traps with the charge-pumping technique. IEEE Electron Dev. Lett., 1992, 43(12), 627-629.

11. Paulsen, R. E. and White, M. H., Theory and application of charge pumping for the characterization of Si-SiO2 interface and near-interface oxide traps. IEEE Trans. Electron Dev., 1994, 41(7), 1213-1216.

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13. Fleetwood, D. M., Shaneyfelt, M. R., Warren, W. L., Schwank, J. R., Meisenheimer, T. L. and Winokur, P. S., Border traps: issues for radiation response and long-term reliability. Microelectron. Reliab., 1995, 35(3), 403-428.

14. Autran, J. L., Ballard, B. and Babot, D., 3 level charge pumping study of radiation-induced defects at Si-SiO2 interface in submicrometar MOS transistors. J. Non-Cryst. Solids, 1995, 187, 211-215.

15. Savi~, Z., Radjenovi6, B., Pejovi6, M. and Stojadinovi6, N., The contribution of border traps to the threshold voltage shift in pMOS dosimetric transistors. IEEE Trans. Nucl. Sci., 1995, 42(4), 1445-1454.

16, Savi6, Z. and Petkovi6, D., Development of MOS dosimeter for personal dosimetry application. In Proc. 18 'h Yugoslav Symposium on Radiation Protection, Beograd, May 1993, pp. 139-142.

17. Holmes-Siedle, A. and Adams, L., The mechanisms of small instabilities in irradiated MOS transistors. IEEE Trans. Nucl. Sci., 1983, NS-30(6), 4135-4140.