hot-hole-induced negative oxide charges in n-mosfets

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 8, AUGUST 1995 1473

Hot-Hole-Induced Negative Oxide Charges in n-MOSFET’ s

Werner Weber, Martin Brox, Roland Thewes, and Nelson S. Saks, Senior Member, IEEE

Abstract- We investigate the generation of electron traps by hole injection during hot-carrier stressing of n-MOSFET’s. These generated electron traps are filled by an electron injection following the primary hole stress. The effect is proven and quan- tified by monitoring the detrapping kinetics in the multiplication factor and the charge pumping current. The traps are located in the oxide within the first few nanometers to the interface. An interaction of those traps with interface states is found in that charged electron traps inhibit charging or uncharging of interface states. The kinetics of hot-carrier-induced fixed negative charges in n- and p-channel MOSFET’s are compared showing significant differences in the properties of the two species of traps. Hole- induced electron traps are located much closer to the interface and their energetic level is deeper. Finally, a method is presented that allows the quantification of the effect for reliability purposes. We conclude that under digital and analog operation conditions in which hole effects cannot completely be ruled out, this effect has to be considered.

I. INTRODUCTION N recent years the possible formation of electron traps I by injection of holes into the gate oxide of n-MOSFET’s

has been discussed [1]-[5]. The speculation was that those traps are neutral under stress conditions with dominating hole injection (at low stress gate voltages) but occupied by electrons under dominating electron injection conditions (at high stress gate voltages) following results from homogeneous injection experiments [6]-[8]. However, clear proofs for the case of inhomogeneous injection in n-MOSFET’s that allow to distinguish the effect unambiguously from stress-induced interface states have not been presented. In this paper such a proof is given. Furthermore, the properties of the traps are investigated, a quantification is given and the significance under operating conditions is evaluated. A clarification of this effect is of importance for analog operation with varying gate voltage conditions and for digital operation under over-voltage conditions.

11. METHODS The generation of fixed charge in the oxide shows as a

shift in various electrical parameters. However, for a definite proof, the influence of stress-induced interface states must be

Manuscript received September 17, 1994; revised February 3, 1995. The

W. Weber and R. Thewes are with Siemens Corporate Research & Devel-

N. S . Saks is with the Naval Research Laboratories, Washington, D.C.

M. Brox is with Siemens Components Inc., Essex Junction, VT 05452 USA. IEEE Log Number 9412371.

revew of this paper was arranged by Associate Editor B. Ricco.

opment, 81730 Munich, Germany.

20375 USA.

screened. In the low gate voltage-high drain voltage regime the Fermi level close to the drain is rather low, leaving the stress-induced acceptor-type interface states essentially uncharged. The multiplication factar A4 = IBUb/I~ measured at a gate voltage little above the threshold voltage (typically at 1.25 V for a threshold voltage of 0.8 V) is thus essentially unaffected by interface states. Since fixed charges influence the channel potential and the electric field responsible for impact ionization, M represents a good parameter for detecting negative fixed charges.

Wide-spread use in detecting fixed positive charges has found the left edge position (&-edge) of the charge pumping effect. Research into detecting negative fixed charges from high gate voltage-high drain voltage electron stress of n- MOSFET’s through the charge pumping at the right edge (flatband edge) was, however, inconclusive. A significant shift of the edge was not detectable unless excessively high densities of negative charges were trapped. As an explanation the position of the negative fixed charge in the region of strong doping gradients very close to the drain junction was shown to be responsible where charge pumping becomes increasingly insensitive [9]. Holes, on the other hand, are injected further away from the drain junction and a detection of negative charges located in hole-induced traps might turn out to be feasible.

Finally, from detrapping experiments [ 101, [ 111 it is known that fixed charges of either polarity can be detrapped if their energetic level is not too deep. By applying a negative gate voltage with source, channel, and drain held at zero potential, an oxide field can be established that leads to detrapping of negative fixed charges and thus to a recovery of the stressed device. In our experiments all three above-described methods for investigating negative fixed charges are utilized in one single experiment that employs three different stress phases. First a hole stress is applied by stressing the n-MOS device with low gate and high drain voltage. Then a high-gate, high- drain voltage electron injection condition is used to neutralize the positive fixed charges and trap electrons at the proposed electron traps formed by the injected holes. In a third phase a moderate negative voltage is applied to the gate with zero volts in the channel (Vs = Vo = V, = 0 V) to detrap the negative fixed charges. The third phase is not really a stressing phase (Fowler-Nordheim conditions are avoided) but leads to an annealing of stress-induced damage. We use a negative polarity of the gate voltage for detrapping. Positive charge possibly residing in the oxide even after electron injection has

0018-9383/95$04.00 0 1995 IEEE

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 8, AUGUST 1995 1474

2.0

1.8

1.6

Stress-induced charge pumping current I 10-9 A 2.5 l ' l ' l ' l ' l ' .

~ .. .. . a

~ .

Base level I V Fig. 1 . Charge pumping data during an electron injection (VG = 8 V, VD = 6 V) following hole injection (VG = 1.3, VD = 8 V, t s t r e s s = 1000 s). One charge pumping curve is depicted per decade of electron stress starting at 0.1 s of electron stress time for curve 1 and finishing at IO4 s for curve 6.

then a reduced probability to detrap and thus influence the electron detrapping data.

Before and/or after each of the three phases charge pumping is performed (constant amplitude of 4 V with varying pulse base and a frequency of 100 kHz) and the multiplication factor (VC = 1.25 V, VD = 4 V) taken. During the detrapping phase the multiplication factor alone is used for characterization as it uses only low voltages over a very short period of time. By this procedure a negligible influence of characterization to the density of trapped charges is ensured.

The electron stress phase has been investigated in detail to determine a suitable length in time. For this task charge pumping has been monitored during an extended electron injection. Results are shown in Fig. 1. Up to curve 4 measured after 100 s of electron injection, the predominant effect is the recharging of oxide traps whereas after that time the formation of electron-induced interface states becomes dominant. We chose a constant value of 100 s for the electron stress in all the experiments presented since we want as little as possible interface state generation by the electron injection phase itself.

111. EXPERIMENTAL DETAILS Samples used are from a number of different processes,

all experimental but in rather mature status. Their oxide thicknesses range from 10-20 nm with minimum gate lengths from 0.5-1.0 pm, all CMOS with LDD structure for the n- channel. Additionally devices are used from a process with 40 nm oxide thickness and 2.2 pm gate length from an n-MOS process with conventional Arsenic sourceldrain implantation.

As will be discussed in the sections below the measured phenomena show rather significant differences. Those differences are process-dependent but unexplained and cannot be correlated to known technological features. All data presented explicitly in this paper are from a CMOS process with to, = 16 nm. The reasons for this choice are given below. Channel lengths are 0.8 pm for LDD n-channel and 0.9 pm for

I ." 10-1 100 101 102 103 104 105

Annealing time / s

Fig. 2. Stress-induced increase of the multiplication factor (measured under condition VG = 1.25 V, Vo = 4 V) during the detrapping period (VG = -6 V, VD = Vs = VB = 0) after hot-hole stress conditions (VG = 1.25 V, VD = 8V) with different lengths in time as given in the figure. The intermediate electron injection is performed with VG = 8 V, VD = 8 V for 100 s. For MO the value of the unstressed device is applied.

p-channel devices which have single implanted drain, buried- channel structure. The process is a single metal process with plasma intermetal oxide isolation. A plasma nitride passivation is deposited as top layer.

Data acquisition is in some cases (Figs. 2, 6, and 7) performed with preselected n-MOSFET's to yield matching multiplication factors before stress. Whereas without preselec- tion a variation over several percent is found, after selection only about 1% remain.

IV. RESULTS

Experimental data of the detrapping phase are shown in Fig. 2. The multiplication factor is depicted in Fig. 2 as a function of time the homogeneous oxide field is applied. It is related to the value before stress which yields a baseline at a ratio

The multiplication factor shows an increased value with respect to the virgin device, demonstrating the existence of negative fixed charges. Positive fixed charges present directly after the hole injection appear to be neutralized during the subsequent electron injection by the time the shown experimental data start. Some may, however, persist. Thus, the least conclusion we can draw is that the number of negative fixed charges is higher than the one of positive.

The time dependence in Fig. 2 shows that after lo4 s about half the trapped electrons have disappeared. Depending on the hole injection times, related increases of the multiplication factor and detrapping effects are obtained. Up to this point, the effect of negative fixed charges has been observed in the increase of the multiplication factor and its annealing as a function of time in which a moderate homogeneous oxide field is applied.

In Fig. 3(a) results of the charge pumping current of the loo0 s hole stress experiment (cf. Fig. 2) are depicted.

M/MO = 1.

WEBER et al.: HOT-HOLE-INDUCED NEGATIVE OXIDE CHARGES 1475

Stress-induced charge pumping current / 10-10 A effect clearest. Other processes show the same mechanisms but the results differ quantitatively. Most similar results are obtained in a process with to, = 10 nm, Lgate = 0.5pm, but the hole-induced negative charge effect is weaker. The conventional process with to, = 40 nm, Lgate = 2.2pm shows a very incomplete hole neutralization leading to the lowest multiplication factor after the longest (1000 s) hole stress. The recovery during the third phase is, however, clear and pronounced. A process with to, = 20 nm, Lgate = 1.0pm shows incomplete recovery of the trapped holes and, additionally, very little annealing during the third phase. Only the 1000 s curve shows some recovery.

V. DISCUSSION 1 I

-6 -5 -4 -3 -2 -1 0 +1 Base level / V A. Quant$cation of the Negative Fixed Charge

(a) In the preceding section the existence of hot-hole-induced

Stress-induced charge pumping current / 10-10 A

I I -6 -5 -4 -3 -2 -1 0 +'

(b)

Base level I V

Fig. 3. (a) Charge pumping signal of the IO00 s hole stress experiment in Fig. 2. Differences between data before stress and after different phases of stress are depicted. However, since a large number of interface states is formed locally during the hole stress, the original signal is negligible in magnitude and the result is essentially equal to the direct charge pumping signals after stress. The signal after hole stress, the one after electron stress and the one after the end of the detrapping experiment are depicted. In (b) data of an experiment with 1 s of hole injection are presented.

During hole stress a huge number of positive fixed charges is formed that shift the whole curve by several volts into the negative direction. This is accompanied by the formation of interface states that dominate largely the charge pumping signal. After electron stress the positive charges disappear, at least predominantly. The annealing experiment shows a pronounced shift into the negative direction, proving clearly that negative fixed charges are lost in this procedure. In Fig. 3(b) data of an experiment with only 1 s of hole injection are shown. In this case very little shift of the charge pumping peak shows proving that indeed the hole injection is responsible for the fixed negative charges.

The experiments shown in Figs. 2 and 3 are performed on a process with to, = 16 nm and Lgate = 0.8 pm that shows the

electron traps was proven in different largely independent parameters. In the following we want to quantify this effect. For this to yield meaningful results, the stress-induced shift of the measured parameter such as the multiplication factor or the charge pumping shift must be related to a density of electron traps. Furthermore, a point in the measured parameter must be identified in which zero density of electron traps is assured.

Both the charge pumping and the multiplication factor results allow the independent calculation of the density of charges after stress. We use

AVO, (1) &Si02 EO NF = ~

@ox

for the area density of charges NF with oxide thickness tox. The local flatband voltage shift AV, is directly read from the middle of the falling edge of the charge pumping signal (see arrows in Fig. 3). For the multiplication factor

is used, which is based on an analytical model of the channel electric field and its stress-induced changes in [ 1 11. For MO we use the multiplication factor before stress and for the length Ld of the damaged region a value of 60 nm. The length 1 is given by

(3)

which has a value of O.lpm in this process for a junction depth xj = 0.2pm. X is the impact ionization length (7.3 nm [12]) and @ i m p / q = 1.5 V is the barrier for impact ionization.

Let us now address the problem of relating the data to the point of zero charge density: for the multiplication factor we chose the value of the unstressed device as a reference for zero negative fixed charge (cf. Fig. 2). During the hole injection positive fixed charges are formed. Surely, most are neutralized during the subsequent electron injection, but some may persist. A nonnegligible amount of remaining positive

1476 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 8, AUGUST 1995

charge would, indeed, obstruct the baseline of the charge density calculation. In fact the existence of positive fixed charge cannot completely ruled out in the devices of the chosen process even though definite proofs for their existence are missing. Quite differently, does the process with to, = 40 nm, Lgate = 2.2 pm show very strong evidences for remaining positive charges in that the 1000 s hole stress curve has the lowest multiplication factor after electron stress. The reason why the presented data are from the process with to, = 16 nm and Lgate = 0.8pm is nothing more than the fact that the positive charge can be essentially neutralized. We have, however, no reason to claim this process to be typical.

For the charge pumping the choice of a good reference value of the flat band voltage for zero charge is equally important. The original curve is dominated by interface states along the whole channel while the post-stress curves are dominated by the locations with high numbers of stress-induced interface states including regions of changing doping profiles. Those have charge pumping signals at different voltage conditions. Thus the original charge pumping curve is unsuitable for a reference. Most of the interface states are formed during the hole stress (cf. Fig. 3). Their distribution in space may be somewhat different from the one of the pure electron-stress induced interface states. The best solution to the baseline question would probably be to try and anneal all hot-hole- induced negative charges from one of the devices. However, this cannot be done exactly since measurement times are limited. A good compromise is using the post-annealing value of 1s-hole-stress curve where on the one hand interface state generation is dominated by the hole stress (cf. Fig. (3b)) and on the other hand little electron trap formation occurs which is even reduced after the detrapping phase. In fact there is little difference between this value and the one of the stress experiment with electron stress alone. For this reason we apply this procedure to calculate the reference value.

Results of this calculation are plotted in Fig. 4. Considering the severe simplifications good agreement between charge pumping and multiplication factor is achieved. Obviously the length of the trapping region is known rather unprecisely. The chosen value of 60 nm yields good agreement but some other value between 50 and 100 nm (cf. [13], [14]) would be equally well justified. This range in trapping lengths relates to a variation of the density of traps by a factor of 1.6.

Values of more than lo1' cm-' as found for the density of trapped negative charges are definitely no negligible quantities and should be visible in many transistor parameters. In fact, in the literature evidence is available that by applying appropriate stress conditions those effects are well-detectable in electrical parameters [ 11-[5]. Moreover, pronounced sensitivity to the trapped charges is expected for the differential analog parameters in saturation where interface states are essentially uncharged.

As already pointed out in Section 11, negative charges in n- MOSFET's seemed too difficult to detect in charge pumping experiments in the past. The fact that the hole-induced negative charges are located further inside the channel has definitely helped to improve the sensitivity. Furthermore, it turns out to be of advantage to relate the charge pumping data to the ones

14.0 13.0

12.0 11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0

3.0

Trapped charge density / 10' cm -2

+ + + + + r

t

Annealing time / s

Fig. 4. Area density of negative fixed charges calculated independently from the charge pumping and the multiplication factors according to (1 ) and (2). Data are taken from the measurements whose multiplication factor is depicted in Fig. 2. For the calculation of the trapped charge density a length of the damaged region L of 60 nm is assumed.

of a stressed device with negligible trapped charge. In this way charge pumping currents of two stressed devices are compared both including interface states in regions of changing doping profiles. By this procedure a much more precise evaluation of fixed charges is made possible.

B. Distribution of Fixed Negative and Positive Charges in Space and Time

Let us now discuss qualitatively at which position the negative fixed charge is located. Holes are injected further inside the channel than electrons. However, since the injected electrons are after all responsible for the formation of the negative fixed charges at trapping positions formed before by the holes we deduce that a certain overlap of the distributions of injected holes and electrons inside the oxide must exist (see schematic drawing in Fig. 5). This region of overlapping distributions of injected holes and electrons in the oxide is where the negative fixed charges are located (hatched area in Fig. 5). Moreover, this explains why certain indications exist in some processes of positive fixed charges remaining after electron injection. Regions further inside the channel may not be reached by electrons leaving a certain density of positive fixed charges after electron injection. This may indeed lead to a dominance of positive charges at positions further inside the channel leading to some effect on the multiplication factor. The right edge of the charge pumping signal, on the other hand, should be less affected by this positive charge since it is more sensitive to channel regions with increased values of the flatband voltage as caused by negative fixed charges. The fact that Fig. 4 shows good agreement between both parameters demonstrates that in the process, data of which are presented here very little positive charge remains after electron injection. In fact, a similar plot of data from the process with to, = 40 nm, Lgate = 2.2pm yields rather uncorrelated results between charge pumping and multiplication factor since the two parameters are differently influenced by the remaining positive charge.


Multiplication factor M 4.80 1

4.70 1

I DRAIN L

Fig. 5. Schematic sketch of the distributions of holes and electrons injected into the oxide consistent with the data obtained from charge pumping and electrical parameters.

* . 4.30 1

* * * * * * * * * * * * * * * * *

C. Correlation of Negative Fixed Charge with Interface States

At this point we want to address a peculiar feature of the charge pumping signal during the electron stress in Fig. 1. Starting from the curve 1, a decrease of the maximum charge pumping current is observed, before beyond curve 4 considerable interface state generation starts. Between 0.1 and 100 s of electron stress this decrease is obvious. During this period of time the hole-induced oxide states are being charged negatively. We speculate that those oxide states are somehow connected to interface states in that the latter become inactive or inhibited during the charging of the first by a screening effect. Another possible explanation is a re- formation of chemical bonds that dislocate the interface states into the interior of the oxide during charging by electrons in the oxide. As soon as the states are located deeper in the oxide their interaction with the channel stops and they become fixed negative charges. The connection between interface state and negative fixed charge is obviously reversible since it can also be observed in Fig. 3. There, the curves after detrapping are higher than before. Detrapping of the negative charges releases the interface states again. This finding indicates that the hole-induced electron traps are located close to the interface within a distance low enough to cause efficient screening of the interface states or re-orientation of the affected chemical bonds. This effect prevents interface states from being charged or uncharged as long as hole-induced electron traps in the vicinity are charged negatively.

D. Comparison of Electron- and Hole-Induced Negative Fixed Charges

In the following we address an interesting difference in the annealing behaviors with and without hole injection included in the stress as depicted in Fig. 6. Both show a pronounced increase indicative of negative fixed charge formation. The hole stress experiment shows a pronounced decrease of the multiplication factor as a function of detrapping time, which can only be explained by negative charge detrapping. On the contrary, detrapping of un-neutralized holes would show as an increase of the multiplication factor. The pure electron stress, on the other hand, shows much less recovery. This indicates that the electron-induced negative fixed charges are of different physical nature which could mean that even their

ul 4.10 ' ' ' "' loo ' ' " ' 1 0 1 ' ' "'102 ' ' "1% ' ' "1& 105 10s

Armealing time / s

Fig. 6. Plots of annealing experiments (with VG = -8 V, VD = Vs = VB = 0) after combined hole + electron injection (dots, hole stress with VG = 1.25 V, VD = 8 V, electron stress with VG = VD = 8 V, tstress = 100 s), only electron injection (sun symbols, VG = VD = 8 V, tstress = 100 s), and no injection at all (asterisks). As parameter the multiplication factor is used.

microscopic nature is different. Quantitatively, those data show that electron trapping by pure electron injection is a small effect as compared to the one by hole injection. The 100 s hole- stress curve in Fig. 2 shows an increase of the multiplication factor higher by a factor of five compared to the 100 s electron stress case in Fig. 6, for electron injection currents higher by many orders of magnitude.

The two stress measurements in Fig. 6 are from two preselected samples with nearly identical multiplication factors before stress. Several of such pairs have been measured. While the differences in the annealing behaviors discussed above were reproducible, the relation of the multiplication factors at the beginning of the de-trapping phases was somewhat different. Here an example is shown with the multiplication factor of the 1 s hole stress sample lower than the 0 s sample but we obtain others in which the inverse happens or nearly identical starting values occur. Since holes and electrons are injected in different stress phases, a sensitive interplay of the different polarities of trapped charges and interface states occurs that leads to considerable scatter in the multiplication factors at the beginning of the detrapping phase.

Finally, the unstressed sample does not show any change at all. This proves that no pre-existing fixed charges are present before stress.

In Fig. 2 the effect of detrapping has been demonstrated. A decrease to about 50% of the original value is found for all measurements monitoring different hole stress times. The time dependence is approximately logarithmic similar to the results found for positive and negative fixed charge in [lo]. No indication is available that part of the fixed negative charges be in traps that cannot detrap. A similar behavior was found before in [ 151 and [ 111 from hot-electron injection-induced negative fixed charges in p-MOSFET's. In order to compare the annealing kinetics of the two species of negative charges we performed stress and detrapping ex-

1478 LEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 8, AUGUST 1995

Fig. 7. Annealing data of n-MOSFET’s after hole-stress (V, = 1.25 V, VD = 8 V, tatress = 1000 s) combined with electron-stress (VG = VD = 8 V, tstress = 100 s) to form hole-induced negative fixed charges and of p-MOSFET’s after formation of hot-electron induced negative fixed charges (VG = -2 V, VD = -7 V, tatress = 5 s). The stress conditions are adjusted to get a doubled multiplication factor in the n-channel case and one half in the p-channel case both directly after negative fixed charge formation. The oxide fields for detrapping are calculated by adding 1 V to negative gate voltages to compensate for work function differences (cf. [IO]). The two sets of data with -5 MV/cm are measured with -9 V at the gate while the ones with 5 MV/cm are measured with 8 V. The trap occupation is defined as ( M - Mo)/(Mi - M O ) with the multiplication factors before stress MO and directly after electron stress MI. This parameter has a value 1 for a complete occupation right after stress and 0 for all traps emptied.

periments on p-channel MOSFET’s on the same wafer. In Fig. 7 the detrapping kinetics of hole- (n-MOSFET) and electron-induced (p-MOSFET) negative charges are compared. Whereas the electron-induced charge detrapping shows very little oxide field polarity dependence [ l l ] the hole-induced shows a strong asymmetry. Negligible annealing is evident for positive fields (the small increase of the multiplication factor can be interpreted as annealing of a small amount of positive fixed charge remaining after hole neutralization), negative field annealing is very fast.

This finding allows two important conclusions: the strong asymmetry of the detrapping effect on oxide field polarity indicates that it originates from states within tunneling distance to the interface in the vicinity of the region where also the injected holes are trapped. In fact a similar asymmetry has been found for the detrapping of hole-induced trapped positive charges in [lo]. The annealing of hole-induced negative charges proceeds at a faster speed for negative oxide fields. At -5 MVIcm 50% of the hole-induced charges anneal within 1 s, while it takes lo4 s for the electron-induced which are known to be distributed essentially uniformly [lo], [ll]. The strong asymmetry of the annealing and its high speed for negative polarities represent the second indication for the hole-induced electron traps to be formed within very short distances of just a few nanometers to the interface, the first being the screening effect of interface states discussed in Section V-C.

The second conclusion is drawn from the fact that nearly no detrapping of hole-induced negative charges is observed for positive oxide fields whereas electron-induced show a significant detrapping effect. We conclude that the energetic level of the hole-induced negative charges is much deeper

Mumpl i in factor M/MO 2

1.8

1.6

1.4

I .2

1

7 u6.5 V / /

10-1 100 10’ 102 103 104 105 106

Hole stregs time I s

Fig. 8. Multiplication factor as a function of the hole stress time for different hole stress drain voltages as given in the figure. An increase by 40% is used to define a lifetime.

indicating differences in the microscopic natures of the two species of traps.

VI. CONCLUSIONS FOR OPERATION

The determination of lifetimes of the hole-induced negative charge effect requires more elaborate data acquisition than for the more common interface state effects in n-MOSFET’s [ 161 or the negative fixed charge effects in p-MOSFET’s [ 1 11. Here for each point in stress time a separate stress experiment has to be performed. In Fig. 8 results are shown relating the multiplication factor directly after electron stress to the one of the unstressed device. A lifetime is obtained by interpolating the data. Rather arbitrarily we chose an increase by 40% just to demonstrate the method. Apparently the 6.5 V curve is rather flat and shows scatter. Thus a precise determination of the lifetime is not possible. We select the point at lo5 s of stress time being aware of the fact that error bars allow much longer times.

Those lifetimes are plotted in Fig. 9 as a function of the inverse hole-stress drain voltage. Similar to other hot- carrier effects an increase of the lifetime for lower voltages is observed. Moreover, the curve bends up, so that at least in the devices investigated here no reliability problem is expected. The reason for this bending is yet unclear.

The hole-detrapping effect connected somehow with the formation of the electron traps, or the approach of the effective source-drain voltage (VD - V D , ~ ~ ~ M ~ V) to the minimum voltage for hole injection in a lucky electron model (x4.8 V)

may be responsible. In Fig. 10 the lifetime is plotted as a function of I s U b / I ~ during stress. The slope in this graph is much steeper than the value of 3 characteristic for electron- induced interface states [16]. We obtain a value around 10. Values much greater than 3 have been found before in charge pumping [17] and electrical parameters [2] and attributed to hole-induced effects. Since the negative fixed charge in this experiment is also hole-induced we infer agreement with those findings in the literature.

The data in Figs. 8 through 10 have been taken from the process with to, = 16 nm and Lgate = 0.8pm as chosen


I . , 1 I I , I

0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18

carrier stressing of n-MOS transistors,” IEEE Trans. Electron Devices, vol. 37, pp. 1869-1876, 1990.

[3] J.-C. Marchetaux, M. Bourcerie, A. Boudou, and D. Vuillaume, “Ap- plication of the floating-gate technique to the study of the n-MOSFET gate current evolution due to hot-carrier aging,” IEEE Electron Device Lett., vol. 11, pp. 406-408, 1990.

[4] D. Vuillaume, J.-C. Marchetaux, and A. Boudou, “Evidence of acceptor- like oxide defects created by hot-carrier injection in n-MOSFETs: A charge-pumping study,” IEEE Electron Device Lett., vol. 12, pp. 60-62, 1991.

[5] M. Bourcerie, J.-C. Marchetaux, A. Boudou, and D. Vuillaume, “Optical spectroscopy and field-enhanced emission of an oxide trap induced by hot-hole injection in a silicon metal-oxide-semiconductor field-effect transistor,” Appl. Phys. Lett., vol. 55, pp. 2193-2195, 1989.

[6] H. Uchida and T. Ajioka, “Electron trap center generation due to hole trapping in Si02 under Fowler-Nordheim tunneling stress,” Appl. Phys.

1 0.19 03

Lett., vol. 51, pp. 433-435, 1987. [7] I. C. Chen, S. Holland, and C. Hu, “Electron-trap generation by

recombination of electrons and holes in SiOz,” J. Appl. Phys., vol. 61, pp. 4544-4548, 1987.

[8] S. Ogawa, N. Shiono, and M. Shimaya, “Neutral electron trap generation in Si02 by hot holes,” Appl. Phys. Lett., vol. 56, pp. 1329-1331, 1990.

191 P. Heremans. J. Witters, G. Groeseneken, and H. E. Maes, “Analysis

I&-vD* /v ~ i ~ . 9, for the effect of hole-induced negative fixed charges in n-MOSFET’s from the data obtained in Fig. 8.

Lifetime as a function of l I ( ~ D - vD,

Lifetime I s _ _

of the charge pumping technique and its application for the evaluaiion of MOSFET degradation,” IEEE Trans. Electron Devices, vol. 36, pp.

[IO] M. Brox and W. Weber, “Dynamic degradation in MOSFET’s-Part I: The physical effects,” IEEE Trans. Electron Devices, vol. 38, pp.

[ 111 M. Brox, A. Schwerin, Q. Wang, and W. Weber, “A model for the time- and bias-dependence of p-MOSFET degradation,” IEEE Trans. Electron Devices, vol. 41, pp. 1184-1196, 1994.

[12] C. Hu, “Hot-carrier effects,” in Advanced MOS Device Physics, N. G. Einspruch and G. Gildenblat, Eds. London: Academic Press, 1989, ch. 3, pp. 119-160.

[13] A. Schwerin, W. Hbsch, and W. Weber, “The relationship between oxide charge and device degradation: A comparative study of n- and p-channel MOSFET’s,” IEEE Trans. Electron Devices, vol. ED-34, pp. 2493-2500, 1987.

[14] M. G. Ancona, N. S. Saks, and D. McCarthy, “Lateral distribution of hot-carrier-induced interface traps in MOSFET’s,” IEEE Trans. Electron

1318-1335, 1989.

1852-1858, 1991.

Devices, vol. 35, pp. 2221-2228, 1988. [15] M. Brox, E. Wohlrab, and W. Weber, “A physical lifetime prediction

method for hot-carrier-stressed p-MOS transistors,” IEDM Tech. Digest,

[I61 C. Hu, S. C. Tam, F.-C. Hsu, P.-K. KO, T.-Y. Chan, and K. W. Temll, “Hot-electron-induced MOSFET degradation-Model, monitor, and immovement.” IEEE Trans. Electron Devices, vol. ED-32, pp.

0.04 0.06 0.1 0.2 0.3 ‘ab AD

pp. 525-528, 1991. Fig. 10. Lifetime as a function of I s u b / I ~ with same data as in Fig. 8.

throughout the paper. A lifetime investigation is possible here since trapped holes are neutralized completely or nearly completely. In other processes the situation is more complicated and the easy procedure presented is not applicable. In those cases the definition of a lifetime through the charge pumping current may be feasible.

In the past the effect of hole-induced electron trap formation has been neglected. From the strength of the effect as found in all processes investigated we conclude, however, that this is not justified in CMOS process developments as long as hole injection cannot be ruled out in principle. Special care has to be taken in analog process development, since hole injection conditions can occur under typical operating conditions in analog CMOS circuits with low gate voltage and high drain voltage conditions despite reduced supply voltages.

REFERENCES

[I] M. Bourcerie, B. S. Doyle, J.-C. Marchetaux, J.-C. Soret, and A. Boudou, “Relaxable damage in hot-carrier stressing of n-MOS transistors-oxide traps in the near interfacial region of the gate oxide,” IEEE Trans. Electron Devices, vol. 37, pp. 708-717, 1990.

[2] B. Doyle, M. Bourcerie, C. Bergonzoni, R. Benecchi, A. Bravis, K. R. Mistry, and A. Boudou, “The generation and characterization of electron and hole traps created by hole injection during low gate voltage hot-

_. 375-385, 1985.

[17] P. Heremans, G. Van Den Bosch, R. Bellens, G. Groeseneken, and H. E. Maes, ‘Temperature dependence of the channel hot-carrier degradation of n-channel MOSFET’s,” IEEE Trans. Electron Devices, vol. 37, pp. 980-993, 1990.

Werner Weber was born in Ruhstorf, Germany, in 1952. He received the Dipl. Phys. degree from the Technische Universitat Munchen, Germany, in 1976, and the Dr. rer. nat. degree from the Ludwig- Maximilians-Universitat Munchen, Germany, in 1981.

In 1981, he was on assignment at the IBM Thomas J. Watson Research Center, Yorktown Heights, NY, where he worked in the field of semiconductor thin films. Since 1983, he has been with the Research Laboratories of Siemens AG,

Munich, Germany. He is engaged in MOS physics and basic circuit design and has authored or co-authored over 70 papers. In 1992, he received the award of the German Information Technology Society for a publication on physical effects of hot-carrier degradation. Presently, he is managing a project on basic circuits in nonvolatile memories.

Dr. Weber is a member of the German Physical Society (DPG) and the German Information Technology Society (ITG).

1480 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 8, AUGUST 1995

Martin Brox received the Diploma and Ph.D. de- grees in physics from the University of Munster, Germany, in 1988 and 1993, respectively. His Ph.D. thesis dealt with oxide properties influencing the hot-carrier degradation of Si-MOS field effect transistors, especially during dynamic operation.

In 1992, he joined an IBWSiemens DRAM development project in Essex Junction, VT, working on circuit design. He is the author or co-author of some 20 publications.

He received the award of the German Information Technology Society for a publication on physical effects of hot-carrier degradation in 1992.

Roland Thewes was bom in Marl, Germany, in 1962. He received the Diploma in electrical engineering from the University of Dortmund, Germany, in 1990. From 1990 to 1994, he was working in the field of hot carrier degradation toward the Dr. Ing. degree in a co-operative program between the Siemens Research Laboratories, Munich, and the University of Dortmund.

In 1994, he joined the Siemens AG, Munich, where he is presently working on nonvolatile memories and analog CMOS circuits. He has authored

Mr. Thewes is a member of the Informationstechnische Gesellschaft im or coauthored 10 publications.

VDE (ITG).

Nelson S. Saks (M’79-SM’89) was bom on March 18, 1946, in Springfield, MA. He received the B.A. degree in physics from Amherst College, Amherst, MA, in 1968 and the M.S. degree in physics from the University of Maryland, College Park, in 1973.

He has held the position of research physicist at the Electronics Science and Technology Divi- sion of the Naval Research Laboratory, Washington, D.C., from 1968 to the present. He has worked on amorphous semiconductors, development of charge- coupled devices, and MOS devices with an em-

phasis on electrical characterization techniques and radiation effects. During the academic year 1984-1985, Mr. Saks spent a sabbatical year at the Department of Electrical Engineering, Catholic University, Leuven, Belgium, where he worked on hot carrier injection in VLSI devices. His present interests include hot carrier injection phenomena in MOSFET’s, techniques for characterization of MOS devices including charge pumping, oxynitrides for MOS gate insulators, and basic mechanisms of radiation damage in MOS structures.

Mr. Saks has been active in the radiation effects community and was technical program chairman of the 1992 IEEE Nuclear and Space Radiation Effects Conference. He currently serves as advisor to the Defense Nuclear Agency regarding basic mechanisms of radiation damage in electron devices.

hot-hole-induced negative oxide charges in n-mosfets

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