High-field mobility effects in reoxidized nitrided oxide (ONO) transistors

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 39, NO. 3. MARCH 1992 607

High-Field Mobility Effects in Reoxidized Oxide (ONO) Transistors

James S. Cable, Member, IEEE, and Jason C. S. Woo, Member, IEEE

Abstract-The high-field mobility behavior of silicon MOS- FET’s fabricated with reoxidized nitrided oxide (ONO) gate dielectrics has been investigated. Measurements have been performed at both room temperature and 77 K on both n- and p-channel FET’s, for both ONO and conventional SiOz films. While the peak electron mobility is much higher for standard SiOz, a crossover occurs in the high-field region beyond which ONO transistors exhibit higher mobility. The crossover voltage is reduced at 77 K. Measurements intended to gain further insight into this phenomena suggest that differences in surface roughness scattering, or the buried-channel nature of an ONO NMOS transistor are the most likely explanations for the high- field mobility behavior observed.

I. INTRODUCTION IGH-FIELD mobility degradation has been widely H quoted as a fundamental limitation for the optimal

scaling of gate oxide thickness for deep-submicrometer devices due to the increase in effective transverse electric field as oxide thickness is reduced [ l ] . This becomes es- pecially important for devices designed for cryogenic temperature operation, where in fact negative transconductance behavior can be observed at high transverse electric fields [2]. Although ONO gate dielectrics are known to result in lower peak mobilities as compared with conventional silicon dioxide films, it has recently been reported that they exhibit lower high-field mobility degradation [3]. Hence, these ONO films look to be an ex- tremely attractive candidate for such deep-submicrometer applications.

This paper presents results of a detailed experimental study into the high-field mobility behavior of ONO films at both room temperature and 77 K. The goal of the work was to investigate the possible physical mechanisms for the mobility behavior. Previous studies have suggested that the improved high-field mobility in ONO transistors can be caused by either reduced concentrations of acceptor-type interface states above the conduction band [3], or a nitrogen-induced donor layer that is present in ONO transistors which tends to make the device behave somewhat as a buried-channel transistor [4]. On the other hand,

Manuscript received March 22, 1990; revised April 10, 1991. The re- view of this paper was arranged by Associate Editor P. Cottrell.

J. S. Cable is with TRW Microelectronics Center, Redondo Beach, CA 90278 and the Solid State Electronics Laboratory, UCLA, Los Angeles, CA 90025.

J. C. S. Woo is with the Solid State Electronics Laboratory, UCLA, Los Angeles, CA 90025.

IEEE Log Number 9105343.

Nitrided

surface roughness scattering has been investigated for Si02 transistors [ 5 ] , and therefore could also be expected to be very important for ONO films. In this study, we attempt to examine the consistencies of these mechanisms with our experimental data on ONO transistors that were subjected to very different processing and stress treat- ments.

11. SAMPLE DESCRIPTION The experimental study was conducted on both NMOS

and PMOS FET’s fabricated with a conventional nf poly- silicon gate, two-level metal, LOCOS isolation, p-well technology. The silicon substrate had (100) orientation. A double-diffused source/drain process was used for substrate current reduction. Theo gate oxide thickness w?s varied between 150 and 250 A . The ONO films (150 A) were fabricated by exposing the furnace-grown gate oxides to dry ammonia using an arc lamp heated RTP system (PEAK ALP5500). The nitridation condition was 1150°C for 60 s. The reoxidation was performed in dry oxygen in the same system at 1000°C for 50 s. The threshold voltages of the p and n devicFs were kept constant (1 V) for both the 250- and 150-A films by tailoring the boron threshold adjust implants. For some samples, additional dopant (as much as 5 X 10I2 cm-2) was added to the threshold adjust implants to compensate for the increased oxide fixed charge known to be introduced as a result of the nitridation process. Following the ONO processing, all wafers were processed together through the remaining steps.

111. EXPERIMENTAL RESULTS Electrical measurements were made on both p- and

n-type capacitors and FET’s. Oxide fixed charge densities were extracted from C- V measurements, interface-state densities were extracted using both the high-low C-V technique on large-area capacitors (200 pm X 200 pm), and charge pumping on relatively small-geometry (12.5 pm x 1.25 pm) FET’s.

Mobility measurements were performed using drain conductance measurements on FET’s with mobility p given by [2]

where gds is the drain conductance, W / L is the aspect ra- tio of the transistor, and Qinv is the inversion charge. Split

0018-9383/92$03.00 O 1992 IEEE

608 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 39. NO. 3. MARCH 1992

T

5 P

z E o_

8 5 n 250A CONTROL 150ACONTROL 15OA Oxy-I U 0 15oAoxy-2

~

0 1 I t I 1 1 I 0 0.2 0.4 0.6 0.8 1.0 12 1

Een ( M V ~ )

are SiO,; 150-A Oxy-1 and 150-A Oxy-2 are ONO with different reoxidation temperatures) Fig. 1. Room-temperature electron mobility as a function of effective field for SiOz and ONO films (250- and 150-A controls

0

U7.a

3

1 l 1 1 1 1 1 1 1 .ow0 11

OAlE V a T m (v)

(a) (b)

Fig. 2 . Transconductance as a function of gate voltage for SiOz and ONO 150-A films. (a) 300 K. (b) 77 K .

C-V techniaues 161 were used on 50 um x 50 um FET's 7Ua I 1 . -

to determine the inversion charge ' e,,, and ' depletion charge Qb as a function of gate bias. The measurements were performed with an HP4274 LCR meter at a frequency of 100 Hz, in order to reduce any potential error due to interface states. The effective field was calculated from the standard expression for an NMOS transistor [7]

Eeff = l/~si(Qb + Q m v / 2 ) where E,, is the dielectric permittivity of silicon.

Results are shown in Fig. 1. While the peak mobility is as much as 30% lower in theooxynitride samples in comparison to the 250- and 150-A silicon dioxide samples, an interesting crossover behavior is observed at an effective field on the order of 0.8 MV/cm. This is similar to the results reported by Hori [3]. Further confirmation of this behavior is illustrated in the g, data shown in Fig. 2(a). The effect is even more pronounced at 77 K as shown in Fig. 2(b). Note that the crossover occurs at a lower gate voltage and that negative g, is not observed in the ONO sample until a voltage of roughly 10 V is reached (EeR = 1.25 MV/cm). Samples which were processed identi- cally, except for the reoxidation temperature and threshold adjust implant were measured as shown in Fig. 3. While the peak mobility was found to be a strong function of both variables due to the difference in oxide fixed

I !

z 0 6 8

1)Q)o 1200 .4ooo MTE VOLTAGE (vl

Fig. 3 . Room-temperature g, as a function of reoxidation temperature for 150-A ONO films.

charge, interface-state density, and ionized impurity scattering, in the high-field region all ONO films behave the same. It should be noted here that the fixed charge on these samples varies from 2 x 10" to 5 x 10" cm-*, the bandgap-averaged interface state density, measured using charge pumping techniques [8], varied from 2 to 6 x 10" cm-2, and the threshold adjustment dose varied by up to 3 x 1 0 ' ~ cm-*.

In order to gain more understanding into this behavior,

CABLE A N D WOO: HIGH-FIELD MOBILITY EFFECTS IN ONO TRANS ; 1 S T 0 R S 609

f - 3

0.0 0.Q

VG M VG (VI (a) (b)

Fig. 4. g, for different substrate bias for 1.50-A SO, film. (a) Room temperature. (b) 77 K .

64.33

9 3

9 3

0.0 9.9 0.0 12.0

vo fv) 0.0

VG M ca) (b)

Fig. 5 . g,, for different substrate bias for 150-A ONO film. (a) Room temperature. (b) 77 K .

9

the g, versus VR characteristics of both the ONO and Si02 350

E films were characterized as a function of substrate bias. Room-temperature and 77 K data for the Si02 samples are

77 K is clearly seen in Fig. 4(b). Similar data for the ONO

fn 5 300 shown in Fig. 4. The high-field mobility degradation at

ence in behavior between the ONO and S O z samples at 77 K. The ONO peak g, is more than 10% higher with a

for the 5-V back-bias case. For the Si02 samples, the peak

E - samples are shown in Fig. 5. There is a dramatic differ- d 250

s k z 0 a 200 substrate back bias of 2.5 V and the peak is slightly less -I W

g, decreases with increasing substrate bias. This behavior 150 0.6 0.8 1.0 1 1 1.4 1.6 1

Eefl (Mv/cm) i s expected based on the increasing transverse gate electric field at the point of peak g , with increasing back bias.

Due to the peculiar substrate bias dependence that was observed, the concept of universal mobility as a function of effective field was investigated by varying substrate bias as well. For the Si02 samples, universal mobility curves- independent of substrate bias-were found to exist, while for the ONO samples a very definite dependence on substrate bias was observed. The room-temperature data for the ONO samples are shown in Fig. 6.

PMOS mobility behavior was also studied for both ONO and SiO, samples. Split C-V techniques were used again to determine the effective field. Universal mobility behavior was observed for both types of samples. No mo-

Fig. 6. Mobility versus effective filed as a function of substrate bias for 150-A ONO film.

bility crossover behavior was observed for the PMOS samples at either 77 or 300 K. Room-temperature g, versus gate voltage data are shown in Fig. 7(a), while 77 K data are shown in Fig. 7(b). While the peak hole mobility is quite a bit less in the ONO transistors than it is in the Si02 transistors, the high-field roll-off is quite similar. Negative g, was found in the PMOS samples as well at 77 K.

Since the same high-field mobility behavior was ob-

-

610 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 39, NO.

202.5 c I lo80.0 I

-22.5 5 0.0 '-9.00

VG M

(a)

3, MARCH 1992

Fig. 7. Comparisons of PMOS SiOz and ONO devices. (a) Room-temperature g, as a function of gate voltage. (b) 77 K mobility as a function of normalized gate voltage.

I I r PRE-RAD

, POST-RAD

600.0 c I 600.0

t \\

VG- V T M V G - V T M

(a) (b) Fig. 8 . Room-temperature mobility as a function of normalized gate voltage pre- and post-radiation stress. (a) ONO. (b) SiOz.

served for two nitridated samples with very different oxide interface properties (Qf and Nss) , the more gradual roll-off is not considered to be due to differences in Cou- lomb scattering from interface changes. Furthermore, such scattering is theoretically expected to be less important at high fields or at cryogenic temperatures due to screening considerations. In order to further investigate the feasibility of this mechanism, additional experiments were performed where large quantities of Coulomb scattering sites were intentionally introduced and the high- field mobility behavior characterized before and after this introduction. Positive charge at the oxide/silicon interface was introduced via ionizing radiation experiments, while interface states were introduced by Fowler-Nordheim stressing.

The irradiation experiments were performed using an ARACOR 4100 X-ray source. The transistor was biased with Vg = 5.0 V and all other terminals grounded. This is known to be the worst case bias conditions for maxi- mum threshold-voltage shift [9]. The part was irradiated to a total dose level of 1 x IO6 rad (Si) at a dose rate of 100 krad(Si)/min. Estimates of the oxide fixed charge from the radiation exposure are 1 X 10I2 (calculated

from the threshold-voltage shift and the oxide thickness), The Fowler-Nordheim stress consisted of the injection of 1 C/cm-2 of charge injected at a current density of 10 mA/cmP2. The gate was biased positive during the stress so that the injection was from the substrate. The post- stress bandgap averaged interface-state density was deter- mined to be 3 x 10i2/eV * cm-2. Very little change in mobility was observed as a result of the ionizing radiation exposure over the full range of gate voltages measured. This data of ONO and Si02 transistors are shown in Fig. 8. The horizontal axis is the normalized gate voltage Vg - V, (where Vg is the gate voltage and V, is the threshold voltage), in order to compare the mobilities at equal transverse gate fields. Transconductance versus gate voltage before and after Fowler-Nordheim stress was also measured. The results for the Si02 films are shown in Fig. 9. From these data, the effective mobility was calculated and plotted as a function of normalized gate voltage, as shown in Fig. 10. While the peak mobility is reduced by more than 40% as a result of the stressing, there is little difference in the high-field region-in fact, the post-stress mobility is actually higher at very high gate voltages. The comparison between pre- and post-stress g, behavior was

CABLE AND woo: HIGH-FIELD MOBILITY EFFECTS IN ONO TRANSISTORS 61 1

0.0 12.0 VG (VI

Fig. 9. Room-temperature g, versus gate voltage pre and post Fowler- Nordheim stress results for 150-A SiO, film.

0.0 10.0

VG- VT (Vl

Fig. 10. Mobility versus normalized gate voltage pre and post Fowler- Nordheim stress for 150-A SiOz film.

made at 77 K as well with the same results observed, thus further confirming that the mobility roll-off is not likely due to the interface charge scattering.

It should be pointed out that the mechanism of a reduction of acceptor-like interface states near the conduction band edge proposed in [3] to be responsible for the reduced high-field mobility does not require Coulomb scattering off these sites in order to be correct. It is perfectly possible that such states might play a role in the reduced mobility observed through removal of inversion-layer electrons. That is to say, that electron trapping at these interface states, rather than Coulomb scattering off them, could be responsible for the observed behavior.

It has previously been observed and reported for ONO transistors that the l/f noise is much larger than for a standard SiOz device [ 1 0 1 . We have made such measurements on our samples and found the same behavior to be true. Room-temperature 1 /f noise comparing SiOz and ONO samples is shown in Fig. 1 1 . Similar results are ob- tained at 77 K. Furthermore, a through study of the l/f noise behavior of ONO transistors have been reported in which measurements have been performed to extract the interfacial electron trap density above the conduction band [ 1 1 1 . The results indicate that the trap density is nearly an

\ 25OAbERMAL

150 A THERMAL

10-17- 100 1000 10.000 100,000

FREQUENCY (Hd

Fig. 11. NMOS 1 /finput referred noise spectra for SiOz and ONO films.

order of magnitude higher in the heavily nitrided samples used in that study. In addition, we have found, in agree- ment with [ 1 1 1 , that reoxidation has a dramatic effect in reducing this trap density. The fact that the high-field mobility behavior is little changed for the various reoxidation processes shown in Fig. 3 tends to suggest that removal of inversion layer electrons in Si02 transistors, mentioned above, is not the mechanism for the lower high-field mobility is Si02 compared to ONO transistors. In addition, we have carefully examined our split C-V data as well to consider this effect. In the region well above threshold we have computed d Q,,,/d Vg and found for the SiOz samples the result to be constant and equal to Cox, as would be expected from the simple expression Q,,, = Cox(Vg - V,). If a carrier removal process were taking place due to interface states, a more complicated expression would be required to fit the experimental data.

Other scattering mechanisms such as surface roughness or surface phonons must evidently be considered in order to account for the differences in the high-field mobility behavior between Si02 and ONO films [5], [12]. Re- cently, high-field mobility degradation has been ex- plained based on the physics of electron population and scattering mechanisms of quantized subbands at (100) surfaces [13]. An alternative explanation might be due to the presence of a nitrogen-induced donor layer in ONO transistors that has recently been reported [4]. Such a do- nor layer would tend to make the NMOS transistor behave as a buried-channel device which would be expected to alter its mobilityheld behavior. Substrate bias would clearly be expected to play a role if such a mechanism was indeed responsible for the unusual behavior, in agree- ment with our experimental results. Mobility measurements performed in [4] indicate that the ONO transistor does exhibit mobility behavior which is similar to a Fow- ler-Nordheim stress-degraded buried-channel device.

We have examined our split C-V data performed at various back biases to try to gain further insight into this possible mechanism. In particular, we have found that in the high-field region that parameter d e,,, / d Vg changes with back bias. Results are shown in Table I. This behav-

612 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 39, NO. 3, MARCH 1992

TABLE I EQUIVALENT OXIDE CAPACITANCE AS A

FUNCTION OF BACK-GATE BIAS

carriers further away from the interface, there is less surface roughness scattering and therefore higher mobility.

0.0 0.224 0.240 -2.5 0.224 0.233 -5.0 0.224 0.217

ior cannot be simply reconciled with the result which is expected from the standard expression Qinv = Cox (V, - V t ) . One immediate possibility can be proposed to explain the observed behavior-that the oxide capacitance is being affected by the finite inversion-layer capacitance which is a function of substrate bias. We have briefly examined the self-consistency of this mechanism.

The issue of finite inversion-layer capacitance has been previously addressed at great length [ 141. In principal, one must replace Cox by CAx where the latter is the series combination of the oxide capacitance and the inversion- layer capacitance. Normally, the inversion-layer capacitance is much greater than the oxide capacitance, and hence has very little impact on the overall expression. At high gate voltages, the inversion-layer capacitance in- creases due to the reduction in the average inversion-layer thickness [14], and hence should become less important. With increasing back bias, the inversion-layer thickness is expected to increase and thus the inversion capacitance is expected to play a more significant role. This is consistent with the results shown in Table I. Since we have observed no change in Cox for the Si02 transistors with substrate bias, evidently the substrate bias has a much greater effect on the ONO transistors. Note that previous results indicate that shallow nitrogen-oxygen donor states can exist in bulk silicon with a range of energies [15]. Other work has found that nitrogen doping of silicon in- duces relatively deep donor states at energy levels E, - 0.19 eV and E, - 0.28 eV [16]. Not all of these deep donors would be ionized even at room temperature. Hence, with a back-gate bias, more ionization will occur due to the sharper resultant band bending [17]. This explanation agrees with the observation that the equivalent gate oxide capacitance CAx is decreasing with increasing back bias, and is consistent with the proposed buried- channel nature of the ONO device.

The most self-consistent explanation for the observed mobility behavior of ONO transistors can be summarized as follows. At low vertical fields and reduced temperatures, the Coulombic scattering due to the higher interface state and fixed charge densities cause a reduction in the ONO transistor mobility. At higher electric fields screening causes these sites to be less effective in carrier scattering. Hence surface roughness is probably the main high-field scattering mechanism. In this regime of operation, ONO transistors have higher mobility since they behave somewhat as buried-channel devices. With the

IV. CONCLUSIONS Detailed measurements have been performed which

suggest that the improvement in high-field mobility degradation observed in n-channel ONO transistors is due to a combination of surface roughness scattering and the buried-channel nature of carrier conduction. This conclu- sion is based on both the temperature and transverse field dependence observed for the mobility degradation.

ACKNOWLEDGMENT The authors would like to thank J. Greenburg for help

with some of the g, and mobility measurements and S . Dacus for help with the split C-V software routines.

REFERENCES

G. Baccarani, M. R. Wordeman, and R. H. Dennard, “Generalized scaling theory and its application to a 1/4 micrometer MOSFET de- sign,” IEEE Trans. Electron Devices, vol. ED-31, p. 452, 1984. T. C. Ong, P. K. KO, and C. Hu, “50-A gate-oxide MOSFETS at 77 K,” IEEE Trans. Electron Devices, vol. ED-34, p. 2129, 1987. T. Hori and H. Iwasaki, “Improved transconductance under high nor- mal field in MOSFET’s with ultrathin nitrided oxides,” IEEE Elec- tron Device Lett., vol. 10, p. 195, 1989. A. Wu, T. Chan, V. Murali, S. Lee, J. Nulman, and M. Gamer, “Nitridation induced surface donor layer and its impact on the characteristics of n- and p-channel MOSFETs,” in IEDM Tech. Dig. , 1989, p. 271. Y. Cheng and E. Sullivan, “On the role of hcattering by surface roughness in silicon inversion layers,” Surface Sci., vol. 34, p. 717, 1973. C. Sodini, T. Ekstedt, and J. Moll, “Charge accumulation and mobility in thin dielectric MOS transistors,” Solid State Electron., vol. 25, no. 9, p. 833, 1982. A. Sabnis and J. Clemens, “Characterization d electron mobility in the inverted (100) Si surface,” in IEDM Tech. P i g . , 1979, p. 18. G. Groeseneken, H. Maes, N. Beltran, and R. F e Keersmaecker, “A reliable approach to charge-pumping measurements in MOS transistors,” IEEE Trans. on Electron Devices, vol. ED-31, no. I , p. 42, 1984. T. P. Ma and P. Dressendorfer, Ionizing Radiation Effects in MOS Circuirs and Devices. W. Yang, R. Jayaraman, and C. Sodini, “Optimization of low pres- sure nitridationlreoxidation of SiOz for scaled MOS devices,” IEEE Trans. Electron Devices, vol. 35, p. 935, 1988. R. Jayaraman and C. Sodini, “l / f noise interpretation of the effect of gate oxide nitridation and reoxidation on dielectric traps,” IEEE Trans. Electron Devices, vol. 37, no. 1, p. 305, 1990. S . Sun and J. Plummer, “Electron mobility in inversion and accumulation layers on thermally oxidized silicon surfaces, ” IEEE Trans. Electron Devices, vol. ED-27, no. 8, p. 1497, 1980. M. S. Lin, “A better understanding of the channel mobility of Si MOSFET’s based on the physics of quantized subbands,” IEEE Trans. Electron Devices, vol. 35, no. 12, pp. 2406, 1988. G. Baccarani and M. Wordeman, “Transconductance degradation in thin-oxide MOSFETs,” IEEE Trans. Electron Devices, vol. ED-30, no. 10, p. 1295, Oct. 1983. M. Suezawa, K. Sumino, H. Harada, and T. Abe, “Nitrogen-oxygen complexes as shallow donors in silicon crystals,” Japan. J . Appl. f h y s . , vol. 25, no. 10, p. 859, 1986. Y. Tokumaru, H. Okushi, T. Matsui, and T. Abe, “Deep levels as- sociated with nitrogen in silicon,” Japan. J. hppl. f h y s . , vol. 21, no. 7, p. 443, 1982. F. Gaensslen and R. Jaeger, “Temperature dependent threshold behavior of depletion mode MOSFETs,” Solid State Electron., vol. 22, no. 4, p. 123, 1979.

New York: Wiley, 1989.

CABLE AND WOO: HIGH-FIELD MOBILITY EFFECTS IN ONO TRANSISTORS 613

James S. Cable (M’89) received the B.S. degree in physics from the University of California, Riverside, in 1981. He received the M.S.E.E. degree from the University of California, Los An- geles, in 1987 and is currently pursuing the Ph.D. degree in electrical engineering at UCLA.

Since 1986 he has been with the TRW Micro- electronics Center, Redondo Beach, CA, where he has been involved in the development of advanced CMOS technologies. His research work at UCLA has focused on the application of ONO gate

dielectrics for CMOS applications.

Jason C. S. Woo (S’81-M’87) was born in Hong Kong, on September 29, 1958. He received the B.A.Sc. degree in engineering science from the University of Toronto, Canada, in 1981, and the M.S. and Ph.D. degrees, both in electrical engineering, from Stanford University, Stanford, CA, in 1982 and 1987, respectively.

He is presently an Assistant Professor in the Electrical Engineering Department at the Univer- sity of California, Los Angeles. His current research interests are in the area of low-temperature

device operation, novel device structures in new semiconductor materials, and new device fabrication technology.

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High-field mobility effects in reoxidized nitrided oxide (ONO) transistors