Si〈1 0 0〉 systems

Microelectronics Reliability 45 (2005) 57–64 www.elsevier.com/locate/microrel

Characterization of interface defects related to negative-bias temperature instability in ultrathin plasma-nitrided SiON/SiÆ1 0 0æ systems Shinji Fujieda a,*, Yoshinao Miura a, Motofumi Saitoh a, Yuden Teraoka b, Akitaka Yoshigoe b a

System Devices Research Laboratories, NEC Corporation, 1120 Shimokuzawa, Sagamihara, Kanagawa 229-1198, Japan b Synchrotron Radiation Research Center, Japan Atomic Energy Research Institute, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo 629-5148, Japan Received 3 November 2003 Available online 17 July 2004

Abstract Interface defects related to negative-bias temperature instability (NBTI) in an ultrathin plasma-nitrided SiON/ SiÆ1 0 0æ system were characterized by using conductance–frequency measurements, electron-spin resonance measurements, and synchrotron radiation X-ray photoelectron spectroscopy. It was confirmed that NBTI is reduced by using D2 -annealing instead of the usual H2 -annealing. Interfacial Si dangling bonds (Pb1 and Pb0 centers) were detected in a sample subjected to negative-bias temperature stress (NBTS). Although we suggest that NBTS also generates non-Pb defects, it does not seem to generate nitrogen dangling bonds. These results show that NBTI of the plasma-nitrided SiON/Si system is predominantly due to Pb depassivation. Plasma nitridation was also found to increase the Pb1 /Pb0 density ratio, modify the Pb1 defect structure, and increase the latent interface trap density by generating Si suboxides at the interface. These changes are likely to be the causes of NBTI in ultrathin plasma-nitrided SiON/Si systems.
2004 Elsevier Ltd. All rights reserved.

1. Introduction Nitrogen is added to the thin gate oxides of metaloxide-silicon field-effect transistors (MOSFETs) in order to suppress boron penetration from the gate poly-Si and increase the gate oxide permittivity. Adding N decreases p-MOSFET reliability, however, by causing negativebias temperature instability (NBTI) [1–3]. Plasma nitridation is used to add N to ultrathin gate oxides because it does not increase the oxide thickness [4,5] and suppresses N penetration into the SiO2 /Si interface. Even when plasma nitridation is used, though, NBTI remains a serious problem.

*

Corresponding author. Tel.: +81-42-771-2394; fax: +81-42771-2481. E-mail address: [email protected] (S. Fujieda).

Theoretical calculations have led several researchers [8–11] to suggest that N increases NBTI because a network N atom efficiently traps Hþ released from interfacial Si dangling bonds (Pb centers [6,7]). Pb depassivation has been assumed to occur in SiON/SiÆ1 0 0æ systems under negative-bias temperature stress (NBTS), evidently by analogy to the SiO2 /SiÆ1 1 1æ systems for which Gerardi et al. identified the NBTS-induced interface defects as Pb centers [12]. The occurrence of Pb depassivation in an ultrathin plasma-nitrided SiON/SiÆ1 0 0æ MOS structure now being developed, however, has not been confirmed. Nor has the mechanism by which introduced N increases NBTI been demonstrated experimentally. The NBTI might result from N–H dissociation at the SiON/Si interface, or from plasma-nitridation-induced interface damage. We have explored these possibilities by carrying out D2 -annealing, conductance–frequency ðGðxÞÞ

0026-2714/$ - see front matter
2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2004.02.017

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S. Fujieda et al. / Microelectronics Reliability 45 (2005) 57–64

measurements, electron-spin resonance (ESR) measurements, and synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) on plasma-nitrided SiON/ Si(1 0 0) samples with equivalent oxide thicknesses (EOTs) less than 1.8 nm. In this article, we report the results of investigations extending our previous report [13].

2. Experimental procedures The samples we examined are listed in Table 1. To characterize NBTS-induced defects, we prepared the MOS diodes for both GðxÞ and ESR measurements on the same substrates. Local-oxidation-of-silicon-isolated pþ poly-Si/SiON/ SiÆ1 0 0æ diodes were prepared. P-type substrates were used to make the ESR measurements easy, and the SiON films were grown by thermal oxidation and subsequent plasma nitridation using N2 as the N source. The typical N content [N ] was 9% and the EOT of the SiON film was 1.6–1.8 nm (type-1 samples). After forming the boron-doped poly-Si electrodes, we annealed the samples at 400 C for 40 min in a gas mixture consisting of 50% N2 and 50% H2 or D2 . We assessed NBTI by applying a bias voltage of )2.0 V to 0.01-mm2 diodes at 150 C. The bias value was corrected for the series resistance ðRs Þ. After the NBTS was applied, the samples were cooled to RT (25 C). The applied negative bias was maintained at least until the temperature became lower than 100 C in order to suppress the recovery of electrical characteristics [14]. Less than 5% of the flatband-voltage shift (DVFB ) was estimated to have recovered when the negative-bias application was stopped at 100 C. DVFB and the interface trap density (DDit ) were evaluated by capacitance–voltage measurements (100 kHz) and GðxÞ measurements (102 –106 Hz) at RT. The GðxÞ curves were corrected by taking into account the tunnel-current Table 1 List of sample types 1 2 3 4 5 6 7 8* 9

Base SiO2 thickness

Nitridation time

Gate

t1 t1 t1 t1 + 0.2 nm t1 + 0.2 nm t2 t2 t2 t1

n1 n1 · 0.4 0 n1 n1 · 0.3 0 n2 n2 n1

Poly-Si Poly-Si Poly-Si None None None None None None

Nitrogen concentration of type-1 samples is 9%. * Type-8 samples were annealed at a temperature above 1000 C after plasma nitridation.

conductance and Rs and were then fitted to the theoretical curve with the interface potential fluctuation ðrs Þ as the parameter [15,16]. The estimated rs was 1:5kB T to 1:6kB T (kB : Boltzmann’s constant), which is larger than that of diodes with SiO2 films (1:3kB T ) because of the positive charges within the SiON films. GðxÞ measurements were also used to evaluate latent Dit present in samples not subjected to a H2 passivation annealing. For the ESR characterization of NBTS-induced defects, a bias of )1.8 V was applied to the H2 -annealed 32-mm2 diode (sample 1) at 150 C for 180 min. A voltage drop along the large-area gate electrodes was avoided by depositing Al on the poly-Si gate. A chip of 2 · 8 mm2 was cut out of the diode and the Al and polySi layers on it were chemically etched off before ESR measurements. To investigate how plasma nitridation influences the interfacial defects, we also prepared another set of samples by plasma-nitriding SiO2 films that were 0.2 nm thicker. The X-band (9.43 GHz) ESR was measured by using a TE1 1 0 cylindrical microwave cavity at 10 K with a microwave power ðPl Þ of 1 mW, an ac magnetic field of 1 G, and a modulation frequency of 100 kHz. ESR measurements at RT would be useful for detecting Pb centers, since the surface Fermi level of Si (EFS ) is located relatively deep in the bandgap at RT, making more Pb centers ESR-active. Temperatures below 30 K, however, were necessary in order to suppress the microwave absorption by the Si substrate (not highly resistive) used in the device structures. This is why we measured ESR at 10 K. Because we thought the actual defect densities would be underestimated because of rather high Pl we used to detect defects at the low densities expected to be generated by NBTS (of 1011 cm
2 order), we estimated the defect density for reference. Spin density was calibrated by using a TEMPOL (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl) standard sample. Although the absolute spin count in ESR is generally thought to have an error of a factor of 2 or more, we estimate our relative spin counts to be accurate to within ±20%. Furthermore, we measured the ESR of a high density of Pb centers (of 1012 cm
2 order) in thermally depassivated samples at Pb values from 10 lW to 1 mW and found the shape of the Pb signals to change little with changes in Pl . It thus seems appropriate to discuss the relative (not absolute) intensities and the shapes of the ESR signals we measured. XPS measurements were used to examine the change in the amount of interfacial Si suboxides due to the plasma-nitridation process. A surface-reaction research system (SUREAC2000) at BL23SU of SPring-8 (Hyogo, Japan) was used to make these measurements, and the 404.1-eV X-ray from the synchrotron radiation monochromator there was used as the excitation light. The energy width of the light was about 40 meV and the energy resolution of the analyzer was about 70 meV. The

S. Fujieda et al. / Microelectronics Reliability 45 (2005) 57–64

take-off angle was set normal to the sample surface in order to enhance the sensitivity for the interfacial suboxide components.

3.1. NBTI dependence on passivation-annealing ambients The DDit and DVFB of type-1 samples (Table 1) annealed under H2 and D2 were investigated. A simple thermal stress at 150 C for 180 min under zero bias (ZBTS) did not cause notable changes in Dit and VFB of a fresh diode. Fig. 1 shows DDit and DVFB plotted against NBTS duration. To get each data point shown in Fig. 2, we measured the C–V and GðxÞ characteristics at RT before and after applying NBTS to a fresh diode. The time dependence factor (n of tn ) for the short stress durations was about 0.29 for DDit and 0.26 for DVFB . These are close to the usually observed n of 0.25 [17,18]. þ We estimated the increase of net oxide charges (DNox ) from DVFB , by assuming flat Dit distributions for the samples subjected to NBTS (see Fig. 3). The estimated þ DNox values were 0.6–0.7 times DNit (¼ DDit  Eg =2). Here, Eg is the bandgap energy of Si and DNit will be equal to the increase in the interface defect density when the interface traps are derived from amphoteric defects like Pb centers. This estimation suggests that the NBTS generated positive oxide charges as well as interface

SiON

ESR INTENSITY (arb. units)

3. Results

SiON

Pb1

3330

E FS-VBM (meV) 300 350 400

250

100

thermal Dit

450

NBTS Dit

o

449 C

60

10800 s o

427 C

3600 s

40 1000 s

o

398 C

20

o

354 C

0 0.25

∆VFB (mV)

3370

Fig. 2. ESR signals (first derivative) of the Pb centers found in (a) a plasma-nitrided SiON/Si diode (type-1) not subjected to NBTS, (b) a plasma-nitrided SiON/Si diode (type-1) subjected to NBTS, (c) a thermally depassivated plasma-nitrided SiON/Si diode (type-1), and (d) a thermally depassivated SiO2 /Si diode (type-3). The magnetic field was set parallel to the [1 0 0] axis.

10

1011 2 10

10

3340 3350 3360 MAGNETIC FIELD (G)

-2

-2 -1

H2 D2

(d) depassivated Pb0

-1

∆D it (eV cm )

0.25

(c) depassivated

SiO2

Dit (10 eV cm )

12

(a) w/o NBTS

(b) after NBTS

80 10

59

10

w/o NBTS -7

10

-6

10

-5

10

-4

10

-3

TIME CONSTANT (s)

1

10

2

10

3

10

4

STRESS DURATION (s) Fig. 1. C–V shift (DVFB ) and increase in interface state density (DDit ) versus NBTS duration for H2 - and D2 -annealed plasmanitrided SiON/Si diodes (type-1 samples in Table 1). The DDit values are for the time constant of 10 ls. The probed surface Fermi level (EFS ) estimated assuming a hole capture cross-section of 10
16 cm2 was 0.3–0.4 eV above the valence band maximum, which corresponds to the (+/0) transition energy of the Pb centers.

Fig. 3. Time-constant distributions of NBTS-induced Dit and thermally activated Dit in type-1 samples. The EFS calculated assuming a hole capture cross-section of 10
16 cm2 is shown for reference on the upper axis.

traps in our SiON samples, but we did not find a sysþ =DNit ratio on the NBTS tematic dependence of the DNox duration. In Fig. 1, the average DDit and DVFB for the samples annealed under D2 are 0.7–0.8 times those for the samples annealed under H2 . This ratio is almost the same as the one for diodes with 3.8-nm-thick SiO2 layers: 0.7

60

S. Fujieda et al. / Microelectronics Reliability 45 (2005) 57–64

[19]. The different NBTI of samples annealed under H2 and D2 suggests that a depassivation of the Pb centers and a transport of the H atoms are involved in the NBTI of the plasma-nitrided SiON. 3.2. Interface defects generated by negative-bias temperature stress ESR measurements were carried out for diodes (type1 samples) subjected or not subjected to NBTS (Figs. 2(a) and (b)). Pb1 and Pb0 centers were clearly found in the diode subjected to NBTS, while Pb signals were not evident in the diode not subjected to NBTS. The estimated Pb density of the diode subjected to NBTS (3.4 · 1011 cm
2 ) was comparable with DDit (4.6 · 1011 eV
1 cm
2 ), but as explained in Section 2, the similarity of these absolute values is only suggestive. The Pb spin count also depends on the surface Fermi level (EFS ) of Si, showing a maximum around the midgap [20]. Because of the high density of positive charges in SiON, the EFS of plasma-nitrided SiON/Si is presumed to be located above the midgap even at 10 K. Since the NBTS-induced positive charges further shift the EFS toward the conduction band, the Pb density would be relatively underestimated after an NBTS application. The NBTSinduced increase of the Pb signal intensity thus seems to have resulted from the generation of Pb centers. The Pb density and Dit in SiO2 /SiÆ1 0 0æ systems annealed above 700 C in vacuum or in an inert gas ambient are 1012 cm
2 [21]. As shown in Fig. 5, we observed such high Dit values for the SiON diodes (sample types 1 and 2). Since the NBTS-induced Dit is less than 1012 cm
2 , we think that some of the initially H-passivated Pb centers were depassivated by the NBTS. However, we did not detect ESR signals that would indicate the presence of N dangling bonds with the Pl of 1 mW or below. The NBTS-induced defects in the plasma-nitrided SiON/Si system thus seem to include Pb centers but not N dangling bonds. 3.3. Interface traps generated by negative-bias temperature stress To investigate whether the NBTI process is the same as the simple Pb –H dissociation that occurs during thermal annealing, we compared the time-constant distribution of the NBTS-induced Dit and that of the thermally activated Dit in type-1 samples (Fig. 3). The EFS calculated assuming a hole capture cross-section of 10
16 cm2 is shown for reference on the upper axis. Low levels of thermal Dit were obtained by carrying out N2 annealing at several temperatures below 500 C. The distributions shown in Fig. 3 evidently differ between the two Dit series: the NBTS-induced Dit distributions are rather flat, while the thermal Dit distributions tend to decrease on the midgap side. This indicates that NBTI

involves processes other than the simple Pb depassivation. The time constant depends strongly on the energy depth ðET Þ and weakly on the cross-section ðrÞ of the traps ðs / 1=r  expðET =kB T ÞÞ. Accordingly, the different time-constant distributions are interpreted as the result of a difference in energy distributions or a considerable change in cross-sections, or both. We therefore need to take into account the possibility that NBTS also induces interface traps other than just the Pb centers. Defects other than the Pb centers (such as N dangling bonds) were not detected by the ESR measurements, however. This could be because the non-Pb traps are ESR-inactive like the H-related traps [22,23]. H atoms can be supplied from Pb –H bonds by the release of H. Additionally, we observed an annealing-out of the NBTS-induced Dit . The Dit (s ¼ 10 ls) we measured just after applying NBTS for 10 800 s was 6.2 · 1011 eV
1 cm
2 . H2 annealing at 400 C decreased it to 9.9 · 1010 eV
1 cm
2 , a value almost equal to that before NBTS application (see Fig. 5, lower graph). Even N2 annealing at 400 C decreased somewhat the Dit to 3.6 · 1011 eV
1 cm
2 , a value similar to that measured after a simple thermal depassivation at 398 C (Fig. 3). Such annealing behaviors suggest that NBTS-induced interface traps are predominantly Pb centers and the others are easily annealed out much like the H-related traps. 3.4. Modification of interfacial defect structure by plasma nitridation Fig. 2 also shows the ESR signals of the Pb centers in plasma-nitrided SiON/Si (type-1) and SiO2 /Si (type-3) samples annealed at 740 C in N2 . The poly-Si gate was etched off before the ESR measurements. The plasmanitrided SiON exhibits a larger Pb1 /Pb0 density ratio (1.72) than the SiO2 does (0.56). Furthermore, the effective g-value of the Pb1 center differed between plasma-nitrided SiON (2.0028) and SiO2 (2.0037), while that of the Pb0 centers was almost the same for plasmanitrided SiON (2.0064) and SiO2 (2.0065). We refer below to the modified Pb1 center as the Pb1 (N) center. The predominance of the Pb1 center and the change in the g-value of Pb1 centers were also observed in the NOannealed oxides [24–26]. Because most of the N added by NO annealing is added to the near-interface region, these results suggest that N atoms can also be incorporated into the interface region of plasma-nitrided SiON. Suppressing N penetration is critical to lowering NBTI, and should be done by using plasma conditions that decrease the ion-to-radical density ratio [4]. To investigate how the Pb modification proceeds during plasma nitridation, we prepared two types of samples (types 4 and 5, Table 1) by plasma-nitriding SiO2 layers 0.2 nm thicker than that in type-1 samples.

S. Fujieda et al. / Microelectronics Reliability 45 (2005) 57–64

ESR INTENSITY (arb. units)

The nitridation time for the type-4 samples was the same as that for the type-1 samples, while the nitridation time for the type-5 samples was only 30% of that for the type1 samples. Poly-Si was not deposited on these samples. The Pb centers were left unpassivated by not subjecting the samples to H2 -annealing. The N concentration at the interface is thought to decrease in the following order: type 1 > type 4 > type 5. Their ESR signals are shown in Fig. 4. The type-4 sample showed three Pb signals with effective g-values of

SiON Pb0 Pb1

(a)

(b)

3350

3360

MAGNETIC FIELD (G)

E FS-VBM (meV)

-1

INTERFACE TRAP DENSITY (eV cm-2)

Fig. 4. ESR signals of the Pb centers found in SiON/Si (sample types 4 and 5 in Table 1) prepared by plasma-nitriding thermal SiO2 layers 0.2 nm thicker than the layer in a type-1 sample. The nitridation time for the type-5 sample (b) was 30% of that for the type-4 sample (a).

1013

250 (a) (b)

300

350

400

450

(c) 1012

2.0062, 2.0037, and 2.0025. The first and second signals are assigned to the Pb0 and Pb1 centers that are usually observed at pure-SiO2 /Si interfaces, while the third signal (g ¼ 2:0025) is assigned to the modified Pb1 (Pb1 (N)) producing the signal that was seen in Fig. 2(c) (type-1 sample, g ¼ 2:0028). The Pb1 (N) signal is less evident in the ESR record of the type-5 sample with a lower [N ]. The proportion of the intensity of the Pb1 (N) signal to the total intensity of the Pb1 signals (Pb1 (N) and unmodified Pb1 ) was 37% for the type-4 sample and less than 4% for the type-5 sample. Pb1 defects thus seem to be changed to Pb1 (N) defects by the plasma nitridation. A similar change in Pb defect structures was also found in a lightly NO-annealed SiO2 /SiÆ1 0 0æ system that will be reported in detail elsewhere [27]. 3.5. Increase of interface trap density by plasma nitridation

Pb1(N)

3340

61

w/o H2 annealing

The present investigation showed that the dehydrogenation of interfacial Si dangling bonds is the predominant NBTI process at a plasma-nitrided SiON/Si interface, where NBTI has been suggested to depend on the density of interfacial Si–H bonds (precursors of Pb centers and interface traps) present before the application of NBTS [17,18]. The Dit values of H2 -annealed samples were reported to be increased by plasma nitridation [4], but NO annealing was also shown to reduce Dit [24–27]. We therefore needed to evaluate the Dit of plasma-nitrided SiON/Si systems not subjected to H2 -annealing. Fig. 5 shows the Dit values of MOS diodes (sample types 1–3) processed with and without H2 -annealing at 400 C. The type-2 samples were prepared with a plasma nitridation having a shorter duration (40% that for type1 samples). The Dit of the plasma-nitrided SiON in samples not subjected to H2 -annealing was either about three times (type 1) or about twice (type 2) that of pure SiO2 (type 3). Plasma-nitridation, in contrast to NOannealing, was thus found to increase the Dit in the asnitrided samples. 3.6. Generation of suboxides by plasma nitridation

1011

(a) (b) (c) after H2 annealing

1010 10-7

10-6 10-5 10-4 TIME CONSTANT (s)

10-3

Fig. 5. Dit values of nitrided (a,b) and un-nitrided (c) samples processed with (lower graph) and without (upper graph) H2 annealing at 400 C. The [N ] of the type-1 sample (a) was 9%, and the type-2 sample (b) was prepared using a shorter plasma nitridation (40% of that used to prepare type-1 samples).

The Dit increase by plasma nitridation is likely to be caused by plasma damage. Keister et al. reported that plasma nitridation of plasma-oxidized SiO2 /SiÆ1 1 1æ increased the amount of Si suboxides at the interface [28]. Subsequent annealing at 700–900 C was shown to decrease the amount of suboxides, especially of the Siþ component. We therefore examined the effects of plasma nitridation of thermal-SiO2 /SiÆ1 0 0æ and of subsequent annealing on Si suboxides by using types 6–9 (Table 1). A typical Si 2p3=2 spectrum for plasma-nitrided SiO2 / Si is shown in Fig. 6, where XPS signals from Si suboxides are seen between the Si0 peak from the Si

62

S. Fujieda et al. / Microelectronics Reliability 45 (2005) 57–64

Si 0

XPS INTENSITY (arb. units)

hν=404.1 eV Si

4+

0 Si -5

-4

-3

3+

Si

2+

-2

Si -1

1+

0

RELATIVE KINETIC ENERGY (eV) Fig. 6. Typical SR-XPS Si 2p3=2 spectrum of a plasma-nitrided SiON/SiÆ1 0 0æ sample. The excitation X-ray energy was 404.1 eV.

substrate and the Si4þ peak from SiO2 . We carried out a curve fitting by assuming Gaussian-shaped peaks and found that the Si2þ /Siþ signal ratio was higher than that of SiÆ1 1 1æ [28], reflecting a difference in the major bond configuration at the interface: Si2 @Si@O2 (Si2þ ) on SiÆ1 0 0æ and Si3 BSi–O (Siþ ) on SiÆ1 1 1æ.

NORMALIZED XPS INTENSITY

0.10

+3

Si

0.08 0.06 0.10

+2

Si

0.08 0.06 0.10

+1

Si

0.08 0.06 (a)

(b)

(c)

(d)

Fig. 7. Suboxide signal intensity variations induced by nitridation and subsequent annealing. All XPS intensities are normalized by that of Si0 2p3=2 peak. The type-6 samples (a) were SiO2 /Si(1 0 0) structures. Type-7 (b) and type-8 (c) samples were lightly plasma-nitrided, and the type-8 samples were also annealed at a temperature above 1000 C. The type-9 samples (d) had a higher [N ] (9%) than the type-8 and type-9 samples had.

The variation of the suboxide signal intensities is summarized in Fig. 7, where the XPS intensity of each peak is normalized by that of the Si0 2p3=2 peak. The amount of the Siþ component in thermal-SiO2 /SiÆ1 0 0æ (type-6 samples) was increased by nitridation (type-7 samples) and decreased by annealing at a temperature above 1000 C (type-8 samples). Furthermore, the Siþ peak intensity became higher for the type-9 sample, which had a larger [N ] of 9%. The changes in the Si3þ and Si2þ intensities in response to nitridation and annealing were smaller than the corresponding changes in the Siþ intensity. In addition, angle-resolved XPS showed that the Siþ component is mainly present at the SiON/Si interface [28]. The increase in the amount of the Siþ component thus seems to be caused by a breaking of Si–O bonds at the interface. Assuming a photoemission cross-section ratio (r(Si)/ r(Siþ )) of unity and an electron escape depth of 1 nm [28,29], we calculated the increase in areal density of Siþ component from the 2–3% increase if the Siþ /Si0 signal ratio of sample types 7 and 9 over that of type-6 samples: it was 1–1.5 · 1014 cm
2 . The increase of latent Dit by plasma nitridation to above 1012 cm
2 (Fig. 5, upper graph) seems to have resulted from this large change in the interface stoichiometry.

4. Discussion The Pb –H dissociation energy under an electric field was suggested to vary with the Pb –H bond direction [30]. Plasma nitridation increases the Pb1 /Pb0 density ratio and generates modified Pb1 centers (Figs. 2 and 4). These are common features of plasma-nitrided SiON and NOannealed SiON [24–27], suggesting that the modification of interfacial defects is caused by N atoms incorporated at the interface. It is possible that the structural modification includes a change in Pb1 –H bond direction [27], which will enhance the Pb1 –H dissociation. Pb1 defects can contribute to NBTI by releasing Hþ . They can also work as interface traps, as recently shown by an electrically detected magnetic resonance measurement [31,32]. Pb1 defects seem to have a greater effect in SiON/ SiÆ1 0 0æ systems than in pure-SiO2 /SiÆ1 0 0æ systems, since the Pb1 /Pb0 ratio is higher there. Adding N to SiO2 is known to decrease the activation energy of NBTI. Jing et al. suggested that H can be transferred from an interfacial Si–H bond to a nearby N site (Si2 @N–H) with a hole [8,9]. Yount et al. reported that a high density of such sites (H-terminated bridging nitrogen) are present in NH3 -nitrided SiON films [33]. Ushio et al. suggested that when a network N atom (Si3 BN) and a H2 O molecule are near the Si–H bond, the bond is easily dehydrogenated by transferring H and a hole to the N atom [10]. And Tan et al. suggested that a network N atom (Si3 BN) can trap Hþ released from

S. Fujieda et al. / Microelectronics Reliability 45 (2005) 57–64

the interfacial Si–H bonds more efficiently than can a network O atom (Si2 @O) [11]. The efficient trapping of Hþ by N atoms is assumed to suppress the re-hydrogenation of the Si dangling bonds, resulting in a greater degree of NBTI. Since we observed neither generation nor annihilation of bulk defects in the present study, we think that a H atom is transferred to an ESR-inactive site that is also ESR-inactive after trapping Hþ . Threefold-coordinated N (and twofold-coordinated O) atoms will work as such sites. This thought is consistent with the above models that treat network N atoms as efficient Hþ -trapping sites. The degree of NBTI has been suggested to be proportional to the square root of the density of interfacial Si dangling bonds initially terminated by hydrogen (DVth / ½Si–H1=2 ) [17,18]. The Dit of plasma-nitrided SiO2 with a [N ] of 9% (type-1 sample without H2 -annealing) was about three times that of pure SiO2 (Fig. 5). This Dit increase will thus increase the DVth by 73% (31=2 ¼ 1.73) and reduce the lifetime by 89% (1(31=2 )1=0:25 ¼ 0.11). Since NO annealing is known to lower Dit [24–26], the Dit increase is not attributable to interfacial N atoms. It is instead likely to result from the damage caused by the plasma nitridation (Fig. 7). Using plasma conditions that decrease the ion-to-radical density ratio is expected to reduce the damage as well as the amount of N penetrating into the interface.

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

5. Conclusions The NBTI of a plasma-nitrided SiON/Si(1 0 0) system with an EOT less than 1.8 nm was shown to be due mainly to Pb depassivation. NBTS seems to also produce non-Pb defects, but not N dangling bonds. ESR did not indicate that bulk defects were generated or annihilated by NBTS, and this is consistent with the models that assume network N to be an efficient Hþ trap. We found an increase in the Pb1 /Pb0 density ratio, structural modification of Pb1 centers, suboxide formation at the interface, and an increased density of latent interface traps, all of which are likely to increase NBTI in plasma-nitrided SiON/Si(1 0 0) systems. Considering such changes in structure as well as the density of interfacial defects is thought to be important in designing the fabrication process used to make ultrathin plasma-nitrided SiON gates.

[11]

[12]

[13]

[14]

[15]

References [16] [1] Kimizuka N, Yamaguchi K, Imai K, Iizuka T, Liu CT, Keller RC, et al. NBTI enhancement by nitrogen incorporation into ultrathin gate for 0.10 lm gate CMOS generation. In: 2000 Symposium on VLSI Technology Digest of Technical papers, 2000. p. 92–3. [2] Liu CH, Lee MT, Lin CY, Chen J, Loh YT, Liou FT, et al. Mechanism of threshold voltage shift (DVth ) caused by

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