On the permanent component profiling of the negative bias temperature instability in p-MOSFET devices

Solid-State Electronics 106 (2015) 54–62

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On the permanent component profiling of the negative bias temperature instability in p-MOSFET devices Boualem Djezzar ⇑, Hakim Tahi, Abdelmadjid Benabdelmoumene, Amel Chenouf, Mohamed Goudjil, Youcef Kribes Microelectronics and Nanotechnology Division, Centre de Développement des Technologies Avancées (CDTA), 20 Août 1956, Baba Hassen, BP: 17, Algiers 16303, Algeria

a r t i c l e

i n f o

Article history: Received 30 July 2014 Received in revised form 25 November 2014 Accepted 10 January 2015

The review of this paper was arranged by Prof. A. Zaslavsky Keywords: Vertical distribution Charge-pumping Border-trap NBTI stress

a b s t r a c t In this manuscript, we have investigated the negative bias temperature instability (NBTI) induced bordertrap (Nbt) depth in the interfacial oxide region of PMOS transistors using multi-frequency charge pumping (MFCP) method. We emphasize on the distribution of the permanent component in the oxide near the interface, giving a clear insight on its effect on NBTI features. According to the experimental data, the extracted effective dipole moment (aeff) and field-independent activation energy (Ea) have revealed a linear relation with depth distance (Z), which consistently explain the variation of n as well as Ea,eff often reported in the literature. In fact, aeff and Ea increase with the depth, indicating the presence of the precursor defects having different effective dipole moments and activation energies. We suggest that such traps are most likely related to O3xSixSi–H (x = 1 and x = 2) family defects (or Pb center hydrogen complex) located in the interfacial sub-oxide region. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In order to capture NBTI phenomenon, different models have been proposed [1–4]. Despite discrepancies on possible structures of the precursor defects, most results indicate that two components are behind NBTI [5–8]. The first one is permanent and often related to the interface trap creation (DNit), while the second one is recoverable and attributed to hole trapping process within the oxide. It has been shown elsewhere [4] that the reaction–diffusion (R–D) model and its variants cannot explain the relaxation phase as well as the duty cycle dependence. In addition, R–D model alone fails to explain the cycling behavior and the relaxation in on the fly (OTF) threshold voltage OTF-Vth and ultra fast Vth experiments observed by Ang et al. [9]. These findings increase doubts over whether the diffusion mechanism is really driving interface state generation and recovery [4,9,10]. On one hand, some research groups consider hole trapping as the prevailing process [5], while other groups still believing that R–D process is central to NBTI [8,11]. On the other hand, the permanent component is not permanent forever, but recovers at long time scale compared to the fast recoverable component [12]. This implies that the permanent component is not only related to ⇑ Corresponding author. Tel.: +213 (0) 21 35 10 40; fax: +213 (0) 21 35 10 21. E-mail address: [email protected] (B. Djezzar). http://dx.doi.org/10.1016/j.sse.2015.01.001 0038-1101/Ó 2015 Elsevier Ltd. All rights reserved.

interface traps (Nit), but also to the near interface oxide traps or border traps (Nbt) even in high frequency CP data [13]. Different values of NBTI parameters, such as the power-law time exponent (n) and the apparent (or effective) activation energy (Ea,eff) with various explanations were proposed by different groups [1–8]. Discrepancies have been reported by several groups [4,9] regarding the relaxation phase [4], and n [9]. Although the improvements brought to the R–D model to match experimental data giving subsequently a steady value of 0.16 for exponent n. Ang et al. [9] have shown that T dependence of n emanates from the superposition of two mechanisms with different thermal activations in cyclic NBTI. In addition, different Ea,eff ascribed to different physical mechanisms have been also reported to explain NBTI phenomenon [14–16]. Schroder et al. [17] have reported dispersion of NBTI Ea,eff values ranging from 0.15 to 0.325 eV, depending on the chemical composition of the dielectric and on the phenomenon that dominates the reaction kinetics as well as the species involved. Published reports on macroscopic NBTI modeling [11] showed that Ea,eff for hole trapping in the pre-existing traps is about 0.04 eV, which is typically expected for tunneling processes without structural relaxation, while the signature of DNit is 0.1 eV in nitrided oxides [9]. In contrast, using time-dependent defect spectroscopy (TDDS) method, Grasser et al. [18] found that the physical microscopic Ea,eff of the individual traps is about 0.5–1.2 eV in small gate

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area devices. They argued that the small overall macroscopic Ea,eff (0.1 eV) [8,9], observed in large gate area devices, is the result of individual trap energy superposition. Thus, the macroscopic Ea,eff in large area is an apparent activation energy resulting from a large number of defects [17]. Therefore, further analysis is required to reconcile contradictions between single trapping event in isolated traps and trapping in a collection of traps. The large spread in NBTI experimental features complicates all attempts to model the NBTI degradation. These disparities suggest that different defects with different structures [3,5,9,16,18,19] are involved in the NBTI phenomenon. These defects do not necessarily have the same Ea. The defect energy distribution may be related to Si/SiO2 interface disorder and bulk SiO2 disorder, especially in the interfacial oxide region [20]. The comprehension of their behavior with NBTI stress is necessary to develop a model capable to interpret a wide number of NBTI experiments. For this purpose, we investigate the depth profiling of the permanent component by exploring the traps lying in the oxide near the interface using the MFCP [21,22]. 2. Experimental details 2.1. Device and process In this work, several p-MOSFET devices were investigated. They were fabricated at ISiT (Institute for Silicon Technology) of Fraunhofer, Germany, with different gate sizes; fixed gate width at 10 lm and varied gate length from 0.8 to 1 lm with a gate oxide thickness of 20 nm and a gate capacitance of 2.12  107 F cm2. We have used thick gate oxide to reduce the parasitic leakage current and tunneling current. This oxide will not hinder the analysis, since the basic mechanisms behind the NBTI are found to be similar in both thin and thick oxides. Indeed, defects induced by NBTI are mainly located at the Si/SiO2 interface and in the interfacial oxide region [5,7,23]. Besides, we have used transistors with short gate lengths (<1 lm) to avoid both geometric and quasi-geometric components at high [24] and low [25] frequencies, respectively. 2.2. Setup details and MSM procedure To investigate the NBTI-induced traps in the interfacial oxide region, we have performed scanning profile of the permanent components. The profiling is obtained using MFCP method [21,22]. The frequency (f) is varied from 500 kHz to 5 kHz by changing high level voltage time (TH) and low voltage time (TL) (TH = TL), while rise and fall time slopes are kept constant (a = 0.1 V/ns). This is to guarantee that the lower and the higher energy boundaries

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contributing to CP remain unchanged, even if the amplitude increases. The trapezoidal pulse is illustrated in Fig. 1. All experiments are performed using fully automated bench, which includes a sensitive Agilent HP 4156C (with maximum f of 500 kHz) for stress and measure. A hot plate is used, inside Karl Suss AP4 micro-manipulator probe station, to vary the temperature of the chip during the experiments. The test circuit chip (non-packaged) within the probe station is isolated from vibration and enclosed in a grounded faraday cage to avoid both RF and light effects. 2.2.1. Before NBTI stress Before starting NBTI stress, we have first measured CP current (ICP) as a function of the frequency for all used transistors. Fig. 2 illustrates the ICP of these transistors at room temperature (27 °C) (Fig. 2(a)) and 120 °C (Fig. 2(b)). The curves are obtained using MFCP method with fixed signal amplitude, DVG at 4 V. As expected from theory, ICP decreases with decreasing f and increasing T [26] (a slight decrease with T). In addition, charge recombined per cycle as a function of the frequency is also plotted in Fig. 2(c) and (d) for 27 °C and 120 °C, respectively. Basically, QCP is the same and fairly constant with frequency, indicating the absence of the pre-existing border-trap at low frequency for all used transistors. In summary, the transistors used in these experiments exhibit the same features regarding ICP and QCP versus f, eliminating any artefact due to the use of different devices. 2.2.2. MSM of NBTI The NBTI permanent component profiling is carried out using MSM protocol, as illustrated on Fig. 3(a). The measurement phase is performed using the MFCP method. During the stress interval, a DC voltage, VGstr is applied to the gate of the device. After each stress time, a gate pulse train with different frequencies is applied. The signal has an amplitude DVG = VH  VGstr, with VL = VGstr duty cycle of 50%. The source and drain voltage (VS = VD = VR) are set at 0 V. The measurement time to scan all frequencies is 30 s and kept the same during all MSM cycles for different T and VGstr. The whole stress time cycle is fixed at 7.5 h. The high frequency is limited by the Agilent HP 4156C capabilities, while the low frequency is chosen the lowest possible as long as the CP signal remains distinguishable from leakage current induced by the amplitude of the signal. Obviously, this limits the scanning depth into the oxide layer. However, this is not an issue in NBTI, since the latter occurs at the vicinity of the interface [5,7,23]. Different DVG are used; 10, 11, and 12 V, VL = 8, 9, and 10 V (used for stress), and VH is fixed at 2 V, respectively, except if otherwise specified. The MSM consists of stressing PMOS

Fig. 1. Gate voltage signal. The pulse is characterized by the amplitude (DVG), the rise time (tr) and fall time (tf). tacc and tinv times correspond to electrons and holes capture times in PMOS transistors. The emission times for electrons and holes are tem,e and tem,h, respectively. a is the rising and falling time slope rates.

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Fig. 2. Charge pumping current, ICP versus frequency for p-channel transistors with fixed gate width and varied gate length at ambient temperature (a) and 120 °C (b). Charges recombined per cycle, QCP as a function of the frequency are given in (c) and (d) for ambient and 120 °C, respectively. The measurements are performed using gate pulse amplitude of 4 V.

transistors with different VGstr at room temperature (27 °C) and higher at 80 and 120 °C. The charge recombined per cycle was measured at different frequencies after 30, 90, 180, 300, and 450 min of stress time, as shown on Fig. 3(b). The first plot illustrates QCP before stress (i.e. at t = 0). All plots show the same features, namely an increase with decreasing f [21,22]. However, for each f, QCP increases with stress time as illustrated by the dashed lines. In other words, QCP is more important after stress than before for the same f (i.e. the same region in the Si/SiO2 interfacial region). This fact indicates that NBTI stress introduces new traps that are lying from Si/SiO2 interface to the interfacial oxide layer. To investigate such kind of traps, we explore their spatial distribution in the next sections. 2.2.3. Extraction method of border-trap density Using MFCP method, the charge recombined by cycle at the interface and in the sub-oxide interfacial layer can be estimated by:

NT ðf Þ ¼

Q CP ðf Þ jICP ðf Þj ¼ qAG qAG f

ð1Þ

Typically, CP current at high f (in our case 500 kHz) is conventionally considered as recombination current at Nit [26]. The additional NT at lower frequencies is attributed to the contribution from bulk traps in the SiO2 dielectric near the interface [21]. These border traps are located in the interfacial region and their densities can be estimated for each f as:

Nbt ðf Þ ¼

jQ CP ðf Þ  Q CP ðf H Þj jQ CP ðf Þ  Q CP ð500 kHzÞj ¼ qAG qAG

ð2Þ

where q (C) is the electron charge, AG (cm2) is the gate area, fH (Hz) is the high frequency, f (Hz) is the frequency, ICP (A) is the CP current, and QCP (C) is the recombined charge by cycle. Nbt (f) (cm2) is given as cumulative areal border-trap density (brought to the Si/SO2 interface area). On the other hand, f is related to the tunneling distance into the oxide [27] by:

Z ¼ kp lnðps rp ð0Þv thp t inv Þ

ð3Þ

where Z(Å) is the maximum tunneling distance, reached by holes of the p-channel inversion regime, in the sub-oxide layer. kp (Å) is the characteristic of hole tunneling distance in the oxide and defined by its energy barrier and the effective mass of holes as follows:

h kp ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2mp EV

ð4Þ

tinv is the inversion time and can be easily estimated from Fig. 1 for tr = tf as:

tinv ¼

  1 V L  V th DV G þ 2 1 2f DV G a

ð5Þ

Then (3) yields:

      1 V L  V th DV G h Z ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ln ps rp ð0Þv thp þ 2 1 2f DV G a 2 2mp EV

ð6Þ

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Fig. 3. (a) Stress/measurement/stress (SMS) protocol used to extract the profile of the NBTI permanent component into the interfacial oxide region. (b) Frequency dependence of QCP in p-channel transistor for different stress times at 120 °C and 8 V. Measurement performed with trapezoidal gate pulse of amplitude 10 V and varied frequency.

where rp(0) is the capture cross section at the Si/SiO2 interface, EV is the energy barrier height for holes, ps is the hole concentration at the silicon surface during inversion time, vthp is the hole thermal velocity, mp is the hole effective mass,  h is the reduced Plank’s constant, Vfb and Vth are the respective flatband and threshold voltages. Since some parameters in Z are T-dependent, it is therefore calculated for each T. However, in (6) rp(0) is assumed independent of T to simplify the calculation of Z. In fact, more accurate rp(0) should be dependent on temperature involving the multi-phonon process [5,28]. This approximation is based on the well established reports from different research groups regarding the relation between defect depth in the oxide and the CP frequency in thick gate oxide [29–31]. Contrarily, Ryan et al. [28] conclude that the relationship is more complex than that expected. They claim that the effective tunneling length to an oxide defect can be different than its actual depth. This constitutes an issue for dual layer SiO2/HfO2 gate stacks regarding the physical location of stress induced traps, whether they are located in SiO2 interfacial layer or in HfO2 bulk. In addition, in very small gate area, each defect presents an individual capture and emission time [18], which can be associated with a single frequency. Therefore, considering a straightforward relationship between frequency and depth is, in fact, an easy way to illustrate a so complicated picture of hazardous energy and space distribution of border traps. That is not an issue for large area transistors with single type thick layer of SiO2, where a large number of defects exist and their pathway tunneling distances can be averaged by a direct relationship between tunneling distance and frequency. From (2) and (6), one can extract trap distribution in the interfacial oxide region. To get the cumulative DNbt(ts) during NBTI stress, we have used the following equation:

DNbt ðts Þ ¼ Nbt ðti Þ  Nbt ðt 0 Þ

ð7Þ

3. Experimental results It is worth to recall that when f is decreased, the applied gate voltage drives the device into inversion regime for a long time. This allows tunneling processes to take place where, not only Nit responses, but also the Nbt within an appropriate distance for a given f [21]. However, the leakage parasitic current can compromise the analysis of CP data as a function of f, especially in NBTI stress using CP-based methods [32,33] where VL (for p-MOSFET) is set at high VGstr resulting in excessive tunneling into oxide [13,34]. In fact, in devices with thick gate oxide, the leakage component is negligible. However, it is not the case in the stress phase and hence it should be deduced from the measured current [35]. Classically, the parasitic current corrections are carried out by either subtracting the current at 0 Hz-f CP [36] or the lower frequency CP current from the high frequency current [34]. Veksler et al. [31] have subtracted 1 kHz-f CP current from all experimental data. In our case, we have subtracted the low frequency (5 kHz) CP current from higher frequencies data. 3.1. NBTI-induced border-trap within interfacial sub-oxide region In Fig. 4(a) and (b), we have respectively presented the NBTIinduced NT density and the cumulative Nbt as a function of the depth into the oxide (Z). They are extracted using (1) and (2). The trap profile has the same shape as that found in the literature using both calculation and measurement [22,37]. The Fig. 4(a)

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Fig. 4. (a) Depth trap profiling before and after NBTI stress for different stress time at 8 V and 27 °C. (b) Cumulative border trap profiling before and after NBTI stress for different stress time at 8 V and 27 °C.

represents the first derivative of the Fig. 4(b). It is clear that NBTI stress modifies the trap density profile. The longer the stress time, the higher the trap density. To estimate trap density distribution, we have subtracted the profile before stress from that obtained after each stress time, as indicated by Eq. (7). The variation of the cumulative DNbt profile with NBTI conditions is depicted in Fig. 5(a) for various T and Fig. 5(b) for VGstr. From the electrical viewpoint, the cumulative DNbt increases rapidly with Z and tends to be almost constant. That means there are no significant additional traps. However, DNbt around 6–7 Å exhibits a strong dependence with T and VGstr. Quantitatively, DNbt is more affected by VGstr than it is by T and tends to saturate with higher stress voltage at 120 °C. All the above illustrated curves in Figs. 4 and 5 start roughly at 5 Å. They are obtained for capture cross sections of electrons and holes at the Si/SiO2 interface rn(0) = 5 . 1015 and rp(0) = 1016 cm2 and mn = mp = 0.5m0 (details are given in [29]). At fH (500 kHz), we have found that CP-signal, corresponding to fast traps, emanates not only from Nit but also from defects located at approximately 5 Å within interfacial oxide layer, even if frequency is increased (for example 1 MHz corresponds to Z = 4.6 Å and 10 MHz to Z = 3.5 Å). That means that the electrical CP-interface cannot be a perfect line but rather extends few angstroms into the transition oxide layer. The same observation has been reported by other research groups [10,13]. 3.2. aeff and Ea distributions in the interfacial oxide layer It is widely accepted that the NBTI degradation is originated from the electrochemical reaction at the Si/SiO2 interface, where

Si–H bond can break under thermal stress due to strong coupling of intrinsic defect activation energy with the local oxide electric field [38]. The dipole-field coupling serves to lower the activation energy required for thermal bond breakage and accelerates the dielectric degradation process [39]. As previously shown, our experimental analysis revealed a strong dependence between Ea,eff and EOX. To experimentally determine the values of aeff and Ea (for EOX = 0) for traps that are at the origin of DNbt behavior, we plot Ea,eff versus EOX. In Fig. 6, we present Ea,eff (EOX) for stress time of 90 min (Fig. 6(a)) and 450 min (Fig. 6(b)). The Ea,eff(EOX) curves give aeff and Ea as the slope and the intercept at EOX = 0, respectively. The extracted parameters versus Z are plotted in Fig. 7(a) for aeff and Fig. 7(b) for Ea. Both parameter data vary linearly with Z (from 5 and 7 Å) and remain roughly constant with stress time. According to (6), the minimum scanned Z is 5 Å for a frequency of 500 kHz. Therefore, to get Ea and aeff corresponding to Z < 5 Å, we have linearly extrapolated Ea(Z) and aeff(Z) functions to Z-axis. We have found Ea(Z) = E0  0.11 eV and aeff(Z) = a0  1.2 q Å and are located at a distance Z  3.2 Å. Interestingly, the latter is in the range of the interface roughness (or the interface thickness, d in R–D model), which is about 0.2–0.3 nm [2,38,40]. The same E0 and a0 were calculated and experimentally extracted for Si–H bond at the interface in R–D model [10,41]. On the other hand, aeff data range between 3 and 10 q Å. The theoretical calculations of McPherson et al. [39,42] have shown that the effective dipole moment of 7 q Å is consistent with an oxygen vacancy (Si–Si) bond, or a hole-captured Si–O bond, or an Si–H bond in the oxide, while the strained Si–O bonds are associated with 13 q Å dipole. Furthermore, Ea ranging between 0.3 and 0.6 eV (see Fig. 7(a)) is in agreement with those reported for an

Fig. 5. NBTI-induced cumulative border trap, DNbt as a function of depth, Z (a) for different stress temperatures at 8 V and (b) for different stress voltages and 120 °C.

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Fig. 6. Ea,eff as a function of electric field, EOX, illustrating extraction procedure of Ea and aeff.

DNbt / A exp ½ðE0  a0 EOX Þ=KT  exp ½ðB  B0 EOX ÞðZ  Z 0 Þ=KT  tn ð8Þ where A(EOX) is a pre-factor depending on the electric field. From (8), it is clear that traps involved in NBTI using MFCP are lying from the interface (represented by the first term) into the near-interfacial oxide region (represented by the second term). In the latter region, different traps having different aeff and Ea coexist. The possible precursor candidates for such kind of traps are explained hereafter. In fact, to identify the origin of Ea and aeff variations with Z, it is important to better understand the local atomic structure of the interfacial region lying between crystalline Si-substrate and non-crystalline SiO2 oxide.

4. Defect within the interfacial Si/SiO2 region

Fig. 7. (a) Activation energy as a function of depth and (b) effective dipole as a function of depth. E0 = 0.11 eV and a0 = 1.2q Å are located at depth of 3.2 Å (roughness thickness) and correspond to Si–H bond at the interface in R–D model.

individual trap in the oxide (between 0.5 and 1 eV) [4,18]. This point will be detailed in the next section. According to our data, DNbt can be expressed as:

Due to Si/SiO2 interface and bulk SiO2 disorders, especially the interfacial region, the Si–H bonds are arbitrarily distributed and oriented in this region, resulting in spread activation energy of different Si–H bonds. Indeed, Si/SiO2 interface is not atomically abrupt, but somewhat shows a transition region of about 0.5– 1 nm thick [40,43,44]. This region (SiOx, x < 2) constitutes the chemical transition between the Si-substrate and the oxide. The interface roughness is determined by the net stress projection across the interface boundary by the oxide. Based on electrical electron paramagnetic resonance (EPR) and spin dependent resonance (SDR) signals, Fleetwood et al. [20] have proposed a classification of traps (such as interface traps, border traps (fast and slow), and oxide traps) for radiation issue at the interface and within interfacial region. Using this categorization of traps as well as the hydrogen species which have a substantial population in this region, we suggest for NBTI phenomenon, the defect precursors with different Si–H bonds, as schematically illustrated in Fig. 8. In addition to the saturated Si dangling bonds at the interface (Pb–H), O3xSixSi–H family defects coexist with x varying from 0 to 3. Their concentration is more important compared to other defect precursors at the vicinity of the interface. For x = 3 (Si3Si– H), the Si–H bond is surrounded by three Si atoms, which is a cluster in SiOx that looks very much like Pb–H. Therefore, when depassivated, it acts like interface Pb, only it switches more slowly, since it is in near interfacial oxide instead of being located at the interface. For x = 1 or 2 (O2SiSi–H or OSi2Si–H), the Si–H is surrounded by two O atoms and one Si atom or by one O and two Si, respectively. It is similar to Pb1–H center at the (1 0 0) Si/SiO2 [45]. For x = 0 (O3Si–H),

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Fig. 8. Schematic representation of defect precursors behind NBTI permanent component. They are mostly located in the interfacial oxide layer.

the Si–H bond is surrounded by three O atoms. Moreover, it has gap states and is similar to E0 c center [46]. In Fig. 9, we plot the volume trap density (nT = dNT/dZ) contributing to the permanent component, as a function of Z. nT profile lies between 5 and 7 Å. The lower the frequency, the thicker the scanned region (for example for fL = 1 kHz, Z = 9 Å). In addition, NBTIinduced nT increases and tends to saturate with stress voltage and looks propagating in the sub-oxide. As schematically illustrated on the top of Fig. 9, the border traps are most likely concentrated in the transition layer, where O3xSixSi–H defect family is dominant. From the above picture, Si–H dipole, which is commonly used to interpret NBTI, is spatially scattered in the interfacial region and differently bonded to various O/Si configurations. Consequently, the basic physical and chemical processes that drive the NBTI phenomenon have different time constants and activation energies. That is why the exponent n is temperature, and field dependent in the time scope of our experiments.

5. Discussion and analysis NBTI-induced DNbt into the interfacial region do not exhibit the signature of hole trapping in the pre-existing bulk traps, but are newly generated during the stress, because they increase with stress time up to 27,000 s. If they were pre-existing traps, they should saturate at <1 s [41]. That is why, they could not be preexisting traps, and they should be something else. Moreover, their apparent activation energies are higher than those reported for hole trapping (0.02–0.06 eV) in the literature, see [3,9,38]. In this work, the apparent energy is in the range of 0.11–0.2 eV, similar to that reported for the interface traps in [2]. Therefore, DNbt traps, measured in timescale of our experiments, look electrically like the interface traps, but they are located in the oxide near the interface, where O3xSixSi–H family precursors are dominant. The distribution of O3xSixSi–H trap precursors could explain the distribution of Ea and aeff as a function of Z, observed in

Fig. 9. Profiling of NBTI-induced traps in the interfacial oxide region as a function of depth. Onset is qualitative schematic position of the scanned traps. Assuming that traps, responding at CP-500 kHz, are interface traps and all those below to 5 kHz are considered as border traps.

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Fig. 7, (Ea (0.6–0.11 eV) and aeff (10–1.2 q Å)). Defect precursors change from the interface Si–H bond surrounded by three Si atoms to Si–H surrounded by three O atoms transiting through the other combination of O3xSixSi–H family defects. However, the linear extrapolation made from data to Z0 (see Fig. 7) is an easy illustrative way to explain the different values of n, Ea, and aeff found here and often reported in the literature [1–9]. Aichinger et al. [19,47] have pointed out, in their studies of NBTI stress on 30 nm and 5 nm gate oxide thicknesses, that NBTI can generate traps which contribute to threshold voltage shift, but do not respond to high frequency CP (500 kHz). Based on EPR studies and theoretical calculations using functional density theory [48], they suggested Pb center-hydrogen complex as a microscopic structure for such traps. The measure of the SDR signal shows a hyperfine (HF) doublet symmetric to Pb line (g = 2.0069). In addition, shallow and deep oxide traps have been identified as a possible source for NBTI [49]. The former is located in the whole channel, while the latter in the LDD region. However, both [47,49] did not specify the atomic structure of defect precursors behind NBTI-induced traps. Based on the aforementioned data from this work and data collected from the literature [20,40,43,44,47], we suggest that O3xSixSi–H complex family is the more probable atomic structure of Pb center-hydrogen complex. Nevertheless, due to the inhomogeneous broadening of trap precursors in space and energy, the NBTI phenomenon could be underestimated if only one Si–H dipole type is considered. Therefore, any attempt to model NBTI has to account for the bonding within SiOx sub-oxide, as it plays a critical role in defining the device performance and reliability. It is important to recall that our results concern primarily the permanent damage component, since the MFCP measurement time is 30 s. This is not really an issue, since during the effective operating (normal) conditions of circuits, only the permanent component is occurring and affects the circuit performances. However, the recoverable component plays an important role during NBTI stress by using high temperature and voltage to accelerate the degradation. It is essential to explain full range of NBTI experiments, including short-time stress data. Despite its long measurement time, the depth scanning method allows in-depth analysis of the NBTI permanent degradation in the interfacial sub-oxide layer. This approach enables profiling the physical parameters of border-trap involved in NBTI; such as the activation energy and the effective dipole moment, necessary for the development of accurate models to predict device lifetime.

6. Conclusion Using MFCP method, we have been able to profile NBTI-induced border-trap in the interfacial oxide layer. We found that the border traps have different effective dipole moments and thermal activation energies. The latter are linearly distributed from the interface to the sub-oxide layer. We have also shown that border traps could be related to Pb center-hydrogen complex precursors, which could correspond to O3xSixSi–H complex defect family. In addition, data suggest that it is very hard to separate the interface traps from border traps, because the Si/SiO2 interface has always a thickness containing O3xSixSi–H complex. On the other hand, we have found that NBTI degradation tends to saturate with T and VGstr. It seems likely that the earlier degradation starts very near the interface and propagates into the interfacial sub-oxide region. Its capability to sense border-trap, makes MFCP method suitable to scan different quasi permanent NBTI components lying into the interfacial oxide region, since the NBTI processes are most likely located in that region. It will be valuable to deeply extend

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