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Electrical characteristic fluctuation of 16nmgate highκ/metal gate bulk FinFET devices in the presence of random interface traps
Nanoscale Research Letters volume 9, Article number: 633 (2014)
Abstract
In this work, we study the impact of random interface traps (RITs) at the interface of SiO_{ x }/Si on the electrical characteristic of 16nmgate highκ/metal gate (HKMG) bulk fintype field effect transistor (FinFET) devices. Under the same threshold voltage, the effects of RIT position and number on the degradation of electrical characteristics are clarified with respect to different levels of RIT density of state (D_{it}). The variability of the offstate current (I_{off}) and draininduced barrier lowering (DIBL) will be severely affected by RITs with high D_{it} varying from 5 × 10^{12} to 5 × 10^{13} eV^{−1} cm^{−2} owing to significant threshold voltage (V_{th}) fluctuation. The results of this study indicate that if the level of D_{it} is lower than 1 × 10^{12} eV^{−1} cm^{−2}, the normalized variability of the onstate current, I_{off}, V_{th}, DIBL, and subthreshold swing is within 5%.
Background
For the last decades, the technology of siliconbased CMOS devices suffered significant fabrication challenges and sizeable characteristic variability [1–5]. Characteristics could be affected by various traps in highκ/metal gate (HKMG) devices [6]. For emerging ultrascaled transistors, the characteristic degradation induced by interface traps at the interface of SiO_{ x }/Si is severe for gigascale circuit designs [7]. Furthermore, random interface traps (RITs) appearing at the interface of SiO_{ x }/Si depend on different fabrication processes of HKMG [8–13]. Except planar MOSFETs, fintype field effect transistors (FinFETs) with HKMG play a key role in sub22nm technology nodes to boost electrical performance [14–16] and suppress various fluctuations. Recent studies reported the density of interface traps (D_{it}) resulting from the orientations of the vertical fin channel of FinFETs [6, 17]. The effects of RITs on sub22nm FinFETs have also been reported and compared between different device structures [18, 19]. Unfortunately, the impact of RITs on 16nmgate HKMG bulk FinFET devices has not been clearly discussed yet.
In this work, we study the DC characteristic fluctuation induced by RITs at the SiO_{ x }/Si interface of 16nm TiN/HfSiON gate stack bulk FinFET devices by using experimentally calibrated threedimensional (3D) device simulation. Under the same threshold voltage, more than 50% suppressions on the standard deviation of threshold voltage and subthreshold swing (SS) are achieved, benefiting from the nature of the vertical channel, compared with the planar MOSFET devices. By considering different levels of D_{it}, the effects of RIT position and number on the degradation of electrical characteristics are also examined. This paper is organized as follows: In the ‘Methods’ section, we illustrate the RIT simulation flow. In the ‘Results and discussion’ section, we report the results and discuss the characteristic fluctuation resulting from RITs on 16nmgate bulk FinFET devices. Finally, the conclusions are drawn.
Methods
RIT simulation method for FinFET devices
We study Sibased 16nmgate HKMG bulk FinFETs and planar MOSFET with amorphousbased titanium nitride/hafnium oxide/silicon oxide (TiN/HfO_{2}/SiO_{ x }) stacks of gate dielectric and an effective oxide thickness (EOT) of around 0.95 nm (EOT = T_{o} + T_{h} × ϵ_{SiO2}/ϵ_{HfO2} = 0.6 + 2 × 3.9/22 = 0.95 nm), where T_{o} is the thickness of SiO_{ x }, T_{h} is the thickness of HfO_{2}, and the dielectric constant of HfO_{2} is assumed to be 22. An aspect ratio of 4 (i.e., H_{f}/W_{f} = 32 nm/8 nm = 4), a 30nmlong source/drain (S/D), and an 8nmlong S/D extension for the explored FinFET are considered, as shown in Figure 1 (a). The doping applied to the channel (N_{ch}), source/drain (N_{S/D}), substrate (N_{B}), and source/drain extension regions is 4 × 10^{18} cm^{−3}, 5 × 10^{20} cm^{−3}, 10^{15} cm^{−3}, and 6 × 10^{18} cm^{−3} for the ntype 16nmgate HKMG bulk FinFET devices, respectively. First, we calibrate the nominal DC characteristic of the studied devices according to the International Technology Roadmap for Semiconductor (ITRS) roadmap for lowpower applications, which was experimentally quantified in our recent study, and fix the threshold voltage at 300 mV. To estimate device characteristics, a set of 3D driftdiffusion equations coupled with the density gradient equation for quantum correction is performed [20–23]. The mobility model used in the 3D device simulation involves fin channel surface roughness, highfield saturation, and impurity scattering. The mobility model was quantified with our device measurements for the best accuracy, and the characteristic fluctuation was validated with the experimentally measured DC baseband data from 15/20nm CMOS and FinFET devices in our earlier work [24].
To perform 3D device simulation with twodimensional (2D) interface trap fluctuation (ITF) for each randomly generated device sample, we assume that the size of each RIT (S_{RIT}) is equal to 2 nm × 2 nm at the interface of SiO_{ x }/Si. Notably, the value of S_{RIT} is numerically set for the simulation of ITF [2, 25]. To generate RITs for the statistical device simulation of ITF, we first generate 3,389 acceptorlike traps marked as pink color in the three large 2D planes for the ntype FinFET device, as shown in Figure 1, and the corresponding concentration of RITs is around 1.5 × 10^{12} cm^{−2}[26, 27]. The total number of generated acceptorlike traps follows the Poisson distribution, as shown in Figure 1 (b)(b”). Then, we partition the large planes into many subplanes and map them to form a surface of RITs, as shown in Figure 1(c), where the number of traps in the subplanes varies from 0 to 6 at the top side and from 0 to 14 at the lateral sides, and the average number of interface traps is 3, 8, and 8, respectively. The concentration of each RIT (N_{it}) on a subplane is randomly assigned according to the RIT’s energy following the relationship, as shown in Figure 1 (d). Then, the level of D_{it} is the total product of (N_{it} × S_{RIT}) divided by the total area of the SiO_{ x }/Si interface. Ultimately, we repeat this process until all subplanes are assigned. Notably, each subplane with RITs is numerically solved with the quantum mechanically corrected device model, where the RITs appear at the righthand side of the Poisson equation.
Notably, the D_{it} varies with respect to the different process treatments on the TiN/HfSiON gate stacks [9, 11, 13], so we also consider the impact of three different D_{it} levels on device performance degradation. The ranges of high, typical, and low D_{it} vary from 5 × 10^{12} to 5 × 10^{13} eV^{−1} cm^{−2}, 1 × 10^{12} to 1 × 10^{13} eV^{−1} cm^{−2}, and 5 × 10^{11} to 5 × 10^{12} eV^{−1} cm^{−2}, respectively. For the ptype devices, we have a similar simulation setting with modification of the acceptorlike traps to donorlike traps. For the planar MOSFET ITF simulation, it follows our recent work, as shown in Figure 1 (a’), (b”’), and (c’), and the details could be found in [7, 28].
Results and discussion
The nominal V_{th} for the fresh device (i.e., device with ultralow D_{it}) is calibrated to 300 mV using a constantcurrent method. To meet the ITRS roadmap for lowpower applications, the voltages applied for the 16nmgate HKMG bulk FinFET and planar MOSFET devices were 0.6 and 0.8 V, respectively. As shown in Figure 2, we firstly simulate the RITfluctuated I_{D}V_{G} curves for the n/ptype bulk FinFET (Figure 2a,b) and n/ptype planar MOSFET (Figure 2c,d) devices with different levels of D_{it}, respectively. For all devices, the magnitude of ITF becomes smaller as the level of D_{it} decreases. The inset tables list the estimated fluctuation of I_{on} (σI_{on}), I_{off} (σI_{off}), and V_{th} (σV_{th}) for devices with different levels of D_{it}. As shown in Figure 3, we compare the ITsfluctuated V_{th} under different levels of D_{it}, where the normalized standard deviation (σ/μ) of V_{th} is calculated, and σ and μ are the standard deviation and average of the fluctuated cases, respectively. The V_{th} shifts and its normalized standard deviation becomes larger when the level of D_{it} is increased. Both the n and ptype FinFET devices, as shown in Figure 3a,c, have comparable magnitudes of σ/μ which are smaller than that of the planar MOSFET devices (about 50% reduction), as shown in Figure 3b,d. Nevertheless, the ITsfluctuated V_{th} is strongly governed by high D_{it} varying from 5 × 10^{12} to 5 × 10^{13} eV^{−1} cm^{−2}. In Figure 4, we show the I_{on} versus I_{off} for all devices with different levels of D_{it}. The results of the normalized standard deviation of I_{on} and I_{off} imply that the advantage of the vertical channel in the suppression of ITF will be weakened when the level of D_{it} is increased; for example, the ellipsoidshape distribution of I_{on} and I_{off} is broadened as the D_{it} increases. For the cases of low D_{it}, as shown in Figure 4a,c, the FinFET σ/μ of I_{on} and I_{off} is about three times smaller than that of the planar device, owing to their significant structural dominance. However, such strength is destroyed with the increasing level of D_{it}; as listed in the inset tables, the normalized standard deviations are considerable and comparable between the two devices for the cases of high D_{it}, in particular, the σ/μ of I_{off}.
The degradation of SS becomes more critical when the level of D_{it} increases. Owing to large gate capacitance (C_{g}) coupling in FinFETs, the dependence relationship of ITsfluctuated SS versus draininduced barrier lowering (DIBL) is reduced, as shown in Figure 5a,c; however, the distribution of SS versus DIBL exhibits a negative dependency in the planar MOSFETs, as shown in Figure 5b,d. The significant dependence relationship of ITsfluctuated SS versus DIBL indicates that the characteristic degradation was caused by an even stronger shortchannel effect [29]. To maximize V_{DD} scaling for logical application, the fluctuations of transconductance (g_{m}) and subthreshold swing must be minimized. As shown in Figure 6, the ITsfluctuated transconductances are calculated for the studied devices with different levels of D_{it}. The flatter normalized standard deviations (within 2%) of the maximum transconductance (g_{m,max}) listed in the inset tables are found for the FinFET devices with high D_{it}, as shown in Figure 6a,c.
To go deep into the physics of the results reported above, as shown in Figure 7, we now examine the advantage of the vertical structure and the effect of random distribution (i.e., the random position) and the random number of ITs on the surface potential profiles of the ITsfluctuated devices. As shown in the inset of Figure 7a, along the channel direction (Zdirection) from the source (S) to the drain (D) at the interface of SiO_{ x }/Si on the top gate, the two profiles of conduction band are extracted and compared for the fresh FinFET device and the FinFET device with high D_{it} under offstate condition, where the barrier difference induced by a single interface trap is about 0.1365 eV. Similarly, as shown in Figure 7b, for the planar MOSFET device, the barrier difference is about 0.2732 eV. Comparison between Figure 7a and 7b indicates the significant structural effect; the planar MOSFET device severely suffers from the impact of RIT compared to the FinFET one. The coupling of gate electrodes from both the lateral sides to the top gate enhances the C_{g}, and thus, it effectively reduces the impact of RITs on the energy band. The findings of this comparison confirm the superiority of a 3D channel structure and the aforementioned results. The random position effect of RITs is further examined for the FinFET device. For similar I_{on} and different I_{off}, the two illustration cases (case A and case B) shown in Figure 7c have the same number of ITs (15 ITs) but different V_{th} owing to the different positions of RITs. For similar I_{off} and different I_{on}, the numbers of ITs for the two illustration cases (case A and case C) shown Figure 7c are 15 and 17. Therefore, according to the random number effect, they have different V_{th} because the effective D_{it} of case C is higher than that of case A. Thus, the device has similar I_{off} and different I_{on}.
Conclusions
In this work, we have investigated the impact of RITs on n/ptype 16nmgate HKMG bulk FinFETs using an experimentally validated device simulation technique. We examined the ITsfluctuated shortchannel effect (SCE) parameters for the bulk FinFET and planar MOSFET devices. Benefiting from the improved gate controllability and stronger gate coupling capability, the estimated normalized standard deviation indicates that the 16nmgate HKMG bulk FinFET devices can effectively suppress the DC characteristic and SCE parameter fluctuations induced by RITs with respect to different levels of D_{it}. The insets of Figures 3, 4, 5, 6 listed the fluctuation magnitudes of I_{off} and DIBL which are severely governed by RITs with high D_{it} level ranging from 5 × 10^{12} to 5 × 10^{13} eV^{−1} cm^{−2}. Due to the strong screening effect for devices under high gate bias, the fluctuation magnitudes of SS and g_{m} induced by different levels of RITs are minimized. To effectively control the magnitude of normalized fluctuation within 5% for the V_{th}, I_{off}, I_{on}, SS, and DIBL, the D_{it} should be lower than 1 × 10^{12} eV^{−1} cm^{−2}. We are currently designing a proper experiment to measure the characteristic fluctuation induced by RITs and study the random bulk traps’ influence together with RITs on device characteristic variability.
Abbreviations
 σI _{off} :

Standard deviation of I_{off}
 σI _{on} :

Standard deviation of I_{on}
 σV _{th} :

Standard deviation of V_{th}
 2D:

Twodimensional
 3D:

Threedimensional
 C _{g} :

Gate capacitance
 DIBL:

Draininduced barrier lowering
 D _{it} :

Density of interface traps
 EOT:

Effective oxide thickness
 FinFET:

Fintype field effect transistor
 g _{m} :

Transconductance
 HKMG:

Highκ/metal gate
 ITs:

Interface traps
 I _{off} :

Offstate current
 I _{on} :

Onstate current
 N _{ch} :

Channel doping
 N _{B} :

Substrate doping
 N _{it} :

RIT concentration
 N _{S/D} :

Source/drain doping
 RITs:

Random interface traps
 SCE:

Shortchannel effect
 S _{RIT} :

RIT size
 SS:

Subthreshold swing
 V _{th} :

Threshold voltage.
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Acknowledgements
This work was supported in part by the Ministry of Science and Technology, Taiwan, under contract no. NSC1022221E009161 and no. MOST1032221 E009180, and by TSMC, Hsinchu, Taiwan, under a 20122013 grant.
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SCH performed the numerical simulation and data analysis. YL conducted the entire study including manuscript preparation. Both authors read and approved the final manuscript.
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Hsu, S., Li, Y. Electrical characteristic fluctuation of 16nmgate highκ/metal gate bulk FinFET devices in the presence of random interface traps. Nanoscale Res Lett 9, 633 (2014) doi:10.1186/1556276X9633
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Keywords
 Density of interface traps
 Random interface traps
 Bulk FinFETs
 Interface trap fluctuation
 Electrical characteristic fluctuation
 Statistical device simulation