N-Type Nanosheet FETs without Ground Plane Region for Process Simplification

Abstract

This paper proposes a simplified fabrication processing for nanosheet Field-Effect Transistors (FETs) part of beyond-3-nm node technology. Formation of the ground plane (GP) region can be replaced by an epitaxial grown doped ultra-thin (DUT) layer on the starting wafer prior to Si_x/SiGe_1−x stack formation. The proposed process flow can be performed in-situ, and does not require changing chambers or a high temperature annealing process. In short, conventional processes such as ion implantation and subsequent thermal annealing, which have been utilized for the GP region, can be replaced without degrading device performance.

1. Introduction

To suppress short-channel effects (SCEs), which are an important concern with aggressive device scaling, semiconductor devices have evolved from 2-dimensional (2-D) structures to 3-dimensional (3-D) architectures. This has further benefits. For example, due to their superior gate controllability, FinFETs have lower subthreshold swing (SS) and off-state leakage (I_OFF) than planar devices [1,2,3]. However, as device scaling approaches extreme levels, it is becoming difficult to control SCEs using FinFETs. As a solution, nanosheet FETs (NS FETs), which have multiple channels with a gate-all-around (GAA) backbone structure, have been introduced as beyond-FinFETs [4,5,6]. However, even though NS FETs have shown better suppression of SCEs than FinFETs, as well as better output performance, there are still many difficulties related to mass production. Producers such as Samsung Electronics Inc. and TSMC Inc. are planning the mass production of NS FETs in 2022 and 2023, respectively. However, it is unknown whether the yield will be sufficient. The reason for this lies in difficulties in the fabrication processing of the NS FETs. For example, surface roughness scattering stemming from germanium diffusion among the nanosheets, striction stemming from adhesion between nanosheets [6], residue formation (e.g., TiN or Si₃N₄) due to uncontrollable wet etching, unwanted void formation during metal gate filling [7], etc., are very challenging problems in current fabrication processing.

In addition, with respect to source/drain (S/D) modules, the contact depth as well as inner spacer thickness needs to be optimized [8,9]. It should be noted in particular that, unlike bulk FinFETs, which have already been mass produced, NS FETs have a parasitic channel underneath the first floor nanosheet [10,11,12,13]. Hence, even though NS FETs have multiple channels that are completely surrounded by metal gates (i.e., GAA), increased I_OFF stemming from the parasitic channel is inevitable. V. Jegadheesan et al., have suggested that applying a ground plane (GP) region can effectively minimize the I_OFF in the parasitic channel [14]. However, as we have mentioned above, the fabrication process flow of NS FETs is very sensitive. For example, ion implantation and rapid thermal annealing (RTA) for GP region formation are associated with an unwanted non-uniform doping profile [15,16]. In this context, it would be better, in terms of device variability and yield, if the conventional processes could be replaced with an alternative process. However, recently, research papers covering the fabrication process of NS FETs have been modest in number.

In this letter, we propose a fabrication process flow for NS FETs. The proposed process flow does not require ion implantation or additional thermal treatment, which are conventionally performed to form the GP region. Alternatively, a doped ultra-thin (DUT) layer is epitaxially grown on the starting wafer in-situ before Si_x/SiGe_1−x stack formation. This achieves process simplification, improved variability, and improved yield during the fabrication of NS FETs.

2. Materials and Method

The proposed fabrication process flow is summarized in Figure 1. In the conventional NS FETs fabrication process, ion implantation and thermal annealing are performed in dashed step two for GP region formation. However, the process can be replaced by Si epitaxial growth for DUT layer formation, as described in the bolded step two. The DUT layer is doped with a p–type dopant (i.e., boron) to minimize the parasitic channel and punch-through underneath the first floor nanosheet. The thickness of the DUT is similar to the thickness of silicon and Si_xGe_1−x layers, which are epitaxially grown as nanosheets and sacrificial layers, respectively. Hence, the formation of the DUT layer is not problematic under current processing technology. Above all, it should be noted that the processing from steps one to three in Figure 1 can be performed in-situ without changing chambers.

A 3-D simulator (Synopsys Sentaurus, Mountain View, CA, USA) was utilized to simulate fabrication processing and device characteristics. The drift-diffusion carrier transport equation was combined with the Poisson equation, and the density-gradient model was considered to reflect the quantum confinement effect of the nanosheet channels [17,18,19,20]. The Slotboom model was included for doping-dependent bandgap narrowing in the overall region [21]. Thin layer models such as inversion and accumulation layer mobility model (IALMob) were included to reflect impurity and phonon scattering [22]. In addition, the Shockley-Read-Hall (SRH) and non-local band-to-band tunneling (BTBT) recombination models were included to reflect gate-induced drain leakage (GIDL) during the simulations [22].

Figure 2 shows the backbone structure of the NS FETs used in the process simulation. Channel thickness (T_CH) and width (W_NS) were 5 nm and 45 nm, respectively. To elaborate, the dielectric constant of the HfO₂ gate dielectric and effective-oxide-thickness (EOT) were assumed to be 25 and 0.7 nm, respectively. In terms of doping concentration, epitaxially grown S/D regions were doped with arsenic at 3 × 10²⁰ cm⁻³ [14,23]. The three nanosheet channels and silicon substrate (N_Sub) were lightly doped with boron at 1 × 10¹⁷ cm⁻³ and 1 × 10¹⁶ cm⁻³, respectively. The doping concentration of the DUT layer was 1 × 10¹⁹ cm⁻³ of boron. The work-function of the titanium nitride for the metal gate was 4.5 eV. Detailed device parameters used for the simulations are summarized in

Table 1. Dimensions and parameters used for the TCAD simulations.

Parameter	Value
Gate Length, LG	12 nm
Nanosheet Width, WNS	45 nm
Inner Spacer Thickness, TSPACER	3 nm
Nanosheet-to-Nanosheet Vertical Space, VSPC	10 nm
Nanosheet Thickness, TNS	5 nm
Doped Ultra-Thin (DUT) Layer Thickness, TDUT	5–100 nm
Doping Concentration of DUT Layer (NDUT)	1019 cm−3
Inter Layer SiO2 Thickness, TIL	0.5 nm
High-k Gate Dielectric Thickness, THK	1.28 nm
Contacted Poly-Si Pitch (CPP)	44 nm

.

Then, the simulated I_D-V_G was carefully calibrated based on fabricated devices (i.e., such as doping concentration, structure, and gate work function) reported in [6], as shown in Figure 3a. Threshold voltage (V_TH) was extracted using a constant current method at I_D of 100 nA. Multiple V_TH characteristic can be visible when series resistance of NS FET is high due to defect existence or low doping concentration of S/D regions, as well as excessively long V_SPC. However, there was no observable multiple value of V_TH observed in the subthreshold region. Hence such concerns were not problematic, as shown in Figure 3b.

3. Results and Discussion

Figure 4a shows the simulated doping profile during the off-state when the GP region as well as the DUT layer were not included. An unwanted parasitic channel and punch-through were formed underneath the first floor nanosheet. These concerns led to increased I_OFF during the off-state (Figure 4b).

Figure 5a shows the simulated transfer characteristics of the NS FETs with various thicknesses of DUT layers (T_DUT) from 0 nm to 7 nm. As the thickness of the DUT layer increases, I_OFF, as well as SS, improve. The I_OFF extracted at V_G = −0.2 V was 21.8 nA with a 0 nm DUT layer but improved to 0.98 pA with a 7 nm DUT layer. Figure 5b shows the current density profile of the NS FETs in Figure 5a extracted at the off-state with V_D = 0.7 V, V_G = −0.2 V. As the T_DUT increases, I_OFF flowing through the parasitic channel (Y–Y′ direction) can be suppressed, aided by the increased energy barrier height (Φ_bi of the parasitic channel), as shown in Figure 5c. In addition, punch-through can be suppressed by applying a DUT layer.

However, when T_DUT is thicker than 7 nm, the I_OFF increases (Figure 6a). Considering the I_OFF value in the range of V_G of –0.2 V to 0 V are identical to the drain–to–substrate leakage current, the source of the I_OFF increase is not the parasitic channel, but rather the BTBT between the drain and the DUT layer. In other words, when T_DUT is thicker than 7 nm, the DUT forms a p-n diode between the drain and the DUT layer, as shown in Figure 6b. During the off-state, the reverse biased p-n diode triggers BTBT of electrons, and increases the I_OFF. Figure 6c shows the simulated rate of electron generation between the drain and the substrate by the BTBT. As the T_DUT increases, there is a noticeable increase in electron generation by the BTBT. As a result, it can be concluded that a 7 nm thickness DUT layer is optimal to avoid unwanted increasing I_OFF.

Figure 7a shows a simulated I_D-V_G curve for various doping concentrations (N_DUT) of the DUT layer. When the N_DUT is a low doping concentration of 1 × 10¹⁸ cm⁻³, the I_OFF cannot be controlled due to increased SS. However, as the N_DUT increases, leakage current through the parasitic channel can be suppressed by the increased built-in potential (Φ_bi) from 0.39 eV to 0.59 eV, as shown in Figure 7b. In addition, suppression of punch-through is possible with increased substrate doping concentration.

Figure 8 shows extracted I_D-V_G curve at V_D = 50 mV and 0.7 V, as well as I_D-V_D curve. The drain-induced barrier lowering (DIBL) of the proposed NS FET is 35 mV, which a low value compared with the value of 33 nm of the planar FET [19] or 15 nm of FinFET [24], as summarized in

Table 2. Comparison of DIBL and SS characteristics with various devices.

	Planar FET [19]	FinFET [24]	NS FET [This Work]
DIBL (mV/V) (VD = 50 mV and 0.7 V)	208	89	35
SS (mV/dec)	-	72	69

. In other words, the superior gate controllability of the NS FET does not degrade even when a DUT layer is applied.

Figure 9 introduces various approaches to suppress the leakage current from the parasitic channel without the formation of a GP region [14,25]. A full bottom dielectric (FBD) can eliminate the parasitic channel perfectly (Figure 9a). However, fabrication of the FBD inevitably requires a starting wafer composed of silicon-on-insulator (SOI) which is vulnerable to self-heating as well as expensive wafer cost. In this context, a steep-retrograde (SSR) region which contains a deep and heavily doped layer is preferred. However, forming an abrupt doping profile for the SSR is impossible using ion implantation and spike annealing. Even though epitaxial growth can be utilized, at least one more epitaxial growth step is required compared to the DUT case. Figure 9d shows the simulated electrical characteristics of the FBD, SSR, and DUT structures, respectively. Even though the DUT layer has a simpler fabrication process than the others, device characteristics in terms of V_TH, SS, and I_ON are not remarkable. Exact device parameters are summarized in

Table 3. Comparison of NS FETs based on FBD, SSR, and DUT structures.

	FBD	SSR	DUT
VTH (0.7 V/50 mV) (mV)	212/229	212/233	216/239
SS (mV/dec)	69	69	69
ION (mA) at VG = 0.7 V, VD = 0.7 V	0.117	0.117	0.102
IOFF (pA) at VG = 0 V, VD = 0.7 V	71.5	71.0	65.3
DIBL (mV/V) (VD = 50 mV and 0.7 V)	32	32	35

.

4. Conclusions

For better process simplification, the fabrication process for nanosheet FETs was newly suggested based on 3-D simulation. The doped ultra-thin (DUT) layer can be epitaxially grown in-situ on the starting wafer. Conventional ground plane (GP) doping implantation as well as annealing process can be excluded while forming the DUT layer. The thickness of the DUT layer has been optimized to suppress parasitic channel leakage, punch-through, and band-to-band tunneling (BTBT). The NS FET with the DUT layer showed comparable performance, but has a simpler fabrication process compared with other NS FETs, including full bottom dielectric (FBD) or steep-retrograde region (SSR).