Electrical Characteristics of Bulk FinFET According to Spacer Length

Abstract

This paper confirms that the electrical characteristics of FinFETs such as the on/off ratio, drain-induced barrier lowering (DIBL), and sub-threshold slope (SS) can be improved by optimizing the FinFET spacer structure. An operating voltage that can maintain a life of 10 years or more when hot-carrier injection is extracted. An excellent on/off ratio ($7.73\times {10}^7$) and the best SS value were found at 64.29 mV/dec with a spacer length of 90 nm. Under hot carrier-injection conditions, the supply voltages that meet the 10-year lifetime condition are 1.11 V, 1.18 V, and 1.32 V for spacer lengths of 40 nm, 80 nm, and 120 nm, respectively. This experiment confirmed that, even at low drain voltages, the shorter is the spacer length, the greater is the deterioration. However, this increasing maximum operating voltage is very small when compared to the increase in the driving voltage required to achieve similar performance when the spacer length is increased; therefore, the effective life is expected to decrease. The results indicate that structural optimization must be performed to increase the driving current of the FinFET and prevent degradation of the analog performance.

1. Introduction

The performance of semiconductors is improved by scaling the gate length up to 10–15 nm [1,2,3]. However, as the distance between source and drain regions decreases, the electric field in the channel increases, while the ability of the gate to control the channel region decreases. This is known as “short channel effect (SCE)”. If the gate length is reduced more than 10–15 nm, it causes source-to-drain direct tunneling, resulting in the degradation of SCE. Previous research work already showed the limitations of the device scaling, and the Drain-Induced-Barrier-Lowering (DIBL) due to SCE [4,5,6,7,8,9,10,11]. As a result, multi-gate FinFETs were introduced to reduce the leakage current and improve the gate controllability of the transistor channel. The FinFET structure also reduces the occurrence of gate-oxide traps and gate critical paths, owing to the reduction of the hot-carrier effect caused by lightly-doped drain (LDD) formation, resulting in a decrease in the gate-induced drain leakage (GIDL). In the off state, the effective channel length is larger than the physical gate length due to the undoped spacer regions. In the on state, the effective channel length is equal to the physical gate length of the device [12,13]. While the introduction of spacer improves the short-channel performance of the devices, drive current is reduced due to higher series resistance in the spacer regions. Furthermore, the parasitic capacitance increases with the increased device transconductance, causing no improvement in unit-gain frequency.

In this work, we tried to solve these problems by optimizing the spacer structure of the FinFET. We present a bulk-Si N-channel FinFET device (bulk N-FinFET) that has desirable transistor characteristics and competitive short-channel performance. The important parameters in bulk N-FinFETs, including DIBL, SS, drive current, and S/D subthreshold off-state leakage current, are combined to reflect the overall performance of the device and provide a criterion for the design of innovative devices.

2. Device Structure and Simulation Setup

A top-view SEM image of the bulk FinFET used for the measurement is shown in Figure 1a. The important geometrical parameters of a FinFET are defined in Figure 1b, namely the height (H_fin), width (W_fin), transistor length (L), and spacer length (LSD). The device design parameters of the FinFET are summarized in

Table 1. Device structure parameters of FinFETs.

Device Parameters	Value
Gate length (L)	70 nm
Fin width (Wfin)	40 nm
Fin height (Hfin)	30 nm
Number of fins	5
Fin pitch	200 nm
Spacer length (LSD)	40~100 nm

. These devices feature a high-k gate dielectric (2.3-nm HfSiON on a 1-nm interfacial oxide) and 100 nm of polycrystalline silicon on top of a 5-nm TiN metal gate. The S/D access region is formed by the selective epitaxial growth of Si on the source and drain areas, followed by NiPt silicidation.

To understand the dependence of the electrical properties of a bulk FinFET on the spacer length, 3D TCAD simulations were performed using Atlas TCAD. For device simulation, along with the inversion layer mobility models of Lombardi CVT, SRH, and Auger recombination, LAT.TEMP models were also included. The S/D regions were N-doped at the concentration $1\times {10}^{21}$ cm⁻³ and the channel regions were P-doped at the concentration $1\times {10}^{17}$ cm⁻³.

After applying voltages of 0.1 and 1 V to the drain, the DC simulation of a five-fin FinFET structure was performed, and the on/off current, DIBL, SS, and lattice temperatures were extracted using the I_ds–V_gs graph for changes in the spacer length.

3. Results and Discussion

3.1. Off-State Leakage Current

Figure 2 shows the transfer curve and on/off current change according to the spacer length change in a bulk N-FinFET. As the spacer length increases, the source and drain resistances also increase. The increase in source resistance causes the degradation in on-state current. For low-power operation, the spacer length should be designed to be as short as possible. However, when the spacer length becomes shorter than 60 nm, the effective channel length decreases rapidly resulting in the increase in leakage current. Therefore, when the spacer length is 80 nm, the on/off ratio will be the highest, at $7.73\times {10}^7$. Further increase in spacer length increases the source and drain resistance and the increased source resistance causes the degradation in on-state current.

Figure 3 shows the changes in the DIBL and SS with changes in the spacer length. As the length of the spacer increases, the effective channel length increases. In addition, the length of the space-charge region formed at the interface between the channel and drain also increases, and, thus, the DIBL decreases. On the other hand, as the size of the electric field from the drain to the channel region decreases and the total volume of the depletion region increases, it is causing the problem of an increase of the SS. As indicated in

Table 2. Electrical properties of bulk FinFET according to spacer length.

LSD (nm)	On/Off Ration	DIBL (mV/V)	SS (mV/dec)
40	3.18 × 105	130.70	107.24
50	2.67 × 106	78.19	78.08
60	3.20 × 107	36.40	70.17
70	6.91 × 107	19.47	67.30
80	7.73 × 107	18.68	64.79
90	6.05 × 107	18.41	64.29
100	3.88 × 107	17.64	66.82

, when the spacer length is 90 nm, the lowest SS value of 64.29 mV/V is obtained.

3.2. Impact Ionization

Hot carriers produced by impact ionization are important components that degrade the device reliability [14,15,16]. Hot carriers, which have sufficient energy to pass through the gate insulator, impact the Si–SiO₂ interface, significantly impairing the transistor performance.

The degree of device damage due to hot carriers is generally analyzed in terms of the substrate current. Figure 4a shows the substrate current according to the spacer-length change. When the spacer length shortens below 60 nm, the substrate current increases rapidly because of impact ionization. The hot carriers generated by impact ionization further accelerate the deterioration of the device by increasing the lattice temperature. Figure 4b shows the correlation between the substrate current and lattice temperature, according to the spacer-length change. The mechanism behind SHE is the impact ionization that occurs at the junction between the P-doped and N-doped regions.

If a positive bias is applied to the n-doped region (i.e., drain region), the energy band in the N-type material will be lower than that in the steady state. On applying a more positive bias, the energy band in the N-type region will be lower [17]. An increased bias attracts electrons, which are minority carriers in the P-type region. Such carriers are accelerated by the positive bias, and the incoming electrons will collide with the other ions. Thus, they will create an electron-hole pair (EHP). The holes generated at this time pass through the substrate and partly flow to the gate to form the gate current. Therefore, when the impact ionization increases, the substrate current will increase.

3.3. Hot-Carrier Degradation

Figure 5 shows a graph of the change in transconductance over time after channel hot-electron injection into real FinFET devices with different spacer lengths. A voltage ranging from 1.2 V to −2.0 V was applied between the drain and source, and the same gate voltage was applied. The stress application time was increased from 100 s to 3800 s. When the spacer length is small, the transconductance deteriorates significantly at a small drain voltage. As the spacer length increases, the deterioration also increases at large drain voltages. As observed previously, even at low drain voltages, when the spacer length is 40 nm, impact ionization occurs because of the effect of a large electric field. Thus, a very fast deterioration rate is observed. However, if a high drain voltage is applied, deterioration due to impact ionization occurs even in a FinFET with 100-nm spacer length. In addition, it can be interpreted that more deterioration occurs at high voltages because the electron interface trap area widens.

This trend is clearly presented in Figure 6, which shows the lifetime of the devices with HCE degradation. We consider the lifetime as the time at which the device shows 10% transconductance degradation. The supply voltage which meets the 10-year lifetime condition at a spacer length of 40 nm is 1.11 V; that for a spacer length of 80 nm is 1.18 V; and that for a spacer length of 120 nm is 1.32 V. As a result of this experiment, it is confirmed that, the shorter is the spacer, the easier is the deterioration, even at low drain voltages. However, this increase is very small when compared to the increase in the driving voltage required to achieve a similar performance when the spacer length is increased; thus, the effective life is expected to decrease.

4. Conclusions

According to Moore’s Law, the channel length of the transistor is reducing continuously; however, due to problems such as deterioration of the subthreshold characteristics of the transistor as a result of the deepening of the SCE, it is no longer possible to expect an improvement in performance because of a reduction in device size. To improve the performance of the transistor, a device having a 3D gate structure in which a transistor structure is changed, rather than simply scaling the channel length, was developed.

In this study, it was confirmed that the characteristics such as on/off ratio, DIBL, and SS were improved through optimization of the FinFET spacer structure. In addition, an operating voltage that could maintain a life of 10 years or more under hot-carrier injection conditions was extracted. As could be seen from the results, structural optimization was required to increase the drive current of the FinFET and prevent degradation of the analog performance.