Implementation of Ambipolar Polysilicon Thin-Film Transistors with Nickel Silicide Schottky Junctions by Low-Thermal-Budget Microwave Annealing

In this study, the efficient fabrication of nickel silicide (NiSix) Schottky barrier thin-film transistors (SB-TFTs) via microwave annealing (MWA) technology is proposed, and complementary metal-oxide-semiconductor (CMOS) inverters are implemented in a simplified process using ambipolar transistor properties. To validate the efficacy of the NiSix formation process by MWA, NiSix is also prepared via the conventional rapid thermal annealing (RTA) process. The Rs of the MWA NiSix decreases with increasing microwave power, and becomes saturated at 600 W, thus showing lower resistance than the 500 °C RTA NiSix. Further, SB-diodes formed on n-type and p-type bulk silicon are found to have optimal rectification characteristics at 600 W microwave power, and exhibit superior characteristics to the RTA SB-diodes. Evaluation of the electrical properties of NiSix SB-TFTs on excimer-laser-annealed (ELA) poly-Si substrates indicates that the MWA NiSix junction exhibits better ambipolar operation and transistor performance, along with improved stability. Furthermore, CMOS inverters, constructed using the ambipolar SB-TFTs, exhibit better voltage transfer characteristics, voltage gains, and dynamic inverting behavior by incorporating the MWA NiSix source-and-drain (S/D) junctions. Therefore, MWA is an effective process for silicide formation, and ambipolar SB-TFTs using MWA NiSix junctions provide a promising future for CMOS technology.


Introduction
Metal silicides, i.e., compounds of metal and silicon (Si), have been widely employed as interconnecting and contact materials in complementary metal-oxide-semiconductor (CMOS) technology due to their low specific resistivity, low contact resistivity towards both types of Si, high thermal stability, good processibility, and excellent process compatibility with standard Si technology [1][2][3][4]. Transition-metal silicide-based Schottky barrier (SB) source-and-drain (S/D) junctions have aroused much interest in nanoscale metal-oxidesemiconductor field-effect transistors (MOSFETs) because the resistive-capacitive (RC) time delay must be reduced by minimizing both parasitic resistance and capacitance components, in order to meet the major requirement of speeding up electronic circuits [5][6][7]. In SB-MOSFETs, the S/D junction consists of silicide in place of conventional impurity-doped silicon, thus enabling lower parasitic series resistance and ultrashallow abrupt junction formation via a simpler process [7][8][9][10]. By contrast, the performance of conventional MOSFETs is determined by doping and activation techniques, which become increasingly difficult for ultra-short-channel devices. Nevertheless, the Ti silicides (TiSi x ) and Co silicides (CoSi x ) that are commonly used in CMOS fabrication have limitations in future, extremely scaled down, ultra-high-density CMOS electronic circuits. In the case of TiSi x , the sheet resistance (R s ) increases as the line width decreases [11][12][13][14], while in the case of CoSi x , junction spiking becomes an issue due to excessive Si consumption [12,15,16]. Meanwhile, Ni silicide (NiSi x ) Nanomaterials 2022, 12, 628 2 of 15 is gaining attention in next-generation deep submicron CMOS devices due to its improved nanoscale performance, and is gradually replacing CoSi x [17]. In particular, there are many advantages, such as low specific resistivity (10-15 µΩ cm) and formation temperature (typically 500 • C), decreased Si consumption (1.83 nm of Si per nanometer of Ni, yielding 2.21 nm of NiSi x ), and little deterioration in resistivity on narrow lines/gates [16,[18][19][20][21][22]. In particular, NiSi x materials are the standard metal contacts in the semiconductor industry for both NMOS and PMOS devices, and are regarded as midgap metals with Schottky barrier heights (SBHs) of 0.45-0.5 eV for holes, and 0.6-0.65 eV for electrons [23,24]. Therefore, when applied to the S/D metallic junctions for Schottky barrier thin-film transistors (SB-TFTs), NiSi x is particularly favorable for obtaining ambipolar operating characteristics without n-type or p-type impurity doping. Consequently, these devices are able to behave as p-type or n-type MOSFETs simply by changing the polarity of the gate bias.
The traditional silicide formation process has generally involved conventional rapid thermal annealing (RTA) using a halogen lamp. However, because the process is typically performed in a vacuum, RTA has the disadvantages of high cost, relatively long processing time, and high thermal budget [25,26]. By contrast, microwave annealing (MWA) does not require a vacuum, is cheaper, and has higher energy-transfer efficiency and consumption, a shorter process time, and a lower thermal budget [27,28]. There are several studies of applying MWA with these advantages to the activation of ion-implanted dopants [29,30]. In addition, MWA can offer quicker volume heating than the RTA, because it interacts directly with individual atoms while inducing dipole rotation in the silicon substrates [31]. Therefore, MWA is employed herein to promote the silicide reaction between Ni and Si. To verify the efficiency of silicidation by MWA, conventional RTA is also applied to NiSi x formation for comparison. The crystallinity and R s values of the NiSi x prepared via MWA at various microwave powers, and by RTA at 500 • C, are evaluated. In addition, NiSi x SB-diodes are fabricated via MWA at various microwave powers, and their electrical characteristics are measured to determine the optimum fabrication conditions. These conditions are then used to fabricate ambipolar NiSi x SB-TFTs, and their electrical characteristics and reliability are evaluated in comparison with identical devices prepared via RTA. Further, CMOS-like inverters are constructed using the MWA-or RTA-NiSi x SB-TFTs, in order to evaluate their voltage transfer characteristics, voltage gains, and dynamic inversion operation.

Nickel Deposition and Silicidation
The substrates were (100)-oriented n-type and p-type Si wafers with resistivities ranging from 10 to 20 Ω·cm. The substrates were cleaned using the process recommended by the Standard Radio Corporation of America (RCA) to remove any surface contamination and native oxides. Active regions were then formed via a photolithographic patterning process and wet etching with a 30:1 buffered oxide etchant (BOE). A 150 nm-thick Ni film was deposited using an electron-beam (E-beam) evaporator. After that, the MWA process was performed at powers of 250-1000 W under a N 2 atmosphere for 2 min for NiSi x formation. For comparison, the RTA process was conducted at 500 • C for 2 min under a N 2 atmosphere. After that, the unreacted Ni was removed using a 1:1 sulfuric acid/hydrogen peroxide mixture (SPM) at room temperature.
The temperature profiles of the MWA and RTA processes are shown in Figure 1. We used an infrared (IR) thermometer to check the temperature during the MW treatment process, because it is hard to determine the temperature inside the MW chamber using metal thermocouples [32]. Thus, after heat treatment at 500 • C via the RTA process, about 15 min are required to return to room temperature, and the thermal budget is 2.84 × 10 5 • C·s. By contrast, the MWA process is a volumetric heating method using electromagnetic waves, which reaches the process temperature within about 20 s and has a very short ramp downtime of 10 s. The thermal budget of the MWA process is 0.47 × 10 5 , 0.54 × 10 5 , 0.56 × 10 5 , 0.59 × 10 5 , and 0.65 × 10 5 • C·s at operating powers of 250, 500, 600, 750, and Nanomaterials 2022, 12, 628 3 of 15 1000 W, respectively. This indicates that the MWA process generally has a lower thermal budget and a higher energy transfer efficiency than the RTA process. waves, which reaches the process temperature within about 20 s and has a very short ramp downtime of 10 s. The thermal budget of the MWA process is 0.47 × 10 5 , 0.54 × 10 5 , 0.56 × 10 5 , 0.59 × 10 5 , and 0.65 × 10 5 °C·s at operating powers of 250, 500, 600, 750, and 1000 W, respectively. This indicates that the MWA process generally has a lower thermal budget and a higher energy transfer efficiency than the RTA process.

Fabrication of the NiSix SB-diodes
As shown schematically in Figure 2, phosphorus-and boron-doped (100) n-type and p-type bulk silicon wafers with resistivities of 3-5 and 7-14 Ω•cm, respectively, were used to fabricate the NiSix-based SB-diodes. After defining the active area of the diode, a 500 nm-thick SiO2 layer was grown via wet oxidation at 980 °C for local oxidation of silicon (LOCOS) isolation. A 150 nm-thick Ni film was deposited using an E-beam evaporator. For NiSix formation, either the MWA process at a power of 250−1000 W for 2 min, or the RTA process at 500 °C for 2 min, was employed under a N2 atmosphere, to investigate the effect of MWA silicidation upon the characteristics of the SB-diodes. Then, unreacted Ni was removed using an SPM solution at room temperature.

Fabrication of the NiSix SB-TFTs
Glass substrates (1737 and EAGLE2000TM, Corning) were each coated with a 160 nm-thick poly-Si layer via excimer-laser annealing (ELA) for use in fabricating the SB-TFTs with NiSix S/D junctions. The resulting ELA poly-Si substrate was then cleaned via the RCA process, after which the active channel regions were patterned via photolithography and wet etching with a 30:1 BOE solution. The channel width (W) and length (L) of the fabricated devices were 20 μm and 10 μm, respectively. Immediately after removing the native oxide film from the poly-Si channel with the 30:1 BOE, a 150 nm-thick Ni film was deposited using an E-beam evaporator. Subsequently, NiSix was selectively formed in the S/D region via the MWA process at 600 W, which is the optimal power condition.

Fabrication of the NiSi x SB-Diodes
As shown schematically in Figure 2, phosphorus-and boron-doped (100) n-type and p-type bulk silicon wafers with resistivities of 3-5 and 7-14 Ω·cm, respectively, were used to fabricate the NiSi x -based SB-diodes. After defining the active area of the diode, a 500 nm-thick SiO 2 layer was grown via wet oxidation at 980 • C for local oxidation of silicon (LOCOS) isolation. A 150 nm-thick Ni film was deposited using an E-beam evaporator. For NiSi x formation, either the MWA process at a power of 250-1000 W for 2 min, or the RTA process at 500 • C for 2 min, was employed under a N 2 atmosphere, to investigate the effect of MWA silicidation upon the characteristics of the SB-diodes. Then, unreacted Ni was removed using an SPM solution at room temperature. waves, which reaches the process temperature within about 20 s and has a very short ramp downtime of 10 s. The thermal budget of the MWA process is 0.47 × 10 5 , 0.54 × 10 5 , 0.56 × 10 5 , 0.59 × 10 5 , and 0.65 × 10 5 °C·s at operating powers of 250, 500, 600, 750, and 1000 W, respectively. This indicates that the MWA process generally has a lower thermal budget and a higher energy transfer efficiency than the RTA process.

Fabrication of the NiSix SB-diodes
As shown schematically in Figure 2, phosphorus-and boron-doped (100) n-type and p-type bulk silicon wafers with resistivities of 3-5 and 7-14 Ω•cm, respectively, were used to fabricate the NiSix-based SB-diodes. After defining the active area of the diode, a 500 nm-thick SiO2 layer was grown via wet oxidation at 980 °C for local oxidation of silicon (LOCOS) isolation. A 150 nm-thick Ni film was deposited using an E-beam evaporator. For NiSix formation, either the MWA process at a power of 250−1000 W for 2 min, or the RTA process at 500 °C for 2 min, was employed under a N2 atmosphere, to investigate the effect of MWA silicidation upon the characteristics of the SB-diodes. Then, unreacted Ni was removed using an SPM solution at room temperature.

Fabrication of the NiSix SB-TFTs
Glass substrates (1737 and EAGLE2000TM, Corning) were each coated with a 160 nm-thick poly-Si layer via excimer-laser annealing (ELA) for use in fabricating the SB-TFTs with NiSix S/D junctions. The resulting ELA poly-Si substrate was then cleaned via the RCA process, after which the active channel regions were patterned via photolithography and wet etching with a 30:1 BOE solution. The channel width (W) and length (L) of the fabricated devices were 20 μm and 10 μm, respectively. Immediately after removing the native oxide film from the poly-Si channel with the 30:1 BOE, a 150 nm-thick Ni film was deposited using an E-beam evaporator. Subsequently, NiSix was selectively formed in the S/D region via the MWA process at 600 W, which is the optimal power condition.

Fabrication of the NiSi x SB-TFTs
Glass substrates (1737 and EAGLE2000TM, Corning) were each coated with a 160 nm-thick poly-Si layer via excimer-laser annealing (ELA) for use in fabricating the SB-TFTs with NiSi x S/D junctions. The resulting ELA poly-Si substrate was then cleaned via the RCA process, after which the active channel regions were patterned via photolithography and wet etching with a 30:1 BOE solution. The channel width (W) and length (L) of the fabricated devices were 20 µm and 10 µm, respectively. Immediately after removing the native oxide film from the poly-Si channel with the 30:1 BOE, a 150 nm-thick Ni film was deposited using an E-beam evaporator. Subsequently, NiSi x was selectively formed in the S/D region via the MWA process at 600 W, which is the optimal power condition. Then, unreacted Ni was removed using the SPM solution. For the gate insulator, a 70 nm-thick SiO 2 film was deposited by radio-frequency (RF) magnetron sputtering at an operating pressure of 3.0 mTorr, an Ar flow rate of 30 sccm, an O 2 flow rate of 2 sccm, and an RF power of 200 W. For the top-gate electrode, a 150 nm-thick Al Nanomaterials 2022, 12, 628 4 of 15 film was deposited using an E-beam evaporator, then patterned by a lift-off method. Finally, post-metallization annealing (PMA) was performed using forming gas (5% H 2 , 95% N 2 ) in a furnace at 400 • C for 30 min to improve the electrical properties. The schematic structure, process flow, and top-view optical microscope image (300×) of the fabricated NiSi x SB-TFTs are presented in Figure 3. Then, unreacted Ni was removed using the SPM solution. For the gate insulator, a 70 nmthick SiO2 film was deposited by radio-frequency (RF) magnetron sputtering at an operating pressure of 3.0 mTorr, an Ar flow rate of 30 sccm, an O2 flow rate of 2 sccm, and an RF power of 200 W. For the top-gate electrode, a 150 nm-thick Al film was deposited using an E-beam evaporator, then patterned by a lift-off method. Finally, post-metallization annealing (PMA) was performed using forming gas (5% H2, 95% N2) in a furnace at 400 °C for 30 min to improve the electrical properties. The schematic structure, process flow, and top-view optical microscope image (300×) of the fabricated NiSix SB-TFTs are presented in Figure 3.

Characterization of the NiSix SB-diodes and SB-TFTs
The electrical characteristics of the fabricated NiSix Schottky junction diodes and the SB-TFTs were measured using an Agilent 4156B precision semiconductor parameter analyzer in a dark box to prevent external effects such as light and electrical noise. In addition, CMOS inverters were constructed using the MWA-and RTA-processed NiSix SB-TFTs, and their voltage transfer characteristics, voltage gains, and dynamic inversion behaviors were evaluated using an RIGOL DG972 function/arbitrary waveform generator and RIGOL MSO5074 oscilloscope in a dark box.

Results and Discussion
The sheet resistances (Rs) of the NiSix samples fabricated at various microwave powers were measured using a four-point probe, and the results are presented in Figure 4a. Here, a significant decrease in Rs is observed at 500 W, thus resulting in low resistance. In particular, the silicidation process at 600 W provides the lowest Rs value of 3.86 Ω/sq, which is lower than the 6.62 Ω/sq obtained via the RTA process at 500 °C. These results are similar to those reported in other literature [33]. Therefore, it is concluded that efficient silicide formation is possible even with a low-power MWA process, by sufficiently reducing the resistance.
The crystallinities of the various NiSix samples are indicated by the XRD spectra in Figure 4b. Here, the XRD pattern of the as-deposited Ni film exhibits peaks corresponding to the (111) and (200) crystal planes of Ni, while various other peaks appear after the RTA

Characterization of the NiSi x SB-Diodes and SB-TFTs
The electrical characteristics of the fabricated NiSi x Schottky junction diodes and the SB-TFTs were measured using an Agilent 4156B precision semiconductor parameter analyzer in a dark box to prevent external effects such as light and electrical noise. In addition, CMOS inverters were constructed using the MWA-and RTA-processed NiSi x SB-TFTs, and their voltage transfer characteristics, voltage gains, and dynamic inversion behaviors were evaluated using an RIGOL DG972 function/arbitrary waveform generator and RIGOL MSO5074 oscilloscope in a dark box.

Results and Discussion
The sheet resistances (R s ) of the NiSi x samples fabricated at various microwave powers were measured using a four-point probe, and the results are presented in Figure 4a. Here, a significant decrease in R s is observed at 500 W, thus resulting in low resistance. In particular, the silicidation process at 600 W provides the lowest R s value of 3.86 Ω/sq, which is lower than the 6.62 Ω/sq obtained via the RTA process at 500 • C. These results are similar to those reported in other literature [33]. Therefore, it is concluded that efficient silicide formation is possible even with a low-power MWA process, by sufficiently reducing the resistance.
ance of a peak corresponding to the (310) crystal plane of NiSix, even at a low microwave power of 250 W, thus confirming the formation of silicide. When the MWA process is performed at 500 W, several strong peaks corresponding to the (211), (220), (310), (221), and (301) crystal planes also appear [34], giving almost the same pattern as that obtained using the 500 °C RTA process. These results indicate that 600 W is the optimal MWA silicidation condition. The current-voltage (I-V) characteristics of the NiSix Schottky junction diodes on ntype and p-type Si substrates according to the various silicidation conditions are presented in Figure 5a and b, respectively, and the corresponding electrical parameters are summarized in Table 1. The as-deposited Ni SB-diode has a low on-current and a high leakage current due to interfacial defects between the unreacted Ni film and Si. However, as silicidation proceeds, the rectification characteristics of the n-type and p-type diodes are improved, and the on/off current ratio increases. In particular, the MWA process improves the operating performance as the microwave power increases, and exhibits the best rectification characteristics at 600 W. Notably, the 600-W MWA SB-diodes on the n-type substrate provide better results than the 500 °C RTA diodes. This is also evidenced by the ideality factors (η) extracted from the I-V curves, and the Schottky barrier heights (ϕb) extracted from the current-voltage (I-V) curves (Table 1); moreover, it is consistent with the Rs and XRD results. In addition, the ϕ b values were extracted by Equation (1) [35]: where V, n are voltage and ideality factor, respectively.  The crystallinities of the various NiSi x samples are indicated by the XRD spectra in Figure 4b. Here, the XRD pattern of the as-deposited Ni film exhibits peaks corresponding to the (111) and (200) crystal planes of Ni, while various other peaks appear after the RTA and MWA silicidation processes. In particular, the MWA treatment leads to the appearance of a peak corresponding to the (310) crystal plane of NiSi x , even at a low microwave power of 250 W, thus confirming the formation of silicide. When the MWA process is performed at 500 W, several strong peaks corresponding to the (211), (220), (310), (221), and (301) crystal planes also appear [34], giving almost the same pattern as that obtained using the 500 • C RTA process. These results indicate that 600 W is the optimal MWA silicidation condition.
The current-voltage (I-V) characteristics of the NiSi x Schottky junction diodes on n-type and p-type Si substrates according to the various silicidation conditions are presented in Figures 5a and 5b, respectively, and the corresponding electrical parameters are summarized in Table 1. The as-deposited Ni SB-diode has a low on-current and a high leakage current due to interfacial defects between the unreacted Ni film and Si. However, as silicidation proceeds, the rectification characteristics of the n-type and p-type diodes are improved, and the on/off current ratio increases. In particular, the MWA process improves the operating performance as the microwave power increases, and exhibits the best rectification characteristics at 600 W. Notably, the 600-W MWA SB-diodes on the n-type substrate provide better results than the 500 • C RTA diodes. This is also evidenced by the ideality factors (η) extracted from the I-V curves, and the Schottky barrier heights (φ b ) extracted from the current-voltage (I-V) curves (Table 1); moreover, it is consistent with the Rs and XRD results. In addition, the φ b values were extracted by Equation (1) [35]: where V, n are voltage and ideality factor, respectively. substrate provide better results than the 500 °C RTA diodes. This is also evidenced by the ideality factors (η) extracted from the I-V curves, and the Schottky barrier heights (ϕb) extracted from the current-voltage (I-V) curves (Table 1); moreover, it is consistent with the Rs and XRD results. In addition, the ϕ b values were extracted by Equation (1) [35]: where V, n are voltage and ideality factor, respectively.    Figure 6 shows schematic energy band diagrams based on V GS and V DS values for ambipolar operation. The current device is mainly related to thermionic emission and tunneling of carriers above the threshold in the SB-TFTs. The electrons are injected into the channel using thermionic emission (e TH ) and tunneling (e T ) from the source-and-drain electrodes at the positive and negative V DS , when the gate bias is positive (n-channel operation), as shown in Figure 6a. In Figure 6b, the holes are injected into the channel region by thermionic emission (h TH ) and tunneling (h T ) at the negative gate bias (p-channel operation). Depending on the operating mode, electrodes or holes from source-and-drain electrodes fill the channel region during the ambipolar operation. As a result, the driving current and I on /I off of p-channel operation can be lower, because the SB height (SBH) for electrons in NiSi x is slightly higher than the SBH for holes, but the effective electron mass of tunneling is lower.
Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 15 electrodes fill the channel region during the ambipolar operation. As a result, the driving current and Ion/Ioff of p-channel operation can be lower, because the SB height (SBH) for electrons in NiSix is slightly higher than the SBH for holes, but the effective electron mass of tunneling is lower. The electrical properties of the NiSix SB-TFTs fabricated via the MWA silicidation process at 600 W, and via the RTA process at 500 °C, are presented in Figure 7. The transfer curves measured at a drain voltage (VD) of 1 V and a gate voltage (VG) range of -25 to +25V (Figure 7a) demonstrate the ambipolar conduction properties of both devices. Meanwhile, the output curves measured at |VG-VTH| = 0-20 V (where VTH is the threshold voltage) in the drain voltage range of -15 to +15 V demonstrate that both devices can behave as ptype (hole channel) or n-type (electron channel) MOSFETs simply by changing the polarity of the gate bias. The drain current (ID) increases linearly in the low VD region, indicating a pinch-off characteristic that gradually reaches the saturation region as VD increases further. Taken together, these results indicate that the 600-W MWA NiSix SB-TFT allows better switching characteristics and higher drive current than the 500-°C RTA-processed device. This is also consistent with the results obtained for the SB-diode.   The electrical properties of the NiSi x SB-TFTs fabricated via the MWA silicidation process at 600 W, and via the RTA process at 500 • C, are presented in Figure 7. The transfer curves measured at a drain voltage (V D ) of 1 V and a gate voltage (V G ) range of −25 to +25 V (Figure 7a) demonstrate the ambipolar conduction properties of both devices. Meanwhile, the output curves measured at |V G − V TH | = 0-20 V (where V TH is the threshold voltage) in the drain voltage range of −15 to +15 V demonstrate that both devices can behave as p-type (hole channel) or n-type (electron channel) MOSFETs simply by changing the polarity of the gate bias. The drain current (I D ) increases linearly in the low V D region, indicating a pinch-off characteristic that gradually reaches the saturation region as V D increases further. Taken together, these results indicate that the 600-W MWA NiSi x SB-TFT allows better switching characteristics and higher drive current than the 500-• C RTA-processed device. This is also consistent with the results obtained for the SB-diode.
Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 15 electrodes fill the channel region during the ambipolar operation. As a result, the driving current and Ion/Ioff of p-channel operation can be lower, because the SB height (SBH) for electrons in NiSix is slightly higher than the SBH for holes, but the effective electron mass of tunneling is lower. The electrical properties of the NiSix SB-TFTs fabricated via the MWA silicidation process at 600 W, and via the RTA process at 500 °C, are presented in Figure 7. The transfer curves measured at a drain voltage (VD) of 1 V and a gate voltage (VG) range of -25 to +25V (Figure 7a) demonstrate the ambipolar conduction properties of both devices. Meanwhile, the output curves measured at |VG-VTH| = 0-20 V (where VTH is the threshold voltage) in the drain voltage range of -15 to +15 V demonstrate that both devices can behave as ptype (hole channel) or n-type (electron channel) MOSFETs simply by changing the polarity of the gate bias. The drain current (ID) increases linearly in the low VD region, indicating a pinch-off characteristic that gradually reaches the saturation region as VD increases further. Taken together, these results indicate that the 600-W MWA NiSix SB-TFT allows better switching characteristics and higher drive current than the 500-°C RTA-processed device. This is also consistent with the results obtained for the SB-diode. Drain The extracted electrical parameters of the 600-W MWA and the 500-• C RTA NiSi x SB-TFTs are summarized in Table 2. The subthreshold swing (SS) and field-effect mobility (µ FE ) values were extracted using Equations (2) and (3) [36]: and  The temperature-dependence of the V TH shift (∆V TH ) during the positive bias temperature stress (PBTS) and negative bias temperature stress (NBTS) tests are indicated for the 600-W MWA and 500-• C RTA NiSi x SB-TFTs in Figure 8. The p-channel behavior is presented in Figure 8a-c, while the n-channel behavior characteristics are shown in Figure 8d-f. For these measurements, the change in V TH was monitored at 25, 55, and 85 • C while applying an electric field of ±20 V to the gate electrode for 10 4 s. Due to the ambipolar nature of the NiSi x SB-TFTs, the p-channel and n-channel behaviors were tested separately. The fitted curves in Figure 8a-f were obtained using Equation (4) [37,38]: where ∆V TH0 is ∆V TH at the initial time, β is the exponent for a stretched-exponential function, and τ is the carrier trapping time from the channel to the dielectric layer, which depends on the temperature. Therefore, V TH and τ are dependent on the thermally activated process. The temperature-dependent effective energy barrier height (E τ ) for carrier transport was calculated using the Arrhenius equation, given as Equation (5): where ν and τ 0 are, respectively, the frequency and the thermal pre-factor for emission over the barrier, and T is the absolute temperature. The results indicate that the ∆V TH increases with increasing stress time, and with increasing stress temperature, in both behavior modes. Moreover, as the ∆V TH of the MWA SB-TFT is smaller than that of the RTA device, MWA silicidation is considered to contribute to the improvement in stability and reliability of the SB-TFT.
where ν and τ0 are, respectively, the frequency and the thermal pre-factor for emission over the barrier, and T is the absolute temperature. The results indicate that the ΔVTH increases with increasing stress time, and with increasing stress temperature, in both behavior modes. Moreover, as the ΔVTH of the MWA SB-TFT is smaller than that of the RTA device, MWA silicidation is considered to contribute to the improvement in stability and reliability of the SB-TFT. The time it takes for the carrier to be trapped in an insulating layer or an insulating layer-channel layer is referred to as the charging trapping time (τ). The τ of the NiSix SB-TFTs in the p-channel and n-channel behavioral modes during the PBTS and NBTS tests are plotted in Figure 9, and the extracted values are summarized in Table 3. In both modes, the trapping time is seen to decrease with increasing bias-stress-temperature. Moreover, the extracted results show that the charge trapping time in the PBTS mode is shorter than that in the NBTS mode, thus indicating that the PBTS is more dominant in charge trapping. Further, the trapping times of the MWA device are greater than those of the RTA The time it takes for the carrier to be trapped in an insulating layer or an insulating layer-channel layer is referred to as the charging trapping time (τ). The τ of the NiSi x SB-TFTs in the p-channel and n-channel behavioral modes during the PBTS and NBTS tests are plotted in Figure 9, and the extracted values are summarized in Table 3. In both modes, the trapping time is seen to decrease with increasing bias-stress-temperature. Moreover, the extracted results show that the charge trapping time in the PBTS mode is shorter than that in the NBTS mode, thus indicating that the PBTS is more dominant in charge trapping. Further, the trapping times of the MWA device are greater than those of the RTA device, thereby indicating that the MWA device is less susceptible to charge trapping in the PBTS and NBTS tests than is the RTA device.
Using the Arrhenius relationship, the logarithm of τ is plotted as a function of the inverse temperature for the p-and n-channels in Figure 10. These results suggest that the charge trapping process is driven by thermal activation. Hence, the trapping process of thermally activated charges is given by a linear relationship in ln(τ) versus 1/T. Thus, from Equation (5), the slope of the Arrhenius plot in the PBTS and NBTS tests represents the average effective barrier height (E τ ) for charge transport. Estimates of E τ for the MWA and RTA NiSi x SB-TFTs during the PBTS and NBTS tests are summarized in Table 4. Lower E τ has been found in several previous studies due to the more organized structure of the channel a-IGZO [39]. As E τ is smaller in MWA NiSi x SB-TFTs than in RTA NiSi x SB-TFTs, MWA processing leads to a more ordered a-IGZO structure than the RTA method. device, thereby indicating that the MWA device is less susceptible to charge trapping in the PBTS and NBTS tests than is the RTA device.  Using the Arrhenius relationship, the logarithm of τ is plotted as a function of the inverse temperature for the p-and n-channels in Figure 10. These results suggest that the charge trapping process is driven by thermal activation. Hence, the trapping process of thermally activated charges is given by a linear relationship in ln(τ) versus 1/T. Thus, from Equation (5), the slope of the Arrhenius plot in the PBTS and NBTS tests represents the average effective barrier height (Eτ) for charge transport. Estimates of Eτ for the MWA and RTA NiSix SB-TFTs during the PBTS and NBTS tests are summarized in Table 4. Lower Eτ has been found in several previous studies due to the more organized structure of the  channel a-IGZO [39]. As Eτ is smaller in MWA NiSix SB-TFTs than in RTA NiSix SB-TFTs, MWA processing leads to a more ordered a-IGZO structure than the RTA method.    Finally, to validate the utility of the ambipolar NiSi x SB-TFTs, the operation of two types of CMOS-like inverter circuits is demonstrated according to the silicidation scheme. Two NiSi x SB-TFTs with identical geometry and channel dimensions were connected in order to construct a single inverter, as shown in the equivalent circuit in Figure 11a. The voltage transfer characteristics (VTCs) of the MWA and RTA devices at various supply voltages (V DD ) are shown in Figure 11b,c, respectively. Here, typical and comparable ambipolar inverter behaviors are shown in the first (positive V DD and V IN ) and third (negative V DD and V IN ) quadrants of the inversion function. This is caused by the exchange of n-and p-channel behavior between the two TFTs, which represents the unique feature of the CMOS-like inverters constructed using the ambipolar NiSi x SB-TFTs. Thus, the SB-TFT connected to the V DD side functions as a pull-up transistor, while the SB-TFT on the ground side operates as a pull-down transistor. for the MWA device than for the RTA device, which is due to the high drive current and low leakage current of the MWA SB-TFTs. Accordingly, the MWA device exhibits superior inverter characteristics, and a steeper switching slope, than the RTA device.  The extracted voltage gains (|∂V OUT /∂V IN |) in the first and third quadrants of the VTC curves at various V DD values are plotted in Figure 12. In both inverter circuits, the voltage gain is seen to increase with increasing V DD . Moreover, the voltage gain is seen to be larger for the MWA device than for the RTA device, which is due to the high drive current and low leakage current of the MWA SB-TFTs. Accordingly, the MWA device exhibits superior inverter characteristics, and a steeper switching slope, than the RTA device.  For circuit applications, it is necessary to understand the dynamic characteristics of the inverter. The dynamic inverting characteristics of the CMOS-like inverters are presented in Figure 13 for square-wave input signals at 1 KHz with various |VDD| values ranging from 1 V to 5 V. Figure 13a,b show the frequency response characteristics in the For circuit applications, it is necessary to understand the dynamic characteristics of the inverter. The dynamic inverting characteristics of the CMOS-like inverters are presented in Figure 13 for square-wave input signals at 1 KHz with various |V DD | values ranging from 1 V to 5 V. Figure 13a,b show the frequency response characteristics in the third and first quadrants, respectively. Here, the output voltage (V OUT ) of the inverter is seen to increase with increasing V DD , and the MWA inverter displays an output waveform that is closer to the input signal (V IN ) than does the RTA device. In addition, the MWA inverter can remain high and low for almost 0.5 ms, while the RTA inverter can only maintain low and difficultto-maintain high-state operation. The MWA inverter has a high-state value of 4.53 V, and the RTA inverter has 4.04 V, which not only maintains a high state but also has better V OUT characteristics of the high state. This is the result of the SS and µ FE characteristics shown in Table 2. Based on excellent electrical properties, the MWA-NiSi x SB-TFTs exhibit superior high-speed response capability than the RTA-NiSi x SB-TFTs, which works the same when configuring inverter circuits. Therefore, Ni silicidation via MWA improves the operating characteristics of the SB-TFT, thereby enabling improved performance of complementary logic gates and faster frequency response. state value of 4.53 V, and the RTA inverter has 4.04 V, which not only maintains a high state but also has better VOUT characteristics of the high state. This is the result of the SS and μFE characteristics shown in Table 2. Based on excellent electrical properties, the MWA-NiSix SB-TFTs exhibit superior high-speed response capability than the RTA-NiSix SB-TFTs, which works the same when configuring inverter circuits. Therefore, Ni silicidation via MWA improves the operating characteristics of the SB-TFT, thereby enabling improved performance of complementary logic gates and faster frequency response.

Conclusions
Herein, high-performance ambipolar nickel silicide (NiSix) Schottky barrier thin-film transistors (SB-TFTs) were fabricated on excimer-laser-annealed (ELA) poly-Si substrates via a microwave annealing (MWA) process. For comparison, the conventional rapid thermal annealing (RTA) process was also utilized for the formation of NiSix. The MWA process was shown to provide advantages such as higher energy transfer efficiency, along with a lower power consumption and thermal budget, than the RTA process. In addition, MWA is effective as a selective heating process, especially for thin-metal films. Prior to manufacturing the NiSix SB-TFTs, SB-diodes were fabricated on bulk-Si substrates in order to evaluate the crystallinity and sheet resistance (Rs) of the NiSix prepared using MWA and RTA. The Rs of the MWA NiSix was shown to decrease with increasing MW power, with the lowest Rs of all (3.86 Ω/sq) being obtained at 600 W. The MWA SB-diodes on ntype and p-type bulk-Si substrates showed better rectification operation and electrical characteristics than the RTA SB-diodes. In addition, NiSix SB-TFTs were fabricated on ELA poly-Si substrates under identical conditions, and their electrical properties were compared. The results indicated that the MWA NiSix SB-TFTs exhibit better electrical characteristics than the RTA devices, including the subthreshold swing (SS), field-effect mobility, threshold voltage (VTH), on/off current ratio (Ion/Ioff) and interface state density (Dit).

Conclusions
Herein, high-performance ambipolar nickel silicide (NiSi x ) Schottky barrier thin-film transistors (SB-TFTs) were fabricated on excimer-laser-annealed (ELA) poly-Si substrates via a microwave annealing (MWA) process. For comparison, the conventional rapid thermal annealing (RTA) process was also utilized for the formation of NiSi x . The MWA process was shown to provide advantages such as higher energy transfer efficiency, along with a lower power consumption and thermal budget, than the RTA process. In addition, MWA is effective as a selective heating process, especially for thin-metal films. Prior to manufacturing the NiSi x SB-TFTs, SB-diodes were fabricated on bulk-Si substrates in order to evaluate the crystallinity and sheet resistance (R s ) of the NiSi x prepared using MWA and RTA. The R s of the MWA NiSi x was shown to decrease with increasing MW power, with the lowest R s of all (3.86 Ω/sq) being obtained at 600 W. The MWA SB-diodes on n-type and p-type bulk-Si substrates showed better rectification operation and electrical characteristics than the RTA SB-diodes. In addition, NiSi x SB-TFTs were fabricated on ELA poly-Si substrates under identical conditions, and their electrical properties were compared. The results indicated that the MWA NiSi x SB-TFTs exhibit better electrical characteristics than the RTA devices, including the subthreshold swing (SS), field-effect mobility, threshold voltage (V TH ), on/off current ratio (I on /I off ) and interface state density (D it ). Furthermore, the MWA NiSi x SB-TFTs exhibited lower threshold voltage shifts during the PBTS and NBTS tests, along with enhanced stability. In addition, complementary metal-oxide-semiconductor (CMOS) inverters with better VTC, gains, and excellent dynamic inversion characteristics in both the first and third quadrants were successfully constructed using the ambipolar MWA SB-TFT NiSi x S/D junctions. Therefore, ambipolar SB-TFTs containing NiSi x junctions prepared via the MWA process provide a prospective CMOS technology, because MWA is an excellent method for silicidation due to its high energy transfer efficiency, low power consumption, low thermal budget, and selective heating capacity.