Device modeling of two-steps oxygen anneal-based submicron InGaZnO back-end-of-line field-effect transistor enabling short-channel effects suppression | Scientific Reports - Nature.com

Amorphous oxide semiconductor (AOS) field-effect transistors (FETs) are successfully utilized successful the backplane panels of commercialized show products, specified arsenic integrated light-emitting diode displays and liquid–crystal displays^1,2. Their advantages see higher mobility than amorphous Si, bully large-area uniformity, and a debased process temperature. Furthermore, AOS FETs are promising candidates for the backmost extremity of enactment (BEOL) FETs; their advantages see a low-temperature process, ultralow leakage current³, scalability, stability, endurance, debased disguise count, compatibility with complementary metal-oxide-semiconductor (CMOS) technology⁴, precocious thermal tolerance⁵, and threshold voltage (V_T) controllability⁶.

Various caller and promising applications person precocious been demonstrated based connected AOS BEOL FETs (e.g., 2T1C gain-cell memory⁴, monolithic three-dimensional (3D) 2T-dynamic random entree memory^7,8, W doping-based high-performance memory⁹, high-density and low-power representation utilizing the incorporation into ferroelectric FET structures¹⁰, and the debased standby powerfulness normally-off microcontroller¹¹). In these applications, electrical properties of AOS FETs are mostly controlled via modulating the attraction of bearer donors, i.e., oxygen vacancy (V_O) oregon hydrogen species, which are employed during the deposition of AOS progressive bladed films oregon by combining the depositions of progressive movie and gross insulator (GI) (especially successful the lawsuit of the self-aligned top-gate coplanar structure^{12,13,14,15,16,17,18}). However, these methods are not really suitable for the BEOL process owing to their complexity and precocious cost. More BEOL-compatible AOS FET exertion is required.

On the different hand, successful these applications, whether the grade of integration density of AOS FETs is comparable to CMOSFETs determines the performance, powerfulness consumption, and cost. In particular, from the viewpoint of powerfulness consumption, AOS FET’s vantage of precise debased leakage existent whitethorn beryllium diluted unless the integration density is importantly improved. Therefore, an AOS FET technology, which is compatible with the BEOL process and facilitates FET scaling-down, should beryllium developed.

Meanwhile, arsenic the AOS FETs are integrated with the CMOSFETs successful the BEOL process and pull much attraction arsenic promising devices that make caller and assorted functionalities, the knowing of the effect of process/structure information connected the short-channel effect (SCE) is urgent. Up to now, the instrumentality modeling and simulation person seldom demonstrated a broad optimization of V_T, SCE, subthreshold plaything (SS), and field-effect mobility (µ_FE) of short-channel AOS FETs, peculiarly successful presumption of the effect of process/structure information connected SCE.

This survey demonstrates the instrumentality modeling of submicron amorphous InGaZnO (a-IGZO) FETs, based connected experimentally extracted parameters. Both the subgap density of states (DOS), which is extracted done the photo-response of the current–voltage (I–V) characteristics, and the lateral illustration of donor doping attraction are considered successful the projected model. In addition, the exemplary parameters are incorporated into the exertion computer-aided plan (TCAD) simulation framework. Through the instrumentality simulation, the effects of oxygen (O) partial unit and instrumentality operation connected the instrumentality parameters are quantitatively investigated, and the feasibility of broad optimization of the instrumentality show parameters, specified arsenic V_T, SCE, SS, and µ_FE, is proved successful the bottom-gate (BG) and double-gate (DG) IGZO FETs fabricated with a two-step O annealing. The scope of the transmission magnitude (L) of utilized instrumentality is L = 0.245–20.2 µm. In particular, which relation the lateral illustration of V_O and O concentrations tin play successful suppressing either the L-dependency of V_T oregon the drain-induced obstruction lowering (DIBL) is explained and validated based connected the projected model-based simulation.

Two-step oxygen anneal-based fabrication of AOS FETs

Figure 1a–e amusement the process of a bottom-gate (BG) IGZO FET fabrication¹⁹. First, 5-nm SiCN and 15-nm Al₂O₃ films are deposited connected a p⁺-Si wafer, serving arsenic backmost gross dielectrics (for BG). Then, a 12-nm-thick a-IGZO movie is deposited by utilizing carnal vapor deposition (PVD). After that, a 180-nm SiO₂ hard disguise is deposited connected apical of the IGZO layer. Thus, the afloat stack is formed (Fig. 1a) and past patterned (Fig. 1b). SiO₂ is past deposited and planarized (Fig. 1c). Next, the source/drain (S/D) interaction trenches are patterned, landing selectively connected IGZO (Fig. 1d). Finally, the metallization is implemented by depositing a 6-nm atomic furniture deposition (ALD) TiN obstruction and ALD/chemical vapor deposition (CVD) W metallic contacts followed by a chemic mechanical polishing (CMP). The archetypal oxygen anneal is past performed astatine 350 °C successful an O₂ ambiance for 1 h successful bid to passivate defects generated during the process fabrication (Fig. 1e). The transmission magnitude of BG FET (L_BG) is determined by the region betwixt the root and the drain interaction holes. The IGZO progressive thickness (t_act) and the equivalent oxide thickness of BG dielectric (t_BGI) are 10 nm and 6.5 nm.

Figure 1

The integration process of the IGZO FET fabrication. Process series for (a–e) BG and (f–m) DG FET fabrication. SEM images of DG FET (n) aft progressive patterning (g) and (o) aft planarization (h). TEM images of DG FET (p) aft gross patterning (j) and (q) aft metallization (m).

On the different hand, Fig. 1f–m amusement the process of the DG IGZO FET fabrication²⁰. First, 15-nm Al₂O₃ movie is deposited connected a p⁺-Si wafer, serving arsenic backmost gross dielectrics (for BG). After a 10-nm-thick a-IGZO movie is past PVD deposited, the archetypal oxygen anneal is performed. A apical gross dielectric (SiO₂, 7 nm) and a TiN gross metallic are past deposited (for top-gate (TG)) and followed by the deposition of SiCN/SiO₂ hard disguise (Fig. 1f). After the progressive patterning (Fig. 1g), SiO₂ spread fill, planarization stopping connected TiN, and etch-back are done (Fig. 1h). Then, the remainder of the gross stack is deposited (TiN/W/SiCN/SiO₂) (Fig. 1i), patterned (Fig. 1j), SiO₂ spread fill, and planarized (Fig. 1k). Next, the S/D interaction trenches are opened down to the IGZO furniture (Fig. 1l). The interaction metals are deposited (TiN-contact barrier/W-metal fill) and planarized. The archetypal oxygen anneal is the aforesaid arsenic the BG devices (Fig. 1m). Additional interconnections made of vias and metallic lines are past implemented to entree tiny devices. The TG transmission magnitude (L_TG) successful the DG operation (unlike the L_BG successful the BG FET) is determined by the magnitude of the TG electrode.

The scanning electron microscope (SEM) images aft progressive patterning (Fig. 1g) and aft planarization (Fig. 1h) of DG FETs are shown successful Fig. 1n,o. In addition, the transmission electron microscopy (TEM) images aft gross patterning (Fig. 1j) and aft metallization (Fig. 1m) of DG FETs are fixed successful Fig. 1p,q. As seen successful Figs. S1–S3 (supplementary information), the wafer representation for the captious magnitude suggests that the full fabrication process is reproducible, and a high-yield process is utilized successful this work.

To set the donor doping attraction of the transmission successful IGZO (N_CH), the 2nd oxygen anneal is performed successful some DG and BG FETs (Fig. 1e,m). According to the information of the 2nd oxygen anneal, 2 types of samples are prepared; (1) 350 °C successful an O₂ ambiance 1 atm for 2 h (called by the modular (STD) sample) and (2) 350 °C successful an O₂ ambiance 1 atm for 2 h + 350 °C successful an O₂ ambiance 20 atm for 2 h (called by the precocious unit (HP) sample).

In the BG structure, the S/D metallic acts arsenic a diffusion obstruction for oxygen during the oxygen annealing process (Fig. 1e). Meanwhile, successful the DG structure, some TG metallic and the S/D metallic enactment arsenic oxygen diffusion barriers during the 2nd oxygen annealing (Fig. 1m). Therefore, oxygen infiltrates locally done the separation (t_s) betwixt the TG borderline and the S/D interaction trenches.

As the BG FET goes done the 2nd oxygen annealing process without TG, the IGZO nether the S/D metallic has a debased O attraction (that is, it has a precocious V_O concentration). Owing to this attraction difference, V_O diffusion occurs from the IGZO nether the S/D metallic toward the halfway of the channel, and the resulting donor doping attraction [n₀(x)] has a laterally non-uniform illustration (shown arsenic a greenish enactment successful Fig. 1e). The lateral illustration of n₀(x) is simply a important origin successful determining the electrical characteristics and the SCE of IGZO FETs, which volition beryllium modeled successful item later. Compared to BG FET, the O attraction of the transmission is comparatively low, and the N_CH is higher successful DG FET (N_{CH_DG} > N_{CH_BG}) due to the fact that the 2nd oxygen annealing measurement proceeds successful the DG operation portion the TG electrode exists. Furthermore, owed to O penetrating the t_s, a section high-concentration O portion (that is, a locally debased n₀ region) is formed connected some edges of the L_TG (shown arsenic the greenish enactment successful Fig. 1m). This portion has a captious relation successful suppressing the SCE of the submicron IGZO BEOL FET, arsenic volition beryllium discussed later.

IGZO FET DC characterization

The instrumentality illustration types are divided into BG and DG according to their structure. They are besides divided into STD and HP according to the partial unit of oxygen during the 2nd O annealing step. The 4 types of samples are named BG-STD, BG-HP, DG-STD, and DG-HP, respectively. The basal characteristics of the devices were analyzed. Figure 2a,b are the transportation characteristics and instrumentality parameters (i.e., V_T, SS, and µ_FE) for the long-channel devices [channel width (W)/channel magnitude (L) = 1/20.2 µm] portion Fig. 2c,d amusement the transportation characteristics and instrumentality parameters for the short-channel devices (W/L = 1/0.5 µm). Here µ_{FE_lin} means the µ_FE extracted successful the linear portion (V_GS = 1 V and V_DS = 0.05 V). Error bars successful Fig. 2b,d were taken from 5 devices. This supports the reproducibility of instrumentality parameters. When measuring the transportation characteristics of the DG FET, the BG voltage (V_BG) was swept, and the TG was floated. L_TG is 20 µm erstwhile L = L_BG = 20.2 µm, and is 0.3 µm erstwhile L = L_BG = 0.5 µm, which is owed to t_s = 0.1 µm.

Figure 2

The (a) transportation characteristics and (b) instrumentality parameters (i.e., V_T, SS, and μ_FE) of long-channel IGZO FETs (L = L_BG = 20.2 μm and L_TG = 20 μm). The (c) transportation characteristics and (d) instrumentality parameters of short-channel IGZO FETs (L = L_BG = 0.5 μm and L_TG = 0.3 μm). Error barroom was taken from 5 instrumentality samples.

As is demonstrated successful Fig. 2b,d, the HP illustration has a higher V_T, a smaller SS, and a little µ_FE than the STD illustration with the aforesaid operation and the aforesaid L, careless of instrumentality operation oregon transmission length. For the HP sample, due to the fact that the magnitude of O wrong the IGZO movie is larger than that of the STD sample, the V_O attraction (n_Vo) is little than the STD sample, and truthful is the N_CH. Since the V_O plays the relation of electron donor wrong the IGZO, the V_T of the HP illustration is comparatively high. In addition, since the electron transport successful IGZO follows a percolation transport²¹, the mobility increases on with electron concentration. The little µ_FE successful the HP illustration is owed to the little electron attraction (relatively little than successful the STD sample). The alteration successful the SS owed to the magnitude of O is related to the trap density astatine the IGZO/GI interface and successful the IGZO thin-film bulk; therefore, it tin beryllium explained by analyzing the DOS.

Subgap DOS extraction

The DOS was extracted utilizing the photo-response of the I–V characteristics of the IGZO FET (inset of Fig. 3a). The elaborate process for the DOS extraction is described successful the supplementary information (including Fig. S4 and S5). Figure 3a demonstrates however the DOS of IGZO, extracted from the long-channel BG FET, depends connected the magnitude of O. The bandgap vigor (E_g) of IGZO is 3 eV, and the extracted DOS consists of the pursuing terms, successful the bid of expanding vigor level from the valence set (VB) maximum level (E_V) to the conduction set (CB) minimum level (E_C): g_TD (valence set process states), g_Vo (neutral V_O states), g_In*−M (undercoordinated In states), g_Vo2+ (ionized V_O states), and g_TA (conduction set process states). Additionally, Fig. 3b shows that the extracted DOS fits good with the projected DOS model. The DOS g(E) exemplary look is calculated successful the pursuing equations, and the extracted DOS exemplary parameters are tabulated successful Table S1 successful the supplementary information.

Figure 3

(a) The DOS depending connected the O content, extracted from the BG IGZO FETs with W/L = 1/ 20.2 μm. The inset shows the measured photo-response of the I–V curve. (b) The extracted DOS (symbol) fitted with the projected DOS exemplary (line).

When the IGZO DOS alteration by the O annealing conditions was evaluated, the g_Vo of the HP illustration with precocious O attraction was confirmed arsenic tiny compared with the STD sample. The alteration successful V_O explains this well. In addition, g_In*−M, which indicates the density of the states by the operation of the undercoordinated In and metallic cation (In^*–M), was besides comparatively little successful the HP illustration case. Consistently with²², this indicates that the In^*–M probability of enslaved enactment increases arsenic the bearer attraction increases. In addition, g_TA and g_Vo2+ conscionable beneath the E_C^23,24,25 are comparatively debased successful the HP illustration case, which is accordant with that the SS of the HP illustration is little than that of the STD illustration (Fig. 2b). Noticeably, the O content-dependent DOS successful long-channel BG FETs and the instrumentality characteristics are good matched; therefore, the extracted DOS is reasonable.

Based connected the consistency betwixt DOS and instrumentality parameters, it is good understood that nether the aforesaid O partial unit information successful a long-channel device, the V_T of the BG FET is higher than that of the DG FET, and the SS and the µ_FE are smaller (Fig. 2a,b). This is due to the fact that the DG instrumentality prevents O from infiltrating (compared with the BG device) into the IGZO during the O annealing process.

On the different hand, the changes by the O contented and instrumentality operation of V_T, SS, and µ_FE go analyzable successful a short-channel instrumentality (Fig. 2c,d). While the O-dependent alteration tendencies of V_T, SS, and µ_FE successful the aforesaid instrumentality operation is the aforesaid arsenic the long-channel device, the inclination of their changes betwixt the BG-STD and the DG-HP samples is reversed successful the short-channel devices (shown arsenic dotted rectangles successful Fig. 2b,d). Undoubtedly, this is due to the fact that the V_T of the sample, which experienced the aforesaid O annealing condition, varied with L.

The knowing of the L-dependence of V_T is yet cardinal to a robust plan of submicron AOS FETs. To recognize either the SCE of IGZO FETs oregon the effect of O contented and operation connected the SCE, due instrumentality modeling and simulation are indispensable and should beryllium supported for a robust plan of a submicron IGZO FET immune to the SCE.

Device exemplary and parameter extraction

To get the robust model for material-process-device co-design of submicron IGZO FETs, we performed instrumentality modeling and incorporated it into a TCAD simulation. First, arsenic is shown successful Fig. 4, the BG FET and DG FET instrumentality structures were implemented with the TCAD, and the n_Vo(x) and n_OX(x) profiles were generated on with the O annealing process conditions and modeled by the equations successful Fig. 4. Here n_OX is the attraction of oxygen which is additionally employed during the 2nd oxygen anneal successful DG FETs.

Figure 4

Models for n_Vo(x) and n₀(x) considering the instrumentality operation and process conditions, which are implemented by the TCAD instrumentality simulator for the (a) BG FET and the (b) DG FET structure.

In our modeling, the n_Vo(x) and n_OX(x) were assumed to beryllium Gaussian profiles done the erstwhile information of the two-step O annealing process and the STD/HP conditions. The L_Vo and L_OX mean diagnostic lengths, which are associated with the respective diffusivity of V_O and O during the oxygen annealing, describing the n_Vo(x) and n_OX(x). Final donor doping illustration [n₀(x)] of the BG FET is determined by the operation of the n_Vo(x) adjacent the S/D hold and the N_{CH_BG} successful the halfway of a channel. In contrast, the last n₀(x) of the DG FET is determined arsenic n_Vo(x)−n_OX(x) adjacent the S/D hold and the N_{CH_DG} successful the transmission part. Then, the N_OX was determined arsenic N_OX = N_{CH_BG} − N_{CH_DG} due to the fact that the further oxygen would compensate the donor concentration.

The elaborate process for extracting the exemplary parameters is described successful Fig. S6 successful the supplementary information. The process is outlined arsenic follows. First, aft inputting the worldly parameters and the instrumentality structural parameters into the TCAD, we designated the experimentally extracted DOS exemplary into the IGZO progressive layer. Based connected the DOS parameter, the electron effectual wide (m_n*) and the CB effectual DOS (N_C) were determined done the formulas below²⁶.

Then, the N_CH and the complaint density successful GI (Q_ox) were fitted by comparing the simulated V_T and level set voltage (V_FB) and the measured ones. After that, the CB mobility (µ_band) was adjusted by the numerical iteration until the simulated ∂I_D/∂V_GS and measured ∂I_D/∂V_GS were successful bully statement astatine assorted V_DS’s. In these fitting processes, the numerical iteration was allowed until the mean mistake betwixt the simulated and measured values fell wrong a circumstantial mistake complaint (ER). The ER tin beryllium selected considering the trade-off betwixt the precision of the exemplary parameter and the computing load of simulation. An ER = 10% was utilized successful this study.

Next, the parameters that defined the lateral illustration of the bearer doping attraction (e.g., N_Vo, L_Vo, N_OX, and L_OX) were decided. These parameters were extracted utilizing the constituent that they affected the V_T roll-off characteristic, i.e., the L-dependency of V_T, of the short-channel FET. Since the n₀(x) of the BG FET was determined by the n_Vo(x), we performed the simulation survey (Fig. 5) to recognize the effect of N_Vo and L_Vo changes connected the V_T roll-off characteristic.

Figure 5

The TCAD simulation results for the L-dependent V_T successful BG FETs with the variations of (a) N_Vo and (b) L_Vo.

As is shown successful Fig. 5, the V_T roll-off occurs arsenic L becomes shorter successful the BG FET. Here, the transmission magnitude astatine which the V_T roll-off begins is defined arsenic L = L_crit, and the V_T astatine the minimum L (L = 0.245 µm) is defined arsenic V_{T_min}. As demonstrated successful Fig. 5a, arsenic the N_Vo becomes higher, the L_crit becomes longer, and the V_{T_min} becomes little simultaneously. It is besides shown successful Fig. 5b that arsenic the L_Vo becomes longer, the L_crit becomes much extended, but V_{T_min} is autarkic of the L_Vo. Therefore, successful the BG FET, the N_Vo tin beryllium extracted done a fitting with the measured V_{T_min} nether a fixed N_{CH_BG} condition, and past the L_Vo tin beryllium extracted done the 2nd fitting with the measured L_crit astatine a fixed N_Vo.

In the DG FET, the changes either successful N_Vo and L_Vo oregon successful N_OX and L_OX impact the V_T roll-off diagnostic since the n₀(x) is determined by the operation of the n_Vo(x) and the n_OX(x). Therefore, successful the DG FET, a much analyzable L-dependency of the V_T appears compared with the BG FET arsenic seen successful Fig. 6. More interestingly, the reverse SCE appears successful the mean L scope wherever the V_T increases arsenic the L decreases. Here, erstwhile the maximum worth of the L-dependent V_T is defined by V_{T_max}, L_crit tin beryllium re-defined peculiarly successful DG FET arsenic the L wherever the L-dependency of V_T interchanges betwixt the summation and alteration successful V_T(L). Then, arsenic the N_Vo increases, V_{T_min} decreases, L_crit increases, and V_{T_max} decreases (Fig. 6a). In addition, arsenic L_OX becomes longer, some L_crit and V_{T_max} increase, but the V_{T_min} is autarkic of the L_OX (Fig. 6b). Moreover, arsenic L_Vo lengthens, some L_crit and V_{T_max} decrease, but the V_{T_min} is immune to the saltation of L_Vo (Fig. 6c).

Figure 6

The TCAD simulation results for the L-dependent V_T successful DG FETs by splitting the (a) N_Vo, (b) L_OX, and (c) L_Vo.

Therefore, successful the DG FET, the parameters, specified arsenic N_Vo, L_Vo, N_OX, L_OX, and N_{CH_DG}, tin beryllium extracted done the narration of N_OX = N_{CH_BG} − N_{CH_DG} and a fitting betwixt the measured V_{T_min,} L_crit, and V_{T_max}, and simulated values nether the fixed N_{CH_BG} condition.

The extracted exemplary parameters are summarized successful Table S1 (supplementary information).

TCAD simulation

The TCAD model was setup for each process information (HP/STD) and operation (BG/DG) by utilizing the parameter acceptable summarized successful Table S1. The simulated and measured I–V characteristics are compared with each different (Fig. 7a,b). In the 4 types of samples (BG-STD, BG-HP, DG-STD, and DG-HP), each the TCAD simulation results reproduce good the measured I–V curves and parameters, i.e., V_T, SS, and µ_FE, for some the long-channel (Fig. 7a) and short-channel FETs (Fig. 7b).

Figure 7

The TCAD simulation (line) vs. the measurement information (symbol). (a) I–V curve astatine W/L = 1/20.2 μm, and (b) I–V curve astatine W/L = 1/0.5 μm.

Considering the discrepancies betwixt the measured and simulated results, the ER scope is 1.16–13.1%. The main origin of this quality is expected to beryllium owed to the bias-dependent mobility of the AOS FETs, suggesting that further studies are needed. In Fig. 7a (L = 20.2 μm), the ER for BG-HP, BG-STD, DG-HP, and DG-STD is 7.35, 5.57, 13.1, and 3.82%, respectively. Meanwhile, successful Fig. 7b (L = 0.5 μm), each ER for BG-HP, BG-STD, DG-HP, and DG-STD is 1.16, 3.67, 8.65, and 9.48%.

These values are accordant with the captious ER (< 10%) utilized successful extracting exemplary parameters. Therefore, the projected exemplary and exemplary parameter extraction are tenable and close capable to expect the electrical characteristics of AOS FETs.

Noticeably, utilizing the L vs. V_T narration successful Figs. 5 and 6, we tin extract the Gaussian doping parameters for each operation and process information (Fig. 4) arsenic a unsocial set. This means that the O and V_O diffusion effects are reflected good on with each L, process condition, and operation (including the minimum L of 245 nm). As seen successful Table S1, successful the agelong L, the N_CH was higher successful the bid of DG-STD, DG-HP, BG-STD, and BG-HP. In the 2nd O annealing measurement of the BG structure, since determination is nary O diffusion barrier, N_{CH_DG} is higher than N_{CH_BG}. In the aforesaid structure, the N_CH of the STD instrumentality is higher than that of the HP device.

Short-channel effects

Understanding wherefore the L-dependency of V_T successful the BG FET changes on the variations of L_Vo and N_Vo (Fig. 5) is cardinal to the plan of the doping illustration of the submicron AOS FET. Similarly, knowing wherefore the L-dependency of V_T of the DG FET changes on with the variations of L_Vo, N_Vo, and L_OX (Fig. 6) is cardinal to the plan of the immunity to SCE successful the submicron AOS FET.

First, for the BG FET, the n₀(x) is composed of N_Vo (S/D region), \(n_{{V_{O} }} (x) = N_{{V_{O} }} \times e^{{ - \left( {\frac{x}{{L{\kern 1pt}_{Vo} }}} \right)^{2} }}\) (S/D hold area), and N_{CH_BG} (the halfway of the channel), arsenic illustrated successful Fig. 4a. At this time, the N_Vo is determined by the magnitude of the V_O employed successful the IGZO film, which is controlled by the magnitude of the O inflow and the somesthesia during the IGZO deposition. The diffusion magnitude of V_Os (L_Vo) is besides determined by either the lateral diffusivity of V_O oregon the thermal fund during the process. In the aforesaid process conditions, erstwhile the L becomes shorter and comparable to the standard of the L_Vo, past the n_Vo(x) profiles of some the root and drain merge successful the halfway of the transmission to efficaciously summation the N_CH; therefore, the V_T starts to decrease. That is, erstwhile the L = L_crit, the N_{CH_BG} increases compared with the long-length device, and the V_T roll-off improvement begins to occur. Accordingly, arsenic the L_Vo lengthens and the N_Vo becomes higher, the longer L_crit becomes (the SCE becomes severe) due to the fact that the n_Vo(x) starts to intersect astatine the transmission halfway astatine the longer L (Fig. 5a). At the minimum L (L = 0.245 µm), the transmission magnitude is excessively short, and the donor doping attraction successful the halfway of the transmission approaches N_Vo, i.e., the highest worth of the Gaussian donor doping n_Vo(x), independently of the L_Vo. Therefore, the V_{T_min} is lone affected by N_Vo (Fig. 5b).

On the different hand, successful the DG FET (Fig. 6), if the locally precocious O attraction country (formed during the 2nd O annealing step) is described arsenic the Gaussian acceptor illustration \(n_{OX} (x) = N_{OX} \times e^{{ - \left( {\frac{x}{{L_{{{\kern 1pt} OX}} }}} \right)^{2} }}\), past the n₀(x) consists of the N_Vo successful the S/D area, the n_Vo(x) − n_OX(x) successful the S/D hold area, and the N_{CH_DG} successful the halfway of the channel. At this time, the O diffusion magnitude L_OX is determined by either the lateral diffusivity of O oregon the thermal fund during the process. Since the n_OX(x) portion is comparatively person to the transmission halfway than the n_Vo(x) region, erstwhile L is shortened, the locally higher n_OX(x) [locally little n₀(x)] regions are merged first, and past V_T increases. That is, arsenic shown successful Fig. 6, a V_T roll-up (the reverse SCE) is observed (V_{T_max} is observed).

Furthermore, arsenic L becomes further shorter, the merge successful the n_Vo(x) conception is overlapped. Then, a V_T roll-off section, wherever V_T decreases again, is observed. Meanwhile, arsenic L_Vo increases, the N_{CH_DG} besides increases and V_{T_max} decreases. In this case, arsenic L_Vo increases, L_crit shortens since the power of the n_OX(x) besides decreases (Fig. 6a). In addition, arsenic L_OX lengthens, V_{T_max} increases due to the fact that the N_{CH_DG} decreases. However, L_crit lengthens due to the fact that the N_{CH_DG}, which has already been lowered, makes the n_Vo(x) overlap followed by the alteration successful V_T hap astatine a longer L (Fig. 6b). Finally, astatine the minimum L (L = 0.245 µm), the N_{CH_DG} (also V_{T_min}) is determined by the operation of the N_Vo and N_OX careless of the L_OX and L_Vo due to the fact that the transmission magnitude is excessively abbreviated (Fig. 6b,c). Thus, erstwhile the N_OX is fixed, L_crit shortens and V_{T_max} decreases arsenic the L_Vo lengthens (Fig. 6c).

Hence it needs to beryllium explained why, successful a short-channel case, the V_T of the DG FET increases wide and the µ_FE decreases wide compared with the BG FET (the dotted boxes successful Fig. 2b,d). As explained above, the N_{CH_DG} is higher than N_{CH_BG} successful the agelong L. Therefore, the DG instrumentality has a little V_T and a higher µ_FE erstwhile compared with the BG device. However, successful a short-channel case, the N_{CH_DG} is little than that of the N_{CH_BG} owed to the merging improvement of the n_OX(x), truthful that the DG instrumentality has a higher V_T and a little µ_FE than the BG instrumentality (Fig. 8a–c). Furthermore, astatine the aforesaid process condition, the SS of the DG instrumentality is higher successful the abbreviated transmission (L = 0.5 µm) than that of the BG device, whereas the SS successful the agelong transmission is immune to the instrumentality operation and process. This is due to the fact that successful the DG instrumentality with a abbreviated L, determination is simply a large quality betwixt the absorption of the transmission and that of the S/D hold (Fig. 8a–c), truthful the lateral electrical tract by the drain voltage becomes much focused connected the transmission center, and the vertical electrical tract by the gross voltage acts comparatively weakly; therefore, this weakens the gross controllability.

Figure 8

The TCAD-simulated n₀(x) profiles successful the 4 types of samples on the structures and processes with varying transmission length. (a) L = 20.2 μm, (b) L = 2.2 μm, and (c) L = 0.5 μm. The TCAD simulated (line) vs. the measured (symbol) L-dependent (d) V_T and (e) DIBL. Error barroom was taken from six instrumentality samples. (f) The vigor set obtained done the TCAD simulation for the BG-HP, BG-STD, DG-HP, and DG-STD devices astatine W/L = 0.5/1 µm.

The symbols successful Fig. 8d,e amusement the L-dependent V_T and DIBL characteristics measured successful the IGZO FET. Error bars were taken from six devices. These 2 characteristics are the show scale representing the SCE. In addition, the lines successful Fig. 8d,e amusement the TCAD simulation results (using the exemplary parameters successful Table S1) reproduce good the L-dependent V_T and DIBL characteristics implicit a wide scope of the L. Based connected these results, the exemplary parameter extraction process and the parameter values corroborate that the O and V_O diffusion effects are reflected good implicit a wide scope of the L. This is besides tenable for the variations of process information and instrumentality structure, some qualitatively and quantitatively.

In Fig. 8d, erstwhile the V_T roll-off occurs successful the BG FET, the HP instrumentality is much immune to the SCE than successful the STD instrumentality (that is, the L_crit is longer). This is due to the fact that the HP instrumentality has a comparatively little N_CH and N_Vo than the STD device. Here, it is worthwhile to enactment that the L_Vo of HP instrumentality is × 1/5 to × 1/3 shorter than that of STD instrumentality (Table S1). It indicates that the much O-poor the IGZO progressive furniture is, the much progressive the lateral diffusion of the V_O and the longer the diffusion magnitude of the V_O volition be. Our uncovering is accordant with the results of studies that bespeak that the diffusion magnitude of the V_O increases erstwhile the N_Vo is high²⁷.

Meanwhile, successful the DG FET (as was already shown successful Fig. 6), the V_T roll-up (the reverse SCE) is observed on with a decreasing L, and the DG-HP instrumentality suppresses the SCE much efficaciously than the DG-STD device. As shown successful N_OX and L_OX successful Table S1, the n_OX(x) spreads much broadly successful the DG-HP device, which lowers N_OX and lengthens L_OX, erstwhile compared with the DG-STD device. This broadening further reduces the N_{CH_DG} (Fig. 8c), and some the V_T roll-off and roll-up are consequently suppressed by weakening the lateral-merge effect of the n_Vo and the n_OX. Therefore, if the DG-HP instrumentality is optimized further, it is expected to efficaciously suppress the SCE portion satisfying a affirmative V_T, which is an indispensable request from the circuit aspect.

Device plan perspective

The pursuing points should beryllium noted: the dispersed of the n_Vo successful the archetypal O annealing measurement increases arsenic it becomes much O-poor (the n_Vo dispersed successful the STD instrumentality is larger than that successful the HP device), and the dispersed of the n_OX successful the 2nd O annealing measurement increases arsenic it becomes much O-rich (broader n_OX successful the DG-HP instrumentality alternatively than successful the DG-STD device). Therefore, if the O unit during the 2nd annealing measurement and the t_s are further optimized successful the two-step O annealing process, we tin expect that the V_T volition go much immune to the scale-down of L.

The TCAD-simulated vigor set is shown successful Fig. 8f. The DIBL is tiny successful the pursuing instrumentality order: DG-HP, BG-HP, DG-STD, and BG-STD, which is accordant with Fig. 8e. The wide inclination shows that the HP instrumentality has a smaller DIBL than the STD device, and the DG operation has a smaller DIBL than the BG structure. The HP operation is much robust to DIBL than the STD operation for the pursuing 2 reasons; first, since the HP operation has a debased N_CH and the Fermi vigor level (E_F) is little than that of the STD structure, a precocious vigor obstruction is formed betwixt the S/D and the channel. Second, the L_Vo of the HP instrumentality is shorter than that of the STD device. If the L_Vo is long, the transmission country wherever the E_F rises gets longer, and the imaginable obstruction lowers betwixt the root and the channel, which makes it much susceptible to the DIBL.

In the DG devices, the voltage driblet by the V_DS is ample due to the fact that of the beingness of a n_OX portion with ample resistance. Thus, the DIBL is suppressed successful DG FET. This effect is further enhanced successful the HP process (the absorption of the n_OX portion becomes larger). The relation of this n_OX portion is precise reminiscent of the relation of halo (or pocket) implantation to artifact SCE successful submicron CMOSFETs. The DG-HP instrumentality is the astir advantageous from the DIBL constituent of presumption (similar to the V_T roll-off), and the DIBL is expected to beryllium suppressed much efficaciously done further optimization.

To comparison the show of devices utilized successful this survey with erstwhile studies, we compared the main instrumentality parameters with those of references, arsenic summarized successful Table 1. It should beryllium noted that, since the operating voltage, instrumentality structure, and size are antithetic for each device, it is indispensable to specify different caller scale for a just comparison. In the lawsuit of mobility, for example, since either the voltage information from which the mobility is extracted oregon the transmission magnitude is different, it is hard to measure arsenic the show indicator of AOS. Therefore, we defined α and β arsenic caller indices. α is defined by \(\alpha = \frac{{I_{D} \times L_{\min } }}{{W \times (V_{GS} - V_{T} ) \times V_{DS} }}\) and means the operating existent normalized by the instrumentality size, gross overdrive voltage, and drain voltage. β is besides defined by \(\beta = \frac{\alpha }{{C_{ox} }}\), means the existent further normalized by C_ox, and has the aforesaid portion arsenic the mobility. That is, if β is used, just examination is imaginable adjacent if the operating voltage, gross capacitive coupling, and instrumentality size are different. In the lawsuit of our device, assessed by β, it is confirmed that the show is fantabulous compared to the devices of the erstwhile studies. Furthermore, successful the viewpoint of L_min, it is recovered that our instrumentality shows bully scalability but for the 3D instrumentality operation specified arsenic FinFET. From the SCE constituent of view, it tin beryllium seen that the erstwhile studies seldom reported connected V_T roll-off oregon DIBL. Noticeably, our survey is simply a uncommon lawsuit that reports some DIBL and V_T roll-off, and it is confirmed that our instrumentality shows precise bully SCE properties contempt not having a 3D structure.

A method that tin optimize V_T, SS, μ_FE and SCE successful of the submicron AOS FET is vital, and the cardinal is to power the n₀(x) with a operation of V_O and O. Figure 9 illustrates the alteration of n₀(x) by the variations of L_Vo, N_Vo, N_{CH_BG}, and L. If we execute the co-optimization of t_s, N_Vo, L_Vo, N_OX, and L_OX by utilizing some a two-step O annealing and DG operation aft we marque the IGZO progressive movie sufficiently O-poor and afloat rise the CB mobility astatine the opening of the process, we volition beryllium capable to instrumentality AOS BEOL FETs that comprehensively fulfill the V_T, SS, μ_FE, and SCE parameters that lucifer the show and specification of the application-dependent circuits.

Figure 9

(a) Change of n₀(x) by the variations of L_Vo, N_Vo, N_{CH_BG}, and L successful the BG structure. (b) Change of n₀(x) by the variations of L, L_OX, and N_OX successful the DG structure. (c) Change of n₀(x) by the variations of N_Vo, N_{CH_DG}, and L_Vo successful the DG structure.

The DOS-based instrumentality exemplary and TCAD simulation model were projected with accent connected the power of n₀(x), and were demonstrated successful the BG/DG IGZO FETs with L = 0.245–20.2 µm. The validity of the instrumentality exemplary and its parameters were proved done a TCAD simulation reflecting the extracted exemplary parameters and the IGZO DOS. Based connected a two-step O annealing, the effect of O partial unit connected not lone the instrumentality show parameters, specified arsenic V_T, SCE, SS, and µ_FE, but besides DOS was elucidated. The simulation results explained successfully the measured V_T roll-off and DIBL characteristics. The results of an investigation based connected the projected exemplary and the extracted parameters bespeak that the SCE of submicron AOS FETs is efficaciously suppressed erstwhile the section precocious O-concentration portion (formed by applying the two-step O annealing to the DG FET) is used. We besides recovered that L_Vo becomes longer arsenic the instrumentality becomes O-poor, and L_OX becomes longer arsenic the instrumentality becomes O-rich.

Our results amusement that the co-optimization of t_s, N_Vo, L_Vo, N_OX, and L_OX is captious to the immunity to SCE successful AOS FETs. Proposed exemplary and simulation model are perchance utile to a broad optimization of the instrumentality show parameters, i.e., V_T, SCE, SS, and µ_FE, for the submicron AOS BEOL FET technology.

IGZO FET characterization

In this study, to analyse the I_D−V_GS characteristics of the IGZO FET, HP4156C (Keithley, Santa Rosa, CA, USA) was used, and the measurements were taken successful a acheronian authorities astatine country temperature. The measurement conditions were arsenic follows; the gate-to-source voltage (V_GS) was swept from 4 to − 6 V successful − 0.05 V steps, and the drain-to-source voltage (V_DS) was fixed astatine 0.05 V. The V_T was extracted by the V_GS astatine I_D/(W/L) = 10⁻⁸ A, and the SS was extracted by I_D/(W/L) = 10⁻¹⁰–10⁻⁹ A. In addition, the µ_{FE_lin} was calculated astatine V_GS−V_T = 3 V by utilizing the pursuing equation.

The DIBL was calculated from the quality betwixt the V_T astatine V_DS = 0.05 V and the V_T astatine V_DS = 1.05 V.

Error evaluation

In the lawsuit of measured data, the mean worth and mistake barroom were calculated by utilizing the basal mean quadrate (RMS) worth arsenic follows (x = measurement value, m = mean, n = number of data):

These mean and mistake barroom were utilized successful Fig. 2b,d, and successful Fig. 8d,e.

In addition, for comparing the measured and simulated data, the mistake complaint (ER) was calculated arsenic follows (y = simulation data, m_abs = average of implicit values of measured data).

This ER was utilized successful Fig. 7.

References

Author notes

1. These authors contributed equally: Donguk Kim and Je-Hyuk Kim.

Authors and Affiliations

1. School of Electrical Engineering, Kookmin University, Seoul, 02707, Republic of Korea

   Donguk Kim, Je-Hyuk Kim, Woo Sik Choi, Tae Jun Yang, Jun Tae Jang & Dae Hwan Kim

2. imec, Kapeldreef 75, 3001, Leuven, Belgium

   Attilio Belmonte, Nouredine Rassoul, Subhali Subhechha, Romain Delhougne & Gouri Sankar Kar

3. Advanced Device Research Lab, Semiconductor R&D Center, Samsung Electronics Co., Ltd., Gyeonggi-do, Hwaseong-si, 18448, Republic of Korea

   Wonsok Lee, Min Hee Cho & Daewon Ha

Contributions

D.K. and D.H.K. designed this enactment and wrote the main portion of manuscript. D.K., J.-H.K., W.S.C., T.J.Y., and J.T.J. performed the instrumentality characterization, modeling and TCAD simulation. A.B., N.R., S.S., R.D., and G.S.K. contributed to the plan and fabrication of the submicron IGZO FETs. D.K., A.B., S.S., R.D., W.L., M.H.C., D.H., and D.H.K. discussed the results. The manuscript was written done contributions of each authors. All authors person fixed support to the last mentation of the manuscript.

Corresponding author

Correspondence to Dae Hwan Kim.

Competing interests

The authors state nary competing interests.

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