Lately, there has been a significant amount of interests in amorphous oxide semiconductor thin-film transistors (TFTs)^[1−3]. Oxide semiconductors composed of metal compounds such as InSnO, InZnO, ZnSnO, InGaZnO, etc. have been widely studied. By adjusting the atomic ratio in the process of deposition^{[4, 5]}, their properties can be modulated. Among these materials, In–Ga–Zn–O (IGZO) is widely recognized as a standard channel layer in flat-panel display field^{[6, 7]}. Additionally, owing to their favorable characteristics such as large area uniformity, low growth temperature, and extremely low off-current consumption, IGZO films are considered as potential channel materials for next-generation BEOL-compatible transistors used in monolithic 3D integration^[8−10].

Meanwhile, to continuously scale down device size and increase integration density, new device structures are always exploited to optimize device performance^{[11, 12]}. Our previous work proposed a novel vertical channel-all-around (CAA) TFT for 2T0C dynamic-random-access memory (DRAM), which offers superior density advantages^[13]. Due to the remarkable merits, such as accurate control of thickness and composition, good step coverage and so on^{[14, 15]}, plasma atomic layer deposition (PEALD) was used as the film growth technology to deposit the channel layer, gate insulator, and gate electrode. However, PEALD technology employ O plasma as the oxidant, which has direction performance, and is incapable to achieve excellent groove coverage and uniformity on complex structures with higher aspect ratios. Thermal atomic layer deposition (TALD) employs ozone (O₃) as oxidant, which is directionless, and is advantaged over PEALD for achieving better groove coverage. Therefore, exploring TALD IGZO is necessary to provide strong support for the application of IGZO channels in 3D structure devices^[16].

In this work, the physical properties of TALD IGZO films were investigated by changing the sub-cycles number during ALD process. Moreover, the effect of TALD IGZO films as channel layers on the performance of vertical CAA TFTs were also explored. The application of TALD methodology greatly contributed to these excellent device properties of our sub-100 nm IGZO CAA-TFTs. These groundbreaking findings offer insights and guidance for future high-density 3D integration technologies.

The structure and fabrication process flow for creating an IGZO FET with a CAA structure are illustrated in Figs. 1(a) and 1(b). Initially, a 70 nm Mo bottom source/drain (S/D) electrode was deposited by sputtering techniques and the pattern was subsequently etched by dry etching. Subsequently, we formed a metal/insulator/metal (MIM) structure by sequentially depositing a SiO₂ insulation layer (50 nm thick) followed by a Mo top S/D electrode (30 nm thick), also via sputtering processes. To create the vertical channel, we used dry etching techniques to sequentially etch the MIM structure until it formed a hole with a diameter measuring 4 μm. After forming the etched hole, 6 nm thick IGZO channel films were deposited by TALD. Without breaking vacuum conditions within the chamber, we then deposited HfO₂ (7 nm thick) to improve the quality of interface between channel layer and gate insulator. Similarly, without breaking vacuum conditions again within the chamber, we proceeded to deposit an InZnO (IZO) film as gate electrode on top of HfO₂ film at an ideal thickness of 16 nm. Finally, device isolation was achieved through the dry etching process, resulting in successful fabrication of an IGZO CAA-TFT device. In accordance with the top view of the device, we respectively define critical dimension (CD) and channel width (W) as the diameter and perimeter of the etched hole. Additionally, the channel length (L) relies upon both SiO₂ insulation layer thickness and angle specifications of the etched hole, which is 65 nm.

Fig. 2 illustrates the schematic representation of the growth process for IGZO thin films deposited by thermal ALD. In this process, we respectively used (3-dimethylamimopropryl)-dimethyl indium (DADI), trimethylgallium (TMGa) and diethylzinc (DEZn) as the indium precursor, gallium precursor and zinc precursor. Ozone acted as the oxidant. The temperature during ALD process was maintained at a constant value of 250 °C, which resulted in self-limiting behaviors for films with respect to substrate temperature. The growth rate per cycle (GPC) for each sub-cycle was determined by high-resolution transmission electron microscopy (HRTEM), yielding values of 0.09 nm/cycle for In₂O₃ sub-cycle, 0.04 nm/cycle for Ga₂O₃ sub-cycle, and 0.15 nm/cycle for ZnO sub-cycle. Purge processes were performed using N₂ after each precursor deposition step. To control our ALD process effectively, we modulated the cyclic ratios of In : Ga : Zn to be 3 : 1 : 1, 4 : 1 : 1, 5 : 1 : 1, 6 : 1 : 1, and 9 : 1 : 1. This means that within each super-cycle, one sub-cycle of Ga₂O₃ and ZnO were deposited sequentially, then followed by n (n = 3, 4, 5, 6, 9) sub-cycles of In₂O₃ on top of ZnO surface. Successive super-cycles were deposited until the set film thickness was reached. For convenience, IGZO films corresponding to different n values are termed as 311, 411, 511, 611, and 911, respectively.

Before analyzing device characteristics, further investigations focused on examining physical characteristics of IGZO films with various In₂O₃ sub-cycles. Firstly, X-ray photoelectron spectroscopy (XPS) was used to investigate elemental compositions and oxygen vacancy in IGZO films. The calculated atomic percentages were summarized in Table 1. It is found that, as the In₂O₃ sub-cycle number increases, the atomic percent of In gradually increases, and that of Zn and Ga gradually decrease. Furthermore, the In/Ga ratio increases from 1.36 to 3.98, while the Zn/Ga ratio almost maintain unchanged. These results indicate that changing the In₂O₃ sub-cycle number is a suitable path to determine the cationic composition of IGZO films during ALD growth. In order to investigate the effect of oxygen-related defects in different IGZO films, the O 1s spectra were measured and shown in Fig. 3. The O 1s spectra were separated into three correlated sub-peaks through deconvolution, specifically at 529.25 eV for the oxygen bonded to completely coordinated metal ions (M–O), 530.25 eV for oxygen bonded to under-coordinated metal ions (V_O), and 531.25 eV for oxygen associated with impurities like hydroxyl groups (O–H)^[17]. The respective figure provides the calculated proportions of these three components in relation to the total area. It can be observed that the areal ratio of the V_O peak consistently increased as the number of In₂O₃ sub-cycles rise. This is attributed to a higher incorporation of In than Ga and Zn, which promotes the formation of V_O due to In–O having weaker bonding strength compared to M–O bonds where M = In, Ga, Zn^[18].

Fig. 4 exhibits the changes of carrier concentration, resistivity and hall mobility for 311, 611 and 911 IGZO films. As expected, the carrier concentration increases from 3.1 × 10²⁰ to 5.6 × 10²⁰ cm⁻³, along with In₂O₃ sub-cycles increasing from 311 to 911. Previous studies have demonstrated that V_O serves as shallow donors in IGZO films and as a source of carriers^[19]. Thus, the increase in carrier concentration with In₂O₃ sub-cycles is consistent with the increase in V_O shown in Fig. 3. The hall mobility of IGZO films also exhibited a monotonic increase from 19.4 to 33.2 cm²/(V∙s) as the number of In₂O₃ sub-cycles increased from 311 to 911. This can be explained by the fact that electron carriers find an effective percolation pathway through the larger atomic radius of In, facilitating enhanced mobility^[18]. Consequently, higher levels of In content lead to improved mobility due to easier percolation and reduced effective electron mass^[20]. Additionally, the resistivity decreases with an increasing number of In₂O₃ sub-cycles owing to elevated carrier concentration and hall mobility.

Based on our analysis about physical properties related to different atom compositions in IGZO films mentioned above, the CAA-TFTs with IGZO as channel films were fabricated, and the electrical properties of the CAA-TFTs were explored. Fig. 5(a) shows the optical microscopic top view of the CAA IGZO FET. Fig. 5(b) shows schematic front view of the CAA IGZO FET, and the red boxed section is observed by TEM, which is shown in Fig. 5(c). As can be seen, each layer demonstrated well-formed structures with flat sidewalls exhibiting nearly vertical slopes, while ensuring uniform deposition on vertical-channel regions. Fig. 5(d) shows energy dispersive spectrometer (EDS) mapping of the elemental distributions of each layer. Both TEM image and EDS mapping confirm that ALD technology is highly suitable for multilayer thin film deposition in vertical structures.

Transfer and output characteristics of the 311, 611 and 911 IGZO CAA-TFTs were shown in Fig. 6. Decent switching characteristics can be seen for 311 and 611 IGZO CAA-TFTs, while 911 CAA-TFT is difficult to be turned off originating from the highest carrier concentration of 911 IGZO. V_TH was determined to be the gate voltage (V_GS), at which drain current (I_DS) is 100 pA × (W/L), and V_DS is 1 V. The V_TH of 311 CAA-TFT and 611 CAA-TFT are –0.5 and –2 V, respectively, consistent with the increasing IGZO carrier concentration from 311 to 611. SS were also extracted from dV_GS/dlogI_DS, and given in the corresponding graph. The field effect mobility of 311 and 611 IGZO CAA-TFTs are calculated to be 0.003 and 0.009 cm²/(V∙s) according to the following formula, far smaller than their corresponding hall mobility. The field effect mobility (Eq. (1)) may be degenerated by serious scattering causing by various scattering sources, such as HfO₂/IGZO such as HfO₂/IGZO interface roughness, HfO₂/IGZO interface dipole, trapped charge in IGZO or HfO₂, and so on. This would lead to small on-current (I_on) and large off-current (I_off), as well as small I_on/I_off ratio. Moreover, the output curves displayed some Schottky characteristic indicating poor contact resistance. Therefore, in order to further improve CAA-TFT device performance, process optimizations to reduce these traps and the contact resistance are very essential.

1μFE=∂Ids∂Vgs1CoxW/L(Vds−2×Ids×RC).

In this work, we proposed a novel vertical CAA structure to fabricate IGZO-based thin film transistors, whose channel length is only 65 nm. In order to obtain better step coverage, thermal atomic layer deposition technique (TALD) was developed to deposit IGZO film in the vertical structure. Firstly, we investigated the IGZO film physical properties via XPS by tuning In₂O₃ sub-cycles during ALD process, including the atomic composition, oxygen vacancy, and OH-related impurity. Then, the corresponding carrier concentration, mobility, and resistivity were obtained by Hall measurement. It can be seen that as the In atomic percent increases, both the carrier concentration and mobility increase, as well as oxygen vacancy. Finally, the TALD IGZO films with different atom compositions were implemented into our short channel CAA-TFTs to explore the impact of IGZO films on the device performance. These findings pave a way to break through the technical problems for the continuous scale of electronic equipment and high-density 3D integration.