ALD/ALE 2024 Session AA1-TuM: Applications in ULSI BEOL: Interconnects, Diffusion Barriers, & DRAM

The scaling of DRAM devices and subsequent increase in memory density requires shrinking memory cell size, which necessitates engineering of the capacitor and transistor of the memory cell. Scaling of capacitor presents challenges in size shrink capacitor size and area shrinking, and reduction in number of stored charges. These factors contribute to increase in capacitor leakage, wherein it is critical to maintain leakage current <10^-7 A/cm² at the operating voltage [1].

ALD oxide films of Hf & Zr, when crystallized to non-monoclinic phase have high dielectric constant, and, therefore, utilized in the DRAM capacitor stack to achieve high capacitance. However, grain boundaries of these crystalline films form dominant carrier conduction paths leading to increased leakage current. Amorphous, thin ALD Al₂O₃ layer in the middle of the high-k stack is known to engineer grain boundary structure in the film creating tortuous carrier conduction path leading to lower leakage current [1]. In this work, we investigated alternate oxide materials for inter-layer that improve leakage performance of the capacitor stack compared to Al₂O₃ inter-layer. Our work demonstrated that ALD Sc₂O₃ or Y₂O₃ can be utilized as inter-layer in the DRAM high-k stack. ALD Sc₂O₃ resulted in 33% improvement and ALD Y₂O₃ resulted in 50% leakage current improvement compared ALD Al₂O₃ (deposited using trimethyl aluminum) inter-layer. Additionally, both Sc₂O₃ and Y₂O₃ inter-layer resulted in lower EOT increase compared to Al₂O₃.

References:

[1] Jeon, W. (2020), Journal of Materials Research, 35(7), 775-794

The 2T0C DRAM has advantages compared to conventional Si channel 1T1C in terms of high cell density (~ 6F²), long retention time (> 400 s), and multi-bit operation using the current sensing mode [1-2]. Oxide semiconductors (OS) have advantages such as extremely low off current (< 10^-18 μA/μm) [3]. Recently, capacitor-less 2T0C DRAM using a low off-current IGZO (In-Ga-Zn-O) channel was proposed. Since the FEOL fabrication includes a high-temperature process, high thermal stability is required to apply OS channel FET to DRAM. However, the electrical properties of the OS channel FET degraded above the 400 ^oC post-annealing process [4]. In general, Ga doping improves the thermal stability of OS. Among the various deposition methods for OS, atomic layer deposition (ALD) is a promising candidate because of its characteristics that allow easy doping concentration adjustment and easy sub-nanometer scale thickness control.

In this study, thermal stability (~ 800 ^oC) of Ga-doped InO channel FET was verified by controlling the thickness using the ALD method, and the 3-line based multi-bit operation of the 2T0C DRAM was confirmed. First, the InO channel FET was evaluated. The 250 ^oC deposited 3-nm thick InO using DBADMIn precursor was adopted to the channel layer. The c-axis aligned C(222) crystalline InO FET exhibits superior electrical properties by lowering the grain boundary effect [5] but degraded after 800 ^oC post-annealing for 3 hours, as shown in Fig 1..To achieve high thermal stability, InO was doped with Ga using the ALD super-cycle method. The thickness of Ga-doped InO (IGO) was also evaluated because it is responsible for crystallinity. As shown in Fig 2., Above the 10 nm thickness of the IGO channel, FET exhibits superior thermal stability even after the post-annealing process at 800 ^oC for 3 hours. The 800 ^oC post-annealed 10 nm thick IGO FET was adopted because the on-current and field-effect mobility were similar to 400 ^oC post-annealed InO channel FET (InO: 55.5 cm²/Vs, IGO: 58.9 cm²/Vs). As shown in Fig 3., the current sensing multi-bit operation is exhibited regardless of channel material, and the RBL current has similar values according to the WBL voltage because the mobility is similar with InO and IGO channel FET.

References

[1] Duan, Xinlv, et al. IEEE Transactions on Electron Devices 69.4 (2022): 2196-2202.

[2]Belmonte, A., et al. 2020 IEEE International Electron Devices Meeting (IEDM). IEEE, 2020.

[3]Kamiya, Toshio, Kenji Nomura, and Hideo Hosono. Science and Technology of Advanced Materials (2010).

[4] Jeong, Hyun-Jun, et al. ACS Applied Electronic Materials 4.3 (2022): 1343-1350.

ZrO₂ has been used as an insulator film for dynamic random-access memory capacitors for a long time due to its compatibility with TiN electrodes and the high maturity of the atomic layer deposition (ALD) process. Tetragonal ZrO₂ thin films with a high dielectric constant (~40) on TiN electrodes could be obtained relatively easily by ALD. However, there are limitations to using only ZrO₂ due to the leakage current problem. The inevitably present grain boundaries acted as the dominant carrier conduction path. Al-doped ZrO₂ effectively suppressed leakage current by reducing the grain size and inducing a winding conduction path, but it also caused the crystallinity of ZrO₂ to deteriorate. Therefore, further scaling requires new dopants that can replace the role of Al without sacrificing the tetragonal crystallinity of ZrO₂.

In this regard, this presentation discusses the results of employing Gd as a dopant to replace Al. The Gd₂O₃ ALD process for doping in ZrO₂ was designed utilizing a novel Gd precursor. We investigated the effectiveness of Gd as a dopant for ZrO₂ by fabricating metal–insulator–metal capacitors with Gd-doped ZrO₂ as an insulator film and analyzing its electrical properties. As the ratio of Gd₂O₃ in the dielectric increased, the capacitance density decreased, but the leakage current characteristics were noticeably improved. Furthermore, as a result of analyzing the crystallinity of the Gd-doped ZrO₂ thin film by X-ray diffraction analysis, it was found that Gd doping did not significantly deteriorate the tetragonal crystallinity of ZrO₂.

Acknowledgments This work was supported by the BK21 Plus program. The authors would like to thank SK Trichem for their support and permission to publish this collaborative work.

References [1] W. Jeon, J. Mater. Res. 35, 7 (2020)

As ULSI becomes more highly integrated, the current density flowing through Cu multilevel interconnects increases, and it has become difficult to ensure device reliability owing to the emergence of wire breakage defects caused by electromigration. In addition, device performance degradation owing to RC delays is a major problem. To solve these issues, it is essential to form a liner layer with a material that exhibits good adhesion to Cu, suppresses electromigration, and crowns the Cu line with a metal cap during the damascene process. It is also important to increase the proportion of Cu in ultrafine Cu interconnects and reduce the effective resistance by employing a thin barrier metal with high Cu diffusion barrier properties.

To address these issues, we have reported that CoW alloy thin films synthesized by ALD have better Cu diffusion barrier properties than existing TaN and higher adhesion to Cu than Ta. We have also shown that Co can be selectively formed on the Cu surface by combining ALD and ALE, and can be used as a metal cap layer.

In this presentation, we report the results of the evaluation of the Cu diffusion barrier properties of CoW films and the development of an AS-ALD process for Co films. To develop an efficient AS-ALD process, we introduce a method to search for novel precursor compounds with appropriate vapor pressures, and a method to estimate and measure the adsorption equilibrium constants of the precursor candidates. We also introduce a method for in situ observation of the incubation cycle by reflectance spectroscopy of the growth surface, and report the results of our investigation of growth conditions and surface treatment methods that exhibit selectivity. These methods are summarized to establish a framework for the ALD process design.

With the scaling of semiconductor integrated circuits, RC delay and power consumption have increased owing to higher line resistance, and reliability degradation due to electromigration (EM) has become a problem. The use of cobalt (Co) in interconnect layers has attracted attention as a solution to these issues. Co liner, Co cap, and Co interconnects replacing Cu, are expected to address these challenges. For these applications, it is necessary to form high-quality, low-resistance Co ultrathin films uniformly on both the macroscale (entire wafer) and microscale (via, trench). Therefore, the use of atomic layer deposition (ALD) is desirable. From the perspective of substrate damage and step coverage, thermal ALD is preferable to plasma-enhanced ALD. However, the chemical options available for thermal ALD are limited. For instance, while there have been reports of Co-ALD utilizing the Co(DAD)₂ precursor,^[1] the issue arises from Co(DAD)₂ being a solid with a low vapor pressure. In contrast, CCTBA is a highly volatile liquid and an appropriate Co precursor. Therefore, in this study, we focus on the thermal ALD of Co using CCTBA.

Hot-wall ALD equipment was used for Co deposition. CCTBA and H₂ were used as the precursor and reactant, respectively, and N₂ was used as the carrier and purge gases, respectively. CCTBA was vaporized at 45°C and bubbled with the carrier gas. The chamber temperature was maintained at 100°C. A sputtered Cu film was used as an underlayer. The composition of the Co film was measured using XPS, and the film thickness was determined using SEM.

Figure 1 shows the growth per cycle (GPC) of the Co film according to the CCTBA supply time. The GPC became saturated after 5 s, and the saturated GPC (0.05 nm/cycle) was smaller than the Co monolayer thickness (0.2 nm). This suggests that the deposition was self-limited via the surface saturation of the adsorbing precursors, which is typical of ALD. Figure 2 shows a cross-sectional SEM image of the Co film deposited on a Si trench structure with an aspect ratio of 8.4. Conformal deposition, a feature of ALD, has been achieved. Figure 3 shows the composition of the Co film versus the H₂ dose. As the H₂ dose increased, the amount of carbon impurities decreased, and the film purity improved. Figure 4 shows the XPS results for the Co film deposited under high H₂ supply conditions. No peaks of C and O, which were present in the CCTBA precursor, were observed, indicating the successful deposition of a high-quality Co film. The Co film resistivity was measured to be 27 µΩ∙cm with a thickness of 36 nm.

Ru emerges as a next-generation interconnect materials capable of overcoming the scaling limits encountered with Cu interconnect [1]. Generally, as the deposition temperature increases, the properties of metal thin films deposited by ALD improve due to the grain size increase and impurities reduction by the enhanced chemical reaction. But at deposition temperatures where the precursor thermal stability isn’t ensured, its thermal decomposition occurs, leading to degraded in thin film properties such as increased resistivity owing to the significant impurities and retarded grain growth. In this study, a novel ALD-Ru process has been developed, which provides additional reactant followed to obtain high-performance ALD-Ru process capable of maintaining excellent properties, even at temperatures where precursor thermal decomposition happen. Here, a Ru metalorganic precursor, tricarbonyl(trimethylenemethane)ruthenium [Ru(TMM)(CO)₃] was used for ALD-Ru. It was reported that by using this precursor, a high-quality film with a very low resistivity (~12.9 μΩ∙cm) was obtained in ALD temperature window (260℃)[2]. But, the precursor thermal stability has been kept to ~260℃ and at 275℃, the thermal decomposition of the precursor started and the resistivity of the film deposited at 300 ℃ was again increased due to the incorporation of carbon impurity. In this study, to improve the properties of Ru thin film, ALD-Ru process was done at higher deposition temperature at 310℃. Moreover, to remove contamination of the film resulting from the precursor thermal decomposition, NH₃ has been provided as an additional reactant followed by providing the main counter reactant, O₂, which is called 6-step ALD process (figure 1). Thus, when only O₂ was used (4-step ALD), the GPC was as high as ~2.4 Å/cycle and with the additional injection of NH₃, the GPC decreased to ~1.3 Å/cycle. The resistivity of the film was as high as 20.1 μΩ∙cm using a conventional 4-step process but with 6-step process, is was decreased to ~13.6 μΩ∙cm. In addition, an increase in peak intensity was confirmed through XRD analysis (figure 2), indicating the improvement of the film crystallinity by 6-step ALD process. An ALD-Ru film with a thickness of 26 nm was obtained with an excellent physical and compositional conformality and nearly 100% step coverage at the hole feature with a high aspect ratio of ~25. In summary, 6-step ALD-Ru process to supply additional reactant followed by the main counter reactant can provide the valuable process option to obtain the high-quality metal film at higher deposition temperature where the precursor thermal decomposition.