(Invited) BEOL-Compatible Oxide Semiconductor Logic and Memory Devices
Authors: Gong Xiao 
Published: February 2024

3D monolithic integration of logic and memory devices has emerged as one of the key enablers for next-generation electronic devices to address the ever-increasing demand for higher integration density, better performance and energy efficiency. BEOL-compatible oxide semiconductors show great potential to revolutionize the field thanks to their unique properties [1].
We present our recent advancement related to BEOL-compatible oxide semiconductors for logic and memory applications. The digital-etch-enabled nanowire transistors and α-IGZO-based eDRAM will first be discussed, followed by the ferroelectric (FE) memories with high performance and novel structures utilizing α-IGZO or ALD-deposited ZnO as the channel.
Ultra-scaled amorphous IGZO (α-IGZO) nanowire field-effect transistors (NW-FETs) hold great potential for applications demanding high performance and integration density. To realize the fabrication of high-quality and aggressively-scaled nanowire structures, a novel digital etching method for amorphous α-IGZO materials has been proposed and demonstrated [2]. Confirmed by the SEM images of an α-IGZO nanowire before and after digital etching, a notable nanowire width W
NW reduction can be observed. We have the thinnest α-IGZO nanowire achieved by the digital etching with a W
NW of approximately 20 nm. By further developing the transistor upon the nanowire, the α-IGZO NW-FET attains a good subthreshold swing (SS) of 80 mV/decade and a high peak extrinsic transconductance (G
m, ext) of 612 μS/μm at V
DS of 2 V (456 μS/μm at V
DS = 1 V). Compared to the previous studies, our IGZO NW-FET achieves one of the highest peak G
m values among all IGZO-based FETs.
Besides the BEOL compatibility and high on-current, ultra-low subthreshold leakage makes α-IGZO FETs extremely promising for eDRAM. We further investigated the potential of α-IGZO eDRAM by developing the charge-domain compute-in-memory (CiM) [3]. Experiments have demonstrated small SS, large on-state current, and long-time charge retention with our dedicated 4T1C memory cell. With differential cell structure, an even higher tolerance for charge loss can be attained. With experiment-calibrated benchmarking in the VGG-8 network for CIFAR-10 image classification tasks, 2092 TOPS/W power efficiency for the CiM core can be expected, outperforming the prior TFT and CMOS-based CiM approaches, exhibiting significant advantages for ultra-low-power applications.
The integration of doped-HfO2 FE material and oxide semiconductor, both BEOL-compatible with large-scale and cost-effective deposition, presents a promising avenue for advancing data storage in the future. We have developed the high-performance α-IGZO Fe-FET with a metal-ferroelectric-metal-oxide-semiconductor (MFMIS) structure as well as the Fe TCAM [4]. The α-IGZO Fe-FET achieves a large memory window of ~3 V with high reliability, while the Fe TCAM reduces the transistor number from 16 to 2 compared to the traditional SRAM-based one. Besides α-IGZO, we believe that ALD-based oxide semiconductors featuring high controllability of film thickness and conformal coverage of the 3D structures can create new opportunities for novel device structure and integration. The fin-gate ZnO Fe-FET stands for a great example while not only outstanding device performance but also suppressed device-to-device variation due to the unique structure have been demonstrated, holding tremendous promise for high-density 3D integration [5].

Acknowledgments: This work is supported by Singapore Ministry of Education (Tier 2: MOE2018-T2-2-154, Tier 1: R-263-000-D65-114).

References: [1] S. Dutta et al., IEDM, p. 36.4., 2020. [2] K. Han et al., VLSI, 2021, p. T10-1. [3] J. Liu et al., IEDM, 2021, p. 46.2. [4] C. Sun et al., VLSI, 2021, p. T7-4. [5] Q. Kong et al., IEDM, 2022, p. 12.3.