(Digital Presentation) Stacked Nanosheet FETs and Beyond
Authors: Liu Cheewee , Tu Chien-Te , Huang Bo-Wei , Cheng Chun-Yi 
Published: November 2022

Si FinFETs have been used in the industry for mass production of advanced technology nodes [1-3]. Recently, the horizontal gate-all-around (GAA) transistors are considered as promising candidates to replace FinFETs due to the excellent electrostatics and short channel control [4-6]. The GAA architecture with vertically channel stacking can further enhance the drive current for a given footprint to achieve high performance and area scaling [7]. Moreover, high mobility channel is an attractive option to boost the drive current thanks to its intrinsic higher mobility as compared with Si [8]. Furthermore, a novel TreeFET architecture [9] as a combination of stacked nanosheets and additional fin interbridges (IBs) between nanosheets can enhance ION per footprint.
In this work, we demonstrate the 8 stacked Ge0.75Si0.25 nanosheets with high inter-channel uniformity and the 7 stacked Ge0.95Si0.05 nanowires with record ION per channel footprint for nFETs [10, 11]. The highly stacked 8 Ge0.9Sn0.1 nanosheet pFET with ultrathin bodies and thick bodies are demonstrated for low power and high performance [12, 13], respectively. Furthermore, TreeFETs can increase Weff per footprint for high performance devices beyond FinFETs and nanosheets.
The epilayers with 8 undoped and fully strained Ge0.75Si0.25 layers sandwiched by 9 P-doped Ge sacrificial layers (SL) were grown on the Ge buffer by CVD epitaxy. The S/D and channels of the 8 stacked Ge0.75Si0.25 nanosheets are fabricated by the same epilayer structures without S/D regrowth. The doping of S/D is obtained by the heavily P-doped Ge SLs by annealing, while the channel is undoped to reduce the impurity scattering. H2O2 wet etching was used to etch the Ge SLs in channel region. The etching selectivity of Ge over Ge0.75Si0.25 is attributed to the Si content in GeSi layers and the enhanced etching rate of Ge SLs by heavily doped phosphorus [14]. To further improve the drive current, [Si] in GeSi channels is decreased to 5% to ensure the electrons populated in L4 valleys with small mt. In addition to channel stacking, TreeFET combining with FinFETs and stacked nanosheets is demonstrated to enhance Weff per footprint. IBs of TreeFETs were formed by well-controlled H2O2 wet etching. With adequate etching selectivity and optimized etching time, the SLs were partially removed to form the IBs between the nanosheets for extra conduction channels.
Strained GeSn has higher hole mobility than Ge to increase ION for pFET [15] due to the hole effective mass reduction under compressive strain. Recently, the radical-based highly selective isotropic dry etching (HiSIDE) was reported to form the stacked GeSn nanosheets [16-18]. However, the challenge of Ge-based channel is the large IOFF due to the small bandgap [15-20]. Ultrathin body device can effectively reduce the IOFF [21], but the mobility degradation is observed as the channel thickness decrease below 5nm due to surface roughness scattering [22]. However, the high mobility GeSn can afford some mobility loss due to the ultrathin body. By combining the high mobility channel and the large number of stacked nanosheets, the ION can be further improved with the suppression of IOFF by the ultrathin body. The 11nm Ge0.9Sn0.1 channel layers sandwiched by 8nm Ge0.97Sn0.03 caps, 3nm Ge caps, and 24nm Ge:B SLs were grown repeatedly on the Ge buffer. The thin double Ge0.97Sn0.03 caps can provide sufficient etching selectivity and prevent the channel bending. The heavily B-doped Ge SLs are used to reduce the S/D resistance. The significant IOFF reduction of the GeSn ultrathin bodies is attributed to the quantum confinement effects. The quantum confinement energy of 170meV in 3nm Ge0.9Sn0.1 ultrathin body is simulated by TCAD. The enlarged bandgap results in the suppression of IOFF and yields the record high ION/IOFF~107 of 8 stacked GeSn ultrathin bodies among GeSn/Ge 3D pFETs.
Simple wet etching can reach the sufficient selectivity to fabricate the highly uniform Ge0.75Si0.25 nanosheets and the Ge0.95Si0.05 nanowires. The 3nm Ge0.9Sn0.1 ultrathin bodies with high inter-channel uniformity are demonstrated to reduce the IOFF. Highly stacked Ge0.9Sn0.1 thick nanosheets can improve the ION. TreeFET with the process flow similar to stacked nanosheets are the promising candidates for technology node scaling. Even higher number of stacked channels (>8) and extremely scaled ultrathin bodies (<3nm) are expected to further enhance the ION and to increase the ION/IOFF for advanced CMOS scaling.

Acknowledgements- This work is supported by MOST (110-2218-E-002-030-, 110-2622-8-002-014-, 110-2218-E-002-034-MBK, and 110-2218-E-002-042-MBK) and MOE (NTU-CC-111L892001), Taiwan. The support from Taiwan Semiconductor Research Institute is also highly appreciated.

Figure 1