Novel Exploration of Flat-Band Voltage Manipulation by NPT for Advanced High-k/Metal-Gate CMOS Technology
Authors: Yao Jiaxin , Hou Zhaozhao , Wu Zhenhua , Yin Huaxiang 
Published: February 2020

Threshold voltage (VT) modulation is crucial for high performance (HP) and low power (LP) applications in the state-of-the-art FinFET and NanowireFET devices with gate-last HKMG integration scheme. Conventional metal-gate work function modulation normally requires a complicated film stack structure and a precise film thickness control in RMG (Replaced Metal Gate) process. As the physical gate length of RMG scaling down to below 14nm for sub-7nm technology node, the gap filling volume becomes rigorous and the VT tuning capability and turns inadequate for CMOS IC applications. This work presents a novel exploration of flat-band voltage (VFB) manipulation scheme with the nitrogen plasma treatment (NPT) in the high-k/metal-gate (HKMG) Metal-Oxide-Semiconductor Capacitor (MOSCAP) for the shrinking scaling of the further CMOS fabrication technology.

HKMG formation is performed via ALD (Atomic Layer Deposition) and CVD (Chemical Vapor Deposition). The gate stacks of fabricated P- and N-MOSCAPs are ALD-HfO2(High-k layer, HK)/ALD-TiN(Capping layer)/ALD-TaN(Etch-stop layer, ESL)/CVD-TiN(Work-function layer, WFM)/ALD-W(Contacted metal) and ALD-HfO2(High-k layer, HK)/ALD-TiN(Capping layer)/ALD-TiAlC(Work-function layer, WFM)/ALD-TiN(Barrier layer)/ALD-W(Contacted metal), respectively. The critical NPT process is developed in chamber of EnduraTM (Applied Materials, Inc) and compatible to the mainstream gate-last HKMG technology. NPT process is applied in the first ALD-TiN capping layer both for P- and N-MOSCAPs. The developed NPT process on the first ALD-TiN capping layer demonstrates significant VFB modulation capability and causes a controllable effective work function of metal gate: 1) shifting to band center with increasing of the radio frequency powers both for P-/N-MOSCAPs; 2) shifting to opposite directions (P- to band edge, N- to band center) with reduced nitrogen flow ratio for P-/N-MOSCAPs; 3) shifting from -220mV to +60mV and from +130mV to +420mV for P-/N-MOSCAPs, respectively. Compared to the samples without NPT, the gate leakage current of both P- and N-MOSCAPs are improved with the NPT process. Based on the different gate stacks of P-/N-MOSCAPs and plasma effect, one unique physical model including N-vacancy/Al-trap effect is successfully proposed to illustrate the complicated shift behaviors both for HKMG P-/N-MOSCAPs. VFB shift for P-MOSCAP is mostly determined by N-vacancies, for the same layer of first capping layer(TiN) and work-function layer(TiN). The VFB shift for N-MOSCAP needs to consider both N-vacancies and Al-traps effect, for the difference between first capping layer(TiN) and the upper work-function layer(TiAlC).

In conclusion, the technology of NPT in the first ALD-TiN capping layer of HKMG MOSCAPs for gate work function modulations is systematically investigated. Significant VFB shift values, such as -220mV/+130mV with different RF powers and +60mV/+420mV with different nitrogen flow ratios are achieved for P-/N-MOSCAPs, respectively. One simple theoretical model by employing the function of N-vacancy in ALD-TiN capping layer with the corresponding Al-trap effect by different WFM atoms in metal-gate is presented, which gives a unique physical explanation to the complex VFB manipulation behaviors with different device types and process conditions. The results demonstrate NPT process is one of the most promising technologies for the VT manipulation in future scaling CMOS technology nodes with less process cost.

Figure 1