Study of Lanthanum Diffusion in HfO2-Based High-k Gate Stack
Authors: Zhu Meng , Kannan Balaji , Zhang Yibin , Medikonda Manasa , Liang Yifan , Li Jinghong , Dasgupta Aritra , Pantisano Luigi , Ozbek Merve , Siddiqui Shahab , Liu Jinping 
Published: February 2020

La2O3 has been selected as a threshold voltage (Vt) tuning material for advanced nodes because of its uniqueness of reducing effective work function towards NFET band edge by creating dipoles at SiO2/HfO2 interface [1,2]. In this paper, we study the factors which influence the diffusion of Lanthanum (La) atoms through HfO2 during anneal. These factors include: (1) Crystallization state of HfO2; (2). HfO2 thickness; (3) Drive-in anneal temperature; (4) TiN capping material during drive-in anneal.
We prepare the following stack: Si/SiO2/ HfO2/ La2O3 /TiN/a-Si, and anneal the stack to diffuse the La atoms into HfO2 layer. Then a-Si, cap TiN and excessive La2O3 were sequentially removed by wet etch. Final structure Si/SiO2/La-doped HfO2 is measured by XPS/XRF to determine the amount of La atoms that were diffused in. The diffusion of La is governed by its initial dosage (La2O3 deposition thickness), its adjacent layers (bottom HfO2, and top cap TiN), and the annealing temperature. By varying these parameters, we systematically study the effects of each factor, and correlate them to final Vt shift in the devices.
The Vt shift in the MOSFET device is found to be proportional to the final diffused La amount in HfO2. The Vt shift shows obvious dependence on initial La dosage, annealing temperature and HfO2 thickness. On the other hand, the crystallization of HfO2 display a surprising blocking effect for La diffusion, indicating that La diffusion doesn’t follow the common grain-boundary diffusion mechanism. We found that HfO2 starts to crystallize when post-deposition anneal temperature is greater than 880C. La diffusion through crystallized HfO2 is significantly reduced compared to the case when HfO2 is still amorphous. On the contrary, Nitrogen atoms, which were incorporated during cap TiN deposition process, shows enhanced diffusion through HfO2. Fig.1 (a) shows the La signal in gate stack prior to and after La diffusion. The final La amount is much less for crystallized HfO2 (Hk PDA temp =900C and 950C) than for amorphous HfO2 (Hk PDA temp =800C and 850C). However, N signal, in Fig.1 (b) shows increased value for crystallized HfO2. Fig. 1(c) (d) show the Vt shift from NFET and PFET devices respectively, confirming less La diffusion (less Vt shift) in crystallized HfO2. The Vt of NFET is reduced while the Vt of PFET is enhanced because of the unipolar nature of the La-Si-O dipoles. We also conducted Angle-resolved XPS (AR-XPS) and TEM/EELS analysis and they both show that in crystallized HfO2, the La distribution is away from HfO2/SiO2 interface, while in amorphous HfO2, more La reside at HfO2/SiO2 interface.
The capping material is also crucial for La diffusion in HfO2. We tested two types of TiN: (1) ALD TiN using a metalorganic precusor, tetrakis (dimethylamido) titanium and Nitrogen (TDMAT-TiN), (2) ALD TiN using TiCl4 precursor and NH3 (TiC4-TiN). We observe significant Vt difference between devices using these two TiN materials. The final diffused La is much less for TiC4-TiN based cap than TDMAT-TiN based cap. This is attributed to different La diffusion rate in the capping TiN material which in turn affects the La diffusion into HfO2.
In summary, we evaluate the factors that influence La diffusion in HfO2-based Hk gate stack. Both crystallization of HfO2 and the absorption of La in top TiN material have great contributions to the effectiveness of La diffusion in HfO2. Some of these factors for La diffusion are especially intertwined with the growth of SiO2 interfacial layer (IL) which affects the Tinv and Toxgl of the MOSFET devices. Cares have to be taken to engineer the stack structure and integration scheme to achieve desired Vt shift and device performance.
REFERENCE:
[1] H. N. Alshareef et.al. Appl. Phys. Lett. 89, 232103 (2006)
[2] Koji Kita and Akira Toriumi, Appl. Phys. Lett. 94, 132902 (2009)

Figure 1