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)