One of the key challenges for Ge gate stack is to achieve an improved interface quality between Ge and the high-K dielectric. In order to overcome this limitation, various interface treatments like ozone pre-gate treatment and ozone ambient annealing (1), nitridation of Ge surface (2),formation of a GeO₂ layer between Ge and high-K dielectric using ECR plasma technique (3) and formation of an ultra-thin SiO₂/GeO₂bilayer have been tried. It has recently been reported that slot plane antenna(SPA) microwave plasma treatment helps to improve the EOT and enhances the oxide quality by reducing impurities and leakage current when the dielectric is exposed during or after deposition (4). Moreover SPA oxidation possesses the unique property of having low electron temperature, which makes it a very low damage process compared to other techniques,either inductively coupled plasma(ICP) and electron cyclotron resonance(ECR) plasma (5).

In this study we investigated the impact of SPA plasma oxidation (SPAO) treatment in improving the interface quality when MOS capacitors were fabricated on 300mm p-type Ge substrates. SPAO plasma treatments were performed at different steps in the high-k deposition process. The substrate was initially subjected to a dry chemical oxide removal (COR) process. Subsequently, three different sets of samples were prepared with the exposure of SPAO plasma at following locations (Fig.1): CASE-1: SPAO plasma treatment after Al₂O₃(1nm)/ZrO₂(3.5nm) deposition; CASE-2: SPAO plasma treatment was carried out in between Al₂O₃(1nm) and  ZrO₂(3.5nm) deposition; CASE-3: the Ge substrate was exposed to SPAO plasma treatment after COR and prior to Al₂O₃(1nm)/ ZrO₂(3.5nm) layer deposition. Al₂O₃(1nm) and ZrO₂(3.5nm) layers were deposited by atomic layer deposition. Capacitance-voltage at multi-frequencies,current-voltage and conductance measurements were performed at room temperature. Parameters like flatband voltage (V_fb), equivalent oxide thickness(EOT), interface state density(D_it),and leakage current were estimated accurately after quantum mechanical corrections and eliminating the series resistance effect.

As shown in Fig. 2 lowest fast interface state density is achieved when SPAO plasma treatment was performed after Al₂O₃ and prior to ZrO₂ deposition (CASE-2). A comparable D_it is also observed when plasma exposure was done after the entire gate stack deposition (CASE-1) (Fig.2). From different experimental results significant degradation to both the dielectric quality and interface was observed when SPAO is performed just after COR (CASE-3).

References:

1)     Zhao Mei et al, Journal of Semiconductors, Volume 34, No. 3, 2013.

2)     A. Dimoulas et al, Appl. Phys. Lett., volume 86, no. 3, 2005.

3)     Yukio Fukuda et al, IEEE transaction on electron devices, volume 57, No. 1, 2010.

4)     M.N. Bhuyian et al, ECS Journal of Solid State Sci and Techn, vol. 3(5), N83, 2014.

5)     C. Tian et al,  J. Vac. Sci. Technol. A 24, 1421, 2006.