[en] In the B_s⁰ → ψφ channel, the phase φ_s is predicted to be significantly small in the Standard Model. This observable is an excellent probe for New Physics because the B_s⁰-B-bar_s⁰ mixing takes place via a loop diagram where new particles are expected to intervene. These two reasons makes the φ_s parameter an excellent probe for New Physics processes. We developed a simplified selection for the 2010 data. It avoids any bias on the proper time distribution in order to reduce systematic uncertainty. In addition, we control the tagging performance for B_s⁰ → J/ψφ events using the similar B_d⁰ → J/ψK^*0 and B⁺ → J/ψK⁺ channels. With the 2010 data, we obtain 570 signal events in 36 pb^-1 of integrated luminosity, a tagging power of (2.2 ± 0.4)% and a proper time resolution of 50 fs. We investigated an alternative selection which maximizes the φs sensitivity using a proper time biasing cuts. We proposed a data-driven method to correct the proper time acceptance. We designed a fitting program to determine the φ_s phase. With the 2010 data, it is found to be: φ_s belongs to [-2.7, -0.5] rad at 68% of confidence. This result is compatible with the Standard Model prediction. (author)