  In this work, we investigate decays of $B_s\to \eta_c\phi$ and $B_s\to
\eta_cf_0(980)$ in a theoretical framework. The calculation is based on the
postulation that $f_0(980)$ and $f_0(500)$ are mixtures of pure quark states
${1\over\sqrt 2} (\bar uu+\bar dd)$ and $\bar ss$. The hadronic matrix elements
for $B_s\to \phi$ and $B_s\to f_0(980)$ are calculated in the light-front quark
model and the important Wilson coefficient $a_2$ which is closely related to
non-perturbative QCD is extracted. However, our numerical results indicate that
no matter how to adjust the mixing parameter to reconcile contributions of
$f_0(980)$ and $f_0(500)$, one cannot make the theoretical prediction on
$B_s\to \eta_c+\pi^+\pi^-$ to meet the data. Moreover, the new measurement of
$BR(B_s\to J/\psi+f_0(500))<1.7\times 10^{-6}$ also negates the mixture
scenario. Thus, we conclude that the recent data suggest that $f_0(980)$ is a
four quark state ( tetraquark or $K\bar K$ molecule ), at least the fraction of
its pure quark constituents is small.
