For a zero order reaction

A general expression for the half, $\left(t_{1/2}\right)$ is given by

$t_{\frac{1}{2}}\times a^{1-n}$

Where ‘n’ is the order of the reaction for zero order  $n=0$

$t_{\frac{1}{2}}\propto a^{1-0}\Rightarrow t_{\frac{1}{2}}\propto a$