  We are interested in entire solutions for the semilinear biharmonic equation
$\Delta^{2}u=f(u)$ in $\R^N$, where $f(u)=e^{u}$ or $-u^{-p}\ (p>0)$. For the
exponential case, we prove that any classical entire solution verifies $-\Delta
u>0$ without any restriction, which completes the results in \cite{Dupaigne,
xu-wei} and yields a nonexistence result in $\R^2$ ; we obtain also a refined
asymptotic expansion of radial separatrix solution for $N=3$, which answers a
question in \cite{Berchio}. For the negative power case, we show the
nonexistence of the classical entire solution for any $0<p\leq1$.
