Growth monitoring during the early stages of film formation is of prime
importance to understand the growth process, the microstructure and thus the
overall layer properties. In this work, we demonstrate that phonons can be used
as sensitive probes to monitor real time evolution of film microstructure
during growth, from incipient clustering to continuous film formation. For that
purpose, a silicon nitride membrane-based sensor has been fabricated to measure
in-plane thermal conductivity of thin film samples. Operating with the
3{\omega}-V\"olklein method at low frequencies, the sensor shows an exceptional
resolution down to {\Delta}({\kappa}*t)=0.065 nm*W/(m*K), enabling accurate
measurements. Validation of the sensor performance is done with organic and
metallic thin films. In both cases, at early stages of growth, we observe an
initial reduction of the effective thermal conductance of the supporting
amorphous membrane, K, related with the surface phonon scattering enhanced by
the incipient nanoclusters formation. As clusters develop, K reaches a minimum
at the percolation threshold. Subsequent island percolation produces a sharp
increase of the conductance and once the surface coverage is completed K
increases linearly with thickness The thermal conductivity of the deposited
films is obtained from the variation of K with thickness
