In situ time‐resolved reflectivity (TRR) measurements have been made on unpatterned 800 nm pure Al films located on top of different underlying layers (e.g., substrate, diffusion barrier, or oxide) under XeCl (λ=308 nm, pulse width 40 ns) excimer laser planarization conditions. The reflectivity change observed when using a HeNe (λ=632.8 nm) probe beam is assumed to indicate layer melting. Results show that Al melt durations increase with laser energy fluence, oxide thickness, and substrate temperature. In addition, under certain processing conditions Al areas with a thick underlying oxide remain molten for over 1000 ns while no Al melting is observed where it directly contacts the silicon substrate. This work explains why substrate heating during excimer laser planarization is necessary in order to completely fill high aspect ratio contact holes.