Al reflow filling in contact holes by annealing after Al deposition at room temperature was investigated with relation to agglomeration properties of very thin films. A strong correlation between filling achievement and agglomeration property was observed when the underlayer TiN film property was changed. When rapid thermal nitridation (RTN) was carried out after reactive sputtering of TiN, excellent filling in submicron contact holes was achieved at annealing temperatures around 400 degrees C for the case of thick Al film, and agglomeration into small islands was observed for thin Al films. The activation energy of atomic mass transport for the filling process was 0.36 eV, which suggests a surface diffusion dominated filling process. On the other hand, when no RTN was carried out for TiN, reflow filling was achieved at temperatures around 600 degrees C which were close to the melting point of Al, and agglomeration into islands in thin films was suppressed at 400 degrees C. The activation energy of the latter case was 0.90 eV, which was between those of lattice diffusion and grain boundary diffusion. The correlation between filling achievement and agglomeration is ascribed to a large surface diffusion atomic flux in the case with RTN.