A new test structure has been developed to identify unambiguously the main mechanism which determines the profiles of thin films deposited by low‐pressure chemical vapor deposition (LPCVD) in structures such as steps, trenches, and via‐holes. The two mechanisms considered are reemission due to a low surface reaction probability and surface diffusion. Experimental results using silane, diethylsilane (DES), tetraethoxysilane (TEOS), and tetramethylcyclotetrasiloxane (TMCTS) as the silicon sources for oxide deposition by LPCVD show that indirect deposition from reemission is the major contributing factor in determining the step coverage.