After the fin-shaped structures 14 are formed, a flush process is conducted by using carbon tetrafluoride (CF₄) to remove polymers on surface of the fin-shaped structures 14 according to step 102, and then a plasma treatment process is conducted to form an oxide layer on a bottom surface of the fin-shaped structures 14 near the surface of the substrate 12 according to step 103. Specifically, the plasma treatment process conducted at this stage preferably injects oxygen gas to the bottom portion of the fin-shaped structures 14, in particular the included angles between the fin-shaped structures 14 and the substrate 12 to form an oxide layer made of silicon oxide. According to a preferred embodiment of the present invention, the formation of the oxide layer could facilitate the angles included by the fin-shaped structures 14 and the substrate 12 to alter from angles made of planar surfaces to angles made of curves or curved surfaces in the later process, which could then be used to prevent fin-shaped structures 14 from falling down as a result of reduced pitch size.

In this embodiment, the temperature of the plasma treatment process conducted to form the aforementioned oxide layer is between 135° C. to 165° C. or more specifically at around 150° C., the pressure of the plasma treatment process is between 0.9 Torr to 1.1 Torr or more preferably at around 1 Torr, and the duration of the plasma treatment process is between 2.7 minutes to 3.3 minutes or more preferably at around 3 minutes.