Method for forming poly-silicon film

A poly-silicon film formation method for forming a poly-silicon film doped with phosphorous or boron, the method comprising:

• performing a film formation process of depositing an amorphous silicon film doped with phosphorous or boron on a target substrate, by setting an interior of a reaction container accommodating the target substrate at a first pressure of a vacuum atmosphere and a first temperature of 400 to 650° C. and supplying into the reaction container a silane family gas at a first flow rate, a doping gas for doping a film with phosphorous or boron at a second flow rate smaller than the first flow rate, and C2H4 as a grain size adjusting gas, which contains a component to retard columnar crystal formation to promote miniaturization of poly-silicon crystals, at a third flow rate smaller than the first flow rate;
• and
• performing an annealing process on the amorphous silicon film, while setting the interior of the reaction container accommodating the target substrate at a second pressure higher than the first pressure and a second temperature of 550 to 1,100° C. and higher than the first temperature, thereby transforming the amorphous silicon film into a poly-silicon film that is doped with phosphorous or boron, and that has an average grain size of 100 nm or less, wherein
• said method further comprises conducting an experiment, which includes performing the film formation process with different values of the third flow rate followed by the annealing process to form poly-silicon film examples, and measuring grain sizes of the examples to obtain values of the third flow rate that make the grain size 100 nm or less.