Method of forming thin film structure with tensile and compressed polysilicon layers
A method for forming a thin film structure, which has small tensile stress due to controlled mechanical stress, and is made to be conductive, is provided. A lower film including polysilicon thin film is formed on a substrate such as Si substrate, then an impurity such as P is doped into the lower film and thermally diffused, thereby the lower film is made conductive. Then, an upper film is deposited on the lower film, the upper film including a polysilicon thin film that is simply deposited and not made to be conductive. The upper film has a tensile stress in an approximately the same level as compressive stress of the lower film, and a thin film structure as a whole, the structure including the lower film and the upper film, is adjusted to have small tensile stress.
1. A method for forming a thin film structure, which includes a lower film and an upper film, on a substrate, comprising:
a step of forming the lower film that includes a polysilicon film on the substrate,
a step of making the lower film to be conductive by doping an impurity into the lower film and thermally diffusing the impurity,
a step of forming the upper film that includes a second polysilicon film on the lower film, the upper film being nonconductive and having a first tensile stress in approximately a same level as a compressive stress of the lower film, and
a step of separating the thin film structure from the substrate by a first etching, leaving at least a part of a periphery of the thin film structure held by the substrate,
wherein the thin film structure as a whole is adjusted to have a final tensile stress, and
wherein after the impurity is doped into the lower film, an anti-defect film is formed on a top of the lower film before the impurity is thermally diffused.
2. The method for forming the thin film structure, according to claim 1 :
wherein after the upper film is formed on the anti-defect film, a portion of the upper film is removed by a second etching with the anti-defect film as an etching stop layer, thereby a part of the lower film is electrically exposed to the upper film.