Method for producing a silicon-germanium film with variable germanium content

A fabrication method of a field effect transistor successively comprising:

• providing a substrate provided with a first semiconducting area partially covered by a first masking pattern to define a protected surface and an open surface,
• depositing a continuous silicon-germanium layer on the first semiconducting area and on the first masking pattern, in non-selective manner, the continuous silicon-germanium layer completely covering the first semiconducting area and the first masking pattern, so as to form an interface between the continuous silicon-germanium layer and the first semiconducting area,
• applying a diffusion annealing configured to make the germanium atoms diffuse from the silicon-germanium layer to the open surface of the first semiconducting area,
• depositing an electrically insulating layer so as to cover the masking pattern and source and drain areas,
• wherein the masking pattern is a gate stack of the field effect transistor or is used to define a shape of a gate stack in the electrically insulating layer so as to form said gate stack which is self-aligned with the source and drain areas and to form germanium-enriched source and drain areas when diffusion annealing is performed.