A method of forming a semiconductor structure includes providing a substrate including a fin at a surface of the substrate, and forming a fin field-effect transistor (FinFET), which further includes forming a gate stack on the fin; forming a thin spacer on a sidewall of the gate stack; and epitaxially growing a epitaxy region starting from the fin. After the step of epitaxially growing the epitaxy region, a main spacer is formed on an outer edge of the thin spacer. After the step of forming the main spacer, a deep source/drain implantation is performed to form a deep source/drain region for the FinFET.