A manufacturing method of a semiconductor structure includes the following steps. A first polysilicon layer is formed on a substrate. A planarization process to the first polysilicon layer is performed. A first etching back process to the first polysilicon layer is performed after the planarization process. A second etching back process to the first polysilicon layer is performed after the first etching back process. A first wet clean process to the first polysilicon layer is performed after the first etching back process and before the second etching back process.