how to change the shape of a buton in flutter to cicular

shape: RoundedRectangleBorder(
  borderRadius: BorderRadius.circular(18.0),
  side: BorderSide(color: Colors.red)
),

Padding(  padding: EdgeInsets.only(left: 150.0, right: 0.0),  child: RaisedButton(    textColor: Colors.white,    color: Colors.black,    child: Text("Search"),    onPressed: () {},    shape: new RoundedRectangleBorder(      borderRadius: new BorderRadius.circular(30.0),    ),  ),)

RawMaterialButton(
  ...,
  shape: CircleBorder(),
)