A method includes forming an inter-layer dielectric over a first source/drain region and a second source/drain region. The first source/drain region and the second source/drain region are of n-type and p-type, respectively. The inter-layer dielectric is etched to form a first contact opening and a second contact opening, with the first source/drain region and the second source/drain region exposed to the first contact opening and the second contact opening, respectively. A process gas is used to etch back the first source/drain region and the second source/drain region simultaneously, and a first etching rate of the first source/drain region is higher than a second etching rate of the second source/drain region. A first silicide region and a second silicide region are formed on the first source/drain region and the second source/drain region, respectively.