The border is very soft. When I use selection tool, I checked anti-alias with feather is zero. I fill in the selection with Alt + backspace at the first time and the result is the image above.
Now, please look closely at the second image.
I keep the selection and fill it again and the border looks very sharp. Could anyone please explain why the sharp border appears at the second time. It still happens if I use paint brush to paint over it with selection tool is running.