1. An extreme ultraviolet radiation source, comprising:a droplet generator configured to output a plurality of target droplets along a target droplet path that is parallel to a horizontal direction; and
a droplet catcher, comprising:
an open end substantially aligned with the target droplet path;
an enclosed end being opposite to the open end;
a pipe wall disposed between the open end and the enclosed end, wherein the pipe wall includes a first pipe wall portion having an inner top surface parallel to the horizontal direction and an inner bottom surface inclined relative to the inner top surface; and
at least one gutter formed on the inner bottom surface and having a long axis extending along the horizontal direction.