The droplet generator is a small vessel. In operation, tin is loaded into the droplet generator and then heated. At that point, a train of tiny tin droplets flow out from the droplet generator, through a filter and into the vacuum chamber in the source. The droplets are 25 microns in diameter and are falling at a rate of 50,000 times a second.
In the vessel, there is a camera. A droplet passes a certain position in the chamber. Then, the camera tells the seed laser in the sub-fab to fire a laser pulse into the main vacuum chamber. This is called the pre-pulse.
Then comes the really hard part. The pre-pulse laser hits the spherical tin droplet and turns it into a pancake-like shape. Then the laser unit fires again, representing the main pulse. The main pulse hits the pancake-like tin droplet and vaporizes it. “We are trying to hit each droplet twice with the pre-pulse and main-pulse at 50,000 times a second,” ASML’s Lercel said.
At that point, the tin vapor becomes plasma. The plasma, in turn, emits EUV light at 13.5nm wavelengths.