Droplet target delivery method for high pulse-rate laser-plasma extreme ultraviolet light source

A laser-plasma extreme ultraviolet (EUV) radiation source comprising:

• a nozzle including a source end and an exit end, said exit end including an orifice having a predetermined diameter, said nozzle receiving liquid xenon at its source end and emitting a stream of xenon droplets from the orifice;
• a xenon excitation source providing a pulsed excitation signal to the nozzle, said excitation source causing the droplets to be emitted from the orifice, said excitation source being pulsed at a frequency that is the natural Rayleigh break-up frequency of the xenon relative to the size of the orifice;
• and
• a laser source providing a pulsed laser beam directed towards a target area, wherein the timing of the xenon excitation source, the diameter of the orifice and the timing of the pulse laser source are designed relative to each other so that the droplets emitted from the orifice of the nozzle have a predetermined speed and spacing therebetween so that target droplets within the droplet stream are ionized by the pulses of the laser beam and a predetermined number of buffer droplets are provided between the target droplets that are not directly ionized by the pulsed laser beam, where the buffer droplets absorb radiated plasma energy from ionized target droplets so as to allow subsequent target droplets to be unaffected by preceding target droplet ionization.

A method of generating target material droplets in a laser-plasma extreme ultraviolet (EUV) radiation source, comprising:

• forcing a liquid target material through an orifice of a nozzle;
• vibrating the nozzle at a pulsed vibration rate so that the target material exits the orifice as a stream of droplets that is directed towards a target area;
• and
• directing a pulsed laser beam towards the target area so that droplets that enter the target area are ionized by the laser beam, wherein the timing of the pulses vibrating the nozzle, the timing of the laser pulses and the diameter of the orifice are selected so that the droplets emitted from the orifice of the nozzle have a predetermined speed and spacing therebetween so that target droplets within the target stream are ionized in the target area by the pulses of the laser beam and a predetermined number of buffer droplets are provided between successive target droplets that are not directly ionized by the pulse beam, where the buffer droplets absorb radiated plasma energy from ionized target droplets so as to allow subsequent target droplets to be unaffected by preceding target droplet ionization.