Apparatus for and method of controlling coalescence of droplets in a droplet stream

A method comprising the steps of:

• providing a stream of droplets of target material for a plasma generating system from a target material dispenser;
• generating a control signal comprising a hybrid waveform including a superposition of a first periodic waveform and a second periodic waveform and controlling a relative phase of the first periodic waveform and the second periodic waveform;
• and
• applying the control signal to an electro-actuatable element mechanically coupled to the target material dispenser, the electro-actuatable element introducing a velocity perturbation on the stream at the exit of the target material dispenser.

A method of determining a transfer function of a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

• providing the stream of liquid target material for a plasma generating system from the droplet generator;
• generating a control signal comprising a hybrid waveform including a superposition of a first periodic waveform and a second periodic waveform;
• applying the control signal to an electro-actuatable element mechanically coupled to the droplet generator to introduce a velocity perturbation into the stream;
• and
• determining a transfer function for the nozzle in response to the control signal based at least in part on a coalescence length of the stream, a velocity profile of the stream, and an amplitude of the first periodic waveform.

A method of determining a transfer function of a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

• providing the stream of liquid target material for a plasma generating system from the droplet generator;
• generating a control signal, the control signal comprising a hybrid waveform including a superposition of a first periodic waveform and a second periodic waveform;
• introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to the droplet generator;
• reducing an amplitude of the first periodic waveform;
• observing the stream at a downstream point to determine whether droplets are fully coalesced;
• and
• determining a transfer function for the droplet generator in response to the control signal based on the amplitude of the first periodic waveform when droplets in the observed stream cease being fully coalesced.

A method of controlling a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

• providing the stream of liquid target material for a plasma generating system from the droplet generator;
• generating a control signal, the control signal comprising a hybrid waveform including a superposition of a first periodic waveform and a second periodic waveform;
• introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to the droplet generator;
• and
• controlling a coalescence length of the stream by adjusting a relative phase of the second periodic waveform with respect to the first periodic waveform.

A method of controlling a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

• providing the stream of liquid target material for a plasma generating system from the droplet generator;
• generating a control signal, the control signal comprising a hybrid waveform including a superposition of a first periodic waveform having a first frequency and a second periodic waveform having a second frequency which is an integral multiple of the first frequency;
• introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to the droplet generator;
• and
• controlling jitter of the stream by controlling an amplitude of the second periodic waveform.

A method of assessing a condition of a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

• providing the stream of liquid target material for a plasma generating system from the droplet generator;
• generating a control signal, the control signal comprising a hybrid waveform including a superposition of a first periodic waveform and a second periodic waveform;
• introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to target material in the droplet generator;
• adjusting a relative phase of the second periodic waveform with respect to the first periodic waveform;
• observing the stream to determine whether coalescence occurs at the relative phase;
• repeating the adjusting step and the observing step to determine a range of relative phases at which coalescence occurs;
• assessing the condition of the droplet generator based on the determined range.