Radiation Source

A method for generating a supercontinuum radiation beam, the method comprising:

• producing a pulsed radiation beam comprising a plurality of pulses;
• passing the pulsed radiation beam through a gain medium of an optical amplifier;
• supplying a pump energy to the gain medium so that as the pulsed radiation beam propagates through the gain medium an intensity of the pulsed radiation beam increases so as to produce an amplified pulsed radiation beam, wherein pump energy is supplied to the gain medium in response to the pulses of the pulsed radiation beam propagating through the gain medium and pump energy is stopped being supplied to the gain medium for at least a portion of the time between the arrival of two consecutive pulses of the pulsed radiation beam at the gain medium;
• and
• passing the amplified pulsed radiation beam through an optical medium configured to broaden a spectrum of that amplified pulsed radiation beam so as to generate a supercontinuum radiation beam.

A lithographic apparatus, comprising:

• an alignment system comprising:
• a supercontinuum radiation source comprising:
• a radiation source configured to produce a pulsed radiation beam comprising a plurality of pulses;
• an optical amplifier configured to receive the pulsed radiation beam and increase an intensity of the pulsed radiation beam so as to produce an amplified pulsed radiation beam, the optical amplifier comprising a gain medium and a pump power source;
• and
• an optical medium configured to receive the amplified pulsed radiation beam and to broaden a spectrum of that amplified pulsed radiation beam so as to generate a supercontinuum radiation beam;
• wherein the pump power source of the optical amplifier is configured to supply pump energy to the gain medium in response to the pulses of the pulsed radiation beam propagating through the gain medium and to stop supply of pump energy to the gain medium for at least a portion of time between arrival of two consecutive pulses of the pulsed radiation beam at the gain medium,
• an optical system configured to project the supercontinuum radiation beam onto an alignment mark on a substrate supported on a substrate table;
• a sensor configured to detect radiation diffracted/scattered by the alignment mark and to output a signal containing information related to a position of the alignment mark;
• and
• a processor configured to receive the signal from the sensor and to determine a position of the alignment mark relative to the substrate table in dependence thereon.

A metrology apparatus for measuring characteristics of structures formed on a substrate, the metrology apparatus comprising a supercontinuum radiation source configured to generate a radiation that illuminates the structures, the supercontinuum radiation source comprising:

• a radiation source configured to produce a pulsed radiation beam comprising a plurality of pulses;
• an optical amplifier configured to:
• receive the pulsed radiation beam;
• and
• increase an intensity of the pulsed radiation beam so as to produce an amplified pulsed radiation beam, the optical amplifier comprising a gain medium and a pump power source;
• and
• an optical medium configured to:
• receive the amplified pulsed radiation beam;
• and
• broaden a spectrum of the amplified pulsed radiation beam so as to generate a supercontinuum radiation beam;
• wherein the pump power source of the optical amplifier is configured to supply pump energy to the gain medium in response to the pulses of the pulsed radiation beam propagating through the gain medium and to stop supplying pump energy to the gain medium for at least a portion of time between arrival of two consecutive pulses of the pulsed radiation beam at the gain medium.