Alignment of light source focus

A method comprising:

• irradiating a target material with an amplified light beam that travels along a propagation direction, an interaction between the target material and the amplified light beam at least partially consuming and converting the target material to a plasma that emits extreme ultraviolet radiation and producing a reflection of the amplified light beam;
• modifying a wavefront of the reflection of the amplified light beam, the modification being a function of a location of a focal plane of the amplified light beam along the propagation direction;
• detecting an image of the modified reflected amplified light beam;
• and
• determining the location of the focal plane of the amplified light beam relative to the target material based on the detected image.

An extreme ultraviolet light system comprising:

• a target material delivery system that delivers a target material to a target location, the target material comprising a material that emits extreme ultraviolet light when converted to plasma;
• a light source configured to provide an amplified light beam, at least a portion of the amplified light beam being sufficient to convert the target material to plasma;
• a detection system comprising at least one detector positioned to detect an image of the amplified light beam reflected from at least a portion of the target material;
• a wavefront modification system in the path of the reflected amplified light beam, the wavefront modification system configured to modify the wavefront of the reflected amplified light beam as a function of a location of a focal plane of the amplified light beam along a propagation direction of the amplified light beam;
• and
• a controller coupled to the detection system and the steering system, the controller configured to:
• access an image of the reflected amplified light beam, and
• determine a location of the focal plane of the amplified light beam relative to the target location along the direction of propagation based on the image of the reflected amplified light beam.