EUV LPP source with improved dose control by combining pulse modulation and pulse control mode

A system for controlling a dose of extreme ultraviolet (EUV) radiation generated by a laser produced plasma (LPP) extreme EUV light source, the LPP EUV light source creating EUV energy by firing source laser pulses capable of hitting droplets of target material with a laser pulse, comprising:

• one or more actuators configured to modulate the energy of a laser pulse, having a range from a minimum pulse energy to a maximum pulse energy, the pulse energy corresponding to a nominal amount of EUV energy to be produced by the laser pulse;
• a sensor configured to measure the EUV energy created by a laser pulse hitting a droplet;
• a controller configured to:
• receive a selected EUV dose target and a number of laser pulses over which the dose target is to be achieved;
• determine the desired average EUV energy per pulse to be generated in order to achieve the dose target over the number of laser pulses;
• cause the light source to fire a laser pulse hit a droplet;
• add the measured EUV energy created by each laser pulse to a running total of the EUV energy generated by laser pulses that have been fired;
• determine the difference between the running total of the EUV dose and the desired average EUV energy per pulse times the number of pulses fired and, from the determined difference, and a desired pulse energy for a next laser pulse;
• if the desired pulse energy for a next laser pulse is within the range of pulse energy that may be produced by the actuators:
• cause the light source to fire a next laser pulse to hit a droplet;
• and
• cause one or more of the actuators to modulate the pulse energy of the next laser pulse to the desired pulse energy;
• and
• if the desired pulse energy for the next laser pulse is not within the range of the actuators:
• cause the light source to fire a next laser pulse to not hit a droplet.

A non-transitory computer readable storage medium having embodied thereon instructions for controlling a dose of extreme ultraviolet (EUV) radiation generated by a laser produced plasma (LPP) EUV light source, the LPP EUV light source creating EUV energy by firing source laser pulses that are modulated in width by an actuator having a range from a minimum width to a maximum width, the width corresponding to a nominal amount of EUV energy to be produced by the pulse, and capable of hitting droplets of target material with a laser pulse, the method comprising:

• receiving, by a controller, a selected EUV dose target and a number of laser pulses over which the dose target is to be achieved;
• determining, by the controller, a desired average EUV energy per pulse to be generated in order to achieve the dose target over the number of laser pulses;
• causing, by the controller, the light source to fire a pulse to hit a droplet;
• measuring, by a sensor, the EUV energy created by the laser pulse hitting the droplet;
• adding, by the controller, the measured EUV energy to a running total of the EUV energy generated by laser pulses that have been fired;
• determining, by the controller, a difference between the running total of the EUV dose and the desired average EUV energy per pulse times the number of pulses fired and, from the determined difference, a desired width for a next laser pulse;
• if the desired pulse energy for the next laser pulse is within the range of pulse energy that may be produced by the actuators:
• causing, by the controller, the light source to fire a next laser pulse to hit a subsequent droplet;
• and
• causing, by the controller, the one or more of the actuators to modulate the pulse energy of the next laser pulse to the desired pulse energy;
• and
• if the desired pulse energy for the next laser pulse is not within the range of the actuator:
• causing, by the controller, the light source to fire a next laser pulse to not hit a droplet.