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

Patent metadata

Field	Value
Publication number	US-9755396-B1
Application number	US-201615363570-A
Country	US
Kind code	B1
Filing date	Nov 29, 2016
Priority date	Nov 29, 2016
Publication date	Sep 5, 2017
Grant date	Sep 5, 2017

Abstract

A method and apparatus for control of a dose of extreme ultraviolet (EUV) radiation generated by a laser produced plasma (LPP) EUV light source that combines pulse control mode and pulse modulation. The EUV energy created by each pulse is measured and total EUV energy created by the fired pulses determined, a desired energy for the next pulse is determined based upon whether the total EUV energy is greater or less than a desired average EUV energy times the number of pulses. If the desired pulse energy for the next droplet is within the range of one or more pulse modulation actuators, the pulse is modulated; otherwise, the pulse is fired to miss the droplet. This provides greater control of the accumulated dose as well as uniformity of the EUV energy over time, greater ability to compensate for pulses that generate EUV energy that is higher or lower than nominal expected values, and ability to provide an average EUV energy per pulse that is less than the nominal minimum EUV energy per pulse of the system.

First claim

What is claimed is:

1. A method 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 energy by one or more actuators such that each pulse has 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 pulse, and capable of hitting droplets of target material with a laser pulse, 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 pulse energy 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 modulated 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, 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:
causing, by the controller, the light source to fire a next laser pulse to not hit a droplet.

2. The method of claim 1 wherein the desired average EUV energy per pulse is less than the nominal amount of EUV energy corresponding to the minimum pulse energy to which a laser pulse may be modulated by any combination of the one or more actuators.

3. The method of claim 1 wherein the LPP EUV light source includes an amplifier, and one of the one or more actuators comprises a radio frequency actuator configured to apply radio frequency energy to the amplifier.

4. The method of claim 1 wherein one of the one or more actuators comprises an acousto-optic modulator configured to modulate the amplitude of source laser pulses.

5. The method of claim 1 wherein one of the one or more actuators comprises an electro-optic modulator configured to modulate the width of source laser pulses.

6. The method of claim 1 wherein causing, by the controller, one or more of the actuators to modulate the pulse energy of the next laser pulse to the desired pulse energy further comprises:
determining, by the controller, whether a first one of the one or more actuators is able to modulate the pulse energy of the next laser pulse to the desired pulse energy, and, if so, causing the first one of the actuators to modulate the pulse energy of the next laser pulse to the desired pulse energy.

7. The method of claim 6 wherein the LPP EUV light source includes an amplifier, and the first one of the one or more actuators comprises a radio frequency actuator configured to apply radio frequency energy to the amplifier.

8. The method of claim 6 wherein, if the first one of the one or more actuators is not able to modulate the pulse energy of the next laser pulse to the desired pulse energy, causing, by the controller, one or more of the actuators to modulate the pulse energy of the next laser pulse to the desired pulse energy further comprises:
determining, by the controller, whether a combination of the first one of the actuators and a second one of the one or more actuators is able to modulate the pulse energy of the next laser pulse to the desired pulse energy, and, if so, causing the first and second ones of the one or more actuators to modulate the pulse energy of the next laser pulse to the desired pulse energy.

9. The method of claim 8 wherein the LPP EUV light source includes an amplifier, the first one of the one or more actuators comprises a radio frequency actuator configured to apply radio frequency energy to the amplifier, and the second one of the one or more actuators comprises an acousto-optic modulator configured to modulate the amplitude of source laser pulses.

10. 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.

11. The system of claim 10 wherein the desired average EUV energy per pulse is less than the nominal amount of EUV energy corresponding to the minimum pulse energy to which a laser pulse may be modulated by any combination of the one or more actuators.

12. The system of claim 10 further comprising an amplifier, and wherein one of the one or more actuators comprises a radio frequency actuator configured to apply radio frequency energy to the amplifier.

13. The system of claim 10 wherein one of the one or more actuators comprises an acousto-optic modulator configured to modulate the amplitude of source laser pulses.

14. The system of claim 10 wherein one of the one or more actuators comprises an electro-optic modulator configured to modulate the width of source laser pulses.

15. The system of claim 10 wherein the controller is further configured to cause one or more of the actuators to modulate the pulse energy of the next laser pulse to the desired pulse energy by:
determining whether a first one of the one or more actuators is able to modulate the pulse energy of the next laser pulse to the desired pulse energy; and,
if the first one of the actuators is able to modulate the pulse energy of the next laser pulse to the desired pu

Assignees

• Asml Netherlands Bv

Inventors

• Frihauf Paul
• Liu Andrew
• Rich Spencer
• Graham Matthew R
• Chang Steven
• Dunstan Wayne J
• Riggs Daniel Jason

Classifications

• H01S3/107

  using electro-optic devices, e.g. exhibiting Pockels or Kerr effect {(H01S3/1061, H01S3/1063, H01S3/1065 take precedence)} · CPC title

• H01S3/1068

  using an acousto-optical device · CPC title

• H01S3/102 Primary

  by controlling the active medium, e.g. by controlling the processes or apparatus for excitation (H01S3/13 takes precedence) · CPC title

• H01S3/10038 Primary

  Amplitude control · CPC title

• H01S3/2232

  Carbon dioxide (CO2) or monoxide [CO] · CPC title