Calibration of photoelectromagnetic sensor in a laser source

Patent metadata

Field	Value
Publication number	US-9239268-B1
Application number	US-201414330488-A
Country	US
Kind code	B1
Filing date	Jul 14, 2014
Priority date	Jul 14, 2014
Publication date	Jan 19, 2016
Grant date	Jan 19, 2016

Abstract

In a laser-produced plasma (LPP) extreme ultraviolet (EUV) system, laser pulses are used to produce EUV light. To determine the energy of individual laser pulses, a photoelectromagnetic (PEM) detector is calibrated to a power meter using a calibration coefficient. When measuring a unitary laser beam comprising pulses of a single wavelength, the calibration coefficient is calculated based on a burst of the pulses. A combined laser beam has main pulses of a first wavelength alternating with pre-pulses pulses of a second wavelength. To calculate the energy of the main pulses in the combined laser beam, the calibration coefficient calculated for a unitary laser beam of the main pulses is used. To calculate the energy of the pre-pulses in the combined laser beam, a new calibration coefficient is calculated. When the calculated energy values drift beyond a pre-defined threshold, the calibration coefficients are recalculated.

First claim

What is claimed is:

1. A system comprising:
an energy monitor within a laser source of a laser-produced plasma (LPP) extreme ultraviolet (EUV) system, the energy monitor configured to measure laser pulses having a same wavelength and occurring in a burst, the energy monitor comprising:
a power meter configured to measure an average power of the laser pulses over a defined period of time, and
a photoelectromagentic (PEM) detector configured to provide a first voltage signal indicative of a temporal profile of the burst of the laser pulses over at least a portion of the defined period of time;

a calibration module configured to determine a calibration coefficient based on the average power and the first voltage signal, the calibration coefficient being a ratio of an energy of the burst of the laser pulses determined from the average power and an integral of the first voltage signal; and
a single pulse energy calculation (SPEC) module configured to determine an energy of a subsequent pulse of the series of the laser pulses based on the calibration coefficient and a pulse integral of a second voltage signal provided by the PEM detector indicating a temporal profile of the subsequent pulse.

2. The system of claim 1 , further comprising a recalibration module configured to calculate an energy of a second burst based on a third voltage signal indicative of a second temporal profile of the second burst.

3. The system of claim 2 , wherein the recalibration module is further configured to compare the energy of the second burst to a second average power sensed by the power meter and to instruct to the calibration module to update the calibration coefficient based on the comparison.

4. The system of claim 3 , wherein the recalibration module is configured to instruct the calibration module to update the calibration coefficient if the comparison exceeds a threshold.

5. The system of claim 1 , wherein the laser pulses are main pulses.

6. The system of claim 1 , wherein the laser pulses are pre-pulses.

7. A method comprising:
measuring laser pulses having a same wavelength and occurring in a burst using an energy monitor within a laser source of a laser-produced plasma (LPP) extreme ultraviolet (EUV) system, the measuring comprising:
receiving, from a power meter, an average power of the laser pulses measured over a defined period of time, and
receiving, from a photoelectromagnetic (PEM) detector, a first voltage signal indicative of a temporal profile of the laser pulses sensed during at least a portion of the defined period of time;

determining a calibration coefficient based on the average power and the first voltage signal, the calibration coefficient being a ratio of an energy of the laser pulses determined from the average power and an integral of the first voltage signal; and
determining an energy of a subsequent pulse of the series of the laser pulses based on the calibration coefficient and an integral of a second voltage signal provided by the PEM detector indicating a temporal profile of the subsequent pulse.

8. The method of claim 7 , further comprising calculating an energy of a second burst based on a third voltage signal indicative of a temporal profile of the second burst.

9. The method of claim 8 , further comprising comparing the energy of the second burst to a second average power sensed by the power meter and updating the calibration coefficient based on the comparison.

10. The method of claim 9 , wherein updating the calibration coefficient is based on the comparison exceeding a threshold.

11. The method of claim 7 , wherein the laser pulses are main pulses.

12. The method of claim 7 , wherein the laser pulses are pre-pulses.

13. A non-transitory computer readable medium having instructions embodied thereon, the instructions executable by one or more processors to perform operations comprising:
measuring laser pulses having a same wavelength and occurring in a burst using an energy monitor within a laser source of a laser-produced plasma (LPP) extreme ultraviolet (EUV) system, the measuring comprising:
receiving, from a power meter, an average power of the laser pulses measured over a defined period of time, and
receiving, from a photoelectromagnetic (PEM) detector, a first voltage signal indicative of a temporal profile of the burst of the laser pulses sensed during at least a portion of the defined period of time; and

determining a calibration coefficient based on the average power and the first voltage signal, the calibration coefficient being a ratio of an energy of the burst of the laser pulses determined from the average power and an integral of the first voltage signal; and
determining an energy of a subsequent pulse of the series of the laser pulses based on the calibration coefficient and an integral of a second voltage signal provided by the PEM detector indicating a temporal profile of the subsequent pulse.

Assignees

• Asml Netherlands Bv

Inventors

• Liu Rong
• Rafac Robert J
• Myers David Wayne
• Bergstedt Robert A
• Mckenzie Paul Alexander

Classifications

• G01J11/00 Primary

  Measuring the characteristics of individual optical pulses or of optical pulse trains · CPC title

• G01J1/0295

  Constructional arrangements for removing other types of optical noise or for performing calibration · CPC title

• H01S5/042

  Electrical excitation {; Circuits therefor (monolithically integrated laser drive components H01S5/0261)} · CPC title

• H01S3/0014

  Monitoring arrangements not otherwise provided for (photometry G01J1/00, e.g. G01J1/4257; radiation pyrometry G01J5/00; measuring coherence of light G01J9/00; measuring wavelength of light G01J9/00, e.g. G01J9/0246; measuring optical pulses G01J11/00; calorimetrically measuring power of laser beams G01K17/003) · CPC title

• G01J2001/4247

  for testing lamps or other light sources · CPC title