Compensating for a physical effect in an optical system

Patent metadata

Field	Value
Publication number	US-10401704-B2
Application number	US-201615349600-A
Country	US
Kind code	B2
Filing date	Nov 11, 2016
Priority date	Nov 11, 2016
Publication date	Sep 3, 2019
Grant date	Sep 3, 2019

Abstract

A wavefront of a light beam that exits an acousto-optic material is estimated; a control signal for an acousto-optic system that includes the acousto-optic material is generated, the control signal being based on the estimated wavefront of the light beam; and
the control signal is applied to the acousto-optic system to generate a frequency-chirped acoustic wave that propagates in the acousto-optic material, the frequency-chirped acoustic wave forming a transient diffractive element in the acousto-optic material, an interaction between the transient diffractive element and the light beam adjusting the wavefront of the light beam to compensate for a distortion of the wavefront of the light beam, the distortion of the wavefront being at least partially caused by a physical effect in the acousto-optic material.

First claim

What is claimed is:

1. A method comprising:
estimating a wavefront of a light beam that exits an acousto-optic material;
generating a control signal for an acousto-optic system that comprises the acousto-optic material, the control signal being based on the estimated wavefront of the light beam; and
applying the control signal to the acousto-optic system to generate a frequency-chirped acoustic wave that propagates in the acousto-optic material, the frequency-chirped acoustic wave forming a transient diffractive element in the acousto-optic material, an interaction between the transient diffractive element and the light beam adjusting the wavefront of the light beam to compensate for a distortion of the wavefront of the light beam, the distortion of the wavefront being at least partially caused by a physical effect in the acousto-optic material.

2. The method of claim 1 , wherein estimating the wavefront of the light beam that exits the acousto-optic material comprises:
receiving at least a portion of the light beam that exits the acousto-optic material, and
estimating the wavefront of the light beam that exits the acousto-optic material based on the received portion of the light beam.

3. The method of claim 1 , wherein the physical effect of the acousto-optic material comprises a thermal distortion of the acousto-optic material, the thermal distortion at least partially causing the distortion of the wavefront of the light beam that exits the acousto-optic material.

4. The method of claim 3 , wherein estimating the wavefront of the light beam that exits the acousto-optic material comprises:
accessing a plurality of temperature measurements of the acousto-optic material, each of the plurality of temperature measurements being a temperature of a different portion of the acousto-optic material,
estimating, based on the accessed plurality of temperatures, a temperature distribution of the acousto-optic material,
estimating, based on the estimated temperature distribution of the acousto-optic material, a spatial distribution of an index of refraction of the acousto-optic material, and
estimating the wavefront of the light beam that exits the acousto-optic material using the estimated index of refraction of the acousto-optic material.

5. The method of claim 1 , wherein the acousto-optic material absorbs one or more of the light beam, the frequency-chirped acoustic wave, and an acoustic wave other than the frequency-chirped acoustic wave that propagates in the acousto-optic material as heat, and the physical effect of the acousto-optic material comprises a thermal distortion arising from the absorbed heat.

6. The method of claim 2 , wherein the transient diffractive element further compensates the light beam for effects other than the physical effect of the acousto-optic material.

7. The method of claim 6 , wherein the effects other than the physical effect of the acousto-optic material comprise a physical effect of an optical element other than the acousto-optic material, the optical element being positioned to interact with the light beam.

8. The method of claim 2 , wherein:
the light beam comprises a pulsed light beam,
receiving at least a portion of the light beam comprises receiving a portion of a first pulse of the light beam,
determining a wavefront of the light beam comprises determining a wavefront based on the received portion of the first pulse of the light beam, and
the generated frequency-chirped acoustic wave propagates in the acousto-optic material and forms the transient diffractive element while a second pulse of the light beam passes through the acousto-optic material, the second pulse of the light beam occurring after the first pulse of the light beam.

9. The method of claim 8 , wherein
the first and second pulses have a temporal duration of 100 nanoseconds (ns) or less, and
the transient diffractive element propagates 500 microns (μm) or less in the acousto-optic material while the second pulse passes through the acousto-optic material such that the second pulse interacts with the transient diffractive element and is diffracted by the transient diffractive element at an angle that depends on a frequency of the frequency-chirped acoustic wave.

10. The method of claim 1 , wherein the physical effect of the acousto-optic material comprises a thermal distortion of the acousto-optic material, the thermal distortion at least partially causing a distortion of the wavefront of the light beam that exits the acousto-optic material.

11. The method of claim 1 , further comprising:
generating an initial control signal for the acousto-optic system, the initial control signal being independent of the estimated wavefront of the light beam; and
applying the initial control signal to the acousto-optic system to generate a constant frequency acoustic wave in the acousto-optic material prior to estimating the wavefront of the light beam, the constant frequency acoustic wave forming an initial transient diffractive element in the acousto-optic material, wherein generation of the initial transient diffractive element is the physical effect that at least partially causes the distortion of the wavefront.

12. A system for an extreme ultraviolet (EUV) light source, the system comprising:
an optical system comprising:
an acousto-optic material in which acoustic waves propagate, the acousto-optic material being configured to be positioned on a beam path, and

an acoustic wave generator comprising:
a transducer configured to couple to the acousto-optic material, and
a waveform generator configured to couple to the transducer;

a sensing apparatus configured to measure data related to a light beam that propagates on the beam path or data related to a condition of the acousto-optic material; and
a control system coupled to the sensing apparatus and the waveform generator, the control system being configured to:
estimate a wavefront of the based on data measured by the sensing apparatus,
generate a control signal based on the estimated wavefront of the light beam, and
provide the control signal to the optical system, the control signal being sufficient to cause the acoustic wave generator to provide a frequency-chirped acoustic wave to the acousto-optic material, the frequency-chirped acoustic wave forming a transient diffractive element in the acousto-optic material, an interaction between the transient diffractive element and the light beam adjusting the wavefront of the light beam to compensate for a physical effect of the acousto-optic material.

13. The system of claim 12 , wherein the physical effect of the acousto-optic material comprises a spatially varying index of refraction in the acousto-optic material.

14. The system of claim 13 , wherein the physical effect of the acousto-optic material comprises a thermal distortion, and the index of refraction at any particular location within the acousto-optic material is related to an amount of heat absorbed by the acousto-optic material and a distance between one or more absorption locations and the particular location, the absorption locations being a region in the crystal where heat is absorbed.

15. The system of claim 12 , wherein the acousto-optic material absorbs one or more of the light beam, the frequency-chirped acoustic wave, and an acoustic wave other than the frequency-chirped acoustic wave that propagates in the acousto-optic material, and the physical effect of the acousto-optic material comprises a thermal distortion arising from the absorbed heat.

16. The system of claim 12 , wherein the acousto-optic material comprise

Assignees

• Asml Netherlands Bv

Inventors

• Rafac Robert Jay

Classifications

• G02F1/11

  based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves (acousto-optical deflection G02F1/33) · CPC title

• G03F7/70941

  Stray fields and charges, e.g. stray light, scattered light, flare, transmission loss · CPC title

• G03F7/706

  Aberration measurement · CPC title

• G02F1/33 Primary

  Acousto-optical deflection devices {(circuit or control arrangements therefor G02F1/113)} · CPC title

• G02F2203/18

  adaptive optics, e.g. wavefront correction · CPC title