Low Noise, High Stability, Deep Ultra-Violet, Continuous Wave Laser

Patent metadata

Field	Value
Publication number	US-2016197449-A1
Application number	US-201615017839-A
Country	US
Kind code	A1
Filing date	Feb 8, 2016
Priority date	Sep 10, 2013
Publication date	Jul 7, 2016
Grant date	—

Abstract

A laser for generating deep ultra-violet (DUV) continuous wave (CW) light includes a second-harmonic generator and a fourth-harmonic generator. The fourth-harmonic generator includes a plurality of mirrors as well as a first non-linear optical (NLO) crystal and a pair of tilted plates. The first NLO crystal generates the light having the fourth harmonic wavelength and a first astigmatism, and is placed in operative relation to the plurality of mirrors. The pair of tilted plates is placed in operative relation to the first NLO crystal such that the light having the second harmonic wavelength passes through both of the tilted plates. Notably, the pair of tilted plates are disposed at substantially equal and opposite angles about respective parallel axes such that they introduce a second astigmatism that corrects for the first astigmatism while minimizing displacement of the circulated light.

First claim

1 . A laser for generating deep ultra-violet (DUV) continuous wave (CW) light, the laser comprising:
a second-harmonic generator for converting light having a fundamental wavelength to light having a second harmonic wavelength; and a fourth-harmonic generator for converting the light having the second harmonic wavelength to light having a fourth harmonic wavelength, the fourth-harmonic generator including: a plurality of mirrors configured to circulate light in the fourth harmonic generator; a first non-linear optical (NLO) crystal having first optical axes that are oriented relative to the light circulated in the fourth harmonic generator such that said first NLO crystal converts a portion of the light having the second harmonic wavelength to light having the fourth harmonic wavelength and having a first astigmatism, the first NLO crystal in operative relation to the plurality of mirrors; and a pair of thin plates, each said thin plate having parallel planar surfaces, said pair of thin plates disposed in operative relation to the first NLO crystal such that the light circulated in the fourth harmonic generator passes through said planar parallel surfaces of the pair of thin plates, wherein the pair of thin plates are disposed in a tilted arrangement at substantially equal and opposite angles about respective parallel axes such that the pair of thin plates introduce a second astigmatism to the light circulated in the fourth harmonic generator that corrects for said first astigmatism while minimizing displacement of said circulated light.

2 . The laser of claim 1 , wherein the first NLO crystal is hydrogen-annealed.

3 . The laser of claim 1 , wherein the first NLO crystal comprises a hydrogen-annealed CLBO (cesium lithium borate) crystal.

4 . The laser of claim 1 , wherein the light having the second harmonic is focused to a substantially elliptical beam waist in, or proximate to, the first NLO crystal, with a long axis of an ellipse substantially parallel to a first e-axis.

5 . The laser of claim 1 , wherein at least the first NLO crystal is controlled in temperature to reduce said first astigmatism created in the first NLO crystal by a focused beam therein.

6 . The laser of claim 5 , wherein a controlled temperature is approximately 100° C. or lower.

7 . The laser of claim 1 , the fourth harmonic generator further including a feedback control loop that automatically adjusts the angles of the pair of thin plates so as to substantially cancel said first astigmatism introduced by the first NLO crystal.

8 . A system for inspecting a wafer, reticle, or photomask, the system comprising:
a deep ultra-violet (DUV) continuous wave (CW) laser including: a second harmonic generator for converting light having a fundamental wavelength to light having a second harmonic wavelength; and a fourth harmonic generator for converting the light having the second harmonic wavelength to light having a fourth harmonic wavelength, the fourth harmonic generator including: a plurality of mirrors configured to circulate light in the fourth harmonic generator; a first non-linear optical (NLO) crystal configured to generate the light having the fourth harmonic wavelength by converting a converted portion of the light having the second harmonic wavelength, the first NLO crystal in operative relation to the plurality of mirrors such that the light circulating in the fourth harmonic generator is directed into the first NLO crystal only in a single light propagation direction; and a pair of thin plates, each said thin plate having parallel planar surfaces, said pair of thin plates disposed in operative relation to the first NLO crystal such that the light circulated in the fourth harmonic generator passes through said planar parallel surfaces of the pair of thin plates, wherein the pair of thin plates are disposed in a tilted arrangement at substantially equal and opposite angles about respective parallel axes such that the pair of thin plates introduce a second astigmatism to the light circulated in the fourth harmonic generator that corrects for said first astigmatism while minimizing displacement of said circulated light.

9 . The system of claim 8 , wherein the first NLO crystal is hydrogen-annealed.

10 . The system of claim 8 , wherein the first NLO crystal comprises a hydrogen-annealed CLBO (cesium lithium borate) crystal.

11 . The system of claim 8 , wherein light having the second harmonic is focused to a substantially elliptical beam waist in, or proximate to, the first NLO crystal, with a long axis of an ellipse substantially parallel to a first e-axis.

12 . The system of claim 8 , wherein at least the first NLO crystal is controlled in temperature to reduce said first astigmatism created in the first NLO crystal by a focused beam therein.

13 . The system of claim 12 , wherein a controlled temperature is approximately 100° C. or lower.

14 . The system of claim 8 , the fourth harmonic generator further including a feedback control loop that automatically adjusts the angles of the pair of thin plates so as to substantially cancel said first astigmatism introduced by the first NLO crystal.

15 . A method of generating deep ultra-violet (DUV) continuous wave (CW) light in a laser, the method including:
converting light having a fundamental wavelength to light having a second harmonic wavelength; converting a portion of the light having the second harmonic wavelength to light having a fourth harmonic wavelength by circulating the light having the second harmonic wavelength in a cavity configured such that said light passes through a first non-linear optical (NLO) crystal only in a single light propagation direction; and cancelling a first astigmatism created in the first NLO crystal using a pair of thin plates, each said thin plate having parallel planar surfaces, said pair of thin plates disposed in operative relation to the first NLO crystal such that the light circulated in the fourth harmonic generator passes through said planar parallel surfaces of the pair of thin plates, wherein the pair of thin plates are disposed in a tilted arrangement at substantially equal and opposite angles about respective parallel axes such that the pair of thin plates introduce a second astigmatism to the light circulated in the fourth harmonic generator that corrects for said first astigmatism while minimizing displacement of said circulated light.

Assignees

• Kla Tencor Corp

Inventors

• Chuang Yung-Ho

Classifications

• G01N2021/95676

  Masks, reticles, shadow masks · CPC title

• G01N2201/067

  Electro-optic, magneto-optic, acousto-optic elements · CPC title

• G01N21/9501

  Semiconductor wafers (manufacturing processes per se of semiconductor devices implementing a measuring step H10P74/20) · CPC title

• H01S3/0085

  Modulating the output, i.e. the laser beam is modulated outside the laser cavity · CPC title

• G01N21/33

  using ultraviolet light (G01N21/39 takes precedence) · CPC title