Reducing the spectral bandwidth of lasers

• US 9,209,589 B2
• Filed: 01/28/2015
• Issued: 12/08/2015
• Est. Priority Date: 07/12/2012
• Status: Active Grant

• Current Assignee: KLA-Tencor Corporation
• Original Assignee: KLA-Tencor Corporation
• Inventors: Armstrong, J. Joseph
• Primary Examiner(s): NGUYEN, DUNG T
• Application Number:
  US14/607,515
• Publication Number:
  US 20150139255A1
• Time in Patent Office: 314 Days
• Field of Search: 372/4, 372/6, 372 21- 22, 372/29.021, 372/69
• US Class Current: 1/1
• CPC Class Codes: G01N 21/00 Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light G01N3/00 - G01N19/00 take precedence
• CPC Class Codes: G01N 21/9501 Semiconductor wafers manufacturing processes per se of semiconductor devices implementing a measuring step H01L22/10
• CPC Class Codes: G01N 21/956 Inspecting patterns on the surface of objects contactless testing of electronic circuits G01R31/308; testing currency G07D; manufacturing processes per se of semiconductor devices implementing a measuring step H01L22/10
• CPC Class Codes: H01S 2301/08 Generation of pulses with special temporal shape or frequency spectrum
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108
• CPC Class Codes: H01S 3/0092 Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity nonlinear frequency conversion per se G02F1/35
• CPC Class Codes: H01S 3/02 Constructional details housings or packages of fibre lasers H01S3/06704
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/06758 Tandem amplifiers
• CPC Class Codes: H01S 3/10 Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
• CPC Class Codes: H01S 3/30 using scattering effects, e.g. stimulated Brillouin or Raman effects
• CPC Class Codes: H01S 3/302 in an optical fibre

Abstract

A laser system for semiconductor inspection includes a fiber-based fundamental light source for generating fundamental light that is then converted/mixed by a frequency conversion module to generate UV-DUV laser light. The fundamental light source includes a nonlinear chirp element (e.g., a Bragg grating or an electro-optic modulator) that adds a nonlinear chirp to the seed light laser system prior to amplification by the fiber amplifier(s) (e.g., doped fiber or Raman amplifiers). The nonlinear chirp includes an x² or higher nonlinearity and is configured to compensates for the Self Phase Modulation (SPM) characteristics of the fiber-based amplifiers such that fundamental light is generated that has a spectral E95 bandwidth within five times that of the seed light. When multiple series-connected amplifiers are used, either a single nonlinear chirp element is provided before the amplifier string, or a chirp elements are included before each amplifier.

Claims

1. A fiber-based fundamental light source for generating fundamental light at a nominal fundamental frequency a laser system, the fiber-based fundamental light source comprising:

one or more seed lasers including means for generating seed light having an initial peak power and an initial spectral bandwidth, and for transmitting the seed light along an optical path;

one or more amplifiers disposed in the optical path and including means for amplifying the seed light generated by said one or more seed lasers such that amplified light output from the one or more amplifiers has a fundamental peak power that is substantially higher than the initial peak power; and

one or more nonlinear chirp elements disposed in the optical path and including means for modifying one of the seed light and the amplified light to include one or more nonlinear chirps that compensate for Self Phase Modulation (SPM) characteristics of the one or more amplifiers,

wherein each of said one or more nonlinear chirp elements comprises one of a Bragg grating, a fiber Bragg grating and an electro-optic modulator, and

wherein said one or more nonlinear chirp elements comprise means for generating said nonlinear pre-chirp with a nonlinearity on the order of x² or higher.

2. The fiber-based fundamental light source of claim 1, wherein said one or more nonlinear chirp elements comprise means for generating said nonlinear pre-chirp U(0,T) having time-based frequency characterized by the equation

3. The fiber-based fundamental light source of claim 1, wherein said one or more seed lasers and said one or more amplifiers comprise means for generating said fundamental light such that said nominal fundamental frequency is approximately 1 μ

m.

4. The fiber-based fundamental light source of claim 1, wherein each of said one or more seed lasers comprises one of a pulsed light oscillator, a solid state laser, a diode laser, a gain-switched laser diode, and a quantum dot laser.

5. The fiber-based fundamental light source of claim 1, wherein said one or more amplifiers comprise one of a doped fiber amplifier and a Raman amplifier.

6. The fiber-based fundamental light source of claim 1, further comprising a pump seed laser for transmitting pump seed light to said one or more amplifiers.

7. The fiber-based fundamental light source of claim 1,

wherein said one or more nonlinear chirp elements are disposed between the one or more seed lasers and the one or more amplifiers in the optical path such that modified seed light exiting the one or more nonlinear chirp elements has a modified power that is substantially equal to said initial peak power and a modified spectral bandwidth, that is different from said initial spectral bandwidth, and

wherein said one or more amplifiers includes means for amplifying the modified seed light such that said nominal fundamental frequency of said fundamental light is in the UV-DUV range.

8. The fiber-based fundamental light source of claim 7, wherein said one or more amplifiers comprises a plurality of amplifiers connected in series in said optical path.

9. The fiber-based fundamental light source of claim 1, wherein said one or more amplifiers comprises a plurality of amplifiers including a first amplifier and a second amplifier, and

wherein said one or more nonlinear chirp elements comprise at least one nonlinear chirp elements disposed between said first and second amplifiers in said optical path.

10. The fiber-based fundamental light source of claim 1,

wherein said one or more seed lasers comprises a continuous wave (CW) laser for generating CW seed light and a pulsed laser for generating pulsed pump seed light,

wherein said one or more amplifiers comprises a Raman amplifier disposed to receive said CW seed light and said pulsed pump seed light,

wherein said one or more nonlinear chirp elements comprises a phase modulator disposed in the optical path between the Raman amplifier and the CW laser such that said Raman amplifier receives both said CW seed light and said nonlinear chirp from said phase modulator.

11. The fiber-based fundamental light source of claim 10,

wherein said CW laser comprises one of a solid state laser, a diode laser and a quantum dot laser, and

wherein said pulsed laser comprises one of a gain-switched laser diode and a pulsed light oscillator.

12. The fiber-based fundamental light source of claim 10,

wherein said said one or more amplifiers comprises a second Raman amplifier disposed in the optical path to receive amplified light output from said Raman amplifier, and

wherein said one or more seed lasers further comprises a second pulsed laser disposed to transmit second pulsed pump seed light into said second Raman amplifier.

13. A fiber-based fundamental light source for generating fundamental light at a nominal fundamental frequency a laser system, the fiber-based fundamental light source comprising:

one or more seed lasers including means for generating seed light having an initial peak power and an initial spectral bandwidth, and for transmitting the seed light along an optical path;

one or more amplifiers disposed in the optical path and including means for amplifying the seed light generated by said one or more seed lasers such that amplified light output from the one or more amplifiers has a fundamental peak power that is substantially higher than the initial peak power; and

one or more nonlinear chirp elements disposed in the optical path and including means for modifying one of the seed light and the amplified light to include one or more nonlinear chirps that compensate for Self Phase Modulation (SPM) characteristics of the one or more amplifiers,

wherein said one or morenonlinear chirp elements includes means for generating said nonlinear chirp such that the nonlinear chirp compensates for said SPM characteristics of the one or more amplifiers such that said fundamental light generated by said laser system has a fundamental spectral bandwidth that is in a range defined by five times said initial spectral bandwidth of said seed light.

14. A method for generating fundamental light using a fiber-based laser system, the method comprising:

generating seed light having an initial spectral bandwidth;

amplifying the seed light using one or more fiber-based amplifiers to produce amplified light having a peak power that is substantially higher than the initial peak power; and

modifying one of the seed light and the amplified light to include a nonlinear chirp,

wherein the nonlinear chirp has a nonlinearity on the order of x² or higher and substantially entirely compensates for Self Phase Modulation (SPM) characteristics of said one or more fiber-based amplifiers such that said fundamental light has a fundamental spectral bandwidth that is within a range defined by five times said initial spectral bandwidth.

15. The method of claim 14, wherein modifying one of the seed light and the amplified light to include a nonlinear chirp comprises generating said nonlinear pre-chirp U(0,T) having time-based frequency characterized by the equation

16. The method of claim 14,

wherein modifying comprises modifying said seed light, and

wherein amplifying comprises amplifying the modified seed light.

17. The method of claim 14,

wherein modifying comprises modifying said amplified light to include said nonlinear chirp, and

wherein the method further comprises amplifying said modified amplified light.

18. The method of claim 14,

wherein generating seed light comprises generating continuous wave (CW) seed light and pulsed pump seed light,

wherein modifying comprises modifying said CW seed light, and

wherein amplifying the seed light comprises directing said modified CW seed light and said pulsed pump seed light into a Raman amplifier.