Patent Publication Number: US-2015063830-A1

Title: Continuous wave ultraviolet laser based on stimulated raman scattering

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/611/994, entitled “Continuous Wave Ultraviolet Laser Based on Stimulated Raman Scattering,” filed on Mar. 16, 2012, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     Numerous material processing and diagnostic applications, such as semiconductor processing and inspection, requires powerful diffraction-limited continuous wave (hereinafter CW) ultraviolet (hereinafter UV) laser light. Presently, the most efficient and practical CW laser sources operate at wavelengths considerably longer than UV wavelengths, thereby requiring harmonic conversion to a desired UV wavelength. For example, CW laser light sources outputting near IR wavelengths or longer may be used as a source. 
     One common CW laser source frequently used in industrial applications is a solid-state Nd laser system configured to output a laser signal at about 1064 nm. Thereafter, the 1064 nm output by the Nd laser system is efficiently converted to 532 nm using intracavity second harmonic generation processes. In some applications, particularly semiconductor inspection and processing applications, the second harmonic signal having a wavelength of about 532 nm undergoes an additional harmonic conversion resulting in a fourth harmonic wavelength of about 266 nm. To be efficient, the additional harmonic conversion requires that the 532 nm second harmonic signal have high optical intensity to produce a harmonic output signal at 266 nm having sufficient power to be useful. As such, often a 532 nm resonant ring cavity is required for the additional harmonic conversion of the 532 nm signal to produce a 266 nm signal having a usable intensity for semiconductor inspection and processing. 
     While the aforementioned method of generating CW UV laser light has proven useful in the past, a number of shortcomings have been identified. For example, the resonant ring cavity used for converting the 532 nm second harmonic signal to 266 nm signal requires very precise locking of the laser and ring resonances. Commonly, active locking of the two cavity lengths to a small fraction of a wavelength is required. In the past, this locking process has proven challenging and expensive. In addition, maintaining this interferometric accuracy for long periods of time has proven difficult. 
     Thus, in light of the foregoing, there is an ongoing need for a system capable of efficient CW wavelength conversion from wavelengths greater than about 500 nm to UV wavelengths without requiring the aforementioned precise locking requirements. 
     SUMMARY 
     The present application is directed to a laser system configured to output a continuous wave output signal. More specifically, the laser system presented herein utilizes Stimulated Raman Scattering (SRS) to generate a Stimulated Raman Scattering output signal at a wavelength (the Stokes wavelength) slightly longer than the pump. Thereafter, the Stimulated Raman Scattering output signal may undergo harmonic conversion to produce a continuous wave ultraviolet wavelength output signal capable of being directed to a work surface or substrate. In one embodiment, the laser system includes at least one pump source configured to generate at least one pump signal having a wavelength of about 500 nm to about 550 nm, at least one resonant cavity in optical communication with the pump source, the resonant cavity resonant at a Stokes wavelength and defined by a first mirror and at least a second minor, at least one SRS gain device positioned within the resonant cavity, the SRS gain device configured to generate at least one SRS output signal at a Stokes wavelength when pumped with the pump signal, and at least one harmonic conversion device positioned within the resonant cavity, the harmonic conversion device configured to produce a second harmonic output signal of the SRS output signal, wherein the second minor is configured to output the second harmonic output signal produced by the harmonic conversion device. 
     In another embodiment, the present application is directed to a laser system and includes at least one pump source configured to generate at least one pump signal having a wavelength of about 400 nm to about 800 nm, at least one resonant cavity in optical communication with the pump source, the resonant cavity resonant at a Stokes wavelength and defined by a first minor and at least a second minor, at least one SRS gain device positioned within the resonant cavity, the SRS gain device configured to generate at least one SRS output signal at a Stokes wavelength when pumped with the pump signal, and at least one harmonic conversion device positioned within the resonant cavity, the harmonic conversion device configured to produce a second harmonic output signal of the SRS output signal, wherein the second minor is configured to output the second harmonic output signal produced by the harmonic conversion device. 
     Further, the present application discloses a method of inspecting a semiconductor wafer. More specifically, the present application discloses providing at least one pump laser configured to produce at least one pump signal having a wavelength of about 500 nm to about 550 nm, irradiating at least one SRS gain medium with the pump signal to produce at least one SRS output signal at a Stokes wavelength, irradiating at least one harmonic conversion device with the SRS output signal to produce a second harmonic output signal having a wavelength of about 270 to about 300 nm, directing the second harmonic output signal to a semiconductor wafer, and detecting light scattered from the semiconductor wafer. 
     Other features and advantages of the embodiments of the continuous wave ultraviolet wave laser system using Stimulated Raman Scattering as disclosed herein will become apparent from a consideration of the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the continuous wave ultraviolet wave laser system using Stimulated Raman Scattering will be explained in more detail by way of the accompanying drawings, wherein: 
         FIG. 1  shows a schematic view of an embodiment of a continuous wave ultraviolet wave laser system using Stimulated Raman Scattering used for semiconductor wafer inspection. 
     
    
    
     DETAILLED DESCRIPTION 
       FIG. 1  shows an embodiment of a CW UV laser system  10  based on Stimulated Raman Scattering. In this process, a pump signal  14  incident on a Raman-active material  24  (hereinafter SRS gain device) generates a SRS output signal  26  (the Stokes wave) at a wavelength longer than that of the pump signal  14 . The wavelength of the Stokes wave is determined by properties of the SRS gain device  24 . As shown, the laser system  10  includes at least one pump laser  12  configured to output a CW pump signal  14  having a wavelength of about 400 nm to about 800 nm. In the illustrated embodiment a single pump laser  12  is used. In an alternate embodiment, multiple pump lasers  12  may be spatially and/or spectrally combined to result in the generation of one or more high power pump signals  14 . Optionally, the pump laser beam may be passed through the SRS gain device  24  multiple times for increased pump intensity. In one embodiment the CW pump signal  14  has a wavelength of about 400 nm to about 800 nm. In another embodiment, the CW pump signal  14  has a wavelength of about 500 nm to about 550 nm. In a more specific embodiment, the CW pump signal  14  has a wavelength of about 532 nm. In one embodiment, the pump laser  12  comprises a CW diode pumped solid state laser. For example, the laser system  12  may comprise a Millennia™ laser manufactured by Spectra-Physics, Inc. Those skilled in the art will appreciate that any variety of laser systems or devices configured to output a CW pump signal  14  having a wavelength of about 400 nm to about 800 nm may be used with the present system. 
     Referring again to  FIG. 1 , one or more optical elements  16  may be used to focus or otherwise condition the CW pump signal  14 . For example, in the illustrated embodiment, a lens or lens system  16  is used to focus the CW pump signal  14  to a point within the laser system  10 . Any variety of optical elements  16  may be used with the present system, including, without limitations, mirrors, lenses, lens systems, gratings, etalons, and the like. 
     As shown in  FIG. 1 , the laser system  10  includes a first mirror  18 , and at least a second mirror  20 , the first and second mirrors  18 ,  20 , respectively, defining at least one resonant cavity  22 . Those skilled in the art will appreciate that any variety of mirrors may be used to form the resonant cavity  22 . In the illustrated embodiment, the first mirror  18  comprises a bandpass mirror configured to transmit optical signals at the pump signal wavelength therethrough while reflecting virtually all optical signals at the Stokes wavelength. Similarly, the second mirror  20  likewise comprises a bandpass mirror configured to reflect the Stokes wavelength and transmit optical signal at a desired wavelength (e.g. a harmonic of a Stokes wavelength generated within the resonant cavity). As such, the second mirror  20  may form an output coupler configured to output CW laser light having a wavelength equal to a harmonic of a wavelength generated within the resonant cavity  22 . Those skilled in the art will appreciate that the first mirror  18 , second mirror  20 , or both may form an output coupler. Further, in the illustrated embodiment, the optical element  16  is located outside the resonant cavity  22 . Optionally, the optical element  16  may be positioned within the resonant cavity  22 . 
     Referring again to  FIG. 1 , at least one SRS gain device  24  is positioned within the resonant cavity  22 . In the illustrated embodiment, the SRS gain device  24  is positioned at the focal point of the optical element  16  positioned outside the resonant cavity  22 . Optionally, the SRS gain device  24  may be positioned at any location within the resonant cavity  22 . The SRS gain device  24  may be configured to generate at least one SRS output signal  26  at a desired Stokes wavelength when pumped with the pump signal  14 . As such, the SRS gain device  24  may be constructed from any variety of materials known to generate a SRS output signal  26  at a desired Stokes wavelength. For example, in one embodiment, the SRS gain device  24  is manufactured from diamond having a single vibrational mode at about 1332 cm −1 , which results in the generation of an SRS optical signal  26  having a Stokes wavelength of about 573 nm In the alternative, the SRS gain device  24  may be constructed from KGW, KYW, Ba(NO 3 ) 2 , BaWO 4 , PbWO 4 , CaWO 4 , YVO 4 , GdVO 4 , LiNbO 3 , SrMO 4 , PbMO 4  or LiIO 3 . Optionally, the SRS gain device  24  may include one or more dopants configured to provide different resonances, fluorescence and/or enhancement to the Raman gain, and the like. In the illustrated embodiment, a single SRS gain device  24  is positioned within the resonant cavity  22 . In an alternate embodiment, multiple SRS gain devices  24  are positioned within the resonant cavity  22 . Further, in the illustrated embodiment, the resonant cavity  22  is resonant at the Stokes wavelength produced by the SRS gain device  24 . Optionally, the resonant cavity may be configured to be resonant at any variety of wavelengths. 
     As shown in  FIG. 1 , the resonant cavity  22  of laser system  10  may include a third minor  28  configured to transmit at least a portion of the pump light  14  having a wavelength of about 532 nm, while reflecting the SRS output signal  26  having a wavelength of about 573 nm Pump light  14  transmitted through minor  28  may optionally be reflected back through minor  28  for one or more additional passes through the SRS gain device  24  resulting in increased optical gain. Those skilled in the art will appreciate that the wavelength transmission/reflection characteristics of the minor  28  may be varied depending on the material used to manufacture the SRS gain device  24  and the resulting Stokes wavelength of the resultant SRS output signal  26 . In short, minor  28  is configured to reflect the SRS output signal  26  at the Stokes wavelength while transmitting at least a portion of other wavelengths therethrough. 
     Referring again to  FIG. 1 , at least one harmonic conversion device  30  may be positioned within the resonant cavity  22 . In the illustrated embodiment a single harmonic conversion device  30  is positioned between third minor  28  and the second mirror  20 , although those skilled in the art will appreciate that the harmonic conversion device  30  may be positioned anywhere within the resonant cavity  22 . In one embodiment, the harmonic conversion device  30  comprises at least one nonlinear optical material. Exemplary nonlinear optical materials include, without limitations, LBO, BBO, CLBO, KABO, DKDP, KTP, PPSLT, KDP, CBO, BIBO, LB4, KBBF, RBBF and the like. In one embodiment, the harmonic conversion device  30  is configured to generate a second harmonic output signal  38  of the SRS output signal  26 . In the present case, the output signal  38  would have a wavelength of about 286 nm, which is the second harmonic of the SRS output signal  26 , which has a Stokes wavelength of about 573 nm As such, the second minor  20  would be configured to transmit the output signal  38  having a wavelength of 286 nm, while reflecting the Stokes wavelength. 
     As shown in  FIG. 1 , one or more additional optical elements  32 ,  34  may be positioned within the resonant cavity. For example, as shown, at least one lens or lens system  32 ,  34  is positioned proximate to the harmonic conversion device  30  and configured to focus the SRS output signal  26  into at least a portion of the harmonic conversion device  30 . Similarly, one or more lenses, lens systems, minor, and the like may be used within the resonant cavity  22 . 
     Referring again to  FIG. 1 , in one embodiment the second harmonic output signal  38  transmitted through the second minor  20  may then be directed to into a semiconductor wafer  50  or into a semiconductor inspection system. More specifically, during use in semiconductor inspection applications, the laser system  10  disclosed herein may be configured such that the pump laser  12  outputs a pump signal  14  having a wavelength of about 500 nm to about 550 nm. Thereafter, the SRS gain device  24  may be irradiated with the pump signal  14  to produce at least one SRS output signal  26  at a Stokes wavelength. The SRS output signal  26  may then be used to irradiate the harmonic conversion device  30  to produce a second harmonic output signal  38  having a wavelength of about 270 to about 300 nm, which is then directed to a semiconductor wafer  50 . Finally, a detector  52  may be used to detect light scattered from the semiconductor wafer  50 . Those skilled in the art will appreciate that the present laser system  10  may be easily adapted for use in any variety of semiconductor inspection or processing system presently available and known in the art. 
     Unlike prior art systems, the present system utilizes a well developed green laser to efficiently and reliably generate CW UV light via stimulated Raman scattering, while avoiding the technical difficulties associated with precise interferometric locking of multiple optical resonators. Moreover, the SRS-based laser system above provides optical gain when pumped at any wavelength where the SRS material has sufficient optical transmission. 
     The embodiments disclosed herein are illustrative of the principles of the invention. Other modifications may be employed which are within the scope of the invention. Accordingly, the devices disclosed in the present application are not limited to that precisely as shown and described herein.