Laser for generating narrow-band radiation

Abstract

A laser, especially an excimer laser, is designed to generate narrow-band radiation and comprises a laser resonator, including two reflecting elements ( 12, 14 ) between which there is disposed a laser active medium ( 10 ), and further comprises a group of several refractive dispersive elements ( 32, 34 ) arranged in the laser beam path and each deflecting, incident light at an angle γ a ,γ b ) which is specific of the wavelength of the incident light. It is provided to reduce variations of the emission wavelength which are temperature responsive and time dependent during burst operation by giving at least one of the refractive dispersive elements ( 32 ) a refractive index which increases as the temperature goes up and at least one of these elements ( 34 ) a refractive index which drops as the temperature rises.

What is claimed is:  
     
       1. A laser for generating narrow-band radiation, comprising: 
       a laser resonator which includes two reflecting elements between which there is disposed a laser active medium, and  
       a plurality of refractive, wavelength selective elements each deflecting incident light at an angle that depends upon the wavelength of the incident light, wherein at least one of the refractive, wavelength selective elements has a refractive index which increases as the temperature rises and at least another one of the refractive, wavelength selective elements has a refractive index which decreases as the temperature rises.  
     
     
       2. The laser as claimed in  claim 1 , wherein at least one of the refractive, wavelength selective elements is made of quartz glass and at least another one of the refractive wavelength selective elements is made of CaF 2 . 
     
     
       3. The laser as claimed in  claim 1 , further comprising: 
       a beam splitter arranged between the laser active medium and one of the reflecting elements, wherein the plurality of refractive, wavelength selective elements being located between the beam splitter and the one reflecting element.  
     
     
       4. The laser as claimed in  claim 1 , wherein the plurality of refractive, wavelength selective elements are dispersion prisms. 
     
     
       5. The laser as claimed in  claim 4 , wherein the plurality of refractive, wavelength selective elements includes at least one Brewster prism. 
     
     
       6. The laser as claimed in  claim 4 , wherein the surfaces of incidence of the prisms are lighted substantially completely. 
     
     
       7. The laser as claimed in  claim 4 , further comprising: 
       a beam expander disposed in the laser resonator adjacent to one of the prisms.  
     
     
       8. The laser as claimed in  claim 4 , wherein the prisms are disposed in the resonator such that radiation is incident onto one of the prisms at Brewster's angle. 
     
     
       9. The laser as claimed in  claim 1 , further comprising: 
       a Fabry-Perot etalon contained in the beam path of the laser ahead of or behind the plurality of refractive, wavelength selective elements.  
     
     
       10. A laser for generating narrow-band radiation, comprising: 
       a laser resonator which includes two reflecting elements between which there is disposed a laser active medium, and  
       a plurality of refractive, wavelength selective elements each deflecting incident light at an angle which is specific of the wavelength of the incident light, at least one of the refractive, wavelength selective elements has a refractive index which increases as the temperature rises and at least another one of the refractive, wavelength selective elements has a refractive index which decreases as the temperature rises,  
       wherein the angles of incidence at the refractive, wavelength selective elements are selected such that the sum of the individual deflection angles of the respective elements at a predetermined wavelength of the incident light is independent of radiation induced temperature variations during operation of the laser.  
     
     
       11. A laser for generating narrow-band radiation, comprising: 
       a laser resonator which includes two reflecting elements between which there is disposed a laser active medium, and  
       a plurality of refractive, wavelength selective elements each deflecting incident light at an angle which is specific of the wavelength of the incident light, at least one of the refractive, wavelength selective elements has a refractive index which increases as the temperature rises and at least another one of the refractive, wavelength selective elements has a refractive index which decreases as the temperature rises,  
       wherein the plurality of refractive, wavelength selective elements contains first and second dispersion prisms, the variation in response to temperature of the refractive index of the first prism having a different sign than the variation in response to temperature of the refractive index of the second prism, and that the apex angle of the second prism is selected such that upon incidence at Brewster's angle on the second prism of the light having passed through the first prism the overall deflection angle determined by the first and second prisms is independent of radiation-induced temperature variations during operation of the laser.  
     
     
       12. A laser for generating narrow-band radiation, comprising: 
       a resonant laser cavity defined by at least two mirrors and having a laser active medium disposed therebetween for generating a laser beam; and  
       a first and a second refractive elements disposed in the laser cavity to select the wavelength of the laser beam, the first and second refractive elements each deflect incident light at an angle that depends upon the refractive index of that refractive element and the wavelength of the incident light, the first refractive element has a refractive index that increases as the temperature thereof increases, and the second refractive element has a refractive index that decreases as the temperature thereof increases:  
       wherein the first and second refractive elements are selected to deflect a predetermined wavelength of the laser beam at a total deflection angle such that variations in the total deflection angle for the predetermined wavelength, due to temperature variations of the first and second refractive elements, are minimized.  
     
     
       13. The laser as claimed in  claim 12 , wherein the first and second refractive elements are dispersion prisms. 
     
     
       14. The laser as claimed in  claim 13 , wherein the first refractive element is made of quartz glass and the second refractive element is made of CaF 2 . 
     
     
       15. The laser as claimed in  claim 13 , wherein the prisms are disposed in the laser cavity such that the laser beam is incident onto one of the prisms at Brewster's angle. 
     
     
       16. The laser as claimed in  claim 13 , wherein at least one of the first and second refractive elements is a Brewster prism. 
     
     
       17. The laser as claimed in  claim 13 , further comprising: 
       a beam expander disposed in the laser cavity such that the surfaces of the prisms are lighted substantially completely.  
     
     
       18. The laser as claimed in  claim 13 , further comprising: 
       a Fabry-Perot etalon disposed in the laser cavity adjacent to one of the first and second refractive elements.  
     
     
       19. A laser generating narrow- band UV radiation for use with a photolithography system, said photolithography system including an imaging system with refractive optics comprising a single material for making integrated circuit structures having sub - micron dimensions, comprising:    
       
         a laser resonator which includes two reflecting elements between which there is disposed a discharge chamber containing a laser active gas mixture;  
       
       
         a plurality of electrodes in the discharge chamber for providing a gas discharge and thereby energizing the gas mixture to produce an output laser beam; and  
       
         a plurality of optical elements in the beam path of the resonator for narrowing the bandwidth of the output laser beam of the laser to less than  1  pm, and wherein the plurality of optical elements includes a prism made of CaF   2    and a diffraction grating arranged to produce a narrow - band output laser beam.   
     
     
       20. A laser as in  claim 19 , wherein the refractive index n of the prism for incident  248  nm radiation is around  1 . 467  and for incident  193  nm radiation is around  1 . 501 . 
     
     
       21. A laser as in  claim 20 , wherein the temperature coefficient dn/dT of the prism for incident  248  nm radiation is around − 7 . 3 × 10   6 K −1    and for incident  193  nm radiation is around − 3 . 0 × 10     6  K −1 . 
     
     
       22. A laser as in  claim 21 , wherein the absorption coefficient α of the prism for incident  248  nm radiation is around  3 . 3 × 10   −3    cm   −1    and for incident  193  nm radiation is around  5 . 6 × 10     −3    cm   −1 . 
     
     
       23. A laser as in  claim 22 , wherein the density of the prism is around  3 . 18  g/cm 3 . 
     
     
       24. A laser as in  claim 23 , wherein the specific heat c of the prism is around  0 . 85  J/g•K. 
     
     
       25. A laser as in  claim 19 , wherein the laser active gas mixture includes fluorine and a buffer gas. 
     
     
       26. A laser as in  claim 19 , wherein the plurality of optical elements disposed in the beam path of the resonator for narrowing the bandwidth of the output beam further includes a second prism arranged to produce a narrow- band output laser beam.   
     
     
       27. A laser as in  claim 26 , wherein the plurality of optical elements disposed in the beam path of the resonator for narrowing the bandwidth of the output beam further includes a third prism arranged to produce a narrow- band output laser beam.   
     
     
       28. A laser as in  claim 19 , wherein the laser active gas mixture includes fluorine, a buffer gas and a rare gas selected from the group of rare gases consisting of argon and krypton. 
     
     
       29. A laser as in  claim 19 , wherein the laser active gas mixture includes fluorine, a buffer gas and argon. 
     
     
       30. A laser generating narrow- band UV radiation for use with a photolithography system, said photolithography system including an imaging system with refractive optics comprising a single material for making integrated circuit structures having sub - micron dimensions, comprising:    
       
         a laser resonator which includes two reflecting elements between which there is disposed a discharge chamber containing a laser active gas mixture;  
       
       
         a plurality of electrodes in the discharge chamber for providing a gas discharge and thereby energizing the gas mixture to produce an output laser beam; and  
       
         a plurality of optical elements in the beam path of the resonator for narrowing the bandwidth of the output laser beam of the laser to less than  1  pm, and wherein the plurality of optical elements includes a prism made of CaF   2    and a diffraction grating arranged to produce a narrow - band output laser beam,    
       
         wherein the laser active gas mixture includes fluorine, a buffer gas and krypton. 
       
     
     
       31. A laser generating narrow- band UV radiation for use with a photolithography system, said photolithography system including an imaging system with refractive optics comprising a single material for making integrated circuit structures having sub - micron dimensions, comprising:    
       
         a laser resonator which includes two reflecting elements between which there is disposed a discharge chamber containing a laser active gas mixture;  
       
       
         a plurality of electrodes in the discharge chamber for providing a gas discharge and thereby energizing the gas mixture to produce an output laser beam; and  
       
         a plurality of optical elements in the beam path of the resonator for narrowing the bandwidth of the output laser beam of the laser to less than  1  pm, and wherein the plurality of optical elements includes a prism made of CaF   2    and a Fabry - Perot etalon arranged to produce a narrow - band output laser beam.   
     
     
       32. A laser as in  claim 31 , wherein the refractive index n of the prism for incident  248  nm radiation is around  1 . 467  and for incident  193  nm radiation is around  1 . 501 . 
     
     
       33. A laser as in  claim 32 , wherein the temperature coefficient dn/dT of the prism for incident  248  nm radiation is around −l 7 . 3 × 10   6    K   −1    and for incident  193  nm radiation is around − 3 . 0 × 10     6    K   −1 .  
     
     
       34. A laser as in  claim 33 , wherein the absorption coefficient α of the prism for incident  248  nm radiation is around  3 . 3 × 10   −3    cm   −1    and for incident  193  nm radiation is around  5 . 6 × 10     −3    cm   −1 .  
     
     
       35. A laser as in  claim 34 , wherein the density of the prism is around  3 . 18  g/cm 3 . 
     
     
       36. A laser as in  claim 35 , wherein the specific heat c of the prism is around  0 . 85  J/g•K. 
     
     
       37. A laser as in  claim 31 , wherein the laser active gas mixture includes fluorine, a buffer gas and a rare gas selected from the group of rare gases consisting of argon and krypton. 
     
     
       38. A laser as in  claim 31 , wherein the laser active gas mixture includes fluorine, a buffer gas and argon. 
     
     
       39. A laser as in  claim 31 , wherein the laser active gas mixture includes fluorine, a buffer gas and krypton. 
     
     
       40. A laser as in  claim 31 , wherein the laser active gas mixture includes fluorine and a buffer gas. 
     
     
       41. A laser as in  claim 31 , wherein the plurality of optical elements disposed in the beam path of the resonator for narrowing the bandwidth of the output beam further includes a second prism arranged to produce a narrow- band output laser beam.   
     
     
       42. A laser generating narrow- band UV radiation for use with a photolithography system, said photolithography system including an imaging system with refractive optics comprising a single material for making integrated circuit structures having sub - micron dimensions, comprising:    
       
         an optical resonator;  
       
       
         a gas discharge compartment located within the resonator and having a laser active gas mixture selected to generate said UV radiation;  
       
       
         a plurality of wavelength selective elements located in the resonator for narrowing the output band of the laser to less than  1  pm, and wherein at least one of said wavelength selective elements is defined by a prism formed from calcium fluoride. 
       
     
     
       43. A laser as recited in  claim 42 , wherein said wavelength selective elements further include an etalon. 
     
     
       44. A laser as recited in  claim 42 , wherein said wavelength selective elements further include a grating. 
     
     
       45. A laser as recited in  claim 42 , wherein said wavelength selective elements further include a second prism. 
     
     
       46. A laser as recited in  claim 45 , wherein said second prism is located between the gas discharge compartment and the calcium fluoride prism. 
     
     
       47. A laser as in  claim 42 , wherein the laser active gas mixture includes fluorine, a buffer gas and argon. 
     
     
       48. A laser as in  claim 42 , wherein the laser active gas mixture includes fluorine, a buffer gas and krypton. 
     
     
       49. A laser as in  claim 42 , wherein the laser active gas mixture includes fluorine and a buffer gas. 
     
     
       50. A laser as recited in  claim 42 , wherein said wavelength selective elements further include a grating. 
     
     
       51. An laser as recited in  claim 45 , wherein said second prism is located between the gas discharge compartment and the calcium fluoride prism.