Low-noise spectroscopic ellipsometer

Abstract

A spectroscopic ellipsometer comprising a light source ( 1 ) emitting a light beam, a polarizer ( 2 ) placed on the path of the light beam emitted by the light source, a sample support ( 9 ) receiving the light beam output from the polarizer, a polarization analyzer ( 3 ) for passing the beam reflected by the sample to be analyzed, a detection assembly which receives the beam from the analyzer and which comprises a monochromator ( 5 ) and a photodetector ( 4 ), and signal processor means ( 6 ) for processing the signal output from said detection assembly, and including counting electronics ( 13 ). Cooling means ( 12 ) keep the detection assembly at a temperature below ambient temperature, thereby minimizing detector noise so as to remain permanently under minimum photon noise conditions. It is shown that the optimum condition for ellipsometric measurement is obtained by minimizing all of the sources of noise (lamps, detection, ambient).

1. A spectroscopic ellipsometer comprising a light source ( 1 ) emitting a light beam, a polarizer ( 2 ) placed on the path of the light beam emitted by the light source, a sample support ( 9 ) receiving the light beam output from the polarizer, a polarization analyzer ( 3 ) for passing the beam reflected by the sample to be analyzed, a detection assembly which receives the beam from the analyzer and which comprises a monochromator ( 5 ) and a photodetector ( 4 ), and signal processor means ( 6 ) for processing the signal output from said detection assembly, and including counting electronics ( 13 ), the ellipsometer being characterized in that it further comprises cooling means ( 12 ) for keeping the detection assembly at a temperature lower than ambient temperature. 
     
     
       2. An ellipsometer according to  claim 1 , characterized in that said cooling means ( 12 ) are suitable for keeping the detection assembly at a temperature of about −15° C. or lower. 
     
     
       3. An ellipsometer according to  claim 1 , characterized in that the source ( 1 ) is constituted by a deuterium lamp. 
     
     
       4. An ellipsometer according to  claim 3 , characterized in that the lamp has power of about 30 watts. 
     
     
       5. An ellipsometer according to  claim 1  or  claim 2 , characterized in that the source is constituted by a cold plasma lamp. 
     
     
       6. An ellipsometer according to  claim 1  or  claim 2 , characterized in that the source is constituted by a halogen lamp. 
     
     
       7. An ellipsometer according to  claim 1 , characterized in that the counting electronics ( 13 ) is suitable for performing amplitude sampling over a number of channels equal to about 1000 or more. 
     
     
       8. An ellipsometer according to  claim 6 , characterized in that the counting electronics ( 13 ) is suitable for implementing amplitude sampling over a number of channels lying in the range 1024 to 8192. 
     
     
       9. An ellipsometer according to  claim 7  or  claim 8 , characterized in that the processor means ( 6 ) apply Fourier analysis to the signal output by the counting electronics. 
     
     
       10. A spectroscopic ellipsometer comprising:
 a light source configured to emit a light beam,   a polarizer placed on the path of the light beam emitted by the light source,   a sample support configured to support a sample such that the sample is arranged to receive the light beam output from the polarizer,   a polarization analyzer arranged to allow passage of the light beam reflected by the sample,   a detection assembly including a monochromator arranged to receive the light beam from the analyzer and a photodetector arranged to receive an output from the monochromator, and   a signal processor configured to process a signal output from said detection assembly,   wherein the ellipsometer further comprises a cooling apparatus configured to maintain the monochromator and the photodetector at a temperature lower than ambient temperature.   
     
     
       11. The ellipsometer as recited in claim 10, wherein said cooling apparatus is configured to maintain the detection assembly at a temperature of about −15° C. or lower. 
     
     
       12. The ellipsometer as recited in claim 10, wherein the light source is a deuterium lamp. 
     
     
       13. The ellipsometer as recited in claim 12, wherein the lamp has power of about 30 watts. 
     
     
       14. The ellipsometer as recited in claim 10, wherein the light source is a cold plasma lamp. 
     
     
       15. The ellipsometer as recited in claim 10, wherein the light source is a halogen lamp. 
     
     
       16. The ellipsometer as recited in claim 10, wherein the signal processor includes counting electronics, and wherein the counting electronics are configured to perform amplitude sampling over a number of channels equal to about 1000 or more. 
     
     
       17. The ellipsometer as recited in claim 16, wherein the counting electronics are configured to implement amplitude sampling over a number of channels lying in the range 1024 to 8192. 
     
     
       18. The ellipsometer as recited in claim 16, wherein the signal processor is configured to apply Fourier analysis to a signal output by the counting electronics. 
     
     
       19. An ellipsometer comprising:
 a detection assembly including a monochromator arranged to receive a beam of light and a photodetector arranged to receive an output from the monochromator and configured to count photons in the output received from the monochromator;   a cooling apparatus, wherein the cooling apparatus is configured to maintain the detection assembly at a temperature lower than ambient temperature.   
     
     
       20. The ellipsometer as recited in claim 19, wherein the cooling apparatus is configured to maintain the detection assembly at a temperature of approximately −15° C. or lower. 
     
     
       21. The ellipsometer as recited in claim 19, further comprising a light source configured to emit the beam of light. 
     
     
       22. The ellipsometer as recited in claim 21, further comprising a polarizer configured to polarize the beam of light. 
     
     
       23. The ellipsometer as recited in claim 22, wherein the polarized beam of light is reflected from a sample material through a polarization analyzer to be received by said detection assembly. 
     
     
       24. The ellipsometer as recited in claim 23, wherein said ellipsometer further comprises a pair of mirrors arranged to focus the beam of light to be received by said detection assembly to an inlet of the monochromator. 
     
     
       25. The ellipsometer as recited in claim 24, wherein the photodetector is configured to count photons. 
     
     
       26. A method of reducing detector noise in an ellipsometer, the method comprising:
 cooling a detection assembly within the ellipsometer so as to maintain the detection assembly at a temperature lower than ambient temperature, the detection assembly including a monochromator and a photodetector coupled to receive an output from the monochromator; and   analyzing optical properties of a sample material using an output signal of the detection assembly of the ellipsometer.   
     
     
       27. The method as recited in claim 26 further comprising maintaining the detection assembly at or below a predetermined temperature. 
     
     
       28. The method as recited in claim 27, wherein the predetermined temperature is −15° C. 
     
     
       29. The method as recited in claim 26, further comprising:
 projecting a beam of light from a light source;   reflecting the projected beam of light off the sample material;   the monochromator receiving the reflected beam of light.   
     
     
       30. The method as recited in claim 29, further comprising polarizing the beam of light prior to said reflecting. 
     
     
       31. The method as recited in claim 30, further comprising using mirrors to focus the reflected beam of light prior to said receiving.