Abstract:
A multilayer polarization sensor (MPS) for measuring the polarization of radiation in the x-ray and extreme UV wavelength regions. The MPS includes a silicon photodiode with a multilayer (e.g. 50 bilayers) interference coating. The interference coating selectively transmits the orthogonal (p) polarization component in the desired wavelength to generate a current. The (s) polarization component is transmitted through a second interference coating to generate another current. The ratio of the difference between the currents to sum of the currents is the measure of polarization of the incident radiation. Radiation outside the desired wavelength can be dispersed out of the incident beam by a transmission or reflection grating.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the measurement of polarization of x-ray and extreme ultraviolet (EUV) radiation. More particularly it relates to a system and method for measuring polarization that can operate over any x-ray and EUV wavelength range where transmissive and reflective multilayer interference coatings can function.  
       BACKGROUND OF THE INVENTION  
       [0002]     The standard technique for measuring the polarization of x-ray and extreme ultraviolet (EUV) radiation is to measure the intensity of the radiation reflected from a mirror at an angle of incidence of 45 degrees. The mirror reflects the component of the radiation with the electric field vector perpendicular to the plane of incidence, the s polarization component. The orthogonal p polarization component is absorbed by the mirror and is not reflected for measurement. The limitation of this technique is that the reflectance of all materials at 45 degrees incidence is very low in the x-ray and EUV regions and decreases drastically with decreasing wavelength. Thus the 45 degree reflection technique has low sensitivity. In addition, the reflectance is susceptible to surface contamination and oxidation of the mirror that can detrimentally affect the sensitivity and accuracy of the polarization measurement.  
       SUMMARY OF THE INVENTION  
       [0003]     An object of this invention is to provide a device for measuring the polarization of x-ray and extreme ultraviolet radiation.  
         [0004]     Another object of this invention is to provide a polarization measurement device that operates over any x-ray or EUV wavelength range where transmissive and reflective multilayer interference coatings can function.  
         [0005]     Another object of this invention is to provide a polarization measurement device that has increased sensitivity in the x-ray region where reflectance is poor.  
         [0006]     Another object of this invention is to provide a polarization measurement device using multilayer interference coatings to greatly enhance reflectance and transmittance compared to bilayer absorption coatings.  
         [0007]     Another object of this invention is to provide a polarization measurement device in which the polarization efficiency of the MPS is essentially 100% within the wavelength range covered by the multilayer interference coating.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows a schematic of the multilayer polarization sensor  FIG. 2   a  shows the current of bilayer coatings on photodiodes, the current recorded by the uncoated photodiode.  
         [0009]      FIG. 2   b  shows the transmittance of a coated photodiode with Fe/Al  
         [0010]      FIG. 2   c  shows the transmittance of a coated photodiode with Mn/Al  
         [0011]      FIG. 2   d  shows the transmittance of a coated photodiode with V/Al  
         [0012]      FIG. 2   e  shows the transmittance of a coated photodiode with Ti/C  
         [0013]      FIG. 2   f  shows the transmittance of a coated photodiode with Pd/Ti  
         [0014]      FIG. 3   a  shows the reflectance of a multilayer polarization sensor  
         [0015]      FIG. 3   b  shows the absorptance of a multilayer polarization sensor  
         [0016]      FIG. 3   c  shows the transmittance of a multilayer polarization sensor  
         [0017]      FIG. 3   d  shows the polarization of a multilayer polarization sensor 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     In the preferred embodiment, a multiple layer polarization sensor as shown in  FIG. 1  includes a silicon photodiode  100  with a multilayer interference coating  110  and bonding wires leading to two electrodes  150 . The silicon photodiode consists of a silicon diode that is sensitive to x-ray and EUV radiation. The multilayer interference coating is deposited onto the surface of the silicon diode using standard vacuum deposition or magnetron sputtering techniques. As x-rays or EUV radiation of less than 0.25 microwatts  120  are directed at the multilayer interference coating  110  at an angle of incidence of approximately 45 degrees, the multilayer interference coating  110  reflects the s polarization component of the incident radiation  130  and transmits the p polarization component  140 . As the p polarization component is selectively transmitted through the MIC and is deposited in the underlying silicon photodiode  100 , the photodiode generates a current that is recorded by connecting the two electrode pins  150  to a standard current measuring device, e.g. a Keithley Model 617 Electrometer  160  which measures current between 1 pA and 1 microA.  
         [0019]     A second multiple layer polarization sensor is also shown in  FIG. 1  and includes a silicon photodiode  105  with a multilayer interference coating  115  and bonding wires leading to two electrodes  155 . The silicon photodiode consists of a silicon diode that is sensitive to x-ray and EUV radiation. The multilayer interference coating is deposited onto the surface of the silicon diode using standard vacuum deposition or magnetron sputtering techniques. As x-rays or EUV radiation of less than 0.25 microwatts are reflected from the first multilayer interference coating and are directed at the second multilayer interference coating  115  at an angle of incidence of approximately 45 degrees, the multilayer interference coating  115  reflects the s′ polarization component of the incident radiation  135  and transmits the p′ polarization component  145 . As the p′ polarization component is selectively transmitted through the MIC and is deposited in the underlying silicon photodiode  105  , the photodiode generates a current that is recorded by connecting the two electrode pins  155  to a second Keithley Model 617 Electrometer  160  which measures current between 1 pA and 1 microA.  
         [0020]     The polarization is determined by using the readouts of the two electrometers by dividing the difference in the two readouts by the sum of the two readouts. For example a readout of 10 pA in electrometer  160  and a readout of 5 pA in electrometer  165  would derive the following polarization: 
 
(10−5)/(10+5)=5/15=0.33 or 33 percent p polarization. 
 
         [0021]     In this manner, the one multilayer polarization sensor senses the p polarized incident radiation and a second multilayer polarization sensor senses the s polarized incident radiation.  
         [0022]     A multiple layer beyond a bilayer coating is the preferred embodiment for this invention since single bilayer coatings shown in  FIGS. 2   a - 2   f  were not found to be effective. These coatings transmitted both polarization components and therefore had no polarization sensitivity.  FIG. 2   a  shows the transmittances of bilayer coatings on photodiodes, the current recorded by the uncoated photodiode.  FIG. 2   b  shows the transmittance of a coated photodiode with Fe/Al.  FIG. 2   c  shows the transmittance of a coated photodiode with Mn/Al.  FIG. 2   d  shows the transmittance of a coated photodiode with V/Al.  FIG. 2   e  shows the transmittance of a coated photodiode with Ti/C.  FIG. 2   f  shows the transmittance of a coated photodiode with Pd/Ti. The wavelength bandpasses were determined by the absorption of the incident radiations in the layers. In contrast, a mulitple bilayer interference coating has greatly enhanced reflectance and transmittance compared to single bilayer absorption coatings. The thickness of the individual layers has been selected to optimize high reflectance of the undesired polarization component and high transmittance of the desired polarization component. In the preferred embodiment, the thickness of the Mo layers are approximately 2.4 to 3.2 nm and the thickness of the Si layers are approximately 5.8 to 7 nm.  
         [0023]     The performance of the multilayer polarization sensor is shown in  FIGS. 3   a - 3   d . For the qualities of reflectance, absorption, transmittance, and polarization, the sensor was constructed from 50 bilayers of Mo and Si with layer thicknesses optimized to reflect s polarized radiation at an angle of 45 degrees and a wavelength of 13.2 nm. The graphs of  FIGS. 4   a - 4   d  show the qualtities for p polarized radiation (p), s polarized radiation (s), and unpolarized radiation (u). The number of bilayers has to be large enough to reflect the unmeasured polarization and small enough to transmit the desired polarization. The reflectance of the s component and p component are shown in  FIG. 3   a . The absorptance of the s component and p component are shown in  FIG. 3   b . The transmittance of the s and p components is shown in  FIG. 3   c . As shown in  FIG. 3   d , the polarization efficiency of the multilayer polarization sensor is essentially 100% within the wavelength range covered by the multilayer polarization sensor, at wavelength 13.2 nm the polarization graph is 100%.  
         [0024]     For wavelengths greater than 13.2 nm and less than 100 nm, the absorption is higher and the number of bilayers required is smaller so that 20 bilayers will be preferable. For wavelengths less than 13.2 nm and greater than 12.5 nm, absorption is lower and the number of bilayers required is greater so that 60 bilayers will be preferable. Because the polarization performance is lower outside the wavelength range covered by the multilayer polarization sensor, the radiation must be dispersed so that only wavelengths within the multilayer interference coating coverage are incident on the multilayer polarization sensor. This dispersion of radiation may be accomplished by using a transmission or reflection grating. Transmission gratings are routinely used to disperse EUV and x-ray radiation from laboratory, solar, and astrophysical radiation sources.  
         [0025]     An advantage of the multilayer polarization sensor is that this device operates in transmission with performance that is greatly enhanced by the multilayer interference coating. In addition the performance of the multilayer polarization sensor is less susceptible to surface contamination and oxidation because the transmission of the p polarization component, the sensed conponent, is a bulk process rather than a surface process as is reflection.  
         [0026]     The present invention as tested in  FIGS. 3   a - 3   d  has been evaluated over the wavelength range of 3 nm to 100 nm. The preferred embodiment for multilayer materials includes Mo and Si, but may also include other material combinations in common usage such as W/C, W/B 4 C, Ir/Si, Sc/Si, Mo/Be, and Mo/Y.  
         [0027]     Although this invention has been described in relation to an exemplary embodiment thereof, it will be understood by those skilled in the art that still other variations and modifications can be affected in the preferred embodiment without detracting from the scope and spirit of the invention as described in the claims: