Patent Publication Number: US-9899197-B2

Title: Hybrid extreme ultraviolet imaging spectrometer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/212,857, filed Sep. 1, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with United States Government support from the National Institute of Standards and Technology. The Government has certain rights in the invention. 
    
    
     BRIEF DESCRIPTION 
     Disclosed is a hybrid extreme ultraviolet (EUV) imaging spectrometer comprising: a radiation source to: produce EUV radiation; subject a sample to the EUV radiation; photoionize a plurality of atoms of the sample; and form photoions from the atoms subject to photoionization by the EUV radiation, the photoions being radiatively desorbed from the sample in response to the sample being subjected to the EUV radiation in a presence of an external electric field; an ion detector to detect the photoions: as a function of a time-of-arrival of the photoions at the ion detector after the sample is subjected to the EUV radiation in the presence of an external electric field; or as a function of a position of the photoions at the ion detector; an electron source to: produce a plurality of primary electrons; subject the sample to the primary electrons; and form scattered electrons from the sample in response to the sample being subjected to the primary electrons; and an electron detector to detect the scattered electrons: as a function of a time-of-arrival of the scattered electrons at the electron detector after the sample is subjected to the EUV radiation or the primary electrons; or as a function of a position of the scattered electrons at the electron detector. 
     Also disclosed is a process for performing hybrid extreme ultraviolet (EUV) imaging of a sample, the process comprising: producing, by a radiation source, EUV radiation; subjecting the sample to the EUV radiation; photoionizing a plurality of atoms of the sample; forming photoions from the atoms subject to photoionization by the EUV radiation; desorbing the photoions from the sample in response to the sample being subjected to the EUV radiation in the presence of an external electric field; detecting, by an ion detector, the photoions: as a function of a time-of-arrival of the photoions at the ion detector after the sample is subjected to the EUV radiation; or as a function of a position of the photoions at the ion detector; producing, by an electron source, a plurality of primary electrons; subjecting the sample to the primary electrons; forming scattered electrons from the sample in response to the sample being subjected to the primary electrons; detecting, by an electron detector, the scattered electrons: as a function of a time-of-arrival of the scattered electrons at the electron detector after the sample is subjected to the EUV radiation or the primary electrons; or as a function of a position of the scattered electrons at the electron detector; and acquiring, by an analyzer, data from the ion detector and the electron detector to image the sample. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike. 
         FIG. 1  shows a hybrid extreme ultraviolet (EUV) imaging spectrometer; 
         FIG. 2  shows a hybrid extreme ultraviolet imaging spectrometer; 
         FIG. 3  shows a hybrid extreme ultraviolet imaging spectrometer; 
         FIG. 4  shows a hybrid extreme ultraviolet imaging spectrometer; 
         FIG. 5  shows production of photoions and electron images by subjecting a sample to primary electrons and EUV radiation; 
         FIG. 6  shows a radiation source; 
         FIG. 7  shows an electron micrograph of a sample; 
         FIG. 8  shows an electron micrograph of the sample shown in  FIG. 7  after the sample was subjected to pulsed EUV radiation in a presence of an external electric field; 
         FIG. 9  shows an extraction electrode in perspective view, an end view, and a cross-sectional view; 
         FIG. 10  shows a plurality of extraction electrodes; 
         FIG. 11  shows an analyzer; and 
         FIG. 12  shows (panel A) a graph of voltage versus time, (panel B) electron diffraction images, (panel C) ion time-of-flight (TOF) mass spectra of photoions, and (panel D) reconstruction of a sample. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments is presented herein by way of exemplification and not limitation. 
     It has been discovered that a hybrid extreme ultraviolet (EUV) imaging spectrometer herein acquires quantitative three-dimensional chemical maps of a sample that can include hard matter or soft matter. Advantageously, the hybrid EUV imaging spectrometer has sub-nanometer spatial resolution for an elemental constituent of a sample subjected to EUV radiation and primary electrons. 
     In an embodiment, with reference to  FIG. 1 , hybrid extreme ultraviolet (EUV) imaging spectrometer  100  includes radiation source  102  to produce EUV radiation  104  and is configured to subject sample  106  to pulsed EUV radiation  104 , to photoionize a plurality of atoms of sample  106 , and to form photoions  112  from the atoms subject to single photoionization by EUV radiation  104 . Photoions  112  are radiatively desorbed from sample  106  in response to sample  106  being subjected to EUV radiation  104 . Also, hybrid EUV imaging spectrometer  100  includes ion detector  114  to detect photoions  112  as a function of a time-of-arrival of photoions  112  at ion detector  114  after sample  106  is subjected to EUV radiation  104 , or as a function of a position of photoions  112  at ion detector  114 . Electron source  108  produces a plurality of primary electrons  110  to subject sample  106  to primary electrons  110  and to form scattered electrons ( 116  or  120 ) from sample  106  in response to sample  106  being subjected to primary electrons  110 . Electron detector ( 118  or  122 ) detects scattered electrons ( 116  or  120 , respectively) as a function of a time-of-arrival of scattered electrons ( 116  or  120 , respectively) at electron detector ( 118  or  122 , respectively) after sample  106  is subjected to EUV radiation  104  or primary electrons  110 , or as a function of a position of scattered electrons ( 116  or  120 ) at electron detector ( 118  or  122 ). 
     As used herein, “radiatively desorbed” (and variants thereof, e.g., radiative desorption and the like) refers to photoions removed from sample  106  in response to sample  106  being subjected to EUV radiation  104  in a presence of an electric field provided by extraction electrode  150  or an external electric field. It should be appreciated that ionization of atoms of sample  106  and desorption of such photoions in a presence of the electric field provided by extraction electrode  150  or an external electric field can occur when using lower energy, a longer wavelength radiation (e.g., ultraviolet or visible radiation) as compared to EUV radiation  104 . This lower energy ionization process can produce multiply charged photoions and local heating of a sample. Subjecting sample  106  to EUV radiation  104  produces photoions  112  that are substantially radiatively desorbed from sample  106 . Without wishing to be bound by theory, it is believed that a contribution to desorption of photoions  112  from sample  106  occurs in a presence of EUV radiation  104  due to radiative desorption in an absence of thermal desorption and in an absence of production of multiply charged photoions, wherein photoions  112  are singly charged, e.g., (Ni − , C + , H 2 O + , C 2 H 3   + , and the like). 
     In an embodiment, with reference to  FIG. 2 , hybrid EUV imaging spectrometer  100  includes chamber  126  (e.g., a vacuum chamber) in which is disposed sample  106 . Ion optics  150  (e.g., an ion extraction electrode) can be interposed between radiation source  102  and ion detector  114 , specifically interposed between sample  106  and ion detector  114  to extract photoions  112  released from sample  106  and to communicate photoions  112  to ion detector  114 . According to an embodiment, extraction electrode  150  is disposed proximate to sample  106  and interposed between sample  106  and ion detector  114 , wherein extraction electrode  150  includes an aperture to transmit photoions  112  from sample  106  to ion detector  114 . 
     Coupler  124  is interposed between radiation source  102  and chamber  126  to optically coupled radiation source  102  to sample  106  disposed in chamber  126 . In this manner, EUV radiation  104  can be produced and transmitted without negatively impacting affluence of EUV radiation  104  such as by absorption of atmospheric gases before EUV radiation  104  interacts with sample  106 . Here, chamber  126  provides a platform to dispose and arrange sample  106  relative to radiation source  102 , electron source  108 , and detectors  114 ,  118 ,  122 . Further, chamber  126  provides a selected environmental condition for sample  106 , EUV radiation  104 , primary electrons  110 , photoions  112 , and scattered electrons  116 ,  120 . The selected environmental condition can include temperature, pressure, gas composition, ultra-high vacuum, and the like. 
     In an embodiment, with reference to  FIG. 3 , hybrid EUV imaging spectrometer  100  includes EUV optic  128  disposed in chamber  126  to communicate and to selectively direct EUV radiation  104  from a radiation source  102  to sample  106 . Stage  152  disposed in chamber  126  receives sample  106 , wherein sample  106  can be mounted on stage  152 . A position of sample  106  relative to components (e.g., EUV optic  128  and ion optics  150 ) of hybrid EUV imaging spectrometer  100  can be controlled statically or dynamically by stage  152 . 
     According to an embodiment, with reference to  FIG. 4 , hybrid EUV imaging spectrometer  100  includes analyzer  131  to acquire data from ion detector  114  and electron detectors ( 118 ,  122 ), to continuously analyze the data for reconstruction of a shape of sample  106  and a chemical composition of sample  106 , and to determine a tomographic shape and composition of sample  106 . Here, analyzer  131  can be in electrical communication with radiation source  102  to receive EUV source data  132  (e.g., a pulse length, duty cycle, power, repetition rate, wavelength, and the like of EUV radiation  104 ) or to control radiation source  102 . Likewise, analyzer  131  can be in electrical communication with electron detector  118  to receive electron data  134  (e.g., a time-of-arrival, diffraction pattern, position of arrival on detector  118 , transmission electron micrograph, energy, and the like of scattered electrons  116 ) or control operation of electron detector  118  (e.g., bias voltage, capture time, and the like). Analyzer  131  can be in electrical communication with electron detector  122  to receive electron data  138  (e.g., a time-of-arrival, position of arrival on detector  118 , energy, scanning electron micrograph, and the like of scattered electrons  116 ) or to control electron detector  122 . Analyzer  131  can be in electrical communication with electron source  108  to receive electron source data  140  (e.g., energy, spatial distribution, flux, and the like of primary electrons  110 ) or to control electron source  108 . Analyzer  131  can be in electrical communication with ion detector  114  to receive ion data  136  (e.g., a time-of-arrival, position of arrival on detector  114 , energy, two-dimensional image, and the like of photoions  112 ) or to control ion detector  114  (e.g., detection on time, bias voltage, and the like). Analyzer  131  can be in electrical communication with sample  106  to receive sample data  160  (e.g., temperature, voltage, and the like of sample  106 ) or to control sample  106  (or stage  152 ) (e.g., position, bias voltage, and the like). Similarly, analyzer  131  can be in electrical communication with extraction electrode  150  to receive extraction electrode data  161  (e.g., voltage and the like of extraction electrode  150 ) or to control extraction electrode  150  (e.g., bias voltage and the like). 
     Although not shown explicitly in  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4 , or  FIG. 5 , hybrid EUV imaging spectrometer  100  can include hardware, instrumentation or software for a fully functional atom probe tomograph. It is contemplated that hardware can include high voltage power supply, timing electronics, and position-sensitive 2D detectors, sample shape and composition reconstruction algorithms, and the like that can be integrated with EUV radiation source  102 , EUV optic  128 , and other components (e.g., detectors ( 122 ,  118 ,  114 ) of hybrid EUV imaging spectrometer  100  to provide operability of hybrid EUV imaging spectrometer  100  as shown in  FIG. 5 . Here, sample  106  having tip  180  is subjected to pulsed EUV radiation  104  such that photoions  112  are produced from tip  180  and propagate from sample  106  two ion detector  114  along ion trajectory  113 . Photoions  112  are communicated through aperture  153  to traverse extraction electrode  150  and impact ion detector  114  (e.g., a two-dimensional particle detector) in a pattern, wherein the position of photoions  112  in the pattern on ion detector  114  depends upon a position of the atoms that were photoionized to produce photoions  112  at tip  180  of sample  106 . Photoions  112  propagating in ion trajectories  113  move in ion time-of-flight mass spectrometry direction  186  from sample  106  to ion detector  114 . 
     Moreover, primary electrons  110  impinge upon tip  180  of sample  106  to produce scattered electrons  116  that arrive at electron detector  118  in electron diffraction pattern  184 . Also, due to primary electrons  110  impinging upon  180  of sample  106 , scattered electrons are detected by electron detector  122  as electron micrograph  182 . It will be appreciated that production of photoions  112  occurs in a presence of EUV radiation  104  and in an absence of primary electrons  110 . It further will be appreciated that production of scattered electrons ( 116 ,  122 ) occurs in a presence of primary electrons  110  and in an absence of EUV radiation  104  or in an absence of an electrical field between sample  106  and extraction electrode  150 . 
     According to an embodiment, hybrid EUV imaging spectrometer  100  includes sample  106  disposed on stage  152  that is disposed in chamber  126 . In a certain embodiment, pulsed EUV radiation  104  is from radiation source  102  enters chamber  126  through coupler  124 . Pulsed EUV radiation  104  is directed and focused by EUV optics  128  disposed in chamber  126  and is incident upon sample  106 . In a certain embodiment, stage  152  adjusts a position of sample  106  with respect to extraction electrode  150 . A voltage difference (produced by a bias voltage applied to extraction electrode  150  or sample  106 ) between extraction electrode  150  and sample  106  provides an electric field therebetween to accelerate and to remove photoions  112  produced in response to EUV radiation  104  of tip  180  of sample  106 . Photoions  112  traverse aperture  152  of extraction electrode  150  and impact ion detector  114 , e.g., a two-dimensional position-sensitive. 
     In an embodiment, voltage difference between extraction electrode  150  and sample  106  is selectively adjustable to control a rate of impact of photoions  112  on ion detector  114 . A position or focus of EUV radiation  104  incident on sample  106  is selectively adjustable to produce EUV radiation-assisted field evaporation of photoions  112  from sample  106 . Sample  106  is irradiated by primary electrons  110  produced by electron source  108 . Scattered electrons  116  transmitted or diffracted by sample  106  are detected by electron detector  118 . Secondary or backscattered electrons  120  emitted from tip  180  of sample  106  are detected by electron detector  122 . An interior of chamber  126  has a pressure that can be ultra-high-vacuum provided, e.g., by a vacuum pumping system. The vacuum pumping system can include a vacuum pump such as a titanium sublimation pump, cryogenic, helium refrigerator, diffusion pump, turbo molecular pump, and the like. Optical camera  130  monitors a position or condition of components disposed in chamber  126 . 
     Hybrid EUV imaging spectrometer  100  includes radiation source  102  to produce EUV radiation  104  for photoionization of atoms of sample  106  to form photoions  112 . In an embodiment, radiation source  102  can include a pulsed laser and a gas cell, wherein the gas cell receives a gas that is subjected to pulsed laser light from the pulsed laser to produce EUV radiation  104 . Here, EUV radiation  104  can be produced as pulsed light by directing the pulsed laser light into the gas cell that includes a gas such as Kr, Xe, Ar, He, Ne, or a combination thereof. Without wishing to be bound by theory, it is believed that EUV radiation  104  is produced by high harmonic generation of the pulsed laser light by the gas in the gas cell. In an embodiment, pulsed EUV radiation  104  is provided by a high voltage electrical discharge of a gas in a capillary cell. In some embodiments, pulsed EUV radiation  104  is provided by synchrotron radiation. In a particular embodiment, pulsed EUV radiation  104  is produced by a free electron laser. 
     According to an embodiment, with reference to  FIG. 6 , radiation source  102  includes pump module  204  to produce pump light  206 . Pump module  204  can include host laser  208  that produces host laser light  210  that is communicated from host laser  208  to frequency doubler  212 . Pump module  204  receives diode light from laser diode  214 , and frequency doubler  212  doubles host laser light  210  (i.e., doubles the energy or halves the wavelength via frequency doubling, e.g., in an optical crystal). Pump light  206  is received by mode-locked laser  218  from pump module  204 , wherein mode-locked laser  218  produces laser output  220  that is received by amplifier  226 . Amplifier  226  also receives amplifier light  222  from amplifier pump  224  to amplify laser output  220  into pulsed light  228 . Pulsed light  228  is communicated from amplifier  226  to high harmonic generator  200 . High harmonic generator  200  receives pulsed light  228  and produces EUV radiation  124 , which is pulsed. 
     Here, laser diode  214  can be a laser diode array that produces diode light  216  with an output power and wavelength (e.g., 800 nanometer (nm) continuous wave (CW) radiation with a selected power sufficient to pump host laser  208 . Host laser  208  can be, e.g., neodymium (Nd)-doped yttrium aluminum garnet (YAG) laser or other laser that produces host laser light  210  that is CW and has a wavelength at about 1000 nm, although a different laser would provide a different wavelength. Frequency doubler  212  can produce doubled pump light  206 , e.g., having CW output and a wavelength of about 500 nm, depending on the wavelength of host laser light  210 . Mode-locked laser  218  can be a passively mode-locked laser such as a titanium sapphire (Ti:sapphire) laser to produce laser output  220  having a wavelength, e.g., of about 800 nm with a repetition rate of about 80 MHz. 
     The resulting output of mode-locked laser  218  is communicated to amplifier  226 , which can be a Ti:sapphire regenerative amplifier, to produce pulsed light  228  at a wavelength of about 800 nm at a selected frequency, e.g., about 10 kiloHertz (kHz) with a selected pulse energy, e.g., of about 0.5 milliJoules (mJ)/pulse. Pulsed light  228  can have a selected pulse width such as 35 femtoseconds (fs). EUV flux from high harmonic generator  200  can be, e.g., 45 eV photons emerging as EUV radiation  104  at a rate of 10 12  photons/pulse with, e.g., 0.72 nJ/pulse. Exemplary high harmonic generation for various gases in high harmonic generator  200  to produce EUV radiation  104  are listed in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 EUV energy 
                 Wavelength 
               
               
                   
                 Gas 
                 (electron volts (eV)) 
                 (nm) 
               
               
                   
                   
               
             
            
               
                   
                 Combination of 
                 10-30 
                 124-41  
               
               
                   
                 krypton and xenon 
               
               
                   
                 Argon 
                 35-45 
                 35-26 
               
               
                   
                 Neon 
                  40-100 
                 31-12 
               
               
                   
                 Helium 
                 100 
                 12 
               
               
                   
                   
               
            
           
         
       
     
     In an embodiment, coupler  124  receives pulsed EUV radiation  104  from high harmonic generator  200  of radiation source  102  and communicates pulsed EUV radiation  104  chamber  126 . Here, an interior of coupler  124  has a pressure that is compatible with propagation of EUV radiation  104  from radiation source  102 , e.g., a high harmonic generator  200 , to chamber  126 . An exemplary pressure of chamber  126  is ultrahigh vacuum. An optical interface between radiation source  102  and coupler  124  can be an EUV optical window (e.g., a metal foil such as an aluminum foil) to transmit EUV radiation  104  therethrough and to transmit EUV radiation  104  into chamber  126 . 
     Disposed in chamber  126  is EUV optic  128  to receive EUV radiation  104  from coupler  124 . EUV optics  128  can include mirror  129  and concave reflector  133 . Mirror  129  can be a flat mirror, curved EUV mirror, and the like or a combination thereof with concave reflector  133  can have a selected position or angle to adjust EUV radiation  104  with respect to sample  106 . It is contemplated that EUV optics  128  (e.g., mirror  129 , concave reflector  133 , and the like) can focus EUV radiation  104  onto sample  106 . In an embodiment, EUV optic  128  is a zone plate to focus EUV radiation  104  on sample  106 . 
     Radiation source  102  can operate at a selected wavelength from 12 nm to 124 nm. The EUV wavelength band provides nearly uniform optical absorption across atoms in sample  106  with absorption depth in sample  106 , e.g., of 10 nm. The absorption depth is selectively tunable and can depend upon the wavelength of EUV radiation  104 . Moreover, the wavelength of EUV radiation  104  provides efficient photoionization of atoms and photo-disassociation of complex ions. EUV radiation  104  substantially photoionizes elemental species of atoms at tip  180  of sample  106  (see, e.g., a micrograph of sample  106  shown in  FIG. 7 ) and contributes to additional photoionization and disassociation of molecular complexes. As used herein, “EUV light” and “EUV radiation” are identical. 
     Unexpectedly, surprisingly, and without wishing to be bound by theory, it is believed that a photoionization pathway for field evaporation of ions of sample  106  occurs through photoionization in a presence of EUV radiation  104  and is significantly different than irradiation of sample  106  with a UV wavelength. In conventional use of UV radiation, e.g., in an atom probe microscope, a UV wavelength from 255 nm to 355 nm (corresponding to photon energy from 4.9 eV to 3.5 eV) is less than a photoionization threshold for inorganic solid materials such that substantial photoionization does not occur. In contrast, hybrid EUV imaging spectrometer  100  produces EUV radiation  104  with a selected wavelength, e.g., from 12 nm to 124 nm to provide an EUV photon energy, e.g., from 10 eV to 100 eV. In this EUV band, photoionization cross-sections are large, and EUV radiation  104  can singly photoionize or multiply ionize atoms in sample  106  that can include a material of technological interest. EUV radiation  104  can have a substantially uniform optical absorption depth in sample  106 , e.g., 10 nm in sample  106 , e.g., a metal, plastic, ceramic, polymer, glass, semiconductor, insulator, conductor, hybrid materials composed of such constituents, and the like. 
     Further, conventional laser atom probes involve thermally-assisted field ion evaporation, wherein field ion evaporation provides removal of atoms from a sample by applying a high electric field (e.g., 10 volts (V)/nm to 60 V/nm) in a presence of UV light that impinges on a sample. In contrast hybrid EUV imaging spectrometer  100  includes EUV radiation  104  that photoionizes atoms of sample  106 . Instead, hybrid EUV imaging spectrometer  100  uses EUV radiation  104  to provide photoionization-assisted field ion evaporation to produce photoions  112  from sample  106  that are produced by electronic excitation. That is, with UV radiation (e.g., 255 nm to 355 nm UV light), thermal pulsing dominates, and significant photoionization does not occur. EUV radiation  104  provides photoionization of constituent atoms of sample  106 . 
     Although photoionization cross-sections can vary among elements of the periodic table, hybrid EUV imaging spectrometer  100  with EUV radiation  104  reduces an element-to-element variation in production of photoions  112  compared to conventional atom probe tomography, which employs UV lasers. Element-to-element differences in the field evaporation rate for a conventional pulsed-laser atom probe, in which absorbed pulsed laser light imparts transient increases in temperature of the specimen tip and involves a difference in a zero barrier evaporation field F. Unexpectedly and surprisingly, element-to-element differences in the evaporation rate in hybrid EUV imaging spectrometer  100  with pulsed EUV radiation  104  are due to differences in photoionization cross-section because surface species that have been photoionized will generally experience reduced bonding strength to the specimen tip. In thermally assisted pulsing with UV light, the evaporation rate is exponentially dependent on both F and temperature such that changes in applied field F lead to changes in evaporation rate with higher temperature thermal transients yielding greater evaporation rates. Therefore, materials or portions of samples that have relatively reduced UV absorption will yield relatively reduced evaporation rates. For pulsed EUV radiation  104 , the contribution to the evaporation rate that is dependent on photoionization is unexpectedly and surprisingly linearly dependent on σ such that element-to-element differences in σ have less influence on the evaporation rate. Additionally, the EUV absorption depth d is unexpectedly and surprisingly comparable or smaller than typical specimen tip diameters for a wide variety of materials of interest, e.g. for 45 eV photons: d=200 nm for Si; d=35 nm for GaAs; d=15 nm for GaN; d=20 nm for Fe; d=35 nm for SiN; d=20 nm for SiO 2 ; d=12 nm for Al 2 O 3 ; d=4 nm for TiO 2 —and so on. Therefore, the thermally-assisted pathway for field evaporation provided by pulsed EUV will generally be far more uniform compared to what is attainable with pulsed UV. Moreover, EUV radiation  104  produces substantially fewer complex ions than UV light, wherein a sample subjected to UV light can produce molecular ions due to thermally-assisted process such as surface diffusion on the sample. EUV radiation  104  provides photoionization and photodissociation of complex ions formed from subjecting sample  106  to EUV radiation  104 . As a result, formation of neutral species or complex ions such as molecular ions from sample  106  are substantially absent in a presence of EUV radiation  104  received by sample  106 . 
     Sample  106  is subjected to EUV radiation  104  and primary electrons  110 . Photoions  112  are produced from photoionization of atoms of sample  106 . Scattered electrons  116  and scattered electrons  120  produced by transmission, diffraction, secondary mission, and the like of electrons in response to subjecting sample  106  to primary electrons  110 . Sample  106  can include any material subject to photoionization by EUV radiation  104 . Exemplary materials include polymers, glasses, other covalently bonded materials, and the like. Sample  106  can have any shape, e.g., a conical shape, spherical, planar, and the like. 
     In an embodiment, with reference to  FIG. 7 , sample  106  include tip  180  and body  181 . As atoms of tip  100  are removed by photoionization under irradiation by EUV radiation  104  in a presence of an external field present between sample  106  and extraction electrode  150 , constituent atoms comprising tip  180  are removed from body  181  as shown in  FIG. 8 . It is contemplated that a composition of sample  106  can change in tip  180  or body  181 . According to an embodiment, sample  106  is prepared by annular milling in the focused ion beam microscope, and sample  106  has axial symmetry. The axial symmetry of tip  180  can be confirmed by of electron tomography. 
     Stage  152  can provide control of a position of sample  106  in chamber  126 , in particular with relation to extraction electrode  150 . Further, stage  152  can control a temperature of sample  106 , remove sample  106  from chamber  126 , insert sample  106  into chamber  126  without substantially changing the vacuum level (e.g., the ultrahigh vacuum) of chamber  126 , or a combination thereof. Chamber  126  can include additional components such as a load-lock chamber, transfer arm, and the like to manipulate (e.g., move or dispose) extraction electrode  150 , sample  106 , and the like in chamber  126 . 
     Ion optics, e.g., extraction electrode  150 , are disposed proximate to sample  106  and interposed between sample  106  and ion detector  114 . In an embodiment, with reference to  FIG. 9  (perspective view, in view, and cross-section along line A-A), ion optics includes extraction electrode  150 . Ion optic  150  includes first end  156  disposed proximate to sample  106  and distal to ion detector  114 , second end  158  disposed proximate to ion detector  114  and distal to sample  106 , entry aperture  153  through which photoions  112  are transmitted and traverse extraction electrode  150  in electrode interior  164  bounded by wall  154  and exit extraction electrode  150  at exit aperture  152 . In an embodiment, the ion optics can include a reflectron disposed to receive photoions  112  transmitted by extraction electrode  150  from sample  106  and to communicate photoions to ion detector by reflecting the photoions in a presence of an electric field in the reflectron. 
     A size and shape of the of extraction electrode and ion optics  150  can be selected to communicate photoions  112  from sample  106  two ion detector  114 . As shown in  FIG. 10 , extraction electrode  150  can be an annular first cone, annulus, stacked annulus, gridded and the like. The annular first to cone form of extraction electrode  150  is shown in both  FIG. 9  and  FIG. 10 . Moreover, as indicated by stacked annulus extraction electrode  150  in  FIG. 10 , a plurality of extraction electrodes  150  can be used in tandem and spaced apart spatially to provide focusing or a selected electric field profile to photoions  112 . In gridded extraction electrode  150 , electrically insulating grid  166  is disposed to cover the entry aperture but provide communication of photoions  112  through extraction electrode  150 . 
     In an embodiment, extraction electrode  150  receives a bias voltage such that a potential difference exists between extraction electrode  150  in sample  106 . Aperture  153  of extraction electrode  150  communicates photoions  112  emitted from sample  106  to ion detector  114 . Extraction electrode  150  can be disposed in or removed from chamber  126  without substantially changing the ultra-high vacuum of chamber  126 . Hybrid EUV imaging spectrometer  100  that includes extraction electrode  150  in combination with sample  106  and ion detector  114  provides acquisition of mass-spectral data and reliable chemical assignments with spatial resolution for mapping materials of sample  106 . 
     Ion detector  114  receives photoions  112  emitted from sample  106 . Here, ion detector  114  records arrival of photoions  114 , a time taken by photoions  112  to traverse a distance between sample  106  and ion detector  114 , a position of arrival on ion detector  114 , or a combination thereof. Exemplary ion detectors  114  is a microchannel plate, an electron multiplier modified to detect cations, a Faraday cup, and the like. Exemplary ion detectors also include a those that can record kinetic energy of detected ions and permit determination of the mass of a detected ion. In an embodiment, ion detector  114  is a two-dimensional position-sensitive detector disposed in chamber  126  to detect or record photoions  112  emitted from sample  106  as a function of time-of-arrival based at ion detector  114  and as a function of a position on ion detector  114 . Ion detector  114  can include ion optics disposed prior to a detecting surface to steer, focus, pulse, mass select, velocity image, and the like photoions  112 . In this manner, ion detector  114  performs time-of-flight mass spectrometry and ion imaging of photoions  112  released from sample  106  in response to sample  106  subjected to EUV radiation  104  that photoionizes atoms of sample  106 . From the time of arrival of photoions  112  at ion detector  114  from sample  106 , a mass of each photoion  112  can be determine based on the potential difference (i.e., electric field strength) present between sample  106  and extraction electrode  150  when photoions  112  are created. EUV radiation  104  singly photoionizes atoms of sample  106  to produce singly charged photoions  112 , and the masses determined from the time-of-flight mass spectrum acquired by ion detector  114  identify the element (i.e., atom) in sample  106  that produced photoions  112 . Multiply charged photoions may yield ambiguous assignments since purely time-of-flight detection schemes are sensitive to the ratio on ionic mass divided by ionic charge—hence, multiply charged heavier species may be indistinguishable from singly charged lighter ones. This ambiguity can be eliminated if the detection scheme records both the ionic time-of-flight and the kinetic energy of the detected ion. In such a scheme, the ionic mass can be unequivocally determined. From the position of photoions  112  incident at ion detector  114 , a position on sample  106  of the atoms that produced photoions  112  can be determined. The mass of the photoions and the position of the atoms on sample  106  are used in part to determine an elemental composition of sample  106  as a function of position on sample  106 . Electron imaging described below in combination with the mass of the photoions in the position of the atoms in sample  106  provide three-dimensional chemical maps of sample  106 . 
     In an embodiment, electron source  108  is in communication with and coupled to chamber  126  to provide primary electrons  110 , e.g., an electron beam, to chamber  126 , wherein primary electrons  110  impinge upon sample  106 . Electron source  108  includes an electron emitter and electron optics. The electron emitter emits electrons that can be subjected to collimation, focusing, energy control (e.g., acceleration, selection, and the like) by the electron optics. Exemplary electron emitters include a field emitter, thermionic emitter, and the like. Primary electrons  110  can have a selected energy, flux, spot size of sample  106  effective to produce scattered electrons ( 116 ,  120 ). The acceleration voltage of primary electrons  110  can be from 0 to 30 kV, e.g., 25 kV. The beam current of primary electrons  110  can be from 5.2 pA to 7.2 nA, with beam current that is, e.g., 20 pA. The spot diameter of primary electrons  110  at sample  106  can be from 4 nm to 100 nm and a typical operational spot diameter is 4 nm. Primary electrons  110  can have a static position at sample  106  or can be scanned (e.g., rastered) over sample  106 . 
     Sample  106  is subjected to primary electrons  110 . In response to primary electrons  110 , scattered electrons ( 116 ,  120 ) are produced. Scattered electrons  116  are received by electron detector  118 . Electron detector  118  can include a transmission electron imaging detector or an electron diffraction detector. Scattered electrons  120  are received by electron detector  122 . Electron detector  122  can be a secondary emission electron detector or an electron backscatter detector. In this manner, in-situ electron imaging of sample  106  is provided by electron source  108 , electron detector  118 , and electron detector  122  in chamber  126  of hybrid EUV imaging spectrometer  100 . As a result, hybrid EUV imaging spectrometer  100  provides near-real-time measurement of a time-varying shape of tip  180  of sample  106  during data acquisition in an absence of dismounting sample  106  or interrupting data acquisition or changing a state of the vacuum in chamber  126 . Accordingly, hybrid EUV imaging spectrometer  100  provides scanning electron microscopy (SEM) and scanning transmission electron microscope (STEM) modes as well as capturing transmission electron diffraction patterns with sample  106  disposed proximate to extraction electrode  150 . Electron imaging and detection with electron detector ( 118 ,  122 ) can occurs in combination with production of photoions  112  from EUV radiation  104  in combination with the external electric field applied between sample  106  and extraction electrode  150 , e.g., by alternating impingement of primary electrons  110  on sample  106  with detection of scattered electrons ( 116 ,  120 ) by electron detector ( 118 ,  122 ) and impingement of EUV radiation  104  on sample  106  with detection of photoions  112  by ion detector  114  and external electric field applied between sample  106  and extraction electrode  150 . 
     In an embodiment, electron detector  118  is disposed on chamber  126  to detect diffraction or imaging of primary electrons  110  impinging on sample  106  from electron source  108  as scattered electrons  116 . 
     In an embodiment, electron detector  122  is disposed on chamber  126  to detect secondary or backscattered electrons emitted by sample  106  as scattered electrons  120  due to primary electrons  110  originating from electron source  108  and impinging on sample  106 . 
     Additionally, hybrid EUV imaging spectrometer  100  includes in-situ real-time electron imaging of scattered electrons ( 116 ,  120 ) to record the shape of tip  180  of sample  106  and uses that information in a reconstruction algorithm to determine the shape and composition of sample  106  as a function of time and space. 
     Electron imaging (e.g., scanning transmission, secondary electron and diffraction) by electron detector ( 118 ,  122 ) to measure the shape of sample  106  at intermediate stages in the data collection process provides the direct, in-situ electron beam imaging of sample  106  to eliminate interpolation of the shape of tip  180  of sample  106  between before and after only images or for ex-situ electron imaging. Beneficially and unexpectedly, hybrid EUV imaging spectrometer  100  provides computational accuracy of three-dimensional (3-D) reconstruction of the shape and composition of sample  106 . 
     Electron data  134  acquired by electron detector  118 , electron data  138  acquired by electron detector  122 , and ion data  136  acquired by ion detector  114  are communicated to analyzer  131 . With reference to  FIG. 11 , analyzer  131  can include control module  143  and data analysis module  141  interconnected and in electrical communication by data path  135 . Control module  143  communicates, controls, monitors, or records functions of and data acquired (e.g., EUV source data  132 , electron data  134 , electron data  138 , electron source data  140 , ion data  136 , sample data  160 , extraction electrode data  161 , pressure of chamber  126 , and the like) by chamber  126 , EUV source  102 , in-chamber inspection camera  130 , electron source  108 , electron detector  118 , electron detector  122 , ion detector  114  using data path  137 . 
     Data accumulated in control module  143  from sample  106  includes position-sensitive time-of-flight mass spectral information (e.g., ion data  136 ) and specimen shape information. These data along with other data are communicated to data analysis module  141  in data path  135 . Data analysis module  141  analyzes this combined data to determine three-dimensional chemical maps of sample  106 . Control module  143  can include hardware (e.g., a microprocessor and the like), software (e.g., an algorithm, script, or other code) to make the determination of the three-dimensional chemical maps of sample  106 . 
     In an embodiment, a process for making hybrid EUV imaging spectrometer  100  includes disposing electron source  108  on chamber  126  to emit primary electrons  110  into chamber  126  to subject sample  106  to primary electrons  110 ; disposing electron detector  118  on chamber  126  to detect electron diffraction or to perform electron imaging of scattered electrons  116  from sample  106  in response to subjecting sample  106  to primary electrons  110  from electron source  108 ; disposing electron detector  122  on chamber  126  to detect secondary electrons or backscattered electrons as scattered electrons  120  emitted from sample  106  in response to subjecting sample  106  to primary electrons  110  from electron source  108 ; disposing radiation source  102 , coupler  124 , and EUV optics  150  on chamber  126  to provide pulsed EUV radiation  104  into chamber  126 , to steer and focus EUV radiation  104 , and to subject sample  106  to EUV radiation  104 ; disposing stage  152  in chamber  126  to receive sample  106 ; disposing extraction electrode  150  and ion detector  114  on chamber  126  such that photoions  114  produced by sample  106  in response to being subjected to EUV radiation  104  and the electric field between sample  106  and extraction electrode  150  are communicated through extraction electrode  150  to ion detector  114  from sample  106  and impact ion detector  114  and a presence a voltage difference between sample  106  and extraction electrode  150 . 
     According to an embodiment, a process for performing hybrid EUV imaging spectrometry of sample  106  includes producing, by radiation source  102 , EUV radiation  104 ; subjecting sample  106  to EUV radiation  104 ; photoionizing a plurality of atoms of sample  104  with EUV radiation  104 ; forming photoions  112  from the atoms subject to single photoionization by EUV radiation  104 ; desorbing photoions  112  from sample  106  in response to sample  106  being subjected to EUV radiation  104  and an electric field between sample  106  and extraction electrode  150 ; detecting, by ion detector  114 , photoions  112 : as a function of a time-of-arrival of photoions  112  at ion detector  114  after sample  106  is subjected to EUV radiation  104 , or as a function of a position of photoions  112  at ion detector  114 ; producing, by electron source  108 , a plurality of primary electrons  110 ; subjecting sample  106  to primary electrons  110 ; forming scattered electrons from sample  106  in response to sample  106  being subjected to primary electrons  110 ; detecting, by the electron detector, the scattered electrons: as a function of a time-of-arrival of the scattered electrons at the electron detector after sample  106  is subjected to EUV radiation  104  or primary electrons  110 , or as a function of a position of the scattered electrons at the electron detector; and acquiring, by analyzer  131 , data from ion detector  114  and the electron detector to image sample  106 . The process can include reconstructing a shape of sample  106 ; and determining a chemical composition of sample  106  as a function of position in sample  106 . The process can include determining a tomographic shape and composition of sample  106 . The process can include transmitting, by extraction electrode  150 , photoions  112  from sample  106  to ion detector  114 , wherein extraction electrode  115  includes aperture  153  to transmit photoions  112 , and extraction electrode  150  is disposed proximate to sample  106  and interposed between sample  106  and ion detector  114 . 
     The process also can include applying a potential difference between sample  106  and extraction electrode  150 ; decreasing the potential difference to detect the scattered electrons; and increasing the potential difference to detect photoions  112 . Sample  106  can be subjected to EUV radiation  104  after the potential difference is increased to form photoions  112 ; and detecting, by ion detector  114 , photoions  112  includes performing time-of-flight mass spectrometry on photoions  112 . 
     According to an embodiment, a process for performing hybrid EUV imaging spectrometry on sample  106  includes disposing sample  106  on stage  152 ; disposed stage  152  and sample  106  in chamber  126 ; applying a voltage bias between sample  106  and extraction electrode  150 ; subjecting sample  106  two pulsed EUV radiation  104 ; optionally monitoring and recording pulsed EUV radiation  104  incident on sample  106 ; producing photoions  112  in response to subjecting sample  106  to the EUV radiation  104 ; receiving photoions  112  at ion detector  114 ; recording a time-of-flight of photoions  112  between sample  106  and ion detector  114  by control module  143 ; recording the positions upon ion detector  114  at which photoions  112  impact ion detector  114  by control module  143 ; providing electronic feedback control to control the voltage bias between sample  106  and extraction electrode  150 ; optionally adjusting the voltage bias to obtain a selected rate of impact of photoions  112  on ion detector  114 ; optionally providing electronic feedback control to control a selected position or focus of pulsed EUV radiation  104  upon sample  106 ; adjusting the bias voltage between sample  106  and extraction electrode  150  zero volts (V) at a selected period during data acquisition while EUV radiation  104  is substantially absent at sample  106 , wherein during the period, subjecting sample  106  to primary electrons  110  and detecting scattered electrons by detector  118  to acquire transmission electron diffraction or electron imaging of sample  106 ; optionally detecting, by electron detector  122 , secondary electrons or backscattered electrons from sample  106  during the interval when the voltage bias is zero V and primary electrons  110  impinge upon sample  106 ; terminating the first interval and terminating subjecting sample  106  to primary electrons  110 ; during a second interval, increasing the voltage bias between sample  106  and extraction electrode  150 , subjecting sample  106  to EUV radiation  104 , and acquiring time-of-flight mass spectra of photoions  112  and positions of impact of photoions  112  on ion detector  114  to perform hybrid EUV imaging spectrometry on sample  106 . The alternating cycle between the first interval and the second interval, wherein EUV radiation-assisted ionic field evaporation of photoions  112  is alternated with electron-sample interactions and scattered electron detection, can be performed a plurality of times. During data acquisition, ion time-of-flight data, ion impact positions on ion detector  114 , diffraction data, electron imaging data, secondary electron data, and electron backscattered data collected from sample  106  by electron source  108  and detectors ( 114 ,  118 ,  122 ) as well as data associated with other components of hybrid EUV imaging spectrometer  100  are acquired by control module  143  or communicated to data analysis module  141  to reconstruct shape and composition of sample  106 . 
     With reference to  FIG. 12 , panel A of  FIG. 12  shows a graph of the potential difference ΔV between sample  106  and extraction electrode  150  versus time. 
     Between time t 0  and time t 1 , potential difference ΔV is first voltage V 1  such that an imaging modality of hybrid EUV imaging spectrometer  100  is electron imaging, wherein primary electrons  110  impinge upon sample  106  and electron imaging and detection by electron detectors ( 118 ,  122 ) occurs to acquire, e.g., electron diffraction images as shown in panel B of  FIG. 12 . As shown in panel A of  FIG. 12 , between time t 1  and time t 2 , potential difference ΔV is second voltage V 2  such that an imaging modality of hybrid EUV imaging spectrometer  100  is ion imaging and the time-time-of-flight mass spectrometry acquisition occurs, wherein EUV radiation  104  impinges upon sample  106 , and ion imaging and detection by ion detector  114  occurs to acquire, e.g., time-of-flight mass spectrometry of photoions  112  emitted from sample  106  as shown in panel C of  FIG. 12 . At time t 2 , analyzer  131  reconstructs the geometrical shape and chemical composition of sample  106  from electron data and ion data as shown in panel D of  FIG. 12 . 
     As shown in panel A of  FIG. 12 , between time t 2  and time t 3 , potential difference ΔV is first voltage V 1  such that an imaging modality of hybrid EUV imaging spectrometer  100  is electron imaging, wherein primary electrons  110  impinge upon sample  106  and electron imaging and detection by electron detectors ( 118 ,  122 ) occurs to acquire, e.g., electron diffraction images as shown in panel B of  FIG. 12 . As shown in panel A of  FIG. 12 , between time t 3  and time t 4 , potential difference ΔV is second voltage V 2  such that an imaging modality of hybrid EUV imaging spectrometer  100  is ion imaging in which the time-of-flight mass spectrometry acquisition occurs, wherein EUV radiation  104  impinges upon sample  106 , and ion imaging and detection by ion detector  114  occurs to acquire, e.g., time-of-flight mass spectrometry of photoions  112  emitted from sample  106  as shown in panel C of  FIG. 12 . At time t 4 , analyzer  131  reconstructs the geometrical shape and chemical composition of sample  106  from electron data and ion data. 
     As shown in panel A of  FIG. 12 , between time t 4  and time t 5 , potential difference ΔV is first voltage V 1  such that an imaging modality of hybrid EUV imaging spectrometer  100  is electron imaging, wherein primary electrons  110  impinge upon sample  106  and electron imaging and detection by electron detectors ( 118 ,  122 ) occurs to acquire, e.g., electron diffraction images as shown in panel B of  FIG. 12 . As shown in panel A of  FIG. 12 , between time t 5  and time t 6 , potential difference ΔV is second voltage V 2  such that an imaging modality of hybrid EUV imaging spectrometer  100  is ion imaging in the fight mass spectrometry acquisition occurs, wherein EUV radiation  104  impinges upon sample  106 , and ion imaging and detection by ion detector  114  occurs to acquire, e.g., time-of-flight mass spectrometry of photoions  112  emitted from sample  106  as shown in panel C of  FIG. 12 . At time t 6 , analyzer  131  reconstructs the geometrical shape and chemical composition of sample  106  from electron data and ion data. Although only three cycles of electron imaging, ion imaging, and sample reconstruction are shown in  FIG. 12 , it should be appreciated that such cycles can be repeated as selected, interrupted, restarted, and the like. Moreover, it is contemplated that sample  106  can be subjected to structural or compositional modification during a cycle or after cycle such that such structural or compositional modification can be ascertained due to electron imaging, ion imaging, or sample reconstruction by hybrid EUV imaging spectrometer  100 . 
     In an embodiment, data stored in control module  143  during data acquisition are communicated to data analysis module  141  via data path  135 . Data analysis module  141  performs computations to generate three-dimensional chemical maps of sample  106 . In performing these computations, data analysis module  141  employs data that includes the shape of sample  106  that is generated using electron source  108 , electron detector  118 , and electron detector  122 ; ion times-of-flight between sample  106  and ion detector  114 ; and photoions  112  impact positions on ion detector  114 ; and the voltage bias between sample  106  and extraction electrode  150 . 
     According to an embodiment, a process for reconstructing the shape and composition of sample  106  includes: during a time interval within which a voltage is applied between sample  106  and extraction electrode  150 , subjecting sample  106  to pulsed EUV radiation  104  such that photoions emitted by sample  106  impact positions on detector  114 ; recording times-of flight of photoions  112 ; recording the detection sequence of photoions  112  on detector  114 ; and recording the impact locations of photoions  112  on detector  114 . In this manner, a dataset is produced that includes a sequential record of photoions  114  that are identified by their respective times-of-flight. Furthermore, the dataset also includes a record of the positions on the specimen tip of sample  106  from which photoions  112  are emitted as computed using back-projection from respective impact locations on detector  114 . At the end of this time interval, when the voltage between sample  106  and extraction electrode  150  is zero, a shape of sample  106  (e.g., tip  181 ) is recorded using electron source  108  and electron detector ( 118  or  122 ). A depth of sample  106  at which photoions  112  resided in sample  106  with respect to an initial position of a terminus of tip  181 , i.e. before photoions  112  are removed, is determined by starting with the first detected photoion  112  as follow: assigning an equivalent volume to identified photoion  112 , wherein the depth increment contributed by this equivalent volume corresponds to the thickness of a thin portion of tip  181  within a field of view of the detector and whose shape and surface area is measured by electron source  108  and detector combinations previously described. Further, in the process, the depth increment is initially assigned as a depth coordinate of the first photoion  112 . Then this depth coordinate is corrected by measuring the shape of tip  181  and determining the additional depth increment that must be added given the location on the surface of sample  106  from which photoion  112  was emitted. The process proceeds for successive photoions in a similar fashion eventually identifying the elemental species and original location of each detected photoion from the specimen tip. 
     The near-real-time measurement of the shape of tip  180  of sample  160  via in-situ electron imaging by hybrid EUV imaging spectrometer  100  provides reconstruction analysis that includes providing dimensional information to analysis algorithms in conjunction with spatial or temporal data from the position-sensitive detectors (e.g.,  118 ,  122 , or  114 ). As used herein, “three-dimensional chemical maps,” “reconstruction analysis,” and “spatial reconstruction” are identical. Accordingly, data acquisition and analysis can be continuous and performed in an absence of terminating acquisition of data, removing sample  106  from chamber  126 , or in other disruption. Beneficially, hybrid EUV imaging spectrometer  100  provides accurate calibration for three-dimensional spatial reconstruction of sample  106 . Without wishing to be bound by theory, it is believed that for tip  180  with axial symmetry, a two-dimensional image of tip  180  is sufficient for determination of the chemical image map of sample  106 . In an embodiment, axial symmetry of tip  180  is provided during application, e.g. by annular milling in a focused ion beam microscope. As a result, geometry parameters of tip  180  for an accurate three-dimensional spatial reconstruction is provided from two-dimensional electron image. 
     Hybrid EUV imaging spectrometer  100  has numerous beneficial uses such as a metrology instrument to provide accurate, subnanometer three-dimensional chemical mapping of sample  106  with high analytical sensitivity across the periodic table of elements. 
     Advantageously and unexpectedly, hybrid EUV imaging spectrometer  100  provides acquisition of mass-spectral data and reliable chemical assignments with spatial resolution for mapping samples  106  whose constituent elements may form large molecular ions in embodiments of atom probe tomography tools that employ ion-emission processes that employ that depend primarily upon thermally driven field evaporation. Additionally, hybrid EUV imaging spectrometer  100  provides highly-local structure and chemical composition at interfaces of different materials in sample  106  to determine structure-properties-processing relationship. Further, hybrid EUV imaging spectrometer  100  provides composition and structure at sub-nanometer length scales with high analytical sensitivity and substantially identical detection efficiency across the periodic table of elements. Hybrid EUV imaging spectrometer  100  provides fast, quantitative, easy to interpret, standards-quality chemical composition and three-dimensional geometrical shape of sample  106 . 
     Embodiments of the subject matter and the operations described in this specification can be implemented in digital electronic circuitry, in tangibly-embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on a computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively, or in addition, the program instructions can be encoded on an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices). 
     The operations described in this specification can be implemented as operations performed by a data processing apparatus on data stored on one or more computer-readable storage devices or received from other sources. 
     The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more computers executing one or more computer programs to perform actions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). 
     Computers suitable for the execution of a computer program include, by way of example, can be based on general or special purpose microprocessors or both, workstations, or any other kind of central processing unit. Generally, a central processing unit will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a central processing unit for performing or executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic; magneto-optical disks, optical disks, USB drives, and so on. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The central processing unit and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), an inter-network (e.g., the Internet), and peer-to-peer networks (e.g., ad hoc peer-to-peer networks). Such interconnects may involve electrical cabling, fiber optics, or be wireless connections. 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. In some embodiments, a server transmits data (e.g., an HTML page) to a client device (e.g., for purposes of displaying data to and receiving user input from a user interacting with the client device). Data generated at the client device (e.g., a result of the user interaction) can be received from the client device at the server. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Thus, particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous. 
     While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation. Embodiments herein can be used independently or can be combined. 
     Reference throughout this specification to “one embodiment,” “particular embodiment,” “certain embodiment,” “an embodiment,” or the like means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of these phrases (e.g., “in one embodiment” or “in an embodiment”) throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. 
     All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. The ranges are continuous and thus contain every value and subset thereof in the range. Unless otherwise stated or contextually inapplicable, all percentages, when expressing a quantity, are weight percentages. The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including at least one of that term (e.g., the colorant(s) includes at least one colorants). “Optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event occurs and instances where it does not. As used herein, “combination” is inclusive of blends, mixtures, alloys, reaction products, and the like. 
     The terms “pulsed EUV light” and “pulsed EUV radiation” are identical. 
     As used herein, “a combination thereof” refers to a combination comprising at least one of the named constituents, components, compounds, or elements, optionally together with one or more of the same class of constituents, components, compounds, or elements. 
     All references are incorporated herein by reference. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. “Or” means “and/or.” Further, the conjunction “or” is used to link objects of a list or alternatives and is not disjunctive; rather the elements can be used separately or can be combined together under appropriate circumstances. It should further be noted that the terms “first,” “second,” “primary,” “secondary,” and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity).