Abstract:
The invention is directed to an arrangement for monitoring the energy radiated by an EUV radiation source with respect to energy variations acting in an illumination beam path, wherein the radiation source has a plasma column emitting extreme ultraviolet radiation. The arrangement includes an energy monitoring unit, and a detection beam path. The detection beam path is separate from the illumination beam path and is arranged with the energy monitoring unit for detecting pulse energy, so that the illumination beam path is not impaired by the energy measurement. The detection beam path is matched to the illumination beam path with respect to bundle extension and optical losses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of German Application No. 102 04 994.7, filed Feb. 5, 2002, the complete disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The invention is directed to an arrangement for monitoring the energy radiated by an EUV radiation source with respect to the energy variations taking place in an illumination beam path, particularly for controlling dose stability in EUV lithography for chip fabrication in semiconductor technology. 
     b) Description of the Prior Art 
     In addition to special lamps, narrow-band excimer lasers with wavelengths of 248 nm and 193 nm are currently used as radiation sources for producing microchips. Scanners based on F 2  lasers (157 nm) are in development at this time. 
     In all photolithography processes, a mask (containing the structure to be imaged) is imaged on a wafer (semiconductor disk) in the scanner in a reduced manner (the reduction is typically 1:5). EUV radiation sources (around 13.5 nm) appear to be the most promising of the various solutions for the next generation of semiconductor lithography. Aside from the characteristics of the optical system (numerical aperture, depth of focus, aberrations or imaging errors of the lenses or mirrors), the image quality of the photolithographic process is essentially determined by how accurately the radiated radiation dose (dose accuracy) can be maintained. This dose stability is determined by:
         a) pulse quantization   b) pulse-to-pulse stability   c) spatial stability of the emitting volume.       

     Pulse quantization is scanner-specific. The quantity of light pulses that can fall into a moving slit during the scan varies. However, this quantity can usually be ignored. 
     The quantities b and c are specific to the EUV radiation source itself. An arrangement which prevents the spatial fluctuations of the emitting region or suitably takes them into account would be useful for regulating pulse energy. 
     The throughput of a photolithography scanner (throughput=quantity of wafers exposed per time unit) is essentially determined by the pulse energy and the pulse repetition frequency of the radiation source as well as by optical losses in the scanner itself. Optical losses occur due to the limited reflectivity of the collector and mirrors and due to so-called geometric losses. The amount of radiation output that can be caught by the collector optics is defined by a quantity specific to the radiation source, the so-called source etendue (the magnitude of the emitting region [mm 2 ] times the usable solid angle [sr]). Strictly speaking, the etendue is determined by the total geometry of the radiation source, by the dimension of the source location, by any exit windows and by the aperture of the optics following it. The etendue quantity also determines how much radiation can be detected by an optical system arranged after it. Further, the etendue represents a measurement of the radiation losses given by the geometric ratios of the radiation source (geometric losses). 
     To prevent geometric losses, the source etendue may not be greater than the etendue of the imaging system in the scanner, as is described in M. Antoni et al., “Illumination optic design for EUV Lithography”,  Proc. of SPIE,  Vol. 4146, August 2000). If this condition can be met, the radiation output in the wafer plane is now only dependent on the reflectivity of all mirrors. Regulation of the pulse energy would require detection of this pulse energy through constant measurement (monitoring) by a radiation detector. 
     However, the scanner optics require an isotropic radiation characteristic with respect to angular distribution. Therefore, mirror optics which couple out light are not desired in the illumination beam path for coupling out at least a part of the radiation to an energy detector. For this reason, previously known EUV radiation sources are usually operated without regulation because measurements impair the energy flux during operation. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is the primary object of the invention to find a novel possibility for detecting variations in the radiation emitted by EUV sources which allows fluctuations in pulse energy as well as spatial fluctuations acting in the illumination beam path to be detected. 
     In an arrangement for monitoring the energy radiated by an EUV radiation source with respect to energy variations acting in an illumination beam path, wherein the radiation source has a plasma column emitting extreme ultraviolet radiation, the object is met according to the invention in that a detection beam path is separated from the illumination beam path with respect to the plasma column and has an energy monitoring unit for measuring pulse energy, so that the illumination beam path is not impaired by the energy measurement, and the detection beam path is matched to the illumination beam path with respect to bundle extension and optical losses. 
     The detection beam path is advantageously arranged opposite to the illumination beam path with respect to the plasma column. The two separate beam paths are advisably adapted to one another in that the etendue of the detection beam path is matched to the aperture of the first collector optics of the illumination beam path by means of a shutter or diaphragm with a defined aperture. It is advantageous when the aperture of the diaphragm is adjustable. 
     All filters required in the illumination beam path are advantageously arranged in the detection beam path in the same manner and in the same quantity. These filters are preferably debris filters and filters for achieving spectral purity of radiation (spectral filters). 
     The energy monitoring unit can be realized in different ways. In a first variant, it can be designed so as to have an energy detector, which is arranged with its light-sensitive surface completely within the light bundle transmitted by the reflection optics of the detection beam path, and a second detector whose light-sensitive surface is only partially illuminated by the edge area of the light bundle of the detection beam path, so that variations in the position of the light bundle due to fluctuations in the spatial distribution of the emitted EUV radiation are quantifiable. For this purpose, a substantial central portion of the light bundle is advisably deflected to the energy detector by means of a plane mirror, this energy detector being arranged along the bent optical axis of the reflection optics. The etendue of the illumination beam path is accordingly simulated in the detection beam path by means of an aperture stop and the effective surface of the plane mirror, and the energy detector registers the effect of an energy fluctuation on the target object (wafer) in the illumination beam path. Due to the spatial fluctuations of the plasma column, proportions of the radiation can travel past the mirror and therefore miss the energy detector and be detected by the second detector behind the mirror as a total spatial radiation variation. 
     The mirror is preferably a multilayer mirror which deflects the light bundle orthogonal to the axis of the reflection optics, but can also be a metal mirror with grazing incidence. 
     Another design possibility for the energy detector unit advantageously comprises a position-sensitive receiver for simultaneous detection of variations in the total pulse energy and spatial distribution of the emitted EUV radiation, wherein the total pulse energy can be determined by summing and the spatial distribution can be determined by centroid calculation from different segments of the receiver. The position-sensitive receiver is preferably a quadrant diode. 
     The special problem which consists in that the isotropic radiation characteristic of the illumination beam path is additionally disrupted as a result of the energy being coupled out for this measurement task is avoided, according to the invention, in that the detection beam path is arranged separate from (preferably opposite to) the illumination beam path with respect to the plasma column, and all of the ratios of the input aperture and optical losses of the illumination beam path are transferred to the detection beam path. The EUV source etendue of the illumination beam path is simulated by a defined diaphragm in front of the reflection optics of the detection beam path. The fluctuations in EUV emission can accordingly be detected by suitable detector arrangements with respect to total pulse energy and with respect to spatial changes as if they were measured in the wafer plane of the illumination beam path. 
     The arrangement according to the invention makes it possible to detect variations in the radiation emitted by EUV sources for purposes of regulating the latter so that it is possible to detect fluctuations in the pulse energy as well as spatial fluctuations acting in the illumination beam path without interfering with the illumination beam path. 
     The invention will be described more fully in the following with reference to embodiment examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic view of the invention with a separate detection beam path arranged opposite to the illumination beam path; 
         FIG. 2  shows an arrangement of the invention with two detectors for separate recording of variations in total pulse energy; 
         FIG. 3  is a schematic view of the radiation pattern striking the energy detector  61 ; 
         FIG. 4  shows a schematic snapshot of a radiation pattern striking the second detector  62 ; 
         FIG. 5  shows an advantageous implementation of the invention for separating the pulse energy variations from the total pulse energy variations; and 
         FIG. 6  is a schematic snapshot of a radiation pattern striking a quadrant detector. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a basic arrangement of an EUV source in which a hot plasma column  1  is generated in an optional manner, the invention comprises, on the one hand, an illumination beam path  12  with collector optics  7  collecting EUV radiation emitted from the plasma column  1  and transmitting it for transfer to a location of use (e.g., a scanner for semiconductor lithography, not shown) and, on the other hand, a detection beam path  13  with an energy monitoring unit  6  to which the emitted EUV radiation  11  is directed by reflection optics  4 . 
       FIG. 1  shows a simplified overview of the invention, wherein the plasma column  1  is generated, for example, by a gas-discharge pumped EUV source, e.g., with Z-pinch (not shown). However, the plasma column  1  can also be generated by laser radiation. 
     As is illustrated in  FIG. 1 , the light path to the scanner (referred to above as illumination beam path  12 ) is opened, according to the invention, to all elements for monitoring the emitted EUV radiation  11  in that the light path to the energy monitoring unit  6  (detection beam path  13 ) is coupled out of the EUV radiation  11  of the plasma column  1  in the direction opposite to the illumination beam path  12 . For reasons pertaining to the symmetry of the plasma column  1 , this is particularly advantageous, but is not compulsory. 
     The EUV radiation  11  exiting from the plasma column  1  to the left-hand side of  FIG. 1  is imaged on the energy monitoring unit  6  in the detection beam path  13  by means of reflection optics  4 . 
     Errors occurring as a result of different out-coupling points and out-coupling optics during monitoring of the plasma function can be compensated by suitable adapting steps. This is carried out by means of the aperture of a diaphragm  3  by altering the etendue in front of the reflection optics  4  to adapt the detection beam path  13  so as to be analogous to the predetermined aperture of the collector optics  7  of an application device (e.g., a scanner, mentioned above) in the illumination beam path  12 . As a result, the light paths of the detection beam path  13  and the illumination beam path  12  are identical with respect to geometric losses. This equality must also be reflected in all of the other elements in the detection beam path  13  in relation to the illumination beam path  12 . Debris filters  2  contained therein, for example, must be adapted in quantity and construction in the same way as all selective spectral filters  5  in both beam paths  12  and  13 . 
     Fluctuations in pulse energy in the wafer plane are undesirable in semiconductor lithography by reason of the required exposure accuracy. However, in EUV emission, such fluctuations are caused by the fluctuation of energy in the plasma column  1  and—because of the limited aperture of the collector optics  7 —by fluctuations in the position and angle of the emitted EUV radiation  11 . 
     By matching the etendues of the detection beam path  13  and illumination beam path  12 , the arrangement shown in  FIG. 1  achieves the effect whereby an energy detector in the energy monitoring unit  6  measures the same energy fluctuations that would be measured in the wafer plane. 
       FIG. 2  shows an advantageous arrangement of the detection beam path  13  by which two different contributions to the total pulse energy fluctuation (pulse energy fluctuation and variations in the spatial distribution of the EUV emission) can be measured separately. For this purpose, the energy monitoring unit  6  has an energy detector  61  and a second detector  62 , and the light bundle  41  transmitted by the reflection optics  4  is deflected onto the energy detector  61  by means of a mirror  63 . Debris filters  2  and spectral filters  5  are arranged in front of and behind the reflection optics  4 , respectively, analogous to the illumination beam path  12  as is described in FIG.  1 . The desired etendue of the illumination beam path  12  is determined in the detection beam path  13  by the aperture of diaphragm  3  and by the effective surface of the mirror  3 . The numerical aperture of the collector optics  7  of the illumination beam path  12  (shown only in  FIG. 1 ) is simulated by the diaphragm  3 . 
     The mirror  3  which can be a multilayer mirror or a metal mirror with grazing incidence reflects the substantial proportion of the light bundle  41 . This portion of the light bundle  41  which is shown in  FIG. 2  by a solid line corresponds to the ideal etendue of the illumination beam path  12  for optimal illumination of the target object (wafer). 
       FIG. 4  shows schematically the ratios of the illumination of the energy detector  61 . The light spot  66  embodies the proportion of the light bundle  41  that is coupled out by the mirror  63 . The energy detector  61  accordingly detects the total pulse energy and its pulse-to-pulse variations analogous to the pulse energy at the location of a wafer (not shown) in the illumination beam path  12 . The imaged light spot  66  of the light bundle  41  deflected by the mirror  63  should not completely fill the light-sensitive surface  67  of the energy detector  61 . 
     Spatial fluctuations in the plasma column  1  lead to a fluctuating bundle edge  42  (shown in dashes) of the light bundle  41  when transmitted by the diaphragm  3  and reflection optics  4 . This fluctuating bundle edge  42  misses the mirror  3 . Therefore, a proportion of the light bundle  41  which has wandered out in this manner does not strike the energy detector  61  and, with the emitted EUV radiation  11  of the plasma column  1  remaining the same per se, leads to a reduction in the pulse energy in the wafer plane of the illumination beam path  12  that is simulated in the detection beam path  13  through the location of the energy detector  61 . 
     The second detector  62  is provided for the purpose of receiving this proportion of the radiation of the fluctuating bundle edge  42  and for subsequently determining from this the actual total pulse energy and the spatial variations in the emitted EUV radiation  11  separately. 
       FIG. 3  shows a schematic snapshot of the light passing the mirror  63  in the form of the fluctuating bundle edge  42 . The light-sensitive surface  67  of the second detector  62  is arranged in such a way that the detector  62  securely detects the “wandering” light spot  66  caused by the fluctuating bundle edge  42 . The black circle in the center represents the cutout bundle core  65  whose light component was completely coupled out by the mirror  63  on the energy detector  61  (light spot  66  shown in white in FIG.  4 ). 
       FIG. 5  shows another arrangement of the detection beam path  13 . In this example, the energy monitoring unit  6  contains only one detector for detecting the total pulse energy variations as well as for detecting fluctuations in position and direction of the emitted EUV radiation  11 . This detector is a position-sensitive receiver  68 , e.g., a quadrant diode, and is arranged behind the reflection optics  4  along its optical axis. The light bundle  41  exiting from the reflection optics  4  must be so dimensioned in relation to the light-sensitive surface  64  that it shows the small detail illustration shown in FIG.  6 . In order to prevent corrupted signals, the light spot  66  can not be permitted to wander out beyond the edge of the position-sensitive receiver  68  even with maximum dimensions of the fluctuating bundle edge  42  (according to FIG.  5 ). 
       FIG. 6  shows a snapshot of the incident light bundle  41 , including a fluctuating bundle edge  42 , so that the light spot  66  occupies an irregular and asymmetrical position on the quadrants  69  of the position-sensitive receiver  68 . 
     In this type of energy monitoring, the sum of the energy contributed by all four quadrants  69  of the receiver  68  gives the total pulse energy and the relative contributions of the individual quadrants  69  gives information about the centroid of the spatial distribution of the radiation emission. 
     There are other possible design variants within the framework of the invention. The examples described above are directed to an energy monitoring unit  6  involving at least the detection of the total pulse energy and of variations in the position and angular position of the EUV radiation  11  emitted from the plasma column  1 . Any other sensor principles not expressly mentioned are also to be clearly understood as belonging to the inventive teaching insofar as they can be arranged without inventive activity in a similar separate detection beam path  13  which is adapted to the aperture ratios and geometric ratios of an illumination beam path  12  which is to be maintained constant (e.g., for achieving an isotropic radiation characteristic for the dose stability in EUV lithography). 
     While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention. 
     Reference Numbers 
     
         
           1  plasma column 
           11  EUV radiation 
           12  illumination beam path 
           13  detection beam path 
           2  debris filter 
           3  diaphragm 
           4  reflection optics 
           41  ideal light bundle 
           42  fluctuating bundle edge 
           5  spectral filter 
           6  energy monitoring unit 
           61  energy detector 
           62  second detector 
           63  (plane) mirror 
           64  reflected bundle 
           65  cutout bundle core 
           66  light spot 
           67  light-sensitive surface 
           68  position-sensitive receiver 
           7  collector optics