Abstract:
The disclosure relates to an illuminating apparatus for illuminating a sample on a work stage, optionally with a relatively narrow illuminating line of relatively controlled energy, as well as methods for controlling energy of a laser source when illuminating a sample on a work stage with a relatively narrow illuminating line.

Description:
FIELD 
     The disclosure relates to an illuminating apparatus for illuminating a sample on a work stage, optionally with a relatively narrow illuminating line of relatively controlled energy, as well as methods for controlling energy of a laser source when illuminating a sample on a work stage with a relatively narrow illuminating line. 
     BACKGROUND 
     Many technical applications in electronics and display technology use a thin polycrystalline silicon (Si) layer on glass. Such panels are typically used for liquid crystal display (LCD), organic light emitting diode (OLED) and solar cell technology. The standard process to produce such panels is to first deposit amorphous Si layers on glass by chemical vapour deposition (CVD) or sputter processes. Subsequently a polycrystalline film is formed by laser annealing such as excimer laser crystallization (ELC) or sequential lateral solidification (SLS) techniques. An overview of these different common techniques is given e.g. in U.S. Pat No. 7,061,959 which is herewith incorporated by reference. 
     A technique for conversion of amorphous silicon into polycrystalline silicon is the so called thin beam directional x-tallization (TDX) process. This process uses a pulsed narrow narrowly focused laser line with a width (so called short axis) dimension of about 10 μm and a longitudinal (so called long axis) dimension of about 500 mm which is scanned in the short axis direction in order to melt the thin Si layer having a thickness of 30 to 100 nm. 
     When applying the ELC, SLS or TDX processes a thin silicon layer on glass is typically melted by an illumination line being emitted by a high energy excimer laser, such as a XeCl excimer laser, and shaped by beam shaping optics which generally perform at least one of the following: 1) changing the shape and/or divergence in one or two directions perpendicular to the direction of beam propagation; 2) homogenizing the intensity at a field and/or pupil plane in one and/or two directions; and/or 3) changing the spatial and/or temporal coherence. 
     After having shaped the beam with the beam shaping optics the beam usually has a rectangular cross section which upon further propagation scales in size in the long and/or the short axis direction. 
     The energy density of the laser line on the silicon layer can be homogenous in the long axis direction and lie within a certain process window. The theoretical process window is reduced by effects like long axis uniformity fluctuations, beam position and pointing fluctuations, variations of the Si film thickness and/or a variation of the overall energy reaching the panel. The latter is often induced by variations of beam parameters of the laser that influence the transmission of the optical system. For these reasons the relevant process parameters have to be measured and stabilized. 
     Usually, the laser energy is stabilized by a feed back loop that measures the energy close to the exit window of the laser. This is the most frequently used laser energy stabilization technique since nearly each commercially available laser is equipped with such a feedback loop. A laser annealing system comprising such a laser energy stabilization system works quite well, however, it does not prevent variations of energy density along the laser line on the silicon layer leading to poor crystal quality when using the TDX process. 
     SUMMARY 
     In one aspect, the disclosure features an illuminating apparatus that includes: a beam shaping optical system configured to shape a laser beam into a line-shaped laser beam, the laser beam being emitted during use by a laser source and propagating through the illuminating apparatus along a beam path; an energy measuring device configured to measure an energy of the line-shaped laser beam; an energy control system configured to generate a control signal based on the measured energy of the laser beam, the energy control system being configured to control energy output of the laser source and/or transmission of one or more components of the beam shaping optical system based on the control signal. The energy measuring device is in the beam path between the beam shaping optical system and a work stage. The illuminating apparatus is configured to illuminate a sample with the line-shaped laser beam when the sample is on the work stage. At the sample, the line-shaped laser beam has a dimension in a first direction that exceeds a dimension in a second direction perpendicular to the first direction. 
     In another aspect, the disclosure features a method that includes: shaping a laser beam into a line-shaped laser beam with a beam shaping optical system, the laser beam being emitted during use by a laser source; measuring an energy of the line-shaped laser beam; generating a control signal based on the measured energy of the line-shaped laser beam; and controlling an energy output of the laser source and/or transmission of one or more components of the beam shaping optical system based on the control signal. The energy of the line-shaped laser beam is measured at a location of a path of the line-shaped laser beam that is between the beam shaping optical system and a work stage. The illuminating apparatus is configured to illuminate a sample with the line-shaped laser beam when the sample is on the work stage. At the sample, the line-shaped laser beam has a dimension in a first direction that exceeds a dimension in a second direction perpendicular to the first direction. 
     In a further aspect, the disclosure features an illuminating apparatus that includes: a beam shaping optical system configured to shape a laser beam into a line-shaped laser beam; an energy measuring device configured to measure an energy of the line-shaped laser beam; and an energy control system configured to generate a control signal based on a measured energy of the line-shaped laser beam, and the energy control system being configured to control energy output of the laser source based on the control signal. The control signal is indicative of an averaged energy of the line-shaped laser beam along a first direction. The illuminating apparatus is configured to illuminate a sample with the line-shaped laser beam when the sample is on the work stage. At the sample, the line-shaped laser beam has a dimension in the first direction that exceeds a dimension in a second direction perpendicular to the first direction. 
     In an additional aspect, the disclosure features a method that includes: shaping a laser beam into a line-shaped laser beam; measuring an energy of the line-shaped laser beam; generating a control signal based on a measured energy of the line-shaped laser beam; and controlling an energy output of a laser source that creates the laser beam based on the control signal. The control signal is indicative of an averaged energy of the line-shaped laser beam along a first direction. The illuminating apparatus is configured to illuminate a sample with the line-shaped laser beam when the sample is on the work stage. At the sample, the line-shaped laser beam has a dimension in the first direction that exceeds a dimension in a second direction perpendicular to the first direction. 
     In some embodiments, the disclosure provides an illuminating apparatus and method for controlling energy of a laser source with improved reduction of variations of energy density of the laser line on the sample being illuminated. 
     Because some beam parameters, such as pointing, divergence, and polarization, can change over time, the transmission through the beam shaping optics is subjected to change. This can influence the energy density on the panel. 
     In certain embodiments, the disclosure provides an illuminating apparatus for illuminating a sample on a work stage with a narrow illuminating line of controlled energy, whereby the illuminating line is emitted from a laser, the illuminating line has an dimension in a first direction and a dimension in a second direction perpendicular to the first direction whereby the first direction dimension exceeds the second direction dimension by a multiple. The illumination apparatus includes a beam shaping optical system for shaping the laser beam into a line shape, an energy measuring device for measuring energy of the laser beam, and an energy control system for generating a control signal upon the measured laser beam energy and for controlling the energy output of the laser source based on the control signal, whereby the energy measuring device is arranged in the beam path after the beam shaping optical system and before the work stage. The energy measuring device, in the following for simplicity called process energy monitor (PEM) measures the energy incident on the panel and feeds a signal back to the control loop of the laser. 
     In some embodiments, the illumination apparatus can be used in laser annealing and/or laser crystallization purposes, such as related to ELC, SLS or TDX processes. In certain embodiments, the illumination apparatus can be used any laser light exposures involving a relatively thin illuminating line having a high aspect ratio of e.g. several hundreds or even thousands. 
     It has been found that, in at least some instances, energy measurement relying only on a fraction of the beam at an outer edge may deliver a useful control signal only if the energy at the outer edge is a good measure of the total beam energy. Often, this will not be the case. Therefore, in some embodiments, the control signal is generated such that it is indicative of an averaged beam energy along the dimension of the line shaped beam in an appropriate direction, e.g., the larger dimension. 
     It can be challenging to place an energy meter directly in the beam path of an illuminating apparatus without disturbing beam performance and/or output energy. One solution may be that the energy measuring device includes a beam splitter for coupling out a fraction of the line shaped beam and a detector for detecting the fraction. In general, the detector may be an array of any kind that is capable of detecting laser light. As an example, the detector may be a two dimensional array having sensors in two dimensions or a one dimensional array having light detecting elements only in one direction, e.g., oriented in the long axis direction. In some embodiments, the detector includes a plurality of photodiodes arranged side by side in a direction, e.g., the long direction, for collecting the fraction. These photodiodes may be arranged such that a few rows of photodiodes form a two dimensional sensor array. Optionally, the photodiodes are in a one dimensional array placed side by side. 
     In some embodiments, the illumination apparatus includes a plurality of spherical lenses for focusing the fraction onto the plurality of photodiodes. This can, for example, reduce the number of photodiodes. Other focusing lenses may optionally be used. Examples include cylindrical lenses and double cylindrical lenses with cylindrical surfaces transversely crossing each other. Nevertheless, spherical lenses may offer certain manufacturability advantages. 
     Optionally, the illuminating apparatus can include reducing optics in the beam path after the beam shaping optical system for reducing the line shaped beam in the second (shorter) direction. The energy measuring device may be located in the beam path direction before or after the reducing optics. In some embodiments, the energy measuring device may also be located in the reducing optics itself. When the beam is narrowed significantly by the reducing optics the collection (and the subsequent detection) of the most relevant part of the beam may be easier. 
     In certain embodiments, the illumination apparatus includes imaging optics for imaging the line shaped beam onto the sample. The imaging optics may include the reducing optics. Beam imaging may improve illuminating line properties on the sample, e.g., the panel. 
     In some embodiments, the beam shaping optical system includes a homogenizer for homogenizing the laser beam at least along its dimension in the first (longer) direction. The energy measuring device is thus located in the beam path after the homogenizer. Typical homogenizers for use in the beam shaping optical system are disclosed in U.S. Pat. No. 5,721,416 A1 or WO 2006/066706 A2. 
     In some embodiments, the beam shaping optical system includes a field defining optical device for defining the dimension of the laser beam at least in the second (shorter) direction. U.S. Pat. No. 5,721,416 A1, U.S. 60/731,539, and U.S. 60/753,829 disclose exemplary arrangements for defining or limiting the dimension of the laser beam at least in the second (shorter) direction. 
     In some embodiments, the disclosure provides a method for controlling the energy of a laser source when illuminating a sample on a work stage with a narrow illuminating line, whereby the illuminating line is generated from a laser beam propagating along a beam path and is emitted from the laser source and has an dimension in a first direction that exceeds a dimension in a second direction perpendicular to the first direction by a multiple. The method includes shaping the laser beam into a line shape, and measuring the energy of the laser beam in the beam path after the beam shaping optical system and before the work stage. The method also includes generating a control signal upon the measured laser beam energy, and controlling energy output of the laser source based on the control signal. 
     In certain embodiments, the control signal is indicative of an averaged beam energy along the dimension of the line shaped beam in the first (longer) direction. In some instances, it is believed that relying only on a fraction at an outer edge of the beam can be disadvantageous because the energy at the outer edge in general might not reflect the energy of the total beam. 
     In some embodiments, the method includes splitting the laser beam in the beam path after the beam shaping optical system and before the work stage and coupling out a fraction and detecting the fraction. 
     Optionally, the method includes averaging a beam energy of the fraction along the dimension of the fraction in the first (longer) direction. In some instances, averaging the beam energy in the second (shorter) direction can be avoided. Averaging the beam energy along the dimension in the first (longer) direction can provide enough information for generating a control signal. The averaging may optionally be done by integrating the beam energy along the long axis direction only. In certain instances, weighing the signal with respect to the length being integrated can be avoided. 
     In certain embodiments, the method includes dividing the fraction into a plurality of beamlets and focusing the plurality of beamlets onto a detector array comprising e.g. a plurality of photodiodes. Optionally, pyroelectric and/or thermoelectric sensors can be used. This can reduce the number of detector array elements such as photodiodes, and/or enhance the efficiency with which collection the beam dimension in total is made. Focusing of a plurality of beams can also provide the advantage that an adjustment of the detector with respect to the beam to be detected may be realized much easier than without having any focusing mechanism. 
     In some embodiments, the method includes reducing the line shaped beam in the second (shorter) direction. The energy may be measured in the beam path direction before, after or during reducing the line shaped beam in the second direction. 
     Optionally, the method can include imaging the line shaped beam onto the sample or panel. Imaging the beam has the advantage of being able to after the illuminating line properties at the intermediate field plane and thus on the sample or panel. 
     In certain embodiments, the method includes homogenizing the laser beam at least along the first (longer) direction. The beam energy may be measured in the beam path after homogenizing the laser beam. 
     In some embodiments, the disclosure provides an illuminating apparatus for illuminating a sample on a work stage is provided with a narrow illuminating line of controlled energy, where the illuminating line is generated from a laser beam propagating along a beam path, emitted from a laser source and has an dimension in a first direction exceeding an dimension in a second direction perpendicular to the first direction by a multiple. The apparatus includes a beam shaping optical system for shaping the laser beam into a line shape, and an energy measuring device for measuring energy of the laser beam. The apparatus also includes an energy control system for generating a control signal, which is indicative of an averaged beam energy along the dimension of the line shaped beam in the first direction and also indicative of the measured laser beam energy and for controlling energy output of the laser source as a result of the control signal. 
     The energy measuring device can include a beam splitter for coupling out a fraction of the line shaped beam and a detector for detecting the fraction. 
     The detector optionally includes a plurality of photodiodes being arranged side by side in the first (longer) direction for collecting the fraction as is explained above. 
     The may include a plurality of spherical lenses for focusing the fraction onto the plurality of photodiodes. This can be used, for example, for simplicity reasons. 
     The at least two of the photodiodes may be electrically coupled in parallel. Coupling of photodiodes in parallel can result in an addition of the photocurrents being generated when illuminating the photodiodes. A current signal formed of an addition of photocurrents can correspond to an (not normalized) averaging of beam energy impinging on the photodiodes. 
     In general, voltage signals are used in order to control laser beam output of the laser. The voltage control signal may be generated by measuring the voltage drop along a shunt resistor that is electrically coupled in series to (at least one of) the photodiodes. 
     To keep the loss for measuring the intensity small, for example, one may optionally use reverse biased photodiodes which may also facilitate time resolved detection of the short excimer laser pulses if desired. 
     Examples for detection of optical signals are given in Dereniak and Crowe: Optical Radiation Detectors (Wiley) which is incorporated by reference herein. 
     The laser output may be controlled by an electronic feed back loop. The input signal may be generated by several photodiodes. The charge that is generated by the laser pulse in each photo diode may be added electronically. In order to keep dark currents low, it is possible that the photodiodes are not biased. 
     In some embodiments, the disclosure provides a method for controlling energy of a laser source when illuminating a sample on a work stage with a narrow illuminating line is provided, whereby the illuminating line is generated from a laser beam propagating along a beam path and being emitted from the laser source and having an dimension in a first direction exceeding an dimension in a second direction perpendicular to the first direction by a multiple. The method includes shaping the laser beam, measuring the energy of the shaped laser beam, and generating a control signal upon the measured laser beam energy, whereby the control signal is indicative of an averaged beam energy along the dimension of the line shaped beam in the first direction and controls the energy output of the laser source based on the control signal. 
     Optionally, a fraction of the line shaped beam is coupled out and subsequently detected. The fraction can be divided into a plurality of beamlets. Each of the beamlets can be detected separately. Each of the beamlets can be focused separately for detection. The method can include converting detection signals generated upon detecting each of the beamlets separately into a sum signal. Such a sum signal can represent an averaging of the detected beam. The sum signal may subsequently be converted into the control signal, for example, in the manner described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be described hereinafter with reference to the drawings, in which: 
         FIG. 1  is a schematic representation of a TDX apparatus; 
         FIG. 2  is a schematic representation of a portion of the TDX apparatus shown in  FIG. 1 ; 
         FIG. 3  shows an energy measuring device incorporated into the TDX apparatus of  FIGS. 1 and 2 ; and 
         FIG. 4  shows exemplary energy fluctuations at a panel/sample when controlling laser output energy in two different systems. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic drawing of an illuminating apparatus, in particular for applying the above mentioned TDX process. This TDX tool includes as a light source an excimer laser  10 , such as a XeCl-excimer laser emitting a pulsed laser beam  12 . Typical pulse widths are 10-30 ns at a typical repetition rate of 100 Hz-10 kHz. The Energy of such a laser pulse is typically in the range of 100 mJ-1000 mJ. 
     This laser beam  12  with typical rectangular cross section of 10×30 mm is directed along a beam path  13  and converted into a narrow illuminating line  70  via an optical system described in detail in the following. The illuminating line  70  on the sample/panel  66  typically has a dimension in a long axis direction transverse to the scanning direction of at least 200 mm and a dimension in a short axis direction, i.e. in scan direction, of 3 to 7 μm. The illuminating line  70  typically has a ratio of the dimension of its long axis to the dimension of its short axis of at least 2.5×10 4  (e.g., at least 5×10 4 , at least 7.5×10 4 , at least 1×10 5 ). The illuminating line  70  with high aspect ratio illuminates a sample such as a panel  66  positioned on a work stage  68 . The work stage  68  is typically moved with a stage scanning speed of around 10 mm/s resulting in a typical scan increment of e.g. 2 μm/pulse. 
     After having left the laser  10  the light beam  12  is directed to a so called beam delivery unit (BDU)  14 . This BDU  14  has an entrance window  16 , a pulse stretcher  18  for lengthening the pulse width by a factor of from 2 to 16. The lengthened laser pulse  12  exits the BDU  14  via an exit window  20 . 
     In the following the cross section of the laser beam  12  is converted from a rectangular one into a line shape. For this purpose the laser beam  12  is first directed to a beam preconditioning unit (BPU)  22 . The BPU  22  has a plurality of optical elements  24 . This arrangement of optical elements  24  serves for flattening the intensity profile of the laser beam  12 . 
     The beam expanding unit (BEU)  26  is arranged in the beam path  13  after the BPU  22  and serves for expanding the laser beam  12  in a lengthwise direction. Hereinafter the lengthwise direction is indicated with the reference character x, the width direction of the laser beam is labelled with the reference character y. In the present case the dimension of the laser beam  12  is carried out via four lenses  28  being arranged one after the other on the beam path  13 . Instead of the use of such lenses also, bent mirrors can be used. 
     For shortening the total dimension of the illuminating apparatus, a plurality of plane mirrors may be used. For example, three mirrors  30 ,  32 ,  34  fold the beam path  13  before the laser beam  12  enters a beam stability metrology unit (BSMU)  36 . The beam stability metrology unit  36  includes an arrangement of optical elements  38  movable in different directions in order to adjust and/or correct e.g. the pointing and/or position of the laser beam  12 . Respective laser beam monitoring devices (not shown here) are located at the exit of the BSMU  36 . Details are e.g. disclosed in U.S. Pat. No. 7,061,959. 
     A homogenizing unite  40  follows the BSMU  36  in the beam path  13 . The homogenizing device  40  is designed to homogenize the expanded line shaped laser beam  12 . Homogenizer  40  in the example according to  FIG. 1  includes a cylindrical lens array  42  followed by a lens  44 , a further cylindrical lens array  46 , a rod  48  and a condenser  50 . U.S. Pat. No. 5,721,416 A1 or WO 2006/066706 A2 disclose a plurality of different homogenizers that are capable of being inserted in addition or alternatively into the beam path  13  where the homogenizer  40  is located. 
     Subsequently, the laser beam  12  is directed to folding mirror  52  and in the following to field defining unit (FDU)  54 . FDU  54  defines dimension of the laser beam  12  in a field plane and in particular in the panel plane  66  in short axis direction y. A FDU  54  may, for example, include an arrangement as described in U.S. Pat. No. 5,721,416 or alternatively one of the arrangements as disclosed in U.S. Ser. No. 60/731,539 or U.S. Ser. No. 60/753,829. 
     Because the optical elements following BDU  14  in optical beam path  13  until the exit of FDU  54  shape the laser beam  12  from a raw laser beam with rectangular cross section into a line beam with a target homogeneous intensity distribution along the long axis direction, the respective arrangement of optical elements in the following is called a beam shaping unit (BSU). The dashed line identified with the reference numeral  80  in  FIG. 1  encircles in the aforementioned optical elements of the BSU. 
     When leaving the BSU  80  the rectangular cross section shaped laser beam  12  is directed to combined imaging, reducing and folding optics  82  that include a plurality of plan or cylindrical mirrors  56 ,  60 ,  62 , respectively. Instead of an arrangement of mirrors  56 ,  60 ,  62 , a plurality of cylindrical lenses or a combination of lenses and mirrors may be used. Typical setups are e.g. disclosed in WO 2006/066706 A2 or in U.S. Pat. No. 5,721,416. The laser beam  12  leaves the imaging, reducing and folding optics  82 , which for simplicity reasons in the following is called beam projection unit (BPU)  82 , through an exit window  64 . The laser beam  12  having an expanded long axis dimension and a reduced short axis dimension as compared to its dimensions when leaving the BSU  80  is focused as the narrow illuminating line  70  on the panel  66 , which is typically covered with an amorphous silicon layer, on the work stage  68 . 
       FIG. 2 , which summarizes certain features of the apparatus shown in  FIG. 1 , shows the light train as a block diagram. In particular, the laser source and the BSU are drawn as rectangular blocks indicated with the reference numerals  10  and  80 , respectively, while the BPU  82  is represented solely by the aforementioned cylindrical mirror  62 . For illustrative purposes the laser beam  12  entering the BSU  80  is indicated as a single straight line having a point cross section while the laser beam with its cross section further expanding in long axis direction when leaving the BSU  80  is indicated by two diverging lines  12   a ,  12   b  and a straight line  70   a  connecting these lines  12   a ,  12   b , respectively. The illuminating line  70  focused on the panel  66  being positioned on the work stage  68  is indicated by a straight line, the long and short axis directions are indicated with reference numerals x and y, respectively. 
     Applicants recognized that because energy density on the panel is the most relevant process parameter, the pulse energy should be measured close to the panel  66  or at a location with corresponding energy density. This signal should be fed back to the stabilization circuit of the laser  10 . 
     The energy density at the panel  66  is mostly given by the laser energy and the optical transmission through the optical system. Changes in system transmission are mostly generated in the beam shaping module  80  when parameters like pointing, beam divergence or polarization are changing. The influence on system transmission from the projection optics  82  is typically relatively small. Therefore suitable positions for energy measurements can be located along the beam path  13  in the projection module  82  or in the reflected beam from the panel. Therefore, an energy meter may be located in the beam path  13  between the exit of the BSU  80  and the panel  66 . 
     There can be some advantages in positioning the sensor in the projection optics over a direct measurement in the panel level. There can be more space available and there is a smaller influence on image quality of the laser line. At some distance from the panel the line usually has not yet reached its full length and so it is easier to collect the light with a beam splitter. 
     Thus, in the specific embodiment shown in  FIG. 1  the energy meter, in the following called process energy monitor (PEM)  58 , is located between the mirrors  56  and  60 . The PEM  58  includes a beam splitter  84 , which might be a semitransparent window or a transparent window, and a detector  86  as is shown in  FIG. 2 . The main portion of the line shaped laser beam  12  hitting the front surface of the beam splitter  84  passes the beam splitter  84  and is further imaged and reduced forming the final illuminating line  70  on the panel  66 . A fraction  70   b  of e.g. 0.05 to 0.5% is reflected on e.g. the rear surface of the beam splitter  84  and directed to the detector  86 . 
     The detector  86  detects the fraction  70   b  (or at least a part thereof) and converts it into a measured signal  74 , e.g. an electrical current or an electrical voltage. The measured signal  74  is fed back via a feed back loop  72  to a control device  76 , such as a master controller which generates a control signal  78  for controlling the output energy of the laser beam source  10 . 
     In order to detect a spatially resolved energy density one would often need a relatively large two-dimensional sensor. The feedback signal for the laser starts as an analog value that determines the laser energy. To get this signal a numerical (adding up recorded values), electrical (adding currents of photo sensitive elements) or optical integration (adding the light with lens elements) should be done. For the current problem with a narrow illuminating line, the solution can involve using an optical and an electrical averaging method. 
       FIG. 3  shows the energy measuring device  86  being part of the TDX apparatus according to  FIGS. 1 and 2 . Approximately 0.2% of the beam energy is coupled out with the aforementioned beam splitter  84  that is realized as a two sided antireflective (AR) coated glass plate. Most of the beam fraction  70   b  is focused onto four photodiodes  90   a ,  90   b ,  90   c ,  90   d  via four (e.g., relatively large) spherical lenses  88   a ,  88   b ,  88   c ,  88   d  dividing the fraction  70   b  into four individual beamlets  92   a ,  92   b ,  92   c ,  92   d . The four photodiodes  90   a ,  90   b ,  90   c ,  90   d  are electrically connected in parallel and in combination electrically connected in series to a shunt resistor R s . The electrical circuit comprising the photodiodes  90   a ,  90   b ,  90   c ,  90   d  and the shunt resistor R s  is reverse biased by the reverse bias voltage V 0 . 
     Upon illuminating the photodiodes  90   a ,  90   b ,  90   c ,  90   d  photocurrents I pha , I phb , I phc , I phd  are generated. The four individual photocurrents I pha , I phb , I phc , I phd  (which are in general not identical) are added on the single shared shunt resistor R s . The control signal  78  needed for the feed back loop  72  is available at the output of this circuit as an output voltage V out . 
     The four photodiodes  90   a ,  90   b ,  90   c ,  90   d  are electrically connected in parallel. The electrical circuit including the photodiodes  90   a ,  90   b ,  90   c ,  90   d  and an electronic circuit that adds up the individual charges, analyzes the sum signal and generates an output Voltage V out . Upon illuminating the photodiodes  90   a ,  90   b ,  90   c ,  90   d  charges Q pha , Q phb , Q phc , Q phd  are generated. The four individual charges Q pha , Q phb , Q phc , Q phd  (which are in general not identical) are added in the electronic circuit. The control signal  78  needed for the feed back loop  72  is available at the output of this circuit as an output voltage V out . 
     Instead of the above sensing device including four photodiodes  90   a ,  90   b ,  90   c ,  90   d  and an electronic circuit for read out, any number of sensor types and their supporting circuitry could be used (photodiode, photomultiplier, pyroelectric, photo resistive, photon drag, etc.). 
     When upgrading the TDX apparatus with the PEM device  58  a significant improvement of stability of energy density on the panel  66  and an enlarged process window is observed.  FIG. 4  shows the normalized line beam energy in the panel plane  66  recorded with an energy meter (such as e.g. disclosed in U.S. Pat. No. 7,061,959). The thin lined curve shows the fluctuations without PEM  58  but an energy meter  94  being located in the beam path  13  between BSMU  36  and homogenizer  40  that are +/−3.5%. Using the PEM  58  in the beam path  13  between mirrors  56  and  60  for feedback control of the laser source  10  the fluctuations (bold curve) were reduced to +/−0.7%. As a result the useful process window could be enlarged by 5.5%. 
     U.S. Pat. No. 7,061,959 and U.S. Pat. No. 5,721,416 are hereby incorporated by reference. Published U.S. patent application 2006-0209310 is hereby incorporated by reference. U.S. Ser. No. 60/731,539, filed Oct. 28, 2005 and U.S. Ser. No. 60/753,829, filed Dec. 23, 2005, are incorporated herein by reference. Published international application WO 2006/066706 is hereby incorporated by reference. 
     A listing of the reference characters and corresponding elements/features follows.
       10  excimer laser     12  laser beam     12   a  line     12   b  line     13  beam path     14  beam delivery unit (BDU)     16  entrance window     18  pulse stretcher     20  exit window     22  beam preconditioning unit (BPU)     24  arrangement of optical elements     26  beam expanding unit (BEU)     28  lens arrangement     30  mirror     32  mirror     34  mirror     36  beam stability metrology unit (BSMU)     38  arrangement of optical elements     40  homogenizer     42  cylindrical lens array     44  lens     46  cylindrical lens array     48  rod     50  condenser     52  mirror     54  field defining unit (FDU)     56  mirror     58  process energy monitor (PEM)     60  mirror     62  mirror     64  exit window     66  panel     68  work stage     70  illuminating line     70   a  line shaped beam     70   b  fraction of line shaped beam     72  feedback loop     74  measured signal     76  master controller     78  control signal     80  illumination system/beam shaping unit (BSU)     82  imaging optics/reducing optics/beam projection unit (BPU)     84  beam splitter/transparent mirror     86  detector     88   a  lens     88   b  lens     88   c  lens     88   d  lens     90   a  photodiode     90   b  photodiode     90   c  photodiode     90   d  photodiode     92   a  focused beam let     92   b  focused beam let     92   c  focused beam let     92   d  focused beam let     94  energy meter (prior art)   x first direction   y second direction   V 0  bias voltage   V out  output voltage   R s  shunt resistor   I pha  photocurrent   I phb  photocurrent   I phc  photocurrent   I phd  photocurrent   

     Other embodiments are in the claims.