Patent Publication Number: US-2015069260-A1

Title: Charged-particle multi-beam apparatus having correction plate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to European Application No. 13183963.1 filed Sep. 11, 2013, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND  
     The invention generally relates to an advanced programmable multi-beam pattern definition device for use in a charged-particle multi-beam processing (in particular nanopatterning or semiconductor lithography) or inspection apparatus. Furthermore, the invention generally relates to a charged-particle multi-beam processing or inspection apparatus (such as a nanopatterning or semiconductor lithography apparatus) including this multi-beam pattern definition device. 
     SUMMARY OF THE INVENTION 
     In particular, said pattern definition device comprises an aperture array field configured to be irradiated with a beam of electrically charged particles and allow passage of the beam through a plurality of apertures thus forming a corresponding number of beamlets, each of the beamlets traversing the aperture array field along a respective beamlet path through the pattern definition device and extending downstream of the pattern definition device to a respective nominal path for each beamlet, wherein the pattern definition device includes a deflection array device positioned across said aperture array field and comprises
         a plurality of blanking openings allowing passage of beamlets through the deflection array device, and   a plurality of deflection devices, each of said deflection devices being associated with a respective blanking opening and comprising at least one electrostatic electrode, the deflection devices being selectively activatable and configured to influence, when activated, the beamlets traversing said respective blanking openings so as to deflect said beamlets by an amount sufficient to divert said beamlets off their nominal paths.       

     A pattern definition device (PDD) of the mentioned kind is used to control the exposure of a target (such as a mask blank or wafer substrate) by means of a multitude of charged electron or ion beamlets in order to expose a desired pattern on the target. Within the multi-beam processing/inspection apparatus, the PDD is irradiated with a wide beam of charged-particles and generates a plurality of beamlets, typically several hundreds of thousands, in a two-dimensional array, such as a 512×512 array. The plurality of beamlets is then imaged onto the target. 
     Charged-particle multi-beam lithography and processing is of high interest for nanolithography and nanopatterning applications, such as for multi-beam mask writing and for maskless multi-beam direct write processes on silicon wafer substrates. With regard to the present application the terms ‘target’ and ‘substrate’ are used without difference in meaning. 
     In particular electron multi-beam writing is a promising concept for the future industrial fabrication of leading-edge photomasks as needed for 193 nm immersion lithography, of EUV-masks for extended ultra-violet lithography (EUVL), and of templates (1× masks) for nano-imprint lithography, in particular for sub-20 nm semiconductor technology nodes, with extendibility to sub-10 nm technology nodes. For the multi-beam mask writer (MBMW) the applicant has coined the acronym eMET (electron Mask Exposure Tool). Configurations of multi electron beam direct write (EBDW) processing on silicon wafers in multi-column PML2 (short for “Projection Mask-Less Lithography”), configurations are described in U.S. Pat. No. 7,214,951 and U.S. Pat. No. 8,183,543 of the applicant. 
     A charged-particle multi-beam apparatus with a multi-beam PDD of the above-mentioned kind is disclosed in the U.S. Pat. No. 6,768,125 of the applicant, which is hereby incorporated into the present disclosure as relevant prior art. That patent describes a charged-particle lithography and processing method and apparatus dubbed PML2 and further publications of the applicant describe eMET (short for “electron multi-beam Mask Exposure Tool”), both of which realize a multi-beam writing concept and use a PDD for structuring a charged-particle beam, which is extracted from a single source of electrically charged particles (electrons or ions). The PDD is, preferably, a programmable multi-aperture device realized as a so called aperture plate system (APS). The implementation based on a programmable APS allows a significant improvement of the achievable productivity in comparison with focused single spot beam systems as well as variable shaped beam (VSB) systems. The reasons for the improved productivity are, firstly, the parallelism of the process using a plurality of beams and, secondly, the increased current (of all beamlets writing in parallel) which can be imaged to a substrate at the same resolution. As compared with a single electron beam system, the current density at target of the electron multi-beam writing apparatus (in the order of A/cm 2 ) is lower by approx. two orders of magnitude when compared to VSB systems, thus reducing instantaneous heating effects which are unavoidable when using single beam systems with high (&gt;100 A/cm 2 ) current density. 
       FIG. 1  illustrates an overview of a charged-particle multi-beam apparatus  100  with a PDD  102  underlying many embodiments of the invention. According to APS architecture, the PDD  102  consists of at least two plates  201 ,  202  in a stacked configuration. The plates have different dedicated functions, namely, an aperture array plate  201  (AAP) and a deflector array plate  202  (DAP). A plurality of beamlets is formed by AAP  201  and pass through larger openings in the DAP  202 ; in the DAP  202  some of the beamlets are deflected from their nominal paths while the other beamlets pass without being deflected; beamlets deflected in the DAP  202  are filtered out at a stopping plate  17  situated at or near the second cross-over c2 of charged-particle projection optics  103 . The beamlets which are not deflected in the DAP  202  continue through the cross-over c2 and are imaged onto a target  14 , thus exposing pixels on the target.  FIG. 2  illustrates the arrangement mf of pixels p 1  which can be exposed by this method on the target  14 . 
     The different kinds of plates in the APS setup of the PDD realize a high integration density of apertures and deflectors. Within the PDD, accurate alignment between the two or more plates and adequate adjustment towards the direction of the incoming beam is required. This alignment can be accomplished in-situ by means of a configuration as disclosed in U.S. Pat. No. 8,546,767. 
     The DAP has electronic circuitry associated with the apertures to allow “switching on ” and “switching off” of the individual beamlets in accordance with the pattern which is to be generated on the target. In particular, the DAP is provided with a deflection array composed of a plurality of deflection devices for deflecting selected beamlets (“switching off”). Each deflection device is located at an opening formed in the DAP and can generate an electrostatic field sufficient to deflect the respective beamlet traversing through the opening from its nominal path. The beamlets which are not deflected are, as already mentioned, imaged onto the target  14 . 
     The DAP is manufactured by three main processing steps: 1) the fabrication of the CMOS circuitry; 2) an etch process for creating the apertures; and 3) a MEMS process for building the capacitive blankers. To date the fabrication process for a DAP is highly reliable, but nonetheless there is always a certain number of the beamlet deflectors in a DAP which are not working in the specified way. The complexity of this manufacturing process may cause a certain failure rate in the functioning of some blankers which can thus not be controlled, resulting in apertures that remain in an open state or apertures that are forever closed. Typically, the number of such beamlet deflector defects is small, below one per mille (i.e., in the order of a hundred for one DAP operated for more than 100-thousand programmable beams). Usually, the beamlet deflector defects are non-clustered, meaning that they are statistically distributed over the field of apertures. Thus, in general one can expect that there are no neighboring beamlet deflector defects, i.e., one can assume that between the locations of two such defects at least one operating beamlet deflector is present, and usually a few. 
     The beamlet deflector defects result in either “always-off” defects or “always-on” defects. An “always-off” defect is present where the beamlet is either blocked from passage through the PDD or deflected from its path so that it will not reach the target, at all times and in particular regardless of the pattern information. An “always-off” defect may be due to a closed aperture in the aperture array plate and/or DAP, or by the presence of a voltage at the deflection electrodes of the deflector device associated with the respective opening in the DAP, where this voltage cannot be turned off during operation of the DAP. Thus, the beamlet affected by this “always-off” defect is always filtered out and cannot be used to contribute to the exposure of a pattern on the target. Thus, during writing a pattern to a substrate, an “always-off” defect will not allow the passage of the beamlet, which would be needed for a correct exposure. 
     An “always-on” defect (also called “always-open”) occurs where a deflection device is defective, for instance due to a defective electrode or non-operative voltage-supply of the electrode, in such a way that the respective beamlet can always pass through the corresponding aperture, without being deflected at all or without being deflected to an amount sufficient for being completely filtered out before the target (for instance at the stopping plate  17 ). 
     It is possible to compensate for the effect of an “always-off” defect within an exposure algorithm which uses redundant writing, such as disclosed in U.S. Pat. No. 8,222,621 of the applicant, by taking into account the location of the beamlet affected by the flawed device and shifting the corresponding amount of exposure to one or more other beamlets within the same redundancy group. The function of the beamlet would thereby be substituted by one or more beamlets at functioning deflection devices. In contrast to such “always-off” defects, an “always-on” defect, besides being a nuisance, is generally more difficult though not impossible to deal with. For an “always-on” defect it is not possible to overcome the effect of unwanted exposure. Thus, it is desired to ensure that a PDD has no “always-on” beam defects. 
     Therefore, it is an aim of many embodiments of the present invention to provide PDD units without “always-on” beam defects, for instance by suitable ways to correct such PDD units which, upon manufacture thereof, exhibit apertures having this type of defect. 
     In prior art approaches for compensating a defect beam are described. For instance, JP 2006-019436 A describes compensating a beam failure of the “always-off” kind by transferring a neighboring beamlet to the position of the beam failure. In particular, arrays of charged-particle beam deflectors are inserted before or after an aperture array; these mufti-deflecting systems enable transferring selected beamlets to the paths of respective neighboring beamlets. This allows to shift a functional beam to the site which the defective beam would have in the aperture array plate. 
     This aim is achieved by a PDD as described in the beginning, in which one or more defective deflection devices are present which are permanently unable to deflect the beamlets traversing blanking openings with which the defective deflection devices are associated, thus leaving such beamlets as “non-deflected beamlets”, by means of an additional filtering device. The filtering device is positioned across the aperture array field and comprises
         a plurality of passage openings allowing passage of beamlets through the filtering device for at least the beamlets passing through blanking openings of the deflection array device where the respective deflection devices are not defective, wherein preferably the paths of the respective beamlets are not affected by said filtering device, and   at least one obstructing device which is able to permanently assume an obstructing state where it prevents at least one of the non-deflected beamlets from traversing the pattern definition device along their respective nominal paths downstream of the pattern definition device.       

     This solution provides a device capable of suppressing the unwanted beamlets, which correspond to “always-on” defects, by physical means. In particular, many embodiments of the invention regard modifying the PDD at the sites where an “always-on” defect is present in such way as to prevent the passage of the respective beamlets towards the target at all times of operation. In other words, all “always-on” defects are converted into “always-off” defects, which are much easier to compensate. It is remarked that the number of defective deflection devices is lower by at least one order of magnitude, but usually two or three orders of magnitude or more, than the complete number of deflection devices in the deflection array device. 
     The solution according to many embodiments of the invention is fundamentally different from the approach of JP 2006-019436 A since the filtering device of many embodiments of the invention does not include a lens array. AH other beamlets are left unaffected so as to propagate along their respective paths, and in particular are not shifted so as to substitute a defective beamlet. Many embodiments of the invention provides a way to turn an “always on” defective beam into an “always off” defective beams, in contrast to JP 2006-019436 A where “always off” beams are not tolerated. 
     In one advantageous aspect of many embodiments of the invention, the (at least one) obstructing device is programmable to take two states, namely, an open state not affecting the respective non-deflected beamlet(s) and the obstructing state. In this case, the filtering device may suitably be realized as a plate-shaped device (“correction plate”) comprising an array of openings which correspond to the plurality of blanking openings of the deflection array device, and a plurality of obstructing devices, each obstructing device being associated with a respective opening and comprising at least one electrostatic electrode. Then, the deflection devices are selectively activatable to realize the obstructing state by diverting the beamlets traversing said respective openings from their nominal path, thus preventing the respective beamlets from traversing the pattern definition device along their respective nominal paths downstream of the pattern definition device for said minimum duration. 
     Whereas the switching times in the deflection array device need to be very fast in order to achieve productivity of the multi-beam writing or inspection apparatus, the switching times in the programmable obstructing device can be very slow. Thus, the obstructing device is configured to be switchable into the obstructing state for a certain minimum duration. This minimum duration may be greater than the product of the typical duration of activating the deflection devices in the deflection array device times the number of openings in a row of openings in the deflection array device; but often, this minimum duration is even longer and holds for the duration of one or more exposure processes of a target. 
     In another aspect of some embodiments of the invention the (at least one) obstructing device may be realized in the form of an opening provided with material obstructing the passage of the respective beamlet. 
     In one useful implementation of the PDD the deflection array device and the filtering device are respectively realized as plate-shaped devices comprising a respective array of openings; in particular, the filtering device may be realized as “correction plate” or “filtering array plate”. Furthermore, it is suitable when the filtering device is positioned across the aperture array field substantially parallel to the deflection array device. 
     In another useful development, the filtering device may also include the function of a (downstream) filed-boundary array plate. In this case, the filtering device may include—as the last component of the pattern definition device as seen along the direction of the beam—a plate-shaped device comprising an array of openings which correspond to the plurality of blanking openings of the deflection array device, and further including a downstream surface of smooth, preferably planar, shape except for said openings, to be oriented towards particle-optical components located downstream of the pattern definition device, such as a projection system for imaging the patterned beam onto the target. 
     Additionally an aperture array device may be provided comprising the plurality of apertures in the aperture array field; these apertures are configured to define the shape of the beamlets formed in the pattern definition device. The aperture array device is, preferably, positioned separate from the filtering device, i.e., in a small but finite distance. 
     In an advantageous alternative, the filtering device may be an aperture array device comprising the plurality of apertures in the aperture array field. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, several embodiments of the present invention are described in more detail with reference to the drawings, which show: 
         FIG. 1  illustrates an overview of a particle-beam exposure apparatus suitable for many embodiments of the invention in a longitudinal sectional view; 
         FIG. 2  illustrates the exposure of pixels accomplished with the apparatus of  FIG. 1 ; 
         FIG. 3  shows a bottom plan view of the POD system in the apparatus of  FIG. 1 ; 
         FIG. 4  shows a simplified sectional view of the POD system in the apparatus of  FIG. 1 ; 
         FIG. 5  illustrates a POD system, shown in a simplified sectional view, having a defective deflection device; 
         FIG. 6A  shows a longitudinal sectional view of a particle-beam exposure apparatus including a POD according to an embodiment of the invention; 
         FIG. 6B  shows an enlarged detail of the POD of  FIG. 6A  provided with a programmable correction plate; 
         FIG. 6C  shows a variant of the PDD of  FIG. 6B  with modified sequence of plates; 
         FIGS. 7A-D  illustrate the electronic layout for programming the correction plate of  FIG. 6B ; 
         FIG. 8  illustrates an embodiment of the invention in a longitudinal sectional view, namely, with a passive filtering plate blocking beamlets of the defective deflection device; 
         FIG. 9  illustrates an embodiment of the invention in a longitudinal sectional view, namely, with a passive filtering plate which also operates as final field-boundary plate; and 
         FIG. 10  illustrates an embodiment of the invention in a longitudinal sectional view, namely, with an aperture array plate (AAP) also serving as passive filtering plate. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments of the invention discussed in the following are based on a development of the eMET-type charged-particle multi-beam exposure apparatus and its PDD system, having a large-reduction projecting system. In the following, first the technical background of the apparatus is discussed as far as relevant to many embodiments of the invention, then certain embodiments of the invention are discussed in detail. It should be appreciated that embodiments of the invention are not restricted to the following embodiments or the particular layout of PDD systems, which merely represent examples of possible implementations of the invention; rather, many embodiments of the invention are suitable for other types of processing systems that employ a charged-particle beam and a multi-beam patterning as well. 
     eMET System 
     A schematic overview of an electron multi-beam mask exposure tool  100  (mask writer) employing an embodiment of the invention is shown in  FIG. 1 . In the following, only those details are given as needed to disclose certain embodiments of the invention such that one of ordinary skill in the art can practice the respective embodiments; for the sake of clarity, the components are not shown to size in  FIG. 1 , particularly the lateral width of the particle beam is exaggerated. Similar to eMET are also the principles of the PML2 system; for more details, the reader is referred to the U.S. Pat. No. 6,768,125 and U.S. Pat. No. 7,781,748, whose teachings with respect to the overall layout of the charged-particle multi-beam apparatus and the PDD system are herewith included by reference. The eMET system implements a suitable source for generating an electron beam. Equivalent principles apply when using an ion source, instead, for an ion multi-beam tool, which the applicant also has realized (CHARPAN, short for “Charged-Particle Nanopatterning”). 
     The main components of the apparatus  100  are—in the order of the direction of the beam lb, pb which in this example runs vertically downward in FIG.  1 —an illumination system  101 , a PDD system  102 , a projecting system  103 , and a target station  104  with the target or substrate  14 . The charged-particle optical systems  101 ,  103  are realized using either electrostatic lenses only (e.g. when using ion multi-beams) or a combination of electrostatic and magnetic lenses (e.g. when using electron multi-beams). The charged-particle optical parts  101 , 102 , 103  of the apparatus  100  are contained in a vacuum housing (not shown) held at high vacuum to ensure an unimpeded propagation of the beam lb, pb along the optical axis of the apparatus. 
     The illumination system  101  comprises, for instance, an electron or ion source  11 , an extractor arrangement defining the location of the virtual source, a general blanker (not shown in  FIG. 1 ), which in the case of using ion beams may also be used as ion mass species filter, and an illumination charged-particle optics realized by a charged-particle optical condenser lens system  13 . In the embodiment shown the particle source  11  emits energetic electrons of a suitable kinetic energy such as, e.g., 5 keV; in other implementations, other electrically charged particles such as ions of primarily a certain species may be used, such as hydrogen or Ar +  ions having a defined (kinetic) energy of typically several keV (e.g. 5 keV at the PDD system  102 ) with a comparatively small energy spread of, e.g., ΔE=1 eV. A velocity/energy dependent filter (not shown) may be provided to filter out other, unwanted particle species that may also be produced in the source  11 ; the filter may also be used to blank out the beam as a whole during repositioning of the beamlets. By means of the condenser lens system  13 , the charged particles emitted from the source  11  are formed into a wide-area, substantially telecentric beam serving as beam lb. 
     The beam lb then irradiates a blanking device which, together with the devices needed to keep its position (not shown), forms the PDD system  102 , which is also shown in a schematic perspective detail view at the left-hand side of  FIG. 1 . The PDD system is held at a specific position in the path of the beam lb, which thus irradiates an aperture array pattern formed by a plurality of apertures  20 . As already mentioned, each of the apertures can be “switched on” or “off”. In a “switched on” or “open” state, an aperture allows the beamlet passing through the respective aperture to reach the target; it is then said, the aperture is transparent to the incident beam. Otherwise, the aperture is “switched off” or “dosed”, in which case the beam path of the respective beamlet is affected (e.g. by means of deflection electrodes to which a transverse voltage is applied) in a way that it will be absorbed or otherwise removed out of the beam path before it can reach the target; thus, the aperture is effectively non-transparent or opaque to the beam. 
     Each switched-off beamlet is absorbed, for instance, on a stopping plate  17 , which is situated at or near the second cross-over c2 of the charged particle projection optics; the other beamlets, which are switched-on, pass through a central opening of the plate  17  and thus are projected onto the target. The filtering of deflected beamlets can also (as e.g. shown in  FIG. 7  of U.S. Pat. No. 6,767,125) take place at a third plate (“terminal plate”) placed in the PDD system, further downstream in proximity of the DAP. In contrast, in the layout illustrated in  FIG. 1  filtering of deflected beamlets at a stopping plate  17  has the advantages that a terminal plate in the DAP is not needed, and the deflection angle as provided by the DAP to accomplish beam blanking can be significantly smaller. 
     In  FIG. 1  only five beamlets are shown in the patterned beam pb, while it will be clear that the actual number of beamlets is very large, i.e. typically many thousands; of the beamlets shown, the first from the left is depicted switched off (marked by an arrow below the position of the active electrode deflecting this beamlet). The PDD system architecture is discussed in further detail below referring to  FIGS. 3 and 4 . 
     The switched-on apertures are the only portions of the PDD transparent to the beam lb, which is thus formed into a patterned beam pb emerging from the apertures (i.e., in  FIG. 1 , below the PDD system  102 ). By virtue of the pattern defined by the PDD  102 , an arbitrary beam pattern can be generated and transferred to the substrate. Depending on the pattern to be exposed on the target there is, from one pixel exposure to the next, a variation of which beamlets are “switched on” or “switched off”. Therefore, while the details of the actual pattern are irrelevant for the description of embodiments of the present invention, it is important to consider that for the proper exposure of a pattern it is required that a considerable number of the beamlets will be switched off in the course of irradiation of the target, with the pattern changing very fast. 
     A charged-particle optical projection system  103  then projects the pattern as represented by the patterned beam pb towards the substrate  14  (such as a 6″ mask blank with resist coating); since the beamlets which are switched off are absorbed at the stopping plate  17 , only the switched-on beamlets will form an image of the switched-on apertures. The projection system  103  implements a demagnification of, for instance, 200:1, as realized by the applicant. The substrate  14  may be, for instance, in the case of an eMET-type system a 6-inch mask blank or a nanoimprint 1× mask or master template, covered with an electron sensitive resist layer, whereas for a PML2 system the substrate  14  may be a silicon wafer covered with a charged-particle sensitive resist layer. The substrate is held and positioned by a substrate stage (not shown) of the target station  104 . 
     The projection system  103  is, for instance, composed of two consecutive charged-particle optical projector sections with a crossover c 1  and c 2 , respectively. The charged-particle optical lenses  30 ,  31  (comprising, for instance, an electrostatic multi-electrode accelerating lens  30  and two magnetic lenses  31 ) used to realize the projectors are shown in  FIG. 1  in symbolic form only as technical realizations of charged-particle imaging systems are well known in the prior art. The first projector section  30  images the plane of the apertures of the PDD system to an intermediate image which in turn is imaged onto the substrate surface by means of the second projector section  31 . Both sections employ a demagnifying imaging through crossovers c1,c2; thus, while the intermediate image is inverted, the final image produced on the substrate is upright (non-inverted). The demagnification factor is about 14:1 for either stage, resulting in an overall demagnification of 200:1. A demagnification of this order is in particular suitable with a lithography setup, in order to elevate problems of miniaturization in the POD system. 
     As a means to introduce a small lateral shift to the image, i.e. along a direction perpendicular to the optical axis cx, deflection means  16  are provided in one or both of the projector sections. Such deflection means can be realized as, for instance, a multipole electrode system, as discussed in the U.S. Pat. No. 6,768,125. Additionally, an axial current coil may be used to generate a an axial magnetic field providing rotation of the pattern in the substrate plane as needed. The lateral deflections are usually quite small in comparison to the lateral width of the beam array field at the substrate which is the width of the structured beam pb leaving the PDD divided by the reduction factor of the charged-particle projection-optics. Thus, the lateral deflections are at least one order of magnitudes below the width of the beam array field (it should be appreciated in this context that the lateral distance between beamlets is considerably smaller than the mentioned width of the beam array field). 
     Referring to  FIG. 2 , the apparatus  100  forms an image field mf on the target  14 , which is, for instance, a resist-covered mask blank or wafer. The image filed mf has a finite size which is usually well smaller than the total area to be exposed on target. Therefore, a scanning stripe exposure strategy is utilized, where the target is moved under the incident beam, so as to change the position of the beam on the target perpetually: the beam is effectively scanned over the target surface along a scanning direction sd. In the embodiment shown, it is the target which effects the (large-scale) movement, so a beam-scanning strategy is no required for the large-scale movement; ft is emphasized that for the purpose of many embodiments of the invention, only the relative motion of the image field mf on the target is relevant. 
       FIG. 2  shows one possible arrangement of a grid of primary pixels p 1  within the basic lattice of pattern pixels px. The image field mf is composed of a plurality of pattern pixels px. It should be appreciated, however, that only a subset of the pixels px can be exposed simultaneously (at one time) since only a finite number of apertures is present in the aperture array field of the PD system. The pixels which can be exposed simultaneously are also referred to as primary pixels p1. The primary pixels p 1  arranged in a regular rectangular grid; in this case, it is a square-like grid (‘quad grid’) with a 3×3 spacing in the basic lattice. The primary pixels p1 are the images of the apertures  20  in the PDD system, so they reproduce the spatial arrangement of the apertures in the AAP of the PDD system (see  FIG. 3 ), though at a different scale, because of the demagnification of the optical system. In a sequence of consecutive pixel exposure cycles, the image field mf is swept over the target along a path a4 (which is achieved by a movement of the target, a movement of the beam, or a proper combination of both) so as to expose all pixels px on the target in successive way. In addition to the basic relative movement along the sd, a transverse movement is made, for instance by means of the deflection means  16  ( FIG. 1 ). After a number of pixels have been exposed in different pixel lines along path a4, the movement shifts back to the original pixel line (plus a linear displacement along the direction sd as effected during the time interval lapsed). Thus, the amplitude of the transverse movement can be small; it will be appreciated that the amplitude is in the order of the mutual distance of beam lets. Other aperture arrangements may implement other geometric figures, such as a staggered grid. 
     PDD and DAP 
       FIGS. 3 and 4  illustrate the above-mentioned PDD  200 , which is basically in conformance with U.S. Pat. No. 6,768,125 and U.S. Pat. No. 7,276,714 of the applicant, in more detail,  FIG. 3  depicts a bottom view of the PDD (view direction against the direction of the illuminating beam) with a regular apertures array, and  FIG. 4  shows a longitudinal sectional view according to a section along a plane corresponding to line  4  in  FIG. 3 . The apertures array may also realize different arrangement, such as a staggered apertures array as shown in U.S. Pat. No. 6,768,125. 
     The PDD system  200  comprises a number of plates  201 ,  202  which are mounted in a stacked configuration, realizing a composite device whose components serve respective specific functions. Each of the plates is preferably realized with silicon microsystems technology in which the structures have been formed by micro-structuring techniques as outlined e.g. in U.S. Pat. No. 7,687,783 and U.S. Pat. No. 8,198,601. 
     A plurality of apertures is located in membranes mb formed by thinned regions of silicon wafers usually cut to a smaller quadratic or rectangular format, forming an aperture array field, designated by reference symbols af in  FIG. 3 ; the membranes mb are stabilized by the surrounding frame fs ( FIG. 4 , the frame is shown hatched in  FIG. 3 ). Each aperture corresponds to a set of consecutive openings which are defined in said plates. The number of apertures visible in  FIGS. 3 and 4  is small for the sake of better clarity, to represent a large number of apertures forming the aperture array field in the membranes mb; in  FIG. 4 , only two apertures are shown. The charged particle beam lb traverses the plates through this array of apertures of the aperture array field af. The AAP  201  is designed to be irradiated with the broad particle beam and allows passage of charged particles through a plurality of apertures  20  only. Thus, the AAP absorbs the majority of the impingent charged particle beam lb, but the charged particles can pass through a number of apertures  20  of defined shape, thus forming a plurality of beamlets, of which only two beamlets b 1 , b 2  are shown. Each of the beamlets traverses the aperture along a respective beamlet  path. Apart from the task of forming the beamlets, the AAP  201  serves to protect the subsequent plate(s) from irradiation damage. In order to avoid local charging the aperture array plate may be coated with an appropriate layer  210 , usually a metallic layer forming no oxide (e.g. iridium). When using ion beams, the layer  210  and method of its formation are chosen suitably to prevent the irradiating particles to be incorporated in the silicon crystalline matrix, which would cause a change of membrane stress, as outlined in U.S. Pat. No. 6,858,118. 
     Referring to  FIG. 3 , the AAP comprises a plurality of apertures ap arranged within an “orthogonal” aperture array field af along a regular rectangular grid. In the embodiment shown, the aperture array field realizes a regular grid wherein the apertures are spaced in a 3×3 grid, for instance, in which the offset between apertures is 3 times the nominal aperture width w1 along either direction of the grid (consequently, the free space between neighboring apertures is 2 w1). This aperture arrangement conforms to the arrangement of primary pixels of  FIG. 2 . In other words, the aperture array field af realizes a regular array of apertures ap, which are arranged in several rows, each row comprising a number of apertures. For instance, the apertures may be arranged in a 512×512 array, i.e., 512 rows of 512 apertures each. 
     Positioned in proximity to the AAP  201 , preferably downstream thereof, a deflector array plate  202  (DAP; also referred to as blanking plate in view of its function in the context of the apparatus  100 ) is provided. The DAP  202  serves to deflect selected beamlets and thus to alter their beam path. The DAP has a plurality of so called blanking openings, which each correspond to a respective aperture of the aperture array plate  201 . As already mentioned and shown in  FIG. 4 , the blanking openings in the DAP are larger than the openings in the aperture array plate. The area between the DAP openings is used for accommodating CMOS electronics (shown cross-hatched in  FIGS. 4 and 5 ) to control and power the beamlet deflection devices. Thus, the beamlets formed with the AAP can pass through the DAP freely (i.e., without geometric obstruction). In the DAP the beamlets can be selectively deflected by an amount sufficient to divert said beamlets off their nominal paths, so that the deflected beamlets cannot reach a target, as described in prior publications of the applicant. 
     Each blanking opening in the DAP is provided with a beamlet deflection device which allows to individually deflect charged particles transgressing through the opening, thus diverting the beamlet traversing the opening off its path. Each beamlet deflection device includes a set of beamlet deflection electrodes, usually a pair. Preferably, each set has electrodes of different types: a first type are ‘ground electrodes’, which are applied at the potential of the PD device, whereas another type, which is called here the ‘active electrodes’, are applied individual potentials in order to deflect the beamlet transgressing the corresponding blanking opening. The ground electrodes may be shared between adjacent beamlet deflection devices. They may be formed so as to have a substantial height over the height of the active electrodes. This is done in order to provide sufficient shielding of the blanking deflection device against cross-talking and other unwanted effects such as lens effects incurred by the deflection electrode geometry, 
     In the DAP illustrated in  FIG. 4 , each beamlet deflection device comprises an active electrode  221 ,  221 ′ and a ground electrode  220 ,  220 ′ respectively. The electrodes are for instance free-standing with respect to the DAP base membrane. Such electrodes may be formed by perpendicular growth employing state of the art techniques as for instance electroplating techniques. 
     For instance, beamlet b 1  transgresses the subsequent larger opening of the pattern definition system  102  without being deflected, since the beamlet deflection device formed by the respective set of beamlet deflection electrodes is not energized, meaning here that no voltage is applied between the active electrode  221  and the associated ground electrode  220 . This corresponds to the “switched-on” state of the aperture. Beamlet b 1  passes the pattern definition system  102  unaffected and is focused by the particle-optical system through the crossovers and imaged onto the target with a reduction as induced by the charged-particle projection optics. For example, in systems implemented by the applicant, a reduction factor as large as 200:1 was realized. In contrast, as shown with beamlet b 2 , a “switched-off” state is realized by energizing the beamlet deflection device of this aperture, Le. applying a voltage to the active electrode  221 ′ with regard to the corresponding ground electrode. In this state, the beamlet deflection device formed by electrodes  220 ′, 221 ′ generates a local electric field across the path of the corresponding beamlet b 2  and thus deflects the beamlet b 2  off its normal path p0 to a deflected direction. As a consequence the beamlet will, on its way through the charged-particle optical system, obey an altered path p1 and be absorbed at an absorbing means provided in the PDD system or the charged-particle optical system (e.g. at the stopping plate  17 ,  FIG. 1 ), rather than reaching the target. Thus, beamlet b 2  is blanked. The beam deflection angle is largely exaggerated in  FIG. 4 ; it is, in general, very small, typically 0.2 to 1 thousands of a radian. 
     In the configuration shown in  FIG. 4 , the plates  201 ,  202  are bonded together at bonding regions  212  in the frame fs by means of known bonding techniques. In other variants, the plates may be mechanically separate and held in their position by adjustable holding apparatus as described for instance in US 201 110204253 A1 of the applicant. This has the advantage that the aperture array plate is not influenced by positional changes of the DAP. Slight positional changes of the DAP with regard to the other plates may be tolerated as the DAP has only the purpose to deflect individual beamlets whereas not deflected beamlets are passing through the DAP and are imaged to the substrate. 
     It is to be noted out that the PDD system may further include an additional plate (not shown, but see the variant of  FIG. 9 ), a so called Field-boundary Array Plate (FAP), which provides a defined interface between the PDD  102  and the charged-particle projection optics  103  in terms of the electric field. 
     Deflection Defects 
       FIG. 5  shows, in a sectional view similar to  FIG. 4 , a simplified example of an PDD system  55  having a defective beamlet deflector producing an “always-on” defect. Realistically, in view of the large number of may thousands of beamlets and deflection devices in a typical PDD system, one has to take into account that some, albeit only very few, of the deflection devices may be defective. The PDD  55  comprises an aperture array plate  51  and a DAP  52 . Five beamlets, which are defined in the apertures  50  of the aperture array plate  51 , are shown, representing a much larger number of beamlets present in a realistic PDD setup. Each of the beamlets traverses the DAP  52  through a respective opening  520 , and at the location of each opening  520 , a deflection device  521  is provided. Each of the deflection devices  521  comprises at least two electrodes, for instance at least one active electrode  522  operates against a counter electrode  523 . The counter electrodes of the deflection devices are held at a common “ground potential”; they may be separate or connected with each other. 
     Furthermore, in  FIG. 5  one of the deflection devices  521  is defective, namely, the device  501  associated with beamlet b. This device  501  forms a defective deflection device (DDD) having an “always-on” defect, for instance due to a broken voltage supply of the active electrode  502 . Thus, the beamlet b will always be in a switched-on state propagating along its nominal path, since regardless of the pattern information supplied to the deflection control unit of this DDD no deflection effect is accomplished, whereas the other beamlets (which are associated with functioning deflection devices  521 ) can be switched between a switched-on and a switched-off state, as illustrated by doubled arrows at the bottom of  FIG. 5 . 
     Such “always-on” defects can be detected by using in-situ beam diagnostics, or it might also be that the position of such beam defect is known from the characterization of the PDD unit prior to inserting it into the multi-beam writing or inspection tool. In particular, each DAP can be tested with a dedicated test bench in order to find openings having a deflector defect. A test bench suitable for this purpose is described in the article of S. Eder-Kapl et al. “Characterization of CMOS programmable multi-beam blanking arrays as used for programmable multi-beam projections lithography and resistless nanopatterning”, J. Micromech. Microeng, 21 (2011) 045038 (doi:10.1088/0960-1317/21/4/045038). This test provides a map of the locations of “always-on” and “always-off” defects. Thus, it is possible to determine the number and positions of DDDs for each DAP. 
     Programmable Correction Plate 
     A first approach to correct “always-on” defects according to some embodiments of the invention is by using a correction plate, which is used as an in-situ device for eliminating “always-on” defects. In one preferable realization this correction plate is an additional plate-shaped device which is inserted to the plate stack of the PD device. 
       FIG. 6A  shows a multi-beam apparatus with a PDD system  7  according to one embodiment of the invention.  FIG. 6B  shows the PDD  7  in an enlarged detail. Besides aperture array plate  51  and DAP  52 , which correspond to those of  FIG. 5 , the PDD  7  includes an additional plate-shaped device  53 , referred to as correction plate, representing one embodiment of a filtering device according to the invention. The correction plate  53  is configured to always act on selected beamlets, i.e. to deflect such (always-on) beamlets in a way that they are then filtered out in the projection optics, for instance at a stopping plate  17  or other absorbing surface, as shown in  FIG. 6A  for beamlet b′; the other beamlets are not affected by the correction plate  53 .  FIG. 6A  shows one specific switching state for the beamlets, in which the leftmost beamlet (which is controlled by an operating deflection devices) and the beamlet b′ are deflected (“switched-off”) while the other beamlets are switched-on and reach the target. 
     The correction plate  53  is a programmable device superficially similar to the DAP, having the ability to deflect or let pass the individual beamlets emanated by the DAP. In contrast to the DAP, the configuration of deflection will be kept during the entire processing of an exposure. The correction plate  53  is positioned within the PDD system, for instance at a short distance downstream the DAP  52 , The correction plate  53  is usually configured with the same number and geometric positions of openings  70  as the DAP  52  with its openings  520 . 
     In particular, the correction plate  53  is provided with a plurality of correction deflection device  71  (ODD), each of which is located at the site of one respective opening  70 , such as to be able to deflect the beamlet traversing that opening  70  when activated. For instance, each of the CDDs  71  comprises at least two electrodes, for instance at least one active electrode  72  operating against a counter electrode  73 . The counter electrodes  73  of the CDDs may be separate or connected with each other; they may be held at a common “ground potential”, or they may be applied a counter potential, for instance of same value but opposite sign as the potential of the active electrode  72 , when the ODD is activated. 
     In  FIG. 6B  the correction of a DDD  501  in the DAP  52  is shown for the example of beamlet b′ corresponding to the ODD  74 . The ODD  74  is activated, for instance, by applying an electrostatic potential to the active electrode  742  which generates an electrostatic field in cooperation with the corresponding counter electrode  743 . Thus, the beamlet b′ will always be deflected off its nominal path, while the other beamlets (which are associated with functioning deflection devices  521 ) can be switched between a switched-on and a switched-off state as before, as illustrated by doubled arrows at the bottom of  FIG. 5B . 
     In the embodiment shown in  FIG. 6B  the correction plate is located downstream of the DAP. However, as illustrated in  FIG. 6C , positioning the correction plate  53  upstream of the DAP  52  is equally well possible in a PDD system  7 ′, since the deflection angle involved is very small, so it is unproblematic that a beamlet deflected by the correction plate will also pass through the opening of the DAP. 
     It is important to note that there are decisive differences between a DAP  52  and a correction plate  53  of an embodiment of the invention: While the DAP  52  has to include a fast CMOS circuitry in order to account for the fast-changing pattern information, the circuitry of the correction plate  53  can be slow since the mode of operation in the correction plate will stay constant over at least the duration of an entire exposure treatment of the target  14 . It is sufficient that the circuitry of the correction plate  53  can be held in a programmed state for a minimum duration which corresponds to the duration of the exposure of a wafer, which is a multiple (by orders of magnitude) of the duration of the exposure of a row of pixels on the target. In terms of the elements in the PDD, this duration is a multiple of product of the duration of activating a deflection device  521  in the DAP  52  times the number of apertures in a row of apertures in the aperture filed af. For instance, an implementation of the correction plate circuitry may include simply wired components or may be realized as simple (flip-flop) slow CMOS circuitry, The latter is discussed more in detail in the following, referring to  FIGS. 7A-D . 
     Referring to  FIGS. 7A to 7D  an implementation of a flip-flop circuitry RR ( FIG. 7D ) of a correction plate  53  includes a plurality of shift registers arranged in rows. Each shift register has a logic output connected to the electrode supply unit of the respective CDD; this supply unit is referred to as “blanker”. Before an exposure takes place, these rows of shift registers will be loaded with a pattern defining which beamlets are to be deflected permanently, where the logic state of each bit of the pattern determines the associated blanker state. The pattern will be held in the shift registers and is kept unaltered during exposure. The example shown here relates to a correction plate for an array of  2   18 =262144 beamlets, which traverse a corresponding number of openings arranged in a 512 by 512 matrix, like the one of  FIG. 3  which shows only a small fraction of the openings. 
       FIG. 7   a  shows two registers  77 ,  77 ′ for programming respective blankers  76 , representing the first two of 64 registers forming a queue sequence S1. A sequence of 8 queues S1, S2, . . . S8 connected serially forms a row R 01  comprising 512 flip flop registers FF1 to FF512, as visible in  FIG. 7B . As depicted in  FIG. 7C , 64 of these register rows R01, R02, . . . R64 are connected in a serial sequence to form a group G1 (=32768 flip flops).  FIG. 7D  shows that 8 groups G1, G2, . . . G8 are combined so as to form a shift register array RR comprising a total of 262144 registers. 
     Prior to operating the PD device, a bit pattern is loaded into the shift register array RR of the correction plate and will then remain there either statically or until a new pattern is loaded (or power is lost). Each bit in one of the shift registers  77  has two states (low and high). The low state of the register sets the blanker  76  of the respective deflection device to “open”, by not applying a voltage and thus allowing the beamlet to pass undeflected, whereas the high state causes the blanker  76  to apply a voltage to the electrodes, so as to effect deflecting the respective beamlet. The bit pattern loaded into the array is by default made up of low bits, allowing all beamlets to pass. Only those bits that correspond to defective, permanently open deflection devices in the DAP, are set high. 
       FIG. 7A  shows two instances of the electrostatic circuitry for programming the CDDs of the correcting plate  53 . Each opening (not shown in  FIG. 7 ) is associated with a corresponding CDD  71  and blanker  76 . For each blanker  76  a single shift register  77  is provided; this shift register  77  is a D-type flip flop with one data input, one data output and one clock input, marked 0, CLK, and Q, respectively. The logic circuit has a 3.3 V supply, which may also be used to supply the blankers; the voltage supply for the electrodes of the COD  71  is not shown. 64 flip flop registers are connected this way into a queue sequence S1. In the queue sequence S1, each data output Q of a register  77  is connected directly to the corresponding blanker  76  and to the data input D of the next flip flop; the output of the last flip flop leaves the queue S1 for feeding a successive queue sequence S2 ( FIG. 7B ). 
     As illustrated in  FIG. 7D  the plurality of 2 18  registers, which are present in the plate  53  of this example, is subdivided into 8 groups, each group comprising 32768 registers. Each group G1 . . . G8 has, as its input, one data input line and clock input line, numbered Data1 in, Clock1 in for group G1 to Data8in, Clock8in for group G8; the inputs Data1 in, Clock1 in of the first group G1 corresponds to the input lines Data, Clock of  FIGS. 7A to 7C . The number of registers in a group exceeds the number of openings in a row on the correction plate, which is 512. As illustrated in  FIG. 7C  for group G1, each group is geometrically arranged in 64 rows R01, R02, . . . R64. Thus, the clock input line Clock1 in is used to drive the registers of 32768 registers (64 rows of 512 flip flops). Since this number exceeds the fanout of the external device providing the clock signal by far, each row R01 . . . R64 comprises a number of repeaters A1, A2, . . . A8, which is shown in  FIG. 7B  for row R01. Thus, in each row, eight repeaters A1 . . . A8 are provided, one for each of queue sequences S1 . . . S8 of 64 flip flops each. 
     Under vacuum conditions, the temperature of the correction plate  53  will be controlled by radiative cooling only. In order to minimize the current being drawn from the power source and to reduce heat production from switching processes during loading of the pattern, the eight groups G1 . . . G8 will be loaded with the pattern data in succession, one at a time, by means of separate data Data 1 in . . . Data8in and clock signals Clock1 in . . . Clock8in. The clock cycle time is chosen suitably so that the propagation delay of the cascaded clock repeaters is accounted for, since the clock signal arriving at the last flip flop takes 8 propagation delays of repeaters A 1  to A 8 . 
     The row-style layout of the circuit array RR shown in  FIGS. 7A-D  allows a simple implementation in a wafer die based device since i is based on a single row of 512 flip flop registers, which is repeated on the die multiple times, with only one line running from the end of a row to the beginning of the next row. 
     Passive Filtering Array Plate 
     In another approach according to many embodiments of the invention a filtering device according to many embodiments of the invention is realized as a passive plate in the APS, where “passive” denotes that the plate does not have a circuitry to deflect beamlets. In the passive filtering plate an array of openings/apertures is provided, but it is modified such that openings are missing, or have been dosed, at those sites which correspond to the locations of the “always-on” defects. 
     To obtain such a modified array, for instance, one possibility is to use a repair system based, e.g., on a focused on beam (FIB) system, in which a gas injection supported deposition is used to dose the openings at the sites of an “always-on” defect. This approach uses the fact that with FB-induced deposition there is not only a deposition onto the surface, but always a side growth as well. This can be exploited to bridge openings, thus covering them, starting the FIB-induced deposition at one or more edges of the opening and then moving on the beam across the area of the opening, until the complete area of the opening is covered by deposited material. Instead of FIB-induced deposition, laser-beam or electron-beam induced deposition methods may be used as well. 
     Thus, referring to  FIG. 8 , the PDD system  8  is provided with a passive “filtering array plate”  83 . The filtering array plate  83  is produced starting from a membrane plate having an arrangement of openings  80  like those in the DAP  82  but without the circuitry layer. Filling selected openings  85 , namely those which correspond to openings with DDDs in the DAP  82 , with suitable material will create “nontransparent sites”  84 . During this process care must be taken that all other openings  80  are kept open. The filling material may be applied to the upper surface of the opening and/or its lower surface, and/or within the opening. Preferably, the width of the openings  80  in the plate  83  is larger than the width of openings in the DAP, in order to ensure unimpeded propagation of the respective beamlets through the openings  85  that are kept open. 
     The filtering array plate  83  is then combined with the (partially defective) DAP  82  while the original aperture array plate  81  is kept. The material in the site  84  will absorb or otherwise obstruct the passage of the respective beamlet b 0  and thus prevents its passage through the PDD system  8 . Thus, a PDD  8  is produced which avoids the “always-on” defects for the beamlets, and the obstruction site  84  serves as obstructing device according to several embodiments of the invention, 
     In an analogous manner as with  FIGS. 6B and 6C , the filtering array plate  83  may be located downstream or upstream of the DAP  82 . 
       FIG. 9  illustrates a variant of the embodiment of  FIG. 8 , wherein the PDD system includes a (downstream) field-boundary array plate (FAP) as final plate, which may suitably realized by the filtering array plate, merging both functions in one component. In the PDD system  8 ′, the filtering array plate  83 ′ is oriented such that it exhibits a smooth, preferably planar, downstream surface  86 , except for the openings through which the beamlets traverse. To realize this surface  86 , the orientation of the plate  83 ′ is suitably inverted (“flipped over”) as compared to the analogous plate  83  of  FIG. 8 . In order to ensure a good definition of an equipotential plane, the surface  86  may include an electrically conducting layer. The surface  86 , by virtue of its downstream orientation, i.e. towards the charged-particle projection optics  103 , provides a defined interface between the PDD  102  and the projection optics  103  in terms of the electric field. In its function as a filtering array plate according to many embodiments of the invention, the plate  83 ′ comprises an array of openings  80  wherein selected openings  85  which correspond to openings with DDDs in the DAP  82 , are filled with suitable material so as to generate nontransparent sites  84 ′. The location of the material in the opening  85 ′ may be chosen as suitable; preferably, the material is applied so as to form a flush surface with the downstream surface  86  of the plate  83 ′. 
     It is to be noted that in all embodiments, the first plate of the POD system can also serve as upstream FAP, by virtue of its planar surface which is oriented towards the illumination system  101 . In order to enhance this function it may suitably include an electrically conducting cover layer (not shown). 
     It is further to be noted that also a programmable correction plate (cf.  FIGS. 6B and 6C ) may be adapted so as to serve as a downstream or upstream FAP. In this case, the CODs will be located at the inner surface of the programmable correction plate (rather than on the opposite to the layout shown in  FIGS. 6B and 6C ), so the opposite surface represents a uniform, suitably planar, surface (possibly including an electrically conducting cover layer) disregarding the opening for the beamlets. The plate is oriented such that the uniform/planar surface is oriented facing outwards, i.e. upstream towards the irradiating beam lb in the case of an upstream FAP, or downstream in the case of a downstream FAP. 
     In another variant shown in  FIG. 10 , the AAP may be modified so as to operate as filtering device according to many embodiments of the invention. This variant starts from the AAP, or a copy of it, which has the full array of openings (see AAP  81  of  FIG. 8 ), and a “filtering aperture array plate”  91  is produced by filling openings  90 ′ corresponding to DDDs with material to create a “nontransparent site”  94  which absorbs or otherwise obstructs the passage of particles of the irradiating beam lb. During this process care must be taken that all other openings  90  are kept open. The absorbing material  94  thus forms an obstructing device which inhibits passage of a beamlet through the PDD system at the relevant location, by suppressing the formation of a beamlet b 9  in the first place. This filtering aperture array plate  91  is then combined with the DAP  92 , replacing the original AAP. 
     Alternatively, a filtering aperture array plate may be produced having an aperture array with pre-defined missing apertures at the sites of “always-on” defects, by means of a fabrication process of state of the art where the plate pattern is written with an electron beam. This kind of fabrication process is straightforward. Therefore, this alternative of producing a modified aperture array plate is expected to be very effective as well.