Abstract:
Charged particle sensing devices and methods of forming charged particle sensing devices are provided. The charged particle sensing device includes a source of charged particles, a plurality of collector electrodes for receiving a first portion of the charged particles and a grid formed around and spaced apart from the plurality of collector electrodes. The grid receives a second portion of the charged particles and directs backscattered charged particles, generated responsive to the second portion, to adjacent collector electrodes.

Description:
FIELD OF THE INVENTION 
     The present invention relates, in general, to charged particle sensing devices and, more specifically, to charged particle collector structures for collecting charged particles and for reducing crosstalk between the collector structures. 
     BACKGROUND OF THE INVENTION 
     Charged particle detectors that sense high energy charged particles are well known in the art and are used for a wide variety of applications, such as mass spectrometry, ion microscopy and night vision. One common charged particle detector includes a micro-channel plate (MCP) to intensify the number of charged particles (by generating secondary charged particles) and a florescent screen to detect the intensified charged particles. Another common charged particle detector includes a solid state active pixel sensor, which typically includes collector electrodes to collect incoming charged particles for a plurality of pixels. The pixel sensor detects the collected charged particles and integrates the charge. The signal charge is then read out by scanning pixels to generate a charged particle image. 
     A consequence of using high energy charged particles is the probability that some of the charged particles may be backscattered upon impact with the surface of the detector. The backscattered particles may produce a loss in signal and in spatial resolution. For solid state active pixel sensors, the collector electrodes are typically electrically isolated from each other, such as by a dielectric material. A consequence of the electrical isolation is that any dielectric material exposed between the collector electrodes may collect charge during bombardment of the charged particles and create crosstalk between the pixels. Another consequence of the electrical isolation is that the fill factor of the solid state pixel sensor (i.e., the ratio of the total charged particle collection surface area to the total contiguous area occupied by the pixel array) may be reduced, typically to be less than about 90%. 
     SUMMARY OF THE INVENTION 
     The present invention relates to charged particle sensing devices. The charged particle device includes a source of charged particles, a plurality of collector electrodes for receiving a first portion of the charged particles from the source and a grid formed around and spaced apart from the plurality of collector electrodes. The grid receives a second portion of the charged particles from the source and directs backscattered charged particles, generated responsive to the second portion, to adjacent collector electrodes. 
     The present invention also relates to charged particle sensing devices which include a source of charged particles and a charged particle collection surface for receiving the charged particles from the source. The charged particle collection surface includes a plurality of collector electrodes and a grid formed around and spaced apart from the plurality of collector electrodes. One of the grid and the plurality of collector electrodes includes a raised edge spaced apart from the charged particle collection surface. 
     The present invention further relates to a method of forming a charged particle sensing device. The method includes disposing a plurality of collector electrodes on a charged particle collection surface and disposing a grid on the charged particle collection surface. The grid is formed around and spaced apart from the plurality of collector electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures: 
         FIG. 1  is a schematic diagram of a charged particle sensing device, according to an exemplary embodiment of the present invention; 
         FIG. 2A  is a cross-section diagram of a portion of the imager shown in  FIG. 1 , according to an exemplary embodiment of the present invention; 
         FIG. 2B  is a cross-section diagram of the particle collection structures shown in  FIG. 2A , illustrating collection of incident and backscattered charged particles, according to an exemplary embodiment of the present invention; 
         FIGS. 3A ,  3 B,  3 C are partial cross-sectional diagrams illustrating fabrication of the imager shown in  FIG. 2 , according to an exemplary embodiment of the present invention; 
         FIGS. 4A and 4B  are partial cross-sectional diagrams illustrating fabrication of an imager according to another exemplary embodiment of the present invention; 
         FIGS. 5A ,  5 B and  5 C are partial cross-sectional diagrams illustrating fabrication of an imager according to a further exemplary embodiment of the present invention; 
         FIGS. 6A ,  6 B,  6 C and  6 D are partial cross-sectional diagrams illustrating fabrication of an imager according to a further exemplary embodiment of the present invention; and 
         FIGS. 7A ,  7 B,  7 C,  7 D,  7 E,  7 F,  7 G,  7 H,  7 I,  7 J and  7 K are partial cross-sectional diagrams illustrating fabrication of an imager according to a further exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aspects of the present invention relate to charged particle sensing devices having charged particle collection structures and methods of collecting charged particles. According to an exemplary embodiment, the charged particle collection structures include a plurality of collector electrodes and a grid formed around and spaced apart from the collector electrodes. The grid may provide shielding to an isolation region between the collector electrodes. The collector electrodes collect a portion of incident charged particles. The grid receives a further portion of the incident charged particles. The charged particles that are received by the grid are channeled away from the isolation region between the collector electrodes. Any backscattered charged particles generated by the grid are directed back to the grid or to neighboring collector electrodes. 
     Exemplary collector electrodes may be formed with a non-uniform shape and may include one or more wells, to provide collection of backscattered charged particles. Collection structures of the present invention provide collection of backscattered charged particles and may reduce the exposure of dielectric material between the collector electrodes and grid to incident and/or backscattered charged particles. Collection structures of the present invention may also increase a fill factor for the sensor, such that the fill factor may be greater than 90%. Charged particle sensing devices of the present invention may be used, for example, for mass spectrometry, including secondary ion mass spectrometry (SIMS), electron microscopy, night vision, medical and life sciences instrumentation and other applications involving low-light imaging areas. 
     Referring to  FIG. 1 , a charged particle sensing device is shown, designated generally as  100 . Device  100  may be used, for example, as an image intensifier. Device  100  includes photocathode  102  having input side  102   a  and output side  102   b . Device  100  also includes imager  106  including charged particle collection surface  106   a . Device  100  may also include micro-channel plate (MCP)  104  which includes input side  104   a  and output side  104   b . MCP  104  is disposed within vacuum gap  118  formed in a housing (not shown) incorporating photocathode  102  and imager  106 . 
     Although MCP  104  is shown disposed between photocathode  102  and imager  106 , it will be understood that MCP  104  may be omitted. Although photocathode  102  is shown, it will be understood that photocathode  102  may be replaced with a cathode, where the cathode provides a source of charged particles. 
     Imager  106  may be any type of solid state sensor capable of detecting charged particles. For example, imager  106  may include a complementary metal oxide semiconductor (CMOS) sensor. As described further below with respect to  FIGS. 2A-7K , charged particle collection surface  106   a  includes a plurality of collection structures, which collect incident charged particles and provide an electrical return path for backscattered electrodes. 
     In operation, light  112  from image  110  enters device  100  through input side  102   a  of photocathode  102 . Photocathode  102  changes the entering light  112  into charged particles  114 , such as electrons, which are output from output side  102   b  of photocathode  102 . Charged particles  114 , exiting photocathode  102 , enter channels  104   c  through input surface  104   a  of MCP  104 . After charged particles  114  bombard input surface  104   a  of MCP  104 , secondary charged particles are generated within the plurality of channels  104   c  of MCP  104 . MCP  104  may generate several hundred charged particles in each of channels  104   c  for each charged particle entering through input surface  104   a . Thus, the number of charged particles  116  exiting channels  104   c  may be significantly greater than the number of charged particles  114  entering channels  104   c . The intensified number of charged particles  116  exit channels  104   c  through output side  104   b  of MCP  104 , and strike charged particle collection surface  106   a  of imager  106 . The output of imager  106  may be stored in a register, then transferred to a readout register, amplified and displayed on video display  108 . 
     Referring to  FIGS. 2A and 2B , imager  106  having particle collection structures  200  is shown. In particular,  FIG. 2A  is a cross-section diagram of a portion of imager  106 ; and  FIG. 2B  is a cross-section diagram of a portion of particle collection structures  200 , illustrating collection of incident and backscattered charged particles. 
     Imager  106  includes pixel circuits  204  formed in substrate  202  and particle collection structures  200  formed on charged particle collection surface  106   a . Imager  106  may also include dielectric layer  206  formed between substrate  202  and particle collection structures  200 . Particle collection structures  200  includes charged particle collector electrodes  210 , associated with respective pixel circuits  204 , and grid  212 . Collector electrodes  210  are electrically connected to respective pixel circuits  204  by connectors  208 . Each pixel circuit  204  may include one or more transistors (not shown) configured to detect charged particles received from respective collector electrode  210  and to integrate the charge. Grid  212  functions to collect and provide an electrical return path for charged particles that are not incident on collector electrodes  210 . An isolation region is formed between grid  212  and dielectric layer  206 , where grid  212  may provide shielding to this isolation region from incident charged particles. 
     Each collector electrode  210  includes an incident surface including recessed surface  210   a , raised surface  210   b  and side walls  210   e . In addition, collector electrode  210  includes a raised edge  210   c  that is spaced apart from charged particle collection surface  106   a . Furthermore, as will be described further below, collector electrode  210  includes side surfaces  210   d  that are used to collect backscattered electrodes from grid  212 . Accordingly, it will be appreciated that collector electrode  210  is formed in a non-uniform shape. Although the incident surfaces  210   a ,  210   b ,  210   c  are shown to form a rectangular-shaped well, it will be appreciated that side walls  210   e  may include a slope, such that incident surfaces  210   a ,  210   b ,  210   c  form a trapezoidal-shaped well. For a trapezoidal-shaped well, the dimensions of the bottom surface (i.e., recessed surface  210   a ), may be less than the dimensions of the top surface (i.e., raised surface  210   b ). 
     Grid  212  surrounds collector electrodes  210  and is spaced apart from side walls  210   d  and raised edges  210   c  of collector electrodes  210  by gap  214  (which includes both a horizontal and vertical gap), with raised edges  210   c  formed above grid  212 . Because collector electrodes  210  and grid are formed on dielectric layer  206  and are spaced apart by gap  214 , collector electrodes  210  and grid  212  are electrically isolated from each other. Because grid  212  is positioned between collector electrodes  210 , fewer backscattered charged particles may be directed to dielectric layer  206 . Accordingly, crosstalk between pixels may be reduced. 
     Substrate  202  may include any suitable semiconductor substrate such as, but not limited to, silicon. Dielectric layer  206  may include any suitable electrically insulating material including, but not limited to, glass, ceramic, and metal oxides. Collector electrodes  210  and grid  212  may include any suitable conductive material, including, but not limited to, aluminum, copper and gold. 
     In operation, a portion  220  of incoming charged particles strike the incident surface of collector  210  (e.g., recessed surface  210   a , raised surface  210   b  or side walls  210   e ) and are collected as collected particles  222  through connector  208 , to produce a signal through the transistors of respective pixel circuit  204 . 
     A further portion  224  of incoming charged particles may pass between collector electrodes  210  and collide with grid  212  in gap  214 . The collision of portion  224  with grid  212  may generate backscattered charged particles  226 . Backscattered charged particles  226  may propagate through gap  214  and be collected by a neighboring collector electrode  210  via side wall  210   d  and/or raised edge  210   c . In general, the amount of backscattered charged particles  226  collected by side wall  210   d  and/or raised edge  210   c  may be controlled by any overlap in the horizontal direction of raised edge  210   c  and a vertical gap between side wall  210   d  and grid  212 . 
     An additional portion  228  of incoming charged particles may collide with recessed surface  210   a  and generate backscattered charged particles  230 . Because collector electrode  210  includes a well, backscattered charged particles  230  may be collected by side walls  210   e  of collector electrode  210 . In general, the probability of backscatter (such as from portions  224  and  228 ) is related to the material properties of collector electrode  210 , the impact energy of the charged particle and the angle of incidence of the charged particle. 
     It will be appreciated that the fill factor may also be controlled by the separation of collector electrodes  210 . Because collector electrodes  210  include raised edges  210   c , collector electrodes  210  may be spaced closer together. Accordingly, the fill factor may be increased, for example, to greater than 90%. 
     Referring next to  FIGS. 3A-C , partial cross-sectional diagrams of imager  106  are shown, illustrating a process for manufacturing particle collection structures  200 . As shown in  FIG. 3A , pixel circuit  204  is formed for each pixel in substrate  202 . Dielectric layer  206  may be formed over substrate  202 . Connectors  208  may be formed to penetrate dielectric layer  206  and substrate  202  and provide signal to respective pixel circuits  204 . Grid  212  is then formed on dielectric layer  206 , such that grid  212  is positioned between pixel circuits  204 . 
     Referring to  FIG. 3B , sacrificial material  302  is formed and patterned to enclose grid  212 . Sacrificial material  302  may be any suitable material including, but not limited to, a polymer, a metal or a semiconductor material. A conductive layer is deposited and patterned to form collector electrodes  210  and to expose sacrificial material  302  in regions corresponding to grid  212 . Collector electrodes  210  are formed to be electrically connected to respective pixel circuits  204  via connectors  208 . 
     Referring to  FIG. 3C , sacrificial material  302  is removed, for example, using a dry or wet etching process. Removal of sacrificial material  302  produces gaps  214  between collector electrodes  210  and grid  212 , thus forming particle collection structures  200  of imager  106 . 
     The formation of pixel circuits  204 , dielectric layer  206 , connectors  208 , grid  212  and collector electrodes  210  may be understood by the skilled person from the description herein. 
     Referring next to  FIGS. 4A and 4B , partial cross-sectional diagrams of imager  106 ′ including particle collection structures  400  are shown, illustrating a process for manufacturing imager  106 ′. As shown in  FIG. 4A , after dielectric layer  206  is formed on substrate  202 , multiple wells  402  are etched or patterned into a top surface of dielectric layer  206 . In addition, grid  212  is formed on dielectric layer  206 , between pixel circuits  204 , as described above with respect to  FIG. 3A . 
     Although  FIG. 4A  illustrates wells  402  shaped as rectangular wells, wells  402  may also be shaped as other geometries, such as inverted pyramids. In this case, the base of the inverted pyramid may be a square at the top surface of dielectric layer  206  such that walls formed in dielectric layer  206  are extended from the base to form an apex of the inverted pyramid. According to an exemplary embodiment of the present invention, wells  402  may be transversely spaced between about 1-2 μm and may have a depth between about 1-2 μm. It can be appreciated that, in general, the spacing and depth of wells  402  may depend on the geometry of collector electrodes  210 ′. 
     Referring to  FIG. 4B , a conductive layer is disposed on dielectric layer  206  to form collector electrodes  210 ′. Because wells  402  are formed in dielectric layer  206 , wells  404  are subsequently formed in collector electrodes  210 ′ which correspond with wells  402  (with respect to their position and shape). Thus, wells  404  may be formed in collector electrodes  210 ′, without any substantial further etching of collector electrodes  210 ′. For example, if wells  402  are formed as inverted pyramids, subsequent wells  404  may also be formed as inverted pyramids. The slopes provided by the pyramids may deflect more of the backscattered charged particles into collector electrodes  210 ′. 
     It will be appreciated that wells  402 ,  404  may provide traps for backscattered charged particles. Although not shown in  FIGS. 4A and 4B , a sacrificial material may also be formed over grid  212  (as discussed above with respect to  FIGS. 3B and 3C ) to form gaps  214 , thus forming particle collection structures  400  of imager  106 ′. Although not shown in  FIG. 4B , collector electrodes  210 ′ may also include recessed regions on the top surface (such as recessed surfaces  210   a  shown in  FIG. 2A ). 
     Referring next to  FIGS. 5A-5C , partial cross-sectional views of imager  106 ″ having particle collection structures  500  are shown, illustrating a process for manufacturing particle collection structures  500 . As shown in  FIG. 5A , collector electrodes  510  are formed over dielectric layer  206  to correspond with respective pixel circuits  204 . In addition, grid  212  is formed on dielectric layer  206  to surround and to be spaced apart from collector electrodes  510 . 
     Referring to  FIG. 5B , sacrificial material  502  is formed over collector electrodes  510 . Sacrificial material  502  is also etched or patterned to form regions  503  corresponding to grid  212 . 
     Referring to  FIG. 5C , conductive layer  504  is formed in regions  503  such that conductive layer  504  is connected to grid  212 . Conductive layer  504  is also patterned or etched to expose sacrificial material  502  in regions corresponding to collector electrodes  510 . Sacrificial material  502  is then removed, to form particle collection structures  500  of imager  106 ″. 
     In imager  106 ″, conductive layer  504  and grid  212  form barriers  512 , with gaps  514  formed between barriers  512  and collector electrodes  510 . Barriers  512  are formed with a thickness greater than a thickness of collector electrodes  510 , such that barriers  512  are spaced above collector electrodes  510 . Because a width of conductive layer  504  is greater than grid  212 , conductive layer  504  overlaps collector electrodes  410 . Particle collection structures  500  are similar to particle collection structures  200  ( FIG. 3C ) except that barriers  512  include a raised edge that extends above and overlaps collectors  510  in a direction normal to the collection surface  106   a.    
     Although not shown in  FIG. 5C , collector electrodes  510  may include a recessed surface (such as recessed surface  210   a  as shown in  FIG. 2A ) and/or wells (such as wells  404  as shown in  FIG. 4B ). 
     Referring next to  FIGS. 6A-6D , partial cross-sectional views of imager  106 ′″ including particle collection structure  600  are shown, illustrating a process for manufacturing particle collection structures  600 . As shown in  FIG. 6A , grid  212  is formed on dielectric layer  206  in regions between pixel circuits  204 . 
     As shown in  FIG. 6B , sacrificial material  302  is formed over grid  212 , as described in  FIG. 3B . In addition, a conductive layer is disposed on dielectric layer  206  in regions corresponding to pixel circuits  204  and patterned or etched to form collector electrodes  210  with recessed surfaces  210   a.    
     Referring to  FIG. 6C , a further sacrificial material  602  is disposed over sacrificial material  302  between collector electrodes  210 . A conductive layer  604  is then formed over alternating collector electrodes  210 . Conductive layer  604  may be patterned or etched to form recessed surface  604 A. 
     Referring to  FIG. 6D , sacrificial materials  302  and  602  are removed, to form particle collection structure  600  of imager  106 ′″. Structures  600  include grid  212  and collector electrodes  210  and  610 . Collector electrodes  210  and  610  alternate with each other, where collector electrode  610  has a greater thickness than collector electrode  210 . Removal of sacrificial material  302  and  602  forms gaps  214  and  614  between respective collector electrodes  210  and  610 . 
     Because collector electrodes  210  and  610  have an alternating thickness (i.e., an alternating height), collector electrodes  210  and  610  can be substantially overlapped to eliminate most areas where charged particles may collect between electrodes (i.e., in gaps  214  and  614 ). In addition, the increased overlap between collector electrodes  210  and  610  may produce a fill factor of about 100%. 
     Referring next to  FIGS. 7A-7K , partial cross-sectional views of imager  700  including particle collection structure  722  are shown, illustrating a process for manufacturing particle collection structure  722 . As shown in  FIG. 7A , imager  700  is formed by disposing a plurality of alternating metal and dielectric layers, designated generally as layer  702 , on substrate  701 . Layer  702  may include vias (V), e.g., V 1 , V 2  and V 3 , and conductive connectors (M), e.g., M 1  and M 2 , for providing signals from respective collector electrodes  722  ( FIG. 7K ) to respective pixel circuits  703 . In an exemplary embodiment, the dielectric material in layer  702  includes silicon dioxide. In general, the dielectric material may include, without being limited to, silicon dioxide, silicon nitride, or any dielectric material suitable for back end of line (BEOL) processes. Grid  704  and conductive layer  706  are defined and formed on layer  702 . Conductive layer  706  is formed in regions corresponding to respective pixels circuits  703 . Grid  704  is formed to surround and to be spaced apart from conductive layer  706 . 
     Referring to  FIG. 7B , dielectric layer  708  is deposited on grid  704  and conductor layer  706 , for electrical isolation. Grid  704  is thus spaced apart from conductive layer  706  by dielectric layer  708 . According to an exemplary embodiment, dielectric layer  708  may be composed of a dielectric material that is different from the dielectric material in layer  702  and any dielectric layers disposed above dielectric layer  708  (such as dielectric layer  710  shown in  FIG. 7D ). For example, the dielectric material of layers  702  and  710  may include silicon dioxide, whereas the dielectric material of layer  708  may include silicon nitride. 
     Referring to  FIG. 7C , dielectric layer  708  is etched or planarized to expose grid  704  and conductive layer  706 . Thus, dielectric layer  708  is formed in the regions between grid  704  and conductive layer  706 . 
     Referring to  FIG. 7D , dielectric layer  710  is deposited above grid  704 , conductive layer  706  and dielectric layer  708 . Referring to  FIG. 7E , portions of dielectric layer  710  are removed in collector regions  712  such that dielectric layer  710  includes side walls in collector regions  712  with a sloped or stepped profile. 
     Referring to  FIG. 7F , conductive layer  714  is deposited on dielectric layer  710  within collector regions  712  and is coupled to conductive layer  706 . Conductive layer  714  is also patterned or etched to create collector regions  713 . In addition, regions  715  of dielectric layers  710  are defined to expose dielectric layer  710 . 
     Referring to  FIG. 7G , dielectric layer  716  is deposited over conductive layer  714  and dielectric layer  710 . Referring to  FIG. 7H , portions of dielectric layer  716  are then removed in collector regions  712 ′ such that dielectric layer  716  includes side walls in collector regions  712 ′ with a sloped or stepped profile. In  FIG. 7H , the combination of dielectric layers  710  and  716  is referred to as dielectric layer  718 . 
     Referring to  FIG. 7I , conductive layer  720  is deposited on dielectric layer  718  within collector regions  712 ′ and is coupled to conductive layer  714 . Conductive layer  720  is also patterned or etched to create collector regions  713 ′. In addition, regions  717  of conductive layer  720  are defined to expose dielectric layer  718 . 
     Referring to  FIG. 7J , dielectric layer  718  is removed in regions  724  removed to expose grid  704 , for example using a dry or wet etching process. In an exemplary embodiment, a dry etching process is used. As shown in  FIG. 7J , the combination of conductive layers  706 ,  714  and  720  form collector electrodes  722 . Referring to  FIG. 7K , the remaining dielectric layer  718  is removed, for example, using a dry or wet etching process. In an exemplary embodiment, a wet etching process is used. The removal of dielectric layer  718  produces gaps  726  between collector electrodes  722  and grid  704 , thus producing particle collection structure  728 . 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.