Patent Publication Number: US-2020279713-A1

Title: Photocathode emitter system that generates multiple electron beams

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to the provisional patent application filed Feb. 14, 2018 and assigned U.S. App. No. 62/630,429, the disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates to electron beam emitters. 
     BACKGROUND OF THE DISCLOSURE 
     Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink, yet the industry needs to decrease time for achieving high-yield, high-value production. Minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for a semiconductor manufacturer. 
     Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices. 
     Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and, thus, higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices. 
     As semiconductor devices become smaller, it becomes more important to develop enhanced inspection and review tools and procedures to increase the resolution, speed, and throughput of wafer and photomask/reticle inspection processes. One inspection technology includes electron beam based inspection such as use of a scanning electron microscope (SEM). An SEM uses an electron source. Electron sources can be divided into two broad groups: thermionic sources and field emission sources. Thermionic sources are usually made of tungsten or lanthanum hexaboride (LaB 6 ). In thermionic emission, electrons are boiled off the material surface when the electron thermal energy is high enough to overcome the surface potential barrier. Even though thermionic emitters are widely used, they typically require elevated temperatures (e.g., &gt;1300 K) to operate, and may have several drawbacks such as inefficient power consumption, wide energy spread, short lifetime, low current density, and limited brightness. The demand for more efficient electron sources has driven the research and development of Schottky emitters and cold electron sources such as electron field emitters. 
     In the Schottky emitters, thermionic emission is enhanced by effective potential barrier lowering due to the image charge effect under an applied external electric field. Schottky emitters are typically made of a tungsten wire having a tip coated with a layer of zirconium oxide (ZrO x ), which exhibits a low work function (e.g., approximately 2.9 eV). Schottky emitters are currently used in some electron beam systems. Despite being quite successful, thermally-assisted Schottky emitters still need to be operated at high temperature (e.g., &gt;1000 K) and high vacuum (e.g., approximately 10 −9  mbar), and have wider than desirable electron emission energy spread due to the high operating temperature. 
     Cold electron sources, particularly electron field emitters, have been used in field emission displays, gas ionizers, x-ray sources, electron beam lithography, and electron microscopes, among other applications. Field emission takes place when the applied electric field is high enough to reduce the potential barrier on the tip-vacuum interface so that electrons can tunnel through this barrier at a temperature close to room temperature (e.g., quantum-mechanical tunneling). A typical field-emitter comprises a conical emitter tip with a circular gate aperture. A potential difference is established across the emitter cathode, the gate and the anode under an applied external field, resulting in high electric field at the surface of the tip. Electrons tunnel through the narrow surface barrier and travel towards an anode, which is biased at a higher potential than the gate. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account the field enhancement factor due to the field emitters. 
     Field emitters, because they can operate near room temperature, have lower energy spread than Schottky and thermionic emitters, and can have higher brightness and electron current than thermionic emitters. However, in practical use, the output current of a field emitter is less stable because contaminants can easily stick to the tip of the emitter and raise its work function, and hence lower the brightness and current. Periodic flashing (i.e., temporarily raising the tip temperature) is required to remove those contaminants. While the tip is being flashed, the instrument is not available for operation. Instruments in the semiconductor industry are required to operate continuously and stably without interruption, so Schottky emitters are usually used in preference to cold field emitters. 
     Previous field emitter arrays (FEAs) had multiple conically shaped electron emitters arranged in a two-dimensional periodic array. These field emitter arrays can be broadly categorized by the material used for fabrication into two broad categories: metallic field emitters and semiconductor field emitters. 
     Thermal field emitters (TFE) were previously used to generate electron beams. An individual electron source was used to form an array. Each electron source requires expensive XYZ stages. The cost of each individual electron source system was expensive and cost-prohibitive for a large array. In addition, the electron current density was low. 
     Photocathodes also have been used to generate electron beams. A single light beam incident on a photocathode system can generate a single electron beam with high brightness that is capable of delivering high electron current density. However, a problem with single electron beam systems is that even with high brightness systems, single electron beam systems still have relative low throughput for inspection. Low throughput is a drawback to electron beam inspection. With current available electron beam sources, thousands of beams would be required. 
     Splitting the single electron beam into numerous beams for a multi-beam SEM system required an array of aperture lenses and/or micro-lenses. The array of aperture lenses and/or micro-lenses are set in small, electrically-charged apertures that are substantially round in design to create lens fields. If the apertures are out-of-round, astigmatism is introduced in the lens fields, which results in a distorted image plane. 
     Therefore, what is needed is an improved system to generate electron beams. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     A system is provided in a first embodiment. The system comprises a diffractive optical element configured to receive an incident radiation beam, an extractor plate, a microlens array that provides collimation and focus of the beamlets, a condenser lens disposed between the diffractive optical element and the microlens array along the path of the beamlets, and a photocathode surface disposed between the microlens array and the extractor plate along the path of the beamlets. The diffractive optical element forms the beamlets from the incident radiation beam. The microlens array is disposed between the diffractive optical element and the extractor plate along a path of the beamlets. The photocathode surface generates a plurality of electron beams from the beamlets. 
     The system can further include a laser light source that generates the incident radiation beam. 
     The beamlets may be in an array. 
     The electron beams can have a density from 2 nA to 5 nA. 
     The electron beams can have a spatial separation from 50 μm to 10 mm. 
     100 to 1000 of the electron beams may be included. 
     The system can further include an electron beam column. In an instance, the electron beams are directed from the extractor plate at the electron beam column. 
     The system can include a plurality of the electron beam columns. In an instance, each of the electron beams is directed at one of the electron beam columns. 
     The beamlets each can have a spot size from 10 μm to 100 μm. The electron beams generated by the photocathode surface each can have a spot size from 10 μm to 100 μm. 
     The incident radiation beam may be ultraviolet radiation. 
     The system can further include a voltage source in electronic communication with the extractor plate. 
     A wafer inspection tool can include the system of the first embodiment. 
     A method is provided in a second embodiment. A radiation beam is generated and received at a diffractive optical element. A plurality of beamlets are formed from the radiation beam using the diffractive optical element. The beamlets are directed through a condenser lens. The beamlets are collimated and focused with a microlens array downstream of the condenser lens with respect to a direction the beamlets are projected. The beamlets from the microlens array are directed to a photocathode surface. A plurality of electron beams are generated from the beamlets using the photocathode surface. The electron beams are extracted from the photocathode surface. 
     The beamlets can be in an array ranging from 4×6 to 48×48 
     The electron beams can have a density from 2 nA to 5 nA. 
     100 to 1000 of the electron beams may be included. 
     The electron beams can have a spatial separation from 50 μm to 10 mm. 
     The beamlets each can have a spot size from 10 μm to 100 μm. The electron beams generated by the photocathode surface each can have a spot size from 10 μm to 100 μm. 
     The radiation beam may be ultraviolet radiation. 
     A pattern of the beamlets can be transmitted to the electron beams. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view of a diffractive optical element photocathode array system to produce multiple electron beams; 
         FIG. 2  illustrates electron beams that are extracted from the photocathode surface by providing voltage to an extractor plate creating an extraction field; 
         FIG. 3  is an embodiment of a method in accordance with the present disclosure; and 
         FIG. 4  is block diagram of an embodiment of a system in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Although claimed subject matter will be described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the disclosure. Accordingly, the scope of the disclosure is defined only by reference to the appended claims. 
     The system is designed for electron beam inspection of semiconductor wafers with high throughput. The system includes a photocathode electron source, diffractive optical element (DOE), lens system to parallelize the beamlets, and microlens array to focus the beamlets. Using a DOE with microlens array to produce collimated parallel beamlets on a photocathode surface can produce a patterned electron beam. 
     A photocathode system can provide high electron density per beam, which reduces the number of beams required. A multiple electron beam system can achieve high resolution with increased throughput. High speed and high resolution electron beams can be provided with a DOE lens system coupled to a photocathode element to generate a multi-electron beam system. A multiple electron beam inspection system using parallel data acquisition may increase the throughput and shorten the time to inspect a wafer or reticle. 
       FIG. 1  is a view of a diffractive optical element photocathode array system  100  to produce a multi-electron beam. A source directs a radiation beam  101  on the DOE  102 , which will produce a beamlet array to be used in combination with a photocathode  111  surface to generate an array of electron beams  110 . In particular, the radiation beam  101  impinging on DOE  102  forms an array of light beamlets  107  that impinge a microlens array  104 . The microlens array  104  focuses the focused parallelized beamlets  109  to a photocathode surface  111 . The photocathode surface  111  generates a multi-electron-beam pattern. 
     The light source that generates the incident radiation beam may be, for example, lasers, diodes, lamps, or broadband (BB) light sources. BB sources can be obtained from, for example, laser-produced plasmas, laser-sustained plasmas, laser-produced supercontinuum sources, white lasers, or largely tunable optical parametric sources. These sources can be continuous-wave (CW) or pulsed and can have wavelengths that range from vacuum ultraviolet (VUV) to the infrared (IR). A single wavelength can be selected with a suitable spectrometer or narrow or wide bands of wavelengths can be selected by filters or other methods. 
     In an embodiment, the light source is a laser light source or a diode. The incident radiation beam may be ultraviolet radiation. In an instance, the radiation beam  101  is light with a wavelength of 266 nm. The radiation beam may be generated such that it has low noise. 
     In another embodiment, the light source is a CW light source. 
     The system  100  includes a DOE  102 , a condenser lens  103 , a microlens array  104 , a photocathode element  105  with a photocathode surface  111 , and an extractor plate  106 . Radiation from the source passes through the DOE  102 , the condenser lens  103 , and the microlens array  104  before being projected on the photocathode surface  111 . The electron beams can be extracted from the photocathode surface  111  using the extractor plate  106 . 
     Optical alignment of the microlens array  104  may impact output of the electron beams  110 . Uniformity across the entire photocathode surface  111  and/or equivalent spot size in the focused parallelized beamlets  109  can affect performance. A light beam may be larger than the electron emitting aperture. A light beam also may be smaller than the electron beam limiting aperture. If the light beam is smaller than the electron beam limiting aperture, then a metrology alignment may be performed. 
     The DOE  102  receives an incident radiation beam  101 . The DOE  102  forms a plurality of beamlets  107  from the incident radiation beam  101 . The number of beamlets  107  produced by the DOE  102  may be the same as the number of beamlets exiting the condenser lens  103  or the number of electron beams  110  that are produced. Four beamlets  107  are illustrated exiting the condenser lens  103  for simplicity. Thus, the number of beamlets  107  can vary from that illustrated in  FIG. 1 . The number of beamlets  107  (and electron beams  110 ) can form an array. The array may include, for example, 4×6, 6×6, 1×10, 10×10, 10×100, 30×100, or other configurations of beamlets  107  and electron beams  110 . 
     The condenser lens  103  is disposed between the DOE  102  and the microlens array;  104 . The condenser lens  103  may adjust trajectory of the beamlets  107  and can be configured to provide an output of parallelized beamlets  108 . The number of parallelized beamlets  108  equals the number of beamlets  107 . 
     The microlens array  104  can provide collimation and focus of the parallelized beamlets  108 . The microlens array  104  is disposed between the DOE  102  and the extractor plate  106 . The parallelized beamlets  108  leaving the microlens array  104  are focused parallelized beamlets  109 , which are focused at the cathode surface  111 . 
     The photocathode surface  111  in the photocathode element  105  is disposed between the microlens array  104  and the extractor plate  106 . The photocathode surface  111  generates a plurality of electron beams  110  from the focused parallelized beamlets  109 . The photocathode surface  111  may be or may include a photosensitive compound. When struck by a photon, the photocathode surface  111  can cause electron emission due to the photoelectric effect. 
     In an embodiment, the photocathode element  105  can include bare metals, coated metals, cesium metal or alloys thereof, negative electron affinity (NEA) materials, a Zintl salt photocathode material, or alkali photocathode materials. 
     While twelve of the electron beams  110  are illustrated, the photocathode surface  111  can generate from 100 to 1000 or from 100 to 2500 the electron beams  110 . The number of electron beams  110  can be scaled depending on the system design or the application. 
     The extractor plate  106  is opposite the DOE  102  with respect to a direction of travel of the beamlets  107 . In an instance, the extractor plate  106  includes 20 μm diameter apertures, though other size apertures are possible. The extractor plate  106  can be in electronic communication with a voltage source  112 . The voltage range can be from 0.1 KV to 50 KV. For high resolution inspection, the voltage range can be from 0.1 KV to 5 KV. Other voltages are possible, and these ranges are merely examples. 
     The DOE  102  can be designed to generate a desired number of beamlets with a desired spatial separation and pattern. Optical lenses can be used to form parallelized beamlets, but also to form and shape the light beams. The beamlets can be aligned to a microlens array  104  optics. The combination of DOE  102 , optical elements, and the microlens array  104  can provide multiple beams of electrons having a desired spatial separation, spot size, and pattern. 
     In an example, radiation, such as ultraviolet radiation, in a collimated beam is directed onto the DOE  102 . A focusing lens can be used to collimate individual beamlets and a microlens array  104  can focus the beamlets on the photocathode surface  111 . 
     The design of the DOE  102  can define the number of the beamlets, a size of the array, and a desired spatial separation and pattern. The pattern imposed onto the beamlets is transmitted to the multiple electron beams  110  as these electron beams  110  are generated by the photocathode surface  111 . Previous designs had difficulty producing an array of electron beams with the same size, arrangement, or other properties as the electron beams  110 . 
     The beamlets, such as the parallelized beamlets  108  or the focused parallelized beamlets  109 , can be in an array. The pattern of the beamlets can be transmitted to the electron beams  110 . Thus, an array of beamlets can be transmitted to an array of electron beams  110 . 
     The beamlets, such as the parallelized beamlets  108  or the focused parallelized beamlets  109 , each can have a spot size from 10 μm to 100 μm. The electron beams  110  generated by the photocathode surface  111  each can have a spot size from 10 μm to 100 μm. 
     The electron beams  110  can have a density from 2 nA to 5 nA. 
     The electron beams  110  can have a spatial separation from 50 μm to 10 mm or more. Spatial separation of the electron beams  110  in the 10&#39;s of mm is possible. 
     The system  100  can include an electron beam column, which may be downstream of the extractor plate  106  relative to the direction of travel of the electron beams  110 . The electron beam column can include components such as, for example, apertures, deflectors, scan coils, electromagnetic lenses, magnetic lenses, or detectors. The configuration of the electron beam column can vary with the particular application of the system. 
     The electron beams  110  can be directed at the electron beam column. In an instance, there are multiple electron beam columns. Each of the electron beams  110  can be directed at one of the electron beam columns. Each electron beam column can individually control one of the electron beams. Simultaneous use of multiple individually-controlled electron beams allows for corrections of each beam. 
     Embodiments disclosed herein allow formation of multiple beams of electrons by directing a small spot size (e.g., 10-100 μm) radiation (e.g., ultraviolet light) onto a suitable photocathode surface  111 . The photocathode surface  111  can produce a small electron spot size (e.g., 10-100 μm). The electrons produced from the photocathode surface  111  can be accelerated by voltage applied to the extractor plate  106 . The final spot of each electron beam  110  can be controlled by the electron optics. 
     Multi-electron beams from photocathodes using DOE with a microlens array can enable inspection of a mask or wafer in a few hours. Conventional methods may take months to perform the same inspection. 
     The photocathode surface  111  may be configured for the wavelength of the radiation beam  101 . For example, the photocathode surface  111  may have different coatings or substrate materials depending on the wavelength of the radiation beam  101 . For UV wavelengths down to 248 nm, fused silica or sapphire may be used as the substrate material. For wavelengths below 193 nm, MgF 2  or CaF 2  may be used. The material of the photocathode surface  111  can be chosen for optimal quantum efficiency (QE) and energy spread at a given wavelength. 
     Design of the DOE  102  may vary depending on the wavelength of the radiation beam  101  or the pitch of the beamlets  107 . Material in the DOE  102  can be selected based on the wavelength that is used in the diffractive optical element photocathode array system  100 . For UV wavelengths down to  248  nm, fused silica or sapphire may be used a material in the DOE  102 . For wavelengths below  193  nm, MgF 2  or CaF 2  may be used. 
     Design of the condenser lens  103  or microlens array  104  can vary depending on the wavelength of the radiation beam  101  or the spot sizes of the beamlets  107  or parallelized beamlets  108 . Material in the condenser lens  103  or microlens array  104  can be selected based on the wavelength that is used in the diffractive optical element photocathode array system  100 . For UV wavelengths down to 248 nm, fused silica or sapphire may be used a material in the condenser lens  103  or microlens array  104 . For wavelengths below 193 nm, MgF 2  or CaF 2  may be used. 
     The system  100  can increase throughput. A larger array of the electron beams  110  and/or a larger spot size of the electron beams  110  can increase throughput. 
       FIG. 2  illustrates electron beams  110  that are extracted from the photocathode surface  111  by providing voltage to an extractor plate  106  creating an extraction field (shown by the arrow  113 ). The extractor plate  106  defines a plurality of extraction apertures  114 . As seen in  FIG. 1 , the electron beams  110  are formed from the focused parallelized beamlets  109 . 
       FIG. 3  is an embodiment of a method  200 . At  201 , a radiation beam is generated. The radiation beam may be ultraviolet radiation or another wavelength of radiation. The radiation beam is received at a DOE at  202 . A plurality of beamlets are formed from the radiation beam using the DOE at  203 . The beamlets are directed through a condenser lens at  204 . The beamlets are collimated and focused downstream of the condenser lens with a microlens array at  205 . The downstream position may be with respect to a direction the beamlets are projected. At  206 , the beamlets are directed from the microlens array to a photocathode surface. A plurality of electron beams are generated from the beamlets using the photocathode surface at  207 . The electron beams are extracted from the photocathode surface at  208 . 
     The beamlets can be in an array ranging from, for example, 4×6 to 48×48. Other array configurations are possible. The array may depend on the pitch of the beamlets. 
     The electron beams can have a density from 2 nA to 5 nA. There may be from 100 to 1000 or from 100 to 2500 electron beams. The electron beams can have a spatial separation from 50 μm to 10 mm or more. Spatial separation of the electron beams  110  in the 10&#39;s of mm is possible. 
     In an instance, the beamlets each have a spot size from 10 μm to 100 μm, and the electron beams generated by the photocathode each have a spot size from 10 μm to 100 μm. 
     A pattern of the beamlets can be transmitted to the electron beams. Thus, the electron beams can have the same pattern as the beamlets impinging the photocathode surface. 
     Embodiments disclosed herein enable designs of multiple electron beams and patterning targets with multiple beams of electrons. Any type of light can be used depending on the photocathode material. For inspection, a CW laser or radiation from lamps, diodes, or laser-produced plasmas may be used as light sources. For most photocathode material with high QE, an ultraviolet light source may be used. 
     Embodiments disclosed herein can be used in reticle or wafer inspection, review, or metrology systems, such as those that use a single electron source or multiple electron sources. Embodiments disclosed herein can be used in systems that use electron sources for generation of x-rays using single or multiple electron sources for use of wafer or reticle, metrology, review or inspection. 
       FIG. 4  is a block diagram of an embodiment of a system  300 . The system  300  includes a wafer inspection tool (which includes the electron column  301 ) configured to generate images of a wafer  304 . 
     The wafer inspection tool includes an output acquisition subsystem that includes at least an energy source and a detector. The output acquisition subsystem may be an electron beam-based output acquisition subsystem. For example, in one embodiment, the energy directed to the wafer  304  includes electrons, and the energy detected from the wafer  304  includes electrons. In this manner, the energy source may be an electron beam source. In one such embodiment shown in  FIG. 4 , the output acquisition subsystem includes electron column  301 , which is coupled to computer subsystem  302 . A chuck (not illustrated) may hold the wafer  304 . 
     As also shown in  FIG. 4 , the electron column  301  includes an electron beam source  303  configured to generate electrons that are focused to wafer  304  by one or more elements  305 . The electron beam source  303  may include, for example, an embodiment of the diffractive optical element photocathode array system  100  of  FIG. 1 . The one or more elements  305  may include, for example, a gun lens, an anode, a beam limiting aperture, a gate valve, a beam current selection aperture, an objective lens, and a scanning subsystem, all of which may include any such suitable elements known in the art. The components of the electron beam column also may be part of the elements  305 . 
     Electrons returned from the wafer  304  (e.g., secondary electrons) may be focused by one or more elements  306  to detector  307 . One or more elements  306  may include, for example, a scanning subsystem, which may be the same scanning subsystem included in element(s)  305 . 
     The electron column also may include any other suitable elements known in the art. 
     Although the electron column  301  is shown in  FIG. 4  as being configured such that the electrons are directed to the wafer  304  at an oblique angle of incidence and are scattered from the wafer  304  at another oblique angle, the electron beam may be directed to and scattered from the wafer  304  at any suitable angles. In addition, the electron beam-based output acquisition subsystem may be configured to use multiple modes to generate images of the wafer  304  (e.g., with different illumination angles, collection angles, etc.). The multiple modes of the electron beam-based output acquisition subsystem may be different in any image generation parameters of the output acquisition subsystem. 
     Computer subsystem  302  may be coupled to detector  307  such that the computer subsystem  302  is in electronic communication with the detector  307  or other components of the wafer inspection tool. The detector  307  may detect electrons returned from the surface of the wafer  304  thereby forming electron beam images of the wafer  304  with the computer subsystem  302 . The electron beam images may include any suitable electron beam images. The computer subsystem  302  includes a processor  308  and an electronic data storage unit  309 . The processor  308  may include a microprocessor, a microcontroller, or other devices. 
     It is noted that  FIG. 4  is provided herein to generally illustrate a configuration of an electron beam-based output acquisition subsystem that may be used in the embodiments described herein. The electron beam-based output acquisition subsystem configuration described herein may be altered to optimize the performance of the output acquisition subsystem as is normally performed when designing a commercial output acquisition system. In addition, the systems described herein may be implemented using an existing system (e.g., by adding functionality described herein to an existing system). For some such systems, the methods described herein may be provided as optional functionality of the system (e.g., in addition to other functionality of the system). Alternatively, the system described herein may be designed as a completely new system. 
     The computer subsystem  302  may be coupled to the components of the system  300  in any suitable manner (e.g., via one or more transmission media, which may include wired and/or wireless transmission media) such that the processor  308  can receive output. The processor  308  may be configured to perform a number of functions using the output. The wafer inspection tool can receive instructions or other information from the processor  308 . The processor  308  and/or the electronic data storage unit  309  optionally may be in electronic communication with another wafer inspection tool, a wafer metrology tool, or a wafer review tool (not illustrated) to receive additional information or send instructions. 
     The computer subsystem  302 , other system(s), or other subsystem(s) described herein may be part of various systems, including a personal computer system, image computer, mainframe computer system, workstation, network appliance, internet appliance, or other device. The subsystem(s) or system(s) may also include any suitable processor known in the art, such as a parallel processor. In addition, the subsystem(s) or system(s) may include a platform with high speed processing and software, either as a standalone or a networked tool. 
     The processor  308  and electronic data storage unit  309  may be disposed in or otherwise part of the system  300  or another device. In an example, the processor  308  and electronic data storage unit  309  may be part of a standalone control unit or in a centralized quality control unit. Multiple processors  308  or electronic data storage unit  309  may be used. 
     The processor  308  may be implemented in practice by any combination of hardware, software, and firmware. Also, its functions as described herein may be performed by one unit, or divided up among different components, each of which may be implemented in turn by any combination of hardware, software and firmware. Program code or instructions for the processor  308  to implement various methods and functions may be stored in readable storage media, such as a memory in the electronic data storage unit  309  or other memory. 
     The system  300  of  FIG. 4  is merely one example of a system that can use the diffractive optical element photocathode array system  100  of  FIG. 1 . Embodiments of the diffractive optical element photocathode array system  100  of  FIG. 1  may be part of a defect review system, an inspection system, a metrology system, or some other type of system. Thus, the embodiments disclosed herein describe some configurations that can be tailored in a number of manners for systems having different capabilities that are more or less suitable for different applications. 
     Each of the steps of the method may be performed as described herein. The methods also may include any other step(s) that can be performed by the processor and/or computer subsystem(s) or system(s) described herein. The steps can be performed by one or more computer systems, which may be configured according to any of the embodiments described herein. In addition, the methods described above may be performed by any of the system embodiments described herein. 
     Although the present disclosure has been described with respect to one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure. Hence, the present disclosure is deemed limited only by the appended claims and the reasonable interpretation thereof.