Photomultiplier tube system and method of operating the same

Abstract

An optical system can be used to increase the effective quantum efficiency of a metal channel photomultiplier tube. This optical system can alternatively guide incident light from a low-efficiency region on the cathode of the metal channel photomultiplier tube to a high-efficiency region on the cathode. These high-efficiency regions of the cathode correspond to a location between dynode structures.

Description

Technical Field

[0001] This invention relates to an optical device for a photomultiplier tube. Prior Technology

[0002] The evolution of the semiconductor manufacturing industry places increasingly higher demands on yield management, and specifically on metrology and inspection systems. As critical dimensions continue to shrink, the industry needs to reduce the time required to achieve high-yield, high-value production. Minimizing the total time from detecting a yield problem to resolving it maximizes a semiconductor manufacturer's return on investment.

[0003] Manufacturing semiconductor devices (such as logic and memory devices) typically involves processing a semiconductor wafer using numerous manufacturing processes to form the various features and multiple layers of the semiconductor device. For example, lithography is a semiconductor manufacturing process that involves transferring a pattern from a photomask to a photoresist disposed on a semiconductor wafer. Additional examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. An arrangement of multiple semiconductor devices manufactured on a single semiconductor wafer can be divided into individual semiconductor devices.

[0004] Inspection procedures are used at various stages of semiconductor manufacturing to detect defects on wafers, thereby promoting higher yields and, consequently, higher profits. Inspection has always been a crucial part of manufacturing semiconductor devices, such as integrated circuits (ICs). However, as the size of semiconductor devices shrinks, inspection becomes even more critical for the successful manufacture of acceptable semiconductor devices, because even small defects can cause device failure. For example, as the size of semiconductor devices decreases, the detection of defects in smaller sizes has become necessary, because even relatively small defects can cause undesirable aberrations in the semiconductor device.

[0005] As the demand for semiconductor devices increases, the demand for improved detection capabilities will also increase. Photocathodes can be used for improved optical detection. In a general sense, a photocathode emits photoelectrons in response to the absorption of photons striking it. A photocathode can be part of a photomultiplier tube (PMT). Previously, light was uniformly incident on the cathode of a PMT, hitting both low-efficiency and high-efficiency regions. Therefore, half of the incident light ended up in the low-efficiency region of the PMT. This affected the overall output of the PMT.

[0006] Therefore, new systems and technologies are needed. Summary of the Invention

[0007] In a first embodiment, a system is provided. The system includes a metal channel photomultiplier tube (MTB). One cathode of the MTB has a high-efficiency region and a low-efficiency region. An optical system is positioned in a path of a beam guided at the MTB. The optical system is configured to guide the majority of the beam to the high-efficiency regions of the MTB.

[0008] The system may include a light source that generates the beam. The optical system is positioned in the path of the beam between the light source and the metal channel photomultiplier tube.

[0009] The optical system can be a microlens array comprising a plurality of cylindrical lens elements. Alternatively, the optical system can be a light guide.

[0010] The metal channel photomultiplier tube may include a dynode structure. The high-efficiency regions of the cathode may correspond to the positions between the dynode structures.

[0011] These high-efficiency regions may be 35% to 65% of the total area of ​​one of the metal channel photomultiplier tubes or 15% to 35% of the total area of ​​one of the metal channel photomultiplier tubes. These regions may refer to the areas of the photocathode exposed to the guided beam.

[0012] In a second embodiment, a method is provided. The method includes generating a light beam; directing the light beam toward an optical system; and using the optical system to direct the light beam onto a plurality of regions of a metal channel photomultiplier tube.

[0013] The optical system can be a microlens array comprising a plurality of cylindrical lens elements. Alternatively, the optical system can be a light guide.

[0014] The metal channel photomultiplier tube may include a dynode structure. The high-efficiency regions of the cathode may correspond to the positions between the dynode structures.

[0015] These high-efficiency regions may be 35% to 65% of the total area of ​​one of the metal channel photomultiplier tubes or 15% to 35% of the total area of ​​one of the metal channel photomultiplier tubes. Simple Explanation of the Diagram

[0016] For a more complete understanding of the nature and objectives of this invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, wherein: Figure 1 illustrates one embodiment of a system according to the present invention; Figure 2 shows an example PMT; Figure 3 shows an efficiency diagram corresponding to the X position of the first exemplary PMT across the first cross-section; Figure 4 shows an efficiency diagram corresponding to the X position of the second exemplary PMT across a cross-section; Figure 5 illustrates one embodiment of a method according to the present invention; and Figure 6 shows a second embodiment of one of the systems according to the present invention. Implementation

[0017] Although the claimed subject matter will be described with reference to specific embodiments, other embodiments (including those not providing all the benefits and features set forth herein) are also within the scope of the invention. Various structural, logical, procedural, and electronic changes may be made without departing from the scope of the invention. Therefore, the scope of the invention is defined only by reference to the claims outlined in the appended patent application.

[0018] The embodiments disclosed herein increase the effective quantum efficiency of a metal channel PMT by using a cylindrical microlens array or other components to guide incident light from a low-efficiency region on the cathode to a high-efficiency region on the cathode. The microlens array increases the effective quantum efficiency of the detector by up to approximately 10%. Defect sensitivity during semiconductor detection can depend on the quantum efficiency of a metal channel PMT. A higher quantum efficiency results in better defect sensitivity.

[0019] A photoelectric sensor (PMT) is constructed using a housing comprising a photocathode, several dynodes, and an anode. Incident photons strike photocathode material deposited on the inner side of the incident window of the housing. Electrons are emitted from the surface of the photocathode using the photoelectric effect. These electrons are guided toward an electron multiplier by a focusing electrode, where they are multiplied through a secondary emission process.

[0020] An electron multiplier includes a dynode. A dynode is an electrode in a vacuum tube within the electron multiplier, used for secondary emission. Each dynode can be maintained at a more positive potential than the previous dynode. When an electron strikes the first dynode, it emits a low-energy electron. These electrons are sequentially accelerated toward the second dynode. The geometry of the dynode chain allows for cascading, in which the number of electrons generated at each stage increases exponentially. This final stage in the series is an anode. This configuration amplifies the current emitted by the photocathode, for example, by a million times.

[0021] Figure 1 illustrates a system 100 having a metal channel PMT 108. The metal channel PMT 108 includes a photocathode 102, a field-forming grid 109, a plurality of dynodes 101, and an anode 107. All these components are contained in a vacuum housing or otherwise incorporated therein. The photocathode 102, dynodes 101, and anode 107 are electrically connected (not shown for simplicity). Each dynode 101 is held at a slightly positive voltage relative to the previous dynode 101 or the photocathode 102 of the first dynode 101. The anode 107 is held at a more positive voltage relative to the last dynode 101. A first dynode and a last dynode refer to the sequence in which electrons strike the dynodes along the electron direction from the photocathode 102 toward the anode 107 after emission from the photocathode.

[0022] When an incident photon from one of the guided beams 106 is absorbed by the photocathode 102, the probability of one or more electrons being emitted from the photocathode 102 is quite high. A focusing electrode can be used to deflect the electrons, causing most of them to strike the first dynode 101. When an electron strikes a dynode 101, it typically causes several (e.g., approximately 10) secondary electrons to be emitted from that dynode 101. Most of the electrons emitted from one dynode 101 strike the next dynode 101. This is repeated multiple times until the amplified signal strikes the anode 107. Therefore, the more dynodes 101 in a metal channel PMT 108, the greater the gain, but the longer it takes for the metal channel PMT 108 to respond to a single photon. Since some electrons from one dynode 101 may miss the next dynode 101 and strike another dynode 101 or anode 107, more dynodes 101 also mean a wider electrical pulse in response to a single photon.

[0023] Although Figure 1 illustrates a transmissive photocathode in which photoelectrons are emitted from the side of the photocathode 102 opposite to the incident photon, reflective photocathodes are also known in this art, in which photoelectrons are emitted from the same side of the photocathode 102 as the incident photon. The embodiments disclosed herein can be applied to either a transmissive or reflective photocathode.

[0024] The quantum efficiency of a metal channel PMT 108 varies with the position of the cathode 102, as shown in Figures 2-4. Due to the structure of the metal channel PMT, there is a periodicity of high-efficiency and low-efficiency regions in the X-direction. This structure is visible in Figure 2. Figure 2 shows a field-forming grid in front of the dynode, with vertical lines. The field-forming grid can have the same periodicity as the dynode.

[0025] The lines in Figures 3 and 4, passing through the X-direction, illustrate the efficiency at different heights relative to cross-sections at different locations in the Y-direction perpendicular to the X-direction. The peaks in Figures 3 and 4 represent the regions where the beam is guided for improved results. For example, depending on the design of the metal channel PMT, these peaks may correspond to dynode positions or positions between dynode positions. In Figures 3 and 4, efficiency decreases at the edges (i.e., near 0 mm and 10 mm along the X-direction) because PMT efficiency decreases near the edge of the circle.

[0026] In one example, the high-efficiency region corresponds to the area between the dynode positions, and the low-efficiency region corresponds to the dynode positions. In another example, the high-efficiency region corresponds to the dynode positions, and the low-efficiency region corresponds to the area between the dynode positions. The electron's line of sight governs which regions correspond to the high-efficiency and low-efficiency regions.

[0027] Referring back to Figure 1, the metal channel PMT 108 has a cathode 102 with both high-efficiency and low-efficiency regions. An optical system 103 is positioned in the path of a beam 105 guided at the metal channel PMT 108. The optical system 103 is configured to guide most of the beam 105 to the high-efficiency region of the metal channel PMT 108. Therefore, the light is deflected towards the high-efficiency region, which increases the effective quantum efficiency of the metal channel PMT 108. The beam 105 is converted into a guided beam 106. In the embodiment of Figure 1, the optical system 103 is a microlens array having cylindrical lens elements that focus the guided beams 106. Each guided beam 106 corresponds to one of the cylindrical lens elements in the optical system 103.

[0028] The amount of light beam 105 that hits the high-efficiency region depends on the degree of collimation in beam 105. If beam 105 is well collimated, almost all the light can hit the high-sensitivity region.

[0029] In one example, the high-efficiency region and the low-efficiency region of the metal channel PMT 108 each constitute half of the total area. Increasing the light fraction in the high-efficiency region will increase the effective quantum efficiency. This result is the same as the efficiency of having more metal channel PMT 108 in the high-efficiency region.

[0030] The high-efficiency region can represent 35% to 65% of the total area of ​​the metal channel PMT 108 (inclusive of all ranges up to 0.1%), but other percentages are possible depending on the configuration of the metal channel PMT 108. Some metal channel PMT 108 configurations may have a smaller high-efficiency region, such as 15% to 35% of the total area of ​​the metal channel PMT 108 (inclusive of all ranges up to 0.1%). Preferentially focusing or otherwise guiding the beam 105 to these high-efficiency regions can provide improved performance of the metal channel PMT 108. For example, the effective quantum efficiency of the detector can be increased by approximately 10%. This is difficult to achieve simply by redesigning the multiplier, so the increase in quantum efficiency is unexpected.

[0031] System 100 also includes a light source 104 that generates a light beam 105. An optical system 103 is positioned in the path of the light beam 105 between the light source 104 and the metal channel PMT 108. The light source 104 can be a laser or other light source. For example, visible or ultraviolet wavelengths can be used. A detector can be used to detect the incoming light.

[0032] Although the optical system 103 is shown to have three cylindrical lens elements, more or fewer cylindrical lens elements may be part of the optical system 103. The lens elements may be etched into a structural surface of the glass or may be discrete lens elements.

[0033] In one example, the low-efficiency region of cathode 102 corresponds to a location of the dynode structure. Therefore, in this example, the low-efficiency region of cathode 102 is where the dynode structure is within the electron's line of sight. Regardless of the relationship between the low-efficiency region of cathode 102 and the location of the dynode structure, the beam 105 can be focused or otherwise guided to the high-efficiency region after determining these regions for a metal channel PMT 108.

[0034] Figure 1 illustrates a cylindrical microlens, but other optical systems are also possible. For example, a light guide can be used. Figure 6 illustrates a light guide 110. A light guide (or optical tube) can direct light to a high-efficiency region.

[0035] Figure 5 is a flowchart of one embodiment of method 200 of system 100. A light beam is generated in step 201. Then, in step 202, the light beam is guided toward an optical system (e.g., a microlens array or a light guide), and in step 203, the optical system is used to guide it onto a metal channel PMT. For example, in step 203, the light beam is focused onto a region of a metal channel PMT using a microlens array. The microlens array may include cylindrical lens elements or other components. Using the metal channel PMT, a dynode structure in the metal channel PMT converts photons in the light beam into electrons.

[0036] The area receiving light can correspond to the high-efficiency region of one of the cathodes of a metal channel photomultiplier tube (PMT). The high-efficiency region of the cathode can correspond to a position between the dynode structures. This position can refer to an alignment in the direction of the beam's travel through the PMT. Therefore, the position of the dynode structures can be within the path of the beam that avoids hitting the cathode.

[0037] In one example, a region rasterized beam can be used across a metal channel PMT.

[0038] The amount of light directed onto the high-efficiency region of the metal channel PMT can be 40% to approximately 100% of the total light in the system, encompassing all values ​​and ranges from 0.1% to the mean. For example, more than approximately 50% of the beam is directed onto the high-efficiency region.

[0039] The efficiency variation across position X is surprising. Previous research had anticipated that there would be minimal efficiency variation across position X and that the efficiency chart should be more uniform.

[0040] Although the invention has been described with respect to one or more specific embodiments, it will be understood that other embodiments of the invention may be made without departing from the scope of the invention. Therefore, the invention is to be considered limited only to the appended claims and their reasonable interpretation.

[0041] 100: System 101: Multiplication Pole 102: Photocathode / Cathode 103: Optical System 104: Light source 105: Beam 106: Guided beam 107: Anode 108: Metal Channel Photomultiplier Tube (PMT) 109: Field-shaped grid 110: Optical Guide 200: Method 201: Steps 202: Steps 203: Steps

Claims

1. A photomultiplier tube system, comprising: A metal channel photomultiplier tube includes a photocathode, an anode, and a dynode structure having three or more rows of dynodes extending from the photocathode to the anode, wherein the photocathode of the metal channel photomultiplier tube has a first region and a second region, wherein the first regions of the photocathode correspond to positions between the rows of dynodes in the dynode structure, and wherein the first regions constitute 15% to 65% of the total area of ​​the metal channel photomultiplier tube; and an optical system positioned in a path of a light beam guided at the metal channel photomultiplier tube, wherein the optical system is configured to guide a majority of the light beam at the first regions of the metal channel photomultiplier tube.

2. The system of claim 1, further comprising a light source for generating the light beam, wherein the optical system is disposed in the path of the light beam between the light source and the metal channel photomultiplier tube.

3. The system of claim 1, wherein the optical system comprises a microlens array of one of a plurality of cylindrical lens elements.

4. The system of claim 1, wherein the optical system is a light guide.

5. The system of claim 1, wherein the first region comprises 35% to 65% of the total area of ​​one of the metal channel photomultiplier tubes.

6. The system of claim 1, wherein the first region comprises 15% to 35% of the total area of ​​one of the metal channel photomultiplier tubes.

7. A method for operating a photomultiplier tube system, comprising: A beam of light is generated; Direct the beam toward an optical system; The optical system guides the beam onto a plurality of regions of a metal channel photomultiplier tube, wherein the metal channel photomultiplier tube includes a photocathode, an anode, and a dynode structure having three or more rows of dynodes extending from the photocathode to the anode, wherein these regions have a higher efficiency than other regions of the photocathode of the metal channel photomultiplier tube, wherein these regions of the photocathode correspond to positions between the rows of dynodes in the dynode structure, and wherein these regions constitute 15% to 65% of the total area of ​​the metal channel photomultiplier tube.

8. The method of claim 7, wherein the optical system comprises a microlens array of one of a plurality of cylindrical lens elements.

9. The method of claim 7, wherein the optical system is a light guide.

10. The method of claim 7, wherein the regions comprise 35% to 65% of the total area of ​​one of the metal channel photomultiplier tubes.

11. The method of claim 7, wherein the regions comprise 15% to 35% of the total area of ​​one of the metal channel photomultiplier tubes.

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