CROSS-REFERENCE TO RELATED APPLICATION

[0001 ] The present application is related to and claims the benefit of the earliest available effective filing date(s) from the following listed application(s) (the "Related Applications") (e.g., claims earliest available priority dates for other than provisional patent applications or claims benefits under 35 USC §1 19(e) for provisional patent applications, for any and all parent, grandparent, great-grandparent, etc. applications of the Related Application(s)).

Related Applications:

For purposes of the USPTO extra-statutory requirements, the present application constitutes a regular (non-provisional) patent application of United States Provisional Patent Application entitled EXTENDED LIFETIME PHOTOMULTIPLIER TUBE, naming Derek Mackay, Paul Donders, Kai Cao, and Jeongsik Lim as inventors, filed October 19, 2013, Application Serial Number 61/893,190. Application Serial Number

61/893,190 is incorporated herein by reference in the entirety.

TECHNICAL FIELD

[0002] The present invention relates to a photomultiplier tube (PMT), and more particularly, to a bias-variant PMT with extended lifetime through the application of a variant dynode bias.

BACKGROUND

[0003] As use of photomultiplier tubes (PMTs) continues, there is an increase in the demand for PMTs with extended lifetimes. PMTs amplify very small light signals. When light hits a cathode of a PMT, a photoelectron is created and accelerated towards a receiving dynode. The receiving dynode then amplifies the photoelectrons and directs them towards a second dynode. This process continues through a series of dynodes until the amplified photoelectron signal is collected at an anode. Typically, gain at each dynode scales with the energy of the incident electrons, which corresponds to the voltage difference between a given dynode and the previous dynode. It is generally observed that larger currents in latter dynodes cause degradation of the alkali coating(s), which enable the secondary emission necessary for signal amplification. The degradation in the coatings decreases the gain of a given stage even in cases where the energy of the incident electrons remains constant. As such, the total gain of the PMT is decreased. Consequently, even in the event an incident light signal stays constant, the signal measured at the anode of a given PMT will decrease as a function of time. Previous methods to mitigate this effect include adjusting the voltage values between the dynodes to restore the gain to its original value. At some point, however, the gain decreases to a level that the voltage cannot be increased sufficiently to counteract the gain loss. Therefore, it is desirable to provide a system and method which cures the deficiencies of the prior art described above.

SUMMARY

[0004] A bias-variant photomultiplier tube (PMT) is disclosed. In one illustrative embodiment, the bias-variant PMT may include, but is not limited to, a photocathode configured to absorb photons and emit photoelectrons responsive to the absorbed photons. In another illustrative embodiment, the bias-variant PMT may include, but is not limited to, a plurality of dynodes configured to receive the photoelectrons emanating from the photocathode. In another illustrative embodiment, the bias-variant PMT may include, but is not limited to, a first pair of dynodes having a first bias difference and at least a second pair of dynodes having a second bias difference different than the first bias difference. In another illustrative embodiment, the bias-variant PMT may include, but is not limited to, the second bias difference between the at least the second pair of dynodes being greater than the first bias difference between the first pair of dynodes. In another illustrative embodiment, the bias-variant PMT may include, but is not limited to, an anode configured to receive photoelectrons directed from the plurality of dynodes.

[0005] A method for biasing a photomultiplier tube (PMT) is disclosed. In one illustrative embodiment, the method for biasing a PMT may include, but is not limited to, generating an initial set of photoelectrons through secondary emission with absorption of photons with a photocathode. In another illustrative embodiment, the method for biasing a PMT may include, but is no limited to, directing the initial set of photoelectrons to a plurality of dynodes. In another illustrative embodiment, the method for biasing a PMT may include, but is not limited to, amplifying the initial set of photoelectrons to form a second set of photoelectrons with a first pair of dynodes having a first bias difference. In another illustrative embodiment, the method for biasing a PMT may include, but is not limited to, amplifying the second set of photoelectrons to form at least a third set of photoelectrons with at least a second pair of dynodes having at least a second bias difference greater than the first bias difference. In another illustrative embodiment, the method for biasing a PMT may include, but is not limited to, receiving the at least a third set of photoelectrons with an anode.

[0006] An inspection system with a bias-variant photomultiplier tube (PMT) sensor is disclosed. In one illustrative embodiment, the inspection system may include, but is not limited to, an illumination source configured to illuminate a portion of a sample surface. In another illustrative embodiment, the inspection system may include, but is not limited to, a bias-variant PMT sensor configured to detect at least a portion of light scattered from the surface of the sample. In another illustrative embodiment, the inspection system may include, but is not limited to, a set of collection optics configured to direct and focus at least a portion of light scattered from the surface of the sample through the bias-variant PMT sensor.

[0007] It is understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which:

FIG. 1A illustrates a simplified schematic view of a bias-variant photomultiplier tube equipped with bias-variant dynodes, in accordance with an embodiment of the present invention.

FIG. 1 B illustrates a simplified schematic view of a bias-variant PMT with a resistive chain equipped differing resistivity, in accordance with an embodiment of the present invention.

FIG. 1 C illustrates the voltage differences in three different biasing schemes that result in the same overall gain, in accordance with an embodiment of the present invention.

FIG. 1 D illustrates the degradation curve of three different biasing schemes, in accordance with an embodiment of the present invention.

FIG. 2 illustrates a block diagram of a method for biasing a photomultiplier tube, in accordance with an embodiment of the present invention.

FIG. 3 illustrates a block diagram of an inspection system equipped with a bias-variant photomultiplier tube sensor, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0009] Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. [0010] Reference generally to FIGS. 1A through 3, a bias-variant photomultiplier tube (PMT) 100 is described in accordance with the present disclosure. Embodiments of the present disclosure are directed to a bias-variant PMT 100 serving to provide resistance to gain degradation. A reduction in gain degradation results in an increase of the lifetime of the given PMT. Embodiments of the present disclosure are further directed to a bias-variant PMT 100 with multiple pairs of dynodes, with a first pair of dynodes having a first bias difference and a second pair of dynodes having a second bias difference. In one embodiment, the second bias difference is greater than the first bias difference (e.g., second bias difference is two times the first bias difference). It is noted herein that such a bias arrangement results in the reduction of gain degradation at the latter dynodes, thereby increasing the lifetime of the PMT.

[001 1 ] FIG. 1A illustrates a bias-variant photomultiplier tube (PMT) 100, in accordance with an embodiment of the present invention. In one embodiment, the bias- variant PMT 100 includes a photocathode 102. In another embodiment, the photocathode 102 is configured to absorb photons 104. In another embodiment, the photocathode emits photoelectrons 108a-d in response to the absorbed photons. In another embodiment, the photocathode 102 includes a transmission-type photocathode suitable for absorbing photons 104 at one surface of the photocathode 102 and emitting photoelectrons 108a-d from the opposite surface of the photocathode 102. In another embodiment, the photocathode 102 includes a reflective-type photocathode suitable for absorbing photons 104 at one surface and emitting photoelectrons 108a-d from the same surface of the photocathode 102. In another embodiment, the photocathode 102 is configured to absorb photons 104 from oblique incident angles and/or normal incident angles.

[001 2] In one embodiment, the bias-variant PMT 100 includes a plurality of dynodes 106a-d. For example, the bias-variant PMT 100 may include a first dynode 106a configured to receive photoelectrons 108a emanating from the photocathode 102. In another embodiment, the bias-variant PMT 100 includes a first dynode 106a configured to amplify the photoelectron current 108a (e.g., via secondary emission) such that the photoelectron current 108b emanating from the first dynode 106a is larger than the current 108a. In another embodiment, the bias-variant PMT 100 includes a second dynode 106b that amplifies the photoelectron current such that the current 108c is larger than the current 108b. In another embodiment, the bias-variant PMT 100 includes multiple dynodes 106a-d that amplify the photoelectron current to desired levels. In another embodiment, the last dynode (e.g., 106d in FIG 1A) in the bias-variant PMT 100 is arranged to direct the amplified photoelectron current output so as to impinge on an anode 1 10.

[001 3] In another embodiment, the bias-variant PMT 100 includes a first set of dynodes with multiple dynode pairs (e.g., 106a and 106b or 106c and 106b) that have different bias differences between them. For example, the bias-variant PMT 100 may include a first dynode 106a with a first bias and a second dynode 106b with a second bias, with the second bias being different than the first dynode 106a bias. In another embodiment, the bias-variant PMT 100 includes a first dynode 106a with a first set bias and second dynode 106b with a second set bias different than the first bias, whereby the difference between the first bias and the second bias creates a first bias difference. In another embodiment, the bias-variant PMT 100 includes a third dynode 106c with a third bias and a fourth dynode 106d with a fourth bias different than the third bias associated with the third dynode 106c. In another embodiment, the bias-variant PMT 100 includes a third dynode 106c with a third set bias and fourth dynode 106d with a fourth set bias different than the third bias, whereby the difference between the third bias and the fourth creates a second bias difference. In another embodiment, the second bias difference between the third dynode 106c and the fourth dynode 106d is greater than the first bias difference between the first dynode 106a and the second dynode 106b. In another embodiment, the bias-variant PMT 100 includes a second dynode pair which shares a dynode with a first dynode pair. For example, the bias- variant PMT 100 may include a first dynode 106a and a second dynode 106b that make up a first dynode pair with a first bias difference and the second dynode 106b and a third dynode 106c make up a second dynode pair with a second bias difference greater than the first bias difference. [0014] In another embodiment, although not depicted, the bias-variant PMT 100 includes at least a second set of dynodes. In another embodiment, the bias-variant PMT 100 includes a first dynode of a second set of dynodes with a first bias configured to receive photoelectrons emanating from the photocathode. In another embodiment, the bias-variant PMT 100 includes a first pair of dynodes of a second set of dynodes having a first bias difference and at least a second pair of dynodes of a second set of dynodes having a second bias difference different than the first bias difference. In another embodiment, the bias-variant PMT 100 includes a second pair of dynodes of a second set of dynodes having a second bias difference greater than a first pair of dynodes of a second set of dynodes first bias difference.

[001 5] In one embodiment, the bias-variant PMT 100 includes an anode 1 10. In another embodiment, the bias-variant PMT 100 includes an anode 1 10 configured to receive photoelectrons directed from the plurality of dynodes 106a-d. In another embodiment, the bias-variant PMT 100 includes an anode 1 10 configured to convert the photoelectrons from the plurality of dynodes 106a-d to light. In another embodiment, the bias-variant PMT 100 includes a detector (not shown) configured to detect illumination emanating from the anode 1 10. For example, the detector may include any detector known in the art, such as but not limited to, a CCD detector, or a TDI-CCD detector.

[0016] FIG. 1 B illustrates a bias-variant PMT 100 with a resistive chain with set resistivity 1 12a and 1 12b, in accordance with an embodiment of the present invention. In one embodiment, the bias-variant PMT 100 includes a resistive chain with a first pair of dynodes 106a and 106b and a second pair of dynodes 106c and 106d. In another embodiment, the resistivity 1 12b is at least greater than resistivity 1 12a. For example, the resistive chain may include having a resistivity 1 12b twice as large as resistivity 1 12a, which results in a greater voltage difference between the second pair of dynodes 106c and 106d than the first pair of dynodes 106a and 106b. [0017] It is noted herein that the number of dynodes used in the bias-variant PMT 100 is not limited to the number of dynodes illustrated in FIG. 1A or 1 B. The number of dynodes depicted in FIG. 1A and 1 B are provided merely for purposes of illustration and it is contemplated that any number of dynodes and dynode pairs may be utilized in the present invention. It is further noted that the choice of the number of dynodes and dynode pairs may ultimately depend on required amplification levels and cost, among other factors. It is further noted that the bias-variant PMT 100 is not limited to the resistive chain illustrated in FIG. 1 B. The resistive chain depicted in FIG. 1 B is provided merely for purposes of illustration and it is contemplated that any resistive chain configuration may be utilized in the present invention.

[0018] FIG. 1 C illustrates the voltage differences in three different biasing schemes that result in the same overall gain, in accordance with an embodiment of the present invention. The curve 122 illustrates the voltage difference between dynodes in a normally biased PMT (constant). The curve 124 illustrates the voltage differences consistent with the bias-variant PMT 100 as discussed throughout the present disclosure with the larger gain at the latter dynodes. The curve 126 illustrates the opposite of the bias-variant PMT 100 where the first dynodes are equipped with a larger bias difference than the latter dynodes used to determine if this shortens the life of a PMT. It is noted herein that all three biasing cases depicted in FIG. 1 C were arranged to produce the same overall PMT gain.

[0019] FIG. 1 D illustrates the degradation curves associated with three different biasing schemes. The Line 128 illustrates the normally biased PMT, Line 130 illustrates the bias-variant PMT 100, and Line 132 illustrates the opposite of the bias-variant PMT 100. Each illustrated line uses the same light source and start with the same gain. All lines have the same anode current, but their degradation lines are vastly different as illustrated.

[0020] FIG. 2 illustrates a flow diagram 200 depicting a method 200 for amplifying a photoelectron signal in a PMT with non-uniform biasing, in accordance with one or more embodiments of the present disclosure. Step 202 generates an initial set of photoelectrons through absorption of photons with a photocathode. In another embodiment, the method for biasing 200 includes emitting (via secondary emission) photoelectrons via the absorption of photons from a photocathode. Step 204 directs the initial set of photoelectrons to a plurality of dynodes. Step 206 amplifies, with a first pair of dynodes having a first bias difference, an initial set of photoelectrons to form a second set of photoelectrons. Step 208 amplifies, with a second pair of dynodes having a second bias difference greater than the first bias difference, the second set of photoelectrons to form a third set of photoelectrons. Step 210, receives the photoelectrons with an anode.

[0021 ] FIG. 3 illustrates an optical system 300 equipped with a bias-variant PMT 100, in accordance with one or more embodiments of the present disclosure.

[0022] In one embodiment, the optical system 300 includes a bias-variant PMT sensor 301 . In another embodiment, the bias-variant PMT sensor 301 of the optical system 300 includes one or more bias-variant PMTs, such as the bias-variant PMT 100 as described previously in the present disclosure. In another embodiment, the bias- variant PMT 100 in the bias-variant PMT sensor 301 includes a photocathode configured to absorb photons and emit photoelectrons responsive to the absorbed photons; a plurality of dynodes configured to receive the photoelectrons emitted from the photocathode, a first pair of dynodes having a first bias difference and at least a second pair of dynodes having a second bias difference greater than the first bias difference; and an anode configured to receive photoelectrons directed from the plurality of dynodes.

[0023] In one embodiment, the optical system 300 includes an illumination source 302 configured to generate illumination. In another embodiment, the illumination source 302 is configured to illuminate a portion of a surface of a sample 304 (e.g., semiconductor wafer) disposed on a sample stage 306. In another embodiment, the illumination source 302 includes one or more broadband light sources, such as a broad band lamp (e.g., xenon lamp). In another embodiment, the illumination source 302 includes one or more narrowband light sources, such as one or more lasers emitting light at a selected wavelength.

[0024] In one embodiment, the optical system 300 includes a set of illumination optics 308 configured to direct and focus the illumination onto the sample surface 304. In another embodiment, the illumination optics 308 of the optical system 300 includes any optical elements known in the art suitable for directing, processing, filtering, polarizing and/or focusing the light beam emanating from the illumination source 302 on to a portion of the surface of the sample 304. For example, the set of illumination optics may include, but are not limited to, one or more lenses, one or more mirrors, one or more beam splitters, one or more polarizer elements, one or more filters and the like.

[0025] In another embodiment, the optical system 300 includes a set of collection optics 310 configured to direct and focus at least a portion of the light scattered from the surface of the sample 304 to an input of the bias-variant PMT sensor 301 . In another embodiment, the collection optics 310 of optical system 300 include any optical elements known in the art suitable for collecting, directing, processing, filtering and/or focusing light scattered, reflected or diffracted from the surface of the sample 304 onto the bias-variant based sensor 301 . For example, the set of collection optics 310 may include, but are not limited to, one or more lenses, one or more mirrors, one or more beam splitters, one or more filters, one or more polarizer elements, and the like.

[0026] In another embodiment, the optical system 300 is an inspection system, or inspection tool. In another embodiment, the optical system 300 is an optical metrology system, or inspection tool. In another embodiment, the illumination source 302, the illumination optics 308, the collection optics 310, and the bias-variant PMT sensor 301 may be arranged in a darkfield configuration such that the optical system 300 operates as a darkfield inspection system. In another embodiment, although not shown, the illumination source 302, the illumination optics 308, the collection optics 310, and the bias-variant PMT sensor 301 may be arranged in a brightfield configuration such that the optical system 300 operates as a brightfield inspection system. [0027] While particular aspects of the present subject matter described herein have been shown and described, it will be apparent to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from the subject matter described herein and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of the subject matter described herein. It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. Furthermore, it is to be understood that the invention is defined by the appended claims.