DETACHABLY COUPLED IMAGE INTENSIFIER AND IMAGE SENSOR

Abstract

A detachably coupled image intensifier and image sensor combination is disclosed along with systems and methods for using the detachably coupled image intensifier and image sensor combination. In one embodiment, there are at least two fiber optic plates aligned between the image intensifier and image sensor, and an oil or a gel is used to fill some or all of the gap(s) between pair(s) of adjacent fiber optic plates. In one embodiment, the detachably coupled image intensifier and image sensor combination is used for sample inspection.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/516,668 filed Sep. 7, 2006, which is a continuation-in-part of U.S. Ser. No. 10/511,092, filed Apr. 26, 2005; itself a national stage application of PCT/US03/28062, filed Sep. 8, 2003; which claims the benefit of U.S. Provisional Patent Application 60/415,082, filed Sep. 30, 2002.

This application also claims the benefit of U.S. Provisional Application 60/715,927, U.S. Provisional Patent Application 60/715,900, and U.S. Provisional Application 60/715,901. Said applications were all filed on Sep. 8, 2005 and are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to image intensifier tubes.

BACKGROUND OF THE INVENTION

Image intensifier tubes (also known as IIT or image intensifiers) are widely used for sensing and amplifying, or intensifying, light images of low intensity. In these devices, light (usually of visible or near infra-red spectra) from an associated optical system is directed onto a photocathode which emits a distribution of photoelectrons in response to the input radiation.

An image intensifier typically includes a vacuum tube with a photocathode unit at one end and a screen unit at the other end. The photocathode unit converts incoming photons to electrons which are accelerated by an electric field (potential difference) in the tube until they hit a screen unit converting them back to photons.

The output of the image intensifier tube is fed into a solid state optical image sensor such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) device. The combination of the image intensifier tube and image sensor is sometimes referred to as an intensified image sensor device ICCD or ICMOS.

In U.S. Pat. No. 4,980,772 to Kawamura et al, some examples of current methods of coupling the image intensifier tube to an image pickup device are disclosed. In one method, a thin fiber plate is interposed between a phosphor layer of the screen of the image intensifier tube and a photosensitive layer of the image pickup device. In another method, two fiber plates are used to couple an image intensifier tube to the image pickup device. In yet another method, a fiber plate on the output surface of the image intensifier tube is bound to an image pickup device by means of an adhesive.

SUMMARY OF THE INVENTION

According to the present invention there is provided an apparatus for intensifying and sensing images, comprising: an image intensifier tube; an image sensor; at least two fiber optic plates aligned between a photo-emitting output area of the image intensifier tube and a photosensitive input area of the image sensor so as to allow light issuing from the image intensifier tube to be transmitted to the image sensor; non-binding filling which fills at least one gap which is between at least one pair of adjacent fiber optic plates among the at least two fiber optic plates; and a detachable attaching medium detachably coupling between the image intensifier tube and the image sensor.

According to the present invention there is also provided a method of separating an image intensifier tube detachably coupled to an image sensor, comprising: a) providing an image intensifier tube detachably coupled to an image sensor; and b) separating the image intensifier tube from the image sensor; wherein the separating does not substantially damage the image intensifier tube nor the image sensor.

According to the present invention there is further provided an apparatus for inspection of a sample, comprising: a radiation source, which is adapted to direct optical radiation onto an area of a surface of the sample; at least one image intensifier, each of which is detachably coupled to an image sensor, so as to receive the radiation from the area over a certain angular range, and to provide intensified radiation to the image sensor; and at least one image sensor, each of which is configured to receive radiation from at least one image intensifier, so as to form at least one respective image of the area.

According to the present invention there is provided a method of inspecting a sample, comprising: a) providing at least one image intensifier tube detachably coupled to an image sensor with non-binding filling; b) directing optical radiation onto an area of a surface of a sample to be inspected; c) receiving and intensifying the radiation scattered from the area using the at least one provided detachably coupled image intensifier tube and image sensor, so as to form a respective images of the area, each of the provided detachably coupled image intensifier tube and image sensor being configured to receive the radiation that is scattered to into a different, respective angular range; and d) processing at least one of the respective images so as to detect a defect on the surface.

According to the present invention there is also provided a method of inspecting a sample comprising: a) providing an image intensifier tube detachably coupled to an image sensor; b) separating the image intensifier tube from the image sensor; c) coupling at least one of the separated image intensifier tube and image sensor in a combination of image intensifier tube and image sensor; d) directing optical radiation onto an area of a surface of a sample to be inspected; e) receiving the radiation scattered from the area using the combination coupled in (c) so as to form a respective image of the area; and f) processing the image so as to detect a defect on the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

FIG. 1A is a schematic, pictorial illustration of an image intensifier detachably coupled to an image sensor, according to an embodiment of the present invention;

FIG. 1B is a schematic pictorial illustration of a magnetically focused image intensifier detachably coupled to an image sensor, according to an embodiment of the present invention;

FIG. 2 is a block diagram that schematically illustrates a system for optical inspection, according to an embodiment of the present invention; and

FIG. 3 is a schematic side view of an optical collection module including image intensifiers detachably coupled to image sensors, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Described herein are embodiments of the present invention relating to an image intensifier and image sensor which are detachably coupled together, and to methods and systems using such a combination.

As used herein, the phrase “for example,” “such as” and variants thereof describing exemplary implementations of the present invention are exemplary in nature and not limiting.

For example, when an image intensifier tube and image sensor are permanently coupled together, the combination can no longer be used once the image intensifier tube and/or image sensor fails (assuming for example, that the failed part cannot be repaired or cannot be partially replaced saving the other part while coupled together). However, if the image intensifier tube and image sensor are detachably coupled together, the image intensifier and image sensor can possibly be detached. For example, say the image intensifier tube fails, the image sensor may be still be used, for example by detaching the failed image intensifier tube from the image sensor and coupling a replacement image intensifier tube to the detached image sensor. Although in this example it is assumed that the image intensifier tube fails, the converse may in some cases be true and when an image sensor fails, the failed image sensor may be detached from the image intensifier and a replacement image sensor may be coupled to the detached image intensifier tube. The example of detaching the detachably coupled together image intensifier tube and image sensor in order to replace a failed element should be understood to be a non-limiting example and detachment for any reason is within the scope of the invention. For example, the image intensifier tube and image sensor may be detached in order to clean, repair, inspect, upgrade, etc. one or the other of the elements. In some cases, the detached image intensifier tube and image sensor may be re-coupled together instead of to replacement image sensor or image intensifier tube respectively. In some cases, the detached image intensifier may be used independently of any image sensor and/or the detached image sensor may be used independently of any image intensifier, for example the detached image intensifier may be used in front of a photomultiplier tube PMT.

Typically although not necessarily, the image sensor is comprised in a camera which also comprises necessary electronics. Therefore it may in some cases be more technically accurate to describe the image intensifier as being detachably coupled to the camera. However since the term camera does not always have a uniform meaning in the art, for clarity of description the terminology used below describes the image sensor as being detachably coupled to the image intensifier or refers to a detachable combination of image intensifier tube and image sensor.

Hereinbelow, in order to improve readability, the abbreviation “DIIS” is used for Detachable combination of image Intensifier tube and Image Sensor.

FIG. 1A is a schematic view of a DIIS 10 comprising an image intensifier 16 detachably coupled to an image sensor 34, according to an embodiment of the present invention.

In the illustrated embodiment, image intensifier tube 16 includes a photocathode unit 14, for example a multialkali photocathode layer on a glass substrate and a screen unit 20, for example a phosphor layer structure on a glass (fiber optic plate) substrate. Any other suitable photocathode and/or screen unit may be used instead.

Depending on the embodiment, image intensifier 16 may be an image intensifier of any generation and/or using any focusing method, as appropriate. As is known in the art, there are several known generations of image intensifiers: The so-called “first generation image intensifiers” are intensifier diodes that utilize only a single potential difference to accelerate electrons from the cathode to the anode (screen). The “second generation image intensifiers” utilize electron multipliers, i.e., not only the energy but also the number of electrons between input and output is significantly increased. Multiplication is achieved by use of a device called a microchannel plate (MCP), i.e. a thin plate of conductive glass containing many small holes. In these holes, secondary electron emission occurs which leads to multiplication factors of up to four orders of magnitude. The “third generation image intensifiers” employ MCP intensifiers with Gallium-Arsenide photocathodes (instead of multialkali photocathodes as Cs, Sb, K, Na, etc. normally used in first and second generation intensifiers or instead of bi-alkali or solar blind (CsTs) sometimes used in first or second generation intensifiers) to increase a luminous sensitivity of approximately 1,200 μA/lm instead of 300 μA/lm found in the multialkali photocathodes. These GaAs photocathodes are also much more sensitive in the NIR region of the light spectrum. Modified third generation image intensifiers which are filmless (i.e. without an ion barrier film) are sometimes termed “fourth generation image intensifiers” or may be grouped under the term “third generation image intensifiers”.

In these intensifiers, focusing is achieved by any of three approaches. The first approach includes placing the screen in close proximity to the photocathode (proximity focus image intensifier). In the second electrostatic approach, electrodes focus electrons originating from the photocathode onto the screen (electrostatic image intensifier or inverter image intensifier). In the third magnetic focus approach, a magnetic field parallel to the optical axis causes electrons to complete exactly one (or complete multiplication of one) full turn (magnetically focused image intensifier).

In some cases, image intensifier 16 may have more than one focusing, for example if an MCP is used, and the more than one focusing may all use the same focusing approach or may use a plurality of different focusing approaches.

Depending on the embodiment, image sensor 34 can be any suitable solid state optical image sensor. Examples of possible solid state optical image sensors which can be used as image sensor 34 include inter-alia: a charge coupled device (CCD) or a complementary-symmetry metal-oxide semiconductor (CMOS) device.

As illustrated in FIG. 1A, a first fiber optic plate 22 is connected to a photo-emitting output area (back face) of screen unit 20. A second fiber optic plate 32 connects to a photosensitive input area (front face) of image sensor 34. In one embodiment each fiber optic plate 22 and 32 is less than 4 fringes (surface quality). In one embodiment, one or both of fiber optic plates 22 and 32 have a mechanism for reduced crosstalk (for example fiber optic plate with extra mural absorption EMA).

First fiber optic plate 22 and second fiber optic plate 32 are aligned (put into the correct relative position) so that the light issuing from screen unit 20 is transmitted to the photosensitive input surface of image sensor 34.

Typically, there is a small gap between first fiber optic plate 22 and second fiber optic plate 32. In one embodiment, the gap is about 0 to 5 microns. Therefore, a non-binding filling 40 is used to fill the gap between the two plates 22 and 34. It should be understood by the reader, that the term “non-binding” filling 40 refers to a filling which allows the two fiber optic plates 22 and 32 to be separated from one another (and therefore image intensifier tube 16 and image sensor 34 to be separated from one another) without substantially damaging the DIIS 10 (for example without substantially damaging any of image sensor 34, image intensifier 16 or fiber optic plates 22 and 32).

Non-binding filling 40 has an index of refraction which is closer to the index of refraction of fiber optic plates 22 and 34, than the index of refraction of air is to the index of refraction of the fiber optic plates, thereby preventing or minimizing Fresnel reflections. (Without non-binding filling 40, Fresnel reflections would probably occur at the interface between the fiber optic plate and air due to the different refractive indices). For example, non-binding filling 40 may have an index of refraction similar to fiber optic plates 22 and 32. Continuing with the example, the index of refraction of non-binding filling 40 may be about 1.8. In another example, the index of refraction of non-binding filling 40 is not an identical match to that of the fiber optic plates. Continuing with the example, the index of non-binding filling 40 may be about 1.5.

Non-binding filling 40 can be for example a gel or an oil. In one embodiment, non-binding filling 40 has minimal outgassing. The size of the gap and the index value stated above are provided solely for further illustration to the reader and should not be construed as limiting.

A detachable attaching medium is used to detachably attach image intensifier tube 16 to image sensor 34. The reader should understand that the term “detachable” refers to an attaching medium which will stop attaching when separation of image intensifier 16 and image sensor 34 from one another is desired. For example, the detachable attaching medium can be removed, released, counteracted, etc. when separation of image intensifier 16 and image sensor 34 from one another is desired. Due to the usage of the detachable attaching medium, the attachment of image sensor 16 and image intensifier 34 as well as the separation of image sensor 16 and image intensifier 34 can be achieved without substantially damaging DIIS 10 (for example without substantially damaging any of image sensor 34, image intensifier 16 or fiber optic plates 22 and 32).

In one embodiment the detachable attaching medium at least includes an elastic material which allows fiber optic plates 22 and 32 to be pushed close together (for example, when attaching image intensifier 16 and image sensor 34 to one another) without substantially damaging DIIS 10 (for example without substantially damaging any of image sensor 16, image intensifier 34, and/or fiber optic plates 22 and 32). Examples of elastic material include inter-alia: spring(s), sponge(s), rubber, etc.

In the illustrated embodiment of FIG. 1A, detachable attaching medium 50 includes: spring(s) 56, screws 51, 53, and 58, and mechanical parts 52 and 54. One or more springs 56 in conjunction with one or more screws 58 are used to connect mechanical part 52 to mechanical part 54. Mechanical part 52 is shown connected to a camera 70 comprising image sensor 34 and mechanical part 54 is shown connected to image intensifier 14. Although mechanical part 52 is shown directly and detachably connected to camera 70 with one or more screws 51, in other embodiments mechanical part 52 can be indirectly and/or permanently attached to camera 70. In other embodiments, mechanical part 52 can be directly or indirectly attached, either detachably or non-detachably to image sensor 34. Although mechanical part 54 is shown detachably and directly connected to image intensifier 16 with one or more screws 53, in other embodiments, mechanical part 54 may be indirectly and/or permanently attached to image intensifier tube 16. In other embodiments mechanical part 52 may be omitted and for example, one or more springs in conjunction with one or more screws may connect for example between camera 70 (or image sensor 34) and image intensifier 16, or for example between camera 70 (or image sensor 34) and mechanical part 54. In other embodiments, mechanical part 54 may be omitted and for example, one or more springs in conjunction with one or more screws may connect for example between camera 70 (or image sensor 34) and image intensifier 16, or for example between mechanical part 52 and image intensifier 16. In another embodiment, detachable attachable medium 50 can include screws and springs, with camera 70 held detachably and firmly (for example with screws) to image intensifier tube 16, and inside camera 70, image sensor 34 “floats” on springs.

In one embodiment, first fiber optic plate 22 and image intensifier 16 are commercially available as one unit and/or permanently coupled together and are thus shown in FIG. 1A. In one embodiment, second fiber optic plate 32, camera 70 and image sensor 34 are commercially available as one unit and/or permanently coupled together and are thus shown in FIG. 1A. However it should be evident that in other embodiments, some or all of these elements may be detachably coupled to one another. For example, in one embodiment, image sensor 34 may be detachably coupled to camera 70.

It should be evident that FIG. 1A illustrates only one example of possible detachable attaching medium. In other embodiments, the detachable attaching medium may comprise less than all of elements 51, 52, 53, 54, 56, and 58. In other embodiments, the detachable attaching medium may comprise additional elements in addition to elements 51, 52, 53, 54, 56, and 58. In other embodiments, the detachable attaching medium may comprise elements different than some or all of elements 51, 52, 53, 54, 56, and 58. In other embodiments, the functionality provided by elements 51, 52, 53, 54, 56, and 58 may be distributed differently among those elements.

Although as mentioned above image the image intensifier in the DIIS may use any focusing approach, in some applications it may be advantageous to have an image intensifier which is magnetically focused. For example, in some cases the usage of a magnetically focused image intensifier in a particular application provides superior optical performance and/or the possibility of enhancing the lifetime of the image intensifier (compared to a proximity focused or electrostatic focused image intensifier). In some of these cases the superior optical performance includes any of the following inter-alia: better resolution, lower halo, and the possibility of having a potential difference greater than 10 to 15 KV in the image intensifier and therefore a higher gain.

For the sake of further illustration to the reader, FIG. 1B illustrates a DIIS which comprises a magnetically focused image intensifier according to an embodiment of the present invention. For simplicity's sake FIG. 1B replicates the DIIS illustrated in FIG. 1A, however also illustrates is a magnet 60 which surrounds image intensifier 16. A magnetic field produced by a magnet 60, parallel to the optical axis, causes electrons to complete exactly one full turn.

Magnet 60 is shown detachably attached to mechanical part 52 with one or more screws 62 in FIG. 1B. In other embodiments magnet 60 can be placed around image intensifier 14 using a different technique.

The illustration of image intensifier 16 as magnetically focused in FIG. 1B should not be construed as binding. The image intensifier in a DIIS of this invention may use any focusing approach, which may vary depending on the embodiment.

The combination of elements in a DIIS may vary depending on the embodiment and is not limited to the combination of elements illustrated in FIG. 1A or 1B. At a minimum, a DIIS comprises an image sensor and an image intensifier in a detachable combination. However, other elements shown in FIG. 1A or 1B may in some embodiments be omitted from a DIIS. In some embodiments, additional elements not shown in FIGS. 1A and 1B may be included in a DIIS.

In some embodiments, the image intensifier tube and the image sensor in the DIIS are later detached from one another. For example, assume an image intensifier tube and an image sensor have been previously detachably connected together into a DIIS, for example as illustrated in FIG. 1A. Assuming the configuration illustrated in FIG. 1A, at a later point in time, image intensifier tube 16 and first fiber optic plate 22 may be detached from image sensor 34 and second fiber optic plate 32, without substantially damaging DIIS 10 (for example without substantially damaging any of image sensor 16, image intensifier 34, and/or fiber optic plates 22 and 32). For example, at least screw(s) 58 may be unscrewed, releasing the connection between mechanical parts 52 and 54. Optionally, depending on the embodiment, any of the other screw(s) 51, 53 may also be unscrewed, and/or any of elements in FIG. 1A which are detachable (for example, any of elements 51, 52, 53, 54, 56, and 58) may be removed.

Once detached, the image intensifier tube and/or image sensor may be retained as is, processed as is (for example inspected), modified (for example repaired, upgraded, or cleaned) or discarded. Optionally, the detached image intensifier tube and/or image sensor may be re-attached together, or one or both may be reattached to another element (e.g. to respectively another image sensor or image intensifier tube, or to a different device). For example, in one embodiment, detachment may occur upon the failure or degradation of the image intensifier tube and the detached image sensor may later be attached to another image intensifier tube, either detachably as described above or permanently. It is also possible that one or both of the detached image intensifier tube and/or image sensor may not be later attached to any other element.

In one embodiment, the non-binding filling which filled the gap between the two fiber optic plates corresponding respectively to the image sensor and image intensifier, is removed any time after detaching the image intensifier tube and image sensor from one another. In this embodiment, if the detached image intensifier and/or image sensor is later detachably attached to each other or to another element, new non-binding filling is applied if necessary. However, in another embodiment, the non-binding filling is not necessarily removed and may optionally be reused when later detachably attaching one or both of the detached image intensifier and/or the image sensor to each other or to another element.

Applications which incorporate one or more DIIS in accordance with embodiments described above are not limited by the invention. For further illumination to the reader, however, it will now be described an application incorporating an embodiment of a DIIS, namely a dark field inspection system for inspecting wafers as described in co-pending and co-assigned U.S. Ser. No. 10/511,092 (U.S. published application number 20050219518), said application hereby incorporated by reference herein FIG. 2 is a block diagram that schematically illustrates a system 220 for optical inspection of a semiconductor wafer 222, in accordance with an embodiment of the present invention. Typically, wafer 222 is patterned, using methods of semiconductor device production known in the art, and system 220 applies dark-field optical techniques to detect defects on the surface of the wafer. Alternatively, however, the principles embodied in system 220 may be applied to unpatterned wafers and to inspection of other types of samples and surfaces as well, such as masks and reticles. Furthermore, although system 220 is dedicated to dark-field inspection, aspects of the present invention may also be applied in bright-field inspection, as well as in other areas of illumination, inspection and imaging.

System 220 comprises an illumination module 224, which illuminates the surface of sample 222 using pulsed laser radiation. Typically, module 224 is able to emit the laser radiation selectably at two or more different wavelengths, either simultaneously or one at a time. The laser radiation at any of the laser wavelengths may be directed by module 224 to impinge on wafer 222 either along a normal to the wafer surface or obliquely, as described in further detail in US Published Application Number 20050219518. The illumination module may be configured to emit optical radiation at wavelengths in the visible, ultraviolet (UV) and/or infrared (IR) ranges. The terms “illumination” and “optical radiation” as used herein should therefore be understood as referring to any or all of the visible, UV and IR ranges.

The radiation scattered from wafer 222 is collected over a large range of angles by an optical collection module 226. Module 226 comprises collection optics 228, which image the surface of wafer 222 onto multiple DIIS 230. Optics 228 may comprise either a single objective with high numerical aperture (NA) or a collection of individual objectives, one for each DIIS 230. Details of both of these alternative optical configurations are described in further detail in US Published Application Number 20050219518, and details of DIIS 230 are described hereinbelow. Optics 228 and DIIS 230 are arranged so that all the DIIS image the same area on the wafer surface, i.e., the area illuminated by illumination module 224, while each DIIS 230 captures the radiation that is scattered into a different angular range. Each DIIS 230 includes a two-dimensional array of detector elements, such as a CCD or CMOS array, as is known in the art. Each detector element of each of the arrays is imaged onto a corresponding spot within the area irradiated by illumination module 224. Thus, the scattering characteristics of any given spot on wafer 222 as a function of angle can be determined based on the signals generated by the corresponding detector elements in the different DIIS 230.

DIIS 230 are typically synchronized with the laser pulses from illumination module by a system controller 232, so that each image output frame generated by each DIIS 230 corresponds to the radiation scattered from a single laser pulse. The output from each DIIS 230 is received, digitized and analyzed by an image processor 234. The image processor which is described in further detail in U.S. Published Application Number 20050219518, typically comprises dedicated hardware signal processing circuits and/or programmable digital signal processors (DSPs). A mechanical scanner, such as an X-Y-Z stage 236 translates wafer 222, typically in a raster pattern, so that each laser pulse from illumination module 224 irradiates a different area of the surface of the wafer, adjacent to (and typically slightly overlapping with) the area irradiated by the preceding pulse. Alternatively or additionally, the illumination and collection modules may be scanned relative to the wafer.

Image processor 234, processes each of the image frames that is output by each DIIS 230 in order to extract image features that may be indicative of defects on the wafer surface. The image features are passed to a host computer 238, typically a general-purpose computer workstation with suitable software, which analyzes the features in order to generate a defect list (or defect map) with respect to the wafer under inspection.

The area irradiated by module 224 and imaged by DIIS 230 can be scanned using stage 236 over the entire wafer surface, or over a selected area of the surface. If the pulses emitted by module 224 are sufficiently short, substantially less than 1 μs, for example, stage 236 may translate wafer 222 continuously in this manner without causing significant blur in the images captured by the DIIS. The irradiated area typically has dimensions on the order of 2×1 mm, although the area can be enlarged or reduced using magnification optics in the illumination module as described in more detail in U.S. Published Application Number 20050219518. Assuming each DIIS 230 includes an array of about 2000×1000 detector elements, the size of each pixel projected onto the wafer surface is then roughly 1×1 μm. With module 224 operating at a repetition rate of 400 pulses/sec, the data output rate of each DIIS 230 to image processor 234 will be 800 Mpixels/sec. At this rate, for instance, an entire 12″ semiconductor wafer can be scanned at 1 μm resolution in less than 2 min. It will be understood, however, that these typical figures of image resolution, size and speed are cited solely by way of example, and larger or smaller figures may be used depending on system speed and resolution requirements.

Controller 232 also adjusts the Z-position (height) of stage 236 in order to maintain the proper focus of DIIS 230 on the wafer surface. Alternatively or additionally, the controller may adjust the DIIS optics for this purpose. Further alternatively or additionally, the controller may instruct image processor 234 and host computer 238 to correct for deviations in the scale and registration of the images captured by different DIIS 230 so as to compensate for height variations.

In order to verify and adjust the focus, controller 232 uses an auto-focus illuminator 240 and an auto-focus sensor module 242. Illuminator 240 typically comprises a laser (not shown), such as a CW diode laser, which emits a collimated beam at an oblique angle onto or adjacent to the area of the surface of wafer 222 that is illuminated by illumination module 224, forming a spot on the wafer surface. Variations in the Z-position of wafer 222 relative to collection module 226 will then result in transverse displacement of the spot. Sensor module 242 typically comprises a detector array (also not shown), which captures an image of the spot on the wafer surface. The image of the spot is analyzed in order to detect the transverse position of the spot, which provides controller 32 with a measurement of the Z-position of the wafer surface relative to the collection module. The controller may drive stage 236 until the spot is in a pre-calibrated reference position, indicative of proper focus.

The beam emitted by illuminator 240 may pass through collection optics 228 on its way to the wafer surface, and sensor module 242 may likewise capture the image of the spot on the surface through the collection optics. In this case, illuminator 240 preferably operates in a different wavelength range from illumination module 224. Thus, appropriate filters may be used to block scatter of the auto-focus beam into DIIS 230, as well as preventing interference of the pulsed beam from module 224 with the auto-focus measurement.

Alternatively, other means of auto-focus detection may be used, as are known in the art. For example, a capacitive sensor may be used to determine and adjust the vertical distance between the optics and the wafer surface.

FIG. 3 is a schematic side view of collection module 226, in accordance with an embodiment of the present invention. In this embodiment and in the embodiment shown in FIG. 2, module 226 is shown as comprising five DIIS 230. Alternatively, module 226 may comprise a smaller or greater number of DIIS, typically as many as ten DIIS. As noted above, all the DIIS image scattered radiation from a common area 348 on the surface of wafer 222, but each DIIS is configured to collect the radiation along a different angular axis (i.e., a different elevation and/or azimuth). Although system 220 is designed mainly for use in dark-field detection, one or more of DIIS 230 may be used for bright-field detection, as well, in conjunction with either the normal-incidence or oblique-incidence illumination beam.

An objective 350 collects and collimates the scattered light from area 348. In order to collect scattered light at low elevation, objective 350 preferably has a high NA, most preferably as high as 0.95. An exemplary design of objective 350, using multiple refractive elements, is described further in U.S. Published Application Number 20050219518. Alternatively, objective 350 may comprise a reflective or catadioptric element, as described, for example, in U.S. Pat. No. 6,392,793 to Chuang et al, which is hereby incorporated by reference herein. Each of DIIS 230 is positioned, as shown in FIG. 3, to receive a particular angular portion of the light collected by objective 350.

For each DIIS 230, a bandpass filter 352 selects the wavelength range that the DIIS is to receive. Typically, filter 352 selects one of the two wavelengths emitted by illumination module 224, while rejecting the other wavelength. Filter 352 may also be implemented as a dichroic beamsplitter, and configured so that one DIIS 230 receives the scattered light along a given angle at one wavelength, while another DIIS receives the scattered light along the same angle at the other wavelength. As a further alternative, filter 352 may be chosen to pass radiation in another wavelength range, such as a band in which wafer 222 is expected to fluoresce. For example, when organic materials, such as photoresist, are irradiated at 266 nm, they tend to fluoresce in the range of 400 nm. Thus, setting filter 152 to pass light in the 400 nm band allows DIIS 230 to detect defects in the organic material or residues thereof.

A spatial filter 354 can be used to limit the collection angle of each DIIS 230, by blocking certain regions of the collimated scattered light. The spatial filter is especially useful in eliminating background diffraction from repetitive features on patterned wafers. The spatial filter is chosen, based on the known diffraction pattern of the features on the wafer surface, to block these strong diffraction nodes, in order to enhance the sensitivity of system 220 to actual defects, as is known in the art. This use of spatial filtering for this purpose is described, for example, in U.S. Pat. No. 6,686,602 to Some, whose disclosure is incorporated herein by reference. This patent describes a method for creating spatial filters adaptively, in response to the diffraction lobes of different sorts of wafer patterns. This method may be implemented in filters 354 in module 226. Alternatively, spatial filters 354 may comprise fixed patterns, as is known in the art.

A rotatable polarizer 356 is provided in the optical path in order to select the direction of polarization of scattered light that is to be received by DIIS 230. The polarizer is useful, for example, in improving detection sensitivity by rejecting background scatter due to rough and/or highly-reflective surface structures on wafer 222. Optionally, polarizer 356 is implemented as a polarizing beamsplitter, which is configured so that two DIIS 230 receive the light scattered along a given angle in orthogonal polarizations.

As a further option (not shown in the figures), the optical path may comprise a beamsplitter, which divides the light scattered along a given collection angle between two or more different DIIS 230. The beamsplitter may be used for wavelength division, as mentioned above, or to divide the same wavelength between the two or more DIIS in a predetermined proportionality. Different spatial filters 354 may be used following the beamsplitter in the beam paths to the different DIIS, in order to filter out diffraction lobes due to different sorts of patterns on the wafer. As a further alternative, the beamsplitter may divide the light scattered along a given angle unequally between two or more of the DIIS, for example, in a ratio of 100:1. This arrangement effectively increases the dynamic range of system 220, since the DIIS receiving the smaller share of the radiation is still able to generate meaningful image data even in areas of bright scatter, in which the DIIS receiving the larger share of the radiation is saturated. An arrangement of this sort is described, for example, in U.S. Pat. No. 6,657,714 to Almogy et al whose disclosure is incorporated herein by reference.

A focusing lens 358 focuses the collected and filtered light onto DIIS 230. Lens 358 may be adjustable, either manually or under motorized control. A variable magnifier 360 may be used to change the size of the magnified image received by the DIIS. Alternatively, the functions of lens 358 and magnifier 360 may be combined within a single optical unit for each DIIS. The magnifier determines the resolution of the image captured by DIIS 230, i.e., the size of the area on the wafer surface that corresponds to each pixel in the output image from the DIIS. Magnifier 360 is typically operated in conjunction with telescopes in illumination module 224, so that size of the illuminated area is roughly equal to the area imaged by the DIIS.

Each DIIS 230 comprises an image intensifier 362, whose photocathode is aligned at the image plane of the focusing lens 358 and magnifier 360. Any suitable type of image intensifier tube of any generation/focusing approach(es) may be used for this purpose. For the sake of further illustration to the reader, non-limiting examples include first and second generation image intensifiers such as the C6654 image intensifier produced by Hamamatsu Photonics K.K. (Shizuoka-ken, Japan) or first generation magnetically focused image intensifiers produced by Photek Ltd (East Sussex, UK). To provide optimal imaging in the demanding environment of system 220, intensifier 362 preferably has high bandwidth and high resolution, and is preferably capable of gated operation, with high current and low phosphor memory, at the repetition rate of laser head 50—typically up to about 1000 pulses per sec. In one embodiment, the useful diameter of intensifier 362 is preferably at least 18 mm. In another embodiment, a larger diameter of intensifier 362, in the range of 25-40 mm, is used.

Although as mentioned above, focusing in intensifier 362 may be achieved by any approach (proximity, electrostatic, magnetic), in one embodiment, intensifier 362 is magnetically focused and results in superior optical performance and/or the possibility of enhancing the lifetime, as described above.

The output of image intensifier 362 is focused by optics 364 onto an image sensor 366. The optics 364 comprises, two fiber optic plates with a non-binding filling in the gap between the two plates as illustrated and described above with reference to FIGS. 1A and 1B. Image sensor 366 comprises a two-dimensional matrix of detector elements, such as a CCD or CMOS array, as is known in the art. For example, the image sensor may comprise a CMOS digital image sensor, such as model MI-MV13, made by Micron Technology Inc. (Boise, Id.). This sensor has 1280×1024 pixels, with 12 μm vertical and horizontal pitch, and a frame rate up to 500 frames per second for full frames. A detachable attaching medium is used to attach image intensifier 362 to image sensor 366 in DIIS 230, as illustrated and described above with reference to FIGS. 1A and 1A.

The use of image intensifiers 362 increases the sensitivity substantially compared to using image sensors 366 alone without intensification. Image intensifiers 362 intensifiers may be gated, in synchronization with the light pulses from illumination module 224, in order to increase the sensitivity of the DIIS and reduce their noise levels still further. Typically, the photocathodes of intensifiers 362 are chosen to have high quantum efficiency at the wavelengths emitted by the illumination module 224, while the phosphors of the intensifiers 362 may be chosen to emit light in a different wavelength range in which image sensors 366 have high responsivity. Thus, the image intensifiers 362, in addition to amplifying the incident scattered light, are also useful in downconverting the ultraviolet (UV) and blue light that is scattered from wafer 222 to the green or red range, to which the silicon image sensors are more responsive. In addition, intensifiers 362 act as low-pass spatial filters, and may thus help to smooth high-frequency structures in the scattered light that might otherwise cause aliasing in the images output by sensors 366.

Intensifiers 362 preferably have high resolution, as dictated by the resolution of sensors 366. For example, to take full advantage of the resolution of the above-mentioned MV13 sensor, intensifier 362 should be designed to provide 1640 distinct pixels along the image diagonal. This resolution criterion may also be expressed in terms of the modulation transfer function (MTF) of the intensifier, giving for example MTF=30% for a test image with 33 line pairs/mm or 30%-40% at 40 line pairs/mm depending on the embodiment of intensifiers 362. Bright points in the image captured by DIIS 230 can result in formation of a bright halo, generally due to reflections inside the image intensifier tube, which may compromise the resolution of the image. Intensifiers 362 are preferably designed to suppress such reflections so that the halo diameter is no more than 0.2 mm in any case. Furthermore, in order to exploit the full range of sensitivity of sensor 366, intensifier 362 should exhibit linear behavior up to high maximum output brightness (MOB), typically on the order of 600 μW/cm².

For brevity of description, other details relating to system 220 as provided in U.S. published application number 20050219518 are hereby incorporated by reference rather than being replicated here.

For ease of understanding, the description above described a single fiber optic plate 22 coupled to a photo-emitting output area of image intensifier tube 16 (or 362) and a single fiber optic plate 32 coupled to a photosensitive input area of image sensor 34 (or 366) with a gap between the two plates filled with a non-binding filling 40. However, it should be evident to the reader that in some embodiments single fiber optic plate 22 may be replaced with a plurality of fiber optic plates 22 and/or single fiber optic plate 32 may be replaced with a plurality of fiber optic plates 32. In addition, or instead there may be fiber optic plates in between image intensifier tube 16 (or 362) and image sensor 34 (or 366) which are not clearly associated with either image intensifier 16 (362) or image sensor 34 (366). Therefore in order to understand the possible embodiments, the reader should recognize that there are at least two fiber optic plates between the photo-emitting output area of image intensifier tubes 16 (362) and the photosensitive input area of image intensifier 34 (366). (For ease of explanation, each fiber optic plate is designated 22/32 because the association or non-association of each plate with image intensifier tube 16 (362) or image sensor 34 (366) may vary with the embodiment). Depending on whether a particular fiber optic plate 22/32 has a neighboring fiber optic plate 22/32 on one side or both sides, that fiber optic plate 22/32 can be considered to belong to one or two pairs of adjacent fiber optic plates, respectively. The gap between one pair of adjacent fiber optic plates 22/32 is filled with non-binding filling 40, but the selection of which pair of adjacent plates 22/32 out of all possible pairs of adjacent plates 22/32 has the gap filled with non-binding filling 40 may vary depending on the embodiment. Also depending on the embodiment, if there are other pairs of adjacent fiber optic plates 22/32, the gap(s) between all other pair(s) of adjacent fiber optic plates 22/32 may be filled with non-binding filling 40, the gap(s) between all other pair(s) of adjacent fiber optic plates 22/32 may be filled with a known in the art adhesive, or some of the gap(s) between the other pair(s) of adjacent fiber optic plates 22/32 may be filled with non-binding filling 40 whereas the gap(s) between other(s) of the other pair(s) of adjacent fiber optic plates 22/32 may be filled with a known in the art adhesive. The reader will recognize that as long as the gap between at least one pair of adjacent fiber optic plates 22/32 is filled with non-binding filling 40, those pair(s) of adjacent fiber optic plates 22/32 may be separated from one another (and therefore image intensifier tube 16 (362) and image sensor 34 (366) may be separated from one another) without substantially damaging the DIIS 10 (for example without substantially damaging any of image sensor 34 (366), image intensifier 16 (362) or fiber optic plates 22/32).

The methods and systems described above, apply to embodiments with more than two fiber optic plates 22/32 between the photo-emitting output area of image intensifier tubes 16 (362) and the photosensitive input area of image intensifier 34 (366), mutatis mutandis. An example is now provided which uses more than two fiber optic plates 22/32. The number of plates, types of bindings between the plates, and other assumptions of the example are provided solely for further illustration to the reader and should therefore not be construed as limiting. It is assumed that a first fiber optic plate is coupled to image intensifier 16 (362) and a second fiber optic plate is attached with an adhesive to the first fiber optic plate. It is further assumed that a third fiber optic plate is coupled to image sensor 34 (366) and a fourth fiber optic plate is aligned between third fiber optic plate and second fiber optic plate. It is further assumed that a non-binding filling 40 fills the gap between the pair of third fiber optic plate and fourth fiber optic plate and a non-binding filling 40 (not necessarily the same filling) fills the gap between the pair of second fiber optic plate and fourth fiber optic plate. In this example, when detachably attaching image sensor 34 (366 to image intensifier 16 (362) using a detachable attaching medium as described above, it is assumed that at least the pair of third fiber optic plate and fourth optic plate are pushed close together and the pair of second fiber optic plate and fourth fiber optic plate are pushed close together. In one embodiment of the example, the detachable attaching medium is assumed to include an elastic material which at least allows the pair of third and fourth fiber optic plates to be pushed close together and the pair of second and fourth fiber optic plates to be pushed close together without substantially damaging any of the fiber optic plates (for example first, second, third and fourth fiber optic plates), image sensor 34 (366) and/or image intensifier 16 (362). In this example if detachment of image sensor 34 (366) from image intensifier 16 (362) is later desired as described above, then in some embodiments the detachment process may include inter-alia the separation of second and fourth fiber optic from one another and/or the separation of third and fourth fiber optic plates from one another.

While the invention has been shown and described with respect to particular embodiments, it is not thus limited. Numerous modifications, changes and improvements within the scope of the invention will now occur to the reader.

Claims

1. An apparatus for inspection of a sample, comprising:

a radiation source, which is adapted to direct optical radiation onto an area of a surface of the sample;

at least one image intensifier, each of which is detachably coupled to an image sensor, so as to receive the radiation from the area over a certain angular range, and to provide intensified radiation to the image sensor; and

at least one image sensor, each of which is configured to receive radiation from at least one image intensifier, so as to form at least one respective image of the area.

2. The apparatus of claim 1, further comprising: an image processor, which is adapted to process at least one of the respective images so as to detect a defect on the surface.

3. The apparatus of claim 1, further comprising at least two fiber optic plates for each image intensifier, wherein said image intensifier is further detachably coupled to one of the image sensors using said at least two fiber optic plates, and wherein non-binding filling fills at least one gap which is between at least one pair of adjacent fiber optic plates among said at least two fiber optic plates.

4. The apparatus of claim 3, wherein said non-binding filling is an oil or gel with an index of refraction that is closer to an index of refraction of said fiber optic plates than an index of refraction of air is to said index of refraction of said fiber optic plates.

5. The apparatus of claim 3, further comprising: a detachable attaching medium detachably coupling each said image intensifier to one of the image sensors, wherein said medium includes an elastic material configured to at least allow said at least one pair of adjacent fiber optic plates to be pushed close together without substantially damaging any of said image intensifier tube, image sensor, and said fiber optic plates.

6. The apparatus of claim 1, further comprising: a detachable attaching medium detachably coupling each said image intensifier to one of the image sensors.

7. The apparatus of claim 6, wherein said detachable attaching medium includes at least one screw configured when screwed in to prevent separation of each said image intensifier tube from said one image sensor.

8. The apparatus of claim 1, wherein at least one of said image intensifiers is magnetically coupled.

9. A method of inspecting a sample, comprising:

a. providing at least one image intensifier tube detachably coupled to an image sensor with non-binding filling;

b. directing optical radiation onto an area of a surface of a sample to be inspected;

c. receiving and intensifying the radiation scattered from the area using said at least one provided detachably coupled image intensifier tube and image sensor, so as to form a respective images of the area, each of said provided detachably coupled image intensifier tube and image sensor being configured to receive the radiation that is scattered to into a different, respective angular range; and

d. processing at least one of the respective images so as to detect a defect on the surface.

10. The method of claim 9, wherein said non-binding filling fills at least one gap which is between at least one pair of adjacent fiber optic plates among at least two fiber optic plates aligned between said image intensifier tube and said image sensor.

11. A method of inspecting a sample comprising:

a. providing an image intensifier tube detachably coupled to an image sensor;

b. separating said image intensifier tube from said image sensor;

c. coupling at least one of said separated image intensifier tube and image sensor in a combination of image intensifier tube and image sensor;

d. directing optical radiation onto an area of a surface of a sample to be inspected;

e. receiving the radiation scattered from the area using said combination coupled in (c) so as to form a respective image of the area; and

f. processing said image so as to detect a defect on the surface.

12. The method of claim 11, wherein said coupling in (c) includes detachably coupling the same or different image intensifier tube to said separated image sensor using at least two fiber optic plates and a non-binding filling which fills at least one gap which is between at least one pair of adjacent fiber optic plates among said at least two fiber optic plates.