Abstract:
A microscope for inspecting a surface in an evacuated volume, including an optical objective assembly which is located in the evacuated volume in proximity to the surface. The assembly is arranged to collect and convey radiation from the surface while focusing the radiation so as to form an image of the surface. The microscope further includes a sensor, located in a space outside the evacuated volume, which is arranged to receive the radiation conveyed from the optical objective assembly so as to generate a signal corresponding to the image.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to surface inspection, and particularly to inspection of surfaces that are in a vacuum. 
   BACKGROUND OF THE INVENTION 
   In a wafer fabrication facility, virtually all of the stages involved in producing a final product are performed on a semiconducting wafer in an evacuated or low pressure chamber. Typically, during and/or after at least some of the stages, the surface of the wafer is inspected. The inspections verify that the wafer is correctly aligned, that expected changes on the surface of the wafer have in fact occurred, and that no unexpected changes have also occurred. The inspections are advantageously performed while the wafer remains in its chamber, and typically so that the chamber remains in substantially the same evacuated or low pressure state used for implementing a stage prior to inspection. 
   Methods for inspecting semiconducting wafer surfaces under evacuated or low pressure conditions are known in the art. For example, Serenity Technologies Inc., of Beaverton, Ore., produce an OPTIVAC™ viewer that is stated to be an entire optical microscope, including a Charged Couple Device (CCD), inside an ultra high vacuum environment. However, the inclusion of electronic parts inside a vacuum chamber is problematic due to space issues and possible contamination of the interior clean vacuum environment. 
   Notwithstanding the above, an improvement to inspection methods is desirable. 
   SUMMARY OF THE INVENTION 
   In an embodiment of the present invention, a microscope, comprising an optical objective assembly and an image sensor, is used for inspecting the surface of an object. The object, typically a semiconducting wafer, is located in an evacuated volume of an object inspection chamber. The optical objective assembly comprises a microscope objective, typically an industry-standard microscope objective. The optical objective assembly is located at least partially in the evacuated volume, and the image sensor is located outside the evacuated volume, typically in a non-evacuated space outside the inspection chamber such as the ambient air. 
   The optical objective assembly collects radiation from a region on the surface and, typically, focuses the radiation to a primary image, usually formed at infinity. In this case the assembly conveys the radiation to the sensor via sensor optics. The sensor optics focus the primary image to a secondary image on the image sensor, and typically comprise a tube lens and coupling elements. Alternatively, the optical objective assembly may focus the primary image directly onto the image sensor. The image sensor generates a signal corresponding to the image formed on the sensor. Locating the objective assembly at least partially within the evacuated volume allows an operator of the microscope more freedom to effectively position the microscope objective relative to the surface being inspected and improves image quality, compared to microscopes which do not have an optical objective assembly located at least partially in the evacuated volume. 
   In one embodiment of the present invention, the objective assembly comprises a substantially plane transparent window which is inset into a wall of the chamber. The microscope objective is completely located in the evacuated volume, collects radiation from the region on the object surface, and transfers the radiation to the image sensor via the window. The radiation from the objective is approximately parallel to an axis of the microscope, so that placing a window after the objective and before the imaging optics leads to virtually no degradation of the image formed on the sensor. 
   In an alternative embodiment of the present invention, the objective assembly comprises a relay lens train. The relay lens train is inset into the cover wall of the inspection chamber. The relay lens train receives radiation from the region, and forms a real “relayed” intermediate image of the region in a location outside the chamber. The relay lens train is typically configured to have a magnification of approximately one. The microscope objective is positioned outside the chamber, so as to collect radiation from the relayed image. 
   The objective transfers the collected radiation to the image sensor (also outside the chamber), upon which is focused the image of the region, according to one of the methods described above. Insetting a relay lens train into the wall of the inspection chamber allows both the objective and the image sensor to be located outside the chamber, thus permitting easy manipulation of these elements by the microscope operator, while allowing the operator freedom to effectively position the objective relative to the surface being inspected. In addition, having the objective outside the chamber allows the microscope operator to use a turret arrangement comprising multiple objectives having different magnifications, fields of view, and/or resolutions. The relay lens train is typically configured to support the magnifications, fields of view, and resolutions of the multiple objectives. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
       FIG. 1  is a schematic diagram of a microscope, according to an embodiment of the present invention; and 
       FIG. 2  is a schematic diagram of an alternative microscope, according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Reference is now made to  FIG. 1 , which is a schematic diagram of a microscope  10 , according to an embodiment of the present invention. An object  16 , herein assumed by way of example to comprise a semiconducting wafer, is mounted on a translation stage  14  for inspection by the microscope. A processing unit (PU)  32 , typically incorporated in a workstation  33 , controls the operations of microscope  10 , and an operator  34  uses the workstation to operate the microscope. In the following description, wafer  16  is assumed to be positioned on the translation stage so that a surface  19  of the wafer is substantially horizontal, and so that elements of microscope  10 , described in detail below, are above the surface. However, it will be understood that in the instant application orientation terms such as “horizontal” and “above” are to clarify the description of microscope  10  and its operation, and that the microscope and wafer may be positioned in orientations other than that specifically referred to herein. Surface  19  is assumed to contain an x-y plane and to define a z-axis normal to the surface. 
   Wafer  16  is located in an internal volume  11  of an inspection chamber  12 , the chamber having a chamber cover wall  13 . In order to effect operations on the wafer, the internal volume of the chamber is sealed from a space  15 , typically ambient air, external to the chamber. The chamber is configured so that volume  11  may be evacuated with a vacuum pump. Volume  11  may also have gases introduced into, and removed from, the volume in a controlled manner. For clarity, connections to chamber  12  for producing a vacuum in the internal volume, and/or for introduction or removal of gases from the internal volume, are not shown in  FIG. 1 . Hereinbelow, unless otherwise stated, volume  11  is assumed to be evacuated. 
   Microscope  10  comprises, as part of an optical objective assembly  23 , a microscope objective  18  which is typically an industry-standard microscope objective. Typically, objective  18  is vacuum compatible, i.e., the objective does not introduce contamination into chamber  12 , and is sufficiently mechanically stable to as to continue to function under frequent evacuations and ventings of the chamber. As is well known in the art, there are a large number of industry-standard microscope objectives available, the objectives being selected according to parameters such as required numerical aperture, working distance, working wavelength or range of wavelengths, flatness of field, extent of correction for aberrations, and resolution. In one embodiment, objective  18  may be an Olympus UMPLFL50x, produced by Olympus America Inc., Melville, N.Y. However, any other suitable objective, typically having a standard short working distance, may be used. In some embodiments of the present invention, objective  18  may comprise an at least partially custom-made objective, or alternatively, may comprise an adapted industry-standard objective. 
   Embodiments of the present invention enable objectives which are not available with long working distances to be easily incorporated into microscope  10 . Such objectives include deep ultra-violet (DUV) objectives, for example the Leica 150x/0.90/248 DUV objective, produced by Leica Microsystems GmbH, of Wetzlar, Germany. 
   An inset  18 I in  FIG. 1  illustrates a cross-section of objective  18  schematically. Objective  18  comprises a front lens  18 A, typically one or more internal lenses  18 C, and a rear lens  18 E. The lenses are separated by one or more spaces  18 B. The lenses of the objective are retained in an objective housing  18 D, which typically terminates close to rear lens  18 E in standard threads  18 F which are used to mount the objective. Those having ordinary skill in the art will be able to adapt the following description, which assumes that objective  18  is configured as shown in inset  18 I, to objectives having configurations other than that shown in inset  18 I, mutatis mutandis. 
   Industry-standard objectives such as objective  18  are typically configured to operate in ambient air pressure, and as illustrated in the inset of  FIG. 1 , typically comprise a multiplicity of lenses, at least some of which may have air spaces between them. In embodiments of the present invention wherein objective  18  is such an industry-standard objective, operator  34  may verify, without undue experimentation, that the unmodified objective functions satisfactorily in the evacuated and/or gas-filled conditions of chamber  12  described above. In some embodiments, the operator may adapt an industry-standard lens, also without undue experimentation, to function satisfactorily in chamber  12 . 
   For example, referring for clarity to inset  18 I, such adaptations may comprise operator  34  providing in objective housing  18 D one or more small holes to spaces  18 B between the lenses, and using cement for the lenses that is vacuum compatible regarding outgassing. The holes facilitate the transfer of gases between the spaces and volume  11 , so that there is no undue pressure on elements of the objective. Alternatively or additionally, operator  34  may adapt objective  18  by having the external lenses of the objective, i.e., front lens  18 A and rear lens  18 E, sealed in place with vacuum-compatible cement, so that spaces within the objective remain air-filled and are sealed from volume  11 . 
   Objective  18  is removably mounted, by standard threads  18 F, in an objective mount  21 . Mount  21  is fixedly positioned in volume  11 , typically by being attached to wall  13 . In some embodiments, mount  21  comprises a translation stage that allows the objective to be moved when it is retained in the mount. Objective  18  collects radiation from a region  17  of surface  19 , and is configured in some embodiments, as described below, so as to form a primary image of the region. The characteristics and position of the primary image formed by the objective is a function of the objective parameters. Typically, the primary image is formed at infinity. Alternatively, the primary image may be formed at a position other than infinity. 
   Typically, microscope  10  also incorporates surface illumination optics. By way of example, microscope  10  comprises a partially transmitting beamsplitter  22 , an illumination source  26 , and a focusing lens  24 , which are configured to transfer radiation from source  26  to surface  19  via objective  18 . This type of illumination may be configured to provide “bright field” illumination on surface  19 , i.e., illumination that is substantially normal to surface  19 , or “dark field” illumination, i.e., illumination that is non-normal to the surface. Alternatively or additionally, illumination on surface  19  may be provided by other radiation sources mounted external to chamber  12 , the other radiation sources not necessarily transferring radiation onto surface  19  via objective  18 . Such other radiation sources typically provide dark field illumination. In some embodiments a ring illuminator  38  provides dark field illumination, and may be located in region  15 , above a window  20  described below. The operating wavelength, or range of wavelengths, of the surface illumination optics typically comprises wavelengths in the visible region, UV, or DUV, although other wavelengths may also be used. The operating wavelength may be selected by operator  34 . Operator  34  also arranges that elements of microscope  10 , such as objective  18  described above and other elements described below, function optimally at the operating wavelength. Except where otherwise indicated, the following description assumes that the surface illumination is provided from source  26  via beamsplitter  22 . 
   Optical objective assembly  23  also comprises a window  20  which is inset into wall  13 . Window  20  is a parallel-sided window which is transparent to the operating wavelengths of microscope  10 , and which is typically anti-reflection coated at the operating wavelength, so that there is little or no reflection at these wavelengths. Operator  34  selects the thickness of the window to be sufficient to withstand any pressure difference between internal volume  11  and external space  15 , while minimally distorting images generated by radiation traversing the window. A typical thickness for window  20  is in the range of approximately 1 mm to approximately 3 mm. The window is inset into wall  13 , usually using O-rings, so that a seal between the window and the wall is gas-tight. 
   Window  20  transfers the surface illumination radiation from beamsplitter  22  to objective  18 . Window  20  also transfers the radiation collected from region  17  by the objective to the beamsplitter, which transmits the collected radiation to imaging optics  31 . Imaging optics  31  comprise a focusing lens  28 , typically a tube lens, and a camera  30 , both of which are selected by operator  34  to function at the operating wavelengths. Camera  30 , comprising an image sensor  27  and one or more coupling elements  25 , forms a real secondary image of region  17  on the sensor. The sensor is typically an imaging array such as an array of charged coupled devices. The camera forms the secondary image by using the primary image from the objective as an object. 
   In an alternative embodiment of the present invention, objective  18  is configured to form its image directly onto image sensor  27 . In this embodiment, lens  28  and coupling elements  25  may be absent. 
   As illustrated schematically by lines  37 , PU  32  controls the operation of camera  30 , and receives signals from the camera in response to the image formed in the camera. In addition, PU  32  controls the operation of source  26 . PU  32  also controls the positioning of region  17  relative to microscope  10 , by operation of stage  14 . Elements of microscope  10 , including objective  18 , beamsplitter  22 , lens  28 , and camera  30 , are arranged to have a common optic axis  36 , which also intersects region  17 , parallel to the z-axis. 
   Typically, in operating microscope  10 , operator  34  introduces wafer  16  via a loading chamber, not shown in  FIG. 1 , into chamber  12 , which has previously been evacuated. The operator adjusts stage  14  and/or mount  21  so that objective  18  aligns with, and is at the correct working distance from, a specific region  17  that the operator is inspecting. The operator performs the adjustments using the image generated by the camera. 
   The configuration of microscope  10  provides a number of significant advantages over prior art microscopes:
         There is no separating element between surface  19  and the external surface of front lens  18 A. Thus the working distance, i.e., the distance between the two surfaces, may be extremely small. Such a small distance allows a higher usable magnification, as well as a higher numerical aperture (NA) and an increase in resolution. In addition, the lack of a separating element means that dark field illumination is relatively easy to configure. For example, unlike systems having a separating element between the surfaces, in embodiments of the present invention there are no interfering reflections from window  20  or its mount, nor from contamination particles on the window. Furthermore, since DUV objectives cannot be manufactured with a window correction and/or a long working distance, the lack of a separating element between the two surfaces allows such objectives to be used. Also, using UV illumination increases the scattered light from small particles, as well as providing improved resolution for the particles.   Window  20  is located in a region of the microscope, after the external surface of objective rear lens  18 E, where the thickness required by the window (because of the difference in pressure between internal volume  11  and space  15 ) leads to minimal reduction in optical performance, so that the image formed on image sensor  27  is substantially undistorted. Positioning window  20  in this region leads to an improvement of approximately two orders of magnitude in image distortion compared to the image distortion caused by positioning a window of the same thickness between objective  18  and surface  19 .       

     FIG. 2  is a schematic diagram of a microscope  40 , according to an alternative embodiment of the present invention. Apart from the differences described below, the operation of microscope  40  is generally similar to that of microscope  10  ( FIG. 1 ), such that elements indicated by the same reference numerals in both microscopes  40  and  10  are generally identical in construction and in operation. Instead of optical objective assembly  23 , microscope  40  comprises an optical objective assembly  43 . Assembly  43  comprises objective  18  and a relay lens train  41  which is inset into wall  13 . Lens train  41  comprises a front lens  48  and a rear lens  46 , retained by a relay housing  42 . Train  41  may comprise an iris  50 , as well as one or more other lenses between lenses  46  and  48 . For clarity the one or more other lenses are not shown in  FIG. 2 . Lens train  41  is configured to form a real “relayed” image of region  17  at a plane  52  above the relay lens train. The relay lens train may be configured so that the size of the image at plane  52  is substantially equal to the size of region  17  that is being imaged, or to generate a dilated, i.e., a magnified or a de-magnified, image. Typical dilations of train  41  are in a range from approximately 1:0.5 to approximately 1:5. 
   In operation, objective  18  is positioned to focus the image formed at plane  52 . The remaining elements of microscope  40 , including the illumination source or sources, are generally positioned with respect to objective  18  substantially as described above for microscope  10 . Thus, bright field or dark field illumination may be provided at region  17  by source  26 . Alternatively, dark field illumination may be provided by ring illuminator  38 , which may advantageously be positioned surrounding objective  18 , as shown in  FIG. 2 . Other methods for conveniently providing dark field illumination will be apparent to those of ordinary skill in the art. As one example, one or more sources which may comprise reflectors and/or shields, or a ring source generally similar to illuminator  38 , may be positioned between objective  18  and lens  46  so that, taking regard of the numerical aperture of the objective, no interfering radiation enters the objective. As a second example, an additional window (not shown in  FIG. 2 ) may be provided in a suitable position in wall  13 , so that radiation via the window gives dark field illumination at surface  19 . 
   As described above for microscope  10 , in microscope  40  objective  18  may be configured to form its image at infinity or at a location different from infinity. Alternatively, as also described with respect to microscope  10 , objective  18  may be configured to form its image directly on sensor  27 . 
   In one embodiment of the present invention, the following distances may be used in microscope  40 . A working distance from an external surface  44  of lens  48  to surface  19  is typically between approximately 2 mm and 4 mm. A relay distance between surface  19  and plane  52  is typically greater than about 100 mm. 
   The configuration of microscope  40  provides the relayed image of region  17  at surface  52 , so that with the relay distance given above there is easy access to the image by objective  18 , and the relayed image is at a convenient height above chamber  12 . By relaying the image to surface  52 , operator  34  may easily check that desired illumination of region  17  is actually achieved. Furthermore, the convenient height and easy access of the relayed image allow objective  18  to be mounted with other objectives, for example an objective  18 ′, in a standard microscope turret  45 , thus providing the operator of microscope  40  with a range of magnifications and resolutions. It will be understood that the numerical aperture, field of view and configuration of the relay lens train  41  should be selected with reference to parameters of objective  18  and other objectives such as objective  18 ′ that may be used, so as not to degrade the relayed image quality, or the quality of the image formed on sensor  27 . 
   It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.