WO2016036250A1

WO2016036250A1 - Compact inspection apparatus comprising a scanning electron microscope and an optical microscope

Info

Publication number: WO2016036250A1
Application number: PCT/NL2015/050616
Authority: WO
Inventors: Andries Pieter Johan EFFTING; Wilhelmus Johannes Adriaan DE PINTH; Gijsbert DE SWART
Original assignee: Delmic B.V.
Priority date: 2014-09-05
Filing date: 2015-09-04
Publication date: 2016-03-10
Also published as: NL2013432B1; EP3189537A1

Abstract

The invention relates to an inspection apparatus comprising a Scanning Electron Microscope (SEM, 201) and an optical microscope (202). The SEM comprises a first chamber (211) comprising a source (203) for emitting a primary electron beam, an electron optical column comprising a lens for focusing the electron beam onto a sample (222) on a sample holder, and an electron detector (208) for detecting electrons from the sample. The inspection apparatus further comprises a second chamber (214) which comprises the sample holder. The second chamber is movably connected to the first chamber for moving the sample holder with respect to the SEM. For imaging, there is an open connection between the first and second chamber for observation of the sample with the SEM. The optical microscope comprises a light collecting device for collecting light from the sample, and a light detector for detecting the light from the sample. The light collecting device is arranged inside the second chamber.

Description

Compact inspection apparatus comprising a Scanning Electron Microscope and an Optical microscope.

BACKGROUND

The invention relates to an inspection apparatus comprising a combination of a Scanning Electron Microscope (SEM) and an Optical Microscope. Such an inspection apparatus is, for example , known form the international publication WO 2007/143736, which is hereby incorporated by reference .

WO 2007/143736 discloses a SEM comprising a source of primary electrons for an electron beam, an electron optical column having a lens for focusing the electron beam and a detector for detecting electrons from a sample. These elements of the SEM are arranged inside a vacuum chamber, which vacuum chamber is provided with an opening for connecting to a removable sample holder for holding the sample for observation with the SEM. Typical for this known inspection apparatus is, that the sample holder is formed as a sample holder cup for holding the sample therein, wherein the sample holder cup is arranged at the opening of the vacuum chamber, and the walls of the sample holder form part of the wall of a vacuum region containing the sample.

In addition the inspection apparatus comprises a movable vacuum connector at the opening in order to establish a movable vacuum connection between the vacuum chamber and the removable sample holder. The movable vacuum connector allows movement of the sample holder cup comprising the sample with respect to the vacuum chamber comprising the electron optical column, in order to image other parts of the sample. Due to this construction, an X-Y adjustable sample stage is arranged outside the vacuum chamber, and is in particular arranged for an X-Y adjustment of the sample holder cup. In the inspection apparatus according to WO 2007/143736 the movable vacuum connector comprises a sliding vacuum seal, which is described in more detail in WO 2007/145712, which is hereby incorporated by reference .

The inspection apparatus according to WO

2007/143736 is further provided with a navigation camera outside the vacuum chamber. Before the removable sample holder cup is arranged at the opening of the vacuum chamber, the sample holder cup moves under the optical camera, which forms and stores a magnified digital image of the sample. The magnification of this image is typically from lOx up to lOOx. The image is typically obtained and stored before the sample holder cup is evacuated, and before the sample holder cup is arranged at the opening of the vacuum chamber of the SEM. This stored optical image is used as an overview of the sample to help the user put a magnified image of the SEM in context .

Although the inspection apparatus according to WO 2007/143736 provides a SEM that is relatively inexpensive, easy to use, and can be of sufficiently small dimensions that it can be placed, for example, on a table-top, it lacks the possibility to obtain optical images of the sample when the sample holder cup is positioned at the opening of the vacuum chamber of the SEM.

It is an object of the present invention to provide an inspection apparatus comprising a SEM as described in WO 2007/143736 and an additional optical microscope which is arranged to obtain optical images of the sample when positioned in a beam path of the electron beam of the SEM, in particular which is arranged to obtain optical images of at least the part of the sample which is irradiated by the electron beam of the SEM, more in particular which is arranged to obtain optical images during the irradiation of the sample by the electron beam of the SEM. SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides an inspection apparatus comprising a Scanning Electron Microscope (SEM) and an optical microscope for inspecting a sample,

wherein the SEM comprises a first chamber comprising a source for emitting a primary electron beam, an electron optical column comprising a lens for focusing the electron beam onto the sample, and an electron detector for detecting electrons from the sample,

wherein the inspection apparatus comprises a second chamber which comprises a sample holder for holding the sample, wherein the second chamber is movably connected to the first chamber for moving the sample holder with respect to the SEM, and wherein there is an open connection between the first and second chamber at least when sample holder is aligned with the electron optical column for observation of the sample with the SEM,

wherein the optical microscope comprises a light collecting device for collecting light from the sample, a light detector for detecting the light from the sample, and light optical elements for directing the light from the light collecting device towards the light detector, wherein the light collecting device is arranged inside the second chamber.

The inspection apparatus of the present invention comprises a first chamber for holding the components of the SEM, which first chamber is in use evacuated to provide a vacuum condition inside said first chamber. In addition, the inspection apparatus comprises a second chamber for holding the sample to be studied with the SEM. The first and second chambers are substantially separate chambers. When the sample holder is aligned with the electron optical column for observation of the sample with the SEM, an open connection is provided between the first and second chamber. The second chamber is, in use, also evacuated, preferably via the open connection to the first chamber, to provide a vacuum condition inside said second chamber. Just as the inspection apparatus described in WO 2007/143736, the sample holder is arranged in a second chamber, which is referred to in WO 207/143736 as a sample holder cup. The second chamber comprises the sample holder and is movably connected to the first chamber for moving the sample holder with respect to the components of the SEM, in order to image different parts of a sample on the sample holder using the SEM. Just as in the prior art, the sample holder can be moved using an X-Y adjustable sample stage is arranged outside the first or second chamber, thus is arranged outside the vacuum environment of the SEM, and is arranged for moving the second chamber with respect to the first chamber.

According to the present invention, the inspection apparatus also comprises an optical microscope for obtaining a light image of the sample on the sample holder when arranged in the beam path of the electron beam of the SEM. The light microscope comprises a light collecting device for collecting light from the sample, and light optical elements for directing the light towards a light detector for detecting the light from the sample. According to the present invention, the light collecting device is arranged inside the second chamber, which is movably connected to the first chamber comprising the components of the SEM. By arranging the light collecting device, such as a microscope objective, inside the second chamber, the light collecting device can suitably be arranged close to the sample, allowing to use light collecting devices with a high numerical aperture, for example to obtain optical images of high resolution and/or to obtain optical images under low light conditions, for example to obtain optical images of luminescence of cathodoluminescence light from the sample.

In a preferred embodiment, the light collecting device is arranged on an adjustable stage for moving said light collecting device with respect to the second chamber, wherein the adjustable stage is arranged inside the second chamber. As stated above, the second chamber is movably connected to the first chamber comprising the components of the SEM. When the second chamber is moved with respect to the first chamber in order to image a different part of the sample on the sample holder, the adjustable stage can be used to compensate the movement of the second chamber in order to substantially maintain the position of the light collecting device with respect to the components of the SEM. In particular, to maintain the position of the light collecting device with respect of the area which is imaged by the SEM, in order to obtain images of the same area of the sample with both the light microscope and the SEM.

In an embodiment, the sample holder is arranged at or near a first side of said second chamber which faces said first chamber, in particular when the second chamber and the first chamber are joined. Said first side of the second chamber is arranged at the open connection between the first and second chamber at least when the sample holder is aligned with the electron optical column for observation of the sample with the SEM. In particular the sample holder is arranged at or near the open connection between the first and second chamber when the sample holder is aligned with the electron optical column of the SEM, in order to allow the electron beam to pass through said open connection to be focused on a sample on the sample holder, and to allow electrons from the sample to move through the open connection to be detected by the electron detector in the first chamber.

In an embodiment, said light collecting device is arranged at a side of said sample holder which faces away from said first chamber. In this embodiment the optical microscope is arranged at a side of the sample opposite to the side of the sample which is illuminated by the electron beam. For example, the SEM is arranged to image an upper surface of the sample, whereas the optical microscope images the same area of the sample from the bottom side. An advantage of this setup is, that the light collecting device of the optical microscope does not interfere with the focusing lens of the SEM.

In an embodiment, the sample holder is fixedly connected to said first side of the second chamber. In an embodiment, the adjustable stage is fixedly connected to said first side of the second chamber. The first side of the second chamber is arranged close to or even abutting the first chamber, at least when the sample holder is aligned with the electron optical column for observation of the sample with the SEM. The position of the first side of the second chamber is well defined with respect to the first chamber holding the constituents of the SEM. By providing a fixed connection of the sample holder and/or the adjustable stage to the first side of the second chamber, the position of the sample holder and/or the adjustable stage, at least in a direction along a central axis or optical axis of the SEM, is substantially equally well defined. In an embodiment, the adjustable stage is connected to a second side of the second chamber by one or more springs, wherein said second side is arranged substantially opposite to the first side.

In order to obtain images of the same area of a sample with both the optical microscope and the SEM, the optical microscope and the SEM must be accurately aligned to each other. One way to align the optical microscope and the SEM with respect to each other is to arrange a fluorescent plate at the position of the sample on the sample holder. When the electron beam of the SEM is illuminating a spot of said fluorescent plate, the spot will emit light which can be detected by the optical microscope. The optical microscope can identify the cathodoluminescent spot on the fluorescent plate as being the position of the electron beam of the SEM on the fluorescent plate. Using this procedure, a correlation between an image of the optical microscope and an image of the SEM can be established.

A more sophisticated method for to determine the relative position of an optical microscope image and a SEM image in an integrated inspection apparatus, featuring the integration of a SEM and a light microscope, is described in WO 2013/151421 of the same applicant, which is hereby incorporated by reference.

In an embodiment, at least one of the sample holder, the first side of the second chamber, and a part of the light collecting device which faces the SEM at least when the first and second chamber are arranged for observation of the sample with the SEM, is provided with alignment markers which can be detected and/or imaged by the SEM. Contrarily to the method as described above, the SEM is used to image the alignment markers on the sample holder, the first side of the second chamber, and/or the part of the light collecting device. The SEM can identify the alignment markers and establish the position of the sample holder, the first side of the second chamber, and/or the light collecting device on the basis of the imaged alignment markers. This procedure can also, or in addition be used to establish a correlation between an image of the optical microscope and an image of the SEM.

In an embodiment, the second chamber is removable from the first chamber. According to this embodiment, the second chamber, comprising the sample holder and the light collecting device of the optical microscope is removable from the first chamber. Preferably the inspection apparatus comprises two or more second chambers, each of which can be placed individually arranged at the open connection of the first chamber to aligned the respective sample holder with the electron optical column for observation of the sample in said second chamber with the SEM. The various second chambers may also be provided with different light collecting devices or microscope objectives, for example to change the magnification of the optical microscope or to use special microscope objectives, such as an immersion objective. The removable second chamber of this embodiment also provides the same advantages as the removable sample holder cup is described in WO 2007/143736.

In an embodiment, at least the light detector is arranged outside the second chamber, and wherein the second chamber is provided with an optical window, which is substantially transparent for light. In an embodiment, the light detector is arranged in the inspection apparatus at a substantially fixed position with respect to the first chamber. In an embodiment, the optical microscope, except the light collecting device thereof, is arranged in the inspection apparatus at a substantially fixed position with respect to the first chamber.

In an embodiment, the source and the electron optical column comprising the lens are arranged along a central axis or optical axis of the SEM, and wherein the second chamber is movable with respect to the first chamber in a plane which extends substantially perpendicular to said central axis. In an embodiment, the adjustable stage is movable with respect to the second chamber in a plane which extends substantially perpendicular to said central axis, wherein the adjustable stage is arranged to compensate a movement of the second chamber with respect to the first chamber .

In an embodiment, the adjustable stage is also movable in a direction substantially parallel to the central axis. This movement can be used for focusing the light collecting device of the optical microscope on the sample.

In an embodiment, the adjustable stage comprises a first translation device which is movable in a first direction, a second translation device which is movable in a second direction, wherein the first direction is arranged at a sharp angle with respect to the second direction, preferably wherein the first and second direction define a plane which extends substantially parallel to the central axis. In an embodiment, the adjustable stage comprises a third translation device which is movable in a third direction, which third direction is arranged substantially perpendicular to the plane.

In an embodiment, the inspection apparatus further comprising a sliding vacuum seal between the first and second chamber for moving the second chamber with respect to the first chamber. According to a second aspect, the present invention provides a use of an inspection apparatus or an embodiment thereof as described above for obtaining a SEM image and an optical image from substantially the same area of the sample.

In an embodiment, the adjustable stage is arranged for compensating a movement of the second chamber with respect to the first chamber. In an embodiment, the adjustable stage is arranged for substantially maintaining the position of the light collecting device at or near the central axis of the SEM, in particular when the second chamber is moved with respect to the first chamber.

According to a third aspect, the present invention provides a method for aligning the optical microscope with respect to the SEM in an inspection apparatus as described above, said method comprising the steps of:

providing a fluorescent plate at the position of the sample on the sample holder or providing the sample holder with a fluorescent plate, wherein the fluorescent plate is arrange to emit cathodoluminescent light from the position where the fluorescent plate is illuminated by an electron beam,

illuminating a spot on said fluorescent plate by the electron beam of the SEM,

detecting the cathodoluminescent light of the spot on the fluorescent plate by the optical microscope, and identifying the cathodoluminescent spot on the fluorescent plate as being the position of the electron beam of the SEM on the fluorescent plate.

Using this procedure, a correlation between an image of the optical microscope and an image of the SEM can be established.

In addition or alternatively, said method comprises the steps of:

providing the sample holder, the first side of the second chamber, and/or a part of the light collecting device which faces the SEM at least when the first and second chamber are arranged for observation of the sample with the SEM, with alignment markers which can be detected and/or imaged by the SEM,

using the SEM to obtain an image of the sample holder, the first side of the second chamber, and/or the light collecting device to image the alignment markers, and identifying the alignment markers in the SEM image to establish the position of the sample holder, the first side of the second chamber, and/or the light collecting device with respect to the SEM on the basis of the imaged alignment markers.

This procedure can also or in addition be used to establish a correlation between an image of the optical microscope and an image of the SEM.

The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, in particular the aspects and features described in the attached dependent claims, can be made the subject of one or more divisional patent applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be elucidated on the basis of an exemplary embodiment shown in the attached drawings, in which :

Figure 1 shows the exterior of a preferred embodiment of the inspection apparatus of the invention;

Figure 2 shows schematically a basic design of the inspection apparatus of the present invention;

Figure 3 shows part of the interior of the optical microscope part of the preferred embodiment of figure 1;

Figure 4, shows a cross section of the optical microscope part of figure 3;

Figure 5 shows a detail of the adjustment stage for the microscope objective; and

Figure 6 shows a removable second vacuum chamber in more detail.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows an overview of a preferred embodiment of an inspection apparatus according to the present invention. The inspection apparatus comprises a Scanning Electron Microscope (SEM) system 101, in particular a table-top SEM, an optical microscope system 102, an external light source 103, a vacuum pump 104, and a display monitor 105.

The SEM system 101 is of a known design as for example described in detail in WO 2007/143736, which SEM system does not require special facilities, that is, the SEM system 101 can operate on conventional power provided by a wall socket, and the SEM system 101 does not require special vibration damping mounting.

According to the present invention, the SEM system 101 is provided with an optical microscope system 102, which is in the example of figure 1 arranged under the SEM system 101. In particular the optical microscope system 102 has substantially the same footprint as the SEM system 101. Preferably the optical microscope system 102 is fixedly attached to the SEM system 101 to form one single unit 100, which makes it easy to install the inspection apparatus; the single unit 100 can be set on any sturdy work surface and connected to a power supply, in particular a wall socket. In an alternative embodiment, the optical microscope system may be arranged adjacent to the SEM system in order to limit the height of the single unit 100.

The optical microscope system shown in the example of figure 1, comprises an external light source 103, which is connected to a light input port 107 via an optical fiber 106. An advantage of an external light source 103 is, that there is no need to provide the single unit 100, in particular the optical microscope system 102, with measures and/or devices to handle any heat generated by the light source 103. In an alternative embodiment, the light source may be incorporated in the optical microscope system 102, in particular when using a light source with a low heat generation such as a Light Emitting Diode (LED) .

The preferred embodiment shown in figure 1 comprises two vacuum pumps, an external pre-vacuum pump 104, and an integral high vacuum pump, such as a turbomolecular pump which is arranged inside the optical microscope system 102, as known from the prior art.

Finally the inspection apparatus comprises a control system for controlling the operation of the inspection apparatus, which control system comprises a display monitor 105, and a user input device such as a keyboard which is known per se. The control system may be incorporated in the display monitor 105 or in the single unit 100. Alternatively, the control system may be a separate processor unit, such as a Personal Computer (PC) .

With reference to figure 2, the basic design of the inspection apparatus 200 of the invention is explained. The apparatus 200 comprises a SEM system 201 and an optical microscope system 202.

The SEM system 201 comprises an electron source 203 which is arranged for emitting an electron beam, and an electron optical column for projecting and focusing said electron beam onto a sample. Said electron optical column comprises:

electrostatic alignment rods 204 for aligning the electron beam from the electron source 203 with the central or optical axis 210,

a condenser lens section 205,

a set of deflector/stigmator rods 206, preferably comprising eight rods in an octupole design to correct for astigmatism of the electron beam and for scanning the electron beam over the surface of the sample, an objective lens section 207 for focusing the electron beam on the surface of the sample, and

an electron detector 208 for detecting electrons from the sample.

These constituents of the SEM system 202 are arranged inside a first chamber 211 which is suitable connected or provided with a vacuum pump in a manner known per se, in order to evacuate the interior of the first chamber 211 to provide a suitable vacuum pressure for operating the SEM. The first chamber 211 comprises a bottom plate 212 which is provided with a through opening 213 through which the electron beam passes to reach the sample.

As shown in figure 2, the inspection apparatus 200 also comprises a second chamber 214. The second chamber

214 is movably connected to the first chamber 211. Such a movable connection may be established using a flexible conduit, such as a vacuum bellow. However, preferably the movable connection comprises a sliding vacuum seal as indicated in figure 2.

The sliding vacuum seal comprises a flexible seal

215 arranged in a groove in a rigid sliding plate 216, which flexible seal 215 is arranged around the second chamber 214. The second chamber 214 is removable arranged in the rigid sliding plate 216 and the flexible seal 215 seals the interior of the opening in the rigid sliding plate 216 which is arranged for holding the second chamber 214. A flexible stainless steel plate 217 is clipped to the sliding plate 216 using clip 218 and moves with the sliding plate 216. The flexible stainless steel plate 217 is provided with a through opening 219 which aligns with the sample when the second chamber 214 is arranged in the sliding plate 216.

To move the second chamber 214 into position under the optical axis 210, the sliding plate 216 slides along the base plate 212 of the first chamber 211. When sliding the second chamber 214 towards or away from the position under the optical axis 210, the through opening 219 passes an opening 220. When the through opening 219 is aligned with the opening 220, said opening 220 is in fluid communication with the interior of the second chamber 214, and can be used to evacuate the second chamber 214 to a pre- vacuum pressure when moving the second chamber towards the position under the optical axis 210, or to vent the second chamber 214 when moving the second chamber away from the position under the optical axis 210, for example for removing the second chamber 214 from the inspection apparatus 200.

In addition, when the second chamber 214 is arranged at the position under the optical 210, the through opening 213 of the base plate 212 and the through opening 219 align in order to provide an open connection between the second chamber 214 and the first chamber 211. In this situation, the first chamber 211 and the second chamber 214 together form a vacuum chamber of the inspection apparatus.

As shown in figure 2, the second chamber 214 comprises a sample holder 221 for holding a sample 222. The sample holder is indicated very schematically in figure 2. Inside the second chamber 214, a light collecting device 223, in particular a microscope objective, is arranged for collecting light from the sample 222. The microscope objective 223 is arranged on an adjustable stage 224 which is connected 225 to the second chamber 214 as describe in more detail below with reference to figure 5 and 6. The light collected from the sample 22 by the microscope objective 223 is directed through an opening in the adjustable stage 224 and leaves the second chamber 214 via a transparent window 226.

Below the second chamber 214, and outside of the second chamber 214, the further components of the optical microscope 202 are arranged. These further components comprises at least a light detector 231, such as a CCD or CMOS camera for obtaining an optical microscope image of the sample 222.

In addition, the optical microscope may contain an illumination assembly, which comprises a light source 227 for emitting light which is directed 228 towards the optical axis 210. A dichroic mirror 229 is used for projecting the illumination light along the optical axis towards a microscope objective 223. The microscope objective 223 projects the illumination light onto the sample 222, collects the reflected light and directs 230 this reflected light towards the light detector 231 for obtaining an optical microscope image of the sample 222.

Figure 3 shows part of the interior of the optical microscope part 301 of the preferred embodiment of figure 1, and figure 4 shows a cross section of the optical microscope part of figure 3. In essence the optical microscope part 301 has a comparable configuration as the optical microscope part 202 as shown in figure 2. It is noted that optical components of the optical microscope are arranged, and the trajectory of the light beams has been arranged so that the optical microscope 301 has substantially the same footprint as the SEM system 101.

Also in this preferred embodiment, the microscope objective 323 is separated from the other constituents of the optical microscope 301, and is arranged in a second chamber 614 as shown in figure 6. The other constituents of the optical microscope 301 are arranged in an optics box 304 which is preferably arranged under the SEM system 101 (see also figure 1) . The optics box 304 is fixedly connected to the SEM system 101 to form one single unit 100. The optics box 304 is provided with a transparent window 305, which is substantially arranged on the optical axis 400 of the inspection apparatus. Below the transparent window 305, projection optics 308 may be provided for projecting the light 330 collected by the microscope objective 323 from the sample 322, via folding mirror 307, onto the detector 331.

In addition, the optics box 304 is provided with an optical fibre connector 327 (more clearly indicated in with reference number 107 in figure 1) for providing illumination light 328. The illumination light 328 is projected towards a dichroic mirror 329 arranged on the optical axis 400, via a set of folding mirrors 306, 306' . The dichroic mirror 329 is used for projecting the illumination light 328 upwards along the optical axis 400 towards a microscope objective 323. The microscope objective 323 projects the illumination light onto the sample 322, and collects the reflected light 330 and directs this reflected light downwards to the dichroic mirror 329. The reflected light 330 passes the dichroic mirror 329 and is projected by the projection optics 308, via the folding mirror 307, onto the light detector 331 for obtaining an optical microscope image of the sample 322.

Any cathodoluminescent light from the sample 322, as induced by the electron beam of the SEM, can also be collected by the microscope objective 323, which directs this cathodoluminescent light downwards to the dichroic mirror 329. The cathodoluminescent light 330 passes the dichroic mirror 329 and is projected by the projection optics 308, via the folding mirror 307, onto the light detector 331 for obtaining an optical microscope image of the cathodoluminescent light from the sample 322.

As shown in figures 3 and 4, the light detector 331, preferably a CMOS camera, is arranged near the backside 303 of the optics box 301. The camera is provided with an air cooling arrangement using a ventilator inside said camera. In order to regulate the air flow of this cooling arrangement, the detector 331 is arranged is a compartment 308 comprising a dividing wall 309. Cooling air is sucked in from the backside 303 of the optics box 301 via an air inlet 310, into the detector 331, and is exhausted out of the detector 331 on the other side of the dividing wall 309 and guided via a substantially separate duct of the compartment 308 again to the backside 303 of the optics box 301. Inside the optics box 301 the air flow of the incoming cooling air and the exhausted air is substantially separated.

It is noted that when combining the optical microscope 102 and the SEM system 101, the whole optics box 301 is aligned with the SEM. In particular, to substantially bring the central axis 210 of the SEM into alignment with the optical axis 400 of the optical microscope. The alignment aims to let the central axis 201 of the SEM and the optical axis 400 of the optical microscope substantially coincide, which allows to obtain a SEM image and an optical image from substantially the same area of the sample. In an embodiment, the folding mirror 307 and/or the projection optics may be used to fine-tune the alignment the optical microscope with respect to the SEM.

Subsequently, the illumination optics may be aligned in order to project the illumination light 328 on the appropriate area on the sample 322. The alignment is suitable performed using a collimator lens arranged to collimate the light from the optical fibre connector 327, and/or the folding mirrors 306, 306' . In an embodiment, the collimator lens may be a lOx microscope objective which is arranged such that the focus spot is arranged substantially at the exit end of the optical fibre, at least when the optical fibre is arranged in the optical fibre connector 327.

In the example shown in figures 3 and 4, the optics box 301 also comprises a controller 311 for, inter alia, driving the adjustable stage 324 in the second chamber 614. This controller 311 may be arranged in the front part of the optics box 301, in order to be easily accessible from the front side 302 for maintenance. In an embodiment, the controller 311 may be connected to the second chamber 214, 614 via a cable connection which is arranged to allow the second chamber 214, 614 to move with respect to the optics box 301.

As clearly indicated in figure 3 and 4, substantially the whole optical microscope, with exception of the microscope objective 323, is arranged in the optics box 301 of the inspection apparatus. The optics box and the components inside are arranged at a substantially fixed position with respect to the SEM system, and thus at a substantially fixed position with respect to the first chamber 211 containing the SEM.

As indicated in figure 2, the microscope objective 323 is arranged inside the movable second chamber 214, and thus will travel with any movement of said second chamber 214. In a preferred embodiment, the second chamber together with the microscope objective is removable from the inspection apparatus.

In order to compensate for small movements of the second chamber 214 with respect to the SEM system on the one hand, and the optical components of the optical microscope in the optics box 311 on the other hand, the microscope objective 323 is arranged on top of an adjustment stage 324. An example of such an adjustment stage is shown in figure 5.

As shown in figure 5, the adjustable stage 324 comprises a first translation device 502 which is movable in a first direction Y, and a second translation device 503 which is movable in a second direction Z+Y. The first direction Y is arranged at a sharp angle a with respect to the second direction Z+Y, wherein the first Y and second Z+Y direction define a plane which extends substantially parallel to the optical axis 400. When moving the first translation device 502, the microscope objective 323 moves in the first direction Y. When moving the second translation device 503, the microscope objective 323 moves both the first direction Y and in a direction Z, substantially parallel to the optical axis 400. In case only a movement of the microscope objective along the optical axis 400 is required, for example for focusing the optical microscope, the first translation device 502 is used to compensate for the movement in de first direction Y by the second translation device 503. Thus, the adjustable stage 324 is movable in a direction Z substantially parallel to the optical axis 400. In an embodiment, the range of travel in the Z direction may be approximately 0,5 mm.

As shown in figure 5, the adjustable stage 324 also comprises a third translation device 501 which is movable in a third direction X, which third direction X is arranged substantially perpendicular to the plane defined by the Z and Y direction. Thus, the adjustable stage 324 is movable with respect to the second chamber 214, 614 in a plane XY which extend substantially perpendicular to said optical axis 400. In an embodiment, the range of travel in both the X and Y direction may be approximately 5 mm.

The adjustable stage 324 is on one side provided with a connector 504 for attaching a microscope objective 323 to the adjustable stage 324. On the other side, facing away from the one side with the connector 504, the adjustable stage 324 is provided with a support plate 505 which is connected to the inside of the second chamber 214, 614. In addition, the adjustable stage 324 is provided with a central opening which connects to the connector 504, and which allows free passage of light between the microscope objective 323 and the optics box 301.

Figure 6 shows a removable second vacuum chamber

614 in more detail. The second vacuum chamber 614 comprises a sample holder 621, which is arranged at or near a first side (top) of said second chamber 614 which, when arranged in the inspection apparatus, faces said first chamber 211. A sample 322 or a fluorescent plate which emits cathodoluminescent light when illuminated by the electron beam, for example comprising a YAG material, may be placed in the sample holder 621 via the top side.

The sample holder 621 is provided with alignment markers 700 which can be detected and/or imaged by the SEM.

When the second chamber 614 is arranged to align with the central axis 210 of the SEM, the second chamber 614 is drawn against the first chamber 211 due do the vacuum pressure inside the first chamber 211, and the first side of the second chamber 614 abuts against the first chamber 211. The first side (top) of the second chamber 614 is therefore substantially accurately aligned in a direction along the central axis 210.

In order to benefit from this accurate alignment, the adjustable stage 624, in particular the bottom plate 605 thereof, is fixedly connected to the inside of the second chamber 614 at or near said first side of the second chamber 614. In an embodiment the bottom plate 605 of the adjustable stage 624 may also be connected to a second side (bottom) of the second chamber 614, preferably by one or more springs, for at least partially supporting the weight of the adjustable stage 624.

The adjustable stage 624 is provided with three actuators (not shown) , preferably stepper motors, for driving the translation devices of the adjustable stage 624. The actuators are connected to the outside of the second chamber 614 via a Printed Wire Board (PWB) which forms part of the walls of the vacuum region inside the second chamber 614, and which is sealed using an O-ring. The PWB is provided with a connector at a side of said PWB which faces to the outside of the second chamber 614. The connector allows connection of a cable to connect the actuators with the controller 311 in the optics box 301.

Since the alignment of the adjustable stage 624 and also of other components inside the second chamber 614 is very delicate, the second chamber 614 is provided with dampers 601 for placing the second chamber onto a work surface. The dampers 601 are made of an elastic material with vibration absorbing properties. The inspection apparatus of the invention is highly suitable for correlative light-electron microscopy. The apparatus is arranged to obtain both light images and SEM images from the same area of a sample. In order to align the optical microscope with respect to the SEM the following steps may be used, with reference to figure 2 :

providing a fluorescent plate at the position of the sample 222 on the sample holder 221 or providing the sample holder 221 with a fluorescent plate, wherein the fluorescent plate is arrange to emit cathodoluminescent light from the position where the fluorescent plate is illuminated by an electron beam,

illuminating a spot on said fluorescent plate by the electron beam of the SEM 201,

- detecting the fluorescent light of the spot on the fluorescent plate by the optical microscope 202, and

identify the fluorescent spot on the fluorescent plate as being the position of the electron beam of the SEM on the fluorescent plate. Thus, the position of the electron beam of the SEM is detected in the image of the optical microscope .

In addition, or alternatively, the alignment method may the steps of:

providing at least one of the sample holder 221, the first side of the second chamber, and a part of the light collecting device 223 which faces the SEM 201 at least when the first 211 and second 214 chamber are arranged for observation of the sample 222 with the SEM 201, with alignment markers (700 in figure 6) which can be detected and/or imaged by the SEM 201,

using the SEM 201 to obtain an image of at least one of the sample holder 221, the first side of the second chamber, and the light collecting device 223 to image the alignment markers, and

- identify the alignment markers in the SEM image to establish the position of at least one of the sample holder 221, the first side of the second chamber, and the light collecting device 223, with respect to the SEM on the basis of the imaged alignment markers. Thus, the position of the optical microscope is detected in the image of the SEM.

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention.

In summary the present invention relates to an inspection apparatus comprising a Scanning Electron Microscope (SEM) and an optical microscope. The SEM comprises a first chamber comprising a source for emitting a primary electron beam, an electron optical column comprising a lens for focusing the electron beam onto a sample on a sample holder, and an electron detector for detecting electrons from the sample. The inspection apparatus further comprises a second chamber which comprises the sample holder. The second chamber is movably connected to the first chamber for moving the sample holder with respect to the SEM. For imaging, there is an open connection between the first and second chamber for observation of the sample with the SEM. The optical microscope comprises a light collecting device for collecting light from the sample, and a light detector for detecting the light from the sample. The light collecting device is arranged inside the second chamber.

Claims

C L A I M S

1. Inspection apparatus comprising a Scanning Electron Microscope (SEM) and an optical microscope for inspecting a sample,

wherein the optical microscope comprises a light collecting device for collecting light from the sample, light optical elements for directing the light towards a light detector for detecting the light from the sample, wherein the light collecting device is arranged inside the second chamber.

2. Inspection apparatus according to claim 1, wherein the second chamber is removable from the first chamber .

3. Inspection apparatus according to claim 1 or 2, wherein the sample holder is arranged at or near a first side of said second chamber which faces said first chamber, wherein said light collecting device is arranged at a side of said sample holder which faces away from said first chamber, in particular when the second chamber and the first chamber are joined.

4. Inspection apparatus according to claim 3, wherein at least one of the sample holder, the first side of the second chamber, and a part of the light collecting device which faces the SEM at least when the first and second chamber are arranged for observation of the sample with the SEM, is provided with alignment markers which can be detected and/or imaged by the SEM.

5. Inspection apparatus according to any one of the previous claims, wherein the light collecting device is arranged on an adjustable stage for moving said light collecting device with respect to the second chamber, wherein the adjustable stage is arranged inside the second chamber .

6. Inspection apparatus according to claim 5, in particular when dependent on claim 3, wherein the adjustable stage is fixedly connected to said first side of the second chamber

7. Inspection apparatus according to claim 6, wherein the adjustable stage is connected to a second side of the second chamber by one or more springs, wherein said second side is arranged substantially opposite to the first side .

8. Inspection apparatus according to any one of the previous claims, wherein at least the light detector is arranged outside the second chamber, and wherein the second chamber is provided with an optical window which is substantially transparent for light.

9. Inspection apparatus according to claim 8, wherein the light detector is arranged in the inspection apparatus at a substantially fixed position with respect to the first chamber.

10. Inspection apparatus according to claim 8 or 9, wherein the optical microscope, except the light collecting device thereof, is arranged in the inspection apparatus at a substantially fixed position with respect to the first chamber.

11. Inspection apparatus according to any one of the previous claims, wherein the source and the electron optical column comprising the lens are arranged along a central axis of the SEM, and wherein the second chamber is movable with respect to the first chamber in a plane which extends substantially perpendicular to said central axis.

12. Inspection apparatus according to claim 11, when depending on claim 3, wherein the adjustable stage is movable with respect to the second chamber in a plane which extend substantially perpendicular to said central axis.

13. Inspection apparatus according to claim 12, wherein the adjustable stage is also movable in a direction substantially parallel to the central axis.

14. Inspection apparatus according to claim 13, wherein the adjustable stage comprises a first translation device which is movable in a first direction, a second translation device which is movable in a second direction, wherein the first direction is arranged at a sharp angle with respect to the second direction, preferably wherein the first and second direction define a plane which extends substantially parallel to the central axis.

15. Inspection apparatus according to claim 14, wherein the adjustable stage comprises a third translation device which is movable in a third direction, which third direction is arranged substantially perpendicular to the plane .

16. Inspection apparatus according to any one of the previous claims, further comprising a sliding vacuum seal between the first and second chamber for moving the second chamber with respect to the first chamber.

17. Use of an inspection apparatus according to any one of the previous claims for obtaining a SEM image and an optical image from substantially the same area of the sample .

18. Use of an inspection apparatus according to claim 17, wherein the adjustable stage is arranged for compensating a movement of the second chamber with respect to the first chamber.

19. Use of an inspection apparatus according to claim 17 or 18, wherein the adjustable stage is arranged for substantially maintaining the position of the light collecting device at or near the central axis of the SEM, in particular when the second chamber is moved with respect to the first chamber.

20. Method for aligning the optical microscope with respect to the SEM in an inspection apparatus according to any one of the claims 1 - 16, said method comprising the steps of:

- illuminating a spot on said fluorescent plate by the electron beam of the SEM,

21. Method for aligning the optical microscope with respect to the SEM in an inspection apparatus according to any one of the claims 1 - 16, said method comprising the steps of:

providing at least one of the sample holder, the first side of the second chamber, and a part of the light collecting device which faces the SEM at least when the first and second chamber are arranged for observation of the sample with the SEM, with alignment markers which can be detected and/or imaged by the SEM,

using the SEM to obtain an image of at least one of the sample holder, the first side of the second chamber, and the light collecting device to image the alignment markers, and

identifying the alignment markers in the SEM image to establish the position of at least one of the sample holder, the first side of the second chamber, and the light collecting device, with respect to the SEM on the basis of the imaged alignment markers.

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BP/HZ