Patent ID: 12191109

DETAILED DESCRIPTION

There are various techniques for inspecting the sample (e.g., a substrate and a patterning device). One kind of inspection techniques is optical inspection, where a light beam is directed to the substrate or patterning device and a signal representing the interaction (e.g., scattering, reflection, diffraction) of the light beam and the sample is recorded. Another kind of inspection techniques is charged particle beam inspection, where a beam of charged particles (e.g., electrons) is directed to the sample and a signal representing the interaction (e.g., secondary emission and back-scattered emission) of the charged particles and the sample is recorded.

FIG.1schematically shows an apparatus100that can carry out charged particle beam inspection. The apparatus100may include components configured to generate and control a beam of charged particles, such as a source10that can produce charged particles in free space, a beam extraction electrode11, a condenser lens12, a beam blanking deflector13, an aperture14, a scanning deflector15, and an objective lens16. The apparatus100may include components configured to detect the signal representing the interaction of the beam of charged particles and a sample, such as an E×B charged particle detour device17, a signal detector21. The apparatus100may also include components, such as a processor, configured to process the signal or control the other components.

In an example of an inspection process, a beam18of charged particle is directed to a sample9(e.g., a wafer or a mask) positioned on a stage30. A signal20representing the interaction of the beam18and the sample9is guided by the E×B charged particle detour device17to the signal detector21. The processor may cause the stage30to move or cause the beam18to scan.

Charged particle beam inspection may have higher resolution than optical inspection due to the shorter wavelengths of the charged particles used in charged particle beam inspection than the light used in optical inspection. As the dimensions of the patterns on the substrate and the patterning device become smaller and smaller as the device manufacturing process evolves, charged particle beam inspection becomes more widely used.

In charged particle beam inspection, electric charges may be applied to a region of a sample before the region is imaged, to better reveal certain features (e.g., voltage contrast defects) in the region. The process of applying the electric charges before imaging may be called pre-charging. Pre-charging may reduce the adverse impact of non-uniform electric charge distribution in the region on imaging, or may enhance contrast by exploiting disparity in the interactions with the electric charges among features in the region.

Pre-charging can be realized by at least two techniques: flooding, and pre-scanning. In the flooding technique, a separate source of electric charges (sometimes called a “flooding gun”) is used to provide a relatively large amount of electric charges to charge the region. After the region is flooded with electric charges, a beam of charged particles is used to image region. The sample may move during flooding. Flooding may last for a time period on the order of minutes. Switching from flooding to imaging may take seconds. The relatively long time period for flooding reduces the throughput of the inspection.

In the pre-scanning technique, a beam of charged particles is used both for pre-charging and for imaging. Using the same beam of charged particles both for pre-charging and for imaging allows precise control of pre-charging. The beam of charged particles in a configuration suitable for pre-charging may be scanned across the region before the same beam in a different configuration suitable for imaging is scanned across the region.

Switching between these configurations should be relatively fast. However, if a much higher current density of the beam is needed for pre-charging (e.g., to reveal a thin void in copper interconnects) than for imaging, quickly switching between these configuration may be difficult. The current of the beam may be increased by increasing the current density of the beam through the aperture. Basically, without changing the aperture size, higher current density leads to more electric charges through the aperture per unit time. The current density may be increased by increasing the strength of the condenser lens (e.g., condenser lens12inFIG.1) or by adding an extra electrostatic round lens before the aperture.

The condenser lens may be a magnetic lens. To increase the strength of a magnetic lens, the excitation current of the magnet in the magnetic lens should be changed. Changing the excitation current is slow and precisely controlling the magnitude of the excitation current may be difficult. In addition, the strength of the magnetic lens as a function of the excitation current has hysteresis.

The condenser lens may be an electrostatic round lens. To increase the strength of an electrostatic round lens, the voltage applied to it should be increased. A change of the voltage by a high magnitude (e.g., >1000 V) may be needed to switch between the configuration for pre-charging and the configuration for imaging, but such a change may be slow.

FIG.2Aschematically shows an apparatus2000that can carry out charged particle beam inspection, where a beam of charged particles may be used both for pre-charging and for imaging, according to an embodiment. The apparatus2000has a source2001of charged particles, a condenser lens2002, and an aperture2007. The source2001is configured to emit a beam of charged particles along the primary beam axis2010of the apparatus2000. The condenser lens2002is configured to cause the beam to concentrate around the primary beam axis2010. Increasing the intensity of the condenser lens2002increases the current density of the beam through the aperture2007. The apparatus2000also has a first multi-pole lens2003and a second multi-pole lens2004. The first multi-pole lens2003is downstream with respect to the condenser lens2002and upstream with respect to the second multi-pole lens2004. The second multi-pole lens2004is downstream with respect to the first multi-pole lens2003and upstream with respect to the aperture2007. The expression “Component A is upstream with respect to Component B” as used herein means that a beam of charged particles would reach Component A before reaching Component B in normal operation of the apparatus. The expression “Component B is downstream with respect to Component A” as used herein means that a beam of charged particles would reach Component B after reaching Component A in normal operation of the apparatus. The apparatus2000may further include a scanning deflector2008configured to scan the beam relative to a sample2030, and an objective lens2009configured to focus the beam onto the sample2030. The apparatus2000may optionally further include a third multi-pole lens2005and a fourth multi-pole lens2006. The third multi-pole lens2005is downstream with respect to the aperture2007. The fourth multi-pole lens2006is downstream with respect to the third multi-pole lens2005. The apparatus2000as shown inFIG.2Adoes not have the first multi-pole lens2003or the second multi-pole lens2004energized. The outer contour2041of the beam shows that only a small portion of the beam goes through the aperture2007. The portion has an outer contour2042. The apparatus2000as shown inFIG.2Bhas the first multi-pole lens2003and the second multi-pole lens2004energized, which causes a much larger portion of the beam to go through the aperture2007. The beam has an outer contour2043. The third multi-pole lens2005and the fourth multi-pole lens2006may be used to shape the beam in the configuration for pre-charging or in the configuration for imaging.

The multi-pole lenses2003-2006in the apparatus2000may be electrostatic lenses. In an embodiment, the multi-pole lenses2003-2006each have at least four poles. For example, the multi-pole lenses2003-2006may be quadrupole lenses.FIG.3schematically shows a cross-section of a quadrupole lens as an example of the multi-pole lenses2003-2006. The quadrupole lens has four poles3001-3004. The poles3001-3004may each have a cross-sectional shape of a section of a ring. Each of the poles3001-3004may be separated from its neighbors by a gap or by a dielectric. The poles3001and3003are disposed opposite to each other; the poles3002and3004are disposed opposite to each other. During use, the poles3001and3003have a positive voltage applied to them; the poles3002and3004have a negative voltage applied to them; the positive voltage and the negative voltage have the same absolute value. Under the assumption that the charged particles carry negative charges (e.g., the charged particles are electrons), the beam is stretched along a line connecting the poles3001and3003and compressed along a line connecting the poles3002and3004.

Namely, the quadrupole lens causes the beam to have an oval-shaped cross-section3999. For example, the first multi-pole lens2003may stretch the beam along a first direction and compress the beam along a second direction, which is different than (e.g., perpendicular to) the first direction; the second multi-pole lens2004may stretch the beam along the second direction and compress the beam along the first direction.

FIG.4schematically shows an example of the cross-sectional shape of the beam in the apparatus2000at the aperture2007(left panel) and at the sample2030(right panel), when the multi-pole lenses2003-2006are energized. Although the beam at the sample2030may not have a uniform current density distribution across the cross-section, the beam may nevertheless be used for pre-charging because beam uniformity is not very important for pre-charging purposes.

FIG.5Aschematically shows a top view of another quadrupole lens as an example of the multi-pole lenses2003-2006.FIG.5Bschematically shows a cross-sectional view of the quadrupole lens ofFIG.5A. This quadrupole lens has four prismatic or cylindrical poles5001-5004. Each of the poles5001-5004may be separated from its neighbors by a gap or by a dielectric. The poles5001and5003are disposed opposite to each other; the poles5002and5004are disposed opposite to each other. During use, the poles5001and5003have a positive voltage applied to them; the poles5002and5004have a negative voltage applied to them; the positive voltage and the negative voltage have the same absolute value. Under the assumption that the charged particles carry negative charges (e.g., the charged particles are electrons), the beam is stretched along a line connecting the poles5001and5003and squeezed along a line connecting the poles5002and5004.

FIG.6Aschematically shows charged particle beam inspection where a beam of charged particles is used both for pre-charging and for imaging, according to an embodiment. For convenience, two directions x and y are defined in a reference frame (“RF”) that has no movement relative to the sample. The x and y directions are mutually perpendicular. In this embodiment, the beam is in a configuration suitable for pre-charging and forms a spot6001on the sample. The spot6001has a diameter of D1. The spot6001is scanned in the −y direction by a length L, thereby pre-charging an area of D1by L. The spot6001quickly snaps back in the y direction by the length L and in the x direction by a suitable length (e.g., (D1−D2)/2) and the beam changes into a configuration suitable for imaging and forms a probe spot6002on the sample; or the beam changes into a configuration suitable for imaging and forms a probe spot6002on the sample and the probe spot6002quickly snaps back in the y direction by the length L and in the x direction by a suitable length (e.g., (D1−D2)/2). D2is the diameter of the probe spot6002. Signals representing the interactions of the beam and the sample are recorded from the probe spot6002while the probe spot6002is scanned across the area of D1by L. For example, the probe spot6002may be scanned in the −y direction by L, and quickly snaps back in the y direction by the length L and in the −x direction by D2. This back-and-forth scanning of the probe spot6002may be repeated until the probe spot6002scans the entirety of the area of D1by L. The probe spot6002quickly snaps back in the y direction by the length L and in the −x direction by a suitable length (e.g., (D1+D2)/2) and the beam changes into a configuration suitable for pre-charging and forms the spot6001on the sample; or the beam changes into a configuration suitable for pre-charging and forms the spot6001on the sample and the spot6001quickly snaps back in the y direction by the length L and in the −x direction by a suitable length (e.g., (D1+D2)/2). This alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging may be repeated to inspect a region on the sample. Dotted arrows represent quick snapping of the spot6001or the probe spot6002. Thick solid arrows represent paths of the spot6001along which some area of the sample is pre-charged. Thin solid arrows represent paths of the probe spot6002along which some area of the sample is imaged.

FIG.6Bshows the paths of the spot6001without showing the paths of the probe spot6002, in the alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging inFIG.6A.FIG.6Cshows the paths of the probe spot6002without showing the paths of the spot6001, in the alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging inFIG.6A.

FIG.7Aschematically shows charged particle beam inspection where a beam of charged particles is used both for pre-charging and for imaging, according to an embodiment. For convenience, two directions x and y are defined in a reference frame (“RF”) that has no movement relative to the sample. The x and y directions are mutually perpendicular. In this embodiment, the beam is in a configuration suitable for pre-charging and forms a spot6001on the sample. The spot6001is scanned in the −y direction by a length L, thereby pre-charging an area of D1by L. The spot6001quickly snaps back in the x direction by a suitable length (e.g., (D1−D2)/2) and the beam changes into a configuration suitable for imaging and forms a probe spot6002on the sample; or the beam changes into a configuration suitable for imaging and forms a probe spot6002on the sample and the probe spot6002quickly snaps back in the x direction by a suitable length (e.g., (D1−D2)/2). Signals representing the interactions of the beam and the sample are recorded from the probe spot6002while the probe spot6002is scanned across the area of D1by L. For example, the probe spot6002may be scanned in the y direction by L, and quickly snaps back in the −y direction by the length L and in the −x direction by D2. This back-and-forth scanning of the probe spot6002may be repeated until the probe spot6002scans the entirety of the area of D1by L. The probe spot6002quickly snaps back in the −x direction by a suitable length (e.g., (D1+D2)/2) and the beam changes into a configuration suitable for pre-charging and forms the spot6001on the sample; or the beam changes into a configuration suitable for pre-charging and forms the spot6001on the sample and the spot6001quickly snaps back in the −x direction by a suitable length (e.g., (D1+D2)/2). This alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging may be repeated to inspect a region on the sample. Dotted arrows represent quick snapping of the spot6001or the probe spot6002. Thick solid arrows represent paths of the spot6001along which some area of the sample is pre-charged. Thin solid arrows represent paths of the probe spot6002along which some area of the sample is imaged.

FIG.7Bshows the paths of the spot6001without showing the paths of the probe spot6002, in the alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging inFIG.7A.FIG.7Cshows the paths of the probe spot6002without showing the paths of the spot6001, in the alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging inFIG.7A.

FIG.8Aschematically shows charged particle beam inspection where a beam of charged particles is used both for pre-charging and for imaging, according to an embodiment. For convenience, two directions x and y are defined in a reference frame (“RF”) that has no movement relative to the sample. The x and y directions are mutually perpendicular. In this embodiment, the beam is in a configuration suitable for pre-charging and forms a spot6001on the sample. The spot6001is scanned in the −y direction by a length L, thereby pre-charging an area of D1by L. The spot6001quickly snaps back in the y direction by a length L and in the x direction by D1. The back-and-forth scanning of the spot6001is repeated until an entire region of nD1by L (n being a positive integer) is pre-charged by the spot6001. The spot6001quickly snaps back in the y direction by the length L and in the x direction by a suitable length (e.g., (n−1)D1+(D1−D2)/2) and the beam changes into a configuration suitable for imaging and forms a probe spot6002on the sample; or the beam changes into a configuration suitable for imaging and forms a probe spot6002on the sample and the probe spot6002quickly snaps back in the y direction by the length L and in the x direction by a suitable length (e.g., (n−1)D1+(D1−D2)/2). Signals representing the interactions of the beam and the sample are recorded from the probe spot6002while the probe spot6002is scanned across the region of nD1by L. For example, the probe spot6002may be scanned in the −y direction by L, and quickly snaps back in the y direction by the length L and in the −x direction by D2. This back-and-forth scanning of the probe spot6002may be repeated until the probe spot6002scans the entirety of the region of nD1by L. Dotted arrows represent quick snapping of the spot6001or the probe spot6002. Thick solid arrows represent paths of the spot6001along which some area of the sample is pre-charged. Thin solid arrows represent paths of the probe spot6002along which some area of the sample is imaged.

FIG.8Bshows the paths of the spot6001without showing the paths of the probe spot6002, in the alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging inFIG.8A.FIG.8Cshows the paths of the probe spot6002without showing the paths of the spot6001, in the alternate scanning of the spot6001for pre-charging and the probe spot6002for imaging inFIG.8A.

FIG.9schematically shows a flowchart for a method for charged particle beam inspection, according to an embodiment. In step9010, a beam of charged particles is set into a configuration suitable for pre-charging a sample. In step9020, an area of the sample is pre-charged using the beam of charged particles in the configuration suitable for pre-charging. In step9030, the same beam of charged particles is set into a configuration suitable for imaging the sample. In step9040, the area of the sample is imaged using the beam of charged particles in the configuration suitable for imaging. The flow may go back to step9010. Step9030does not have to be performed immediately after step9020. In an example in which the beam of charged particles is generated using the apparatus2000describe above, setting the beam into the configuration suitable for pre-charging may include energizing the first multi-pole lens2003and the second multi-pole lens2004, and optionally the third multi-pole lens2005and the fourth multi-pole lens2006if they are present. Setting the beam into the configuration suitable for imaging may include de-energizing at least one of the first multi-pole lens2003and the second multi-pole lens2004, and optionally at least one of the third multi-pole lens2005and the fourth multi-pole lens2006if they are present.

The embodiments may further be described using the following clauses:1. An apparatus comprising:a source of charged particles configured to emit a beam of charged particles along a primary beam axis of the apparatus;a condenser lens configured to cause the beam to concentrate around the primary beam axis;an aperture;a first multi-pole lens;a second multi-pole lens;wherein the first multi-pole lens is downstream with respect to the condenser lens and upstream with respect to the second multi-pole lens;wherein the second multi-pole lens is downstream with respect to the first multi-pole lens and upstream with respect to the aperture.2. The apparatus of clause1, further comprising a scanning deflector configured to scan the beam relative to a sample.3. The apparatus of any one of clauses1or2, further comprising an objective lens configured to focus the beam onto a sample.4. The apparatus of any one of clauses1through3, further comprising a third multi-pole lens and a fourth multi-pole lens;wherein the third multi-pole lens is downstream with respect to the aperture;wherein the fourth multi-pole lens is downstream with respect to the third multi-pole lens.5. The apparatus of any one of clauses1through4, wherein the first multi-pole lens and the second multi-pole lens are electrostatic lenses.6. The apparatus of any one of clauses1through5, wherein the first multi-pole lens and the second multi-pole lens each have at least four poles.7. The apparatus of clause6, wherein the poles each have a cross-sectional shape of a section of a ring.8. The apparatus of clause6, wherein the poles each are prismatic or cylindrical.9. The apparatus of clause6, wherein each of the poles is separated from its neighbors by a gap or by a dielectric.10. The apparatus of clause6, wherein the poles are configured to have different electric voltages applied thereto.11. The apparatus of any one of clauses1through10, wherein the first multi-pole lens is configured to stretch the beam in a first direction and to compress the beam in a second direction; wherein the second multi-pole lens is configured to stretch the beam in the second direction and to compress the beam in the first direction.12. A method comprising:setting a beam of charged particles into a first configuration suitable for pre-charging a sample;pre-charging an area of the sample using the beam of charged particles in the first configuration;setting the beam of charged particles into a second configuration suitable for imaging the sample;imaging the area using the beam of charged particles in the second configuration.13. The method of clause12, wherein the beam of charged particles is generated in an apparatus comprising:a source of charged particles configured to emit the beam of charged particles along a primary beam axis of the apparatus;a condenser lens configured to cause the beam to concentrate around the primary beam axis; an aperture;a first multi-pole lens;a second multi-pole lens;wherein the first multi-pole lens is downstream with respect to the condenser lens and upstream with respect to the second multi-pole lens;wherein the second multi-pole lens is downstream with respect to the first multi-pole lens and upstream with respect to the aperture.14. The method of clause13, wherein setting the beam into the first configuration comprises energizing the first multi-pole lens and the second multi-pole lens.15. The method of clause13, wherein setting the beam into the second configuration comprises de-energizing at least one of the first multi-pole lens and the second multi-pole lens.16. The method of clause13, wherein the apparatus further comprises a third multi-pole lens and a fourth multi-pole lens;wherein the third multi-pole lens is downstream with respect to the aperture;wherein the fourth multi-pole lens is downstream with respect to the third multi-pole lens.17. The method of clause16, wherein setting the beam into the first configuration comprises energizing the third multi-pole lens and the fourth multi-pole lens.18. The method of clause16, wherein setting the beam into the second configuration comprises de-energizing at least one of the third multi-pole lens and the fourth multi-pole lens.19. A computer program product comprising a non-transitory computer readable medium having instructions recorded thereon, the instructions when executed by a computer implementing the method of any of clauses12through18.

While the concepts disclosed herein may be used for inspection on a sample such as a silicon wafer or a patterning device such as chrome on glass, it shall be understood that the disclosed concepts may be used with any type of samples, e.g., inspection of samples other than silicon wafers.

The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below.