Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2002-05-13
2004-01-06
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S307000, C250S311000, C250S3960ML, C250S3960ML, C250S492100
Reexamination Certificate
active
06674075
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a beam directing device and a charged particle beam apparatus utilizing the same for inspecting samples by irradiating them with a focused beam of electrically charged particles, such as electrons, positrons, or ions. Such apparatus is used in the form of a scanning electron microscope (SEM), particularly for inspection of semiconductor wafers.
BACKGROUND OF THE INVENTION
Scanning electron microscopy is a known technique widely used in the manufacture of semiconductor wafers, being utilized in a Critical Dimension metrology tool, the so-called CD-SEM (critical dimension scanning electron microscope) and a defect review metrology tool SEM, the so-called DR-SEM (defect review scanning electron microscope). In a SEM, the surface region of a sample to be examined is two-dimensionally scanned by means of a primary beam of electrically charged particles, usually electrons, which travel along an optical axis of the apparatus. Irradiation of the sample with the primary electron beam releases secondary (or backscattered) electrons thereby defining a secondary electron beam. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected secondary electron beam. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
An SEM typically includes such main constructional parts as an electron beam source, an electron beam column, and a detection unit. The electron beam column comprises inter alia a beam aligning means (the so-called “alignment coils”) and a beam shaping means (stigmator) arranged along an anode tube that defines a primary beam drift space, and comprises a focusing means for directing a primary electron beam onto a sample and directing secondary electrons towards one or more detection units. The focusing assembly typically includes an objective lens arrangement and scanning coils.
To increase the image resolution and improve image acquisition, the primary electron beam should be affected as little as possible, and secondary electrons should be completely detected. The increase of the image resolution can be achieved by reducing chromatic aberration of focusing and deflection. WO 01/45136 assigned to the assignee of the present application discloses a deflection and focusing technique, wherein chromatic aberrations are compensated by using one or two deflections of the primary electron beam propagating towards the sample, i.e., the pre-lens deflection, in-lens deflection, or both. The complete detection of the secondary electrons requires spatial separation between the primary and secondary electrons and the effective detection of the secondary electrons (with minimal losses of electrons).
In many cases, the detector is accommodated above the objective lens outside the path of the primary beam propagation through the column. To direct secondary electrons to the detector, a generator of orthogonal electric and magnetic fields (known as Wien-filter) is used (e.g., U.S. Pat. Nos. 5,894,124; 5,900,629). To ensure detection of those secondary electrons that are not sufficiently deflected by the Wien-filter, a target or extracting electrode made of a material capable of generating a secondary electron when an electron collides therewith is additionally used. Such a target is formed with an aperture and is located such that the axis of the primary beam propagation towards the focusing means intersects with this aperture, which thereby serves as a primary beam hole.
To eliminate the use of the Wien-filter, which requires extensive care and is difficult to adjust, WO 99/26272 suggests scanning a sample with one or more angles of incidence of the primary beam. According to this technique, the primary beam is directed to run diagonally to the optical axis of the focusing assembly, and is redirected into the optical axis by a redirection unit arranged below the plane of a detector accommodated outside the primary beam path. The redirection unit also affects secondary electrons in the sense that it separates primary and secondary electrons.
The technique of the above-indicated publication WO 01/45136, assigned to the assignee of the present application, utilizes a secondary electrons' detector formed with a primary beam hole that is located in the path of a primary electron beam propagating towards the focusing assembly. Here, a deflection system is located downstream of the detector (with respect to the direction of the primary electron beam propagation towards the sample) and operates to affect the trajectory of the primary electron beam such that the primary electron beam impinges onto a sample along an axis forming a certain angle with the sample's surface (the so-called “tilt mode”). This is aimed at solving another problem of the inspection systems of the kind specified associated with inspecting and/or measuring on patterned surfaces. The pattern is typically in the form of a plurality of spaced-apart grooves. To detect the existence of a foreign particle located inside a narrow groove, it is desirable to tilt the scanning beam with respect to the surface. Generally, a tilt mechanism can be implemented by mechanically tilting either the sample carrier relative to the charged particle beam column (e.g., U.S. Pat. Nos. 5,734,164; 5,894,124; 6,037,589) or the column (e.g., U.S. Pat. No. 5,329,125). The technique of WO 01/45136 achieves a tilt mechanism by affecting the trajectory of the primary electron beam using single- or double-deflection. However, the column's configuration of WO 01/45136 while providing effective detection of secondary electrons with the above-described tilt mode of operation, will be problematic for detecting secondary electrons, especially fast electrons (the so-called HAR mode), when operating with normal incidence of the primary beam, i.e., beam incidence substantially perpendicular to the sample's surface.
SUMMARY OF THE INVENTION
There is accordingly a need in the art to improve inspection of samples with a charged particle beam by providing a novel beam directing method and device, and a charged particle beam apparatus utilizing the same.
The main idea of the present invention consists of providing effective detection of a secondary charged particle beam with a detector, which is made with an opening and has detecting regions outside this opening, and which is accommodated in the path of the primary charged particle beam such that the primary beam propagation axis intersects with the opening, which therefore serves as a primary beam hole. This is the so-called “in-column detector”. The present invention provides for affecting the trajectories of primary and secondary charged particle beams propagating through a beam directing device to cause a desired incidence of the primary charged particle beam onto a sample, and to cause propagation of the secondary beam to a region of the detector outside the primary beam hole.
The term “primary beam” or “primary charged particle beam” used herein signifies a charged particle beam, which is formed by charged particles generated by a source (cathode), and which is to be directed to a sample to knock out charged particles forming a “secondary beam” (also referred to as “secondary charged particle beam”, which is to be detected.
The above is implemented by deflecting the primary beam entering the beam directing device along a first axis of beam propagation, so as to cause the primary beam incidence onto the sample along a second axis that is spaced-apart from the first axis, thereby causing the secondary beam propagation towards a region of the detector outside the primary beam hole.
The present invention enables operation in both the “normal” and “tilt” operational modes without the need for inclining the sample with respect to the charged particle beam apparatus or vice versa. The term “normal mode” used herei
Petrov Igor
Rosenberg Zvika
Applied Materials Inc.
Blakely & Sokoloff, Taylor & Zafman
Hughes James P.
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