Sectored magnetic lens and method of use

Details Sectored magnetic lens and method of use Sectored magnetic lens and method of use

2001-06-15

2003-02-04

Anderson, Bruce (Department: 2881)

Radiant energy

With charged particle beam deflection or focussing

Magnetic lens

C250S398000

Reexamination Certificate

active

06515287

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a magnetic lens that may be configured to apply a magnetic field to a charged particle beam. Certain embodiments relate to a sectored magnetic lens that may be incorporated into a scanning electron microscope system.
2. Description of the Related Art
As the dimensions of semiconductor devices continue to shrink with advances in semiconductor materials and processes, the ability to examine microscopic features and to detect microscopic defects has become increasingly important in the successful fabrication of advanced semiconductor devices. Significant research continues to focus on increasing the resolution limit of metrology tools that are used to examine microscopic features and defects. Optical microscopes generally have an inherent resolution limit of approximately 200 nm and have limited usefulness in current manufacturing processes. Microscopes that utilize electron beams to examine devices, however, may be used to investigate feature sizes as small as, e.g., a few nanometers. Therefore, tools that utilize electron beams to inspect semiconductor devices are increasingly becoming integral to semiconductor fabrication processes. For example, in recent years, scanning electron microscopy has become increasingly popular for the inspection of semiconductor devices. Scanning electron microscopy generally involves scanning an electron beam over a specimen and creating an image of the specimen by detecting electrons that are reflected, scattered, and/or transmitted by the specimen.
The electron optical system of a scanning electron microscope generally includes an electron source, a device or a plurality of devices configured to focus an electron beam generated by an electron source, a detector or a plurality of detectors configured to detect electrons reflected, scattered, or transmitted by the specimen, and a control system. A thermal field emission source may typically be used as an electron source, and the energy of the electron source may be controlled by an emission control electrode and an anode. The electron beam may pass through a magnetic condenser lens configured to collimate the electron beam. An initial deflection system may also be located near the electron source. An initial deflection system may be configured to correct alignment, stigmation and blanking of the beam. Prior to passing through a magnetic objective lens, the beam may also be passed through a beam limiting aperture and one or more electrostatic pre-lens deflectors. The magnetic objective lens may further focus the electron beam to a spot size of, for example, approximately five nanometers. As used herein, the term “spot size” is generally defined as a lateral dimension of an electron beam incident upon a specimen. A magnetic objective lens may typically include a lower pole piece, an intermediate electrode, and an upper pole piece.
An electron beam exiting a magnetic objective lens may be scanned across a specimen. Typically, the electron beam may be scanned in a first direction while the stage supporting the specimen may be moved in a direction perpendicular to the first direction. A plurality of detection systems may be used to detect secondary electrons, back-scattered electrons, and transmitted electrons that may be produced when the electrons contact the specimen. Examples of scanning electron microscope systems are illustrated, for example, in U.S. Pat. No. 4,928,010 to Saito et al., U.S. Pat. No. 5,241,176 to Yonezawa, U.S. Pat. No. 5,502,306 to Meisburger et al., U.S. Pat. No. 5,578,821 to Meisburger et al., U.S. Pat. No. 5,665,968 to Meisburger et al., U.S. Pat. No. 5,717,204 to Meisburger et al., U.S. Pat. No. 5,869,833 to Richardson et al., U.S. Pat. No. 5,872,358 to Todokora et al., and U.S. Pat. No. 5,973,323 to Adler et al., and are incorporated by reference as if fully set forth herein.
The performance of a scanning electron microscope may vary depending on, for example, the capability to focus an electron beam on a small target area. High voltage electrons may penetrate deep into a semiconductor substrate or a portion of a semiconductor formed upon a semiconductor substrate thereby damaging the substrate or the device and rendering it unsuitable as a working device such as an integrated circuit. Therefore, low voltage electron beams may typically used to analyze delicate semiconductor specimens that otherwise might be damaged by high voltage electron sources. The primary factor that reduces resolution in the low acceleration voltage region is blur of the electron beam due to chromatic aberration. Dispersion in the energy of the electron beam emitted from the electron source typically causes chromatic aberration. As such, significant effort has been focused on improving the performance of a scanning electron microscope by enhancing the ability of the magnetic objective lens to reduce chromatic aberrations in an electron beam source especially in low voltage particle beams.
Traditionally, magnetic lenses may be axially symmetric and may produce axially symmetric magnetic potentials and magnetic fields. An example of such a magnetic lens is illustrated, for example, in U.S. Pat. No. 6,002,135 to Veneklasen et al. and is incorporated by reference as if fully set forth herein. A magnetic lens may include an inner pole piece that may have a cylindrical upper portion and a conical lower portion that may be substantially enclosed by an outer pole piece. The outer pole piece may also have a cylindrical upper portion and a conical lower portion corresponding to the inner pole pieces. A solenoidal excitation coil may be disposed between the inner pole piece and the outer pole piece. When a current is applied to the excitation coil, an axial focusing field may be generated within the lens by magnetic flux from the inner and outer pole pieces. The axial focusing field may be used to focus an electron beam. Shielding rings may be arranged between the upper and lower portions of the inner pole piece to reduce the air gap between the pole portions. The shielding rings may also provide a return path for deflection flux that may otherwise radiate through the gap and induce eddy currents in outer pole pieces and excitation coil. Deflection coils may also be included within the lens along the beam path.
Variable axis lenses have also been developed to focus electron beams. Variable axis lenses incorporate supplementary lenses or supplementary deflectors in the magnetic lens to provide some correction of electron beam paths that may be laterally displaced from an optical axis of the lens. The supplementary lenses and deflectors may be energized based on the lateral displacement of the beam path. Although electron beams may be deflected by this lens, astigmation may still be a problem. Therefore, a separate astigmation compensator may also be included in such a lens. Alternatively, an astigmatism-correction deflector system may be arranged within a variable axis lens adjacent the internal surface of the supplementary deflectors. Such deflectors may be constructed of an octapole three-stage coil in which each octapole includes two tetrapole sets. A deflection field coil may be added to one of the tetrapole coil sets of the octapole. An example of a variable axis lens is illustrated in U.S. Pat. No. 5,952,667 to Shimizu and is incorporated by reference as if fully set forth herein. The incorporation of a separate astigmator octapole forces the beam to pass through the center of this octapole. The overall alignment of the lens system, however, may be non-colinear due to the incorporation of such a separate feature. Therefore, complexity of the overall alignment of the system increases when the charged particle beam is forced to pass through successive non-colinear points.
There are, however, several disadvantages to the lens systems described above. For example, axially symmetric lenses may typically suffer from hysteresis, large inductance of the excitation coil, and thermal stability problems. Hysteresis may cause a r

Affiliated with

Also associated with

Rating

Sectored magnetic lens and method of use does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Sectored magnetic lens and method of use, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Sectored magnetic lens and method of use will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3120333