Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Patent
1998-03-09
2000-04-18
Anderson, Bruce C.
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
250310, 250311, H01J 37141
Patent
active
060518391
DESCRIPTION:
BRIEF SUMMARY
1. BACKGROUND OF THE INVENTION
1.1 Field of the Invention
The present invention relates to magnetic lens apparatus comprising magnetic monopoles or magnetic dipoles useful for focusing a beam of charged particles, such as electrons, onto a target in front of the lens. More particularly, the invention concerns magnetic lenses which may be positioned below the specimen in a scanning electron microscope, which have magnetic field strengths on the order of between about 0.5 and 10 Tesla. Disclosed are lenses optimized for high-resolution focusing of an electron beam having an accelerating voltage of between about 10 and about 50,000 V. In a preferred embodiment, the lens apparatus disclosed herein provide the sole focusing lens for a low-voltage (10 to 10,000 V) high-resolution scanning electron microscope. Alternative embodiments employing superconducting coil technologies for producing a magnetic field in such lens apparatus are also disclosed as are methods for their use in variable-probe lithographic etching processes.
1.2 Description of the Related Art
1.2.1 Electron Microscopy
The scanning electron microscope (SEM) is the instrument of choice for the investigation of irregular surfaces both in biology materials sciences and the semiconductor industry. The SEM forms an image by focusing an electron probe onto the surface of the specimen and the image contrast is formed using the secondary electrons or the high energy backscattered electrons which are generated at or near to the surface. Since the depth of focus may be quite large there is no penalty to be paid for deep indentations or sharp projections and as a result the SEM has been used very effectively for the study of such diverse specimens as the surface of cell membranes or semiconductor circuits.
The normal form of the SEM is an instrument which uses electrons of around 30 kV to form the probe. The reason for this choice of voltages is that it is in this range where the electron probe can take on dimensions of relevance to the investigations of these various specimens. Probes can be formed with dimensions of the order of about 0.5 nm, producing effective resolutions in the range of about 0.7 nm.
It should also be noted that these very high resolving powers have been obtained by inserting the specimen inside the magnetic field of the lens which forms the probe and for best operation the specimen is placed at the point of maximum magnetic field at the center of the lens. The maximum magnetic field which is attainable with conventional materials is about 2 or 2.5 T and it is this which limits the focal length which can be obtained with these lenses and in turn it is the focal length which determines the aberration coefficients of the lens.
1.2.2 Unsuitability of High-Voltage Electrons for Delicate Specimens
Unfortunately, there is a penalty to be paid for using high voltage electrons, particularly when biological or other fragile specimens are examined. Such electrons penetrate deeply into specimens of these types to a distance on the order of a micron or two and yet the secondary electrons which form the image are generated only within the top 2 nm or so of the specimen. The consequence of this is that although the probe is small and high resolution can be obtained, the majority of the electrons penetrate deeply into the specimen and cause substantial damage. In the case of biological materials this can cause significant mass loss and even collapse of the specimen. In the case of semiconductors it has the consequence that the area that is being investigated can no longer be used in a working circuit.
1.2.3 Limitations of Conventional Magnetic Lenses
Focusing a beam of charged particles, e.g., electrons, by causing it to pass axially through a magnetic field of symmetrical distribution produced by a current-carrying coil positioned around the beam is well known in the mechanical arts. The focusing of charged particle beams, and particularly electron beams, is of paramount important in the illuminating systems of scanning- and transmission-type elect
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Anderson Bruce C.
ARCH Development Corporation
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