Radiant energy – Inspection of solids or liquids by charged particles – Analyte supports
Reexamination Certificate
2003-02-03
2004-10-26
Wells, Nikita (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Analyte supports
C250S289000, C250S310000, C250S3960ML
Reexamination Certificate
active
06809322
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the technical field of electron and ion microscopy as well as to electron and ion beam technologies in general.
BACKGROUND ART
Electron microscopes in general and scanning electron microscopes (SEM) in particular use an electron beam probe to examine specimens. The electron beam requires a good vacuum where it is generated by an electron gun and propagated through focussing lenses all the way to the specimen. In addition, many of the detection means used to detect the emerging signals from the beam-specimen interaction also require a vacuum condition in which the specimen is severely limited. In the past, this meant that only dehydrated specimens could be used. In addition, because the electron beam delivers an electric current, the specimen should generally have a conductive surface to prevent accumulation of charge that hinders normal operation of the instrument. This meant that generally an insulating surface could not be examined. However, the more recent technology of an environmental scanning electron microscope (ESEM) has made it possible to examine specimens in a gaseous environment. The presence of a gaseous envelope around the specimen at sufficient pressure makes it possible to maintain moist conditions so that hydrated specimens can be observed in their natural state. Also, the ionised gas dissipates the electron beam current away from the surface of insulating specimens and, therefore, these specimens need not have the pre-treatments conventionally used to render their surface conductive. Additionally, the gas is used as detection medium to detect the signals generated in the gaseous envelope around the specimen. Such signals are usually the secondary electrons and the backscattered electrons from the specimen, which ionise the surrounding gas and become detected by appropriate means.
This prior art is comprehensively described by U.S. Pat. No. 4,596,928, DE Patent No. 0 022 356 B1, U.S. Pat. No. 4,992,662, U.S. Pat. No. 4,785,182, U.S. Pat. No. 4,823,006, U.S. Pat. No. 4,897,545 and U.S. Pat. No. 5,945,672.
One basic feature of this technology is the use of at least two pressure limiting apertures (PLA) to separate the high vacuum of the electron optics column (where the beam is generated and propagated) from the high pressure environment of the specimen chamber. The gas leaking through the first of the apertures, PLA1, is quickly pumped out of the system before any significant amount of gas leaks through the second of the apertures, PLA2, into the vacuum of the electron optics column.
Now, despite the many and important advantages created with the advent of ESEM over SEM, this technology has its own limitations which are amenable to improvement.
One particular limitation in the ESEM is with respect to the permissible field of view described herein. In a SEM operating totally in vacuum, the electron beam is focussed into a very small probe, which is scanned in a raster form over the specimen area observed, in a similar manner as in an ordinary television set (i.e. a cathode ray tube—CRT). However, the scanned raster over the specimen is very small in absolute terms, which is of the order of a few mm or much less as opposed to the large screen of an ordinary television set. The very small scanned raster is possible to obtain because of the extremely fine electron probe used, which allows, for example, one thousand lines to be stacked in the raster. Through appropriate electronics, a synchronous raster in a CRT creates an image corresponding point by point to the variation of signals emanating from the scanned specimen surface. The ratio of the size of the image on the CRT screen over the size of specimen raster defines the magnification. Thus, the maximum magnification corresponds to the minimum raster on the specimen possible to scan without distortion, noise or blurring (defining the ultimate resolving power of the instrument). At the other extreme, the maximum scanned area over the specimen defines the largest possible field of view (corresponding to the lowest magnification). The largest field of view possible is a useful specification of a microscope, because it is usual to survey a large area before a particular feature of interest is magnified. Now, the maximum field of view in an ESEM is limited by the size of the first pressure limiting aperture used. Usually, a 0.5 mm diameter first pressure limiting aperture is used and hence the field of view is much smaller than in a SEM, where there is no pressure limiting aperture and the field of view is limited by other electron optical considerations, but nevertheless being superior to that of an ESEM.
The limited field of view in an ESEM is a consequence of the use of two pressure limiting apertures in a position, relative to the scanning coils, that creates a collimating effect on the probe within the scanned cone.
Because this is an important limitation, attempts have been disclosed in prior art to rectify the problem. U.S. Pat. No. 5,362,964 describes two approaches. According to the first approach, light optics is used to obtain a light image at low magnification with lenses and mirrors located below the first pressure limiting aperture. With appropriate electronics controls, the operator switches between the light image and electron image at different magnifications. Whilst this provides one solution to the problem, it is characterised by considerable cost, cumbersome usage and technological complexity.
According to the second approach of the same U.S. Pat. No. 5,362,964, a third scan coil is positioned between the two pressure limiting apertures to bend the electron beam below the first pressure limiting aperture beyond the boundary of the first pressure limiting aperture. This should be a better solution than the first because of the apparent simplicity. However, the latter solution is difficult in reality to achieve, because the third scan coil has limited space to operate between the two pressure limiting apertures, it requires a high current and the resulting multi-deflection system is difficult to materialise. This prior art has not yet been implemented on any commercial instrument yet. The lack of such an otherwise useful system from all commercial ESEMs available to date is indicative of the difficulties involved. Notwithstanding the reasons for such a lack, the present invention discloses a much simpler solution to remedy the limited field of view of an ESEM.
Another device of U.S. Pat. No. 5,485,008 also aims at improving the limited field of view in an ESEM. This attempt further demonstrates the need to overcome the disadvantage of field of view. However, a person skilled in the art of ESEM, gas dynamics and electron optics immediately recognises that the device of this patent does not readily achieve the aim either. The increase of the aperture size at the expense of pumping volume and all other suggested improvements would come at a high cost, they would create other technical problems to the system, and no technical reports are known showing the feasibility, implementation, or use of this device. Notwithstanding the reasons for the lack of use of the device as described in this patent, it will be appreciated that the present invention discloses a radically different solution from this device with regard to the remedy of the limited field of view of an ESEM.
Another limitation of an ESEM is that it generally requires large pumping capacity between the apertures as the first pressure limiting aperture is increased in order to increase the field of view.
A further limitation of an ESEM is that it generally requires high accelerating voltage in order to avoid the large electron beam losses taking place above the first pressure limiting aperture where a supersonic gas jet and shock waves inevitably form. A large first pressure limiting aperture creates a catastrophic amount of electron scattering at low accelerating voltage.
There is also a class of SEM instruments, which allow low pressure gas of the order of 100 Pa. This class of SEM is often refe
Bednarek Michael D.
Shaw Pittman LLP
Wells Nikita
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