Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2000-09-08
2003-07-08
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S397000
Reexamination Certificate
active
06590210
ABSTRACT:
DESCRIPTION
This invention relates to a scanning electron microscope (SEM), in particular a scanning electron microscope operating under a slightly elevated pressure, or retrofitting a scanning electron microscope that operates in vacuo for operation with gas in the specimen chamber, and in particular it relates to an improved detection efficiency of such a microscope (i.e., improving the signal-to-noise ratio of the images recorded with it), in particular in operation with a low primary energy.
With a scanning electron microscope that operates under a slightly elevated pressure (pressure SEM), a maximum operating pressure of a few hectopascals to a few kilopascals is usually allowed in the specimen chamber. At this pressure, the primary electrons have only a short mean free path length. Therefore, the microscope column is sealed with respect to the specimen chamber by a pressure stage aperture (or a pressure limiting aperture) through which the primary electron beam enters the specimen chamber. Above this pressure stage aperture, the pressure is reduced by several powers of ten.
The backscatter electrons emitted by the specimen can be detected with a scintillator-light guide combination arranged between the specimen and the pressure stage aperture. An improved resolution, however, is obtained when using the secondary electrons which are emitted by the specimen and can be detected with the help of a collector electrode (PCT Patent application WO 88/09564 A1). The bottom side of the pressure stage aperture is usually designed as a collector electrode or a separate collector electrode may be arranged beneath the pressure stage aperture.
With other scanning electron microscopes which operate under a slightly elevated pressure, the secondary electrons are detected through the opening in the pressure stage aperture in a type of antechamber which is sealed toward the top with respect to the lens by another pressure stage aperture. Here again, a collector electrode is used as the detector for the secondary electrons (PCT Patent application WO 90/04261 A1). Although this design has been tested (G. D. Danilatos, “Design and Construction of an Environmental SEM; Part 4” Scanning, vol. 12 (1990) p. 23), it has not been successful in practice.
Detector systems with collector electrodes have a poor detection sensitivity because of the noise with the subsequent electronic amplification, and therefore they require pre-amplification of the secondary electron signal before it reaches the collector electrode. This pre-amplification takes place with the help of an electric field between the specimen and the collector electrode which accelerates the secondary electrons emitted by the specimen so that they can ionize gas molecules. After colliding with the gas molecules, the secondary electrons thus generated in the gas and the secondary electrons already present previously are accelerated again through the electric field and generate additional secondary electrons in the gas. In this way, a secondary electron cascade is induced by the secondary electrons emitted by the specimen and ultimately reaches the collector electrode. Even when using a light guide with a downstream photomultiplier as the gas scintillation detector, a secondary electron cascade is used as the pre-amplifier.
Despite this cascade pre-amplification, in both cases the signal-to-noise ratio of the images recorded at a slightly elevated pressure are much worse at the same beam amperage than with the images recorded with conventional secondary electron detectors without elevated pressure. Therefore, improving detection efficiency and reducing detector noise of pressure SEMs are important goals when investigating sensitive specimens in particular (e.g., semiconductor components, plastics, biological and medical specimens).
In addition, the use of a low primary energy is also advantageous in investigating sensitive specimens so that less energy is applied to the specimen and damage to the specimen due to the electron beam is limited to a thin surface layer. The pressure SEMs known so far need a secondary electron cascade in the gas for their collector electrode and are therefore unsuitable for operation with a low primary energy (1 keV, for example), in particular in observation of wet specimens. For operation with a low primary energy, the shortest possible gas path between the specimen and the pressure stage aperture above it is necessary, as well as the lowest possible pressure above the pressure stage aperture, because with a lower primary energy, the mean free path length of the primary electrons in the gas also decreases. Under these conditions, however, no satisfactory cascade pre-amplification is possible so that the pressure SEMs known in the past can be used in operation with gas in the specimen chamber for observation of wet specimens only above a primary energy of 3 keV. However, even at 3 keV a signal background is produced due to the large amount of scattered primary electrons, resulting in an even worse signal-to-noise ratio in the images than at a higher primary energy. The same is also true in operation with a low primary energy (1.5 keV, for example) which is used with the pressure SEMs known in the past at a pressure up to approximately 1.5 hPa for the purpose of combatting a charge buildup.
The pressure SEMs known today are also not very suitable for a low beam amperage due to the poor detection efficiency and the time constant of the operational amplifier even when using a collector electrode. When investigating wet specimens at a high magnification in particular, a lower beam amperage and a lower primary energy would be important, however, to prevent local heating and the resulting drying out of the specimen location observed.
In addition, there is a demand for a pressure SEM with a good signal-to-noise ratio which would also be suitable for a low primary energy to permit better reproduction of fine surface structures and to prevent the edge effect.
Preventing a charge buildup in operation with a low primary energy is another important problem which is not solved satisfactorily by today's pressure SEMs because of the resulting poor signal-to-noise ratio. Possible applications for corresponding pressure SEMs include, for example, imaging sensitive plastics, use in electron beam lithography and in metrology equipment, such as that used in the semiconductor industry for automated monitoring in production. Instead of that, the influence of charge buildup is reduced today in other metrology equipment by using backscatter electrons for imaging.
The object of this invention is to provide an improved SEM that operates under a slightly elevated pressure (hereinafter: pressure scanning electron microscope or pressure SEM) which does not have the above disadvantages of traditional pressure SEMs, and in particular to improve the detection efficiency of pressure SEMs, where detection takes place through the pressure stage aperture through which the microscope column is closed with respect to the specimen chamber (or the signal-to-noise ratio of the images recorded with it), in particular in operation with a low primary energy.
This object is achieved by an SEM having the features according to Patent Claim
1
. Advantageous embodiments of this invention are defined in the dependent claims.
This invention is based in general on the idea of improving upon a scanning electron microscope with the features according to the definition of the preamble of Patent Claim
1
so that a high-sensitivity detector under a positive bias with respect to the specimen is used as the detector.
According to this invention, this object is achieved according to a first aspect in particular by the fact that one or more electrode elements (solid electrodes or thin electrode layers) are arranged above the pressure stage aperture and are at a positive potential with respect to the pressure stage aperture, not using a collector electrode as the detector for the secondary electrons generated in the specimen and in the gas but instead using one or
Berman Jack
Buchanan Ingersoll P.C.
Koons, Jr. Robert A.
McWilliams Matthew P.
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