Scanning electron microscope

Details Scanning electron microscope Scanning electron microscope

2002-10-22

2004-12-21

Lee, John R. (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

Electron probe type

C250S397000

Reexamination Certificate

active

06833546

ABSTRACT:

The present invention relates to a scanning electron microscope, and in particular, a scanning electron microscope which uses an imaging gas.
Presently, a number of types of scanning electron microscope are provided, such as the ESEM (Environmental Scanning Electron Microscope), the VPSEM (Variable Pressure Scanning Electron Microscope) and the LPSEM (Low Pressure Scanning Electron Microscope) which operate using an imaging gas to amplify secondary electrons produced by the target sample. Such systems operate by irradiating the target sample with an electron beam which in turn causes the emission of electrons from the sample, which can include the emission of secondary electrons. The secondary electrons are then accelerated towards an anode, which can be used as a detector.
The target sample is placed in a low pressure sample chamber which includes a gas such as water vapour. As the secondary electrons are accelerated towards the detector, the electrons collide with water vapour causing the production of ions and additional electrons. The ions fall back to the surface of the sample and act to compensate for any negative charge which has been induced by the irradiating electron beam. Meanwhile, the additional electrons are accelerated towards the anode, which causes a cascade effect such that for each secondary electron emitted by the sample, a number of electrons are incident on the anode.
By scanning the irradiating electron beam across the sample surface and measuring an appropriate signal, such as the number of electrons incident on the anode at each position, an image of the sample surface can be generated.
However, not all the ions produced operate to neutralise the build up of charge on the sample. Accordingly, a number of ions are typically present in the sample chamber and this can lead to a reduction in the quality of the image generated, for example due to recombination of the ions with electrons, and/or the presence of a space charge within the sample chamber which can lead to a reduction in the acceleration of the electrons towards the anode.
A solution to this has been proposed in U.S. Pat. No. 5,396,067 which describes the addition of a ion collector plate. The ion collector plate is positioned in the sample chamber between a pressure limiting aperture and the sample. The plate is charged to a positive potential below that of the pressure limiting aperture, to ensure amplification between the plate and the sample. As a result, any ions generated within the sample chamber fall back on to and are collected by the ion collector plate.
However, with the ion collector positively charged, not all the ions will be collected and accordingly, the sample will still be subject to a build-up of positive charge. Furthermore, the collector plate includes only a single aperture through which the irradiating beam must pass. This therefore can restrict the field of view to the sample. Furthermore, the presence of the collector plate can obstruct electrons emitted from the sample surface, as well as preventing additional detectors positioned in the sample chamber from monitoring the sample.
An alternative system has been proposed in U.S. Pat. No. 5,466,936 in which an ion detector is provided in the roof of the sample chamber adjacent the secondary electron detector. In this example, ions generated in the region of the secondary electron detector are collected by the ion detector and are then used in the generation of the image signal. This utilises the fact that the ions are generated by the secondary electrons, and accordingly the number of ions incident on the ion detector is also representative of the number of secondary electrons emitted from the sample.
Accordingly, by adding the signal representative of the number of ions collected by the ion collector to the signal obtained from the electron detector can lead to the production of a combined signal which can be used in the generation of an image. However, this combined signal merely increases the magnitude of the signal used by the image processing system. Accordingly, this combined signal does not result in an enhanced image which includes additional information, it is merely stated that the signal-to-noise ratio is improved when compared to an image obtained using secondary electron detection only.
In accordance with a first aspect of the present invention, we provide a scanning electron microscope for imaging a sample, the microscope comprising:
a. a sample chamber containing a gas in which the sample is positioned in use,
b. a bias member which is maintained at a predetermined electrical potential so as to accelerate electrons emitted from the sample; and,
c. an ion collector comprising one or more electrically conductive elongate members extending into a region between the sample and the bias member, the or each elongate member being maintained at a potential below the predetermined electrical potential to thereby collect the ions.
Accordingly, the present invention provides apparatus for reducing the effect of ion build up in the sample chamber of a scanning type electron microscope. This is achieved using a ion collector which is held at a predetermined potential and therefore operates to decouple the field generated by the bias member from the sample. This ensures that electrons emitted from the sample are always accelerated by a constant field between the ion collector and the bias member. Furthermore, excess ions generated within the sample chamber are discharged by the ion collector thereby reducing the effects of electron recombination and charge cloud formation in the sample chamber. In addition to this, because the ion collector is formed from one or more elongate members it therefore has a relatively small cross-sectional area compared to the sample. As a result, the ion collector does not obstruct the irradiating electron beam or prevent secondary electrons being accelerated away from the sample, whilst still allowing the absorption of ions. Furthermore, by using an ion collector with a relatively small cross-sectional area, this ensures that the view of the sample is not blocked from elsewhere within the sample chamber. This allows additional detector equipment, such as X-ray detectors, backscatter electron detectors, and cathode luminescence detectors to be used within the sample chamber.
In one example, an image of the sample is generated by a detection system which uses a sensor coupled to the bias member for determining the number of electrons incident thereon; and, a processing system responsive to the sensor to generate an image of the sample. In this case, because the ion collector decouples the field from the sample, this ensures that the secondary electrons emitted from the sample surface are accelerated towards the bias member thereby causing a cascade effect which ensures a large number of electrons are incident on the bias member. Furthermore, with space charge and recombination effectively diminished, the image generated by the signal obtained from the bias member is greatly enhanced over the image obtained without the ion collector present. This enhancement allows features to be viewed which are not normally visible using this form of detection.
Alternatively, the detection system may also comprise:
i. the ion collector;
ii. a sensor coupled to the ion collector for determining the number of ions collected thereon; and,
iii. a processing system responsive to the sensor to generate an image of the sample.
In general, the image generated by determining the number of ions collected by the ion collector results in an image of similar quality to the enhanced electron image (i.e. an image of far greater quality than is normally detected when only a secondary electron detector is present). Both the enhanced electron image and the ion image allow the features to be viewed which are not normally discernable on images produced by a secondary electron detector.
In accordance with a second aspect of the present invention, we provide a scanning electron microscope for imaging a sample, the microscope comp

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