GB2066618A - Electron microscope image recording system - Google Patents

Abstract

In an image recording system, for example an electron microscope, a specimen 44 is scanned by an electron beam and a resulting video signal from a semiconducting photosensitive surface 46, such as a solar cell, in contact with the specimen is displayed on a monitor 52 in synchronism with the scanning. The solar cell detects both electrons and X-rays produced in the specimen. To produce an entirely X-ray image, the specimen is covered with a thin metal foil. <IMAGE>

Description

SPECIFICATION rnproved image recorder When an electron microscope is used in conventional transmission mode, it is essential to use a very thin specimen. This may be difficult; for example filled rubber and polymeric materials are not sectioned easily. This problem can be overcome by use of the technique known as microradlography, in which a conventional electron microscope is used in a non-scanning role. The electron beam is focused at a stationary position on a target of thin metal foil and X-rays generated in the foil diverge and pass through a sample to a photographic film. The spacing of the foil, sample and film determines the magnification of the photographically recorded image. The apparatus is described in a paper by D. A. Hemsley and M.Hayles in the Institute of Physics Conference Series No. 36, 1977, chapter 1, pages 53 to 56.

In the arrangement described, a solar cell is placed between the sample and the photographic film. It is known that such a cell is responsive to Xrays and electrons; the overall electrical response of the cell to the radiation from the sample is measured and used to control the exposure time of the film, but is not directly involved in the image recording process.

According to the invention an image recording system comprises generating means for generating a focused electron beam; a semiconducting photosensitive surface arranged to receive the scanned beam; positioning means for positioning a specimen between the electron beam and said surface in contact with the surface and at the focus of the electron beam; scanning means for causing relative movement in two dimensions between the beam focus and the specimen and photosensitive surface; sensing means for sensing the magnitude of any electrical response of the photosensitive surface as the beam is scanned; and means for recording the magnitude of the response of the surface and the corresponding position of the scanned electron bsam.

The generating means and scanning means may comprise a conventional scanning electron microscope when the beam focus is scanned across the specimen. Alternatively, the beam focus may be fixed and the specimen and photosensitive surface scanned relative to the focal point.

The semiconducting photosensitive surface may be any material which generates electrons in response to an input of electromagnetic radiation in the range of wavelengths including infra red, visible, ultra violet and X-ray radiation, and may be a photovoltaic or photoconductive material.

Usually the surface will be a plane surface of p or n doped silicon. An example of suitable surface is a solar cell.

Usually the sensed response will be used to provide the intensity control signal of a display device such as the video monitor of the electron microscope, the beam of which is scanned in synchronism with the beam of the electron microscope so that an image of the sample is provided; alternatively intensity and position signals can be stored for use at a later time or for analysis by image analysis apparatus.

In the accompanying drawings, a prior at arrangement of microradiographic apparatus is shown in Figure 1. The invention will be described by way of example with reference to: Figure 2, which is a schematic diagram of apparatus according to the present invention; Figure 3 which illustrates, in hiyhly schematic form, three possible image-generating modes according to the present invention; and Figure 4 which is a sketch of a modified imageproducing arrangement.

In conventional microradiography, an image is generated as shown in Figure 1. The electron beam 10 from a scanning electron microscope is limited by an aperture in an aperture plate 12, the aperture diameter being typically 400 microns, and the beam is focused at a stationary position on a metal foil target 14 in which the electron beam generates X-rays which diverge from the foil as indicated by the undulating lines 1 to. The X-rays pass through a specimen 18 and are monitored by a solar cell 20 which is connected by wires 22 to an exposure control device; the X-rays then pass to a photographic emulsion 24 on a carrier film 26 and protected by a sheet of polyethylene 28.

The function of the solar cell is to sense the strength of the X-rays in the diverging beam; the sensor output controls a shutter (not iilustrated) containing the solar cell, which swings out to allow exposure of the photographic film and then swings in again after an exposure time which is appropriate for the X-ray intensity.

In Figure 2, a scanning electron microscope indicated generally by reference 30 comprises an electron gun 32, condenser lenses 34, 36 and final lens and scan coils 38. The electron beam 40 passes through an aperture plate 42 and impinges on a thin plane specimen 44 supported by a plane photosensitive semiconducting material 46, such as a solar cell. The specimen 44 and photosensor 46 are carried by a removable holder 47. One side of the photosensor 46 is connected to earth, the other to a preamplifier, voltage level adjustment and variable gain circuit 48 which supplies a video amplifier 50 connected to a cathode ray tube video monitor 52. A scan generator circuit 54 is connected to the scan coils 38 and to the video monitor 52.

In operation, the electron beam 40 is generated and scanned, as indicated by the arrow 56, over the surface of the specimen; any electrons passing through the specimen or X-rays generated in it causes a voltage to be generated at the solar cell 46 which is amplified and supplied as a brightness control signal to the video monitor 52. The beam in the monitor is scanned in synchronism with the electron beam in the electron microscope 30 to control the position of the monitor beam. Thus an image of the specimen 44 is displayed on the monitor 52.

The solar cell 46 can be stimulated in two basic modes which merge gradually; three distinct image generating systems can be distinguished as shown in Figure 3. The specimen 44 is shown as varying in thickness and the electron beam 40 impinges over the whole surface.

Consider the part of the electron beam 40A falling on the thin part of the specimen. Electrons from the beam pass through the specimen and are received by the solar cell; an electron image of the specimen is displayed by the video monitor. The part of the electron beam 40C falling on the thickest part of the specimen will be entirely converted to X-rays within the specimen, and the X-rays will pass to the solar cell; an X-ray image is provided. At an intermediate thickness, indicated by the part of the beam 40B, both electrons and X-rays reach the solar cell and a I mixed image results.

The type of image depends on specimen thickness, atomic number, and the intensity of the stimulating electron beam; the maximum input and minimum beam diameter are required. As an example, using a Cambridge Stereoscan microscope at 30 kilovolts with a 400 micron final aperture, with the first and second microscope lenses on half scale, a 2000 Angstrom unit thick carbide steel specimen and a Plessey SC4 silicon solar cell, an X-ray image is produced, with a maximum solar cell output voltage of 0.4 volts.

If an entirely X-ray image is required with a thin specimen or a specimen of low atomic number.

the arrangement shown in Figure 4, can be used; the surface of specimen 44 is covered with a thin metal foil 58; for example the foil may be 2 to 4 microns thick. When the electron beam impinges, X-rays are generated in the foil, and pass through the specimen 44 to the solar cell 46.

The appropriate metal and metal thickness can be calculated using the Monte Carlo computer programme.

It is expected that an X-ray will be required in most applications, showing bulk composition of a sample of metal or metal alloys, but an electron image may be useful for studying materials of low atomic number as polymers and biological materials; it is also applicable to the study of thin conducting films, and for detecting pinholes.

Further advantages are that the surface of an insulator can be viewed without the need to apply a conductive coating, and that precise information on size is easily available.

While the inventive apparatus has been described with reference to the production of an image by a directly transmitted beam, a sensor as described can be used to provide back scattered images. Further it is a great advantage that stereo pairs of photographs can be produced simply by tilting the solar cell and the sample while keeping their planes parallel, i.e. by tilting the holder 47, and recording one image at each tilt position; relative rotation of sample and sensor is not involved.

Another advantage is that the image recording method according to the invention does not interfere with the conventional microscope operation; the secondary electron mode image can be recorded in addition to the image produced by use of the solar cell; thus both the surface topography and the internal morphology of a sample can be studied without removal from the microscope.

In general, because the sensor is a solid state device, noise levels are very low, certainly lower than a conventional electron multiplier detector.

While a photovoltaic device has been described, a photoconductive material could also be used as a sensor. A highly polished surface is preferable, otherwise surface defects are incorporated in the image of the object.

Claims (6)

1. An image recording sysem comprising generating means for generating a focused electron beam; a semiconducting photosensitive surface arranged to receive the beam; positioning means for positioning a specimen between the electron beam and said surface in contact with the surface and at the focus of the electron beam: scanning means for causing relative movement in two dimensions between the beam focus and the specimen and photosensitive surface; sensing means for sensing the magnitude of any electrical response of the photosensitive surface as the beam is scanned; and means for recording the magnitude of the response of the surface and the corresponding position of the scanned electron beam.

2. A system according to Claim 1 in which the generating means and the scanning means comprise a scanning electron microscope.

3. A system according to Claim 1 or Claim 2 in which the semiconducting photosensitive surface is a plane surface of p or n doped silicon.

4. A system according to Claim 3 in which the semiconducting photosensitive surface is the surface of a solar cell.

5. A method of recording an image comprising generating a focused electron beam; positioning a specimen at the focus of the beam and in contact with a semiconducting photosensitive surface remote from the beam source; causing relative movement in two dimensions between the focus of the beam and the specimen and photosensitive surface; and recording the magnitude of the response of the photosensitive surface at each relative position of the beam focus and the specimen.

6. An image recording system substantially as hereinbefore described with reference to Figures 2 and 3 or Figures 2 and 4 of the accompanying drawings.