Detector devices

Details Detector devices Detector devices

1998-12-23

2001-04-03

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiation responsive nonelectric signalling

Luminescent device

C250S483100, C250S310000, C250S397000, C250S399000

Reexamination Certificate

active

06211525

ABSTRACT:

This invention relates to detector devices, and concerns more particularly devices for detecting electrons emanating from the specimen being examined by a scanning electron microscope or the like.
In a conventional light microscope the specimen to be examined is illuminated with visible light—that is, photons with wavelengths in the range to which the human eye is sensitive—and then viewed either by gathering and using the reflected light, to provide information about its surface and shape, in the way that any ordinary object is viewed by the eye, or by gathering and using the light that has been transmitted through it, to disclose details of its internal construction. Magnifying lenses and mirrors can be employed to increase the size of the image, and so make the specimen seem larger, and thus reveal features that the unaided eye cannot see, but inevitably there comes a point—at about 2000 times magnification—where further magnification is impossible because the items to be seen are much the same size as, or even smaller than, the wavelength of the light being used (and so in effect the light travels round them rather than impinging upon them, and without any interaction is unable to reveal anything about them). The “average” wavelength of visible light is around 600 nanometer (0.6 micrometer); by way of illustration, a light microscope can be employed to examine a bacterium or bacillus, because these are relatively large (about 10 micrometer and more in length), but it cannot be utilised to provide clear pictures of viruses, which, at a mere 1 micrometer or less across, are too small to be seen in any fine detail using light.
The electron microscope partially solves this problem by using electrons instead of light to illuminate the specimen. Electrons can behave as a waveform, much like the photons of light, but with a very much shorter wavelength (commonly around 0.001 micrometer and below); they can, therefore, be used to “look at” objects, such as viruses, and details of objects, that are much too small to be seen clearly with a conventional light microscope.
There are two main types of electron microscope. In the first, and older, known as a Transmission Electron Microscope (TEM), the whole of a very thin specimen is illuminated with electrons just as in a light microscope it is illuminated with light—this is rather like turning on a light bulb in a room—and those electrons that pass through it are allowed to fall on a screen which converts them into a visible image. In the second, and newer, type, known as a Scanning Electron Microscope (SEM), which is the sort with which the present invention is primarily concerned, a spot made from a very narrow beam of electrons is scanned in fine strips across the specimen, rather like waving a torch around in a room, and an image is built up from the electrons that emanate from the scanning spot (in the same sort of way that a complete television picture is made up from the light emitted from the TV screen's phosphors as they are scanned in strips with a spot-forming electron beam).
The beam of electrons impinging upon the target—the specimen—in an SEM can give rise to several different varieties of emanating electron. Most obviously there are those electrons that are transmitted through the specimen and are “viewed” from the reverse side; in this mode the microscope is acting as a Scanning Transmission Electron Microscope, or STEM. Next, there are the electrons from the beam that get reflected, or “back-scattered”, off the specimen's surface; the invention is mainly concerned with these. Finally, there are the electrons that originate in the atoms from which the specimen is made, and which have been knocked free from those atoms by collisions with the scanning beam electrons; the freeing of these electrons is known as “secondary electron emission”.
The electron beam can also cause other useful, and usable, energy forms to be emitted by the specimen. For example, some specimens will be of a material that, when struck by electrons, gives out electromagnetic radiation; depending on the material this radiation might take the form of X-rays or it might manifest itself as visible light photons. The emission of such photons is known as Cathodoluminescence Emission (CE).
In order to benefit from the effects of the electron beam the SEM must have some way of gathering the emanating electrons and converting them into a visible image. Moreover, because the intensity of the gathered electrons is inevitably rather low—in simple terms the specimen does not seem very bright—it is common directly or indirectly to amplify either the electrons themselves or the image they form until it is convenient for the human eye to see. One conventional method of achieving this is to cause the electrons to strike a scintillator—a device (or material) that gives off flashes of light when hit by electrons—and then to direct the thus-formed light to a photomultiplier which converts the flashes into significantly large pulses of electricity (“back”, as it were, into electrons, but now in such large quantities that they can be used to drive or control ordinary equipment such as television or cathode ray screens). It is this, the primary detection of the gathered electrons (and specifically the back-scattered electrons) by their conversion into light using a scintillator, and the feeding of the thus-formed light to a photomultiplier tube or the like, with which the invention is primarily concerned.
At the moment one of the more successful detector systems available utilises a solid finger-like light guide (usually of a transparent acrylic plastic, though glass or quartz can also be employed) on the end side surface of which is a layer of scintillator material (typically a phosphor). The finger is mounted by its other end adjacent and projecting from the input screen of a photomultiplier tube (PMT), or the like, orientated with the scintillator layer facing the specimen, and poked into the path of the back-scattered electrons such that they impinge upon the layer to cause it to emit light. This light then passes through the layer into the body of the finger, which guides it (by total internal reflection) along to the far end where it shines out into the PMT.
This type of detector system, which has been in use for several years, is quite good, but nevertheless suffers from a number of disadvantages, of which perhaps the most serious arises from the conflicting requirements for the scintillator layer. The problem is that the layer needs to be thick, and relatively opaque to electrons, so as to have the best chance of capturing, and thus making light pulses from, most of the electrons that hit it (rather than letting them wastefully pass through without generating light), and yet at the same time it is most preferably thin enough to let those very pulses of light pass though it into the body of the light guide finger rather than being absorbed in and attenuated by the layer and so squandered.
It is this problem that the present invention seeks to solve—and to do so it proposes a solution so simple as at first sight to seem obvious and yet which has thus far been completely overlooked. More specifically, in accordance with the invention there is suggested an improved version of the light-directing finger having, instead of the side surface layer of scintillator material at the end to be inserted into the stream of back-scattered electrons, an electron receptor disposed so as to have a face angled as though to reflect received electrons along the finger to the PMT and having the scintillator material coating on that face. It will be appreciated that in this way the PMT is actually “looking at”, and so receiving light directly from, the front, or input side, of the scintillator layer, rather than, as in the Prior Art detector described above, effectively looking at the back, or output side of the layer. Thus, the light that is emitted by the layer does not need to travel through the layer to get to the PMT, and is therefore not attenuated by that layer—and a det

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