SUMMARY OF THE PRIOR ART Scanning beam instruments operating at high pressures, may incorporate two or more apertures to achieve differential pumping to maintain a large pressure gradient across the specimen area (chamber) and the electron beam column of the scanning beam instrument.

(See for instance GB 1477458 and GB2186737). Hereafter, these apertures will be referred to as differential apertures.

Some such instruments also incorporate a biased electrode as a detector. Previously used configurations of the electrodes within the scanning beam instruments are described in patents GB2186737, AU-8945284, US 5250808 AU 799433 (EP22356), AU 8817900 (US 4785182), US 4880976, AU 867221 (EP 275306), AU 8778023 (EP 275306), US 5412211, US 4897545, US 4824006 and US 5362964.

According to the above specifications, the electrode may be in the form of a plate, cone, ring or a wire. The electrode, when biased with respect to the specimen, produces an electric field. This performs an important task, namely; It aids electrons, especially low energy electrons, to acquire higher energies. These accelerated electrons and backscattered electrons have sufficient energy to ionise the gases present in the ambience of the specimen. Because ionisation produce's further charges, the signal is amplified.

Normally the biasing electrode is either placed in the immediate vicinity of one of the differential apertures or on the aperture itself (see the specifications cited above). The electrode usually faces the surface of the specimen on which the primary beam of the scanning instrument is incident. Thus, in all of the previous patent specifications the electrode configurations are such that the electric field produced is either parallel to the axis of the primary beam or is concentrically spread in the form of a cylinder or a cone. The biasing electrode may also be configured as the collecting electrode (detector). Then it will detect charge carriers accelerated towards it. The detecting electrodes in all the previous specifications, except that described in (GB 2186737), are positively biased.

They detect a mixture of the varying amounts of secondary

emitted electrons (SE) and backscattered electrons (BSE) and the products of ionisation caused by the passage or the above electrons through the gases of the atmosphere.

A moving electrode has also been described in US 500523.

In the patent specification US 5362964 an additional biased ring'electrode, placed concentric to the electron beam has been used. The ring electrode is also placed facing the specimen. It is claimed that the detection via the ring electrode reduces the noise and optimises the signal amplification of the secondary electrons.

SUMMARY OF THE INVENTION Configuration of the new detector systems, disclosed herein, offer better collection efficiency and therefore larger signals of the various types. They are simpler, more flexible and offer improved control on the manipulation of the signal collection, not only in vacuum, but also at higher pressures. They make efficient use of the additional charge carriers produced by ionisation at higher pressures, surrounding the specimen within the scanning beam instrument.

Electrons and ions being charged particles, their movements can be influenced by both electric and magnetic field. For instance, if we apply an electric field (E) in the Z direction and a magnetic field in the X direction as shown in Fig. 1; then the movement of a particle with

mass m will be influenced by Lorentz force and the equation of the force is given by the equation: m*a =e* (E+ (1/c) *vxB) where, m is the mass of the particle, a is the acceleration of the particle in the x direction, e is the electric charge, E is the electric field vector, c is the velocity of light, v is the velocity of the charged particle and B is the Magnetic field vector.

One can write down similar equations for combinations of electric and magnetic fields in different directions. It is important to note that both the electric field and magnetic fields on their own, and independently of each other, exert forces, so are capable of changing the directions of the charged carriers. When a charged particle is subjected to a n electric field and a magnetic field simultaneously, it will be subject to the combined forces as shown by the equation cited above.

The equation also shows that, when the electric and magnetic fields are orthogonal to each other the forces generated are vectorially additive. The present invention relies on the above effects and makes it possible to change the trajectories of charged particles and direct them specifically towards detector means.

At its most general the invention proposes that one

or more of the following features is provided in a scanning beam instrument, or in a structure for incorporation into a scanning beam instrument in the proximity of the specimen: (a) Detecting means on one or more sides of the specimen with the detecting face substantially perpendicular to the surface of the specimen and parallel to the axis of the beam.

(b) Means for producing an electric field perpendicular to the beam axis and substantially parallel to the specimen surface.

(c) Means for producing a magnetic field that is perpendicular to the beam axis, substantially parallel to the specimen surface (and orthogonal to the electric field, if present); and magnetic shielding to minimise the effects of the magnetic field on the primary beam.

The new arrangement discussed herein provide means of producing an electric field on its own (in configurations not used for this purpose before), or magnetic field on its own, or a combination of the magnetic and electric field in the vicinity of the specimen. This influences the trajectories of the charge carriers, which constitute an image signal.

The detector systems described herein are well suited to be integrated within: (A) instruments in which the specimen is viewed in

high vacuum, and (B) instruments in which specimens can be viewed at pressures relatively much higher than the operational pressures in the electron beam column of the instrument.

Scanning electron microscopes (SEMs) of this kind will be referred to as atmosphere-containing SEMs, but are also sometimes referred to as environmental SEMs.

The three aspects (a) to (c) referred to above may be embodied alone, or in any combination.

Thus, the present invention may provide a scanning beam instrument comprising: a beam column for generating a focussed beam of electrically charged particles along a beam axis, a specimen Chamber for containing a specimen to be scanned having an inlet for the focussed beam; means for causing the focussed beam to scan a surface of the specimen; and means for detecting charged particles from the specimen and the vicinity of the specimen due to the scanning of the surface of the specimen by the focussed beam; wherein: the specimen chamber contains: means and for generating a magnetic field magnetic shielding for containing said magnetic field within the vicinity of the specimen; and

a detector electrode of said means for detecting charged particles.

The present invention may further provide a scanning beam instrument comprising a beam column for generating g a focussed beam of electrically charged particles along a beam axis, a specimen Chamber for containing a specimen to be scanned having an inlet for the focussed beam; means for causing the focussed beam to scan a surface of the specimen; and means for detecting charged particles from the specimen and the vicinity of the specimen due to the scanning of the surface of the specimen by the focussed beam; wherein: the specimen chamber contains: means for generating an electric field substantially parallel to the surface of the specimen; and means for generating magnetic field substantially parallel to the surface of the specimen, the magnetic and electric field being crossed; wherein said means for detecting charged particle includes detector electrode extending substantially perpendicular to the surface of the specimen.

The present invention may also provide a scanning

beam instrument comprising: a beam column for generating a focussed beam of electrically charged particles along a beam axis, means for causing the focussed beam to scan the surface of a specimen contained in a specimen Chamber for containing a specimen to be scanned having an inlet for the focussed beam; means for causing the focussed beam to scan a surface of the specimen; and means for detecting charged particles from the specimen and the vicinity of the specimen due to the scanning of the surface of the specimen by the focussed beam; wherein: the specimen chamber contains: means for generating an electric field substantially parallel to the surface of the specimen; and a detector electrode of said means for detecting charged particles, said detector electrode being substantially perpendicular to the said specimen; wherein the electric field and any magnetic field, if any, in the specimen chamber, is such as to direct charge particles from the specimen and in the vicinity of the specimen to the detector electrode.

As was mentioned above, the aspects (a) to (c) of the present invention discussed above are incorporated

into the scanning beam instrument proximate to the specimen. Thus, the invention is not concerned with features of electric and magnetic fields used in the scanning beam instrument to focus the beam and/or to cause it to scan. A scanning beam instrument will normally incorporate a column comprising a plurality of magnetic and or electrostatic lenses but the present invention is not concerned with such lenses. There will also be plurality of apertures in the parts of the beam column. The column is usually joined to the part containing the specimen which is commonly referred to as the specimen chamber. Any or all of the aspects of the invention may be permanently incorporated proximate the point of focus to form a part of the scanning beam instrument in the same way as the beam column is also a part. However the present invention is not limited to an arrangement in which any or all of the features (a) to (c) referred to above are incorporated on a structure which is non removable from the specimen chamber. The aspects of the invention may be incorporated in a structure which is removable and insertable into the specimen chamber of a scanning beam instrument in a position proximate the specimen such that the signal may be obtained when the beam is focussed on the specimen.

Thus, the invention includes a specimen chamber for a scanning beam instrument,

having means for supporting a specimen; an inlet for a focussed beam to define a beam axis from the inlet to a specimen on said supporting means ; means for detecting charged particles from the specimen and the vicinity of the specimen due to the scanning of the specimen by the focussed beam; wherein: the specimen chamber further contains: means for generating a magnetic field; and magnetic shielding for containing said magnetic field within the vicinity of the specimen the magnetic shielding having an inlet for the focussed beam; and a detector electrode of said means for detecting charged particles.

The invention further includes a specimen chamber for a scanning beam instrument having: means for supporting a specimen ; an inlet for a focussed beam to define a beam axis from the inlet to the specimen on said supporting means; means for detecting charged particles from the specimen and the vicinity of the specimen due to the scanning of the specimen by the focussed beam; wherein: the specimen chamber further contains: means for generating an electric field substantially

parallel to the surface of the specimen; and means for generating a magnetic field substantially parallel to the surface of the specimen, the magnetic and electric fields being crossed; wherein said means for detecting charged particles includes a detector electrode extending perpendicular to the surface of the specimen.

The invention also includes a specimen chamber for a scanning beam instrument, having: means for supporting a specimen; an inlet (Al) for a focussed beam to define a beam axis from the inlet (Al) to the specimen on said supporting means; means for detecting charged particles from the specimen and the vicinity of the specimen due to the scanning of the specimen by the focussed beam; wherein: the specimen chamber contains: means for generating an electric field substantially parallel to the surface of the specimen; and a detector electrode of said means for detecting charged particles, said detector electrode being substantially perpendicular to the surface of said specimen; wherein the electric field, and any magnetic field, if any, in the specimen chamber, is such as to direct charged particles from the specimen and the vicinity of

the specimen to the detector electrode.

The present invention in any or all of its aspects, may be used either in an instrument in which the specimens are viewed at high vacuum or in which the specimens are maintained at a pressure higher than the pressures within the beam column itself. In the latter case there may be a plurality of pressure limiting apertures between the beam column and the sample, although it should be noted that it is possible to form an atmosphere containing scanning beam instrument which contain only one pressure limiting aperture between the electron beam column and the higher pressure region containing the specimen. However the differential apertures referred to previously are preferred. It is also possible to have one or more reservoirs of fluid proximate the sample, e. g. as in GB 2186737.

Where aspect (c) of the invention is used, possibly with one or more of the other aspects of the invention, in a separable structure distinct from the body of the beam column and or the body of the scanning beam instrument, it may include a separate means (e. g. an envelope surrounding the specimen and the other aspects of the invention) providing magnetic shielding.

Alternatively, the walls of the chamber enclosing the embodiment may provide the magnetic shielding means.

Also, where any or all of aspects (a) to (c) are used in

the invention within a separable sample chamber, that sample chamber may be made interchangeable with a plurality of different beam columns.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described in detail, by way of example, with a reference to the accompanying drawings in which: Fig. 1 illustrates the effect of electric and magnetic fields on the charged particle, and has already been described; Figs. 2 (a) to 2 (o) show different instruments of sample, detecting means and field producing means; Fig. 2p shows means for tilting together the specimen and the various means shown in Figs 2 (a) to 2(o); Fig. 3 shows a first embodiment of a scanning beam instrument in which one or more aspects of the invention may be embodied.

Fig. 4 is a second embodiment of a scanning beam instrument in which one or more aspects of the invention may be embodied.

Fig. 5 is a third embodiment of a scanning beam instrument of which one or more aspects of the invention may be embodied.

Fig. 6a, 6b and 6c are schematic diagrams showing magnetic shielding which may be used in embodiments of

the invention; and Fig. 7a, 7b and 7c show schematic arrangements for screening the detector in embodiments of the invention.

DETAILED DESCRIPTION By way of the example only, the embodiments of the invention incorporating different ways of detecting means, together with the placements of means of producing electric field and magnetic fields are described below.

Various electrodes in the description can be either, in a wire, or a block, or a rigid strip/ribbon, or a ring form. Unless otherwise stated, it is preferable to have a strip/ribbon form so that the side with the largest surface area is used either for the purpose of application of an electric field, or for the purpose of detection. The following embodiments, may be an integral part of the scanning beam instrument. They can also be configured as units placed within in the scanning beam instrument when in use. These are schematically described in Fig. 2 (a) to (o) by way of examples only. To make the descriptions below less ambiguous it is assumed that the specimen (6) has a smooth planar surface. Furthermore this specimen surface is assumed to be placed perpendicular to the axis of the beam. The assumptions enable one to better describe the geometrical relationships between the various means of producing electric and magnetic fields with respect to the specimen

placement and the axis of the beam. However, it. should be recognised that, in practice, the specimen surface may not always be smooth.

Fig. 2 (a): (i) A positively or negatively biased electrode, preferably in a thin plate form, with a central aperture for the passage of electrons with its flat side facing the surface of the specimen so that the applied electric field is either parallel or anti- parallel, or concentric to the primary beam axis; and (ii) a detecting electrode, preferably in a strip or a plate form, perpendicular to the surface of the specimen on one side of the specimen. An amplifier is connected to the detecting electrode.

Fig. 2 (b): (i) A positively or negatively biased electrode, preferably in a thin plate form, with a central aperture for the passage of electrons with its flat side facing the surface of the specimen so that the applied electric field is either parallel or anti- parallel, or concentric to the primary beam axis; (ii) Placement of one or more permanent magnets, or electromagnets in the vicinity of the specimen which produces a magnetic field orthogonal to the axis of the primary beam; and (iii) a detecting electrode, preferably in a strip or a plate form, perpendicular to the surface of the specimen on one side of the specimen. An amplifier is connected to the detecting electrode.

Fig. 2 (c): (i) A positively or negatively biased electrode, preferably in a thin plate form, with a central aperture for the passage of electrons with its flat side facing the surface of the specimen so that the applied electric field is either parallel or anti- parallel, or concentric to the primary beam axis; (ii) Placement of one or more permanent magnet, or electromagnets, in the vicinity of the specimen, which produces a magnetic field orthogonal to the axis of the primary beam; and (iii) The specimen, connected to an amplifier so that it itself acts as a detector.

The novel feature of the above systems in Fig. 2 (b) and (c) disclosed herein, is that they use magnetic field and electric field simultaneously. This offers an advantage over the electric configurations previously used. The collecting signal is significantly bigger.

Fig. 2 (d): (i) Placement of permanent magnets, or electromagnets, in the vicinity of the specimen, which produces a magnetic field orthogonal to the axis of the primary beam; and (ii) The specimen, connected to an amplifier so that it itself acts as a detector.

Fig. 2 (e): (i) Placement of permanent magnets, or electromagnets, in the vicinity of the specimen so that it produces a magnetic field orthogonal to the axis of the primary beam; and (ii) a detecting electrode, preferably in a strip or a plate form, and preferably

placed perpendicular to the surface of the specimen on one side of the specimen. An amplifier is then connected to the detecting electrode.

Fig. 2 (f): (i) A biasing electrode placed near the specimen and a detecting electrode placed on the other side of the specimen, diametrically opposite to the biasing electrode. Both electrodes are perpendicular to the surface of the specimen so that the applied electric field is concentrated near the surface of the specimen and is substantially parallel to it. (ii) The specimen, connected to an amplifier so that it itself acts as a detector.

Fig. 2 (g): (i) A biasing electrode placed near the specimen and a detecting electrode placed on the other side of the specimen, diametrically opposite to the biasing electrode. Both electrodes are perpendicular to the surface of the specimen so that the applied electric field is concentrated near the surface of the specimen and is substantially parallel to it. (ii) The detecting electrode is connected to an amplifier.

Fig. 2 (h) : (i) The specimen surface is biased either positively or negatively so that the electric field is parallel to the surface of the specimen (ii) A detecting electrode places on one side of the specimen; collecting face of which being orthogonal to the electric field direction. (iii) The detecting electrode is

connected to an amplifier.

Fig. 2 (i) : (i) The specimen surface is biased either positively or negatively so that the electric field is parallel to the surface of the specimen; (ii) Placement of permanent magnets, or electromagnets in the vicinity of the specimen which produces a magnetic field orthogonal to the axis of the primary beam; (iii) A detecting electrode placed on one side of the specimen; collecting face of which being orthogonal to the electric field direction. (iv) The detecting electrode is connected to an amplifier.

Fig. 2 (j): (i) A biasing electrode placed near the specimen and a detecting electrode placed on the other side of the specimen, diametrically opposite to the biasing electrode. Both electrodes are perpendicular to the surface of the specimen so that the applied electric field is concentrated near the surface of the specimen and is substantially parallel to it. (ii) Placement of permanent magnets, or electromagnet in the vicinity of the specimen which produces a magnetic field orthogonal to the axis of the primary beam; and (iii) The detecting electrode is connected to an amplifier.

Fig. 2 (k): (i) A biasing electrode placed near the specimen and a detecting electrode placed on the other side of the specimen, diametrically opposite to the biasing electrode. Both electrodes are perpendicular to

the surface of the specimen so that the applied electric field is concentrated near the surface of the specimen and is substantially parallel to it. (ii) Placement of permanent magnets, or electromagnets in the vicinity of the specimen which produces a magnetic field orthogonal to the axis of the primary beam; and (iii) The specimen, connected to an amplifier so that it itself acts as a detector.

Fig. 2 (1): (i) A biasing electrode placed near the specimen, which also acts as a detector (i. e. (54) & (56) combined) The field produced is concentrated near the surface of the specimen and is substantially parallel to it. (ii) The combined electrode is connected to a voltage source and a suitable amplifier (80).

Fig. 2 (m): (i) A biasing electrode placed near the specimen, which also acts as a detector (i. e. (54) & (56) combined) The field produced is concentrated near the surface of the specimen and is substantially parallel to it. (ii) The combined electrode is connected to a voltage source and a suitable amplifier (80); and (2) Placement of permanent magnets, or electromagnets, in the vicinity of the specimen which produces a magnetic field orthogonal to the axis of the primary beam.

Fig. 2 (n): (i) A biasing electrode placed near the specimen, perpendicular to the surface of the specimen so that the applied electric field is concentrated near the

surface of the specimen and is substantially parallel to it. (ii) Placement of permanent magnet, or an electromagnet, in the vicinity of the specimen so that it produces a magnetic field orthogonal to the axis of the primary beam; and (iii) The specimen, connected to an amplifier so that it itself acts as a detector.

Fig. 2 (o): (i) A biasing electrode placed near the specimen and a detecting electrode placed on the other r side of the specimen, diametrically opposite to the biasing electrode. Both electrodes are perpendicular to the surface of the specimen so that the applied electric field is concentrated near the surface of the specimen and is substantially parallel to it. (ii) Placement of permanent magnet, or an electromagnet, in the vicinity of the specimen so that it produces a magnetic field orthogonal to the axis of the primary beam ; and (iii) The detecting electrode is connected to an amplifier.

The mode of action of the invention will be described with a special reference to the scanning electron microscope (SEM). In the SEM the primary electron beam, which is focussed, is made incident on the surface of the specimen. As it strikes the specimen there is an interaction of the beam with the specimen. This results in producing emission of electrons (backscattered electrons and secondary electrons) from the specimen.

These electrons, in the presence of gas and an electric

field would produce further charged carriers due to the process of ionisation. The trajectories of these carriers will be under the influence of both, magnetic and electric fields. Any embodiment of this invention may be an integral part of the SEM; or placed as separate units in the SEM, so that the detector parts of the embodiments are in a strategic place within the SEM to intersect the trajectories of the carriers of the selected polarity to deliver a signal for image generation.

The embodiments can work either in vacuum or in the high pressure environment, surrounding the specimen. For the embodiments, which require magnetic field, the invention has a provision for magnetic shielding so that the extent of the path of the primary beam within the instrument is minimally affected by the magnetic field.

Electric shielding is also provided to the detector electrode to minimise pickup of noise.

Practical examples of the integration of the various embodiments in the SEM operating in vacuum and the SEM, which includes differential apertures, to operate at higher pressures. Any of the detector systems from Fig.

2 (a) to 2 (o) can also be placed in the specimen chambers such as those described in GB 1477458 and GB 2186737 Fig. 3 schematically shows a first embodiment of the instrument which may contain any of the arrangements of Figs. 2 (a) to 2 (o). Additionally it may contain

arrangements of Fig. 6 and or Fig. 7. It comprises an electron beam column (2). The column is attached to a specimen chamber (4). In this instrument, the specimen chamber is evacuated along with the beam column to approximately the same pressure, below 10-4 Torr. In the upper part of the column there is a beam source (the electron gun (40)). There are many types of guns including tungsten filament gun, Lanthanum hexaboride gun and field emission guns. The lower part of the column contains lenses (42,44), apertures (46,48) and scanning coils (50) for focussing and scanning the beam. The specimen chamber has provision of inserting the specimen (6) in the specimen chamber and placing it in such a way that the focussed beam (8) from the column becomes incident on the specimen surface.

Fig. 4 is a schematic diagram of a second embodiment of an atmosphere containing scanning beam instrument which may contain any of the arrangement of Figs. 2 (a) to 2 (o). Additionally it may contain arrangements of Fig. 6 and or Fig. 7. As shown in Fig. 4 creation and preservation of the high pressure area is achieved by means of insertion of at least two differentially pumped apertures (Al & A2). However, the actual geometrical configuration of these apertures can be of different form. For example, the apertures may form an integral part of the specimen chamber, as shown in Fig. 4 and in

the embodiments described in (GB2186737). Alternatively, the apertures may be configured completely independently of the specimen stge in a third embodiment as shown in Fig. 5. They are housed in a tube (30)-with appropriate vacuum sealing to separate the volume of the beam column (2) and the volume of specimen chamber-as an attachment protruding in column (2).

This embodiment of the apertures may be completely detachable from the electron beam column. One of the differential apertures used in the embodiment (e. g. Al) may also act as an'objective aperture'of the scanning beam instrument. The space between the apertures (10) (here after referred to as the intermediate chamber) is capable of being pumped, through a duct (12) and appropriate valving, independent from the volume of the beam column (2) and the volume of the rest of the specimen chamber, including region 14, which contains the specimen (6). In this volume (14) there is also a provision of ducting (16) for leaking gases (e. g.

Nitrogen, water vapour) in the chamber with a controlled leak valve. By controlling the pumping rate and the leak rate and suitable choice of apertures, it is possible to maintain a designated pressure within the range 0.1 Torr. to 750 Torr. The ducting for leaking may be connected to a secondary reservoir (18) of water or other liquids. It may contain a large area sponge just underneath the

specimen (6). This reservoir (18), preferably includes ducting (20) and valves (not shown) and means of connecting to a vacuum pump for the purpose of evacuation prior to, or contemporaneous with controlled leaking of air or a gas. For obtaining good signal for image formation, it is desirable to have the working pressure, surrounding the specimen, in the range 0.1-30 Torr.

In Fig. 2 are schematic diagrams of the specimen environs to illustrate the configuration of electrodes and magnet (s) to produce desirable electric and magnetic fields. Earthing provisions are also shown. For the sake of clarity in Fig. 2; the details of the SEM shown in Fig.

3,4 and 5 are omitted. They will be referred to when and if necessary. It must be understood that embodiments depicted in Fig. 2 are situated within the interior of the SEM in the specimen chamber area. The other features such as ; vacuum lead through'terminal blocks for electrical connections (82,84), apertures (Al and A2,) various ducting provisions for evacuating and leaking in, secondary reservoir of water, or other liquids (18) and various embodiments described herein and depicted in Fig.

3,4, and 5, where appropriate can be present and functioning, at the same time as the detector assemblies of Fig. 2. It is not necessary to have all the configurations together. However, the description below will describe how the various elements of the embodiments

for producing the aforementioned fields and collecting signals, can be configured. For configurations in Figs.

2 (a), ( b), and (c), the electrode (50) is mounted directly above the specimen. so that the electron beam can pass through it. Its biasing face is perpendicular to the beam (and the Z-axis). It can be preferably mounted below the aperture A2, concentric to the electron beam axis. The electrode (50)., in the embodiments of the Fig. 2 (a), (b) and (c) acts as the biasing electrode. It produces an electric field when a voltage is applied to it. The magnetic field is produced by the poles (70,72) of a magnet. Its magnitude can be estimated from the use of equations of the type indicated above. The magnitude is preferably in the range 5-50mT. It can be easily produced within the environs of the SEM) As indicated in the diagrams the direction of the magnetic field is orthogonal to the Z-axis and, at the same time, perpendicular to the direction of the electric field if present i. e. in the x-direction. In Figs. 2 (a) and (c), the magnetic field is penetrating in to the plane of the diagrams. The direction of the field may be changed by 180 degrees by swapping the poles, so that the field direction comes out of the plane of diagrams. In embodiments 2 (c), (d), (f), (k), and (n) the specimen itself acts as a detector. In this case a connection is taken from the specimen to an amplifying means (80)-see

Figs. 3,4 and 5 within the specimen chamber via a terminal block (82) and, possibly through, another terminal block (84) outside the SEM. In the configurations of Fig. 2 (a), (b), (e), (g), (h), (i), (j), (1), (m) and (o), a detecting electrode (54) is incorporated. It is preferably placed on the specimen mount but is insulated from the specimen (6) and (electrical) earth. The plane of the detecting face of the electrode is parallel to the Z-axis. In Fig. 2 biasing electrodes (56) as placed parallel to the Z-axis.

In cases where, both biasing electrode (56) and the detecting electrode (54) are used together, they are preferably mounted in the close proximity of the specimen, on the diametrically opposite sides of the specimen (e. g. Y direction). The two electrodes (54) and (56) are facing each other and are perpendicular to the incident surface of the specimen. It is preferable that the relative geometry between the electrodes, the specimen, and the magnetic poles does not change when the specimen is moved relative to the beam. This can be simply achieved by mounting the specimen, electrodes and magnetic poles on a platform. It can be tilted with respect to the beam axis. This is shown schematically in fig. 2 p by way of example only. The positions of the biasing electrode (54) and the detecting electrode (56) could be swapped. The electrodes could also be mounted,

parallel to each other, in X-direction i. e. in positions to orthogonal positions to those shown in the relevant figures. In all the electrodes configurations described, the electric field generated by applying voltage to either electrode and is orthogonal to the beam direction but parallel to surface of the specimen on which the beam is incident. The width of the electrodes should be such that they completely cover the width of the specimen. It is preferable to have the vertical height of the electrodes in such a way that the electric field is highly concentrated above the specimen surface in a volume covering the scanned area of the specimen surface.

In an atmosphere containing SEM, it is desirable to reduce the scattering of the primary beam. In these circumstances, the height of the electrode could be reduced as much as possible. The electric field is preferably produced in any of the following ways: (a) By keeping the detecting electrode near earth potential (including virtual earth configuration) and applying either positive or negative voltage to the other electrode.

(b) Keeping both electrodes electrically insulated from the earth and by connecting a bipolar voltage supply between the electrodes.

(c) Biasing the specimen itself, as shown in Fig. 2 (h) and (i).

The signal for generating image of the specimen could be extracted and amplified in any one or more manners described below: (I) By connecting the specimen (which is insulated from the earth) to an amplifier (80) In this case it is preferable that either the specimen mount is earthed, or an earthed flat ring of a conductor (90) is surrounding the specimen where the specimen (6) cannot be earthed (Fig. 7a). It must be understood that the screening and earthing means can be provided wherever appropriate in any of the embodiments described herein.

(II) By earthing the specimen and connecting the detecting electrode to an amplifier (80).

In order to increase the signal to noise ratio and reduce other'pickups'via the detector, it may be necessary to, electrically, screen (shown as (92) in Fig.

7 (a)) the detector. The part of the screen in the vicinity of the detecting face may preferably be in a grid (or a gauze form) with thin grid bars shown in Fig.

7 b. However, in order to keep the signal yield high, this part of the screen may be removed.

Alternatively, a blanket screening (94), shown in Fig. 7c, may be provided to electrically screen many aspects of the invention together. In such a case the screening may be combined with the magnetic shielding.

For example, the magnetic shielding embodiment (74),

shown in Fig. 6c, may also perform the function of electrical screening (94) in Fig. 7c.

In cases such as those shown in Fig. 2 (1) and (m) the biasing and detecting function is performed by the same electrode (i. e. the same electrode is labelled as (54) and (56) together), it is connected to the voltage source and a suitable amplifier.

It is observed that the application of the electric field via the above configurations of the electrodes may give rise to shift of the image along the direction of the field. In practice, this shift should be such that it can be compensated by a counter movement produced mechanical means and or electromagnetic means.

A big advantage of using these systems is that it gives a much bigger signal to noise ratio than the application of the field via the electrode placed above the specimen. The magnitude of the signal depends upon the following parameters: (i) The distance between the components where the field is applied (i. e. electrodes, or an electrode and the specimen).

(ii) The direction of the field (i. e. where the positive potential is applied) (iii) The magnitude of the potential (iv) The pressure in the system (v) The path length of the primary beam (in high

pressure environment).

In practice, it is found that, depending upon the distance between the biasing electrode from the specimen and the detector, beneficial effects can be achieved by applying the voltages in the range of a few volts to about one Kilovolt. 'However, the actual magnitude of the voltage applied, depends upon the distance between the embodiments between which it is desired to create the field. The important criterion that must be observed is that any of the electric potentials used should be well below the sparking potential. It may be beneficial to coat electrodes in the disclosed embodiments by a thin insulating layer to avoid sparking and short circuiting to the other components of the system.

The various embodiments described herein are found to deliver relatively bigger signals. In some cases, at high pressures, (where the field produced is substantially parallel to the surface of the specimen and perpendicular to the Z-axis) signals as big as 30 times that obtained in GB 2186737 has been produced.

In cases where magnetic field is present, it is desirable to provide a magnetic shield (schematically shown in Fig. 7) to minimise its effect on the primary beam (8). This objective may be achieved in a number of preferred ways.

(I) The provision of a plate as shown in Fig. 6 (a),

made of a magnetic shielding material (for example mu metal), with a central hole (78) to allow the passage of the primary beam on to the specimen. The plate is preferably placed perpendicular to the beam direction.

The area of the plate is such that it covers the top faces of the magnet and the other embodiments including the specimen (6) so that in the space above the plate, the magnetic field is sufficiently weaker to affect the beam minimally. The central hole in the plate is such that it allows the ingress of the beam for viewing the specimen at all magnifications.

(II) The provision of magnetic shields (74), as shown in Fig. 6 (b), in the proximity of the magnetic means so that the magnetic flux lines, which are substantially perpendicular to the beam axis, are confined in a narrow volume just above the surface of the specimen.

(III) Yet another preferable method is to provide an envelope of mu metal to cover a volume surrounding the specimen, the magnet and the electrodes. This envelope has preferably a detachable lid (76) with a suitably narrow hole or aperture (78) to allow the passage of the beam. The magnetic shield can be incorporated in any of the embodiments of a scanning beam instrument.

For example in the embodiment shown in Fig. 3 the entire magnetic shield assembly (74) and (76) may be

inserted so as to surround the specimen and the other aspects of the invention so that the magnetic field is confined to the volume surrounding the specimen and is delineated from the rest of the scanning beam instrument.

The size of the magnetic shield need not be the same as that of the specimen chamber.

When the magnetic shield is incorporated in the embodiment represented by Fig. 3, it need not have vacuum sealing. The volume enclosed by the magnetic shield should have the same degree of vacuum as the beam column and the rest of the volume of the instrument in Fig. 3.

From this point of view the hole in the beam should be sufficiently large to be pumped efficiently to equalise the pressure through the entire instrument; and at the same time sufficiently small so that the leakage of magnetic field is not too large.

When the magnetic shield is incorporated in the embodiment of Fig. 4, it may be used to enclose the volume (14) along with the specimen (6) and the reservoir (18). The shield may integrally incorporate ducting (16) and (20) and the lead through terminal block (82). The lid of the magnetic shield (76) may need to have vacuum sealing to properly separate the high pressure zone volume (14). Also the aperture (A2) may be merged or combined with the hole (78).

For the embodiment by Fig. 5, the magnetic shield

may be configured as described above. However, vacuum sealing may not be necessary because the volume 14 may be kept at a desired pressure, higher than that in the volume (10), and or that in the volume of the beam column (2) without vacuum sealing. Similarly ducting (16) may not be necessarily connected to the actual magnetic shield. It is however desirable in the configurations used in the Figs. 4 and 5 to keep the path length of the primary beam through the high pressure region to a minimum to minimise the beam scattering. The magnetic poles (70) and (72) and the electrodes (54) and (56) are situated within the magnetic shield. Thus, the passage of the primary beam outside this magnetic field shielding envelope is not affected. However, the beam once it enters the envelope will be subject to magnetic and electric forces. This will result in shifting of the image. In practice, this shift should be such that it can be compensated by a counter movement produced by mechanical means and or electromagnetic means. The dimensions of the magnet are such that the magnetic field is confined to roughly the same volume over the specimen surface in which the electric field acts. The entire shield may, preferably, be earthed to obtain beneficial effects of electrical screening.

It is found that a substantial separation of SE from the BSE can best be achieved using embodiments

incorporating the arrangement of Fig. 2 (b), 2 (e), 2 (i), 2 (j) and 2 (m). The polarity of the magnetic field and that of the accelerating electric field, and their magnitudes, can be appropriately adjusted so that SE (which have low energy) are substantially directed towards the collecting electrode. In high vacuum, the detected contrast is similar to that detected by the conventional SE detector e. g. Everhart-Thornley detector.

Thus, the embodiments of this invention can easily replace currently used SE detectors in the high vacuum scanning electron microscope. A further advantage of the detectors of this invention is that they can be can be used for detecting a signal which is substantially a BSE signal. This is effected by simply changing the polarity of the field, i. e. by making the field on the specimen surface negative and sufficiently strong so that the emission of the secondary electrons from the specimen surface is totally suppressed.

These detector arrangements, in the presence of gas take advantage of another beneficiary effect. Additional electrons produced by ionisation caused due to the accelerated electrons also bend towards the detector. On the other hand electrons produced by the primary beam ionisation have their velocity vector more inclined downwards towards the specimen. These electrons therefore bend in the opposite direction to that in which the

emission related electrons produced by ionisation. In the presence of gas, reversing the sign of the electric field causes the ions to move in towards the detector. This results in the reversal of contrast of the image at some value of the voltage applied. If the voltage is further increased than the reversed contrast also correspondingly increases. Thus, this electromagnetic detection system can take full advantage of the amplification due to ionisation. By simply changing the polarity of the potential, it is possible to detect electrons emitted from the specimen and subsequent ionisation.

The collection efficiency in each case can be changed by the angle of the detector surface with respect to the specimen. For instance, it can be positioned so that its detecting surface is parallel to the surface of the specimen. However, the criterion for the maximum signal efficiency is that the detector surface intersects the maximum number of trajectories of the charged particles, which constitutes the signal. This angle can be estimated by calculating the equations of trajectories from the force equations of the type described above. It is found in practice that maximum (or near maximum) efficiency can be achieved by the'90 degree orientation' illustrated in figures 2 (e), (g) and (i). Irrespective of the polarity of the potential applied; the signals detected by the detector systems described herein are

substantially bigger than that obtained by the detection system in GB 1477458 and GB 2186737.

This has a very practical advantage that the degree of amplification required is reduced. So an amplifier of smaller gain can be used. The signal to noise ratio of the amplifier can also be improved. The frequency bandwidth of the amplifier can be in the region of MHz.

This applies even if one is detecting an ion current because the velocities of ions substantially increase due to accelerating fields.

The uses of the detector systems, described herein, are illustrated with scanning electron beam instruments.

The detector systems can also be used in the instruments in which ion beams are used, since in such instruments, specimen ion interaction results in production of charged particles of positive and negative polarity from which signal can be detected.