WO2010008307A2

WO2010008307A2 - Electron detection unit and a scanning electron microscope

Info

Publication number: WO2010008307A2
Application number: PCT/PL2009/000073
Authority: WO
Inventors: Witold Slowko
Original assignee: Politechnika Wroclawska
Priority date: 2008-07-14
Filing date: 2009-07-13
Publication date: 2010-01-21
Also published as: WO2010008307A3; PL217173B1; PL385656A1

Abstract

The subject of the invention is a electron detector unit and a scanning electron microscope equipped with this unit, destined particularly for detection of low energy secondary electrons at the pressures range from values lower than 0.1 Pa to values exceeding 1000 Pa in the sample chamber. Electron detector unit, destined particularly for a scanning electron microscope with variable gas pressure P₁ in the sample chamber contains the intermediate chamber (13 ) of a differential vacuum system where the differential pressure P₂ is maintained. The intermediate chamber (13 ) is arranged under the objective lens ( 9 ) of the scanning electron microscope and sealed with a gasket ( 30 ). In a hole made in a side part of the intermediate chamber (13 ), a main light guide ( 11 ) in the form of a bar made of poly methyl methacrylate (PMMA) is fastened hermetically. Inside the intermediate chamber (13 ), a screening sleeve ( 4 ) in a shape of a cut cone is secured with use of insulating insertions. In the neighborhood of the lower hole of the screening tube ( 4 ), an anode ( 5 ) in the form of a metal ring covered with a thin layer of scintillator ( 5 ) a is arranged and electrically separated. In the lower part of the intermediate chamber ( 13 ) is a hole screened by a lower throttling aperture ( 5 ) of small diameter being sealed with an insulating interlay ( 29 ). The screening tube ( 4 ) and anode ( 5 ) and the lower throttling aperture (15 ) are placed coaxially with the electron optical axis EOA along which the electron beam EB passes. On the lower throttling aperture ( 15 ), a repelling aperture ( 31 ) made of metal foil well reflecting light is fastened with use of an insulating interlay ( 29 ). Diameter of the opening of the repelling aperture ( 31 ) is larger than that of the lower throttling aperture (15 ) and its symmetry axis is shifted aside the electron optical axis EOA toward the main light guide (11 ).

Description

Electron detection unit and a scanning electron microscope

The subject of the present invention is an electron detection unit and a scanning electron microscope comprising a corresponding unit, destined particularly for detection of low energy secondary electrons in a range of variable pressures in a sample chamber from values lower than 0.1 Pa to values exceeding 1000 Pa.

From the patent application nr US 2005/0173644A1, by Gnauck P. et al., entitled "Detector for variable pressure areas and an electron microscope comprising a corresponding detector", a detector capable to work at various pressures in the sample chamber is known. This detector is designed similarly to the standard scintillator detector of the Everhart-Thornley type and it is placed in the sample chamber in vicinity of the specimen. In contrast to the Everhart-Thornley detector, the detector discussed comprises a scintillator covered with a conducting layer transparent for light, so it can be applied classically for electron detection in high vacuum or for detection of photons arising in the process of gas ionization at elevated pressures.

A disadvantage of the both mentioned detectors is a relatively narrow range of gas pressures in the sample chamber which they can accept. A detector of the Everhart- Thornley type can work only at pressures below 0.1 Pa because of the high voltage biasing its scintillator. The essence of the Gnauck 's invention has been adaptation of the detector also to photon detection which additionally makes possible electron detection in the pressure range 10 Pa to 100 Pa. in which the Townsend discharge can be intensive enough. However, the range of working pressures is relatively narrow and even in this range the signal intensity and other discharge parameters are varying seriously causing deterioration of the image. Then, there is a need of stabilization of the detector working conditions irrespective of the gas pressure in the sample chamber which should be changed in a much wider range exceeding 1000 Pa, respectively to the sample properties. Besides, gas filling the sample chamber - usually water vapor - is a little effective converter of the electron energy into a light signal, thus the detector output signal is also poor. So the electron detection system demonstrates the need of application of less noisy and more sensitive solutions.

The patent application nr US 2006/0027748 Al by Slόwko W.: Secondary electron detector unit for a scanning electron microscope, is an attempt on elimination of the above disadvantages both in the aspect of acceptable pressures in the sample chamber and stable detection conditions at these pressures. However, the system described there is a two stage system, very complex in vacuum and electrical terms. At the first place this concerns the detection process which is realized at the first stage in the intermediate chamber and consists in collecting and amplifying the electron stream by an electron multiplier in the form of the microsphere plate (MSP). The second detection stage is realized in the low pressure or rather high vacuum zone, and consists in detection of the amplified stream of electrons by a typical detector of the Everhart- Thornley type with a high voltage biased scintillator. The mentioned principles of operation imply arranging two pressure zones inside the detector: the intermediate pressure zone about 10 Pa and the low pressure zone below 0.1 Pa. Both zones demand arranging proper connections with the differential pumping system which is particularly troublesome for the low pressure. However, there is no room for so complex system and its optimization, because a so called working distance between the sample stage and the objective lens is limited as this affects the microscope resolution. A high cost of the detector manufacturing is also a disadvantage to be reduced.

According to the invention, the above demands can be fulfilled by the electron detection unit and a scanning electron microscope containing a corresponding unit, comprising: an intermediate chamber placed in the way of the electron beam to a sample stage and enabling internal gas pressure different than in a sample chamber where emission of detected electrons take place, at least one aperture throttling gas flow to the intermediate chamber, at least one screening sleeve, at least one anode for electron flow and conversion of the electron signal into light signal and at least one photo-detector. The essence of the invention consists in that inside the intermediate chamber the screening sleeve and anode are placed, that said screening sleeve and anode show rotational symmetry at least in the part turned toward the lower throttling aperture and together with the opening in the lower throttling aperture are disposed on a common electron optical axis along which electron beam goes, that an input window of at least one photo detector set is arranged near the anode.

The electron detection unit has advantageously the photo detector set containing at least one photoniultiplier. The electron detection unit advantageously contains the photo detector set with at least one semiconductor photo detector.

The electron detection unit has advantageously a main light pipe leading toward the anode connected to the photomultiplier. The electron detection unit has advantageously a light pipe protrusion with the front surface turned toward the sample stage and arranged in surroundings of the electron optical axis.

The electron detection unit has advantageously a scintillator layer arranged on the front surface the light pipe protrusion and covered with thin conductive film. The electron detection unit has advantageously a cover with opening attached movably between the scintillator layer and the sample stage for screening the scintillator layer against electron bombardment.

The electron detection unit has advantageously the main light pipe and/or the light pipe protrusion having a hole inside which the screening sleeve and anode are arranged.

The electron detection unit advantageously contains a fragment of the main light pipe surface and/or a fragment of the light pipe protrusion surface covered with thin light reflecting layers.

The electron detection unit has advantageously a fragment of the main light pipe surface and/or a fragment of the light pipe protrusion surface surrounded with a foil of high light reflectivity.

The electron detection unit has advantageously the anode covered with a thin layer of scintillator.

The electron detection unit advantageously has the anode ring-shaped, made of a conductive material and fixed on the lower throttling aperture with use of an insulating separator.

The electron detection unit has advantageously the anode made in the form of a thin conductive layer transparent for light, deposited on the side wall of the light pipe hole. The electron detection unit has advantageously the screening sleeve and the anode made in the form of thin layers deposited on the inner and outer surface of the insulating sleeve. The electron detection unit has advantageously the screening sleeve arranged in the form similar to a truncated cone with the smaller opening turned toward the lower throttling aperture.

The electron detection unit has advantageously the screening sleeve showing deviations in the rotational symmetry of shape and position of the lower opening with respect to the electron optical axis lesser than 10% of said lower opening diameter.

The electron detection unit has advantageously a diameter of the lower throttling aperture as well as a distance of the lower throttling aperture from the screening sleeve smaller than the diameter of the lower opening of the screening sleeve. The electron detection unit has advantageously a reflecting aperture arranged between the lower throttling aperture and the anode.

The electron detection unit has advantageously the intermediate chamber connected immovably with the objective lens.

The electron detection unit has advantageously the intermediate chamber connected movably with the objective lens and is equipped with a desired position blockage.

The electron detection unit has advantageously more than one photo detector set.

The electron detection unit has advantageously more than one electron detector system. The electron detection unit has advantageously the intermediate chamber with an opening in the front wall in which the scintillator and the auxiliary light pipe leading to the photomultiplier are arranged.

The electron detection unit has advantageously both the main light pipe and the auxiliary light pipe connected to the same photomultiplier. The electron detection unit has advantageously the inlet valve with an actuator for opening or closing the gas and electron flow, arranged in the hole in the front wall of the intermediate chamber.

The electron detection unit has advantageously the intermediate chamber with an opening in the upper wall positioned on the electron optical axis, in which the upper throttling aperture is fixed hermetically.

The electron detection unit has advantageously a gas feed to the region of the light pipe protrusion arranged on the lower surface of the intermediate chamber. The electron detection unit has advantageously the screening aperture divided into two segments at least, which are mutually isolated and have individual electrical outlets, arranged beneath the lower throttling aperture.

The electron detection unit has advantageously at least one semiconductor backscattered electron detector BSE arranged on the face of the light pipe protrusion.

The essence of the invention consists in that a scanning electron microscope comprising an objective lens, a sample chamber and a sample stage is equipped with the electron detection unit where inside the intermediate chamber the screening sleeve and anode are placed, that said screening sleeve and anode show rotational symmetry at least in the part turned toward the lower throttling aperture and together with the opening in the lower throttling aperture are disposed on a common electron optical axis along which electron beam goes, that an input window of at least one photo detector set is arranged near the anode.

The microscope has advantageously a lens throttling aperture with the opening diameter less than 1 mm fixed hermetically in the bore of the objective lens.

The microscope has advantageously the intermediate chamber connected immovably with the objective lens.

The microscope has advantageously the intermediate chamber connected movably with the objective lens and equipped with a desired position blockage. An advantage of the electron detection unit according to the invention, is its capability of working in a wide range of gas pressures in the sample chamber, from pressures below 0,1 Pa to pressures exceeding 1000 Pa. The electron detection unit demonstrates a relatively simple structure enabling its miniaturization and integration with electron detectors of other types into a single unit, showing high sensitivity and a low noise level both for secondary and backscattered electron detection. The electron detection unit according to the invention, can be combined with an autonomous intermediate vacuum system and applied in a standard scanning electron microscope of the high vacuum type, as a kind of an additional equipment. In this case it enables widening of the microscope capabilities to higher gas pressures for examining dielectrics or biological samples at their natural state.

The object of the present invention is depicted in the drawing in which, Fig. 1 shows in a vertical section, the electron detection unit with a cylindrical screening sleeve and an upper throttling aperture, as well as some parts of a scanning electron microscope to which the detector has been fastened, - Fig. Ia shows in a vertical section, the electron detection unit with a cylindrical screening sleeve and a lens throttling aperture, as well as some parts of a scanning electron microscope to which the detector has been fastened, - Fig. 2 shows in a horizontal section the electron detection unit integrated with a scintillation electron detector of the Everhart-Thornley type, as well as some parts of a scanning electron microscope to which the detector has been fastened, - Fig. 3 shows in a vertical section, the electron detection unit with a funnel- shaped screening sleeve, - Fig. 4 shows in a vertical section, a magnified fragment of the electron detection unit with an anode deposited on a side wall of a light pipe hole, - Fig. 5 shows in a vertical section and in a view from a Z direction, a magnified fragment of the electron detection unit with an anode placed on a lower throttling aperture, - Fig. 6 shows in a vertical section, the electron detection unit with a miniaturized intermediate chamber, - Fig. 7 shows in a vertical section and in a view from a Z direction, the electron detection unit with a semiconductor photo-detector and a semiconductor backscattered electron (BSE) detector, and Fig. 7a shows in a vertical section and in a view from a Z direction, the electron detection unit with two semiconductor photo-detectors and four semiconductor backscattered electron (BSE) detectors. Embodiment 1.

The electron detection unit shown in Fig. 1 and Fig. 2, is arranged inside a intermediate chamber 13 constituting a part of the differential vacuum system. The intermediate chamber 13 is placed under the objective lens 9 of the microscope and sealed with the gasket 10 in an upper wall of the intermediate chamber 13, where also the upper throttling aperture 8 with a small hole on the electron optical axis EOA is fixed hermetically. Inside the intermediate chamber 13, the main light pipe 11 made of poly methyl methacrylate in the form of a bar with a cylindrical light pipe protrusion 11a at the electron optical axis EOA sector, is arranged. The light pipe protrusion 1 Ia is sealed with the light pipe gasket 3 in the hole made in the lower wall of the intermediate chamber 13. An initial sector of the main light pipe 11 is covered with a thin layer of aluminum constituting the light reflecting layer 12. At the electron optical axis EOA, the main light pipe 11 with the light pipe protrusion 11a have the light pipe hole l ib, and the holder 6 in the form of a plate with a hole, fastened on a chamfered upper side of the main light pipe 11. In the hole of the holder 6, the screening sleeve 4 in the form of a light-wall metal tube is centered on the electron optical axis EOA with use of the insulating sleeve 7. A conductive layer constituting anode 5 covered with a thin layer of scintillator is arranged on the outer surface of the insulating sleeve 7. The surface of the anode 5 is smooth and well reflecting light. The light pipe hole 1 Ib is shut out by the lower throttling aperture 15 in the form of a metal foil disc with a small opening, glued hermetically at the face of the light pipe protrusion 11a. At the face of the light pipe protrusion 11a, the scintillator layer 2 in the form of a plate having an opening at the electron optical axis EOA is also arranged, and on its surface the screening aperture 1 is coaxially deposited in the form of a thin aluminum layer. On a lower surface of the intermediate chamber 13, the plate 14 is fastened in such way that a small gap is left. The plate 14 is equipped with the gas feed G and sealed along its edges to the intermediate chamber 13 body, except the edge positioned on the verge of the light pipe protrusion 11a.

The intermediate chamber 13 is connected to a pumping system by means of the connecting pipe 24 with a nipple 17 and vacuum lines. The connecting pipe 24 is fastened movably to a wall of the sample chamber 25 by means of the sealing block 26. The inside of the intermediate chamber 13 kept at an intermediate pressure P₂, is arranged in the form of three cylindrical bores slightly overlapping each other at their sides. In the central bore, the main light pipe 11 is placed. The main light pipe 11 leads to the photo multiplier 27 which is fixed vacuum-tightly at the end of the connecting pipe 24 in a proper casing. One of the side bores passes through a frontal wall of the intermediate chamber 13. Inside this bore, a standard scintillator electron detector composed of the scintillator 21 connected to the auxiliary light pipe 19 and the fixing insulator 20 fastening these elements vacuum-tightly in the bore. The auxiliary light pipe 19 leads to the photo multiplier 27. In the opening of the side bore, the inlet valve 22 is arranged giving a possibility to be opened or closed thanks to the pusher 23 with the spring 18. The electron detection unit arranged in the described way works as follows.

The electron beam EB runs along the electron optical axis EOA of a scanning electron microscope column and passes through successive apertures of the electron detection unit hitting a surface of the specimen placed on the sample stage 16, which causes emission of secondary electrons SE as well as backscattered electrons BSE. Secondary electrons SE have low initial energies, so they are attracted and focused by an extracting electric field produced by the lower throttling aperture 15 and the screening aperture 1 biased properly. The mentioned electrodes are arranged in the light pipe protrusion 11a which is sealed in a hole made at the corner of the intermediate chamber 13. This location enables the sample stage 16 tilting in a great range of angles despite a small distance from the lower throttling aperture 15.

At the elevated pressure Pj maintained in the sample chamber of the microscope, secondary electrons SE experience multiple collisions with gas molecules, which leads to their scattering and loses on the apertures. However, when the voltage biasing the lower throttling aperture 15 is high enough, there are many ionization collisions causing electron multiplication and amplification of the electron signal as a result. Positive ions arising in the process flow to a specimen placed on the sample stage 16 and to the screening aperture 1 in a proportion depending on the screening aperture 1 bias. The bias voltage should take low positive or negative values, depending on a type of the sample. The screening aperture 1 also ensure such an electric field distribution in the space between the sample stage 16 and the lower throttling aperture 15, that electron flow in this space tends to be focused on the electron optical axis EOA. These electrons that have reached an opening of the lower throttling aperture 15 are attracted by the electric field formed thanks to electrodes arranged inside the light pipe hole l ib and biased with voltages depending on a gas pressure. The mentioned electrodes, namely: the lower throttling aperture 15, the screening sleeve 4 and the anode 5 create a system of the electrostatic electron lens which can influence also the electron beam EB. The electric field produced by the electrodes should not cause astigmatic deformations of the electron beam EB, thus the electrodes should show a rotational symmetry with respect to the electron optical axis EOA. The main task of the electrode system is a possibly efficient conversion of the electron signal into a light signal subject to the photo- detection in a further part. In order to transport electrons efficiently through the lower throttling aperture 15 toward the anode 5, the electric field intensity in the aperture opening should be possibly high. Advantageously, in the surroundings of the lower throttling aperture 15, the electric field on the intermediate chamber 13 side should be stronger than that on the sample chamber side. For this purpose it is advantageous to bias the screening sleeve 4 with a voltage similar to that biasing the lower throttling aperture 15, i.e. an order of magnitude of 100 V, while the anode 5 bias should be possibly high, advantageously higher than 1000 V. In these conditions and at an intermediate pressure P₂ present in the light pipe hole l ib, the electron flow to the anode 5 leads to ionizing and exciting gas molecules and to photons generation during their relaxations. Additionally, the anode 5 is covered with a thin layer of scintillator which emits a light signal due to bombardment by electrons that preserved necessary high kinetic energies. This increases the total efficiency of conversion of the electron signal into the light one. Here, the photo-detector unit is basically composed of the photomultiplier 27 known as a most sensitive photo device, and the main light pipe 11 with the light pipe protrusion 11a. In this case, the input window of the photo-detector unit is defined by the surface through which the light signal penetrates into the main light pipe 11, i.e. the lateral surfaces of the light pipe hole 1 Ib in the main light pipe 11 and the light pipe protrusion 1 Ia. A region of the light generation is placed inside the main light pipe 11 and the light pipe protrusion 11a which gives a large area input window of the photo-detector unit covering almost a full solid angle of the light signal diffusion. Thus, the light signal is introduced very efficiently to the main light pipe 11 and transported to the photomultiplier 27, where it is once more converted into an electric signal and amplified. Efficiency of the light signal transport is additionally increased thanks to the reflective layer of a high reflection coefficient, covering an outer surface of the main light pipe 11 in the surroundings of the light pipe hole 1 Ib. It is also advantageous when the holder 6 and the lower throttling aperture 15 have a smooth and well light reflecting surface.

The electron detection unit can also work when high vacuum is present both in the sample chamber and the intermediate one. In this case, the bias voltage of the screening sleeve 4 should be lowered to negative values to create an electron mirror reflecting electrons, while the anode bias voltage is to be enlarged to increase efficiency of the conversion of an electron kinetic energy into the light energy which occurs in the scintillator layer covering the anode 5. The light signal is introduced into the main light pipe 11 and transported to the photo-multiplier 27 to be converted back into an electric signal and amplified.

The described electron detection unit also enables detection of backscattered electrons BSE similarly to the Robinson detector. For this purpose, the scintillator layer 2 is arranged on a head of the light pipe protrusion l la. The scintillator layer 2 is covered with a aluminum layer thin enough to be transparent for backscattered electrons BSE but carrying away the electric charge induced by them. Backscattered electrons BSE penetrating the scintillator layer 2 generate the light signal flowing to the light pipe protrusion l la and the main light pipe 11 and next being transported to the photo- multiplier 27 where it is converted back into an electric signal and amplified. In the main light pipe 11, the light signal generated by backscattered electrons BSE is added to the one generated by secondary electrons SE. Hence, the bias of electrodes arranged in the light pipe hole l ib should be put off, to obtain a sole backscattered electron signal. In turn, a dominant signal of secondary electrons SE can be obtained when bias voltages on the electrodes arranged in the light pipe hole 1 Ib are increased properly to have the SE signal amplified enough. In the case when the dominant SE signal should be free from the BSE signal component, it is advantageous to put a cover with an opening at the electron optical axis EOA on the scintillator plate 2. Advantageously, the cover is movable and can be slid over the scintillator plate 2 with use of an actuating mechanism with a remote control.

The lower throttling aperture 15 plays a double role: the role of the electrode of an electrostatic electron lens and that of the component part limiting gas flow from the sample chamber with an elevated pressure P₁ exceeding frequently 1000 Pa, to the intermediate chamber 13 with the intermediate pressure P₂ about 10 Pa usually. The upper throttling aperture 8 plays a similar function to the lower one, and it limits gas flow from the intermediate chamber 13 to the electron column region with a low pressure P₃ below 0.1 Pa.

On the verge of the the scintillator plate 2, a gap arranged between a lower surface of the intermediate chamber 13 and the plate 14 serves for the gas injection G toward the sample stage 16. It is particularly useful for biological samples containing water and observed on the sample stage 16 cooled to suppress a vapor pressure. The water vapor injection directly at the investigated place enables to attain a state of the vapor-liquid dynamic equilibrium, even though more distant regions of the stage have a temperature lower than the sample and cause condensation of the vapor.

The electron detector arranged in the central part of the intermediate chamber 13 and based on the main light pipe 11, is destined for work in low vacuum conditions. However the described electron detection unit can work very efficiently also in high vacuum conditions, thanks to a classical scintillator detector of the Everhart-Thornley type arranged in a side bore of the intermediate chamber 13. In this case, the intermediate chamber 13 should be shifted a few centimeters aside the electron optical axis EOA to facilitate access of secondary electrons SE emitted from the sample to the scintillator 21. This is possible thanks to a movable fixing of the connecting pipe 24 in the sealing block 26, and a proper shifting gear equipped with a desired position blockage. The scintillator 21 metallized and biased with a high voltage about 10 kV generates an electric field which flows out through the bore in a frontal wall of the intermediate chamber 13 and attracts secondary electrons SE. A stream of secondary electrons SE is converted into a light signal in the scintillator 21 and transported through the auxiliary light pipe 19 to the photomultiplier 27 where it is once more converted into an electric signal and amplified. In this case, the voltages biasing the electron detector in the main light pipe 11 are put off, so this part of the electron detection unit does not introduce any own signal to the common photomultiplier 27. The fixing insulator 20 is sealed in the bore of the intermediate chamber 13 and serves for preventing from electric discharge but also is used for sealing and fixing the auxiliary light pipe 19 and the scintillator 21 in a proper position. The sealing mentioned above prevents from gas leakage from the sample chamber to the intermediate chamber 13 in low vacuum conditions, when the low vacuum electron detector arranged in the main light pipe 11 is working. However, the gaseous atmosphere present in the sample chamber may be harmful to the scintillator 21 of the high vacuum detector. To protect against such gas influence, an application of the inlet valve 22 is advantageous. The inlet valve 22 connected to the pusher 23 with the spring 18 have a fixed position with respect to the electron optical axis EOA, however with a minute margin of shift in a range of mm for better fitting while the valve is being closed. As a result, when the intermediate chamber 13 is shifted aside the electron optical axis EOA, the inlet valve 22 and access of secondary electrons SE to the detector is opened and vice versa, the inlet valve 22 gets closed hermetically when the intermediate chamber 13 comes back to the initial position.

The electron detection unit is connected through the nipple 17 to an autonomous intermediate vacuum system containing a vacuum pomp, vacuum meters and vacuum valves. The unit arranged in the form of an additional equipment for a standard high vacuum scanning electron microscope is mounted to the sample chamber wall 25. In this case, the unit enables an extension of capabilities of the microscope toward elevated pressures exceeding 1000 Pa for examination of dielectrics or biological samples in a natural state. Embodiment 2.

The electron detection unit shown in Fig. Ia and Fig 2 is arranged similarly as the first embodiment, with such differences that in the in the bore of the objective lens 9, the lens throttling aperture 8 a with a small opening at the electron optical axis EOA is fixed hermetically instead the upper throttling aperture 8. This time, a foil showing high reflectivity is used as the reflecting layer 12 surrounding an initial part of the main light pipe 11. At the electron optical axis EOA in the hole of the holder 6, the insulating sleeve 7 is mounted, inside which the screening sleeve 4 made in a form of a thin conductive layer is arranged. Besides, the scintillator plate 2 is screened with the movable cover If in the form of a hood with an opening at the electron optical axis EOA.

The electron detection unit for a scanning electron microscope arranged in the described way works analogously as it was explained in the first embodiment, with such a difference that the lens throttling aperture 8a of a very small diameter can be placed in a proper principal plane of the objective lens 9, which enables effective throttling the gas flow without serious limitations of the microscope view field. Simultaneously it may also fulfill a role of the microscope main aperture. In turn, the cover If screening the scintillator plate 2 fully eliminates participation of the backscattered electrons BSE signal from the final light signal transported by the main light pipe 11. The cover If is fixed movably at the head of the light pipe protrusion 1 Ia it may be removed in order to expose the scintillator plate to an influence of the backscattered electrons BSE if information contained in this signal is desired. An opening in the cover If ensures flow of the electron beam EB to the sample stage 16 as well as the flow of secondary electrons SE generated there, toward the lower throttling aperture 15. Advantageously, the cover If is arranged in the form of a movable aperture shifted with use of an actuating mechanism with a remote control.

Embodiment s. The electron detection unit shown in Fig. 3 is arranged similarly as the first embodiment, with such differences that the screening sleeve 4 is arranged in the form similar to a truncated cone, advantageously a funnel, with the smaller opening turned toward the lower throttling aperture 15 and kept at a mutual distance comparable to a diameter of the smaller opening of the screening sleeve 4, or less than the diameter. Besides, the anode 5 is placed closely to the edge of the screening sleeve 4 and has the shape of a ring. The insulating sleeve 7 has a similarly reduced size, or advantageously can be eliminated at all if a space between the anode 5 and the screening sleeve 4 is large enough. It is also advantageous if the diameter of the lower throttling aperture 15 as well as the distance of the lower throttling aperture 15 from the screening sleeve 4 are smaller than the diameter of the lower opening of the screening sleeve 4.

The electron detection unit for a scanning electron microscope arranged in the described way works analogously as it was described in the first embodiment. The only differences come from the changed shape and size of electrodes arranged in the light pipe hole l ib. They have a quantitative character rather than a qualitative one and concern a gas pressure and an electric field distribution between the anode 5, the screening sleeve 4 and the lower throttling aperture 15. When the electrodes are brought closer together, the electric field in the opening of the lower throttling aperture 15 gets stronger, though a strength of influence on the electron beam EB exerted by the electron lens created by this field does not increase. As a result, a transport efficiency of secondary electrons through the lower throttling aperture 15 and a signal noise ratio S/N can be improved.

On the other hand, a properly shaped opening in the lower throttling aperture 15 is a nozzle, through which a stream of gas is blown into the lower opening of the screening sleeve 4. It creates a system of the classic ejector pump, which suck in gas from neighborhoods of the gap between the two electrodes. As a result, a local pressure drop appears around the anode 5, which enables an anode voltage increase leading to the rise of efficiency of the electron signal conversion into the light signal in a thin layer of the scintillator covering the anode 5. Embodiment 4.

The electron detection unit shown in Fig. 4 as a fragment covering the light pipe protrusion 11a region, is arranged similarly as the third embodiment, with such a difference that the anode 5 is arranged in the form of a ring shaped layer being conductive and transparent for light. The anode layer is deposited on a side wall of the light pipe hole l ib in the light pipe protrusion 11a, leaving a proper distance to the lower throttling aperture 15. The anode 5 surface is covered with the thin scintillator layer 5a, advantageously transparent for light. A lower opening of the screening sleeve 4, an opening of the lower throttling aperture 15 and the anode 5 ring as well as an opening of the screening aperture 1 are positioned coaxially on the common axis EOA, along which the electron beam EB runs, similarly as in the first embodiment, the second embodiment and the third embodiment. The electron detection unit arranged in the described way works analogously as it was described in the first embodiment, the second embodiment and the third embodiment. The only differences come from the changed form of the anode 5 arranged in the light pipe hole l ib. Also in this case, the anode 5 biased with a relatively high voltage exceeding 1 kV produces a strong electric field showing a rotational symmetry, which causes effective extraction of electrons through an opening of the lower throttling aperture 15. On the way to the anode 5, the electrons experience collisions with gas molecules causing their excitations. A light signal arises as an effect of relaxations of the excitations and penetrates the light pipe protrusion 1 Ia as well as the main light pipe 11 reaching finally the photomultiplier 27, where it is converted back into an electric signal and amplified. On the other hand, the anode 5 surface is covered with a thin scintillator layer 5 a which also generates a light signal under bombardment by electrons with energies high enough. Accessible solid state scintillators have much higher light efficiencies than gases like air or water, then it is advantageous to convert into light a possibly great part of the electron energy in the scintillator layer 5 a. The light signal generated in the scintillator layer 5a passes through a transparent anode 5 to the light pipe protrusion 11a and the main light pipe 11 reaching finally the photomultiplier 27, where it is also converted back into an electric signal and amplified. This increases the summary efficiency of the electron signal conversion into the light signal particularly when the scintillator layer 5a is also transparent for light and does not cause losses of this part of the signal which is generated in gas.

Embodiment 5. The electron detection unit shown in Fig. 5 as a part section and a local view from Z direction of the fragment covering the light pipe protrusion 11a region, is arranged similarly as the third and forth embodiment, with such difference that the anode 5 is made of a conductive material in the annular form and fixed on the lower throttling aperture 15 with use of the insulating separator 29. The anode 5 is covered with a thin scintillator layer 5a. The annular anode 5 has the diameter of its opening greater than that of the lower throttling aperture 15 and positioned coaxially both with the lower throttling aperture 15 and the screening sleeve 4 on the electron optical axis EOA, like the former embodiments. It is advantageous when the anode 5 is deposited on the insulating separator 29 in the form of a thin light reflecting layer. Unlike the embodiments described formerly, the screening aperture 1 is deposited on the scintillator layer 2 as a thin conductive layer of the many folded rotational symmetry shape, advantageously with a circular contour, divided into four equal segments disposed at 90° intervals symmetrically around the electron optical axis EOA. Segments of the screening aperture 1 are electrically isolated and equipped with individual electrical leads.

The electron detection unit arranged in the described way works analogously as those explained in the first, second, third and forth embodiment. Main differences result from the changed design and placement of the anode 5, fixed on the surface of the lower throttling aperture 15 with use of the insulating separator 29. Also in this case, the anode 5 is biased with a high voltage exceeding 1 IcV, and produces a strong electric field causing extraction of electrons through the opening of the lower throttling aperture 15. However, a location of the anode 5 on the lower throttling aperture causes that the most intensive electric field occurs in surroundings of the lower throttling aperture opening 15, which facilitates effective electron extraction through this aperture. Mechanical integration of the anode 5 and the lower throttling aperture 15 into a single piece facilitates making these elements fully coaxial, which contributes to elimination of astigmatism and other errors connected with disturbed axial symmetry of the electron lens field. The screening sleeve 4 is relatively distant from the anode 5 and a potential gradient in its surroundings is low. Then, this part of the lens field affects the electron beam EB weakly and small disturbances in adjustment of the screening sleeve 4 and the rest of elements are not very important. When the screening sleeve 4 shows disturbances in axial symmetry of the shape and position of its lower opening smaller than 10% of the diameter, they may be even advantageous particularly in a high vacuum range. According to the description of the first embodiment, in the high vacuum range the electric field in the screening sleeve 4 region works as an electron mirror. Thanks to properly introduced asymmetry of this field, reflected electrons can be directed toward the anode 5 at the photo-detector side which contributes to a more efficient acquisition of the light signal generated by them in the scintillator layer 5 a.

The rest of processes, concerning the gas flow or the flow of secondary electrons SE and backscattered electrons BSE as well as their conversion into a light signal and back into an electric signal, are realized analogously as it has been described for previous embodiments. However, the change consisting in division of the screening aperture 1 into segments creates additional abilities of the backscattered electrons BSE detection in the range of higher pressures P₁ in the sample chamber. When the pressure P₁ and the distance of the electron detection unit from the sample stage 16 is large enough, backscattered electrons BSE will suffer multiple ionizing collisions with gas molecules on their way to the detector, and losing their kinetic energy generate secondary electrons. Such backscattered electrons BSE are unable to generate photons in the scintillator layer 2, but secondary electrons created by them can be further multiplied in gas ionizing collisions when the screening aperture 1 is biased with a properly high positive voltage. There, signal currents proportional to BSE streams flowing toward segments of the screening aperture 1, will arise in the particular segments Ia, Ib, le i Id. Then the segments signals have a directional character, and they enable to obtain quantitative information about morphology and topography of the sample, after their amplifying and processing by proper electronic circuits. The segment Ie deposited on the remaining part of the scintillator layer 2 does not take part in the directional BSE signals creation and serves only for carrying away electric charges from this part of the plate. Embodiment 6. The electron detection unit shown in Fig. 6 is a much simpler structure than those presented in the embodiment 1, the embodiment 2, the embodiment 3, the embodiment 4, and the embodiment 5. It has been arranged inside the intermediate chamber 13 of the differential vacuum system. The intermediate chamber 13 interior is polished and advantageously covered with a high reflectivity material. The intermediate chamber 13 is placed under the objective lens 9 of a scanning electron microscope and sealed with the gasket 30 in a hole of the objective lens 9. In the opening in a side wall of the intermediate chamber 13, the main light pipe 11 made in the form of a poly methyl methacrylate bar has been cemented hermetically. Inside the intermediate chamber 13, the screening sleeve 4 in a form of a cut cone is fastened with the insulating spacers 28, and in the vicinity of a lower opening of the screening sleeve 4, the anode 5 in the shape of a metal ring covered with a thin layer of the scintillator 5 a is positioned and electrically separated. At the axis of the hole made in a lower part of the intermediate chamber 13, the lower throttling aperture 15 in the form of a thin foil with a small opening is sealed with the insulating separator 29. The screening sleeve 4, the anode 5 and the lower throttling aperture 15 are positioned coaxially at the electron optical axis EOA, along which the electron beam EB runs. Further, the reflecting aperture 31 made of a foil with a well light reflecting surface is fastened with the insulating separator 29 on the lower throttling aperture 15. An opening in the reflecting aperture 31 is larger than that in the lower throttling aperture 15 and its symmetry axis is shifted out of the electron optical axis EOA toward the main light pipe 11. Electric throughputs arranged in a wall of the intermediate chamber 13 enable biasing the electrodes with proper voltages.

The electron detection unit arranged in the described way is a successive variant of the low pressure electron detector arranged in different varieties in the central channel of the intermediate chamber 13 and described in previous embodiments. It is destined for a low pressure scanning electron microscope, in which the intermediate chamber is arranged inside the objective lens 9 and pumped out to the intermediate vacuum P₂. In order to enable tilting the sample stage 16 in various directions and facilitate unlimited application of different detectors, the intermediate chamber 13 of the electron detection unit is miniaturised to dimensions below 1 cm³, however at the cost of resigning from backscattered electrons BSE detection and a separate scintillator detector of secondary electrons SE. Secondary electrons SE have low initial energies, so they are attracted by an extracting electric field created by the lower throttling aperture 15 positively biased. The electrons reaching an opening of the lower throttling aperture 15 are attracted by an electric field generated by electrodes arranged inside the intermediate chamber 13 and biased with voltages depending on the gas pressure. Electrodes: the lower throttling aperture 15, the screening sleeve 4 and the anode 5 create a typical electrostatic electron lens influencing also the electron beam EB. Thus, they should create an unit rotationally symmetric with respect to the electron optical axis EOA to prevent the electric field produced by them from causing astigmatic deformations of the electron beam EB. In these conditions, the reflecting aperture 31 is biased with the same voltage as the lower throttling aperture 15 and is placed approximately in the same plane, thus it does not disturb the symmetry of the mentioned electron lens field. To obtain effective electron transport through the lower throttling aperture 15 and electron flow toward the anode 5, electric field intensity at the aperture should be possibly high, similarly to the former embodiments. Therefore the anode 5 is biased with a high voltage advantageously exceeding 1000 V, though the screening sleeve 4 voltage does not differ much from the lower throttling aperture 15 bias, i.e. it is of order of 100 V. In these conditions and at the intermediate pressure P₂ in the intermediate chamber 13, the electron flow to the anode 5 causes excitation and ionization of gas molecules and photon generations accompanying their relaxations. Additionally the anode 5 is covered with thin layer of scintillator 5a, which generates a light signal under high energy electron bombardment. This increases an efficiency of the electron signal conversion into light one. Here, the input window of the photo- detector unit is determined by a face of the main light pipe 11. Thanks to location of the light generation region in a well light reflecting interior of the intermediate chamber 13, the light signal is introduced into the main light pipe 11 very effectively and transported to the photomultiplier where it is converted back into an electric signal and amplified.

The electron detection unit can also work when high vacuum conditions are maintained in the sample chamber and the intermediate one. In this case, the screening sleeve 4 voltage is reduced to negative values to create an electron mirror field diverging electrons. This electron mirror reflects electrons toward the lower throttling aperture 15, but on its surface the reflecting aperture 31 also biased with a negative voltage is arranged to reflect electrons in an opposite direction. This electron reflection is done toward the main light pipe 11 side of the anode 5, because the biased reflecting aperture 31 causes a certain field asymmetry at its opening which makes the electron stream enter the intermediate chamber 13 under certain angle with respect to the electron optical axis EOA. At vacuum conditions, the anode 5 voltage may be higher to increase an efficiency of conversion of the electron kinetic energy of electrons hitting the scintillator layer 5a on the anode 5, into light energy. Like of the former embodiments, the light signal is introduced to the photo-detector unit where it is converted back into an electric one. Similarly to the former embodiments, the lower throttling aperture 15 fulfils a double role: the role of the electrode of an immersion electron lens and that of the element throttling gas flow from the sample chamber with the elevated pressure P₁ to the intermediate chamber 13 where the intermediate pressure P₂ is maintained at a lower level. Embodiment ?.

The electron detection unit shown in Fig. 7 is a much simpler structure than those presented in the embodiment 1, the embodiment 2, the embodiment 3, the embodiment 4, and the embodiment 5. Like the embodiment 6 it is destined for cooperation with a low pressure scanning electron microscope equipped with the intermediate chamber 13 pumped through the objective lens 9. The intermediate chamber 13 of the detection unit contains a metal part placed under the objective lens 9 of the scanning electron microscope and sealed with the gasket 30 in the opening of the objective lens 9. This part of the intermediate chamber 13 is combined with the screening sleeve 4 of a cut cone shape with a smaller opening on the lower throttling aperture 15 side. The light pipe protrusion 1 Ia with polished walls, made of poly methyl methacrylate and connected hermetically with the screening sleeve 4 creates a lower part of the intermediate chamber 13. Advantageously, an upper surface of the light pipe protrusion 11a is made askew to the electron optical axis EOA. The light pipe protrusion 1 Ia is shut out by the lower throttling aperture 15 having an annular anode 5 fastened with use of the insulating separator 29 on its upper surface, and covered with a thin layer of the scintillator 5 a. Openings in the lower throttling aperture 15 and the anode 5 as well as the screening sleeve 4 are adjusted on the common electron optical axis EOA₃ along which the electron beam runs. On the face of the light pipe protrusion 11a, the semiconductor detector BSE 33 with four sector signal electrodes a, b, c, d, disposed symmetrically around the electron optical axis EOA is fastened with use of the insulating plate 34. On a side wall of the light pipe protrusion 11a, the input window of the semiconductor photo-detector 32 advantageously in the form of a photo-diode PIN is cemented, while the rest of the side wall of the light pipe protrusion 11a is covered with the reflecting layer 12.

The electron detection unit arranged in the described way is an equivalent of the low pressure electron detector arranged in several variants, inside a central channel of the intermediate chamber 13 described in the first second, third, forth and fifth embodiment. Like the sixth embodiment, the present one is destined for cooperation with a low pressure scanning electron microscope in which the intermediate chamber 13 is pumped to the intermediate pressure P₂ through the objective lens 9. In order to enable tilting the sample stage 16 in various directions and unlimited application of different detectors, the intermediate chamber 13 of the detection unit is miniaturized to dimensions essentially lower than 1 cm³ and deprived of the separate high vacuum scintillator detector system for secondary electrons SE. The electron optical system enabling conversion of the electron signal into the light one has been arranged similarly as in the embodiment 5, and the secondary electron SE detection is also conducted in a similar way. An essential difference with respect to the former embodiments is application of the semiconductor photo-detector 32, advantageously a PIN photo- diode. As it was mentioned, such a solution ensure further miniaturization of the electron detection unit though at the cost of lower sensitivity. When high sensitivity is necessary, it is advantageous to equip the unit with a photo-detector composed of the main light guide 11 and the photo-multiplier 27 as it was shown in the former embodiments.

A next particular feature of the electron detection unit in this embodiment is application of the semiconductor detector BSE 33 in a form of the four quadrant detector containing four signal electrodes a, b, c, d, disposed symmetrically around the electron optical axis EOA. The application of the semiconductor detector BSE 33 ensures good sensitivity of the BSE detection but the four quadrant variant of the detector enables directional detection of BSE in four directions defined by the four signal electrodes a, b, c, d. Four electric signals from these electrodes properly amplified and processed, may be a source of microscope images with quantitative material and topographic contrasts as well as a three-dimensional surface reconstruction. Embodiment 8.

The electron detection unit shown in Fig. 7a is build similarly as the unit explained in the embodiment 7 with such an exception that two semiconductor photo- detectors 32 of the PIN type advantageously, has been placed on two opposite sides of the light pipe protrusion 11a. In turn, on the face of the light pipe protrusion 11a four semiconductor detectors of backscattered electrons BSE 33 have been disposed symmetrically around the electron optical axis EOA.

The electron detection unit arranged in the described way works analogously as that described as the embodiment 7 with some little changes resulting from the two differences in its structure mentioned above. A light signal, arising when secondary electrons flowing toward the anode 5 collide with gas molecules and the scintillator layer 5a, is generated symmetrically around the electron optical axis EOA in the anode 5 vicinity. In turn, detection of the light signal is performed at the place where an input window of the semiconductor photo-detector 32 has been fixed to a side of the light pipe protrusion 1 Ia. A light signal generated in the detector vicinity reaches it more efficiently than that coming from the space at the other side of the electron optical axis EOA. As the output signal of the semiconductor photo-detector 32 is proportional to the light flux reaching its input window, it is advantageous to use two semiconductor photo-detectors 32 at opposite sides of the light pipe protrusion 11a and add their output signals. This way, an utilization of the light flux generated inside the light pipe protrusion 11a can be better while loses caused by multiple reflections from its internal surface can be reduced.

In the embodiment 7 an application of the four quadrant semiconductor detector BSE 33 has been discussed. Such detector enables directional electron detection and microscopy images with different kinds of quantitative contrasts as the result. However, small size four quadrant BSE detectors, which fulfill miniaturization requests of the electron detection unit described, are inaccessible in the market. In the embodiment discussed, the four quadrant detector has been advantageously substituted by four single small size semiconductor detectors BSE 33 symmetrically disposed around the electron optical axis EOA on the face of the light pipe protrusion 11a. Similarly as in the former embodiment, signals generated in single detectors contain information about an angular distribution of the backscattered electrons BSE 33 but manufacturing costs of the described detection unit is much lower. Embodiment 9.

The scanning electron microscope fragmentary shown in Fig. Ia and Fig. 2 comprises among others: an electron optical column with the objective lens 9, the sample chamber 25 and the sample stage 16, and is equipped with the electron detection unit destined for work in a variable pressure range which inside the intermediate chamber 13 has the screening sleeve 4 and the anode 5 showing rotational symmetry at least in the part turned toward the lower throttling aperture 15 and together with the opening in the lower throttling aperture 15 are disposed on a common electron optical axis EOA along which electron beam EB goes. Near the anode 5 an input window of at least one photo detector set is arranged which is composed of the photomultiplier 27 and the main light pipe 11 with the light pipe protrusion 11a. The scanning electron microscope has the lens throttling aperture 8a of a diameter lesser than 1 mm fixed hermetically in a channel of the objective lens 9. On a wall of the sample chamber 25 of the scanning electron microscope, main elements of the electron detection unit are fastened movably by means of the sealing block 26. These include the intermediate chamber 13 connected by means of the connecting pipe 24 with the nipple 17 and vacuum lines. The gear shifting the detection unit contains also elements blocking it in a desired position.

The scanning electron microscope arranged in the described way works as follows. The scanning electron microscope equipped with the electron detection unit destined for work in the variable pressure range may also be a high vacuum microscope so this electron detection unit should contain an intermediate chamber 13 separating a region of the low pressure P₃, necessary for right functioning of the electron optical column, from a region of the elevated pressure P₁ in the sample chamber 25. In the intermediate chamber 13, an intermediate pressure P₂ is maintained which differs over two orders of magnitude from pressures in adjacent regions, thus to prevent a rapid gas flow the neighboring regions are separated from the intermediate chamber 13 by two throttling apertures with small openings which let the electron beam EB pass through. These are the lens throttling aperture 8a, and the lower throttling aperture placed in a side of the detection unit turned toward the sample stage 16. Advantageously the lens throttling aperture 8a is placed in a central plane of the objective lens 9 and throttles efficiently the gas flow without limitation of the microscope view field. It also can play the role of a microscope electron optical aperture.

The electron detection unit is connected with use of the nipple 17 to an autonomous intermediate vacuum system containing also a vacuum pump, gas pressure meters as well as vacuum valves. The unit as the form of an additional equipment can be mounted to a wall of the sample chamber 25 in a standard scanning electron microscope of the high vacuum type. In this case it enables extension of the microscope capabilities toward elevated pressures exceeding 1000 Pa and examining dielectrics as well as biological samples in a natural state. The movable connection of the electron detection unit in a wall of the sample chamber 25 of the microscope enables displacement of the intermediate chamber 13 aside of the electron optical axis EOA and restitution of the typical functions of the scanning electron microscope as a high vacuum instrument.

Embodiment 10.

The scanning electron microscope fragmentary shown in Fig. 6 is an microscope of the low vacuum type which contains among others an electron optical column with the objective lens 9 and the sample stage 16, and is equipped with the electron detection unit destined for work in a variable pressure range which inside the intermediate chamber 13 has the screening sleeve 4 and the anode 5 showing rotational symmetry at least in the part turned toward the lower throttling aperture 15 and together with the opening in the lower throttling aperture 15 are disposed on a common electron optical axis EOA along which electron beam EB goes. Near the anode 5 covered with the scintillator layer 5 a, an input window of the detector set composed of the main light pipe 11 connected with a photomultiplier is arranged. The intermediate chamber 13 is fixed under the objective lens 9 of the scanning electron microscope and sealed in an opening of the objective lens 9 with use of the gasket 30. The scanning electron microscope has its intermediate chamber combined with the objective lens 9 inside which the intermediate pressure P2 is maintained. The scanning electron microscope arranged in the described way works as follows. The scanning electron microscope equipped with the electron detection unit destined for work in the variable pressure range is a low vacuum microscope with its intermediate chamber combined with the objective lens 9 in which the intermediate pressure P₂ is maintained. In such an microscope the lower throttling aperture separating the intermediate chamber from the sample chamber is usually fixed hermetically in an opening of the objective lens 9 at the top of a conical mount. Thus, the electron detection unit substitutes in vacuum terms the mentioned aperture mount and contains a miniaturized intermediate chamber 13 of the height corresponding with the mount length, and at the unit top turned toward the sample stage 16 it has the throttling aperture 15 hermetically fixed which limits gas flow from the sample chamber to the intermediate chamber. Simultaneously the electron detection unit mounted in the scanning electron microscope ensures efficient detection of secondary electrons, which does not collide with other types of detectors applied in the microscope and does not disturb other manners of work implied by the microscope design.

Claims

1. Electron detection unit, destined particularly for a scanning electron microscope with variable pressure in a sample chamber comprising: an intermediate chamber placed in the way of the electron beam to a sample stage and enabling internal gas pressure different than that in a sample chamber where emission of detected electrons take place, at least one aperture throttling gas flow to the intermediate chamber, at least one screening sleeve, at least one anode for electron flow and conversion of the electron signal into light signal and at least one photo-detector, characterized in that it contains at least one electron detector system, that the screening sleeve (4) and the anode (5) are placed inside the intermediate chamber (13), that said screening sleeve (4) and anode (5) show rotational symmetry at least in the part turned toward the lower throttling aperture (15) and together with the opening in the lower throttling aperture (15) are disposed on a common electron optical axis (EOA) along which electron beam (EB) goes, that an input window of at least one photo detector set is arranged near the anode (5).

2. The electron detection unit of claim 1, characterized in that the photo detector set contains at least one photomultiplier (27).

3. The electron detection unit of claim 1, characterized in that the photo detector set contains at least one semiconductor photo detector (32).

4. The electron detection unit of claim 2, characterized in that it has a main light pipe (11) leading toward the anode (5) connected to the photomultiplier (27).

5. The electron detection unit of claim 3 or 4, characterized in that a light pipe protrusion (Ha) with the front surface turned toward the sample stage (16) is arranged in surroundings of the electron optical axis (EOA).

6. The electron detection unit of claim 5, characterized in that the light pipe protrusion (Ha) has a scintillator layer (2) fixed on the front surface and covered with thin conductive film.

7. The electron detection unit of claim 6, characterized in that a cover (If) with opening is attached movably between the scintillator layer (2) and the sample stage (16) for screening the scintillator layer (2) against electron bombardment.

8. The electron detection unit of claim 4 or 5, characterized in that the main light pipe (11) and/or the light pipe protrusion (Ha) have a hole (l ib) inside which the screening sleeve (4) and anode (5) are arranged.

9. The electron detection unit of claim 4 or 5, characterized in that a fragment of the main light pipe (11) surface and/or a fragment of the light pipe protrusion (Ha) surface are covered with light reflecting layers (12).

10. The electron detection unit of claim 4 or 5, characterized in that a fragment of the main light pipe (11) surface and/or a fragment of the light pipe protrusion (Ha) surface are surrounded with a foil of high light reflectivity.

11. The electron detection unit of claim 1, characterized in that the anode (5) is covered with a scintillator layer (5 a).

12. The electron detection unit of claim 1, characterized in that the anode (5) is ring- shaped, made of a conductive material and fixed on the lower throttling aperture (15) with use of an insulating separator (29).

13. The electron detection unit of claim 1, characterized in that the anode (5) is made in the form of a thin conductive layer transparent for light deposited on the side wall of the light pipe hole (1 Ib).

14. The electron detection unit of claim 1, characterized in that the screening sleeve (4) and the anode (5) are made in the form of thin layers deposited on the inner and outer surface of the insulating sleeve (7).

15. The electron detection unit of claim 1, characterized in that the screening sleeve (4) is arranged in the form similar to a truncated cone with the smaller opening turned toward the lower throttling aperture (15).

16. The electron detection unit of claim 1, characterized in that the screening sleeve (4) shows deviations in the rotational symmetry of shape and position of the lower opening with respect to the electron optical axis (EOA) advantageously lesser than 10% of said lower opening diameter.

17. The electron detection unit of claim 1 or 15, characterized in that the diameter of the lower throttling aperture (15) as well as the distance of the lower throttling aperture (15) from the screening sleeve (4) are smaller than the diameter of the lower opening of the screening sleeve (4).

18. The electron detection unit of claim I₅ characterized in that a reflecting aperture (31) is arranged between the lower throttling aperture (15) and the anode (5).

19. The electron detection unit of claim I₅ characterized in that the intermediate chamber (13) is connected immovably with the objective lens (9).

20. The electron detection unit of claim I₅ characterized in that it the intermediate chamber (13) is connected movably with the objective lens (9) and is equipped with a desired position blockage.

21. The electron detection unit of claim I₅ characterized in that the intermediate chamber (13) has an opening in the front wall in which the scintillator (21) and the auxiliary light pipe (19) leading to the photomultiplier (27) are arranged.

22. The electron detection unit of claim 21, characterized in that both the main light pipe (11) and the auxiliary light pipe (19) are connected to the same photomultiplier (27).

23. The electron detection unit of claim 21, characterized in that it has the inlet valve (22) with an actuator opening or closing the gas and electron flow, arranged in the opening made in the front wall of the intermediate chamber (13).

24. The electron detection unit of claim I₅ characterized in that the intermediate chamber (13) has an opening in the upper wall positioned on the electron optical axis (EOA), in which the upper throttling aperture (8) is fixed hermetically.

25. The electron detection unit of claim I₅ characterized in that a gas feed (G) to the region of the light pipe protrusion (l la) is arranged on the lower surface of the intermediate chamber (13).

26. The electron detection unit of claim 1, characterized in that it has the screening aperture (1) arranged beneath the lower throttling aperture (15) and divided into two segments at least which are mutually isolated and have individual outlets.

27. The electron detection unit of claim 5, characterized in that at least one semiconductor backscattered electron detector BSE (33) is arranged on the face of the light pipe protrusion (1 Ia),

28. A scanning electron microscope comprising an objective lens, a sample chamber and a sample stage characterized in that it is equipped with the electron detection unit that the screening sleeve (4) and anode (5) are placed inside the intermediate chamber (13), that said screening sleeve (4) and anode (5) show rotational symmetry at least in the part turned toward the lower throttling aperture (15) and together with the opening in the lower throttling aperture (15) are disposed on a common electron optical axis (EOA) along which electron beam (EB) goes, that an input window of at least one photo detector set is arranged near the anode (5).

29. The microscope of claim 28, characterized in that a lens throttling aperture (8a) with the opening diameter less than 1 mm is fixed hermetically in the bore of the objective lens (9).

30. The microscope of claim 28, characterized in that the intermediate chamber (13) is connected immovably with the objective lens (9).

31. The microscope of claim 28, characterized in that the intermediate chamber (13) is connected movably with the objective lens (9) and is equipped with a desired position blockage.

List of designations in the drawings:

1 — screening aperture, la,b,c,d,e - sectors of the screening aperture, If- cover,

2 - scintillator layer,

3 - light pipe gasket,

4 - screening sleeve,

5 - anode, 5a - scintillator layer,

6 — holder,

7 - insulating sleeve,

8 - upper throttling aperture, 8a - lens throttling aperture, 9 - obj ective lens,

10 - gasket,

11 - main light pipe,

1 Ia - light pipe protrusion,

1 Ib - light pipe hole, 12 - reflecting layer,

13 - intermediate chamber,

14 -plate,

15 - lower throttling aperture,

16 - sample stage, 17 — nipple,

18 - spring,

19 - auxiliary light pipe,

20 - fixing insulator,

21 - scintillator, 22 - inlet valve,

23 - pusher,

24 — connecting pipe, 25 - sample chamber wall,

26 - sealing block,

27 - photo-multiplier,

28 - insulating spacer, 29 - insulating separator,

30 - gasket,

31 - reflecting aperture,

32 - semiconductor photo-detector,

33 - semiconductor backscattered electron detector BSE, 34 - insulating plate,

BSE - backscattered electrons, EB - electron beam, EOA - electron optical axis, SE - secondary electrons, G - gas feed,

P₁ - elevated pressure, P₂ - intermediate pressure, P₃ - low pressure.