SPECIMEN CHAMBER FOR SCANNING ELECTRON BEAM INSTRUMENTS

Abstract: A specimen chamber (2) for use in a scanning electron beam instrument has an electrode (52) mounted above the specimen (32) so as to intercept the cone of emitted charge or scattered carriers and back scattered electrons produced from anywhere on the specimen by the electron beam (16). The electrode (52) is held at a few volts but is placed close to the specimen so that a substantial electric field is generated between the specimen and the electrode. The image of the specimen is produced by a specimen current collected from the specimen by a fine wire (46). The specimen chamber may be an integral part of a scanning electron beam instrument.

Description: SPECIMEN CHAMBER FOR SCANNING ELECTRON BEAM INSTRUMENTS The present invention relates to improvements in specimen chambers for scanning electron beam instruments. Scanning electron beam instruments, such as microscopes, can be used to examine both biological and non-biological specimens at magnification in the range 20 to 50,000 times. The specimen is normally maintained in an evacuated specimen chamber, which is normally an integral part of the instrument. The pressure in the specimen chamber is 10"5 Torr or less, since interactions between the high energy electrons of the beam with atmospheric gases adversely effect the image formed and prevent a high energy beam being maintained. Where the specimen to be examined contains water vapour or other gases it becomes increasingly difficult to maintain a low enough atmospheric pressure in the region of the specimen to prevent it being damaged or out gassed. Therefore conventional microscopes cannot be used for examining such specimens. Various specimen chambers have been proposed for overcoming this problem. Suitable chambers are described, for example, in GB-A-1 477 458 and EP-B-0 022 356. These constructions use the principles of differential pumping in an attempt to isolate the specimen environment from the electron beam column. In these arrangements a greater pressure is allowed to exist in the environment of the specimen itself. Some deterioration of the image arises because of interactions between the electrons of the beam and the atmosphere as the beam travels towards the specimen. There are three methods of forming an image cf a specimen in a scanning electron beam instrument. Firstly, it is possible to collect electrons which are back-scattered from the specimen. Secondly, it is possible to operate the instrument in an emissive mode whereby secondary electrons emitted by the specimen are collected. Thirdly, the specimen current can be measured. This specimen current arises because electrons are knocked off the surface of the specimen leaving a residual charge, and also because electrons are absorbed by the specimen. The specimen current image may be considered as an inverted image of the image obtained using the emissive mode. Thus the specimen current mode can produce good topological and morphological information, provided a high quality specimen current amplifier of appropriate bandwidth is used. Secondary emitted electrons have energies in the range 1-50 eV, while back-scattered electrons have energies greater than 50eV. Therefore it is possible to distinguish between these two types of electrons for imaging purposes. Conventionally imaging is usually carried out in scanning electron beam microscopes in the emissive mode as the secondary electrons produce better topological and morphological information. However this technique does not produce as good results where there is a specimen atmosphere. This is because the extra carriers generated by ionisation of the gasses surrounding the specimen by the beam electrons, back scattered electrons and secondary emitted electrons, have an energy range which is similar to the energy range of the secondary-emitted electrons. Also some of the information carrying electrons recombine with carriers of the opposite charge. Thus a considerable loss of topological information occurs. The specimen current mode is also susceptible to deterioration which can be caused by subsequent electron collisions with atoms in the environment around the specimen. Difficulties also arise when the specimen to be examined by a scanning electron beam instrument is made of insulating, semiconducting or poorly conducting material. With such specimens charge build-up can take place on the material surface. This alters the trajectories of the primary electrons of the electron beam and also of the secondary emitted electrons. The charge on the surface can also affect the process of emission of secondary electrons. All these factors distort the image grossly and result in a loss of detail and resolution. Further, the charge build-up on the surface can induce electrical breakdown in the material of the specimen. In order to overcome this problem of image degradation, it has been proposed to increase the surface conductivity of the specimen by depositing a thin layer of a highly conductive material onto the specimen surface. Materials such as gold, gold/palladium alloys or carbon have been used. This solution has various associated problems. Even a moderately thin deposited layer can mask the topographical features of the specimen surface. If attempts are made to deposit a very thin layer, this can result in electrical discontinuities in the layer, which, in turn, cause local charge buildup. In order to ensure that good topographical contrast is obtained in the image, the deposited layer must be a good secondary electron emitter. This is the reason for the choice of heavy metal elements such as gold. However, this results in the atomic contrast inherent in the specimen being masked by the deposited layer. Additionally, it becomes very difficult to obtain information about the elemental composition of the specimen by analysis of x-rays emitted from the bulk of the specimen. Other techniques have been proposed for examining such poorly conducting specimens. One technique is described in US-A-2 890 342, which describes a method of neutralising the charge by electrons which impinge on the surface. Charge neutralisation can also be achieved by the use of ions. The main disadvantage of the charge neutralisation technique is that it superimposes unwanted effects on the resulting image due to the additional electrostatic or magnetic fields associated with the equipment for producing the neutralising charge particles. These distort the interaction between the primary electron beam and the specimen. It is also difficult to maintain a balance between the neutralising current and other electron currents responsible for charge buildup and image production. The present invention attempts to solve the technical problem of providing an improved method of forming an image in a scanning electron beam instrument of a specimen that is made of an insulating semiconducting or poorly conducting material and/or requires or produces a gas atmosphere. The present invention provides a specimen chamber for use in a scanning electron beam instrument comprising a housing having an aperture in one wall for the electron beam, means for mounting the specimen opposite said aperture, means for collecting the specimen current generated by the electron beam from the specimen, and biasing means for producing a substantial electric field at the surface of the specimen and in the region through which the electron beam passes. It is found that the provision of the biasing means considerably improves the contrast of the image produced from the specimen current. This is thought to be because ionised and other charge carriers are rapidly removed from the surface and sub-surface of the specimen by the electric field at the surface of the specimen and cannot blur the image by making any further contributions to the specimen current. The electric field also removes negative and positive charges from the atmosphere of the specimen. The electric field is also though to generate additional carriers which contribute to the topological and morphological contrast in the image. In one embodiment, the biasing means comprises an electrode in the form of an annular bias plate disposed on and insulated from the apertured wall of the specimen chamber facing the specimen. The biasing means may alternatively or, preferably, additionally comprise- a generally f rustro-conical bias member adapted to surround the specimen. Particularly advantageous results can be obtained using such a bias member at a potential of one polarity and an annular bias plate facing the specimen at a potential of the opposite polarity. This arrangement enables charge carriers of either sign to be readily removed. Preferably the spacing of the bias plate from the specimen is such as to enable an electric field of the order of a few thousand volts per metre to be produced by applying only a few volts to the bias plate. In this way the capacitance between the bias plate and the specimen is maintained at a low level, typically a few picofarads. The specimen chamber is also preferably provided with evacuating means and means including a moisture reservoir as described in GB-A-1 477 458 in order to prevent drying out of the specimen, for supplying an ambient gas so as to maintain the desired pressure, typically 0.1 to 15 Torr or above, within the specimen chamber itself. The invention also includes a scanning electron beam instrument comprising a specimen chamber as defined above. Specimen chambers embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a sectional view through a first embodiment of a specimen chamber ; Figure 2 is a plot showing relative signal strength versus the field between a bias plate and the specimen ; Figure 3 is a sectional view through a second embodiment of a specimen chamber; Figure 4 is a diagrammatic plan view illustrating means for adjusting the position of an intermediate chamber and upper wall of the specimen chamber relative to the remainder of the chamber; Figure 5 is a diagram showing an alternative arrangement of the biasing means in the specimen chamber ; Figure 6 is a diagram showing another alternative arrangement of the biasing means; and Figure 7 is a set of images produced by a scanning electron microscope incorporating a specimen chamber in accordance with the invention and showing how the image quality is enhanced with increasing electric field produced by the biasing means, for each image a typical sample of the specimen current produced is shown displayed on an oscilloscope. Referring first to Figure 1 , a specimen chamber 2 is located in a column 4 of an electron beam instrument. The specimen chamber is defined by a housing which also defines an intermediate chamber 8. The intermediate chamber is defined by an upper wall 10 and a side wall 12 of the housing. The upper wall has a small pressure limiting aperture 14 to allow an electron beam 16 to enter the chamber. A duct 18 connects the intermediate chamber 8 to a vacuum pump (not shown) . The specimen chamber 2 is defined by an upper domed wall 20 which is sealed at its periphery by seals 22, 24 to the side wall 12 of the intermediate chamber and to a side wall 26 of the specimen chamber which is connected to a base 28 so that the specimen chamber 2 is completely closed. A duct 30 is connected to a vacuum pump (not shown). A pressure limiting aperture 29 is formed centrally in the wall 20 beneath aperture 14 to allow entry of the electron beam. The pressure limiting apertures 14, 29 have diameters in the range 100 to 500 micrometers. A terminal block 31 is mounted in the wall 26 to allow electrical connections to be made to the interior of the chamber without allowing gas to flow between the chamber and the column 4. A specimen 32 is mounted on a specimen support 34. The support 34 includes a moisture-absorbing pad 36. An inlet duct 38 is connected via the pad to the interior of the specimen chamber 2 and is connected to a suitable source of ambient gas. The differential pressure between the intermediate chamber and the specimen chamber 2 may be adjusted in accordance with the specimen being examined. The arrangements for differential pressure production may be as described in GB-A-1 477 458. A suitable pressure for the specimen chamber is, for example, 15 torr. The specimen chamber and intermediate chamber are mounted in a section of the column 4 which has a plate 40 forming part of the wall of the column. The plate 40 sealingly receives the various ducts 18, 30 and 38 and also a terminal block 42 for electrical connections. The plate 40 is sealed by seal means 44 to the wall of the column 4. A fine, preferably gold, wire 46 is connected to the specimen 32 and passed out of the specimen chamber through terminal block 31 to a preamplifier 48 positioned just outside the specimen chamber. The wire 46 collects the specimen current produced when the electron beam is incident on the specimen. The output of the preamplifier 48 is fed out of the column 4 via terminal block 42 to further image processing and display circuitry 50 of any suitable type. The positioning of the preamplifier 48 just outside the specimen chamber helps to keep the lead capacitance to a minimum. An electrode, here in the form of an annular bias plate 52 is mounted to the inner surface of the specimen chamber wall 20 facing the specimen. The bias plate 52 is mounted on an insulating annular member 54, preferably made of PTFE, which is mounted directly onto the chamber wall 20. Both the insulating member 54 and bias plate 52 have central holes 56, 58 aligned with the central hole 29 in the wall 20 so as to allow the electron beam to enter. These holes 56, 58 are either of the same diameter or of slightly larger diameter than the limiting aperture 29 in the wall 20. The surface area of the bias plate is such that it is capable of intercepting the cone of electrons emitted or scattered from anywhere on the scanned surface of the specimen 32. In practice, it is found that a copper foil with a thickness of 100 - 200 mu and a diameter of about 5 - 8 mm is sufficient. The bias plate 52 is connected via a wire 60 through the terminal block 42 to a potential source capable of providing up to 100 volts potential. The plate is preferably held at a few volts positive or negative with respect to the specimen. In order to avoid electric discharge between the specimen and the bias plate, it is desirable that the magnitude of the potential be low. However, it is necessary to maintain a substantial electric field of a few thousand volts per metre between the specimen and the plate for effective image enhancement. This requires that the specimen and bias plate should be separated by a distance typically in the range of 0.5 to 1.5 mm. This distance also keeps the capacitance between the specimen and the bias plate low. The bias plate 52 can be held at either positive or negative with respect to the specimen. The effect of positive or negative biasing on the specimen current in the presence of water vapour at a pressure of 15 torr is illustrated in Figure 2. The curve will be different at different pressures and will also vary with temperature and with the gases in the specimen environment. However, the general shape of the curve remains substantially the same. In particular it will be noted that when either a positive or negative field gradient is produced between the specimen and the bias plate, the relative signal changes rapidly and then more slowly to reach a plateau. The image quality and contrast are normally best when the field gradient is such that the relative signal is on the plateau. For the given example, it is apparent that the field gradient is desirably set to be either plus or minus substantially 2,000 volts per metre. At the point I in Figure 2, the relative signal changes sign from positive to negative. This point on the curve represents image inversion. Thus, by appropriately selecting a negative bias voltage it is possible to obtain an image in specimen current mode with the same contrast as that in the emissive mode. Therefore using a sufficient negative field gradient effectively inverts the image normally produced in specimen current mode. The specimen chamber illustrated in Figure 3 is essentially similar to that described with reference to Figure 1 and like reference numerals have been used to identify corresponding parts. In this embodiment the main body of specimen chamber 2 is fixed relative to the column 4. However, the intermediate chamber 8, which includes the upper wall of the specimen chamber, is mounted to the remainder of the specimen chamber via adjusting means 80 which allow the position of the intermediate chamber to be adjusted in a plane perpendicular to the path of the electron beam 16. The specimen chamber 2 is defined by a housing 68 which is formed integrally with a plate 40' which is sealed into an opening in the side wall of the column 4 by means of seal means 44'. The specimen 32 is mounted on support 3 which is provided with adjusting apparatus 70 which allows the position of the specimen to be adjusted in the X, Y and Z directions. The electrical connections to the specimen and the bias plate 52, together with the mechanical connections to the adjusting apparatus 70 pass out of the specimen chamber 2 via a further wall member 72 which is sealingly connected to the outside of the plate 40 * . A duct 76 connects the interior of the specimen chamber 2 to a source of air or gas such as nitrogen. A controlled leak valve 78 is provided in the duct 76 so that the pressure inside the specimen chamber can be controlled. The chamber is preferably evacuated via tne duct 30 to a vacuum pump prior to a controlled leaking of air or gas into the chamber so that a desired working pressure can be maintained. The selected pressure may vary in dependence on the type of specimen under consideration from 0.1 to 15 Torr or above. This pressure may be maintained continuously by using differential pumping as described in GB-A-1 477 458 between the intermediate chamber 8 via the duct 18 and the main specimen chamber 2 via the duct 30. In this embodiment, the upper wall 20' of the specimen chamber is defined by a flat plate sealingly mounted on an upper support wall 82 of the housing 68 defining the specimen chamber. The duct 18 connecting the intermediate chamber 8 to a vacuum pump is provided with a bellows section 74 so as to accommodate the adjustments of the position of the intermediate chamber within the column 4 made by the adjusting means 80. In order to adjust the position of the intermediate chamber relative to the specimen chamber, the adjusting means 80 (indicated only diagrammatically in Figure 3) mounted between a support wall 82 of the intermediate chamber 2 and the upper wall 20' is used. This adjusting mechanism 80 is shown in more detail in Figure 4. The intermediate chamber assembly is connected to the upper support wall 82 by means of four legs 100 each connected lo the intermediate chamber, for example wall 20'. Each leg is provided with an elongate slot, through which a nut and bolt arrangement pass to secure each leg to the support wall 82. Springs are mounted round the bolts and retained by the nuts to hold the intermediate chamber in its adjusted position. The side wall of the intermediate chamber is coupled to one arm 104 of a lever arrangement which also includes a knee lever 106 which has one end slidingly received in an elongate slot in arm 104. The knee lever 106 has a fixed fulcrum at 110. The end of the arm 104 remote from the intermediate chamber is coupled to a further lever 112 which has a fixed fulcrum 108. The remote end of lever 112 is coupled to a lead screw 114. A further lead screw 116 is coupled to the other end of the knee lever 106. The lead screws 114 and 116 project outside the wall of the column 4. Rotation of these lead screws allows fine adjustments to be made of the position of the intermediate chamber relative to the specimen chamber 2 in a horizontal plane perpendicular to the direction of the main electron beam 16. The adjusting mechanism 80 described is only one example of a suitable mechanism for aligning the intermediate chamber relative to the fixed specimen chamber 2. The adjusting apparatus 70 associated with the specimen support 34 enables the specimen to be correctly aligned with the adjusted, intermediate chamber 8 and, via adjustment in the Z direction allows the electric field between the bias electrode 52 and the specimen to be adjusted. In order to prevent short circuiting which may occur if the specimen stage is raised too fast so that the specimen makes contact with electrode 52, a microswitch is preferably provided to operate a warning system when the specimen support reaches a minimum distance from the upper wall of the specimen chamber. The image produced using the specimen chamber described with reference to Figures 3 and 4 is essentially the same as that described with reference to Figures 1 and 2. It will be noted that since the specimen chamber 2 is of a larger size than that illustrated in Figure 1, a first preamplifier 48' is mounted within the specimen chamber itself and a further preamplifier 48' ' located outside of the column 4. Figure 5 illustrates an alternative arrangement of the biasing means for use with specimen chambers as described with reference to Figure 1 or Figure 3. In this arrangement an additional biasing member in the form of a hollow f rustro-conical member 62 is arranged symmetrically surrounding the specimen 32. This second biasing member is electrically isolated from the bias plate 52 and the specimen 32. The member 62 is placed such that the electric field around the specimen is symmetrical about tne axis of the electron beam. The simultaneous application of a positive bias to the bias plate 52 and a negative bias to the second biasing member 62 further improves image quality. Adjustment of the field produced by the biasing member 62 may be used to provide a finer control over image contrast. Figure 6 shows another alternative arrangement where the second biasing member is a ring member 64 disposed around the bias plate 52 and insulated from the bias plate by a ring 66 of insulating material, for example PTFE. The ring member 64 is also insulated from the wall 20 of the specimen chamber by an extension of the PTFE layer 54. Figure 7 illustrates how the contrast and image quality of the specimen current image of a typical specimen are improved as the voltage gradient is increased from -0.38 x lOBvm- "1 in Figure 5a to minus or plus 3.8 x lO^ m" "1 in Figures 5b and 5c respectively. In this case the specimen is a fine grid and the image is produced in a scanning electron microscope fitted with biasing means as shown in Figure 1.

Claims: 1. A specimen chamber (2) for use in a scanning electron beam instrument comprising a housing (20,26) having an aperture (29) in one wall (20, 20') for the electron beam (16), means (34, 36) for mounting the specimen (32) opposite said aperture (29), and means (46) for collecting the specimen current generated by the electron beam (16) from the specimen (32), characterised in that it further comprises biasing means (52, 62, 64) for producing a substantial electric field at the surface of the specimen (32) and in the region through which the electron beam passes. 2. A specimen chamber as claimed in claim 1, characterised in that the biasing means comprises an electrode (52) in the form of an annular bias plate disposed on and insulated from the apertured wall (20, 20') of the specimen chamber facing the specimen (32). 3. A specimen chamber as claimed in claim 2, characterised in that the bias plate (52) is positioned relative to the specimen such that the application of less than 100 volts to the bias plate (52) produces an electric field of the order of a few thousand volts per metre between the specimen (32) and the bias plate (52). 4. A specimen chamber as claimed in claim 1, characterised in that the biasing means comprises a generally frustro-conical bias member. (62) adapted to surround the specimen (32), 5. A specimen chamber as claimed in claim 1, characterised in that it further comprises an intermediate chamber (8) partially defined by said one wall (20, 20') of the specimen chamber. 6. A specimen chamber as claimed in claim 5, further comprising means (18, 30) for establishing a differential pressure between said intermediate chamber (8) and said specimen chamber. 7. A specimen chamber as claimed in claim 5, characterised in that it further comprises means (80) for adjusting the position of the intermediate chamber (18) including said one wall (20') in a plane substantially perpendicular to said electron beam (16) relative to the remainder of said specimen chamber. a . An,-— electroi beam instrument compri iJπLg -a specimen chamber as claimed in claim 1. 9. A method of forming an image of a specimen in a scanning electron beam instrument, comprising the steps of establishing a substantial electric field of a few thousand volts per metre adjacent the surface of said specimen, and collecting the specimen current resulting from an electron beam impinging the surface of said specimen.

Inventors: SHAH JITENDRA SHANTILAL, SHAH, JITENDRA, SHANTILAL
Applicants: CAMBRIDGE INSTR LTD, CAMBRIDGE INSTRUMENTS LIMITED
Application year: 1987
Main classification: H01J37
IPC codes: H01J37