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A gaseous detector device for an environmental SEM
Micron and Microscopica Acta, Vol. 14, No. 4, pp. 307 318, 1983.
0739-6260/83 $3.00+0.00 © 1983PergamonPressLtd.
Printedin Great Britain.
A GASEOUS
DETECTOR
DEVICE FOR AN ENVIRONMENTAL
SEM
G. D. DANILATOS CSIRO Division of Textile Physics, 338 Blaxland Road, Ryde, NSW 2112, Australia (Received 5 Sepetember 1983)
Abstract--A new detection system has been used in the environmental SEM. The presence of gaseous and liquid phases in the microscope has made possible the formation of images of both insulators and conductors using the current mode. In addition, the ionizing radiations create in the gas negative and positive charge carriers which contain information from the beam-specimen interaction. These carriers, when collected by suitable means, such as a biased wire away from the specimen, can modulate the display signal to form images. Thus, the gas itself constitutes the basic component of a signal detector device, apart from its use as a conditioning medium.
INTRODUCTION The SEM has been modified so that specimens can be examined in a gaseous environment. The design and construction of an environmental or atmospheric SEM (ESEM or ASEM) is being reported in a separate series (Danilatos, 1981; Danilatos and Postle, 1983) and the reader will find it useful to refer to previous work as an aid to follow the current advances. The main objective in the previous work was to establish the conditions in the SEM under which the electron beam suffers a minimum amount of electron scattering due to gas. Imaging was performed by using backscattered electron detectors of specific forms. The present paper describes new possibilities of forming images in the SEM as a result of the presence of gas around the specimen. The electrical conductivity of the specimen and the surrounding gas is altered in the ESEM and ASEM and it is possible to use various currents as carriers of the information arising from the electron beam-specimen interactions.
USING THE CURRENT MODE OF DETECTION The absorbed current mode of detection is used in a conventional SEM with specimens which are good electrical conductors or made so, for example by depositing a conductive layer on 307
the specimen surface. However, in the conditions of ESEM or ASEM where the specimen is surrounded by gas it has been possible to obtain images of both conductors and insulators without any pretreatment by using the current mode. This possibility arises from the fact that the presence of gas creates large currents which are non-existent in the conditions of conventional SEM. The current used does not always correspond to the current absorbed by the specimen. Several examples below illustrate the main observations made. In Figs. 1-10 a comparison is made of the micrographs obtained under different conditions of imaging. A polystyrene diffraction grating with a saw-tooth spacing of 10 gm and about 1 gm groove depth was held by a metal clip which was connected to the input of the micro-micro ammeter of the JEOL JSM-2 SEM. Thus the current collected by the metal clip could be used to form images which could be compared with the backscattered electron images formed by using a plastic scintillator detector as described in previous work. A backscattered electron (BSE) image is shown in Fig. 1 under the conditions indicated. The current given in the captions is the average magnitude recorded in the presence of gas, which is different from the primary beam current as will be shown later. The captions of the following figures indicate only the changes made from the preceding figure. The scanning lines are horizontal and run from top to
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Fig. 1. BSE image of polystyrene blazed diffraction grating with 10 p,m spacing and about 1 ~.tmgroove depth at 21 mbar water vapour pressure, 15 kV, 2 x 10 -~ A, 50 s. Fig. 2. Current image, 100 s. Fig. 3. BSEimageat 1.7mbar l x l 0 sA,50s. Fig. 4. Current image, 100 s.
b o t t o m of the m i c r o g r a p h . T h e light pipe of the BSE d e t e c t o r was in the one o'clock direction a n d this gave rise to directional emission c o n t r a s t in the same direction. The c u r r e n t i m a g e of Fig. 2 shows the s a m e basic characteristics with less directional c o n t r a s t when it is f o r m e d at a pressure of 21 mbar. At a pressure of 1.7 m b a r the differences between the BSE i m a g e (Fig. 3) a n d the current i m a g e (Fig. 4) are significantly m o r e p r o n o u n c e d . T h e m i c r o g r a p h s were o b t a i n e d
sequentially in the o r d e r presented a n d it is believed that the differences shown are not due to i r r a d i a t i o n effects which can exist. T o confirm this, six m i c r o g r a p h s are p r e s e n t e d of the s a m e specimen which was r o t a t e d b y 90 ° . T h e y were o b t a i n e d u n d e r v a r y i n g c o n d i t i o n s as i n d i c a t e d in Figs. 5-10. It is o b s e r v e d that the specimen surface presents a distinct s q u a r e at the lower left corner. This c o r r e s p o n d s to a s c a n n i n g raster d u r i n g previous o b s e r v a t i o n . It was n o t e d that
Gaseous Detector Device
309
. . . .
'
~
T
I t
Fig. 5. Current image of the same sample as in Fig. 1 rotated by 90° at 20 mbar pressure, 100 s. Fig. 6. BSE image at 10 mbar, 50 s. Fig. 7. Current image at 10 mbar, 100 s. Fig. 8. BSE image at 7.3 mbar, 50 s.
such rasters appear when the pressure becomes very low, for example 0.1 mbar, but they do not form above about 1 mbar and once they appear at low pressure they result in a durable effect on this particular specimen. The basic characteristics of the BSE images remain unchanged with the variation of pressure whilst the current images show considerable variation with change of pressure. The current image tends to resemble the
BSE image at high pressure. A further nine micrographs were recorded after the one in Fig. 9, the last of which is shown in Fig. 10. The BSE image in Fig. 10 appears the same as the one in Fig. 6. Similarly the current images at 21 mbar, not shown here, are reproductions of the one shown in Fig. 5. Therefore the differences appearing in this series of micrographs should not be attributed to beam irradiation effects.
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Fig. 9. Current image at 3.5 mbar, 100 s. Fig. 10. BSE image at 21 mbar, 50 s. Fig. 11. Current image of wool fibres partially in water at 20 mbar of water vapour pressure, 15 kV, 2 x 10 ~ A, 100 s, horizontal field width = 250 jam. Fig. 12. BSE image, 50 s.
F i g u r e s 11 a n d 12 d e m o n s t r a t e the possibility of f o r m i n g images of wet specimens by using the c u r r e n t mode. A w o o l fibre b u n d l e w r a p p e d a r o u n d the s t r i p p e d end o f a wire a n d s o a k e d in w a t e r was examined. A c o m p a r i s o n between the c u r r e n t m o d e a n d BSE images indicates characteristics of c o n t r a s t inversion which is not u n i f o r m across the whole image. It is believed that s o m e differences between the t w o images are
d u e to detection of different signals a n d s o m e are due to p a r t i a l e v a p o r a t i o n of water a n d to i r r a d i a t i o n effects present u n d e r the relatively high c u r r e n t used. N o i m a g e signal processing is a v a i l a b l e a n d within the limits of o r d i n a r y level suppression a n d c o n t r a s t e n h a n c e m e n t m u c h i n f o r m a t i o n is lost on the images having extreme v a r i a t i o n s of signal intensity. In all the a b o v e cases the specimen was
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13
Fig. 13. Top view diagram of a copper grid specimen A and a current collector wire B placed 2 mm below the specimen. Arrow L indicates the direction of the light pipe of the BSE detector. Fig. 14. BSE image of the 50 lam copper grid aperture at 20 mbar of vapour pressure, 15 kV, 50 s. (The vertical stripe appearing on the left side of some micrographs was due to a fault of the electronics of the scanning circuit.) Fig. 15. Specimen A current image with electrode B being earthed, 2 × 10-8 A, 100 s. Fig. 16. Electrode B current image with A being earthed, 0.8 x 10 -8 A, 100 s.
a t t a c h e d to the e l e c t r o d e c o n d u c t i n g the signal current. In the following examples, s o m e alternative m e a n s a r e examined. A 50 lam single hole c o p p e r grid g l u e d with silver d a g o n t o a n a k e d wire was used as specimen. T h e specimen (see Fig. 13) was s u r r o u n d e d b y a wire f o r m i n g a l o o p at 2 m m b e l o w the level of the grid. F i g u r e 14 shows the BSE i m a g e of the c o p p e r
grid hole. All m i c r o g r a p h s are m o u n t e d in the s a m e w a y in relation to the direction of the scanning lines as in Fig. 1 a n d in a d d i t i o n their o r i e n t a t i o n c o r r e s p o n d s to t h a t of the d i a g r a m in Fig. 13. T h e d i r e c t i o n a l i t y of the BSE d e t e c t o r is readily seen. T h e c o r r e s p o n d i n g i m a g e b y using the current collected b y the specimen itself is shown in Fig. 15. T h e characteristics of inver-
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Fig. 17. Electrode B current image with A not being earthed, 1.2 × 10- s A. Fig. 18. BSE image of the 50 tam copper grid aperture at 0.2 mbar of vapour pressure, 50 s. Fig. 19. Specimen A current image with electrode B being earthed, 1.6 x 10 8 A, 100 s. Fig. 20. Electrode B current image with A not being earthed.
sion can be observed. However, when the c u r r e n t collected b y electrode B was used whilst the specimen A was e a r t h e d the result was different as is s h o w n in Fig. 16. It seems that there is a n a n a l o g y between the images showing the surface of the c o p p e r grid in Figs. 14 a n d 16 but this is n o t true for the regions inside a n d near the rim of the aperture. U s i n g exactly the same c o n d i t i o n s except with the specimen n o t being
earthed, the result is s h o w n in Fig. 17. The difference from t h a t of the previous m i c r o g r a p h is a d a r k h o r i z o n t a l zone which b e c o m e s slowly b r i g h t e r as the b e a m is scanned from the right rim of the a p e r t u r e a n d continues on from the left edge of the raster to the left rim of the aperture. A similar sequence of m i c r o g r a p h s is now presented at low pressure (0.2 m b a r of water v a p o u r ) in Figs. 18, 19 a n d 20. T h e same
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22
Fig, 21. Electrode B current image. Fig. 22. Top view of a copper grid specimen A, a current collector wire B placed opposite a second current collector C at 2 m m below the specimen A. Arrow indicates the direction of the light pipe of the BSE detector. Fig. 23. Unbiased e•ectr•de B current image with C earthed and A n•t earthed at 22 mbar pressure• •.8 x 10 8 A, 50 s. Fig. 24. Electrode C current image, C biased with + 2 V while A and B were earthed, 1.5 × 10 -8 A, 100 s.
specimen is used in exactly the same configuration as in Fig. 13 except that it appears rotated since it had to be remounted after it was unstuck accidentally. In addition the magnification used is lower but these changes are not important for the purposes of this article. The special effect described for Fig. 17 is also observed in Fig. 20 but in an inverted manner. Furthermore, the contrast inside the hole is inverted. The same
effect appears very weakly in Fig. 19. The zone effect appearing in Fig. 20 is not seen when the pressure is raised to 25 mbar (see Fig. 21) even with the specimen not earthed. Instead, two very weak horizontal dark stripes extend from the top and bottom rim. One more electrode configuration was tested as is shown in Fig. 22. The loop electrode was placed on one side of the specimen 2 mm below
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the specimen level and a second naked electrode (a metal clip) was placed at the opposite side again 2 mm below the specimen. The dimensions accompanying these diagrams are given for the accuracy of the report but they are not necessarily imperative. Figure 23 shows an image made by the current collected by electrode B without bias while C was earthed and A not earthed. Figure 24 shows an image made by the current collected by electrode C having a positive bias of + 2 V, while A and B were earthed. A significant amplification of the collected negative current was recorded. A comparison between Figs. 23 and 24 reveals that (a) there is an analogy of the surface contrast but this contrast becomes more enhanced with biasing the electrode, (b) there is a directionality of the contrast as can be seen by the dark and bright regions near the rim of the aperture relating to the orientation of the electrodes in Fig. 22 and (c) the current intensity is increased when the beam is scanned inside the aperture. These micrographs can be finally compared with the BSE image given in Fig. 25. Lastly images were formed by using the positive electric current collected by the electrode C when this electrode was biased with a small negative voltage. Figure 26 shows an image of the same specimen when the electode was biased with - 0 . 5 V at 22 mbar vapour pressure, Fig. 27 shows an image with - 0 . 5 V at a continuously decreasing pressure from 4.5 to 2.3 mbar during scanning of the micrograph and Fig. 28 shows the same specimen when the electrode was biased with - 3 V at 0.5 mbar of pressure. The vertical lines shown at regular intervals in these micrographs and particularly in the last micrograph are believed to be an artefact of the electronics of the micro-micro ammeter designed to measure normally negative currents. When a positive current flows at the input of the ammeter a deflection below the zero indication occurred and this restricted the amount of negative bias applied on the electrode C. No record of the current level was taken. During the progress of the above experiments it was found that images of the upper surface of the grid of the pressure limiting aperture (rough vacuum region) could be made by collecting the current below the aperture (high pressure region). THEORY In the previous section a compilation of
representative micrographs gives an illustration of new possibilities for imaging in the SEM. The principle which is believed to explain in general terms this new approach is now described supported with some further experimental evidence. In the vacuum SEM, the well known equation l=lh+ls+I
a
(1)
holds under steady state conditions, where I is the primary beam current, I b the backscattered electron current, Is the secondary electron current and la the absorbed specimen current. Each of the components of the right-hand side of the above equation produces an inverted image in relation to the image produced by the sum of the other two components. However, the equation (1) does not hold in the presence of gas. This can be seen from the graphs of Fig. 29 where the current was recorded as a function of gas pressure when the electron beam was directed inside the hole of a conducting cup or outside the hole as is shown in the diagram. The cup was made by drilling a 1 mm diameter by 0.5 mm depth hole into carbon material and gluing a 100 pm copper grid on top of the hole with silver dag. The current was collected either by the cup electrode, or by a second circular electrode wire placed 1 mm above the cup. In both cases the collecting electrode was biased with + 1 V while the other was earthed. It is observed that the current is increased in all cases with increase of pressure above 0.1 mbar, the minimum specimen chamber pressure attainable with the present set up. The number of primary electrons which arrive at the total cup area (5 mm in diameter) is expected to decrease due to scattering with increase of pressure and therefore the difference between the expected current and the measured current represents a net increase of negative current. This increase can be attributed to the negative ions and electrons which are captured by the electrode and which were created by ionizing radiations. Such ionizing radiations can be for example the primary, backscattered and secondary electrons as well as the X-rays. At low enough pressures the current collected by the cup is higher when the beam is directed inside the hole than when it is directed against the top surface of the copper grid. This is the expected trend because a fraction of the primary electrons is lost in the form of emitted BSE and SE electrons when the beam strikes the outside
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Fig. 25. BSE image of the 50 gm copper grid aperture. Fig. 26. Positive current image by electrode C at -0.5 V while B was earthed and A not earthed at 22 mbar, 100 s. Fig. 27. Positive current image with continuous variation of pressure from 4.5 to 2.3 mbar during the scanning time. Fig. 28. Positive current image by electrode C at - 3 V and 0.5 mbar.
surface. However, this trend is reversed as the pressure increases. This can be attributed primarily to the emitted electrons which can ionize the surrounding gas over a considerable distance from the cup. The emitted electrons from the copper surface outside the cup create an equal number of positive and negative ions but only (or mainly) the negative ions (or electrons) arrive at the cup because of its positive bias; inside the cup all ions, positive and negative, are trapped in the
main and they do not create a net current. Furthermore, the emitted electrons from outside the cup travel over much longer distances than inside the cup, thus creating a far greater number of ion pairs. The current level that is recorded when the beam is directed inside the hole is composed of primary electrons and ions formed by the primary electrons and therefore it contributes only to background noise level. The difference between
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O
1
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20
30
40
50
60
SPECIMENCHAMBER PRESSURE, tabor
29
Fig. 29. Graphs of the variation of current collected by electrode A while B was earthed or by B while A was earthed. The numerals 1 or 2 indicate that the beam was directed inside or outside the aperture of the cup respectively. Aperture of A was placed 1.5 mm below the pressure limiting aperture. Fig. 30. Micrograph of the 100 I,tm aperture of the cup A using the current collected by A at 0.5 mbar, 15 kV, 1.8 × 10 s A, 100 s, electrode A at + 1 V, B earthed. Fig. 31, Current image of A at 1.5 mbar, other conditions same as previous micrograph. Fig. 32. Current image of A at 5 mbar, other conditions same.
Gaseous Detector Device
the curves A1 and A2, or B1 and B2 is the component of current which contributes to the contrast formation for the copper surface. The number of ions formed increases rapidly initially with increase of pressure. This results in an increase of the difference B2-B1. In the case of the electrode A the difference A2-A1 is initially negative at low pressures, becomes zero at 1.7 mbar and then becomes positive. Figures 30, 31 and 32 show three micrographs taken in the regions below, near and above the crossover pressure by using the current of electrode A. The micrographs show the polished surface of the 100 lam aperture copper grid and the change of its contrast in relation to the contrast of the hole. The second crossover point of the curves A1-A2 and B1-B2 at the high pressure range does not seem to be of significance for contrast formation because the primary beam suffers severe scattering. The behaviour of these curves when the scattering index (product of gas pressure x beam travel distance) is high depends also on the size and geometry of A and its relative position to the beam path. The Observations and arguments presented up to now lead to the concept that the gas surroundingthe specimen in an electron microscope can 15e viewed in a similar way as a solid state backscattered electron detector. In the latter, the backscattered electrons create electron-hole pairs which flow across the junction barrier. In the present case, all ionizing radiations create pairs of positive and negative charges which can be collected independently by suitable electrodes. In this sense we may speak of a gaseous detector device t'or use in an environmental SEM. The scope and efficiency of the gaseous detector device depends on a number of variables such as: (a) nature of gas, (b) pressure of gas, (c) temperature of gas, (d) electrode configuration, (e) electrode bias, (f) accelerating voltage, (g) intensity of primary beam current, (h) scanning speed, (i) nature of specimen. A detailed study of the above variables is now needed to determine the origin of ion-pair generation, the rates of ion-pair creation and recombination as well as the ion-pair spatial distribution. This study will determine the ionpair populations and hence the intrinsic amplification of the gaseous detector device as well as the time constant of it. In addition, it will determine the extent to which the different ionizing signals can be separated from each other or mixed in a controlled way.
317
DISCUSSION The gas in the ESEM and ASEM can be used not only to maintain the specimen in a chosen state but also to act as a detection medium at the same time. Lane (1970) has considered it likely that more secondary electrons would be produced in the very thin low pressure gas layer surrounding the specimen in his device and these electrons would contribute to the signal in the conventional detector. However, it has now been found that charge carriers in general, i.e. free or quasi-free electrons, positive and negative ions are created in sufficient numbers and they can be collected by the application of only a low or even no external bias. The theory proposed in the previous section can help in the understanding of the basic mechanism which operates to produce the images presented in this work. However, many of the characteristics shown on the micrographs cannot be fully explained at this stage. For example, the origin of certain characteristics on the micrographs in Figs. 1-10 is not certain. The resemblance of Figs. 1 and 2 may indicate that the BSE are mainly responsible for the ionic current image at high pressure. The image inversion characteristics between Figs. 3 and 4 may indicate either a specimen absorbed current image or a secondary electron image if the absorbed current is negligible, or both. It is not known how conduction takes place with an insulator specimen. A conductive surface layer containing quasi-free electrons or a conductive adsorbed gas layer may be assumed but these hypotheses have to be proved. Conductivity could be easier to assume with the wet specimens of Fig. 1i. The details revealed to the right-hand side of the vertical ridges in Fig. 9 are not revealed under the conditions of the other micrographs in the group and they may be associated with the secondary electrons. The same applies for the detail shown around the ridges in Fig. 7. However, it was not possible to obtain a secondary electron image for a direct comparison. Alternatively some sort of trajectory contrast might be present. Furthermore, no external bias was applied, and it is not known to what extent the specimen and the attached electrode might have been self-biased. The results presented indicate that additional information can be obtained in the new detection system but the precise nature of this information requires further investigation.
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(i. Danilat os
Observing the contrast inside the hole of the specimen in Figs. 16, 17, 24 and 26, it is noted that the current intensity is maximum in this region. This can be attributed to the maximum ionic current created by the primary beam travelling over a long distance through the hole. The same is not observed for example in Figs. 20, 21 and 23 perhaps because the established selfbias and electrode configuration at the given pressure did not allow maximum collection of current. The zone effect observed such as in Figs. 17, 20, 26, 27 and 28 may be attributed to the variable bias that could be established during scanning. Transient phenomena have been observed during current measurements and these can be attributed to capacitance or inductance effects of a given time constant characterizing each electrode configuration. The electron beam crossing the rim of the aperture behaves like being switched on and off and the electrodes charge or discharge with time through some effective resistance. To the same cause may be attributed the gradual increase of signal intensity from left to right edge of micrographs frequently observed such as in Figs. 19 and 21. This variation of signal intensity is separate from the variation due to directionality of detection. The intensity of primary beam currents used in this work is rather high because of the electronic limitations of the given microscope. The optimum operating conditions of the new detection system are yet to be established but as low as 800 pA beam current has been used successfully. Current images have been obtained by using water vapour or air but the effect of the nature of the gas is a continuing study. The precise dependence of the collected ionic current on the external bias and the pressure of gas together with other quantitative investigations will be reported in the series on the design and construction of an ASEM or ESEM. The full implications of the developments in ESEM and ASEM now in progress will become clear gradually as more studies are performed on the interactions (~-~) between the members of the system below. beam 4
spectmen 4--
.. gas
,. signals
Several of the above interactions have been studied in detail by various workers in the past but the presence of gas added a new dimension to these studies. In particular, the interactions specimen ~ signals ~-~ gas ~-~ specimen play a determining role for the gaseous detector which is associated with a special scheme of electrodes, filters, windows etc. to achieve a required result.
CONCLUSION The gas and liquid phases in the ESEM and ASEM can be used as detection means. Absorption and emission signals can be detected in the presence of gas. It is proposed that the ionizing radiations in the electron microscope generate charge carriers which can be readily collected by one or more electrodes. The type of charge for a given system collected depends on the electrode configuration and bias. The charge collection forms the current which, after amplification, directly modulates the oscilloscope in the present work, but it is envisaged that other intermediate aids may be used to improve efficiency. It is believed that the gaseous detector device constitutes an alternative means to the backscattered electron detector which has been the only detector employed up to now in our ESEM.
Acknowledgement This work was supported by a grant from the Wool Research Trust Fund on the recommendation of the Australian Wool Corporation.
REFERENCES Danilatos, G. D., 1981. Design and construction of an atmospheric or environmental SEM (Part 1 ). Scanning, 4: 9 20. Danilatos, G. D. and Postle, R., 1983. Design and construction of an atmospheric or environmental SEM (Part 2). Micron, 14, 41 52. Lane, W. C., 1970. The environmental control stage. In: Scanning Electron Microscopy 1970, IITRI, Chicago, 43 48.
0739-6260/83 $3.00+0.00 © 1983PergamonPressLtd.
Printedin Great Britain.
A GASEOUS
DETECTOR
DEVICE FOR AN ENVIRONMENTAL
SEM
G. D. DANILATOS CSIRO Division of Textile Physics, 338 Blaxland Road, Ryde, NSW 2112, Australia (Received 5 Sepetember 1983)
Abstract--A new detection system has been used in the environmental SEM. The presence of gaseous and liquid phases in the microscope has made possible the formation of images of both insulators and conductors using the current mode. In addition, the ionizing radiations create in the gas negative and positive charge carriers which contain information from the beam-specimen interaction. These carriers, when collected by suitable means, such as a biased wire away from the specimen, can modulate the display signal to form images. Thus, the gas itself constitutes the basic component of a signal detector device, apart from its use as a conditioning medium.
INTRODUCTION The SEM has been modified so that specimens can be examined in a gaseous environment. The design and construction of an environmental or atmospheric SEM (ESEM or ASEM) is being reported in a separate series (Danilatos, 1981; Danilatos and Postle, 1983) and the reader will find it useful to refer to previous work as an aid to follow the current advances. The main objective in the previous work was to establish the conditions in the SEM under which the electron beam suffers a minimum amount of electron scattering due to gas. Imaging was performed by using backscattered electron detectors of specific forms. The present paper describes new possibilities of forming images in the SEM as a result of the presence of gas around the specimen. The electrical conductivity of the specimen and the surrounding gas is altered in the ESEM and ASEM and it is possible to use various currents as carriers of the information arising from the electron beam-specimen interactions.
USING THE CURRENT MODE OF DETECTION The absorbed current mode of detection is used in a conventional SEM with specimens which are good electrical conductors or made so, for example by depositing a conductive layer on 307
the specimen surface. However, in the conditions of ESEM or ASEM where the specimen is surrounded by gas it has been possible to obtain images of both conductors and insulators without any pretreatment by using the current mode. This possibility arises from the fact that the presence of gas creates large currents which are non-existent in the conditions of conventional SEM. The current used does not always correspond to the current absorbed by the specimen. Several examples below illustrate the main observations made. In Figs. 1-10 a comparison is made of the micrographs obtained under different conditions of imaging. A polystyrene diffraction grating with a saw-tooth spacing of 10 gm and about 1 gm groove depth was held by a metal clip which was connected to the input of the micro-micro ammeter of the JEOL JSM-2 SEM. Thus the current collected by the metal clip could be used to form images which could be compared with the backscattered electron images formed by using a plastic scintillator detector as described in previous work. A backscattered electron (BSE) image is shown in Fig. 1 under the conditions indicated. The current given in the captions is the average magnitude recorded in the presence of gas, which is different from the primary beam current as will be shown later. The captions of the following figures indicate only the changes made from the preceding figure. The scanning lines are horizontal and run from top to
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Fig. 1. BSE image of polystyrene blazed diffraction grating with 10 p,m spacing and about 1 ~.tmgroove depth at 21 mbar water vapour pressure, 15 kV, 2 x 10 -~ A, 50 s. Fig. 2. Current image, 100 s. Fig. 3. BSEimageat 1.7mbar l x l 0 sA,50s. Fig. 4. Current image, 100 s.
b o t t o m of the m i c r o g r a p h . T h e light pipe of the BSE d e t e c t o r was in the one o'clock direction a n d this gave rise to directional emission c o n t r a s t in the same direction. The c u r r e n t i m a g e of Fig. 2 shows the s a m e basic characteristics with less directional c o n t r a s t when it is f o r m e d at a pressure of 21 mbar. At a pressure of 1.7 m b a r the differences between the BSE i m a g e (Fig. 3) a n d the current i m a g e (Fig. 4) are significantly m o r e p r o n o u n c e d . T h e m i c r o g r a p h s were o b t a i n e d
sequentially in the o r d e r presented a n d it is believed that the differences shown are not due to i r r a d i a t i o n effects which can exist. T o confirm this, six m i c r o g r a p h s are p r e s e n t e d of the s a m e specimen which was r o t a t e d b y 90 ° . T h e y were o b t a i n e d u n d e r v a r y i n g c o n d i t i o n s as i n d i c a t e d in Figs. 5-10. It is o b s e r v e d that the specimen surface presents a distinct s q u a r e at the lower left corner. This c o r r e s p o n d s to a s c a n n i n g raster d u r i n g previous o b s e r v a t i o n . It was n o t e d that
Gaseous Detector Device
309
. . . .
'
~
T
I t
Fig. 5. Current image of the same sample as in Fig. 1 rotated by 90° at 20 mbar pressure, 100 s. Fig. 6. BSE image at 10 mbar, 50 s. Fig. 7. Current image at 10 mbar, 100 s. Fig. 8. BSE image at 7.3 mbar, 50 s.
such rasters appear when the pressure becomes very low, for example 0.1 mbar, but they do not form above about 1 mbar and once they appear at low pressure they result in a durable effect on this particular specimen. The basic characteristics of the BSE images remain unchanged with the variation of pressure whilst the current images show considerable variation with change of pressure. The current image tends to resemble the
BSE image at high pressure. A further nine micrographs were recorded after the one in Fig. 9, the last of which is shown in Fig. 10. The BSE image in Fig. 10 appears the same as the one in Fig. 6. Similarly the current images at 21 mbar, not shown here, are reproductions of the one shown in Fig. 5. Therefore the differences appearing in this series of micrographs should not be attributed to beam irradiation effects.
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Fig. 9. Current image at 3.5 mbar, 100 s. Fig. 10. BSE image at 21 mbar, 50 s. Fig. 11. Current image of wool fibres partially in water at 20 mbar of water vapour pressure, 15 kV, 2 x 10 ~ A, 100 s, horizontal field width = 250 jam. Fig. 12. BSE image, 50 s.
F i g u r e s 11 a n d 12 d e m o n s t r a t e the possibility of f o r m i n g images of wet specimens by using the c u r r e n t mode. A w o o l fibre b u n d l e w r a p p e d a r o u n d the s t r i p p e d end o f a wire a n d s o a k e d in w a t e r was examined. A c o m p a r i s o n between the c u r r e n t m o d e a n d BSE images indicates characteristics of c o n t r a s t inversion which is not u n i f o r m across the whole image. It is believed that s o m e differences between the t w o images are
d u e to detection of different signals a n d s o m e are due to p a r t i a l e v a p o r a t i o n of water a n d to i r r a d i a t i o n effects present u n d e r the relatively high c u r r e n t used. N o i m a g e signal processing is a v a i l a b l e a n d within the limits of o r d i n a r y level suppression a n d c o n t r a s t e n h a n c e m e n t m u c h i n f o r m a t i o n is lost on the images having extreme v a r i a t i o n s of signal intensity. In all the a b o v e cases the specimen was
Gaseous Detector Device
311
13
Fig. 13. Top view diagram of a copper grid specimen A and a current collector wire B placed 2 mm below the specimen. Arrow L indicates the direction of the light pipe of the BSE detector. Fig. 14. BSE image of the 50 lam copper grid aperture at 20 mbar of vapour pressure, 15 kV, 50 s. (The vertical stripe appearing on the left side of some micrographs was due to a fault of the electronics of the scanning circuit.) Fig. 15. Specimen A current image with electrode B being earthed, 2 × 10-8 A, 100 s. Fig. 16. Electrode B current image with A being earthed, 0.8 x 10 -8 A, 100 s.
a t t a c h e d to the e l e c t r o d e c o n d u c t i n g the signal current. In the following examples, s o m e alternative m e a n s a r e examined. A 50 lam single hole c o p p e r grid g l u e d with silver d a g o n t o a n a k e d wire was used as specimen. T h e specimen (see Fig. 13) was s u r r o u n d e d b y a wire f o r m i n g a l o o p at 2 m m b e l o w the level of the grid. F i g u r e 14 shows the BSE i m a g e of the c o p p e r
grid hole. All m i c r o g r a p h s are m o u n t e d in the s a m e w a y in relation to the direction of the scanning lines as in Fig. 1 a n d in a d d i t i o n their o r i e n t a t i o n c o r r e s p o n d s to t h a t of the d i a g r a m in Fig. 13. T h e d i r e c t i o n a l i t y of the BSE d e t e c t o r is readily seen. T h e c o r r e s p o n d i n g i m a g e b y using the current collected b y the specimen itself is shown in Fig. 15. T h e characteristics of inver-
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Fig. 17. Electrode B current image with A not being earthed, 1.2 × 10- s A. Fig. 18. BSE image of the 50 tam copper grid aperture at 0.2 mbar of vapour pressure, 50 s. Fig. 19. Specimen A current image with electrode B being earthed, 1.6 x 10 8 A, 100 s. Fig. 20. Electrode B current image with A not being earthed.
sion can be observed. However, when the c u r r e n t collected b y electrode B was used whilst the specimen A was e a r t h e d the result was different as is s h o w n in Fig. 16. It seems that there is a n a n a l o g y between the images showing the surface of the c o p p e r grid in Figs. 14 a n d 16 but this is n o t true for the regions inside a n d near the rim of the aperture. U s i n g exactly the same c o n d i t i o n s except with the specimen n o t being
earthed, the result is s h o w n in Fig. 17. The difference from t h a t of the previous m i c r o g r a p h is a d a r k h o r i z o n t a l zone which b e c o m e s slowly b r i g h t e r as the b e a m is scanned from the right rim of the a p e r t u r e a n d continues on from the left edge of the raster to the left rim of the aperture. A similar sequence of m i c r o g r a p h s is now presented at low pressure (0.2 m b a r of water v a p o u r ) in Figs. 18, 19 a n d 20. T h e same
Gaseous Detector Device
313
22
Fig, 21. Electrode B current image. Fig. 22. Top view of a copper grid specimen A, a current collector wire B placed opposite a second current collector C at 2 m m below the specimen A. Arrow indicates the direction of the light pipe of the BSE detector. Fig. 23. Unbiased e•ectr•de B current image with C earthed and A n•t earthed at 22 mbar pressure• •.8 x 10 8 A, 50 s. Fig. 24. Electrode C current image, C biased with + 2 V while A and B were earthed, 1.5 × 10 -8 A, 100 s.
specimen is used in exactly the same configuration as in Fig. 13 except that it appears rotated since it had to be remounted after it was unstuck accidentally. In addition the magnification used is lower but these changes are not important for the purposes of this article. The special effect described for Fig. 17 is also observed in Fig. 20 but in an inverted manner. Furthermore, the contrast inside the hole is inverted. The same
effect appears very weakly in Fig. 19. The zone effect appearing in Fig. 20 is not seen when the pressure is raised to 25 mbar (see Fig. 21) even with the specimen not earthed. Instead, two very weak horizontal dark stripes extend from the top and bottom rim. One more electrode configuration was tested as is shown in Fig. 22. The loop electrode was placed on one side of the specimen 2 mm below
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the specimen level and a second naked electrode (a metal clip) was placed at the opposite side again 2 mm below the specimen. The dimensions accompanying these diagrams are given for the accuracy of the report but they are not necessarily imperative. Figure 23 shows an image made by the current collected by electrode B without bias while C was earthed and A not earthed. Figure 24 shows an image made by the current collected by electrode C having a positive bias of + 2 V, while A and B were earthed. A significant amplification of the collected negative current was recorded. A comparison between Figs. 23 and 24 reveals that (a) there is an analogy of the surface contrast but this contrast becomes more enhanced with biasing the electrode, (b) there is a directionality of the contrast as can be seen by the dark and bright regions near the rim of the aperture relating to the orientation of the electrodes in Fig. 22 and (c) the current intensity is increased when the beam is scanned inside the aperture. These micrographs can be finally compared with the BSE image given in Fig. 25. Lastly images were formed by using the positive electric current collected by the electrode C when this electrode was biased with a small negative voltage. Figure 26 shows an image of the same specimen when the electode was biased with - 0 . 5 V at 22 mbar vapour pressure, Fig. 27 shows an image with - 0 . 5 V at a continuously decreasing pressure from 4.5 to 2.3 mbar during scanning of the micrograph and Fig. 28 shows the same specimen when the electrode was biased with - 3 V at 0.5 mbar of pressure. The vertical lines shown at regular intervals in these micrographs and particularly in the last micrograph are believed to be an artefact of the electronics of the micro-micro ammeter designed to measure normally negative currents. When a positive current flows at the input of the ammeter a deflection below the zero indication occurred and this restricted the amount of negative bias applied on the electrode C. No record of the current level was taken. During the progress of the above experiments it was found that images of the upper surface of the grid of the pressure limiting aperture (rough vacuum region) could be made by collecting the current below the aperture (high pressure region). THEORY In the previous section a compilation of
representative micrographs gives an illustration of new possibilities for imaging in the SEM. The principle which is believed to explain in general terms this new approach is now described supported with some further experimental evidence. In the vacuum SEM, the well known equation l=lh+ls+I
a
(1)
holds under steady state conditions, where I is the primary beam current, I b the backscattered electron current, Is the secondary electron current and la the absorbed specimen current. Each of the components of the right-hand side of the above equation produces an inverted image in relation to the image produced by the sum of the other two components. However, the equation (1) does not hold in the presence of gas. This can be seen from the graphs of Fig. 29 where the current was recorded as a function of gas pressure when the electron beam was directed inside the hole of a conducting cup or outside the hole as is shown in the diagram. The cup was made by drilling a 1 mm diameter by 0.5 mm depth hole into carbon material and gluing a 100 pm copper grid on top of the hole with silver dag. The current was collected either by the cup electrode, or by a second circular electrode wire placed 1 mm above the cup. In both cases the collecting electrode was biased with + 1 V while the other was earthed. It is observed that the current is increased in all cases with increase of pressure above 0.1 mbar, the minimum specimen chamber pressure attainable with the present set up. The number of primary electrons which arrive at the total cup area (5 mm in diameter) is expected to decrease due to scattering with increase of pressure and therefore the difference between the expected current and the measured current represents a net increase of negative current. This increase can be attributed to the negative ions and electrons which are captured by the electrode and which were created by ionizing radiations. Such ionizing radiations can be for example the primary, backscattered and secondary electrons as well as the X-rays. At low enough pressures the current collected by the cup is higher when the beam is directed inside the hole than when it is directed against the top surface of the copper grid. This is the expected trend because a fraction of the primary electrons is lost in the form of emitted BSE and SE electrons when the beam strikes the outside
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315
Fig. 25. BSE image of the 50 gm copper grid aperture. Fig. 26. Positive current image by electrode C at -0.5 V while B was earthed and A not earthed at 22 mbar, 100 s. Fig. 27. Positive current image with continuous variation of pressure from 4.5 to 2.3 mbar during the scanning time. Fig. 28. Positive current image by electrode C at - 3 V and 0.5 mbar.
surface. However, this trend is reversed as the pressure increases. This can be attributed primarily to the emitted electrons which can ionize the surrounding gas over a considerable distance from the cup. The emitted electrons from the copper surface outside the cup create an equal number of positive and negative ions but only (or mainly) the negative ions (or electrons) arrive at the cup because of its positive bias; inside the cup all ions, positive and negative, are trapped in the
main and they do not create a net current. Furthermore, the emitted electrons from outside the cup travel over much longer distances than inside the cup, thus creating a far greater number of ion pairs. The current level that is recorded when the beam is directed inside the hole is composed of primary electrons and ions formed by the primary electrons and therefore it contributes only to background noise level. The difference between
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8 xl0 3-
Z CI: c)
tlJ (Z O (.J t,i t~
O
1
~ A
0
10
20
30
40
50
60
SPECIMENCHAMBER PRESSURE, tabor
29
Fig. 29. Graphs of the variation of current collected by electrode A while B was earthed or by B while A was earthed. The numerals 1 or 2 indicate that the beam was directed inside or outside the aperture of the cup respectively. Aperture of A was placed 1.5 mm below the pressure limiting aperture. Fig. 30. Micrograph of the 100 I,tm aperture of the cup A using the current collected by A at 0.5 mbar, 15 kV, 1.8 × 10 s A, 100 s, electrode A at + 1 V, B earthed. Fig. 31, Current image of A at 1.5 mbar, other conditions same as previous micrograph. Fig. 32. Current image of A at 5 mbar, other conditions same.
Gaseous Detector Device
the curves A1 and A2, or B1 and B2 is the component of current which contributes to the contrast formation for the copper surface. The number of ions formed increases rapidly initially with increase of pressure. This results in an increase of the difference B2-B1. In the case of the electrode A the difference A2-A1 is initially negative at low pressures, becomes zero at 1.7 mbar and then becomes positive. Figures 30, 31 and 32 show three micrographs taken in the regions below, near and above the crossover pressure by using the current of electrode A. The micrographs show the polished surface of the 100 lam aperture copper grid and the change of its contrast in relation to the contrast of the hole. The second crossover point of the curves A1-A2 and B1-B2 at the high pressure range does not seem to be of significance for contrast formation because the primary beam suffers severe scattering. The behaviour of these curves when the scattering index (product of gas pressure x beam travel distance) is high depends also on the size and geometry of A and its relative position to the beam path. The Observations and arguments presented up to now lead to the concept that the gas surroundingthe specimen in an electron microscope can 15e viewed in a similar way as a solid state backscattered electron detector. In the latter, the backscattered electrons create electron-hole pairs which flow across the junction barrier. In the present case, all ionizing radiations create pairs of positive and negative charges which can be collected independently by suitable electrodes. In this sense we may speak of a gaseous detector device t'or use in an environmental SEM. The scope and efficiency of the gaseous detector device depends on a number of variables such as: (a) nature of gas, (b) pressure of gas, (c) temperature of gas, (d) electrode configuration, (e) electrode bias, (f) accelerating voltage, (g) intensity of primary beam current, (h) scanning speed, (i) nature of specimen. A detailed study of the above variables is now needed to determine the origin of ion-pair generation, the rates of ion-pair creation and recombination as well as the ion-pair spatial distribution. This study will determine the ionpair populations and hence the intrinsic amplification of the gaseous detector device as well as the time constant of it. In addition, it will determine the extent to which the different ionizing signals can be separated from each other or mixed in a controlled way.
317
DISCUSSION The gas in the ESEM and ASEM can be used not only to maintain the specimen in a chosen state but also to act as a detection medium at the same time. Lane (1970) has considered it likely that more secondary electrons would be produced in the very thin low pressure gas layer surrounding the specimen in his device and these electrons would contribute to the signal in the conventional detector. However, it has now been found that charge carriers in general, i.e. free or quasi-free electrons, positive and negative ions are created in sufficient numbers and they can be collected by the application of only a low or even no external bias. The theory proposed in the previous section can help in the understanding of the basic mechanism which operates to produce the images presented in this work. However, many of the characteristics shown on the micrographs cannot be fully explained at this stage. For example, the origin of certain characteristics on the micrographs in Figs. 1-10 is not certain. The resemblance of Figs. 1 and 2 may indicate that the BSE are mainly responsible for the ionic current image at high pressure. The image inversion characteristics between Figs. 3 and 4 may indicate either a specimen absorbed current image or a secondary electron image if the absorbed current is negligible, or both. It is not known how conduction takes place with an insulator specimen. A conductive surface layer containing quasi-free electrons or a conductive adsorbed gas layer may be assumed but these hypotheses have to be proved. Conductivity could be easier to assume with the wet specimens of Fig. 1i. The details revealed to the right-hand side of the vertical ridges in Fig. 9 are not revealed under the conditions of the other micrographs in the group and they may be associated with the secondary electrons. The same applies for the detail shown around the ridges in Fig. 7. However, it was not possible to obtain a secondary electron image for a direct comparison. Alternatively some sort of trajectory contrast might be present. Furthermore, no external bias was applied, and it is not known to what extent the specimen and the attached electrode might have been self-biased. The results presented indicate that additional information can be obtained in the new detection system but the precise nature of this information requires further investigation.
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Observing the contrast inside the hole of the specimen in Figs. 16, 17, 24 and 26, it is noted that the current intensity is maximum in this region. This can be attributed to the maximum ionic current created by the primary beam travelling over a long distance through the hole. The same is not observed for example in Figs. 20, 21 and 23 perhaps because the established selfbias and electrode configuration at the given pressure did not allow maximum collection of current. The zone effect observed such as in Figs. 17, 20, 26, 27 and 28 may be attributed to the variable bias that could be established during scanning. Transient phenomena have been observed during current measurements and these can be attributed to capacitance or inductance effects of a given time constant characterizing each electrode configuration. The electron beam crossing the rim of the aperture behaves like being switched on and off and the electrodes charge or discharge with time through some effective resistance. To the same cause may be attributed the gradual increase of signal intensity from left to right edge of micrographs frequently observed such as in Figs. 19 and 21. This variation of signal intensity is separate from the variation due to directionality of detection. The intensity of primary beam currents used in this work is rather high because of the electronic limitations of the given microscope. The optimum operating conditions of the new detection system are yet to be established but as low as 800 pA beam current has been used successfully. Current images have been obtained by using water vapour or air but the effect of the nature of the gas is a continuing study. The precise dependence of the collected ionic current on the external bias and the pressure of gas together with other quantitative investigations will be reported in the series on the design and construction of an ASEM or ESEM. The full implications of the developments in ESEM and ASEM now in progress will become clear gradually as more studies are performed on the interactions (~-~) between the members of the system below. beam 4
spectmen 4--
.. gas
,. signals
Several of the above interactions have been studied in detail by various workers in the past but the presence of gas added a new dimension to these studies. In particular, the interactions specimen ~ signals ~-~ gas ~-~ specimen play a determining role for the gaseous detector which is associated with a special scheme of electrodes, filters, windows etc. to achieve a required result.
CONCLUSION The gas and liquid phases in the ESEM and ASEM can be used as detection means. Absorption and emission signals can be detected in the presence of gas. It is proposed that the ionizing radiations in the electron microscope generate charge carriers which can be readily collected by one or more electrodes. The type of charge for a given system collected depends on the electrode configuration and bias. The charge collection forms the current which, after amplification, directly modulates the oscilloscope in the present work, but it is envisaged that other intermediate aids may be used to improve efficiency. It is believed that the gaseous detector device constitutes an alternative means to the backscattered electron detector which has been the only detector employed up to now in our ESEM.
Acknowledgement This work was supported by a grant from the Wool Research Trust Fund on the recommendation of the Australian Wool Corporation.
REFERENCES Danilatos, G. D., 1981. Design and construction of an atmospheric or environmental SEM (Part 1 ). Scanning, 4: 9 20. Danilatos, G. D. and Postle, R., 1983. Design and construction of an atmospheric or environmental SEM (Part 2). Micron, 14, 41 52. Lane, W. C., 1970. The environmental control stage. In: Scanning Electron Microscopy 1970, IITRI, Chicago, 43 48.