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Vacuum micro-chambers in electron microscopes
Vacuum/volume 41/numbers 7-9/pages 2055 to 2057/1990
0042-207X/9053.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
Vacuum micro-chambers in electron microscopes M D r e c h s l e r , CRMC2-CNRS, Campus de Luminy, Case 913-13288 Marseille, France A C l a v e r i e , CRNS, Laboratoire d'Optique, 31055 Toulouse, France and J Chazelas,
Thomson-CSF, Corbeville-910401 Orsay, France
In the course of in-situ transmission electron microscope studies, we found and analyzed a phenomenon of self-formation of small evacuated chambers. The chamber wall is partly metal and partly graphite film. The chamber formation is explained as an interface diffusion transport induced by capillarity. The chamber is resistant against atmospheric pressure. Such chambers can be used to study kinetic phenomena by in-situ electron microscopy.
Introduction
In order to produce a vacuum in a chamber a pump is usually required to transport gas atoms to the outside. Another possibility might be to find and use a force which drives atoms from the inner bulk of a solid by diffusion to the surface, thus producing a small vacuum chamber inside the solid. One might be sceptical as to whether such a process exists. Astonishingly, we found such a process in the course of experimental studies of composite solids inside a transmission electron microscope (TEM). This process and its interpretation are described in the following*. The composite structure of the solid The solid in which a vacuum chamber can be formed has a structure composed by a metal (Cu) tip, which is covered by a carbon or graphite film. The steps to form the composite structure are: (1) electrolytic etching of a finite 99.99 copper wire to a sharp conical tip having the desired cone angle 1"2, (2) cleaning, smoothing and some diffusion blunting of the tip by heating in vacuum 2, and (3) inside an electron microscope (TEM), growth of a 102-103 A thick carbon film (which becomes graphitized). Then a metal/carbon tip is obtained (Figures 1 and 2a), which is the basis for the formation of a vacuum chamber. The technique to produce and visualize the carbon film is described elsewhere 3. Formation of a vacuum chamber
A vacuum chamber is formed inside the microscope (TEM) simply by heating the described metal/carbon tip. The forma* The micro-chambers described here are produced by recrystallization processes and act as vacuum pumps. Such chambers should not be confused with manufactured micro-chambers, which might be located in a vacuum but which do not produce a vacuum (as those of A MAAS: Rhein Westf Akad d Wiss, V, 301, Westdeutscher, pp. 51-124 (1981).
Figure 1. Carbon film (0.5/am thick) on a copper tip.
tion is visible on the screen of the microscope and registered by video. An example is shown in Figure 2. As result of the heating, the inner tip end becomes bright (Figure 2b), which indicates that a vacuum has been formed by diffusion of copper towards the tip basis (see Interpretation). During the vacuum chamber formation, the vacuum/copper interface is continuously moving with an average speed of about 5 A/s (see Figure 2). Nevertheless, this speed is not constant, but decreases from ~ 2 0 A / s at the beginning (Figure 2a) to ~2A,/s for the situation shown in Figure 2b. A vacuum chamber formed at a higher temperature is shown in Figure 3. The length of this chamber is ~ 3/am. Inside the chamber are visible crystals, in particular a copper whisker of ~400 A, diameter and a compact copper crystal of ~3000 ,/k diameter. The whisker was growing on the Cu surface during its displacement. The compact crystal (on the right) has a size and a position which agrees with the known formation of 'solid drops' by surface self-diffusion2'4, a phenomenon which is limited to small (half) cone angles ( < 3°) 2"4. The half-cone angle near the initial tip end is in fact smaller than 3°. 2055
M Drechsler et al: Vacuum micro-chambers in electron microscopes
Figure 4. Blunting of a metal tip by self-diffusion of surface atoms (calculated shape change which is experimentally confirmed2,4): (a) tip of 4° cone angle; (b) tip of (a) after annealing.
Figure 2. Beginning of the formation of a vacuum chamber (C/Cu): (a) Initial state; (b) A micro-chamber is formed in 8 min by heating (600°C).
Figure 3. C/Cu vacuum chamber formed at a higher temperature (850°C, 15 min).
For a test, samples with vacuum chambers were taken out of the TEM and then evacuated again. The result was that the chambers are resistant against atmospheric pressure. The TEM pressure was either 10 -6 mbar or 3 x 10 -9 mbar (VG ultrahigh vacuum TEM). Such a pressure change has no recognizable influence on the vacuum chamber. Controls by S T E M - E E L S (scanning transmission electron microscope-electron energy loss spectroscopy) have shown that some of the films contain only carbon. In other cases the carbon film contains Cu crystallites (the small dark points in Figure 2 are probably such crystallites of a size of the order of 300 A). If the etched tip is not well cleaned (as in the experiment of Figure 2), impurities on the tip surface migrate at elevated temperature from the tip shank to the tip end where the impurities form a tip end bonnet (grey in Figure 2) to be described and explained elsewhere. The carbon layer is then formed on top of the bonnet substance. This experiment also shows that it is not difficult to bring a foreign substance into the chamber.
probably also for the vacuum chamber formation) is capillarity, i.e. the existence of a gradient of the surface curvature (1/R~+ l/R2), where R~ and R 2 are the principle radii of curvature. Atoms from regions of very small curvature at the tip end migrate to regions of larger curvature radii. This migration is always correlated with a reduction of the total surface area, i.e. a reduction of the surface free energy of the system. In the case of the composite structure (Figures 2 and 3), the atom migration is somewhat different, as schematized in Figure 5. During the chamber formation, the copper crystal has minimum curvature radii, always near the tip apex. Consequently, it exists always as a strong driving force to transport Cu atoms from the apex region to regions of larger curvature. What regions are those? Experience and interpretation show that not one but three different large-radii regions can act as a storage place for the diffusing Cu atoms. Where the Cu atoms are really stored depends essentially on the type of the formed graphite film as shown in the following: (1) No vacuum chamber is formed if the graphite film thickness is in the order of 10 A or less. Then the Cu atoms diffuse along the interface, and are stored on practically the same places as without the graphite film (as in Figure 4), i.e. diffusion and storage of the Cu atoms deform and displace this graphite film. A rough measurement has shown that the coefficient of the Cu diffusion along the Cu/C interface is of the same order of magnitude as the coefficient of Cu surface self-diffusion. (2) Vacuum chambers are formed when the graphite film is thicker than 102-103A. Then the film is not displaced or deformed in a measurable manner, i.e. the Cu atoms diffuse but are not stored along the interface. In this case, it is important that the graphite layer is prepared in a way that its length is DIFFUSION AND STORAGE OF THE CU ATOMS LOW x /-'--CURVATURE
,'~
2056
GRAPHITE FIL~
CROWING VACUUM
¢-
On the mechanism of the vacuum chamber formation
The disappearance of the copper crystal part near the tip end (Figures 2 and 3) must be the result of a diffusion transport initiated by a driving force. The crystal shape change is more or less similar to the blunting of a fine metal tip characterized by the known increase of the tip radius and a reduction of the tip length (Figure 4 ) 2 , 4 . The driving force for this blunting (and
CU SURFACE SELFFUSION
q ,/ INITIAL CU SURFACE
~
END OF GRAPHITE FILM
CURVATURE \~ CU DIFFUSION TRANSPORT ALONG THE CU/C INTERFACE
, HI H CURVATURE AT THE INITIAL CU TIP END
Figure 5. Scheme of the formation of a vacuum chamber.
M Drechsler et al: Vacuum micro-chambers in electron microscopes
limited (see Figure 5). Then, the diffusing Cu atoms pass the interface region and are stored on the graphite-free part of the tip shank. The carbon layer on the tip is limited by the electron impact which induces the layer formation. The layer limit on the tip is not yet directly visualized, but there is hope that this will be possible. (3) The third case corresponds to case (2), except that the graphite film covers a much longer part of the tip shank. Then, the driving force for the transport of Cu atoms must be considerably smaller, so that much more time would be necessary to form a vacuum chamber (which is usually not of interest).
Discussions Is bulk diffusion of importance? It is imaginable that the Cu transport may occur not along the interface but along the Cu bulk. This hypothesis is probably not correct because it is known that bulk diffusion becomes important only in regions of larger tip shank diameters (roughly above 102-10 3 pm). Consequently, it has to be assumed that bulk diffusion is negligible during the process of the vacuum chamber formation. On the vacuum pressure. The vacuum pressure in a chamber has not been determined so far. It can be assumed that this pressure is influenced by: ( l ) impurities on the metal tip surface, (2) the vapor pressure of the metal, (3) diffusion of gas atoms into the chamber (if the TEM vacuum is not UHV), and (4) atoms desorbing from substances which are brought into the chamber for special studies. It might be possible to produce ultra-high vacuum chambers and chambers of a definite pressure if improved preparation techniques are used (for example, improved substance cleaning).
On the chamber size. The maximum size of a vacuum chamber seems to be limited by the value of the driving force. Unfortunately, this force decreases considerably with increasing chamber size. The chamber in Figure 3 has a length of ~ 3 #m. Perhaps it might be possible to produce chambers of a length of 10 or even 20/~m, but one has to be aware that a limit of this order must exist. Outlook. The described vacuum chamber experiments may be regarded only as first examples of a variety of possible future experiments in this direction. Not only are controls of the described interpretations desirable, but so are studies using other substances, attempts to obtain quantitative informations, and tests of possible applications.
Acknowledgements The authors would like to acknowledge the contribution of Dr A Maas (University of Bonn, F R G ) who found in 1987, in collaboration with one of the authors (M D), by accident the first signs of a vacuum chamber formation. The authors would also like to thank Mr F Bel for technical assistance.
References =E W Miiller and T T Tsong, Field Ion Microscopy, Elsevier, New York (1969). 2V T Binh, A Piquet, H Roux, R Uzan and M Drechsler, Rev Physique Appl, 5, 648 (1970), and Surface Science, 25, 348 (1971). 3 M Drechsler, S Ramdani, A Claverie and A Mass, J Physique, C6-T48, 209 (1987), and Surface Science, 1891190, 1076 (1987). 4 F A Nichols and W W Mullins, J Appl Physics, 36, 1826 (1965).
2057
0042-207X/9053.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
Vacuum micro-chambers in electron microscopes M D r e c h s l e r , CRMC2-CNRS, Campus de Luminy, Case 913-13288 Marseille, France A C l a v e r i e , CRNS, Laboratoire d'Optique, 31055 Toulouse, France and J Chazelas,
Thomson-CSF, Corbeville-910401 Orsay, France
In the course of in-situ transmission electron microscope studies, we found and analyzed a phenomenon of self-formation of small evacuated chambers. The chamber wall is partly metal and partly graphite film. The chamber formation is explained as an interface diffusion transport induced by capillarity. The chamber is resistant against atmospheric pressure. Such chambers can be used to study kinetic phenomena by in-situ electron microscopy.
Introduction
In order to produce a vacuum in a chamber a pump is usually required to transport gas atoms to the outside. Another possibility might be to find and use a force which drives atoms from the inner bulk of a solid by diffusion to the surface, thus producing a small vacuum chamber inside the solid. One might be sceptical as to whether such a process exists. Astonishingly, we found such a process in the course of experimental studies of composite solids inside a transmission electron microscope (TEM). This process and its interpretation are described in the following*. The composite structure of the solid The solid in which a vacuum chamber can be formed has a structure composed by a metal (Cu) tip, which is covered by a carbon or graphite film. The steps to form the composite structure are: (1) electrolytic etching of a finite 99.99 copper wire to a sharp conical tip having the desired cone angle 1"2, (2) cleaning, smoothing and some diffusion blunting of the tip by heating in vacuum 2, and (3) inside an electron microscope (TEM), growth of a 102-103 A thick carbon film (which becomes graphitized). Then a metal/carbon tip is obtained (Figures 1 and 2a), which is the basis for the formation of a vacuum chamber. The technique to produce and visualize the carbon film is described elsewhere 3. Formation of a vacuum chamber
A vacuum chamber is formed inside the microscope (TEM) simply by heating the described metal/carbon tip. The forma* The micro-chambers described here are produced by recrystallization processes and act as vacuum pumps. Such chambers should not be confused with manufactured micro-chambers, which might be located in a vacuum but which do not produce a vacuum (as those of A MAAS: Rhein Westf Akad d Wiss, V, 301, Westdeutscher, pp. 51-124 (1981).
Figure 1. Carbon film (0.5/am thick) on a copper tip.
tion is visible on the screen of the microscope and registered by video. An example is shown in Figure 2. As result of the heating, the inner tip end becomes bright (Figure 2b), which indicates that a vacuum has been formed by diffusion of copper towards the tip basis (see Interpretation). During the vacuum chamber formation, the vacuum/copper interface is continuously moving with an average speed of about 5 A/s (see Figure 2). Nevertheless, this speed is not constant, but decreases from ~ 2 0 A / s at the beginning (Figure 2a) to ~2A,/s for the situation shown in Figure 2b. A vacuum chamber formed at a higher temperature is shown in Figure 3. The length of this chamber is ~ 3/am. Inside the chamber are visible crystals, in particular a copper whisker of ~400 A, diameter and a compact copper crystal of ~3000 ,/k diameter. The whisker was growing on the Cu surface during its displacement. The compact crystal (on the right) has a size and a position which agrees with the known formation of 'solid drops' by surface self-diffusion2'4, a phenomenon which is limited to small (half) cone angles ( < 3°) 2"4. The half-cone angle near the initial tip end is in fact smaller than 3°. 2055
M Drechsler et al: Vacuum micro-chambers in electron microscopes
Figure 4. Blunting of a metal tip by self-diffusion of surface atoms (calculated shape change which is experimentally confirmed2,4): (a) tip of 4° cone angle; (b) tip of (a) after annealing.
Figure 2. Beginning of the formation of a vacuum chamber (C/Cu): (a) Initial state; (b) A micro-chamber is formed in 8 min by heating (600°C).
Figure 3. C/Cu vacuum chamber formed at a higher temperature (850°C, 15 min).
For a test, samples with vacuum chambers were taken out of the TEM and then evacuated again. The result was that the chambers are resistant against atmospheric pressure. The TEM pressure was either 10 -6 mbar or 3 x 10 -9 mbar (VG ultrahigh vacuum TEM). Such a pressure change has no recognizable influence on the vacuum chamber. Controls by S T E M - E E L S (scanning transmission electron microscope-electron energy loss spectroscopy) have shown that some of the films contain only carbon. In other cases the carbon film contains Cu crystallites (the small dark points in Figure 2 are probably such crystallites of a size of the order of 300 A). If the etched tip is not well cleaned (as in the experiment of Figure 2), impurities on the tip surface migrate at elevated temperature from the tip shank to the tip end where the impurities form a tip end bonnet (grey in Figure 2) to be described and explained elsewhere. The carbon layer is then formed on top of the bonnet substance. This experiment also shows that it is not difficult to bring a foreign substance into the chamber.
probably also for the vacuum chamber formation) is capillarity, i.e. the existence of a gradient of the surface curvature (1/R~+ l/R2), where R~ and R 2 are the principle radii of curvature. Atoms from regions of very small curvature at the tip end migrate to regions of larger curvature radii. This migration is always correlated with a reduction of the total surface area, i.e. a reduction of the surface free energy of the system. In the case of the composite structure (Figures 2 and 3), the atom migration is somewhat different, as schematized in Figure 5. During the chamber formation, the copper crystal has minimum curvature radii, always near the tip apex. Consequently, it exists always as a strong driving force to transport Cu atoms from the apex region to regions of larger curvature. What regions are those? Experience and interpretation show that not one but three different large-radii regions can act as a storage place for the diffusing Cu atoms. Where the Cu atoms are really stored depends essentially on the type of the formed graphite film as shown in the following: (1) No vacuum chamber is formed if the graphite film thickness is in the order of 10 A or less. Then the Cu atoms diffuse along the interface, and are stored on practically the same places as without the graphite film (as in Figure 4), i.e. diffusion and storage of the Cu atoms deform and displace this graphite film. A rough measurement has shown that the coefficient of the Cu diffusion along the Cu/C interface is of the same order of magnitude as the coefficient of Cu surface self-diffusion. (2) Vacuum chambers are formed when the graphite film is thicker than 102-103A. Then the film is not displaced or deformed in a measurable manner, i.e. the Cu atoms diffuse but are not stored along the interface. In this case, it is important that the graphite layer is prepared in a way that its length is DIFFUSION AND STORAGE OF THE CU ATOMS LOW x /-'--CURVATURE
,'~
2056
GRAPHITE FIL~
CROWING VACUUM
¢-
On the mechanism of the vacuum chamber formation
The disappearance of the copper crystal part near the tip end (Figures 2 and 3) must be the result of a diffusion transport initiated by a driving force. The crystal shape change is more or less similar to the blunting of a fine metal tip characterized by the known increase of the tip radius and a reduction of the tip length (Figure 4 ) 2 , 4 . The driving force for this blunting (and
CU SURFACE SELFFUSION
q ,/ INITIAL CU SURFACE
~
END OF GRAPHITE FILM
CURVATURE \~ CU DIFFUSION TRANSPORT ALONG THE CU/C INTERFACE
, HI H CURVATURE AT THE INITIAL CU TIP END
Figure 5. Scheme of the formation of a vacuum chamber.
M Drechsler et al: Vacuum micro-chambers in electron microscopes
limited (see Figure 5). Then, the diffusing Cu atoms pass the interface region and are stored on the graphite-free part of the tip shank. The carbon layer on the tip is limited by the electron impact which induces the layer formation. The layer limit on the tip is not yet directly visualized, but there is hope that this will be possible. (3) The third case corresponds to case (2), except that the graphite film covers a much longer part of the tip shank. Then, the driving force for the transport of Cu atoms must be considerably smaller, so that much more time would be necessary to form a vacuum chamber (which is usually not of interest).
Discussions Is bulk diffusion of importance? It is imaginable that the Cu transport may occur not along the interface but along the Cu bulk. This hypothesis is probably not correct because it is known that bulk diffusion becomes important only in regions of larger tip shank diameters (roughly above 102-10 3 pm). Consequently, it has to be assumed that bulk diffusion is negligible during the process of the vacuum chamber formation. On the vacuum pressure. The vacuum pressure in a chamber has not been determined so far. It can be assumed that this pressure is influenced by: ( l ) impurities on the metal tip surface, (2) the vapor pressure of the metal, (3) diffusion of gas atoms into the chamber (if the TEM vacuum is not UHV), and (4) atoms desorbing from substances which are brought into the chamber for special studies. It might be possible to produce ultra-high vacuum chambers and chambers of a definite pressure if improved preparation techniques are used (for example, improved substance cleaning).
On the chamber size. The maximum size of a vacuum chamber seems to be limited by the value of the driving force. Unfortunately, this force decreases considerably with increasing chamber size. The chamber in Figure 3 has a length of ~ 3 #m. Perhaps it might be possible to produce chambers of a length of 10 or even 20/~m, but one has to be aware that a limit of this order must exist. Outlook. The described vacuum chamber experiments may be regarded only as first examples of a variety of possible future experiments in this direction. Not only are controls of the described interpretations desirable, but so are studies using other substances, attempts to obtain quantitative informations, and tests of possible applications.
Acknowledgements The authors would like to acknowledge the contribution of Dr A Maas (University of Bonn, F R G ) who found in 1987, in collaboration with one of the authors (M D), by accident the first signs of a vacuum chamber formation. The authors would also like to thank Mr F Bel for technical assistance.
References =E W Miiller and T T Tsong, Field Ion Microscopy, Elsevier, New York (1969). 2V T Binh, A Piquet, H Roux, R Uzan and M Drechsler, Rev Physique Appl, 5, 648 (1970), and Surface Science, 25, 348 (1971). 3 M Drechsler, S Ramdani, A Claverie and A Mass, J Physique, C6-T48, 209 (1987), and Surface Science, 1891190, 1076 (1987). 4 F A Nichols and W W Mullins, J Appl Physics, 36, 1826 (1965).
2057