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Direct determination of microstructures by IAP FIM
Surface Science 246 (1991) 465-461 North-Holland
Direct determination
of microstructures
by IAP FIM
II. W bee lattice determined by a manual method D.M. Rena, W. Liu b and B.Y. Hu b a Center of Fundamental Physics, University of Scieme and Technology of China, Hefei, Anhui 230026, People’s Republic of China b Physics Department, Huazhong Normal Unrversity, Wuhan, Hubei 430070, People’s Republic of China Received
31 July 1990; accepted
for publication
27 August
1990
A manual procedure to determine the body-centered cubic structure of tungsten directly by its FIM micrographs and by field evaporation is reported here. It was performed to obtain a general method for determining the three-dimensional microstructure of FIM specimens, and as a first step of characterizing the method as well. It mainly consists of the following steps: The selection of the region of exploration; the localization of imaged atoms; the determination of the diameter of the tungsten atom model; simulation of the field ion micrographs; acquisition of the three-dimensional microstructure of the examined area.
1. Introduction It is a longing desire to acquire three-dimensional structural information of a sample directly by its field ion image, since the AP FIM technique has atomic scale resolution in the lateral plane and field evaporation has the same in-depth resolution. We have reported a scheme to attempt this aim [l]. The first step to confirm the feasibility of such idea and to acquire basic knowledge of performing the procedures has been fulfilled by manual operation. The result is reported here.
2. Experiment The experiment was carried out with our newly developed combination system /2]. The Chevron image intensifier of the system was at a distance of 17 cm from the tip, so it had a bigger magnification factor and less spherical distortion than in usual FIM. After the field evaporation and acquisition of a good field ion image of the W tip, the next step was to choose a proper exploration area of the surface with a sample rotation mechanism. In order to locate the atoms in a plane, the intervals 0039-602S/91/$03.50
0 1991 - Elsevier Science Publishers
between the neighboring atoms have to be as big as possible so that every atom can be distinguished from its image; but this requirement is hard to satisfy and the result is not as good as it seems because the change in the symmetry of ligand arrangement of one atom during field evaporation may cause it to displace significantly and lead to misjudgment of its location. We chose an intermediate case, a stepped plane in the vicinity of the W(110) plane (fig. l), as the exploration area: the atom chains could be separated distinctly, but not every atom of a chain could be discriminated without difficulty. Then the photographs were taken as the field evaporation went on. The following graphs were specially taken: The pictures of the complete plane were examined layer by layer to locate the atoms of the different layers, the images of the fieldevaporation process of the top layer help to locate some indistinguishable atoms, and the micrographs of the last few remaining atoms of a chain or a plane are used to examine the dislocation driven by external high field when the symmetry of their ligand arrangements significantly changed. The micrographs were projected on a screen to map the atom locations of every layer within the examined area. The size of the circular shadow of
B.V. (North-Holland)
466
D. M. Ren et al. / Direrf determination of microstructures
of the explored plane of W Fig. I. A fiefd ion micrograph imaged with He at 4.5 kV, which is in the vicinity of the (110) plane.
the projected detector screen was used as a yardstick to keep the projecting distance unchanged, and the defects of the detector were used as positioning marks for the mapping. After mapping and checking, seven maps of different layers symbolized with different colors were obtained. In order to get the basic parameters, some short atom chains consisting of two or three closely neighboring atoms (if too long, it is hard to know how many atoms it contains), were measured to estimate the diameter of the tungsten atom model. The average distance between the neighbors of an atom chain in the examined area was also calculated and taken as the interval between the nearest atom models. Modeling of the atoms and atom chains was done in accordance with the measured diameter and interval. Then a glass disk, of which the diameter was the same as the projected screen rim, was put on the map of the last layer to mark the orientation symbol and to model the explored plane with ball chains of a certain color (fig. 2). In the reverse order of field evaporation, the glass disk was moved onto the map of the former layer, oriented according the “orientation symbol”, and the second plane was modeled with ball chains of a different color. When this process continued, different color chains were stacked up into a set of
by IAP FIM. II
i Fig. 2. One-layer
model of the examined
plane.
separated tilted planes (fig. 3). When these layers were brought together along the direction normal to the glass-base to contact each other, their relative coordinates in the three dimensions were solved and the three-dimensional microstructure of the exploration region was obtained (fig. 4).
Fig. 3. A set of tilted planes was formed according to the layer maps. Because of the uncertainty of their coordinates in the normal direction, these planes were widely separated.
D.M. Ren et al. / Direct determination of microstructures by IAP FIh4. II
Fig. 4. When these planes were moved in the vertical direction until they make contact, a body-centered cubic lattice structure appeared. In order to clarify the bee structure, the balls were changed into two different colors.
3. Results and analysis The results are summarized as follows: (1) The size of the modeling atom was about 1 cm in our case, and the average interval of the nearest atoms in the examined chains was about 1.2 cm. Taking into account the error in the measurement, these atom chains were along the [loo] direction. That is, the average interval was the lattice parameter of the tungsten crystal. (2) The dislocation of an atom, due to the change in the symmetry in arrangement of its ligand, was found to be less than one half of the radius of the tungsten atom. (3) When the separately observed layer models were assembled in the right order according to their relative position, differently colored atomic chains were stacked up into a set of (110) planes. This result is consistent with the fact that these chains were in the [loo] direction. (4) When these (110) planes were brought into contact with each other vertically, a body-centered cubic lattice structure was obtained, as expected. Although the manual procedures cannot be
467
precise, its results confirmed the effectiveness of such a method convincingly. The second result was to be expected unless diffusion had taken place. This argument leads us to the reasonable assumption that the total error in the location of an atom caused by any possible factor, such as field distortion, dislocation, parallax, etc., except diffusion, is less than the radius of the atoms on which the atom image is situated. This implies that the method of determining the microstructure of a FIM sample by its FI image can provide at least a general idea about its real structure, even though the precise microstructure cannot be obtained without correction and modification. As the foregoing statement has pointed out, the gray level cannot be taken as a criterion to measure the depth of atoms, but the gray level pattern, bright at a plane edge and dim inside, can help to determine whether the atom images belong to a certain plane. According to this, the examined plane was pictured and its orientation adjusted. Another question concerning the gray level is whether the position of maximum gray level can be considered as the center of the atom image. To our knowledge the point of maximum brightness of an atom image is often off its center position unless its ligand arrangement is symmetric. So it is better to find out the rim of the atom image and to take its geometric center as the atom position, as we did. We leave the discussion of all the errors in the measurement to a following paper, for a digital processing procedure can provide more precise and meaningful data.
Acknowledgment This project is partly supported by The tional Science Commission of China.
Na-
References [I] W. Liu, D.M. Ren and B.Y. Hu, Surf. Sci. 246 (1991) 462. [2] W. Liu and D.M. Ren, J. Phys. (Paris) Colloq. 6 (1988) 35.
Direct determination
of microstructures
by IAP FIM
II. W bee lattice determined by a manual method D.M. Rena, W. Liu b and B.Y. Hu b a Center of Fundamental Physics, University of Scieme and Technology of China, Hefei, Anhui 230026, People’s Republic of China b Physics Department, Huazhong Normal Unrversity, Wuhan, Hubei 430070, People’s Republic of China Received
31 July 1990; accepted
for publication
27 August
1990
A manual procedure to determine the body-centered cubic structure of tungsten directly by its FIM micrographs and by field evaporation is reported here. It was performed to obtain a general method for determining the three-dimensional microstructure of FIM specimens, and as a first step of characterizing the method as well. It mainly consists of the following steps: The selection of the region of exploration; the localization of imaged atoms; the determination of the diameter of the tungsten atom model; simulation of the field ion micrographs; acquisition of the three-dimensional microstructure of the examined area.
1. Introduction It is a longing desire to acquire three-dimensional structural information of a sample directly by its field ion image, since the AP FIM technique has atomic scale resolution in the lateral plane and field evaporation has the same in-depth resolution. We have reported a scheme to attempt this aim [l]. The first step to confirm the feasibility of such idea and to acquire basic knowledge of performing the procedures has been fulfilled by manual operation. The result is reported here.
2. Experiment The experiment was carried out with our newly developed combination system /2]. The Chevron image intensifier of the system was at a distance of 17 cm from the tip, so it had a bigger magnification factor and less spherical distortion than in usual FIM. After the field evaporation and acquisition of a good field ion image of the W tip, the next step was to choose a proper exploration area of the surface with a sample rotation mechanism. In order to locate the atoms in a plane, the intervals 0039-602S/91/$03.50
0 1991 - Elsevier Science Publishers
between the neighboring atoms have to be as big as possible so that every atom can be distinguished from its image; but this requirement is hard to satisfy and the result is not as good as it seems because the change in the symmetry of ligand arrangement of one atom during field evaporation may cause it to displace significantly and lead to misjudgment of its location. We chose an intermediate case, a stepped plane in the vicinity of the W(110) plane (fig. l), as the exploration area: the atom chains could be separated distinctly, but not every atom of a chain could be discriminated without difficulty. Then the photographs were taken as the field evaporation went on. The following graphs were specially taken: The pictures of the complete plane were examined layer by layer to locate the atoms of the different layers, the images of the fieldevaporation process of the top layer help to locate some indistinguishable atoms, and the micrographs of the last few remaining atoms of a chain or a plane are used to examine the dislocation driven by external high field when the symmetry of their ligand arrangements significantly changed. The micrographs were projected on a screen to map the atom locations of every layer within the examined area. The size of the circular shadow of
B.V. (North-Holland)
466
D. M. Ren et al. / Direrf determination of microstructures
of the explored plane of W Fig. I. A fiefd ion micrograph imaged with He at 4.5 kV, which is in the vicinity of the (110) plane.
the projected detector screen was used as a yardstick to keep the projecting distance unchanged, and the defects of the detector were used as positioning marks for the mapping. After mapping and checking, seven maps of different layers symbolized with different colors were obtained. In order to get the basic parameters, some short atom chains consisting of two or three closely neighboring atoms (if too long, it is hard to know how many atoms it contains), were measured to estimate the diameter of the tungsten atom model. The average distance between the neighbors of an atom chain in the examined area was also calculated and taken as the interval between the nearest atom models. Modeling of the atoms and atom chains was done in accordance with the measured diameter and interval. Then a glass disk, of which the diameter was the same as the projected screen rim, was put on the map of the last layer to mark the orientation symbol and to model the explored plane with ball chains of a certain color (fig. 2). In the reverse order of field evaporation, the glass disk was moved onto the map of the former layer, oriented according the “orientation symbol”, and the second plane was modeled with ball chains of a different color. When this process continued, different color chains were stacked up into a set of
by IAP FIM. II
i Fig. 2. One-layer
model of the examined
plane.
separated tilted planes (fig. 3). When these layers were brought together along the direction normal to the glass-base to contact each other, their relative coordinates in the three dimensions were solved and the three-dimensional microstructure of the exploration region was obtained (fig. 4).
Fig. 3. A set of tilted planes was formed according to the layer maps. Because of the uncertainty of their coordinates in the normal direction, these planes were widely separated.
D.M. Ren et al. / Direct determination of microstructures by IAP FIh4. II
Fig. 4. When these planes were moved in the vertical direction until they make contact, a body-centered cubic lattice structure appeared. In order to clarify the bee structure, the balls were changed into two different colors.
3. Results and analysis The results are summarized as follows: (1) The size of the modeling atom was about 1 cm in our case, and the average interval of the nearest atoms in the examined chains was about 1.2 cm. Taking into account the error in the measurement, these atom chains were along the [loo] direction. That is, the average interval was the lattice parameter of the tungsten crystal. (2) The dislocation of an atom, due to the change in the symmetry in arrangement of its ligand, was found to be less than one half of the radius of the tungsten atom. (3) When the separately observed layer models were assembled in the right order according to their relative position, differently colored atomic chains were stacked up into a set of (110) planes. This result is consistent with the fact that these chains were in the [loo] direction. (4) When these (110) planes were brought into contact with each other vertically, a body-centered cubic lattice structure was obtained, as expected. Although the manual procedures cannot be
467
precise, its results confirmed the effectiveness of such a method convincingly. The second result was to be expected unless diffusion had taken place. This argument leads us to the reasonable assumption that the total error in the location of an atom caused by any possible factor, such as field distortion, dislocation, parallax, etc., except diffusion, is less than the radius of the atoms on which the atom image is situated. This implies that the method of determining the microstructure of a FIM sample by its FI image can provide at least a general idea about its real structure, even though the precise microstructure cannot be obtained without correction and modification. As the foregoing statement has pointed out, the gray level cannot be taken as a criterion to measure the depth of atoms, but the gray level pattern, bright at a plane edge and dim inside, can help to determine whether the atom images belong to a certain plane. According to this, the examined plane was pictured and its orientation adjusted. Another question concerning the gray level is whether the position of maximum gray level can be considered as the center of the atom image. To our knowledge the point of maximum brightness of an atom image is often off its center position unless its ligand arrangement is symmetric. So it is better to find out the rim of the atom image and to take its geometric center as the atom position, as we did. We leave the discussion of all the errors in the measurement to a following paper, for a digital processing procedure can provide more precise and meaningful data.
Acknowledgment This project is partly supported by The tional Science Commission of China.
Na-
References [I] W. Liu, D.M. Ren and B.Y. Hu, Surf. Sci. 246 (1991) 462. [2] W. Liu and D.M. Ren, J. Phys. (Paris) Colloq. 6 (1988) 35.