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Removal Characteristics of Processing with SPM
Removal Characteristics of Processing with SPM T. Miyazaki’, S. Yoshiokal, Y. Shirail, T. Misul, N. Taniguchiz (1) Chiba Institute of Technology, Tsudanuma, Narashino, Chiba 275 Japan 2ScienceUniversity of Tokyo, Yamazaki, Noda, Chiba 278 Japan Received on January 2,1998
Abstract SPM (Scanning Probe Microscope) can be used as a processing tool. In the present paper, processings are carried out with AFMand STM in air at room temperature. In case of AFM processing with silicon tips, no depression is formed in gold workpiece; however, it is formed in silicon workpiece, which seems to be caused by strong adhesion between the same materials. In case of STM processing with tungsten tips, although depressions are formed in the gold workpiece, the reproducibility is lowwith hand-made tips; however, field evaporation processing is carried out with high reproducibility. Keywords :Removal, Processing, SPM
1 . Introduction
Scanning probe microscope (SPM) has become a common observation tool in various fields of science and technology. In the manufacturing technology, it has been used for evaluation of the processed surfaces (Ichida et al., Carneiro et al.); however, this can be used as a processing tool. The manipulation of single atom was done (Eigler et al.). Atomic scale surface modification with SPM has been made owing to field evaporation, van der Waals’ force, mechanical force, reaction due to tunneling electrons and so forth. The atomic scale processing has been done in ultrahigh vacuum or ultralow temperature. h e experimental results are very interesting because the nature of materials on atomic scale is revealed and they present possibilities of ultrafine processing of materials for making nanometer scale devices. Howver, a purpose of nanoprocessing is to make small devices which can be operated at room temperature and in air (Garcia et al., Kaneko et al.). The processing is required to be done in such conditions. The present paper deals with the processing characteristics in removal processings with AFM and STM in air at room temperature. 2 . Processina equipment and t ips
A conventional AFM is used in a contact mode of observation with high pressure force. As to STM processing, a conventional STM was remodeled so that the additional voltage can be forcibly applied to the piezoelectric element for position controlling unit and workpiece. By interrupting the observation of the
Annals of the ClRP Vol. 47/1/1998
workpiece surface, the processing due to movement of the tip or applying the additional electrical field can be done at a prescribed position. In this kind of processing, the processing and observation should be made with the same equipment without interruption because it is difficult to find the processed portion with a different observation unit. Tips used for the AFM processing are commercially obtained, which are made of single crystal silicon. I p s used for the STM processing are made by electrochemical etching of a tungsten wire of 0.3 mm in diameter in NaOHsolution (8%) (Bryant et al.). From the estimation of the SEM images, the curvature radius of the pointed end of the tip is less than 200 nm. An example of the tip is shown in Fig.1. Workpiece surface is required to be smooth on nanometer scale in this kind of processing so that gold and silicon were selected. Droplet of gold with smooth surface is easily realized and the surface is kept clean in air. The droplet of gold (111) is made of wire of 0.5 mm in diameter with melting by a hydrogen flame and sequential cooling in air. Single crystal silicon (100) is commercially obtained.
3. Experimenta I results and discussion (1) Processing of gold with AFM The tip is pressed into the surface with a prescribed force and is scanned. Traces of the tip scanning have not been observed with a scanning on a line: thus, the tip has been scanned on a wide range, that is, the range of width of 200 nm, scanning distance of 200 nm and pitch of 0.78 nm. Traces of upheavals at the starting and ending points of the scanning are observed; however, clear traces of material removal
153
have not been observed. The AFM images of the scanned region are almost the same as those of the original workpiece surface. The upheaval height at the ending position of the scanning is always obtained. Relations hips between the upheaval height, pressing force and scanning speed are shown in Figs. 2 and 3. It is clearly seen that the upheaval height does not depend on the pressing force. The height of 0.2 - 0.3 nm is nearly the same as the lattice distance of the gold workpiece, which is estimated from the STfvl image; thus, the upheaval is formed by one atomic layer. However, the height becomes higher at lower scanning speed. The SEM image of the tip is shown in Fig.4, after scanning with a pressing force of 850 nNand distance of 10.6 mm (200nm X 5.3 x lo'times). The image is almost the same as that of the original tip. The radius of the pointed end of the tip is estimated less than 50nm. When the radius is assumed to be 50 nm, the Hertzian stress is 4.1 GPa for the pressing force of 680 nN; the contact radius is 7.3 nm and the normal deformation between the tip and workpiece is 1.1 nm. The yield strength of Au (60 % reduction) is 2.1 Gpa (Lyman,ed.). The stress is higher than the yield strength; the clear material removal due to scanning cannot be observed, but upheaval is formed. It is said that the tip is sliding on the workpiece with only very small amount of gold atoms scratched by the tip. The scratched amount is so small that there is no clear trace of removal. Those gold atoms adhere to the workpiece when the tip leaves the workpiece; thus, the upheaval is formed at the end of scanning. Some atoms adhere to the workpiece at the starting point of scanning. For increasing the amount of material removal, the scanning of the tip is repeated. A non-contact AFM image of the workpiece, which is repeatedly processed under the conditions of a pressing force of 850 nN, scanning speed of 2p,m/s, scanning pitch of 0.78 nm, width of 200 nm, and scanning distance of 200 nm, is shown in Fig.5. The image is obtained by scanning in the same direction of the processing. The piezoelectric element for the position control of the workpiece has a hysteresis characteristic so that the reverse movement of the piezoelectric element causes a false image of the surface. Figures 6(a) and (b) showthe cross section images of the center of the processed region; the former (a) shows the image obtained by scanning in the same direction as the processing and the latter (b) the image in the reverse direction. In the figures, (A) corresponds to (F) and (C) corresponds to (H). Depressions (B) and (0)in Fig.6(a) are not observed in Fig.G(b) and those (E)and (G) in Fig.G(b) are not observed in Rg.G(a). Thus, the dark portion indicating the depression in Fig.5 is a false image. There are no depressions around the upheavals; therefore, the plastic deformation is not the cause of the upheaval. The cause of the upheavals at the starting and ending points of the processing has been already described previously. 154
(2) Processina of aold with STM The surface is observed for determining the processing point. Then, the additional voltage is applied to the piezoelectric element for pressing the tip into the workpiece and scanning the tip. A groove is formed, which is shown in Fig.7. In this case, the displacement distance of the tip from the measuring position is 4.8 nm and scanning speed is 3*m/s. The width and depth of the groove are 12 nm and 2 nm, respectively. However, the dimensions of the processed region have strong dependency on the tips, which are hand-made. For measurement, one atom at the pointed end of the tip seems to be used: thus, relatively clear STM images can be obtained with hand-made tips. However, the shape of the tip seems to have strong effect on the mechanical processing reproducibility. Field evaporation processingis also carried out, which is done by applying a voltage between the tip and workpiece. When positive voltage is applied to the workpiece with respect to the tip, atoms of the workpiece surface layer are removed and a depression is formed. When negative voltage is applied to the workpiece, atoms of the tip are removed and deposited on the workpiece. Relationships between the applied voltage and processed diameter are s h o w in Rg.8 for different hand-made tips. For reference, deposition data are also shown. For this kind of processing, the shape of the tip has little effect on the processing reproducibility. 431 Processina of silicon with AFM In micromachining systems, silicon is used as a mechanical material. Therefore, in the present experiments, single crystal silicon (100) is also selected for the workpiece. One scanning with the pitch of 0.78 nm, width of 200 nm and distance of 200 nm generates upheavals at the ending point of the processing. This is the same as in case of the gold workpiece. The height is also shown in Figs.2 and 3; the height does not depends on the force or speed. The amount of the material removal is very small so that the removed depth is not distinguished from the original roughness of the workpiece. The height corresponds to 3 to 4 atomic layer because the lattice distance of single crystal silicon is about 0.3 nm. Repeated processing is also carried out under the same conditions as for the gold workpiece. A noncontact image of the cross section is shown in Fig.9, which is obtained by scanning the tip in the same direction of the processing. A SEM image of the tip used for repeated processing of silicon is shown in Fig.10. The image is also influenced by the hysteresis effect. It is however obvious that there is no upheaval at the starting point of the processing; furthermore, the material removal is generated at the scanned region and the upheaval is formed at the ending position. On the other hand, in case of the processing of gold, the material removal is not observed. The Hertzian stress for silicon under the experimental conditions (assumingthe curvature radius of the tip 50 nm) is 6.0 GPa and the yield strength is 7.0 Gpa
(Petersen et al.). The yield strength of silicon is much higher than that of gold; however, the material removal is clearly observed in silicon. The difference in removalcapability in these two materials s e e m to be caused by the higher adhesion effect between the same materials. This is indicated in Fig.10, where the pointed end of the tip is also worn. This will be a difficult problem in the micromachine systems constructed by silicon. 4 . Summary
Processings with AFM and STM are carried out in air at room temperature. Mechanicalprocessing with silicon tip with AFM cannot generate clear material removal from the gold work piece; however depressions are formed in the silicon workpiece. This seems to be caused by adhesion effect between the same materials. The mechanical processing capability with STM is not stable; however, the electrical field processing is stably carried out.
Bryant,P.J., Kim,H.S., Zheng,Y.C., Yang,R., 1987,Technique for shaping tunneling microscope tips, Rev.Sci.lnstrum., 58: 1115. Carneiro, K., Jensen, C.P., J4rgensen, J.F., Graina?s,J., 1995, Roughness Parameters of Surfaces by Atomic Force Microscopy, Annals Of CIRP, 44: 517-522. Eigler,D.M., Schweizer, E.K.,1990, Positioning single atoms with a scanning tunneling microscope, Nature, 344: 524-526. Garcia.R.G., 1992, Atomic scale manipulation in air with the scanning tunneling microscope, Appl. Phys. Lett., 60: 1960-1962. Ichida,Y., Kishi,K., 1993, Nanotopography of UltraprecisionGround Surface of Fine Ceramics Using Atomic Force Microscope,Annals of CIRP, 42: 647-650. Kaneko,R., Umemura, S. , Hirano, M., Andoh, Y., Miyamoto,T. and Fukui, S., 1996, Recent progress in microtribology,Wear, 200: 296304. Lyrnan,T., (ed.), Metals Handbook, Vol.1, 1961, American Society for Metals, 8th ed., p.1186. Petersen, Kurt E., 1982, Silicon as a MechanicalMaterial, ProcJEEE, 70:420-457.
1.5
-
1.0
cbo
0
Si 0
0
A 0.5
fi A Au
A
0.5 1.o 2.0 Scanning speed pm/s Fig.3 Relationships between scanning speed and upheaval height at ending position of scanning in Au and Si workpieces with Si tip Pressing force: 85 nN, Scannina Ditch: 0.78 nm 0
Fig:l SEM image of tungsten tip
0
200
400
Pressing force
A
600 nN
Fig.2 Relationships between pressing force and upheaval height at ending position of scanning in Au and Si workpieces with Si tip Scanning speed: 0.5 p d s , Scanning pitch: 0.78 nm
Fig.4 SEM image of silicon tip after processing of gold
155
Fig.5 AFM image of processed gold with AFM Pressing force: 850 nN, Scanning speed: 2 pm/s, Scanning pitch: 0.78 nm, Scanning width: 200 nm, Scanning distance: 200 nm, Repeated number: 66
-6
-4
-2
0
2
Voltage between tip (grounded) and workpiece Fig.8
6
4
V
Relationships between applied voltage and processed diameter in Au workpiece with W tip mean that the data are Symbols 0,0,0 obtained with different tips.
Fig.9 AFM image of silicon processed with AFM Pressing force: 850 nN, Scanning speed: 2 pm/s, Scanning pitch: 0.78 nm, Scanning width: 200 nm, Scanning distance: 200 nm, Repeated number: 66 Fig.6
Cross sections of gold processed with AFM Arrows mean the tip scanning directions in measurement.
Fig.7 STM image of gold processed with STM Scanning speed: 3pm/s,Displacement of tip: 4.8nm 156
Fig.10 SEM image of silicon tip after processing of silicon
Abstract SPM (Scanning Probe Microscope) can be used as a processing tool. In the present paper, processings are carried out with AFMand STM in air at room temperature. In case of AFM processing with silicon tips, no depression is formed in gold workpiece; however, it is formed in silicon workpiece, which seems to be caused by strong adhesion between the same materials. In case of STM processing with tungsten tips, although depressions are formed in the gold workpiece, the reproducibility is lowwith hand-made tips; however, field evaporation processing is carried out with high reproducibility. Keywords :Removal, Processing, SPM
1 . Introduction
Scanning probe microscope (SPM) has become a common observation tool in various fields of science and technology. In the manufacturing technology, it has been used for evaluation of the processed surfaces (Ichida et al., Carneiro et al.); however, this can be used as a processing tool. The manipulation of single atom was done (Eigler et al.). Atomic scale surface modification with SPM has been made owing to field evaporation, van der Waals’ force, mechanical force, reaction due to tunneling electrons and so forth. The atomic scale processing has been done in ultrahigh vacuum or ultralow temperature. h e experimental results are very interesting because the nature of materials on atomic scale is revealed and they present possibilities of ultrafine processing of materials for making nanometer scale devices. Howver, a purpose of nanoprocessing is to make small devices which can be operated at room temperature and in air (Garcia et al., Kaneko et al.). The processing is required to be done in such conditions. The present paper deals with the processing characteristics in removal processings with AFM and STM in air at room temperature. 2 . Processina equipment and t ips
A conventional AFM is used in a contact mode of observation with high pressure force. As to STM processing, a conventional STM was remodeled so that the additional voltage can be forcibly applied to the piezoelectric element for position controlling unit and workpiece. By interrupting the observation of the
Annals of the ClRP Vol. 47/1/1998
workpiece surface, the processing due to movement of the tip or applying the additional electrical field can be done at a prescribed position. In this kind of processing, the processing and observation should be made with the same equipment without interruption because it is difficult to find the processed portion with a different observation unit. Tips used for the AFM processing are commercially obtained, which are made of single crystal silicon. I p s used for the STM processing are made by electrochemical etching of a tungsten wire of 0.3 mm in diameter in NaOHsolution (8%) (Bryant et al.). From the estimation of the SEM images, the curvature radius of the pointed end of the tip is less than 200 nm. An example of the tip is shown in Fig.1. Workpiece surface is required to be smooth on nanometer scale in this kind of processing so that gold and silicon were selected. Droplet of gold with smooth surface is easily realized and the surface is kept clean in air. The droplet of gold (111) is made of wire of 0.5 mm in diameter with melting by a hydrogen flame and sequential cooling in air. Single crystal silicon (100) is commercially obtained.
3. Experimenta I results and discussion (1) Processing of gold with AFM The tip is pressed into the surface with a prescribed force and is scanned. Traces of the tip scanning have not been observed with a scanning on a line: thus, the tip has been scanned on a wide range, that is, the range of width of 200 nm, scanning distance of 200 nm and pitch of 0.78 nm. Traces of upheavals at the starting and ending points of the scanning are observed; however, clear traces of material removal
153
have not been observed. The AFM images of the scanned region are almost the same as those of the original workpiece surface. The upheaval height at the ending position of the scanning is always obtained. Relations hips between the upheaval height, pressing force and scanning speed are shown in Figs. 2 and 3. It is clearly seen that the upheaval height does not depend on the pressing force. The height of 0.2 - 0.3 nm is nearly the same as the lattice distance of the gold workpiece, which is estimated from the STfvl image; thus, the upheaval is formed by one atomic layer. However, the height becomes higher at lower scanning speed. The SEM image of the tip is shown in Fig.4, after scanning with a pressing force of 850 nNand distance of 10.6 mm (200nm X 5.3 x lo'times). The image is almost the same as that of the original tip. The radius of the pointed end of the tip is estimated less than 50nm. When the radius is assumed to be 50 nm, the Hertzian stress is 4.1 GPa for the pressing force of 680 nN; the contact radius is 7.3 nm and the normal deformation between the tip and workpiece is 1.1 nm. The yield strength of Au (60 % reduction) is 2.1 Gpa (Lyman,ed.). The stress is higher than the yield strength; the clear material removal due to scanning cannot be observed, but upheaval is formed. It is said that the tip is sliding on the workpiece with only very small amount of gold atoms scratched by the tip. The scratched amount is so small that there is no clear trace of removal. Those gold atoms adhere to the workpiece when the tip leaves the workpiece; thus, the upheaval is formed at the end of scanning. Some atoms adhere to the workpiece at the starting point of scanning. For increasing the amount of material removal, the scanning of the tip is repeated. A non-contact AFM image of the workpiece, which is repeatedly processed under the conditions of a pressing force of 850 nN, scanning speed of 2p,m/s, scanning pitch of 0.78 nm, width of 200 nm, and scanning distance of 200 nm, is shown in Fig.5. The image is obtained by scanning in the same direction of the processing. The piezoelectric element for the position control of the workpiece has a hysteresis characteristic so that the reverse movement of the piezoelectric element causes a false image of the surface. Figures 6(a) and (b) showthe cross section images of the center of the processed region; the former (a) shows the image obtained by scanning in the same direction as the processing and the latter (b) the image in the reverse direction. In the figures, (A) corresponds to (F) and (C) corresponds to (H). Depressions (B) and (0)in Fig.6(a) are not observed in Fig.G(b) and those (E)and (G) in Fig.G(b) are not observed in Rg.G(a). Thus, the dark portion indicating the depression in Fig.5 is a false image. There are no depressions around the upheavals; therefore, the plastic deformation is not the cause of the upheaval. The cause of the upheavals at the starting and ending points of the processing has been already described previously. 154
(2) Processina of aold with STM The surface is observed for determining the processing point. Then, the additional voltage is applied to the piezoelectric element for pressing the tip into the workpiece and scanning the tip. A groove is formed, which is shown in Fig.7. In this case, the displacement distance of the tip from the measuring position is 4.8 nm and scanning speed is 3*m/s. The width and depth of the groove are 12 nm and 2 nm, respectively. However, the dimensions of the processed region have strong dependency on the tips, which are hand-made. For measurement, one atom at the pointed end of the tip seems to be used: thus, relatively clear STM images can be obtained with hand-made tips. However, the shape of the tip seems to have strong effect on the mechanical processing reproducibility. Field evaporation processingis also carried out, which is done by applying a voltage between the tip and workpiece. When positive voltage is applied to the workpiece with respect to the tip, atoms of the workpiece surface layer are removed and a depression is formed. When negative voltage is applied to the workpiece, atoms of the tip are removed and deposited on the workpiece. Relationships between the applied voltage and processed diameter are s h o w in Rg.8 for different hand-made tips. For reference, deposition data are also shown. For this kind of processing, the shape of the tip has little effect on the processing reproducibility. 431 Processina of silicon with AFM In micromachining systems, silicon is used as a mechanical material. Therefore, in the present experiments, single crystal silicon (100) is also selected for the workpiece. One scanning with the pitch of 0.78 nm, width of 200 nm and distance of 200 nm generates upheavals at the ending point of the processing. This is the same as in case of the gold workpiece. The height is also shown in Figs.2 and 3; the height does not depends on the force or speed. The amount of the material removal is very small so that the removed depth is not distinguished from the original roughness of the workpiece. The height corresponds to 3 to 4 atomic layer because the lattice distance of single crystal silicon is about 0.3 nm. Repeated processing is also carried out under the same conditions as for the gold workpiece. A noncontact image of the cross section is shown in Fig.9, which is obtained by scanning the tip in the same direction of the processing. A SEM image of the tip used for repeated processing of silicon is shown in Fig.10. The image is also influenced by the hysteresis effect. It is however obvious that there is no upheaval at the starting point of the processing; furthermore, the material removal is generated at the scanned region and the upheaval is formed at the ending position. On the other hand, in case of the processing of gold, the material removal is not observed. The Hertzian stress for silicon under the experimental conditions (assumingthe curvature radius of the tip 50 nm) is 6.0 GPa and the yield strength is 7.0 Gpa
(Petersen et al.). The yield strength of silicon is much higher than that of gold; however, the material removal is clearly observed in silicon. The difference in removalcapability in these two materials s e e m to be caused by the higher adhesion effect between the same materials. This is indicated in Fig.10, where the pointed end of the tip is also worn. This will be a difficult problem in the micromachine systems constructed by silicon. 4 . Summary
Processings with AFM and STM are carried out in air at room temperature. Mechanicalprocessing with silicon tip with AFM cannot generate clear material removal from the gold work piece; however depressions are formed in the silicon workpiece. This seems to be caused by adhesion effect between the same materials. The mechanical processing capability with STM is not stable; however, the electrical field processing is stably carried out.
Bryant,P.J., Kim,H.S., Zheng,Y.C., Yang,R., 1987,Technique for shaping tunneling microscope tips, Rev.Sci.lnstrum., 58: 1115. Carneiro, K., Jensen, C.P., J4rgensen, J.F., Graina?s,J., 1995, Roughness Parameters of Surfaces by Atomic Force Microscopy, Annals Of CIRP, 44: 517-522. Eigler,D.M., Schweizer, E.K.,1990, Positioning single atoms with a scanning tunneling microscope, Nature, 344: 524-526. Garcia.R.G., 1992, Atomic scale manipulation in air with the scanning tunneling microscope, Appl. Phys. Lett., 60: 1960-1962. Ichida,Y., Kishi,K., 1993, Nanotopography of UltraprecisionGround Surface of Fine Ceramics Using Atomic Force Microscope,Annals of CIRP, 42: 647-650. Kaneko,R., Umemura, S. , Hirano, M., Andoh, Y., Miyamoto,T. and Fukui, S., 1996, Recent progress in microtribology,Wear, 200: 296304. Lyrnan,T., (ed.), Metals Handbook, Vol.1, 1961, American Society for Metals, 8th ed., p.1186. Petersen, Kurt E., 1982, Silicon as a MechanicalMaterial, ProcJEEE, 70:420-457.
1.5
-
1.0
cbo
0
Si 0
0
A 0.5
fi A Au
A
0.5 1.o 2.0 Scanning speed pm/s Fig.3 Relationships between scanning speed and upheaval height at ending position of scanning in Au and Si workpieces with Si tip Pressing force: 85 nN, Scannina Ditch: 0.78 nm 0
Fig:l SEM image of tungsten tip
0
200
400
Pressing force
A
600 nN
Fig.2 Relationships between pressing force and upheaval height at ending position of scanning in Au and Si workpieces with Si tip Scanning speed: 0.5 p d s , Scanning pitch: 0.78 nm
Fig.4 SEM image of silicon tip after processing of gold
155
Fig.5 AFM image of processed gold with AFM Pressing force: 850 nN, Scanning speed: 2 pm/s, Scanning pitch: 0.78 nm, Scanning width: 200 nm, Scanning distance: 200 nm, Repeated number: 66
-6
-4
-2
0
2
Voltage between tip (grounded) and workpiece Fig.8
6
4
V
Relationships between applied voltage and processed diameter in Au workpiece with W tip mean that the data are Symbols 0,0,0 obtained with different tips.
Fig.9 AFM image of silicon processed with AFM Pressing force: 850 nN, Scanning speed: 2 pm/s, Scanning pitch: 0.78 nm, Scanning width: 200 nm, Scanning distance: 200 nm, Repeated number: 66 Fig.6
Cross sections of gold processed with AFM Arrows mean the tip scanning directions in measurement.
Fig.7 STM image of gold processed with STM Scanning speed: 3pm/s,Displacement of tip: 4.8nm 156
Fig.10 SEM image of silicon tip after processing of silicon