- Email: [email protected]
Data processing in force curve mapping
Applied Surface Science 144–145 Ž1999. 613–617
Data processing in force curve mapping Katsumi Sugisaki ) , Nobuyuki Nakagiri
1
Tsukuba Research Laboratory, Nikon, 5-9-1 Tokodai, Tsukuba, Ibaraki 300-2635, Japan
Abstract Force curve mapping is expected to become a useful technique for material analysis. In order to analyze the huge amount of force curve data involved in such mapping, we have developed specialized data processing software. This software can extract three types of surface properties: adhesion force, topographic height and elasticity. We applied our data processing technique to force curve mapping of, first, particles on a substrate and, second, a silicon nitride membrane window. Our data processing successfully extracted those surface properties for these samples, demonstrating its effectiveness. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Force curve; Adhesion force; Topographic height; Elasticity
1. Introduction Force curves obtained by atomic force microscopy ŽAFM. are force–distance plots measured by monitoring the deflection of a cantilever as the cantilever tip approaches or retracts from a sample. Force curves include various information about tip– sample interaction. Both physical and chemical surface properties of the sample surface, including adhesion force and elasticity can be extracted by analyzing force curves. These properties extracted from force curves obtained at various locations can then be mapped to study their distribution over the sample surface. This is referred to as force curve mapping. Force curve mapping has been applied to various
)
Corresponding author. Group 3, 1st R & D Department, R & D Headquarters, Nikon, 1-6-3 Nishi-ohi, Shinagawa, Tokyo 1408601, Japan. Tel.: q81-3-3773-1111 Žext. 2732.; Fax: q81-33773-6376; E-mail: [email protected] 1 Present address: 1st R & D Department, R & D Headquarters, Nikon, 1-6-3 Nishi-ohi, Shinagawa, Tokyo 140-8601, Japan.
applications research fields including industrial w1–4x and biological w5–9x fields. Although much information can be derived by force curve mapping, it requires the analysis of a huge amount of force curve data. Analyzing these data and extracting surface properties from them are complicated and time-consuming. To overcome this, we have developed an automatic data processing algorithm to more efficiently analyze force curves and extract surface properties w10x. In this article, we describe how this data processing extracts surface properties from force curves. We also present our applications of this data processing to two kinds of samples: first, particles on a substrate and, second, a silicon nitride membrane window.
2. Data processing Our algorithm makes use of a differentiated force curve derived from an original force curve to recognize the condition of the cantilever ŽFig. 1.. Since
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 8 7 6 - 9
614
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
ferentiated curve. Since the force acting on its tip decreases with increasing z-position while the tip is in contact with the sample surface, C–E and E–F in Fig. 1a, the differentiated force values are negative, as shown by the corresponding CX –EX and EX –FX in Fig. 1b. Consequently, contact of the tip with the sample can be recognized by a negative differentiated force value. Adhesion force is derived from the difference between the force before snap-out and the null force, seen as F–G in Fig. 1a. The z-position at point D is used as the topographic height at the point where the cantilever is in contact but not in a state of deflection. Therefore this z-position indicates the sample
Fig. 1. Algorithm for force curve analysis. Ža. The force curve, Žb. the differentiated force curve.
this algorithm has been described in detail elsewhere w10x, here we describe it only briefly. During force curve measurements, the AFM tip suddenly jumps to make contact with the sample surface Žsnap-in, B–C in Fig. 1a. or to detach from it Žsnap-out, F–G.. The differentiated force curve has peaks at the snap-in and snap-out positions ŽPin and Pout in Fig. 1b.. The z-positions on the force curve where the snap-in and snap-out occurred can be automatically determined by these peaks on the dif-
Fig. 2. Schematic diagrams of the two kinds of samples analyzed showing Ža. latex particles of 1 mm on a Si substrate and Žb. a silicon nitride membrane window.
Fig. 3. Force curve mapping images of latex particles: Ža. topographic and Žb. adhesion force images.
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
height without any deformation due to pressing or pulling. The elastic properties are obtained by averaging the negative differentiated force values in the z-position region where the tip is pressing against the sample surface, seen as DX –EX in Fig. 1b. The elastic properties thus obtained represent the spring constant of the combined spring of the cantilever and the sample. The spring constant of the sample itself can then be calculated by using this combined spring constant and the cantilever’s known spring constant. Several snap-outs have been observed in the retracting process w11x indicating that the detachment process is actually composed of more than one snapout. Because our algorithm locates and identifies the snap-outs themselves, it can obtain the corresponding force of each. For statistical data collection using force curves such as protein rupture force measurements, the application of this algorithm is superior to other analysis methods which measure adhesion force at the peak of maximum negative cantilever deflection w4x. Furthermore, our algorithm can be employed in a real time system based on an analog circuit.
615
3. Experimental We employed a custombuilt AFM w12x which was controlled by a computer system. After identifying an interesting location by optical microscope, the AFM control system acquired an entire set of force curve data for mapping at that location and then stored the data set into the computer memory. Surface properties were extracted by our software developed on the basis of the above algorithm, and then mapped. We used two types of samples as illustrated in Fig. 2. Our first sample consisted of latex particles on a Si substrate ŽFig. 2a.. Uniformly sized latex particles Ž1.0 mm. were mixed with alcohol and diffused by ultrasonic vibration. A single drop of this mixture was applied to a Si substrate. After drying, the latex particles remained. The cantilever used for analysis of this sample had a spring constant of 0.1 Nrm. Our second sample was a silicon nitride membrane window ŽFig. 2b.. Since this sample had two
Fig. 4. Detailed analysis of a single particle’s motion. Ža. and Žb. show topographic and adhesion force images obtained by force curve mapping, respectively. Žc. Shows the force curve obtained at position A and Žd. that obtained at position B.
616
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
different regions, a free-standing membrane window and a support substrate, different elastic properties were expected to be derived from the different regions. The membrane had a thickness of 0.1 mm. The spring constant of the cantilever used for this analysis was 18 Nrm.
4. Results and discussions Fig. 3 shows force mapping images of the latex particles observed. A topographic image of the particles was clearly obtained and the measured heights of the particles were roughly equal to their actual sizes. Adhesion force at particle locations was lower than that on the surrounding Si substrate. The adhesion force was greatest around the edges of the particles. The contact area between the tip and the sample increases around a particle’s edge, resulting in higher adhesion force. The appearance of greater adhesion force at the edge of a particle has been previously reported by Torii et al. w3x. During our observations, the latex particles were sometimes moved by the AFM tip. Some of the latex particles appear to not have been tightly fixed on the substrate. Fig. 4 shows an example of such an occurrence. Using our stored force curve data, we were able to further analyze the force curves after automatic analysis. Judging from Fig. 4a and b, the latex particle indicated seemed to start moving at position A. The force curve at that position, seen in Fig. 4c, indicates that the AFM tip did not detach from the sample as it normally would. The force curve in Fig. 4d corresponding to position B, where the particle was still in motion, shows several snap-outs. These anomalous force curves suggest that this particle movement was caused by the AFM tip pulling or pushing. Additionally, since our data processing software algorithm locates and identifies each snap-out as mentioned earlier, the corresponding force of each snap-out in Fig. 4d was detected. These are indicated in the figure. Fig. 5a and b shows respectively topographic and elasticity images of the silicon nitride membrane window. This sample surface was observed to be flat in its topographic image ŽFig. 5a.. In contrast to this,
Fig. 5. Force curve mapping images of the silicon nitride membrane window. Ža. and Žb. show the topographic and elasticity images, respectively. Žc. Shows a cross section of the image indicated by A–B in Žb..
the elasticity image in Fig. 5b shows the window region to be softer Ždarker. than the support region. The topographic heights of the image were extracted from the force curves when the cantilever does not
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
deflect, that is, the tip applied no force to the sample. The featureless topographic image indicates no deformation of the window. Therefore, our height extraction can be applied to softer samples including biological materials. Fig. 5c shows the cross section at A–B in the elasticity image. This indicates that the interior of the window was softer than near its edge. In other words, deformation was greater at the center of the window than at its edge. Assuming that the spring constant of the support region was infinitely higher than the cantilever’s spring constant, the elasticity of 42 Nrm was obtained at the center of the window.
5. Conclusions Our data processing successfully extracted surface properties including topographic height, adhesion force and elasticity from force curves. It was also able to measure the corresponding force of each snap-out in a multiple snap-out process, indicating its applicability to the study of protein rupture forces where multiple snap-out often occur. Since a whole set of force curve data is stored during this data processing, further detailed analysis can be performed if any anomalous behavior is seen in the mapping images automatically obtained, as demonstrated with the additional detailed analysis of parti-
617
cle movement on a substrate. The elasticity analysis of the silicon nitride membrane window implies that this process could be applied to the characterization of mechanical properties of thin membrane micromachine parts.
References w1x R. Kaneko, K. Nonaka, K. Yasuda, J. Vac. Sci. Technol. A 6 Ž1988. 291. w2x H.A. Mizes, K.-G Loh, R.J.D. Miller, S.K. Ahuja, E.F. Grabowski, Appl. Phys. Lett. 59 Ž1991. 2901. w3x A. Torii, M. Sasaki, K. Hane, S. Okuma, Sens. Actuators A 44 Ž1994. 153. w4x K.O. van der Werf, C.A.J. Putman, B.G. de Grooth, J. Greve, Appl. Phys. Lett. 65 Ž1994. 1195. w5x M. Radmacher, J.P. Cleveland, M. Fritz, H.G. Hansma, P.K. Hansma, Biophys. J. 66 Ž1994. 2159. w6x M. Radmacher, M. Fritz, J.P. Cleveland, D.A. Walters, P.K. Hansma, Langmuir 10 Ž1994. 3809. w7x D.R. Baselt, J.D. Baldeschwieler, J. Appl. Phys. 76 Ž1994. 33. w8x M. Radmacher, M. Fritz, C.M. Kacher, J.P. Cleveland, P.K. Hansma, Biophys. J. 70 Ž1996. 556. w9x D.E. Laney, R.A. Garcia, S.M. Parsons, H.G. Hansma, Biophys J. 72 Ž1997. 806. w10x K. Sugisaki, K. Nakano, H. Sugimura, N. Kandaka, N. Nakagiri, Jpn. J. Appl. Phys. 37 Ž1998. 3820. w11x A. Ikai, K. Mitsui, Y. Furutani, M. Hara, J. McMurty, K.P. Wong, Jpn. J. Appl. Phys. 36 Ž1997. 3887. w12x K. Nakano, Rev. Sci. Instrum. 69 Ž1998. 1406.
Data processing in force curve mapping Katsumi Sugisaki ) , Nobuyuki Nakagiri
1
Tsukuba Research Laboratory, Nikon, 5-9-1 Tokodai, Tsukuba, Ibaraki 300-2635, Japan
Abstract Force curve mapping is expected to become a useful technique for material analysis. In order to analyze the huge amount of force curve data involved in such mapping, we have developed specialized data processing software. This software can extract three types of surface properties: adhesion force, topographic height and elasticity. We applied our data processing technique to force curve mapping of, first, particles on a substrate and, second, a silicon nitride membrane window. Our data processing successfully extracted those surface properties for these samples, demonstrating its effectiveness. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Force curve; Adhesion force; Topographic height; Elasticity
1. Introduction Force curves obtained by atomic force microscopy ŽAFM. are force–distance plots measured by monitoring the deflection of a cantilever as the cantilever tip approaches or retracts from a sample. Force curves include various information about tip– sample interaction. Both physical and chemical surface properties of the sample surface, including adhesion force and elasticity can be extracted by analyzing force curves. These properties extracted from force curves obtained at various locations can then be mapped to study their distribution over the sample surface. This is referred to as force curve mapping. Force curve mapping has been applied to various
)
Corresponding author. Group 3, 1st R & D Department, R & D Headquarters, Nikon, 1-6-3 Nishi-ohi, Shinagawa, Tokyo 1408601, Japan. Tel.: q81-3-3773-1111 Žext. 2732.; Fax: q81-33773-6376; E-mail: [email protected] 1 Present address: 1st R & D Department, R & D Headquarters, Nikon, 1-6-3 Nishi-ohi, Shinagawa, Tokyo 140-8601, Japan.
applications research fields including industrial w1–4x and biological w5–9x fields. Although much information can be derived by force curve mapping, it requires the analysis of a huge amount of force curve data. Analyzing these data and extracting surface properties from them are complicated and time-consuming. To overcome this, we have developed an automatic data processing algorithm to more efficiently analyze force curves and extract surface properties w10x. In this article, we describe how this data processing extracts surface properties from force curves. We also present our applications of this data processing to two kinds of samples: first, particles on a substrate and, second, a silicon nitride membrane window.
2. Data processing Our algorithm makes use of a differentiated force curve derived from an original force curve to recognize the condition of the cantilever ŽFig. 1.. Since
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 8 7 6 - 9
614
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
ferentiated curve. Since the force acting on its tip decreases with increasing z-position while the tip is in contact with the sample surface, C–E and E–F in Fig. 1a, the differentiated force values are negative, as shown by the corresponding CX –EX and EX –FX in Fig. 1b. Consequently, contact of the tip with the sample can be recognized by a negative differentiated force value. Adhesion force is derived from the difference between the force before snap-out and the null force, seen as F–G in Fig. 1a. The z-position at point D is used as the topographic height at the point where the cantilever is in contact but not in a state of deflection. Therefore this z-position indicates the sample
Fig. 1. Algorithm for force curve analysis. Ža. The force curve, Žb. the differentiated force curve.
this algorithm has been described in detail elsewhere w10x, here we describe it only briefly. During force curve measurements, the AFM tip suddenly jumps to make contact with the sample surface Žsnap-in, B–C in Fig. 1a. or to detach from it Žsnap-out, F–G.. The differentiated force curve has peaks at the snap-in and snap-out positions ŽPin and Pout in Fig. 1b.. The z-positions on the force curve where the snap-in and snap-out occurred can be automatically determined by these peaks on the dif-
Fig. 2. Schematic diagrams of the two kinds of samples analyzed showing Ža. latex particles of 1 mm on a Si substrate and Žb. a silicon nitride membrane window.
Fig. 3. Force curve mapping images of latex particles: Ža. topographic and Žb. adhesion force images.
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
height without any deformation due to pressing or pulling. The elastic properties are obtained by averaging the negative differentiated force values in the z-position region where the tip is pressing against the sample surface, seen as DX –EX in Fig. 1b. The elastic properties thus obtained represent the spring constant of the combined spring of the cantilever and the sample. The spring constant of the sample itself can then be calculated by using this combined spring constant and the cantilever’s known spring constant. Several snap-outs have been observed in the retracting process w11x indicating that the detachment process is actually composed of more than one snapout. Because our algorithm locates and identifies the snap-outs themselves, it can obtain the corresponding force of each. For statistical data collection using force curves such as protein rupture force measurements, the application of this algorithm is superior to other analysis methods which measure adhesion force at the peak of maximum negative cantilever deflection w4x. Furthermore, our algorithm can be employed in a real time system based on an analog circuit.
615
3. Experimental We employed a custombuilt AFM w12x which was controlled by a computer system. After identifying an interesting location by optical microscope, the AFM control system acquired an entire set of force curve data for mapping at that location and then stored the data set into the computer memory. Surface properties were extracted by our software developed on the basis of the above algorithm, and then mapped. We used two types of samples as illustrated in Fig. 2. Our first sample consisted of latex particles on a Si substrate ŽFig. 2a.. Uniformly sized latex particles Ž1.0 mm. were mixed with alcohol and diffused by ultrasonic vibration. A single drop of this mixture was applied to a Si substrate. After drying, the latex particles remained. The cantilever used for analysis of this sample had a spring constant of 0.1 Nrm. Our second sample was a silicon nitride membrane window ŽFig. 2b.. Since this sample had two
Fig. 4. Detailed analysis of a single particle’s motion. Ža. and Žb. show topographic and adhesion force images obtained by force curve mapping, respectively. Žc. Shows the force curve obtained at position A and Žd. that obtained at position B.
616
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
different regions, a free-standing membrane window and a support substrate, different elastic properties were expected to be derived from the different regions. The membrane had a thickness of 0.1 mm. The spring constant of the cantilever used for this analysis was 18 Nrm.
4. Results and discussions Fig. 3 shows force mapping images of the latex particles observed. A topographic image of the particles was clearly obtained and the measured heights of the particles were roughly equal to their actual sizes. Adhesion force at particle locations was lower than that on the surrounding Si substrate. The adhesion force was greatest around the edges of the particles. The contact area between the tip and the sample increases around a particle’s edge, resulting in higher adhesion force. The appearance of greater adhesion force at the edge of a particle has been previously reported by Torii et al. w3x. During our observations, the latex particles were sometimes moved by the AFM tip. Some of the latex particles appear to not have been tightly fixed on the substrate. Fig. 4 shows an example of such an occurrence. Using our stored force curve data, we were able to further analyze the force curves after automatic analysis. Judging from Fig. 4a and b, the latex particle indicated seemed to start moving at position A. The force curve at that position, seen in Fig. 4c, indicates that the AFM tip did not detach from the sample as it normally would. The force curve in Fig. 4d corresponding to position B, where the particle was still in motion, shows several snap-outs. These anomalous force curves suggest that this particle movement was caused by the AFM tip pulling or pushing. Additionally, since our data processing software algorithm locates and identifies each snap-out as mentioned earlier, the corresponding force of each snap-out in Fig. 4d was detected. These are indicated in the figure. Fig. 5a and b shows respectively topographic and elasticity images of the silicon nitride membrane window. This sample surface was observed to be flat in its topographic image ŽFig. 5a.. In contrast to this,
Fig. 5. Force curve mapping images of the silicon nitride membrane window. Ža. and Žb. show the topographic and elasticity images, respectively. Žc. Shows a cross section of the image indicated by A–B in Žb..
the elasticity image in Fig. 5b shows the window region to be softer Ždarker. than the support region. The topographic heights of the image were extracted from the force curves when the cantilever does not
K. Sugisaki, N. Nakagirir Applied Surface Science 144–145 (1999) 613–617
deflect, that is, the tip applied no force to the sample. The featureless topographic image indicates no deformation of the window. Therefore, our height extraction can be applied to softer samples including biological materials. Fig. 5c shows the cross section at A–B in the elasticity image. This indicates that the interior of the window was softer than near its edge. In other words, deformation was greater at the center of the window than at its edge. Assuming that the spring constant of the support region was infinitely higher than the cantilever’s spring constant, the elasticity of 42 Nrm was obtained at the center of the window.
5. Conclusions Our data processing successfully extracted surface properties including topographic height, adhesion force and elasticity from force curves. It was also able to measure the corresponding force of each snap-out in a multiple snap-out process, indicating its applicability to the study of protein rupture forces where multiple snap-out often occur. Since a whole set of force curve data is stored during this data processing, further detailed analysis can be performed if any anomalous behavior is seen in the mapping images automatically obtained, as demonstrated with the additional detailed analysis of parti-
617
cle movement on a substrate. The elasticity analysis of the silicon nitride membrane window implies that this process could be applied to the characterization of mechanical properties of thin membrane micromachine parts.
References w1x R. Kaneko, K. Nonaka, K. Yasuda, J. Vac. Sci. Technol. A 6 Ž1988. 291. w2x H.A. Mizes, K.-G Loh, R.J.D. Miller, S.K. Ahuja, E.F. Grabowski, Appl. Phys. Lett. 59 Ž1991. 2901. w3x A. Torii, M. Sasaki, K. Hane, S. Okuma, Sens. Actuators A 44 Ž1994. 153. w4x K.O. van der Werf, C.A.J. Putman, B.G. de Grooth, J. Greve, Appl. Phys. Lett. 65 Ž1994. 1195. w5x M. Radmacher, J.P. Cleveland, M. Fritz, H.G. Hansma, P.K. Hansma, Biophys. J. 66 Ž1994. 2159. w6x M. Radmacher, M. Fritz, J.P. Cleveland, D.A. Walters, P.K. Hansma, Langmuir 10 Ž1994. 3809. w7x D.R. Baselt, J.D. Baldeschwieler, J. Appl. Phys. 76 Ž1994. 33. w8x M. Radmacher, M. Fritz, C.M. Kacher, J.P. Cleveland, P.K. Hansma, Biophys. J. 70 Ž1996. 556. w9x D.E. Laney, R.A. Garcia, S.M. Parsons, H.G. Hansma, Biophys J. 72 Ž1997. 806. w10x K. Sugisaki, K. Nakano, H. Sugimura, N. Kandaka, N. Nakagiri, Jpn. J. Appl. Phys. 37 Ž1998. 3820. w11x A. Ikai, K. Mitsui, Y. Furutani, M. Hara, J. McMurty, K.P. Wong, Jpn. J. Appl. Phys. 36 Ž1997. 3887. w12x K. Nakano, Rev. Sci. Instrum. 69 Ž1998. 1406.