- Email: [email protected]
Etching characteristics for tracks of carbon cluster ions in polycarbonate
Nuclear Instruments and Methods in Physics Research B 217 (2004) 621–626 www.elsevier.com/locate/nimb
Etching characteristics for tracks of carbon cluster ions in polycarbonate Ziqiang Zhao *, Dongchen Qi, Zhiyu Guo Institute of Heavy Ion Physics, Peking University and Key Laboratory of Heavy Ion Physics, Ministry of Education, Beijing 100871, PR China Received 23 July 2003; received in revised form 19 November 2003
Abstract A series of chemical etching experiments were carried out on polycarbonate foils irradiated by carbon cluster ions with an energy of 0.6 MeV/atom. The bulk etching rate was calculated from weight loss. The transversal etching rate was obtained by using linear fit to the pore diameters under a time sequence. It was found that the transversal etching rate depends on the rate of electronic energy deposition of projectiles. A distinctly irregular pore size distribution was found on the image of etched pores after Cþ 4 irradiation and after one hour etching and explained as the contribution of dissociation of the carbon cluster. 2003 Elsevier B.V. All rights reserved. PACS: 61.81.jh; 61.82.pv; 79.20 Keywords: Latent tracks; Energy loss; Carbon cluster; Polycarbonate; Etching rate; Pores; Distribution
1. Introduction The formation of tracks induced by the electronic deposited energy of energetic ions has been studied experimentally and theoretically since the early 1950s [1–4]. Although a lot of efforts have been made on this subject, there is still an incomplete understanding of tracks concerning the fundamental atomic structure of damage as well as macroscopic observable properties. Among all the materials, track formation in polymers is most complex and attracting since not only primary but *
Corresponding author. Tel.: +86-10-6275-5363; fax: +8610-6275-1875. E-mail address: [email protected] (Z. Zhao).
also secondary processes such as the formation of radicals and chemical processes like cross-linking in polyethylene terephthalate (PET) [6] are involved. The selective chemical etching has become by far the most general and useful means of investigating tracks of energetic charged particles in polymers. It is based on the fact that the damaged region possesses a higher chemical activity than the undamaged matrix, so the etching process allows the tracks can be observed directly by optical or scanning electron microscopy. This technique has numerous application in applied science ranging from solid state nuclear detector to micromachining such as growth of nanostructure [5]. Successful track etching requires that the damaged material along the track is preferentially
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.091
622
Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
etched at a much higher rate than the surrounding undamaged matrix; in another word the track etching rate Vt has to be higher than the bulk etching rate Vb . The ratio of the two etching rates Vb =Vt determines the geometry of the etched pores. It has been experimentally verified that the bulk etching rate remains constant for a given material and etchant under a specific etching condition including the temperature and concentration [7]. In contrast, the track etching rate which reflects the extent of damage along the ion path in material depends greatly on the energy loss of the trackforming ions. For fundamental understanding and practical considerations, we studied the etching properties of polycarbonate after irradiated of individual carbon ion and cluster ions. The reason polycarbonate was chosen from numerous available materials is that these films offer a high differential etching rate between the irradiated and nonirradiated parts of the membrane which is very crucial for the geometry of the etched tracks. In general, the electronic energy loss (dE=dx) of the ion in the materials plays a main role [2] and the radial extension of the track implies that the energetic secondary electrons (d-rays) created in the path of ion in materials also contribute to the energy transportation and track formation [8]. Based on this we choose carbon cluster ions in our investigation because clusters have higher local deposited energy densities in materials than individual ions at same velocity. For comparison, individual carbon ions are also used to irradiation. A series of traces etched after irradiated of carbon clusters Cþ n (n ¼ 1–5) at same velocity are performed the characteristic of tracks are defined and compared.
2. Experimental details 2.1. Irradiation Polycarbonate films are commercial available films with thickness of 138 lm. They were cut into rectangle 25 mm in length and 20 mm in width. These films were then irradiated by carbon cluster ions at room temperature at normal incidence in the 2 · 1.7 MeV tandem accelerator (5SDH-2
NEC) at the Key Laboratory of Heavy Ion Physics, Peking University. The energy of carbon cluster ions is 0.6 MeV/atom. The ion beam was scanned and average ion beam current was typically in the order of 1011 A in the irradiated area of 11 cm2 . The track density of 106 –107 ions/cm2 in the irradiated sample will be made and no overlapping of tracks after being etched to round pores. After irradiation, the samples were kept in air for several days. 2.2. Etching conditions In our experiment, irradiated films were etched in a 6 M/l solution of NaOH containing 10% of the methanol at a constant temperature of 45 C, with etching time in a range from 0.5 to 3.0 h with a step of 0.5 h. After etching the films were rinsed in deionized water and then dried in air for days. 2.3. Microscopy and measurements After etching, the number and the size of the etched pits on the surface of the samples were determined using a scanning electron microscope (SEM) of KYKY-1000B. The weight loss of the samples were also measured with an analytical electrobalance.
3. Results and discussion 3.1. Weight loss measurements The weight of one unirradiated film and other five films irradiated by carbon cluster ions Cþ n (n ¼ 1–5) respectively were measured by a sensitive analytical electrobalance with an accuracy of ±0.01 mg before and after 1.0 h etching in order to determine the weight loss during etching process. The result can be seen in Table 1, where M1 is the weight before etching and M2 is the weight after etching, the projected range was calculated by using the TRIM code [10], and the dose calculated from the ion beam current and the area of irradiation surface was expected to be in the order of 1 · 107 ions/cm2 . The real track density was expected in the same order and also were listed in
Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
623
Table 1 The weight loss of samples irradiated by carbon cluster ions and after 1.0 h etching Ion
Projected range [lm]
Track density [ions/cm2 ]
M1 [g]
M2 [g]
M1 M2 M1
Nonirradiated Cþ Cþ 2 Cþ 3 Cþ 4 Cþ 5
1.40 1.40 1.40 1.40 1.40 1.40
– 2.3 · 107 2.0 · 107 9.3 · 106 3.4 · 107 3.0 · 107
0.04345 0.02080 0.04117 0.04203 0.03417 0.03571
0.04325 0.02067 0.04085 0.04171 0.03386 0.03548
0.4603 0.6250 0.7773 0.7614 0.9072 0.6441
Table 1. It is obvious that the weight loss of the irradiated films are larger than that unirradiated film under the same etching condition. It can be understood the additional track etching including that transversal and along the track contributes to the more weight loss. The bulk etching rate Vb can be obtained using the following relation [9]: ðM1 M2 Þ D1 =2 M1 ¼ Vb t;
ð1Þ
where M1 and M2 are the initial and final weights respectively, D1 is the initial thickness of the films and was measured using a regular sensitive device with an accuracy of ±1 lm. t is the etching time. Using Eq. (1) and the data from Table 1, a constant bulk etching rate of Vb ¼ 0:318 0:002 lm/h was calculated. 3.2. Shape, size and characteristics of the etched pores 3.2.1. General description Spohr [4] simply divides the full process of track formation from the ion irradiation to the observation of the etched track into the following three distinct steps, as shown in Fig. 1. The first step is well understood that energy transfer from projectile to target electrons and
ion
Atomic Level Energy transter from projectile to target electrons and nuclei
[%]
nuclei. The second step, solid state level, latent tracks are formed as a result of the electronic and atomic collision-cascades adjacent to the ion path. The atomic defects reorganize in the form of a track core about 0.01 lm in diameter along the ion path and the electronic defects lead to a track halo consisting of chemically activated sites (radicals) up to a distance of about 1 lm (the value varies in different materials and under different energy loss) [4]. It is known that the latent track can be observed by using the selective chemical etching which is the final step in the track-forming process. In this chemical level, the damaged zone of the latent track induced by electron excitation to higher energy levels which can lead to break the long chain molecules and to free radical production is either transformed chemically or dissolved by the etchant, and then the visible track is formed. There are two kinds of phase transformation starting preferentially along the latent track in the track development: internal phasetransformations and external phase transformations are characterized by the range of diffusion process. After the three steps, the visible ion track are photographed by the SEM. The pore number was checked to be a good agreement with the ion beam
Solid State Level latent track formation by reorganization of electronic and atomic defects
Chemical Level track development by removal or transformation of target material
Fig. 1. Three steps approach for track formation.
observed ion track
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Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
Fig. 2. SEM images with different magnification taken from the same sample irradiated by Cþ 3 and etched for 2.0 h. Hole density 2 · 107 cm2 . Mean hole diameter 1.24 ± 0.15 lm.
density used for irradiation and almost all tracks were revealed by chemical etching. Two typical SEM images after Cþ 3 irradiation and 2.0 h etching are indicated in Fig. 2.
Diameter (µm)
3.2.2. Transversal etching rate measurements Before the discussion of the transversal etching rate, it should be stated that the over etching occurred in all films, which means the etch tip has reached the endpoint of the ion range. However, since the transversal etching rate we discussed is only related to the surface of films, the over etching occurring at the end of ion path does not influence our result of the transversal etching rate. In order to determine the relation between the electronic deposited energy density of projectiles and the transversal etching rate, we measured a time sequence the etched pore diameter for irradiation of carbon cluster ions with different etching time (from 0.5 to 3.0 h with step of 0.5 h). And the mean transversal etching rate Vtrans calculated by using the linear fit to the diameters versus etching time were listed in Table 2. The pore diameters of samples irradiated by carbon cluster ions Cþ n (n ¼ 1–5) is shown in Fig. 3. The difference
2.0 +
1.8
C
1.6
C 2+
1.4
C 3+
C5
+
+
C4
1.2
C
1.0
+
C
+ 5
0.8 0.6 0.4 0.2 0.5
1.0
1.5
2.0
2.5
3.0
Etching Time (hr)
Fig. 3. Pore size measured from SEM images versus etching time for the samples irradiated by Cþ n (n ¼ 1–5) with energy of 0.6 MeV/atom.
between diameters of pores induced by individual carbon ion and carbon cluster ions increased with etching time going. After 2 h, the difference decreased gradually and the etching rate would become the same as the bulk etching rate. It should be noticed that the electronic energy loss of cluster projectiles is still not clear and thus it cannot be obtained directly using the TRIM code [10]. But
Table 2 Characteristics of the various irradiations with cluster ions: incident energies, linear rates of energy deposition by the clusters in electronic processes, the transversal etching rate through linear fit Ion
Cþ
Cþ 2
Cþ 3
Cþ 4
Cþ 5
Incident energy (MeV/atom) ðdE=dxÞe (keV/nm) Vtrans (lm/h)
0.6 0.618 0.30 ± 0.02
0.6 1.236 0.32 ± 0.01
0.6 1.854 0.34 ± 0.02
0.6 2.472 0.36 ± 0.01
0.6 3.090 0.39 ± 0.01
Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
Fig. 4. SEM images of two polycarbonate samples after an etching time of 1.0 h. (a) is from 0.6 MeV/atom Cþ 4 irradiation and (b) is from 0.6 MeV/atom Cþ irradiation.
some experimental results [11,12] show that the energy loss per carbon in cluster equals to that of an individual carbon with the same velocity, so the
625
energy loss of Cn is estimated as the sum of n individual carbon atoms which can be calculated by using TRIM code. It is easy to obtain a determinate dependence of diameter on the energy loss, ðdE=dxÞe , comparing the transversal etching rate listed in Table 2, it indicated that the transversal etching rate increases with increasing rate of electronic energy deposition. The two main factors play an important role. One is that the higher local deposited energy will induce more defects in a certain area adjacent to the ion path which effect directly the chemical etching rate. The other is that higher energy deposition will lead to a larger size of both the track core and track halo. Previous work [13] indicated that the diameters of the track core and the track halo are approximately proportional to the square root of the energy loss. In can general, only relatively large pores (d > 500 A) be observed by SEM, so the Vtrans should be in the same order of the bulk etching rate Vb . Since Vb ¼ 0:318 0:002 lm/h, both measurements are in close agreement. Because of the fuzziness on the edge of the pores, the diameter measuring accuracy was expected within ±5%. In addition to error factor, the deposited conducting layer required for the SEM investigation also contributes to the error of the diameter measurements. It has been experimentally verified that [14] the measured pore diameters decrease with increasing the thickness of the conducting layer.
35
(a)
(b)
25
30 20
Frequency
Frequency
25 20 15
15 10
10 5
5
0
0 0.2
0.4
0.6
Diameter (µm)
0.8
1.0
0.2
0.4
0.6
0.8
1.0
Diameter (µm)
þ Fig. 5. The histogram of measured pores diameters. (a) is from 0.6 MeV/atom Cþ 4 irradiation and (b) is from 0.6 MeV/atom C irradiation.
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Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
3.2.3. Pore size distribution From measurements of the diameter of pores on the SEM images, it is also possible to estimate the distribution of pore sizes around the mean value. In most cases, the distribution is fairly narrow, around 10%, which is comparable to the measuring accuracy. But in some images of SEM for samples irradiation by cluster ions, it becomes apparently much larger up to 20% for the etched pores from Cþ 4 after 1 h etching as shown in Fig. 4. By comparing the two images of SEM, it is easy to find that the pore sizes in Fig. 4(a) have a much larger distribution than that in (b). Their respective distribution histograms were presented in Fig. 5. This phenomenon was considered as a result of the dissociation of the Cþ 4 before reaching the surface of polycarbonate sample. The fragments separated from the cluster should only carry a very small part of the cluster energy and thus have very low ðdE=dxÞe , so the pores possibly caused by these fragments were much smaller than the normal ones.
4. Conclusion For irradiation of carbon clusters and chemical etching used in our experiment, the pores with different sizes up to 1.8 lm were observed in polycarbonate membranes. The weight loss of sample was measured using a very precise electrobalance and the bulk etching rate Vb ¼ 0:318 0:002 lm/h was calculated from the weight loss. From the diameter measured on the SEM images we calculated the mean transversal etching rate Vtrans and found a general dependence of Vtrans on the energy loss ðdE=dxÞe . The difference between diameters of pores induced by individual carbon ion and carbon cluster ions increased with etching time going. After 2 h, the difference decreased gradually and the etching rate would become the same as the bulk etching rate. Most of the pore size distributions were found regular and about 10%
around the mean value, but for the etched pores after Cþ 4 and 1 h etching the distribution was distinctly large. This phenomenon is attributed by the dissociation of the cluster before hitting the surface and the fragments of the cluster carrying different portion of the cluster energy induced larger distribution of pores in the polycarbonate.
Acknowledgements This work was supported by Key Laboratory of Heavy Ion Physics, Ministry of Education, China.
References [1] D.A. Young, Nature (9 August 1958) 375. [2] R.L. Fleischer, P.B. Price, R.M. Walker, Nuclear Tracks in Solids, Principles and Application, Univeristy of California Press, Berkeley, 1975. [3] S.A. Durrani, R.K. Bull, Solid State Nuclear Track Detection, Pergamon Press, 1987. [4] R. Spohr, Ion Tracks and Microtechnology, Basic Principles and Applications, Vieweg, 1990. [5] D. Dobrev, J. Vetter, N. Angert, GSI Sci. Rep. 1994 (1995) 201. [6] P. Apel, A. Schulz, R. Spohr, C. Trautman, V. Vutsadakis, Nucl. Instr. and Meth. B 146 (1998) 429. [7] M. Fromm, These, Besancßon, 1990. [8] R. Katz, Nucl. Track Detection 2 (1978) 1. [9] Z. Lounis, S. Djeffal, K. Morsli, M. Allab, Nucl. Instr. and Meth. B 179 (2001) 543. [10] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257; J.F. Ziegler, J.P. Biersack, U. Littmark, in: Stopping Power and Ranges of Ions in Matter, Vol. 1, Pergamon, New York, 1985. [11] K. Baudin, A. Brunelle, M. Chabot, S. Della-Negra, J. Depauw, D. Gardes, P. H akansson, Y. Le Beyec, A. Billebaud, M. Fallavier, J. Remillieux, J.C. Poizat, J.P. Thomas, Nucl. Instr. and Meth. B 94 (1994) 341. [12] Ch. Tomaschko, D. Brandl, R. K€ ugler, M. Schurr, H. Voit, Nucl. Instr. and Meth. B 103 (1995) 407. [13] P. Apel, A. Schulz, R. Spohr, C. Trautmann, V. Vutsadakis, Nucl. Instr. and Meth. B 146 (1998) 468. [14] G. Guillot, F. Rondelez, J. Appl. Phys. 52 (12) (1981).
Etching characteristics for tracks of carbon cluster ions in polycarbonate Ziqiang Zhao *, Dongchen Qi, Zhiyu Guo Institute of Heavy Ion Physics, Peking University and Key Laboratory of Heavy Ion Physics, Ministry of Education, Beijing 100871, PR China Received 23 July 2003; received in revised form 19 November 2003
Abstract A series of chemical etching experiments were carried out on polycarbonate foils irradiated by carbon cluster ions with an energy of 0.6 MeV/atom. The bulk etching rate was calculated from weight loss. The transversal etching rate was obtained by using linear fit to the pore diameters under a time sequence. It was found that the transversal etching rate depends on the rate of electronic energy deposition of projectiles. A distinctly irregular pore size distribution was found on the image of etched pores after Cþ 4 irradiation and after one hour etching and explained as the contribution of dissociation of the carbon cluster. 2003 Elsevier B.V. All rights reserved. PACS: 61.81.jh; 61.82.pv; 79.20 Keywords: Latent tracks; Energy loss; Carbon cluster; Polycarbonate; Etching rate; Pores; Distribution
1. Introduction The formation of tracks induced by the electronic deposited energy of energetic ions has been studied experimentally and theoretically since the early 1950s [1–4]. Although a lot of efforts have been made on this subject, there is still an incomplete understanding of tracks concerning the fundamental atomic structure of damage as well as macroscopic observable properties. Among all the materials, track formation in polymers is most complex and attracting since not only primary but *
Corresponding author. Tel.: +86-10-6275-5363; fax: +8610-6275-1875. E-mail address: [email protected] (Z. Zhao).
also secondary processes such as the formation of radicals and chemical processes like cross-linking in polyethylene terephthalate (PET) [6] are involved. The selective chemical etching has become by far the most general and useful means of investigating tracks of energetic charged particles in polymers. It is based on the fact that the damaged region possesses a higher chemical activity than the undamaged matrix, so the etching process allows the tracks can be observed directly by optical or scanning electron microscopy. This technique has numerous application in applied science ranging from solid state nuclear detector to micromachining such as growth of nanostructure [5]. Successful track etching requires that the damaged material along the track is preferentially
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.091
622
Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
etched at a much higher rate than the surrounding undamaged matrix; in another word the track etching rate Vt has to be higher than the bulk etching rate Vb . The ratio of the two etching rates Vb =Vt determines the geometry of the etched pores. It has been experimentally verified that the bulk etching rate remains constant for a given material and etchant under a specific etching condition including the temperature and concentration [7]. In contrast, the track etching rate which reflects the extent of damage along the ion path in material depends greatly on the energy loss of the trackforming ions. For fundamental understanding and practical considerations, we studied the etching properties of polycarbonate after irradiated of individual carbon ion and cluster ions. The reason polycarbonate was chosen from numerous available materials is that these films offer a high differential etching rate between the irradiated and nonirradiated parts of the membrane which is very crucial for the geometry of the etched tracks. In general, the electronic energy loss (dE=dx) of the ion in the materials plays a main role [2] and the radial extension of the track implies that the energetic secondary electrons (d-rays) created in the path of ion in materials also contribute to the energy transportation and track formation [8]. Based on this we choose carbon cluster ions in our investigation because clusters have higher local deposited energy densities in materials than individual ions at same velocity. For comparison, individual carbon ions are also used to irradiation. A series of traces etched after irradiated of carbon clusters Cþ n (n ¼ 1–5) at same velocity are performed the characteristic of tracks are defined and compared.
2. Experimental details 2.1. Irradiation Polycarbonate films are commercial available films with thickness of 138 lm. They were cut into rectangle 25 mm in length and 20 mm in width. These films were then irradiated by carbon cluster ions at room temperature at normal incidence in the 2 · 1.7 MeV tandem accelerator (5SDH-2
NEC) at the Key Laboratory of Heavy Ion Physics, Peking University. The energy of carbon cluster ions is 0.6 MeV/atom. The ion beam was scanned and average ion beam current was typically in the order of 1011 A in the irradiated area of 11 cm2 . The track density of 106 –107 ions/cm2 in the irradiated sample will be made and no overlapping of tracks after being etched to round pores. After irradiation, the samples were kept in air for several days. 2.2. Etching conditions In our experiment, irradiated films were etched in a 6 M/l solution of NaOH containing 10% of the methanol at a constant temperature of 45 C, with etching time in a range from 0.5 to 3.0 h with a step of 0.5 h. After etching the films were rinsed in deionized water and then dried in air for days. 2.3. Microscopy and measurements After etching, the number and the size of the etched pits on the surface of the samples were determined using a scanning electron microscope (SEM) of KYKY-1000B. The weight loss of the samples were also measured with an analytical electrobalance.
3. Results and discussion 3.1. Weight loss measurements The weight of one unirradiated film and other five films irradiated by carbon cluster ions Cþ n (n ¼ 1–5) respectively were measured by a sensitive analytical electrobalance with an accuracy of ±0.01 mg before and after 1.0 h etching in order to determine the weight loss during etching process. The result can be seen in Table 1, where M1 is the weight before etching and M2 is the weight after etching, the projected range was calculated by using the TRIM code [10], and the dose calculated from the ion beam current and the area of irradiation surface was expected to be in the order of 1 · 107 ions/cm2 . The real track density was expected in the same order and also were listed in
Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
623
Table 1 The weight loss of samples irradiated by carbon cluster ions and after 1.0 h etching Ion
Projected range [lm]
Track density [ions/cm2 ]
M1 [g]
M2 [g]
M1 M2 M1
Nonirradiated Cþ Cþ 2 Cþ 3 Cþ 4 Cþ 5
1.40 1.40 1.40 1.40 1.40 1.40
– 2.3 · 107 2.0 · 107 9.3 · 106 3.4 · 107 3.0 · 107
0.04345 0.02080 0.04117 0.04203 0.03417 0.03571
0.04325 0.02067 0.04085 0.04171 0.03386 0.03548
0.4603 0.6250 0.7773 0.7614 0.9072 0.6441
Table 1. It is obvious that the weight loss of the irradiated films are larger than that unirradiated film under the same etching condition. It can be understood the additional track etching including that transversal and along the track contributes to the more weight loss. The bulk etching rate Vb can be obtained using the following relation [9]: ðM1 M2 Þ D1 =2 M1 ¼ Vb t;
ð1Þ
where M1 and M2 are the initial and final weights respectively, D1 is the initial thickness of the films and was measured using a regular sensitive device with an accuracy of ±1 lm. t is the etching time. Using Eq. (1) and the data from Table 1, a constant bulk etching rate of Vb ¼ 0:318 0:002 lm/h was calculated. 3.2. Shape, size and characteristics of the etched pores 3.2.1. General description Spohr [4] simply divides the full process of track formation from the ion irradiation to the observation of the etched track into the following three distinct steps, as shown in Fig. 1. The first step is well understood that energy transfer from projectile to target electrons and
ion
Atomic Level Energy transter from projectile to target electrons and nuclei
[%]
nuclei. The second step, solid state level, latent tracks are formed as a result of the electronic and atomic collision-cascades adjacent to the ion path. The atomic defects reorganize in the form of a track core about 0.01 lm in diameter along the ion path and the electronic defects lead to a track halo consisting of chemically activated sites (radicals) up to a distance of about 1 lm (the value varies in different materials and under different energy loss) [4]. It is known that the latent track can be observed by using the selective chemical etching which is the final step in the track-forming process. In this chemical level, the damaged zone of the latent track induced by electron excitation to higher energy levels which can lead to break the long chain molecules and to free radical production is either transformed chemically or dissolved by the etchant, and then the visible track is formed. There are two kinds of phase transformation starting preferentially along the latent track in the track development: internal phasetransformations and external phase transformations are characterized by the range of diffusion process. After the three steps, the visible ion track are photographed by the SEM. The pore number was checked to be a good agreement with the ion beam
Solid State Level latent track formation by reorganization of electronic and atomic defects
Chemical Level track development by removal or transformation of target material
Fig. 1. Three steps approach for track formation.
observed ion track
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Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
Fig. 2. SEM images with different magnification taken from the same sample irradiated by Cþ 3 and etched for 2.0 h. Hole density 2 · 107 cm2 . Mean hole diameter 1.24 ± 0.15 lm.
density used for irradiation and almost all tracks were revealed by chemical etching. Two typical SEM images after Cþ 3 irradiation and 2.0 h etching are indicated in Fig. 2.
Diameter (µm)
3.2.2. Transversal etching rate measurements Before the discussion of the transversal etching rate, it should be stated that the over etching occurred in all films, which means the etch tip has reached the endpoint of the ion range. However, since the transversal etching rate we discussed is only related to the surface of films, the over etching occurring at the end of ion path does not influence our result of the transversal etching rate. In order to determine the relation between the electronic deposited energy density of projectiles and the transversal etching rate, we measured a time sequence the etched pore diameter for irradiation of carbon cluster ions with different etching time (from 0.5 to 3.0 h with step of 0.5 h). And the mean transversal etching rate Vtrans calculated by using the linear fit to the diameters versus etching time were listed in Table 2. The pore diameters of samples irradiated by carbon cluster ions Cþ n (n ¼ 1–5) is shown in Fig. 3. The difference
2.0 +
1.8
C
1.6
C 2+
1.4
C 3+
C5
+
+
C4
1.2
C
1.0
+
C
+ 5
0.8 0.6 0.4 0.2 0.5
1.0
1.5
2.0
2.5
3.0
Etching Time (hr)
Fig. 3. Pore size measured from SEM images versus etching time for the samples irradiated by Cþ n (n ¼ 1–5) with energy of 0.6 MeV/atom.
between diameters of pores induced by individual carbon ion and carbon cluster ions increased with etching time going. After 2 h, the difference decreased gradually and the etching rate would become the same as the bulk etching rate. It should be noticed that the electronic energy loss of cluster projectiles is still not clear and thus it cannot be obtained directly using the TRIM code [10]. But
Table 2 Characteristics of the various irradiations with cluster ions: incident energies, linear rates of energy deposition by the clusters in electronic processes, the transversal etching rate through linear fit Ion
Cþ
Cþ 2
Cþ 3
Cþ 4
Cþ 5
Incident energy (MeV/atom) ðdE=dxÞe (keV/nm) Vtrans (lm/h)
0.6 0.618 0.30 ± 0.02
0.6 1.236 0.32 ± 0.01
0.6 1.854 0.34 ± 0.02
0.6 2.472 0.36 ± 0.01
0.6 3.090 0.39 ± 0.01
Z. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 621–626
Fig. 4. SEM images of two polycarbonate samples after an etching time of 1.0 h. (a) is from 0.6 MeV/atom Cþ 4 irradiation and (b) is from 0.6 MeV/atom Cþ irradiation.
some experimental results [11,12] show that the energy loss per carbon in cluster equals to that of an individual carbon with the same velocity, so the
625
energy loss of Cn is estimated as the sum of n individual carbon atoms which can be calculated by using TRIM code. It is easy to obtain a determinate dependence of diameter on the energy loss, ðdE=dxÞe , comparing the transversal etching rate listed in Table 2, it indicated that the transversal etching rate increases with increasing rate of electronic energy deposition. The two main factors play an important role. One is that the higher local deposited energy will induce more defects in a certain area adjacent to the ion path which effect directly the chemical etching rate. The other is that higher energy deposition will lead to a larger size of both the track core and track halo. Previous work [13] indicated that the diameters of the track core and the track halo are approximately proportional to the square root of the energy loss. In can general, only relatively large pores (d > 500 A) be observed by SEM, so the Vtrans should be in the same order of the bulk etching rate Vb . Since Vb ¼ 0:318 0:002 lm/h, both measurements are in close agreement. Because of the fuzziness on the edge of the pores, the diameter measuring accuracy was expected within ±5%. In addition to error factor, the deposited conducting layer required for the SEM investigation also contributes to the error of the diameter measurements. It has been experimentally verified that [14] the measured pore diameters decrease with increasing the thickness of the conducting layer.
35
(a)
(b)
25
30 20
Frequency
Frequency
25 20 15
15 10
10 5
5
0
0 0.2
0.4
0.6
Diameter (µm)
0.8
1.0
0.2
0.4
0.6
0.8
1.0
Diameter (µm)
þ Fig. 5. The histogram of measured pores diameters. (a) is from 0.6 MeV/atom Cþ 4 irradiation and (b) is from 0.6 MeV/atom C irradiation.
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3.2.3. Pore size distribution From measurements of the diameter of pores on the SEM images, it is also possible to estimate the distribution of pore sizes around the mean value. In most cases, the distribution is fairly narrow, around 10%, which is comparable to the measuring accuracy. But in some images of SEM for samples irradiation by cluster ions, it becomes apparently much larger up to 20% for the etched pores from Cþ 4 after 1 h etching as shown in Fig. 4. By comparing the two images of SEM, it is easy to find that the pore sizes in Fig. 4(a) have a much larger distribution than that in (b). Their respective distribution histograms were presented in Fig. 5. This phenomenon was considered as a result of the dissociation of the Cþ 4 before reaching the surface of polycarbonate sample. The fragments separated from the cluster should only carry a very small part of the cluster energy and thus have very low ðdE=dxÞe , so the pores possibly caused by these fragments were much smaller than the normal ones.
4. Conclusion For irradiation of carbon clusters and chemical etching used in our experiment, the pores with different sizes up to 1.8 lm were observed in polycarbonate membranes. The weight loss of sample was measured using a very precise electrobalance and the bulk etching rate Vb ¼ 0:318 0:002 lm/h was calculated from the weight loss. From the diameter measured on the SEM images we calculated the mean transversal etching rate Vtrans and found a general dependence of Vtrans on the energy loss ðdE=dxÞe . The difference between diameters of pores induced by individual carbon ion and carbon cluster ions increased with etching time going. After 2 h, the difference decreased gradually and the etching rate would become the same as the bulk etching rate. Most of the pore size distributions were found regular and about 10%
around the mean value, but for the etched pores after Cþ 4 and 1 h etching the distribution was distinctly large. This phenomenon is attributed by the dissociation of the cluster before hitting the surface and the fragments of the cluster carrying different portion of the cluster energy induced larger distribution of pores in the polycarbonate.
Acknowledgements This work was supported by Key Laboratory of Heavy Ion Physics, Ministry of Education, China.
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