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Electrophysical properties of organic materials irradiated with accelerated ions
Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 650±654
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Electrophysical properties of organic materials irradiated with accelerated ions F.F. Komarov *, A.V. Leontyev, V.V. Grigoryev Institute of Applied Physics Problems, Belorussian State University, 7 Kurchatova Street, 220080 Minsk, Byelorussia
Abstract The results of ion irradiation in¯uence on electrical conductivity of some polymers are presented in this study. It was shown that irradiation of photoresist FP-383 and polyimide with Ar ions at an energy of 100 keV permits to get the polymer layers with sheet resistivity of 100±200 X=. A simple model allowing to forecast polymer qs values under irradiation conditions with various ions is suggested. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 02.70 Lq; 41.75 Ak; 61.72 Ww Keywords: Implantation; Photoresist; Resistivity; Cryogenic temperature sensors
1. Introduction The results of investigation of the in¯uence of high-dose ion irradiation on polymer material conductivity are presented in this paper. Analysing available literature data [1±4], we can note some principal moments. At ®rst, under speci®c irradiation regimes, it is possible to achieve an increase of the conductivity of polymer materials by a factor of 1012 ±1013 . Secondly, dependence of conductivity r on basic irradiation parameters (ion types and its energies, ion beam density, substrate types) is not suciently studied up to now. Thirdly, the procedure of electrophysical parameter prediction for irradiated polymer layers is practically *
Corresponding author. Tel.: +375-172-774833; fax: +375172-780417. E-mail address: k@rfe.bsu.unibel.by (F.F. Komarov).
unknown, which makes it dicult to apply it to technological processes of microelectronics [2±4]. These problems are treated in this paper. The problems concerning correlation of electrophysical characteristics with structure and composition of irradiated polymers and at the same time detailed discussion of conductivity mechanisms in such systems will be carried out in another paper.
2. Experimental The ®lms of wide-spread positive photoresist FP-383 (based on novolac resins and naphthoquinone diazide) and polyimide (PI) C22 H10 O5 N2 n synthesized in our laboratory on the basis of pyromellitic dyanhydride and 4; 40 -diaminodiphenyl ether have been chosen as initial objects of investigations. Photoresist ®lms (thickness of about
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 1 2 1 3 - 6
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
1.0 lm) were spin coated onto SiO2 /Si and alumina substrates. Before irradiation the samples were prebaked in a three-zone IR-oven for 15±30 min at 120±140±160°C. In an analogous way, the ®lms of polyimide were formed. Prebaking was carried out at 100°C for 30±40 min and then at 210°C for 20± 30 min. The above-mentioned ®lms were irradiated with H 2 ; N2 ; Ar ; As ; Sb and In ions at energies of 100±300 keV in a dose range of 1015 ±1018 cmÿ2 . The implants were performed on a KASPER medium current implanter and 0.5 MeV Van de Graa accelerator with the ion beam incidence at 7° from normal to the wafer. The four-point probe method was used to measure sheet resistance of polymers irradiated by accelerated ions. Low temperature experiments were performed using a closed cycle helium refrigerator and a two-point probe method. Electrical contacts to the implanted polymer layer were performed by evaporating 1 lm gold ®lms. The thickness of the modi®ed polymer layers was determined using Monte-Carlo calculations [5]. The electrically active layer is usually related to the mean range hRi of ions in polymers.
651
sheet resistivity on ion current qs (I) gives an evidence of an essential in¯uence of thermal processes on structural phase transformations in the irradiated organic layers. A high stability of electrophysical properties of irradiated polymers is observed in our experiment. A sample keeping in the atmospheric conditions for one full year did not lead to any change of qs value. The irradiated layers have also high chemical resistivity against treatments with strong inorganic acids (HCl, H2 SO4 ) and organic solvents (dimethylformamide, dioxane, trichloroethylene). The experimental results of the in¯uence of light ion irradiation on surface conductivity may be interpreted in the following manner. Under implantation of H 2 ; He ; N2 ions in the energy range of 100±300 keV sheet resistivity qs values approach 100 kX= only at dose levels of D > 2 1017 cmÿ2 . Therefore, implantation of
3. Results and discussion The results of surface resistivity measurements for ®lms of the FP-383 resist are presented in Fig. 1. In view of limitations for experimental registration of qs values for implantation doses less than 1016 cmÿ2 , it was not possible to perform reliable measurements. It may be only roughly extrapolated that in the dose range 1015 < D < 1016 cmÿ2 the value of qs > 100 kX=. The dependencies of qs on ion beam current, I (lA), are illustrated in Table 1. As can be seen from the data of Table 1 and Fig. 1, qs values at the same irradiation dose are strongly sensitive to the ion beam intensities. The registered dependence of
Fig. 1. The qs kX= data for (FP-383) photoresist ®lms implanted with Ar ions at an energy of 100 keV versus irradiation dose: 1, 2 for I 300 lA; 3, 4 for I 600 lA. Curves 1 and 3 correspond to as-irradiated specimens, curves 2 and 4 are results for these specimens kept in atmospheric conditions for 1 year.
Table 1 The qs (kX=) data for photoresist FP-383 ®lms on alumina modi®ed by Ar ions at an energy of 100 keV versus ion beam current density Beam current density (lA/cm2 ) qs (kX=)
3 100
4 42.0
5 17.8
6 10.8
8 4.7
12 3.2
13 1.6
652
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
these ions at the mentioned regimes is not eective from the practical point of view. Analogous results were obtained in [1]. Irradiation of polymers with heavy ions (As ; Sb ; In ) at energy values of 100±300 keV permits to achieve a high surface conductivity. However, the ®lm surface is extremely damaged in this case. Cracks and traces of gas bubbles are observed at the ®lm surface. Therefore, irradiation with heavy ions requires special regimes of preimplantation processing and does not give particular advantages as compared to irradiation with argon ions.
Table 2 Dose-dependence of relative intensity of the EPR signal for (FP-383) photoresist ®lms irradiated with Ar ions at an energy of 100 keV
3.1. Low-temperature conductivity
T0
It is well known that organic ®lms exposed to ion irradiation are prospective materials for the creation of cryogenic temperature sensors [6±9]. Therefore, investigation of low-temperature conductivity in these systems is of special interest. The conductivity of irradiated polymers is usually considered as a succession of 1D and 3D jumps of carrieres. Low-temperature conductivity r(T) may be described on the basis of MottÕs notion about conductivity of disordered systems [10]. In polymer ®lms exposed to ion bombardment two types of disorder may be present: positional and topological ones. For the latter, availability of free bonds is characteristic, forming additional localized defects. The information about availability of free bonds may be obtained from EPR spectroscopy data. The EPR spectra of the FP-383 photoresist ®lms irradiated with Ar ions were characteristic of randomly oriented free radicals (g value decreases from g 2:0030 for D 5 1015 cmÿ2 to g 2:0025 for D 1 1017 cmÿ2 ) ÔtrappedÕ in an amorphous polymer matrix. The dose-dependence of the relative intensity of EPR signal lines is presented in Table 2. It can be seen that value Irelative (Irel ) rises with increase of irradiation dose, reaching a maximum at D 3 1016 cmÿ2 and then declines. Therefore, the increase may be considered as the evidence of the existence of an essential quantity of free bonds in implanted samples. MottÕs theory [10] can be adequate for explanation of low-temperature dependencies r(T) for these samples:
Dose (cmÿ2 )
Irelative
Dose (cmÿ2 )
Irelative
7 1015 1 1016 2 1016
0.29 0.40 0.65
3 1016 5 1016 1 1017
1.00 0.74 0.71
r r0 exp ÿ T0 =T 0:25 ; 2:5 ; 9p3 kkb g
1 2
where k is the localization length, g is the state density, kb is the Boltzman constant, r0 is the preexponential factor. The experimental low-temperature (20±2.5 K) dependencies r(T) for samples of FP-383 photoresist and polyimide (PI) irradiated with Ar (100 keV) ions at doses of 1 1016 and 2 1016 cmÿ2 , respectively, are presented in Fig. 2. T0 values determined from Fig. 2 on the basis of the formula (1) are 2:2 105 K (for FP-383) and 4:1 105 K (for PI). The value of g was taken from [8] and the value of k k 0:5 nm was calculated according
Fig. 2. Low-temperature dependencies of conductivity for FP383 and PI ®lms irradiated by Ar (100 keV) ions: 1 for FP-383, D 1 1016 cmÿ2 , 2 for PI, D 2 1016 cmÿ2 .
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
to formula (2). It can be noted that under irradiation doses of D > 3 1016 cmÿ2 dependencies of r(T) similar to those represented in Fig. 1 were not detected. The Raman spectra (514.5 nm) from FP383 photoresist ®lms irradiated with Ar (100 keV) ions at the doses of 5 1015 ; 1 1016 and 1 1017 cmÿ2 are presented in Fig. 3. For all doses, a broad peak near 1550 cmÿ2 (G peak) can be observed, which is assigned to an amorphous carbon-like structure. Another peak at 1350 cmÿ2 (D peak) is less pronounced and turns into a plateau at the doses of D > 5 1016 cmÿ2 . The ®lms having a r(T) dependence as described by formula (1) could be used as working elements for cryogenic temperature sensors. Sensors of this class have been developed at the Institute of Physics (St. Petersbourg) [8,9], but they are massive samples. Usage of the described technology would allow to create thin sensitive ®lms and reduce the inertness of the converter. 3.2. Predicting of electrophysical properties of ionimplanted organic materials From the practical point of view, it would be interesting to predict the electrophysical parameters of modi®ed layers for successful application of organic materials irradiated with accelerated ions
Fig. 3. Raman spectra for Ar (100 keV) ± implanted FP-383 resist ®lms: curve 1 for D 5 1015 cmÿ2 , curve 2 for D 1 1016 cmÿ2 , curve 3 for D 1 1017 cmÿ2 .
653
in microelectronics technological processes. The simplest and at the same time suciently reliable approach, has to do with calculation of some critical irradiation parameters at which a qs value in the required range will be achieved. The average value of energy deposited by implanted ions in elastic Gn and inelastic Ge collisions can be treated as the main critical parameter, Gn DEn D=Rp ; Gne Gn Ge :
Ge DEe D=Rp ;
3
Here DEe and DEn are the values of ion energy dissipated in electronic and nuclear subsystems, D is the dose of implanted ions, Rp is the projected range. Because of spatial separation of maxima for elastically and inelastically deposited energies, formulae (3) are only a most probable rough approach. However, it permits us to correctly evaluate the implantation dose needed for a conductive graphite-like phase formation with accuracy of the order of magnitude. More explicitly, the value of Rp in expression for Gn must be replaced by the following expression: DX 1=2 2DRp 2 ln 2 . In this case, however, the simple sum of two contributions for Gne is not correct. In order to determine the values of density of evolved energy necessary for the formation of a conductive Ôgraphite-likeÕ phase (Gne ) it is possible to apply experimental results of irradiation with dierent ions in a wide range of doses and energies on the surface conductivity. It was shown [1,4] that conductive organic layers with qs values equal approximately 100 X= may be obtained by implantation of Ar (100 keV) ions below D 5 1016 cmÿ2 . This is the best currently available result for medium energy ion irradiation of polymers. On the basis of given results it is possible to estimate the critical value of Gne . The calculated means of Gne obtained under the most wide-spread irradiation conditions are listed in Table 3. Calculations of values listed in Table 3 were carried out by the Monte-Carlo procedure. The calculated values correspond to the irradiation dose required for reaching the critical value of DGne . On the basis of data of Table 3, it is rather easy to substantiate the results of experiments on
654
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
Table 3 List of parameters of PMMA irradiation with dierent types accelerated ions 3
) Ge (eV/A
3
) Gn (eV/A
D (1017 cmÿ2 )
± ±
4.2 4.2
Ion type
E (keV)
DEe (keV)
H
100 300
99.3 299.9
0.7 0.1
1160 3460
43.1 43.3
He
100 300
95.6 296.1
4.6 3.9
858.5 1770
55.6 83.6
2.7 1.4
3.1 2.4
N
100 300
81.9 273.0
18.1 27.0
318.0 782.4
128.8 174.5
28.4 17.3
1.1 1.0
Ar
100 200 300
63.3 141.8 228.9
36.7 58.2 71.7
139.1 275.6 409.6
227.5 257.3 279.4
131.9 105.6 86.8
0.5 0.5 0.5
Kr
150
84.0
66.6
124.2
339.4
265.5
0.3
DEn (keV)
the in¯uence of irradiation by protons and He ions on conductivity of organic materials. It is evident that the Gne value for protons is lower nearly by an order of magnitude than the required value, and therefore very large irradiation doses are needed for obtaining the conducting layers. Irradiation with B , N , C and Ne ions permits to get the required value of evolved energy density below a dose of the order of D 1:2 1017 cmÿ2 . In this sense, Ar ions are the most favorable type for polymer conductivity modi®cation.
4. Conclusion It has been shown that irradiation of FP-383 and polyimide ®lms with Ar ions permits to get high conductive, chemically stable, graphite-like samples with a limit of qs about 100 X=. The low-temperature conductivity (under 20± 2.5 K) of thin FP-383 photoresist and polyimide ®lms under irradiation doses of 1 1016 ± 2 1016 cmÿ2 is connected with carrier jumps having dierent lengths (Mott's mechanism). A simple model for predicting the doses of implanted ions in polymers for graphite-like structure formation has been discussed.
Rp (nm)
Acknowledgements This work has been supported in part by Soros Fund (grant B6-17-2710-7) and the Fundamental Research Fund of Belarus (grant F98-146). The authors wish to acknowledge Prof. A.N. Aleshin for low-temperature measurements. References [1] J. Davenas, G. Boiteux, X.L. Xu, E. Adem, Nucl. Instr. and Meth. B 32 (1988) 136. [2] T. Trigaud, J.P. Moliton, C. Jussiaux, B. Maziere, Nucl. Instr. and Meth. B 107 (1996) 323. [3] U.S. Sias, G. Sanchez, J.R. Kaschny, L. Amaral, M. Behar, D. Fink, Nucl. Instr. and Meth. B 141 (1998) 187. [4] I.H. Loh, R.B. Oliver, P. Sioshansi, Nucl. Instr. and Meth. B 34 (1988) 337. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.P. Ziegler, (Ed.), The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [6] T. Okade, T. Nishijima, S. Takahata, M. Kunori, K. Iwamoto, Nucl. Instr. and Meth. B 37/38 (1989) 720. [7] R.E. Giedd, M.G. Moss, M.M. Craig, D.E. Robertson, Nucl. Instr. and Meth. B 59/60 (1991) 1253. [8] A.N. Aleshin, A.V. Gribanov, A.V. Suvorov, Fiz. Tverd. Tela. 31 (1989) 12 (in Russian). [9] A.N. Aleshin, A.V. Suvorov, Fiz. Tverd. Tela. 32 (1990) 1717 (in Russian). [10] O. Madelung. Fizika tverdogo tela. Lokalizovannye sostoiania, Nauka, Moskva, 1985, p. 183 (in Russian).
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Electrophysical properties of organic materials irradiated with accelerated ions F.F. Komarov *, A.V. Leontyev, V.V. Grigoryev Institute of Applied Physics Problems, Belorussian State University, 7 Kurchatova Street, 220080 Minsk, Byelorussia
Abstract The results of ion irradiation in¯uence on electrical conductivity of some polymers are presented in this study. It was shown that irradiation of photoresist FP-383 and polyimide with Ar ions at an energy of 100 keV permits to get the polymer layers with sheet resistivity of 100±200 X=. A simple model allowing to forecast polymer qs values under irradiation conditions with various ions is suggested. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 02.70 Lq; 41.75 Ak; 61.72 Ww Keywords: Implantation; Photoresist; Resistivity; Cryogenic temperature sensors
1. Introduction The results of investigation of the in¯uence of high-dose ion irradiation on polymer material conductivity are presented in this paper. Analysing available literature data [1±4], we can note some principal moments. At ®rst, under speci®c irradiation regimes, it is possible to achieve an increase of the conductivity of polymer materials by a factor of 1012 ±1013 . Secondly, dependence of conductivity r on basic irradiation parameters (ion types and its energies, ion beam density, substrate types) is not suciently studied up to now. Thirdly, the procedure of electrophysical parameter prediction for irradiated polymer layers is practically *
Corresponding author. Tel.: +375-172-774833; fax: +375172-780417. E-mail address: k@rfe.bsu.unibel.by (F.F. Komarov).
unknown, which makes it dicult to apply it to technological processes of microelectronics [2±4]. These problems are treated in this paper. The problems concerning correlation of electrophysical characteristics with structure and composition of irradiated polymers and at the same time detailed discussion of conductivity mechanisms in such systems will be carried out in another paper.
2. Experimental The ®lms of wide-spread positive photoresist FP-383 (based on novolac resins and naphthoquinone diazide) and polyimide (PI) C22 H10 O5 N2 n synthesized in our laboratory on the basis of pyromellitic dyanhydride and 4; 40 -diaminodiphenyl ether have been chosen as initial objects of investigations. Photoresist ®lms (thickness of about
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 1 2 1 3 - 6
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
1.0 lm) were spin coated onto SiO2 /Si and alumina substrates. Before irradiation the samples were prebaked in a three-zone IR-oven for 15±30 min at 120±140±160°C. In an analogous way, the ®lms of polyimide were formed. Prebaking was carried out at 100°C for 30±40 min and then at 210°C for 20± 30 min. The above-mentioned ®lms were irradiated with H 2 ; N2 ; Ar ; As ; Sb and In ions at energies of 100±300 keV in a dose range of 1015 ±1018 cmÿ2 . The implants were performed on a KASPER medium current implanter and 0.5 MeV Van de Graa accelerator with the ion beam incidence at 7° from normal to the wafer. The four-point probe method was used to measure sheet resistance of polymers irradiated by accelerated ions. Low temperature experiments were performed using a closed cycle helium refrigerator and a two-point probe method. Electrical contacts to the implanted polymer layer were performed by evaporating 1 lm gold ®lms. The thickness of the modi®ed polymer layers was determined using Monte-Carlo calculations [5]. The electrically active layer is usually related to the mean range hRi of ions in polymers.
651
sheet resistivity on ion current qs (I) gives an evidence of an essential in¯uence of thermal processes on structural phase transformations in the irradiated organic layers. A high stability of electrophysical properties of irradiated polymers is observed in our experiment. A sample keeping in the atmospheric conditions for one full year did not lead to any change of qs value. The irradiated layers have also high chemical resistivity against treatments with strong inorganic acids (HCl, H2 SO4 ) and organic solvents (dimethylformamide, dioxane, trichloroethylene). The experimental results of the in¯uence of light ion irradiation on surface conductivity may be interpreted in the following manner. Under implantation of H 2 ; He ; N2 ions in the energy range of 100±300 keV sheet resistivity qs values approach 100 kX= only at dose levels of D > 2 1017 cmÿ2 . Therefore, implantation of
3. Results and discussion The results of surface resistivity measurements for ®lms of the FP-383 resist are presented in Fig. 1. In view of limitations for experimental registration of qs values for implantation doses less than 1016 cmÿ2 , it was not possible to perform reliable measurements. It may be only roughly extrapolated that in the dose range 1015 < D < 1016 cmÿ2 the value of qs > 100 kX=. The dependencies of qs on ion beam current, I (lA), are illustrated in Table 1. As can be seen from the data of Table 1 and Fig. 1, qs values at the same irradiation dose are strongly sensitive to the ion beam intensities. The registered dependence of
Fig. 1. The qs kX= data for (FP-383) photoresist ®lms implanted with Ar ions at an energy of 100 keV versus irradiation dose: 1, 2 for I 300 lA; 3, 4 for I 600 lA. Curves 1 and 3 correspond to as-irradiated specimens, curves 2 and 4 are results for these specimens kept in atmospheric conditions for 1 year.
Table 1 The qs (kX=) data for photoresist FP-383 ®lms on alumina modi®ed by Ar ions at an energy of 100 keV versus ion beam current density Beam current density (lA/cm2 ) qs (kX=)
3 100
4 42.0
5 17.8
6 10.8
8 4.7
12 3.2
13 1.6
652
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
these ions at the mentioned regimes is not eective from the practical point of view. Analogous results were obtained in [1]. Irradiation of polymers with heavy ions (As ; Sb ; In ) at energy values of 100±300 keV permits to achieve a high surface conductivity. However, the ®lm surface is extremely damaged in this case. Cracks and traces of gas bubbles are observed at the ®lm surface. Therefore, irradiation with heavy ions requires special regimes of preimplantation processing and does not give particular advantages as compared to irradiation with argon ions.
Table 2 Dose-dependence of relative intensity of the EPR signal for (FP-383) photoresist ®lms irradiated with Ar ions at an energy of 100 keV
3.1. Low-temperature conductivity
T0
It is well known that organic ®lms exposed to ion irradiation are prospective materials for the creation of cryogenic temperature sensors [6±9]. Therefore, investigation of low-temperature conductivity in these systems is of special interest. The conductivity of irradiated polymers is usually considered as a succession of 1D and 3D jumps of carrieres. Low-temperature conductivity r(T) may be described on the basis of MottÕs notion about conductivity of disordered systems [10]. In polymer ®lms exposed to ion bombardment two types of disorder may be present: positional and topological ones. For the latter, availability of free bonds is characteristic, forming additional localized defects. The information about availability of free bonds may be obtained from EPR spectroscopy data. The EPR spectra of the FP-383 photoresist ®lms irradiated with Ar ions were characteristic of randomly oriented free radicals (g value decreases from g 2:0030 for D 5 1015 cmÿ2 to g 2:0025 for D 1 1017 cmÿ2 ) ÔtrappedÕ in an amorphous polymer matrix. The dose-dependence of the relative intensity of EPR signal lines is presented in Table 2. It can be seen that value Irelative (Irel ) rises with increase of irradiation dose, reaching a maximum at D 3 1016 cmÿ2 and then declines. Therefore, the increase may be considered as the evidence of the existence of an essential quantity of free bonds in implanted samples. MottÕs theory [10] can be adequate for explanation of low-temperature dependencies r(T) for these samples:
Dose (cmÿ2 )
Irelative
Dose (cmÿ2 )
Irelative
7 1015 1 1016 2 1016
0.29 0.40 0.65
3 1016 5 1016 1 1017
1.00 0.74 0.71
r r0 exp ÿ T0 =T 0:25 ; 2:5 ; 9p3 kkb g
1 2
where k is the localization length, g is the state density, kb is the Boltzman constant, r0 is the preexponential factor. The experimental low-temperature (20±2.5 K) dependencies r(T) for samples of FP-383 photoresist and polyimide (PI) irradiated with Ar (100 keV) ions at doses of 1 1016 and 2 1016 cmÿ2 , respectively, are presented in Fig. 2. T0 values determined from Fig. 2 on the basis of the formula (1) are 2:2 105 K (for FP-383) and 4:1 105 K (for PI). The value of g was taken from [8] and the value of k k 0:5 nm was calculated according
Fig. 2. Low-temperature dependencies of conductivity for FP383 and PI ®lms irradiated by Ar (100 keV) ions: 1 for FP-383, D 1 1016 cmÿ2 , 2 for PI, D 2 1016 cmÿ2 .
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
to formula (2). It can be noted that under irradiation doses of D > 3 1016 cmÿ2 dependencies of r(T) similar to those represented in Fig. 1 were not detected. The Raman spectra (514.5 nm) from FP383 photoresist ®lms irradiated with Ar (100 keV) ions at the doses of 5 1015 ; 1 1016 and 1 1017 cmÿ2 are presented in Fig. 3. For all doses, a broad peak near 1550 cmÿ2 (G peak) can be observed, which is assigned to an amorphous carbon-like structure. Another peak at 1350 cmÿ2 (D peak) is less pronounced and turns into a plateau at the doses of D > 5 1016 cmÿ2 . The ®lms having a r(T) dependence as described by formula (1) could be used as working elements for cryogenic temperature sensors. Sensors of this class have been developed at the Institute of Physics (St. Petersbourg) [8,9], but they are massive samples. Usage of the described technology would allow to create thin sensitive ®lms and reduce the inertness of the converter. 3.2. Predicting of electrophysical properties of ionimplanted organic materials From the practical point of view, it would be interesting to predict the electrophysical parameters of modi®ed layers for successful application of organic materials irradiated with accelerated ions
Fig. 3. Raman spectra for Ar (100 keV) ± implanted FP-383 resist ®lms: curve 1 for D 5 1015 cmÿ2 , curve 2 for D 1 1016 cmÿ2 , curve 3 for D 1 1017 cmÿ2 .
653
in microelectronics technological processes. The simplest and at the same time suciently reliable approach, has to do with calculation of some critical irradiation parameters at which a qs value in the required range will be achieved. The average value of energy deposited by implanted ions in elastic Gn and inelastic Ge collisions can be treated as the main critical parameter, Gn DEn D=Rp ; Gne Gn Ge :
Ge DEe D=Rp ;
3
Here DEe and DEn are the values of ion energy dissipated in electronic and nuclear subsystems, D is the dose of implanted ions, Rp is the projected range. Because of spatial separation of maxima for elastically and inelastically deposited energies, formulae (3) are only a most probable rough approach. However, it permits us to correctly evaluate the implantation dose needed for a conductive graphite-like phase formation with accuracy of the order of magnitude. More explicitly, the value of Rp in expression for Gn must be replaced by the following expression: DX 1=2 2DRp 2 ln 2 . In this case, however, the simple sum of two contributions for Gne is not correct. In order to determine the values of density of evolved energy necessary for the formation of a conductive Ôgraphite-likeÕ phase (Gne ) it is possible to apply experimental results of irradiation with dierent ions in a wide range of doses and energies on the surface conductivity. It was shown [1,4] that conductive organic layers with qs values equal approximately 100 X= may be obtained by implantation of Ar (100 keV) ions below D 5 1016 cmÿ2 . This is the best currently available result for medium energy ion irradiation of polymers. On the basis of given results it is possible to estimate the critical value of Gne . The calculated means of Gne obtained under the most wide-spread irradiation conditions are listed in Table 3. Calculations of values listed in Table 3 were carried out by the Monte-Carlo procedure. The calculated values correspond to the irradiation dose required for reaching the critical value of DGne . On the basis of data of Table 3, it is rather easy to substantiate the results of experiments on
654
F.F. Komarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 650±654
Table 3 List of parameters of PMMA irradiation with dierent types accelerated ions 3
) Ge (eV/A
3
) Gn (eV/A
D (1017 cmÿ2 )
± ±
4.2 4.2
Ion type
E (keV)
DEe (keV)
H
100 300
99.3 299.9
0.7 0.1
1160 3460
43.1 43.3
He
100 300
95.6 296.1
4.6 3.9
858.5 1770
55.6 83.6
2.7 1.4
3.1 2.4
N
100 300
81.9 273.0
18.1 27.0
318.0 782.4
128.8 174.5
28.4 17.3
1.1 1.0
Ar
100 200 300
63.3 141.8 228.9
36.7 58.2 71.7
139.1 275.6 409.6
227.5 257.3 279.4
131.9 105.6 86.8
0.5 0.5 0.5
Kr
150
84.0
66.6
124.2
339.4
265.5
0.3
DEn (keV)
the in¯uence of irradiation by protons and He ions on conductivity of organic materials. It is evident that the Gne value for protons is lower nearly by an order of magnitude than the required value, and therefore very large irradiation doses are needed for obtaining the conducting layers. Irradiation with B , N , C and Ne ions permits to get the required value of evolved energy density below a dose of the order of D 1:2 1017 cmÿ2 . In this sense, Ar ions are the most favorable type for polymer conductivity modi®cation.
4. Conclusion It has been shown that irradiation of FP-383 and polyimide ®lms with Ar ions permits to get high conductive, chemically stable, graphite-like samples with a limit of qs about 100 X=. The low-temperature conductivity (under 20± 2.5 K) of thin FP-383 photoresist and polyimide ®lms under irradiation doses of 1 1016 ± 2 1016 cmÿ2 is connected with carrier jumps having dierent lengths (Mott's mechanism). A simple model for predicting the doses of implanted ions in polymers for graphite-like structure formation has been discussed.
Rp (nm)
Acknowledgements This work has been supported in part by Soros Fund (grant B6-17-2710-7) and the Fundamental Research Fund of Belarus (grant F98-146). The authors wish to acknowledge Prof. A.N. Aleshin for low-temperature measurements. References [1] J. Davenas, G. Boiteux, X.L. Xu, E. Adem, Nucl. Instr. and Meth. B 32 (1988) 136. [2] T. Trigaud, J.P. Moliton, C. Jussiaux, B. Maziere, Nucl. Instr. and Meth. B 107 (1996) 323. [3] U.S. Sias, G. Sanchez, J.R. Kaschny, L. Amaral, M. Behar, D. Fink, Nucl. Instr. and Meth. B 141 (1998) 187. [4] I.H. Loh, R.B. Oliver, P. Sioshansi, Nucl. Instr. and Meth. B 34 (1988) 337. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.P. Ziegler, (Ed.), The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [6] T. Okade, T. Nishijima, S. Takahata, M. Kunori, K. Iwamoto, Nucl. Instr. and Meth. B 37/38 (1989) 720. [7] R.E. Giedd, M.G. Moss, M.M. Craig, D.E. Robertson, Nucl. Instr. and Meth. B 59/60 (1991) 1253. [8] A.N. Aleshin, A.V. Gribanov, A.V. Suvorov, Fiz. Tverd. Tela. 31 (1989) 12 (in Russian). [9] A.N. Aleshin, A.V. Suvorov, Fiz. Tverd. Tela. 32 (1990) 1717 (in Russian). [10] O. Madelung. Fizika tverdogo tela. Lokalizovannye sostoiania, Nauka, Moskva, 1985, p. 183 (in Russian).