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Ion bombardment effects in conducting polymers
Nuclear Instruments and Methods in Physics Research B 91(1994) 473-477 North-Holland
Ion bombardment S. Schiestel,
NIOMI B
Beam Interactions with Materials&Atoms
effects in conducting polymers
W. Ensinger and G.K. Wolf *
Institut fiir Physikalische Chemie, Im Neuenheimer Feld 500, 69120 Heidelberg, Germany
The modification of properties of conducting polymers by ion bombardment offers interesting possibilities for their application in the field of microelectronics and electrochemistry. Therefore foils of intrinsically conducting polymers (polypyrrole and polythiophene) were modified by noble gas ion implantation. By ion bombardment the resistivity can be increased by several orders of magnitude. This effect depends on the species, energy and ion fluence. Cyclovoltammetric experiments show the same electrochemical features for the modified foils as the untreated polymers, but the observed currents are smaller by two orders of magnitude. The conductivity of the virgin foil ((T= 100 S/cm> is sufficient for galvanic metal deposition. The bombardment induced change of resistivity causes an inhibition of electrochemical deposition. By using masks lateral structures in the range of several km can be produced. During implantation the black colour of the untreated foil changes to brown depending on the implanted dose. The associated changes of the chemical structures, investigated by XPS, are discussed.
1. Introduction More than ten years ago the first intrinsically conducting polymer (polyacetylene) was synthesized. An extended doped (oxidized or reduced) r-system is responsible for the conductivity and also for the optical, magnetic and chemical behaviour [l]. The most simple conducting polymer and the one with the highest conductivity (up to lo5 S/cm) is polyacetylene [2]. The main problem for the application is its poor stability against oxidation. The conducting polymers which consist of aromatic rings are more stable but the conductivity is lower (up to some hundred S/cm). Ion implantation in polymers destroyes the initial structure by cross-linking, scission and emission of atoms, molecules and molecule fragments [3,4]. This leads to changes of their properties like density [5], conductivity [6], optical absorption [7], molecular weight distribution [8] and solubility. The effectiveness of these changes produced in the polymer depends on the structure of the polymer and the experimental conditions of the ion implantation like ion energy, fluence and beam current. Conducting polymers show the same features as isolating polymers during ion bombardment: darkening, carbonization, change of solubility and conductivity. Until now ion implantation in conducting polymers was only carried out for doping [9], and determination of the distribution of the dopants [lo]. In this contribution ion bombardment is used to modify the chemical and electrical properties and to
* Corresponding 6221 563082.
author. Tel. +49 6221 562505, fax +49
produce lateral structures. The changes of the electrical properties were investigated by resistivity measurements, chemical changes by cyclovoltammetric measurements and changes of the structure by XPS experiments.
2. Experimental
For the present study the conducting polymers polypyrrole (PPY) and poly-3-ethoxythiophene (PEOT), which differ only by the heteroatom, were used. The PPY (BASF AG, Ludwigshafen) was obtained as a black foil with a thickness of 120 pm, a conductivity of about 100 S/cm and a doping level of 20%, the counter anion being phenylsulfonate. PEOT (HOECHST AG, Frankfurt) was obtained as a black powder, soluble in nitromethane or acetonitrile, with a doping level of 15%, a conductivity of $3 S/cm and tetrafluoroborate as counter anion. PEOT films were prepared by dipping glass substrates in a PEOT/nitromethane solution. The thickness of the films was about 400 nm (determinated by Talystep) corresponding roughly to the range of the implanted ions [ll]. The resulting PEOT layers show a blue colour, and were used for the resistivity measurements whereas the electrochemical experiments were carried out with the PPY foils. The implantation of the noble gas ions with energies up to 120 keV were made in our lab with the implanter described in ref. [12]. The doses were varied from 1013 to 1016 ions/cm2, the beam current was kept below*500 nA/cm2, and the pressure was about 10W4 Pa. The implantation of nitrogen ions with 2 MeV was
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E1061-P
VI. POLYMERS
474
S. Schiestelet al. / Nucl. Instr. and Meth. in Phys.Res. B 91 (1994) 473-477
performed at MPI Heidelberg with the tandem accelerator described in ref. [13]. The fluence reached from lOi to 101’ ions/cm* and the beam current was less than 20 nA/cm’. The resistivity measurements were also done at MPI Heidelberg with the four-pointmethod (van der Pauw) 1141. For the electrochemical experiments a glass cell with a three electrode configuration was used (platinum counter electrode, saturated calomel (SCE) reference electrode). Electrolyte solution was 1N H,SO,, the potential scan rate was 10 mV/s, and the potential ranged from - 1000 mV to 1000 mV. The X-ray photoelectron spectra were taken with an ESCASCOPE spectrometer (V.G.), described in ref. [15] using Al Ko radiation for excitation.
3. Results During all implantations the polymers darken depending on the implanted dose. Even at low doses (1013 ions/cm*) irradiated PEOT was no longer soluble in nitromethane or acetonitrile. 3.1. Change of resistivity The first implantation experiments were carried out with noble gas ions of 120 keV energy to study the effects of resistivity change. One part of the sample was covered with a mask to compare the resistivity R, without and R, with implantation. Fig. 1 shows the relative changes of resistivity RJR, in dependence on the fluence. The curves have for each ion the same shape: at low doses (I 1014 ions/cm’) the resistivity increases by several orders of magnitude; at higher doses it decreases again. It will be shown that the
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fluence [ions/cma] Fig. 1. Relative change of resistivity (resistivity with implantation R,; resistivity without implantation R,) by ion irradiation of PEOT with noble gas ions with an energy of 120 keV versus fluence l : Ne, A : Ar, l : Kr. The lines are drawn to gide the eyes.
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fluence [ion&ma] Fig. 2. Dependence of resistivity change on ion energy and penetration depth of PEOT, the relative change of resistivity is plotted versus fluence; 100 nm: 0: He 5 keV, +: Ne 36 keV, +:120keVKr,200nm: r:HellkeV, r:75keVNe, x:Kr 240 keV. n : N, 2 MeV. The lines are drawn to gide the eyes. increase is mainly due to the destruction of the conducting T-system. The conjugation is interrupted by irradiation induced bond breaking. Upon further irradiation with increasing dose the resistivity decreases again due to carbonization as known from experiments with non-conducting polymers [16]. Attention must be payed to the different penetration depths of the implanted ions. For evaluating the influence of the ion range an experiment with two different ion energies was carried out. The energies were chosen in such a way that the range amounted to = 100 nm and = 200 nm for the three different rare gas ions (Fig. 2). The same shape of the curve as in Fig. 1 can be seen depending again on the implanted dose, but the resistivity changes for the same dose differ by two orders of magnitude, not only by a factor of two like the projected ranges. This proves that the penetration depth is not the dominant factor. In the used ion energy range the amount of energy transfer is not uniform throughout the modified thickness for every ion. For He ions for example, the electronic stopping power is dominant whereas for Kr ions the nuclear stopping power is much stronger. To have a rough indication for the importance of the energy transfer effects nitrogen ions with an energy of two MeV were implanted (Fig. 2, W>. For this value the electronic energy loss is reasonably uniform throughout the film thickness. Only the part of the curve where the resistivity increases is shown. It can be seen that high energy implantation causes the same effect at lower dose as implantation with lower energy (keV) at higher dose. This demonstrates for the present case that mainly electronic energy transfer is responsible for the changes of the resistivity of several orders of magnitude. Comparing the high-energy implantation with the keV implantation, the total energy input seems to be the one dominant factor for the change of resistivity.
475
Ins&, and Me& in Phys. Res. B 92 (1994) 473-477
S. S~hi~~t~~ et al. /Nucl.
3.2. XPS results
lites. During the ion implantation C3 and C4 decrease in dependence on the doses. In general the FWIIIH decreases whereas the intensity increases considerably. This corresponds to an enrichment of carbon at the surface and a lower number of C species. 2) The fitting of the curves implies six peaks. S 1, S 3 originate from S in the neutral thiophene ring, S 2 and S 4 from the oxidized form. S 5 and S 6 are assigned to (~-~*-transition) shake-up satellites. During implantation S 5, S 6 increase up to a dose of 1015 ions/cm2 and decreases after further implantation again. In general the intensity of the S 2p peak decreases with increasing dose. This is also a proof for the decomposition of the conducting r-system. 3) Both the F Is- and the B Is-peak (not shown in Fig. 31, originating from the counter anion, undergo a considerable loss even at low doses (lOi ions/cm’). 4) For the 0 1s peak nearly no change could be observed. A possible explanation is oxygen from the residual gas in the impl~~tion chamber 1173,another
The information depth of the X-ray photoelectron spectroscopy is quite small (some nm) compared to the penetration depth of the implanted ions. Sputtering for depth profiling is not possible for polymers because the sputtering process will also change the structure of the polymer. Fig. 3 shows the XP spectra from PEOT, which contain the elements C, S, 8, F, B. The peaks of all these elements shift about 3-4 eV to higher binding energies for implanted samples. These shifts correspond to the change of resistivity and are caused by charging of the sample. As can be noticed for 10r6 ions/cm’ (low resistivity again) the C 1s peak is in the same position as for the untreated sample. In Fig. 3 the spectra of C 1s and S ‘2p are summerized. 1) The C 1s peak of the virgin sample is composed of four peaks: C 1 corresponds to C-C and C-S bonds, C 2 to C-O and C-C, C-S positively charged (due the o~dation) and C 4 to (W-T *-transition) shake-up satel-
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Fig. 3. XP spectra (only C Is and S 2p peaks) of PEOT, ~mparison of the untreated and irradiated PEOT (Ar2+, 120 keV). VI. POLYMERS
S. Schiestel et ial./NucE lnstr. and Meth. in Whys.Res. B 91 (1994) 473-477
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potential U [rnq vs SCE Fig. 4. Cyclic voltammograms of PPY ~planted with AT'+, 120 keV in 1N H,SO,, at a potential scan rate of 10 mV/s; 1(J15 pristine, ..... ‘ lOi4 ions/cm2, --ions/cm’.
the exposition to air during the change from the implantation chamber to the analysis chamber. Sputter experiments in the UHV chamber showed a loss of oxygen.
The two polymers PPY and PEOT can also be produced electrochemically. They can be reduced or
potential U [mV] vs SCE Fig. 5. Cyclic volta~o~a~ of PPY, implanted with Ar ions (120 keV), lOi ions/cm’, taken in the multi cycle mode; cycle, -----41st cycle, 1st cycIe, . . . . . . -2lst --61st cycle, * * - - - ’ ’ Slst cycle.
oxidized reversibly. Fig. 4 shows the cyclic voltammograms of PPY implanted with A?+, 120 keV. The peak 2 0 mV SCE corresponds to the oxidation, the other one I 0 mV to the reduction of the polymer. Performing muIticycling a saturation in current density is reached. For the implanted samples two effects can be observed:
Fig. 6. Micrograph of PPY after implantation with Ar ions (120 keV) through a mask and subsequent galvanic copper deposition.
S. Schiestelet al./NucZ. In.+. and Meth. in Phys. Rex B 91 (1994) 473-477
411
1) The general electrochemical features are the same (see Fig. 4), but the current density is smaller by one to two orders of magnitude compared to the untreated sample depending on the implanted dose. 2) When the number of cycles increases, the current density rises again (Fig. 5). During the first cycles the thin ion beam modified layer is dominant. With an increasing number of cycles it will be more and more removed. It is well known that carbon can be oxidized to CO, during electrochemical cycling [18]. After a certain number of cycles, depending on the implanted energy and dose, the current density reaches the one of the untreated samples, and the colour of the modified sample changes from brown to the original black. After implantation with MeV ions one observes the same effect, however already at lower doses.
the conducting P-system and of the counter anion. Therefore the possibility for producing lateral structures for electronic devices exists and the electrochemical behaviour is affected.
3.4. Galvanic copper deposition
References
The conductivity of PPY is sufficient to deposite copper galvanically. A current density of 10 mA/cm’ and a commercial acidic copper bath (Doduco, Pforzheim) were used. The deposit can be removed again by dissolution in nitric acid. The structure of the polymer does not change during this procedure and copper deposition can be carried out again. Because ion bombardment increases the resistivity of the PPY, it leads to an inhibition of the electrochemical copper deposition. Performing ion implantation through a mask, structures in the urn range can be produced which are revealed by the Cu-deposition process. The micrograph (Fig. 6) shows a PPY foil after implantation through a mask and subsequent galvanic copper deposition. The width of the copper stripes is about 4 pm.
4. Conclusions In general conducting polymers show the same effects as isolating polymers during ion bombardment: they darken, change their conductivity and solubility and carbonize, accompanied by loss of their heteroatoms. Opposite to isolators the resistivity can be increased at low doses, as shown by four point resistivity measurements. XP spectra show a degradiation of
Acknowledgements
The authors gratefully acknowledge Ch. Klatt for the MeV implantation, M. Grunze for support with XPS measurements and K. Holldack for valuable discussions. We are also very grateful to Dr. H. Naarmann (BASF AG) and Dr. H. Millauer (Hoechst AG) for the gift of the conducting polymers.
[l] T.A. Scotheim (ed.), Handbook of Conducting Polymers, Vols. I and II (Dekker, New York, 1986). [2] H. Naarmann and N. Theophilou, Synth. Met. 22 (1987) [3] k. Marletta, Nucl. Instr. and Meth. B 46 (1990) 295. [4] A. Chapiro, Nucl. Instr. and Meth. B 32 (1988) 111. [5] L. Calgagno and G. Foti, Nucl. Instr. and Meth. B 19/20 (1987) 895. [6] J. Davenas, G. Boiteux and X.L. Xu, Nucl. Instr. and Meth. B 32 (1988) 136. [7] J. Davenas, P. Thevenard, G. Boiteux, M. Fallavier and X.L. Xu, Nucl. Instr. and Meth. B 46 (1990) 317. [8] 0. Puglisi, A. Licciardello, L. Calgagno and G. Foti, Nucl. Instr. and Meth. B 19/20 (1987) 865. [9] A. Kuczkowski and I. Liwo, Synth. Met. 18 (1987) 575. [lo] H. Isotalo and H. Stubb, Synth. Met. 28 (1989) C305. [ll] J.F. Ziegler and J.P. Biersack, TRIM code (91.04) (1991). [12] G. Frech, W. Ensinger, H. Miiller, M. Hans and G.K. Wolf, Nucl. Instr. and Meth. B 68 (1992) 402. [13] P. Oberschachtsieck, M. Weiser and S. Kalbitzer, Nucl. Instr. and Meth. B 56/57 (1991) 1010. [14] T.A. Scotheim (ed.), Handbook of Conducting Polymers, Vol. I (Dekker, New York, 1986) p. 224. [15] M. Kinzler, M. Gnmze, N. Blank, H. Schenkel and L. Scheffler, J. Vat. Sci. Technol. A 10 (1990) 2691. [16] L. Calgano, G. Compagnini and G. Foti, Nucl. Instr. and Meth. B 65 (1992) 413. [17] T. Venkatesan, Nucl. Instr. and Meth. B 7/S (1985) 461. 1181 M. Pourbaix, Atlas d’Equilibres electrochemique (Gauthier-Villars, 1963) p. 449.
VI. POLYMERS
Ion bombardment S. Schiestel,
NIOMI B
Beam Interactions with Materials&Atoms
effects in conducting polymers
W. Ensinger and G.K. Wolf *
Institut fiir Physikalische Chemie, Im Neuenheimer Feld 500, 69120 Heidelberg, Germany
The modification of properties of conducting polymers by ion bombardment offers interesting possibilities for their application in the field of microelectronics and electrochemistry. Therefore foils of intrinsically conducting polymers (polypyrrole and polythiophene) were modified by noble gas ion implantation. By ion bombardment the resistivity can be increased by several orders of magnitude. This effect depends on the species, energy and ion fluence. Cyclovoltammetric experiments show the same electrochemical features for the modified foils as the untreated polymers, but the observed currents are smaller by two orders of magnitude. The conductivity of the virgin foil ((T= 100 S/cm> is sufficient for galvanic metal deposition. The bombardment induced change of resistivity causes an inhibition of electrochemical deposition. By using masks lateral structures in the range of several km can be produced. During implantation the black colour of the untreated foil changes to brown depending on the implanted dose. The associated changes of the chemical structures, investigated by XPS, are discussed.
1. Introduction More than ten years ago the first intrinsically conducting polymer (polyacetylene) was synthesized. An extended doped (oxidized or reduced) r-system is responsible for the conductivity and also for the optical, magnetic and chemical behaviour [l]. The most simple conducting polymer and the one with the highest conductivity (up to lo5 S/cm) is polyacetylene [2]. The main problem for the application is its poor stability against oxidation. The conducting polymers which consist of aromatic rings are more stable but the conductivity is lower (up to some hundred S/cm). Ion implantation in polymers destroyes the initial structure by cross-linking, scission and emission of atoms, molecules and molecule fragments [3,4]. This leads to changes of their properties like density [5], conductivity [6], optical absorption [7], molecular weight distribution [8] and solubility. The effectiveness of these changes produced in the polymer depends on the structure of the polymer and the experimental conditions of the ion implantation like ion energy, fluence and beam current. Conducting polymers show the same features as isolating polymers during ion bombardment: darkening, carbonization, change of solubility and conductivity. Until now ion implantation in conducting polymers was only carried out for doping [9], and determination of the distribution of the dopants [lo]. In this contribution ion bombardment is used to modify the chemical and electrical properties and to
* Corresponding 6221 563082.
author. Tel. +49 6221 562505, fax +49
produce lateral structures. The changes of the electrical properties were investigated by resistivity measurements, chemical changes by cyclovoltammetric measurements and changes of the structure by XPS experiments.
2. Experimental
For the present study the conducting polymers polypyrrole (PPY) and poly-3-ethoxythiophene (PEOT), which differ only by the heteroatom, were used. The PPY (BASF AG, Ludwigshafen) was obtained as a black foil with a thickness of 120 pm, a conductivity of about 100 S/cm and a doping level of 20%, the counter anion being phenylsulfonate. PEOT (HOECHST AG, Frankfurt) was obtained as a black powder, soluble in nitromethane or acetonitrile, with a doping level of 15%, a conductivity of $3 S/cm and tetrafluoroborate as counter anion. PEOT films were prepared by dipping glass substrates in a PEOT/nitromethane solution. The thickness of the films was about 400 nm (determinated by Talystep) corresponding roughly to the range of the implanted ions [ll]. The resulting PEOT layers show a blue colour, and were used for the resistivity measurements whereas the electrochemical experiments were carried out with the PPY foils. The implantation of the noble gas ions with energies up to 120 keV were made in our lab with the implanter described in ref. [12]. The doses were varied from 1013 to 1016 ions/cm2, the beam current was kept below*500 nA/cm2, and the pressure was about 10W4 Pa. The implantation of nitrogen ions with 2 MeV was
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E1061-P
VI. POLYMERS
474
S. Schiestelet al. / Nucl. Instr. and Meth. in Phys.Res. B 91 (1994) 473-477
performed at MPI Heidelberg with the tandem accelerator described in ref. [13]. The fluence reached from lOi to 101’ ions/cm* and the beam current was less than 20 nA/cm’. The resistivity measurements were also done at MPI Heidelberg with the four-pointmethod (van der Pauw) 1141. For the electrochemical experiments a glass cell with a three electrode configuration was used (platinum counter electrode, saturated calomel (SCE) reference electrode). Electrolyte solution was 1N H,SO,, the potential scan rate was 10 mV/s, and the potential ranged from - 1000 mV to 1000 mV. The X-ray photoelectron spectra were taken with an ESCASCOPE spectrometer (V.G.), described in ref. [15] using Al Ko radiation for excitation.
3. Results During all implantations the polymers darken depending on the implanted dose. Even at low doses (1013 ions/cm*) irradiated PEOT was no longer soluble in nitromethane or acetonitrile. 3.1. Change of resistivity The first implantation experiments were carried out with noble gas ions of 120 keV energy to study the effects of resistivity change. One part of the sample was covered with a mask to compare the resistivity R, without and R, with implantation. Fig. 1 shows the relative changes of resistivity RJR, in dependence on the fluence. The curves have for each ion the same shape: at low doses (I 1014 ions/cm’) the resistivity increases by several orders of magnitude; at higher doses it decreases again. It will be shown that the
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fluence [ions/cma] Fig. 1. Relative change of resistivity (resistivity with implantation R,; resistivity without implantation R,) by ion irradiation of PEOT with noble gas ions with an energy of 120 keV versus fluence l : Ne, A : Ar, l : Kr. The lines are drawn to gide the eyes.
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fluence [ion&ma] Fig. 2. Dependence of resistivity change on ion energy and penetration depth of PEOT, the relative change of resistivity is plotted versus fluence; 100 nm: 0: He 5 keV, +: Ne 36 keV, +:120keVKr,200nm: r:HellkeV, r:75keVNe, x:Kr 240 keV. n : N, 2 MeV. The lines are drawn to gide the eyes. increase is mainly due to the destruction of the conducting T-system. The conjugation is interrupted by irradiation induced bond breaking. Upon further irradiation with increasing dose the resistivity decreases again due to carbonization as known from experiments with non-conducting polymers [16]. Attention must be payed to the different penetration depths of the implanted ions. For evaluating the influence of the ion range an experiment with two different ion energies was carried out. The energies were chosen in such a way that the range amounted to = 100 nm and = 200 nm for the three different rare gas ions (Fig. 2). The same shape of the curve as in Fig. 1 can be seen depending again on the implanted dose, but the resistivity changes for the same dose differ by two orders of magnitude, not only by a factor of two like the projected ranges. This proves that the penetration depth is not the dominant factor. In the used ion energy range the amount of energy transfer is not uniform throughout the modified thickness for every ion. For He ions for example, the electronic stopping power is dominant whereas for Kr ions the nuclear stopping power is much stronger. To have a rough indication for the importance of the energy transfer effects nitrogen ions with an energy of two MeV were implanted (Fig. 2, W>. For this value the electronic energy loss is reasonably uniform throughout the film thickness. Only the part of the curve where the resistivity increases is shown. It can be seen that high energy implantation causes the same effect at lower dose as implantation with lower energy (keV) at higher dose. This demonstrates for the present case that mainly electronic energy transfer is responsible for the changes of the resistivity of several orders of magnitude. Comparing the high-energy implantation with the keV implantation, the total energy input seems to be the one dominant factor for the change of resistivity.
475
Ins&, and Me& in Phys. Res. B 92 (1994) 473-477
S. S~hi~~t~~ et al. /Nucl.
3.2. XPS results
lites. During the ion implantation C3 and C4 decrease in dependence on the doses. In general the FWIIIH decreases whereas the intensity increases considerably. This corresponds to an enrichment of carbon at the surface and a lower number of C species. 2) The fitting of the curves implies six peaks. S 1, S 3 originate from S in the neutral thiophene ring, S 2 and S 4 from the oxidized form. S 5 and S 6 are assigned to (~-~*-transition) shake-up satellites. During implantation S 5, S 6 increase up to a dose of 1015 ions/cm2 and decreases after further implantation again. In general the intensity of the S 2p peak decreases with increasing dose. This is also a proof for the decomposition of the conducting r-system. 3) Both the F Is- and the B Is-peak (not shown in Fig. 31, originating from the counter anion, undergo a considerable loss even at low doses (lOi ions/cm’). 4) For the 0 1s peak nearly no change could be observed. A possible explanation is oxygen from the residual gas in the impl~~tion chamber 1173,another
The information depth of the X-ray photoelectron spectroscopy is quite small (some nm) compared to the penetration depth of the implanted ions. Sputtering for depth profiling is not possible for polymers because the sputtering process will also change the structure of the polymer. Fig. 3 shows the XP spectra from PEOT, which contain the elements C, S, 8, F, B. The peaks of all these elements shift about 3-4 eV to higher binding energies for implanted samples. These shifts correspond to the change of resistivity and are caused by charging of the sample. As can be noticed for 10r6 ions/cm’ (low resistivity again) the C 1s peak is in the same position as for the untreated sample. In Fig. 3 the spectra of C 1s and S ‘2p are summerized. 1) The C 1s peak of the virgin sample is composed of four peaks: C 1 corresponds to C-C and C-S bonds, C 2 to C-O and C-C, C-S positively charged (due the o~dation) and C 4 to (W-T *-transition) shake-up satel-
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288 286 284 282 280 Binding energy [eV]
286
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Binding energy[eV]
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Fig. 3. XP spectra (only C Is and S 2p peaks) of PEOT, ~mparison of the untreated and irradiated PEOT (Ar2+, 120 keV). VI. POLYMERS
S. Schiestel et ial./NucE lnstr. and Meth. in Whys.Res. B 91 (1994) 473-477
475
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8.0
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4.0
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1200
30
potential U [rnq vs SCE Fig. 4. Cyclic voltammograms of PPY ~planted with AT'+, 120 keV in 1N H,SO,, at a potential scan rate of 10 mV/s; 1(J15 pristine, ..... ‘ lOi4 ions/cm2, --ions/cm’.
the exposition to air during the change from the implantation chamber to the analysis chamber. Sputter experiments in the UHV chamber showed a loss of oxygen.
The two polymers PPY and PEOT can also be produced electrochemically. They can be reduced or
potential U [mV] vs SCE Fig. 5. Cyclic volta~o~a~ of PPY, implanted with Ar ions (120 keV), lOi ions/cm’, taken in the multi cycle mode; cycle, -----41st cycle, 1st cycIe, . . . . . . -2lst --61st cycle, * * - - - ’ ’ Slst cycle.
oxidized reversibly. Fig. 4 shows the cyclic voltammograms of PPY implanted with A?+, 120 keV. The peak 2 0 mV SCE corresponds to the oxidation, the other one I 0 mV to the reduction of the polymer. Performing muIticycling a saturation in current density is reached. For the implanted samples two effects can be observed:
Fig. 6. Micrograph of PPY after implantation with Ar ions (120 keV) through a mask and subsequent galvanic copper deposition.
S. Schiestelet al./NucZ. In.+. and Meth. in Phys. Rex B 91 (1994) 473-477
411
1) The general electrochemical features are the same (see Fig. 4), but the current density is smaller by one to two orders of magnitude compared to the untreated sample depending on the implanted dose. 2) When the number of cycles increases, the current density rises again (Fig. 5). During the first cycles the thin ion beam modified layer is dominant. With an increasing number of cycles it will be more and more removed. It is well known that carbon can be oxidized to CO, during electrochemical cycling [18]. After a certain number of cycles, depending on the implanted energy and dose, the current density reaches the one of the untreated samples, and the colour of the modified sample changes from brown to the original black. After implantation with MeV ions one observes the same effect, however already at lower doses.
the conducting P-system and of the counter anion. Therefore the possibility for producing lateral structures for electronic devices exists and the electrochemical behaviour is affected.
3.4. Galvanic copper deposition
References
The conductivity of PPY is sufficient to deposite copper galvanically. A current density of 10 mA/cm’ and a commercial acidic copper bath (Doduco, Pforzheim) were used. The deposit can be removed again by dissolution in nitric acid. The structure of the polymer does not change during this procedure and copper deposition can be carried out again. Because ion bombardment increases the resistivity of the PPY, it leads to an inhibition of the electrochemical copper deposition. Performing ion implantation through a mask, structures in the urn range can be produced which are revealed by the Cu-deposition process. The micrograph (Fig. 6) shows a PPY foil after implantation through a mask and subsequent galvanic copper deposition. The width of the copper stripes is about 4 pm.
4. Conclusions In general conducting polymers show the same effects as isolating polymers during ion bombardment: they darken, change their conductivity and solubility and carbonize, accompanied by loss of their heteroatoms. Opposite to isolators the resistivity can be increased at low doses, as shown by four point resistivity measurements. XP spectra show a degradiation of
Acknowledgements
The authors gratefully acknowledge Ch. Klatt for the MeV implantation, M. Grunze for support with XPS measurements and K. Holldack for valuable discussions. We are also very grateful to Dr. H. Naarmann (BASF AG) and Dr. H. Millauer (Hoechst AG) for the gift of the conducting polymers.
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VI. POLYMERS