- Email: [email protected]
Ion implantation of polymers for electrical conductivity enhancement
Nuclear Instruments and Methods in Physics Research B56/57 (1991) 656-659
656
North-Holland
Ion implantation of polymers for electrical conductivity enhancement Lynn B. Bridwell, Center
R.E; Giedd, Y.Q. Wang, S.S. Mohite
forScientific Research, Southwest Missouri State University, Springfield
and Tamera
Jahnke
MO 65804, USA
I.M. Brown McDonnell
Douglas Research Loboratory,
St. Louis MO 63166, USA
C.J. Bedell and C.J. Sofield AEA
Technology Intec, Hat-well Laboratory
Didcot, Oxon, UK
The polymers PET, PAN, PES and PEEK were implanted with ions of He, B, C, N, Ar and As at an energy of 50 keV. PEEK was implanted with I at 22.5 MeV, Xe at 24 MeV, and Ni at 47 MeV. Surface resistivity for electrical conduction indicated a plateau effect in the dose range lOI -10” ions/cm’. The more aliphatic polymer, PET, indicated the lowest resistance of the low energy implants. The high energy ions produced much lower resistivities at much lower doses. The temperature dependence of the resistivities indicate a quasi-one-dimensional variable range hopping mechanism for electrical conduction.
1. Introduction
In the recent past we have undertaken a series of experiments aimed at increasing the electrical conductivity of polymers by ion implantation [l-4]. In the work by Bedell et al. [l] large increases in conductivity of poly-ether-ether-ketone (PEEK) were observed with both high energy ions (I and Xe at 22 MeV and Ni at 47 MeV) and low energy ions (e.g. He, B, C, N, Ar and As at 50 keV). The advantage of the high energy ions was that much higher conductivities were achieved at lower fluxes. An incidental observation showed that implanted surfaces can be made as hard as stainless steel. In the work by Giedd et al. [2] poly(ethylene terephthalate) (PET) was implanted with 50 keV Ar and B. Later Giedd et al. [3] reported similar work with polyacrylonitrile (PAN). Bridwell et al. [4] reported a more comprehensive study of ion implanted poly-ethersulfone (PES) in an attempt to relate electrical conductivity to specific radiation damage mechanisms. This study is aimed at understanding the relationship between polymer structure and electrical conductivity, as well as point out the contrast between high energy and low energy implants.
2. Experiment Commercially available sheets of PEEK and PES were cut into circular targets. PET and PAN targets 0168-583X/91/$03.50
were formed by spin-coating from solution followed by baking to remove all traces of solvents. The targets were implanted at Southwest Missouri State University with beams of He, B, C, N, Ar and As at an energy of 50 keV. Final doses in the range of 10’6-10’7 ions/cm’ were achieved. Beam currents of 300 uA over an area of 1 cm X 2 cm were used to implant a circular target assembly with a total area of approximately 500 cm’. Six targets of 8.6 cm diameter were mounted on the assembly which rotated and reciprocated simultaneously to prevent severe local heating. As each target dose was achieved, the target chamber was opened and one target removed. The final target assembly temperature, measured by a standard thermocouple at the end of the run, did not exceed SO’C. Instantaneous target temperatures could not be determined. The target chamber was pumped with an 8 in. cryopump at a vacuum of lop6 Torr. Pressures were slightly higher in the case of the He beam. Beam doses were determined with a digital current integrator that measured the electron current required to neutralize the target by electron flooding. High energy implants of Ni at 47 MeV, Xe at 24 MeV, and I at 22.5 MeV into amorphous target of PEEK were performed on the variable energy cyclotron at the AEA Technology Laboratory at Harwell. The beam spot was approximately 1 cm diameter and the beam current was measured in a long, well suppressed Faraday cup. The target chamber was pumped by a water cooled diffusion pump to a vacuum of 10m6 Torr.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
L.B. Bridwell et al. / Ion implantation
Surface resistance of the circular samples was measured by determining the current flow between two annular probes with dimensions 57 mm o.d. and 51 mm i.d. Current was measured with a high precision electrometer. The voltage used was 1 V across the gap of 6 mm. Resistance versus temperature was measured with two planar electrodes consisting of evaporated Al films, 2.5 cm wide with a linear gap of 1 cm. Contact resistance was of the order of 1 k0. The sample was cooled by a closed cycle liquid helium refrigeration system with an operating range that extends down to about 10 K. The sample chamber was filled with gaseous helium to promote rapid thermal equilibrium between the sample and the platinum resistance thermometer. Surface resistance and temperature dependence of the high energy implants were measured with a standard four-point probe at Harwell in a similar liquid helium refrigeration system. It is important to note that the implant currents and target removal sequence were held constant throughout. Wang et al. [5] have shown significant dose rate effects on surface conductivity.
651
of polymers
--• -A
PAN
B Ar
“1, , , ,::_: 12
3
4
5
6
7
8
910
Beam Dose (xl 016)10NS/CM2 Fig. 1. Surface resistivity of PAN vs target dose for B and Ar ions implanted at 50 keV.
3. Results
50% 5 Figs. l-4 show surface resistance for PAN, PET, PES and PEEK, respectively, for various ions implanted in the dose range 1016-10’7 ions/cm2 at an energy of 50 keV under the conditions described above. Connecting lines were drawn to show general trends. These results indicate a plateau effect in the dose range reported here. Earlier results of ESR studies [4] indicate that the ion tracks achieve significant overlap in this same dose range. Wasserman [6] has hypothesized that the conducting mechanism is a variable range quasi-linear hopping of electrons between grains or “blobs” rendered conducting in the track left by the ion. As the dose increases, these grains become more dense and the links between grains become shorter. It is evident from figs. 3 and 4 that He does not produce as obvious a plateau effect as the heavier ions. This is probably due to the smaller number of lateral recoils within the target about each ion path. Even at this low energy, He ions deposit 85% of their energy by electronic processes [7]. Fig. 5 shows the surface resistance of PEEK versus dose for the high energy implants. Xe and I were chosen to see if implant doping effects could be seen. While the energies of the two ions are not exactly the same, it is clear that there are no significant contributions to the conductivity due to chemical activity of I. The resistivity of the Ni implant is the lowest of all the results reported here. The dose at which these results were achieved were much lower than for the 50 keV implants. In order to determine more details about the conduction mechanism, we examined the temperature dependence of the resistance. Fig. 6 shows the log of the ratio
5
5
:_--___t 106
12
I
I
I
I
I
I
I
3 4 5 6 7 8 910 Beam Dose (xl 016)10NS/CM2
.
Fig. 2. Surface resistivity of PET vs target dose for B and Ar ions implanted at 50 keV.
10'5, -I-
PES
Helium
==o=Boron C,--Carbon --o--
Nitroger
- +-
Arsenic
50 keV
1081 123456789 Beam Dose (x~O~~)IONS/CM* Fig. 3. Surface resistivity of PES for He, B, C, N and As ions implanted at 50 keV. VII. ION IMPLANTATION
658
L.B. Bridwell et al. / Ion implantation
--*B PEEK
ofpolymers
50 keV
PET 50 keVAr
-.C .----. 0N ---0 He
12345678
9
Beam Dose
10
(x10i6)10NS/CM2
T-l/Z
Fig. 4. Surface resistivity of PEEK for He, B, C and N ions implanted at 50 keV.
Fig. 7. Plot of the log,,(R/R,,) vs T-l/‘, where R,, is the resistance at room temperature, for PET implanted with Ar at 50 keV.
---@ Xe 24 MeV ---A 122.5MeV -m Ni 47 MeV
10. s
. v)lOO 10'3
I
I
10'4 10'5 Beam Dose (IONS/CM*)
Fig. 5. Surface resistivity of PEEK implanted with 22.5 MeV I, 24 MeV Xe and 47 MeV Ni.
8-
zs iE 2 %
4_
$ J
2_
6-
00.04
0.06
0.08
0.10 T-1 /2
0.12
0.
Fig. 8. Plot of the log,,(R/R,,) vs T-‘j2, where R,, is the resistance at room temperature, for PAN implanted with B at 50 keV.
7
PEEK
3.0
1 2.2 _
.05
.06
.07
.08
.09
.lO
T-l/2
Fig. 6. Plot of the log,,(R/Rxw) vs T-‘j2, where rsoo is the resistance at room temperature, for PES implanted with As at 50 keV.
2.0 fl-” .05
..-I//’ I .I0
.15
.20
.25
T-‘/Z
Fig. 9. Plot of the logIOp, where p is the bulk resistivity of PEEK after implantation by I at an energy of 22.5 MeV.
659
I_ B. Bridwell et al. / Ion impI~~tati~~ of polymers
the sample resistance at temperature T to that at room temperature versus the reciprocal of the square root of the temperature for PES implanted with 50 keV As. Fig. 7 indicates similar results for Ar-implanted PET. Fig. 8 shows the same for B-implanted PAN. Fig. 9 shows the resistivity versus temperature dependence for the 22 MeV I-implanted PEEK. Redfield j8] has treated the problem of quasi-one-Dimensions electronic conduction in disordered, nonmetallic materials. Such a variable range hopping conduction process should be linearly dependent on T -‘/’ . This is clearly the case for all the data presented here within certain temperature ranges. We should make two observations. The 22 MeV I-implanted PEEK shows the same temperature dependence as the low energy implanted samples. In experiments just completed, the temperature range was increased to 400 K. The same temperature dependence was observed over the entire range. In the case of B-implanted PAN we see a departure from linearity at about 70 K. This is consistent with a decrease in the coupling between the phonons and electrons in the hopping process. of
4. Conclusion Surface resistivity versus dose in the range 10’6-10’7 ions/cm* indicates a plateau effect at final values that are dependent on the incident ion and the target polymer. This work indicates that aliphatic or partially aliphatic polymers such as PET and PAN will reach lower resistivity than polymers that have more fully aromatic structures. Although both PEEK and PES are fully aromatic, the coupling within the monomer is somewhat different. PEEK reaches the lower surface
resistivity. With regard to the conduction mechanism, our results are consistent with a quasi-one-dimensional variable range hopping mechanism for the conduction process. Although the high energy implants show much greater conductivity at much lower doses, the mechanism
for that
conductivity
is the same.
This work was funded jointly by Southwest Missouri State University, the AEA Technology Corporate Research Fund, and the McDonnell Douglas Independent Research Program. Travel support for L. Bridgwell was provided in part by NSF Grant No. INT-8620233.
References 111 C.J. Bedell, C.J. Sofield, LB. Bridwell
and I.M. Brown, J. Appl. Phys. 67 (1990) 1736. 121 R.E. Giedd, J. Shipman and M. Murphy, Mater. Res. Sot. symp. Pxoc. 147 (1989) 377. 131 RE. Giedd, M.G. Moss and M. Craig, 7th Int. Conf. on Ion Beam M~ification of Materials, Knoxville, TN, 1990, to be published in Nucl. Instr. and Meth. B. 141 L.B. Brldwell, R.E. Giedd, Y.Q. Wang, S.S. Mohite, Tamera Jahnke and I.M. Brown, ibid. 151 Y.Q. Wang, L.B. Bridwell, R.E. Giedd and M. Murphy, these Proceedings (11th Int. Conf. on the Applications of Accelerators in Research and Industry, Denton, TX, 1990) Nucl. Instr. and Meth. B56/57 (1991) 660. 161 B. Wasserman, Phys. Rev. B34 (1986) 1926. 171J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, Oxford, 1985). 181 David Redfield, Phys. Rev. Lett. 30 (1973) 1319.
VII. ION IMPLANTATION
656
North-Holland
Ion implantation of polymers for electrical conductivity enhancement Lynn B. Bridwell, Center
R.E; Giedd, Y.Q. Wang, S.S. Mohite
forScientific Research, Southwest Missouri State University, Springfield
and Tamera
Jahnke
MO 65804, USA
I.M. Brown McDonnell
Douglas Research Loboratory,
St. Louis MO 63166, USA
C.J. Bedell and C.J. Sofield AEA
Technology Intec, Hat-well Laboratory
Didcot, Oxon, UK
The polymers PET, PAN, PES and PEEK were implanted with ions of He, B, C, N, Ar and As at an energy of 50 keV. PEEK was implanted with I at 22.5 MeV, Xe at 24 MeV, and Ni at 47 MeV. Surface resistivity for electrical conduction indicated a plateau effect in the dose range lOI -10” ions/cm’. The more aliphatic polymer, PET, indicated the lowest resistance of the low energy implants. The high energy ions produced much lower resistivities at much lower doses. The temperature dependence of the resistivities indicate a quasi-one-dimensional variable range hopping mechanism for electrical conduction.
1. Introduction
In the recent past we have undertaken a series of experiments aimed at increasing the electrical conductivity of polymers by ion implantation [l-4]. In the work by Bedell et al. [l] large increases in conductivity of poly-ether-ether-ketone (PEEK) were observed with both high energy ions (I and Xe at 22 MeV and Ni at 47 MeV) and low energy ions (e.g. He, B, C, N, Ar and As at 50 keV). The advantage of the high energy ions was that much higher conductivities were achieved at lower fluxes. An incidental observation showed that implanted surfaces can be made as hard as stainless steel. In the work by Giedd et al. [2] poly(ethylene terephthalate) (PET) was implanted with 50 keV Ar and B. Later Giedd et al. [3] reported similar work with polyacrylonitrile (PAN). Bridwell et al. [4] reported a more comprehensive study of ion implanted poly-ethersulfone (PES) in an attempt to relate electrical conductivity to specific radiation damage mechanisms. This study is aimed at understanding the relationship between polymer structure and electrical conductivity, as well as point out the contrast between high energy and low energy implants.
2. Experiment Commercially available sheets of PEEK and PES were cut into circular targets. PET and PAN targets 0168-583X/91/$03.50
were formed by spin-coating from solution followed by baking to remove all traces of solvents. The targets were implanted at Southwest Missouri State University with beams of He, B, C, N, Ar and As at an energy of 50 keV. Final doses in the range of 10’6-10’7 ions/cm’ were achieved. Beam currents of 300 uA over an area of 1 cm X 2 cm were used to implant a circular target assembly with a total area of approximately 500 cm’. Six targets of 8.6 cm diameter were mounted on the assembly which rotated and reciprocated simultaneously to prevent severe local heating. As each target dose was achieved, the target chamber was opened and one target removed. The final target assembly temperature, measured by a standard thermocouple at the end of the run, did not exceed SO’C. Instantaneous target temperatures could not be determined. The target chamber was pumped with an 8 in. cryopump at a vacuum of lop6 Torr. Pressures were slightly higher in the case of the He beam. Beam doses were determined with a digital current integrator that measured the electron current required to neutralize the target by electron flooding. High energy implants of Ni at 47 MeV, Xe at 24 MeV, and I at 22.5 MeV into amorphous target of PEEK were performed on the variable energy cyclotron at the AEA Technology Laboratory at Harwell. The beam spot was approximately 1 cm diameter and the beam current was measured in a long, well suppressed Faraday cup. The target chamber was pumped by a water cooled diffusion pump to a vacuum of 10m6 Torr.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
L.B. Bridwell et al. / Ion implantation
Surface resistance of the circular samples was measured by determining the current flow between two annular probes with dimensions 57 mm o.d. and 51 mm i.d. Current was measured with a high precision electrometer. The voltage used was 1 V across the gap of 6 mm. Resistance versus temperature was measured with two planar electrodes consisting of evaporated Al films, 2.5 cm wide with a linear gap of 1 cm. Contact resistance was of the order of 1 k0. The sample was cooled by a closed cycle liquid helium refrigeration system with an operating range that extends down to about 10 K. The sample chamber was filled with gaseous helium to promote rapid thermal equilibrium between the sample and the platinum resistance thermometer. Surface resistance and temperature dependence of the high energy implants were measured with a standard four-point probe at Harwell in a similar liquid helium refrigeration system. It is important to note that the implant currents and target removal sequence were held constant throughout. Wang et al. [5] have shown significant dose rate effects on surface conductivity.
651
of polymers
--• -A
PAN
B Ar
“1, , , ,::_: 12
3
4
5
6
7
8
910
Beam Dose (xl 016)10NS/CM2 Fig. 1. Surface resistivity of PAN vs target dose for B and Ar ions implanted at 50 keV.
3. Results
50% 5 Figs. l-4 show surface resistance for PAN, PET, PES and PEEK, respectively, for various ions implanted in the dose range 1016-10’7 ions/cm2 at an energy of 50 keV under the conditions described above. Connecting lines were drawn to show general trends. These results indicate a plateau effect in the dose range reported here. Earlier results of ESR studies [4] indicate that the ion tracks achieve significant overlap in this same dose range. Wasserman [6] has hypothesized that the conducting mechanism is a variable range quasi-linear hopping of electrons between grains or “blobs” rendered conducting in the track left by the ion. As the dose increases, these grains become more dense and the links between grains become shorter. It is evident from figs. 3 and 4 that He does not produce as obvious a plateau effect as the heavier ions. This is probably due to the smaller number of lateral recoils within the target about each ion path. Even at this low energy, He ions deposit 85% of their energy by electronic processes [7]. Fig. 5 shows the surface resistance of PEEK versus dose for the high energy implants. Xe and I were chosen to see if implant doping effects could be seen. While the energies of the two ions are not exactly the same, it is clear that there are no significant contributions to the conductivity due to chemical activity of I. The resistivity of the Ni implant is the lowest of all the results reported here. The dose at which these results were achieved were much lower than for the 50 keV implants. In order to determine more details about the conduction mechanism, we examined the temperature dependence of the resistance. Fig. 6 shows the log of the ratio
5
5
:_--___t 106
12
I
I
I
I
I
I
I
3 4 5 6 7 8 910 Beam Dose (xl 016)10NS/CM2
.
Fig. 2. Surface resistivity of PET vs target dose for B and Ar ions implanted at 50 keV.
10'5, -I-
PES
Helium
==o=Boron C,--Carbon --o--
Nitroger
- +-
Arsenic
50 keV
1081 123456789 Beam Dose (x~O~~)IONS/CM* Fig. 3. Surface resistivity of PES for He, B, C, N and As ions implanted at 50 keV. VII. ION IMPLANTATION
658
L.B. Bridwell et al. / Ion implantation
--*B PEEK
ofpolymers
50 keV
PET 50 keVAr
-.C .----. 0N ---0 He
12345678
9
Beam Dose
10
(x10i6)10NS/CM2
T-l/Z
Fig. 4. Surface resistivity of PEEK for He, B, C and N ions implanted at 50 keV.
Fig. 7. Plot of the log,,(R/R,,) vs T-l/‘, where R,, is the resistance at room temperature, for PET implanted with Ar at 50 keV.
---@ Xe 24 MeV ---A 122.5MeV -m Ni 47 MeV
10. s
. v)lOO 10'3
I
I
10'4 10'5 Beam Dose (IONS/CM*)
Fig. 5. Surface resistivity of PEEK implanted with 22.5 MeV I, 24 MeV Xe and 47 MeV Ni.
8-
zs iE 2 %
4_
$ J
2_
6-
00.04
0.06
0.08
0.10 T-1 /2
0.12
0.
Fig. 8. Plot of the log,,(R/R,,) vs T-‘j2, where R,, is the resistance at room temperature, for PAN implanted with B at 50 keV.
7
PEEK
3.0
1 2.2 _
.05
.06
.07
.08
.09
.lO
T-l/2
Fig. 6. Plot of the log,,(R/Rxw) vs T-‘j2, where rsoo is the resistance at room temperature, for PES implanted with As at 50 keV.
2.0 fl-” .05
..-I//’ I .I0
.15
.20
.25
T-‘/Z
Fig. 9. Plot of the logIOp, where p is the bulk resistivity of PEEK after implantation by I at an energy of 22.5 MeV.
659
I_ B. Bridwell et al. / Ion impI~~tati~~ of polymers
the sample resistance at temperature T to that at room temperature versus the reciprocal of the square root of the temperature for PES implanted with 50 keV As. Fig. 7 indicates similar results for Ar-implanted PET. Fig. 8 shows the same for B-implanted PAN. Fig. 9 shows the resistivity versus temperature dependence for the 22 MeV I-implanted PEEK. Redfield j8] has treated the problem of quasi-one-Dimensions electronic conduction in disordered, nonmetallic materials. Such a variable range hopping conduction process should be linearly dependent on T -‘/’ . This is clearly the case for all the data presented here within certain temperature ranges. We should make two observations. The 22 MeV I-implanted PEEK shows the same temperature dependence as the low energy implanted samples. In experiments just completed, the temperature range was increased to 400 K. The same temperature dependence was observed over the entire range. In the case of B-implanted PAN we see a departure from linearity at about 70 K. This is consistent with a decrease in the coupling between the phonons and electrons in the hopping process. of
4. Conclusion Surface resistivity versus dose in the range 10’6-10’7 ions/cm* indicates a plateau effect at final values that are dependent on the incident ion and the target polymer. This work indicates that aliphatic or partially aliphatic polymers such as PET and PAN will reach lower resistivity than polymers that have more fully aromatic structures. Although both PEEK and PES are fully aromatic, the coupling within the monomer is somewhat different. PEEK reaches the lower surface
resistivity. With regard to the conduction mechanism, our results are consistent with a quasi-one-dimensional variable range hopping mechanism for the conduction process. Although the high energy implants show much greater conductivity at much lower doses, the mechanism
for that
conductivity
is the same.
This work was funded jointly by Southwest Missouri State University, the AEA Technology Corporate Research Fund, and the McDonnell Douglas Independent Research Program. Travel support for L. Bridgwell was provided in part by NSF Grant No. INT-8620233.
References 111 C.J. Bedell, C.J. Sofield, LB. Bridwell
and I.M. Brown, J. Appl. Phys. 67 (1990) 1736. 121 R.E. Giedd, J. Shipman and M. Murphy, Mater. Res. Sot. symp. Pxoc. 147 (1989) 377. 131 RE. Giedd, M.G. Moss and M. Craig, 7th Int. Conf. on Ion Beam M~ification of Materials, Knoxville, TN, 1990, to be published in Nucl. Instr. and Meth. B. 141 L.B. Brldwell, R.E. Giedd, Y.Q. Wang, S.S. Mohite, Tamera Jahnke and I.M. Brown, ibid. 151 Y.Q. Wang, L.B. Bridwell, R.E. Giedd and M. Murphy, these Proceedings (11th Int. Conf. on the Applications of Accelerators in Research and Industry, Denton, TX, 1990) Nucl. Instr. and Meth. B56/57 (1991) 660. 161 B. Wasserman, Phys. Rev. B34 (1986) 1926. 171J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, Oxford, 1985). 181 David Redfield, Phys. Rev. Lett. 30 (1973) 1319.
VII. ION IMPLANTATION