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Modification of the electronic properties of ion-beam-deposited diamond-like carbon on polymers
Diamond and Related Materials, 3 (1994) 1265-1269
1265
Modification of the electronic properties of ion-beam-deposited diamond-like carbon on polymers A. R. M c C a b e , A. M . J o n e s * , S. J. Bull a n d C. J o h n s t o n AEA Technology, Surface Technologies Department, 8 Harwell, Oxon. O X I 1 0 R A (UK) Received October 1, 1993: accepted in final form January 19, 1994)
Abstract Ion-beam-deposited diamond-like carbon (DLC) is an attractive coating material because of its combination of low friction, high hardness and chemical inertness. In this process a 50 keV nitrogen bucket-type ion source is used both to modify the substrate surface and to crack a low vapour pressure oil which is evaporated and condensed onto the substrate surface. The technique is of special interest owing to the low deposition temperature; components never exceed a temperature of 80 °C during the coating process. Further, the ability to modify the substrate surface in the same process is of particular importance for polymers for electronic applications. DLC coatings have been applied to a range of polymers, with and without surface modification. The assessment of the electrical properties of these coatings is reported, including variations in resistivity with implant dose. These are correlated with chemical and mechanical changes due to ion implantation.
1. Introduction Surface engineering by an appropriate treatment or coating to give a number of complementary mechanical and electrical properties is clearly desirable. In an ideal case, a process may be envisaged which could fulfil three requirements, namely (a) the modification of the mechanical properties of soft polymer substrates to increase the load support that the surface gives to subsequent treatments, (b) the deposition of a hard, low friction inert coating which can mechanically and environmentally protect the substrate and (c) the selection of a surface resistivity to permit the item to be relatively insulating, to allow static discharge, or to achieve limited conductivity. The present authors have previously investigated the mechanical properties of diamond-like carbon (DLC) ion-beam deposited onto polymers [1], showing that considerable improvements in mechanical and tribological properties can be achieved, particularly with aromatic polymers. In this paper these results are extended to include electrical properties and are correlated with the earlier observations.
*Author to whom correspondence should be addressed.
SSDl 092 5- 9635 ( 94 ~00191 -S
2. Electrical properties of polymers modified by ion bombardment It is well known that the surface of polymers can be modified by ion bombardment using both ion beams and plasmas. Such treatments can be used to modify the surface chemistry, wetting, electrical or mechanical properties of the material. A number of workers have studied the effects of ion bombardment on the electrical properties of polymers. Svorcik et al. [2] showed that medium energy (50 150 keV) implantation of polypropylene with iodine or fluorine ions resulted in a substantial reduction in resistivity. Doses of less than 1015 ions cm -2 gave a reduction of a factor of 10s 107 with fluorine, and 107-109 with iodine. Bridwell [3] demonstrated a decrease in resistance with increasing dose rate with polyimide (using 150 keV copper and zinc ions). A reduction of up to six orders of magnitude was observed using 50 keV arsenic, iodine, xenon and helium ions (doses of 101s-101Sionscml2) for poly(ether ether ketone) (PEEK), and a similar decrease in surface resistance for poly(ether sulphone) (PES) (from 1012 to 109~/[~ using a dose of 101~ionscm 2 of boron, carbon, nitrogen or arsenic ions, and from 1014 to 10 l° f2/[E using a dose of 1016 ions cm 2 of helium ions). At higher energies, Lee et al. [4] used 1.0 and 0.5 MeV argon and demonstrated a decrease in the resistivity of polystyrene by a factor of 10 I° but only a factor of 106 decrease using 200 keV argon.
Elsevier Science S.A.
1266
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
Other workers have investigated the effect of modification of various forms of DLC. Demichelis et al. [5] compared the effect of r.f. magnetron sputtering of graphite with and without hydrogen, finding a 10 5 times difference in conductivity. The behaviour was not attributed directly to the hydrogen content but to the sp2/ sp 3 ratio. Tochitsky et al. [6], using a pulsed-vacuumarc-discharge method, also attributed a decrease in resistivity to an increase in sp 2 content. Wang et al. [-7] used a dual-ion-beam sputter deposition method to produce a form of DLC with a resistivity of 880 f~ cm. They found that ion implantation could be used both to increase and to decrease this resistivity. Whereas resistivity was decreased to 38f~cm by implanting with carbon (ll4keV; 1017ions cm-2), it was increased to 1400f~cm when implanted with hydrogen (112 keV; 101Tions cm-2). Frauenheim et al. [-8] compared the resistivity of DLC-type coatings produced by a range of deposition techniques. The variations are quite interesting, some of the values quoted being as follows: arc evaporation of graphite in argon (0.1-100 fl cm); magnetron sputtering of carbon in argon (3 fl cm); dual-beam sputtering of graphite by argon (104-106~cm); laser pulse vapour deposition (105-108 fl cm); mass-separated hydrogen ion bombardment (106-1011fl cm). Annealing the various coatings was also shown to change the resistivity by several orders of magnitude in certain cases. Most studies have used high energy (megaelectronvolt) ion beams or rather exotic and expensive ion beam source materials. The present work uses medium energy (kiloelectronvolt) ion beams with a more economical ion source gas, nitrogen.
vaporized at a pressure of approximately 5 x 10 - 6 Torr and condenses onto any cooler surface. Consequently, a flux of oil vapour is continually condensing onto the substrates which are simultaneously irradiated by a 40-80 keV beam of mostly molecular nitrogen ions. As a high energy ion penetrates its way through the surface layer of condensed oil, it imparts its energy to the oil molecules, mostly by electronic excitation [12]. This results in a dramatic number of bond breakages, the release of volatile species such as oxygen and hydrogen and the solidification of the local area as a result of the subsequent bonding of unsatisfied carbon atoms [ 13]. Adjusting the rate of vapour deposition and ion bombardment allows control of the composition of the final material deposited on the substrates, from a semisolid hydrocarbon material to dense DLC, without the need for any substrate heating.
3. Diamond-like carbon by ion implantation
4. Experimental details
D L C coatings have been produced by a range of techniques which deposit coatings with variable properties. Most techniques are not, however, suitable for largearea deposition onto temperature-sensitive substrates such as polymers. The production of DLC at temperatures under 100 °C has been developed and patented [9] and involves the cracking of a low vapour pressure oil precursor by a large-area ion beam from a bucket ion source [ 10]. This method allows the coating of a wide range of substrate materials with large surface areas (greater than 1 m2). Further, the mechanical, electrical and optical properties of the coatings can be modified by the selection of the process parameters and the use of a wide range of oil precursors and additions. This is the principle of the ion-assisted DLC process [ 11], illustrated in the schematic diagram in Fig. 1. The oil precursor, typically poly(phenyl ether), is
4.1. Sample production
2.5 m
Fig. 1. Harwell'sblue tank ion implantationfacility.
Samples were produced using the following substrates: two types of polyethylene, polyimide, polypropylene, poly(methyl methacrylate) (PMMA), three types of polycarbonate, poly(ethylene terephthalate) (PET), polyamide (PA), PES, poly(vinyl difluoride) (PVDF), polytetrafluoroethylene (PTFE), PEEK, carbon-fibrereinforced polymer (CFRP) and glass. These were implanted with a range of doses of nitrogen ions from 4 x 1013 to 1 x 1017 N~- ions c m -2. DLC of 1 gm thickness was then applied to samples which had been prebombarded to a dose of 2.5 x 1015 N~- ions c m -2. 4.2. Resistivity measurements
Sheet resistance measurements were made using a standard four-point probe. When using this technique to measure very high resistance samples, factors such as the contact resistance can become important and hence
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
some caution is required both in making measurements and in interpreting the results. In our experiments, we took simple precautions such as making measurements in both current directions and making repeated measurements to ensure reproducibility. Measured values below about 10~1 ~/Z~ are reliable; above this, the trends in the data are qualitatively correct, although the absolute values of sheet resistance should be treated with caution. Sheet resistance values were converted to resistivities of the thin surface layers by including a value for the thickness of the coating or surface-implanted layer. In the case of DLC, this was found using a Dektak IIA surface profilometer on regions of the substrate surface that had been masked during coating deposition. For the thin implanted layers, the thickness of the implanted nitrogen ion distributions were determined using the computer simulation code PROFILE [14].
5. Results
•
PET--
•
m [] [] [] []
Polyimide " Polycarbonate Polyethylene
PES'
::
:::
DLC coated lx1017 lx1016 1x1015 lxlO14 4x1013 Untreated
...........
Polypropylene PMMA
::-.=-:.
PEEK •
10 a
'..'..'_,
. :.....
.... : ........
10 x
10 °
10 2
: ........... l0 s 10 4
. ....... 10 5 10 6
Resistivity (~ em) Fig. 2. The variation in resistivity with ion beam treatment for a range of polymers.
Table 1. Range of resistivities achieved by processing of polymer materials and glass Material
Fractional resistivity change from untreated (range after treatmentl
5.1. Appearance In the untreated condition, the materials were generally divided up into two classes. The majority of materials (glass, PMMA, polycarbonate and PET) were optically transparent with a shiny surface finish whilst PTFE, polyethylene and polypropylene had a matt finish and were white in colour. In all cases, DLC coating of these materials produced a silvery black surface layer which retained the surface texture of the substrate. Implantation to doses above 1015 ions cm -2 produced a shiny finish on all materials, regardless of the initial surface appearance. All the polymers showed an increasing colouration with dose, from transparent to black.
1267
PET 10 -3 101 CFRP 10 -5 10° Polypropylene 10- 4-10° Polyethylene 10- 3__10° PTFE 10-3-100 PES 10-3-10 ° PEEK 10-2-10 o Polycarbonate (type 1) 10-3-100 Polycarbonate {type 2) 10-6-10 o PVDF 10-2-100 PA 10 -2 10° Polyimide 10 3 10o PMMA 10 -6 100 Glass 10- 6 100
Order-of-magnitude change in resistivity
4 5 4 3 3 3 2 3 6 2 2 3 6 6
5.2. Resistivity' At low implant doses the resistivity is reduced for all of the polymers tested (Fig. 2). However, rather than saturating at a given value once a sufficient implant dose is reached (as observed previously; see for example ref. 3) there is an increase in resistivity at the highest doses for most polymers. A summary is given in Table 1 of the range of resistivity values measured, and the extent of variation over this selection of surface treatments. A comparison between the mechanical and electrical effects produced by the different surface treatments is shown in Figs. 3-5 for glass, polypropylene and PET respectively. Changes in resistivity can occur without changes in hardness for the P E T since the most apparent resistivity change is at very low doses where little effect on hardness is observed. For the polypropylene there is little improvement in hardness after implantation, but considerable changes in resistivity are observed. It is
10000 ~ 1000" ~
1000 ---K)-'- Resistivity [ -Knoop hardness I
,._:
lOO ~ '~ ~
10
lO0
,~
.12 .Ol I
. 1014
.
. 101s
. 1016
t lO lOXV
Dose (ions]cm2 ) Fig. 3. The effect of ion b o m b a r d m e n t on resistivity and hardness for glass•
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
1268
10000
10 Resistivity Knoop hardness
1000 '
~.
100 ~
10
a l
lo"
lo TM lo'"
lo"
la
Dose (ions/em) Fig. 4. The effect of ion bombardment on resistivity and hardness for polypropylene.
1000~
klO00
100
.
,o1\
_/
•"r i-~-_ " Res,s~,,
i
~,
10 a4 10 a5 1016 10 n Dose (ions/em) Fig. 5. The effect of ion b o m b a r d m e n t on resistivity and hardness for PET.
thus apparent that different mechanisms are responsible for generating the changes measured.
6. Discussion The present authors have previously shown that the difference in mechanical response of the various polymers to ion bombardment can be explained in terms of the structure and composition of those polymers [-1]. Polymers containing aromatic groups (such as PET and polycarbonate) all increase in hardness with increasing ion dose, while aliphatic polymers (such as polyethylene and PMMA) show negligible change in hardness with ion bombardment. Chain scission and cross-linking are more likely for aromatics, whilst the recombination of broken chain ends is the main mechanism occurring in aliphatic polymers. Such simple structural distinctions do not explain the electrical results. As an ion comes to rest in a material it can lose energy by two main mechanisms: (1) inelastic collisions with the band electrons of the target by excitation or
ionization; (2) elastic collisions with nuclei, causing displacements. Electronic energy loss mechanisms dominate at high ion energies whereas displacement damage becomes increasingly important as the ion comes to rest. At the implantation energies used in this study, around 60% of the energy loss occurs by electronic processes. Previous work has shown that both electronic and displacement damage can cause densification and changes to the mechanical properties of ion-implanted glasses [15]. The mechanisms based on electronic energy loss processes dominate at low implantation doses (about 1014ions cm -2) whereas displacement damage is not significant until above 5 x 1015 ions cm -2. Interestingly four-point probe measurements on ion-implanted glass (Fig. 3) show a very similar double-peak structure as is observed for the mechanical properties, with a low and high dose minimum in conductivity. It appears that a similar explanation could be used for the electrical measurements reported here; the low ion dose increase in conductivity is caused by an electronic energy loss mechanism whereas displacement damage causes a slight increase in resistivity at high doses. The explanation for the different responses in electrical properties for different polymers is not always clear. Comparison of a typical aromatic and aliphatic polymer (Figs. 4 and 5) reveals different behaviours. Nitrogen implantation into PET (an aromatic) leads to a fall in resistivity at the lowest dose. The resistivity then remains at a constant level with increasing dose. The addition of a DLC layer then increases the resistivity. It would appear that the structural changes caused by breaking up the aromatic rings dominate from very low doses and there is little change in conductivity at higher doses as the cross-linked material formed retains much of its sp 2 bonding character. However, polypropylene (an aliphatic) shows an initial collapse in resistivity and then a steady increase with dose, the addition of a DLC layer making virtually no difference to the resistivity. The gradual cross-linking of the polymer as the ion dose increases leads to an increase in the amount of sp 3 bonding. In this case the effects of displacement damage depend on the implantation dose, the change in polymer structure is more gradual and this is reflected in the resistivity. Hydrogen is removed very effectively by low dose implantation, leaving a disordered graphitic structure with predominantly sp 2 bonding I16]. This leads to the high conductivities observed and is related to the electronic energy loss processes. More ionizing ions such as hydrogen would thus be expected to give larger conductivity improvements. However, as the nitrogen dose increases, the surface is converted to a form of "DLC" with a much higher sp 3 bonding percentage. This has a higher resistivity than the graphitic material produced at lower doses.
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
The same effect may occur for aromatics at very high doses (e.g. polyimide) but is masked by the changes due to break-up of the aromatic rings. However, the present authors believe that the behaviour is dependent on the chemical composition and manufacturing route in any given material, as well as on the overall structure.
7. Conclusions Three main conclusions may be drawn from this work. (i) The resistivity of polymers may be changed by ion bombardment; this effect also results in hardening of aromatics. (ii) Ion implantation causes cross-linking and removal of hydrogen to convert the surface to a form of DLC. (iii) There are a number of polymers where the electrical properties can be controlled while improving the mechanical properties.
Acknowledgments The authors wish to thank Dr. R. E. Harper and Mr. G. J. Stockford for assistance with the four-point probe facility. This work is part of the long term Corporate Research Programme of AEA Technology.
1269
References 1 A. R. McCabe, A. M. Jones and S. J. Bull, Diamond Relat. Mater., 3 ( 1994} 205. 2 V. Svorcik, V. Rybka, K. Volka, V. Hnatowicz, J. Kvitek and V. Perina, Appl. Phys. Lett., 61 (10) (1992) 1168. 3 L. B. Bridwell, Solid State Phenom., 27 (1992) 163. 4 E. H. Lee, G. R. Rao, M. B. Lewis and L. K. Mansur, Nucl. lnstrum. Methods. B, 74 (1993) 326. 5 F. Demichelis, A. Tagliaferro and D. Das Gupta, Surji Coat. Technol., 47 (1991) 218. 6 E. I. Tochitsky, A. V. Stanishevskii, 1. A. Kapustin, V. V. Akulich and O. V. Selifanov, Surf. Coat. Technol., 47 (1991) 292. 7 W. Wang, T. Wang and B. Chen, J. Appl. Phys., 72 ( 1 ) (1992) 69. 8 Th. Frauenheim, U. Stephan, K. Bewilogua, F. Jungnickel, P. Blaudeck and E. Fromm, Thin Solid Films, 182 (1989) 63. 9 P. D. Goode, W. Hughes and G. W. Proctor, UK Pat. GB2122224B, 1986. 10 W. L. Stirling, P. M. Ryan, C. C. Tsai and K. N. Leung, Rev. Sci. lnstrum.. 50 (1979) 102. 11 C.J. Bedetl, A. M. Jones and G. Dearnaley, Applications of Diamond Fihns and Related Materials, Elsevier, Amsterdam, 1991, pp. 827 83l. 12 J. F. Ziegler, J. P. Biersack and U. Littmark, The Stopping and Ranges ~f Ions in Solids, Pergamon, Oxford, 1985. 13 M. L. Kaplan, S. R. Forrest, P. H. Schmidt and T. Venkatesan, J. Appl. Phys., 55 (1984) 732. 14 PROPILECode, Implant Sciences, Danvers, MA 01923, USA, 1991. 15 S. J. Bull and T. F. Page, J. Mater. Sci., 27 (1992) 3605. 16 C. J. Sofield. S. Sugden, J. Ing, L. B. Bridwell and Y. Q. Wang, Vacuum, 44 {3 4} (1993) 285.
1265
Modification of the electronic properties of ion-beam-deposited diamond-like carbon on polymers A. R. M c C a b e , A. M . J o n e s * , S. J. Bull a n d C. J o h n s t o n AEA Technology, Surface Technologies Department, 8 Harwell, Oxon. O X I 1 0 R A (UK) Received October 1, 1993: accepted in final form January 19, 1994)
Abstract Ion-beam-deposited diamond-like carbon (DLC) is an attractive coating material because of its combination of low friction, high hardness and chemical inertness. In this process a 50 keV nitrogen bucket-type ion source is used both to modify the substrate surface and to crack a low vapour pressure oil which is evaporated and condensed onto the substrate surface. The technique is of special interest owing to the low deposition temperature; components never exceed a temperature of 80 °C during the coating process. Further, the ability to modify the substrate surface in the same process is of particular importance for polymers for electronic applications. DLC coatings have been applied to a range of polymers, with and without surface modification. The assessment of the electrical properties of these coatings is reported, including variations in resistivity with implant dose. These are correlated with chemical and mechanical changes due to ion implantation.
1. Introduction Surface engineering by an appropriate treatment or coating to give a number of complementary mechanical and electrical properties is clearly desirable. In an ideal case, a process may be envisaged which could fulfil three requirements, namely (a) the modification of the mechanical properties of soft polymer substrates to increase the load support that the surface gives to subsequent treatments, (b) the deposition of a hard, low friction inert coating which can mechanically and environmentally protect the substrate and (c) the selection of a surface resistivity to permit the item to be relatively insulating, to allow static discharge, or to achieve limited conductivity. The present authors have previously investigated the mechanical properties of diamond-like carbon (DLC) ion-beam deposited onto polymers [1], showing that considerable improvements in mechanical and tribological properties can be achieved, particularly with aromatic polymers. In this paper these results are extended to include electrical properties and are correlated with the earlier observations.
*Author to whom correspondence should be addressed.
SSDl 092 5- 9635 ( 94 ~00191 -S
2. Electrical properties of polymers modified by ion bombardment It is well known that the surface of polymers can be modified by ion bombardment using both ion beams and plasmas. Such treatments can be used to modify the surface chemistry, wetting, electrical or mechanical properties of the material. A number of workers have studied the effects of ion bombardment on the electrical properties of polymers. Svorcik et al. [2] showed that medium energy (50 150 keV) implantation of polypropylene with iodine or fluorine ions resulted in a substantial reduction in resistivity. Doses of less than 1015 ions cm -2 gave a reduction of a factor of 10s 107 with fluorine, and 107-109 with iodine. Bridwell [3] demonstrated a decrease in resistance with increasing dose rate with polyimide (using 150 keV copper and zinc ions). A reduction of up to six orders of magnitude was observed using 50 keV arsenic, iodine, xenon and helium ions (doses of 101s-101Sionscml2) for poly(ether ether ketone) (PEEK), and a similar decrease in surface resistance for poly(ether sulphone) (PES) (from 1012 to 109~/[~ using a dose of 101~ionscm 2 of boron, carbon, nitrogen or arsenic ions, and from 1014 to 10 l° f2/[E using a dose of 1016 ions cm 2 of helium ions). At higher energies, Lee et al. [4] used 1.0 and 0.5 MeV argon and demonstrated a decrease in the resistivity of polystyrene by a factor of 10 I° but only a factor of 106 decrease using 200 keV argon.
Elsevier Science S.A.
1266
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
Other workers have investigated the effect of modification of various forms of DLC. Demichelis et al. [5] compared the effect of r.f. magnetron sputtering of graphite with and without hydrogen, finding a 10 5 times difference in conductivity. The behaviour was not attributed directly to the hydrogen content but to the sp2/ sp 3 ratio. Tochitsky et al. [6], using a pulsed-vacuumarc-discharge method, also attributed a decrease in resistivity to an increase in sp 2 content. Wang et al. [-7] used a dual-ion-beam sputter deposition method to produce a form of DLC with a resistivity of 880 f~ cm. They found that ion implantation could be used both to increase and to decrease this resistivity. Whereas resistivity was decreased to 38f~cm by implanting with carbon (ll4keV; 1017ions cm-2), it was increased to 1400f~cm when implanted with hydrogen (112 keV; 101Tions cm-2). Frauenheim et al. [-8] compared the resistivity of DLC-type coatings produced by a range of deposition techniques. The variations are quite interesting, some of the values quoted being as follows: arc evaporation of graphite in argon (0.1-100 fl cm); magnetron sputtering of carbon in argon (3 fl cm); dual-beam sputtering of graphite by argon (104-106~cm); laser pulse vapour deposition (105-108 fl cm); mass-separated hydrogen ion bombardment (106-1011fl cm). Annealing the various coatings was also shown to change the resistivity by several orders of magnitude in certain cases. Most studies have used high energy (megaelectronvolt) ion beams or rather exotic and expensive ion beam source materials. The present work uses medium energy (kiloelectronvolt) ion beams with a more economical ion source gas, nitrogen.
vaporized at a pressure of approximately 5 x 10 - 6 Torr and condenses onto any cooler surface. Consequently, a flux of oil vapour is continually condensing onto the substrates which are simultaneously irradiated by a 40-80 keV beam of mostly molecular nitrogen ions. As a high energy ion penetrates its way through the surface layer of condensed oil, it imparts its energy to the oil molecules, mostly by electronic excitation [12]. This results in a dramatic number of bond breakages, the release of volatile species such as oxygen and hydrogen and the solidification of the local area as a result of the subsequent bonding of unsatisfied carbon atoms [ 13]. Adjusting the rate of vapour deposition and ion bombardment allows control of the composition of the final material deposited on the substrates, from a semisolid hydrocarbon material to dense DLC, without the need for any substrate heating.
3. Diamond-like carbon by ion implantation
4. Experimental details
D L C coatings have been produced by a range of techniques which deposit coatings with variable properties. Most techniques are not, however, suitable for largearea deposition onto temperature-sensitive substrates such as polymers. The production of DLC at temperatures under 100 °C has been developed and patented [9] and involves the cracking of a low vapour pressure oil precursor by a large-area ion beam from a bucket ion source [ 10]. This method allows the coating of a wide range of substrate materials with large surface areas (greater than 1 m2). Further, the mechanical, electrical and optical properties of the coatings can be modified by the selection of the process parameters and the use of a wide range of oil precursors and additions. This is the principle of the ion-assisted DLC process [ 11], illustrated in the schematic diagram in Fig. 1. The oil precursor, typically poly(phenyl ether), is
4.1. Sample production
2.5 m
Fig. 1. Harwell'sblue tank ion implantationfacility.
Samples were produced using the following substrates: two types of polyethylene, polyimide, polypropylene, poly(methyl methacrylate) (PMMA), three types of polycarbonate, poly(ethylene terephthalate) (PET), polyamide (PA), PES, poly(vinyl difluoride) (PVDF), polytetrafluoroethylene (PTFE), PEEK, carbon-fibrereinforced polymer (CFRP) and glass. These were implanted with a range of doses of nitrogen ions from 4 x 1013 to 1 x 1017 N~- ions c m -2. DLC of 1 gm thickness was then applied to samples which had been prebombarded to a dose of 2.5 x 1015 N~- ions c m -2. 4.2. Resistivity measurements
Sheet resistance measurements were made using a standard four-point probe. When using this technique to measure very high resistance samples, factors such as the contact resistance can become important and hence
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
some caution is required both in making measurements and in interpreting the results. In our experiments, we took simple precautions such as making measurements in both current directions and making repeated measurements to ensure reproducibility. Measured values below about 10~1 ~/Z~ are reliable; above this, the trends in the data are qualitatively correct, although the absolute values of sheet resistance should be treated with caution. Sheet resistance values were converted to resistivities of the thin surface layers by including a value for the thickness of the coating or surface-implanted layer. In the case of DLC, this was found using a Dektak IIA surface profilometer on regions of the substrate surface that had been masked during coating deposition. For the thin implanted layers, the thickness of the implanted nitrogen ion distributions were determined using the computer simulation code PROFILE [14].
5. Results
•
PET--
•
m [] [] [] []
Polyimide " Polycarbonate Polyethylene
PES'
::
:::
DLC coated lx1017 lx1016 1x1015 lxlO14 4x1013 Untreated
...........
Polypropylene PMMA
::-.=-:.
PEEK •
10 a
'..'..'_,
. :.....
.... : ........
10 x
10 °
10 2
: ........... l0 s 10 4
. ....... 10 5 10 6
Resistivity (~ em) Fig. 2. The variation in resistivity with ion beam treatment for a range of polymers.
Table 1. Range of resistivities achieved by processing of polymer materials and glass Material
Fractional resistivity change from untreated (range after treatmentl
5.1. Appearance In the untreated condition, the materials were generally divided up into two classes. The majority of materials (glass, PMMA, polycarbonate and PET) were optically transparent with a shiny surface finish whilst PTFE, polyethylene and polypropylene had a matt finish and were white in colour. In all cases, DLC coating of these materials produced a silvery black surface layer which retained the surface texture of the substrate. Implantation to doses above 1015 ions cm -2 produced a shiny finish on all materials, regardless of the initial surface appearance. All the polymers showed an increasing colouration with dose, from transparent to black.
1267
PET 10 -3 101 CFRP 10 -5 10° Polypropylene 10- 4-10° Polyethylene 10- 3__10° PTFE 10-3-100 PES 10-3-10 ° PEEK 10-2-10 o Polycarbonate (type 1) 10-3-100 Polycarbonate {type 2) 10-6-10 o PVDF 10-2-100 PA 10 -2 10° Polyimide 10 3 10o PMMA 10 -6 100 Glass 10- 6 100
Order-of-magnitude change in resistivity
4 5 4 3 3 3 2 3 6 2 2 3 6 6
5.2. Resistivity' At low implant doses the resistivity is reduced for all of the polymers tested (Fig. 2). However, rather than saturating at a given value once a sufficient implant dose is reached (as observed previously; see for example ref. 3) there is an increase in resistivity at the highest doses for most polymers. A summary is given in Table 1 of the range of resistivity values measured, and the extent of variation over this selection of surface treatments. A comparison between the mechanical and electrical effects produced by the different surface treatments is shown in Figs. 3-5 for glass, polypropylene and PET respectively. Changes in resistivity can occur without changes in hardness for the P E T since the most apparent resistivity change is at very low doses where little effect on hardness is observed. For the polypropylene there is little improvement in hardness after implantation, but considerable changes in resistivity are observed. It is
10000 ~ 1000" ~
1000 ---K)-'- Resistivity [ -Knoop hardness I
,._:
lOO ~ '~ ~
10
lO0
,~
.12 .Ol I
. 1014
.
. 101s
. 1016
t lO lOXV
Dose (ions]cm2 ) Fig. 3. The effect of ion b o m b a r d m e n t on resistivity and hardness for glass•
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
1268
10000
10 Resistivity Knoop hardness
1000 '
~.
100 ~
10
a l
lo"
lo TM lo'"
lo"
la
Dose (ions/em) Fig. 4. The effect of ion bombardment on resistivity and hardness for polypropylene.
1000~
klO00
100
.
,o1\
_/
•"r i-~-_ " Res,s~,,
i
~,
10 a4 10 a5 1016 10 n Dose (ions/em) Fig. 5. The effect of ion b o m b a r d m e n t on resistivity and hardness for PET.
thus apparent that different mechanisms are responsible for generating the changes measured.
6. Discussion The present authors have previously shown that the difference in mechanical response of the various polymers to ion bombardment can be explained in terms of the structure and composition of those polymers [-1]. Polymers containing aromatic groups (such as PET and polycarbonate) all increase in hardness with increasing ion dose, while aliphatic polymers (such as polyethylene and PMMA) show negligible change in hardness with ion bombardment. Chain scission and cross-linking are more likely for aromatics, whilst the recombination of broken chain ends is the main mechanism occurring in aliphatic polymers. Such simple structural distinctions do not explain the electrical results. As an ion comes to rest in a material it can lose energy by two main mechanisms: (1) inelastic collisions with the band electrons of the target by excitation or
ionization; (2) elastic collisions with nuclei, causing displacements. Electronic energy loss mechanisms dominate at high ion energies whereas displacement damage becomes increasingly important as the ion comes to rest. At the implantation energies used in this study, around 60% of the energy loss occurs by electronic processes. Previous work has shown that both electronic and displacement damage can cause densification and changes to the mechanical properties of ion-implanted glasses [15]. The mechanisms based on electronic energy loss processes dominate at low implantation doses (about 1014ions cm -2) whereas displacement damage is not significant until above 5 x 1015 ions cm -2. Interestingly four-point probe measurements on ion-implanted glass (Fig. 3) show a very similar double-peak structure as is observed for the mechanical properties, with a low and high dose minimum in conductivity. It appears that a similar explanation could be used for the electrical measurements reported here; the low ion dose increase in conductivity is caused by an electronic energy loss mechanism whereas displacement damage causes a slight increase in resistivity at high doses. The explanation for the different responses in electrical properties for different polymers is not always clear. Comparison of a typical aromatic and aliphatic polymer (Figs. 4 and 5) reveals different behaviours. Nitrogen implantation into PET (an aromatic) leads to a fall in resistivity at the lowest dose. The resistivity then remains at a constant level with increasing dose. The addition of a DLC layer then increases the resistivity. It would appear that the structural changes caused by breaking up the aromatic rings dominate from very low doses and there is little change in conductivity at higher doses as the cross-linked material formed retains much of its sp 2 bonding character. However, polypropylene (an aliphatic) shows an initial collapse in resistivity and then a steady increase with dose, the addition of a DLC layer making virtually no difference to the resistivity. The gradual cross-linking of the polymer as the ion dose increases leads to an increase in the amount of sp 3 bonding. In this case the effects of displacement damage depend on the implantation dose, the change in polymer structure is more gradual and this is reflected in the resistivity. Hydrogen is removed very effectively by low dose implantation, leaving a disordered graphitic structure with predominantly sp 2 bonding I16]. This leads to the high conductivities observed and is related to the electronic energy loss processes. More ionizing ions such as hydrogen would thus be expected to give larger conductivity improvements. However, as the nitrogen dose increases, the surface is converted to a form of "DLC" with a much higher sp 3 bonding percentage. This has a higher resistivity than the graphitic material produced at lower doses.
A. R. McCabe et al. / Modification of electronic properties of DLC on polymers
The same effect may occur for aromatics at very high doses (e.g. polyimide) but is masked by the changes due to break-up of the aromatic rings. However, the present authors believe that the behaviour is dependent on the chemical composition and manufacturing route in any given material, as well as on the overall structure.
7. Conclusions Three main conclusions may be drawn from this work. (i) The resistivity of polymers may be changed by ion bombardment; this effect also results in hardening of aromatics. (ii) Ion implantation causes cross-linking and removal of hydrogen to convert the surface to a form of DLC. (iii) There are a number of polymers where the electrical properties can be controlled while improving the mechanical properties.
Acknowledgments The authors wish to thank Dr. R. E. Harper and Mr. G. J. Stockford for assistance with the four-point probe facility. This work is part of the long term Corporate Research Programme of AEA Technology.
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References 1 A. R. McCabe, A. M. Jones and S. J. Bull, Diamond Relat. Mater., 3 ( 1994} 205. 2 V. Svorcik, V. Rybka, K. Volka, V. Hnatowicz, J. Kvitek and V. Perina, Appl. Phys. Lett., 61 (10) (1992) 1168. 3 L. B. Bridwell, Solid State Phenom., 27 (1992) 163. 4 E. H. Lee, G. R. Rao, M. B. Lewis and L. K. Mansur, Nucl. lnstrum. Methods. B, 74 (1993) 326. 5 F. Demichelis, A. Tagliaferro and D. Das Gupta, Surji Coat. Technol., 47 (1991) 218. 6 E. I. Tochitsky, A. V. Stanishevskii, 1. A. Kapustin, V. V. Akulich and O. V. Selifanov, Surf. Coat. Technol., 47 (1991) 292. 7 W. Wang, T. Wang and B. Chen, J. Appl. Phys., 72 ( 1 ) (1992) 69. 8 Th. Frauenheim, U. Stephan, K. Bewilogua, F. Jungnickel, P. Blaudeck and E. Fromm, Thin Solid Films, 182 (1989) 63. 9 P. D. Goode, W. Hughes and G. W. Proctor, UK Pat. GB2122224B, 1986. 10 W. L. Stirling, P. M. Ryan, C. C. Tsai and K. N. Leung, Rev. Sci. lnstrum.. 50 (1979) 102. 11 C.J. Bedetl, A. M. Jones and G. Dearnaley, Applications of Diamond Fihns and Related Materials, Elsevier, Amsterdam, 1991, pp. 827 83l. 12 J. F. Ziegler, J. P. Biersack and U. Littmark, The Stopping and Ranges ~f Ions in Solids, Pergamon, Oxford, 1985. 13 M. L. Kaplan, S. R. Forrest, P. H. Schmidt and T. Venkatesan, J. Appl. Phys., 55 (1984) 732. 14 PROPILECode, Implant Sciences, Danvers, MA 01923, USA, 1991. 15 S. J. Bull and T. F. Page, J. Mater. Sci., 27 (1992) 3605. 16 C. J. Sofield. S. Sugden, J. Ing, L. B. Bridwell and Y. Q. Wang, Vacuum, 44 {3 4} (1993) 285.