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Fluorocarbon films sputtered under various conditions
Thin Solid Films, 55 (1978) L 11 - L 13 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
L 11
Letter
Fluorocarbon films sputtered under various conditions H. BIEDERMAN
Faculty of Mathematics and Physics, Charles University, Prague 2, Ke Karlovu 5 (Czechoslovakia} (Received September 8, 1978: accepted September 18, 1978)
Fluorocarbon polymer films have been studied extensively for several years ~' 2. Most investigations have been concerned with films prepared by the sputtering of polytetrafluoroethylene (PTFE) 3'4 and by the plasma polymerization of a fluorocarbon gas 5' 6 or polyether vapour 7. The two types of films and their modes of preparation have been compared 8' 9 and show certain differences9. It is the purpose of this short report to present several observations yielding more information about the processes involved in the sputtering of PTFE. We sputtered P T F E using a target 75 mm in diameter supported by an insulated water-cooled electrode and connected to an r.f. supply (20 MHz) with one side grounded. Argon was admitted to the continuously pumped vessel after initial evacuation to 1 x 10 - 3 Torr using a rotary pump. The pressure of argon was maintained at 0.05 Torr during the deposition. Some researchers shut off the argon supply after a time and allow the discharge to continue in its own P T F E vapour. It has been shown 9 that when argon is admitted continuously at a pressure of 0.05 Torr with the same power input the deposit has similar properties but the growth rate is higher. A constant r.f. power of 180 W was delivered during all our experiments. Measurements of the probe characteristics showed the plasma and floating potentials to be + 145 V and +65 V respectively relative to ground. Soda lime silicate microscope glass was used as the substrates. The experiments were first performed with the substrates in positions 1 and 3 (Fig. 1) and then position 1 was removed and further experiments were carried out using positions 2 and 3. The average deposition rate over a 30 min period was found to be 120 A min - ~ for position 3 in all experiments. In position 2 the rate was about 75 A min- ~ with a bias of + 265 V relative to ground and 65 A min- ~ when the support was at floating potential. Rates lower by about 10% were obtained in position 1 in these cases. Here some non-uniformity of the deposit appeared mainly in the part of the substrate placed near the target sheath. In position 2 the layer had approximately the same thickness over the whole substrate (26 mm x 30 mm). In position 3 the thickness decreased with increasing distance from the axis of the target and we therefore determined its average value in the centre of the sample using Tolansky's method of multiple beam interferometry. In this case the deposits were yellow in agreement with previous results 2'3'9. The yellow deposit produced in positions I and 2 when they were grounded became more transparent as the potential applied to the supporting electrode was raised. When the potential was about + 265 V relative to ground (120 V higher than the plasma potential) the yellow shade had almost disappeared.
L 12
l .HTERS
The transmission spectra in the region 300 600 nm were measured with a CF 4Optica Milano spectrophotometer with uncoated glass in the reference beam. On the basis of complementary reflection measurements we can consider that absorption is responsible for the transmission data with a systematic error to about 3"~;, which arises from neglect of the differences between the reflectances of the uncoated reference glass and the coated glass. The transmission of films deposited in positions 2 and 3 [Fig. 2) shows that the films fabricated on a grounded support are more absorbing for near-UV light. Similar results were obtained for positions I and 3. Our transmission data are in good agreement with those for sputtered fluorocarbon films published by Holland et al. '~ From their transmission measurements at 320 nm, for example, the absorption constants obtained according to Lambert's law are 0.327 × 104 and 1.5 × 104 cm ~ for plasma-polymerized and sputtered films respectively. At the same wavelength we obtained 2.586 x 104 and 2.213 x 104 cm ~ for grounded and biased films respectively. Similarly to the conclusion given in ref. 9 we consider that an excess of carbon in the layers causes the higher absorption. The films obtained in positions 1 and 2 at floating or bias potential appeared to be harder than those from position 3. The friction properties of all the films were comparable with previous findings-'.
80 ~
/ //
~5
2O
SUPPLY
~
"\ ~ T E R COOLED RF TARGET
GAUGL 300
400
"50C
600 [rim,
Fig. 1. The arrangement of the experiments. The distance from the target to the substrates in positions 2 and 3 was 45 ram. Fig. 2. The transmission of the films obtained on glass substrates: curve a, in position 2, biased at + 265 V relative to ground (film thickness 2200 Ak curve b, in position 3, grounded (film thickness 3500 AI.
According to the data produced by Mathias and Muller 1" CF 2 radicals are likely to be present in r.f. fluorocarbon discharges in large concentrations. Among other species they give carbon and neutral or negative fluorine by disproportionation reactions. We assume that these are drawn out of the plasma to the positively biased receiver [positions 1 and 2 in Fig. 1) and improve the F: C ratio in the growing deposit. However, we cannot stop the exposed surface of the growing film on the glass substrate from developing a potential near to the floating potential. This
LETTERS
L13
worsens the supply of negative fluorine ions and probably does not allow the stoichiometry in the film composition to be improved. Some experiments were done using positions 1 and 3 with substrates previously covered with about 1000 A of aluminium and conductively coupled to the support. In position 1 at a potential of + 265 V relative to ground the aluminium layer was converted into a transparent dielectric film probably as a result of a reaction with fluorine. The resulting deposit appeared to be a mixture of the polymer and a cryolite-like compound. Further investigations are in progress but it appears to be difficult to prepare the same or very similar fluorocarbon films by the plasma polymerization of a fluorocarbon gas and by the sputtering of PTFE. The results presented show that the properties of fluorocarbon films prepared in the same apparatus can be changed by altering the physical conditions at their receiver. Thus our results have fully confirmed the previous conclusion 9 that the composition of fluorocarbon films depends not only on the technique used but also on the specific design of the apparatus. The author is grateful to Professor L. Holland for critical comments on the manuscript. He is also indebted to Dr. J. Phigek for technical assistance in the transmission measurements. I 2 3 4 5 6 7 8 9 10
L. Holland, J. Vae. Sci. Teehnol., 14(1977) 5. H. Biederman, S. M. Ojha and L. Holland, Thin Solid Films, 41 (1977) 329. D.T. Morrison and T. Robertson, Thin Solid Films, 15 (1973) 87. 1. H. Pratt and T. C. Lausman, Thin Solid Films, 10 (1972) 151. S.M. Lee, lnsul./Cireuits(1971) 33. H. Kobayashi, M. Shen and A. T. Bell, J. Macromol. Sei., Chem., 8 (8) (1974) 1345. L. Holland, L. Laurenson, R. E. Hurley and K. Williams, Nucl. lnstrum. Methods, 3 (1973) 555. J.M. Tibbit, M. Shen and A. T. Bell, Thin Solid Films, 29 (1975) L43. L. Holland, H. Biederman and S. M. Ojha, Thin Solid Films, 35 (1976) LI9. E. Mathias and C. H. Muller, J. Phys. Chem., 71 (8) (1967) 2673.
L 11
Letter
Fluorocarbon films sputtered under various conditions H. BIEDERMAN
Faculty of Mathematics and Physics, Charles University, Prague 2, Ke Karlovu 5 (Czechoslovakia} (Received September 8, 1978: accepted September 18, 1978)
Fluorocarbon polymer films have been studied extensively for several years ~' 2. Most investigations have been concerned with films prepared by the sputtering of polytetrafluoroethylene (PTFE) 3'4 and by the plasma polymerization of a fluorocarbon gas 5' 6 or polyether vapour 7. The two types of films and their modes of preparation have been compared 8' 9 and show certain differences9. It is the purpose of this short report to present several observations yielding more information about the processes involved in the sputtering of PTFE. We sputtered P T F E using a target 75 mm in diameter supported by an insulated water-cooled electrode and connected to an r.f. supply (20 MHz) with one side grounded. Argon was admitted to the continuously pumped vessel after initial evacuation to 1 x 10 - 3 Torr using a rotary pump. The pressure of argon was maintained at 0.05 Torr during the deposition. Some researchers shut off the argon supply after a time and allow the discharge to continue in its own P T F E vapour. It has been shown 9 that when argon is admitted continuously at a pressure of 0.05 Torr with the same power input the deposit has similar properties but the growth rate is higher. A constant r.f. power of 180 W was delivered during all our experiments. Measurements of the probe characteristics showed the plasma and floating potentials to be + 145 V and +65 V respectively relative to ground. Soda lime silicate microscope glass was used as the substrates. The experiments were first performed with the substrates in positions 1 and 3 (Fig. 1) and then position 1 was removed and further experiments were carried out using positions 2 and 3. The average deposition rate over a 30 min period was found to be 120 A min - ~ for position 3 in all experiments. In position 2 the rate was about 75 A min- ~ with a bias of + 265 V relative to ground and 65 A min- ~ when the support was at floating potential. Rates lower by about 10% were obtained in position 1 in these cases. Here some non-uniformity of the deposit appeared mainly in the part of the substrate placed near the target sheath. In position 2 the layer had approximately the same thickness over the whole substrate (26 mm x 30 mm). In position 3 the thickness decreased with increasing distance from the axis of the target and we therefore determined its average value in the centre of the sample using Tolansky's method of multiple beam interferometry. In this case the deposits were yellow in agreement with previous results 2'3'9. The yellow deposit produced in positions I and 2 when they were grounded became more transparent as the potential applied to the supporting electrode was raised. When the potential was about + 265 V relative to ground (120 V higher than the plasma potential) the yellow shade had almost disappeared.
L 12
l .HTERS
The transmission spectra in the region 300 600 nm were measured with a CF 4Optica Milano spectrophotometer with uncoated glass in the reference beam. On the basis of complementary reflection measurements we can consider that absorption is responsible for the transmission data with a systematic error to about 3"~;, which arises from neglect of the differences between the reflectances of the uncoated reference glass and the coated glass. The transmission of films deposited in positions 2 and 3 [Fig. 2) shows that the films fabricated on a grounded support are more absorbing for near-UV light. Similar results were obtained for positions I and 3. Our transmission data are in good agreement with those for sputtered fluorocarbon films published by Holland et al. '~ From their transmission measurements at 320 nm, for example, the absorption constants obtained according to Lambert's law are 0.327 × 104 and 1.5 × 104 cm ~ for plasma-polymerized and sputtered films respectively. At the same wavelength we obtained 2.586 x 104 and 2.213 x 104 cm ~ for grounded and biased films respectively. Similarly to the conclusion given in ref. 9 we consider that an excess of carbon in the layers causes the higher absorption. The films obtained in positions 1 and 2 at floating or bias potential appeared to be harder than those from position 3. The friction properties of all the films were comparable with previous findings-'.
80 ~
/ //
~5
2O
SUPPLY
~
"\ ~ T E R COOLED RF TARGET
GAUGL 300
400
"50C
600 [rim,
Fig. 1. The arrangement of the experiments. The distance from the target to the substrates in positions 2 and 3 was 45 ram. Fig. 2. The transmission of the films obtained on glass substrates: curve a, in position 2, biased at + 265 V relative to ground (film thickness 2200 Ak curve b, in position 3, grounded (film thickness 3500 AI.
According to the data produced by Mathias and Muller 1" CF 2 radicals are likely to be present in r.f. fluorocarbon discharges in large concentrations. Among other species they give carbon and neutral or negative fluorine by disproportionation reactions. We assume that these are drawn out of the plasma to the positively biased receiver [positions 1 and 2 in Fig. 1) and improve the F: C ratio in the growing deposit. However, we cannot stop the exposed surface of the growing film on the glass substrate from developing a potential near to the floating potential. This
LETTERS
L13
worsens the supply of negative fluorine ions and probably does not allow the stoichiometry in the film composition to be improved. Some experiments were done using positions 1 and 3 with substrates previously covered with about 1000 A of aluminium and conductively coupled to the support. In position 1 at a potential of + 265 V relative to ground the aluminium layer was converted into a transparent dielectric film probably as a result of a reaction with fluorine. The resulting deposit appeared to be a mixture of the polymer and a cryolite-like compound. Further investigations are in progress but it appears to be difficult to prepare the same or very similar fluorocarbon films by the plasma polymerization of a fluorocarbon gas and by the sputtering of PTFE. The results presented show that the properties of fluorocarbon films prepared in the same apparatus can be changed by altering the physical conditions at their receiver. Thus our results have fully confirmed the previous conclusion 9 that the composition of fluorocarbon films depends not only on the technique used but also on the specific design of the apparatus. The author is grateful to Professor L. Holland for critical comments on the manuscript. He is also indebted to Dr. J. Phigek for technical assistance in the transmission measurements. I 2 3 4 5 6 7 8 9 10
L. Holland, J. Vae. Sci. Teehnol., 14(1977) 5. H. Biederman, S. M. Ojha and L. Holland, Thin Solid Films, 41 (1977) 329. D.T. Morrison and T. Robertson, Thin Solid Films, 15 (1973) 87. 1. H. Pratt and T. C. Lausman, Thin Solid Films, 10 (1972) 151. S.M. Lee, lnsul./Cireuits(1971) 33. H. Kobayashi, M. Shen and A. T. Bell, J. Macromol. Sei., Chem., 8 (8) (1974) 1345. L. Holland, L. Laurenson, R. E. Hurley and K. Williams, Nucl. lnstrum. Methods, 3 (1973) 555. J.M. Tibbit, M. Shen and A. T. Bell, Thin Solid Films, 29 (1975) L43. L. Holland, H. Biederman and S. M. Ojha, Thin Solid Films, 35 (1976) LI9. E. Mathias and C. H. Muller, J. Phys. Chem., 71 (8) (1967) 2673.