- Email: [email protected]
Electron-assisted deposition of thin organic layers
ARTICLE IN PRESS
Vacuum 76 (2004) 223–226 www.elsevier.com/locate/vacuum
Electron-assisted deposition of thin organic layers D. Dimova,, E. Spassovaa, I. Karamanchevab, I. Zhivkova, G. Daneva a
Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. G. Bonchev St., bl. 109, 1113 Sofia, Bulgaria b University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria
Abstract The present paper forwards a study related to a new type of process for production of thin organic layers. Energystimulated processes based on electron-assisted deposition in vacuum are implemented. A system for electron bombardment of the molecular flow of the (polyimide) PI precursors–ODA (oxidianiline) and PMDA (pyromellitic dianhydride) is being developed. By means of electron-microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) the surface structure of the deposited layers is studied under the conditions of differing densities of the electron beam. A morphology modification and formation of films of a better defined composition are established. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electron-assisted deposition; Nanocomposite films; Polyimide layers
1. Introduction Upon an energy action of the electron flow on the organic monolayers [1] due to the cleavage of the chemical bonds or active radical formation, a modification in the place of influence occurs. The obtained results are of importance for the optimization of self-assembled monolayer-based lithography. Preparation of highly adhesive thin films [2] or change of the thin film orientation [3] by the electron shower method is also well-known. The formation of polymer layers by deposition of their components from a gas phase yields considerable advantages and possibilities for obtaining new Corresponding author. Fax:+359-2-722-465.
E-mail address: [email protected] (D. Dimov).
materials in the field of nano-technology. Polymide (PI) is especially suitable for the role of a nano-composite matrix due to their chemical inertia and high thermal stability [4,5]. The aim of the paper is to sum up our investigations of the possibilities for application of a new type of energy-stimulated processes based on electron-assisted deposition (EAD) in vacuum in the formation of thin organic layers of the PI precursors ODA and PMDA.
2. Experimental 1. Substrates: soda–lime glass plates. 2. Substances: PI precursors: ODA (oxidianiline) and PMDA (pyromellitic dianhydride).
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.061
ARTICLE IN PRESS D. Dimov et al. / Vacuum 76 (2004) 223–226
224
Fig. 2. SEM micrographs of ODA layers: (a) without electron influence; (b) with electron influence: Ua=4 kV and Uk=7.5 V. Fig. 1. A detailed scheme of the system for EAD.
3. Methods of investigation: SEM-Philips 515 and FTIR spectroscopy. FTIR spectra (PI on KBr substrate) were recorded on a Perkin-Elmer 1600 spectrometer in the 3600–450 cm1 range; resolution 4 cm1; interval 0.2. The size of the IR beam was 15 mm in diameter. 4. Film preparation: the experimental set-up is shown in Fig. 1. Evaporation rate is 2.5–3.5 A˚/s on static substrate; Uk=0–12 V; Ua=0–4.5 kV; charge current is up to 250 mA; the potential difference of the focussing electrode is 500–1000 V.
3. Results and discussion Electron microscopy investigation results (Figs. 2 and 3) show a thickening of the surface morphology. The layers acquire a smoother, more orderly surface structure. With the augmentation of the Ua , the effect is enhanced. The FTIR spectroscopy investigation results (Figs. 4 and 5) indicate a modification of the surface morphology expressed in a thickened and more orderly surface structure most probably due to the diminution of the degree of free rotative movement of the linearly built molecules, i.e., ‘‘molecule fixation’’ [6]. This effect is enhanced with the increase of the electron flow energy in the experimentally fixed limits. The intensity of the peaks from 1620 to 3387 cm1 indicates the degree of humidity (Fig. 4). When EAD is applied, signals for water are not registered and the peak at
Fig. 3. SEM micrographs of PMDA layers: (a) without electron influence; (b) with electron influence: Ua=2 kV and Uk=7.5 V; (c) with electron influence: Ua=4 kV and Uk=7.5 V.
1499 cm1 strongly increases its intensity. A decrease in the number of signals in the area of the skeletal vacillations of the benzene nucleus is observed (1398 cm1). This fact can be assigned to the decrease in the degree of free rotative movement of the linearly built molecules. As a result, the film obtained is more homogeneous, orderly and better oriented. There is no data about decomposition and emergence of new fragments in the identification range of FTIR. With the augmentation of Ua (Fig. 5) a decrease of the rotative vacillations of the benzene nucleus (1560 cm1) is observed, the free acid/anhydride
ARTICLE IN PRESS D. Dimov et al. / Vacuum 76 (2004) 223–226
drides, which normally appears in the 1780–1850 cm1 range, is now not established. In all the cases in PMDA, a characteristic triplet at 1257, 1281 and 1302 cm1 is found. It is due to the C–O–C stretch for anhydrides and the C¼O stretch for acids. Their intensity and ratios change depending on the anhydride and acid content. In all the cases when EAD is applied the intensity of this triplet is decreased which is again an indication of decrease of the rotative vacillations of single C–O bonds.
3387
(a) 1499
A
1223 1618 1398 3387
(b)
824
1499
1223
824
1618 3600
3000
2000
1500
225
1000
450
cm-1
Fig. 4. FTIR spectra of ODA films, 0.8 mm thick: (a) 0 kV; (b) 4 kV.
1279 1701
1410
(a) 1731 3529
1117 1560
1769
A
1701
(b)
753
1279 1410
1731
3529
816,
1117 816753
1769 1701 1279
(c)
1410
1731 3529 3600
1117
816 753
1769 3000
2000
1500
1000
450
cm-1
Fig. 5. FTIR spectra of PMDA films, 0.8 mm thick: (a) 0 kV; (b) 2 kV; (c) 4 kV.
ratio is changed (the dehydration effect as in ODA) and the dimerization of pyromellitic acid (PA) is decreased—the hydrogen bond between the –COOH groups is broken. In the 3500 cm1 range the stripes typical of the carboxyl acid dimers are observed. In the carbonyl range 3 absorption stripes of 1701, 1730 and 1770 cm1 are observed. The peak at 1701 cm1 is the most intense and is due to the –COOH groups present. The stripes at 1730 and 1770 cm1 are due to a valently symmetrical vacillation of the anhydrides. The valently asymmetrical vacillation for anhy-
4. Conclusions Energy-stimulated processes based on electronassisted deposition in vacuum are implemented. A system for electron bombardment of the molecular flow of the polyimide precursors is developed. A modification of the surface morphology of the ODA and PMDA layers is presented, expressed in a thickened and more orderly surface structure most probably due to the diminution of the degree of free rotative movement of the linearly built molecules, i.e., ‘‘molecule fixation’’. This effect is enhanced with the increase of the electron flow energy in the experimentally fixed limits. The films are with a better-defined composition—a decrease of the –OH groups and the number of the carboxyl acid dimers are shown. The implementation of the above—described process in a simultaneous deposition of the PI precursors and the formation of a thin-layer PI matrix will be the objective of a future endeavour of ours. Possibly, radical formation or carbon cluster formation of nano-sizes and their homogeneous embedding into the matrix in the EAD would render opportunities for the production of novel materials in the nano-tech area.
Acknowledgements The financial support of the National Fund of Ministry of Education and Science, Bulgaria— contract X-1322, is gratefully acknowledged.
ARTICLE IN PRESS 226
D. Dimov et al. / Vacuum 76 (2004) 223–226
References [1] Mueler HU, Zharnikov M, Voelkel B, Scherel A, Harder P, Grunze M. J Phys Chem B 1998;102/41: 7949–59. [2] Yumoto H, Kaneko K, Ishihara M, Kato Y, Akashi K. Thin Solid Films 1996;281–282:311–3.
[3] Ishihara M, Yumoto H, Tsuchiya T, Akashi K. Thin Solid Films 1996;281–282:321–3. [4] Spassova E. Vacuum 2003;70/4:551–61. [5] Danev G, Spassova E, Assa J, Karamancheva I, Paskaleva A, Popova K, Ihlemann J. Vacuum 2003;70:37–45. [6] Lambert Y, Shuvell H, Vevleit L, Cooks RG, Stout G. Org struct anal. New York: Macmillan; 1986.
Vacuum 76 (2004) 223–226 www.elsevier.com/locate/vacuum
Electron-assisted deposition of thin organic layers D. Dimova,, E. Spassovaa, I. Karamanchevab, I. Zhivkova, G. Daneva a
Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. G. Bonchev St., bl. 109, 1113 Sofia, Bulgaria b University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria
Abstract The present paper forwards a study related to a new type of process for production of thin organic layers. Energystimulated processes based on electron-assisted deposition in vacuum are implemented. A system for electron bombardment of the molecular flow of the (polyimide) PI precursors–ODA (oxidianiline) and PMDA (pyromellitic dianhydride) is being developed. By means of electron-microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) the surface structure of the deposited layers is studied under the conditions of differing densities of the electron beam. A morphology modification and formation of films of a better defined composition are established. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electron-assisted deposition; Nanocomposite films; Polyimide layers
1. Introduction Upon an energy action of the electron flow on the organic monolayers [1] due to the cleavage of the chemical bonds or active radical formation, a modification in the place of influence occurs. The obtained results are of importance for the optimization of self-assembled monolayer-based lithography. Preparation of highly adhesive thin films [2] or change of the thin film orientation [3] by the electron shower method is also well-known. The formation of polymer layers by deposition of their components from a gas phase yields considerable advantages and possibilities for obtaining new Corresponding author. Fax:+359-2-722-465.
E-mail address: [email protected] (D. Dimov).
materials in the field of nano-technology. Polymide (PI) is especially suitable for the role of a nano-composite matrix due to their chemical inertia and high thermal stability [4,5]. The aim of the paper is to sum up our investigations of the possibilities for application of a new type of energy-stimulated processes based on electron-assisted deposition (EAD) in vacuum in the formation of thin organic layers of the PI precursors ODA and PMDA.
2. Experimental 1. Substrates: soda–lime glass plates. 2. Substances: PI precursors: ODA (oxidianiline) and PMDA (pyromellitic dianhydride).
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.061
ARTICLE IN PRESS D. Dimov et al. / Vacuum 76 (2004) 223–226
224
Fig. 2. SEM micrographs of ODA layers: (a) without electron influence; (b) with electron influence: Ua=4 kV and Uk=7.5 V. Fig. 1. A detailed scheme of the system for EAD.
3. Methods of investigation: SEM-Philips 515 and FTIR spectroscopy. FTIR spectra (PI on KBr substrate) were recorded on a Perkin-Elmer 1600 spectrometer in the 3600–450 cm1 range; resolution 4 cm1; interval 0.2. The size of the IR beam was 15 mm in diameter. 4. Film preparation: the experimental set-up is shown in Fig. 1. Evaporation rate is 2.5–3.5 A˚/s on static substrate; Uk=0–12 V; Ua=0–4.5 kV; charge current is up to 250 mA; the potential difference of the focussing electrode is 500–1000 V.
3. Results and discussion Electron microscopy investigation results (Figs. 2 and 3) show a thickening of the surface morphology. The layers acquire a smoother, more orderly surface structure. With the augmentation of the Ua , the effect is enhanced. The FTIR spectroscopy investigation results (Figs. 4 and 5) indicate a modification of the surface morphology expressed in a thickened and more orderly surface structure most probably due to the diminution of the degree of free rotative movement of the linearly built molecules, i.e., ‘‘molecule fixation’’ [6]. This effect is enhanced with the increase of the electron flow energy in the experimentally fixed limits. The intensity of the peaks from 1620 to 3387 cm1 indicates the degree of humidity (Fig. 4). When EAD is applied, signals for water are not registered and the peak at
Fig. 3. SEM micrographs of PMDA layers: (a) without electron influence; (b) with electron influence: Ua=2 kV and Uk=7.5 V; (c) with electron influence: Ua=4 kV and Uk=7.5 V.
1499 cm1 strongly increases its intensity. A decrease in the number of signals in the area of the skeletal vacillations of the benzene nucleus is observed (1398 cm1). This fact can be assigned to the decrease in the degree of free rotative movement of the linearly built molecules. As a result, the film obtained is more homogeneous, orderly and better oriented. There is no data about decomposition and emergence of new fragments in the identification range of FTIR. With the augmentation of Ua (Fig. 5) a decrease of the rotative vacillations of the benzene nucleus (1560 cm1) is observed, the free acid/anhydride
ARTICLE IN PRESS D. Dimov et al. / Vacuum 76 (2004) 223–226
drides, which normally appears in the 1780–1850 cm1 range, is now not established. In all the cases in PMDA, a characteristic triplet at 1257, 1281 and 1302 cm1 is found. It is due to the C–O–C stretch for anhydrides and the C¼O stretch for acids. Their intensity and ratios change depending on the anhydride and acid content. In all the cases when EAD is applied the intensity of this triplet is decreased which is again an indication of decrease of the rotative vacillations of single C–O bonds.
3387
(a) 1499
A
1223 1618 1398 3387
(b)
824
1499
1223
824
1618 3600
3000
2000
1500
225
1000
450
cm-1
Fig. 4. FTIR spectra of ODA films, 0.8 mm thick: (a) 0 kV; (b) 4 kV.
1279 1701
1410
(a) 1731 3529
1117 1560
1769
A
1701
(b)
753
1279 1410
1731
3529
816,
1117 816753
1769 1701 1279
(c)
1410
1731 3529 3600
1117
816 753
1769 3000
2000
1500
1000
450
cm-1
Fig. 5. FTIR spectra of PMDA films, 0.8 mm thick: (a) 0 kV; (b) 2 kV; (c) 4 kV.
ratio is changed (the dehydration effect as in ODA) and the dimerization of pyromellitic acid (PA) is decreased—the hydrogen bond between the –COOH groups is broken. In the 3500 cm1 range the stripes typical of the carboxyl acid dimers are observed. In the carbonyl range 3 absorption stripes of 1701, 1730 and 1770 cm1 are observed. The peak at 1701 cm1 is the most intense and is due to the –COOH groups present. The stripes at 1730 and 1770 cm1 are due to a valently symmetrical vacillation of the anhydrides. The valently asymmetrical vacillation for anhy-
4. Conclusions Energy-stimulated processes based on electronassisted deposition in vacuum are implemented. A system for electron bombardment of the molecular flow of the polyimide precursors is developed. A modification of the surface morphology of the ODA and PMDA layers is presented, expressed in a thickened and more orderly surface structure most probably due to the diminution of the degree of free rotative movement of the linearly built molecules, i.e., ‘‘molecule fixation’’. This effect is enhanced with the increase of the electron flow energy in the experimentally fixed limits. The films are with a better-defined composition—a decrease of the –OH groups and the number of the carboxyl acid dimers are shown. The implementation of the above—described process in a simultaneous deposition of the PI precursors and the formation of a thin-layer PI matrix will be the objective of a future endeavour of ours. Possibly, radical formation or carbon cluster formation of nano-sizes and their homogeneous embedding into the matrix in the EAD would render opportunities for the production of novel materials in the nano-tech area.
Acknowledgements The financial support of the National Fund of Ministry of Education and Science, Bulgaria— contract X-1322, is gratefully acknowledged.
ARTICLE IN PRESS 226
D. Dimov et al. / Vacuum 76 (2004) 223–226
References [1] Mueler HU, Zharnikov M, Voelkel B, Scherel A, Harder P, Grunze M. J Phys Chem B 1998;102/41: 7949–59. [2] Yumoto H, Kaneko K, Ishihara M, Kato Y, Akashi K. Thin Solid Films 1996;281–282:311–3.
[3] Ishihara M, Yumoto H, Tsuchiya T, Akashi K. Thin Solid Films 1996;281–282:321–3. [4] Spassova E. Vacuum 2003;70/4:551–61. [5] Danev G, Spassova E, Assa J, Karamancheva I, Paskaleva A, Popova K, Ihlemann J. Vacuum 2003;70:37–45. [6] Lambert Y, Shuvell H, Vevleit L, Cooks RG, Stout G. Org struct anal. New York: Macmillan; 1986.