Structure and morphology of ion-implanted polyimide films

878

STRUCTURE

Nuclear Instruments and Methods in Physics Research B19/20 (1987) 878-881 North-Holland, Amsterdam

A N D M O R P H O L O G Y OF I O N - I M P L A N T E D

POLYIMIDE

FILMS

Kazuo YOSHIDA* Torav Ind. Inc., Sonoyama 1-1-10tsu, Siga, 520, Japan Masaya IWAKI RIKEN, Hirosawa 2-1, Wako, Saitama, 351-01, Japan

The surface structure and morphology of Ar +, Cu +, and Zn + implanted polyimide films have been studied by transmission electron microscopy (TEM), Raman spectroscopy and Rutherford backscattering spectroscopy. TEM observation of the surface reveals a formation of porous structure in the high-dose Ar + implantation, and the formation of tiny metallic islands in the high-dose Cu ÷ and Zn + implantation. This suggests migration and aggregation of implanted atoms. It is found that the implanted surface layers of the parent polymer are carbonized as a result of the polymer scission and the out-diffusion of oxygen and nitrogen atoms.

1. Introduction Attempts to modify organic materials (polymer and nonpolymer) by ion implantation began recently. In the research field of ion implantation into insulating polymers, the most distinct feature is the production of electrically conductive regions in the insulator. Extensive work has been performed with respect to the modification of parent polymers by high energy Ar + implantation [1-3]. This has shown that an amorphous carbon-like structure was produced by ion implantation. The electrical properties characteristic to the ion beam induced conductivity have also been discussed. Polymer films much thinner than the penetration depth of the projectiles were used. Therefore the effects studied in these experiments were mainly due to radiation damage, and not due to the projectiles themselves. The ion energies used were typically 2 MeV, which were higher than those utilized in the conventional implanters, e.g. 5-200 keV. Hioki et al. [4] implanted a variety of ion species into polyimide films and measured the electrical conductivity as a function of energy and dose. The results showed that polyimide films irradiated to high doses at energies of 0.3-2 MeV exhibit metallic luster and a low resistivity of about 5 × 10 -4 I2cm. Mazurek et al. [5] have studied the effect of As +, Kr +, and Br + implantation to poly(p-phenylene sulfide) (PPS) by using 100-200 keV energies. Under these conditions, the penetration depth of the projectiles is close to the center of the film thickness (0.1-0.2 /~m). They showed that the ion ira*Present address: Toray Research Center Inc. Sonoyama 1-1-1 Otsu, Siga, 520, Japan.

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plantation rendered the PPS conductive, and suggested the chemical doping effect of Br + implantation in comparison to the Kr + implantation. Since the distribution of implanted atoms and the radiation damage is dependent on the depth, ion implantation at a low energy will cause an inhomogeneous modification in the surface. In this study, we implanted Ar +, Cu +, and Zn + ions in polyimide films at an energy of 150 keV, and studied the surface morphological structure and composition in relation to electrical properties by means of transmission electron microscopy (TEM), laser Raman spectroscopy (LRS) and Rutherford backscattering spectrometry (RBS).

2. Experiments The substrates used were polyimide films (poly(N,N'(p,p'-oxydiphenylene)pyromellitimide), Cl2N205 Hto, K a p t o n - H T M produced by Toray Ind. Inc.) of 50 /~m thickness. The samples were 35 × 40 mm 2 in size. The ion implantation of Ar +, Cu +, and Zn + was performed from a dose of 1 × 1015 to 5 x 1017 ions/era 2 at an energy of 150 keV. The four edges of a sample were held on the aluminum target holder by stainless steel sheets. The ion beam current density was 1-5 ~ A / c m 2 and rastered over an area of 3.3 x 3.3 cm2. Although the samples mounted on the holder were not temperature controlled, the holder temperature did not exceed 80°C even at a high dose of 2.5 × 1017 ions/era 2. The pressure during the ion implantation rose to 5 × 10 -4 Pa at the initial stage of implantation due to outgassing from the polymer but it was normally around 2 × 10 =4 Pa.

K. Yoshida, M. Iwaki / Structure of ion-irnplanted polyirnidefilms The sheet resistivity measurements at room temperature were carried out in dry N 2 atmosphere by the two-electrode method, using silver paste for the electrodes. The current-voltage relation indicated ohmic resistivity. The Raman spectra were measured by using an argon ion laser (514.5 rim, Jobin-Yvon Ramanor U1000). In order to prevent an increase in sample temperature, a 50 mW laser beam was used under defocused condition. Transmission electron microscopy with a 100 keV electron beam (Hitachi, H-100) was employed in the observation of the cross section of the film (perpendicular to the film surface). The film was wrapped up in plastic resin and thinned to about 80 rim thick using a diamond blade. The Rutherford backscattering measurements were performed by using a Tandetron (General Ionex Corp.). The 1 MeV 4He+ ion beam was bombarded on a specimen with a fluence of 5 /~C. The samples were mounted on a 3-axis goniometer in a target chamber, which was at a pressure below 5 × 10 -4 Pa. Backscattered particles at an angle of 150 ° were detected and analyzed by a solid state detector and conventional electronics.

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3. Results and discussion

lustrous black and revealed electrical conductivity. Fig. 1 shows the sheet resistivity of Ar+-implanted Kapton at various ion beam current densities as a function of the dose. The sheet resistivity depends on both the dose and the ion beam current density. The sheet resistivity is lower at higher doses. In addition, at a given dose, the sheet resistivity is lower for a higher beam current.

3.1. Sheet resistivity of the implanted samples

3.2. Raman spectra

With increasing doses, the surface of the implanted region of the Kapton changed color from dark brown to

Fig. 2 shows the Raman spectra for the Ar+-im planted Kapton. For both doses, a broad peak near 1550 cm -1 can be observed. This is assigned to an amorphous carbon-like structure. This peak broadened with increasing dose, which is understood to be the result of heavier damage at higher doses. At lower doses, a relatively sharp peak can be observed near 1400 cm-1. The peak is different from the reported ones for the single crystal diamond (1332 cm -1) [7] or for the glassy carbon (near 1350 cm -1) [8], and is not understood. However, it seems that the peak suggests an intermediate stage before the formation of the heavily damaged amorphous structure since it disappears with increasing dose. The Raman spectra for the Cu +, and Zn + implanted samples were similar to those for Ar+-implanted samples. The peaks indicating the presence of Cu20 (220 cm -x) and ZnO (440 cm-1) were not detected.

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3.3. The structure of the sample cross section Fig. 3 shows the TEM micrographs that demonstrate the difference in the structure of the cross section of Ar+-implanted samples at different implantation conditions. The darker areas correspond to the regions of VI. INSULATORS

880

K. Yoshida, M. lwaki / Structure of ion -implanted polyimide films

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Fig. 3. TEM micrographs of Ar+-implanted Kapton films at various ion beam currents (IBC) and doses (D); (a) IBC= 3.5, #A/cm 2, D = 1 × 1016cm- 2, (b) IBC = 3.5 #A/cm 2, D = 2 × 1017cm- 2, (C) IBC = 1.0 #A/cm 2, D = 5 × 1016cm- 2, (d) IBC = 4.5 /LA/cm2, D= 5 × 1016cm -2.

b Fig. 4. TEM micrographs of Cu +- and Zn+-implanted Kapton films; (a) 1 × 1017 Cu+/cm 2, (b) 5 × 1017 Zn+/cm 2

higher free electron density, which are conductive layers. The thickness of these layers is almost of the same order as the projected range predicted by range theory [9] (assuming the atomic number and the density of the target to be 6 and 1.0, respectively). The Kapton film can be carbonized by pyrolysis [6]. Under such implantation conditions where the projectiles stop in the polymer, most of the ion energy will be dissipated generating heat. However, the boundary between the carbonized layer and the parent polymer was rather sharp and the thickness of the carbonized layer was almost the same as the ion penetration depth, implying

that the carbonization is caused mainly by the damage effect and not by pyrolysis. In the high dose case (fig. 3b), the conductive layers consist of three layers, which can be explained in terms of the difference in the damage distribution. The sample implanted at the high ion beam current density (fig. 3d) is lower in resistivity than that implanted at low ion beam current density (fig. 3c), and exhibits a surface structure similar to the sample implanted with high dose. Figs. 4a and b are the TEM micrographs of the Cu ÷ and Zn + implanted Kapton, respectively. Many tiny

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On the other hand, the amount of retained Cu atoms agrees with the implanted dose within experimental error. Therefore, the tiny particles observed by TEM probably consist mainly of Cu atoms. The relative amount of O and N atoms to C atoms decreases. This, together with the results of the LRS measurements, supports the idea of scission of the chemical bonds and the out-diffusion of the N and O atoms.

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The atoms implanted in polyimide films have been proved to migrate and aggregate to form many tiny islands in the surface region. The implanted surface region of the parent polymer is carbonized as a result of the scission of the chemical bonds and the subsequent out-diffusion of nitrogen and oxygen atoms• The authors gratefully acknowledge Mr. T. Kobayashi of The Institute of Physical and Chemical Research ( R I K E N ) for the RBS measurement, and Drs. Y. Murata and G. Katagiri of Toray Research Center Inc. for the T E M and LRS measurements.

(a) 1 × 1017Ar+/cm2, (b) 1 x 10 w Cu+/cm 2.

particles in the surface layer can be observed. They are regarded as the aggregation of implanted Cu atoms. The diameters of most particles are 20-30 nm. An increase in the dose did not cause swelling of the particles, but did increase their number• At lower doses, below 5 x 1016 cm -2, no such particles were observed. These results suggest that the implanted atoms have migrated, and part of them have aggregated• It seems plausible that, in the high dose At-implantation case, similar spheres of AS atoms (bubbles) form and their explosion results in the porosity of the structure. 3.4. R B S measurements Fig. 5a and b are the RBS spectra for Ar ÷ and Cu ÷ implanted Kapton, respectively. In Ar ÷ implantation, the amount of As atoms retained in the surface layer is determined to be approximately 1.5 × 1015 cm -2 for an implantation dose of I × 1017 cm-2. Thus the implanted A r atoms diffuse out of the surface resulting in the formation of the porous structure.

References

[1] T. Venkatesan, S.R. Follest, M.L. Kaplan, C.A. Murray, P.H. Schmidt and B.J. Wilkens, J. Appl. Phys. 54 (1983) 3150. [2] T. Venkatesan, M. Feldman, B.J. Wilkens and W.E. Willenbrock, Jr., J. Appl• Phys. 55 (1984) 1212. [3] T. Venkatesan, D. Edelson and W.L. Brown, Nucl. Instr. and Meth. B1 (1984) 286. [4] T. H.ioki, S. Noda, M. Sugiura, M. Kakeno, K. Yamada and J. Kawamoto, Appl. Phys. Lett. 43 (1983) 30. [5] H. Mazurek, D.R. Day, E.W. Maby, J.S. Abel, S.D. Senturia, M.S. Dresselhause and G. Dresselhause, J. Polym. Sci.: Polym. Phys. Ed. 21 (1983) 537. [6] H.B. Brom, Y. Tomkiewicz, A. Aviram, A. Broers and B. Surmers, Solid State Commun. 35 (1980) 135. [7] Y. Sato, M. Kamo, H. Kanda and N. Setaka, Hyoumenkagaku 1 (1980) 60, in Japanese. [8] M. Nakamura and K. Tamai, Tanso no. 117 (1984) 94, in Japanese. [9] J.F. Gibbons, W.S. Johnson and S.W. Mylorie, Projected Range Statistics; Semiconductor and Related Materials, 2nd ed. (Wiley, New York, 1975).

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