Vacuum lithography using plasma polymerization and plasma development

Thin Solid Films, 83 (1981) 189-194 ELECTRONICS AND OPTICS

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VACUUM LITHOGRAPHY USING PLASMA POLYMERIZATION AND PLASMA DEVELOPMENT* SHUZO HATTORI, JUNJI TAMANO, MASAO YAMADA AND MASAYUKI IEDA

Department of Electrical Engineering, Nagoya University, Chikusa-ku, Nagoya 464 (Japan) SHINZO MORITA, KATSUMI YONEDA AND SHINTARO ISHIBASHI

Department of Electrical Engineering, Meijo University, Tempaku-ku, Nagoya 468 (Japan) (Received March 20, 1981 ; accepted April 7, 1981)

A completely dry lithography has been proposed which involves plasma polymerization of methyl methacrylate (MMA) and plasma development with CC14. It was called vacuum lithography because all processes were performed in a vacuum. However, the developed pattern had a lower resolution than patterns produced by conventional lithography with a wet process. After several technical refinements, the quality of the resist and the developed pattern was markedly improved. In this paper, recent results will be reported. A gas-flow-type reactor was used instead of a bell-jar-type reactor because the morphology of plasma-polymerized MMA (PPMMA) varied with each experimental run which was performed with the same gas and discharge parameters. The monomer vapour was introduced downstream of the argon discharge, and the polymerized film was formed on the substrate placed further downstream in the mixed gas. The development of pattern was performed by etching with an Ar-O2 mixture and with hydrogen gas instead of C f l 4 gas, because the etching rate of the resist was too high in a CC14 plasma and a clear pattern was not obtained. The evaluated sensitivity and 7 value of PPMMA were 1000 IxC cm -2 and 1 respectively. MMA containing 5~ tetramethyltin was also used as a monomer gas for plasma polymerization downstream of the argon discharge. In this case the sensitivity and ,/ value were 10 ~tC cm-2 and 2 respectively.

1. INTRODUCTION Fine lithographic technology in a submicron design regime is necessary for the fabrication of very-large-scale integrated circuits. In such lithography, fine pattern delineation is performed by electron beam, ion beam and X-ray lithography instead of photolithography. It is also believed that a completely dry process is necessary for fine lithography. Because of recent progress in dry process technology1'2 most lithographic processes are now carried out under dry conditions except for resist * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A., April 6-10, 1981. 0040-6090/81/0000-0000/$02.50

© Elsevier Sequoia/Printed in The Netherlands

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coating and pattern development. It has recently become possible to undertake the pattern development process under dry conditions using plasma etchinga'4. If the resist coating can also be carried out under dry conditions, completely dry lithography will be possible. We have previously proposed a completely dry lithography, using plasma polymerization and plasma development, in relation to photomask fabrication 5. In that study the resist layer was prepared by plasma polymerization of methyl methacrylate (MMA) on a chromium-coated glass substrate, the pattern of lines and spaces was delineated with an electron beam, and the pattern was developed by a CC14 plasma and subsequently transferred onto the chromium film by CCl 4 plasma etching. However, in that primitive trial the developed pattern was of poor resolution. After several technical refinements6-8, the quality of the plasmapolymerized resist was significantly improved. In this paper we shall give an outline of our technical improvements and discuss an additional new aspect. 2.

EXPERIMENTAL DETAILS

An argon-glow-type reactor was used for the plasma polymerization8. It comprised a glass discharge tube of i.d. 22 mm and length 270 mm connected directly to a reaction chamber of i.d. 75 mm and length 170 mm. Argon was introduced into the discharge tube, in which the discharge was maintained by means of an external coil. Monomer vapour was introduced downstream of the argon discharge and the polymerized film was formed on a substrate placed in the reaction chamber further downstream of the discharge. A generator of frequency 13.56 MHz was used to produce the discharge and the substrate was glass coated with an evaporated chromium film5. MMA was used as the monomer for the plasma polymerization. As the first step of an experiment to find a lithography process which was free from the influence of electrons back scattered from the chromium film on the substrate during the electron beam delineation, a material containing atoms of high atomic number was co-polymerized in the resist 7. Tetramethyltin (TMT) was the first monomer selected. A liquid mixture of TMT and MMA containing 4-5 vol.% TMT was used and vapour from this mixture was introduced into the reactor for the plasma polymerization. A typical plasma polymerization was performed at a gas pressure of 1.0 Torr, an argon flow rate of 40 cm 3 min- 1, a monomer flow rate of 3 cm 3 min- 1 and a discharge power of 20 W 6 For comparison a conventional spin-coated polymethyl methacrylate (PMMA) film (Tokyo-oka, OEBR-1000) was also investigated. The pattern of lines and spaces was delineated using an electron-beampatterning machine (Elionix ELS-2A) at an acceleration voltage of 20 kV and a dose of between 5 and 1000 IxC cm -2 (refs. 7 and 8). The delineated pattern in the resist was developed with a plasma-etching apparatus (ULVAC SBR-1104) which contains parallel plane electrodes of diameter 100 mm. An Ar-O2 mixture and hydrogen were used as the etching gases. The development was conducted at a gas pressure of 2-3 mTorr and a discharge power of 100 W.

VACUUMLITHOGRAPHY

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Transfer of the pattern from the resist to the chromium could be accomplished by plasma etching with CC14. However, this process is not described here because it is a well-established technique. 3. EXPERIMENTAL RESULTS AND DISCUSSION In the case of plasma-polymerized MMA (PPMMA), the surface of the resist did not show any significant change after the delineation by an electron beam 5. On plasma etching with the Ar-O2 gas mixture, a visible pattern was developed on the resist. However, for conventional PMMA a visible pattern was delineated by an electron beam before it was subjected to etching for development. This phenomenon has previously been reported as a vapour development9' 10, but we shall call it selfdevelopment to distinguish it from an external thermal effect. In order to study the self-development, the thicknesses removed from three samples were measured and plotted against exposure dose in Fig. 1. The removed thickness for PPMMA was smaller than that for conventional PMMA at the same exposure dose. The small rem6ved thickness for PPMMA could be understood in terms of cross-linking in PPMMA.

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Fig. 1. The dependence of the thickness removed on the exposure dose: I'q, PP(MMA-TMT); A. PPMMA; O, spin-coated conventional PMMA.

The phenomenon of self-development was most significant for PP(MMATMT), while the sensitivity of the resist is expected to be higher than that of conventional PMMA. The high sensitivity is partly due to the production of lowenergy electrons when an electron is scattered by a tin atom doped in PpMMA. A scanning electron micrograph of a self-developed pattern on PP(MMATMT) is shown in Fig. 2. Each square is 4 lain x 4 Ixm which corresponds to the area of a single shot by the flat square beam of the electron-beam-patterning machine. For doses higher than 20 laC cm- 2, a hole-like crater is formed in each square area. This suggests that self-development is due to the thermal effect of the electron beam. At a low dose of 20 ~tC cm-2, a two-dimensional array of dimples was clearly visible

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(a)

(b) (c) Fig. 2. Scanningelectron micrographs of self-developedpatterns on PP(MMA-TMT) for three doses: (a) 20 ~tCcm- 2(the surfacelayer of the resist was gently removed; see text for an explanation of the pattern); (b) 40 laC cm- 2(a hole-likecrater was produced by each electron beam shot; the angle of incidenceof the scanning electron beam was 45° to the surfaceof the resist; a cross section of the resist,the chromium film and the glass substrate can be seen); (c) 70 IxC cm -2 (evaporation of the resist by the shot was more intense; the angle of incidenceof the scanning electron beam was again 45°).

o n the resist, each c o r r e s p o n d i n g to the electron energy d i s t r i b u t i o n in the square electron beam. The latent p a t t e r n o n P P M M A was successfully developed by p l a s m a etching with an A r - O 2 gas mixture. This implies that p l a s m a etching is selective to the exposure dose. The r e m o v e d film thicknesses before a n d after plasma etching for P P M M A a n d c o n v e n t i o n a l P M M A were plotted against the exposure dose as s h o w n in Fig. 3. The difference in the r e m o v e d film thickness before a n d after plasma

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VACUUM LITHOGRAPHY

etching is the thickness removed by plasma etching. The plasma-etching rate is determined from the difference in film thickness divided by the duration of plasma etching. The plasma-etching rate of PPMMA is apparently lower than that of conventional PMMA. The relative etching rates of PPMMA and conventional PMMA are shown in Fig. 4(a). The low etching rate of PPMMA can be attributed to the occurrence of cross-linking in PPMMA. 0.6

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Fig. 4. The dependence of the relative plasma-etching rate (the etching rate of unexposed film is normalized to unity) on the exposure dose (A, PPMMA; C), spin-coated conventional PMMA) for etching with (a) an Ar-O 2 gas mixture and (b) hydrogen gas.

The mechanism of the plasma etching involves two processes: chemical etching by activated oxygen and a thermal effect due to energetic particle bombardment. In order to distinguish between the two processes, the effect of plasma etching with hydrogen was studied. The relative etching rates of PPMMA and PMMA were

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plotted against the exposure dose, as shown in Fig. 4(b). The relative etching rate with a hydrogen plasma is higher than that with an oxygen plasma. The effect of chemical etching is assumed to be small in a hydrogen plasma compared with that in an oxygen plasma. Therefore, we concluded that energetic particle bombardment of the resist is the most important process in plasma etching and that chemical etching with an oxygen plasma is not sensitive to the exposure dose, at least for the plasma conditions and in the parallel electrode type of etching machine that we used. The sensitivity and y value of resist can be determined from the characteristic curve of the relative residual film thickness against the exposure dose a. According to Fig. 1, the sensitivity of PP(MMA-TMT) will be 20-30 ~tC cm -2 without development. Iffhe selectivity for hydrogen plasma etching is the same as that in Fig. 4(b), the sensitivity will exceed 10 laC cm- 2. If our model of the development process is correct, the value of 10 ~tC cm- 2 is not an overestimate. By means of the technical refinements outlined here, we markedly improved the sensitivity of the plasma-polymerized resist from 1000 to 10 ~tC cm -2, which is sufficient for an electron beam resist. However, several problems remain to be solved: (1) to obtain homogeneity of polymer character and film thickness over a wide substrate; (2) to ensure that the etching resistivity is adequate for the transfer of the resist pattern to a chromium film; (3) to achieve higher y values which would be desirable for overcoming the spreading of the pattern due to backscattering of electrons. However, the results obtained in our experiments are sufficient to encourage further research on completely dry electron beam lithography. Because plasma polymerization was found to give resists with similar properties to those of conventional resist polymers of known molecular structure, it provides an easy method of investigating new polymer resists based on simple molecular design considerations. REFERENCES 1 J.L. VossenandW. Kern(eds.), Thin Film Processes, AcadenficPress, NewYork, 1978. 2 T.Sugan•(ed.)•Hand•taiP•asma-pr•cessGijutsu•Sangy•-t•sh••T•ky•••98•(inJapanese). 3 H. Hughes, W. Goodner, T. Wood, J. Smith and J. Keller, Proc. 8th Technical ConJ~ on Photopolymer Principles, Processes and Materials, Ellenville, N Y, October 10-12,1979, pp. 207-224. 4 H. Nakane, W. Kanai, A. Yokota, I. Hijikata, A. Uehara, S. Oikawa and M. Tsuda, Oyo Butsuri, 50 (1981) 145-156 (in Japanese). 5 S. Morita, J. Tamano, S. Hattori and M. Ieda, J. Appl. Phys., 51 (1980) 3938-3941. 6 S. Morita, S. Hattori and M. Ieda, Proc. 80/2 Micro Syrup. on Plasma Polymerization, Tokyo, June 26, 1980, pp. 10-15. 7 M. Yamada, J. Tamano, K. Yoneda, S. Morita and S. Hattori, Proc. 2nd Symp. on Dry Processes, Tokyo, October 29-30, 1980, pp. 79-86. 8 J. Tamano, S. Hattori, S. Morita and Y. Yoneda, Plasma Chemistry and Plasma Processing, Plenum, New York, in the press. 9 M.J. B o w d e n a n d L F . Thompson, J. AppLPolym. Sci.,17(1973)3211-3221. 10 L.F. Thompson and M. J. Bowden, J. Electrochem. Soc., 120 (1973) 1722-1726.