SELF-DEVELOPMENT OF NITROCELLULOSE AND ITS MECHANISM

543. Microelectronic Engineering 9(1989)543-546 North-Holland. SELF-DEVELOPMENT. Y.Yasuoka,. OF NITROCELLULOSE. K.Kaneko,. K.Gamo*. AND ITS MECHANISM. and S.Namba*. Department of Electrical Engineering, The National Defense Academy, Hashirimizu, Yokosuka 239, Japan *Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan. Dependence of the self-development process of nitrocellulose film on the atomic mass of an irradiated ion, and the energy of incident ion is studied experimentally using seven different kinds of ions. The experimental results are compared with the electronic and nuclear stopping powers calculated by using the LSS theory. It is found that the etch rate of the nitrocellulose film depends upon the electronic stopping power, but the complete self-development can not be accomplished without the sufficient nuclear stopping power.. 1.. INTRODUCPION. Nitrocellulose is known as a self-developing resist material when which is volatilized exposed to the radiation of ion beams, eliminating the need for the development process.l1,2] In particular, by using nitrocellulose with a focused ion beam (FIB), the advantage of the self-development properties can be shown.131 Previously, we reported a method of drawing self-developed patterns with reproducibility, and demonstrated the aluminum line patterns with a line width of less than 0.a. [41. obtain a uniformly self-developed wide area for surface observation. Six different kinds of ion N+, Ne+. AT+. Kri and Xe+ species such as He+, The 50keV ions were used in the experiments. were directed onto the samples through a copper the mask. In order to investigate mesh ion energy dependence of the selfincident development properties of the nitrocellulose, 1 In the and 2keV Ar+ and 2OOkeV Si++ were used.. In the present work, the ion species dependence of the self-development properties of nitrocellulose films was studied experimentally. From the experimental results and the nuclear and electronic stopping powers calculated by using LSS theory[Sl, the self-development mechanism of nitrocellulose was investigated.. 2.. EXPERIMENTS. In the experiments, nitrocellulose with a 13.5% nitrogen content was used. It was spun on a silicon wafer, and the residual solvent was removed by baking it at 8O'C for 30 min in a flowing nitrogen atmosphere_ The thickness of the nitrocellulose film was controlled at 500550nm by the use of a spin speed of the spinner. The self-development of nitrocellulose film was formulated in a conventional (flood) type ion implantor system in order to. 0167.9317/89/$3.50 0 1989,Elsevier Science Publishers B.V. (North-Holland). DOSE (x ld31cm2,. Fig-l. Change of the thickness of nitrocellulose films with dose, when the films were exposed to the irradiation of 50keV ions such as N+, Ar+ and Xe+..

544. Y. Yasuoka. et al. / Self-development. of nitrocellulose. case. of the irradiation of 2OOkeV Si++, the focused ion beam system (JIBL-100) was used.. self-developed depth of the samples was The measured by Sloan Decktak surface propfiler. The surfaces of the self-developed samples were Spectroscopy observed with the Auger Electron (AES) in order to confirm the existence of nonvolatilized residue.. 3. RESULTS. Figure 1 shows the change of the thickness of films with dose when the the nitrocellulose films were exposed to the irradiation of 5OkeV In the ion beams such as N+, Ar+, and Xe+. the case of the irradiation of Ar+ and Xe+, of the nitrocellulose film decreases thickness in proportion to the dose through the whole The same range of the self-developing process. results were obtained for self-development with Kr+. On the other hand, in the case of the the etch rate gradually irradiation of N+, decreases with the increase of the dose. Figure 2 shows the etch rate at the beginning from the of self-developing process obtained In the figure, of the curves in Fig-l. slope the results obtained for the other ions such as It is found Ne+ and Kr+ are also shown. He+, the etch rate of the nitrocellulose that mass increases with the increase of the atomic of the ions among the light ions such as He+, N+, Ne+ and Ar+, but among the heavy ions such Kr+ and Xe+, the etch rate decreases as Ar+, with the increase of the atomic mass of the rate is ions. Further, the maximum etch obtained at the irradiation of Ar+. depenIn order to investigate the ion energy dence of the self-development process, the experiments using the 1 and 2keV Ar+ and 2OOkeV Si++ were carried out. Figure 3 shows the change of the thickness of the nitrocellulose film when the films were exposed to the irradiation of 1 and 2keV Ar+. The etch rate of the the film is very low in comparison with self-development with 50keV Ar+. However, the etch rate remains constant through the whole range of the self-development process as well as the case of self-development with 50keV Ar+. Figure 4 shows the results when the films were exposed to the 2OOkeV Si++ focused ion beam. The thickness of the 52Onm thick nitrocellulose film quickly decreases as the film was exposed to the ion beam. However, the etch rate slows In the case down a little at the final step. of the nitrocellulose film with the thickness during of lum, the slow down of the etch rate the second half of the experiment becomes distinguishable. Figure 5 shows the Auger electron spectra of the samples which were with self-developed. t;. Eb. Li. ION. lC&-. ENERGY. : 50 keV -= 10’. I I I I I I I I I II 1 I1 I. 150 50 0 100 ATOMIC MASS (A M.U.) Fig.2. Relationship between etch rate and the atomic mass of the irradiated ions. The electronic and nuclear stopping powers culculated by using LSS theory are also shown for comparison.. enough dose to self-develop all of the nitrocellulose film_ curve (al is In the figure, the spectrum observed at the surface of Si wafer before it is spin-coated with the nitrocellulose. With the signals of Si (920eV and 1619eV), the signals of C (272eV) and 0 (510eV) are observed; these signals would be caused by the adsorption of carbon and oxygen in the vacuum system or in the atmosphere, or caused by Si02 formed on the surface of the Si while it was in the atmosphere. Curves (d) and (e.) the samples show the spectra observed with which and Xe+. were self-developed with Ar+ These spectra (a). are the same as spectrum nitrocellulose These facts indicate that the on the surface of the Si wafer would have been removed completely by being self-developed with This idea the heavy ions such as Ar+ and Xe+. which is supported by the experimental results show that the signals of C (272eV) and 0 disappeared by sputtering the samples (510eV) with Ar+ in situ, Curves (b) and (c) show the which were spectra observed with the samples respectively. with N+ and Nei, self-developed In these cases, only the signal of C (272eV) is of Si (92eV and observed and the signals though the are not observed, even 1619eV) were ions for long samples exposed to the that These facts indicate the period. irradiation of the light ions such as N+ and not remove the nitrocellulose film Ne+ could rich instead, left a carbon completely but, residue..

Y.Yasuoka et al. I Self-development. II. 6’. I. II. 1. “I”. 1. DOSE. 545. ‘I. INITIAL THICKNESS OF NITROCELLULOSE : 1200 nm ION. of nitrocellulose. (a). Substrate(Si). : A?. % t. (d). Ar*. u. ( x 10?crn*). Change of the thickness of nitrocelluFig.3. films were lose films with dose, when the exposed to Ar+ of 1 and 2keV.. i. ’. I. I. I. I. I. -I. 0 lym thick Nitrocellulose l. 1500. ELECTRON. ENERGY. (eV. ). Auger electron spctra of Si wafer Fig.5. self-developed nitrocellulose films.. and. 520nm thick Nitrocellulose Figure 6 shows the Auger spectra of the Si wafer and the samples which were self-developed with 1 and 50keV Ar+. It is found from the figure that the nitrocellulose film was completely removed when it was self-developed with 1keV Ar+ as well as when it was self-developed with 50keV or+. These facts indicate that the condition, whether the nitrocellulose film is removed completely or not, does not depend on the incident ion energy, but, instead, depends on the atomic mass of the incident ions.. 4. DISCUSSION. 1 6. Line. Dose. (xlOg/cm). Fig.4. Change of the thickness of nitrocellulose films with line dose, when the films were exposed to the focused 2OOkeV Si++ beam.. In order to interpret the experimental results, we calculated the nuclear and electronic stopping power by using the LSS theory[Sl. The calculated powers are plotted in FIg.2 by solid and broken curves. By comparing the change of the etch rate with that of the stopping powers, it is found that the change of the etch rate is similar to that of the electronic stopping power rather than that of the nuclear stopping power. This suggests that the initial decrease.

546. Y. Yasuoka et al. / Self-development. (a) Substrate(Si). of nitrocehlose. fore, the etch rate decreased a little at the final step of the etching process, although the Si++ was accelerated with strong electric field.. 5.CONCLUSION (b). 1 keV. 500 1000 ELECTRON ENERGY. Ar*. 1500. ( eV ). Fig.6. Auger electron spectra of Si wafer and self-developed nitrocellulose films.. in the thickness of the nitrocellulose film depends on the electronic stopping power. This, however, does not mean that the nuclear stopping power does not contribute to the selfdevelopment process of the nitrocellulose film. Although the electronic stopping power is the etch almost the same between Ne+ and Ar+, rate with Ar+ is larger than that with Ne+. In addition, when the nitrocellulose films were exposed to AT+ and to ions heavier than Ar+, selfthe nitrocellulose films are completely On the developed without leaving any residue. other hand, the nonvolatile residue is formed after self-development, films are when the exposed to Ne+ and to ions lighter than Ne+. from these facts that the It is considered nitrodecrease of the thickness of virgin cellulose film depends on the electronic stopping power. However, in nitrcellulose decomthere is some posed by electronic collision, residue which is difficult to be volatilized by electronic only collision. In such a case, this residue is gradually accumulated during and as such it the self-development process, the decrease of the etch rate of the raises nonThe removal of this nitrocellulose film. volatilized residue is the role of the nuclear collision. The results shown in Figs.3 and 4 are explained by the above idea: The nuclear stopping power of 1keV Arf is 9.8xlO9eV/cm-particle, and it is almost the same as that of 50keV Ar+(Fig.2). This would be the reason why 1keV Ar+ could completely self-developed nitrocellulose film in spite that the etch rate is much lower than In the case of 2OOkeV Si++, that of 50keV AT+. stopping power is 3.8x10geV/cmthe nuclear particle. This is smaller than that of 1keV Ar+ Therealthough it is larger than that of Ne+.. the above From experiments, the following results were obtained: (1) The etch rate of the nitrocellulose film initially increased with the irraincrease of the atomic mass of the diated ion, and then decreased. The maximum etch rate was obtained at the irradiation of Ar+. (2) The irradiation of ions heavier than Ar+ self-developed the nitrocellulose film completely. On the other hand, the irradiation of ions lighter than Ne+ left nonvolatilized residue during self-development. (3) Although the etch rate strongly depended on the ion energy, the condition whether the irradiation of ions left nonvolatilized residue or not did not depend on the ion energy but, instead, depended on the atomic mass of the irradiated ions. These experimental results are explained by the following model: (1) The initial decrease of the thickness of nitrocellulose film strongly depends on the electronic stopping power of incident ions. (2) However, there exists a residue which is difficult to be removed with only the electronic collision, and this residue accumulates during the self-development (3) In order to remove the residue, process. nuclear collision is required. fairly large (41 In order to give the fairly large nuclear collision, the ions with heavy atomic mass need to be used.. REFERENCES. [II. r21 [31 [41 [51. P.andall,J.N., Deutsch,T.F., Geis , M-W., Efremow,N.N., Donnelly,J.P. and WoodhOUSe, J-D., J. Vat. Sci. Technol. Bl (1983) 1178. Harakawa,K., Yasuoka,Y., Gamo,K. and Namba, 355. S., J. Vat. Sci. Technol. B4 (19861 Yasuoka,Y., Harakawa,K., Gamo,K. and Namba, 405. S., J. Vat. Sci. Technol. B5 (1987) Yasuoka,Y., Gamo,K. and Namba, Kaneko,H., 982. S., J. Vat. Sci. Technol. B6 (1988) Lindhard, J., Scharff,M. and Schiott,H.E., K. Dan. Vidensk. Mat. Fys. Medd. 36 (1968) N0.14..