The evolution of microstructure and surface bonding in SiO2 aerogel film after plasma treatment using O2, N2, and H2 gases

Thin Solid Films 384 Ž2001. 236᎐242

The evolution of microstructure and surface bonding in SiO 2 aerogel film after plasma treatment using O 2 , N2 , and H 2 gases Jean-Jong Kim, Hyung-Ho ParkU , Sang-Hoon Hyun Department of Ceramic Engineering, Yonsei Uni¨ ersity, 134, Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea Received 21 February 2000; received in revised form 3 August 2000; accepted 13 October 2000

Abstract In this work, we investigated the effects of various gases ŽO 2 , N2 , and H 2 . plasma treatment on SiO 2 aerogel films in order to strengthen the film and improve the surface chemical bonding nature of the film. The plasma treatments could reduce the density of silanol ŽSi᎐OH. and ethoxy ŽSi᎐OR. groups. The physical, chemical, and electrical properties of SiO 2 aerogel film through curing with various plasma gases were evaluated. The modification of SiO 2 aerogel film by plasma gas treatment was related to the physical impingement effect of ions, chemical reaction, and irradiated vacuum UV. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: IMD; SiO 2 ; Aerogel film; Plasma treatment; VUV

1. Introduction Development in ultra large scale integration ŽULSI. requires intermetal dielectric ŽIMD. materials of low-␬ Ždielectric constant. because these materials permit us to obtain lower power consumption and reduced crosstalk. Porous SiO 2 aerogel films which contain large internal surface area, typically in the range of 500᎐1000 m2rg, are promising ultra-low dielectric materials with high thermal stability for ULSI interconnecting application. However, SiO 2 aerogel films have been found to have a number of surface terminal bonds that would influence the dielectric property w1x. Plasma treatment has been widely used in industrial treatment to clean or improve the properties of materials surface. In the past, organics and contaminants were removed by harsh chemical methods. Most wet

U

Corresponding author. Tel.: q82-2-2123-2853; fax: q82-2-3655882. E-mail address: [email protected] ŽH. Park..

processes rely on dilution to remove contaminants from the surface. However, even if the greater part of the contaminant is removed, trace amounts of residue are still left on the surface. Residues can be reduced by multiple rinses, but this approach generates an even higher volume of waste and higher disposal expense. Unless the process is carefully controlled, the wet cleaning method can damage conformal coating and other sensitive parts w2x. In contrast, the plasma process is a combination of chemical reactions between surface bonds and radicals formed in the discharge volume, and sputtering effects by ion bombardment w3,4x. The plasma medium consists of radicals, electrons, ions, UV, and soft X-rays. The treatment effects of plasma can include the transformation of liquid films or solid surface contaminants into volatile reaction products that are evacuated by the pumping system. The plasma process uses inexpensive, easily available and easy-to-handle gases only. Most of all, an important characteristic of a plasma is its penetrating power. The gas penetrates into small pores that are difficult or impossible for liquids to

0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 2 7 - 7

J. Kim et al. r Thin Solid Films 384 (2001) 236᎐242

access so that parts with complex shapes can be easily processed with plasma. Due to the technological, environmental, and economical advantages, plasma treatment can be said to be a powerful tool for cleaning applications w5x. The efficiency depends on gas species, RF power, gas pressure, and the properties of materials surface. In the fabrication of sub-micron devices, SiO 2 aerogel film applied by spin-coating and supercritical drying is one of the possible candidates due to its inherent low dielectric constant which result from the high porosity of its internal structure. However, the film contains a lot of surface terminal bonds such as ethoxy group, silanol group, and adsorbed moisture. These terminal groups contribute to an increase in the dielectric constant and leakage current of the film. Therefore, the evaluation of chemical composition and porosity of SiO 2 aerogel film is most important from a microelectronic point of view. We have tried to control the microstructure and surface terminal bonds of SiO 2 aerogel film to be used as intermetal dielectric w6,7x. In this study, we investigated the effects of O 2 , N2 , and H 2 plasma treatments on SiO 2 aerogel films. These gases have different chemical reactivity and atomic mass. The present study aims to investigate the effects of various plasma gases on the film because it is very important to know how plasma gas will improve or degrade SiO 2 aerogel films. 2. Experimental procedures SiO 2 sol was prepared by a two-step process involving acid and base catalysts with tetra-ethoxylane ŽTEOS. as a precursor. We used iso-propyl alcohol ŽIPA. as a solvent. The composition of sol was TEOSrH 2 OrNH 4 OHrHClrIPA ᎏ 1:4:8.2 = 10y3 : 1.8= 10y4 :3 molar ratio. The sol was spin-deposited on a p-type SiŽ100. wafer. Then each spun-on film was immersed in IPA for 24 h and subsequently placed in an autoclave apparatus. The solvent could be extracted from the internal pores between skeletal solid fraction without any shrinkage through the supercritical drying treatment. Plasma treatments were performed in a parallel-plate reactor ŽDUAL-ACE, RIE-type, TEL, Japan.. RF power Ž13.56 MHz. was applied to a top electrode. The bottom substrate electrode was grounded. The two electrodes had a diameter of 17 cm and were mounted with a 6.5 cm-spacing. The temperature of samples during the treatment was kept at approximately 21⬚C. The feed gases were admitted into the reactor through a shower head in the top electrode. High purity oxygen Ž99.995%., nitrogen Ž99.999%., and hydrogen Ž99.999%. gases were used for plasma treatments. The operational parameters were as follows. Plasma exposure time was fixed at 1 min and RF power was varied from 100

237

to 500 W at 50 mt. Gas pressure was varied from 30 to 100 mt at 100 W. Prior to the treatment, the chamber was pumped down to 1 mt. Surface morphology and thickness of films were observed using scanning electron microscope ŽSEM, Hitachi S-4200, Japan. and ␣-step, respectively. Composition and chemical bonding state of films were evaluated using Fourier transform infrared spectroscopy ŽFT-IR. and X-ray photoelectron spectroscopy ŽXPS.. Capacitance᎐voltage ŽC᎐V. measurement was made at 1 MHz on a MOS capacitor structure obtained by depositing Au electrodes using a shadow mask. The backside of Si wafer was etched with HF solution for making a good contact. 3. Results and discussion 3.1. Oxygen plasma treatment Although a lot of chemical reactions are possible in O 2 plasma, oxygen atoms are generally regarded as being the primary reactive species in conjunction with vacuum UV for surface activation w8x. A great degree of oxidation may occur with the oxygen plasma. Fig. 1 shows FT-IR spectra of the SiO 2 aerogel film before and after oxygen plasma treatment. An intensive peak of approximately 618 cmy1 from the Si substrate and a broad peak of approximately 1070 cmy1 from the Si᎐O asymmetric and stretching vibration were contained. This implied that the film had a regular SiO 2 network on the Si substrate. Peaks at 1100 and 1200 cmy1 were due to ᎐OR ŽRs C 2 H 5 . bonds and peaks at 2932᎐2986 cmy1 and 1382᎐1455 cmy1 corresponded to stretching and deformation vibration of C᎐H bonds, respectively w9x. This organic material originated from the incomplete hydrolysis of TEOS and residual IPA. A peak near 962 cmy1 and broad peaks of approximately 3400᎐3750 cmy1 and

Fig. 1. FT-IR spectra of SiO 2 aerogel films: Ža. before and after various O 2 plasma treatment conditions; Žb. 30 mtr100 W, Žc. 50 mtr100 W, and Žd. 50 mtr500 W.

J. Kim et al. r Thin Solid Films 384 (2001) 236᎐242

238

Table 1 Energy of VUV irradiation from O 2 , N2 , and H 2 plasmas w12,13x Plasma gas

Oxygen

Nitrogen

Hydrogen

VUV wavelength Žnm. Energy ŽKcalrmole.

130.5 219.0

120.0r149.2 237.5r191.3

121.5 234.8

1640 cmy1 indicated the presence of Si᎐OH bonds and adsorbed H 2 O. With increasing RF power and pressure, the cleaning process became more effective by supplying higher ion energies. However, the intensity of the ᎐OR peak decreased significantly with increasing RF power and gas pressure, while that of the moisture peak increased. Oxygen radicals from the O 2 plasma broke Si᎐OR bonds and were absorbed to form Si᎐O᎐ dangling bonds that finally induced the generation of Si᎐OH bonds with adsorbed moisture w10x. It is also known that vacuum UV ŽVUV. causes the scission of various chemical bonds in polymer molecules including C᎐C, C᎐H, and C᎐O w11x. As shown in Tables 1 and 2, VUV radiated from oxygen plasma has enough energy to dissociate surface terminal ᎐OH and ᎐OR groups w12᎐14x. As a result, it could be said that the increase in RF power and gas pressure during oxygen plasma treatment was effective for the removal of ᎐OR groups in SiO 2 aerogel films due to the increase of high-energetic ion bombardment, reactive oxygen radicals, and VUV. Fig. 2 shows typical XPS wide scan spectra containing all the elements present on the film. We could easily confirm that the film consisted of Si, O, and C and showed a drastic decrease of carbon after O 2 plasma treatment. Before the plasma treatment, the composition ratio of Si:O:C was 1:2.48:0.9. High O and C content in the film reflected the incomplete hydrolysis of TEOS and residual IPA. However, after the treatment, composition was changed to 1:2.27:0.07. From these results, it could be concluded that O 2 plasma was very effective for removing surface terminal groups and the film content was enough to establish a complete SiO 2 network. Fig. 3 shows the thickness variation of SiO 2 aerogel film with RF power and gas pressure. Generally, the thickness of the SiO 2 aerogel film can be reduced by oxygen plasma treatment due to condensation reaction w15x. The film thickness after the treatment for 1 min at 500 W was reduced from 1000 to 676 nm. With increasTable 2 Bond dissociation energy of various bonding types in ᎐OR and ᎐OH groups w14x Bonding

Si᎐O

C᎐O

C᎐C

C᎐H

O᎐H

Bond dissociation energy ŽKcalrmole.

90

86

88

104

119

Fig. 2. XPS wide scan spectra of SiO 2 aerogel films: Ža. as-prepared and Žb. O 2 plasma-treated Ž50 mtr100 W..

ing RF power, many electrons were generated and electron-impact-process increased the density of ions, radicals, metastable particles, and photons in the plasma. These could diffuse into the porous SiO 2 aerogel film and break Si᎐OR and Si᎐OH bonds so that the film shrunk. Also the microstructure of the film may change due to VUV light irradiation w16,17x. Therefore, the particle size of the film increased and the surface was covered with a dense network of macroscopic chains as shown in Fig. 4. In SEM analysis, the image is constructed by the secondary electrons emitted from the sample. Therefore, in Fig. 4 the dark regions correspond to voids in the SiO 2 aerogel films. In the images of the films treated under 30 and 50 mt at 100 W, the surface was smoother than that of as-prepared films. The surface morphology of the film treated at 500 W under 50 mt showed similar particle coarsening as the film treated at 100 W under the same pressure. However, the surface structure became more porous and this reflected the effect of energetic ions impacting upon the film, i.e. partial surface etching of the film. Fig. 5 shows the dielectric constant of SiO 2 aerogel films as a function of O 2 plasma RF power. As previously described, according to the increase in RF power, a decrease in the amount of surface ᎐OR groups and a reduction in the film thickness were observed. These two changes affected oppositely on the dielectric con-

J. Kim et al. r Thin Solid Films 384 (2001) 236᎐242

239

Fig. 5. Dielectric constant of SiO 2 aerogel films as a function of RF power under 50 mt: Ža. O 2 plasma; and Žb. O 2 plasma and subsequent thermal treatment Ž150⬚C, 30 min, Ar..

Fig. 3. Thickness variation of SiO 2 aerogel films with O 2 plasma according to the variation in Ža. RF power Žpressure is fixed at 50 mt. and in Žb. pressure ŽRF power is fixed at 100 W..

stant of the film. The removal of polar ᎐OR groups induced a decrease in dielectric constant but the reduction in the film thickness corresponding to the decrease in the film porosity induced an increase in dielectric

Fig. 4. SEM images of O 2 plasma treated SiO 2 aerogel films: Ža. as-prepared, Žb. 30 mtr100 W, Žc. 50 mtr100 W, and Žd. 50 mtr500 W.

constant. Furthermore, generated Si᎐OH bonds between Si᎐O᎐ dangling bonds and adsorbed moisture played an important role in increasing the dielectric constant of the film. Therefore, the oxygen plasmatreated film was thermally annealed under an argon atmosphere for 30 min at 150⬚C before C᎐V measurement to eliminate the effect of adsorbed moisture on the measured dielectric constant. The results are given in Fig. 5b. Even though at 500 W the thickness of the plasma-treated film reduced to 68% of the value of the as-prepared film, the dielectric constant still decreased. This showed how undesirable was the adsorption of moisture after oxygen plasma treatment. 3.2. Nitrogen plasma treatment FT-IR spectra in Fig. 6 indicate that the intensities of C᎐H vibrational peaks in SiO 2 aerogel films treated by N2 plasma were reduced, but remained larger than those of the O 2 plasma-treated samples. This phenomenon was assumed to be due to the low reactivity of

Fig. 6. FT-IR spectra of SiO 2 aerogel films: Ža. before and after various N2 plasma treatment conditions; Žb. 30 mtr100 W, Žc. 50 mtr100 W, and Žd. 50 mtr500 W.

240

J. Kim et al. r Thin Solid Films 384 (2001) 236᎐242

Fig. 8. Dielectric constant of SiO 2 aerogel films as a function of RF power under 50 mTorr: Ža. N2 plasma; and Žb. N2 plasma and subsequent thermal treatment Ž150⬚C, 30 min, Ar..

Fig. 7. Thickness variation of SiO 2 aerogel films with N2 plasma according to the variation of Ža. RF power Žpressure is fixed at 50 mt. and in Žb. pressure ŽRF power is fixed 100 W.; solid lines correspond to the N2 plasma treated films and broken lines correspond to the O 2 plasma-treated films.

N2 plasma. N2 plasma mainly consisted of Nq and Nq 2 ions. Nq 2 ions were produced in the plasma bulk and in the sheath by charge exchange collisions. In contrast, Nq ions were generated only in the plasma bulk however in the sheath, they underwent only elastic collisions w18x. The variation of thickness in the N2 plasma-treated samples was smaller than that of O 2 plasma-treated samples as shown in Fig. 7. The thickness of SiO 2 aerogel film treated by O 2 plasma was reduced as a result of the chemical reaction, VUV, and ion bombardment of oxygen. However, the thickness change in N2 seemed to be due to the ion bombardment and VUV of nitrogen. As given in Table 1, the ionic mass and VUV radiation energy of oxygen and nitrogen were almost the same. Then it could be said that a difference in the thickness reduction of two different plasma-treated films was mainly due to a difference in chemical reactivity of the gases. The surface morphologies of N2 plasma-treated SiO 2 aerogel films were found to be similar to those of O 2 plasma treated films by SEM observation.

Fig. 8 shows the variation of dielectric constant of SiO 2 aerogel films as a function of RF power in N2 plasma treatment. Even though the thickness reduction was smaller than in the case of O 2 plasma treatment, the increase in the dielectric constant was larger than for the O 2 plasma. This seemed to be due to the residual ᎐OR and ᎐OH groups as confirmed with the results of FT-IR ŽFigs. 1 and 6.. With N2 plasma-treated SiO 2 aerogel films, there was no improvement in the dielectric constant through the thermal treatment. This was also expected from the FT-IR results because the more ᎐OR and ᎐OH groups that remained, the less moisture was adsorbed. 3.3. Hydrogen plasma treatment Fig. 9 shows FT-IR spectra of H 2 plasma-treated SiO 2 aerogel films with varying RF power and gas pressure. The change in the peak intensity of ᎐OR groups was small compared with those of O 2 and N2

Fig. 9. FT-IR spectra of SiO 2 aerogel films: Ža. before and after various H 2 plasma treatment conditions; Žb. 30 mtr100 W, Žc. 50 mtr100 W, and Žd. 50 mtr500 W.

J. Kim et al. r Thin Solid Films 384 (2001) 236᎐242

plasma-treated films. Physical bombardment effects of hydrogen on the film could be neglected because of the light atomic mass of hydrogen. However, the VUV energy of hydrogen plasma was in a similar range to that in oxygen and nitrogen plasmas. Hence, the chemical reactivity of hydrogen to remove surface terminal ᎐OR and ᎐OH groups could not be distinguished. Fig. 10 shows the thickness variation of SiO 2 aerogel films with increasing RF power and gas pressure of H 2 plasma treatment. As we could expect from the FT-IR results, the reduction in the thickness due to H 2 plasma treatment was smaller than for the other gas plasma treatments. However, for varying gas pressure, no difference was found in the reduced film thickness between 50 and 100 mt. This was due to the fact that, for a light element such as hydrogen, as the gas pressure increased, the plasma intensity decreased owing to the higher collision rate of ions which induced the formation of gas molecules w12x. The change in surface morphology by plasma treatment was similar to that for the other gases and somewhat smoother.

241

Fig. 11. Dielectric constant of SiO 2 aerogel films as a function of RF power under 50 mt: Ža. H 2 plasma; and Žb. H 2 plasma and subsequent thermal treatment Ž150⬚C, 30 min, Ar..

The dielectric constant of SiO 2 aerogel films as a function of RF power in H 2 plasma treatment is given in Fig. 11. The dielectric constants with increasing RF power were slightly reduced. As previously described, removal of ᎐OR and ᎐OH groups during plasma treatment produced Si᎐O᎐ dangling bonds that adsorbed moisture to form Si᎐OH bonds. However, in the case of H 2 plasma treatment, this dangling bonded oxygen could be replaced with hydrogen which prevented moisture from absorbing during the exposure of plasmatreated SiO 2 aerogel film to air. This was why the H 2 plasma-treated films showed a reduced dielectric constant. The H 2 plasma-treated film with RF power of 500 W at 50 mt had almost the same FT-IR spectrum and reduction in film thickness as the N2 plasma-treated film with RF power of 100 W at 50 mt. However, the H 2 plasma-treated film had a smaller dielectric constant than that of the N2 plasma treated film. After thermal treatment at 150⬚C under an argon atmosphere, the dielectric constant of the H 2 plasma-treated films increased. This seemed to be due to the thermal desorption of dangling bonded hydrogen and subsequent moisture adsorption. 4. Conclusions

Fig. 10. Thickness variation of SiO 2 aerogel films with H 2 plasma according to the variation of Ža. RF power Žpressure is fixed at 50 mt. and in Žb. pressure ŽRF power is fixed at 100 W.; solid lines correspond to the H 2 plasma treated films and broken lines correspond to the O 2 and N2 plasma treated films.

Gaseous plasma species could easily penetrate into 1-␮m-thick SiO 2 aerogel films due to their porous nature. SiO 2 aerogel films contained a lot of ᎐OR and ᎐OH groups as surface terminal bonds. Through oxygen plasma treatment, these surface terminal bonds were removed. The removal process was more effective with increasing RF power and gas pressure. Physical bombardment by energetic ions, chemical reaction of reactive oxygen radicals, and VUV irradiation were the main causes for this dissociation of surface terminal

242

J. Kim et al. r Thin Solid Films 384 (2001) 236᎐242

bonds. Removal of surface terminal bonds induced the coarsening of particles due to a condensation reaction. A denser, flatter, and smoother surface was obtained for an increase in RF power and gas pressure. The thickness of the plasma-treated films decreased according to the increase of RF power and gas pressure due to the condensation reaction and physical bombardment effects. However, the removal of surface terminal bonds generated Si᎐O᎐ dangling bonds acting as moisture adsorption sites and Si᎐OH bonds could be formed. Measurement of the dielectric constant of oxygen plasma treated and subsequently thermallytreated film revealed that adsorbed moisture played a very important role on the dielectric property of the film. Nitrogen plasma treatment showed a similar behavior as oxygen plasma treatment. However, because of the lower chemical reactivity of nitrogen than oxygen, the treatment was not as effective as oxygen plasma. Hydrogen plasma was the least effective for the removal of surface terminal groups due to its lightest atomic mass and insignificant reactivity. However after the treatment, the film might be passivated with dangling bonded hydrogen and no more moisture adsorption occurred. Acknowledgements The authors of this paper would like to thank the Samsung Electronics Corporation for its support of this research and Brain Korea 21 project.

References w1x M.H. Jo, J.K. Hong, H.H. Park, J.J. Kim, S.H. Hyun, Microelectron. Eng. 33 Ž1997. 343. w2x F. Fazlin, Solid State Technol. 39 Ž1996. 117. w3x H.F. Dylla, J. Vac. Sci. Technol. A6 Ž1988. 1276. w4x E. Taglauer, Appl. Phys. A51 Ž1990. 238. w5x W. Petasch, B. Kegel, H. Schmid, K. Lendenmann, H.U. Keller, Surf. Coat. Technol. 97 Ž1997. 176. w6x M.H. Jo, H.H. Park, D.J. Kim, S.H. Hyun, S.Y. Choi, J.T. Paik, J. Appl. Phys. 82 Ž1997. 1299. w7x H.R. Kim, H.H. Park, Jpn. J. Appl. Phys. 37 Ž1998. 6955. w8x A.G. Shard, J.P.S. Badyal, J. Phys. C65 Ž1989. 5096. w9x K. Usami, S. Sugahara, K. Sumimura, M. Matsumura, in: C. Chiang, T. T. Wetzel, T-M. Lu, P. S. Ho ŽEds.., Low Dielectric Constant Material IV, Sanfrancisco, U.S.A., April 13᎐17, Materials Research Society Symposium Proceeding 511 Ž1998. Ž1998. 27. w10x P.T. Liu, T.C. Chang, Y.S. Mor, S.M. Sze, Jpn. J. Appl. Phys. 38 Ž1999. 482. w11x V.N. Vasilets, I. Hirata, H. Iwata, Y. Ikada, J. Polym. Sci. 35 Ž1998. 2215. w12x C. Cismaru, J.L. Shohet, Appl. Phys. Lett. 74 Ž1999. 2599. w13x A. Ikeda, T. Sadou, H. Nagashima, K. Kouno, N. Yoshikawa, K. Tshukamoto, Y. Kuroki, Thin Solid Films 345 Ž1999. 172. w14x Lawrence H. Van Vlack, Elements of Materials Science and Engineering, Addison-Wesley Publishing Co. Ž1989.. w15x H.R. Kim, H.H. Park, S.H. Hyun, G.Y. Yeom, Thin Solid films 332 Ž1998. 444. w16x Y. Morimoto, Y. Yasuda, J. Appl. Phys. 85 Ž1999. 7385. w17x F. Poncin-Epaillard, J.C. Brosse, T. Falher, Macromolecules 30 Ž1997. 4415. w18x F. Becker, I.W. Rangelow, K. Mabeli, R. Kassing, Surf. Coat. Technol. 74 Ž1995. 485.