Low-Dielectric-Constant SiOC(-H) Films Prepared from DMDMS and O 2 Precursors by Using Plasma Enhanced Chemical Vapor Deposition

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1 Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007, pp Low-Dielectric-Constant SiOC(-H) Films Prepared from DMDMS and O 2 Precursors by Using Plasma Enhanced Chemical Vapor Deposition Seung Hyung Kim, R. Navamathavan, An Soo Jung and Yong Jun Jang Nano-Thin Film Materials Laboratory, Department of Physics, Cheju National University, Jeju Kwang-Man Lee Faculty of Electrical and Electronics Engineering, Cheju National University, Jeju Chi Kyu Choi Research Institute for Basic Sciences, Nano-Thin Film Materials Laboratory, Department of Physics, Cheju National University, Jeju (Received 6 September 2006) SiOC(-H) films were deposited on p-type Si(100) substrates from dimethyldimethoxysilane (DMDMS; C 4H 12O 2Si) and oxygen precursors with different flow rate ratios by using plasma enhanced chemical vapor deposition. The refractive index and the dielectric constant of the SiOC(-H) film decreased with increasing flow rate ratio. The concentration of the carbon content increased from 5 to 11 % when the precursor flow rate ratio was increased from 50 to 90 %. However, a slight decrease in the carbon concentration was observed when the films were annealed at temperatures up to 400 C. The SiOC(-H) film prepared with a lower flow rate ratio showed a higher dielectric constant. However, the annealed films shows lower dielectric constant than the as-deposited SiOC(- H) films. The lowest dielectric constant of about 2.15 was obtained for the SiOC(-H) film annealed at 400 C at a DMDMS/O 2 flow rate ratio of 70 %. PACS numbers: f, d, Gh, Mb Keywords: SiOC(-H) films, Low-k material, PECVD, DMDMS I. INTRODUCTION Dielectric materials with lower permittivity are required for isolation to solve the problem of resistancecapacitance (RC) delay, cross talk, and increased power consumption in integrated circuit (IC) interconnections. In order to decrease the capacitance between wires, it is necessary to reduce the dielectric constant (or relative dielectric constant) of an interlayer insulating film. These problems can be overcome by choosing proper materials with low dielectric constants (low-κ), so there is a driving demand for researchers to develop new materials with low dielectric constants to solve the shortcomings in ultra large scale integration (ULSI) technology. The introduction of Cu and low-k dielectrics has incrementally improved the situation as compared to the conventional Al/SiO 2 technology by reducing both the resistivity and the capacitance between interconnects [1]. Therefore, development of advanced low-κ interlayer material has become very important. Carbondoped silicon-oxide (SiOCH) dielectrics prepared by us- cckyu@cheju.ac.kr ing the chemical vapor deposition (CVD) technique were reported to be some of the most suitable candidates due to their low-k values and their electrical and integration processing being similar to those of SiO 2 [2, 3]. Because the chemical structure of the SiOC(-H) film is basically composed of a silicon-oxygen backbone with methyl (CH 3 ) group incorporation. These methyl groups have lower polarization and can reduce the film density considerably, hence decreasing the dielectric constant. Various organic and inorganic materials have been proposed as alternatives to SiO 2, including hydrogensilsequioxane (HSQ), silsesquioxane (SSQ), methylsilsisequioxane (MSQ), fluorine doped silicon oxide, fluorinated amorphous carbon, and carbondoped silicon oxide (SiOC) [4 14]. In order to fabricate SiOC(-H) thin films, many inorganic precursors such as methyltrimethoxysilane (MTMS), bistrimethylsilymethane (BTMSM), methyltrimethoxysilane (MTES), and trimethylsilane (TMS), have been used. Obviously, the precursor plays an important role in the fabrication of a low-κ thin film with desirable characteristics for specific device applications. At the same time, the amount of each constituent in the precursor material is

2 Low-Dielectric-Constant SiOC(-H) Films Prepared from Seung Hyung Kim et al responsible for the specific properties of the SiOC(-H) films. In our previous studies, we reported detailed analyzes of SiOC(-H) films deposited by using MTMS, TMS, BTMSM, and MTES precursors [15 18]. Therefore, a detailed study on various precursors is necessary to understand the formation mechanisms in SiOC(-H) films. In this study, we investigated the structural and the bonding configuration of low-dielectric-constant SiOC(- H) thin films deposited using dimethyldimethoxysilane (DMDMS) and oxygen as precursors. The effect of the annealing temperature on the structural properties of the films was studied systematically. II. EXPERIMENTS SiOC(-H) thin films were deposited on p-si(100) substrates by using a mixture of DMDMS (C 4 H 12 O 2 Si) and oxygen gases as precursors in a PECVD system at room temperature. The plasma was generated using a radiofrequency (rf) power supply with a frequency of MHz between the two electrodes. Before depositing the SiOC(-H) thin films, we evacuated the chamber to a pressure less than 10 6 Torr, and we fixed the working pressure at 150 mtorr. The rf power supply was kept as 700 W. The total flow rate of the precursors was maintained as 50 sccm, and the flow rate ratio of R(%) = [DMDMS/(O 2 + DMDMS)] 100 was varied as 50, 60, 70, 80 and 90 (%). The DMDMS precursor is a colorless transparent liquid with a purity of 99 %, a density of g/cm 3, and a boiling point of 82 C. The DMDMS precursor was supplied to the chamber by using Ar as a carrier gas, through a bubbler heated to 40 C, and the gas delivery lines were heated and kept at a constant temperature of 40 C to avoid the DMDMS vapor condensation. The refractive index and the thickness of the deposited film were measured by using an ellipsometer at a wavelength of nm and field emission scanning electron microscopy (FESEM, JSM-6700F), respectively. After deposition, the films were annealed at 100, 200, 300 and 400 C for 30 min in vacuum. Fourier-transform infrared (FTIR) spectroscopy (Bruker, IFS-120HR/FRA- 106S) was carried out in absorbance mode to characterize the chemical bonds in the film. In order to determine the dielectric constant of SiOC(-H) films, we measured the capacitance-voltage (C-V) characteristic for the metalinsulator-semiconductor (MIS) structure [Al/SiOCH/p- Si(100)] at 1 MHz by using a semiconductor parameter analyzer (HP4280A). III. RESULTS AND DISCUSSION Figure 1 shows the FTIR spectra over a wavenumber range between 700 and 4000 cm 1 of as-deposited Fig. 1. FTIR spectra of SiOC(-H) films deposited at room temperature for flow rate ratios of 50, 60, 70, 80 and 90 %. SiOC(-H) films for different flow rate ratios. The characteristics of the SiOC(-H) film showed absorption bands due to Si-O-Si (around 1055 cm 1 ), Si-O-C (around 1105 cm 1 ), Si-CH 3 (around 950 and 1250 cm 1 ), and Si- C 2 H 5 (around 750 cm 1 ) in addition to those of Si substrate. The bonding mode at 1100 cm 1 is due to Si-O-C asymmetric stretching modes in an open-link and Si-C cage-link [19]. There is shoulder at about 1150 cm 1 in all the absorption spectra, assigned to the broad Si-O-C peak, which corresponds to the Si-O-C cage-link structure resulting in more porous SiOC(-H) films [20]. This shoulder at lower frequency indicates an increased number of Si-O-C groups in the film. As expected, fractional methyl groups are incorporated in the film with increasing DMDMS flow rate ratio (see Figure 1). As the flow rate ratio of the precursor was increased, the characteristic band of Si-O-C was well separated in the SiOC(- H). The broad peak between 3700 and 3800 cm 1 corresponds to a stretching of -OH and physisorbed moisture on the surface in several modes. The precursors oxygen and DMDMS molecules are dissociated into highly reactive species that form the SiOC(-H) films with clearly separated absorbance peaks of Si-O-Si and Si-O-C band structures due to complete dissociation of the radicals and ions of the precursors. Figure 2 shows the FTIR spectra of as-deposited and annealed SiOC(-H) films deposited at a flow rate ratio of 70 %. Upon annealing, the Si-O-Si become steeper, and the peak corresponding to Si-O-C band was reduced slightly as shown in Figure 2. No significant spectral change was observed in the absorption spectra of samples annealed up to 400 C, indicating that the thermally treated film retained a substantial structural stability up to this temperature. However, as the annealing temperature was increased from 100 C to higher temperatures, the relative intensity of the Si-O-Si peak also

3 Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007 Fig. 2. FTIR spectra of annealed SiOC(-H) films for a flow rate ratio of 70 %. increased. A distinguishable separation of peak position in the range of 950 to 1250 cm 1 for the Si-O-Si and the Si-O-C bonding structures was consistently observed for all the samples after the annealing treatment. The peak structures of the prominent and clearly separated Si-O- Si and Si-O-C bonds indicate that the existence of caged Si-C bonds and is evidence of enhanced porosity in the film [21]. A way to reduce the dielectric constant of a material is to reduce its weight density by increasing the free volume in the Si-O network [22, 23]. After annealing treatment, numerous Si-O-Si networks are generated due to the rearrangement of the bonding structures in the SiOC(-H) films, as shown in Figure 2. All the films exhibit the same bonding structure, and it is noted that the intensity of the Si-O-Si bond increases and that of Si-O-C bond remains unchanged as the annealing temperature increases. This is attributed to a rearrangement of the chemical bonds in SiOC(-H) during the post-annealing process. Figure 3 shows a deconvolution of the Si-O stretching bond in the region from 950 to 1250 cm 1 of the as-deposited and the annealed films of SiOC(-H) for a flow rate ratio of 70 %. The peak can be decomposed into its four constituents by using Gaussian peak fitting. These four peaks correspond to the Si-O-Si asymmetric stretching (peak position appearing between 1019 and 1023 cm 1 ), the Si-O-C ring-link (between 1057 and 1060 cm 1 ), the open-link (at 1102 and 1108 cm 1 ), and the cage-link (between 1141 and 1155 cm 1 ) structures. The peak position of the Si-O-Si bonding mode slightly shifted with annealing temperature, and this frequency shift in the FTIR spectra is related to changes in the bonding characteristics, such as bond length and bonding structure [24]. Figure 4 shows the relative absorption areas of the Si-O-Si asymmetric stretching, ring-link, open-link, and cage-link modes of the Si-O-C bond for the as-deposited Fig. 3. Deconvolution of the Si-O stretching absorption bond of SiOC(-H) film for a flow rate ratio of 70 % in the region of cm 1 for as-deposited and annealed samples. Fig. 4. Relative absorption areas of the Si-O-Si and the Si-O-C bonding modes of SiOC(-H) films deposited at a flow rate ratio of 70 %. and the annealed SiOC(-H) films for a flow rate ratio of 70 %. The relative absorption area of each bonding structure was deduced from the deconvoluted region between 950 and 1250 cm 1, as depicted in Figure 2. Based on the FTIR spectra presented above for SiOC(-H) films prepared using the DMDMS precursor, we could calculate the relative absorption area for the as-deposited and the annealed samples. The relative absorption areas of the Si-O-Si bond and the Si-O-C ringlink modes increased slightly with annealing as shown in

4 Low-Dielectric-Constant SiOC(-H) Films Prepared from Seung Hyung Kim et al Fig. 5. Relative absorption area (%) of the Si-CH 3 bonds of the SiOC(-H) films deposited at different DMDMS/ (DMDMS+O 2) flow rate ratios. Fig. 7. Dielectric constant of as-deposited SiOC(-H) films as a function of annealing temperature for five flow-rate ratios. Fig. 6. Refractive index of as-deposited SiOC(-H) films as a function of the flow rate ratio. Figure 4. However, the relative absorption area of the Si-O-C open-link structure decreased, in contrast with increasing of Si-O-C cage-link structure, with increasing annealing temperature. From this results, the annealing process of SiOC(-H) films led to a rearrangement of the bonding configurations, resulting in more open-link structures being converted into cage-link structures [19, 21]. These results demonstrate that the increase in the Si-O-C ring-link structure with annealing results in an enhanced porosity in the film. We can infer that the increases in the Si-O-C ring-link and the cage-link structures with annealing results in an enhanced porosity in the film. It is believed that the cage structures of Si-O-Si can produce a nanoporous structure, resulting in a lower film density [22]. Figure 5 shows the carbon content as a function of an- nealing temperature for the SiOC(-H) films deposited at different DMDMS/(DMDMS+O 2 ) flow rate ratios. The relative carbon content is calculated by normalizing the peak height of the Si-O-Si stretch mode over the FTIR spectra from cm 1 as [Si-CH 3 /(Si-O)+(Si- CH 3 )]. The absorption intensities corresponding to the Si-CH 3 wavenumber regions cm 1 and 1250 and 1350 cm 1 were taken into account for the calculations. The amount of carbon content in the SiOC(-H) film increased with the flow rate of the precursors. The relative amount of carbon content in the film deposited at a flow rate ratio of 90 % was found to be two times greater than that of the film deposited at a flow rate ratio of 50 %. Figure 6 shows the refractive index of the as-deposited SiOC(-H) films for different flow rate ratios. The films with a larger fraction of DMDMS, in which more methyl groups were incorporated into the silica network had lower refractive indices because the Si-CH 3 group has larger molar volume and a smaller polarizability than the Si-O group does. It is well known that a lower refractive index may indicate a higher film porosity [25]. Obviously, more cage structures are incorporated in the film at lower deposition temperatures, so the film s density should be smaller. As the flow rate ratio was increased, the refractive index gradually decreased to a value of corresponding to a flow rate ratio of 90 %, as shown in Figure 6. Figure 7 shows the dielectric constant of the asdeposited and the annealed SiOC(-H) films for different flow rate ratios. It is clearly observed that for a higher flow rate ratio results in a low dielectric constant SiOC(- H) films due to abundant incorporation of carbon atoms in the film. A film with a dielectric constant of about 2.15 could be obtained by annealing at 400 C. From a combination of the structural and electrical behaviors

5 Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007 shown in Figures 2, 3, 4, and 5, it is evident that an enhancement of the cage-link mode due to rearrangement of bonding configurations within SiOC(-H) film reduces the dielectric constant. This is attributed to the lower polarizability of Si-CH 3 bonds, which gives rise to the lower dielectric constant. A possible mechanism responsible for the decrease in the dielectric constant in the SiOC(- H) film is an incorporation of an abundant amount of carbon atoms into the film via the -CH 3 network. It is concluded that the bonding rearrangement due to annealing and to the DMDMS flow rate ratio in the films is crucial for obtaining a lower dielectric constant. IV. CONCLUSIONS SiOC(-H) thin films with low dielectric constants were deposited on p-si(100) substrates by means of PECVD with a mixture of DMDMS and oxygen gases at different flow rate ratios. The bonding structure and the dielectric properties of the SiOC(-H) thin films were analyzed as functions of the post-annealing temperature. The FTIR analysis revealed that the SiOC(-H) film was comprised of clearly distinguishable bonds of Si-O-Si and Si-O-C. When the DMDMS flow rate was increased, more and more -CH 3 and -H groups were introduced into the Si- O-Si network. The relative absorbance of carbon in the SiOC(-H) film was increased from 5 to 11 % when the DMDMS flow rate ratio was increased from 50 to 90 %. The results indicated that the abundant incorporation of carbon atoms into the SiOC(-H) film with increasing flow rate ratio causes a reduction in the dielectric constant of the film. A lower refractive index and dielectric constant could be obtained for the SiOC(-H) films prepared at higher flow rate ratios. With increasing annealing temperature, the number of cage-linked Si-O-C structures increased; consequently, the dielectric constant of the films decreased. 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