Electrochemical and Solid-State Letters, 4 (1) F3-F5 (2001)

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S1099-0062/2001/4(1)/F3/3/$7.00 © The Electrochemical Society, Inc.

Controllable Change of Porosity of 3-Methylsilane Low-k Dielectric Film D. G. Shamiryan,z M. R. Baklanov, S. Vanhaelemeersch, and K. Maex IMEC, B-3001 Leuven, Belgium A method for controllable increase of porosity of low-k silicon oxycarbide films (SiOCH), deposited by oxidation of 3-methylsilane, has been developed using etching of the SiOCH film by diluted HF solution. The modified SiOCH film is characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectrscopy, and ellipsometric porosimetry. It is found that the chemical composition of the modified SiOCH film remains almost the same during etching. No significant thickness loss is observed, while the pore radius and the film porosity increase with HF dip time. It was concluded that the increase of the pore radius is caused by isotropic etching inside the pores as well as at the film surface. The very low etch rate of SiOCH film by diluted HF and the large difference between the pore radius and the film thickness allows an increase in the porosity without significant thickness loss. This method is a way to prepare ultralow-k dielectric films with higher chemical stability as compared to oxide and silsesquioxane-based porous materials. © 2000 The Electrochemical Society. S1099-0062(00)08-010-X. All rights reserved. Manuscript submitted August 7, 2000; revised manuscript received September 25, 2000. Available electronically November 16, 2000.

Chemical vapor deposited (CVD) silicon oxycarbide films are becoming very popular low-k materials for the advanced interconnects because of their compatibility with the traditional ultralarge scale integration technology and their high chemical stability. In this material, part of the oxygen atoms in the SiO4/2 structure is replaced by -CHx- groups. Because the Si-C bond has less polarizability than the Si-O bond, SiOCH has a lower dielectric constant than SiO2. Moreover, a SiOCH film has a microporous structure1 that is probably related to a partial termination of the Si-O-Si network by a -CH3 radical. The film porosity results in a further decrease of the film permittivity. An as-deposited SiOCH film typically has a k value in the 2.6-2.8 range, which is less than the k value of SiO2 and is comparable with the popular organic low-k films like SiLK. The SiOCH films are more chemically stable than most porous inorganic low-k films like Nanoglass and porous hydrogen- and methylsilsesquioxane (HSSQ and MSSQ) based porous films.2 Therefore, the issues related to dry etching and post-dry-etch cleaning could find more simple solutions. The goal of this work is the controllable increase of porosity (to decrease the k value) of the CVD SiOCH film without changing the chemical composition and material properties. An ambitious goal is preparation of ultralow-k dielectric film with higher chemical stability compared to Nanoglass and porous SSQ-based materials. Several basic ideas were used to develop this method. 1. It was established3 that a diluted HF solution can etch the top surface and the pore walls in porous SiO2 at the same rate. This process leads to a significant change of the film porosity and pore size. The most important physical requirement for such type of modification is that the diffusion rate of active species and reaction products in pores must be much higher than the etch rate of the SiO2 by HF. 2. A CVD SiOCH film is microporous and more resistant to HF. However, SiOCH contains siloxane-like Si-O-Si groups that are attacked by HF.4 Therefore, the process described above can be realized. The uniform etching and/or modification of the SiOCH surface and the pore walls decreases the SiOCH thickness. Although the thickness loss (a few nanometers) is negligible, but this process will have a huge impact on the pore radius and dielectric constant of the film. Using these facts, we supposed that a diluted HF solution should increase the film porosity and the pore radius of the SiOCH film without significant loss of thickness. Experimental The SiOCH (deposited by oxidation of 3-methylsilane) low-k films were deposited by a plasma-enhanced oxidation of (CH3)SiH z E-mail: [email protected]

by N2O at 400°C in the Applied Material P5000 CVD tool.5 An asdeposited SiOCH film had a dielectric constant close to 2.7 and refractive index 1.41-1.43. Then, these films were etched in a diluted HF (2%) solution for various times (up to 10 min). Samples were etched at room temperature in a relatively large volume of solution (500 mL per 1 cm2 sample) and dried by compressed nitrogen. After the etching, the refractive index and the thickness of the films were measured by ellipsometry (Sentech automatic single wavelength SE401 ellipsometer). The chemical composition of the SiOCH films was analyzed by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). The FTIR spectra were recorded on a Bio-Rad FTIR spectrometer to investigate the chemical composition of the HF-modified films. The XPS analysis were done on a Fison SSX100 spectrometer equipped with a monochromatic Al Kα source and concentric hemispherical electron energy analyzer. The depth profiles of the chemical elements were obtained using the built-in ion sputter gun. The porous structure of the films was studied by the ellipsometric porosimetry (EP).6 This method allows the film porosity and pore size distribution (PSD) to be determined by analyzing the change of the refractive index that occurs during the adsorption and desorption cycle of vapors of some organic adsorbates. Toluene vapor was used as an adsorbate. Results and Discussion Initial refractive index (n) and thickness (d) of the SiOCH film were equal to 1.42 and 1000 nm, respectively (Fig. 1). This film has a chemical composition typical for the CVD SiOCH films: IR absorption peaks corresponding to Si-O, C-H, Si-CH3, Si-H, and SiC bonds were observed (Fig. 2).7 The change of the n and d values during the HF treatment is shown in Fig. 1. After a short incubation period of about 1-2 min the refractive index decreases linearly with HF dip time, while the thickness remains almost constant up to 6 min of the HF treatment. Change of the composition of the SiOCH film is not significant even after 8 min etching in a HF solution when thickness of the film begins to decrease. The FTIR spectra before and after HF etching are shown in Fig. 2. After HF treatment, a small peak appeared at about 900 cm-1; it can be identified as a Si-F bond. Moreover, the largest peak identified as a Si-O bond for pristine SiOCH film is slightly shifted toward the higher wavenumber. This shift can be explained by appearance of a small peak at about 1200 cm-1, which corresponds to a C-F bond. It should be noted that no water peak at 3500 cm-1 appeared after the HF treatment. This means that the SiOCH

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Electrochemical and Solid-State Letters, 4 (1) F3-F5 (2001)

Figure 1. The refractive index (a) and the thickness of the SiOCH film as functions of etch time in 2% HF solution.

Figure 2. The FTIR spectra of the SiOCH film before and after 8 min etching in a 2% HF water solution.

Figure 3. The results of the porosity measurements. (a) The adsorbate volume vs. relative pressure for initial SiOCH film (X) and for the same film after 4 min etching in a 2% HF water solution. (b) The pore radius distribution calculated as a derivative of the adsorbate volume with respect to the pore radius. The distributions have been calculated for the adsorption (X) as well as for the desorption (X).

film remains hydrophobic. This statement is also supported by thermodesorption (TDS) data.8 Some increase of adsorbed water has been found by TDS, however, this increased value is less than that after etching the film in O2/CF4/CHF3 plasma. Further, no increase of a k value of the plasma-etched film was found,5 therefore, SiOCH film modified in an HF solution was expected to have no k value increase due to water adsorption. The decrease in the intensities of the peaks is explained by a decrease in the infrared absorption due to the increase of the film porosity (absolute value of porosity is equal to 57% for this film). The concentration profiles of Si, C, O, and F were analyzed by XPS after the layer-by-layer etching by the built-in ion gun. The atomic concentration of these elements normalized to Si content in the blanket SiOCH film were equal to Si:O:C=1.00:0.70:0.53. The surface concentration of silicon is less than in the film volume, and the oxygen concentration is higher (Si:O:C=1.00:1.09:0.72 for the surface). The surface concentration of carbon is almost equal to the volume concentration. The enrichment of the film surface by oxygen and decrease of the silicon concentration are probably related to partial oxidation of the SiOCH film by atmospheric oxygen. Etching of this film in a HF solution slightly changes both surface and volume concentration of the elements. The elements concentration equal to Si:O:C=1.00:0.79:0.74 was found in the film volume and Si:O:C=1.00:1.09:1.00 on the film surface. Therefore, only some increase of the carbon concentration and decrease of the Si

concentration is caused by the HF etching. Additionally, some fluorine (Si:F=1:0.08) was detected both on the film surface and inside the film after the HF etching. An insignificant change of the film composition (increase of carbon concentration and appearance of small Si-F and C-F peaks in the FTIR spectra) can be explained by the partial removal of siloxane groups from the film surface (both top surface and pore sidewalls), and formation of chemisorbed and nonsoluble CFx and SiFx groups. These insignificant changes of the film composition cannot provide the observed decrease of the refractive index. Based on the Lorentz-Lorentz equation, one can conclude that the HF etching changes the film density.6 Therefore, an examination of the film porosity is an important issue. The film porosity and PSD were measured after the different HF etching times. The film porosity was measured by determination of the toluene volume condensed in porous film (open porosity).6 The results of these measurements fit very well to a single-film model even for 2 min HF treatment. This means that HF penetrates throughout a whole film at the early stage of etching. The typical results for 4 min HF etching are shown in Fig. 3. Figure 3a shows the change of the adsorbtive volume as a function of the toluene relative pressure. The adsorption/desorption isotherm for an as-deposited film is typical for a microporous film. The toluene adsorption and desorption occur at the relative toluene pressure P/P0 (where P0 is saturated toluene pressure) below 0.1 and almost no hysteresis loop

Electrochemical and Solid-State Letters, 4 (1) F3-F5 (2001)

Figure 4. The SiOCH film porosities (full, calculated from the refractive index; and open, measured by porosimetry) as functions of etch time in 2% HF solution. Inset: The mean pore radius (calculated from desorption) vs. the full porosity.

is observed. This behavior suggests that the pore radius in the SiOCH is less than 1 nm.6 The relative volume of the open pores is close to 10% of the film volume. The adsorption/desorption isotherms dramatically change after HF etching. The relative volume of the open pores has increased up to 30% and the hysteresis loop between the adsorption and desorption curves becomes typical for a mesoporous film.6,9 However, the low-pressure branch related to the micropores is still observed (Fig. 3b). The mean pore radius calculated from the desorption curve has increased up to 1.6 nm. The pore radius that was calculated from the adsorption curve is two times higher. According to the porosimetry theory,6,9 this difference suggests that the pores can be described well by a model of cylindrical pores (differences in effective radius of curvature of cylindrical and spherical meniscus formed during the vapor adsorption and desorption, respectively). Figure 4 shows the dependence of the film porosity on the HF dip time. The two types of porosity are plotted on the same graph. The first one mentioned above as the open porosity is the relative volume of the toluene adsorbed by the film. The open porosity is related to pores available for the toluene penetration. Therefore, this value gives us information related to the concentration of open pores. Some closed pores may not be available for toluene adsorption. Therefore, the real (full) film porosity that defines the value of a dielectric constant can be higher than the open porosity. The full film porosity was calculated with an assumption that there are no closed pores at the maximal measured toluene porosity (66.7%). There are two reasons supporting this assumption. 1. Analysis of different types of mesoporous low-k films shows that normally all pores are open (interconnected) if the film porosity is higher than 50%6 (the pore volume is higher than the percolation threshold). 2. The adsorption/desorption isotherm of the modified SiOCH film does not have a low-pressure branch related to micropores. If all pores are open, EP allows the refractive index of the film skeleton to be calculated.10 The refractive index of the film skeleton calculated for the film with highest porosity (66.7%) was n = 1.533 that is, an intermediate one between SiO2 (1.46) and CVD SiC (~2.00). This value allows the full porosity of the film to be calculated using equations described in Ref. 6 and 10. According to these

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calculations, the full porosity of the pristine SiOCH film was 18% while the porosity calculated from the amount of adsorbed toluene is 10%. Therefore, 45% of pores in the pristine film were closed (not interconnected). The degree of pore interconnection increases monotonically with HF etch time. The mean pore radius vs. full film porosity is plotted in the inset in Fig. 4. The pore radius increases linearly with increasing film porosity. Thus, the porosity and the mean pore size in a SiOCH film can be changed by controllable etching in a HF solution, without significant change to the film composition or its chemical properties. Such modified films can be used as an ultralow-k dielectric with chemical properties similar to an as-deposited SiOCH film. Conclusions A method for the controllable change of the low-k SiOCH film porosity by diluted HF has been developed. The modified SiOCH film was characterized by FTIR, XPS, and EP. Change of the chemical composition of the modified SiOCH film is insignificant: only a small amount of CFx and SiFx groups remains on pore sidewalls and the top surface. There is no significant thickness loss, and the pore radius and porosity increase with HF dip time. We conclude that the increase of the pore radius is caused by the isotropic etching inside pores as well as the film surface. Due to the very low etch rate (0.6 nm/min) and the large difference between the pore radius (several nanometers) and the film thickness (hundreds of nanometers), this process allows the porosity to be increased without significant thickness loss. The open and full film porosity calculated from the EP measurements. The refractive index of the dense matrix material (film skeleton) was calculated using the assumption that all the pores are interconnected at the highest measured porosity (67%). The refractive index for the film skeleton is 1.533. The mean pore radius increases linearly with the total porosity. The pores are almost cylindrical. It was shown that the total film porosity can be increased until 70% as a function of an etch time without significant thickness loss. Therefore, this process allows the SiOCH film with different porosity to be obtained in a simple and controllable way. The chemical stability of these films is similar to the nonmodified SiOCH films. They are stable in most of the industrially available polymer removing chemicals. Results of this kind of chemical analysis, dry etch behavior, and electrical evaluation of the modified SiOCH film will be reported in future publications. IMEC assisted in meeting the publication costs of this article.

References 1. C. K. Ryu, S.-B. Kim, S.-D. Kim, and C.-T. Kim, Proc. AMC, 41 (1999). 2. J. van Aelst, M. R. Baklanov, and W. Boullart, Unpublished results. 3. M. R. Baklanov, L. L. Vasilyeva, T. A. Gavrilova, F. N. Dultsev, K. P. Mogilnikov, and L. A. Nenasheva, Thin Solid Films, 171, 43 (1989). 4. H. H. Born and M. Prigogine, J. Chem. Phys., 76, 538 (1979) 5. W. D. Gray, M. Loboda, H. Struyf, M. Lepage, M. Van Hove, R. A. Donaton, E. Sleckx, M. Stucchi, F. Lanckmans, T. Gao, W. Boullart, B. Coenegrachts, M. Maenhoudt, S. Vanhaelemeersch, H. Meynen, and K. Maex, Mater. Res. Soc. Symp. Proc., 612 (2000). 6. F. N. Dultsev and M. R. Baklanov, Electrochem. Solid-State Lett., 2, 192 (1999). 7. G. Socrates, Infrared Characteristic Group Frequencies, John Wiley & Sons, New York (1994). 8. D. Shamiryan, M. R. Baklanov, G. Vereecke, S. Vanhaelemeersch, and K. Maex, Paper presented at UCPSS conference (2000) 9. S. J. Gregg and S. W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, New York (1982). 10. M. R. Baklanov and K. P. Mogilnikov, Mater. Res. Symp. Proc., 612, D4.2.1 (2000).