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Jun 28, 2007 - Kyuyoung Heo,Sung-Gyu Park,Jinhwan Yoon,Kyeong Sik Jin ... and tetravinylsilane as silane precursors and oxygen gas as an oxidant and ...
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10848

J. Phys. Chem. C 2007, 111, 10848-10854

Quantitative Structure and Property Analysis of Nanoporous Low Dielectric Constant SiCOH Thin Films Kyuyoung Heo,†,‡ Sung-Gyu Park,‡,§ Jinhwan Yoon,† Kyeong Sik Jin,† Sangwoo Jin,† Shi-Woo Rhee,*,§ and Moonhor Ree*,† Department of Chemistry, National Research Lab for Polymer Synthesis & Physics, Pohang Accelerator Laboratory, Center for Integrated Molecular Systems, Polymer Research Institute, and BK21 Program, Pohang UniVersity of Science and Technology, Pohang 790-784, Republic of Korea, and Department of Chemical Engineering and National Research Lab of Materials and Processes for New Memory, Pohang UniVersity of Science and Technology, Pohang 790-784, Republic of Korea ReceiVed: March 16, 2007; In Final Form: May 16, 2007

We have carried out grazing incidence X-ray scattering measurements and specular X-ray reflectivity analysis of the nanoporous structures of low dielectric constant (low k) carbon-doped silicon oxide (SiCOH) films, which were prepared with plasma-enhanced chemical vapor deposition (PECVD) from vinyltrimethylsilane, divinyldimethylsilane, and tetravinylsilane as silane precursors and oxygen gas as an oxidant and then thermally annealed under various conditions. In addition, we measured the refractive indices and dielectric constants of the dielectric films. The nanoporous SiCOH thin films produced in the present study were homogeneous and had well-defined structures, smooth surfaces, and excellent properties and, thus, are suitable for use as low k interdielectric layer materials in the fabrication of advanced integrated circuits. In particular, the vinyltrimethylsilane precursor, which contains only one vinyl group, was found to produce SiCOH films after PECVD and annealing at 450 °C for 4 h with the highest population of nanopores and the lowest electron density, refractive index, and dielectric constant.

Introduction Continuous improvements in device density and performance have been achieved through feature size reduction and the scaling down of device dimensions to the deep submicrometer level. The coupling of the intermetal capacitance effect with line resistivity is now a limiting factor for the ultra-large-scale integration of electric circuits. To reduce this problem, low dielectric constant (low k) materials are required for use as interlayer dielectric and low resistivity conductors as metal lines, such as copper, are required to replace aluminum and tungsten, which have been widely used in the electric circuits.1,2 Thus, much research effort has been devoted to developing new low k dielectric materials to replace current workhorse dielectrics such as silicon dioxide (k ) 3.9-4.3) and silicon nitride (k ) 6.07-7.0).2-7 Carbon-doped silicon oxide (SiCOH) films have recently gained much attraction from both academia and the microelectronic industry because of their low dielectric constant as well as their good mechanical strength and high thermal stability. Many precursors for the plasma enhanced chemical vapor deposition (PECVD) of SiCOH films have been tested: trimethylsilane,8 tetramethylsilane,9 hexamethyldisiloxane,10 bis* To whom correspondence should be addressed. E-mail: ree@ postech.edu (M.R.), [email protected] (S.W.R.). Tel: +82-54-279-2120. Fax: +82-54-279-3399. † Department of Chemistry, National Research Lab for Polymer Synthesis & Physics, Pohang Accelerator Laboratory, Center for Integrated Molecular Systems, Polymer Research Institute, and BK21 Program, Pohang University of Science and Technology. ‡ K. Heo and S.-G. Park contributed equally to this study. § Department of Chemical Engineering and National Research Lab of Materials and Processes for New Memory, Pohang University of Science and Technology.

(trimethylsilyl)methane,11 tetramethylcyclotetrasiloxane,12,13 vinyltrimethylsilane,14 divinyldimethylsilane,15,16 andtetravinylsilane.15 The chemical compositions and the chemical, physical, and dielectric properties of the resulting SiCOH dielectric films have been extensively studied.8-15 However, only a few studies have reported the film structures of SiCOH dielectrics, which are critical to understanding their dielectric, physical, and mechanical properties.13,17 In the present study, a series of SiCOH dielectric films were prepared on silicon substrates by carrying out the PECVD processing of vinyltrimethylsilane (VTMS), divinyldimethylsilane (DVDMS), and tetravinylsilane (TVS) precursors with oxygen (O2) gas as an oxidant and subsequently annealing in argon ambient under various conditions. The resulting dielectric films were quantitatively investigated using grazing incidence X-ray scattering (GIXS) and specular X-ray reflectivity (SXR) with synchrotron radiation sources. This combined GIXS and SXR analysis provided details of the film structure, electron density, and electron density gradient along the thickness direction, as well as the pore shape, size distribution, and porosity of the films. In addition, the surface morphologies, refractive indices, and dielectric constants of the dielectric films were measured. Moreover, we investigated the effects of thermal annealing on the structures and properties of the dielectric films. Experimental Section A series of SiCOH dielectric films were prepared from VTMS, DVDMS, and TVS precursors (Figure 1) using the PECVD technique as follows. A capacitively coupled plasma reactor was used where a silane precursor was introduced together with O2 gas into the plasma reactor through the shower head, which is also an upper electrode with 6 in. diameter. Both

10.1021/jp072125x CCC: $37.00 © 2007 American Chemical Society Published on Web 06/28/2007

Nanoporous Low Dielectric Constant Thin Films

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Figure 1. Chemical structures of vinyltrimethylsilane (VTMS), divinyldimethylsilane (DVDMS), and tetravinylsilane (TVS).

of the gases were flown vertically toward the substrate on the bottom electrode. The film was deposited on (100) oriented p-type silicon substrates and platinum-coated silicon substrates. A flow rate of the silane precursor was fixed at 10 sccm (standard cubic centimeter per min) and that of O2 gas was varied with total flow rate adjusted at 210 sccm with helium. The film deposition temperature was 25 °C, and the chamber pressure remained constant at 1 Torr (133.322 Pa). After the deposition, as-deposited films were annealed at 400, 450, and 450 °C for 0.5, 0.5, and 4 h in argon ambient, respectively. GIXS measurements were carried out at the 4C1 beamline18 of the Pohang Accelerator Laboratory (PAL).19 The sample-todetector distance was 1163 mm, and an X-ray radiation source of λ ) 0.1608 nm (λ, wavelength) and a two-dimensional charge-coupled detector (2D CCD) (Mar USA) were used (Figure 2a). Samples were mounted on a homemade z-axis goniometer equipped with a vacuum chamber. The incidence angle Ri of the X-ray beam was set at 0.20°, which is between the critical angles of the films and the silicon substrate (Rc,f and Rc,s). Scattering angles were corrected for the variations in the positions of the X-ray beams reflected from the silicon substrate interface with changes in the incidence angle Ri and by a precalibrated copolymer, polystyrene-b-polyethylene-bpolybutadiene-b-polystyrene. A set of aluminum foil strips was employed as semitransparent beam stops because the specular reflection from the substrate was much more intense than the GIXS near the critical angle. SXR data were measured at the 3C2 and 4C2 beamlines of the PAL. Samples were mounted on a HUBER four-circle goniometer, and a scintillation counter with an enhanced dynamic range (Bede Scientific, EDR) was used as a detector. The horizontal beam size at the sample position was ca. 2 mm, and the full width at half-maximum of the direct beam profile measured by a detector scan was 0.015°. The measured reflected intensities were normalized to the intensity of the incident beam, which was monitored using an ionization chamber. Ellipsometric measurements were additionally performed using a spectroscopic ellipsometer (model VASE, Woollam) to obtain a refractive index of the films. The film surface morphology was measured using an atomic force microscope (model Multimode AFM Nanoscope IIIa, Digital Instruments) in tapping mode. The film surface was scanned using an ultralever cantilever (with a 26 N/m spring constant and 268 kHz resonance frequency). Image processing and data

Figure 2. (a) Geometry of GIXS: Ri is the incident angle at which the X-ray beam impinges on the film surface; Rf and 2θf are the exit angles of the X-ray beam with respect to the film surface and to the plane of incidence, respectively, and qx, qy, and qz are the components of the scattering vector q. (b) 2D GIXS pattern measured at Ri ) 0.20° for a SiCOH film deposited with a VTMS precursor and subsequently annealed at 450 °C for 4 h.

analysis were performed using a software program provided by Digital Instruments. The dielectric constant was obtained by capacitance-voltage measurements of metal-insulator-metal structures (Al/0.3 µm thick film/Pt) with Al electrode area of 5.0 × 10-3 cm2 at 1 MHz. Results and Discussion The SiCOH dielectrics prepared as thin films on silicon substrates were examined with GIXS in order to investigate their structures. Figure 2b shows a representative two-dimensional (2D) GIXS pattern, which was obtained from a SiCOH dielectric film prepared from VTMS precursor and subsequently annealed at 450 °C for 4 h. For each measured 2D GIXS pattern, onedimensional (1D) in-plane and out-of-plane GISAXS profiles were extracted at Rf ) 0.18° and 2θf ) 0.22°, respectively, where Rf is the angle between the scattered beam and the film surface and 2θf is the angle between the scattered beam and the plane of incidence. Representative in-plane and out-of-plane GIXS profiles are displayed in Figure 3, which were extracted from the 2D GIXS patterns obtained from the dielectric films prepared from VTMS, DVDMS, and TVS precursors and annealed at 450 °C for 4 h. We attempted to quantitatively analyze the extracted scattering profiles using the following recently derived GISAXS formula4,5

IGIXS(Rf,2θf) =

[

1 1 - e-2Im(qz)d 16π2 2Im(qz)

|TiTf|2I1(q|,Re(q1,z)) + |TiRf|2I1(q|,Re(q2,z)) + |TiRi|2I1(q|,Re(q3,z)) + |RiRf|2I1(q|,Re(q4,z))

]

(1)

where Im(qz) ) |Im(kz,f)| + |Im(kz,i)|, Re(x) is the real part of x, d is the film thickness, Ri and Ti are the reflected and transmitted amplitudes of the incoming X-ray beam, respectively, and Rf and Tf are the reflected and transmitted amplitudes

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Figure 4. Pore radius distribution obtained from the GIXS data analysis of a SiCOH film deposited with a VTMS and subsequently annealed at 450 °C for 4 h.

the monodisperse hard sphere model.20 n(r) is the log normal size distribution function of the pores

n(r) )

1 1/2

e-(ln(r/ro) /2σ ) 2

σ2/2

2

(3)

(2π) roσe

Figure 3. 1D scattering profiles extracted from the 2D GIXS patterns of the SiCOH dielectric films prepared with using VTMS, DVDMS, and TVS, and subsequently annealed at 450 °C for 4 h: (a) in-plane scattering profiles extracted at Rf ) 0.18°; (b) out-of-plane scattering profiles at 2θf ) 0.22°. The symbols are the experimental data and the solid lines were obtained by fitting the data with the GIXS formula for spherical scatterer.

of the outgoing X-ray beam, respectively. In addition, q| ) (qx2 + qy2)1/2, q1,z ) kz,f - kz,i, q2,z ) -kz,f - kz,i, q3,z ) kz,f + kz,i, and q4,z ) -kz,f + kz,i; here, kz,i is the z-component of the wave vector of the incoming X-ray beam, which is given by kz,i ) ko(nR2 - cos2 Ri)1/2, and kz,f is the z-component of the wave vector of the outgoing X-ray beam, which is given by kz,f ) ko(nR2 - cos2 Rf)1/2, where ko ) 2π/λ, λ is the wavelength of the X-ray beam, nR is the refractive index of the film given by nR ) 1 - δ + iξ with dispersion δ and absorption ξ, Ri is the out-of-plane grazing incident angle of the incoming X-ray beam, and Rf is the out-of-plane exit angle of the out-going X-ray beam. qx, qy, and qz are the components of the scattering vector q. I1 is the scattering intensity of the pores in the film, which can be calculated kinematically. To analyze the scattering profiles in the present study, we examined all possible scattering models (sphere, ellipsoid, cylinder, and so on) for the I1 term. We found that a sphere model is the most suitable for the structures in the dielectric film prepared with VTMS precursor and annealed at 450 °C for 4 h

I1 ) c

∫0∞ n(r)υ2(r)|F(qr)|2S(qr) dr

(2)

where c is a constant, υ(r) is the volume of each pore, F(qr) is the spherical form factor, and S(qr) is the structure factor for

where r is the pore radius and r0 and σ are the pore radius corresponding to the peak maximum and the width of the pore radius distribution, respectively. As can be seen in Figure 3, the scattering profiles can be satisfactorily fitted with the GIXS formula for spherical scatterers (eqs 2 and 3). The concordance between theory and experiment indicates that the pores generated in the films are spherical and have a sharp interface with the dielectric SiCOH matrix. For this SiCOH film, the structural parameters determined by analyzing the in-plane scattering profile are identical with those obtained from the out-of-plane scattering profile, indicating that the pores are randomly dispersed within the film. The determined structural parameters are summarized in Figure 4 and Table 1. In contrast, the dielectric films prepared from DVDMS and TVS and subsequently annealed at 450 °C for 4 h produce featureless scattering profiles (Figure 3). These scattering profiles cannot be fitted with the GIXS formula for spherical, ellipsoidal, cylindrical, or disk scatterers buried in films. These results suggest that these dielectric films do not contain any structures or pores or that if such structures are present, they are too small to be detected within the measured q range, or that their quantity is insufficient to produce a scattering profile. We attempted to analyze the featureless scattering profiles, in particular the in-plane scattering profiles, using Guinier’s law.21 The Guinier plots of the scattering profiles and the results of the fitting are shown in Figure 5 and Table 1. As shown in Figure 5, all the Guinier plots can be divided into two regions, a low q region and a high q region, which satisfy the Guinier condition (qRg e 1.0 where Rg is the average radius of pores in a SICOH film). From the Guinier fits of the scattering profiles in the low and high q regions, the Rg values of the pores in the films prepared from DVDMS and TVS were determined to be 1.64-1.85 nm ()Rg(low q)) and 0.49-0.55 nm ()Rg(high q)), respectively, i.e., strongly dependent on the silane precursor (Table 1). The scattering profiles and analysis results indicate that pores with at least two different sizes (Rg(high q) ) 0.490.55 nm and Rg(low q) ) 1.64-1.85 nm) were generated in the dielectric films prepared from DVDMS and TVS precursors but that their populations are very low. The Guinier analysis was extended to the in-plane scattering profile of the dielectric film

Nanoporous Low Dielectric Constant Thin Films

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TABLE 1: GIXS Analysis Results for SiCOH Dielectric Thin Films Prepared from VTMS, DVDMS, and TVS Precursors with PECVD pore size and size distribution a

annealing silane precursor VTMS

DVDMS

TVS

temp (°C) 25 400 450 450 25 400 450 450 25 400 450 450

time (h) 0 0.5 0.5 4 0 0.5 0.5 4 0 0.5 0.5 4

radius of gyration b

roc (nm)

σd

jr e (nm)

Rgf (nm)

Rg(low q)g (nm)

Rg(high q)h (nm)

1.20 (0.03) j -

0.20 (0.01) -

1.21 -

1.28 -

1.33 (0.03) 1.33 (0.03) 1.30 (0.02) 1.28 (0.03) 1.62 (0.02) 1.64 (0.03) 1.85 (0.03)

0.55 (0.04) 0.65 (0.02) 0.63 (0.02) 1.08 (0.03) 0.47 (0.02) 0.49 (0.02) 0.55 (0.01)

-i

a Determined by the best fits of scattering profiles using the GIXS formula. b Determined from the Guinier analysis of the scattering profile. Pore radius determined from the peak maximum of the radius r and the number distribution of pores. d Width of the radius r and the number distribution of pores. e Average pore radius determined from the radius r and the number distribution of pores. f Average radius of gyration determined from the radius r and the number distribution of pores. g Determined from the Guinier analysis of the scattering profile in a low q region (qRg(low q) e 1.0). h Determined from the Guinier analysis of the scattering profile in a high q region (qRg>(high q) e 1.0). i Could not be determined. j Standard error in the determination of pore parameter. c

Figure 5. Guinier plots [ln I vs qy2 (nm-2)] from the in-plane scattering profiles extracted at Rf ) 0.18°. The symbols are the experimental data and the solid lines were obtained by fitting the data with the Guinier’s law: the red and blue lines were obtained by fitted the data at the low and high q regions respectively, which satisfied the Guinier condition.

prepared from VTMS precursor and annealed at 450 °C for 4 h. There are two q regimes in the Guinier fits of the scattering profile of this film, as for the other dielectric films (Figure 5). Rg was determined to be 1.28 nm from the Guinier fit of the scattering profile in the low q region and 1.08 nm from the analysis of the scattering profile in the high q region. The determined Rg values are close to each other, in contrast to the results for the films prepared from DVDMS and TVS precursors, and further the average value is comparable to that (1.28 nm) obtained from the GIXS analysis of the scattering profile. The GIXS measurements and data analysis were also carried out for as-deposited dielectric films and for films annealed for 0.5 h at 400 and 450 °C. The dielectric films were found to produce featureless scattering profiles regardless of the silane precursor (data not shown), i.e., to be similar to those observed for the films prepared from DVDMS and TVS precursors and subsequently annealed at 450 °C for 4 h. Thus, only Guinier analysis was carried out on their extracted in-plane scattering profiles. The findings of the analyses are summarized in Table 1. The scattering profiles and Guinier analysis results show that pores of at least two different sizes (Rg(high q) ) 0.47-0.65 nm

Figure 6. (a) A representative specular X-ray reflectivity (SXR) profile of a SiCOH film deposited with a VTMS and subsequently annealed at 450 °C for 4 h. The symbols are the measured data, and the solid line represents the fit curve assuming a homogeneous electron density distribution within the film except for a thin film surface skin layer, in which the electron density is slightly different. The inset shows a magnification of the region around the two critical angles: Rc,f is the critical angle of the film and Rc,s the critical angle of the Si substrate. (b) A model of the electron density distribution across the film thickness between the silicon substrate and air, which gives the best fit for the SXR profile in (a).

and Rg(low q) ) 1.30-1.62 nm) were generated in the asdeposited dielectric films and the films annealed for 0.5 h at 400 and 450 °C but that their populations are very low. The SiCOH dielectric films were further examined by carrying out SXR measurements and data analysis. Figure 6a displays a representative SXR profile, which was obtained from a dielectric film prepared from VTMS precursor and subsequently annealed at 450 °C for 4 h. The SXR profile clearly

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TABLE 2: SXR, AFM, Refractive Index, and Dielectric Constant Analysis Results for Nanoporous SiCOH Dielectric Thin Films Prepared from VTMS, DVDMS, and TVS Precursors with PECVD annealing silane precursor VTMS

DVDMS

TVS

thickness a (nm)

surface roughness (nm)

Fed (nm-3)

porosity (%)

T (°C)

time (h)

bulk

skin

τSXRb

τrmsc

bulk

skin

film

PSO e

PPMSSQ f

nRg

kh

25 400 450 450 25 400 450 450 25 400 450 450

0 0.5 0.5 4 0 0.5 0.5 4 0 0.5 0.5 4

156.4 156.3 115.8 117.7 133.1 117.5 106.4 89.9 198.3 175.0 134.9 222.5

0.8 0.8 0.8 0.7 1.2 1.3 1.1 0.5 1.6 1.6 2.0 2.4

0.6 0.7 0.7 0.7 0.6 0.8 0.6 0.3 0.3 0.4 0.4 0.4

0.39 0.40 0.40 0.40 0.37 0.40 0.40 0.42 0.43 0.40 0.38 0.36

388 353 339 321 402 377 367 364 436 426 422 415

401 362 359 354 588 560 557 455 560 589 624 624

388 353 339 321 404 379 369 365 437 426 425 417

42.9 47.9 50.0 52.7 40.4 44.1 45.6 46.2 35.5 37.0 37.3 38.5

3.0 11.5 15.0 19.5 -i 5.0 7.5 8.5 -

1.405 1.369 1.356 1.340 1.425 1.397 1.382 1.373 1.512 1.488 1.453 1.432

2.65 2.55 2.53 2.16 3.20 2.86 2.83 2.21 3.40 3.12 3.05 2.50

a Thicknesses of bulk and skin layers. b Surface roughness of film determined from SXR analysis. c Root-mean-square surface roughness of film determined over an area of 3.0 × 3.0 µm2 by AFM analysis. d Electron densities of the bulk layer and the skin layer, and averaged over the entire film. e Relative porosity estimated from the electron density of the film with respect to the electron density (678 nm-3) of silicon oxide (SO). f Relative porosity estimated from the electron density of the film with respect to the electron density (399 nm-3) of polymethylsilsesquioxane (PMSSQ). g Refractive index measured at 633 nm using spectroscopic ellipsometry. h Dielectric constant obtained at 1 MHz by capacitancevoltage measurements. i Relatively smaller porosity than that of polymethylsilsesquioxane (PMSSQ).

shows the critical angles of the film and the substrate (Rc,f and Rc,s) over the qz range 0.2-0.35 nm-1; here qz is the magnitude of the scattering vector along the direction of the film thickness and is defined by qz ) (4π/λ) sin Ri, where λ is the wavelength of the X-ray beam and Ri is the grazing incidence angle. Oscillations between Rc,f and Rc,s are also clearly discernible, which are due to the waveguide modes of the X-ray beam confined in the film. Thus, the reflected intensity is close to the incident intensity, although slightly lower due to a certain degree of X-ray beam absorption in the film. Once Ri exceeds the critical angle of the substrate, the reflected intensity drops sharply. The steeply decaying reflectivity curve is modulated by high-frequency oscillations, which are commonly referred to as Kiessig fringes.22 These fringes are due to the interference between the X-ray beams reflected from the film-air surface and those reflected from the film-substrate interface. As the angle increases, there is an overall decay of the reflected intensity and of the modulation amplitudes, which is due to the film-substrate interface and the surface roughness. The SXR data were quantitatively analyzed using the Parratt formalism to determine structural parameters such as the electron density, the electron density gradient across the film thickness, the surface roughness, and the film thickness.23 As can be seen in Figure 6a, all features of the experimental data are well fitted with the curve based on the Parratt formalism and a structural model (Figure 6b) that assumes the silicon substrate has infinite thickness, a homogeneous electron density distribution throughout the film, and a certain amount of surface roughness. It is noteworthy, however, that the minima of the Kiessig fringes are considerably shallower in the experimental data (Figure 6a) than in the theoretical curve. The shallower depth of the minima in the experimental SXR data may be due to several factors, including slight inhomogeneities in the film thickness on the length scale of the projected beam size, the curvature of the substrate induced by residual stress built up at the substratefilm interface, and so on. The SXR data analysis provides the following structural details. The film thickness was precisely determined to be 118.4 nm, with an average film electron density Fe,f of 321 nm-3; here Fe,f was obtained from the determined Rc,f using the relationship Rc,f ) λ(Fe,fre/π)1/2, where re is the classical electron radius. The film surface roughness is only 0.7 nm (Table 2),

indicative of a very smooth surface. A very thin skin layer (0.7 nm thick) was detected, with Fe,f ) 354 nm-3, which is larger than that (321 nm-3) of the bulk layer. The SXR data and analysis results collectively indicate that a well-defined structure is present in the SiCOH film prepared from VTMS precursor and subsequently annealed at 450 °C for 4 h. The above SXR measurements and data analysis were carried out for the other SiCOH films. The findings of the analyses are summarized in Table 2, and selected SXR data and corresponding analysis results are shown in Figure 7. As can be seen in Table 2, all the dielectric films have a well-defined structure similar to that observed in the film prepared from VTMS precursor and annealed at 450 °C for 4 h. All the films have a very smooth surface with roughnesses (τSXR) in the range 0.40.8 nm. These smooth surfaces were confirmed by atomic force microscopy (AFM) analysis (Figure 8 and Table 2); the rootmean-square surface roughness (τrms) over a surface area of 3.0 × 3.0 µm2 ranged 0.36-0.42 nm, depending on the used silane precursors and the annealing history. A very thin skin layer was observed for all the films, with thicknesses varying in the range 0.5-2.4 nm depending on the precursor and the thermal annealing history. Figure 7 displays the SXR profiles around the Rc,f and Rc,s of the as-deposited dielectric films and the films annealed at 450 °C for 4 h. All of the fits adequately describe the locations of the maxima and minima in the experimental data, giving a precise determination of the films’ critical angles Rc,f. Overall, the Rc,f shift to the low angle region as the number of vinyl groups in the silane precursor is decreased and as the annealing temperature and time are increased (Figure 7). From the measured Rc,f values, the electron densities Fe,f of the dielectric films were determined and the results are summarized in Table 2. The average Fe,f value of the films varies from 321 to 437 nm-3, depending on the precursor and the thermal annealing history (Table 2). For all the films, the Fe,f of the skin layer is always larger than that of the bulk layer (Table 2). The relative porosities of the dielectric films with respect to those of thermally grown silicon oxide and poly(methylsilsesquioxane) (PMSSQ) films, which are used as dielectric interlayer materials in the semiconductor industry, were estimated from the determined Fe,f values.1-4 The results are summarized in Table 2.

Nanoporous Low Dielectric Constant Thin Films

Figure 7. (a) SXR profiles around the two critical angles (Rc,f and Rc,s) measured for the as-deposited SiCOH films using VTMS, DVDMS, and TVS precursors. (b) SXR profiles around the Rc,f and Rc,s measured for the SiCOH films deposited using VTMS, DVDMS, and TVS precursors with subsequent annealing at 450 °C for 4 h. The symbols are the measured data and the solid lines represent the fit curves.

The above GIXS and SXR measurements and analyses provide important information about the structures of the SiCOH dielectric thin films prepared from VTMS, DVDMS, and TVS precursors with PECVD as follows. First, the PECVD processing of the silane precursors with the aid of oxygen gas as the oxidant produces SiCOH dielectric thin films with good quality that have a well-defined structure, a very smooth surface, and a very thin skin layer with a higher electron density than the bulk layer. Second, the Fe,f values of the as-deposited dielectric films are in the range 388-437 nm-3, depending on the silane precursor used in the film formation process. A silane precursor with a higher number of vinyl groups results in a denser dielectric film. As can be seen in Figure 1, all the precursors belong to the same silane family and consist of methyl and vinyl groups in various ratios. Thus the observed results indicate that the vinyl group in the silane precursor makes a more significant, positive contribution to the density of the resulting dielectric film than the methyl group. Moreover, the electron density results and the structural data obtained from the GIXS analysis suggest that a lower population of pores is generated in a film prepared from a silane with a higher number of vinyl groups. Finally, for all the dielectric films, Fe,f is always reduced by thermal annealing. The reduction in Fe,f becomes significant as

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10853

Figure 8. Surface AFM images of SiCOH films prepared from VTMS, DVDMS, and TVS precursors before and after annealing at 450 °C for 4 h.

the annealing temperature and time are increased. Furthermore, this thermal annealing induced reduction in Fe,f is more significant in films prepared from VTMS precursor, which is a silane with only one vinyl group, i.e., the lowest number of vinyl groups of the silanes used in the present study. In addition, our GIXS analysis found that pores were generated with a higher population in the film prepared from VTMS and annealed at a higher temperature for a longer time. These results indicate that the as-deposited film produced from VTMS precursor which has only one vinyl group contains a relatively large amount of thermally labile components (i.e., thermally degradable CxHy phases), and that these labile components are removed by thermal annealing, generating a relatively large number of pores in the film and consequently reducing the film’s Fe,f value. In contrast, the films prepared from DVDMS and TVS precursors, which have two and four vinyl groups respectively, contain smaller amounts of thermally labile components, and so generate pores with low populations in the films and result in only a small reduction in the film’s Fe,f value. These results might be due to the high thermal stability of the vinyl-based cross-links that possibly form in the films during the PECVD process. Such cross-link formation is less likely in a dielectric film prepared from VTMS precursor, which has only one vinyl group. The structural characteristics determined above are directly reflected in the properties of the dielectric films. As shown in Table 2, the as-deposited films have dielectric constants k in the range 2.65-3.40 and refractive indices nR in the range 1.405-1.512, depending on the silane precursor (Table 2). A silane precursor with a higher number of vinyl groups produces a dielectric film with larger k and nR values. However, these

10854 J. Phys. Chem. C, Vol. 111, No. 29, 2007 measured k and nR values are still lower than those of films prepared from tetramethylsilane (4MS).14 The films’ k and nR values were found to be reduced by thermal annealing; the reductions in the k and nR values increase when the thermal annealing is carried out at a higher temperature and for a longer time (Table 2). The k and nR values of the annealed films are lower than those of annealed films prepared from 4MS precursor.14 The above structure and property measurements and analysis results show that that vinyl group substitution in the 4MS precursor helps produce low k SiCOH dielectric films but that the lowest k film can be achieved with a silane precursor with only one vinyl group. The presence of only one vinyl group in methylsilane was found to result in nanopores of Rg ) 1.28 nm with a reasonably high population in the dielectric films resulting from film deposition and subsequent annealing at a high temperature, 450 °C for 4 h, consequently producing porous low k dielectric films. Otherwise, the presence of more vinyl groups results in a higher degree of cross-linking associated with these vinyl groups in the resulting films and restricts the formation of nanopores in the films, which results in only a limited reduction in the film’s k value. Conclusions Quantitative GIXS and SXR analyses were successfully carried out for a series of SiCOH dielectric films with nanometer-scale thicknesses prepared on silicon substrates through the PECVD processing of VTMS, DVDMS, and TVS precursors with O2 and subsequent annealing in argon ambient under various conditions. In addition, spectroscopic ellipsometric and capacitance-voltage measurements were performed on the dielectric films. These analyses found that the PECVD processing of the silane precursors with O2 produces high-performance low k SiCOH dielectric films with homogeneous, well-defined structures and smooth surfaces. Moreover, the films contain nanopores and, as a result, have electron densities and refractive indices that are much lower than those of the current workhorse dielectric, silicon dioxide. Thermal annealing at higher temperatures for a longer time was found to further reduce the electron density, refractive index, and dielectric constant of the films, and to increase the population of the nanopores. In particular, the VTMS precursor, which contains only one vinyl group, was found to produce SiCOH films with the highest population of nanopores and the lowest electron density, refractive index and dielectric constant with PECVD and subsequent annealing at 450 °C for 4 h. The SiCOH thin films produced in this study are homogeneous and have well-defined structures, smooth surfaces, and excellent properties, and so are suitable for use as low k interdielectric layer materials in the fabrication of advanced integrated circuits. Acknowledgment. This work was supported by the National Research Lab Program and the Science Research Center Program (Center for Integrated Molecular Systems at Postech) of the Korea Science and Engineering Foundation, by the Ministry of Commerce, Industry and Resources and the Ministry of Science and Technology (MOST) (System IC 2010 Project),

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