Subnanoporosity Development in Hydrocarbon-siliconoxide Hybrid PECVD Films Elucidated by Variable-energy Positron Annihilation

Hybrid PECVD Films Elucidated by Variable-energy Positron Annihilation Kenji Ito1,∗, Toshitaka Oka2, Chunqing He3, and Yoshinori Kobayashi1 1National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305–8565, Japan 2Institute for Excellence in Higher Education, Tohoku University, Sendai, Miyagi 980–8576, Japan 3Key Laboratory of Nuclear Solid State Physics Hubei Province, School of Physics and Technology, Wuhan University, Wuhan 430072, China


Introduction
Formation of nanometer-scaled pores in silicon oxide films is essential for the development of various functional materials such as gas sensors and separation membranes with high performances [1][2][3].In practical applications, pore interconnectivity is one of the key factors to control target properties such as permeability and flux on the molecular transport through the films.Thus a reliable method for characterizing the interconnectivity is necessary in order to fabricate films with superior performance.
Our group have reported a series of research results on nanoporosity of silicon-oxide-backboned films, fabricated by various methods such as magnetron sputtering [4][5][6][7], sol-gel [8][9][10], plasma enhanced chemical vapor deposition (PECVD) [11][12][13][14], etc., by means of positron annihilation (PA) techniques.Every result demonstrated significantly that PA techniques, especially PA lifetime measurement and Doppler broadening of PA radiation, are quite useful for exploring the nano-scaled holes formed in those films [15].In silicon oxide and related substances, considerable amounts of positrons may form positronium (Ps).When located in open interconnected pores, Ps moves from one pore to another, and it may reach the surface and escape into vacuum, followed by the intrinsic annihilation into 3γ rays.Therefore it is expected that the fraction of 3γ annihilation of the positrons I 3γ is associated with the pore interconnectivity.
In this work, we applied the variable-energy PA technique to the characterization of the interconnectivity of sub-nano-scaled pores formed in hydrocarbon-siliconoxide hybrid films fabricated through PECVD.The prepared PECVD films, containing a sacrificial carbonaceous porogen, were annealed at different temperatures up to 600 • C so that successive nanopore formation due to the decomposed porogen by pyrolysis is examined.I 3γ for the annealed films were measured as a func-tion of positron incident energy E in .The pore interconnectivity is discussed with the Ps diffusion length, evaluated from the E in dependence of I 3γ , in comparison to the amount of decomposed porogen, in order to test the feasibility of the I 3γ measurement for elucidating the interconnectivity of the sub-nano-scaled pores during their formation in thin films.

Experiments
A hydrocarbon-siliconoxide hybrid film was deposited on a 8-inch silicon wafer at 80 • C in a capacitivecoupled PECVD reactor with parallel plate electrodes.Mixtures of TEOS and cyclohexane diluted in an argon gas were used as precursors.The deposition was carried out under a pressure of 150 Pa with 200 W RF power.The obtained film was prebaked at 150 • C under TEOS vapor, followed by annealing at various temperatures up to 600 • C under a dried N 2 flow.The thickness of the annealed PECVD films were 600 nm to 900 nm, determined by means of spectroscopic ellipsometry.More details of the film preparation were reported elsewhere [13].
The chemical structure of the films was analyzed by Fourier transform infrared (FT-IR) spectroscopy.The variation of the refractive index n at a wavelength of 630 nm for the hybrid film during the heating and cooling processes was observed by means of thermoellipsometry.
Positron lifetime measurements were carried out by utilizing an intense pulsed-positron beam generated with an electron linear accelerator [16].A multi-exponential analysis was applied to the recorded positron lifetime data, accumulated for ∼2 M counts, to deduce the longest-lived ortho-Ps (o-Ps) lifetimes.Prior to the measurements, the films were capped with 30-nm-thick nonporous silica by depositing it on the top of the films.
PA γ-ray spectra were measured by using a 22 Na-source-based magnetically-guided beam at various positron incident energies E in from 0 to 20 keV [17].To quantify the o-Ps self-annihilation process, the fraction of positron 3γ annihilation I 3γ , was determined from the low-energy γ-ray counts, accumulated for 0.5 M counts-0.6M counts, in the range between 413 keV and 493 keV.

Results and discussion
Figure 1 shows typical FT-IR spectra for the PECVD films before and after annealing at 600 • C. The pronounced absorption peak around 1100 cm −1 is due to the silicon-oxide network.Incorporation of a hydrocarbonous component into the pre-baked film is confirmed by the absorption peak over  2800 cm −1 to 3000 cm −1 due to the CH stretching vibration.This absorption range is enlarged in Fig. 1(b) with the spectra for the PECVD films annealed at various temperatures T a .As seen in the figure, the absorption intensity due to the CH groups gradually decreases with increasing T a up to 480 • C. Above 500 • C it is unobservable, suggesting the complete decomposition of the hydrocarbonous porogen at that T a range.
Figure 2 shows the result of thermoellipsometry for the pre-baked film, displaying the temperature T dependence of the film refractive index n at a wavelength of 630 nm, observed on ascending and descending T processes.In the heating process, n gradually decreases with increasing T from 20 • C to 400 • C. Above 400 • C n decreases more rapidly as T increases, it approaches ∼1.28 at T = 600 • C, and keeps that value after the cooling down to room temperature.According to the Lorentz-Lorenz equation [18], n is inversely proportional to the overall density of a substance, meaning that n can be a measure of the porosity.Thus the obtained result signifies that the film porosity gradually increase up to 400 • C, and at T > 400 • C the porosity is intensified more along with the remarkable decomposition of the hydrocarbonous porogen as evident in Fig. 1(b).
In order to evaluate the dimension of the porosity introduced into the present films by the annealing at different T , positron lifetime measurements were performed at a positron incident energy E in = 4 keV, corresponding to a mean implantation depth from 160 nm to 240 nm for the present system [13].Table I lists the longest-lived o-Ps lifetimes τ o-Ps , their relative intensities I o-Ps for the present PECVD films.τ o-Ps ranges from ∼3 ns to ∼16 ns, and it increases with increasing annealing temperature T a , indicating that the pores are enlarged along with the increased decomposition of the porogen.In the table is also tabulated the pore radius r, calculated from τ o-Ps based on the following equation [19,20], .
(1) r increases from 0.4 nm to 0.8 nm with increasing T a , illustrating that the pores formed by annealing are in the micropore range, according to the IUPAC classification [21], and their size is enlarged as the amount of the decomposed porogen is increased.
In a film with open, interconnected pores, energetic o-Ps atoms may diffuse back to the surface through those pores continuously losing kinetic energy via collisions with the pore walls [22].Eventually they escape from the film into vacuum, so that they undergo 3γ annihilation there.Hence the probability of 3γ annihilation of the positrons, associated with the amounts of o-Ps emitted from the surface, is dependent on the pore interconnectivity [9] and/or pore tortuosity [22,23].For the present system with pores smaller than 1 nm, a fraction of o-Ps, formed near the surface region, may survive long enough to diffuse out into the vacuum by finding interconnected open pores.
Figure 3 shows the variation of the 3γ decay probability I 3γ for the film samples annealed at various T a up to 600 • C as a function of incident positron energy E in .For the three films annealed with T a < 400 • C I 3γ gradually decreases with increasing E in from 0.1 keV to 1.0 keV, while for Positron incident energy, E in [keV] 3γ decay probability, the films annealed with T a > 440 • C I 3γ keeps a constant value at ∼0.1 keV < E in 0.5 keV, and decreases with increasing E in above ∼0.5 keV.For all the samples I 3γ approaches zero at E in = 10 keV.The higher I 3γ near the surface at E in < 1 keV for the higher T a means more o-Ps emit from the film surface at higher T a .
From the I 3γ -E in curves of Fig. 3, the o-Ps diffusion length L o-Ps is estimated through a fitting to the data based on one-dimensional diffusion models of Eldrup [24] and Xu [25].Here, L o-Ps is ascribed to the average distance that o-Ps travels normal to the surface plane.The fitting results are shown in Fig. 3 by the red broken lines, and the obtained L o-Ps is plotted in Fig. 4 as a function of T a .Fig. 4 illustrates that L o-Ps below 300 • C is in the range of a few nm, comparable with that for bulk polymers [26], and it increases with increasing T a from 400 • C to 500 • C and above that temperature it saturates around 45 nm.We can reasonably divide the T a dependence of L o-Ps into three regions with respect to T a , namely, (I) 150 (III) 500 • C < T a ≤ 600 • C.This T a range is somewhat associated with the case of the FT-IR result.
In comparison with the FT-IR results in Fig. 1(b), we propose a possible illustration for the o-Ps escaping along with the subnanopore formation, according to the three stages on T a (Fig. 5).In the first stage (I), the pore development is limited with small amounts of the decomposed porogen, hence most of the o-Ps escaping occurs in the shallower region.In the second stage (II), as the porosity is further developed with more decomposed porogen, the pore interconnectivity involved with the o-Ps diffusion surpasses the o-Ps percolation threshold [27], so that L o-Ps rapidly extends with the further porogen decomposition.Finally, in the third stage (III), after the complete decomposition of the porogen, L o-Ps saturates around 45 nm.

Summary
We have applied variable-energy PA spectroscopy to the investigation of nanopore formation in hydrocarbon-siliconoxide hybrid films fabricated through PECVD.o-Ps lifetime and IR absorption measurements revealed that the formed pore size increases from 0.4 nm to 0.8 nm in radius with increasing annealing temperature, illustrating that the pores are in the micropore range, and their size is enlarged as the amount of the decomposed hydrocarbon constituent is increased.The o-Ps diffusion length, estimated from the I 3γ -E in curves based on the one-dimensional diffusion model, rapidly increased above an annealing temperature of 400 • C, and saturates around 45 nm after the complete decomposition of the porogen above 500 • C.This suggests that the pore interconnectivity during pore formation by annealing depends on the gradual decomposition of the porogen component.It was demonstrated that the o-Ps 3γ annihilation technique is a useful tool to elucidate the interconnectivity of sub-nano-scale pores in thin films.

Fig. 1 .
Fig.1.FT-IR spectra for the present PECVD films.The absorbance for each spectrum is normalized to the peak intensity at 1080 cm −1 .

Fig. 2 .
Fig. 2. Variation of the refractive index at a wavelength of 630 nm for the hybrid film in the heating and cooling processes, observed by means of thermoellipsometry.The arrows guide to the direction of the temperature change.

Fig. 3 .
Fig.3.Variation of the 3γ decay probability as a function of incident positron energy.The relative uncertainty of I 3γ due to the statistics is smaller than 0.2 % for each point.

Fig. 4 .Fig. 5 .
Fig. 4. Variation of the o-Ps diffusion length, estimated through fitting based on a 1-D diffusion model, as a function of annealing temperature.

Table I .
ortho-Positronium lifetimes and their relative intensities for the hydrocarbon-siliconoxide CVD films.