Dec 12, 2013 - (PA) cross-section ÏPA and effective quantum yield Ï of SiâCH3 photodissociation. ..... anode, which was biased at high voltage (â¼3â4 kV).
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Low-k films modification under EUV and VUV radiation
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys. D: Appl. Phys. 47 025102 (http://iopscience.iop.org/0022-3727/47/2/025102) View the table of contents for this issue, or go to the journal homepage for more
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Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 47 (2014) 025102 (14pp)
doi:10.1088/0022-3727/47/2/025102
Low-k films modification under EUV and VUV radiation T V Rakhimova1 , A T Rakhimov1 , Yu A Mankelevich1 , D V Lopaev1 , A S Kovalev1 , A N Vasil’eva1 , S M Zyryanov1,2 , K Kurchikov1,2 , O V Proshina1 , D G Voloshin1 , N N Novikova3 , M B Krishtab4 and M R Baklanov4 1 2 3 4
Skobeltsyn Institute of Nuclear Physics, Moscow State University, SINP MSU, Moscow, Russia Faculty of Physics, Moscow State University, MSU, Moscow, Russia Institute of Spectroscopy of Russian Academy of Science, ISAN, Troitsk, Russia Interuniversity Microelectronic Centre, IMEC, Leuven, Belgium
Received 24 August 2013, revised 17 October 2013 Accepted for publication 11 November 2013 Published 11 December 2013 Abstract
Modification of ultra-low-k films by extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission with 13.5, 58.4, 106, 147 and 193 nm wavelengths and fluences up to 6 × 1018 photons cm−2 is studied experimentally and theoretically to reveal the damage mechanism and the most ‘damaging’ spectral region. Organosilicate glass (OSG) and organic low-k films with k-values of 1.8–2.5 and porosity of 24–51% are used in these experiments. The Si–CH3 bonds depletion is used as a criterion of VUV damage of OSG low-k films. It is shown that the low-k damage is described by two fundamental parameters: photoabsorption (PA) cross-section σPA and effective quantum yield ϕ of Si–CH3 photodissociation. The obtained σPA and ϕ values demonstrate that the effect of wavelength is defined by light absorption spectra, which in OSG materials is similar to fused silica. This is the reason why VUV light in the range of ∼58–106 nm having the highest PA cross-sections causes strong Si–CH3 depletion only in the top part of the films (∼50–100 nm). The deepest damage is observed after exposure to 147 nm VUV light since this emission is located at the edge of Si–O absorption, has the smallest PA cross-section and provides extensive Si–CH3 depletion over the whole film thickness. The effective quantum yield slowly increases with the increasing porosity but starts to grow quickly when the porosity exceeds the critical threshold located close to a porosity of ∼50%. The high degree of pore interconnectivity of these films allows easy movement of the detached methyl radicals. The obtained results have a fundamental character and can be used for prediction of ULK material damage under VUV light with different wavelengths. Keywords: low-k materials, nanoporous organosilicate glasses, EUV and VUV emission absorption, plasma and radiation damage of low-k films (Some figures may appear in colour only in the online journal)
k materials are porous organosilicate glasses (OSGs), which are deposited by plasma-enhanced chemical vapour deposition (PECVD) and spin-on glass (SOG) technology. The matrix structure of OSG films is similar to conventional SiO2 but contains terminating Si–CH3 groups to keep the materials hydrophobic. Detailed reviews on different low-k materials structure and recent advances in their deposition technology can be found in [1–3]. In semiconductor processing, the
1. Introduction
Dielectric materials with a low dielectric constant (k-value) are needed for interconnects of ULSI devices to reduce resistance– capacitance (RC) delay, dynamic power consumption and crosstalk noise. The reduction of the k-value is achieved by using low-polar chemical bonds (such as C–C, C–H, etc) and introducing porosity. At present, the most popular low0022-3727/14/025102+14$33.00
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© 2014 IOP Publishing Ltd Printed in the UK
J. Phys. D: Appl. Phys. 47 (2014) 025102
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low-k materials are exposed to different plasmas during the patterning and photoresist removal, barriers deposition, post CMP cleaning etc. Many different active and energetic species, such as ions, radicals, vacuum ultraviolet (VUV) and ultraviolet (UV) photons are formed in these plasmas and they can induce modification and degradation (damage) of low-k materials, and change their hydrophobicity due to the removal of methyl groups. The porous structure of low-k materials makes them even more sensitive to modification because of the deep penetration of active species into the films and low density. In order to reduce the plasma damage to an acceptable level, a fundamental understanding of the damage mechanisms is needed. In the Plasma Roadmap 2012 [4], the problem of plasma damage is mentioned as one of the most important topics in low temperature plasma investigations. When the k-value of low-k materials drops below k = 2.3, the pore radius and porosity increase up to ∼3–5 nm and ∼50%, respectively. Large pore size and high porosity create severe problems with plasma processing and integration. These problems are discussed in detail in a recent review paper [5]. If the ion and radical related modifications can be partially controlled by applying optimized plasma chemistry and plasma reactors [5], it is much more difficult to control the damage induced by photons because the plasma species can radiate over a wide spectral range. In a low-pressure plasma, the photons can be emitted in the range of wavelengths starting from VUV and extreme ultraviolet (EUV) to infrared (IR), and their relative intensities can noticeably vary in dependence on the discharge type, input power, gas composition and pressure. Particularly, the most extensive damage is generated by the high-energy photons in the VUV spectral range where OSG materials have significant absorption. For instance, He and Ar plasma used as carrier gases and for the wafer de-chucking process emit the most intensive lines at 58.4 and 106 nm, while a 130 nm line of atomic oxygen is characteristic for the oxygen plasma used in the ash process. The fluorocarbonbased plasmas typically used for etching purposes also have rather intensive emissions in the VUV and UV range: ∼150– 350 nm [6]. The effects of UV radiation on low-k materials have been extensively studied when UV light has been introduced for the curing of OSG materials [2]. It was found that UV light with wavelength shorter than 190 nm damages OSG low-k materials, reduces Si–CH3 concentration and makes the films hydrophilic. At present, broadband light with wavelength longer than 200 nm is considered as being more suitable for this purpose and widely used for UV-assisted thermal curing [2, 7]. In addition to the chemical modification, VUV radiation with photon energy higher than 7 eV (λ < 190 nm) is able to induce different physical effects in OSG films such as photoconduction, photoemission, and photoinjection. It generates trapped charges within the dielectric films, which can seriously degrade electrical properties and reliability of the films [8]. The damage of OSG films by VUV radiation appears to be rather critical during patterning in fluorocarbonbased plasma, when the feature sidewalls are damaged due to the long exposure to VUV photons and radicals. The thickness of the damaged layer may become comparable with the width of etched pattern itself.
The degree and depth of photon-induced modification of OSG films depends on the absorption coefficient and, therefore, the photon’s penetration length (PPL). PPL depends on the material density, structure and photon energy. Since the OSG films are SiO2 -based, it is reasonable to expect that their photon absorption spectra should be similar to fused SiO2 [9]. At the same time, the OSG films also have other chemical bonds, mostly like Si–CH3 [1, 5]. The presence of Si–CH3 groups changes the absorption edge and generates new absorbing states in the band gap [6, 10]. The reduced density of porous OSG films increases PPL in these films and leads to deeper damage. In real plasma etch/strip conditions, the separation of the effects of photon, ions and radicals is challenging. For this reason, experimental studies of plasma damage were carried out both in regular plasma reactors by using special windows (so-called small gap techniques [5, 11, 12]) and specially designed experimental chambers [13–19]. For instance, VUV damage of low-k films was studied by using low-pressure rf discharge (ICP, CCP, TCP) as well as other special sources such as lasers or VUV lamps [17–20]. A special onwafer monitoring technique with neural network processing was reported [6] for tracking and monitoring the low-k film exposure to UV photons during plasma processing. Since OSG films damage is mainly related to the Si–CH3 concentration reduction and the following moisture uptake, most of the theoretical models are based on an evaluation of CH3 removal. So Lee and Graves [16, 17] developed a onedimensional (1D) model to predict the removal of CH3 groups from OSG for low-k films (k = 2.4 and porosity ∼30%) during their exposure to VUV photons and O atoms as a function of the photon fluence and atom flux. This 1D model assumes that the total absorption coefficient is determined by the absorption of two types of bonds: Si–C and Si–O. Shoeb et al [21] developed a phenomenological model of OSG film damage during treatment in He/H2 and Ar/O2 plasmas to describe the damage induced by photons as well as H and O atoms during the plasma cleaning of the film. Both photon transport and atom motion in the pores were described on the base of the Monte-Carlo method and two sorts of absorbers were considered [17]. Since the O–Si–O structure in OSG films is supposed not to absorb at 190 nm wavelength, the Si–CH3 bond dissociation at this wavelength was studied in detail by using the quantum-chemical calculations on model SiOCH clusters [22]. It was concluded that the light absorption at that wavelength leads to the excitation of the first singlet cluster’s state. Then the excitation due to the intersystem crossing transforms to the excited triplet state and after that the Si–CH3 bond dissociation can happen. The details about how the Si–CH3 dissociation will occur with increasing photon energy depend on many factors, such as absorption mechanism and excitation energy transformation channels, structure, porosity of SiOCH films, etc. The effects of EUV and VUV photons on the chemical modification of OSG low-k materials having different structure and porosity (24%–46%) have recently been reported in [18]. The absorption cross-sections averaged on the low-k films and the effective quantum yield of Si–CH3 dissociation were 2
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at temperatures higher than 350 ◦ C may also contribute to an improvement of PMO-material hydrophobicity by the bridge transition to a terminal organic group consuming silanols.
obtained for the photon spectral range from 13.5 to 147 nm. It was shown that the most significant damage happens at 147 nm due to the deeper penetration comparable to the film thickness. This paper reports the results of an extensive study of ULK films damage by VUV radiation with wavelengths 13.5, 58, 106, 147 and 193 nm. Special attention was paid to the influence of porosity on the films damage. A 1D phenomenological model of OSG film damage after the photon absorption and Si–CH3 bond decay was developed to analyse the experimental data on changes in chemical bond concentrations and carbon depth profiles in the films. The paper is organized as follows. Section 2 describes the materials, diagnostics and experimental methods. In section 3, the experimental results on Si–CH3 group depletion in OSG films at various VUV and EUV exposures are analysed. The hydrocarbon decomposition and abstraction under VUV radiation are discussed in section 4. The depth profiles of C, O and Si atoms in VUV damaged films are presented in section 5. The 1D model of CH3 group depletion owing to VUV and EUV radiation absorption and a general comparison with the experimental data are discussed in section 6. Section 7 is devoted to the porosity effects on the OSG film damage. The final conclusions are given in section 8.
2.2. Methods of analysis
The as-deposited low-k films were characterized using Fourier transform infrared (FTIR) spectroscopy, ellipsometric porosimetry (EP) [26] and capacitance measurements with metal (Pt) dots [27]. The modified films after exposure to different VUV photon fluences were evaluated by FTIR spectroscopy (Brucker IFS-66/v), x-ray fluorescence (XRF) spectroscopy and spectroscopic ellipsometry (SENTECH 800). An energy-dispersive x-ray spectroscopy (EDS) tool was integrated with a scanning electron microscope (SEM). The energy of the beam was lower than 3 keV providing the limited beam penetration depth (∼200 nm) for the beam electrons. Thereby it allowed probing changes occurring mostly in the top part of the films. The depth profiles of the modified films were studied by secondary ion mass spectroscopy (SIMS) and Rutherford backscattering spectroscopy (RBS). The SIMS measurements were carried out on a ‘Hiden Analytical’ SIMS complex integrated with an XPS ‘KRATOS AXIS Ultra DLD’ (in the SRC of collective usage in the Chemical Department of MSU) in the raster scanning mode and detecting only the central area at energy of bombarding Ar+ ions 1 keV. RBS measurements were done using a Van der Graaf accelerator (in SINP MSU): ion beam—He2+ , E = 2 MeV, ion fluence 106 pulses. The measurements were made for C, O and Si atoms.
2. Experiment 2.1. Materials
The most important properties of the low-k films studied in this work are presented in table 1. The OSG films, CVD1, CVD2, were prepared by using PECVD technology and broadband UV-assisted thermal curing [2]. The porogen residue free ALK B films were deposited using similar precursors as CVD1 and CVD2 but then they were exposed in downstream He/H2 plasma before UV curing [23]. This approach allows achieving more efficient porogen removal without producing amorphous carbon-like porogen residue. Nanocrystalline silica (NCS) is a thermally cured spin-on low-k film [24]. OP B is pure organic low-k material having only carbon and hydrocarbon bonds in its structure due to it having a rather low k-value and relatively small pore radius and porosity [25]. The porosity effect was studied by using OSG materials with similar chemical compositions and different porosity. The advanced OSG films (SBA) represent periodic mesoporous organosilicates (PMO) and they were prepared by using spinon technology with thermal curing. Preparation of these low-k materials starts from mixture of an organosilicate precursor, representing a certain ratio between tetraethylorthosilicate (TEOS), terminally alkylated silicate ester and alkyl-bridged silicate, and surfactant, th type and concentration of which are largely responsible for the pore structure of the resulting material. Then that mixture undergoes soft baking at 150 ◦ C for 2 min. This step is crucial since in the course of that short annealing the solvent is evaporating and inducing the selfassembly of the surfactant and the formation of the templated organosilicate skeleton. One common way to remove the organic template is to apply another high-temperature annealing. Moreover it has been shown that thermal curing
2.3. VUV sources and experimental procedure
As already mentioned, the wavelengths used in this work were 13.5 nm (EUV) and 58.4, 106, 147 nm and 193 nm (VUV). Discharged Z-pinch in Sn vapour was used as a pulsed source of EUV emission at 13.5 nm. It was initiated by evaporating the required amount of Sn from the surface of melted tin (cathode) by a Nd–YAG laser pulse. Sn vapour further short circuits the 3 mm interelectrode’s gap with the anode, which was biased at high voltage (∼3–4 kV). Energy inputted into the discharge was ∼1–2 J while CE (conversion efficiency) to 13.5 nm emission in 2% spectral band was about 1%. The Z-pinch emission was collected by six grazingincidence cylindrical Mo mirrors. The EUV-treated sample was located in the focus of this cylindrical collector. Special systems of debris and plasma mitigation in front of the collector allowed the long-term operation of the EUV source during hundreds of Mshots. The low-k samples were treated in a special ‘clean’ chamber in the place of a collector focus spot having an approximately homogeneous distribution of EUV intensity on ∼6 mm diameter. The ‘clean’ chamber was separated from the EUV source chamber by a multilayer Zr/Si SPF (spectral purity filter) having transmittance ∼40% at 13.5 ± 0.3 nm. Before the experiments, the clean chamber was evacuated to ∼(3–5) × 10−8 Torr. The experimental set up is schematically shown in figure 1(a). EUV photon fluence 3
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Table 1. Characteristics of the investigated OSG materials.
Sample
CVD1
CVD3
NCS
ALK B
OP B
SBA 2.2
SBA 2.0
SBA 1.8
Type Porosity (%) Pore radius (nm)/neck radius (nm) Thickness (nm) k-value [Si]/1022 (cm−3 ) [Si–CH3 ]/1022 (cm−3 )
PECVD 24 0.8/∼0.7 500 2.5 1.10 0.34
PECVD 28 0.8/∼0.7 185 2.3 0.93 0.30
Spin-on 35 1.2/1 200 2.2 1.08 0.50
PECVD 46 2/1.5 105 2.1 0.9 0.41
Organics 17 0.6/0.6 205 2.3 — —
Spin-on 40 3.6/2.1 218 2.2 1.0 0.41
Spin-on 44 2.8/2.1 217 2.0 0.93 0.41
Spin-on 51 3.2/2.7 214 1.8 0.81 0.41
(a)
(b) MLM Zr/Mo SPF
(c)
ULK Plasmaconfining grid ICP discharge: He plasma – 58.4 nm Ar plasma - 106 nm Xe plasma - 147 nm
Mo collector
Detector of EUV power
ULK samples
ULK samples UV condenser Detector of VUV power
EUV 13.5 nm flux: ~5⋅1012
VUV fluxes: 58.4nm ~4.4⋅1014 ph/(cm2s) 106nm ~4.7⋅1014 ph/(cm2s) 147nm ~1.6⋅1015 ph/(cm2s)
Debris mitigation system Sn Z-pinch
ArF laser Detector of UV power UV 193 nm flux: 15 2 ~8*10 ph/(cm pulse)
Figure 1. Scheme of experiments: (a) with EUV (13.5 nm) source; (b) with VUV 58.4 nm, 106 nm, 147 nm source and (c) with VUV (193 nm) source.
level. The estimates including also resonant trapping of He, Ar and Xe atom emissions under the studied experimental conditions show that more than ∼80–85% of energy emitting by atoms is concentrated in the resonant lines: 58.4 for He, 106 (and 104) for Ar and 147 nm for Xe. In our experiments, it was indirectly confirmed by the measurements of absolute VUV photon flux incident from plasma in the sample direction. The absolute photon flux in the VUV range was measured using an absolutely calibrated detector AXUV100G and sodium salicylate (C7 H5 NaO3 ), which was homogeneously deposited on a glass plate. The sodium salicylate has an approximately constant fluorescence quantum yield close to unity in a range of 40–300 nm and is often used for absolute measurements in the VUV spectral area. The absolute calibration of AXUV100G and the fluorescence quantum yield of the sodium salicylate film (since the quantum yield may partly depend on the deposition method) were validated again at ∼300 nm by using a radiometer (EGEG model 550–1). The VUV photon fluxes from He, Ar and Xe plasmas were measured by using the AXUV100G detector and sodium salicylate in the ‘sample’ scheme when the detector was located in the holder instead of the sample, and the only difference between the plasma emissions with and without using quartz plate in front of detector was taken into account. These measurements were done at different distances from the grid and repeated few times. It allowed providing a quite accurate correction on sodium salicylate damage due to VUV photons exposure and also showing that the radiation trapping actually takes place. Since in our experimental conditions only resonant lines are possible, it indirectly proves that VUV emission from He, Ar and Xe plasmas is mainly concentrated in the respective resonant lines. Moreover, to validate the above-mentioned statement about the prevailing resonant lines emission in the VUV spectra, experiments with filtering the resonant 58.4 nm
per shot was measured by using an absolutely calibrated detector AXUV100G (Opto Diode Corporation) and was ∼5×1012 ph cm−2 . The shot repetition rate of the EUV source was ∼1.6 kHz, which allowed high photon fluences for quite a reasonable time with ∼3–5% stability. The scheme of experiments with VUV sources at 58.4, 106 and 147 nm wavelengths is shown in figure 1(b). As sources of VUV emission at wavelengths of 58.4 nm, 106 nm and 147 nm, magnetized dense ICP plasma in pure He, Ar and Xe respectively was used. The ICP discharge was produced in a quartz tube (56 mm inner diameter) at pressure of about of 50 mTorr and confined between the grounded grid and gas inlet flange. The grid has transparency of ∼50% with an elementary cell size of ∼70 µm, which was comparable with the Debye radius and allowed effective separation of plasma from the downstream area. The OSG film samples were placed in the plasma downstream in 20 mm behind the grid. The sample holder was designed in such a way that only half of the sample was exposed to VUV photons from the plasma, whereas the other half of the sample was covered with a thin quartz plate and thereby could be exposed only to UV photons with λ > 190 nm. The films modification was estimated by evaluating the data for three samples, namely: a pristine one, one exposed to plasma emission in the total spectral range, and one in the range of λ > 190 nm. It is worth noting that no difference or only an extremely small difference was observed between the spectra of pristine films and the films exposed to photons with λ > 190 nm while the difference for the films exposed to VUV photons with shorter wavelengths was so big that often it could be seen with the naked eye. It is well known [28], that VUV emission of noble gas plasma at low pressure is mainly determined by resonant atom emission when the atom ground state is the lower state of the emitting transition and emission goes from the first excited 4
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Table 2. Photon fluences (photons number per second or pulse incident on cm2 ) at the used wavelengths.
Wavelength (nm)
Photon fluence
193 147 106 58 13.5
8 × 1015 ph cm−2 pulse−1 1.6 × 1015 ph cm−2 s−1 4.7 × 1014 ph cm−2 s−1 4.4 × 1014 ph cm−2 s−1 5 × 1012 ph cm−2 pulse−1
line in He plasma were carried out. A filter from thin 150 nm Al foil on the supporting grid with a total transmittance of ∼60% in a range of ∼24–80 nm was used to select the He 58.4 nm line. The radiation flux measurements using the AXUV100G detector and sodium salicylate film were repeated a few times and showed that the emission from the used He plasma in the range λ < 190 is practically totally concentrated in this line and thus the applicability of the resonant line wavelength as VUV emission is quite justified in the given research. An ArF laser was used for exposing the OSG films to 193 nm photons (figure 1(c)). The photon fluences on the samples’ surfaces are presented in table 2.
Figure 2. Example of FTIR data (CVD1 film on Si substrate)—pristine sample and after the VUV 147 nm exposure at 5.2 × 1018 ph cm−2 .
sp3 CH2 (2850 and 2920 cm−1 ) bonds concentrations. Similar phenomena and anticorrelation between Si–CH3 and Si–H groups concentration was reported by Marsik et al [29] after UV curing OSG low-k materials by using monochromatic light with λ = 172 nm. Hydrogen atoms formed during the decomposition and removal of CH3 groups saturate the dangling bonds and form Si–H and Si=CH2 groups. PECVD low-k materials often contain a certain amount of amorphous carbon-like porogen residue. This residue is almost not visible in FTIR spectra because of the lack of polar bonds [7]. However, their hydrogenation can form CHx groups, which are able to absorb IR light [30]. Therefore, the following analysis was focused on the evolution of the fraction of Si–CH3 bonds remaining after OSG films exposure to VUV radiation with increasing photon fluence at each studied wavelength. The evolution of Si–CH3 groups in CVD1, ALK B, CVD3 and NCS films after exposing them to different photon fluences is shown in figures 3(a)–(d), respectively. The experimental data are represented by symbols while the curves are results of modelling (see below section 6). As expected [22], the Si–CH3 depletion is very small at 193 nm light, which is a direct consequence of low photoabsorption (PA). At shorter wavelengths the Si–CH3 depletion increases but tends to get saturated at high fluences. The saturation is related to the limited penetration depth, indirectly confirming the strong absorption of the films. It is seen that the penetration depth is small and correspondingly absorption is high for VUV emissions at 58.4 and 106 nm. The EUV emission at 13.5 nm as well as VUV emission at 147 nm have weak absorption and therefore penetrate deeper into the films. No Si–CH3 bonds are present in organic carbon-based low-k films. Therefore to analyse modifications occurring in organic OP B film the relative changes in intensity of C–Hx peaks at 2750–3000 cm−1 were used. The structure of C–Hx IR bands in OP B film is rather complex indicating the presence of different hydrocarbon groups. Several peaks in this region
3. ULK films damage under VUV/EUV radiation: reduction of Si–CH3 concentration at different photon fluences
The modification of OSG films under exposure to VUV– EUV radiation was mainly evaluated from the changes in FTIR transmission spectra. All changes in FTIR spectra were normalized on the changes in the film thickness. It is worth noting that significant thickness reduction was observed only for organic low-k film OP-B, which became thinner up to 15–20% at the highest VUV photon fluences. The thickness changes in OSG films were negligible and comparable with error in the thickness measurement (∼1–2 nm). The different photon fluences at each wavelength were used to provide a clear observation of VUV radiation exposure to the films. As an example, typical FTIR spectra of CVD1 film before (pristine) and after exposure to 147 nm light are shown in figure 2. The spectrum of Si substrate with a native oxide is also presented. The analysis of the films modification was carried out by tracking the changes in the main features only, namely: O–Si–O suboxide, sp3 CH3 , sp3 CH2 , O–H and Si–CH3 bonds. It was revealed that the evolution of water uptake with increasing VUV photon fluence reproduces the evolution of Si–CH3 bonds depletion well, but with the opposite sign. The change of O–H and Si–CH3 groups’ concentration has similar dynamics and they simultaneously saturate at the same fluence. The evolution of the O–Si–O suboxide peak also reproduces the Si–CH3 evolution since O–Si–O suboxide in OSG materials mainly corresponds to an O3 ≡ Si–CH3 structure on the pore surface. Thus, damage to all of the studied OSG films (that is usually understood as water uptake) due to VUV emission exposure can be evaluated in a first approximation by the degree of Si–CH3 depletion. Reduction of Si–CH3 groups concentration leads to a simultaneous increase in Si–H (2200 and 890 cm−1 ) and 5
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Figure 3. The fraction of Si–CH3 bonds remaining after the OSG films exposure to radiation at different wavelengths as functions of the photon fluence: (a) CVD1, (b) ALK B, (c) CVD3, (d) NCS. Symbols represent experimental data; solid lines are results of modelling. The solid line for 193 nm is a fit of the experimental data.
correspond to few (CH3 )asym around ∼2960 cm−1 , (CH3 )sym around ∼2878 cm−1 , (CH2 )asym around ∼2918 cm−1 , and (CH2 )sym around ∼2854 cm−1 vibration modes [31]. No any significant redistribution in C–Hx peak intensities was observed after exposure to VUV radiation but the total integral C–Hx intensity gradually drops with the increasing photon fluence. The evolution of the remaining fraction of C–Hx bonds in OP B film with the photon fluence at each wavelength is shown in figure 4. The relative depletion of C–Hx bonds in OP B film at 193 nm is notably higher than in OSG films. It is a consequence of the increased PA of organic OP B film at this wavelength. The damage to the OP B film structure by VUV photons also leads to water uptake that became essential at the higher photon fluence. Thus even such a soft VUV emission (in fact, UV emission) was able to have a notable effect on the damage of organic low-k materials. However, the transition to the shorter wavelengths up to 13.5 nm leads to decreasing OP B damage compared with OSG films because of the significant decrease in PA. It is easily seen, even visually, by comparing the same exposures to 193 nm and 13.5 nm emissions.
Figure 4. The fraction of residuary CHx bonds in OP B film after the exposure to the different wavelengths as functions of the photon fluence.
groups. These groups can be related to the remaining porogen and newly formed carbon-rich porogen residues [7]. The evolution of total concentration of hydrocarbon CHx bonds (mainly sp3 CH2 in FTIR spectra) and C/O ratio (from XRF data on C and O densities) with changing photon fluence at each wavelength was analysed. The example of the data for EUV 13.5 nm emission is shown in figure 5.
4. Evolution of hydrocarbon bonds and C/O ratio under exposure to VUV/EUV radiation
As already mentioned, the OSG films in addition to Si–CH3 groups can also contain an amount of other carbon-containing 6
J. Phys. D: Appl. Phys. 47 (2014) 025102
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Figure 5. The evolution of CHx bonds (FTIR data) and C/O ratio (XRF data) with photon fluence (in kilopulses) at the wavelength 13.5 nm. The inset is a schematic picture of EUV/VUV curing.
It is seen that the evolution of CHx bonds (FTIR) correlates well with C/O ratio measured by XRF. As already mentioned, the energy of electron beam during XRF analysis was low providing the limited beam penetration depth smaller than ∼200 nm. Therefore XRF analysis had a higher sensitivity to the top part of the film. The data shown in figure 5 suggest the enrichment of the upper film layers by carbon after exposure to EUV emission. Most probably, it occurs owing to the transport of –CHx (or even –Cx Hy ) fragments upward to the surface after the carbon-containing structure decomposition by EUV photons. The response of the films to EUV emission depends on the absorption rate as well as the carbon content and transport along pores, i.e. size and interconnectivity of the pores. So the maximum response is observed for ALK B film having notable absorption, big pores, high porosity and carbon in its structure. Organic OP B film does not have a similar response because of its low absorption, small pores and low porosity. The response of other OSG films is smaller than for ALK B because of the content of carbon-carbon bonds in the matrix structure. This process is somewhat similar to the known UV curing used for producing pores in films. Apparently VUV photons can lead to the photodecomposition of the hydrocarbons remaining in pores after the pores production. It is worth noting that the rate of VUV curing sharply drops (for a few times and even more) with increasing wavelength from EUV to VUV. EUV photons generate a high-energy tail in the spectrum of secondary electrons almost up to the photon energy. Therefore the hydrocarbons decompose and correspondingly the observed carbon transport inside the interconnected pores is assisted to the greatest extent by the energetic secondary electrons which have larger decomposition cross-sections than photons. Finally it should be specially emphasized that the absorption of porogen residue and any other hydrocarbon fragments is noticeably lower than the absorption of the basic SiOCH material (O–Si–O matrix mainly) because of their small concentration. Therefore both damage and water uptake in OSG SiOCH films following exposure to EUV/VUV radiation is mainly defined by Si–CH3 depletion.
5. The depth profiles of OSG damage following exposure to VUV radiation
To validate the integral picture of OSG films damage following exposure to VUV radiation, the depth profiles of OSG films damage were evaluated using SIMS and RBS. The OSG films undergo a strong chemical modification after exposure to VUV photons. The modification leads to notable matrix effects in SIMS when the yield of ions of any element depends on the surrounding chemical structure. The matrix effects were observed in SIMS of VUV-exposed OSG films: the ion yields for exposed (upper layers) and unexposed (bottom layer) were essentially different. To minimize these effects, the ratio of the measured SIMS signal for C and O atoms was used for the characterization of the carbon depletion profile. As an example, the depth profiles of C/O ratio from SIMS data and C, O, Si profiles (shown as a fraction to the pristine atom concentrations) from RBS data for ALK B film exposed during 7200 s to 58.4 nm, 106 nm and 147 nm emissions are shown in figures 6(a) and (b), respectively. The depth resolution using RBS method was not very high, ∼30 nm (the apparatus resolution function is also shown in figure 6(b)) as in SIMS method but it is sufficient to characterize the penetration depth of VUV light. It should be noted that the Si–CH3 groups concentration is smaller than the total C concentration even in the pristine unexposed OSG films because carbon also presents in other forms [30]. It explains the difference in the levels of signals from Si–CH3 in FTIR and C/O in SIMS and RBS. Even if Si–CH3 bonds are completely depleted in the exposed region, SIMS and RBS show that an amount of carbon is still present. As can be seen in figure 6, there is a clear correlation between the measured profiles of C/O ratio in SIMS and C density in RBS. Since, according to spectroscopic ellipsometry, no change of the films thickness was observed after exposure to VUV emission, SIMS and RBS data can be used for characterizing the PPL and thereby absorption crosssection for each VUV wavelength. The PPLs for 58.4 and 106 nm are comparable and approximately half of the film 7
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Figure 6. The depth profiles of C/O ratio from SIMS data (a) and C, O and Si atoms (shown as a fraction to the pristine atom concentrations) from RBS data (b), in ALK B film exposed to 58.4, 106 and 147 nm emissions during 7200 s (maximal used photon fluence at each wavelength).
thickness, i.e. ∼50 nm. It shows that the absorption crosssections at these wavelengths are similar while PPL at 147 nm is much larger. Such dependence on the wavelength similar to SiO2 absorption since the OSG matrix is based on O–Si–O (siloxane) groups. The deep penetration of 147 nm light into the film produces the strong reduction in carbon concentration over the total film thickness at the high photon fluence. In this sense the VUV emission near the O–Si–O absorption edge can be the most damaging factor during plasma processing if the plasma process allows the accumulation of high photon fluence. It is worth noting that an increase of oxygen concentration is also observed near the surface region of exposed samples (see figure 6(b)). The depth profile of the increased oxygen looks like the carbon depletion profile but with the opposite sign. One possible explanation is water uptake in the damaged layer because of the loss of hydrophobic Si–CH3 groups. It is in agreement with FTIR measurements where an intensive O–H peak is observed after exposure to VUV emission and then room atmosphere.
of two different absorbers in wide range of photons energy (8.4–92 eV) may be an occasional coincidence. However, another scenario of the PA mechanism seems to be more probable. The porous OSG films are amorphous solids. Therefore for considering both PA and radiation-induced damage of OSG film it is quite reasonable to use the standard approach usually applied to solids. Namely, one can suppose that there is a single type of absorber, e.g. Ox SiCy H3y complexes (‘elementary cell’ of solid in the given case), y = 0 or 1, x = 4 − y, where concentration in the films is equal to the Si atoms concentration (further [Si]). Let us assume the same PA crosssections for these complexes σOx SiCy H3y = σPA and use the symbol σPA for the PA cross-section normalized on the total concentration of Si atoms, [Si]. Starting from certain threshold photon energy the PA could lead to the electron excitation of the Ox SiCy H3y complex. For instance, Prager et al [22] have shown that octamethyl- and tetramethylcyclotetrasiloxane as well as hexamethyl- and tetramethyldisiloxane are excited into the first excited singlet state by photons with wavelengths λ < 189–198 nm with further intersystem crossing in an excited triplet state and the scission of the Si–CH3 bond. Similarly, except for ionization (if photon energy is high enough for ionization), the absorption of VUV/EUV photon could lead to the electron excitation of the Ox SiCy H3y complex in a certain excited state. The further decay of this state can happen in various ways, including bond(s) breaking, e.g. fragmentation of Ox SiCH3 while the terminating bonds will break down with the most probability:
6. 1D Model of Si–CH3 bonds depletion in OSG films following exposure to VUV radiation
As has been shown above, the OSG films damage essentially depends on the wavelength. A 1D model for describing the radiation absorption and the Si–CH3 depletion evolution in OSG low-k films was developed to find out the cross-sections of photons absorption and the yield of photo-stimulated Si–CH3 destruction in 13.5–147 nm wavelength range. Initially, the experimental results (see figure 3) related to the Si–CH3 depletion’s dependence on wavelength and photon fluence were analysed by the model using two kinds of the absorbers: Si–O and Si–C bonds, similar to the approach [17, 19]. However it was revealed that the PA due to SiOx bonds was dominating for all OSG films and absorption crosssections σSiOx and σSiCH3 as functions of wavelength had very similar functional behaviour. Such consistent behaviour
Ox SiCH3 + hv → (Ox SiCH3 )∗ → SiOx + CH3 ,
(1)
Si–CH3 bond breaking is treated further as a photodissociation process with cross-section σSiCH3 (λ) = σPA (λ) × ϕ(λ) and effective photodissociation quantum yield ϕ(λ) to be determined. The effective quantum yield ϕ in the range 0 < ϕ < 1 includes the efficiency of CH3 escaping from the porous film with a possible reattachment of –CH3 back to dangling Si-bonds. Thus here the decay of the Si–CH3 bond is treated 8
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Figure 7. (a) PA cross-sections and (b) effective quantum yield of Si–CH3 photodissociation for various radiation wavelengths and different OSG films.
as an irretrievable loss of CH3 . In reality (especially in high vacuum conditions) CH3 groups have a chance to reattach to dangling Si-bonds during CH3 random walk inside the porous channels. The presence of even a small admixture of oxygen can affect the CH3 escaping as it was shown in paper [18]. EUV emission PA is formally considered in the same manner as VUV, but the EUV photons absorption mechanism can be more complex and involve various channels: ionizations, excitations and secondary effects of hot photoelectrons. The approach described above can be realized with the appropriate model equations for radiation flux (intensity) I (z) (photons/(cm2 s)) inside the OSG film (0 < z < L) and the dynamics of Si–CH3 depletion with exposure time t and depth z inside the film: dI (z)/dz = −σPA × [Si] × I (z)
(2)
d[CH3 ](t, z)/dt = −I (z) × σPA × ϕ × [CH3 ](t, z).
(3)
numerical experiments with varying sets of parameters allows establishing (for each radiation wavelength) the true values of PA cross-sections σPA with an error of ten per cent and effective quantum yield ϕ within a factor of 2. The calculated evolution of Si–CH3 bonds in OSG films (CVD1, CVD3, NCS and in ALK B) at exposures to various VUV wavelengths are shown by solid lines in figure 3 along with experimental data. The PA cross-sections σPA and effective quantum yield of Si–CH3 photodissociation process ϕ = σSiCH3 /σPA for studied OSG films are shown in figures 7(a) and (b), respectively. As can be seen in figure 7(a), the PA cross-sections are sensitive to the films’ structure and composition. Nevertheless, the PA cross-sections behave similarly to SiO2 -based materials (like quartz) excepting only 147 nm where too high PA crosssections are observed. The averaged over the studied OSG films PA cross-section at 58.4 and 106 nm wavelengths scaled to the non-porous OSG gives PA coefficients αPA ∼ 8 × 105 cm−1 , which is very close to αPA ∼ 106 cm−1 absorption typical for SiO2 -based materials for these wavelengths [9, 32]. The obtained PA of the non-porous OSG at 147 nm is αPA ∼ 3 × 105 cm−1 . This value is notably higher than αPA ∼ 2 × 104 cm−1 for crystalline SiO2 while essentially lower than αPA ∼ 4 × 106 cm−1 for fused SiO2 [33], but gives a cross-section that is very close to σPA ∼ 10−17 cm2 for the SiO molecule [34]. It should be noted that our model also allows reproducing well the Si–CH3 depletion curve from Lee and Graves’s experiment [17] (PECVD low-k films with k = 2.4 exposed to Xe VUV 147 nm) with PA coefficient αPA = σPA × [Si] = (1.27 ± 0.25) × 105 cm−1 (σPA = (1.28±0.26)×10−17 cm2 ) and photodissociation cross-section σSiCH3 = (7.2 ± 1.8) × 10−18 cm−2 . In contrast to it, Lee and Graves [17] approximated the observed Si–CH3 depletion by photodissociation cross-section σSiC = 8.8 × 10−18 cm−2 with the dominated PA of Si–C bonds αSiO αSiC = 3.6 × 105 cm−1 at this wavelength. It is seen in figure 7(b) that the effective quantum yield ϕ for all OSG films is in the range of ∼0.15–0.4 with some trend to drop slightly with decreasing wavelength. The observed differences in ϕ for different OSG films could be induced by various factors. For example, the drop of ϕ for the shorter wavelengths could be caused by the redistribution of
Here z = 0 corresponds to the surface of OSG film of thickness L, I0 = I (z = 0) is an radiation intensity on a sample surface, [CH3 ] (in cm−3 ) is a concentration of Si–CH3 bonds. It is a challenge to describe numerous experimental data for different wavelengths, photon fluences and OSG films using only two fundamental parameters σPA (λ) and ϕ(λ). The model equations (2) and (3) can be integrated analytically to obtain the depth profiles of radiation intensity and CH3 concentration: I (z) = I0 × exp(−σPA × [Si] × z)
(4)
[CH3 (t, z)] = [CH3 ]0 × exp(−t × σPA ×ϕ × I0 × exp(−σPA × [Si] × z)).
(5)
Here [CH3 ]0 = [CH3 (t = 0)] is the initial concentration of Si– CH3 bonds in the OSG film. The evolution of Si–CH3 column density {CH3 (t)} (Si–CH3 density over the film thickness) can be calculated from equation (5) {CH3 (t)} = [CH3 (t, z)] dz and directly compared with the experimentally measured behaviour of {CH3 (t)}. Numerical solution of equations (4) and (5) does not allow approximating all experimental Si–CH3 depletion curves by using the biunique values of parameters σPA and ϕ (or αPA = σPA × [Si] and σSiCH3 = σPA × ϕ). As a rule, there is an acceptable range of the parameters variations. However, the 9
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absorbed photon energy between the different channels such as photodissociation and photoionization. It is worth noting that the porosity of the used various OSG films was varied from 24% to 46% and it was hard to trace a clear dependence of the film damage on their porosity because of the different structure and composition of the films. The porosity dependence was specially studied for the OSG SBA films of the same type and structure (but with the different porosity) and the results of this study are presented in section 7. The model allows calculating the profiles of radiation intensities I (z) and time behaviour of Si–CH3 profiles in OSG films. As an example, the calculated relative profiles I (z)/I0 in CVD1 film for the different wavelengths are presented in figure 8(a). The time evolution of Si–CH3 depth profiles for two wavelengths 147 and 58.4 nm is shown in figure 8(b). As can be seen, the penetration depth (estimated on the level I /I0 = 0.05) for emissions at 58.4 nm and 106 nm is only ∼50–75 nm whereas 147 nm emission penetrates deeper ∼220–230 nm. As a result, the Si–CH3 depletion layer has a different thickness depending on the emission wavelength. It is important to note that the most damage is expected to be for those OSG films and those wavelengths for which the photon penetration depth correlates well with the film thickness. It also should be noted that there is some uncertainty in the model caused by the fact that the same experimentally observed Si–CH3 evolution curve can be fitted by using a few slightly different sets of the biunique parameters σPA and ϕ. It increases uncertainties in establishing values of σPA and ϕ. First of all, it is connected with the really unknown profiles of photon absorption and Si–CH3 depletion in the depth. Estimations of Si–CH3 depletion depth profile made by using SIMS and RBS techniques (see figure 6) allowed normalizing and fixing more carefully a choice of the σPA and ϕ set. For comparison with the experimental data in figure 6 the model depth profile of Si–CH3 bonds in ALK B film exposed to VUV emissions at 58.4, 106 and 147 nm over 2 h is shown in figure 9. As can be seen, there is a good correlation between the model Si– CH3 depths profile and the measured C/O ratio and C density profiles. The difference in Si–CH3 and C profiles is caused by the presence of other carbon bonds different from Si–CH3 . As mentioned above, FTIR data allow determining some range rather than the exact values of σPA and ϕ. Thus the observed agreement between model and SIMS (RBS) data indirectly proves the model approach used and the obtained values of σPA and ϕ. The developed model of low-k OSG damage by VUV radiation allows predicting both the depth profile and the degree of the damage. And, vice versa, knowing the PA cross-sections and effective quantum yields this model can be used for estimating the VUV radiation intensity and fluence incident on low-k film surface from plasma. Measuring the Si–CH3 depletion dynamics in the film under exposure to VUV radiation in some environment and experiments (for instance, during the plasma treatment) with the unknown VUV radiation intensity I0 one can estimate I0 from fitting the measured data by using equation (5).
Figure 8. (a) Depth profile of VUV radiation in CVD1 film for different wavelengths. Evolution of Si–CH3 depletion in the depth in CVD1 film with exposure time for (b) 147 nm (Xe) and (c) 58.4 nm (He) emissions. The initial Si–CH3 density in the film ∼3.4 × 1021 cm−3 .
7. Effect of porosity on OSG films damage following VUV photons exposure
The porosity of OSG materials is the key property allowing an aggressive reduction of their dielectric constant. However, high porosity and large pore size are the most critical factors obstructing and limiting successful integration of 10
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figures 11(a)–(c), respectively. As can be seen, the degree of Si–CH3 depletion increases with porosity for each VUV wavelength at the same photon fluence. The evolution of the Si–CH3 depletion is similar to that in other OSG films studied previously (see figure 3). Indeed, Si–CH3 depletion in SBA films changes with the wavelength in the same manner as for other OSG films: it saturates at the higher fluence due to the limited photon penetration depth, i.e. because of the strong absorption, whereas the emission at 147 nm penetrates deeper than at 58.4 and 106 nm. However, despite the behaviour of Si–CH3 depletion being functionally the same for SBA 2.2, SBA 2.0 and SBA 1.8 films, the rates of this depletion are essentially different. Modelling of the Si–CH3 depletion evolution in SBA films (see figure 11) has shown that the large difference in the depletion rate cannot be explained only by the difference in the films density (absorber concentrations). The PA cross-sections for SBA 1.8 and SBA 2.0 films appeared to be the same, i.e. in fact not depending on porosity: σPA /(10−17 cm2 ) = 3.75, 2.5 and 2.1 for 58.4 nm, 106 nm and 147 nm, respectively. But the effective quantum yield ϕ of Si–CH3 photodissociation increases with porosity: ϕ (SBA 2.0)/ϕ (SBA 1.8) = 0.6/1, 0.4/0.8, 0.6/0.7 for 58.4 nm, 106 nm and 147 nm, respectively. The averaged VUV spectral range effective quantum yield is shown as function of porosity of all the OSG films studied in figure 12. It is seen that the data obtained for SBA films correlate well with the data obtained with other OSG films. It shows that the chemical structure of OSG films slightly influence the effective quantum yield while the porosity effect is clearly observed. The observed sharp increase in ϕ at reaching porosity about of 50% indicates some critical limit in the reconstruction of the pore’s structure beyond which the enhanced damage occurs. Thus, porosity (also pores interconnectivity) and VUV emission wavelength determine the degree of OSG damage at the same photon fluence. The OSG films having sufficiently high porosity close to the limit value undergo stronger degradation owing to the highest pores interconnectivity. It should also be valid for radicals, since the high interconnectivity will allow deeper penetration of active species into the film. The threshold character of OSG films damage by VUV emission is important from the viewpoint of future strategy in development of ultra-low-k materials. Similar behaviour was first observed when using positron life time spectroscopy (PALS) and interpreted as pores percolation threshold [41]. In PALS [42], positronium (Ps) can diffuse through thick films if the pores are fully interconnected, escape from the film, and annihilate in vacuum with the tell-tale lifetime of ∼140 ns. Partially interconnected pores will result in a much smaller measured Ps diffusion length that is determined by the pore interconnectivity. By straightforwardly measuring the fraction of Ps that escape from the film Fesc as a function of mean positron implantation depth one can deduce the pore interconnection length Lint of the mesopores. PALS depth profiling method allows defining the average depth over which porogen-induced pores (mesopores) are connected to the surface that is a physically simple and direct interpretation of
Figure 9. The depth profiles of model Si–CH3 bonds depletion in ALK B film exposed to 13.5, 58.4, 106 and 147 nm emissions for 7200 s (corresponding to the maximum photon fluence at each wavelength).
ultra-low-k materials into advanced ULSI devices [5]. Porosity deteriorates the mechanical and electrical characteristics of dielectric materials, reduces their compatibility with diffusion barriers, stimulates their degradation during technological processing and reduces the reliability of integrated structures. To meet ULSI technology’s interconnects requirements for 10 nm technology nodes and beyond, a new generation of low-k materials with the k-value smaller than 2.3 (porosity up to ∼45–55%) should be developed. As already mentioned, to obtain valuable data about the porosity influence on OSG film damage, the films with the same (or similar) chemical composition but different porosity should be used. Presently, periodic mesoporous organosilicates (PMO) [35–37] prepared by spinon technology are considered as possible candidates for future technology nodes because the application of self-assembly approaches and chemistries allows the control of properties and characteristics to a certain extent [38, 39]. Three different experimental materials were prepared by SBA Materials, Inc. by spin-on technology and thermal curing5 . Films SBA 2.0 and SBA 1.8 have a similar chemical composition while SBA 2.2 has slightly different composition, with a higher concentration of suboxide and bridging CH2 groups on a pore surface. The major film parameters are shown in table 1. One principal feature is related to their pore structure. These films have remarkably different neck and void sizes of interconnected pores, as determined from the adsorption and desorption isotherms using ellipsometric porosimetry [40] (figure 10). The neck sizes of SBA 2.0 and SBA 2.2 are very similar (2.1 nm), while SBA 2.2 has a larger void size. At the same time, SBA 1.8 has a larger neck size but the void size is intermediate between SBA 2.0 and SBA 2.2. The evolution of Si–CH3 depletion in SBA 2.2, SBA 2.0 and SBA 1.8 films during exposure to different photon fluences at wavelengths of 58.4 nm, 106 nm and 147 nm is shown in 5 It is necessary to note that the standard SBA material with k = 2.2 and developed for the integration purpose (trademark name is uLK 222TM ) has pore radius (both neck and void) of about 1.5 nm.
11
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Figure 10. The pore shape of thermally cured SBA films.
pore connection. It has been noted that once Ps is in these mesopores it is energetically trapped from returning to the micropores of the low-k matrix and therefore Ps diffusion is governed solely by the mesopore morphology [43]. In our experiments, the CH3 radicals formed after Si–CH3 bonds breaking diffuse through the interconnected pores to the gas phase. From this viewpoint the situation is similar to Ps in PALS. If the pores are large enough, the CH3 transport through the interconnected pores is easy and the effective quantum yield of Si–CH3 depletion is large. If the degree of pores interconnectivity is small, CH3 radicals can be reattached back to Si-bonds or re-deposited on the pore wall and, therefore, the measured quantum yield is reduced. The critical threshold observed in our experiments is close to 50% porosity while PALS, normally, gives the threshold at 25–30% of porosity in the most porous OSG films. A possible explanation is that both pore size and interconnectivity of the pores can be reduced during the VUV light exposure because of re-deposited carbon. One pertinent remark is that in the case of PALS and in our case the percolation threshold does not mean transition from absolutely not connected to completely interconnected pores. The nature of this threshold is more complicated, as was demonstrated in the case of PALS [43] and, in fact, depends on the methods that were used for its evaluation.
The values of photoabsorption cross-sections σPA and effective quantum yields ϕ at different wavelengths were determined for all studied OSG films from fitting of experimental data by using the developed model of Si–CH3 evolution. The VUV emissions at 58.4 and 106 nm have the highest photoabsorption cross-sections and therefore cause the most significant Si–CH3 damage in the upper layers ∼50– 100 nm. Only partial damage will occur in the deeper layers of OSG films because of the poor penetration depth of this emission. The most significant damage integrated on the whole film thickness occurs following exposure to 147 nm since this emission, having the smaller photoabsorption cross-section (by factor 1.5–3 than σPA at 58.4 and 106 nm), penetrates deeper into OSG films while the effective quantum yield ϕ stays approximately the same ∼0.3. The quantum yield sharply rises up to the limit value ‘1’ when reaching critical porosity ∼50% and approaching the percolation limit when the pores interconnectivity is close to maximum and methyl radicals can leave pores without too many collisions with the pore walls. SIMS and RBS measurements of the depth profiles of the film elements have shown a reasonable agreement with the model profiles. It confirms the validity of the model approach used and suggests that the integrated damage could be obtained with using somewhat different sets of σPA , ϕ and correspondingly different depth profiles. The OSG damage mechanism under exposure to EUV high-energy photons and the role of energetic secondary electrons require an additional study. The typical fluences in 13.5 nm EUV lithography are lower by orders of magnitude than the fluences used in this study and it is unlikely that damage to OSG films will occur during the EUV lithography process. The revealed peculiarities of VUV/EUV emission interaction with OSG films may also find practical application, for instance, for selective modification (hydrophilization) of a low-k surface for adhesion improvement, generation of active sites for atomic layer deposition (ALD), SAM (self-assembling monolayer) deposition, etc. For instance, the limited depth of low-k modification during the exposure to VUV light emitted by He/H2 plasma was recently demonstrated in ALD of TiO2 on porous OSG films [44]. The depth of observed modification in this work was shown to be close to 40–50 nm, which is in excellent agreement with our estimations for emission from He plasma.
8. Conclusion
Several OSG low-k films with different porosity and dielectric constants have been damaged by EUV and VUV emissions from five different sources in the range of wavelengths from 13.5 to 193 nm and photons fluences up to 6 × 1018 photons cm−2 . The obtained experimental results and their theoretical analysis revealed the damage mechanism, rates, photon penetration depths and the most dangerous spectral region. It is shown that the damage to OSG films is very small at 193 nm because of the low photoabsorption. To analyse the OSG damage a model based on the classical concept of radiation absorption by solid is developed. According to the model, the Si–CH3 depletion can be described by two fundamental parameters: photoabsorption crosssection σPA and effective quantum yield ϕ of Si–CH3 bonds photodissociation as some relaxation method of absorbed photon energy. 12
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Figure 12. The average effective quantum yield for breaking Si–CH3 bonds at VUV photon absorption (a range: 58 nm–147 nm) depending on OSG films porosity.
opportunity to apply the RnD ISAN’s EUV tool for exposing low-k films to 13.5 nm emission. The authors are also grateful to Dr Khashayar Pakbaz from SBA Materials, Inc. for providing experimental materials. References [1] Maex K, Baklanov M R, Shamiryan D, Iacopi F, Brongersma S H and Yanovitskaya Z S 2003 J. Appl. Phys. 93 8793 [2] Jousseaume V, Zenasni A, Gourhant O, Favennec L and Baklanov M R 2012 Advanced Interconnects for ULSI Technology ed M Baklanov et al (New York: Wiley) [3] Volksen W, Miller R D and Dubois G 2010 Chem. Rev. 110 56 [4] Seiji Samukawa et al 2012 The 2012 Plasma Roadmap J. Phys. D: Appl. Phys. 45 253001 [5] Baklanov M R, de Marneffe J-F, Shamiryan D, Urbanowicz A M, Shi H, Rakhimova T V, Huang H and Ho P S 2013 J. Appl. Phys. 113 041101 [6] Jinnai B, Fukuda S, Ohtake H and Samukawa S 2010 J. Appl. Phys. 107 043302 [7] Marsik P, Verdonck P, De Roest D and Baklanov M R 2010 Thin Solid Films 518 4266–72 [8] Sinha et al 2012 J. Appl. Phys. 112 111101 [9] Woodworth R, Riley M E, Amatucci V A, Hamilton T W and Aragon B P 2001 J. Vac. Sci. Technol. A 19 45 [10] Eslava S, Iacopi F, Urbanowicz A M, Kirschhock C E A, Maex K, Martens J A and Baklanov M R 2008 J. Electrochem. Soc. 155 G231 [11] Uchida S, Takashima S, Hori M, Fukasawa M, Ohshima K, Nagahata K and Tatsumi T 2008 J. Appl. Phys. 103 073303 [12] Bao J, Shi H, Huang H, Ho P S, McSwiney M L, Goodner M D, Moinpour M and Kloster G M 2010 J. Vac. Sci. Technol. A 28 207 [13] Goldman M A, Graves D B, Antonelli G A, Behera S P and Kelber J A 2009 J. Appl. Phys. 106 013311 [14] Braginsky V, Kovalev A S, Lopaev D V, Malykhin E M, Mankelevich Yu A, Rakhimova T V, Rakhimov A T, Vasilieva A N, Zyryanov S M and Baklanov M R 2010 J. Appl. Phys. 108 073303 [15] Braginsky O V et al 2011 J. Appl. Phys. 109 043303 [16] Lee J and Graves D B 2010 J. Phys. D: Appl. Phys. 43 425201 [17] Lee J and Graves D B 2011 J. Phys. D: Appl. Phys. 44 325203
Figure 11. The evolution of Si–CH3 depletion in SBA 2.2, SBA 2.0, and SBA 1.8 films after exposing them to the different photon fluences at wavelengths of 58.4 (a), 106 (b) and 147 nm (c). Symbols represent experimental data; solid lines are the modelling results.
Acknowledgments
This research is supported by SRC programme Contract 2012KJ-2280, and the Russian Foundation of Basic Research (1202-00536-a). The authors are very thankful to Dr O Yalushev and Dr V Krivtsun from RnD ISAN (Troitsk, RAS) for the 13
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