Comprehensive ex-situ and in-situ investigations of thermal curing processes in spin-on ultra-low-k thin films conducted by positron annihilation spectroscopy and Fourier transform infrared spectroscopies are presented. Positron annihilation lifetime spectroscopy of ex-situ cured samples reveals an onset of the curing process at about 200 ◦C, which advances with increasing curing temperature.
Microporous and Mesoporous Materials 308 (2020) 110457 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso Thermal kinetics of free volume in porous spin-on dielectrics: Exploring the network- and pore-properties A.G Attallah a, b, *, N Koehler c, M.O Liedke a, **, M Butterling a, E Hirschmann a, R Ecke d, S E Schulz c, d, A Wagner a a Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiation Physics, 01328, Dresden, Germany Physics Department, Faculty of Science, Minia University, P.O 61519, Minia, Egypt Center for Microtechnologies, Chemnitz University of Technology, 09126, Chemnitz, Germany d Fraunhofer ENAS, Technology-Campus 3, 09126, Chemnitz, Germany b c A R T I C L E I N F O A B S T R A C T Keywords: In-situ curing Positron annihilation spectroscopy Porogen removal Porosimetry FTIR Ultra-low-k Dielectric Dielectrics Pore size distribution Positronium Comprehensive ex-situ and in-situ investigations of thermal curing processes in spin-on ultra-low-k thin films conducted by positron annihilation spectroscopy and Fourier transform infrared spectroscopies are presented Positron annihilation lifetime spectroscopy of ex-situ cured samples reveals an onset of the curing process at about 200 ◦ C, which advances with increasing curing temperature Porogen agglomeration followed by diffusive migration to the surface during the curing process leads to the generation of narrow channels across the film thickness The size of those channels is determined by a pore size distribution analysis of positron lifetime data Defect kinetics during in-situ thermal curing has been investigated by means of Doppler broadening spectroscopy of the annihilation radiation, showing several distinct partially superposed and subsequent curing stages, i.e., moisture and residual organic solvents removal, SiOx network cross-linking, porogen decomposition, and finally creation of a stable porous structure containing micropore channels interconnecting larger mesopores formed likely due to micelle like interaction between porogen molecules, for curing temperatures not larger than 500 ◦ C Static (sequencing curing) states captured at specific temperature steps confirm the conclusions drawn during the dynamic (continuous curing) measurements Moreover, the onset of pore inter-connectivity is precisely estimated as pore interconnectivity sets in at 380–400 ◦ C Introduction The functioning operation and performance of Integrated Circuits (IC) strongly depend on the properties of their main building blocks, i.e., conductors, transistors, and insulators (dielectrics) Due to the contin uous increase of transistor densities integrated in modern microchips and their decreasing size, the main research focused on methods to lower the dielectric constant (k) [1–5] The scaling-down of microchip dimensions towards Ultra Large-Size Integration (ULSI) results in an enlarged Resistance-Capacitance (RC) delay time, which strongly limits the microchip’s functionality [6–8] Cu wiring in the Back-End-Of-the-Line (BEOL) process replaced Al [9,10] reduced the electrical resistance by 40% [1] Moreover, different strategies have been proposed in order to reduce the dielectric constant of the interlayer dielectric (ILD) materials [2], e.g., adding less polarized organic groups (F or C doped) with widening of the network and incorporating porosity into SiO2 [11] Notably, the introduction of porosity (effective reduction of density) is crucial for fabricating ultra-low-k (ULK) films with k ≤ 2.0, due to presence of air (kair/vacuum = 1.0) in the solid phase Porous low-k materials could be deposited by plasma enhanced chemical vapor deposition (PECVD) or by spin-on processes [5,12–16] For both processes, there are different precursors for the network building and for the formation of porogen [5,17] In every case, the curing process is critical for achieving a good control of the final film structure and the resulting film properties The curing enhances the matrix crosslinking structure which is correlated to better mechanical stability of the ULK and the formation of pore structure by removal of porogen Mainly in industry fabrication process, the curing is a combi nation of thermal processes – necessary for the network formation, and UV treatment – for more efficient porogen removal [18,19] However, * Corresponding author Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiation Physics, 01328, Dresden, Germany ** Corresponding author E-mail addresses: a.elsherif@hzdr.de (A.G Attallah), m.liedke@hzdr.de (M.O Liedke) https://doi.org/10.1016/j.micromeso.2020.110457 Received 27 May 2020; Received in revised form 30 June 2020; Accepted July 2020 Available online 29 July 2020 1387-1811/© 2020 The Authors Published by Elsevier Inc This is (http://creativecommons.org/licenses/by-nc-nd/4.0/) an open access article under the CC BY-NC-ND license A.G Attallah et al Microporous and Mesoporous Materials 308 (2020) 110457 the experiences in the introduction of porous low-k materials in the interconnect systems identify operational and reliability problems [20–22] On the hand these problems are attributed to the weaker network and the porosity in principle, but on the other hand on not optimal curing processes Subsequent following processes after ULK deposition and curing with temperature and/or UV radiation leads to further out diffusion of gaseous components and film cracking Because of that it is necessary to have a deeper insight in the kinetic of the curing process Contrary to CVD, the porogen is not chemically incorporated and is much easier removable by thermal activation In this manner a single kinetic processes, i.e., thermal curing, is sufficient for the network (vitrification) and pore formation - with pore agglomeration, porogen decomposition and out diffusion – which are partly superposed Conventional porosimetry methods like gas adsorption and mercury intrusion are not reliable for characterizing ~500 nm-thick films because they lose their sensitivity in thin films (less than μm thickness) which are applied on a Si wafer [23] Ellipsometric porosimetry (EP) and X-ray porosimetry (XRP) can be used for characterizing such thin films While the EP method quantifies the refraction index of the absorbent (toluene) to get the pore size distribution [24], XRP detects absorbent density increase EP and XRP are only suitable to determine open porosity and depth profiling is not possible In order to overcome such drawbacks, we chose positron annihilation spectroscopy (PAS) using positron beam-based sources, which allow depth-profiling porosimetry of open and closed pores in thin films [25,26] The presented work explores thermal curing processes in spin-on porous dielectrics utilizing ex-situ and in-situ experimental methods The fundamental issues will be addressed here: (i) the mechanism of pore formation during porogen removal, (ii) the temperature threshold of porogen diffusion and agglomeration, as well as (iv) volumetric diffusion restrictions, e.g channels connecting pores in the bulk with the surface Addressing these questions is extremely important in order to formulate recipes for controlling pore sizes, to prevent pore inter connectivity and accessibility to the surface, and even to support the introduction of state-of-the-art porous dielectrics for future integration processes studied as a function of temperature from T = 100 ◦ C–450 ◦ C The curing time was fixed to 30 2.2 Methods 2.2.1 FTIR Fourier-transform infrared spectroscopy (FTIR) was used to deter mine the chemical and structural changes after ex-situ annealing at different temperatures The measurements were performed in trans mission mode in the spectral mid-range from 400 to 4000 cm− 1, using a Bruker Tensor 27 spectrometer The optical response was given as absorbance after a baseline subtraction According to the Beer-Lambert law, the absorbance is proportional to the molar concentration of chemical species and the sample thickness Therefore, all spectra were normalized by the initial thickness in order to quantify changes in bonding arrangements, which are important since curing introduces a high loss in thickness The thickness was measured with a Sentech SE 850 spectral ellipsometer in a wavelength range from 380 nm to 830 nm at a constant angle of 70◦ The film thickness was calculated by a Cauchy model [28] Selected peak areas were integrated for the characterization of temperature driven processes 2.2.2 PAS In materials, the journey of injected positrons (e+) starts with ther malization, followed by diffusion, then annihilation realized by emission of two 511-keV γ quanta During e+ diffusion in porous materials, it has the ability to form a positron-electron (e+-e-) hydrogen-like bound state known as Positronium (Ps) [29–31] Depending on the relative spin orientations of e+ and e− , a singlet state (para-Ps; p-Ps) and a triplet state (ortho-Ps (o-Ps) are formed The intrinsic vacuum lifetimes of p-Ps and o-Ps are 125 ps and 142 ns, respectively The lifetime of the short-lived p-Ps is not significantly affected by molecular electrons but that of the long-lived o-Ps is o-Ps collides with pore walls and when it exchanges the electron with an e− of antiparallel spin orientation, the annihilation lifetime is reduced as an inverse function of the pore size A correlation between this collisionally-reduced o-Ps lifetime and the pore size was firstly described in the Tao-Eldrup (TE) model [32,33] for small pores (R < nm) and later expanded for large pore sizes and at different tem peratures in the rectangular TE (RTE) model [34] Considering the energy balance of the annihilation process, p-Ps annihilates by 2γ photons mode (each of 511 keV) while o-Ps annihila tion (under vacuum or in large pores) gives 3γ photons with each of them having an energy distribution extending from to 511 keV [35] In 2γ annihilation, the energies of the annihilation photons are broadened due to the electronic momentum at the annihilation site (assuming zero velocity of the thermalized positrons) This energy broadening is measured by Doppler broadening spectroscopy (DBS) which is charac terized by two shape parameters, S and W The S-parameter is a measure of the ratio between the central region of the photopeak and the com plete broadened peak area, while the W-parameter represents the counts in the wings (tails) of the spectrum divided by the total area below the peak In defective sites, the electronic density is low and hence the probability of annihilation with valence electrons is higher than with core electrons Accordingly, the yield is increased in the central region of the spectrum because of the larger fraction of low momentum electrons (valence electrons) [36] causing a higher S-parameter By definition, the S-parameter represents annihilation of free and bound positrons [35], where the latter is related to the pick-off and p-Ps annihilation [37] On the other hand, the W-parameter describes positron annihilation with core (high momentum) electrons and it characterizes the chemical sur rounding at the annihilation site For better understanding of the chemical environment at the annihilation site we employed coincidence Doppler broadening spectroscopy (cDBS) cDBS is able to show very small changes in W-parameter as discussed later In case of connected pores towards the samples surface or relatively large pores (>50 nm), the self-annihilation (3γ) probability of o-Ps increases and the counts in Experimental details The curing process of spin-on ULK films has been investigated by positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS), coincidence DBS (cDBS), and Fourier-transform infrared spectroscopy (FTIR) First, the results of the ex-situ curing by PALS, cDBS, and FTIR at several different temperatures are presented in order to estimate the onset of porogen decomposition and effects of temperature on the created pore sizes However, the ex-situ curing gives not so deep insights about kinetics of the curing process Therefore, in the second part of this paper, the in-situ curing of the ULK films by DBS is presented For better understanding of the film’s chemical composition, cDBS of annihilation radiation has been utilized 2.1 Materials The chemicals used for the spin-on organo-silicate glasses were provided by SBA Materials, Inc with k = 2.2 The liquid precursor consists of silicon alkoxide esters dissolved in a suitable organic solvent and an amphiphilic block copolymer acting as pore generator [27] The solute was spin-coated on 6-inch silicon wafers with 2000 rpm for 60 s for 500 nm thick films The spin-coated samples were then soft baked for 120 s at 150 ◦ C on a hot plate at ambient air The soft bake remove the majority of spinning solvent and the tackiness of the film Then the wafers were cut in small samples of × cm before the curing process The proper curing process were carried out in a quartz glass furnace under nitrogen atmosphere and with a heating ramp of 10 ◦ C/min The investigation of pore- and network formation during thermal curing was A.G Attallah et al Microporous and Mesoporous Materials 308 (2020) 110457 the energy spectrum for energies well below 511 keV increases So, the 3γ/2γ ratio of energy spectra [38] shows variations in pore sizes (qualitatively) and it can visualize the pore interconnectivity [38] Depth-profiling of ex-situ and in-situ cured ULK thin films has been performed at the slow positron beam-based facilities; MePS1 (for PALS measurements), and SPONSOR2 and AIDA3 (for DBS and cDBS measurements) PALS: The MePS system is a beamline at the user-facility ELBE dedicated to probe open volume defects in thin films by means of PALS The positron beam is generated from a 35 MeV electron beam via bremsstrahlung and pair production in a W converter [39] The positron lifetime measurement utilizes a CeBr3 scintillation detector with an overall timing resolution down to ~210 ps at FWHM and count-rates of about 100 kcps A Y2O3-stabilized ZrO2 (YSZ) reference sample with a well-known single positron annihilation lifetime of ~181 ps has been used to determine the timing resolution PALS measurements of the ULK films were performed by using positron implantation energies, EP, from keV to 12 keV for depth profiling Prior to PALS experiments, all samples were in-situ annealed at 150 ◦ C for 30 at ~1 × 10− mbar to purge the pores from moisture adsorbed during or after preparation A discrete data analysis has been performed by the PALSfit3 routine [40] while the MELT code [41] has been used for calculating continuous lifetime distributions Spectra with × 107 total counts were used for the former and × 107 counts for the latter, respectively The pore size was derived using the EELViS4 code [42] DBS and cDBS: A high-purity Ge detector with an energy-resolution of (1.09 ± 0.01) keV at 511 keV was used for DBS at the AIDA [43] chamber for in-situ curing The shape parameters S were calculated from the central region of the peak with E = 511 ± 0.70 keV and the wing parameter W was chosen as E = 511 ± 2.13 keV to E = 511 ± 2.74 keV Two-collinear high-purity Ge detectors (energy resolution of 780 ± 20 eV) of the SPONSOR [44] setup have been used to perform cDBS Fig FTIR spectra of uncured and cured samples at different curing tem peratures The baseline is corrected and data are normalized to a thickness of 500 nm Si–OH stretching in silanol This peak undergoes a strong decrease until 250 ◦ C and disappears after 350 ◦ C curing in the FTIR spectra The silanol condensation contributes mainly to the crosslinking reaction by: -Si–OH + -Si– OH → -Si–O–Si + H2O This is also reflected in Fig 2b, the behavior of Si–OH and OH (>3100 cm− 1) vibration peaks indicates that the Si–OH condensation process takes place mainly between 100 ◦ C and 200 ◦ C and probably completing around 300 ◦ C The porogen removal becomes manifest in the reduction of the peak area in the range of 3000 cm− to 2800 cm− [45–48] There, different symmetrical and asymmetrical stretching modes of CHx-bonds occur, which are indicative for porogen composi tions It must be kept in mind that spin-on solutions are a complex mixture of chemicals, containing not only the network and porogen precursor For instance, chemicals to improve rheology during spin coating are necessary These chemicals mostly consist of similar struc tures as the porogen itself and contributes to the symmetrical and asymmetrical stretching modes of CHx-bonds inside the material Nevertheless, unlike the porogen, these chemicals are less temperature stable and are removed at lower temperature regimes Fig a shows that the porogen removal is divided into two parts, where until T = 200 ◦ C more that 80% of CHx-bonds are removed from the material From 200 ◦ C until the final curing temperature is reached, a further >10% decrease can be observed Since from literature it is known, that the porogen removal usually takes places at higher curing temperature [49–52], the second part of the CHx-bond reduction is dedicated to the porogen removal, whereas the first part until 200 ◦ C contributes to the removal of remaining rheological chemicals The Si–CH3 peak between 1300 cm− and 1200 cm− was added, because the relationship between Si–CH3 to Si–O absorption area in dicates a change of mechanical properties This happens due to the SiCH3 bonds breakage, which potentially can initiate further conden sation reactions [53] The absorption from 900 cm− to 700 cm− be longs to the fingerprint region, a complex structure of different Si-(CH3)x and Si–O bonds [45] This part of the spectrum is not included in the discussion The Si–CH3 peak in Fig d also increases with temperature At 450 ◦ C a small drop could be explained by CH group loss Results and discussion 3.1 Ex-situ curing 3.1.1 FTIR The ULK films have been ex-situ cured in the temperature of T = 100–450 ◦ C (50 ◦ C steps) and then investigated utilizing FTIR technique Fig.S1 (supplementary materials) shows the thickness change of the cured ULK samples as a function of temperature The initial thickness of ~509 nm at the uncured state is reduced to ~336 nm after curing at 450 ◦ C The shrinkage of films is more than 30% making thickness normal ization for FTIR spectroscopy analysis mandatory Fig presents the normalized FTIR spectra revealing a detailed overview of the temperature-induced material changes The obtained peaks where in tegrated and compared to the uncured state to qualitatively demonstrate the material changes A strong bond is observed between 1000 and 1200 cm− mostly assigned to SiO bond vibrations in Si–O–Si groups and is equated with the matrix crosslinking structure These bonds are strongly developed by curing The peak could be deconvoluted in peaks, for a cage like structure, suboxide and network [45] In stoichiometric ther mal oxides the bonding angle is reported to be ~144◦ with single FTIR absorption around 1080 cm− The material here has different bonding angle from the ideal stoichiometry, ~140◦ for network peak around 1063 cm− 1, 200 ◦ C (Fig 2a) Two processes su perimpose: (i) porogen decomposition and for largest temperatures – porogen removal and (ii) micropore and network formation take place at the same time The fifth LT τ5 reflects mesopore evolution as a function of temper ature Their signature has been not detected in the uncured state, but exists in all cured samples LT τ5 and hence the size of mesopores (D5) monotonically increases with T The increase of mesopore size is a consequence of the network (matrix) microstructure evolution, porogen mobility, agglomeration, and out diffusion It mostly represents free volume generated by porogen agglomerates due to mutual attraction between porogen molecules generating micelle like formations [54] The relative intensity I5 grows monotonically until 400 ◦ C From 400 ◦ C to 450 ◦ C I5 increases from 10% to over 30%, whereas the intensity of LT τ3 decreases in a same abrupt way This abrupt increase in I5 can be interpreted as thermal decomposition of remaining residual porogen in channel like structures between mesopores and is an evidence of inter connectivity The rise of I5 is a physical consequence of increased trap ping cross-section at these largest free volumes The positron energy scans of τi, Ii, and Di, where i = 3, 4, denotes the order of LTs, detected across the ULK films thickness are depicted in supplementary materials as Fig S5 Taking into account that each discrete o-Ps LT component is in fact a weighted average of LT in a certain group of similar pore sizes, the width of the broadening of the pore size distribution (PSD) calculated from oPs LT should be investigated in order to examine pore size uniformity Accumulating high statistics per spectrum (30 million) allows calcu lating lifetime distributions using the MELT code which provides the intensity distribution per o-Ps LT [57] The conversion of this distribu tion of o-Ps intensity into PSD has been done successfully for other low-k films where PSD with nm (FWHM) broadening has been found [58] because of dispersed pores with different sizes in PECVD films Since the spin-on coated ULK films should yield uniform pore sizes, PSD is determined here to confirm this assumption (see Fig 4) Four well separated groups of free volumes or pores have been identified The first two distributions centered at ~0.8 nm and ~1.8 nm reflect the Fig Spherical pore size distribution (PSD) derived from PALS results by MELT of ULK samples cured at different temperatures at EP = keV broadening of τ3 and τ4 from discrete analysis (compare to bottom panel of Fig 3) However, the uncured sample measured at 25 ◦ C (black dis tribution) shows a broad PSD extending from 0.4 nm to 0.8 nm, being most likely an overlapping of the o-Ps annihilation in the matrix (D3 in Fig 3) and in the porogen (D4 in Fig 3) This overlapping takes place because the difference in LT is not large enough to separate these two close components This broad PSD has been split into two PSD for all the cured samples In the cured samples, the intensity of the first distribution (defined as the area under the curve; not shown), which corresponds to the matrix, increases up to 400 ◦ C and then drops at 450 ◦ C, which agrees well with the behavior of I3 presented in Fig Next two distri butions represent micropore formation, i.e., first and second micropore distribution The first distribution of micropores centered at about 0.9 nm can be ascribed to free volumes existed in and left after the porogen decomposition and its integrated area decreases with the curing tem perature similarly to I4 shown in Fig The second micropore distri bution ranges from 1.45 nm to 1.95 nm Both micropore distributions are likely associated with free volume connecting mesopores, e.g neck like channels created by the porogen during its out diffusion from the film, which transport o-Ps once residual porogen concentration is suf ficiently low Interestingly, the PSD of mesopores is very narrow (~0.1 nm width) and the integrated area increases with the curing tempera ture The maximum and narrowest mesopore size with the highest in tensity is obtained for the sample cured at 450 ◦ C Such a narrow PSD suggests that the free volume microstructure consists of very well defined free volume blocks without any dispersion across the overall sample thickness The results presented in Fig have been obtained for a depth of ~140 nm Similar narrow distributions have been found for depths of ~46 nm and ~88 nm, too (see supplementary Fig.S6) Both discrete (PALSfit) and continuous (MELT) analysis combined provide similar values of positron lifetime, evidencing moreover basi cally no dispersion of pore sizes It is plausible to expect that the increasing of D5 value as a function of curing temperature reflects increased porogen mobility during curing, which on the other hand enables porogen agglomeration in the miscible phase hence formation of larger pores Consequently, bottle neck like interlinked pore networks would be generated Here, very slow 10 ◦ C/min curing rate was used, which likely enhances porogen clustering In order to prevent porogen clustering other means of treatments like faster curing rates with a glass furnace or optimized UV curing in a couple of minutes, or possibly even faster curing rates in the sub-second range are presently investigated Another solution to prevent porogen agglomeration is to pre-heat the film to moderate temperatures in order to initiate and stabilize network cross-linking and densification earlier than the onset of porogen A.G Attallah et al Microporous and Mesoporous Materials 308 (2020) 110457 decomposition Also, the existence of the 1.45–1.95 nm distribution (linked to channels) suggests that the porogen removal from the film is only possible via these channels and it reflects the size of porogen molecules after migration from original sites Whereas the larger mes opores most likely originate from local porogen agglomerations, stabi lized be developing with temperature network In addition, the formation of these channels leads to surface-accessible pores, which, if hydrophilic, will adsorb moisture or other impurities and in turn leading to higher leakage currents and lower breakdown voltages in devices Employing materials with larger free volumes than the size of the decomposed porogen as a matrix or using porogen with sizes smaller than the free volumes in the matrix could pave a new way to solve this problem It would allow for porogen removal via the free volume of the matrix (matrix intrinsic porosity) By preventing the formation of these channels, self-sealed pores form vitreous C reference is depicted in Fig for the ULK films (cured for 90 at temperatures from 150 ◦ C to 450 ◦ C in 100 ◦ C steps) as well as for the glass reference The low-momentum region (pL < 10− m0c) is mostly a representation of the S-parameter (EP = keV), hence the free volume of pores (Fig 5) The momentum region pL > 10− m0c shows the annihilation with core-electrons of porogen and the matrix The general shape of the uncured curve resembles to a large extent the SiO2 reference sample in the low and intermediate momentum re gions (minimum at pL ~5 × 10− m0c and maximum at pL ~15 × 10− m0c) and at the same time the high momentum part overlaps with the Creference The latter, is a signature of dominant positron annihilation with electrons in carbon With increasing curing temperature the amplitude (ratio to vitreous C) at the high momentum region increases monotonically, suggesting decrease of carbon decoration of the defect site, which can be linked to porogen decomposition and out diffusion from the annihilation site Such a dependence of the C content on the curing temperature confirms the results of PALS and FTIR for porogen removal and general reduction of the carbon content with curing temperature The variation of cDBS curves in the higher momentum region (pL > 10− m0c) represents the development of the matrix as a function of curing temperature, too The remaining backbone of the fully cured and porogen-free ULK films does not consist of pure SiO2, but rather Si–O bonds with methyl-groups It is obvious that for larger temperatures the ratio curves are dissimilar to the electronic structure of amorphous glass since the annihilation site is decorated by other elements Especially, samples cured at 350 ◦ C and 450 ◦ C deviate strongly at pL > 17 × 10− m0c Beside the stronger matrix crosslinking with the curing tempera ture, -Si-CH3 bond strengthen increases, too (FTIR- Fig 2d) In conclu sion, the disagreement between the ratio curves of the cured samples at 350 ◦ C and 450 ◦ C and the glass reference spectrum evidences that the structure is not formed solely by SiOx but it contains SiCH3 bonds as well, which are a part of the matrix, rather than the pore wall 3.1.3 cDBS The core electrons possess high momentum acting as fingerprints for each element Due to the higher chance of positrons to annihilate with valence electrons, the overlap of these high momenta-electrons with positrons is small Consequently, the counts in the tails of the spectrum which probe the high-momentum electron distributions are low Since, only one detector is used during DBS the noise-to-signal ratio is in turn high The so-called ”coincidence Doppler broadening spectroscopy” (cDBS) utilizes two detectors during the measurement, which greatly reduces the noise-to-signal ratio [59] cDBS detects both annihilation photons and reveals the contribution to the positron annihilation with electrons originating from different elements located at the annihilation site Elemental information is derived from cDBS by analyzing the photon intensity in the high-momentum region (similarly to the W-parameter) Uncured samples containing both porogen and SiO2 show a distinguished dependence of the electron momentum, pL, which is referenced to a given reference sample [see Fig 5] Such ratio curves will be different for uncured and completely cured samples Since the uncured system have a high carbon content due to the presence of the porogen, vitreous carbon has been used as a reference for porogen content The fully cured sample should contain a large concentration of SiOx (Si–O bonds), which electronically are to a large extent similar to amorphous glass, hence a glass substrate has been used as a reference for fully cured samples Such references have been used to register their characteristic shapes of the annihilation line Comparative measure ments between these references and samples cured at different tem peratures show the evolution of the curing process The electronic momenta variation obtained at EP = keV and normalized to the 3.2 In-situ curing at AIDA system as a probe of free volume kinematics PALS measurements on ex-situ cured films provided important hints regarding porogen removal In this section the kinetic evolution of porosity during the curing process are discussed, which has been demonstrated to our knowledge for the first time using the all in-situ approach This experiment gives insights into the mechanism of pore formation and the onset of pore inter-connectivity Two different sample series were probed: (i) capped (with 20 nm-thick carbon layer) and (ii) uncapped films and treated with exactly the same thermal conditions One film series (with and without cap layer) have been cured sequen tially at AIDA [43] using different curing temperatures and a dwell time of 1h in ultra-high vacuum (10− - 10− mbar) and measured by Doppler broadening PAS at room temperature The sequentially cured uncapped (supplementary materials Fig.S7a) and caped (supplementary materials Fig.S7b) films both showed a monotonic increase of the S-parameter and a decrease of the W-parameter as functions of curing temperature, a clear signature of increasing open and free volume In addition, a thickness reduction is evidenced in both systems as a shorter plateau of the material sensitive W-parameter for the 200 ◦ C curing step compare the pristine film, which is in accordance with FTIR The thickness reduction is further progressing at larger curing temperatures The re sults of the continuous in-situ curing experiment, where both type of films (with and without cap layer) have been cured at a constant heating ramp of ◦ C/min with sampling period of min, are summarized in Fig 6, which shows the variation of the S-parameter (Fig 6a) and the normalized (with respect to the Si substrate) Nvalley/Ntotal ratio (Fig 6b) at EP = keV The meaning of the Nvalley/Ntotal is explained later in the text The comparison between the capped and uncapped samples serves to disentangle contributions of a pure pick-off process from o-Ps escaping the films through the pore network, respectively At the same time, the porogen decomposition could feature two feasible scenarios: Fig Dependence of the ratio to vitrous carbon of ULK thin films cured from 150 ◦ C to 450 ◦ C on the electronic momenta where glass has been used as references of fully cured samples A.G Attallah et al Microporous and Mesoporous Materials 308 (2020) 110457 examined using the Nvalley/Ntotal parameter (Fig 6.b) The Nvalley de scribes mostly the number of 3γ annihilation events and the Ntotal ac counts for both the number of 3γ and 2γ annihilation events [35] Notably, Nvalley/Ntotal shows five stages in the uncapped samples while these five stages are combined into only two stages in the capped one Through stage I in the uncapped sample, the normalized Nvalley/Ntotal ratio is constant and slightly higher than unity This means, that the removal of residual organic solvents and mositure could already clean some openings to the surface allowing for o-Ps to annihilating via 3γ emission The increase of Nvalley/Ntotal during the stage II represents additional openings to the surface created during the cross-linking process In stage III only a gentle increase of Nvalley/Ntotal is observed, which indicates that the fraction of the accessible from the surface pores, created at the beginning of this stage, is unaffected by temperature and most likely the created pores are still isolated from each other and from the surface Here, the slight increase of Nvalley/Ntotal as a function of temperature represents matrix modifications allowing o-Ps to travel more undisturbed, e.g., due to the size and concentration increase of intrinsic open volume as shown from PALS results The transition from isolated pores into interconnected pores starts at ~380 ◦ C (stage IV) as displayed by the steeper increase of Nvalley/Ntotal It can be explained as increasing probability of 3γ annihilation, which takes place in the case of infinite long channels that can extend up to the film surface and taking into account the constancy of S-parameter along the stage IV (Fig a and Fig.S1) The rate of pore interconnectivity is lower along the stage V, starting at 450 ◦ C, which shows a smaller slope of Nvalley/Ntotal with respect to that of the stage IV Interestingly, Nvalley/Ntotal is nearly constant along stages I-III in the capped sample and it is ~1.0, which proves that the 20 nm carbon cap works nicely and prevented o-Ps from escaping The capping also hides all internal curing-related processes (residual organic solvents and moisture removal, cross-linking, and porogen removal) form being detected from Nvalley/Ntotal At ~400 ◦ C, Nvalley/Ntotal starts to increase which means the onset of interconnectivity inside the sample itself in stages IV + V of the capped sample similar to the uncapped sample Here, due to interconnectivity long enough channels are created allowing a small fraction of o-Ps to annihilate not via the pick-off process but with intrinsic vacuum lifetime In case o-Ps could leave the film across cracks in the cap layer, which we cannot unambiguously exclude, Nvalley/Ntotal the parameter should be much larger, more in the range for uncapped films The transition from closed (isolated) to open pores (inter connectivity) can be monitored using o-Ps 3γ annihilation simply because o-Ps can travel a long distance inside the pore network and finally out-diffuse into vacuum via open to surface pores where finally it annihilates emitting 3γ photons [60] We have utilized this property to estimate an interconnectivity length, LPs, as a function of curing tem perature The overall 3γ annihilation process of o-Ps besides the contribution from out-diffused into vacuum, it consists in addition of a small fraction, which annihilates inside the pore network [60,61] Ac cording to Ref [61], the experimental 3γ annihilation fraction, F3γ (definition of F3γ is given in the supplementary materials section S.F3γ), can be evaluated using the following equation: Fig S-parameter (a) and normalized Nvalley/Ntotal ratio (b) of capped (red squares) and uncapped (black squares) ULK samples at different temperature (from RT to 500 ◦ C) during the curing in-situ at AIDA system (For interpre tation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) (i) porogen diffuses inside the matrix but is not mobile enough and re mains there or (ii) it, unavoidably, creates paths to the surface In the latter case, the cap layer would hinder or delay the removal of the decomposed porogen from the film In Fig a, the S-parameter shows four stages of curing along tem peratures from 30 ◦ C to ~500 ◦ C for both the capped and uncapped samples It should be noted, that the S- parameter is a weighted average of different open volume contribution, i.e., free annihilation at inter stitial positions and in vacancy like defects as well as bound annihilation as p-Ps, and o-Ps in pores due to pick-off process and outside the films in case of escape throughout the pore network In the uncapped sample, the linear increase of S-parameter in the stage I can be attributed to the removal of residual organic solvents and absorbed from ambient mois ture During stage II from ~100 ◦ C to ~160 ◦ C, matrix cross-linking starts to takes place as well as the removal of remaining rheological chemicals continues, the latter reflected by increase of S, hence free volume The slow increase of S-parameter during stage III illustrates the time and temperature dependence of porogen removal and matrix for mation In stage IV, starting at 400 ◦ C, the mean pore size is formed, since it does not change in size anymore and a curing process is close to completion In the capped sample (red squares in Fig 6a), the S-parameter is constant along stage I, which represents most probably hindering of residual organic solvents and moisture removal, which is delayed and starts at ~ 100 ◦ C during the stage II The stage II for the capped film has a similar slope as stage I in the uncapped sample, hence one can say that sages I and II in the uncapped sample are combined into one single stage, II, in the capped sample Hence, in the capped sample, the stage II re sembles the residual organic solvents and moisture removal, crosslinking onset, and porogen decomposition onset Again as in the uncapped sample, stage III represents the removal of porogen residues Here, the slope of stage III is slightly smaller than that in the uncapped sample and also the stage IV, which reflects a complete pore formation and the initiation of interconnectivity, has been shifted due to the presence of the cap layer It seems that the porogen tries to leave the sample by making channels to the surface but since the cap is there, it diffuses into the matrix or it incorporates into the carbon layer or leaves across it Therefore, we believe that the capping hinders porogen removal to a large extend The analysis of the S-parameter illustrated the change of created porosity, whereas the onset of the pore inter-connectivity has been (F3γ) total = (F3γ) vacuum + (F3γ) mesopores (2) The fraction of o-Ps annihilation in vacuum has been calculated for uncapped films (see supplementary materials, section S.F3γ) The frac tion of o-Ps annihilation in mesopores calculated for the same samples but capped is estimated by fitting the intensity I5 of τ5 Table shows the calculated values of F3γ in mesopores, vacuum, as well as the inter connectivity length, LPs, and its ratio to film thickness D, LPs/D, as functions of curing temperature For 3γ annihilation in mesopores positron implantation energies Ep = keV and keV have been chosen to indicate the o-Ps fraction close to the middle of the film and at the film-substrate interface, respectively The energies Ep = keV and keV A.G Attallah et al Microporous and Mesoporous Materials 308 (2020) 110457 curing by Doppler broadening PAS, which realized the most deep insight into free volume and porogen kinetics, hints that the creation of path ways (micropores) to the surface is the only way for the porogen removal The in-situ curing also nicely shows that the pore inter connectivity occurs at ~380–400 ◦ C The interconnectivity length in creases nonlinearly as a function of curing temperature reaching a value of ~180 nm at final 450 ◦ C curing temperature, which is ~50% of the film thickness Table Fractions of o-Ps annihilating in mesopores (at Ep = and keV) and vacuum (at Ep = and keV), interconnectivity length, and its ratio to film thickness as functions of curing temperature Curing temperature (◦ C) 200 300 400 450 3γ fraction (%) in mesopores 3γ fraction (%) in vacuum Ep = keV Ep = keV Ep =1 keV Ep =6 keV 0.015 0.01 0.062 0.062 0.018 0.011 0.067 0.067 30 39 47 51 12 17 15 15 Interconnectivity length, LPs, (nm) LPs/D 1.76 24.91 74.17 179.92 0.004 0.06 0.21 0.49 CRediT authorship contribution statement A.G Attallah: Conceptualization, Writing - original draft, Writing review & editing N Koehler: Resources, Software, Data curation, Validation M.O Liedke: Supervision, Resources, Software, Data cura tion, Validation M Butterling: Resources, Visualization, Software, Formal analysis E Hirschmann: Resources, Visualization, Software, Data curation, Funding acquisition R Ecke: Resources, Visualization, Supervision S.E Schulz: Resources, Supervision, Visualization A Wagner: Resources, Supervision, Visualization have been selected for the 3γ fraction in vacuum to show the highest and lowest escaped portions, respectively but still within the film region The obtained 3γ fraction in mesopores is very low most likely due to their small size and characteristic microstructure as shown in the sketch of Fig 3, i.e worm-like micropore channels leading to larger mesopores Such a microstructure suggests that o-Ps annihilates dominantly by pickoff and probability of 3γ annihilation is low Since the chance of o-Ps escape to vacuum reduces with increasing Ep, the 3γ fraction in meso pores at keV is the same or larger than at keV Moreover, the 3γ vacuum fraction of o-Ps is the largest at keV (sub-surface region) and clearly increases with curing temperature Similarly, the inter connectivity length LPs raises with the curing temperature reaching ~180 nm at 450 ◦ C curing temperature The variation of LPs is nonlinear and a steeper, more pronounce increase is visible after curing at T > 300 ◦ C in agreement with in-situ curing results (Fig 6b) The LPs/D ratio shows that ~50% of the film thickness is interconnected after final curing at 450 ◦ C Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgments This research was funded by the DFG project No 398216953 (WA 2496/1-1 and SCHU1431/9-1) Part of this research was carried out at ELBE at the Helmholtz-Zentrum Dresden - Rossendorf e V., a member of the Helmholtz Association We would like to thank the facility staff for assistance This work was partially supported by the Impulse-und Networking fund of the Helmholtz Association (FKZ VH-VI-442 Memriox) and the Helmholtz Energy Materials Characterization Platform (03ET7015) We thank to J Hickman from SBA Materials for providing chemicals utilized to manufacture ULK films Conclusions Monoenergetic positron beam based spectroscopic methods (lifetime and Doppler broadening) have been employed in conjunction with FTIR to study the thermal curing process in-situ and ex-situ by varying the curing temperature of spin-on low-k thin films FTIR spectroscopy shows, that the crosslinking of the network material by silanol condensation starts at low temperatures and is finished by the complete disappearance of the Si–OH peak at 350 ◦ C However, the Si–O peak region continually grows, most likely originated by a densification of the material, which also causes the thickness reduction of 30% At 400 ◦ C Si–CH3 bond break starts to occur, which can cause the transition from isolated pores into interconnected pores, started at ~380 ◦ C, which is explicitly detected by PALS and in-situ DB PAS The porogen removal starts at 200 ◦ C and slowly decrease to less than 5% at 450 ◦ C In-situ DB PAS and ex-situ PALS suggest that porogen removal and pore formation kinematics is not a single step mechanism but rather a continuous multistage process, which is initiated at 200 ◦ C ending at 450 ◦ C Because of the incomplete vitrification process of the matrix in the initial stages of the curing, porogen molecules can approach each other, forming micelle like aggregations, which in turn contribute to the formation of openings (channels) towards the surface and inside the film This leads to the increased pore size accompanied by increased positron annihilation intensity as a function of the curing temperature Hence, a channel-like free volume structure is obtained as a consequence of porogen migration with a channel cross-section of about 1.6 nm and numerous larger mesopores across the pathways, former porogen agglomerations It is unambiguously confirmed by distributional analysis of PALS data, which revealed a narrow pore size distribution becoming even narrower with increasing curing temperature Mesopores with about 3.1 nm size and 35% intensity were obtained after curing the sample at 450 ◦ C Moreover, PALS indicates that the matrix free volume increases both in size and concentration with the curing temperature as well The in-situ Appendix A Supplementary data Supplementary data to this article can be found online at https://doi org/10.1016/j.micromeso.2020.110457 References [1] A Grill, S.M Gates, T.E Ryan, S.V Nguyen, D Priyadarshini, Progress in the development and understanding of advanced low k and ultralow k dielectrics for very large-scale integrated interconnects - state of the art, Appl Phys Rev (2014), https://doi.org/10.1063/1.4861876, 011306 [2] D.J Michalak, J.M Blackwell, J.M Torres, A Sengupta, L.E Kreno, J.S Clarke, D Pantuso, Porosity scaling strategies for low-k films, J Mater Res 30 (2015) 3363–3385, https://doi.org/10.1557/jmr.2015.313 [3] H.S Rathore, Electrochemical Society Dielectric science and technology division, in: Proceedings of the Second International Symposium on Low and High 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tackiness of the film Then the wafers were cut in small samples of × cm before the curing process The proper curing process were carried out in a... with FTIR to study the thermal curing process in- situ and ex-situ by varying the curing temperature of spin-on low-k thin films FTIR spectroscopy shows, that the crosslinking of the network material