Organosilicate Glass Research Articles

Multi-step reaction mechanism for F atom interactions with organosilicate glass and SiOx films

An ab initio approach with the density functional theory (DFT) method was used to study F atom interactions with organosilicate glass (OSG)-based low-k dielectric films. Because of the complexity and significant modifications of the OSG surface structure during the interaction with radicals and etching, a variety of reactions between the surface groups and thermal F atoms can happen. For OSG film etching and damage, we propose a multi-step mechanism based on DFT static and dynamic simulations, which is consistent with the previously reported experimental observations. The important part of the proposed mechanism is the formation of pentavalent Si atoms on the OSG surface due to a quasi-chemisorption of the incident F atoms. The revealed mechanism of F atom incorporation into the OSG matrix explains the experimentally observed phenomena of fast fluorination without significant modification of the chemical structure. We demonstrate that the pentavalent Si states induce the weakening of adjacent Si–O bonds and their breaking under F atom flux. The calculated results allow us to propose a set of elementary chemical reactions of successive removal of CH3 and CH2 groups and fluorinated SiOx matrix etching.

Effect of porosity and pore size on dielectric constant of organosilicate based low-k films: An analytical approach

An analytical approach allowing to analyze effect of porosity, pore size, and interconnectivity on dielectric constant of organosilicate based low-k materials is developed. Within the framework of this approach, a good agreement between the calculated and experimentally measured dielectric constants for several porogen (template) based organosilicate glasses low-k films is demonstrated. It is shown that the best agreement between the calculated and measured k-values corresponds to low-k structure with CH3 groups localized on pore wall surface. The results also demonstrate a good agreement with recently published results of similar analysis based on numerical approach.

Surface sealing using self-assembled monolayers and its effect on metal diffusion in porous low-k dielectrics studied using monoenergetic positron beams

Surface sealing effects on the diffusion of metal atoms in porous organosilicate glass (OSG) films were studied by monoenergetic positron beams. For a Cu(5nm)/MnN(3nm)/OSG(130nm) sample fabricated with pore stuffing, C4F8 plasma etch, unstuffing, and a self-assembled monolayer (SAM) sealing process, it was found that pores with cubic pore side lengths of 1.1 and 3.1nm coexisted in the OSG film. For the sample without the SAM sealing process, metal (Cu and Mn) atoms diffused from the top Cu/MnN layer into the OSG film and were trapped by the pores. As a result, almost all pore interiors were covered with those metals. For the sample damaged by an Ar/C4F8 plasma etch treatment before the SAM sealing process, SAMs diffused into the OSG film, and they were preferentially trapped by larger pores. The cubic pore side length in these pores containing self-assembled molecules was estimated to be 0.7nm. Through this work, we have demonstrated that monoenergetic positron beams are a powerful tool for characterizing capped porous films and the trapping of atoms and molecules by pores.

Mechanical characterization of porous nano-thin films by use of atomic force acoustic microscopy

The indentation modulus of thin films of porous organosilicate glass with a nominal porosity content of 30% and thicknesses of 350nm, 200nm, and 46nm is determined with help of atomic force acoustic microscopy (AFAM). This scanning probe microscopy based technique provides the highest possible depth resolution. The values of the indentation modulus obtained for the 350nm and 200nm thin films were respectively 6.3GPa±0.2GPa and 7.2GPa±0.2GPa and free of the substrate influence. The sample with the thickness of 46nm was tested in four independent measurement sets. Cantilevers with two different tip radii of about 150nm and less than 50nm were applied in different force ranges to obtain a result for the indentation modulus that was free of the substrate influence. A detailed data analysis yielded value of 8.3GPa±0.4GPa for the thinnest film.The values of the indentation modulus obtained for the thin films of porous organosilicate glasses increased with the decreasing film thickness. The stiffening observed for the porous films could be explained by evolution of the pore topology as a function of the film thickness. To ensure that our results were free of the substrate influence, we analyzed the ratio of the sample deformation as well as the tip radius to the film thickness. The results obtained for the substrate parameter were compared for all the measurement series and showed, which ones could be declared as free of the substrate influence.

Thermal expansion coefficients of ultralow-k dielectric films by cube corner indentation tests at elevated temperatures

This paper demonstrates the use of cube corner indentation tests performed at elevated temperatures to measure the coefficient of thermal expansion (CTE) of ultralow-k dielectric films. Using this approach, the CTE of organo-silicate glass low-k films with different intrinsic film stresses is estimated to vary between 8.2 ± 0.8 ppm/ °C and 10.9 ± 1.1 ppm/ °C. The advantages and limitations of the proposed test methodology are discussed.

Vacuum ultra-violet damage and damage mitigation for plasma processing of highly porous organosilicate glass dielectrics

Porous organosilicate glass thin films, with k-value 2.0, were exposed to 147 nm vacuum ultra-violet (VUV) photons emitted in a Xenon capacitive coupled plasma discharge. Strong methyl bond depletion was observed, concomitant with a significant increase of the bulk dielectric constant. This indicates that, besides reactive radical diffusion, photons emitted during plasma processing do impede dielectric properties and therefore need to be tackled appropriately during patterning and integration. The detrimental effect of VUV irradiation can be partly suppressed by stuffing the low-k porous matrix with proper sacrificial polymers showing high VUV absorption together with good thermal and VUV stability. In addition, the choice of an appropriate hard-mask, showing high VUV absorption, can minimize VUV damage. Particular processing conditions allow to minimize the fluence of photons to the substrate and lead to negligible VUV damage. For patterned structures, in order to reduce VUV damage in the bulk and on feature sidewalls, the combination of both pore stuffing/material densification and absorbing hard-mask is recommended, and/or the use of low VUV-emitting plasma discharge.

Cryogenic etching processes applied to porous low-k materials using SF6/C4F8 plasmas

Cryogenic etching processes in SF6 and SF6/C4F8 plasmas were successfully applied to porous organosilicate glasses. Such materials are low-k candidates for advanced interconnects. Their integration is very challenging because of plasma induced damage. These two chemistries (SF6 and SF6/C4F8) have demonstrated a promising capability of significantly reducing the damage caused by plasma etching. Desorbed species were analyzed during the wafer warm-up from cryogenic to room temperature by in situ mass spectrometry. An equivalent damage layer (EDL) was evaluated by ex situ Fourier transform infrared (FTIR) spectroscopy and in situ ellipsometry. An anneal step at 350 °C seems efficient to completely desorb the remaining CFx species. Anisotropic profiles were obtained using both chemistries. The selectivity is enhanced using SF6/C4F8 process at low temperature.

Surface reactions and structural evolution of organosilicate glass under Ar plasma bombardment

Ab initio molecular dynamics (AIMD) simulations with a variable time step setup were used to investigate collision reactions, collision cascades and subsequent structural changes of organosilicate glass (OSG) low-k dielectric surfaces under Ar plasma bombardment with kinetic energies up to 100eV. Reactions that result in carbon loss and glass network structure changes during plasma treatment were carefully examined, since carbon loss increases the dielectric constant of the low-k dielectrics and the susceptibility to water attack, thus adversely affecting the performance of these dielectric materials in microelectronic devices. Results from AIMD based collision cascade simulations have provided mechanistic understanding of collision reactions during plasma bombardment. It was found that Si–CH3 bond scission and CH3 abstraction reactions are the primary methods of carbon loss. Two distinct mechanisms were identified for CH3 removal from 100eV plasma bombardment, one due to direct momentum transfer from the high-energy incident Ar atom to a Si–O bond on the OSG surface, and the other due to generation of highly reactive atomic oxygen radicals during collision cascades. Atomic oxygen radicals generated during cascades were found to occur at a rate of 1–3 per 100eV Ar impact with radical kinetic energies ranging from 2.2 to 42.6eV, well above the minimum energy of 0.1eV for Si–CH3 bond breakage. Both reaction mechanisms were examined through atomic charge analysis and the results indicated that the Ar atom ionizes when it impacts the OSG surface, causing temporary charge transfer from the Ar atom to the Si–O bond and playing a role in CH3 removal. The cascading atomic oxygen radicals and the ionization of the Ar atom during the Si–CH3 bond scission and CH3 removal demonstrate the secondary effects of Ar bombardment which must be considered during plasma etching of low-k dielectrics.

Optimization of the SEM Working Conditions: EsB Detector at Low Voltage

Material contrast in scanning electron microscopy (SEM) is studied for several kinds of samples with an energy selective backscattered (EsB) electron detector using low voltage. The working conditions are optimized for every specimen and the contrast is quantified, in order to investigate the influence of the most important parameters on the performance: the primary beam voltage, the grid voltage of the EsB detector, and the working distance. Furthermore, strategies to avoid undesirable beam‐induced sample damage like shrinkage in organosilicate glass (OSG) thin films or charging in polymer samples are discussed.

Molecular dynamics simulations of nanoporous organosilicate glasses using Reactive Force Field (ReaxFF)

The structure and properties of nanoporous organosilicate glass (OSG) structure models with 30–70% porosities and different pore morphologies were simulated using molecular dynamics simulations with Reactive Force Field (ReaxFF) potential. The OSG structures were created from nanoporous silica structures generated using classical molecular dynamics (MD) simulations with partial charge pairwise potentials and a subsequent step of adding hydroxide and methyl groups to the dangling bonds and coordination defects. The nanoporous OSG systems were then fully relaxed using molecular dynamics simulations with ReaxFF and detailed structure analysis performed and properties calculated. Analysis of the OSG systems indicated that the structural features (bond distances and angles) as well as the Qn distribution of the nanoporous silica backbone structure are consistent with the features of dense silica and that the geometry of the added methyl groups is experimentally accurate, even after ReaxFF relaxation. The elastic modulus of the nanoporous silica was calculated and found to be 24–31GPa for system with 30% porosity and 0.5–2.5GPa for those with 70% porosity, consistent with previously reported experimental results.

Impact of carbon-doping on time dependent dielectric breakdown of SiO2-based films

Impact of carbon-doping on time dependent dielectric breakdown (TDDB) of three SiO2-based films was investigated under two different breakdown mechanisms, one involving Cu ion injection and the other caused by intrinsic dielectric degradation without Cu injection. In the case of breakdown dominated by dielectric degradation, an undoped SiO2 film shows better TDDB performance than the two other carbon-doped SiO2 or organo-silicate glass films, suggesting that carbon-doping makes the films weaker for dielectric breakdown. In contrast, in the case of breakdown involving Cu ion injection, the two carbon-doped films show better TDDB performance than the undoped SiO2, suggesting that the presence of the carbon slows down Cu ion injection and therefore leads to less TDDB degradation.

Improved Plasma Resistance for Porous Low-k Dielectrics by Pore Stuffing Approach

The pore stuffing method is studied with the objective of improving the plasma induced damage for porous organo-silicate glass low-k dielectrics. Experiments on blanket films show that, pore stuffing reduces the low-k degradation compared to non-protected porous films during plasma etch. The post etch surface roughness is also improved. The protection mechanism is attributed to reduced radical penetration, mainly fluorine and oxygen. The resistance against vacuum UV degradation is also improved by pore stuffing, since polymers used as filling agents have a higher VUV absorption coefficient than Si–O based dielectric network. The protection effect against fluorocarbon-based gas discharges is extensively studied with both blanket and patterned samples. For patterning of porous, non-stuffed low-k films, the addition of polymerizing gas reduces surface plasma damage but this polymerizing protection effect doesn't work well on trench sidewalls. Pore stuffing enables an efficient sidewall protection even with oxidizing gas discharges.

Post Etch Residue Removal and Material Compatibility in BEOL Using Formulated Chemistries

A possible way to realize a 22.5 nm 1⁄2 pitch and beyond BEOL interconnect structures within the low-kmaterial is the partial-trench via first with self-aligned double patterning (SADP) integration approach. A scheme of this BEOL integration stack with the different materials used after patterning is described in Figure 1. In BEOL processing, fluorocarbon-containing plasma is commonly used to pattern silica-based dielectric layers. During the patterning of the low-kdielectric layer, a thin layer of fluoropolymer (CFx-type residues) is intentionally deposited on the dielectric sidewalls and TiN hardmask to ensure anisotropic etching and prevent/minimize dielectric degradation. This polymer layer must be removed from the sidewall and the via bottom prior to the subsequent processing steps to achieve good adhesion and coverage of materials deposited in the etched features. The compatibility requirement is even more stringent for advanced low-kdielectrics, i.e. materials with lowerk-value and higher porosity. The post etch residue (PER) amount and properties are specific and depend on the stack structure and the plasma that is used for patterning. The low-kmaterials and hardmasks that are used in this work are respectively an organo-silicate glass (OSG) type of low-kmaterial withk= 2.4 (~20 % open porosity) and low-stress TiN. Recent results clearly showed the presence of a highly fluorinated layer deposited on the trench sidewalls during the plasma etch based on a fluorocarbon plasma [1-3]. Commodity aqueous cleaning solutions, such as diluted HF, do not efficiently remove the sidewall polymers without etching the underlying layer (lift-off). Therefore, there is a need for commercially available chemicals that can be easily tuned to deal with the different requirements. This study focuses on the use of FOTOPUR® R 2300 mixed with H2O2 for polymer residue removal selectively to other materials (presented in the stack) such as MHM, metals (Cu, W), and porous low-k dielectrics. We will show that TiN etch can be easily tuned by changing the concentration of H2O2.

Effects of plasma and vacuum-ultraviolet exposure on the mechanical properties of low-k porous organosilicate glass

The effects of plasma exposure and vacuum-ultraviolet (VUV) irradiation on the mechanical properties of low-k porous organosilicate glass (SiCOH) dielectric films were investigated. Nanoindentation measurements were made on SiCOH films before and after exposure to an electron-cyclotron-resonance plasma or a monochromatic synchrotron VUV beam, to determine the changes of film hardness, elastic modulus, and crack threshold due to these exposures. This permits the effects of ion bombardment and photon bombardment to be analyzed separately. The role of energetic ions was examined with a variety of inert plasma-exposure conditions. The role of VUV photons was analyzed as a function of synchrotron photon energy. It was found that both energetic ions and VUV photons with energies larger than the bond energy of the Si-O bond cause a significant increase in film hardness along with a smaller increase in elastic modulus and crack threshold. Differential Fourier transform infrared spectra and x-ray photoemission spectroscopy results show that the energetic ions affect the SiCOH properties mainly through physical bombardment, during which the ions transfer their momentum to the Si-O-Si backbone and transform them into more energetically stable Si-O-Si network structures. This results in the Si-O-Si network structures becoming densified. VUV photons assist reaction that increase the number of bridging O3≡Si-O-Si≡O3 bonds and deplete nonbridging O3≡Si-O and C-Si≡O3 bonds. This increased degree of cross linking in porous organosilicate dielectrics can substantially enhance their hardness and elastic modulus while showing no significant film shrinkage or densification.

Mechanisms of oxygen plasma damage of amine and methyl terminated organosilicate low-k dielectrics from ab initio molecular dynamics simulations

Ab initio molecular dynamics simulations based on density functional theory were used to probe the interactions of O(1D) and O(3P) radicals on trimethylcyclictrisiloxane (TMCTS) and aminated TMCTS (TMCTS-NH2)—a model system for vicinal and aminated organosilicate glass (OSG) low-k dielectric materials. The roles of triplet, O(3P), and singlet, O(1D), interactions were investigated, as well as the directional and energetic dependence of the plasma on the product species. O(1D) attack resulted in the development of complex bonding states including Si–CH3O and Si–O–NH2. In contrast the O(3P) bombardment resulted in the removal of H, CH3, C2HO or OH− from the TMCTS molecule. In the amine terminated model OSG molecule, TMCTS-NH2, H and OH–groups were removed but the Si–NH2 bond remained stable over the energy range 0.1–5.0 eV for all but one of the directions investigated. Therefore, the Si–NH2 bond is shown to be significantly more stable than the Si–CH3 and is resistant to changes in bombardment angles and energies. The data explains recent experimental results demonstrating enhanced resistance of aminated OSG films to O2 plasma-induced carbon loss, and suggest that these methods can provide a predictive tool for understanding plasma damage in OSG films with both terminal and methyl carbon species.

Ar ions and oxygen plasma interactions of amine terminated organosilicate glass: A combined experimental and ab initio simulations study

Ar+ ion bombardment (900 eV) of organosilicate glass (OSG) in the presence of NH3 (1 × 10−6 Torr) yields an overlayer containing Si-NHx bonds. The NHx layer decreases the rate of carbon loss from the remaining film upon subsequent oxygen plasma exposure, due to preferential removal of N from the surface region. Ab initio density functional theory calculations have been performed to investigate the stability of the bonds present in low-k dielectrics utilizing a trimethyltrisiloxane model system. Calculated bond energies are 6.30 eV (Si-NH2), 6.27 eV (Si-OH), 5.69 eV (Si-CH3), and 5.54 eV (Si-H). The slightly higher calculated Si-NH2 bond energy is consistent with experiment and indicates that the nitrided OSG surface layer inhibits carbon loss in part by inhibition of O2 diffusion into the bulk.

Corrosion in low dielectric constant Si-O based thin films: Buffer concentration effects

Organosilicate glass (OSG) is often used as an interlayer dielectric (ILD) in high performance integrated circuits. OSG is a brittle material and prone to stress-corrosion cracking reminiscent of that observed in bulk glasses. Of particular concern are chemical-mechanical planarization techniques and wet cleans involving solvents commonly encountered in microelectronics fabrication where the organosilicate film is exposed to aqueous environments. Previous work has focused on the effect of pH, surfactant, and peroxide concentration on the subcritical crack growth of these films. However, little or no attention has focused on the effect of the conjugate acid/base concentration in a buffer. Accordingly, this work examines the “strength” of the buffer solution in both acidic and basic environments. The concentration of the buffer components is varied keeping the ratio of acid/base and therefore pH constant. In addition, the pH was varied by altering the acid/base ratio to ascertain any additional effect of pH. Corrosion tests were conducted with double-cantilever beam fracture mechanics specimens and fracture paths were verified with ATR-FTIR. Shifts in the threshold fracture energy, the lowest energy required for bond rupture in the given environment, GTH, were found to shift to lower values as the concentration of the base in the buffer increased. This effect was found to be much larger than the effect of the hydroxide ion concentration in unbuffered solutions. The results are rationalized in terms of the salient chemical bond breaking process occurring at the crack tip and modeled in terms of the chemical potential of the reactive species.

Effect of water uptake on the fracture behavior of low-k organosilicate glass

Water uptake in porous low-k dielectrics has become a significant challenge for both back-end-of-the-line integration and circuit reliability. This work examines the effects of water uptake on the fracture behavior of nanoporous low-k organosilicate glass. By using annealing dehydration and humidity conditioning, the roles of different water types and their concentrations are analyzed in detail. For as-deposited SiCOH films, annealing dehydration can enhance the resistance to crack occurrence, and these enhancements can be offset by higher humidity conditioning. It was found that the film-cracking threshold can be lowered by in-diffused water in the film as well as by water at the SiCOH/subtract interface. This occurs because the water decreases the film fracture energy and adhesion energy, respectively. By conditioning at high humidity, the variation of the film cracking threshold agrees well with the behavior of the film hardness and modulus of elasticity as a function of relative humidity. The crack morphologies of low-k porous films are also implicitly related to water uptake in the materials. Film cracking thresholds and crack morphologies of UV-cured low-k materials exhibit a weaker dependence on the water uptake, indicating a low degree of hydrophilicity of the SiCOH film after UV curing, which corroborates the previous results. Furthermore, by measuring the surface crack length, the material–fracture toughness can be found. The results demonstrate that neither annealing dehydration nor water uptake have significant effects on fracture toughness of as-deposited SiCOH, while for UV-cured SiCOH, annealing enhances the film-fracture toughness.

Low-k films modification under EUV and VUV radiation

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.

Plasma damage mechanisms in low k organosilicate glass and their inhibition by Ar ion bombardment

In-situ x-ray photoelectron spectroscopy and ex-situ Fourier transform infrared spectroscopy studies of vacuum ultraviolet (VUV) photons with or without O2, and O radicals point to distinct mechanisms of carbon abstraction in nanoporous organosilicate glass (OSG) films. VUV alone in the absence of O2 results in Si-CH3 bond scission and recombination preferentially at silicon monomethyl sites, obeying diffusion kinetics. In contrast, the presence of O2 interferes with recombination, resulting in diffusion-dominated carbon loss kinetics, enhanced Si oxidation, and greatly accelerating the rate of carbon loss in both the near surface and bulk regions of the OSG, at both monomethyl and dimethyl sites. Carbon abstraction due to exposure to (O(3P)) does not follow diffusion kinetics, and such interactions yield a SiO2-like surface layer inhibiting further O diffusion. Results indicate that diffusion-dominated carbon abstraction kinetics previously observed for OSG exposure to O2 plasma damage is primarily attributable to the diffusion of O2 down OSG nanopores, reacting at photoactivated sites, rather than the diffusion of O radicals. OSG pretreatment by 900 eV Ar+ bombardment effectively inhibits both VUV + O2 and O damage mechanisms by formation of ∼1 nm thick SiO2-like surface region that inhibits both O and O2 diffusion.