Porogen Approach Research Articles

SiOCH Hits the Gas

There is an increasing need for on-site and real-time analyses of complex gas mixtures in many application fields such as industry, air quality, security and medicine. One challenge consists in developing complete and portable multi-gas analysis systems allowing in situ and real-time quantitative analysis of complex gas mixtures. A promising approach is based on the integration of resonating devices on silicon chips by using “standard” microelectronic technologies. The fabrication of these gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer to ensure a better collection and concentration of the target molecules. This enhances the sensitivity in conjunction with the limit of detection. The integration of a chemical sensitive layer on to NEMS fabricated on Si wafers leads to specific material requirements. The use of solvent is prohibited in order to avoid the degradation of the Si nanocantilevers due to capillarity forces,Very thin film thickness (less than few hundred of nanometers) should be deposited on the Si beam in order to avoid filling the gap between the resonant cantilever and the electrodes,The deposition technique should provide good uniformity over a large surface (typically on 200 mm Si wafers) for a collective sensor fabrication.And the material should be optimized to absorb the highest level of targeted gas for a given thickness, reversibly and quickly. In this work, the objective was to develop materials deposited using microelectronic compatible techniques in order to detect light alkanes and aromatic volatile organic compounds such as BTEX (for Benzene, Toluene Ethylbenzene and Xylene). Organosilicate materials are potentially good candidates for this application especially because they are nonpolar or weakly polar materials. In order to better understand the impact of the material chemistry on the detection of hydrocarbon gas and optimize the sensitivity of the chemical layer, a large panel of organosilicate thin films were deposited by different chemical vapor deposition (CVD) techniques. First, SiOCH were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), this technique being the most used in the microelectronic industry for dielectric thin-film deposition. Several precursors (diethoxymethylsilane, trimethylsilane or octamethyltrisiloxane) and deposition conditions were used in order to vary their chemical composition and their physical properties. In these SiOCH, the carbon content is mainly under the form of methyl groups bonded to silicon. At the same time, to allow a better flexibility in term of chemical functions, filament assisted chemical vapor deposition (FACVD) was used for the deposition of organosilicate thin films. Indeed, FACVD allow a better control over the precursor fragmentation pathway in comparison to plasma-based techniques. Then, it is possible to add polar groups (such as ethoxy to in case of FACVD of methyltriethoxysilane) or to introduce siloxane rings in the materials (in case of initiated CVD of cyclosiloxane). Finally, the introduction of additional porosity was investigated as another way to potentially improve the sensitivity of the chemical sensitive layers. By using a porogen approach, it is possible to introduce up to 40 % porosity in a PECVD SiOCH. Using a foaming approach, we can go even further (> 50 %). Film properties after deposition and after potential annealing were investigated using multiple characterization techniques (ellipsometry, FTIR and Tof-SIMS). Porosity was characterized by ellipsometric porosimetry and grazing incidence small angle X-ray Scattering. In these material, pores (if any) are nanometrics and porosity is mainly open and interconnected. Thin film response under toluene or pentane exposures was studied using Quartz Crystal Microbalance functionalized with the different SiOCH.Through the synthesis and characterization of these various SiOCH thin films, the role of hydrophobicity, carbon content and specific chemical bonding can be highlighted. It is shown that the hydrophobic nature of SiOCH materials composed of a Si-O-Si backbone with methyl groups bonded to silicon, combined with the presence nanoporosity, lead to very high sensibility. The presence of isolated polar bonds such as ethoxy groups seems beneficial especially for BTEX detection. High partition coefficients toward toluene and pentane can be obtained, at least ten time higher than those measured on more classical polymers such as PDMS. Both deposition techniques (PECVD or FACVD) are able to produce good chemical sensitive layers for the detection of light alkanes and aromatics VOCs and these organosilicates constitute promising solution for the functionalization of NEMS-based gas sensors.Acknowledgment: Developments on FACVD were performed in the frame of a collaboration with TEL, US-Technology Development Center, America.

Graded-Index Fluoropolymer Antireflection Coatings for Invisible Plastic Optics.

Plastic optics are used in an ever-expanding range of applications and yet a durable, high performance antireflection (AR) coating remains elusive for this material class. Here, we introduce a sacrificial porogen approach to produce ultralow refractive index nanoporous fluoropolymer AR coatings via thermal coevaporation of Teflon AF and the small molecule N, N'-bis(3-methylphenyl)- N, N'-diphenylbenzidine (NPD). Using this approach, we demonstrate a five-layer, step-graded AR coating that reduces the solar spectrum-averaged (400 < λ < 2000 nm) reflectance of acrylic plastic to <0.5% for incidence angles up to 40° and withstands over 3 months of outdoor rooftop exposure with minimal degradation. A trilayer coating optimized for the visible range yields luminous reflectivity down to ∼0.1%, effectively rendering double-side coated acrylic plastic invisible under room lighting conditions. Strong adhesion to most optical plastics, an outstanding combination of mechanical, chemical, and environmental durability, and compatibility with commercial vacuum coating systems should enable this AR technology to find widespread practical use.

Tuning the ion channel network of perfluorosulfonated membranes via a facile sacrificial porogen approach

The morphology in terms of the ion channel size, apparent porosity and tortuosity of perfluorosulfonated membranes was modified by using sulfonated polystyrene (SPS) as a sacrificial porogen. The precursor polymers were fabricated via solution-casting of the commercially available Nafion solution together with SPS. Upon swelling SPS was leached from the membrane matrix resulting in the formation of larger nanosized ion channels and clusters and hence a more open membrane matrix in fully hydrated conditions. Below a threshold value of 30% SPS, the membranes maintained high permselectivity and the porogen mainly increased the apparent porosity and network interconnectivity resulting in increased hydraulic permeability, streaming potential coefficient and membrane conductivity. Higher porogen contents resulted in larger characteristic hydrophilic domain sizes, quantified from SAXS/WAXS, and enhanced membrane conductivity up to 300% with only a slight decrease in the permselectivity in dilute LiCl solutions. These results together confirm the effective modification of the charged nanoscopic ion network and clearly indicate that SPS can efficiently be used for precise tuning the PTFE-based membrane matrix to achieve tailored transport properties for specific electrochemical applications.

Functionalized Doubly Porous Networks: From Synthesis to Application in Heterogeneous Catalysis

SummaryThis paper addresses the functionalization of doubly porous poly(2‐hydroxyethyl methacrylate) (PHEMA)‐based networks obtained from the double porogen approach, and their subsequent utilization as catalytic supports in heterogeneous catalysis. Advantage of the availability of hydroxyl groups within the PHEMA‐based frameworks was taken through a chemical modification with carbonyldiimidazole. This key activation led to three main functionatization reactions of the materials via nucleophilic substitution or thiol‐yne “click” addition of amino molecules. Further, in‐situ generation of gold nanoparticles (GNPs) allowed for the preparation of innovative functionalized biporous GNP@PHEMA materials. The as‐obtained hybrid frameworks could efficiently be used as catalytic supports in the reduction of 4‐nitrophenol.

Biporous Crosslinked Polymers With Controlled Pore Size and Connectivity

SummaryA critically selected overview of different parameters affecting the porous features of doubly porous polymeric materials based on poly(2‐hydroxyethyl methacrylate) (PHEMA) and designed via a double porogen approach is addressed. A careful investigation was undertaken so as to highlight the main advantages of the proposed double porogen approach which relied on the distinct and independent control of both porosity levels, i.e. nano‐ and macro‐porosity. The nanoporosity level was produced via a phase separation phenomenon that occurred during the polymerization process in the presence of a porogenic solvent. To generate an interconnected macroporosity, two straightforward and versatile strategies were implemented through the use of sodium chloride (NaCl) macro‐sized particles. They involved the sintering of NaCl particles prior to the generation of PHEMA‐based networks. NaCl particles were fused through two different methods, either by sintering at 730 °C or by Spark Plasma Sintering. Upon porogen removal, doubly porous PHEMA‐based materials were obtained with macropores having NaCl particle imprints in the 100 μm order of magnitude, while the second porosity level laid within the 0.1–1 μm orders of magnitude, as evidenced by mercury intrusion porosimetry and scanning electron microscopy.

Facile fabrication of doubly porous polymeric materials with controlled nano- and macro-porosity

A critical overview of the different parameters affecting the porous features of doubly porous polymeric materials based on poly(2-hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate) (poly(HEMA-co-EGDMA)) and designed via a double porogen templating approach is presented. A thorough investigation was accomplished so as to highlight the main advantages of the proposed double porogen approach which relied on the distinct and independent control of both porosity levels, i.e. nano- and macro-porosity. The nanoporosity level was produced via a phase separation phenomenon that occurred during the polymerization of the comonomers in the presence of various porogenic solvents. To generate the macroporosity, three straightforward and versatile strategies were implemented through the use of sodium chloride (NaCl) macro-sized particles. The first one implied the use of non-sintered NaCl particles that allowed for the creation of non-interconnected macropores. The other two strategies involved the sintering of NaCl particles prior to the generation of the poly(HEMA-co-EGDMA) network. In the latter case, NaCl particles were fused through two different methods, either through sintering at 730 °C or by Spark Plasma Sintering. Upon porogen removal, doubly porous PHEMA-based materials were obtained with macropores having NaCl particle imprints in the 100 μm order of magnitude, while the second porosity level laid within the 10 nm to 10 μm order of magnitude, as evidenced by mercury intrusion porosimetry and scanning electron microscopy, depending on the solvent structure, its volume proportion, and the cross-linker concentration in the polymerization feed. A full porous characterization was carried out in order to clearly understand the effect of such structural parameters on the porosity of the as-obtained doubly porous polymeric materials and their nanoporous counterparts.

Effect of thermal annealing on the structure and properties of plasma enhanced chemical vapor deposited SiCOH film

The development of high-performance integrated circuit chips and the shrinkage of feature sizes according to Moore’s law bring forward continuously the requirements for low dielectric constant (low-k) materials with various excellent properties in the back-end-of-the-line (BEOL) interconnect. Porous SiCOH films prepared by plasma enhanced chemical vapor deposition (PECVD) through a porogen approach are widely applied to industry and extensively studied. Thermal annealing is an important process for fabricating the porous low-k films, which has a great influence on film structure as well as properties. SiCOH films are deposited by PECVD using tetraethoxysilane and limonene as precursors, and annealed at 450 ℃ for 1.5 h under nitrogen atmosphere. The evolutions of film structure and properties during thermal annealing are revealed, and the reaction mechanism for structure change is also proposed. Fourier transform inferred spectroscopy and solid state nuclear magnetic resonance results show that the as-deposited film is an organic-inorganic hybrid film composed of various kinds of Si-O-Si, -CHx, Si-O-CH2CH3, etc. The organic component is removed almost completely during thermal annealing, making a porous film with a Si-O-Si inorganic skeleton. The skeleton is also rearranged at the same time. Deconvolution of the Si-O-Si absorption band of the FTIR spectrum reveals that the cage-like Si-O-Si occupies the major part for both as-deposited and annealed films, while the amount of silicon suboxide Si-O-Si decreases and that of network Si-O-Si increases during thermal annealing, making the film more robust. More C=C and Si-C are formed through chemical reactions between Si-H, -CHx and Si-O-CH2-CH3, and crosslinking is further enhanced. Nitrogen adsorption/desorption isothermal measurement reveals that a large number of micropores with diameter less than 2-3 nm are created during thermal annealing, which is consistent with the removal of organic groups and the existence of cage-like Si-O-Si. As a result, both the refractive index and dielectric constant decrease significantly from 1.476 (λ =630 nm) and 3.45 to 1.365 and 2.60, respectively. Because of the increase of C=C after annealing, extinction coefficient and leakage current density increase. Although there is a shrinkage of 14.7% in film thickness and a reduction of mechanical properties after annealing, the Young’s modulus is still larger than 4 GPa. Considering all kinds of properties, the obtained film appears to be a competitive candidate as inter layer dielectrics in the BEOL interconnect of integrated circuits.

A 6-month in vivo study of polymer/mesenchymal stem cell constructs for cranial defects

Two biodegradable polymers, poly(L-lactide) and poly(ε-caprolactone) were blended (50/50) and used to produce polymeric scaffolds by the dual porogen approach using a salt leaching technique to create pores within the matrix, while supercritical-CO 2 treatment was used to enhance the interconnectivity and to remove impurities from synthesis steps. The scaffolds were highly porous (porosity &gt;90%) with interconnected pore morphologies. These biodegradable scaffolds were evaluated in Sprague Dawley rats for osteoconductive properties over a 6-month period. Bone specimens were analyzed after 1, 3, and 6 months, for bone healing and tissue response. The cortical bone remodeling by controlled osteoblastic and osteoclastic activities as well as the bone marrow elements recovery were semi-quantitatively examined for each group. Excellent integration and biocompatibility behavior was observed in all groups. No adverse tissue responses were observed.

Crosslinking of porous SiOCH films involving Si–O–C bonds: Impact of deposition and curing

This paper focuses on the properties of nanoporous SiOCH thin films deposited using a porogen approach by plasma enhanced chemical vapor deposition. The impact of deposition temperature, porogen loading and porogen removal treatment is investigated using Fourier transform infrared spectroscopy, solid-state nuclear magnetic resonance analysis, and electrical and mechanical measurements. This work shows that a higher deposition temperature allows limiting the film shrinkage during the porogen removal treatment and leads to the best compromise in term of electrical and mechanical properties. Beside, the effect of Si–O–C bonds on the enhancement of mechanical properties is promoted since a typical crosslinking mechanism is highlighted in case of ultraviolet curing.

Grazing incidence small angle x-ray scattering study of the structure of nanoporous ultralow-k dielectrics prepared by plasma enhanced chemical vapor deposition

This paper focuses on the structure of nanoporous SiOCH thin films deposited using a porogen approach by plasma enhanced chemical vapor deposition (PECVD). The grazing incidence small angle x-ray scattering signal demonstrates the existence of a biphase pattern in hybrid films, deposited by PECVD. After porogen removal, there are few differences between pore pattern of optimized ultraviolet (UV) illuminated and thermally treated samples: anisotropy of the pore pattern is observed in both samples, probably due to the porogen degradation. Finally, a kinetic study of porogen degradation by UV shows that the porous structure develops in only a few minutes.

Macroporous photocrosslinked elastomer scaffolds containing microposity: Preparation and in vitro degradation properties

The engineering of soft tissue would benefit from the development of effective biodegradable scaffolds capable of dynamic, elastic loading. For this purpose, highly porous, elastomeric scaffolds containing microporous struts were prepared using a dual porogen approach and a photocrosslinkable elastomer. The combination of paraffin microbeads distributed through a water-in-[star-poly(lactide-co-epsilon-caprolactone) triacrylate dissolved in ethyl acetate] emulsion followed by photocrosslinking generated a macroporous foam scaffold of average porosities between 90% to 93%, with an average pore diameter of 104 +/- 31 microm with struts containing micropores of 3.1 +/- 2 microm average diameter. The mechanical properties of the scaffolds were readily manipulatable by altering the molecular weight of the star-poly(lactide-co-epsilon-caprolactone) triacrylate prepolymer used. The elastomer scaffolds degraded at the same rate as nonporous polymer samples of the same molecular weight, and exhibited similar changes in mass loss, mechanical properties, and sol fraction during in vitro degradation as found with the nonporous scaffolds. The modulus and stress at break of the scaffolds decreased continuously during degradation while the strain at break remained constant. These scaffolds show potential for use in the engineering of soft tissues.

Ultralow k films by using a plasma-enhanced chemical vapor deposition porogen approach: Study of the precursor reaction mechanisms

As interconnects are scaled down, much effort is made to achieve ultralow k material with a dielectric constant lower than 2.5. Thus, many new precursors are investigated in plasma-enhanced chemical vapor deposition. This is particularly true with the porogen approach where two molecules are used: an organosilicon to create the silicon matrix and an organic molecule “porogen” that creates material porosity during a post-treatment such as annealing. In this article, the influence of the organosilicon molecular structure is investigated. Two “matrix precursors” with different structures are therefore compared. The first one, referred to as D5, has a ring structure (decamethyl pentacyclosiloxane); the second one, referred to as DEOMS, has a star structure (diethoxymethyl silane). The porogen organic molecule, referred to as CHO, is cyclohexen oxide. The fragmentation paths of the precursor molecules in the plasma are investigated by quadrupole mass spectroscopy and the film structure is studied by Fourier transform infrared spectroscopy. The mass spectroscopy analysis shows that the fragmentation in plasma is highest for DEOMS, intermediate for CHO, and lowest for D5 in comparable process conditions. At the maximum plasma power setting, the loss rate, which yields molecule consumption, is 43%–81% for the D5-CHO mixture, respectively, and 73%–37% for the DEOMS-CHO mixture, respectively. This is related to higher bond-dissociation energy for the siloxane (SiOSi) link in D5 than silane (SiH), silylethoxyde (SiOC2H5) in DEOMS, or CC and epoxy cycle in CHO. Indeed, a higher electron-energy relative threshold for dissociation under electron impact is measured for D5 (around 7eV) than for DEOMS and CHO (around 4eV). Moreover, the fragment structures differ from one precursor to another. Methyl groups are abstracted from D5 and a few polysiloxane chains are produced from pentacycle opening and fragmentation. In the case of DEOMS, many single silicon-atom-bearing species are produced. Consequently, the D5-based films have significant retention of siloxane cycles and a less diverse silicon environment than DEOMS-based films. The porogen incoporation (organic phase) was evidenced through alkyl group absorption and is more important with DEOMS than D5 as a matrix precursor. Moreover, the epoxy moiety of the porogen seems scavenged by the plasma and is not retained in the films. These results confirm other studies that discarded D5-CHO chemistry for porous dielectric achievement in an industrial reactor, whereas DEOMS-CHO leads to porous films with an ultralow dielectric constant. Eventually, this study shows that the usefulness of cyclosiloxane precursors is not straightforward.

Ultralow k using a plasma enhanced chemical vapor deposition porogen approach: Matrix structure and porogen loading influences

To improve integrated circuit performance, microelectronic chip interconnections need dielectric materials with ultralow k (ULK) values. Porous a-SiOCH, an ULK material, can be created using a two step strategy called a porogen approach. The first step consists of a hybrid film deposition, which is an a-SiOCH matrix containing an organic sacrificial phase. Afterwards, the organic phase is removed during a post-treatment to generate the porosity. In this work, hybrid deposition was performed by plasma enhanced chemical vapor deposition and the post-treatment was a thermal annealing. Firstly, hybrid films with different a-SiOCH matrix structures were created using two matrix precursors [decamethylcyclopentasiloxane (DMCPS) and diethoxymethylsilane (DEMS)] and an O2 addition in a plasma gas feed. For the same porogen loading, the shrinkage behavior during the porogen removal is correlated to the matrix structure. Fourier transform infrared spectroscopy and Si29 solid nuclear magnetic resonance are both used to determine the structure. The O3SiMe1 is favorable to prevent high film shrinkage and the O2SiMe2 leads to high film shrinkage and absence of porosity generation. To prevent the existence of this type of structure, DEMS+O2 appears more appropriate as a matrix precursor than DEMS only or DMCPS. Secondly, using the appropriate matrix structure, the influence of the porogen loading on the porosity creation is studied. Hybrids with different porogen loadings are achieved by changing the porogen precursor ratio in the plasma gas feed. The porogen conversion into porosity is studied for different porogen loadings and the results indicate the existence of a porogen loading threshold. Above it, there is no more porosity generation because of the too high film shrinkage during the porogen removal. This behavior is explained by a too low matrix ratio in the hybrid film. For the high porogen loadings, the matrix skeleton (mainly constituted by SiOSi bridging bonds) is not in sufficient quantity to prevent such film shrinkage. The porous a-SiOCH created with the most favorable matrix structure and porogen loading has the ultralow k value of 2.3.

Thermal furnace and Ultraviolet assisted curing impact on SiOCH spin-on ultra low dielectric constant materials

In recent years, the continuous progression of ultra-large scale integration has driven the emergence of technological solutions. In particular, major challenges have been faced for the fabrication of interconnect structures, where ultra low dielectric constants are required to decrease the parasitic capacitances between metal lines. Porous material, obtained using the porogen approach, is the main candidate investigated. The curing process is critical for achieving a good control of final film structure. The integration of such material requires a good chemical and mechanical stability, particularly to maintain the structure integrity during the stressing steps: chemical mechanical polishing and packaging. In this work, Ultraviolet assisted thermal cure (or UV curing) is investigated as an alternative solution to the conventional thermal curing. Chemical and physical analyses reveal that the best porogen removal efficiency and the enhancement of matrix crosslinking are achieved when the material is UV cured. This crosslinking improvement (as indicated by higher Si–O–Si bond density in the fourier transformed infra-red spectra) can be correlated to better mechanical properties. Significantly better electrical properties (dielectric constant, leakage current and breakdown voltage) are obtained with better integrity (no moisture uptake after 1 week storage in humid atmosphere 85 °C/85% relative humidity) when the dielectric is optimally cured. Porosity evaluation reveals similar results between both curing processes with slightly larger pore size in the case of the UV cured film. Finally, a basic model is described to illustrate how the UV assisted thermal cure may improve the crosslinking in comparison to the thermal curing. Selective UV action is proposed to explain the curing process kinetics.

Porous ultra low k deposited by PECVD: From deposition to material properties

A new challenge for semiconductor industry is to reach low permittivity ( k < 2.4) required for advanced microelectronic interconnections. Porosity introduction in SiOCH matrix is the main research field investigated. Several synthesis ways can be used to deposit these nanoporous SiOCH layers. In this work, plasma enhanced chemical vapor deposition (PECVD) of this material using three different concepts (porogen approach, cyclic precursor, foaming) is studied and compared to spin-coated films. It is shown that the porogen approach, which consists in the co-deposition of a matrix precursor and a sacrificial organic porogen followed by a post-treatment to remove the organic porogen phase, allows to create porosity in a thin SiOCH film. The impact of deposition process and curing on basic film properties such as dielectric constant, porosity and elastic properties are studied. Foaming appears as another attractive way to deposit nanoporous SiOCH using PECVD.

Etch mechanisms of hybrid low-k material (SiOCH with porogen) in fluorocarbon based plasma

This study is dedicated to the etching of a low-k material using the late porogen removal approach. In this approach, the porogen is removed by thermal annealing or UV curing after patterning or copper filling. Starting from a CF4 based plasma, the etch rate decreases with an Ar dilution or by adding a polymerizing gas such as CH2F2. On the other hand, the etch rate increases with O2 or N2 addition in the fluorocarbon gas mixture. X-ray photoelectron spectroscopy analyses show that the etching is controlled by the thickness of the fluorocarbon layer formed at the top surface of the hybrid material during the steady state etch regime. An etch stop phenomenon is even observed with highly polymerizing etch chemistries due to the formation of a thick fluorocarbon layer. Infrared spectroscopy analysis shows that the hybrid film is not damaged by the different etching chemistries investigated in contrast to the porous material.

Comparison Between E-beam and Ultraviolet Curing to Perform Porous a-SiOC : H

Porous ultralow-k films are required by the microelectronics industry as interlayer dielectrics for technologies and below. These porous insulating films can be deposited by plasma-enhanced chemical vapor deposition using a porogen approach. It consists of the codeposition of a matrix precursor and a sacrificial organic porogen, and then on a post-treatment to remove the organic porogen phase and create porosity in the film. In this work, an e-beam assisted thermal curing was compared to an ultraviolet-assisted thermal curing. Basic film properties such as , film shrinkage, porosity, pore size, and pore size distribution were evaluated. NMR and Fourier transform infrared analyses were used to study the chemical modifications induced by the post-treatment. These analyses show that the post-treatment impact depends on the radiation used. Both treatments lead to a removal of terminal nonbridging bonds such as , , and and can contribute to a subsequent formation of crosslinks. Both treatments remove methyls from , but the e-beam induces a bond increase while the UV bulb used decreases the contribution. The cross-linking improvement induces an increase of Young’s modulus, the elastic properties being mainly correlated to the volumic bond concentration in the film.

The double porogen approach as a new technique for the fabrication of interconnected poly(L-lactic acid) and starch based biodegradable scaffolds

One of the most widely used fabrication methods of three dimensional porous scaffolds involves compression moulding of a polymer salt mixture, followed by salt leaching. However, the scaffolds prepared by this technique have typically limited interconnectivity. In this study, besides salt particles, an additional polymeric porogen, poly(ethylene oxide), PEO, was added to poly(L-lactic acid), PLLA, to enhance the interconnectivity of the scaffolds. Compression moulded specimens were quenched and put into water, where PEO crystallized and phase separated. Following the leaching of PEO fraction, the permeability and interconnectivity among the macropores formed by salt leaching could be observed. The porosities obtained in the prepared scaffolds were between 76 to 86%. Moreover, the highest porosity of 86% was obtained with minimum fraction of total porogen. The water absorption of the porous scaffolds prepared with PEO could vary between 280 to 450% while water uptake of pure PLLA scaffolds was about 93%. The increase of interconnectivity induced by compounding PLLA with PEO could also be obtained in porous PLLA/starch blends and PLLA/hydroxyapatite composites demonstrating the versatility and wide applicability of this preparation protocol. The simplicity of this organic solvent free preparation procedure of three-dimensional porous scaffolds with high interconnectivity and high surface area to volume ratio holds a promise for several tissue engineering applications.

Structural Transformation in PECVD Ultralow-k Material during Porogen Removal by UV Assisted Thermal Curing

ABSTRACTNext-generation microelectronic interconnects require the use of dielectrics with continuously lower permittivity (k) to overcome limitations induced by crosstalk parasitic signal delay. Using PECVD, Ultralow-k film (ULK, k ≤ 2.5) can be developed by creating pore inclusions within an organosilicate matrix through porogen approach. Both ULK deposition and subsequent curing process has to be adjusted in order to achieve optimized mechanical and electrical properties and maintain stability during integration. For this concern, the attention was recently focused on ultraviolet (UV) radiation to sustain the thermal curing. In the present work, a fundamental understanding of structural transformations occurring during porogen extraction from as-deposited ULK materials when exposed to thermal-assisted UV radiation is proposed. This thermal-assisted UV cure technique is very efficient in porogen removal since in a few minutes the desired porosity is reached. During the first stage of curing, the film shrinks strongly whereas the porosity is created. After porogen removal step, the porous film continues shrinking under UV radiation leading to an increase of SiOSi bonds concentration (film densification). The normalized FTIR SiOSi peak increase during UV curing (related in literature to an improvement of mechanical properties) is mainly due to the film densification, in addition to the SiOSi bridging bond creation. In this case, correlation is found between shrinkage and elastic modulus.

Extendibility of the PECVD Porogen Approach for ULK Materials

ABSTRACTThe increase of integrated circuits performances requires ultra-low dielectric constant (ULK) materials to minimize the drawbacks of miniaturization. Amorphous SiOCH are promising candidates for ULK materials as porosity can be introduced via a two steps elaboration. In a first step, organo-silicon species and organic species are co-deposited by PECVD. Then, a thermal annealing, alone or assisted by UV radiation, removes the organic labile phase and creates pore inclusions into the final material. In this work, the extendibility of this porogen approach is investigated in order to lower the dielectric constant. An increase of the porogen loading in hybrid film is studied by tuning the precursors ratio injected in the plasma gas feed. The increase of organic species amount is operated in order to create more pores sites. However, the post-treatment does not lead automatically to higher porosity. Actually, an increase of the porosity is observed only until a porogen loading limit and decreases above this limit. The shrinkage of the film during the post-treatment can explain this limitation. For high ratios of porogen, the film shrinkage increases drastically and leads to a decrease of the porosity finally created. At last, the link between porosity and dielectric constant is enlightened and a minimum in term of K value is reached with both post-treatments: dielectric constant of 2.1 and 2.3 are obtained using respectively thermal treatment and UV curing.