Molecular Network Reinforcement of Sol–Gel Glasses

Abstract

Nanoporous materials are often accessed by a supramolecular approach using soft matter templates. Porous silicates and organosilicates synthesized by wet chemistry processes have been intensively studied for the last 20 years. Due to the versatility of the sol-gel chemistry, thermally sensitive bonds, organic molecules [1] and polymers [2] can be introduced either at the pore surfaces or in the network, and pore size, distribution and organization can be controlled as well.[3] The combination of the most important properties of the inorganic and organic components, coupled with the control of the "empty space" has suggested many potential applications for these materials.[4] For example, several thin-film applications in emerging optical, electronic, biological and filtration technologies have been envisioned. [5,6] Until now, a major roadblock in the realization of technological applications for these materials seems to be their poor mechanical properties. Indeed, the incorporation of pores into brittle glasses often has a catastrophic effect on their mechanical and fracture properties. This behavior is even more pronounced when moving from silicate to organosilicate materials (ORMOSILS) due to a decrease in network atom connectivity, impacting potential applications for these materials. A case in point is the observation that the fracture energy value, G c , for dense methylsilsesquioxane (MSSQ) glass is typically less than 5 J.m -2 , as compared to 10 J.m -2 for SiO 2 . [7] The presence of methyl substituents, while useful to lower the dielectric constant, generate free volume and impart hydrophobicity, simultaneously reduces the density, modulus, hardness and fracture energy. [7][8][9][10] We have recently demonstrated that these organosilicates show a strong power relationship dependence between film density and mechanical properties (e.g. modulus and fracture resistance), resulting in a rapid decay of the latter with increase in porosity. [7] For MSSQ materials, unfortunate consequences of this are: a) at low level of porosity the modulus decreases extremely rapidly (e.g. 50 % reduction at 15 vol.% porosity), b) at higher porosities (> 30 %), the modulus vs density curves tend to converge, abrogating any anticipated benefits by starting from dense materials with higher modulii.[11] Without acceptable fracture resistance, porous organosilicate glasses would be of little utility in any of the applications cited above. This statement is especially applicable to microelectronics, where dense organosilicate materials containing varying amount of Si-Me substitutents (k = 2.7-3.0) are currently used as the insulating layers in 90 nm interconnect technology, [12,13] and porous analogs will be implemented in future electronic devices. [14,15] Escalating integration issues are anticipated for materials with increased porosity since many of the integration processes require good mechanical properties (e.g. chemical mechanical polishing, chip dicing, wire bonding, etc). [16,17] Mechanical instability and the associated...

Structure–property tuning in hydrothermally stable sol–gel-processed hybrid organosilica molecular sieving membranes

“…The higher flexibility of ethylenebridged silica as compared to pure silica allows fabrication of films with a thickness over 1 lm without crack formation during drying [67]. Its adhesive and cohesive fracture resistance is considerably higher than for carbon-doped silica in a non-bridged configuration [68], and it maintains a high Young's modulus at high porosities [68]. Theoretical models have been developed to predict elastic and fracture properties for other organosilica precursors [69].…”

Organic groups influencing microporosity in organosilicas

“…replacement of Si-O-Si linkages with Si-R or Si-R-Si linkages, influences various functional properties such as mechanical flexibility and fracture resistance [1,2], hydrophobicity [3][4][5] and hydrothermal stability [5][6][7][8]. For applications that make use of the microporosity of organosilica materials, such as separation membranes, varying the nature and location of the organic groups allows tuning of micropore sizes and surface chemistries.…”

Laser anneal of oxycarbosilane low-k film

“…Nevertheless, these studies did not consider laser anneal for porogen removal or investigate its effect on oxycarbosilane films which contain backbone carbon as bridging alkylene (Si-(CH2)x-Si) groups. The latter have been shown [5,6] to be more promising than the conventional carbon-doped oxides where carbon is present solely as a -CH3 group.…”