Theo J. Frot

Application of the Protection/Deprotection Strategy to the Science of Porous Materials

The strategy of introducing protecting groups in organic chemistry has enabled major progress in multistep synthesis. [ 1 , 2 ] We believe that applying such a powerful concept to the processing of porous materials could signifi cantly extend the range of their applications. In microelectronics, porous thin fi lms have been used to both increase microprocessor performance and reduce power consumption. The extendibility of this approach relies on the successful integration of highly porous materials (ultralowk , ULK) at ever smaller dimensions. As of today, materials with a dielectric constant ( k ) as low as 2.4 are commonly used in manufacturing. [ 3 ] However, below this dielectric constant, increased pore size and interconnectivity lead to high sensitivity to wet and dry processes, which ultimately results in device failure. Here, we show that the porous structure can be fi lled with an organic polymer, which then acts as a protective agent during the various aggressive processing steps. We have demonstrated the effi cacy of this approach using a highly porous ( k = 2.0) organosilicate dielectric fi lm. The fi lled composite fi lm was almost impervious to processing damage and its porosity was fully regenerated after integration, while the unprotected, porous fi lm suffered severe structural and physical damage. We believe that this protection strategy will facilitate the integration of materials at k < 2.4 in future electronic devices. [ 4 ] In addition, using this concept it is possible to selectively block pores of different size and/or further independently modify the external and internal thin-fi lm surfaces. Therefore, it holds great promise for the selective functionalization of porous materials in areas such as membranes, biosensors, and catalysis. [ 5 , 6 ]

Atmospheric Plasma Deposited Dense Silica Coatings on Plastics

We explore the application of a high-temperature precursor delivery system for depositing high boiling point organosilicate precursors on plastics using atmospheric plasma. Dense silica coatings were deposited on stretched poly(methyl methacrylate), polycarbonate and silicon substrates from the high boiling temperature precursor, 1, 2-bis(triethoxysilyl)ethane, and from two widely used low boiling temperature precursors, tetraethoxysilane and tetramethylcyclotetrasiloxane. The coating deposition rate, molecular network structure, density, Youngs modulus and adhesion to plastics exhibited a strong dependence on the precursor delivery temperature and rate, and the functionality and number of silicon atoms in the precursor molecules. The Youngs modulus of the coatings ranged from 6 to 34 GPa, depending strongly on the coating density. The adhesion of the coatings to plastics was affected by both the chemical structure of the precursor and the extent of exposure of the plastic substrate to the plasma during the initial stage of deposition. The optimum combinations of Youngs modulus and adhesion were achieved with the high boiling point precursor which produced coatings with high Youngs modulus and good adhesion compared to commercial polysiloxane hard coatings on plastics.

Impact of low-k structure and porosity on etch processes

The fabrication of interconnects in integrated circuits requires the use of porous low dielectric constant materials that are unfortunately very sensitive to plasma processes. In this paper, the authors investigate the etch mechanism in fluorocarbon-based plasmas of oxycarbosilane (OCS) copolymer films with varying porosity and dielectric constants. They show that the etch behavior does not depend on the material structure that is disrupted by the ion bombardment during the etch process. The smaller pore size and increased carbon content of the OCS copolymer films minimize plasma-induced damage and prevent the etch stop phenomenon. These superior mechanical properties make OCS copolymer films promising candidates for replacing current low-k dielectric materials in future generation devices.

Investigation of plasma etch damage to porous oxycarbosilane ultra low-k dielectric

There has been much interest recently in porous oxycarbosilane (POCS)-based materials as the ultra-low k dielectric (ULK) in back-end-of-line (BEOL) applications due to their superior mechanical properties compared to traditional organosilicate-based ULK materials at equivalent porosity and dielectric constant. While it is well known that plasma etching and strip processes can cause significant damage to ULK materials in general, little has been reported about the effect of plasma damage to POCS as the ULK material. We investigated the effect of changing the gas discharge chemistry and substrate bias in the dielectric trench etch and also the subsequent effect of the cap-open etch on plasma damage to POCS during BEOL integration. Large differences in surface roughness and damage behaviour were observed by changing the fluorocarbon depositing conditions. These damage behaviour trends will be discussed and potential rationalizations offered based on the formation of pits and craters at the etch front that lead to surface roughness and microtrenching.

Post Porosity Plasma Protection a new approach to integrate k ≤ 2.2 porous ULK materials

Integration of porous low dielectric constant materials constitutes a major roadblock in the reliable manufacturing of back end of the line (BEOL) wiring for the advanced technology nodes. The two main issues for Ultra low-k (ULK) materials are their low mechanical properties and high sensitivity to plasma induced damage (PID). We have developed a new class of bridged oxycarbosilane (OCS) type materials with unique stiffness, and a novel process to enable their integration. The Post Porosity Plasma Protection (P4) consists of refilling the pores of the fully cured porous ULK with an organic material prior to patterning, integrating the protected ULK and thermally removing the filler at the end of the process. We demonstrate the enormous potential of our integrated solution (materials at k≤2.2 and P4 process) on blanket films and its compatibility with integration of single damascene structures at relaxed ground rules.

Hyperconnected molecular glass network architectures with exceptional elastic properties

Hyperconnected network architectures can endow nanomaterials with remarkable mechanical properties that are fundamentally controlled by designing connectivity into the intrinsic molecular structure. For hybrid organic–inorganic nanomaterials, here we show that by using 1,3,5 silyl benzene precursors, the connectivity of a silicon atom within the network extends beyond its chemical coordination number, resulting in a hyperconnected network with exceptional elastic stiffness, higher than that of fully dense silica. The exceptional intrinsic stiffness of these hyperconnected glass networks is demonstrated with molecular dynamics models and these model predictions are calibrated through the synthesis and characterization of an intrinsically porous hybrid glass processed from 1,3,5(triethoxysilyl)benzene. The proposed molecular design strategy applies to any materials system wherein the mechanical properties are controlled by the underlying network connectivity.Organic—inorganic glasses can possess unique properties and functionalities, but their poor mechanical strength and stiffness typically limit their applicability. Here the authors demonstrate that inducing hyperconnectivity into silicon-based glass networks endows them with exceptional elastic stiffness.

Future of PECVD and spin-on ultra low-k materials

A few years ago, we developed at the IBM Almaden Research Center, the concept of introducing carbon in low-k materials in the form of bridging units between the silicon atoms and not as a pendant methyl group. Since then, this strategy has been widely adopted among the semi-conductor industry and the most advanced spin-on and PECVD ULK materials are now based on this model. This paper addresses the concept of designing ultra low-k bridged materials to achieve the best mechanical properties, to control the pore size and connectivity and to prevent plasma damage.