Geraud Dubois

Low Dielectric Constant Materials

Modern computer microprocessor chips are marvels of engineering complexity. For the current 45 nm technology node, there may be nearly a billion transistors on a chip barely 1 cm2 and more than 10 000 m of wiring connecting and powering these devices distributed over 9-10 wiring levels. This represents quite an advance from the first INTEL 4004B microprocessor chip introduced in 1971 with 10 μm minimum dimensions and 2 300 transistors on the chip! It has been disclosed that advanced microprocessor chips at the 32 nm node will have more than 2 billion transistors.1 For instance, Figure 1 shows a sectional 3D image of a 90 nm IBM microprocessor, containing several hundred million integrated devices and 10 levels of interconnect wiring, designated as the back-end-of-the-line (BEOL). Since the invention of microprocessors, the number of active devices on a chip has been exponentially increasing, approximately doubling every two years. This trend was first described in 1965 by Gordon Moore,2 although the original discussion suggested doubling the number of devices every year, and the phenomenon became popularly known as Moore’s Law. This progress has proven remarkably resilient and has persisted for more than 50 years. The enabler that has permitted these advances is known as scaling, that is, the reduction of minimum device dimensions by lithographic advances (photoresists, tooling, and process integration optimization) by ∼30% for each device generation.3 It allowed more active devices to be incorporated in a given area and improved the operating characteristics of the individual transistors. It should be emphasized that the earlier improvements in chip performance were achieved with very few changes in the materials used in the construction of the chips themselves. The increase of performance with scaling * Corresponding author. E-mail: gdubois@us.ibm.com. † IBM Almaden Research Center. ‡ Stanford University. Willi Volksen received his B.S. in Chemistry (magna cum laude) from New Mexico Institute of Mining and Technology in 1972 and his Ph.D. in Chemistry/Polymer Science from the University of Massachusetts, Lowell, in 1975. He then joined the research group of Prof. Harry Gray/Dr. Alan Rembaum at the California Institute of Technology as a postdoctoral fellow and upon completion of the one-year appointment joined Dr. Rembaum at the Jet Propulsion Laboratory as a Senior Chemist in 1976. In 1977 Dr. Volksen joined the IBM Research Division at the IBM Almaden Research Center in San Jose, CA, where he is an active research staff member in the Advanced Materials Group of the Science and Technology function.

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 ]

Metallocorroles as sensing components for gas sensors: remarkable affinity and selectivity of cobalt(III) corroles for CO vs. O2 and N2

Most commercially available CO detectors are based upon metal oxides or electrochemical cell technologies. None of these approaches use the selective adsorption of CO gas on a molecular complex. Conversely, cobalt(III) corroles can bind small gaseous molecules allowing them for an application as sensing components for gas detectors. Here we describe the ability of cobalt corroles to selectively coordinate carbon monoxide vs. dinitrogen and dioxygen. The coordination properties were determined in the solid state and the adsorption characteristics were compared to those of the reference compound (To-PivPP)Fe(1,2-Me2Im), known for its remarkable CO binding properties. The adsorption data evidence that the selectivity, affinity and capacity of the cobalt(III) corroles for CO are larger than those of the porphyrin complex. However, from a chemical point of view, the selectivity of cobalt(III) corroles for CO vs. O2 is infinite since these derivatives do not bind O2 while (To-PivPP)Fe(1,2-Me2Im) does with an M value (PO2(1/2)/PCO(1/2)) equal to 51. In this manuscript we also show that the affinity of cobalt(III) corroles for CO is closely related to the Lewis acid character of the central cobalt(III) ion and therefore to the nature of the substituents at the periphery of the corrole macroring.

Cyclam complexes containing silica gels for dioxygen adsorption

Several cyclam incorporating silica gels have been synthesised using three different methods, including a sol–gel approach. These various materials show different textures and the macrocycle contents can reach values up to 1.5 mmol g−1. Cu(II) and Co(II) complexes of these modified silica gels have been studied. Finally, the efficiency of [Co(cyclam)]2+ grafted onto silica for binding dioxygen has been determined using ESR spectroscopy and static volumetric gas uptake measurements. The most efficient material behaves as a high-performance dioxygen binding system, showing a very high affinity for dioxygen ((P1/2)1 = 1.91 Torr) coupled with a large total volume of gas adsorbed at 1 atm (3.0 cm3 g−1).

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.

Moisture-Insensitive Polycarbosilane Films with Superior Mechanical Properties

We report cross-linked polycarbosilane (CLPCS) films with superior mechanical properties and insensitivity to moisture. CLPCS are prepared by spin-coating and thermal curing of hexylene-bridged disilacyclobutane (DSCB) rings. The resulting films are siloxane-free and hydrophobic, and present good thermal stability and a low dielectric constant of k = 2.5 without the presence of supermicropores and mesopores. The elastic stiffness and fracture resistance of the films substantially exceed those of traditional porous organosilicate glasses because of their unique molecular structure. Moreover, the films show a remarkable insensitivity to moisture attack, which cannot be achieved by traditional organosilicate glasses containing siloxane bonds. These advantages make the films promising candidates for replacing traditional organosilicate glasses currently used in numerous applications, and for use in emerging nanoscience and energy applications that need protection from moisture and harsh environments.

Improved adhesion of dense silica coatings on polymers by atmospheric plasma pretreatment.

Oxygen atmospheric plasma was used to pretreat polycarbonate (PC) and stretched poly(methyl methacrylate) (PMMA) surfaces in order to enhance the adhesion of the dense silica coatings deposited by atmospheric plasma on the polymer substrates. The treatment time and chemical structure of the polymers were found to be important factors. For PC, a short treatment increased the adhesion energy, while longer treatment times decreased the adhesion. In contrast, plasma pretreatment monotonically decreased the adhesion of PMMA, and pristine PMMA exhibited much higher adhesion than the PC counterpart. We found that adhesion enhancement was achieved through improved chemical bonding, chain interdiffusion, and mechanical interlocking at the coating/substrate interface, after a short atmospheric plasma treatment. Decreased adhesion resulted from overoxidation and low-molecular-weight weak layer formation on the polymer surface by prolonged atmospheric plasma treatment. The dramatic differences in the behavior of PC and PMMA in relation to the plasma treatment time were due to their dissimilar resistance to atmospheric plasma exposure.

Fundamental limits of material toughening in molecularly confined polymers.

The exceptional mechanical properties of polymer nanocomposites are achieved through intimate mixing of the polymer and inorganic phases, which leads to spatial confinement of the polymer phase. In this study we probe the mechanical and fracture properties of polymers in the extreme limits of molecular confinement, where a stiff inorganic phase confines the polymer chains to dimensions far smaller than their bulk radius of gyration. We show that polymers confined at molecular length scales dissipate energy through a confinement-induced molecular bridging mechanism that is distinct from existing entanglement-based theories of polymer deformation and fracture. We demonstrate that the toughening is controlled by the molecular size and the degree of confinement, but is ultimately limited by the strength of individual molecules.

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.

Independent control of adhesive and bulk properties of hybrid silica coatings on polycarbonate.

Transparent polymers are widely used in many applications ranging from automotive windows to microelectronics packaging. However, their intrinsic characteristics, in particular their mechanical properties, are significantly degraded with exposure to different weather conditions. For instance, under humid environment or UV-irradiation, polycarbonate (PC) undergoes depolymerization, leading to the release of Bisphenol A, a molecule presumed to be a hormonal disruptor, potentially causing health problems. This is a serious concern and the new REACH (Registration, Evaluation, Authorization and Restriction of Chemical substances ) program dictates that materials releasing Bisphenol A should be removed from the market by January 1st, 2015 (2012-1442 law). Manufacturers have tried to satisfy this new regulation by depositing atop the PC a dense oxide-like protective coating that would act as a barrier layer. While high hardness, modulus, and density can be achieved by this approach, these coatings suffer from poor adhesion to the PC as evidenced by the numerous delamination events occurring under low scratch constraints. Here, we show that the combination of a N2/H2-plasma treatment of PC before depositing a hybrid organic-inorganic solution leads to a coating displaying elevated hardness, modulus, and density, along with a very high adherence to PC (> 20 J/m(2) as measured by double cantilever beam test). In this study, the sol-gel coatings were composed of hybrid O/I silica (based on organoalkoxysilanes and colloidal silica) and designed to favor covalent bonding between the hybrid network and the surface treated PC, hence increasing the contribution of the plastic deformation from the substrate. Interestingly, double-cantilever beam (DCB) tests showed that the coatings adhesion to PC was the same irrespective of the organoalkoxysilanes/colloidal silica ratio. The versatility of the sol-gel deposition techniques (dip-coating, spray-coating, etc.), together with the excellent mechanical properties and exceptional adherence of this hybrid material to PC should lead to interesting new applications in diverse fields: optical eye-glasses, medical materials, packaging, and so forth.