Darshan D. Gandhi

Annealing-induced interfacial toughening using a molecular nanolayer

Self-assembled molecular nanolayers (MNLs) composed of short organic chains and terminated with desired functional groups are attractive for modifying surface properties for a variety of applications. For example, organosilane MNLs are used as lubricants, in nanolithography, for corrosion protection and in the crystallization of biominerals. Recent work has explored uses of MNLs at thin-film interfaces, both as active components in molecular devices, and as passive layers, inhibiting interfacial diffusion, promoting adhesion and toughening brittle nanoporous structures. The relatively low stability of MNLs on surfaces at temperatures above 350–400 °C (refs 12, 13), as a result of desorption or degradation, limits the use of surface MNLs in high-temperature applications. Here we harness MNLs at thin-film interfaces at temperatures higher than the MNL desorption temperature to fortify copper–dielectric interfaces relevant to wiring in micro- and nano-electronic devices. Annealing Cu/MNL/SiO2 structures at 400–700 °C results in interfaces that are five times tougher than pristine Cu/SiO2 structures, yielding values exceeding ∼20 J m-2. Previously, similarly high toughness values have only been obtained using micrometre-thick interfacial layers. Electron spectroscopy of fracture surfaces and density functional theory modelling of molecular stretching and fracture show that toughening arises from thermally activated interfacial siloxane bridging that enables the MNL to be strongly linked to both the adjacent layers at the interface, and suppresses MNL desorption. We anticipate that our findings will open up opportunities for molecular-level tailoring of a variety of interfacial properties, at processing temperatures higher than previously envisaged, for applications where microlayers are not a viable option—such as in nanodevices or in thermally resistant molecular-inorganic hybrid devices.

Surfactant-Directed Synthesis of Branched Bismuth Telluride/Sulfide Core/Shell Nanorods†

Branched core/shell bismuth telluride/bismuth sulfide nanorod heterostructures are prepared by using a biomimetic surfactant, L-glutathionic acid. Trigonal nanocrystals of bismuth telluride are encapsulated by nanoscopic shells of orthorhombic bismuth sulfide. Crystallographic twinning causes shell branching. Such heteronanostructures are attractive for thermoelectric power generation and cooling applications.

Suppression of chemical and electrical instabilities in mesoporous silica films by molecular capping

Mesoporous silica (MPS) thin films are attractive for achieving low relative dielectric permittivity (low k) interlayer isolation in integrated circuit wiring, but are susceptible to instabilities in electrical behavior due to water uptake and copper diffusion. Here, we show that capping MPS films with a trimethyl-group terminated organosilane layer irreversibly suppresses moisture-induced capacitance instabilities, and decreases the relative dielectric permittivity and Cu-induced leakage currents. Analysis of capacitance-voltage and current-voltage characteristics along with infrared spectroscopy shows that the trimethyl organosilanes inhibit hydrogen bonding of water molecules by rendering the dielectric surfaces hydrophobic. These features are promising for tailoring the chemical and interfacial properties and reliability of porous dielectric materials for insulation in device wiring applications.

Molecular-nanolayer-induced suppression of in-plane Cu transport at Cu-silica interfaces

Recent reports have shown that molecular nanolayers (MNLs) can be used to inhibit Cu diffusion across Cu-dielectric interfaces in nanodevice wiring. Here, we demonstrate that MNLs can curtail in-plane interfacial Cu transport. Cu lines embedded in SiO2 in interdigitated comb configurations were passivated by organosilane MNLs with thiol, amino-phenyl, and amino-propyl termini. Leakage current and breakdown voltage measurements at 0–1.4MV∕cm electric fields reveal that amino-phenyl-terminated MNLs are the most effective in inhibiting in-plane leakage, likely due to Cu–N complex formation. Our results suggest that MNLs with appropriate termini could be used to tailor the stability and reliability of device wiring structures.

Ultraviolet-oxidized mercaptan-terminated organosilane nanolayers as diffusion barriers at Cu-silica interfaces

We demonstrate the use of UV-exposed molecular nanolayers (MNLs) of 3-mercaptan-propyl-trimethoxysilane to inhibit copper-transport across Cu–SiO2 interfaces more efficiently than the pristine MNLs. Bias-thermal-annealing tests of Cu∕MNL∕SiO2∕Si(001)∕Al capacitors, with MNLs exposed to 254nm UV radiation, exhibit enhanced barrier properties to Cu diffusion, when compared with capacitors with MNLs not exposed to UV light. X-ray photoelectron spectroscopy reveals that UV exposure converts the mercaptan termini to sulfonates, which are more effective in inhibiting Cu diffusion. Our findings are of importance for tailoring the chemical and mechanical integrity of interfaces for use in applications such as nanodevice wiring and molecular electronics.

Copper diffusion and mechanical toughness at Cu-silica interfaces glued with polyelectrolyte nanolayers

We demonstrate the use of polyallylamine hydrochloride (PAH)-polystyrene sulfonate (PSS) nanolayers to block Cu transport into silica. Cu/PSS-PAH/SiO2 structures show fourfold enhancement in device failure times during bias thermal annealing at 200 °C at an applied electric field of 2 MV/cm, when compared with structures with pristine Cu-SiO2 interfaces. Although the bonding at both Cu-PSS and PAH-SiO2 interfaces are strong, the interfacial toughness measured by the four-point bend tests is ∼2 Jm−2. Spectroscopic analysis of fracture surfaces reveals that weak electrostatic bonding at the PSS-PAH interface is responsible for the low toughness. Similar behavior is observed for Cu-SiO2 interfaces modified with other polyelectrolyte bilayers that inhibit Cu diffusion. Thus, while strong bonding at Cu-barrier and barrier-dielectric interfaces may be sufficient for blocking copper transport across polyelectrolyte bilayers, strong interlayer molecular bonding is a necessary condition for interface toughening. T...

Pore orientation and silylation effects on mesoporous silica film properties

Low dielectric permittivity mesoporous silica (MPS) films with high mechanical and chemical stability are attractive for electrically isolating multilevel wiring in future nanodevices. Here, we show that pore structure is a crucial determinant of chemically induced leakage currents in pristine and silylated MPS films and strongly influences film stiffness and hardness in silylated MPS films. Films with three-dimensional pore networks exhibit superior mechanical properties than films with cylindrical pores oriented exclusively parallel to the surface. The latter, however, exhibit a fourfold higher resilience to copper diffusion. These differences are attributed to the pore structure and its influence on silylation-induced bond-breaking and passivation.

Stabilization of mesoporous silica films using multiple organosilanes

Mesoporous silica (MPS) thin films are attractive for electrically isolating Cu wiring in nanodevices. While porosity is conducive for realizing low-dielectric permittivity k necessary for low signal propagation delays, it renders the MPS susceptible to moisture uptake and metal diffusion. Here, we show that passivating MPS with more than one organosilane with different molecular termini provides several fold greater protection against such instabilities than improvements observed by functionalizing MPS with either type of organosilane individually. MPS films functionalized with bis[3-(triethoxysilyl)propyl] tetrasulfide (BTPTS) and trimethylchlorosilane (TMCS) exhibit at least three orders of magnitude greater time to dielectric breakdown. Bias thermal annealing and infrared spectroscopy measurements indicate that the increased stability is due to Cu blocking by the tetrasulfide groups in BTPTS and decreased moisture uptake is caused by hydrophobic passivation with TMCS. These findings are germane for re...

Effects of silylation on fracture and mechanical properties of mesoporous silica films interfaced with copper

Mesoporous silica (MPS) films are attractive for isolating Cu wiring in nanodevices but are susceptible to pore wall collapse and water and metal uptake. Pore-sealing and chemical passivation with molecular surfactants are potential solutions that could address these challenges. Here, we show that silylated MPS films capped with a Cu overlayer fracture near the Cu/MPS interface at a distance that correlates with the Cu penetration depth into MPS. Pristine MPS films fracture farther from the MPS/Cu interface than silylated MPS, where silylation-induced pore passivation hinders Cu penetration. Silylation also lowers the tensile stress and the fracture toughness of MPS films, but the relative extent of the decreases in these properties decreases the overall driving force for cracking. Such effects of molecular passivation on metal penetration, film stress, and fracture toughness and pathways are important for engineering stable porous dielectrics for nanodevice wiring structures.

Thermal stability of molecularly functionalized mesoporous silica thin films

We report the stability of ordered mesoporous silica (MPS) thin films functionalized with mercaptan- and cyanide-terminated organosilanes upon annealing at temperatures up to 500 °C in vacuum, nitrogen, and air. Electron spectroscopy analyses indicate that the molecules are attached to the surfaces of the films as well as the pores inside the films. The cyanide-functionalized MPS films are stable up to 500 °C in vacuum, N2, and air ambient. In contrast, mercaptan-functionalized MPS films are stable only up to 400 °C in vacuum due to the higher reactivity of mercaptan with oxygen. Our results provide insights into effects of temperature and gas environments on the properties of molecularly functionalized porous dielectrics, and would be important considerations for developing new options for interlayer electrical isolation of nanodevice wiring.