David W. Gidley

Determination of pore-size distribution in low-dielectric thin films

Positronium annihilation lifetime spectroscopy is used to determine the pore-size distribution in low-dielectric thin films of mesoporous methylsilsesquioxane. A physical model of positronium trapping and annihilating in isolated pores is presented. The systematic dependence of the deduced pore-size distribution on pore shape/dimensionality and sample temperature is predicted using a simple quantum mechanical calculation of positronium annihilation in a rectangular pore. A comparison with an electron microscope image is presented.

Contributions of the nanovoid structure to the moisture absorption properties of epoxy resins

Epoxy resins absorb significant quantities of moisture, typically 1 to 7% by weight for various formulations, which can greatly compromise their physical properties. It is known that polarity of the epoxy is a significant factor in determining the ultimate moisture uptake. However, the contribution from molecular topology still remains vague. In this work, the effects of molecular topology are elucidated by synthesizing novel epoxies where the polarity is maintained constant but the topology is systematically altered. The molecular topology is quantified in part via Positron Annihilation Lifetime Spectroscopy (PALS) in terms of the nanometer-sized voids, or nanovoids, that are also commensurate with typical interchain distances. The nanovoids are separated into their absolute zero and thermally fluctuating fractions by performing PALS measurements over a wide range of temperatures. A strong correlation is observed between the absolute zero hole volume fraction and the ultimate moisture uptake. Although the correlation is clear, the absolute zero hole volume fraction alone is not sufficient to predict the ultimate moisture uptake, and network polarity must also be considered. It is surmised that the role of the nanovoids is to open the epoxy matrix and alleviate steric hindrances that may prevent a water molecule from associating with a polar group.

High thermal conductivity in amorphous polymer blends by engineered interchain interactions

Thermal conductivity is an important property for polymers, as it often affects product reliability (for example, electronics packaging), functionality (for example, thermal interface materials) and/or manufacturing cost. However, polymer thermal conductivities primarily fall within a relatively narrow range (0.1-0.5 W m(-1) K(-1)) and are largely unexplored. Here, we show that a blend of two polymers with high miscibility and appropriately chosen linker structure can yield a dense and homogeneously distributed thermal network. A sharp increase in cross-plane thermal conductivity is observed under these conditions, reaching over 1.5 W m(-1) K(-1) in typical spin-cast polymer blend films of nanoscale thickness, which is approximately an order of magnitude larger than that of other amorphous polymers.

Contributions of the nanovoid structure to the kinetics of moisture transport in epoxy resins

Absorbed moisture can degrade the physical properties of an epoxy resin, jeopardizing the performance of an epoxy-based component. Although specific water- epoxy interactions are known to be very important in determining transport behavior, the role of network topology is not clear. In this article, a series of epoxies in which the topology is systematically varied (and the polarity held constant) is used to explore how topology influences the kinetics of moisture transport. The topology is quantified via the positron annihilation lifetime spectroscopy technique in terms of the size and volume fraction of electron density heterogeneities 5- 6 A in diameter, a dimension comparable to the 3-A kinetic diameter of a water molecule. Surprisingly, the volume fraction of such nanopores does not affect the diffusion coefficient (D) of water in any of the resins studied. For temperatures at and below 35 °C, there is a mild exponential dependence of D on the average nanopore size observed. Otherwise, the kinetics of moisture transport do not appear to depend on the nanopores. However, the initial flux of moisture into the epoxy does appear to correlate with the intrinsic hole volume fraction. That this correlation persists only in the initial stages of absorption is partially understood in terms of the ability of the water to alter the nanopore structure; only in the initial stages of uptake are the nanopores, as quantified in the dry state, relevant to transport. The role of specific epoxy-water interactions are also discussed in terms of transport kinetics. The lack of a correlation between the topology and transport sug- gests that polar interactions, and not topology, provide the rate-limiting step of trans- port.

Reconciling the discrepancies between crystallographic porosity and guest access as exemplified by Zn-HKUST-1.

There are several compounds for which there exists a disconnect between porosity as predicted by crystallography and porosity measured by gas sorption analysis. In this paper, the Zn-based analogue of Cu(3)(btc)(2) (HKUST-1), Zn(3)(btc)(2) (Zn-HKUST-1; btc = 1,3,5-benzenetricarboxylate) is investigated. Conventional analysis of Zn-HKUST-1 by powder X-ray diffraction and gas sorption indicates retention of crystalline structure but negligible nitrogen uptake at 77 K. By using positron annihilation lifetime spectroscopy, a densified surface layer preventing the entry of even small molecular species into the crystal framework is revealed. The material is shown to have inherent surface instability after solvent removal, rendering it impermeable to molecular guests irrespective of handling and processing methods. This previously unobserved surface instability may provide insight into the failure of other microporous coordination polymers to exhibit significant porosity despite crystal structures indicative of regular, interconnected, microporous networks.

Impact of reductive N2∕H2 plasma on porous low-dielectric constant SiCOH thin films

Porous low-dielectric constant (low-κ) SiCOH thin films deposited using a plasma-enhanced chemical-vapor deposition have been comprehensively characterized before and after exposure to a reactive-ion-etch-type plasma of N2 and H2 chemistry. The low-κ film studied in this work is a carbon-doped silicon oxide film with a dielectric constant (κ) of 2.5. Studies show that a top dense layer is formed as a result of significant surface film densification after exposure to N2∕H2 plasma while the underlying bulk layer remains largely unchanged. The top dense layer is found to seal the porous bulk SiCOH film. SiCOH films experienced significant thickness reduction, κ increase, and leakage current degradation after plasma exposure, accompanied by density increase, pore collapse, carbon depletion, and moisture content increase in the top dense layer. Both film densification and removal processes during N2∕H2 plasma treatment were found to play important roles in the thickness reduction and κ increase of this porous ...

Adjusting the Skeleton and Pore Structure of Porous SiCOH Dielectrics

The addition of a carbosilane skeleton precursor to a plasma-enhanced chemical vapor deposition process producing porous SiCOH dielectrics (pSiCOH) is shown to affect the carbon content and the pore structure of pSiCOH. Positron annihilation lifetime spectroscopy (PALS) reveals that added carbosilane reduces pore connectivity. PALS also shows evidence of both smaller ultramicropores ( 0.7 nm) in these materials. The method of N 2 adsorption porosimetry and the alpha-plot analysis provide an understanding of the PALS connectivity results through a correlation of high pore connectivity with a high fraction of porosity as the larger supermicropores. Low pore connectivity correlates with a low fraction of the porosity as supermicropores. A potential figure of merit for pSiCOH is then obtained, which is the amount of the porous volume present as the smallest ultramicropores. This is derived as the difference in porous volumes measured by N 2 adsorption vs toluene adsorption. The addition of a carbosilane precursor to pSiCOH deposition results in pSiCOH with a smaller average pore size and reduced pore connectivity.

Probing diffusion barrier integrity on porous silica low-k thin films using positron annihilation lifetime spectroscopy

The technique of positron annihilation lifetime spectroscopy (PALS) has been used to investigate the continuity and thermal stability of thin barrier layers designed to prevent Cu atom diffusion into porous silica, low-dielectric constant (k) films. Nanoglass™ K2.2-A10C (A10C), a porous organosilicate film, is determined to have interconnected pores with an average tubular-pore diameter of (6.9 ± 0.4) nm. Cu deposited directly on the A10C films is observed to diffuse into the porous structure. The minimum necessary barrier thickness for stable continuity of Ta and TaN layers deposited on A10C is determined by detecting the signal of positronium (Ps) escaping into vacuum. It is found that the 25 nm thick layers do not form continuous barriers. This is confirmed by the presence of holes observed in such films using a transmission electron microscope. Although 35 nm and 45 nm Ta and TaN layers perform effectively at room temperature as Ps barriers, only the Ta-capped samples are able to withstand heat treatm...

Water sensitivity in Zn4O-based MOFs is structure and history dependent.

Moisture can cause irreversible structural collapse in metal-organic frameworks (MOFs) resulting in decreased internal surface areas and pore volumes. The details of such structural collapse with regard to pore size evolution during degradation are currently unknown due to a lack of suitable in situ probes of porosity. Here we acquire MOF porosity data under dynamic conditions by incorporating a flow-through system in tandem with positronium annihilation lifetime spectroscopy (PALS). From the decrease in porosity, we have observed an induction period for water degradation of some Zn4O-based MOFs that signals much greater stability than commonly believed to be possible. The sigmoidal trend in the degradation curve of unfunctionalized MOFs caused by water vapor has been established from the temporal component of pore size evolution as characterized by in situ PALS. IRMOF-3 is found to degrade at a lower relative humidity than MOF-5, a likely consequence of the amine groups in the structure, although, in contrast to MOF-5, residual porosity remains. The presence of an induction period, which itself depends on previous water exposure of the sample (history dependence), and sigmoidal temporal behavior of the moisture-induced degradation mechanism of MOFs was also verified using powder X-ray diffraction analysis and ex situ gas adsorption measurements. Our work provides insight into porosity evolution under application-relevant conditions as well as identifying chemical and structural characteristics influencing stability.

Depth-profiling plasma-induced densification of porous low-k thin films using positronium annihilation lifetime spectroscopy

Positronium annihilation lifetime spectroscopy (PALS) has been used to depth profile the densification induced in a porous low-dielectric constant (k) thin film by typical device integration processing, including exposure to plasmas and oxygen ashing. Such “integration damage” has previously been observed as an undesirable increase in k accompanied by shrinkage in the porous film thickness. PALS confirms that the structural damage is confined to a surface layer of collapsed pores with the underlying pores being undamaged. The dense layer thickness determined by PALS increases with plasma exposure time.