W. E. Frieze

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.

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...

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.

Evolution of nanoscale pore structure in coordination polymers during thermal and chemical exposure revealed by positron annihilation.

[*] Prof. A. J. Matzger, Dr. A. G. Wong-Foy, J. K. Schnobrich Department of Chemistry, University of Michigan 930N. University St., Ann Arbor, MI 48109 (USA) E-mail: matzger@umich.edu Prof. D. W. Gidley, Dr. M. Liu, Dr. W. E. Frieze Department of Physics, University of Michigan 450 Church St. Ann Arbor, MI 48109 (USA) E-mail: gidley@umich.edu Prof. R. S. Vallery Department of Physics, Grand Valley State University 151 Padnos Hall, Allendale, MI 49401 (USA)

Pore Sealing by NH3 Plasma Treatment of Porous Low Dielectric Constant Films

Porous interlayer dielectric films with interconnected pores pose a serious challenge for their integration into next-generation microchips. The opening of interconnected pores in the surface region needs to be sealed to prevent intrusion of atomic layer deposition precursors used to create metal diffusion barriers. In this paper, we report the formation of a thin, nonporous surface layer on a porous methyl-silsesquioxane-based dielectric film by NH 3 plasma treatment. Depth-profiled beam positronium annihilation lifetime spectroscopy was applied to conveniently examine the formation of the dense layer. A nonporous surface layer was readily identified by the curtailment of positronium escape into vacuum through the surface. Among plasma treatments at temperatures ranging from 25 to 300°C for duration of 3-600 s, the best result was achieved at 300°C for 10 s. A very thin skin layer, ∼ 10 nm, could be formed with little damage to the bulk of the low-κ film. This thin skin layer further proved to improve the performance of Ta barriers for Cu diffusion. Chemical analysis, infrared spectroscopy, and sputtering secondary ion mass spectroscopy were also performed to examine how the plasma treatment altered the dielectric film.

How Pore Size and Surface Roughness Affect Diffusion Barrier Continuity on Porous Low-k Films

The structural features that affect continuity of ultrathin diffusion barriers on porous low-dielectric-constant (k) thin films has been investigated. The dimensions of interconnected nanopores in a series of Dow Corning XLK films are found to increase as the dielectric constant becomes smaller. The minimum thickness required for tantalum (Tal to form a continuous barrier layer is affected by the pore size and surface roughness of the porous low-k films according to positronium annihilation lifetime spectroscopy analysis. The films with large pores require thick barrier layers to form effective diffusion harriers, The surface roughness of the porous films as observed by atomic force microscopy has a significant influence on the continuity of diffusion barriers.

Revealing hidden pore structure in nanoporous thin films using positronium annihilation lifetime spectroscopy

The highly inhomogeneous pore morphology of a plasma-enhanced-chemical-vapor-deposited ultralow-k dielectric film (k=2.2) has been revealed using depth-profiled positronium annihilation lifetime spectroscopy (PALS) combined with progressive etch back of the film surface. The film is found to have a dense surface layer, an intermediate layer of 1.8nm diameter mesopores, and a deep region of ∼3nm diameter mesopores. After successively etching of the sealing layer and the isolated 1.8nm pore region, PALS reveals that the underlying large pores are highly interconnected. This inhomogeneous pore structure is proposed to account for observed difficulties in film integration.

Deducing nanopore structure and growth mechanisms in porogen-templated silsesquioxane thin films

Adjusting the functional group of a porogen is found to have a tremendous effect on the pore structre of porous low dielectric constant films with silsesquioxane as the matrix precursor. The pore size and interconnection length measured by positronium annihilation lifetime spectroscopy can be used to deduce the pore shape and its evolution with porosity from templates of isolated porogen molecules through film percolation. Inert, self-linkable, and amphiphilic porogens are demonstrated to randomly aggregate three-dimensionally, linearly polymerize, and form micelles, respectively.

Probing Pore Characteristics in Low-K Thin Films Using Positronium Annihilation Lifetime Spectroscopy

Depth profiled positronium annihilation lifetime spectroscopy (PALS) has been used to probe the pore characteristics (size, distribution, and interconnectivity) in thin, porous films, including silica and organic-based films. The technique is sensitive to all pores (both interconnected and closed) in the size range from 0.3 nm to 300 nm, even in films buried under a diffusion barrier. PALS may be particularly useful in deducing the pore-size distribution in closed-pore systems where gas absorption methods are not available. In this technique a focussed beam of several keV positrons forms positronium (Ps, the electron-positron bound state) with a depth distribution that depends on the selected positron beam energy. Ps inherently localizes in the pores where its natural (vacuum) annihilation lifetime of 142 ns is reduced by collisions with the pore surfaces. The collisionally reduced Ps lifetime is correlated with pore size and is the key feature in transforming a Ps lifetime distribution into a pore size distribution. In thin silica films that have been made porous by a variety of methods the pores are found to be interconnected and an average pore size is determined. In a mesoporous methyl-silsesquioxane film with nominally closed pores a pore size distribution has been determined. The sensitivity of PALS to metal overlayer interdiffusion is demonstrated. PALS is a non-destructive, depth profiling technique with the only requirement that positrons can be implanted into the porous film where Ps can form.

Antihydrogen: Production and Applications

The formation of antihydrogen (\(\overline H\)) is of interest for a variety of reasons. Properties of the \(\overline H\) such as the electronic energy levels, fine structure, Lamb shift, and hyperfine structure can be measured and compared to the corresponding quantities in hydrogen as tests of CPT invariance. Novel investigations of the interactions of \(\overline H\) with atoms and with gravitation can be undertaken. Finally, applications such as the production of polarized antiprotons or the storage of macroscopic quantities of \(\overline H\) can also be pursued.