Re‐emitted positron spectroscopy of cobalt and nickel silicide films

The techniques of re‐emitted positron spectroscopy (RPS) have been employed in the first systematic investigation of the positronic properties of the various stoichiometric phases (M2Si, MSi, and MSi2) of Co and Ni silicide films grown in situ on Si substrates. The positron work function is found to be negative for all of the different phases; thus implanted positrons may be re‐emitted. The energy of the re‐emitted positrons is found to have a surprisingly large variation for the different phases. This feature should provide the image contrast necessary to observe each phase on a microscopic scale using the positron re‐emission microscope (PRM). The positron deformation potential, E+d≡V(∂Σ/∂V), was determined for CoSi2 films; it can be used to estimate the size of the positron diffusion constant, which is found to be comparable to that of other metals. Thus the short positron diffusion length (of order 150 Å) determined from depth‐profiling measurements of CoSi2 films must be a result of positron trapping in either the film or at the interface with the Si substrate. RPS results considered as a function of film thickness support the conclusion that defects in the film (misfit dislocations and/or vacancies) represent the major source of positron trapping.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87603/2/264_1.pd

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. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70582/2/APPLAB-81-8-1447-1.pd

Positron tunneling microscopy

A new technique for analyzing thin film growth processes, called positron tunneling microscopy (PTM), is proposed as an extension of the recently developed positron reemission microscope. The unique feature of a PTM is that image contrast is provided by the exponential reemission probability for positrons tunneling through thin-film overlayers that present an energy barrier to reemission. Results of positron tunneling experiments show that PTM should have monolayer thickness resolution to processes that locally affect either the tunneling barrier's width, such as islanding and subsurface roughness, or the barrier's energy, such as lattice strain in pseudomorphic growth and compositional mixing in interdiffusion alloying. In the case of these latter effects where there may be no topological contrasts at all, experimental results are discussed in greater detail. Comparisons of PTM with existing electron microscopies are presented where appropriate.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28554/1/0000356.pd

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-kk dielectric film (k = 2.2)(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 1.8 nm1.8nm diameter mesopores, and a deep region of ∼ 3 nm∼3nm diameter mesopores. After successively etching of the sealing layer and the isolated 1.8 nm1.8nm 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87843/2/121904_1.pd

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 treatments up to 500 °C500 °C without breakdown or penetration into the porous film. TaN interdiffusion into the silica pores is indicated by the reduction of the Ps lifetime after high annealing temperatures. The validity of using Ps diffusion to test barrier layers designed to inhibit Cu diffusion is discussed. The procedures to standardize the testing of barrier layer integrity and thermal stability using PALS are proposed. Extension to probing barrier layers in realistic vias and trenches should be straightforward. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71187/2/JAPIAU-89-9-5138-1.pd

An overview of the Michigan Positron Microscope Program

An overview of the Michigan Positron Microscope Program is presented with particular emphasis on the second generation microscope that is presently near completion. The design and intended applications of this microscope will be summarized.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87602/2/391_1.pd

Fabrication of 58Co positron sources

A technique for producing 58Co positron sources for use in slow positron beams has been developed. The method has been successfully tested at the [mu]Ci and mCi level. Scaling up of the technique is under way to consistently produce 1-2 Ci sources from 60 g of irradiated nickel.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31106/1/0000002.pd

Positron re-emission microscopy and its applications

Positron re-emission microscopes (PRMs) are distinctly different from electron microscopes in the physical origin of their image contrasts. In a PRM, positrons of several keV energy are implanted into a sample and those positrons that are subsequently re-emitted at several eV are accelerated, focused and imaged. Contrast is produced by any process that affects the transport to, or re-emission from, the sample surface. After an introduction to the basic features of positron microscopy, applications of a PRM in four broad areas of research will be considered. These areas include: materials research, surface catalysis, microelectronic devices and biological systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28264/1/0000009.pd

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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87828/2/161903_1.pd