Depth-profiling Pore Morphology in Nanoporous Thin Films Using Positronium Lifetime Annihilation Spectroscopy
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Depth-profiling Pore Morphology in Nanoporous Thin Films Using Positronium Lifetime Annihilation Spectroscopy Richard S. Vallery, Hua-Gen Peng, William E. Frieze, David W. Gidley, 1 Darren L. Moore and 1 Richard J. Carter Department of Physics, University of Michigan, Ann Arbor, MI 48109 1 LSI Logic Corporation, 23400 NE Gilsan Street, Gresham, OR 97030 ABSTRACT Positronium annihilation lifetime spectroscopy (PALS) using a positron beam is a proven technique to characterize porosity in amorphous thin film materials. The capability to control the depth of the implanted positrons is unique to beams as compared to traditional bulk PALS techniques. By increasing the positron beam energy, positrons are implanted deeper into the film. Control of the positron implantation depth in beam-PALS allows analysis of sub- micron films, investigation of depth-dependent film inhomogeneities, determination of pore interconnection lengths, and access to buried films under barrier layers. Details on PALS depth profiling and an example of applying the technique to a plasma-enhanced-chemical- vapor- deposited (PECVD) porous film are presented. INTRODUCTION Advances in thin- film porous materials which have a wide range of applications have generally trended toward engineering novel films with pores at the nanometer level. Various techniques are used to perform metrology of porous materials including x-ray and neutron scattering, ellipsometric porosimetry, and gas absorption [1]. In the last few years beam-based positronium annihilation lifetime spectroscopy (PALS) has proven to be a novel technique able to characterize the porous structure of thin films at the nanometer scale. For example, beam PALS has been used to analyze advanced low dielectric constant (low-k) films, diffusion barriers, and thin polymer films. The application of PALS to investigate defects and voids is well known and has been used for several decades to characterize bulk materials [2]. Polymers in particular, with their subnanometer voids, have been extensively studied. The bulk PALS technique uses positrons directly emitted from a radioactive source, such as Na-22, to study a millimeter thick specimen. The positrons will thermalize in the material and may eventually form positronium (Ps, the bound state of an electron and its anti-particle, the positron) which will tend to be localized in open volume defects, such as pores. The Ps lifetime in these small pores is substantially reduced from the nominal vacuum lifetime of approximately 140 ns (typical lifetimes in polymers are 2 – 4 ns). The shortened Ps lifetime is directly related to the size of the pore in which the Ps annihilates. The Tao-Eldrup model has long been used to calibrate the lifetime to the pore volume [3,4]. A major limitation of the bulk PALS technique arises from the high energy of the beta decay positrons, which penetrate hundreds of microns in the material before annihilation. This limits the technique to samples that are millimeters thick in order to sufficiently stop all of the positrons
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