Brian E. O'Rourke

Slow Positron Beam Apparatus for Surface and Subsurface Analysis of Samples in Air

A technique for investigating atomic-scale defects and/or nanometer (sub-nanometer)-order pores near the surface of samples mounted in air (without vacuum) by positron annihilation spectroscopy (PAS) has been developed. The method relies on the extraction of slow positron beams from the vacuum chamber to air through a thin SiN membrane vacuum window. Using a positron beam with an injection energy of 2.6 keV and a vacuum window with a thickness of 30 nm, samples mounted in air can be investigated by PAS to a depth of ~100 nm.

Recent Developments and Future Plans for the Accelerator Based Slow Positron Facilities at AIST

We describe the recent installation of a new slow positron beamline at AIST and our plans to develop a dedicated superconducting accelerator for positron production. The new positron beamline is already installed and should be operational by the end of this fiscal year (March 2012). Initially positrons will be generated using a 70 MeV electron beam from the existing accelerator directed onto a newly installed converter and moderator assembly. The beamline has two experimental ports both dedicated to positron lifetime spectroscopy, one port with a focused beam (diameter ~ 30 microm) and the other unfocussed (~ 10 mm). A superconducting accelerator for positron production is currently under development. When completed, it will deliver a high frequency (~ MHz), high current (~ mA), short pulse length (< 100 ps) beam to the positron production target. We investigate the possibility of transporting the positron pulses thus produced directly onto samples for lifetime measurement. Such a scheme would remove the necessity for pulse stretching and chopping which is required with the existing LINAC and should allow for greatly increased slow positron transport efficiency.

Development of a dedicated superconducting accelerator for positron production

We report on the current status of a project to develop a dedicated superconducting accelerator for slow positron production at AIST. Two 500 MHz, 5 cell cavities will form the basis of the new accelerator. Initial set-up and preliminary design activities are reported.

Subnanopore filling during water vapor adsorption on microporous silica thin films as seen by low-energy positron annihilation

Positron annihilation lifetime spectroscopy (PALS) using a low-energy positron microbeam extracted into air was applied to elucidating molecular-level pore structures formed in silicon-oxide-backboned microporous thin films under controlled humidity conditions; as a result, a direct observation of the interstitial spaces in the micropores filled with water molecules was achieved. It was demonstrated that PALS using a microbeam extracted into air in combination with water vapor adsorption is a powerful tool for the in-situ elucidation of both open and closed subnanoscaled pores of functional thin materials under practical conditions.

Evaluation of vacancies in positive-tone non-chemically and chemically amplified euv / eb resists ∼ relationship between free-volume and LER ∼

EUV, X-ray and EB mainly induce ionization in resist materials and its energy deposition process is different from ArF exposure. Linear energy transfer (LET) effect for resist sensitivity is very important issue from the viewpoint of radiation induced chemical reactions for high-volume nanofabrication. Therefore the knowledgebase of radiation chemistry is required for understanding the resist performances for EUV lithography. In order to acquire the knowledge of resist materials for EUV / EB lithography from a viewpoint of the free-volume, the positron annihilation lifetime spectroscopy was carried out using positron probe microanalyzer (PPMA) installed at AIST. The size of free-volume can be evaluated from the lifetime of ortho-Positronium (o-Ps). The lifetime and intensity of o-Ps in EB-exposed positive-tone non-chemically amplified (non-CA, ZEP) and CA (UV-III) EUV / EB resists were observed. Moreover, to evaluate the relationship between line edge roughness (LER) and free-volume, EB lithography was carried out, and then sensitivities (E0 and Esize) and LER were measured. For both non-CA and CA resist materials, the changes of free-volume due to evaporation of outgas, polarity change or chain scission would hardly influence on their LER and resolution.

Development of a sample chamber with humidity control for an atmospheric positron probe microanalyzer

In order to perform positron lifetime measurements on thin films under atmospheric conditions, a slow positron microbeam was extracted into air using silicon nitride thin films (30 nm and 200 nm) as a vacuum window. Even the thinner window (30 nm) was found to reliably withstand a differential pressure of 1 atm under various stress tests. By placing the sample in an enclosed chamber through which gas with a fixed, controllable relative humidity (RH) was continuously passed, the RH dependence of the ortho-positronium lifetime for bulk fused silica was examined.

Production and Applications of Intense Pulsed, Slow Positron Beams

Positron annihilation lifetime spectroscopy (PALS) is an experimental technique whereby the lifetime spectrum of positrons injected into a material is measured. Analysis of this spectrum can be used to characterize defects in the material. While radioisotope positron sources are often used for PALS, there are several advantages of using mono-energetic, slow positron beams. In order to measure lifetime spectra with such beams it is necessary to produce a pulsed beam with an extremely short pulse length (~ 200 ps). In this article we discuss the advantages of using pulsed, slow-positron beams, the various methods of beam production and pulsing methods. We focus in particular on intense beams generated by electron accelerators and describe the current status and future plans for the accelerator based facility at AIST.