Atsuki Matsumura

Superconductive radiation detector with large sensitive area (series‐connected STJ detector)

One of the serious problems in the development of particle detectors with superconducting tunnel junctions (STJ) is the difficulty in devising a large area that is sensitive to nuclear radiations. The problem has been solved by using an assembly of micro STJs connected in series (series‐connected STJ detector), which is designed for detecting nonthermal phonons created by irradiating the rear substrate (sapphire) with nuclear radiations. The present detector is sensitive not only to ionizing events but also to nonionizing events, providing us with a new method to detect nuclear radiations. Some features of the series‐connected STJ detector are described in this article.

Nb/Al‐AlOx/Nb superconductor detector using a single‐crystal Nb layer

Single‐crystal Nb films coated with a thin polycrystalline Nb film were adopted as the bottom layer of Nb/Al‐AlOx/Nb superconducting tunnel junctions. The junctions were applied to detection of x rays from 55Fe. The signal charge induced by each radiation was one order of magnitude larger than that from usual junctions and was two orders of magnitude larger than that from semiconductor Si detectors.

X-Ray Detection with Nb/AlOx/Nb Superconductor Detectors

We have detected 5.9 keV X-rays with Nb/AlOx/Nb superconducting tunnel junctions which are resistant to thermal cycling. Using an anodization method to fabricate the junctions, X-ray signals could be detected. We propose an experimental method to determine the fundamental limit of energy resolution which comes from the statistical fluctuation of signal charges. The limit obtained for one of our samples was smaller than 135 eV for 5.9 keV radiation.

Influence of Crystal Originated Particles on Gate Oxide Breakdown.

Defects causing gate oxide breakdown in Czochralski silicon were evaluated using a location marking technique by copper plating. The cross sectional structure of the defect was observed by a combination technique of focused ion beam (FIB) etching at the marked location and transmission electron microscopy (TEM) and found to have similar appearances and sizes to those of crystal originated particles (COPs) observed by atomic force microscopy (AFM). Correlation between locations of the defects at a wafer surface determined using the copper plating and those of COPs determined using a particle counter was investigated for various electric field applied for the copper plating. We found that COPs are one of the major factors causing gate oxide breakdown at fields around 4 MV/cm, while other factors become dominant when a higher electric field is applied.

Analysis of Buried-Oxide Dielectric Breakdown Mechanism in Low-Dose Separation by Implanted Oxygen (SIMOX) Substrates Fabricated by Internal Thermal Oxidation (ITOX) Process

Buried-oxide (BOX) dielectric breakdown behavior of low-dose separation by implanted oxygen (SIMOX) substrates fabricated by the internal thermal oxidation (ITOX) process was analyzed. From the time-zero dielectric breakdown (TZDB) characterization of the metal-oxide-semiconductor (MOS) capacitors using BOX as a dielectric, of various areas, BOX breakdown was found to be dominated by the electrically weak spots (EWSs) distributed randomly in the BOX layer. The densities of EWSs show good correlation with those of Si islands in the BOX for several samples, indicating that the Si islands are the main cause of BOX breakdown. A model for extracting the EWS density as a function of breakdown field is proposed, the appropriateness of which is verified by its application to the experimental results. Using the proposed model, the dependence of Si island density and their thickness distribution on oxygen ion dose and ITOX layer thickness was investigated, indicating that both dose reduction and ITOX enhancement can effectively reduce the Si island density. By combining the dose reduction and ITOX enhancement, BOX breakdown characteristics, almost comparable to that of the thermally grown oxide were attained, even for a relatively large capacitor area of 7.85×10-3 cm2, revealing the high performance of ITOX-SIMOX technology.

X-ray detection with an Nb-based junction and investigation of series-junction detectors

Two topics on our recent investigations into superconducting tunnel junction (STJ) detectors are discussed: the single Nb/Al-AlOx/Al/Nb junction for high-resolution x-ray detection and the series-connected superconducting tunnel junction detectors (SCSDs) with high detection efficiency. Using a single Nb/Al-AlOx/Al/Nb junction with rather large area of 178 X 178 micrometers 2, we obtained a high energy resolution of 88 eV for 5.9-keV x- rays. The signal-to-noise ratio of the SCSD is discussed from a simple theoretical formulation of electronics, showing that the series-connected STJs can be developed as a nuclear radiation detector both with high energy resolution and high efficiency. Recent experimental results of our SCSD are also given in this paper.

Gate oxide integrity on ITOX-SIMOX substrates and influence of test device geometry on characterization

The integrity of gate oxides on low-dose separation by implanted oxygen (SIMOX) substrates fabricated by the internal-thermal-oxidation (ITOX) process, so-called ITOX-SIMOX substrates, was evaluated, and the influence of test device geometry on the characterization was investigated. Characterization of time-dependent dielectric breakdown (TDDB) was performed for a gate oxide of 8.6-nm thick using lateral test devices. Experimental results show considerable influence of gate electrode geometry on the gate oxide integrity (GOI) characteristics. This can be explained by a model that includes a lateral parasitic resistance in the superficial Si layer beneath the gate electrode. Based on analysis using this model, a test device with a small gate array was proposed to reduce the influence of lateral parasitic resistance, and the advantage of the device was verified.

A new data processing method to determine accurate density and size distribution of defects by bright field infrared laser interferometer (optical precipitate profiler)

We have addressed essential problems for the determination of the size distribution and volume density of crystal defects by bright field infrared laser interferometer [optical precipitate profiler (OPP)]. By taking into consideration the influences of the intensity profile and phase shift of the incident laser beam along the optical axis, we developed a new statistical data processing method to determine the true size distribution and volume density of the crystal defects from the raw OPP data. Using this method we evaluated the size distribution and volume density of oxygen precipitates introduced in a CZ–Si wafer and the validity of the new method was confirmed by comparing the obtained results with those obtained by the etching method and transmission electron microscopy.