BUNSEKI KAGAKU Abstracts

Vol. 52 No. 6

June, 2003

Accounts

Various methods for X-ray pulse generation using a femtosecond laser and

Koji Hatanaka¹, Toshifumi Miura¹, Hideho Odaka¹, Hiroshi Ono¹ and Hiroshi Fukumura¹

(Received 28 January 2003, Accepted 17 February 2003)

Three different methods for X-ray pulse generation with femtosecond laser pulses were outlined. The first method was with a photo-excitable X-ray tube irradiated by femtosecond laser pulses. The X-ray emission spectrum was composed of a broad component and some characteristic X-ray lines. The X-ray pulse width was calculated to be about 20 ps, based on an equation of the electron motion in the tube. The second method was by the irradiation of focused femtosecond laser pulses onto electrolyte aqueous solution jets, such as a cesium chloride aqueous solution. The X-ray emission spectra showed solute-dependent characteristic X-ray lines and broad bremsstrahlung backgrounds up to 40 keV. Under double-pulse excitation onto a distilled water jet, the X-ray intensity was about 10³-times higher than the case excited by single pulse. The third method was by the irradiation of focused femtosecond laser pulses onto cassette tapes. The characteristic X-ray lines of Fe Kα and Kβ were clearly observed in addition to a broad bremsstrahlung background. By using this X-ray pulse source, an X-ray diffractometer was developed. An X-ray diffraction image of LiF (200) was clearly taken by an X-ray image intensifier and a cooled CCD camera.

Keywords : X-ray pulse; femtosecond laser; electrolyte aqueous solution; photo-excitable X-ray tube; cassette tape.

Electrical measurements for single molecules using scanning probe microscopes combined with laser excitation

Hiroshi Sakaguchi¹

(Received 27 January 2003, Accepted 3 April 2003)

Electrical measurements for single molecules using scanning probe microscopes combined with laser excitation are reviewed in the present article. Techniques using a conductive atomic force microscope and a scanning tunneling microscope could realize a quantitative analysis of the tunnel conduction through a single molecule of a self assembled monolayer fabricated on Au(111) as well as conductive imaging with a molecular resolution. Additionally, new scanning probe microscopes combined with laser excitation, like the photoconductive atomic-force microscope and the laser-assisted scanning-tunneling microscope, were developed to investigate the local probe analysis of the photoelectrical properties on organic thin films. These new techniques have proven to attain a molecular special resolution as well as a femtosecond time resolution.

Keywords : laser; scanning probe microscope; molecular electronics; self assembled monolayer.

New techniques for controlling the excitation source in glow discharge optical emission spectrometry

Kazuaki Wagatsuma¹

(Received 24 December 2002, Accepted 26 February 2003)

This paper describes three advanced detection techniques in glow discharge optical emission spectrometry: (a) an applied voltage modulation method for a d.c.-powered glow discharge emission source, (b) a bias-current introduction method and (c) an amplitude modulation method for an r.f.-powered glow discharge emission source. In the applied voltage modulation method, an alternating component superimposed on a d.c. bias voltage is applied to the excitation source. Only the emission component corresponding to the alternating frequency can be selectively detected at very low noise levels with a lock-in amplifier, thus contributing to an improvement in the signal-to-noise ratio. In the bias-current introduction method, a d.c. current driven by the self-bias voltage is conducted through the plasma body by connecting a low-pass filter circuit and a load resistor with the glow discharge lamp. The electrons introduced into the plasma can cause various excitation processes more actively and, as a result, the emission intensities increase, which yields a better detection limit in the quantitative analysis. In the amplitude modulation of an r.f. voltage, the emission detection with a lock-in amplifier enables the measurement to be performed with a better signal-to-noise ratio, whereas the sputtering rate and the sampling amount are reduced. This method can be successfully applied to determine depth elemental composition of nm-order thin films.

Keywords : atomic emission spectrometry; glow discharge plasma; lock-in amplifier; bias current; voltage modulation; amplitude modulation; signal-to-noise ratio; signal-to-background ratio; detection limit.

Solvation structure of ions at solution surfaces as studied by total-reflection total-conversion-electron yield X-ray absorption fine structure

Makoto Harada¹, Tetsuo Okada¹, Hajime Tanida² and Iwao Watanabe³

(Received 24 January 2003, Accepted 12 April 2003)

X-ray absorption fine structure (XAFS) is an efficient tool to probe the local structure of an absorbing atom, and has been applied to various samples. Although the total reflection of X-ray has allowed us to study solid surfaces by XAFS, the accessibility of this approach to a solution surface has been restricted because of a difficulty to make precise measurements of X-ray absorption. We proposed a novel method, named total-reflection total-conversion-electron yield (TRTCY) XAFS, where the Auger electrons generated by X-ray absorption are detected with high sensitivity. The detection depth was typically <10 nm, and the detection volume was 2~3 nl. Solution surfaces can be modified by monolayers, and model the various interfaces having separation, sensing, and molecular recognition functions. TRTCY-XAFS provides information on the local concentrations and structures of ions attracted by the surface monolayers from a subphase. The principle of this approach, the instruments used in a series of studies, and some obtained results are discussed in this review. In addition to our prototype instruments, a new version, which has allowed automatic control of the surface molecular density, is also outlined.

Keywords : solution surface; solvation structures of ions; X-ray absorption fine structure; surface monolayers.

Research Papers

Application of photoionizable crowned spirobenzopyrans to flow-injection extraction-spectrophotometry for lithium ion

Makoto Nakamura¹, Takeshi Fujioka¹, Hidefumi Sakamoto¹ and Keiichi Kimura¹

(Received 24 January 2003, Accepted 24 March 2003)

A system for the flow-injection analysis of Li⁺ was designed for the first time, using (nomoaza-12-crown-4)-spirobenzopyran and (diaza-12-crown-4)-bis(spirobenzopyran) as extraction-spectrophotometric reagents. The photoirradiation effect on Li⁺ extraction was studied. In this flow-injection system with (diaza-12-crown-4)-bis(spirobenzopyran), which has a high stability constant for Li⁺, the extractability of Li⁺ was increased under UV irradiation (300~400 nm) conditions compared with under dark conditions, while being decreased under visible-light irradiation (>500 nm) conditions. This behavior was proved quantitatively by using flame spectrophotometry and conductometry. For Li⁺ flow-injection analysis with (diaza-12-crown-4)-bis(spirobenzopyran), UV irradiation, therefore, amplified the sensitivity owing to an increase in the Li⁺ extractability.

Keywords : crowned spirobenzopyran; lithium ion; flow-injection analysis; liquid-liquid extraction; photoinduced amplification.

Super-sensitive detection of sodium in water using dual-pulse laser-induced breakdown spectroscopy

Yutaka Uchida¹, Akira Kuwako¹, Chikara Konagai¹ and Katsuji Maeda²

(Received 23 January 2003, Accepted 12 April 2003)

Laser-induced breakdown spectroscopy (LIBS) has been used for the detection of trace species in water. LIBS has the capability to be used for a real-time, in-situ analysis during power-plant operation. To improve the detection limit of the LIBS technique, a dual-pulse cross-beam scheme (DP-XB) was adopted. In this paper, we discuss the effects of the detector gate delay time, dual pulse interval, laser irradiating energies, and irradiating position for DP-XB LIBS. The signal-to-noise ratio (S/N) was optimized when the laser irradiating energies were 240/200 mJ, while the detector gate delay and pulse interval were 4~8 µs and 8 µs, respectively. The linear correlation between the Na concentration and the fluorescence intensity was obtained from 1 to 100 ppb and the limit of detection (LOD) for Na was 0.14 ppb.

Keywords : laser; spectroscopy; LIBS; Na.

Analyses of alcohols using a gold-deposited optical-fiber refractive index sensor system

Masaru Mitsushio¹, Shusaku Higashi² and Morihide Higo¹

(Received 4 December 2002, Accepted 28 February 2003)

A gold-deposited optical-fiber sensor system using surface plasmon resonance (SPR) without a rotation stage or spectrometer was constructed, and the system was tested for the quantitative analyses of various alcohols. The sensor was fabricated by depositing a thin (45 nm) gold film around the unclad core of an optical fiber. When a sample solution was introduced into the sensor cell, the intensity of the transmitted light through the sensor decreased and became constant within 0.4 min. The detection limit was less than 0.5%, v/v and a change of 0.0012 refractive index (RI) units in the refractivity of a methanol solution of phenethyl alcohol could be measured. A calibration curve with a correlation coefficient of 0.998 was obtained in the concentration range of 0 to 5%, v/v. The relative standard deviation was 2% in 5 repeated measurements of a 60%, v/v aqueous ethanol solution. Though the curves of the transmitted light intensities vs. the concentrations of 12 alcohols differed from each other, the curve of the transmitted light intensity vs. RIs of the alcohols had the minimum at about 1.37 RI units and the minimum shifted with increasing the thickness of the gold film. Thus, the properties of the sensor can be controlled by the thickness of the gold film.

Keywords : optical fiber; gold film; surface plasmon resonance (SPR); analysis of alcohol; refractive index.

Emission characteristics of a radio-frequency powered helium glow-discharge plasma excitation source associated with bias current introduction

Hideyuki Matsuta¹, Tariq Mahmood Naeem¹ and Kazuaki Wagatsuma¹

(Received 24 January 2003, Accepted 24 February 2003)

In a radio-frequency (r.f.) powered He glow-discharge plasma excitation source, sputtering of the sample atoms scarcely occurs when the bias current is not conducting. However, when the bias current is conducting, the sputtering rate of the sample increases rapidly with increasing the bias current. The maximum sputtering rate of the He excitation source reaches about 1/10 of the sputtering rate of the usual 13.56 MHz Ar glow-discharge plasma excitation source. The emission yield of the r.f. He glow-discharge plasma is found to be over 10-times larger than that of the usual 13.56 MHz Ar glow-discharge plasma. Although a bias current conducting method with an r.f. Ar glow-discharge excitation source becomes ineffective to enhance the excitation of atomic lines having an energy higher than 6.2 eV, the method with r.f. He glow-discharge excitation source is effective for the excitation of emission lines having a high excitation energy. The atomic emission lines of fluorine having an excitation energy larger than 14 eV can be observed with an r.f.-powered He glow-discharge plasma excitation source.

Keywords : glow-discharge optical emission spectrometry; helium; glow-discharge plasma; radio-frequency; bias-current introduction.

Determination of ultratrace metallic impurities in metal-oxide thin films for ferroelectric random-access memories by precipitation separation/ICP-MS and ETAAS

Motonaka Yabuki¹

(Received 27 January 2003, Accepted 14 March 2003)

A new sensitive analytical method has been developed to evaluate metallic impurities in ferroelectric capacitor films (PbZr_xTi_(1-x)O₃ and SrBi₂Ta₂O₉) and conductive oxide electrode films (SrRuO₃) used in semiconductor ferroelectric random-access memories. It was found that precipitation separation using several acids as precipitants was successful to reduce matrix elements, such as Pb, Zr, Ti, Sr, Ta, and Bi, which decrease ion intensities during an ICP-MS measurement. In the case of PbZr_xTi_(1-x)O₃ and SrBi₂Ta₂O₉ thin films, 80 to 99% of the matrix components could be eliminated as PbSO₄, H₂ZrO₃, H₂TiO₃ and so forth from metallic impurities using HClO₄ and H₂SO₄ as precipitants. As for SrRuO₃, 99% of the matrix components could be separated, removing Sr as SrSO₄ by H₂SO₄ after evaporating Ru as RuO₄ by HClO₄. The above-mentioned precipitation separation could be used determine impurities, such as Al, Co, Cr, Cu, Fe, and Mg, on the 1E10 to 1E12 atoms/cm² levels. The impurity levels of these thin films indicated that the target material's purity and sputtering codition are very seriously affected by the extant impurities in the films. The technique reported herein gives low detection limits and a good recovery for an ultra-trace metallic determination for semiconductor materials.

Keywords : precipitation separation; semiconductor materials; ICP-MS; ETAAS; trace impurities.

Technical Papers

Characterization of epitaxial semiconductor layers using terahertz time-domain spectroscopy

Mamoru Usami^1,2, Toshiyuki Iwamoto^1,2,³, Hiromichi Akahori¹, Naoki Tsumura¹, Ryoichi Fukasawa¹ and Kiyomi Sakai²

(Received 24 January 2003, Accepted 14 March 2003)

Terahertz (THz) light, which is located at the boundary between radio waves and light, was not easily accessible until the last decade of the 20th century. Since the invention of a photoconductive antenna with ultrafast excitation by femtosecond laser pulses, THz pulse light has attracted much attention. Not only inorganic materials, such as semiconductor wafers, but also biomedical materials have been studied by THz time-domain spectroscopy (THz-TDS). In the future this method will be recognized as a standard tool for studies in the THz region. Using THz-TDS, the electric fields of THz pulses are measured in the time domain. Each THz pulse that is multiply reflected inside a planar sample is separately observed in the time domain because of a difference in the optical path length. Therefore, by data windowing, it is possible to pick out only a pulse which is reflected off the surface of a sample. This is not possible by conventional Fourier-transform spectroscopy, whose spectrum is always affected by multiple reflection, because a continuous wave light source is used and measurements are not made in the time domain on such systems. This feature is useful for studying thin films on substrates; we can neglect any reflection on the back surface of a substrate. In this paper, we demonstrate the characterization of n-type GaAs epitaxial layers on semi-insulating GaAs substrates by THz-TDS. The carrier densities and mobilities of the epitaxial layers are determined by a non-contact, non-destructive method.

Keywords : terahertz time-domain spectroscopy; thin film; epitaxial layer; the Drude theory.

Introduction of imaging detection into a time-of-flight mass spectrometer to improve the dynamic range in the isotope ratio measurement

Atsushi Katayama¹, Katsutoshi Furukawa¹ and Kazuo Watanabe¹

(Received 30 January 2003, Accepted 22 April 2003)

An imaging ion-detection system was introduced to a time-of-flight mass spectrometer (TOFMS) in order to increase the dynamic range in isotope ratio measurements. The new detection system utilizes a position-sensitive micro channel plate (MCP) with a phosphor plate and a CCD camera which records light spots on the MCP. Mass discriminated ion beams produced by laser resonance ionization were swept on the surface of the MCP by a time-dependent electric field located in front of the MCP. The resulting light spots were recorded as images by the CCD camera. A mass spectrum was obtained from the images. The present method was applied to an isotope analysis of calcium. A dynamic range of more than 5 orders of magnitude was achieved.

Keywords : resonance ionization; time-of-flight mass spectrometer; isotope ratio measurement; imaging detection.

Forensic discrimination of headlight glass by analysis of trace impurities with synchrotron radiation X-ray fluorescence spectrometry and ICP-MS

Yasuhiro Suzuki¹, Masaaki Kasamatsu¹, Ritsuko Sugita¹, Hikoto Ohta^1,2, Shinich Suzuki¹ and Yoshiteru Marumo¹

(Received 24 January 2003, Accepted 14 April 2003)

The application of synchrotron radiation X-ray fluorescence spectrometry (SR-XRF) to the analysis of trace impurities in headlight glass was investigated in order to discriminate headlight glass samples from different origins. Glass fragments, smaller than 1 mm in length and 0.5 mg by weight, were taken from 6 kinds of headlight samples and used for analysis. The X-ray fluorescence (XRF) spectrum of these fragments was measured with monochromatic X-rays of 116 keV as an excitation source. Impurity elements, such as Zn, As, Sr, Zr, Sb, Ba, Ce and Hf, in headlight glass could be found using only one fragment and a comparison of the XRF spectrum was effective for the discrimination, since a significant difference in the X-ray intensities of the characteristic lines of these elements was observed between different samples. The impurity elements in the headlight glass samples were determined also by inductively coupled plasma mass spectrometry (ICP-MS); the obtained analytical values were compared with those by SR-XRF. Considerably good correlation was found between the concentration of the elements by ICP-MS and the X-ray intensity per sample weight by SR-XRF, which indicates that the comparison of the X-ray fluorescence intensity per sample weight is nearly as effective for the dicrimination of glass fragments as quantitative analysis by ICP-MS. Comparison of the analytical results by SR-XRF has made it possible to discriminate 6 headlight samples from each other using only one fragment smaller than 0.5 mg and with no destruction of the sample.

Keywords : analysis of trace elements; headlight glass; ICP-MS; synchrotron radiation X-ray fluorescence spectrometry; forensic science.

Determination of tantalum and nitrogen for semiconductor copper metallization technology by ICP-OES and its application to XRF

Masayuki Onuma¹, Miyuki Takenaka¹, Motonaka Yabuki¹ and Masaru Hayashi¹

(Received 27 January 2003, Accepted 1 April 2003)

The evaluation method concerning the precise composition ratio of tantalum nitride (TaN), one of the barrier metals examined for use in semiconductor copper metallization technology, was investigated by wet chemical digestion. TaN thin films could be dissolved by using a mixture of HF and HCl acids in PTFE pressure vessels at 180°C for 2 hours. In the proposed method, both tantalum and nitrogen could be measured from the same solution: tantalum by ICP-OES and nitrogen by spectrophotometry with Nessler's reagent. Moreover, several samples determined by the proposed method were successfully applied to standard materials for X-ray fluorescence analysis, indicating that both wet chemical digestion and an XRF measurement were in good agreement with each other. The results of a sample evaluation showed that the distribution of the tantalum-nitrogen composition ratio varied markedly throughout the wafer. The semiconductor process appraisal method reported herein is a reliable and rapid evaluation technique for tantalum nitride films.

Keywords : barrier metals; TaN; wet chemical digestion; X-ray fluorescence analysis.

Notes

Spectrophotometric determination of cyanide ion in waste water using copper(II) and N,N-diethyl-p-phenylenediamine

Mitsuyoshi Hoshino¹, Kunio Nagashima¹, Minori Kamaya¹ and Nobuo Nakano²

(Received 24 January 2003, Accepted 11 March 2003)

Spectrophotometric determination of cyanide ions in water was studied using Cu(II) ions and N, N-diethyl-p-phenylenediamine(DPD). A cyanide ions reacted with Cu(I) ions, producing a stable complex, CuCN (s.p.=3.5×10^-20). Then, Cu(II) ions were reduced to Cu(I) ions with DPD owing to a decrease of Cu(I) ions. The concentaration of the coloring oxidant of DPD was proportional to the concentration of cyanide ions between 0.1 and 2.0 mg CN^- l^-1. The coloring development by this method was attained in one minute. A copper formate was the most suitable copper salt for a reaction in copper chloride, copper sulfate and copper acetate because of its highest sensitivity and lowest blank. The influences of co-existing metal ions [(Cr(III): 40 mg l^-1), (Ag(I): 0.3 mg l^-1), (Fe(II): 0.4 mg l^-1), (Fe(III): 3 mg l^-1), (Ni(II): 0.16 mg l^-1), (Co(II): 0.01 mg l^-1) and (Zn(II): 100 mg l^-1)] were less than 10% for 0.4 mg l^-1 of cyanide ions. No influences of co-existing metals on the linearity of calibration curve were observed. This method is very simple, and can be used for the pre-detection of cyanide ions in waste water.

Keywords : spectrophotometric determination of cyanide ion; copper(II); DPD; waste water; cyano-complex.

Technical Letters

Direct spectrophotometric determination of traces of fluoride in rain water

Minoru Okumura¹, Tomohiro Yoshimura¹, Kaoru Fujinaga¹ and Yasushi Seike¹

(Received 24 January 2003, Accepted 3 April 2003)

A simple and direct method for the spectrophotometric determination of trace amounts of fluoride in rain waters using a commercially available reagent, “Alfusone (DOJINDO)”, has been developed. In order to increase the sensitivity in the determination of fluoride in water samples, a 5-cm glass cell was used for spectrophotometric measurements instead of a 1-cm glass cell, which was used in conventional methods. The absorbances of the reagent blank in the 5-cm cell increased in proportion to the concentration of Alfusone in a coloring solution. As an optimal coloring solution, a 1% Alfusone solution was adopted. The other optimum conditions, such as the standing time for color development and the effect of other ions, were also examined. The determination range was 10~500 µg/l of fluoride by using 25 or 10 ml of a water sample. The established method was successfully applied to the determination of fluoride in rain water samples as well as fresh river and ground water samples.

Keywords : fluoride ion; rain water; spectrophotometric determination; alizarine complexone; lanthanum.

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