OPTICAL REVIEW Vol. 9, No. 6 (2002) 277-281 Near Field Stimulated Time of Flight MaSS Surface AnalyZer* Yu DING, Ruggero MICHELETTol, Hiroaki HANADA2, Toshihiko NAGAMURA2, Satoshi OKAZAKI and Koji OTSUKA Department of Material Che,nistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan l Department of Electronic Science, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan 2 Unisoku Co., Ltd., 2-4-3 Kasugano, Hirakata, Osaka 573-0J3J, Japan (Received June 28, 2002; Accepted August 15, 2002) This work describes a groundbreaking process that provides a direct highly localized measurement of the atomic mass on surfaces at room temperature. Employing an original system that joins a scanning tunneling microscopy (STM) device and a time of flight (TOF) mass analyzer, we could previously ionize surface atoms by the combination of an optical laser pulse and an electric pulse at the STM tip. Desorbed ions from a localized area were accelerated and detected by a TOF chamber. We will demonstrate in this paper that high localization and mass discrimination can be obtained even without the aid of an electric pulse from the tip. We reduced the angle of incidence of the laser beam to zero (laser beam parallel to the sample surface). In this condition we were able to demonstrate for the first time ionic desorption at a confinement level of the order of 5-10 nm, an order of magnitude better than previous configurations. Key words: TOF, STM, near field, mass spectrometer, ionization, Iaser, surface analysis, surface characterization 1. Introduction With scanning tunneling microscopy (STM)1) it is possible at atomic scale resolution to obtain topological information of a surface under investigation. Detected images contain structural data that give the researcher several clues as to the chemical composition of the sample, however, no knowledge of the atomic species is directly available. This fact implies that the interpretation of STM images is not a straightforward process, and in many cases deep theoretical investigation is necessary in order to fully understand them 2,3) To overcome these limitations, we have developed an integrated device of new design, to analyze the sample surface with STM and simultaneously measure the time of flight (TOF) mass. Our previously developed STM-TOF system4) employed an electric pulse, delivered directly through the STM tip, and a near simultaneous optical laser pulse irradiated on the sample surface. A specially adapted electric field then guides the extracted ions to a TOF chamber where the analysis is carried out.5~8) This document describes a totally new configuration of this system, where a particular optical enhancement phenomenon occurs in the proximity of the tip and the electric pulse becomes unnecessary. We will describe the apparatus and present our first results with this latest experimental approach. 2. Methods, Results and Discussion The setup is basically arranged as a conventional STM system (Unisoku, Ltd.). The STM head is enclosed in a specially designed vacuum chamber where the sample is 10cated at the center of the main compartment. Two electrode stages provide a local field to accelerate the desorbed ions. One is placed at the entrance to the TOF tube, *This paper was originally presented at the 1 Ith Optical Near Field Workshop, which was held on June 28, 2002 at Tokyo Institute of Technology, Yokohama, organized by the Optical Near Field Group of the Optical Society of Japan, an affiliate of the Japan Society of Applied Physics. whilst a smaller electrode is positioned in the proximity of the sample, to collect the first emitted ions and guide them to the subsequent stages. A glass window is placed on the side of the main chamber to allow an external laser pulse (Nd- YAG Iaser, 266 nm, 5 ns) to illuminate the tip. In earlier experiments4) the laser pulse was used in conjunction with an electrical pulse on the tip of a conventional STM system. See Fig. I for a diagram of the whole system and the STM head compartment. The intensity of the laser light was adjusted to the maximum value that would not induce ionization on the sample when the electric pulse is not applied. The voltage pulse duration was limited to 100 ns, as longer duration pulses damaged the tip. The angle of incidence of the laser beam to the sample surface was 45'. Figure 2(a) shows a scan of a sample of gold. Three gold clusters are present; we moved the tip to the central cluster and applied an 8 V extraction pulse. In the next scan, Fig. 2(b), the cluster was moved away without disturbing the surrounding gold film. Figure 2(c) shows the TOF spectrum associated with the extraction process. We performed several tests of this kind with various materials; depending on the electrical pulse intensity, the diameter of the circular ionization region differed. The plot in Fig. 3 shows the dependence of pulse intensity and desorption area we obtained for three test materials. To improve and simplify the setup, we tried out several optical configurations. After a number of tests we noticed that by reducing the angle of incidence, initially set at 45', the electrical pulse applied to the STM tip became less significant. We found that at an incidence angle of zero, the laser light was enhanced in a conflned region by the sole presence of the STM probe, and the additional electrical pulse in this optical configuration became superfluous. Thus we set the new optical configuration with an angle of incidence of O'. To test this new configuration we examined gold film on a silicon substrate crystal Si(111). The film was obtained through a conventional evaporation process to a thickness of about 100 nm. First, the surface was imaged through the 277