Room temperature observation of point defect on gold surface using thermovoltage mapping Arijit Roy a, * , Cher Ming Tan a , Sean J. O’Shea b , Kedar Hippalgaonkar b , Wulf Hofbauer b a School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639 798, Singapore b Institute of Materials Research and Engineering, 3 Research Link, Singapore 117 602, Singapore Received 4 July 2007 Available online 4 September 2007 Abstract In this work, we apply thermovoltage imaging using scanning tunneling microscope to observe atomic scale surface imperfections at room temperature. Thermovoltage mapping can provide high resolution (down to 1 nm) images of standing waves in metal at room temperature, thus avoiding the need for low temperature scanning tunneling microscopy for the investigation of the standing waves. In order to generate a thermovoltage between the sample and tip, the sample (Au(1 1 1)) is heated to about 40 °C above the room tem- perature and surface scanning is performed. Heating the sample is simpler than heating the tip by laser irradiation. The thermovoltage technique can be applied to estimate surface defect density and the severity of the surface defects in materials, which can be a useful tool for the reliability study of nano-scale materials and devices. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction With the continuous down-scaling towards the nano- scale in ULSI technology, surface imperfection becomes important for the electrical performance of electronic devices. In fact, the critical reliability issues of present day electronic devices are due more to the surface defects rather than the bulk defects. For example, the electromi- gration reliability of Cu interconnect is lowered than expected due to the surface defects generated during the chemical mechanical polishing [1]. Crystal imperfections such as point defects, grain boundaries, adatoms, surface steps disrupt the periodicity of crystal potential and act as scattering centers for electron waves. Electrons in the two dimensional surface states are subjected to scattering at surface imperfection leading to spatial oscillations of the electronic local density of states (LDOS). This oscillation is analogue to the well known Friedel oscillation of the total charge density [2]. The LDOS oscillation around the surface imperfection is the result of the interference of the electron wave traveling toward the scattering center with the back scattered electron wave. Scanning tunneling microscope (STM) is a surface- analytical tool for real-space imaging of surface structure and chemistry at the atomic scale. Direct observation of quantum mechanical characteristics of electrons using STM provides Camera-Ready Manuscript insights into the solid state phenomena such as electron scattering, screening, electron confinement and quantum transport [3]. Conventionally, the operational temperature to observe electron waves under STM investigation is about 253 °C with an electrical bias between sample and probe tip. The necessity for the low temperature is mainly to enhance the mean free path of the conduction electrons. With this method, standing wave pattern around point defect, sur- face step or adatoms on the various surfaces are observed [4–6]. However, such a low temperature requirement 0026-2714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2007.07.010 * Corresponding author. Tel.: +65 6790 5129; fax: +65 6792 0415. E-mail address: aroy@ntu.edu.sg (A. Roy). www.elsevier.com/locate/microrel Microelectronics Reliability 47 (2007) 1580–1584