Nondestructive Localization of Surface Particles for Elemental Compositional Analysis by TOF-SIMS

Abstract

A surface particle localization system based on oblique angle dark field optical scattering has been incorporated into time-of-flight secondary ion mass spectrometry. ͑TOF-SIMS͒ In this design, oblique incident laser light would be scattered to all directions by surface particles on silicon wafer and be collected at oblique angle to determine the x-y coordinate of the particle detected. This fast and nondestructive technique is an effective alternative to the standard method for imaging and localization of surface particles using primary ion bombardment by TOF-SIMS which might result in the alteration or loss of physiochemical information from the surface particles.Silicon wafers with surface areas virtually free of external contaminants are of utmost importance for achieving acceptable process yield in ultralarge scale integrated circuit fabrication. Effective localization of surface particles on silicon wafers for accurate elemental composition analysis 1,2 could provide essential information for identifying the source of contamination and thus allow preventive control measures to be implemented before process yield is significantly compromised during manufacturing, wafer storage and handling processes. In this respect, time-of-flight secondary ion mass spectrometry ͑TOF-SIMS͒ has emerged as one useful analytical tool for the elemental composition diagnosis of surface particles due to the high transmission and high mass resolution capability for ions with different masses simultaneously. 3 In a typical surface particles analysis using TOF-SIMS, secondary electrons generated 4 from material surface by the bombardment of high energy primary ions are collected to indirectly localize surface particles during timeconsuming and exhaustive scanning process on selected wafer surface area. Once the particles of interest are located, detailed elemental composition analysis by TOF-SIMS is carried out. However, the bombardment of solid surface by high energy primary ions could remove material from solid surface 5 and thus may result in inaccurate acquisition or alteration of physiochemical information from the surface particles.An optical microscope coupled with a charge coupled device ͑CCD͒ camera and an external light source for illumination are usually attached to TOF-SIMS for alignment of wafers on x-y stage with respect to the center position of the ion gun. This simple optical microscopy setup could provide fast and nondestructive localization of surface particles with size larger than the optical resolution of the microscope prior to the primary ion bombardment onto the wafer surface in TOF-SIMS. However, optical microscopy is limited by the resolving power of the microscope 6 and thus a limited magnification with sufficient depth of field. 7 The performance requirement for the optical microscopy system might even be more stringent if the surface particles are optically transparent.Modern TOF-SIMS system can analyze microareas and generate surface images with a high lateral resolution of 0.1 m or less 8-10 by using ...

Atomic spectrometry update. Atomic mass spectrometry

“…The use of obliqueangle dark-field optical scattering was explored for the localization of particles smaller than the resolution of the optical visualization microscope in SIMS instruments. 151 When the feature of interest could not be positioned within the analysis spot of the focused primary ion gun, time-consuming rasteranalysis of a large area had to be performed. Therefore, deposition of the particles on a perfectly polished surface allowed scattering of oblique incident laser light to be used for the localization of particles as small as 200 nm.…”