An Acoustic Microscope for Subsurface Defect Characterization

Abstract

Abstmct-A scanning acoustic microscope operating in the frequency range 0.1 - L GHz has been developed. The acoustic micrographs obtained have clearly demonstrated that this device can be used nondestructively to observe spike defects at the edge of the local oxidation of silicon structures in semiconductor devices and hydrogenion-doped regions in silicon crystals. The acoustic data have been compared with results obtained through the scanning electron microscope and the optical microscope. ICROANALYSIS techniques used to measure and examine microscopic regions in materials with highfrequency ultrasound waves have recently received a great deal of attention as a new and highly promising means for measurement and observation. Typical of these new methods is the mechanical scanning acoustic microscope developed by Professor C. F. Quate at Stanford University in 1973 [l]. This device directs a narrow focused acoustic beam at a specimen being scanned two-dimensionally, and detects acoustic waves that are reflected from or transmitted through the specimen to obtain a two-dimensional image. The image contrast obtained reflects changes in the mechanical properties of materials in the specimen, such as elasticity, density, and viscosity. Applications of the acoustic microscope include, for example, fault detection in materials, the examination of semiconductor devices, and materials evaluation using surface acoustic waves [2]-161. In this paper we report the results of studies conducted on spike defects that arise in isolation regions between elements in semiconductor devices and regions bombarded by hydrogen ions on silicon substrates. 11. CONSTRUCTION OF THE ACOUSTIC MICROSCOPE The operating principles and construction of acoustic microscopes in general have already been described in a number of papers and will be omitted here. We intend to discuss here only several specific features particular to our reflection scanning acoustic microscope. The most important technical problems that had to solved during development of this device were the development of a process for forming high-performance piezoelectric film; a process for fabricating microspherical