The qualification of semiconductor surfaces is an important aspect of IC lechnology, from checking the quality of the
substrate material right through to the dimensioning of the finished device. Etching the surface of the substrate reveals details
concerning chemical and crystal defects. Surface roughness measurements are required before preparing epitaxial layers and
device processing. The height, width and shape of component features need to be checked throughout the device processing
stage.
In this paper we describe a three dimensional NDT optical profiler we have developed for the nanometrology of
semiconductor surfaces. The system is based on interference microscopy, using phase stepping for automatic fringe analysis.
Illumination is by high intensity LED or white light and detection is by a CCD camera. A vertical resolution of mm is
achieved, with the lateral resolution being better than O.5i.m. Comparison of a two dimensional profile of a chemically
etched surface using PSM with that obtained by a stylus profiler are within 10% agreement.
Results are shown of three dimensional profiles of chemically etched InP and an MOCVD grown epitaxial layer, surface
roughness of polished InP, and profiles of a recess in a combined laser/photodiode device.
KEYWORDS: Microscopy, Super resolution microscopy, Particles, Tomography, Semiconducting wafers, 3D image processing, Silicon, Point spread functions, Diffraction, 3D metrology
Nanoscopy is a term that we use to describe optical techniques using digital image processing that are capable of nanometric
observation and measurement. Laser Scanning Tomography (LST) is used for defect analysis in the bulk of semiconductor
wafers for revealing particles as small as mm and for measuring densities of uptolO13 cm3 The unusually high contrast of
the system allows us to observe submicron particles which are more than three orders of magnitude smaller than the Rayleigh
criterion for the optical system. Recent work using deconvolution of point image functions enables us to perform sub-micron
optical serial sectioning, for determining the depth of defects.
The best conditions for classical LST (using laser illumination perpendicular to the viewing direction) are when operating
further than a few microns below the surface in semiconductor wafers; ie it is ideal for bulk defect studies. The study of
imperfections inside epilayers in the top ijim layer requires a modified technique. Instead of illuminating at 900 the
viewing direction, the infra red laser beam is introduced obliquely to the front suffe to illuminate the defects in the epilayer
while still in the dark-field mode. Combining this method with high resolution sectioning will be the basis for a technique of
three dimensional submicron defect analysis in epilayers.
Results are given of defect studies in annealed GaAs and silicon to demonstrate the capabilities of LST for naiiometer analysis
in bulk materials. Sub-micron depth measurement is shown for single particles using the PSF of the system for the out-offocus
case. Some initial results are given of studies of defects in a highly lattice mismatched epilayer using IR transmission
microscopy, phase stepping microscopy (PSM, used in surface profiling) and the new dark field oblique laser illumination
technique.
LST, and the new nanoscopy techniques are non-destructive, operate under normal room conditions, and give sub-micron
observation and distance measurement of defects over large areas with the possibility of 3D image synthesis for defect
analysis.
From the recent developments in Laser Scanning Tomography it appears that dark field microscopy methods make it possible to observe very small microprecipitates which are bound to grow in the bulk of semi conductor materials or epitaxial layers. Improvements in such investigations rely on reducing the effective "sectioning' thickness of the optical observation in order to be able to measure the axial position of the point sources with submicron precision. This is possible owing to the fact that the recorded individual images of the point sources (considered as Point Spread Functions) are separable diffraction patterns. This communication aims at presenting the first attempt to explore this typical reverse problem of "micro ranging". It will be shown using a series of images from precipitates in Indium Phosphide bulk material that numerical interpolation can lead to the relative position of each particle with a convenient precision. Optical perturbations such as spherical aberration and coma are also to be taken into account if the optical system is not already corrected.
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