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Edge Determination for Polycrystalline Silicon Lines on Gate Oxide

IP.com Disclosure Number: IPCOM000125637D
Original Publication Date: 2001-Aug-01
Included in the Prior Art Database: 2005-Jun-09

Publishing Venue

National Institute of Standards and Technology

Related People

John S. Villarrubia: INVENTOR [+4]

Abstract

In a scanning electron microscope (SEM) top-down secondary electron image, areas within a few tens of nanometers of the line edges are characteristically brighter than the rest of the image. In general, the shape of the secondary electron signal within such edge regions depends upon the energy and spatial distribution of the electron beam and the sample composition, and it is sensitive to small variations in sample geometry. Assigning edge shape and position is done by finding a model sample that is calculated, on the basis of a mathematical model of the instrument-sample interaction, to produce an image equal to the one actually observed. Edge locations, and consequently line widths, are then assigned based upon this model sample. In previous years we have applied this strategy to lines with geometry constrained by preferential etching of single crystal silicon. With this study we test the procedure on polycrystalline silicon lines. Polycrystalline silicon lines fabricated according to usual industrial processes represent a commercially interesting albeit technically more challenging application of this method. With the sample geometry less constrained a priori, a larger set of possible sample geometries must be modeled and tested for a match to the observed line scan, and the possibility of encountering multiple acceptable matches is increased. For this study we have implemented a data analysis procedure that matches measured image line scans to a precomputed library of sample shapes and their corresponding line scans. Linewidth test patterns containing both isolated and dense lines separated from the underlying silicon substrate by a thin gate oxide have been fabricated. Line scans from test pattern images have been fitted to the library of modeled shapes.

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Proc. SPIE 4344, pp. 147-156 (2001).

     J. S. Villarrubia, A. E. Vladár, J. R. Lowney and M. T. Postek National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

ABSTRACT

In a scanning electron microscope (SEM) top-down secondary electron image, areas within a few tens of nanometers of the line edges are characteristically brighter than the rest of the image. In general, the shape of the secondary electron signal within such edge regions depends upon the energy and spatial distribution of the electron beam and the sample composition, and it is sensitive to small variations in sample geometry. Assigning edge shape and position is done by finding a model sample that is calculated, on the basis of a mathematical model of the instrument-sample interaction, to produce an image equal to the one actually observed. Edge locations, and consequently line widths, are then assigned based upon this model sample. In previous years we have applied this strategy to lines with geometry constrained by preferential etching of single crystal silicon. With this study we test the procedure on polycrystalline silicon lines. Polycrystalline silicon lines fabricated according to usual industrial processes represent a commercially interesting albeit technically more challenging application of this method. With the sample geometry less constrained a priori, a larger set of possible sample geometries must be modeled and tested for a match to the observed line scan, and the possibility of encountering multiple acceptable matches is increased. For this study we have implemented a data analysis procedure that matches measured image line scans to a precomputed library of sample shapes and their corresponding line scans. Linewidth test patterns containing both isolated and dense lines separated from the underlying silicon substrate by a thin gate oxide have been fabricated. Line scans from test pattern images have been fitted to the library of modeled shapes.

Keywords: critical dimension (CD), independent-edges approximation; library-based metrology, linewidth metrology, line shape, scanning electron microscopy (SEM)

1. INTRODUCTION

A line's width, by definition, is the distance between its left and right edges. Thus, the accuracy with which edge positions can be measured determines the uncertainty in linewidth or critical dimension (CD) measurements. Determining edge positions in a scanning electron microscope (SEM) accurately and reproducibly enough to satisfy the increasingly demanding International Technology Roadmap for Semiconductors1 is a continuing challenge.

In previous studies,2,3 we focused attention upon lines preferentially etched into single crystal silicon. These samples were intended to have nearly ideal rectangular cross sections and thereby provide simple cases for testing a measurement system. However, it is recognized that features of industrial interest differ from these. For example, transistor gates are typically poly- cryst...