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Through-Focus Phase-Sensitive Technique for Determining Feature Dimensions Using Optics of Optical Microscopy

IP.com Disclosure Number: IPCOM000030863D
Original Publication Date: 2004-Aug-30
Included in the Prior Art Database: 2004-Aug-30
Document File: 5 page(s) / 2M

Publishing Venue

National Institute of Standards and Technology

Related People

Richard Silver: INVENTOR [+4]

Abstract

This method can be used to evaluate line widths, line heights and pitches of a single line to an array of lines and multilevel structures. This method can also be used to evaluate overlay-offset values for multilevel structures. Broadly, sub wavelength feature dimensions of the illuminating light can be evaluated using regular optical microscopes with exceptional sensitivity and accuracy. The method is likely best applied to feature width dimensional metrology.

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Invention Disclosure                                                 

Inventors: *Ravikiran Attota, **Richard Silver, ***Robert Larrabee, and **Egon Marx

Title:

Through-focus phase-sensitive technique for determining feature dimensions using optics or optical microscopy.

Purpose:

This method can be used to evaluate line widths, line heights and pitches of a single line to an array of lines and multilevel structures. This method can also be used to evaluate overlay-offset values for multilevel structures. Broadly, sub wavelength feature dimensions of the illuminating light can be evaluated using regular optical microscopes with exceptional sensitivity and accuracy.  The method is likely best applied to feature width dimensional metrology.

Method:

In an optical microscope incident light scatters off the target (or features). Optical images are formed by the interference of scattered light and reflected light (normally the incident light is not considered). The image formed at the image plane depends on the relative height or focus position between the sample and the objective.  With appropriate design of the optical set up and target geometry, the variations in the images formed at different focus positions are unique, for a given physical feature. By evaluating the variations of the different captured images through analysis of the optical image profiles obtained at various heights, and by comparing these images with a library of theoretically simulated images, it is possible to determine the dimensions of the features. This comparison is between a library of simulated images and a set of experimentally acquired images.  A particular metric, as described shortly, is used to compare the images obtained at different focus heights.

The method may also be applied without comparison to the simulation-based library for use in process control.  In this application, the change in metric or measured value through-focus is used as a direct monitor of feature geometry or characteristic feature quality.

One method of identifying variations in the images is by integrating variations in the image intensity across the entire image. This is also equivalent to the integration of variation in the slopes of the optical intensity profile and is referred to as a “Focus Metric” (FM). There are several of these types of methods such as integrated energy, average slope, contrast or frequency content, used to evaluate the focus metric.  Each image at a given height provides a value of FM. Variations in the FM with focus height (FH) (both above and below the feature) provide a characteristic, unique pattern for a given feature at a given incident wavelength. By comparing the FM at different focus heights from the experimentally obtained images with the library of simulated images, dimensions of the measured feature can be evaluated.  These FM values are plotted as a function of change in height and form a “signature”.

This method of evaluating a metric...