The present invention is generally in the field of optical measurement techniques, and relates to a method and system for optical critical dimension (OCD) metrology, particularly useful for measuring in patterned structures, such as semiconductor wafers.
Over the last few decades, semiconductor devices become ever smaller and more elaborate, and fabrication tolerances increasingly challenging. One solution to this growing challenge is found through improved metrology capabilities, allowing better control and monitoring of the fabrication process.
One of the strongest technologies in use today for semiconductor characterization is Optical Critical Dimension (OCD) metrology. This title is in fact used to describe a set of optical techniques implementing scatterometry: measurement of light scattering properties of the sample. Through accurate numerical models, it is then possible to relate the measured scattering properties to the geometrical and material characterizations of the sample under test.
Today, optical critical dimension (OCD) metrology is largely based on spectral reflectometry (SR) and spectral ellipsometry (SE). Additional forms of metrology in use are full Mueller matrix characterization and dome scatterometry. All these techniques are based on measuring the reflected intensity of differently polarized light for different incident directions.
For example, U.S. Pat. No. 8,848,185 and US 2014/168646, both assigned to the assignee of the present application, describe an optical system for use in measuring a via profile parameters in a patterned structure.
According to U.S. Pat. No. 8,848,185, the system comprises an illumination channel for propagating illuminated light onto the structure being measured, a detection channel for collecting light returned from the illuminated structure to a detection unit, and a modulating assembly configured and operable for implementing a dark-field detection mode by carrying out at least one of the following: affecting at least one parameter of light propagating along at least one of the illumination and detection channels, and affecting propagation of light along at least the detection channel.
According to US 2014/168646, the optical system comprises an illumination channel for propagating illuminated light onto the structure being measured; a detection channel for collecting light returned from the illuminated structure to a detection unit; and an attenuation assembly accommodated in the illumination and detection channels and being configured and operable for selectively attenuating light propagating along the detection channel, the attenuation creating a predetermined condition for the selectively attenuated light, said predetermined condition being defined by a predetermined ratio between a first light portion corresponding to a dark field condition and a second light portion corresponding to a bright field condition in said selectively attenuated light, detected selectively attenuated light being therefore indicative of at least one parameter of the via being illuminated.