1. Field of the Invention
The present invention relates to a shape measurement apparatus and method, and more particularly to an apparatus and method for measuring a nano-sized shape through optical interference.
2. Description of the Related Art
A three-dimensional shape measurement apparatus illuminates a specific-shaped light beam on an object to be measured to form an interference pattern, measures the interference pattern, and analyzes the measured interference pattern, such that it acquires information indicating the height of the object. This measurement method can easily acquire a three-dimensional (3D) shape of the object, such that it has been widely used for medical and industrial fields. Specifically, the rapid technical development in all industrial fields requires micro-fabrication in semiconductors, microelectromechanical systems (MEMS), flat panel displays, and optical components. Presently, the micro-fabrication has been rapidly introduced to nano-sized super-precision fabrication technologies. Fabrication shapes required for the nano-sized super-precision fabrication technologies have been changed from two-dimensional (2D) shapes to three-dimensional (3D) shapes. Thus, the importance of the three-dimensional micro-fabrication measurement technology is also rapidly increasing.
Conventionally, a three-dimensional (3D) shape measurement method based on an optical phase shifting interferometer (PSI) has been widely used. The basic measurement principles of the above-mentioned PSI technology are as follows. In more detail, a light beam generated from a light source is illuminated on individual reference planes and measurement planes, and the individual light beams are collected by a beam splitter, such that images of measurement surfaces and striped interference signals are formed. Thereafter, the above-mentioned conventional PSI method calculates phases of the interference signals generated from optical detection elements are calculated such that the height of the object is measured. The above-mentioned PSI method called an interference signal tracking method considers that an interval between the interference signals corresponds to a half-wavelength of a light-source wavelength, and interpolates a variation of interference signals with a harmonic function, such that it indirectly calculates the phase of the interference signal.
The above-mentioned PSI performs a measured-modulo operation on the value of 2π, and assumes that an optical path difference (OPD) between neighboring pixels is one-half of a wavelength so as to remove inconsistency of 2π between measured phase data. Therefore, if a slope of an object surface is very high such that a phase variation between neighboring pixels is higher than a specific value of π, the phase measurement result may be deteriorated.
The conventional PSI continuously moves a reference plane at least three times by a predetermined phase simultaneously while allowing the optical detection element to measure an interference pattern at least three times, and compares a relative phase difference in wavefront between a measurement light speed and a reference light speed, such that it measures the height of an object surface. Therefore, the conventional PSI must measure the height of the object surface simultaneously while moving the reference plane in the direction of an upper part of the measurement surface at regular intervals, such that it requires a long period of a measurement time and is unable to correctly and precisely measure a shape of the object due to a position error caused by mechanical movement.