Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-03-01
2004-05-11
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S497000
Reexamination Certificate
active
06734978
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-267786, filed Sep. 4, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a profile measuring method and apparatus for use in the field of manufacturing of an ultra high degree precision machine components, the field of managing their quality and/or the field of measuring them, the surface profile of a preformed product, such as a magnetic recording component and an LSI substrate, or a liquid crystal substrate for computers, the method and apparatus capable of accurately measuring, by uses of the interference of light, with the ultra high degree precision ranging from several nanometers to several hundreds of micrometers.
2. Description of the Related Art
Techniques for measuring the shapes, in particular, surface profiles of industrial products, are important industrial techniques, and profile measurement apparatuses employing various measuring methods are now being used. In particular, profile measurement apparatuses use of the interference of light capable of non-contact measurements are widely used. However, most apparatuses of this type are not made in JAPAN.
In general, the profile measuring methods by uses of the interference of light, white light is applied to the surface of a sample through a band-pass filter, interference light formed by light reflected from the surface of a sample and light reflected from a reference surface (a reference mirror) is picked up by a camera, and an image output from the camera is subjected to image processing (the image output is analyzed). In the profile measuring methods for acquiring interference light include, for example, the phase shift method or multipoint simultaneous pickup method. The phase shift method include various types of making an optical-path difference for the acquisition of interference light. Further, there are various types of output image processing methods.
Wyco Corporation (USA), for example, provides a profile measurement apparatus in which an interference phase is obtained for a smooth surface having an unevenness of about several microns, using the phase shift method that employs a shift amount of &pgr;/2, while the profile of a surface having a greater unevenness is obtained from the peak position of an envelope indicative of variations in the sine wave of an interference signal appearing when an optical-path difference is changed.
In the Wyco profile measurement apparatus, to enable both a smooth surface and a rough surface to be measured, phase shifting is executed in units of less than &pgr;, such as &pgr;/2 or &pgr;/3. A number of interference images are picked up by a two-dimensional camera, and phase data of each pixel of the camera is obtained from a signal output from the camera, using the phase shift algorithm. Further, the envelope of variations in the sine wave of an interference signal is extracted from the signal, thereby calculating the profile of the surface.
Zygo corporation (USA) provides a measurement apparatus for picking up an interference image using a shift amount of &pgr;/2, and simultaneously extracting interference contrast data and phase data, using a data analysis in frequency domain.
In the Zygo profile measurement apparatus and method, it is not necessary to measure the peak position of an envelope for each pixel, and both a smooth surface and a relatively rough surface with a step can be measured using the same data processing method. Further, this method incorporates a technique for minimizing the required memory capacity.
In addition to the above-described two measuring apparatuses, a further method has been proposed. In this method, the shape of an envelope of variations in the sine wave of an interference signal, which appears when an optical-path difference is changed, is estimated from a small number of sampling points (in terms of an amount of shift (shift amount) in the phase shift method, image pickup is executed in units of about 10&pgr;) using a band-pass type sampling theorem, thereby extracting the peak position and obtaining a surface profile.
Moreover, a method for increasing measurement speed has been proposed, which employs multipoint simultaneous image pickup using a confocal microscope effect and a microlens array (Journal of the Japan Society for Precision Engineering, Vol. 64, No. 7, pp 1022 to 1028, 1998).
In the case of using light interference based on light of a wavelength &lgr;, if an optical-path difference is differed with &lgr;×n′ (n′ is an integer), the intensity of interference light is substantially identical for different values of n′, and hence the optical difference cannot be correctly determined. Accordingly, a sample that has a rough surface with discontinuous surface level variations cannot be measured simply by a method for obtaining a phase value.
In the aforementioned Wyco profile measurement apparatus, phase shifting is executed in units of less than &pgr;, such as &pgr;/2 or &pgr;/3, and a number of light interference images are picked up by a two-dimensional camera. In this case, usually, about {fraction (1/30)} second is required to transfer data corresponding to one image from the camera to a computer, and the speed, at which the optical-path difference of the interferometer is changed, is set to, at maximum, (&lgr;/4)/({fraction (1/30)} sec.). In other words, if a phase shift amount is just &pgr;/2, the image pickup operation must be executed four times in order to change the optical-path difference by one wavelength. This means that the interferometer is of a reflection type, and the movement speed of an incorporated movable table is about 2.0 &mgr;m/sec., and much time is required for measuring a rough surface.
In this measuring method, even if a high-speed camera is used to increase the transfer rate of image data, much time is required for data processing since the amount of image data itself cannot be reduced, thereby making it difficult to considerably increase the measurement speed.
Furthermore, in the Zygo measuring method and measurement apparatus, a phase shift amount employed in its three-dimensional profile measuring method is about &pgr;/2, as in the Wyco apparatus, and hence it is difficult to significantly increase the measurement speed.
On the other hand, in the method in which the shape of an envelope of variations in the sine wave of an interference signal, which appears when an optical-path difference is changed, is estimated from a small number of sampling points (in terms of a shift amount in the phase shift method, image pickup is executed in units of about 10&pgr;), using a band-pass type sampling theorem, thereby extracting the peak position and obtaining a surface profile, the time required for measurement can be shortened (the measurement speed can be increased), but the measurement precision is about {fraction (1/10)} to {fraction (1/100)} of that of the phase shift method.
The other method, which employs multipoint simultaneous image pickup using a confocal microscope effect and a microlens array, merely provides a measurement precision as low as that obtained by a microscope using confocal focusing (confocal microscope).
BRIEF SUMMARY OF THE INVENTION
It is the object of the invention to provide a profile measurement apparatus and method for measuring, at high speed and in a non-contact manner, the surface profile of a to-be-measured object with a precision as high as that of the phase shift method.
According to an aspect of the present invention, there is provided a profile measuring method of calculating, using a phase shift method, a phase value of an interference image formed by a light beam reflected from an object and a light beam reflected from a reference mirror, thereby obtaining an optical path difference from the calculated phase value and obtaining a profile of the object from the optical path difference, c
Lyons Michael A.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
President of Kanazawa University
Turner Samuel A.
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