Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
2001-03-05
2003-06-10
Kim, Robert H. (Department: 2882)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S484000, C356S485000, C356S489000, C356S511000, C356S513000, C356S515000
Reexamination Certificate
active
06577400
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an interferometer for measuring a phase difference between a reference beam and an object beam transformed by an optical element, comprising light source means, for generating the object beam and the reference beam, an optical device, for allowing the transformed object beam and the reference beam to interfere in a detection plane, and detection means, for detecting the phase difference between wave fronts of the transformed object beam and the reference beam in the detection plane.
An interferometer of this type is disclosed in U.S. Pat. No. 5,076,695. The known interferometer is intended for measuring the surface accuracy of the spherical surface of an object with a high degree of accuracy, without making use of a comparison surface of an accurately known shape. In the past a comparison surface of this type was placed in the position of the spice to be tested, with the aim of calibrating the interferometer. The known interferometer has first optical means for an essentially monochromatic light beam, originating from a light source, along a first optical axis towards a surface to be tested and directing the reflected beam in the opposite direction along the first axis Furthermore, second optical means are present in order to direct the light beam from the light source as a reference beam along a second axis which crosses the first axis and interference means for directing the reflected beam from the surface to be tested towards the second axis in order to interfere with the reference beam. Detection means are arranged on the second axis in order to measure the interference patterns which are produced by interference of the reflected beam and the reference beam. The Interferometer also comprises point diaphragms, arranged at the point of intersection of the first and the second axis to transform the light beam to the surface to be tested and the reference beam directed towards the detection means into spherical waves. The accuracy that is achieved with this known interferometer is between &lgr;/100 and &lgr;/1000.
Increasingly more refined techniques are being used for the production of semiconductor chips, the semiconductor structures becoming ever smaller. An extreme ultraviolet (EUV) lithographic technique will probably be used for the production of structures having a resolution of 0.1 &mgr;m, with which technique an optical mask is projected with the aid of a system of mirror as an image reduced in scale onto the substrate on which the structures are produced. The mirrors in this system must be of highly accurate shape in order to obtain the desired effect. The required accuracy of measurement of the shape of the mirrors is approximately 0.1 nm, which at a wavelength of approximately 630 nm (for example an He—Ne laser) corresponds to &lgr;/6300. The known interferometer for testing a surface is thus insufficiently accurate for this purpose.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide an interferometer which has a very high accuracy compared with the known interferometers.
Said aim is achieved by means of an interferometer of the type defined in the preamble, wherein the optical device comprises:
a first light conductor having an input surface that couples the object beam generated by the light source means into the first light conductor and having a first output surface that generates an object beam having a spherical wave front,
a second light conductor having an input surface that couples the reference beam generated by the light source means into the second light conductor and having a second output surface that generates a reference beam having a spherical wave front, the reference beam being directed onto the detection plane, and
wherein the arrangement of the first light conductor and the optical element is such that the transformed object beam interferes with the reference beam in the detection plane.
Because the optical device of the interferometer does not contain any optical elements between the location where the spherical wave fronts are generated and the detection plane, except for the optical element that transforms the object beam, the (transformed) object beam and the reference beam are not additionally distorted and the interference between the transformed object beam and the reference beam is very clean. Consequently, in principle, a highly accurate measurement of the phase difference between the beam transformed by the optical element and the reference beam in the detection plane is possible, the accuracy being higher than the mum required accuracy (&lgr;/6300). Furthermore, the interferometer can be used for both reflecting and transmitting optical elements.
One embodiment of the interferometer according to the present invention further comprises processing means for calculating the phase distribution over a specific cross-section of the transformed object beam on the basis of the phase difference between the wave fronts, the positions of the first and second output surfaces of the first and second light conductors, respectively, the position of the detection plane and the position of the specific cross-section.
With this embodiment of the interferometer it is possible to determine the phase difference with respect to the reference beam in a specific cross-section of the transformed object beam. Because both the object beam and the reference beam have spherical wave fronts, this provides information on the transformation of the object beam by the optical element and thus information on the optical characteristics of the optical element.
In an alternative embodiment of the interferometer according to the present invention, the latter comprises processing means for calculating a spatial plane where the phase difference between the wave fronts has a specific shape on the basis of the phase difference between the wave fronts, the positions of the first and second output surfaces of the first and second light conductors, respectively, the position of the detection plane, the position of the spatial plane in general and of a specific point on the spatial plane. Instead of determining the phase difference in a specific cross-section of the transformed object beam, it is possible, with the aid of this embodiment, to determine a spatial plane on which the phase difference has a specific shape. A particular case of this is a shape of the phase difference for which the phase difference is constant. In this case it is then necessary to determine a position of the spatial plane, as well as a point on said spatial plane that defines the constant phase difference.
In a preferred embodiment of the interferometer according to the present invention, the spatial plane is formed by the surface of a reflecting body. Using the interferometer according to this embodiment it is then possible to determine the precise shape of the reflecting body and specifically to do so as the spatial plane in which the phase difference between the object beam and the transformed object beam is equal to the phase difference that is produced by reflection at the reflecting body. In the case of a reflecting body of dielectric material this phase difference is exactly &pgr;. If the reflecting body is not made of dielectric material, the phase rotation is dependent on the angle of incidence of the light beam. The processing means can take this into account. The shape of a reflecting body is also referred to by the term “form figure” or the height z as a function of x and y, which is a dimensionless parameter, which is a measure of the shape or asphericity of the reflecting body.
In a preferred embodiment according to the invention, the said positions are determined by a position determination device which is present in the optical device. An output of the position determination device is connected to the processing means. A laser positioning system is used in order to achieve the required accuracy of position determination.
In a preferred embodiment the light source means and detection means of the interfero
Ho Allen C
Kim Robert H.
Stichting Voor de Technische Wetenschappen
Young & Thompson
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