Optics: measuring and testing – By light interference – Having wavefront division
Reexamination Certificate
2001-04-26
2003-11-18
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having wavefront division
Reexamination Certificate
active
06650421
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting an optical device and, more particularly, it relates to a method and an apparatus for inspecting an optical device, which are suitable for inspecting a wavefront of ultraviolet rays, X-rays, etc., passing through a predetermined optical system.
2. Description of the Related Art
For exposure apparatuses so far employed in a lithography process for producing semiconductor elements and so on, there have been increasing demands to effect improvements in imaging features of an optical projection system for transcribing an image of a mask pattern onto a board such as a wafer, in order to enable finer and finer detail to be produced. In order to achieve such improvements, it is required to measure a status of wavefront aberration of an optical projection system with a high degree of precision so that a Twyman-Green interferometer and a Fizeau interferometer have hitherto been employed for inspecting a wavefront of laser beams, each of which uses He—Ne laser beams as an inspecting light as opposed to an exposing light employed conventionally. These measuring methods comprise measuring the status of a wavefront of light flux passed through an optical system to be inspected by causing the light flux passed through the optical device to be inspected to interfere with light flux from another reference plane by taking advantage of a very high interference performance inherent in He—Ne laser beams.
However, a measuring method using two light fluxeses with different light paths suffers from the disadvantage that interference fringes are likely to undergo an irregular distortion due to influences of vibration, large-scale optical parts of high precision are required to form a reference plane of high precision, and it is difficult to adjust the two light paths. Therefore, a point diffraction interferometer (PDI) is proposed (e.g. as disclosed in Japanese Patent Application Laid-open (Kokai) No. 57-64,139) which is so adapted as to create a reference light from a portion of the light flux passed through the optical projection system to be inspected. The prior art point diffraction interferometer can use even an exposing light intact as an inspecting light, which is not so high in interference performance, and can adjust a light path with ease, because the light path of a measuring light is almost equal to the light path of a reference light.
Particularly, as an exposing light (exposing light energy beams), there have recently been employed far ultraviolet rays such as ArF excimer laser beams (having wavelength of 193 nm), too, in addition to ultraviolet rays such as KrF excimer laser beams (having wavelength of 248 nm). Further, reviews have been made to use F
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laser beams (having wavelength of 157 nm), soft X-rays (having wavelength of several tens nm to about 1 nm) or hard X-rays (having wavelength of below about 1 nm) as an exposing light. As the kind of glass materials having high transmission which can be applied to an exposing light with such short wavelengths are limited, the use of an optical projection system has been reviewed which combines e.g. a concave mirror or a plane mirror covered with a reflecting coating composed of a predetermined number of layers. Although quartz, fluorite and the like are known to be employed as glass materials for transmission of e.g. ArF excimer laser beams, it is required to improve performance of a transmission coating in order to enhance transmission of those materials when they are to be used as such glass materials. In order to inspect a wavefront of the optical projection system including characteristics of a reflecting coating or transmission coating, it is required to use an exposing light as an inspecting light. To this end, a point diffraction interferometer is suitable.
Such a point diffraction interferometer can present the advantages that an exposing light can be employed intact as an inspecting light and a light path can be readily adjusted upon effecting inspection of the wavefront of the optical projection system of the exposure apparatuses as stated above. Using this conventional point diffraction interferometer, however, it is difficult to measure a phase difference of interference fringes between certain two measurement points with high precision because such interference fringes yielded therewith remain static. Therefore, the conventional point diffraction interferometer poses the problems that it is difficult to further improve precision of inspecting the wavefront of the optical projection system and it is rather poor in the ability of reproducing results of inspection. In other words, as the conventional point diffraction interferometer can be provided with various advantages, it nevertheless is not sufficiently precise.
Likewise, among interferometers other than the point diffraction interferometer, an interferometer of the type (e.g. a zone plate interferometer) capable of producing a reference light from a portion of light flux passed through an optical system by taking advantage of an illuminating light (e.g., an exposing light, etc.) with which the optical projection system to be inspected is actually irradiated suffers from the defect that a SN ratio of interference fringes may be likely to become worse. Therefore, demands have been made to develop a technique for inspecting a phase of interference fringes at desired measurement points with high precision.
SUMMARY OF THE INVENTION
In light of the above, the present invention has a primary object to provide an inspection method and an inspection apparatus (an interferometer) for inspecting an optical device, which are so adapted as to detect a phase of interference light between light flux passed through the optical device as an object to be inspected and reference light to be produced from a portion of the light flux with high precision.
Further, the present invention has a second object to provide an inspection method and an inspection apparatus (an interferometer) for inspecting an optical device, which are adapted so as to detect a two-dimensional distribution of phases of interference light with high precision, even when there is employed a light flux having a lower interference performance as an illuminating light for detection.
The inspection method for inspecting the optical device in accordance with the present invention is directed to an inspection method for effecting inspection of a wavefront of light flux passed through the optical device, which comprises the step of passing the light flux through the optical device as an object to be inspected, and the step of producing a diffracting light from a portion of the light flux passed through the optical device through a diffracting element in the process of movement. The inspection method further comprises the step for detecting an interference light between the diffracting light and another light flux passed through the optical device during the movement of the diffracting element.
The diffracting element is a diffracting member having a pinhole shape, and it is preferred to generate the diffracting light with the pinhole-shaped diffracting member. The movement of the pinhole-shaped diffracting member may include vibration of the diffracting member in a direction intersecting the light flux passed through the optical device or vibration of the diffraction member in a direction along or parallel to the light flux passed through the optical device.
The inspection method for the optical device in accordance with the present invention comprises varying phases of the diffracting light generated by diffraction with the diffraction member by effecting the movement of the diffraction member. The diffracting light is a reference light having an ideal wavefront (an undistorted spherical plane). The movement of the diffraction member can periodically vary a phase difference between the diffracting light as a reference light and another light flux (having a wavefront distorted by influences of aberra
Armstrong Westerman & Hattori, LLP
Font Frank G.
Natividad Phil S.
Nikon Corporation
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