Optics: measuring and testing – By light interference – Having wavefront division
Reexamination Certificate
2002-08-13
2004-07-20
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having wavefront division
Reexamination Certificate
active
06765683
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an interferometer system for measuring the shape of an aspheric surface of an optical element in an optical system and for measuring the wavefront aberration of such an optical system, particularly in connection with manufacture of a projection optical system suited to for use in an exposure apparatus employing soft-X-ray (EUV) exposure light.
BACKGROUND OF THE INVENTION
Light of wavelength 193 nm or longer has hitherto been used as the exposure light in lithographic equipment used when manufacturing semiconductor devices such as integrated circuits, liquid crystal displays, and thin film magnetic heads. The surfaces of lenses used in projection optical systems of such lithographic equipment are normally spherical, and the accuracy in the lens shape is 1 to 2 nm RMS (root mean square).
With the advance in microminiaturization of the patterns on semiconductor devices in recent years, there has been a demand for exposure apparatus that use wavelengths shorter than those used heretofore to achieve even greater microminiaturization. In particular, there has been a demand for the development and manufacture of projection exposure apparatus that use soft X-rays of wavelength of 11 to 13 nm.
Lenses (i.e., dioptric optical elements) cannot be used in the EUV wavelength region due to absorption, so catoptric projection optical systems (i.e., systems comprising only reflective surfaces) are employed. In addition, since a reflectance of only about 70% can be expected from reflective surfaces in the soft X-ray wavelength region, only three to six reflective surfaces can be used in a practical projection optical system.
Accordingly, to make an EUV projection optical system aberration-free with just a few reflective surfaces, all reflective surfaces are made aspheric. Furthermore, in the case of a projection optical system having four reflective surfaces, a reflective surface shape accuracy of 0.23 nm RMS is required. One method of forming an aspheric surface shape with this accuracy is to measure the actual surface shape using an interferometer and to use a corrective grinding machine to grind the surface to the desired shape.
In a conventional surface-shape-measuring interferometer, measurement repeatability is accurate to 0.3 nm RMS, the absolute accuracy for a spherical surface is 1 nm RMS, and the absolute accuracy of an aspheric surface is approximately 10 nm RMS. Therefore, the required accuracy cannot possibly be satisfied. As a result, a projection optical system designed to have a desired performance cannot be manufactured.
So-called null interferometric measurement using a null (compensating) element has hitherto been conducted for the measurement of aspheric surface shapes. Null lenses that use spherical lenses comprising spherical surfaces, and zone plates wherein annular diffraction gratings are formed on plane plates have principally been used as null elements.
FIG. 1
shows a conventional interferometer system
122
arrangement for null measurement using a null (compensating) element
132
. The interferometric measurement described herein is a slightly modified version of a Fizeau interferometric measurement. Namely, a plane wave
126
emitted from an interferometric light source
124
is partially reflected by a high-precision Fizeau surface
130
formed on a Fizeau plane plate
128
. The component of plane wave
126
transmitted through Fizeau surface
130
is converted into measurement wavefront (null wavefront)
134
by null element
132
and assumes a desired aspheric design shape at a measurement reference position RP, following which it arrives at a test surface
138
of a test object
136
previously set at the reference position. The light arriving at test surface
138
is reflected therefrom and interferes with the light component reflected from Fizeau surface
130
, and forms monochromatic interference fringes inside interferometer system
122
. These interference fringes are detected by a detector such as a CCD (not shown). A signal outputted by the detector is analyzed by an information processing system (not shown) that processes the interferometer information contained in the output signal. Similar measurements can be performed using a Twyman-Green interferometer.
To accurately ascertain the shape of test surface
138
, the null element
132
must be manufactured with advanced technology since there must be no error in the null wavefront. Specifically, this means that the optical characteristics of the null element
132
must be measured beforehand with high precision. Based on these measurements, the shape of null wavefront
134
is then determined by ray tracing. This results in the manufacture of null element
132
taking a long time. Consequently, the measurement of the desired aspheric surface takes a long time.
FIG. 2
shows another example of a conventional Fizeau interferometer
222
. Referring to
FIG. 2
, laser light from laser
224
passes through a lens system
226
to become a collimated light beam of a prescribed diameter and is incident Fizeau plate
228
. Rear side
230
of Fizeau plate
228
is accurately ground to a highly flat surface, and the component of the incident light reflected by rear side
230
of Fizeau plate
228
becomes a reference beam having a plane wavefront. The component of incident light transmitted through a Fizeau plate
228
passes through null element
232
, where the plane wavefront where the plane wavefront is converted to a desired aspheric wavefront. The aspheric wavefront is then incident in perpendicular fashion an aspheric test surface
238
. The light reflected by test surface
238
returns along the original optical path, is superimposed on the reference light beam, reflects off a beam splitting element
256
in lens system
226
, and forms interference fringes on a CCD detector
260
. By processing these interference fringes by a computer (not shown), the shape error can be measured.
A problem with interferometer
222
is deterioration, in absolute accuracy, due to null element
232
. A null element comprising a number of high-precision lenses (e.g., lenses
234
and
236
) a CGH (computer-generated hologram), or the like is ordinarily used as null element
232
, and manufacturing errors on the order of 10 nm RMS typically result.
Since interferometer
222
tends to be affected by vibration and air fluctuations due to the separation of reference surface
230
(i.e., rear side of Fizeau plate
228
) and test surface
238
. Repeatability is also poor, at 0.3 nm RMS. Furthermore, in measuring an aspheric surface, alignment of null element
232
and test surface
238
is critical. Measurement repeatability deteriorates by several nanometers if alignment accuracy is poor.
SUMMARY OF THE INVENTION
The present invention relates to an interferometer system for measuring the shape of an aspheric surface of an optical element in an optical system and for measuring the wavefront aberration of such an optical system, particularly in connection with manufacture of a projection optical system suited to for use in an exposure apparatus employing soft-X-ray (EUV) exposure light.
The goal of the present invention is to overcome the above-described deficiencies in the prior art so as to permit fast and accurate calibration of a null wavefront corresponding to an aspheric surface accurate to very high dimensional tolerances.
Another goal of the present invention is to manufacture a projection optical system having excellent performance.
Additional goals of the present invention are to provide an aspheric-surface-shape measuring interferometer having good reproducibility, to measure wavefront aberration with high precision and to permit calibration of an aspheric-surface-shape measuring interferometer so as to improve absolute accuracy in precision surface measurements.
Accordingly, a first aspect of the invention is an interferometer capable of measuring a surface shape of a target surface as compared to a reflector standard. The interferometer comprises a light source ca
Oliff & Berridg,e PLC
Turner Samuel A.
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