Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-04-27
2001-12-18
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S017000
Reexamination Certificate
active
06331368
ABSTRACT:
The invention relates to a method of detecting aberrations of an optical image system, comprising the steps of:
arranging a test object in the object plane of the system;
providing a photoresist layer in the image plane of the system;
imaging the test object by means of the system and an imaging beam;
developing the photoresist layer, and
detecting the developed image by means of a scanning detection device having a resolution which is considerably larger than that of the imaging system.
The fact that the resolution of the scanning detection device is considerably larger than that of the imaging system means that the detection device allows observation of details which are considerably smaller than the details that can still be separately imaged by the imaging system.
An optical imaging system in the form of a projection lens system having a large number of lens elements is used in photolithographic projection apparatuses which are known as wafer steppers or as wafer step-and-scanners. Such apparatuses are used, inter alia, for manufacturing integrated circuits, or ICs. In a photolithographic projection apparatus, a mask pattern present in the mask is imaged a large number of times, each time on a different area (IC area) of the substrate by means of a projection beam having a wavelength of, for example, 365 nm in the UV range, or a wavelength of, for example, 248 nm in the deep UV range, and by means of the projection lens system.
The method mentioned above is known from the opening paragraph of EP-A 0 849 638, relating to a method of measuring the comatic aberration of projection lens systems in lithographic projection apparatuses.
The aim is to integrate an ever-increasing number of electronic components in an IC. To realize this, it is desirable to increase the surface area of an IC and to decrease the size of the components. For the projection lens system, this means that both the image field and the resolution must be increased, so that increasingly smaller details, or line widths, can be imaged in a well-defined way in an increasingly larger image field. This requires a projection lens system which must comply with very stringent quality requirements. Despite the great care with which such a projection lens system has been designed and the great extent of accuracy with which the system is manufactured, such a system may still exhibit aberrations such as spherical aberration, coma and astigmatism which are not admissible for the envisaged application. In practice, a lithographic projection lens system is thus not an ideal, diffraction-limited system but an aberration-limited system. Said aberrations are dependent on the positions in the image field and are an important source of variations of the imaged line widths occurring across the image field. When novel techniques are used to enhance the resolving power, or the resolution, of a lithographic projection apparatus, such as the use of phase-shifting masks, as described in, for example, U.S. Pat. No. 5,217,831, or when applying an off-axis illumination as described in, for example, U.S. Pat. No. 5,367,404, the influence of the aberrations on the imaged line widths still increases.
Moreover, the aberrations are not constant in modem lithographic projection lens systems. To minimize low-order aberrations, such as distortion, curvature of the field, astigmatism, coma and spherical aberration, these systems comprise one or more movable lens elements. The wavelength of the projection beam or the height of the mask table may be adjustable for the same purpose. When these adjusting facilities are used, other and smaller aberrations are introduced. Moreover, since the intensity of the projection beam must be as large as possible, lithographic projection lens systems are subject to aging so that the extent of the aberrations may change with respect to time.
Based on the considerations described above, there is an increasing need for a reliable and accurate method of measuring aberrations.
It has also been proposed to use for the projection beam a beam of extreme UV (EUV) radiation, i.e. radiation at a wavelength in the range of several nm to several tens of nm. The resolution of the projection lens system can thereby be enhanced considerably without increasing the numerical aperture (NA) of the system. Since no suitable lens material is available for EUV radiation, a mirror projection system instead of a lens projection system must then be used. A lithographic mirror projection system is described in, inter alia, EP-A 0 779 258. For reasons analogous to those for the lens projection system, there is a need for an accurate and reliable method of measuring aberrations for this EUV mirror projection system as well.
The opening paragraph of said EP-A 0 849 638 rejects the method in which the image of a test mask formed in the photoresist layer is scanned with a scanning detection device in the form of a scanning electron microscope. Instead, it is proposed to detect said image with optical means. To this end, a test mask having one or more patterns of strips which are alternately radiation-transmissive and radiation-obstructive, i.e. an amplitude structure, is used. The comatic aberration of a projection system can be detected with such a pattern. The detection is based on measuring the widths of the light or dark strips in the image formed and/or measuring the asymmetry between the strips at the ends of the image of the patterns.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of the type descrobed on the opening paragraph, which is based on a different principle and with which different aberrations can be measure independently of each other. This method is characterized in that use is made of a test object which comprises at least one closed single figure having a phase structure, and in that the image of this figure observed by the scanning detection device is subjected to an image analysis in order to ascertain at least one of different types of changes of shape in the image of the single figure, each type of shape change being indicative of a given kind of aberration.
A single figure is understood to mean a figure having a single contour line which is closed in itself. The contour line is the boundary line between the figure and its ambience.
The invention is based on the recognition that the contour line of a figure having a phase structure is not imaged in a single line but in a first and a second image line, the second image line being located within the first image line, and the distance between the first and the second image line is determined by the point spread function, or Airy distribution, of the imaging system. In the method according to the invention, useful use is thus made of the point spread function, or Airy distribution, of the imaging system. If this system has given aberrations, given deviations of the ideal image occur, such as deviations of the shape of the image lines themselves and/or changes of the mutual position of the two image lines. The novel method thus allows detection of aberrations which cannot be detected when using a test object in the form of an amplitude, or black-white, structure. When using a test object with an amplitude structure, its contour line is imaged in a single line. Consequently, only the aberrations of the imaging system which cause deviations of the imaged single contour line can be detected when using such a test object, and this even less accurately. When using a test object having a phase structure, different aberrations occurring simultaneously can be detected separately because the effects of the different aberrations remain well distinguishable in the image formed, in other words, the different aberrations do not exhibit any mutual crosstalk.
It is to be noted that, in one embodiment described in U.S. Pat. No. 5,754,299, relating to a method and a device for measuring an asymmetrical aberration of a lithographic projection system, the test object is denoted as phase pattern. However, this pattern is not a closed single figure, but a
Dirksen Peter
Juffermans Casparus A. H.
Biren Steven R.
U.S. Philips Corporation
Young Christopher G.
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