Radiation imagery chemistry: process – composition – or product th – Registration or layout process other than color proofing
Patent
1994-06-27
1996-03-05
Rosasco, S.
Radiation imagery chemistry: process, composition, or product th
Registration or layout process other than color proofing
430 30, 430394, 356123, 356372, 356394, G03F 900
Patent
active
054966698
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an optical system for forming an image of a mask pattern in photolithography and especially to a device for determining the image plane with maximum sharpness in a system of this type.
DESCRIPTION OF THE PRIOR ART
Technical development in past years has shown that the boundaries in respect of the efficiency of photolithography are still evolving. Hitherto, inter alia x-ray, electron and ion lithography have been used to meet the demands of mass production of integrated semiconductor devices. In recent mass production developments, half-micron structures have been fabricated by means of the application of so-called wafer steppers which transfer the contents of a conventional chromium mask by means of protection objectives whose numerical aperture, hereinafter indicated as NA, is within the range of 0.45 and 0.56, and ultraviolet light having a wavelength of 365 nm. A so-called global alignment was used for improving the accuracy of the mutual superposition between various masks with a direct mutual coverage between a so-called mask feature or mask marker and wafer marker. In the process, optical filtering takes place of the optical signals which are generated upon exposure of the wafer by means of light from the alignment detection system.
In order to achieve a further decrease of the structure width, a still further increase of the NA and a further decrease of the wavelength in the UV region has been considered. This would, however, result in a considerable reduction of the sharpness. Nor can this situation be significantly altered by improving the image by means of phase-shifting or exposure methods.
In order to be able to apply said new technical possibilities of further structure refinements successfully in mass production, a further reduction is required of lithographic parameters such as overlay faults and fluctuations of the line widths on wafers being processed. These are primarily determined by fluctuations of the energy injected into the photographic material or resist, and thus in particular the unsharpness of the image, such as the defocusing of the aerial image in the resist. As a result, the decrease in the influence of technical parameters on the position of the aerial image having optimum sharpness in the resist (defocusing) is becoming increasingly important. Therefore, attempts were made to determine and to correct, by means of a latent image, the optimum position of the aerial image in the resist as well as the position-dependent fluctuations of the injected energy.
In a publication "SPIE, part 1464" on "Integrated Circuit Metrology, Inspection and Process Control V(1991), pp. 245 to 257 inclusive, and pp. 294 et seq", measuring devices having a projection system are described which are based on the absorption of the light which is diffracted from the latent image of a grid which has equidistant lines and grooves and an image of which is formed in the resist upon exposure to non-actinic light. This can be used to draw inferences with respect to the position of the focus, and photolithographic and image parameters of the projection system can be determined. A grid is generated by forming the image of a matching mask structure, by means of a so-called stepper, in the resist which is located on a wafer. For the purpose of recording light diffracted by the image of the grid, objectives having a relatively high NA can be used without having to satisfy oversevere constraints with respect to optical defects. Consequently, strong measured signals were obtained, even for extremely small changes in the absorption of the resist and in its refractive index. A first drawback in this case is that an additional device is required which means additional cost. A further drawback is that the desired information, such as optimum focus position, is obtained after a considerable delay and that said device can only be used on test wafers. Thus said device does not permit technically desirable checks on-line and in situ, nor, consequently, direct measurements
REFERENCES:
patent: 5124927 (1992-06-01), Hopewell et al.
Pforr Rainer
Seltmann Rolf
Wittekoek Steve
Interuniversitair Micro-Elektronica Centrum vzw
Rosasco S.
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