Patent Number: 056129866
Section: summary

BACKGROUND OF THE INVENTION As feature sizes on integrated circuits approach the minimum size possible with visible or ultraviolet wavelengths optics, other technologies are considered for circuit manufacturing techniques. X-ray proximity lithography is one alternative technique which has been developed, and while its use in circuit production with minimum linewidth 250 nanometers (nm) seems likely, blurring through diffraction at practical mask-to-wafer separation distances complicates extension of the technique to significantly finer line widths. Recently, X-ray projection lithography has begun to be considered. J. E. Bjorkholrn et al, in Jour. Vac. Sci. Technol., vol. B8 (1990) pp. 1509-1513, have demonstrated the printing of 50 nm line width features using a 20:1 reduction system based on near-normal incidence optics, coated with multilayers for good reflectivity at .lambda.=14 nm wavelength. Along with other developments, this has led to considerable activity aimed at the development of X-ray projection lithography systems capable of printing features of size 100 nm or finer over a large field with high throughput for use in circuit production. Most of the optical systems discussed in the literature for use in X-ray projection lithography have been based on the use of multilayer-coated, near-normal incidence optics. At present, multilayer-coated optics are able to deliver good normal incidence refractivity only at relatively long wavelengths (.lambda.&gt;10 nm), where X-ray penetration in photoresist materials is low and the contrast of likely contaminants is high compared to the .lambda..apprxeq.1 nm wavelengths used for X-ray proximity lithography. Further, while 1:1 systems using only flat and spherical optics have been proposed which would have curved image fields, T. E. Jewell et al, in Jour. Vac. Sci Technol., vol. B8 (1990) pp. 1519-1523, have found that at least 4-aspherical optics are required for a 20:1 reduction system with the required field and resolution. The figure tolerances of such optics are in the 0.5-1 mn range over a diameter of many cm, which is well beyond current fabrication limits, even for spherical optics, as noted in W. Silfvast, ed., Workshop on High Precision Soft X-ray Optics, Rockville, Md., October 1989. These and other considerations indicate that the challenges involved in the development of X-ray projection lithography by optical reduction are daunting, or even insurmountable, even considering the commercial payoff expected of such systems. Holographic optics were first proposed over twenty years ago for visible light lithography, by K. A. Stetson, Appl. Phys., Lett., vol. 12 (1967) pp. 362-364, by E. B. Champayne et al, Appl. Optics, vol. 8 (1969) pp. 1879-1885, and by M. J. Beesley et al, Electronics Lett., vol. 4 (1970) pp. 49-50. These methods did not become popular, because better and easier alternatives were developed based on the use of lenses for optical reduction. However, once one considers soft X-ray projection lithography, for which refracting lenses cannot be used, the technology for reduction imaging outlined above involves the enormous technological challenges referred to above, and it becomes worthwhile to consider again the potential contribution of holographic techniques. What is needed is a technique for optical lithography that: (1) allows definition of integrated circuit feature sizes of the order of 0.25 .mu.m and below; (2) is simple, preferably requiring only one or two optical components; (3) is relatively free of optical aberrations; (4) is relatively easy to fabricate; (5) allows reasonably uniform illumination of the desired image area; and (6) provides some means of dealing with high incoming power loads. SUMMARY OF THE INVENTION These needs are met by the invention, which provides a method that projects a holographic image of the desired circuit pattern onto tile wafer or other image-receiving substrate to allow recording of the desired image in photoresist material, for use in subsequent microfabrication steps. In a first embodiment, the method uses on-axis transmission and requires the following steps: (1) providing a high flux X-ray source, whose X-ray monochromaticity and coherence requirements are modest and would, for example, be satisfied by a standard X-ray tube; (2) providing a layer of light-sensitive photoresist material on the wafer with a selected surface to receive the image(s); (3) determining a hologram having variable optical thickness and variable associated optical phase angle and amplitude attenuation, for transmission of X-rays through the hologram, where irradiation of the hologram produces the desired X-ray image at the wafer with minimum X-ray image linewidths that can be smaller than 0.25 .mu.m; (4) positioning the hologram at a suitable object plane; and (5) irradiating the selected surface with X-rays received by transmission through (and diffraction from) the hologram. In a second embodiment of the invention that uses off-axis holography, the wafer receives the holographic image by grazing incidence reflection from a hologram printed on a flat metal or other highly reflecting surface or substrate, and the zero order diffraction beam passes to the side of the wafer. In this embodiment, an X-ray beam with a high degree of monochromaticity and spatial coherence is required. The following advantages can be adduced for the use of holographic methods. (1) The holographic real image is aberration-free in either embodiment, if the hologram is illuminated by the conjugate of the reference beam that was used to form (or compute) the hologram. This is exact within diffraction limits. (2) One can achieve aberrationless imaging, using only a single optical component, a computer-generated hologram that replaces both the imaging optics and the mask of a conventional projection system. (3) An on-axis hologram (first embodiment), can modulate both the phase and the amplitude of the transmitted X-ray beam, to project the desired intensity pattern on the wafer. The coherence length required is modest (.lambda./.DELTA..lambda..apprxeq.20-45); a coherence width of 20 .mu.m or less is also required. (4) The off-axis method (second embodiment) does not require that the hologram be used at normal incidence, and there are advantages to using grazing incidence X-ray beams: (a) the power load of the X-ray bearn is spread over a larger area, and (b) the tolerances for surface figure and finish are relaxed compared to norrnal incidence. (5) An off-axis hologram (second embodiment) is rigid and can be cooled. (6) Only one optical component is needed for either embodiment, and this component can be flat, which is good for fabrication and for stray light reduction. (7) The hologram in the second embodiment would be larger than the final image, but the difficulty in terms of resolution and distortion tolerances would be similar to that in mask making for proximity printing at the same feature size. (8) No special effort is required to illuminate the hologram uniformly in the second embodiment. Slowly varying non-uniformities of illumination have an effect only on the shape of the resolution function, and this can be corrected in forming the hologram if the illumination forms are known. (9) In both embodiments, the use of multi-layer coatings is avoided and the required hologram writing resolution is similar to that of a 1:1 proximity mask for the same pattern. A disadvantage of the second embodiment is that a highly monochromatic, single-mode X-ray beam is needed to illuminate the off-axis hologram. The monochromaticity value .lambda./.DELTA..lambda. needs to be of the order of the number of resolvable features within the image width, which is .gtoreq.5.times.10.sup.4 for useful images.