Patent Number: 051877260
Section: summary

FIELD OF THE INVENTION This invention pertains generally to the field of micro-electronic processing techniques, particularly to X-ray lithography and to X-ray lithography masks. BACKGROUND OF THE INVENTION In the formation of micro-electronic devices using photolithographic techniques, the wavelength of the "light" utilized to form the image on the target photoresist imposes a fundamental limit on the available image definition. The image resolution, or minimum linewidth that can be imaged, is limited by the diffraction of the light at the edges of the features of the masks through which the light is projected. The commonly used figure of merit is the Fresnel number f calculated as f=W.sup.2 /G.lambda., where G is the gap distance between the mask and the target surface, .lambda. is the wavelength of the light being utilized and W is the feature size. The Fresnel number f provides a guide in assessing the obtainable resolution, with f=0.5 corresponding to about what is usually considered the resolution limit. To allow the creation of smaller micro-electronic structures than are attainable utilizing visible or ultraviolet optical systems, X-ray sources are being utilized. Synchrotrons are particularly suitable as X-ray sources for X-ray lithography since the synchrotron provides an intense, steady beam of substantially collimated X-ray photons having a mixture of wavelengths spanning soft to hard X-rays. Because the wavelength .lambda. of X-rays is smaller than the wavelength of optical or ultraviolet light, X-ray lithography inherently allows smaller features to be created. However, the feature size for X-rays is also ultimately limited, as the Fresnel number criterion also applies to X-ray lithography systems. The formula for the Fresnel number is approximate since it does not include physical effects, and resolutions in X-ray lithography systems less than 1,000 Angstroms (A) have been demonstrated with gaps larger than those that might be expected from the Fresnel number calculated for such systems. Nonetheless, the Fresnel number provides an approximate criterion for determining the ultimate resolution. Using this criterion, for example, it is found that to image 0.25 micrometer (.mu.m) lines with one nanometer (nm) radiation would allow a maximum gap of only 12.5 .mu.m. Thus, as the required line resolution shrinks, so does the available working distance between the mask and the target surface. It is generally considered difficult to perform X-ray lithography exposures at distances between the mask and target of less than 10 .mu.m. It may be noted that there are two types of images that can be considered in determining the resolution in X-ray lithography, the aerial image (the X-ray intensity at the target surface) and the latent image (the image recorded in the target photoresist resist material). Phase shifting masks have been used to increase the image definition in projection optical systems, and their use has been proposed to allow extension of the resolution limit of conventional visible and ultra-violet light optical lithographic systems. The phase shift mask includes a transparent layer of suitable thickness defining certain features which introduces a half wavelength (.pi.) phase shift of the field E.sub.1 of the light transmitted through the layer relative to the field E.sub.2 of the light transmitted through an area without the transparent layer. The total field E.sub.t is obtained by addition of the two fields, i.e., E.sub.t =E.sub.1 +E.sub.2, so that at some position along the image plane, the total field must become zero because of the continuity requirement. This creates a sharp modulation in the intensity pattern. A judicious choice of the phase shifting overlayer can improve the image even for complex patterns, although the technique works best for regular and repetitive cases such as those used in the manufacture of dynamic random access memories (DRAMs). It has been suggested that an X-ray mask having an absorbent thickness appropriate for yielding a .pi. phase shift can improve image sharpness. See Y. C. Ku, et al., "Use of a Pi-Phase Shifting X-Ray Mask to Increase the Intensity Slope at Feature Edges," J. Vac. Sci. Technol. B, Vol. 6, No. 1, January/February 1988, p. 150-153. The masks described therein are absorbing masks, and the phase effects were used to refine the image rather than to define it. SUMMARY OF THE INVENTION In accordance with the present invention, a phase shift mask for X-ray lithography is provided having at least one phase shifting feature having at least one sharply defined sidewall which is upright with respect to the surface of the carrier on which the feature is supported. The material of the phase shifter feature introduces a half wavelength shift to X-ray photons passed therethrough compared to photons passed through a region of the mask having no phase shift material. The region of phase shift material is preferably of relatively low attenuation of X-rays passed through it, i.e., it is substantially "transparent" to X-rays, and is mounted on a carrier substrate which itself is substantially transparent to X-rays. The phase shift mask is placed in close proximity (e.g., preferably within five to ten micrometers) of a target structure which includes a layer of photoresist material. A beam of preferably collimated X-rays is then passed through the mask to expose the resist beneath the mask. Where a positive photoresist is used, the interaction of the X-ray photons with the photoresist causes the photoresist to be susceptible to dissolution in a developer. The target with the exposed photoresist thereon is then treated with a developer to remove all of the exposed photoresist, which leaves photoresist structures on the carrier which were not exposed to X-ray photons sufficiently to render such regions of the photoresist removable. Such structures occur underneath the intersection beneath the upright sidewalls of the phase shift features on the phase shift mask because of destructive interference of the spatially coherent X-rays which are passed through the mask. The interaction of the photons phase shifted by 1/2 of a wavelength and the photons that are not so phase shifted leaves a zone between the two regions where there is very little effective X-ray energy deposited, preferably at a level below a sharply defined resist exposure threshold so that the resist is left unremoved in those areas. The phase shift mask of the present invention allows very thin wall structures to be formed in the photoresist on the target substrate, in the range of a few hundred Angstroms wide. Typical X-ray photoresists such as polymethyl methacrylate (PMMA), may be utilized to provide such structures. It is found that the unexposed region is very sharply defined because of the rapid variation of the electrical field. The width of the region, i.e., the resolution, is dependent on the gap between the mask and the photoresist layer, while the modulation is not. In accordance with the present invention, closed figure structures can readily be formed by providing a feature comprising a bounded region of phase shift material on the mask carrier with sharply defined upright sidewalls. However, the structures of the present invention can also be formed in other than closed figures. For example, by combining a phase shift mask region with a pure absorber region, (which substantially blocks the X-rays), an exposure of the target resist can be carried out in such a way that the region covered by the absorber of the target resist, after developing, is connected to the lines formed by the phase shift feature mask. Unconnected lines may also be formed utilizing a phase shift mask having some side walls which are substantially upright and also slanting sidewalls in which the material of the phase shift mask at the sidewall slants inwardly or outwardly. When the X-ray beam is passed through such a phase shift mask, the phase shift effect will cause destructive interference of the X-ray beam at the region underneath the upright sidewalls, but the area under the slanting sidewalls will not have substantial cancellation of the X-ray beam thereunder, resulting in substantially total exposure of the photoresist under these regions. The result is isolated thin walls formed in the target resist which correspond to the upright sidewalls in the phase shift feature on the mask. The slanting sidewalls of the phase shift mask can be produced, for example, by exposing a PMMA photoresist material with a beam of X-rays passed at an angle through an X-ray mask having an absorber on it, so that the absorption of X-rays into the photoresist takes place at an angle. The width of the lines left unexposed on the target photoresist can also be selected as desired by varying the gap between the mask structure and the target photoresist, or by varying the gap between selected regions of phase shifter material on the carrier of the mask and the underlying target photoresist material. Further objects, features, and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.