Yuli Vladimirsky

Demagnification in proximity x-ray lithography and extensibility to 25 nm by optimizing Fresnel diffraction

This new understanding and demonstration of features printed by proximity x-ray lithography allows a revolutionary extension and simplification of otherwise established processes for microfabrication. The ability to produce fine features is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature bias. In the classical approach the bias is minimized. Bias optimization in terms of mask/wafer gap and resist processing allows the formation, on a wafer, of features smaller than those on the mask: thus producing local demagnification. This demagnification ( ? 3- ? 6) is achieved without lenses or mirrors, but it offers the same advantages as projection optical lithography in terms of critical dimension control. The photoelectron blur is more or less pronounced depending on exposure dose and development conditions. Resist exposure and process can be optimized to utilize a ~ 50% photoelectron energy loss range. In consequence proximity x-ray lithography is extensible to feature sizes below 25 nm, taking advantage of comparatively large mask features (> 100 nm) and large gaps (30-15 ? m). The method is demonstrated for demagnification values down to ? 3.5. To produce DRAM half-pitch fine features, techniques such as multiple exposures with a single development step are proposed.

Near field x-ray lithography simulations for printing fine bridges

By using the near field in proximity x-ray lithography (PXL), a technique is demonstrated that extends beyond a resolution of 25 nm print feature size for dense lines. ‘Demagnification by bias’ of clear mask features is positively used in Fresnel diffraction together with multiple exposures of sharp peaks. Exposures are performed without lenses or mirrors between the mask and wafer, and ‘demagnification’ is achieved in a selectable range, 1×–9×. The pitch is kept small by multiple stepped exposures of sharp, intense image peaks followed by single development. Low pitch nested lines are demonstrated. The optical field is kept compact at the mask. Since the mask–wafer gap scales as the square of the mask feature size, the mask feature sizes and mask–wafer gaps are comparatively large. Because the features are themselves larger, the masks are more easily manufactured. Meanwhile, exposure times for development levels high on sharp peaks are short, and there are further benefits including defect reduction, virtual elimination of sidebands, etc. A critical condition (CC) has been identified that is typically used for the highest resolution. Many devices, including batches of microprocessors, have been demonstrated previously by traditional 1× PXL, which is the only next generation lithography developed and which is now further extended. For two-dimensional near field patterning, temporal and spatial incoherence at the CC have been used to show not only that peculiarities in the aerial pattern, such as ‘ripple’ and ‘bright spots’, can be virtually eliminated but also that there is an optimum demagnification, around 3×, in the Fresnel diffraction, where the contrast and, therefore, critical dimension control are highest. In the simulation of a bridge pattern, ‘ripple’ is likewise controlled. Blur and run-out are compared for various sources. Magnification corrections can be applied by various means. Extension to 15 nm printed features is predicted.

A critical condition in Fresnel diffraction used for ultra-high resolution lithographic printing

The adoption of a novel method for producing fine features by 1 nm proximity x-ray lithography would solve all of the current technical limitations to its extensibility. These limitations include the fabrication of fine features on masks and the maintenance of narrow mask-wafer gaps. Previously, with demagnification by bias, we described line features of 43 nm width produced with comparatively large clear mask features and large mask-wafer gaps. The method is generally applicable and has been shown to be extensible to beyond 25 nm printed features sizes on the wafer. The demagnification, ×1-×6, is a result of Fresnel diffraction and occurs without lenses or mirrors. The method takes advantage of the modern control of resist processing and has good exposure stability. We now expand on the optimization of the process by defining and explaining the critical condition and by demonstrating the consistency of various types of simulation. The simulations demonstrate the effects of the gap width, non-symmetric rectangular masks, spectral bandwidth, outriggers, T junctions, blur, etc. In two-dimensional images, the spectral bandwidth allows sharp features due to interference and effectively eliminates ripple parallel to the longer dimension. Demagnification by exposure near the critical condition extends the most mature of the next generation lithographies which we define generically—following actual current lithographic practice—in terms of the departure from the classical requirement for fidelity in the reproduction of masks. Specifically, for 1 nm proximity lithography, demagnification of critical features greatly facilitates the printing of fine features.

Near-field x-ray lithography to 15 nm

It is time to revisit X-ray. By enhancing, in the Near Field, Proximity X-ray Lithography (PXL), the technique is demonstrated that extends to 15nm printed feature size with 2:1 ratio of pitch to line width. Demagnification by bias of clear mask features is positively used in Fresnel diffraction together with rapid, multiple exposures of sharp peaks. Pitch is kep small by multiple, stepped exposures of the intense image followed by single development. The optical field is kept compact at the mask. Since the mask-wafer gap scales as the awaure of the mask feature size, mask feature sizes and mask-wafer gaps are comparatively large. A Critical Condition has been identified which is typically used for the highest resolution. Many devices, including batches of microprocessors, have been demonstrated previously by traditional 1X PXL which is the most mature of the Next Generation Lithographies and which is now further extended. Throughput and cost are conventional.

22-nm lithography using near-field x rays

By using the Near Field in Proximity X-ray Lithography (PXL), the technique is demonstrated that extends beyond a resolution of 25 nm print featuer size with 2:1 pitch to line width. Demagnification by bias of clear mask features is positively used in Fresnel diffraction together with multiple exposures of sharp peaks. Exposures are performed without lenses or mirrors between mask and wafer, and the demagnification is achieved in the selectable range 1X to 9X. Pitch is kept small by multiple, stepped exposures of sharp, intense, image peaks followed by single development. Low pitch nested lines are demonstrated. The optical field is kep compact at the mask. Since the mask-wafer gap scales as the square of the mask feature size, mask feature sizes and mask-wafer gaps are comparatively large. Because the features are themselves larger, the masks are more easily manufactured. Meanwhile exposure times, for development levels high on sharp peaks, are short, and there are further benefits including defect reduction. Many devices, including batches of microprocessors, have been demonstrated previously by traditional 1X PXL which is the most mature of the Next Generation Lithographies and which is now further extended. For 2D Near field patterning, temporal and spatial incoherence at the Critical Condition are used to show, not only that peculiarities in the aerial pattern, such as ripple and bright spots, can be virtually eliminated, but also that there is an optimum demagnification, around 3X, in the Fresnel diffraction, where the contrast is highest. At this demagnification, patterns of various dimensions can be printed using various and appropriate demagnifications.

Demagnification by bias in proximity x-ray lithography

The ability to produce fine features using X-ray proximity lithography is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature bias. The classical approach is to attempt to minimize the bias; that is, to print features which are 1:1 images of those on the mask. However, bias can also be exploited to print features smaller than those on the mask. This demagnification-by-bias technique can be optimized with respect to mask-wafer gap and resist processing, and can provide reductions of 3X to 6X. Demagnification offers many of the same advantages as projection optical lithography in terms of critical dimension control: relaxed mask features CD. In addition, it provides a very large depth of focus and wide dose latitude. In consequence proximity X-ray lithography is extendible to feature sizes below 25 nm, taking advantage of comparatively large mask features (> 0.1 nm) and large gaps (10 -25 micrometer). The method was demonstrated for demagnification values down to X3.5. To produce DRAM half- pitch fine features techniques such as multiple exposures with a single development step are proposed.

Phase zone plates for hard X-ray focusing

The major obstacle for constructing a deep UV or X-ray lens is the low refraction and high absorption of materials in this region. Only a few Fresnel zones are utilized, so the diameter of an X-ray lens is of the order few microns, and the resolution is hardly better than that of a pinhole. The thickness of a phase zone plates (ZP) is determined by the refraction of the material. To provide π phase shift and form a hard x-ray phase ZP a thickness of a few microns is required. Approaches and techniques developed at the Center for X-ray Lithography (CXrL) for producing thick phase ZPs are presented.