James P. Levin

Mitigation of extreme ultraviolet mask defects by pattern shifting: Method and statistics

Currently the mask blanks used in extreme ultraviolet lithography cannot be fabricated free of defects. A rapid method of determining the optimum placement of mask patterns on the blank to avoid these defects is described. Using this method, the probability of fabricating defect-free masks, when the pattern is (1) randomly placed on the mask blank or (2) positioned optimally to avoid defects, is determined for a variety of integrated circuit designs, defect densities, and defect sizes. In addition to circular defects, oval and clusters of defects are also considered. Finally, simple analytical expressions for the probability of obtaining a defect-free mask in the case of random placement of the mask pattern is presented and compared to Monte Carlo simulations.

Diffraction effects in x‐ray proximity printing

The influence of diffraction on the shape and size of features printed using x‐ray proximity printing with a collimated x‐ray source (measured beam divergence of ∼0.2 mrad full width at half‐maximum) at mask to wafer gaps of 25 μm and above is described. Three major conclusions can be drawn from the results: (1) Diffraction can distort the shape of a printed feature, and the systematic shape changes observed in resist images can be explained using simple scaling based on Fresnel diffraction; (2) The linewidth change with exposure dose is independent of feature type and size, and depends only on the square root of the mask to wafer gap; and (3) The bias of each printed feature varies when all of the feature types and sizes are printed at the same dose. Resist images of 2.0–0.25 μm contact holes printed at mask to wafer gaps ranging from 25 to 515 μm are presented. The square contact holes on the mask print diamond shaped at a Fresnel number of 2.5. Isolated lines, spaces, and line‐space arrays ranging from...

High-Accuracy Defect-Free X-Ray Mask Technology

There are many material and processing options for building highly accurate defect-free X-ray masks that meet the 0.25-µ m and smaller lithography groundrules. IBMs path and rationale for reducing the key mask parameters of image size, image placement and defects is covered. For image size resolution and control, high voltage e-beam lithography (greater then 50 kV) is the preferred technique for X-ray masks. For tighter image placement control, special writing schemes that reduce the e-beam lithography systematic and random placement errors must be used. Special absorber electroplating conditions and thermal controls were implemented to control process-induced distortion. For tight defect control, identifying and eliminating sources of defect is key. Clearly, for IBM, most of the defect sources were process rather than foreign material related. Our defect reduction work has resulted in the fabrication of a fully functional 64-Mb DRAM (single chip) mask.

Diffraction effects and image blurring in x‐ray proximity printing

The influence of diffraction on the shape and size of features printed using x‐ray proximity printing is reviewed, and the effect of image blurring on these results is described. Diffraction can alter the shape of a printed feature, and the systematic shape changes observed in resist images can be explained using simple scaling based on Fresnel diffraction. In addition, the linewidth change with exposure dose is independent of feature type and size, and depends only on the square root of the mask to wafer gap. The shape of printed features and the linewidth change with dose can be modified by smearing the aerial image at the wafer plane. This can be achieved by adding beam divergence or by varying the angle of incidence of the x‐ray beam onto the mask (wobbling). A technique for incorporating wobble into an exposure system is described, and exposures of contact holes, spaces, lines, and line‐space arrays using this technique are presented. For example, 0.35 μm square contact holes normally print diamond shaped at a 40 μm gap. However, the same contact holes are round in resist when 4 mrad of wobble is incorporated into the exposure. The linewidth change for a 10% increase in dose is 24 nm at a 40 μm gap with 5 mrad of wobble. This linewidth change with exposure dose is larger than the 20 nm measured for exposures at a 40 μm gap without wobble. Finally, wobbling during exposure can either increase or decrease the absolute linewidth of a feature in resist at a given exposure dose.

Model for focused ion beam deposition

A model of focused ion beam deposition of materials is described. Decomposition of organometallic molecules by ion beams 50-300 nm in diameter allow localized deposition of a variety of metals. The large current density (approximately I A/cm2) and the inherent sputtering of a focused ion beam can result in no net deposition for a variety of process conditions. The major process parameters of current density, beam dwell time, and readsorption time are introduced. Experimental examples of gold depositions from dimethylgold hexafluoro acetylacetonate or DMG(hfac) are presented to illustrate the effect of the process parameters on size and shape of the depositions.

Status of x-ray mask inspection and repair

The status of x-ray mask inspection and repair at the IBM Advanced Mask Facility is presented. Defect classification and sources are presented along with some preliminary results from a defect printing study done at the Advanced Lithography Facility.