Inspection and Defectivity

As a technology that uses masks with the same feature CDs as the resulting resist pattern, Imprint presents some new challenges to the current 4x mask inspection infrastructure. Indeed, while the resolution requirements are not the only item to consider in imprint mask inspection, it is clearly one of the most important. Working with partners and leaders in the inspection industry such as KLA we have found that today’s leading-edge inspection systems perform extremely well at detecting programmed defects, in a variety of 32nm pattern types, such as those illustrated below:

32nm Half-Pitch lines on mask with detected program defect:

Work to further develop mask inspection continues with our partners. In the meantime, use has also been made of e-beam systems such as the KLA-Tencor eS35, Hermes Microvision eScan 315 and the NGR 2100 to analyze defect sizes deep sub-CD. These wafer systems are used to inspect resist patterns to obtain both an indirect inspection of the mask (from early imprints) and a characterization of the imprint process itself. For example, using the eS35, 8-12nm programmed defects can be detected reliably in many cases:

As the results obtained from both direct and indirect mask inspection indicate, the capability is near to consider implementing a production inspection process for imprint.

With the inspection capabilities in hand, the next task is to tackle the process defectivity of imprint lithography. Any new lithographic technology, particularly one with the unique fidelity of imprint, brings with it a new set of defect sources and mechanisms that must be addressed. Such was the case with immersion lithography, which introduced bubble, water mark, residue and other defects to the lithographer’s lexicon. Fortunately, as we learned with immersion, with appropriate resources, these defect issues can be addressed.

Using the inspection techniques described above, together with more conventional optical mask inspection tools, Molecular Imprints has characterized the defectivity of the imprinting process. Such defectivity typically stems from a small group of sources, the main contributors being:

• Mask fabrication (writing and process).Mask defects come from both e-beam writing and the use of pilot, rather than full production process flows. In conjunction with our strategic partner, Dai Nippon Printing (DNP), sub-32nm masks are now being made routinely using traditional variable shaped e-beam pattern generation systems, while the overall mask process flow is now being introduced to the manufacturing environment to ensure process defectivity meets the rigorous standards of commercial masks.
• Incoming particles on wafers/coatings etc. Particles obviously lead to defect locations and in imprint it is important to minimize the incoming particles on the wafer. Such requirements are consistent with the increasingly stringent cleanliness requirements of the immersion lithography process and therefore represent a known and well understood issue for fabs. Work at customer locations, where there is access to state of the art clean rooms and cleaning facilities has demonstrated that such particles can be controlled well for the imprint process.
• Resist feature failure (adhesion/release). It is critical that the wafer adhesion coatings have uniform coverage and that the correct release treatment is utilized to minimize this defect. Other considerations include such items as the feature aspect ratio and the mechanical performance of the resist. Molecular Imprints has consistently and successfully focused on the resist and its adhesion/release properties in order to minimize this defect source in relation to other defect sources.

Using the techniques and processes described above, we have been able to bring defectivity down over time to the level of a ~1 defect/cm2, the chart below illustrating the progress made in the last year. To continue this progress toward manufacturing levels of defectivity, we are now focusing with customers and partners to take advantage of their leading-edge facilities and equipment infrastructure. The near term goal is to extend the imprint process to 50,000 imprints with a single mask at defect densities less than 1 cm-2.

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