<title>Multilayer optics for an extreme-ultraviolet lithography tool with 70-nm resolution</title>

Abstract

One of the most critical tasks in the development of extreme ultraviolet lithography (EUVL) is the accurate deposition of reflective multilayer coatings for the mirrors comprising the EUVL tool. The second set (Set 2) of four imaging optics for an alpha-class EUVL system has been coated successfully. All four mirrors (M1, M2, M3, M4) were Mo/Si- coated during a single-deposition run with a production- scale DC-magnetron sputtering system. Ideally, the multilayer coatings should not degrade the residual wavefront error of the imaging system design. For the present EUVL camera, this requirement is equivalent to depositing multilayer coatings that would add a figure error of less than 0.11 nm rms. In addition, all mirrors should be matched in centroid wavelength, in order to insure maximum throughput of the EUVL tool. In order to meet these constraints, the multilayer deposition process needs to be controlled to atomic precision. EUV measurements of the coated mirrors determined that the added figure errors due to the multilayer coatings are 0.032 nm rms (M1), 0.037 nm rms (M2), 0.040 nm rms (M3) and 0.015 nm rms (M4), well within the aforementioned requirement of 0.11 nm rms. The average wavelength among the four projection mirrors is 13.352 nm, with an optic-to-optic matching of 1(sigma) =0.010 nm. This outstanding level of wavelength matching produces 99.3% of the throughput of an ideally matched four-mirror system. Peak reflectances are 63.8% (M1), 65.2% (M2), 63.8% (M3) and 66.7% (M4). The variation in reflectance values between the four optics is consistent with their high frequency substrate roughness. It is predicted that the multilayer coatings will not introduce any aberrations in the lithographic system performance, for both static and scanned images of 70 nm - dense features.