Recently, in order to make it possible to transfer micropatterns having a size of 40 nm or less, EUV exposure technology has been promising in place of the existing ArF exposure technology that uses ArF excimer laser light having a wavelength of 193 nm. The EUV exposure technology uses, as the exposure light, EUV (Extreme Ultra-Violet) light having a shorter wavelength than the ArF excimer laser light. The EUV light includes a soft X-ray and vacuum UV light, and is specifically light having a wavelength of from about 0.2 to 100 nm. At present, as the exposure light, EUV light having a wavelength of about 13.5 nm is mainly investigated.
In the EUV lithography (EUVL) technology, a reflective photomask is used. The reflective photomask includes a multilayer reflective film and an absorber layer formed in this order on a substrate with the absorber layer being partially removed. The absorber layer is formed in a prescribed pattern. EUV light incident on the reflective photomask is absorbed in an area of the photomask with the absorber layer being present and is reflected by the multilayer reflective film in the other area with the absorber layer being absent such that an image is formed on the front side of an exposure material by an optical system. In that manner, the pattern of the absorber layer is transferred to the front side of the exposure material.
The multilayer reflective film has a periodic structure such that a plurality of films having different refractive indexes are repeatedly stacked on a substrate in a prescribed order. For example, the multilayer reflective film has Mo layers as low-refractivity layers and Si layers as high-refractivity layers alternately and repeatedly stacked therein.
In a case where a multilayer reflective film is contaminated with a foreign substance during production of the multilayer reflective film by stacking, or in a case where a defect (e.g. a foreign substance, a scratch or a pit) is present on the front side of a substrate to be provided with the multilayer reflective film thereon, the periodic structure of the multilayer reflective film is disordered to cause a defect (so-called phase defect) in the multilayer reflective film. Such a defect causes a problem in that the pattern of a reflective photomask is not correctly transferred to a wafer. It is extremely technically difficult to absolutely remove such a defect from the multilayer reflective film (e.g. see 2010 International Symposium on Extreme Ultraviolet Lithography, S. Huh et. al., “Printability and Inspectability of Programmed and Real Defects on the Reticle in EUV Lithography”).
From this point of view, investigation has been made to develop a technique of controlling the position and the direction of the pattern of an absorber layer in accordance with the position of such a defect in the multilayer reflective film (e.g. see EUVL Mask Fiducial SEMI Standard Discussion January 2006, P. Seidel and P. Y. Yan).
Further, in order to accurately identify the position of such a defect in a multilayer reflective film, there has been proposed a technique of forming fiducial marks, in advance, on a front side of a substrate with the multilayer reflective film deposited thereon or on a mark formation thin film formed on the substrate (e.g. see WO-A-2010-110237). The fiducial marks are transferred to the multilayer reflective film such that the position of such a defect in the multilayer reflective film can be identified based on the positions of transferred fiducial mark as reference positions.
Apart from the above, there has also been proposed a technique of identifying the position of a defect in a multilayer reflective film and repairing the defect in the multilayer reflective film (e.g. see WO-A-2008-129914). This publication recites that, when an absorber layer is deposited on a multilayer reflective film, fiducial marks are formed on the absorber layer but are not formed on the substrate nor the multilayer reflective film.
WO-A-2009-130956, WO-A-2012-121159 and WO-A-2013-031863 also disclose that fiducial marks or positioning marks are formed on an absorber layer. WO-A-2013-031863 also discloses that fiducial marks are formed on a reflective layer or a protective layer of the reflective layer, followed by forming an absorber layer on the reflective film or the protective layer with the marks formed thereon. WO-A-2012-121159 and WO-A-2013-031863 also disclose that temporary fiducial marks are formed on a substrate to identify the position of a defect on the substrate using temporary fiducial marks as reference positions.