1. Field of the Invention
The invention generally relates to reflective masks, and more particularly to a light scattering and radiation reflective EUVL mask.
2. Description of the Related Art
The optical lithographic technique that is used to image wafers throughout the semiconductor industry relies on transparent masks to transfer an image from the mask to the wafer. As wafer images shrink, new ways of imaging the wafer resist are needed. One likely candidate for the Next Generation Lithography uses Extreme Ultraviolet (EUV) light to image. At the 13.4 nm EUV wavelength, materials are too absorptive to build a transmissive mask, so reflective ones are used instead. Conventional Extreme Ultraviolet Lithography (EUVL) masks, such as the mask illustrated in FIG. 1, are built by depositing a reflective film onto an ultra low expansion (ULE) substrate 10. Material properties of ULE substrates are well known in the art. This film can be composed of many different materials. The most commonly deployed reflective Bragg mirror for EUVL mask applications is created with multiple (as many as forty or more) alternating bilayers of molybdenum (Mo) and silicon (Si), finishing with a protective Si cap shown collectively as a Mo/Si multilayer 20. A buffer layer 30 and absorber layer 40 are then deposited on the multilayer stack 20. Additional layers can be deposited anywhere within the capping/buffer/absorber stack for different purposes, such as to provide an etch stop or conductive inspection/repair layer. The mask pattern is written onto a resist layer using standard mask patterning processes. A dry etch transfers the pattern through the absorber layer. Inspection and repair are performed to ensure that the absorber pattern matches the design data and then the final pattern is transferred through the buffer layer to expose the reflective multilayer surface.
There are many material challenges inherent in building and using an EUVL mask. One fundamental mask issue is the selection of absorber and buffer materials that combine ideal chemical durability, adhesion, dry etch characteristics and optical. Moreover, maintaining the quality (and hence reflectivity) of the capping layer's reflective surface during mask processing is difficult.
Generally, conventional optical masks include transmissive regions that permit light to pass onto the wafer and absorptive regions that block the light. However, the masks used in the EUVL system, introduce a new set of challenges. Because an EUVL mask is reflective, the EUV radiation must be exposed to the mask surface at an angle such that the pattern will reflect onto the surface of the wafer. Specifically, light incident on the exposed reflective surface is reflected. Light incident on the patterned absorber film is absorbed, not reflected; an essential component to imaging. A by-product of this absorption is that the radiation heats the mask and must be controlled to avoid pattern distortion and also to limit heat-induced wear that would decrease mask lifetime. Experiments have shown that 5 degrees is the optimal angle of exposure.
The absorber stack height is finite and creates a shadow under the angle of illumination which blurs the edge of the raised absorber when imaged. This reduction in contrast is a function of the angle of the incident exposure light and both the absorber and buffer layer thickness. Reduced contrast at the pattern edges is a significant issue since it can result in shifted or mis-sized images on the wafer.
The industry has sought to overcome these identified challenges, yet a solution has not been adequately defined. Therefore, due to the limitations of the conventional devices and processes, there is a need for a novel EUVL mask which overcomes the problems associated with the standard techniques.