Nano Lithography

Chapter 1

X-ray Lithography: Fundamentals and Applications ¹

1.1. Introduction

The invention of X-ray proximity lithography [SPE 72] dates back to the early 1970s, when the declared objective was overcoming the resolution of the lithographic techniques then employed in the semiconductor industry. At that time, UV projection lithography was the leading technology, having reached a scale resolution of one micrometer [WIL 29]. Nevertheless, a very problematic future was forecast for UV-lithography, given the expected requirements of the microelectronics industry to achieve sizes as small as 250 nm. In fact, the physical barrier represented by diffraction was believed to be insuperable. Therefore, the quite obvious idea of using radiation of shorter wavelengths for exposures was seen as the only viable option to keep pace with Moore's law [MOO 65] and the semiconductor industry roadmaps [ITRS] for device miniaturization.

However, shifting to shorter wavelengths, down to the region of Extreme UV (EUV) which extends between approximately 30 and 250 eV, raises new problems. One fundamental problem is represented by the low transparency of most materials. At these photon energies, the radiation is so strongly absorbed by any dense material that it is difficult or even impossible to find suitable materials to be used as transparent substrates for photomasks and for the sophisticated demagnification optics of the projection system.

One viable solution to the low transparency problem considered ...