Low energy electrons in photoresists and underlayers for EUV lithography

The exponential increase in density and computational power of integrated circuits that we have been witnessing during the last five decades – also known as Moore’s law – is underpinned by the astonishing advancements of patterning technology of which optical lithography has been and still is the main enabler. Miniaturization (or scaling) of semiconductor devices brings not only higher density but lower energy consumption and faster operation at the same time. Pattern dimensions – which nowadays are in the few tens of nanometers – depend, among other parameters, on the light source used. Extreme ultra-violet (EUV) lithography, at a wavelength of 13.5 nm, is the leading-edge technology for the tightest critical dimension (CD): recently it has been introduced for high-volume manufacturing (HVM) in the semiconductor industry for the 7 nm technology node (N7). However, to enable the future technology nodes (N3, N2 and beyond), higher resolution, higher sensitivity and lower roughness photoresists are required. Photosensitive materials play a key role in the lithography process by switching solubility or condensing upon exposure to light, therefore expressing the topography of the mask onto the wafer.
 
Experimental evidence shows that the sensitivity and the resolution of some EUV photoresists depend on their interactions with underlayer, while others seem unaffected. More specifically, low energy electrons of energy below 10 eV are mostly responsible for the chemical changes in the material, producing volatile species, both in the resist and in the rest of the layers of the stack. Because of the limited thickness of these films, interaction effects at the interfaces begin to dominate, causing variation of dose (and productivity/throughput), and of resolution (electron blur). To understand these effects, we aim to describe the energy distribution of electrons in materials exposed to EUV light, which is a typical out-of-equilibrium condition with added complexity of interface effects in multilayer stacks. Electrons energy loss in particular could explain the efficiency and mean free path of electrons in photoresist and explain its ultimate resolution limit (dictated by the energy-dependent electron blur). This kind of understanding is lacking not only for the photosensitive layers but to the dielectrics and all layers (underlayers/stack) used in a real manufacturing process. All these effects play a very concrete role in the scaling of semiconductor manufacturing. In this project we will explore the patterning limits of the lithographic process (including dose, resolution, roughness) and ways to control or mitigate the detrimental effects of low energy electrons, so as to link the fundamental science to industrial applications.
 
In this PhD project the student will have access to a broad variety of experimental techniques including (1) EUV-X-ray/ultraviolet photoemission spectroscopy, (2) electron energy loss spectroscopy, and (3) patterning in the EUV scanner and measurement using electron microscope. Understanding the root causes of the variations of dose in EUV lithography with the support of external research centers and collaborations, the student will devise an experimental approach to study and predict the effect of underlayers and ambient on the performance of several types of photoresist chemistries.