A Student's Blog: My PhD topic - New material chemistry exploration for Extreme Ultraviolet (EUV) Lithography

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Wednesday, 23 May 2018

My PhD topic - New material chemistry exploration for Extreme Ultraviolet (EUV) Lithography

Hello folks, 

I am currently a doctoral researcher at Interuniversity Mictroelectronics Center (IMEC) located in Leuven, Belgium. My PhD project is part of a Marie-Curie funded European project with the acronym ELENA (Low energy ELectron driven chemistry for the advantage of emerging NAnofabrication methods). Here is the link to the ELENA website: ELENA-eu (currently under construction).

Under this consortium, a team of 15 Early Stage Researchers (ESRs) have been hired by different European research institutes and universities for their PhD, with the research focus on two important nano-fabrication techniques namely, FEBID and EUV Lithography.

I am ESR number 12 and my work will be on EUV Lithography. The topic of my research is "New material chemistry exploration for Extreme Ultraviolet Lithography". I have presented a synopsis of my project work below. 

Photolithography has been the major workhorse responsible for the advancement of the electronics industry over the past few years. It is a nanofabrication process in which light is passed through a patterned mask onto a substrate (Silicon wafer) coated with light-sensitive material called ‘photoresist’. Then using a solvent called ‘developer’, the exposed part of the photoresist is removed and the pattern (from mask) is replicated onto the substrate. The substrate is further processed to produce integrated circuits (IC).

Photolithography is controlled by Rayleigh’s formula. Currently, a deep ultraviolet (DUV) light source (wavelength λ=193 nm) is used to produce patterns at a maximum resolution of 40 nm. To further push the resolution down to 10 nm range, the wavelength of the source light needs to be reduced to as low as 13.5 nm. This is when the process becomes Extreme Ultraviolet (EUV) lithography.

EUV lithography, which is still in the research phase, is deemed to be the future of the semiconductor industry. However, as we reduce the wavelength to 13.5 nm, the energy of the photons becomes so high, that we move from excitation chemistry (in DUV lithography) to radiation chemistry (in EUV lithography). This changes the chemical interactions happening between the photons and the photoresist. The obscure change in the chemistry results in the underperformance of the currently available state-of-the-art photoresists.

The major problem associated with the current systems of EUV resist is something known as RLS tradeoff. R stands for the resolution, which is the smallest feature size that can be printed using that material. L stands for line-edge-roughness, which is the deviation of line-space feature from an ideal smooth shape. And S stands for Sensitivity, which is the minimum exposure dose required to reach the resolution. It is proving to be impossible to improve two of the parameters without exacerbating the third (hence, a trade-off). This RLS trade-off is caused due to chemical variability at nanoscale level. Only through the fundamental understanding of the chemistry of the process, it will be possible to produce robust photoresist systems that can work efficiently for EUV lithography. The objective of this research project is to bridge this understanding.

The approach of this project is a combination of two ways: 1) To enhance our knowledge of fundamental chemistry happening at the nanoscale level during EUV-patterning (through fundamental experiments such as solid- and gas-phase reactions) and 2) To use that understanding to design and characterize novel and robust EUV photoresists systems (through understanding synthesis of novel EUV systems, EUV patterning using ASML NXE scanner, CD-SEM analysis and RLS characterization).

Expected results from the project is to get a better understanding of the fundamental chemistry and correlating that to the critical parameters of novel EUV resist systems.

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