Exposure dose control in Extreme Ultraviolet Lithography (EUVL)

Topic description: This PhD project aims to understand and predict the dose variability in EUV lithography, using a broad approach: from fundamental interaction of photons with multi-layer stacks, to the patterning of actual industrially relevant use-cases using imec's EUV tool. 

Abstract: 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. Photoresists play a key role in the lithography process by switching solubility upon exposure and therefore expressing the topological information of the mask onto the wafer.

While the solubility switch was well understood in past ultraviolet-based lithography, EUV light is substantially different from the latter: in this case, for the first time, ionizing radiation is used to expose the photoresist, which leads to a variety of side effects such as photoemission, charging, outgassing. Moreover, the requirements on photoresist materials for high-numerical aperture (0.55, High NA) EUVL in terms of resolution, etch resistance, film thickness, and roughness obviously place a heavy burden on the shoulder of material suppliers. One of such requirements is the smaller depth of focus that will force lithographers to use even thinner photoresist and stack. As we approach single-digit nanometer thickness of layers, more complex phenomena take place. The most problematic of these phenomena is that the apparent exposure dose required to pattern the a given CD with EUV seems to depend not just on the photosensitive layer, but on the rest of the stack beneath as well. In addition, ultrathin films are more susceptible to environmental contaminations and modifications, before, during and after process: to mention a few, formation of adventitious carbon layers, absorption of water vapor from ambient, temperature variations, outgassing, agglomeration and so forth. All these fundamental effects have a very practical impact on the manufacturability of semiconductor devices because they introduce systematic or stochastic fluctuations in CD vs. dose. At the moment, it is believed that the generation and diffusion of low energy electrons from the substrate are responsible for these fluctuations, but a comprehensive framework is still missing.

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.

Main Duties and Responsibilities: The PhD candidate will · Acquire a broad knowledge of lithography techniques for advanced patterning, ranging from optical to e-beam, spectroscopic and X-ray scattering metrology · Acquire a wide knowledge on EUVL, EUV resists and their characterization/integration relevant to the work program outlined above · Be able to devise a workplan with timeline, and identify the key problems to tackle and the methodologies to do so · Collaborate closely with experts in the field of lithography, physics, chemistry and materials science available at imec and at collaborating organization · Learn experimental techniques of photoemission spectroscopy in EUV and soft X-rays both at imec and at the synchrotron during extended assignments · Disseminate the results of such activities to internal colleagues/partners and capture relevant intellectual property via patent applications and/or publish results in high impact journals and conferences · Bring an energetic and enterprising approach to the execution of the research program · Potentiate inter-personal skills suitable for playing a role at the center of a complex multidisciplinary teams · Be alert to unexpected opportunities arising during the research · Assist with the training of graduated or undergraduate students working in the area of the project · Develop written and oral communication skills.

Type of work: The candidate must be willing to relocate abroad for extended periods of time to carry our experiments at the beamline, while at the same time developing a fundamental understanding light/electron-matter interaction and proposing new mechanistic models for low energy electron induced chemistry.