PhD - Leuven | About a week ago
This PhD research uses Extreme-Ultraviolet (EUV), Infrared, X-ray, and electron ultrafast spectroscopy techniques within the newly installed imec AttoLab to explore, for the first time anywhere, the femto- and attosecond range of the radiolysis process, of both polymeric and metal-containing resist materials, that occurs during EUV exposure, at 13.5 nanometers (nm). The acquired knowledge paves the way for mitigation of stochastic print defects while increasing throughput by reducing the effective exposure dose of next-generation, high-NA EUV projection lithography step and scan systems.
Semiconductor projection lithography is a nano-imaging technique where a radiation-sensitive material called a photoresist is exposed with a pattern that, once developed, provides an etch mask needed to transfer a circuit layout into a germane device layer that, when combined with other imaging steps on other layers, ultimately form a computer chip. The achievable resolution to meet the demands of producing advanced semiconductor devices with smaller features requires reduction of the exposure wavelength and increasing of the projection lens numerical aperture (NA) to capture the information contained within the diffraction pattern that is formed when the actinic exposure illuminates a mask holding the pattern to be replicated. To achieve features smaller than 10 nm with spacings less than 10 nm between them requires the use of 13.5 nm, called Extreme-Ultraviolet (EUV) and a projection lens NA of 0.55. Prior to EUV, deep UV (DUV, 193 nm), lithographers used immersion lens with an NA of 1.35. At 193 nm, the photochemistry by absorption induces nuclear motion [in electronically excited states] and bond scission that leads to the formation of photoproducts that then enables the delineation between exposed and unexposed areas that define the desired pattern after development and plasma etch. Unlike, DUV, EUV lithography exposure of resist imaging materials covers a complex reaction spectrum of first photon absorption, the release of a photo-electron, and multiple generations of thermalizing secondary electrons that span from 92 eV to 0.025 eV, where ionization and radiation chemistry first occurs at energies above 10 eV that is then followed by resonant low energy electron – molecular orbital interaction chemistry. Here, each part of the reaction spectrum has the potential to form permanent reaction products whose chemical nature may or may not be compatible with later lithographic processing steps. In organic polymer-based chemically amplified resists (CARs) most products formed are identical to that formed by DUV exposure at 193 or 248 nm (6.4 and 5.0 eV, respectively). The desired resultant chemical reaction product is therefore linked to low energy electron resonant interaction within the valence shell of the reactive components. But there is a minority of radiolysis products that form upon EUV exposure including active (but stable) free-radical cations and solvated electrons. Those radiolysis products may drive unwanted product formation that gives rise to sources of stochastic print defects like microbridges between resist features and microbreaks within resist features that lead to semiconductor product failures.
The radiolysis of the imaging material occurs faster than one picosecond thus it is instrumental to deploy ultrafast measurement techniques to observe, understand, and ultimately alter the kinetics in order to mitigate the occurrence of stochastic defects while at the same time increasing performance so that information transfer of the projected circuit image occurs at lower exposure doses, thus allowing for higher productivity of the EUV lithography tool. The same is also true for metal-containing resist where imaging results often show no enhancement in sensitivity even though they have significantly higher absorption cross-sections for EUV light. Further, their exposure dynamics have even been studied less than their CAR counterparts.
For the student, besides peering into the sub-picosecond domains of the EUV exposure process for the first time, this research provides access to learn the exciting and challenging area of semiconductor lithography and lithographic materials via the equally fascinating area of ultrafast high energy physics and its application to soft matter. With this PhD there will be opportunities to pursue careers in both academia and industry, such as ultrafast laser laboratories, semiconductor and photonic fabrication, and lasers and spectroscopy.
Required background: physical chemistry, physics, ultrafast time-resolved spectroscopy, laser physics
Type of work: 70% experimental, 20% simulation, 10% literature
Supervisor: Stefan De Gendt
Co-supervisor: John Petersen
Daily advisor: Fabian Holzmeier, Kevin Dorney
The reference code for this position is 2021-047. Mention this reference code on your application form.