Study of 3D pattern etch mechanisms into inorganic layers for optoelectronic applications
Optoelectronic devices such as CMOS Image Sensors (CIS) require the realization of 3D structures, convex microlenses, in order to focus photons towards the photodiodes defining the pixels. These optical elements are mandatory for the device efficiency. Their shape and dimension are critical for device performances. In the same way, devices based on diffractive optic and hyperspectral sensors are looking for complex multi-height structures. Finally, recent micro-display technologies for augmented reality (AR) and virtual reality (VR) require 3D structures difficult to achieve with conventional micro-fabrication technics.
Leti is at the state of the art on an alternative photolithography technics, so-called Grayscale. This process can produce a whole range of 3D structures not available with standard photolithography, such as concave, elliptic, pyramids and asymmetrical shapes. These structures could be used in a large number of application fields, like photonics and micro-displays (AR/VR). Once these structures achieved in photoresist, it is necessary to transfer them in an adapted functional layer using plasma etching. The etch mechanisms behind the transfer of micrometric 3D patterns into a polymer layer have been recently studied at Leti. To address new application needs, it is interesting to transfer these structures into silicon based inorganic layers because of their optical properties. Furthermore, the 3D pattern dimensions, currently few micrometers, need to be sub-micrometric for the most advanced technologies. In these condition, pattern transfer fidelity of 3D structures is even more challenging and it underlines why the etch mechanisms need to be well understood.
Currently the transfer into inorganic layers by plasma etching of submicronic 3D patterns obtained with Grayscale photolithography is not well studied in literature. Consequently, this thematic is innovative and has a real benefit. The goal of this PhD thesis is to study and understand the etch mechanisms in order to control the shape and dimension of the transferred structures. The work will be very experimental and will be mainly performed in Leti’s 300mm cleanroom. You will have access to a last generation plasma etch tool and numerous characterization technics. This thesis is in collaboration with the photolithography department and in interaction with different teams, such as the silicon platform and application department.
Development of algorithms and modeling tools of Low-Energy Critical Dimension Small Angle X-ray Scattering
This PhD will take place at the CEA–LETI, a major European actor in the semiconductor industry, and more precisely, at the Nanocharacterization platform of the CEA–LETI witch offer world-class analytical techniques and state-of-the-art instruments. Our team aims to accompany the industry in the development of new characterization tools and so to meet the metrological needs of future technological nodes. Over the past few years, pioneer developments on a new metrology technique based on hard x-ray scattering called CD-SAXS were done at the PFNC. This technique is used to reconstruct the in-plane and out-of-plane structure of nanostructured thin-films with a sub-nm resolution. In this project, we are looking to extend the CD-SAXS approach leveraging the recent breakthrough in the development of low-energy x-ray sources (A. Lhuillier et al. 1988, Nobel prize 2023) called High Harmonics Generation (HHG) sources. Therefore, you will participate in the development of a new and promising characterization methods called Low-energy critical dimension small angle x-ray scattering. The very first proof of concept of this new measurement was conducted in November 2023.
Mission:
In order to include in the data reduction the measurement specificities of this new approach (multi-wavelength, low energy, …) your mission will focus on several aspects to explore in parallel:
- Develop new modeling tools to analyze the data:
o Finite element simulations with Maxwell solver
o Analytical Fourier Transform (similar to standard CD-SAXS) vs dynamical theory
o Comparison between the two approaches
- Build new models dedicated to lithography problematic (CD, overlay, roughness)
- Define the limitations of the technique through the simulation (in term of resolution (nm), uncertainty)
This work will support the development of CD-SAXS measurements with a laboratory HHG (High Harmonic Generation) source lead by a Postdoctoral fellow.
Thermomechanical study of heterostructures according to bonding conditions
For many industrial applications, the assembly of several structures is one of the key stages in the manufacturing process. However, these steps are generally difficult to carry out, as they lead to significant increases in warpage. Controlling stresses and strains generated by heterostructures is however imperative. We proposes to address this subject using both experimental exploration and simulation through thermomechanical studies in order to predict and anticipate problems due to high deformations.
Optimisation of advanced mask design for sub-micrometer 3D lithography
With the advancement of opto-electronic technology, 3D patterns with sub micrometer dimensions are more and more integrated in the device, especially on imaging and AR/VR systems. To fabricate such 3D structures using standard lithography technique requires numerous process steps: multiple lithography and pattern transfer, which is time and resource consuming.
With optical grayscale lithography, such 3D structures can be fabricated in single lithography step, therefore reducing significantly the number of process steps required in standard lithography. For high volume manufacturing of such 3D patterns, optical grayscale lithography with Deep-UV (DUV), 248nm and 193nm are the most relevant, as it is compatible with industrial production line. This technique of 3D lithography is however more complex than it seems, which requires advance lithography model and data-preparation flow to design optical mask corresponding to the desired 3D pattern.
Low temperature selective epitaxial growth of SiGe(:B) for pMOS FD-SOI transistors
As silicon technologies for microelectronics continue to evolve, processes involved in device manufacturing need to be optimized. More specifically, epitaxy, a crystal growth technique, is being used to fabricate 10 nm technological node FD-SOI (Fully Depleted-Silicon On Insulator) transistors as part of CEA-Leti's NextGen project. Doped and undoped Si and SiGe semiconductor epitaxy is being developed to improve the devices' electrical performances. The thesis will focus on selective SiGe(:B) epitaxy for channels and source/drains of pMOS transistors. A comparison of SiGe and SiGe:B growth kinetics will be made between growth under H2, the commonly used carrier gas, and N2. Innovative cyclic deposition/etching (CDE) strategies will also be evaluated, with the aim of lowering process temperatures.
Advanced Surface Analysis of Ferroelectrics for memory applications
CEA-Leti has a robust track record in memory technology. This PhD project aims to contribute to the development of HfO2-based ferroelectric devices. One of the major challenges in this field is to stabilize the orthorhombic phase while reducing film thickness and thermal budget. To gain a deeper understanding of the underlying mechanisms, a novel sample preparation method will be adapted from a previous PhD project and further developed for application to ferroelectric memories. This method involves creating a beveled crater that exposes the entire thickness of the film, allowing for access by multiple characterization techniques (XPS, TOF-SIMS, SPM) on the same area. This approach will enable the correlation of compositional and chemical measurements with electrical properties. Furthermore, heating and biasing within advanced surface characterization instruments (TOF-SIMS, XPS) will provide insights into how device performance is influenced by compositional and chemical changes.
You possess strong experimental skills and a keen interest in state-of-the-art surface analysis instruments. You excel in team environments and will have the opportunity to collaborate with experts across a wide range of techniques on the nanocharacterization platform, including advanced numerical data treatment. Proficiency in Python or similar programming languages is highly desirable.
ALD materials for FE and AFE capacitances
Ultrathin HfO2-based materials are regarded as promising candidates for embedded non-volatile memory (eNVM) and logic devices. The CEA-LETI has a leadership position in the field of BEOL-FeRAM memories ultra-low consumption (<100fj/bit) at low voltage (<1V). In this context, the developments expected in this thesis aim to evaluate the impact of HfO2-based ferroelectric FE and antiferroelectric AFE layers (10 to 4 nm fabricated by Atomic Layer Deposition ALD) on the FeRAM properties and performances.
In particular, the subject will permit a deep understanding of the crystallographic phases governing the FE/AFE properties using advanced measurements techniques offered by the CEA-LETI nano-characterization platform (physico-chemical, structural and microscopy analysis, electrical measurements). Several integration solutions for ferroelectric capacitances FeCAPs using ALD FE/AFE layers will be studied including doping, interface layers, sequential fabrication w/wo air break…
Thus, the developments based on FeCAPs stack fabricated using 300mm ALD deposition tool aspires to explore the following items:
1-Doping incorporation in FE/AFE layers (La, Y…)
2-Engineering of the interface between FE/AFE layers and top/bottom electrode
3-Plasma in-situ treatment of bottom electrode surface
4-Sequential deposition with and without air break
[1] S. Martin et al. – IEDM 2024
[2] Appl. Phys. Lett. 124, 243508 (2024)
SCO&FE ALD materials for FeFET transistors
Ferroelectric Field Effect Transistors FeFET is a valuable high-density memory component suitable for 3D DRAM. FeFET concept combines oxide semiconductors SCO as canal material and ferroelectric metal oxides FE as transistor gate [2, 3]. Atomic layer deposition ALD of SCO and FE materials at ultrathin thickness level (<10 nm) and low temperature (10 cm2.Vs); ultrathin (<5nm) and ultra-conformal (aspect ratio 1:10). The PhD student will beneficiate from the rich technical environment of the 300/200mm CEA-LETI clean-room and the nano-characterization platform (physico-chemical, structural and microscopy analysis, electrical measurements).
The developments will focus on the following items:
1-Comparison of SCO layers (IGZO Indium Gallium Zinc Oxide) fabricated using ALD and PVD techniques: implementation of adapted mesurements techniques and test vehicles
2-Intrinsec and electrical characterization of ALD-SCO (IWO, IGZO, InO) and ALD-EF (HZO) layers: stoichiometry, structure, resistivity, mobility….
3-Co-integration of ALD-SCO and ALD-FE layers for vertical and horizontal 3D FeFET structures
[1]10.35848/1347-4065/ac3d0e
[2]https://doi.org/10.1109/TED.2023.3242633
[3]https://doi.org/10.1021/acs.chemmater.3c02223
Direct metal etch mechanisms study for the BEOL of ultimate SOI nodes
The topic fits into the deployment of silicon technologies at the European level (European chips act), led by CEA-Leti. The focus will be on providing advanced technological building blocks for electrical routing (Back End of Line) of logic and analog devices. The development of increasingly high-performance circuits requires interconnections with more aggressive dimensions. The use of traditional routing materials such as copper is therefore being questioned, as is the conventional back-end of line (BEOL) architecture. This thesis topic will address a breakthrough approach, necessary to achieve these ultimate dimensions.
The objective of this PhD is to develop a BEOL technological building block for the advanced SOI (Silicon on Insulator) nodes through a direct metal etching approach. After a preliminary simulation of the electrical properties of interconnections made with different metals, the work will consist in proposing and implementing an innovative integration. In the first phase, the task will be to determine the design of the electrical test structures and establish an integration scheme. In the second phase, the research work will focus on studying the direct etching of the selected metal using sustainable processes while maintaining the performance of both the processes and the final device. The candidate will be able to rely on the eco-innovation team to perform a comparative life cycle analysis (LCA) of this building block.
The PhD contract is for a duration of 3 years and the research work will take place in the clean rooms of CEA-Leti. To successfully carry out this study, the candidate will have access to state-of-the-art equipment and a cutting-edge work environment.
Study of grayscale photoresists and lithography process optimizations for submicron optical applications
Grayscale lithography process has been used for several years to obtain complex tridimensional structures on semiconductors substrates. This process is particularly adapted for optical and opto-electronics applications.
CEA-LETI has developed a strong expertise on I-line (365nm) grayscale lithography, and is now willing to expand its capabilities and explore grayscale process with DUV (248nm and 193nm) lithography. The objective is to be able to obtain complex 3D structures with critical dimensions less than 1µm.
This PhD work will focus on acquiring a better understanding of the chemical and physical phenomena involved in grayscale photoresists, allowing the optimization of lithography processes. This work will also help with the development of etching processes and new optical models for mask designs.
You will join the lithography team of CEA-LETI, and you will exchange as well with other teams working on this topic (etching, optical simulation). You will have access to a wide range of state of the art equipments installed in LETIs cleanrooms, as well as a world class nanocharacterization platform (PFNC).