CE09 - Nanomatériaux et nanotechnologies pour les produits du futur

Planar Assembly of NAnowires for SidewallS Emission – PANASSE

PANASSÉ

Planar Assembly of NAnowires for SidewallS Emission

Project objectives

The main goal of the PANASSÉ project is to develop cold emission cathode technology based on an innovative planar nanowire structure, the production of which is controlled by a buried electrode in the carrier substrate.<br />For this goal, we have to develop several bricks, namely the production of nanowires networks on large surfaces (active surface of the order of 1mm², total surface on wafer of the order of several cm²). We also need to develop a dedicated semi-encapsulation process for the production of these components. The material used for this purpose is a high permittivity dielectric (high-k) which will be developed and selected from among the materials developed by the project partners. A critical point is the local electrostatic control of the component and to assess this point we will base ourselves on near field charge microscopy techniques under ultra-vacuum conditions.

We have chosen to test two paths in parallel for the realization of nanowire networks: a top-down approach and a bottom-up approach. The top-down approach is based on the realization of nanowires networks (or unit nanowires) on an SOI type substrate with a layer of polysilicon as active layer where the nanowires are etched after electronic lithography. This achievement was subcontracted to a silicon platform (CEA-LETI) on TRT's own funds to overcome the recruitment problems on the IEMN side, which was to take over this task. On the bottom-up development side of cathodes, we are basing ourselves on the experience of LMGP to produce non-oriented deposits of silver nanowires on which we must produce a contact technology. Regardless of the approach, we then develop the semi-encapsulation methodology using ALD materials developed at TRT on a commercial frame or s-ALD materials developed at LMGP on a tool developed in-house. First, we test the unit components on a near field characterization frame (IEMN) or on a UHV-HV frame (TRT) and we compare the results with the electrostatic (TRT) and quantum (IEMN) models developed in the context of of the project.

Today we have received the basic devices outsourced with critical dimensions of extended nanowires down to 30nm. We will develop a size reduction step to 20nm or even 15nm if necessary in the rest of the project, starting from these basic wafers (today 2000 chips available). Note that we now have access to IEMN manufacturing technology thanks to the recruitment at the end of 2020 of a post-doc which will allow us to test other design variants if necessary. We also have results in the realization of metallic nanowires obtained by lift-off with critical dimensions down to 20nm.
At the same time, we succeeded in producing a technology for making contacts on silver nanowires after having come up against this point at the start of the project. This point being resolved, we were able to develop semi-encapsulation from ALD and s-ALD materials with convincing technological results.
Electrostatic and quantum models are now available and can be compared to experimental results to understand the results obtained.
Near-field microscopy tests performed on non-semi-encapsulated nanowires were carried out, demonstrating local charge effects with low temporal dynamics as we expected.
The UHV-HV test frame is now fully functional and has already been used to test components (not semi-encapsulated) in real conditions with success.

The assembly of the complete basic bricks is underway to test the components in near-field microscopy in order to compare the effects of charges (localization, dynamics) with and without semi-encapsulation. This will be done with the different structures (polysilicon nanowires and silver nanowires) with the different ALD material (s) validated during the preliminary tests. These tests on unit components will allow us to target the most relevant material assemblies and to understand their effects on load dynamics in order to possibly adjust the optimal operating mode for the field emission (continuous, pulsed mode with adapted duty cycle ).
At the same time, ALD material (s) continue to be developed to provide a more comprehensive library that can be used to resolve any charge flow problems encountered.
The deposition of silver nanowires will also be optimized to reduce the effects of topology (overlapping nanowires and inducing local problems of ALD deposition (s)). This innovative method will be developed and compared to other sintering methods involving higher thermal budgets.

10.1016/j.cej.2020.126234
10.1039/d0na00895h
10.1039/c9nr05658k

In PANASSE project, we aim at developing planar networks of nanowires as vacuum electron sources. By adopting a planar integration scheme, our approach will allow numerous technological bolts to be unlocked in order to produce efficient sources. Indeed, compared to classical tip emitter based cold cathode technology, planar nanowires based cold cathode would offer enhanced thermal management, variability control, efficiency, cost reduction, large surface production, etc.
We already demonstrated proof-of-concept about such planar emission structures with main features being efficient backgate control of current emission and need for high permittivity material partial passivation of nanowires. In PANASSE project, we will focus on single nanowire device fabrication in order to define best routes for size reduction and control as well as passivation optimization. In parallel, this structure will allow for testing quantum confinement effect related with nanowire dimensions. Planar configuration will give access to a new panel of near field characterization, otherwise impossible considering tip emitter with out-of-plane configuration. Field emission characterization on single elements will be performed in parallel. Results combination on these two methods will be valuable for making decision about key features to be taken into account for rest of the project. After this first step, arrays of nanowires will be processed to form collective emission cathode components that will allow for ameliorating process routes on large emitter numbers. Device performances will be correlated with process in order to assess technological options. Progressive upscaling and cathodes emission performance will be continuously pursued and compared to single emitters performances in order to maximize chances to reach high current delivering cathodes. Post emission characterizations will be performed at different scale levels (from single emitters to full emission zone) with conclusions being highly valuable inputs for whole processes control and improvement. After this step reached,partners efforts will be dedicated to produce cathodes on large surface (4 inches) wafers while maintaining high performance on individual processed cathodes, paving the way to large production scheme.
PANASSE work focuses on production of innovative field emission structure for future compact X-ray imaging systems, and in particular computed tomography equipment for 3D reconstruction. Various applications in industry, security and medicine could emerge from this concept with no alternative based on nowadays technologies. But if field emission is set as a user case for defining PANASSE framework, related results and developments can be basis for numerous other applications because we treat about generic topics of large area processing for planar nanoelements assemblies and high-quality thin dielectric films. These outputs will be valorized by each partners and the presence on a worldwide leader company in critical systems (electronics, optics, radiofrequency, etc.) such as Thales offers effective opportunities for technological transfer in industry with large impact on society.

Project coordination

Laurent GANGLOFF (Thales Research & Technology)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

TRT Thales Research & Technology
IEMN Institut d'électronique, de microélectronique et de nanotechnologie
LMGP Laboratoire des Matériaux et du Génie Physique

Help of the ANR 492,110 euros
Beginning and duration of the scientific project: December 2018 - 36 Months

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