Developing an array of field emitters as a stationary digital tomosynthesis source

Abstract

In a world with a dynamic, growing, and ageing population, a new generation of medical imaging devices is needed to meet the demands of early preventive diagnosis, sustainability, and mobility. Static Digital Tomosynthesis offers low-dose 3D imaging with small tumour detection, low power consumption, minimal maintenance, and a compact, mobile form. The core enabling technology is field emission, where a strong electric field (F > 1 GV/m) at a high aspect ratio tip induces quantum electron emission (see Chapter 1).

For medical imaging, the emitter must deliver large currents (I > 100 μA), but in this regime, heating effects (Joule and Nottingham) can cause emitter failure via vacuum breakdown. This thesis focuses on identifying a material that withstands high current, temperature, and electric fields, while remaining compatible with cleanroom mass production for use by an industrial partner.

In the effort to develop a high-power field emitter GETELEC (General Tool for Electron Emission Calculations) has been expanded to include the physics of field emission from semiconductors (see Chapter 2). To provide the inputs to GETELEC-2.0, a finite element method Multiphysics model has been developed. This model solves self consistently all the required differential equations, taking GETELEC-2.0 outputs as boundary conditions, to compute the electric field, the temperature, the band structure, and the emitted current. To the knowledge of the author this is the first full self-consistent 3D model for thermal-field emission from semiconductors (see Chapter 2). With this new model, it is predicted that 4H-SiC should be able to withstand from ×4 to ×10 more current than Si (reference material). In order to validate the theoretical predictions, three types of emitters have been fabricated both in Si and 4H-SiC, keeping the same geometry but changing the material - emitter twins – (see Chapter 3). The experimental data obtained from each emitter type shows that 4H-SiC emitters yield currents between ×3.5 and ×9.5 higher than those from their Si twins (see Chapter 4).

The theoretical and experimental evidence gathered over the course of this PhD research supports the idea that 4H-SiC is a material that can be used to meet the current needs that medical imaging devices require. Improvements on the emitter cleaning protocol (e.g., thermal treatment or laser treatment) and taking the emitter to ultra-high vacuum conditions (P<10-9 Torr) is likely to improve the performance the emitters; and yet these have yield up to ×12 more current (in DC mode) than any other SiC reported emitters to the knowledge of the author (caution when comparing field emission experiments is to be reminded). These results are sufficient justification for the industrial partner of this PhD research to start the development of 4H-SiC field emitters for their high current devices.