Stable Field Emitters Using Inverse Opal Structures

For the first time since they have been used in photonics applications, Inverse Opal structures are being applied to field emission applications.  JPL-pioneered Carbon Nanotubes (CNTs) bundle arrays are ideal for field emission devices due to their sharp tips, however their stochastic microstructure leads to deformation at low applied mechanical/electrical loads and frequent failure when used as vacuum microelectronic devices. Inverse opal structures offer the salient geometric and field emission features of CNTs, but with greater control of array height and uniformity. Our objective is to create inverse opal field emission devices with enhanced reliability and robustness compared to CNT devices, while preserving desirable performance properties and reducing packaging challenges for sustaining harsh Venus/Jovian-like environments.

There is a need for alternate cold cathode materials that are as efficient, robust, and poor-vacuum tolerant as CNTs, but with improved rigidity to enable high-reliability, close-gap (<5μm) electrode integration for low voltage operation. Inverse opal structures offer many of the desired geometric/electronic features for field emission applications, such as sharp tips (~10 nm) and high aspect ratios (25); they also offer more uniform array height, similar to Spindt-type cathodes [Spindt, C., et.al., J. Appl. Phys. (1976)], and are decidedly more robust than CNTs. Inverse opals can be fabricated using any conductive material, which allows for optimization of both field emission and structural properties. While significant advances are being made in solid-state devices involving SiC, SiGe, and SoI (silicon on insulator), which enjoy higher maturity and integration levels, challenges concerning failing Ohmic contacts, and the existence of finite dark current make them unsuitable for certain niche field emission applications. Vacuum microelectronics with field emission cathodes, when developed using an innovative planar vacuum packaging technology, can become the desired technology to fill this need. This development will lead to structurally robust and reliable miniaturized field emitting devices capable of sustaining harsh Venus or Jovian-like environments. This is the first time inverse opal structures are being applied for field emission devices/sensors, and can lead to a new class of devices for NASA-JPL. 

The objective is to create inverse opal field emission vacuum electronics devices with enhanced reliability and structural robustness compared to CNT devices while preserving desirable properties, such as low threshold operation and robustness to poor vacuums. Inverse opals can be used to create an ordered 2D array of conducting sharp tips suitable for high temperature applications. These can provide emission at low threshold fields while also offering a rigid structure that does not deform under mechanical/electrical loads. The variation in inverse opal tip height is expected to be 2-3% (~100s of nm). The high reliability in tip height means the gap between components can be <5 μm, resulting in low threshold operation and lower likelihood of device shorting/failure. This work aims to produce triode assemblies using semi-monolithic fabrication processes to achieve: 1) operating voltages of <10 V on gate electrodes and <50 V on anodes, 2) transconductance of >1 μSiemens, and 3) gains of ~10 at the end of year 2. Additionally, the triode assembly will be tested in a vacuum chamber up to 300 C (or higher depending on equipment availability).