Fabrication of nano-scale metal-insulator-metal cathode: control of film structure and thickness

Nano-scale metal-insulator-metal (MIM) structures have potential application in vacuum electron sources. When voltage is applied across the two metals of an MIM structure, electrons from the metal at one side (M1) tunnel through a thin insulator and reach the other metal (M2). If the potential of the electrons coming through is higher than the Fermi level of M2, electrons can penetrate M2 without energy loss and be emitted to a vacuum. As a wide gap insulating material, alumina has been widely utilized in microelectronics. Fluctuation-free electron emission was obtained from an MIM cathode based on anodic alumina. However, the energy loss in such an MIM structure was found to be large, electron emissivity was very low, and heat generated by the energy loss process damaged the structure. It is known that 5A of well-ordered alumina thin film can be prepared by oxidation of an NiAl(110) surface. The insulating property of the alumina layer is a key factor which determines the performance of the MIM structure. Nano-scale MIM structure based on well-ordered alumina might show better electron transport property than amorphous alumina. In an MIM structure, uniformity of each layer is also essential for electron emission. Such MIM devices with uniform metal and oxide over-layer thickness are expected to provide better performance than non-uniform devices. Thus, it is of great importance to control of the film structure and thickness of alumina layer and M2 layer. In this study, oxidation of NiAl(110) surface was chosen to fabricating the well-ordered ultra-thin alumina layer, and palladium was chosen as the top electrode. Here, we report our recent progress on the control of the film structure and thickness during the MIM structure fabrication.