Development of ballistic hot electron emitter and its applications to parallel processing: Active-matrix massive direct-write lithography in vacuum and thin films deposition in solutions

Making the best use of the characteristic features in nanocrystalline Si (nc-Si) ballistic hot electron source, the alternative lithographic technology is presented based on the two approaches: physical excitation in vacuum and chemical reduction in solutions. The nc-Si cold cathode is a kind of metal-insulator-semiconductor (MIS) diode, composed of a thin metal film, an nc-Si layer, an n⁺-Si substrate, and an ohmic back contact. Under a biased condition, energetic electrons are uniformly and directionally emitted through the thin surface electrodes. In vacuum, this emitter is available for active-matrix drive massive parallel lithography. Arrayed 100×100 emitters (each size: 10×10 μm², pitch: 100 μm) are fabricated on silicon substrate by conventional planar process, and then every emitter is bonded with integrated complementary metal-oxide-semiconductor (CMOS) driver using through-silicon-via (TSV) interconnect technology. Electron multi-beams emitted from selected devices are focused by a micro-electro-mechanical system (MEMS) condenser lens array and introduced into an accelerating system with a demagnification factor of 100. The electron accelerating voltage is 5 kV. The designed size of each beam landing on the target is 10×10 nm² in square. Here we discuss the fabrication process of the emitter array with TSV holes, implementation of integrated ctive-matrix driver circuit, the bonding of these components, the construction of electron optics, and the overall operation in the exposure system including the correction of possible aberrations. The experimental results of this mask-less parallel pattern transfer are shown in terms of simple 1:1 projection and parallel lithography under an active-matrix drive scheme. Another application is the use of this emitter as an active electrode supplying highly reducing electrons into solutions. A very small amount of metal-salt solutions is dripped onto the nc-Si emitter surface, and the emitter is driven without using any counter electrodes. After the emitter operation, thin metal (Cu, Ni, Co, and so on) and elemental semiconductors (Si and Ge) films are uniformly deposited on the emitting surface. Spectroscopic surface and compositional analyses indicate that there are no significant contaminations in deposited thin films. The implication is that ballistic hot electrons injected into solutions with appropriate kinetic energies induce preferential reduction of positive ions in solutions with no by-products followed by atom migration, nuclei formation, and the subsequent thin film growth. The availability of this technique for depositing thin SiGe films is also demonstrated by using a mixture solution. When patterned fine emission windows are formed on the emitter surface, metal and semiconductor wires array are directly deposited in parallel.