5.1 Fabrication Techniques

  Charge quantization effects - single-electron tunneling - was first observed in thin granular metal films [45] [73] [90] [110]. Since the size and position of individual grains could not be controlled the effects were observed in an averaged way, which denied a more thorough and detailed study. However, R. Shekhter [99] and I. Kulik and R. Shekhter [70] devised a comprehensive theory for these systems. It took several years until single ultra small tunnel junctions could be manufactured reproducibly. The first process for metal tunnel junctions was developed by T. Fulton and G. Dolan [35] and is referred to as `shadow mask evaporation'. Today with this technique researchers routinely produce Al-Al $_{\text{2}}$O $_{\text{3}}$-Al tunnel junctions with 30 nm $\times$ 30 nm. In semiconductor structures a laterally patterned two-dimensional electron gas was used to form quantum dots. The size of the dots and tunnel junctions were still too big to observe single-electron phenomena at room temperature. The junctions had to be studied at cryogenic temperatures. Consequently a search for production techniques for even smaller tunnel junctions started, which brought about various techniques with Scanning-Tunneling-Microscopes and Atomic-Force-Microscopes [64] [83] [104]. These experiments showed that single-electron effects are present at room temperature if the structure is sufficiently small. However, these laboratory procedures are not suitable for industrial mass production. Today the trend goes back to granular films, because their nano-meter size grains with self-assembling properties provide the small feature sizes required for room temperature operation without the need for atomic precision lithography for the definition of individual grains. Granular films have been produced and used for SET devices in metals [19] [20] and semiconductors [108] [109]. Another promising approach is to fabricate polymer coated metal clusters [6] [55] [93] which are assembled to planar grain films. Electron-beam lithography, ion-beam lithography and dry etching are preferred patterning techniques for the larger device structures. Following the fabrication methods, possible applications are given. We focus on SET memories, since we believe that they are the most promising and most likely applications to appear in the near future.

From a physical point of few single-electron devices work fine and are understood well. Their characteristics are promising and their production would mean a huge step in the miniaturization of electronic devices. However, whether they will have an economical impact depends on the successful industrial mass production. In the following sections some of the many proposed and used production techniques are described.