5.1.2 Step Edge Cut Off

  A simpler method for metal tunnel junctions is the step edge cut-off process shown in Fig. 5.2. The particular process shown was proposed by W. Langheinrich and H. Ahmed [74]. A similar technique is from S. Altmeyer et al. [1].

  

Figure 5.2: Step edge cut-off method. A metal line is evaporated over a step edge.

A step or groove is formed, over which a metal line is deposited. The capacitance of this tunnel junctions are small compared to the junctions produced by the shadow mask technique, since there is no overlap of the electrodes at all. This process is especially suitable for material systems with lower barrier heights than Al/Al₂O₃(about 2 eV), such as Pb/Cr₂O₃ (0.02 eV) and Cr/Cr₂O₃ (0.06 eV). The lower barrier height allows a larger distance, about 10 nm compared to 1 nm, between the electrodes by equal barrier strength. This results in a further reduced tunnel junction capacitance and larger process tolerances.