Fabrication of suspended fully metallic ultra‐small capacitance nano‐junctions

Abstract

The concept of using carrier transport across junctions of different or similar materials lies at the heart of semiconductor devices. Immediately one thinks of Schottky and Shockley who first understood and applied the resulting non-linear I-V characteristics. Schottky diodes formed from metal-semiconductor junctions are probably the most fundamental diodes which are typically applied as rectifiers [1]. As the basic element of so-called rectennas -i.e. arrays of Schottky diodes -they are used for DC power generation via broadband rectification of electromagnetic radiation [2].The reason for using Schottky diodes is their ease of integration into antenna structures and more importantly their high cutoff frequency. The figure of merit for this is the response time typically given as τ = R j C j with the junction impedance R j and the junction capacitance C j . Clearly, a reduction of the total junction capacitance has the potential to maximize the bandwidth of an antenna or on the same token a rectenna. While the antenna impedance commonly is matched to the free-space impedance of 377 Ω, the junction impedance is often in the range of several 10 kΩ with the junction capacitance minimized by making use of nano-technology.The structure we report on here represents the realization of a lateral diode with ultra-small metal junction capacitance. The diode design itself is oriented on the bynow classical metal-insulator-metal (MIM) contacts or to be more precise on MIMIM junctions [1,3]. In short we show how to fabricate a nano-MIMIM diode, which is freely suspended between emitter and collector (see Fig. 1(a)). The metallic island in the center forms a floating base contact with the insulator's symmetry and thickness determining the details of the I-V characteristics, as will be discussed below.Key to suspend the island is the deposition of a thin teflon layer, which holds the island in place and serves as an insulator between emitter, base, and collector (see top view in Fig. 1(b)). With dimensions of only 80 nm × 80 nm × 60 nm (length × width × height) the total capacitance can be approximated to be C j ≈ 10 aF. This yields a value of τ ≈ 10 kΩ × 10 aF = 100 THz [3], which can be regarded as the upper operating frequency of suspended MIMIM diodes.Fabrication of the nano-MIMIMs starts with patterning a Silicon-On-Insulator (SOI) substrate by using optical lithography. The SOI only serves as a template for the actual device with the buried oxide layer providing a sufficient device-to-substrate insulation for the leads. We used slightly n-doped SOI wafers with a resistivity at 300 K of 5-20 Ω cm (Shin-Etsu Chemical Co.). The top layer has a 500 nm thick silicon layer with a 1 µm thick insulating layer underneath. The device design and lithography are very straightforward: the optical lithography step defines lead electrodes to which bond wires are connected. TheWe demonstrate the fabrication of freely suspended metallic islands using an SF 6 etching and deposition technique. The resulting structure consists of nanoscale ...