With miniaturization (scaling law: Moore's law) of semiconductor devices (particularly, silicon devices), device integration and power reduction have advanced at a rate of four times in three years. In recent years, gate lengths of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) have been decreased to 20 nm or less. In addition, because of rising costs of the lithography process (costs for devices and mask sets) and of physical limits (operation limit/variation limit) of the device size, improvement in device performance demands an approach different from conventional approaches based on the scaling law.
In recent years, a rewritable programmable logic device referred to as FPGA has been developed as a device positioned between a gate array and a standard cell. With the FPGA, customers can design an arbitrary circuit configuration after the chip is manufactured. The FPGA includes a variable resistance element in a multilevel wiring layer structure, and customers can make electrical connection of wirings arbitrarily. By using a semiconductor device including such FPGA, circuit freedom can be improved. Examples of the variable resistance element include a ReRAM (Resistance Random Access Memory) using a transition metal oxide and NanoBridge (registered trademark of NEC Corporation) using an ion conductor.
As a variable resistance element that is likely to satisfy relevant requirements, Non-Patent Document 1 discloses a switching element using metal ion movement and electrochemical reaction in an ion conductor (solid substance in which ions can move freely when an electric field is applied). This switching element disclosed in Non-Patent Document 1 includes three layers of an ion conducting layer and first and second electrodes that are in contact with the ion conducting layer and are arranged on opposing surfaces. The first electrode serves to supply metal ions to the ion conducting layer. Metal ions are not supplied from the second electrode.
An operation of this switching element will be described briefly. When the first electrode is connected to ground and a negative voltage is applied to the second electrode, metal of the first electrode is dissolved in the ion conducting layer as metal ions. The metal ions in the ion conducting layer are precipitated as metal, and the precipitated metal forms a metal bridge connecting the first and second electrodes. Since the first and second electrodes are electrically connected by the metal bridge, the switch is brought in an on-state. On the other hand, in the above on-state, if the first electrode is connected to ground and a positive voltage is applied to the second electrode, part of the metal bridge is cut off. As a result, the electrical connection between the first and second electrodes is cut off, and the switch is brought in an off-state. Before the electrical connection is completely cut off, electric characteristics are changed. For example, the resistance between the first and second electrodes is increased or the capacitance between the electrodes is varied. After such electric characteristics are changed, the electrical connection is finally cut off. If the first electrode is connected to ground and a negative voltage is applied to the second electrode again, the switch is brought from the off-state to the on-state.
Non-Patent Document 1 discloses a configuration and operation of a 2-terminal-type switching element that includes two electrodes sandwiching an ion conductor and controls the conduction state between the two electrodes. Further, Non-Patent Document 1 proposes a 3-terminal-type switching element that includes, in addition to the above electrodes, a single control electrode (third electrode). According to this document, by applying a voltage to the control electrode, the conductive state of the ion conductor between the first and second electrodes is controlled.
Such switching element is smaller in size and on-resistance, compared with conventionally-used semiconductor switches (MOSFETs and the like). Thus, the switching element is considered to be a promising technique to be applied to programmable logic devices. Further, based on this switching element, even after the applied voltage is turned off, the conduction state (on or off) is maintained. Thus, the switching element can be also considered as a nonvolatile memory element. For example, by arranging a plurality of memory cells, each of which includes a single selection element such as a transistor and a single switching element as a basic unit, in vertical and horizontal directions and by using word and bit lines, an arbitrary memory cell can be selected from among the plurality of memory cells. Thus, a nonvolatile memory that can sense the conduction state of the switching element of the selected memory cell and can read whether information 1 or 0 is stored based on the on-state/off-state of the switching element can be realized.
Non-Patent Document 1: Shunichi Kaeriyama et al., “A Nonvolatile Programmable Solid-Electrolyte Nanometer Switch”, IEEE Journal of Solid-State Circuits, Vol. 40, No. 1, pp. 168-176, January 2005.
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