et al. reported that they moved Xe atoms via the tip of a scanning tunneling
microscope (STM), and thus called it as “Atomic switch” [11]. Li and Tao demonstrated metal bridge deposition and dissolution in electrolyte [12], and Terabe et al.
succeeded in making a reversible atomic bridge at the crossing point between a metal
electrode and a solid electrolyte, showing the possibility of atomic switch for “nanodevice” fabrication [13]. Applying these results, the atom switches were successfully
integrated into read-only memories (ROMs) and field programmable gate arrays
(FPGAs) for their configuration memory cells, thanks to compatibility with the
fabrication process for complementary metal oxide semiconductor (CMOS) devices.
The features (i.e., small size on the nanometer-scale order, non-volatility, high on/off
resistance ratio) make the devices attractive. Details are well described separately
[14, 15]. Atom switches in non-volatile memory applications are known as conductive bridge random access memory (CBRAM), electrochemical metallization cell
(EMC) or programmable metallization cell (PMC) memory [16].
Non-volatile memories are widely used as data storage memory in the commercial market for terrestrial applications, as well as for space applications. However,
these memories are in fact sensitive to ionizing particles, known as Single Event
Effects (SEEs). Such vulnerability is becoming a more serious problem as scaling
techniques become increasingly smaller. In particular, single event upset (SEU),
single event latch-up (SEL) and single event transient (SET) should be considered in
memories and logic devices [17]. Atom switches that exhibits non-volatility are
totally different from the conventional types of memory cells that stores the memory
state as the amount of electric charges contained in them. Instead of such electric
charge, the resistive changing switches that employing electrochemical mechanisms
and ion transportation are used for these cells. Therefore, it is expected to have
non-vulnerability to the ionizing particles.
Previous studies have reported gamma-ray tolerance [18] for Ag/GeS2/W-based
CBRAM [19]. And other studies have reported on an Ag-based test chip that
exhibited SEUs due to upsets of the access transistors [20]. The Ag-based standalone memory chip was vulnerable to bit upsets during dynamic write/read tests [21].
This work investigated a Cu-based conductive bridge or atom switches in terms
of SEEs. Radiation test results were presented as a preliminary evaluation for
applying these technologies for space applications.
2 Atom Switch
2.1 Overview
The atom switch is a nano-scale switch that electrochemically controls the connection/disconnection of an electrically conductive Cu-bridge. Figure 7a shows a crosssectional illustration (left) and transmission electron microscopy (TEM) image
(right) of an atom switch cell that is fabricated through the Cu Back-end-of-theline (BEOL) process in the CMOS fabrication process. Figure 7b shows schematic
64
K. Takeuchi et al.
microscope (STM), and thus called it as “Atomic switch” [11]. Li and Tao demonstrated metal bridge deposition and dissolution in electrolyte [12], and Terabe et al.
succeeded in making a reversible atomic bridge at the crossing point between a metal
electrode and a solid electrolyte, showing the possibility of atomic switch for “nanodevice” fabrication [13]. Applying these results, the atom switches were successfully
integrated into read-only memories (ROMs) and field programmable gate arrays
(FPGAs) for their configuration memory cells, thanks to compatibility with the
fabrication process for complementary metal oxide semiconductor (CMOS) devices.
The features (i.e., small size on the nanometer-scale order, non-volatility, high on/off
resistance ratio) make the devices attractive. Details are well described separately
[14, 15]. Atom switches in non-volatile memory applications are known as conductive bridge random access memory (CBRAM), electrochemical metallization cell
(EMC) or programmable metallization cell (PMC) memory [16].
Non-volatile memories are widely used as data storage memory in the commercial market for terrestrial applications, as well as for space applications. However,
these memories are in fact sensitive to ionizing particles, known as Single Event
Effects (SEEs). Such vulnerability is becoming a more serious problem as scaling
techniques become increasingly smaller. In particular, single event upset (SEU),
single event latch-up (SEL) and single event transient (SET) should be considered in
memories and logic devices [17]. Atom switches that exhibits non-volatility are
totally different from the conventional types of memory cells that stores the memory
state as the amount of electric charges contained in them. Instead of such electric
charge, the resistive changing switches that employing electrochemical mechanisms
and ion transportation are used for these cells. Therefore, it is expected to have
non-vulnerability to the ionizing particles.
Previous studies have reported gamma-ray tolerance [18] for Ag/GeS2/W-based
CBRAM [19]. And other studies have reported on an Ag-based test chip that
exhibited SEUs due to upsets of the access transistors [20]. The Ag-based standalone memory chip was vulnerable to bit upsets during dynamic write/read tests [21].
This work investigated a Cu-based conductive bridge or atom switches in terms
of SEEs. Radiation test results were presented as a preliminary evaluation for
applying these technologies for space applications.
2 Atom Switch
2.1 Overview
The atom switch is a nano-scale switch that electrochemically controls the connection/disconnection of an electrically conductive Cu-bridge. Figure 7a shows a crosssectional illustration (left) and transmission electron microscopy (TEM) image
(right) of an atom switch cell that is fabricated through the Cu Back-end-of-theline (BEOL) process in the CMOS fabrication process. Figure 7b shows schematic
64
K. Takeuchi et al.