results in a large circuit size. The two-terminal atomic switch with a higher V SET
needs a 3.3-V-tolerant transistor [11]. When the CAS is applied to the programmable
switch, both high off-state reliability and low V SET are realized. The circles in
Fig. 4b show the operation points of CAS. The highly reliable off-state is achieved
even when the atomic switch with a lower V SET (or a thinner solid electrolyte) is
used, because the stress voltage during the logic operation is complementally shared
by two elements. A 1.8-V-tolerant transistor with small foot print is used
[12, 13]. Recently, a logic transistor is available to program CAS, resulting in further
reduction of circuit area [14].
The on-state reliability of the atomic switch depends on the on conductance or the
thickness of the filament, which is controlled by a programming current. To get a
higher on-state reliability, lower on-conductance or larger programming current is
needed. The reliability also depends on the fabrication process or materials of the
atomic switch. We have explored various Ru alloy materials instead of simple Ru,
which is the inert electrode of atomic switch [15]. Then, the on-state reliability is
improved by two orders of magnitudes. It is noted that Ru is used for implementing
atomic switch in IC to avoid a metal contamination due to Pt. Moreover, when the
Cu filament is induced to form at the edge of the Cu electrode, the on-state reliability
is improved further. We also optimize the process steps of switch stack, fail bit count
can be eliminated for millions of switches. Highly reliable on-state and low programming current can be obtained by optimizing the material, device structure and
process conditions.
4 Atomic-Switch Based FPGA
This section describes the circuit architecture and the performance evaluation results
of atomic-switch based FPGA [11–13, 16]. Figure 5 shows the schematic circuit
diagram of atomic-switch based FPGA. FPGA is composed of array of configurable
logic block (CLB), Input-output (IO) cell, and programming drivers for atomic
switch (Fig. 5a). The routing tracks spread on the CLB arrays (Fig. 5b). Each of
4 routing tracks with 4 segment length has 3 lanes in all 4 directions (vertical or
horizontal direction). Since the tracks are unidirectional, they include the tracks for
positive and negative directions. Each signal from the CLB can then reach the CLB
at 4 units away in either direction by using only one wire. This contributes to the
routability and the reduction in the signal delay. The segment wires are buffered by
AND gate at the section between the second and third CLBs. The AND gate also
terminates unused wires to reduce the parasitic wire capacitance. The segment length
and the lane number are chosen to have the capability of routing all of the MCNC
“golden 20” benchmark circuits [17]. CLB has a crossbar switch for signal routing
and two basic logic-elements (BLEs) for logic operation, which is composed of
4-input look-up-table (LUT) and D-flip flop (DFF) (Fig. 5c). The crossbar has
51 inputs comprising of 48 routing tracks, 2 feedback lines, and the one fixed-low
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T. Sakamoto et al.
needs a 3.3-V-tolerant transistor [11]. When the CAS is applied to the programmable
switch, both high off-state reliability and low V SET are realized. The circles in
Fig. 4b show the operation points of CAS. The highly reliable off-state is achieved
even when the atomic switch with a lower V SET (or a thinner solid electrolyte) is
used, because the stress voltage during the logic operation is complementally shared
by two elements. A 1.8-V-tolerant transistor with small foot print is used
[12, 13]. Recently, a logic transistor is available to program CAS, resulting in further
reduction of circuit area [14].
The on-state reliability of the atomic switch depends on the on conductance or the
thickness of the filament, which is controlled by a programming current. To get a
higher on-state reliability, lower on-conductance or larger programming current is
needed. The reliability also depends on the fabrication process or materials of the
atomic switch. We have explored various Ru alloy materials instead of simple Ru,
which is the inert electrode of atomic switch [15]. Then, the on-state reliability is
improved by two orders of magnitudes. It is noted that Ru is used for implementing
atomic switch in IC to avoid a metal contamination due to Pt. Moreover, when the
Cu filament is induced to form at the edge of the Cu electrode, the on-state reliability
is improved further. We also optimize the process steps of switch stack, fail bit count
can be eliminated for millions of switches. Highly reliable on-state and low programming current can be obtained by optimizing the material, device structure and
process conditions.
4 Atomic-Switch Based FPGA
This section describes the circuit architecture and the performance evaluation results
of atomic-switch based FPGA [11–13, 16]. Figure 5 shows the schematic circuit
diagram of atomic-switch based FPGA. FPGA is composed of array of configurable
logic block (CLB), Input-output (IO) cell, and programming drivers for atomic
switch (Fig. 5a). The routing tracks spread on the CLB arrays (Fig. 5b). Each of
4 routing tracks with 4 segment length has 3 lanes in all 4 directions (vertical or
horizontal direction). Since the tracks are unidirectional, they include the tracks for
positive and negative directions. Each signal from the CLB can then reach the CLB
at 4 units away in either direction by using only one wire. This contributes to the
routability and the reduction in the signal delay. The segment wires are buffered by
AND gate at the section between the second and third CLBs. The AND gate also
terminates unused wires to reduce the parasitic wire capacitance. The segment length
and the lane number are chosen to have the capability of routing all of the MCNC
“golden 20” benchmark circuits [17]. CLB has a crossbar switch for signal routing
and two basic logic-elements (BLEs) for logic operation, which is composed of
4-input look-up-table (LUT) and D-flip flop (DFF) (Fig. 5c). The crossbar has
51 inputs comprising of 48 routing tracks, 2 feedback lines, and the one fixed-low
22
T. Sakamoto et al.