In this work, we explore the implementation of the energy efficient I/O interface circuits, namely the analog-to-digital converter (ADC), using back-end-of-line (BEOL) Nano-electromechanical (NEM) relays, for ultra-low power VLSI and IoT applications.
Nowadays, integrated circuits (IC) are mostly implemented using CMOS transistor technology. Due to the inherent leakage property of transistors, whenever they are connected to the power supply, even when the circuit is not in operation, the system suffers from leakage current that passes through the switches and gets wasted. In other words, by nature, the transistors can never be totally turned off.
In the IC fabrication process, metal wires and VIAs (the vertical connection between metal layers) are used as interconnections on the chip for connecting millions of transistors. With the recently developed air gap technology, on the back side of the chip, or back-end of the line, an air gap can be introduced in between two metal layers, such that, there is a spatial movement potential for those metal wires. Hence a mechanical relay using metal layers can be designed and implemented on the back side of the transistor chip, helping to save energy by eliminating the leakage current and reducing the cost since it is inherently CMOS fabrication compatible.
The NEM relay has two parts, the input gate (G) and the channel are connected through an isolating via as one part that will move. The other part is the source (S), the drain (D) and the body (B) are connected through isolating VIA as the fixed part. The electrodes of the NEM relay are separated by an air gap and the initial state is off. When a high voltage is applied to the gate, the gate will bend over to the body due to the electrostatic force generated in between, eventually resulting in the channel connecting the source and the drain and current passes between them. And there is zero leakage when it is turned off since the current path is isolated by the air gap (Illustration #1).
Our implementation of ADC based on BEOL NEM relay utilizes multiple optimization strategies, such as custom tuning of the operation voltage of devices in comparators (Illustration #2), eliminating static power by adopting a reference-less design, body biasing for low voltage operation (Illustration #3).
We show that the proposed design can achieve at least one order of magnitude improved FOM compared to CMOS counterparts. This paves the way for a new generation of ultra-low power data converters, a critical component of the internet of things (IoT).