Ultra-low-power logic gates based on capacitive based on MEMS devices operating in adiabatic regime

Research topics >>

Summary:

Logic gates are devices that act as building blocks for digital circuits. Based on the combination of digital signals coming from their inputs, logic gates perform logical functions based on Boolean algebra that are fundamental to digital circuits. In modern practice, most gates, e.g. in microprocessors, are made from MOSFETs (metal–oxide–semiconductor field-effect transistors) [1].

In the sixties in the last century Rolf Landauer has proposed the principle describing the smallest amount of energy that is used to switch between two logic states – “0” and “1”, that is, to erase one bit of information. This principle is now known as the Landauer’s limit and claims that the energy requirement, at room temperature for one operation, should be of the order of magnitude of zepto (10-21) joules. However, this limit is still far from being reached. The state-of-the-art MOSFET transistors dissipate ~1000 kBT even in advanced CMOS nodes during a single logic operation and therefore remain orders of magnitude above Landauer’s limit [1][2]. The energy dissipation of digital circuits has been widely studied and can be explained by three different aspects:

  • Dynamic losses, which occur during abrupt switching between two logic states,
  • Static losses which are due to the non-zero leakage currents when the switch is “off”,
  • In order to be able to distinguish two different states their energetic difference has to be well above the thermal noise level (kBT), which in turn increases the dynamic losses.

In our work, we aim to eliminate static and dynamic losses from logic circuits and therefore approach zero power dissipation during the logic operations. Lower dissipations however come at a cost of lower processing speed as this approach requires operating at low frequencies (speed limit comes from the device’s time constant). Some previous works in the field have made use of N/MEMS relays to pursue this goal [3-6]. Here, we propose capacitive (contactless – to overcome the issue of N/MEMS relays) MEMS devices based on comb-drive actuators for storing and transmission of the logic information in a differential architecture. 

The capacitance between the interdigitated combs is governed by their physical position and controlled by the electrostatic forces that act between two electrically conductive combs. By using capacitive divider instead of a resistance divider to define the logic level (Figure 1a and 1b), the electrical connection between the power and the ground is omitted – therefore avoiding leakage currents. Furthermore, using the AC power supply- acting as the power-clock (VPC), allows for recovery of the electrical charges invested in the encoding of the logic state (adiabatic functioning [1][7]).

The proposed geometry (Figure 1c) of the device is such that it consists of two pairs of comb drive actuators – input and output pair. Each pair consists of a fixed (active) and a movable (passive) electrode while the movable electrodes are connected by a mechanical spring of a stiffness k. Active and passive electrodes are connected to the power-clock VPC and to the ground, respectively. They are electrostatically coupled by n interdigitated fingers separated by an air gap g.

Figure 1: a) Resistor-based (CMOS) logic with VDD being the DC supply voltage, Vin and Vout being the input and output voltages, respectively, and C being the input load of the next gate; b) Adiabatic contactless logic based on variable capacitors with VPC being the power clock – a variable power supply capable of energy recovery; c) Implementation of variable capacitors with comb-drive actuators in a differential configuration with Cf being the fixed capacitor that (together with the input load C) allows reading of the output voltage Vout through the capacitive divider.

The voltage that is set on the fixed comb in the input pair (blue in Figure 1b) causes the movement of the “input”-movable comb (and therefore the “output” movable comb as well). The consequential movement of the output comb, coding the logic state, results in a changed capacitance between the combs of the output actuator (green in Figure 1c) which can then be read through a capacitive divider formed with the fixed capacitor (Cf in Figure 1c) and the load (from the interconnection and the next gate). The output voltages Vout and Vout serve as the input voltages for the control of the next device.

Our proposed system follows the adiabatic logic style [1][7]. The information encoding and reading is done in phases governed by the power clock VPC which has a trapezoidal waveform for the sake of energy efficiency and recovery of the electrical charges. 

Behavioral modeling and extensive FEM analysis of the device, including the comb drive actuators, have been done in order to provide the necessary information for the design of the comb pairs and to enable the operation of logic functions in cascaded gates. In particular, geometrical parameters as the length of the fingers, their width and thickness, gap between consecutive fingers, their overlap and number were investigated. Their influence on the variable, fixed and parasitic capacitances of the actuator, including fringe effects, and the induced force between the combs were analyzed in detail.

These analyses provided the fundamental information that was necessary for the design of the logic gate and the subsequent floor planning for the device fabrication on SOI wafers. The next steps in the project would be realization of the devices in LAAS followed by their characterization. 

This work is performed in the framework of the ANR project ZerÔuate, involving 4 laboratories: CEA-LETI, LAAS, ESYCOM and G2ELAB.

 

References

[1] H. Fanet, Ultra Low Power Electronics and Adiabatic Solutions. Hoboken, NJ, USA: Wiley, 2016, doi: 10.1002/9781119006541

[2] R. Landauer, “Irreversibility and heat generation in the computing process,” IBM J. Res. Dev., vol. 5, no. 3, pp. 183–191, Jul. 1961, doi: 10. 1147/rd.53.0183

[3] H. Samaali, Y. Perrin, A. Galisultanov, H. Fanet, G. Pillonnet, et P. Basset, « MEMS four-terminal variable capacitor for low power capacitive adiabatic logic with high logic state differentiation », Nano Energy, vol. 55, p. 277-287, janv. 2019, doi: 10.1016/j.nanoen.2018.10.059.

[4] Z. A. Ye et al., “Demonstration of 50-mV digital integrated circuits with microelectromechanical relays,” in IEDM Tech. Dig., Dec. 2018, pp. 411–414, doi: 10.1109/IEDM.2018.8614663.

[5] G. Pillonnet, H. Fanet and S. Houri, "Adiabatic capacitive logic: A paradigm for low-power logic," 2017 IEEE International Symposium on Circuits and Systems (ISCAS), 2017, pp. 1-4, doi: 10.1109/ISCAS.2017.8050996.

[6] Y. Perrin, A. Galisultanov, L. Hutin, P. Basset, H. Fanet and G. Pillonnet, "Contact-Free MEMS Devices for Reliable and Low-Power Logic Operations," in IEEE Transactions on Electron Devices, vol. 68, no. 6, pp. 2938-2943, June 2021, doi: 10.1109/TED.2021.3070844.

[7] P. Teichmann, “Adiabatic Logic: Future Trend and System Level Perspective,” Springer, 2012, DOI. 10.1007/978-94-007-2345-0.

 

ContactB.Legrand
 

Collaborations: 

  • CEA-LETI in Grenoble (Contact G.Pillonnet, H.Fanet)
  • LAAS-CNRS (Contact B.Legrand)
  • ESYCOM in Paris (Contact P.Basset, A.Karami, F.Marty)
  • G2ELAB in Grenoble (Contact O.Gallot, F.Aitken)