Samer Houri

Limits of CMOS Technology and Interest of NEMS Relays for Adiabatic Logic Applications

In this paper, a detailed analysis and comparison of nanoelectromechanical systems (NEMS) and CMOS technologies for low power adiabatic logic implementation is presented. Fundamental limits of CMOS-based adiabatic logic are identified. Analytic relations describing the energy-performance for sub-threshold adiabatic logic are also explicitly derived and optimized. The interest of combining NEMS technology and adiabatic logic is described, and the key NEMS switch parameters that govern the dissipation-performance relationship are identified as the switch commutation frequency, its actuation voltage, and the contact resistance between the switch electrodes. Furthermore, NEMS-based adiabatic gates architectures are described. Finally, the contribution of the power-clock or energy recovery generator is estimated in order to compare CMOS and NEMS-based adiabatic architectures at the system level. The paper concludes with a detailed comparison of the energy-performance of the different explored technologies.

Comparing CMOS-Based and NEMS-Based adiabatic logic circuits

In this paper, a detailed comparison between the expected performance of CMOS-based and nanoelectromechanical systems (NEMS) based adiabatic logic circuits is presented. The modeling of the NEMS devices is done using a 1-dimensional reduced order model (1d ROM) of the electromechanical switches. This model will give an honest analytical depiction of the NEMS-based adiabatic circuits. The performance of NEMS-based circuits compares favorably with that of CMOS-based circuits. To the best knowledge of the authors, this is the first reported detailed comparison between NEMS and CMOS devices for adiabatic circuits.

Colorimetry Technique for Scalable Characterization of Suspended Graphene

Previous statistical studies on the mechanical properties of chemical-vapor-deposited (CVD) suspended graphene membranes have been performed by means of measuring individual devices or with techniques that affect the material. Here, we present a colorimetry technique as a parallel, noninvasive, and affordable way of characterizing suspended graphene devices. We exploit Newtons rings interference patterns to study the deformation of a double-layer graphene drum 13.2 μm in diameter when a pressure step is applied. By studying the time evolution of the deformation, we find that filling the drum cavity with air is 2-5 times slower than when it is purged.

Adiabatic capacitive logic: A paradigm for low-power logic

Although CMOS technology scaling combined with efficient frequency and voltage scaling strategies offer femto Joule per logic operation, energy consumption remains orders of magnitude above the limit given by information theory. To alleviate this inherent energy dissipation, this paper introduces a new paradigm: the adiabatic capacitive logic. Based on adiabatic operation, the principle also relies on a smooth capacitance modulation to achieve a quasi zero-power logic dissipation. This method limits leakage by using metal-metal junctions instead of semiconductor one. It also avoids dynamic power consumption by adiabatic transitions. The contact-less operation promises a better reliability compared to logic based on nano-mechanical relays.

Direct and parametric synchronization of a graphene self-oscillator

We explore the dynamics of a graphene nanomechanical oscillator coupled to a reference oscillator. Circular graphene drums are forced into self-oscillation, at a frequency f osc, by means of photothermal feedback induced by illuminating the drum with a continuous-wave red laser beam. Synchronization to a reference signal, at a frequency f sync, is achieved by shining a power-modulated blue laser onto the structure. We investigate two regimes of synchronization as a function of both detuning and signal strength for direct ( f sync ≈ f o s c ) and parametric locking ( f sync ≈ 2 f osc ). We detect a regime of phase resonance, where the phase of the oscillator behaves as an underdamped second-order system, with the natural frequency of the phase resonance showing a clear power-law dependence on the locking signal strength. The phase resonance is qualitatively reproduced using a forced van der Pol-Duffing-Mathieu equation.

Very large scale characterization of graphene mechanical devices using a colorimetry technique

We use a scalable optical technique to characterize more than 21 000 circular nanomechanical devices made of suspended single- and double-layer graphene on cavities with different diameters (D) and depths (g). To maximize the contrast between suspended and broken membranes we used a model for selecting the optimal color filter. The method enables parallel and automatized image processing for yield statistics. We find the survival probability to be correlated with a structural mechanics scaling parameter given by D4/g3. Moreover, we extract a median adhesion energy of Γ = 0.9 J m-2 between the membrane and the native SiO2 at the bottom of the cavities.

Mechanical characterization and cleaning of CVD single-layer h-BN resonators

Hexagonal boron nitride is a 2D material whose single-layer allotrope has not been intensively studied despite being the substrate for graphene electronics. Its transparency and stronger interlayer adhesion with respect to graphene makes it difficult to work with, and few applications have been proposed. We have developed a transfer technique for this extra-adhesive material that does not require its visual localization, and fabricated mechanical resonators made out of chemical vapor-deposited single-layer hexagonal boron nitride. The suspended material was initially contaminated with polymer residues from the transfer, and the devices showed an unexpected tensioning when cooling them to 3 K. After cleaning in harsh environments with air at 450 °C and ozone, the temperature dependence changed with f0Q products reaching 2 × 1010 Hz at room temperature. This work paves the way to the realization of highly sensitive mechanical systems based on hexagonal boron nitride, which could be used as an alternative material to SiN for optomechanics experiments at room temperature.Nanofabrication: optimized transfer enables hexagonal boron nitride mechanical resonatorsAn improved transfer method allows easy placement of highly transparent and strongly adhesive hexagonal boron nitride on target substrates. A team led by Santiago J. Cartamil-Bueno at Delft University of Technology developed a technique that enables the transfer of large-area, single-layer hexagonal boron nitride films grown by chemical vapor deposition onto a substrate of choice, whilst not requiring optical visualization. Following an additional cleaning step, the atomically thin membranes were transferred onto circular microcavities patterned on a silicon oxide substrate, resulting in the formation of suspended drums. Cleaning in harsh environments using a mixture of air and ozone is instrumental to a substantial improvement in the quality factor of the drums, indicating that undesired contamination causes damping of the mechanical motion. These results show promise for the development of sensitive hexagonal boron nitride resonators.

Optomechanics for thermal characterization of suspended graphene

Thermal properties of suspended single-layer graphene membranes are investigated by characterization of their mechanical motion in response to a high-frequency modulated laser. A characteristic delay time

Opto-thermally excited multimode parametric resonance in graphene membranes

\tau

Power-Clock Generator Impact on the Performance of NEM-Based Quasi-Adiabatic Logic Circuits

between the optical intensity and mechanical motion is observed, which is attributed to the time required to raise the temperature of the membrane. We find, however, that the measured time constants are significantly larger than the predicted ones based on values of the specific heat and thermal conductivity. In order to explain the discrepancy between measured and modeled tau, a model is proposed that takes a thermal boundary resistance at the edge of the graphene drum into account. The measurements provide a noninvasive way to characterize thermal properties of suspended atomically thin membranes, providing information that can be hard to obtain by other means.