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4. NANOMEMS APPLICATIONS: CIRCUITS AND SYSTEMS 151

4.2.2 NanoMEMS SoC Building Blocks

4.2.2.1 Interfaces

The idea behind NanoMEMS is that of creating a system that, in order to
accomplish a given function, avails itself of devices and techniques spanning
the range from the micro- down to the nano-scale and beyond. In the most
general case, the input signal to a NanoMEMS SoC will be analog, i.e., will
exhibit continuous amplitude and will exist at all times, see Figure 3-1.
Processing this signal, therefore, will entail deciding whether it is feasible to
act on it as received/detected, or to transform it to a more convenient state.
The nature of the interface sensor, in particular, its sensitivity, bandwidth,
and dynamic range, will come into play here and will dictate the need for
transduction, amplification, digitization, filtering, etc., thus determining the
rest of the architecture. In this context, the doubly-anchored Si beam has
been considered as a potential mechanical sensing element in future
NanoMEMS SoCs, and impressive estimates for its intrinsic force sensitivity
(S F), dynamic range (DR), mass sensitivity (M), and bandwidth (BW) have
been obtained by Roukes [174]. For instance, a beam of length, width, and
thickness 0.1 x 0.01 x 0.01 microns and active mass 10ag would exhibit
S F / 1 2 (ω 0 ) = 3× 10 − 17 N / Hz , DR = 35 dB , M = 7 . 1 × 10 − 21 g , and
BW = 7 . 7 GHz , assuming a temperature of 300K and a Q of 10,000.

Unfortunately, it is unclear whether the full extent of these parameters will
be accessible due to various practical difficulties such as mass variation due
to unpredictable adsorbates, and the impossibility of realizing a noiseless
read-out. This latter theme is also common to electrostatic- and optically-
based sensing interfaces as well. In the former case, which according to
− 18
Roukes [174] may attain a minimum capacitance of 10 F , the parasitic
capacitance would preclude resolving it. In the latter case, the fact that the
spot size of the light delivered by the optical fiber used in AFM
displacement-sensing is much greater than nanoscale dimensions, precludes
its resolution and, hence, proper detection.
In systems with an electronic input signal sensing scheme, however, the
sensor may take the form of a quantum superlattice-based analog-to-digital
converter, Fig. 4-2 [175]. Here, the pulsating nature of the superlattice’s
current-voltage characteristic directly samples/quantizes the voltage axis.
The resulting current is used to generate pulses that drive a counter whose
output is a digital representation of the input voltage. For highest resolution,
the superlattice may be realized with molecular devices.