Propagation analysis for molecular FCN neural
5.4 Final choice for d and connection with the second neuronneuron
In the previous sections, were delivered several ground considerations to understand the final structure of the axon of the molecular neural network. Different techniques were presented to work with an intermolecular distance equal to 1nm. The analog propagation of the information was not achieved, and the wire had to be saturated, implementing a binary network. The remaining problem related to a high separation is the one concerning the low weights of the interfaces. For this reason, provided that the final neural network will be a fully digital one, it is better to work with 0.9nm within the molecules. This way, saturation is ensured along the wire, and the weight coefficients have proper values for neuromorphic purposes.
It is also possible to complicate the wire structure to analyze better the different compo-nents’ behavior in a whole system. The design of the adopted molecular wire is reported in figure 5.20a. There are three clock regions. The first two split the propagation along the wire according to the analysis performed in the previous sections, while the third clock influences only the bis-ferrocene cell connected at the end of the wire. The interface cell is
Propagation analysis for molecular FCN neural networks
localized in the second zone. Moreover, a further cell is added at the far end to simulate propagation after the computation at the neuron site. It is possible to appreciate the waveforms and time relations of these signals in figure 5.20b.
(a)
(b)
Fig. 5.20: Clock regions organization and timing. Preliminary assumptions Notice how the three regions behave: CK1 and CK2 are in complete phase rotation, i.e,. the first’s maximum value corresponds to the second’s minimum. The third region takes the value present in the second region and propagates it to the first one again, so to the last cell of the wire. Focusing the attention on the propagation behavior, it is possible to study the results from a simulation in which the input voltage was set to be 0.65V and the distance to 0.9nm. These are reported in figures5.21a-5.21c. In5.21a, it is possible to notice the moment in which the information reaches the bis-ferrocene cell. Consider that the last cell, governed by the first clock signal, presents some small charge distribution.
The information propagates through the wire and arrives at the interface cell, which is characterized by α = 1V . The logic value is correctly evaluated after the interface, and the propagation can proceed. From a timing point of view, this simulation confirms the correctness of the adopted clock distribution. However, some problems can be highlighted if the goal is to transport multiple successive binary values. This can be easily understood from a practical example, reported in figures 5.22a-5.22f.
In this last case, the objective was to correctly transport two different binary values with corresponding input values from the drivers equal to -0.6V and 0.2V. Bis-ferrocene molecules form the wire, and the interface has α = 1V molecules. As in the previous example, the neuron central cell and the output molecules are connected at the wire end.
Propagation analysis for molecular FCN neural networks
(a) (b) (c)
Fig. 5.21: Potential plots: propagation along the complete molecular wire. Vin= 0.65V , d = 0.9nm
(a) (b) (c)
(d) (e) (f)
Fig. 5.22: Multiple inputs propagation. The clash event is evident from the charge distri-bution plots
The first snapshot is related to propagating the first logic value; in the second one, the input changes, and the corresponding logic value starts to move. The main problems are evident in 5.22c in which a clash occurs, making the propagation through the wire extremely unstable with the possibility of complete loss of the correct information.
This behavior derives from the back-propagation from the second clock region while the first is trying to transport an opposite binary value. A possible solution is to introduce the saturator molecules at the beginning of each wire to solve this problem. In this way,
Propagation analysis for molecular FCN neural networks
the strong polarization obtained in the saturation cell will overcome the effects of back-propagation and the new charge configuration. In figures 5.23a-5.23f are reported the results of the simulation. The other parameters remain unchanged.
(a) (b) (c)
(d) (e) (f)
Fig. 5.23: Multiple inputs propagation with the introduction of saturator molecules
The issue seems to be solved. However, the introduction of molecules with such a small saturation voltage will lead to important issues if an interface with a high α parameter is present. Indeed, as soon as the first clock zone gets active, the saturator molecules polarize randomly in one of the two stable configurations, biasing their charges depending on spurious electric field influences. This effect combines with the low polarization of interface cells with high saturation voltage. The final result is that the information could be completely lost also in this case since the system is not reliable. Simulation results are proposed in figures5.24a-5.24f. The idea of saturating the wire at the front end is positive and is not mandatory to renounce to it. Indeed, it can be shown that by increasing Vsat
of the saturator up to 0.3V, thus maintaining it lower than 0.5V, it is possible to save the benefits related to the use of such molecules and, at the same time, avoid information loss. The results showing this concept are reported in the pictures 5.25a-5.25f. In figure 5.26is reported the VACT of the new saturator molecules.
Propagation analysis for molecular FCN neural networks
(a) (b) (c)
(d) (e) (f)
Fig. 5.24: Multiple input propagation with interface cell having α = 1.5V
(a) (b) (c)
(d) (e) (f)
Fig. 5.25: Multiple input propagation with interface cell having α = 1.5V and α = 0.3V on the saturator cells
Propagation analysis for molecular FCN neural networks
Fig. 5.26: VACT α = 0.3V , clk = 2nmV
The only problem remaining is the possible presence of clash events in propagation.
Even though the introduction of low Vsat molecules allow for the correct value recon-struction; this is an unstable situation that is better to avoid. Moreover, the sudden configuration change within a molecule would increase the overall power dissipated by the structure. The main reason this situation can occur is, as already explained, the back-propagation from the second region. It is due to the absence of a reset state in the clock configuration adopted so far. For this reason, the clock signals must be modified. The change has to combine with the saturator molecules’ effects, providing a stable and safe transport of the information through the entire molecular wire for every interface.