# THz Rectennas and Their Design Rules
## 7 Fabrication Aspects
### Nano Transfer Print  
Nano transfer print (nTP) is also another fabrication process that incorporates printing as NIL, but is different in two aspects: (a) it is suggested for printing on a substrate layer; and (b) the whole feature is printed; meaning that the whole diode is printed as shown in Figure 15. In the stack of Figure 15d, the first deposited layer is the organic self-assembled monolayer (SAM) to provide weak adhesion to the stamp surface; this is followed by AuPd alloy that is weakly attached to the SAM surface; the aluminum deposition follows this as the first metal in the actual MIM diode; the insulating layer is then deposited followed by another AuPd alloy layer; and finally, an adhesion promoter, which can be titanium or chrome. The gold palladium alloy has been used because it provides surface roughness of less than 1 nm [109], which is already known for its impact on the predictability of the diode functionality [37].  
Figure 14: The suggested roll to roll chamber to produce rectennas (Reproduced with permission from [107], [PERSON], 2013).  
The same group [96] managed to use the same method to fabricate the diode integrated with an antenna network at the nano-scale. It should be noted, however, that there have been flaws in the fabrication process, which resulted in sections that did not follow the tunneling rule. Little information is available in that paper about how many sectors have the unpredicted response. [PERSON] et al. [110], on the other hand, managed to reliably fabricate metal oxide substrate (MOS) junctions of Au/Ti/TiO\({}_{\mathrm{x}}\)/p\({}^{+}\)-Si measuring below 70 nm. Furthermore, [PERSON] et al. [111] have addressed the use of this method in the fabrication of circuits measuring below 50 nm using a stamp upon which the rectennas can be deposited. Their method looks similar to that of [PERSON] discussed above in the sense of reproducing the stamp from the master template. The difference is that here, the whole device is deposited before stamping. They, quantitatively, evaluate the performance of the method where over 70% of the produced devices are operative if the printing time is about 40 min under set conditions as shown in ([111], Figure 4d).  
In the end, the nTP method provides a deep understanding of the adhesion of stacked materials, but its literature is still based on the printing on silicon or MOS substrates where the biggest cost for device fabrication is concentrated. Another practical aspect also refers to the fabrication of extremely tiny nanometer structures such as that described by [PERSON] et al. [36], who propose a metal-vacuum-metal tunneling diode structure able to rectify wavelengths up to green light with high efficiency?The structure relies on a metal tip protruding in the vacuum opposite a flat anode. The fabrication of such a device with the available technology is a big challenge, although there are ongoing trials on this matter at an experimental scale.  
Figure 15: The nano transfer printing method. (**a**) shows how the stamp looks before the printing where diodes are still on the stamp protrusions; (**b**) since the organic self-assembled monolayer (SAM) in the figures provides weak adhesion to the stamp surface, the diodes are transferred to the substrate when the stamp is applied to the substrate; the diode stack between the stamps remains there without getting printed because it has no contact with the substrate surface; (**c**) close-up view of the stamp; (**d**) shows the diode stack up reaching the stamp protrusion (Reproduced with permission from [109], [PERSON], 2013).