A Test Vehicle for RF/​DC Evaluation and Destructive Testing Of Vertically Grown Nanostructures (VGCNS)

We have developed an RF test vehicle suit­able for meas­ur­ing DC and microwave per­form­ance of ver­tic­ally grown car­bon nano­struc­tures (VGCNS) as via-inter­con­nects. A first ver­sion of the inter­con­nect test vehicles devices were designed, fab­ric­ated and char­ac­ter­ized. The RF small sig­nal (S‑parameter) and large sig­nal meas­ure­ments show that car­bon nan­ofibres can be used as inter­con­nects in microwave cir­cuit, even for power devices. The design of test vehicle employs a three met­al lay­er concept, form­ing sequen­tially the ground, sig­nal and device under test struc­tures for char­ac­ter­iz­a­tion in a microstrip con­fig­ur­a­tion. The struc­tures as such con­sisted of inter­con­nects of dimen­sions ran­ging from 50 nm to 100 µm dia­met­er made of VGCNS. In the first ver­sion of the inter­con­nect test vehicles, the inter­con­nects were made of car­bon nan­ofibers grown at 450 C. From SEM meas­ure­ment we found that the res­ult­ing height was around 1.5–2 µm. Epoxy poly­mer SU‑8 was employed by spin­ning on the device and a sub­sequent etch back pro­cess was car­ried out to open up the tip of the fibres to con­nect to con­sec­ut­ive inter­con­nects with the third met­al­lisa­tion lay­er. After grow­ing the nan­ofibres, it was observed, using SEM, that inter­con­nect sizes smal­ler than 10 µm dia­met­er suffered from para­sit­ic growth and there­fore the effect­ive device dimen­sion devi­ated from the ini­tial design. We car­ried out small sig­nal meas­ure­ments using a vec­tor net­work ana­lys­er for fre­quency ran­ging from 1 to 25 GHz, in order to char­ac­ter­ise the trans­mis­sion and reflection/​absorption of the devices as func­tion of their dia­met­er size. The large sig­nal eval­u­ation was per­formed by meas­ur­ing the gain com­pres­sion of the devices. In addi­tion destruct­ive tests, aim­ing at test­ing the cur­rent car­ry­ing cap­ab­il­ity of the inter­con­nect, have also been per­formed. The res­istiv­ity of inter­con­nects was meas­ured to vary var­ies from 0.2–1.3 mΩ·mm. Appar­ently, the device per­form­ance is con­sid­er­ably influ­enced by the fill factor of the inter­con­nect with VGCNS. Small vari­ations in fill factor (in %) provided a large vari­ations in device res­istiv­ity. Fur­ther­more, it was also observed that the res­ist­ance drops at high­er power levels. RF con­duct­iv­ity of inter­con­nects ranges from 5×10³ S/​m to 7×10⁵ S/​m. The aver­age input power before inter­con­nect destruc­tion is lar­ger than 25W with effect­ive device dia­met­er ran­ging from 3 µm to 100 µm inter­con­nects. In addi­tion, the aver­age gain com­pres­sion before inter­con­nect destruc­tion was found to be 0.6 dB. It was not pos­sible to extract the con­duct­iv­ity value of an indi­vidu­al nan­ofiber using com­par­is­on to sim­u­la­tion data, since the devices might have suffered from para­sit­ic growth as well as pin­hole met­al dif­fu­sion dur­ing top met­al con­tact form­a­tion. This cer­tainly affects the actu­al device dimen­sion and prop­er­ties. Nev­er­the­less, the proof of concept of design and man­u­fac­tur­ing a test vehicle for RF meas­ure­ments of ver­tic­ally grown nano­struc­tures was achieved. We will report the find­ings and anom­alies in the meas­ured devices. Fur­ther improve­ment is expec­ted in the com­ing test vehicle version.