Sign up for our newsletter!

Subscribe form (en)

No spam. Simply good reading. Get your free subscription to Smoltek Newsletter infrequently delivered straight to your inbox.

Your data will be handled in compliance with our privacy policy.

Vertically Aligned Carbon Nanofibers

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

Research paper in the proceedings of NT11 International Conference on the Science and Application of Nanotubes, University of Cambridge, United Kingdom, July 10–16, 2011.

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.

Sign up for our newsletter!

Subscribe form (en)

No spam. Simply good reading. Get your free subscription to Smoltek Newsletter infrequently delivered straight to your inbox.

Your data will be handled in compliance with our privacy policy.

Related news and insights