On-chip solid-state CMOS compatible micro-supercapacitors

Research paper pub­lished in the pro­ceed­ings of 2018 IEEE 68th Elec­tron­ic Com­po­nents and Tech­nol­o­gy Con­fer­ence (ECTC), 2018, pp. 1382–1388.

Using a fab­ri­ca­tion process and mate­ri­als that are com­plete­ly CMOS com­pat­i­ble, on-chip inte­grat­ed sol­id-state micro-super­ca­pac­i­tors using ver­ti­cal­ly aligned car­bon nanofibers (CNFs) and car­bon nan­otubes (CNTs) as elec­trode mate­r­i­al and an iono­gel as elec­trolyte have been man­u­fac­tured and char­ac­ter­ized. The car­bon nanos­truc­tures are grown direct­ly on the devices at tem­per­a­tures below 400 °C using a cat­alyt­ic CVD process. Build­ing on a pre­vi­ous study, an inter­dig­i­tat­ed capac­i­tor design was used with vary­ing size of the gaps between the dig­its, and nov­el elec­trolyte mate­ri­als were used to ensure oper­at­ing volt­ages of above 2 V. The devices were char­ac­ter­ized elec­tro­chem­i­cal­ly using cyclic voltam­me­try sweep­ing up to 2 V, gal­vano­s­ta­t­ic charg­ing and dis­charg­ing, and elec­tro­chem­i­cal imped­ance spec­troscopy. A capac­i­tance of 0.45 mF/​cm2 and 0.31 mF/​cm2 (per device foot­print area) was achieved for CNF based devices and CNT based devices respec­tive­ly. Both types of devices show a max­i­mum capac­i­tance for when the dis­tance between the dig­its are ca 30–50 μm, and low­er capac­i­tance val­ues for larg­er gap sizes. Cycle life mea­sure­ments show that the devices are sta­ble up to at least 2,000 cycles, and the high­est char­ac­ter­is­tic fre­quen­cies achieved are 223 Hz and 1,023.7 Hz for the CNF based and the CNT based devices respec­tive­ly. The char­ac­ter­is­tic fre­quen­cy is shown to decrease as the gap size increas­es. An equiv­a­lent cir­cuit mod­el is pre­sent­ed and used to show that the CNT based devices could be fur­ther improved by improv­ing the wet­ting of the elec­trode by the elec­trolyte. dense growth.

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