On-chip solid-state CMOS compatible micro-supercapacitors

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.