Carbon nanomaterial-based interconnects, integrated capacitors and supercapacitors

PhD the­sis, Chalmers Uni­veristy of Tech­nol­o­gy, 2017.

The con­stant minia­tur­iza­tion and steady per­for­mance improve­ment of elec­tron­ics devices have gen­er­at­ed inno­v­a­tive ideas such as inter­net of thing (IoT), which also includes devices with inte­grat­ed ener­gy sources.

The high per­for­mance is con­ceived by the high den­si­ty of the devices on a chip lead­ing to a high den­si­ty of inter­con­nects, to con­nect these devices to out­side world. Since the size and the pitch of the inter­con­nects have decreased, the cur­rent den­si­ty in inter­con­nect has increased, pos­ing chal­lenges on the exist­ing cop­per pil­lar inter­con­nect tech­nol­o­gy, such as inter­metal­lic com­pound for­ma­tion and elec­tro-migra­tion result­ing in open cir­cuit. The chal­lenges are fore­cast­ed to increase on fur­ther down scal­ing due to bridg­ing of the sol­der between pil­lars. More­over, the envi­ron­men­tal pol­lu­tion and the threat of van­ish­ing of fos­sil fuel have prompt­ed to find cheap and effi­cient alter­nat­ing ener­gy sources and ener­gy stor­age systems.

Car­bon nano­ma­te­ri­als such as car­bon nan­otubes and car­bon nanofibers have unprece­dent­ed elec­tri­cal, mechan­i­cal and ther­mal prop­er­ties, high resis­tance to cor­ro­sion and high sur­face area have been pro­posed for the solu­tion of above men­tioned challenges.

In this the­sis, ver­ti­cal­ly aligned car­bon nanofibers (VAC­N­Fs) have been grown by direct cur­rent plas­ma enhanced chem­i­cal vapor depo­si­tion (dc-PECVD) at com­ple­men­tary met­al oxide semi­con­duc­tor (CMOS) com­pat­i­ble tem­per­a­tures for on chip appli­ca­tion. In addi­tion, the cat­a­lyst to grow VAC­N­Fs is deposit­ed using inno­v­a­tive low-cost polymer–Pd nanohy­brid col­loidal solu­tions by an effec­tive coat­ing method.

Also, due to con­trol­lable DC behav­ior and good mechan­i­cal rein­force­ment prop­er­ties of sol­der-CNFs, the sol­der­able micro-bumps of VAC­N­Fs have been shown to poten­tial­ly yield the accept­able elec­tri­cal resis­tances. More­over the CNFs bumps can be made in sub­mi­cron size range, which can com­ply with fur­ther down scal­ing of inter­con­nect. In addi­tion, advanced CNF based adhe­sives, pro­duced by coat­ing CNFs with low tem­per­a­ture poly­mers, have been inves­ti­gat­ed as alter­nat­ing anisotrop­ic con­duct­ing film for anisotrop­ic con­nec­tion, using a ther­mo-com­pres­sion bond­ing. The shear­ing strength of the bond­ed chip qual­i­fies the MIL-STD-883 stan­dards of bond­ing strength in micro­elec­tron­ics devices.

Fur­ther, super­ca­pac­i­tor are the ener­gy stor­age devices hav­ing high ener­gy den­si­ty, and high pow­er den­si­ty due to quick intake and release of charges and long cycles life of about 1 mil­lion. On-chip inte­grat­ed sol­id-state par­al­lel-plate capac­i­tor and super­ca­pac­i­tor are demon­strat­ed based on VAC­N­Fs. The pre­lim­i­nary capac­i­tance of the par­al­lel-plate capac­i­tor and super­ca­pac­i­tor are 10–15 nF/​mm2 and 10 μF/​mm2, respec­tive­ly. The pro­file of par­al­lel plate capac­i­tor is below 10 microm­e­ters, which enables inte­gra­tion even in advance 2.5 and 3D het­ero­ge­neous pack­ag­ing. The on-chip capac­i­tor can work as decou­pling capac­i­tor to resolve the ener­gy fluc­tu­a­tion relat­ed issues and also pow­er up devices on the chip.

Then, along with oth­er prop­er­ties, high aspect ratio and ease of fab­ri­ca­tion, the car­bon nan­otubes (CNTs) are con­sid­ered as poten­tial elec­trode mate­r­i­al for future high per­for­mance super­ca­pac­i­tor. The CNTs are direct­ly grown on elec­tro­spun CNFs giv­ing the spe­cif­ic capac­i­tance of 92 F/​g, i.e. twice the capac­i­tance of bare CNFs. Final­ly, a com­plete ener­gy stor­age device coin-cell super­ca­pac­i­tor is made by direct­ly grow­ing VAC­N­Fs on the cur­rent col­lec­tor and the capac­i­tance is 15 times high­er than the capac­i­tance with­out CNFs. Thus such super­cap­caitor is suit­able to be com­bined with har­vester to col­lect ener­gy to the lev­el of oper­at­ing pow­er of the devices and can pro­vide durable solu­tion to the fre­quent change of bat­tery in the devices mount­ed at sen­si­tive or air­borne locations.

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