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{1250}
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ref: -0 tags: polyimide electrodes thermosonic bonding Stieglitz adhesion delamination date: 03-06-2017 21:58 gmt revision:7 [6] [5] [4] [3] [2] [1] [head]

IEEE-6347149 (pdf) Improved polyimide thin-film electrodes for neural implants 2012

  • Tested adhesion to Pt / SiC using accelerated aging in saline solution.
  • Targeted at retinal prostheses.
  • Layer stack:
    • 50nm SiC deposited through PECVD @ 100C using SPS, with low frequency RF modulation.
    • 100nm Pt
    • 100nm Au
    • 100nm Pt
      • These layers will alloy during cure, and hence reduce stress.
    • 30nm SiC
    • 10nm DLC (not needed, imho; PI sticks exceptionally well to clean SiC)
  • Recent studies have concluded that adhesion to PI is through carbon bindings and not through oxide formation.
    • Adhesion of polyimide to amorphous diamond-like carbon and SiC deteriorates at a minimal rate.
  • Delamination is caused by residual stress, which is not only inevetable but a major driving force for cracking in thin films.
    • Different CTE in layer stack -> different contraction when cooling from process temperature.
  • Platinum, which evaporates at 1770C, and is deposited ~100C (photoresists only withstand ~115C) results in a high-stress interface.
    • Pt - Carbon bonds only occur above 1000C
  • After 9 and 13 days of incubation the probes with 400 nm and 300nm of SiC, respectively, which were not tempered, showed complete delamination of the Pt from the SiC.
    • 60C, 0.9 M NaCl, 1 year.
    • The SiC remained attached to the PI.
      • Tempering: repeated treatment at 450C for 15 min in a N2 atmosphere.
    • All other probes remained stable.
  • Notably, used thermosonic bonding to the PI films, using sputtered (seed layer) then 12um electroplated Au.
  • Also: fully cured the base layer PI film.
  • Used oxygen plasma de-scum after patterning with resists to get better SiC adhesion to PI.
    • And better inter-layer adhesion (fully cured the first polyimide layer @ 450C).
  • Conclusion: "The fact that none of the tempered samples delaminated even after ~5 years of lifetime (extrapolated for 37 C) shows a tremendous increase in adhesion.

{1253}
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ref: -0 tags: polyimide stieglitz stability date: 01-22-2017 05:35 gmt revision:1 [0] [head]

PMID-20144477 In vitro evaluation of the long-term stability of polyimide as a material for neural implants

  • PI degrades at 85C in PBS; otherwise, it's stable.
  • mechanical tests only; no electrical tests.
  • Durimide 7510 contains a photo-initiator and an adhesion promoter. Spin-coatable.
    • Adhesion can be inhibited with C4F8
    • notably softer.
  • Dupont Kapton is PMDA-ODA (phenol linkage in the amide); PI-2611 is BPDA-PPD (aromatic carbon-carbon in the dicarboxcylic acid). The latter resists water uptake better.

{1266}
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ref: -0 tags: polyimide adhesion delamination Stieglitz date: 08-18-2015 22:19 gmt revision:1 [0] [head]

Thin films and microelectrode arrays for neuroprosthetics

  • Juan Ordonez, Martin Schuettler, Christian Boehler, Tim Boretius and Thomas Stieglitz
  • Discussion of adhesion & ideas of using siliconcarbides as opposed to adhesion promoters (Silane A-174) to maintain good metal-polymer adhesion even with an equilibrium water vapor pressure.
  • Transition metals form carbide bonds with polyimide, but noble metals do not.
  • A one-metal (preferably noble) system is advantageous, as two metals will form a galvanic cell and eventually corrode.
  • Therefore it's best to develop non-metallic non-toxic adhesion promotion technologies.

{1251}
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ref: -0 tags: microflex interconnect polyimide Stieglitz date: 03-03-2015 00:33 gmt revision:1 [0] [head]

IEEE-938305 (pdf) High Density Interconnects and flexible hybrid assemblies for active biomedical implants

  • Idea: make vias in your metallized PI film. Bump-bond through these vias to a chip below.
  • Achieve center-to -center distances of 100um.
  • No longer using this? See {1250}, which uses thermosonic bonding.

{1252}
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ref: -0 tags: parylene metal adhesion Stieglitz date: 08-15-2013 17:22 gmt revision:0 [head]

PMID-20119944 Characterization of parylene C as an encapsulation material for implanted neural prostheses.

  • On Si3N4, platinum, and a first film of parylene-C, satisfactory adhesion was achieved with silane A-174, even after steam sterilization. (>1 N/cm)
  • higher adhesion for the parylene that was deposited at lower pressures.
  • but: higher deposition pressures results in lower crystalinity.
  • [33] parylene can be used to build freestanding nanowires.
  • Parylene does not stick to polyimide.
  • Parylene sticks to parylene well if left untreated.
  • Annealing parylene dramatically increased crystalinity / decreases elongation to break.
  • The deposited parylene C layers on untreated and with oxygen plasma-treated samples delaminated immediately after contact with saline. The behavior was also observed at two out of three samples of the A-174 treated wafers, but not in this magnitude.
    • A potential reason for these results could be contamination of the samples during assembly or excessive treatment with the adhesion promoter.