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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.

{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.

{1258}
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ref: -0 tags: polyimide platinum electrodes Spain longitudinal intrafasicular adhesion delamination date: 10-05-2013 22:24 gmt revision:4 [3] [2] [1] [0] [head]

PMID-17278585 Assessment of biocompatibility of chronically implanted polyimide and platinum intrafascicular electrodes. 2007

  • Designed platinum/polyimide longitudinal intrafasicular electrodes (LIFEs)
    • 25um PT/Ir, insulated to 60-75um diameter. PT/IR has a young's modulus of 202 Gpa.
      • Plated with platinum black under sonication, as this forms a tougher surface than without sonication.
      • See also: PMID-20485478 Improving impedance of implantable microwire multi-electrode arrays by ultrasonic electroplating of durable platinum black. Desai SA, Rolston JD, Guo L, Potter SM. 2010
    • Polyimide PI2611, 10um thick, 50mm long, 220um wide in the electrode segment.
  • Implanted into rat sciatic nerve for 3 months.
  • These electrodes have been tested in people for two days:
    • Electrical stimulation through the implanted electrodes elicited graded sensations of touch, joint movement, and position, referring to the missing limb. This suggested that peripheral nerve interfaces could be used to provide amputees with prosthetic limbs with sensory feedback and volitional control that is more natural than what is possible with current myoelectric and body-powered prostheses.
  • CMAPs = compound muscle action potentials.
  • CNAPs = compound nerve action potentials.
  • Platinum wire LIFE performed very similarly to the thin-film polyimide LIFE in most all tests, with slightly higher potentials recorded by the larger polyimide probe.
  • 'Higher encapsulation with the polyimide probes! Geometry?
  • However, the polyimide LIFEs induced less functional decline than the wire LIFEs.
  • Other polyimide studies [14] [16] [24] -- one of which they observed a 70% reduction of tensile strength after 11 months of implantation.
    • [14] F. J. Rodríguez, D. Ceballos, M. Schüttler, E. Valderrama, T. Stieglitz, and X. Navarro, “Polyimide cuff electrodes for peripheral nerve stimulation,” J. Neurosci. Meth., vol. 98, pp. 105–118, 2000.
    • [16] N. Lago, D. Ceballos, F. J. Rodríguez, T. Stieglitz, and X. Navarro, “Long term assessment of axonal regeneration through polyimide regenerative electrodes to interface the peripheral nerve,” Biomaterials, vol. 26, pp. 2021–2031, 2005.
    • [24] M. Schuettler, K. P. Koch, and T. Stieglitz, “Investigations on explanted micromachined nerve electrodes,” in Proc. 8th Annu. Int. Conf. Int. Functional Electrical Stimulation Soc., Maroochydore, Australia, 2003, pp. 306–310.
      • The technology of sandwiching a metallization layer between two layers of polyimide seems to be suitable, because no delamination of the polyimide layers was observed even after 11 months. The right choice of metals for building the electrical conductive elements of the microelectrodes is crucial. Ti/Au/Ti/Pt layers tend to flake off from polyimide while delamination of Ti/Pt layers was not observed. However, adhesion of Ti/Pt layers was investigated after 2.5 months of implantation while Ti/Au/Ti/Pt layers were exposed after 11 months to the biological system. In previous research projects, surgeons also reported on delamination of Ti/Au layers from polyimide substrate after three months. Unfortunately, we had no possibility of inspecting these microelectrodes in our laboratory.
      • See also {1250}

{1248}
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ref: -0 tags: polyimide adhesion silver surface treatment adhesion delamination date: 10-04-2013 01:30 gmt revision:8 [7] [6] [5] [4] [3] [2] [head]

Improved polyimide/metal adhesion by chemical modification approaches

  • Suggest fuming sulfuric acid (H2S04) + Ag2SO4 for 30s as the most effective treatment.
  • 1 minute in 1M KOH also effective.
  • Silver was magnetron-sputtered on; peel test performed with tape.

IEEE-4936772 (pdf) Studies of adhesion of metal films to polyimide

  • Suggest Ar / O2 plasma treatment of surface to increase Cr/Cu adhesion (mechanical effect?)
  • Used two different polyimides: one derived from (BPDA‐PDA) polyamic acid, and pyromellitic dianhydride‐4,4’‐oxydianiline (PMDA‐ODA).

IEEE-670747 (pdf) Adhesion evaluation of adhesiveless metal/polyimide substrate for MCM and high density packaging

  • Adhesion of Cr / polyimide interface is degraded significantly upon exposure to high temperature and humidity environment due to the hydrolysis of polyimide.
  • There is also some worry of Cu diffusion into the polyimide.
  • All used a Cr tie layer, 200A thick (20nm).
  • Deposited photoresist, electroplated copper, then etched to define pattern.
  • Testing performed at 121C 100% RH, +15psi. (tough!)

On polyimide-polyimide interlayer adhesion: Diffusion and self-adhesion of the polyimide PMDA-ODA (1987)

  • Diffusion occurred during the curing process of the second layer and was controlled by the cure schedule.
  • It was found that a large diffusion distance, at least 200 nm, was required to obtain a bond whose strength was equal to that of bulk material.
  • Good protocol:
    • Dry first layer at 80C for 30 minutes.
    • 150C (or lower?) bake of first layer. "as the polyamic acid imidizes (and the solvent is lost) its diffusive mobility decreases rapidly; very little diffusion occurs after the first few minutes of the second bake.
    • Spin coat second layer.
    • 400C second bake.
  • Ductility is increased for polyimide that has experienced a series of increasing cure temperatures.
  • In this context it is worth noting that the contour length of a PMDA-ODA of 30,000 molecular weight is about 130nm, a value very similar to the diffusion distances measured when T1 (first layer bake) was 150C.