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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.

{1365}
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ref: -0 tags: polyimide precursors date: 01-22-2017 06:03 gmt revision:1 [0] [head]

Dianhydride:

Dianiline / diamine:

{1364}
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ref: -0 tags: polyimide aqueous degradation kapton date: 01-22-2017 05:51 gmt revision:0 [head]

Aqueous degradation of polyimides

  • Above ph 2, Kapton (PMDA-ODA) test specimens decreased both tensile strength and elongation to break with water, with a rate that increased with temperature.
  • No samples completely degraded, however; tensile strength decreased by about 2x, and elongation from 30% to 5%.
  • The authors suspect that ortho (off-molecular axis) amide bonding, at about 0.6% of the total number of imide bonds, is responsible for this (otherwise the film would completely fall apart.)
  • Imide bonds themselves are robust to all but strong bases and acids.
  • See also {1253}.

{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.

{1305}
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ref: -0 tags: graphene polyimide polymerization date: 01-22-2017 05:20 gmt revision:3 [2] [1] [0] [head]

Preparation and properties of graphene oxide/polyimide composite films with low dielectric constant and ultrahigh strength via in situpolymerization

  • The GO/PI composite films provide ultrahigh tensile strength (up to 844 MPa) and Young's modulus (20.5 GPa).
    • Almost 10x increase in tensile strength!
    • And even larger increase in modulus.
  • Also, you can reduce graphene / graphite oxide with an infrared laser: http://pubs.acs.org/doi/abs/10.1021/nn204200w

{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.

{1322}
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ref: -0 tags: polyimide silicon carbide adhesion DBS syle electrodes date: 07-22-2015 18:01 gmt revision:0 [head]

PMID-25571176 Fabrication and characterization of a high-resolution neural probe for stereoelectroencephalography and single neuron recording.

  • Layer stack:
    • 5um PI (UBE U-varnish S)
    • 50nm SiC
      • Deposited at 100C.
    • 300nm Pt
    • 30nm SiC
    • 10nm DLC
    • 5um PI
      • Cured at 450C
    • 100nm Al hard mask (removed)
    • Cytop dry adhesion layer
      • softbake to remove solvent,
      • then hardbake at 290C for 4 hours to anneal the PI and adhere the Cytop to it.

{1319}
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ref: -0 tags: polyimide epoxy potassium hydroxide etch adhesion date: 06-25-2015 00:28 gmt revision:0 [head]

Improvement in the adhesion of polyimide/epoxy joints using various curing agents

  • Used 1M KOH, ~2min, followed by 0.2M HCl for 6 min to ring-open the imide.
  • PMDA/ODA polyimide (Pyromellitic Dianhydride, single aromatic ring + 4,4 diamino diphenyl ether )
  • Epoxy of the DGEBA + linear amide or aromatic (3,3 methylenedianiline)
  • Best result was with a polyamide curing agent, and high-temp curing profile. Unlikely that this will work for us, parylene will decompose..

{1317}
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ref: -0 tags: tantalum chromium polyimide adhesion date: 06-24-2015 23:20 gmt revision:1 [0] [head]

Tantalum and chromium adhesion to polyimide. Part 2. Peel and locus of failure analyses

  • CF4 etch followed by Ar sputter yielded the strongest bond to the PI.
  • Suggest that failure may be within the PI (cohesive), not between the PI and metal (adhesive).

Tantalum, tantalum nitride, and chromium adhesion to polyimide: effect of annealing ambient on adhesion

  • The peel adhesion at T-0 (initial) shows the following order: TaNx∼ TaN < Ta∼ Cr, with all samples failing in apparently virgin PI.
  • After ten thermal cycles to 400°C
    • in forming gas the peel adhesion showed the following trend: TaNx < TaN∼ Ta ∼ Cr,
    • whereas if the annealing was done in N2 the order changed to TaNx∼ TaN « Ta < Cr.
  • The peel locus of failure was
    • always in the apparently virgin PI in the Cr/PI samples,
    • while the Ta/PI samples failed in the modified PI,
    • and the TaN/PI and TaNx/PI samples failed between the Ta-nitride and the Cu peel backing film after thermal cycling.

{1316}
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ref: -0 tags: polyimide adhesion chromium copper tie layer upilex date: 06-24-2015 23:14 gmt revision:3 [2] [1] [0] [head]

Adhesion Evaluation of Adhesiveless Metal/Polyimide Substrate for MCM and high density packaging

  • Adhesion degradation after thermal and humidity stresses can occur for a number of reasons.
    • Copper diffusion can promote adhesion loss at elevated temperatures and can be inhibited by coating a barrier layer of metal – tie layer2.
    • Oxygen diffusion through polyimide film to the metal/polyimide interface plays a critical role in promoting degradation too3. Adhesion of Cr/polyimide interface is degraded significantly upon exposure to high temperature and humidity environment due to the hydrolysis of polyimide4,5 .
    • Catastrophic adhesion loss has been linked to moisture induced oxidation of chromium interfaces based on studies using radioactively tagged water4, 5.
  • That said, most of these vendors use Cr (20nm) as and adhesion layer, and Cu (200nm) as the conductor.
  • Upilex A faired very well after the pressure cooker test -- > 60% retention after 192 hours.
  • Seemingly Ta and Cr both adhere similarly to PI -- {1317}
    • Though Ta is much more ductile, and forms a stronger carbide, Cr is preferred... cheaper?

{1306}
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ref: -2008 tags: tantalum chromium polyimide tungsten flexible neural implants adhesion layer date: 06-24-2015 22:53 gmt revision:2 [1] [0] [head]

PMID-18640155 Characterization of flexible ECoG electrode arrays for chronic recording in awake rats.

  • Yeager JD1, Phillips DJ, Rector DM, Bahr DF.
  • We tested several different adhesion techniques including the following: gold alone without an adhesion layer, titanium-tungsten, tantalum and chromium.
  • All films were DC magnetron sputtered, without breaking vacuum between the adhesion layer (5nm) and gold counductor layer (300nm).
  • We found titanium-tungsten to be a suitable adhesion layer considering the biocompatibility requirements as well as stability and delamination resistance.
  • While chromium and tantalum produced stronger gold adhesion, concerns over biocompatibility of these materials require further testing.
    • Thought: use tantalum directly, no Ti needed.
    • Much better than Cr -- much more ductile and biocompatible.
    • Caveat: studies showing reduction to stociometric Ta results in delamination.
  • Ta conductivity: 1.35e-7 Ohms * m; Ti 4.2e-7; 3x better (film can be 3x thinner..)

{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.

{1308}
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ref: -0 tags: polyimide polyamide basic reduction salt surface modification date: 02-27-2015 19:45 gmt revision:0 [head]

Kinetics of Alkaline Hydrolysis of a Polyimide Surface

  • The alkaline hydrolysis of a polyimide (PMDA-ODA) surface was studied as a function of time, temperature and hydroxide ion concentration.
  • Quantification of the number of carboxylic acid groups formed on the modified polyimide surface was accomplished by analysis of data from contact angle titration experiments.
  • Using a large excess of base, pseudo-first-order kinetics were found, yielding kobs ≈ 0.1−0.9 min-1 for conversion of polyimide to poly(amic acid) depending on [OH-].
  • From the dependence of kobs on [OH-], a rate equation is proposed.
  • Conversion of the polyimide surface to one of poly(amic acid) was found to reach a limiting value with a formation constant, K, in the range 2−10 L·mol-1.

{1307}
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ref: -2000 tags: polyimide acrylic aluminum electro deposition imide insulation ultra thin date: 02-27-2015 19:42 gmt revision:0 [head]

Ultrathin, Layered Polyamide and Polyimide Coatings on Aluminum

  • Alternating polyelectrolyte deposition of layered poly(acrylic acid)/poly(allylamine hydrochloride) (PAA/PAH) films on Al produces ultrathin coatings that protect Al from Cl--induced corrosion.
  • Resistance goes from 5 MOhm/cm^2 at 10nm thickness to ~50MOhm/cm^2 following imidization of the monolayer-applied polymer films.

{1265}
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ref: -0 tags: nickel chrome polyimide adhesion date: 10-11-2014 00:13 gmt revision:7 [6] [5] [4] [3] [2] [1] [head]

Adhesiveless copper on polyimide substrate with nickel-chromium tiecoat

  • Chrome works the best, with Nichrome lagging slightly behind. Thicker tie layers (20nm) work slightly better.
  • 17 nm Cr and 5nm NiCr both work well after gold plating
    • in aggressive cyanide solution -- without tie layer, the copper was released.
    • note how thin the layers are!
  • Surface benefits from oxygen plasma pre-treatment. (de-scum?)
  • Still not sure how to get second layer of polyimide to adhere to top layer of Cr.

Adhesion Between Polymers and Other Substances - A Review of Bonding Mechanisms, Systems and Testing

  • The adhesion between the polyimide, PMDA-ODA and metals such as copper or chromium has received considerable attention due to its importance in the microelectronics industries.
  • As mentioned, the PMDA-ODA is normally deposited from solution as the polyamic acid and cured in-situ to the imide form.
  • Adhesion of the polyimide deposited on a metal is therefore a different problem than adhesion of a metal deposited on the cured polyimide.
  • The former situation (polyimide on metal) tends to give stronger adhesion than the latter (metal on polyimide) but there can be problems of metal, particularly copper, dissolution.
  • Great! (is this a reliable source?)
  • The interaction between the metals and the polyimide has been studied in great detail using x-ray photoelectron spectroscopy (XPS) and other surface analysis techniques but there is not complete agreement on the form of the interaction.
    • It is clear that strong interaction and electron transfer occurs when the metal is deposited from vapour onto the polyimide.
    • When the polyamic acid is deposited on the metal and cured then reaction occurs between the acid and the metal.
  • The strong interface formed between chromium and the polyimide is clearly a result of the strong chemical interaction but there is still considerable interest in making it more resistant to water and oxidation.

High-Performance Polymers (book) Guy Rabilloud (via google books.)

  • Order of metals by increasing adhesion:
    • Cu, Pd, Ni, V, Cr, Nb, Ti [140]
  • The adhesion between chromium and polyimide is degraded sharply as the interface is exposed to temperature-humidity stressing (85C, 81% RH [612]
  • Polyimide-polyimide self-adhesion strongly benefits from partial cure of the first layer (which is not possible with lithographic processes, TMAH etches uncured film). Plasma and adhesion treatments would likely help, due to molecular tangling (?). Presumably VM-651 helps. We'll cross that bridge when we get to it.
  • PMDA-PPD or PMDA-PDA is perhaps the most rigid of all the polyimides, but due to the extremely hydrophillic nature of PMDA & associated electron affinity of the dianhydride ( E a ), and the fact that it tends to crystalize & not be tough/plastic, it's infrequently used.

{1277}
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ref: -0 tags: polyimide anodic release 2005 date: 06-16-2014 23:58 gmt revision:1 [0] [head]

IEEE-1416914 (pdf) Partial release and detachment of microfabricated metal and polymer structures by anodic metal dissolution

  • recommend 100nm Cr/Al release layer.
  • finished devices just 'float to the surface' of saline solution.

{1276}
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ref: -0 tags: polyimide silicon oxide aluminum adhesion pressure cooker date: 06-16-2014 21:28 gmt revision:2 [1] [0] [head]

Interfacial adhesion of polymeric coatings for microelectronic encapsulation

  • Find that, after a pressure-cooker test, adhesion of polyimide PI-2610 (what we use) to SiO2 was weaker than to Al, SiN, and copper.
  • Aluminum adhesion is quite good, at least to (only) 15 days @ 85C / 85% RH. Reference studies that find the adhesion to be 'acceptable' for the microelectronics industry.
    • Should we use an aluminum adhesion layer? Less biocompatible metal than Ti, and more likely to degrade in saline.
  • Found that copper adhesion actually went up with water exposure!
  • Polyimide adheres more strongly to glass than epoxy following accelerated aging.

{1282}
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ref: -0 tags: polyimide adhesion aluminum integrated circuit date: 05-07-2014 19:29 gmt revision:0 [head]

Polyimide insulators for multilevel interconnections Arthur M. Wilson

  • Old article (1981), but has useful historical information on the development of various PI insulators and their adhesion to aluminum, SiOx, etc.
  • Suggests that a higher-temperature cure (400C) is needed to fully drive water from the PI & cause a glass-transition. Might want to do this for the second PI layer.

{1275}
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ref: -0 tags: polyimide adhesion oxygen nitrogen plasma surface energy date: 03-10-2014 22:33 gmt revision:0 [head]

Adhesion Properties of Electroless-Plated Cu Layers on Polyimide Treated by Inductively Coupled Plasmas

  • O2 then N2/H2 ICP treatment of polyimide surfaces dramatically lowers the surface energy (as measured by contact angle), and increases the adhesion of palladium-catalyzed electroless copper.
  • Particularly, C-N bonds are increased as revealed by XPS.
  • No peel-strength measurements given.

{1274}
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ref: -0 tags: flexible neural probe polyimide silicon polyethylene glycol dissolvable jove livermore loren frank date: 03-05-2014 19:18 gmt revision:0 [head]

http://www.jove.com/video/50609/insertion-flexible-neural-probes-using-rigid-stiffeners-attached-with

  • details the flip-chip bonding method (clever!)
  • as well as the silicon stiffener fabrication process.

{1203}
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ref: Cheung-2007.03 tags: flexible electrode array Michigan probe histology Vancouver current source density EPFL polyimide date: 12-21-2013 21:07 gmt revision:5 [4] [3] [2] [1] [0] [head]

PMID-17027251[0] Flexible polyimide microelectrode array for in vivo recordings and current source density analysis.

  • Polyimide -- PI-2611 precusor.
  • 50nm Ti adhesion, 200nm Pt, both sputtered.
  • Electrodes etched via RIE in Cl2.
    • Sputtered and photo-patterned SiO2 etch mask.
  • Used regular solder to connect to a Samtec.
  • 15um total thickness.
  • 25um electrode diameter.
  • They were inserted directly (no carrier nor guide) into the brain; can be re-used.
  • Tested to 8 weeks.
  • No figure comparing silicon and polyimide, though they claim minimal GFAP response to the electrodes.

____References____

[0] Cheung KC, Renaud P, Tanila H, Djupsund K, Flexible polyimide microelectrode array for in vivo recordings and current source density analysis.Biosens Bioelectron 22:8, 1783-90 (2007 Mar 15)

{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}

{1259}
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ref: -0 tags: polyimide flexible cable frontiers florida date: 10-04-2013 01:55 gmt revision:0 [head]

PMID-24062716 A highly compliant serpentine shaped polyimide interconnect for front-end strain relief in chronic neural implants.

  • Sankar V, Sanchez JC, McCumiskey E, Brown N, Taylor CR, Ehlert GJ, Sodano HA, Nishida T.
  • 20um polyimide / gold / 20um polyimide.
    • No tie layer; then again, no longevity testing either.
  • Used sacrificial aluminum coating to release polyimide.

{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.

{1242}
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ref: -0 tags: ACF chip bonding parylene field's metal polyimide date: 07-10-2013 18:34 gmt revision:10 [9] [8] [7] [6] [5] [4] [head]

We're making parylene electrodes for neural recording, and one critical step is connecting them to recording electronics.

Currently Berkeley uses ACF (anisotropic conductive film) for connection, which is widely used for connecting flex tape to LCD panels, or for connecting driver chips to LCD glass. According to the internet, pitches can be as low as 20um, with pad areas as low as 800um^2. source

However, this does not seem to be a very reliable nor compact process with platinum films on parylene, possibly because ACF bonding relies on raised areas between mated conductors (current design has the Pt recessed into the parylene), and on rigid substrates. ACF consists of springy polymer balls coated in Ni and Au and embedded in a thermoset epoxy resin. The ACF film is put under moderate temperature (180C) and pressure (3mpa, 430psi), which causes the epoxy to cure in a state that leaves the gold/nickel/polymer balls to be compressed between the two conductors. Hence, even if the conductors move slightly due to thermal cycling, the small balls maintain good mechanical and electrical contact. The balls are dispersed sufficiently in the epoxy matrix that there is little to no chance of conduction between adjacent pads.

(Or so I have learned from the internet.) Now, as mentioned, this is an imperfect method for joining Pt on parylene films, possibly because the parylene is so flexible, and the platinum foil is very thin (200-300 nm). Indeed, platinum does not bond very strongly to parylene, hence care must be taken to allow sufficient overlap to prevent water ingress. My proposed solution -- to be tested shortly -- is to use a low-melting temperature metal with strong wetting ability -- such as Field's metal (bismuth, tin, indium, melting point 149F, see http://www.gizmology.net/fusiblemetals.htm) to low-temperature solder the platinum to a carrier board (initially) or to a custom amplifier ASIC (later!). Parylene is stable to 200C (392F), so this should be safe. One worry is that the indium/bismuth will wet the parylene or polyimide, too; however I consider this unlikely due to the difficulty in attaching parylene to any metal.

That said, there must be good reason why ACF is so popular, so perhaps a better ultimate solution is to stiffen the parylene (or ultimately polyimide) substrate so that it can support both the temperature/pressure of ACF bonding and the stress of a continued electrical/mechanical bond to polyimide fan-out board or ASIC. It may also be possible to gold or nickel electroplate the connector pads to be slightly raised instead of recessed.


Update: ACF bond to rigid 1/2 oz copper, 4mil trace / space connector (3mil trace/space board):

Note that the copper traces are raised, and the parylene is stretched over the uneven surface (this is much easier to see with the stereo microscope). To the left of the image, the ACF paste has been sqeezed out from between the FR4 and parylene. Also note that the platinum can make potential contact with vias in the PCB.


Update 7/2: Fields metal (mentioned above) does stick to platinum reasonably well, but it also sticks to parylene (somewhat), and glass (exceptionally well!). In fact, I had a difficult time removing traces of field's metal from the Pyrex beakers that I was melting the metal with. These beakers were filled with boiling water, which may have been the problem.

When I added flux (Kester flux-pen 951 No-clean MSDS), the metal became noticeably more shiny, and the contact angle increased on the borosilicate glass (e.g. looked more like mercury); this leads me to believe that it is not the metal itself that attaches to glass, but rather oxides of indium and bismuth. Kester 951 flux consists of:

  • 2-propanol 15% (as a denaturing agent) boiling point 82.6C
  • Ethanol 73% (solvent) boiling point 78.3C
  • Butyl Acetate 7% boiling point 127C, flash point 27C
  • Methanol <3% b.p. 64.7C
  • Carboxylic acids < 3% -- proton donors? formic or oxalic acid?
  • Surfacants < 1% -- ?
Total boiling point is 173F.

After coating the parylene/platinum sample with flux, I raised the field's metal to the flux activation point, which released some smoke and left brown organic residues on the bottom of the glass dish. Then I dipped the parylene probe into the molten metal, causing the flux again to be activated, and partially wetting the platinum contacts. The figure below shows the result:

Note the incomplete wetting, all the white solids left from the process, and how the field's metal caused the platinum to delaminate from the parylene when the cable was (accidentally) flexed. Tests with platinum foil revealed that the metal bond was not actually that strong, significantly weaker than that made with a flux-core SnPb solder. also, I'm not sure of the activation temperature of this flux, and think I may have overheated the parylene.


Update 7/10:

Am considering electrodeless Ni / Pt / Au deposition, which occurs in aqueous solution, hence at much lower temperatures than e-beam evaporation Electrodeless Ni ref. On polyimide substrates, there is extensive literature describing how to activate the surface for plating: Polyimides and Other High Temperature Polymers: Synthesis ..., Volume 4. Parylene would likely need a different possibly more aggressive treatment, as it does not have imide bonds to open.

Furthermore, if the parylene / polyimide surface is *not* activated, the electrodeless plating could be specific to the exposed electrode and contact sites, which could help to solve the connector issue by strengthening & thickening the contact areas. The second fairly obvious solution is to planarize the contact site on the PCB, too, as seen above. ACF bonds can be quite reliable; last night I took apart (and successfully re-assembled) my 32" Samsung LCD monitor, and none of the flex-on-glass or chip-on-flex binds failed (despite my clumsy hands!).

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ref: -0 tags: polyimide aging deadhesion humidity water absorption date: 06-28-2013 02:07 gmt revision:1 [0] [head]

Environmental Aging and Deadhesion of Polyimide Dielectric films

  • At 35C, 85% RH (not immersion!) there was little degradation in the polyimide to 2000 hours.
  • Suggest chromium or titanium as an adhesion promoter & to prevent copper from diffusing into the polyimide.
  • Plasma treatment of polyimide is commonly used prior to metal deposition in order to improve adhesion of polyimide to metallization [20].
    • Clearfield, Furman, Callegari 1994 "The Role of Physical and Chemical Structure in the Long-term Durability of Metal/Polyimide Interfaces" International Journal of Microcircuits and electronic Packaging 17(3), pp. 228-35.

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ref: -0 tags: polyimide electrodes ecog japan photosensitive date: 06-28-2013 01:50 gmt revision:0 [head]

PMID-22719725 Photosensitive-polyimide based method for fabricating various neural electrode architectures

  • Yasuhiro X. Kato,1,* Shigeto Furukawa,2 Kazuyuki Samejima,1 Naoyuki Hironaka,2 and Makio Kashino2
  • many typos in this paper (not that I should talk..) Yet still, it's informative.
  • 12um thick photosensitive polyimide + Cr/Au fabrication.
  • Wet etch (photodeveloper).
  • Wet release (ferric chloride) from glass substrate.
  • Soldered a connector to the polyimide w/ stiffener.
  • Note that polyimide tends to shrink (up to 29%) during baking, unlike parylene!
  • Suggest 20-40um diameter neural recording sites; they did not coat.

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ref: -0 tags: polyimide platinum nanowire recording electrode plating date: 06-28-2013 00:46 gmt revision:2 [1] [0] [head]

IEEE-5734597 (pdf) A novel platinum nanowire-coated neural electrode and its electrochemical and biological characterization

  • Young-Hyun Jin ; IMTEK, Univ. of Freiburg, Freiburg, Germany ; Daubinger, P. ; Fiebich, B.L. ; Stieglitz, T.
  • 10um thick RIE etched polyimide and platinum electrodes.
  • polyimide was spin coated onto wafers.
  • Used relatively simple wet chemistry to plate platinum onto electrodes:
    • 0.14 M-% chloroplatin acid hexahydrate (H2PtCl6·6H2O, Sigma-Aldrich) and 7.4 M-% formic acid (HCOOH, Sigma-Aldrich) were mixed in de-ionized (DI) water. The fabricated device was floated upside down on the solution.
  • Let this plate for 7 days & effective site was enlarged by 617 times!