An Assessment of Surface Treatments for Adhesion of Polyimide Thin Films

Abstract

Polyimide films are currently of great interest for the development of flexible electronics and sensors. In order to ensure a proper integration with other materials and PI itself, some sort of surface modification is required. In this work, microwave oxygen plasma, reactive ion etching oxygen plasma, combination of KOH and HCl solutions, and polyethylenimine solution were used as surface treatments of PI films. Treatments were compared to find the best method to promote the adhesion between two polyimide films. The first selection of the treatment conditions for each method was based on changes in the contact angle with deionized water. Afterward, further qualitative (scratch test) and a quantitative adhesion assessment (peel test) were performed. Both scratch test and peel strength indicated that oxygen plasma treatment using reactive ion etching equipment is the most promising approach for promoting the adhesion between polyimide films.

Towards Long-Term Stable Polyimide-Based Flexible Electrical Insulation for Chronically Implanted Neural Electrodes

“…The samples which were not treated before the second polyimide coating (0 min) achieved only a marginal stability of 4.0 ± 3.8 days. This corresponded to the low adhesion strength between the two pristine polyimide layers [9]. Using the oxygen RIE plasma, the stability could be clearly increased; however, the deviation within the same plasma process was relatively large with up to 33.5 days (5 min at 25 W).…”

“…For neural implants, the electrical conducting paths composed of biocompatible and noble metals such as gold or platinum are embedded between two thin polyimide layers, which encapsulate the implant against the saline environment of the neural tissue [4]. For a long-term stable electrical insulation of the conducting paths, the adhesion between the two polyimide layers must be strong enough to prevent delamination, which is a challenge due to the chemical inertness of polyimide after it is completely polymerized [9]. This long-term stability is especially important for active neural implants, where electrical current is also applied for electrical stimulation of neural activity.…”

“…In a previously published paper, we assessed different surface treatments of polyimide to evaluate the adhesion strength between two polyimide layers [9]. As a result, the samples treated with oxygen reactive ion etching plasma were not peelable at all thus indicating the best approach to promote a strong adhesion between the PI layers.…”

“…This can lead to an increased signal crosstalk and, in the worst case, to a signal loss. To improve this electrical insulation stability of a polyimide/metal/polyimide sandwich layer in a saline environment, different methods, e.g., the chemical or oxygen plasma activation of the first polyimide layer surface, are used [8,9]. The current state-of-the-art method is still the use of a short oxygen reactive ion etching (RIE) treatment process to increase the polyimide to polyimide adhesion [10].…”

“…polyimide layers, which increased the parasitic capacitance be paths. This can lead to an increased signal crosstalk and, in the To improve this electrical insulation stability of a polyimide/m layer in a saline environment, different methods, e.g., the chem tivation of the first polyimide layer surface, are used [8,9]. T method is still the use of a short oxygen reactive ion etching increase the polyimide to polyimide adhesion [10].…”