Electrochemical Deposition of Polyetherimide (PEI) Thin Films from Aqueous Emulsions for Structured Substrates: Towards Advanced Integrated Capacitors in Power Electronics

In the pursuit of miniaturized high-power electronic devices, the demand for robust high-power capacitors capable of withstanding electric fields in the range of several hundred volts across micrometric dielectric layers has emerged. To miniaturize these capacitors, one possibility is to create 3D structures in a silicon wafer to maximize capacitance density by expanding the dielectric surface area. However, this approach encounters difficulties when trying to combine it with conventional inorganic dielectrics (such as SiO₂), due to the mechanical stress induced by thick films. Among suitable dielectric materials to build such devices, polyetherimide (PEI) is highly promising due to its exceptional chemical resistance and thermal stability up to 170°C. The conventional technique of spin-coating for polymer thin film deposition, however, falls short in achieving conformal deposits on 3D-structured substrates with large developed surface area. To address this limitation, we explore an electrochemical deposition process for PEI, employing an aqueous emulsion of organic droplets containing the polymer as the electrolyte.[1]<br/>In this study, we grafted N-methylpiperazine onto the imide groups of raw PEI polymeric chains within a solvent mixture. The grafting reaction is confirmed to be quantitative through FTIR and NMR measurements. The addition of an aqueous lactic acid solution, followed by ultrasonic mixing, led to the formation of emulsions encompassing modified PEI organic droplets. These droplets exhibited an approximate diameter of 40 nm, as observed through Cryo-TEM, and zeta potentials exceeding 50 mV. These emulsions were diluted by a factor of 10 with the addition of deionized water, and were stable for at least several weeks.<br/>Deposition experiments were conducted within a 3-electrode electrochemical cell, under mild conditions (room temperature, pH ranging from 3 to 4), utilizing Cu metal surfaces. A pulsed DC waveform with an amplitude below 2 V/SCE was employed to mitigate hydrogen evolution. Within a timeframe of 600 seconds, a PEI coating with a thickness of a few micrometers was successfully obtained.<br/>Remarkably, our results demonstrate comparable film thickness to existing literature [1], despite using an emulsion with 20 times dilution and voltages 40 times lower. Our comprehensive investigation into the influence of deposition factors on coverage and uniformity has revealed the significant impact of hydrodynamic conditions and the pivotal role played by the pulse/tension combination in achieving distinct morphology and thickness. SEM inspection showcased a uniform film at the center with iridescence.<br/>Following deposition, the film was thermally cured under vacuum conditions to eliminate solvents and restore the imide groups of PEI.[1] FTIR analyses confirmed excellent agreement between IR spectra of the cured and pristine polymers, as well as the elimination of amide (I and II) peaks.<br/>This work establishes the feasibility of electrochemically depositing insulating polymeric films under mild conditions, enabled by the formation of stable emulsions containing charged polymeric droplets. This innovative electrodeposition process holds immense promise for coating substrates with complex 3D structures, contrasting with conventional dielectric deposition techniques. Importantly, this original methodology presents a potential avenue for fabricating integrated capacitors on silicon wafers, paving the way for the next generation of power electronics devices.<br/>[1] H. QARIOUH <i>et</i> <i>al</i>. Polym Int. 1999, Vol. 48, p. 1183-1192