Toward Thin Dielectric Films of Polyetherimide Electrodeposited from Aqueous Emulsions of the Polymer
Authors: Seurot Alexandre , Hoang Antoine , Jousseaume Vincent , Haumesser Paul-Henri 
Published: September 2023

Miniaturized, high power electronic devices are needed for a variety of applications. Although, the miniaturization of these devices is challenging. For instance, high power capacitors need to sustain high electric fields, with typically several hundred volts across micrometric dielectric layers. Among the most promising materials for this application, polyetherimide (PEI) has a number of desired characteristics such as adequate electrical properties, good chemical and mechanical resistance with thermal stability up to 170°C.

Spin-coating is the conventional technique used in the microelectronics industry for polymer thin film deposition. However, this technique is not appropriate to form conformal deposits on 3D-structured substrates. To overcome this limitation, we investigate an electrochemical deposition process of PEI in which the electrolyte is an aqueous emulsion formed from organic droplets containing the polymer [1]. In this work, a pulsed voltage below 2 V/ECS, was applied for the deposition of PEI on Cu metal surfaces. The influence of deposition conditions on the thickness and surface smoothness of the deposit was thoroughly investigated, identifying the processing window.

PEI aqueous emulsions were synthesized as follows, after the raw material dissolved in a mixture of N-methylpyrrolidone and acetophenone, N-methylpiperazine was added to be grafted on the imide groups of the polymeric chains. The PEI emulsions were subsequently produced by ultrasonic mixing [2] of the reaction crude with an aqueous solution of lactic acid. During mixing, nanoscale droplets were formed and electrostatically charged by protonation of ternary amine groups of N-methylpiperazine.

Critical for the application are the droplet size and stability of the emulsion. These properties were modulated upon stoichiometry of both N-methylpiperazine for the degree of modified imide cycles and lactic acid for the degree of protonation of the amine [1,2]. FTIR and NMR were used to screen the quantity of modified imide groups, displaying that the PEI imide function attack by N-methylpiperazine is quantitative. The protonation was indirectly characterized by measuring the zeta potential of the emulsion [3]. As a result, in an optimized sequence, droplets with hydrodynamic diameters below 200 nm were repeatedly formed with zeta potentials greater than 50 mV. For emulsions containing 3.5 wt% PEI with 64 % modified imide groups and 50 % quaternisation, representing 32 % of the imide cycle initial population. The emulsions were stable for at least twenty-one days at 4°C. This is a significant improvement as compared to the stability reported in the literature [1].

Deposition tests were conducted in a 3-electrode electrochemical cell, using a 10-time diluted emulsion, at relatively low voltage, below 2 V/ECS with pulsed DC waveform under mild conditions (room temperature, pH between 3 and 5). The pulses allowed the movement of droplets in the emulsion towards the cathode while minimizing H2 evolution. Under these conditions, a PEI coating was obtained with a thickness of a few micrometers within 600 s. The impact of deposition parameters on coverage and uniformity was also studied in detail.

The last process step by removing the N-methylpiperazine and the residual solvents consists in recovering the initial unmodified structure of PEI grafted on the substrate. Curing conditions were chosen according to the previous literature [1] under vacuum. The FTIR spectra of the coated substrate were recorded after curing. From the FTIR spectra, the disappearance of the amide (I and II) peaks after curing and a good peak-for-peak correspondence between the spectrum of the cured polymer and the pristine polymer are observed.

This work demonstrates that polymer films can be deposited by an electrochemical process under mild conditions, assuming that a stable emulsion of charged polymer droplets can be formed. The process is well suited to coat 3D structured substrates, in contrast with more conventional techniques for polymer deposition. This original electrodeposition process should allow the realization of integrated capacitors on silicon wafers for the next generation of power electronics devices.

References:

[1] H. QARIOUH et al. Polym Int. 1999, Vol. 48, p. 1183-1192

[2] Y. LEE et al. J. Coll. Interf. Sci. 2019, Vol. 536, p. 281-290

[3] L. YANG et al. Surf. Eng. 2012, Vol. 28, nᵒ 8, p. 585-589