Scale-up of enface electrochemical reactor systems

Abstract

Photolithography, the standard pattern transfer technique, has many sustainable issues due to the application of a mask to the substrate. A ‘maskless’ pattern transfer method, called the Enface technique, has recently been proposed for metal plating and etching. This method introduces the idea of bringing a patterned tool and a substrate together in close proximity and a current or voltage is passed between them enabling metal to be selectively deposited or removed from the substrate. The process requires sufficient electrolyte agitation within a narrow inter-electrode gap and has previously been shown to hold in a vertical flow channel reactor. However, the process has to be adapted for tank-type systems for industrial implementation. Mass transfer during electrodeposition can be enhanced by ultrasonic waves. It has therefore been investigated whether this would be an appropriate agitation method for Enface. In order to scale-up the process, 3 types of Enface reactors were investigated; a vertical flow cell, a 500 ml lab-scale tank-type cell and an 18 L ultrasound plating tank. The limiting current technique was used to study the mass transfer in these systems. Electrodeposition of copper pattern features in 0.1 M CuSO4 was achieved in each of these geometries. The scalability was quantified by measuring the uniformity of deposit roughness and deposit thickness of the features across the substrate using profilometry. The lab-scale tank-type cell with a 20 kHz ultrasound probe was used to investigate the effect of ultrasound agitation within narrow inter-electrode gaps. Mass transfer correlations showed that turbulent flow becomes fully developed when using ultrasound in this narrow geometry. Limiting current experiments showed that relatively low ultrasound powers of 9 – 18 W/cm2 should be used and current distribution modelling showed that the ultrasound source should be placed no less than 30 mm from the substrate. Copper pattern features were deposited onto 10 mm diameter substrates and using long current pulses with bursts of ultrasound during the off-time was the most suitable plating mode. Specially designed electrode holders in the large-scale 18 L ultrasound tank was used to deposit copper patterns onto larger substrates. Features of μm-scale were deposited onto A7 size substrates, but there was an unacceptable variation in deposit thickness of ±80% due to the non-uniformity of the electrode gap across the plate. However, mm-scale features were successfully deposited onto A7 size substrates with an acceptable deposit thickness uniformity and deposit roughness uniformity of ±18% and ±40% respectively across the plate. Enface is therefore currently scalable for mm-scale features on substrates of this size.