CupraEtch DE - Recyclable Anisotropic Etch: Differential Etch for SAP Manufacturing
Authors: Hotz Stephan , Haidar Rami , Lamprecht Sven , Luetzow Norbert 
Published: October 2015

The demand for ever finer circuitry especially for IC-substrate manufacturing has lead the way from the traditional subtractive circuit formation to additive, semi-additive, and modified semi-additive technology. Fully additive processing remains a niche technology, while semi-additive (SAP) and especially modified semi-additive processing (mSAP) are already widely used in the IC-substrate manufacturing business.

Both SAP and mSAP require a copper seed layer in order to be able to pattern plate the desired circuitry. In SAP this seed layer consists only of a layer of electroless copper, with a thickness ranging from 0,3 μm to 1,5 μm depending on the design and manufacturer. Therefore after pattern plating and resist stripping only the thin electroless copper seed layer needs to be removed for circuit formation. Considering mSAP several different variations exist. In some cases the seed layer consists only of sputtered copper, in others it is a layer of electroless copper with strike copper plating as protective layer, and in other cases half-etched copper panels are being used. Depending on which type of mSAP was applied the seed layer thickness can be in the nanometer range but also up to 10 μm.

Nevertheless for both SAP and mSAP the copper seed layer has to be removed through etching to finalize the circuit formation. Typical etching solutions contain sulfuric acid and hydrogen peroxide in addition to organic stabilizers and banking agents. Two draw-backs have been observed with peroxide based etchants; firstly the solution requires feed and bleed operation to maintain the maximal copper content and to replenish spent oxidizer, and secondly peroxide based etchants etch three-dimensionally with the same etching speed no mater if sprayed or in immersion. The first draw back has economical as well as ecological effects, since considerable amounts of chemical waste is being generated and thereby requires waste treatment. The second drawback has functional effects, since the three-dimensional etching causes undercut of the conductor tracks of several micrometers, thereby affecting the mechanical stability of the track as well as the electrical properties (i.e. impedance control).

In order to prevent these two draw-backs of the typical peroxide based etchants a different etchant system has been developed. The novel etchant is based on ferric sulfate and thereby offers the possibility to regenerate the solution in bypass equipment, therefore eliminating the need for feed and bleed operation. Furthermore, besides regenerating the oxidizer pure copper is plated, which could either be re-used internally or sold to recyclers. In addition this ferric sulfate based etchant causes minute to none undercut eliminating the second draw back of peroxide based etchants.

This paper describes the newly developed ferric sulfate based etchant. The focus will be on the etch performance in comparison to hydrogen peroxide etchants. In addition regeneration equipment suitable for this application will be illustrated and discussed, especially under economical and ecological aspects.