Microfabrication By High Rate Anodic Dissolution: Fundamentals and Applications
Authors: Datta Madhav 
Published: February 2020

Electrochemical metal shaping and finishing processes involve controlled material removal from an electrically conducting workpiece which is made an anode in an electrolytic cell. These processes are employed in a variety of applications ranging from through-mask fabrication of microstructures in thin films and foils to high speed electrochemical machining (ECM) of complex shaped large components such as turbine blades.

    Mass transport mechanism and diffusion layer thickness play an important role in high rate anodic dissolution processes (1,2). Mass transport controlled limiting current density influences the dissolution rate and the morphology of dissolved surface.  Below the limiting current, surface etching reveals crystallographic steps, etch pits, preferred grain boundary attack, or finely dispersed microstructure. At or above the limiting current, salt films formed at the surface suppress the influence of crystallographic orientation and surface defects yielding electropolished surfaces.  Component-specific precision tool design is essential for achieving the desired machining performance.  The tool must provide high electrolyte flow velocity in order to achieve high rate of mass transport at the anode.  The use of pulsating current provides an additional possibility of influencing the mass transport conditions at the anode surface (3). 

    Microfabrication by through-mask electrochemical micromachining (EMM) requires an understanding of some of the complexities and challenges associated with the process. They include: (I) elimination of the loss of electrical contact due to island formation in one sided through-mask EMM (ii) uniformity of metal removal on the sample scale and on the feature scale, and (iii) minimized photoresist undercutting for fine features. The problem of island formation, which occurs in large openings, can be resolved by insertion of dummy photoresist art work thereby preventing premature stoppage of the EMM process (4). Electrolyte impingement minimizes undercut (5), and the conditions leading to salt formation at the dissolving surface provide uniform dissolution independently of pattern spacing (6). 

    Through-mask EMM processes have been employed in the fabrication of several microelectronic and biomedical components (5,7-9). Some selected examples include fabrication of ink-jet nozzle plates (7), metal masks for screening and evaporation (5), and electrochemical fabrication of C4 (flip-chip) solder bumps (8). For the fabrication of ink-jet nozzle plates, the metallic foil is laminated with photoresist on both sides and the photoresist on one side is patterned with an array of circular opening.  The EMM tool consists of an electrolytic-jet shower head that is placed over the moving patterned metallic foil.  Pulsating voltage EMM with extremely high peak voltage (current) provides directionality, dimensional uniformity, and microsmooth surfaces required for maintaining ink drop size uniformity through the nozzles.  Fabrication of metal masks by two-sided through-mask EMM represents an alternative environmentally friendly processing technology with significant cost savings due to elimination of several waste treatment and disposal process steps usually associated with commonly employed chemical etching process (5).  In electrochemical fabrication of C4s (flip-chip), an array of solder bumps are electrodeposited on a photoresist patterned BLM (ball limiting metallurgy) layer. This is followed by photoresist stripping and BLM etching to electrically isolate the C4 bumps.  In a typical BLM layer consisting of Cr/phased CrCu /Cu or TiW/phased CrCu/Cu layers, the phased CrCu layer is extremely hard to etch for which an EMM (electroetching) tool has been designed and implemented in manufacturing (10).  The EMM tool uses four wafers that are mounted vertically, two wafers on each side back-to-back on a wafer holder. An electrolyte delivery system in the form of a multi-nozzle assembly is attached to a linear motion and scanned over the wafer during EMM.  The EMM tool and process have been successfully employed in the manufacturing of electrochemically fabricated C4s.

References

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