Etching forward - new etching process
Metals and alloys such as titanium, shape memory alloy NiTi and Inconel are renowned for their corrosion resistance and are used in the biomedical, electronic, marine, chemical processing, automotive and aerospace industries. Photochemical machining (PCM), a controlled corrosion process that enables selective removal of material from thin-sheet metal stock (- two millimetres thick), seems an ideal process to fabricate the intricate geometries required.
The process combines photolithography (using a patterned resist) with chemical etching to achieve simultaneous machining of features over the surface of a substrate.
However, the combination of an advanced corrosion process and corrosion-resistant metals and alloys introduces a number of issues, the chief of which is that highly aggressive etchants, such as hydrofluoric acid, are required. This has serious implications for the safe storage, use and disposal of such chemicals, especially in large quantities. These chemicals also producea large degree of undercutting as the acid permeates beneath the resist layer exhibiting low etch factors (the ratio of etch depth to undercut).
To counter the effects, more robust resists are required. Alternatives havebeen investigated, although by being more resistant to the aggressive etching conditions, they are also more difficult to remove afterwards.
Electrolytic etching is one way to enable ‘safer etchants’ and conventional resists to be used. Investigations at Cranfield University, UK, haved emonstrated that innocuous chemicals, such as sodium chloride, can etchthrough a variety of highly corrosion-resistant materials.
In electrolytic etching, the substrate is imaged using conventional lithography and connected as the anode in an electrolytic cell (see diagrambelow). Depending on whether single or double-sided etching is required, a cathode or cathodes are connected as the negative electrode/s to a DC powersupply. Both anode and cathode/s are immersed in an electrolyte, which is the etching medium. The current enhances the etching capabilities according to Faraday’s Laws of Electrolysis.
Potential advantages over PCM include a higher etch factor therefore better for deep, narrow features, comparable or faster etch rates, ambient temperature processing and the use of less aggressive etchants. A significant disadvantage is achieving uniform current distribution and dimensional accuracy.
Potential sources of disruption are the electric field concentration at high points, edges and discontinuities, and the electrolyte conductance – influenced by parameters such as temperature, pH, dissolved metal content within the electrolyte and the volume of hydrogen bubbles generated at the cathode. Other considerations include the build up of waste products in the interelectrode gap (IEG) and resistive heating that arises due to the charge trying to pass through blockage of the IEG. These effects can be minimised by continually refreshing the electrolyte – however, high electrolyte flow rates are generally incompatible with the use of a thin resist.
There is a trade-off between etch rate and dimensional control. The current should be higher than the limiting current to achieve smooth etched surfaces and directional etching, but a low etch rate results in better process control. However, if a pulsed mode is used to apply current, directionality is achieved by applying a high peak current (at low average current) and both by-productand heat can be removed from the IEG by flushing or agitating the electrolyte during the ‘off’ cycle (see diagram, right). In the electrolytic etching of molybdenum (Mo) with sodium hydroxide/potassium ferricyanide, it has been demonstrated that the pulsed mode eliminates the smutting produced in the DC mode.
The maintenance of more uniform electrolytic and hydrodynamic conditions during pulsed etching enables the IEG to be smaller and the electrolyte flowrate to be reduced, therefore localising the current and improving lateral control thereby dimensional accuracy and surface quality. However, pulsed etching alone does not suit passive metals and alloys such as the Ni-based superalloys, Mo and Ti alloys.
If sufficient oxygen is present, by gas evolution at the anode, the oxidefilm, responsible for the substrate’s anti-corrosion capabilities, can ‘reheal’ during the off-time of the cycle. This means that only partial breakdown of the film occurs on the next cycle, resulting in a pitted and rough surface. Also, dissolved material from the anode may precipitate out at the cathode due to the high pH region generated as a result of hydrogen evolution at the cathode. This can cause a size- or shape-change in the cathode.
If a pulse reverse cycle is used, where the polarity is periodically swapped, hydrogen can be anodically ‘consumed’ at the cathode, reducing the pH and avoiding precipitation of reaction products and subsequent cathode geometry changes. Simultaneously, the oxide layer’s rehealing in the presence of oxygen is prevented on the anode (substrate) during the cathodic cycle, thereby enabling lower currents to break down the film. The oxygen’s cathodic consumption leads to a reduction in the degree of cavitation experienced there. Again, the removal of both heat and by-product enables a narrower IEG to be used. These factors combine to produce smoother surfaces,more uniform machining and better dimensional accuracy. The process must be predominantly anodic to encourage active etching.
Reports of electrolytic etching investigations appear to be increasing. While electrolytic etching has a history stretching back to the 1960s, advances in computerised electrical and electronic control equipment have helped researchers to better understand the phenomena involved and are responsible for the renewed interest in non-conventional hybrid machining technology applied to recently-developed difficult-to-machine metals and alloys.
Further information: Cranfield University