Comparison of Photochemical Etching with Conventional Wet Etching
Wet etching is the process of using aqueous chemical solutions of acids, bases, or other chemicals to dissolve and remove unwanted areas of materials to achieve the desired form. This process can be used on metals, semiconductors, and glass. The process involves the use of a masking material to cover the entire part, followed by removing the masking from areas that need to be etched. The masked components are then immersed in a reactive solution for the etching to occur and are washed on completion of machining.
Photochemical etching is a simple, high quality microfabrication process that is quite similar to conventional wet etching and involves the use of a photoresist in place of the masking agent. This process is a low-cost alternative to various processes such as stamping and punching. Though photochemical etching can be used to machine components made from any commercially available metal or alloy, it is limited to the machining of materials up to 0.080" thick. This process is also known as photochemical machining or photo etching.
Comparison of Photochemical Etching with Conventional Wet Etching:
While both the photochemical etching processes are quite similar, the only differences between them lie in the processes. Typically, both are used for microfabrication and etching of various metals, semiconductors, and even glass. Some of metals that are commonly etched are aluminum, copper, and various types of steel, including stainless steel. These processes can also be used to etch silica as well, a fact that has made them the primary fabrication process in the microprocessor industry for the production of integrated circuits. However, the toxic by-product created by the etchants used in wet etching has led to its reduced use in the industry.
Wet Etching Process:
The wet etching process begins with the removal of any residual stresses present in the part. This prevents any warping during the machining process. The part undergoes a pre-processing operation of extensive cleaning and degreasing, which ensures that the masking agent used in the machining process adheres well to the part. The masking agent is then applied to the part.
A variety of masking agents are used in wet etching, and include special tapes or paints known as maskants, Some of the other masking materials that are commonly used are elastomers such as neoprene and rubber, as well as different plastics, including polyvinyl chloride (PVC), polystyrene, and polyethylene.
After applying the masking agent, the areas of the part selected for etching are to be cleared of any masking. This is usually facilitated by the scribe and peel technique of removing masking. On removal of the masking, the areas that are to be etched are exposed and the part is ready to be etched. The part is immersed in a tank filled with a reagent that is used to etch the exposed areas of the part. To ensure a good rate of material removal from the part, the temperature of the reagent and its agitation should be considered.
Reagents used in the etching process are selected based on the material of the part to be etched. For example to etch aluminum parts, a commonly used etchant is sodium hydroxide, while nitric and hydrochloric acids are usually used to etch steel parts. Some of the other etchants used based on the part material are:
Nital (a mixture of nitric acid and methanol, methylated spirits, or ethanol)
Ferric Chloride, Ammonium Persulfate, Ammonia, 25-50% Nitric Acid
After the machining process is completed, the masking is removed from the part and the part is thoroughly washed. This ensures that the reagent is completely removed, thus preventing etching by any remaining etchant. In some cases, the etched component may also be finished with additional machining processes.
Photochemical Etching Process:
The photochemical etching process begins with the shape of the part being printed on two sheets of Mylar or photographic film. The two film sheets are printed with a negative image of the part, with the areas to be etched clear and the areas not being etched being dark. The metal parts are cut to the required size and cleaned. The part is then laminated on both sides with a layer of UV light sensitive photoresist.
The two sheets of film are then slipped over the metal part. Using vacuum, the two films are set close to the metal part. The film-covered part is then exposed to UV-light, which causes the photoresist exposed by the clear areas of the film to harden. After exposure to UV light, the unexposed photoresist is washed away.
The plate is now ready to be etched, which is typically performed on a conveyor belt that passes through multiple chambers. In the beginning, the heated etchant is sprayed under pressure on the part. The etchant in most cases is the aqueous solution of an acid, typically ferric chloride. The plate is etched by the acid, on completion of which the etchant is neutralized and rinsed off. The resist that remains on the part after the etching process is removed and the part is cleaned, ready for final use.
Some industrial applications for photochemical etching include fine screens and meshes, apertures and masks, battery grids, sensors, heat sinks, flexible heating elements, semiconductor lead frames, EMI/RFI shields and washers. The short lead times and intricate geometry designing achieved by photochemical milling make it an ideal option over conventional wet etching. Plus, wet etching produces a large amount of waste materials, including hazardous waste.
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