Photo Etching

Photo Etching

Photo etching, or photo engraving, often called "photochemical milling", is a process very similar to regular chemical etching or chemical milling. It is still a chemical process and utilizes the same masking and etching technique, with one significant difference: instead of using masking material and cutting out the area to be etched, a photoresist is applied and developed through a patterned exposure to light. The result is the same; the undeveloped areas of the photoresist protect the metal's surface from being etched, while the developed areas are washed away and exposed to etching agents. Photo engraving is often used not only on metals, but on glass as well; this technique allows photos or complex images to be engraved rather inexpensively into decorative items.

Like chemical milling and etching, the first step to photo etching is to clean the metal surface. Next a thin layer of photoresist is laid on the area to be etched. This photoresist serves two purposes: to protect the metal surface not to be engraved and to break down under targeted ultraviolet light exposure over the metal surfaces which are to be engraved. It is essentially the same technology used for photo development, where a negative is produced by light exposure then chemicals are used through the negative pattern to create a positive image. The developing process, as with photography, can be reversed; positive or negative working photoresists may be used.

After the photoresist has been applied, the metal is exposed to a precise light pattern. If the photoresist is negative working, the exposed portion will protect the metal while the unexposed portion will dissolve away when chemicals are applied. If the photoresist is positive working, the metal portion exposed to light will dissolve under chemicals while the rest remains. Negative and positive working photoresists achieve different types of images and desired results. After the metal has been exposed to a light pattern, a specific chemical will be applied in varying strength and force, again, depending on the desired depth of cut, sideways etch, et cetera. The most common chemical used for photochemical milling is ferric chloride dissolved in hydrochloric acid (the amount of HCl determines the speed and depth of the cut). After the acid has achieved its desired etch, both the ferric chloride and the remaining photoresist are stripped. The metal is polished, any irregularities in the cut are burnished, and the etching is finished.

The benefits of using photo etching as opposed to regular chemical milling or manual engraving are substantial. Chemical milling requires parts of the masking to be cut and removed, whereas with photo engraving, the surface merely need be exposed to a pattern of light and then the developed photoresist rinsed. Photo etching allows for much more complex patterns and images to be laid; this is often the only way to etch photos or intricate designs, and the etching is achieved much quicker and more cost effectively than even regular chemical milling.