Both methods employ photolithography to fabricate nano-scale structures. But where one adds material to exposed surfaces, the other subtracts — with far different results.
For a very long time component makers have widely applied photolithography as the go-to technique for fabricating extremely small features. The tiny circuits in a computer chip are the most obvious example. One reason is simplicity. To create a pattern on a piece of silicon, metal, glass or other surface you shine a light through a stencil (photo mask) onto a surface covered by a layer of opaque photo resistant chemical. Openings will then appear in the photoresist corresponding to the transparent areas of the mask. Those exposed areas are then available — on which to add material to build up features on the surface or from which to subtract material, leaving the desired features behind.
Extreme precision is another reason for photolithography’s appeal. The ability to image very small features is only limited by the wavelength of light itself, which in the case of ultraviolet light can be as small as 10 nanometers or 0.01 microns. But imaging the feature and actually fabricating the feature are two different things. So even though additive and subtractive methods both start with photo imaging’s extremely high precision as a base, the actual precision achieved — in terms of feature tolerances, i.e., minimum error allowed — will be different.
We see that difference when comparing electroforming (additive) with photochemical (subtractive). Even though both employ photo imaging, electroforming achieves feature tolerances of ±2 microns versus ±25 microns for photochemical machining (PCM) — an order of magnitude difference.
That difference is explained by comparing how the two methods work.
How Electroforming Works
Electroforming adds material to photo imaged metal by electrical attraction. The process starts by coating a glass mandrel with a very thin metal layer that’s been covered by a photoresist. The photo resist is then exposed to light through a photo mask a designer created on a CAD system — exposing the metal in areas where material is to be added. That happens by immersing the mandrel in an electrolytic bath containing ionizing salts, and a positively charged cathode made of a metal such as nickel. The metal from the cathode passes to and bonds with the exposed parts of the negatively charged metal on the mandrel. As more and more metal is deposited, a three-dimensional structure rises from the photoresist pattern.
Depending on the resolution of the mask image, electroformed features can be extremely tiny — down to 5 microns (or less) in the Metrigraphics’ process. The size of openings, like a nozzle orifice, can be as small as 5 microns and in some cases even smaller.
Electroforming is therefore ideal for fabricating micron-scale metallic components as well as for making injection molds used for forming nonmetallic microstructures with nano-scale features. Benefits include low-cost, high-quality production plus high repeatability.
How Photochemical Machining (PCM) Compares
PCM also starts with photolithography, but instead of employing electrical attraction to add more layers to the exposed metal surface, it subtracts material from the metal with a chemical etching agent.
PCM consists of three steps:
- Creating a photo tool
- Using the photo tool to image a photoresist-covered metal plate; and
- Exposing the metal plate to chemical agents to etch features by removing material from exposed metal.
The photo tool consists of two sheets of film optically aligned to form the top and bottom halves of the tool. Both sheets are optically imprinted with an image of the part so that areas with features are clear and the rest are opaque. Placing a photoresist-covered metal plate between the two sheets and exposing it to UV light hardens the photoresist, forming a protective layer over those exposed areas.
Photoresist from the unexposed areas is then washed away. The metal plate is then immersed in a corrosive chemical bath (typically ferric chloride) that corrodes metal away from the unprotected areas and leaves the photoresist-covered features intact.
The plate is then clean, dried and singulated into individual components.
The reason electroforming achieves such smaller tolerances is because chemical corrosion can’t provide the same level of control as an electric current can. With an electrolytic additive process it is possible to add more or less material to a feature at almost an atomic level just by dialing the voltage up or down. Features are also less uniform with PCM since the amount of time the chemical agent stays in contact with the metal varies based on how deep the agent is allowed to penetrate the metal — so feature size will vary too.
Leverage the Precision of Photolithography
Every day advancing photolithographic techniques continue to take component makers closer to the edge of what’s possible in achieving ever-smaller feature tolerances. That is not an opportunity they’ll want to miss by marrying those techniques to suboptimal processes that underutilize photolithography’s full potential. Electroforming’s additive process means manufacturers won’t have to. They can continue to ride photolithography’s opportunity curve today and long into the future.