One of the fabrication processes used in producing intricate dental implants is a method called photochemical machining or etching. Photochemical etching is a non-conventional machining method wherein the process does not involve cutting tools. Photographic and chemical techniques are used to remove material from the workpiece to shape the final product. The process involves exposing the workpiece to a special chemical solution and selectively corrode defined areas. These areas are defined through photoresist imaging.
Fundamental Steps of Photochemical Etching
1. Preparation for the Photo-tool Plotting.
The pattern used for this process is designed using CAD software that will be converted to the appropriate format needed for printing. The pattern is printed on a photographic film, in the form of a diazo or silver halide film, by a photoplotter or a laser imaging system. Common compensation factors affect the photo-tool dimensions; these are the etch factor, the temperature, and the humidity variations.
2. Preparing the Workpiece
The work part will be cut and cleaned prior to loading it in for the etching process. It is important to ensure that there should be no contaminants on the metal surface for successful adhesion of the photoresist. The parts may be cleaned by two methods (mechanical and chemical). Mechanical cleaning involves the application of a degreasing solution and subjecting it to a certain form of scrubbing. On the other hand, in chemical cleaning, the work part is suspended into a degreasing solution made up of combining mild acids and agents. Most fabrication shops prefer chemical cleaning methods because of their minimal damage applied to the work part.
3. Photoresist Processing
Photoresists are usually set down on the surface of the work part. These can withstand the strong effects of the etching solution, leaving behind a masked and defined image. The photo-tool is responsible for protecting and exposing the desired photoresist regions. This compound exposure to UV light makes it either soluble or insoluble (depending on what was used) to the developer agent.
Below are the different classifications of photoresists:
● Classification according to the type of image produced (Positive Photoresist and Negative Photoresist)
● Classification according to chemical structure (Photopolymer, Photodecomposition, and Photocrosslinking)
● Classification according to form (Dry Film and Liquid or Wet Film)
Soft Bake (90°C to 110°C)
The next process, after applying the photoresist material, is “soft baking.” This is done to heat, vaporize, and release residual solvents. This process should be supervised to avoid high evaporation, causing bubbles and voids within the photoresist. Additionally, a low evaporation rate might inhibit the necessary evaporation of the residual solvents, which will result in a film formation on the surface.
Before proceeding with this process, it is vital to ensure proper alignment of the photo-tools in a multi-layered pattern. After everything has been set, the work part may now be subjected to exposure. The exposure process is where the photo-tool image will be relayed to the work part with the photoresist. This is typically done with wavelengths less than 400nm (Ultra-violet rays). Different techniques are used for exposing the photoresist, and these are through contact exposure, proximity exposure, projection, direct laser imaging, and electron beam.
Post Exposure Bake (110°C to 120°C)
This process involves thermally catalyzing the chemical reactions, which completes the photoreaction initiated by the Ultraviolet light exposure.
This process is made to remove unnecessary parts of the photoresist, leaving behind the desired pattern on the work part.
Hard Baking (120°C)
This process adds to the physical stability of the workpiece to withstand the etching process.
4. Etching Proper
This is when the unnecessary materials are subtracted from the work part to form it into the desired shape. Etching can be classified into two main types, namely, wet etching and dry etching.
This method uses liquid chemicals to erode the unprotected part. It starts with oxidizing components through hydrogen peroxide or nitric acid. Next is the oxidized part's dissolution using chemicals like hydrofluoric acid, phosphoric acid, and hydrochloric acid. Finally, the dissolved oxidized substrate is removed from the part, ensuring a homogenous solution in contact.
This method relies on high-velocity gaseous ions in removing and corroding the material. The collision of ions with the part is responsible for removing the material in one direction only, removing the undercut problems arising from wet etching.
5. Photoresist Removal or Stripping
After the desired shape has been created, the photoresist materials are removed. This process can be made with two methods: by using solvents and by using combustion. Solvents have the ability to break down the structure of the photoresist layer, and oxygen combustion, on the other hand, can deal with materials that are not easily removed by chemical agents.