(9) Electroforming

Substrates used for electroforming are electrically conductive. To make it easier to separate after electroforming, the resist separation treatment is necessary. Potassium dichromate solution mentioned in previous volumes is used for the resist separation treatment.

If you use a glass substrate, it is necessary to make its surface conductive. To add conductive properties, it is common to form silver, copper, or nickel coating by electroless plating, cathode DC sputtering, or vacuum deposition. Copper and nickel are frequently used for electrodeposition in photo electroforming. The bath composition and electrolysis conditions are same as those for general electroforming. [Fig.1] illustrates the electrodeposition onto a substrate using a photoresist.

To separate the electrodeposited metal from a substrate, use the tip of a knife to peel it off or paste adhesive tape over it and strip the tape. When you separate the metal, be careful not to damage the protective film. The photoresist layer still exists on the substrate even after separating the electrodeposited metal. You can use this substrate repeatedly and produce a number of electrocast products.

[Fig.2] shows how the electrodeposited metal grows. The deposited metal grows up to the height of the resist layer; however, the excess metal deposited after that starts growing in the lateral direction. This lateral growth becomes the margin of error for the pattern. The length of lateral growth on both sides (δ) is almost equal to the thickness of the electrodeposited layer (t). Therefore, this dimensional adjustment must be incorporated into the original drawing.

The thinner resist makes the photo electroforming more accurate. The resist layer thickness varies from 1 to 5 µm depending on the application method. Since the resist layer can be built up to only a given thickness in a single application, a double coating method using two different resists is available if you want to form a thicker layer.

We have seen an electroforming method of depositing a metal layer on a resist-applied substrate up to a certain height and separating the deposited metal. In addition, there is a way to deposit metal higher than the resist layer height. First, electrodeposit metal on a substrate until it becomes several micrometers high. Separate the deposited metal from the substrate and place this layer into another frame. Put this back into the electroforming bath and wait for metal to be deposited on both sides for a certain thickness.

How to remove the resist after separating from the mandrel is different for resist to resist. In general, dip it into alkali solution such as sodium hydroxide or organic solvent as ketones, and brush the resist off from the substrate.