(5)Pretreatment of base plate

The accuracy of patterns created by photoresist is affected by surface conditions such as roughness and oxides on a base plate where photoresist will be applied. The pretreatment is extremely important because contaminants, such as oil, dust, grease content, and fingerprints, may result in poor adhesion, uneven thickness of the photoresist layer or leave pinholes. The pretreatment is generally performed in the following steps: degreasing -> surface polishing -> water washing -> acid treatment -> water washing -> drying. These steps are same as those for plating pretreatment.

(6)Photoresist

Resins sensitive to ultraviolet and visible light rays are dissolved into photoresists. Once the applied photoresists dry up, they will form a photosensitive film.

For negative resists, a cross-linking reaction of a photosensitive resin is initiated by light exposure, which makes the linear molecule form polymers with a three-dimensional network structure. After the light exposure, these resists become insoluble and resistant to acids and alkalis. Polyvinyl cinnamate is a well-known photosensitive resin.

On the contrary, exposed portions of positive resists become soluble upon exposure to light. In that sense, positive resists are more flexible since they can be exposed to light repeatedly.

Photoresists are available in water-soluble and solvent-based forms. Water-soluble resists are made from casein, gelatin, and polyvinyl alcohol with a mixture of dichromate.

Compared to negative resists, positive resists have lower chemical resistance and adhesion performance to base plates. However, they have better resolving power, which makes them suitable for forming fine patterns.

(7)How to apply photoresist

It is necessary to select an appropriate application method of photoresists according to the processing accuracy. Use a high-resolution photoresist for high accuracy processing. Apply a thin layer evenly over the substrate.

Photoresists are applied in the following methods: 1) dip coating (one by one); 2) spray coating; 3) liquid discharge by nozzle (for continuous sheet); 4) spin coating; 5) roller coating (for application without requiring accuracy), etc.

A film formed by dip coating is only about 1 µm thick. This is too thin for electroforming in terms of corrosion resistance. Even if you make it thicker by repeating the dipping process, the layer becomes unevenly thick. Application by dipping or liquid discharge will make the layer thickness uneven on the top and bottom sides because photoresists are dried with the application plate standing in the vertical direction. However, you can still make the photoresist film evenly distributed if you slowly pull up the application plate from the solution.

Spin coating is a procedure used to deposit an even layer of photoresists on a substrate by centrifugal force of the rotation. An even layer with the thickness of 3 to 5 µm will be deposited if you dry the photoresist using infrared rays during the rotation. In this procedure, secure the base plate onto the spinning disk and rotate the disk for about 80 to 180 spins per minute. It is a popular method because the application layers deposited by this procedure are most accurate and consistent.