The first step in electronic packaging substrate fabrication is coating the substrate with a photosensitive polymer. There are several coating procedures such as spray, dip, roller, spin and meniscus. The two procedures utilized at the Packaging Research Center are spin coating and meniscus coating.

Once the film is deposited, using one of the above techniques, its film thickness controls or influences:

• Resolution
• Polymer filtration
• Polymer thickness
• Pinhole size
• Exposure time
• Pre-bake time
• Develop time and etch time

A thick film provides better adhesion, reduced pinholes, and greater protection for reactive ion erosion (RIE).
Thinner films are favored for higher resolution because of the reduced forward and backscattered radiation.
Many performance characteristics are crucial to the coating of substrates:

• Material Efficiency: percentage of liquid that becomes a film
• Edge Effects: impact of the area not usable due to thickness variations, exlusion zones, or edge bead
• Throughput: number of panels coated per hour
• Particle Defects: surface defects per square centimeter
• Homogeneity: area excluded due to internal voids or other microscopic inhomogeneity
• Uniformity: RMS thickness variation
• Robustness: Mean curvature of the process response surface

Coating techniques utilized at the Packaging Research Center:

• Spin Coating
• Meniscus Coating

	Advantages	Disadvantages
Spin Coating	Efficient process optimization Works well for circular substances Infinite number of materials can be used to coat manually 2-3 Materials can be used in automated coating, with a cost increase of about 30K per robot	Waste Rotational motion causes stress Non-uniformity of edges caused by air current
Meniscus Coating	Flexible substrate size Little waste Easy clean-up Can easily coat up to four materials without cost increase	Difficult to optimize the variable parameters involved in meniscus coating