Virtual Electronics Packaging Laboratory

Photolithography refers to a process whereby images are created by exposure to light. Photolithography is composed of two procedures:

Exposure
Development

Click on the icon to view an applet that demonstrates the affect of the exposure and development processes on a substrate.

Before the photolithography process is begun, the oxidized substrate must first be coated with a light-sensitive polymer. (See Dielectric Deposition for process steps and theory).

The criteria for photosensitive polymers is as follows:

It must be able to produce patterns at the desired resolution, consistently.
It must provide an edge profile consistent with the processing requirementes.
It must not introduce unwanted impurities into the etching process.
It must survive and protect the underlying film during the etching process.
It must be readily removable after the etching process.

Photosensitive polymers can be either positive or negative. The exposure and development steps of photolithography depend on which type of polymer is used.

For positive polymers, the polymer is exposed with UV light wherever the underlying material is to be removed. For these polymers, exposure to the UV light causes a change in chemical structure so that it becomes more soluble in the developer. The exposed polymer is then washed away by a developer solution, leaving windows of bare underlying material. The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer. A saying used to remember this process by is; "What shows goes."

Negative polymers behave in just the opposite manner. Exposure to the UV light causes the polymer to become polymerized, and more difficult to dissolve. Consequently, the polymer remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. Masks used for negative photosensitive polymers, contain the inverse (or photographic "negative") of the pattern to be transferred.

The figure below shows the pattern differences generated from the use of positive and negative photosensitive polymers.

The following table highlights some of the key differences between positive and negative polymers.

	Positive Polymers	Negative Polymers
Molecular weight average in film	1,000	400,000
Solvents normally used for Polymers	Xylene and 2. Ethoxy ethyl acetate	Xylene, Cyclohexanone and Butyl acetate
Chemical Resistance of film	Poor: easily oxidized and soluble in strong alkaline/acid solutions	Good
Development process and adhesion	Solubility differential achieved in alkaline solutions, can have adhesion problems because molecular weight too high to achieve low film stress.	The low uncrosslinked polymer adhesion is good due to low molecular weight before exposure.
Problems with resolution	Better resolution can normally be achieved in positive polymers than negative polymers. Exposure for positive polymers is usually slower and suffers less from humidity problems than negative polymers.	Spreading of the image can occur if over-exposed. Negative polymers suffer from standing waves during printing.