Photoresists An Application of Polymers to Microelectronics

Photoresists have been nsed for many years in lithography for the prodnction of printing plates. Strictly speaking, these generally are applied from a solution. 

Chromated gelatin is one classical example that still finds some use in making masks for silk screens. Somewhat newer are films made from poly(vinyl alcohol) 

The microelectronics industry is built around the ability to assemble transistors that have dimensions on the order of 100 nm or less. Almost aU lithography involves the creation of patterns in photopolymers made possible by a radiation-induced solubility change in the polymer. A photoresist needs to adhere to a substrate, undergo a radiation-induced solubility change, possess etch resistance, be developable in aqueous base (or other solvent), and disappear when not wanted. Photoresists are classified as positive tone if they become more soluble in the exposed region and negative tone if they are less soluble in the exposed area. 

A silicon wafer as much as 300 mm in diameter has one surface oxidized to a controlled depth. 

Polymer is applied by placing a few drops of solution at the center of the oxide side of the wafer and then spinning the wafer to obtain a thin, uniform coating that when dry is about 1 )jm thick. The polymers used are those that will change in solubility upon irradiation. Polymers such as poly(vinyl cinnamate) 

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