Optical Lithography; Photolithography; Light Lithography

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Optical Lithography

The fabrication of circuits on a wafer requires a process by which specific patterns of various materials can be deposited on or removed from the wafer's surface. The process of defining these patterns on the wafer is known as lithography. Lithography uses photoresist materials to cover areas on the wafer that will not be subjected to material deposition or removal.

Optical Lithography refers to a lithographic process that uses visible or ultraviolet light to form patterns on the photoresist through printing. Printing is the process of projecting the image of the patterns onto the wafer surface using a light source and a photo mask. There are three types of printing - contact, proximity, and projection printing, each of which will be described below. Equipment used for printing are known as printers or aligners.

Patterned masks, usually composed of glass or chromium, are used during printing to cover areas of the photoresist layer that shouldn't get exposed to light. Development of the photoresist in a developer solution after its exposure to light produces a resist pattern on the wafer, which defines which areas of the wafer are exposed for material deposition or removal.

Figure 1. Example of a mask aligner from Suss; source: www.suss.com

There are two types of photoresist material, namely, negative and positive photoresist. Negative resists are those that become less soluble in the developer solution when exposed to light, forming negative images of the mask patterns on the wafer. On the other hand, positive resists are those that become more soluble in the developer when exposed to light, forming positive images of the mask patterns on the wafer.

Commercial negative photoresists normally consist of two parts: 1) a chemically inert polyisoprene rubber; and 2) a photoactive agent. When exposed to light, the photoactive agent reacts with the rubber, promoting cross-linking between the rubber molecules that make them less soluble in the developer. Such cross-linking is inhibited by oxygen, so this light exposure process is usually done in a nitrogen atmosphere.

Positive resists also have two major components: 1) a resin; and 2) a photoactive compound dissolved in a solvent. The photoactive compound in its initial state is an inhibitor of dissolution. Once this photoactive dissolution inhibitor is destroyed by light, however, the resin becomes soluble in the developer.

A disadvantage of negative resists is the fact that their exposed portions swell as their unexposed areas are dissolved by the developer. This swelling, which is simply volume increase due to the penetration of the developer solution into the resist material, results in distortions in the pattern features.

This swelling phenomenon limits the resolution of negative resist processes. The unexposed regions of positive resists do not exhibit swelling and distortions to the same extent as the exposed regions of negative resists. This allows positive resists to attain better image resolution.

Lithography/Etch; IC Manufacturing

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