Optical Lithography; Photolithography; Light Lithography

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.

Contact printing refers to the light exposure process wherein the photomask is pressed against the resist-covered wafer with a certain degree of pressure. This pressure is typically in the range of 0.05-0.3 atmospheres. Light with a wavelength of about 400 nm is used in contact printing.

Contact printing is capable of attaining resolutions of less than 1 micron. However, the presence of contact between the mask and the resist somewhat diminishes the uniformity of attainable resolution across the wafer. To alleviate this problem, masks used in contact printing must be thin and flexible to allow better contact over the whole wafer.

Contact printing also results in defects in both the masks used and the wafers, necessitating the regular disposal of masks (whether thick or thin) after a certain level of use. Mask defects include pinholes, scratches, intrusions, and star fractures.

Despite these drawbacks, however, contact printing continues to be widely used. After all, good contact printing processes can achieve resolutions of 0.25 micron or better.

Proximity printing is another optical lithography technique. As its name implies, it involves no contact between the mask and the wafer, which is why masks used with this technique have longer useful lives than those used in contact printing. During proximity printing, the mask is usually only 20-50 microns away from the wafer.

The resolution achieved by proximity printing is not as good as that of contact printing. This is due to the diffraction of light caused by its passing through slits that make up the pattern in the mask, and traversal across the gap between the mask and the wafer.

This type of diffraction is known as Fresnel diffraction, or near-field diffraction, since it results from a small gap between the mask and the wafer. Proximity printing resolution may be improved by diminishing the gap between the mask and the wafer and by using light of shorter wavelengths.

Projection printing is the third technique used in optical lithography. It also involves no contact between the mask and the wafer. In fact, this technique employs a large gap between the mask and the wafer, such that Fresnel diffraction is no longer involved. Instead, far-field diffraction is in effect under this technique, which is also known as Fraunhofer diffraction.

Projection printing is the technique employed by most modern optical lithography equipment. Projection printers use a well-designed objective lens between the mask and the wafer, which collects diffracted light from the mask and projects it onto the wafer. The capability of a lens to collect diffracted light and project this onto the wafer is measured by its numerical aperture (NA). The NA values of lenses used in projection printers typically range from 0.16 to 0.40.

The resolution achieved by projection printers depends on the wavelength and coherence of the incident light and the NA of the lens. The resolution achievable by a lens is governed by Rayleigh's criterion, which defines the minimum distance between two images for them to be resolvable. Thus, for any given value of NA, there exists a minimum resolvable dimension.

Using a lens with a higher NA will result in better resolution of the image, but this advantage has a price. The depth of focus of a lens is inversely proportional to the square of the NA, so improving the resolution by increasing the NA reduces the depth of focus of the system. Poor depth of focus will cause some points of the wafer to be out of focus, since no wafer surface is perfectly flat. Thus, proper design of any aligner used in projection printing considers the compromise between resolution and depth of focus.

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