Optical Lithography - Page 2 of 2: Types of Printing: Contact; Proximity; Projection

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Optical Lithography (Page 2 of 2)

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.

Lithography/Etch; IC Manufacturing

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