Masks

For many miniaturization tasks, lithography, the technique used to transfer copies of a master pattern onto the surface of a solid material such as a silicon wafer, is of central importance. In this chapter, we review different forms of lithography, detailing those that differ most from the miniaturization processes used to fashion integrated circuits (ICs).

The image of an IC, micro-, or nanomachine pattern is transferred to a substrate in a process known as photolithography. Substrates are first coated with a thin emulsion of photoactive organic material called a photoresist. A pattern is then aligned and projected onto the substrate using a light source (e.g., deep UV). Subsequently, the resulting image is developed and inspected.

The stencil used to repeatedly generate a desired pattern on resist-coated wafers is called a mask. In typical use, a photomask—a nearly optically flat glass [transparent to near ultraviolet (UV)] or quartz plate (transparent to deep UV) with an absorber pattern metal (e.g., an 800-Å-thick chromium layer)—is placed above the photoresist-coated surface, and the mask/wafer system is exposed to UV radiation (Figure 1.4).

The absorber pattern on the photomask is opaque to UV light, whereas the glass or quartz is transparent. The absorber pattern on the mask is generated by e-beam lithography, a technique that yields higher resolution than photolithography. In e-beam lithography, a pattern drawn on a computer-aided design (CAD) system is exposed onto the mask. Like resists, masks can be positive or negative.

A positive or dark field mask is a mask on which the pattern is clear with the background dark. A negative or clear field mask is a mask on which the pattern is dark with the background clear. A light field or dark field image, known as mask polarity (Figure 1.5), is then transferred to the semiconductor surface. This procedure results in a 1:1 image of the entire mask onto the silicon wafer.

Masks making direct physical contact (also referred to as hard contact) with the substrate are called contact masks. Unfortunately, these masks degrade faster because of wear than noncontact, proximity masks (also referred to as soft contact masks), which are slightly raised, e.g., 10–۲۰ μm, above the wafer.