4.5 Optical issues in photolithography

Introduction

Photolithography is one of the most important technology in the production of advanced integrated circuits. It is through photolithography that semiconductor surfaces are patterned and the circuits formed. In order to make extremely small features, on the order of the wavelength of the light, advanced optical techniques are used to transfer a pattern from a mask onto the surface. A polymeric film or resist , is modified by the light and records the information in a process not dissimilar to ordinary photography.

An illustration of the photolithographic process is shown in [link] . The process follows the following basic steps:

1. The wafer is spin coated with resist to form a uniform ~1 µm thin film of resist on the surface.
2. The wafer is exposed with ultraviolet light through a mask which contains the desired pattern. In the simplest processes the mask is simply placed over the wafer, but advanced sub-micron technologies require the pattern to imaged through a complex optical system.
3. The photoresist is developed and the irradiated area is washed away (positive resist) or the unirradiated area is washed away (negative resist).
4. Processing (etching, deposition etc.)
5. Remaining resist is stripped.

In addition to being possibly the most important semiconductor process step, photolithography is also the most expensive technology in semiconductor manufacturing. This expense is the result of two considerations:

1. The optics in photolithography tools are expensive where a single lens can cost a $1 million or more
2. Each chip (often referred to as a "dye") must be exposed individually unlike other semiconductor processes such as CVD where an entire wafer can be processed at a time or oxidation processes where many wafers can be processed simultaneously.

This means that not only are photolithography machines the most expensive of semiconductor processing equipment, but more of them are needed in order to maintain throughput.

Optical issues in photolithography

The critical dimension and depth of focus

A semiconductor process technology is often described by a characteristic length known as the critical dimension. The critical dimension (CD) is the smallest feature that needs to be patterned on the surface. The exact definition varies from process to process but is often the channel length of the smallest transistor (typical of a memory chip) or the width of the smallest metal interconnection line (logic chips). This critical dimension is defined by the photolithographic process and is perhaps the most important figure of merit in the manufacture of integrated circuits. Making the critical dimension smaller is the primary focus of improving semiconductor technology for the following reasons:

1. Making the CD smaller dramatically increases the number of devices per unit area and this increase goes with the square of the CD (i.e., a reduction in CD by a factor of 2 generates 4 times the number of devices).
2. Making the CD smaller of a device already in production will make a smaller chip. This means that the number of chips per wafer increases dramatically, and since costs generally scale with the number of wafers and not the number of chips to a wafer, costs are dramatically reduced.
3. Smaller devices are faster.