An optical-lithography imaging system includes the following key components: a light source that provides the exposure photons with the desired energy spectrum; an illuminator that collects light from the source, adjusts its coherence and incident angles, and delivers light to the mask uniformly; a mask containing the circuit image to be replicated to the photoresist; an imaging lens of the desired NA and field size to reproduce the mask image in the resist by exposure; the resist layer coated on the wafer; the thin-film stack to be delineated by the pattern transfer process or implantation; the wafer held by a chuck on a wafer stage, which can be moved for alignment and field stepping; and devices to guide the alignment movement of the stage. These components are described consecutively in the following sections.
5.1 Light Source
There are two types of light sources for use in optical-lithography exposure tools: the mercury arc lamp and the excimer laser. They are both bright and efficient in their respective wavelength spectrum.
5.1.1 Mercury arc lamp
The mercury arc lamp has been the light source of choice for optical lithography because of its many usable emission lines in the near-UV (350-460 nm), mid-UV (280-350 nm), and deep-UV (200-280 nm) regions. There is even an emission line at 184 nm. Its high brightness, compared to other nonlaser sources, is another reason for its popularity. Figure 5.1 shows the radiation spectrum of a typical mercury arc lamp. Figure 5.2 shows a schematic drawing of a mercury arc lamp. There are pointed and rounded electrodes enclosed in a quartz envelope that contains a noble gas and a small quantity of mercury. During ignition, electric discharge takes place between the two electrodes. As the heat builds up, the mercury is vaporized, and the radiation spectrum becomes dominated by that of the mercury. The pointed electrode becomes a bright radiating spot.