Lithography using photons,1 otherwise called photolithography or optical lithography, has been and continues to be the preeminent lithographic technique used in the fabrication of integrated circuit devices in the semiconductor industry since the invention of the transistor and the integrated circuit. Spanning a period of over 50 years and many generations of device technology nodes that have witnessed critical dimension reduction of over three orders of magnitude, and up to now comprising largely photons with wavelengths from the visible (436 nm) through mid-UV (365 nm), DUV (248 nm and 193 nm), and VUV (157 nm) regions of the spectrum, it has demonstrated remarkable longevity that is expected to continue into the foreseeable future.
The remarkable longevity of optical lithography is a direct consequence of the advancements that have been made in optical exposure tools, and resist and process technologies. In particular, technological advancement in lithographic exposure tools has evolved across four broad categories, i.e., how the pattern is transferred from mask to wafer, the type of optics, the exposure medium and area exposed, and the wavelength of light used. Figure 13.1 shows the technologies that fall into these categories. All optical lithographic exposure tools in use today use one technology from each of these categories.
The key factors driving the advancements in exposure tool technology are line width resolution, registration, and depth of focus. The ability to decrease device feature line width is beneficial in that it lowers cost, increases the number of dies produced per wafer, and also increases device speed. Registration helps to increase device yield and speed by accurately overlaying one layer on another. Depth of focus control helps to determine CD control and consequently device speed and yield.