Semiconductor devices (as of 2005) are built by depositing and patterning many thin layers. The patterning steps, or lithography, define the function of the device and the density of its components.
For example, in the interconnect layers of a modern microprocessor, a conductive material (copper or aluminum) is inlaid in an electrically insulating matrix (typically fluorinated silicon dioxide or another low-k dielectric). The metal patterns define multiple electrical circuits that are used to connect the microchip's transistors to one another and ultimately to external devices via the chip's pins.
The most common patterning method used by the semiconductor device industry is photolithography -- patterning using light. In this process, the substrate of interest is coated with photosensitive resist and irradiated with short-wavelength light projected through a photomask, which is a specially prepared stencil formed of opaque and transparent regions - usually a quartz substrate with a patterned chromium layer. The shadow of opaque regions in the photomask forms a submicrometer-scale pattern of dark and illuminated regions in the resist layer -- the areal image. Chemical and physical changes occur in the exposed areas of the resist layer. For example, chemical bonds may be formed or destroyed, inducing a change in solubility. This latent image is then developed for example by rinsing with an appropriate solvent. Selected regions of the resist remain, which after a post-exposure bake step form a stable polymeric pattern on the substrate. This pattern can be used as a stencil in the next process step. For example, areas of the underlying substrate that are not protected by the resist pattern may be etched or doped. Material may be selectively deposited on the substrate. After processing, the remaining resist may be stripped. Sometimes (esp. during MEMS fabrication), the patterned resist layer may be incorporated in the final product. Many photolithography and processing cycles may be performed to create complex devices.
A resist is not always necessary. Several materials may be deposited or patterned directly using techniques like soft lithography, Dip-Pen Nanolithography, evaporation through a shadow mask or stencil...