The sequence of process operations that is required to manufacture a given device is called a process flow. A process flow is only valid for a given technology (usually named after the minimum achievable feature size), and reads a fixed number of lithography masks which are derived from layout information provided by the device designer. Figure 5.1 gives a schematic overview of the fabrication of a semiconductor device.
Figure 5.1:
Fabrication of an integrated circuit from layout and process flow.
In this text, a process step refers to a primary process operation as described above and reflects the fact that an operation is applied to achieve a desired effect in the wafer or in the behavior of the semiconductor device. It is perceived as an atomic building block of a process flow and is usually carried out in a single piece of equipment. As a matter of fact, its granularity depends on what a given piece of equipment is expecting. Although a process step usually represents an indivisible unit of fabrication, it should rather be perceived as a milestone on the run traveler.
Most types of equipment are controlled by recipes that specify the internal operation of the machine during a process step. A recipe in itself contains a sequence of statements and can be viewed as a flow similar to a process flow. To clearly mark the difference from statements appearing in a process flow, we define an equipment step to be the basic building block of a recipe. Examples of equipment steps include temperature ramp commands, gas flow switches, wafer pull operations, etc.