The photolithography process consists of transferring a pattern optically from a chrome on quartz reticle onto a partially processed wafer that has been coated with photoresist. The image is transferred onto an underlying blanket of material after developing the resist by some sort of chemical and/or thermal step (etch, sinter, implant, growth, dope, etc.). This is repeated about 15 to 20 times, with each step seeing different pattern transfer steps, using different equipment. These steps may distort the wafer and its patterns, causing differences between wafers and lots. Despite this variance, each new step must precisely align to the previous layers. To accomplish this, each step of the process and every piece of equipment must be characterized. In this paper, the primary parameters and data analysis techniques that are most commonly tracked and used by photolithography engineers are reviewed.
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Lynda C. Hannemann-Mantalas, Harry J. Levinson, "Semiconductor process control," Proc. SPIE 10274, Handbook of Critical Dimension Metrology and Process Control: A Critical Review, 102740E (1 July 1994);