Spectrophotometry has been applied to optimizing photolithography processes in semiconductor manufacturing. For many years thin film measurement systems have been used in manufacturing for controlling film deposition processes. The combination of film thickness mapping with photolithography modeling has expanded the applications of this technology. Experimental measurements of dose-to-clear, the minimum light exposure dose required to fully develop a photoresist, are described. It is shown how dose-to-clear and photoresist contrast may be determined rapidly and conveniently from measurements of a dose exposure matrix on a monitor wafer. Such experimental measurements may underestimate the dose-to- clear because of thickness variations of the photoresist and underlying layers on the product wafer. Online modeling of the photolithographic process together with film thickness maps of the entire wafer can overcome this problem. Such modeling also provides maps of dose-to- clear and resist linewidth that can be used to estimate and optimize yield.
Measurement of the thickness of the top layer of a wafer having many film layers using microreflectometry normally requires detailed and rather accurate values of the thicknesses and optical properties of the underlying layers and substrate. In many cases such knowledge is unavailable; for example, the layers may be deposited sequentially in a process that does not allow the thickness measurements at each stage. A process is described to characterize the underlying layers and to determine an effective substrate that allows the accurate calculation of the top layer thickness in subsequent measurements. The procedure has been applied to antireflection coated aluminum, which is used in one stage of semiconductor manufacturing. Silicon dioxide thicknesses of 0.1 to 2.2 micrometers determined using the procedure predict reflectances to within 0.6% of the measured values for wavelengths throughout the entire spectrum of 400 to 750 nm.
Visible and ultraviolet light reflectometry provides a fast, convenient, and nondestructive method of characterizing multilayer film structures that include polycrystalline silicon. Reflectance measurements of silicon wafers containing such films have provided information as to the roughness of the poly surface, the thickness of the films, and the optical properties of the poly.