Selectively removing or depositing materials from a semiconductor wafer to form integrated circuit structures from wafers is well known in the art of semiconductor processing. Removal of material from a semiconductor wafer is accomplished by employing some type of etching process, such as, reactive ion etching and plasma etching. Depositing material on a wafer may involve processes such as chemical and physical vapor deposition, and molecular beam epitaxy. Other removal and deposition processes are also known. Such processes are tightly controlled and are often performed in a sealed process chamber.
Because exact amounts of material must be deposited onto or removed from the semiconductor wafer, its progress must be continually and accurately monitored to precisely determine the stopping time or endpoint of a particular process. Optically monitoring the process is one very useful tool for determining the stage or endpoint for an ongoing process. For instance, gasses within the interior of the process chamber may be optically monitored for certain known emission lines by spectrally analyzing predetermined wavelengths of light emitted or reflected from the wafer in the chamber. Conventional methods include optical emission spectroscopy (OES), absorption spectroscopy, reflectometry, etc.
A customary way to monitor the optical spectrum in semiconductor plasma process tools is to use an optical monitoring system which consists of an array-based optical spectrometer, and an optical coupling system to bring the light from the plasma in the interior of the chamber to the spectrometer. The optical spectrum is typically recorded as a series of light intensity measurements in a set of narrow spectral bands or over a broad spectrum, typically repeated at specific time intervals.