Figure 1: Change in reflectivity as a function of time for the deposition of polymer residue in the chamber. Cassettes and wafers were changed as indicated. Note the cosine like functional form of response. The initial reflectivity is 6.5% since additional wafers had previously been etched and polymer deposited on the fiber end.

Modern semiconductor production lines rely heavily on "dry" plasma processing to allow formation of sub-micron features and to reduce the environmental problems associated with wet processing steps using chemical baths. One of the primary problems associated with "dry" processes is the production of residue polymeric materials that are byproducts of the processes. These polymeric materials pose several threats to the process since they are deposited on the walls and fixturing of the tool. First, they are chemically unstable and can interfere with the process chemistry. Second and more importantly, they can lead to particle deposition onto the product wafers reducing yields. Currently, specific recipes are operated in the tools that remove the residue polymer from the tool reducing the risk of particles; however, these processes are not perfect and the result is a buildup of material on the tool that eventually has to be hand cleaned by a technician. This results in very costly down time for the tool and reduced yield from the fab.

An instrument to measure the total deposited material on the tool walls without disrupting the process would allow the system to be hand cleaned only when needed rather than the current practice of cleaning on fixed time intervals. Conventional film monitoring techniques such as quartz crystal microbalances are unacceptable for application in many dry processes, particularly oxide etch processes, due to material incompatibilities and the electric field disruptions that result from their application. In addition, knowledge of the residue buildup rate and index of refraction is useful in determining the state of health of the chamber process. We have developed a novel optical fiber-based robust sensor that allows measurement of the residue polymer buildup rate while not effecting the plasma process.

A single ended, optical fiber-based sensor has been developed that can be inserted into a semiconductor processing chamber to measure the buildup of residue polymers from the process. The sensor functions by monitoring the reflectivity of the fiber tip as the residue polymer is deposited onto the tip. Since the residue polymers are generally transparent at near infrared wavelengths (860 nm), the reflectivity changes as a function of polymer thickness as a micro-Fabry-Perot cavity is formed by the polymer. Prior knowledge of the probe wavelength allows measurement of the film thickness simply by counting the number of minima and maxima that are observed (Figure 1). More detailed information about the film, such as index of refraction, can also be obtained from the sensor by fitting the data from the sensor to common Fabry-Perot reflectivity equations. Index of refraction is a measure of the film properties from which one can infer the health of the plasma process. Data obtained from measurement of a Dry Tech poly-silicon etching tool indicate excellent reproducibility and signal-to-noise ratio for the sensor.

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