One type of vacuum wafer processing chamber includes a plasma source comprising a source of gas to be converted into a plasma and an electrically driven electrode for exciting the gas into an a.c. plasma state. The gas is supplied to the chamber by an appropriate gas line, while the chamber is maintained at a vacuum by a vacuum pump connected to the chamber interior by a foreline. In one arrangement, the electrode is a horn excited by r.f. energy via a matching network. The matching network includes variable reactances having values which are controlled to obtain an impedance match between a source of the r.f. energy and a dynamic r.f. impedance seen by the horn. The r.f. dynamic impedance varies unpredictably as a function of many parameters of the r.f. plasma. A sheath exists between metal walls of the electrode and the plasma to cause the electrode to be maintained at a DC bias voltage that is also a function of the plasma parameters.
Vacuum wafer processing chambers of all types, including the type previously described, become contaminated with various byproducts of the wafer process. Typically, the contamination is so great that the chamber must be cleaned after several (e.g., five) wafers have been processed. Preferably, the chambers are cleaned in situ, without breaking the vacuum of the chamber, to minimize processing down time. Typically, the chamber is cleaned by supplying nitrogen trifluoride (NF.sub.3) to the interior of the chamber via the same line that supplies the processing gas to the chamber interior. The NF.sub.3 is excited to a plasma including free nitrogen and fluorine ions that etch contaminant particles from various interior parts of the chamber, particularly the horn, the chamber walls and other hardware, such as rivets and bolts. The particles are etched from the chamber parts as a result of a chemical reaction of ions in the NF.sub.3 plasma with the contaminants. The particles are removed by the vacuum pump via the foreline.
While it is desirable to clean the chamber as thoroughly as possible, it is also desirable to minimize the time required for the cleaning operation. Since no wafer processing can occur while the chamber is being cleaned, minimizing the chamber cleaning time increases chamber wafer processing throughput. In addition, overcleaning the chamber has deleterious effects on the cleaned parts because the cleaned parts are etched by ions of the NF.sub.3 plasma. Hence, overcleaning the chamber shortens the life of the cleaned parts. In addition, overcleaning can actually increase contamination problems because additional particulates are outgassed and sputtered into the chamber as a result of the reaction of the NF.sub.3 plasma ions with the cleaned parts in the chamber interior.
Presently, an optical emission spectroscopy (OES) technique is used to determine when a vacuum wafer processing chamber is in situ cleaned by the NF.sub.3 plasma ions. The OES technique requires a relatively expensive optical emission spectrometer to have a hand-held probe positioned to be responsive to optical energy emitted by the plasma and coupled through a window of the chamber. The optical emission spectrometer monitors the intensities of fluorine and nitric oxide peaks of the emitted plasma spectra resulting from the NF.sub.3 cleaning process. When the intensities of the fluorine (F) and nitric oxide (NO) peaks are stabilized, the chamber interior is assumed to be clean.
However, the prior art OES technique is quite vague and subjective. Very often the intensities of the F and NO peaks never stabilize, but keep changing at a rate lower than the peak rate. In addition, localized plasma conditions that sometimes occur in the processor can cause unreliable OES results. Further, the OES spectra are affected by the location of the spectrometer probe which is frequently positioned at different points relative to the window. The different positions of the spectrometer probe result in inconsistent measurements of the fluorine and nitric oxide peaks. Because the OES technique is incapable of distinguishing between contaminant products resulting from quartz and silicon dioxide etchings, the F and NO peak intensities frequently are never completely stabilized and the results are often convoluted. A further disadvantage of the OES technique is that it requires a specialized, relatively expensive piece of equipment to determine a characteristic completely unrelated to wafer processing.
It is accordingly an object of the present invention to provide a new and improved method of and apparatus for determining when a vacuum processing chamber and/or a part therein is clean.
Another object of the present invention is to provide a new and improved method of and apparatus for controlling the end of a cleaning cycle of a vacuum processing chamber.
A further object of the present invention is to provide a new and improved method of and apparatus for determining when a vacuum processing chamber and/or a part therein is clean and/or a cleaning cycle is ended, wherein equipment and a computer program associated with workpiece processing are the only apparatus required.
An additional object of the present invention is to provide a new and improved method of and apparatus for determining when a vacuum processing chamber and/or a part therein is clean and/or ending a cleaning cycle, wherein overcleaning of the chamber parts and its attendant disadvantages is avoided.
An added object of the present invention is to provide a new and improved method of and apparatus for determining when a vacuum processing chamber and/or a part therein is clean and/or ending a cleaning cycle, wherein the end of the clean cycle is detected with relatively great precision.