Ultrahigh purity gases are extremely important in the manufacture of many materials and devices such as state of the art semiconductors and optical fibers. The properties of these devices are so critically dependent upon the absolute purity of the process gases that successful manufacture requires continuous monitoring for impurities in the process gas stream. Reducing gases such as hydrogen and carbon monoxide are examples of impurities that may be present in a process gas stream of, for example, nitrogen. These impurities, most notably carbon monoxide, are present in bulk liquid nitrogen. Nitrogen, from an on-site plant, will contain low levels of hydrogen and carbon monoxide. From the prospective of the semiconductor manufacturer trace impurity levels as low as 200 parts per billion (ppb) of a contaminant representing either a gaseous impurity or solid particulates is significant and detrimental to the manufacturing process. At these contaminant levels of impurity conventional process analyzers require considerable operator skill and expertise to evaluate the data. The operator must have considerable experience in the areas of instrument fault diagnosis, analyzer calibration and data analysis to evaluate the data generated by conventional analyzers in order to distinguish between data identifying a "real" problem from data which is simply erroneous.
Continuous monitoring of process gases currently involves the use of discrete analyzers to monitor impurities of interest. An analysis system for monitoring a feed gas stream within a process reactor is taught and described in U.S. Pat. No. 4,891,186 the disclosure of which is incorporated herein by reference. The system employs a plurality of gas analyzers for separately analyzing individual gas samples from a feed gas stream and includes flow control valves to direct gas samples for individual analysis and to discharge the gas samples that are not being analyzed. A typical analytical system may also include a computer for data acquisition and display.
Although sophisticated analytical equipment is commercially available for impurity detection in a process gas stream at the impurity levels of interest the capability to distinguish process upsets from analyzed phenomenon without extensive operator intervention is severely limited. Any undetected increase in trace impurities in the process gas can be extremely detrimental and an "apparent" increase in trace impurities from extraneous events or momentary "glitches" in data transmission can be even more detrimental in that the manufacturing operation may have to be shut down to correct the non-existent problem. A false or "apparent" problem may be due to a computer or analyzer malfunction, improper calibration, operation outside of design parameters or simply incorrect data analysis. In essence, the ability to assess the validity of analytical data, generated from commercially available process analyzers, is as important as the analytical data itself.
The present invention relates to a process for the continuous analysis of trace contaminants in a process gas stream of O.sub.2, N, Ar or H.sub.2 and for identifying, storing and recording data representative of such trace contaminants in the process gas stream, for analyzing the stored data to identify erroneous analysis data and for identifying remedial actions to remedy the conditions causing said erroneous analysis data. The process broadly comprises the steps of:
sampling a process gas stream to provide a stream of sample gas;
passing the stream of sample gas through a plurality of analyzers to determine the presence of one or more trace contaminants selected from the group consisting of O.sub.2, H.sub.2, CO, CO.sub.2, hydrocarbons, moisture (H.sub.2 O) and particulate matter;
generating an output signal for each analyzer corresponding to the level of impurity for each trace contaminant in the process gas stream;
generating a status signal representative of preselected parameters of analyzer operation corresponding to the operating status of one or more of said analyzers;
transferring said status signals and output signals to a computer for storage in the form of data values;
analyzing said data values for the existence of a problem using an expert system rule base program consisting of a multiplicity of rules arranged to form statements corresponding to different problems;
executing said rule base program using an expert system shell with each problem recognized when said data values fall outside defined limits or are not present,
storing a file of remedial actions for a preselected number of problem conditions;
directing the expert shell to select the examination of the rules in the rule base program in a predetermined hierarchy and in a linear sequence; and
matching problems identified by execution of said rule base program with preselected remedial actions in said remedial action file.