Multiple gas impurity analyses and verification for semiconductor process tools

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Courtesy of CIC Photonics, Inc.

With the increased emphasis on the purity of cleaning, etching and treatment gases by the semiconductor industry, advanced analysis instrumentation is required to monitor the electronic specialty gases and to verify the performance of process tools and components. This paper describes new instrumentation and software specifically designed to accomplish ppb level gas analyses in times as short as 15 seconds.

The overall system description is called IRGAS, which stands for “Integrated Real-Time Gas Analysis Solution.” The IRGAS is a turnkey industrial gas analysis system. It is composed of a FTIR spectrometer, a stainless steel long path gas cell, optics, sample and purge gas lines and manifold, and two software components known as SPGAS and SpectraStream. The SPGAS software provides two major functions: (1) total operational management and monitoring of all the hardware components; and (2) weighted, multiband, multicomponent chemometricsbased quantitative analysis. The SpectraStream module adds the capability for a fast-response early-warning detection of sudden changes in gas purity or composition. A block diagram is shown in Figure 1.

The hardware components are well-established tools.

The FTIR spectrometer is a Bomem WorkIR, which is a very compact and ruggedized unit frequently used in industrial applications; and the long path gas cell is a stainless steel cell with customized gold-coated stainless steel mirrors enhanced to resist corrosive and toxic gases, to provide for fast exchanges of gas samples, and to offer very high energy throughput.

The SPGAS software offers gas calibrations based upon fundamental HITRAN data for up to 36 gaseous species, weighted multiband CLS (classical least squares) data analysis, protection against unknown species, and infinite calibration sets. It automatically matches, in real time, the recorded sample gas spectra with calibration spectra and then displays the absolute concentrations as a function of time, as illustrated in Figure 2. Multiple species can be
simultaneously displayed. Two constraints of SPGAS are that the HITRAN data base is limited to those gaseous species found in atmospheric air and that the calibration sets must be regenerated for optimal performance if either the spectrometer or gas cell is changed; however, additional
gas calibrations can be incorporated using standard calibration procedures.

The major benefits of the SpectraStream module are (1) that it provides high sensitivity detection of impurities at the low ppb level, (2) that it reduces the time response typically associated with FTIR spectroscopy from minutes to seconds, and (3) it reduces the effects of spectrometer drift.

Among other tests, SpectraStream has been applied to the rapid detection (</= 20 sec) of gas surges due to cylinder tank openings, including moisture surges from UHP nitrogen, and more importantly to the detection of air leaks in process gas lines. While FTIR can not detect O2 from an air leak, it can detect both H2O and CO2 and use the rates of signal increase and the absolute ratio of the two species to prove the occurrence of an air leak. An actual result from a fab plant operation is shown in Figure 3. In this case, the increase in the CO2 concentration (right scale) is the most obvious indicator of a leak. Then the identical growth rate pattern for H2O is the concurrent indicator. But the proof of an air leak is given by the H2\O to CO2 ratio, which corresponds approximately to the air ratio at the plant site.

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