a High Sensitivity, High Resolution Mass Spectrometer Beam System for Trace Contaminant Measurements

The purpose of developing a mass spectrometer molecular beam system, the subject of this thesis, is to measure trace contaminants in gas mixtures at part-per -billion range demanded by many microelectronic fabrication processes. The design of this instrument is based on the modifications of an existing stratospheric mass spectrometer beam system. Two innovative techniques have been employed in the new system design. They are the focusing glass capillary array molecular beam sample inlet and the closed cycle Helium cryopump ultrahigh vacuum generation. In addition to an available double focusing magnetic sector mass analyzer, a Nier type electron bombardment ion source with mutual perpendicular electron, neutral, and ion beam configuration is used in the new system. The upper operating pressure level for the focusing glass capillary array is found to be within 30 mTorr dictated by the molecular flow condition. For the low pressure sample direct input mode, the sensitivity of the new system can be as high as 10^{10} counts/sec./Torr. Beyond 30 mTorr, the sample transmission rate of the glass capillary array levels off rapidly. Due to the necessary pressure reduction, the obtained system normal input mode sensitivity is about 10^7 counts/sec./Torr which is comparable to that of the existing stratospheric MS system. The system detection limit tested is about 10 ppb and below. The repeatability is about +/-10%. A complementary device, the cryogenic moisture sampler, has been developed along with the mass spectrometer. A pre-concentration factor of 10^4 has been obtained with a sampling time of about 2 minutes. The detection limit of the mass spectrometer for extremely low concentration of moisture can therefore be extended considerably to ppt range using the pre-concentration technique. One application of the new mass spectrometer system is to analyze the outgassing characteristics of mylar materials used in proton decay detectors. The contamination species identified is methylene chloride. A drawback of the new system is the hour long response time for water vapor measurements. It is believed that the low operating pressure dictated by the glass capillary array molecular flow condition is the main cause of this problem.