Measuring and testing – Instrument proving or calibrating – Gas or liquid analyzer
Reexamination Certificate
1999-08-31
2003-03-04
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Gas or liquid analyzer
Reexamination Certificate
active
06526803
ABSTRACT:
1. FIELD OF THE INVENTION
The present invention relates to an apparatus and method for generating moisture standards of known water concentrations in gases. The method and apparatus of the invention utilize volumetric measurement of water delivered to the gas to calculate the resulting water concentration. In particular, the method and apparatus of the invention provide for delivery of a preselected amount of water vapor to a flowing gas stream, thus enabling quick and reliable calculation of the resulting concentration of water in the gas.
2. BACKGROUND OF THE INVENTION
Measurement of low levels of moisture content in dry gases is critical for applications in which high or ultrahigh purity gases are used. For example, the measurement of ppb levels of moisture in ultrahigh purity gases used in the microelectronics or semiconductor industries is crucial. Moisture is one of the most ubiquitous and serious contaminants in the fabrication of microelectronic devices, such as wafers. Its presence in the gas phase can lead directly to impaired wafer yield. Moisture in certain gases used in fabrication of microelectronic devices can also accelerate the corrosion of tubing, regulators and valves used in handling these gases, and the corrosion products can negatively impact wafer yields and quality. Therefore, stringent measurement and control of moisture as a contaminant is required.
Instruments which can be used to measure moisture at such low levels include electrolytic cells, dielectrics of polymer/ceramic, vibration crystals, Fourier transform infrared (FTIR) spectrometers, atmospheric pressure ionization mass spectrometers, and chilled-mirror frost-point hygrometers. These instruments generally require calibration using gases with known moisture content. For example, FTIR analysis of moisture in a gas requires the generation of a calibration curve of absorbance due to water at a selected wavelength, versus known concentration of water in that gas. In order to generate such a calibration curve, gas standards with known concentrations of water are necessary. In addition, it is often necessary to check the response of a moisture meter used to monitor the moisture level in a gas, requiring a gas having a known water concentration, or “humidity challenge.” Moreover, measurement of the corrosion of steel as a function of the moisture contamination in hydrogen chloride gas requires the generation of gas standards having known concentrations of water vapor.
Such calibration gases are typically produced using moisture standard generators. There are currently two primary methods for generating known concentrations of water vapor in a flowing gas: permeation tubes and diffusion vials.
Permeation tubes are containers having a permeable polymeric membrane and that are filled with water. The tubes are placed in a flowing dry gas stream, and water vapor permeates through the membrane and into the dry gas. Permeation tubes operate on the principle that the amount of water permeating through the membrane is constant at a constant temperature. The resultant water vapor concentration in the flowing dry gas is determined by multiplying the molar gas constant for water vapor by the permeation rate, and dividing by the gas flow rate. To produce standards having different moisture concentrations, either tubes having membranes with different emissivities must be employed, or the flow rate of the gas must be varied.
Permeation tubes are the most widely employed moisture standard generators, perhaps because of their convenience. However, problems with the reliability of these devices often occur. One problem is that the moisture emitted from the permeation tube is highly dependent on the control of temperature and pressure. Thus, slight variations in the temperature or pressure of the flowing dry gas will cause the resultant moisture content of the gas to vary significantly.
A potentially more serious problem with permeation tubes is the deviation of the tubes from their claimed accuracy. In experiments or calibrations using moisture standards, complex ancillary equipment is often used, making it difficult to ascribe irregularities that may arise in the measurements to a particular causative agent. Therefore, the moisture standard generator itself should be as reliable as possible. Although initial calibration of the manufacturer's equipment is performed with National Institute of Standards and Technology (NIST) traceable standards, a user employing a particular permeation tube must depend upon the continued validity of the manufacturer's calibration. It has been shown that permeation tubes may be in error of their reported values by as much as about 30% around 1 ppm when compared to NIST humidity standards validated by an optical frost-point hygrometer. See Huang, Peter A., “Accuracy of PPM Humidity Standard Based on Permeation Method,” Proceedings of the Sixth International Meeting on Chemical Sensors, National Institute of Standards and Technology, Gaithersburg, Md. (Jul. 22-25, 1996).
If the reliability of permeation tubes is suspect, calibration may be verified using commercially available moisture meters. However, these meters are expensive and are themselves often inaccurate, unreliable and responsive over only narrow concentration ranges. Another recourse is to send the tube back to the manufacturer for re-calibration, but this is very time consuming and inconvenient. A third alternative is that the calibration could be performed in-house. However, this would be impractical and extremely inconvenient, as the calibration would require NIST traceable standards, proper equipment and experienced personnel.
Diffusion vials offer a significant increase in certainty at the expense of convenience and flexibility. A diffusion vial system consists of a bottle of water having a small hole at the top and contained in a temperature-controlled housing. The water is introduced into an inert dry gas by flowing the gas over the vial opening. The system operates on the principle that the vapor pressure of water, and therefore the rate of release of water vapor from the vial, is constant at a constant water temperature. Thus, once the system reaches a steady temperature and constant flow rate, the rate of water introduced into the dry gas is theoretically constant.
The main advantage of this system is that the total amount of water introduced into the dry gas can be accounted for gravimetrically, i.e. the exact quantity of water introduced into the flowing gas stream can be determined by weighing the vial before and after use. However, a significant problem with the diffusion vial is that it is impractical to obtain values of the water concentration before the end of the experiment for two reasons. First, obtaining water consumption rates essentially requires stopping the experiment because one must stop the gas flow, disconnect the gas lines, remove the vial and weigh it. Hence, any measurement of concentration will necessarily be performed at the end of the experiment, making intermediate water consumption rates impossible to obtain. Second, the diffusion vial is sensitive to temperature, pressure and water level, and disturbing these conditions by stopping the gas flow and disconnecting the gas lines will affect the rate of diffusion. Therefore, the diffusion vial method of generating moisture standards is more suited to providing an accurate average, rather than instantaneous, water concentration.
Another problem with the diffusion vial system is that this method is useful only over a relatively high concentration range, i.e. thousands of ppm. To achieve lower concentrations of water, one could attempt to pass higher flow rates of gas over the vial, but this leads to temperature control problems. Another solution is to dilute the highly concentrated vapor and reject the excess flow. However, this requires dilution apparatus and flow meters, each having uncertainties of their own, and these errors would combine additively thereby diminishing the accuracy of the moisture concentration.
U.S. Pat. No. 3,59
Cooper Gerald
Fraenkel Dan
Hogan & Hartson LLP
Matheson Tri-Gas, Inc.
O'Rourke Sarah S.
Petersen Steven C.
Raevis Robert
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