APPARATUS FOR MEASURING VERY LOW WATER CONTENT IN GAS

Abstract: A method is disclosed for measuring the amount of water in a gas with a very low water content which comprises using a dew-point meter or a frost-point meter comprising: a reflecting mirror whose temperature is variable in the range of from room temperature to the temperature of liquid nitrogen; a gas ejecting nozzle disposed so as to face said reflecting mirror; means for projecting either a condensed light ray or a laser beam onto that portion at which dew and/or frost is formed; and means for detecting a sudden increase in the intensity of scattered light, the process comprising: a step of cooling a gas to be measured so that its temperature becomes close to that of the mirror; a step of ejecting the gas from the said nozzle so as to hit said reflecting mirror; a step of projecting either a condensed light ray or a laser beam onto that portion of the mirror surface against which the gas to be measured is blown; and a step of detecting a sudden increase in the intensity of scattered light, thereby measuring a low dew point in the range of from -80°C to the temperature of liquid nitrogen.

Description: The present invention relates to an apparatus for measuring a very low water content in a non-condensible gas which has a dew point in the range of from ordinary temperature to -80°C or less. The invention also relates to the use of the said apparatus. As a result of the rapid developments in technology, utilization of inert gases, for example, N₂, Ar, H₂ and He, which have very low water contents, i.e., 0.5 ppm or less, (dew points of -80°C or less) has increased. In particular, material and carrier gases which are used for epitaxial growth and CVD in the semiconductor industry are required to have ultrahigh purities, and there is therefore a need to measure dew points of -80°C or less with a high degree of precision. Many means for measuring the water content of a gas have heretofore been known. For example, as a means for measuring a dew point, one method has been known for a long time in which a change in the intensity of light which occurs when moisture contained in the gas to be measured has condensed on a cooled mirror surface is optically detected. As one type of optical detection method, absolute measurement is described in, for example, Industrial Instrumentation Handbook (Asakura Shoten, 1976), p.297. According to this method, a variation in the intensity of the light reflected from a mirror surface is detected by the use of a freezing medium, a refrigerating machine and a thermoelectric cooler, and the temperature of the mirror surface at the time when any change in the light intensity is detected is measured to thereby obtain a humidity value which is basically taken from the dew point. The apparatus employed to practice this method has a cooled surface onto which light is projected from a light-emitting diode or a white light source, and the reflected light from the cooled mirror surface is received by a photodetector. In general, the mirror surface is cooled by a Peltier element, and as the surface is cooled, moisture contained in the gas to be measured becomes condensed or frozen on the cooled surface, so that the light projected on the adsorbed water molecules is partially absorbed or scattered, resulting in a decrease in the intensity of the reflected light. The change in the intensity of the reflected light is detected by the photodetector, and the temperature of the cooled mirror surface at the time when the intensity change occurs is measured to thereby measure the dew point of the gas concerned. There have been a rather large number of studies of dew-point instruments designed to measure the water content of a gas for industrial purposes with even more precision and on a continuous basis by using changes in the intensity of reflected light, e.g., Japanese Patent Publication (Kokai) Nos. 58-113840 (1983), 61-75235 (1986), USP 4335597, USP 4345455. etc. Attempt to measure a dew point in the range of from 0 °C to -75 °C is disclosed in Control and Instrumentation, vol. 15, No. 2, 1983, London GB, pages 41-43. The reference discloses that a mirror is maintained at -75 °C by using Peltier multi-stage cooler, and then a gas to be measured is previously cooled to -30 °C by a precooler and a dew point is measured by using a reflecting light detecter and a scatter light detector. However, the reference does not disclose measurement of a dew point of less than -80 °C. Japanese Patent Publication (Kokai) No. 58-113839 (1983) also shows a method of measuring a dew point by detecting a change in the intensity of light scattered at an angle different from the reflection angle, rather than by measuring a change in the intensity of reflected light. More specifically, when moisture becomes condensed or frozen on a mirror surface, a sudden change occurs in the intensity of the light scattered by the condensate (or ice). Therefore, the temperature of the cooled mirror surface at the time when such an intensity change occurs is measured to thereby measure the dew point of the gas concerned. The principle of this method has been known for a long time and is described in Humidity and Moisture (Vol. 1), p.165, Reinhold Publ. Co., New York, 1965. However, the measurable range of this method is shown to be between -73°C and 49°C, and it is therefore impossible with this prior art to realize measurement of low temperature dew points, as in the case of the present invention. In addition to the methods wherein the amount of water in a gas with a very low water content is measured by detecting an optical change that occurs when moisture contained in the gas becomes frozen on a cooled mirror surface, other methods have also been put into practical use wherein the amount of water in a gas with a very low water content is measured by detecting a change in the frequency of a crystal oscillator or a change in the electrostatic capacity which is caused by adsorption of water. However, none of these conventional methods is capable of measuring a dew point of -80°C or less with high accuracy. The above-described method wherein the dew point of a gas is measured by detecting a decremental change in the intensity of the reflected light at the time when moisture contained in the gas has become condensed or frozen on a cooled mirror surface suffers from the problem that, as the water content in the gas becomes lower, that is, as the dew point becomes lower, the amount of condensate (or ice) decreases to an extreme degree, so that it is difficult to measure the dew point with a high degree of precision. Further, the prior art has the disadvantage that, when a gas flows slowly on a cooled mirror surface in the conventional arrangement, condensation (or freezing) of an extremely small amount of moisture takes place very slowly, and therefore no distinct change in the reflected light is shown, particularly when the dew point to be measured is -80°C or less. In the case of a very low water content, i.e., 10 [ppb] or less, water molecules are adsorbed on the mirror surface very gradually, so that it has heretofore been difficult to read any sudden change in the intensity of the reflected light. It is therefore effective to blow the gas to be measured against a cooled mirror surface from a nozzle-shaped (or pipe-shaped) member. However, with respect to a gas having a dew point of -80°C, which has heretofore been considered capable of measurement with a substantially good degree of accuracy, it is necessary to precisely read the temperature of the cooled mirror surface at the time when the moisture contained in the gas in an extremely low amount, i.e., about 1 ppm, becomes frozen on the mirror surface; for a dew point of -110°C, this amount is about 1/1000 of 1 ppm; and for a dew point of -136°C, 1/1000 of 1/1000 of 1 ppm, that is, 1 ppt. There has accordingly been a demand for a technique and apparatus for precisely measuring such a freezing point. In view of the above-described problems of the prior art, it is a primary object of the, present invention to provide an apparatus for measuring a dew point in the range of from -80°C to the temperature of liquid nitrogen with a satisfactorily high degree of precision for the purpose of measuring the amount of water in a gas with a very low water content. The invention thus relates to an apparatus for measuring a dew-point and/or a frost-point comprising: a reflecting mirror means for gradually cooling the mirror, means for cooling the gas to be measured so that its temperature becomes close to that of the mirror, a gas ejecting nozzle disposed so as to face said reflecting mirror; means for projecting either a condensed light ray or a laser beam onto that portion of the mirror surface against which a gas to be measured is ejected; and means for detecting a sudden increase in the intensity of scattered light, characterized in that the temperature of the mirror is variable in the range of from room temperature to the temperature of liquid nitrogen, and the reflecting surface of said mirror is formed of a silicon wafer. The invention also relates to the use of the said apparatus for measuring the amount of water in a gas with a very low water content comprising the steps of: (a) gradually cooling the mirror and simultaneously or sequently cooling a gas to be measured so that its temperature becomes close to that of the mirror, (b) ejecting the gas from the said nozzle so as to hit said reflecting mirror; (c) projecting either a condensed light ray or a laser beam onto that portion of the mirror surface against which the gas to be measured is ejected; and (d) detecting a sudden increase in the intensity of scattered light. Further, the present invention enables industrial measurement of the water content of gases, for example, nitrogen, argon, hydrogen, helium, etc., which have dew (frosting) points of -80°C or less, by precisely measuring the condensation (freezing) temperature with a combination of a cooling means such as a helium refrigerating machine and a heater, although it is intermittent measurement based on rises and falls in temperature Japanese Patent Publication (Kokai) No. 56-142794 (1981) has already disclosed a method of precisely measuring a dew point with a reduced liklihood of error by means of an apparatus which is provided with a pre-cooling section for previously cooling a gas to be measured before it is blown against a cooled mirror surface and a nozzle for blowing the gas cooled in the pre-cooling section against the mirror surface. It is also a critical factor in precise measurement of a dew point to maintain the temperature inside the measuring chamber as close to the temperature of the cooling section as possible during any measurement carried out at low temperatures. In a method wherein a gas to be measured flows slowly on a cooled mirror surface, if the dew (frosting) point which is to be measured is very low, i.e., -80°C or less, it is not easy to cool the gas down to the temperature of the cooled mirror surface unless the gas is previously cooled down to a temperature as close to the dew (frosting) point of the gas as possible. If no pre-cooling is carried out, the temperature of the cooled mirror surface which is measured when moisture contained in the gas becomes condensed or frozen on the mirror surface may be considerably lower than the actual dew (frosting) point of the gas, which results in a large error. In addition, in order to read accurately the temperature at which an extremely small amount of moisture contained in a gas starts to freeze, it is necessary to previously cool the gas down to a temperature close to the dew (frosting) point thereof, blow the gas against the mirror surface from a nozzle-shaped member, and irradiate a limited portion of the mirror surface with either a light ray condensed as much as possible or a laser beam. In this way, a sudden increase in the intensity of the scattered light which is generated when the light ray or laser beam is reflected from the mirror surface is measured to thereby enable measurement of the dew (frosting) point of a gas having a very low water content. The ejecting angle of the gas to be measured onto the mirror may be in the range of 30°C - 60°C with respect to the mirror surface. The above and other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings. Figs. 1 and 2 are schematic views respectively showing apparatuses employed to practice the present invention; Fig. 3 is a graph showing the relationship between the temperature of the mirror surface and the output of the photodetector; Fig. 4 is a graph showing the relationship between the incident angle with respect to a line perpendicular to the mirror surface and the output of the photodetector; Fig. 5 is a graph showing the relationship between the surface precision of the condensation surface and the output of the photodetector when no condensation occurs; Figs. 6 to 9 are graphs showing the relationship between the temperature of the mirror surface and the output of the photodetector; and Fig. 10 is a graph showing the relationship between the output of a conventional electrostatic condenser type hygrometer and the output of the photodetector of the water content measuring apparatus according to the present invention. One embodiment of the present invention will be described hereinunder in detail with reference to Fig. 1. It should be noted that the present invention is not necessarily limited to the arrangement shown in Fig. 1. Light which is emitted from a light source 1 is converged in the form of parallel rays by a parabolic mirror 19 and then applied to a mirror surface 4 through an entrance window 3 after passing through a condenser lens 2 so that the parallel rays are condensed on the mirror surface 4 as much as possible. The mirror surface 4 is held on the cold surface of a helium refrigerating machine 5 with a thermally adequate degree of contact therewith. The arrangement is such that the temperature of the mirror surface 4 is capable of being raised or lowered as desired by a combination of a heater 6 and a temperature controller 7. The incident light is reflected by the mirror surface 4, and the light which is scattered at the mirror surface 4 is measured by a photodetector 8 which is installed on an optical path having an angle which is different from that of the optical path of the light reflected from the mirror surface 4. The temperature of the mirror surface 4 is measured by a thermocouple 9. A gas which is to be measured is introduced as shown by the reference numeral 13 through a filter 20 which is formed from a hydrophobic material which can be heated up to 200°C such that any particulate foreign matter alone can be removed. After the gas to be measured has previously been cooled in a precooler 14 to a temperature slightly higher than that of the mirror surface 4, the gas is blown against the mirror surface 4 from the opening of a nozzle 10 and then discharged from the system as shown by the reference numeral 15. Since all the above-described constituent elements are cooled down to low temperatures, the measuring chamber needs appropriate heat insulating means (including a vacuum). The mirror surface 4 is made of silicon wafer. Further, it is important that the incident light be focused on the mirror surface 4 as precisely as possible. The present inventors have found experimentally that employment of a ray of light which is itself condensed, such as helium-neon laser light, is, of course, one of the means that can be used for effectively improving the sensitivity. It is also a matter of course that improving the hardness of the mirror surface 4 by coating it with a thin film of titanium nitride, aluminum nitride, silicon oxide, aluminum oxide or the like is one of the effective means for maintaining the smooth surface condition of the mirror surface 4 for a long period of time. Since silicon wafer is a good helium-neon laser light absorbing material, its surface is not a good reflecting surface. However, the surface smoothness of silicon wafer is considerably high; according to the experiment carried out by the present inventors, it was possible to measure a dew point near -120°C. From this point of view, a high reflectivity is not an absolutely necessary condition. In order to measure effectively a very low water content in even a gas having a dew point of -80°C or less on the basis of the frosting point, it is absolutely necessary to apply condensed light to that position on the mirror surface 4 where the gas to be measured is blown from the nozzle 10. The present inventors measured the incident angle of light under various conditions and, as a result, have found that, as the incident angle becomes closer to 0° with respect to a line perpendicular to the mirror surface 4, the degree of sensitivity with which a sudden change in the intensity of scattered light can be read becomes higher. When a light-emitting diode is employed as the light source 1, the sensitivity becomes higher as the incident angle becomes closer to 0° with respect to the line perpendicular to the mirror surface 4, as shown in Fig. 4. The results of the measurement show that, as the incident angle becomes closer to 0° with respect to the perpendicular, more condensed light is applied to the mirror surface 4. It has been shown that the incident angle is preferably set at 60° or less, particularly preferably, 10° or less. In order to enable measurement of a very low water content, it is essential to heat the sampling piping by means of a heater. With a view to precisely measuring a very low water content by the dew (frosting) point method, it is particularly important to heat the gas to a temperature near 200°C together with a filter which is capable of removing foreign particles flowing together with the gas and thereby constantly maintain the sampling piping and the like within the system in a condition wherein the amount of adsorbed water is reduced. Compressor for the refrigerator is shown at 11. A heater for the precooler is shown at 16. A temperature controller for the precooler is shown at 17. A thermocouple for the precooler is shown at 18. A heater for the heating filter is shown at 21. The present invention will be explained more specifically below by way of Examples. However, the present invention is in no way restricted to these Examples. Example 1: With the arrangement shown in Fig. 1, a gas generated from liquid nitrogen by evaporation was supplied as the gas to be measured to the mirror surface 4 from the nozzle 10, light from the light-emitting diode 1 that had been condensed as much as possible by the parabolic mirror 19 and the condenser lens 2 then being projected on the mirror 4, and the light scattered at the mirror surface 4 received by the photodetector 8. As the mirror surface 4 was gradually cooled by means of the small-sized helium refrigerating machine 5, moisture contained in the gas to be measured became frozen on the mirror surface 4, resulting in an increase in the intensity of the scattered light received by the photodetector 8. The temperature of the mirror surface 4 at this time was measured by means of the thermocouple 9. The temperature was -118°C, and thus the dew point of this gas was found to be -118°C. Thus, it was possible to measure a dew point so low that it had heretofore been impossible to measure. The relationship between the temperature of the mirror surface 4 and the output of the photodetector 8 is shown by the line a in Fig. 3. It should be noted that a silicon wafer employed in the semiconductor industry was employed in this Example as the mirror surface and the light from the light-emitting diode that was condensed as much as possible was made incident on the mirror surface at 5° with respect to the perpendicular. Example 2: An experiment was conducted with the same arrangement as in Example 1 except that the light-emitting diode 1 and the parabolic mirror 2 were installed inside the measuring chamber 12 as shown in Fig. 2. In such a case, since the temperature inside the chamber 12 changes, the luminance of the light-emitting diode 1 changes, and this may hinder precise measurement of the scattered light by means of the photodetector 8. The small-sized helium refrigerating machine 5 and the light-emitting diode 1 were therefore brought into thermal contact with each other by the use of an appropriate copper wire 22, and the temperature of the light-emitting diode 1 was maintained at a constant temperature, i.e., -20°C, by temperature control effected by means of a heater 23, thereby preventing any change in the luminance of the light-emitting diode 1 due to temperature changes. With this arrangement, an experiment similar to that in Example 1 was carried out. As a result, it was possible to conduct measurement with a higher sensitivity than that in the case of Example 1. The line b in Fig. 3 shows the relationship between the temperature of the mirror surface 4 and the output of the photodetector 8. Thus, it was confirmed that the arrangement shown in Fig. 2 also enables precise measurement of a low dew point, i.e., -80°C or less. Comparative Example 1: With the arrangement of Example 1, a copper surface was ground so that the surface precision corresponded to the measuring wavelength λ (λ = 10.6 µm), and light from the light-emitting diode 1 that was condensed as much as possible was projected on the mirror surface 4 in the same way as in Example 1. As a result, even when the moisture contained in a gas to be measured had not yet frozen, the intensity of the scattered light was higher than that measured when the moisture became frozen in Example 1. Further, even when the moisture became frozen, there was no change in the output of the photodetector 8. Similar experiments were conducted with the surface precision of the copper surface serving as a cooled mirror surface changed to varying degrees. The results, showed that it is impossible to measure the dew point (e.g., -120°C) of a gas containing an extremely small amount of moisture when the surface precision corresponds to 1/4 or more of the measuring wavelength λ (λ = 10.6 µm). Fig. 5 shows changes in the output of the photodetector 8 occurring when the surface precision was changed from λ ( λ= 10.6 µm) to λ/4. Comparative Example 2: Experiments similar to Example 1 were conducted with the same arrangement as in Example 1 except that the parabolic mirror 19 and the condenser lens 2 were removed from the arrangement shown in Fig. 1. When the light from the light-emitting diode 1 was not focused on the mirror surface 4, it was possible to measure a dew point as low as -78°C, but it was impossible to detect an increase in the intensity of the scattered light in the case of moisture-containing gases having respective dew points lower than -78°C. Figs. 6 and 7 show results obtained in the case where the dew points of the gases tested were -120°C and -78°C respectively. As will be understood from these figures, in order to measure a low dew point, i.e., -80°C or less, the light which is projected on the mirror surface 4 must be condensed as much as possible. Comparative Example 3: Experiments similar to Example 1 were conducted with the same arrangement as in Example 1 except that the photodetector 8 was installed in the optical path of the reflected ray. Helium-neon laser light and light from a light-emitting diode that was condensed as much as possible were employed as the light projected on the mirror surface 4. In this case, the decrease in the intensity of the reflected light was very small irrespective of the type of light source. The results of these experiments are shown in Figs. 8 and 9. It will be understood from these results that, in the case where a dew (frosting) point is measured by detecting a decrease in the intensity of the reflected light, it is impossible to measure a dew (frosting) point of -75°C or less. Comparative Example 4: One method which does not employ the technique of measuring the amount of moisture contained in a gas by causing the gas to become attached to a mirror surface by condensation of freezing to allow optical detection of the attached condensate or ice is the method wherein the water content of a gas is measured on the basis of changes in the electrostatic capacity of a sensor comprising a thin film of aluminum oxide which is coated with gold by deposition. This known method is shown in JIS-K0226 as a method of measuring a water content of 2 [ppm] or more. With this method, nitrogen gases having respective dew points of from 0°C to -90°C were measured. The results showed that no change was to be seen in the electrostatic capacity in the case of nitrogen gases having dew points of -80°C or less. Fig. 10 shows a comparison between the output of the electrostatic condenser type hygrometer and the output of the water content measuring apparatus according to the present invention. As has been described in detail, the apparatus according to the present invention is provided with a pre-cooling section for previously cooling a gas to be measured before it is blown against a mirror surface and a nozzle for blowing the precooled gas against the mirror surface. Further, a light ray which is condensed as much as possible is employed as a light source to be projected onto the mirror surface, and the condition of moisture condensed or frozen on the mirror surface is detected by the light scattered at the mirror surface. Thus, it is possible to measure electrically and precisely the water content of a gas having a dew point in the range of from ordinary temperature to -80°C or less.

Claims: An apparatus for measuring a dew-point and/or frost-point comprising: a reflecting mirror means for gradually cooling the mirror, means for cooling the gas to be measured so that its temperature becomes close to that of the mirror, a gas ejecting nozzle disposed so as to face said reflecting mirror; means for projecting either a condensed light ray or a laser beam onto that portion of the mirror surface against which a gas to be measured is ejected; and means for detecting a sudden increase in the intensity of scattered light, characterized in that the temperature of the mirror is variable in the range of from room temperature to the temperature of liquid nitrogen, and the reflecting surface of said mirror is formed of a silicon wafer. The apparatus of claim 1, wherein the cooling source employs a low-temperature liquified gas. The apparatus of claim 1, wherein the cooling source is a small-sized helium refrigerating machine. The apparatus of claim 1, wherein the light source for projecting light onto the mirror surface is a light-emitting diode. The apparatus of claim 1, wherein the incident angle of light projected onto the mirror surface is within 60° with respect to a line perpendicular to the mirror surface. The apparatus of claim 1, wherein a heat resistant hydrophobic filter is provided at the inlet for the gas to be measured. The apparatus of claim 1, wherein the mirror surface is coated with a thin film of titanium nitride or an other hard material. The apparatus of claim 1, wherein the ejecting angle of the gas to be measured onto the mirror surface is in the range of 30° - 60° with respect to the mirror surface. The use of the apparatus of claim 1 for measuring the amount of water in a gas with a very low water content comprising the steps of: (a) gradually cooling the mirror and simultaneously or sequently cooling a gas to be measured so that its temperatur becomes close to that of the mirror, (b) ejecting the gas from the said nozzle so as to hit said reflecting mirror; (c) projecting either a condensed light ray or a laser beam onto that portion of the mirror surface against which the gas to be measured is ejected; and (d) detecting a sudden increase in the intensity of scattered light.

Inventors: KIJIMA, TAKAHIKO, TAGAWA, TAICHI
Applicants: NISHIZAWA, JUNICHI, OSAKA SANSO KOGYO LIMITED
Application year: 1988
Main classification: G01N25
IPC codes: G01N25, G01N21