SILANE GAS SENSOR AND A METHOD OF MAKING THE SAME

Abstract: A sensor for detecting low concentrations of silane gas in air and a process for making the sensor are disclosed. The sensor comprises a silane-sensitive element and a heater to heat the element to 200 - 400°C. The element comprises an electrode-equipped alumina procelain tube coated with a mixture of SnO₂, SbOCl, and Pt. The tube is calcined in air or in an oxidized antimony atmosphere at 600 - 850°C and is exposed to a sailne atmosphere at 50 - 850°C to stabilize its properties.

Description: This invention relates to a sensor for detecting low concentrations of silane gas in the air according to patent claim 1 and to a process of manufacturing same according to patent claim 3. In semiconductor factories, chemical factories, laboratories and the like, large quantities are used of certain gases which can spontaneously ignite and combust upon contacting or mixing with air or other gases even when existing in concentrations of a few percent. Examples of these gases are monosilane (SiH4), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), phosphine (PH3), diborane (B2H6), and arsenic hydride (AsH3). There are increasing incidents of fires caused by the leaking of such gases into the atmosphere followed by their spontaneous ignition. For this reason, there is a great need for a gas sensor which can detect low concentrations of these gases. However, up to the present time no such sensor has been developed. From DE-A-28'16'331 is known a device for detecting carbon monoxide with an element comprising stannic oxide as bulk material, platinum to increase the sensitivity to carbon monoxide, and antimony trioxide to improve the temperature stability. An improved carbon monoxides sensing element can be prepared by mixing stannic oxide, antimony oxychloride and platinum in ratios of Sb/Sn = 2-8 mol%, Pt/Sn = 2-10 mol% and calcining said mixture at a temperature of from 600 to 850°C. Such sensing element is claimed in the not prepublished EP-A2-0'114'310. In the course of experiments, the present inventors discovered that if a heating means were added to the carbon monoxide sensing element described above and the element was thereby heated to 260 o C, the resulting apparatus had a high selectivity for monosilane gas (SiH4). Specifically, in their experiments, the present inventors prepared a mixture of SnO₂ H₂PtCl₆, and SbOCl with a Pt/Sn molar ratio of .04 and an Sb/Sn molar ratio of .06 and coated this mixture on two alumina porcelain tubes each having a pair of electrodes. The tubes were then calcined at 700 ±5 o C for 15 minutes either in air or in a quartz tube containing an oxidizing anitmony gas prepared by calcining 4.0 mg of SbOCl. Electrical heating means were then inserted into the alumina porcelain tubes and a voltage was applied to the heaters to heat the tubes to 260 o C. The electrical resistance between the electrodes of each tube was then measured in clean air at 25 o C and then again in each of eight different samples of air at 25 o C each containing 100ppm of either CO, CH₄, C₂H₄, C₂H₆, H₂, NH₃, EtOH, or SiH₄. Measurement in SiH₄ was carried out last. The results of these measurements are shown in Table 1 in the form R o /R g , where R o is the initial electrical resistance in air and R g is the electrical resistance in a particular gas mixture. Tube A is the tube which was calcined in air and Tube B is the tube which was calcined in the aforementioned oxidized antimony gas. The initial resistance in air R o was 135 kilohms for Tube A and 74 kilohms for Tube B. It was found that exposure to the first seven gases in the table did not significantly effect the resistance in air, but that after exposure to SiH₄ gas, the resistance in air (R air ) upon the second measurement fell to 28 kilohms for Tube A and to 16.5 kilohms for Tube B. After the second measurement of resistance in clean air, the resistance in each gas was again measured. This time, for Tube A, R air /R g was 36 in SiH₄ , 5.8 in EtOH, and 1.1 - 3.6 in the other gases, while for Tube B, R air /R g was 33 in SiH₄ , 6.3 in EtOH, and 1.1 - 3.8 in the other gases. For the third and subsequent measurements of resistance, the resistance in air R air showed almost no change between measurements, R air/ R g in SiH₄ was 18 - 27 for both Tube A and Tube B, and the values of R air /R g in the other gases were substantially the same as or lower than those given above. The above experiment was repeated using different calcining temperatures and different compositions for the mixture coated on the tubes. The value of R air /R g in SiH₄ was 110 - 300 for the first measurement, 30 - 50 for the second measurement, and 16 - 35 for subsequent measurements, while the values of R air /R g in other gases were 1/4 - 1/6 of the values in SiH₄. The silane gas sensing apparatus achieved by adding a heater to the carbon monoxide sensing element of Japanese Application No. 57-226510 and heating the apparatus at 200 - 400 o C thus has a high selectivity for gaseous silanes. Although the change over time of the value of R air /R g for gaseous silanes is initially large, if the apparatus is once exposed to gaseous silanes, the change over time of R air /R g becomes small and tends to reach a stable value. SUMMARY OF THE INVENTION It is the object of the present invention to provide a sensor for detecting silane gases which can detect even low concentrations of gaseous silanes in the air. It is another object of the present invention to provide a sensor for silane gases which exhibits only a small change in its sensitivity over use. The apparatus for sensing silane gases according to the present invention is basically the carbon monoxide sensing element of Japanese Application No. 57-226510 to which a heater has been added and which has been exposed to silane gas in order to stabilize its sensitivity to silanes. A sensor for silane gas according to the present invention comprises a silane-sensitive element prepared by mixing SnO₂, SbOCl, and Pt such that the molar ratio of Sb to Sn is approximately .02 - .08 and the molar ratio of Pt to Sn is approximately .02 - .10, and calcining the mixture at 600-850°C, and exposing said element to a silane atmosphere comprising from 25 to 1000 ppm silane gas (SiH4) for approximately 12 hours, while heating the element to a temperature in the range of 50 to 850°C; and means for heating said silane-sensitive element to a desired temperature. A preferred silane gas sensor is obtained when the element is heated during the exposure to the silane gas to a temperature in the range of 325±5°C. A process for making a silane gas sensor according to the present invention comprises the steps of: (a) adding SnO2 to an aqueous solution of PtCl₄ so that the molar ratio of Pt/Sn is approximately .02 - .10, thoroughly dispersing said SnO2 in said PtCl₄-solution, followed by freezing and vacuum drying of the resulting mixture; (b) mixing SbOCl with the product of step (a) so that the molar ratio of Sb to Sn in the resulting mixture is approximately .02 - .08; (c) adding an organic solvent to the product of step (b), forming the resulting mixture into a paste, coating the paste on an electrical insulator having electrodes, and drying said coated paste; (d) calcining said coated insulator produced in step (c) at approximately 600-850°C in air or an oxidized antimony atmosphere; and (e) exposing said calcined insulator of step (d) for approximately 12 hours, to a silane atmosphere comprising from 25 to 1000 ppm silane gas (SiH4) while heating the element to a temperature in the range of 50 to 850°C, prefarably to a temperature in the range of approximately 325±5°C. Figure 1 is a graph of the electrical resistance in various concentrations of SiH4 gas of the two different gas sensors according to the present invention. Figure 2 is a graph of the change in electrical resistance as a function of the number of times used of a silane gas sensor according to the present invention which was calcined in air. Curve (b) shows the resistance in SiH₄ gas and Curve (a) shows the resistance in clean air. Figure 3 is a graph of the change in electrical resistance as a function of the number of times used of a silane gas sensor according to the present invention which was calcined in an oxidized antimony gas atmoshphere. Curve (b) shows the resistance in SiH₄ gas and Curve (a) shows the resistance in clean air. DESCRIPTION OF THE PREFERRED EMBODIMENTS A silane gas sensor according to the present invention and a method for manufacturing this apparatus will now be explained in connection with the following illustrative examples. Example 1 An aqueous solution of H₂PtCl₆ was added to SnO₂ in an amount such that the molar ratio of Pt to Sn was 0.04 and was well dispersed using ultrasonic waves. The dispersed solution was quick frozen at -40 o C and then dried in a vacuum freeze dryer. SbOCl was then added to the dried product in an amount such that the molar ratio of Sb to Sn was .04, and the two were mixed with a mortar for 30 minutes. Isopropyl alcohol was then added to the mixture to form a paste, and the paste was then coated on an alumina porcelain tube equipped with electrodes and dried by air drying. After the paste had dried, the tube was placed into a quartz tube containing air preheated to 700 o ± 5 o C and calcined for 30 minutes. After sintering, an electric heater was installed inside the porcelain tube, and a current was supplied to the heater so as to heat the tube to 300 o C ± 50 o C. In this heated condition, the tube was aged in air for 12 hours. After aging, the tube was heated to 325 ± 5 o C using the heater and was exposed to a silane gas atmosphere containing 100ppm SiH₄ for 10 minutes in order to stabilize the characteristics of the sensor. Finally, to complete the gas sensor, the tube was heated to 300 ± 50 o C by the heater and the tube was aged for 12 hours in air. Sixteen identical gas sensors were prepared according to the above method. The tubes of the apparatuses were then heated to 325 o C by the heaters and were exposed to clean air at 25 o C and to each of 8 different gas mixtures containing either 100 ppm of SiH₄, EtOH, or CO, or 1000ppm of H₂, CH₄, C₂H₄, C₂H₆, or NH₃. Exposure to air was carried out first, and exposure to the SiH₄ gas atmosphere was carried out last. The electrical resistance between the electrodes of each of the apparatuses was measured in each of the atmospheres. The results of the measurements are shown in Table 2 in the form R o /R g , where as before R o is the initial resistance in air and R g is the resistance in the gas. The values in the table are the averages for 16 sensors. The maximum value of R o was 91 kilohms and the minimum was 35 kilohms. The maximum value of R g in the SiH₄ gas atmosphere was 3.3 kilohms and the minimum value was 0.9 kilohms. Next, the 16 sensors were exposed to different concentrations of SiH₄ gas. The values of R g in the different concentrations are plotted as Curve (a) of Figure 1. Examples 2 -24 Using the same method as for Example 1, sensors having various values of the Pt/Sn molar ratio, the Sb/Sn molar ratio, the sintering temperature in air, the temperature for exposure to the SiH₄ gas atmosphere, and the concentration of the SiH₄ during exposure were prepared. The characteristics and preparation conditions of the examples are shown in Table 3. Examples 17-24 were comparative examples. For the comparative examples, either the composition or the preparation conditions were outside the limits prescribed for the present invention. The asterick marks in Table 3 indicate which item was not in accordance with the present invention. Example 1 of Table 2 and Example 1 of Table 3 represent different batches of sensors prepared using the same method. For each example, 16 sensors were prepared. As was done in Example 1, each of the sensors was preheated to 325 o C by its heater and then exposed to clean air at 25 o C and then to 100ppm of SiH₄ gas at 25 o C, and the electrical resistance between the electrodes of each sensor was measured in both atmospheres. The process of measurement in air followed by measurement in SiH₄ gas was carried out ten times for each sensor. The results of measurement are shown in the Table 3 in the form R o /R g1 ,R g10 /R g1 , and R o /R g10 , where R o is the electrical resistance upon the first measurement in air, R g1 is the electrical resistance upon the first measurement in SiH₄ gas, and R g10 is the electrical resistance in SiH₄ upon the 10th measurement. R o /R g1 is an indication of the initial sensitivity of the sensor to SiH₄ gas. A value of 10.0 or above is desirable. R g10 /R g1 is an indication of the stability of the sensor and is desirably 5.5 or less. R o /R g10 is an indication of the sensitivity of an sensor to SiH₄ after being used numerous times, and is desirably 3.0 or above. The values shown in Table 3 are the averages of 16 sensors. It can be seen that all of the examples of gas sensors according to the present invention (Examples 1 - 16) shown in Table 3 had good sensitivity to SiH₄ gas, both initially and after 10 measurements, and thus showed good stability. However, as can been seen from the results for the comparative examples, if the Pt to Sn molar ratio is greater than approximately .10 (Example 21), the resulting sensors have a large change in sensitivity to SiH₄ gas, as indicated by a large value for R g10 /R g1 . On the other hand, if the Pt to Sn molar ratio is less than approximately .02 (Example 22), the initial sensitivity to SiH₄ gas, expressed by R o /R g1 , is too low. If the molar ratio of Sb to Sn is less than approximately .02 (Example 19), the value of R g10 /R g1 is too large, whereas if the molar ratio is greater than approximately .08 (Example 20), the value of R o /R g1 is too small. If the calcining temperature in air of a sensor is below approximately 600 o C (Example 17), the value of R g10 /R g1 is too high, whereas if the calcining temperature is above approximately 850 o C (Example 18), the value of R o /R g1 is too low. The concentration of silane gas to which an sensor prepared according to the present invention is exposed is defined as being approximately 25ppm - 1000ppm. If the concentration falls below this range (Example 23), the value of R g10 /R g1 is too high, whereas if too high a concentration is used (Example 24), the value of R o /R g1 is too low. All of the preceding examples involved calcining in air. However, a silane gas sensor according to the present invention can also be prepared by calcining in an oxidized antimony atmosphere. The following examples illustrate silane gas sensors according to the present invention prepared by this second method. Example 25 In a manner similar to that used for Example 1, an aqueous solution of H₂PtCl₆ was added to SnO₂ in an amount such that the molar ratio of Pt to Sn was 0.04 and was well dispersed using ultrasonic waves. The dispersed solution was quick frozen at -40 o C and then dried in a vacuum freeze dryer. SbOCl was then added to the dried product in an amount such that the molar ratio or Sb to Sn was .04, and the two were mixed with a mortar for 30 minutes. Isopropyl alcohol was then added to the mixture to form a paste, and the paste was then coated on an alumina porcelain tube equipped with electrodes and dried by natural drying. Up until this point, the method was identical to that used in Example 1. However, instead of performing calcining in air as in Example 1, calcining was instead performed in an oxidized antimony atmosphere. The oxidized antimony atmosphere was prepared by preheating a quartz tube (I.D. = 40mm, length of the portion of tube lying within an electric furnace = 500mm) to 700 ± 5 o C in an electric furnace. An alumina boat holding 2.5mg of SbOCl was then encapsulated in the quartz tube and heated for 30 minutes to produce an atmosphere of oxidized antimony therein. Although the ends of the quartz tube extended outside of the electric furnace, the oxidized antimony atmosphere was substantially confined to the 500mm inside the furnace, so that the effective volume of the quartz tube was 628 cm³. The concentration of oxidized antimony in the quartz tube was 2.3 x 10⁻⁸ moles/cm³. The naturally dried alumina porcelain tube was then encapsulated in the quartz tube and calcined in the oxidized antimony atmosphere at 700 ± 5 o C for 30 minutes. After calcining, the alumina porcelain tube was removed from the quartz tube and an electric heater was installed therein. Then, as in the method used for Example 1, the porcelain tube was heated to 300 ± 50 o C by the heater and aged in air for 12 hours. After aging, the tube was heated to 325 ± 5 o C using the heater and was exposed to a silane gas atmosphere containing 100ppm SiH₄ for 10 minutes in order to stabilize the characteristics of the sensor. After exposure to SiH₄ gas, the porcelain tube was again heated to 300 ± 50 o C and aged in air for 12 hours. At the end of aging, the silane gas sensor was complete. Sixteen sensors were prepared in this manner. The sensors were then heated to 325 o C by their heaters and then exposed to clean air at 25 o C and then to eight different atmospheres at 25 o C containing 100ppm of gaseous EtOH, CO, H₂, CH₄, C₂H₄, C₂H₆, NH₃, or SiH₄. In each atmosphere, the electrical resistance between the electrodes of each sensor was measured. The average results are shown in Table 4 in the form R o /R g , where R o is the initial electrical resistance in clean air and R g is the electrical resistance in a particular atmosphere. Measurement was carried out first in air and last in the SiH₄ gas atmosphere. The maximum value of R o for the 16 sensors was 110 kilohms and the minimum value was 43 kilohms. The maximum value of R g in 100ppm of SiH₄ gas was 3.7 kilohms and the minimum was 1.1 kilohms. As can be seen from Table 4, the sensors had excellent sensitivity to SiH₄ gas. The electrical resistance of these sensors was also measured in different concentrations of SiH₄ gas. The results are plotted as Curve (b) of Figure 1. Examples 26 - 48 Using basically the same method as for Example 25, examples of gas sensors were prepared using various values for the Pt to Sn molar ratio, the Sb to Sn molar ratio, the calcining temperature in the oxidized antimony atmosphere, the temperature of the silane gas atmosphere to which each sensor was exposed during preparation, and the concentration of SiH₄ in the silane gas atmosphere. The composition and preparation conditions for each of the examples are shown in Table 5. Examples 41 - 48 were comparative examples in which either the composition or preparation conditions fell outside of the bounds prescribed for the present invention. Those items falling outside the prescribed bounds are marked by asterick marks in the table. For each example, sixteen sensors were prepared. As for the previous examples, each sensor was heated to 325 o C by its heater and measurement of the electrical resistance of the sensor in clean air at 25 o C and the electrical resistance in an atmosphere containing 100ppm SiH₄ gas at 25 o C was carried out 10 times. The results of measurements (avergae values for the 16 sensors of each example) are shown in Table 5. As before, R o is the initial resistance in air, R g1 is the resistance upon the first exposure to SiH₄, and R g10 is the resistance upon the 10th exposure. As can be seen from the results of measurements on the comparative examples, if the molar ratio of Pt to Sn is below approximately .02 ( Example 45) or above approximately .10 (Example 46), the value of R g10 /R g1 is too large, indicating a large change in the sensitivity of the sensor over use. If the molar ratio of Sb to Sn is less than approximately .02 (Example 43), the value of R g10 /R g1 is too large, and if the molar ratio is greater than approximately .08 (Example 44), the value of R o /R g10 is too small. If the calcining temperature in the oxidized antimony atmosphere is less than approximately 600 o C (Example 41), the value of R g10 /R g1 is too large, and if the calcining temperature is greater than approximately 850 o C (Example 42), the value of R o /R g10 is too small. If the SiH₄ gas atmosphere to which an sensor is exposed during preparation contains less than approximately 25ppm of SiH₄ (Example 47), the value of R o /R g10 is too small, and if the SiH₄ concentration is greater than approximately 1000ppm (Example 48), the value of R g10 /R g1 is too large. In contrast, all of examples 26 - 40 prepared according to the present invention had good initial sensitivity to SiH₄ (R o /R g1 ≧ 10), an acceptable change in sensitivty over use (R g10 /R g1 ≦ 5.5), and a good sensitivity to SiH₄ after being used 10 times (R o /R g10 ≧ 3). In order to measure the sensitivity of sensors according to the present invention to silane gases other than SiH₄, the resistance of sensors identical to Example 27 was measured in either SiH₂Cl₂ or (CH₃)₃SiCl. Sixteen sensors were prepared using the method of Example 27. The sensors were heated to 325 o C with their heaters, and the electrical resistance of each sensor measured in clean air at 25 o C (R₀) was measured. Eight of the sensors were then placed in an atmosphere containing 100ppm SiH₂Cl₂ at 25 o C and the other eight sensors were placed in an atmosphere containing 100ppm (CH₃)₃SiCl. and the resistance of each sensor (R g ) was measured. The results of measurements are shown in Table 6 and Table 7. Examples 49 - 60 To illustrate the effects of different concentrations of oxidized antimony in the oxidized antimony atmosphere used in calcining, Examples 49 - 60 of Tables 8 and 9 were prepared. Examples 49 - 54 were identical in composition and method of preparation to Example 32 of Table 5 except that the amount of SbOCl which was calcined to produce an oxidized antimony atmosphere was varied from 0.25mg to 7.5mg. (Example 52 exactly corresponds to Example 32). The effective volume in the quartz tube in which the SbOCl was calcined was 628 cm³, so the concentrations of oxidized Sb in the quartz tube ranged from 2.3 x 10⁻⁹ to 6.9 x 10⁻⁸ moles/cm³. The examples of Table 9 were similar to those of Table 8 except that Sb₂O₃ was used in place of SbOCl. In this case the concentration of oxidized antimony ranged from 2.27 x 10⁻⁹ to 8.2 x 10⁻⁸ moles/cm³. For each example, 6 elements were prepared. Each of the elements was then heated to 325 o C by its heater and measurement of its electrical resistance in clean air at 25 o C (R o ) and its electrical resistance in an atmosphere containing 100ppm SiH₄ at 25 o C (R g ) was performed. As can be seen from both tables, if the concentration of oxidized antimony in the oxidized antimony atmosphere lies between approximately 1 x 10⁻⁹ to 3 x 10⁻⁸ moles/cm³, the resulting element has good sensitivity to SiH₄ gas (R o /R g ≧ 10). For this reason, the concentration of (oxydized antimony) gas is defined as being 2 x 10⁹ to 6 x 10⁻⁸ moles/cm³, and preferably 4 x 10⁻⁹ to 4 x 10⁻⁸ moles/cm³. In all of the preceding examples, when the sensors were exposed to a silane gas atmosphere subsequent to calcining, the silane gas atmosphere was at 25 o C and the sensors were heated by the electrical heaters installed therein. However, if during exposure the sensor is not heated but instead the silane gas atmosphere is heated to 150 - 850 o C, a sensor having the same characteristics as those described above can be obtained. In the above-described examples, calcining either in air or in an oxidized antimony atmosphere was carried out for 30 minutes, and exposure to a silane gas atmosphere was carried out for 10 minutes. However, calcining may be carried for 5 to 50 minutes, and exposure to a silane gas atmosphere may be carried out for 2 to 60 minutes. Whereas in the above examples the sensors were dried by natural drying after the application of a paste, drying may be performed in a constant temperature bath. In the above examples, aging of the sensors was performed in unheated air and the sensors were heated by their heaters. Alternatively, a sensor may be aged in a constant temperature bath without use of the sensor heater. As the solvent for preparing a paste, isopropyl alcohol was used in the above examples. However, other organic solvents may be used, such as an organic solvent comprising 25% by weight of beta-terpineol, 72% by weight of ethyl cellulose, and 3% by weight of butyl-carbitol acetate. Whereas an alumina porcelain tube was used as a base for the above-mentioned paste in the above examples, an electrical insulator which can withstand calcining having a tubular, plate-like, or other shape can be used instead. The heating means provided in the sensor may be a conventional electrical heater or may be an infrared lamp or the like. In the above examples, the oxidized antimony atmosphere was prepared by calcining SbOCl or Sb₂O₃` Alternatively, the atmosphere may be prepared from a solid such as SbCl₃ or a gas such as SbH₃. As the silane gas atmosphere to which sensors were exposed following calcining, SiH₄ in air was used in the above examples. However, SiH₂Cl₂, (C₂H₅O)₄Si, SiCl₄, (CH₃)₂SiCl₂, or other silane may be used instead of SiH₄.

Claims: A silane gas sensor comprising: a silane-sensitive element prepared by mixing SnO2,SbOCl, and Pt such that the molar ratio of Sb to Sn is approximately .02 - .08 and the molar ratio of Pt to Sn is approximately .02 - .10, calcining the mixture at 600-850°C, and exposing said element to a silane atmosphere comprising from 25 to 1000 ppm silane gas (SiH4) for approximately 12 hours, while heating the element to a temperature in the range of 50 to 850°C; and means for heating said silane-sensitive element to a desired temperature. A silane gas sensor according to claim 1, wherein said element is exposed to a silane atmosphere comprising from 25 to 1000 ppm silane gas (SiH4) for approximately 12 hours, while heating the element to a temperature in the range of 325±5°C. A silane gas sensor according to claim 1, wherein said desired temperature is approximately 200-400°C. A process for making a silane gas sensor comprising the steps of: (a) adding SnO2 to an aqueous solution of PtCl so that the molar ratio of Pt/Sn is approximately .02 - .10, thoroughly dispersing said SnO2 in said PtCl-solution, followed by freezing and vacuum drying of the resulting mixture; (b) mixing SbOCl with the product of step (a) so that the molar ratio of Sb to Sn in the resulting mixture is approximately .02 - .08; (c) adding an organic solvent to the product of step (b), forming the resulting mixture into a paste, coating the paste on an electrical insulator having electrodes, and drying said coated paste; (d) calcining said coated insulator produced in step (c) at approximately 600-850°C in air or an oxidized antimony atmosphere; and (e) exposing said calcined insulator of step (d) for approximately 12 hours, to a silane atmosphere comprising from 25 to 1000 ppm silane gas (SiH4) while heating the element to a temperature in the range of 50 to 850°C. A process according to claim 4, characterized in that said calcined insulator of step (d) is exposed to a silane atmosphere comprising from 25 to 1000 ppm silane gas (SiH4) for approximately 12 hours, while heating the element to a temperature in the range of approximately 325±5°C. A process according to any of claims 4 and 5, characterized in that said oxidized antmony atmosphere is produced by calcining an antmony compound so that the concentration of oxidized antimony in said oxidized antimony atmosphere is approximately 2 x 10 exp -9 to 6 x 10 exp -8 moles/cm³, and preferably approximately 4 x 10 exp -9 to 4 x 10 exp -8 moles/cm³.

Inventors: OKAYAMA, YOSHIAKI
Applicants: NOHMI BOSAI KOGYO CO., LTD.
Application year: 1984
Main classification: G01N27
IPC codes: G01N27