Apparatus and method for the removal of impurity gases from inert gases and the ensurance of extremely low levels of hydrogen

Abstract: An apparatus is described for the removal of impurity gases such as O₂, CH₄, CO, CO₂ and H₂ from impure inert gases such as rare gases and N₂. The apparatus comprises an impure inert gas inlet, a housing containing first and second gas sorbing materials and a purified gas outlet. The first gas sorbing material may be a Zr-V-Fe getter alloy if the gas to be purified is a rare gas, whereas it may be a Zr-Fe alloy if the gas to be purified is N₂. The second gas sorbing material is a Zr-Al alloy which ensures that the purified inert gas has an extremely low level of hydrogen. A process for the removal of impurity gases from inert gases and ensuring an extremely low level of hydrogen in the purified gas is also described.

Description: Many manufacturing processes require the use of pure inert gases such as the rare or noble gases, helium, neon, argon, krypton, xenon or nitrogen. One particular example of their use is in the manufacture of semiconductor devices. Before use impure inert gases must be purified by removing contaminant gases such as CO, CO₂, CH₄, O₂ and H₂. GB-A-2,177,079 and GB-A-2,177,080 describe methods for the purification of inert gases to remove these impurity gases. However these two GB Patent Application Publications give no information relative to the problems of the maintenance of extremely low levels of hydrogen. US-A-4 306 887 and US-A-4 312 669 as well as BE-A-792 561 disclose various sorbing materials, also in the presence of H₂, but they do not teach how to reach a hydrogen partial pressure of 10⁻³ Pascal or less, as required in applications to semiconductor industry. Although there is no quantitative measure available which relates the percentage of defects found during the manufacture of semiconductor devices with the partial pressure of hydrogen present during the manufacturing process, it is generally believed that the lower the hydrogen partial pressure, the lower the number of defects found. It is therefore an object of the present invention to provide an improved apparatus for the removal of impurity gases from impure inert gases such as rare gases and nitrogen. It is another object of the present invention to provide an apparatus for the purification of inert gases which ensures that the purified inert gas has an extremely low level of hydrogen. It is yet another object of the present invention to provide a process for the removal of impurity gases from impure inert gases while ensuring an extremely low level of hydrogen in the purified outlet gas. These and other objects and advantages of the present invention will become evident to those skilled in the art with reference to the following drawings and descriptions wherein; Fig. 1 is a partially cutaway cross section of an apparatus of the present invention for the removal of impurity gases from inert gases which ensures that the purified gas has an extremely low level of hydrogen. Fig. 2 is a graph showing the concentration of the impurity CO as distributed throughout the apparatus shown in Fig. 1 after said apparatus has been purifying inert gas for a certain period of time. Fig. 3 is a graph having a curve similar to that of Fig. 2 but shows the concentration of the hydrogen impurity. Fig. 4 is a graph showing the partial pressure of hydrogen throughout the apparatus of Fig. 1. Fig. 5 is a graph showing the H₂ partial pressure in a purified inert gas of a prior art purifier and a purifier of the present invention. The apparatus and the process for the removal of impurity gases from impure inert gases have the features of appended claims 1 and 5 and are herebelow described. Referring now to the drawings and in particular to Fig. 1 there is shown a gas purification apparatus 100 for the removal of impurity gases from inert gases. By inert gases is meant the rare or noble gases helium, neon, argon, krypton and xenon as well as nitrogen. Gas purification apparatus 100 comprises a housing 102 in the form of a cylindrical sheath 104, which is preferably of a metal such as stainless steel. Cylindrical sheath 104 is provided with end caps 106, 106′ which are attached thereto in a gas tight manner. End caps 106, 106′ are again preferably of stainless steel. End cap 106 has attached thereto an inert gas inlet 108 in the form of hollow cylinder, again preferably of stainless steel. End cap 106′ is provided with a purified inert gas outlet 110. Gas outlet 110 is again in the form of a cylinder and preferably made of stainless steel. Cylindrical sheath 104 and end caps 106, 106′ define a hollow space 112 which contains a first gas sorbing material 114 and a second gas sorbing material 116. The first gas sorbing material 114 is preferably a non-evaporable getter alloy chosen from the group consisting of a) an alloy of Zr-V-Fe and; b) an alloy of Zr-Fe. Said first gas sorbing material 114 should consist of a powder having an average particle size of less than about 125 µm in the form of a multiplicity of pellets formed by compressing and/or thermally sintering the powder. These pellets of first gas sorbing material 114 are located in a first region 118 adjacent to inert gas inlet 104. If the gas to be purified is a rare or noble gas such as argon then the first gas sorbing material 114 should be an alloy of Zr-V-Fe. In this case the first gas sorbing material is a non-evaporable getter alloy of Zr-V-Fe whose weight composition is such that percentages by weight of the three elements when plotted on a ternary composition diagram, lie within a polygon having as its corners the points defined by: a) 75% Zr - 20% V - 5% Fe; b) 45% Zr - 20% V - 35% Fe; c) 45% Zr - 50% V - 5% Fe. If the inert gas to be purified is nitrogen then the first gas sorbing material 114 is a non-evaporable getter alloy of Zr-Fe and preferably consists of from 15 to 30% by weight of Fe, balance Zr. The second gas sorbing material 116 is a non-evaporable getter alloy having a composition 5-30% by weight Al, balance Zr. The second gas sorbing material 116 is in the form of pellets of compressed powder, the powder having an average particle size of less than 125 µm, said pellets being located in the housing in a second region 120 adjacent to purified inert gas outlet 110. The process of the present invention for the removal of impurity gases from impure inert gas and ensuring an extremely low level of hydrogen in the purified outlet gas comprises the steps of passing the impure inert gas through an inlet to a housing and contacting the impure inert gas with the first gas sorbing material chosen from the group consisting of: a) a non-evaporable getter alloy of Zr-V-Fe, and; B) a non-evaporable getter alloy of Zr-Fe, to produce a purified inert gas. The purified inert gas is then passed through and contacts a second gas sorbing material to produce a purified inert gas having an extremely low level of hydrogen. The second gas sorbing material 116 is a non-evaporable getter alloy having a composition 5 to 30% by weight aluminium balance zirconium. The purified inert gas having an extremely low level of hydrogen then passes through the purified inert gas outlet 110. EXAMPLE 1 This example is not representative of the present invention but has been designed to aid in the understanding of the present invention. An inert gas purification apparatus comprising a stainless steel housing of 7 cm length and 1 cm diameter, provided with an inert gas inlet and an inert gas outlet was filled with pellets of compressed powder of Zr₂Fe having a particle size less than 125 µm. The pellets were cylindrical in shape having a diameter of 4 mm and a height of 3 mm. The total weight of pellets used was 14 g. The apparatus was heated to 330°C and impure N₂ gas was allowed to flow through the apparatus. The impurity levels of the nitrogen gas were as follows: IMPURITY LEVELS ppm O₂ CH₄ H₂ CO CO₂ A 10 10 9 9 B 9 8.5 7.5 9.5 C 9 9 8.5 8.5 A, B and C are arbitrary identification letters for successive cylinders of impure N₂ gas used in the present example. After a total of about 350 hours of flowing impure N₂ gas through the apparatus, samples of the Zr₂Fe were taken at various positions along the length of the apparatus and analyzed for their content of C , O₂, N₂ and H₂. Fig. 2 shows the results of the C content measurements plotting the C content in cm³ Pascal.mg⁻¹ on the y axis as a function of position 1 in cm of the sample from the inert gas inlet 108. Fig. 3 is a similar curve for H₂. Fig. 4 shows the expected profile of the hydrogen partial pressure within the apparatus. This example shows that sorption of the impurities takes place in a progressive manner through the apparatus. EXAMPLE 2 This example is not representative of the present invention but has been designed to aid in the understanding of the present invention. An apparatus similar to that of Fig. 1 was filled with 720g of pellets of compressed powder of 70% Zr - 24.6% V - 5.4% Fe having a particle size less than 125 µm. The pellets were cylindrical in shape having a diameter of 6 mm and a height of 4 mm. Argon gas of five nines declared purity was passed through the apparatus held at a fixed temperature, at an inlet pressure of 1.3 10⁵ Pascal and an outlet pressure of 1·10⁵ Pascal. The Ar gas flow was 1 litre min⁻¹. After approximately 1 hour the H₂ partial pressure was measured and plotted on the graph of Fig. 5. The temperature was changed and the test repeated. The points were plotted for the temperatures 300°C, 350°C, 400°C and 450°C leading to line A on Fig. 5. EXAMPLE 3 This example represents the present invention. Example 2 was repeated except that a portion of the housing adjacent to the gas inlet was filled with 650 g of Zr-V-Fe pellets identical to those used in Example 2. The remaining space adjacent the gas outlet was filled with 70 g of identical sized pellets of 95% by weight of 84% Zr - 16% Al alloy of particle size less than 125 µm intimately mixed with 5% by weight of Al powder and compressed. The test was repeated as for Example 2 at 400°C and 450°C. During each test the Zr-V-Fe alloy and the Zr-Al alloy were at the same temperature. The results are shown as line B on Fig. 5. As can be seen the resulting hydrogen impurity level of the outlet gas has been reduced by two orders of magnitude i.e., by a factor of 100. EXAMPLE 4 Examples 2 and 3 are repeated except that the Zr-V-Fe pellets are replaced by pellets of Zr₂Fe and the gas to be purified is N₂. The outlet partial pressure of H₂ is considerably reduced when Zr-Al pellets are present adjacent the purified gas outlet.

Claims: An apparatus for the removal of impurity gases, comprising hydrogen, from impure inert gases comprising an impure inert gas inlet (108) in fluid communication with a housing (102) containing a first gas sorbing material (114) and a second gas sorbing material (116), said housing being in fluid communication with a purified gas outlet (110), characterized by the fact that: A. the first gas sorbing material (114) is chosen from the group consisting of: a) a non-evaporable getter alloy of Zr-V-Fe b) Zr-Fe, and B. the second gas sorbing material (116) is a non-evaporable getter alloy having a composition 5-30% Al balance Zr. An apparatus according to claim 1, wherein said inert impure gas is argon, characterized by the fact that: A. The first gas sorbing material (114) is a non-evaporable getter alloy of Zr-V-Fe whose weight composition is such that percentages by weight of the three elements, when plotted on a ternary composition diagram, lie within a polygon having as its corners the points defined by: a) 75% Zr - 20% V - 5% Fe b) 45% Zr - 20% V - 35% Fe c) 45% Zr - 50% V - 5% Fe and B. the second gas sorbing material is a non-evaporable getter alloy having a composition of 16% Al balance Zr. An apparatus according to claim 1, wherein said inert impure gas is nitrogen, characterized by the fact that: A. the first gas sorbing material (114) is a non-evaporable getter alloy consisting of from 15 to 30% by weight of Fe, balance Zr, and B. the second gas sorbing material (116) is a non-evaporable getter alloy having a composition of 16% Al balance Zr. An apparatus according to claim 1, characterized by the fact that: A. the first gas sorbing material (114) is in the form of a multiplicity of pellets of compressed powder the powder having an average particle size of less than 125 µm, said pellets being located in the housing (102) in a region (118) near to said inert gas inlet (108), and B. the second gas sorbing material (116) is in the form of pellets of compressed powder the powder having an average particle size of less than 125 µm, said pellets being located in the housing (102) in a region (120) near to said purified inert gas outlet (110). A process for the removal of impurity gases, comprising hydrogen, from impure inert gases and ensuring an extremely low level of hydrogen in the purified outlet gas, characterized by comprising the steps of: A. passing the impure inert gas through an inlet to a housing, B. contacting the impure inert gas with a first gas sorbing material chosen from the group consisting of a) a non-evaporable getter alloy of Zr-V-Fe, b) Zr-Fe, to produce a purified inert gas, C. passing the purified inert gas through a secod gas sorbing material to produce a purified inert gas having an extremely low level of hydrogen said second gas sorbing material being a non-evaporable getter alloy having a composition 5-30% Al balance Zr, and D. passing the purified inert gas having an extremely low level of hydrogen through a purified gas outlet. A process of claim 5 in which the gas to be purified is argon and the first gas sorbing material is a non-evaporable getter of Zr-V-Fe whose weight composition is such that the percentages by weight of the three elements, when plotted on a ternary composition diagram, lie within a polygon having as its corners the points defined by: a) 75% Zr - 20% V- 5% Fe b) 45% Zr - 20% V - 35% Fe c) 45% Zr - 50% V - 5% Fe. A process of claim 5 in which the gas to be purified is nitrogen and the first gas sorbing material is a non-evaporable getter alloy consisting of from 15 to 30% by weight of Fe, balance Zr. A process of claim 5 in which the second gas sorbing material is a non-evaporable getter alloy having a composition of 16% Al balance Zr. A process according to claim 6, wherein said first gas sorbing material is in the form of multiplicity of pellets of compressed powder the powder having an average particle size of less than 125 µm, said pellets being located in the housing in a region near to said inert gas inlet, and being maintained at a temperature of from 350°C to 450°C; and said second gas sorbing material is in the form of a multiplicity of pellets of compressed powder the powder having an average particle size of less than 125 µm, said pellets being located in the housing in a region near to said purified inert gas outlet, and being maintained at a temperature substantially equal to that of the first gas sorbing material.

Inventors: FURLAN VALERIO, SUCCI MARCO, FURLAN, VALERIO, SUCCI, MARCO
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1989
Main classification: C01B21
IPC codes: C01B21, C01B23, B01D53, B01J20