Method for the transfer of a gas of high purity
United States Patent Sifre et a1.  
 [ Apr. 29, 1975 METHOD FOR THE TRANSFER OF A GAS OF HIGH PURITY [75] Inventors: Gaston Sifre, Orsay; Rene Barandon, Morangis, both of France [73] Assignee: Societe Generale de Constructions Electriques et Mecaniques (Alsthom), Paris, France [22] Filed: May 25, 1972 [21] Appl. No.: 256,765  
 [30] Foreign Application Priority Data May 27, 1971 France 71.19398 [52] US. Cl. 148/175; 23/232; 29/575; 117/106 R; 134/22 R; 134/22 C; 134/28; 134/30; 137/15;148/186;148/189; 156/17 [51] Int. Cl. 110117/00; C23g 1/08; B08b 9/06 [58] Field of Search 117/106 A, 106 C, 106 R; l48/174,1.6, 175; 134/2, 21, 22 R, 37, 22 C, 134/28, 30; 23/232 R; 75/224; 156/17; 29/575; 137/15 [56] References Cited UNITED STATES PATENTS 2,935,429 5/1960 Grotlisch 134/28 2,948,642 8/1960 MacDonald 148/l.5 2,965,523 12/1960 Englc [34/28 3,168,422 2/1965 Allegretti 148/175 3,461,003 8/1969 Jackson 148/175 3,511,727 5/1970 Hays 156/17 GAS USE 3,645,812 2/1972 Sussmann 148/175 OTHER PUBLICATIONS J. F. Pudvin and F. .I. Biondi, Transistor Technology, Vol. 111, D. Van Nostrand Company, Inc., New York, N.Y., 1958, pages 102-116.  
 Primary Examiner-Morris O. Wolk Assistant Examiner-Dale Lovercheck Attorney, Agent, or Firm-Craig &amp; Antonelli [57] ABSTRACT Method for transferring very pure ultimate use gas from a cylinder to a user apparatus through a tube fitted with valves and other distributing and control elements, the tube being fitted, more particularly stated, for regulating flow and pressure. The tubing, valves and other elements are subjected, before the transferring, and before assembly as a flow line, to chemical etching by aqueous solutions of volatile noncontaminating acids, then to decontamination by extensive sweeping by a very pure hot gas to volatilize the acids. The ultimate use gas is subsequently transferred, after assembly and additional sweeping by a pure hot gas, through the decontaminated tubing, valves and other elements, to the ultimate user apparatus, as for example in the transferring of dopantcontaining gas in the manufacturing of semiconductors by epitaxy or diffusion.  
 9 Claims, 1 Drawing Figure GAS SUPPLY SOURCE METHOD FOR THE TRANSFER OF A GAS OF HIGH PURITY The present invention concerns a method and a device for transferring very pure gas from a supply to a user apparatus through a pipe fitted with valves and other distributing and control elements, more particularly for regulating flow and pressure. It applies more particularly to the transferring of gas intended for providing the depositing of doping impurities at a predetermined low percentage on pellets made of a basic semiconductor material, for example, silicon, germanium, gallium arsenide, silicon carbide. etc., with a view to manufacturing semiconductors, in installations for treating by epitaxy or by diffusion. or else for any other operation for the treatment of semiconductors requiring the presence of very pure gas. It is also suitable for other applications, such as the transferring of carrier gas for analysis by chromatography or the trans ferring of gas for medical uses.  
  The very pure gases supplied in industry are generally stored either in a cylinder under very high pressure, or sometimes. in the liquid state in a heat-proof container. Their impurity content is in the order of l volume per million (10*). After possible vaporization and expansion, they are often subjected to additional purifying in a purifying apparatus, for example, in the case of hydrogen, to a treatment by selective diffusion through a palladium or palladium alloy membrane. which further reduces their impurity contents.  
  Nevertheless, it has been observed that known transferring methods and devices do not enable this high purity to be maintained. In the case of the manufacturing of semiconductors. the length of the life of minority carriers in the basic substance, which is originally in the order of 1000 microseconds, can fall to a value in the order of 2 to 5 microseconds after manufacturing of a junction&#39;by epitaxy or diffusion. A treatment for trapping the impurities enables it to be raised again, but it generally remains less than 100 microseconds.  
  The aim of the present invention is to overcome the above disadvantages. and to provide a method and a device for transferring very pure gas which ensure the maintaining of that purity up to the using apparatus,  
 and which ensure, more particularly, in the case of gas intended .for the manufacturing of semiconductors, after treatment of the basic substance by epitaxy by means of a very pure carrier gas charged with a dosed quantity of the impurity chosen, the producing of semiconductors having a length oflife of the minority carriers which is substantially greater than 100 microsec onds.  
  The method according to the invention is characterized in that the tubing, valves and other distributing and control elements, are subjected, prior to the transferring of gas. to chemical etching by means of acids which are not liable to introduce harmful impurities into the gas, then to a decontamination treatment by extensive sweeping by means of a very pure gas.  
  It comprises, moreover, preferably at least one of the following characteristics.  
  For the transferring of gas intended for depositing very small quantities of a doping impurity in a pellet made of basic material with a view to manufacturing semiconductors, chemical etching is effected by means of acids free from doping impurities, preferably by means of a mixture of nitric. hydrochloric and hydrofluoric acids having electronic purity.  
  Each tubing element, each valve and each other element. are subjected, after etching, to extensive sweeping by means of a very pure hot gas; the elements are assembled in a place sheltered from impurities; then &#39;the assembly is subjected to sweeping by means of a very pure gas.  
  The device according to the invention is characterized in that the valves it comprises are manually or pneumatically operated. and in that the tubing, the valves and the other distributing and control elements have their internal surfaces made of stainless steel, metal or alloy, silica glass, polymonochlorotrifluorethylene, or any other polymer having analogous mechanical strength, chemical inertia characteristics and favorable characteristics under vacuum conditions, are etched by means of acids free from impurities liable to be drawn in with the gas to be transferred, subjected to a decontamination treatment by extensive sweeping by means of a very pure hot gas. then assembled. after the fluid-tight sealing, under vacuum conditions. of each component has been ascertained. 1n the case of the use of stainless steel tubing or valves, there are preferably made of steel containing approximately 18 percent chromium and 12 percent nickel.  
  It has been observed, more particularly, that the use of electromagnetic valves was to be abandoned. for they have certain disadvantages such as vibrations or overheating, and that the use of any elements comprising glass other than silica glass, as well as the use of grease in vacuum conditions was also to be abandoned.  
  1n the case of the transferring of carrier gases for manufacturing semiconductors by epitaxy or by diffusion, the use of any phosphoric acid. which would be liable to contaminate the gas. for the etching operation is more particularly to be avoided, and nitric hydrochloric and hydrofluoric acids having what is known as electronic purity, such as they are used for treating silicon, which is the usual basic material in production, should be used.  
  The assembling of a device for the transferring of very pure hydrogen from a storage cylinder under high pressure to a treatment furnace for depositing by epitaxy is described herebelow by way of an example with reference to the accompanying FIGURE which shows a schematic representation of an embodiment of gas supply connection tubing and use apparatus according to the present invention.  
  The elements of the gas transferring device from a gas supplying source 1 to a gas use apparatus 2 include tubing 4, connections 5, valves 3, flowmeters 6 which comprise, for example at least on their internal surface, stainless steel containing approximately 18 percent chromium and 12 percent nickel, such as that corresponding to AFNOR standard Z3 CND 18-12 or to designation 316L of US standard AlSl, are subjected, after removal of grease with a hot chlorated solvent such as trichlorethylene, to chemical etching in a bath at 60C having a volumetric composition of about 16 percent nitric acid at adensity of 1.42, 3 percent hydrochloric acid at a density of 1.189, 4 percent by volume of hydrofluoric acid at a density of 1.18 and 77 percent de-ionized water, for several minutes. then they are washed, rinsed and dried. They are then passivated by immersion in a cold nitric acid bath at a density of 1.1 15 during about 15 minutes, rinsed then subjected to a decontamination treatment by sweeping by means of a very pure hot gas. for example nitrogen at 150C. for about an hour.  
  All the above operations must be performed in a fume cupboard of the kind known as controlled laminar flow type. and in usual conditions for the treating of the semiconductor substance itself.  
  The various elements are then stored in filtered air and assembled. after the fluid-tight sealing of each of them under vacuum conditions has been ascertained. After the fluid-tight sealing of the assembly under vacuum conditions has been ascertained. the latter is subjected to a decontamination treatment by sweeping by means of a very pure gas. for example. hydrogen containing not more than 0.1 volume per million of oxygen and dampness. during several days.  
  Although the method and device which have just been described are to be considered as preferable. it will be understood that various modifications may be made thereto without going beyond the scope of the invention. it being possible to replace certain operations of the method by others which would have the same technical function therein. and to replace certain substances of the device by others having equivalent properties.  
 We claim:  
  I. A method for transferring very pure use gas from a supply to a user apparatus through tubing fitted with valves and other distributing and control elements for regulating the flow and pressure of said use gas, said method comprising:  
 chemically etching the tubing. valves and other distributing and control elements prior to assembly with an aqueous solution of volatile acids which are free from impurities which might be introduced into the use gas:  
 decontaminating the chemically etched tubing.  
 valves and other distributing and control elements so formed by extensively sweeping them with a very pure sweeping gas:  
 assembling said tubing. valves and other distributing and control elements together; and passing said very pure use gas from said supply to said user apparatus through the thus etched and decontaminated tubing.  
  2. The method according to claim I, wherein said tubing. valves and other distributing and control elements are further decontaminated after assembly but prior to the passage of said very pure use gas therethrough by sweeping with another very pure sweeping gas.  
 3. The method according to claim 1, wherein said another very pure sweeping gas is hydrogen containing not more than about 0.l volume per million of oxygen and water vapor. and further wherein said very pure sweeping gas is nitrogen.  
  4. The method according to claim 1, wherein said volatile acid is selected from the group consisting of nitric acid, hydrochloric acid and hydrofluoric acid.  
  5. A method according to claim 1, wherein said tubing. valves and other distributing and control elements are washed. rinsed and dried after chemical etching and prior to sweeping with decontaminating gas.  
  6. A method according to claim 5, wherein said tubing. valves and other distributing and control elements are passivated by immersion in a cold nitric acid bath after washing. rinsing and drying and prior to sweeping with decontaminating gas.  
  7. In a process for making a semiconductor in which a use gas consisting essentially ofa very pure carrier gas charged with a doping quantity of a doping impurity is transferred from a use gas supply to a user apparatus where said doping impurity is deposited on a basic semiconductor material at a predetermined low percentage. said use gas being transferred from said use gas supply to said user apparatus via tubing fitted with valves and other distributing and control elements for regulating the flow and pressure of said use gas. the improvement for maintaining the purity of said very pure use gas comprising chemically etching the tubing. valves and other distributing and control elements prior to assembly together in a bath of an aqueous volatile acid selected from the group consisting of nitric acid. hydrochloric acid and hydrofluoric acid and mixtures thereof; decontaminating the chemically etched tubing. valves and other distributing and control elements by extensively sweeping them with a first very pure decontaminating gas; assembling said tubing. valves and other distributing and control elements together&#34;. and sweeping said tubing. valves and other distributing and control elements with a second very pure decontaminating gas prior to the transfer of said use gas therethrough.  
  8. The method according to claim 7, further comprising passivating the tubing. valves and other distributing and control elements in nitric acid after the chemical etch but prior to the sweeping with decontaminating gas with the first very pure decontaminating gas.  
  9. The method according to claim 8, further comprising washing, rinsing and drying the tubing. valves and other distributing and control elements after the chemical etch and prior to passivation.