Method for the selective deposition of epitaxial germanium stress-suspension alloys

Abstract

Method (100, 200) for forming germanium stress-affected layers on a substrate, comprising: positioning the substrate in a processing chamber (302); introducing a germanium precursor material into the processing chamber; forming a metal halide stress-affected precursor material in a reaction volume connected to the processing chamber, wherein the formation of the metal halide stress-affected precursor material comprises sublimation of metal halide crystals into a flowing carrier gas stream, wherein the carrier gas stream comprises N2, H2, Ar or He; introducing the metal halide stress-affected precursor material into the processing chamber; and epitaxially depositing the germanium stress-affected layer onto the substrate.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The technology described here relates to the production of semiconductor components. More specifically, methods for the production of field-effect transistors using stressed materials are described. Description of the related prior art

[0002] Germanium was one of the first materials used for semiconductor applications such as CMOS transistors. However, due to the abundance of silicon compared to germanium, silicon became the predominant semiconductor material of choice for CMOS fabrication. Since component geometries are constantly decreasing according to Moore's Law, the size of transistor components poses challenges for engineers working to create devices that are smaller, faster, consume less energy, and generate less heat. For example, as the size of a transistor decreases, the transistor's channel region also becomes smaller, and the channel's electronic properties become less favorable at higher resistivity and threshold voltages.Charge carrier mobility is increased in the silicon channel region by using silicon-germanium stress-inducing agents embedded in the source / drain regions, as some manufacturers have done for the 45 nm node. However, future nodes will require components with even higher charge carrier mobility. Therefore, there is a continuing need for methods and devices for fabricating semiconductor devices with high charge carrier mobility. SUMMARY OF THE INVENTION

[0003] A method and apparatus for forming stress-sensitive layers on a semiconductor substrate are provided. A germanium stress-sensitive layer can be formed on a substrate by positioning the substrate in a processing chamber, infusing a germanium precursor material into the processing chamber, forming a stress-sensitive precursor material outside the processing chamber, infusing the stress-sensitive precursor material into the processing chamber, and epitaxially depositing the germanium stress-sensitive layer onto the substrate.A device for forming such layers comprises a rotating substrate holder arranged in an enclosure, several gas inlets formed in a wall of the enclosure, at least one gas outlet formed in a wall of the enclosure, a reactive or non-reactive starting material for generating a stress agent precursor material connected to a gas inlet via a first line, a non-reactive starting material for providing a germanium precursor material connected to a gas inlet via a second line, and a heated exhaust system. The heated exhaust system may have a coating applied to reduce the adhesion of exhaust gas components and may include a condensation trap.

[0004] The germanium precursor can be a hydride, and the voltage-inducing precursor can be a metal halide. A selectivity-controlling agent, such as a halide gas, can be incorporated into the reaction mixture to control deposition selectivity on semiconductor and dielectric zones of the substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] To gain a more detailed understanding of how the features of the invention described above can be understood, a more comprehensive description of the invention summarized above, with reference to embodiments, some of which are illustrated in the accompanying drawings, can be found. It should be noted, however, that the accompanying drawings represent only typical embodiments of this invention and should therefore not be considered to limit its scope, as the invention may also include other equally effective embodiments.

[0006] Fig. Figure 1 is a flowchart that summarizes a procedure according to one embodiment.

[0007] Fig. 2 is a flowchart that summarizes a procedure according to another embodiment.

[0008] Fig. Figure 3 is a schematic representation of a device according to another embodiment.

[0009] To facilitate understanding, identical reference numerals have been used wherever possible to designate identical elements common to the figures. It is assumed that elements disclosed in one embodiment can also be used effectively in other embodiments without further discussion. DETAILED DESCRIPTION

[0010] Fig. 1 is a flowchart that describes a procedure 100 according to one embodiment. A semiconductor substrate is used in 102The semiconductor substrate is positioned in a processing chamber. The semiconductor substrate can be any semiconducting material on which a stress-filling layer is to be formed. A silicon substrate on which a transistor structure is to be formed can be used in one example. The semiconductor substrate can have dielectric regions, which in some embodiments are formed on a surface of it. For example, a silicon substrate can have transistor-gate structures and dielectric spacers adjacent to semiconducting source / drain zones, which can be zones of doped silicon or zones in which source / drain materials are to form. Thus, the source / drain zones can comprise the stress-filling layers described here in addition to, or instead of, doped silicon layers.

[0011] The stress-affected layers described here typically comprise metal atoms embedded in a germanium matrix (Ge). x M yLarge metal atoms, for example, group IV metals larger than germanium, such as tin or lead, are useful for adding compressive stress to a germanium matrix. A germanium crystal typically has a cubic structure with a unit cell size of approximately 570 pm. Each germanium atom has a radius of approximately 125 pm, with tin atoms having a radius of approximately 145 pm and lead having a radius between 155 and 180 pm. Adding the larger metal atoms to a germanium crystal matrix results in a larger lattice size, which exerts uniaxial compressive stress on lateral germanium atoms and / or biaxial tensile stress on overlying germanium atoms. Such stress increases the energy of local electrons and narrows the band gap of the germanium, resulting in higher charge carrier mobility compared to unstressed germanium.

[0012] In one aspect, the silicon substrate can have a germanium channel layer, with the voltage-bearing layer being formed adjacent to this as part of a transistor-gate structure. The Ge x M y The stress-inducing agent applies a uniaxial stress to the adjacent germanium layer in this case. In another aspect, the germanium channel layer is deposited above the stress-inducing layer, so that a biaxial tensile stress is applied to the germanium channel layer.

[0013] At 104 A germanium precursor is supplied to the processing chamber containing the semiconductor substrate. The germanium precursor is typically a germanium hydride such as germanium hydrogen (GeH4) or germanium hexahydride (Ge2H6), or higher hydrides (Ge2H6). x H 2x+2) or a combination thereof. The germanium precursor can be mixed with a carrier gas, which can be a non-reactive gas such as nitrogen, hydrogen, or a noble gas such as helium or argon, or a combination thereof. The ratio of the germanium precursor volume flow rate to the carrier gas flow rate can be used to control the gas flow rate through the chamber. This ratio can be any ratio from approximately 1% to approximately 99%, depending on the desired flow rate. In some embodiments, a relatively high velocity can improve the uniformity of the formed layer. In one embodiment of a 300 mm single wafer, the germanium precursor flow rate can range from approximately 0.1 slm (standard liters per minute) to approximately 2.0 slm. For a chamber with a volume of approximatelyWith the above flow rates for the germanium precursor material, 50 l provides a carrier gas flow rate of between approximately 5 slm and approximately 40 slm, resulting in a uniform layer thickness.

[0014] At 106 A metal halide is supplied to the processing chamber to react with the germanium precursor and deposit a layer of metal-doped germanium. The metal halide can be a tin or lead halide gas, for example SnCl4, SnCl2, PbCl4 or PbCl2, or an organometallic chloride with the formula R x MCl y trade, wherein R is methyl or t-butyl, x 1 or 2, M is Sn or Pb and y 2 or 3, such that the formed layer consists primarily of elements of group IV.

[0015] The degree of mobility increase achieved in the adjacent germanium layer depends on the lattice mismatch and the resulting stress communicated by the stress-transfer layer. This, in turn, depends approximately linearly on the concentration of metal atoms in the stress-transfer matrix. As the metal concentration in the stress-transfer layer increases, the energy of valence electrons in the adjacent stressed germanium rises due to orbital bending and stress loading, and the conduction band energy decreases. At a sufficiently high concentration, the semiconductor / metal alloy becomes a direct bandgap material (i.e., metallic). In some embodiments, it may be advantageous to limit the metal concentration so that the alloy remains an indirect bandgap material.In transistor applications, maintaining an indirect bandgap material in the source / drain zones can reduce stray loss.

[0016] The metal halide is supplied to the processing chamber at a throughput of between approximately 10 sccm (standard cubic centimeters per minute) and approximately 300 sccm, for example, between approximately 50 sccm and approximately 200 sccm, or approximately 100 sccm. The metal halide can also be mixed with a carrier gas to achieve a desired velocity and / or mixing efficiency in the processing chamber. The metal halide can be obtained from a solid starting material, from metal halide crystals sublimated in a flowing carrier gas stream such as N₂, H₂, Ar, or He, or the metal halide can be generated by passing a halogen gas, optionally with one of the aforementioned carrier gases, over a solid metal in a contact chamber to allow the reaction M + 2Cl₂ → MCl₄ to proceed, where M is Sn or Pb.The contacting chamber can be connected to the processing chamber adjacent to it via a line, which is preferably short to reduce the likelihood of metal halide particles settling in the line.

[0017] The metal halide and the germanium precursor are typically supplied to the processing chamber via different routes. The germanium precursor is supplied via a first route, and the metal halide is supplied via a second route. The two routes are generally distinct and are kept separate until they enter the processing chamber. In one embodiment, both streams enter through a side wall of the chamber near an edge of the substrate holder, flow across the substrate holder from one side to the other, and into an exhaust system. The substrate holder may rotate during the formation of the stressed layer to improve uniformity.The first path generally connects to a first entry point into the processing chamber, which may include one or more openings in a wall of the chamber or a gas distributor, such as a showerhead, connected to a wall of the chamber. The one or more openings may be located near an edge of the substrate holder, as described above, or may be outlets in a dual or multi-way gas distributor. The second path similarly connects to a second entry point, comparable to the first. The first and second entry points are arranged such that the two streams mix, providing a deposition or layer growth mixture in a region above the substrate holder. The use of a gas distributor may, in some embodiments, reduce or eliminate the need to rotate the substrate during processing.

[0018] For high structural quality, the growth of the stress-affected layer is generally epitaxial. The pressure in the processing chamber is maintained between approximately 5 Torr and approximately 200 Torr, for example, between approximately 20 Torr and approximately 80 Torr, or at approximately 40 Torr. The temperature is maintained between approximately 150°C and approximately 500°C, for example, between approximately 200°C and approximately 400°C, or at approximately 300°C. The temperatures are maintained below the deposition temperature of the metal halide precursor, generally at approximately 600°C or below. The pressures can be below approximately 5 Torr in some embodiments, but reduced pressures also decrease the deposition rate. Under these conditions, the deposition rate is between approximately 50 Å / min and approximately 500 Å / min.

[0019] At 108 A germanium stress-affected layer or germanium / metal alloy layer is formed according to the following reactions. MCl4 + GeH4 → MH2Cl2 + GeH2Cl2 MH2Cl2 + H2 → M + 2HCl + H2 GeH2Cl2 + H2 → Ge + 2HCl + H2 where M is Sn or Pb. Similar reactions occur with the organometallic chlorides described above. Higher-order germanes give a mixture of chlorgerman intermediates that dissolve similarly in germanium deposits. Hydrogen gas can be supplied to the chamber to promote the deposition reactions. A hydrogen gas flow rate of between approximately 5 slm and approximately 40 slm may be included with some or all of the precursors to provide an ambient hydrogen concentration.

[0020] The layer is typically deposited in a thickness between approximately 300 Å and 800 Å. The concentration of tin atoms in a germanium matrix can be determined according to the method. 100The concentration of lead atoms in the germanium matrix can range from approximately 1% to approximately 12%, such as from approximately 3% to approximately 9%, for example, approximately 6%. If lead is used, the concentration of lead atoms in the germanium matrix can range from approximately 0.2% to approximately 5%, such as from approximately 1% to approximately 3%, for example, approximately 2%. If desired, a mixture of lead and tin can be used. Lead can achieve a higher band gap reduction than tin at lower dosages, and the use of a mixture of lead and tin can be advantageous in some embodiments to provide machinability (i.e., tin halides are more stable at elevated temperatures than lead halides) with some increase in band gap reduction.

[0021] Fig. 2 is a flowchart that describes a procedure 200 according to another embodiment. The method 200 is in many respects similar to the procedure 100similar and can be used to achieve similar results when processing substrates with semiconducting and dielectric zones. 202 A substrate with semiconducting and dielectric components is placed in a processing chamber with the characteristics described above, in conjunction with Fig. 1 described arranged. 204 A germanium precursor material, which is one of those associated with Fig. The germanium precursor substances described in section 1 are supplied to the chamber via a first pathway. 206 A tin or lead precursor or a mixture of tin and lead precursors, in which one or more of the following are associated with Fig. The chamber may contain tin or lead precursor materials as described in section 1, provided via a second route.

[0022] At 208A deposition control agent is provided to the processing chamber. This agent is used to control the deposition of germanium, tin, and / or lead on the substrate surface. The deposition control agent selectively removes deposited substances from the dielectric regions of the substrate more rapidly than from the semiconducting regions. Therefore, the deposition control agent can be considered a selectivity control agent because, in some embodiments, the selectivity can be controlled by adjusting the amount of the selectivity control agent relative to the reactive substance in the reaction mixture.

[0023] The deposition control agent is typically a halogenated substance, such as a halide, for example, HCl, HF, or HBr. In one embodiment, the deposition control agent is HCl. The deposition control agent can be supplied at a flow rate between approximately 10 sccm and approximately 1000 sccm, such as between approximately 100 sccm and approximately 500 sccm, for example, approximately 200 sccm. The layer growth selectivity and layer deposition rate can be controlled by adjusting the volume ratio of the deposition control agent to the germanium precursor. A higher ratio reduces the overall deposition rate but improves the selectivity. The volume flow ratio of the deposition control agent to the germanium precursor is between approximately 0.01 and approximately 0.1 in most embodiments, such as between approximately 0.02 and approximately 0.06, for example, approximately 0.04. At the upper end of the range, the deposition rate is approximately...50 Å / min, while the deposition rate at the lower end of the range is approximately 500 Å / min. However, no layer growth can be observed in the dielectric zones of the substrate at the upper end of the range, whereas at the lower end of the range, the deposition rate in the semiconducting zones is approximately 50 times the deposition rate in the dielectric zones.

[0024] The amount of compressive stress introduced through the stress-bearing layer can be controlled at low metal concentrations by varying the concentration of metal incorporated into the stress-bearing matrix. The metal concentration can be controlled by adjusting the ratio of metal precursor to germanium precursor in the reaction mixture. In most embodiments, the volumetric throughput ratio of metal precursor to germanium precursor supplied to the processing chamber will be between approximately 1% and 20%, for example, between approximately 4% and 12%, or approximately 8%.

[0025] Fig. Figure 3 is a schematic representation of a device 300 according to another embodiment. The device 300It can be used to practically implement the methods described here for forming stress-loaded layers. A processing chamber 302 features a substrate holder 308 , which may be a rotating substrate holder arranged within an interior space. A heat source 306 is one side of the substrate holder 308 arranged facing the substrate. Alternatively, a heat source can be placed in the substrate holder. 308be embedded. A chamber with a heated substrate holder, as described in jointly allocated US patent 7172792 entitled "Method for forming a high quality low temperature silicon nitride film," granted on February 6, 2007, can be adapted to form the device described herein and to implement the methods described herein. A chamber with a lamp heating module, as described in jointly allocated US patent publication number 2008 / 0072820 entitled "Modular CVD Epi 300 mm Reactor," published on March 27, 2008, can be adapted to form the device described herein and to implement the methods described herein. An Epi TM 300 mm reactor or a 300 mm xGen TMThe two chambers, both of which can be obtained from Applied Materials, Inc., Santa Clara, California, can be adapted to manufacture and use the embodiments described herein. The machining chamber 302 can be accessed via a spray head 304 The chamber can be equipped with a gas inlet. Alternatively, gas can enter the processing chamber through a side inlet. 320 be provided, which is attached to a side wall 360 the chamber 302 is connected.

[0026] A supply system 328 , which is a chemical dispensing system 310 and a metal precursor material contact chamber 312 includes, is connected to the chamber via various lines 302 connected. A first line 322 and a second line 324 can the supply system 328 with the optional spray head 304 connect. To carry out the procedures described here, the spray head can be used. 304It should be a two-way spray head to ensure mixing of the precursor substances before they enter the chamber. 302 to prevent this. An exemplary two-way spray head is described in the jointly allocated US patent 6,983,892 entitled “Gas distribution showerhead for semiconductor processing”, granted on January 10, 2006.

[0027] Alternatively or additionally, cross-flow gas injection can be practiced by using a first and a second cross-flow gas line. 316 and 318 at the side entrance point 320 can be provided for. An example of a cross-flow gas injection configuration is described in US Patent 6,500,734. The device 300 It can include both a spray head configuration and a cross-flow injection configuration, or only one or the other configuration.

[0028] The chemical dispensing system 310delivers germanium precursor materials to the chamber, optionally with carrier gases such as nitrogen and / or hydrogen. 302 The chemical dispensing system 310 Deposition or selectivity control substances can also be added to the chamber. 302 deliver. The chemical dispensing system 310 May include liquid or gaseous feedstocks and control elements (not shown) assembled in a gas control panel.

[0029] The contact chamber 312 can either go to the side entrance point 320 or the spray head 304 be connected, specifically via a cable 314 , which is arranged to deliver a metal precursor material to the chamber 302 to transport. The lines 314 , 316 and 322They can be heated to a temperature between approximately 50°C and approximately 200°C to control or prevent condensation of the metal halide substances they contain. The contact chamber 312 It typically contains a bed of solid metal or metal halide crystals. The metal halide crystals may be sublimated in a carrier gas supplied through one or both of the gas supply lines. 362 and 364 The solid metal can be brought into contact with a halogen gas precursor supplied through one or both of the gas supply lines. 362 and 364 is provided. In one embodiment, a halogen gas starting material is supplied via a first gas supply line. 362 provided, while a carrier gas is supplied via a second gas supply line 364The gases can be passed through a powdered metal or metal halide fluid bed to enhance contact, either to sublimate or react. A mesh screen or filter can be used to prevent particles from entering the chamber. 302 to prevent this. Alternatively, the gases can flow over a fixed, solid metal or metal halide bed.

[0030] An exhaust system 330 is addressed to the chamber 302 connected. The exhaust system 330 It can be connected to the chamber at any suitable point, which may depend on the point of gas entry into the chamber. For gas entry through the spray head. 304 Can the exhaust system be attached to a bottom wall of the chamber around the heat source? 306It may be connected, for example, via an inlet or multiple inlets, or via an annular opening. In some embodiments, an annular collecting pipe can be arranged near an edge of the substrate holder and connected to the exhaust system. 330 be connected. For cross-flow designs, the exhaust system can be 330 to a side wall of the chamber opposite the side entrance point 320 be connected.

[0031] An exhaust pipe 340 connects an exhaust cap 332 via a throttle valve 366 with a vacuum pump 352 A casing 368 surrounds the exhaust pipe 340 and the throttle valve 366 from the exhaust cap 332 up to an entrance 350 the vacuum pump 352 The casing 368 enables heat control of the exhaust pipe 340To prevent condensation of exhaust gases within the duct, any heat-emitting medium, such as steam, hot air, hot water, or another hot liquid, can be used to maintain the exhaust duct at a temperature above the exhaust gas's dew point. Alternatively, the ductwork can incorporate resistance heating elements (i.e., a heating blanket). If desired, a condensate trap can be included. 336 via a valve 338 to the exhaust pipe 340 to be connected to prevent the capture of any condensate in the exhaust system 330 to increase it even further. The vacuum pump 352 carries via a disposal line 354 , which is typically not heated or insulated, to a disposal system 356 out, and purified gas is produced at 358 drained. To prevent wetting or bacterial growth in the exhaust pipe. 340 To further reduce the exhaust pipe 340be coated with quartz or an inert polymer material.

[0032] Cleaning agents activated by plasma or ultraviolet light can be introduced into the exhaust system. 330 be coupled in, specifically through an active source 334 , which may be connected to a microwave or RF chamber to generate active cleaning agents. A cleaning gas line 326 can remove cleaning gases from the chemical delivery system 310 the exhaust pipe 340 through the active source 334 Provide continuously if desired. The use of active cleaning agents allows the cleaning process to take place at reduced temperatures.

[0033] A method for cleaning a chamber used to carry out the procedures described herein, such as the chamber 302 or any other chamber that is responsible for carrying out the proceedings 100 and 200The process involves supplying the chamber with a halogen gas, which converts residues into volatile halides. During purification, the chamber temperature is typically kept below approximately 600°C, and metal deposits are removed in MCl₂. x , typically SnCl x or PbCl x The halogen gas can be chlorine gas, fluorine gas, HCl, or HF. The chamber can be heated to such an extent that separate heating of the exhaust pipe, especially if the exhaust pipe is insulated, is unnecessary. Alternatively, if desired, the chamber temperature can be kept below approximately 400°C and the exhaust pipe... 340 They must be heated to prevent condensation.

[0034] Alternative embodiments for forming a stress-affected layer may include cyclic processes to form an essentially pure epitaxial germanium layer and then to form a metal-doped epitaxial germanium layer, wherein the pure and the doped layers are formed by generally starting and stopping the influx of the metal precursor material according to the above-described recipes, while maintaining the influx of the germanium precursor material.In other embodiments, a staggered stress layer can be formed by initiating an influx of the germanium precursor material over a period of time to form an epitaxial starting layer of essentially pure germanium, initiating an influx of the metal precursor material with an initial throughput, and then increasing the throughput of the metal precursor material to a final throughput according to any desired linear or non-linear pattern. Such a staggered stress layer can exhibit stronger adhesion to underlying layers, thereby providing enhanced electron mobility.

[0035] Although the foregoing is directed towards embodiments of the invention, other and further embodiments of the invention are also conceivable without deviating from its basic scope.

Claims

[1] Method for forming germanium stress-affected layers on a substrate, comprising the following: Positioning the substrate in a processing chamber; Introducing a germanium precursor material into the processing chamber; Formation of a metal halide stress-inducing precursor in a reaction volume connected to the processing chamber; Introducing the stress-inducing precursor material into the processing chamber; and epitaxial application of the germanium stress-inducing layer to the substrate. [2] The method of claim 1, further comprising introducing a selectivity control substance into the chamber. [3] Method according to claim 2, wherein the substrate comprises dielectric regions and semiconducting regions, and the selectivity control material controls a relative growth rate of the germanium voltage-applied layer on the dielectric and semiconducting regions. [4] Method according to claim 1, wherein forming the stress precursor material comprises allowing a halogen-containing carrier gas to flow over a solid metal material. [5] Method according to claim 1, wherein the stress-inducing precursor material organotin chlorides with the general formula R x SnCl y comprising, wherein R is methyl or t-butyl, x 1 or 2 and y 2 or 3, and the stress-inducing precursor material is sublimated from anhydrous solid crystals into a carrier gas stream comprising N2, H2, Ar or He. [6] Method according to claim 1, wherein the germanium precursor is monogerman or digerman, and the stress-exercise precursor is an organotin chloride. [7] Method according to claim 1, wherein the germanium precursor material and the stress-inducing precursor material are guided through the chamber from one side of the chamber to an opposite side of the chamber. [8] Method according to claim 1, wherein the germanium precursor material and the stress-inducing precursor material are introduced into the chamber by means of a spray head. [9] Method according to claim 3, wherein the selectivity control material selectively removes material deposited on the dielectric regions of the substrate. [10] Method according to claim 2, wherein the epitaxial application of the germanium stress-affected layer to the substrate comprises maintaining the processing chamber at a pressure between approximately 5 Torr and approximately 80 Torr and a temperature between approximately 150°C and approximately 400°C. [11] Method according to claim 10, wherein the throughput ratio of the voltage-applying precursor material to the selectivity control material is between approximately 2:1 and approximately 100:

1. [12] Device for forming a stress-affected layer on a substrate, comprising: a rotating substrate holder arranged in an enclosure; several gas inlets formed in a wall of the enclosure; at least one gas outlet formed in a wall of the enclosure; a generator of a reactive precursor substance connected to a gas inlet via a first line; a source of a non-reactive precursor material connected to a gas inlet via a second line; and a heated exhaust system that includes a condensate trap. [13] Device according to claim 12, wherein at least one gas inlet is formed in a wall of the enclosure near the substrate holder, and the reactive source is connected to a halide source. [14] Device according to claim 12, wherein the heated exhaust system comprises sheathed pipes and valves and an adhesion-reducing coating. [15] Device according to claim 14, wherein the heated exhaust system further comprises a vacuum pump, and the sheathed lines and valves terminate at an inlet of the vacuum pump. [16] Device for forming a stress-affected layer on a substrate, comprising the following: a rotating substrate holder arranged in an enclosure; a heat source located in the enclosure beneath the substrate holder; a gas inlet formed in a side wall of the enclosure near an edge of the substrate holder and having a flow direction substantially parallel to a top of the substrate holder; a gas outlet formed in a side wall opposite the gas inlet and near the edge of the substrate holder; a first precursor pathway connected to the gas inlet and a germanium hydride source; a second precursor pathway connected to the gas inlet and a metal halide or organometallic halide source; and an exhaust system that includes sheathed pipes and valves connecting the gas outlet to a vacuum pump.