Abstract:
Joint assembly for connecting two ceramic material tubes. One of the tubes is provided with a cup and the other tube with a ball to be received in said cup. To prevent atmospheric gases entering at the joint of the cup and ball inert gas in introduced at super atmospheric pressure at the innerface between cup and ball.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to connecting two tubes through a ball-cup connection. This connection should be tight with regard to diffusion of gasses from the surrounding atmosphere.  
         BACKGROUND ART  
         [0002]    In processes at elevated temperatures in furnaces, process tubes are used from ceramic material. Such tubes are connected to feed/discharge tubes outside the furnace or other tubes such as tubes in which a thermocouple or other sensor is introduced. Connection between the two tubes should prevent the ingress of gasses from the surrounding atmosphere into the process tube and leakage of process gasses to the surroundings.  
           [0003]    In the prior art, ball-cup connections are used to achieve this purpose. When the cup and the ball are dimensioned correctly and are perfectly mating, a high quality seal is achieved, However, in practice it is difficult to control the dimensional tolerances of the cup and the ball. Further, when the ball is larger than the cup, breakage of the cup can easily occur. Therefore the nominal diameter of the ball should always be slightly smaller than the diameter of the cup. Consequently, the bail and cup connection is not a perfect seal. Furthermore, if the ball and the cup are made of different materials some tolerances need to be allowed to accommodate differences in thermal expansion when the furnace is heated from room temperature to process temperature. Because the requirements regarding the leak integrity of such a connection have become more and more stringent, in the art the ball-cup connection is provided with an additional O-ring seal. However, in situations where the temperature of the connection is fairly high, the lifetime of the O-ring is impractical short due to aging of the material of the O-ring.  
           [0004]    In U.S. Pat. No. 5,653,479 a ball-cup connection is proposed to connect a low pressure chemical vapor deposition (LPCVD) tube to a vacuum system via a vacuum exhaust tube. The cup, attached to the vacuum exhaust tube, contains annular shaped grooves, cut into the inside surface of the cup. Each groove contains a connection to a second vacuum system. When the ball, attached to the LPCVD process tube, is inserted into the cup, the open side of the grooves is bounded by the surface of the ball. The grooves are evacuated by the second vacuum system and this differentially pumped vacuum seal is sufficient to contain a rough vacuum, down to 1 mTorr.  
           [0005]    This design has several limitations and disadvantages. The design of the cup is rather complex. All embodiments show three grooves, machined into the cup, which cup is of stainless steel. Stainless steel is not allowed a the inlet side of an oxidation furnace to connect e.g. an outside torch system to the process tube because it would lead to metallic contamination. Cutting the grooves in a ceramic cup, like quartz, is much more complicated and costly than cutting the grooves in stainless steel. Furthermore, for reasons given above, the diameter of the ball will always be smaller than die diameter of the cup. The more so as in the embodiment shown in U.S. Pat. No. 5,653,479 the cup is of stainless steel and the ball of ceramic material. Even when at room temperature the cup and ball fit exactly together, when heated the cup will be larger than the ball because of the larger thermal expansion coefficient of stainless steel as compared to quartz. The actual contact between cup and ball will be a line rather than a plane. Over the line contact, gas transport through diffusion can easily take place. Another disadvantage of the proposed connection is that it is directed towards vacuum applications. When the process tube is at atmospheric pressure, just as the surrounding atmosphere, it is not convenient to apply a vacuum to the connection because then air from outside and process gas from the inside are sucked into the coupling and meet each other there, resulting in potentially undesired reactions. Finally, the design requires a second vacuum system with all its connections and supplies, adding to die complexity of the installation.  
           [0006]    It is the object of the invention to avoid the above disadvantages and provide in a high temperature resistant ball-cup connection that prevents the ingress of gasses from the surrounding atmosphere into the process tube and that is in particular suited for atmospheric process tubes.  
         SUMMARY OF THE INVENTION  
         [0007]    According to one aspect of the invention a joint assembly is provided for connecting two tubes, one of said tubes comprising at its extremity a cup shaped member and the other of said tubes comprising at its extremity a ball shaped member to be received in said cup shaped member, said cup shaped member being provided with a gas feed channel discharging at one end into an annular groove at the cup/ball interface and at the other end connected to a conduit outside said cup shaped member, said conduit being connected to an inert gas source having a pressure above atmospheric pressure.  
           [0008]    Because of the use of overpressure of the inert gas introduced between the parts of the ball joint, inert gas will be flowing from the annular groove through the gap between the cup shaped member and the ball shaped members in one direction towards the surrounding atmosphere and in the opposing direction towards the interior of the process tube. The supply of inert gas should be such that in the gap an inert gas flow speed is maintained at a sufficiently high level such that back diffusion of atmospheric gasses such as oxygen and water vapor through the ball joint is reduced to an acceptable minimum.  
           [0009]    The joint assembly according to the invention can be used in any application wherein two tubes have to be connected to each other and where the connection should be heat resistant and prevent the ingress of gasses from the surrounding atmosphere. More particular this joint is used in a furnace for processing semiconductor wafers.  
           [0010]    According to a preferred aspect of the invention the process tube is provided with a ball at its extremity whilst the tube connecting thereto is provided with a cup shaped member. The tube connecting to the furnace tube can be a gas feed/discharge tube. However, the invention can also comprise the connection with a sensor tube. Typically, the process tube comprises a ceramic material like quartz or SiC. Preferably, the connecting tube comprises also a ceramic material like quartz or SiC, although for some applications it is possible that the connecting tube comprises a metal like stainless steel.  
           [0011]    According to a further aspect, the invention relates to a method for operating of a furnace comprising a ceramic material furnace tube in which a substrate is processed and through which a processing gas is flowed, a ceramic material feed/discharge tube connected to said furnace tube through a ball joint and through which said processing gas and possible reactants are fed/discharged, wherein between the cup and the ball of said ball joint an inert gas having a super atmospheric pressure is introduced.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]    [0012]FIG. 1 is an illustration of a first embodiment of a ball-cup connection, comprising a groove in the cup.  
         [0013]    [0013]FIG. 2 is an illustration of a second embodiment of a ball-cup connection, comprising a groove in the ball. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    A process tube, connecting to a connecting tube by a ball-cup connection is shown in FIG. 1. The process tube  1  comprises a gas feed/discharge tube  2  which is provided at its extremity with the ball member  3 . The connecting tube  10  is provided at its extremity with the cup member  11 . Of connecting tube  10 , only the part close to the extremity provided with the cup member  11  is shown. The walls of connecting tube  10  delimit a process gas feed/discharge channel. The cup is provided with an inert gas feed channel  12 , discharging at one en into an annul groove  13 , cutted-out into the cup and at the other end provided with a connection  15  to be connected to a conduit, said conduit being connected to an inert gas source having a pressure above atmospheric pressure. Both said conduit and said inert gas source are not shown in FIG. 1. Although it is not essential, it is standard practice to provide the process tube with the ball member because the cap member is more fragile than the ball member is. Repair or replacement of the sconnecting tube in case of a broken cup is easier and less costly than repair or replacement of the process tube. The lower end of process tube  1  is provided with a tube flange  4  and rests on a support structure, not shown. The support structure can be provided with a further tube, such that process gas is supplied into process tube  1  via connecting tube  10 , and process gas is exhausted via the further tube. Supply and exhaust can also be reversed. As an alternative, the process tube itself can be provided with a further tube to provide for supply and exhaust functions of the process gas.  
         [0015]    In FIG. 2, the ball member  10  is provided with an annular groove  14  in which the gas feed channel  12  discharges. The other end of channel  12  is provided with connection  15 , to be connected to a conduit, said conduit being connected to an inert gas source having a pressure above atmospheric pressure. This is a convenient embodiment because balls, provided with a groove, are commercially available. The intention of the groove in these commercially available parts is to accommodate an O-ring to provide a non-heat resistant seal according to the prior art. Nevertheless, they can be used as building blocks to provide a coupling according to the invention.  
         [0016]    Although not shown in FIG. 1 and  2 , the joint needs to be provided with a clamp to keep the ball and the cup together. In particular because the inert gas in the groove  13  or  14  is at superatmospheric pressure, such a clamp is required to prevent ball and cup from moving away from each other. This clamp can be any clamp known in the art for this purpose. Both in the embodiment of FIGS. 1 and 2, the uppermost part of the ball is truncated to provide for a more convenient and reliable fit. Usual materials for process tubes and connecting tubes are quartz and Silicon Carbide but other ceramic materials are also possible.  
         [0017]    In the method according to the invention for operating of a furnace as described above, it is essential that the supply of the inert gas, to be introduced between the cup and ball at superatmospheric pressure, it at such a rate as to maintain a gas velocity that is sufficient to counteract the in-diffusion of ambient air into the process tube. This will be illustrated by the example below.  
       EXAMPLE  
       [0018]    A ball joint is used having a Silicon Carbide ball diameter of 28.222 mm. The diameter of the quartz cup is 28.582 mm. Further taking into account the tolerances in these dimensions the difference in diameter between ball and cup can be 0.36 mm maximum, resulting in an annular gap between ball and cup of 0.18 mm maximum. The cross section of the gap is then the width×length=0.18×π×28=16 mm2. Assuming a temperature of the joint of 400° C., the diffusivity of O2 in N2 is 9.2×10 −5 . Choosing a path length of the gas of 10 mm, which is the distance that the inert gas has to travel from the groove  13 ,  14  through the gap to the atmospheric ambient, and a gas velocity of 100 mm/s, the concentration of O2 in N2 can be kept below 4 ppm. To maintain this gas velocity of 100 mm/s over the gap cross section of 16 mm2, a gas flow §400° C. is required of 100×16 mm3/s which equals about 100 cm3/min. Converting this to a gas flow at standard conditions results in 40 sccm (standard cubic centimeter per minute). Depending on the dimensioning and the size of the cup and ball, a different gas flow is required. However, a gas velocity of 20 mm/s seems a practical lower limit that should be maintained.