Abstract:
A double directional check valve and flow restrictor combination including a housing which contains a pair of axially-aligned check valves and a flow restrictor disposed between the check valves. Dry purge air flows into the housing through the flow restrictor, which facilitates smooth and uniform flow of the air, and out of one of the check valves to a dessicant chamber to dry or regenerate dessicant in the chamber. The housing is fitted with flanges or other connection structures for quick, easy and convenient attachment of the housing to a piping network in an air, nitrogen or other process drying system.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to drying compressed air and more particularly, to a double directional check valve and flow restrictor combination which is capable of drying compressed air, nitrogen or other process gas for use in the production of semiconductors.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the semiconductor production industry, clean dry air, nitrogen and other gases are frequently used in the processing of semiconductor wafers. These processing gases typically must contain a very low moisture content in order to prevent manufacturing defects in the integrated circuits on the wafers. For example, during the semiconductor fabrication process, a semiconductor die is attached to a leadframe and encapsulated in plastic. Any moisture remaining on the semiconductor wafer from the semiconductor processing steps can be absorbed from the semiconductor wafer into the package interior and lead to one of several failure mechanisms.  
           [0003]    A minimum package moisture content accepted in the semiconductor industry is 0.04 percent, or {fraction (1/2500)} of the total weight of the package. Any moisture content which exceeds this level may cause a “popcorn” failure, in which the moisture is vaporized and increases the internal pressure of the package. Consequently, the package may pop or fracture along the seams. Replacing the semiconductor device is expensive and time-consuming, and may result in damage to the printed circuit board.  
           [0004]    A moisture absorbing-type drying system is commonly used in the semiconductor industry for drying compressed air, nitrogen or other process gas. This type of drying system uses dessicant to produce dry air from air having a relatively dry moisture content. The system uses two check valves and a flow restrictor which are installed as separate units. The valve housings, flow restrictor housing and connecting piping for the system occupy a relatively large volume of valuable space. Furthermore, poor purge efficiency in such systems increases operating costs due to waste of purge air.  
         SUMMARY OF THE INVENTION  
         [0005]    It is therefore an object of the present invention to consolidate usage of space in the production of clean, dry air, nitrogen or other process gas.  
           [0006]    Another object of the present invention is to provide a flow restrictor and check valve combination in a single housing.  
           [0007]    Still another object of the present invention is to provide a double directional check valve and flow restrictor combination which is capable of drying compressed air, nitrogen or other process gas for use in the production of semiconductors.  
           [0008]    Yet another object of the present invention is to provide a double directional check valve and flow restrictor combination which can be easily and conveniently installed.  
           [0009]    A still further object of the present invention is to provide a double directional check valve and flow restrictor combination which is simple in construction.  
           [0010]    Yet another object of the present invention is to provide a double directional check valve and flow restrictor combination which enhances purge efficiency to save purge air volume and cost in a compressed air drying system.  
           [0011]    Another object of the present invention is to provide for constant flow of air for the drying of desiccant in an air drying system.  
           [0012]    In accordance with these and other objects and advantages, the present invention comprises a T-shaped housing which contains a pair of axially-aligned check valves and a flow restrictor disposed between the check valves. Dry purge air flows into the housing through the flow restrictor, which facilitates smooth and uniform flow of the air, and out of one of the check valves to a dessicant chamber to dry or regenerate dessicant in the chamber. The housing is fitted with flanges or other connection structures for quick, easy and convenient attachment of the housing to a piping network in an air, nitrogen or other process drying system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0014]    [0014]FIG. 1 is a schematic view of a conventional air drying system suitable for implementation of the present invention;  
         [0015]    [0015]FIG. 2 is a perspective view of an illustrative embodiment of the double directional check valve and flow restriction combination of the present invention;  
         [0016]    [0016]FIG. 3 is a sectional view, taken along section lines  3 - 3  in FIG. 2, of the present invention;  
         [0017]    [0017]FIG. 4 is a cross-sectional view of an air flow restrictor assembly component of the present invention illustrated in FIG. 3;  
         [0018]    [0018]FIG. 5 is a bottom view, in section, of a valve assembly component of the present invention illustrated in FIG. 3;  
         [0019]    [0019]FIG. 6 is a sectional view of the air flow restrictor assembly component of the present invention, as shown in FIG. 4, more particularly illustrating partial restriction of air flow through the assembly;  
         [0020]    [0020]FIG. 7 is a schematic view of an air drying system which incorporates the present invention; and  
         [0021]    [0021]FIG. 8 is a cross-sectional view, partially in section, of a poppet valve element of the present invention, more particularly illustrating opening of the poppet valve responsive to air or gas pressure against the valve. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The present invention has particularly beneficial utility in application to drying air, nitrogen and other process gases in the semiconductor production industry. However, the invention is not so limited in application and while references may be made to semiconductor processing and production, the invention may be more generally applicable to drying air or other gases in a variety of industrial and product applications.  
         [0023]    Referring initially to FIG. 1 of the drawings, a conventional air drying system for drying air, nitrogen or other process gases in a semiconductor production facility is generally indicated by reference numeral  45  and includes a source (not illustrated) of relatively humid air which is initially introduced under pressure into diverging wet air lines  47  through a common valve  46 . One of the wet air lines  47  feeds a first dessicant chamber  48 , and the other of the wet air lines  47  feeds a second dessicant chamber  49 . The first dessicant chamber  48  and the second dessicant chamber  49  each includes a working chamber (not illustrated) which contains a supply of dessicant (not illustrated), typically dialuminum trioxide (Al 2 O 3 ), for capturing moisture in the wet air that flows into the first dessicant chamber  48  and second dessicant chamber  49  through the respective wet air lines  47 . The first desiccant chamber  48  and second desiccant chamber  49  each further includes a regenerating chamber (not illustrated) which dries or regenerates saturated desiccant, as hereinafter described. Dry air emerges from the working chamber of each first dessicant chamber  48  and second dessicant chamber  49  through respective dry air lines  50 , and the dry air is fed through a valve  51  in a dry air dispensing line  52  for delivery to a desired destination such as a semiconductor wafer process chamber or furnace (not illustrated).  
         [0024]    After prolonged operation of the conventional air drying system  45 , the saturation level of the desiccant in the working chamber of the respective first desiccant chamber  48  and second desiccant chamber  49  approaches or becomes equal to the saturation level of the wet air entering the chambers through the wet air lines  47 , such that transfer of moisture from the wet air to the desiccant is minimal or nonexistent. Therefore, the wet desiccant in the working chamber of each desiccant chamber  48 ,  49  must be dried, or regenerated, in order for effective drying of the air in the chambers  48 ,  49 . Accordingly, a dry air purge line  53  extends from the dry air dispensing line  52  for diverting some of the dry air back to the first dessicant chamber  48  or the second dessicant chamber  49  to facilitate drying the dessicant in the regeneration chamber of the respective first desiccant chamber  48  or second desiccant chamber  49 . Because the desiccant in both chambers generally does not become saturated at the same time, the saturated desiccant in one of the desiccant chambers  48 ,  49  can be regenerated while the other desiccant chamber remains functional.  
         [0025]    An air flow restrictor  55  is provided in the dry air purge line  53 , and connecting lines  58  connect the air flow restrictor  55  to a first chamber check valve  56 , which leads to the first desiccant chamber  48  through a purge air return line  59  and the dry air line  50 , and to a second chamber check valve  57 , which leads to the second desiccant chamber  49  through a purge air return line  59  and the dry air line  50 . The air flow restrictor  55  regulates the flow of dry air through the dry air purge line  53 , to provide a smooth or uniform flow of the dry air to the first desiccant chamber  48  or second desiccant chamber  49 . The first chamber check valve  56  and second chamber check valve  57  prevent backflow of air from the purge air return lines  59  to the dry air purge line  53 . After regenerating the saturated desiccant in the regeneration chamber of the first desiccant chamber  48  or second desiccant chamber  49 , the dry purge air is fed back through the corresponding wet air line  47 , through a purge air exhaust line  60  and through a purge air exhaust valve  61  to a purge exhaust muffler  62 .  
         [0026]    In the conventional air drying system  45 , the air flow restrictor  55 , first chamber check valve  56  and second chamber check valve  47  are installed as separate units. Because these separate units must be connected to each other via the dry air purge line  53  and connecting lines  58 , valuable space is occupied and installation and/or removal is complex and time-consuming.  
         [0027]    Referring next to FIGS.  2 - 8  of the drawings, an illustrative embodiment of the double directional check valve and flow restrictor combination, hereinafer referred to as the combination, of the present invention is generally indicated by reference numeral  1 . The combination  1  includes a T-shaped housing  2 , having a first poppet valve arm  4 , a second poppet valve arm  7  and a flow restrictor arm  10  disposed in perpendicular relationship to the first poppet valve arm  4  and the second poppet valve arm  7 . As illustrated in FIG. 3, the first poppet valve arm  4 , the second poppet valve arm  7  and the flow restrictor arm  10  define a first poppet valve arm interior  5 , a second poppet valve arm interior  8  and a flow restrictor arm interior  11 , respectively. One of a pair of annular poppet valve retainer flanges  15  separates the first poppet valve arm interior  5  from a housing interior  3 , and the other poppet valve retainer flange  15  separates the second poppet valve arm interior  8  from the housing interior  3 . An annular spring retainer  16  separates the flow restrictor arm interior  11  from the housing interior  3 . The first poppet valve arm  4 , the second poppet valve arm  7  and the flow restrictor arm  10  of the housing  2  is each typically fitted with a housing flange  13  for purposes hereinafter described.  
         [0028]    An air flow restriction assembly for the combination  1  is generally indicated by reference numeral  19  in FIGS. 3 and 4. The air flow restriction assembly  19  includes a taper cone  25  having a base  26  which, as illustrated in FIG. 3, is mounted on a holding block  30  typically by means of taper cone base lock screws  40 . As further illustrated in FIG. 3, the holding block  30  is secured to the interior surface of the housing  2 , inside the housing interior  3  typically by means of a pair of holding block lock screws  41 . The air flow restrictor assembly  19  further includes a typically cylindrical or disc-shaped orifice poppet  20 , provided with a central air flow orifice  21  including a cone seat  22 , and a restrictor cylinder  23  extends from a surface of the orifice poppet  20 . The orifice poppet  20  is slidably mounted in the flow restrictor arm interior  11 , and the stationary taper cone  25  is positioned just beneath the central air flow orifice  21 , as illustrated in FIG. 4. A restrictor spring  24  in the flow restrictor arm interior  11  is interposed between the annular spring retainer  16  and the orifice poppet  20 , and encircles the restrictor cylinder  23 . The restrictor spring  24  normally biases the orifice poppet  20  away from the tip of the taper cone  25  and against a retainer ring  27  provided in the interior walls of the flow restrictor arm  10 . Accordingly, the orifice poppet  20  is capable of movement in the flow restrictor arm interior  11  against the restrictor spring  24  such that the air flow orifice  21  approaches and may receive the upper end of the taper cone  25  to restrict flow of air through the air flow orifice  21 , as illustrated in FIG. 6 and hereinafter described.  
         [0029]    A valve assembly for the combination  1  is generally indicated by reference numeral  29  in FIGS. 3 and 5, and includes an elongated valve mount rod  37  which slidably extends through the holding block  30 . A first poppet valve  31  and a second poppet valve  32  are mounted on respective end portions of the valve mount rod  37 , inside the first poppet valve arm interior  5  and the second poppet valve arm interior  8 , respectively. The first poppet valve  31  and the second poppet valve  32  each includes a valve disc  33 , slidably mounted on the valve mount rod  37 . A retainer disc  35 , typically held in place by a securing nut  36 , is mounted on each end of the valve mount rod  37 . A valve spring  34  is interposed between the valve disc  33  and the retainer disc  35 . As illustrated in FIG. 3, the valve spring  34  normally biases the valve disc  33  against the corresponding poppet valve retainer flange  15  of the housing  2  to seal the housing interior  3  from the corresponding first poppet valve arm interior  5  and second poppet valve arm interior  8 . The valve assembly  29  further includes a pair of spacer springs  38 , each of which is interposed between the holding block  30  and a retainer disc  35  mounted on the valve mount rod  37  typically by means of a securing nut  36 .  
         [0030]    An air drying system which incorporates the combination  1  of the present invention is generally indicted by reference numeral  43  in FIG. 7. Accordingly, the flow restrictor arm  10  of the housing  2  is connected to the dry air purge line  53 , and the first poppet valve arm  4  and the second poppet valve valve arm  7  of the housing  2  are connected to the respective purge air return lines  59 , of the air drying system  43 . Under circumstances in which the desiccant in the working chamber of the first desiccant chamber  48  becomes saturated with moisture and needs drying, the second desiccant chamber  49  remains functional and continues to dry air and feed the dried air to the dry air dispensing line  52 , while flow of wet air from the wet air line  47  into the working chamber of the first desiccant chamber  48  stops. A drop in air pressure is simultaneously induced in the regenerating chamber of the first desiccant chamber  48 , and this drop in air pressure causes dry purge air to flow from the dry air purge line  53  and into the flow restrictor arm interior  11  and housing interior  3  of the combination  1 , through the air flow orifice  21  as illustrated in FIGS. 3 and 4. The air in the housing interior  3  pushes against the valve disc  33  of the first poppet valve  31 , disengaging the valve disc  33  from the poppet valve retainer flange  15  as illustrated in FIG. 8 and facilitating flow of air from the housing interior  3 , through the first poppet valve arm interior  5  and to the regenerating chamber of the first desiccant chamber  48 . Due to substantially equal air pressure between the housing interior  3  and the regenerating chamber of the second desiccant chamber  49 , however, the valve disc  33  of the second poppet valve  32  remains firmly seated against the corresponding poppet valve retainer flange  15  and prevents backflow of air from the second desiccant chamber  49  into the housing interior  3  of the combination  1 .  
         [0031]    As the dry purge air flows from the dry air purge line  53  and into the flow restrictor arm interior  11  of the housing  2 , the air pushes against the orifice poppet  20  of the air flow restrictor assembly  19 , thereby causing the air flow orifice  21  approach the upper end of the taper cone  25 , as illustrated in FIG. 6. This restricts the flow area available for the flowing dry purge air through the air flow orifice  21 , and the resulting restriction in air flow is directly proportional to the pressure differential or gradient between the flow restrictor arm interior  11  and the regenerating chamber of the first desiccant chamber  48 . Accordingly, when the pressure gradient or differential is relatively low, the restrictor spring  24  is in the extended configuration of FIGS. 3 and 4, and the air flows freely through the flow restrictor arm interior  11 , housing interior  3 , open first poppet valve  31  and first poppet valve arm interior  5 , and to the regenerating chamber of the first desiccant chamber  48 . When the air pressure differential between the flow restrictor arm interior  11  and the regenerating chamber of the first desiccant chamber  48  is relatively large, the air pushes against the orifice poppet  20  and compresses the restriction spring  24  as the air flow orifice  21  approaches and may receive the stationary tip of the taper cone  25 , as illustrated in FIG. 6, to increasingly retard the flow of air through the flow restrictor arm interior  11 . Conversely, as the air pressure differential decreases, the restrictor spring  24  partially or completely overcomes the opposing air pressure against the orifice poppet  20  and pushes the orifice poppet  20  and the air flow orifice  21  away from the tip of the taper cone  25 , as illustrated in FIG. 4. Consequently, the air flow restriction assembly  19  facilitates a substantially constant, uniform and smooth flow of dry purge air through the housing  2  and to the first desiccant chamber  48  to dry or regenerate the desiccant in the regenerating chamber of the first desiccant chamber  48 . Such a constant air flow serves to both increase purge efficiency and decrease purge time and air volume.  
         [0032]    At the end of the desiccant drying or re-generating process in the regenerating chamber of the first desiccant chamber  48 , the air pressure between the regenerating chamber and the flow restrictor arm interior  11  is substantially equal, and the valve spring  34  of the first poppet valve  31  biases the valve disc  33  against the poppet valve retainer flange  15  to again seal the housing interior  3  from the first poppet valve arm interior  5 . Simultaneously, the restrictor spring  24  of the air flow restrictor assembly  19  becomes fully extended and urges the orifice poppet  20  against the retainer ring  27 . Normal air-drying operation of the first desiccant chamber  48  resumes, and saturated desiccant in the second desiccant chamber  49  can then be dried in the regenerating chamber thereof. This is accomplished by facilitating flow of dry purge air from the dry air purge line  53  through the air flow orifice  21 , open second poppet valve  32  and second poppet valve arm interior  8  of the combination  1  and to the regenerating chamber of the second desiccant chamber  49 , in the manner and sequence heretofore described with respect to the desiccant-regenerating process of the first desiccant chamber  48 .  
         [0033]    While the prefered embodiments of the invention have been described above, it will be recognized and understood that various modifications may be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.