Abstract:
Pick-up tools and other devices for handling semiconductor wafers are require to be capable of withstanding corrosive chemical substances and extreme temperatures, and the valve unit for use in such devices must meet these requirements. The valve unit is also desired to be free from electrostatic charging. To this end, the valve unit comprises a valve main body, having a valve seat defined therein, which is made of relatively electroconductive and self-lubricating material such as PTFE, and a valve case which is injection molded around the valve unit and made of a material having some electroconductivity and a high resistance against deformation such as PEEK mixed with carbon fibers. Provision of annular grooves on the outer surface of the valve main body improves the attachment between the valve main body and the valve case so as to improve both the sealing performance and the mechanical integrity of the valve unit.

Description:
TECHNICAL FIELD 
     The present invention generally relates to a pick-up tool for releasably holding an article such as a semiconductor wafer by using vacuum suction, and more particularly relates to a valve unit which is suitable for use in such a pick-up tool. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices are most often made by chemically and physically processing silicon wafers, and pick-up tools using vacuum suction are often used for handling the silicon wafers, instead of more conventional pincers. Such pick-up tools are required to be resistant to various chemicals and high temperatures which are used to etch or otherwise process the silicon wafers, and to be free from electrostatic charges because electrostatic charges can damage the silicon wafers. Examples of such pick-up tools are disclosed in Japanese patent publication (kokoku) No. 3-50672, and Japanese patent publication (kokoku) No. 5-75554. 
     A pick-up tool is equipped with a valve unit for controlling the vacuum suction, and the valve unit also must meet the above-mentioned requirements which are associated with the handling of silicon wafers. The valve unit typically includes a valve main body having a valve seat defined therein, and a valve member which is slidably received in the valve seat so that a passage defined inside the valve main body may be selectively opened and closed as required by turning or otherwise moving the valve member in relation with the valve seat. The valve member must be closely fitted in the valve seat so that a required fluid-tightness may be obtained, and it is essential that the valve main body has a sufficient rigidity not to deform and to maintain the fluid-tightness under substantial external forces. The material for the valve unit must be also capable of withstanding various chemical substances, moisture and temperatures of up to 250° C. 
     The requirements for the material of the valve unit can thus be summarized as given in the following. 
     (1) The material should be electroconductive so as to avoid electrostatic charging of the valve unit components which could cause damages to the silicon wafers and lead to deposition of foreign matters. 
     (2) The material should be resistant to chemical substances which are used for chemically processing and washing the silicon wafers, and should not release any metallic ions when subjected to such chemical substances. 
     (3) The material should be self-lubricating so that smooth operation may be ensured and emission of particles may be avoided even after long use. 
     One of the most preferred materials for such valve units as well as such article pick-up tools is fluoride resins such as tetrafluoroethylene. Fluoride resins can meet most of the required properties, but are relatively readily deformable. Therefore, when subjected to significant external forces, a valve unit made of a fluoride resin may not be able to operate smoothly or may lose fluid-tightness. In particular, the screws used for securing a fluoride resin member may become loose in time due to the combined effect of repeated application of fluid pressure and the lack of the mechanical strength of the fluoride resin material. Loosening of the screws, which are typically made of metal or alloy, is also a cause of the rusting of the screws because it increases the chance of exposing the screws to various chemical substances. Additionally, fluoride resins are less immune to electrostatic charging than other resin materials, and the article pick-up tool could become undesirably electrically charged as it is handled by hand. 
     Fluoride resins such as tetrafluoroethylene are also known to be unsuitable for injection molding and extruding, and are therefore not suited to be molded into complicated shapes. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide a valve unit which is both mechanically and chemically stable. 
     A second object of the present invention is to provide a valve unit which is suitable for use in extreme environments which arise in the processing of semiconductor wafers. 
     A third object of the present invention is to provide a valve unit which is economical to manufacture, and reliable in use. 
     A fourth object of the present invention is to provide a valve unit which is suitable for use in a pick-up tool for handing semiconductor wafers. 
     According to the present invention, these and other objects are accomplished by providing a valve unit, comprising: a valve main body having a communication passage, and a valve seat defined in an intermediate part of the communication passage; a valve member received in the valve seat for selective communication of the communication passage; and a valve case which is injection molded around the valve main body; the valve member being provided with a valve stem extending out of the valve main body and the valve case. 
     Thus, the valve main body may be made of a material suitable for the operation of the valve member while the valve case may be made of a material suitable for ensuring a required mechanical strength of the valve unit. For instance, the valve main body may be made of a resin material having a self-lubricating property, and the valve case may be made of a resin material which is resistant to deformation. Also, the valve case may be made of a material suited to be molded into a desired complicated shape. This is particularly advantageous when the valve case is provided with a passage communicating with the communication passage of the valve main body for external communication. 
     In applications where the electrostatic charging of the valve body may become a problem, for instance in semiconductor wafer pick-up tools, the valve case may be made of a relatively electroconductive resin material. According to a preferred embodiment for such an application, the valve case is made of polyether-etherketone resin material mixed with carbon fibers while the valve main body is made of tetrafluoroethylene resin material. 
     Because the injection molding process can achieve a highly close attachment between the valve main body and the valve case, the leakage of fluid can be avoided. If the valve main body is provided with a plurality of grooves or projections for restraining relative deformation between the main body and the valve case, an even closer attachment between the two parts can be achieve. In particular, the valve main body can be favorably reinforced by the valve case so that the valve seat defined in the valve main body can be maintained even under significant external forces, and the satisfactory operation of the valve unit can be ensured at all times. 
     According to a preferred embodiment of the present invention which is suited to be installed in semiconductor wafer pick-up tools, the valve main body comprises a cylindrical member, the communication passage consisting of a longitudinal passage passed longitudinally through the cylindrical member, the valve seat being defined by a valve guide hole passed laterally across the cylindrical member. Typically, the valve member is slidably or rotatably received in the valve guide hole, and a valve stem extends out of the valve case coaxially with the valve guide hole. To achieve a close contact between the valve case and the valve main body, and to favorably reinforce the valve main body against deformation, an annular groove may be defined in a surface part of the valve main body surrounding the valve guide hole, and a pair of annular grooves may be formed circumferentially around the valve main body on either axial side of the valve guide hole. 
     According to a particularly preferred embodiment of the present invention, the valve member is slidably received in the valve guide hole, and a compression coil spring is interposed between a bottom end of the valve member and a bottom of the valve guide hole, the valve member being provided with an annular groove for selectively communicating with the communication passage. Furthermore, the valve case may be provided with an upstream passage communicating with one end the communication passage and a downstream passage communicating with the other end of the communication passage, the bottom end of the valve guide hole being provided with a vent hole for communication with the exterior, the valve main body being provided with a bypass passage extending between the valve guide hole and one of the upstream passage and the downstream passage of the valve case so as to be closed by the valve member when the valve member is opening the communication passage, and to be in communication with the vent hole when the valve member is closing the communication passage. In the case of a normally open valve unit, the vent hole should be provided in an upper end of the valve guide hole. 
     The valve stem must be sealed while allowing the required movement of the valve member for the operation of the valve unit. To achieve this goal both reliably and economically, the valve case may be provided with an annular boss surrounding the valve stem, and a cap may be fitted on the annular boss for retaining the valve member inside the valve seat, the valve stem being passed through the cap. Alternatively, the valve case may be provided with an annular boss surrounding the valve stem, and a plug may be fitted into the annular boss for retaining the valve member inside the valve seat, the valve stem being passed through the plug. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Now the present invention is described in the following with reference to the appended drawings, in which: 
     FIGS.  1 ( a ) and  1 ( b ) are side and front views of a semiconductor wafer pick-up tool to which the present invention is applied; 
     FIGS.  2 ( a ) and  2 ( b ) are perspective views of the suction adapter showing the front and reverse sides thereof, respectively; 
     FIG.  3 ( a ) is a front view of the suction adapter; 
     FIGS.  3 ( b ),  3 ( d ) and  3 ( c ) are sectional views taken along lines A-B, C-D and E-F, respectively; 
     FIG. 4 is a perspective view showing the core pin and the tubular insert used for molding the suction adapter; 
     FIG. 5 is a sectional view showing a first embodiment of the valve unit according to the present invention; 
     FIGS.  6 ( a ) and ( b ) are a perspective view and a sectional view of the valve block used for forming the valve main body respectively; 
     FIG. 7 is a sectional view of an essential part of the molding die assembly for injection molding the valve case over the valve main body; 
     FIG. 8 is a sectional view of the valve housing immediately after being taken out of the die cavity; 
     FIGS.  9 ( a ) and  9 ( b ) are fragmentary sectional views showing parts of FIG. 8 indicated by V and H, respectively; 
     FIG. 10 is an exploded sectional view of the valve unit of the first embodiment; 
     FIGS.  11 ( a ) and  11 ( b ) are sectional views of a second embodiment of the valve unit according to the present invention during and after the assembling process, respectively; 
     FIG. 12 is a view similar to FIG. 5 showing a third embodiment of the present invention constructed as a normally open type valve unit; and 
     FIGS.  13 ( a ) and  13 ( b ) are sectional views of a fourth embodiment of the valve unit according to the present invention during and after the assembling process, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS.  1 ( a ) and  1 ( b ) show a wafer pick-up tool  1  to which the present invention is applied. The pick-up tool  1  is generally elongated in shape, and comprises a valve unit  6  which is provided with an operation knob  3 , an extension tube  8  connected to the rear end of the valve unit  6  and having an internal bore  7  defined therein, and a connector  10  attached to the rear end of the extension tube  8 . The connector  10  can be releasably connected to a corresponding connector  81  of a hose  82  which leads to a vacuum source not shown in the drawing. The pick-up tool  1  further comprises a suction adapter  2  which is detachably connected to the front end of the valve unit  6  via a connector  9 . 
     Referring to FIGS.  2 ( a ) and  2 ( b ), the suction adapter  2  comprises a connecting tube  61  adapted to be connected to the front end of the valve unit  6  via the connector  9  and provided with a circular cross section, a conduit section  62  formed in the front end portion of the connecting tube  61  and provided with an elliptic cross section, and a planar suction tip  64  formed in the front end portion of the conduit section  62 . The suction tip  64  is provided with a suction plate  68  (FIG.  3 ( a )) which defines a peripheral suction surface, and provided with a comb-shaped recess  5  recessed from the suction surface, and a suction hole  4  which communicates the recess  5  with an inner bore  66  passed through the connecting tube  61  and the conduit section  62 . 
     Referring to FIGS.  3 ( a ) to  3 ( d ), a tubular insert  67  (FIG.  3 ( c )) is insert molded from a front end portion of the conduit section  62  to a base end portion of the suction tip  64 . Using the tubular insert  67  substantially reduces the difficulty in forming the internal passage extending from the internal bore  66  of the conduit section  62  to the suction hole  4 . This is particularly advantageous because the material suitable for the suction tip  64  such as polyimide resin involves some difficulty in being injection molded into a complicated shape, and because the structure of the die assembly can be simplified. A particularly suitable material for the suction tip  64  is totally aromatic polyimide resin such as Vespel (tradename) commercially available from DuPont. Polybenzimidazole resin is also suitable as a material for the suction tip  64 . The remaining part of the suction adapter  2  as well as the tubular insert  67  may be made of more economical and more injection moldable materials such as PEEK (polyether-etherketon) which is preferably mixed with carbon fibers by approximately 15% so as to increase the electroconductivity as well as the mechanical strength of the material. 
     FIG. 4 shows the tubular insert  67  as well as a molding core  70  having two sections  71  and  72  for defining the different sections of the internal bore  66  (FIG. ( 3 ( b )) of the connecting tube  61  and the conduit section  62  (FIG.  3 ( a )), respectively. The tubular insert  67  is preferably made of the same or a similar material as the conduit section  62  so that these two members may be integrally joined together by the insert molding process consisting of an injection molding process. The tubular insert  67  is provided with a plurality of projections  69  so that the tubular insert  67  may be securely attached to the suction tip  64  and the conduit section  62  after the insert molding process. 
     Referring to FIG. 5, the valve unit  6  is provided with a valve housing  13  consisting of a valve main body  11  and a cylindrical valve case  12  which surrounds the valve main body  11 . The internal bore of the valve case  12  is blocked by the valve main body  11  which consists of a substantially solid block member, and thus defines an upstream passage  14 A and a downstream passage  14 B on either longitudinal end of the valve main body  11 . The valve main body  11  is provided with a longitudinal passage  15  passed completely and centrally through the valve main body  11 , and a valve guide hole  16  is passed laterally and centrally across the valve main body  11 . The longitudinal passage  15  and the valve guide hole  16  therefore cross each other at a right angle. A bypass passage  25  is provided below the longitudinal passage  15 , and extends between the upstream passage  14 A and the valve guide hole  16 . 
     The valve guide hole  16  receives therein a valve member  17  which is urged upward as seen in FIG. 5 by a compression coil spring  18 . The valve member  17  is generally cylindrical in shape, and is provided with a recess  20  at its bottom end for receiving an end of the compression coil spring  18 , and is provided with an annular groove  19  around its circumference. The other end or the lower end of the compression coil spring  18  abuts the wall of the valve case  12  which is provided with a vent hole  21  communicating the valve chamber defined by the bottom end of the valve member  17  and the valve guide hole  16  with the atmosphere. The upper end of the valve guide hole  16  is integrally provided with an annular boss  27  which extends out of the valve case  12 , and is closed by a cap  23  threadably engaged with an annular boss  22  which is integrally formed with the valve case  12  and closely surrounds the annular boss  27  of the valve main body  11 . The cap  23  thus restrains the upward movement of the valve member  17 . A valve rod  24  is securely attached to the upper end of the valve member  17 , and is passed through the cap  23 . The outer end of the valve rod  34  is attached to the knob  3 . 
     The valve unit  6  may be either a normally open type which normally opens the longitudinal passage  15  and communicates the vacuum suction to the suction adapter  2 , or a normally closed type which normally closes the longitudinal passage  15  and interrupts communication of the vacuum suction to the suction adapter. The valve unit  6  illustrated in FIG. 5 is a normally closed type, and the upstream passage  14 A and the downstream passage  14 B are normally discommunicated from each other by the valve member  17  closing the longitudinal passage  15 . 
     Initially, the longitudinal passage  15  is closed by the valve member  17 , and the upstream passage  14 A is communication with the atmosphere via the bypass passage  25 , and the vent hole  21 . The downstream passage  14 B is communicated only with the vacuum source not shown in the drawings. When the knob  3  is pressed downward against the biasing force of the compression coil spring  18  to push down the valve member  17 , the longitudinal passage  15  is now communicated by the annular groove  19  while the bypass passage  18  is closed by the valve member  17 . Therefore, the vacuum suction is transmitted from the downstream passage  14 B to the upstream passage  14 A. 
     When the knob  3  is released, the valve member  17  moves upward to the position illustrated in FIG. 5 under the spring force of the compression coil spring  18 . As a result, the initial state is restored. In other words, the longitudinal passage  15  is now closed by the valve member  17  while the bypass passage  25  is communicated with the atmosphere via the vent hole  21 . Therefore, the vacuum suction would not reach the upstream passage  14 A, and any vacuum suction that may be remaining in the upstream passage  14 A is relieved by the communication with the atmosphere via the bypass passage  25  and the vent hole  21 . 
     The process of manufacturing valve unit  6  is described in the following with reference to FIGS.  6 ( a ),  6 ( b ) and  7 . Initially, a valve block  11 A for forming the valve main body  11  is prepared, and is formed the shape illustrated in FIGS.  6 ( a ) and  6 ( b ). This valve block  11 A is fitted into a die cavity  45  of a die assembly  26  of an injection molding machine as illustrated in FIG. 7, and the valve case  12  is injection molded around the valve block  11 A. Some additional and optional machining, such as drilling the valve guide hole  16 , the longitudinal hole  15  and the bypass passage  25 , is conducted on the thus prepared valve housing  13 , and the valve member  17  and other components are assembled to the valve housing  13  as illustrated in FIG.  10 . 
     The valve block  11 A is typically made of heat resistant, corrosion resistant and self-lubricating synthetic resin material such as fluoride resin. In the present embodiment, tetrafluoroethylene (PTFE) is used for the valve block  11 A. The material for the valve case  12  is desired to have a suitable electroconductivity to avoid electrostatic charging as well as being heat resistant and corrosion resistant. In the present embodiment, PEEK containing approximately 15% of carbon fibers is used to form the valve case  12 . PEEK reinforced by carbon fibers is particularly preferred as it is highly heat resistant and has a high mechanical strength combined with a relatively high electroconductivity. 
     The block  11 A consists of an axially elongated block having a rectangular cross section as illustrated in FIGS.  6 ( a ) and ( b ), and is provided with a circular projection  27 A which is surrounded by an annular groove  30 . A similar annular groove  31  is provided on the bottom side of the block  11 A. The two axial ends of the block  11 A are provided with circular recesses  28  and  29 , and a pair of annular grooves  32  and  33  are formed circumferentially around the block  11 A. 
     FIG. 7 illustrates a part of the molding die assembly  26  for molding the valve case  12  which comprises a fixed die section  26 A and a moveable die section  26 B. The fixed die section  26 A is provided with an ejector plate  34 , a fixed die plate  35  and angular pins  36  while the moveable die section  26 B comprises a moveable die plate  37 , a pair of split die blocks  38  and  39 , and . ejector pins  40 . The split die blocks  38  and  39  are provided with guide holes  41  for receiving the angular pins  36  when closing the die assembly  26 . 
     Additionally, the fixed die section  26 A is provided with a fixed end support pin  42  projecting vertically downward from the ejector plate  34 , and the moveable die section  26 B is similarly provided with a moveable end support pin  43  projection vertically upward from the fixed die plate  37 . These support pins  42  and  43  oppose each other along the central axial line of the die cavity  45  inside the die cavity  45 , and the die cavity  45  is defined by threaded sections  44  and recesses  45   a  and  45   b.    
     According to this molding die assembly  26 , the block  11 A is held in the die cavity  45  by fitting the support pins  42  and  43  into the corresponding circular recesses  28  and  29  provided in the axial ends of the block  11 A, and as the split die blocks  38  and  39  are pushed into the fixed die plate  35  of the fixed die section  26 A by the ejector pins  40  projecting from the split die blocks  38  and  39 , the die cavity  45  is closed by the split die blocks  38  and  39  guided by the angular pins  36  passed through the guide holes  41 . As a result, the valve block  11 A is completely enclosed inside the cavity  45 . 
     The fixed and moveable die sections  26 A and  26 B are heated in advance, and polyether-etherketone (PEEK) mixed with carbon fibers and heated to a prescribed temperature is filled into the die cavity  45  from a gate via a spruce runner and a runner. The injected resin material is thus molded into the valve case  12  which closely surrounds the valve block  11 A. At the same time, the outer thread  12   a  is formed by a thread surface formed on the wall surface of the die cavity  45 , and the upper surface of the circular extension  27  is exposed out from a side of the valve case  12 . The injected resin also fills into the annular grooves  30  and  31 , and the circumferential grooves  32  and  33 , and forms ribs, complementary to these grooves, which reinforce the valve case  12  on the one hand, and increase the force of attachment between the valve case  12  and the valve block  11 A. The support pins  42  and  43  similarly form the upstream and downstream passages  14 A and  14 B of the valve housing  13 . 
     In this embodiment, the die assembly was heated to the temperature of approximately 180° C., and polyether-etherketone (PEEK) which was mixed with carbon fibers and heated to the temperature of approximately 380° C. was filled into the cavity  45 . The injected resin material is attached around the valve block  11 A which is made of tetrafluoroethylene (PTFE) and held inside the cavity  45 , and the thus prepared valve housing  13  incorporates the valve block  11 , serving as an insert member, integrally with the valve case  12 . 
     Because the valve case  12  is injection molded over the valve block  11 A made of a fluoride resin which is somewhat thermally expanded immediately after the molding process, as soon as the injection molding process is completed, the valve case  12  starts thermally shrinking. Therefore, when the molded assembly is removed from the cavity and placed in an ambient temperature, a gap would be produced between the valve block  11 A and the valve case  12  because of the difference in the thermal expansion coefficients of these two parts, but owing to the engagement between the ribs and the annular grooves  30  to  33 , the valve block  11 A and the valve case  12  remain securely attached to each other. Also, it is possible to eliminate any gaps which would reduce the air-tightness of the valve unit. Such a reduction in air-tightness could cause the loss of the force that is available in securing the article at the suction adapter, and is therefore detrimental to the satisfactory operation of the pick-up tool. 
     During the injection molding process for the valve housing  13 , the valve block  11 A placed inside the die cavity  45  of the molding die assembly  26  initially expands thermally owing to the heat supplied from the die assembly  26 , and expands further owing to the heat supplied by the hot molten resin material injected into the die cavity  45 . Such an expansion of the valve block  11 A is opposed by the pressure of the injected resin material, and if necessary, the temperature condition may be selected such that the surface of the valve block  11 A is slightly melted, and that the outer profile of the valve block  11 A is made somewhat reduced from the initial conduction by the pressure applied to it. In any case, it is essential to properly select the pressure at which the molten resin is injected into the cavity. If the pressure is excessive, upon removal of the molded assembly, the valve block  11 A which is released from the pressure may apply an excessive internal pressure to the valve case  12 , and could damage the valve case  12 . If the injection pressure is insufficient, some gaps may be formed in the interface between the two parts, and a desired air-tightness may not be obtained. 
     Upon removing the valve housing after completion of the injection molding process, the valve case  12  is immediately exposed to the ambient air and shrinks to its prescribed size in a relatively short time period. However, the valve block  11 A would not immediately shrink even when the surface has been cooled to the ambient temperature because of the heat trapped inside the valve block  11 A. Therefore, there are some differences in the history of contraction between the valve block and the valve case after removal form the die cavity, but owing to the resilience of the valve case  12 , the two parts eventually become closely attached to each other substantially without any gap between them. Because the valve case  12  tends to shrink from its molded size as it cools, it is desirable to define the cavity slightly larger than the final intended size of the valve housing  13 . 
     Referring to FIG. 8, the shrinking of the valve block  11 A occurs inwardly both longitudinally (X-axis) and radially (Y-axis). Although this shrinking is extremely minor, it still could affect the operation of the valve unit when controlling the flow of high pressure fluid. Were it not for the annular grooves  30  to  33 , there would be a high tendency for a relative displacement between the inner surfaces of the valve case  12  which is made of a material having a relatively large thermal expansion coefficient, and the outer surfaces of the valve block  11 A, and gaps would be created between them. Such gaps can cause leakage of fluid out of the valve chamber. The annular grooves are not essential for the present invention, but are highly beneficial in controlling leakage in the valve unit. 
     It is also essential that the guide hole  16  maintains its shape for satisfactory operation of the valve because the valve main body  11  is made by drilling the guide hole  16  from the end surface of the circular projection, and the inner surface of this guide hole  16  serves as a valve seat which cooperates with the valve member  17  which is slidably received in the guide hole  16 . To this end, not only the valve main body  11  must be sufficiently resistant to deformation but also the valve case  12  should protect the valve body  11  from external forces. 
     When the valve main body  11  is made of a synthetic resin material which has a high thermal coefficient and is relatively deformable, the valve main body  11  may expand or contract according to the changes in the ambient temperature. The valve case  11  may also deform when the valve case  12  is firmly held by a hand to operate the knob  3 , and the valve case  12  is itself readily deformable. From such considerations, the valve case should be free from shrinking which could apply pressure upon the valve body  11 , and the valve case  12  is also desired to be resistant to deformation so that it prevents any external force applied thereto from being transmitted to the valve body  11 . 
     However, when the valve block  11 A is provided with the circumferential grooves  32  and  33 , and the valve case  12  is provided with the corresponding circumferential ribs  48  and  49  as illustrated in FIG. 8, an axial contraction of the valve block  11 A is opposed by the engagement between the circumferential grooves  32  and  33  and the circumferential ribs  48  and  49 . FIG.  9 ( a ) shows the axial contraction of circumferential rib  48  against circumferential groove  32 . When there is any relative axial contraction between the valve block  11 A and the valve case  12 , the circumferential grooves  32  and  33  are pushed firmly against the circumferential ribs  48  and  49  as indicated by numeral  54  so that the gap  52  which may develop between the valve block  11 A and the valve case  12  is closed by this circumferential contact region  54 , and the required air tightness of the valve housing  13  can be ensured. The sealing effect afforded by this circumferential contact region  54  is maintained even when the valve block  11 A laterally contracts relative to the valve case  12  as indicated by the arrow B. 
     Moreover, when the valve block  11 A is provided with the annular grooves  30  and  31 , and the valve case  12  is provided with the corresponding annular ribs  46  and  47  as illustrated in FIGS. 8, an axial contraction of the valve block  11 A is opposed by the engagement between the annular grooves  30  and  31  and the annular ribs  46  and  47 . FIG.  9 ( b ) shows the axial contraction of annular rib  46  against annular groove  30 . When there is any relative axial contraction between the valve block  11 A and the valve case  12 , the annular grooves  30  and  31  are pushed firmly against the annular ribs  46  and  47  as indicated by numeral  55  so that the gap  52  which may develop between the valve block  11 A and the valve case  12  is closed by this annular contact region  55 , and the required air tightness of the valve housing  13  can be ensured. The sealing effect afforded by this annular contact region  55  is maintained even when the valve block  11 A laterally contracts relative to the valve case  12  as indicated by the arrow D. 
     Furthermore, this contact region  55  attaches the part of the valve case  12  surrounding the circular projection  27  to the same, thereby reinforcing the circular projection  27  against a radial deformation, and preventing the deformation of the valve guide hole  16  that will be drilled in the projection  27 . 
     Referring to FIG. 10, valve housing assembly  13  is taken out of the molding die assembly  26 , and is suitable drilled and machined. More specifically, the valve guide hole  16  is drilled centrally from the outer end surface of the circular projection  27 , and a thread  22   a  is formed around the outer end of the circular projection  27  to form the annular boss  22 . The vent hole  21  is drilled in the valve case  12  to communicate the bottom end of the valve guide hole  16  with the atmosphere. 
     The longitudinal hole  15  is drilled axially through the valve case  11  between the upstream passage  14 A and the downstream passage  14 B, and the bypass passage is drilled between the upstream passage  14 A and the valve guide hole  16 . An inner thread  12   b  is formed in the inner wall of the outer end of the downstream passage  14 B, and the outer thread  12   a  on the outer circumference of the upstream end of the valve case  12  is finished to a required precision. 
     Then, a compression coil spring  18  and the valve member  17  are fitted into the valve guide hole  16 , in that order and with the recessed end of the valve member  17  first, from the upper end of the annular boss  22 , and the cap  23  is threadingly fitted over the annular boss  22 . The upper stem end  50  of the valve member  17  projects out of the cap  23  is attached to the knob  3  with a valve shaft  24  which is press fitted into the valve member  17  and secured to the knob  3  by a set screw  51 . 
     FIGS.  11 ( a ) and  11 ( b ) show an alternate arrangement. In these drawings, the parts corresponding to those of the previous embodiment are denoted with like numerals. The valve case  13  is provided with an annular boss  56  having an internal thread  56   a  while the valve main body  11  is not provided with the circular projection  17 . A flanged bush  57  is placed inside the annular boss  56 , and is secured therein by a tubular plug  58  provided with an outer thread  58   a  which is engaged with the outer thread  56   a.  A cap  59  may be optionally fitted over the annular boss  56 . 
     FIG. 12 shows a normally open type valve unit, and, in this drawing, the parts corresponding to those of the previous embodiments are denoted with like numerals. This embodiment is similar to the valve unit illustrated in FIG.  5 , but the bypass passage  25 ′ extends from the upstream passage  14 A to a part of the valve guide hole  16  adjacent the valve stem end of the valve guide hole  16 . The valve member  17  is retained in the valve guide hole  16  by a collar  57  and a C-ring  58 . The collar  57  is provided with a vent hole  57 A for communicating the chamber defined between the upper end of the valve member  17  and the collar  57  to the atmosphere. A lateral hole  73  is passed through a downstream end of the longitudinal passage  15 , and a shut-off valve member  74  is slidably but closely fitted in this lateral hole  73 . The two ends of the shut-off valve member  74  extend out of the valve housing so that an annular groove  74 A formed in an intermediate part of the shut-off valve member  74  may move and in and out of the longitudinal passage  15 . 
     Therefore, according to this embodiment, when the knob  3  is not operated, the longitudinal passage  15  is communicated, however, provided that the annular groove  74 A is placed in longitudinal passage  15 . By pushing down the knob  3 , the longitudinal passage  15  is closed by the valve member  17 , and the upstream passage  14 A is communicated with the atmosphere via the bypass passage  25 ′ and the vent hole  57 A so that any negative pressure that may be remaining in the upstream passage  14 A can be removed. When the valve unit is not being used, and the suction is not required to be transmitted to the upstream passage  14 A, the shut-off valve member  74  may be pushed in either direction so that the longitudinal passage  15  can be closed without operating the knob  3 . 
     In this embodiment, the outer surface of the valve main body  11  is not provided with any grooves for more closely securing the valve main body  11  and the valve case  12  together. If desired, the grooves similar to the grooves  30  to  33  of the previous embodiments, or, alternatively, annular projections can be formed in the valve main body  11  so that corresponding annular projections or grooves, as the case may be, may be formed in the inner surface of the valve case  12 . 
     In the above described embodiments, the valve member  17  was moved axially inside a valve seat defined by the valve guide hole  16 , but other known arrangements of valves can be applied to the present invention. FIGS.  13 ( a ) and  13 ( b ) show an embodiment of the present invention applied to a cock valve having a frusto-conical valve member which is received in a complementary valve seat, and is adapted to be turned around its axial center line. 
     Referring to FIGS.  13 ( a ) and  13 ( b ), according to this embodiment, similarly to the previous embodiments, a valve housing  113  is formed by injection molding a valve case  112  around an insert member serving as a valve main body  111 . The valve main body  111  is provided with annular grooves  130  to  133 , similar to the annular grooves  30  to  33  of the previous embodiments, which cause corresponding annular projections  146  to  149  inside the valve case  112 . 
     A longitudinal passage  115  is passed axially through the valve main body  111  so as to communicate an upstream passage  114 A with a downstream passage  114 B. A frusto-conical valve guide hole  116  is formed laterally across the valve main body  111 , and rotatably receives the complementary valve member  117  which is provided with a lateral through hole  119 . The valve member  117  is provided with a valve stem  124  extending out of the valve case  112 , and the valve member  117  is retained in the valve guide hole  116  by a collar  157  and a cap  123 , with the cap threadably engaged to an annular boss forced in the valve case  112  around the valve guide hole  116  and the collar  157  interposed between the upper end of the valve member  117  and the inner surface of the cap  123 . The outer end of the valve stem  124  is fitted with a knob  103  which is secured thereto by a set screw  151 . By turning the valve member  117  around its axial center, the lateral through hole  119  can be selectively aligned with the longitudinal passage  115 , and the upstream passage  114 A can be thereby selectively communicated with the downstream passage  114 B. 
     Although the present invention has been described in terms of specific embodiments thereof, it is possible to modify and alter details thereof without departing from the spirit of the present invention.