Abstract:
An enclosure for containment of chemical distribution lines and methods for installation and modification of devices installed along such chemical distribution lines are provided. The enclosure contains two or more separable portions. Openings within the enclosure through which the chemical distribution lines or conduits extend are formed along a boundary between two separable portions. Devices, such as valves, regulators, gauges and filters may be installed along a conduit before the enclosure is formed around the portion of the conduit containing the installed devices. The enclosure may therefore be made smaller, because excess working room within the enclosure is not needed. Installed devices may be modified by removing portions of the enclosure to expose the relevant devices. The conduit portion containing the devices may be removed for device modification, before re-assembly of the enclosure around the conduit portion.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to integrated circuit fabrication, and more particularly, to an enclosure for valves, fittings and other devices which may be associated with chemical gas/liquid distribution lines found generally in a piping industry such as, for example, semiconductor fabrication. 
     2. Description of the Relevant Art 
     Integrated circuit fabrication is a complex process involving many steps. To form a metal-oxide-semiconductor (MOS) integrated circuit, for example, a gate dielectric, typically formed from silicon dioxide (“oxide”), is formed on a semiconductor substrate which is doped with either n-type or p-type impurities. For each MOS field effect transistor (MOSFET) being formed, a gate conductor is formed over the gate dielectric, and dopant impurities are introduced into the substrate to form a source and drain. Such transistors are connected to each other and to terminals of the completed integrated circuit using conductive interconnect lines. 
     Process steps such as those described above often require the use of liquid or gaseous chemicals. For example, the gate conductor formation described above typically involves deposition of a polycrystalline silicon (“polysilicon”) layer using silane gas. The gate conductor may be etched from this polysilicon layer using a dry etch process involving chlorine and/or fluorine-containing gases. The chemicals needed are typically distributed using pipes or tubing between the corresponding sources (such as bulk gas, gas cylinders or liquid chemical tanks) to the tools (such as deposition or etching reactors) which use the chemicals. 
     This distribution of chemicals is subject to various constraints. For example, purity of the chemicals should be maintained so that contamination of the fabricated circuits is avoided. Another important constraint is that the chemicals be distributed according to proper safety procedures. Some of the chemicals used in semiconductor fabrication may be toxic, corrosive, and/or pyrophoric (prone to combustion when in contact with air). Such chemicals may therefore be subject to specific distribution requirements. For example, double containment is typically used for toxic, corrosive, or pyrophoric chemicals. Double containment refers to the use of an inner container or conduit containing the chemical to be distributed, surrounded by an outer container or conduit which is connected to an exhaust system. In this way, any gaseous chemical which is leaked from the inner container or conduit is drawn into the exhaust system rather than escaping into the ambient of the fabrication facility. The exhaust system may include a scrubber or other means of reducing any hazards associated with the exhaust gas. For double containment of liquid chemicals, the outer container or conduit typically includes a drain in addition to an exhaust connection, so that any liquid which is leaked from the inner container or conduit moves into the drain system of the fabrication facility. 
     The double containment described above may be implemented in various ways. For example, a chemical source such as a gas cylinder is typically enclosed in a cabinet which is connected to an exhaust system. The chemical may be delivered from the cabinet to the tool using a double-walled conduit, such as a tube or pipe, having a space between the inner and outer walls. The chemical is transported within the inner wall of the tube, while the space within the outer wall is connected to an exhaust system (and a drain, in the case of a liquid chemical). Measurement and/or control devices are generally installed somewhere along the path between the chemical source and the tool. Examples of such devices include valves, pressure gauges, pressure regulators, and filters. Because these measurement or control devices control or sample the chemical being transported through the conduit, the outer wall of the conduit is removed at the installation point, and the device is installed between portions of the inner wall. Installation of a measurement or control device therefore disrupts the double containment of the double-walled conduit. Such devices, and the portions of the conduit in which they are installed, must therefore be surrounded by an enclosure if double containment of the system is to be maintained. 
     An example of an enclosure which may be used to provide double containment in the vicinity of measurement and/or control devices installed along a gas conduit is shown in FIG.  1 . Conduit  10 , including outer conduit  12  and inner conduit  14 , is connected to either side of enclosure  16  (the connector used is not shown in FIG.  1 ). In the embodiment of FIG. 1, enclosure  16  includes box  18  and door  20 . Outer conduit  12  is connected to box  18  at wall  19 . Inner conduit  14  extends through a hole such as hole  28  in wall  19  into box  18 , where its path is interrupted by one or more installed devices such as valve  22  and pressure gauge  24 . External to enclosure  16 , inner conduit  14  is surrounded by outer conduit  12 . Enclosure  16  surrounds devices  22  and  24  and a portion of inner conduit  14  unsheathed from outer conduit  12 . The volume between conduits  12  and  14  and the volume within enclosure  16  combine to form a containment volume which surrounds inner conduit  14  and through which the ambient gas (typically air) flows to an exhaust system through one or more exhaust connections  30 . Door  20  of enclosure  16  provides access to devices  22  and  24 . 
     There are some problems associated with the use of enclosures such as enclosure  16  of FIG. 1, however. For example, inner conduit  14  of FIG. 1 must be threaded through holes  28  in either side of box  18  before devices  22  and  24  can be installed. Installation of and access to devices such as  22  and  24  must therefore be accomplished with the devices inside the enclosure. Such measurement and control devices must be accessed for many reasons, including routine maintenance (such as changing filter cartridges), repair of faulty devices, and reconfiguration or upgrading of the chemical distribution system (e.g., for installation of new or replacement tools). The requirement that such maintenance, repair, or reconfiguration must be performed with the devices inside of the enclosure can cause problems in the fabrication facility. The designer of the distribution system is often forced to choose between undesirable options, such as making the enclosure oversized to allow adequate working room within it, or having to discard and replace some sections of conduit in the event that repair or reconfiguration is required, because there is insufficient room to properly disassemble or repair components within the enclosure. Making the enclosure oversized is undesirable because this takes up extra space within the fabrication facility. Fabrication facility space is limited and costly, and space used up for chemical distribution is space made unavailable for other functions, such as placement of processing tools. If the enclosure is made more compact to save space, however, multiple devices and/or multiple conduits may be spaced so closely together that a particular device cannot be accessed. In this case, an entire conduit, and possibly adjacent conduits, may have to be cut, removed and replaced in order to perform a repair on one device. In addition to the associated direct costs, such conduit replacement may result in otherwise unnecessary tool downtime. 
     It would therefore be desirable to develop a means of providing containment of measurement and control devices installed in a chemical distribution system, while allowing improved access to the installed devices. 
     SUMMARY OF THE INVENTION 
     The problems outlined above are in large part addressed by a compound enclosure formed from two or more separable enclosure portions. Openings, or holes, through which conduit enters and leaves the enclosure are formed along a boundary between two of the enclosure portions, such that a portion of each hole is in one of the two enclosure portions, while the remainder of each hole is in the other of the two enclosure portions. In this way, the enclosure may be assembled around one or more conduits, rather than the conduits having to be threaded into the enclosure. Measurement and control devices can therefore be installed along a conduit without the constraint of working within an enclosure. The compound enclosure is formed such that the enclosure portions are separable, preferably using latches. A portion of the enclosure can therefore be removed after installation, so that a conduit may be removed from the box for maintenance, reconfiguration, or repair. The freedom from working within the enclosure allows the enclosure to be made smaller, thereby saving space in the fabrication facility. 
     An exhaust connection is formed in a wall of the enclosure, so that any gases which are leaked into the enclosure may be pulled into an exhaust system. If the enclosure is to be used with a liquid chemical distribution conduit, a drain is formed in the bottom of the enclosure, so that any liquids leaked into the enclosure may flow into a drain system. The enclosure is typically used with a double-walled conduit having an inner conduit and an outer conduit. One portion of the enclosure preferably includes a mating surface with a protruding ledge around the periphery of the boundary between the portion and its adjacent enclosure portion, to aid in fitting the portions together when the enclosure is assembled. Such a mating surface and ledge is preferably discontinuous around the periphery of the boundary, however, being absent in the vicinity of the conduit holes. In this way, the assembled enclosure has a substantially uniform thickness in the vicinity of the conduit holes, and a connector such as a bulkhead connector may be used to attach the outer conduit to the enclosure. The inner conduit extends through the connector and the hole in the enclosure, while the outer conduit surrounds the portions of the inner conduit external to the enclosure. A door, which typically includes a window, is formed in a portion of the enclosure for inspection of and/or access to the devices and conduits within the enclosure. 
     In some embodiments, the enclosure may be formed from more than two portions. Openings for conduits may be formed along each boundary between enclosure portions, such that multiple conduit “layers” may be installed within an enclosure. Because portions of the enclosure, and the associated conduits, may be readily removed for maintenance, repair, or reconfiguration, even conduits which are behind or beneath other conduits may be readily accessible. 
     In addition to the enclosure described above, a method for providing containment of a device installed along a conduit is contemplated herein. A device, such as a regulator, valve, gauge, or filter, is installed along a conduit. The conduit is positioned within notches formed in each end of a first portion of a containment enclosure, such that the device is positioned between the notches. A second portion of the containment enclosure is then attached to the first portion, such that notches formed in each end of the second portion align with the notches in the first portion to form openings around the conduit. The conduit typically has a circular cross-section, and the notches are therefore preferably semicircular, such that circular openings are formed. Furthermore, the conduit typically includes an inner conduit and an outer conduit. In this case, the device is installed along a section of inner conduit not surrounded by outer conduit. An end of the outer conduit is connected to each conduit opening of the enclosure, such that the outer conduit surrounds portions of the inner conduit external to the enclosure. 
     A method for modifying a device installed along a conduit according to the method described above is also contemplated herein. The modification may be routine maintenance, such as changing a filter cartridge, or repair of a defective device. In addition, the modification may involve reconfiguration of the devices installed along the conduit. Such reconfiguration may be associated with installation of a new or replacement tool, for example. The second portion of the containment enclosure is removed, so that the conduit containing the device is exposed. If there is sufficient working room, the modification may be performed with the conduit in place. Alternatively, the conduit may be removed from the remaining first portion of the containment enclosure for modification of the device. After the modification is performed, the conduit is replaced within the notches of the first portion of the containment enclosure, and the second portion of the enclosure is then replaced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
     FIG. 1 is a perspective view of an enclosure used to provide containment for devices installed along an inner conduit of a double-walled conduit; 
     FIGS. 2 a - 2   d  are perspective and elevation views from various angles of a two-portion enclosure as recited herein for use with gas distribution lines; 
     FIG. 3 a  is a perspective view of a two-portion enclosure as recited herein for use with liquid or gas distribution lines; 
     FIG. 3 b  is a view of the enclosure of FIG. 3 a  when disassembled; 
     FIG. 4 is a perspective view showing portions of a three-portion enclosure for use with gas distribution lines; 
     FIGS. 5 a - 5   d  are perspective views illustrating a method for providing containment of devices installed along an inner conduit of a double-walled conduit; and 
     FIGS. 6 a - 6   d  are perspective views illustrating a method for modifying a device installed along a conduit within a containment enclosure as shown in FIGS. 5 a - 5   d.    
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning to the drawings, FIG. 2 a  is a perspective view of an embodiment of the enclosure as recited herein for containment of gas distribution lines. The enclosures shown in FIGS. 1-6 have the shape of a rectangular box. This is believed to be a convenient, readily-available shape in terms of ease of fabrication and ease of placement in the chemical distribution system. However, other shapes, such as cylindrical shapes, for example, may also be used. As in the case of all of the enclosures recited herein, enclosure  40  of FIG. 2 is made from a chemical-resistant material, or from a material coated with a chemical-resistant material. Stainless steel is believed to be a suitable material; the 316 alloy is often used for chemical distribution. Other suitable materials may include various chemical-resistant plastics. 
     Enclosure  40  is a two-portion enclosure, including a first portion  42  and a second portion  44 . Second portion  44  includes door  46 . Latches  48  are used to connect portions  42  and  44 . Other fasteners having a reversible action may also be suitable, so that the enclosure may be repeatedly assembled and disassembled along the boundary between portions  42  and  44 . Conduit openings  50  are formed along the boundary between portions  42  and  44  on each of the small ends of the enclosure of FIG. 2, such that two conduits may be installed running lengthwise through the enclosure. Exhaust connection  52  is formed in a wall of second portion  14 , but may be formed in a different wall or a different portion of the enclosure, depending on the design and planned mounting of the enclosure. Exhaust connection  52  is typically a short tube connected to the edge of a hole in a wall of the enclosure, such that gases within enclosure  40  may be pulled through exhaust connection  52  and into an exhaust system connected to exhaust connection  52 . Such an exhaust system typically uses fans to pull air and other gases through the system, and may include a scrubber or other means of mitigating or removing toxic or hazardous components of the exhaust gas. The exhaust system may also include sensors associated with a gas monitoring system. Depending on the design of the chemical distribution and exhaust systems, such sensors may also be positioned on or within exhaust connection  52  and/or enclosure  40 . 
     Door  46  includes a window  54 , which allows inspection of the conduit and devices within enclosure  40  without opening door  46 . Although the use of such a window is currently preferred, the window may be omitted in some embodiments, and the door opened for viewing the inside of enclosure  40 . Window  54  is formed from a transparent, shatter-resistant, and chemical-resistant material. For example, glass reinforced with a wire grid may be a suitable window material. Chemical-resistant transparent plastics may also be suitable for window  54 . Latch  56  is used to open and close door  46  in the embodiment of FIG.  2 . Other fasteners which allow the door to be opened and closed repeatedly may also be used. Enclosure  40  may be mounted in any orientation which allows access to latches  48  and  56 , opening of door  46 , and removal of portion  44  for installation and/or modification of conduits. 
     FIGS. 2 b ,  2   c  and  2   d  are elevation views of the right, left, and back sides, respectively, of the enclosure of FIG. 2 a , with portions  42  and  44  separated. Notches  58  of portion  44  and notches  60  of portion  42 , as shown in FIGS. 2 b  and  2   c , combine to form openings  50  of FIG. 2 a  when portions  42  and  44  are fit together. Ledges  62  extending from portion  42  are also shown in FIGS. 2 b ,  2   c  and  2   d . Ledges  62  are slightly displaced from the wall of portion  42 , toward the interior of enclosure  40 . A mating surface, shown in FIG. 3 b  below, connects each of ledges  62  to the edge of portion  42  which mates to the corresponding edge of portion  44 . Ledges  62  help maintain the positioning of portions  42  and  44  with respect to each other when the portions are connected together, and could alternatively be formed along the mating edge of portion  44  rather than that of portion  42 . Rather than connecting to form a ledge extending completely around the mating edge of portion  42 , ledges  62  are preferably absent in the vicinity of notches  60 , as shown in FIGS. 2 b  and  2   c . In this way, the wall of enclosure  40  surrounding openings  50  has a uniform thickness. This uniform wall thickness facilitates the use of bulkhead connectors to attach conduit to the enclosure. Such bulkhead connectors are discussed in more detail in the description of FIG. 5 below. The view of FIG. 2 d  includes hinge  64  with which door  46  is attached to portion  44 . 
     An embodiment of the enclosure for containment of liquid or gas distribution lines is shown in FIG. 3 a . Differences between enclosure  66  of FIG. 3 a  and enclosure  40  of FIG. 2 a  include drain  68 , formed in the bottom of enclosure  66 , and the configuration of first portion  70  such that a liquid containment  76  is formed at the bottom of enclosure  66 . The components of enclosure  66  are shown more clearly in the exploded view of FIG. 3 b . Drain  68  is formed in the bottom wall of first portion  70 , and side walls of portion  70  are connected to and extend upward from the bottom wall such that a liquid containment  76  is formed at the bottom of portion  70 . In this way, any liquids which may leak into enclosure  66  can flow out of the enclosure through drain  68 . Although the configurations of first portion  70  and second portion  72  are believed to be convenient for providing proper drain orientation when enclosure  66  is mounted upon a wall, other geometries may also be used, subject to the constraint of having a liquid containment and a drain at the bottom of the enclosure. Drain  68  includes a hole in the bottom of the enclosure, and may also include a pipe connected to the edge of the hole. Drain  68  allows connection of enclosure  66  to a drain system which includes piping and may include a scrubber or other means of mitigating or removing toxic or hazardous liquids from the waste fluid stream. Sensors which detect the presence of liquid and/or the presence of particular chemicals may be used as part of a monitoring system, and may be included in the drain system. Such sensors may also be used upon or within drain  68  and/or enclosure  66 . 
     Ledges  78  of FIG. 3 b  are similar to ledges  66  of FIGS. 2 b  and  2   c . Breakout  3  of FIG. 3 b  shows ledges  78  in greater detail. In the embodiment of FIG. 3 b  , each of ledges  78  is substantially parallel to the wall of portion  70  to which it is attached. As shown in breakout  3 , edge  78  is offset from mating edge  84  of portion  70  by mating surface  80 . Ledges  78  and mating surfaces  80  help to maintain the positioning of portions  70  and  72  with respect to each other when the enclosure is assembled. As shown in breakout  3 , ledges  78  are preferably absent in the immediate vicinity of notches  82 . In this way, a substantially uniform wall thickness in the vicinity of conduit openings  86  may be achieved when the enclosure is assembled. As noted above in the description of FIG. 2, this uniform wall thickness may facilitate the attachment of conduit to the enclosure using a bulkhead connector. Alternate ledge configurations or other means of maintaining position of portions  70  and  72  with respect to each other may also be used. For example, ledges such as ledges  78  may be formed on portion  72  instead of on portion  70 . Notches  82  are semicircular in shape in the embodiment of FIG. 3, so that openings  86  are circular. This geometry is preferred for use with conduit having circular cross-section. In the event that conduit having a different cross-sectional shape is used, the shape of notches  82  should be chosen accordingly. For all embodiments of the enclosure recited herein, the enclosure dimensions and number of notches are chosen to contain a desired number of conduits over a desired length, depending on the details of the chemical distribution application. 
     In the embodiment of FIG. 3, second portion  72  includes exhaust connection  88 . Exhaust connection  88  is similar to exhaust connection  52  of FIG. 2, and may alternatively be formed in other positions on the enclosure, subject to mounting and drainage constraints. Portion  72  includes door  74 , which is similar to door  46  in FIG.  2 . In some embodiments, a gasket may be included to form a tighter seal between the door and the enclosure, or between enclosure portions. The tightness of the seal desired depends in part on the design of the exhaust system. It is typically not desired, for example, to have a vacuum created within an enclosure such as enclosures  40  and  66 . Such a vacuum may be created by the exhaust system, however, if connections within the enclosure are too tightly sealed. It may be preferable, therefore, to reduce gasket use so that ambient air may be pulled in through seams of the enclosure and the desired flow rate into the exhaust system realized. 
     Turning now to FIG. 4, an alternative embodiment of an enclosure for containment of gas distribution lines is shown. Enclosure portions  90 ,  92 , and  94  may be connected together using fasteners such as latches  96  to form a three-portion enclosure. When the portions are connected together, notches  100  of portion  90  align with notches  102  of portion  92  to form a layer of conduit openings at the boundary between portions  90  and  92 . Similarly, notches  104  of portion  92  and notches  106  of portion  94  align to form a second layer of conduit openings. Because the conduit portions can be added and/or removed one by one to allow installation or modification of a layer of conduit, multiple layers of conduit may be contained while maintaining accessibility. In other respects, including, for example, exhaust connection  98  and door  99 , design considerations for the enclosure of FIG. 4 are similar to those for gas conduit enclosure  40  of FIG.  2 . The concept of the enclosure of FIG. 4 may also be extended to. multiple-portion enclosures having more than  3  portions, and more than  2  layers of conduit. A multiple-portion enclosure for containment of liquid lines may also be formed, by configuring the portions such that a liquid containment and drain are included at the bottom of the enclosure. For example, a configuration similar to that of FIG. 3 could be used, wherein the bottom part of portion  70  is extended, and multiple portions connected together in place of portion  72 . 
     A method for using the enclosure recited herein to provide containment around a device installed along a conduit is illustrated in FIGS. 5 a - 5   d.  The device is first installed in the conduit to form conduit assembly  107 , as shown in FIG. 5 a . In the embodiment of FIG. 5, outer conduit  108  surrounds inner conduit  110 . Outer conduit  108  is cut to expose the portion of inner conduit  110  which is to be contained by the enclosure. Inner conduit  110  is cut for installation of valve  112  and bulkhead connectors  114 , used to connect conduit assembly  107  to the enclosure. Locknuts  116  mate with bulkhead connectors  114  when the enclosure is assembled. Bulkhead connectors  114  typically include a nut portion  118  permanently connected to threaded portions  120  and  122  on either side. Connector  114  has a hollow bore which inner conduit  110  extends through. Outer conduit  108  may be connected to connector  114  using a compression fitting including nut  124  which tightens onto threaded portion  120 . Outer conduit  108  may extend within the bore of threaded portion  120 , but is typically blocked by the diameter of the bore through nut portion  118  from extending all the way through connector  114 . Other types of connectors may also be suitable for connection of the conduit assembly to the enclosure. Although one valve is installed along inner conduit  110  in the embodiment of FIG. 5, different devices and/or multiple devices may be installed, depending on application requirements. Although not shown, various fittings, including pipe fittings and compression fittings, may be used to install devices such as valve  112  along inner conduit  110 . 
     Conduit assembly  107  of FIG. 5 a  is then placed within notches formed in mating edge  126  of first enclosure portion  128 , as shown in FIG. 5 b . In the embodiment of FIG. 5, threaded portions  122  of connectors  114  are placed within the notches, with nut portions  118  just outside of the enclosure, and locknuts  116  within the enclosure. The notches are therefore preferably sized such that the resulting conduit openings have a diameter slightly larger than that of portions  122 . If other types of connector are used, the orientation of the connector parts with respect to the enclosure may be different. First enclosure portion  128  of FIG. 5 b  could represent, for example, portion  42  of FIG. 2, portion  70  of FIG. 3, or portions  90  or  92  of FIG.  4 . Second enclosure portion  130  is then connected to first portion  128 , using fasteners such as latches  132 , to form enclosure  134  as shown in FIG. 5 c.  Notches in portion  130  align with those in portion  128  so that openings are formed around the conduit assembly. Portion  130  of FIG. 5 c  could represent, for example, portion  44  of FIG. 2, portion  72  of FIG. 3, or portions  92  or  94  of FIG.  4 . After formation of enclosure  134 , locknuts  116  are tightened over threaded portions  122  of connectors  114 , so that the conduit assembly is securely attached to enclosure  134 . This tightening is done by opening door  136 , as shown in FIG. 5 d.    
     A method for modifying a device installed within a containment enclosure as described above is shown in FIGS. 6 a - 6   d.  “Modifying a device” could refer to performing routine maintenance, such as changing a filter cartridge, repairing a device, or reconfiguring the conduit assembly to include more or different devices. In some installations, sufficient working room for modification may be obtained by opening door  136  and reaching into enclosure  134 . In situations for which this is not workable, however, portion  130  of enclosure  134  is removed, as shown in FIG. 6 a.  Before removal of portion  130 , locknuts  116  are loosened by reaching in through opened door  136 , in a reversal of the tightening illustrated in FIG. 5 d.  In some installations, removal of portion  130  may provide sufficient working room for the modification to be performed. If more room is needed, conduit assembly  107  may be removed from portion  128 , as shown in FIG. 6 b.  After modification of the conduit assembly is performed, the assembly is replaced within the notches formed in portion  128 , as shown in FIG. 6 c.  In the embodiment of FIG. 6, the modification includes installation of a regulator  138  to form new conduit assembly  140 . Portion  130  is then replaced and locknuts  116  tightened so that conduit assembly  140  is contained by enclosure  134 , as shown in FIG. 6 d.    
     It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide an enclosure formed from two or more separable portions for containment of chemical distribution lines, and methods for installation and modification of devices along such chemical distribution lines. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. For example, if a connector which can be tightened from the outside of the enclosure is used to connect conduit to the enclosure, a door in the enclosure may not be needed. It is intended that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.