Abstract:
A releasable anode liner that is fitted within the interior of the anode of an ion source. The cover permits electrons to be projected into the anode wherein any insulating deposits adhere to the interior of the anode liner, thereby increasing the effective life of the anode without premature replacement or repair.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the field of mass analyzers, and in particular to a replaceable anode liner for an ion source, such as those used in semiconductor process monitoring.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is known that certain semiconductor wafer monitoring processes utilize mass spectrometers or other apparatus in order to determine the presence and relative amount of process gases. A number of these processes, such as those, for example, utilizing Chemical Vapor Deposition (CVD) techniques, contain volatile silicon and/or other species which can cause a mass spectrometer monitoring the process to lose sensitivity in a relatively short period of time; that is, as compared to the average lifespan of an ion source typically used in conjunction with the spectrometer. More succinctly, the resulting problem that ensues is that the ion source can lose required sensitivity in a matter of days, as opposed to the normal or typical lifetime (e.g., months) of the ion source, thereby necessitating premature replacement of same.  
         [0003]     This loss in sensitivity noted above is attributable to the accumulation of insulating deposit on the interior of the anode of the ion source. Typical ion sources are depicted in  FIGS. 1 and 2 , while a mass analyzer system  31  incorporating same is illustrated in  FIG. 3 . For purposes of better describing the problems, reference is made to each of these Figs.  
         [0004]     First and with regard to  FIGS. 1 and 2 , a pair of ion sources  10 ,  30  is shown. Components commonly used in each of these sources and referred to herein are labeled with the same reference numerals for the sake of clarity.  
         [0005]     As to the differences between the depicted ion sources  10 ,  30 , some ion source manufacturers have used replaceable anodes in which the whole element is replaced or removed for cleaning, such as those, for example, in an ion source that was manufactured by Leybold Inficon of East Syracuse, N.Y. for their Q-Mass sensor system. Typically, these organic mass spectrometer units have gas entry extending from a gas chromatograph or other form of output that enters the side of the anode (i.e., laterally),as shown in  FIG. 1 , representative of a portion of the known ion source  10 .  
         [0006]     However and for vacuum processing applications, process analyzers based on residual gas analyzers (RGAs) such as the Compact Process Monitor manufactured by Inficon, Inc., typically have a closed ion source  30 , such as shown in  FIG. 2 .  
         [0007]     Each of the ion sources  10 ,  30  commonly include an electron stream producing means, in this case a heated filament  14 , typically made from tungsten or a similar material that forms an electron stream which projects into the structure of the anode  18 ,  32 , respectively. As noted above, the anode  18  according to the ion source  10  of  FIG. 1  is replaceable, the anode being shown in both the assembled and unassembled positions in the figure, while the closed ion source  30  of  FIG. 2  includes a fixed anode  32  with supporting structure such as a sealed disk  34  at the upper end thereof.  
         [0008]     Electrons that are formed from the heated filament  14  of each ion volume  10 ,  30  are expelled into an ionization volume or region within the interior of the anode  18 ,  32 . The potential of the anode  18 ,  32  is positive with respect to the filament and an electron repeller (not shown). Reagent gases from a deposition chamber or other source to be monitored are provided into the ionization volume. As noted above and in the instance of the ion source  10 , the gases are provided laterally through a port  22  while in the ion source  30 , the gases are provided axially; that is, the gases are introduced in a direction  27  that is substantially perpendicular to the direction of the electron stream through the anode  32 .  
         [0009]     An example mass analysis system  31  is shown in  FIG. 3  in which a sensor  33 , that houses the ion detector and Quadrupole mass detector, is arranged relative to a vacuum test chamber  35  and a vacuum pump  37  that draws the reagent gases into the ionization volume. Gas, from process  20  is supplied to the closed ion source  30  by means of a flow control orifice  21 . Additional details concerning the above system are provided in U.S. Pat. No. 5,889,281, the entire contents of which are herein incorporated by reference.  
         [0010]     In each ion source  10 ,  30 , the ions resultingly formed in the confines of the ionization volume are pulled by appropriate potential through an ion lens assembly that comprises at least one focus plate or extractor  24  and a parallel and concentric exit lens  29 . The plate  24 , having less positive potentials to that of the anode  18 ,  32 , serves to accelerate the formed positive ions as a focused ion beam  26  through concentric openings  28  in the ion lens assembly along an axis  25  to a mass filter or other apparatus (not shown in  FIGS. 1 and 2 ), such as a quadrupole. Insulators  38  are provided in the lens assembly of each ion source  10 ,  30  to prevent gas leakage. In quadrupole mass spectrometers (hereinafter referred to QMS) especially, the sensitivity (that is, the ion current that is detected in ratio to the ion source partial pressure) is extremely dependent upon ion energy.  
         [0011]     In either instance, the electron beam heating the anode surface can induce the formation of an insulating deposit layer  39  from the CVD reagent gases that are being monitored. Subsequently, the same electron beam accumulates electrons on the insulated deposit layer surface  39 , forming a negative surface charge and generating an electrical potential that is negative with respect to the anode.  
         [0012]     Typically, a closed ion source  30 , such as shown in  FIG. 2 , that is used for process monitoring is operated to produce ions with approximately 6-8 electron volts of ion energy. The ion energy of the resulting ions entering the mass analyzer (not shown in  FIG. 2 )is reduced by the negative potential that is produced by the insulating layer effect described above, drastically reducing sensitivity for closed ion source QMS units.  
         [0013]     There are two traditional solutions for solving the above problem that are currently practiced in accordance with the known art. The first solution is a total replacement of the ion source. This solution is extremely expensive in that the ion source includes a number of components in addition to the anode. This first solution is also time consuming. The second solution is replacement of the standard anode. The latter solution requires a disassembly of the ion source in addition to a replacement of the anode. In all likelihood, the latter solution also requires a replacement of the filament, thereby incurring additional repair costs.  
         [0014]     In the ion source  10 , the side or lateral entry of reagent gas through port  22  lends itself to removal of the anode  18  along the axis  25  of the ion beam  26  for removal thereof. In the closed ion source  30  in which the reagent gases enter the source along the ion beam axis  25 , the anode  32  is typically an integral part of the ion source  30 . The disassembly sequence for replacing the anode  32  requires the removal of a number of component parts including the sealing disk  34 , a compression spring (not shown), the heated filament  14 , and then the actual anode structure prior to replacement. Replacement of the anode  32  for axial gas entry closed ion sources is therefore a major rework of the ion source assembly. As noted, minimally the anode assembly is replaced but also the filament  14  more than likely also requires replacement. This is especially true if the filament is made from tungsten, due to its brittle nature and the risk of fracture of the filament on assembly. A new (e.g., unheated) tungsten filament is much less brittle than one that has already been heated. Often, a user may opt to replace the complete ion source other than to perform disassembly in the field.  
       SUMMARY OF THE INVENTION  
       [0015]     It is a primary object of the present invention to overcome the above noted problems of the prior art.  
         [0016]     It is another primary object of the present invention to increase the useful life of an ion source for a mass spectrometer or similar apparatus by permitting field replacement of a disposable component that can be introduced relative to the anode structure without compromising the overall sensitivity of the ion source.  
         [0017]     Therefore and according to a preferred aspect of the present invention, there is provided an ion source for a mass analysis system, said ion source comprising: 
        means for forming an electron stream;     an anode having an interior region into which said formed electron stream is injected, said electron stream terminating within the anode region and in which ions are formed; and     a releasable anode liner, said anode liner being insertable into said interior anode region and configured to receive said electron stream therein.        
 
         [0021]     According to another preferred aspect of the present invention, there is disclosed a replaceable anode liner for an ion source, said ion source comprising having means for creating an electron stream disposed in relation to the interior of an anode support structure, said liner being releasably engageable with said ion source and configured to fit within said anode support structure.  
         [0022]     Preferably, the replaceable or sacrificial anode liner comprises a sleeve-like portion that is fitted within the interior of the fixed anode of the ion source, said liner further including indexing means for orienting said liner with respect to the electron stream creating means, such as a filament, when said liner is placed onto said anode. According to one preferred embodiment, the liner has an indexing means and a tensioning means, each accomplished by means of a T-shaped slot formed on one end of the liner that is aligned with a reference feature on the anode structure. A lateral slot formed on the opposing end of the liner is indexed automatically relative to the electron stream creating means, such as a filament, in the case of a closed ion source, when the T-shaped slot is initially aligned with the reference feature on the anode structure.  
         [0023]     The liner includes means to permit insertion and removal thereof, without requiring disassembly of the ion source; that is, the liner can be assembled to and removed directly from the fixed anode using a removal tool.  
         [0024]     Preferably, the liner is designed to maintain a close sliding fit within the exterior of the anode, such that gas does not leak along a path between the interior of the ion source anode and the exterior of the liner to the low-pressure side of the ion source anode.  
         [0025]     According to yet another preferred aspect of the present invention, there is provided an ion source assembly for a gas analysis system, said assembly comprising: 
        an ion source including at least one filament, an anode structure into which a formed electron beam from said filament enters, a gas port that permits the entry of process gases for analysis and a plurality of anode liners wherein an anode liner is insertable into the interior of said anode structure, each of said liners being made from an electrically conductive material and having means for permitting at least a portion of said electron stream to enter the interior of said anode structure.        
 
         [0027]     According to yet another aspect of the present invention, there is disclosed a method for improving the sensitivity of a contaminated ion source, said ion source including an anode structure defining an interior region, said interior anode region receiving an electron stream wherein ions are formed in said region, said method comprising the steps of: 
        inserting a replaceable anode liner into the anode structure such that said liner is disposed in said interior anode region and receives said electron stream, said liner being made from an electrically conductive material permitting insulating deposits from said electron stream to form on an interior surface thereof in lieu of the interior of said anode structure.        
 
         [0029]     An advantage of the present invention is that the anode liner, as herein described, permits the entire useful life of the ion source to be realized without significant disassembly or replacement of critical componentry.  
         [0030]     Another direct advantage that is realized by the present invention is that the herein described anode liner(s) can be fabricated in a manner that can effectively control the emission of the electron beam into the anode region, depending on the application of the ion source of the hardware (e.g., mass spectrometer) that is being utilized.  
         [0031]     Yet another advantage is that the liner as herein described does not significantly affect the sensitivity of the ion source when a liner is initially installed, that is, prior to contamination. Moreover, a methodology and design is described that effectively centers and aligns the liner relative to the formed electron beam of the ion source automatically upon insertion thereof.  
         [0032]     Yet another advantage of the present invention is that effective contamination control is performed using a disposable component without sacrificing or significantly affecting the overall sensitivity of the ion source.  
         [0033]     The preferred embodiment accomplishes restoration of ion source sensitivity with a low cost replacement element and time-saving replacement method over the known techniques of replacing the complete ion source or anode.  
         [0034]     These and other objects, features and advantages will become readily apparent from the following Detailed Description which should be read in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a partial side elevational view, taken in section, of a prior art ion source;  
         [0036]      FIG. 2  is a partial side elevational view, taken in section, of another prior art ion source;  
         [0037]      FIG. 3  depicts an ion source as used in a mass spectrometer system for use in a semiconductor monitoring process;  
         [0038]      FIG. 4 ( a ) is a partial side elevational view, taken in section, of an ion source having a replaceable anode liner fabricated in accordance with a preferred embodiment of the present invention;  
         [0039]     FIGS.  4 ( b ) and  4 ( c ) represent side elevational views of the anode liner of  FIG. 4 ( a );  
         [0040]      FIG. 5  is a perspective view illustrating the removal of the liner of FIGS.  4 ( a )- 4 ( c ) from an ion source in accordance with a particular embodiment of the invention;  
         [0041]      FIG. 6  depicts a perspective view of the attachment/replacement of the anode liner of FIGS.  4 ( a )- 4 ( c ) onto the ion source of  FIG. 5 ;  
         [0042]      FIG. 7  illustrates a side view of an anode liner in accordance with a second embodiment of the present invention;  
         [0043]      FIG. 8  illustrates a side view of an anode liner made in accordance with a third embodiment of the present invention; and  
         [0044]      FIG. 9  illustrates a side view of an anode liner that is fabricated in accordance with a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0045]     The present invention is herein described in terms of certain preferred embodiments in terms of a replaceable anode liner, as well as the forms of ion sources that the herein described covers can be used in conjunction with. It will be readily apparent from the discussion that follows to those of sufficient skill in the field, however, that other modifications and variations are possible within the spirit and scope of the intended invention. In addition, certain terms are used repletely throughout the discussion such as “top”, “bottom”, “lateral”, “above”, “beneath”, “side” and the like. These terms are used in order to provide a frame of reference with regard to the accompanying drawings and are not intended to be overly limiting, except where specifically indicated to the contrary.  
         [0046]     Turning to  FIG. 4 ( a ), there is shown a closed ion source  40 , such as that previously represented in  FIG. 2 . For purposes of the discussion herein, similar parts are labeled with the same reference numerals. As in the preceding, the ion source  40  includes an anode structure  32  that is aligned relative to a heated filament  14  serving to form electrons that are projected into an interior portion of the anode. The ion source  40  further includes an ion lens assembly that includes a focus plate  24  and a concentric exit lens, each having openings  28  that focus and direct an extracted ion beam  26  from the anode region to a mass filter (not shown). Reagent gases enter the anode region axially; that is, from the upper portion of the anode structure in a direction that is parallel to the axis  25  of the ion beam  26 . The assembly is sealed by means of a sealing disk  34  mounted to the top of the anode structure, and insulators  38  mounted in the ion lens assembly.  
         [0047]     The assembly further includes a sacrificial anode liner  44 , shown in FIGS.  4 ( a )- 4 ( c ), that is made in accordance with a first embodiment of the present invention. The anode liner  44  according to this embodiment is defined by a cylindrical sleeve-like housing  48  comprising a pair of open ends  52 ,  56  that further define a hollow interior  60 . The liner  44  is constructed from any electrically conductive material, though according to this specific embodiment, the liner is constructed from 304 stainless steel with gold plating. The liner  44  is thin-walled, for reasons better explained below and is relatively light weight, the liner being sized to tightly fit within the interior of the fixed anode structure  32  of the ion source  40  and more particularly the anode region into which ions are formed, as shown in  FIG. 4 ( a ).  
         [0048]     Referring particularly to FIGS.  4 ( b ) and  4 ( c ), the anode liner  44  further includes a T-shaped slot  64  having a vertical portion  68  and a horizontal or lateral portion  72 , the slot extending in the proximity of a first or top open end  52  as well as a lateral slot  76  that is formed proximate to an opposing second or bottom open end  56  thereof. The T-shaped slot  64  is shaped to a large diameter relative to the remainder of the liner outer diameter as a means for both tensioning and holding the liner  44  in place when inserted. The T-shaped slot  64  is configured in order to permit engagement by an insertion/removal tool  80 ,  FIG. 5 , for assembling/mounting of the liner  44  relative to the anode structure, as described in greater detail below. The lateral slot  76  of the liner  44  is sized for alignment with the electron producing source (in this instance, the heated filament  14 ,  FIG. 4 ( a )) of the ion source  40 ,  FIG. 4 ( a ) in order to permit electrons to penetrate the interior of the anode  32 ,  FIG. 4 ( a ) in the usual manner. However and due to the presence of the anode liner, any insulating deposits that would typically form from either or both of the surface adsorbed species and the gas phase species on the interior of the anode will now form on the interior surface of the conductive interior surface of the anode as electrons strike the interior wall of the liner  44 , depositing ion energy, raising the wall temperature thereof and allowing deposits to form.  
         [0049]      FIGS. 5 and 6  depict the removal and the subsequent replacement of a sacrificial anode liner  44  in accordance with the invention in relation to a closed ion source  40 A, similar to that described above. An insertion/removal tool  80  used therewith is defined by a cylindrical member having a pair of opposing ends; namely, an insertion end  88  and a removal end  84 , respectively.  
         [0050]     Referring first to  FIG. 5 , the apparatus depicted therein already assumes that a sacrificial anode liner  44 , as described above, is already in place relative to the fixed anode structure  32 A of the closed ion source  40 A. The insertion/removal tool  80  of this specific embodiment has a diameter that is sized to engage the interior of the anode structure  32 A and the interior of the already inserted anode liner  44 . The tool  80  is inserted into the anode until an alignment removal pin  92  projecting from the tool bottoms out on the horizontal portion  72  of the T-shaped slot  64 . The tool  80  is then rotated about its center axis until it meets with the end of the horizontal portion  72  of the T-shaped slot  64 . It does not matter for purposes of liner removal whether the tool  80  is rotated clockwise or counterclockwise. Once the tool alignment removal pin  92  is engaged with the lateral portion  72  of the T-shaped slot  64 , the liner  44  can be pulled from the anode interior by retraction of the insertion/removal tool  80  as shown in direction  101 .  
         [0051]     Referring to  FIG. 6 , a new anode liner  44  can then replace the removed liner of  FIG. 5 . Insertion is made using the tool  80  and more specifically a tool alignment insertion pin  94  that projects radially from the exterior of the tool. The alignment insertion pin  94 , according to this embodiment, is initially aligned along the vertical portion  68  of the T-shaped slot  64  of the sacrificial anode liner  44 . The lateral slot  76  of the liner  44  is aligned, in accordance with this embodiment, automatically with the filament (not shown) of the ion source  40 A by providing a small circumferential notch  102  in the uppermost point of the fixed anode  32 A. This notch  102  is provided such that engagement of the tool alignment insertion pin  94  of the removal/insertion tool  80  therewith will automatically align or index the lateral slot  76  in the bottom of the liner  44  with the electron stream source (e.g., the filament) of the ion source  40 A. Insertion is then performed axially in direction  108 , the insertion end permitting insertion to a predetermined axial distance within the anode structure by means of a shoulder  105 . The height of the anode liner  44  is set to be slightly higher than that of the anode  32  such that, when fully inserted, the liner projects outwardly above the top of the anode very slightly, thereby ensuring that the liner is fully inserted.  
         [0052]     As such, insertion effectively aligns and centers the electron entrance slot of the liner  44  relative to the filament  14  automatically without the need for additional aids or inspection.  
         [0053]     Preferably and in operation, the herein described sacrificial or replaceable anode liner  44  would be initially incorporated into the interior of the anode structure of an ion source, the anode structure further including the circumferential notch  102 . The thickness of the liner  44  must be sufficiently thin in order to preserve the sensitivity of the ion source, partially controlled by the dimensions of the ionization region within the anode.  
         [0054]     Verification testing was performed to verify the use of a prototype sacrificial liner, such as that described above, in an ion source assembly. For purposes of this testing, the ion source was a CVD version closed ion source manufactured by Inficon, Inc. Testing was performed using a Phase 2 Compact Process Monitor which was equipped with a quadrupole mass filter to determine the effect of sensitivity as measured both without the presence of a sacrificial anode liner and with the inclusion of a said liner  44 , as described above.  
                                                       Sensitivity           Configuration   (A/Torr)                           Closed Ion Source without an Anode   1.20 × 10 −5             Liner           Closed Ion Source with an Anode Liner   1.15 × 10 −5             inserted           Closed Ion Source without an Anode   0.95 × 10 −5             Liner (Removed)                      
 
         [0055]     A second comparison was performed using a contaminated ion source measured before and after insertion of a sacrificial anode liner, as described above.  
                                                       Sensitivity           Configuration   (A/Torr)                           Closed Ion Source Contaminated with   0.45 × 10 −5             SiO 2  from SiCl 4  Operation           Contaminated Closed Ion Source with an    1.4 × 10 −5             Anode Liner inserted                      
 
         [0056]     According to yet another embodiment of the present invention, the sacrificial anode liner can be designed so as to control the flow of electrons into the ionization volume. A multi-purpose or “universal” ion source  110  is depicted in  FIGS. 7-9  that can individually accommodate a plurality of multiple sized or designed anode liners. The ion source  110  is of the closed form type and includes an anode structure  114  as well as a filament  115  serving as an electron source. The source  110  further includes an ion lens assembly that includes a conductive focus plate  118  and an ion exit lens  122 , each having a concentric opening  126  that permits an ion beam  130  to pass therethrough. Process reagent gases enter the ion source  110  axially (with respect to the formed ion beam  130 ) through the anode structure  114  and exit through the ion lens assembly as well as the filament. The ion source  110  is otherwise sealed for gas leakage by means of a sealing disk  135  disposed at the top of the anode structure  114  and insulators  139  provided at the ion lens assembly.  
         [0057]     According to one variation shown in  FIG. 7 , a sacrificial anode liner  140  is defined as a cylindrical sleeve member designed and sized to fit within the interior of the anode structure  114 . The liner  140  is a thin-walled structure made from an electrically conductive material and includes a pair of opposite open ends that define a hollow interior. An electron entrance slot  147  is provided at the bottom end thereof which aligns with the filament  115  in order to permit formed electrons to enter the interior of the anode structure  114 . As in the preceding, and rather than forming on the interior of the anode structure  114 , any insulating deposit from the reagent gases will subsequently form as a layer  149  instead on the interior surface of an opposite wall of the liner  140 , that is, opposite from the entrance electron slot  147 , the liner being electrically conductive thereby promoting same. The liner  140  is shown in the figure in both the assembled and unassembled condition, the liner being insertable and removable in the direction  145 .  
         [0058]     Referring to  FIG. 8 , another version of a sacrificial liner  150  is illustrated for use with the ion source  110 . In this specific embodiment, the design of the liner  150  is literally identical to that of  FIG. 7 , other than that the lateral electron entrance slot at the bottom end of the liner is replaced with a smaller opening  154  that controls the admission of electrons into the closed ion source, such as for use in PVD (Physical Vapor Deposition) processes. The smaller electron entrance opening  154  reduces the conductance of gas out the electron entrance and therefore raises the pressure inside the anode region.  
         [0059]     A third liner  160 , illustrated in  FIG. 9 , is similar in design to the previous liners  140 ,  150  but in this liner the lateral electron entrance slot is removed and the open lower end of the liner is replaced by a single or multiple gas effusion opening  164  in the lower end of the liner  160 . The latter design is useful in that only a molecular beam of gas is flowed through the anode region. In each of the above liner designs, however, only a single anode structure and ion optics assembly is required.  
         [0060]     The remainder of the design of each of the above liners commonly includes an upper open end that includes a T-shaped slot  166 , as described above, wherein the anode structure  114  can similarly be configured with a circumferential notch  116 , shown only in  FIG. 9 , to permit indexing of each liner  140 ,  150 ,  160  relative to the filament  115 . An insertion tool, such as shown in  FIGS. 5 and 6 , can therefore be used to easily import and remove liners  140 ,  150 ,  160 , as needed, relative to the ion source  110 , either for contamination control and improved life of the ion source or for utilizing different applications, such as PVD, among others.  
       PARTS LIST FOR FIGS.  1 - 9   
       [0000]    
       
           10  ion source  
           14  electron source (heated filament)  
           18  anode  
           20  gas supply  
           21  flow control valve  
           22  gas port  
           23  calibration pressure gauge  
           24  focus plate  
           25  ion beam axis  
           26  on beam  
           27  direction  
           28  openings  
           29  exit lens  
           30  ion source  
           31  mass analysis system  
           32  anode  
           32 A anode structure  
           33  sensor  
           34  sealing disk  
           35  vacuum chamber  
           37  vacuum pump  
           38  insulators  
           39  insulating deposit layer  
           40  ion source  
           40 A ion source  
           44  anode liner  
           52  open end  
           56  open end  
           60  interior  
           64  T-shaped slot  
           68  vertical portion  
           72  horizontal or lateral portion  
           76  lateral slot  
           80  insertion/removal tool  
           84  removal end  
           88  insertion end  
           92  tool removal alignment pin  
           94  tool insertion alignment pin  
           101  direction  
           102  circumferential notch  
           105  shoulder  
           108  direction  
           110  ion source  
           114  anode structure  
           115  filament  
           116  circumferential notch  
           118  focus plate  
           122  ion exit lens  
           126  openings  
           130  ion beam  
           135  sealing disk  
           139  insulators  
           140  sacrificial anode liner  
           145  direction  
           147  electron entrance slot  
           149  insulating deposit layer  
           150  sacrificial anode liner  
           154  electron entrance opening  
           160  liner  
           164  gas effusion opening  
           166  T-shaped slot  
       
     
         [0122]     It will be readily apparent that there are many variations and modifications that are possible within the ambits of the herein described invention to those of sufficient skill in the field according to the following claims. For example, there are other forms of ion source where the anode is neither cylindrical nor is its long axis concentric with the long axis of the sensor. The above anode liner concept can also be useful in these ion sources. In such cases, a retention spring could be integrated into the liner section itself or other means such as a screw or the like could retain the liner in position. Similarly, a spring effect could be realized by slightly crushing the top of the liner until it is slightly oval in cross section. A spring could also be formed by placing two parallel cuts in the long axis of the cylinder, forming a tab, which could be bent outwardly slightly to improve retention force.  
         [0123]     Additionally, other alignment features could similarly be realized using the tab, for example, or no alignment other than visually may be necessary.