Abstract:
A method and apparatus for trapping AlCl 3  from an aluminum etch effluent includes a housing containing disposable trapping medium with a first trapping stage and a second trapping media positioned radially outward and axially downward, respectively, from the strap inlet at respective distances to take advantage of differences in heat exchange efficiencies between the trapping media and solid AlCl 3  build-up on the trapping media and of resulting changes in partial vapor pressure of AlCl 3  adjacent condensation surfaces as solid AlCl 3  build-up occurs to initially induce condensation and build-up near the inlet, but then preferentially flow vapor to more distant trapping media as build-up occurs before the build-up clogs the inlet. The first stage trapping media has less surface density than the second stage trapping media, so that the first stage trapping media collect more than half, preferably 90-95% of the AlCl 3  vapor with the media at ambient temperature, and so that the second stage trapping media remove the remaining AlCl 3  vapor with the media at ambient temperature.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a Division of co-pending U.S. patent application Ser. No. 09/489,374, filed Jan. 21, 2000, and entitled “Method and Apparatus for Removing Condensable Aluminum Chloride from Aluminum Etch Effluent”, which is a Continuation-in-Part of U.S. patent application Ser. No. 09/250,928, filed Feb. 18, 1999, and entitled “Method and Apparatus for Controlling Polymerized TEOS Build-Up in Vacuum Pump Lines.” 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to traps for collecting and removing condensable aluminum chloride vapor from a gas stream to control the build-up of aluminum chloride in vacuum pump lines, valves, and other components downstream from aluminum etching chambers, and more particularly to an improved aluminum chloride trap containing a disposable element for cooling and condensing condensable aluminum chloride vapor byproducts from an aluminum etch system and retaining the condensed aluminum chloride solids within the disposable element, wherein the disposable element can be easily and quickly removed for the safe and rapid removal and disposal of the condensed aluminum chloride solids.  
           [0004]    2. Description of the Prior Art  
           [0005]    In a typical aluminum etching process for producing components for semiconductor devices, a silicon wafer or other substrate having a film of aluminum on its top surface is positioned in a reaction chamber, and the chamber is evacuated to a vacuum of about 10 millitorr via a turbo pump and a mechanical pump which are connected to a reaction chamber via a foreline. A photoresist having a particular pattern is placed on the aluminum surface to protect part of the aluminum film. The exposed part of the aluminum film that is not protected by the photoresist is then etched by employing a reactive, chlorine-containing gas such as chlorine (Cl 2 ) or boron trichloride (BCl 3 ). Typically, the etching reaction is plasma-enhanced, where the reaction between the chlorine-containing gas and the aluminum film is enhanced by applying rf power to the reaction chamber to create a plasma comprising the atomic constituents of the reactive gas in high energy states in the chamber. The generation of the plasma also causes the reaction chamber to heat up, typically to a temperature of 100 to 150° C. or in some cases, 150-200° C. The plasma-assisted reaction between the aluminum film and the chlorine-containing reaction gas etches aluminum from the exposed areas of the aluminum film, resulting in the formation of condensable aluminum chloride vapor (AlCl 3 ) byproducts. The reaction chamber effluent, which contains the condensable aluminum chloride vapor in addition to excess chlorine-containing reaction gas, is then removed from the reaction chamber by an applied vacuum from a vacuum pump. An exhaust line leading from the vacuum pump then directs the effluent to a scrubber, where the condensable aluminum chloride vapor and any excess chlorinated reaction gases are destroyed. A wet scrubber is often employed to treat the effluent with water to remove the condensable aluminum chloride vapor and excess chlorinated reaction gas from the effluent. Alternatively, a dry scrubber may be employed to destroy excess chlorine-containing reaction gases from the effluent, however, the dry scrubber is not able to destroy the condensable aluminum chloride vapor byproduct.  
           [0006]    The condensable aluminum chloride vapor byproduct in the conventional aluminum etching systems described above cause problems downstream from the reaction chamber, because they condense, solidify, and deposit upon contact with cool surfaces, such as the cooler interior surfaces of the vacuum forelines and exhaust lines that are used to convey the effluent gas away from the reaction chambers, as well as in other components of the vacuum conduit system of the etching system. Such buildup of solid aluminum chloride downstream from the etching chamber can partially or even entirely clog the pipes, thus reducing vacuum conductance, and can cause piping, pumps, scrubbers and other equipment used in the etching system to be functionally impaired or inoperative and in need of frequent maintenance. Solid aluminum chloride buildup can also flake apart and fall off the piping surfaces and migrate back into the reaction process chamber to become a source of contamination in the semiconductor device manufacturing process.  
           [0007]    Typically, the vacuum in the foreline of an aluminum etching system is approximately 500 millitorr, and consequently it is necessary to heat the forelines to a temperature of about 70° C. in order to keep condensable aluminum chloride vapor in the vapor phase so that the condensable aluminum chloride vapor can be removed from the chamber and the foreline by the applied vacuum. However, the pressure in the exhaust line between the pump and the scrubber is typically 760 torr, and therefore it is necessary to heat the exhaust lines to even higher temperatures, typically around 105° C., to keep the condensable aluminum chloride vapor in the vapor phase as the effluent flows through the exhaust lines. If either the foreline, the exhaust line, or both are not maintained at the proper temperature, the condensable aluminum chloride vapor will cool, condense, and solidify, and the condensed aluminum chloride solids will build up along the interior surfaces of the vacuum conduit system, resulting in the diminished function or clogging of the vacuum source.  
           [0008]    In order to prevent condensation and solidification and build-up of condensed aluminum chloride solids in the forelines and exhaust lines in aluminum etching systems, heater jackets can be wrapped around such piping to maintain the forelines and exhaust lines at an elevated temperature, thereby preventing condensation and solidification of condensable aluminum chloride vapor on the inside surfaces of such piping. However, depending on the length of the forelines and exhaust lines in the etching system, the use of heating jackets can be quite costly. For example, in some etching systems the reaction chamber may be located on one floor of a building while the vacuum pump and scrubber may be located one or more floors above or below the reaction chamber. Consequently, the foreline, the exhaust line, or both may be 10 feet or longer. In such situations, the use of heating jackets could be cost prohibitive. Therefore, as a less costly alternative, heating tape is often used in place of heating jackets to heat the forelines and exhaust lines. However, the use of heating tape also has drawbacks in that it can be difficult to completely and effectively wrap the lines, and consequently gaps are often left between sections of the heating tape. Such gaps between sections of heating tape on a poorly wrapped pipe result in “cold spots,” where the condensable aluminum chloride vapor condenses on the inside of the pipe.  
           [0009]    Additional measures used to control the buildup of solid aluminum chloride in vacuum forelines and exhaust lines in vacuum systems of etching systems can include installing a trap just after the heated section of the piping line for trapping and removing aluminum chloride vapors from the effluent gas flow. As a result, the condensable aluminum chloride vapor is condensed and collected in the trap instead of depositing and building up in the piping lines. The trap can then be removed from the piping line whenever necessary for cleaning and removal of the deposited solid aluminum chloride.  
           [0010]    The use of traps to remove condensable vapor from piping lines is well-known in the art. Conventional traps for trapping condensable vapor are often designed on the principle that lowering the temperature of the condensable vapor in the trap will cause the condensable vapor to condense as a solid. For example, U.S. Pat. No. 5,422,081, issued to Miyagi et al., discloses a trap device for a vapor phase reaction apparatus having an adjustable number of interior surfaces upon which the condensable aluminum chloride vapor cools on contact with the interior surfaces and condense on such surfaces. However, the Miyagi et al. trap requires a plurality of plate-shaped members assembled in layers, which can be difficult and time-consuming to manufacture and assemble. In addition, the large number of parts makes the Miyagi et al. trap difficult and time-consuming to disassemble for cleaning and removal of condensed aluminum chloride solids. Further, the close proximity between the plate-shaped members and the intake opening can cause the trap to clog prematurely, thus wasting a significant portion of the trap&#39;s volume.  
           [0011]    Nor-Cal, Inc., of Yreka, Calif., has developed and manufactured a number of water-cooled traps for semiconductor processing equipment, including traps having coaxial and right angle configurations between the entrance ports to the traps and the exit ports to the traps. In the FTWA and FTWS series of traps manufactured by Nor-Cal, Inc., baffles redirect the gas flow between cooling coil tubes arranged cylindrically to increase the surface area for condensation of the condensable vapor flowing through the trap. Gas flowing into the trap is redirected either ninety degrees or one hundred eighty degrees by impacting either an interior surface of the trap or a cooling coil tube located in the trap. However, the Nor-Cal, Inc. trap, similar to many other conventional traps, becomes clogged near the entrance pipe to the trap, which results in low capacity and the need for frequent maintenance and cleaning.  
           [0012]    U.S. Pat. No. 5,820,641 to Gu et al. describes a liquid cooled trap for collecting condensable vapor in a chemical vapor reaction system that comprises a first stage, which is a very poor heat exchanger, in order to avoid condensation of the vapor and the resulting solid deposits that could clog the entrance port to the trap, and a second stage which is a good heat exchanger and comprises cooling coil tubes and cooling cones or fins. However, due to the expense of the cooling coil tubes and cooling cones in the Gu et al. trap, the trap cannot be discarded when removal of the build-up of aluminum chloride is necessary. Therefore economic considerations require cleaning of the deposition surfaces in the Gu et al. trap so that the trap can be reused.  
           [0013]    Since it is difficult to maintain all parts of an entire vacuum conduit system of an aluminum etching system at the proper temperature or to efficiently trap condensable aluminum chloride vapor with conventional traps, the buildup of solid aluminum chloride will inevitably occur throughout the vacuum conduit system of an aluminum etch system. Thus, at some point, the vacuum conduit system will require cleaning to remove the buildup of condensed aluminum chloride solids. This cleaning is usually carried out using water which, when contacted with aluminum chloride, results in the generation of tremendous amounts of toxic and corrosive hydrogen chloride (HCl) fumes and also generates a large amount of heat, both of which create hazardous conditions for workers.  
           [0014]    Consequently, in spite of the heating jackets, heating tape, and various types of aluminum chloride traps already available, there is still a need for an improved trap that condenses and collects condensable aluminum chloride vapor produced in aluminum etching systems in an efficient manner and which allows for safer, easier, and more rapid removal and disposal of the deposited aluminum chloride solids from the trap, especially if such removal and disposal could eliminate the need to clean solid aluminum chloride deposits from deposition surfaces in such traps and from pipes in the vacuum forelines and exhaust lines of aluminum etch systems.  
         SUMMARY OF THE INVENTION  
         [0015]    Accordingly, it is a general object of the present invention to provide an improved trap for aluminum etching systems wherein condensable aluminum chloride vapor byproducts of an aluminum etch reaction can be removed quickly and easily from such etching systems and safely discarded without having to clean solid aluminum chloride deposits from deposition surfaces in the trap.  
           [0016]    A more specific object of the invention is to provide a trap for removal of condensable aluminum chloride vapor from an etching effluent, wherein the trap comprises a disposable element for cooling, condensing, and retaining the condensable aluminum chloride vapor, and wherein the disposable element is easy to remove from the trap with minimal down time and labor and wherein the disposable element is inexpensive to replace.  
           [0017]    Another specific object of the invention is to provide a trap having a disposable element for trapping and collecting condensable aluminum chloride vapor, wherein the disposable element prevents deposition of condensed aluminum chloride solids on the inner walls of the trap.  
           [0018]    It is also an object of this invention to provide an improved method and apparatus for removing condensable aluminum chloride vapor from the exhaust line downstream of the vacuum pump and/or from the foreline upstream of the vacuum pump.  
           [0019]    It is a further object of the present invention to decrease time, labor, and costs required to deal with removal of aluminum chloride downstream from a reaction chamber in semiconductor processing systems.  
           [0020]    It is a further object of the present invention to provide a trap for an aluminum etching apparatus wherein condensable aluminum chloride vapor byproducts of an aluminum etch reaction can be quickly and easily removed from such etching systems and subsequently disposed of, thus reducing or eliminating the need to remove build-up of condensed aluminum chloride solids from the interior of a vacuum conduit system of the aluminum etching system, thereby reducing or eliminating exposure of humans to hazardous and toxic hydrogen chloride (HCl) fumes.  
           [0021]    Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.  
           [0022]    To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method of the present invention may comprise condensing, solidifying, and retaining condensable aluminum chloride vapor from a reaction chamber effluent onto trapping media in an aluminum chloride trap. The apparatus for achieving the foregoing and other objects in accordance with this invention may comprise a trap having a disposable, replaceable trapping element contained within a chamber, wherein the disposable element comprises trapping media for condensing and trapping condensable aluminum chloride vapor as condensed aluminum chloride solids. The trap is designed such that the disposable element containing the condensed and deposited solid aluminum chloride can be easily removed from the trap for rapid and safe disposal of the aluminum chloride solids and subsequently replaced with a new disposable element. The disposable element efficiently traps condensable aluminum chloride vapor such that condensable aluminum chloride vapor is prevented from depositing as condensed aluminum chloride solids and building up on the interior walls of a vacuum conduit system (e.g., the vacuum forelines and exhaust lines) of the aluminum etching system or on the interior walls of the trap chamber, thus eliminating the hazardous conditions associated with cleaning the disposable element and the trap to remove condensed aluminum chloride solids from the interior surfaces of the vacuum conduit system (i.e., the forelines and exhaust lines) of the etching system and/or from the interior surfaces of the trap.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the preferred embodiments of the present invention, and together with the descriptions serve to explain the principles of the invention.  
         [0024]    In the Drawings:  
         [0025]    [0025]FIG. 1 is a representative diagrammatic view of a typical aluminum etching system with a trap of the present invention positioned in a foreline downstream of the reaction chamber and between the turbo pump and the vacuum pump, with the trap and part of the foreline shown in cross-section;  
         [0026]    [0026]FIG. 2 is an isometric view of the trap of the present invention with a portion of the housing cut away to reveal the trapping media, and with a portion of the trapping media cut away to reveal the inner core and guide;  
         [0027]    [0027]FIG. 3 is an elevation view of the trap shown in FIGS. 1 and 2;  
         [0028]    [0028]FIG. 4 is a top plan view of the trap shown in FIGS. 1 and 2, as indicated by line  4 - 4  in FIG. 3;  
         [0029]    [0029]FIG. 5 is a bottom plan view of the trap shown in FIGS. 1 and 2, as indicated by line  5 - 5  in FIG. 3;  
         [0030]    [0030]FIG. 6 is a longitudinal cross-sectional view of the trap of the present invention take along section line  6 - 6  of FIG. 3;  
         [0031]    [0031]FIG. 7 is a transverse cross-sectional view of the trap shown in FIGS. 2 and 3, taken along section line  7 - 7  of FIG. 3;  
         [0032]    [0032]FIG. 8 is a longitudinal cross-sectional view similar to FIG. 6, but of an alternative embodiment of the trap of the present invention wherein the disposable element includes an outer core;  
         [0033]    [0033]FIG. 9 is an isometric view of a section of a preferred trapping medium according to the present invention;  
         [0034]    [0034]FIG. 10 is an elevation view of a single layer of an interlaced metal fabric mesh used in the preferred trapping media according to this invention;  
         [0035]    [0035]FIG. 11 is an elevation view of two layers of the interlaced metal fabric mesh of FIG. 10 laminated together;  
         [0036]    [0036]FIG. 12 is an elevation view of four layers of the interlaced metal fabric mesh of FIG. 10 laminated together;  
         [0037]    [0037]FIG. 13 is a diagrammatic view of a strip of the interlaced metal fabric mesh of FIG. 10 folded over on itself to make two layers laminated together;  
         [0038]    [0038]FIG. 14 is a graph of the vapor pressure curve for aluminum chloride (AlCl 3 );  
         [0039]    [0039]FIG. 15 is an alternative representative diagrammatic block view of a typical aluminum etching system showing a reaction chamber, a turbo pump, an aluminum chloride trap, a vacuum pump, vacuum system forelines and exhaust lines, and scrubber, wherein the trap is positioned between the vacuum pump and the scrubber; and  
         [0040]    [0040]FIG. 16 is an enlarged view of a segment of a wire in the preferred embodiment mesh trapping media shown in FIG. 9.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    An aluminum chloride trap  10  according to the present invention is shown diagrammatically in FIG. 1 in a preferred mounting placement for use in trapping the aluminum chloride byproduct of an aluminum etching process in which aluminum is etched from exposed surfaces of an aluminum film  23  in a reaction furnace or chamber  14  by exposure to a chlorinated reaction gas in chamber  14  with the assistance of an rf plasma. In such etching systems, a turbo pump  13  is connected to a foreline  11  and is used to evacuate etching chamber  14  to a low pressure and to maintain such vacuum in etching chamber  14  in a range typically of about 5 to 100 mtorr, often about 10 mtorr, throughout the aluminum etching process. In the etching process for producing, for example, a semiconductor device, a substrate  21  having a film of aluminum  23  deposited on its surface is positioned in the chamber  14 . A mask  25  (also referred to as a photoresist) is formed on the aluminum film  23  in a desired pattern by methods known in the art. A chlorinated reaction (i.e., etching) gas, such as chlorine (Cl 2 ) or boron trichloride (BCl 3 ), is then introduced through feed gas inlet  20 , as indicated by flow arrow  27 , into the approximately 5 to 100 mtorr vacuum etching chamber  14 . An rf voltage field is applied to chamber  14  to create a plasma, which assists in the reaction (i.e., etching) of exposed aluminum film  23  with the chlorinated reaction gas. This plasma-assisted etching reaction between the exposed aluminum film  23  and the chlorinated reaction gas produces condensable aluminum chloride vapor (AlCl 3 ) byproducts and can be illustrated by the following equation:  
         2Al+3Cl 2 →2AlCl 3   (1)  
         [0042]    where Al is the aluminum film  23 , Cl 2  is the chlorinated reaction gas, and AlCl 3  is the condensable aluminum chloride vapor byproduct. The condensable aluminum chloride vapor exits the reaction chamber  14  along with excess chlorinated reaction gas through chamber outlet  24 , as indicated by flow arrow  22 . Vacuum pump  16  then pumps the chamber effluent (i.e., the condensable aluminum chloride vapor and excess chlorinated reaction gas) through forelines  11  and  12  and then through trap  10 , where the condensable aluminum chloride vapor is condensed, solidified and trapped on disposable element  42 , while the remaining effluent comprising the excess chlorinated reaction gas is directed out of trap i 0 , as indicated by flow arrow  84 , and along exhaust lines  18  and  32  to scrubber  37 , where the effluent containing the excess chlorinated reaction gas is treated for safe disposal.  
         [0043]    Since turbo pump  13  must operate continuously to maintain the vacuum in chamber  14  as new reaction gas flows into the feed gas inlet  20 , substantial amounts of the condensable aluminum chloride vapor byproducts created in the etching reaction are drawn out of the etching chamber  14 , as indicated by flow arrow  22 , and into vacuum conduit foreline segments  11  and  12  and vacuum conduit exhaust line segments  18  and  32  of the vacuum system. The condensable aluminum chloride vapor begins to decrease in temperature immediately after exiting the etching chamber  14  at 100-150° C. or, in some cases, 150-200° C., and upon contact with cooler components of the vacuum conduit system (e.g., forelines segments  11  and  12  and exhaust line segments  18  and  32 ). Thus, if the vacuum conduit system foreline segments  11  and  12  and/or exhaust line segments  18  and  32  downstream of chamber  14  are not sufficiently heated, or if the condensable aluminum chloride vapor is not sufficiently trapped, the condensable aluminum chloride vapor will cool on contact with the cooler interior surfaces downstream of the chamber  14 , causing the condensable aluminum chloride vapor to condense and deposit solid aluminum chloride along the inside surfaces of the foreline segments  11  and  12  and exhaust line segments  18  and  32 . The position of trap  10  of this invention in an etching system will determine the number of heaters  33  and  34 , such as heating jackets or heating tape, needed to heat foreline segments  11  and  12  and/or exhaust line segments  18  and  32  to prevent deposition of solid aluminum chloride in a vacuum conduit system of an aluminum etch system. Therefore, trap  10  is preferably, but not necessarily, positioned as close as practical to chamber  14  to minimize the number of heaters  33  needed, such as between turbo pump  13  and vacuum pump  16  as shown in FIG. 1. Trap  10  cools, condenses, solidifies, and traps the condensable aluminum chloride vapor byproducts created in reaction chamber  14 , as indicated at  40  and  41 , thus preventing condensable aluminum chloride vapor from cooling, condensing, solidifying and building up in vacuum pump  16  and in vacuum conduit exhaust line segments  18  and  32 .  
         [0044]    To prevent the condensable aluminum chloride vapor from condensing in and clogging the foreline segments  11  and  12  when trap  10  is positioned between turbo pump  13  and vacuum pump  16 , as shown in FIG. 1, heaters, such as heating jackets  33  and  34 , are usually placed around the piping foreline segments  11  and  12 , respectively, to keep the temperature in the foreline segments  11 ,  12  elevated, preferably at a temperature above 70° C., to prevent the condensable aluminum chloride vapor in the effluent from cooling, condensing, solidifying, and accumulating before the effluent containing the condensable aluminum chloride vapor reaches the trap  10 . Such condensation and solidification can also occur in valves and other piping components (not shown) upstream of trap  10 , so it is not unusual to also keep such components heated as well. Through the use of heater  34 , it is possible to control the temperature of the effluent as it enters trap  10 , and, since the heater  34  may be positioned such that it abuts the inlet pipe  30  of trap  10 , the heater  34  can also be used to help control the temperature of the inlet pipe  30  of trap  10  to prevent accumulation of the solid aluminum chloride on the inside walls of inlet pipe  30 .  
         [0045]    As a result of the placement of trap  10  in the preferred embodiment shown in FIG. 1, condensable aluminum chloride vapor is efficiently and completely trapped, so there is no need to heat outlet pipe  110  or exhaust line segment  18  leading from trap  10  to vacuum pump  16  or exhaust line  32  leading from vacuum pump  16  to the scrubber  37 . In an industrial setup employing the embodiment illustrated in FIG. 1, the number of heaters which would be required upstream of trap  10  is relatively small compared to the number of heaters that would be necessary to heat an entire vacuum conduit system in a situation where trap  10  was not employed in an etching system. Consequently, trap  10  significantly reduces the cost associated with heating long sections of a vacuum conduit system in an etching system, which is required in conventional etching systems.  
         [0046]    As discussed above, trap  10  of the present invention, when positioned as illustrated in FIG. 1, is designed to prevent build-up of condensed aluminum chloride solids in vacuum pump  16  and in exhaust lines  18 ,  32 , as well as in pressure gauges, valves, and other components downstream of trap  10 . Briefly, trap  10  comprises a disposable element  42  contained within a housing  60 , wherein disposable element  42  contains trapping media (discussed below in detail), that create ideal conditions for condensing condensable aluminum chloride vapor as solid aluminum chloride and condensing and trapping the condensable aluminum chloride vapor in disposable element  42  of trap  10 , thereby removing the condensable aluminum chloride vapor from the chamber effluent before the condensable aluminum chloride vapor can cause problems farther downstream. The remainder of the effluent that exits trap  10  will be substantially free of condensable aluminum chloride vapor, and thus will comprise primarily excess chlorinated reaction gas, which passes through the vacuum pump  16  and exhaust line segments  18 ,  32  harmlessly and without build-up of solid aluminum chloride, and then continues on to the scrubber  37  for safe treatment and disposal of such excess chlorinated reaction gas. Disposable element  42  of trap  10  is further designed so that it may be removed easily and quickly from trap  10  after an appropriate period of time, allowing for safe and rapid disposal of the solid aluminum chloride buildup collected within disposable element  42 , and subsequently replaced with a new disposable element.  
         [0047]    Referring now to FIGS.  2 - 6 , the preferred embodiment of the aluminum chloride trap  10  according to this invention has a metal housing  60 , an upstream end wall  94  and a downstream end wall  108 . The upstream end wall  94  has an inlet opening  35 , and the downstream end wall has an outlet opening  114 . One of the end walls  94 ,  108 , preferably, but not necessarily, the upstream end wall  94 , is removable and fastenable in place, such as with clamp  104  on flanges  98 ,  100 . The upstream end wall  94  has a pipe-fitting  96 , and the downstream end wall  108  has a pipe-fitting  112  for the removably fastening the trap  10  in the forelines  12 ,  18  of the system illustrated in FIG. 1.  
         [0048]    As shown in FIGS. 2 and 6, the housing  60  and end walls  94 ,  108  of trap  10  enclose an interior chamber  90 , and a removable, disposable trap element  42  is positioned in the chamber  90  for condensing and trapping the aluminum chloride in the effluent from the reaction chamber  14  of FIG. 1. The trap element  42  comprises a cylindrical outer trapping medium  48  surrounded by a solid, cylindrical shield  53 , a smaller diameter cylindrical screen column  52  inside the outer trap medium  48 , a core trapping medium  44  disposed in the cylindrical screen column  52 , and an annular intermediate trapping medium  46  disposed in the annular space  54  between the screen column  52  and the outer medium  48 . The trapping media  44 ,  46 ,  48  comprise mesh material, preferably metal mesh, that allows flow of gases therethrough, but that also provides many surfaces that facilitate condensation and deposition of aluminum chloride  40 ,  41  in FIG. 6. A primary or first trapping stage  200  comprises a first medium positioned closer to the inlet opening  35  for initially more condensation and build-up  40  and a second medium positioned farther from the inlet opening  35  for initially less condensation and build-up  41  than the build-up  40 , but gradually increasing condensation and solid build-up  41  of AlCl 3  as heat transfer between AlCl 3  in the gas flow and the solid AlCl 3  build-up  40  becomes less efficient, as described in more detail below. In the preferred embodiment of the trap  10 , an outer cylindrical medium  48  and the cylindrical screen column  52 , especially those portions of outer medium  48  and screen column  52  that are upstream of core medium  44  and annular medium  46 , comprise the first medium and second medium, respectively, of the primary or first trapping stage  200  of the removable, disposable trapping element  42 . The core medium  44  and annular medium  46 , along with the lower portions of the outer cylindrical medium  48  and cylindrical screen column  52 , comprise a secondary or second trapping stage  205  of the disposable trapping element  42 .  
         [0049]    Essentially, the aluminum chloride component of the gaseous effluent flowing into chamber  90 , as illustrated by flow arrows  64 ,  66  in FIG. 6, condenses and deposits first on the upper portions of outer trapping medium  48 , as illustrated by solid aluminum chloride build-up  40 , and on the upper end of inner screen column  52 , as illustrated by solid aluminum chloride build-up  41 . Most of the aluminum chloride gas in the effluent (approximately 90 to 95 percent or more) condenses and deposits in this primary or first trapping stage  200 . The rest of the aluminum chloride in the effluent (approximately 5 to 10 percent or less) that does not condense and deposit in this primary or first trapping stage will condense and deposit in the core medium  44  and annular medium  46  of the secondary or second trapping stage  205 . The outer shield  53  prevents any gaseous aluminum chloride from passing through the outer medium  48  and condensing and depositing on the inside surface of the housing  60  of the trap  10 .  
         [0050]    When the solid aluminum chloride build-up  40 ,  41  in the first trapping stage  200  and/or solid aluminum chloride build-up in the core medium  44  and annular medium  46  of the second trapping stage  205  accumulate enough to inhibit effluent flow into or through the trap  10 , the trapping element  42  with the aluminum chloride build-up  40 ,  41  can be removed as a unit from chamber  90  and replaced with a new trapping element  42 . The removable end wall  94  facilitates such removal and replacement of the trapping element  42 .  
         [0051]    Typically, an aluminum etch system employing trap  10  of this invention may be operated continuously for about 6 to 12 months before it is necessary to replace the disposable trapping element  42 . Such capacity and longevity of trap  10  of this invention, combined with the relatively inexpensive materials used to construct the replaceable, disposable trapping element  42 , as will be described in more detail below, makes trap  10  far more inexpensive and practical than any aluminum chloride trap previously used in the art.  
         [0052]    Trap  10 , according to the present invention and as illustrated in a preferred embodiment in FIGS. 1 and 2, is designed to efficiently and thoroughly condense condensable aluminum chloride vapor produced in an aluminum etching system onto disposable element  42  of trap  10  and to retain the condensed aluminum chloride solids in disposable element  42 . Such trapping and easy disposal of aluminum chloride is achieved by the novel design of disposable element  42 , which is positioned in chamber  90  of trap  10  between inlet pipe  30  and outlet pipe  110 . Disposable element  42  comprises aluminum chloride trapping media  44 ,  46 ,  48  (described in more detail below), which provide large surface areas for the efficient condensation and trapping of condensable aluminum chloride vapor that builds up as condensed aluminum chloride solids on the surfaces of the trapping media as illustrated by build-up  40  and  41  in FIG. 1, while allowing other molecules, such as chlorinated reaction gases in the effluent, to pass unimpeded through the disposable element  42 . In order to cause the condensable aluminum chloride vapor to condense on the components of disposable element  42 , disposable element  42  must lower the temperature of the condensable aluminum chloride vapor. Disposable element  42  comprises trapping media  44 ,  46 ,  48  (discussed below in detail) which act as heat exchangers where heat is transferred from the condensable aluminum chloride vapor in the effluent to the trapping media  44 ,  46 ,  48 . A significant feature of disposable element  42  of trap  10  is that the physical geometries and properties of the trapping media  44 ,  46 ,  48  in disposable element  42  can be optimized to maximize the cooling, condensation, and solidification of condensable aluminum chloride vapor within disposable element  42  without the need for any external or internal cooling mechanism. Since the amount of heat exchanged between the molecules of the condensable aluminum chloride vapor and the trapping media of disposable element  42  is largely dependent on the physical impact or collision of the vapor molecules onto the ambient temperature interior surfaces of the trapping media  44 ,  46 ,  48 , the surface areas of the trapping media  44 ,  46 ,  48  of disposable element  42  are optimized to create sufficient heat exchange surfaces, without impeding effluent flow though trap  10  (discussed below).  
         [0053]    To maximize the condensation, deposition, and trapping of condensable aluminum chloride vapor, disposable element  42  according to this invention preferably has a combination of features. First, the trapping media  44 ,  46 ,  48  of disposable element  42  will preferably have sufficient surface areas for condensing and trapping condensable aluminum chloride vapor. Second, while providing the many surface areas for condensation and deposition of the condensable aluminum chloride vapor, the trapping media  44 ,  46 ,  48  of disposable element  42  should nevertheless have high flow conductance for chlorinated etching gas molecules in the effluent so as not to inhibit the ability of the vacuum pump to remove the chamber effluent from etching chamber  14 . Third, the trapping media  44 ,  46 ,  48  of disposable element  42  should also have a large collection capacity to hold substantial volumes of condensed aluminum chloride solids build-up  40  without clogging the trap  10 . Fourth, there should be sufficient distance between trap inlet opening  35  and the upper surfaces of the trapping media  44 ,  46 ,  48  so that solid aluminum chloride buildup  40  and  41  will not clog inlet opening  35  after only a short period of time. Finally, disposable element  42  should be easy to remove for safe and rapid disposal of the deposited aluminum chloride solids.  
         [0054]    A preferred, albeit not the only, embodiment of trap  10  of this invention will now be described in detail. Trap  10 , as stated broadly, is a structure that contains disposable element  42  within a housing and between an inlet and an outlet, and wherein disposable element  42  has sufficient transverse thickness and sufficient density of surfaces to condense and trap substantially all of the aluminum chloride gas molecules on the surfaces. Preferably, trap  10  has a substantially cylindrical structure. However, trap  10  may comprise many other shapes and structures that can also be used according to the principles of this invention.  
         [0055]    Referring now to FIGS.  2 - 7 , the preferred embodiment for trap  10  of this invention has an elongated, substantially cylindrical housing  60  having an inner diameter D in the form of a canister that encloses a trap chamber  90 . The inlet end  92  of the housing is enclosed by a removable inlet fitting  94  with an inlet opening  35  and a suitable flange  96  adapted for connection to a pipe fitting in the pump line  12  (FIG. 1). With continuing reference to FIGS.  2 - 7 , a suitable flange  98  is affixed to the inlet end  92  of the housing  60  to mate and seal with a similar flange  100  on the inlet fitting  94 . A gasket  102  can be positioned between the mating flanges  98 ,  100  to assist in providing a vacuum-tight seal. Suitable clamps  104  or any other suitable fastener can be used to squeeze and retain the two flanges  98 ,  100  together, as is common and well-known to persons skilled in the art. The outlet end  106  of housing  60 , as shown in FIGS.  2 - 5 , is enclosed by an end wall  108  with an outlet opening  114  that terminates with a suitable pipe fitting flange  112  connected to outlet pipe  110 .  
         [0056]    With continued reference to FIGS.  2 - 7 , the preferred, but certainly not necessary, structure of the disposable element  42  (described in detail below) is cylindrical with a height h 5 , outer diameter d 4 , and inner core diameter d (FIG. 6). Guide  120  for mounting disposable element  42  in housing  60  is affixed to end wall  108  and has a width that is approximately equal to, or slightly smaller than, the inner core  56  diameter d of disposable element  42 . The lower end  122  of disposable element  42  slips around guide  120 , which centers and retains the disposable element  42  within the housing  60 , and abuts the end wall  108 . Therefore, the disposable element  42  is centered and held in place by guide  120 . Guide  120  is preferably a U-shaped strap, as best seen in FIGS. 2 and 6, or may be any other structure provided that it does not occlude the outlet opening  114  of trap  10 . As previously discussed, a heating jacket  34  or heating tape can be used to heat inlet pipe  30  (see FIG. 1) to control the temperature of the effluent from reaction chamber  14 , as the effluent enters trap  10  at inlet pipe  30 , and since the heater  34  abuts inlet pipe  30 , the heater  34  can also be used to help control the temperature of inlet pipe  30 .  
         [0057]    For purposes of explanation, but not limitation, of the structure of trap  10  of the present invention, housing  60  can have an outer diameter of approximately 6 inches, an inner diameter D of approximately 5.75 inches (FIG. 6), and a height h 5  of approximately 8.5 inches.  
         [0058]    To remove disposable element  42  from housing  60 , such as when disposable element  42  is clogged with condensed aluminum chloride solids, trap  10  is first removed from the pump lines  12  and  18  (see FIG. 1). Then, the clamps  104  are removed so the inlet fitting  94  can be removed from the inlet end  92  of the housing  60  (see FIG. 6), and disposable element  42  can be slid off the guide  120  and removed from the trap chamber  90  of housing  60 . A new disposable element  42  can be installed by reversing this procedure.  
         [0059]    It goes without saying that the terms “upper” and “lower” or “top” and “bottom” as used in this description are for convenience only. The “upper” and “lower” or “top” and “bottom” are in reference to the vertical orientation of trap  10  illustrated in FIGS. 1 and 3. Obviously, trap  10  can also be used in other mounting positions, such as horizontal, upside down, or any position in between, without changing the substance of this invention. Also, housing  60  and trapping media shapes other than cylindrical can be used according to this invention. Further, there are many other structures for opening the trap  10  to facilitate removal and replacement of the disposable element  42  within the scope of this invention, such as an openable housing  60 , a threaded end wall  94  or  108 , and many others, as will be apparent to persons skilled in the art once they understand the principles of this invention.  
         [0060]    Disposable element  42  of trap  10  will now be described in detail. Referring now primarily to FIG. 6 but also with supplemental reference to FIGS.  2 - 5  and  7 , disposable element  42  comprises outer trapping medium  48  of height h 1  and transverse thickness t 1  and which preferably is a substantially cylindrical structure, and inner screen column  52 , also preferably a cylindrical hollow structure and having a height h 2 , contained within outer trapping medium  48  and in spaced relation to outer trapping medium  48  such that an annular space  54  is defined between outer trapping medium  48  and inner screen column  52 . Disposed within annular space  54  is intermediate trapping medium  46  having a transverse thickness t 2  and height h 3 . Inner cylindrical screen column  52  further defines inner core  56  within which is disposed trapping medium  44  having transverse thickness t 3  and height h 4 . A solid protective shield  53  having a height h 5  surrounds outer trapping medium  48 . Optionally, an outer screen column  50 , preferably cylindrical in shape, may be disposed between trapping media  46  and  48 , as shown in FIG. 8, to provide additional structural support to disposable element  42 . Trapping media  44 ,  46 , and  48  are made of metal (preferably stainless steel) mesh  130 , as illustrated in FIG. 9, and provide the preferred surface structures and functions for disposable element  42  as described above. Such metal mesh  130  can be formed in a variety of ways with a variety of structures, including, but not limited to, stacked or compounded layers of metal fabric made, for example, with interlaced metal wire or thread  134 , as illustrated in FIGS.  9 - 13  and  16 , or with multiple layers of woven metal screens, or some other material with many tangled or ordered metal micro-surfaces  210  (FIG. 16), to create the required transverse thickness and surface area density through which the effluent from the reaction chamber  14  (FIG. 1) must pass to condense and trap all the aluminum chloride gas molecules.  
         [0061]    Inner cylindrical screen column  52  serves not only as a trapping medium for condensing, solidifying, and trapping condensable aluminum chloride vapor, but also serves to anchor disposable element  42  within chamber  90  of trap  10 . Therefore, inner cylindrical screen column  52  comprises a material that is somewhat stiffer than trapping media  44 ,  46 ,  48 , and preferably comprises a wire screen such as a 4×4 to 8×8 mesh wire screen. The term “screen” as used throughout this invention is not limited to wire screens and includes other structures such as pierced or perforated materials. Further, inner column shapes other than cylindrical may be employed. As discussed above, outer cylindrical screen column  50  (FIG. 8) may optionally be incorporated into disposable element  42  for additional structural support. Optional outer cylindrical screen column  50  preferably comprises a wire screen such as a 4×4 to 8×8 mesh wire screen. Other materials, such as perforated or pierced metal, may also be utilized as outer screen column  50 . Further, outer column shapes other than cylindrical may be employed.  
         [0062]    Trapping media  44 ,  46 ,  48 , inner cylindrical screen column  52 , and solid shield  53  of disposable element  42  preferably all have sufficient heights and diameters to accommodate a large enough volume of condensed aluminum chloride solids, such that the etching chamber  14  can be operated for substantial periods of time before the disposable element  42  becomes clogged with build-up  40 ,  41  to an extent that its capacity to condense and trap aluminum chloride gas molecules is diminished, or to an extent that conductance of the effluent gas through the vacuum conduit system is diminished. Such heights and diameters will, of course, depend on the amount of condensable aluminum chloride vapor in the effluent and the length of time it is desired to operate the etching chamber  14  before requiring service. Before the build-up of condensed aluminum chloride solids in disposable element  42  accumulates to such a volume that the condensed aluminum chloride solids diminish the capacity of disposable element  42  to condense and trap aluminum chloride gas molecules or to conduct non-condensed gas, the system can be shut down and the clogged or partially clogged disposable element  42  can be simply removed from the trap  10  and replaced with a new disposable element as discussed above.  
         [0063]    The primary functions of trapping media  44 ,  46 ,  48  and of inner cylindrical screen column  52  and outer (gas-impervious) shield  53  are to provide large surface areas which are at approximately ambient temperatures, such that aluminum chloride gas molecules in the effluent can be efficiently cooled, condensed, deposited, and trapped in trap  10  without the need for additional internal or external active cooling of trap  10 . A significant feature of trap  10  of this invention is that disposable element  42  can be designed in a manner that allows for the manipulation of the deposition profile of the condensed aluminum chloride solids in the disposable element  42 , and thus enables an efficient method of condensing, solidifying, and trapping aluminum chloride vapor that is present in the etching effluent. The deposition profile may be manipulated by adjusting the heights and densities of trapping media  44 ,  46 ,  48  and of inner cylindrical screen column  52  and outer shield  53 , as discussed in more detail below.  
         [0064]    The condensation process of the aluminum chloride vapor in disposable element  42  is a phase change process. The condensable aluminum chloride vapor changes from the vapor or gaseous phase to the solid phase as the aluminum chloride vapor flows through disposable element  42 . A condensable vapor or gas in a gas stream will condense when its partial pressure in the vapor phase is greater that the equilibrium vapor pressure. More specifically, the partial pressure of a gas comprising two or more different gaseous components (molecular species) is the cumulative total of the individual pressures of each such component in the gas. Therefore, for an effluent of an aluminum etching system, such as the effluent from the reaction chamber  14  (FIG. 1), comprising condensable aluminum chloride vapor (AlCl 3 ) and chlorine (Cl 2 ) reaction gas, each of the two components, aluminum chloride (AlCl 3 ) and chlorine (Cl 2 ), has its own partial pressure. The total pressure of the gas comprising the mixture of the two components, aluminum chloride (AlCl 3 ) and chlorine (Cl 2 ), is equal to the sum of the partial pressures of the two components aluminum chloride (AlCl 3 ) and chlorine (Cl 2 ). The equilibrium vapor pressure for aluminum chloride (AlCl 3 ) is the pressure at which the rate of condensation of the aluminum chloride (AlCl 3 ) from a vapor to a solid is equal to the rate of evaporation or vaporization of the aluminum chloride (AlCl 3 ) from a solid to a vapor.  
         [0065]    The vapor pressure of a condensable vapor is related to the temperature of the condensable vapor, which can be expressed by the Antoine equation:  
               ln                 p     =     A   -     B     T   +   C                 (   2   )                               
 
         [0066]    where A, B, and C are constants, p is the vapor pressure measured in Torr, and T is the temperature measured in degrees Celsius. For aluminum chloride (AlCl 3 ), A is approximately equal to 31.431, B is approximately equal to 950436, and C is approximately equal to 202.39. The vapor pressure curve for aluminum chloride is shown in FIG. 14. Continuing the example discussed above in relation to equation (1) and with reference to the vapor pressure curve shown in FIG. 15, if it is assumed that the temperature of the aluminum chloride vapor entering the trap  10  is 100° C. and the partial pressure of the aluminum chloride is 100 milliTorr, an initial temperature decrease of approximately 18° C. will result in a condensation of approximately 90% of the condensable aluminum chloride vapor in the effluent flowing through disposable element  42  of trap  10 . A second temperature decrease of approximately 16° C. will result in a condensation of approximately ninety percent (90%) of the remaining ten percent (10%) aluminum chloride vapor in the effluent flowing through disposable element  42 . Thus, as previously discussed above, controlling the physical profile of disposable element  42  (i.e., by adjusting the densities of the trapping media  44 ,  46 ,  48  of disposable element  42  and thus the amount of surface area  210  available) will significantly control the deposition profile of the trap  10 . Therefore, in order to obtain a trap  10  that has a high trap capacity in a reasonable physical size, uses most of its available trap volume, and does not become prematurely clogged at the inlet opening  35  to the trap  10 , the cooling of the condensable aluminum chloride vapor entering trap  10  through inlet opening  35  should be limited so that condensation of the condensable aluminum chloride vapor at these locations is minimized. In other words, the profile of the trapping media  44 ,  46 ,  48  of disposable element  42  of trap  10  should be such that the trap  10  does not clog prematurely at the inlet opening  35 , as will be discussed in more detail below.  
         [0067]    With reference now to FIG. 6, the deposition of aluminum chloride solids in disposable element  42  of trap  10  and the manipulation of the deposition profile will be discussed. In general, disposable element  42  of trap  10  may be considered as comprising at least two deposition stages or areas, first deposition stage  200  and second deposition stage  205 , where deposition of solid aluminum chloride will occur. A majority (approximately 90 to 95 percent or more) of the condensable aluminum chloride vapor contained in the chamber effluent will be collected and trapped in trapping media provided in the first or primary deposition stage  200  of disposable element  42 . The above-described trapping media of the first deposition stage  200  will preferably be sufficiently spaced from the inlet opening  35  of trap  10  such that inlet opening  35  will not become clogged with condensed aluminum chloride solids after only a short period of use. The remainder (approximately 5% to 10% or less) of the condensable aluminum chloride vapor will be condensed and trapped in the second deposition stage  205  of disposable element  42 , where denser and highly efficient trapping media are provided. The denser trapping media provided in the second deposition stage  205  of disposable element  42  contain significantly higher amounts of surface areas provided to maximize molecular contact with the remaining condensable aluminum chloride vapor molecules (approximately 5 to 10 percent or less), which were not trapped in the first deposition stage  200 , and thus maximizing heat exchange, cooling, condensing, and trapping of the remaining (approximately 5 to 10 percent or less) condensable aluminum chloride vapor molecules.  
         [0068]    With continued reference to FIG. 6, a portion of the aluminum chloride gas molecules in the effluent entering trap  10  through inlet opening  35  will impact outer trapping medium  48  (and optionally outer cylindrical screen column  50  if outer cylindrical screen column  50  is included in the disposable element, as shown in FIG. 8) as indicated by flow arrows  64  and  66  in FIG. 6. The impact of the aluminum chloride gas molecules creates a heat transfer between the aluminum chloride gas molecules and outer trapping medium  48 , thus reducing the temperature of the aluminum chloride gas molecules, which in turn causes the aluminum chloride molecules to condense and solidify on outer trapping medium  48 , as indicated by the solid aluminum chloride buildup  40  in FIG. 6. Since the gas volume flow rate in aluminum etch systems is typically fairly low (approximately 100-200 sccm), and the mass flow rate is also relatively low (approximately 0.12 g/min), cooling of the aluminum chloride vapor by physical collision with outer trapping medium  48  is very efficient, and consequently an external or internal cooling mechanism is not necessary in trap  10  of the present invention. Therefore, as the aluminum chloride gas molecules come into contact with outer trapping medium  48 , the temperature of the aluminum chloride gas molecules will decrease due to contact cooling, and therefore a large amount of aluminum chloride gas molecules will condense, solidify, and accumulate as solid aluminum chloride build-up  40  on outer trapping medium  48 . In order to avoid early clogging of inlet opening  35 , it is preferred that the upper surface of outer trapping medium  48  be of a sufficient distance from trap inlet opening  35  so that the initial aluminum chloride build-up  40  on outer trapping medium  48  will not clog inlet opening  35 . Preferably height h 1  of outer trapping medium  48  is at least one-half to one inch less than the height h 5  of disposable element  42 .  
         [0069]    As the build-up  40  of condensed aluminum chloride accumulates on the upper portion of outer trapping medium  48 , as indicated in FIG. 6, the heat transfer from aluminum chloride gas molecules impinging such deposition build-up  40  becomes less efficient, because the deposition medium  40  itself does not conduct heat as well as the trapping medium  44 , which is preferably metal. Therefore, with such build-up  40  on the upper portion of outer trapping medium  48 , the effluent flow will then be gradually re-directed in the direction shown by flow arrows  68 ,  70 , and  72 . Essentially, heat transfer, thus condensation, is initially more efficient on outer trapping medium  48 , and the condensation of aluminum chloride on the outer trapping medium  48 , as described above, decreases partial pressure adjacent the outer trapping medium  48 , thereby causing a decreasing partial pressure gradient of aluminum chloride gas with a resulting preferential flow of aluminum chloride gas toward the outer trapping medium  48 , as indicated by flow arrows  64 ,  66 . However, as solid aluminum chloride  40  builds up on outer trapping medium  48 , heat transfer, thus condensation, becomes less efficient, and the resulting partial vapor pressure of aluminum chloride adjacent the solid aluminum chloride outer trapping medium  48  increases to something more than the partial vapor pressure of aluminum chloride adjacent the inner screen column  52 . Thus, the remainder of the effluent containing the condensable aluminum chloride vapor will be drawn through disposable element  42  in the directions indicated by flow arrows  68 ,  70 , and  72  due to the influence of the vacuum and resulting partial pressure gradients, and will next come increasingly into contact with inner cylindrical screen column  52 , which is also part of the first deposition stage  200  of the disposable element  42 , where a second large portion of condensable aluminum chloride vapor will condense and deposit as solid aluminum chloride build-up  41  in a manner similar to that discussed above for the build-up  40  on outer trapping medium  48 . In this way the effluent gas flow is balanced and overall efficiency and longevity of trap  10  is increased.  
         [0070]    After the effluent has come into contact with the first deposition stage  200  (i.e., upper portions of the outer trapping medium  48  and inner cylindrical screen column  52 ) of disposable element  42  and has deposited on outer trapping medium  48  and screen column  52  of disposable element  42  as discussed above, approximately 90-95% of the original condensable aluminum chloride vapor present in the effluent has deposited as solid aluminum chloride build-up  40 ,  41  in the first deposition stage  200 . The effluent containing the remaining 5 to 10% of the condensable aluminum chloride vapor then flows through and is condensed in the second deposition stage  205  of disposable element  42 . The second deposition stage  205  comprises inner trapping medium  44  and middle trapping medium  46 , each of which contain significantly more surface areas  210  for providing maximum heat exchange surfaces while still allowing for the flow of the effluent through trapping media  44  and  46 , as indicated by flow arrows  74 ,  76 ,  78  and  80 , under the influence of the vacuum. Thus, the densities and heights h 4  and h 3  of trapping media  44  and  46 , respectively, are important to the efficient trapping of condensable aluminum chloride vapor. If trapping media  44  and  46  are not of sufficient density, the majority of the effluent would flow though trapping media  44 ,  46  under the influence of the vacuum, resulting in inefficient trapping of condensable aluminum chloride vapor, thereby allowing the condensable aluminum chloride vapor to pass through the trap  10  and condense, solidify, and accumulate in exhaust lines  18  and  32  downstream of trap  10 . Preferably, the density of trapping medium  44  is approximately 10 in 2 /in 3 , and the density of trapping medium  46  is approximately 8 in 2 /in 3 , as described below in detail.  
         [0071]    As discussed above, the height h 2  of inner cylindrical screen column  52  is preferably spaced from the inlet opening  35  such that only a minimum amount of aluminum chloride is deposited near the inlet opening  35  in order to avoid clogging of the inner core. Preferably, the height h 2  of inner cylindrical screen column  52  is approximately 0.5 h 5  to 0.99 h 5 , preferably about ⅔ h 5 , where h 5  is the height of housing  60 . Outer trapping medium  48  preferably has a height h 1  that is just about the same as, or is just slightly less than the height h 5  of housing  60 . Essentially, the protective shield  53  should be of sufficient height to provide protection of the interior surface of housing  60  but is not of a height that would prohibit closing and sealing housing  60 .  
         [0072]    As indicated above, trapping media  44  and  46  of the second deposition stage  205  are of sufficient density to provide enough surface areas to trap the remaining condensable aluminum chloride vapor (approximately 5-10% or less) in the effluent after the effluent has passed through the first deposition stage  200  of disposable element  42 , but at the same time trapping media  44  and  46  are not so dense as to impede the flow of the effluent through trapping media  44  and  46 . Therefore the remaining condensable aluminum chloride vapor is able to flow through and deposit within trapping media  44  and  46 , as indicated by flow arrows  74 ,  76  and  78 . Further, trapping media  44  and  46  have a greater surface areas and therefore a greater capacity to trap the remaining condensable aluminum chloride vapor (approximately 5-10% or less) in the effluent. Thus, disposable element  42  of trap  10  may be designed to maximize the trapping efficiency of condensable aluminum chloride vapor, such that the majority (i.e., more than half and preferably 90-95%) of the AlCl 3  in the gas flow deposits in the primary stage, as illustrated by deposition  40 ,  41 , so that the denser trapping media  44  and  46  won&#39;t become clogged after a short period of time, and so vacuum pump  16  will be able to pull a vacuum through disposable element  42  for a greater length of time while efficiently trapping the condensable aluminum chloride vapor. In addition, the denser trapping media  44  and  46  are sufficiently dense such that trapping media  44  and  46  are able to efficiently trap the remaining condensable aluminum chloride vapor (approximately 5-10% or less) in the effluent that was not trapped in the first stage  200  of trap  10 .  
         [0073]    Thus, a significant feature of disposable element  42  of trap  10  is that the deposition of aluminum chloride solids can be manipulated by varying the heights and densities of trapping media  44 ,  46 ,  48  and the height of inner cylindrical screen column  52  in order to maximize the amount of aluminum chloride solids that can be trapped before disposable element  42  can no longer efficiently trap condensable aluminum chloride vapor, such that the etch chamber  14  can be operated for a substantial period of time before it is necessary to replace disposable element  42  laden with aluminum chloride deposits with a new disposable element  42 .  
         [0074]    As a result of the efficient trapping of aluminum chloride by disposable element  42  of trap  10 , the effluent exiting outlet opening  114  (flow arrow  84 ), is essentially free of condensable aluminum chloride vapor. Consequently, it is not necessary to heat any of the lines, pumps, valves, or other parts of the vacuum system downstream of trap  10  to prevent condensation of aluminum chloride vapor in those components.  
         [0075]    As discussed briefly above, the trapping media  44 ,  46 ,  48  comprises a sufficient density of microsurfaces  210  to cool, condense, and solidify substantially all of the aluminum chloride vapor in the effluent. At the same time, the microsurfaces  210  are not so dense as to inhibit the ability of the vacuum pump to maintain the required vacuum in the reaction furnace  14 . In other words, excess reaction gas molecules such as Cl 2  or BCl 3  should be able to pass substantially unimpeded through the trapping media  44 ,  46 ,  48 .  
         [0076]    A preferred, but not essential, embodiment of the trapping media  44 ,  46 ,  48  comprises a mesh  130  of one or more layers of crimped metal fabric made with intertwined or interlaced metal wire  134  to form a maze or tangle of metal microsurfaces as illustrated in FIGS.  9 - 12  and  16  and in more detail in the enlarged section of such trapping media in FIG. 9. As stated above, trapping media  44  and  46  are denser than trapping medium  48 . Consequently, in the following discussion it is to be understood that while the general characteristics of the mesh  130  apply to all trapping media  44 ,  46 , and  48 , preferably the density of trapping medium  46  is less than the density of trapping medium  44 , and preferably the density of trapping medium  48  is less than the density of trapping medium  46 .  
         [0077]    As shown in FIG. 9, the mesh  130  that forms the preferred embodiment trapping media is comprised of a loose tangle of intertwined or interlaced metal wires  134 . The word “tangle” as used herein does not imply that the wires are not assembled or laced in an ordered manner or pattern, but only that they are shaped and positioned in a manner that substantially prevents condensable aluminum chloride vapor from flowing straight through the trapping medium without contacting the trapping medium, resulting in the condensable aluminum chloride vapor being cooled and condensing as a solid on the trapping medium.  
         [0078]    In the preferred embodiment of trapping media  44 ,  46 ,  48 , the tangle of wires  134  provides enough surface area in the transverse thickness t so that substantially all aluminum chloride gas molecules are not only cooled, but also condensed as solids and retained on wires  134 . Of course, too many wires  134  in the thickness t of the trapping media would impede flow of other gas molecules such as the chlorinated reaction gas molecules, and thus interfere with the ability of the vacuum pump to maintain the required vacuum in the chamber, as described above.  
         [0079]    Accordingly, a significant feature of this invention is to place enough wire  134  in the mesh  130  to provide a density (Surface Area/Unit Volume) in a range of about 2 in 2 /in 3  to 15 in 2 /in 3 , preferably about 8 in 2 /in 3 . In other words, in each cubic-inch volume of mesh  130 , there is about 2 in 2  to 15 in 2 , preferably about 8 in 2 , of surface area. The surface area As for a cylindrical wire  134  in the mesh  130 , can be determined by the formula in Equation (3):  
           A   s =(π)×( dia. )×( l )  (3)  
         [0080]    where (dia.) is the diameter of the wire  134  and l is the length of the wire  134  in a volume of mesh  130  (see also FIG. 16).  
         [0081]    Stainless steel wire  134  is preferred, but other common metals, such as copper, bronze, and aluminum would also provide satisfactory condensation and trapping of condensable aluminum chloride vapor molecules, as would ceramic strands or threads in a mesh  130 . While wire  134  with circular cross-section is preferred, mostly because of its availability, strips of wire  134  with flat or other cross-sections could be used to provide the micro-surface density within the range described above.  
         [0082]    An example of a single layer of crimped wire fabric  140  is shown in FIG. 10, wherein strands of the wire  134  are interlaced to form the open, single layer metal fabric  140 . The density of metal fabric  140  can be increased by stacking or laminating multiple layers of such metal fabric  140  together, as shown in FIG. 11. Even greater density can be obtained by stacking or laminating four metal fabric  140  layers together, as illustrated in FIG. 12.  
         [0083]    The preferred embodiment mesh  130  for the trapping media  44 ,  46 ,  48  can, therefore, be fabricated quite easily by stacking together layers of the metal fabric  140  until the desired density is attained. For example, but not for limitation, a long strip of the metal fabric  140  can be folded over on itself, as shown in FIG. 13, to create a double density stack similar to that shown in FIG. 11. The metal fabric  140  can also be crimped to add some three-dimensional depth to the fabric  140 , as indicated by the crimped convex and concave bends  141 ,  142 , respectively, in FIG. 13. Further, when the crimped bends  141 ,  142  are formed diagonally, as shown in FIG. 13, the concave bends  142  of the top layer  143  bridge against the concave bends  142  of the bottom layer  144  to maintain the three-dimensional depth of the composite of the two layers  143 ,  144 , which creates a higher surface density than if the two metal fabric layers were not crimped. It follows, therefore, that the surface density of the composite mesh  130  can be a function of the sharpness or depth from convex bends  141  to adjacent concave bends  142 . The folded composite metal fabric  140  of FIG. 13, can then be rolled as many turns as necessary to make the desired thickness t of the mesh  130  trapping media as described above. Of course, any of the finished trapping media may have the metal fabric  140  wrapped as tightly as desired.  
         [0084]    Preferably, trapping medium  44  has a height h 4  in a range of about 1 to 3 inches, preferably about 2 inches and a transverse thickness t 3  in a range of about 1 to 3 inches, preferably about 2 inches. Trapping medium  46  preferably has a height h 3  in a range of about 3 to 5 inches, preferably about 4 inches and a transverse thickness t 2  in a range of about 1 to 4 inches, preferably about 2.5 inches. Trapping medium  48  preferably has a height h 1  in a range of about six to ten inches, preferably about eight inches and a transverse thickness t 1  in a range of about 0.1 to 1 inch, preferably about 0.5 in. Such sizing of trapping media  44 ,  46 ,  48  provides a disposable element  42  having a diameter d 4  in a range of about four to eight inches, preferably about 4.75 inches, which, with a micro-surface density in the range described above, provides for the trapping capacity for aluminum chloride molecules, while allowing conductance of other gas molecules, to prevent downstream deposition of aluminum chloride solids. In order for disposable element  42  to fit into housing  60  of trap  10 , it is necessary that the diameter d 4  of disposable element be slightly smaller than the inside diameter D of chamber  90 . Consequently, a narrow annular space  148  may be present between solid shield  53  and inside surface of side wall  124  of housing  60 . Preferably, annular space  148  is less than one-eighth to one-sixteenth of an inch.  
         [0085]    Further, it is preferred that the distance of upper surface  85  of trapping medium  44  from chamber inlet end  92  be approximately the same as the distance of the upper surface  86  of trapping medium  46  from chamber inlet end  92 . Of course the height h 4  of trapping medium  44  will depend on the size of guide  120 .  
         [0086]    Solid gas-impervious shield  53  primarily serves to protect the interior surface housing  60  from deposition of aluminum chloride as well as to provide structural support to disposable element  42 , and therefore is preferably a solid, stainless steel metal having a height h 5  that is sufficiently high to substantially block effluent molecules from reaching inner surface of side wall  124  of housing  60 , but is not of a height that would prevent disposable member  42  from fitting into housing  60 .  
         [0087]    An alternative mounting position of trap  10  is shown in FIG. 15, in which trap  10  is placed along exhaust pipe line  32  downstream of pump  16 . In this embodiment, all components upstream of trap  10 , such as foreline  12 , pump  16 , and all other components such as valves, etc., must be heated to prevent deposition of aluminum chloride solids in such components. In addition, since nitrogen (N 2 ) is added to pump  16  via nitrogen inlet  150  in order to improve the pump performance, part of the aluminum chloride will be condensed as a fine powder because of gas phase precipitation  165 , rather than the more manageable solid deposit  40 . This fine powder can be difficult to contain, and consequently a trap such as Y-shaped trap  160  may be necessary to collect the fine powder aluminum chloride. The powder  165  will fall down trap  160 , while the effluent will flow on to scrubber  37 .  
         [0088]    While such installation as that shown in FIG. 15 with the trap  10  positioned post-pump, is not as desirable, such installations as that illustrated in FIG. 15 are still very effective at aluminum chloride deposition and removal.  
         [0089]    The foregoing description is considered as illustrative only of the principles of the invention. The words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups thereof. Furthermore, since a number modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown described above. Accordingly, all suitable modifications and equivalents may be resorted to falling within the scope of the invention as defined by the claims which follow.