Abstract:
A cold trap having improved trapping efficiency for particles, particularly ammonium chloride fine powder particles. The cold trap has a dual-stage design, including an upstream stage fitted with at least one trap plate and a downstream stage fitted with a typically cup- or cone-shaped trap core in which is provided a conically-spiraled water cooling coil. A decreasing temperature gradient is provided along the length of the cold trap, such that exhaust gases flowing through the cold trap are gradually cooled and ammonium chloride or other particles which form from the gas are trapped by the trap plate or plates and collect in the trap core. The exhaust gases exiting the downstream end of the trap core are substantially or completely devoid of particles.

Description:
FIELD OF THE INVENTION 
   The present invention relates to chemical vapor deposition (CVD) chambers used in the deposition of conductive and resistive layers on a semiconductor wafer substrate. More particularly, the present invention relates to a multi-stage cold trap for cooling exhaust gases from a CVD chamber and trapping particulate precipitates in the gas. 
   BACKGROUND OF THE INVENTION 
   In the semiconductor production industry, various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include the deposition of layers of different materials including metallization layers, passivation layers and insulation layers on the wafer substrate, as well as photoresist stripping and sidewall passivation polymer layer removal. In modern memory devices, for example, multiple layers of metal conductors are required for providing a multi-layer metal interconnection structure in defining a circuit on the wafer. Chemical vapor deposition (CVD) processes are widely used to form layers of materials on a semiconductor wafer. 
   Silicon nitride has been an important material in various semiconductor applications. For instance, silicon nitride has been used as a mask against oxygen diffusion during a local oxidation (LOCOS) process; as a passivation layer for its superior barrier property to contaminants; as a gate dielectric layer in memory devices; and as an interlevel dielectric layer in an oxide-nitride-oxide (ONO) stacked-gate structure. Silicon nitride also has superior barrier properties against metal ions and moisture. 
   Silicon nitride has been widely used as a passivation layer for protecting a semiconductor component. Silicon nitride can be formed by either a LPCVD or PECVD technique. The LPCVD technique, where dichiorosilane is used as the reactant gas, can be carried out in a hot-wall LPCVD system, such as in a vertical furnace. The chemical reaction can be described as follows:
 
3SiH 2 Cl+10NH 3 →Si 3 N 4 +6NH 4 Cl+6H 2 
 
   The hot-wall LPCVD system is normally carried out at a temperature between about 750°˜800° C., and the chamber pressure is kept at several hundred m Torr. A layer of stoichiometric silicon nitride can thus be deposited on a wafer surface. A typical deposition equipment utilizing a vertical furnace is shown in FIG.  1 . 
   During a vertical furnace silicon nitride process, as described by the above mechanism for the chemical reaction, a reaction by-product such as ammonium chloride (NH 4 Cl) in the form of a fine powder can easily deposit on any cold surface in the furnace or in the ducting system for the furnace. The ammonium chloride powder must be captured by a cold trap such that it does not form on the inner walls of the ducting system or in the furnace and present a serious contamination source. For instance, fine powder in the ducts may be syphoned back into the furnace during a deposition process if the pressure in the furnace is not carefully controlled. The capture efficiency of the cold trap for the ammonium chloride fine powder is therefore an important factor in the successful deposition of silicon nitride films in a furnace technique. 
   As shown in  FIG. 1 , a vertical furnace unit  12  is the heart of a silicon nitride deposition system  10 . During the deposition of a silicon nitride film on a plurality of wafers  16  positioned in the vertical furnace, the furnace exhaust gas  14 , which contains unreacted reactant gases such as dichlorosilane, ammonium and reaction byproduct ammonium chloride powder, is drawn through a cold trap  22 , via an exhaust conduit  20 , by a vacuum pump  18  before the furnace exhaust gas enters into a gas treatment unit (not shown) and is released into a factory exhaust system (not shown). The capture of substantially all of the ammonium chloride fine powder in the cold trap  22  is therefore an important step in a successful exhaust gas treatment process for depositing silicon nitride. 
   A schematic view of a typical conventional cold trap  22  is shown in FIG.  2  and includes a bellow or pipe  24  which receives the exhaust gases from the exhaust conduit  20  of the system  10  shown in  FIG. 1. A  jacket heater  23  on the pipe  24  prevents the temperature of the exhaust gases flowing through the pipe  24  from dropping below a temperature of typically about 150° C. A trap housing  25  is provided in fluid communication with the outlet end of the pipe  24 . The exhaust gases flow through a typically constant-diameter flow bore  26  extending through the trap housing  25 , and a water cooling coil  27  winds through the flow bore  26 . Cooling water is introduced into the water cooling coil  27  through a cooling water inlet  28 , and is conducted from the water cooling coil  27  and the trap housing  25  through a cooling water outlet  29 . Typically, the cooling water inlet  28  is located at the same vertical position as the cooling water outlet  29 , as shown. Accordingly, as the exhaust gases flow through the flow bore  26 , the exhaust gases contact the water cooling coil  27 , which cools the exhaust gases. Ammonium chloride powder in the exhaust gases become trapped on the water cooling coil  27  and in the flow bore  26 , such that the exhaust gases leave the bottom of the trap housing  25  substantially or completely devoid of the ammonium chloride. 
   One of the problems frequently associated with the conventional cold trap  22  is that the ammonium chloride powder accumulates in the flow bore  26  and on the water cooling coil  27  after a relatively short period of time of operation of the silicon nitride deposition system  10 . This tends to restrict or completely block flow of the exhaust gases through the flow bore  26 , and consequently increases loading on the vacuum pump  18 . Consequently, damage to or failure of the vacuum pump  18  may result. Thus, the cold trap  22  must be subjected to preventative maintenance about every 4 months to remove the ammonium chloride powder from the trap housing  25 . This results in unnecessary downtime in operation of the silicon nitride deposition system  10 . Accordingly, a new and improved cold trap which is capable of operating for longer periods of time between periodic maintenance is needed. 
   Accordingly, an object of the present invention is to provide a new and improved cold trap that can be efficiently used in a semiconductor fabrication process for collecting unwanted particles and which does not have the drawbacks or shortcomings associated with conventional cold traps. 
   Another object of the present invention is to provide a cold trap that can be used effectively in a semiconductor material deposition system such that the cleaning frequency required for the cold trap can be reduced. 
   A further object of the present invention is to provide a cold trap for use in a semiconductor fabrication process and which does not require frequent cleaning. 
   Still another object of the present invention to provide a new and improved cold trap which enhances operational efficiency and throughput of a semiconductor fabrication process. 
   Yet another object of the present invention is to provide a cold trap which may be provided with one or multiple trap plates for trapping particles. 
   Still another object of the present invention is to provide a cold trap which may be provided with a cone or cup shaped trap core which prevents premature particle blockage of the cold trap. 
   Yet another object of the present invention is to provide a cold trap which includes a first stage provided with one or multiple trap plates and a second stage which may be provided with a cup- or cone-shaped trap core for preventing premature particle blockage of the cold trap. 
   A still further object of the present invention is to provide a cold trap for use in a vertical furnace for depositing silicon nitride films wherein the trap has greatly improved efficiency for trapping ammonium chloride fine powder. 
   Another object of the present invention is to provide a cold trap which is particularly suitable for trapping ammonium chloride or other particles in a semiconductor fabrication process but which may be equally adapted to a variety of other industrial applications. 
   SUMMARY OF THE INVENTION 
   In accordance with these and other objects and advantages, the present invention is generally directed to a cold trap having improved trapping efficiency for particles, particularly ammonium chloride fine powder particles. The cold trap has a dual-stage design, including an upstream stage fitted with at least one trap plate and a downstream stage fitted with a typically cup- or cone-shaped trap core in which is provided a conically-spiraled water cooling coil. A decreasing temperature gradient is provided along the length of the cold trap, such that exhaust gases flowing through the cold trap are gradually cooled and ammonium chloride or other particles which form from the gas are trapped by the trap plate or plates and collect in the trap core. The exhaust gases exiting the downstream end of the trap core are substantially or completely devoid of particles. The trap plate or plates in the upstream stage, alone or in combination with the cone-shaped trap core in the downstream stage, facilitate effective removal of particles from the flowing gas while preventing premature clogging of the trap with particles which otherwise requires frequent cleaning of the trap. 
   The cold trap may include a bellow or pipe on which is fitted a primary jacket heater. A trap plate housing, which forms the upstream stage of the cold trap and contains the trap plate or plates, is fitted with a tertiary jacket heater. A secondary jacket heater is fitted typically on a connector which connects the pipe to the trap plate housing. A cooling housing, which forms the downstream stage of the cold trap and contains the typically cone-shaped trap core and spiraled cooling coil, is connected to the downstream end of the trap plate housing. A connector provided on the cooling housing connects the cooling housing to the trap plate housing, and is typically fitted with a quaternary jacket heater. 
   As the exhaust gases flow downstream through pipe or bellows of the cold trap, the primary jacket heater on the pipe or bellows prevents the temperature of the gases from dropping beneath typically about 150° C. The secondary jacket heater on the connector between the pipe or bellows and the trap plate housing maintains the temperature of the gases at or above about 120° C. The tertiary jacket heater on the trap plate housing maintains the temperature of the gases at or above about 80° C. The quaternary jacket heater on the connector between the trap plate housing and the cooling housing maintains the temperature of the gases at or above about 30° C. Finally, the water or other cooling fluid flowing through the cooling coil in the cooling housing cools the exhaust gas to room temperature, or about 25° C. 
   The gradual cooling of the exhaust gases as they traverse the cold trap facilitates a gradual, thermally-induced deposit of ammonium chloride particles first on the trap plate or plates in the trap plate housing of the upstream stage, and then on the cooling coil and interior walls of the typically cone-shaped trap core in the cooling housing of the downstream stage, of the cold trap. Accordingly, the gradual, rather than sudden, cooling of the exhaust gases among two stages, rather than one stage, of the cold trap prevents deposit of an excessive quantity of the ammonium chloride particles in one portion of the trap which would otherwise cause premature clogging of the trap and frequent trap cleanings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic view of a typical conventional furnace deposition system equipped with a furnace unit, a cold trap and a vacuum pump; 
       FIG. 2  is a side view of a typical conventional cold trap; 
       FIG. 3  is an exploded view, partially in cross-section, of a cold trap in accordance with the present invention; 
       FIG. 3A  is a cross-sectional view, taken along section lines  3 A— 3 A in  FIG. 3 ; 
       FIG. 3B  is a sectional view illustrating an alternative configuration for the cooling housing element of the present invention; 
       FIG. 4  is a partially schematic view of the cold trap of  FIG. 3 , in implementation of the present invention; and 
       FIG. 5  is a graph illustrating long-term performance of the cold trap of the present invention as compared to the long-term performance of a conventional cold trap. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention discloses a cold trap for use in a semiconductor film deposition system for collecting reaction byproducts in an exhaust gas from a deposition furnace. The present invention cold trap can be used in any furnace exhaust system, but is particularly suitable for use in a silicon nitride furnace deposition system wherein the exhaust gas contains ammonium chloride fine powder that should be collected by an efficient cold trap device. 
   Referring to  FIGS. 3-3B , an illustrative embodiment of the cold trap of the present invention is generally indicated by reference numeral  35  and typically includes a bellow or pipe  38 , the upstream inlet end of which may be provided with a pipe flange  38   a  to facilitate confluent attachment of the cold trap  35  to an exhaust conduit such as the exhaust conduit  20  heretofore described with respect to the silicon nitride deposition system  10  of  FIG. 1. A  pipe clamp  56  may be provided on the pipe  38  for removable attachment of the pipe  38  to the exhaust conduit. A pipe bore  37  extends through the pipe  38 . A trap plate housing  41 , which may have a tapered body  42  that defines a housing interior  43 , is provided in fluid communication with the pipe  38 . Accordingly, the downstream outlet end of the pipe  38  may be fitted with a threaded connector  40  to facilitate confluent attachment of the upstream inlet end of the trap plate housing  41  to the pipe  38 . As shown in  FIGS. 3 and 3A , at least one, and preferably, a pair of trap plates  44  extends from the interior surface of the body  42  and into the housing interior  43 . Each of the trap plates  44  typically has a substantially semicircular shape, as shown in  FIG. 3A , and may be characterized by a screen or mesh, or may have any other alternative construction which is suitable for trapping fine particles therein, as hereinafter described. As shown in  FIG. 3 , the trap plates  44  typically extend from opposite sides of the body  42 , and may be disposed in vertically-offset or staggered relationship with respect to each other. 
   A cooling housing  46  is provided in fluid communication with the trap plate housing  41 . Accordingly, the upstream inlet end of the cooling housing  46  typically includes a connector  48  which may threadably or otherwise sealingly engage the outlet end of the trap plate housing  41 . The cooling housing  46  includes a cooling housing wall  47  in which is provided a trap core  49  that is disposed in fluid communication with the housing interior  43  of the trap plate housing  41 . As shown in  FIG. 3 , the trap core  49  typically has an inverted cone shape, the wide end of which trap core  49  is disposed at or adjacent to the upstream or inlet end of the cooling housing  46  and the narrow end of which trap core  49  is disposed at the downstream or outlet end of the cooling housing  46 . A cooling coil  50  enters the cooling housing  46  at a cooling coil inlet  51  and winds in a conical spiral configuration from the wide end of the cone-shaped trap core  49  to the narrow end of the trap core  49 . The cooling coil  50  exits the cooling housing  46  at a cooling coil outlet  52 , which may be disposed in offset relationship with respect to the cooling coil inlet  51  on the cooling housing  46 , as shown in FIG.  3 . Alternatively, as shown in  FIG. 3B , the cooling coil outlet  52  may be disposed at the same level on the cooling housing  46  as the cooling coil inlet  51 . An outlet bore  54  extends from the narrow end of the trap core  49  and typically through a connector  55  which facilitates confluent attachment of the cold trap  35  to the remaining segment of the exhaust conduit, such as the exhaust conduit  20  heretofore described with respect to FIG.  1 . 
   As further shown in  FIG. 3 , in typical application of the cold trap  35  as hereinafter described, a primary jacket heater  36 , which may be conventional and typically includes heating coils  36   a , is typically fitted on the pipe  38 . A secondary jacket heater  39  is fitted on the pipe  38 , typically on the connector  40  thereof. A tertiary jacket heater  45  is fitted on the body  42  of the trap plate housing  41 , and a quaternary jacket heater  53  is fitted on the cold trap  35 , typically at the junction of the trap plate housing  41  with the cooling housing  46 . 
   As hereinafter further described, the primary jacket heater  36 , the secondary jacket heater  39 , the tertiary jacket heater  45 , the quaternary jacket heater  53  and the water cooling coil  50  impart a decreasing temperature gradient to the cold trap  35  as exhaust gases flow through the pipe  38 , the trap plate housing  41  and the cooling housing  46 , respectively. The primary jacket heater  36  heats the pipe  38  to a temperature of typically about 150° C.; the secondary jacket heater  39  heats the connector  40  of the pipe  38  to a temperature of typically about 120° C.; the tertiary jacket heater  45  heats the body  42  of the trap plate housing  41  to a temperature of typically about 80° C.; and the quaternary jacket heater  53  heats the junction of the trap plate housing  41  and the cooling housing  46  to a temperature of typically about 30° C. The cooling water or other liquid or fluid flowing through the cooling coil  50  in the trap core  49  of the cooling housing  46  cools the exhaust gas to a temperature of typically about 25° C., or room  5  temperature. The resulting decreasing temperature gradient along the length of the cold trap  35  facilitates controlled deposit of ammonium chloride particles from the gas onto the interior surfaces of the trap plate housing  41  and the cooling housing  46 , thereby preventing excessive deposit of the ammonium chloride particles in the cooling housing  46 , and thus, premature blockage of the cooling housing  46  during operation of the silicon nitride deposition system. 
   Referring next to  FIG. 4 , in typical application the cold trap  35  of the present invention is mounted in an exhaust conduit such as the exhaust conduit  20  of the silicon nitride deposition system  10  heretofore described with respect to FIG.  1 . Accordingly, the pipe flange  38   a  of the pipe  38  is attached to one segment of the exhaust conduit, and the connector  55  of the cooling housing  46  is connected to another segment of the exhaust conduit. However, it is understood that the cold trap  35  may be mounted in fluid communication with the exhaust conduit according to any of a variety of methods known by those skilled in the art. 
   During operation of the silicon nitride deposition system, the trap plate housing  41  of the cold trap  35  functions as a first stage particle trap, and the cooling housing  46  of the cold trap  35  functions as a second stage particle trap. A stream of furnace exhaust gas  57 , which contains unreacted reactant gases such as dichlorosilane, ammonium and reaction byproduct ammonium chloride powder, is drawn through the cold trap  35 , via the exhaust conduit. The exhaust gas  57  cools as it flows through the exhaust conduit, and continues to gradually cool as it flows through the pipe bore  37  of the pipe  38 . However, it is necessary to maintain the temperature of the interior surfaces of the pipe  38  at an elevated level, preferably about 150° C., to prevent premature deposit and excessive accumulation of the ammonium chloride particles on those surfaces as the exhaust gas  57  flows through the pipe  38 . Accordingly, the primary jacket heater  36  heats the pipe  38  to a temperature of typically about 150° C., and this relatively high temperature of the pipe  38  prevents premature deposit and accumulation of the ammonium chloride particles on the interior surfaces of the pipe  38 . The secondary jacket heater  39  maintains the temperature of the connector  40  at a temperature of at least about 120° C., and the tertiary jacket heater  45  maintains the temperature of the interior surfaces of the body  42  and the trap plates  44  of the trap plate housing  41  at a temperature of at least about 80° C. As the exhaust gas flows from the pipe  38  into the trap plate housing  41  and through the trap plates  44 , ammonium chloride particles collect primarily on the trap plates  44 . Accordingly, the trap plates  44  filter the exhaust gas and remove many of the ammonium chloride particles therefrom as the ammonium chloride particles accumulate on the trap plates  44 . As it then flows from the trap plate housing  41  and into the trap core  49  of the cooling housing  46 , the exhaust gas further cools to a temperature not exceeding typically about 30° C., since the quaternary jacket heater  53  maintains the connector  48  of the cooling housing  46  at a temperature of no less than 30° C., to prevent excessive deposit of the ammonium chloride particles on the interior surfaces of the connector  48 . Finally, the cooling coil  50 , at a temperature of typically about 25° C., provides a sufficiently cool surface to facilitate deposit of a relatively large quantity of ammonium chloride particles on the cooling coil  50  and the cone-shaped interior surfaces of the cooling housing wall  47  in the trap core  49 . Finally, the exhaust gas, substantially devoid of ammonium chloride particles, exits the trap core  49  and the cooling housing  46  through the outlet bore  54 . 
   Referring next to  FIG. 5 , it will be appreciated by those skilled in the art that the cold trap of the present invention is capable of substantially increasing the time required between successive periodic maintenance (PM) of the cold trap for the removal of ammonium chloride particles therefrom, as compared to the time required between PM cleanings of conventional cold traps. As can be seen from the graph, the conventional cold trap, indicated by the numeral  60 , becomes clogged with ammonium chloride particles and thus, requires a periodic maintenance cleaning, after about 48 runs of operation. In contrast, the cold trap of the present invention, indicated by reference numeral  61 , requires a periodic maintenance cleaning after about 144 runs of operation. Referring again to  FIG. 4 , due to the cone-shaped configuration of the trap core  49 , the ammonium chloride particles are capable of forming a layer of up to about 21 μm on the interior surfaces of the cooling housing wall  47  before PM cleaning is required, necessitating PM cleanings about every eight months. In contrast, PM cleanings of the conventional cold trap are required after the ammonium chloride particles form a layer of about 7 μm, necessitating PM chamber cleanings about every eight months. 
   While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.