Abstract:
A method is described of treating a gas stream exhausted from an atomic layer deposition (ALD) process chamber to which two or more gaseous precursors are alternately supplied. Between the process chamber and a vacuum pump used to draw the gas stream from the chamber, the gas stream is conveyed to a gas mixing chamber, to which a reactant is supplied for reacting with one of the gaseous precursors to form solid material. The gas stream is then conveyed to a cyclone separator to separate solid material from the gas stream. By deliberately reacting a non-reacted precursor to form solid material upstream from the pump, reaction within the pump of the non-reacted precursor and a second non-reacted precursor subsequently drawn from the chamber by the pump can be inhibited.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method of, and apparatus for, treating a gas stream, and finds particular use in treating a gas stream exhausted from a process chamber in which a pulsed gas delivery system is used to supply gases to the process chamber.  
         [0002]     Pulsed gas delivery systems are commonly used in the formation of multi-layer thin films on a batch of substrates located in a process chamber. One such technique for forming thin films on substrates is atomic layer deposition (ALD), in which gaseous reactants, or “precursors”, are sequentially delivered to a process chamber to form very thin layers, usually on an atomic-layer scale, of materials on the substrates.  
         [0003]     By way of example, a high dielectric constant capacitor may be formed on a silicon wafer using an ALD technique through the sequential deposition of hafnium oxide (HfO 2 ) and aluminium oxide (Al 2 O 3 ) thin films. HfO 2  thin films may be formed by the sequential supply of a hafnium precursor, such as tetrakis(ethylmethylamino)hafnium (TEMAH), and an oxidant, such as ozone (O 3 ), to the process chamber, and Al 2 O 3  thin films may be formed by the sequential supply of an aluminium precursor, such as trimethyl aluminium (TMA), and O 3  to the chamber.  
         [0004]     In overview, the first precursor delivered to the process chamber is adsorbed on to the surfaces of the substrates within the process chamber. The non-adsorbed first precursor is drawn from the process chamber by a vacuum pumping system, and the second precursor is then delivered to the process chamber for reaction with the first precursor to form a layer of deposited material. In the deposition chamber, the conditions immediate to the substrates are optimised to minimise gas-phase reactions and maximise surface reactions for the formation of a continuous film on each substrate. Any non-reacted second precursor and any by-products from the reaction between the precursors is then removed from the process chamber by the pumping system. Depending on the structure being formed within the process chamber, the first precursor, or a third precursor, is then delivered to the process chamber.  
         [0005]     A purge step is typically carried out between the delivery of each precursor, for example by delivering a purge gas, such as N 2  or Ar, to the chamber between the delivery of each precursor. The purpose of the purge gas delivery is to remove any residual precursor from the process chamber so as to prevent unwanted reaction with the next precursor supplied to the chamber.  
         [0006]     In practice, only around 5% or less of the precursors supplied to the process chamber are consumed during the deposition process, and so the gas drawn from the chamber during the process chamber will, between supplies of purge gas to the chamber, alternately be rich in the first precursor, and then rich in the second precursor.  
         [0007]     In convention vacuum pumping systems, the gases drawn from the process chamber enter a common foreline leading to a vacuum pump. In the event that the non-reacted precursors meet within the vacuum pumping system, cross-reaction of the precursors can occur, and this can result in both the deposition of solid material and the accumulation of powders within the foreline and the vacuum pump. Particulates and powders that have accumulated within the pump can effectively fill the vacant running clearance between the rotor and stator elements of the pump, leading to a loss of pumping performance and ultimately pump failure. Periodic pump cleaning or replacement is then required to maintain pumping performance, resulting in costly process downtime and increasing manufacturing costs.  
         [0008]     One aim of the present invention is to seek to solve this problem.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     In a first aspect, the present invention provides a method of treating a gas stream exhausted from a process chamber to which a first gaseous precursor and a second gaseous precursor are alternately supplied, the method comprising the steps, upstream from a vacuum pump used to draw the gas stream from the chamber, of conveying the gas stream to a gas mixing chamber, supplying to the gas mixing chamber a reactant for reacting with one of the first and second gaseous precursors to form solid material, and subsequently conveying the gas stream to a separator to separate solid material from the gas stream.  
         [0010]     By deliberately reacting, say, unconsumed first gaseous precursor to form solid material, for example particulates and/or dust, before it reaches the pump, reaction within the pump of the unconsumed first gaseous precursor and unconsumed second gaseous precursor subsequently drawn from the chamber by the pump can be inhibited. The separator serves to separate from the gas stream solid material produced from the reaction between the unconsumed first gaseous precursor and the reactant. The separator may be provided by any trap device for removing solid material from a gas stream. One example is a dead-leg type of trap device. In the preferred embodiment, the separator is provided by a cyclone separator. An advantage associated with the use of a cyclone separator to separate the solid material from the gas stream is that the solid material will settle out in the bottom of the cyclone separator without increasing the impedance of the separator to the flow of the gas stream. Two or more cyclone separators may be provided in parallel to increase gas conductance.  
         [0011]     The reaction between the reactant and the first gaseous precursor will take a finite amount of time, and so the gas mixing chamber is preferably configured to define a tortuous path for the mixture of the reactant and the gas stream to increase the residence time of the gas mixture within the gas mixing chamber, and to optimise mixing. At least one of the gas mixing chamber and the reactant may be heated to increase the rate of reaction between the reactant and the first gaseous precursor. The separator may also be heated to complete the reaction between the reactant and the first gaseous precursor. In one preferred embodiment, the gas mixing chamber is integral with the separator.  
         [0012]     The reactant is preferably a gas that is relatively cheap, safe and readily available. In the preferred embodiment, in order to minimise the number of gas supplies the reactant is the second gas precursor, which is supplied from a second precursor supply both to the process chamber and to the gas mixing chamber for reaction with the first gaseous precursor. In this case, the reactant may be either an oxidising agent or a reducing agent used in the process conducted within the process chamber. In the preferred embodiment, the reactant is an oxidising agent, such as ozone, and the first gaseous precursor is an organometallic precursor, which may comprise one of hafnium and aluminium. Examples include tetrakis(ethylmethylamino)hafnium (TEMAH) and trimethyl aluminium (TMA).  
         [0013]     In a second aspect the present invention provides a method of treating a gas stream exhausted from a process chamber, the method comprising the steps of adding to the gas stream a reactant for reacting with a gaseous component of the gas stream to form solid material, and subsequently conveying the gas stream to a separator to separate solid material from the gas stream.  
         [0014]     In a third aspect, the present invention provides apparatus for treating a gas stream exhausted from a process chamber to which a first gaseous precursor and a second gaseous precursor are alternately supplied, the apparatus comprising a gas mixing chamber for receiving the gas stream from the process chamber, a reactant supply for supplying to the mixing chamber a reactant for reacting with one of the first and second gaseous precursors to form solid material, and a separator for receiving the gas stream from the gas mixing chamber and separating solid material from the gas stream.  
         [0015]     In a fourth aspect, the present invention provides an atomic layer deposition apparatus comprising a process chamber, a first gaseous precursor supply for supplying a first gaseous precursor to the chamber, a second gaseous precursor supply for supplying a second gaseous precursor to the chamber, a vacuum pump for drawing a gas stream from the process chamber, and, between the process chamber and the vacuum pump, a gas mixing chamber for receiving the gas stream from the process chamber and second gaseous precursor from the second precursor gas supply for reacting with first gaseous precursor within the gas stream to form solid material, and a separator for receiving the gas stream from the gas mixing chamber and separating solid material from the gas stream.  
         [0016]     Features described above in relation to method aspects of the invention are equally applicable to apparatus aspects, and vice versa. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0018]      FIG. 1  illustrates schematically an atomic layer deposition apparatus;  
         [0019]      FIG. 2  illustrates the sequence of supply of gases to the process chamber of the apparatus of  FIG. 1 ;  
         [0020]      FIG. 3  illustrates an external view of a trap device comprising a gas mixing chamber and a cyclone separator; and  
         [0021]      FIG. 4  illustrates a cross-sectional view of the trap device of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     With reference first to  FIG. 1 , an atomic layer deposition (ALD) apparatus comprises a process chamber  10  for receiving a batch of substrates to be processed simultaneously within the process chamber  10 . The process chamber  10  receives separately and alternately two or more different gaseous reactants or precursors for use in forming layers of material on the exposed surfaces of the substrates. In the example illustrated in  FIG. 1 , a first precursor supply  12  is connected to the process chamber  10  by a first precursor supply line  14  for supplying a first precursor to the process chamber  10 , and a second precursor supply  16  is connected to the process chamber  10  by a second precursor supply line  18  for supplying a second precursor to the process chamber  10 . A purge gas supply  20  is also connected to the process chamber  10  by a purge gas supply line  22  for supplying a purge gas such as nitrogen or argon to the process chamber  10  between the supply of the precursors to the process chamber  10 .  
         [0023]     The supply of the precursors and the purge gas to the process chamber  10  is controlled by the opening and closing of gas supply valves  24 ,  26 ,  28  located in the supply lines  14 ,  18 ,  22  respectively. The operation of the gas supply valves is controlled by a supply valve controller  30  which issues control signals  32  to the gas supply valves to open and close the valves according to a predetermined gas delivery sequence. A typical gas delivery sequence involving two gaseous precursors and a purge gas is illustrated in  FIG. 2 . The first trace  40  represents the stepped delivery sequence for the first gaseous precursor, and the second trace  42  represents the stepped delivery sequence for the second gaseous precursor. As described above, the first and second precursors are alternately supplied to the chamber to form layers of solid material on the batches of substrates located within the process chamber  10 . The duration of each pulsed delivery of precursor to the process chamber  10  is defined for the particular process to be performed within the process chamber  10 ; in this example, the duration of each pulsed delivery of the second precursor to the process chamber  10  is longer than that for the first precursor. The third trace  44  represents the stepped delivery sequence for the purge gas that is introduced into the process chamber  10  between the delivery of first and second gaseous precursors to flush the process chamber  10  before introducing the next gaseous precursor.  
         [0024]     Returning to  FIG. 1 , a vacuum pumping system is connected to the outlet  50  of the process chamber  10  for drawing a gas stream from the process chamber  10 . The pumping system comprises a vacuum pump  52  for receiving the gas stream through an inlet  54  thereof and exhausting the gas stream at an elevated pressure through an exhaust  56  thereof. The gas stream exhausted from the vacuum pump  52  is conveyed to an inlet  58  of an abatement device  60 , for example a thermal processing unit or a wet scrubber, for removing one or more species from the gas stream before it is exhausted to the atmosphere.  
         [0025]     In one example, the first gaseous precursor is an organometallic precursor containing one of hafnium and aluminium, such as tetrakis(ethylmethylamino)hafnium (TEMAH) or trimethyl aluminium (TMA), and the second gaseous precursor is an oxidant, such as ozone. The second precursor supply  16  may therefore be provided by an ozone generator. Currently available ozone generators can be difficult to start and stop in synchronisation with the pulsed delivery sequence of ozone to the process chamber  10 . In view of this, the ozone generator  16  may be continuously generating ozone during the ALD process, and when ozone is not being delivered to the process chamber  10  the ozone may be diverted along ozone supply line  62  to a location downstream from the vacuum pump  52 , for example to the inlet of a backing pump (not illustrated) provided between the vacuum pump  52  and the abatement device  60 , or directly to a second inlet of the abatement device  60 , where the ozone may assist in the abatement of the gas stream exhausted from the vacuum pump  52 .  
         [0026]     In view of the alternating supply of first and second gaseous precursors to the process chamber  10 , the gas stream drawn from the process chamber  10  will alternate between a first precursor-rich gas stream, comprising unconsumed first precursor and by-products from the reaction between the precursors, and a second precursor-rich gas stream, comprising unconsumed second precursor and the by-products, with a purge gas-rich gas stream being drawn from the process chamber  10  between these precursor-rich gases. Each of the precursor-rich gas streams is also likely to contain traces of purge gas and other contaminants.  
         [0027]     In order to inhibit mixing of the unconsumed precursors within the vacuum pump  52 , which could lead to undesirable reaction between the precursors and the formation of dust and/or powders within the vacuum pump, apparatus is provided between the outlet  50  of the process chamber  10  and the inlet  54  of the vacuum pump  52  to treat the gas stream exhausted from the process chamber  10  so as to reduce the amount of one of the first and second precursors that enters the vacuum pump  52 . In the example illustrated in  FIG. 1 , the amount of the first precursor entering the vacuum pump  52  is reduced.  
         [0028]     The apparatus for treating the gas stream exhausted from the process chamber  10  comprises a gas mixing chamber  70  having a first inlet  72  for receiving the gas stream exhausted from the process chamber  10 , and a second inlet  74  for receiving a reactant for reacting with the chosen precursor to be at least partially removed from the gas stream. In the illustrated example, a reactant supply for supplying the reactant to the gas mixing chamber  70  is conveniently provided by the ozone generator  16 , with a reactant supply line  76  being connected between the ozone supply line  62  and the second inlet  74  of the gas mixing chamber  70  to supply ozone to the gas mixing chamber  70  as the reactant. The supply of reactant to the gas mixing chamber  70  is controlled by the opening and closing of reactant supply valve  78  located in the reactant supply line  76 . The operation of the reactant supply valve  78  is controlled by a supply valve controller  30 , which issues control signals  32  to the reactant supply valve  78  to open and close in synchronisation with the delivery of the first gaseous precursor to the process chamber  10 , so that the reactant is supplied to the gas mixing chamber  70  with a stepped delivery sequence that is similar to that for the first gaseous precursor. The amount of reactant periodically delivered to the gas mixing chamber  70  is preferably at least sufficient to react with the amount of the first gaseous precursor that is supplied to the process chamber  10 .  
         [0029]     In order to increase the reaction rate between the reactant and the first gaseous precursor within the gas mixing chamber  70 , at least one of the reactant and the gas mixing chamber  70  may be heated. With reference to  FIG. 1 , a first heater  80  may optionally extend about the reactant supply line  76  for heating the reactant before it is supplied to the gas mixing chamber  70 , and a second heater  82  may optionally extend about the gas mixing chamber  70  for heating the gas mixing chamber  70 .  
         [0030]     The reactant is preferably chosen to replicate a reaction that would occur between unconsumed first and second gaseous precursors within the vacuum pump  52 . Therefore, a product from the reaction between the reactant and the first gaseous precursor is the solid material, such as a dust and/or powder, that would otherwise be formed in the vacuum pump  52  through the reaction between the unconsumed precursors. Consequently, the apparatus for treating the gas stream exhausted from the process chamber  10  comprises a separator  84  having an inlet  86  connected to an outlet  88  of the gas mixing chamber  70 . In one embodiment, the separator is a cyclone separator, which receives the solid material-laden gas stream from the gas mixing chamber  70 , and, in a manner known in the art, separates the solid material from the gas stream, retaining the solid material therewithin and exhausting the gas stream from an outlet  90  thereof to the inlet  54  of the vacuum pump  52 . As illustrated in  FIG. 1 , a third heater  92  may optionally be provided around the separator  84  for heating the separator  84  to promote reaction between any remaining unconsumed first gaseous precursor and the reactant.  
         [0031]     In the example illustrated in  FIG. 1 , the gas mixing chamber  70  is separate from the separator  84 . However, the gas mixing chamber  70  may be mounted on, or integral with, the separator  84 . An example of a trap device  100  comprising a gas mixing chamber  102  mounted on a cyclone separator  104  is illustrated in  FIGS. 3 and 4 . The trap device  100  comprises a first inlet  106  for receiving the gas stream drawn from the process chamber  10  by the vacuum pump  52 , and a second inlet  108  for receiving the gaseous reactant for reacting with the chosen unconsumed precursor contained within the gas stream. As illustrated in  FIG. 1 , a heater (not shown) may be located around the trap device  100  for heating at least one of the gas mixing chamber  102  and the cyclone separator  104 .  
         [0032]     The gas stream and the gaseous reactant enter the gas mixing chamber  102 , wherein the gas streams combine. A baffle  110  located within the gas mixing chamber  102  causes the combined gas stream to turn through 180° as it passes through the gas mixing chamber  102 , and thereby defines a tortuous flow path  112  for the combined gas stream passing through the gas mixing chamber  102 . The turbulence created in the combined gas stream by the baffle  110  serves to both increase the residence time for the combined gas stream within the gas mixing chamber  102 , and optimise the mixing of the gaseous reactant with the unconsumed gaseous precursor to promote reaction between the reactant and the unconsumed precursor to form solid material, such as particulates and/or powder.  
         [0033]     The now solid material-laden gas stream passes through an outlet  114  of the gas mixing chamber  102  and enters the cyclone separator  104  through an inlet  116  thereof. The cyclone separator  104  comprises at least one separating chamber  118  for receiving the solid material-laden gas stream from the inlet  116 . In one preferred embodiment, the cyclone separator  104  comprises at least six separating chambers  118  located circumferentially within the cyclone separator  104  each for receiving a portion of the solid material-laden gas stream from the inlet  116 .  
         [0034]     Each of the separating chambers  118  comprises a cylindrical upper portion  120  and a conical lower portion  122 . A removable, annular particulate collecting chamber  124  is located beneath the separating chambers  118  for receiving solid material in the form of particulates, powder and/or dust that is separated from the solid material-laden gas stream within the separating chamber  118 . As is well known in the art of cyclone separators, the solid material-laden gas stream that enters the separating chamber  118  spirals downwardly along the inner wall surface of the separating chamber  118 . At the lower portion of the separating chamber  118 , the flow of gas is turned upwardly to form a spiral upward flow along the centre of the separating chamber  118 . During this process, the solid material in the gas stream is separated from the gas under the influence of the centrifugal force of the spiral gas flow, and falls down along the inner wall surface of the separating chamber  118  to be collected in the particle collecting chamber  124 .  
         [0035]     An outlet pipe  126  extends downwardly into the cylindrical upper portion  120  of the separating chamber  118  for receiving the gas stream from the separating chamber  118  and conveying the gas stream into an annual plenum chamber  128 . The plenum chamber  128  receives gas from each of the separating chambers  118  and exhausts the gas into a central outlet pipe  130  through which the gas stream flows to the outlet  132  of the trap device  100 .  
         [0036]     A sensor (not shown) may be provided in association with the particle collecting chamber  124 . The sensor  34  may be in the form of a level sensor for outputting a signal indicative of the level of solid material collected within the particle collecting chamber  124 . Alternatively, the sensor may be in the form of a load cell for monitoring the weight of solid material collected by the particle collecting chamber  124 , a temperature sensor, a vibration sensor or any other suitable sensor. In use, the sensor outputs a signal to a controller indicative of the amount of solid material collected by the trap device  100 . When the monitored amount reaches or exceeds a predetermined value, the controller may generate a visual or audible alert to a user to indicate that servicing of the trap device  100  is required. The particle collecting chamber  124  may be disconnected from the trap device  100  by the user. Alternatively, the particle collecting chamber  124  may be provided with an access port selectively closable by a door or other movable member for providing external access to the particulates collected by the trap device  100 . For example, the bottom wall of the particle collecting chamber  124  may be pivotally connected to the side wall of the chamber so that, when the trap device  100  requires servicing, the particle collecting chamber  124  can be opened to allow the collected particulates to fall from the particle collecting chamber  124  into a suitable receptacle placed thereunder. Alternatively, safety may be improved by providing a closable access port or slot at a position above the particulate level at which the alert is generated, and inserting a suction device into the particle collecting chamber  124  via the access port to remove the collected particulates therefrom.  
         [0037]     While the description above and the system shown in  FIG. 1  show a single separator, the present invention also contemplates the use of two or more separators, or two or more trap devices that may be provided in parallel to enable one of the separators, or trap devices, to be serviced while the other is operational, thereby enabling continuous operation.  
         [0038]     It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description and examples, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the appended claims.