Abstract:
This invention relates to a method of producing B 2 H 6  (diborane) in semiconductor wafer processing apparatus. In particular, although not exclusively, this invention relates to producing a dopant gas species containing a desired dopant element, and then producing dopant ions for implanting in semiconductor wafers using an ion implanter. The present invention provides such a method by passing a flow of a boron containing gas such as BF 3  over a hydrogen containing solid such as NaH thereby forming an outflow of B 2 H 6 .

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a method of producing B 2 H 6  (diborane) in semiconductor wafer processing apparatus. In particular, although not exclusively, this invention relates to producing a dopant gas species containing a desired dopant element, and then producing dopant ions for implanting in semiconductor wafers using an ion implanter. 
       BACKGROUND OF THE INVENTION 
       [0002]    The semiconductor industry has a requirement for the production of semiconductor devices that is most often met by fabrication of arrays of many devices on a single wafer. These wafers undergo a range of treatments, some of which may involve the use of boron. For example, semiconductor devices often require doping to very fine tolerances to achieve desired characteristics, and boron is a commonly used dopant. Such doping may be performed using an ion implanter that comprises an ion source to generate ions corresponding to, or containing, boron or another dopant. Optics then form the ions into a focussed ion beam that is incident upon the wafer. Control of the ion beam (e.g. beam current, ion content, energy, size, scanning, etc.) is of paramount importance as this determines the dopant concentration in the wafer and also the depth of implant, thereby determining the conductive properties of the devices. 
         [0003]    Bottles are generally used as a supply of a boron-containing gas, e.g. B 2 H 6 . These bottles are connected to the ion source such that the gas is allowed to enter an arc chamber where an arc discharge ionises the gas to form a plasma. An extraction electrode is used to extract ions from the arc chamber through an aperture provided therein. Further electrodes are used to form an ion beam that is directed at the wafer to be implanted. Generally, the ion beam passes through a mass-analysing magnet that selects only ions with the desired mass-to-charge ratio: put another way, the mass-analysing magnet effectively rejects unwanted ions that are inevitably produced in the arc chamber/plasma or otherwise generated. 
       SUMMARY OF THE INVENTION 
       [0004]    Against this background, and from a first aspect, the present invention resides in a method of producing B 2 H 6  in situ in semiconductor processing apparatus, comprising passing a flow of a boron containing gas over a hydrogen containing solid thereby forming an outflow of B 2 H 6 . 
         [0005]    As a result, more commonly available gases may be used as a feed gas for producing B 2 H 6  in situ. For example, a gaseous boron halide such as BF 3  or BCl 3  may be used as a feed gas. 
         [0006]    Various hydrogen containing solids may be used with the method, such as solid hydrides. Metal hydrides, and the alkali metal hydrides are currently preferred, with NaH or KH being the most preferred. 
         [0007]    Optionally, the method further comprises heating the hydrogen containing solid while passing the gas thereover. Preferably, the hydrogen containing solid is heated to 150° C. to 200° C., substantially 180° C. being particularly preferred. 
         [0008]    The outflow of B 2 H 6  may be cooled. This mitigates against the chances of decomposition or polymerisation. 
         [0009]    Optionally, the method may comprise purging a vessel in which the hydrogen containing solid is contained by passing a flow of an inert gas through the vessel. 
         [0010]    From a second aspect, the present invention resides in a method of producing B 2 H 6  in situ in semiconductor processing apparatus, comprising: heating a vessel containing NaH, and passing a flow of BF 3  over the NaH contained in the vessel thereby forming a flow of B 2 H 6  out of the vessel. Hence, commonly available BF 3  may be used as a feed gas for producing B 2 H 6  in situ. The BF 3  may be supplied from an SDS type bottle. 
         [0011]    Preferably, the NaH in the vessel is heated to 150° C. to 200° C., 180° C. being particularly preferred. 
         [0012]    Optionally, the flow of BF 3  may be regulated using a mass flow controller. 
         [0013]    Preferably, the flow of B 2 H 6  is cooled after it leaves the vessel. This mitigates against the chances of decomposition or polymerisation. 
         [0014]    Optionally, the vessel may be purged by passing a flow of an inert gas such as Ar over the NaH in the vessel. This assists in removing any deposits that may have formed. 
         [0015]    The semiconductor processing apparatus may be an ion implanter. The B 2 H 6  produced according to the above methods may be used for B 2  diamer implants. 
         [0016]    According to a third aspect, a method of implanting a semiconductor wafer, comprising: producing B 2 H 6  according to any preceding claim; introducing the B 2 H 6  into an ion source; operating the ion source thereby producing an ion beam; and guiding the ion beam to a semiconductor wafer to be incident thereon. 
         [0017]    The NaH in the vessel may be heated to 150° to 200° C., substantially 180° C. being particularly preferred. 
         [0018]    Optionally, the flow of BF 3  is regulated using a mass flow controller. The flow of B 2 H 6  may be cooled after it leaves the vessel. The BF 3  may be supplied from a SDS bottle. 
         [0019]    Optionally, the method may further comprise purging the vessel by passing a flow of an inert gas such as Ar over the NaH in the vessel. 
         [0020]    Optionally, the ion source comprises an arc chamber, and the method comprises generating a plasma in the arc chamber. An indirectly-heated cathode may be used. The method may comprise guiding the ion beam through a mass analyser placed on the ion beam path to the semiconductor wafer, and mass selecting ions of a desired mass to charge ratio. For example, B 2  diamer ions may be mass selected. 
         [0021]    From a fourth aspect, the present invention resides in semiconductor processing apparatus including a B 2 H 6  source comprising: a source of a boron containing gas; a vessel containing a hydrogen containing solid that is coupled to the source of the boron containing gas via a gas flow regulator; a heater for heating the hydrogen containing solid in the vessel; and a conduit arranged to convey the outflow of B 2 H 6  from the vessel. 
         [0022]    Optionally, the source of a boron containing gas is a boron halide source, such as a BF 3  or BCl 3  source. 
         [0023]    The vessel may contain a solid hydride, such as a metal hydride like an alkali metal hydride. NaH and KH are particularly preferred. 
         [0024]    The vessel may be a heated column or a conversion column. The apparatus may further comprise a cooler for cooling the flow of B 2 H 6  through the conduit. The gas flow regulator may be a mass flow controller. The BF 3  source may be a SDS type bottle. 
         [0025]    The apparatus may further comprise an inert gas source coupled to the vessel via a gas flow regulator. The inert gas source may be an argon source, optionally a SDS bottle of Ar. 
         [0026]    The semiconductor processing apparatus may be an ion implanter. The ion implanter may further comprise an ion source arranged to receive the flow of B 2 H 6  from the conduit and to generate ions therefrom, and means for guiding ions generated by the ion source to a semiconductor wafer to be incident thereon. The ion source may comprise an arc chamber and, optionally, an indirectly heated cathode. The ion implanter may further comprise a mass analyser positioned on the ion path from the ion source to the semiconductor wafer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    In order that the invention can be more readily understood, reference will now be made, by way of example only, to the accompanying drawings, in which: 
           [0028]      FIG. 1  is a schematic view of an ion implanter according to an embodiment of the present invention; and 
           [0029]      FIG. 2  is a simplified view of an ion source according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    An ion implanter  10  for implanting ions in semiconductor wafers  12  is shown in  FIG. 1  that includes an ion source  14  according to the present invention. Ions are generated by the ion source  14  to be extracted and passed through a mass analysis stage  30 . Ions of a desired mass-to-charge ratio are selected to pass through a mass-resolving slit  32  and then to strike a semiconductor wafer  12 . 
         [0031]    The ion implanter  10  contains an ion source  14  for generating an ion beam of a desired species that is located within a vacuum chamber  15 . The ion source  14  generally comprises an arc (or discharge or ionisation) chamber  16  containing an indirectly-heated cathode  20  located at one end thereof and an anode that is provided by the walls  18  of the arc chamber  16 . The cathode  20  is heated sufficiently to generate thermal electrons. 
         [0032]    Thermal electrons emitted by the cathode  20  are attracted to the anode, i.e. the adjacent chamber walls  18 . The thermal electrons ionise gas molecules as they traverse the arc chamber  16 , thereby forming a plasma and generating the desired ions. The gas molecules are produced at  21 , as will be described in more detail with reference to  FIG. 2 , and drift into the arc chamber  16  through gas feed  22 . 
         [0033]    The path followed by the thermal electrons is controlled to prevent the electrons merely following the shortest path to the chamber walls  18 . A magnet assembly  46  provides a magnetic field extending through the arc chamber  16  such that thermal electrons follow a spiral path along the length of the arc chamber  16  towards a counter-cathode  44  located at the opposite end of the arc chamber  16 . 
         [0034]    A gas feed  22  fills the arc chamber  16  with a precursor gas species. The thermal electrons travelling through the arc chamber  16  ionise the precursor gas molecules and possibly also crack the precursor gas molecules as well to form other ions. The ions created in the plasma will also contain trace amounts of contaminant ions (e.g. generated from the material of the chamber walls). 
         [0035]    Ions from within the arc chamber  16  are extracted through an exit aperture  28  using a negatively-biased extraction electrode  26 . A potential difference is applied between the ion source  14  and the following mass analysis stage  30  by a power supply  21  to accelerate extracted ions, the ion source  14  and mass analysis stage  30  being electrically isolated from each other by an insulator (not shown). The mixture of extracted ions are then passed through the mass analysis stage  30  so that they pass around a curved path under the influence of a magnetic field. The radius of curvature traveled by any ion is determined by its mass, charge state and energy. The magnetic field is controlled so that, for a set beam energy, only those ions with a desired mass and charge state exit along a path coincident with the mass-resolving slit  32 . The emergent ion beam  34  is then transported to the target, i.e. one or more semiconductor wafers  12  to be implanted or a beam stop  38  when there is no wafer  12  in the target position. In other modes, the beam  34  may also be decelerated using a lens assembly positioned between the mass analysis stage  30  and the target position. 
         [0036]    An ion source  14  suitable for use in the ion implanter  10  of  FIG. 1  is shown in schematic form in  FIG. 2 . The ion source  14  includes an arc chamber  16  connected to a gas supply  21  by gas feed  22 . The gas supply  21  comprises apparatus for in situ production of B 2 H 6 . The feed gas is a SDS-type bottle shown at  52 , and contains BF 3  in this embodiment. The BF 3  bottle  52  is connected to a conversion column  54  via line  56 . Flow of the BF 3  through the line  56  to the conversion column  54  is controlled using a mass flow controller  58 . 
         [0037]    In this embodiment, the conversion column  54  contains NaH and is heated to a temperature of 180° C. Passing the BF 3  feed gas over the NaH at the elevated temperature results in the following reaction: 
         [0000]      2BF 3 +6NaH|B 2 H 6 +6NaF 
         [0038]    Accordingly, an outflow of B 2 H 6  is obtained from the conversion column  54  that is conveyed to the arc chamber  16  via gas feed  22 . The flow of B 2 H 6  in gas feed  22  is cooled, for example by using a water chiller, or other means, shown at  60 . Cooling the B 2 H 6  is advantageous as it prevents decomposition or polymerisation while being conveyed to the arc chamber  16 . 
         [0039]    The by-product of the above reaction (NaF) is a stable, low-volatility salt that remains within the conversion column  54 . This is advantageous as it removes the need to separate and purify the product gas. As NaH will be consumed during operation of the gas supply  21 , the conversion column  54  will need to be replaced from time to time. 
         [0040]    In addition, it is preferred to be able to purge the system and, to this end, an SDS bottle  62  containing Ar gas is provided. Flow of Ar gas from the bottle  62  is regulated by a mass flow controller  64 , as shown in  FIG. 2 . A line  66  connects the Ar bottle  62  and mass flow controller  64  to the line  56  leading from the BF 3  bottle  52  to the conversion column  54 . With this arrangement Ar may be admitted into the system, thereby purging the system. This may be performed, for example, following a change of conversion column  54  or BF 3  gas bottle  52 . Purging may also be performed periodically. 
         [0041]    Those skilled in the art will appreciate that variations may be made to the above embodiments without departing from the scope of the present invention. 
         [0042]    While the above embodiment sets the present invention in the context of an ion implanter  10 , it will be appreciated that the present invention may find application in other semiconductor wafer processes. 
         [0043]    In addition to using BF 3  as the feed gas, other boron containing gases may be used such as other boron halides like BCl 3 . Similarly, alternatives to NaH may be used. Alternatives include hydrogen containing solids such as metal hydrides. Alkali hydrides like KH would make good alternatives. 
         [0044]    While Ar has been described as the purge gas, other inert gases could be suitable alternatives. While the conversion column  54  has been described as being heated to 180° C., other temperatures could be used. Although cooling the B 2 H 6  flow in the gas feed  22  is described above, this is but merely a preferred feature and may be omitted if desired. 
         [0045]    The ion source  14  comprises an arc chamber  16  having an indirectly heated cathode  20 . Details of the actual ion source may be varied, for example a Bernas cathode may be used instead. The inclusion or not of a counter cathode  44  is also optional, and various biasing arrangements may be used in the arc chamber  16 .