Abstract:
This invention relates to a method of producing a dopant gas species containing a required dopant element for implanting in a target and to an ion source for implementing such a method. In particular, although not exclusively, this invention relates to producing dopant ions for implanting in semiconductor wafers using an ion implanter. The present invention provides a method of producing a dopant gas species containing a required dopant element for implanting in a target, the method comprising: exposing a source mass of the element to gaseous bromine and element react to form a reactant product, and ionising the reactant product to produce ions of the dopant gas species.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of producing a dopant gas species containing a desired dopant element for implanting in a target and to an ion source for implementing such a method. In particular, although not exclusively, this invention relates to producing dopant ions for implanting in semiconductor wafers using an ion implanter. 
     BACKGROUND OF THE INVENTION 
     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. The semiconductor devices often require doping to very fine tolerances to achieve desired characteristics. Such doping may be performed using an ion implanter that comprises an ion source to generate ions corresponding to, or containing, the required 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. 
     Typical dopants include boron, phosphorus, arsenic, aluminium, antimony and indium. Ions of these dopants are often produced in the ion source by obtaining a gas containing the required dopant, allowing the gas 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/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. 
     The gas supplied to the arc chamber may be obtained in a variety of ways. One method is to heat the elemental form of the dopant (invariably a solid) in an oven. The vapour so produced is allowed to pass into the arc chamber. However, many of the dopants are metals with low vapour pressures meaning the oven must be operated at high temperatures to produce the required vapour. 
     Alternatively, compounds containing the dopant of interest may be heated in an oven. U.S. Pat. No. 2002/0029746 discloses heating indium fluoride to achieve indium doping. Adjusting the beam current requires an adjustment of the temperature of the oven and control is therefore limited by the thermal response time of the oven (as much as 30 minutes). Moreover, control is unpredictable because the true temperature of the contents of the oven cannot be known precisely. The low vapour pressure of indium fluoride poses another problem in the condensation of the vapour so produced. Thus, transport of the vapour becomes difficult. 
     These methods of producing a dopant gas species pose a problem because they show great sensitivity to variations in temperature, i.e. a graph showing how their vapour pressure varies with temperature is particularly steep around the operating temperature of the ovens. As a result, there is a burden in that fine control of the oven temperature is necessary. Typically, the oven must be controlled to better than 1° C. 
     Another approach to producing the required dopant in gaseous form is to pass a gas over the dopant (or a compound thereof) such that the two react to form the required gas that drifts into the arc chamber. This technique has been known for quite some time, both in ion implantation and in other fields. For example, Sidenius and Stilbreid in E.M. Separations with High Efficiency of Microgramme Qualities (E.M. Separation of Radioactive Isotopes, Proceedings of the International Symposium, Vienna, May 1960, Springer-Verlag, pp 244–249) discloses passing carbon tetrachloride over heated rare-earth oxides to form gaseous chlorides of the rare earth. Halides such as carbon tetrachloride are often used because of their high vapour pressure. More recently, U.S. Pat. No. 6,001,172 discloses passing a variety of gases such as fluorides (NF 3 , ClF 3 , BF 3  and fluorine itself) over heated indium or antimony to produce fluorides of indium or antimony that are then ionised. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve on the methods of producing dopant gas species described above. 
     From a first aspect, the present invention resides in a method of producing a dopant gas species containing a desired dopant element for implanting in a target, the method comprising: exposing a source mass of the element to gaseous bromine such that the bromine and element react to form a reactant product, and ionising the reactant product to produce ions of the dopant gas species. 
     Use of a bromine gas to create volatile bromide, rather than using chlorides and fluorides previously suggested, has been found to have many benefits, as follows. 
     (1) Bromine reactions have been found to have a much reduced temperature sensitivity than other halides thereby removing the onerous requirement for tight temperature control of the oven. Instead, the temperature of the oven can be allowed to drift around a desired temperature without the amount of bromine production changing appreciably. The flow of bromine vapour may then be adjusted to control an ion beam flux, for example. 
     (2) When used with indium as the source element, bromine is beneficial as indium bromide has an acceptably high vapour pressure at temperatures as low as around 400° C. This lower oven temperature results in a less reactive environment. Use of fluorine instead requires substantial heating of the indium as indium fluoride requires temperatures in excess of 800° C. for a sufficient amount of vapour to be liberated. 
     (3) Bromine is less reactive than fluorine making it easier to handle with less safety precautions to consider. Also, its higher vapour pressure means less condensation. Thus transporting the bromine vapour from bottle to reaction chamber is easier. 
     (4) Bromine is a liquid at room temperature with a high vapour pressure that allows vapour released by the liquid to be passed along ordinary gas handling systems (by gaseous bromine, we mean pure gas vapours, mists, atomised suspensions, etc. that are essentially gas like and so can be handled like a gas). 
     (5) Bromine is relatively inexpensive to buy. 
     (6) Unlike carbon tetrachloride, the use of bromine does not result in carbon deposits being left in the source. 
     (7) Bromine has two isotopes at 79 and 81 amu, both separated from other popular elements used in the semiconductor industry. Indium has isotopes at 113 and 115 amu producing double-charge ions at 56.5 and 57.5 amu, and antimony has isotopes at 129 and 131 amu producing double-charge ions at 64.5 and 65.6 amu. This separation ensures that any bromine ions extracted from the ionisation chamber will be rejected by the mass-analysing magnet. 
     Preferably, the method further comprises regulating the supply of gaseous bromine to the element. This, in turn, leads to a regulation of the rate of ion production in the ionisation chamber and hence the ion beam current. Moreover, this method of control is far more responsive than the prior art method of varying the temperature of an oven. 
     In fact, only the flow rate may be regulated to control ion beam current and the oven temperature need not be altered. For example, there is a delay between turning off an oven and ion production coming to a stop corresponding to the time the oven takes to cool. Clearly, interrupting the supply of the bromine gas may be effected rapidly such that cessation of ion production follows very quickly. 
     Optionally, the supply of gaseous bromine is regulated such that a reservoir of reactant product does not build up in the ionisation chamber. This “starved” mode of operation ensures the reactant product has a very short mean lifetime in the ionisation chamber prior to being ionised and extracted. Having a reservoir of reactant in the ionisation chamber is less desirable because the response time following adjustment of the flow rate of the bromine gas is degraded. The flow of gaseous bromine may be regulated by a flow control device; i.e. a mass flow controller or a valve. 
     From a second aspect, the present invention resides in a method of producing ions from a source material in an ion implanter comprising an ion source having an ionisation chamber and an oven supplied with a regulated gas supply, the method comprising providing a mass of the source metal in the oven, connecting a container containing liquid bromine to the gas supply, regulating the flow of bromine vapour produced by the liquid bromine along the gas supply, exposing the source material to the regulated supply of bromine vapour thereby to react to produce a gaseous reactant product, the gaseous reactant product being free to pass into the ionisation chamber, and operating an arc discharge in the ionisation chamber thereby to ionise the reactant product and produce the ions. 
     From a third aspect, the present invention resides in a method of implanting a semiconductor wafer including any of the methods of producing a dopant gas species described above. 
     From a fourth aspect, the present invention resides in an ion source comprising: a vessel containing liquid bromine, means defining a path from the vessel through an ionisation chamber to an exit aperture of the ionisation chamber, along which path bromine vapour produced by the liquid bromine may pass, a mass of source material positioned in the path and electrodes operable to provide an arc discharge within the ionisation chamber. 
     The means defining a path may be a conduit such as a pipe or the like. 
     Preferably, the ion source further comprises a regulator disposed along the path operable to regulate the flow of bromine vapour along the path. Optionally, the regulator is located upstream of the mass of source material although, alternatively, the regulator may be located between the mass of source material and the ionisation chamber. 
     Optionally, the ion source further comprises a controller operable to receive an input indicative that the ion source should be shut down and to effect shutting down of the ion source by closing the regulator. The controller may be a dedicated device, or it may be shared with other components of an ion implanter in which it may be located. 
     The input may come from a variety of sources. For example, the input may be derived from a manual shutdown, say following an operator pushing an “off” button or effecting shutdown using a computer-controlled graphical user interface. The input may also be generated automatically, possibly at the end of a pre-programmed procedure or as the result of a fault being detected automatically. 
     In addition merely to effecting shutdown of the ion source, the controller may also regulate flow of gaseous bromine. This may be performed in response to the controller determining the flow rate of bromine vapour. 
     From a fifth aspect, the present invention resides in an ion implanter comprising the ion source described above, means for extracting ions through the exit aperture of the ionisation chamber and means for guiding the extracted ions along an ion beam path to be incident upon a target to be implanted. 
     The means for extracting ions may be a structural element containing an electric charge to attract or repel ions out of the ionisation chamber, e.g. an electrode supplied with an appropriate voltage. Alternatively, the means for extracting ions may be a magnet operable to produce a magnetic field to cause ions to exit the ionisation chamber. 
     The means for guiding the extracted ions may comprise ion optics such as electrodes supplied with appropriate voltages or other charged structural members, or such as magnets operable to produce suitable focussing magnetic fields. 
     Optionally, the ion implanter operates using a feedback loop such that the flow of bromine vapour is regulated to achieve a desired ion beam current. One possible implementation is for the ion implanter further to comprise a monitor for monitoring the ion beam as it traverses the ion beam path and a controller operable to receive a signal from the monitor that is indicative of the ion beam current. The controller may determine whether the current corresponds to a desired current and, if not, to adjust the regulator to achieve the desired current. 
     Other preferred, but optional, features of the above method and apparatus are set out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic view of an ion implanter according to an embodiment of the present invention; 
         FIG. 2  is a simplified view of an ion source according to an embodiment of the present invention; 
         FIG. 3  is a side sectional view of an oven according to an embodiment of the present invention; and 
         FIG. 4  is a mass spectrum of the ions produced according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 selected to pass through a mass-resolving slit  32  and then to strike a semiconductor wafer  12 . 
     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 a 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. 
     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 in an oven  21  and drift into the arc chamber  16  through gas feed  22 . 
     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 . 
     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). 
     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 travelled 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. 
     An ion source  14  suitable for use in the ion implanter  10  of  FIG. 1  is shown in a simplified schematic form in  FIG. 2 . The ion source  14  includes an arc chamber  16  connected to an oven  21  by gas supply  22 . The oven  21  is heated by a heating element  50  supplied with current from a current source  52  (that may be either a dc or ac source). The heating element  50  is located adjacent the oven  21  which contains the source material reactant  54 . In this case, the reactant  54  is a strip of indium, although other metals such as antimony or other source materials may be used, whether in strip-form or otherwise (e.g. powdered). 
     Bromine vapour  56  is introduced into the oven  21  by gas feed  58  that includes an adjustable regulator  60  such as a mass flow controller. A bottle  62  of liquid bromine  64  is connected to another end of the gas feed  58  through a valve  66 . 
     A second line  70  is provided to merge with gas feed  58 . This line  70  is connected to an argon gas supply  72  via valve  74 . The argon gas supply  72  allows the ion source  14  to be purged with argon (or with any other suitable inert gas). This gas may also be used to maintain an arc within the arc chamber  16  when the bromine supply  56  is inhibited. 
     Thus, the ion source  14  operates as follows. Bottle  62  contains liquid bromine  64  as well as bromine vapour  56  due to its high vapour pressure. Valve  66  is left open so that bromine vapour  56  passes along gas feed  58  to regulator  60 . The regulator  60  is set to allow only a desired flow of bromine vapour  56  onwards to the oven  21 . 
     The bromine vapour  56  drifts into oven  21 , where it may contact the indium  54  and react to form gaseous indium bromide  68 . The oven  21  is operated to maintain a temperature of around 380° C. As mentioned previously, the present invention advantageously does not require careful control of the temperature of the oven  21 . This is because the bromine/indium reaction does not show great sensitivity to temperature and so the temperature of the oven  21  may drift without the flow rate of indium bromine  68  into the arc chamber  16  being adversely affected. 
     Gas from the oven  21 , including both bromine vapour  56  and gaseous indium bromide  68 , drifts along gas supply  22  into arc chamber  16  where it may be ionised before subsequent extraction through aperture  28 . The cathode  20  and counter-cathode  44  are not shown in the arc chamber  16  of  FIG. 2  for the sake of clarity. 
     The regulator  60  is set to allow a flow of bromine vapour  56  such that only enough indium bromide  68  collects in the arc chamber  16  to meet the required indium ion beam current (a “bromine starved” mode of operation). Put another way, the arc chamber  16  is not flooded to create a reservoir of indium bromide/indium ions ready for extraction. Operation of the regulator  60  to obtain the correct rate of flow can be found empirically or may be performed automatically using a negative feedback loop from an ion beam current monitor or the like. 
       FIG. 3  shows in greater detail an example of an oven  21  having a body  75  of generally cylindrical shape. The oven  21  is mounted to the arc chamber  16  via a flange  79  such that the gas supply  22  projects into the arc chamber  16 . The body  75  is hollow with a central wall  77  that partly defines the oven  21 . The body  75  also defines a stand-off chamber  76 . 
     Oven  21  contains the strip of indium  54  and has an inlet corresponding to the gas feed  58 . The gas feed  58  enters the oven  21  opposite the indium  54  so that bromine vapour  56  passes over the indium  54  after exiting the gas feed  58 . A spiral groove  80  is provided in the unitary body around the oven  21  that receives the heating element  50  (not shown in  FIG. 3  for the sake of clarity). Thus, the heating element  50  heats the oven  21  when a current is passed therethrough. 
     EXAMPLE 
     By way of example,  FIG. 4  shows the results of an experiment conducted using an ion source  14  exemplified by  FIGS. 2 and 3 . In the experiment, 100 g of bromine liquid  64  was placed in a stainless steel container  62 . The flow of bromine vapour  56  was regulated at 0.24 sccm, which has been found to optimise the production of In ++  ions. 
     The oven  21  contained 8 g of indium metal strip  54  and was heated to 380° C. The arc chamber  16  was operated to arc at 75V/1A. An extraction current of 12 mA was used with an extraction voltage of 50 kV. This extraction voltage is generally low, but allowed the observation of In +  ions. These conditions produced an In ++  beam current of 1 mA, as shown in  FIG. 4  that shows the beam currents of the different ions extracted from the arc chamber  16 . During implantation, the bromine ions and undesired indium ions may be rejected using the mass analysing magnet  30 . 
     In addition, the effect of varying the oven temperature on a 1 mA In ++  beam was investigated. An initial temperature of 400° C. was stepped down to 380° C., 360° C. and then 340° C. before being stepped back up to 400° C. and beyond to 420° C., 440° C. and 460° C. Only at 340° C. was the ion beam current observed to change (it decreased): at all other temperatures the ion beam current remained steady thus showing the process to be insensitive to temperature over a wide range. As noted before, this is particularly beneficial as it removes the requirement for a high degree of control over the oven temperature. 
     Finally, the response times following operation of the regulator  60  to switch off, then switch on the bromine vapour  56  flow was tested. The regulator  60  was shut rapidly and the In ++  beam was seen to extinguish in less than 30 seconds. After closing the regulator  60 , the argon supply  70  was turned on to purge the ion source  14  and keep the arc going. The argon was then switched off and the regulator  60  turned back on rapidly. The In ++  beam was seen to re-establish within 30 seconds. 
     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. For example, the oven  21  of  FIG. 3  is but merely one design that may be used. The length of the gas supply  22  of  FIG. 3  is advantageously short to achieve quicker response times when the regulator  60  is adjusted. 
     While it is convenient merely to use liquid bromine  64  from which bromine vapour  56  is collected, other arrangements are possible. For example, the liquid bromine  64  may be warmed or a supply of gaseous bromine derived from any other source may be provided. 
     Of course, how the indium  54  or other source material is heated is immaterial. In fact, heating can be omitted altogether although this will result in reduced ion beam currents. 
     The foregoing embodiments show the source material reactant  54  to be located in an oven  21  that is separate from the arc chamber  16 : this need not be the case. The reactant  54  need not be located in an oven  21  when heating is not required and, irrespective of that consideration, the reactant  54  may be located within the arc chamber  16  itself. In this latter arrangement the gas feed  58  and line  70  used to provide bromine vapour  56  and argon respectively in the foregoing embodiments may be connected directly to the arc chamber  16 .