Abstract:
A method for improving the ion beam quality in an ion implanter is disclosed. In some ion implantation systems, contaminants from the ion source are extracted with the desired ions, introducing contaminants to the workpiece. These contaminants may be impurities in the ion source chamber. This problem is exacerbated when mass analysis of the extracted ion beam is not performed, and is further exaggerated when the desired feedgas includes a halogen. 
     The introduction of a diluent gas in the ion chamber may reduce the deleterious effects of the halogen on the inner surfaces of the chamber, reducing contaminants in the extracted ion beam. In some embodiments, the diluent gas may be germane or silane.

Description:
[0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 61/847,776, filed Jul. 18, 2013, the disclosure of which is incorporated by reference. 
     
    
     FIELD 
       [0002]    Embodiments of the present disclosure relate to methods for improving ion beam quality in an ion implantation system, and more particularly, improving boron ion beam quality. 
       BACKGROUND 
       [0003]    Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. For example, solar cells may be implanted with a dopant species to create an emitter region. This implant may be done using a variety of different mechanisms. In one embodiment, an ion source is used. This ion source may include a chamber in which source gasses are ionized. The ions from these source gasses may be extracted through an aperture in the chamber, using one or more electrodes. These extracted ions are directed toward a workpiece, where they are implanted in the workpiece to form the solar cell. 
         [0004]    In an effort to improve process efficiency and lower cost, in some embodiments, the ions extracted from the ion source are accelerated directly toward the workpiece, without any mass analysis. In other words, the ions that are generated in the ion source are accelerated and implanted directly into the workpiece. A mass analyzer is used to remove undesired species from the ion beam. Removal of the mass analyzer implies that all ions extracted from the ion source will be implanted in the workpiece. Consequently, undesired ions, which may also be generated within the ion source, are then implanted in the workpiece. 
         [0005]    This phenomenon may be most pronounced when the source gas is a halogen-based compound, such as a fluoride. Fluorine ions and neutrals (metastable or excited) may react with the inner surfaces of the ion source, releasing unwanted ions, such as silicon, oxygen, carbon, and aluminum and heavy metals present as impurity elements. 
         [0006]    Therefore, a method which improves beam quality, particular for embodiments in which halogen based source gasses are employed, would be beneficial. 
       SUMMARY 
       [0007]    A method for improving the ion beam quality in an ion implanter is disclosed. In some ion implantation systems, contaminants from the ion source are extracted with the desired ions, introducing contaminants to the workpiece. These contaminants may be impurities in the ion source chamber. This problem is exacerbated when mass analysis of the extracted ion beam is not performed, and is further exaggerated when the desired feedgas includes a halogen. The introduction of a diluent gas in the ion chamber may reduce the deleterious effects of the halogen on the inner surfaces of the chamber, reducing contaminants in the extracted ion beam. In some embodiments, the diluent gas may be germane or silane. 
         [0008]    In one embodiment, a method of implanting dopant into a workpiece is disclosed. The method comprises introducing a feedgas and a diluent gas into a chamber of an ion source, the feedgas comprising molecules comprising a dopant and fluoride, wherein the dopant comprises a Group 3 or Group 5 element, and the diluent gas comprises molecules comprising hydrogen and a Group 4 element or molecules comprising hydrogen and a species having an opposite conductivity as the dopant, where between 3% and 40% of a total volume of gas introduced comprises the diluent gas and a remainder of the total volume comprises the feedgas; ionizing the feedgas and the diluent gas in the chamber; and extracting ions from the chamber and accelerating the ions toward the workpiece. 
         [0009]    In another embodiment, a method of implanting boron into a workpiece is disclosed. The method comprises introducing a feedgas and a diluent gas into a chamber of an ion source, the feedgas comprising molecules containing boron and fluorine, the diluent gas comprising molecules containing hydrogen and a Group V, where between 3% and 20% of a total volume of gas introduced comprises diluent gas and a remainder of the total volume comprises feedgas; ionizing the feedgas and the diluent gas in the chamber; and extracting ions from the chamber and accelerating the ions toward the workpiece, wherein the extracted ions are not mass analyzed before implanting into the workpiece. 
         [0010]    In another embodiment, a method of implanting boron into a workpiece is disclosed. The method comprises introducing a feedgas and a diluent gas into a chamber of an ion source, the feedgas comprising molecules containing boron and fluorine, the diluent gas comprising molecules containing hydrogen and germanium, where between 10% and 20% of a total volume of gas introduced comprises diluent gas and a remainder of the total volume comprises feedgas; ionizing the feedgas and the diluent gas in the chamber; and extracting ions from the chamber and accelerating the ions toward the workpiece, wherein the extracted ions are not mass analyzed before implanting into the workpiece. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0012]      FIGS. 1A-C  show the implant system according to different embodiments; and 
           [0013]      FIG. 2  is a representative graph of dopant current and contaminant level as a function of diluent gas concentration. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    As described above, ionization of halogen-based species, such as fluorides, may cause particles released from the inner surfaces of the ion source to be implanted in the workpiece. These contaminants may include aluminum, carbon, oxygen, silicon, fluorine-based compounds, and other unwanted species (including heavy metals present as impurity elements). One approach to address the damage caused by free halogen ions may be to introduce a second source gas. 
         [0015]      FIGS. 1A-1C  show various embodiments in which a second source gas may be introduced to a chamber  105  of an ion source  100 . In each of these figures, the ion source  100  includes a chamber  105  defined by several walls  107 , which may be constructed from graphite or another suitable material. This chamber  105  may be supplied with one or more source gasses, stored in a source gas container  170 , via a gas inlet  110 . This source gas may be energized by an RF antenna  120  or another mechanism. The RF antenna  120  is in electrical communication with a RF power supply (not shown) which supplies power to the RF antenna  120 . A dielectric window  125 , such as a quartz or alumina window, may be disposed between the RF antenna  120  and the interior of the ion source  100 . The ion source  100  also includes an aperture  140  through which ions may pass. A negative voltage is applied to extraction suppression electrode  130  disposed outside the aperture  140  to extract the positively charged ions from within the chamber  105  through the aperture  140  and toward the workpiece  160 . A ground electrode  150  may also be employed. In some embodiments, the aperture  140  is located on the side of the ion source  100  opposite the side containing the dielectric window  125 . Ions extracted from the chamber  105  are formed into an ion beam  180 , which is directed toward the workpiece  160 . As described above, no mass analyzer is used to filter the ions before they strike the workpiece  160 . In one particular embodiment, shown in  FIG. 1A , the second source gas may be stored in a second gas container  175  and introduced to the chamber  105  through a second gas inlet  111 . In another embodiment, shown in  FIG. 1B , the second source gas may be stored in a second gas container  176  and introduced to the chamber  105  through the same gas inlet  110  used by the first source gas. In yet another embodiment, shown in  FIG. 1C , the second source gas may be mixed with the first source gas in a single gas container  178 . This mixture of gasses is then introduced to the chamber  105  through gas inlet  110 . 
         [0016]    In any of these embodiments, the first source gas and the second source gas may be introduced simultaneously or sequentially to the chamber  105 . 
         [0017]    The first source gas, also referred to as the feed gas, may comprise a dopant, such as boron, in combination with fluorine. Thus, the feed gas may be in the form of DF n  or D m F n , where D represents the dopant atom, which may be boron, gallium, phosphorus, arsenic or another Group 3 or Group 5 element. The second source gas, also referred to as the diluent gas, may be a molecule having a chemical formula of XH n or  X m H n , where H is hydrogen. X may be a dopant species, such as any of those described above. Alternatively, X may also be an atom that does not affect conductivity of the workpiece  160 . For example, if the workpiece  160  comprises silicon, X may be a Group 4 element, such as silicon and germanium. 
         [0018]    In other words, the feedgas may be BF 3  or B 2 F 4 , while the diluent gas may be, for example, PH 3 , SiH 4 , NH 3 , GeH 4 , B 2 H 6 , or AsH 3 . This list represents some possible species that may be used. It is understood that other feedgas species and diluent species are also possible. 
         [0019]    By combining the feedgas with the diluent gas, the deleterious effects of the fluorine ions may be reduced. For example, without being limited to any particular theory, the introduction of hydrogen may create a film or coating on the dielectric window  125 . This serves to protect the dielectric window  125 , which reduces the amount of contaminants originating from the dielectric window  125  that are contained in the extracted ion beam  180 . In addition, the diluent gas may coat the inner surfaces of the chamber walls  107 , which may be another source of contaminants. This coating may reduce the interaction between fluorine ions and the inner surfaces of the chamber walls  107 , reducing the amount of contaminants generated. 
         [0020]    The introduction of a diluent gas may reduce the creation of contaminants and the incorporation of these contaminants in the ion beam. Conversely, the introduction of a large amount of diluent gas may negatively impact the production of dopant ions to be used in the ion beam. For example, the introduction of an excessive amount of diluent gas may reduce the dopant beam current created by the ion source. 
         [0021]    Unexpectedly, it has been determined that contaminant reduction does not decrease proportionally with increased diluent concentration. In other words, if the amount of diluent is increased above a certain threshold, the ratio of contaminant current to dopant current actually increases. This may be caused by the fact that above a certain threshold, the additional coating on the inner surfaces of the chamber walls  107  provides little or no additional protection against fluorine ions. Additionally, plasma parameters, such as high plasma potential, will be changed with high diluent gas percentage, which may cause additional sputtering of the chamber walls  107  by diluent gas ions. The additional sputtering of the chamber walls  107  may cause increased contamination level. Thus, if the dopant current decreases as a function of diluent concentration, and contaminant concentration remains constant or increases after a certain threshold, the percentage of contaminant in the ion beam necessarily increases. 
         [0022]      FIG. 2  shows a representative graph, showing the effects of diluent gas concentration on both dopant beam current and the percentage of contaminant as compared to dopant in the ion beam. As described above, the contaminant may be an ion species containing silicon, oxygen, hydrogen, aluminum, carbon, carbon-based compounds, fluorine, fluorine-based compounds, or other non-dopant species. 
         [0023]    As can be seen in  FIG. 2 , the dopant current, which is represented by the bar graph, is greatest when no diluent is present. There is a nearly linear reduction in dopant current as the concentration of diluent gas, which in this example is GeH 4 , is increased. Although this graph shows a particular relationship between dopant current and diluent concentration, it is noted that this relationship may be specific to the test conditions used. For example, a different diluent gas, a different RF power level, or a different pressure (or flow rate) inside the plasma chamber may create different results. Thus, this bar graph is intended to represent a general trend between dopant current and diluent concentration. 
         [0024]    Line  300  shows a measure of beam impurity, defined as the percentage of contaminant compared to dopant in the ion beam, where the contaminant may be one or more of the species identified above. As expected, the beam impurity decreases as the diluent concentration is increased from 0% to 10%. As stated above, this may be due to the coating action of the hydrogen in the diluent gas. It may be possible that the other species in the diluent gas may affect the coating action. For example, in the case of GeH 4 , hydrogen molecules are light, therefore may pumped out quickly. However, GeH 4  is a heavy molecule with hydrogen attached, therefore may have long transit time and high possibility to react with chamber surfaces and coat these surfaces. 
         [0025]    However, unexpectedly, the beam impurity stays relatively flat until the diluent concentration reaches about 30%. In other words, despite the introduction of more diluent gas, the amount of the contaminant relative to the amount of dopant remains relatively constant. Throughout the range from about 5% to 30%, the beam impurity is less than about 1%. Surprisingly, if the diluent concentration is increased beyond about 30%, the beam impurity increases rather dramatically, reaching levels in excess of 5% if the gas mixture is 60% diluent gas. Beam impurity may be minimized when the concentration of diluent gas is between 5% and 30%. 
         [0026]      FIGS. 1A-1C  utilize an ion source having a RF antenna  120  and an RF power supply to generate the necessary ions. However, it can be appreciated that other ion sources may be used. For example, an indirectly heated cathode (IHC) which uses heat to cause thermionic emission of electrons may also be used in some embodiments. Other ion sources are also within the scope of the disclosure. 
         [0027]    Thus, an extracted ion beam  180  having reduced beam impurity can be created by using two source gasses. The first source gas, or feedgas, may be a species than contains both boron and fluorine, such as BF 3  or B 2 F 4 . The second source gas, or diluent, may be a species that contains hydrogen and either silicon or germanium, such as silane (SiH 4 ) or germane (GeH 4 ). These two source gasses are introduced into a chamber  105  of an ion source  100 , either simultaneously or sequentially, where they are ionized. The ion source may use RF energy generated by RF antenna  120 . In another embodiment, the ion source may utilize the thermionic emission of electrons using an IHC. Other methods of ionizing a gas may also be used by the ion source. These two source gasses may be introduced such that between 5%-30% of the total gas (by volume) is diluent gas, while the remainder is feedgas. Ions from both source gasses are extracted through aperture  140  through use of electrodes  130 ,  150  and accelerated toward a workpiece  160 , where they are implanted into the workpiece  160 . As described earlier, these ions may not be mass analyzed, meaning that all extracted ions are implanted into the workpiece  160 . 
         [0028]    In another example, the diluent gas may include a dopant having the opposite conductivity. For example, the first source gas, or feedgas, may be a species than contains both boron and fluorine, such as BF 3  or B 2 F 4 . The second source gas, or diluent, may be a species that contains hydrogen and a Group V element, such as phosphorus, nitrogen or arsenic. 
         [0029]    The above disclosure discusses diluent ranges between about 5%-30% of the total volume of gas. However, in some embodiments, this range may different. For example, in some embodiments, the diluent concentration may be up to 10%, such as 1-10%, 2-10%, 3-10% or 5-10%. In another embodiment, the diluent concentration may be up to 15%, such as 1-15%, 2-15%, 3-15%, 5-15%, or 7-15%. In another embodiment, the diluent concentration may be up to 20%, such as 2-20%, 3-20%, 5-20%, 7-20%, or 10-20%. In another embodiment, the diluent concentration may be up to 30%, such as 3-30%, 5-30%, 7-30%, 10-30% or 15-30%. In another embodiment, the diluent concentration may be up to 40%, such as 3-40%, 5-40%, 7-40%, 10-40%, 15-40% or 20-40%. In another embodiment, the diluent concentration may be up to 50%, such as 5-50%, 7-50%, 10-50%, 15-50%, 20-50%, or 25-50%. Finally, in another embodiment, the diluent concentration may be as high as 60%, such as 5-60%, 7-60%, 10-60%, 15-60%, 20-60%, 25-60% or 30-60%. 
         [0030]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.