Patent Publication Number: US-2013240753-A1

Title: Ion Source and Ion Implanter Including the Same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2012-0026202, filed on Mar. 14, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     The inventive concept relates to an ion source and an ion implanter including the ion source, and more particularly, to an ion source including a cathode for emitting electrons for ionization and an ion implanter including the ion source. 
     Generally, an ion implantation method is used to add dopants to a semiconductor wafer. The principle of the ion implantation method is that an ion implanter accelerates ions and then implants the accelerated ions, in the form of beam, into a semiconductor wafer on which an ion implantation mask has been formed. Normally, the amount of dopants to be implanted is determined according to the size of dopant atoms, an acceleration speed of ions, and a time during which a semiconductor wafer is exposed to an ion implantation beam. Typical, ion implantation processes that are mainly used in semiconductor device manufacturing processes include an impurity implantation process for forming a diode and source/drain regions in a substrate, an impurity implantation process for providing conductivity when depositing polysilicon to form a gate electrode, and an impurity implantation process for increasing a threshold voltage. 
     SUMMARY 
     Some embodiments of the present invention provide an ion source including a filament configured to emit thermoelectrons and a cathode having a first side proximate the filament and a second side opposite the first side. The cathode includes a first layer that includes a first material on the first side of the cathode and a second layer on the second side of the cathode. The first layer is between the filament and the second side. The second layer is configured to limit discharge of the first material of the first layer from the ion source when the filament emits themoelectrons to generate ions from the ion source. 
     In further embodiments, the first layer is heated by the thermoelectrons that are emitted from the filament and the second layer emits secondary electrons responsive to energy transmitted from the first layer when the first layer is heated by the thermoelectrons. 
     In other embodiments, the second layer includes a second material that is different from the first material. The first material may be a metal and the second material may be a nonmetallic material. The second material may include at least one of carbon, black lead, graphite and a high melting point material. The second material may include at least one of carbon, black lead and graphite. 
     In further embodiments, the second material is a nonmetallic material that has a melting point of at least 3000° C. The first material may be a high melting point metal that has a melting point of at least 2000° C. 
     In other embodiments, an ionization energy of the second material is larger than an ionization energy of the first material. 
     In other embodiments, the second material is a material that generates a shallow-level trap in silicon compared to a trap generated by the first material. The second material may generate a trap within 0.45 eV from a valence band or a conduction band in silicon. The second material may generate a trap within 0.25 eV from a valence band or a conduction band in the silicon. 
     In yet further embodiments, the second layer has a thickness in a range of about 0.1 mm to about 1 mm. The cathode may include a supporting unit that supports the first layer and the second layer. 
     In other embodiments, the ion source further includes an arc chamber that defines an ionizing space. The arc chamber includes a first opening configured to be connected to a gas supplying unit and a second opening configured to extract ions generated by the ion source. The cathode is disposed between the filament and an internal area of the arc chamber at an end of the arc chamber. The second layer is positioned between the cathode and the internal area of the arc chamber and the first layer is positioned between the cathode and the second layer. The second layer may cover an upper side and lateral sides of the first layer so that the first layer is not exposed to the internal area of the arc chamber. 
     In further embodiments, an ion implanter includes an ion source as described above and further includes a mass analyzer configured to sort the ions generated by the ion source to provide a sorted ion beam. The ion implanter further includes an ion transmitter configured to accelerate the sorted ion beam and an end station configured to hold a substrate in a location where ions in the sorted ion beam are implanted into the substrate when the sorted ion beam is generated by the ion transmitter. 
     In yet other embodiments, an ion source includes an electron emission unit for emitting thermoelectrons and a cathode. The cathode includes a nonmetallic secondary electron emission unit that is heated by the thermoelectrons emitted from the electron emission unit and emits secondary electrons. The ion source may include a conductive intermediate unit that is disposed between the electron emission unit and the nonmetallic secondary electron emission unit and transmits energy obtained from the thermoelectrons emitted by the electron emission unit to the nonmetallic secondary electron emission unit. 
     In further embodiments, an ion source includes a cathode and a filament for emitting thermoelectrons. The cathode includes a first layer, which is heated by the thermoelectrons that are emitted from the filament, and a second layer of which at least a portion is adjacent to the first layer and which emits secondary electrons by using energy transmitted from the first layer and prevents a discharge of a material of the first layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the inventive concept will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic cross-sectional view illustrating an ion implanter according to some embodiments of the inventive concept; 
         FIG. 2  is a schematic cross-sectional view illustrating an ion source according to some embodiments of the inventive concept; 
         FIGS. 3A through 3D  are schematic cross-sectional views illustrating cathodes according to some embodiments of the inventive concept; 
         FIG. 4  is a diagram illustrating energy levels of impurities in silicon; 
         FIG. 5  is a schematic cross-sectional view of an image sensor manufactured by using the ion implanter of  FIG. 1 ; and 
         FIG. 6  is a graph illustrating a characteristic of an image sensor manufactured by using the ion implanter of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Advantages and features of the present inventive concept and methods of accomplishing the same may be understood more readily by reference to the following detailed description of preferred embodiments and the accompanying drawings. The present inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art, and the present inventive concept will only be defined by the appended claims. 
     The terms used in the present specification are merely used to describe particular embodiments, and are not intended to limit the inventive concept. An expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. In the present specification, it is to be understood that the terms such as “including,” “comprises/comprising” or “having,” etc., are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, numbers, steps, actions, components, parts, or combinations thereof may exist or may be added. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to” or “coupled to” another element or layer, it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on”, “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Other words used to describe the relationship between elements or layers should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” “on” versus “directly on”). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present inventive concept. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Embodiments are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, these embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. Thus, embodiments of the present inventive concept should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, right-angled etched regions may actually show rounded regions or regions having predetermined radii of curvatures. Thus, the regions illustrated in the figures are conceptual in nature and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the present inventive concept. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Like numbers refer to like elements throughout this specification. Thus, the same or similar numbers may be described with reference to other drawings even if they are neither mentioned nor described in the corresponding drawing. Also, even elements that are not denoted by reference numbers may be described with reference to other drawings. 
       FIG. 1  is a schematic cross-sectional view illustrating an ion implanter  100  according to some embodiments of the inventive concept. 
     Referring to  FIG. 1 , the ion implanter  100  includes an ion source  10 , a mass analyzer  20 , an ion transmitter  30 , and an end station  40 . Ions that are provided from the ion source  10 , which is an ion generator, may be sorted by the mass analyzer  20 , and the sorted ions may be transmitted to the end station  40  by the ion transmitter  30  to implant them in a specific portion of a semiconductor substrate, such as a silicon wafer. A transmission path of the ions is illustrated by a dashed line in  FIG. 1 . 
     A source gas is supplied to the ion source  10 , and ions may be generated by ionizing the supplied source gas. Electrons may be emitted from a filament of the ion source  10  by letting a high current pass through the filament and the emitted electrons may be transmitted to a cathode to generate secondary electrons. The secondary electrons that are generated in the cathode may generate ions by colliding source gas molecules. The cathode may have a double structure including a first layer and a second layer, as will be further described with reference to  FIG. 2  below. Ions generated from the ion source  10  may be extracted by an ion extractor  15 , and the extracted ions may have the form of an ion beam. The ion extractor  15  may include a polarity converter for converting a polarity of the extracted ions. 
     The mass analyzer  20  may sort predetermined ions to be implanted into a substrate from among ions constituting the ion beam by using differences in masses of ions. The mass analyzer  20  may include, for example, an analyzer magnet having a 90° bend angle and a magnetic field B, that is formed by the analyzer magnet, provides power for deflecting an ion beam, which includes impurities and various kinds of positive ions, to form a curve orbit. With respect to the intensity of the magnetic field B, ions having comparatively heavy mass may be deflected with a small angle compared to ions having a comparatively light mass. A curvature radius of such a curve orbit may be determined according to the intensity of the magnetic field B of the analyzer magnet. Thus, by adjusting the intensity of the magnetic field B of the analyzer magnet, ions having a desired mass may preferentially be selected to reach the ion transmitter  30  located in an end of the mass analyzer  20 . 
     The ion transmitter  30  may include a quadrupole type lens  32  for focusing an ion beam  32  and an accelerator  34  for accelerating ions. Ions may have higher motive energy when a movement thereof becomes faster and may be more deeply implanted into a substrate when colliding with the surface of the substrate. The accelerator  34  may form an electric field that adds additional energy to positive ions. The positive ions are attracted to an electric field having a negative polarity when the positive ions have positive charges. An accelerating voltage is applied to form an electric field having a negative polarity, and the positive ions move faster if the intensity of the electric field is increased. On the other hand, ions sorted from the mass analyzer  20  may be decelerated if the polarity of the electric field is changed by applying a decelerating voltage. 
     In the end station  40 , an accelerated ion beam is guided to a substrate. The end station  40  includes a supporting unit  42  for supporting one or more substrates and a driving unit  44  for rotating or moving the supporting unit  42 . The one or more substrates are loaded on the supporting unit  42 , and the supporting unit  42  may be rotated at high speed so that ions may be more uniformly implanted in a substrate. 
     The ion implanter  100  may include one or more vacuum pumps. In addition, the ion implanter  100  may further include a Faraday cup system for measuring an ion current of an ion beam and the amount of ions that are implanted into a substrate may be calculated by the Faraday cup system. Although  FIG. 1  schematically illustrates only some components that may be included in the ion implanter  100 , the inventive concept is not limited thereto. 
       FIG. 2  is a schematic cross-sectional view illustrating an ion source  1000  according to some embodiments of the inventive concept.  FIG. 2  may correspond to a portion A of the ion source  10  of  FIG. 1 . Referring to  FIG. 2 , the ion source  1000  may include an arc chamber  110 , a filament  120 , a cathode  130  and a repeller  140 . Electrons emitted from the filament  120  may transmit energy to the cathode  130 , and thus, secondary electrons may be emitted from the cathode  130 . The emitted secondary electrons quickly move toward the repeller  140 , and may ionize a source gas by colliding with the source gas supplied to the arc chamber  110 . 
     The arc chamber  110  may limit a space for ionizing the source gas. The arc chamber  110  may include a plurality of side walls to limit the space, and plasma may be formed therein. A first opening  112 , that is connected to a gas supplying unit from which a source gas is supplied, and a second opening  114 , through which generated ions are extracted, may be disposed at a first side wall  110   a  and a second side wall  110   b , respectively, that are opposite each other in a certain direction, for example, a Y-direction, from among the plurality of side walls. The cathode  130  and the repeller  140  may be disposed adjacent to a third side wall  110   c  and a fourth side wall  110   d , respectively, that are opposite each other in another particular direction, for example, a X-direction, from among the plurality of side walls. 
     A vacuum pump may be connected to the arc chamber  110  and, thus, a vacuum state thereof may be maintained. In addition, a predetermined positive voltage may be applied to the arc chamber  110  to improve ionization efficiency. In addition, the activity of electrons may be accelerated by applying an electric field to the arc chamber  110 . 
     The filament  120  may be heated by a current applied from a power supply and then may emit thermoelectrons. The filament  120  may include nichrome or tungsten. The emitted electrons may be transmitted to the cathode  130  and, thus, the cathode may emit secondary electrons. 
     The cathode  130  may include a cathode cap  130   a  and a cathode supporting unit  130   b . The cathode cap  130   a  may include a first layer  132  and a second layer  134 . The first layer  132  may be heated by colliding with electrons that are emitted from the filament  120  and the second layer  134  may emit secondary electrons, which are generated by energy received from the first layer  132 , toward the repeller  140 . The cathode  130  in some embodiments may have a cylindrical shape or a square pillar shape. 
     The material of the first layer  132  may be different from that of the second layer  134 . The first layer  132  may include a conductive material, for example, a high melting point metal such as tungsten (W) or molybdenum (Mo) that has a melting point of 2000° C. or more. The second layer  134  may include a nonmetallic material, for example, a nonmetallic material that has a melting point of 3000° C. or more. In addition, the second layer  134  may include, for example, a high melting point material that has a melting point of 3000° C. or more. 
     In the above ionization process of the source gas, a phenomenon in which a portion of a material constituting the cathode  130  is also ionized together with the source gas may occur. In this case, an ionized cathode material may be combined with an ionized source gas material, and then may be implanted into a substrate together with the ionized source gas material. The ionized cathode material implanted into the substrate may act as impurities that are not desired and, thus, may affect electrical characteristics of a device that is manufactured by using the substrate. In particular, if a high melting point metal is implanted together with the ionized source gas material into the substrate, a defect may be caused in the device by forming a deep-level trap in a semiconductor substrate, such as a silicon substrate. However, according to some embodiments of the inventive concept, metal ions may be limited or even prevented from being implanted into a substrate as the nonmetallic second layer  134  is disposed at the outer wall of the cathode  130 . 
     In addition, the second layer  134  may include a material that is not well combined with the ionized source gas. That is, the material of the second layer  134  may be a material of which a possibility to be implanted into the substrate is relatively low compared to the material of the first material  132 . In particular, the material of the second layer  134  may be a material of which an ionized energy is larger than that of the material of the first layer  132 . 
     For example, if the material of the cathode  130  is tungsten (W) and the source gas is boron trifluoride (BF 3 ), a portion of the tungsten (W) is ionized and then is combined with positive source gas ions, and thus, positive ions such as tungsten fluoride (WF x   + ) may be formed. WF x   +  ions may be accelerated together with other positive ions of BF 3 , and then may be implanted into the substrate as stated above with reference to  FIG. 1 . However, the second layer  134  may include a material that has less tendency to form positive ions combined with the positive source gas ions than that of the first layer  132 . That is, an ionization tendency in the material of the second layer  134  is relatively low, and a tendency to combine with the positive source gas ions may be relatively low even when the material of the second layer  134  is ionized. 
     In addition, the second layer  134  may include a material that generates a shallow-level trap in silicon (Si) compared to the material of the first layer  132 . For example, the second layer may include a material that generates a trap within 0.45 eV from a valence band or a conduction band in silicon (Si). In addition, the second layer  134  may include a material that generates a trap within 0.25 eV from the valence band or the conduction band in silicon (Si). Thus, although the material of the second layer  134  is implanted into the substrate, defects that are generated in a device manufactured by using the substrate may be decreased. With respect to this, a detailed description will be given with reference to  FIG. 4  below. The second layer  134  may include, for example, carbon (C), black lead, or graphite. In addition, the second layer  134  may include titanium (Ti) and an oxide thereof. 
     The first layer  132  may have a first thickness T 1 , and the second layer  134  may have a second thickness T 2  that is smaller than the first thickness T 1 . For example, the second thickness T 2  may be in the range of about 0.1 mm to about 1 mm. A mechanical stability may be lowered if the second thickness T 2  is relatively small, and efficiency in generating secondary electrons may be lowered if the second thickness T 2  is relatively large. 
     The cathode supporting unit  130   b  may include a material having enhanced processability, and, for example, may include molybdenum (Mo). 
     The repeller  140  may improve ionization efficiency by repelling electrons emitted into the arc chamber  110 . The repeller  140  may have a negative polarity and, thus, electrons may be repelled in the arc chamber  110  by the repeller  140 , thereby improving the ionization efficiency. 
       FIGS. 3A through 3D  are cross-sectional views illustrating cathodes according to embodiments of the inventive concept. 
     Referring to  FIG. 3A , a cathode  230  may include a cathode cap  230   a  and a cathode supporting unit  230   b . The cathode cap  230   a  may include a first layer  232  and a second layer  234 . The cathode cap  230   a  may correspond to the cathode cap  130   a  of  FIG. 2 , and the cathode supporting unit  230   b  may correspond to the cathode supporting unit  130   b  of  FIG. 2 . Thus, a description thereof will not be repeated here. 
     In the embodiments of  FIG. 3A , unlike the embodiments of  FIG. 2 , the cathode supporting unit  230   b  may extend to cover lateral sides of the cathode cap  230   a . In modified embodiments, the cathode supporting unit  230   b  may extend to cover only portions of the lateral sides of the cathode cap  230   a . In the embodiments of  FIG. 3A , as the second layer  234  is disposed on the upper side of the first layer  232 , a deep-level trap may be effectively prevented from being formed due to implantation of metal ions generated from a high melting point metal of the first layer  232  into a substrate. 
     Referring to  FIG. 3B , a cathode  330  may include a cathode cap  330   a  and a cathode supporting unit  330   b . The cathode cap  330   a  may be formed of a single layer. The cathode cap  330   a  may emit secondary electrons by receiving energy generated through collisions with electrons that are emitted from the filament  120  of  FIG. 2 . 
     The cathode cap  330   a  may include a material that generates a shallow-level trap in silicon (Si) compared to the material of the first layer  232 . For example, the cathode cap  330   a  may include carbon (C), black lead, or graphite. In addition, the cathode cap  330   a  may include titanium (Ti) and oxide thereof. The cathode cap  330   a  may have a third thickness T 3 . The third thickness T 3  may be equal to or larger than the second thickness T 2  illustrated in  FIG. 2 . A mechanical stability may be lowered if the third thickness T 3  is relatively small, and efficiency in generating secondary electrons may be lowered if the third thickness T 3  is relatively large. 
     The cathode supporting unit  330   b  may include a material having enhanced processability and, for example, may include molybdenum (Mo). 
     In the embodiments of  FIG. 3B , as the cathode cap  330   a  does not include a high melting point metal, a deep-level trap may be effectively prevented from being formed due to implantation of metal ions generated from the cathode cap  330   a  into a substrate. 
     Referring to  FIG. 3C , a cathode  430  may include a first layer  432  and a second layer  434 . The first layer  432  may correspond to the first layer  132  of  FIG. 2 , and the second layer  434  may correspond to the second layer  134  of  FIG. 2 . Thus, a description thereof will not be repeated here. 
     The cathode  430  according to the embodiments of  FIG. 3C , unlike the embodiments of  FIG. 2 , may have a structure in which the cathode supporting unit  130   b  and the cathode cap  130   a  of  FIG. 2  are unified. In the embodiments of  FIG. 3C , as the second layer  434  is disposed on the upper side of the first layer  432 , a deep-level trap may be effectively prevented from being formed due to an implantation of metal ions generated from a high melting point metal of the first layer  432  into a substrate. 
     Referring to  FIG. 3D , a cathode  530  may include a first layer  532  and a second layer  534 . The first layer  532  may correspond to the first layer  132  of  FIG. 2 , and the second layer  534  may correspond to the second layer  134  of  FIG. 2 . Thus, a description thereof will not be repeated here. 
     The cathode  530  according to the embodiments of  FIG. 3D , unlike the embodiments of  FIG. 2 , may have a structure in which the cathode supporting unit  130   b  and the cathode cap  130   a  of  FIG. 2  are unified. In addition, unlike the embodiments of  FIG. 3C , the second layer  534  may be disposed to surround lateral sides of the first layer  532  as well as the upper side of the first layer  532 . 
     In the embodiments of  FIG. 3D , as the second layer  534  is disposed on the upper side and the lateral sides of the first layer  532 , a deep-level trap may be effectively prevented from being formed due to an implantation of metal ions generated from a high melting point metal of the first layer  532  into a substrate. 
       FIG. 4  is a diagram illustrating energy levels of impurities in silicon. Referring to  FIG. 4 , a conduction band Ec and a valance band By of the silicon are shown. The silicon has a bandgap energy of about 1.12 eV, and the center of the bandgap energy is indicated by a dotted line. Energy levels of impurity atoms in a solid may be determined by interaction due to formation of a hybrid orbit between an orbit of an impurity atom and orbits of adjacent atoms that are adjacent to the impurity atom. 
     The upper portion on the basis of the center of the bandgap energy corresponds to donor levels, and energy differences from the conduction band Ec are indicated in the upper portion. The lower portion on the basis of the center of the bandgap energy corresponds to acceptor levels except for the case that is shown by “D” indicating a donor level, and energy differences from the valence band Ev are indicated in the lower portion. 
     As illustrated in  FIG. 4 , impurities in the silicon may have respective specific energy levels. The energy levels of the impurities may be different according to the impurities. Molybdenum (Mo) that is used as the material of the first layer  132  of the cathode  130  (refer to  FIG. 2 ) may have a plurality of energy levels including an acceptor level of about 0.34 eV and a donor level of about 0.33 eV. In addition, tungsten (W) that is used as the material of the first layer  132  of the cathode  130  may have a plurality of energy levels including an acceptor level of about 0.34 eV and a donor level of about 0.37 eV. 
     In the cathode  130  according to the embodiments of  FIG. 2 , carbon (C), black lead, or graphite that is used as the material of the second layer  134  facing the arc chamber  110  includes carbon (C) atoms, and the carbon (C) atoms have an acceptor level of about 0.25 eV and a donor level of about 0.35 eV (here, 0.35 eV is an energy difference from the valence band Ev). In addition, titanium (Ti) has a donor level of 0.21 eV. Thus, in the cathode  130 , the second level  134  may include a material having a relatively shallow donor level compared to the first level  132 . Thus, although the material of the second layer  134  is implanted into a silicon substrate, as the material of the second layer  134  forms a relatively shallow impurity level compared to molybdenum (Mo) and tungsten (W), a recombination probability is decreased and, thus, defects in electrical characteristics of a device manufactured by using a silicon substrate may be decreased. 
     In addition, metal ions of metals, such as molybdenum (Mo) and tungsten (W), may form various energy levels as well as energy levels indicated in  FIG. 4  by combining with source gas ions. For example, a plurality of energy levels may be formed with a small energy interval if a plurality of metal ions are implanted into a silicon substrate and, thus, electrons may be easily captured or emitted. 
       FIG. 5  is a schematic cross-sectional view of an image sensor  10000  manufactured by using the ion implanter of  FIG. 1 . Referring to  FIG. 5 , the image sensor  10000  includes a plurality of photodiodes  2400  formed in a substrate  2000  and a plurality of transistors  2500  formed in the substrate  2000 . The plurality of photodiodes  2400  and the plurality of transistors  2500  may be arranged adjacent to each other. The image sensor  10000  may be, for example, a complementary metal oxide silicon (CMOS) image sensor. 
     The substrate  2000  may be a semiconductor substrate such as a silicon wafer, and a device isolation layer  2100  is formed in the substrate  2000  to limit active regions. 
     The photodiodes  2400  receive light and then generate charges and each of the photodiodes  2400  may have a form of a PN junction diode. Each of the photodiodes  2400  may include a first well  2420  in which P-type impurities such as boron (B), gallium (Ga), indium (In), or the like are implanted and a second well  2440  in which N-type impurities such as phosphorus (P), arsenic (As), antimony (Sb), or the like are implanted. 
     Each of the transistors  2500  may include a gate insulation layer  2520 , a gate electrode layer  2550 , and a side wall spacer  2580 . At a lateral side of each of the transistors  2500 , a source/drain region  2200  in which impurities are doped may be formed in the substrate  2000 . 
     Dopants may be implanted into the substrate  2000  of the image sensor  10000  by using the ion implanter as shown in  FIG. 1 , which includes the ion source described above with reference to  FIGS. 2 through 3D . For example, the first well  2420  and the second well  2440  of each of the photodiodes  2400  may be formed by using the ion source according to an embodiment of the inventive concept. 
       FIG. 6  is a graph illustrating a characteristic of the image sensor manufactured by using the ion implanter of  FIG. 1 . Referring to  FIG. 6 , data indicating a generation amount of white spots in the image sensor as described with reference to  FIG. 5  is illustrated. The white spots are defects that are generated in the image sensor, and are a result of electrical signals being generated even if light is not incident. Data of an image sensor manufactured by using an ion implanter in which the cathode cap  130  (refer to  FIG. 2 ) including a single layer formed of tungsten (W) is used is illustrated in the left side of the graph of  FIG. 6 . Data of an image sensor manufactured by using the ion implanter according to the embodiment of the inventive concept, in which the cathode cap  130  including a double layer formed of tungsten (W) of the first layer  132  and carbon C of the second layer  134  is used, is illustrated in the right side of the graph of  FIG. 6 . 
     As illustrated in  FIG. 6 , in the image sensor manufactured by using the ion implanter according to the embodiment of the inventive concept, less white spots are generated compared to the image sensor manufactured by using the ion implanter in which the cathode cap  130  including a single layer formed of tungsten (W) is used. According to the inventive concept, metal ions may be limited or even prevented from being implanted into a semiconductor substrate for forming an image sensor and, thus, the amount of white spots may be decreased by preventing a phenomenon in which electrons are captured and emitted due to an impurity level that is formed by the metal ions. 
     As described above, embodiments of the inventive concept provide an ion source for preventing electrical characteristics from deteriorating due to impurities. Embodiments of the inventive concept also provide an ion implanter for preventing electrical characteristics from deteriorating due to impurities. 
     According to an aspect of the inventive concept, there is provided an ion source including: a filament for emitting thermoelectrons; and a cathode comprising a first layer, which is heated by the thermoelectrons that are emitted from the filament, and a second layer of which at least a portion is adjacent to the first layer and which emits secondary electrons by using energy transmitted from the first layer and prevents a discharge of a material of the first layer. 
     The second layer may include a material that is different from that of the first layer. 
     The first layer may include a metal, and the second layer may include a nonmetallic material. 
     The second layer may include carbon, black lead, graphite, or a high melting point material. 
     The second layer may include a nonmetallic material that has a melting point of 3000° or more. 
     The ion second layer may include carbon, black lead, or graphite. 
     The first layer may include a high melting point metal that has a melting point of 2000° C. or more. 
     Ionization energy of a material of the second layer may be larger than that of the material of the first layer. 
     The second layer may include a material that generates a shallow-level trap in silicon compared to a material of the first layer. 
     The second layer may include a material that generates a trap within 0.45 eV from a valence band or a conduction band in silicon. 
     The second layer may include a material that generates a trap within 0.25 eV from a valence band or a conduction band in the silicon. 
     The second layer may have a thickness in a range of about 0.1 mm to about 1 mm. 
     The cathode may further include a supporting unit for supporting the first layer and the second layer. 
     The ion source may further include an arc chamber that provides an ionizing space and is connected to a gas supplying unit and an ion beam path, wherein the cathode is disposed between the filament and an internal area of the arc chamber at one end of the arc chamber. 
     The second layer may cover upper side and lateral sides of the first layer so as not to expose the first layer to the internal area of the arc chamber. 
     According to another aspect of the inventive concept, there is provided an ion implanter including: the ion source; a mass analyzer for sorting ion beams extracted from the ion source; an ion transmitter for accelerating the sorted ion beams; and an end station in which a substrate, into which the sorted ion beams are implanted, is disposed. 
     According to another aspect of the inventive concept, there is provided an ion source including: an electron emission unit for emitting thermoelectrons; and a cathode comprising a nonmetallic secondary electron emission unit that is heated by the thermoelectrons emitted from the electron emission unit and emits secondary electrons. 
     The ion source may further include a conductive intermediate unit that is disposed between the electron emission unit and the nonmetallic secondary electron emission unit and transmits energy obtained from the thermoelectrons emitted by the electron emission unit to the nonmetallic secondary electron emission unit. 
     The foregoing is illustrative of the present inventive concept and is not to be construed as limiting thereof. Although a few embodiments of the present inventive concept have been described, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of the present inventive concept. Accordingly, all such modifications are intended to be included within the scope of the present inventive concept as defined in the claims. Therefore, it is to be understood that the foregoing is illustrative of the present inventive concept and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The present inventive concept is defined by the following claims, with equivalents of the claims to be included therein.