Abstract:
An oxygen ion containing plasma is generated using a hot filament ion source. The oxygen ions in the plasma come from an oxide source (e.g., a metal oxide) which has a lower free energy of formation than that of the filament metal oxide (e.g., WO 3 ) at the operating temperatures of the ion source. Consequently, oxidation of the filament and other metal components of the arc chamber is limited, or even prevented. Thus, the invention can advantageously lead to longer filament lives as compared to certain conventional processes that generate oxygen plasmas using hot filament sources.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional patent application Ser. No. 60/275,425, filed Mar. 13, 2001, and entitled “Oxygen Implant From Solid Source”, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to ion implantation and, more particularly, to an apparatus and method for generating oxygen ions. 
     BACKGROUND OF THE INVENTION 
     Ion implantation is a conventional technique for introducing dopants into semiconductor materials. A desired dopant may be ionized to form a plasma in an ion source. The ions are accelerated from the source to form an ion beam of selected energy which is directed at the surface of a wafer. The ions in the beam penetrate into the bulk semiconductor material to form an implanted region. 
     In some processes, oxygen is implanted into a semiconductor material. Some conventional implantation processes have generated oxygen plasmas by introducing oxygen gas into an arc chamber of the ion source using a gaseous source feed. If the ion source uses an RF or microwave source to generate the plasma, the presence of oxygen gas in the arc chamber may not be problematic. However, if the ion source uses a hot filament formed of an oxidizable metal (e.g., tungsten) to generate the plasma, the presence of oxygen gas in the arc chamber can cause oxidation of the hot filament. This oxidation can significantly reduce filament life and may result in filament metal deposits on the insulators, both inside and outside the arc chamber. 
     One approach to address the oxidation problem has been to use a less oxidizing source gas, such as CO 2  or CO. However, some oxidation of the filament still may occur and carbon deposits may form inside the ion source. Another approach has been to use a very dilute mixture of an oxygen-containing gas (e.g., NO, N 2 O, and the like) and an inert gas, such as Argon. This approach, however, is generally effective only for low ion dose requirements because of the low oxygen concentration. 
     It would be desirable to have an ion implantation method and apparatus which can generate an oxygen ion beam from a hot filament source, while limiting oxidation of the filament. 
     SUMMARY OF THE INVENTION 
     According to the methods and apparatus of the invention, an oxygen ion containing plasma is generated using a hot filament ion source. The oxygen ions in the plasma come from an oxide source (e.g., a metal oxide) which has a lower free energy of formation than that of the filament metal oxide (e.g., WO 3 ) at the operating temperatures of the ion source. Consequently, oxidation of the filament and other metal components of the arc chamber is limited, or even prevented. Thus, the methods and apparatus of the invention can advantageously lead to longer filament lives as compared to certain conventional processes that generate oxygen plasmas using hot filament sources. 
     In one aspect, the invention provides an ion source comprising a chamber, a metal filament housed within the chamber, and an oxygen source component housed within the chamber. The oxygen source component comprises an oxide compound which has a lower free energy of formation than the oxide of the filament metal over the operating temperature range of the ion source. 
     In another aspect, the invention provides an ion source. The ion source comprises a chamber, a metal filament housed within the chamber, and an oxygen source component housed within the chamber. The oxygen source component comprises a metal oxide of the group consisting of Ga 2 O 3 , B 2 O 3 , SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , and MgO. The ion source further comprises a source gas supply connected to the chamber. 
     In another aspect, the invention provides a repeller unit for an arc chamber of an ion source. The repeller unit comprises a conductor plate and a connector connecting the conductor plate to a voltage source. The repeller unit further comprises an ion source component mounted on the conductor plate and exposed to the interior of the arc chamber when the repeller unit is mounted therein. The ion source component comprises a solid oxide compound which is a member of the group consisting of Ga 2 O 3 , B 2 O 3 , SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , and MgO. 
     In another aspect, the invention provides a method of generating oxygen ions in an arc chamber of an ion source having a metal filament therein. The method comprises the step of providing a solid oxygen source within the arc chamber. The solid oxygen source comprises an oxide compound which has a lower free energy of formation than the oxide of the filament metal over the operating temperature range of the arc chamber. The method further comprises the steps of feeding a source gas into the chamber, passing current through the filament, and causing electrons to be emitted from the filament. The electrons collide with source gas molecules to create fluoride ions that chemically interact with the solid oxygen source to form oxygen ions. 
     In another aspect, the invention provides a method of generating an ion beam of a desired species, selected from the group consisting of Al, Ga, B, Si, Ti, Zr, and Mg ions, in an ion implanter apparatus comprising an ion source including an arc chamber having an electron emitting filament therein of an oxidizable metal. The method comprises the steps of providing a solid oxide of the desired species in the arc chamber; feeding a source gas into the chamber; generating an arc in the arc chamber by thermionic electron emission from the filament; extracting an ion beam comprising ions of the desired species from the arc chamber; and separating the desired species ions from other components of the ion beam to form the ion beam of the desired species. 
     Other aspects, features and advantages will become apparent from the following detailed description and drawings when considered in conjunction with the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic block diagram of an ion implanter. 
     FIG. 2 is a schematic top sectional view of an ion source chamber for an ion implanter employing a repeller according to an embodiment of the invention. 
     FIG. 3 is a perspective side sectional view of an ion source chamber according to another embodiment of the present invention. 
     FIG. 4 is a graph showing the free energy of formation versus temperature for WO 3  and a number of other metal oxides which can be used as the oxide source component according to embodiments of the invention. 
     FIG. 5 is a plot of a setup cup beam spectrum obtained from an arc SiF 4  arc discharge using an Al 2 O 3  repeller disk in accordance with an example of the invention. 
    
    
     DETAILED DESCRIPTION 
     The invention is intended to enable the implantation of oxygen ions using a hot filament ion source (e.g., a Bernas source) without creating an oxidizing atmosphere inside the source. This limits oxidization of metal components inside the arc chamber and, in particular, the hot filament. As described further below, filament oxidation is limited by using an oxygen source formed of an oxide compound which has a lower free energy of formation than the oxide of the filament metal (e.g., WO 3 ). In some embodiments of the invention, the oxide compound is formed of a solid metal oxide and may be mounted within an arc chamber of the ion source. Advantageously, the invention enables production of an O +  ion beam for ion implantation using hot filament ion sources, while limiting detrimental effects on the filament life. 
     A schematic block diagram of a typical ion implantation system  1  is shown in FIG.  1 . An ion source  10  generates an ion beam  12  which is directed along a beam path toward a target such as a semiconductor wafer  14 . The ion source  10  includes an arc chamber in which an ionized plasma is generated by thermionic electron emission from a filament, as described further below. The ion beam  12  is deflected and focused by a mass analyzing magnet  20 . Downstream of the mass analyzing magnet  20 , the ion beam may be focused in the plane of a mass resolving slit assembly  22 . A variety of slit assembly configurations are known, including the rotating cylinder configuration of U.S. Pat. No. 5,629,528 and the prior slit assembly configurations described in the background thereof. Slit assembly  22  may be fashioned according to any such known configuration. 
     The ion beam  12  is accelerated to a desired energy by an accelerator  24  and impinges on wafer  14  located within an end station  26 . The entire region between the ion source  10  and the wafer is evacuated during ion implantation. 
     The ion beam  12  may be distributed over the surface of wafer  14  by mechanically scanning the wafer with respect to the beam, by scanning the ion beam with respect to the wafer or by a combination of these techniques. The wafers may be, for example, mounted on a rotating disk during ion implantation. End station  26  may include a system for automatically loading semiconductor wafers into one or more wafer positions for implantation and for removing the wafers from the wafer chamber after ion implantation. The ion implantation system may include other components, not shown but known to the skilled person in the art, such as a dose measuring system, an electron flood system, and a tilt angle monitoring system, among others. 
     Referring to FIG. 2, a simplified schematic of an arc chamber  28  of an ion source is shown according to one embodiment of the invention in which an ionized oxygen plasma is generated and from which an oxygen ion beam may be extracted. Arc chamber  28  is maintained under vacuum. A source gas, which typically includes no oxygen or a very small percentage of oxygen (e.g., less than 1% oxygen by weight), is fed into the chamber through a port  30 . Current is passed through filament  32 , causing it to heat up and to thermionically emit electrons from its surface. Filament  32  may be any type of hot filament used in the art. In some embodiments, a tungsten filament is used. In other embodiments, a tantalum filament is used. 
     A voltage, for example between about 30 and about 150 volts, may be applied between the filament  32  and a positive electrode (e.g., chamber  28 ). The electrons emitted from the filament collide with source gas molecules to generate positive ions. In some embodiments, a magnetic field may be applied perpendicular to the electric field within the chamber to increase the electron path and, thus, increase the probability of collisions between electrons and ion source gas molecules within the chamber. 
     An oxygen source component  36 , which may be in the form of a plate, is mounted to a repeller unit  34  so that it is exposed to the interior of the arc chamber. The source gas ions in the arc discharge chemically interact with the oxygen source component to form oxygen ions. The oxygen ions are introduced into the plasma within the chamber. The chemical interaction may result in erosion of the oxygen source component. Thus, the oxygen source component plate may require periodic replacement. 
     The oxygen ions in the plasma may be extracted from the chamber to form a beam. It should be understood that the beam also may include other types of ions, such as source gas ions. As the beam passes downstream through the ion implantation system, conventional techniques may be used to eliminate unwanted ionic species from the beam and, thus, select a specific ionic species for implantation. Further downstream the ion beam, may be monitored for “set up current,” accelerated or decelerated as desired, scanned and directed at the wafer. 
     FIG. 3, shown in a slight perspective view, depicts an arc chamber  50  according to another embodiment of the invention. In this embodiment, arc chamber  50  includes two filaments  52  and two repeller units  54 . Each repeller unit  54  includes an oxygen source component  61 , which may be in the form of a plate, that is held on a metallic repeller conductor  62  by a retaining ring  63 . The repeller conductor  62  is disk-shaped with a rearwardly extending shaft  64  which extends to the outside of the chamber  50 . Other components of the repeller units  54  include inner insulator  65 , repeller shield  66 , and outer insulator  67 , all of conventional design. 
     Oxygen source components  36  and  61  in FIGS. 2 and 3, respectively, function as the oxygen ion sources according to the invention. As shown in the illustrative embodiments, the oxygen source component is formed of a solid metal oxide. Suitable metal oxides include those that have a lower free energy of formation than the oxide of the filament metal (e.g., WO 3 ) over the operating temperature of the ion source (e.g., between about 290 K to about 2500 K). Examples of suitable metal oxides include Ga 2 O 3 , B 2 O 3 , SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , and MgO. In some cases, Al 2 O 3  is particularly preferred for the oxygen source component. FIG. 4 is a plot of the free energy of formation versus temperature showing that WO 3  has a higher free energy of formation over the ion source operating temperature than Ga 2 O 3 , B 2 O 3 , SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , and MgO. It should be understood, however, that other oxide materials which have lower free energies than suitable hot filament elements (e.g., WO 3 ) within the operating temperature of the ion source may also be used. 
     It should be understood that, in some embodiments, the plasma generated using metal oxides as the oxygen source component may also include substantial quantities of ions of the metal or metalloid used to form the oxide. If desired, these ions may be eliminated from the ion beam using conventional techniques as described above. 
     In some embodiments of the present invention, a fluoride gas is used as the source gas. Suitable fluoride gases may be, for example, BF 3 , SiF 4 , F 2 , or HF, or mixtures thereof. In some embodiments, BF 3  or SiF 4  may be preferred. Conditions for generating plasmas in an arc chamber using such gases are well known. It should be understood that the source gas includes no oxygen, or a very small percentage of oxygen, so as to limit filament oxidation. 
     The invention may be implemented in any filament electron emitter arc chamber including those employed in ion implanters such as the VIISion, VIISta and EHPi series ion implanters manufactured by Varian Semiconductor Equipment Associates, Inc., Gloucester Mass., USA. 
     The present invention will be further illustrated by the following example, which is intended to be illustrative in nature and is not to be considered as limiting the scope of the invention. 
     EXAMPLE 
     This example illustrates the generation of an ion beam that includes oxygen ions using the methods and apparatus of the present invention. 
     A VIISion Ion Implanter from Varian Semiconductor Equipment Associates, Inc. was modified to include a repeller unit and an arc chamber similar to those shown schematically in FIG.  3 . An SiF 4  source gas was fed into the arc chamber at a flow rate of 3 sccm. An Al 2 O 3  repeller plate was used as the oxygen source component. An arc voltage of 80 V was used to generate a plasma containing oxygen ions and aluminum ions from the Al 2 O 3  repeller plate. An ion beam was extracted from the arc chamber. 
     FIG. 5 shows the beam current spectrum of an ion beam, measured at the setup cup of the implanter. The inset portion of FIG. 5 is an enlargement of the region of the spectrum between 0 and 38 AMU. The spectrum in FIG. 5 shows that approximately 1.5 mA of O +  beam current may be obtained. As also shown in FIG. 5, the beam also includes aluminum ions as a result of the use of an Al 2 O 3  source component. It should be understood that higher oxygen beam currents may be obtainable with optimization and that the aluminum ions can be eliminated from the beam using known techniques, if desired. 
     The above description is intended to be illustrative and not exhaustive. The description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to be specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto. Further, the particular features presented in the independent claims below can be combined with each other in other manners within the scope of the invention such that the invention should be recognized as also specifically directed to other embodiments having any other possible combination of the features of the dependent claims.