Decaborane ion source

#The present invention relates generally to ion sources for ion implantation equipment and more specifically to an ion source for ionizing decaborane.

#Ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.

#Ion dose and energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable up to about 1 mA beam current) are used for lower dose applications. Ion energy is used to control junction depth in semiconductor devices. The energy of the ions which make up the ion beam determine the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below 1 thousand electron volts (keV).

#FIG. 1 is a perspective, partially cross sectional view of a conventional ion source for an ion implanter;

#FIG. 2 is a schematic, partially cross sectional view of a first embodiment of an ion source for an ion implanter constructed according to the principles of the present invention;

#FIG. 3 is a cross sectional view of a connecting tube of an alternative embodiment of the ion source of FIG. 2, taken along the lines 3—3; and

#Referring now to FIGS. 2-4 of the drawings, and initially to FIG. 2, an ion source 50 comprising a vaporizer 51 and an ionizer 53 are shown, constructed according to the present invention. The vaporizer 51 comprises a non-reactive, thermally conductive sublimator or crucible 52, a heating medium reservoir 54, a heating medium pump 55, a temperature controller 56, and a mass flow controller 60. Ionizer 53 is shown in more detail in FIG. 3. The crucible 52 is located remotely from the ionizer 53 and connected thereto by a feed tube 62, constructed of quartz or stainless steel. In the disclosed embodiment, the feed tube 62 is surrounded by an outer single-chamber annular sheath 90 along substantially the entire length thereof.

#The crucible 52 provides a container 64 enclosing a cavity 66 for containing a source material 68. The container is preferably made of a suitable non-reactive (inert) material such as stainless steel, graphite, quartz or boron nitride and which is capable of holding a sufficient amount of source material such as decaborane (B10H14). Although the invention is described further below only in terms of decaborane, it is contemplated that the principles of the present invention may be used for other molecular solid source materials, such as indium trichloride (InCl3), which are characterized as having both low melting points (i.e., sublimation temperatures of between 20° C. and 250° C.) and significant room temperature vapor pressures (i.e. between 10−2 Torr and 103 Torr).

#The decaborane is vaporized through a process of sublimation by heating the walls of the container 64 with a heating medium 70 contained in reservoir 54. A wire mesh 71 prevents non-vaporized decaborane from escaping the crucible 52. Completely vaporized decaborane exits the crucible 52 via feed tube 62 and enters mass flow controller 60, which controls the flow of vapor, and thus meters the amount of vaporized decaborane which is provided to the ionizer 53, as is known in the art.