Abstract:
An improved ion source head for use with an ion implantation machine includes an arc chamber within which a heated filament creates an ion plasma from a source gas. The source gas is introduced into the chamber evenly through at least four, but preferably six through hole openings in a bottom liner in the chamber. Even distribution of the gas entering the chamber reduces build-up and flaking of material in the chamber that can result in short circuits.

Description:
TECHNICAL FIELD 
     The present invention generally relates to ion implantation machines used in fabricating semiconductor devices, and deals more particularly with an improved ion source head that reduces material build-up and flaking in the arc chamber. 
     BACKGROUND OF THE INVENTION 
     Semiconductor fabrication processes often use a high current ion implantation machine to implant impurity ions into semiconductor substrates in order to form doped regions, such as sources and drains. The ion implanter delivers a beam of ions of a particular type and energy to the surface of a silicon substrate. Such machines typically include an ion source supply, normally a gas source, and an ion source power supply which is connected to an ion source head. A small quantity of the gas is passed through a vaporizer oven and then into an arc chamber which includes a heated filament, and an anti-cathode. The filament is directly heated by passing electric current through it, derived from the power supply. This heating causes thermionic emission of electrons from the surface of the filament. An electric field, typically 30 to 150 volts is applied between the filament and the arc chamber walls using the power supply. The field accelerates the electrons in the filament area to the arc chamber walls. A magnetic field is then introduced to perpendicular to the electric field and causes the electrons to spiral outward, increasing the path length and chances for collisions with the gas molecules. The collisions break apart many of the molecules and ionize the resultant atoms and molecules by knocking outer shell electrons out of place. As charged particles, these atomic or molecular ions can now be controlled by magnetic and/or electric fields. Source magnets are employed to change the ion path from a straight path to a helicoid path. With one or more electrons missing, the particles carry a net positive charge. An extraction electrode (anti-cathode) placed in proximity to a slit and held at a negative potential attracts and accelerates the charged particles out of the chamber through the slit opening in the top of the chamber. Ions exiting the chamber are passed through an acceleration tube where they are accelerated to the implantation energy as they move from high voltage to ground. The accelerated ions form a beam well collimated by a set of apertures. The ion beam is then scattered over the surface of a wafer using electrostatic deflection plates. 
     After operation over a period of time, the processing of gasses in the arc chamber results in the accumulation of materials deposited from the gas, causing the formation of a conductive coating on the filament, chamber walls and anti-cathode. This coating eventually flakes, causing the filament to short out such that its can no longer produce electrons and the implantation machine becomes inoperable. This shorting phenomena is a result of arc-outs and typically occurs during boron implanting. When the arc chamber shorts out, it is necessary to clean the chamber, the anti-cathode and filament, which is a time consuming procedure since the machine is operated at a high vacuum pressure. This procedure is not only time consuming and costly, but the machine down time reduces throughput. 
     Accordingly, there is a clear need in the art to provide an improved arc chamber that reduces or eliminates arcing and shorting within the arc chamber. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an improved ion source head is provided for use in an ion implantation machine comprising an arc chamber, a source of gas, arc means for generating an ion plasma within the chamber, and a bottom liner in the chamber having the plurality of spaced apart through holes therein for allowing the evenly distributed introduction of gas into the chamber, thus tending to avoid concentrated material buildup or coating on elements within the chamber. The bottom liner includes at least four, but preferably six thru-holes therein communicating with a feed chamber beneath the bottom liner which is coupled with the gas source. Each of the through-holes is preferably greater than 4 mm but less than 8 mm in diameter. The through-holes are desirably arranged in two groups thereof respectively lying along parallel axes, and thus are disposed symmetrically in a bottom of the arc chamber. 
     According to another aspect of the invention, an ion source is provided for generating ions, comprising: and arc chamber defined by a top, a bottom, opposing sides and opposing ends; a source of ions in the form of a selected gas; a heated filament on one of the chamber ends; and an anti-cathode on the other of the chamber ends; and a plurality of openings in the bottom of the chamber coupled with the gas source and allowing evenly distributed introduction of gas into the chamber between the filament and the anti-cathode. 
     Accordingly, it is a primary object to provide an improved ion source head which reduces arcing and shorting within the arc chamber caused by coating build-up on elements within the chamber. 
     Another object of the invention is to provide an improved ion source head of the type described above which reduces down time necessary for cleaning the arc chamber. 
     A still further object of the invention is to provide an improved ion source head of the type described above which allows for the even distribution of an ion source gas flowing into the arc chamber. 
     Another object of the invention is to provide an improved ion source head which significantly reduces coating and flaking of ion source materials on the anti-cathode, and undesired arcing which results from such flaking. 
     These, and further objects and advantages of the invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which form an integral part of the specification, and are to be read in conjunction therewith, and in which like reference numerals are employed to designate identical components in the various views: 
     FIG. 1 is a side view of an ion implantation machine, parts being broken away in section, and showing a prior art source head; 
     FIG. 2 is an exploded, perspective view of the arc chamber body and liner components; 
     FIG. 3 is an enlarged, fragmented view of the prior art source head shown in FIG. 1; 
     FIG. 4 is a view similar to FIG. 3 but depicting the improved ion source head of the present invention; 
     FIG. 5 is a plan view of the bottom liner that forms part of the head shown in FIG. 4; 
     FIG. 6 is an end view of the bottom liner shown in FIG.  5 ; 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIGS. 1-3, a conventional prior art ion source head, generally indicated with the numeral  10 , is employed as part of a conventional high current ion implantation machine. The ion source head  10  broadly includes a rigid body  12 , and two or more elongated legs  14  supporting an ion head assembly that includes a cooled plate  18  and an arc chamber assembly  25 . A pre-selected gas which provides the source of ions is supplied to the arc chamber  25  by supply lines  20 ,  62  and  64  that respectively terminate in nozzles communicating with a feed chamber  60 . One of the above-mentioned designated by the numeral  42 . The supplied gas may be any of a number of types, depending upon the particular application, such as boron, antimony, or other known gasses used in ion implantation processes. Ovens  16  surrounding at least certain of the tubes  20 ,  64  may be provided for heating the supplied gasses. The ovens  16  are cooled by cooling water introduced into jacket surround the ovens  16  through quick connect fittings  35 . The arc chamber assembly  25  includes an arc chamber body  26  provided with a central, generally rectangular arc chamber  45  having a liner consisting of a pair of opposed side walls  42 ,  48 , a pair of opposing end walls  36 ,  46  and a top wall  34  having an elongate slit  51  (FIG. 2) therein through which ions may escape. 
     An anti-cathode  24  extends through an aperture in the liner end  36  at one wall of the liner  28  and is held in place by a clamp  22 . An electrical filament  30  also extends into the chamber  45  and includes a pair of legs which pass through a pair of apertures  54  in the and the other liner end  46 , and corresponding through-holes in the opposite end of the chamber body  28 . The filament  30  is held in place by a filament clamp  37  which is adjustable by means of a adjustment assembly including a thumb screw  38 . Filament  30  is provided with a electron reflector  32  which extends into the chamber  45  and assists in reflecting electrons away from the filament  30 . As best seen in FIGS. 1 and 3, the bottom wall of the body  26  includes a recessed area that defines the supply chamber  60 , immediately beneath the liner bottom  44 . The liner end  36  includes a generally rectangular opening  50  along the bottom edge thereof which communicates with the supply chamber  60 . 
     Accordingly, gas supplied through any one of the lines  20 ,  62 ,  64  passes into the supply chamber  60 , and thence through opening  50  into the interior of the chamber  45 . This gas flow arrangement results in a concentration of the freshly supplied gas in one end of the chamber  45 , adjacent the anti-cathode  24 . As a result of this gas concentration, there is a tendency for parts of the chamber assembly  25 , but particularly the anti-cathode  24  to become heavily coated with material which is an electrically conductive precipitant of the supplied gas. This coating eventually flakes, and the flaked coating causes an electrical short within the chamber  45 . 
     As previously discussed, the electrical filament  30  is coupled with a suitable power supply source which heats the element  30  and causes the thermionic emission of electrons from the surface of the filament  30 . An electric field, typically 150 volts, is applied between the filament  30  and the liner walls which are electrically conductive, using the power supply. This field accelerates electrons from the area of the filament  30  to the liner walls. A magnetic field is introduced perpendicular to the electric field and causes the electrons to spiral outward increasing the path thereof and chances for collisions with the molecules of the gas introduced into the chamber  45 . These collisions break apart many of the molecules and ionize the resultant atoms and molecules by knocking outer shell electrons out of place. As charged particles, these atomic or molecular ions can then be controlled by magnetic and/or electric fields, and using magnets or the like, these charged particles, which have a positive charge, are attracted by the anti-cathode  24  so that the charged particles are accelerated out of the chamber  45  through the slit  51  in the top wall  34 . 
     Referring now also to FIGS. 4-6, it has been discovered that by changing the path of the flow of gas into the chamber  45 , concentration of the coating build-up on the interior of the chamber  45  are reduced, in turn reducing the probability of flaking and resultant short circuits. In accordance with the present invention, the opening  52  in the prior art liner end  36  (FIG. 2) is eliminated, and the liner bottom  48  is provided with a plurality of through holes  58 . At least four of such through holes  58  should be provided, although six holes has been found to be most desirable. 
     The through holes  58  are arranged in two groups of three holes each, respectively aligned along two parallel axes to form a symmetrical, grid pattern in order to uniformly distribute gas flowing into the chamber  45 . The diameter of each of the through holes  58  is preferably greater than 4 mm, but less than 8 mm, although the exact diameter will vary depending on the total number of through holes  58  and their relative spacing. In the preferred embodiment of the invention, the spacing between the centers of the through holes  58  and each group thereof is approximately 15 mm (dimension “x” in FIG. 5) and the spacing between the center lines of the through holes  58  and the two groups is approximately 30 mm (dimension “y” in FIG.  5 ). 
     Surprisingly, it has been found that the improved arrangement for supplying gas into the arc chamber  45  as described above can increase the life of the ion source head 10 by 20%, while significantly reducing down time caused by equipment failure due to shorting. 
     From the foregoing, it is apparent that the present invention not only provides for the reliable accomplishment of the objects of the invention but does so in a peculiarly efficient and economical manner. It is recognized, of course, that those skilled in the art may make various modifications or additions to the preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.