Patent Number: 048250872
Section: summary

CROSS REFERENCE TO RELATED CASES The apparatus and system and methods of the present invention are applicable to ion implantation systems such as the PI 9000 system available from Applied Implant Technology of Horsham, England, a subsidiary of the Assignee, Applied Materials, Inc. of Santa Clara, Calif. The PI 9000 system is described in Aitken U.S. Pat. No. 4,578,589, which issued Mar. 25, 1986, and is assigned to Applied Materials, Inc. This patent is incorporated by reference in its entirety. BACKGROUND OF THE INVENTION The present invention relates to wafer charge reduction systems for ion implanters, and to so-called electron flood guns for introducing negative charge into the ion beam to reduce positive charging of ion implanted wafers. In particular, the invention relates to an electron flood gun and to methods of operation which provide a hitherto unattainable combination of large magnitude flood electron current and low energy characteristics which are required to control or eliminate both local and bulk positive charging by the ion implant beam. EXAMPLE OF USE OF ION IMPLANTATION FIGS. 1-3 illustrate the use of a sequence of ion implantation steps in fabricating CIS (conductor-insulator-semiconductor) integrated circuit devices on a semiconductor wafer. FIG. 1 illustrates a first ion implantation step which may be performed on the P-type wafer 10 to produce a light implant in the field regions 14 of the wafer. The field regions 14 at this point are not covered by the region of photoresist mask 11. The photoresist 11 is formed using a standard lithography process in which a thin layer of resist is applied over the entire surface of the wafer. After the layer of resist has been exposed and developed, a thin layer of thermal oxide 12 typically is grown over the exposed surfaces of the semiconductor wafer so that the implant in the field regions 14 will be made through the thin oxide layer. Next, the light field implantation of ions of a P-type material such as boron is done to provide greater electrical isolation between the active device regions which lie under the regions 11 of photoresist material. Then, thick field oxide regions 15 are grown using a wet oxidation process. See FIG. 2. During this oxidation process, the implanted ions 14 are driven into the semiconductor substrate to underlie the field oxide regions 15. The mask 11 is then removed, a thin gate oxide 17 is formed in the active device regions 18, and a second ion implantation step is performed to implant N-type dopant ions 16 through the gate oxide layer 17. This light implant step creates the implanted region 18 and tailors the threshold voltage of the MOS (metal oxide semiconductor) silicon gate field effect transistor. See FIG. 3. After this light threshold-setting implant, the silicon gate regions 19 of the field effect transistor devices are formed on the wafer to produce the device topology shown in FIG. 3. Then, a heavy implantation of N-type ions may be performed to simultaneously dope the silicon gate element 19 and the source and drain regions 21 and 22 to complete the basic structure of the silicon gate field effect transistor device. Of course, additional fabrication steps are required to complete typical integrated circuits, including additional ion implantation steps. ION BEAM-INDUCED TARGET CHARGE-UP The present invention is directed to device performance degradation and the concomitant decrease in yields which can result from positive charging of the target semiconductor wafer during ion implantation steps such as are described above. Positive charging typically manifests itself in two ways, as bulk charging or as localized charging. Bulk charging occurs during ion implantation because limited charge mobility causes the whole surface to become charged. Localized charging manifests itself when conductive regions or layers (such as the gate electrodes 19 shown in FIG. 3) which are isolated from the conducting substrate by a dielectric (such as gate oxide 17, FIG. 3), charge up. The positive charge which is induced in a semiconductor wafer target during ion implantation usually can readily exceed a few volts. However, depending upon the device architecture, development of a positive charge of only a few volts magnitude on a dielectrically isolated conductor "island" such as a gate electrode 19 can create a field across the underlying dielectric which is sufficient to cause breakdown and loss of dielectric integrity and, as a consequence, render the device inoperative. While local charging can be a problem for bi-polar circuits, it presents very difficult problems for MOS and CMOS (complementary metal oxide semiconductor) circuits, more so as the technology implements thinner gate oxides and high dose implants. To our knowledge, the prior art does not suggest an adequate solution to the positive charging problem. Simple diode electron flood guns which introduce electrons into the ion beam have been available for some time. See, for example, Bower U.S. Pat. No. 3,507,709, issued Apr. 21, 1970. However, to be effective for contemporary and future ion implant systems, such flood guns must be able to provide low energy electrons at high current levels. This is so because, first, contemporary so-called medium current implanters and high current implanters utilize high ion implant beam current levels, within the approximate range 0.1-5 milliamps (mA) for medium current operation and 5-100 milliamps for high current operation. Clearly, effective neutralization of wafers which are implanted using such large magnitude currents requires much larger electron flood currents, current levels roughly comparable to the ion beam current. Second, the flood electrons must have low energy in order to have sufficient "selectivity" to the wafer surface which, as mentioned, is charged positive with respect to earth or ground. Clearly, for the electrons to be attracted to the positively charged regions of the wafer surface their trajectories must be affected by the small electric fields associated with the low voltage positive charge. This can only happen if the energy of the electrons is low in comparison to the potential of the charge regions. Unfortunately, the flood guns known to us are incapable of providing the high flux currents of low energy electrons which are necessary to neutralize wafers without device damage. In particular, the emission current is limited by space charge effects. The energy spread is small, influenced predominantly by the potential difference across the filament and the thermal distribution. Consequently, under vacuum this type of system (1) produces electrons with unacceptably high energy concentrated in a narrow band about the filament bias voltage, V.sub.bias, and (2) requires unacceptably high values of V.sub.bias to generate large quantities of flood electrons. Regarding (1), not only are high energy electrons insufficiently selective to the relatively low voltage (&lt;10 volts) locally charged regions of the wafer, but in fact may charge up the wafer to a high negative voltage. This merely replaces the positive charging problem with a negative charging problem with the same result, namely, breakdown and the loss of dielectric integrity. Regarding (2), heretofore there has been no known way to provide sufficiently large quantities of flood electrons to neutralize wafer charge-up. Consider, for example, the flood gun disclosed in the above-mentioned Bower U.S. Pat. No. 3,507,709. There, the electron energy associated with the simple diode emitter is equal to the potential difference between the cathode and wafer. Col. 4, lines 45-50, thereof says energies in the range 4 to 40 eV can be produced but that optimum performance is at 4 eV. The flood gun drive characteristics can be approximated with reference to FIG. 9, which depicts the inter-dependency of bias voltage and bias current for our flood gun 50, FIGS. 4-7. Curve 91 illustrates theoretically the effect of bias voltage on bias current during operation in a vacuum. It is seen that operation at the maximum Bower level of 40 volts would provide at most inadequate quantities of flood electrons with reasonable size guns, whereas operation at the optimum 4 volt level would provide a much lower bias current, perhaps at the microamp level. More recently, secondary electron emission from a metallic surface has been used in an attempt to neutralize positive charge build-up. Using this technique, typically the electron flux from a flood gun is aimed at the metallic surface so that secondary electron emission, presumably of lower energy, provides neutralization. In fact, however, secondary emission can also be characterized by an unacceptably large percentage of high energy electrons, as well as by difficulty in achieving consistent reproducible control of the process. In short, to our knowledge the existing flood gun technology and the secondary electron emission technology have not afforded sufficient control of the electron energy distribution or of the neutralization process to be considered a solution to the problem of positive charging during ion implantation. SUMMARY OF THE INVENTION Objects In view of the above discussion, it is one object of the present invention to prevent potentially catastrophic positive charging of semiconductor wafers during ion implantation. It is another object of the present invention to prevent such charging by the introduction of flood electrons into the ion beam used for implanting, and without negative charge build up. It is yet another object of the present invention to introduce flood electrons into the ion beam at a high flux/current and at low electron energies and with precise control of these and other characteristics including trajectory. SUMMARY In one aspect, the present invention is embodied in an electron flood gun for neutralizing positive charge induced in a target such as a semiconductor wafer by an ion beam, comprising: diode means comprising an anode and a cathode adapted for receiving a bias voltage for emitting a flux of electrons into the ion beam; means for introducing an inert gas into the region adjacent the cathode for amplifying the electron flux or current and lowering the peak electron energy to a level commensurate with the voltage level of the positive charge on the target; and means for applying an adjustable bias voltage to the cathode for controlling the electron current. In another aspect, our invention relates to the combination of (1) a system for irradiating a target with an ion beam in a system end station comprising post-analysis electrode means for accelerating the ion beam to a given velocity incident upon a target located at a selected position downstream from the post-analysis electrode means and (2) a flood gun inserted between the post-analysis electrode means and the target position for neutralizing positive charge build-up induced in the target by the ion beam. The flood gun of this combination comprises: a spiral wire grid anode having coil turns spaced a distance selected for admitting gas therethrough; a filament cathode extending lengthwise within the grid anode and being adapted for receiving a bias voltage to stimulate the emission of electrons into the ion beam; means for introducing an inert gas through the grid anode for magnifying the flux of emitted electrons and lowering the peak electron energy to a value commensurate with the positive voltage level induced by the ion beam in the target; and means for supplying an adjustable bias voltage to the filament for amplifying the current of emitted electrons and for controlling the electron peak energy. In still another aspect, the present invention involves the combination, with a process of implanting ions into a semiconductor wafer using an incident ion beam, of a method of neutralizing low magnitude voltage positive charge up of the wafer resulting from the ion implant process, comprising: providing an electron flood gun having a filament for directing electrons into the beam; bleeding inert gas into the flood gun for amplifying the electron current and lowering the average peak flood electron energy; and controlling the voltage applied to the electron gun filament to control the magnitude of the electron current and limit the average peak electron voltage to a value commensurate with the magnitude of the positive wafer charge.