System and methods for wafer charge reduction for ion implantation

A diode flood gun for introducing an amplified current of low energy electrons into an ion beam for neutralizing charge build up on a target such as a semiconductor wafer during irradiation by the beam. The low energy, amplified current is effected by introducing an inert gas into the flood gun.

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.

DETAILED DESCRIPTION 
Flood Gun Structure and Operation 
FIGS. 4-7 depict a presently preferred embodiment of our electron flood gun 
50 and its use in the system end station 60 of the PI 9000 Ion Implant 
System disclosed in the referenced Aitken U.S. Pat. No. 4,578,589. 
In the system end station 60, the post-acceleration electrode means 52 
(comprising six electrodes individually designated E1-E6) accelerates the 
analyzed ion beam along the system beam line optic axis 51, through the 
ground electrode means 53 (individual electrodes E6, E7, E10) and 
suppression electrode means 54 (electrodes E8, E9) for implanting wafers 
56--56 mounted on the arms 57--57 of a scanning wheel assembly. The two 
suppression electrodes E8 and E9 are connected electrically in common at 
OV to -30kV and, typically, -2 kV to -5 kV. 
As discussed in the referenced Aitken U.S. Pat. No. 4,578,589, the scanning 
of the arms 57--57 consists of a combination of a linear radial "slow" 
scanning movement and a rotational "fast" scanning movement, relative to 
the fixed beam axis 51. Beam stop assembly 58 absorbs the beam when it is 
not intercepted by a wafer 56, or any portion of the beam which is not 
intercepted by a wafer 56. The beam stop assembly 58 includes a conical 
shaped water cooled beam stop comprising mating jackets 96 and 97 and a 
magnet 98, all of which are mounted as shown to end station door 99. 
Flood gun 50 is inserted between electrode E10 of the ground electrode 
means 53 and the walls of the differential pumping box 55 closely adjacent 
the beam 51 and the scanned wafers 56. The ground electrode means 53, and 
the differential pumping box 55 are maintained at ground potential for 
reasons related to beam optical control, arcing, etc., which are not 
specifically related to this invention. 
As is perhaps best shown in FIGS. 4-6, the flood electrons are produced by 
a diode electron gun 50 of cylindrical geometry which is enclosed by a 
semi-cylindrical water-cooled cooling chamber or shield 61 (FIG. 5) having 
a longitudinal exit slot 62 for the electrons. (Shield 61 is not shown in 
FIG. 6.) The hot cathode tungsten filament 63 extends along the 
longitudinal axis of the cylindrical gun and is biased negatively with 
respect to ground at a potential of -V.sub.bias, typically within the 
range OV to -300V, by a computer-controlled adjustable power supply 72 
(FIG. 7). The anode 64 is a spiral tungsten wire grid having a large open 
area (that is, a large distance between adjacent coil turns). Filament 63 
extends along the grid's longitudinal axis. The cooling shield 61 (FIG. 5) 
not only absorbs radiant heat energy from the filament, but also acts as a 
ground electrode and collects the electrons which propagate away from the 
ion beam. The flood gun assembly 50 also includes a semi-cylindrical 
grounded electrical shield 68 for the filament and bias voltage electrical 
wiring. 
The filament 63, grid 64 and electrical shield 68 are mounted to a pair of 
molybdenum end flanges 67--67 and optional graphite end heat shields 
670--670 within the cooling chamber of shield 61. The resulting assembly 
is mounted on a support rod (not shown) which extends generally along axis 
69 and is connected to the end flanges 67--67 for mounting the flood gun 
assembly in the ion implanter system end station 55 in the orientation and 
position shown in FIGS. 4 and 5. 
As mentioned previously, emission from simple diode filament and grid flood 
guns is well known. Such simple diode guns do not satisfy the above 
requirements of high electron flux, low electron energy and controlled 
trajectory. In particular, under vacuum conventional diode-type flood gun 
systems produce electrons with unacceptably high energy concentrated in a 
narrow band about V.sub.bias (eV) and require unacceptably high values of 
V.sub.bias to generate sufficient quantities of flood electrons. 
In contrast, our flood gun 50 incorporates gas bleed which eliminates the 
problems of high energy and low flood electron current. Referring 
particularly to FIGS. 5 and 12, inert gas such as argon is bled into the 
flood gun via a bleed line 66. As used here, "bleed" means the inert gas 
is admitted directly into the flood gun region, thereby increasing the 
flood gun pressure. This is done with a minimum of effect on the target 
chamber pressure, particularly since the differential pumping box 55 is 
pumped by a 10 inch cryo pump. As mentioned, the grid anode 64 has a large 
open area, which permits electron flow from the gun via the exit slit 62 
and allows gas molecules to be readily admitted into the gun from the 
bleed line 66. In this improved flood gun, collisions of electrons emitted 
by the filament 63 with gas neutrals provide low energy electrons in large 
quantities and thus achieve wafer neutralization readily without harmful 
large magnitude negative charge build up. 
FIG. 7 is a block diagram of one suitable control system for controlling 
the operation of the flood gun 50. As shown, and as discussed above, the 
flood gun 50 is mounted within the ion implant system end station 60 
adjacent to the ion beam 51 for supplying flood electrons, as indicated 
schematically at 76, into the beam and/or onto wafers 56--56. The wafers 
are supported on the wafer support paddles 57--57 of a wheel assembly 60 
which, as is discussed in the referenced Aitken U.S. Pat. No. 4,578,589, 
is mounted for scanning the wafers through the ion beam. A power supply 71 
supplies current to the filament 63 for raising it to the desired electron 
emitting operational temperature. A second power supply 72 is connected 
across the filament 63 and grid 64 for biasing the filament negatively 
with respect to the grounded grid at a potential of -V.sub.bias, which as 
mentioned, is typically within the range zero volts to -300 volts. The 
operation of the two variable power supplies 71 and 72 is controlled by a 
programmable power supply controller 73. Also, the bias power supply 
voltage 72 is connected in common to chamber ground along with flood gun 
emission current return line 76 as well as grid current return line 74. 
The inert gas is applied to the flood gun 50 via inlet line 66 from a gas 
supply reservoir 77. The gas inlet flow is regulated by a valve 78 which 
presently is manually operated but quite obviously could be an automatic 
valve operating under the control of the flood gun control system computer 
73. Operation of the flood gun control system computer 73 can readily be 
controlled by the ion implanter's system computer 79. 
In operation, with the filament at a voltage -V.sub.bias which preferably 
is within the range of -50 to -150 volts, the grid maintained at zero 
volts, the inert argon gas admitted at a flow rate of 0.4 atmospheric 
liter/hr. (typically 0.1-1 atmospheric liter/hr), a flood gun pressure of 
10.sup.-4 to 10.sup.-1 mbar and an end system pressure of 10.sup.-7 to 
10.sup.-4 mbar, electrons propagate from the hot filament cathode 63, 
colliding with the argon gas as indicated schematically at 86, FIG. 11. 
Each collision generates an electron/ion pair, the electron of which is 
accelerated toward the anode 64 (and may cause further ionization) while 
the ion is accelerated towards and collected by the filament 63. In short, 
the argon gas causes the electron current to be amplified greatly above 
the magnitude of the vacuum current by a mechanism which allows the total 
flood current to be controlled and monitored. Typical amplification is 
1-10 times. The current can be maintained at the desired level by the 
simple expedient of controlling V.sub.bias. The amplified flood electron 
current traverses from the gun exit slit 62 into the beam as a high, 
readily controllable current. 
The second effect of the argon bleed is on the energy of the emergent 
electrons. As shown schematically at 86 and 87, FIG. 11, ionization and 
excitation mechanisms reduce the average energy of the flood electrons 
dramatically, typically to 5 percent to 50 percent of the bias voltage. 
Contemporary prior art flood guns also use relatively high bias voltage, 
-300V, as compared to an exemplary -100V for the present system 50. Thus, 
as an example, the energy of the electrons emerging from our flood gun 50 
is 0.05.times.100V=-5V, as compared to -300V for a conventional flood gun. 
Like the current level, the electron energy is readily controlled. The 
energy level is generally an inverse function of the flow rate of argon to 
the gun. As a consequence, increasing or decreasing the flow rate of the 
inert gas decreases or increases the electron energy. 
Thus, the combination of gas bleed and V.sub.bias give an efficient and 
repeatable means of controlling electron current and energy. As the 
electron energy is decreased, by increasing the rate of flow of the inert 
gas, the flood electron current increases. This current is controllable as 
desired and is maintained at the required level by the adjustment of 
V.sub.bias. 
The effects of pressure on bias voltage, flood gun current and flood 
electron energy are shown in FIGS. 8-10. Curves 91-95 of FIG. 8 depict 
bias voltage as a function of internal flood gun bias current and vice 
versa at different flood gun pressures. Curves 101-104 of FIG. 9 depict 
bias voltage as a function of flood gun output current and vice versa, 
also at different flood gun pressures. It can be seen that increasing 
flood gun pressure systematically increases the bias current (FIG. 8) and 
flood gun output current (FIG. 9), for a given bias voltage, and that the 
bias current and flood gun output current (which is the amount of the 
total bias current which is transmitted through the grid) may be 
controlled over a wide range. The bias voltage for a given current may 
also be varied by using the gas bleed to change the flood gun pressure. 
The flood electron energy spectra shown in FIG. 10 were measured by 
collecting the current on a negatively charged, biased wafer in the 
absence of an ion beam and external electric and magnetic fields. The 
resulting curves 111-114 illustrate the effectiveness of the increased 
flood gun pressure due to the inert gas in decreasing the percentage of 
electrons (ordinate) above a selected energy level or wafer bias voltage 
(abscissa). As an aid to understanding FIG. 10, consider the curve 111 
associated with a pressure of 1E-6 mbar and curve 114 associated with a 
relatively higher pressure of 5E-5 mbar. For a wafer bias voltage of -10 
volts, approximately 90 to 95 percent of the electrons have energy levels 
equal to or greater than the bias voltage at the lower pressure, whereas 
the percentage is reduced to 25 percent at the higher pressure. As 
evidenced by curves 111-114, the energy of the flood electrons is 
sequentially and greatly reduced by increasing the flood gun pressure. 
It is important to realize that this inert gas mechanism for producing 
electrons is not dependent on secondary emission from a bombarded surface. 
It is not dependent on surface states which are hard to maintain in an 
implanter environment. Moreover, the distribution of electron energies 
does not have a high (primary) energy peak. 
FLOOD GUN CONTROL SYSTEM 
1. Filament Control 
The filament is controlled by the bias voltage, V-.sub.bias. 
Advantageously, if the filament is sufficiently hot 
(.gtoreq..about.2500.degree. K. for a tungsten filament), emission is 
predominantly limited by space charge effect and hence by the bias 
voltage. Preferably, the filament current is kept constant during an 
implant to maintain the flood electron emission level constant. When the 
gun is filament temperature limited, the emitted current depends on bias 
voltage and filament temperature. At sufficiently high temperatures, the 
emission depends only on bias voltage. We seek to operate flood gun 50 
slightly above the temperature limited regime so that emission is only 
dependent on bias voltage. 
However, filament control is not simply a matter of maintaining a constant 
value of V.sub.bias, but involves as well the consideration of several 
competing factors. First, as the filament wears and its size reduces, a 
lesser current is required to achieve a given emission temperature. 
Secondly, high filament currents reduce the filament lifetime. Also, a 
magnetic field is generated by the current flowing through the filament 
which affects the electron trajectory and consequently the efficiency at 
which the emitted current can be transmitted through the grid (the ratio 
of bias current to flood current). Generally, this efficiency is highest 
when the filament current is low. 
Taking into consideration the above factors, the optimum filament current 
is the lowest filament current which achieves the required emission 
levels. This current level provides an optimum combination of electron 
current and operating efficiency, without undue reduction of the filament 
lifetime. 
In one presently preferred working embodiment, a 0.5 millimeter diameter 
tungsten filament 63 is used, the target emission temperature is 
2500.degree. kelvin, and the nominal filament current required to achieve 
this emission temperature is 17.5 amps. Implementation of the above 
objectives is achieved using the filament current controller and bias 
voltage controller as follows. Initially, the filament current is set at 
the desired value of 17.5 amps. Then, the filament current is reduced in 
small decrements while the bias voltage V.sub.bias is adjusted to maintain 
the flood gun current constant. At the same time, V.sub.bias is monitored. 
An increase in V.sub.bias as the result of the decrement in filament 
current indicates that the filament current has gone through an optimum 
value and the onset of a thermally limited regime. That is, emission has 
become thermally limited. The filament current is then reset to it next 
previous value, to the value before the decrement which resulted in the 
increase in the bias voltage V.sub.bias. The current is maintained at this 
level by adjusting V.sub.bias as necessary. 
2. Bias Voltage 
As discussed in the previous section, the filament control system is 
designed to provide a flood gun current at a constant level which is 
consistent with the maximum operating efficiency and filament lifetime and 
independent of external changes. This is done by raising and lowering the 
bias voltage V.sub.bias. 
However, during the course of an implant the flood gun pressure typically 
will rise from the initial value, for example, due to out-gassing from the 
wafers during implantation. This is particularly true where polymer 
photoresist masks are used. Photoresist may outgas hydrogen and nitrogen 
into the vacuum system as a pressure burst of ions which at least 
temporarily increases the ambient pressure at the flood gun. If the bias 
voltage were maintained constant, such increases in pressure would cause 
higher flood gun currents and, possibly, would increase the flood gun 
current above the optimum range. 
However, in establishing this desired filament current level as described 
in the previous section, the filament controller sets up the system at the 
beginning of the implant when the flood gun pressure is at a minimum and, 
as a result, emission from the flood gun is not limited by the filament 
temperature. Since emission is not temperature limited, the flood gun 
current can be decreased by decreasing the bias voltage. Thus, as the 
pressure increases the bias voltage can be reduced to maintain a constant 
gun current and to keep the flood electron energy as low as possible. 
3. Flood Gun Pressure; Dosimetry Error 
Because of the relatively high pressure range which is used during 
operation of the flood gun 50 (&lt;5E-5 mbar), the mean energy of the flood 
electrons is reduced when pressure is increased and/or when the bias 
voltage required to generate a particular gun current level is reduced. 
However, excessive target chamber pressure (&gt;2E-5 mbar) can cause 
significant beam neutralization which causes unacceptable dosimetry 
errors. 
The excessive pressure threshold for given operating conditions may be 
determined by monitoring the beam current on the beam stop 58 while the 
pressure is increased. The pressure or bleed gas feed rate which causes an 
unacceptable reduction in beam current can then be used as the maximum 
value for the flood gun operating pressure range. 
Once the allowable operating range for the flood gun pressure is 
established, the flood gun pressure may be controlled in any one of 
several ways. 
Presently, a manually-operated needle valve (FIG. 7) is incorporated into 
the bleed line 66. 
Alternatively, an automatic constant pressure controller (not shown) can be 
appropriated into the system to control the bleed gas flow in response to 
pressure variations monitored by a pressure gauge located in the target 
chamber or, for greater accuracy, off the differential pumping box. 
Still another alternative approach for controlling the flood gun pressure 
involves the use of an automatic constant energy controller. This approach 
would use a small electron collector plate mounted on the side of the 
flood gun 50 away from the beam. The operator would specify a peak flood 
electron energy, E(eV). The system then control the pressure to keep a 
given percentage such as 99 percent of the flood electrons at an energy 
below the selected peak value. The electron current arriving at the 
collector is monitored as the collector is pulled alternatively to ground 
and a negative voltage, e.g., -100V. If I.sub.e and I.sub.o are, 
respectively, the currents measured when the collector is biased 
negatively and when it is grounded, then the requirement is that I.sub.e 
/I.sub.o .ltoreq.0.01. If the measured ratio is too high, then the 
controller would increase the gas flow rate. Conversely, if the ratio were 
low, the flow rate would be decreased. At no time is the flow rate be 
permitted to exceed the maximum for neutralization described earlier. 
4. Emission Temporal Control 
It is desirable to operate the flood gun only when the ion beam is incident 
on a wafer. This prevents negative charge damage due to flood electrons 
irradiation in the absence of an incident ion beam. 
In addition, the PI 9000 Ion Implanter System disclosed in the referenced 
Aitken U.S. Pat. No. 4,578,589 periodically measures the ion beam dosage. 
It is desirable to interrupt the operation of the flood gun during this 
measurement to prevent error in the reading. Specifically, the beam 
current is measured by the PI 9000 software at the end of each slow scan. 
The reading is initiated by a signal from a sensor on the linear 
transducer attached to the slow scan arm indicating that the wafer is at 
its most distant position from the ion beam. 
According to the present invention, this so-called min-scan signal is also 
used to temporarily terminate the operation of the flood gun during the 
dosimetry reading process. That is, when the ion implanter and the flood 
gun are being operated, the bias controller monitors the linear transducer 
output. When the signal is received from the min-scan sensor indicating 
the wheel is at the min-scan position, the bias controller drops 
V.sub.bias to zero, thereby stopping the emission of electrons while the 
beam current is measured. Then, as the next scan commences and the wheel 
arm moves off the min-scan sensor, the min-scan signal is terminated and 
the bias controller responsively restores V.sub.bias to operate the flood 
gun during the next wafer scan. 
Alternatively, the following temporal control approach may be used not only 
to disable flood emission during measurement of the wafer beam but also to 
disable flood emission when an ion beam is not impinging on a wafer. 
This alternative approach uses capabilities which are incorporated in the 
PI 9000 implanter. First, the voltage output from the linear transducer on 
the slow scan arm depends upon position of the wafer (more precisely, on 
the center of rotation of the wafers) with respect to the beam line optic 
axis. This voltage output may be translated by the implanter software into 
the distance from the beam line optic axis. 
Second, the implanter may include a beam profiler which measures beam 
current density distributions across a plane in front of the beam stop 58 
and behind the wafer implant position shown in FIGS. 4 and 5. This 
profiler comprises five faraday cups located on an arm which is pivoted 
above the beam stop 58. The arm is reciprocally driven by a stepper motor 
through the beam line optic axis 51. The associated cup currents are 
recorded at each step. In this way, the beam current density distribution 
may be measured along five arcs through the beam. This data can be 
processed by a local processor located in the target chamber as well as by 
the main system software to provide the beam size and position relative to 
the beam line optic axis. 
Based upon the voltage output from the linear transducer associated with 
the slow scan arm, the position of the edges of the beam may be translated 
by the software into two voltages which define the positions of the wafer 
corresponding to the opposite edges of the beam and the position of the 
wafer as it enters and leaves the beam during the slow (radial) scan. 
These voltages are stored in the memory of the flood gun bis controller. 
Then, in the same way that flood emission is disabled when the wheel is at 
the min-scan position, the bias controller disables emission except when 
the linear transducer output is between the limiting voltages 
corresponding to the opposite edges of the beam. As a consequence, the 
flood gun is operated only when the wafer is positioned between the outer 
edges of the beam. Flood emission is, thus, disabled (1) when the wafer is 
at the min-scan position and the ion beam is being measured, and (2) 
whenever the beam is not incident upon the wafer. As mentioned, these two 
flood emission interrupts are used, respectively, to decrease errors in 
the dosimetry calculations and to reduce the likelihood of negative charge 
damage to the wafer. 
5. Trajectory Control; Summary of Charge Neutralization 
The trajectories of flood electrons are controlled and the emission of 
secondary electrons is suppressed by the electric field generated by 
suppression electrodes E8 and E9 and by the magnetic fields generated by 
the magnet 98 in the beam stop 58. 
Specifically, the beam stop magnet 98, FIG. 4, generates a magnetic field 
82, FIG. 11, between the two poles of the magnet generally transverse to 
the beam line optic axis 51. This transverse magnetic field forces the 
secondary electrons emanating from the beam stop 58 to return to the beam 
stop and thereby prevents these secondary electrons from affecting the 
measured beam current. In addition the magnetic field 82 inhibits the flow 
of flood electrons to the wafer, so that, in the absence of an ion beam 
and the electric field generated by the suppression electrodes E8 and E9, 
negligible electron current is transmitted to the wafer. In the unlikely 
event of the beam dropping out during an implant this system thus is 
prevented from inducing significant negative charging of the wafer. 
A potentially variable negative voltage V.sub.supp is applied to the 
suppression electrodes E8 and E9 so that the resulting electrostatic 
suppression field 83 prevents secondary electrons created by the beam 51 
impinging upon the wafer 56 or paddle 57 from entering the 
post-acceleration system. Also, in conjunction with the external magnetic 
field 82 developed by the beam stop magnet and the internal electric field 
of the beam 51, this electrostatic suppression field 83 precisely controls 
the trajectories of the flood electrons, directing the flood electrons to 
the wafer 56. By varying V.sub.supp applied to the suppression electrodes 
E8 and E9 to vary the suppression electric field 83, the user is able to 
control the flood electron trajectories, and optimize the elimination of 
positive charge on wafer device structures. 
Referring further to FIG. 11, the overall charge neutralization system and 
methodology employed in our present invention can now be summarized as 
follows. Inert gas inlet from line 66 flows through the grid 64, as 
indicated by arrows 72--72, and creates a high pressure region 73 of inert 
gas within the flood gun 50. High energy electrons emitted by the filament 
63 collide with the argon gas and thereby undergo energy loss and 
multiplication by ionization, as indicated schematically at 74. The 
electrons also experience energy loss by excitation, as indicated 
schematically at 75. The result is a highly amplified flow of low energy 
flood electrons which propagate past the grid and through the surrounding 
lower pressure region, as indicated at 76, and which are pulled into the 
ion beam by its associated electric field. There, the internal ion beam 
field in combination with the electric field 71 applied by the suppression 
electrodes sweep the flood electrons toward the wafer. At the same time, 
the magnetic field 70 generated by the beam stop magnet prevents secondary 
electrons from the beam stop from affecting the measured beam current and 
inhibits the flow of flood electrons to the wafer in the event the beam 
drops out during implantation, while the electric suppression field 71 
prevents secondary electrons generated at the wafer paddle from entering 
the post-acceleration system. 
6. Charge Sensor 
Referring to FIG. 7, a charge sensor 80 is mounted on the slow scan arm so 
that its position with respect to the wafer locus is constant. The PI 9000 
system uses capacitive coupling to provide an output which is 
approximately proportional to the mean wafer surface potential. The peak 
voltage may be written to a sample and hold buffer. Software in system 
computer 79 may read this buffer and then reset to allow for subsequent 
readings. The peak wafer potential may, therefore, be monitored during an 
implant. Also, the output from the charge sensor may be monitored on an 
oscilloscope 81. 
Thus, there has been described preferred and alternative embodiments of our 
flood gun and the method of operating the flood gun. The flood gun was 
conceived with respect to an ion implantation system and, in particular, a 
system for ion implanting silicon integrated circuit wafers. However, the 
flood gun is applicable as well to other implantable materials in addition 
to silicon including but not limited to glass, quartz, gallium, arsenide 
and silicon on sapphire. In addition, the principles disclosed here are 
applicable to the application of flood guns, for example, to plasma 
etching of electronically programmable read-only memories (EPROM's) and 
other structures and, in more general terms, wherever it is desired to use 
electrons or an ion beam to control the charge level associated with or 
induced by another ion or electron beam used in irradiation processing 
equipment.