Ion implantation and surface processing method and apparatus

A capacitor is charged to a high potential or voltage from a power source. A plasma switch, preferably a CROSSATRON modulator switch, is periodically closed and opened to discharge the capacitor into an object for implantation with ions from a plasma in a plasma source ion implantation apparatus. The periodic discharge results in the application of high voltage negative pulses to the object, causing ions from the plasma to be accelerated toward, and implanted into the object. A pulse transformer is preferably provided between the plasma switch and capacitor, and the object to step up the voltage of the pulses and enable the plasma switch to operate at lower voltage levels. The plasma switch enables high duty factor and power operation, and may be combined with arc detection and suppression circuitry to prevent arcing between the object and plasma. A second power source, capacitor, and plasma switch may be provided to apply positive pulses to the object in alternation with the negative pulses to cause generation of the plasma, or to accelerate electrons into the object for performing thermally assisted ion implantation, surface annealing, and the like.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an ion implantation and surface processing 
apparatus including an improved pulse modulator for applying electrical 
pulses to an object for plasma source ion implantation (PSII). 
2. Description of the Related Art 
PSII is an ion implantation technique which circumvents the line-of-sight 
restriction inherent in conventional ion implantation. The basic technique 
is disclosed in U.S. Pat. No. 4,764,394, entitled "METHOD AND APATUS 
FOR PLASMA SOURCE ION IMPLANTATION", issued Aug. 16, 1988, to J. Conrad 
(University of Wisconsin Alumni Association); and in an article entitled 
"Plasma source ion-implantation technique for surface modification of 
materials", by J. Conrad, in the Journal of Applied Physics, vol. 62, no. 
11, 1 Dec. 1987, pp. 4591-4596. Ion implantation into surfaces of 
three-dimensional target objects is achieved by forming a plasma about the 
target within an enclosing container and applying repetitive pulses of 
negative high voltage between the target and the conductive walls of the 
container. Ions from the plasma are driven into the target object surface 
from all sides simultaneously and omnidirectionally without the need for 
manipulation of the target object. The plasma may be formed of a neutral 
gas introduced into the evacuated container and ionized therein with 
ionizing radiation so that a constant source of plasma is provided which 
surrounds the target object during the implantation process. Significant 
increases in the surface hardness and wear resistance characteristics of 
various materials are obtained with ion implantation using the PSII 
technique. 
An apparatus for performing PSII as taught by Conrad is illustrated in FIG. 
1 and generally designated as 10. An object 12 which is to be implanted 
with ions is supported in a container or chamber 14 on a target stage or 
support 16. The container 14 is electrically grounded, and evacuated by a 
vacuum pump 17 to a pressure on the order of 2.times.10.sup.-4 Torr. A 
working gas which may be, for example, nitrogen, helium, or argon, is 
supplied from a tank 18 into the container 14. The gas is ionized to 
produce a plasma 19 by means of filament discharge using voltages from a 
discharge bias source 20 and a filament supply 22. The plasma density can 
be varied between approximately 10.sup.6 and 10.sup.11 ions/cm.sup.3 by 
adjusting the filament current and bias applied by the sources 20 and 22. 
A pulse modulator 24 which is supplied with a direct current voltage or 
potential from a high voltage power source 26 applies negative pulses to 
the object 12 through the stage 16 of up to approximately 100 kV. A 
Langmuir probe 28 is used to measure the plasma density and electron 
temperature, which are displayed on a curve tracer 30. Target temperatures 
during implantation are monitored by an infrared pyrometer 32, and 
recorded on a chart recorder 34. 
The high voltage negative pulses applied from the pulse modulator 24 to the 
object 12 attract the positively charged ions from the plasma 19 and cause 
them to be accelerated toward and implanted into the object 12. A 
schematic diagram of a conventional pulse modulator 24 is shown in FIG. 2. 
A capacitor 36 is negatively charged from the power source 26 through a 
resistor 38. The negatively charged end (connected to the resistor 38) of 
the capacitor 36 is connected through a resistor 40 to the cathode of a 
high voltage "hard" vacuum tube 42. The anode of the vacuum tube 42 is 
connected through a resistor 44 to the stage 16 and thereby to the object 
12. Further illustrated is a filament supply 46 for the vacuum tube 42. 
The tube 42 is normally turned off or electrically non-conductive, thereby 
electrically disconnecting the capacitor 36 from the object 12. In 
response to pulses from a pulse generator 48 applied to the control grids 
of the tube 42, the tube 42 becomes conductive for the duration of each 
pulse and connects the capacitor 36 to the object 12. This causes the 
capacitor 36 to discharge through the tube 42 into the object 12, 
resulting in the application of a high voltage negative pulse to the 
object. The capacitor 36 recharges between pulses to provide a high level 
of discharge current. 
In PSII as disclosed in the above reference patent to Conrad, the 
implantation time is independent of the object size. The implantation time 
depends only on the duty factor and power handling capability of the pulse 
modulator, without excessively heating the object. For a typical plasma 
ion current density of 1 mA/cm.sup.2, implantation of any size object at a 
maximum dose of 10.sup.18 ions/cm.sup.2 (typically corresponding to 
stoichiometry) will require over 44 hours, using the conventional pulse 
modulator 24 utilizing the hard vacuum tube 42, and operating at a duty 
factor of 0.1 %. This is impractical for cost-effective use of PSII in a 
manufacturing environment. A factor of ten reduction in the implantation 
time, corresponding to an increase of at least ten times in the duty 
factor and power-handling capability of the pulse modulator 24 (for fixed 
plasma density), is required to make PSII processing at 10.sup.18 
ions/cm.sup.2 dose cost-effective. This must be accomplished by increasing 
the duty factor and power handling capability of the pulse modulator. 
The conventional pulse modulator 24 has several limitations restricting its 
ability to achieve these goals. The conventional vacuum tube 42 of the 
pulse modulator 24 is connected in a "floating" circuit arrangement, and 
must be very large and thereby expensive since it must hold off the full 
implantation voltage between pulses. For PSII operation at voltages above 
100 kV, it is very difficult to obtain vacuum tubes with such a high 
voltage rating. 
For implantation of large objects at high ion dose (approximately 10.sup.18 
/cm.sup.2), high duty factor (average power) pulse modulator operation is 
required. For high duty factor operation using the hard vacuum tube 42, a 
large cooling system (not shown) is required to dissipate the heat 
generated due to the high voltage drop across the tube. In addition, the 
vauuum tube, filament power supply, and grid driver circuitry must float 
at the full implantation voltage during operation, requiring additional 
costly isolation transformers to be provided for the circuit. Floating the 
vacuum tube and control circuitry at high voltage makes it very 
complicated to implement arc-suppression circuitry in the apparatus to 
protect the object from being damaged in the event of an arc. In addition, 
vacuum tubes have short operating lifetimes due to their inherent hot 
cathode operation, and generate large amounts of undesirable x-rays and 
electromagnetic interference. 
SUMMARY OF THE INVENTION 
The present invention overcomes the limitations of the conventional PSII 
technique as disclosed in the above referenced patent to Conrad by 
providing an ion implantation and surface processing method and apparatus 
including an improved pulse modulator having greatly increased duty factor 
and power handling capabilities over conventional PSII apparatus including 
pulse modulators utilizing hard vacuum tubes. 
In accordance with the present invention, a capacitor is charged to a high 
potential or voltage from a power source. A plasma switch, preferably a 
CROSSATRON, Modulator Switch, is periodically closed and then opened to 
discharge part of the energy stored in the capacitor into an object for 
implantation with ions from a plasma in a plasma source ion implantation 
apparatus. The periodic discharge results in the application of high 
voltage negative pulses to the object, causing ions from the plasma to be 
accelerated toward, and implanted into the object. A pulse transformer is 
preferably provided between the plasma switch/capacitor and the object to 
step up the voltage of the pulses and enable the plasma switch to operate 
at lower voltage levels. The plasma switch enables high duty factor and 
power operation, and may be combined with arc detection and suppression 
circuitry to prevent arcing between the object and plasma. A second power 
source, capacitor, and plasma switch may be provided to apply positive 
pulses to the object in alternation with the negative pulses to cause 
generation of the plasma, or to accelerate electrons into the object for 
performing surface processing such as thermally assisted ion implantation, 
surface annealing, and the like. 
The CROSSATRON modulator switch is capable of operating with a duty factor 
substantially in excess of ten times higher than that of a hard vacuum 
tube, and with much greater power handling capability. The CROSSATRON 
switch may be operated in a grounded cathode, rather than a floating 
configuration, therefore facilitating arc suppression and protection of 
expensive objects during implantation. The CROSSATRON switch has a very 
low forward voltage drop, thereby dissipating a relatively small amount of 
power in the closed or conducting state and generating very little heat. 
These factors enable ion implantation of large objects to be economically 
performed on a commercial production basis. 
These and other features and advantages of the present invention will be 
apparent to those skilled in the art from the following detailed 
description, taken together with the accompanying drawings, in which like 
reference numerals refer to like parts.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides an ion implantation and surface processing 
method and apparatus which enable improved performance and capabilities 
not possible with the conventional apparatus disclosed by Conrad. The 
present apparatus has the general configuration illustrated in FIG. 1, 
including the container or chamber 14 for enclosing the object 12 for ion 
implantation and surface processing, and the elements described above for 
generating the plasma 19 which surrounds the object 12. 
As illustrated in FIG. 3, a pulse modulator which replaces the prior art 
pulse modulator 24 in the present apparatus is generally designated as 50, 
and includes a source 52 of high voltage or potential on the order of 100 
kV. The negative end of the source 52 is grounded, whereas the positive 
end of the source 52 is connected through a surge limiting series 
impedance or inductor 54 to the anode of a plasma switch 56. The switch 56 
is preferably a CROSSATRON modulator switch, but may be embodied by any 
other applicable type of plasma switch within the scope of the invention. 
The CROSSATRON modulator switch is disclosed in U.S. Pat. No. 4,596,945, 
entitled "MODULATOR SWITCH WITH LOW VOLTAGE CONTROL", issued June 24, 
1986, to R. Schumacher. CROSSATRON is a trademark of the Hughes Aircraft 
Company, the assignee of the present invention as well as the patent to 
Schumacher. 
The cathode of the switch 56 is grounded. The anode of the switch 56 is 
further connected through a capacitor 58 and resistor 60 to the anode of a 
diode 62. The cathode of the diode 62 is connected to ground. The anode of 
the diode 62 is further connected through a current sensor 64 to the stage 
16 shown in FIG. 1, and thereby to the object 12 for ion implantation. 
Further illustrated in FIG. 3 are a power source 66 and resistor 68 which 
apply a voltage of approximately 500 V to the keeper grid of the switch 
56. A power source 70 provides a reservoir heater voltage of approximately 
2.5 V. A power source 72 and capacitor 74 apply a voltage of approximately 
1 kV to the control grid of the switch 56 through a thyristor 76 and 
resistor 78 to turn on or close the switch 56. A power source 80 and 
capacitor 82 apply a voltage of approximately -400 V to the control grid 
of the switch 56 through a thyristor 84. A timing pulse generator 86 
applies ON pulses to the gate of the thyristor 76, and OFF pulses to the 
gate of the thyristor 84 through a diode 88. The sensor 64 is constructed 
to apply ARC pulses to the gate of the thyristor 84 through a diode 90. 
It will be noted that the plasma switch 56 is connected in parallel across 
the power source 52, and that the capacitor 58 is connected in series with 
the object 12 across the switch 56. 
In operation, the capacitor 58 charges through the inductor 54, resistor 
60, and diode 62 when the switch 56 is in an open or non-conductive state. 
More specifically, the end of the capacitor 58 which is connected to the 
anode of the switch 56 is charged positive with respect to ground. Closing 
of the switch 56 has the effect of connecting the positive end of the 
capacitor 58 to ground through the low resistance path of the electrically 
conducting switch 56. The end of the capacitor 58 which is connected to 
the resistor 60 becomes negative with respect to ground, thereby reverse 
biasing the diode 62. The capacitor 58 discharges through the resistor 60 
into the object 12, resulting in the application of a negative pulse to 
the object 12 to affect ion implantation as described above. Opening of 
the switch 56 terminates the negative pulse to the object 12. 
The operation of the timing pulse generator 86 is illustrated in FIG. 4. 
The leading edge of each ON pulse turns on the thyristor 76, thereby 
gating the 1 kV voltage from the power source 72 to the control grid of 
the switch 56 to turn on or close the switch. The OFF pulses alternate 
with the ON pulses, such that the leading edge of each OFF pulse gates the 
-400 V voltage from the power source 82 to the control grid of the switch 
56, thereby turning off or opening the switch. The switch 56 is closed or 
electrically conductive during the periods between the leading edges of 
the ON and OFF pulses, thereby resulting in the capacitor 58 periodically 
charging from the source 52 and discharging into the object 12. As 
illustrated in FIG. 4, negative pulses OUT are applied to the object 12 
during the periods when the switch 56 is closed. 
Although a single capacitor 58 is shown in the drawing for simplicity of 
illustration, a large bank of capacitors will generally be employed in a 
practical application to provide the required discharge current. It will 
be understood, however, that if the power source 52 where made large 
enough to provide sufficient pulse current on a continuous basis, the 
capacitor 58 would be unnecessary, and the switch 56 could directly 
connect and disconnect the power source 52 and the object 12 for 
application of the implantation pulses. 
The sensor 64 is designed to sense or detect current flow therethrough in 
excess of a predetermined value above which arcing between the object 12 
and plasma 19 would be imminent. In response to detection of an imminent 
arcing condition, the sensor 64 feeds an ARC pulse to the thyristor 84 
through the diode 90. The leading edge of an ARC pulse causes the switch 
56 to open immediately, terminate discharge of the capacitor 58 into the 
object 12, and thereby suppress the imminent arcing condition. Arc 
detection and suppression circuitry as presently described is commercially 
available for the CROSSATRON modulator switches manufactured by the Hughes 
Aircraft Company, and is capable of opening a CROSSATRON switch within one 
microsecond of imminent arc detection. Arc detection and suppression are 
facilitated since the cathode of the plasma switch 56 is grounded, rather 
than floating as with the hard vacuum tube 42 illustrated in FIG. 2. 
FIG. 4 illustrates three ON pulses and three OFF pulses. No ARC pulse is 
shown as being generated between the first ON pulse and the first OFF 
pulse. Thus, the first OUT pulse as applied to the object 12 has a normal 
period or duration. An ARC pulse is illustrated as being generated between 
the second ON pulse and the second OFF pulse. The leading edge of the ARC 
pulse causes the switch 56 to open immediately, and the second OUT pulse 
to have a duration which is shorter than normal. No ARC pulse is generated 
between the third ON and OFF pulses, and the third OUT pulse has a normal 
duration. 
FIG. 5 illustrates a modified pulse modulator 50' embodying the present 
invention, with like elements designated by the same reference numerals 
used in FIG. 3, and corresponding but modified elements designated by the 
same reference numerals primed. The pulse modulator 50' differs from the 
pulse modulator 50 in that it additionally includes a pulse transformer 92 
for stepping up the voltage of the pulses applied to the object 12. More 
specifically, a power source 52' may be constructed to provide a voltage 
on the order of 50 kV. Where the transformer 92 has a step-up ratio of 
2.5:1, the pulse modulator 50' will apply pulses with a voltage of over 
120 kV to the object 12. 
The pulse transformer 92 enables the switch 56 to operate at a lower 
voltage than in the modulator 50. A CROSSATRON modulator switch which may 
be incorporated directly into the modulator 50' is commercially available 
from the Hughes Aircraft Company under product designation 8454H. This 
particular switch is capable of continuous operation at 50 kV. The rise 
and fall times of the current switch has a forward voltage drop of less 
than 500 V, and will dissipate a maximum of 2500 W of power. The heat 
generated during operation may be removed by a small fan. The switch is 
capable of 750 A peak current, 50% duty factor, pulse widths in excess of 
one millisecond, and an average power capacity in excess of 250 kW. These 
capabilities are substantially in excess of the requirements for large 
scale, commercial PSII application. 
A current sensor 64' is connected in the primary winding of the pulse 
transformer 92, and may therefore have a lower voltage rating than the 
sensor 64. The transformer 92 is further advantageous in that it provides 
electrical isolation between the other components of the pulse modulator 
50' and the high voltage applied to the object 12. The pulse modulator 50' 
may be adapted to apply positive, rather than negative, pulses to the 
object 12 merely by reversing the connections to the secondary winding of 
the transformer 92 if desired. 
The apparatus illustrated in FIG. 5 was fabricated and tested for 
implantation of nitrogen ions using negative pulses applied to a steel 
object 12. A nitrogen plasma was produced in a plasma chamber 14 which was 
1.22 meters in diameter and 1.22 meters long. With the plasma switch 56 
operating at 1% duty factor and at an input voltage of 35 kV, an 
implantation voltage of 75 kV was applied to the object 12, suitable for 
ion implantation of nitrogen into the steel object 12 with collection of 5 
amperes of ion current. 
FIG. 6 illustrates the configuration of a CROSSATRON modulator switch which 
is preferably employed as the plasma switch 56. The CROSSATRON is a cold 
cathode, crossed-magnetic-field plasma discharge switch, and is based upon 
a crossed-field discharge in a four element, coaxial system consisting of 
a cold cathode 93, an anode 94, a keeper grid 95 and a control grid 96 
disposed between the cathode 93 and anode 94. Charges for conduction are 
generated by a plasma discharge near the cathode 93. The plasma is 
produced by a crossed-field cold cathode discharge in a gap located 
between the keeper grid 95 (which serves as an anode for the local 
crossed-field discharge) and the cathode 93. The gap is magnetized with a 
cusped field supplied by permanent magnets 97 attached to the outside of 
the switch. 
A source plasma 98 is generated by pulsing the potential of the keeper grid 
to a level above 500 V for a few microseconds to establish a crossed-field 
discharge. When equilibrium is reached, the keeper grid potential drops to 
the low discharge level about 500 V above the potential of the cold 
cathode 93. With the control grid 96 remaining at the cathode potential, 
the switch 56 remains open and the full anode voltage appears across the 
vacuum gap between the control grid 96 and anode 94. 
The switch 56 is closed by releasing the control grid 96 potential, or by 
pulsing it momentarily above the 500 V plasma potential (1 kV in the 
present example). This allows plasma 98 to flow through the keeper grid 95 
and control grid 96 to the anode 94. Electrons from the plasma 98 are 
collected by the anode 94, the switch 56 conducts, and the anode voltage 
falls to the 500 V level. To open the switch 56, the control grid 96 is 
returned to the cathode potential or below (-400 V in the present 
example), thereby terminating collection of electrons from the plasma 98 
by the anode 94. 
FIG. 7 illustrates a bipolar pulse modulator embodying the present 
invention which is generally designated as 100, and includes two pulse 
modulators 102 and 104 which are essentially similar to the modulator 50' 
shown in FIG. 5. 
A modified pulse transformer 106 has a secondary winding 106a connected to 
the stage 16 and thereby the object 12, and two primary windings 106b and 
106c connected to the outputs of the modulators 102 and 104 respectively. 
The primary winding 106b is connected to the modulator 102 in the same 
manner as illustrated in FIG. 5. However, the connections of the primary 
winding 106c to the modulator 104 are reversed. Whereas the output pulses 
from the modulator 102 induce negative output pulses in the secondary 
winding 106a as described above, the output pulses from the modulator 104 
induce positive pulses in the secondary winding 106a. The modulators 102 
and 104 may be operated individually to selectively apply only negative or 
positive pulses to the object, or in combination to apply negative pulses 
alternating with positive pulses. 
Application of positive pulses to the object 12 causes electrons, rather 
than positively charged ions, to be accelerated into the object. One 
purpose for bombarding the object with electrons is to provide thermally 
assisted ion implantation as described in a paper entitled "New 
application for multipolar plasmas: High-temperature treatment of 
materials", by J. Pelletier et al, Rev. Sci. Instrum., vol. 55, no. 10, 
Oct. 1984, pp. 1636-1638. Other uses of electron collection by the object 
include surface processing operations such as target heating for surface 
annealing, bulk annealing, material outgassing in the vacuum system, 
surface cleaning, and enhancement of coating formation by temperature 
control. A yet further application is surface cleaning by electron induced 
desorption. 
The power sources 52' may be made variable so that the magnitudes of the 
negative and positive pulses can have different values. The pulse 
generators 86 may also be adjusted so that the negative and positive 
pulses have different durations and/or waveforms. 
As illustrated in FIG. 8 by way of example, the negative pulses may have a 
shorter duration and a larger magnitude than the positive pulses. Another 
example is shown in FIG. 9, in which the negative pulses have a ramp 
shaped waveform whereas the positive pulses have a nonlinear waveform. 
This enables the pulses to be programmed to provide optimal implantation 
profiles for individual object materials. 
It will be noted that the present invention may also be utilized to 
generate negative pulses for applications including surface cleaning by 
sputtering, surface modification by ion bombardment, and reactive ion 
deposition for target coating by alternative materials. 
FIG. 10 illustrates another bipolar pulse modulator 120 embodying the 
present invention, with like elements again designated by the same 
reference numerals. The bipolar modulator 120 includes a positive pulse 
modulator 122 including a power source 124 having its negative end 
connected to ground, and its positive end connected to ground through the 
inductor 54, charging diode 62, and capacitor 58. The junction of the 
diode 62 and capacitor 58 is connected through a resistor 126 to the anode 
of a plasma switch 128, which may be a Hughes 8454 CROSSATRON modulator 
switch as described above. The cathode of the switch 128 is connected to 
the primary winding of the pulse transformer 92. 
The bipolar modulator 120 further includes a negative pulse modulator 130 
including a power source 132 and a plasma switch 134. The modulator 130 is 
similar to the modulator 122, except that the connections of the power 
source 132 and switch 134 are reversed relative to the corresponding 
elements in the modulator 122. It will be noted that the plasma switches 
128 and 134 are electrically connected in series with the object across 
the respective capacitors 58. 
In operation, the capacitor 58 of the modulator 122 charges positively with 
respect to ground during the periods between positive pulses. This occurs 
because the switch 128 is open during these periods, and electrically 
disconnects the respective capacitor 58 from the pulse transformer 92. 
Closing the switch 128 causes the capacitor 50 of 58 of the the modulator 
122 to be electrically connected to and discharge into the primary winding 
of the pulse transformer 92, thereby inducing a positive pulse in the 
secondary winding of the transformer. The modulator 130 operates in the 
same manner as the modulator 122, with reversed polarity. 
Although two separate plasma switches 128 and 134 are illustrated in FIG. 
10, it is within the scope of the invention to replace the two switches 
with a single bidirectional CROSSATRON or other plasma switch (not shown). 
A suitable bidirectional CROSSATRON switch is disclosed in U.S. patent 
application Ser. No. 454,675, entitled "COMT HIGH VOLTAGE POWER 
SUPPLY", filed Dec. 2, 1989, now U.S. Pat. No. 5,008,798, issued on Apr. 
16, 1991, by R. Harvey, which is assigned to the Hughes Aircraft Company, 
the assignee of the present application. 
The bipolar modulator 100 or 120 may be operated in such a manner as to 
alternatingly generate positive pulses which cause plasma generation and 
negative pulses which cause ion implantation. The positive pulses initiate 
glow discharge between the walls of the chamber 14 and the object 12, 
thereby eliminating the necessary of providing a separate plasma 
generating source and associated components. 
The waveform, including the duration and voltage profile of the positive 
pulses, are selected to produce the correct plasma density, and maintain 
adequate plasma density after the positive pulses are terminated and the 
negative pulses applied to cause ions from the plasma to be accelerated 
into the object 12. An exemplary pulse train including positive and 
negative pulses having waveforms selected to accomplish this operation is 
illustrated in FIG. 11. The pulse train includes +10 kV, 100 microsecond 
pulses for plasma generation, followed closely by -100 kV, 10 microsecond 
pulses for ion implantation. The pulse repetition frequency may typically 
vary between approximately 50 Hz to 500 Hz. 
The bipolar operations described above may alternatively be performed using 
thyratron or other plasma switches, or with other types of switches such 
as vacuum tube switches as illustrated in FIG. 12. Assuming that the 
bipolar plasma generation and implantation operations described with 
reference to FIG. 11 are to be performed, a bipolar pulse modulator 140 
may include a positive power source 142 for supplying a positive voltage 
or potential of +5 kV. A capacitor 144 charges positively from the source 
142 through a resistor 146. A power source 148 supplies a negative 
potential of -50 kV to charge a capacitor 150 through a resistor 152. 
The capacitor 144 is connected to the primary winding of a pulse 
transformer 154 through a tetrode Vacuum tube 156, resistor 158 and diode 
160. The capacitor 150 is similarly connected to the primary winding of 
the pulse transformer 154 through a vacuum tube 162, resistor 164 and 
diode 166. The vacuum tubes 156 and 162 are configured to function as 
switches, and have maximum voltage ratings on the order of 50 kV. Further 
illustrated are filament voltage supplies 168 and 170 for the vacuum tubes 
156 and 162 respectively. 
In response to trigger pulses from a pulse generator 172, the tube 156 is 
turned on to connect the capacitor 144 to the pulse transformer 154 for 
100 microseconds and thereby apply the positive +10 kV pulse to the object 
12. Then, the tube 156 is turned off and the tube 162 is turned on for 10 
microseconds to connect the capacitor 150 to the pulse transformer 154 and 
thereby apply the negative -100 kV pulse to the object 12. 
While several illustrative embodiments of the invention have been shown and 
described, numerous variations and alternate embodiments will occur to 
those skilled in the art, without departing from the spirit and scope of 
the invention. Accordingly, it is intended that the present invention not 
be limited solely to the specifically described illustrative embodiments. 
Various modifications are contemplated and can be made without departing 
from the spirit and scope of the invention as defined by the appended 
claims.