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Challenges and progress toward a 250 kV, 100 kW plasma ion implantation facility
Surfaceand Coatings Technology85 (1996) 111-119
Challenges and progress toward a 250 kV, 100 kW plasma ion implantation facility JesseN. Matossian, Ronghua Wei Plasma Physics Laboratory, Hughes Research Laboratories, Malibu, CA 90265, USA
Abstract Plasma ion implantation (PII) is a large-scale cost-effective technique for modifying the surface properties of materials via omnidirectional ion implantation. The implantation voltage and ion species define the energy-to-ion ratio which, together with the ion dose, determines the effectiveness of an implant in achieving successful tribological improvements in materials. For most metal treatment applications of PII, modest energy-to-ion ratios and high ion doses are required to achieve respectable improvements in hardness and wear life. For example, implantation of N: ions into metal tools and diesvia PI1 at 100kV (energy-to-ionratio of 50keV per N+ ion) at a doseof 3 x 1Ol7N’ cm-’ can result in an increaseof wear life by a factor of 2-8. In contrast with metals,polymers require high energy-to-ion ratios and low dosesto achievesign&ant improvementsin hardnessand wear life. For example,ion beamimplantation of N+ ions into Kapton at 300kV (energy-to-ionratio of 300keV per N+ ion) and a dose of 3 x 10” N+ cm-’ can increasehardnessby a factor of 13. This energy-to-ion ratio is six times higher than that achievable using presentPII technology. A provocative questionto ask is how this can be achievedin a PI1 system.Work is under way at the Hughes ResearchLaboratories (HRL) to addressthis issue.The approach being used involves first scaling PI1 voltage technology from 100 to 250kV, and then implanting doubly and triply chargedatomic nitrogen ions at 250kV to achieve an energy-to-ion ratio range of 500-750keV per Nf ion. To date single-pulseion implants have been conductedat 250kV in the HRL PI1 facility. A plasmasourcehas beenbuilt for the production of multiply chargednitrogen ions. A techniquefor treating non-conductingobjectsin a PI1 facility hasbeendeveloped,and a methodof reducingX-ray production in a PI1 systemoperating at 250kV hasbeeninvestigated. Keywords:Plasmaion implantation; Surfacemodification; Tribology
1. Introduction Plasma ion implantation (PII) is a large-scale implementation [l-7] of conventional ion beam implantation. Much of the work reported has involved the use of PI1 to improve the tribological (friction and wear) properties of materials [7]. In many of these applications N: ions have been implanted into metals, specifically metal tools and dies. For example, using a voltage range of 50-100 kV and a dose range of (2-4) x 1O1’ Nf cme2, PI1 has been used to achieve (1) an improvement of two to three times in the wear life of Co/WC drill bits used in printed wiring board fabrication [ 51, (2) an improvement of five to eight times in the wear life of TiN-coated cutting tools [4], (3) an improvement of four times in the wear life of tool steel dies used for producing bolts [2] and (4) an improvement of three to four times in the wear life of M-2 steel pierce punches [Cl. Other ions that have been used in PI1 treatments of metal tools include methane and ammonia; however, molecular 0257~8972/96/$15.00 0 1996ElsevierScience S.A.All riahtsreserved
nitrogen remains one of the most common ion species used to date for improving the friction, wear and hardness properties of metal materials. There are a few reports describing the use of PI1 for improving the tribological properties of non-metal materials such as polymers. These include modelling [8,9] the charging effects of dielectric surfaces and actual implantation of polymer materials [lo]. This is in contrast with the relatively large number of reports describing conventional ion beam implantation [ 1 l-141. In this paper we address the implantation treatment requirements of PII, contrasting metals with non-metals (polymers), and discuss work that is under way at the Hughes Research Laboratories (HRL) to pave the way for PI1 processing of non-metal polymer materials. There are four parameters that affect the successful
use of ion implantation for improving the tribological properties of metal and non-metal materials: (1) the implantation voltage, (2) the ion species, (3) the charge state of the ion species and (4) the ion dose. The
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implantation voltage and the charge state of the ion define the energy-to-ion ratio. The ion speciesand the ion dose determine the type of surface modification that is achieved.For a given material, the energy-to-ion ratio, the ion speciesand the dose must be selectedproperly in order to achieve the tribological improvement that is desired. In the examplescited above for PI1 treatment of metal tools and dies [l-7], respectableimprovements in wear life have been achieved by implanting Ni ions at modest energy-to-ion ratios and relatively high ion doses.For example, implantation of N,C via PI1 at 50-100 kV (energy-to-ion ratio range of 25-50 keV per Nt ion) results in an implanted ion depth range of 0.05-0.1 pm, The dose range of (2-4) x 1O1’ Nf cm-’ produces a peak implanted nitrogen atom concentration of about lo%-40%. This results in significant improvements in the surface wear resistanceand hardness of metals. The physical mechanismsresponsiblefor this include dislocation formation, hard inclusion compound formation and solid solution hardening [ 15-171. In contrast with metals, significant improvements in the surface wear resistance and hardness of polymers are achieved using relatively high energy-to-ion ratio values, but with relatively low ion doses.For example, ion beam implantation of NS ions into Kapton at 300 kV (an energy-to-ion ratio of 300keV per Nf ion) and a dose of 3 x 1015Ni cmm2increases the surface hardnessby a factor of 13 [ 111. This dose,which is two orders of magnitude lower than that required for hardening of metals,results in a peak implanted nitrogen atom concentration of only about O.l%-0.3%. The physical mechanism responsible for surface hardening of polymers is ion-induced cross-linking which produces a three-dimensionally connected carbon-rich rigid-surface network of atoms [ 11-141. In addition to atomic nitrogen, other atomic ions such as carbon are effective in achieving similar improvements in polymer hardness when implanted at even higher energy-to-ion ratios (1000 keV per ion). A provocative question to ask is how high energy-toion ratio values (300-1000 keV per ion) for nitrogen and carbon can be achieved in a PI1 system.At HRL, work is under way to address this issue.The approach being used involves first scaling PI1 voltage technology from 100 to 250 kV, and then implanting multiply charged ion speciesto achieve the desired energy-to-ion ratio. The aim is to implant doubly and triply charged atomic nitrogen ions (N2 ’ and N3 ‘) at 250 kV for an ion energy range of 500-750 keV, and an energy-to-ion ratio range of 500-750 keV per Nf ion. At high power levels (100 kW) this would pave the way for unparalleled PI1 processing capabilities for polymers, such as hardening large-scale Kapton and Teflon parts used in aerospace, defenseand commercial applications. In this paper, we
report on the work that is under way toward achieving a 250 kV 100kW PI1 facility to achieve this goal. To date, voltage scale-upto 250 kV has beenachieved and single-pulse ion implants have been conducted at this voltage. The development of a plasma source for the production of multiply charged nitrogen ions is in progress.A novel technique for implanting non-conducting objects in a PI1 systemhas been developed, as well as a technique for minimizing X-ray production in PII systems.In the sectionsbelow, we report on the progress that has been made in each of these areas. It should be noted that significant improvements in the tribological properties of metals (specifically tools and dies) can also be obtained by implanting ions at high energy-to-ion ratios [S]. However, it is believed that the requirement of both a high energy-to-ion ratio and a high ion dose may make PI1 processing of metals impractical. In the caseof polymers, it is believed that the requirement of both a high energy-to-ion ratio and a low ion dose (a reduction of two orders of magnitude in the ion dose compared with metals)allow for practical and tractable treatment of thesematerials via PI1 technology. It is for this reason, as well as the potentially large market for cost-effectivehardening of polymers via PII, that work is under way at HRL to develop such a capability.
2. PII technologyrequirementsfor polymer processing Four areas need to be addressed in order to develop PI1 technology for hardening polymers: (1) plasma processing technology; (2) high-voltage technology; (3) plasma production technology; (4) secondary-electron production and generation of X-rays. The plasma processingtechnology requirements include the development of treatment techniques for non-conductive (polymer) objects in a PI1 system.The high-voltage technology requirements include scaling up the present PI1 voltage capability from 100 to 250 kV and then upgrading the vacuum chamber design to accommodate this voltage increase,Plasma production requirements to implant polymers at high energy-to-ion ratios will require the development of a plasma source to produce multiply charged ions. Finally, secondaryelectron production and X-ray generation become significant for ion implantation at 250 kV. Therefore tractable techniques for minimizing X-ray production as well as handling secondary-electronpower deposition will be required. In the sections below, we describe progress that has been made in each of theseareas of technology as a step toward developing a PI1 processing capability for polymers.
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3. PII plasma processing technology
VACUUM CHAMBER
3.1. Treatment of non-conducting objects
In order to implant a polymer object at high energyto-ion ratio levels, a technique must be developed that allows the implantation of polymer (non-conducting) objects in a PI1 system. To understand the issues that must be addressed to accomplish this, consider first how a metal (conducting) object is implanted in a PI1 system. Fig. 1 shows a schematic diagram of a metal object in a PI1 system. The object is placed on a conducting support table in a vacuum chamber and then electrically connected to the output of a pulse modulator which provides implantation voltage pulses - V, to the support table and therefore to the object. A conducting object is an equipotential surface, and therefore the applied implantation voltage -V, develops uniformly over its entire surface. There is no voltage drop inside the object and the electric field (E,) lines across the plasma sheath terminate normal to the surface of the object as shown. Ions are implanted into the surface at the full implantation voltage independent of the size, thickness or shape of the object. When a non-conducting object is placed in a PI1 system for implantation, the situation is different as shown in Fig. 2. A non-conducting object is not an equipotential surface, and therefore the implantation voltage (-I$) applied to the support table does not develop uniformly over the surface of the object. A nonuniform voltage -V’ develops over the surface of the object. There is a voltage drop inside the object that results in an electric field ED inside the object. The electric field (Eb) lines across the plasma sheath do not terminate normal to the surface as they do with a conducting object, and ions are not implanted with the full implantation voltage. Surface charging during a
J
2 -vo‘K%DUCTlNG OBJECT
Fig. 2. Schematic diagram implanted via PII.
of how
object is
single pulse during PI1 can occur, causing severe arcing along the surface or through the bulk of the object. A technique developed to ion implant non-conducting objects in a PI1 system [lo] is shown schematically in Fig. 3. The non-conducting object to be implanted is placed on a conducting support table and then covered with a transparent conducting grid that is electrically connected to the support table as shown. The grid transparency is equal to or greater than 70% and it is displaced from the surface of the object by at least 0.5 inch or more to eliminate shadowing effects of the grid. The grid is made of a material that has a low sputter yield and is sufficiently flexible to conform easily to the same general surface features of the part. With the grid in place, the non-conducting object to be implanted is enclosed in a volume that is free of electric fields. When an implantation voltage pulse (V, = 100 kV) is applied to the support table, this voltage also develops along the whole grid surface that conforms to the whole surface of the non-conducting object. Because VACUUM CHAMBER
VACUUM CHAMBER
-L x
Fig. 1. Schematic diagram of how a metal (conducting) implanted via PII.
a non-conducting
object is
-Vo
METAL’ GRID
Fig. 3. Overlayer grid technique to implant a non-conducting via PII.
object
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it is transparent, a fraction (given by the open area of the grid) of the plasma ions that are accelerated toward the grid are implanted in the object’s surface with the full implantation voltage. Surface charging of the part surface still occurs when the positive ions are implanted in the part surface. However, secondary electrons are ejected from the metal grid by plasma ions intercepted by the metal and provide the necessary negative charge to discharge the positive-ion-implanted surface of the part. A systematic set of experiments were conducted to determine the grid transparency and the grid position with respect to the part surface to ensure a uniform implant over the non-uniform surface features of parts [lo]. Without the use of a grid, surface charging and arcing over the part surface was inevitable and often catastrophic. The amount of surface charging experienced was dependent on the voltage level used for implantation, the plasma density and the pulse repetition rate. No quantitative measurements of surface charging were made. Instead, surface charging was inferred from the arcing observed during an application of the applied voltage pulses. For example, for a plasma density set near 5 x lo8 cmm3, implantation could be conducted using 10 ps, 20 kV voltage pulses at a repetition rate of 500 Hz for small non-conductive polymer samples (1 cm wide x 1 cm wide x 0.5 cm thick) without observing arcing. However, at 100 kV, significant surface charging was observed in a single pulse that resulted in surface and volume arcing that severely damaged the surface. When the overlayer grid described above was used, all arcing phenomena (and presumably surface charging) were eliminated. The implantation voltage and the plasma density could be set at any arbitrary value without observing arcing phenomena. Dose profiling of small stainless steel coupons placed under the grid on the surface of samples of varying thickness (0.5-30 cm) confirmed that the overlayer grid functioned properly, i.e. ions were implanted with the full applied voltage of 100 kV. The overlayer grid technique was successfully used to implant a three-piece polymer-composite object; each piece weighing about 2300 lb, giving a total weight of 7000 lb [4,10]. The largest piece had dimensions 3 ft wide x 6 ft long x 1 ft thick. The three-piece polymer object was implanted in the HRL PI1 facility shown in Fig. 4. It consists of a vacuum chamber 4 ft in diameter x 8 ft long and a high-voltage (100 kV) highpower (100 kW) pulse modulator. The design and operating characteristics of this facility are presented in two companion papers [ 18,191. Fig. 5 shows a photograph of one piece of the polymer-composite object to be implanted in the facility with a stainless steel grid placed over its surface. Molecular nitrogen ions were implanted into the object at a voltage of 100 kV (an energy-to-ion ratio of
VACUUMCHAMBER
PULSE MODULATOR
Fig. 4. The HRL PI1 facility. TRANSPARENT GRID PLACED OVERPARTSURFACE
Fig. 5. One section of a three-piece polymer object to be implanted via the overlayer grid technique.
50 keV per N* ion) and a dose of 3 x 1015 N* cmT2. The total time required to achieve this dose level was 5 min. This represents the first demonstration of largescale fast ion implantation of a polymer object in a PII facility. The depth and dose of the implanted nitrogen ions were validated using a technique that is described in a separate publication [20]. The surface hardness of the PII-treated polymer object was inferred from hardness measurements of small polymer coupons that were placed in benign iocations of the actua1 object and implanted at the same time as the object. Coop indentor measurements [4] of these coupons conducted at the Oak Ridge National Laboratory (ORNL) indicated an improvement of a factor of 2 in the hardness compared with pristine (unimplanted) polymer material. This improvement in hardness was insufficient to improve the wear resistance of the polymer object significantly when it was evaluated under actual manufacturing conditions. The reason attributed to this was the low energy-to-ion ratio (50 keV per N’ ion) of
J.N. Matossian,
R. WeiJSurface
and Coatings
implanted nitrogen. A subsequent ion beam implant study was conducted with ORNL to define the energyto-ion ratio required for significant surface hardening and wear-resistance improvement of the polymer material. It was found that the implantation of atomic nitrogen (or carbon) ions at 300 kV (energy-to-ion ratio of 300 keV per N+) and a dose of 3 x lOi Nf cm-’ results in an improvement in hardness by a factor of 20 and a significant (but not quantifiable) improved surface wear resistance. To achieve this result via PII, the implantation voltage capability must be scaled from 100 to 250 kV and a plasma source must be developed for the production of multiply charged nitrogen ions.
4. PII high-voltage technology 4.1. Scaling from 100 to 250 kV
To scale the voltage capability of PI1 technology, the design of the pulse modulator and the vacuum chamber must be upgraded. The present state-of-the-art PI1 voltage capability [4,18] is 100 kV. Fig. 6(a) shows a schematic diagram of the HRL 100 kV pulse modulator connected to the 4 ft diameter x 8 ft long PI1 vacuum chamber. A 0.5 uF capacitor is resistively charged to 100 kV and then voltage modulated using the Hughes 8455H CrossatronTM switch to produce 100 kV negative polarity square-wave voltage pulses. A 5 kR resistive load is connected at the output of the pulse modulator, which is in parallel with the plasma load inside the vacuum chamber during implantation. To upgrade from 100 to 250 kV, a 2.5 : 1 step-up pulse transformer was installed at the output of the 100 kV pulse modulator circuit as shown in Fig. 6(b) and the 5 kR resistive load was moved to the output of the pulse transformer, again in parallel with the plasma load. The pulse transformer uses an autotransformer design and was developed by Stangenes Industries (Sunnyvale, CA) to produce near-square-wave voltage pulses with a pulse-width range of 2-30 us and a rise and fall time of 4 us. The total rms current capability of the transformer is 29 A. At 250 kV, the maximum peak pulsed output current is 920 A at a frequency of 100 Hz. Some design features of the vacuum chamber were upgraded to accommodate the increased voltage capability provided by the pulse transformer. To make the electrical connection from the output of the pulse transformer to the vacuum chamber, a high-voltage (300 kV) epoxy cable termination was installed in the base of the vacuum chamber. The cable termination had provision for four cooling lines of diameter 1.27 cm (0.5 inch) for cooling the support table in the vacuum chamber. Details of the design features of the support table are presented in a companion paper [ 181. The dimensions of the table are 3 ft x 3 ft and it is supported by four
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alumina rods of diameter 2 inches which also provide electrical isolation (up to 400 kV dc) from the grounded vacuum chamber walls. The perimeter of the table is surrounded by a corona ring of diameter 1 inch, and there is a 4 inch gap between this and the chamber walls. We conducted preliminary implantation tests at various voltage levels using the pulse modulator and vacuum chamber PI1 facility shown in Fig. 4. These preliminary tests were intended to compare the tribological properties of polymer materials implanted with argon and nitrogen. Fig. 7 shows the time-phased current and voltage waveforms for 180 kV (Fig. 7(a)) and 250 kV (Fig. 7(b)) voltage pulses via PI1 using argon ions. No actual parts were implanted for these preliminary shakedown tests which were intended to demonstrate the feasibility of conducting 250 kV implants via the PI1 process. Since the implanted dose levels of polymers are two to three orders of magnitude smaller (of the order of 1014-1015 cm-“) compared with metals (of the order of 1Ol7 cmm2), the required current per pulse can be two to three orders of magnitude smaller (i.e. 0. 3-3 A for polymers compared with 300 A for metals) to achieve the required dose level in comparable times. Fig. 7 shows a peak pulsed current of 2 A (ion current plus secondaryelectron current) for voltage levels of 180 and 250 kV. Stable operation was demonstrated for pulse repetition rates up to 10 Hz for 180 kV voltage pulses; however, only single-shot pulses were demonstrated for 250 kV voltage pulses. The reason for the limitation to singleshot pulses at 250 kV is that arcing was occasionally experienced from pulse to pulse, suggesting that the vacuum gap between the support table and the vacuum chamber wall (originally set for stable operation at 100 kV) must be increased for 250 kV operation. The necessary upgrades to accomplish this are under way. For these voltage levels, we limited the pulse width to a total of 10 us. Since the pulse transformer was designed for a 3-4 us rise and fall time, the current and voltage pulse waveforms in Fig. 7 show a near-semisinusoidal shape instead of a near-square-wave shape. Experiments will continue this year with the aim of demonstrating stable repetitive operation at voltage levels of 200-250 kV and at higher current levels and longer pulse durations using argon and nitrogen ions implanted into actual polymer parts. High-volt,age high-power diodes are also planned for installation at the output of the pulse transformer to eliminate the ‘voltage (and current) ringing shown in Fig. 7 during the period when the pulse is off. 5. PI1 plasma production technology 5.1. Plasma source design for multiply charged ions
With the feasibility of conducting 200-250 kV PI1 implants established, the next step ‘in conducting high
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J.N. Matossian, R Wei/Surface and Coatings Technology 8.7 (1996) 111-119 9512-15-054a fin--------)
8455H CROSSATRON
/
100 kV POWER SUPPLY
kR
PULSE MQDULATOR
VACUUM
CHAMBER
(a)
PLASMA SHYTH
OBJECT ,
8455H CROSSATRON
Pf 100 kV POWER SUPPLY
---
PULSE MODULATOR
VACUUM
CHAMBER
04 Fig. 6. Schematic diagram of (a) the 100 kV pulse modulator and (b) the upgrading and modification required to achieve 250 kV voltage levels.
energy-to-ion ratio implants of nitrogen ions into polymersin a PI1 systemconsistsin developing the capability to produce multiply charged nitrogen ions. It should be noted that alternative approachesare also being pursued. These include the development of a single charged Nf plasma source [21] and the implantation of NH; ions. In the caseof N+ ions, a 250 kV implant would result in an energy-to-ion ratio of 250 KeV per Nf ion, which should be suitable for hardening polymers. Implantation at 250 KV using NH: ions, would result in a slightly lower energy-to-ion ratio level. Although these approaches are being investigated, the main emphasisis on developing a multiply charged atomic nitrogen ion plasma source,sincethis approach would allow for more
flexibility and capability to implant over a wider range of energy-to-ion ratio values. The approach we are using for a multiply charged ion plasma source is based on PIG ion source technology [22]. Other work on the development of multiply charged ion plasma sources is also under way at other research organizations [ 231, In collaboration with the General PhysicsInstitute of the Russian Academy of Sciences(PLASMAIOFAN, Moscow, Russia, Agreement Sl 402809-2, 1994), we have designed and built a PIG plasma ion source (Fig. 8). The source operates in the pulsed mode and is designed to produce over 0.3 A of double and triply charged atomic nitrogen ions. This current should be
J.N. Matossian, R WeiJSurface and Coatings Technology 85 (1996) 111-119 04
40 kV div. 2A div.
-180 -
-
kV 2A
2 p secldiv. f=lOHt 04
o9
1
div.
2A div.
-250
117
duced via multistep ionization [24]. The source has no extraction electrodes; plasma is designed to diffuse out of the source and into the vacuum chamber. The source has been operated off-line at PLASMAIOFAN and is planned for preliminary testing as a remotely located plasma source [ 25,261 in the HRL PI1 vacuum chamber in the near future. These experiments will measure the ion species fraction of nitrogen ions using an EXB filtered analyzer. Charge exchange effects have not yet been studied analytically for multiply charged nitrogen ions, although they have been investigated experimentally for singly charged nitrogen ions [27]. It has been found that, in the anticipated operating pressure range of mid 10m4 Torr, singly charged atomic nitrogen ions can be transported over a distance of 1 m from the plasma source without significant (20%) degradation in density. It is planned to make measurements of this type for the multiply charged ion plasma source in an offline chamber in order to define the charge exchange characteristics of the source prior to its installation in the PI1 chamber. These experiments are scheduled for 1996.
kV
-2A
t O-
2 p secldiv. SINGLE SHOT
6. X-ray production in a PIT system 6.1. Techniques to minimize X-rays
Fig. 7. Time-phased current and voltage waveforms for (a) 180 kV and (b) 250 kV ion implants via PII.
sufficient to achieve a dose of 1 x 1015 cm-’ in a time period of less than 1 min for 100 kV implantation voltages with a duty factor of 0.01% (pulse duration of 10 us and a frequency of 100 Hz). The multiply charged atomic nitrogen ions are pro-
In addition to the formidable challenges involved in achieving a 250 kV voltage level in a PI1 facility, there are considerable problems in handling X-rays that are produced during 250 kV implants. In PII, implanted ions eject secondary electrons from the surface of the object being implanted. These electrons are re-accelerated across the plasma sheath surrounding the
Fig. 8. The PIG ion plasma source for producing multiply charged nitrogen ions for PII.
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object and then impact the vacuum chamber walls to produce X-rays. At voltages of 100kV, singly charged ions are accelerated across the plasma sheath at an energy of 100keV and secondary electrons are re-accelerated across the plasma sheath at the same energy. At this energy,0.25 inches of lead shielding is required to ensure the safetyof laboratory and personnel. This thicknessof lead shielding completely surrounds the exterior of the vacuum chamber shown in Fig. 4. At voltages of 250 kV, triply charged ions will be accelerated across the plasma sheath at an energy of 750 keV, but secondary electrons are re-accelerated across the plasma sheath at 250 keV. In addition, the yield of secondary electrons for multiply charged ions is greater (50% or more) than that for singly charged ions [27] and will contribute to a higher X-ray intensity than that obtained with singly charged ions. Thus, the X-ray shielding technique that is used when operating at 250 kV implantation voltage levels for either singly or doubly charged ions must contend with 250 keV electrons. Above 100keV, the K-shell absorption edge for lead is exceeded,and the absorption cross-section for X-rays drops exponentially. At 250 kV, the absorption cross-sectionof lead is reduced by almost a factor of 15, which would have a significant effect on the thickness of lead required for 250 kV operation in a PI1 facility. We have investigated the use of a new proprietary technique [28] for minimizing X-ray production in a PI1 system.It consists of electrostatic confinement of the secondary electrons in the chamber volume and is shown schematically in Fig. 9. An enclosure is placed along the perimeter of the chamber that is isolated from the chamber walls and electrically connected to the support table. A grounded plasma is produced via a grounded remotely located plasma source as shown. When the voltage pulse is applied to the part, it is also
applied to the enclosure. Ions implanted into the part (and the enclosure) eject secondary electrons that are reflected at the enclosure boundary and are prevented from impacting a ground-potential surface other than the small area of the plasma source. At a voltage of 50 kV, the technique was successful in reducing the normal X-ray level from 20 mrem h-’ to a level that was below the minimum detection limit of the dosimeter. At 80 kV operation, the X-ray level was reduced by a factor of four. This may be due to the increased probability of electron interception by the grounded plasma source at the higher voltages. Experiments to confirm this explanation, as well as testing the technique at voltages of loo-250 kV, are planned for 1997. The use of multiply charged ions can also be used to reduce X-ray production at low implantation voltage levels. Consider the use of triply charged nitrogen ions implanted at 33 kV. The effective energy of the nitrogen ion would be 99 keV, while the secondary-electron energy would be only 33 keV. At this energy, the X-ray production is quite low and would be adequately handled by X-ray reduction techniques [ZS], Furthermore, the requirement of a 33 kV PII pulse modulation would significantly simplify the overall systemdesign and would be very attractive for commercial applications. This implantation scenario is being explored. The effect of a higher secondary-electron yield for multiply charged ions has not been accounted for in this simple analysis, and this will be necessaryin actual practice. 7. Conclusion
Ion implantation for hardening metals requires modest energy-to-ion ratio levels (25-50 keV per Nt ion) and high ion doses(2-4 x 1017N’ cm-“). In contrast, ion implantation for hardening polymers requires high energy-to-ion ratios (300 keV per N’ ion) and low PLASMA
SOURCE
METAL ENCLOSURE AT SAME POTENTIAL AS PART \
REFLECTED ELECTRON PATH
PLASMA k SHEATH AT PART AND ENCLOSURE
Fig. 9. Schematic diagram of the electrostatic confinement of secondary electrons.
J.N. Matossian,
R Wei]Surface
and Coatings
ion doses (3 x 1OL5 N+ cm-“). Achieving this in a PI1 system requires scaling the voltage to 250 kV and producing multiply charged nitrogen ions. We have reported on the challenges faced and the progress toward achieving this capability in a PI1 facility. To date, single-pulse 250 kV implants of ions have been demonstrated and a plasma source for producing multiply charged nitrogen ions has been built. A new proprietary technique based on electrostatic confinement of secondary electrons has been developed to reduce X-ray production at the 250 kV voltage level. Full system integration for conducting 250 kV implants of multiply charged nitrogen ions is under way.
Acknowledgments The authors wish to thank Dr. Dan Goebel for the transformer design, and Mr. John Elverum for the design and fabrication of the pulse modulator and vacuum chamber hardware.
References [1] J.R. Reeber and K. Sridharan, Adu. Muter. Processes, 146(6) (1994) 21. [2] S.M. MaLik, K. Sridharan, R.P. Fetherston, A. Chen and J.R. Conrad, J. Vat. Sci. Technol. B, 12(2) (1994) 873. [S] B.P. Wood, I. Henins, R.J. Gribble, W.A. Reass, R.J. Faehl, M.A. Natasi and D.J. Rej, .J. T/UC.Sci. Technol. B, 12(2) (1994) 870. [4] J.N. Matossian, J. Vuc. Sci. Technol. B, 12(2) (1994) 850. [S] J.N. Matossian, J.J. Vajo, J.A. Wysocki and M. Bellon, Surf. Coat.
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62(11) (1987) 4591. c71 J. Conrad and K. Sridharan (eds.), J. Vuc. Sci. Technol. B, I2(2) (1994). Bl W. En and N.W. Cheung, Nucl. [email protected]. B, 96 (1995) 435. cg1 G.A. Emmert, J. Vat. Sci. Technol. B, 12(2) (1994) 880. e101 J.N. Matossian, R.W. Schumacher and D.M. Pepper, US Patent 5,347,456, 20 December 1994. Cl11 E.H. Lee, M.B. Lewis, P.J. Blau and L.K. Mansur, J. Mater. Res., 6(3) (1991) 610. 1121 E.H. Lee, G.R. Rao and L.K. Mansur, J. Muter. Res., 7(7) (1992) 1900. Cl31 G.R. Rao, E.H. Lee and L.K. Mansur, Mater. Res. Sot. Symp. Proc., 239 (1992) 189. [141 R. Dagani, Chern. Eng. News, (9 Jan 1995) 24. Cl51 R. Wei, P.J. Wilbur, W.S. Sampath, D.L. Williamson, U. Qu and L. Wang, ASME J. Tribal., 112 (1990) 27. Cl61 D.L. Williamson, L. Wang, R. Wei and P.J. Wilbur, Mater. Lett., 9 (1990) 302. Cl71 R. Wei, P.J. Wilbur, 0. Ozturk and D.L. Williamson, Nucl. Instwn. Meth. B, 59-60 (1991) 731. 1181 J.N. Matossian and D.M. Goebel, Surf. Coat. Technol., this volume. [I91 J.N. Matossian and R. Wei, SurL Coat. Technol., this volume. 1201 J.N. Matossian and J.J. Vajo, US Patent 5,455,061, 3 October 1995. c2.11D. Gregoire, J.N. Matossian and J.D. Williams, to be published. ix4 B.F. Gavin, in LG. Brown (Ed.), Physics and Technology of Ion Sources, Wiley, New York, 1989. 1231 CC. Tsai, G.C. Barber, E.H. Lee, L.K. Mansur, D.E. Schechter, W.L. Stirling, J.H. Whealton and J.M. Williams, Bull. APS 37th Annual Meeting, Div. Plasma Physics, 6-10 Nov. 1995, Louisville, KY, Poster 2Rl. [241 A. Li-Scholz (Ed.), At. Data Nucl. Dam Tables, 36(2) (1987). c251 J.N. Matossian and D.M. Goebel, US Patent 5,218,179, 8 June 1993. C261J.N. Matossian and D.M. Goebel, US Patent 5,296,272, 22 March 1994. on Metal c271 M. Kaminsky, Atomic and Ionic Impact Phenomena Surfaces, Academic Press, New York, 1965. C281J.N. Matossian and J.D. Williams, US Patent 5,498,290, 12 March 1996.
Challenges and progress toward a 250 kV, 100 kW plasma ion implantation facility JesseN. Matossian, Ronghua Wei Plasma Physics Laboratory, Hughes Research Laboratories, Malibu, CA 90265, USA
Abstract Plasma ion implantation (PII) is a large-scale cost-effective technique for modifying the surface properties of materials via omnidirectional ion implantation. The implantation voltage and ion species define the energy-to-ion ratio which, together with the ion dose, determines the effectiveness of an implant in achieving successful tribological improvements in materials. For most metal treatment applications of PII, modest energy-to-ion ratios and high ion doses are required to achieve respectable improvements in hardness and wear life. For example, implantation of N: ions into metal tools and diesvia PI1 at 100kV (energy-to-ionratio of 50keV per N+ ion) at a doseof 3 x 1Ol7N’ cm-’ can result in an increaseof wear life by a factor of 2-8. In contrast with metals,polymers require high energy-to-ion ratios and low dosesto achievesign&ant improvementsin hardnessand wear life. For example,ion beamimplantation of N+ ions into Kapton at 300kV (energy-to-ionratio of 300keV per N+ ion) and a dose of 3 x 10” N+ cm-’ can increasehardnessby a factor of 13. This energy-to-ion ratio is six times higher than that achievable using presentPII technology. A provocative questionto ask is how this can be achievedin a PI1 system.Work is under way at the Hughes ResearchLaboratories (HRL) to addressthis issue.The approach being used involves first scaling PI1 voltage technology from 100 to 250kV, and then implanting doubly and triply chargedatomic nitrogen ions at 250kV to achieve an energy-to-ion ratio range of 500-750keV per Nf ion. To date single-pulseion implants have been conductedat 250kV in the HRL PI1 facility. A plasmasourcehas beenbuilt for the production of multiply chargednitrogen ions. A techniquefor treating non-conductingobjectsin a PI1 facility hasbeendeveloped,and a methodof reducingX-ray production in a PI1 systemoperating at 250kV hasbeeninvestigated. Keywords:Plasmaion implantation; Surfacemodification; Tribology
1. Introduction Plasma ion implantation (PII) is a large-scale implementation [l-7] of conventional ion beam implantation. Much of the work reported has involved the use of PI1 to improve the tribological (friction and wear) properties of materials [7]. In many of these applications N: ions have been implanted into metals, specifically metal tools and dies. For example, using a voltage range of 50-100 kV and a dose range of (2-4) x 1O1’ Nf cme2, PI1 has been used to achieve (1) an improvement of two to three times in the wear life of Co/WC drill bits used in printed wiring board fabrication [ 51, (2) an improvement of five to eight times in the wear life of TiN-coated cutting tools [4], (3) an improvement of four times in the wear life of tool steel dies used for producing bolts [2] and (4) an improvement of three to four times in the wear life of M-2 steel pierce punches [Cl. Other ions that have been used in PI1 treatments of metal tools include methane and ammonia; however, molecular 0257~8972/96/$15.00 0 1996ElsevierScience S.A.All riahtsreserved
nitrogen remains one of the most common ion species used to date for improving the friction, wear and hardness properties of metal materials. There are a few reports describing the use of PI1 for improving the tribological properties of non-metal materials such as polymers. These include modelling [8,9] the charging effects of dielectric surfaces and actual implantation of polymer materials [lo]. This is in contrast with the relatively large number of reports describing conventional ion beam implantation [ 1 l-141. In this paper we address the implantation treatment requirements of PII, contrasting metals with non-metals (polymers), and discuss work that is under way at the Hughes Research Laboratories (HRL) to pave the way for PI1 processing of non-metal polymer materials. There are four parameters that affect the successful
use of ion implantation for improving the tribological properties of metal and non-metal materials: (1) the implantation voltage, (2) the ion species, (3) the charge state of the ion species and (4) the ion dose. The
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implantation voltage and the charge state of the ion define the energy-to-ion ratio. The ion speciesand the ion dose determine the type of surface modification that is achieved.For a given material, the energy-to-ion ratio, the ion speciesand the dose must be selectedproperly in order to achieve the tribological improvement that is desired. In the examplescited above for PI1 treatment of metal tools and dies [l-7], respectableimprovements in wear life have been achieved by implanting Ni ions at modest energy-to-ion ratios and relatively high ion doses.For example, implantation of N,C via PI1 at 50-100 kV (energy-to-ion ratio range of 25-50 keV per Nt ion) results in an implanted ion depth range of 0.05-0.1 pm, The dose range of (2-4) x 1O1’ Nf cm-’ produces a peak implanted nitrogen atom concentration of about lo%-40%. This results in significant improvements in the surface wear resistanceand hardness of metals. The physical mechanismsresponsiblefor this include dislocation formation, hard inclusion compound formation and solid solution hardening [ 15-171. In contrast with metals, significant improvements in the surface wear resistance and hardness of polymers are achieved using relatively high energy-to-ion ratio values, but with relatively low ion doses.For example, ion beam implantation of NS ions into Kapton at 300 kV (an energy-to-ion ratio of 300keV per Nf ion) and a dose of 3 x 1015Ni cmm2increases the surface hardnessby a factor of 13 [ 111. This dose,which is two orders of magnitude lower than that required for hardening of metals,results in a peak implanted nitrogen atom concentration of only about O.l%-0.3%. The physical mechanism responsible for surface hardening of polymers is ion-induced cross-linking which produces a three-dimensionally connected carbon-rich rigid-surface network of atoms [ 11-141. In addition to atomic nitrogen, other atomic ions such as carbon are effective in achieving similar improvements in polymer hardness when implanted at even higher energy-to-ion ratios (1000 keV per ion). A provocative question to ask is how high energy-toion ratio values (300-1000 keV per ion) for nitrogen and carbon can be achieved in a PI1 system.At HRL, work is under way to address this issue.The approach being used involves first scaling PI1 voltage technology from 100 to 250 kV, and then implanting multiply charged ion speciesto achieve the desired energy-to-ion ratio. The aim is to implant doubly and triply charged atomic nitrogen ions (N2 ’ and N3 ‘) at 250 kV for an ion energy range of 500-750 keV, and an energy-to-ion ratio range of 500-750 keV per Nf ion. At high power levels (100 kW) this would pave the way for unparalleled PI1 processing capabilities for polymers, such as hardening large-scale Kapton and Teflon parts used in aerospace, defenseand commercial applications. In this paper, we
report on the work that is under way toward achieving a 250 kV 100kW PI1 facility to achieve this goal. To date, voltage scale-upto 250 kV has beenachieved and single-pulse ion implants have been conducted at this voltage. The development of a plasma source for the production of multiply charged nitrogen ions is in progress.A novel technique for implanting non-conducting objects in a PI1 systemhas been developed, as well as a technique for minimizing X-ray production in PII systems.In the sectionsbelow, we report on the progress that has been made in each of these areas. It should be noted that significant improvements in the tribological properties of metals (specifically tools and dies) can also be obtained by implanting ions at high energy-to-ion ratios [S]. However, it is believed that the requirement of both a high energy-to-ion ratio and a high ion dose may make PI1 processing of metals impractical. In the caseof polymers, it is believed that the requirement of both a high energy-to-ion ratio and a low ion dose (a reduction of two orders of magnitude in the ion dose compared with metals)allow for practical and tractable treatment of thesematerials via PI1 technology. It is for this reason, as well as the potentially large market for cost-effectivehardening of polymers via PII, that work is under way at HRL to develop such a capability.
2. PII technologyrequirementsfor polymer processing Four areas need to be addressed in order to develop PI1 technology for hardening polymers: (1) plasma processing technology; (2) high-voltage technology; (3) plasma production technology; (4) secondary-electron production and generation of X-rays. The plasma processingtechnology requirements include the development of treatment techniques for non-conductive (polymer) objects in a PI1 system.The high-voltage technology requirements include scaling up the present PI1 voltage capability from 100 to 250 kV and then upgrading the vacuum chamber design to accommodate this voltage increase,Plasma production requirements to implant polymers at high energy-to-ion ratios will require the development of a plasma source to produce multiply charged ions. Finally, secondaryelectron production and X-ray generation become significant for ion implantation at 250 kV. Therefore tractable techniques for minimizing X-ray production as well as handling secondary-electronpower deposition will be required. In the sections below, we describe progress that has been made in each of theseareas of technology as a step toward developing a PI1 processing capability for polymers.
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3. PII plasma processing technology
VACUUM CHAMBER
3.1. Treatment of non-conducting objects
In order to implant a polymer object at high energyto-ion ratio levels, a technique must be developed that allows the implantation of polymer (non-conducting) objects in a PI1 system. To understand the issues that must be addressed to accomplish this, consider first how a metal (conducting) object is implanted in a PI1 system. Fig. 1 shows a schematic diagram of a metal object in a PI1 system. The object is placed on a conducting support table in a vacuum chamber and then electrically connected to the output of a pulse modulator which provides implantation voltage pulses - V, to the support table and therefore to the object. A conducting object is an equipotential surface, and therefore the applied implantation voltage -V, develops uniformly over its entire surface. There is no voltage drop inside the object and the electric field (E,) lines across the plasma sheath terminate normal to the surface of the object as shown. Ions are implanted into the surface at the full implantation voltage independent of the size, thickness or shape of the object. When a non-conducting object is placed in a PI1 system for implantation, the situation is different as shown in Fig. 2. A non-conducting object is not an equipotential surface, and therefore the implantation voltage (-I$) applied to the support table does not develop uniformly over the surface of the object. A nonuniform voltage -V’ develops over the surface of the object. There is a voltage drop inside the object that results in an electric field ED inside the object. The electric field (Eb) lines across the plasma sheath do not terminate normal to the surface as they do with a conducting object, and ions are not implanted with the full implantation voltage. Surface charging during a
J
2 -vo‘K%DUCTlNG OBJECT
Fig. 2. Schematic diagram implanted via PII.
of how
object is
single pulse during PI1 can occur, causing severe arcing along the surface or through the bulk of the object. A technique developed to ion implant non-conducting objects in a PI1 system [lo] is shown schematically in Fig. 3. The non-conducting object to be implanted is placed on a conducting support table and then covered with a transparent conducting grid that is electrically connected to the support table as shown. The grid transparency is equal to or greater than 70% and it is displaced from the surface of the object by at least 0.5 inch or more to eliminate shadowing effects of the grid. The grid is made of a material that has a low sputter yield and is sufficiently flexible to conform easily to the same general surface features of the part. With the grid in place, the non-conducting object to be implanted is enclosed in a volume that is free of electric fields. When an implantation voltage pulse (V, = 100 kV) is applied to the support table, this voltage also develops along the whole grid surface that conforms to the whole surface of the non-conducting object. Because VACUUM CHAMBER
VACUUM CHAMBER
-L x
Fig. 1. Schematic diagram of how a metal (conducting) implanted via PII.
a non-conducting
object is
-Vo
METAL’ GRID
Fig. 3. Overlayer grid technique to implant a non-conducting via PII.
object
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it is transparent, a fraction (given by the open area of the grid) of the plasma ions that are accelerated toward the grid are implanted in the object’s surface with the full implantation voltage. Surface charging of the part surface still occurs when the positive ions are implanted in the part surface. However, secondary electrons are ejected from the metal grid by plasma ions intercepted by the metal and provide the necessary negative charge to discharge the positive-ion-implanted surface of the part. A systematic set of experiments were conducted to determine the grid transparency and the grid position with respect to the part surface to ensure a uniform implant over the non-uniform surface features of parts [lo]. Without the use of a grid, surface charging and arcing over the part surface was inevitable and often catastrophic. The amount of surface charging experienced was dependent on the voltage level used for implantation, the plasma density and the pulse repetition rate. No quantitative measurements of surface charging were made. Instead, surface charging was inferred from the arcing observed during an application of the applied voltage pulses. For example, for a plasma density set near 5 x lo8 cmm3, implantation could be conducted using 10 ps, 20 kV voltage pulses at a repetition rate of 500 Hz for small non-conductive polymer samples (1 cm wide x 1 cm wide x 0.5 cm thick) without observing arcing. However, at 100 kV, significant surface charging was observed in a single pulse that resulted in surface and volume arcing that severely damaged the surface. When the overlayer grid described above was used, all arcing phenomena (and presumably surface charging) were eliminated. The implantation voltage and the plasma density could be set at any arbitrary value without observing arcing phenomena. Dose profiling of small stainless steel coupons placed under the grid on the surface of samples of varying thickness (0.5-30 cm) confirmed that the overlayer grid functioned properly, i.e. ions were implanted with the full applied voltage of 100 kV. The overlayer grid technique was successfully used to implant a three-piece polymer-composite object; each piece weighing about 2300 lb, giving a total weight of 7000 lb [4,10]. The largest piece had dimensions 3 ft wide x 6 ft long x 1 ft thick. The three-piece polymer object was implanted in the HRL PI1 facility shown in Fig. 4. It consists of a vacuum chamber 4 ft in diameter x 8 ft long and a high-voltage (100 kV) highpower (100 kW) pulse modulator. The design and operating characteristics of this facility are presented in two companion papers [ 18,191. Fig. 5 shows a photograph of one piece of the polymer-composite object to be implanted in the facility with a stainless steel grid placed over its surface. Molecular nitrogen ions were implanted into the object at a voltage of 100 kV (an energy-to-ion ratio of
VACUUMCHAMBER
PULSE MODULATOR
Fig. 4. The HRL PI1 facility. TRANSPARENT GRID PLACED OVERPARTSURFACE
Fig. 5. One section of a three-piece polymer object to be implanted via the overlayer grid technique.
50 keV per N* ion) and a dose of 3 x 1015 N* cmT2. The total time required to achieve this dose level was 5 min. This represents the first demonstration of largescale fast ion implantation of a polymer object in a PII facility. The depth and dose of the implanted nitrogen ions were validated using a technique that is described in a separate publication [20]. The surface hardness of the PII-treated polymer object was inferred from hardness measurements of small polymer coupons that were placed in benign iocations of the actua1 object and implanted at the same time as the object. Coop indentor measurements [4] of these coupons conducted at the Oak Ridge National Laboratory (ORNL) indicated an improvement of a factor of 2 in the hardness compared with pristine (unimplanted) polymer material. This improvement in hardness was insufficient to improve the wear resistance of the polymer object significantly when it was evaluated under actual manufacturing conditions. The reason attributed to this was the low energy-to-ion ratio (50 keV per N’ ion) of
J.N. Matossian,
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and Coatings
implanted nitrogen. A subsequent ion beam implant study was conducted with ORNL to define the energyto-ion ratio required for significant surface hardening and wear-resistance improvement of the polymer material. It was found that the implantation of atomic nitrogen (or carbon) ions at 300 kV (energy-to-ion ratio of 300 keV per N+) and a dose of 3 x lOi Nf cm-’ results in an improvement in hardness by a factor of 20 and a significant (but not quantifiable) improved surface wear resistance. To achieve this result via PII, the implantation voltage capability must be scaled from 100 to 250 kV and a plasma source must be developed for the production of multiply charged nitrogen ions.
4. PII high-voltage technology 4.1. Scaling from 100 to 250 kV
To scale the voltage capability of PI1 technology, the design of the pulse modulator and the vacuum chamber must be upgraded. The present state-of-the-art PI1 voltage capability [4,18] is 100 kV. Fig. 6(a) shows a schematic diagram of the HRL 100 kV pulse modulator connected to the 4 ft diameter x 8 ft long PI1 vacuum chamber. A 0.5 uF capacitor is resistively charged to 100 kV and then voltage modulated using the Hughes 8455H CrossatronTM switch to produce 100 kV negative polarity square-wave voltage pulses. A 5 kR resistive load is connected at the output of the pulse modulator, which is in parallel with the plasma load inside the vacuum chamber during implantation. To upgrade from 100 to 250 kV, a 2.5 : 1 step-up pulse transformer was installed at the output of the 100 kV pulse modulator circuit as shown in Fig. 6(b) and the 5 kR resistive load was moved to the output of the pulse transformer, again in parallel with the plasma load. The pulse transformer uses an autotransformer design and was developed by Stangenes Industries (Sunnyvale, CA) to produce near-square-wave voltage pulses with a pulse-width range of 2-30 us and a rise and fall time of 4 us. The total rms current capability of the transformer is 29 A. At 250 kV, the maximum peak pulsed output current is 920 A at a frequency of 100 Hz. Some design features of the vacuum chamber were upgraded to accommodate the increased voltage capability provided by the pulse transformer. To make the electrical connection from the output of the pulse transformer to the vacuum chamber, a high-voltage (300 kV) epoxy cable termination was installed in the base of the vacuum chamber. The cable termination had provision for four cooling lines of diameter 1.27 cm (0.5 inch) for cooling the support table in the vacuum chamber. Details of the design features of the support table are presented in a companion paper [ 181. The dimensions of the table are 3 ft x 3 ft and it is supported by four
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alumina rods of diameter 2 inches which also provide electrical isolation (up to 400 kV dc) from the grounded vacuum chamber walls. The perimeter of the table is surrounded by a corona ring of diameter 1 inch, and there is a 4 inch gap between this and the chamber walls. We conducted preliminary implantation tests at various voltage levels using the pulse modulator and vacuum chamber PI1 facility shown in Fig. 4. These preliminary tests were intended to compare the tribological properties of polymer materials implanted with argon and nitrogen. Fig. 7 shows the time-phased current and voltage waveforms for 180 kV (Fig. 7(a)) and 250 kV (Fig. 7(b)) voltage pulses via PI1 using argon ions. No actual parts were implanted for these preliminary shakedown tests which were intended to demonstrate the feasibility of conducting 250 kV implants via the PI1 process. Since the implanted dose levels of polymers are two to three orders of magnitude smaller (of the order of 1014-1015 cm-“) compared with metals (of the order of 1Ol7 cmm2), the required current per pulse can be two to three orders of magnitude smaller (i.e. 0. 3-3 A for polymers compared with 300 A for metals) to achieve the required dose level in comparable times. Fig. 7 shows a peak pulsed current of 2 A (ion current plus secondaryelectron current) for voltage levels of 180 and 250 kV. Stable operation was demonstrated for pulse repetition rates up to 10 Hz for 180 kV voltage pulses; however, only single-shot pulses were demonstrated for 250 kV voltage pulses. The reason for the limitation to singleshot pulses at 250 kV is that arcing was occasionally experienced from pulse to pulse, suggesting that the vacuum gap between the support table and the vacuum chamber wall (originally set for stable operation at 100 kV) must be increased for 250 kV operation. The necessary upgrades to accomplish this are under way. For these voltage levels, we limited the pulse width to a total of 10 us. Since the pulse transformer was designed for a 3-4 us rise and fall time, the current and voltage pulse waveforms in Fig. 7 show a near-semisinusoidal shape instead of a near-square-wave shape. Experiments will continue this year with the aim of demonstrating stable repetitive operation at voltage levels of 200-250 kV and at higher current levels and longer pulse durations using argon and nitrogen ions implanted into actual polymer parts. High-volt,age high-power diodes are also planned for installation at the output of the pulse transformer to eliminate the ‘voltage (and current) ringing shown in Fig. 7 during the period when the pulse is off. 5. PI1 plasma production technology 5.1. Plasma source design for multiply charged ions
With the feasibility of conducting 200-250 kV PI1 implants established, the next step ‘in conducting high
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J.N. Matossian, R Wei/Surface and Coatings Technology 8.7 (1996) 111-119 9512-15-054a fin--------)
8455H CROSSATRON
/
100 kV POWER SUPPLY
kR
PULSE MQDULATOR
VACUUM
CHAMBER
(a)
PLASMA SHYTH
OBJECT ,
8455H CROSSATRON
Pf 100 kV POWER SUPPLY
---
PULSE MODULATOR
VACUUM
CHAMBER
04 Fig. 6. Schematic diagram of (a) the 100 kV pulse modulator and (b) the upgrading and modification required to achieve 250 kV voltage levels.
energy-to-ion ratio implants of nitrogen ions into polymersin a PI1 systemconsistsin developing the capability to produce multiply charged nitrogen ions. It should be noted that alternative approachesare also being pursued. These include the development of a single charged Nf plasma source [21] and the implantation of NH; ions. In the caseof N+ ions, a 250 kV implant would result in an energy-to-ion ratio of 250 KeV per Nf ion, which should be suitable for hardening polymers. Implantation at 250 KV using NH: ions, would result in a slightly lower energy-to-ion ratio level. Although these approaches are being investigated, the main emphasisis on developing a multiply charged atomic nitrogen ion plasma source,sincethis approach would allow for more
flexibility and capability to implant over a wider range of energy-to-ion ratio values. The approach we are using for a multiply charged ion plasma source is based on PIG ion source technology [22]. Other work on the development of multiply charged ion plasma sources is also under way at other research organizations [ 231, In collaboration with the General PhysicsInstitute of the Russian Academy of Sciences(PLASMAIOFAN, Moscow, Russia, Agreement Sl 402809-2, 1994), we have designed and built a PIG plasma ion source (Fig. 8). The source operates in the pulsed mode and is designed to produce over 0.3 A of double and triply charged atomic nitrogen ions. This current should be
J.N. Matossian, R WeiJSurface and Coatings Technology 85 (1996) 111-119 04
40 kV div. 2A div.
-180 -
-
kV 2A
2 p secldiv. f=lOHt 04
o9
1
div.
2A div.
-250
117
duced via multistep ionization [24]. The source has no extraction electrodes; plasma is designed to diffuse out of the source and into the vacuum chamber. The source has been operated off-line at PLASMAIOFAN and is planned for preliminary testing as a remotely located plasma source [ 25,261 in the HRL PI1 vacuum chamber in the near future. These experiments will measure the ion species fraction of nitrogen ions using an EXB filtered analyzer. Charge exchange effects have not yet been studied analytically for multiply charged nitrogen ions, although they have been investigated experimentally for singly charged nitrogen ions [27]. It has been found that, in the anticipated operating pressure range of mid 10m4 Torr, singly charged atomic nitrogen ions can be transported over a distance of 1 m from the plasma source without significant (20%) degradation in density. It is planned to make measurements of this type for the multiply charged ion plasma source in an offline chamber in order to define the charge exchange characteristics of the source prior to its installation in the PI1 chamber. These experiments are scheduled for 1996.
kV
-2A
t O-
2 p secldiv. SINGLE SHOT
6. X-ray production in a PIT system 6.1. Techniques to minimize X-rays
Fig. 7. Time-phased current and voltage waveforms for (a) 180 kV and (b) 250 kV ion implants via PII.
sufficient to achieve a dose of 1 x 1015 cm-’ in a time period of less than 1 min for 100 kV implantation voltages with a duty factor of 0.01% (pulse duration of 10 us and a frequency of 100 Hz). The multiply charged atomic nitrogen ions are pro-
In addition to the formidable challenges involved in achieving a 250 kV voltage level in a PI1 facility, there are considerable problems in handling X-rays that are produced during 250 kV implants. In PII, implanted ions eject secondary electrons from the surface of the object being implanted. These electrons are re-accelerated across the plasma sheath surrounding the
Fig. 8. The PIG ion plasma source for producing multiply charged nitrogen ions for PII.
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object and then impact the vacuum chamber walls to produce X-rays. At voltages of 100kV, singly charged ions are accelerated across the plasma sheath at an energy of 100keV and secondary electrons are re-accelerated across the plasma sheath at the same energy. At this energy,0.25 inches of lead shielding is required to ensure the safetyof laboratory and personnel. This thicknessof lead shielding completely surrounds the exterior of the vacuum chamber shown in Fig. 4. At voltages of 250 kV, triply charged ions will be accelerated across the plasma sheath at an energy of 750 keV, but secondary electrons are re-accelerated across the plasma sheath at 250 keV. In addition, the yield of secondary electrons for multiply charged ions is greater (50% or more) than that for singly charged ions [27] and will contribute to a higher X-ray intensity than that obtained with singly charged ions. Thus, the X-ray shielding technique that is used when operating at 250 kV implantation voltage levels for either singly or doubly charged ions must contend with 250 keV electrons. Above 100keV, the K-shell absorption edge for lead is exceeded,and the absorption cross-section for X-rays drops exponentially. At 250 kV, the absorption cross-sectionof lead is reduced by almost a factor of 15, which would have a significant effect on the thickness of lead required for 250 kV operation in a PI1 facility. We have investigated the use of a new proprietary technique [28] for minimizing X-ray production in a PI1 system.It consists of electrostatic confinement of the secondary electrons in the chamber volume and is shown schematically in Fig. 9. An enclosure is placed along the perimeter of the chamber that is isolated from the chamber walls and electrically connected to the support table. A grounded plasma is produced via a grounded remotely located plasma source as shown. When the voltage pulse is applied to the part, it is also
applied to the enclosure. Ions implanted into the part (and the enclosure) eject secondary electrons that are reflected at the enclosure boundary and are prevented from impacting a ground-potential surface other than the small area of the plasma source. At a voltage of 50 kV, the technique was successful in reducing the normal X-ray level from 20 mrem h-’ to a level that was below the minimum detection limit of the dosimeter. At 80 kV operation, the X-ray level was reduced by a factor of four. This may be due to the increased probability of electron interception by the grounded plasma source at the higher voltages. Experiments to confirm this explanation, as well as testing the technique at voltages of loo-250 kV, are planned for 1997. The use of multiply charged ions can also be used to reduce X-ray production at low implantation voltage levels. Consider the use of triply charged nitrogen ions implanted at 33 kV. The effective energy of the nitrogen ion would be 99 keV, while the secondary-electron energy would be only 33 keV. At this energy, the X-ray production is quite low and would be adequately handled by X-ray reduction techniques [ZS], Furthermore, the requirement of a 33 kV PII pulse modulation would significantly simplify the overall systemdesign and would be very attractive for commercial applications. This implantation scenario is being explored. The effect of a higher secondary-electron yield for multiply charged ions has not been accounted for in this simple analysis, and this will be necessaryin actual practice. 7. Conclusion
Ion implantation for hardening metals requires modest energy-to-ion ratio levels (25-50 keV per Nt ion) and high ion doses(2-4 x 1017N’ cm-“). In contrast, ion implantation for hardening polymers requires high energy-to-ion ratios (300 keV per N’ ion) and low PLASMA
SOURCE
METAL ENCLOSURE AT SAME POTENTIAL AS PART \
REFLECTED ELECTRON PATH
PLASMA k SHEATH AT PART AND ENCLOSURE
Fig. 9. Schematic diagram of the electrostatic confinement of secondary electrons.
J.N. Matossian,
R Wei]Surface
and Coatings
ion doses (3 x 1OL5 N+ cm-“). Achieving this in a PI1 system requires scaling the voltage to 250 kV and producing multiply charged nitrogen ions. We have reported on the challenges faced and the progress toward achieving this capability in a PI1 facility. To date, single-pulse 250 kV implants of ions have been demonstrated and a plasma source for producing multiply charged nitrogen ions has been built. A new proprietary technique based on electrostatic confinement of secondary electrons has been developed to reduce X-ray production at the 250 kV voltage level. Full system integration for conducting 250 kV implants of multiply charged nitrogen ions is under way.
Acknowledgments The authors wish to thank Dr. Dan Goebel for the transformer design, and Mr. John Elverum for the design and fabrication of the pulse modulator and vacuum chamber hardware.
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