KDC 10 DC ION SOURCE MANUAL WITH 1 CM TWO-GRID GRAPHITE OPTICS

Transcription

1 KDC 10 DC ION SOURCE MANUAL WITH 1 CM TWO-GRID GRAPHITE OPTICS Kaufman & Robinson, Inc Blue Spruce Drive Fort Collins, Colorado Tel: , Fax: Internet: Copyright 2011 by Kaufman & Robinson, Inc. All rights reserved. No part of this publication may be reproduced without written permission. January 2011, Version 2

2 CONTENTS i CONTENTS 1 Safety Specifi cations Ion Source Dimensions Ion beam Ion optics Power Supplies Inspection and Installation Unpacking and Inspection Physical Description Power Supplies Ion Source Installation Operation Standard Cathode Neutralizer Maximum Discharge-Chamber Power Continuous operation Short-term operation Gas Flow Electron-Backstreaming Limit Maximum Ion Beam Current Ion-Beam Profi les Startup and Operation Maintenance Remove Ion Source From Socket Place Ion Source on Maintenance Stand Replace neutralizer Remove ion optics Remove outer shell Remove anode Replace cathode Clean anode Clean the back wall of the discharge chamber Reassemble the ion source Maintenance of the Ion Optics Remove nuts Remove lock washers and insulators

3 ii CONTENTS 5.33 Remove screen grid support Remove screen grid Remove ball insulators and accelerator grid Clean screen and accelerator grids Clean screen-grid support and ion-optics support M and ball insulators Reassemble the ion optics Spares Warranty Figures 2-1 Dimensions of the KRI KDC 10 Ion Source Simplifi ed electrical schematic of KRI KDC 10 Ion Source together with the power supplies necessary to operate this ion source Electrical connections of the KRI KDC 10 Ion Source KDC /4 Confl ate installation drawing Flange mount KDC40 installation drawing Fabrication of a cathode for the KRI KDC 10 Ion Source Short-term, high-discharge-power operation Variation in ion beam current for a variation in argon gas fl ow when the discharge voltage, discharge current, beam voltage, and accelerator voltage are held constant Evaluation of the backstreaming limit for a 400 V beam. The accel voltage is varied while the discharge voltage, discharge current, and argon gas fl ow are held constant Argon ion-beam profi le for collimated and focused graphite 2-grid ion optics at a source distance of 5.08 cm. Beam Voltage of 600V Argon ion-beam profi le for collimated graphite 2-grid ion optics at a source distance of cm. Beam voltage ranges from V Argon ion-beam profi le for collimated graphite ion optics at a source distance of cm. Beam voltage ranges from V Argon ion-beam profi le for collimated graphite ion optics at a source distance of cm. Beam voltage ranges from V Argon ion-beam profi le for collimated graphite ion optics at a source distance of cm. Beam voltage ranges from V Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V

4 CONTENTS iii 4-12 Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V Assembled ion source installed in socket Ion source removed from socket Ion source placed on the maintenance stand Ion optics removed from ion source Outer shell removed Anode removed Cathode Ion optics Ion optics with nuts removed Lock washers and insulators removed Screen grid support removed Screen grid removed Ball insulators and accelerator grid removed, leaving the ion-optics support and four 4-40 screws

5 SAFETY SAFETY Only technically qualifi ed personnel should install, maintain, and troubleshoot the equipment described herein. Troubleshooting and maintenance should be carried out only after grounding the components to be worked on and assuring that power cannot be applied to those components while working on them.

6 SPECIFICATIONS SPECIFICATIONS The Kaufman & Robinson, Inc., (KRI ) KDC 10 Ion Source is a small gridded ion source that uses a direct-current (dc) discharge to generate ions. It comes with two-grid graphite optics. These optics can be either collimated, focused or defocused. Except for the ion beam profi le, ion-source performance is generally similar for the three ion optics. This manual covers the three types of ion optics. The source described also incorporates a hot-fi lament neutralizer, although other types of neutralizers can be used. 2.1 Ion Source 2.11 Dimensions The dimensions of the KRI KDC 10 Ion Source are shown in Fig The insulated wires and the gas tube are shown coming out of the socket cover on the axis of the ion source. An optional socket cover is available where the wires and tube come out of the side. The dimensions shown assume a hot-fi lament immersed neutralizer, which is incorporated in this ion source. The use of an alternate neutralizer (see Section 4.2) would have an impact on the footprint of this ion source. The diameter of this source is small enough that it can be mounted on a 2 3/4 Confl at fl ange with a tube inside diameter of 1.63 inches or more Ion beam Useful argon ion-beam currents are generated from about 100 ev up to 1200 ev. The typical argon gas fl ow is 4 sccm, but lower fl ows can be used with some reduction in ion-generation effi ciency. Continuous operation of this ion source is limited by discharge-chamber heating. With the standard tungsten cathode (0.25 mm diameter, 15 mm long), the maximum continuous discharge power (discharge voltage X discharge current) is 15 W. This discharge power is suffi cient to generate an argon ion beam of about 10 ma at a beam voltage of 500 V or more and a gas fl ow of 4 sccm. The ion beam current can be limited to less than 10 ma by the ion optics at beam voltages less than 500 V. An increased discharge power and beam current can be obtained by reducing the cathode diameter, with a corresponding reduction in cathode lifetime. An increased discharge power and beam current is also possible for time-limited operating times.

7 2-2 SPECIFICATIONS 2.13 Ion optics The KRI KDC 10 Ion Source can be confi gured with various ion optics. The ion optics options include, but are not limited to: (1) Collimated two-grid graphite optics with a minimum half-angle of about 5 degrees. (2) Focused two-grid graphite optics with a focal length of about 5 cm. (3) Defocused two-grid graphite optics covering about twice the diameter of collimated optics at a distance of 5 cm. 2.2 Power Supplies The suggested minimum power supplies for the different functions of the KRI KDC 10 Ion Source are given below. Although this ion source can be operated with discrete power supplies for these different functions, it is better if the power supplies are integrated into an ion source controller. Some of the sequencing and balance problems that can be encountered by using discrete power supplies are described in Section 4.7. Cathode: 6 V, 20 A. Discharge: 100 V, 1 A. Beam: 1000 V, 100 ma. Accelerator: 500 V, 100 ma.* Neutralizer: 18 V, 20 A. *A current capacity of the accelerator supply equal to that of the beam supply is not required for steady-state operation, but reduces the likelihood of electrostatic charging damage to the substrate during ion-optics arcs.

8 SPECIFICATIONS 2-3 Figure 2-1 Dimensions of the KRI KDC 10 Ion Source

9 INSPECTION AND INSTALLATION INSPECTION AND INSTALLATION This section describes how to install the Kaufman & Robinson, Inc., KRI KDC 10 Ion Source. Information on unpacking and inspection, physical description, and hardware inventories is also provided to assist in a successful installation. 3.1 Unpacking and Inspection Prior to shipment, the ion source was inspected, tested, and shipped free of physical defects. As soon as the equipment has been removed from all packing materials a visual inspection should be made to determine if there has been any damage during shipment. If any damage has occurred contact both Kaufman & Robinson, Inc. and the shipping company to report the damage (see Warranty, Section 6). Retain packing materials in case equipment must be returned to Kaufman & Robinson. All ion source hardware was cleaned prior to shipment. Use clean lintfree gloves while handling components to prevent contamination. 3.2 Physical Description This product consists of two sections, the ion source and the socket into which the ion source is plugged. The socket can remain attached to a vacuum chamber or a fl ange while the ion source is removed for maintenance. The ion optics are at the opposite end of the ion source from the socket. The ion optics were protected for shipment and should also be examined for shipping damage. The ion source can then be removed from the socket. This is done, after removing any packing material attached to the ion source, by removing the two hold-down rods (with acorn nuts attached to them) at the ion-optics end of the ion source and separating the two. A maintenance stand (made of either acrylic resin or aluminum) is included for carrying out maintenance on the ion source. A torque wrench is included for carrying out maintenance on the ion optics. 3.3 Power Supplies If the user has purchased a KRI Kaufman Source Controller with this ion source, unpacking and inspection instructions in that manual should be followed.

10 3-2 INSPECTION AND INSTALLATION 3.4 Ion Source Installation A simplifi ed electrical schematic of the ion source and associated power supplies is shown in Fig Power supplies must include provisions for cathode, discharge, beam, accelerator, and neutralizer functions. An ion source controller integrates these power-supply functions into one unit. If the user has purchased a KRI Kaufman Source Controller with this ion source, installation instructions in that manual should be followed. Depending on details of the equipment purchased, the correct connections between the controller and the ion source may, or may not, be provided for. Figure 3-2 shows the locations of the different ion-source connections on the socket. Figures 3-1 and 3-2 together should provide the information necessary for making the correct electrical connections between the controller and the ion source. Fig 3-3 shows the installation drawing for a remote mounted KDC 10, using a 2-3/4 confl ate fl ange. Note: The sleeve needs to be slide back to expose the in-line connector. Any wiring used in these connections should have insulation rated for 1200 V or more. The gas-line connections should use clean stainless steel tubing. Stainless steel tubing is not clean on the inside unless certi f ed so by the seller. There are multiple mistakes that can be made in providing clean gas to the ion source. If the user is not familiar with this activity, he should review the Kaufman & Robinson publication, Technical Note KRI-04, Gas Cleanliness. If the user has not purchased a controller with this ion source, these functions must be provided for by the user. Figures 3-1 and 3-2 should again be used. Note that the multiple ground connections between the different power supplies and the grounded vacuum chamber are necessary to provide electrical isolation between the different supplies. The user should keep in mind that the outer envelope of the ion source is electrically connected to the accelerator grid, which operates at a negative potential relative to ground (see Fig. 2-1). This means that the ion source should not touch a grounded surface while it is operating. The back of the socket, including the socket cover, is electrically isolated from the ion source and can be connected to ground. The installation described in this section assumes a hot-fi lament neutralizer that is immersed in the ion beam, which is the standard neutralizer provided with the Model KDC 10 Ion Source. Information describing alternate neutralizers and their installation can be found in their manuals.

11 INSPECTION AND INSTALLATION 3-3 Gas Cathode Screen grid Anode Accelerator grid Neutralizer e d Ion source Vacuum chamber Operating cables Operating cables Ion source controller AC CT CT AC Cathode module Discharge module Beam module Accelerator module Neutralizer module Ground to cabinet Fig. 3-1 Simplifi ed electrical schematic of KDC 10 Ion Source together with the power supplies necessary to operate this ion source.

12 3-4 INSPECTION AND INSTALLATION Fig. 3-2 Electrical connections of the KRI KDC 10 Ion Source

13 INSPECTION AND INSTALLATION 3-5 Connector Actual feethrough with vacuum cables Fig. 3-3 KDC 10 Internal mount installation with 2-3/4 inch confl at fl ange

14 3-6 INSPECTION AND INSTALLATION Figure 3-4 Flange mount KDC10 installation drawing

15 OPERATION OPERATION With its small size and high output for its size, the Kaufman & Robinson, Inc., KRI KDC 10 ion source is suitable for a wide range of research, high precision,instrumentation, and other specialized applications. Such applications often have special requirements. This section is written in considerable detail to help the user evaluate and tailor the operation for such special requirements. The experimental performance of different sources will vary slightly, due to small differences in construction. Some of the variations may also be due to normal errors in the various measurements, impurities present, or details of the particular installation. The performance presented in this section should therefore be used as only an approximate guide. The user may want to start operation quickly, without spending time on a detailed evaluation. In that case the user should go directly to Section 4.8, Startup and Operation. Except for slightly different profi les, the operation of the focused, defocused, and collimated optics is similar. The following sections therefore apply to both optics, except for the section on profi les. 4.1 Standard Cathode The cathode recommended for most operation is tungsten wire, 0.25 mm (0.010 inch) in diameter and about 1.5 cm (0.6 inch) between supports. The length of wire clamped between the nuts on the cathode supports does not contribute to the heating. These cathodes can be purchased from KRI, and are listed as one of the spares options in Section 5.4. The cathodes can be fabricated by the user, if that is preferred for some reason. The fabrication of this cathode uses the cathode jig (supplied with the ion source) and follows the sequence shown in Fig The fi nished cathode should have the shape shown in Section B-B of Fig The lifetime of this cathode should be hr when generating a 10 ma argon ion beam, in the absence of reactive gases or reactive materials sputtered on the cathode. The length of the cathode between supports is important. If it is signifi cantly shorter than 1.5 cm, the required heating power will increase while the lifetime will drop. If it is much longer, the heating power will also increase, with little increase in lifetime. With limits on the maximum discharge-chamber power (Section 4.3), an increase in cathode heating power may decrease the maximum

16 4-2 OPERATION ion beam current that can be obtained. 4.2 Neutralizer The neutralizer options are described in some detail here because the lifetime of a hot-fi lament neutralizer can be short in this ion source. This is because of the high ion current densities that can be obtained, which result in rapid erosion of the hot fi lament. As described in Section 4.61, at a moderate ion beam current of 10 ma, the ion current density at the ion optics is about 9 ma/cm2, which is very high for a gridded ion source. This ion source is designed to use a hot-fi lament neutralizer. Tungsten wire up to 0.38 mm (0.015 inch) in diameter may be used. The neutralizer may be stretched from one support to the other, and held in place by tightening two nuts, as the final step in ion source assembly. The neutralizer can use either 0.25 mm (0.010 inch) or 0.38 (0.015 inch) tungsten wire. The heavier wire will provide a longer lifetime. Bending the neutralizer wire so that it passes through a less-dense region of the ion beam will also increase the lifetime. If the ion beam is directed at a grounded conducting surface, the neutralizer may be bent so that it only passes through the edge of the ion beam, or is even slightly outside of it. This is because the beam will lose neutralizing electrons slowly and will need a neutralizer emission that is only a fraction (10-20%) of the ion beam current. But if the ion beam is directed at a surface that is insulating or electrically isolated from ground, the neutralizer emission must equal the ion beam current or be slightly higher. Such a high electron emission can be conducted to the ion beam from a hot-fi lament neutralizer only when the neutralizer is immersed in a fairly dense region of the ion beam. Alternative non-immersed neutralizers, which couple to the ion beam through a plasma bridge, can also be used with this ion source. Such neutralizers will provide extended operation without maintenance. A hollow-cathode is one example of a non-immersed neutralizer. Information about these neutralizers and their operation can be found in their manuals. 4.3 Maximum Discharge-Chamber Power Ion source operation can be limited by the discharge-chamber power, the sum of the discharge power and the cathode heating power. Exceeding this limit may overheat the magnet and demagnetize it, resulting in a noticeable drop in the ion beam current generated at an otherwise similar operating condition.

17 OPERATION Continuous operation The maximum discharge-chamber power for continuous operation is 50 Watts. With the recommended cathode, described above, the cathode heating power is about 35 Watts and up to 15 Watts may be used for the discharge power. With a 40 V discharge, the discharge current is then limited to about 0.38 A. A cathode with a smaller diameter will reduce the cathode heating power and permit an increase in continuous discharge power. But the smaller diameter will also result in a decrease in the cathode lifetime. A cathode with a larger diameter will usually result in an excessive cathode heating power for continuous operation, but is a possibility for short-term operation (see below) Short-term operation 4.4 Gas Flow Operation at a power higher than the continuous power limit is possible for a limited duration. To avoid overheating the magnet, the operating time in minutes X the discharge-chamber power in Watts should not exceed 1400 W-min. For example, a discharge power of 100 Watts could be used for 14 minutes, assuming the source is near room temperature when it is started. For cycled on-off operation, the power X time depends on the cool-down time since the last operation and is shown in Fig For example, after 50 minutes of cooling, short-term operation of 1000 W-min could be used. This could be operation at 100 Watts for 10 minutes, or operation at 140 Watts for 7 minutes. The data for Fig. 4-2 were obtained with a vacuum chamber near room temperature. High-power operation would be more limited for a vacuum chamber at a higher temperature. Figure 4-3 shows the variation in ion beam current for a variation in argon gas fl ow when the discharge voltage, discharge current, beam voltage, and accelerator voltage are all held constant. If vacuum-chamber pumping is suffi cient and a pressure of 0.5 mtorr or less can be maintained, a gas fl ow of 4 sccm is recommended for most operation.

18 4-4 OPERATION If vacuum-chamber pumping is limited, Fig. 4-3 can indicate the tradeoff between gas fl ow and ion beam current. Remember that reducing the gas fl ow will reduce the ion beam current for a given discharge power, so that reducing the gas flow will reduce the ion beam current generated at maximum discharge power. Gas fl ows for ion sources with larger beam diameters are usually given as a function of ion beam current and background pressure in the vacuum chamber. The effect of these variables is greatly reduced in an ion source as small as the KRI KDC 10 Ion Source. For example, a 10 ma ion beam ionizes (singlycharged ions) only 0.14 sccm of gas fl ow, so that the ion beam generated has little effect on the gas density in the discharge chamber. In a similar manner, the gas pressure in the discharge chamber is several mtorr, so that a background pressure of 0.5 mtorr in the vacuum chamber will cause little neutral gas backfl ow into the discharge chamber. To a fi rst approximation, then, the variation in beam current with gas fl ow shown in Fig. 4-3 can be used as a relative variation for all operating conditions. 4.5 Electron-Backstreaming Limit The accelerator grid (see Fig. 3-1) serves as a barrier to neutralizing electrons, preventing them from fl owing backward through the ion optics and giving a false contribution to the indicated ion beam current. Because the potential at the center of an accelerator-grid aperture is more positive than the accelerator potential, the accelerator grid must be negative of ground to provide such a barrier. A sample of the data used to evaluate the backstreaming limit is shown in Fig With the gas flow, discharge voltage, discharge current, and beam voltage (400 V) all held constant, the magnitude of the negative accelerator voltage is reduced. At fi rst there is a slow reduction in indicated ion beam current, because the reduction in negative accelerator voltage results in some reduction in ion extraction effi ciency. As the accelerator grid approaches ground (0 V), however, backstreaming electrons from the neutralizer start fl owing back through the ion optics. To avoid this false contribution to indicated ion beam current, the accelerator should be negative of ground by about 60 V or more for a 400 V beam. The backstreaming limits are different for different beam voltages, but are a nearly constant fraction of whatever beam voltage is used. For the KRI KDC 10 ion optics, the negative accelerator voltage should, in general, be at least 15 percent of the beam voltage to avoid electron backstreaming. There are small differences in this backstreaming limit for different ion optics and operating

19 OPERATION 4-5 conditions. If the application is for a focused ion beam, the optimum focusing is usually found with the minimum negative value of accelerator potential. For that reason, there may be some advantage in making a more detailed study of the backstreaming limit for a particular focused ion beam application. 4.6 Maximum Ion Beam Current In normal operation, the accelerator current results from the collection of low energy charge-exchange ions. The maximum ion beam current is the maximum value that can be obtained without the direct impingement of energetic ions on the accelerator grid. This maximum beam current is a function of beam and accelerator voltages and is separate from the maximum power limit described in Section 4.3. Depending on the operating condition, it is possible to have the ion beam current limited by either the ion optics or the maximum discharge-chamber power. Table 4-1 gives the maximum argon ion beam currents for the graphite ion optics over a range of ion beam voltages. The accelerator voltages (negative) are 15 percent of the beam voltages (see Section 4.5), except for the 100 V beam. Table 4-1. Maximum argon ion beam current for 2-grid graphite ion optics. Beam,V Accel,V Beam, ma (11*) (14*) (20*) (26*) (32*) (38*) * See Figure 4.2 and section 4.32 Short-term operation The best focusing will usually be found between about percent of the maximum ion beam current. For example, operation with a 10 ma ion beam would only be 26 percent of the maximum ion current at 1000 V. Using such a small fraction of the ion-optics capability at 1000 V would result in broading the

20 4-6 OPERATION beam, even with focused ion optics. The maximum ion beam current can be increased at a given beam voltage by increasing the negative accelerator voltage. But the user should be aware that the beam will diverge more as the negative voltage is increased. If the increase in negative voltage is moderate, the maximum ion beam current can be calculated from the total voltage - the sum of the beam voltage and the absolute value of the accelerator voltage. For example, the maximum beam current in Table 4-1 is 11 ma for a 500 V beam. The total voltage for this 500 V beam is = 575 V. The same maximum beam current of 11 ma can be obtained with a 300 V beam by increasing the accel voltage to = 275 V. With this large accelerator voltage, however, the 300 V ion beam will be much broader than the 500 V beam. The broad ion beam may be acceptable if a large area is being processed, or unacceptable if a tightly focused ion beam is desired. The maximum beam currents given in Table 4-1 include a safety factor for variation in individual sets of ion optics. The user may be familiar with the procedure for evaluating the maximum beam current, and wish to work closer to the limit for the specifi c ion optics being used. The user is cautioned that it is easy to damage the grids in this ion source. Not only are the grids thin and fragile compared to the grids in larger ion sources, but the ion current densities are much higher. For example, even at a moderate ion beam current of 10 ma, the ion current density at the grids is about 9 ma/cm2, a value that is very high for a gridded ion source. 4.7 Ion-Beam Profiles Two ion-beam profi les were obtained at a distance of 5 cm from the ion source. The use of a conventional screened probe (to exclude the effects of chargeexchange ions) was not possible at this distance due to the small size of the ion beam. As an alternative, etch profi les in fused quartz were used to determine the shape of the profi les. The depth at each radius was compared to the total material removed to calculate the ion-beam profi le at the 5-cm distance. Focused and collimated ion-beam profi les calculated in this manner are presented in Fig The maximum current density was also estimated from the maximum etch rate, and was found to agree with the maximum current density calculated from the etch profi le within about 15%. Because of the greater number of steps in calculating these profi les, compared to the conventional approach using electrical probes, the uncertainty of the profi les shown in Fig. 4-5 is estimated at ±20%.

21 OPERATION 4-7 An additional set of ionbeam profi les were collected using the conventional approach with a guarded planar probe. Fig. 4-6 through show collimated and defocused profi les taken at a distance of 15.24cm for various beam voltages and currents. 4.8 Startup and Operation The preceding sections describe a number of choices and tradeoffs that can be made in operating the KRI KDC 10 Ion Source. This section describes the startup and operation for a typical operating condition: 4 sccm Ar, 600 V beam, 90 V accel, and 10 ma beam. The discharge power is low enough that for this condition that the 10 ma beam can be maintained indefi nitely. Use the following steps: 1. If the ion source does not have a cathode and neutralizer, install them (see Sections 5.21 and 5.25). Cathodes and neutralizers can be purchased from KRI. If preferred, they can also be fabricated by the user (see Sections 4.1 and 4.2). 2. With the ion source installed in the vacuum chamber and connected as described in Section 3 of this manual, start an argon fl ow of 4 sccm to the ion source. 3. Start the discharge. This is done by setting the discharge voltage at 40 V and increasing the cathode heater current until a discharge current is obtained. Adjust the cathode current to give a discharge current of 0.1 A or less. (A slightly higher voltage may be required for the initiation of the discharge. An automatic increase in discharge voltage is built into KRI Kaufman Source Controllers for very low discharge currents when using the auto mode.) 4. Turn on the hot-fi lament neutralizer. For a 0.25 mm tungsten neutralizer, use a neutralizer heating current of 9 A. If a 0.38 mm tungsten neutralizer is used, use a neutralizer current of 14 A. (If the ion beam is directed at an insulating or electrically isolated surface, it is important to have suffi cient electron emission available when the ion beam is turned on - see Section 4.2). 4(alternate). Turn on the hollow cathode or other non-immersed neutralizer. 5. If the beam and accel voltages can be preset, set the beam at 600 V and the accel at 90 V. If the two voltages can be turned on at the same time,

22 4-8 OPERATION do so. If they cannot be turned on at the same time, turn on the accel fi rst and the beam second. You should now have an ion beam. Increase the cathode current until the beam current is 10 ma. (If you have been to this condition previously, you can increase the cathode current in Step 3 to give the expected discharge current for a 10 ma beam. This will decrease the adjustment required after turning on the beam and accelerator voltages.) 5(alternate). If the beam and accel voltages cannot be preset, you should make sure the discharge current is low (less than 0.1 A) and turn on the accel fi rst. Adjust the accel to 90 V. Turn on the beam next and rapidly increase it to 600 V. You should now have an ion beam. Increase the cathode current until the beam current is 10 ma. (The discharge current is kept low while increasing the beam voltage because you are passing through low beam voltages where a 10 ma beam current would result in the direct impingement of energetic ions on the accelerator grid. The exposure to direct impingement is greatly reduced with the more rapid increase to 600 V when using a preset beam voltage in Step 5.) 6. The neutralizer should have an emission greater than 10 ma. Reduce the emission to a value between 10 and 11 ma. For the hot-fi lament immersed neutralizer, this is done by adjusting the neutralizer heating current. You now have the ion source operating at the selected operating condition. 7. The beam current and the neutralizer emission can be maintained at the selected values by adjusting the cathode and neutralizer currents. Depending on the power supplies being used, this can be done either manually or automatically. 8. In turning off the ion source, there is often a need for a well-defi ned termination of the ion beam, with no voltage or current excursions that may cause damage to sensitive targets. Several options are possible, listed in a generally decreasing order of preference. a. When the power supplies are integrated into an ion source controller, the main power can be turned off. The argon gas fl ow can then be turned off. b. When separate power supplies are used for the different functions, the beam and accel supplies can be turned of simultaneously. If they cannot be turned off simultaneously, turn off the beam supply fi rst then the accel supply. Turn off the other supplies next - the order shouldn t be important. Then turn off the argon fl ow. If you turn off the accel supply first, the beam potential will be elevated to near the ion-beam voltage, possibly damaging substrates sensitive to electrostatic fi elds. You will also cause

23 OPERATION 4-9 a surge in electron current from the neutralizer, back through the ion optics, possibly overloading the beam power supply.) c. Turn off the cathode or discharge supply fi rst. Turn off the beam supply next, followed by the other supplies in any order. Then turn off the argon fl ow. The preceding steps include options that depend on the power supplies used. There should be no diffi culty fi nding power supplies that can satisfy the current and voltage requirements for continuous operation. The options presented for Step 8 show that the improper sequencing of power supplies can cause problems. There are other problems that can result from the use of supplies with characteristics that are improperly balanced for ion-optics arcs. The possibility of encountering these problems is greatly reduced when the power supplies have been properly integrated into an ion source controller, which is the case for all KRI Kaufman Source Controllers. Occasional ion-optics arcs are a part of normal operation of a gridded ion source. Frequent or repetitive arcs can indicate: a. A longer warmup (discharge-only operation) may be required if the ion source has been exposed to atmosphere for a long time, the ion optics are new, or have recently undergone maintenance. b. A gradual conditioning to higher beam and accelerator voltages may be required if there is a large increase in these voltages. c. Maintenance of the ion optics may be required. d. A cathode or anode conductor may be exposed in the vacuum chamber. (The charge-exchange plasma that fi lls the vacuum chamber can provide a conductive path between the neutralizer and any positivepotential conductor that is not properly shielded.)

24 4-10 OPERATION Cathod e jig Step 1. Wrap 0.25 mm (0.010 inch) tungsten wire around screws in cathode jig. 3 mm 3 mm Step 2. Cut ends. 3 mm, ~ rad 2 pl 4mm 2 pl Step 3. Bend ends. r d ~ 4 mm a Step 4. Bend center. Fig. 4-1 Fabrication of a cathode for the KRI KDC 10 Ion Source

25 OPERATION PowerX time, W-min Operating time, min Fig. 4-2 Short-term, high-discharge-power operation. See Section 4.32.

26 4-12 OPERATION Ion beam current, ma Recommended argon flow Argon flow, sccm Fig. 4-3 Variation in ion beam current for a variation in argon gas fl ow when the discharge voltage, discharge current, beam voltage, and accelerator voltage are held constant.

27 OPERATION Beam, 400 V Backstreaming electrons Indicated Ion beam current, ma Ion beam current Accelerator potential, V Fig. 4-4 Evaluation of the accelerator backstreaming limit for a 400 V beam. The accel voltage is varied while the discharge voltage, discharge current, and argon gas fl ow are held constant.

28 4-14 OPERATION Ion current density, j, ma/cm V b, V V a, V I b, ma focused graphite ion optics collimated graphite ion optics V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig. 4-5 Argon ion-beam profi le for collimated and focused graphite 2-grid ion optics at a source distance of 5.08 cm. Beam Voltage of 600V.

29 OPERATION 4-15 Ion current density, j, ma/cm V b, V V a, V I b, ma V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig. 4-6 Argon ion-beam profi le for collimated graphite ion optics at a source distance of cm. Beam voltage ranges from V.

30 4-16 OPERATION V b, V V a, V I b, ma Ion current density, j, ma/cm V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig. 4-7 Argon ion-beam profi le for collimated graphite ion optics at a source distance of cm. Beam voltage ranges from V.

31 OPERATION V b, V V a, V I b, ma Ion current density, j, ma/cm V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig. 4-8 Argon ion-beam profi le for collimated graphite ion optics at a source distance of cm. Beam voltage ranges from V.

32 4-18 OPERATION 2.5 V b, V V a, V I b, ma Ion current density, j, ma/cm V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig. 4-9 Argon ion-beam profi le for collimated graphite ion optics at a source distance of cm. Beam voltage ranges from V.

33 OPERATION 4-19 Ion current density, j, ma/cm V b, V V a, V I b, ma V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V.

34 4-20 OPERATION V b, V V a, V I b, ma Ion current density, j, ma/cm V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V.

35 OPERATION 4-21 Ion current density, j, ma/cm V b, V V a, V I b, ma V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V.

36 4-22 OPERATION 1 V b, V V a, V I b, ma Ion current density, j, ma/cm V b = beam voltage, V a = accelerator voltage, I b = beam current Radius from ion source axis, r, (cm) Fig Argon ion-beam profi le for defocused graphite ion optics at a source distance of cm. Beam voltage ranges from V.

37 MAINTENANCE MAINTENANCE Maintenance on the KRI KDC 10 Ion Source should be carried out in a clean environment where the ion source is protected from accidental damage. Several things should be kept in mind to help avoid maintenance problems. a. Don t tighten most threaded parts more than fi nger tight. A wrench should be necessary only if there is an obstruction on the threads. Routinely overtightening threaded parts will cause seizing problems when the source is disassembled. b. Most nuts are 4-40 small pattern (with outside dimensions the same as standard 2-56 nuts). Small-pattern nuts permit the use of 4-40 threads throughout this source instead of using more delicate threads and parts in many places. If you cannot fi nd 4-40 small-pattern stainless-steel nuts, order them from Kaufman & Robinson, Inc. (KRI ). The only exceptions are the bottom nuts on the neutralizer supports, which are standard stainless-steel 4-40 nuts. c. The ion optics provide precise and repeatable alignment without an inherently imprecise manual alignment step. The maintenance instructions for the ion optics are not diffi cult to follow, but it is important to follow them if you wish to obtain the precision and repeatability of which these ion optics are capable. Never tighten the nuts that hold the ion optics together without a torque wrench. A torque wrench is supplied with the ion source. The most frequent maintenance required is often the hot-fi lament neutralizer. The frequency of neutralizer maintenance can be greatly reduced by using a non-immersed neutralizer. But the footprint of the ion source would be increased by the use of an alternate neutralizer. The next most frequent maintenance is usually the cathode. Maintenance of theanode, back wall of the discharge chamber, and the ion optics can be much less frequent than for the cathode. Although all normal maintenance is described in this section, not all maintenance is necessary every time the ion source is taken apart to replace a cathode. Experience with a particular application should be used to construct a preventative maintenance schedule for this ion source that reduces unscheduled down-time.

38 5-2 MAINTENANCE 5.1 Remove Ion Source From Socket The ion-source/socket assembly is shown in Fig The two hold-down screws are removed and the ion source separated from the socket in Fig In an actual installation, the socket would probably be attached to either the vacuum chamber or a fl ange on the vacuum chamber. 5.2 Place Ion Source on Maintenance Stand The ion source is shown placed on the maintenance stand in Fig All maintenance except that on the ion optics can be carried out with the ion source mounted on the maintenance stand. There is a large notch on the maintenance stand that is visible in Fig When the pins extending from the back of the ion source are aligned with the holes in the maintenance stands, that large notch is circumferentially aligned with the all the alignment notches in the source parts Replace neutralizer (This section assumes that the KRI KDC 10 is equipped with a hot f lament neutralizer. If a non-immersed neutralizer is used instead, there are two threaded rods and nuts in place of the neutralizers. These rods and nuts are in electrical contact with the outer shell of the ion source, not isolated from it as would be the case with a hot-f lament neutralizer.) Indication: End of life for hot-filament neutralizer. Maintenance: Replace neutralizer. If this is the only maintenance to be performed, do it now. If there is other maintenance to be performed, go to the next step and replace the neutralizer when the ion source is reassembled. Even if the neutralizer hasn t failed, it is unlikely that it can be removed and replaced without damage. To replace the hot-fi lament neutralizer, remove the two small-pattern 4-40 nuts that hold the neutralizer in place. Remove the old neutralizer. Finger-tighten the two standard size 4-40 nuts left until they are stopped by the 4-40MK insulators under those nuts. Then back off 1/2 turn with each nut. The neutralizer supports are a different material from the holddown rods (Fig. 5-2) and the outer shell. Differential expansion can cause problems without the intentional loosening. (The intentional loosening described above is not necessary when a non immersed neutralizer is used. The threaded rods that replace the neutral-

39 MAINTENANCE 5-3 izer supports have the same thermal expansion coef shell.) f cient as the outer Place a new neutralizer on the neutralizer supports. Without rotating the standard 4-40 nuts you have just loosened, hold the neutralizer in place by tightening the two small-pattern 4-40 nuts you removed previously Remove ion optics (For an ion source used with a non-immersed neutralizer, the ion optics can be removed after removing two 6-32 nuts that are located where the nuetralizer supports would be located.) To perform ion-source maintenance, other than replacing the hot-fi lament neutralizer, it is necessary to remove the ion optics. Two small-pattern 4-40 nuts, the neutralizer, two standard 4-40 nuts, two inch OD washers, and two 4-40MK insulators (all on the neutralizer supports) are removed. The ion optics can then be removed, as shown in Fig The ion optics should be placed where they are protected from damage until maintenance can be carried out on them. (See Section 5.3.) 5.23 Remove outer shell The outer shell can be lifted off. The removal of the outer shell exposes most of the ion-source parts that require maintenance, as shown in Fig Remove anode The removal of the anode exposes the cathode as shown in Fig The anode is held in place only with magnetism and friction. This is suffi cient to hold the anode in place during normal handling of the ion source. If there appears to be insuffi cient friction, the bottom of the anode (the end nearest the openings in the anode) can be squeezed in a vise to make it slightly out of round Replace cathode Indication: End of life for cathode. To help in arriving at a replacement schedule, end of life often occurs when the heating current has dropped to

40 5-4 MAINTENANCE about 60 percent of the initial value. Maintenance: Remove and replace the cathode. If there is other maintenance to be performed, replace with a new cathode when the ion source is reassembled - even if the cathode has not reached end of life. The cathode is shown installed in the ion source in Fig. 5-6, and by itself in Fig The cathode can be removed by fi rst removing the top two smallpattern 4-40 nuts that hold it in place. Tungsten recrystallizes when it is heated to electron-emission temperature, so that it is usually brittle when it is removed. The cathode can be replaced with the new cathode (Fig. 5-7) and held in place with the two nuts that were removed with the old cathode. Before tightening the top nuts, make sure that the bottom nuts are spaced about one mm from the plate behind them (the back wall of the discharge chamber). This spacing prevents shorting between the anode and cathode. Replacement cathodes can be purchased from KRI or, if preferred, they can be made by the user, as shown in Fig Clean anode Indication. There are fl akes peeling from the anode or there is an insulating layer on the anode. Flakes can cause shorting of the anode to the screen grid or shorting between the screen and accelerator grids. An insulating layer is indicated if it is diffi cult or impossible to start a discharge or, after starting the discharge, the discharge current is very low until the anode heats up. An insulating layer is also indicated if there is no continuity shown by an ohmmeter when the rounded ends of the probe tips are gently placed on the inside surface of the anode. (It doesn t require much pressure for the tips to break through a thin insulating layer and indicate continuity, so don t press the tips against the anode.) Maintenance. The anode can be cleaned with a light, low-pressure grit blast. (A high-pressure grit blast will rapidly erode the anode.) A soft abrasive material such as Scotch Brite or Bear Tex can also be used. Alternate maintenance. If there is no tendency to form an insulating layer, a small wire brush can be used to remove fl akes, followed by a blast of clean air or nitrogen to remove small remaining particulates.

41 MAINTENANCE Clean the back wall of the discharge chamber Indication. Cleaning is indicated if fl akes are peeling from this part. An insulating layer should not be a problem on this part because most of the discharge current is to the anode. Maintenance. The back wall of the discharge chamber can be removed for cleaning. This is the plate immediately behind the cathode. To remove this part, remove the two bottom nuts on the cathode supports. (The top nuts and the cathode have already been removed.) Then remove the two nuts (also small-pattern 4-40 nuts) that hold the plate in place. A light (lowpressure) grit blasting can be used for the cleaning. The back plate can be re-installed after cleaning, followed by installing the new cathode. Alternate maintenance. A small wire brush can be used to remove fl akes, followed by a blast of clean air or nitrogen to remove small remaining particulates. This can be done each time the cathode is replaced and doesn t require that the back wall be removed from the rest of the assembly shown in Fig Reassemble the ion source The preceding sections describe the disassembly and normal maintenance of the ion source - except for the ion optics. The maintenance of the ion optics is described below. The reassembly of the ion source generally follows the disassembly, but in reverse order. 5.3 Maintenance of the Ion Optics The ion optics should not be disassembled except to perform required maintenance. Indication. Frequent arcs are observed in the ion optics, involving the beam and accelerator supplies. Contra-indication. These arcs are avoided or greatly reduced if a longer discharge-chamber warmup is used, or if a gradual conditioning to higher beam and accel voltages is used. (A warmup or gradual conditioning may be helpful if the ion optics have been exposed to atmosphere for a long time, are new, have recently undergone maintenance, or have been operated for a long time at lower voltages. )

42 5-6 MAINTENANCE Maintenance. Carry out the following maintenance procedure Remove nuts The ion optics are shown in Fig The alignment notches are shown at the bottom of the picture (the foreground). Note that some of the notches are on the front surfaces of the parts, not on the outer-diameter surfaces. The four small-pattern 4-40 nuts are removed fi rst, leaving the ion optics as shown in Fig It may be necessary to keep the other ends of the 4-40 screws from rotating while the nuts are removed. This is done by sliding the ion-optics assembly to the edge of a work surface, so that the head of one of these screws is exposed from the bottom and it can be kept from rotating with a socket wrench. This can be repeated with each nut and screw, if necessary. The removal of these nuts leaves the assembly as shown in Fig. 5-10, 5.32 Remove lock washers and insulators The four helical lock washers and four 4-40M insulators are shown on the ion-optics assembly in Fig. 5-9 and removed from that assembly in Fig The helical lock washers are non-standard hardware and provide a spring function to maintain grid alignment during thermal cycling. Standard stainless-steel lock washers are not acceptable because they lose their spinginess in one run at maximum discharge-chamber power (see Section 4.3. These helical lock washers can be purchased from KRI. If necessary, they can be made by wrapping a coil of 0.51 mm (0.020 inch) tungsten wire around the shank of a No. 35 drill, then using an abrasive disk in a hand power tool to cut off individual lock washers Remove screen-grid support The screen-grid support is shown removed in Fig This exposes the graphite screen grid Remove screen grid The screen grid is shown removed in Fig This exposes the graphite

43 MAINTENANCE 5-7 accelerator grid and the eight ball insulators that are used to separate and align the screen and accelerator grids Remove ball insulators and accelerator grid Removal of the ball insulators and the accelerator grid leaves the ionoptics support and the four 4-40 screws that are used to hold the ion optics together (Fig. 5-13) Clean screen and accelerator grids The screen grid and accelerator grid are fragile and should not be subjected to any mechanical cleaning such as scraping, abrasive paper, or grit blasting. Use only ultrasonic or chemical cleaning. The specifi c cleaning procedure will depend on the material being deposited on the grids. A deionized or distilled water rinse is recommended after cleaning. A minute bake at about 400 F should remove any adsorbed liquids. In the absence of other equipment, a clean toaster oven can be used for the bake. If the grids are not going to be used immediately in an ion source, store them where they will be protected against accidental damage or atmospheric particulates Clean screen-grid support and ion-optics support A light (low-pressure) grit blast may be used for most of the surfaces of the screen-grid support (Fig. 5-10) and the ion-optics support (Fig. 5-13) - but do not grit blast the surfaces that come in contact with the screen grid and the accelerator grid. If cleaning of the surfaces that come in contact with the screen grid and accelerator grid is necessary use a fi ne abrasive paper (at least 600 grit). A deionized or distilled water rinse is recommended after cleaning, followed by drying in air. If the grids are not going to be used immediately in an ion source, store them where they will be protected against accidental damage or atmospheric particulates M and ball insulators Do not re-use insulators. A thin coating that is almost invisible to the eye can promote electrical breakdowns. Cost-effective maintenance requires that used ion-optics insulators be discarded and new ones used during

44 5-8 MAINTENANCE reassembly Reassemble the ion optics 5.4 Spares The reassembly of the ion optics generally follows the disassembly, but in reverse order. Several things should be kept in mind. a. Use new insulators. b. Keep the alignment notches in the same circumferential location. Remember that some of the notches are on the front surfaces of the parts, not on the outer-diameter surfaces. c. If you re-use lock washers, make sure that they haven t been damaged. You can purchase new lock washers from KRI. d. When replacing the four 4-40 nuts that hold the ion optics together, use the torque wrench. Follow the tightening sequence shown in Fig. 5-8 and tighten fi rst to 0.6 cm-kg (0.5 inch-pound), then follow the same sequence and tighten to 1.2 cm-kg (1.0 inch-pound). If the ion optics are not going to be used immediately in an ion source, store them where they will be protected from accidental damage and atmospheric particulates. Kits of spares are available from KRI.

45 MAINTENANCE 5-9 Fig. 5-1 Assembled ion source installed in socket.

46 5-10 MAINTENANCE Hold-down rods { Rods replacing neutralizer supports Socket Ion Source Fig. 5-2 Ion source removed from the socket. The two hold-down screws that held the ion source are next to the ion source.

47 MAINTENANCE 5-11 Alignment notch Notch in maintenance stand Fig. 5-3 Ion source placed on the maintenance stand. The alignment notches in the source are circumferentially aligned with the notch in the stand.

48 5-12 MAINTENANCE Outer shell Ion optics Fig. 5-4 Ion optics removed from ion source.

49 MAINTENANCE M insulator Anode 4-40MK insulator Fig. 5-5 Outer shell removed.

50 5-14 MAINTENANCE Cathode Back wall of discharge chamber Fig. 5-6 Anode removed.

51 MAINTENANCE 5-15 Fig. 5-7 Cathode.

52 5-16 MAINTENANCE Alignment notch Fig. 5-8 Ion optics. The numbers show the tightening sequence for re-assembly.

53 MAINTENANCE 5-17 Lock washer 4-40M insulator Fig. 5-9 Ion optics with nuts removed.

54 5-18 MAINTENANCE Screen-grid support Fig Lock washers and insulators removed.

55 MAINTENANCE 5-19 Screen-grid Fig Screen-grid support removed.

56 5-20 MAINTENANCE Ball insulators Accelerator-grid Fig Screen-grid removed.

57 MAINTENANCE x 3/4 SS screw 4 places Front plate Fig Ball insulators and accelerator-grid removed, leaving the ion-optics support and four 4-40 screws.