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Operating procedure for improving ion source lifetime for the 80-180XP ion implanter
465
Nuclear Instruments and Methods in Physics Research B55 (1991) 465-468 North-Holland
Operating procedure ion implanter S.R. Walther
for improving
ion source lifetime for the 80-18OXP
and R.F. Outcault
Varian Ion implant Systems, Blackburn Industrial Park, Gloucester, MA 01930, USA
The limited lifetime of the Freeman ion source filament has been the cause of equipment downtime, for filament replacement, in the use of bigb current ion implanters. The ion source lifetime of the 160XP ion implanter has been improved by a factor of four for bigb cnrrent operation, where source lifetime, especially with boron, has been very limited. The key to obtaining this result is a reduction of physical sputteringof the tungsten filament. This is achievedby minimizingthe arc voltage used by the ion source, while still delivering the same boron performance. The reduction in filament wear is also applicable to other dopants, and has been demonstrated with arsenic. Calculations of the sputtering rate, as a function of arc voltage, are consistent with the improvement in filament lifetime noted experimentally and imply that further improvements may be possible.
1. Introduction The limited ion source lifetime of the Freeman ion source used on the 80-18OXP ion implanter [l] has been a problem, contributing to the overall downtime of the machine. This is particularly true during high current “B+ ion beam operation. To address this issue, it is important to know the mechanisms by which the source filament is eroded. The most likely mechanism is sputtering of the filament material by plasma ions. The rate at which this process progresses is determined by the plasma density (roughly proportional to the discharge power) and the arc voltage (which determines the plasma ion energy, along with a small contribution from the plasma potential). A second mechanism that reduces filament life is cycling of the source on and off. Although it is not completely understood, the filament appears to fail prematurely when subjected to complete’. thermal cycling (the source is allowed to cool and is then restarted). The Freeman ion source [2] has been used for many years in the ion implantation industry. The use of the externally generated axial magnetic field, in combination with the magnetic field produced by the filament itself, provides the electron confinement for the discharge. The electrons emitted from the filament have a cycloidal orbit [3] in the region near the filament. This results in a substantial plasma density gradient between the filament and the ion extraction slot of the ion source. The limited filament lifetime of the Freeman source is due to the high plasma densities near the filament. In order to improve the filament lifetime, either the plasma density must be reduced, or the sputtering rate per plasma ion striking the filament must be reduced. Unfortunately, reducing the plasma 0168-583X/91/$03.50
density also reduces the beam current available and is therefore unacceptable. It is well known that lowering the arc voltage of the discharge increases the source lifetime by reducing sputtering. The improvement in filament life is due to the much lower sputtering coefficient for incoming ions at reduced arc voltages, as well as the reduction in plasma density. The sputtering coefficient is a very strong function of the ion energy (arc voltage). The rate can be calculated.using an empirical relation [4], given below, as a function of ion mass (m,), target atom mass (m,), ion energy (W), and the .surface binding energy (W,) of the filament material: A value of 8.4 eV is used here for the W,, of tungsten. The ion energy (W) is given approximately by the sum of the arc voltage and the plasma potential: Rate = 0.0064m,P5/3(
W/Wti,)1’4(1 - W,/W)7’2,
where /3 is given by 4mlm,/(ml + m2)2, and W,, is given by W,/p(l - p), for ml/m2 I 0.3, or 8W,(mi/m2) 2P for ml/m2 > 0.3. In the case of boron, the discharge contains B’, F+, BF+, and BF2+ as the dominant ion species in the plasma. The minor difference in sputtering yield of i”B+ (versus i’B+) is neglected here. The ion beam fractions of the different species were measured on numerous occasions for high current boron operation. An average of 33% B+, 21% F+, 11% BF+, and 35% BFC was used to model the discharge, even though there is some variation over time and from one test to another. The sputtering rate for each species was calculated and summed. Fig. 1 shows the relative sputtering rate as a function of ion energy for a tungsten filament in a BF, discharge. This graph details the significance of the arc voltage to the sputtering rate. A change of ion
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
V. MACHINES
S.R. Walther, R.F. Outcault / Improving
Ion
Energy
the ion source lifetime
time during which the source is producing the specified i’B+ ion beam, until the filament fails. To determine if higher arc currents and lower arc voltages would improve source life, a production machine was operated at a constant 5 A of arc current, normally the upper limit for this parameter, and the arc voltage was varied in a manner to maintain the constant 5 mA of “B+ ion current. The use of the larger arc current allowed a reduction in the arc voltage, while still providing the same performance. These tests lasted several days, which included cycling the source on and off, with the result of a source life of 27 h. The test was repeated twice to confirm this, and lifetimes of 26.5 and 28.5 h were obtained. This includes the effect of the source cycling on and off, where the filament often fails even though the diameter is relatively large. This premature filament failure is most likely caused by mechanical stresses induced by rapid thermal expansion and contraction of ion source parts during startup and shutdown. This can be alleviated through continuous operation of the ion source. The arc voltage was reduced to 60 V towards the end of these tests as the filament became thinner. Additional testing was undertaken, using a constant 6 A arc current and 24 h/d operation, to determine if further improvements in source life were possible. It should be noted that operation at 6 A of arc current exceeds the rated power supply capacity and is not recommended. The results of these tests show the impact a large reduction in arc voltage and continuous operation can achieve. Source lifetimes of 66, 43, 39.5, and 60+ h were attained during operation at 5 mA of i’B+ current. The 60 h test was terminated, due to personnel constraints, before the filament failed. The true filament life for that test is estimated to be 2 75 h using an extrapolation based on the filament current.
(eV)
Fig. 1. Calculated physical sputtering rate versus ion energy for a BF, discharge. energy from 120 to 60 V results in a tenfold reduction in the sputtering rate. To arrive at an actual rate, the results must be multiplied by the local current density of the plasma. If this value is known, the erosion due to physical sputtering of the filament as a function of time, and hence filament lifetime, can be calculated.
2. 16OXF’ lifetime experiments Initial testing was conducted to determine the actual source lifetime using standard operating procedures, which specify an arc voltage of - 120 V, while the arc current is used as a free parameter to adjust the beam current. The implanter was operated at a constant 5 mA of “B+ beam current (100 keV beam energy) in the end station, since 5 mA “B+ can be achieved immediately and maintained over the course of the test. In this case, the source lifetime was between 12 and 14 h during standard operation, with no thermal cycling of the ion source. Lifetime is defined here as the actual operating
60
Standard
Trial 1
Standard
Trial 2
Standard
Trial 3
5 A Arc Trial 1 5 A Arc Trial 2 5 A Arc Trial 3 6 A Arc Trial 1 6 A Arc Trial 2 6 A Arc Trial 3 6 A Arc Trial 4’ 1
Filament
3
2
Life
Operating
Fig. 2. Source life results for testing with a 5 mA “B+
Procedures
ions beam. * Test terminated prior to filament failure.
461
S.R. Walther, R.F. Outcault / Improving the ion source lifetime
40"'. 0
'. 20
10
/ 30
Time
* 40
I 50
"
60
~
' 70
I
0
10
20
30
Time
(Hours)
Fig. 3. Arc voltage as a function of time for the three cases tested.
Fig. 2 is a graph illustrating the improvement in source life obtainable by reducing the arc voltage, while producing the same beam current for three cases: (1) standard operation, (2) 5 A arc current operation, and (3) 6 A arc current operation. It should be emphasized that high current boron operation ordinarily represents the worst case for source lifetime. These experiments have demonstrated that operation of the 16OXP at reduced arc voltages and higher arc currents than are currently standard has substantially improved the source lifetime during high current BF, operation, when source life is a serious problem. This is due to a reduction in filament sputtering by plasma ions at the lower arc voltages. Fig. 3 shows the arc voltage used as a function of time for the three cases tested. The major difference is the lower arc voltage used, when the arc current is maintained at a constant value. Arc current for the three cases is detailed in fig. 4. The relative rate of filament wear for these three cases can be judged from the slope of the filament current as a function of time, shown in fig. 5. This indicates the improvement due to the reduction in arc voltage used. Operation at a constant 5 A of arc current resulted in a 2 times improvement in source life versus standard operation. If the effect of thermal cycling of the source
40
50
60
70
(Hours)
Fig. 5. Filament current as a function of time for the three cases tested.
is accounted for, the real improvement is even larger. For continuous operation at 6 A of arc current, a source lifetime of - 4 times (an average lifetime of over 52 h) that attained during standard operation was achieved. Using the measured arc voltages, and assuming a plasma potential of 5 V, the filament lifetime results are compared with physical sputtering theory. Fig. 6 uses the standard 120 V arc data to determine a plasma density consistent with the measured 13 h lifetime. The data from the 66 h 6 A arc test was then modeled for comparison. The experimental values of the final filament radii are denoted by solid triangles on the graph. The theory predicts a somewhat lower wear rate (larger radius) than that found experimentally. However the predicted lifetime, determined by extrapolating the physical sputtering model, was - 6 X the standard value (- 84 h), versus the actual value of - 5 x (66 h). This represents relatively good agreement, considering the strong dependence of sputtering rate on ion energy and the assumptions made regarding plasma density and potential. This result does not rule out chemical effects, since chemically enhanced physical sputtering could also be taking place. This process would have a similar dependence on ion energy as physical sputtering, but the rate would be increased through a chemical reaction on the filament surface.
6
0” 0
I
10
s
s
20
”
30
Time
”
40
9 50
”
60
70
(Hours)
Fig. 4. Arc current as a function of time for the three cases tested.
Time
(Hours)
Fig. 6. Calculated filament radius versus time using the physical sputtering model. V. MACHINES
468
S.R. Walther, R.F. Outcault / Improving
the ion source lifetime
3. Operation witharsenic While improving the source life for boron operation is critical overall, it is certainly advantageous to be able to extend these results to other species such as arsenic and phosphorous. Given the long lifetimes expected, testing until filament failure is not feasible. Instead the rate of filament wear is measured based on operation for 10 h continuously at m~mum beam. Two trials, for both standard operation and extended life operation, have been completed for arsenic using amine gas. It was expected that there would be little improvement in life due to the already low arc voltage (55-65 V) which is standard. However, the results show that reducing the arc voltage further does provide a sig~fic~t reduction in filament wear. Fig. 7 shows the filament current as a function of time for the case of standard operation (constant 65 V arc voltage) and extended life operation (constant - 35 V arc voltage), which used the lowest arc voltage possible from the power supply. The rate of filament wear can be roughly judged by the slope of the data points. It is clear that the rate of wear is much lower at the reduced arc voltage, which is consistent with physical sputtering as the dominant wear mechanism. This is also born out by the filament diameters at the end of the test. The measured filament diameters after these tests were 1.45 and 1.60 mm for standard operation, and 1.93 and 1.96 mm for the test conducted at low arc voltages. As usual, the initial filament diameter was 2.03 mm. Fig. 8 details the relative sputtering yield for As+ as a function of ion energy, calculated in the same manner as fig. 1. It is apparent that a reduction in arc voltage to 35 V from 65 V still provides a substanti~ reduction in sputtering yield. In fact, only a reduction to a voltage under - 50 V appears to be necessary. Similar results are expected for operation with phosphorous and other dopants. Hence, there is now reason to believe that the strategy of reducing are voltages to improve source life will be applicable to all dopant gases including operation with a vaporizer.
150
AAdA
(eV)
4. Conclusion Filament lifetime for high current boron operation has been improved by an average of a factor of 4, by reducing the arc voltage, with no loss of performance. This filament lifetime improvement is also applicable to other dopants and has been demonstrated during operation with high current As t ion beams at reduced arc voltages. The dominant filament wear mechanism is physical sputtering; in the case of boron, chemically enhanced physical sputtering is also a possibility. In general and particularly for high current operation, the new operating procedure [5], utilizing a reduction in arc voltage, will result in a substanti~ decrease in implanter downtime. These results have been verified through over 500 h of tests on production model ,160XP ion implanters. Future work will expand these results to include other dopants, and to determine the effect on filament life during low and medium current operation. Although physical sputtering has been determined to be the dominant filament wear mechanism, in the case of BP, operation more work should be done to ascertain whether this is chemically enhanced. Automation software to incorporate these improvements to the operating procedure must be developed in order to provide the most ~e~bility to the user. Further improvements in filament life, particularly during high current boron operation, are possible with a hardware upgrade to increase the power supply capacity. References
n!3
[I] R. Liebert, B. Pedersen, C. Ehrlich and W. Callahan, Nut.
Time
Fig. 7. Filament
Energy
~AAAAAAAAAAAAAA
140 130
Ion
Fig. 8. Calculated physical sputtering rate versus ion energy for an arsenic discharge.
(hours)
current versus time for standard extended life operation.
and for
Instr. and Meth. B6 (1985) 16. [2] J.H. Freeman, Nut. Inst. and Meth. 22 (1963) 306. [3] II. Hinkel, Nut. Instr. and Meth. 139 (1976) 1. [4] M.D. Gabovich, N.V. Pleshivtsev and N.N. Semashko, Ion and Atomic Beams for Controlled Fusion and Technology (Consultants Bureau, New York, 1989). [5] S.R. Walther and S. Hays, Varian Associates Product Support Bulletin (January, 1991).
Nuclear Instruments and Methods in Physics Research B55 (1991) 465-468 North-Holland
Operating procedure ion implanter S.R. Walther
for improving
ion source lifetime for the 80-18OXP
and R.F. Outcault
Varian Ion implant Systems, Blackburn Industrial Park, Gloucester, MA 01930, USA
The limited lifetime of the Freeman ion source filament has been the cause of equipment downtime, for filament replacement, in the use of bigb current ion implanters. The ion source lifetime of the 160XP ion implanter has been improved by a factor of four for bigb cnrrent operation, where source lifetime, especially with boron, has been very limited. The key to obtaining this result is a reduction of physical sputteringof the tungsten filament. This is achievedby minimizingthe arc voltage used by the ion source, while still delivering the same boron performance. The reduction in filament wear is also applicable to other dopants, and has been demonstrated with arsenic. Calculations of the sputtering rate, as a function of arc voltage, are consistent with the improvement in filament lifetime noted experimentally and imply that further improvements may be possible.
1. Introduction The limited ion source lifetime of the Freeman ion source used on the 80-18OXP ion implanter [l] has been a problem, contributing to the overall downtime of the machine. This is particularly true during high current “B+ ion beam operation. To address this issue, it is important to know the mechanisms by which the source filament is eroded. The most likely mechanism is sputtering of the filament material by plasma ions. The rate at which this process progresses is determined by the plasma density (roughly proportional to the discharge power) and the arc voltage (which determines the plasma ion energy, along with a small contribution from the plasma potential). A second mechanism that reduces filament life is cycling of the source on and off. Although it is not completely understood, the filament appears to fail prematurely when subjected to complete’. thermal cycling (the source is allowed to cool and is then restarted). The Freeman ion source [2] has been used for many years in the ion implantation industry. The use of the externally generated axial magnetic field, in combination with the magnetic field produced by the filament itself, provides the electron confinement for the discharge. The electrons emitted from the filament have a cycloidal orbit [3] in the region near the filament. This results in a substantial plasma density gradient between the filament and the ion extraction slot of the ion source. The limited filament lifetime of the Freeman source is due to the high plasma densities near the filament. In order to improve the filament lifetime, either the plasma density must be reduced, or the sputtering rate per plasma ion striking the filament must be reduced. Unfortunately, reducing the plasma 0168-583X/91/$03.50
density also reduces the beam current available and is therefore unacceptable. It is well known that lowering the arc voltage of the discharge increases the source lifetime by reducing sputtering. The improvement in filament life is due to the much lower sputtering coefficient for incoming ions at reduced arc voltages, as well as the reduction in plasma density. The sputtering coefficient is a very strong function of the ion energy (arc voltage). The rate can be calculated.using an empirical relation [4], given below, as a function of ion mass (m,), target atom mass (m,), ion energy (W), and the .surface binding energy (W,) of the filament material: A value of 8.4 eV is used here for the W,, of tungsten. The ion energy (W) is given approximately by the sum of the arc voltage and the plasma potential: Rate = 0.0064m,P5/3(
W/Wti,)1’4(1 - W,/W)7’2,
where /3 is given by 4mlm,/(ml + m2)2, and W,, is given by W,/p(l - p), for ml/m2 I 0.3, or 8W,(mi/m2) 2P for ml/m2 > 0.3. In the case of boron, the discharge contains B’, F+, BF+, and BF2+ as the dominant ion species in the plasma. The minor difference in sputtering yield of i”B+ (versus i’B+) is neglected here. The ion beam fractions of the different species were measured on numerous occasions for high current boron operation. An average of 33% B+, 21% F+, 11% BF+, and 35% BFC was used to model the discharge, even though there is some variation over time and from one test to another. The sputtering rate for each species was calculated and summed. Fig. 1 shows the relative sputtering rate as a function of ion energy for a tungsten filament in a BF, discharge. This graph details the significance of the arc voltage to the sputtering rate. A change of ion
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
V. MACHINES
S.R. Walther, R.F. Outcault / Improving
Ion
Energy
the ion source lifetime
time during which the source is producing the specified i’B+ ion beam, until the filament fails. To determine if higher arc currents and lower arc voltages would improve source life, a production machine was operated at a constant 5 A of arc current, normally the upper limit for this parameter, and the arc voltage was varied in a manner to maintain the constant 5 mA of “B+ ion current. The use of the larger arc current allowed a reduction in the arc voltage, while still providing the same performance. These tests lasted several days, which included cycling the source on and off, with the result of a source life of 27 h. The test was repeated twice to confirm this, and lifetimes of 26.5 and 28.5 h were obtained. This includes the effect of the source cycling on and off, where the filament often fails even though the diameter is relatively large. This premature filament failure is most likely caused by mechanical stresses induced by rapid thermal expansion and contraction of ion source parts during startup and shutdown. This can be alleviated through continuous operation of the ion source. The arc voltage was reduced to 60 V towards the end of these tests as the filament became thinner. Additional testing was undertaken, using a constant 6 A arc current and 24 h/d operation, to determine if further improvements in source life were possible. It should be noted that operation at 6 A of arc current exceeds the rated power supply capacity and is not recommended. The results of these tests show the impact a large reduction in arc voltage and continuous operation can achieve. Source lifetimes of 66, 43, 39.5, and 60+ h were attained during operation at 5 mA of i’B+ current. The 60 h test was terminated, due to personnel constraints, before the filament failed. The true filament life for that test is estimated to be 2 75 h using an extrapolation based on the filament current.
(eV)
Fig. 1. Calculated physical sputtering rate versus ion energy for a BF, discharge. energy from 120 to 60 V results in a tenfold reduction in the sputtering rate. To arrive at an actual rate, the results must be multiplied by the local current density of the plasma. If this value is known, the erosion due to physical sputtering of the filament as a function of time, and hence filament lifetime, can be calculated.
2. 16OXF’ lifetime experiments Initial testing was conducted to determine the actual source lifetime using standard operating procedures, which specify an arc voltage of - 120 V, while the arc current is used as a free parameter to adjust the beam current. The implanter was operated at a constant 5 mA of “B+ beam current (100 keV beam energy) in the end station, since 5 mA “B+ can be achieved immediately and maintained over the course of the test. In this case, the source lifetime was between 12 and 14 h during standard operation, with no thermal cycling of the ion source. Lifetime is defined here as the actual operating
60
Standard
Trial 1
Standard
Trial 2
Standard
Trial 3
5 A Arc Trial 1 5 A Arc Trial 2 5 A Arc Trial 3 6 A Arc Trial 1 6 A Arc Trial 2 6 A Arc Trial 3 6 A Arc Trial 4’ 1
Filament
3
2
Life
Operating
Fig. 2. Source life results for testing with a 5 mA “B+
Procedures
ions beam. * Test terminated prior to filament failure.
461
S.R. Walther, R.F. Outcault / Improving the ion source lifetime
40"'. 0
'. 20
10
/ 30
Time
* 40
I 50
"
60
~
' 70
I
0
10
20
30
Time
(Hours)
Fig. 3. Arc voltage as a function of time for the three cases tested.
Fig. 2 is a graph illustrating the improvement in source life obtainable by reducing the arc voltage, while producing the same beam current for three cases: (1) standard operation, (2) 5 A arc current operation, and (3) 6 A arc current operation. It should be emphasized that high current boron operation ordinarily represents the worst case for source lifetime. These experiments have demonstrated that operation of the 16OXP at reduced arc voltages and higher arc currents than are currently standard has substantially improved the source lifetime during high current BF, operation, when source life is a serious problem. This is due to a reduction in filament sputtering by plasma ions at the lower arc voltages. Fig. 3 shows the arc voltage used as a function of time for the three cases tested. The major difference is the lower arc voltage used, when the arc current is maintained at a constant value. Arc current for the three cases is detailed in fig. 4. The relative rate of filament wear for these three cases can be judged from the slope of the filament current as a function of time, shown in fig. 5. This indicates the improvement due to the reduction in arc voltage used. Operation at a constant 5 A of arc current resulted in a 2 times improvement in source life versus standard operation. If the effect of thermal cycling of the source
40
50
60
70
(Hours)
Fig. 5. Filament current as a function of time for the three cases tested.
is accounted for, the real improvement is even larger. For continuous operation at 6 A of arc current, a source lifetime of - 4 times (an average lifetime of over 52 h) that attained during standard operation was achieved. Using the measured arc voltages, and assuming a plasma potential of 5 V, the filament lifetime results are compared with physical sputtering theory. Fig. 6 uses the standard 120 V arc data to determine a plasma density consistent with the measured 13 h lifetime. The data from the 66 h 6 A arc test was then modeled for comparison. The experimental values of the final filament radii are denoted by solid triangles on the graph. The theory predicts a somewhat lower wear rate (larger radius) than that found experimentally. However the predicted lifetime, determined by extrapolating the physical sputtering model, was - 6 X the standard value (- 84 h), versus the actual value of - 5 x (66 h). This represents relatively good agreement, considering the strong dependence of sputtering rate on ion energy and the assumptions made regarding plasma density and potential. This result does not rule out chemical effects, since chemically enhanced physical sputtering could also be taking place. This process would have a similar dependence on ion energy as physical sputtering, but the rate would be increased through a chemical reaction on the filament surface.
6
0” 0
I
10
s
s
20
”
30
Time
”
40
9 50
”
60
70
(Hours)
Fig. 4. Arc current as a function of time for the three cases tested.
Time
(Hours)
Fig. 6. Calculated filament radius versus time using the physical sputtering model. V. MACHINES
468
S.R. Walther, R.F. Outcault / Improving
the ion source lifetime
3. Operation witharsenic While improving the source life for boron operation is critical overall, it is certainly advantageous to be able to extend these results to other species such as arsenic and phosphorous. Given the long lifetimes expected, testing until filament failure is not feasible. Instead the rate of filament wear is measured based on operation for 10 h continuously at m~mum beam. Two trials, for both standard operation and extended life operation, have been completed for arsenic using amine gas. It was expected that there would be little improvement in life due to the already low arc voltage (55-65 V) which is standard. However, the results show that reducing the arc voltage further does provide a sig~fic~t reduction in filament wear. Fig. 7 shows the filament current as a function of time for the case of standard operation (constant 65 V arc voltage) and extended life operation (constant - 35 V arc voltage), which used the lowest arc voltage possible from the power supply. The rate of filament wear can be roughly judged by the slope of the data points. It is clear that the rate of wear is much lower at the reduced arc voltage, which is consistent with physical sputtering as the dominant wear mechanism. This is also born out by the filament diameters at the end of the test. The measured filament diameters after these tests were 1.45 and 1.60 mm for standard operation, and 1.93 and 1.96 mm for the test conducted at low arc voltages. As usual, the initial filament diameter was 2.03 mm. Fig. 8 details the relative sputtering yield for As+ as a function of ion energy, calculated in the same manner as fig. 1. It is apparent that a reduction in arc voltage to 35 V from 65 V still provides a substanti~ reduction in sputtering yield. In fact, only a reduction to a voltage under - 50 V appears to be necessary. Similar results are expected for operation with phosphorous and other dopants. Hence, there is now reason to believe that the strategy of reducing are voltages to improve source life will be applicable to all dopant gases including operation with a vaporizer.
150
AAdA
(eV)
4. Conclusion Filament lifetime for high current boron operation has been improved by an average of a factor of 4, by reducing the arc voltage, with no loss of performance. This filament lifetime improvement is also applicable to other dopants and has been demonstrated during operation with high current As t ion beams at reduced arc voltages. The dominant filament wear mechanism is physical sputtering; in the case of boron, chemically enhanced physical sputtering is also a possibility. In general and particularly for high current operation, the new operating procedure [5], utilizing a reduction in arc voltage, will result in a substanti~ decrease in implanter downtime. These results have been verified through over 500 h of tests on production model ,160XP ion implanters. Future work will expand these results to include other dopants, and to determine the effect on filament life during low and medium current operation. Although physical sputtering has been determined to be the dominant filament wear mechanism, in the case of BP, operation more work should be done to ascertain whether this is chemically enhanced. Automation software to incorporate these improvements to the operating procedure must be developed in order to provide the most ~e~bility to the user. Further improvements in filament life, particularly during high current boron operation, are possible with a hardware upgrade to increase the power supply capacity. References
n!3
[I] R. Liebert, B. Pedersen, C. Ehrlich and W. Callahan, Nut.
Time
Fig. 7. Filament
Energy
~AAAAAAAAAAAAAA
140 130
Ion
Fig. 8. Calculated physical sputtering rate versus ion energy for an arsenic discharge.
(hours)
current versus time for standard extended life operation.
and for
Instr. and Meth. B6 (1985) 16. [2] J.H. Freeman, Nut. Inst. and Meth. 22 (1963) 306. [3] II. Hinkel, Nut. Instr. and Meth. 139 (1976) 1. [4] M.D. Gabovich, N.V. Pleshivtsev and N.N. Semashko, Ion and Atomic Beams for Controlled Fusion and Technology (Consultants Bureau, New York, 1989). [5] S.R. Walther and S. Hays, Varian Associates Product Support Bulletin (January, 1991).