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High-performance low-maintenance sputter source
Nuclear Instruments and Methods in Physics Research A287 (1990) 176-179 North-Holland
176
HIGH-PERFORMANCE LOW-MAINTENANCE SPUTTER SOURCE T.A. TRAINOR, G.C . HARPER and D.J. HODGKINS
Nuclear Physics Laboratory, University of Washington, Seattle, WA 98195, USA
A Model 860 sputter ion source obtained from General Ionex Corporation has been extensively modified to improve beam quality and reduce maintenance . Modifications include improving the cesium vapor delivery and pumping systems, eliminating the target cathode insulator, alteration of the ionizer coil to minimize the cesium beam spot size, changing the source biasing scheme to optimize beam energy homogeneity, and adding a large diameter gap lens to improve the negative ion optics. Beams from hydrogen to tin have been produced. A carbon beam of 650 WA was limited in intensity only by the current capacity of the target rod bias supply . It is observed with carbon that the principal emittance source is the space charge of secoadary electrons produced at the target cathode by the cesium beam.
1 . Hardware modifications Early expeg iencc a Model 860 sputter source purchased from General lonex Corporation indicated several problem areas . Supply of cesium vapor to the ionizer was unreliable, recovery from cesium oversupply was slow or not possible, the sputter target insulator was subject to frequent breakdown and failure by tracking, and the ionizer delivered a substantial amount of cesium beam to the cathode structure which was unproductive but loaded the cesium focus supply . The measures taken to solve these problems can be seen in part in fig . 1 . To improve recovery from cesium overshoot four large rectangular holes were milled in the cylinder surrounding the end of the target rod (cathode) . This increases the pumping speed of cesium out of the cathode region, but has not noticeably affected the relationship between cesium temperature and
negative ion output except for -- 2596 lower yield for copper. In the case of an oversupply of cesium (usually during startup) the cesium reservoir is rapidly cooled for a short time with air . The source recovers in less than five minutes and bias voltages can be reapplied . The IT in . diameter cesium feed tube (not shown) was extended, without coupling, through the source vacuum wall into an external coaxial vacuum jacket. A Cajon fitting at the end of the jacket couples the tube to the cesium reservoir . The heater was changed to two 150 W Rama rod * heaters in a copper block clamped to the reservoir miniflange . Copper strips run from the block to the vacuum jacket and to the base of the reservoir, thus ensuring uniform heating of the entire cesium system . The orit inal cathode insulator has been removed and support for the end of the target rod transferred to three macor rods located in a very well shielded region . The pathway from the ionizer region to these insulators now has a very low conductance . There has been no failure during a 12 month period . It was determined that cesium from the last two turns of the ionizer (nearest the cathode) was mainly sputtering the conical cathode support and the
aluminum sample holder . These turns were first blocked and then removed with good result . With the target flush with the end of the conical support and a cesium current of 1-2 mA the cesium waist is located at the target and < 7 r7m diameter . With reduced cesium current this waist moves back toward the ionizer as one would expect . Space charge effects are produced by the positive cesium beam, the negative heavy ion beam and secondary electrons produced on the cathode surface . Such
fig . 1 . Model 1160 sputter source with modification . 0168-9(?412, yf)/S03 .50 , F.lsevicr Science Publishers B N . (North-Holland)
RAMA Corporation . 600 W . Esplanade, San Jacinto, CA 12383, USA (=31 All).
T.A . Trainor et al. / High-performance low-maintenance sputter source
effects are greatest where the ions move most slowly. Thus, the cesium space charge mainly affects where the cesium beam waist will occur. More current moves the waist axially toward the cathode. At the cathode space charge effects are produced mainly by the high current of secondary electrons produced there with many samples (e.g. carbon) which can be several times the heavy ion current. The space charge increases the negative ion beam divergence, and, since the electrons are quickly bent off axis by magnetic suppression, also produces radially asymmetric beam distortions. Sample holders are aluminum cylinders with 3 mm diameter x 3 mm deep samples. A $ mm thick lead washer is used to ensure good thermal contact with the cooled target rod. During production of 400 tLA 12 C beam the cesium was incident at 10 keV with 1-2 mA true cesium current and - 10 mA secondary electrons . The negative ions were accelerated across the 10 kV target potential and focused at th.a final large diameter gap lens to a final energy of .i0 keV . Focusing is optimum when there is little or no voltage drop across the small middle gap lens . Total source output was 1 .2 mA. The 400 RA beam component was - the total analyzed beam, the rest being mostly higher carbon polymers. Thus, transmission through the analysis system was 676 assuming the entire source output was carbon . t2 C
7'
2. Electrical system The present electrical system for the Model 860 is shown in fig. 2. As delivered the source was intended to have two bias supplies, a 0-20 kV bias for the source body and a 0-10 kV bias for the cathode, referenced to the source body. The negative ion energy would be determined by the sum of the two bias voltages. We HIGH VOLTAGE RACK
177
considered this scheme inappropriate for our application for two reasons . (1) The cathode bias supply generally provides a 5-10 mA current. This loading degrades the voltage stability to 1/10 3 for a good switching power supply or about 10 V. Also, the voltage noise from two power supplies contributes to the final energy spread of the beam. (2) Because the ion source beam immediately passes through a high resolution mass analysis system a change in either bias supply to alter the beam focus would require a corresponding change in either the magnet current or the other bias supply to restore t;-ansmission of the beam through the analysis system. To avoid these problems we added a third bias supply . The 30 kV supply is referenced to injector deck ground and directly determines the cathode potential and negative ion beam energy . The loading on this supply is only the total negative heavy ion beam current ( s 1 mA). The supply operates at a fixed 30 kV, and the magnet mass calibration never changes. To accommodate this scheme we modified the Model 860 by adding an additional larger gap lens near the output flange. The upstream element of this new gap lens is also the downstream element of the original smaller gap lens . The cesium focus supply now determines the source 1?,)dy (and ionizer) potential with respect to the fixed cathode bias (extraction) supply . The cesium focus supply is misnamed because except for space charge effects the cesium ion trajectories are independent of this bias voltage. This supply does affect the negative ion yield and beam heating of the cathode surface, and is adjusted to maximize the sputter yield for a given element consistent with maintaining an adequate neutral cesium deposit on the sample . The ion focus supply determines the potential of the intermediate focusing element, and hence the relative 860 SOURCE CAGE
Fig. 2. Model 860 sputter source electronics. IV . PARTICLE BEAMS
T.A . Trainor et al. / High-performance low-maintenance sputter source
178
strengths of the two gap lenses. If the two focus supply voltages add up to the extraction potential then the source is operating as originally intended, and the larger gap lens is off. However, considerably more beam is produced and the output is maximized when the ion focus supply is near or at zero. This result confirms a suspicion that the smaller upstream gap lens is much too small to provide reasonable matching into a typical analysis system acceptance, and spherical aberations would be larger than necessary . The ionizer and reservoir heaters are both powered with DC currents . The ionizer is run open loop with 22 A current . The cesium reservoir heater is powered by a servo loop. The thermocouple difference signal is chopper amplified, compared to a reference and the difference fed to an active filter high gain stage designed specifically for the response of this heater system. The output is fed directly to the gate pulse generator of an SCR controlled (Sorenson) current supply, bypassing all internal regulation circuits . This system provides fast heating with negligible overshoot and stability "o 5 1 ° provided some modest convection shield is placed around the heater .
Fig. 4. Analyzed 27A12 beam.
3. Beam properties The mass analysis system for the Model 860 consists of a 90' analyzing magnetic preceded by a 45' switch magnet for a 135 ° total bend. An electrostatic quadrupole doublet upstream of the 45 ° magnet provides for double focusing at the 90' magnet image point . Horizontal slits are provided at object and image points . It was expected that with slit widths of 2-4 mm and some beam loss a mass resolution m/,gym - 100 would be achieved . However, ti, emittance of the source appears to be considerably better than expected for at least some elements. Figs . 3-7 show the quality of the beam achieved after some experience with the optics. Figs. 3-5 show
Fiâ. 3 . Analyzed
16 0
beam.
Fig. 5. Analyzed 27Al l(, O - beam.
Fig. 6. Analyzed Moo - beam with peaks at 4 = 96, 95. 94, 93 for Mo.
T. A . Trainor et aL / High-performance tow-maintenance sputter source
Table l Currently available beams Ion
1H 6. l_i
Li q Deo
C 1;C le- 0 12
NH `'AI 'KSi Fig. 7. Analyzed
12C -
beam .
beam scanner traces at the image slit position for magnet settings which transmit 1602, 27A12 and 2'A1 16 0, respectively . No other transport parameters were changed. One can see that aluminum is being produced from an annular region of the sample holder around the M903 sample, oxygen is being produced at the sample itself (in a spot <_ 3 mm) and Al0 is produced in an intermediate region, perhaps involving two or more sputte!ing events. In fig. 6 is shown another scanner trace, of a Moo beam, showing Mo isotopes with A = 93, 94, 95 and 96 from right to left . The mass resolution from this figure is m/Am - 650, but the scanner wire width contributes a significant fraction to the peak widths . The peak heights do not reflect normal isotopic abundances because the dispersed beams are partially blocked by a circular aperture just upstream of the scanner. Fig. 7 sho- ., a 300 ~tA carbon beam . The base width of the peaks corresponds to the 3 nin1 sample diameter . The
32s
"C1 4" CaH a8TiH "Ni "oZrH
"moo
`*Moo, 1'12 Ru
Analyzed current (N A) 85 2 10
15 400 50 50 2 6 25 25 25 5 15 25 1 0.2 1 2
Composition 'Ti H (50 '50s Lili,'10`ï A g LO/20% Ag
BeO/Ag graphite a'C(amorph . --., graph.) MgO/Ag AIN AI Si/50% Ag Li,S AgCI CaCl 2 /Ag TiH/l0% Ag Ni MOO, Ru/TiH
scanner traces reveal the intensity profile of the Cs' beam across the carbon sample . These profiles are consistent with the observed < ' mm holes sputtered in carbon t, trgets . For all of these figures the object slits are unused . The object slits were moved in to determine the initial tune and fix the object waist at the proper point. th"~-i pulled back well out of the beam . The s,.an ier is upstream from the image slits, so these fi,ur`,s represent slitless operation of the mass analysis sysazm . Table I presents a partial list of currently available beams with intensities and target compositions.
IV . PARTICLE- BEAMS
176
HIGH-PERFORMANCE LOW-MAINTENANCE SPUTTER SOURCE T.A. TRAINOR, G.C . HARPER and D.J. HODGKINS
Nuclear Physics Laboratory, University of Washington, Seattle, WA 98195, USA
A Model 860 sputter ion source obtained from General Ionex Corporation has been extensively modified to improve beam quality and reduce maintenance . Modifications include improving the cesium vapor delivery and pumping systems, eliminating the target cathode insulator, alteration of the ionizer coil to minimize the cesium beam spot size, changing the source biasing scheme to optimize beam energy homogeneity, and adding a large diameter gap lens to improve the negative ion optics. Beams from hydrogen to tin have been produced. A carbon beam of 650 WA was limited in intensity only by the current capacity of the target rod bias supply . It is observed with carbon that the principal emittance source is the space charge of secoadary electrons produced at the target cathode by the cesium beam.
1 . Hardware modifications Early expeg iencc a Model 860 sputter source purchased from General lonex Corporation indicated several problem areas . Supply of cesium vapor to the ionizer was unreliable, recovery from cesium oversupply was slow or not possible, the sputter target insulator was subject to frequent breakdown and failure by tracking, and the ionizer delivered a substantial amount of cesium beam to the cathode structure which was unproductive but loaded the cesium focus supply . The measures taken to solve these problems can be seen in part in fig . 1 . To improve recovery from cesium overshoot four large rectangular holes were milled in the cylinder surrounding the end of the target rod (cathode) . This increases the pumping speed of cesium out of the cathode region, but has not noticeably affected the relationship between cesium temperature and
negative ion output except for -- 2596 lower yield for copper. In the case of an oversupply of cesium (usually during startup) the cesium reservoir is rapidly cooled for a short time with air . The source recovers in less than five minutes and bias voltages can be reapplied . The IT in . diameter cesium feed tube (not shown) was extended, without coupling, through the source vacuum wall into an external coaxial vacuum jacket. A Cajon fitting at the end of the jacket couples the tube to the cesium reservoir . The heater was changed to two 150 W Rama rod * heaters in a copper block clamped to the reservoir miniflange . Copper strips run from the block to the vacuum jacket and to the base of the reservoir, thus ensuring uniform heating of the entire cesium system . The orit inal cathode insulator has been removed and support for the end of the target rod transferred to three macor rods located in a very well shielded region . The pathway from the ionizer region to these insulators now has a very low conductance . There has been no failure during a 12 month period . It was determined that cesium from the last two turns of the ionizer (nearest the cathode) was mainly sputtering the conical cathode support and the
aluminum sample holder . These turns were first blocked and then removed with good result . With the target flush with the end of the conical support and a cesium current of 1-2 mA the cesium waist is located at the target and < 7 r7m diameter . With reduced cesium current this waist moves back toward the ionizer as one would expect . Space charge effects are produced by the positive cesium beam, the negative heavy ion beam and secondary electrons produced on the cathode surface . Such
fig . 1 . Model 1160 sputter source with modification . 0168-9(?412, yf)/S03 .50 , F.lsevicr Science Publishers B N . (North-Holland)
RAMA Corporation . 600 W . Esplanade, San Jacinto, CA 12383, USA (=31 All).
T.A . Trainor et al. / High-performance low-maintenance sputter source
effects are greatest where the ions move most slowly. Thus, the cesium space charge mainly affects where the cesium beam waist will occur. More current moves the waist axially toward the cathode. At the cathode space charge effects are produced mainly by the high current of secondary electrons produced there with many samples (e.g. carbon) which can be several times the heavy ion current. The space charge increases the negative ion beam divergence, and, since the electrons are quickly bent off axis by magnetic suppression, also produces radially asymmetric beam distortions. Sample holders are aluminum cylinders with 3 mm diameter x 3 mm deep samples. A $ mm thick lead washer is used to ensure good thermal contact with the cooled target rod. During production of 400 tLA 12 C beam the cesium was incident at 10 keV with 1-2 mA true cesium current and - 10 mA secondary electrons . The negative ions were accelerated across the 10 kV target potential and focused at th.a final large diameter gap lens to a final energy of .i0 keV . Focusing is optimum when there is little or no voltage drop across the small middle gap lens . Total source output was 1 .2 mA. The 400 RA beam component was - the total analyzed beam, the rest being mostly higher carbon polymers. Thus, transmission through the analysis system was 676 assuming the entire source output was carbon . t2 C
7'
2. Electrical system The present electrical system for the Model 860 is shown in fig. 2. As delivered the source was intended to have two bias supplies, a 0-20 kV bias for the source body and a 0-10 kV bias for the cathode, referenced to the source body. The negative ion energy would be determined by the sum of the two bias voltages. We HIGH VOLTAGE RACK
177
considered this scheme inappropriate for our application for two reasons . (1) The cathode bias supply generally provides a 5-10 mA current. This loading degrades the voltage stability to 1/10 3 for a good switching power supply or about 10 V. Also, the voltage noise from two power supplies contributes to the final energy spread of the beam. (2) Because the ion source beam immediately passes through a high resolution mass analysis system a change in either bias supply to alter the beam focus would require a corresponding change in either the magnet current or the other bias supply to restore t;-ansmission of the beam through the analysis system. To avoid these problems we added a third bias supply . The 30 kV supply is referenced to injector deck ground and directly determines the cathode potential and negative ion beam energy . The loading on this supply is only the total negative heavy ion beam current ( s 1 mA). The supply operates at a fixed 30 kV, and the magnet mass calibration never changes. To accommodate this scheme we modified the Model 860 by adding an additional larger gap lens near the output flange. The upstream element of this new gap lens is also the downstream element of the original smaller gap lens . The cesium focus supply now determines the source 1?,)dy (and ionizer) potential with respect to the fixed cathode bias (extraction) supply . The cesium focus supply is misnamed because except for space charge effects the cesium ion trajectories are independent of this bias voltage. This supply does affect the negative ion yield and beam heating of the cathode surface, and is adjusted to maximize the sputter yield for a given element consistent with maintaining an adequate neutral cesium deposit on the sample . The ion focus supply determines the potential of the intermediate focusing element, and hence the relative 860 SOURCE CAGE
Fig. 2. Model 860 sputter source electronics. IV . PARTICLE BEAMS
T.A . Trainor et al. / High-performance low-maintenance sputter source
178
strengths of the two gap lenses. If the two focus supply voltages add up to the extraction potential then the source is operating as originally intended, and the larger gap lens is off. However, considerably more beam is produced and the output is maximized when the ion focus supply is near or at zero. This result confirms a suspicion that the smaller upstream gap lens is much too small to provide reasonable matching into a typical analysis system acceptance, and spherical aberations would be larger than necessary . The ionizer and reservoir heaters are both powered with DC currents . The ionizer is run open loop with 22 A current . The cesium reservoir heater is powered by a servo loop. The thermocouple difference signal is chopper amplified, compared to a reference and the difference fed to an active filter high gain stage designed specifically for the response of this heater system. The output is fed directly to the gate pulse generator of an SCR controlled (Sorenson) current supply, bypassing all internal regulation circuits . This system provides fast heating with negligible overshoot and stability "o 5 1 ° provided some modest convection shield is placed around the heater .
Fig. 4. Analyzed 27A12 beam.
3. Beam properties The mass analysis system for the Model 860 consists of a 90' analyzing magnetic preceded by a 45' switch magnet for a 135 ° total bend. An electrostatic quadrupole doublet upstream of the 45 ° magnet provides for double focusing at the 90' magnet image point . Horizontal slits are provided at object and image points . It was expected that with slit widths of 2-4 mm and some beam loss a mass resolution m/,gym - 100 would be achieved . However, ti, emittance of the source appears to be considerably better than expected for at least some elements. Figs . 3-7 show the quality of the beam achieved after some experience with the optics. Figs. 3-5 show
Fiâ. 3 . Analyzed
16 0
beam.
Fig. 5. Analyzed 27Al l(, O - beam.
Fig. 6. Analyzed Moo - beam with peaks at 4 = 96, 95. 94, 93 for Mo.
T. A . Trainor et aL / High-performance tow-maintenance sputter source
Table l Currently available beams Ion
1H 6. l_i
Li q Deo
C 1;C le- 0 12
NH `'AI 'KSi Fig. 7. Analyzed
12C -
beam .
beam scanner traces at the image slit position for magnet settings which transmit 1602, 27A12 and 2'A1 16 0, respectively . No other transport parameters were changed. One can see that aluminum is being produced from an annular region of the sample holder around the M903 sample, oxygen is being produced at the sample itself (in a spot <_ 3 mm) and Al0 is produced in an intermediate region, perhaps involving two or more sputte!ing events. In fig. 6 is shown another scanner trace, of a Moo beam, showing Mo isotopes with A = 93, 94, 95 and 96 from right to left . The mass resolution from this figure is m/Am - 650, but the scanner wire width contributes a significant fraction to the peak widths . The peak heights do not reflect normal isotopic abundances because the dispersed beams are partially blocked by a circular aperture just upstream of the scanner. Fig. 7 sho- ., a 300 ~tA carbon beam . The base width of the peaks corresponds to the 3 nin1 sample diameter . The
32s
"C1 4" CaH a8TiH "Ni "oZrH
"moo
`*Moo, 1'12 Ru
Analyzed current (N A) 85 2 10
15 400 50 50 2 6 25 25 25 5 15 25 1 0.2 1 2
Composition 'Ti H (50 '50s Lili,'10`ï A g LO/20% Ag
BeO/Ag graphite a'C(amorph . --., graph.) MgO/Ag AIN AI Si/50% Ag Li,S AgCI CaCl 2 /Ag TiH/l0% Ag Ni MOO, Ru/TiH
scanner traces reveal the intensity profile of the Cs' beam across the carbon sample . These profiles are consistent with the observed < ' mm holes sputtered in carbon t, trgets . For all of these figures the object slits are unused . The object slits were moved in to determine the initial tune and fix the object waist at the proper point. th"~-i pulled back well out of the beam . The s,.an ier is upstream from the image slits, so these fi,ur`,s represent slitless operation of the mass analysis sysazm . Table I presents a partial list of currently available beams with intensities and target compositions.
IV . PARTICLE- BEAMS