- Email: [email protected]
The negative ion sputter source MISS-585
Nuclear Instruments and Methods in Physics Research A244 (1986) 155-157 North-Holland, Amsterdam
155
T H E NEGATIVE ION S P U T r E R S O U R C E MISS-585 H. M A T T H E S
and W. P F E S T O R F
Academy of Science of the GDR, Central Institute for Nuclear Research, Rossendorf, GDR
A brief description of the newly designed negative ion sputter source MISS-585 is given. The sputter source MISS-585 represents a more powerful and reliable version of the model MISS-483. In discussing the source performance emphasis is given on the effect of the temperature of the sputter surface on the negative ion yield in a wide range of temperatures and for different species of ions.
MISS-483. The primary aim of our new development was to attain a high current of negative ions. In comparison with the version MISS-483 the source MISS-585 is characterized by an increased ionizer surface (up to 100%) and an essentially improved heat insulation. Early papers [1,2] show that the effect of the sputter surface temperature on the negative ion yield is not fully understood. In connection with the design of the source MISS-585 this temperature effect has been investigated.
I. Introduction
Since 1983 the ion source MISS-483 [1] has been used successfully at different tandem accelerators. It operates reliable and produces a sufficient intense beam of low emittance. However, for special applications and for target materials of low electron affinities a more powerful ion source would be desirable. F r o m this point of view the ion sputter source MISS-585 (fig. 1) has been developed keeping the proven features of the source
9
13 1211 16
I/.
/
/'
,,
/:
r 1
2
.I
3
! m
'i I .I
4
5
GAS SPRAY 2 COOLING 3 SLIDING SEAL CATH00E ADJUSTMENT 5 GATE VALVE 6 Cs - RESERVlOR 7 Cs-VAPOR FEED 8 METERING VALVE" 9 CATHODEINSULATOR 10 IONIZERHEATER FFFD-TI-ROUGH 11 IONIZER 12 TARGET ALL 13 IONIZERCARRIER TUBE 14 RADIATION SHIELD 15 PUMP OPENI~ 16 EXIRACIION ELECTRODE 17 ACC LENS
10
9 , , , , 6
B
7 V.
Fig. 1. Ion sputter source MISS-585. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
111. ION SOURCES
H. Matthes, Hi. Pfestorf / MISS-585
156
2. The effect of the sputter surface temperature on the negative ion yield
IB-
Applying different cooling agents (air, water, liquid nitrogen) and changing the thermal conductivity between the sputter target and the target holder, the temperature at the sputter surface could be varied between 80°C and 700°C. The lower temperature was limited by the sealing system of the gate valve, the upper one by the inner cathode insulator. During operation of the source the temperature of the sputter surface was measured by a small thermocouple. It was taken through the gas spray equipment and touched the sputter target 0.2 mm behind the sputter surface. The tests were carried out at the injector of the tandem accelerator EGP-10-1 of the Central Institute for Nuclear Research Rossendorf. The results achieved are plotted in figs. 2 and 3. Silver and copper show a weak maximum at temperatures of approximately 400°C and 450°C, respectively. By increasing the partial pressure of cesium, these maxima shift to higher temperatures. Also the carbon target provides a maximum of the ion current between 300°C and 400°C. No essential dependence of temperature was visible for the gold target. (The increase in intensity corresponds to an increase in pressure in this case). From the target materials investigated only chlorine showed an increase of the current with raising temperature. In contrast to that the yield in 160- ions was rather constant up to temperatures of about 200°C and distinctively decreased with continuously raising
LIO
I
I
I
I
I
I
:
,_~o
+
0
:
*
÷~ +
+
.
4
~"-'~
SPU'ITERTARC,~r TEMPERATURE
Fig. 2. Ion current vs sputter target temperature for A u (v), A g ( + ) and C u ions (x), U~p,u~r=2.0 kV. Intensity increased with raising cesium partial pressure.
__0------"'%0
I
too
20o
360
4oo
s0o
SPUrfERTARGET
1
s~o lit~ TEMPERATURE
Fig. 3. Ion current vs sputter target temperature for CI-(O), O - (zx) and C - ions (121), U~pu,c, = 2.0 kV. Intensity increased with raising cesium partial pressure.
temperature. The result obtained agrees with the results of ref. [1]. The ion source operation experiments showed that the source would operate over a surprisingly wide range of sputter surface temperature and the need for a special cathode cooling became questionable. The optimum target temperature of about 200°C up to 450°C can be easily achieved by using a moderate air or water cooling. With the exception of target materials with a low melting temperature no special cooling of the target is required for the M1SS-585 ion source.
3. Ion optics and preliminary results The ion source geometry was calculated by means of the program E G U N . It solves the Laplace equation (and the Poisson equation if space charge is included) [3,4]. The sputtering yield considerations are estimated following ref. [5]. Differing from the source MISS-483 the cathode diameter was enlarged from 5.5 to 7 mm. The front surface of the sputter cathode is concavely shaped in order to improve the focussing of the negative ion beam. The results calculated are plotted in fig. 4. They are in good agreement with the geometry of the sputter pit shown in fig. 5. In spite of an accurate centre position of the sputter target ionizer and extractor electrode a deviation of the beam from the optical axis can be observed on the extractor electrode. The reason for that is still unclear because it could not be observed at the very similar source MISS-483. The increase in beam current amounts to a factor of 2 in average compared with the source MISS-483. The activities to optimize the ion beam parameters of the source MISS-585 are continuing.
H. Matthes, W. Pfestorf / MISS- 585
157 ^rr-~l~rTP0D E
cvTo^rTno-
2O E E 10
0
10
20
30
40
50
60
mm
Fig. 4. Calculated trajectories of positive and negative ions of the sputter source MISS-585 for a concavely shaped sputter surface (space charge is included).
Fig. 5. Cross section of a copper sputter target after 20 h of running (diameter 7 ram).
Fig. 6. Off axis beam traces on the extractor electrode of the source MISS-585.
References
[3] A. Seifert, Diplomarbeit TU Dresden (1985). [4] W.B. Herrmannsfeldt, SLAC-266 (Nov. 1979). [5] J.H. Billen, Proc. 3rd Int. Conf. on Electrostatic Accelerator Technology, IEEE Catalog No. 81 Ch. 1639-4, p. 238.
[1] H. Matthes, W. Pfestorf and L. Steinert, Nucl. Instr. and Meth. 220 (1984) 112. [2] R. Middleton, Nucl. Instr. and Meth. 214 (1983) 139.
II!. ION SOURCES
155
T H E NEGATIVE ION S P U T r E R S O U R C E MISS-585 H. M A T T H E S
and W. P F E S T O R F
Academy of Science of the GDR, Central Institute for Nuclear Research, Rossendorf, GDR
A brief description of the newly designed negative ion sputter source MISS-585 is given. The sputter source MISS-585 represents a more powerful and reliable version of the model MISS-483. In discussing the source performance emphasis is given on the effect of the temperature of the sputter surface on the negative ion yield in a wide range of temperatures and for different species of ions.
MISS-483. The primary aim of our new development was to attain a high current of negative ions. In comparison with the version MISS-483 the source MISS-585 is characterized by an increased ionizer surface (up to 100%) and an essentially improved heat insulation. Early papers [1,2] show that the effect of the sputter surface temperature on the negative ion yield is not fully understood. In connection with the design of the source MISS-585 this temperature effect has been investigated.
I. Introduction
Since 1983 the ion source MISS-483 [1] has been used successfully at different tandem accelerators. It operates reliable and produces a sufficient intense beam of low emittance. However, for special applications and for target materials of low electron affinities a more powerful ion source would be desirable. F r o m this point of view the ion sputter source MISS-585 (fig. 1) has been developed keeping the proven features of the source
9
13 1211 16
I/.
/
/'
,,
/:
r 1
2
.I
3
! m
'i I .I
4
5
GAS SPRAY 2 COOLING 3 SLIDING SEAL CATH00E ADJUSTMENT 5 GATE VALVE 6 Cs - RESERVlOR 7 Cs-VAPOR FEED 8 METERING VALVE" 9 CATHODEINSULATOR 10 IONIZERHEATER FFFD-TI-ROUGH 11 IONIZER 12 TARGET ALL 13 IONIZERCARRIER TUBE 14 RADIATION SHIELD 15 PUMP OPENI~ 16 EXIRACIION ELECTRODE 17 ACC LENS
10
9 , , , , 6
B
7 V.
Fig. 1. Ion sputter source MISS-585. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
111. ION SOURCES
H. Matthes, Hi. Pfestorf / MISS-585
156
2. The effect of the sputter surface temperature on the negative ion yield
IB-
Applying different cooling agents (air, water, liquid nitrogen) and changing the thermal conductivity between the sputter target and the target holder, the temperature at the sputter surface could be varied between 80°C and 700°C. The lower temperature was limited by the sealing system of the gate valve, the upper one by the inner cathode insulator. During operation of the source the temperature of the sputter surface was measured by a small thermocouple. It was taken through the gas spray equipment and touched the sputter target 0.2 mm behind the sputter surface. The tests were carried out at the injector of the tandem accelerator EGP-10-1 of the Central Institute for Nuclear Research Rossendorf. The results achieved are plotted in figs. 2 and 3. Silver and copper show a weak maximum at temperatures of approximately 400°C and 450°C, respectively. By increasing the partial pressure of cesium, these maxima shift to higher temperatures. Also the carbon target provides a maximum of the ion current between 300°C and 400°C. No essential dependence of temperature was visible for the gold target. (The increase in intensity corresponds to an increase in pressure in this case). From the target materials investigated only chlorine showed an increase of the current with raising temperature. In contrast to that the yield in 160- ions was rather constant up to temperatures of about 200°C and distinctively decreased with continuously raising
LIO
I
I
I
I
I
I
:
,_~o
+
0
:
*
÷~ +
+
.
4
~"-'~
SPU'ITERTARC,~r TEMPERATURE
Fig. 2. Ion current vs sputter target temperature for A u (v), A g ( + ) and C u ions (x), U~p,u~r=2.0 kV. Intensity increased with raising cesium partial pressure.
__0------"'%0
I
too
20o
360
4oo
s0o
SPUrfERTARGET
1
s~o lit~ TEMPERATURE
Fig. 3. Ion current vs sputter target temperature for CI-(O), O - (zx) and C - ions (121), U~pu,c, = 2.0 kV. Intensity increased with raising cesium partial pressure.
temperature. The result obtained agrees with the results of ref. [1]. The ion source operation experiments showed that the source would operate over a surprisingly wide range of sputter surface temperature and the need for a special cathode cooling became questionable. The optimum target temperature of about 200°C up to 450°C can be easily achieved by using a moderate air or water cooling. With the exception of target materials with a low melting temperature no special cooling of the target is required for the M1SS-585 ion source.
3. Ion optics and preliminary results The ion source geometry was calculated by means of the program E G U N . It solves the Laplace equation (and the Poisson equation if space charge is included) [3,4]. The sputtering yield considerations are estimated following ref. [5]. Differing from the source MISS-483 the cathode diameter was enlarged from 5.5 to 7 mm. The front surface of the sputter cathode is concavely shaped in order to improve the focussing of the negative ion beam. The results calculated are plotted in fig. 4. They are in good agreement with the geometry of the sputter pit shown in fig. 5. In spite of an accurate centre position of the sputter target ionizer and extractor electrode a deviation of the beam from the optical axis can be observed on the extractor electrode. The reason for that is still unclear because it could not be observed at the very similar source MISS-483. The increase in beam current amounts to a factor of 2 in average compared with the source MISS-483. The activities to optimize the ion beam parameters of the source MISS-585 are continuing.
H. Matthes, W. Pfestorf / MISS- 585
157 ^rr-~l~rTP0D E
cvTo^rTno-
2O E E 10
0
10
20
30
40
50
60
mm
Fig. 4. Calculated trajectories of positive and negative ions of the sputter source MISS-585 for a concavely shaped sputter surface (space charge is included).
Fig. 5. Cross section of a copper sputter target after 20 h of running (diameter 7 ram).
Fig. 6. Off axis beam traces on the extractor electrode of the source MISS-585.
References
[3] A. Seifert, Diplomarbeit TU Dresden (1985). [4] W.B. Herrmannsfeldt, SLAC-266 (Nov. 1979). [5] J.H. Billen, Proc. 3rd Int. Conf. on Electrostatic Accelerator Technology, IEEE Catalog No. 81 Ch. 1639-4, p. 238.
[1] H. Matthes, W. Pfestorf and L. Steinert, Nucl. Instr. and Meth. 220 (1984) 112. [2] R. Middleton, Nucl. Instr. and Meth. 214 (1983) 139.
II!. ION SOURCES