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Electrostatic reflex plasma source as a plasma bridge neutralizer
Vacuum/volume36/numbers 11/12/pages 857 to 860/1986
0042-207X/8653.00 + .00 Pergamon Journals Ltd
Printed in Great Britain
E l e c t r o s t a t i c reflex plasma source as a plasma bridge n e u t r a l i z e r C Lejeune, J P
Grandchamp and O Kessi, Institut d'Electronique fondamentale, Unit6 Associ6e au CNRS
(UA 22), Universit6 Paris-Sud, Orsay, France
A new type of plasma source is described, to be used as a plasma bridge neutralizer (PBN) for space and current compensation of low energy ion beams. In contrast to existing hollow cathode PBN, the present design has a discharge which operates down to a low neutral pressure ( I - 5 x 1 0 - 3 tort), so that the plasma jet n o w effuses through an aperture which may be quite large (5-12 mm), according to the beam current to be compensated. Two anodes are provided: one is located within the source chamber whereas the second is outside, near the plasma emission aperture. Thus two different coupling modes of the PBN are possible. Typical performance and lifetime are reported for the neutralization of reactive beams ( 0 2 or CF4). With a 6 mm emission aperture and the external anode it is possible to neutralize 230 mA ion beams, the operation parameters being: coupling voltage 40 V; discharge current--2 A; argon flow rate 1.3 sccm; total beam chamber pressure I x 10 - 4 torr. For these conditions the filament lifetime is: 60 h for 02; 150 h for Ar and 250 h for CF4.
1. Introduction In the past few years 'ion beam processing' of materials has grown rapidly because of the improved control over process parameters, as compared to 'plasma processing, t. Broad and low energy ion beams are generally used2; space charge and current compensation of such beams by electron injection is necessary 3. The concept of 'plasma bridge neutralizer' (PBN) has been developed for ion thrusters 4 and more recently made suitable for ion beam reactors s). The electron source is no longer a thermoionic emitter but is a plasma effusing from a discharge chamber located outside the periphery of the beam. Because the edge of this plasma jet is not a priori defined, this latter--the plasma bridge---may insure an efficient coupling between the neutralizer discharge (the electron source) and the ion beam (the electron collector). For material processing applications, the PBN avoids the dramatic drawbacks of the commonly used immersed thermoionic emitter: short lifetime because of ion physical and chemical sputtering; target contamination by sputtered material and target damage by photon irradiation. For industrial facilities, the requirements for the PBN, to be added and compared to those concerning the main broad-beam source, are: the lifetime, the extra-argon flow rate and also the simplicity and ease for automation. Classical PBN, as those referred to above, used a 'hollow cathode discharge' (HCD) fed either with mercury vapour (space propulsion) or argon (ground application) which operate at rather high pressure ( > 1 torr), the plasma bridge effusing out of the HCD through a small diameter aperture (0.25-1 mm) bored in a hot tungsten tip (1000°C for usual operation) 4. Argon HCD requires a rather large flow rate to operate and starting the discharge is not easyS; for operation in a reactive atmosphere the emission aperture corrosion is a cause of reduced lifetime. More recently the addition of a hot internal emitter was reported in order to
'facilitate discharge starting and to eliminate the need for an internal coating of a low work function oxide '6; unfortunately no details were given with regard to the discharge principle and the gas flow rate which is required for steady operation of this PBN, having also a small emissive aperture. As an alternative solution we developed an electrostatic reflex plasma source (ERPS), the discharge of which may be operated down to a rather low neutral pressure (typically-5 x 10- 3 torr for argon, and for a 5 cm diameter chamber). This feature results from the combined action of an internal hot emitter and of the electrostatic containment of the electrons which, accelerated from this cathode, are responsible for the plasma creation. This concept of electrostatic countainment has already been used to produce small or large ion sources known as MINI-ERIS 7 and MAXI_ERIS s. 9. In this paper we present the performance of such an ERPS which has been designed especially for electron injection into low energy broad ion beams.
2. Plasma source design and experimental set-up 2.1. The ERPS structure. The principle and an elementary model of the hot cathode structure which ensures the electrostatic countainment of the ionizing electrons have already been reported 1° . The discharge chamber has to be negatively biased with respect to the hot cathode (K), i.e. it is an anticathode (AK), and the anode (A) must have a small effective collecting area SA, because the electron free path length Lf within the structure is calculated as:
Lr=4V/SA
(1)
where F is the chamber volume. Generally, in particular if one wants to use the discharge plasma as an ion source, the small 857
C Lejeune et al:
Electrostaticreflex plasmasource
anode is located inside the discharge chamber; on the other hand if one wants to use the discharge plasma as an electron source it might be interesting to have at one's disposal an external anode because the discharge electrons (either emitted by the hot cathode or created within the plasma by ionizing collisions) are supposed to be shared between the 'material anode' and the 'virtual anode', this latter being represented by the positive ion beam to be compensated. Thus, with an external anode one might expect to have a better control over the electron share between the two competitive 'anodes' which are then very close one from the other. As a consequence we have tested the potentialities of an ERPS having two anodes: one A 1 inside the discharge chamber and the other one A2 outside the chamber but very close to the plasma emission aperture. A schematic diagram of the source electrodes together with their typical associated circuitry is shown in Figure 1. The electron collector C is, for preliminary tests of available electron current, a collector consisting of a mesh grid with large gas conductance. The chamber housing is a cylindrical stainless steel shell (diameter 50 mm and height 50 mm) which is cooled by a liquid circulating in a surrounding pipe. The two end walls of the chamber are easily dismantled so as to allow chamber cleaning. The hot cathode is made from 0.8 mm tantalum wire, the active length of which is 8 cm, in order to deliver an electron current up to 2 A without excessive heating; the power is then 85 W. The plasma emission aperture which is bored on the axis of the opposite end of the housing has a diameter which may be chosen as a function of the required electron current; 6, 8 and 12 mm have been tested, but the present data correspond to a 6 mm diameter aperture. The internal anode A~ is a 25 mm diameter ring made from 0.8 mm tantalum wire; it has the same axis as the chamber and its plane is slightly displaced with respect to the chamber median plane. The anode lead is insulated by alumina tubing. The external anode A 2 is a 2 mm thick stainless steel disc; it is separated from the chamber housing by a 0.6 mm mica sheet which ensures both the insulation and the cooling of A 2. Both pieces have a 6 mm diameter axial aperture.
2.2. Experimental set-up for ion beam compensation. For ion beam compensation data, the electron collector C shown in Figure 1 is now replaced by the ion beam and occasionally by the surrounding vessel. Figure 2 is a sketch of the apparatus designed to study the current compensation of a low energy broad ion beam with either the PBN described above or an immersed thermoionic emitter. This latter is stretched across the beam 1 cm downstream of the three grid formation optics. This optics and the main plasma sources have been discussed at this conference9. The emergent beam has a rectangular section (150 x 28 mm) the PBN is located 20 cm downstream and can be displaced transversally, its axis being perpendicular to the beam large dimension. The ion beam collector C is located 37 cm downstream of the optics. Five cylindrical and identical Langmuir probes solid with this collector give information about electron energy distribution and local potential and their variations across the small dimension of the beam-plasma. C is grounded and its current Ic is monitored. A mesh grid screen with a rectangular section surrounds the beam; it may be independently biased in order to prevent the neutralizing electrons from escaping towards the grounded vessel wall. The coupling efficiency is then close to unity. Generally this screen, the third electrode of the optics and the PBN, are biased at the same negative potential - VN, which is the 'coupling voltage' with respect to ground potential; the neutralizer current I N delivered by the PBN (or hot filament) is monitored. 3. PBN behaviour and performances The behaviour of the ERPS--as an electron emitter--is quite different according to the anode, either A~ or A2, which is used to sustain the discharge• This difference has been observed when the emitted electrons are either collected by a solid and fixed material or injected into a beam-plasma; on the other hand there is no significant difference between these two collection modes• The
/
(A,)
/ I
~
]A2
--
I
Mica
Collector (C)
?, ,~m,?
•
VD,
VD2
~ ~I c
x ~ vN r7/7- ~77~
0 0.5 1cm
Figure 1. Schematic diagram of the electrostatic reflex plasma source (ERPS) and associated circuitry; C is a material electron collector.
858
"/1'
"External(A2) Anode
Figure 2. Schematic diagram of the experimental set up for ion beam compensation from a plasma bridge neutralizer; the filamentneutralizer is used for beam bebaviour comparison.
C Lejeune et al: Electrostatic reflex plasma source
detail of these data will be reported in a forthcoming paper. We present here the main features and conclusions, considering separately the ERPS with internal and with external anode.
IcOon beam current,50 mA ) 5E . . . .
~
/
Ic(Ion beam current=2OmA_)
3.1. ERPS with internal anode. Discharge current IA. The discharge with A t is very easy to fire if a convenient gas flow rate Q is provided. Argon is usually used and it is monitored by a mass flow-meter, but any other gas which does not react with the hot cathode may be used (N 2 or H 2 for example), according to the subsequent material process. The discharge current is either limited by the hot cathode emission (lower heating power) or by space charge phenomena at the cathode sheath. These latter are governed both by the discharge voltage VA~ and the neutral pressure: it is the pressure limitation mode 1°. Because, a minimum flow rate is required for the overall facility optimization, Q is adjusted to its minimum value, i.e. that which allows the expected value for IA, to be achieved with the cathode being then slightly overkeated. Quantitatively (with the present design and VAa=30V, Q=0.4SCCM) IAx may be increased up to 2.5 A, and within this range, is controlled by the cathode power. The Q value may be lowered as compared to the above value, but at the expense of IrA, which must be increased; this is not to a low coupling voltage. Available neutralizer electron current. The electron current I s available from the PBN has about the same value either if it is collected on a solid material (Figure 1). or injected into an ion beam (Figure 2). Its value saturates as the coupling voltage V N becomes higher than VA,. For operation with A 1, both A 2 and AK are connected to the cathode. The saturation value, say (Is)sat , is proportional to IA~and (as a first approximation) does not depend on VA~and Q as long as these quantities allow IA~to be reached, as discussed above. Furthermore, quantitatively, an empirical equation may be proposed (IN)~at~ IA, ( S ~ / S A ) × G
(2)
whereS~ is the section of emission aperture, SA is the area of the internal anode and G is a geometrical correcting factor which depends mainly on the emission channel thickness. For the present design G ~ 0.25 because the channel thickness is 4 mm and thus is large compared to its diameter (6 mm). Other experiments have shown that G becomes close to unity when the channel thickness is only 1 mm and the diameter larger than 8 ram; such a thickness is obtained when the external anode is removed, i.e. for the normal geometry if one wants to use the ERPS only in its 'internal anode mode'. Figure 3 shows the influence of the ERPS discharge current on the neutralizer current and the collector current for two values of the positive ion beam (20 and 50 mA); this latter is obtained from a triplasmatron reflex ion source operating with CF 4 (ref 9). The neutralizer current does not depend on the intensity of the ion beam which ensures electron extraction from the plasma of the PBN. Ifthe 'coupling ion beam' is turned off a residual electron current is directly collected by C; it is about 15% of the current extracted by the ion beam (note that the distance between C and PBN is only 17 cm). From this graph, it appears that the present design of the PBN may compensate ion beam currents up to about 65 mA, with 40 V for the coupling voltage and 0.4 SCCM for the Ar flow rate. Larger neutralizing currents might be achieved from an increase of the emission aperture and the removal of the external anode. With a material collector, 400 mA have been collected from a 8 mm dia aperture
~
0
.
.
.
.
.
~
-
.
.
.
.
2"'~"
~ , ~ without the beam
--
~
z
~, -50 g ~-
Q=O.4 SCCM
o
VA~: 3 s v
VN:-40 V -100
1
I
0.5 Discharge
I
1 1.5 current IA,( A )
2
Figure 3. ERPS plasma bridge neutralizer with internal anode. Neutralizer electron current I N extracted by a positive ion beam of fixed intensity (20 and 50 mA) and resulting net current I~ on collector C, vs ERPS discharge current; positive ion energy: 700 eV; gas in main source: CF 4. PBN, the argon gas flow rate being then 1.6 SCCM, and all other parameters being unmodified. 3.2. ERPS with external anode. Discharge current 1,2. To fire this discharge mode with A 2, a plasma has to be previously created inside the chamber with the aid of an auxiliary anode, because the electric field applied between A 2 and K is screened by AK. This first step is obtained by the biasing of either A 1, or AK if there is no internal anode. If the proper gas flow rate is chosen, the discharge is then fired between K and A2, with increasing VA~. The electrodes A I and AK may then be kept floating (At) or connected to K (AK). Once the discharge is fired the behavior is similar to that reported forA 1, but the quantitative values are different; typical examples are Q=0.75SCCM Q=l.3
SCCM
~VA2= 7 0 V - - , I A < 0 . 3 A [VA2=120V__,IA< 1A VA=
70V--~IA2<2A.
Neutralizer electron current I N . When the discharge is fired with A 2 a plasma ball protrudes from the PBN emission aperture which really consitutes a plasma bridge, the coupling mechanisms of which with either a beam or a collector are quite different from the previous case. In particular a large fraction of IA~ may be injected into the beam even for coupling voltages smaller than VA~; but this fraction is very sensitive to the gas flow rate which must be adjusted close to its minimum value necessary to obtained the IA2 expected value. However there is no simple relationship as that given in equation (2) for the previous mode. Figure 4 shows the influence of 1^2 (as controlled by the cathode power) on the same quantities as in Figure 3 and for the same ion beam intensities. For this mode, and a 2 A discharge, the PBN is able to neutralize ion beams up to 230 mA with 40 V for the coupling voltage; but the Ar flow rate is now 1.3 SCCM. The coupling efficiency is improved as compared to the previous mode, but if the gas flow rate is taken into account it appears that the potentialities of the two modes are about the same. It must 859
C Lejeune et al. Electrostatic reflex plasma s o u r c e
< 100 E v o
Ic(Ion beam current:50 mA )
"-..<
Ic
(
Ion beam current:20 mA )
/-- . . . . .
20 o
~'.. Iz
°~..~ "
~
IN=5 m A L . without IN 21 mA J the beam" = =
-50
shows semi-logarithmic plots of electron current vs Langmuir probe voltage for a 50 mA argon beam compensated ion current from the hot filament, the ERPS with internal anode and the ERPS with external anode. A much larger fraction of high energy electrons is obtained from the internal anode mode. Such a difference may be important, according to any subsequent ion beam process, in particular for microelectronics applications. On the other hand, for these three neutralizers no significant variations have been detected as concerns the beam current density profile.
%% `%% %%% %%%
-100
o-15(3
`%~]'%% `%%% `%`% x%%
Q=1.3 SCCM VA2=70 V ~:-40 V 0..5 1 Discharge current IA2( A )
I
-200 0
~ I 1.5
,
`%,~ NN% xxx
Figure 4. ERPS with external anode; variables and parameters as in Figure 3, excepted V^2 =70 V and argon gas flow rate Q = 1.3 SCCM. 1000
(~) (external anode) ( ~ (internal anode)
4. Conclusion Argon plasma bridge neutralizers using an electrostatic reflex plasma source have been described which are able to compensate positive ion beams up to 230 mA with a low coupling voltage (40 V) and a low gas consumption (1.3 SCCM); extrapolation to larger beam currents is possible from an increase of the plasma emission aperture. The lifetime of the present PBN is determinated only by the plasma source hot cathode corrosion. For the reported design and discharge conditions corresponding to the above compensated current i.e. IA2=2 A and VA2=70V, the PBN has been operated in reactive atmospheres obtained when the main source was operated either in 0 2 and CF 4 (50 mA ion beam), the total pressure in the beam chamber being 10 -4 torr, corresponding to3× 10-Storr of argon and 7× 10-Storr of either 0 2 or CF 4. The filament lifetime has been: 60 h for 0 2 ; 150 h for Ar and 250 h for CF 4. This latter very high value has been correlated with the formation of tantalum carbide having a low sputtering coefficient.
(~Filament)
Acknowledgements The authors are very appreciative of the technical skills of C Mardirossian in this work.
'- IC
References
5volt~ 1L
• Probe potential
.--I
Figure 5. Typical semi-logarithmic plots of Langmuir probe I - V characteristics as recorded for a 50 mA argon beam compensated in current either from: (1) the hot thermoionic emitter; (2) the ERPS with external anode and (3) the ERPS with internal anode.
however be emphasized that the external anode is not so easy to operate. Another difference should also be noted which concerns the energy distribution of the beam neutralizing electrons. Figure 5,
860
1 j M E Harper, J J Cuomo and H R Kaufman, J Vac Sci Technol, 21,737 (1982)• 2 H R Kaufman, J J Cuomo and J M E Harper, J Vac Sci Technol, 21, 725 (1982). 3 T S Green, Rep Proo Phys, 37, 1257 (1974). 4 H R Kaufman, in Advances in Electronics and Electron Physics (Edited by L Marton), Vol 36, p 265, Academic Press, New York (1974). 5 L R Rehn, NASA Report, CR 135102 (1976). 6 p D Reader, D P White and G C Isaacson, J Vac Sci Technol, 15, 1093 (1978). 7 C Lejeune and G Gautherin, Vacuum, 34, 251 (1984). 8 C Lejeune, C r hebd skanc Acad Sci Paris, 296, 1391 (1983). 9 C Lejeune, J P Grandchamp, O Kessi, J P Gilles, Vacuum, 36, 85l (1986). lo C Lejeune, Nucl Instrum Meth, 116, 417 and 429 (1974).
0042-207X/8653.00 + .00 Pergamon Journals Ltd
Printed in Great Britain
E l e c t r o s t a t i c reflex plasma source as a plasma bridge n e u t r a l i z e r C Lejeune, J P
Grandchamp and O Kessi, Institut d'Electronique fondamentale, Unit6 Associ6e au CNRS
(UA 22), Universit6 Paris-Sud, Orsay, France
A new type of plasma source is described, to be used as a plasma bridge neutralizer (PBN) for space and current compensation of low energy ion beams. In contrast to existing hollow cathode PBN, the present design has a discharge which operates down to a low neutral pressure ( I - 5 x 1 0 - 3 tort), so that the plasma jet n o w effuses through an aperture which may be quite large (5-12 mm), according to the beam current to be compensated. Two anodes are provided: one is located within the source chamber whereas the second is outside, near the plasma emission aperture. Thus two different coupling modes of the PBN are possible. Typical performance and lifetime are reported for the neutralization of reactive beams ( 0 2 or CF4). With a 6 mm emission aperture and the external anode it is possible to neutralize 230 mA ion beams, the operation parameters being: coupling voltage 40 V; discharge current--2 A; argon flow rate 1.3 sccm; total beam chamber pressure I x 10 - 4 torr. For these conditions the filament lifetime is: 60 h for 02; 150 h for Ar and 250 h for CF4.
1. Introduction In the past few years 'ion beam processing' of materials has grown rapidly because of the improved control over process parameters, as compared to 'plasma processing, t. Broad and low energy ion beams are generally used2; space charge and current compensation of such beams by electron injection is necessary 3. The concept of 'plasma bridge neutralizer' (PBN) has been developed for ion thrusters 4 and more recently made suitable for ion beam reactors s). The electron source is no longer a thermoionic emitter but is a plasma effusing from a discharge chamber located outside the periphery of the beam. Because the edge of this plasma jet is not a priori defined, this latter--the plasma bridge---may insure an efficient coupling between the neutralizer discharge (the electron source) and the ion beam (the electron collector). For material processing applications, the PBN avoids the dramatic drawbacks of the commonly used immersed thermoionic emitter: short lifetime because of ion physical and chemical sputtering; target contamination by sputtered material and target damage by photon irradiation. For industrial facilities, the requirements for the PBN, to be added and compared to those concerning the main broad-beam source, are: the lifetime, the extra-argon flow rate and also the simplicity and ease for automation. Classical PBN, as those referred to above, used a 'hollow cathode discharge' (HCD) fed either with mercury vapour (space propulsion) or argon (ground application) which operate at rather high pressure ( > 1 torr), the plasma bridge effusing out of the HCD through a small diameter aperture (0.25-1 mm) bored in a hot tungsten tip (1000°C for usual operation) 4. Argon HCD requires a rather large flow rate to operate and starting the discharge is not easyS; for operation in a reactive atmosphere the emission aperture corrosion is a cause of reduced lifetime. More recently the addition of a hot internal emitter was reported in order to
'facilitate discharge starting and to eliminate the need for an internal coating of a low work function oxide '6; unfortunately no details were given with regard to the discharge principle and the gas flow rate which is required for steady operation of this PBN, having also a small emissive aperture. As an alternative solution we developed an electrostatic reflex plasma source (ERPS), the discharge of which may be operated down to a rather low neutral pressure (typically-5 x 10- 3 torr for argon, and for a 5 cm diameter chamber). This feature results from the combined action of an internal hot emitter and of the electrostatic containment of the electrons which, accelerated from this cathode, are responsible for the plasma creation. This concept of electrostatic countainment has already been used to produce small or large ion sources known as MINI-ERIS 7 and MAXI_ERIS s. 9. In this paper we present the performance of such an ERPS which has been designed especially for electron injection into low energy broad ion beams.
2. Plasma source design and experimental set-up 2.1. The ERPS structure. The principle and an elementary model of the hot cathode structure which ensures the electrostatic countainment of the ionizing electrons have already been reported 1° . The discharge chamber has to be negatively biased with respect to the hot cathode (K), i.e. it is an anticathode (AK), and the anode (A) must have a small effective collecting area SA, because the electron free path length Lf within the structure is calculated as:
Lr=4V/SA
(1)
where F is the chamber volume. Generally, in particular if one wants to use the discharge plasma as an ion source, the small 857
C Lejeune et al:
Electrostaticreflex plasmasource
anode is located inside the discharge chamber; on the other hand if one wants to use the discharge plasma as an electron source it might be interesting to have at one's disposal an external anode because the discharge electrons (either emitted by the hot cathode or created within the plasma by ionizing collisions) are supposed to be shared between the 'material anode' and the 'virtual anode', this latter being represented by the positive ion beam to be compensated. Thus, with an external anode one might expect to have a better control over the electron share between the two competitive 'anodes' which are then very close one from the other. As a consequence we have tested the potentialities of an ERPS having two anodes: one A 1 inside the discharge chamber and the other one A2 outside the chamber but very close to the plasma emission aperture. A schematic diagram of the source electrodes together with their typical associated circuitry is shown in Figure 1. The electron collector C is, for preliminary tests of available electron current, a collector consisting of a mesh grid with large gas conductance. The chamber housing is a cylindrical stainless steel shell (diameter 50 mm and height 50 mm) which is cooled by a liquid circulating in a surrounding pipe. The two end walls of the chamber are easily dismantled so as to allow chamber cleaning. The hot cathode is made from 0.8 mm tantalum wire, the active length of which is 8 cm, in order to deliver an electron current up to 2 A without excessive heating; the power is then 85 W. The plasma emission aperture which is bored on the axis of the opposite end of the housing has a diameter which may be chosen as a function of the required electron current; 6, 8 and 12 mm have been tested, but the present data correspond to a 6 mm diameter aperture. The internal anode A~ is a 25 mm diameter ring made from 0.8 mm tantalum wire; it has the same axis as the chamber and its plane is slightly displaced with respect to the chamber median plane. The anode lead is insulated by alumina tubing. The external anode A 2 is a 2 mm thick stainless steel disc; it is separated from the chamber housing by a 0.6 mm mica sheet which ensures both the insulation and the cooling of A 2. Both pieces have a 6 mm diameter axial aperture.
2.2. Experimental set-up for ion beam compensation. For ion beam compensation data, the electron collector C shown in Figure 1 is now replaced by the ion beam and occasionally by the surrounding vessel. Figure 2 is a sketch of the apparatus designed to study the current compensation of a low energy broad ion beam with either the PBN described above or an immersed thermoionic emitter. This latter is stretched across the beam 1 cm downstream of the three grid formation optics. This optics and the main plasma sources have been discussed at this conference9. The emergent beam has a rectangular section (150 x 28 mm) the PBN is located 20 cm downstream and can be displaced transversally, its axis being perpendicular to the beam large dimension. The ion beam collector C is located 37 cm downstream of the optics. Five cylindrical and identical Langmuir probes solid with this collector give information about electron energy distribution and local potential and their variations across the small dimension of the beam-plasma. C is grounded and its current Ic is monitored. A mesh grid screen with a rectangular section surrounds the beam; it may be independently biased in order to prevent the neutralizing electrons from escaping towards the grounded vessel wall. The coupling efficiency is then close to unity. Generally this screen, the third electrode of the optics and the PBN, are biased at the same negative potential - VN, which is the 'coupling voltage' with respect to ground potential; the neutralizer current I N delivered by the PBN (or hot filament) is monitored. 3. PBN behaviour and performances The behaviour of the ERPS--as an electron emitter--is quite different according to the anode, either A~ or A2, which is used to sustain the discharge• This difference has been observed when the emitted electrons are either collected by a solid and fixed material or injected into a beam-plasma; on the other hand there is no significant difference between these two collection modes• The
/
(A,)
/ I
~
]A2
--
I
Mica
Collector (C)
?, ,~m,?
•
VD,
VD2
~ ~I c
x ~ vN r7/7- ~77~
0 0.5 1cm
Figure 1. Schematic diagram of the electrostatic reflex plasma source (ERPS) and associated circuitry; C is a material electron collector.
858
"/1'
"External(A2) Anode
Figure 2. Schematic diagram of the experimental set up for ion beam compensation from a plasma bridge neutralizer; the filamentneutralizer is used for beam bebaviour comparison.
C Lejeune et al: Electrostatic reflex plasma source
detail of these data will be reported in a forthcoming paper. We present here the main features and conclusions, considering separately the ERPS with internal and with external anode.
IcOon beam current,50 mA ) 5E . . . .
~
/
Ic(Ion beam current=2OmA_)
3.1. ERPS with internal anode. Discharge current IA. The discharge with A t is very easy to fire if a convenient gas flow rate Q is provided. Argon is usually used and it is monitored by a mass flow-meter, but any other gas which does not react with the hot cathode may be used (N 2 or H 2 for example), according to the subsequent material process. The discharge current is either limited by the hot cathode emission (lower heating power) or by space charge phenomena at the cathode sheath. These latter are governed both by the discharge voltage VA~ and the neutral pressure: it is the pressure limitation mode 1°. Because, a minimum flow rate is required for the overall facility optimization, Q is adjusted to its minimum value, i.e. that which allows the expected value for IA, to be achieved with the cathode being then slightly overkeated. Quantitatively (with the present design and VAa=30V, Q=0.4SCCM) IAx may be increased up to 2.5 A, and within this range, is controlled by the cathode power. The Q value may be lowered as compared to the above value, but at the expense of IrA, which must be increased; this is not to a low coupling voltage. Available neutralizer electron current. The electron current I s available from the PBN has about the same value either if it is collected on a solid material (Figure 1). or injected into an ion beam (Figure 2). Its value saturates as the coupling voltage V N becomes higher than VA,. For operation with A 1, both A 2 and AK are connected to the cathode. The saturation value, say (Is)sat , is proportional to IA~and (as a first approximation) does not depend on VA~and Q as long as these quantities allow IA~to be reached, as discussed above. Furthermore, quantitatively, an empirical equation may be proposed (IN)~at~ IA, ( S ~ / S A ) × G
(2)
whereS~ is the section of emission aperture, SA is the area of the internal anode and G is a geometrical correcting factor which depends mainly on the emission channel thickness. For the present design G ~ 0.25 because the channel thickness is 4 mm and thus is large compared to its diameter (6 mm). Other experiments have shown that G becomes close to unity when the channel thickness is only 1 mm and the diameter larger than 8 ram; such a thickness is obtained when the external anode is removed, i.e. for the normal geometry if one wants to use the ERPS only in its 'internal anode mode'. Figure 3 shows the influence of the ERPS discharge current on the neutralizer current and the collector current for two values of the positive ion beam (20 and 50 mA); this latter is obtained from a triplasmatron reflex ion source operating with CF 4 (ref 9). The neutralizer current does not depend on the intensity of the ion beam which ensures electron extraction from the plasma of the PBN. Ifthe 'coupling ion beam' is turned off a residual electron current is directly collected by C; it is about 15% of the current extracted by the ion beam (note that the distance between C and PBN is only 17 cm). From this graph, it appears that the present design of the PBN may compensate ion beam currents up to about 65 mA, with 40 V for the coupling voltage and 0.4 SCCM for the Ar flow rate. Larger neutralizing currents might be achieved from an increase of the emission aperture and the removal of the external anode. With a material collector, 400 mA have been collected from a 8 mm dia aperture
~
0
.
.
.
.
.
~
-
.
.
.
.
2"'~"
~ , ~ without the beam
--
~
z
~, -50 g ~-
Q=O.4 SCCM
o
VA~: 3 s v
VN:-40 V -100
1
I
0.5 Discharge
I
1 1.5 current IA,( A )
2
Figure 3. ERPS plasma bridge neutralizer with internal anode. Neutralizer electron current I N extracted by a positive ion beam of fixed intensity (20 and 50 mA) and resulting net current I~ on collector C, vs ERPS discharge current; positive ion energy: 700 eV; gas in main source: CF 4. PBN, the argon gas flow rate being then 1.6 SCCM, and all other parameters being unmodified. 3.2. ERPS with external anode. Discharge current 1,2. To fire this discharge mode with A 2, a plasma has to be previously created inside the chamber with the aid of an auxiliary anode, because the electric field applied between A 2 and K is screened by AK. This first step is obtained by the biasing of either A 1, or AK if there is no internal anode. If the proper gas flow rate is chosen, the discharge is then fired between K and A2, with increasing VA~. The electrodes A I and AK may then be kept floating (At) or connected to K (AK). Once the discharge is fired the behavior is similar to that reported forA 1, but the quantitative values are different; typical examples are Q=0.75SCCM Q=l.3
SCCM
~VA2= 7 0 V - - , I A < 0 . 3 A [VA2=120V__,IA< 1A VA=
70V--~IA2<2A.
Neutralizer electron current I N . When the discharge is fired with A 2 a plasma ball protrudes from the PBN emission aperture which really consitutes a plasma bridge, the coupling mechanisms of which with either a beam or a collector are quite different from the previous case. In particular a large fraction of IA~ may be injected into the beam even for coupling voltages smaller than VA~; but this fraction is very sensitive to the gas flow rate which must be adjusted close to its minimum value necessary to obtained the IA2 expected value. However there is no simple relationship as that given in equation (2) for the previous mode. Figure 4 shows the influence of 1^2 (as controlled by the cathode power) on the same quantities as in Figure 3 and for the same ion beam intensities. For this mode, and a 2 A discharge, the PBN is able to neutralize ion beams up to 230 mA with 40 V for the coupling voltage; but the Ar flow rate is now 1.3 SCCM. The coupling efficiency is improved as compared to the previous mode, but if the gas flow rate is taken into account it appears that the potentialities of the two modes are about the same. It must 859
C Lejeune et al. Electrostatic reflex plasma s o u r c e
< 100 E v o
Ic(Ion beam current:50 mA )
"-..<
Ic
(
Ion beam current:20 mA )
/-- . . . . .
20 o
~'.. Iz
°~..~ "
~
IN=5 m A L . without IN 21 mA J the beam" = =
-50
shows semi-logarithmic plots of electron current vs Langmuir probe voltage for a 50 mA argon beam compensated ion current from the hot filament, the ERPS with internal anode and the ERPS with external anode. A much larger fraction of high energy electrons is obtained from the internal anode mode. Such a difference may be important, according to any subsequent ion beam process, in particular for microelectronics applications. On the other hand, for these three neutralizers no significant variations have been detected as concerns the beam current density profile.
%% `%% %%% %%%
-100
o-15(3
`%~]'%% `%%% `%`% x%%
Q=1.3 SCCM VA2=70 V ~:-40 V 0..5 1 Discharge current IA2( A )
I
-200 0
~ I 1.5
,
`%,~ NN% xxx
Figure 4. ERPS with external anode; variables and parameters as in Figure 3, excepted V^2 =70 V and argon gas flow rate Q = 1.3 SCCM. 1000
(~) (external anode) ( ~ (internal anode)
4. Conclusion Argon plasma bridge neutralizers using an electrostatic reflex plasma source have been described which are able to compensate positive ion beams up to 230 mA with a low coupling voltage (40 V) and a low gas consumption (1.3 SCCM); extrapolation to larger beam currents is possible from an increase of the plasma emission aperture. The lifetime of the present PBN is determinated only by the plasma source hot cathode corrosion. For the reported design and discharge conditions corresponding to the above compensated current i.e. IA2=2 A and VA2=70V, the PBN has been operated in reactive atmospheres obtained when the main source was operated either in 0 2 and CF 4 (50 mA ion beam), the total pressure in the beam chamber being 10 -4 torr, corresponding to3× 10-Storr of argon and 7× 10-Storr of either 0 2 or CF 4. The filament lifetime has been: 60 h for 0 2 ; 150 h for Ar and 250 h for CF 4. This latter very high value has been correlated with the formation of tantalum carbide having a low sputtering coefficient.
(~Filament)
Acknowledgements The authors are very appreciative of the technical skills of C Mardirossian in this work.
'- IC
References
5volt~ 1L
• Probe potential
.--I
Figure 5. Typical semi-logarithmic plots of Langmuir probe I - V characteristics as recorded for a 50 mA argon beam compensated in current either from: (1) the hot thermoionic emitter; (2) the ERPS with external anode and (3) the ERPS with internal anode.
however be emphasized that the external anode is not so easy to operate. Another difference should also be noted which concerns the energy distribution of the beam neutralizing electrons. Figure 5,
860
1 j M E Harper, J J Cuomo and H R Kaufman, J Vac Sci Technol, 21,737 (1982)• 2 H R Kaufman, J J Cuomo and J M E Harper, J Vac Sci Technol, 21, 725 (1982). 3 T S Green, Rep Proo Phys, 37, 1257 (1974). 4 H R Kaufman, in Advances in Electronics and Electron Physics (Edited by L Marton), Vol 36, p 265, Academic Press, New York (1974). 5 L R Rehn, NASA Report, CR 135102 (1976). 6 p D Reader, D P White and G C Isaacson, J Vac Sci Technol, 15, 1093 (1978). 7 C Lejeune and G Gautherin, Vacuum, 34, 251 (1984). 8 C Lejeune, C r hebd skanc Acad Sci Paris, 296, 1391 (1983). 9 C Lejeune, J P Grandchamp, O Kessi, J P Gilles, Vacuum, 36, 85l (1986). lo C Lejeune, Nucl Instrum Meth, 116, 417 and 429 (1974).