- Email: [email protected]
Negative-ion sources for ion implantation
323
Nuclear Instruments and Methods in Physics Research B55 (1991) 323-327 North-Holland
Negative-ion
sources for ion implantation
A.J.T. Holmes and G. Proudfoot AEA Technology, Cufham Laboratov, Abingdon, Oxon, UK
The development of negative-ion sources has now reached the state where they can be considered for implanter applications. In the following paper a conceptual negative-ion source is described, based on production by dissociative attachment in the plasma. This type of source reduces some of the problems encountered xvi& positive-ion sources such as surface charging and impurity ions and has the additional advantage of variable beam current at fixed beam energy.
Table 1 List of negative-ion states for elements of interst for implanters
1. Introduction The implantation of ions into semiconductors can be readily achieved by the impact of a high-energy beam of low current density on the semiconductor surface. The beam energy determines the depth of penetration and is typically in the range 5-200 keV. However, the electric charge carried by the beam leads to a buildup of charge on the surface which modifies the beam energy and can lead to electrical breakdown problems. The usi of positive-ion beams always requires highresolution mass analysis in order to achieve the desired low impurity levels. However, a negative-ion beam has a natural low impurity level. As a result substantially different system concepts can be considered for negative-ion implantation which could influence both process cost and reliability. I. 1. Charging effects
State
Electron affinity
H 0
H00; LiP-
0.75 1.46 0.5 0.61 0.77
AS-
0.80
Cl-
3.7 “6 ;:2 1.3 ? 0.92
Comments
Ievl
Li P As Cl F Ge C N B
FGe CN-
B-
only stable state known higher states exist only state known only state known only state known
only state known
only state known
have observed that y is similar for positive and negative ion beams of similar electronic configurations and ion velocities. Hence the effective charging current for low values of y (i.e. around 1 to 2) is much reduced for negative ion beams but this relative effect becomes of lower importance if y is large. [lJ
In the case of positive-ion beams, which have been used so far for ion implantation, this charge buildup is enhanced, by secondary-electron emission from the surface so that the charging current is (1 + y+)l,, where y+ is the secondary-electron emission coefficient per incident ion. Typically for normal incidence y+ lies on the range of 1 to 10 but takes on lower values for clean surfaces. There exist two solutions to this problem. We could use a neutral-atom beam for the implant formed by passing the positive-ion beam through a gas cell. Hotiever, this solution has unattractive aspects; the gas load formed by the cell would cause pumping difficulties, beam control and mass analysis would be almost impossible and the conversion efficiency is low at high implanter beam energies. Alternatively we could use a negative ion beam for implantation. In this case the charging current is (1 -y-)1,. There are very few experimental measurements of y_ but Dixit and Ghosh 0168-583X/91/$03.50
Element
@ 1991 - asevier
1.2. Beam purity In a typical positive-ion discharge, the.plasma contains many species of ion, imp~ties being present both as molecular ions and m~ticharged ions. Elements with stable negative polarity ions are listed in table 1 and include O-, P-, As-, Ge- and B-, all of which are routinely used dopants in the semiconductor industry. I&like positive ions, there is virtually no evidence for the existence of stable molecular negative ions apart from oxygen which has 0; and 0; ionic states [2]. Some unstable molecular negative ions do exist, however, which have lifetimes in the picosecond range. It has been reported (but not in open literature) that some
Science Publishers B.V. worth-Holland)
IV. SOURCES & BEAM TRANSPORT
324
A.J. T. Holmes, G. Proudfoot / Negative-ion sources for ion implantation
S
I
s
S
N
N
S
S
::,s :f
N
i
Fig. 1. A schematic of a volume negative-ion source and triode accelerator.
experiments have seen evidence for molecular negative ions arising from magnetic moments analysis. These ions could be impurities associated with oxygen or OH radicals. However, they are likely to be well separated from the ion mass of interest in momentum and hence easily analysed. In practise, therefore, negative-ion extraction and analysis can select the ion of interest with minimal impurity levels, provided the correct source materials are used. One aspect of beam purity special to negative-ion systems concerns the electron beam co-extracted with the negative ions from the discharge. Extensive work has shown that to a large extent this electron current in the beam can be reduced to a small fraction of the ion current, provided steps are taken to suppress and trap electrons within the accelerator. 1.3. Conceptual design A conceptual design of a heavy-negative-ion source is shown in fig. 1. While similar in many ways to its positive-ion counterpart, there are significant differences in the design of the discharge chamber to enhance negative-ion formation and in the accelerator design to reduce electron extraction. This concept is based on an II- accelerator which has been developed over the last few years in the Fusion and Strategic Defense Initiative Programmes. However, for heavy ions subtle changes are needed to optimise the performance to that needed for industry. In the rest of this paper we discuss the design of
negative-ion sources suitable for implanter applications and the likely performance that can be achieved.
2. Formation of negative ions Tmee major methods of producing negative ions have emerged over the past few years. The first of these is the production of negative ions by double electron capture by a low-energy positive-ion flux passing thorugh a vapour. The second is by single-electron capture by absorbed neutral atoms on a low-work-function surface, such as a monolayer of cesium or bulk barium metal. The third is direct formation within a plasma discharge via dissociative attachment of vibrationalIy excited molecules in the gas phase of the element to be implanted. All three techniques have been used to produce intense beams of H- but there is a fundamental difference between the latter two techniques. The second method forms negative ions with initial energies of typically 200-400 eV, of which 2-5% is transverse to the beam axis [3]. This leads to low-quality beam optics with high transverse temperatures - 10 eV compared with 0.5 eV for dissociative attachment with resultant mass-analysis problems downstream when the desired ion species is separated from imp~ity ions. In addition, there is an effusion of the converter material (i.e. caesium or barium) at a low rate from the source. Neither situation is desirable. The dissociative-attachment tech-
325
A.J. T. Holmes, G. Proudfoot / Negative-ion sourcesfor ion implantation nique has the advantage that the ions are formed at low energy and this yields highly collimated beams [4] with consequent high transmission to the target. Furthermore, there is virtually no effusion of impurities from the source. In the rest of this paper we will focus on sources based on dissociative attachment. The fundamental process involved in the production of the negative ion occurs in essentially two steps. The first of these is the formation of ~bration~ly excited
Grid1
Grid 2
u
-USV
molecules via fast electron impact on unexcited molecules, e,,, +
X2
--$ XJIy>
+
efastt
where X, is the molecule of the desired element in the ground state and the electron energy is typically in excess of 20 eV. The term v indicates that the molecule is elevated to a vibrational level 8. The second stage is the d~s~iative-attac~ent collision itself, eSIoW f X,(Y)
+X;
+ X,-C X.
In this instance the electron energy is usually about 1 eV and only the vibrational levels with internal energies which exceed about 0.6 of the dissociation energy of the X, molecule have a significant cross section. As well as the production processes, there are various destruction processes for negative ions both in the plasma and on the walls of the discharge chamber. The low binding energy of the extra electron leads to electron-detachment collisions,
I:; -ii-l
e+X--+X+2e,
dl
which have a very large cross section (typically lOO-fold the production cross section) for energetic electrons (q > 10 eV). There is also ion-ion recombination, x+x--+x+x*, where X* denotes an excited state. This cross section falls rapidly with increasing ion energy.
Fl=J*dl
3. Negative-ion source design The above processes indicate the dilemma faced by the designers of negative-ion sources. Fast electrons are
Fig. 2. Thin-lens model of the triode accelerator.
a/d Iv0.57
a/dN0.73 1.5
.
I
1.4 1.3 1.2 1.1 1 0.9 0.6 OJ 0.6 0.5
*
AXCEL -
0.4
AXCEL. J.=lSmA/cmP
0.3
Expwlment
0.2
J-“16Wti
Up*rlmont m J--6mAJsm’
+
0.1
,
0-l 4
6
6
ExtnctlonVoitagNkV Fig. 3.
A
comparison of AXCEL
10
12
f4
1.=SmNcml
. *. j.=lJmA/cmp
0
, 2
‘
,
4
,
‘
6 l%tradlon
,
,
I
6
,
10
1.“lSmA/ae ,
,
12
,
,
14
voltage/kv
predictions of beam divergence vs extraction voltage to experimental data for two values of a/d (triode, H-, U = 70 kv). IV. SOURCES
& BEAM TRANSPORT
326
A.J.T. Holmes, G. Proudfoot
/ Negatiue-ion sources for ion implantation
needed for vibrational molecule production (in particular for large values of v) but at the same time only slow electrons are needed for XP production and the presence of fast electrons would reduce the density of XP These problems have been overcome by use of the so-called “magnetic filter”. The basic concept is shown in fig. 1 for an H-/Dsource. The discharge chamber is a magnetic multipole confinement volume where fast electrons are emitted from hot wire filaments at the back. Here the plasma is ionised and vibrational molecules are formed. The magnetic filter is a long-range field created by the arrangement of magnets on the sides of the source [5] but ‘can be also created by internal magnets within the discharge [6]. This field preferentially transmits low-energy electrons as only these electrons have the necessary collisionality to escape being trapped on field lines. The effective temperature change AT is given by
where n, and n2 are the electron densities on the production and extraction sides of the filter and T2 is the electron temperature on the extraction side of the filter and should be not more than 1 eV in a good source design. The dc arc discharge is not essential and a comparable approach would be possible with, for example, and rf-excited discharge of reactive ions. The accelerator is also shown in fig. 1 and contains two accelerating gaps with bar magnets in all three electrodes. The function of the magnets in the plasma electrode facing the discharge is to suppress electrons from being extracted from the plasma. The transverse magnetic flux F created by the magnets, for the plasma electrode at plasma potential, attenuates the electrons by a factor
where j, is the random electron flux in the plasma (i.e. j, = en+,) and v, is the thermal-electron speed and j, is the electron current which is eventually extracted. It is not practical to totally suppress the electrons as the stray fields of this structure interfere with the discharge so some electrons do enter the accelerator. These are swept out of the beam path by magnets in the upstream face of the second electrode, as shown in fig. 1, and are collected in the recess in the bulk of this electrode. Thus the maximum electron energy is just the first gap potential. The magnets in the downstream face of the second electrode and the third electrode serve to correct the trajectories of the X- ions so that they exit the accelerator on the mechanical axis of the system. The magnetic flux is cancelled to the second order so that there is neither beam deflection or displacement.
4. Beam transport Collimation of a beam produced from the negative ion accelerator described discribed above is straightforward; it is achieved by merely adjusting the ratio of the voltages across the two gaps. An explanation of this voltage ratio can be seen in fig. 2. There exist three lenses in the system where each lens located at a step change in axial electric field; a strong focusing and defocusing lens on either side of the second electrode and a weak defocusing lens at the third electrode. Only the focusing lens is variable over a significant range by altering the second electrode potential and collimation is achieved for a unique value of this voltage. Fig. 3 shows experimental and numberical calculations of the beam divergence for different values of this voltage vor various beam currents of H- ions. The final beam energy is constant, indicating that negative-ion triode accelerators do not have a unique beam perveance for collimation (unlike positive ion systems) which allows the ion dose rate to varied without readjusting the final beam energy. This could be a significant advantage.
5. Ion species Many, but not all, of the elements of the periodic table can form stable negative ions, the major exceptions being the noble gases (except He). Table 1 lists the elements of interest for implantation and their binding energies. Because of its low binding energy the more stable ion (if two or more states or species can form) is highly preferentially formed. Thus a small impurity of oxygen in a hydrogen discharge leads to virtually only Obeing created with the HP ions being a minor fraction. This rules out using gaseous compounds of elements to produce discharges of the desired element if the ion of the combining element is m&e stable. It is preferable to create discharges by evaporation from an oven containing the required ion element. At present H- and D- are formed from discharges of this type at energies of up to 100 keV and beam currents from a single aperture of more than 100 mA [7,8]. Negative-lithium beams have been formed at low energies and modest currents from a small discharge chamber [lo] and O- beams have been produced. In the latter case filament erosion is a serious problem and an rf discharge technique has been successfully developed ,[91. In the case of the other elements no experimental work has been done but the technology of positive-ion sources in this area can be ,transferred directly to the design of a negative-ion source, as the only major difference is the addition of a magnetic filter within the
A.J. T. Holmes, G. Proudfoot
/ Negative-ion
discharge chamber. However, the potential of negativeion-beam-based systems is significant - not only could charging effects be reduced but mass analysis could be considerably simplified and the attendant impurities caused by beam impingement substantially reduced. Such a capability could be particularly important for the low-energy beams required for shallow doping. Alternatively, crude mass analysis could produce veryhip-purity beams.
References [l] S.D. Dixit &nd S.N. Ghosh, [Z] H.S.W.‘“Massey, Negative Press, 1976).
Indian J. Phys. B55 (1981) 87. Ions (Cambridge University
sources for ion implantation
327
[3] C.F.A. Van OS, A.W. Kleyn, L.M. Lea and A.J.T. Holmes, Rev. Sci. Instr. 60 (1989) 539. [4] A.J.T. Holmes and M.P.S. Ni~ting~e, Rev. Sci. Instr. 57 (1986) 2402. [S] A.J.T. Holmes, T.S. Green, M. Inman, A. Walker and N. Hampton, Proc. 3rd Conf. on Heating in Toroidal Plasmas, .Grenoble, EUR 7979 EN (Commission of European Communities, 1982). [6] K.N. Leung, K.W. Ehlers and M. Bacal, Rev. Sci. Instr. 54 (1983) 56. [7] R. McAdams, A.J.T. Holmes and M.P.S. Nightgale, Rev. Sci. Instr. 59 (1988) 895. [8] J.W. Kwan, Private communication. [9] G. Proudfoot, C.M.O. Mahony and R. Perrin, Nucl. Instr. and Meth. B37/38 (1989) 103. [IO] S.R. Walter, K.N. Leung and W.B. Kunkel; Appl. .Phys. Lett. 51 (1987) 566.
IV. SOURCES
& BEAM TRANSPORT
Nuclear Instruments and Methods in Physics Research B55 (1991) 323-327 North-Holland
Negative-ion
sources for ion implantation
A.J.T. Holmes and G. Proudfoot AEA Technology, Cufham Laboratov, Abingdon, Oxon, UK
The development of negative-ion sources has now reached the state where they can be considered for implanter applications. In the following paper a conceptual negative-ion source is described, based on production by dissociative attachment in the plasma. This type of source reduces some of the problems encountered xvi& positive-ion sources such as surface charging and impurity ions and has the additional advantage of variable beam current at fixed beam energy.
Table 1 List of negative-ion states for elements of interst for implanters
1. Introduction The implantation of ions into semiconductors can be readily achieved by the impact of a high-energy beam of low current density on the semiconductor surface. The beam energy determines the depth of penetration and is typically in the range 5-200 keV. However, the electric charge carried by the beam leads to a buildup of charge on the surface which modifies the beam energy and can lead to electrical breakdown problems. The usi of positive-ion beams always requires highresolution mass analysis in order to achieve the desired low impurity levels. However, a negative-ion beam has a natural low impurity level. As a result substantially different system concepts can be considered for negative-ion implantation which could influence both process cost and reliability. I. 1. Charging effects
State
Electron affinity
H 0
H00; LiP-
0.75 1.46 0.5 0.61 0.77
AS-
0.80
Cl-
3.7 “6 ;:2 1.3 ? 0.92
Comments
Ievl
Li P As Cl F Ge C N B
FGe CN-
B-
only stable state known higher states exist only state known only state known only state known
only state known
only state known
have observed that y is similar for positive and negative ion beams of similar electronic configurations and ion velocities. Hence the effective charging current for low values of y (i.e. around 1 to 2) is much reduced for negative ion beams but this relative effect becomes of lower importance if y is large. [lJ
In the case of positive-ion beams, which have been used so far for ion implantation, this charge buildup is enhanced, by secondary-electron emission from the surface so that the charging current is (1 + y+)l,, where y+ is the secondary-electron emission coefficient per incident ion. Typically for normal incidence y+ lies on the range of 1 to 10 but takes on lower values for clean surfaces. There exist two solutions to this problem. We could use a neutral-atom beam for the implant formed by passing the positive-ion beam through a gas cell. Hotiever, this solution has unattractive aspects; the gas load formed by the cell would cause pumping difficulties, beam control and mass analysis would be almost impossible and the conversion efficiency is low at high implanter beam energies. Alternatively we could use a negative ion beam for implantation. In this case the charging current is (1 -y-)1,. There are very few experimental measurements of y_ but Dixit and Ghosh 0168-583X/91/$03.50
Element
@ 1991 - asevier
1.2. Beam purity In a typical positive-ion discharge, the.plasma contains many species of ion, imp~ties being present both as molecular ions and m~ticharged ions. Elements with stable negative polarity ions are listed in table 1 and include O-, P-, As-, Ge- and B-, all of which are routinely used dopants in the semiconductor industry. I&like positive ions, there is virtually no evidence for the existence of stable molecular negative ions apart from oxygen which has 0; and 0; ionic states [2]. Some unstable molecular negative ions do exist, however, which have lifetimes in the picosecond range. It has been reported (but not in open literature) that some
Science Publishers B.V. worth-Holland)
IV. SOURCES & BEAM TRANSPORT
324
A.J. T. Holmes, G. Proudfoot / Negative-ion sources for ion implantation
S
I
s
S
N
N
S
S
::,s :f
N
i
Fig. 1. A schematic of a volume negative-ion source and triode accelerator.
experiments have seen evidence for molecular negative ions arising from magnetic moments analysis. These ions could be impurities associated with oxygen or OH radicals. However, they are likely to be well separated from the ion mass of interest in momentum and hence easily analysed. In practise, therefore, negative-ion extraction and analysis can select the ion of interest with minimal impurity levels, provided the correct source materials are used. One aspect of beam purity special to negative-ion systems concerns the electron beam co-extracted with the negative ions from the discharge. Extensive work has shown that to a large extent this electron current in the beam can be reduced to a small fraction of the ion current, provided steps are taken to suppress and trap electrons within the accelerator. 1.3. Conceptual design A conceptual design of a heavy-negative-ion source is shown in fig. 1. While similar in many ways to its positive-ion counterpart, there are significant differences in the design of the discharge chamber to enhance negative-ion formation and in the accelerator design to reduce electron extraction. This concept is based on an II- accelerator which has been developed over the last few years in the Fusion and Strategic Defense Initiative Programmes. However, for heavy ions subtle changes are needed to optimise the performance to that needed for industry. In the rest of this paper we discuss the design of
negative-ion sources suitable for implanter applications and the likely performance that can be achieved.
2. Formation of negative ions Tmee major methods of producing negative ions have emerged over the past few years. The first of these is the production of negative ions by double electron capture by a low-energy positive-ion flux passing thorugh a vapour. The second is by single-electron capture by absorbed neutral atoms on a low-work-function surface, such as a monolayer of cesium or bulk barium metal. The third is direct formation within a plasma discharge via dissociative attachment of vibrationalIy excited molecules in the gas phase of the element to be implanted. All three techniques have been used to produce intense beams of H- but there is a fundamental difference between the latter two techniques. The second method forms negative ions with initial energies of typically 200-400 eV, of which 2-5% is transverse to the beam axis [3]. This leads to low-quality beam optics with high transverse temperatures - 10 eV compared with 0.5 eV for dissociative attachment with resultant mass-analysis problems downstream when the desired ion species is separated from imp~ity ions. In addition, there is an effusion of the converter material (i.e. caesium or barium) at a low rate from the source. Neither situation is desirable. The dissociative-attachment tech-
325
A.J. T. Holmes, G. Proudfoot / Negative-ion sourcesfor ion implantation nique has the advantage that the ions are formed at low energy and this yields highly collimated beams [4] with consequent high transmission to the target. Furthermore, there is virtually no effusion of impurities from the source. In the rest of this paper we will focus on sources based on dissociative attachment. The fundamental process involved in the production of the negative ion occurs in essentially two steps. The first of these is the formation of ~bration~ly excited
Grid1
Grid 2
u
-USV
molecules via fast electron impact on unexcited molecules, e,,, +
X2
--$ XJIy>
+
efastt
where X, is the molecule of the desired element in the ground state and the electron energy is typically in excess of 20 eV. The term v indicates that the molecule is elevated to a vibrational level 8. The second stage is the d~s~iative-attac~ent collision itself, eSIoW f X,(Y)
+X;
+ X,-C X.
In this instance the electron energy is usually about 1 eV and only the vibrational levels with internal energies which exceed about 0.6 of the dissociation energy of the X, molecule have a significant cross section. As well as the production processes, there are various destruction processes for negative ions both in the plasma and on the walls of the discharge chamber. The low binding energy of the extra electron leads to electron-detachment collisions,
I:; -ii-l
e+X--+X+2e,
dl
which have a very large cross section (typically lOO-fold the production cross section) for energetic electrons (q > 10 eV). There is also ion-ion recombination, x+x--+x+x*, where X* denotes an excited state. This cross section falls rapidly with increasing ion energy.
Fl=J*dl
3. Negative-ion source design The above processes indicate the dilemma faced by the designers of negative-ion sources. Fast electrons are
Fig. 2. Thin-lens model of the triode accelerator.
a/d Iv0.57
a/dN0.73 1.5
.
I
1.4 1.3 1.2 1.1 1 0.9 0.6 OJ 0.6 0.5
*
AXCEL -
0.4
AXCEL. J.=lSmA/cmP
0.3
Expwlment
0.2
J-“16Wti
Up*rlmont m J--6mAJsm’
+
0.1
,
0-l 4
6
6
ExtnctlonVoitagNkV Fig. 3.
A
comparison of AXCEL
10
12
f4
1.=SmNcml
. *. j.=lJmA/cmp
0
, 2
‘
,
4
,
‘
6 l%tradlon
,
,
I
6
,
10
1.“lSmA/ae ,
,
12
,
,
14
voltage/kv
predictions of beam divergence vs extraction voltage to experimental data for two values of a/d (triode, H-, U = 70 kv). IV. SOURCES
& BEAM TRANSPORT
326
A.J.T. Holmes, G. Proudfoot
/ Negatiue-ion sources for ion implantation
needed for vibrational molecule production (in particular for large values of v) but at the same time only slow electrons are needed for XP production and the presence of fast electrons would reduce the density of XP These problems have been overcome by use of the so-called “magnetic filter”. The basic concept is shown in fig. 1 for an H-/Dsource. The discharge chamber is a magnetic multipole confinement volume where fast electrons are emitted from hot wire filaments at the back. Here the plasma is ionised and vibrational molecules are formed. The magnetic filter is a long-range field created by the arrangement of magnets on the sides of the source [5] but ‘can be also created by internal magnets within the discharge [6]. This field preferentially transmits low-energy electrons as only these electrons have the necessary collisionality to escape being trapped on field lines. The effective temperature change AT is given by
where n, and n2 are the electron densities on the production and extraction sides of the filter and T2 is the electron temperature on the extraction side of the filter and should be not more than 1 eV in a good source design. The dc arc discharge is not essential and a comparable approach would be possible with, for example, and rf-excited discharge of reactive ions. The accelerator is also shown in fig. 1 and contains two accelerating gaps with bar magnets in all three electrodes. The function of the magnets in the plasma electrode facing the discharge is to suppress electrons from being extracted from the plasma. The transverse magnetic flux F created by the magnets, for the plasma electrode at plasma potential, attenuates the electrons by a factor
where j, is the random electron flux in the plasma (i.e. j, = en+,) and v, is the thermal-electron speed and j, is the electron current which is eventually extracted. It is not practical to totally suppress the electrons as the stray fields of this structure interfere with the discharge so some electrons do enter the accelerator. These are swept out of the beam path by magnets in the upstream face of the second electrode, as shown in fig. 1, and are collected in the recess in the bulk of this electrode. Thus the maximum electron energy is just the first gap potential. The magnets in the downstream face of the second electrode and the third electrode serve to correct the trajectories of the X- ions so that they exit the accelerator on the mechanical axis of the system. The magnetic flux is cancelled to the second order so that there is neither beam deflection or displacement.
4. Beam transport Collimation of a beam produced from the negative ion accelerator described discribed above is straightforward; it is achieved by merely adjusting the ratio of the voltages across the two gaps. An explanation of this voltage ratio can be seen in fig. 2. There exist three lenses in the system where each lens located at a step change in axial electric field; a strong focusing and defocusing lens on either side of the second electrode and a weak defocusing lens at the third electrode. Only the focusing lens is variable over a significant range by altering the second electrode potential and collimation is achieved for a unique value of this voltage. Fig. 3 shows experimental and numberical calculations of the beam divergence for different values of this voltage vor various beam currents of H- ions. The final beam energy is constant, indicating that negative-ion triode accelerators do not have a unique beam perveance for collimation (unlike positive ion systems) which allows the ion dose rate to varied without readjusting the final beam energy. This could be a significant advantage.
5. Ion species Many, but not all, of the elements of the periodic table can form stable negative ions, the major exceptions being the noble gases (except He). Table 1 lists the elements of interest for implantation and their binding energies. Because of its low binding energy the more stable ion (if two or more states or species can form) is highly preferentially formed. Thus a small impurity of oxygen in a hydrogen discharge leads to virtually only Obeing created with the HP ions being a minor fraction. This rules out using gaseous compounds of elements to produce discharges of the desired element if the ion of the combining element is m&e stable. It is preferable to create discharges by evaporation from an oven containing the required ion element. At present H- and D- are formed from discharges of this type at energies of up to 100 keV and beam currents from a single aperture of more than 100 mA [7,8]. Negative-lithium beams have been formed at low energies and modest currents from a small discharge chamber [lo] and O- beams have been produced. In the latter case filament erosion is a serious problem and an rf discharge technique has been successfully developed ,[91. In the case of the other elements no experimental work has been done but the technology of positive-ion sources in this area can be ,transferred directly to the design of a negative-ion source, as the only major difference is the addition of a magnetic filter within the
A.J. T. Holmes, G. Proudfoot
/ Negative-ion
discharge chamber. However, the potential of negativeion-beam-based systems is significant - not only could charging effects be reduced but mass analysis could be considerably simplified and the attendant impurities caused by beam impingement substantially reduced. Such a capability could be particularly important for the low-energy beams required for shallow doping. Alternatively, crude mass analysis could produce veryhip-purity beams.
References [l] S.D. Dixit &nd S.N. Ghosh, [Z] H.S.W.‘“Massey, Negative Press, 1976).
Indian J. Phys. B55 (1981) 87. Ions (Cambridge University
sources for ion implantation
327
[3] C.F.A. Van OS, A.W. Kleyn, L.M. Lea and A.J.T. Holmes, Rev. Sci. Instr. 60 (1989) 539. [4] A.J.T. Holmes and M.P.S. Ni~ting~e, Rev. Sci. Instr. 57 (1986) 2402. [S] A.J.T. Holmes, T.S. Green, M. Inman, A. Walker and N. Hampton, Proc. 3rd Conf. on Heating in Toroidal Plasmas, .Grenoble, EUR 7979 EN (Commission of European Communities, 1982). [6] K.N. Leung, K.W. Ehlers and M. Bacal, Rev. Sci. Instr. 54 (1983) 56. [7] R. McAdams, A.J.T. Holmes and M.P.S. Nightgale, Rev. Sci. Instr. 59 (1988) 895. [8] J.W. Kwan, Private communication. [9] G. Proudfoot, C.M.O. Mahony and R. Perrin, Nucl. Instr. and Meth. B37/38 (1989) 103. [IO] S.R. Walter, K.N. Leung and W.B. Kunkel; Appl. .Phys. Lett. 51 (1987) 566.
IV. SOURCES
& BEAM TRANSPORT