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Experimental study of electrode materials for use in a cold-cathode oxygen discharge
and loon Physics, 52 (1983) 299-309 Elsevier Science Publishers B-V., Amsterdam - Printed in The Netherlands
International Journal of Mass Spectrometry
EXPERIMENTAL STUDY OF ELECTRODE A COLD-CATHODE OXYGEN DISCHARGE
M.G.
DOWSETT
Solid
State
MATERIALS
299
FOR USE IN
and E.H.C. PARKER
Research
Street, London EC3N
Group, 2EY
Department
of Physics,
City of London
Polytechnic,
31 Jewry
(Great Britain)
(First received 28 January 1983; in final form 27 March 1983)
ABSTRACT A large range of elements has been evaluated for use as cathode, anticathode and anode materials in an oxygen cold-cathode discharge. The criterion for suitability of a particular material/electrode combination was based on the ability to extract from the source an ion beam suitable for high dynamic range SIMS. The effect of materials on long and short term stability and total beam current was investigated. The best materials were found to be Al (all electrodes), Ni/Fe (cathode) and Ti (anticathode). Magnesium was found to be totally unsuitable as a cathode material in an oxygen discharge and caused extreme instability in both the extracted beam and the discharge. The reasons for beam current instability are reIated to departure from cylindrical symmetry in the source, and are discussed qualitatively.
INTRODUCTION
Penning-type cold-cathode discharges form a useful and, in some ways, superior alternative to the duoplasmatron as sources of ion beams for surface physics experiments. Much larger total currents may be extracted from the duoplasmatron (tens of milliamps compared with hundreds of microamps). However, this parameter is irrelevant since for the beam energies ( -C 10 keV) necessary for surface studies and high dynamic range depth profiling the amount of usable current reaching the working point is usually determined by the transport properties of the column rather than the total source emission. As a generalisation, Penning discharges require - 5% of the power input, need simpler power supplies and cooling arrangements, and operate at around 1% of the gas load of the duoplasmatron. The latter parameter is most important as lower column pressures enable scattering losses and neutral beam generation to be minimised, and lower working pressures can be obtained in the analysis chamber, especially when operating at low ion currents. 0020-7381,‘83/$03.00
0 1983 Elsevier Science Publishers B.V.
300
The literature on ion sources deals mostly with unmodified emission properties measured at some point close to the extraction electrodes. Such measurements give little indication of the sources’ suitability for feeding a complex ion column. In this paper, in contrast, we consider the performance of various electrode materials in an ion source for supplying a beam-forming system to produce a beam suitable for high dynamic range SIMS depth profiling and imaging. EXPERIMENTAL
The 160,+ beam extracted from an A-DIDA ion source (Wittmaack [l-3], manufactured by Atomika Gmbh) was studied. The primary-ion column employed (Fig. 1) was fitted to the high-performance EVA 2000 SIMS system designed and built at the City of London Polytechnic and is similar to a design published by Wittmaack and Clegg [4]. However, it contains extra electrostatic alignment stages which improve the current transport, and a Septier-type einzel lens [5] for low spherical aberration final demagnification. The ion column is differentially pumped by a 1000 1 s- ’ cryopump (Air Products UHV-202C). With a 0.5 X 0.5 mm extraction canal in the discharge anticathode and a 1 mm diameter pressure step between the ion column and ihe analysis chamber, the partial pressure in the analysis chamber due to the
Anahrsis
ton column
chamber
c13 I
a
?300& LN, diffusion 70000 lb
cryopump
EVA 2000
PRIMARY
trawed pump + Ti sub.
/ON COLUMN
Fig. 1. EVA 2000 SIMS primary-ion dolunm schematic diagram: a, Atom&a discharge; b, anticathode (extraction canal 0.5 X0.5 mm); c, extraction and acceleration “Telefocus” optics; d, scatter suppression aperture stops (Ta) 1-6, 2-3, 3-3 and 4-2 mm diameter; e, quadrupole alignment plates; f, alignment and Wien filter; g, pressure step 1 mm diameter; h, beam deflection (4”) for neutral beam suppression; i, objective aperture 1 mm diameter; j, quadrupole beam scanning plates; k, Septier lens.
301
working gas is = 10d9 Torr for a 1 E_LAbeam at the sample. The pumping speed in the analysis chamber of > 500 1 SK’. The Atomika discharge chamber consists of a pointed cylindrical cathode tip mounted on a cylindrical body inside a tubular anode. The anticathode (maintained at the same potential as the cathode) is a flat disc with a hole 0.5-0.8 mm diameter in the centre through which the beam effuses. The assembly fits inside a 38 x 100 nun conflat port around which a coil is wound which provides an axial field - 0.02 T (typical operating conditions). When the discharge is operating, the cathode tip is shrouded in plasma and a volume of plasma extends up around the axis of the anode to the anticathode [6]. Ions are extracted through the anticathode by a “ telefocus” lens system [l] which uses a small extraction voltage to control the focal length of the emission system ( - lo-30 cm). The discharge was found to run continuously for up to several months before cathode and anode needed replacement. At this stage, operation of the discharge is inhibited by either a thick, insulating oxide layer on the anode which must be-machined off, or by erosion of the cathode tip which necessitates replacement of the cathode. Our original A-DIDA discharge (purchased in 1978) was constructed entirely in a commercial grade of aluminium probably 5 99% pure. The resolution of the Wien filter was not sufficiently high to separate enough of the 27A1’ ions generated from sputtered cathode material from the selected i60c (32 a-mu.) ions. In addition to problems of beam purity, the current at the sample exhibited both short (- 0.1 s) and long ( >, 0.5 h) term fluctuations of up to 3%. For these reasons a search for other discharge electrode materials was started. I. Cathode material’s
The performance of a cold-cathode discharge depends critically on the interaction between the working gas (in this case 99.9999% O,, BOC Special Gases, Ltd.) and the electrode materials. Materials suitable for cathode construction are usually divided into two categories according to whether they need low ( -E 800 V) or high ( - 15003000 V) operating voltages [6]. However, most published data apply to clean, or oxide-covered materials in inert gas discharges. In an oxygen discharge the dynamic equilibrium level of surface oxygen may be expected to modify the electron emissivity of the cathode surfaces. The following materials are listed as low voltage ones in ref. 6: Al (oxidised), Mg (oxidised), Be (oxidised), Fe, U and Ti. The materials studied here are Al, Mg, Fe, Ni, Ti, Ta, MO and Ni. Ni, Ta and MO are all listed in ref. 6 as high-voltage cathodes, but were found to operate at 600-800 V in an 0, discharge. Be and U were not studied because of their toxicity. Table 1 shows the materials
302 TABLE
1
Electrode materials and source of supply Material
Purity (%)
Fe MO Ta Ti Ni Al (anode) Al (cathode) Stainless Steel
Source or specification
99.95
Goodfellow Metals, Ltd.
99.999 > 98
Metal Crystals, Ltd. ’ Commercial Grade Dual Origin& Atomika 316 (EN58J)
used with details of purity and source of supply. Tables 2 and 3 show the various combinations of these materials with anodes of aluminium and stainless steel together with properties of the resulting discharge and ion beams, discussed in more detail later. 2. Optimisation
criteria
The ion beams extracted from the discharge are used for a wide range of profiling tasks in the EVA 2000 SIMS system, particularly on layer structures consisting of highly conducting (l-100 Q cm) or poorly conducting (103-lo6 Q cm) Si epilayers on insulating (A1203, > 1O’4 Q cm) substrates. This work demands very high beam stability because it is necessary to achieve simultaneous dynamic surface potential stabilisation with a focussed beam of 500-1000 eV electrons [7]. For high dynamic range depth profiling (i.e., > 5 decades in concentration) the EVA 2000 instrument requires a gaussian current distribution at the sample with a f.w.h.m. -=z70 pm in all planes. This condition is found ‘to be satisfied when the current collected on a flat Cu SEM mesh with 100 pm holes and 30 pm bars, held at + 12 V with respect to system earth, modulates by a factor of at least 2 as the beam is swept across in the x and y directions. This latter parameter is therefore used as the main criterion for suitability when evaluating the discharge performance. In addition, in the energy range 4- 12 keV, a minimum current of 100 nA per keV is. required. 3. Modifications
to geometry
In the course of the experiments the diameter and shape of the anode were varied slightly to overcome shorting (see later). A set of Al anodes 28 mm 0-d. (cf., A-DIDA 32 mm o.d.), and - 2 mm shorter than the standard
303 TABLE
2
Unsuccessful combinations a Material Anode
Cathode body/tip
v, (v)
P (Torr) I,(A)
i,(pA)
Anticathode Formation of loose tantalum oxide particles inhibits discharge and causes shorting.
Al or Stainless Steel
Ni TaTa
Al
unstable
4 x IO-’
Al
Ni
MO
670
3x 1O-6 1.8
1.5
Al
Ni
Ta
640
3x lo+
1.8
1.5
Al (lipped) Fe
Ta
680
3x 1O-6 1.8
1.2
Al (~PP~
Ta
490 5x1o-7 (unstable)
1.5
1
Al (lipped) Mg/Fe
Ta
640
2x1o-6
1.7
1.3
Al (lipped) Ni/Fe
Mg
430
2 x 1o-6
1.4
1
Mg
Comments
Formation of ~XXX molybdenum or tantalum oxide particles less serious than above but occasional shorting still encountered. Fe body oxidises at end; stable. Very unstable-long and short term fluctuations > 50%. Discharge voltage unstable; Mg completely oxidised. Mg anticathode ran for 2 days before causing gross instability.
a For criterion of usability, see Experimental, sectiun 3. V, = discharge voltage, P = column pressure (discharge on), I, = magnet current, and i, = maximum usable current at target.
replacement were made, with an internal 0.5-mm high lip at the anticathode end. These anodes had, incidentally, a wall thickness of 2.5-3 mm making remachining of the bore to remove the oxide easier than with the A-DIDA standard replacement (wall - 1 mm). Cathode tips from 10 to 30 mm long (3-3.2 mm diameter) were tested. To save material and ease fabrication the cathode tip was threaded- and screwed into the cathode body, rather than making the two in one piece. In this way it was normally only necessary to replace the tip itself after - 1 month, and tips and bodies of different materials could be employed.
304 TABLE
3
Successful
combinations
a
Material
V,(v)
P(Torr)
I,,,{A)
i,(pA)
Comments
Al
350420
3x
1.6
I.0
Slight fluctuations ( - 1%) in beam current. Lowest column pressure.
Al/Ni
Al
460
4x10-’
1.6
2.0
Highest useful current; slight nickel oxide particle formation if discharge turned on/off.
Ni/Fe Ni/Ni
Ti
680
5 x lo-’ 2x 1O-6
1.x2.4
1.0
Best overall performante for Al-free beam. Overall stability better than 1% b.
Anode
Cathode body/tip
Anticathode
Al
Al
Al
Al
lo-’
a See footnote to Table 2. b 2 x 10m6 Torr gave best foci.
RESULTS
Cathode
AND
DISCUSSION
and anticathode
The main purpose of this investigation was to find materials for electrode construction which produced both a stable 0, discharge, and an Al+-free beam. Tables 2 and 3 summarise the maximum usable beam currents at the sample, the operating pressures in the ion column and the target current stability for the various combinations tested. It can be seen that the running pressure and voltage are generally determined by the characteristics of the lowest voltage material present in the cathode or anticathode. With all Al or Mg cathodes present, in combination with various tips and anticathodes or with an Al anticathode in combination with various cathodes the operating voltage is typically -C 500 V and the operating pressure -=z5 X 10m7 Torr. With these materials absent from the discharge, running voltages are > 600 V and operating pressures are - 2-7 X lop6 Torr. With Ti electrodes present the operating pressure was - 5 X 10m7 Torr (although smallest spot sizes are achieved at 2 x 1 0p6 Torr). (Pressures in the discharge volume are - 100 times greater than those in the column quoted here [2].)
305
Fig. 2. Comparison of tip wear in similarly aged non-magnetic (Al) cathodes. The Fe cathode is on the left.
and magnetic (Fe)
The effect of the ferromagnetic cathodes Ni and Fe is to intensify the magnetic field at the cathode end of the discharge and (possibly) to stabilise the position of the plasma, which improves the stability of the target current. Figure 2 shows shadow-graphs of Fe and Al tips after approximately the same running time. It is evident that the Fe tip wears from the end and the side, whilst the Al tip wears predominantly from the front. This indicates both a redistribution of the plasma for a ferromagnetic cathode tip and (possibly) a larger cathode fall for Fe and Ni, leading to faster sputtering. The best overall performance both in terms of usable current and long and short term beam stability was obtained with either an Al cathode with an Ni tip, Al anticathode and an Al anode, or a Ni or Fe cathode, Ti anticathode and Al anode. Mg was almost useless as a cathode or anticathode due to the formation of a thick insulating oxide on its surface. In addition magnesium oxide flaked off the anode rather readily. The optimum length for the cathode tip was found to be 15-20 mm. With cathodes longer than this the limiting factor on discharge lifetime was the coating of the anode with cathode oxidation products whilst with shorter tips the tip wear became the limiting factor. In addition, as the tip eroded to < 4 mm in length the plasma position became increasingly unstable inside the discharge, and the magnetic field needed to sustain the discharge became
306
higher. These effects resulted in beam current instability and increased dispersion in the Wein filter and neutral beam stop due to an increase in the energy spread of ions emitted from the discharge. The spot shape achieved on. the sample became progressively more elongated in the plane of the Wien filter and beam stop deflection. The optimum dimensions for the Ti anticathode exit canal were found to be 0.5 x 0.5 mm. Anode material and shape Only Al and stainless steel were tried as anode materials. No erosion of the anode due to sputtering was observed; indeed, there was invariably a rapid coating with cathdde and anticathode oxidation products. The most important characteristic of the anode was how well the cathode (and anticathode) oxide adhered to its inner surface. None of the oxides adhered well to stainless steel and the formation of loose particles of oxide, from dust up to strips or flakes 0.5 X 10 mm, occurred for Ni, Fe and Ta cathodes with both Al and stainless anodes. Ti and Al however produced coatings which remained firmly attached to the Al anodes. Loose particles interfered with the operation of the discharge both by dangling from the anode into the plasma and producing a varying field configuration in the discharge, and by sliding down the anode and bridging the gap between the anode and anticathode causing parasitic discharges to form at this point, loading the power supply and quenching the main discharge_ A set of smaller-diameter lipped anodes was made (see Experimental, section 3) and both the larger clearance and the lip prevented the problems encountered with loose particles. In addition, it was found that these particles (except for Ta cathodes) usually formed when the discharge was switched off and the anode cooled and contracted, cracking off the ridged oxide coating. If the discharge was run continuously between service periods, almost no loose material was generated. Fluctuation and drift in target current It was found that almost any adjustment of the source conditions (discharge current, pressure, magnetic field) caused the current at the sample to be reduced. Adjustment of the alignment electrodes in the column, however, enabled the target current to be repeaked, often to a higher value. It is clear that ions do not in general leave the source with a uniform distribution about the mechanical axis, but with some asymmetric distribution which varies slightly as a function of source conditions. Similarly for cathode and anticathode materials with a poor stability performance, it was found that
307
continual readjustment of the column alignment could temporarily recover lost current so that much of the instability was related to the directional properties of the source and exacerbated by the small. widely-separated apertures in the ion column. It is known that the plasma potential is close to the anode potential [8] so that extracted ions which have been accelerated by the cathode fall between the plasma and the anticathode canal. If this field lacks cylindrical symmetry then the ion emission will be skewed. Examination of the cathodes after some period of running revealed that Ni and Fe tips were polished and free from oxide, Al tips were perhaps slightly oxidised, whilst Mg and Ta tips were oxide coated. Mg cathodes and anticathodes including the tip were entirely coated with an insulating oxide sheath after only a few hours running time. The Fe cathode bodies became slightly oxidised across the front surface. It is likely that the instability and drift in the beam current are caused by the localised charging of insulating oxides forcing the plasma column to wander to a fresh site. This results in a deflection in the extracted beam which varies as a function of time {plasma position). Formation of local fields due to charging of the anticathode surface probably has an even more pronounced effect. The excellent performance of Ni and Ni/Al cathodes is therefore probably due to the low steady-state oxide concentration on the tip and surface.
Fig. 3. Depth profile of an SiO,/Si/Al,O, (silicon-on-sapphire) multilayer structure showing the Al profile in the epilayer and at the interface. The chain dotted line shows apparent “Al levels in Si before removal of the Al cathode and anticathode. Open and closed circles show the “Al profile obtained with an Al-free discharge. Primary ions: 8 keV I602 1 PA, 600 X600 ,um raster, gated area 250 x 250 pm.
308
Performance of Al free discharge in depth profiiing Figure 3 shows a comparison of the dynamic range achieved (for 27A1in a silicon-on-sapphire epilayer consisting of 70 nm SiO, on 0.6 pm Si epimaterial on an Al,O, substrate) when profiled using Al-containing and Al-free cathodes. The whole structure was treated as an insulator and primary-ion charge compensation was achieved using a beam of 600 eV electrons focussed into the 600~pm square crater. The chain dotted line shows apparent Al levels in Si before the removal of all Al electrodes from the discharge, whilst the full and open circles show the 27A1profile recorded with an Al-free discharge. The open circles correspond to zero Al counts accumulated per frame, and the average Al channel count in the Si-epilayer is -=z1 count min- ‘. This corresponds roughly to the background residual count in the instrument. The beam-induced 27Al level has therefore been reduced by more than three decades. SUMMARY
AND
CONCLUSION
A variety of cathode and anode materials have been evaluated for use in a A-DIDA-type cold-cathode ion source. The best performance in terms of maximum beam current is achieved using an Al cathode body with a Ni tip - 15 mm long with an Al anticathode. Alternatively, for long and short term stability combined with freedom from Al’ in the beam, a Ni or Fe cathode with a Ti anticathode gives the best performance. The increased stability achieved with Ni is probably due to the absence of local insulating patches on the cathode tip -and body rather than its effect of concentrating the magnetic field. Al proved to be the best anode material tested in all cases. ACKNOWLEDGEMENTS
The authors would like to thank Dr. K. Wittmaack for valuable advice and suggestions and Mr. A. Aldridge and Mr. V. Manning for making the electrode structures tested. Support from the following establishments is gratefully acknowledged: SERC (grant GR/A 71141), GEC Hirst Research Centre, and US Office of Naval Research (grant NOOO14-80-9-0072). REFERENCES 1 2 3 4
K. K. K. K.
Wittmaack, Wittmaack, Wittmaack, Wittmaack
Nucl. Instrum. Methods, 118 (1974) 99. Nucl. Instrum. Methods, 143 (1977) 1. Adv. Mass Spectrom., 7 (1977) 758. and J.B. Clegg, Appl. Phys. Lett., 37 (1980)
285.
309 5 6 7 8
A. L. K. K.
Septier, CERN Report No. 69-39 (1960). Valyi, Atom and Ion Sources, Wiley, New York, 1977. Wittmaack, J. Appl. Phys., 50 (1979) 493. Wittmaack, private communication, 1982.
International Journal of Mass Spectrometry
EXPERIMENTAL STUDY OF ELECTRODE A COLD-CATHODE OXYGEN DISCHARGE
M.G.
DOWSETT
Solid
State
MATERIALS
299
FOR USE IN
and E.H.C. PARKER
Research
Street, London EC3N
Group, 2EY
Department
of Physics,
City of London
Polytechnic,
31 Jewry
(Great Britain)
(First received 28 January 1983; in final form 27 March 1983)
ABSTRACT A large range of elements has been evaluated for use as cathode, anticathode and anode materials in an oxygen cold-cathode discharge. The criterion for suitability of a particular material/electrode combination was based on the ability to extract from the source an ion beam suitable for high dynamic range SIMS. The effect of materials on long and short term stability and total beam current was investigated. The best materials were found to be Al (all electrodes), Ni/Fe (cathode) and Ti (anticathode). Magnesium was found to be totally unsuitable as a cathode material in an oxygen discharge and caused extreme instability in both the extracted beam and the discharge. The reasons for beam current instability are reIated to departure from cylindrical symmetry in the source, and are discussed qualitatively.
INTRODUCTION
Penning-type cold-cathode discharges form a useful and, in some ways, superior alternative to the duoplasmatron as sources of ion beams for surface physics experiments. Much larger total currents may be extracted from the duoplasmatron (tens of milliamps compared with hundreds of microamps). However, this parameter is irrelevant since for the beam energies ( -C 10 keV) necessary for surface studies and high dynamic range depth profiling the amount of usable current reaching the working point is usually determined by the transport properties of the column rather than the total source emission. As a generalisation, Penning discharges require - 5% of the power input, need simpler power supplies and cooling arrangements, and operate at around 1% of the gas load of the duoplasmatron. The latter parameter is most important as lower column pressures enable scattering losses and neutral beam generation to be minimised, and lower working pressures can be obtained in the analysis chamber, especially when operating at low ion currents. 0020-7381,‘83/$03.00
0 1983 Elsevier Science Publishers B.V.
300
The literature on ion sources deals mostly with unmodified emission properties measured at some point close to the extraction electrodes. Such measurements give little indication of the sources’ suitability for feeding a complex ion column. In this paper, in contrast, we consider the performance of various electrode materials in an ion source for supplying a beam-forming system to produce a beam suitable for high dynamic range SIMS depth profiling and imaging. EXPERIMENTAL
The 160,+ beam extracted from an A-DIDA ion source (Wittmaack [l-3], manufactured by Atomika Gmbh) was studied. The primary-ion column employed (Fig. 1) was fitted to the high-performance EVA 2000 SIMS system designed and built at the City of London Polytechnic and is similar to a design published by Wittmaack and Clegg [4]. However, it contains extra electrostatic alignment stages which improve the current transport, and a Septier-type einzel lens [5] for low spherical aberration final demagnification. The ion column is differentially pumped by a 1000 1 s- ’ cryopump (Air Products UHV-202C). With a 0.5 X 0.5 mm extraction canal in the discharge anticathode and a 1 mm diameter pressure step between the ion column and ihe analysis chamber, the partial pressure in the analysis chamber due to the
Anahrsis
ton column
chamber
c13 I
a
?300& LN, diffusion 70000 lb
cryopump
EVA 2000
PRIMARY
trawed pump + Ti sub.
/ON COLUMN
Fig. 1. EVA 2000 SIMS primary-ion dolunm schematic diagram: a, Atom&a discharge; b, anticathode (extraction canal 0.5 X0.5 mm); c, extraction and acceleration “Telefocus” optics; d, scatter suppression aperture stops (Ta) 1-6, 2-3, 3-3 and 4-2 mm diameter; e, quadrupole alignment plates; f, alignment and Wien filter; g, pressure step 1 mm diameter; h, beam deflection (4”) for neutral beam suppression; i, objective aperture 1 mm diameter; j, quadrupole beam scanning plates; k, Septier lens.
301
working gas is = 10d9 Torr for a 1 E_LAbeam at the sample. The pumping speed in the analysis chamber of > 500 1 SK’. The Atomika discharge chamber consists of a pointed cylindrical cathode tip mounted on a cylindrical body inside a tubular anode. The anticathode (maintained at the same potential as the cathode) is a flat disc with a hole 0.5-0.8 mm diameter in the centre through which the beam effuses. The assembly fits inside a 38 x 100 nun conflat port around which a coil is wound which provides an axial field - 0.02 T (typical operating conditions). When the discharge is operating, the cathode tip is shrouded in plasma and a volume of plasma extends up around the axis of the anode to the anticathode [6]. Ions are extracted through the anticathode by a “ telefocus” lens system [l] which uses a small extraction voltage to control the focal length of the emission system ( - lo-30 cm). The discharge was found to run continuously for up to several months before cathode and anode needed replacement. At this stage, operation of the discharge is inhibited by either a thick, insulating oxide layer on the anode which must be-machined off, or by erosion of the cathode tip which necessitates replacement of the cathode. Our original A-DIDA discharge (purchased in 1978) was constructed entirely in a commercial grade of aluminium probably 5 99% pure. The resolution of the Wien filter was not sufficiently high to separate enough of the 27A1’ ions generated from sputtered cathode material from the selected i60c (32 a-mu.) ions. In addition to problems of beam purity, the current at the sample exhibited both short (- 0.1 s) and long ( >, 0.5 h) term fluctuations of up to 3%. For these reasons a search for other discharge electrode materials was started. I. Cathode material’s
The performance of a cold-cathode discharge depends critically on the interaction between the working gas (in this case 99.9999% O,, BOC Special Gases, Ltd.) and the electrode materials. Materials suitable for cathode construction are usually divided into two categories according to whether they need low ( -E 800 V) or high ( - 15003000 V) operating voltages [6]. However, most published data apply to clean, or oxide-covered materials in inert gas discharges. In an oxygen discharge the dynamic equilibrium level of surface oxygen may be expected to modify the electron emissivity of the cathode surfaces. The following materials are listed as low voltage ones in ref. 6: Al (oxidised), Mg (oxidised), Be (oxidised), Fe, U and Ti. The materials studied here are Al, Mg, Fe, Ni, Ti, Ta, MO and Ni. Ni, Ta and MO are all listed in ref. 6 as high-voltage cathodes, but were found to operate at 600-800 V in an 0, discharge. Be and U were not studied because of their toxicity. Table 1 shows the materials
302 TABLE
1
Electrode materials and source of supply Material
Purity (%)
Fe MO Ta Ti Ni Al (anode) Al (cathode) Stainless Steel
Source or specification
99.95
Goodfellow Metals, Ltd.
99.999 > 98
Metal Crystals, Ltd. ’ Commercial Grade Dual Origin& Atomika 316 (EN58J)
used with details of purity and source of supply. Tables 2 and 3 show the various combinations of these materials with anodes of aluminium and stainless steel together with properties of the resulting discharge and ion beams, discussed in more detail later. 2. Optimisation
criteria
The ion beams extracted from the discharge are used for a wide range of profiling tasks in the EVA 2000 SIMS system, particularly on layer structures consisting of highly conducting (l-100 Q cm) or poorly conducting (103-lo6 Q cm) Si epilayers on insulating (A1203, > 1O’4 Q cm) substrates. This work demands very high beam stability because it is necessary to achieve simultaneous dynamic surface potential stabilisation with a focussed beam of 500-1000 eV electrons [7]. For high dynamic range depth profiling (i.e., > 5 decades in concentration) the EVA 2000 instrument requires a gaussian current distribution at the sample with a f.w.h.m. -=z70 pm in all planes. This condition is found ‘to be satisfied when the current collected on a flat Cu SEM mesh with 100 pm holes and 30 pm bars, held at + 12 V with respect to system earth, modulates by a factor of at least 2 as the beam is swept across in the x and y directions. This latter parameter is therefore used as the main criterion for suitability when evaluating the discharge performance. In addition, in the energy range 4- 12 keV, a minimum current of 100 nA per keV is. required. 3. Modifications
to geometry
In the course of the experiments the diameter and shape of the anode were varied slightly to overcome shorting (see later). A set of Al anodes 28 mm 0-d. (cf., A-DIDA 32 mm o.d.), and - 2 mm shorter than the standard
303 TABLE
2
Unsuccessful combinations a Material Anode
Cathode body/tip
v, (v)
P (Torr) I,(A)
i,(pA)
Anticathode Formation of loose tantalum oxide particles inhibits discharge and causes shorting.
Al or Stainless Steel
Ni TaTa
Al
unstable
4 x IO-’
Al
Ni
MO
670
3x 1O-6 1.8
1.5
Al
Ni
Ta
640
3x lo+
1.8
1.5
Al (lipped) Fe
Ta
680
3x 1O-6 1.8
1.2
Al (~PP~
Ta
490 5x1o-7 (unstable)
1.5
1
Al (lipped) Mg/Fe
Ta
640
2x1o-6
1.7
1.3
Al (lipped) Ni/Fe
Mg
430
2 x 1o-6
1.4
1
Mg
Comments
Formation of ~XXX molybdenum or tantalum oxide particles less serious than above but occasional shorting still encountered. Fe body oxidises at end; stable. Very unstable-long and short term fluctuations > 50%. Discharge voltage unstable; Mg completely oxidised. Mg anticathode ran for 2 days before causing gross instability.
a For criterion of usability, see Experimental, sectiun 3. V, = discharge voltage, P = column pressure (discharge on), I, = magnet current, and i, = maximum usable current at target.
replacement were made, with an internal 0.5-mm high lip at the anticathode end. These anodes had, incidentally, a wall thickness of 2.5-3 mm making remachining of the bore to remove the oxide easier than with the A-DIDA standard replacement (wall - 1 mm). Cathode tips from 10 to 30 mm long (3-3.2 mm diameter) were tested. To save material and ease fabrication the cathode tip was threaded- and screwed into the cathode body, rather than making the two in one piece. In this way it was normally only necessary to replace the tip itself after - 1 month, and tips and bodies of different materials could be employed.
304 TABLE
3
Successful
combinations
a
Material
V,(v)
P(Torr)
I,,,{A)
i,(pA)
Comments
Al
350420
3x
1.6
I.0
Slight fluctuations ( - 1%) in beam current. Lowest column pressure.
Al/Ni
Al
460
4x10-’
1.6
2.0
Highest useful current; slight nickel oxide particle formation if discharge turned on/off.
Ni/Fe Ni/Ni
Ti
680
5 x lo-’ 2x 1O-6
1.x2.4
1.0
Best overall performante for Al-free beam. Overall stability better than 1% b.
Anode
Cathode body/tip
Anticathode
Al
Al
Al
Al
lo-’
a See footnote to Table 2. b 2 x 10m6 Torr gave best foci.
RESULTS
Cathode
AND
DISCUSSION
and anticathode
The main purpose of this investigation was to find materials for electrode construction which produced both a stable 0, discharge, and an Al+-free beam. Tables 2 and 3 summarise the maximum usable beam currents at the sample, the operating pressures in the ion column and the target current stability for the various combinations tested. It can be seen that the running pressure and voltage are generally determined by the characteristics of the lowest voltage material present in the cathode or anticathode. With all Al or Mg cathodes present, in combination with various tips and anticathodes or with an Al anticathode in combination with various cathodes the operating voltage is typically -C 500 V and the operating pressure -=z5 X 10m7 Torr. With these materials absent from the discharge, running voltages are > 600 V and operating pressures are - 2-7 X lop6 Torr. With Ti electrodes present the operating pressure was - 5 X 10m7 Torr (although smallest spot sizes are achieved at 2 x 1 0p6 Torr). (Pressures in the discharge volume are - 100 times greater than those in the column quoted here [2].)
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Fig. 2. Comparison of tip wear in similarly aged non-magnetic (Al) cathodes. The Fe cathode is on the left.
and magnetic (Fe)
The effect of the ferromagnetic cathodes Ni and Fe is to intensify the magnetic field at the cathode end of the discharge and (possibly) to stabilise the position of the plasma, which improves the stability of the target current. Figure 2 shows shadow-graphs of Fe and Al tips after approximately the same running time. It is evident that the Fe tip wears from the end and the side, whilst the Al tip wears predominantly from the front. This indicates both a redistribution of the plasma for a ferromagnetic cathode tip and (possibly) a larger cathode fall for Fe and Ni, leading to faster sputtering. The best overall performance both in terms of usable current and long and short term beam stability was obtained with either an Al cathode with an Ni tip, Al anticathode and an Al anode, or a Ni or Fe cathode, Ti anticathode and Al anode. Mg was almost useless as a cathode or anticathode due to the formation of a thick insulating oxide on its surface. In addition magnesium oxide flaked off the anode rather readily. The optimum length for the cathode tip was found to be 15-20 mm. With cathodes longer than this the limiting factor on discharge lifetime was the coating of the anode with cathode oxidation products whilst with shorter tips the tip wear became the limiting factor. In addition, as the tip eroded to < 4 mm in length the plasma position became increasingly unstable inside the discharge, and the magnetic field needed to sustain the discharge became
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higher. These effects resulted in beam current instability and increased dispersion in the Wein filter and neutral beam stop due to an increase in the energy spread of ions emitted from the discharge. The spot shape achieved on. the sample became progressively more elongated in the plane of the Wien filter and beam stop deflection. The optimum dimensions for the Ti anticathode exit canal were found to be 0.5 x 0.5 mm. Anode material and shape Only Al and stainless steel were tried as anode materials. No erosion of the anode due to sputtering was observed; indeed, there was invariably a rapid coating with cathdde and anticathode oxidation products. The most important characteristic of the anode was how well the cathode (and anticathode) oxide adhered to its inner surface. None of the oxides adhered well to stainless steel and the formation of loose particles of oxide, from dust up to strips or flakes 0.5 X 10 mm, occurred for Ni, Fe and Ta cathodes with both Al and stainless anodes. Ti and Al however produced coatings which remained firmly attached to the Al anodes. Loose particles interfered with the operation of the discharge both by dangling from the anode into the plasma and producing a varying field configuration in the discharge, and by sliding down the anode and bridging the gap between the anode and anticathode causing parasitic discharges to form at this point, loading the power supply and quenching the main discharge_ A set of smaller-diameter lipped anodes was made (see Experimental, section 3) and both the larger clearance and the lip prevented the problems encountered with loose particles. In addition, it was found that these particles (except for Ta cathodes) usually formed when the discharge was switched off and the anode cooled and contracted, cracking off the ridged oxide coating. If the discharge was run continuously between service periods, almost no loose material was generated. Fluctuation and drift in target current It was found that almost any adjustment of the source conditions (discharge current, pressure, magnetic field) caused the current at the sample to be reduced. Adjustment of the alignment electrodes in the column, however, enabled the target current to be repeaked, often to a higher value. It is clear that ions do not in general leave the source with a uniform distribution about the mechanical axis, but with some asymmetric distribution which varies slightly as a function of source conditions. Similarly for cathode and anticathode materials with a poor stability performance, it was found that
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continual readjustment of the column alignment could temporarily recover lost current so that much of the instability was related to the directional properties of the source and exacerbated by the small. widely-separated apertures in the ion column. It is known that the plasma potential is close to the anode potential [8] so that extracted ions which have been accelerated by the cathode fall between the plasma and the anticathode canal. If this field lacks cylindrical symmetry then the ion emission will be skewed. Examination of the cathodes after some period of running revealed that Ni and Fe tips were polished and free from oxide, Al tips were perhaps slightly oxidised, whilst Mg and Ta tips were oxide coated. Mg cathodes and anticathodes including the tip were entirely coated with an insulating oxide sheath after only a few hours running time. The Fe cathode bodies became slightly oxidised across the front surface. It is likely that the instability and drift in the beam current are caused by the localised charging of insulating oxides forcing the plasma column to wander to a fresh site. This results in a deflection in the extracted beam which varies as a function of time {plasma position). Formation of local fields due to charging of the anticathode surface probably has an even more pronounced effect. The excellent performance of Ni and Ni/Al cathodes is therefore probably due to the low steady-state oxide concentration on the tip and surface.
Fig. 3. Depth profile of an SiO,/Si/Al,O, (silicon-on-sapphire) multilayer structure showing the Al profile in the epilayer and at the interface. The chain dotted line shows apparent “Al levels in Si before removal of the Al cathode and anticathode. Open and closed circles show the “Al profile obtained with an Al-free discharge. Primary ions: 8 keV I602 1 PA, 600 X600 ,um raster, gated area 250 x 250 pm.
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Performance of Al free discharge in depth profiiing Figure 3 shows a comparison of the dynamic range achieved (for 27A1in a silicon-on-sapphire epilayer consisting of 70 nm SiO, on 0.6 pm Si epimaterial on an Al,O, substrate) when profiled using Al-containing and Al-free cathodes. The whole structure was treated as an insulator and primary-ion charge compensation was achieved using a beam of 600 eV electrons focussed into the 600~pm square crater. The chain dotted line shows apparent Al levels in Si before the removal of all Al electrodes from the discharge, whilst the full and open circles show the 27A1profile recorded with an Al-free discharge. The open circles correspond to zero Al counts accumulated per frame, and the average Al channel count in the Si-epilayer is -=z1 count min- ‘. This corresponds roughly to the background residual count in the instrument. The beam-induced 27Al level has therefore been reduced by more than three decades. SUMMARY
AND
CONCLUSION
A variety of cathode and anode materials have been evaluated for use in a A-DIDA-type cold-cathode ion source. The best performance in terms of maximum beam current is achieved using an Al cathode body with a Ni tip - 15 mm long with an Al anticathode. Alternatively, for long and short term stability combined with freedom from Al’ in the beam, a Ni or Fe cathode with a Ti anticathode gives the best performance. The increased stability achieved with Ni is probably due to the absence of local insulating patches on the cathode tip -and body rather than its effect of concentrating the magnetic field. Al proved to be the best anode material tested in all cases. ACKNOWLEDGEMENTS
The authors would like to thank Dr. K. Wittmaack for valuable advice and suggestions and Mr. A. Aldridge and Mr. V. Manning for making the electrode structures tested. Support from the following establishments is gratefully acknowledged: SERC (grant GR/A 71141), GEC Hirst Research Centre, and US Office of Naval Research (grant NOOO14-80-9-0072). REFERENCES 1 2 3 4
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Wittmaack, Wittmaack, Wittmaack, Wittmaack
Nucl. Instrum. Methods, 118 (1974) 99. Nucl. Instrum. Methods, 143 (1977) 1. Adv. Mass Spectrom., 7 (1977) 758. and J.B. Clegg, Appl. Phys. Lett., 37 (1980)
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Septier, CERN Report No. 69-39 (1960). Valyi, Atom and Ion Sources, Wiley, New York, 1977. Wittmaack, J. Appl. Phys., 50 (1979) 493. Wittmaack, private communication, 1982.