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A combined time-of-flight spectrometer using field desorption and ion impact sputtering
::
Surface Science 266 (1992)517-522 North-ttolland
::::
.
surface
!:
:
:
science
A combined time-of-flight spectrometer using field desorption and ion impact sputtering M. L e i s c h a n d K.D. R e n d u l i c Technische Unit'ersitiit Graz, bzstitut fiir Festk6rperphy,~'ik, Petersgasse 16, A-8010 Graz, Attstrit, Received 5 August 1991; accepted for publication 2 September 1991
A differentially pumped ion sputter gun has been mounted to operate the energy comper.,ated atom probe in the secondary ion mass spectrometer (SIMS) mode. In one and the same instrument, field-ion mass spectrometry (FIMSI and SIMS investigations can be performed and the additional benefits of the SIMS technique which allows analysis independent of a particular surface geometry can be achieved. In a single-ion counting mode, SIMS analyses can be performed with very low primary ion dose well imo the static SIMS mode even on poorly conducting samples. The field desorption meast,,'ements provide a quantitative depth profiling amalysis. Collecting comparable amounts of ions in the SIMS-mode provides informaiion only of a part of a mon'qayer and exhibits extreme surface sen¢:tivity. Preliminary studies on microalloyed steel, superconducting oxide films and ceramic targets will be discussed.
1. Introduction
The increasing importance of surface and interface properties in modern technologies is strongly related to an exact and comprehensive knowledge of the mate:'iai composition. Among the wide variety of surface analytical techniques, secondary ion mass spectrometry (SIMS) has become an expanding technique for the analysis of surfaces [1]. Offering high sensitivity, the technique is ideally suited for problems which arise in the semiconductor industry where impurity detection on surfaces plays an important role. In the past decade, time-of-flight analyzers have been favored in the design of high performance secondary ion spectrometers. Especially, time-offlight SIMS has been shown to have unique capabilities for the surface anaiysis of delicate materials [2]. This suggests the use of the electrostatic sector atom probe in a seconda~' ion mode of operation and performing FIMS and SIMS investigations in one and the same time-of-flight apparatus [3]. The field-ion microscopist could get the additional benefits of the SIMS-technique, which al!ows, for example analysis independent of a
particular surface geomctry. The operation of 1hc detector in tl~c single ion counting mode leads to a further advantage. Analyses can be performcd with very low primaw ;:m doses well into thc static SIMS regimc with negligible surface damage. As an additional adwmmge, the charging of poorly conducting samples is very lo~ and ,mai~sis becomes possible without any means of charge compensation. In this work, we report on the modifications on the time-of-flight instrument which allows experiments in the SIMS as well as in the FIMS-mode. First tests on microalloyed steel, high temperature superconducting oxide films and ceramic targets will be presented.
2. lnslrumentation
Te operate thc time-of-flight in.~trumcnt h~ the S~MS-modc, only minor modifications had to be made on the used apparatus. The field-ion mass spectrometer is ap energy compensating instrument equipped with an electrostatic sector (1',~4': after Poschenricder [4]). The total flight path is 966 mm long. The electrostatic sector has a maxi-
1)[)39-6028/92/$05.1)0 e~ 1992 - Elsevier Science Publishers B.V. All rights rcse~cd
518
M. Leisch, K.D. Rendulic / A combined time-of-flight spectrometer
mum acceptance angle [5] of 67 mrad, which allows the collection of desorbed ions from a surface area of approximatcly 10 nm 2. The specimen tip is mounted on a manipulator and can be cooled by liquid nitrogen. A specimen interlock allows fast changeover of the samples. The time measurement is performed by a four channel digital counter [6]. A chevron micro-channelplate is used as a detector. In order to reduce the influence of residual gas, the system is fully bakeable and provides U H V conditions. The spectrometer and detection system operates automatically under computer control. For operation of the spectrometer in the SIMS-mode, a differentially pumped ion-sputter gun (VG-IONEX EX 05) is mounted in an angle of 60 ° with respect to the axis of the spectrometer. The ion gun is an electron impact type with a spot diameter of 100/xm and equipped with a TV scan unit. The maximum beam energy is 5 keV, the maximum current is 10/zA. By recording the secondary electron image in the scan mode a controlled positioning of the sample is provided. The sample stage is the same as used in the FIMS-mode. The specimen is mounted on the sapF, hire insulator and can be rotated and positioned by means of the X-Y-Z stage. For SIMS analysis, the tip holder is exchanged fcr a flat cup which can hold samples up to a size of 15 mm in diameter. Cooling of the specimen by means of the copper braid is usually not used in the SIMSmode. A schematic diagram of the instrumental arrangement is depicted in fig. 1. The primary ion pulses are created in a very simple way by deflecting the beam over an adjustable pulse slit using the built-in Y-Y deflection plates. The pulse slit is shaped like a skimmer and made from tantalum foils. The open width is l(;t, ,tm. Normally the beam is blanked by a steady DC potential on one Y-plate. The deflection is pertormed by a negative HV pulse provided by a Krytron pulser and tran:;mitted via capacitor to the second 5"plate. The X-X plates are used for additional beam steering. To extract the created secondary ions the sample is kept on a DC potential (typically 500-1000 V) which corresponds to the transmission condition ot the electrostatic sector. Additionally, an cxtractor (on ground potential) is
- . F =r \
;i
/
I
[ ,OTFIRy~OIFFUSIO~N~ ION~-~ I '°"' I
(b] ,
I 'U"'l
I "u"' I
' ~ [ Old-GUN
~~VOLTRGE
HIGH
EXTflACTOfl ,I IMEN LOCK
Fig. ]. Schematic diagram of the combined time-of-flight FIMS-SIMS system shown in the SIMS-mode. (a) Side ,,'Jew; (b) lop view, with ion gun mounted.
fitted in front of the sample. The extractor can be moved along the axis of the spectrometer to change the aperture and to avoid a cut-off of a part of the ion image in the FIMS-mode. On the chevron channeiplate detector with phosphorescent screen, the spatial beam profile can be monitored and the focus and spot size of the gun as well as other instrumental settings can be optimized. Ion detection and time measurement is performed in the same manner as in the field-desorption mode. The operation of the detector in a single ion counting mode leads to a further advantage. Combined with the high transmission of a time-of-flight system an analysis can be performed with very lo~: primary :on doses well in the static SIMS mode.
M. Leisch, K.D. Rendulic / A combined time-of-flight spectrometer
3. E x p e r i m e n t a l
519
2500
results
!Fe
To obtain information of the performance characteristics of the modified instrument, tests were first performed on mieroalloyed HSLA-steel specimens. On this material, FIM and atom probe depth profiling analyses have been carried out in order to study size, density and composition of small well-dispersed precipitates, and data sets have been available for comparison. The samples used for this study were obtained from continuous cast slabs of a commercial microalloyed steel (in wt.%: 0.05 C; 0.012 N; 0.09 Mo; (}.38 Si; 1.85 Mn; 0.045 AI; 0.35 Cr; 0.05 Nb; 0.07 V; 0.03 Ti). The submitted steel samples were cut with a diamond saw into slices and additionally into small bars for tip preparation. In fig. 2, a typical FI mass spectrum of a microalloyed steel specimen is depicted for comparison, containing the information of about 300 desorbed atomic layers. The analysis was performed at a low evaporation rate (below 0.1 100000
IONS 2000-
1500
H2
CO Si
1000
500
o
0 Na"
, ......
=C,
Ca
!1
.,..I,, ! ..... I I,,.I
lO
FeC
.b.o
, Ii..,11 ],...,,511...... ,q.,,h,. 2o ao 40 so 60 ro 8o 90 lOO ATOMIC MASS UNITS
Fig. 3. Positive secondary ion spectrum of a microalloyed steel sample, containing the information of about 10% of a monolayer. Sample temperature 300 K, primary Ar-ion beam 5 keV, total ion dose approx. 10 ~, probed surface area I00 ~ m in diameter.
IONS I Fe
i
10000 :
Si
1000
H
M
N C
IO0
~lo
I
,1:1t' o;i li
Nb lTi Fe
. . . . . . . . . .
!11~1
'
,
0
10
20
30
i o n / p u l s e ) and high pulse fraction which fulfills the conditions for good quantitative analysis [7]. The tip temperature was kept at 80 K. To obtain a sufficient high number of ions from one individual surface layer, the electrostatic sector was operated with full acceptance angle without additional aperture. A certain decrease in mass resolution has been accepted for better statistics in this case. Additionally it should be noted that the plot in fig. 2 has been scaled to give a direct comparison to fig. 3 and does not represent, of course, the actual resolution, obtained in the energy compensated atom probe. The measured concentration of camun . . . . . .cox . . . .~c~pu~u~J- very. we, to the nominal composition checked by thermogravimetric analysis. The quantitative separation of the metal ions is more complex, different charge states of field desorbed ions as well as the natural abundance of several isotopes of alloy elements lead to a signal overlap in the vicinity of the Fe ++-peak. Nevertheless the concentration of
i
,,!il+ 40
50
60
70
80
90
100
ATOMIC MASS UNITS Fig, 2. Field desorption mass spectrum of a microalloyed steel specimen, containing the ions from about 300 atomic layers in depth. The probed surface area is about 10 nm-'; specimen temperature 80 K.
I!
520
M. Leisch, K.D. Rendulic / A combined time-@flight spectrometer
the microalloy-elements like Nb, V, Ti, Mo obtained in the spectrum differs significantly from the bulk composition. In this particular ar, atysis, a prccipitate region was located in the analyzed volume. For th~s reason, a rather high proportion of non-iron ions are detected. It is, of course, for this reason that this particular experimental run was chosen as an example for an FIMS analysis: local atomic concentrations can be obtained with high accuracy. For the SIMS analysis, flat steel samples from the same material with a dimension of 10 × 10 × 0.5 mm were used. The surface has been polished mechanically, cleaned with alcohol and tlried before inserting into the vacuum system. In comparison to the very small area probed in the field desorption mode (10 nm 2) the SIMS-mode information is gained from a surface area of about 100 ~tm in diameter, defined by the spot size of the ion gun. Because of the large area probed, one can expect to obtain the average composition of the sample. The individual precipitate regions are too small to be detected individually. One of our preliminary results is shown in fig. 3. Here a mass spectrum of positive secondary ions of the same steel material as in fig. 2 is depicted. The electronic timing circuitry allows a simultaneous detection of four ions from one dcsorption event. l'he primm-y dose of the argon ion beam has been adjusted to a rather low value (approximately 1O00-10000 primary Atdons) to avoid overloading of the timing unit. It is estimated that for collecting the above spectrum, secondaries in an amount of about 10% of a monolayer have been removed from the sampled area. The spectrum therefore gives only information of the outermost surface region. Besides the dominating Fe-peak, adsorbates like H, N, O, CO and hydrocarbons are obser'v'ed. The intensities of the microalioying elements Nb, V, Ti and Mo are only slightly above the background level. Additionally distinct ~lgl~il~,~ ~:ollc~lJomtuHlg . . . . ~O . l'~u, K iauu~-J ~a,""'- Or igi haling from contaminants, arc present in the SIMS spectra. The total yield is extremely high for these elements. With respect to the signal-to-noise ratio, the SIMS spectrum in fig. 3 cannot compete with a FIMS analysis. However, with respect to dynamic SIMS measurement on a similar steel :
.....
' ...................
:
target with a quadrupole system [8], improvements by using the time-of-flight mode can be ob:;erved. The mass resolution in the SIMS-mode in the prcscnt configuration is principally limited by the duration of the primary ion pulse which ranges between 50-100 nanoseconds and has to be taken into account for the simplicity of the arrangement. With optimal settings, a mass resolution of M/&M near 100 is obtained. An improvement can of course be obtained by fitting an additional ion optics system [1] or by using a liquid metal ion gun [2,9]. These first measurements exhibit the known extreme surface sensitivity of this technique. Collecting comparable amounts of ions as in the field desorption mode provides information on only a part of a monolayer. Further tests have been carried out on Y~Ba 2Cu3 O7_, high-temperature superconducting oxide films prepared by reactive sputtering [10] using a SrTiO 3 single crystal as the target. The film, with an average thickness of 1/~m, has been previously characterized by calibrated energy-dispersive X-ray measurements and has almost 1 : 2 : 3 composition. The complete superconducting transition was found at 70 K. Before starting the SIMS analysis, the surface has been cleaned in-situ by sputtering the surface in the scanning mode. A typical positive secondary ion spectrum is shown in fig. 4. Besides the distinct signals of Ba, Cu and Y and their oxides, hydrogen and CO are found, which are the main components in the residual gas and obviously adsorbed on the surface. Tile intensity of the Na signal is very low, however the Ca signal indicates a certain level of contaminants. The small signal about mass 48 is probably Ti. It can be explained by redeposition of the sputtered target material. Contrary to FIMS measurements [11], a quantitative evaluation of the composition from the spectra is very complex. The measured iotensities of £3~
i:lllU
I
dl~
dpplUAlllldL~ly
ill
~
K,U I I E ; k , L
ldLIU.
The copper concentration is too low by one order of magnitude, but this observation is in rough agreement with experimentally found positive ion yields for oxygen covered Cu and Ba surfaces [1]. The unique features of time-of-flight SIMS measurements in an ion counting mode can be shown
M. Leisch, K.D. Rendulic / A combined time-of-flight spectrometer
by the analysis of poorly conducting san,ples. The use of very low primary ion doses reduces the magnitude of charging of the sample to negligible levels. In fig. 5 the positive secondary ion spectrum of a BaTiO 3 ceramic target sample is depicted. The resistivity of this material is in the order of 1013 ~'~ cm. The measurement of the surface composition in the SIMS mode has been obtained without any means of charge compensation. The signals corresponding to Ba and Ti and their oxide compounds can be observed clearly. The intensities of impurities like Na and Ca are extremely low compared to the other SIMS measurements discussed above. Additional signals are found around mass 120 which were finally assigned to Sn. The occurrence of Sn on the ceramic surface has a trivial reason: before it was analyzed the sample had been stored with soldered parts. Accidentally traces of tin are transferred by rubbing of the parts. Complementary measurements show no traces of tin. Nevertheless this example shows that this technique is well suited for surface char-
.
1000 -
.
.
.
.
521 .
.
.
.
.
.
IONS !H 2
Ti 800
600
Ba
400
Na I
[1 c,
200 O
,~ !i,
il
ii
[ir,~~ .
Sn
,ll "l'lO
I
" 0
!
co I1I
20
~ I!1,
,Ii
i -- A ~ =at.,
lliil
I=' 40
'!i
60
80
100
120 140 160
ATOMIC
MASS
180 2 0 0
UNITS
Fig. 5. Positive secondary, ion spectrum of a BaTiO3-ceramie target. Surface contaminated with Sn. Primary. ion beam: Ar. 5 keV.
1200
acterization and quick inspection even on insulating materials where commonly used methods (e.g. with electron beams) can be applied only with major restrictions.
IONS 1 0 0 0 ]i H2 S; CO 800
Ca
4. Summary 600
The main result of this investigation is that, with rather small modifications on the time-offlight apparatus, experiments in the SIMS and in the FIMS mode can be obtained. Both of these modes provide information on different aspects of the sample composition. The combined instrume,,i of course cannot compete with the characteristics of several hi-h performance time-of-flight
Cu Ba
Y 400
2ooi
II
O i!
!
I! ,ii,
CuO
[1~i
' ='
iq
il
YO
BaO
,i
[ J~]i,
i ,i
I
0 0
20
40
60
80
100
120 140
1G0
180 200
ATOMIC MASS UNITS Fig. 4. Positive secondary- ion spectrum of a sputtered Y1BazCu)O7_x film deposed on a SrTiO3-target. Primar), ion beam: At, 5 keV.
theless, this rather simple, low-cost instrumental arrangement has already been successfully used in accompanying surface inspection during thinfilm deposition processes where even qualitative data give valuable information for process control.
522
M. Leisch. K.D. Rendulic / A combined time-of-flight spectrometer
Acknowledgement This work has been supported by the Austrian "Fonds zur F6rderung der wissenschaftlichen Forschung".
References [1] A. Benninghoven, F.C. Riidenauer and H.W. Werner, Secondary h)n Mass Speclrometry (Wiley, New York, 1987). [2] A.J. Eccles and J.C. Vickerman, J. Vac. Sci. Technol. A 7 (1989) 234. [3] A.R. Waugh, D.J. Fathers and D.R. Kingham, Microbcam Analysis - 1986, eds, A.D. Romig Jr. and W.F. Chambers (San Francisco Press, 1986) p. 38.
[4] W,P. Poschenrieder, Int. J. Mass. Spectrom. Ion Phys. 6 (1971) 413. [5] I!.-O. AndrEn, J. Phys. (Paris) 47, Suppl. 11 (1986) C7-483. L6] I]. RcinmiJllcr, J. Phys. E ~6 (1983) 1228. [7] M.K. Miller and G.D.',;v. Smith, Atom Probe Microanalysis (Material Research, Pittsburgh, PA, 1989). [8] J. Tiimpner, R. Wiisch and A. Benninghoven, J. Vac. Sci, Technol. A 5 (1987) 11860. [9] R. Levi Setti, Y.L. Wang and B, Crow, J. Phys. (Paris) 45, Suppl. 12 (1984) C9-197. [10] G. Springholz, K. Aichholzer, R. Abt, G. Leising, O. Leitner, P. Kranebitter and P. P61I, J. Less-Comm. Met. 151 (1989) 3770. [11] M. Leisch, M. Eisl, E. Schweiger and G. Leising, J. Phys. (Paris) Colloq. (1989) C8-481. [12] A. Benninghoven, Surf. Sci. 53 (1975) 569.
Surface Science 266 (1992)517-522 North-ttolland
::::
.
surface
!:
:
:
science
A combined time-of-flight spectrometer using field desorption and ion impact sputtering M. L e i s c h a n d K.D. R e n d u l i c Technische Unit'ersitiit Graz, bzstitut fiir Festk6rperphy,~'ik, Petersgasse 16, A-8010 Graz, Attstrit, Received 5 August 1991; accepted for publication 2 September 1991
A differentially pumped ion sputter gun has been mounted to operate the energy comper.,ated atom probe in the secondary ion mass spectrometer (SIMS) mode. In one and the same instrument, field-ion mass spectrometry (FIMSI and SIMS investigations can be performed and the additional benefits of the SIMS technique which allows analysis independent of a particular surface geometry can be achieved. In a single-ion counting mode, SIMS analyses can be performed with very low primary ion dose well imo the static SIMS mode even on poorly conducting samples. The field desorption meast,,'ements provide a quantitative depth profiling amalysis. Collecting comparable amounts of ions in the SIMS-mode provides informaiion only of a part of a mon'qayer and exhibits extreme surface sen¢:tivity. Preliminary studies on microalloyed steel, superconducting oxide films and ceramic targets will be discussed.
1. Introduction
The increasing importance of surface and interface properties in modern technologies is strongly related to an exact and comprehensive knowledge of the mate:'iai composition. Among the wide variety of surface analytical techniques, secondary ion mass spectrometry (SIMS) has become an expanding technique for the analysis of surfaces [1]. Offering high sensitivity, the technique is ideally suited for problems which arise in the semiconductor industry where impurity detection on surfaces plays an important role. In the past decade, time-of-flight analyzers have been favored in the design of high performance secondary ion spectrometers. Especially, time-offlight SIMS has been shown to have unique capabilities for the surface anaiysis of delicate materials [2]. This suggests the use of the electrostatic sector atom probe in a seconda~' ion mode of operation and performing FIMS and SIMS investigations in one and the same time-of-flight apparatus [3]. The field-ion microscopist could get the additional benefits of the SIMS-technique, which al!ows, for example analysis independent of a
particular surface geomctry. The operation of 1hc detector in tl~c single ion counting mode leads to a further advantage. Analyses can be performcd with very low primaw ;:m doses well into thc static SIMS regimc with negligible surface damage. As an additional adwmmge, the charging of poorly conducting samples is very lo~ and ,mai~sis becomes possible without any means of charge compensation. In this work, we report on the modifications on the time-of-flight instrument which allows experiments in the SIMS as well as in the FIMS-mode. First tests on microalloyed steel, high temperature superconducting oxide films and ceramic targets will be presented.
2. lnslrumentation
Te operate thc time-of-flight in.~trumcnt h~ the S~MS-modc, only minor modifications had to be made on the used apparatus. The field-ion mass spectrometer is ap energy compensating instrument equipped with an electrostatic sector (1',~4': after Poschenricder [4]). The total flight path is 966 mm long. The electrostatic sector has a maxi-
1)[)39-6028/92/$05.1)0 e~ 1992 - Elsevier Science Publishers B.V. All rights rcse~cd
518
M. Leisch, K.D. Rendulic / A combined time-of-flight spectrometer
mum acceptance angle [5] of 67 mrad, which allows the collection of desorbed ions from a surface area of approximatcly 10 nm 2. The specimen tip is mounted on a manipulator and can be cooled by liquid nitrogen. A specimen interlock allows fast changeover of the samples. The time measurement is performed by a four channel digital counter [6]. A chevron micro-channelplate is used as a detector. In order to reduce the influence of residual gas, the system is fully bakeable and provides U H V conditions. The spectrometer and detection system operates automatically under computer control. For operation of the spectrometer in the SIMS-mode, a differentially pumped ion-sputter gun (VG-IONEX EX 05) is mounted in an angle of 60 ° with respect to the axis of the spectrometer. The ion gun is an electron impact type with a spot diameter of 100/xm and equipped with a TV scan unit. The maximum beam energy is 5 keV, the maximum current is 10/zA. By recording the secondary electron image in the scan mode a controlled positioning of the sample is provided. The sample stage is the same as used in the FIMS-mode. The specimen is mounted on the sapF, hire insulator and can be rotated and positioned by means of the X-Y-Z stage. For SIMS analysis, the tip holder is exchanged fcr a flat cup which can hold samples up to a size of 15 mm in diameter. Cooling of the specimen by means of the copper braid is usually not used in the SIMSmode. A schematic diagram of the instrumental arrangement is depicted in fig. 1. The primary ion pulses are created in a very simple way by deflecting the beam over an adjustable pulse slit using the built-in Y-Y deflection plates. The pulse slit is shaped like a skimmer and made from tantalum foils. The open width is l(;t, ,tm. Normally the beam is blanked by a steady DC potential on one Y-plate. The deflection is pertormed by a negative HV pulse provided by a Krytron pulser and tran:;mitted via capacitor to the second 5"plate. The X-X plates are used for additional beam steering. To extract the created secondary ions the sample is kept on a DC potential (typically 500-1000 V) which corresponds to the transmission condition ot the electrostatic sector. Additionally, an cxtractor (on ground potential) is
- . F =r \
;i
/
I
[ ,OTFIRy~OIFFUSIO~N~ ION~-~ I '°"' I
(b] ,
I 'U"'l
I "u"' I
' ~ [ Old-GUN
~~VOLTRGE
HIGH
EXTflACTOfl ,I IMEN LOCK
Fig. ]. Schematic diagram of the combined time-of-flight FIMS-SIMS system shown in the SIMS-mode. (a) Side ,,'Jew; (b) lop view, with ion gun mounted.
fitted in front of the sample. The extractor can be moved along the axis of the spectrometer to change the aperture and to avoid a cut-off of a part of the ion image in the FIMS-mode. On the chevron channeiplate detector with phosphorescent screen, the spatial beam profile can be monitored and the focus and spot size of the gun as well as other instrumental settings can be optimized. Ion detection and time measurement is performed in the same manner as in the field-desorption mode. The operation of the detector in a single ion counting mode leads to a further advantage. Combined with the high transmission of a time-of-flight system an analysis can be performed with very lo~: primary :on doses well in the static SIMS mode.
M. Leisch, K.D. Rendulic / A combined time-of-flight spectrometer
3. E x p e r i m e n t a l
519
2500
results
!Fe
To obtain information of the performance characteristics of the modified instrument, tests were first performed on mieroalloyed HSLA-steel specimens. On this material, FIM and atom probe depth profiling analyses have been carried out in order to study size, density and composition of small well-dispersed precipitates, and data sets have been available for comparison. The samples used for this study were obtained from continuous cast slabs of a commercial microalloyed steel (in wt.%: 0.05 C; 0.012 N; 0.09 Mo; (}.38 Si; 1.85 Mn; 0.045 AI; 0.35 Cr; 0.05 Nb; 0.07 V; 0.03 Ti). The submitted steel samples were cut with a diamond saw into slices and additionally into small bars for tip preparation. In fig. 2, a typical FI mass spectrum of a microalloyed steel specimen is depicted for comparison, containing the information of about 300 desorbed atomic layers. The analysis was performed at a low evaporation rate (below 0.1 100000
IONS 2000-
1500
H2
CO Si
1000
500
o
0 Na"
, ......
=C,
Ca
!1
.,..I,, ! ..... I I,,.I
lO
FeC
.b.o
, Ii..,11 ],...,,511...... ,q.,,h,. 2o ao 40 so 60 ro 8o 90 lOO ATOMIC MASS UNITS
Fig. 3. Positive secondary ion spectrum of a microalloyed steel sample, containing the information of about 10% of a monolayer. Sample temperature 300 K, primary Ar-ion beam 5 keV, total ion dose approx. 10 ~, probed surface area I00 ~ m in diameter.
IONS I Fe
i
10000 :
Si
1000
H
M
N C
IO0
~lo
I
,1:1t' o;i li
Nb lTi Fe
. . . . . . . . . .
!11~1
'
,
0
10
20
30
i o n / p u l s e ) and high pulse fraction which fulfills the conditions for good quantitative analysis [7]. The tip temperature was kept at 80 K. To obtain a sufficient high number of ions from one individual surface layer, the electrostatic sector was operated with full acceptance angle without additional aperture. A certain decrease in mass resolution has been accepted for better statistics in this case. Additionally it should be noted that the plot in fig. 2 has been scaled to give a direct comparison to fig. 3 and does not represent, of course, the actual resolution, obtained in the energy compensated atom probe. The measured concentration of camun . . . . . .cox . . . .~c~pu~u~J- very. we, to the nominal composition checked by thermogravimetric analysis. The quantitative separation of the metal ions is more complex, different charge states of field desorbed ions as well as the natural abundance of several isotopes of alloy elements lead to a signal overlap in the vicinity of the Fe ++-peak. Nevertheless the concentration of
i
,,!il+ 40
50
60
70
80
90
100
ATOMIC MASS UNITS Fig, 2. Field desorption mass spectrum of a microalloyed steel specimen, containing the ions from about 300 atomic layers in depth. The probed surface area is about 10 nm-'; specimen temperature 80 K.
I!
520
M. Leisch, K.D. Rendulic / A combined time-@flight spectrometer
the microalloy-elements like Nb, V, Ti, Mo obtained in the spectrum differs significantly from the bulk composition. In this particular ar, atysis, a prccipitate region was located in the analyzed volume. For th~s reason, a rather high proportion of non-iron ions are detected. It is, of course, for this reason that this particular experimental run was chosen as an example for an FIMS analysis: local atomic concentrations can be obtained with high accuracy. For the SIMS analysis, flat steel samples from the same material with a dimension of 10 × 10 × 0.5 mm were used. The surface has been polished mechanically, cleaned with alcohol and tlried before inserting into the vacuum system. In comparison to the very small area probed in the field desorption mode (10 nm 2) the SIMS-mode information is gained from a surface area of about 100 ~tm in diameter, defined by the spot size of the ion gun. Because of the large area probed, one can expect to obtain the average composition of the sample. The individual precipitate regions are too small to be detected individually. One of our preliminary results is shown in fig. 3. Here a mass spectrum of positive secondary ions of the same steel material as in fig. 2 is depicted. The electronic timing circuitry allows a simultaneous detection of four ions from one dcsorption event. l'he primm-y dose of the argon ion beam has been adjusted to a rather low value (approximately 1O00-10000 primary Atdons) to avoid overloading of the timing unit. It is estimated that for collecting the above spectrum, secondaries in an amount of about 10% of a monolayer have been removed from the sampled area. The spectrum therefore gives only information of the outermost surface region. Besides the dominating Fe-peak, adsorbates like H, N, O, CO and hydrocarbons are obser'v'ed. The intensities of the microalioying elements Nb, V, Ti and Mo are only slightly above the background level. Additionally distinct ~lgl~il~,~ ~:ollc~lJomtuHlg . . . . ~O . l'~u, K iauu~-J ~a,""'- Or igi haling from contaminants, arc present in the SIMS spectra. The total yield is extremely high for these elements. With respect to the signal-to-noise ratio, the SIMS spectrum in fig. 3 cannot compete with a FIMS analysis. However, with respect to dynamic SIMS measurement on a similar steel :
.....
' ...................
:
target with a quadrupole system [8], improvements by using the time-of-flight mode can be ob:;erved. The mass resolution in the SIMS-mode in the prcscnt configuration is principally limited by the duration of the primary ion pulse which ranges between 50-100 nanoseconds and has to be taken into account for the simplicity of the arrangement. With optimal settings, a mass resolution of M/&M near 100 is obtained. An improvement can of course be obtained by fitting an additional ion optics system [1] or by using a liquid metal ion gun [2,9]. These first measurements exhibit the known extreme surface sensitivity of this technique. Collecting comparable amounts of ions as in the field desorption mode provides information on only a part of a monolayer. Further tests have been carried out on Y~Ba 2Cu3 O7_, high-temperature superconducting oxide films prepared by reactive sputtering [10] using a SrTiO 3 single crystal as the target. The film, with an average thickness of 1/~m, has been previously characterized by calibrated energy-dispersive X-ray measurements and has almost 1 : 2 : 3 composition. The complete superconducting transition was found at 70 K. Before starting the SIMS analysis, the surface has been cleaned in-situ by sputtering the surface in the scanning mode. A typical positive secondary ion spectrum is shown in fig. 4. Besides the distinct signals of Ba, Cu and Y and their oxides, hydrogen and CO are found, which are the main components in the residual gas and obviously adsorbed on the surface. Tile intensity of the Na signal is very low, however the Ca signal indicates a certain level of contaminants. The small signal about mass 48 is probably Ti. It can be explained by redeposition of the sputtered target material. Contrary to FIMS measurements [11], a quantitative evaluation of the composition from the spectra is very complex. The measured iotensities of £3~
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The copper concentration is too low by one order of magnitude, but this observation is in rough agreement with experimentally found positive ion yields for oxygen covered Cu and Ba surfaces [1]. The unique features of time-of-flight SIMS measurements in an ion counting mode can be shown
M. Leisch, K.D. Rendulic / A combined time-of-flight spectrometer
by the analysis of poorly conducting san,ples. The use of very low primary ion doses reduces the magnitude of charging of the sample to negligible levels. In fig. 5 the positive secondary ion spectrum of a BaTiO 3 ceramic target sample is depicted. The resistivity of this material is in the order of 1013 ~'~ cm. The measurement of the surface composition in the SIMS mode has been obtained without any means of charge compensation. The signals corresponding to Ba and Ti and their oxide compounds can be observed clearly. The intensities of impurities like Na and Ca are extremely low compared to the other SIMS measurements discussed above. Additional signals are found around mass 120 which were finally assigned to Sn. The occurrence of Sn on the ceramic surface has a trivial reason: before it was analyzed the sample had been stored with soldered parts. Accidentally traces of tin are transferred by rubbing of the parts. Complementary measurements show no traces of tin. Nevertheless this example shows that this technique is well suited for surface char-
.
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Fig. 5. Positive secondary, ion spectrum of a BaTiO3-ceramie target. Surface contaminated with Sn. Primary. ion beam: Ar. 5 keV.
1200
acterization and quick inspection even on insulating materials where commonly used methods (e.g. with electron beams) can be applied only with major restrictions.
IONS 1 0 0 0 ]i H2 S; CO 800
Ca
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The main result of this investigation is that, with rather small modifications on the time-offlight apparatus, experiments in the SIMS and in the FIMS mode can be obtained. Both of these modes provide information on different aspects of the sample composition. The combined instrume,,i of course cannot compete with the characteristics of several hi-h performance time-of-flight
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ATOMIC MASS UNITS Fig. 4. Positive secondary- ion spectrum of a sputtered Y1BazCu)O7_x film deposed on a SrTiO3-target. Primar), ion beam: At, 5 keV.
theless, this rather simple, low-cost instrumental arrangement has already been successfully used in accompanying surface inspection during thinfilm deposition processes where even qualitative data give valuable information for process control.
522
M. Leisch. K.D. Rendulic / A combined time-of-flight spectrometer
Acknowledgement This work has been supported by the Austrian "Fonds zur F6rderung der wissenschaftlichen Forschung".
References [1] A. Benninghoven, F.C. Riidenauer and H.W. Werner, Secondary h)n Mass Speclrometry (Wiley, New York, 1987). [2] A.J. Eccles and J.C. Vickerman, J. Vac. Sci. Technol. A 7 (1989) 234. [3] A.R. Waugh, D.J. Fathers and D.R. Kingham, Microbcam Analysis - 1986, eds, A.D. Romig Jr. and W.F. Chambers (San Francisco Press, 1986) p. 38.
[4] W,P. Poschenrieder, Int. J. Mass. Spectrom. Ion Phys. 6 (1971) 413. [5] I!.-O. AndrEn, J. Phys. (Paris) 47, Suppl. 11 (1986) C7-483. L6] I]. RcinmiJllcr, J. Phys. E ~6 (1983) 1228. [7] M.K. Miller and G.D.',;v. Smith, Atom Probe Microanalysis (Material Research, Pittsburgh, PA, 1989). [8] J. Tiimpner, R. Wiisch and A. Benninghoven, J. Vac. Sci, Technol. A 5 (1987) 11860. [9] R. Levi Setti, Y.L. Wang and B, Crow, J. Phys. (Paris) 45, Suppl. 12 (1984) C9-197. [10] G. Springholz, K. Aichholzer, R. Abt, G. Leising, O. Leitner, P. Kranebitter and P. P61I, J. Less-Comm. Met. 151 (1989) 3770. [11] M. Leisch, M. Eisl, E. Schweiger and G. Leising, J. Phys. (Paris) Colloq. (1989) C8-481. [12] A. Benninghoven, Surf. Sci. 53 (1975) 569.