- Email: [email protected]
A time-of-flight atom-probe field-ion microscope for the study of defects in metals
Scripta
METALLURGICA
V o l . I0, pp. 4 8 5 - 4 8 8 , 1976 Printed in t h e U n i t e d S t a t e s
Pergamon
Press,
Inc.
SUMMARY A TIME-OF-FLIGHT
ATOM-PROBE FIELD-ION MICROSCOPE FOR THE STUDY OF DEFECTS IN METALS
+
Thomas M. Hall, Alfred Wagner, Arnold S. Berger and David N. Seidman Cornell University, Bard Hall Department of Materials Science and Engineering and the Materials Science Center, Ithaca, New York 14853 (Received
March
25,
1976)
ABSTRACT An ultra-high vacuum time-of-flight (TOF) atom-probe field-ion microscope (FIM) specifically designed for the study of defects in metals is described. Performance experiments show that this instrument can clearly resolve the seven stable isotopes of molybdenum, the five stable isotopes of tungsten, and the two stable isotopes of rhenium in a tungsten-25at.% rhenium alloy. The entire process of applying the evaporation pulse to the FIM specimen, measuring the dc and pulse voltages, and analyzing the T0F data is controlled by a Nova 1220 computer. With this automated system we can presently record and analyze 600 TOF events min -1.
This note summarizes a very detailed report (1) which describes an ultra-high vacuum (UHV) TOF atom-probe FIM specifically designed for the study of defects in metals. The T0F atom-probe FIM (or more simply the atom probe) was first described by ~/ller et al. (2) and combines an FIM with a TOF mass spectrometer. With this instrument it is possible both to image the microstructural features of a metal specimen on an atomic scale and to measure the mass-to-charge ratios (m/n) of individual ions from pre-selected regions of a specimen. The atom-probe is ideally suited for the study of the interaction of both substitutional and interstitial impurity atoms with lattice defects such as vacancies, self-interstitial atoms, dislocations, grain boundaries and voids. The potential of the atom-probe for studying a wide range of materials science problems had been demonstrated by Brenner and co-workers (3,4) and by Turner, Southon and co-workers
(5). A schematic d~agram illustrating thg main features of the atom probe is shown in Fig. i. A specimen with a radius of 50 to 400A is maintained at a positive potential (6-20kV) so that gas atoms surrounding the specimen are ionized over individual atomic sites and are projected radially outward to produce a visual image on the internal-image-intensification system. When a short high-voltage pulse is applied, atoms on the surface of the specimen are field evaporated in the form of ions. Those ions projected into the probe hole at the center of the internal-imageintensification system will pass down the flight tube to the ion detector. The TOFs of the ions and the voltage on the specimen are measured and the (m/n) ratios are calculated employing the equation: m/n = 2e(Vdc +
aVpulse) (t-to)2/d 2
Research supported by the U.S. Energy Research and Development Administration; additional support was received from the National Science Foundation through the use of the technical facilities of the Materials Science Center at Cornell University. +Now at: Argonne National Laboratory, Argonne, Illinois 60h39.
485
486
FIELD-ION
MICROSCOPE
Vol,
i0,
No,
S
as discussed by Panitz et al. (6), where e is the charge on an electron, Vdc the steady-state imaging voltage, Vouls e the pulse-evaporation voltage, a the so-called pulse factor, d the fligh distance, and (t-to) the actual TOF of the ion. The quantity t is the observed TOF and t o is th total delay time. The procedure we have developed to determine (m/n) is based on making Vpuls e a constant fraction of Vdc; i.e., Vpuls e = fVdc where f is a constant that is much less than uni and is as small as possible to maximize the mass resolution (7). By the controlled pulse-field evaporation of successive atomic layers it is possible to examine the bulk of the specimen and to reconstruct in three dimensions the correspondence between special microstructural features and chemical composition. The details of the atom-probe are now summarized. The specimen is mounted on a liquidhelium-cooled goniometer stage similar to Brenner's (3) which provides rotation about two orthogonal axes, thus allowing any portion of the specimen's surface to be projected into the probe hole for mass analysis. The goniometer stage is also translatable in three mutually orthogonal directions to facilitate alignment of the specimen with respect to the probe hole. The specimen is cooled by liquid helium in order to improve the quality of the FIM image and to control the diffusivity of point defects; the temperature of the specimen is continuously variable from 13 to h50K (8). The specimen is inserted into the goniometer stage via a high-vacuum (<10-6Torr) specimen exchange device which allows rapid transfer of specimens without breaking the vacuum in the FIM. The specimen can also be irradiated in-situ with low energy gas ions (100eV to ~10keV) employing a specially constructed ion-gun. The internal-image-intensification system consists of a 75mm diameter channel-electron-mult~ plier array (CEMA) and a phosphor screen with 3mm diameter holes through their centers. The distance from the FIM specimen to the front surface of the internal-image-intensification system is continuously variable so that the magnification of the FIM image, as well as the size of the region projected onto the probe hole, can be varied by an areal magnification factor of %64X. An electrostatic lens immediately behind the internal-image-intensification system serves to focus those ions which pass through the probe hole down to a lmm diam. spot on the ion detector at the end of a 2.22m long flight tube. The ion detector consists of two CEMAs placed back-to-back in the Chevron configuration and a phosphor screen which provides a visual image of the ion beam. The atom probe was constructed to routinely operate in UHV (~5xl0-1OTorr) in order to minimize the interaction of residual gas atoms with the specimen.
FIM SPECIMEN--
IMAGE OF
/-INTERNAL IMAGE I NTENSIFIC AT ION
r". . . . . . . . ~ n ~ v=ON nu,~
SYSTEM
OETECTOR METAL
:ON~ .--
OC + PULSE HIGH VOLTAGE
FRONT-SURFACED GLASS MIRROR
~
PICK-OFF PULSE
I
TRIGGER ,=, PULSER SPECIMEN
NOVA 1220
VOLTAGE SYSTEM
COMPUTER im
VOLTAGE
TOF
I START
DATA
I
STOP
DIGITAL TIMER
DATA
FIG. i Schematic diagram of the TOF atom-probe FIM. Shown at the top are the internal elements of the atom-probe including the FIM specimen, the internal-image-intensification system, the focusing lens, the 45 ° glass mirror, and the Chevron ion-detector. As indicated in the lower part of the figure, the specimen voltage system and the digital timer of the TOF mass spectrometer are operated automatically by a Nova 1220 minicomputer.
Vol.
i0,
No.
5
FIELD-ION
MICROSCOPE
487
The mass-spectrometer electronics, consisting of the specimen-voltage system and the digital timer are operated by a Nova 1220 minicomputer. This computerized system can automatically analyze up to 600 T0F events min -1 so that statistically significant results can be readily obtained even for small solute concentrations. As shown in the lower part of Fig. 1 the computer triggers the Vpuls e to the specimen which causes atoms on the surface of the specimen to be field-evaporated. A fraction of V ulse is picked off and used to start a modified version of an eight-channel digital timer whicE has a ±lOnsec resolution (9). The pulses produced when ions strike the detector are used to stop the timer. A total of eight ion species from a single evaporation
u~ 6 0 Iz IJJ ILl
40
w II1
20
Mo~M ~
Mo9e
MO+2
Mo'=
z 0 45
46
L
i
i
47
MOIoo nI
M°~l
i "[}~J~ I \ ll~l~ 48
49
MASS-TO-CHARGE
50
RATIO
WI82
600
W 'o4 W t~
FIG. 3a The W+3 spectrum of Westinghouse as-received tungsten pulse-field evaporated at = 2 5 K w i t h f=O.05 for Vde varied continuously from 13 to 15kV. The ions were collected from the (551) plane and the background pressure in the atom probe was 6xl0-1OTorr. The calibration parameters used were a=2.0, to=0.56~sec and d=l.6003m. The total number of W+3 events in this histogram is 60h5.
W+3
400
3a 200
O~ Iz iJJ > i.=J h 0
WI~
I o
1
-i__1
W*3ond
n~ 150 IJJ m :E
Z
FIG. 3b The W +3 and Re +3 spectrum of W-25at.% Re thermocouple wire. The spectrum was recorded at a specimen temperature of =25K with f=0.10 at a pressure of 5xl0-gTorr. The calibration parameters used were a=l.5, to=0.56Wsee and d=2.232m. The total number of W+3 and Re+3 events in this histogram is 1755.
Re+5
Re187
IO0
3b 50
0 59
FIG. 2 Spectrum of Mo +2 obtained at a background pressure of 5xl0-9Torr, a specimen temperature of =60K and with the probe hole in the internal image intensification system near the (llO) pole. The pulse fraction, f, was set at 0.025 and the calibration parameters used were ~=1.482, to=0.56psec , and d=2.213m. The total number of Mo +2 events in this histogram is 696.
60
61
62
M A S S - T O - CHARGE
63 RATIO
64
65
488
FIELD-ION
MICROSCOPE
Vol,
I0,
No,5
event can be identified. The controls of the dc and pulse power supplies as well as the power supply for the focusing lens are coupled, so that the pulse and lens voltages are maintained at a constant fraction of the dc voltage (7). The values of Vdc and Vouls e applied to the specimen are measured by an analog-to-digital converter and read into the computer along with the TOF" data. The (m/n) ratios are calculated by the computer and stored in the computer memory in the form of a histogram of the number of events versus m/n. In addition, the TOF and voltage data are stored on magnetic tape so that the results of the run can be re-analyzed in the future. The computer is interfaced to a Tektronix 4010 graphics display terminal and Tektronix hard copy unit, so that a copy of the histogram can be obtained in ~20 see. Several performance experiments were conducted with the atom probe to test the mass resolution of the instrument. As shown in Fig. 2 the 7 stable isotopes of molybdenum are clearly separated in the Mo +2 spectrum. The peaks associated with the 5 stable isotopes of tungsten are readily distinguished in the W +3 spectrum shown in Fig. 3a and the 2 stable isotopes of rhenium are clearly separated from the 5 stable isotopes of tungsten in a tungsten-25at.% rhenium alloy (see Fig. 3b.). In all cases the isotopic abundances were in good agreement with the handbook values. The concentration of rhenium in the W-25at.%-Re alloy was determined by atom-probe analysis to be 22±2at.% and there was no apparent segregation or clustering of rhenium. Thus, the present atom-probe appears to have mass resolution which is satisfactory for many materials science type problems. ACKNOWLEDGEMENTS We are indebted to Dr. S.S. Brenner of the U.S. Steel Corporation for providing the plans of the goniometer stage and for helpful advice and discussions concerning the TOF atom-probe FIM technique. We also thank Prof. R. W. Balluffi for encouragement, Mr. R. Whitmarsh and Mr. J. Hart for technical assistance, and Mr. A. Babbaro for machining the liquid-helium cooled goniometerstage. REFERENCES i. T.M. Hall, A. Wagner, A.S. Berger, and D.N. Seidman, Cornell University Materials Science Center Report No. 2357 (1975). 2. E.W. M'u_ller, J.A. Panitz, and S.B. McLane, Hey. Sei. Instrum. 39, 83 (1968). 3. S. S. Brenner and J.T. McKinney, Surf. Sci. 23, 88 (1970). 4. S.S. Brenner and J.T. McKinney, Appl. Phys. Letters 13, 29 (1968); S.S. Brenner and S.R. Goodman, Scripta Met. ~, 865 (1971); S.R. Goodman, S.S. Brenner, and J.R. Low, Jr., Met. Trans. k, 2371 (1973). 5. P.J. Turner, B.J. Regan, and M.J. Southon, Vacuum, 22, 443 (1972); P.J° Turner, B.J. Regan, and M.J. Southon, Surface Sci. 35, 336 (1973); A. Youle, P.J. Turner and B. Ralph, J. Microscopy i01, 1 (1973)~ P.J. Turner and M.J. Papazian, Met. Sci. ~, 81 (1973). 6. J.A. Panitz, S.B. McLane, and E.W. ~thler, Rev. Sci. Instrum. 40, 1321 (1969). 7. A. Wagner, T.M. Hall, and D.N. Seidman, Rev. Sci. Instrum. 46, 1032 (1975). 8. D.N. Seidman, R.M. Scanlan, D.L. Styris and J.W. Bohlen, J. Sci. Instrum. [, h73 (1969). 9. A.S. Berger, Rev. Sci. Instrum. hh, 592 (1973).
METALLURGICA
V o l . I0, pp. 4 8 5 - 4 8 8 , 1976 Printed in t h e U n i t e d S t a t e s
Pergamon
Press,
Inc.
SUMMARY A TIME-OF-FLIGHT
ATOM-PROBE FIELD-ION MICROSCOPE FOR THE STUDY OF DEFECTS IN METALS
+
Thomas M. Hall, Alfred Wagner, Arnold S. Berger and David N. Seidman Cornell University, Bard Hall Department of Materials Science and Engineering and the Materials Science Center, Ithaca, New York 14853 (Received
March
25,
1976)
ABSTRACT An ultra-high vacuum time-of-flight (TOF) atom-probe field-ion microscope (FIM) specifically designed for the study of defects in metals is described. Performance experiments show that this instrument can clearly resolve the seven stable isotopes of molybdenum, the five stable isotopes of tungsten, and the two stable isotopes of rhenium in a tungsten-25at.% rhenium alloy. The entire process of applying the evaporation pulse to the FIM specimen, measuring the dc and pulse voltages, and analyzing the T0F data is controlled by a Nova 1220 computer. With this automated system we can presently record and analyze 600 TOF events min -1.
This note summarizes a very detailed report (1) which describes an ultra-high vacuum (UHV) TOF atom-probe FIM specifically designed for the study of defects in metals. The T0F atom-probe FIM (or more simply the atom probe) was first described by ~/ller et al. (2) and combines an FIM with a TOF mass spectrometer. With this instrument it is possible both to image the microstructural features of a metal specimen on an atomic scale and to measure the mass-to-charge ratios (m/n) of individual ions from pre-selected regions of a specimen. The atom-probe is ideally suited for the study of the interaction of both substitutional and interstitial impurity atoms with lattice defects such as vacancies, self-interstitial atoms, dislocations, grain boundaries and voids. The potential of the atom-probe for studying a wide range of materials science problems had been demonstrated by Brenner and co-workers (3,4) and by Turner, Southon and co-workers
(5). A schematic d~agram illustrating thg main features of the atom probe is shown in Fig. i. A specimen with a radius of 50 to 400A is maintained at a positive potential (6-20kV) so that gas atoms surrounding the specimen are ionized over individual atomic sites and are projected radially outward to produce a visual image on the internal-image-intensification system. When a short high-voltage pulse is applied, atoms on the surface of the specimen are field evaporated in the form of ions. Those ions projected into the probe hole at the center of the internal-imageintensification system will pass down the flight tube to the ion detector. The TOFs of the ions and the voltage on the specimen are measured and the (m/n) ratios are calculated employing the equation: m/n = 2e(Vdc +
aVpulse) (t-to)2/d 2
Research supported by the U.S. Energy Research and Development Administration; additional support was received from the National Science Foundation through the use of the technical facilities of the Materials Science Center at Cornell University. +Now at: Argonne National Laboratory, Argonne, Illinois 60h39.
485
486
FIELD-ION
MICROSCOPE
Vol,
i0,
No,
S
as discussed by Panitz et al. (6), where e is the charge on an electron, Vdc the steady-state imaging voltage, Vouls e the pulse-evaporation voltage, a the so-called pulse factor, d the fligh distance, and (t-to) the actual TOF of the ion. The quantity t is the observed TOF and t o is th total delay time. The procedure we have developed to determine (m/n) is based on making Vpuls e a constant fraction of Vdc; i.e., Vpuls e = fVdc where f is a constant that is much less than uni and is as small as possible to maximize the mass resolution (7). By the controlled pulse-field evaporation of successive atomic layers it is possible to examine the bulk of the specimen and to reconstruct in three dimensions the correspondence between special microstructural features and chemical composition. The details of the atom-probe are now summarized. The specimen is mounted on a liquidhelium-cooled goniometer stage similar to Brenner's (3) which provides rotation about two orthogonal axes, thus allowing any portion of the specimen's surface to be projected into the probe hole for mass analysis. The goniometer stage is also translatable in three mutually orthogonal directions to facilitate alignment of the specimen with respect to the probe hole. The specimen is cooled by liquid helium in order to improve the quality of the FIM image and to control the diffusivity of point defects; the temperature of the specimen is continuously variable from 13 to h50K (8). The specimen is inserted into the goniometer stage via a high-vacuum (<10-6Torr) specimen exchange device which allows rapid transfer of specimens without breaking the vacuum in the FIM. The specimen can also be irradiated in-situ with low energy gas ions (100eV to ~10keV) employing a specially constructed ion-gun. The internal-image-intensification system consists of a 75mm diameter channel-electron-mult~ plier array (CEMA) and a phosphor screen with 3mm diameter holes through their centers. The distance from the FIM specimen to the front surface of the internal-image-intensification system is continuously variable so that the magnification of the FIM image, as well as the size of the region projected onto the probe hole, can be varied by an areal magnification factor of %64X. An electrostatic lens immediately behind the internal-image-intensification system serves to focus those ions which pass through the probe hole down to a lmm diam. spot on the ion detector at the end of a 2.22m long flight tube. The ion detector consists of two CEMAs placed back-to-back in the Chevron configuration and a phosphor screen which provides a visual image of the ion beam. The atom probe was constructed to routinely operate in UHV (~5xl0-1OTorr) in order to minimize the interaction of residual gas atoms with the specimen.
FIM SPECIMEN--
IMAGE OF
/-INTERNAL IMAGE I NTENSIFIC AT ION
r". . . . . . . . ~ n ~ v=ON nu,~
SYSTEM
OETECTOR METAL
:ON~ .--
OC + PULSE HIGH VOLTAGE
FRONT-SURFACED GLASS MIRROR
~
PICK-OFF PULSE
I
TRIGGER ,=, PULSER SPECIMEN
NOVA 1220
VOLTAGE SYSTEM
COMPUTER im
VOLTAGE
TOF
I START
DATA
I
STOP
DIGITAL TIMER
DATA
FIG. i Schematic diagram of the TOF atom-probe FIM. Shown at the top are the internal elements of the atom-probe including the FIM specimen, the internal-image-intensification system, the focusing lens, the 45 ° glass mirror, and the Chevron ion-detector. As indicated in the lower part of the figure, the specimen voltage system and the digital timer of the TOF mass spectrometer are operated automatically by a Nova 1220 minicomputer.
Vol.
i0,
No.
5
FIELD-ION
MICROSCOPE
487
The mass-spectrometer electronics, consisting of the specimen-voltage system and the digital timer are operated by a Nova 1220 minicomputer. This computerized system can automatically analyze up to 600 T0F events min -1 so that statistically significant results can be readily obtained even for small solute concentrations. As shown in the lower part of Fig. 1 the computer triggers the Vpuls e to the specimen which causes atoms on the surface of the specimen to be field-evaporated. A fraction of V ulse is picked off and used to start a modified version of an eight-channel digital timer whicE has a ±lOnsec resolution (9). The pulses produced when ions strike the detector are used to stop the timer. A total of eight ion species from a single evaporation
u~ 6 0 Iz IJJ ILl
40
w II1
20
Mo~M ~
Mo9e
MO+2
Mo'=
z 0 45
46
L
i
i
47
MOIoo nI
M°~l
i "[}~J~ I \ ll~l~ 48
49
MASS-TO-CHARGE
50
RATIO
WI82
600
W 'o4 W t~
FIG. 3a The W+3 spectrum of Westinghouse as-received tungsten pulse-field evaporated at = 2 5 K w i t h f=O.05 for Vde varied continuously from 13 to 15kV. The ions were collected from the (551) plane and the background pressure in the atom probe was 6xl0-1OTorr. The calibration parameters used were a=2.0, to=0.56~sec and d=l.6003m. The total number of W+3 events in this histogram is 60h5.
W+3
400
3a 200
O~ Iz iJJ > i.=J h 0
WI~
I o
1
-i__1
W*3ond
n~ 150 IJJ m :E
Z
FIG. 3b The W +3 and Re +3 spectrum of W-25at.% Re thermocouple wire. The spectrum was recorded at a specimen temperature of =25K with f=0.10 at a pressure of 5xl0-gTorr. The calibration parameters used were a=l.5, to=0.56Wsee and d=2.232m. The total number of W+3 and Re+3 events in this histogram is 1755.
Re+5
Re187
IO0
3b 50
0 59
FIG. 2 Spectrum of Mo +2 obtained at a background pressure of 5xl0-9Torr, a specimen temperature of =60K and with the probe hole in the internal image intensification system near the (llO) pole. The pulse fraction, f, was set at 0.025 and the calibration parameters used were ~=1.482, to=0.56psec , and d=2.213m. The total number of Mo +2 events in this histogram is 696.
60
61
62
M A S S - T O - CHARGE
63 RATIO
64
65
488
FIELD-ION
MICROSCOPE
Vol,
I0,
No,5
event can be identified. The controls of the dc and pulse power supplies as well as the power supply for the focusing lens are coupled, so that the pulse and lens voltages are maintained at a constant fraction of the dc voltage (7). The values of Vdc and Vouls e applied to the specimen are measured by an analog-to-digital converter and read into the computer along with the TOF" data. The (m/n) ratios are calculated by the computer and stored in the computer memory in the form of a histogram of the number of events versus m/n. In addition, the TOF and voltage data are stored on magnetic tape so that the results of the run can be re-analyzed in the future. The computer is interfaced to a Tektronix 4010 graphics display terminal and Tektronix hard copy unit, so that a copy of the histogram can be obtained in ~20 see. Several performance experiments were conducted with the atom probe to test the mass resolution of the instrument. As shown in Fig. 2 the 7 stable isotopes of molybdenum are clearly separated in the Mo +2 spectrum. The peaks associated with the 5 stable isotopes of tungsten are readily distinguished in the W +3 spectrum shown in Fig. 3a and the 2 stable isotopes of rhenium are clearly separated from the 5 stable isotopes of tungsten in a tungsten-25at.% rhenium alloy (see Fig. 3b.). In all cases the isotopic abundances were in good agreement with the handbook values. The concentration of rhenium in the W-25at.%-Re alloy was determined by atom-probe analysis to be 22±2at.% and there was no apparent segregation or clustering of rhenium. Thus, the present atom-probe appears to have mass resolution which is satisfactory for many materials science type problems. ACKNOWLEDGEMENTS We are indebted to Dr. S.S. Brenner of the U.S. Steel Corporation for providing the plans of the goniometer stage and for helpful advice and discussions concerning the TOF atom-probe FIM technique. We also thank Prof. R. W. Balluffi for encouragement, Mr. R. Whitmarsh and Mr. J. Hart for technical assistance, and Mr. A. Babbaro for machining the liquid-helium cooled goniometerstage. REFERENCES i. T.M. Hall, A. Wagner, A.S. Berger, and D.N. Seidman, Cornell University Materials Science Center Report No. 2357 (1975). 2. E.W. M'u_ller, J.A. Panitz, and S.B. McLane, Hey. Sei. Instrum. 39, 83 (1968). 3. S. S. Brenner and J.T. McKinney, Surf. Sci. 23, 88 (1970). 4. S.S. Brenner and J.T. McKinney, Appl. Phys. Letters 13, 29 (1968); S.S. Brenner and S.R. Goodman, Scripta Met. ~, 865 (1971); S.R. Goodman, S.S. Brenner, and J.R. Low, Jr., Met. Trans. k, 2371 (1973). 5. P.J. Turner, B.J. Regan, and M.J. Southon, Vacuum, 22, 443 (1972); P.J° Turner, B.J. Regan, and M.J. Southon, Surface Sci. 35, 336 (1973); A. Youle, P.J. Turner and B. Ralph, J. Microscopy i01, 1 (1973)~ P.J. Turner and M.J. Papazian, Met. Sci. ~, 81 (1973). 6. J.A. Panitz, S.B. McLane, and E.W. ~thler, Rev. Sci. Instrum. 40, 1321 (1969). 7. A. Wagner, T.M. Hall, and D.N. Seidman, Rev. Sci. Instrum. 46, 1032 (1975). 8. D.N. Seidman, R.M. Scanlan, D.L. Styris and J.W. Bohlen, J. Sci. Instrum. [, h73 (1969). 9. A.S. Berger, Rev. Sci. Instrum. hh, 592 (1973).