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A compact double-focusing mass spectrometer
Nuclear Instruments and Methods in Physics Research A258 (1987) 323-326 North-Holland, Amsterdam
323
A COMPACT DOUBLE-FOCUSING MASS SPECTROMETER H. LIEBL
Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-8046 Garching/München, FRG This paper describes a mass spectrometer consisting of an einzel lens, a magnetic and an electric sector field with 30 ° deflection each in opposite directions, matched to provide energy focusing at the position of the exit slit. Since these sector fields exert only weak focusing action, the einzel lens serves for angle focusing. A compact mass spectrometer of this type was constructed. Its housing is a straight vacuum chamber about 30 cm long, it weighs about 20 kg and it can be mounted on any 6 in. conflat attachment port . It is in use for the investigation of ion sources and for SIMS applications . 1. Introduction The aim of this work was to develop a compact magnetic mass spectrometer comparable in size to a quadrupole mass spectrometer, without its own pump, for attachment to existing ports of UHV systems. Since one of the applications in mind was secondary ion mass spectrometry (SIMS), a double-focusing (energy and angle focusing) mass spectrometer was called for in view of the rather broad energy distribution of sputtered ions . Moreover, the acceptance defined as entrance slit area times solid angle accepted, as well as the transmitted energy width, should be better than for other instruments of the size . 2. Geometry
X
v
Fig. 1. Ion trajectory through a uniform magnetic sector field with normal entry and exit axis .
2.1 . Magnetic sector field For a uniform magnetic sector field with boundaries normal to the beam axis (fig . 1), the equation of an ion trajectory after passage through the field is (in first approximation) Y=L'a+X[(1- L'If )a+v(S+y) ], where L' is the object distance from the deflection centre, a the entrance angle, f = r/sin 0 the focal length of the sector field, P = sin 0/2 the dispersion factor, S=dU/U the relative energy spread of the ions, and y = d M/M the relative mass difference (ions with energy U and mass M have a deflection radius r) . 2.2 . Electric sector field Analogously, for an electric sector field (fig. 2) the exit equation is 0168-9002/87/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 2. Ion trajectory through an electric sector field (cylindrical condenser) . I. MASS SPECTROMETERS/SEPARATORS
324
H. Liebl / A compact doublefocusing mass spectrometer
Ye = Lea,+Xe[( 1- L; (2) where L,, is the object distance from the principal plane, a e the entrance angle, fe = re / sin Y" `Ye the focal length of the cylindrical sector field, and X = sin(vl2 ¢e)/vr2 its dispersion factor. Here, only energy dispersion is exerted but no mass dispersion. There are two principal planes at distance p = re tan(Oe/ vr2 )/ from the field boundaries .
Fig . 4 . Angle focusing.
2 .3 . Energy focusing
To achieve a compact geometry, a case without an intermediate image of the entrance slit was chosen, resulting in opposite deflection (fig. 3). In order to find the condition for energy focusing, trajectories are considered of ions of mass M (y = 0) entering the magnetic field on axis (a = 0) with a certain energy deviation S. The energy dispersion of the magnetic field causes an entrance angle a.= - PS for the electric field, the object point being the deflection centre of the magnetic field at the distance D from the principal plane. By inserting Le = D and a e = - vS into eq. (2), the distance Xe = Le' is found where the trajectories cross the exit abscissa (Ye = 0): D L = e Dlfe + À/P - 1 . The parameters were chosen such that the whole assembly would fit into a straight rectangular tube welded between 15.2 cm (6 in .) o.d. conflat flanges : ¢ =Oe =30 °, D=7 .1cm . r=re =12cm, This choice results in v = 0.26, À = 0.48, 24 cm, fe = 12.6 cm, and from eq. (3) Le = 4.8 cm. This means that the energy focus is located close (1.5 cm) behind the electric field exit boundary. There the exit slit is placed.
f=
2.4. Angle focusing
In order to obtain double-focusing, the angle focus, i.e. the image of the entrance slit, must coincide with the
energy focus . The calculation yields a negative object distance L'(= -39 cm) with the above parameters (fig. 4). This means that the lens action of the two sector fields is too weak to form a real image of an entrance slit situated anywhere in front of the magnetic field at the location of the exit slit. An einzel lens is therefore placed in front of the magnetic field, which can be tuned to image the entrance slit on the exit slit. In the direction normal to the deflection plane, the einzel lens also focuses the beam, so that no loss due to cutoff occurs in that direction between the einzel lens and the detector placed behind the exit slit. 2.5. Mass resolution
The mass dispersion at the exit slit is obtained by introducing into eq. (2) L' = D, ae = - vy, Xe = L', 5=0 : Ye(y) =- [D+Lë( 1- Dlfe)]vy _ -2.3y(cm) . (4) The imaging ratio from the entrance to the exit slit is found by the transfer matrix method to be s
-~
+1
fl)(ffe
f
fe
)
f(1
fe
)]
D _ I _ 1 ) + I_ D (5) f fe fe ( Ae By equating the mass dispersion, eq. (4), with the image width, s", from eq. (5) one obtains the theoretical mass resolution (M/4 M) 1h = 1/y . +e
3. Technical details
Fig . 3. Energy focusing.
The whole mass spectrometer assembly consisting of entrance slit, two pairs of beam adjustment plates, einzel lens, pole pieces, cylindrical condenser, exit slit and channeltron detector is mounted on a 15.2 cm (6 in.) o.d. conflat flange. The flange carries all electrical feedthroughs, so that all connections can be made on the bench outside the vacuum (fig. 5). The assembly slides into its UHV housing consisting of a straight
H. Liebt / A compact double focusing mass spectrometer
32 5
Fig. 5 . Assembled mass spectrometer with its vacuum housing and magnet.
rectangular tube with 3 X 10 cmz inner dimensions, welded between 15 .2 cm o .d. conflat flanges . The magnet with copper coils, cores and yokes is attached to one of the conflat flanges. It appears as a block with edge dimensions of 13 X 15 X 15 cm3 . It is capable of generating a field strength of up to 10 kG inside the gap with a coil power of 450 W . This corresponds to a maximum product of mass and voltage of nearly 7 X 10 5 amu V, i.e ., at an acceleration voltage of 1000 V, the mass range is up to 700 . For prolonged operation in the higher power range, water cooling plates can be attached to the magnet. Owing to the compact shielded construction, the stray field outside the magnet block is very low ; it is < 10 -3 of the field strength in the gap . The vacuum housing has a pocket reaching into the magnet gap, so that the field may be monitored or controlled with a Hall probe. The distance e from the einzel lens to the magnet is 6 cm . The distance a of the entrance slit from the einzel lens was chosen as 14 cm . With these values, the focal length of the einzel lens, fl , must be 10.7 cm for imaging the entrance slit on the exit slit . The imaging ratio, eq . (5), is then 0.76, i .e ., slight demagnification occurs . The einzel lens was operated in the accel-decel mode in order the keep the spherical and chromatic aberrations sufficiently low. For a =14 cm, f1 =10.7 cm a voltage ratio of 1 .85 is required with the present construction, i.e ., with the ion source at +2 kV the lens voltage is -1 .7 kV.
The entrance slit height is 2 mm, the exit slit height being larger than the beam height, so that no part of the beam with a certain limited energy spread filling the entrance slit area and passing the 4 mm diameter einzel lens aperture is cut off. For the test measurements, the entrance and exit slits were both adjusted to a width of 50 ,am. With this value, a comparison of eqs . (4) and (5) predicts a mass resolution of 250.
4. Test results The mass spectrometer was tested using a beam of 2 keV Xe + ions from a unoplasmatron gun with a hollow cathode . Fig . 6a shows the Xe + spectrum as recorded by ramping the magnetic field . The isotopes are completely resolved, the fwhm resolution is about 250 . The energy spread AU of the ions from the plasma source is estimated to be about 4 eV, so that S = 2 X 10 -3 . In order to check the effectiveness of the energy focusing, an ac voltage was superimposed on the accelerating do voltage . This results in an additional artificial energy spread (fig. 6, inset) . Figs. 6b-e show the effect of increasing AU on the mass resolution. With DU= 20 eV (S =10 -Z ) no effect is noticeable. With AU= 40 eV (S = 2 X 10 -Z ) there is a slight deterioration, while with 4U=100 eV (S = 5 X 10 -2 ) and AU= 200 eV (S =10 -1 ) the mass resolution really shows increasing deterioration . l . MASS SPECTROMETERS/SEPARATORS
H. Debt / A compact doublefocusing mass spectrometer
326
Xe
6 = 10-2
dN du
ràù d rcut.ti i~ ni w
Fig. 7. Peak switching with the electric sector field. 132 M
r
6=2x10-2 (d)
6=5x10-2 (e)
6=10-
toi
,1ll P
M
Fig. 8. Xenon spectrum obtained with constant magnetic field by ramping the electric field. Fig. 6. Mass spectra of xenon ions from a unoplasmatron source (energy spread AU). In order to test the energy focusing, an ac voltage of increasing amplitude AU., (see inset) was superimposed on the acceleration voltage U to yield an artificial total energy spread AU,,,. One advantage of the field sequence chosen is that, for a limited relative mass range, electric peak switching with the electric sector field is possible (fig . 7) . The electric field gap is wide enough to accommodate ions leaving the magnetic field that cover a relative mass range of y = 0.1 . Most of the elemental isotopes can therefore be scanned or switched with constant magnetic field by ramping or stepping the deflection voltage of the cylindrical condenser . This is demonstrated in fig. 8, which shows the Xe + spectrum obtained with a constant magnetic field of 6000 G (the nominal value for the 130Xe isotope) by varying the electric field voltage from +5% to -5% of the nominal value.
5. Summary The compact mass spectrometer developed comes up to expectation. At an fwhm resolution of 260, it transmits ions filling an entrance slit 0.1 mmz in area and a solid angle of 6.4 X 10 -° sr and having a relative energy spread of 1% . The maximum product of mass and energy is 7 X 10 5 amu V. With a nominal ion energy of 2 keV and an energy spread of 20 eV, the mass range is therefore up to mass number 350. Fast electric peak switching is possible within a relative mass range of 10%. The authors wishes to thank H. Weiss and A. Schlamp for technical assistance and A. Klekamp for help with the test measurements .
323
A COMPACT DOUBLE-FOCUSING MASS SPECTROMETER H. LIEBL
Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-8046 Garching/München, FRG This paper describes a mass spectrometer consisting of an einzel lens, a magnetic and an electric sector field with 30 ° deflection each in opposite directions, matched to provide energy focusing at the position of the exit slit. Since these sector fields exert only weak focusing action, the einzel lens serves for angle focusing. A compact mass spectrometer of this type was constructed. Its housing is a straight vacuum chamber about 30 cm long, it weighs about 20 kg and it can be mounted on any 6 in. conflat attachment port . It is in use for the investigation of ion sources and for SIMS applications . 1. Introduction The aim of this work was to develop a compact magnetic mass spectrometer comparable in size to a quadrupole mass spectrometer, without its own pump, for attachment to existing ports of UHV systems. Since one of the applications in mind was secondary ion mass spectrometry (SIMS), a double-focusing (energy and angle focusing) mass spectrometer was called for in view of the rather broad energy distribution of sputtered ions . Moreover, the acceptance defined as entrance slit area times solid angle accepted, as well as the transmitted energy width, should be better than for other instruments of the size . 2. Geometry
X
v
Fig. 1. Ion trajectory through a uniform magnetic sector field with normal entry and exit axis .
2.1 . Magnetic sector field For a uniform magnetic sector field with boundaries normal to the beam axis (fig . 1), the equation of an ion trajectory after passage through the field is (in first approximation) Y=L'a+X[(1- L'If )a+v(S+y) ], where L' is the object distance from the deflection centre, a the entrance angle, f = r/sin 0 the focal length of the sector field, P = sin 0/2 the dispersion factor, S=dU/U the relative energy spread of the ions, and y = d M/M the relative mass difference (ions with energy U and mass M have a deflection radius r) . 2.2 . Electric sector field Analogously, for an electric sector field (fig. 2) the exit equation is 0168-9002/87/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 2. Ion trajectory through an electric sector field (cylindrical condenser) . I. MASS SPECTROMETERS/SEPARATORS
324
H. Liebl / A compact doublefocusing mass spectrometer
Ye = Lea,+Xe[( 1- L; (2) where L,, is the object distance from the principal plane, a e the entrance angle, fe = re / sin Y" `Ye the focal length of the cylindrical sector field, and X = sin(vl2 ¢e)/vr2 its dispersion factor. Here, only energy dispersion is exerted but no mass dispersion. There are two principal planes at distance p = re tan(Oe/ vr2 )/ from the field boundaries .
Fig . 4 . Angle focusing.
2 .3 . Energy focusing
To achieve a compact geometry, a case without an intermediate image of the entrance slit was chosen, resulting in opposite deflection (fig. 3). In order to find the condition for energy focusing, trajectories are considered of ions of mass M (y = 0) entering the magnetic field on axis (a = 0) with a certain energy deviation S. The energy dispersion of the magnetic field causes an entrance angle a.= - PS for the electric field, the object point being the deflection centre of the magnetic field at the distance D from the principal plane. By inserting Le = D and a e = - vS into eq. (2), the distance Xe = Le' is found where the trajectories cross the exit abscissa (Ye = 0): D L = e Dlfe + À/P - 1 . The parameters were chosen such that the whole assembly would fit into a straight rectangular tube welded between 15.2 cm (6 in .) o.d. conflat flanges : ¢ =Oe =30 °, D=7 .1cm . r=re =12cm, This choice results in v = 0.26, À = 0.48, 24 cm, fe = 12.6 cm, and from eq. (3) Le = 4.8 cm. This means that the energy focus is located close (1.5 cm) behind the electric field exit boundary. There the exit slit is placed.
f=
2.4. Angle focusing
In order to obtain double-focusing, the angle focus, i.e. the image of the entrance slit, must coincide with the
energy focus . The calculation yields a negative object distance L'(= -39 cm) with the above parameters (fig. 4). This means that the lens action of the two sector fields is too weak to form a real image of an entrance slit situated anywhere in front of the magnetic field at the location of the exit slit. An einzel lens is therefore placed in front of the magnetic field, which can be tuned to image the entrance slit on the exit slit. In the direction normal to the deflection plane, the einzel lens also focuses the beam, so that no loss due to cutoff occurs in that direction between the einzel lens and the detector placed behind the exit slit. 2.5. Mass resolution
The mass dispersion at the exit slit is obtained by introducing into eq. (2) L' = D, ae = - vy, Xe = L', 5=0 : Ye(y) =- [D+Lë( 1- Dlfe)]vy _ -2.3y(cm) . (4) The imaging ratio from the entrance to the exit slit is found by the transfer matrix method to be s
-~
+1
fl)(ffe
f
fe
)
f(1
fe
)]
D _ I _ 1 ) + I_ D (5) f fe fe ( Ae By equating the mass dispersion, eq. (4), with the image width, s", from eq. (5) one obtains the theoretical mass resolution (M/4 M) 1h = 1/y . +e
3. Technical details
Fig . 3. Energy focusing.
The whole mass spectrometer assembly consisting of entrance slit, two pairs of beam adjustment plates, einzel lens, pole pieces, cylindrical condenser, exit slit and channeltron detector is mounted on a 15.2 cm (6 in.) o.d. conflat flange. The flange carries all electrical feedthroughs, so that all connections can be made on the bench outside the vacuum (fig. 5). The assembly slides into its UHV housing consisting of a straight
H. Liebt / A compact double focusing mass spectrometer
32 5
Fig. 5 . Assembled mass spectrometer with its vacuum housing and magnet.
rectangular tube with 3 X 10 cmz inner dimensions, welded between 15 .2 cm o .d. conflat flanges . The magnet with copper coils, cores and yokes is attached to one of the conflat flanges. It appears as a block with edge dimensions of 13 X 15 X 15 cm3 . It is capable of generating a field strength of up to 10 kG inside the gap with a coil power of 450 W . This corresponds to a maximum product of mass and voltage of nearly 7 X 10 5 amu V, i.e ., at an acceleration voltage of 1000 V, the mass range is up to 700 . For prolonged operation in the higher power range, water cooling plates can be attached to the magnet. Owing to the compact shielded construction, the stray field outside the magnet block is very low ; it is < 10 -3 of the field strength in the gap . The vacuum housing has a pocket reaching into the magnet gap, so that the field may be monitored or controlled with a Hall probe. The distance e from the einzel lens to the magnet is 6 cm . The distance a of the entrance slit from the einzel lens was chosen as 14 cm . With these values, the focal length of the einzel lens, fl , must be 10.7 cm for imaging the entrance slit on the exit slit . The imaging ratio, eq . (5), is then 0.76, i .e ., slight demagnification occurs . The einzel lens was operated in the accel-decel mode in order the keep the spherical and chromatic aberrations sufficiently low. For a =14 cm, f1 =10.7 cm a voltage ratio of 1 .85 is required with the present construction, i.e ., with the ion source at +2 kV the lens voltage is -1 .7 kV.
The entrance slit height is 2 mm, the exit slit height being larger than the beam height, so that no part of the beam with a certain limited energy spread filling the entrance slit area and passing the 4 mm diameter einzel lens aperture is cut off. For the test measurements, the entrance and exit slits were both adjusted to a width of 50 ,am. With this value, a comparison of eqs . (4) and (5) predicts a mass resolution of 250.
4. Test results The mass spectrometer was tested using a beam of 2 keV Xe + ions from a unoplasmatron gun with a hollow cathode . Fig . 6a shows the Xe + spectrum as recorded by ramping the magnetic field . The isotopes are completely resolved, the fwhm resolution is about 250 . The energy spread AU of the ions from the plasma source is estimated to be about 4 eV, so that S = 2 X 10 -3 . In order to check the effectiveness of the energy focusing, an ac voltage was superimposed on the accelerating do voltage . This results in an additional artificial energy spread (fig. 6, inset) . Figs. 6b-e show the effect of increasing AU on the mass resolution. With DU= 20 eV (S =10 -Z ) no effect is noticeable. With AU= 40 eV (S = 2 X 10 -Z ) there is a slight deterioration, while with 4U=100 eV (S = 5 X 10 -2 ) and AU= 200 eV (S =10 -1 ) the mass resolution really shows increasing deterioration . l . MASS SPECTROMETERS/SEPARATORS
H. Debt / A compact doublefocusing mass spectrometer
326
Xe
6 = 10-2
dN du
ràù d rcut.ti i~ ni w
Fig. 7. Peak switching with the electric sector field. 132 M
r
6=2x10-2 (d)
6=5x10-2 (e)
6=10-
toi
,1ll P
M
Fig. 8. Xenon spectrum obtained with constant magnetic field by ramping the electric field. Fig. 6. Mass spectra of xenon ions from a unoplasmatron source (energy spread AU). In order to test the energy focusing, an ac voltage of increasing amplitude AU., (see inset) was superimposed on the acceleration voltage U to yield an artificial total energy spread AU,,,. One advantage of the field sequence chosen is that, for a limited relative mass range, electric peak switching with the electric sector field is possible (fig . 7) . The electric field gap is wide enough to accommodate ions leaving the magnetic field that cover a relative mass range of y = 0.1 . Most of the elemental isotopes can therefore be scanned or switched with constant magnetic field by ramping or stepping the deflection voltage of the cylindrical condenser . This is demonstrated in fig. 8, which shows the Xe + spectrum obtained with a constant magnetic field of 6000 G (the nominal value for the 130Xe isotope) by varying the electric field voltage from +5% to -5% of the nominal value.
5. Summary The compact mass spectrometer developed comes up to expectation. At an fwhm resolution of 260, it transmits ions filling an entrance slit 0.1 mmz in area and a solid angle of 6.4 X 10 -° sr and having a relative energy spread of 1% . The maximum product of mass and energy is 7 X 10 5 amu V. With a nominal ion energy of 2 keV and an energy spread of 20 eV, the mass range is therefore up to mass number 350. Fast electric peak switching is possible within a relative mass range of 10%. The authors wishes to thank H. Weiss and A. Schlamp for technical assistance and A. Klekamp for help with the test measurements .