Secondary ion mass spectrometry on the helium ion microscope:A feasibility study of ion extraction

David Dowsett,a) Tom Wirtz, Nico Vanhove, and Lex PillatschDepartment “Science and Analysis of Materials” (SAM), Centre de Recherche Public—Gabriel Lippmann,41 rue du Brill, L-4422 Belvaux, Luxembourg

Sybren Sijbrandij and John NotteCarl Zeiss NTS LLC, One Corporation Way, Peabody, Massachusetts 01960

(Received 28 June 2012; accepted 4 September 2012; published 21 September 2012)

The combination of the high-brightness Heþ/Neþ atomic level ion source with secondary ion

mass spectrometry detection capabilities opens up the prospect of obtaining chemical information

with high lateral resolution and high sensitivity on the Zeiss ORION helium ion microscope

(HIM). The analytical performance in terms of lateral resolution and sensitivity was investigated.

The effect of the secondary ion extraction field on the probe size of the HIM and the transmission

of the extraction system were studied using SIMION. Probe sizes <10 nm and sensitivities in the

ppm range are possible using a set of extraction electrodes consistent with the geometry of the

ORION instrument. VC 2012 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4754309]

I. INTRODUCTION

Dedicated nanometrology is crucial in order to support

the ongoing development of nanotechnology products and

processes in both material and life sciences. In recent years,

the ORION helium ion microscope has become a well-

established tool for high-resolution imaging and nanofabri-

cation.1 The ORION instrument is based on the atomic level

ion source (ALIS) gas field ion source, which has a bright-

ness of 4� 109 A cm�2 sr�1, leading to probe sizes of less

than 0.5 nm. The source is normally operated with helium

but has also been demonstrated to operate with neon2 for

accelerating voltages of 10–35 keV. While secondary elec-

trons are used for high-resolution high-contrast imaging, the

detection of backscattered helium or neon atoms can provide

only limited compositional information on well-selected

specimens.3 By contrast, secondary ion mass spectrometry

(SIMS) is an extremely powerful technique for analyzing

surfaces due to its excellent sensitivity, high dynamic range,

very high mass resolution, and ability to differentiate

between isotopes.4–6 Adding SIMS capability (extraction

optics, mass filter, and detector) to the ORION HIM opens

up the prospect of obtaining chemical information with a

high lateral resolution and a high sensitivity. Currently, the

lateral resolution in commercial SIMS instruments is limited

to approximately 50 nm due to aberrations in the ion column

and/or limited ion source brightness.7 Due to the small spot

size of the ALIS Heþ/Neþ ion source, its lateral resolution in

terms of SIMS is determined by the physical limit corre-

sponding to the lateral dimensions of the collision cascades

induced in the sample. Previous studies on the Heþ/Neþ

beam-sample interactions have focused on the stopping pro-

cess in the sample,8 the secondary electron (SE) emission

process,9 and the SE imaging resolution.10 Due to their low

mass and/or chemically inert character, Heþ or Neþ ions

have rarely been used as primary ion species for SIMS. Con-

sidering the small voxel that can be analyzed with the ALIS

Heþ/Neþ beam, special attention has to be paid to sensitivity

issues. Bernheim found similar useful yield for Heþ, Neþ,

and Arþ primary ions, which were some orders of magnitude

lower compared to the conventional cesium or oxygen

beams.11 However, the yields may be increased by up to sev-

eral orders of magnitude by using reactive gas flooding dur-

ing analysis, namely oxygen flooding for positive secondary

ions12 and cesium flooding for negative secondary ions.13–15

These fundamental aspects related to ionization mecha-

nisms were investigated in the first part of a feasibility study16

investigating the prospect of adding SIMS to the HIM. This

first study concluded that detection limits for silicon in the

10�6 and 10�5 range are possible for Neþ and Heþ bombard-

ment, respectively. Simulations using transport and range of

ion in matter (TRIM) also showed that secondary ions were

emitted from areas <10 nm (FW50). This implies that corre-

sponding lateral resolutions should be possible, albeit at

reduced sensitivity as there is a trade-off between achievable

lateral resolution and sensitivity. These initial fundamental

studies are very encouraging; however, further work needs to

be done to assess more practical instrumental concerns.

This study aims to extend the previous work and investi-

gate whether it is practically possible to perform SIMS on the

HIM. The practical limitations imposed by adding an extrac-

tion system for secondary ions to the HIM will be studied in

detail as the addition of an extraction field above the sample

will perturb the primary beam, potentially increasing the spot

size through aberrations. The transmission of any extraction

system will also have a significant influence on the sensitivity

of any subsequent mass spectrometer. These more practical

aspects are studied in detail here with a view to developing an

extraction system compatible with the geometry of the

ORION system while maintaining the highest levels of per-

formance. In particular, the aberrations in the primary beam

caused by the extraction field are studied by charged particlea)Electronic mail: dowsett@lippmann.lu

06F602-1 J. Vac. Sci. Technol. B 30(6), Nov/Dec 2012 2166-2746/2012/30(6)/06F602/7/$30.00 VC 2012 American Vacuum Society 06F602-1

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optics simulations, and transmissions of prospective extrac-

tion systems are investigated using a combination of TRIM

calculations and Monte Carlo techniques.

II. DESIGN OF EXTRACTION SYSTEM

On the HIM, SIMS would have to operate as a separate

mode from microscopy. The secondary ion extraction field

would, both in positive and negative secondary ion modes,

prevent any secondary electrons from reaching a SE detec-

tor. In the microscope mode, the extraction field would be

switched off, and the system operate as normal. In SIMS

mode, the extraction field would be switched on, and second-

ary ions extracted into the mass spectrometer. In the SIMS

mode, the total secondary ion current could be used to per-

form microscopy, but with reduced lateral resolution com-

pared with secondary electron images as the ions are emitted

from a larger area on the surface.

The geometry of the sample region of the ORION system

places several constraints on the design of the extraction sys-

tem. The number of available ports for mounting a mass

spectrometer on the ORION system is limited. The most

appropriate port is orientated at an angle of 65� with respect

to the primary beam. To prevent discharges and interference,

the nosecone of the extraction system must be grounded.

This also avoids the requirement of floating the mass spec-

trometer to achieve the required beam energy. Under these

conditions, the extraction field is provided by biasing the

sample. The sample holder is only insulated to 500 V; there-

fore, additional insulation is required for the 5 kV typically

required to extract secondary ions. Consistent with its use as

a microscope, any modification to the geometry of the sam-

ple holder should leave it as flexible as possible in terms of

sample mounting and positioning. The extraction field

should modify the primary beam as little as possible, both in

terms of position and spot size/shape. Any increase in spot

size will reduce the lateral resolution in SIMS mode and any

deflection will have to be corrected. The transmission of the

extraction system should be as high as possible (at least 10s

of percent) to ensure good sensitivity of the SIMS analysis.

Previous studies have shown that the transmission of typical

extraction systems decreases rapidly as the sample normal is

tilted away from the axis of the extraction system.17 To min-

imize this loss, the sample should be oriented toward the

extraction system.

By careful consideration of all these constraints, two

potential extraction geometries have been developed. The first

geometry, shown in Fig. 1, is based on a straight axis system

with rotational symmetry, consisting of an angled sample

stub, extraction electrode, and focussing lens. The axis of this

geometry coincides with the port axis at 65�. The tilted sam-

ple stub is necessary to ensure that the normal to the sample

points along the axis of the SIMS extraction to guarantee high

transmission. However, the tilted sample stub reduces the

flexibility of sample mounting and introduces an unwanted

image projection in the microscope mode. For these reasons,

a second more complex system (shown in Fig. 2) with a

curved axis is also considered. It consists of an extraction

electrode followed by two pairs of spherical sectors. A

grounded electrode above the spherical sectors screens the

nose cone of the ORION column. Secondary ions exit the sec-

ond pair of spherical sectors at the required 65�. A double

bend was used for two reasons: first, a single bend is not pos-

sible at the small radii required, and second, if the correct

combination of sector angles radii is chosen, the energy dis-

persion introduced by the system is minimized. As the axis of

the extraction field coincides with that of the ORION column,

a tilted sample stub is not required. Also, most of the electric

fields in this geometry are parallel to the axis of the primary

beam and therefore should not result in large deviations or

aberrations. However, the transmission of this geometry is

likely to be lower than that of the straight axis.

III. SIMULATION

A. Transmission

To determine the transmission of each extraction system,

Monte Carlo techniques were used to generate secondary

ions with appropriate angular and energy distributions. To

determine the appropriate angular and energy distributions

for sputtered matter, TRIM simulations have been carried

out. For both Heþ and Neþ bombardment, 105 impacts were

carried out at 30 keV on a bulk silicon target for incidence

angles of 0� and 65�. The resultant angular and energy

FIG. 1. (Color online) Monte Carlo simulation of 5000 ions flying through

straight axis extraction system. To orient the sample normal along the axis

of the extraction system, an angled sample stub must be used.

FIG. 2. (Color online) Monte Carlo simulation of 5000 ions flying through

curved axis extraction system. The system consists of a sample biased to

64500 V, grounded extraction electrode and two pairs of spherical sectors.

06F602-2 Dowsett et al.: Secondary ion mass spectrometry on the helium ion microscope 06F602-2

J. Vac. Sci. Technol. B, Vol. 30, No. 6, Nov/Dec 2012

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distributions of sputtered matter of 0� incidence are shown

in Figs. 3 and 4, respectively. The distribution of azimuthal

angles (not shown) was uniform (or extremely close) in all

cases and was therefore modeled as such in the Monte Carlo

simulations. The distribution of the polar angle may be fitted

with a function of the form

pðhÞ � cosnðh� h0Þ: (1)

The energy distribution of secondary ion is described by

the well known Sigmund-Thompson distribution.18,19 The

form used here is slightly modified to take into account the

threshold energy E0 present in the distributions from the

TRIM simulations

pðEÞ � ðE� E0ÞðE� E0 þ UÞ3

; (2)

where U is the surface binding energy. The fit parameters for

the different bombardment conditions are listed in Table I.

The exponents in angular distributions of material sputtered

with He and Ne are significantly higher than the typical values

of 1–2 reported for Gaþ, Csþ, and Oþ, resulting in a very nar-

row angular spread. The fit parameters were used to generate

corresponding distributions of secondary ions during charged

particle simulations.17,20 For each extraction system, bundles

of 5000 ions were generated using the angular and energy dis-

tributions. These ions were launched from the sample surface

at the point where the axis of the ORION intersected with the

sample surface and the ions flown through the system. The

number of ions reaching a detector electrode placed immedi-

ately after the extraction system was recorded.

For the curved axis system, a retarding potential can be

superimposed on the deflection voltages of the two pairs of

spherical sectors. This controls the deflection field to which

the primary beam is exposed as the required potential differ-

ence between each pair of spherical sectors is dependent on

the energy of the secondary ions in this region. To investi-

gate the effect on primary beam deflection and transmission,

a series of 5000 ion bundles was flown through the system

for retarding potentials from 0 to 3500 V.

B. Primary beam aberrations

To investigate potential effects on the primary beam, 105

ions with a 2D Gaussian spatial distribution were flown down

the axis of the HIM and through the extraction field at a pri-

mary beam energy of 30 keV and their XY positions in the

FIG. 3. (Color online) Angular distribution of particles sputtered from a sili-

con target by bombardment at 30 keV and normal incidence using (a) helium

and (b) neon.

FIG. 4. (Color online) Energy distribution of particles sputtered from a sili-

con target by bombardment at 30 keV and normal incidence using (a) helium

and (b) neon.

TABLE I. Fit coefficients for angular and energy distributions calculated from

TRIM for 0� and 65� primary ion impact angles.

Angle He Ne

0� n: 9.78 n: 9.00

h0: 29.8� h0: 30.4�

E0: 4.36 eV E0: 4.6 eV

U: 4.6 eV U: 4.7 eV

65� n: 9.40 n: 8.67

h0: 30.4� h0: 31.7�

E0: 4.5 eV E0: 4.6 eV

U: 5.1 eV U: 5.0 eV

06F602-3 Dowsett et al.: Secondary ion mass spectrometry on the helium ion microscope 06F602-3

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sample plane recorded first with the extraction field off

and then with on. The widths of the distribution were

rx¼ry¼ 0.26 nm for helium. The energy spread of the

source was modeled as a Gaussian with FWHM 1 eV. The

XY positions were used to determine the deflection of the pri-

mary beam from the axis of the ORION and the aberrations/

spot size increase. In each case, the intensity distribution on

the sample was fitted with an elliptical Gaussian of the form

Iðx; yÞ ¼ A exp �0:5x� x0

rx

� �2

þ y� y0

ry

� �2" # !

; (3)

where x0 and y0 are the beam deflections and rx and ry the

widths of the intensity distribution. Equation (3) was then

used to calculate the contour corresponding to A/2 which

was used to determine the FWHM in the x and y directions.

FIG. 5. Intensity distribution on the sample mounted on a tilted stub at 65� with respect to the incident beam for a 30 keV He primary beam. (a) Straight axis

extraction field off, spot size (FWHM) 1.45� 0.6 nm. (b) Straight axis extraction field on (4.5 kV), spot size (FWHM) 110� 0.6 nm. The contour lines corre-

spond to 75%, 50%, and 25% of the maximum intensity of the fitted distribution.

06F602-4 Dowsett et al.: Secondary ion mass spectrometry on the helium ion microscope 06F602-4

J. Vac. Sci. Technol. B, Vol. 30, No. 6, Nov/Dec 2012

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IV. RESULTS AND DISCUSSION

A. Straight axis

After optimization of the extractor-lens voltage, the trans-

missions of the straight axis system were 94.4% and 93.6%

for Ne and He bombardment, respectively. The slight differ-

ence in transmission between He and Ne bombardment

arises from the differences in angular and energy distribution

of the sputtered secondary ions. These values are slightly

higher than the transmission of the extraction system of the

Cameca IMS-4f.20 As this instrument was the basis of the

sensitivity calculations in Ref. 16 detection limits of 10�5

and 10�6 for silicon using He and Ne, respectively, are

indeed achievable for a mass spectrometer with this extrac-

tion geometry.

However, this type of extraction system introduces both a

large deflection (2 mm) and as Fig. 5 shows a large distortion

in spot shape as a result of the primary beam traversing the

extraction field at an angle [the ellipticity in Fig. 5(a) is a

result of the projection of the beam onto the tilted sample

stub]. The spot size in the deflection direction is over

100 nm. Without correction, this would seriously limit the

lateral resolution achievable during SIMS. The aspect ratio

of the spot on the sample is 183:1. While Oral and Lencov�ahave shown that it is possible to correct for such distortions21

when the aspect ratio is as large as 14:1, it requires signifi-

cantly larger voltages than are typically available to be

applied to the sigmator corrector electrodes and refocusing

of the objective lens. Correction of an aspect ratio of 183:1

would certainly require modification of the ORION sigma-

tor/deflector system. Therefore, this geometry seems unsuit-

able for use on the HIM.

B. Curved axis

The beam deflections and secondary ion transmissions of

the curved axis geometry are shown in Fig. 6. The deflection

of the primary beam can be minimized to 220 lm while

retaining the maximum transmission of 80% if a retarding

voltage of 2.5 kV is applied to the spherical sectors. While

this deflection is slightly larger than is currently correctable

on the HIM, only a modest increase in the deflector voltages

would be required for complete correction. In fact, correc-

tion of the beam deflection is not strictly necessary in this

case. For a given retarding potential on the spherical sectors,

the beam deflection is known; thus, the sample can be shifted

mechanically by a corresponding amount to reposition the

area of interest under the beam. Alternatively, a scanning ion

microscopy mode using the total secondary ion current could

be used for sample navigation. The acceptance of the extrac-

tion geometry is such that the origin of secondary ions being

220 lm away from the axis of the system does not signifi-

cantly affect the transmission.

While the transmission of this system is slightly lower

than that of the straight axis geometry and the IMS-4f, the

effect on detection limits is minimal (they increase by a fac-

tor of 1.125). Thus, detection limits in the ppm range are still

possible for silicon using neon bombardment.

The intensity distribution in Fig. 7 shows that this geome-

try introduces some aberrations into the beam; however, the

uncorrected spot size is still smaller than the dimensions of

the collision cascade (�10 nm). As the aspect ratio of the

spot is only 13:1, the correction of the spot shape should be

possible at a small increase in spot size. A corrected probe

size of <2 nm should be possible following the scheme of

FIG. 6. (Color online) Primary beam deflection and transmission through the curved axis extraction system.

06F602-5 Dowsett et al.: Secondary ion mass spectrometry on the helium ion microscope 06F602-5

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Oral and Lencov�a. The resolution limiting factor in either

case would be the dimensions of the collision cascade rather

than the probe size.

V. SUMMARY AND CONCLUSIONS

SIMS is practically feasible on the ORION helium ion

microscope. While the geometry of the ORION system

imposes many constraints on the design of the extraction

system, the curved axis system proposed here is probably

close to optimal for an instrument based on the ALIS source

optimized for high lateral resolution SIMS. This is because

the primary and secondary ion beams are very close to shar-

ing the same axis. Thus, the extraction field has minimal

effect on the primary beam in terms of introducing aberra-

tions, and the secondary ions are extracted with high effi-

ciency. Using this geometry, detection limits in the ppm

range are possible for silicon due to its high transmission.

FIG. 7. Intensity distribution on the sample for a 30 keV He primary beam. (a) Curved axis extraction field off, spot size (FWHM) 0.6� 0.6 nm. (b) Curved

axis extraction field on (Extractor 4.5 kV, retarding voltage 2.5 kV), spot size (FWHM) 7.9� 0.6 nm. The contour lines correspond to 75%, 50%, and 25% of

the maximum intensity of the fitted distribution.

06F602-6 Dowsett et al.: Secondary ion mass spectrometry on the helium ion microscope 06F602-6

J. Vac. Sci. Technol. B, Vol. 30, No. 6, Nov/Dec 2012

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Lateral resolutions of <10 nm are also possible. The extrac-

tion field introduces some aberration into the He/Ne beam at

30 keV; however, correction schemes exist for removing

such aberrations with only minimal effect on the final spot

size. The deflection of the primary beam in the sample plane

can be reduced to 220 lm by adjusting the retarding potential

superimposed on the deflection voltages of the two pairs of

spherical sectors without reducing the transmission. While

this deflection is currently beyond the range that can be

directly corrected by the HIM’s deflection system, it could

be simply corrected either by shifting the sample position

between the microscope and SIMS mode or increasing the

voltage output by the ORION scan unit.

ACKNOWLEDGMENT

This work was supported by the National Research Fund,

Luxembourg (C10/MS/801311).

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