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A large current scanning electron microscope with MEMS-basedmulti-beam optics

Takashi Ichimura ⇑ ,1 , Yan Ren, P. KruitFaculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

a r t i c l e i n f o

Article history:Received 24 June 2013Accepted 16 July 2013Available online 25 July 2013

Keywords:MEMS electron opticsMulti-electron beamScanning electron microscope

a b s t r a c t

Recently a multi-beam scanning electron microscope (MBSEM) has been developed, which delivers 196focused beams to a sample, each of which has around 1 nA. In this article a design for an optical system isdescribed and analyzed which can focus all these beams onto a single spot, using an array of micro elec-tron lenses. Although each individual micro lens will be of lower quality than a single macro objectivelens, a system is obtained with larger beam current than the conventional SEM. The goal set in an exam-ple design is to focus a total current of 200 nA within 50 nm at a landing energy of 500 eV.

Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction

Moore’s law has reached the point where sub-50 nm featuresare routinely produced both in laboratories and semiconductorfabs. To inspect such small structures, light based systems cannotalways be used. Electron microscopes can easily provide the highresolution, but the current in a single beam, and thus the through-put, is limited due to the availablebrightness of the electron sourceand the presence of aberrations of the lenses. Another limitation insingle beam systems is the broadening by stochastic Coulombinteractions. The limitation by brightness and aberrations can di-rectly be derived from the denition of (reduced) source brightnessBr as expressed in terms of the angular current density I X and thevirtual source size dv, given by

Br ¼I X

p4 d2

v V beam ð1Þwhere V beam is the beam potential. As the electron optical column

demagnies the virtual source down to a geometrical spot size dsam-ple as perceived in the sample plane, the current in the electronbeam at the sample is given by

I ¼Br Áp 2

4d2

sample a 2sample V beam ð2Þ

The beam potential V beam is determined by requirements of theinspection process, such as the necessity to have a secondaryelectron emission coefcient of one. The spot size dsample has a

maximum determined by the required resolution. The half-open-ing angle a sample is limited by the spherical and chromatic aberra-tion contribution of the electron optical system to the total spotsize. For large currents it is usually the spherical aberration thatdominates:

ds ¼0:18 ÁC sa 3 ð3Þwith C s the spherical aberration coefcient of the system, usuallyequal to the spherical aberration of the last lens in the system.

One way of increasing the current in an inspection machine is touse multiple beams. A multi beam system usually has separatebeams, which makes it necessary to use multiple detectors: oneper beam. This is a practical problem, which we try to circumventin the solution proposed in this paper. We will look at the possibil-ity of focusing multiple beams in a single point on the sample anddetect the signal with one detector. Fig. 1 shows the basic cong-uration of our large current system. In the basic conguration,there are an electron source, 2 micro lens arrays, 1 macro lens, asample and a detector. The rst micro lens array is used to createthe multiple beamlets from a single source and focus each beamleton the macro lens. The macro lens is used to change the directionof all beamlets towards a single point on the sample. The secondmicro lens array is used to focus the beamlets on the sample. Be-cause the plane of the macrolens is conjugate with the sample,the aberration has no inuence on the spot size. The only effectof the macrolens aberration is to shift the beamlet in the secondmicrolens array, but the microlens will still focus all electrons inthe same point on the sample [1] .

Recently a multi-beam scanning electron microscope (MBSEM)has been developed, which delivers 196 focused beams (array of 14 Â 14), each of which has around 1 nA [2–5] . Of course this is amore complicated system than sketched in Fig. 1 , but it is also

0167-9317/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.mee.2013.07.008

⇑ Corresponding author. Tel./fax: +81 29 276 6353.E-mail address: ichimura-takashi@naka.hitachi-hitec.com (T. Ichimura).

1 On leave from Research and Development Div, Hitachi High-Technologies Co.,Ibaraki 312–8504, Japan.

Microelectronic Engineering 113 (2014) 109–113

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more exible and gives a good experimental platform to try thenew idea. The key challenge is to focus all beams that exit fromthe last lens into a single spot, using an array of electron lenses.Our goal is to focus a total current of 200 nA within 50 nm at alanding energy of 500 eV. The objective of this article is to furtheranalyze the optics of the new multi-beam-single-probe system andto describe the electron optical design of the required micro lensarray (MLA) for the experimental set-up.

2. Electron optics of multi-beam-single-probe system

In an optimized system, the size of the aberration disc is aboutequal to the size of the geometrical image of the source:

dgeo ¼c 1 ÁC sa3 ð4Þwhere c 1 is a factor depending on the defocus and choice of size

denition (e.g. full width 50%, full width half maximum). The res-olution of the microscope is equal to the total spot size. Thus, theaccepted angle at the sample is limited to

a 6dgeo

c 1 ÁC s 1=3

ð5ÞThe current in the spot is proportional to a 2 .In the system of Fig. 1 , the same restriction applies for the angle

in the microlenses. However, at rst sight it seems that we can

make this angle arbitrarily small and compensate for the loss of current by increasing the number of beams. But this is not true:if the microlens size is too small, the spherical aberration of themacrolens may deect the outer beamlets out of the correspondingmicrolens altogether. Let us call the half full acceptance angle of the system at the sample b and the diameter of the microlensopening dm . If we allow the spherical aberration of the macrolensto shift the most outer beamlet a fraction c 2 of the microlens diam-eter, then

C smacro b3

¼c 2 Ádm ð6Þb 6 ð

c 2 Ádm

C smacro Þ1=3

ð7Þwhere we assume, for simplicity, the distance from the macrolensto the microlens array to be about equal to the distance from the

macrolens to thesample as fromestimation object distance is muchlarger than the image distance, see Fig. 3 .

In order to have the maximumcurrent in the spot, we will try toll the microlenses as fully as possible, thus there is a relation be-tween a micro , dm and f m , the focal distance of the microlens:

dm ¼2 f ma ð8ÞThe maximum d

mis thus determined by the spherical aberra-

tion of the microlens:

dm ¼2 f mdgeo

c 1 ÁC smicro 1=3

ð9ÞFor single aperture lenses there is the following scaling relation

between lens size, focal distance and spherical aberration [ref: EODsimulation program]:

C smicro ¼c 3 f 2mdm ð10Þ

With that, we nd

dm ¼8dgeo Á f m

c 1Ác 3

1=2

ð11 ÞCombining this with the expression for the full acceptance an-

gle, we nd a new limitation:

b 6c 2 Á ffiffiffiffiffiffiffiffiffiffi8dgeo Á f mp ffiffiffiffiffiffiffiffiffiffic 1 Ác 3p ÁC smacro !

1=3

ð12ÞSo if we compare the single beam system with the new multi-

beam-single-spot systemand assuming that we can make a macro-lens with the same spherical aberration as in the single beam sys-tem, the gain in current is

bnew

bold 2

¼8c 1 c 22 f mc 3 dgeo

1=3

ð13ÞThe constant c 2 is a choice and should be about 0.25. c 1 for the

FW 50 size is 0.18 and c 3 is 1.0 [ref]. For a focal distance fm = 2 mmand a resolution of 50 nm, this could lead to a gain in current of 15(reduced to about 10 because the microlens array is not 100%open). We could add more microlenses which would only be par-tially lled, we could try to design for a larger fm, get the precisionto make it work for dgeo = 25 nm and with all that reach anotherfactor of 2 extra current. A nal remark in this analysis must bethat it is possible, in principle, to correct for the spherical aberra-tion of the macrolens because an electrode can be placed insidethe lens with only small holes to transmit the focused electronbeamlets.

3. Multi beam SEM

The MBSEM on which we will perform the initial experimentsconsists of three subsystems [2,3] : the multi-electron beam source(MBS), a commercial SEM column, and the micro lens array (MLA)systemthat we need to design. Fig. 2 shows a schematic drawing of the electron optical system of the MBSEM.

In the MBS system the standard source module is replaced toproduce 196 beamlets (array of 14 Â 14). The emission from a highbrightness Schottky emitter is split into an array by an aperturelens array (ALA). This ALA consists of a thin Si membrane withapertures of 18 l m diameter at 25 l m pitch. Two macro electrodesare combined with the extractor electrode of the electron sourceand create a so called ‘‘zero-strength lens’’, which means that theoff-axis beams are avoiding the problemassociated with chromatic

deection errors. The eld from the macro electrodes ends on theALA, forming low aberration single micro aperture lenses for the

Fig. 1. Basic conguration of large current system.

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beamlets which focus the beams on the principle plane of theaccelerating (ACC) lens. The MBS creates an array of focused beamsin the principle plane of ACC lens with a geometrical probe size of 95 nm at a pitch of 70 l m. For a landing energy of 1500 eV in theACC plane, the total current delivered by the MBS is around 1 nAper a beamlet.

The commercial SEM column consists of a condenser ( C 2 ) lens, avariable aperture (VA), the intermediate(INT)lens andthe high res-olution(HR)/the magnetic immersion ultra-high resolution(UHR)objective lens. The C 2 lens images the common cross-over of theACC lens onto the VA. The VA acts as a current limiting apertureand determines the aperture angle for all beams. By changing thestrength of theACC lens and C 2 lens, the magnication of the systemcan be changed. Further demagnication is done by the INT andeitherthe HRlens orUHR lens. The design principles of theMBS sys-temand the commercial SEM column have been described in detailelsewhere [2] .

To achieve our goal (200 nA within the 50 nm at the landingenergy of 500 eV) we will direct all beamlets towards a single spoton the sample at the same time that each beamlet is focused by amicrolens.As wesawin thebasic congurationin Fig. 1 , a macrolensis needed to direct all beamlets towards a single point. Here wedecided not to use the UHR lens because of the complications of itsmagnetic eld, which extends all the way to the sample. In suchan immersion lens, secondary electron detection can only be donethrough-the-lens, which will be difcult with the MLA in the way.In principle the HR lens can then be used, which is inside the SEMcolumn and keeps the MLA region eld free. There is a mechanicallimitation, however, in the present SEM column. The minimumworking distance between the HR lens and the sample is about40 mm, while the diameter of the lens opening is only 2 mm. Thislimits the maximum angle at the sample to about 25 mrad, whilewewantto go to largervalues.So thedesignfor theMLAwillinclude

an electrostatic macrolens to replace the magnetic HR lens, givingthe opportunity to also decelerate the electrons inside this lens.Fig. 3 shows a schematic overview of the proposed MLA system.

4. The micro lens array (MLA) rst order design

The system is aiming for a total current of 200 nA within 50 nmat a landing energy of 500 eV. In order to avoid Coulomb interac-tions in the SEM we will transport the beams at 10 kV and deceler-ate in the MLA to 500 V, possibly in two steps. The SEM will focusthe beamlets in the plane of the macrolens. Since this plane is animage of the principle plane of the accelerating lens in the sourcesubsystem, the ratio between pitch and geometrical probe size isconserved at

P macro : dmacro ¼70 l m : 95 nm : ð14ÞOur design will consist of an electrostatic macro lens, an array

of single aperture microlenses and two macro electrodes whichsupply the electrostatic eld on the apertures and distribute thiseld such that the curvature of eld is corrected [4] . When we de-note the pitch size of the MLA as P MLA, the distance between thenew electrostatic macrolens and the MLA lo, the distance betweenthe MLA and the sample li, we can write a relation between thepitch size of the beamlets in the macro objective lens and the pitchof the MLA:

P macro ¼P MLA Âlo þ li

li ð15ÞThe magnication of the microlenses is given by

M ¼ ffiffiffiffiffiffiffiffiffiV 500r Á

lilo

: ð16Þwhere V is the electron energy with which the electrons emerge

from the macro lens.The spot size of the beamlets in the macro objective lens is then

dmacro ¼dgeo

M : ð17ÞFrom the requirement we have set ourselves (200 nA, 50 nm,

500 V), we can calculate the sample angle b . Let us take Br = 5 -Â 10 7 A/m 2 srV, and a 50% lling of the MLA, then

b ¼ ffiffiffiffiffiffiffiffiffiffiI 0:5 ÁBr Ápd2

sample V beams ¼45mrad : ð18ÞBecause it is a square array, the maximum angle will be

64 mrad. For the choice li = 2.5mm and lo = 10 mm, we ndP macro = 87 micron, P MLA = 17 micron. Because of the given relation

between pitch and geometric size, dgeo in the macrolens is 120 nm.Thus M = 0.42 from which it follows that

Fig. 3. Schematic drawing of Micro lens array system.

Fig. 2. Schematic drawing of multi beam SEM with the proposed MLA system.

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Fig. 4. The electrical eld in front of MLA (shownin purple). Thesample is on theright (gray). Thedecelerating macrolens is also shown. (a) Theon-axis beam fromthe macrolens area is focused on the sample. (b) The trajectories of the off-axis beamlets are far apart in the macro lens and come to a common cross-over on the sample.

Fig. 4 (continued )

Fig. 4 (continued )

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V ¼500 ÁM Áloli

2

¼1400 V : ð19ÞThus, the macrolens needs to be a decelerating lens taking the

electron energy from 10 to 1.4 kV. The microlenses take the energyfrom 1400 to 1500 V.

The eld at the single aperture lenses follows from

f %li ¼4 ÁV finalE : ð20Þ

So the eld is 800 V/mm and the distance between the macroelectrode and the aperture array is in the order of 1 mm, ttingvery well with the other dimensions.

With our analysis in Section 2, we can check if the lens aberra-tions will allow this design, Eq. (11) only needs a value for thespherical aberration coefcient of the electrostatic macrolens. Letus take twice the focal distance, that is 25 mm. Then, withdgeo = 50nm, we nd b 6 78 mrad while the design employs64 mrad. The microlenses have a pitch of 17 l m, so a maximumdiameter of 15 l m and a focal length of 2.5 mm. This would givea microlens spherical aberration of 1.2 m. Eq. (5) yields the maxi-mum aperture angle for the microlens: a 6 6 mrad while the de-

sign employs 3 mrad. From these estimates we conclude that ourdesign will not be limited by the spherical aberrations of thelenses.

5. Simulation

Fig. 4 shows the MLA system conguration as modeled by theEOD simulation program. It is composed of a decelerating macrolens, two macroelectrodes, the MLA and the sample. By manipulat-ing the shape of the eld in front of the MLA, creating the ‘‘zerostrength macro lens’’ and forming single aperture micro lensessimultaneously, it is possible to fulll all requirements. For thesimulation of the beamlet focusing by the micro lens, the MLAplate has an aperture on axis to create the micro lens. The trajecto-

ries for this situation are shown in Fig. 4 a. For the simulation of thecenter trajectories of the off-axis beams (which are not inuenced

by the micro lenses), the MLA plate in the simulation has no aper-ture, thus no lens effect. These trajectories are shown in Fig. 4 b.

6. Conclusion and discussion

It should be possible to obtain substantially more current in theprobe of an electron beam inspection system when the electron

beam from the source is rst split up in many beamlets whichare transported through the system separately and brought to-gether again in new optical conguration as described in this pa-per. As an example, we have designed an optical system to bemounted in an existing multi beam SEM, which delivers 196 fo-cused beams (array of 14 Â 14) to a sample, each of which hasaround 1 nA. The results of a computer simulation seem to supportthe promise that we can fabricate a system that delivers a totalcurrent of 200 nA within 50 nm at a landing energy of 500 eV.

We still have to perform a more detailed aberration analysisand design a mechanical construction. One of the challenges is toposition the single aperture lenses exactly around the many beamsbecause any displacement of an individual lens is reected in theposition of that beam in the nal probe. This requires either veryaccurate manufacturing or a way to individually deect the beam-lets towards the nal common probe. The presence of the microlens array will also make scanning anddetection of secondary elec-trons more difcult.

In conclusion: The proposed system opens a new direction to-wards higher throughput electron beam inspection, but there arestill many practical difculties to overcome.

References

[1] B. van Someren, P. Kruit, Method and apparatus for imaging, WO 2008/002132A1.

[2] A. Mohammadi-Gheidari, C.W. Hagen, P. Kruit, JVST B 28 (6) (2010). C6G5–C6G10 .

[3] A. Mohammadi-Gheidari, P. Kruit, Nucl. Instr. Meth. A645 (2011) 60–67 .[4] Y. Zhang, P. Kruit, JVST B 25 (6) (2007) 2239–2244 .

[5] M.J. van Bruggen, Multi-electron beam system for high resolution electronbeam induced deposition, PhD thesis, Delft University, 18th February, 2008.

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