- Email: [email protected]
A large current scanning electron microscope with MEMS-based multi-beam optics
Microelectronic Engineering 113 (2014) 109–113
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A large current scanning electron microscope with MEMS-based multi-beam optics Takashi Ichimura ⇑,1, Yan Ren, P. Kruit Faculty 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 2013 Accepted 16 July 2013 Available online 25 July 2013 Keywords: MEMS electron optics Multi-electron beam Scanning electron microscope
a b s t r a c t Recently a multi-beam scanning electron microscope (MBSEM) has been developed, which delivers 196 focused beams to a sample, each of which has around 1 nA. In this article a design for an optical system is described and analyzed which can focus all these beams onto a single spot, using an array of micro electron lenses. Although each individual micro lens will be of lower quality than a single macro objective lens, a system is obtained with larger beam current than the conventional SEM. The goal set in an example 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 features are routinely produced both in laboratories and semiconductor fabs. To inspect such small structures, light based systems cannot always be used. Electron microscopes can easily provide the high resolution, but the current in a single beam, and thus the throughput, is limited due to the available brightness of the electron source and the presence of aberrations of the lenses. Another limitation in single beam systems is the broadening by stochastic Coulomb interactions. The limitation by brightness and aberrations can directly be derived from the definition of (reduced) source brightness Br as expressed in terms of the angular current density IX and the virtual source size dv, given by
Br ¼
IX p d2 V 4
v
ð1Þ
beam
where Vbeam is the beam potential. As the electron optical column demagnifies the virtual source down to a geometrical spot size dsample as perceived in the sample plane, the current in the electron beam at the sample is given by
I ¼ Br
p2 4
2
dsample a2sample V beam
ð2Þ
The beam potential Vbeam is determined by requirements of the inspection process, such as the necessity to have a secondary electron emission coefficient of one. The spot size dsample has a ⇑ Corresponding author. Tel./fax: +81 29 276 6353. E-mail address: [email protected] (T. Ichimura). On leave from Research and Development Div, Hitachi High-Technologies Co., Ibaraki 312–8504, Japan. 1
maximum determined by the required resolution. The half-opening angle asample is limited by the spherical and chromatic aberration contribution of the electron optical system to the total spot size. For large currents it is usually the spherical aberration that dominates:
ds ¼ 0:18 C s a3
ð3Þ
with Cs the spherical aberration coefficient of the system, usually equal to the spherical aberration of the last lens in the system. One way of increasing the current in an inspection machine is to use multiple beams. A multi beam system usually has separate beams, which makes it necessary to use multiple detectors: one per beam. This is a practical problem, which we try to circumvent in the solution proposed in this paper. We will look at the possibility of focusing multiple beams in a single point on the sample and detect the signal with one detector. Fig. 1 shows the basic configuration of our large current system. In the basic configuration, there are an electron source, 2 micro lens arrays, 1 macro lens, a sample and a detector. The first micro lens array is used to create the multiple beamlets from a single source and focus each beamlet on the macro lens. The macro lens is used to change the direction of all beamlets towards a single point on the sample. The second micro lens array is used to focus the beamlets on the sample. Because the plane of the macrolens is conjugate with the sample, the aberration has no influence on the spot size. The only effect of the macrolens aberration is to shift the beamlet in the second microlens array, but the microlens will still focus all electrons in the 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 14), each of which has around 1 nA [2–5]. Of course this is a more 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
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T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
macrolens to the sample as from estimation object distance is much larger than the image distance, see Fig. 3. In order to have the maximum current in the spot, we will try to fill the microlenses as fully as possible, thus there is a relation between amicro, dm and fm, the focal distance of the microlens:
dm ¼ 2f m a
ð8Þ
The maximum dm is thus determined by the spherical aberration of the microlens:
dm ¼ 2f m
dgeo c1 C smicro
1=3 ð9Þ
For single aperture lenses there is the following scaling relation between lens size, focal distance and spherical aberration [ref: EOD simulation program]:
C smicro ¼ c3
fm2 dm
ð10Þ
With that, we find
dm ¼ Fig. 1. Basic configuration of large current system.
more flexible and gives a good experimental platform to try the new idea. The key challenge is to focus all beams that exit from the 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 a landing energy of 500 eV. The objective of this article is to further analyze the optics of the new multi-beam-single-probe system and to describe the electron optical design of the required micro lens array (MLA) for the experimental set-up.
In an optimized system, the size of the aberration disc is about equal to the size of the geometrical image of the source:
ð4Þ
where c1 is a factor depending on the defocus and choice of size definition (e.g. full width 50%, full width half maximum). The resolution of the microscope is equal to the total spot size. Thus, the accepted angle at the sample is limited to
a6
dgeo c1 C s
1=3 ð5Þ
The current in the spot is proportional to a2. In the system of Fig. 1, the same restriction applies for the angle in the microlenses. However, at first 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 the macrolens may deflect the outer beamlets out of the corresponding microlens altogether. Let us call the half full acceptance angle of the system at the sample b and the diameter of the microlens opening dm. If we allow the spherical aberration of the macrolens to shift the most outer beamlet a fraction c2 of the microlens diameter, then
C smacro b3 ¼ c2 dm c2 dm b6ð Þ C smacro
ð6Þ
1=3
ð7Þ
where we assume, for simplicity, the distance from the macrolens to the microlens array to be about equal to the distance from the
ð11Þ
Combining this with the expression for the full acceptance angle, we find a new limitation:
b6
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !1=3 c2 8dgeo fm pffiffiffiffiffiffiffiffiffiffiffiffiffi c1 c3 C smacro
ð12Þ
So if we compare the single beam system with the new multibeam-single-spot system and assuming that we can make a macrolens with the same spherical aberration as in the single beam system, the gain in current is
2 1=3 bnew 8c1 c22 fm ¼ bold c3 dgeo
2. Electron optics of multi-beam-single-probe system
dgeo ¼ c1 C s a3
1=2 8dgeo fm c1 c3
ð13Þ
The constant c2 is a choice and should be about 0.25. c1 for the FW 50 size is 0.18 and c3 is 1.0 [ref]. For a focal distance fm = 2 mm and 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 partially filled, we could try to design for a larger fm, get the precision to make it work for dgeo = 25 nm and with all that reach another factor of 2 extra current. A final remark in this analysis must be that it is possible, in principle, to correct for the spherical aberration of the macrolens because an electrode can be placed inside the lens with only small holes to transmit the focused electron beamlets. 3. Multi beam SEM The MBSEM on which we will perform the initial experiments consists of three subsystems [2,3]: the multi-electron beam source (MBS), a commercial SEM column, and the micro lens array (MLA) system that 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 to produce 196 beamlets (array of 14 14). The emission from a high brightness Schottky emitter is split into an array by an aperture lens array (ALA). This ALA consists of a thin Si membrane with apertures of 18 lm diameter at 25 lm pitch. Two macro electrodes are combined with the extractor electrode of the electron source and create a so called ‘‘zero-strength lens’’, which means that the off-axis beams are avoiding the problem associated with chromatic deflection errors. The field from the macro electrodes ends on the ALA, forming low aberration single micro aperture lenses for the
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T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
beamlets which focus the beams on the principle plane of the accelerating (ACC) lens. The MBS creates an array of focused beams in the principle plane of ACC lens with a geometrical probe size of 95 nm at a pitch of 70 lm. For a landing energy of 1500 eV in the ACC plane, the total current delivered by the MBS is around 1 nA per a beamlet. The commercial SEM column consists of a condenser (C2) lens, a variable aperture (VA), the intermediate (INT) lens and the high resolution(HR)/the magnetic immersion ultra-high resolution(UHR) objective lens. The C2 lens images the common cross-over of the ACC lens onto the VA. The VA acts as a current limiting aperture and determines the aperture angle for all beams. By changing the strength of the ACC lens and C2 lens, the magnification of the system can be changed. Further demagnification is done by the INT and either the HR lens or UHR lens. The design principles of the MBS system and the commercial SEM column have been described in detail elsewhere [2]. To achieve our goal (200 nA within the 50 nm at the landing energy of 500 eV) we will direct all beamlets towards a single spot on the sample at the same time that each beamlet is focused by a microlens. As we saw in the basic configuration in Fig. 1, a macro lens is needed to direct all beamlets towards a single point. Here we decided not to use the UHR lens because of the complications of its magnetic field, which extends all the way to the sample. In such an immersion lens, secondary electron detection can only be done through-the-lens, which will be difficult with the MLA in the way. In principle the HR lens can then be used, which is inside the SEM column and keeps the MLA region field free. There is a mechanical limitation, however, in the present SEM column. The minimum working distance between the HR lens and the sample is about 40 mm, while the diameter of the lens opening is only 2 mm. This limits the maximum angle at the sample to about 25 mrad, while we want to go to larger values. So the design for the MLA will include
Fig. 3. Schematic drawing of Micro lens array system.
an electrostatic macrolens to replace the magnetic HR lens, giving the 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) first order design The system is aiming for a total current of 200 nA within 50 nm at a landing energy of 500 eV. In order to avoid Coulomb interactions in the SEM we will transport the beams at 10 kV and decelerate in the MLA to 500 V, possibly in two steps. The SEM will focus the beamlets in the plane of the macrolens. Since this plane is an image of the principle plane of the accelerating lens in the source subsystem, the ratio between pitch and geometrical probe size is conserved at
Pmacro : dmacro ¼ 70 lm : 95 nm:
ð14Þ
Our design will consist of an electrostatic macro lens, an array of single aperture microlenses and two macro electrodes which supply the electrostatic field on the apertures and distribute this field such that the curvature of field is corrected [4]. When we denote the pitch size of the MLA as PMLA, the distance between the new electrostatic macrolens and the MLA lo, the distance between the MLA and the sample li, we can write a relation between the pitch size of the beamlets in the macro objective lens and the pitch of the MLA:
Pmacro ¼ PMLA
lo þ li li
ð15Þ
The magnification of the microlenses is given by
rffiffiffiffiffiffiffiffiffi V li M¼ : 500 lo
ð16Þ
where 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 ¼
dgeo : M
ð17Þ
From the requirement we have set ourselves (200 nA, 50 nm, 500 V), we can calculate the sample angle b. Let us take Br = 5 107 A/m2srV, and a 50% filling of the MLA, then
b¼
Fig. 2. Schematic drawing of multi beam SEM with the proposed MLA system.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I 2
0:5 Br pdsample V beam
¼ 45mrad:
ð18Þ
Because it is a square array, the maximum angle will be 64 mrad. For the choice li = 2.5 mm and lo = 10 mm, we find Pmacro = 87 micron, PMLA = 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
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T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
Fig. 4. The electrical field in front of MLA (shown in purple). The sample is on the right (gray). The decelerating macrolens is also shown. (a) The on-axis beam from the macro lens 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)
T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
2 lo V ¼ 500 M ¼ 1400 V: li
ð19Þ
Thus, the macrolens needs to be a decelerating lens taking the electron energy from 10 to 1.4 kV. The microlenses take the energy from 1400 to 1500 V. The field at the single aperture lenses follows from
f li ¼
4 V final : E
ð20Þ
So the field is 800 V/mm and the distance between the macro electrode and the aperture array is in the order of 1 mm, fitting very well with the other dimensions. With our analysis in Section 2, we can check if the lens aberrations will allow this design, Eq. (11) only needs a value for the spherical aberration coefficient of the electrostatic macrolens. Let us take twice the focal distance, that is 25 mm. Then, with dgeo = 50 nm, we find b 6 78 mrad while the design employs 64 mrad. The microlenses have a pitch of 17 lm, so a maximum diameter of 15 lm and a focal length of 2.5 mm. This would give a microlens spherical aberration of 1.2 m. Eq. (5) yields the maximum aperture angle for the microlens: a 6 6 mrad while the design employs 3 mrad. From these estimates we conclude that our design will not be limited by the spherical aberrations of the lenses. 5. Simulation Fig. 4 shows the MLA system configuration as modeled by the EOD simulation program. It is composed of a decelerating macro lens, two macro electrodes, the MLA and the sample. By manipulating the shape of the field in front of the MLA, creating the ‘‘zero strength macro lens’’ and forming single aperture micro lenses simultaneously, it is possible to fulfill all requirements. For the simulation of the beamlet focusing by the micro lens, the MLA plate has an aperture on axis to create the micro lens. The trajectories for this situation are shown in Fig. 4a. For the simulation of the center trajectories of the off-axis beams (which are not influenced
113
by the micro lenses), the MLA plate in the simulation has no aperture, thus no lens effect. These trajectories are shown in Fig. 4b. 6. Conclusion and discussion It should be possible to obtain substantially more current in the probe of an electron beam inspection system when the electron beam from the source is first split up in many beamlets which are transported through the system separately and brought together again in new optical configuration as described in this paper. As an example, we have designed an optical system to be mounted in an existing multi beam SEM, which delivers 196 focused beams (array of 14 14) to a sample, each of which has around 1 nA. The results of a computer simulation seem to support the promise that we can fabricate a system that delivers a total current of 200 nA within 50 nm at a landing energy of 500 eV. We still have to perform a more detailed aberration analysis and design a mechanical construction. One of the challenges is to position the single aperture lenses exactly around the many beams because any displacement of an individual lens is reflected in the position of that beam in the final probe. This requires either very accurate manufacturing or a way to individually deflect the beamlets towards the final common probe. The presence of the micro lens array will also make scanning and detection of secondary electrons more difficult. In conclusion: The proposed system opens a new direction towards higher throughput electron beam inspection, but there are still many practical difficulties to overcome. References [1] B. van Someren, P. Kruit, Method and apparatus for imaging, WO 2008/002132 A1. [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 electron beam induced deposition, PhD thesis, Delft University, 18th February, 2008.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A large current scanning electron microscope with MEMS-based multi-beam optics Takashi Ichimura ⇑,1, Yan Ren, P. Kruit Faculty 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 2013 Accepted 16 July 2013 Available online 25 July 2013 Keywords: MEMS electron optics Multi-electron beam Scanning electron microscope
a b s t r a c t Recently a multi-beam scanning electron microscope (MBSEM) has been developed, which delivers 196 focused beams to a sample, each of which has around 1 nA. In this article a design for an optical system is described and analyzed which can focus all these beams onto a single spot, using an array of micro electron lenses. Although each individual micro lens will be of lower quality than a single macro objective lens, a system is obtained with larger beam current than the conventional SEM. The goal set in an example 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 features are routinely produced both in laboratories and semiconductor fabs. To inspect such small structures, light based systems cannot always be used. Electron microscopes can easily provide the high resolution, but the current in a single beam, and thus the throughput, is limited due to the available brightness of the electron source and the presence of aberrations of the lenses. Another limitation in single beam systems is the broadening by stochastic Coulomb interactions. The limitation by brightness and aberrations can directly be derived from the definition of (reduced) source brightness Br as expressed in terms of the angular current density IX and the virtual source size dv, given by
Br ¼
IX p d2 V 4
v
ð1Þ
beam
where Vbeam is the beam potential. As the electron optical column demagnifies the virtual source down to a geometrical spot size dsample as perceived in the sample plane, the current in the electron beam at the sample is given by
I ¼ Br
p2 4
2
dsample a2sample V beam
ð2Þ
The beam potential Vbeam is determined by requirements of the inspection process, such as the necessity to have a secondary electron emission coefficient of one. The spot size dsample has a ⇑ Corresponding author. Tel./fax: +81 29 276 6353. E-mail address: [email protected] (T. Ichimura). On leave from Research and Development Div, Hitachi High-Technologies Co., Ibaraki 312–8504, Japan. 1
maximum determined by the required resolution. The half-opening angle asample is limited by the spherical and chromatic aberration contribution of the electron optical system to the total spot size. For large currents it is usually the spherical aberration that dominates:
ds ¼ 0:18 C s a3
ð3Þ
with Cs the spherical aberration coefficient of the system, usually equal to the spherical aberration of the last lens in the system. One way of increasing the current in an inspection machine is to use multiple beams. A multi beam system usually has separate beams, which makes it necessary to use multiple detectors: one per beam. This is a practical problem, which we try to circumvent in the solution proposed in this paper. We will look at the possibility of focusing multiple beams in a single point on the sample and detect the signal with one detector. Fig. 1 shows the basic configuration of our large current system. In the basic configuration, there are an electron source, 2 micro lens arrays, 1 macro lens, a sample and a detector. The first micro lens array is used to create the multiple beamlets from a single source and focus each beamlet on the macro lens. The macro lens is used to change the direction of all beamlets towards a single point on the sample. The second micro lens array is used to focus the beamlets on the sample. Because the plane of the macrolens is conjugate with the sample, the aberration has no influence on the spot size. The only effect of the macrolens aberration is to shift the beamlet in the second microlens array, but the microlens will still focus all electrons in the 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 14), each of which has around 1 nA [2–5]. Of course this is a more 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
110
T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
macrolens to the sample as from estimation object distance is much larger than the image distance, see Fig. 3. In order to have the maximum current in the spot, we will try to fill the microlenses as fully as possible, thus there is a relation between amicro, dm and fm, the focal distance of the microlens:
dm ¼ 2f m a
ð8Þ
The maximum dm is thus determined by the spherical aberration of the microlens:
dm ¼ 2f m
dgeo c1 C smicro
1=3 ð9Þ
For single aperture lenses there is the following scaling relation between lens size, focal distance and spherical aberration [ref: EOD simulation program]:
C smicro ¼ c3
fm2 dm
ð10Þ
With that, we find
dm ¼ Fig. 1. Basic configuration of large current system.
more flexible and gives a good experimental platform to try the new idea. The key challenge is to focus all beams that exit from the 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 a landing energy of 500 eV. The objective of this article is to further analyze the optics of the new multi-beam-single-probe system and to describe the electron optical design of the required micro lens array (MLA) for the experimental set-up.
In an optimized system, the size of the aberration disc is about equal to the size of the geometrical image of the source:
ð4Þ
where c1 is a factor depending on the defocus and choice of size definition (e.g. full width 50%, full width half maximum). The resolution of the microscope is equal to the total spot size. Thus, the accepted angle at the sample is limited to
a6
dgeo c1 C s
1=3 ð5Þ
The current in the spot is proportional to a2. In the system of Fig. 1, the same restriction applies for the angle in the microlenses. However, at first 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 the macrolens may deflect the outer beamlets out of the corresponding microlens altogether. Let us call the half full acceptance angle of the system at the sample b and the diameter of the microlens opening dm. If we allow the spherical aberration of the macrolens to shift the most outer beamlet a fraction c2 of the microlens diameter, then
C smacro b3 ¼ c2 dm c2 dm b6ð Þ C smacro
ð6Þ
1=3
ð7Þ
where we assume, for simplicity, the distance from the macrolens to the microlens array to be about equal to the distance from the
ð11Þ
Combining this with the expression for the full acceptance angle, we find a new limitation:
b6
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !1=3 c2 8dgeo fm pffiffiffiffiffiffiffiffiffiffiffiffiffi c1 c3 C smacro
ð12Þ
So if we compare the single beam system with the new multibeam-single-spot system and assuming that we can make a macrolens with the same spherical aberration as in the single beam system, the gain in current is
2 1=3 bnew 8c1 c22 fm ¼ bold c3 dgeo
2. Electron optics of multi-beam-single-probe system
dgeo ¼ c1 C s a3
1=2 8dgeo fm c1 c3
ð13Þ
The constant c2 is a choice and should be about 0.25. c1 for the FW 50 size is 0.18 and c3 is 1.0 [ref]. For a focal distance fm = 2 mm and 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 partially filled, we could try to design for a larger fm, get the precision to make it work for dgeo = 25 nm and with all that reach another factor of 2 extra current. A final remark in this analysis must be that it is possible, in principle, to correct for the spherical aberration of the macrolens because an electrode can be placed inside the lens with only small holes to transmit the focused electron beamlets. 3. Multi beam SEM The MBSEM on which we will perform the initial experiments consists of three subsystems [2,3]: the multi-electron beam source (MBS), a commercial SEM column, and the micro lens array (MLA) system that 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 to produce 196 beamlets (array of 14 14). The emission from a high brightness Schottky emitter is split into an array by an aperture lens array (ALA). This ALA consists of a thin Si membrane with apertures of 18 lm diameter at 25 lm pitch. Two macro electrodes are combined with the extractor electrode of the electron source and create a so called ‘‘zero-strength lens’’, which means that the off-axis beams are avoiding the problem associated with chromatic deflection errors. The field from the macro electrodes ends on the ALA, forming low aberration single micro aperture lenses for the
111
T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
beamlets which focus the beams on the principle plane of the accelerating (ACC) lens. The MBS creates an array of focused beams in the principle plane of ACC lens with a geometrical probe size of 95 nm at a pitch of 70 lm. For a landing energy of 1500 eV in the ACC plane, the total current delivered by the MBS is around 1 nA per a beamlet. The commercial SEM column consists of a condenser (C2) lens, a variable aperture (VA), the intermediate (INT) lens and the high resolution(HR)/the magnetic immersion ultra-high resolution(UHR) objective lens. The C2 lens images the common cross-over of the ACC lens onto the VA. The VA acts as a current limiting aperture and determines the aperture angle for all beams. By changing the strength of the ACC lens and C2 lens, the magnification of the system can be changed. Further demagnification is done by the INT and either the HR lens or UHR lens. The design principles of the MBS system and the commercial SEM column have been described in detail elsewhere [2]. To achieve our goal (200 nA within the 50 nm at the landing energy of 500 eV) we will direct all beamlets towards a single spot on the sample at the same time that each beamlet is focused by a microlens. As we saw in the basic configuration in Fig. 1, a macro lens is needed to direct all beamlets towards a single point. Here we decided not to use the UHR lens because of the complications of its magnetic field, which extends all the way to the sample. In such an immersion lens, secondary electron detection can only be done through-the-lens, which will be difficult with the MLA in the way. In principle the HR lens can then be used, which is inside the SEM column and keeps the MLA region field free. There is a mechanical limitation, however, in the present SEM column. The minimum working distance between the HR lens and the sample is about 40 mm, while the diameter of the lens opening is only 2 mm. This limits the maximum angle at the sample to about 25 mrad, while we want to go to larger values. So the design for the MLA will include
Fig. 3. Schematic drawing of Micro lens array system.
an electrostatic macrolens to replace the magnetic HR lens, giving the 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) first order design The system is aiming for a total current of 200 nA within 50 nm at a landing energy of 500 eV. In order to avoid Coulomb interactions in the SEM we will transport the beams at 10 kV and decelerate in the MLA to 500 V, possibly in two steps. The SEM will focus the beamlets in the plane of the macrolens. Since this plane is an image of the principle plane of the accelerating lens in the source subsystem, the ratio between pitch and geometrical probe size is conserved at
Pmacro : dmacro ¼ 70 lm : 95 nm:
ð14Þ
Our design will consist of an electrostatic macro lens, an array of single aperture microlenses and two macro electrodes which supply the electrostatic field on the apertures and distribute this field such that the curvature of field is corrected [4]. When we denote the pitch size of the MLA as PMLA, the distance between the new electrostatic macrolens and the MLA lo, the distance between the MLA and the sample li, we can write a relation between the pitch size of the beamlets in the macro objective lens and the pitch of the MLA:
Pmacro ¼ PMLA
lo þ li li
ð15Þ
The magnification of the microlenses is given by
rffiffiffiffiffiffiffiffiffi V li M¼ : 500 lo
ð16Þ
where 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 ¼
dgeo : M
ð17Þ
From the requirement we have set ourselves (200 nA, 50 nm, 500 V), we can calculate the sample angle b. Let us take Br = 5 107 A/m2srV, and a 50% filling of the MLA, then
b¼
Fig. 2. Schematic drawing of multi beam SEM with the proposed MLA system.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I 2
0:5 Br pdsample V beam
¼ 45mrad:
ð18Þ
Because it is a square array, the maximum angle will be 64 mrad. For the choice li = 2.5 mm and lo = 10 mm, we find Pmacro = 87 micron, PMLA = 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
112
T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
Fig. 4. The electrical field in front of MLA (shown in purple). The sample is on the right (gray). The decelerating macrolens is also shown. (a) The on-axis beam from the macro lens 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)
T. Ichimura et al. / Microelectronic Engineering 113 (2014) 109–113
2 lo V ¼ 500 M ¼ 1400 V: li
ð19Þ
Thus, the macrolens needs to be a decelerating lens taking the electron energy from 10 to 1.4 kV. The microlenses take the energy from 1400 to 1500 V. The field at the single aperture lenses follows from
f li ¼
4 V final : E
ð20Þ
So the field is 800 V/mm and the distance between the macro electrode and the aperture array is in the order of 1 mm, fitting very well with the other dimensions. With our analysis in Section 2, we can check if the lens aberrations will allow this design, Eq. (11) only needs a value for the spherical aberration coefficient of the electrostatic macrolens. Let us take twice the focal distance, that is 25 mm. Then, with dgeo = 50 nm, we find b 6 78 mrad while the design employs 64 mrad. The microlenses have a pitch of 17 lm, so a maximum diameter of 15 lm and a focal length of 2.5 mm. This would give a microlens spherical aberration of 1.2 m. Eq. (5) yields the maximum aperture angle for the microlens: a 6 6 mrad while the design employs 3 mrad. From these estimates we conclude that our design will not be limited by the spherical aberrations of the lenses. 5. Simulation Fig. 4 shows the MLA system configuration as modeled by the EOD simulation program. It is composed of a decelerating macro lens, two macro electrodes, the MLA and the sample. By manipulating the shape of the field in front of the MLA, creating the ‘‘zero strength macro lens’’ and forming single aperture micro lenses simultaneously, it is possible to fulfill all requirements. For the simulation of the beamlet focusing by the micro lens, the MLA plate has an aperture on axis to create the micro lens. The trajectories for this situation are shown in Fig. 4a. For the simulation of the center trajectories of the off-axis beams (which are not influenced
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by the micro lenses), the MLA plate in the simulation has no aperture, thus no lens effect. These trajectories are shown in Fig. 4b. 6. Conclusion and discussion It should be possible to obtain substantially more current in the probe of an electron beam inspection system when the electron beam from the source is first split up in many beamlets which are transported through the system separately and brought together again in new optical configuration as described in this paper. As an example, we have designed an optical system to be mounted in an existing multi beam SEM, which delivers 196 focused beams (array of 14 14) to a sample, each of which has around 1 nA. The results of a computer simulation seem to support the promise that we can fabricate a system that delivers a total current of 200 nA within 50 nm at a landing energy of 500 eV. We still have to perform a more detailed aberration analysis and design a mechanical construction. One of the challenges is to position the single aperture lenses exactly around the many beams because any displacement of an individual lens is reflected in the position of that beam in the final probe. This requires either very accurate manufacturing or a way to individually deflect the beamlets towards the final common probe. The presence of the micro lens array will also make scanning and detection of secondary electrons more difficult. In conclusion: The proposed system opens a new direction towards higher throughput electron beam inspection, but there are still many practical difficulties to overcome. References [1] B. van Someren, P. Kruit, Method and apparatus for imaging, WO 2008/002132 A1. [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 electron beam induced deposition, PhD thesis, Delft University, 18th February, 2008.