Charging suppression in focused-ion beam fabrication of visible subwavelengthdielectric grating reflector using electron conducting polymerMohd Sharizal Alias, Hsien-Yu Liao, Tien Khee Ng, and Boon Siew Ooi Citation: Journal of Vacuum Science & Technology B 33, 06F701 (2015); doi: 10.1116/1.4929152 View online: http://dx.doi.org/10.1116/1.4929152 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/33/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Focused-ion beam patterning of organolead trihalide perovskite for subwavelength grating nanophotonicapplications J. Vac. Sci. Technol. B 33, 051207 (2015); 10.1116/1.4927542 Advanced experimental applications for x-ray transmission gratings spectroscopy using a novel gratingfabrication method Rev. Sci. Instrum. 83, 083109 (2012); 10.1063/1.4746771 Templated fabrication of large area subwavelength antireflection gratings on silicon Appl. Phys. Lett. 91, 231105 (2007); 10.1063/1.2821833 High-frequency properties of 1.55 μm laterally complex coupled distributed feedback lasers fabricated byfocused-ion-beam lithography Appl. Phys. Lett. 77, 325 (2000); 10.1063/1.126965 1.55 μm single mode lasers with complex coupled distributed feedback gratings fabricated by focused ion beamimplantation Appl. Phys. Lett. 75, 1491 (1999); 10.1063/1.124732

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Charging suppression in focused-ion beam fabricationof visible subwavelength dielectric grating reflectorusing electron conducting polymer

Mohd Sharizal Alias,a) Hsien-Yu Liao, Tien Khee Ng, and Boon Siew Ooib)

Photonics Laboratory, Computer, Electrical, and Mathematical Sciences and Engineering (CEMSE),King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

(Received 26 June 2015; accepted 10 August 2015; published 19 August 2015)

Nanoscale periodic patterning on insulating materials using focused-ion beam (FIB) is challenging

because of charging effect, which causes pattern distortion and resolution degradation. In this pa-

per, the authors used a charging suppression scheme using electron conducting polymer for the

implementation of FIB patterned dielectric subwavelength grating (SWG) reflector. Prior to the

FIB patterning, the authors numerically designed the optimal structure and the fabrication tolerance

for all grating parameters (period, grating thickness, fill-factor, and low refractive index layer thick-

ness) using the rigorous-coupled wave analysis computation. Then, the authors performed the FIB

patterning on the dielectric SWG reflector spin-coated with electron conducting polymer for the

anticharging purpose. They also performed similar patterning using thin conductive film anticharg-

ing scheme (30 nm Cr coating) for comparison. Their results show that the electron conducting

polymer anticharging scheme effectively suppressing the charging effect during the FIB patterning

of dielectric SWG reflector. The fabricated grating exhibited nanoscale precision, high uniformity

and contrast, constant patterning, and complied with fabrication tolerance for all grating parameters

across the entire patterned area. Utilization of electron conducting polymer leads to a simpler anti-

charging scheme with high precision and uniformity for FIB patterning on insulator materials.VC 2015 Author(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution 3.0 Unported License. [http://dx.doi.org/10.1116/1.4929152]

I. INTRODUCTION

High reflectivity and broadband mirror with polarization

selectivity can be achieved by utilizing subwavelength gra-

ting (SWG). Recently, the SWG has been demonstrated as

reflector for vertical-cavity surface-emitting laser,1–9 quan-

tum cascade laser (QCL),10 photodetector,11 solar cell,12 and

optical waveguide.13 Depending on the operating wave-

length of interest, SWG reflector can be patterned using

micro- and nanofabrication methods, such as optical lithog-

raphy,14 laser interferometric lithography,12 holographic li-

thography,6 nanoimprint lithography,9 and electron beam

lithography (EBL),2–5,7,8 or focus ion beam (FIB).10

SWG patterning using FIB has thus far demonstrated the

SWG antireflector for QCL at infrared (IR) spectrum,10 plas-

monic collimator for QCL at IR spectrum,15 and membrane

SWG for GaN at visible spectrum.16 The first two examples

are demonstrated at IR spectrum. As for application in the

visible spectrum, the GaN membrane SWG is still patterned

using EBL while FIB is only used for direct milling of the

membrane layer. All these studies are based on III–V com-

pound semiconductor materials. For dielectric materials,

realizing SWG structure using the FIB method at visible

wavelength is inherently challenging. This is due to the

nanoscale patterning dimension (subwavelength require-

ment) and poor dielectric conductivity (sample charging)

under the FIB environment. The ion charging affects the fea-

ture size and uniformity even more so for visible photonic

devices due to the nanoscale dimension requirement for

shorter wavelength operation. To counter the charging

effect, several charge neutralization schemes have been

reported, such as the simultaneous electron and beam irradia-

tion,17 thin conductive film coating,18 microprobing,19 elec-

tron flood gun,20 ultraviolet (UV) flood gun,21 and lower

energy irradiation.22 However, these anticharging schemes

introduced complexity in terms of mathematical calculation,

additional sample processing steps and require hardware/

software modification to the FIB equipment. In the simulta-

neous electron and beam irradiation scheme, mathematical

calculation is required to couple both electron and ion beam

spots. The charging mitigation using the thin film coating

and microprobing schemes require conductive material dep-

osition (using evaporation or sputtering) on the sample and

later this material need to be removed (using wet- or dry-

etching) for subsequent sample processing. For the electron-

and UV-flood guns, both anticharging schemes require FIB

hardware modification to insert the tungsten filament (for the

electron gun scheme) and UV light-emitting-diodes (for the

UV gun scheme). As for the lower energy irradiation

scheme, the FIB operation require both hardware and soft-

ware modifications to monitor and program the patterning

using memory device.

We present here a simpler charging suppression scheme

in FIB fabrication of visible dielectric SWG reflector using

electron conducting polymer. The electron conducting poly-

mer scheme has been recently used for charging suppression

in EBL,23–25 but has not been demonstrated for FIB yet. In

this scheme, we spin-coated the electron conducting polymer

a)Electronic mail: drsharizal@gmail.comb)Electronic mail: boon.ooi@kaust.edu.sa

06F701-1 J. Vac. Sci. Technol. B 33(6), Nov/Dec 2015 2166-2746/2015/33(6)/06F701/7 VC Author(s) 2015 06F701-1

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on the front and back surfaces of the sample prior to FIB pat-

terning. We compared the FIB patterning using the electron

conducting polymer scheme with the thin conductive film

coating scheme. Using the electron conducting polymer

scheme, we obtained nanoscale precision with high uniform-

ity and sharp contrast for the FIB patterning of visible dielec-

tric SWG reflector. We demonstrate that the electron

conducting polymer scheme can provide straightforward

charging control for FIB patterning, eliminating complicated

processing steps or equipment modification.

II. EXPERIMENT

A. SWG reflector design and fabrication

Figure 1 shows the schematic of the dielectric SWG reflec-

tor on top of GaN substrate with sapphire template. It is com-

posed of binary grating of Si3N4 layer as high refractive index

(RI) material embedded in between two low RI layers of SiO2

layer (bottom) and air (top). We employed rigorous-coupled

wave analysis (RCWA) method to design the grating parame-

ters for the visible dielectric SWG reflector. The grating pa-

rameters consist of period (K), high RI layer thickness (HId),

low RI layer thickness (LId), and fill-factor (FF), which is

defined as ratio of grating width to period.

We assumed that the incident light propagate bottom up

to the surface-normal plane through the SWG structure

(z-direction). The transverse electric (TE)- and transverse

magnetic (TM)-polarization are considered parallel to

(y-direction) and perpendicular to (x-direction) the grating

lines, respectively. The dielectric SWG reflector is designed

to maximize reflectivity at visible wavelength of 425 nm

with TE-polarization. The Si3N4 and SiO2 dielectrics layers

were deposited on a generic GaN-sapphire template substrate

using plasma-enhanced chemical vapor deposition (PECVD)

technique. From ellipsometry measurement, the refractive

index at 425 nm wavelength for each layer is Si3N4¼ 1.92,

SiO2¼ 1.47, GaN¼ 2.52, and sapphire¼ 1.77, respectively.

These refractive index values are used in the RCWA

computation.

B. FIB patterning processing and measurement

The FIB patterning was performed using a FEI Quanta

3DFEG dual beam system that consisted of both electron- and

ion-columns. Prior to the FIB patterning, we spin-coated

approximately 20 nm electron conducting polymer (Espacer

300Z from Showa Denko) on the surface and backside of the

dielectric deposited sample to suppress surface and subsurface

charging. The electron conducting polymer was spin-coated

at 2000 rpm for 60 s. As for the thin conductive coating film

sample, we sputter-deposited approximately 30 nm of Cr on

the top of another dielectrics deposited sample. Using FIB,

we patterned the dielectric SWG reflector on a square area of

30.6� 30.6 lm2 (consisted of 75 grating lines) for both sam-

ples. The FIB parameters were fixed at Gaþ ion source accel-

erated voltage of 30 kV, probe current of 30 pA (beam

diameter of approximately 17 nm), dwell time of 100 ns,

beam overlap of 50%, and using serial scan mode (left to right

direction). The patterning time was around 4 h per sample.

After FIB patterning is completed, the dielectric sample with

the electron conducting polymer was immersed in deionized

(DI) water for 60 s, followed by DI water rinsing for 120 s and

finally N2 drying. We purposely extended the electron con-

ducting polymer removal processing where we immersed and

rinsed with DI water for total 150 s longer time instead of

only rinsing with DI water for 30 as described in the manufac-

turer technical datasheet for Espacer 300Z. From the optical

microscopy, we observed no significant residue existence on

the sample surface due to the extended electron conducting

polymer removal processing. Thus, FIB patterning in our case

should be straightforward without residue hindrance.

Furthermore, our patterning resolution is larger (>200 nm).

Using the EBL technique, the electron conducting polymer

has succeeded in demonstrating around 20 nm resolution pat-

terning.25 As for the Cr-coated sample, we removed the Cr by

dipping the sample in Cr etchant (from Sigma Aldrich) at

room temperature for 20 s. In addition, we performed a cross-

section milling for the dielectric SWG reflector using the FIB

to investigate the grating cross-section profile. We utilized the

scanning electron microscope (SEM) capability in the FEI

Quanta 3DFEG FIB/SEM dual beam system to measure the

grating dimensions. All samples were sputter-coated with

3 nm Ir to neutralize charging during the SEM imaging.

III. RESULTS AND DISCUSSION

A. SWG optimal design and fabrication tolerance

Figures 2(a)–2(d) exhibit the TE-polarized reflectivity

contour plot for all grating parameters (K, HId, LId, and

FF) as a function of wavelength computed by the RCWA.

By varying these grating parameters, we can design the

optimal dielectric SWG reflector at 425 nm operating wave-

length. In addition, we can investigate the fabrication toler-

ance (defined at �99% reflectivity) for all grating

parameters. For each of the result in Figs. 2(a)–2(d), we

computed the corresponding grating parameter and fixed

the remaining grating parameters at the respective optimal

values. Figure 2(a) indicates that high reflectivity (>90%)FIG. 1. (Color online) Schematic of the visible dielectric SWG reflector.

06F701-2 Alias et al.: Charging suppression in focused-ion beam fabrication 06F701-2

J. Vac. Sci. Technol. B, Vol. 33, No. 6, Nov/Dec 2015

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is obtained whenever the K parameter is at the subwave-

length of the designated operating wavelength. By design-

ing the SWG period at subwavelength, all diffraction

orders except for the zero-order are suppressed, resulting in

the incident light to be either reflected or transmitted in

the normal direction.26 We chose the K¼ 408 nm for the

dielectric SWG reflector, which cross-over the operating

wavelength of 425 nm, thereby resulted in 630 nm fabrica-

tion tolerance for �99% reflectivity. In Fig. 2(b), the effect

of varying the HId parameter to the dielectric SWG reflec-

tivity is illustrated. To achieve �99% reflectivity, we pro-

posed the HId¼ 165 nm with the fabrication tolerance

approximately 630 nm. A high reflectivity and symmetric

SWG reflector is obtained for the FF parameter as shown in

Fig. 2(c). We suggested FF¼ 0.4 for the optimal dielectric

SWG reflector design to accommodate high fabrication

tolerance (60.19 for �99% reflectivity). This result in gra-

ting width of 163 nm and air separation of 245 nm for the

fabrication of each grating period. Figure 2(d) exhibits that

LId parameter has negligibly effect on the reflectivity spec-

trum of the dielectric SWG reflector. We selected the

LId¼ 100 nm for broad reflectivity spectrum and to reduce

the PECVD deposition duration. Furthermore, thinner LId

layer resulted in broader reflectivity bandwidth.27 Based on

the grating parameters results, the SWG operating wave-

length is controlled using the K parameter whereas the

reflectivity intensity is regulated using the HId and FFparameters.

Figure 3 shows the computed reflectivity spectrum for the

optimized visible dielectric SWG reflector for both TE- and

TM-polarizations. It exhibits high reflectivity >99.9% at

425 nm wavelength for the TE-polarization while the TM-

polarization was suppressed below 40% reflectivity, indicat-

ing the SWG reflector capability for polarization selectivity.

The optimized TE-polarized dielectric SWG reflector also

exhibited 65 nm broadband for reflectivity of >90%.

Table I summarized all optimized grating parameters

design with the respective fabrication tolerance (�99%

FIG. 2. (Color online) Contour plot of reflectivity as function of wavelength from RCWA computation for (a) K, (b) HId, (c) FF, and (d) LId, respectively. The

dotted lines in each figure correspond to the selected grating parameter for the design optimization. The solid lines defined the fabrication tolerance of �99%

reflectivity for all grating parameters.

FIG. 3. (Color online) Reflectivity spectrum for the optimized visible dielec-

tric SWG reflector for TE- and TM-polarizations.

06F701-3 Alias et al.: Charging suppression in focused-ion beam fabrication 06F701-3

JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

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reflectivity) for the FIB patterning of the dielectric SWG

reflector. The period and fill-factor can be precisely con-

trolled using FIB patterning whereas the grating- and low

RI-thicknesses can be controlled through accurate FIB mill-

ing and PECVD deposition.

B. Charging suppression

To demonstrate the severity of charging in realizing nano-

scale structure for the visible dielectric SWG reflector, we

intentionally patterned a dielectric deposited sample using a

basic charging mitigation scheme. In this basic scheme, we

simply used Ag tape and Ag paste on the sample edges and

back surfaces. The sample consists of similar layers with the

Cr-coated sample and the electron conducting polymer-

coated sample. Figure 4 exhibits the SEM image after the

completion of FIB patterning. No uniform and continuous

grating lines can be patterned under the influence of charging

even at low current FIB operation (30 pA). We notice a high

charges built-up at the beginning of the patterning (top-left

of the image) from the bright contrast. As the milling contin-

ues, the pattern is severely deformed and distorted toward

the bottom-right from the designated patterning area due to

the extreme charging effect and charge accumulation.

Figure 5 shows the SEM image for the Cr-coated sample

(for the thin conductive film coating anticharging scheme).

The Cr film managed to improve the FIB patterning by

dissipating most of the charges. However, pattern distortions

are observed at the end of the patterning (from left to right)

due to charge accumulation. Furthermore, although the pat-

terned SWG demonstrated decent contrast, the patterned Kand FF parameters are not uniform across the whole pattern-

ing area. The insets in Fig. 5 exhibited examples of the gra-

ting period variations at several spots on the FIB patterned

area.

To investigate the effectiveness of the Cr-coated sample

in dissipating the charges, we measured K and FF parame-

ters for every single patterned grating lines (from left to

right, 75 grating lines in total). These measurements are

baselined against the fabrication tolerance for K and FF pa-

rameters, as plotted in Fig. 6. For the K parameter, more

than 50% of the patterned dielectric SWG is beyond the fab-

rication tolerance of 630 nm. To ensure the dielectric SWG

reflected the light at the designated 425 nm operating

TABLE I. Optimal design and fabrication tolerance for the visible dielectric

SWG reflector grating parameters.

Grating parameter Optimal Minimum Maximum Tolerance

Period (K) 408 nm 390 nm 420 nm 630 nm (7%)

Grating thickness (HId) 165 nm 155 nm 180 nm 625 nm (15%)

Fill-factor (FF) 0.40 0.3 0.49 60.19 (48%)

Low index thickness (LId) 100 nm — — —

FIG. 4. (Color online) SEM image of FIB patterning with Ag tape and Ag

paste. The inset shows the bright contrast from charging and a period of gra-

ting line being patterned.

FIG. 5. (Color online) SEM image of FIB patterning for Cr-coated sample. The insets show the variations of period across the patterned SWG.

06F701-4 Alias et al.: Charging suppression in focused-ion beam fabrication 06F701-4

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wavelength and only allowed fundamental mode propagation

(the subwavelength physic), we require precise FIB pattern-

ing for the K parameter. However, we observed that more

than 20% of the grating lines exhibit K> 420 nm, especially

for the gratings patterned near the end. Thus, these gratings

are no longer considered as a subwavelength structure, and

the reflected wavelength will be red-shifted. As for the FFparameter, although we designed a large fabrication toler-

ance (60.19 or 48%), almost 20% of the patterned grating

exceeded the fabrication tolerance. This will reduce the

reflectivity intensity. As a result, the dielectric SWG reflec-

tor will not uniformly reflect light above 99%. For both Kand FF parameters, we observed that the patterning intoler-

ance occurred at several spots of the patterned area (grating

lines 10–30 and 40–60) and significantly increased as the

FIB patterning approaching completion (grating lines

68–75). This observation confirms that charging is still

occurs across the Cr-coated sample. It will accumulate sub-

stantially as the FIB patterning is nearing the end, resulting

in pattern distortion.

Figure 7 shows the SEM image of the FIB patterning on

the electron conducting polymer-coated sample. We

achieved a high uniformity and high contrast patterning for

the dielectric SWG reflector. No significant pattern distortion

is observed on the electron conducting polymer-coated sam-

ple. The inset in Fig. 7 indicates high contrast with nanoscale

precision gratings were fabricated for the dielectric SWG

reflector.

Similar to the Cr-coated sample, we also measured K and

FF parameters for every single patterned grating lines on the

electron conducting polymer-coated sample. In Fig. 8, we

plotted the results against the fabrication tolerance for each

grating parameter. In comparison to the Cr-coated sample,

we achieved a highly uniform dielectric SWG reflectorFIG. 6. (Color online) Periodicity and fill-factor of all grating lines for the

patterned dielectric SWG reflector with Cr coating. The dotted lines are the

minimum and maximum fabrication tolerance for each grating parameter.

FIG. 7. (Color online) SEM image of FIB patterning for the electron con-

ducting polymer-coated sample. The inset shows the sharp contrast and

nanoscale precision achieved for the patterned gratings.

FIG. 8. (Color online) Periodicity and fill-factor of all grating lines for the

patterned dielectric SWG reflector with electron conducting polymer coat-

ing. The dotted lines are the minimum and maximum fabrication tolerance

for each grating parameter.

06F701-5 Alias et al.: Charging suppression in focused-ion beam fabrication 06F701-5

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patterning across the whole patterned area. Both K and FFparameters indicate nanoscale precision patterning within

the optimal design dimensions (408 nm for K, and 0.40 for

FF, respectively) for all grating lines. For both grating pa-

rameters, no single grating line is patterned beyond the

specified fabrication tolerance. Based on the constant pattern

preservation within the whole range of patterned area, the

electron conducting polymer anticharging scheme is efficient

for charging suppression in FIB patterning. The patterned

dielectric SWG will reflect uniformly with >99% reflectivity

at the designated 425 nm operating wavelength across the

entire patterned area.

Our results demonstrate that the electron conducting poly-

mer scheme is more efficient in suppressing charging and

patterning constantly compared to the thin conductive film

coating scheme. This finding is similar to the reported pat-

terning using EBL,25 where the utilization of electron con-

ducting polymer anticharging scheme realized the finest

nanopatterning resolution within larger dose range compared

to thin metal coatings of Al and Cr. We attributed the inef-

fectiveness of the thin conductive film coating scheme for

FIB patterning due to charge existence and broadening of

the ion beam by elastic scattering from metallic material.

For the charge existence factor, we assumed that certain

charges still trapped on the grainy surface (due to sputtering-

deposition process) of the coated Cr. This is supported by

similar observation for the Cr sputter-deposited anticharging

scheme for EBL patterning25 (referred to Cr islands as in the

paper). As for the ion beam broadening factor, we consid-

ered the same scattering effect as occurred in EBL pattern-

ing.18 In the paper, Monte Carlo simulation of point spread

functions of electrons in Al, Cr, and Cu reveals that heavier

material coating for anticharging scheme would scatter the

electrons broader in EBL, resulting in larger patterning vari-

ation. As the atomic number of the coating material

increased, it would increase the elastic scattering of elec-

trons.25 Besides atomic number, the coating material density

also would increase the electrons scattering.18 In our case,

the incoming ions and the secondary electrons generated

during FIB patterning were scattered widely, thus distorted

the patterned profile when heavier and denser material coat-

ing using Cr is employed as the anticharging scheme. Using

a lighter and less dense material coating like polymer would

generally result in less scattering than the heavier and denser

metallic coating. For this reason, we achieved a constant pat-

terning of K and FF parameters when using the electron con-

ducting polymer as the anticharging scheme. It has also been

demonstrated that electron conducting polymer anticharging

scheme would provide constant patterning preservation

(640 nm) for low energy exposure in EBL at nanoscale re-

gime.24 In contrast, larger patterning variation (6150 nm) is

reported for the thin conductive film scheme (Al in this

case). Therefore, this supports our constant patterning

achievement for the electron conducting polymer anticharg-

ing scheme at low current (30 pA) FIB operation.

In addition to the fabrication tolerance investigation for Kand FF parameters, we also performed cross-section milling

to investigate the HId parameter (grating thickness). Figure 9

shows the cross-section microscopy for the electron conduct-

ing polymer-coated sample. The patterned structure exhibits

a filleted profile that is typical for a direct writing process.

We obtained approximately HId of 167.5 nm from the FIB

patterning. This is within the nanoscale precision of the des-

ignated thickness (165 nm) and comply with the fabrication

tolerance (625 nm). The cross-section examination also

shows that the dielectric SWG reflector sidewalls are fabri-

cated almost vertically. The slight angled slope variation

obtained is expected because of the fabrication imperfection.

IV. CONCLUSIONS

In this paper, we presented a charging suppression scheme

for FIB patterning using the electron conducting polymer for

the first time. By fabricating dielectric SWG reflector, we

demonstrated the effectiveness of the electron conducting

polymer as an anticharging scheme for nanoscale FIB. Prior

to the FIB experiment, we designed the optimal structure and

the fabrication tolerance for all grating parameters (period,

grating thickness, fill-factor, and low RI layer thickness) using

the RCWA computation. Our results show that the FIB pat-

terning of dielectric SWG reflector has nanoscale precision

with high contrast and uniform gratings formed. The pat-

terned gratings are also constant within the fabrication toler-

ance across the entire patterned area. These findings indicate

that the electron conducting polymer is effective in suppress-

ing potential charging built up in FIB patterning. This will

lead to a simpler anticharging control for future FIB process-

ing (deposition or milling) without the need of additional

processing steps or equipment modification.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding support

from KAUST and King Abdulaziz City for Science and

Technology Technology Innovation Center (TIC) for Solid-

State Lighting at KAUST.

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