Application of double patterning technology to fabricate optical elements:Process simulation, fabrication, and measurementS. Babin, G. Glushenko, T. Weber, T. Kaesebier, E.-B. Kley et al. Citation: J. Vac. Sci. Technol. B 30, 031605 (2012); doi: 10.1116/1.3698315 View online: http://dx.doi.org/10.1116/1.3698315 View Table of Contents: http://avspublications.org/resource/1/JVTBD9/v30/i3 Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Related ArticlesHigh-resolution nondestructive patterning of isolated organic semiconductors J. Vac. Sci. Technol. B 30, 06FB04 (2012) Dual applications of free-standing holographic nanopatterns for lift-off and stencil lithography J. Vac. Sci. Technol. B 30, 06FF04 (2012) High resolution patterning on nonplanar substrates with large height variation using electron beam lithography J. Vac. Sci. Technol. B 30, 06F303 (2012) Fast and continuous patterning on the surface of plastic fiber by using thermal roller imprint J. Vac. Sci. Technol. B 30, 06FB01 (2012) Creating nanoscale Ag patterns on the Si(111)–(√3×√3)R30°-Ag surface via guided self-assembly J. Vac. Sci. Technol. B 30, 050601 (2012) Additional information on J. Vac. Sci. Technol. BJournal Homepage: http://avspublications.org/jvstb Journal Information: http://avspublications.org/jvstb/about/about_the_journal Top downloads: http://avspublications.org/jvstb/top_20_most_downloaded Information for Authors: http://avspublications.org/jvstb/authors/information_for_contributors

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Application of double patterning technology to fabricate optical elements:Process simulation, fabrication, and measurement

S. Babina) and G. GlushenkoAbeam Technologies, Inc., 5286 Dunnigan Ct., Castro Valley, California 94546

T. Weber, T. Kaesebier and E.-B. KleyFriedrich-Schiller-University Jena, Institute of Applied Physics, Albert-Einstein-Straße 15,D-07743, Jena, Germany

A. SzeghalmiMax Planck Institute of Microstructure Physics, Weinberg 2, Halle (Saale) D-06120, Germany

(Received 12 July 2011; accepted 12 March 2012; published 3 April 2012)

Double patterning is an important technique for the improvement of spatial resolution in fabricated

micro and nanostructures. In this paper, we investigated and applied the double patterning

technique to fabricate diffractive optical elements. Simulations of multiple dry etch and film

deposition steps were performed to study and optimize the vertical profiles of the fabricated

patterns. Etch and deposition characteristics were varied to study their impact on the resulting

vertical profile of the metal layers. The influence of the linewidth of the initial resist pattern and

the process-induced tapering of the grating tops on the optical performance were investigated

in particular. A variably shaped electron-beam lithography system was used for the fabrication of

the initial resist pattern. The spatial frequency was then doubled by means of double patterning.

Broadband aluminum and iridium wire grid polarizers were fabricated for applications down to the

UV range with a feature size of 30 nm, a period of 100 nm, and a vertical aspect ratio of about 5:1.

Optical measurements have confirmed the designed optical properties. VC 2012 American VacuumSociety. [http://dx.doi.org/10.1116/1.3698315]

I. INTRODUCTION

Double patterning is an important technique for the

improvement of spatial resolution in fabricated patterns.

Furthermore, it is an attractive way to fabricate patterns at

resolutions far beyond the limits of traditional optical lithog-

raphy. The principle of double patterning was demonstrated

in 1983.1 Recent progress has led to its application in micro-

electronic manufacturing, and the technique is being adopted

by major semiconductor companies for the volume production

of semiconductor devices.2–5 Among multiple variations of

the double patterning technique, the technology based on the

fabrication of a spacer by conformal deposition of a material,

followed by anisotropic etch of this material, is of high inter-

est. The process is also called spacer patterning technology or

self-aligned double patterning.6 This method is also attractive

for the fabrication of diffractive optical elements, where the

topological elements are mostly repetitive.

In the fabrication process, there are two competing

requirements for a large area of the optical element: high

writing speed and high resolution. Electron-beam lithogra-

phy (EBL) can normally provide either high resolution with

slow writing speed, or can write a large area with high speed

at modest resolution. The double patterning technique per-

fectly suits these two contravening requirements, thereby

extending the manufacturing capabilities of EBL. In semi-

conductor manufacturing, the spacer technique is used to

make a hard mask for ensuing manufacturing processes. In

optical elements, these properly fabricated spacers could

work as wire grid polarizers. They consist of a parallel

arrangement of conductive wires and are characterized by a

high transmission of TM-polarized light, and a suppression

of TE-polarized light.7 A further characteristic is the extinc-

tion ratio defined by the ratio of TM- and TE-polarized light.

In this paper, we investigate the suitability of the double

patterning process for the fabrication of wire grid polarizers.

The investigation is carried out by analyzing the influence of

the fabrication-based deviations from a perfect binary gra-

ting profile on the optical properties of wire grid polarizers.

The TRAVIT software package was used to simulate the com-

plex multistep fabrication process with several deposition

and etching steps, based on an analytical model of dry etch.8

The simulation predicts etch profile, linewidth, variation of

critical dimensions (CD), and placement errors in double

patterning. TRAVIT has been successfully applied to simu-

lations of 22 nm and 11 nm half-pitch designs. Simulations

of the process should complement time-consuming experi-

ments so as to reduce costs and shorten development time.

The influence of linewidth variation of the initial resist pat-

tern and the shape of the tapered grating tops on the grating

optical properties were investigated. To determine the effect

of these deviations from the ideal binary grating profile, the-

oretical calculations using the software GRATING SOLVER9 and

measurements of the optical performance of fabricated sam-

ples were performed. Using a multistep process, including

ultrafast electron-beam lithography and different etching and

deposition techniques, aluminum and iridium gratings with a

period of 100 nm and a vertical aspect ratio of about 5:1a)Electronic mail: sb@abeamtech.com

031605-1 J. Vac. Sci. Technol. B 30(3), May/Jun 2012 2166-2746/2012/30(3)/031605/5/$30.00 VC 2012 American Vacuum Society 031605-1

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were fabricated. The fabricated optical elements work as

broadband wire grid polarizers from the infrared to the ultra-

violet spectral region.

II. MODELING

First demonstration of the double patterning process for

the fabrication of wire grid polarizers was shown by the

authors in previous works.10,11 However, the effects of the

double patterning process on the device’s optical properties

have not been discussed. A better understanding of the evo-

lution of the grating structure during fabrication can lead to

an optimization of the grating profile and an enhancement of

the optical properties of the polarizer. Simulating the dry

etch process is a complex problem that requires detailed

knowledge of plasma physics, the interaction of plasmas

with solids, plasma chemistry, kinetics, etc.12–15 Because of

the complexity of this task, it is impractical to expect any

simulation software to be universally applicable to all dry

etch situations. On the other hand, addressing the problem

for specific cases using defined boundary conditions with

given tolerances is feasible and cost effective, and can

deliver usable information.

TRAVIT simulation software, presented by the authors in an

earlier publication,16 uses an analytical model of dry etch.

The software tool is focused on the simulation of the dynam-

ics of etched profiles. In addition, it is capable of calculating

CDs and CD variations resulting from dry etch; these are not

addressed by currently known models.16 The software can

model complex processes involving the combination of dry

etch, film deposition, and electroplating. Furthermore, the

software predicts the etch profile, linewidth, variation of crit-

ical dimensions (CD), and placement errors in double

patterning.

The geometry of the diffractive optical element (DOE) is

defined by the optical design and simulation. This geometry

is the final target of the fabrication process. Double pattern-

ing involves multiple processing steps; each of them has

many parameters. Optimization of this process to achieve

the required geometry can be done experimentally. How-

ever, because of the multidimensional space of parameters

involved, such work is costly and time consuming. Process

simulations can guide the fabrication process and set up tar-

gets for each separate process step; for example, the shape of

the resist profile after development or the thickness of the

spacer layer in the deposition process. Once the target

requirements for each specific process step are known, it is

much easier to develop the double patterning process as

compared to its development when only the final desired ge-

ometry is known.

The vertical resist profile and its thickness after EBL and

resist development were the input parameters for the simula-

tion; they were varied in order to study their effects on the

final shape of the grating. The resist trim process was also

varied in order to optimize the full process. For a specific

resist profile, etch and deposition characteristics were varied

to study their impact on the resulting vertical profile of the

metal layers. The simulations involved are setup of a GDSII

resist pattern, assigning material thicknesses and resist pro-

file after development; resist trim; conformal deposition of

sacrificial layer; anisotropic etch of the sacrificial layer; and

etch of the resist. Each process step includes many variables.

Simulations were repeated with variations of the process pa-

rameters in order to obtain the desired linewidth and vertical

profiles of the metal gratings after the final fabrication step.

When the simulations yielded the desired geometry of the

final grating, the pattern geometries at each intermediate

processing step were used as targets in the respective steps

of the actual fabrication process.

III. EXPERIMENT

The double patterning technique was used to fabricate

high-aspect-ratio metal gratings with a pitch of 100 nm.

First, a multilayered resist was deposited on a fused silica

substrate. The multilayers involved a tempered resist,

chrome, and an e-beam resist. The 200 nm pitch pattern was

written in the resist using a Vistec SB 350 OS electron-beam

lithography system. A lattice aperture was used for the ultra-

fast writing of a high-frequency pattern. The lattice aperture

is located in the electron column and allows for the writing

of a defined character with a single shot; for example, a gra-

ting pattern. Here, the area of the grating determines the

writing time in the system, but the pitch of the grating does

not.17 When using the ZEP 520 resist of Nippon Zeon

Corporation, the writing speed was about 0.4 mm2/s. The

resist pattern was transferred into the chromium layer by

means of an inductively coupled plasma (ICP) etching pro-

cess. The chrome pattern served as a hard mask for the struc-

turing of the tempered resist layer in another ICP etching

process. In the ensuing processing, the patterned tempered

resist grating was covered with the functional grating mate-

rial. Iridium and aluminum were chosen because of their

suitable optical properties. To coat the grating with about 30

nm, atomic layer deposition (ALD) with iridium18 was used,

and sputtering under oblique incidence was chosen as the

deposition technique for aluminum. Afterwards, the metal

was ion beam etched. It was preferentially removed in the

grooves and on top of the ridges due to the high anisotropy

of the ion beam etching. Finally, the polymer was removed

in an ICP etching process. The metal deposited on the side-

walls of the polymer formed a grating with doubled fre-

quency compared to the grating fabricated in the tempered

resist.

IV. RESULTS AND DISCUSSION

Broadband iridium and aluminum gratings for UV appli-

cations were designed and fabricated. Figures 1 and 2 show

the results of the process simulations at multiple fabrication

steps and the corresponding SEM images of the fabrication

process. Figures 1(a)–1(g) show the fabrication of the irid-

ium grating using ALD. In Fig. 2, selected fabrication steps

of the aluminum grating fabricated using sputter deposition

are demonstrated. An initial resist profile after EBL, resist

development, and ICP etching is displayed in Fig. 1(a). The

ALD deposition of the iridium layer is shown in Figs. 1(b)

031605-2 Babin et al.: Application of double patterning technology to fabricate optical elements 031605-2

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and 1(e). In the fabrication process, the thickness of the de-

posited metal layer (30 nm) plays an important role, because

it determines the ridge width of the final grating. The follow-

ing etch process was mostly anisotropic; see Figs. 1(c) and

1(f), and the resist was removed at the final step as shown in

Figs. 1(d) and 1(g). In this way, the fabrication process of a

grating with half the pitch compared to the resist pattern was

simulated and the results are in good agreement with the

cross-section SEM images of the process. The measured

spectral transmission and extinction ratio of the fabricated

element are shown in Fig. 3. At a wavelength of 300 nm in

the UV spectral range, the measured transmission for TM-

polarized light is 63% and the extinction ratio is 150. For

comparison, the values of a commercial broadband alumi-

num wire grid polarizer have a transmission of 50% and an

extinction ratio of 30 at a wavelength of 300 nm.19

The effects of the etch process variation on the vertical

profile of the grating is demonstrated in Fig. 2. The thickness

of the deposited material on the sidewalls of the grating was

relatively small due to shadowing effects during the sputter

deposition. The etch duration of the ion beam etching step

was varied, resulting in significant variations of the grating

profile. The resist was removed after etching in both fabrica-

tion processes. If the etch time is insufficient, the thickness

of the metal and its profile are far from expected, as shown

in Figs. 2(a) and 2(c). Continued etching results in a grating

with a shape close to the desired cross-section; see Figs. 2(b)

and 2(d). It is obvious that the resulting grating profiles

shown in Figs. 1(g) and 2(d) are different. Specifically, the

tapering of the grating top is more distinctive in Fig. 2(d).

Tapered tops are characteristic for the double patterning pro-

cess and depend on the previous fabrication steps.20,21 The

influence of this effect on the optical performance, namely

the extinction ratio of the polarizer, is theoretically calcu-

lated for an iridium wire grid polarizer, as shown in Fig. 4.

Grating 1 has a period of 100 nm, a height of 150 nm, and a

binary profile, while gratings 2 and 3 have tapered tops with

heights of 20 and 40 nm, respectively. It can be seen that the

extinction ratio decreases with the increasing height of the

tapered top. One reason is that the amount of grating mate-

rial is decreasing, leading to a higher transmission of TE-

polarized light, resulting in a reduction of the extinction ra-

tio. Another reason is that the nonhomogeneous grating

ridges are periodic with a period of 200 nm, leading to the

appearance of higher diffraction orders. A wire grid polarizer

usually works as a zero order grating with only the zeroth

diffraction order occurring.22 Higher diffraction orders com-

promise the optical performance of the polarizer. Hence, the

fabrication process should be optimized so that a nearly per-

fect binary grating profile is produced. The profile is better

in the process shown in Fig. 1 than in Fig. 2.

Moreover, the linewidth of the initial resist pattern gener-

ated using EBL is an important factor for the optical per-

formance of the grating. The distance between the ridges of

the 100 nm period grating is supposed to be equal, as shown

in Figs. 5(a) and 5(d). If the initial resist linewidth after

FIG. 1. (Color online) Simulated profiles and SEM images of the double pat-

terning process for the fabrication of iridium wire grid polarizers using

ALD. The initial resist pattern (a) is coated by ALD with iridium [(b) and

(e)] and afterwards anisotropically etched [(c) and (f)]. The resulting grating

with 100 nm period is depicted in (d) and (g).

FIG. 2. (Color online) Simulation and SEM images of the fabrication of alu-

minum wire grid polarizers using sputter deposition after the removal of the

tempered resist [(a) and (c)] and further ion beam etching [(b) and (d)].

FIG. 3. Spectral measured transmission and extinction ratio of the fabricated

iridium wire grid polarizer.

031605-3 Babin et al.: Application of double patterning technology to fabricate optical elements 031605-3

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exposure and development is incorrect, and the ridge width

of the 200 nm polymer grating is also incorrect the structure

may appear as two enclosed gratings, each with a pitch equal

to the pitch in the initial resist (200 nm). Resulting patterns

with initial resist lines that are too wide and too narrow are

displayed in Figs. 5(b) and 5(e) and in Figs. 5(c) and 5(f). To

avoid such a misalignment, the width of the metal ridge and

the linewidth of the initial resist pattern have to be coordi-

nated. The optical performance decreases considerably with

an increase in the misalignment of the grating ridges.

Thereby, the effective grating period for the incoming light

rises from 100 to 200 nm, leading to the occurrence of higher

diffraction orders. This effect is shown in Fig. 6, where the

extinction ratios of iridium wire grid polarizers with differ-

ent exposure doses of the initial resist pattern are compared.

The exposure dose rises from dose 1 to dose 5, leading to a

decreased ridge width of the resist pattern as demonstrated

in Figs. 5(b) and 5(e).

These considerations show that the accuracy of the double

patterning process has a major impact on the optical proper-

ties of the wire grid polarizers. To effectively study the pro-

cess, a computer-based simulation is a good tool.

V. SUMMARY AND CONCLUSIONS

In conclusion, we investigated the suitability of the dou-

ble patterning process for the fabrication of wire grid polar-

izers. The influence of the selected fabrication steps on the

development of the grating profile and placement of the gra-

ting ridges were studied by simulating the dry etch process

using the software program TRAVIT. Furthermore, the optical

function of wire grid polarizers was calculated and compared

for different resulting grating configurations. The investiga-

tions showed that precise control of the width of the initial

resist pattern and of the height of the tapered grating tops are

necessary to achieve a high extinction ratio. Structural devia-

tions from the ideal binary grating structure lead to a reduced

extinction ratio. Aluminum and iridium wire grid polarizers

for applications in the UV spectral range were fabricated by

means of double patterning and characterized by their optical

properties. Polarizers fabricated using ALD show a better

grating profile than those fabricated using sputter deposition.

Furthermore, the simulated etch profiles are in good agree-

ment with the fabricated grating profiles. The fabricated irid-

ium wire grid polarizer has a transmission of 63% and an

extinction ratio of 150 at a wavelength of 300 nm.

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FIG. 4. (Color online) Calculation of the extinction ratio of three gratings

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FIG. 5. (Color online) Simulations and SEM images of the influence of the

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FIG. 6. (Color online) Spectral measured extinction ratio of gratings with dif-

ferent initial resist linewidths. The linewidth decreases from dose 1 to dose 5.

031605-4 Babin et al.: Application of double patterning technology to fabricate optical elements 031605-4

J. Vac. Sci. Technol. B, Vol. 30, No. 3, May/Jun 2012

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031605-5 Babin et al.: Application of double patterning technology to fabricate optical elements 031605-5

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