Proceedings of the 13th IEEE International Conference on Nanotechnology Beijing, China, August 5-8, 2013

Fabrication of Aperiodic Subwavelength Nanostructures by Grayscale Interference Lithography (GIL)

Hyungryul J. Choi, Student Member, Jeong-Gil Kim, and George Barbastathis, Member, IEEE

Abstract- We propose a novel and simple method to fabricate aperiodic subwavelength nanostructures by Grayscale Interference Lithography (GIL) with the conventional Lloyd's mirror interferometer and a movable aperture plate.

I. INTRODUCTION

Interference lithography (IL) is one of the most effective and low-cost approaches for large-area nano-patterning and nanostructure fabrication of periodic nanostructures, and it has been utilized for years to build multifunctional surfaces [1], magnetic data storage media [2, 3], and photonic materials [4, 5]. IL is based on interference between two or more coherent laser beams in a Lloyd's mirror interferometer [6] and Mach-Zehnder interferometer [7]. Because of the nature of interference, periodicity is guaranteed if the beam wavefront quality and uniformity are sufficiently good over the required surface area of overlap. Thus, 1 dimensional (lD) gratings, 2 dimensional (2D) gratings, and hexagonal hole/dot arrays can be easily fabricated.

For the same reason conventional IL is appealing for periodic structures, it has not hitherto been considered appropriate for aperiodic nanostructures. There are of course exceptions, e.g. a small-area scanning beam IL system ("nano ruler") that continuously varies the pattern period and orientation, but it's not a simple and low-cost setup and it also requires complex control devices [8]. Thus, other less costly but still expensive approaches such as electron-beam lithography [9, 10] are used for aperiodic patterns.

Here, we propose a novel and simple method to fabricate aperiodic subwavelength nanostructures by Grayscale Interference Lithography (GIL) with the conventional Lloyd's mirror interferometer and a movable aperture plate, which allows spatially varying duty cycle of gratings with a single exposure. The key requirement for being able to successfully switch the duty cycle of the grating is the movable aperture plate, which modulates exposure dose on a photoresist layer.

The advantage of GIL compared to conventional IL is that it requires a simple modification only, namely the additional aperture plate; consequently, it presents itself as a practical fabrication process of aperiodic subwavelength

*Research supported by the MIT Institute for Soldier Nanotechnologies (ISN) under Contract DAAD-19-02D-0002 with the U.S. Army Research Office.

H. J. Choi and J. G. Kim are with Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: hr _ choi@mit.edu, astis@mit.edu).

G. Barbastathis is with Massachusetts Institute of Technology, Cambridge, MA 02139 USA, and with the Singapore-MIT Alliance for Research and Technology (SMART), Singapore 117543 Singapore (phone: +65-6516-8592; fax: +65-6778-5654; e-mail: gbarb@mit.edu).

nanostructures with significantly lower manufacturing time than the alternatives.

II. ANALYSIS

A moving aperture plate

.. �� ........ =-=-�--- ----+ ,-......,.......,"-'L_----;---;-_..,....,-____ -; gap distant (d)

After development

Substrate

Figure I. Schematic of Grayscale Intelference Lithography (GIL)

processes for fabricating aperiodic subwavelength nanostructures.

Aperiodic subwavelength nanostructures can be created by the conventional intelference lithograph combined with a moving aperture plate.

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The proposed GIL fabrication process is illustrated in Fig. 1. The process is based on the conventional interference lithography with exposure dose modulation by the moving aperture plate. The aperture plate can move at a speed v as a function of time. In our GIL demonstration experiment, the conventional Lloyd's mirror interferometer was used.

Exposure dose is determined by the relative intensity of the interfering laser beams and the exposure time as the plate moves. To analyze the dependence, consider two monochromatic mutually coherent plane waves at

wavelength A. propagating at a relative angle 2 e . The interference pattern is a sinusoidal intensity pattern with spatial period P, which is given by

P = A. 1(2sin e). (1)

Interference lithography is typically performed with an anti-reflective coating layer deposited below a photoresist layer to maximize contrast of the sinusoidal intensity pattern by minimizing the reflection from the interface between the coating and the resist layer. If reflection from the interface is successfully eliminated by the anti-reflective coating, a sinusoidal exposure dose recorded by the interfering laser beams has perfect contrast, which leads to improvement of the photoresist sidewall.

<l) <Fl o

""0 <l) .... ::l <Fl o 0.. X

u.J

Dose by interfering laser beams 2n

: D, � I oF (l+cos(-x))' t P

Average exposure dose (over one period): Dm ..

·········1···· . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ··· . . · · · · . . . · . . · · . . · . . ····· . . ·· . . · . . · . . ····· . . · ·�d

Dose reslIltlllg from shade region: D, � I 'YcosO -v '- [ a-n-O

L-� ______________ ��_�+ X

P (period)

Figure 2. Exposure dose distribution with respect to lateral position x over

one period P in Grayscale Intelference Lithography (GIL). The total

exposure dose D is the sum of the dose exposed by the intelfering beams D, and the dose exposed by single beam under the shade region D,.

GIL, however, has one difference feature from the normal interference lithography, which is that a gap between the photoresist and the aperture plate creates shade region, as depicted in Fig. l. In the shadow region, the photoresist is exposed by only one laser beam instead of interference intensity pattern. This partial exposure results in lowering the contrast of the dose distribution, as plotted in Fig. 2. Since the dose exposed by single beam under the shade region Ds is is proportional to the product of intensity times dwell time, it is estimated as follows:

Ds�I-Fcos e·2·d/(v·tan e) (2)

where 1 is the incident intensity normal to the propagation direction of the light (measured with a power meter), F is the fraction of the incident light that is transmitted into the resist, and d is the gap distance between the aperture plate and resist layer. The dose exposed by interfering laser beams is

Di � l·Ft '(l+cos(2rrxl P» (3)

where t is the dwell time.

The total exposure dose D is the sum of the dose exposed by the interfering beam Di and the dose exposed by single beam under the shade region Ds. Due to the existence of the gap in GIL, the minimum value of the total dose is the same as the dose exposed by the single beam Ds, and thus, the contrast C of the exposure dose distribution IS lowered according to

(4)

In practice, the values of C should be greater than 0.9 to fabricate good grating profiles [11]. To maximize the contrast C, the minimum value of the total dose, which is the dose exposed by the single beam Ds on the shade region, can be minimized by controlling the gap distance d and velocity v.

III. F ABRlCA TlON PROCESS

In our GIL experiment, hydrogen silsequioxane (HSQI4, Dow Corning, 310 nm) films are first spun on a silicon wafer and hard-baked at 500°C in an oven for 4 hours to be cured, because a cured HSQ layer is optically similar to Si02 layer. After RCA cleaning, sonication, and plasma oxygen etching for 30 seconds in order to remove organic particulates, an antireflective coating layer (I-con 7, Brewer Science, 105 nm) is spun on top of the HSQ layer and baked at 180°C on a hot plate for 1 minute. The thicknesses of the multiple coated layers on the wafer are individually optimized for interference lithography in order to remove reflected beams from the interfaces between the stacked films below a photoresist layer, as well as increase contrast of the sinusoidal intensity pattern [12], as mentioned previously. A positive photoresist (PFi-88A2, Sumitomo, 240 nm) is spun on the stacked layers in the end, and baked on the hot plate at 90 °C for 90 seconds.

Using Lloyd's mirror, the laser beam of wavelength A. =

325 nm is split into two, and the incidence angles are adjusted for creating a standing wave of 200 nm spatial period. This standing wave is projected onto the photoresist to produce a ID grating. For 2D gratings, two separate orthogonal laser exposures are projected onto the photoresist, with an interval time of 1 minute. During the exposure, a motorized linear stage (Zaber Technologies) is utilized to control the speed of the aperture plate and modulate the exposure dose. The gap distance d is fixed to 1 mm to maximize the contrast C of the exposure dose distribution. The exposed photoresist is finally developed to form a pattern of lines and posts.

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IV. FABRICATION RESULTS

By using the process described above, ID and 2D aperiodic subwavelength gratings with varying duty cycles were successfully fabricated by single and double exposure methods. (The duty cycle of the grating is defined as the ratio of the width to the period of the grating.) Fig. 3a and 3b show the image of the fabricated ID grating sample, and cross-sectional micrographs of the 200 nm period ID grating fabricated by GIL with the single exposure, respectively. The moving aperture was moved from viii region to i region at a constant speed of 3l.25 um/s for 16 minutes to fabricate the ID grating. Before the exposure, the aperture plate covered the whole prepared sample. After the exposure begun, it started to move away from the sample to the left side of Fig. 3a. As shown in Fig. 2b, the duty cycle of the aperiodic ID grating decreases from i to viii since the average value of the total exposure dose over one period Dave increases from 4.4 mJ/cm2 (i region) to 76.4 mJ/cm

2 (viii region) due to the linear movement of the aperture.

a

b

Figure 3. a) Image of the subwavelength I D grating fabricated by GIL with

the single exposure. During the exposure, the aperture plate moves to the lefl

side of the sample (-x direction) at the constant speed. b) Cross-sectional

micrographs of the grating with respect to the position./i'om i to viii region marked in Fig. 2a.

The duty cycle of the fabricated lD grating (Fig. 3) with respect to the coordinate in the x-dimension is plotted in Fig. 4. Since the speed of the aperture is fixed during the experiment, the coordinate in the x-dimension is directly proportional to the average exposure dose over one period Dave. The width of the grating appears after 10 mm distance away from the starting point, because the low dose underexposes the resist. The behavior of the duty cycle for the ID grating fabricated by GIL is found to match well the experimental results previously described in the literature [11, 12], because we adjusted the gap distance d such that Ds was much smaller than the average total dose over the whole area. The minimum width of the ID grating fabricated by GIL was 20 nm.

1.0

0.8

<l) 0.6 U ;>, <.)

€ 0.4 Cl

0.2

0.0

• • • • • •

III, i ••

o 5 10 15

.. , II i

20 25

f • •

30

Coordinates in the x-dimension (mm)

Figure 4. Duty cycle of the 1 D grating with respect to coordinate in the

x-dimension in Fig. 3a. Error bars mean the standard deviation of the measured duty cycle.

We also conducted a feasibility test of 2D aperiodic gratings. The results are as shown in Fig. Sa. The average exposure dose Dave of the single exposure varied linearly from 22 mJ/cm2 to 31 mJ/cm2 with the aperture plate travelling 2.5 cm long at a constant speed of 0.20Smm/s. The lowest total average exposure dose Dave was 44 mJ/cm2 on the top left region i and the highest total average exposure dose Dave was 62 mJ/cm2 on the bottom right region ii.

a

44 mJ/cm2

2.Scm

b iii • • • • • �

, • • • '. • 4

• • • • • • t

• • '. • • • 4

• • • • 2(2nm4

Figure 5. 2D aperiodic subwavelength gratings fabricated by GIL with the

double exposures. a) schematic of the double exposures on a 2.5 cm by 2.5

cm sample. The darker color means higher average exposure dose. b)

top-view micrographs of the grating on the i region (lefl picture) and on the

ii region (right picture).

Fig. 5b shows top-view micrographs of the 2D grating on two regions i and ii of Fig. Sa. Due to the characteristic of the positive photoresist, the duty cycle of the 2D grating with lower exposure dose on the i region is greater than that with higher exposure dose on the ii region. This demonstrates that the duty cycle of the aperiodic 2D grating also can be well controlled by the modulation of the exposure dose with the moving aperture plate.

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V. DISCUSSION

We have introduced a simple and novel fabrication process, Grayscale Interference Lithography (GIL), for aperiodic subwavelength nanostructures. The ID and 2D

aperiodic gratings with the varying duty cycles were successfully created by GIL. The proposed process consists of the conventional interference lithography and moving aperture plate, and it delivers simple ways not only to make subwavelength gradient index (GRIN) optical elements such as Luneburg lens [9, 10], but also to characterize and understand the behavior of a photoresist to changes in exposure dose.

Future work will be to fabricate other type of periodic gratings with different exposure dose modulations and different materials, and extend this process with different aperture plates, e.g., motorized iris diaphragms.

ACKNOWLEDGMENT

We also gratefully acknowledge the staff and facility

support from the Nano Structures Laboratory and

Microsystems Technology Laboratory at MIT for fabricating

and characterizing the nanostructured surfaces. This research

was funded by the Army Research Office through the

Institute of Soldier Nanotechnology at MIT. H. J. Choi

thanks Kwanjeong Educational Foundation Scholarship for

partial financial support, and J. G. Kim thanks Samsung

Fellowship for financial support.

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