Generation of nanoscale anticounterfeiting patterns on silicon by optical trap assistednanopatterningT.-H. ChenY.-C. TsaiR. Fardel and C. B. Arnold

Citation: J. Laser Appl. 29, 012006 (2017); doi: 10.2351/1.4966590View online: http://dx.doi.org/10.2351/1.4966590View Table of Contents: http://lia.scitation.org/toc/jla/29/1Published by the Laser Institute of America

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Generation of nanoscale anticounterfeiting patterns on silicon by optical trapassisted nanopatterning

T.-H. ChenDepartment of Mechanical and Aerospace Engineering, Princeton Institute for Science and Technology ofMaterials, Princeton University, Princeton, New Jersey 08544

Y.-C. Tsaia)

Fischell Department of Bioengineering, University of Maryland, College Park, Maryland 20742

R. Fardel and C. B. Arnoldb)

Department of Mechanical and Aerospace Engineering, Princeton Institute for Science and Technology ofMaterials, Princeton University, Princeton, New Jersey 08544

(Received 13 July 2016; accepted for publication 14 October 2016; published 8 November 2016)

Among the different strategies aimed at protecting products from counterfeiting, hidden security

patterns are used by manufacturers to mark their products in a unique way. However, most

anticounterfeiting patterns bear the risk of being reproduced by an unauthorized party who has

gained knowledge of the exact technique and process parameters. In this paper, we use optical trap

assisted nanopatterning to create unique security markings by taking advantage of statistical

fluctuations when generating nanoscale features within the pattern. We image the patterns by

optical microscopy, scanning electron microscopy, and atomic force microscopy and propose a

three-level examination process that allows for an efficient yet highly secure authentication.VC 2016 Laser Institute of America. [http://dx.doi.org/10.2351/1.4966590]

Key words: OTAN, nanopatterning, solid-immersion lens

I. INTRODUCTION

Brand protection and product authentication rely on the

ability to apply a distinctive marking on original products.1

Among these markings, covert security features are used by

manufacturers to identify their products as authentic and rely

on the creation of a hidden security pattern on the product to

protect. Several techniques have been developed to generate

covert security patterns such as invisible printing, laser coding,

and digital watermarking.2–7 These techniques require special-

ized equipment for fabricating and authenticating security fea-

tures, and therefore, present a barrier to counterfeiting.

However, in all these techniques, the exact parameters to cre-

ate the features must be kept confidential, because knowledge

of the fabrication conditions is sufficient to reproduce them.

One way to mitigate this issue is to take advantage of statisti-

cal fluctuations that are intrinsic to the fabrication method. An

ideal technique would introduce statistical fluctuations that

make each pattern unique no matter how well the fabrication

conditions are reproduced, yet create features that cannot be

easily made by other methods. One technique that fulfills these

requirement is optical trap assisted nanopatterning (OTAN),

which uses a Bessel-beam optical trap to position a micro-

sphere to be used as a near-field optical element.8–12 Besides

introducing statistical fluctuation, the Bessel beam optical trap

has the advantage of self-positioning the bead at the optimal

distance from the substrate, which eliminates the complicated

step of precise vertical focusing in the alignment process.

Upon illumination with an appropriate pulsed laser, a micro-

sphere focuses the laser light to a subwavelength-sized spot

and creates surface features as small as 100 nm. The feature

creation is dependent on the exact position of the microsphere

relative to the laser and to the substrate, and small fluctuations

in position lead to small fluctuations in shape and size of the

features. In this case, the Bessel-beam nature of the optical

trap means that the statistical fluctuations associated with

Brownian motion of the bead are significantly greater along

the optical axis than orthogonal to the optical axis.13 Most

OTAN literature has dealt with the fundamental principle and

the development of the OTAN technique. In this paper, for the

first time, we demonstrate nanopatterns on silicon and present

an application for OTAN that specifically takes advantages of

the technique’s unique features. More precisely, we exploit

fluctuations along the optical axis of an optically trapped

microsphere to create unique patterns that are virtually impos-

sible to reproduce even when using the identical processing

conditions. We utilize OTAN to create anticounterfeiting pat-

terns on silicon, a technologically important material, and a

substrate for high added-value devices. This approach takes

advantage of the multiscale nature of the patterns by enabling

a three-level authentication process with increasing levels of

spatial resolution based on optical microscopy, scanning elec-

tron microscopy (SEM), and atomic force microscopy (AFM).

II. EXPERIMENTS’ DETAILS

A. Sample preparation

The sample on which patterns are created is a silicon

substrate cleaned in a Piranha solution and hydrogen

a)Present address: ASML Brion, 339 W. Trimble Road, San Jose, California

95131.b)Electronic mail: cbarnold@princeton.edu

1938-1387/2017/29(1)/012006/5/$28.00 VC 2016 Laser Institute of America012006-1

JOURNAL OF LASER APPLICATIONS VOLUME 29, NUMBER 1 FEBRUARY 2017

terminated in a hydrofluoric acid solution according to stan-

dard practices.14 OTAN requires the sample to be immersed

in a liquid in which microspheres are optically trapped. To

that end, the substrate is assembled into a wet cell: a hollow

square of double-sided tape is applied to the substrate, creat-

ing a cavity in which we pipette a suspension containing

4000 particles/ll of 2 lm diameter polystyrene micro-

spheres. The wet cell is sealed by placing a glass cover slip

on the top. Finally, the sample is placed upside-down on a

MATLAB-controlled piezostage.

B. Nanopatterning by OTAN

The experimental setup for OTAN system is shown in

Fig. 1. Two lasers are used: a continuous wave laser (Spectra

Physics, Model VHP80-106Q) with a wavelength of

1064 nm for trapping particles and a pulsed nanosecond laser

(Spectra Physics, Model VSL-337ND-S) with a wavelength

of 337 nm for material processing. The pulse duration is 4 ns.

The stage speed is constant (5 lm/s), and the repetition rate

varies with the distance between pulses (0.4–10 Hz). The

trapping laser is sent through an axicon lens with a micro-

scope objective and imaged to the focal plane of the objec-

tive to create a Bessel beam optical trap. Then, the

processing laser is focused by the objective to a spot of

1.45 lm in diameter on the substrate and aligned with the

trapping beam. To make a nanofeature, we trap a bead with

the trapping laser and bring it to the selected location on the

substrate. We then illuminate the bead with a processing

laser pulse, which induces material modification underneath

the bead. A pattern made of many individual features is real-

ized by moving the piezostage, and repeating the operation

until the desired pattern is completed. At the end of the

experiment, the substrate is separated from the cover slip and

tape and rinsed with deionized water to eliminate any resi-

due. The features and patterns are imaged with an optical

microscope, a Quanta 200 FEG Environmental Scanning

Election Microscope (ESEM) and a Digital Dimension

Atomic Force Microscope (AFM).

III. RESULTS AND DISCUSSION

A. Nanofeatures on silicon

At first, we investigate the creation of individual nano-

scale features on silicon. We find that two different types of

features are created: nanocones and nanoholes, depending on

the laser fluence. SEM images (Fig. 2(a)) show the differences

between a nanocone (left) and nanohole (right), but the

detailed height and width of the features cannot be obtained.

Rather, AFM characterization is needed to determine the pre-

cise depth or height of the feature (Fig. 2(b)). In order to quan-

titatively compare the features, we fit the cones and holes to a

two-dimensional (2D) Gaussian function, from which we

extract the height and full-width-at-half-maximum (FWHM).

In the case of Fig. 2(b), the nanocone has a height of 22 nm

and the nanohole is 27 nm deep. The average height and

FWHM from 60 data points at each fluence is shown in Fig.

3. The plot of height versus fluence reveals three zones (Fig.

3(a)). At low fluences, no modification of the silicon occurs

and the height is zero. When the fluence increases, there is a

range where the height is positive and cones are formed.

Finally, in the highest fluence range, the height becomes nega-

tive as nanoholes are created on the silicon surface. The

boundaries between the three regions are not clearly defined,

because the fluence resolution of our experimental setup is not

sufficient to observe sharp transitions. However, we believe

this transition occurs with continuous change from one regime

to another based on the mechanism proposed below. The crea-

tion of nanocones and nanoholes on silicon has been described

previously in the literature, but only separately, and two dif-

ferent mechanisms have been proposed. The nanocone fea-

tures might be created by the difference in density between

the liquid and solid silicon during phase transformation.15–19

Upon irradiation with a laser pulse, silicon is locally melted.

The liquid pool of silicon cools down laterally faster than

FIG. 1. Experimental setup and the sample.

FIG. 2. AFM images and cross sections of (a) a nanocone created by a laser

fluence of 8.7 mJ/cm2 and (b) a nanohole made at 10.3 mJ/cm2.

012006-2 J. Laser Appl., Vol. 29, No. 1, February 2017 Chen et al.

depthwise. Solid silicon has a lower density than liquid sili-

con, therefore, silicon expands upon resolidification and

pushes the molten liquid silicon to the center and forms a

hump. Although this mechanism is still not fully verified,

most researchers accept this hypothesis based on the experi-

mental results and available simulation.19 This explanation is

valid for silicon thin films. The case in bulk silicon has not

been investigated, but we believe that the result would be sim-

ilar to the thin film case. In the case of the nanohole, the

Marangoni effect dominates.20,21 The surface tension of liquid

silicon has a negative temperature dependency. Therefore,

after the laser pulse, the surface tension at the edge is higher

than at the center, which drives the liquid silicon to the edge

and forms a volcano crater. The surface tension effect

becomes significant only when a higher laser fluence is

applied to the substrate because the gradient of the laser inten-

sity increases with the laser fluence. These two effects are pre-

sent simultaneously but compete with each other, so that at

low fluence the resolidification dominates, therefore creating

nanocones, whereas at high fluences, the surface tension effect

is more important and craters are formed.

B. User-designed pattern on silicon

In order to serve as a security pattern, multiple distin-

guishable features encoded in the pattern are needed. The

OTAN system is a direct-write technique, so one can design

the spacing and pattern shape as needed without resorting to

multiple layers of photomasks with different spacing and

patterns. We outline a Princeton shield with nanocones on

silicon for demonstrating the direct-write ability, as shown in

Fig. 4. Since the pattern is composed of several nanofeatures,

the distance between each nanofeature has to be defined. It is

important to take the resolution of the nanopattern into con-

sideration when we design and generate security markings.

Therefore, we investigate different spacings between indi-

vidual pulses. The smallest spacing that one can choose

before an individual nanocone begins to overlap with its

neighbor is the diameter at base, about 500 nm according to

the 2D Gaussian fit in our case. Figure 5 shows the average

height and FWHM of the nanocones in a 10 � 10 array

determined by fitting a 2D Gaussian to each feature. When

the feature separation is between 500 and 1000 nm, the aver-

age height of nanocones is around 10–20 nm, but when the

separation increases to 1500–2000 nm the average height of

the nanocones increases to around 30–40 nm. The FWHM

appears to be less sensitive to the feature separation.

The phenomenon of feature spacing dependency has not

been fully studied, but we propose that the decrease in height

at smaller spacings is due to the fact that the heat affected

zone of subsequent laser pulses overlaps with existing nano-

cones. We propose that the existing nanocones are remelted

and reform a new structure after the succeeding laser pulse

illuminates the nearby area. In order to verify the hypothesis,

we used a 3D Maxwell solver from Lumerical Computational

Solution, Inc. The simulation domain contains a 2 lm polysty-

rene bead in water and a silicon substrate. The bead-substrate

spacing is set to be 50 nm, which based on our former

research, is the typical separation distance of the OTAN sys-

tem.8 The refractive indices for the polystyrene bead, water,

and silicon substrate are 1.62, 1.34, and 5.64, respectively.

We assume absorbing boundary conditions at the left, right,

and bottom boundaries. At the top boundary, a linearly polar-

ized beam with a plane wave front and a wavelength of

337 nm propagates normally to the silicon surface, which has

a silicon nanocone 500 nm away from the center of the beam

to simulate the situation when the subsequent laser pulse cre-

ates the next silicon nanocone. The intensity enhancement

factor is calculated based on the time-averaged Poynting vec-

tor at the last interaction cycle. We slice the simulation

domain and show the intensity result on the top of silicon sub-

strate in Fig. 6, which indicates the area where one sees signif-

icant intensity enhancement is in the center of the beam,

within 500 nm in radius. This enhancement profile suggests

FIG. 3. The average height and FWHM extracted from fitting a 2D Gaussian

function to the nanofeatures, in function of the laser fluence.

FIG. 4. AFM image of the Princeton shield created by OTAN.

J. Laser Appl., Vol. 29, No. 1, February 2017 Chen et al. 012006-3

that the succeeding processing laser pulse will illuminate an

existing silicon nanocone and remelt it to form a modified

structure smaller in height.

C. Anticounterfeit pattern

An ideal anticounterfeiting strategy should prevent the

exact reproduction of a security marking even when using

the same parameters and the same technique. As we have

discussed, we are able to generate a user-designed anticoun-

terfeit pattern with feature size smaller than the diffraction

limit by OTAN. The Brownian motion associated with the

trapped bead varies the z-position of the bead while the

nature of the Bessel beam trap retains a high resolution in

the x-y plane.18 Thus, the morphology of each feature is

slightly free to vary from spot to spot. In addition, the

remelting mechanism for closely spaced features adds uncer-

tainty as demonstrated in Fig. 5. These size and shape varia-

tions ensure that the nanofeatures on the genuine item are

truly unique.

Pattern authentication by examining nanoscale features

accurately can be time consuming and expensive. In order to

reduce the effort, we propose that the OTAN-created pattern is

suitable for a multilevel authentication procedure such that

each succeeding level brings an additional layer of security

while being more time and cost intensive than the previous

one. In practice, after the pattern is produced on the genuine

item, an optical, an SEM, and an AFM image of the pattern

are recorded and stored as reference. When authentication of

the item is needed, one can compare the measured images with

the reference images. In Fig. 7, we repeat the creation of a pat-

tern in multiple locations on the silicon wafer where each fea-

ture in the pattern is created using the identical conditions. In

this case, we can compare the two patterns directly. In the first

level, the optical image provides the outline of the pattern, as

seen in the top box of Fig. 7. The laser pulse causes a change

in reflectivity in the heat-affected zone,22 which is significantly

larger than the actual nanofeature and is therefore able to be

resolved in the optical image. The size of the individual fea-

tures making up the pattern is smaller than the diffraction limit,

FIG. 5. Average height and FWHM of the Gaussian fitting for the nanocone

in function of the separation distance between pulses.

FIG. 6. Finite-Difference Time-Domain (FDTD) simulation result of the

intensity enhancement of the pulsed laser induced by the microsphere on the

silicon surface.

FIG. 7. Optical microscope image, SEM image, and AFM image of the anti-

counterfeiting pattern created by OTAN. Each feature is produced using the

identical conditions of fluence¼ 7.3 mJ/cm2 and spacing¼ 1250 nm. The

inset shows a line scan along the red line in the figure.

012006-4 J. Laser Appl., Vol. 29, No. 1, February 2017 Chen et al.

thus microscope cannot resolve this information. However, the

general shape and location of the pattern can be obtained

through optical methods. Based on this image, one would infer

that the two patterns and all features within are identical within

the uncertainty of this verification protocol. In the second level

examination involves the use of SEM to obtain a high resolu-

tion image of the features. SEM is able to elucidate the shape

(nanohole versus nanocone) and location of each nanofeature

but not able to obtain the detailed height information. Figure 7

demonstrates that all the features in both patterns are nano-

cones. However, the difference between the two patterns is

slight and difficult to quantify. For instance, differences

between the two features in the circled region cannot easily be

distinguished in this method. At the final level of examination,

we use AFM to obtain the height information of the features.

This type of investigation can be expensive and time consum-

ing but is necessary only for ultimate verification if desired.

Here, one can easily see the differences between the individual

nanocones. In particular, a line scan through the two similar

nanocones from the SEM reveals that in fact, their heights dif-

fer by a factor of 3. This multilevel verification process ensures

that a pattern is genuine and not reproducible even if the exact

same system parameters are used. However, this particular

example was perhaps the worst case scenario in that we used

the same system to create the features. In general, if one were

to reproduce our setup and acquire similar microbeads to pro-

duce patterns, it would be likely that the first two screening

techniques would reveal variation in the patterns.

IV. CONCLUSION

In this paper, we investigated the use of OTAN to create

unique covert security features on silicon for the protection of

high added-value products. We exploit the statistical fluctua-

tion intrinsic to OTAN to create truly unique patterns on sili-

con that cannot be reproduced even with the identical

conditions. One can produce either nanocones or nanoholes by

simply changing the laser fluence, and combined with the

direct-write nature of the OTAN process, a multilevel encod-

ing system can record binary information in the pattern. The

patterns themselves can be imaged by optical microscopy,

SEM, and AFM to provide a three-level authentication process

where each level increases the resolution and provides addi-

tional information about the pattern. Thus rapid, inexpensive

screening or more expensive high resolution characterization

can be selected based on the user needs for authentication.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial

support from NSF (Grant No. CMMI-1235291) and AFOSR

(Grant No. FA9550-11-0208). Also, the authors

acknowledge the usage of PRISM Imaging and Analysis

Center, which is supported in part by the NSF MRSEC

program through the Princeton Center for Complex

Materials (Grant No. DMR-0819860). The authors sincerely

thank Joshua A. Spechler for help with acquiring SEM

images. In addition, T.-H. Chen thanks Elizabeth Davison

for helpful discussions.

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