Directed dewetting of amorphous silicon film by a donut-shaped laser pulse
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Nanotechnology Nanotechnology 26 (2015) 165303 (8pp)
Directed dewetting of amorphous silicon ﬁlm by a donut-shaped laser pulse Jae-Hyuck Yoo1, Jung Bin In2,5, Cheng Zheng1, Ioanna Sakellari3, Rajesh N Raman4, Manyalibo J Matthews4, Selim Elhadj4 and Costas P Grigoropoulos1 1
Department of Mechanical Engineering, Laser Thermal Laboratory, University of California Berkeley, Berkeley, CA 94720-1740, USA 2 Department of Nanomechanics, Nano-Convergence Mechanical Systems Research Division, Korea Institute of Machinery & Materials (KIMM), Daejeon 305-343, Korea 3 4th Physics Institute, University of Stuttgart, Stuttgart, D-70550, Germany 4 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA E-mail: [email protected]
Received 8 January 2015, revised 13 February 2015 Accepted for publication 25 February 2015 Published 1 April 2015 Abstract
Irradiation of a thin ﬁlm with a beam-shaped laser is proposed to achieve site-selectively controlled dewetting of the ﬁlm into nanoscale structures. As a proof of concept, the laserdirected dewetting of an amorphous silicon thin ﬁlm on a glass substrate is demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon ﬁlm melts and dewets on the substrate surface. The irradiation with the donut beam induces an unconventional lateral temperature proﬁle in the ﬁlm, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot. Upon solidiﬁcation, the ultrathin amorphous silicon ﬁlm is transformed to a crystalline silicon nanodome of increased height. This morphological change enables further dimensional reduction of the nanodome as well as removal of the surrounding ﬁlm material by isotropic silicon etching. These results suggest that laser-based dewetting of thin ﬁlms can be an effective way for scalable manufacturing of patterned nanostructures. S Online supplementary data available from stacks.iop.org/NANO/26/165303/mmedia Keywords: donut beam, dewetting, thermocapillary, crystallization, nanosecond laser (Some ﬁgures may appear in colour only in the online journal) Introduction
practical use of these methods is still challenging since additional fabrication steps are required to build substrate templates and ﬁlm patterns. Furthermore, complicated experimental apparatus should be integrated, in order to apply designed electrical ﬁeld or heating. On the contrary, laser irradiation can directly control dewetting of thin ﬁlms in a facile manner. Upon irradiation with a focused laser beam, the absorbed heat can locally melt the target material inducing a steep temperature gradient across the melt surface. This temperature gradient also drives a gradient in surface tension and hence generates thermocapillary force-induced material transport. When the ﬁlm is ultrathin, laser irradiation can lead to dewetting through this process. Negative line patterns were written in a polystyrene polymer ﬁlm based on this mechanism by scanning a laser
Dewetting of thin ﬁlms into nanoscale structures has been actively exploited for scalable nano-manufacturing of various materials such as polymers , metals [2–7], and semiconductors [8, 9]. In general, uncontrolled, spontaneous dewetting yields random arrangement of the nanostructures, with the well-known instability [5–7]. Therefore, spatially well-ordered nanostructures were achieved by various methods such as using substrate templates , pre-patterning thin ﬁlms [2, 8, 9, 11–13], and applying electric ﬁelds  or temperature gradients . However, 5
Present address: School of Mechanical Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 156-756, Korea.
© 2015 IOP Publishing Ltd Printed in the UK
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beam of Gaussian intensity proﬁle to induce transport of the material away from the center of the beam spot . In this paper, we used a shaped laser beam for siteselective fabrication of nanostructures via thermocapillaryinduced dewetting of thin ﬁlms. We applied a donut-shaped laser beam for spot-wise dewetting of an amorphous silicon ﬁlm on a fused silica substrate. Each pulse of the donut beam could produce a nanoscale crystalline silicon dot (or nanodome) at the center of the beam spot. Furthermore, we could reduce the size of the nanodome by isotropic dry etching of silicon. Our laser-based approach for thermocapillary-induced dewetting can provide a promising pathway for scalable manufacturing of silicon nanodome structures that can be useful for photonic applications [17–19].
Due to the morphology of the silicon nanodome that is different than the 2D type ﬁlm structure, an isotropic silicon etching could shrink down the silicon nanodome with maintaining its morphology simply by the etching time control. Simultaneously, the rim and the initial amorphous ﬁlm were removed since they were thinner than the silicon nanodome. As a result, smaller silicon nanodomes selectively remained on the substrate. For the isotropic etching, we employed the reactive ion etching with 50 sccm SF6 (with 10% O2) at the power of 50 W and the pressure of 100 mTorr. For reproducible results, 10:1 BHF treatment for 2 s was performed before the selective etching to remove any oxide layer (that can be a mask layer for the dry etching) on the silicon nanodome, whereby the original nanodome shape engraved in the fused silica substrate could be observed in the SEM pictures of ﬁgure 5(b).
Ultrathin amorphous film preparation
20 nm thick amorphous silicon ﬁlm was deposited on a fused silica substrate by the standard low-pressure chemical vapor deposition with a silane (SiH4) precursor at the deposition temperature of 550 °C.
SEM imaging was performed on a FEI-Nova SEM. In order to avoid the charging effect due to the dielectric substrate, 2 s 10:1 BHF treatment and DIW rinsing were performed, followed by sputtering of conductive material. A lab built Raman system was used for the Raman spectrum measurement. For the wavenumber calibration, a HgNe calibration lamp (Newport) was used. The continuous Raman laser was of 532 nm wavelength and a 100× objective lens (NA: 0.8, Nikon) was employed. Liquid nitrogen cooled spectrometer with 1200 grating was used.
Donut beam shaping
The input laser beam was of 2 nm wavelength, 22.7 ns fullwidth-half-maximum pulse duration, and 1 kHz pulse repetition rate. The input Gaussian laser beam was modulated via the vortex phase plate (RPC Photonics) by the destructive interference that was established in the beam center, resulting in the donut-shaped laser beam. The modulated beam was delivered on the sample via a 20× objective lens (NA:0.42, Mitutotyo) and the 1/e2 beam radius of the input beam without the vortex phase plate was 2.42 μm, which was measured by the standard knife edge method. The morphology transformation from the ultrathin amorphous ﬁlm to the silicon nanodome occured upon irradiation of isolated single pulses as the sample was translated at 10 mm s−1 by a motorized stage and the laser ﬁred at 1 kHz. As a result, the train of pulses was spatially distributed on the amorphous silicon ﬁlm with 10 μm inter-distance. The laser power was controlled by the power attenuator set consisting of a half wave plate and a polarizing beam splitter.
For the temperature proﬁle simulation in ﬁgure S2 (supporting information) of the amorphous ﬁlm under the donutshaped laser beam irradiation, FEM (COMSOL) was used to solve the diffusive heat transfer model that is valid since the phonon mean free path of amorphous silicon ﬁlm is of atomic scale and the time scale of interest is in the nanosecond regime. Due to the radial symmetry of the donut-shaped laser beam intensity proﬁle (ID(r)), axisymmetric ﬁnite element method (FEM) was carried out for a 2 mm diameter computational domain. For materials properties such as K(T) and Cp(T), temperature-dependent values were applied to the simulation with accounting for the latent heat . In order to estimate light energy absorption throughout the thin ﬁlm for the temperature proﬁle simulation, FDTD (Lumerical) simulation was performed in ﬁgure S3 (supporting information).
The SECCO etchant consists of HF, K2Cr2O7, and H2O . The oxidizing agent (K2Cr2O7) converted the amorphous silicon to a soluble oxide relatively faster than the crystallized silicon. The oxide was removed by the HF, and then the continuous chemical process delineated the grain boundary of multi-crystalline structure. For the grain boundary inspection of the silicon nanodome in ﬁgure 4, 20 s etching was performed with the diluted SECCO etchant by DIW (DIW: SECCO = 4:1).
Results and discussion Directed dewetting by donut-shaped laser beam pulse
Figure 1(a) describes morphology change of a bulk silicon substrate (left) and a silicon thin ﬁlm on a fused silica substrate (right) upon irradiation of a Gaussian beam and a donutshaped laser beam, respectively . Since the surface 2
Nanotechnology 26 (2015) 165303
J-H Yoo et al
Figure 1. Silicon nanodome array by donut-shaped laser beam pulses (a) schematics of morphology transformations of bulk silicon upon
Gaussian laser beam irradiation and ultrathin silicon ﬁlm upon donut-shaped laser beam irradiation. IGaussian and Idonut are the laser intensity proﬁles for the Gaussian laser beam and the donut-shaped laser beam, respectively. The temperature proﬁle (Tﬁlm) is presented with a dashed gray line. The solid arrows indicate the transport directions of molten silicon upon the laser irradiation. (b) Schematics of experimental conﬁguration for producing silicon nanodome array by donut-shaped laser beam pulses with stage translation. (c) SEM pictures of silicon nanodome array in top view (left) and tilted view (right). The laser ﬂuence used for the silicon nanodomes is 0.74 J cm−2. The pitch between silicon nanodomes is 10 μm. Scale bars are 5 μm. The location of a donut-shaped laser beam pulse is illustrated on the SEM pictures in green. (d) Magniﬁed SEM pictures of silicon nanodome in top view (left) and tilted view (right). Scale bars are 1 μm.
standard deviation of 41.3, which is 20 times larger than the initial ﬁlm thickness. The peripheral rim that is marked with a white solid arrow was also produced by the same mechanism, although in this case, the height increase was mild since the molten silicon spread radially outwards.
tension of liquid silicon decreases with increasing temperature , the temperature gradient imposed by the Gaussian beam on the molten silicon layer induces the surface tension gradient that drives the molten silicon ﬂuid from the hot center (lower surface tension) to the cold periphery (higher surface tension). The mass transfer direction is indicated by the solid arrows in the ﬁgure 1(a). In contrast, when the donut-shaped temperature proﬁle is developed, the material transport can be in the opposite direction—i.e. the material in the hot melt region is transported radially inward to the cold center as shown in ﬁgure 1(a). More importantly, in the case of ultrathin (