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Nanopatterning using a simple bi-layer lift-off process for the fabrication of a photonic crystal nanostructure
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 Nanotechnology 24 085302 (http://iopscience.iop.org/0957-4484/24/8/085302) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 085302 (6pp)
doi:10.1088/0957-4484/24/8/085302
Nanopatterning using a simple bi-layer lift-off process for the fabrication of a photonic crystal nanostructure A Mao1 , C D Schaper2 and R F Karlicek Jr1 1 2
Smart Lighting Engineering Research Center, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Transfer Devices, Incorporated Santa Clara, CA 95054, USA
E-mail:
[email protected]
Received 16 October 2012, in final form 9 January 2013 Published 1 February 2013 Online at stacks.iop.org/Nano/24/085302 Abstract A simple and versatile method for fabricating nanopatterns by a lift-off procedure is demonstrated. The technique involves the use of molecular transfer lithography based on water-soluble templates to form a nanopatterned UV-curable material on a PMGI layer, which serves as an underlying resin suitable for lift-off processes. This bi-layer procedure is used for the fabrication of nickel patterns, which are subsequently used as a hard mask for plasma etch processing. Using this procedure, a two-dimensional TiO2 photonic crystal layer with a 450 nm lattice constant is fabricated on Y3 Al5 O12 :Ce3+ (YAG:Ce) yellow ceramic plate phosphor to enhance its forward emission. The yellow emission in the forward direction is improved by a factor of 3.5 compared to that of a conventional non-scattering YAG:Ce phosphor plate excited by a blue LED. (Some figures may appear in colour only in the online journal)
1. Introduction
the deposited metal material solely within the patterned areas [11]. To ensure well formed patterns following stripping, a slightly re-entrant or undercut photoresist sidewall profile is needed to prevent bridging between the material to be removed and the remaining patterned material. Common bi-layer procedures that produce suitable lift-off profiles typically use a polymeric material such as PMGI resin to form an undercut beneath the patterned top photoresist layer prior to material deposition [12]. Such lift-off procedures have been extended to the formation of high-resolution structures using NIL, where a patterned mask is pressed into a deformable material to form the patterned layer [13]. The challenges of NIL for lift-off relate to the issues NIL generally faces. It is difficult to obtain uniform, clean release of the NIL templates from the imprinted material during physical removal of the mask from the deformed material. To address such issues for large-area low-cost patterning, this paper describes a simple lift-off procedure which includes in its novelty the use of nanopattern formation in a bi-layer resist structure by molecular transfer lithography (MxL) based on water-soluble templates [14–16].
Nanoimprint lithography (NIL) is a useful technology for fabricating high-resolution patterns at nanometer scales. It is widely used in optical and semiconductor device fabrication as well as high-performance membrane and biomimetic materials development [1–6]. Other techniques such as e-beam lithography and nanosphere lithography are also used to obtain nanoscale patterns. However, e-beam lithography is expensive and time-consuming and nanosphere lithography limits the type of patterns that can be generated [7–9]. In many cases, nanopatterning hard materials (i.e. semiconductors and ceramics) requires pattern transfer with an etch resistant material using a lift-off process, followed by subsequent pattern formation by etching. Lift-off processes are a widely used patterning method for manufacturing semiconductor and biological sensor devices as well as structured optical materials [10]. A standard procedure for lift-off involves the use of optical photolithography to form a pattern in photoresist into which a material, generally a metal, is deposited, followed by removal of the resist by wet chemical stripping, leaving 0957-4484/13/085302+06$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 085302
A Mao et al
Figure 1. (a) Roll a PVA nanostructured template coated with UV-curable resist onto a substrate coated with PMGI lift-off layer; (b) expose to UV light; (c) after the resist is cured, use water to dissolve the PVA template; (d) the excess resist (residual layer) is removed by RIE etch; (e) develop the PMGI resist to get recessed sidewalls; (f) nickel deposition and lift-off to get a metal mask; (g) pattern is transferred to the substrate by ICP–RIE etch.
To demonstrate the performance of this lift-off technique, photonic crystal structures are produced for use with non-scattering phosphors excited by LEDs. Specifically, a photonic crystal structure is fabricated on top of the Czochralski grown YAG:Ce polished ceramic plate phosphor (CPP) with a refractive index around 1.82 for visible light to increase the directional extraction efficiency of the yellow light. Since YAG:Ce cannot be easily etched by conventional ICP–RIE dry etching methods, the patterns were developed in a thin layer of sputtered TiO2 deposited on top of the phosphor to demonstrate enhanced directional emission of the yellow light generated upon excitation of the YAG:Ce CPP with a blue LED. The novelty and simplicity of this bi-layer lift-off process is easily extended to the fabrication of nanostructures useful for many other applications, and should
be easily scaled to the fabrication of large-area nanostructures in rigid or flexible materials.
2. Experimental procedure The fabrication procedure for the formation of a Ni hard mask by lift-off is shown in figure 1. In this procedure, a UV-curable photoresist mr-UVcur21 (from micro resist technology Gmbh) and a standard lift-off resin of PMGI SFG 2S (from MicroChem Corporation) make up the bi-layer stack. The method begins with PMGI spin coated onto the substrate, and as in MxL processes, mr-UVcur21 is spin coated onto water-soluble templates with a diameter of 50 mm. The water-soluble templates of the photonic crystal pattern are composed of polyvinyl alcohol (PVA) and were 2
Nanotechnology 24 (2013) 085302
A Mao et al
Figure 2. (a) Pattern on the surface of the water-soluble PVA template; (b) after applying the MxL process using the PVA water-soluble template, the pattern in cured mr-UVcur21 is shown.
Figure 3. (a) Patterned bi-layer stack with thick mr-UVcur21 residual layer; (b) deformed pattern of bi-layer resist after MxL.
fabricated and provided by Transfer Devices Inc. (TDI) using previously described processing methods [12]. The resist side of the coated water-soluble PVA template is brought into contact with the PMGI coated substrate by a six-roller laminator similar to a commercial MxL patterning product offered by Transfer Devices Inc. [13] that has temperature control of the lamination rollers. After lamination, the mr-UVcur21 resist is cured by broadband ultraviolet light and then the PVA template is removed by dissolving it in water to produce a negative copy of the template nanostructure in the resist on the PMGI layer on the substrate. There is a residual layer of mr-UVcur21 at the base of the nanostructure that requires removal, which is performed by anisotropic plasma etching, which also removes any underlying PMGI layer that is exposed to the plasma. The thickness of the PMGI resist used depends on the pattern size and height of the template. If the thickness is not great enough, the thickness of the hard mask that can be deposited to the substrate will be small, while if the thickness is too great, the underlying PMGI layer will not be fully removed during the anisotropic plasma etching process. In these experiments, the etch pressure is 45 mTorr, RIE power is 75 W, O2 is 15 sccm, CHF3 is 60 sccm. After that, the PMGI is developed using diluted MIF 300 (1:2 dilution) to achieve the recessed sidewalls needed to complete the lift-off structure (figure 1(e)). Then nickel film is deposited by electron-beam evaporation and lift-off is performed using Remover PG (from MicroChem Corporation) in an ultrasonic bath. To ensure the success of the lift-off process, the thickness of the nickel that is deposited should be no more than 75% of
the thickness of the PMGI. The PVA templates manufactured by Transfer Devices Inc. contained triangular lattice pillars with a thickness (h) of 150 nm (figure 2(a)). The diameter (d) of the pillars is 300 nm; the lattice constant (a) is 450 nm. Figure 2(b) shows the replicated pattern of the master template.
3. Results and discussion Selection and optimization of the specific processing conditions used for the lift-off procedure are related to the design of the targeted nanostructure as well as the PVA templates which form the pattern in the resist. If the aspect ratio of the structures on the PVA template is low (
