Silicon-based photonic crystals fabricated using proton beam writing combined with electrochemical etching method
© Dang et al.; licensee Springer. 2012
Received: 27 April 2012
Accepted: 23 July 2012
Published: 23 July 2012
A method for fabrication of three-dimensional (3D) silicon nanostructures based on selective formation of porous silicon using ion beam irradiation of bulk p-type silicon followed by electrochemical etching is shown. It opens a route towards the fabrication of two-dimensional (2D) and 3D silicon-based photonic crystals with high flexibility and industrial compatibility. In this work, we present the fabrication of 2D photonic lattice and photonic slab structures and propose a process for the fabrication of 3D woodpile photonic crystals based on this approach. Simulated results of photonic band structures for the fabricated 2D photonic crystals show the presence of TE or TM gap in mid-infrared range.
Periodically modulated refractive-index structures, i.e., a photonic crystal which can modulate the flow of electromagnetic waves, exhibit photonic band gaps under certain conditions [1, 2]. As electron mobility in a semiconductor can be controlled by engineering the electronic bands of these materials, electromagnetic wave propagation inside a photonic crystal may be manipulated by machining its photonic bands . Silicon-based photonic crystals are one of the most promising choices due to their easy integration in silicon technology, allowing applications in several fields, such as optical devices, including waveguides [4, 5], resonators , etc. A lot of work has been reported on the fabrication and theoretical study of two-dimensional (2D) silicon-based photonic crystals  because of the advantages of easy integration and applications in planar platforms , such as planar waveguides [9, 10]. Porous silicon-based photonic crystal is another promising candidate to be integrated in silicon technology [7, 8, 11]. To completely manipulate the flow of electromagnetic waves, a three-dimensional (3D) photonic crystal with complete band gap is required. Many methods have been reported to be able to fabricate 3D silicon-based photonic crystals with 3D complete band gap, such as double-angled reactive ion etching , macropore formation in silicon , glancing-angle deposition , and colloidal self-assembly . One type of 3D photonic crystal that has attracted great attention is the 3D woodpile structure. Several techniques have been reported on the fabrication of 3D silicon-based woodpile structures, such as silicon double-inverse method  and layer-by-layer approach . However, in the year 2000, Chow et al. reported the fabrication of a 2D photonic crystal slab capable of fully controlling light in all three dimensions , where the periodic dielectric structure is in only two dimensions, and index guiding is used to confine light in the third one. Most of the reported silicon-based photonic slabs are based on silicon-on-insulator platform [4–6].
In the present work, a method to fabricate 2D/3D silicon-based photonic crystals that uses high-energy proton beam writing and subsequent electrochemical etching of p-type bulk silicon wafers is presented. This technique uses either the whole defect regions at high ion fluence to completely inhibit the etching process or localized high defect density at the end-of-range region of high-energy protons at moderate fluence for the fabrication of silicon structures within the bulk silicon at certain depths, based on selective formation of porous silicon in other regions during subsequent anodization. A finely focused, high-energy ion beam  is scanned over the silicon wafer surface. As the ion beam penetrates the silicon, the crystal lattice is damaged, producing additional defects, which reduces the localized hole density and hole current [20, 21]. The defect density for light ions, with energies greater than about 50 keV, peaks close to the end of their range . By pausing the focused beam of different energy for different amounts of time at different locations, any pattern of localized damage can be built up. The irradiated wafer is then electrochemically anodized in an electrolyte of Hydrofluoric acid(HF). At a high ion fluence, the irradiated regions completely inhibit the formation of porous silicon and remain as silicon, based on which, Teo et al. has reported that fabrication of a periodic array of sub-micron diameter pillars is potentially important for the fabrication of photonic crystals . While at a moderate ion fluence, only the buried regions with high defect density inhibit the porous silicon formation process. Thus, as the sample is etched beyond the depth of the ion range, the structure starts to become undercut due to isotropic etching, producing a silicon core that is surrounded by porous silicon. Multiple-energy proton irradiation can be used to create localized defects at different depths within the silicon wafer to fabricate multilevel 3D structures . By varying the proton energy, the penetration depth changes, and subsequent etch steps enable the fabrication of true 3D silicon freestanding structures.
Additionally in this work, some sample structures of 2D photonic crystals are shown: a square lattice of silicon pillars in an air matrix, which utilize the complete inhibition of etching in irradiated regions, and a 2D photonic slab of air holes in silicon matrix, which utilizes the highly damaged regions at the end of range of ions in silicon. Theoretical photonic band structures of these structures were calculated, showing a complete transverse magnetic (TM) gap for the first structure and several complete transverse electric (TE) gaps for the second one. To further explore the fabrication of 3D photonic crystal structure using this approach, the fabrication of 3D silicon-based woodpile structure is proposed, and its initial result is shown.
A nuclear microprobe at the Centre for Ion Beam Applications, National University of Singapore, was used . High-energy protons with 200 keV to 1 MeV can be focused down to 100 nm. By controlling the duration time of protons on different points of the surface of a 0.02- or 0.4-Ω·cm p-type silicon wafer, designed defect distributions in the silicon wafers were built. Etching for irradiated silicon wafers was performed in an electrolyte of HF (48%): ethanol in the ratio of 1:1, with a current density of 40 mA/cm2 for 0.02-Ω·cm and 60 mA/cm2 for 0.4-Ω·cm wafers. A further step of dipping in KOH solution removed the porous silicon.
2D photonic lattice on silicon substrate: silicon pillars with square lattice
2D freestanding photonic slab: a photonic slab with square lattice of air hole in silicon matrix
3D woodpile structure
Here, we propose the fabrication of a 3D woodpile structure using proton beam writing to generate end-of-range regions with high defect density at different depths. In order to fabricate woodpile structure with one period, protons with four different energies, E1, E2, E3, and E4, will be required to irradiate lines in the same area with suitable fluence and alignment, as shown in Figure 6b. Figure 6c shows an initial result on two-layer freestanding silicon wires in two directions using 250 and 200-keV protons, respectively, in 0.02-Ω·cm silicon wafers supported by thick walls from 1-MeV protons. By carefully controlling the alignment, ion energy, ion fluence, and etching current density, a 3D woodpile structure with complete band gap lying in the mid-infrared (IR) range should be possible.
Results and discussions
The MIT photonic band package  has been used to study the photonic band structure (PBS) of these photonic crystal structures. The software is based on conjugate-gradient minimization of the Rayleigh quotient in a plane-wave basis .
PBS for silicon pillars with square lattice in an air matrix on silicon substrates
On the other hand, in Figure 7a, other gaps open at higher frequencies. Between the third and fourth bands, a gap opens from 0.345 to 0.365 of the normalized frequency, and between the sixth and seventh, from 0.513 to 535. However, no TE gap opens in either structure. The flexibility of the fabrication process allows varying the radius of the Si pillar and the lattice period to tune the frequency ranges where the gaps can be opened for a specific application.
PBS for photonic slab with square lattice of air hole in silicon matrix
In this case, as a freestanding structure was studied, novel approaches were needed. First, the eigenstates of the slab were calculated using a z-supercell approach, where guided bands were unaffected. Then, the light cone was obtained and overlapped .
We have shown a highly flexible approach of using electrochemical etching following proton beam writing of bulk p-type silicon wafers to fabricate 2D/3D silicon-based photonic crystals. Simulation studies show that the structure of silicon pillars with square lattice has TM gaps in the mid-IR range, while the structure of air holes with square lattice in silicon matrix has TE gaps in the same wavelength range. Photonic crystals fabricated using this approach have an extra degree of tuning from porous silicon. Based on the platform of bulk wafers and being compatible with silicon technology, this flexible fabrication method is a promising candidate for the development of silicon-based photonic crystals.
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