Microfabrication of freestanding porous silicon particles containing spectral barcodes

Abstract

Experiments in biomedical research require the need to screen for large numbers of analytes in very small volume samples. This need has led to the development of two main classes of multiplexing technologies: microarrays and encoded microparticles [1]. While microarrays separate the individual assays in a fixed, spatially differentiated array, the individual assays in an encoded microparticle system are identified by placing a specific label on each microparticle. Specific labels such as fluorescent molecular tags [1], reflective metal stripes [2], and photonic structures [3,4] have been used. The present work is based on onedimensional photonic crystals, microfabricated from thin films of porous silicon. These materials can be functionalized with biomolecules and their use in biomolecular screening assays have been demonstrated [5]. The approach uses photolithography to define the particle dimensions in the xand y-coordinates [6]. The z-coordinate contains the encoding information, defined using a temporally modulated current waveform during electrochemical etch.The photonic crystal encoding information is produced by anodically etching p-type, boron-doped, (100) oriented silicon with <1 mΩ cm resisitivity (Siltronix, Inc.) in a solution of 48% aqueous HF:ethanol (3 :1 by volume). A computer-generated waveform containing the encoding information is used to control the electrochemical reaction. The encoding technique is described in detail in previous work [4]. Figure 1 summarizes the photolithographic process used to define the xand y-dimensions of the particle. First, an aluminum layer is sputter-coated onto the surface of the porous silicon film surface at a nominal thickness of 40 nm. The substrate is then heated at 200 °C for 3 minutes and SU8-25 photoresist (Microchem) is spin-coated on the aluminum layer, producing a ~20 µm-thick film. The function of the aluminum layer is to allow for clean removal of the photoresist from the porous silicon particles at the end of the process. The substrate is soft-baked on a hot plate in two steps: 65 °C for 3 min followed by 95 °C for 7 min. After the soft-bake, the photoresist-covered substrate is exposed to UV light (λ = 365-405 nm) for 25 s, through a contact mask. The mask consists of a square array of circles, each 25 µm in diameter with a pitch of 35 µm. The mask is aligned to the center of the porous silicon sample. This reduces spectral non-uniformity of the particles by eliminating the edges of the film from the microfabrication process. The porosity at the edges of the film varies due to restricted electrolyte diffusion and nonuniform current density in the vicinity of the o-ring seal during the electrochemical etch. The UV-exposed sample is then baked at 65 °C for 1 min and at 95 °C for 1 min. The substrate is then submerged in SU8 developer (Microchem) for 4 min. At this point the sample consists of an array of SU8 features, 25 µm in diameter, bonded to an alu-A method to microfabricate micron-scale freestanding porous silicon photonic crystal particles is d...

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Porous silicon in drug delivery devices and materials☆

“…Because they can concentrate molecules within their nanostructure and protect them from the body's natural immunological responses, the particles can potentially carry a much higher dose than could be allowed with a free drug injection. The particles can also be manufactured with well-defined shapes and dimensions, allowing the reproducible loading and release of precise quantities [44][45][46].…”

“…The film can then be converted into microparticles by ultrasonic fracture. Conventional lithography [44,45] or microdroplet patterning [46,47] methods can also be used if particles with more uniform shapes are desired.…”

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“…Very few reports on PSi patterning using dry etching techniques can be found in literature. The processes proposed are based on fluorine (Arens-Fischer et al, 2000;Tserepi et al, 2003) or chlorine plasmas (Meade & Sailor, 2007) and have been used to realize patterns with widths in the 10-100 µm range. In spite of these encouraging achievements, PSi patterning at submicrometer scale with high aspect ratios remains a real challenge for many reasons: the porous nanostructure of the material and its anisotropic morphology leading to poor efficiency in the case of such directional etching processes, the large internal surface of PSi favouring high sensitivity to contaminations such as polymer deposition during plasma etching, as well as the strongly insulating nature of the material.…”