다중신호로 암호화된 다공성 실리콘

Abstract

Photonic crystals containing rugate structure result in a mirror with high reflectivity in a specific narrow spectral region and are prepared by applying a computer-generated pseudo-sinusoidal current waveform. Strategies to encode multiple rugate structures have been investigated. Multiple rugate structures can be etched on a silicon wafer and placed in the same physical location, showing that many sharp spectral lines can be obtained in the optical reflectivity spectrum. The method used to generate multiple rugate structures containing all the encoding information displays five rugate peaks corresponding to the each of the sine components varied from 0.16 to 0.24 Hz, with a spacing of 0.02 Hz between each sine component. The complete deletion of a peak has been achieved and demonstrates the capacity of this method to create binary codes. The development of a new technology to build one-dimensional photonic structure is of great interest because it is too complex to fabricate by using conventional lithographic method and could be useful for a variety of applications such as chemical and biological sensors or medical diagnostics. Since the discovery of porous silicon from silicon wafer, research has been associated with emerging technologies, such as photonic crystals for optical band pass filters and micro chemical reaction applications in micro chemical reactors and micro fuel cells. Their importance is due to their very high surface area for reactions, their unique photonic properties and process compatibility to semiconductor technology. This high surface area material has been shown to be useful in a variety of analytical applications; in MEMS (micro electro mechanical systems), as a matrix for MALDI (matrix assisted laser desorption ionization) mass spectroscopy and especially in chemical and biological sensors. Multiplexed assays are one of the powerful tools to study genomes, to identify candidate drugs, and in a variety of other applications in which a large number of parallel experiments should be performed in a short period of time. Recently, beads or particles containing many independent codes by using quantum dots, fluorescent molecules, and metal rods have been used for these applications. Here we report a method of preparing one-dimensional photonic crystals with multiple peaks in their optical reflectivity spectrum. Sample Preparation: Porous silicon samples were prepared by electrochemical etch of heavily doped p++-type silicon wafers (boron doped, polished on the (100) face, resistivity of 0.8-1.2 mΩ-㎝, Siltronix, Inc.). The etching solution consisted of a 3:1 volume mixture of aqueous 48% hydrofluoric acid (ACS reagent, Aldrich Chemicals) and absolute ethanol (ACS reagent, Aldrich Chemicals). Galvanostatic etch was carried out in a Teflon cell by using a two-electrode configuration with a Pt mesh counter electrode. A sinusoidal current density waveform varying between 51.5 and 74.6 mA/㎠ is applied. The anodization current was supplied by a Keithley 2420 high-precision constant current source which is controlled by a computer to allow the formation of PSi multilayers. To prevent the photogeneration of carriers, the anodization is performed in the dark. After formation the samples are rinsed with pure ethanol and dried with nitrogen gas. Instrumentation and Data Acquisition: Samples were illuminated with a tungsten lamp, and the reflected light spectrum was measured using an Ocean Optics S2000 CCD spectrometer fitted with a fiber optic input. The reflected light collection end of the fiber optic is positioned at the focal plane of the optical microscope. Photonic crystals containing rugate structure result in a mirror with high reflectivity in a specific narrow spectral region and are prepared by applying a computer-generated pseudo-sinusoidal current waveform. The Rugate PSi generated in this method exhibits a very sharp line in the optical reflectivity spectrum. This reflectivity can be tuned to appear anywhere in the visible to near-infrared spectral range, depending on the programmed etch waveform. One of the most unique features for multilayer porous silicon is that its reflective spectral band is much narrower than the fluorescence spectrum obtained from an organic dye or core-shell quantum dot[9]. Thus more spectral lines can be placed in a narrower spectral window with the photonic structures. Rugate filters possess a sinusoidally varying porosity gradient in the direction perpendicular to the plane of the filter. The waveform used in the present work involves an individual sine component, which is represented by Equation 1. Yi = Aisin(kit) + B (1) Where Yi represents a temporal sine wave of amplitude, Ai ; frequency, ki ; time, t; applied current density, B. Strategies to encode multiple rugate structures have been investigated. Multiple rugate structures can be etched on a silicon wafer and placed in the same physical location, showing that many sharp spectral lines can be obtained in the optical reflectivity spectrum. Two methods are used to generate multiple rugate structures. First, a set of five sine components (Fig. 1A) combined each individual sine component has been used to create multiple rugate structure, Equation 2. (Fig. 1B) Ycomp = (y1, y2, … , yn) (2) Second, all of the individual sine components are summed together to create a composite waveform shown in Fig. 1C, Equation 3. Ycomp = A1sin(k1t) + A2sin(k2t) + … + Ansin(knt) + B (3) Equation 2 and 3, containing all the encoding information, can be converted to an analog current-time waveform to etch using a computer-controlled digital galvanostat. Multiple rugate structures have been generated by using different parameters in Equation 1 and displayed five peaks shown in Fig. 2. A waveform containing five separate frequency components has been investigated. The values of ki for each of the sine components are varied from 0.16 to 0.24 Hz, with a spacing of 0.02 Hz between each sine component. The values of Ai and B for every sine components are 11.55 and 63.05 mA, respectively. Reflectivity spectra of multiple rugate-structured porous silicons etched continuously with five separate periodicities using a former method are obtained and displayed five peaks as shown in Fig. 2 (top), but the rugate peaks are not placed in the same physical location. Multiple rugate structures have been successfully generated by using an Equation 3 and displayed five peaks as shown in Fig. 2 (bottom). The resulting rugate porous silicon film exhibits a porosity depth profile which relates directly to the current-time profile used in etch. Each of the main peaks in an optical reflectivity spectrum corresponds to one of the sine components of the composite waveform, indicating that the reflectivity spectrum represents the Fourier transform of the composite current-time waveform. Reflectance spectra of five encoded porous silicon samples are prepared and showed five bit encoding. The spectra shown in Fig. 3 are divided up to five regions, and the bit representation of each peak in the spectrum is superimposed over each peak. In latter case, both the wavelength and the amplitude of the spectral peaks are controllable by changing the etch parameters and could be useful for an encoding information. The complete deletion of a peak shown in Fig. 3 has been achieved and demonstrates the capacity of this method to create binary codes, while the wavelengths and relative amplitudes of the remaining peaks has been fixed. The presence or absence of a spectral line can be recognized as an on state or an off state, respectively. A robust method used to prepare optically encoded rugate porous silicon which could be useful to create multiplexed assays has been demonstrated. A composite waveform summed all of the individual sine components results multiple rugate peaks placed in the same physical location. Each individual spectral peak in an optical reflectivity spectrum corresponds to one of the sine components of the composite waveform and is controllable to create binary codes.