Monolithic integration of microfluidic channels and optical waveguides in silica on silicon

Sealing of the flow channel is an important aspect during integration of microfluidic channels and optical waveguides. The uneven topography of many waveguide-fabrication techniques will lead to leakage of the fluid channels. Planarization methods such as chemical mechanical polishing or the etch-back technique are possible, but troublesome. We present a simple but efficient alternative: By means of changing the waveguide layout, bonding pads are formed along the microfluidic channels. With the same height as the waveguide, they effectively prevent leakage and hermetically seal the channels during bonding. Negligible influence on light propagation is found when 10-mum-wide bonding pads are used. Fabricated microsystems with application in absorbance measurements and flow cytometry are presented.     

Calcium-assisted glass-to-glass bonding for fabrication of glass microfluidic devices

Glass is a desired material for many microfluidics applications. It is chemically resistant and has desirable characteristics for capillary electrophoresis. The process to make a glass chip, however, is lengthy and inconvenient, with the most difficult step often being the bonding of two planar glass substrates. Here we describe a new glass bonding technique, which requires only washing of the glass surfaces with a calcium solution and 1-2 h of bonding at 115 degrees C. We found calcium uniquely allows for this simple and efficient low-temperature bonding to occur, and none of the other cations we tried (e.g., Na (+), Mg (2+), Mn (3+)) resulted in satisfactory bonding. We determined this bond is able to withstand high applied field strengths of at least up to 4 kV x cm (-1). When intense pressure was applied to a fluid inlet, a circular portion of the coverslip beneath the well exploded outward but very little of the glass-glass interface debonded. In combination with the directed hydrofluoric acid etching of a glass substrate using a poly(dimethylsiloxane) (PDMS) etch guide, we were able to make glass chips with better than 90% yield within 6 h. This technique is compatible with inexpensive unpolished glass and is limited in resolution by the PDMS etch guide used and the intrinsic properties of isotropic etching.     

Fabrication of silicon based glass fibres for optical communication

Silicon based glass fibres are fabricated by conventional fibre drawing process. First, preform fabrication is carried out by means of conventional MCVD technique by using various dopants such as SiCl₄, GeCl₄, POCl₃, and FeCl₃. The chemicals are used in such a way that step index single mode fibre can be drawn. The fibre drawing process consists of various steps such as heating the preform at elevated temperature, diameter monitor, primary and secondary coating, and ultra violet radiation curing. The fibres are then characterized for their geometrical and optical properties. The drawn fibre has diameter of core and cladding to be 8.3 μm and 124.31 μm, respectively whereas non-circularity is found to be 4.17% for core and 0.26% for cladding as seen from phase plot. Mode field diameter is found to be 8.9 μm and 9.2 μm using Peterman II and Gaussian method, respectively. The fabricated fibres showed the signal attenuation of 0.35 dB/km and 0.20 dB/km for 1310 nm and 1550 nm, respectively as measured by the optical time domain reflectometer (OTDR).     

Numerical and experimental study of microfluidic devices in step-index optical fibers

Microfluidic devices composed of microslits in step-index optical fibers are thoroughly investigated. Numerical simulations are performed to explain scattering and power loss in such devices. Experimental results based on microslits fabricated by femtosecond laser processing corroborate theoretical data. Dependency of the device performance on the refractive index of fluid in the slit is further utilized to construct a refractive index sensor and an in-fiber attenuator.     

Monolithic integration of microfluidic channels, liquid-core waveguides, and silica waveguides on silicon

The fabrication of embedded microchannels monolithically integrated with optical waveguides by plasma-enhanced chemical vapor deposition of doped silica glass is reported. Both waveguide ridges and template ridges for microchannel formation are patterned in a single photolithography step. The microchannels are formed within an overlay of borophosphosilicate glass (BPSG), which also serves as the top cladding layer of the silica waveguides. No top sealing of the channels is required. Surface accessible fluid input ports are formed in a BPSG layer, with no additional steps, by appropriate design of template layers. By tightly controlling the refractive index of the waveguide layer and the microchannel-forming layer, fully integrated structures facilitating optical coupling between solid waveguides and liquids segments in various geometries are demonstrated. Applications in liquid-filled photonic device elements for novel nonlinear optical devices and in optical sensors and on-chip spectroscopy are outlined.     

Superlocalization of single molecules and nanoparticles in high-fidelity optical imaging microfluidic devices

Superlocalization of single molecules and nanoparticles with a precision of subnanometer to a few tens of nanometers is crucial for elucidating nanoscale structures and movements in biological and chemical systems. A novel design of ultraflat and ultrathin glass/polydimethylsiloxane (PDMS) hybrid microdevices is introduced to provide almost uncompromised optical imaging quality for on-chip superlocalization and super-resolution imaging of single molecules and nanoparticles under a variety of microscopy modes. The performance of the high-fidelity (Hi-Fi) optical imaging microfluidic device was validated by precisely mapping micronecklaces made of fluorescent microtubules and 40 nm gold nanoparticles and by demonstrating the activation and excitation cycles of single Alexa Fluor 647 dyes for direct stochastic optical reconstruction microscopy in PDMS-based microchannels for the first time. Furthermore, the microdevice's feasibility for multimodality microscopy imaging was demonstrated by a vertical scan of live cells in epi-fluorescence and differential interference contrast (DIC) microscopy modes simultaneously.     

Femtosecond pulsed laser micromachining of glass substrates with application to microfluidic devices

We describe a technique for surface and subsurface micromachining of glass substrates by using tightly focused femtosecond laser pulses at a wavelength of 1660 nm. A salient feature of pulsed laser micromachining is its ability to drill subsurface tunnels into glass substrates. To demonstrate a potential application of this micromachining technique, we fabricate simple microfluidic structures on a glass plate. The use of a cover plate that seals the device by making point-to-point contact with the flat surface of the substrate is necessary to prevent the evaporation of liquids in open channels and chambers. Methods for protecting and sealing the micromachined structures for microfluidic applications are discussed.     

Characterization of borophosphosilicate glass soot fabricated by flame hydrolysis deposition for silica-on-silicon device applications

The use of flame hydrolysis deposition (FHD) to fabricate porous silica glass soot in the B₂O₃-P₂O₅-SiO₂ glass system (BPSG) is described for silica-on-silicon device applications. The deposition conditions with a Si substrate temperature (200 °C) and a flame temperature (1300–1500 °C) are appropriate to synthesize the SiO₂ and P₂O₅-SiO₂ non-crystalline glass soot. However, further investigations for the B₂O₃-P₂O₅-SiO₂ glass soot are needed to obtain complete amorphous phases. The densification process of porous silica glass soot in the three systems of SiO₂, P₂O₅-SiO₂ and B₂O₃-P₂O₅-SiO₂ is also described to estimate the onset of sintering temperature. The OH absorption measurements are performed to try to identify incorporation of hydroxyl contaminants in the systems of P₂O₅-SiO₂ and B₂O₃-P₂O₅-SiO₂.     

Fabrication and optical characterization of light trapping silicon nanopore and nanoscrew devices

We have fabricated nanotextured Si substrates that exhibit controllable optical reflection intensities and colors. Si nanopore has a photon trapping nanostructure but has abrupt changes in the index of refraction displaying a darkened specular reflection. Nanoscrew Si shows graded refractive-index photon trapping structures that enable diffuse reflection to be as low as 2.2% over the visible wavelengths. By tuning the 3D nanoscale silicon structure, the optical reflection peak wavelength and intensity are changed in the wavelength range of 300-800?nm, making the surface have different reflectivity and apparent colors. The relation between the surface optical properties with the spatial features of the photon trapping nanostructures is examined. Integration of photon trapping structures with planar Si structure on the same substrate is also demonstrated. The tunable photon trapping silicon structures have potential applications in enhancing the performance of semiconductor photoelectric devices.     

Optical properties of fluorinated silicon oxide films by liquidphase deposition for optical waveguides

Optical properties of fluorinated silicon oxide (SiOF) films for optical waveguide in optoelectronic devices were investigated. The SiOF films are formed at 25°C by a liquid phase deposition (LPD) technique using a supersaturated hydrofluosilicic acid (H₂SiF ₆) aqueous solution. Two main absorption peaks corresponding to Si-O and Si-F bonds were observed at the wavenumbers of 1090 and 930 cm^-1 in Fourier transform infrared (FTIR) spectrum, respectively. The LPD-SiOF films show very little content of water components such as Si-OH bonds and OH group. Although the transmittance for 600-nm-thick LPD-SiOF film gradually decreased from the wavelength around 700 nm, the relative transmittances to quartz glass are over 98% in the wavelength region from 350-2500 nm. The concentration of fluorine atoms in the LPD-SiOF film was about 5%, and the calculated composition was SiO_1.85F_0.15. The calculated refractive index from the polarizability for LPD-SiOF film was 1.430, and agrees very well with the measured value at the wavelength of 632.8 nm by ellipsometry. The dispersion of refractive index was evaluated and fitted to a three-term Sellmeier's dispersion equation. The zero dispersion wavelengths for the LPD-SiOF and thermally grown SiO₂ films were 1.271 and 1.339 μm, respectively     

Electrophoretic analysis of N-glycans on microfluidic devices

We report rapid and efficient electrophoretic separations of N-glycans on microfluidic devices. Using a separation length of 22 cm and an electric field strength of 750 V/cm, analysis times were less than 3 min, and separation efficiencies were between 400,000 and 655,000 plates for the N-glycans and up to 960,000 plates for other sample components. These high efficiencies were necessary to separate N-glycan positional isomers derived from ribonuclease B and linkage isomers from asialofetuin. Structural isomers of N-glycans derived from a blood serum sample of a cancer patient were also analyzed to demonstrate that clinically relevant, complex samples could be separated on-chip with efficiencies similar to those derived from model glycoproteins. In addition, we compared microchip and capillary electrophoresis under similar separation conditions, and the microchips performed as well as the capillaries. These results confirmed that the noncircular cross section of the microchannel did not hamper separation performance. For all experiments, the glycan samples were derivatized with 8-aminopyrene-1,3,6-trisulfonic acid to impart needed charge for electrophoresis and a fluorescent label for detection.     

Preparation and characterization of TiO2-BaO-ZnO-B2O3 glass systems for optical devices

We study the TiO₂-BaO-ZnO-B₂O₃ glass system, where the ZnO and B₂O₃ compositions were constant and the ratio TiO₂/BaO was varied from 0.87 to 1.76. A super kanthal resistance furnace was used to melt the compounds inside an alumina crucible, at 1200 °C, for 10 min. After melting, the glasses were poured out into steel moulds and rapidly cooled by quenching. The glasses obtained were homogeneous, bubble free and transparent. They were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy (FTIR), UV-VIS spectroscopy, dilatometry, density and linear refractive index. An infrared cut off caused by the composition influence was found in both IR and UV-vis spectra. From dilatometry T _d and T _g were verified as being anomalous. The linear thermal expansion coefficient presented an anomalous behaviour in relation to TiO₂ concentrations. The density and linear refractive index increased with increasing TiO₂/BaO ratio arriving at their peak value of TiO₂/BaO=1.5 and then decreasing. The dependence of softening point T _d on the ratio TiO₂/BaO exhibited the same behaviour. It is suggested that Ti⁴⁺ plays a dual part in the glass system, assuming a predominantly tetrahedral coordination in the low titania region and a predominantly octahedral coordination in the high titania region. With a heat treatment of the glass around 600 °C, we observed a rapid change of refractive index with increasing temperature.     

Linear and nonlinear optical properties of plasma-enhanced chemical-vapour deposition grown silicon nanocrystals

We provide a systematic study on the linear and nonlinear optical properties of silicon nanocrystals (Si-nc) grown by plasma-enhanced chemical vapour deposition (PECVD). Linear optical properties, namely absorption, emission and refractive indices are reported. The sign and magnitude of both real and imaginary parts of third-order nonlinear susceptibility X(³) of Si-nc are measured by the Z-scan method. Closed aperture Z-scan reveals a positive nonlinearity for all the samples. From the open aperture measurements, nonlinear absorption coefficients are evaluated and attributed to two-photon absorption. Absolute values of X(³) are in the order of 10^-9 esu and show systematic correlation with the Si-nc size, due to quantum confinement related effects. A correlation has been made between X(³), nanocrystalline size, linear refractive index and optical band gap.     

Surface-reactive acrylic copolymer for fabrication of microfluidic devices

A surface-reactive acrylic polymer, poly(glycidyl methacrylate-co-methyl methacrylate) (PGMAMMA), was synthesized and evaluated for suitability as a substrate for fabrication of microfluidic devices for chemical analysis. This polymer has good thermal and optical properties and is mechanically robust for cutting and hot embossing. A key advantage of this polymeric material is that the surface can be easily modified to control inertness and electroosmotic flow using a variety of chemical procedures. In this work, the procedures for aminolysis, photografting of linear polyacrylamide, and atom-transfer radical polymerization on microchannel surfaces in PGMAMMA substrates were developed, and the performance of resultant microfluidic electrophoresis devices was demonstrated for the separation of amino acids, peptides, and proteins. Separation efficiencies as high as 4.6 x 10(4) plates for a 3.5-cm-long separation channel were obtained. The results indicate that PGMAMMA is an excellent substrate for microfabricated fluidic devices, and a broad range of applications should be possible.     

Determination of the optical properties of non-uniformly thick non-hydrogenated sputtered silicon thin films on glass

An improved envelope method (EM) is presented in this paper that allows the determination of the refractive index (n_f) and absorption coefficient (_f) of non-uniformly thick, absorbing films on a slightly absorbing substrate from a single transmission measurement. The limitation of the previous version of the EM [R. Swanepoel, J. Phys. E: Sci. Inst. 17 (1984) 896] only permitted the evaluation of samples that exhibited a transparent region in the near infrared (NIR). As an initial test of the improved EM, n_f and _f of a 0.5-μm thick epitaxially grown silicon-on-sapphire (SOS) film were determined over the range of 1.1–3.2 eV, with increased absorption being observed at the silicon: sapphire interface. Subsequently, sputtered amorphous silicon (a-Si) films, which exhibit absorption throughout the visible–NIR spectrum, were successfully characterised and a definite trend towards lower absorption coefficients for films deposited at higher temperatures was observed. After the a-Si films were subjected to solid phase crystallisation (SPC), increased sub-bandgap absorption was attributed to higher defect levels in the films, which also resulted in amorphous features remaining in the Raman spectra.     

Chemotaxis assays of mouse sperm on microfluidic devices

Sperm chemotaxis is an area of significant interest to scientists involved in reproductive science. Understanding how and when sperm cells are attracted to the egg could have profound effects on reproduction and contraception. In an effort to systematically study this problem, we have fabricated and evaluated a microfluidic device to measure sperm chemotaxis. The device was designed with a flow-through configuration using a spatially and temporally stable chemical gradient. Mouse sperm cells were introduced into the chemotaxis chamber between confluent flows of mouse ovary extract and buffer. The sperm experiencing chemotaxis swam toward the extract and were counted relative to those that swam toward the buffer. The ovary extracts were diluted from 10(2) to 10(7) times, and each extract dilution was screened for chemotaxis. Four out of six ovaries showed a strong chemotactic response at extract dilutions of 10(-3) to 10(-5). This device provided a convenient, disposable platform on which to conduct chemotaxis assays, and the flow-through design overcomes difficulties associated with distinguishing chemotaxis from trapping.     

Effects of bombardment on optical properties during the deposition of silicon nitride by reactive ion-beam sputtering

Thin silicon nitride (Si(1_x)N(x)) films were synthesized without substrate heating by means of reactive argon-ion sputtering of either silicon or a silicon nitride target in the 1000-1500-eV energy range at a nitrogen partial pressure of 1.3 × 10(-2) Pa and with simultaneous nitrogen ion-assisted bombardment in the 300-500-eV low energy range. The extinction coefficient and refractive index of the films were directly dependent on the N(+) ion-to-atom arrival ratio, assisted ion energy, film growth rate, and indicated a correlation with film stoichiometry and disorder. Si(3)N(4) films were obtained for N(+) ion/Si atom arrival ratios from 0.6 to 1.7 and for different Si:N atom arrival rates and had a refractive index as high as 2.04 (633 nm) and a low hydrogen content as indicated by IR spectra.     

Patterned solvent delivery and etching for the fabrication of plastic microfluidic devices

A very simple method for micropatterning flat plastic substrates that can be used to build microfluidic devices is demonstrated. Patterned poly(dimethylsiloxane) elastomer is used as a template to control the flow path of an etching solvent through a channel design to be reproduced on the plastic substrate. The etching solvent was a acetone/ethanol mixture for poly(methyl methacrylate) substrates or a dimethylformamide/acetone mixture for polystyrene. The method is extremely fast in that duplicate plastic substrates can be patterned in just a few minutes each. We identified conditions that lead to smooth channel surfaces and characterized the rate of etching under these conditions. We determined that, for sufficiently short etching times (shallow channel depths), the etch rate is independent of the linear flow rate. This is very important since it means that the etch depth is approximately constant even in complex channel geometries where there will be a wide range of etchant flow rates at different positions in the pattern to be reproduced. We also demonstrate that the method can be used to produce channels with different depths on the same substrate as well as channels that intersect to form a continuous fluid junction. The method provides a nice alternative to existing methods to rapidly fabricate microfluidic devices in rigid plastics without the need for specialized equipment.     

Structural and optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition

Silicon nanocrystals (Si-nc) embedded in SiO2 matrix have been prepared by high temperature thermal annealing (1000-1250 degrees C) of substoichiometric SiOx films deposited by plasma-enhanced chemical vapor deposition (PECVD). Different techniques have been used to examine the optical and structural properties of Si-nc. Transmission electron microscopy analysis shows the formation of nanocrystals whose sizes are dependent on annealing conditions and deposition parameters. The spectral positions of room temperature photoluminescence are systematically blue shifted with reduction in the size of Si-nc obtained by decreasing the annealing temperature or the Si content during the PECVD deposition. A similar trend has been found in optical absorption measurements. X-ray absorption fine structure measurements indicate the presence of an intermediate region between the Si-nc and the SiO2 matrix that participates in the light emission process. Theoretical observations reported here support these findings. All these efforts allow us to study the link between dimensionality, optical properties, and the local environment of Si-nc and the surrounding SiO2 matrix.