Parylene to silicon nitride bonding for post-integration of high pressure microfluidics to CMOS devices

High pressure-rated channels allow microfluidic assays to be performed on a smaller footprint while keeping the throughput, thanks to the higher enabled flow rates, opening up perspectives for cost-effective integration of CMOS chips to microfluidic circuits. Accordingly, this study introduces an easy, low-cost and efficient method for realizing high pressure microfluidics-to-CMOS integration. First, we report a new low temperature (280 °C) Parylene-C wafer bonding technique, where O(2) plasma-treated Parylene-C bonds directly to Si(3)N(4) with an average bonding strength of 23 MPa. The technique works for silicon wafers with a nitride surface and uses a single layer of Parylene-C deposited only on one wafer, and allows microfluidic structures to be easily formed by directly bonding to the nitride passivation layer of the CMOS devices. Exploiting this technology, we demonstrated a microfluidic chip burst pressure as high as 16 MPa, while metal electrode structures on the silicon wafer remained functional after bonding.     

Biocompatible "click" wafer bonding for microfluidic devices

We introduce a novel dry wafer bonding concept designed for permanent attachment of micromolded polymer structures to surface functionalized silicon substrates. The method, designed for simultaneous fabrication of many lab-on-chip devices, utilizes a chemically reactive polymer microfluidic structure, which rapidly bonds to a functionalized substrate via"click" chemistry reactions. The microfluidic structure consists of an off-stoichiometry thiol-ene (OSTE) polymer with a very high density of surface bound thiol groups and the substrate is a silicon wafer that has been functionalized with common bio-linker molecules. We demonstrate here void free, and low temperature (< 37 °C) bonding of a batch of OSTE microfluidic layers to a silane functionalized silicon wafer.     

Sol–Gel SiO2-ZrO2 Coatings for Optical Applications

SiO₂-ZrO₂ based nanostructured multilayers films have been prepared by sol–gel processing from metallorganic precursors by low temperature inorganic polymerization reactions. Simultaneous gelation of both precursors was realized. Homogeneous and transparent films were obtained at room temperature by dip- and spin-coating on glass and silicon wafer substrates. Samples with successively deposited layers (1–3 layers) and successive thermal treatments have been also studied. Each deposited layer was thermally treated for 1 h at 300°C. The coatings were characterized by XRD, spectroellipsometry (SE), UV-VIS spectroscopy and AFM methods. The influence of substrates, number of coatings and number of thermal treatments on the optical and structural properties of the films was established. The thickness of three deposited SiO₂-ZrO₂ layers is about 496 nm on glass substrates and 413 nm on the silicon wafer substrate. The films deposited on glass are more porous than those deposited on silicon. The properties of optical waveguide prepared from SiO₂-ZrO₂ layers on silicon substrates will be discussed.     

Field-effect flow control in a polydimethylsiloxane-based microfluidic system.

The application of the field-effect for direct control of electroosmosis in a polydimethylsiloxane (PDMS)-based microfluidic system, constructed on a silicon wafer with a 2.0 microm electrically insulating layer of silicon dioxide, is demonstrated. This microfluidic system consists of a 2.0 cm open microchannel fabricated on a PDMS slab, which can reversibly adhere to the silicon wafer to form a hybrid microfluidic device. Aside from mechanically serving as a robust bottom substrate to seal the channel and support the microfluidic system, the silicon wafer is exploited to achieve field-effect flow control by grounding the semiconductive silicon medium. When an electric field is applied through the channel, a radial electric potential gradient is created across the silicon dioxide layer that allows for direct control of the zeta potential and the resulting electroosmotic flow (EOF). By configuring this microfluidic system with two power supplies at both ends of the microchannel, the applied electric potentials can be varied for manipulating the polarity and the magnitude of the radial electric potential gradient across the silicon dioxide layer. At the same time, the longitudinal potential gradient through the microchannel, which is used to induce EOF, is held constant. The results of EOF control in this hybrid microfluidic system are presented for phosphate buffer at pH 3 and pH 5. It is also demonstrated that EOF control can be performed at higher solution pH of 6 and 7.4 by modifying the silicon wafer surface with cetyltrimethylammonium bromide (CTAB) prior to assembly of the hybrid microfluidic system. Results of EOF control from this study are compared with those reported in the literature involving the use of other microfluidic devices under comparable solution conditions.     

Femtosecond laser processing for optofluidic fabrication

Femtosecond laser direct writing is a promising technique for fabricating optofluidic devices since it can modify the interior of glass in a spatially selective manner through multiphoton absorption. The chemical properties of laser-irradiated regions in glass are modified allowing them to be selectively etched by subsequent wet etching using aqueous solutions of etchants such as hydrofluoric (HF) acid. This technique can be used to directly form three-dimensional microfluidic systems. The two-step process can also be used to fabricate free-space optical components such as micromirrors and microlenses inside glass. In addition, femtosecond laser direct writing can alter the optical properties of a substrate to create a wide range of micro-optical components inside glass, including optical waveguides, Mach-Zehnder interferometers, and optical attenuators. The unique ability of femtosecond laser direct writing to simultaneously alter the chemical and optical properties of glass opens up a new avenue for fabricating a variety of optofluidic microchips for biological analysis. Optofluidic microchips fabricated using femtosecond lasers have been used to determine the functions of living microorganisms, determine the concentrations of liquid samples, detect and manipulate single cells, and rapidly screen algae populations. This paper presents a comprehensive review of optofluidic devices for biological analysis fabricated by femtosecond laser processing.     

Determination of metallic impurities in a silicon wafer by local etching and electrothermal atomic absorption spectrometry.

An etching technique for the determination of the metallic impurities distribution in silicon wafers has been developed. An area of 10 mmphi and 10 microm depth was etched by 100 microL of an etching solution with a HF and HNO3 mixture. The acid matrix was evaporated on the wafer surface by IR lamp illumination and vacuum exhaust. Metallic impurities remaining on the wafer surface were redissolved into the collection solution, which was measured by electrothermal atomic absorption spectrometry (ET-AAS). The recovery invested by local etching/ET-AAS was within 95 - 112% for Fe, Cu and Ni. The detection limit (3sigma) for Fe, Cu and Ni in silicon was 1 x 10(13) atoms/cm3. To confirm the applicability, local etching was applied to evaluate the effects of metallic impurities in a gettering study and the electronic properties of semiconductor devices. It was found that local etching is a useful sample preparation technique for the analysis of metallic impurities in a specific area on a silicon wafer.     

PDMS微流体系统的加工制作

目前,微流体装置越来越多地应用到分析系统、生物医学、化学等基础研究领域。传统的微流体系统制作方法是对玻璃和硅片进行刻蚀。用软刻法制作PDMS(Poly(dimethylsiloxane)：聚二甲基硅氧烷)微流体装置比传统的制作方法更快速,成本更低廉,并且对于通道的密封也不需要玻璃或硅芯片键合密封等复杂工艺。这类软刻法的核心技术是快速原样制作法和复制压模技术。相对于微电子加工工艺,软刻法制作过程不需要超静环境,化学家和生物学家可在普通的实验室实现加工制作。本文介绍了PDMS微装置在分离和生物材料模式化等方面的应用。     

Simple and cheap microfluidic devices for the preparation of monodisperse emulsions

Droplet microfluidics, which can generate monodisperse droplets or bubbles in unlimited numbers, at high speed and with complex structures, have been extensively investigated in chemical and biological fields. However, most current methods for fabricating microfluidic devices, such as glass etching, soft lithography in polydimethylsiloxane (PDMS) or assembly of glass capillaries, are usually either expensive or complicated. Here we report the fabrication of simple and cheap microfluidic devices based on patterned coverslips and microscope glass slides. The advantages of our approach for fabricating microfluidic devices lie in a simple process, inexpensive processing equipment and economical laboratory supplies. The fabricated microfluidic devices feature a flexible design of microchannels, easy spatial patterning of surface wettability, and good chemical compatibility and optical properties. We demonstrate their utilities for generation of monodisperse single and double emulsions with highly controllable flexibility.     

High-resolution direct patterning of gold nanoparticles by the microfluidic molding process

A novel microfluidic molding process was used to form microscale features of gold nanoparticles on polyimide, glass, and silicon substrates. This technique uses permeation pumping to pattern and concentrate a nanoparticle ink inside microfluidic channels created in a porous polymer template in contact with a substrate. The nanoparticle ink is self-concentrated in the microchannels, resulting in dense, close-packed nanoparticle features. The method allows for better control over the structure of printed features at a resolution that is comparable to inkjet printing, and is purely additive with no residual layers or etching required. The process uses low temperatures and pressures and takes place in an ambient environment. After patterning, the gold nanoparticles were sintered into continuous and conductive gold traces.     

Fabrication of microfluidic systems in poly(dimethylsiloxane)

Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.     

Fabrication of microfluidic devices using photopatternable hybrid sol-gel coatings

UV-patternable organic-inorganic hybrid sol-gel coating was used to develop microchannel on silicon and glass wafers by photolithography processing. The sol-gel coating was formed with 3-methacryloxypropyltrimethoxysilane (MPTS) as photosensitive component and zirconium propoxide as property modifier. In order to enhance the UV light efficiency during photolithography processing for glass substrates, a thin copper layer with thickness of 100–200 nm was deposited on one side of the glass wafer. The closed microchannels were formed by bonding of two developed surface channels of compensating patterns by wafer bonding technology. The bonding material is the same as the channel body ensuring a uniform surface property for the microchannels. A thin layer of about 2 μm was applied on the developed channels by spin coating. The depth of the surface channels on each wafer is about 10–12 μm, and the height of the closed channels is therefore in the range of 22–24 μm. Different microfluidic devices such as chaotic micromixers and microsplitters were fabricated.     

Effect of Wafer Temperature on High Aspect Ratio Hardmask Etching

Fluorocarbon-based chemistries were used to study the effect of wafer temperature on the etch of high aspect ratio hardmasks composed of SiO₂ and SiN_x layers. It is found that etch stop can occur easily at high temperature. The rate of polymer deposition plays an important role in etch stop. The etching rates were found to be inversely proportional to the wafer temperature. Such a relation indicates a negative activation energy in the rate expression of hardmask etching using fluorocarbon plasma. It also implies that in hardmask etching, complicated gas-surface, but not simple one-step, reactions are involved. Different wafer surface temperature can provide different degree of activation for etching reactions. Analysis of etching rate and optical emission trends indicates that CF_x may contribute more than F does in the etch of SiO₂ and SiN_x, since polymer-rich etching chemistries were used. Based on the temperature-dependent etching rate, we propose a reaction mechanism for the reaction trends observed in hardmask etching.     

Active particle control through silicon using conventional optical trapping techniques

Functional integration of optical trapping techniques with silicon surfaces and environments can be realized with minimal modification of conventional optical trapping instruments offering a method to manipulate, track and position cells or non-biological particles over silicon substrates. This technique supports control and measurement advances including the optical control of silicon-based microfluidic devices and precision single molecule measurement of biological interactions at the semiconductor interface. Using a trapping laser in the near infra-red and a reflective imaging arrangement enables object control and measurement capabilities comparable to trapping through a classical glass substrate. The transmission efficiency of the silicon substrate affords the only reduction in trap stiffness. We implement conventional trap calibration, positioning, and object tracking over silicon surfaces. We demonstrate control of multiple objects including cells and complex non-spherical objects on silicon wafers and fabricated surfaces.     

Die Herstellung von Reinstsiliciumscheiben: Mehr Chemie als man glaubt

Monocrystalline semiconductor wafers made of silicon represent the base material for microelectronic devices. The transfer of the precursor material quartz into a 300 mm wafer deposited with an epitaxial layer requires a multitude of process steps, part of which are determined by chemical reactions. This article has introduced into the manufacturing of semiconductor silicon and the processes etching, polishing, cleaning and epitaxy. This technology branch, though, is distinguished by extreme requirements regarding surface perfection and cleanliness, which may be expressed in atomic layers (surface roughness of the wafers) and ppt (contamination of chemicals)     

Surface engineering of microchannel walls for protein separation and directed microfluidic flow

The preparation of surfaces in microfluidic devices that selectively retain proteins may be difficult to implement due to the incompatibility of derivatization methods with microdevice fabrication techniques. This review describes recently reported developments in simple and rapid methods for engineering the surface chemistries of microchannels based on construction of press-fit microdevices. These devices are fabricated by placing a glass fiber on a PDMS film and pressing the film on a silicon wafer or a microscope slide that has been derivatized with octadecyltrichlorosilane (ODS). The film adheres to the slide and forms an elliptically shaped channel around the fiber. The combination of surface wettability of a hydrophilic glass microfiber and the surrounding hydrophobic microchannel surfaces directs a narrow boundary layer of liquid next to the fiber in order to bring the sample in contact with the separation media and results in selective retention of proteins. This phenomenon may be exploited to enable microscale separation applications since there are a wide variety of fibers available with different chemistries. These may be used to rapidly fabricate microchannels that serve as stationary phases for separation at a microscale. The fundamental properties of such devices are discussed.     

玻璃微流控芯片廉价快速制作方法的研究

研究了一种玻璃微流控芯片的快速、低成本制作工艺和方法. 该方法采用商品化的显微载玻片(soda-lime玻璃)作为芯片基质材料, 利用AZ 4620光刻胶代替传统工艺中的溅射金属层或多晶硅/氮化硅层作为玻璃刻蚀的掩膜层, 同时利用一种紫外光学胶键合方法代替传统熔融键合方法实现芯片的键合, 整个工艺对玻璃基质材料要求低, 普通微流控芯片(深度小于50 μm)制作流程仅需约3.5 h, 可降低制作成本, 缩短制作周期. 还系统地研究了光刻胶厚度、光刻胶硬烘时间和玻璃腐蚀液配比对玻璃微流控芯片制作的影响, 获得了优化的工艺参数.     

Manual cleaving of anodically bonded silicon/glass wafers

Packaging technologies are a great issue in MEMS/NEMS fabrication. Moreover, silicon to glass anodic bonding is a common boding technique for MEMS/NEMS packaging. Hence, dicing the wafer into individual devices is a real challenge for fragile structures. This investigation shows an extension of the manual cleaving dicing for bonded silicon/glass wafers. The designed structure with no coincident anchors between silicon and glass avoids that the fracture expands toward the interior of the device during manual cleaving. Highest stresses are localized on the anchors, causing the fracture in those points. This method was experimentally tested to dice anodically bonded silicon-glass wafers.     

Glass microfabricated nebulizer chip for mass spectrometry

A microfluidic nebulizer chip for mass spectrometry is presented. It is an all-glass device which consists of fusion bonded Pyrex wafers with embedded flow channels and a nozzle at the chip edge. A platinum heater is located on the wafer backside. Fabrication of the chip is detailed, especially glass deep etching, wafer bonding, and metal patterning. Various process combinations of bonding and metallization have been considered (anodic bonding vs. fusion bonding; heater inside/outside channel; metallization before/after bonding; platinum lift-off vs. etching). The chip vaporizes the liquid sample (0.1-10 microL min(-1)) and mixes it with a nebulizer gas (ca. 100 sccm N2). Operating temperatures can go up to 500 degrees C ensuring efficient vaporization. Thermal insulation of the glass ensures low temperatures at the far end of the chip, enabling easy interconnections.     

A Novel Silicon Etching Method Using Vapor of Tetramethylammonium Hydroxide Solution

Silicon bulk etching is an important part of micro-electro-mechanical system (MEMS) technology. In this work, a novel etching method is proposed based on the vapor from tetramethylammonium hydroxide (TMAH) solution heated up to boiling point. The monocrystalline silicon wafer is positioned over the solution surface and can be anisotropically etched by the produced vapor. This etching method does not rely on the expensive vacuum equipment used in dry etching. Meanwhile, it presents several advantages like low roughness, high etching rate and high uniformity compared with the conventional wet etching methods. The etching rate and roughness can reach 2.13 μm/min and 1.02 nm, respectively. Furthermore, the diaphragm structure and Al-based pattern on the non-etched side of wafer can maintain intact without any damage during the back-cavity fabrication. Finally, an etching mechanism has been proposed to illustrate the observed experimental phenomenon. It is suggested that there is a water thin film on the etched surface during the solution evaporation. It is in this water layer that the ionization and etching reaction of TMAH proceed, facilitating the desorption of hydrogen bubble and the enhancement of molecular exchange rate. This new etching method is of great significance in the low-cost and high-quality micro-electro-mechanical system industrial fabrication.     

Automated and parallel microfluidic DNA extraction with integrated pneumatic microvalves/pumps and reusable open-channel columns

A novel microfluidic DNA extraction protocol based on integrated diaphragm microvalves/pumps and silica-deposited open-channel columns was developed specifically for automated and parallel DNA solid-phase extraction (SPE). The method uses microfluidic chips with a sandwiched structure containing three layers, which are the upper fluidic layer with surface-deposited silica on glass open channels as the extraction phase, the lower actuation layer with valve actuation channels on a glass wafer, and the middle poly(dimethylsiloxane) (PDMS) membrane for reversible bonding of the two glass substrates. These two glass substrates can be reused after thoroughly cleaning and the PDMS membrane can be replaced conveniently, which could effectively decrease the time and cost of chip manufacturing. The normally closed microvalves/pumps were used to automatically control all processes of the on-chip DNA SPE without cross-contamination and leakage, enabling the processing of multiple samples in parallel without changing the microvalve control module. Using the microchip device with integrated microvalves/pumps, automated, programmable, and simultaneous λ-DNA extractions from different samples could be attained, even from complex solutions such as human blood, and the silica-deposited open-channel columns could be reused stably and reliably. Results have demonstrated that most of the eluted λ-DNA was recovered in the second 2 µL of elution buffer with high-purity suitable for successful polymerase chain reaction amplification, making it possible for further integration into microfluidic devices for fully functional and high-throughput genetic analysis.