Heat-Transfer-Enhancement Measurement for Microfabricated Electrostatic Fluid Accelerators

Air cooling, because of its simplicity, remains as the most popular cooling solution for microelectronics in the consumer market. However, the trend of increasing heat generation in microelectronics and the demand for compact devices result in heat fluxes approaching the limit of conventional rotary-fan air-cooling technology. Electrostatic fluid accelerators (EFAs), also known as electrohydrodynamic ionic wind pumps, have the potential of becoming a critical element of electronic thermal-management solutions. In this technique, application of voltage to a sharp electrode ionizes air molecules, which are propelled by the electric field, transferring part of their energy to neutral air molecules, thus creating airflow and cooling effects. The airflow, so-called “corona wind,” can be used discretely for hot-spot cooling or integrated into a compact thermal-exchange surface to decrease the fluid boundary layer and increase heat transfer. The EFA investigated in this paper consists of a microfabricated atomic force microscopy (AFM)-cantilever corona electrode and a flat collecting electrode that doubles as the thermal-exchange surface. The fabrication results, as well as electrical and thermal performance characterization of microfabricated EFAs, are presented in this paper. Air-gap separation distances of 2, 3, 4, and 5 mm between the corona electrode and the thermal-exchange surface were examined under constant surface-to-ambient-temperature difference of approximately 38.3 $^{circ}hbox{C}$. $hfill$[2008-0173]      

Thermal isolation of high-temperature superconducting thin filmsusing silicon wafer bonding and micromachining

Using a new micromachining technology, thermally isolated thin films of high-temperature superconductor have been microfabricated. The intended application for these structures is in infrared bolometers. A silicon wafer bonding process produces a low thermal mass island of single-crystal silicon on a silicon nitride membrane which provides thermal isolation. The silicon can act as a seed for the epitaxial growth of YBa₂Cu₃O₇ on a yttria-stabilized zirconia buffer layer. This paper describes the overall concept of the thermally isolated device, and demonstrates that the micromachined structure can be fabricated with high-quality superconducting films     

Evaluation of Si liquid cooling structure with microchannel and TSV for 3D application

The development of 3D integration has caused a major technology paradigm shift to all integrated circuit (IC) devices, interconnects, and packages. Despite the benefits of 3D integration, this process faces the key challenge of thermal management, especially for high power and high density IC devices. Due to the limitations of conventional thermal solutions, liquid cooling technology has become a field of great interest for IC thermal management. In this study, an on-chip liquid cooling module with three different through Si vias (TSVs) and a fixed microchannel structure has been fabricated on an Si wafer using deep reactive ion etching and anodic bonding, followed by a grinding and dicing process. Pressure drop, coolant flow, and temperature difference before and after liquid flow were experimentally measured. TSV depth and diameter have been shown to have little effect on the change of pressure drop; however, shallower TSV depth and larger TSV diameter led to improved liquid cooling performance. The trapezoidal TSV showed slightly more effective cooling than did the scalloped TSV or the straight TSV.

     

High-speed microfabricated silicon turbomachinery and fluid film bearings

A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This work presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.     

Temperature-Regulated Nonlinear Microvalves for Self-Adaptive MEMS Cooling

In this paper, thermal buckling of doubly clamped microfabricated nickel beams is implemented as a passive actuation mechanism to drive temperature-regulated nonlinear microvalves for adaptive microcooling applications. The nonlinear buckling phenomenon is combined with the nonlinear change in flow rate through parallel plates with a variable spacing. The thermal buckling mechanism and parallel plate flow are modeled analytically, and nondimensional characteristic design curves have been generated. Passive flow-control microvalves were fabricated using deep reactive ion etching and a through-mold nickel electroplating process over a thin sacrificial layer. The model is validated with experimental results from the microfabricated temperature-regulated microvalves. Experimental characterization using an integrated micromachined heat exchanger with air as the working fluid shows the desired nonlinear valving behavior with mass flow rates of up to 5 mg/s for a temperature increase of 50 $^{circ}hbox{C}$, corresponding to 0.25 W of heat removal. It is shown that temperature-induced elastic instabilities in microfabricated structures can be modeled and manipulated to create a nonlinear adaptive valving mechanism. The modeling approach, microfabrication process, and full characterization of the microvalves are presented.$hfill$ [2008-0008]      

Micromachined flat-walled valveless diffuser pumps

The first valveless diffuser pump fabricated using the latest technology in deep reactive ion etching (DRIE) is presented. The pump was fabricated in a two-mask micromachining process in a silicon wafer polished on both sides, anodically bonded to a glass wafer. Pump chambers and diffuser elements were etched in the silicon wafer using DRIE, while inlet and outlet holes are etched using an anisotropic etch. The DRIE etch resulted in rectangular diffuser cross sections. Results are presented on pumps with different diffuser dimensions in terms of diffuser neck width, length, and angle. The maximum pump pressure is 7.6 m H₂O (74 kPa), and the maximum pump flow is 2.3 ml/min for water     

Formation of homogenous nanofibers using silicon microneedle spinnerets

We report the use of a silicon microfabricated device as a new spinneret for electrospinning purposes. This device has been realized on silicon substrates using a deep reactive ion etching process. To make proper holes in the center of microneedles, a rotating angle deposition method followed by vertical etching of silicon is employed. By using these needles as fluid nozzles in the electrospinning process, poly vinyl alcohol solution with a concentration of 7?% has been converted into nanofibers. The formation of nanofibers has been investigated using field emission scanning electron microscopy. Using this process, nanofibers with a diameter of 100–200?nm are realized where the dispersion is less than 50?nm. Finally, the effects of needle size and the applied voltage have been investigated on the diameter of nanofibers.     

Fabrication of Magnetooptical Atom Traps on a Chip

Ultracold atoms can be manipulated using microfabricated devices known as atom chips. These have significant potential for applications in sensing, metrology, and quantum information processing. To date, the chips are loaded by transfer of atoms from an external macroscopic magnetooptical trap (MOT) into microscopic traps on the chip. This transfer involves a series of steps, which complicate the experimental procedure and lead to atom losses. In this paper, we present a design for integrating a MOT into a silicon wafer by combining a concave pyramidal mirror with a square wire loop. We describe how an array of such traps has been fabricated, and we present magnetic, thermal, and optical properties of the chip.     

Post-CMOS processing for high-aspect-ratio integrated siliconmicrostructures

Presents a new fabrication sequence for integrated-silicon microstructures designed and manufactured in a conventional complementary metal-oxide-semiconductor (CMOS) process. The sequence employs a post-CMOS deep silicon backside etch, which allows fabrication of high aspect ratio (25:1) and flat (greater than 10 mm radius of curvature) MEMS devices with integrated circuitry. A comb-drive resonator, a cantilever beam array and a z-axis accelerometer were fabricated using this process sequence. Electrical isolation of single-crystal silicon was realized by using the undercut of the reactive ion etch (RIE) process. Measured out-of-plane curling across a 120-μm-wide 25-μm-thick silicon released plate was 0.15 μm, which is about ten times smaller than curl of the identical design as a thin-film CMOS microstructure. The z-axis DRIE accelerometer structure is 0.4 mm by 0.5 mm in size and has a 25-μm-thick single-crystal silicon proof mass. The measured noise floor is 1 mG/√Hz, limited by electronic noise. A vertical electrostatic spring "hardening" effect was theoretically predicted and experimentally verified     

A Microfabricated Planar Electrospray Array Ionic Liquid Ion Source With Integrated Extractor

This paper reports the design, fabrication, and experimental characterization of a fully microfabricated planar array of externally fed electrospray emitters that produces heavy molecular ions from the ionic liquids $hbox{EMI-BF}_{4}$ and EMI-Im. The microelectromechanical systems (MEMS) electrospray array is composed of the following two microfabricated parts: 1) an emitter die with as many as 502 emitters in 1.13 $hbox{cm}^{2}$ and 2) an extractor component that provides assembly alignment, electrical insulation, and a common bias voltage to the emitter array. The devices were created using Pyrex and silicon substrates, as well as microfabrication techniques such as deep reactive ion etching, low-temperature fusion bonding, and anodic bonding. The emitters are coated with black silicon, which acts as a wicking material for transporting the liquid to the emitter tips. The extractor electrode uses a 3-D MEMS packaging technology that allows hand assembly of the two components with micrometer-level precision. Experimental characterization of the MEMS electrospray array includes current–voltage characteristics, time-of-flight mass spectrometry, beam divergence, and imprints on a collector. The data show that with both ionic liquids and in both polarities, the electrospray array works in the pure ionic regime, emitting ions with as little as 500 V of bias voltage. The data suggest that the MEMS electrospray array ion source could be used in applications such as coating, printing, etching, and nanosatellite propulsion. $hfill$[2008-0270]      

Fabrication and characterization of a passive silicon-based direct methanol fuel cell

This paper reports on the fabrication and characterization of a passive silicon microfabricated direct methanol fuel cell (μDMFC). The main characteristics of the device are its capability to work without complex pumping systems, only by capillary pressure, and the fact that its performance is not affected by the device orientation. A simple fabrication process based in deep reactive ion etching (DRIE), allows obtaining a reliable and low-cost final device. The device consists of two silicon microfabricated plates mounted together with a commercial membrane electrode assembly (MEA). The impact of current collector design on microfuel cell performance is explored and current–voltage (I–V) and current–power (I–P) curves of the device at different methanol concentration and orientation are presented. Optimal performance was obtained for methanol concentrations between 3 and 5 M, achieving a maximum power density of 12 mW/cm². The results obtained in this work demonstrate the feasibility of the device and give a guideline for design and conditions optimization.     

The surface/bulk micromachining (SBM) process: a new method forfabricating released MEMS in single crystal silicon

This paper presents the surface/bulk micromachining (SBM) process to allow fabricating released microelectromechanical systems using bulk silicon. The process starts with a (111)-oriented silicon wafer. The structural patterns are defined using the reactive ion etching technique used in surface micromachining. Then the patterns, as well as sidewalls, are passivated with an oxide film, and bare silicon is exposed at desired areas. The exposed bare silicon is further reactive ion etched, which defines sacrificial gap dimensions. The final release is accomplished by undercutting the exposed bulk silicon sidewalls in aqueous alkaline etchants. Because {111} planes are used as etch stops, very clean structural surfaces can be obtained. Using the SBM process, 5-, 10-, and 100-μm-thick arbitrarily-shaped single crystal silicon structures, including comb-drive resonators, at 5-, 30-, and 100-μm sacrificial gaps, respectively, are fabricated. An electrostatic actuation method using p-n junction isolation is also developed in this paper, and it is applied to actuate comb-drive resonators. The leakage current and junction capacitance of the reversed-biased p-n junction diodes are also found to be sufficiently small for sensor applications. The developed SBM process is a plausible alternative to the existing micromachining methods in fabricating microsensors and microactuators, with the advantage of using single crystal silicon     

A novel microneedle array for the treatment of hydrocephalus

We present a microfabricated 10 by 10 array of microneedles for the treatment of a neurological disease called communicating hydrocephalus. Together with the previously reported microvalve array, the current implantable microneedle array completes the microfabricated arachnoid granulations (MAGs) that mimic the function of normal arachnoid granulations. The microneedle array was designed to enable the fixation of the MAGs through dura mater membrane in the brain and thus provide a conduit for the flow of cerebrospinal fluid. Cone-shaped microneedles with hollow channels were fabricated using a series of microfabrication techniques: SU-8 photolithography for tapered geometry, reactive ion etching for sharpening the microneedles, 248 nm deep UV excimer laser machining for creating through-hole inside the microneedles, and metal sputtering for improved rigidity. Puncture tests were conducted using porcine dura mater and the results showed that the fabricated microneedle array is strong enough to pierce the dura mater. The in vitro biocompatibility test result showed that none of the 100 outlets of the microneedles exposed to the bloodstream were clogged significantly by blood cells. We believe that these test results demonstrate the potential use of the microneedle array as a new treatment of hydrocephalus.     

DRIE based technology for 3D silicon barcodes fabrication

Microworld barcoding has become a promising tool for cell biology. Individual and subpopulation cell tracking is of great interest to evaluate cell behaviour. Nowadays, many micrometer and even nanometer size silicon structures can be fabricated using microelectronics techniques. In this work we report for first time the development of 3D barcodes based on silicon substrate. The proposed silicon micromachining technology based on deep reactive ion etching (DRIE) allows to obtain micrometer-sized cylindrical structures with vertical etch profile that defines a bit = 1 and non-vertical etch profile that defines a bit = 0. Although this technology will allow more than 15 bits representation, only 4-8 bits are necessary for cell labelling. The results of this work show that DRIE has become a versatile technique to produce high aspect 3D biocompatible silicon-based barcodes structures for cell studies.     

A nanoporous silicon membrane electrode assembly for on-chip micro fuel cell applications

Silicon-based fuel cells are under active development for chip-scale electrical power supply. One of the greatest challenges in micro-fuel-cell research is the development of a suitable proton conducting membrane material that is compatible with standard silicon microfabrication technology. In this paper, the use of nanoporous silicon as a novel proton conducting membrane material in a microscale fuel cell membrane electrode assembly (MEA) is demonstrated. The devices were fabricated by first creating 100-/spl mu/m-thick silicon windows in a standard silicon wafer, anodizing to create pores in the windows, and then painting catalyst layers and insulators onto the porous structures. Using 5 M formic acid and 0.5 M sulfuric acid as the fuel, the fuel cell peak power density reached about 30 mW/cm/sup 2/ at current density level of about 120 mA/cm/sup 2/. These results represent the successful integration of a new class of protonic conductor into a microfabricated silicon fuel cell.     

Improved process flow for buried channel fabrication in silicon

The fabrication of microchannels using MEMS technology always attracted the attention of researchers and designers of microfluidic systems. Our group focused on realizing buried fluidic channels in silicon substrates involving deep reactive ion etching. To meet the demands of today’s complex microsystems, our aim was to create passive microfluidics in the bulk Si substrate well below the surface, while retaining planarity of the wafer. Therefore additional lithographic steps for e.g. integrating circuit elements are still possible on the chip surface. In this paper, a more economic process flow is applied which also contains a selective edge-masking method in order to eliminate under-etching phenomenon at the top of the trenches to be filled. The effect of Al protection on the subsequent etch steps is also discussed. Applying the proposed protection method, our group successfully fabricated sealed microchannels with excellent surface planarity above the filled trenches. Due to the concept, the integration of the technology in hollow silicon microprobes fabrication is now available.     

Mechanical Properties of Epitaxial 3C Silicon Carbide Thin Films

Microscale tensile specimens of epitaxial 3C silicon carbide (3C-SiC) thin films were fabricated on Si substrates and tested to provide measurements of strength and elastic modulus. Samples were fabricated using both micromolding and reactive ion etching (RIE) processes to pattern the 3C-SiC films. All specimens were on the     

Integrated polysilicon and DRIE bulk silicon micromachining for anelectrostatic torsional actuator

This paper presents a fabrication process that integrates polysilicon surface micromachining and deep reactive ion etching (DRIE) bulk silicon micromachining. The process takes advantage of the design flexibility of polysilicon surface micromachining and the deep silicon structures possible with DRIE. As a demonstration, a torsional actuator driven by a combdrive moving in the out-of-plane direction, consisting of polysilicon fingers and bulk silicon fingers, has been fabricated. The integrated process allows the combdrive to be integrated with any structure made by polysilicon surface micromachining     

Polymer microstructure generated by laser stereo-lithography and its transfer to silicon substrate using reactive ion etching

Micro-fabrication combining stereo-lithography with reactive ion etching is proposed. Three-dimensional polymer structures smaller than 1 mm are fabricated on silicon wafer by He-Cd (325.0 nm) laser stereo-lithography. Using the polymer structure having a high-aspect ratio as resist for deep reactive ion etching, the microstructure is transferred to the silicon substrate with an etching ratio of 0.5. The proposed technique has been demonstrated by the fabrication of lens-like structures.     

Silicon microneedle array with biodegradable tips for transdermal drug delivery

This paper presents the fabrication process, characterization results and basic functionality of silicon microneedle array with biodegradable tips for transdermal drug delivery. In order to avoid the main problems related to silicon microneedles; the breaking of the top part of the needles inside the skin, a simple solution can be the fabrication of microneedle array with biodegradable tips. A silicon microneedle array was fabricated by deep reactive ion etching (RIE), using the photoresist reflow effect and RIE notching effect. The biodegradable tips were successfully realized using the electrochemical anodization process that selectively generated porous silicon only on the top part of the skin. The porous tips can be degraded within a few weeks if some of them are broken inside the skin during the insertion and release process. The paper presents also the results of in vitro release of calcein with animal skins using a microneedle array with biodegradable tips. Compared to the transdermal drug delivery without microneedle enhancer, the microneedle array had presented significant enhancement of drug release.