Replication and bonding techniques for integrated microfluidic systems

From the technical and economic points of view, systems integration, and packaging represent a crucial step in the production of microsystems. Compared to purely silicon- or glass-based systems, the variety of materials and geometries available for purely polymer microfluidic systems is much larger, due to the outstanding material properties. Moreover, polymers may be shaped and joined by comparably simple methods. Examples are polymer microreplication as well as various bonding methods. With them, complete polymer microsystems can be integrated. In addition, a number of established, compatible processes are available for the integration of functional elements that may also be made of other materials.

     

Materials selection in micromechanical design: an application of the Ashby approach

The set of materials available to microsystems designers is rapidly expanding. Techniques now exist to introduce and integrate a large number of metals, alloys, ceramics, glasses, polymers, and elastomers into microsystems, motivating the need for a rational approach for materials selection in microsystems design. As a step toward such an approach, we focus on the initial stages of materials selection for micromechanical structures with minimum feature sizes greater than 1 /spl mu/m. The variation of mechanical properties with length scale and processing parameters is discussed. Bounds for initial design values of several properties are suggested and the necessity for the measurement of other properties (especially residual stresses and intrinsic loss coefficients) is discussed. Adapting the methods pioneered by Ashby et al., materials indices are formulated for a number of properties and materials selection charts are presented. These concepts are applied to illustrate initial materials selection for shock-resistant microbeams, force sensors, micromechanical filters, and micromachined flexures. Issues associated with the integration of materials into microsystems are briefly discussed.     

Hot embossing of thermoplastic multilayered stacks

The combination of different polymer materials during replication offers additional opportunities for fabrication and functionality of microsystems. Different surface and structural properties of polymers allow for improvements in microsystems for example by means of hydrophilic and hydrophobic combinations in microfluidic devices. Due to its high flexibility and precision hot embossing as one of the established micro replication processes facilitates processing of several polymer layers in one single process step. By this multi-component process micro structured systems consisting of thin layers of different polymers with adapted surface properties are fabricated. In this paper we describe the challenge of molding different types of polymers and some applications for multi-component micro systems.     

Bonding-less (B-less) fabrication of polymeric microsystems

A new technology of the microsystems fabrication was developed. The process is based on a so-called capillary film, which is a commercially available material. A matrix, master pattern corresponding to the microchannels structure, made of the capillary film emulsion is embedded inside a block of polydimethylsiloxane (PDMS). After polymer cross-linking, the matrix is dissolved and removed as a solution from the PDMS structure leaving network of empty spaces that can act as microchannels. In the paper, the fabrication methods of the microsystems with 2D or sandwich-like microchannels architecture were described.     

Electric impedance spectroscopy using microchannels with integratedmetal electrodes

Microelectric impedance measurement systems containing microchannels with integrated gold electrodes were fabricated to enable EI measurements of femtoliter (10^-15) volumes of liquid or gas. The microinstruments were characterized using samples of air, partially deionized water, and saline solutions with various ionic concentrations over the frequency range of 100 Hz to 2 MHz. Resulting spectral patterns varied systemically as a function of ionic concentration. In addition to industrial sensing applications, this technology may prove to be beneficial in monitoring microsystems utilizing on-chip fluid chemistry, measuring the dielectric dispersion of polymer solutions, and determining the electrical properties of isolated biological materials     

An intermediate-layer lithography method for generating multiple microstructures made of different conducting polymers

An intermediate-layer lithography (ILL) method has been developed in this work to generate multiple microstructures of different conducting polymers on the same substrate. Previous and current efforts in developing conducting polymer microsystems mainly focus on generating a device of a single function. When multiple micropatterns of different conducting polymers are produced on the same substrate, many microsystems of multiple functions can be envisioned. However, existing techniques present significant technical challenges of degradation, low throughput, low resolution, depth of field, and/or residual layer in producing conducting polymer microstructures. To circumvent these challenges, the ILL method has been explored to generate multiple micropatterns of different conducting polymers in a parallel manner. In this method, conducting polymer materials and a non-conducting polymer intermediate layer are first coated on a substrate, and are then patterned through a mold insertion at a raised temperature. In this work, the ILL has been used to successfully pattern three types of commonly used conducting polymers on the same substrate under a single mold insertion, and simulation has been conducted to gain a good understanding of the molding process. Due to distinctive advantages of simplicity, low cost and high throughput, the ILL has promising applications in fabricating micropatterns for polymer-based microsystems.     

Polymer Microfabrication for Microarrays, Microreactors and Microfluidics

Polymer microfabrication methods are becoming increasingly important as low-cost alternatives to the silicon or glass-based MEMS technologies. Polymer hot embossing and injection molding are replication methods applicable to microreplication of a diversity of materials and microstructures.

Equipment with high precision control of pressure and temperature for hot embossing of polymer materials is now available commercially. These systems have made possible the replication of chips containing microchannels for capillary electrophoresis (CE) and microfluidics devices, microoptical components and microreactors. Stable and reproducible polymer microstructures have been demonstrated in several types of materials with structural and optical properties meeting other biocompatibility and detection requirements. The process involves few variable parameters and results in high structural accuracy suited for a wide range of microfabrication applications.

After demonstrating equivalent and, in cases, improved performance, the alternative use of plastic as the microdevice material addresses needs for rapid prototyping in product development and provides cost advantages in product commercialization. Thus an increasing number of devices have been reported recently in the literature, fabricated on a variety of polymer substrates and using different fabrication methods such as laser ablation, injection molding, silicone rubber casting or embossing for microfabrication.     

Thesaurus federations: loosely integrated thesauri for document retrieval in networks based on Internet technologies

As a result of the distribution of interrelated information over several different information systems, the interconnection of information systems has increased in recent years. However, a purely technical interconnection is insufficient for users who need to find their way to information they are looking for. Thesauri are a proven means to identify documents, e.g., books of interest in a library. For different domains, different thesauri are available, which can be used in information systems as well, e.g., for the indexing and retrieval of data objects. Thus, the interconnection of information systems raises the need to integrate related thesauri. Furthermore, recent advances in open interoperability technologies (World Wide Web, CORBA, and Java) offer the potential for completely new technical solutions for employing thesauri. This paper presents an approach for integrating multiple thesaurus databases. It concentrates on the integration of distributed and heterogeneous thesaurus databases and the integration of multilingual and monolingual thesauri. The software architecture takes advantage of the most advanced Internet and CORBA technology currently available in public domain and in commercial implementations.     

New soldering processes and solder systems for hybrid microsystems: developments and applications

One of the promising joining technologies for the assembly of hybrid microsystems is soldering. This technique meets the specific requirements of the microstructures, like a high flexibility concerning materials and geometry and low process temperatures ensuring the function of the joined components. Soldering offers the possibility of joining different materials in an easy way. Regarding the temperature aspects, the soldering processes can be carried out by low temperature to guarantee any damages in the joined components. Two soldering technologies are examined in this study: transient liquid phase (TLP) bonding and active soldering. Active solders based on Sn and different TLP systems have been developed and optimised according to the requests of the microsystems being joined. The application of the solder systems by magnetron sputter physical vapour deposition has been investigated. The deposition temperature has to be lower than the later joining temperature and lower than the melting points of the solder materials. This fact is a great challenge for the PVD process. The presented investigations and results are acquired within the project SFB 440 “Assembly of Hybrid Microsystems”, financed by the “German Research Foundation” (DFG).     

Biosensors for analytical microsystems

A modular concept for analytical microsystems with separately developed and optimized components is proposed, where parts can be easily substituted. Since the system should be universal hybrid integration is preferred over monolithic fabrication technology.The biosensor as an essential part of an analytical microsystem is commonly restricted in its functional stability due to the lifetime of the biological component and the adhesion of the sensitive layer at the transducer. Optimum design requirements have to be derived from the particular system to be measured. For the optimization of the (bio)sensor the polymer matrix carrying or covering the enzyme has been modified, the transducer design itself has been varied, and enzymes have been coupled.     

Integrating testability into microsystems

 The integration of sensors and actuators with microelectronics into either compact packages or onto a single silicon die is likely to be of major technological importance over the next decade. These systems are referred to as Microsystems or Micro-Electro-Mechanical-Systems (MEMS). One obstacle to mass-market introduction are difficulties with quality and reliability verification. This paper outlines the difficulties of testing microsystems, shows approaches of test generation and verification transferable from the mixed-signal Integrated-Circuit (IC) domain, and demonstrates an on-line test designed for bridge-type, micromachined accelerometer and pressure sensors [1]. Received: 31 October 1996 / Accepted: 14 November 1996     

High functionality of a polymer nanocomposite material for MEMS applications

We present on a carbon nanoparticle-filled SU-8 photosensitive polymer nanocomposite for use in microelectromechanical systems (MEMS) or microsystems. Exposure and fabrication of the material was carried out using X-ray lithography. The polymer nanocomposite was studied for its electrical, thermal and mechanical characteristics. It was found that at low filler weight percentages, the SU-8 polymer became electrically and thermally conductive. A comparative study of the lithography performance of this functionalized SU-8 to pure SU-8 was also performed. It was determined that UV lithography of the PNC was not suitable for thick films and that by using X-rays, thick film high-aspect-ratio microstructures were achievable. Such results are favorable for many applications such as monolithically integrated polymeric micro-resistive heating elements and polymeric micro-heat sinks.     

Fluidic interconnections for microfluidic systems: A new integrated fluidic interconnection allowing plug‘n’play functionality

A crucial challenge in packaging of microsystems is microfluidic interconnections. These have to seal the ports of the system, and have to provide the appropriate interface to other devices or the external environment.

Integrated fluidic interconnections appear to be a good solution for interconnecting polymer microsystems in terms of cost, space and performance. Following this path we propose a new reversible, integrated fluidic interconnection composed of custom-made cylindrical rings integrated in a polymer house next to the fluidic network. This allows plug‘n’play functionality between external metal ferrules and the system.

Theoretical calculations are made to dimension and model the integrated fluidic interconnection.

Leakage tests are performed on the interconnections, in order to experimentally confirm the model, and detect its limits.     

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.     

Biological cardio-micro-pumps for microbioreactors and analytical micro-systems

Bio-hybrid microsystems actuated by living cells, as micro-bio-actuators and micro-bio-pumps have been developed recently. In these devices biological cells may be powered without external energy sources and the movement or the contraction of muscle cells trigger off the flow of fluid (i.e. culture medium or blood) through microchannels in micro-multi-bioreactor systems. Isolated and in vitro cultured cardiomyocytes (cardiac cells) are the most promising bio-material, which can be used to design a micro-bio-pump/actuator. These spontaneously contracting cells are autonomously powered with glucose as an energy source without any external power supply or stimulus, unlike conventional micro-actuators/micro-pumps. Cardio-micro-bio-pumps/actuators are using collective, synchronous contracting forces of cardiac cells or cardiac cell sheets to drive the flow of fluid. The feasibility of building such actuators was demonstrated in a few examples of bio-hybrid microsystems actuated by single or sheeted cardiomyocytes.     

New approach for batch microfabrication of silicon-based micro fuel cells

This paper reports a novel and straightforward approach to the development of a compact micro direct methanol fuel cell. The device consists of a hybrid polymer membrane as a feasible microintegrable electrolyte to be used together with silicon current collectors. These current collectors consist in microfabricated silicon chips that incorporate a fine electrode grid. The membrane combines two polymers with different functionalities, Nafion^® as a proton conducting material and PDMS as a flexible mechanical support. The compatibility of this membrane with MEMS fabrication processes lies in the acknowledged bonding capabilities of the PDMS polymer to materials typically used in microsystems technologies—such as silicon, silicon dioxide and glass—as well as its ability to withstand variations of the Nafion^® volume. The compatibility of all the components with microfabrication processes will permit the application of batch fabrication techniques for the whole device, so contributing to a significant lowering of the fabrication costs.     

Shape-memory polymers for microelectromechanical systems

This paper investigates the use of shape-memory polymer thin films in microelectromechanical systems (MEMS). shape-memory polymers possess the capacity to recover large-strain deformations by the application of heat and are candidates for small-scale transduction. The key advantages of shape-memory polymers are their low material/fabrication cost coupled with their simplicity of integration/operation. In the present study, shape-memory polymers are spin coated onto a standard Si wafer and polymerized by thermal annealing. The thermomechanics of strain storage and recovery in the polymer films are studied using instrumented microindentation. The sharp microindents demonstrate full recovery at all load levels, establishing the feasibility of microscale actuation. The microindentation response of the polymer film is shown to depend on temperature and the cooling cycle during indentation. In turn, the subsequent recovery behavior of an indent depends on the thermal history during indentation. Indents performed at higher temperatures are larger in size, but have smaller stored strain energy compared to indents performed at low temperature. The larger stored strain energy in low temperature indents results in lower shape recovery temperatures. The effects of indentation temperature and load are systematically investigated to provide a framework for the use of shape-memory polymers in microsystems. Application of shape-memory polymers is demonstrated through the development of an active microfluidic reservoir. The reservoir was created by indentation at the end of a microfluidic channel and was activated by local heating. The collapse of the filled reservoir caused the motion of fluid down the microfluidic channel.     

Fabrication, metallographic and tribologic investigations of LIGA-microstructures made of nickelphosphorous alloys

 The performance and lifetime of micro electro mechanical systems (MEMS) is strongly affected by friction and wear. Because of the restrictions due to the fabrication process the variety of materials used for micro systems is not very manifold. This results often is very poor tribological properties, since the tribological pairings are disadvantageous with regard to friction and especially wear. We therefore investigated materials which can be fabricated by the process currently used for LIGA-microstructures, and have the potential of better properties concerning friction and wear. The results of our tribologic experiments showed, that nickel phosphorous alloys are a promising material for microsystems suffering wear, especially at high surface pressures (high loading). Their absolute values of the wear intensity, are at least one order of magnitude lower than those of nickel and copper, which are the materials mostly used today. Received: 27 June 1996/Accepted:12 July 1996     

A Polymeric Paraffin Microactuator

Paraffin wax is a promising material in microactuators not only because of its ability of producing large displacements and high forces at the same time but also because of the variety of manufacturing techniques available. In this paper, a simple actuator based on paraffin wax as the active material is fabricated and tested. Ultraviolet-curable epoxy is used in a technique combining simultaneous moulding and liquid-phase photopolymerization in a single-process step to build the stiff part of the actuator body. A heater is integrated in the paraffin reservoir, and a polyimide tape is used as the deflecting membrane. Thermomechanical analysis of the paraffin wax shows that it exhibits a volume expansion of 10%, including phase transitions and linear expansion. As for the actuator, a stroke of 90 mum is obtained for the unloaded device, whereas 37 mum is recorded with a 0.5-N contact load at a driving voltage of 0.71 V and a frequency of 1/32 Hz. The actuator can be used in microsystems, where both large strokes and forces are needed. The low-cost materials and low driving voltage also makes it suitable for disposable systems.     

Fast spin dynamics algorithms for classical spin systems

We have proposed new algorithms for the numerical integration of the equations of motion for classical spin systems. In close analogy to symplectic integrators for Hamiltonian equations of motion used in Molecular Dynamics, these algorithms are based on the Suzuki-Trotter decomposition of exponential operators and unlike more commonly used algorithms exactly conserve spin length and, in special cases, energy. Using higher order decompositions we investigate integration schemes of up to fourth order and compare them to a well-established fourth order predictor-corrector method. We demonstrate that these methods can be used with much larger time steps than the predictor-corrector method and thus may lead to a substantial speedup of computer simulations of the dynamical behavior of magnetic materials.