Electrochemical post-processing of LIGA structures

 The Louisiana State University (LSU) Microsystems Engineering Team (μSET) is developing techniques for producing microstructures with predictable 3-D geometry. One approach is based on controlled anodic dissolution of electroplated structures to obtain desired shapes. A finite element model was developed to predict the resulting shapes. As a demonstration of the shaping process, development of LIGA-based scanning probe microscope tips was investigated. An array of micro-posts 1.8 micrometers in diameter and 12 micrometers high was produced using x-ray lithography and subsequent electrodeposition. The posts were sharpened electrochemically. Other geometries are being investigated. Received: 30 October 1995/Accepted: 18 December 1995     

The formation of moulds for 3D microstructures using excimer laser ablation

 Fabrication by the ‘Laser-LIGA’ process of three-dimensional structures for micro-fluidic devices is investigated. Polymer moulds are formed by projection ablation using an excimer laser operating at 248 nm wavelength. The moulds are replicated by electroforming –as in conventional X-ray LIGA – to produce fixed and freely moving nickel microturbine parts with heights of 150 μm, wall angles of 1.5° from vertical and surface roughnesses below 100 nm. Received: 30 October 1995/Accepted: 4 March 1996     

High aspect ratio structures obtained by electroforming in microstructured glass

 The application of glass in the microsystems technology becomes more and more important. Therefore, it is necessary to combine special microstructurable glasses with various other materials (e.g. joining with silicon by anodic bonding, joining with metals by electroforming, joining with polymers by glueing, joining with various other glasses by brazing or diffusion welding). In order to increase the use of glass in the microsystems technology especially four fields of research are important: –development of special glasses with various properties –technologies of microstructuring –technologies of joining –application These four topics are main research fields performed at the Institute of Glass/Ceramics-Technology and will be described in this paper. Received: 30 October 1995 / Accepted: 12 January 1996     

The fabrication of LIGA mask using photolithography and synchrotron radiation lithography

 The fabrication of LIGA mask is a very important step in LIGA process. Usually an intermediate mask with gold absorber pattern of 2 μm thickness is fabricated firstly using gold electroplating for absorber pattern in the resist structure written by Electron Beam (e-Beam), then the LIGA mask can be copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Recently, we use photolithography (instead of e-beam) to make the primary structure, and produce the intermediate mask with gold absorber pattern of 1.5 μm thickness produced by etching gold film with 1.5 μm thickness under the photoresist structure using Ar⁺. The LIGA mask with absorber pattern of 13 μm thickness is copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Received: 30 October 1995/Accepted: 9 December 1995     

Development of a solder bump technique for contacting a three-dimensional multi electrode array

 The application of a solder bump technique for contacting a three-dimensional multi electrode array is presented. Solder bumping (or C4: Controlled Collapse Chip Connections, also called Flip Chip contacting) is the most suitable contacting technique available for small dimensions and large numbers of connections. Techniques adapted from the literature could successfully be scaled down to be used for 55x55 μm pads at 120 μm heart-to-heart spacing, yielding well-conducting, reasonably strong bonds. Received: 30 October 1995/Accepted: 16 September 1996     

Fabrication of LIGA-Acceleration Sensors by Aligned Molding

 A technology for the fabrication of movable LIGA-Microstructures by molding was developed, which enables the cheaper production of e.g. LIGA-Acceleration Sensors [1]. For this purpose an aligned molding process had to be developed. The realized experimental setup consists of two subsystems, the molding machine and the alignment arrangement [2]. After aligning a substrate it is transported into the molding machine. An effective and extremely precise dimension translation system is required. Although during molding temperature changes appear and high forces are applied, the dimension stability during the molding process has to be guaranteed. The presented system and setup deals successfully with these conditions. An alignment quality of ±10 μm is realized. Using the aligned molding technology temperature compensated LIGA-acceleration sensors [1] were fabricated. The proper function of the sensors was demonstrated. Received: 30 October 1995/Accepted: 11 December 1995     

On the interdependence between shuffle and crossing-over operations

 We investigate the link between the crossing-over operation on languages as a formal model of the linkage and recombination of genes in chromosomes, and the shuffle operation on languages. The obtained results indicate a strong connection between the two operations. Received: August 31, 1995/January 24, 1996     

Forgetting automata and context-free languages

 It is shown that context-free languages can be characterized by linear bounded automata with the following restriction: the head can either move right without rewriting or move left with erasing the current cell (i.e. rewriting it with a special, nonrewriteable, symbol). If, instead of erasing, we consider deleting (complete removing of the cell), the corresponding automata are less powerful. Received June 6, 1994/April 12, 1995     

LIGA and LIGA-like processing with high energy photons

 High energy X-rays, in the present case photons near 20,000 eV, can be used in LIGA-like processing. Heating problems with this type of photon flux can be avoided by proper filtration via transmission filters. The exposure of PMMA with high energy photons leads to cost effective exposures with full cost recovery estimates of $0.10 per square centimeter at 200 μm photoresist thickness. The cost advantage originates from increased PMMA absorption lengths and the ability to fabricate large area masks without diaphragms which permit large exposure fields. Received: 30 October 1995 / Accepted: 17 January 1996     

Fabrication of monolithic integrated fiber-lens connector arrays by deep proton irradiation

 Deep proton irradiation in poly (methyl methacrylate) (PMMA) is a fabrication method for monolithic integrated micro optics which offers high stability and interesting autoalignment features. The process consists of three basic steps: irradiation of a PMMA substrate followed by either a development of the irradiated regions or a swelling of the irradiated regions by organic vapor or both applied to different regions. With this technique a variety of elementary refractive microoptical components and monolithically integrated combinations can be fabricated: microlenses, microprisms, beam splitters, fiber connectors with selfaligned microlenses on top of each fiber. Received: 30 October 1995 / Accepted: 8 December 1995     

Ceramic microstructures and potential applications

 The developed process of tape casting with subsequent stamping is a promising way to make three-dimensional ceramic microstructures. Examples of different ceramic materials illustrate, that a good quality of the moulding with high form fit can be attained after optimisation of the slurry and the stamping process. The sintered microstructures are characterised by lateral dimensions in the micron range and high aspect ratios. The unique properties of ceramic materials in combination with this new process can be used to produce microstructured ultrasonic transducers or micro heat exchangers/reactors. Received: 30 October 1995 / Accepted: 12 January 1996     

Fabrication of collimating grids for an x-ray solar telescope using LIGA methods

 We are fabricating sub-collimating X-ray grids that are to be used in an orbiting solar X-ray telescope. The telescope optics consist of twelve rotating pairs of high aspect ratio grids. The pitch for the grids ranges from 34 μm to 317 μm. The grid thickness-to-grid-slit ratio must be approximately 50:1, resulting in grid thicknesses of 1 to 10 millimeters. We are implementing a design in which a 34 μm pitch, free-standing PMMA grid is fabricated with 20 μm wide slits through a 800 μm thickness. After exposure and developing, metal is electrodeposited into the slits in the PMMA grid and the PMMA is left in place to hold the individual metal pieces. For optimum imaging performance, the root-mean-square pitch of the two grids of each pair must match to within 1 part in 10000 and simultaneous exposures of stacked sheets of PMMA have insured that this requirement is met. Received: 30 October 1995 / Accepted: 25 January 1996     

Efficient network authentication protocols: Lower bounds and optimal implementations

Summary.  Research in authentication protocols has focused largely on developing and analyzing protocols that are secure against certain types of attacks. There is little and only scattered discussion on protocol efficiency. This paper presents results on the lower bounds on the numbers of messages, rounds, and encryptions required for network authentication. For each proven lower bound, an authentication protocol achieving the bound is also given, thus proving that the bound is a tight bound if the given optimal protocol is secure. Moreover, we give impossibility results of obtaining protocols that are simultaneously optimal with respect to the numbers of messages and rounds. Received: October 1994 / Accepted: July 1995     

The study of LIGA technique at NSRL

 The activity of LIGA technique was started in the fall of 1993 at National Synchrotron Radiation Laboratory (NSRL), some primary results have been obtained. Micro gearwheels were produced by deep-etch lithography using Hefei light source. The largest diameter of gearwheel is 200 μm, the smallest one only 35 μm, both with thickness of 50 μm. The first metallic structure of nickel was fabricated recently. The mask used in experiments was made by ourselves, it is a composite of kapton membrane as a mask blank, and gold as an absorber. Received 30 October 1995/Accepted: 25 January 1996     

Development of ceramic microstructures

 For the production of microstructured ceramic parts, development efforts are necessary in the field of powder synthesis as well as in the area of shaping. Several processes have been developed. Powder synthesis was performed starting from metal organic precursors and using a two stage thermal process. Shaping was performed by sol-gel methods, by pressing of a ceramic slurry or an organic precursor paste as well as by direct electrophoretic deposition of gels in a microstructured form. An overview is given on the methods and the results. Received: 30 October 1995 / Accepted: 12 January 1996     

Self-stabilizing token circulation in uniform networks

Summary.  A self-stabilizing protocol for token circulation in a connected, uniform network of nodes with prime size is proposed. A network of nodes is said to be uniform if all nodes are logically equivalent and identically programmed. The protocol has the ability to handle any arbitrary initial state in which more than one token or no token at all exist in the network and makes the network eventually have one and only one token fairly circulating among the nodes of the network. The protocol is deterministic, its self-stabilization property is proven under the assumption of a serial and fair scheduler. Received: July 1995 / Accepted: February 1997     

Syntactic complexity of context-free grammars over word monoids

 The syntactic complexity of context-free grammars defined over word monoids is investigated. It is demonstrated that every recursively enumerable language can be defined by a ten-nonterminal context-free grammar over a word monoid generated by an alphabet and six words of length two. Open problems are formulated. Received October 10, 1994/February 23, 1995     

Optical DUV-lithography for high microstructures

 A new technology called 3D UV-Microforming consisting of an advanced resist preparation process, a UV lithographic step, resist development, a moulding procedure by electrodeposition, and finally stripping and cleaning for finishing the structures was developed for application in microsystem technology. It enables the low-cost fabrication of a wide variety of micro components for many different users. During resist preparation, layers up to two hundred μm thickness were obtained until now. By using a standard UV mask aligner as an exposure tool followed by immersion development, thick resist layers up to 100  μm could be patterned in a single step on pre-processed silicon wafers. Repeated exposure and development were successfully used for structuring resist layers of up to 200  μm thickness. High aspect ratios of more than 10 as well as steep edges of more than 88° could be fabricated. The resist patterns were moulded by using pulse or DC electroplating. For microsystem applications some metals and alloys were deposited, Three-dimensional micro components were fabricated as demonstrators for the new technique. It allows the use of materials with interesting properties which could not be provided by standard processes. Received: 30 October 1995 / Accepted: 9 December 1995     

High winding density micro coils for magnetic actuators

 A high winding density micro coil technology is reported here which has been used to reduce the power requirements of several classes of micro actuators. [Christenson (1995)] Actuator design sets the required magnetic field and power. At low frequencies coil design translates these requirements to a total winding conductor cross-sectional area. The winding fill fraction of the coil technology sets the final coil size. Using the design equations, coil technologies may be compared. Received: 25 August 1997 / Accepted: 20 October 1997     

Synchronous,asynchronous, and causally ordered communication

Summary.  This article studies characteristic properties of synchronous and asynchronous message communications in distributed systems. Based on the causality relation between events in computations with asynchronous communications, we characterize computations which are realizable with synchronous communications, which respect causal order, or where messages between two processes are always received in the order sent. It is shown that the corresponding computation classes form a strict hierarchy. Furthermore, an axiomatic definition of distributed computations with synchronous communications is given, and it is shown that several informal characterizations of such computations are equivalent when they are formalized appropriately. As an application, we use our results to show that the distributed termination detection algorithm by Dijkstra et al. is correct under a weaker synchrony assumption than originally stated. Received: November 1992/Accepted: July 1995