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.     

Enhanced adhesion of PMMA to copper with black oxide for electrodeposition of high aspect ratio nickel-iron microstructures

 Copper is widely used as a plating base for soft magnetic alloy electrodeposition in sensors and actuators. PMMA, the X-ray resist used in the LIGA process, typically has poor adhesion with copper. The use of black oxide of copper to enhance PMMA-copper adhesion was investigated. In this work, peel strength as a function of treatment time and the method of bonding was evaluated using an ASTM standard T-peel test. Peel strength increased with increasing treatment time. The feasibility of producing microstructures with predictable 3-D geometry for use in resonating sensors was investigated using the process developed. Nickel-iron structures of 100-1000 micrometers wide and 500 micrometers tall were successfully electrodeposited. Growth of other representative microstructures is being investigated. Received: 25 August 1997/Accepted: 3 September 1977     

Injection molding of polymeric LIGA HARMs

The primary goal of an ongoing research effort at LSU is to develop the three-step LIGA process to inexpensively manufacture high aspect ratio microstructures (HARMs). The first two steps of the process (lithography and electroplating) produce a metallic mold insert that can be used as a template for molding microstructures. The final step of LIGA is molding. This paper focuses on injection molding of thermoplastics to produce surfaces covered with HARMs. The resulting microstructures are hundreds of micrometers in height, tens of micrometers in width, and separated by gaps on the order of tens of micrometers. Injection molding experiments using high density polyethylene were performed using a commercially available injection molding machine. Experimental variables included injection speed, the tool temperature, and air pressure in the mold cavity. Elevating the tool temperature above the melting point ensured that the polymer completely filled the mold, producing microstructures with the desired geometry. As the temperature of the mold was reduced, higher injection speeds did not necessarily ensure filling of the mold cavity. The cycle time is shorter than the values previously reported in the literature [Madou (1996)]. Received: 30 March 1999 / Accepted: 12 April 1999     

Movable microstructures made by a sub-micron LIGA process

 Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied. Received: 30 October 1995 / Accepted: 17 January 1996     

Quasi-3D microstructure fabrication technique utilizing hard X-ray lithography of synchrotron radiation

 Quasi-three-dimensional (3D) microstructure fabrication technique utilizing hard X-ray lithography (HXL) has been developed. In this technique, as the intensity distribution of the X-rays is controlled by a newly developed bending mirror, the exposure residual depth of polymethyl methacrylate (PMMA) resist is controlled over the exposed area. The maximum difference of depths was approximately 50 μm over the large area more than 60 mm (horizontal) × 5 mm (vertical). We also investigated the effects of controlling the beam intensity distribution for exposure changing X-ray mask absorber shapes and angle on the obtained quasi-3D resist pattern shapes. As the results, Quasi-3D PMMA patterns with inclined shape sidewall and graded depths were successfully fabricated. We believe this technique greatly expands applications of LIGA process. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

The micromilling process for high aspect ratio microstructures

 High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.). The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures. Received: 30 October 1995 / Accepted: 4 March 1996     

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     

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     

Tribological investigations of LIGA-microstructures

 The performance and lifetime of micro electro mechanical systems (MEMS) is strongly affected by friction and wear. We therefore analysed the friction and wear characteristics of microstructures produced by the LIGA- process. The measurements were carried out in a special designed microtribometer, which is capable to work inside a scanning electron microscope (SEM), and hence offering the possibility of a high resolution in-situ observation of the microscopic tribological mechanism during operation of the system. The material combinations investigated were chosen in order to represent the most important tribological junctions occurring in the ‘micro-motor’ and ‘micro-turbine’ currently produced by the LIGA-technique. We studied the tribological properties of nickel–nickel micro components (LIGA-sidewall/LIGA-sidewall and LIGA-bottom side/LIGA -top side), as well as nickel micro components (bottom side)– Al₂O₃-ceramic substrates (contact between rotor-base). The results indicate a strong influence of the relative humidity level (RHL) on the friction and wear properties of the metal/ceramic as well as of the metal/metal contacts appearing in the LIGA-MEMS, showing the lowest level of friction and wear at high RHL-values. We also found out that the friction and wear parameters were highly influenced by the applied surface pressure. Changing the surface pressure results in the formation of different kinds of wear debris accumulating or smearing out in the wear track. The metal oxides produced hereby in the wear track can lower adhesion and thus reduce the friction of the system. Received: 30 October 1995/Accepted: 18 December 1995     

Micro-droplet formation in non-Newtonian fluid in a microchannel

Micro-droplet formation from an aperture with a diameter of micrometers is numerically investigated under the cross-flow conditions of an experimental microchannel emulsification process. The process involves dispersing an oil phase into continuous phase fluid through a microchannel wall made of apertured substrate. Cross-flow in the microchannel is of non-Newtonian nature, which is included in the simulations. Micro-droplets of diameter 0.76–30 μm are obtained from the simulations for the apertures of diameter 0.1–10.0 μm. The simulation results show that rheology of the bulk liquid flow greatly affects the formation and size of droplets and that dispersed micro-droplets are formed by two different breakup mechanisms: in dripping regime and in jetting regime characterized by capillary number Ca. Relations between droplet size, aperture opening size, interfacial tension, bulk flow rheology, and disperse phase flow rate are discussed based on the simulation and the experimental results. Data and models from literature on membrane emulsification and T-junction droplet formation processes are discussed and compared with the present results. Detailed force balance models are discussed. Scaling factor for predicting droplet size is suggested.     

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     

Microgrippers created in microstructurable glass

 In this paper a new microgripper will be presented. The specific feature is the microfabrication based on a UV-lithographic process in microstructurable, photosensitive glass. Technological and manufacturing problems of the gripper will be described. The developed microgripper is actuated by a piezoelectric ceramic (monomorph). Glass microstructures are used as solid state hinges. With the special design of the gripper it is possible to realise a high distance ratio. The deflection of the gripping arms is some hundred micrometers. The gripping forces are a few mN up to 50 mN. The new grippers were fabricated and tested successfully. Received: 20 December 1996/Accepted: 9 January 1997     

Superplastic extrusion of microgear shaft of 10 μm in module

 The goal of this study is to establish microforming technologies for MEMS parts such as microgears of 10 micrometers in module. For this purpose, it is important to develop new methods to fabricate microdies and to select suitable materials for microforming. Photochemically machinable glass has been used to fabricate microdies by photolithography and anisotropic etching techniques. Al-78Zn superplastic material has been employed in microextrusion. This material has the great advantage of deformation under very low stresses in comparison with conventional plastic deformation. Furthermore, the material exhibits good microformability. A new superplastic forward extrusion machine has been developed for this study. Specimens were placed in a container and extruded by a linear actuator in a vacuum or an argon gas atmosphere. Microextrusion has succeeded in forming microgear shafts. The gear of Al-78Zn superplastic alloy is 10 micrometers in module and the diameter of the pitch circle is 100 micrometers. Received: 7 July 1999/Accepted: 29 September 1999     

Population Shape Regression from Random Design Data

Regression analysis is a powerful tool for the study of changes in a dependent variable as a function of an independent regressor variable, and in particular it is applicable to the study of anatomical growth and shape change. When the underlying process can be modeled by parameters in a Euclidean space, classical regression techniques (Hardle, Applied Nonparametric Regression, 1990; Wand and Jones, Kernel Smoothing, 1995) are applicable and have been studied extensively. However, recent work suggests that attempts to describe anatomical shapes using flat Euclidean spaces undermines our ability to represent natural biological variability (Fletcher et al., IEEE Trans. Med. Imaging 23(8), 995–1005, 2004; Grenander and Miller, Q. Appl. Math. 56(4), 617–694, 1998).     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999     

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     

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     

An Automatic Correction of Ma’s Thinning Algorithm Based on <Emphasis Type="Italic">P</Emphasis>-simple Points

The notion of P-simple points was introduced by Bertrand to conceive parallel thinning algorithms. In ‘A 3D fully parallel thinning algorithm for generating medial faces’ (Pattern Recogn. Lett. 16:83–87, 1995), Ma proposed an algorithm for which there are objects whose topology is not preserved. In this paper, we propose a new application of P-simple points: to automatically correct Ma’s algorithm.     

Sensor guided handling and assembly of active micro-systems

Positioning accuracies in the range of a few micrometers and below are necessary for the assembly of active micro-systems. In order to reach these accuracies, an assembly system for sensor guided micro-assembly is developed in the Collaborative Research Centre 516 “Design and manufacturing of active micro-systems”. The combination of a parallel robot with an integrated 3D vision sensor and a micro-gripper enables to reach relative positioning accuracies below 1 μm.