Todd R. Christenson

EBSD studies on wear-induced subsurface regions in LIGA nickel

Abstract The application of focused ion beam techniques to prepare cross-sections of wear tracks is presented. Electron backscatter diffraction analysis of wear scars on electroformed Ni revealed the formation of two subsurface zones, each with its own characteristic features. Formation of low-angle grain boundaries and spread in the orientation of pole figures were also observed.

Mechanical and metallographic characterization of LIGA fabricated nickel and 80%Ni-20%Fe Permalloy

A table top servohydraulic load frame equipped with a laser displacement measurement system was constructed for the mechanical characterization of LIGA fabricated electroforms. A drop in tensile specimen geometry which includes a pattern to identify gauge length via laser scanning has proven to provide a convenient means to monitor and characterize mechanical property variations arising during processing. In addition to tensile properties, hardness and metallurgical data were obtained for nickel deposit specimens of current density varying between 20 and 80 mA/cm{sup 2} from a sulfamate based bath. Data from 80/20 nickel-iron deposits is also presented for comparison. As expected, substantial mechanical property differences from bulk metal properties are observed as well as a dependence of material strength on current density which is supported by grain size variation. While elastic modulus values of the nickel electrodeposit are near 160 GPa, yield stress values vary by over 60%. A strong orientation in the metal electrodeposits as well as variations in nucleating and growth morphology present a concern for anisotropic and geometry dependent mechanical properties within and between different LIGA components.

Deep x-ray lithography for micromechanics

Extensions of the German LIGA process have brought about fabrication capability suitable for cost effective production of precision engineered components. The process attributes a low fabrication of mechanical components which are not capable of being made via conventional subtractive machining methods. Two process improvements have been responsible for this extended capability which involve the areas of thick photoresist application and planarization via precision lapping. Application of low-stress x-ray photoresist has been achieved using room temperature solvent bonding of a preformed photoresist sheet. Precision diamond lapping and polishing has provided a flexible process for the planarization of a wide variety of electroplated metals in the presence of photoresist. Exposure results from the 2.5 GeV National Synchrotron Light Source storage ring at Brooklyn National Laboratory have shown that structural heights of several millimeter and above are possible. The process capabilites are also well suited for microactuator fabrication. Linear and rotational magnetic microactuator have been constructed which use coil winding technology with LIGA fabricated coil forms. Actuator output forces of 1 milliNewton have been obtained with power dissipation on the order of milliWatts. A rotational microdynamometer system which is capable of measuring torque-speed data is also discussed.

High-G MEMS integrated accelerometer

This paper describes the design and implementation of a surface micromachined accelerometer for measuring very high levels of acceleration (up to 50,000 G). Both the mechanical and electronic portions of the sensor were integrated on a single substrate using a process developed at Sandia National Laboratories. In this process, the mechanical components of the sensor were first fabricated at the bottom of a trench etched into the wafer substrate. The trench was then filled with oxide and sealed to protect the mechanical components during subsequent microelectronics processing. The wafer surface was then planarized in preparation for CMOS processing using Chemical Mechanical Polishing. Next, the CMOS electronics were fabricated on areas of the wafer adjacent to the embedded structures. Finally, the mechanical structures were released and the sensor tested. The mechanical structure of the sensor consisted to two polysilicon plate masses suspended by multiple springs (cantilevered beam structures) over corresponding polysilicon plates fixed to the substrate to form two parallel plate capacitors. The first polysilicon plate mass was suspended using compliant springs (cantilever beams) and acted as a variable capacitor during sensor acceleration. The second polysilicon plate mass was suspended using very stiff springs and acted as a fixed capacitor during acceleration. Acceleration was measured by comparing the capacitance of the variable capacitor (compliant suspension) with the fixed capacitance (stiff suspension).

Understanding and Tailoring the Mechanical Properties of Liga Fabricated Materials

LIGA fabricated materials and components exhibit several processing issues affecting their metallurgical and mechanical properties, potentially limiting their usefulness for MEMS applications. For example, LIGA processing by metal electrodeposition is very sensitive to deposition conditions which causes significant processing lot variations of mechanical and metallurgical properties. Furthermore, the process produces a material with a highly textured lenticular rnicrostructural morphology suggesting an anisotropic material response. Understanding and controlling out-of-plane anisotropy is desirable for LIGA components designed for out-of-plane flexures. Previous work by the current authors focused on results from a miniature servo-hydraulic mechanical test frame constructed for characterizing LIGA materials. Those results demonstrated microstructural and mechanical properties dependencies with plating bath current density in LIGA fabricated nickel (LIGA Ni). This presentation builds on that work and fosters a methodology for controlling the properties of LIGA fabricated materials through processing. New results include measurement of mechanical properties of LIGA fabricated copper (LIGA Cu), out-of-plane and localized mechanical property measurements using compression testing and nanoindentation of LIGA Ni and LIGA Cu.

Application of Diamond-Like Nanocomposite Tribological Coatings on LIGA Microsystem Parts

The major focus of this study was to examine the feasibility of applying diamond-like nanocomposite (DLN) coatings on the sidewalls of Ni alloy parts fabricated using lithographie, galvanoformung and abformung (LIGA: a German acronym that means lithography, electroforming, and molding) for friction and wear control. Planar test coupons were employed to understand the friction mechanisms in regimes relevant to LIGA microsytems. Friction tests were conducted on planar test coupons as well as between LIGA-fabricated test structures in planar-sidewall and sidewall-sidewall configurations. Measurements were made in dry nitrogen and air with 50% relative humidity by enclosing the friction tester in an environmental chamber. In contrast to bare metal-metal contacts, minimal wear was exhibited for the DLN-coated LIGA NiMn alloy parts and test coupons. The low friction behavior of DLN was attributed to its ability to transfer to the rubbing counterface providing low interfacial shear at the sliding contact. The coating coverage and chemistry on the sidewalls and the substrate-coating interface integrity were examined by transmission electron microscopy, Automated eXpert Spectral Image Analysis, and electron backscatter diffraction on cross sections prepared by focused ion beam microscopy. The role of novel characterization techniques to evaluate the surface coatings for LIGA microsystems technology is highlighted.

Parallel assembly of high-aspect-ratio microstructures

In this paper, a prototype robotic workcell for the parallel assembly of LIGA components is described. A Cartesian robot is used to press 386 and 485 micron diameter pins into a LIGA substrate and then place a 3-inch diameter wafer with LIGA gears onto the pins. Upward and downward looking microscopes are used to locate holes in the LIGA substrate, pins to be pressed in the holes, and gears to be placed on the pins. This vision system can locate parts within 3 microns, while the Cartesian manipulator can place the parts within 0.4 microns.

LIGA Microsystems: Surface Interactions, Tribology, and Coatings

Abstract Deep X-ray lithography based techniques such as LIGA (German acronym representing Lithographie, Galvanoformung, and Abformung) are being currently used to fabricate net-shape components for microelectromechanical systems (MEMS). Unlike other microfabrication techniques, LIGA lends itself to a broad range of materials, including metals, alloys, polymers, as well as ceramics and composites. Currently, Ni and Ni alloys are the materials of choice for LIGA microsystems. While Ni alloys may meet the structural requirements for MEMS, their tribological (friction and wear) behavior poses great challenges for the reliable operation of LIGA-fabricated MEMS. Typical sidewall morphologies of LIGA-fabricated parts are described, and their role in the tribological behavior of MEMS is discussed. The adaptation of commercial plasma-enhanced chemical vapor deposition to coat the sidewalls of LIGA-fabricated parts with diamond-like nanocompositeis described.

Strength and tribology of bulk and electroformed nickel amorphized by implantation of titanium and carbon

Abstract Dual ion implantation of titanium and carbon was shown to produce an amorphous layer of exceptional strength within annealed bulk Ni and also within electroformed Ni and Ni 80 Fe 20 materials used in micro-electromechanical systems. The intrinsic elastic and plastic mechanical properties of the implanted region were quantified using nanoindentation testing in conjunction with finite-element modeling, and the results were interpreted in the light of microstructures observed by transmission electron microscopy. The implantation treatment was found to produce substantial reductions in unlubricated friction and wear.

LIGA for lobster: First observation of lobster-eye focusing from lithographically produced optics

The prospect of making a lobster-eye telescope is drawing closer with recent developments in the manufacture of microchannel-plate optics. This would lead to an x-ray all-sky monitor with vastly improved sensitivity and resolution over existing and other planned instruments. We consider a new approach, using deep etch x-ray lithography, to making a lobster-eye lens that offers certain advantages even over microchannel-plate technology.