Precision Molding of Metallic Micro-Components

Michael J. Kaufman ¹
W. Gregory Sawyer ²
Tony L. Schmitz ²
John C. Ziegert ²

¹Materials Science and Engineering,
²Mechanical and Aerospace Engineering

University of Florida
Gainesville, Fl., 32611

Proof of Concept

Preliminary experiments were carried out to evaluate the feasibility of molding micron sized features using an amorphous alloy.  A silicon substrate with raised features generated by scanning probe lithography, where the desired pattern is obtained by controlled oxidation of the substrate surface using an applied voltage between an atomic force microscope (AFM) cantilever tip and the substrate, was pressed into a heated (360ºC) metallic glass blank using a small laboratory press.  After a short dwell time under a pressure of 14 MPa, the ram was retracted and the sample was removed and quenched in water.  

Topograph of bulk metallic glass after being pressing into patterned silicon substrate.

Subsequent examination revealed good replication of various complicated features including posts, ‘bow ties’, rectangular wells, and filleted rectangles with sizes at the micron scale and below. These results are encouraging and suggest that this material is ideally suited for mass production of three dimensional geometries using a micro-molding process.  It is expected that this process will produce high aspect ratio features with dimensions on the order of microns or less, combined with excellent surface finish and tight tolerances.

State of the Art in MEMS

   Current MEMS fabrication is heavily influenced by, and largely dependent on, technologies and processes originally developed for microelectronics manufacturing.  These processes impose severe limitations on the materials used, and are primarily limited to silicon in combination with sputtered and etched thin metallic coatings.  The layered nature of the process imposes very severe limits on the types and range of component geometries which can be produced, and thus on the types of mechanical motion that can be realized.  Further these processes are extremely slow and not amenable to mass production.

Potential Impacts on Device Components

   If it were possible to fabricate miniature components with complex, three-dimensional geometries from high strength materials, one could envision fabricating micro-resonators, high frequency microwave components such as waveguides, connectors, and enclosures, micro-flexures, micro-surgical devices, micro-motors, micro-transmission components, micro-fluidic arrays, and non-planar reflective micro-optics.

Advantages of Bulk Metallic Glasses

   Many of the bulk metallic glasses have several fundamental characteristics which make them ideal for net-shape forming of micro-components. First are comparatively low glass transition temperatures (in some cases as low as approximately 350ºC). Above this temperature, the material becomes essentially a supercooled liquid, although it has not truly “melted” in the sense of a phase transformation. Second, since no phase change occurs when the material is cooled, the shrinkage is very small, on the order of 0.5% or less, compared to 4 to 8% for conventional metallic alloys upon solidification.  This gives exceptional tolerance control for the molded features.  Finally, due to its lack of crystallinity, bulk metallic glasses exhibit excellent surface finish upon vitrification; this is important because of the difficulty in performing secondary surface finishing operations on micro-components.

   The low molding temperatures and pressures required for forming these bulk metal glasses permits molds to be made of conventional materials such as tool steels or copper.  This suggests that it may be possible to slightly modify conventional thermo-plastic molding equipment for this application, resulting in relatively low capital costs. 

   When molding fine features with high aspect ratios, where the high surface area to volume ratios lead to high heat transfer rates, the ability to control the viscosity of the material by controlling temperature should allow additional flexibility in optimizing processing parameters.  The components made from the bulk metallic glasses will have exceptional mechanical strengths, high flexibilities, good fracture toughnesses and fatigue strengths, and will be both electrically and thermally conductive.

Research Challenges and Approach

   We anticipate that the ability to flow the bulk metallic glasses in the supercooled liquid state is ideally suited for mass production using micro-molding processes.  There are, however, a series of challenges that must be overcome before such a vision becomes a reality.  These challenges and the innovative approaches to overcome these challenges are described in the following table.

Key variables and technical challenges

Personnel

Dr. Michael Kaufman (mkauf@eng.ufl.edu) is a Professor in the Department of Materials Science and Engineering.  His primary interests are in the structure-property-processing relationships in structural metallic alloys with an emphasis on advanced characterization.

Dr. Gregory Sawyer (wgsawyer@ufl.edu) is an Assistant Professor in the Department of Mechanical and Aerospace Engineering.  His primary interests are in tribology, particularly solid-lubrication and sliding contacts in extreme environments, where the use of fluid lubrication is not available.

Dr. Tony L Schmitz (tschmitz@ufl.edu) is an Assistant Professor in the Department of Mechanical and Aerospace Engineering.  His primary interests are in manufacturing metrology and process dynamics.

Dr. John Ziegert (johnz@ufl.edu) is a Professor in the Department of Mechanical and Aerospace Engineering.  His research background is in precision manufacturing and machining operations, and precision dimensional metrology, and is the President of the American Society for Precision Engineering.