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From 1999 to 2001 I worked on the DARPA-funded 3DMEMS program at the University of Maryland. My advisor was Prof. Don DeVoe.Macro-scale spatial parallel manipulator 

Pictured at left is a 3-DOF parallel manipulator originally designed by Prof. Lung-Wen Tsai and prototyped by Rick Stamper at the University of Maryland. The 3DMEMS effort began as a collaboration between Prof. DeVoe's MEMS group and Prof. Tsai's mechanisms group. 

As envisioned by DeVoe and Tsai, the primary goal of the 3DMEMS program is to develop, demonstrate, and apply a batch fabrication process to construct 3-D microsystems. The process takes advantage of recent developments in silicon-based fabrication technology such as fusion bonding and deep reactive ion etching (DRIE). This research is expected to enable batch fabrication of mechanically robust micromechanisms that are capable of true 3-D motion

The essential ideas of the 3DMEMS process are:

  • Robust, true 3-D capability: A true 3-D fabrication process must be capable of producing devices with characteristics such as overhangs, out-of-plane joints, and enclosed cavities. The traditional criticism of MEMS is that devices are simply extrusions of 2-D patterns.  This is certainly the case when bulk micromachining processes such as DRIE are used without any modifications. While multi-layer processes such as MUMPsŪ* and SUMMiT can be used to produce true 3-D devices, the thin structural layers associated with surface micromachining are extremely compliant. The 3DMEMS process combines the mechanical strength of high-aspect-ratio devices with the true 3-D capability of multi-level processes.

*Note: this overview was written in 2001. A MUMPs process (SOIMUMPs) now exists wherein users can create high-aspect-ratio structures. 

  • Silicon-on-insulator (SOI) wafers: Single crystal silicon (SCS) was chosen as the 3DMEMS structural material because of its favorable mechanical and electrical properties. For example, SCS has no grain boundaries, making it a perfectly elastic material. Additionally, silicon possesses a stable, insulating oxide. SOI wafers feature a thin silicon layer separated from a thick silicon substrate by an oxide layer. Because the 3DMEMS process uses two SOI wafers, only one aligned bonding step is required to produce several structural layers. 

  • Batch fabrication: Once masks are produced for photolithography, subsequent process steps fabricate all the devices on the wafer simultaneously. Although many 3-D microfabrication processes have been explored by other groups, most require individual or sequential device fabrication, which increases manufacturing time and expense. The use of batch fabrication permits the simultaneous fabrication of hundreds or thousands of devices.

  • Parallel manipulation: In general, manipulators can be fabricated in either serial or parallel configurations. A parallel manipulator allows multiple degrees of freedom to be attained while all of the actuators remain on the base. In contrast, a multiple-DOF serial manipulator requires some form of actuation at the intermediate joints. Since it is not practical in our case to mount microactuators on moving links, all control inputs to the micromanipulator must originate from the fixed base.

  • Linear actuation: Tsai's 3-DOF parallel manipulator employs rotational electromagnetic actuators to position the platform. At the micro-scale, however, it is easier to design and fabricate a linear actuator. Our design uses linear 1-DOF slider inputs. We chose electrothermal motors as an actuation method because they can produce relatively high forces and are relatively simple to fabricate. In fact, our electrothermal linear motor design is very compatible with the 3DMEMS fabrication process, requiring only the addition of a metallization step to create bond pads.Fabricated compliant joint micrograph

  • Compliant joints: The macro-scale version uses hinges to achieve in-plane and out-of-plane motion. Our design replaces these hinges with compliant, or flexural, beams. A close-up view of an out-of-plane joint is shown at right. Compliant beams can be used to mimic the behavior of hinges without the associated "slop" resulting from clearances between hinge components. Part of the 3DMEMS research program involves modeling the behavior of compliant joints.

U-beam video screenshot
Compliant joint video (40 KB .rm file)

A large portion of my graduate work involved developing and refining the 3DMEMS process. Around April 2001, when I graduated, we wrapped up the first fabrication run, completing the first out-of-plane joints. This joint was constructed by bonding together the patterned active layers of two SOI wafers. The actual joint, which is only 2 microns thick, is part of the active layer of the top SOI wafer. The substrate of that wafer is used to form caps that hold the sliders in place. 

Shown below are conceptual views of a three-degree-of-freedom (3-DOF) spatial platform micromanipulator (images rendered in AutoCAD 2000 from the original design files). This device was designed by Zhongzhou Tang and me. A spatial manipulator is just one example of a device that can be fabricated using the 3DMEMS process. Envisioned applications for similar devices include sample positioners for scanning electron microscopes, tools for minimally-invasive surgery, and dexterous linkages for microrobots. 

Complete view of spatial platform micromanipulator with integrated thermal actuators

One of the single-DOF, thermally actuated, linear inputs to the manipulator

Closeup of platform showing three single-DOF inputs and several compliant joints

All 3-D microfabrication was conducted with the help of my colleagues Dave Schreiber and Brett Piekarski of the Maryland MEMS Lab. Some fabrication was performed at the Army Research Laboratory. The assistance of Tom Loughran and Nolan Ballew is gratefully acknowledged.