Precision Compliant Systems Laboratory
Prof. Martin Culpepper
The purpose of the Precision Compliant Systems Laboratory (PCSL) is to generate the knowledge required to create and integrate machine elements (flexures, mechanisms and actuators) to form small-scale, multi-axis nanopositioning systems (MNS). Nanopositioners are electromechanical systems, composed of integrated actuators, bearings and mechanisms, that position and orient components with nm-level accuracy. MNS are relevant to (a) instruments that make measurements of small-scale geometries/phenomena and (b) equipment that fabricates parts that rely on small-scale geometries/phenomena. Advances in MNS technology enable many scientific discoveries (via instruments) and help put these discoveries into practice (via equipment).
New machine element concepts, synthesis methods and design tools are required to realize small-scale MNS. The PCSL aims to create and grow a body of knowledge that supports the design and fabrication of small-scale MNS. Six-axis systems present the most challenging problems and therefore they serve as platforms for validating the research. Although this work is inspired by specific applications, the results are applicable to a wide array of small-scale MNS problems.
Two current research focuses are: (1) the development of a high force, high speed, and long stroke micro-scale thermomechanical actuator (TMA) for a three-axis millimeter scale compliant endoscopic scanner, and (2) the design of small scale non-linear optics for an endomicroscope capable of performing optical biopsy based two-photon excitation (in collaboration with Prof. Peter So’s research group). Two-photon microscopic imaging is based on the non-linear excitation of fluorophores using infrared radiation. A two-photon fluorescence microscope has the advantage of having inherent 3-D resolution, allowing fluorescence contrast to identify tissue biochemical signatures, providing deep image penetration into highly turbid tissues, and minimizing tissue photodamage.
In 2003, the µ-HexFlex, a PCSL invention, was awarded a R&D 100 Award, and a US patent in conjunction with MIT is pending. The µ-HexFlex is a six-axis micro-mechanism with nanometer level resolution, composed of stacked layers of silicon and silicon dioxide. The integrated TMAs are capable of exerting in-plane and out-of-plane forces on the central stage and flexure bearings.
Chemical-Mechanical Planarization Group
Chemical-Mechanical Planarization (CMP) is a critical process in the manufacture of high-performance, ultra-large-scale-integrated (ULSI) electronics. CMP involves two components, a chemical element and a mechanical element. The chemical component of the CMP process is the reaction of the wafer surface with slurry chemicals to form a soft layer, and the mechanical component is the removal of the softened surface by hard abrasive particles in the slurry. The CMP Group specifically studies copper (Cu) CMP which involves removing excess, deposited Cu as a result of the damascene Cu metallization process. Ideally, the goal of CMP is the complete removal of excess Cu, leaving a planar surface for the deposition of the next oxide layer. In practice, the process is far from ideal due to interactions from a multitude of process variables. As the semiconductor industry moves towards the next generation of semiconductors, with features of finer resolution and materials with lower dielectric constants, the demand to understand and control the CMP process will become greater than ever. The CMP Group has recently developed a comprehensive, tribological Cu CMP model to minimize dishing and erosion. Current research involves the development of a novel CMP architecture to experimentally validate the multi-scale model as well as the modeling of nano-scale scratching of the barrier layer during CMP.
Fuel Cells for Portable Electronics
Fuel cells are used to convert chemical energy stored in a fuel directly to electrical work without the need for combustion. Direct methanol fuel cells (DMFCs) use methanol, an energy dense liquid, as the fuel in place of the more common hydrogen. The simplicity of liquid fuel storage and the large energy density of methanol make DMFCs a leading candidate to replace lithium-ion batteries as the power source of choice for power-hungry portable electronics of the future. Fuel cell research in the LMP is focused on the design and characterization of novel DMFC electrodes. These electrodes are designed such that they will be highly catalytically active, inexpensive, and durable over the life of the fuel cell.
Despite the appeal of fuel cells as energy conversion devices, there are many obstacles to their commercialization. Primary amongst these obstacles is that the electrochemical reduction of oxygen is an inherently slow reaction, and that the best available catalyst for this reaction is platinum. The result of these two factors is that fuel cells are expensive and have limited power densities. Absent a catalyst superior to platinum, we are left to engineer a fuel cell to maximize platinum surface area and utilization so as to achieve the higher power densities with lower amounts of platinum catalyst. Much of fuel cells research has focued on achieving these ends by engineering electrodes with higher internal surface areas. However, this is often to the detriment of the mass transport or reactants within the electrode. The focus of our research in this area is on the engineering of fuel cells with improved mass transfer as compared with fine pore gas diffusion electrodes. With improved mass transfer it becomes possible to increase the utilization of platinum and therefore generate higher overall reaction rates from a given mass of platinum catalyst, ultimately resulting in a fuel cell with higher power density at a lower cost.
Led by Chun and Suh and funded by the KIMM-MIT Collaborative Research Program.
Large scale manufacturing for microfluidics is still in its infancy, but in recent years there has been a shift from materials and equipment typically used for semiconductor fabrication, to polymers and polymer processing. The eventual goal of polymer processing is to mass manufacture economically, but today polymers are more often chosen for their material properties than for their manufacturabilty. The ability to use and combine a range of polymers is essential to keep polymer processing attractive for design, yet many are orders of magnitude more compliant than glass or silicon and need to be handled differently. Semiconductor equipment used for wafer bonding and alignment are no longer adequate many polymers. My research is focused on identifying the factors that make compliant materials difficult to align to one another, and their consequential limitation to alignment accuracy. This project is supported by the SMA MST Flagship Research Program.
Micro and Nano Systems Laboratory (MNSL)
Prof. Sang-Gook Kim
In order to establish a systematic framework for product realization at micro and nanoscales, MNSL is working on the development of new manufacturing and assembly processes and design of devices with nano-enabled functionalities. Kim group has developed strain-tunable optical micro-photonic devices, 100 billion cycle MEMS contact switches and energy-harvesting piezoelectric MEMS devices. Systems approach for MEMS now extends to multi-scale systems, which includes transplanting CNT assembly, muscle inspired MEMS actuators and digital printing of piezoelectric MEMS.