KEYWORDS: Micromirrors, Mirrors, Microelectromechanical systems, Oxides, Digital signal processing, Transmitters, Space operations, CMOS cameras, Charge-coupled devices, Imaging systems
MEMX Corporation in collaboration with Johns Hopkins University Applied Physics Laboratory (JHU/APL) has developed micro-mirror technology applicable to free-space multi-access optical communications terminals. Based on their previously developed micro-electro-mechanical systems (MEMS) optical switches, these new units are being evaluated for applications on spacecraft. These devices must operate within very accurate digitally-controlled pointing and tracking subsystems, which are an essential adjunct to the long-haul optical communication channels that would be operated potentially from geosynchronous earth orbit (GEO) to ground. For such spacecraft applications high-powered laser diodes are likely be the required transmitter. Coupled with their potential operation in a vacuum or at partial atmospheric pressures, MEMS mirror shape stability and fabrication tolerances are of key concern to a system designer. To this end we have measured the performance of preliminary micro-mirror units in terms of angular jitter, focal spot stability, and open and closed-loop response versus laser transmitter power in both ambient air and at low partial pressures. We will describe the fabrication process as well as the experimental test configurations and results in the context of optical beamsteering. We will also discuss the applicability and scalability of this technology to multi-access terminals.
SUMMiT at the Sandia National Laboratories' MDL is a standardized MEMS technology that allows designers to fabricate concept prototypes. This technology provides four polysilicon layers plus three sacrificial oxide layers to enable fabrication of complex mechanical systems-on-a-chip. Quantified reproducibility of the SUMMiT process is important for process engineers as well as designers. Summary statistics for critical MEMS technology parameters such as film thickness, line width, and sheet resistance will be reported for the SUMMiT process. Additionally, data from Van der Pauw test structures will be presented. Data on film thickness, film uniformity and critical dimensions of etched line widths are collected from both process and monitor wafers during manufacturing using film thickness metrology tools and SEM tools. A standardized diagnostic module is included in each SUMMiT run to obtain post- processing parametric data to monitor run-to-run reproducibility such as Van der Pauw structures for measuring sheet resistance. This characterization of the SUMMiT process enables design for manufacturability in the SUMMiT technology.
The integration of miniaturized mechanical components has spawned a new technology known as microelectromechanical systems (MEMS). Surface micromachining, defined as the fabrication of micromechanical structures from deposited thin films, is one of the core technological processes underlying MEMS. Surface micromachined structures have a large ratio of surface area to volume which makes them particularly vulnerable to adhesion to the substrate or adjacent structures during release on in use- a problem is called stiction. Since microactuators can have surfaces in normal or sliding contact, friction and wear are critical issues for reliable operation of MEMS devices. Surface modifications are needed to reduce adhesion and friction in micromechanical structures. In this paper, we will present a process used to selectively coat MEMS devices with Tungsten using a CVD process. We will discuss the effect of wet and vapor phase cleans along with different process variables. Endurance of the W coating is important, especially in applications where wear due to repetitive contacts with the film may occur. Further, tungsten is hard and chemically inert. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable.
Surface polysilicon micromachined micromirrors require ultra-flat surfaces for advance optical applications such as adaptive optics. This paper details the planarization of micromirrors using chemical-mechanical polishing. We show that the increase in topography is due to a high temperature anneal step downstream for the CMP process itself. Two process alternatives were investigated: (1) perform a CMP step after the high temperature anneal step, and (2) perform a CMP step on a final polysilicon mirror surface. Both process alternatives produced acceptable flatness requirements for micromirror applications.
We recently reported on the development of a 5-level polysilicon surface micromachine fabrication process consisting of four levels of mechanical poly plus an electrical interconnect layer and its application to complex mechanical systems. This paper describes the application of this technology to create micro-optical systems-on-a-chip. These are demonstration systems, which show that give levels of polysilicon provide greater performance, reliability, and significantly increased functionality. This new technology makes it possible to realize levels of system complexity that have so far only existed on paper, while simultaneously adding to the robustness of many of the individual subassemblies.
This work presents film properties and initial reliability studies for thin Teflon-like films applied to a unique test vehicle, the Sandia-designed and fabricated microengine. Results on microengines coated with the film show a factor of three improvement in their lifetime and an order of magnitude reduction in the coefficient of friction when compared to uncoated samples. Coefficients Of Friction (COF) of 0.07 for the Teflon-like film and 1.0 for uncoated samples are extracted from models which match the measured behavior of working microengines. These films, deposited from a plasma source, exhibit the ability to penetrate into very narrow, deep channels common to many MEMS devices. For as-deposited film, both the refractive index at 1.4 and the contact angle with water at 108 degrees show the film to be very similar to bulk Teflon PTFE. Film stability as a function of temperature has been examined using Fourier Transform Infrared (FTIR) spectroscopy. The film structure as observed by the fluorine- carbon (F-C) peak is stable up to 200 C, but starts decomposing above 250 C. Film composition has been examined using X-ray photoelectron spectroscopy (XPS) and is quite different for directly exposed surfaces compared with deep, narrow channels where the deposition process is diffusion limited.
The National Institute of Standards and Technology (NIST) is exploring the feasibility of using artifacts fabricated on silicon-on-insulator (SOI) materials to quantify methods divergence, for critical dimension (CD) metrology applications. Test structures, patterned on two types of (110) SOI materials, SIMOX (Separation by IMplantation of OXygen) and BESOI (Bonded-and-Etched-back Silicon-on-Insulator), have been compared. In this paper, we describe results of electrical critical dimension (ECD) measurements and the relative performance of the test structures fabricated on the two SOI materials.
We discuss tow examples of integration of micro- electromechanical system (MEMs) and a photonic device. In the first instance, a MEMs locking device pin is driven by a voltage generated by photovoltaic cells connected in series, which are driven by a laser. In the second case, a VCSEL emitting at 1.06 micrometers is packaged together with a metallized MEMs shutter. By appropriate alignment to the opening in the shutter, the VCSEL is turned on and off by the movement of the Si chopper wheel.
Quad-level polysilicon surface micromachining technology, comprising three mechanical levels plus an electrical interconnect layer, is giving rise to a new generation of micro-electromechanical devices and assemblies. Enhanced components can now be produced through greater flexibility in fabrication and design. New levels of design complexity that include multi-level gears, single-attempt locks, and optical elements have recently been realized. Extensive utilization of the fourth layer of polysilicon differentiates these latter generation devices from their predecessors. This level of poly enables the fabrication of pin joints, linkage arms, hinges on moveable plates, and multi-level gear assemblies. The mechanical design aspects of these latest micromachines will be discussed with particular emphasis on a number of design modifications that improve the power, reliability, and smoothness of operation of the microengine. The microengine is the primary actuation mechanism that is being used to drive mirrors out of plane and rotate 1600-micrometers diameter gears. Also discussed is our most advanced micromechanical system to date, a complex proof-of-concept batch-fabricated assembly that, upon transmitting the proper electrical code to a mechanical lock, permits the operation of a micro-optical shutter.
The SAMPLE (Sandia Agile MEMS prototyping, layout tools, and education) service makes Sandia's state-of-the-art surface- micromachining technology, known as SUMMiT, available to U.S. industry for the first time. The Sandia ultra-planar multi- level MEMS technology (SUMMiT) offered through SAMPLE is the world's most advanced surface-micromachining technology, with three independently patternable ultra-low-stress mechanical polysilicon levels (in addition to the electrical polysilicon layer), one-micron design rules, flanged hubs, and CMP planarization of the third sacrificial oxide to provide planar structures in the third mechanical polysilicon layer (MMPoly3). Participants in the SAMPLE program learn about MEMS and SUMMiT process through the Sandia MEMS short course and then use Sandia's specialized design and layout tools to design their own micromachines to be fabricated in SUMMiT.
Gear systems rotating on hubs have been operated to failure using Sandia's microengine as the actuation device. Conventional failure modes such as fatigue induced fracture did not occur, indicating that the devices are mechanically extremely robust. The generic route to failure observed for all rotating devices involves sticking of structures that are in sliding contact. This sticking evidently results from microscopic changes in the sliding surfaces during operation. The rate at which these changes occur is accelerated by excessive applied forces, which originate from non-optimized designs or inappropriate drive voltages. Precursors to failure are observed, enabling further understanding of the microscopic changes that occur in the sliding surfaces that ultimately led to failure.
The SAMPLE (Sandia agile MEMS prototyping, layout tools, and education) service makes Sandia's state-of-the-art surface micromachining fabrication process, known as SUMMiT, available to U.S. industry for the first time. The service provides a short course and customized computer-aided design (CAD) tools to assist customers in designing micromachine prototypes to be fabricated in SUMMiT. Frequent small-scale manufacturing runs then provide SAMPLE designers with hundreds of sophisticated MEMS (microelectromechanical systems) chips. SUMMiT (Sandia ultra-planar, multi-level MEMS technology) offers unique surface-micromachining capabilities, including four levels of polycrystalline silicon (including the ground layer), flanged hubs, substrate contacts, one-micron design rules, and chemical-mechanical polishing (CMP) planarization. This paper describes the SUMMiT process, design tools, and other information relevant to the SAMPLE service and SUMMiT process.
Recently, a great deal of interest has developed in manufacturing processes that allow the monolithic integration of microelectromechanical systems (MEMS) with driving, controlling, and signal processing electronics. This integration promises to improve the performance of micromechanical devices as well as lower the cost of manufacturing, packaging, and instrumenting these devices by combining the micromechanical devices with a electronic devices in the same manufacturing and packaging process. In order to maintain modularity and overcome some of the manufacturing challenges of the CMOS-first approach to integration, we have developed a MEMS-first process. This process places the micromechanical devices in a shallow trench, planarizes the wafer, and seals the micromechanical devices in the trench. Then, a high-temperature anneal is performed after the devices are embedded in the trench prior to microelectronics processing. This anneal stress-relieves the micromechanical polysilicon and ensures that the subsequent thermal processing associated with fabrication of the microelectronic processing does not aversely affect the mechanical properties of the polysilicon structures. These wafers with the completed, planarized micromechanical devices are then used as starting material for conventional CMOS processes. The circuit yield for the process has exceeded 98 percent. A description of the integration technology, the refinements to the technology, and wafer- scale parametric measurements of device characteristics is presented. Additionally, the performance of integrated sensing devices built using this technology is presented.
The planarization technology of chemical-mechanical- polishing (CMP), used for the manufacturing of multilevel metal interconnects for high-density integrated circuits, is also readily adaptable as an enabling technology in micro- electro-mechanical systems (MEMS) fabrication, particularly polysilicon surface micromachining. CMP not only eases the design and manufacturability of MEMS devices by eliminating several photolithographic and film issues generated by severe topography, but also enables far greater flexibility with process complexity and associated designs. Thus, the CMP planarization technique alleviates processing problems associated with fabrication of multilevel polysilicon structures, eliminates design constraints linked with non- planar topography, and provides an avenue for integrating different process technologies. Examples of these enhancements include: a simpler extension of surface micromachining fabrication to multiple mechanical layers, a novel method of monolithic integration of electronics and MEMS, and a novel combination of bulk and surface micromachining.
An electromechanical model of Sandia's microengine is developed and applied to quantify critical performance tradeoffs. This is done by determining how forces impact the mechanical response of the engine to different electrical drive signals. To validate the theoretical results, model- based drive signals are used to operate actual engines, where controlled operation is achieved for the following cases: 1) spring forces are dominant, 2) frictional forces are dominant, 3) linear inertial forces are dominant, 4) viscous damping forces are dominant, and 5) inertial load forces are dominant. Significant improvements in engine performance are experimentally demonstrated in the following areas: positional control, start/stop endurance, constant speed endurance, friction load reduction,and rapid actuation of inertial loads.
Measurements of the linewidths of submicrometer features made by different metrology techniques have frequently been characterized by differences of up to 90 nm. The purpose of the work reported here is to address the special difficulties that this phenomenon presents to the certification of reference materials for the calibration of linewidth-measurement instruments. Accordingly, a new test structure has been designed and fabricated, and has undergone preliminary tests. Its distinguishing characteristics are assured cross-sectional profile geometries with known side-wall slopes, surface planarity, and compositional uniformity when it is formed in mono-crystalline material at selected orientations to the crystal lattice. To allow the extraction of electrical linewidth, the structure is replicated in a silicon film of uniform conductivity which is separated from the silicon substrate by a buried oxide layer. The utilization of a silicon-on-insulator (SOI) substrate further allows the selective removal of substrate material from local regions below the reference features, thus facilitating measurements by optical and electron-beam transmission microcopy. The combination of planar feature surfaces having known side-wall slopes is anticipated to eliminate factors which are believed to be responsible for methods divergence in linewidth measurements, a capability which is a prerequisite for reliable certification of the linewidths of features on reference materials.
We detail a new monolithically integrated silicon mold/surface-micromachining process which makes possible the fabrication of stiff, high-aspect-ratio micromachined structures integrated with finely detailed, compliant structures. An important example, which we use here as our process demonstration vehicle, is that of an accelerometer with a large proof mass and compliant suspension. The proof mass is formed by etching a mold into the silicon substrate, lining the mold with oxide, filling it with mechanical polysilicon, and then planarizing back to the level of the substrate. The resulting molded structure is recessed into the substrate, forming a planar surface ideal for subsequent processing. We then add surface-micromachined springs and sense contacts. The principal advantage of this new monolithically integrated mold/surface-micromachining process is that it decouples the design of the different sections of the device: in the case of a sensitive accelerometer, it allows us to optimize independently the proof mass, which needs to be as large, stiff, and heavy as possible, and the suspension, which needs to be as delicate and compliant as possible. The fact that the high-aspect-ratio section of the device is embedded in the substrate enables the monolithic integration of high- aspect-ratio parts with surface-micromachined mechanical parts, and, in the future, also electronics. We anticipate that such an integrated mold/surface micromachining/electronics process will offer versatile high-aspect-ratio micromachined structures that can be batch- fabricated and monolithically integrated into complex microelectromechanical systems.
KEYWORDS: Microelectromechanical systems, Reliability, Actuators, Solids, Francium, Data modeling, Systems modeling, Temperature metrology, Dielectrics, Lanthanum
Understanding the frictional properties of advanced Micro-Electro-Mechanical Systems (MEMS) is essential in order to develop optimized designs and fabrication processes, as well as to qualify devices for commercial applications. We develop and demonstrate a method to experimentally measure the forces associated with sliding friction of devices rotating on a hub. The method is demonstrated on the rotating output gear of the microengine recently developed at Sandia National Laboratories. In-situ measurements of an engine running at 18300 rpm give a coefficient of friction of 0.5 for radial (normal) forces less than 4 (mu) N. For larger forces the effective coefficient of friction abruptly increases, suggesting a fundamental change in the basic nature of the interaction between the gear and hub. The experimental approach we have developed to measure the frictional forces associated with the microengine is generically applicable to other MEMS devices.
The monolithic integration of micromechanical deviecs with their controlling electronics offers potential increases in performance as well as decreased cost for these devices. Analog devices has demonstrated the commercial viability of this integration by interleaving the micromechanical fabrication steps of an accelerometer with the microelectronic fabrication steps of its controlling electronics. Sandia's Microelectronics Development Laboratory has integrated the micromechanical and microelectronic processing sequences in a segregated fashion. In this CMOS-first, micromechanics-last approach, conventional aluminum metalization is replaced by tungsten metalization to allow the CMOS to withstand subsequent high-temperature processing during the micromechanical fabrication. This approach is a refinement of an approach originally developed at UC Berkeley. Specifically, the issues of yield, repeatability, and uniformity of the tungsten/CMOS approach are addressed. Also, material issues related to the development of high-temperature diffusion barriers, adhesion layers, and low-stress films are discussed. Processing and material issues associated with alternative approaches to this integration such as micromechanics-first, CMOS-last or the interleaved process are also discussed.
An overview of the major sensor and actuator projects using the micromachining capabilities of the Microelectronics Development Laboratory at Sandia National Laboratories will be presented. Development efforts are under way for a variety of micromechanical devices and control electronics for those devices. Surface micromachining is the predominant technology under development. Pressure sensors based on silicon nitride diaphragms have been developed. Hot polysilicon filaments for calorimetric gas sensing have been developed. Accelerometers based upon high-aspect ratio surface micromachining are under development. Actuation mechanisms employing either electrostatic or steam power are being combined with a three-level active (plus an additional passive level) polysilicon surface micromachining process to couple these actuators to external devices. The results of efforts toward integration of micromechanics with the driving electronics for actuators or the amplification/signal processing electronics for sensors is also described. This effort includes a tungsten metallization process to allow the CMOS electronics to withstand high-temperature micromechanical processing.
Several authors have given overviews of microelectromechanical systems, including microactuators. In our presentation we review some of these results, and provide a brief description of the basic principles of operation, fabrication, and application, of a few selected microactuators (electrostatic and surface tension driven). We present a description of a three- level mechanical polysilicon surface-micromachining technology with a discussion of the advantages of this level of process complexity. This technology is capable of forming complex, batch-fabricated, interconnected, and interactive, microactuated micromechanisms which include optical elements. The inclusion of a third deposited layer of mechanical polysilicon greatly extends the degree of complexity available for micromechanism design. Two examples of microactuators fabricated using this process are provided to illustrate the capabilities and usefulness of the technology. The first actuator is an example of a novel actuation mechanism based on the effect of surface tension at these micro-scale dimensions and of a microstructure within a microstructure. The second is a comb-drive-based microengine which has direct application as a drive and power source for micro optical elements, specifically, micro mirrors and micro shutters. This design converts linear oscillatory motion from electrostatic comb drive actuators into rotational motion via a direct linkage connection. The microengine provides output in the form of a continuously rotating output gear that is capable of delivering drive torque to a micromechanism.
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