Papers

AVS2004 Session MN+MS+PS+TF-TuA: Nano/MEMS Manufacturing and Plasmas

Tuesday, November 16, 2004 1:20 PM in Room 213C

Tuesday Afternoon

Start	Invited?	Item
1:20 PM		MN+MS+PS+TF-TuA-1 Wafer-Level, Low-Cost, High-Vacuum Packaging of MEMS Devices Using Nanogetter TM N. Najafi, D.S. Sparks (Integrated Sensing Systems, Inc. (ISSYS)) As part of its development effort to commercialize a Micro-Density Meter, ISSYS Inc. invented a new technology for long-term, low-cost, wafer-level, high-vacuum, hermetic encapsulation of MEMS devices. This technology is now commercially available through a spin-off company: Nanogetter Inc. At the system level perspective, one of the most attractive features that NanogetterTM offers to the MEMS community is a "Total Solution" to an important problem facing many emerging MEMS products. Nanogetters Inc. technology offers: Wafer-level, high-vacuum (< 1mTorr) encapsulation, Long-term vacuum stability, Hermetic electrical lead transfer, Compatibility with all MEMS technologies (polysilicon surface, bulk, silicon-on glass, and LIGA micromachining technologies), High yield, Low cost In addition to high-vacuum packaging applications, NanogetterTM will be further developed to provide ambient environments suitable for applications requiring higher pressures. For example, for micro-switches and accelerometers, the technology will absorb humidity and oxygen. As a testbed for using this wafer-level, high-vacuum technology, the performance of a micro-density meter will be presented.
2:00 PM		MN+MS+PS+TF-TuA-3 Low-Pressure and Plasma-Enhanced Chemical Vapor Deposition Modeling at the Feature Length Scale of MEMS Devices L.C. Musson (Sandia National Laboratories); P. Ho (Reaction Design); R.C. Schmidt (Sandia National Laboratories) Theoretical modeling of the surface chemistry and concomitant surface evolution during MEMS fabrication processes has great potential for improving surface micromachining (SMM) process technologies. A greater understanding of the fundamental factors leading to surface non-uniformities and other non-ideal geometric artifacts can lead to better device designs and assist in process optimization. We are developing ChISELS, a parallel code to model material deposition and etch processes at the feature scale. ChISELS uses the level-set method which was chosen for its natural ability to handle substantial changes in topology that occur when fabricating MEMS devices. We describe the algorithm by which the surface is evolved in process models, the transport model, the tools used for modeling chemical reactions and dynamic balancing of the computational load in a parallel environment. The capabilities of the ChISELS code are demonstrated by models of low-pressure deposition of SiO2 from TEOS and from a silane/oxygen/argon plasma. The uniformity of deposition into various geometries has been studied and will be presented in both 2-D and 3-D models. Some comparisons between the predicted deposition geometries and experimental SEMs will also be shown.
2:20 PM		MN+MS+PS+TF-TuA-4 Detection of Metal Film Deposit Smoothness by a MEMS-NEMS Structure via Surface Plasmon Effects D.T. Wei (Wei & Assoc.); A. Scherer (California Institute of Technology) A thin metal film under strong illumination, from uv to visible, will induce a quantum effect of electron plasma called surface plasmon effect. When the film is a deposit on a semiconductor surface, it takes additional structure in submicron scale to make an electronic detector. This detection method has high potential for controlling the smoothness of metal coating by traditional plasma or by ion beam deposition. Such an integrated structure is effective to detect the surface roughnesses vs. plasmon modes not often obtainable through other means, such as their decay products. A NEMS device is designed and fabricated for collecting electrons from the decaying surface plasmons in avalanche mode. The signal responds to the degree of the metal deposit surface roughness down to nano, even subnano sizes. Imperfections in metal film resulted from thermal plasma deposition are theoretically analyzed and relevant data are presented from the nano structure with new insights. Assembled unit will be applicable to monitoring the metal coating smoothness. Applications in transparent electroding and adaptive optics are sought.
2:40 PM		MN+MS+PS+TF-TuA-5 Etching of High Aspect Ratio Structures in Si using SF₆-O₂-HBr and SF₆-O₂-Cl₂ Plasmas S. Gomez, J. Belen (University of California, Santa Barbara); M. Kiehlbauch (Lam Research Corporation); E.S. Aydil (University of California, Santa Barbara) Plasma etching of high aspect ratio (depth-to-width) structures in Si is a crucial step in manufacturing trench capacitors for memory devices, and integrated components for microelectromechanical systems (MEMS). We have investigated etching of deep (~3-10 µm) and narrow (~0.2-0.5µm) features with high aspect ratios (~10-50) using plasmas maintained in mixtures of SF₆, O₂ and HBr gases, and in mixtures of SF₆, O₂ and Cl₂ gases as an alternative to the Bosch process. Experiments were conducted in a low pressure (25 mTorr), high density, inductively coupled plasma etching reactor with a planar coil to maintain the discharge and with radio frequency (rf) biasing of the substrate electrode to achieve independent control of the ion flux and ion energies. Specifically, we have studied HBr and Cl₂ addition to SF₆/O₂ plasmas and O₂ addition to SF₆/HBr and to SF₆/Cl₂ plasmas. We have analyzed the effect of these additions on the etch rate and feature profile using Si wafers patterned with 0.2 µm diameter holes in a SiO₂ mask. Visualization of the profiles using SEM is complemented by plasma diagnostics such as optical emission and mass spectroscopies to understand the key factors that control the anisotropy and etch rate. Upon adding HBr to an SF₆/O₂ plasma, a silicon oxybromide film forms on the sidewall, reducing undercut and increasing taper. However, subsequent reduction of O₂ gas increases mask undercut and isotropic etching by reducing sidewall oxidation. On the other hand, adding Cl₂ to an SF₆/O₂ plasma causes a reduction of O density and a weak silicon oxychloride film forms on the sidewall. This chlorinated film is more easily etched by F, therefore increasing mask undercut. Subsequent reduction of O₂ gas further increases mask undercut and isotropic etching.
3:00 PM		MN+MS+PS+TF-TuA-6 Deep Reactive Ion Etching of Silicon Structures for Profile and Morphology Control R.J. Shul, M.G. Blain, S.G. Rich, S.A. Zmuda (Sandia National Laboratories) Deep reactive ion etching (DRIE) of Si or the Bosch process relies on an iterative etch/deposition process where a sidewall etch inhibitor is formed to prevent lateral etching of the Si thus resulting in highly anisotropic etch profiles at reasonably high etch rates. The formation of deep, high-aspect ratio, straight-wall Si structures achieved with this process has been used to fabricate chemical and biological sensors, micro-fluidic devices, and mechanical actuators and gears. However as device designs become more complicated and aspect ratios increase, conventional DRIE processes often cannot meet the demands. For example, high-aspect ratio features etched to depths greater than 150 microns often become tapered and rough with unacceptably slow etch rates. This observation is often referred to as RIE lag or aspect ratio dependent etching and is attributed to reduced diffusion of neutral reactants and etch product species and reduced ion transport to the feature bottom as the depth increases. In many cases the etch will actually terminate due to either inefficient etching or polymer deposition dominating the process. We will report on the use of the DRIE platform to fabricate deep, high-aspect ratio Si features incorporating a process in which etch parameters are incrementally varied during each cycle of the process. The use of this in-situ variable etch process has resulted in a high degree of profile control and smooth etch morphologies while maintaining reasonably fast etch rates for high aspect ratio features. Etch results using this process will be reported as a function of cathode power, etch and deposition time, and reactive gas flow. These results will be compared to results obtained using conventional DRIE processes. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:20 PM		MN+MS+PS+TF-TuA-7 Fabrication of Wide-IF 200-300 GHz SIS Mixers with Suspended Metal Beam Leads Formed on SOI A.B. Kaul, B. Bumble, K.A. Lee, H.G. LeDuc (Jet Propulsion Laboratory, California Institute of Technology); F. Rice, J. Zmuidzinas (California Institute of Technology) We report on a novel fabrication process that uses SOI substrates and micromachining techniques to form wide-IF SIS mixer devices that have suspended metal beam leads for RF grounding. The mixers are formed on thin 25 µm membranes of Si, and are designed to operate in the 200 - 300 GHz band. Potential applications are in tropospheric chemistry, where increased sensitivity detectors and wide-IF bandwidth receivers are desired. They will also be useful in astrophysics to monitor absorption lines for CO at 230 GHz, to study distant, highly red-shifted galaxies by reducing scan times. Aside from a description of the fabrication process, electrical measurements of these Nb/Al-AlNx/Nb trilayer devices will also be presented. Since device quality is sensitive to thermal excursions, the new process appears to be compatible with conventional SIS device fabrication technology.
3:40 PM		MN+MS+PS+TF-TuA-8 Characterization of Polycrystalline AlN Film Quality Using Variable Angle Spectroscopic Ellipsometry L.-P. Wang, D.S. Shim, Q. Ma, V.R. Rao, E. Ginsburg, A. Talalyevsky (Intel Corp) Aluminum nitride (AlN) thin films have been investigated for piezoelectric, wide band gap, high-k dielectric and other applications. Recently, AlN films for bulk acoustic wave (BAW) resonators and filters have been studied extensively, driven by the fast growth of wireless communications. For this application, AlN films are mostly prepared by reactive sputtering, a technique with the advantage of low deposition temperature, easy process control and low cost when compared to alternatives such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Highly c-axis oriented AlN films are desirable for optimal piezoelectric and crystal properties. Currently, X-ray diffraction (XRD) rocking curve is the predominate method for characterizing the crystal and piezoelectric properties. In this study, optical constants of AlN films, refractive index (n) and extinction coefficient (k), were determined by a variable angle spectroscopic ellipsometry (VASE). The microstructure of the sputtered polycrystalline films is well reflected in the VASE optical model, which includes cylindrical symmetry, effective medium approximation (EMA), index gradient, and surface roughness. For the first time, the film optical constants were correlated to the full width at half maximum (FWHM) of XRD rocking curve. It was found that the films with smaller FWHM, an indication of better crystal and piezoelectric properties, had higher n and lower k. This is consistent with the general observation that higher n and k of polycrystalline films typically have fewer defects and better microstructures. The correlation between the optical parameters and the film quality leads to a simpler and faster method for characterizing sputtered AlN films. Furthermore, such optical tools can be integrated in a sputter deposition system for in-situ monitoring of AlN film thickness and quality simultaneously.