We report on long-term stability experiments on a novel MEMS radio frequency (RF) resonator fabricated in Aluminum
Nitride technology. The AlN fabrication process allows for the realization of resonators, filters, and resonant sensors
operating over the frequency range from 500 kHz to in excess of 10 GHz using CMOS compatible materials. The 100
MHz resonators used in these experiments were a ring design with 140-micron outer diameter and 100-micron inner
diameter. Electrodes on the top and bottom of this AlN ring enable measurement of resonance. Wafer sections were
stored in air and vacuum and tested daily. We observed a steady degradation in the resonant frequency (600 ppm over
the 800 hours) for the devices stored in a vacuum. Small degradation was observed in the air experiment (50 ppm over
1200 hours). Failure analysis using secondary emission microscopy (SEM) revealed no differences between control
devices and devices on test. However, subsequent investigation of blank wafer sections by Time-of-Flight secondary ion
mass spectrometry (ToF-SIMS) found small levels of silicone surface contamination from vacuum chamber exposure.
This contamination added enough mass to shift the resonant frequency. These experiments demonstrate the need for
clean environments for future wafer-level testing and also packaging for these small-mass resonators.
Optical actuation of microelectromechanical systems (MEMS) is advantageous for applications for which electrical isolation is desired. Thirty-two polycrystalline silicon opto-thermal actuators, optically-powered MEMS thermal actuators, were designed, fabricated, and tested. The design of the opto-thermal actuators consists of a target for laser illumination suspended between angled legs that expand when heated, providing the displacement and force output. While the amount of displacement observed for the opto-thermal actuators was fairly uniform for the actuators, the amount of damage resulting from the laser heating ranged from essentially no damage to significant amounts of damage on the target. The likelihood of damage depended on the target design with two of the four target designs being more susceptible to damage. Failure analysis of damaged targets revealed the extent and depth of the damage.
Failure analysis tools and techniques that identify root cause failure mechanisms are key components to improving MEMS technology. Failure analysis and characterization are relatively simple at the wafer and die level where chip access is straightforward. However, analysis and characterization of packaged parts or components encapsulated with covers, caps, etc may be more cumbersome and lead to problems assessing the root cause of failure. This paper will discuss two methods used to prepare the backside of the package/device to allow for failure analysis and inspection of different MEMS components without removing the cap, cover, or lid on the device and/or the package. One method for backside preparation was grinding and polishing the package for IR inspection. This method involved backfilling the package cavity with epoxy to hold the die in place. The other method involved opening a window through the backside of the package, exposing the die for IR inspection. Failure analysis results showed both methods of backside preparation were successful in revealing the failure mechanisms on two different MEMS technologies.
Shallow V type symmetric electrothermal actuators which have a central shuttle and overall lengths of ~610 μm, leg widths between 3 and 4.5 μm, and offset angles between 0.7 and 2.3° have been subjected to short term, high stress drive currents under different environmental conditions. For all the devices and all test conditions, ~200 mW power levels lead to plastic deformation both for DC actuation and square wave modulation at the limit of the device’s bandwidth. Also, it is noted that under vacuum conditions the hottest portions of the surface roughen significantly and there is significant discoloration of the silicon nitride under the device. SEM analysis of cleaved surfaces of these vacuum actuated devices shows significant near surface pitting.
Spatial microstages are microfabricated controlled platforms that can be popped out of the fabrication plane and are free to move in three-dimensional (3D) space. Spatial microstages have shown promise for use in MOEMS for adaptive optics, automatic focusing systems, fiber optic alignment/precision positioning, real time optical alignment, interconnects, and a host of other applications. These devices were designed and fabricated to position a controllable stage in 3D space from microassembly and microfabrication. Microstages can be designed and fabricated to move in plane (x, y) and out of plane (z). Advanced microstages are designed to move in plane, out-of-plane, rotate, and tilt about x, y, and z.
Design and fabrication of the rotational and tilt components are critical in performing the three-dimensional pop up and tilting action needed for precise micropositioning. The device used for analysis contains linear racks driven by electrostatic actuators. The actuators are attached to a microstage through a hinge component with revolving, rotating, and tilting joints. The actuators allow x, y, and z positioning while the hinge allows rotational motion along the stage. Failure analysis of the Sandia fabricated microstage was performed on released and as fabricated microstages. Failure analysis of these devices revealed design and fabrication irregularities along the revolving components of the hinge. This paper will discuss the design and functionality of the microstage, failure analysis activities and failure mechanisms found in polysilicon fabricated microstages, corrective actions and design improvements.
Anodic oxidation can be a catastrophic failure mechanism for MEMS devices that operate in high humidity environments. Shea and coworkers have shown that positively charged polysilicon traces can fail through a progressive silicon oxidation reaction whose rate depends critically on the surface conductivity over the silicon nitride. We have found a related anodic oxidation-based failure mechanism: progressive delamination of Poly 0 electrodes from silicon nitride layers, which then mechanically interfere with device function well before the electrode is fully oxidized. To explain this effect, we propose that the silicon oxide which initially forms at the electrode edge has insufficient strength to hold the local Poly 0 / silicon nitride interface together. This low-density silicon oxide also creates a bilayer system, which curls the edge of the 300 nm thick Poly 0 electrode away from the nitride. As delamination progresses more nitride surface is exposed and more of the interface is then attacked. This process continues cyclically until the electrode edge pushes against other device components, catastrophically and irreversibly interfering with normal operation. Additionally, we observe that the delamination only starts at electrode edges directly under cantilevers, suggesting the oxidation rate also depends on the perpendicular electric field strength.
MEMS are rapidly emerging as critical components in the telecommunications industry. This enabling technology is currently being implemented in a variety of product and engineering applications. MEMS are currently being used as optical switches to reroute light, tunable filters, and mechanical resonators. Radio frequency (RF) MEMS must be compatible with current Gallium Arsenide (GaAs) microwave integrated circuit (MMIC) processing technologies for maximum integration levels. The RF MEMS switch discussed in this paper was fabricated using various layers of polyimide, silicon oxynitride (SiON), gold, and aluminum monolithically fabricated on a GaAs substrate. Fig. 1 shows a metal contacting series switch. This switch consists of gold signal lines (transmission lines), and contact metallization. SiON was deposited to form the fixed-fixed beam, and aluminum was deposited to form the top actuation electrode. To ensure product performance and reliability, RF MEMS switches are tested at both the wafer and package levels. Various processing irregularities may pass the visual inspection but fail electrical testing. This paper will focus on the failure mechanisms found in the first generation of RF MEMS developed at Sandia National Laboratories. Various tools and techniques such as scanning electron microscopy (SEM), resistive contrast imaging (RCI), focused ion beam (FIB), and thermally-induced voltage alteration (TIVA) have been employed to diagnose the failure mechanisms. The analysis performed using these tools and techniques led to corrective actions implemented in the next generation of RF MEMS metal contacting series switches.
We describe the design, fabrication, test and preliminary analysis of a polycrystalline silicon MEMS inchworm actuator fabricated in a five level surface micromachining process. Large force generation (500 micronewtons), large range of motion (+/- 100 microns), small area requirements (600 X 200 um), small step size (10, 40 or 120 nanometers), and a large velocity range (0 to 90 microns per second) are demonstrated. We characterize force with a load cell whose range is calibrated on a logarithmic scale from micronewtons to millinewtons. We characterize out-of-plane displacement with interferometry, and in-plane displacement with Moire metrology sensitive to approximately 60 nm. The actuator serves well for testing friction under conditions of well- known applied pressure. We found that our surfaces exhibited a static coefficient of friction (cof) of approximately 0.3, and a dynamic cof of approximately 0.2. We also present initial wear studies for this device.
Electrostatic discharge (ESD) and electrical overstress (EOS) damage of Micro-Electrical-Mechanical Systems (MEMS) has been identified as a new failure mode. This failure mode has not been previously recognized or addressed primarily due to the mechanical nature and functionality of these systems, as well as the physical failure signature that resembles stiction. Because many MEMS devices function by electrostatic actuation, the possibility of these devices not only being susceptible to ESD or EOS damage but also having a high probability of suffering catastrophic failure doe to ESD or EOS is very real. Results from previous experiments have shown stationary comb fingers adhered to the ground plane on MEMS devices tested in shock, vibration, and benign environments [1,2]. Using Sandia polysilicon microengines, we have conducted tests to establish and explain the EDS/EOS failure mechanism of MEMS devices. These devices were electronically and optically inspected prior to and after ESD and EOS testing. This paper will address the issues surrounding MEMS susceptibility to ESD and EOS damage as well as describe the experimental method and results found from EDS and EOS testing. The tests were conducting using conventional IC failure analysis and reliability assessment characterization tools. In this paper we will also present a thermal model to accurately depict the heat exchange between an electrostatic comb finger and the ground plane during an ESD event.
Failure analysis (FA) tools have been applied to analyze tungsten coated polysilicon microengines. These devices were stressed under accelerated conditions at ambient temperatures and pressure. Preliminary results illustrating the failure modes of microengines operated under variable humidity and ultra-high drive frequency will also be shown. Analysis os tungsten coated microengines revealed the absence of wear debris in microengines operated under ambient conditions. Plan view imagine of these microengines using scanning electron microscopy (SEM) revealed no accumulation of wear debris on the surface of the gears or ground plane on microengines operated under standard laboratory conditions. Friction bearing surfaces were exposed and analyzed using the focused ion beam (FIB). These cross sections revealed no accumulation of debris along friction bear surfaces. By using transmission electro microscopy (TEM) in conjunction with electron energy loss spectroscopy (EELS), we were able to identify the thickness, elemental analysis, and crystallographic properties of tungsten coated MEMS devices. Atomic force microscopy was also utilized to analyze the surface roughness of friction bearing surfaces.
Failure analysis tools have been applied to analyze failing polysilicon microengines. These devices were stressed to failure under accelerated conditions in both oxidizing and non-oxidizing environments. The dominant failure mechanism of these microengines was identified as wear of rubbing surfaces. This often results in either seized microengines or microengines with broken pin joints. Analysis of these failed polysilicon devices found that wear debris was produced in both oxidizing and non-oxidizing environments. By varying the relative percent humidity (%RH), we observed an increase in the amount of wear debris with decreasing humidity. Plan view imaging under scanning electron microscopy revealed build-up of wear debris on the surface of microengines. Focused ion beam (FIB) cross sections revealed the location and build-up of wear debris within the microengine. Seized regions were also observed in the pin joint area using FIB processing. By using transmission electron microscopy in conjunction with energy dispersive x- ray spectroscopy and electron energy loss spectroscopy, we were able to identify wear debris produced in low (1.8% RH, medium and high (39% RH) humidities.