This project investigated a recently patented Sandia technology known as visible light Laser Voltage Probing (LVP). In this effort we carefully prepared well understood and characterized samples for testing. These samples were then operated across a range of configurations to minimize the possibility of superposition of multiple photon carrier interactions as data was taken with conventional and visible light LVP systems. Data consisted of LVP waveforms and Laser Voltage Images (LVI). Visible light (633 nm) LVP data was compared against 1319 nm and 1064 nm conventional LVP data to better understand the similarities and differences in mechanisms for all wavelengths of light investigated. The full text can be obtained by reaching the project manager, Ed Cole or the Cyber IA lead, Justin Ford.
Polycrystalline silicon (polysilicon) surface micromachining is a new technology for building micrometer ({micro}m) scale mechanical devices on silicon wafers using techniques and process tools borrowed from the manufacture of integrated circuits. Sandia National Laboratories has invested a significant effort in demonstrating the viability of polysilicon surface micromachining and has developed the Sandia Ultraplanar Micromachining Technology (SUMMiT V{trademark} ) process, which consists of five structural levels of polysilicon. A major advantage of polysilicon surface micromachining over other micromachining methods is that thousands to millions of thin film mechanical devices can be built on multiple wafers in a single fabrication lot and will operate without post-processing assembly. However, if thin film mechanical or surface properties do not lie within certain tightly set bounds, micromachined devices will fail and yield will be low. This results in high fabrication costs to attain a certain number of working devices. An important factor in determining the yield of devices in this parallel-processing method is the uniformity of these properties across a wafer and from wafer to wafer. No metrology tool exists that can routinely and accurately quantify such properties. Such a tool would enable micromachining process engineers to understand trends and thereby improve yield of micromachined devices. In this LDRD project, we demonstrated the feasibility of and made significant progress towards automatically mapping mechanical and surface properties of thin films across a wafer. The MEMS parametrics measurement team has implemented a subset of this platform, and approximately 30 wafer lots have been characterized. While more remains to be done to achieve routine characterization of all these properties, we have demonstrated the essential technologies. These include: (1) well-understood test structures fabricated side-by-side with MEMS devices, (2) well-developed analysis methods, (3) new metrologies (i.e., long working distance interferometry) and (4) a hardware/software platform that integrates (1), (2) and (3). In this report, we summarize the major focus areas of our LDRD project. We describe the contents of several articles that provide the details of our approach. We also describe hardware and software innovations we made to realize a fully automatic wafer prober system for MEMS mechanical and surface property characterization across wafers and from wafer-lot to wafer-lot.
A new instrument to accurately and verifiably measure mechanical properties across an entire MEMS wafer is under development. We have modified the optics on a conventional microelectronics probe station to enable three-dimensional imaging while maintaining the full working distance of a long working distance objective. This allows standard probes or probe cards to be used. We have proceeded to map out mechanical properties of polycrystalline silicon along a wafer column by the Interferometry for Material Property Measurement (IMaP) methodology. From interferograms of simple actuated cantilevers, out-of-plane deflection profiles at the nanometer scale are obtained. These are analyzed by integrated software routines that extract basic mechanical properties such as cantilever curvature and Young's modulus. Non-idealities such as support post compliance and beam take off angle are simultaneously quantified. Curvature and residual stress are found to depend on wafer position. Although deflections of cantilevers varied across the wafer, Young's modulus E - 161 GPa is independent of wafer position as expected. This result is achieved because the non-idealities have been taken into account.
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 of tungsten coated microengines revealed the absence of wear debris in microengines operated under ambient conditions. Plan view imaging 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 bearing surfaces. By using transmission electron 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.
In order to determine the susceptibility of the MEMS (MicroElectroMechanical Systems) devices to shock, tests were performed using haversine shock pulses with widths of 1 to 0.2 ms in the range from 500g to 40,000g. The authors chose a surface-micromachined microengine because it has all the components needed for evaluation: springs that flex, gears that are anchored, and clamps and spring stops to maintain alignment. The microengines, which were unpowered for the tests, performed quite well at most shock levels with a majority functioning after the impact. Debris from the die edges moved at levels greater than 4,000g causing shorts in the actuators and posing reliability concerns. The coupling agent used to prevent stiction in the MEMS release weakened the die-attach bond, which produced failures at 10,000g and above. At 20,000g the authors began to observe structural damage in some of the thin flexures and 2.5-micron diameter pin joints. The authors observed electrical failures caused by the movement of debris. Additionally, they observed a new failure mode where stationary comb fingers contact the ground plane resulting in electrical shorts. These new failure were observed in the control group indicating that they were not shock related.
MicroElectricalMechanical Systems (MEMS) were subjected to a vibration environment that had a peak acceleration of 120g and spanned frequencies from 20 to 2000 Hz. The device chosen for this test was a surface-micromachined microengine because it possesses many elements (springs, gears, rubbing surfaces) that may be susceptible to vibration. The microengines were unpowered during the test. The authors observed 2 vibration-related failures and 3 electrical failures out of 22 microengines tested. Surprisingly, the electrical failures also arose in four microengines in the control group indicating that they were not vibration related. Failure analysis revealed that the electrical failures were due to shorting of stationary comb fingers to the ground plane.
The burgeoning new technology of Micro-Electro-Mechanical Systems (MEMS) shows great promise in the weapons arena. We can now conceive of micro-gyros, micro-surety systems, and micro-navigators that are extremely small and inexpensive. Do we want to use this new technology in critical applications such as nuclear weapons? This question drove us to understand the reliability and failure mechanisms of silicon surface-micromachined MEMS. Development of a testing infrastructure was a crucial step to perform reliability experiments on MEMS devices and will be reported here. In addition, reliability test structures have been designed and characterized. Many experiments were performed to investigate failure modes and specifically those in different environments (humidity, temperature, shock, vibration, and storage). A predictive reliability model for wear of rubbing surfaces in microengines was developed. The root causes of failure for operating and non-operating MEMS are discussed. The major failure mechanism for operating MEMS was wear of the polysilicon rubbing surfaces. Reliability design rules for future MEMS devices are established.