Richard A. Plass

Nanostructures: Self-assembled domain patterns

The ordered domain patterns that form spontaneously in a wide variety of chemical and physical systems as a result of competing interatomic interactions can be used as templates for fabricating nanostructures. Here we describe a new self-assembling domain pattern on a solid surface that involves two surface structures of lead on copper. The evolution of the system agrees with theoretical predictions, enabling us to probe the interatomic force parameters that are crucial to the process.

Surface Morphology Changes During Pb Deposition on Cu(100): Evidence for Surface Alloyed Cu(100)-c(2x2) Pb

Using Low Energy Electron Microscopy (LEEM), the authors have followed Cu(100) surface morphology changes during Pb deposition at different temperatures. Surface steps advance and 2-D islands nucleate and grow as deposited Pb first alloys, and then dealloys, on a 125 C Cu(100)surface. From LEEM images, they determine how much Cu is being displaced at each stage and find that the amount of material added to the top layer for a complete Pb/Cu(100) c(4x4) reconstruction (a surface alloy) is consistent with the expected c(4x4) Cu content of 0.5 monolayer. However, as the surface changes to the Pb/Cu(100) c(2x2) overlayer, they find that the displaced material from surface dealloying, 0.22ML, is more than a factor of two lower than expected based on a pure Pb c(2x2) overlayer. Further, they find that in the 70 to 130 C range the amount of Cu remaining in c(2x2) increases with increasing substrate temperature during the deposition, showing that surface Cu is alloyed with Pb in the c(2x2) structure at these temperatures. When holding the sample at 125 C, the transformation from the c(2x2) structure to the higher coverage c(5{radical}2 x{radical}2)R45{degree} overlayer structure displaces more Cu, confirming the c(2x2) surface alloy model. They also find the c(2x2) surface has characteristically square 2-D islands with step edges parallel to the (100) type crystallographic directions, whereas the c(5{radical}2 x{radical}2)R45{degree} structure has line-like features which run parallel to the dislocation double rows of this surfaces atomic structure and which expand into 2-D islands upon coarsening.

Anodic oxidation-induced delamination of the SUMMiT poly 0 to silicon nitride interface

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.

Electrothermal Actuator Reliability Studies

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.

The Relationship Between the Growth Shape of Three-Dimensional Pb Islands on Cu(100) and the Domain Orientation of the Underlying c(5v2xv2)R45 Degree Structure

The authors use low energy electron microscopy to identify a correlation between the growth shape of three-dimensional Pb islands on Cu(100)and the domain structure of the underlying Pb overlayer. Deposition of 0.6 monolayer Pb on Cu(100) produces a compressed c(2x2) overlayer, designated c(5{radical}2x{radical}2)R45{degree}, with periodic rows of anti-phase boundaries. They found that heating the surface to temperatures above 100 C coarsens the orientational domains of this structure to sizes that are easily resolved in the low energy electron microscope. Three-dimensional Pb islands, grown on the coarsened domains, are found to be asymmetric with orientations that correlate with the domain structure. Once nucleated with a preferred growth orientation, islands continue to grow with the same preferred orientation, even across domain boundaries.

Unified characterization of surfaces and gases in MEMS devices

Chemical and physical materials-aging processes can significantly degrade the long-term performance reliability of dormant microsystems. This degradation results from materials interactions with the evolving microenvironment by changing both bulk and interfacial properties (e.g., mechanical and fatigue strength, interfacial friction and stiction, electrical resistance). Eventually, device function is clearly threatened and as such, these aging processes are considered to have the potential for high (negative) consequences. Sandia National Laboratories is developing analytical characterization methodologies for identifying the chemical constituents of packaged microsystem environments, and test structures for proving these analytical techniques. To accomplish this, we are developing a MEMS test device containing structures expected to exhibit dormancy/analytical challenges, extending the range of detection for a series of analytical techniques, merging data from these separate techniques for greater information return, and developing methods for characterizing the internal atmosphere/gases. Surface analyses and data extraction have been demonstrated on surfaces of various geometries with different SAMS coatings, and gas analyses on devices with internal free volumes of 3 microliters have also been demonstrated.