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SEM/EDS Based Trace Element Sensitivity (TSA) Analysis for Reducing Defects in MEMS and MOEMS (Micro-(Opto)-Electro-Mechanical Systems) Assembly
Keywords: Reliability, Yield, Quality Control
The transition of MEMS and MOEMS devices from the laboratory to laminate based (PCB), high-volume manufacturing is challenging. Devices now must be manufactured with low defect rates and high reliability. As device density and complexity increase, new analytical techniques are required to understand and correct problems. This is even more critical in manufacturing MEMS and MOEMS devices where moving parts and both fluidic and optical pathways may be present. These features can be fatally affected by particulate contaminates that can increase friction and jam movement capabilities. Fluidic components can leak and must be sealed to be effective. Microscopic burrs and particulate contamination can result in leaking seam failures. Techniques that build in defect free manufacturing require rigorous attention to details, that must first be identified and understood. A new Electron Dispersive Spectroscopy (EDS) technique is now available, Trace Element Sensitivity Analysis (TSA) provides a new tool for improved chemical mapping of surfaces. This technique enables pinpointing contamination at levels that would be lost by conventional mapping techniques. It is capable of determining individual particle chemistry with excellent accuracy and traceability. Identifying the contamination and its source (whether its source is wear, contamination, or material inclusion related) are the first steps toward troubleshooting and corrective action. The result is higher yields and defect free product. Mapping the chemistry of a surface requires highly sensitive equipment and fast software. For each pixel of the SEM image the X-Ray spectra is stored and analyzed. Good X-Ray spectra requires a high X-Ray detection count rate, commonly measured as counts/second (cps). Normal instruments commonly detect 1000-3000 cps. In order to capture sufficient information to adequately map a surface they may require an hour or more of real detection time. Newly introduced, digital Ultra Thin Window (UTW) light element detectors, with onboard DSP circuitry, are capable of significantly higher detection rates (>1,000,000cps), enabling not only faster, higher resolution mapping but also higher sensitivity at low beam voltage. Low kV beam voltage enables shallower beam penetration (depth of beam penetration of the surface) and means the spectra describes the surface rather than the bulk chemistry of the sample. In addition, beam interaction with the surface can burn sensitive materials such as thin films, and disrupt sensitive MEMS mechanical components. Less exposure to the beam is beneficial. Trace Element Sensitivity Analysis (TSA) with higher resolution, faster mapping, and light element detection, combined with advanced software tools provides better analytical capability and enables higher yields. SEM analysis has been a necessary tool for semiconductor assembly for a long time. Its the best method to visualize surface and near surface defects within the structure and to determine corrective actions. Optical microscopy just doesnt have the resolution and the depth of focus for imaging the sub-micron defects that can affect yield. MEMS and MOEMS devices, with large aspect ratios and sub-micron size mechanical components require SEM imaging capability. Tiny burrs and flaws, which might not matter at a larger scale may completely disable functionality of a sub-micron micro-fluidic or mechanical component. SEM photos and TSA results will be demonstrated and results discussed.
Lee Levine, Sr. Consultant
Process Solutions Consulting
New Tripoli, PA


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