Micralyne Inc. Newsletter - March 2007

MEMS Tip List - Choosing Substrate Materials

The following MEMS tips focus on how to choose the appropriate substrate materials for MEMS development and manufacturing.

From a design point of view the following elements should be considered in choosing a substrate: mechanical, electrical, and thermal properties. The designer should also make substrate decisions in the context of the part's fabrication, assembly, and function so that all requirements are identified. Written by Micralyne's engineering team, these tips cover a variety of topics ranging from quality issues and silicon doping to handling and thickness implications.

Transparency - glass is transparent but will exhibit fluorescence if used in UV, so microfluidic projects may need to use fused silica if detection methods are sensitive to this. Both can be wet etched to make channels or fused silica may be dry etched to make vertical channels. The high temperature bonding of fused silica makes using embedded electrodes more problematic than with glass.
Silicon doping - silicon doping is a major consideration for active electronics, and is also important for MEMS devices even if the substrate is not used electrically; charging and carrier considerations are still important.
Silicon and glass flatness - if the substrate is to be bonded, the total bow and TTV (total thickness variation) over the wafer need to be considered.
Quality issues - without getting into brand issues, surface quality of glass can be influenced by polishing, handling and other specifications that are easily overlooked and only show up as magnified surface features after etching. Similarly, silicon can be procured as "Prime Grade" or "Test Grade", and may only show a difference because inclusions and crystal plane defects show up as magnified errors later in processing.
Crystal plane - if the silicon crystal plane is to be used for etching, the flat alignment to the plane is available in a variety of tolerances. It may be easier to etch reference marks first to determine the exact plane location.
SOI (Silicon On Insulator) thickness specification - if the membrane used in device layers is sensitive to thickness change, a higher precision polished SOI may have to be specified, or a combination of SOI and epitaxial silicon may be required.
Wafer flat and size - wafer diameter can vary by a few millimeters and the wafer flat size and location can also vary slightly. Some considerations for matching to bonded wafers and equipment restrictions come into play when deciding how to specify these criteria. Looking at the bigger picture, the decision to move to a larger wafer to increase die counts is not straightforward. It requires qualification and consideration, of all the items mentioned above, all over again.
Handling - glass can be ordered in round or square formats. Round is preferred for automated handlers and cleaning systems. Transparent substrates may not work well on track systems, so either system has to be calibrated or coatings applied to aid sensors in handling.
Thickness implications - for design reasons, some MEMS devices need to be thicker or thinner than the standard available material, but this could change process and tool requirements.
Substrate change through processing - one may start with standard wafers, but after cutting through holes, bonding different materials, and using various materials and layers, the substrate can become difficult to handle and require special fixtures. It also can be a source of particulate as edge pieces and areas, which did not process well, become a problem for remaining areas of the substrate being processed.

These tips are just a few examples of what should be considered for your MEMS project. Please contact us to learn more about this topic or Micralyne's MEMS capabilities.

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1911-94 Street, Edmonton, Alberta, Canada, T6N 1E6
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