MICROMIRROR DESIGN USING SANDIA SUMMIT TOOLS

GE 4230: Design and Fabrication of MEMS Dr. Osama Jadaan

University of Wisconsin – Platteville Jon Zickermann

November 28, 2011

Abstract: A fabrication layout for a micromirror was designed and developed using the SUMMiT V design and visualization tools provided by Sandia National Laboratories for the first project of the year. In addition to the tools, the dimensions for the spring of the micromirror were provided along with a basic layout and PowerPoint slides from Sandia to give an example to model after. After a 2d layout was drawn in AutoCAD 2010, a 3d model was created and imported into ANSYS Workbench for FEA simulation. Using Workbench the behavior of the mirror was observed when put under static loading and modal analysis.

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TABLE OF CONTENTS Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Fabrication Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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OBJECTIVE Previously in EP3930 (Introduction to MEMS/Nanotech) students were introduced to the concepts of the microscale, including the physics, applications and the fabrication of devices at this scale. One method included surface microfabrication, which is used to create MEMS devices like micromotors and micromirrors. Currently in GE4230 a more in -depth look has been taken with the focus on the Sandia SUMMiT V tools, which allow a user to create a MEMS device that can be fabricated using the SUMMiT batch surface microfabricat ion method. In addition, the tools allow for the user to preview the design while in AutoCAD to verify if the design will work even without any loads present. Given the dimensions for the spring of the micromirror along with a basic layout and PowerPoint slides from Sandia, students in GE4230 were challenged to design a micromirror with the SUMMiT tools and simulate in ANSYS workbench to verify the design.

DEVELOPMENT Fabrication Layout:

The basis behind the design was to fabricate a mirror template that would be strong with a minimal usage of material. With the basic shape of the Poly2 layer given and the dimensions for the structure of Poly3 (the spring) given, the first focus was to secure the respective layers down to the substrate. At the ends of the spring, a SacOx3 layer was offset from the Poly3 end to secure to the Poly2 structure, while another square was offset and assigned to the SaxOx1

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Layer to secure the Poly2 structure to the base. The Poly2 structure was further supported by additional SaxOx1 layers equally spaced along the base.

Fig i: Cross section at the spring end

Fig ii: Cross section through the center of the Poly2 “arm”

The next task was to create etch holes in the spring and supporting structure. Originally the holes were circles, however due to excessive node counts in the meshes for the FEA simulations, the circles were changed to squares. Following this, the focus shifted to the spring. The first task was to secure the plate to the Poly2 structure to further su pport the spring. After, a base for the Poly4 layer was need. As with the etch holes, the original ideas contained circles protruding from the Poly3 layer, however, the design was changed to a more typical structure due to a bad FEA outcome with very high stress concentrations at the supports.

Fig iii: Cross section through the center of the Poly3 support

Fig iv: Cross section through the center spring with Poly4 layer added

The last task in fabrication was to add the Poly4 layer. The dimensions were a square based from the maximum width of the Poly2 structure. Etch holes were added in the similar style as the previous designs and two slots were added to further match the Sa ndia design.

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Layers: Fig1: Fabrication layout dimensions:

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Fig 2: Poly 2 Layer

Fig 3: SaxOx1 Layer on Poly2, all dimensions same for other side

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Fig 4: Poly3 Layer

Fig 5: SacOx3 Layer

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Fig 6: Poly4 Layer

Simulations Fig 7: Deformation Analysis *Using a load of 0.0054µNm – see discussion

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Fig 8: Stress Analysis

Fig 9: Modal Analysis Diagrams:

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DISCUSSION For the FEA simulation, the substrate was chosen as the fixed support and a 0.0054µNm moment load was applied. The moment load was estimated from a paper on a torsional micromirror estimating the attraction force of the mirror to the electrode would be 5 µN (Zhang, Chau, Quan, Lam, & Liu, 2001) and the width calculated from half of the length of the Poly4 layer structure. To prepare for the FEA simulation, several steps were taken including many face and body sizing operations to cut down on the number of nodes from over 350,000 to about 60,000. Without the reduction of nodes, no solution would be provided. Overall, the mirror deflected as expected that should prove durable according to the FEA simulation.

REFERENCES Zhang, X., Chau, F., Quan, C., Lam, Y., & Liu, A. (2001). A study of the static characteristics of

a torsional micromirror. Elsevier Science B.V.