MECH 466 Micro Electromechanical Systems Laboratory Manual Laboratory #: Stiction of MEMS and Strength of MEMS Materials Department of Mechanical Engineering, University of Victoria N. Dechev, 2011, University of Victoria Page 1 of 7
Report: The laboratory report must be in the following format: (i) (ii) (iii) (iv) (v) Title Page i. Include your name, student number, date of lab, etc Objective Data i. Include data and images you collected. ii. Comment on any Sources of Error that may have influenced the data, or the collection of the data. Discussion i. Compare and contrast the experimental results in comparison to the theoretical/analytical results. ii. Include answers to the questions that pertain to the experiments, as listed in the Lab Write-up section. Conclusion Please organize the laboratory report into the five divisions indicated above (Title Page, Objective, Data/Images, Discussion (with questions answered), and Conclusion). The report should be a maximum of 5 pages (double spaced text), including figures, and data. Appendices may be added as extra pages. N. Dechev, 2011, University of Victoria Page 2 of 7
Laboratory #: Stiction of MEMS and Strength of MEMS Materials Purpose: The purpose of this laboratory is to investigate the microscale phenomenon of stiction. The second purpose of this laboratory is to investigate the strength of polysilicon, which is typically used as a structural material in MEMS devices. Introduction: Part (I): Stiction is an effect observed when micro-scaled objects come into direct contact with each other. To a casual observer, it appears that when the micro-objects are brought into contact, they stick together with relatively high adhesion. The stiction effect is actually a combination of a number of different micro-phenomena [1,2,], including: (a) electrostatic attraction (transient), (b) fluid surface tension due to a nano-scale water layer on objects, and (c) Van der Vaals force. Research into the stiction phenomena is on going and there are a number of other possible causes under investigation [4]. This laboratory will attempt to experimentally quantify the stiction that exists between a polysilicon microtool tip shown in Fig. A-1, and the substrate of a MUMPs chip. Part (II): The knowledge of the strength of MEMS materials is an important aspect to their design. This laboratory will destructively test the beams of a microtool by using the robotic micromanipulator. The purpose is to determine the ultimate strength of polysilicon beams fabricated by the MUMPs process, and compare these results with the results reported by others [5,6]. Laboratory Preparation: Review this document and the procedures prior to performing the laboratory. Note: Appendix A provides an illustration and the geometry of the microtool that will be used in these experiments. Procedure: Setup: (1) The lab technician/ta will mount a glass slide containing MEMS chips onto the worktable of the micromanipulator. (2) The lab technician/ta will bond a microtool to the pin-probe of the micromanipulator. (Observation #1): Bonding of a MEMS microtool: (1) Mount the probe pin holder (with a tungsten probe installed) on the Beta Axis of the micromanipulator (2) Locate the tip of the probe pin using the Manual XYZ Stage, i.e. bring it into the field of view of the camera. () Locate the desired microtool for a given task on the MEMS chip. (4) Localize the probe pin tip with respect to the microtool bond-pad (Fig. A-1), by touching down on the bond-pad with the probe pin. Be careful not to crush the bond-pad with too much pressure. (5) Zero the coordinate system in software. (6) Raise the probe pin by 10 mm (10,000 microns) and apply UV adhesive to the tip. (7) Return the probe pin to 100 microns above the bond-pad, and slowly lower the pint until the encoder displays the zero position N. Dechev, 2011, University of Victoria Page of 7
(8) Watch/Verify that the adhesive flows from the probe pin onto the bond-pad. (9) Put on your UV-Glasses. If you are unable to use UV-Glasses, you will be required to exit the laboratory until the UV-bonding step is completed. (10) The lab technician/ta will expose the adhesive with the UV spot light system. (11) After UV adhesive has cured, slowly command the probe pin upwards along the z-axis. This will raise the microtool away from the substrate, thereby breaking off the tethers that hold it down. (12) The microtool is now bonded to the probe pin, and is ready to be used for the experiments. (1) Since the microtool is parallel to the substrate, it is necessary to tilt it by degrees, to ensure that the square-tip is the lowest point. Do this by commanding the beta-axis to tilt 0.9 (x ) in the appropriate direction. Experiment #1: Static Test of Stiction (1) Locate the long polysilicon tracks along the edge of the chip. The tracks are approximately 50 microns wide and 2000 microns long. (2) Line up the microtool square-tip such that it is perpendicular to the poly-track. () Gently place the microtool square-tip onto the poly-track. Since the microtool is tilted at, you may need to move down (z-direction) an extra micron, to ensure the square-tip is flat onto the poly-track. NOTE: It may be difficult to determine when contact occurs, so ask your TA or the instructor for help. (4) Record an image (Starting position). (5) Using the micromanipulator controller, move the microtool laterally (i.e. sideways in comparison to it s longitudinal axis) in 1 um increments. (6) Record the # of microns you can move, before the microtool square-tip slips relative to the poly-track. (7) Try to record an image showing the deflected microtool (Final Position). (8) Lift the microtool off the substrate. (9) Repeat steps # ( to 8) at two other locations along poly-track. (10) Repeat steps # ( to 8) with the square-tip touching the silicon nitride substrate. Experiment #2: Dynamic Test of Stiction (1) Line up the microtool square-tip such that it is perpendicular to the poly-track. (2) Gently place the microtool square-tip onto the poly-track. Since the microtool is tilted at, you may need to move down (z-direction) an extra micron, to ensure the square-tip is flat onto the poly-track. NOTE: It may be difficult to determine when contact occurs, so ask your TA or the instructor for help. () Record an image (Starting position). (4) Using the micromanipulator controller, move the microtool laterally (i.e. sideways in comparison to it s longitudinal axis) at a constant velocity. Specify a speed of 10 microns/second for the velocity, and a move distance of 50 microns. This should provide 5 seconds of motion. (5) As the microtool is in motion, Record an image. (6) Lift the microtool off the substrate. (7) Repeat steps # (1 to 6) at one other location along the poly-track. (8) Repeat steps # (1 to 6) with the square-tip touching the silicon nitride substrate. N. Dechev, 2011, University of Victoria Page 4 of 7
Experiment #: Ultimate Strength of Polysilicon Beams (1) Locate a corner of the chip, (or a poly-1 feature on the substrate). (2) Gently place one microtool square-tip against the corner of the chip (or against the selected poly-1 feature). Ensure that the square-tip is just touching the edge/poly-1-feature, without any noticeable deflection. () Record an image (Starting position). (4) Using the micromanipulator controller, move the microtool laterally in 1 um increments. (5) Record an image of the microtool tips as they bend, for each increment. (6) Record the # of microns you can move, before the microtool tip breaks off. (7) Record an image (Final position). (8) Move the microtool back to the starting position and this time gently place the remaining microtool square-tip against the corner of the chip (as instructed by the TA). Ensure that the square-tip is just touching the edge/poly-1-feature, without any noticeable deflection. (9) Repeat steps # ( to 7). Laboratory Write-up: In relation to Experiment #1, answer the following questions in the Discussion Section of your laboratory report. You may use the supplementary information in the next section, to help you answer these questions. (Q1) What is the static stiction force holding the microtool to the substrate? In relation to Experiment #2, answer the following questions in your laboratory report: (Q2) What is the dynamic stiction force between the micrgripper and the substrate? (Q) Comment on any unexpected results you may have observed for both experiment #1 and #2. What may have caused these? In relation to Experiment #, answer the following questions in your laboratory report: (Q4) Calculate the ultimate strength (stress) of the polysilicon due to bending, using the flexture formula. (Q5) What is the ultimate strength (stress) of polysilicon, as reported in the literature? Comment on the difference/agreement with your result, as compared to the literature. Provide possible explanation for differences. (Q6) Comment on any sources of error for Experiment #1, #2 or #. Supplementary Information for the Microtool: The geometry of the microtool is given in Appendix A. The polysilicon material properties can be assumed as: E (Young s Modulus) = 160 GPa, and other values as listed in references [5,6]. To analytically determine the forces applied to the square-tip, for a given tip deflection from its rest position, we can use the following: For the lateral forces, the beams can be modeled as follows: Referring to the microtool illustration of Fig. A-1(a), and assuming loading force F 2 as shown in Fig. A-1(c), we can model the microtool as three parallel fixed-guided beams. N. Dechev, 2011, University of Victoria Page 5 of 7
For each beam, the distance from the bond-pad to the end of the square-tip is 120 um. Also, the width of the beams is w = 2 um and the thickness is t = 2 um. Using the flexture formula: σ = My I We can substitute values to re-write this as (where we approximate moment M = FL, which is valid for cantilever beams, where M is maximum at the root of the beam): ( σ = FL) w 2 = 6FL tw tw 2 12 For a single fixed-guided beam, the deflection is: δ = FL b 12EI Where E is assumed to be 160 GPa. Re-arranging the above equation in terms of F, we have: 12δE tw F = 12 = δetw L b L b For the case of three parallel beams the lateral force developed for a given deflection is: F = δetw L b Note: for two parallel beams, the constant in the numerator is 2. Therefore, the maximum lateral stress in any one of the beams (for three-beam configuration) can be approximated as: 6 F L b σ = tw 2 References: [1] N. Tas, T. Sonnenberg, H. Jansen, R. Legtenberg, and M. Elwenspoek, Stiction in Surface Micromachining, Journal of Micromechanics and Microengineering, vol. 6, 1996, pp. 85-97 [2] C. H. Mastrangelo, and C. H. Hsu, Mechanical stability and adhesion of microstructures under capillary forces part I: basic theory, Journal of Microelectromechanical Systems, vol. 2, 199, pp. -4 [] W. Merlijn van Spengen, R. Puers, and I. De Wolf, A Physical Model to Predict Stiction in MEMS, Journal of Micromechanics and Microengineering, vol. 12, 2002, pp. 702-71 N. Dechev, 2011, University of Victoria Page 6 of 7
[4] Philip Ball, Fundamental physics: Feel the force, Nature, http://www.nature.com/nature/journal/v447/n7146/full/447772a.html, [Online, last cited July 2, 2008] [5] W. N. Sharpe and K. Jackson, Tensile testing of MEMS materials, in Proc. 2000 SEM IX International Congress, Orlando, FL, June 5 8, 2000 [6] W. N. Sharpe, B. Yuan, R. Vaidyanathan, and R. L. Edwards, Measurements of Young s modulus, Poisson s ratio, and tensile strength of polysilicon, in Proc. Tenth IEEE International Workshop on Microelectromechanical Systems, Nagoya, Japan, 1997, pp. 424 429. Appendix A: Figure A-1: Microtool Geometry N. Dechev, 2011, University of Victoria Page 7 of 7