MECH 466 – Microelectromechanical Systems

© N. Dechev, 2016, University of Victoria Page 1 of 8

MECH  466    

Micro  Electromechanical  Systems  

Laboratory  #1:  Testing  of  Electrostatic  Microactuators  

   

   

Department  of  Mechanical  Engineering,  University  of  Victoria  

                           

 

MECH 466 – Microelectromechanical Systems

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Report: The laboratory report must be in the following format:

(i) Title Page A. Include your name, student number, date of lab, etc…

(ii) Objective (iii) Data

A. Include data and images you collected. B. Comment on any ‘Sources of Error’ that may have influenced the data, or the

collection of the data. (iv) Discussion

A. Compare and contrast the experimental results in comparison to the theoretical/analytical results.

B. Include answers to the questions that pertain to the experiments, as listed in the ‘Lab Write-up’ section.

(v) 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 8 pages (double spaced text), including figures, and data. Appendices may be added as extra pages.

MECH 466 – Microelectromechanical Systems

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Laboratory #1: Testing of Electrostatic Microactuators Purpose: This lab will investigate the operation of MEMS-based micro-scale electrostatic actuators. Additionally, the lab will familiarize the student with the testing of MEMS devices using a motorized micromanipulator system and a digital microscope system, as shown in Appendix B. Introduction: There are numerous MEMS sensors and actuators that are based on the electrostatic ‘comb’ device. These are devices that operate on the principle of electrostatic charge, and the forces that arise from this charge. They can alternatively sense motion or produce motion. For example, an accelerometer senor can sense motion when a proof mass supported by flexible beams, experiences an acceleration that causes a change in the spacing between a set of interdigitated fingers. Alternatively, an electrostatic comb-drive can force a proof mass to move toward or away from a set of interdigitated fingers that are attracted or repulsed by each other, respectively. The driving voltage applied to electrostatic actuators is generally large (50V to 200V), as compared to conventional electronic parts/circuits. In this laboratory, we will apply various voltages to the device, and examine the response and behavior of the device. Procedure: Setup:

(1) The TA/technician will mount a MEMS chip (attached to an aluminum disc) onto the rotational worktable (alpha-stage) of the micromanipulator.

(2) The TA/technician will mount a 5 pin-probe system (shown in Appendix C-1) to the manual x, y, z probe-stage (shown in Appendix B-1).

(3) Using the micromanipulator system (microscope, robot and control software) the student will locate the electrostatic actuator that is to be tested.

(4) Locate the central tip of the 5 pin-probe using the manual x, y, z microscope-stage. (5) Carefully place the central probe tip (pin with the green wire coming out) onto the central pad

of the electrostatic actuator. Ensure that the other probe tips are touching the ground pad of the electrostatic actuator.

(6) The electrostatic actuator is to be driven using various voltages, from a variable voltage DC power supply (VVDCPS, shown in Appendix C-2). Simply dial in the desired voltage, and press the momentary switch. Check the actual voltage using a multimeter, since the dial only shows approximate voltage. The VVDCPS will on deliver the voltage while the momentary switch is held down. The TA/technician will instruct you in the safe use of the VVDCPS.

(7) Connect the appropriate wires from the 5 pin-probe (center pin and left or right pin) to the VVDCPS, and conduct the experiment as explained next.

Experiment: Electrostatic Actuator Characterization Two different geometries of electrostatic actuators can be tested. The first electrostatic actuator uses 250um long beams, and the second uses 275 um long beams. Attempt the experiment with the 275 um long device first.

(1) Test the electrostatic actuator using 60-80 VDC, to ensure that the device appears to operate properly, and is not ‘stuck down’ due to stiction effects.

(2) With the power off, measure the electrical resistance of the electrostatic actuator. You may

MECH 466 – Microelectromechanical Systems

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use the multi-meter to do this. The measurement should be infinite, to indicate that there is no short circuit between the fingers of each comb.

(3) The VVDCPS can provide voltages from 0 VDC to 200 VDC. (4) Begin the experiment with the 30 VDC value. (5) Apply the voltage to the electrostatic device, by dialing in the desired voltage on the VVDCPS

and pressing the momentary switch. Use a multi-meter to verify the voltage (prior to application), and record the value applied.

(6) Using the ‘video capture software’, capture an image of the resulting displacement of the electrostatic actuator, while the voltage is applied.

(7) Increment the voltage by 10 VDC. (8) Repeat step (5 to 7) for a range of voltage values from 30 VDC up to 150 Volts VDC. Record

all data and images. (9) Locate an electrostatic actuator with 250 um long beams, and repeat Steps (1 to 8).

Laboratory Write-up: Please answer the following questions, and include them in the ‘Discussion Section’ of your laboratory report. You may use the ‘supplementary notes’ in the next section, to help you answer these questions.

(Q1) Prepare a graph of the experimental values for applied voltage vs. displacement for the electrostatic actuators.

(Q2) The diagrams of the electrostatic actuators are provided in Appendix A. Determine the ‘mechanical stiffness’, km(total), for each of the two devices.

(Q3) Using the experimental data of deflection vs. applied voltage, estimate the ‘fringe capacitance’ that causes the electrostatic actuator to deflect (for each device).

(Q4) Using the diagram of Appendix A, determine the ‘normal capacitance’ of each device. (Q5) Based on your results for (Q3-Q4), estimate the ‘total capacitance’ of each device. (Q6) Discuss any ‘other observations’ you made during the experiment, and suggest possible

causes. (Q7) Discuss any sources of error.

Supplementary Notes: Estimation of fringe capacitance: - Define a relationship between d (displacement) and V (voltage). - We know: 𝐹 = 𝑘𝛿, where: 𝑘 = 𝑘!(!"!#$) from (Q2). - Recall from Lecture 6,

𝐹 =𝜕𝑈𝜕𝑥

=𝜕 12𝐶𝑉

!

𝜕𝑥

Therefore, since voltage does not vary with x, we can write:

MECH 466 – Microelectromechanical Systems

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𝐹 =12𝜕𝐶𝜕𝑥

𝑉!

This is solved in the lecture notes as:

𝐹 =𝜀𝐴𝑉!

2𝑑!=𝐶𝑉!

2𝑑

We can see that the force must be entirely balanced by the mechanical force due to the structure. Also, the longitudinal force must only be due to the fringe capacitance (as the normal capacitance will not contribute to longitudinal force), therefore:

𝐹 = !!!!

!!, and hence: 𝐶! =

!!"!

!!

We have a plot of displacement vs. applied voltage, therefore we know the corresponding value of d, for any given V. Also, we know the value of k, since we can compute this from the geometry. Therefore, Cf, can be determined for any value of d. References: [1]  D. Koester, A. Cowen, R. Mahadevan, M. Stonefield, and B. Hardy, “PolyMUMPs Design Handbook Revision 9.0”, MEMSCAP, MEMS Business Unit (CRONOS), Research Triangle Park, N.C., USA, 2001. Appendix A: Correct as of Summer 2016:

Figure A-0: Overall Imageof Electrostatic Actuator. Lbeam = 250 um or 275 um.

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Figure A-1: Flexible beams (close-up). The beams are 2 um wide, by 2 um thick (out of page).

Figure A-2: Scale diagram of interdigitated fingers (close-up).

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Appendix B:

Figure B-1: Image of micromanipulator system, with main components labeled.

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Appendix C:

Figure C-1: Image of PicoProbe (GBB Industries, Naples, FL, Model MCW-15-9547). 5-pin probe with pins separated by 200 microns center to center. Pin Position Cable Color Pin # Far Left Brown 5 Left White-Green 6 Center Green 7 Right White-Orange 8 Far Right Orange 9

Figure C-2: Image of Variable Voltage DC Power Supply (0 to 200VDC, 100 uA max).