Crableg Resonator Tutorial (Part 2)
  From Design to Testing
     
    Purpose of this document

This second part of the tutorial provides an overview of using the MIT Microvision system to test the crableg resonators that were designed and simulated as described in Part 1. As such, this may serve as a reference for new users in learning the Microvision system. Data collected on a crableg resonator die at Carnegie Mellon is detailed in Part 3.

The document 'Matisse Testing for Standard Device' provides an overview of the testing plan of a standard device laid out for the Matisse project.

Release and Packaging

After the wafer was manufactured by AMS and returned to the Carnegie Mellon MEMS Lab, the dies were released by Dr. Xu Zhu through a series of etching steps involved in the CMOS MEMS process. One may find the dies divided into two groups. The dies in Group 2 were subject to an additional cleaning step during the release process. This may slightly shift the resonant frequency and is of interest to those studying the release process.

The dies were then packaged into a 40 lead, 0.310 sq. cavity, Side Braze package (#KD-78300) from Addison Engineering. A silver epoxy was used in this step obtained from the cleanroom - BIPAX TRA-DUCT BA-2958 (www.tra-con.com). The packaged dies were then heated in an oven and then allowed to sit overnight.

The dies were bonded to the package using a Kulicke and Soffa Model 4123 Universal Wedge Bonder. The document 'Test Structure Resonator Bond Pattern' shows the pinout for the bonding. More infomation about the bond pad layout can be found by looking at Salil Desai's bond pattern for dMachine1 and dMachine2. (The document 'Naming Convention' provides more details about his six degree-of-freedom resonators.)

Description of Tests

Several tests can be performed to characterize the crableg resonator. One test is to measure the resonant frequency. Finding the resonant frequency may involve a measurement with a rough frequency sweep (10 KHz steps) followed by more measurements focusing in on the resonant frequency with finer steps (i.e., 1 Khz, 100 Hz, 50 Hz).

Continued Work:Can this process of focusing in on a resonant frequency be automated with a script that redefines the frequency parameter list with a guess resonant frequency?

The device response can be further characterized by a frequency sweep in reverse order (hysteresis), a measurement with either AC or DC voltage descreased by half, and by stimulating the resonator at a very low frequency (a couple order of magnitudes less than the resonant frequency) to measure the DC motion.

Microvision Acquisition

Information on the MIT Microvision and Testbed System at Carnegie Mellon can be found on the MEMS Lab Intranet here.

The packaged die is now mounted onto the Microvision system using the zero-force insertion socket. The following electrical connections are made:

  • Ground: m3Field, substrate, and one of the two combs of interest Note: (the power supply ground and Microvision ground are conected together)
  • Microvision AC: the other comb not connected to ground
  • Power Supply DC: the plate
We visually center the packaged die under the ojective but we cannot focus on it until after starting the Microvision software and going to the 'Focus Image' command.

The window shown below will open when we start the Microvision software ('cmacq' at the command line). Note: The following window shows all of the fields filled out as described in the following steps.

Microvision Acqusition
Microvision Acquisition (click on image for larger view)

First, we can include a reminder in the 'Experiment Description' field such as what comb drive is connected and what the external DC voltage is.

To specify the test parameters, we click on the 'Hardware Settings.' The important settings to note include the Illum Time (max of 0.066), Phase Div, Peak Amplitude, DC Offset, Frequency, Waveform, Objective, and External Amplification (set to 1x). The Frequency specified here will be the default frequency used for the 'Watch Motion' and 'Test Motion.' To see the maximum displacement, choose a frequency close to the resonant frequency of the device. For the crableg resonator, that is approximately 55.7 KHz.

Hardware Settings
Hardware Settings (click on image for larger view)

After the specifying the hardware settings, click on 'Focus Image' to turn on the LED to focus on the device. Take note of the position of the 'Image Select' rod on the top right side of the microscope. The light can go to the eyepiece (all the way in), the CCD camera (all the way out), or both (in the middle). After focusing through the eyepiece using the mircoscope knob, fine focus can be done using the camera image. The slider bar along the top of the window controls the PZT adjusting the objective. Note: The final position of the slider must be included in the list of Focal Planes as specified later. Otherwise, the Analysis will not work properly.

We skip 'Sample Image.'

By clicking 'Watch Motion,' we'll see the device moving if everything is set-up properly. If there is no motion, there are several troubleshooting steps to try:

  • Is the proper frequency set? (Away from the resonant frequency, motion may be too small to see.)
  • Is the DC power supply turned on?
  • Are all of the electrical connections complete (wires, socket into the protoboard, cables, etc.)?
  • Does connecting the opposite comb fingers to the AC signal help?
  • Has the bonds for this device been checked?
  • Is the device released (i.e., no stiction)?
The 'Focal Planes' allows us to pick a range for the z-axis. The z-axis range needs to be large enough and contain enough planes to allow the Analysis to work properly. Otherwise, an error will occur stating the z-axis motion is out of range. Although the software defaults to a range of plus or minus 4 microns from the position chosen during the 'Focus Image', I tried plus or minus 1 micron from this position. By choosing this range and 4 steps, the step size is 0.5 micron. (The number of planes is one more than the number of steps.)

The 'Test Motion' allows us to see a recorded movie of our device moving. This is similar to 'Watch Motion' except that it is recorded and looped rather than a constant excitation of the device. (Great for visual demos.)

To see a Bode plot response of our resonator to find the resonant frequency, our test parameter is Frequency. The other option is Voltage. We then specify a list of frequency in the Parameter List. Over a range from 10 KHz ro 90 KHz, our first measurement may involves steps of 10 KHz. To focus in on the resonant frequency, we can decrease that step size to 1 KHz, 100 Hz, even 50 Hz, as we get a better idea of the resonant frequency.

The 'Plane List' and 'Phase List' should already be set from the earlier steps. That last thing to choose is the Region of Interest (ROI) as the 'Crop Region.' The data set for these measurement can become large considering the number of focal planes, the number or phases, and the number of frequencies. Rather than saving the whole image, we'll choose a smaller portion of the comb finger where we'll measure the response. I selected the region near the anchor to have a reference point as shown in the following image.

Microvision ROI Interface
Microvision ROI Interface (click on image for larger view)

At this point, we can save our test set-up. All of the parameters and settings are listed in a .cmg file that can be loaded by the software later. An example .cmg file from one of the crableg resonaters is available here. This measurement can be taken now by clicking 'Run Now.' Alternatively, we can add this measurement to the Queue and specify another measurement to run. The Queue works well for setting up another where the frequency is swept backwards or a voltage is descreased by half. One should perform a simple test to make sure the current settings work fine for the analysis (in particular, the focal plane z-axis range). After the measurement is taken, we are ready to do the Microvision Analysis.

A comment on loading .cmg files: I have experienced some difficulty in copying a file outside of the software and trying to load it again. However, saving an old file under a new name using the 'Save As...' command avoids that problem. If a measurement has already been taken, the field 'dataTaken' in the .cmg file will have a 1 instead of a 0. When this is the case, only the Analysis can be performed; the measurement settings cannot be changed. However, by changing that field back to a 0, the measurement can be taken again under the same name. After you hit the 'Run Now' button, there will be an error saying the previous partial data set has been deleted. After saying ok, run the measurement again and it'll be fine. This could cause a delay if the measurements are included in a Queue and it is not noticed.

Microvision Analysis

After a measurement is taken, we launch the Microvision Analysis tool from the 'Tools' menu. The following window will appear.

Microvision Analysis
Microvision Analysis (click on image for larger view)

The first step is to perform an analysis by giving it a name, choosing the type (Single or Dots), and then specifying the ROIs. The following images choosing an ROI for both types of analysis. After choosing the ROIs, click Run to perform the analysis. Note: You might try selecting a different ROI if you have a z-axis out of range error before running a new measurement.

Single ROI Selection
ROI Selection for Single Analysis (click on image for larger view)



Dots ROI Selection
ROI Selection for Dots Analysis (click on image for larger view)

We can study the results of the analysis using the Visualize tools included here. Another option is to view the corresponding text file containing the data and load it into another program.

The 'View Response' gives a plot of the magnitude or phase response for the range of the chosen parameter. For our case with the frequency parameter, the magnitude respone will generate a Bode plot along x, y, and z directions. An example plot of the crableg resonator is shown below. The red crosshair shows roughly the resonant frequency.

Response Plot
Plot of the Magnitude Response (click on image for larger view)

The other visualize options are specific for frequency selected in the list. The 'View Raw' will give you a video clip of your device at that frequency. The 'View Motion' (Position or Raw) shows you the motion for each phase division along a period of the waveform. The 'View Dots' will highlight the video clip with moving dots that you can increase in amplitude. The following shows examples of this data.

Response at 55.6 KHz
Response at 55.6 KHz



Motion Plot
Plot of Motion Response at 55.6 KHz (click on image for larger view)

When the analysis is performed, a textfile is generated containing all the data that was plotted above. The measurement data is stored in a directory with the same name as the .cmg file. The text files called 'response_analysis name_ROI_n.anl' and '..._f2.anl' where analysis name is the one chosen above and the n represent the ROI (0 for single; 0 and 1 for dots). The file lists the data in seven columns corresponding to the frequency, x position, x phase, y position, y phase, z position, and z phase. An example .anl is available here. One program that can be used to analyze the data is Matlab. An example .m file to load the data is available here.

Back to Part 1

Continue to Part 3