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:
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.
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.
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:
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.
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.
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.
|