Engineering
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Compact Lens-less Digital Holographic Microscope for MEMS Inspection and Characterization
Chapters
Summary July 5th, 2016
We present a compact reflection digital holographic system (CDHM) for inspection and characterization of MEMS devices. A lens-less design using a diverging input wave providing natural geometrical magnification is demonstrated. Both static and dynamic studies are presented.
Transcript
The overall goal of this procedure is to test micro-electro-mechanical structures to be used in static and dynamic applications using optical full-field measurements with computational and experimental techniques. This method can help answer key questions in the semiconductor industry and more specifically for micro-electro-mechanical system inspection such as the characterization of the mechanical properties of MEMS structures at different stages of manufacturing. The main advantage of this technique is that it is full-field, non-contact, real-time, high-resolution, and it provides a fully quantitative 3D map of the reflective object.
The system have been made with no lens and, hence, is really compact. So this method can provide insight into MEMS characterization. It can also be used for wafer inspection or optics reference testing.
If set up in a transparent geometry, it is also possible to inspect micro-optics, diffractive optics, or even used for bioimaging. This procedure uses a compact digital holographic microscope, or CDHM, to characterize a micro-electro-mechanical system or MEMS device. For this demonstration, an 11 square millimeter silicon wafer with square gold electrodes positioned every 0.25 millimeters will be characterized.
Using tweezers, place the MEMS sample on the sample holder. Adjust the sample holder so that the laser beam targets the electrodes. The largest possible field of view is limited by the camera sensor, and in this case, is 2.3 by 1.8 millimeters.
Vertically position the system about 1.5 centimeters from the sample, and proceed with setting up the 3D View software. This package was developed in C+Begin with clicking on the green box icon to select the video imaging device. Choose the DMx 41BU02 imaging source camera in the drop-down menu.
Next, under the Device Settings, Select the Y800 Video Format option, and set the capture rate to 15 Frames Per Second. Press OK, and start the camera using the yellow play button. A live video image of the sample should appear.
The displayed image should be a defocus image of the sample. Adjust the exposure time and contrast of the image if needed. Now, center the sample as best as possible and access the Settings options.
There, set the System Type to Reflection or Transmission. Check that the wavelength of the laser is set to 633 nm, The Pixel size of the camera to 4, 650 nm, and the magnification is times two. Next, select the Convolution Reconstruction algorithm, and set the Reconstruction Distance to 100 mm and the Reconstruction Step to one.
The reconstruction distance can be adjusted later to adjust the intensity image to a sharp focus, while the Step parameter indicates how many times the convolution operation is performed to simulate the beam propagation. This parameter can be changed to fine-tune the reconstruction distance in the intensity image. Lastly, under the Post Processing options, set the Unwrapping algorithm to the Quality mapped option.
Make sure that the Intensity filter and the Phase filter options are set to None. Then, press OK.Begin with accessing the 3D View software and opening the Fourier spectrum window. If one zero order and two plus one minus one order spectrums do not appear, check that the sample is placed below the red laser beam.
The laser reflection can easily be seen from the sample. Now, stop the live measurement mode and select one of the diffracted orders using the filter tool. Select an area large enough to encompass all the frequencies needed for the phase retrieval.
Select the SET option to apply the filter. Then, restart the live measurement mode. With the selection made, and imaging live, open the Phase window and make sure that the unwrapped mode is not enabled.
Next, adjust the vertical stage to reduce the number of fringes in the phase image so only one or two fringes remain. Let the system adjust after each stage movement. To find the best reconstruction distance, use the auto-focus option.
Several refocus steps may be needed to get to the optimal reconstruction distance. Ultimately, the image should become sharp and clear. Fine focus adjustments can be made using the focus slider bar if the auto-focus is not providing the best result.
For very fine focus adjustments, the Reconstruction Step can be changed in the settings. With the image prepared, enable the unwrapped mode to see the unwrapped phase image. In the 3D View software, open the 3D image window to see the final image of sample, and use the available image adjustments to observe the result.
On the unwrapped phase image, select the ruler icon and draw a line on the area of interest to get a cross-sectional plot in the line plot window. Now, in the line plot window, use the two green line markers to get an approximate height of the object. To arrange the windows for simultaneous viewing, select the Tile Windows option.
Surface roughness can also be calculated on the flat part of the sample using MATLAB software. Save the final phase image as a bitmap for further viewing in other software. To prepare the sample for dynamic analysis, position it on the sample stage for analysis.
In this case, the sample is a micro diaphragm. Connect the sample electrodes to the crocodile clips of the generator. Following the described procedures, record a hologram of the micro diaphragm at the ambient temperature for reference.
Click on the Delta icon to remove the initial phase, and thus only observe deformation. Then, turn on the DC generator and gradually increase the voltage from zero to 12 volts, taking phase map images at every 1 volt increment. Later, using a simple MATLAB code, plot the different phase map deformations into one graph to better observe the total deformation, and characterize the MEMS deformation to electrical loading.
A CDHM system was used to characterize a MEMS device as described, using a monomode fiber coupled to a diode laser operating at a 633 nanometer wavelength. The object beam and reference beam path were matched to obtain a hologram image of the MEMS device. The yellow line represents the cross-section location on the sample, and the two green marker lines were used to estimate the sample height.
To validate the results of the digital holographic system, an atomic force microscope was used to measure the same structure. A height difference of 2.1 nanometers was found between the atomic force microscope measurement and the CDHM measurement. A MEMS electrode made using a lift-off process is subject to sample morphology variance that needs quantification.
Using the described protocol, a def map was made for this purpose. A plot in the other dimension shows that surface roughness of the electrode is also observable using the system. In a dynamic system, with the temperature rising from 50 to 300 degrees Celsius, morphological changes were measured in a micro diaphragm fabricated by bonding a thin-plate onto an SOI wafer sample.
This thermal deformation was then summarized in a line plot showing a cross-sectional view of the different deformation states. After watching this video you should have a clear understanding of how to set up the system and carry out morphological studies related to reflective samples such as 3D profile, deformation maps, and surface roughness. Once trained, anyone can use the software and the system so that it can be easily implemented on production chains and operated by technicians.
While attempting this procedure, it's important to remember to place the sample at the right distance from the system. This is an important step. And it has a real impact on the final results.
After it's development, this technique b-v for researchers in the field of optical and electrical engineering to categorize the behavior of MEMS samples for static and dynamic conditions. Following this procedure, other experiments like observation of resonant mode while your playing high-frequency alternative current or quantifying cantilever deflection can be performed in order to provide a calibration of a packed column MEMS device.
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