The authors have no acknowledgements.

1. Preliminary Preparation of the Measurement
Note: The sample used for the experiment is a MEMS electrode. The gold electrodes are fabricated on a silicon wafer using lift off process. The sample is an 18 mm x 18 mm wafer with periodic structures (electrodes) with 1 mm period
<ol>
	<li>Sign into the logbook before using the system.</li>
	<li>Turn on the computer, LASER and translation stage power.</li>
	<li>Place the MEMS electrode/micro-diaphragm sample.
	<ol>
		<li>Place the MEMS sample in the middle of the s...

<ol>
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The authors have nothing to disclose.

In this review, we provide a protocol to accurately recover the quantitative morphology of different MEMS devices by using a compact system relying on digital holography. MEMS characterization in both static and dynamic mode is demonstrated. Quantitative 3D data of a micro channel MEMS is obtained. In order to validate the accuracy of the system, results have been compared between the CDHM and the AFM. Good agreement is found meaning that digital holography can be a reliable technique for 3D imaging. Results indicate tha...

Metrology of micro and nano objects is of great importance for both industry and researchers. Indeed, miniaturization of objects represents a new challenge for optical metrology. Micro electro mechanical systems (MEMS) are generally defined has miniaturized electromechanical systems and usually comprises components such as micro sensors, micro actuators, microelectronics and microstructures. It has found many applications in diverse field such as biotechnology, medicine, communication and sensing1. Recently, the increasing complexity as well as the progressive miniaturization of test object features call for the development of suitable characterization techniques for MEMS. High throughput manufacturing of these complex microsystems requires the implementation of advanced inline measurement techniques, to quantify characteristic parameters and related defects caused by the process conditions2. For instance, the deviation of geometrical parameters in a MEMS device affects the system properties and has to be characterized. In addition, industry requires high resolution measurement performance, such as full three dimension (3D) metrology, large &#64257;eld of view, high imaging resolution, and real time analysis. Thus, it is essential to ensure a reliable quality control and inspection process. Moreover, it requires the measuring system to be easily implementable on a production line and thus relatively compact to be installed on existing infrastructures.
Holography, which was first introduced by Gabor3, is a technique that allows the recovery of the full quantitative information of an object by recording the interference between a reference and an object wave into a photosensitive medium. During this process known as recording, the amplitude, phase and polarization of a field are stored in the medium. Then the object wave field can be recovered by sending the reference beam onto the medium, a process known as optical reading of the hologram. Since a conventional detector only records the intensity of the wave, holography has been a subject of great interest in the past fifty years since it gives access to additional information on the electric field. However, several aspects of conventional holography make it unpractical for industry applications. Indeed, photosensitive materials are expensive and the recording process generally requires a high degree of stability. Advances in high resolution camera sensors such as charged coupled devices (CCD) have opened a new approach for digital metrology. One of those techniques is known as digital holography4. In Digital Holography (DH), the hologram is recorded on a camera (recording medium) and numerical processes are used to reconstruct the phase and intensity information. As with conventional holography, the result can be obtained after two main procedures: the recording and reconstruction as shown in Figure 1. However, if the recording is similar to conventional holography, the reconstruction is only numerical5. The numerical reconstruction process is shown in Figure 2. Two procedures are involved in the reconstruction process. Firstly, the object wave field is retrieved from the hologram. The hologram is multiplied with a numerical reference wave to get the object wavefront at the hologram plane. Secondly, the complex object wavefront is numerically propagated to the image plane. In our system, this step is performed using the convolution method6. The reconstructed field obtained is a complex function and thus phase and intensity can be extracted providing quantitative height information on the object of interest. The capability of whole &#64257;eld information storage in holography method and the use of computer technology for fast data processing offer more flexibility in experimental configuration and significantly increase the speed of the experimental process, opening up new possibilities to develop DH as a dynamic metrological tool for MEMS and micro-systems7,8.
Use of digital holography in phase contrast imaging is now well established and was first presented more than ten years ago9. Indeed, investigation of microscopic devices by combining digital holography and microscopy has been performed in many studies10, 11, 12, 13. Several systems based on high coherence14 and low coherence15 sources as well as different types of geometry13, 16, 17 (in line, off axis, common path&#8230;) have been presented. In addition, in line digital holography has been used previously in characterization of MEMS device18, 19. However, those systems are generally difficult to implement and bulky, making them unsuitable for industrial applications. In this study, we propose a compact, simple and lens free system based on off axis digital holography capable for real time MEMS inspection and characterization. The Compact Digital Holographic microscope (CDHM) is a lens less digital holographic system developed and patented to obtain the 3D morphology of micro-size specular objects. In our system, a 10 mW, highly stable, temperature controlled diode laser operating at 638 nm is coupled into a mono-mode fiber. As shown in Figure 3, the diverging beam emanating from the fiber is split into a reference and an object beam by a beam splitter. The reference beam path comprises a tilted mirror to realize the off axis geometry. The object beam is scattered and reflected by the sample. The two beams interfere on the CCD giving the hologram. The interference pattern imprinted onto the image is called a spatial carrier and permits the recovery of the quantitative phase information with only one image. The numerical reconstruction is performed using a common Fourier transform and convolution algorithm as stated previously. The lens-less configuration has several advantages making it attractive. As no lenses are used, the input beam is a diverging wave providing a natural geometrical magnification and thus improving the system resolution. Moreover, it is free of aberrations encountered in typical optical systems. As can be seen in Figure 3B, the system can be made compact (55x75x125 mm3), lightweight (400 g), and thus can be easily integrated into industrial production lines.

<table border="0" cellpadding="0" cellspacing="0" width="1228">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">2 MP Camera</td>
			<td style="width:251px;">Imaging Source</td>
			<td style="width:251px;">DMX 41BU02</td>
			<td style="width:512px;">used to record the hologram. 4.65 microns pixel size</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Motorized X,Y,Z Translation Stage</td>
			<td style="width:251px;">Zaber Technology&#160;</td>
			<td style="width:251px;">TLS28-M</td>
			<td>Holder for the system&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Beam splitter</td>
			<td style="width:251px;">Edmund optics</td>
			<td style="width:251px;">49-003</td>
			<td>Cube Beam splitter. Separate and recombine the object and reference beam</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Laser&#160;</td>
			<td style="width:251px;">Micro Laser Systems, Inc.</td>
			<td>SRT-F635S-20/OSYS</td>
			<td>Diode laser</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Mirror</td>
			<td style="width:251px;">Edmund Optics</td>
			<td style="width:251px;">#43-412-566</td>
			<td>1&#34; Dia. Protected Gold, &#955;/20 Flat Zerodur</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">monomode Fiber</td>
			<td style="width:251px;">Thorlabs</td>
			<td style="width:251px;">S405-XP</td>
			<td style="width:512px;">Single Mode Optical Fiber, 400 - 680 nm, &#216;125 &#181;m Cladding</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sample holder</td>
			<td style="width:251px;">Edmund Optics</td>
			<td style="width:251px;">#39-930</td>
			<td>Ideal Positioning Platform,&#177;35mm Travel in Both X and Y</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Hotplate</td>
			<td style="width:251px;">Thermolyne Mirak hotplate</td>
			<td style="width:251px;">Barnstead International HP72935-60</td>
			<td>temperature range&#160;&#160; 40-370 &#176;C</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Holoscope Software</td>
			<td style="width:251px;">d&#39;Optron Pte Ltd</td>
			<td style="width:251px;">NA</td>
			<td>software developed by the NTU researchers&#160;</td>
		</tr>
	</tbody>
</table>

compact lens-less digital holographic microscope for mems inspection and characterization

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and automotive. However, effective inspection and characterization metrology systems are needed to ensure the functional reliability of MEMS. This study presents a system based on digital holography as a tool for MEMS metrology. Digital holography has gained increasing attention in the past 20 years. With the fast development and decreasing cost of sensor arrays, resolution of such systems has increased broadening potential applications. Thus, it has attracted attention from both research and industry sides as a potential reliable tool for industrial metrology. Indeed, by recording the interference pattern between an object beam (which contains sample height information) and a reference beam on a CCD camera, one can retrieve the quantitative phase information of an object. However, most of digital holographic systems are bulky and thus not easy to implement on industry production lines. The novelty of the system presented is that it is lens-less and thus very compact. In this study, it is shown that the Compact Digital Holographic Microscope (CDHM) can be used to evaluate several characteristics typically consider as criteria in MEMS inspections. The surface profiles of MEMS in both static and dynamic conditions are presented. Comparison with AFM is investigated to validate the accuracy of the CDHM.

The protocol described above was designed to inspect and characterize MEMS and Micro devices using CDHM system. In our system, a mono-mode fiber is coupled to a diode laser operating at a 633 nm wavelength. Due to the diverging beam configuration, it is important to match the object beam and reference beam path in order to obtain a hologram that can be reconstructed. This is achieved through careful vertical positioning of the sample with respect to the system. In the calculated wrapped p...

Watch this Scientific Journal Video about Compact Lens-less Digital Holographic Microscope for MEMS Inspection and Characterization at JoVE.com

Compact Lens-less Digital Holographic Microscope for MEMS Inspection and Characterization

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.

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and ...

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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10.2K ビュー.  Nanyang Technological University. 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 ...