Name: compact lens-less digital holographic microscope for mems inspection and characterization
Uploaded: 2016-07-05T15:00:00
Description: Watch this Scientific Journal Video about Compact Lens-less Digital Holographic Microscope for MEMS Inspection and Characterization at JoVE.com

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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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 ...

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