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

Summary

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.

Abstract

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.

Introduction

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 tech....

Protocol

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

1. Sign into the logbook before using the system.
2. Turn on the computer, LASER and translation stage power.
3. Place the MEMS electrode/micro-diaphragm sample.
   1. Place the MEMS sample in the middle of the sample holder using a tweezer.
   2. Adjust the sample holder to position the electrodes in the beam path. The maximum measurement field of view i....

Results

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.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2 MP Camera	Imaging Source	DMX 41BU02	used to record the hologram. 4.65 microns pixel size
Motorized X,Y,Z Translation Stage	Zaber Technology	TLS28-M	Holder for the system
Beam splitter	Edmund optics	49-003	Cube Beam splitter. Separate and recombine the object and reference beam
Laser	Micro Laser Systems, Inc.	SRT-F635S-20/OSYS	Diode laser
Mirror	Edmund Optics	#43-412-566	1" Dia. Protected Gold, λ/20 Flat Zerodur
monomode Fiber	Thorlabs	S405-XP	Single Mode Optical Fiber, 400 - 680 nm, Ø 125 µm Cladding
Sample holder	Edmund Optics	#39-930	Ideal Positioning Platform, ±35 mm Travel in Both X and Y
Hotplate	Thermolyne Mirak hotplate	Barnstead International HP72935-60	temperature range 40-370 °C
Holoscope Software	d'Optron Pte Ltd		software developed by the NTU researchers