Digital holographic microscope with automatic focus tracking by detecting sample displacement in real time (original) (raw)
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A digital holographic microscope for complete characterization of microelectromechanical systems
Measurement Science & Technology, 2004
Digital holographic microscopy (DHM) can be described as a non-invasive metrological tool for inspection and characterization of microelectromechanical structures (MEMS). DHM is a quick, non-contact and non-invasive technique that can offer a high resolution in both lateral and vertical directions. It has been employed for the characterization of the undesired out-of-plane deformations due to the residual stresses introduced by technological processes. The characterization of these deformations is helpful in studying and understanding the effect of residual stress on the deformation of a single microstructure. To that end, MEMS with different geometries and shapes, such as cantilever beams, bridges and membranes, have been characterized. Moreover, DHM has been applied efficiently to evaluate variations of the structure profile due to some external effects. As an example, the characterization of a cantilever subjected to a thermal process has been described. The results reported show that DHM is a useful non-invasive method for characterizing and developing reliable MEMS.
Fringe 2005, 2006
The main microscopic systems for industrial inspection are optical microscopes, including confocal scanning instruments, Scanning Electron Microscopes (SEM), Atomic Force Microscopes (AFM), and a few interferometers. With the recent technological advances, the demand for nanometer scale resolution of full wafers of micro lenses, biochips, Micro Electro-Mechanical and Opto-Mechanical Systems (MEMS and MOEMS), and of large quantities of individual samples increases. The use of current systems in industrial environments is limited either by their limited resolution (optical microscopes), or by their strong sensitivity to vibrations due to their long measurement time associated with the scanning or phase shifting mechanisms, or by the lengthy measurement protocols (SEM), or by the difficulty arising from the precise automated positioning and focalization of the sample (white light interferometers). An ideal system should offer high measurement rates, robustness, ease of use, non contact measurement, no specific preparation of the samples simultaneously with nanometer scale resolution. We have developed Digital Holographic Microscopes (DHM), presently used in production environments This paper describe the principles of the technology and show up the potential of DHM for industrial application in with an interferometer resolution. Ease of use, associated the use of numerical procedure to a level never reach so fair in microscopy with, is demonstrated on two features: focusing and sample tilt adjustment.
Digital Holographic Microscopy for MEMS/MOEMS Device Inspection and Complete Characterization
Journal of the Indian Institute of Science, 2013
Digital holography became feasible since the availability of charged coupled devices (CCDs) with smaller pixel sizes, higher pixel numbers and high speed computers. Fresnel or Fourier holograms are recorded directly by the CCD and stored digitally in the computer. The reconstruction of the wavefield, which is done optically by illumination of a hologram in conventional holography, is performed by numerical methods in digital holography. In the numerical reconstruction process, not only the intensity but also the phase distribution of the stored wavefield can be computed from the digital hologram. This offers new possibilities for a variety of applications. This review article describes the principle of digital holographic microscopy (DHM) and its major applications in microelectro-optomechanical systems (MEMS/MEOMS) inspection and characterization. MEMS structures are generally realized using different material layers involving various process steps. Static and dynamic characterization of MEMS devices form an important part in carrying out their functional testing and reliability analyses. Development of a digital holography (DH) based system for micro-device inspection and characterization is presented in this review. A reflection mode hand-held type compact DH microscope system is developed based on the lensless magnification configuration. Application of the developed system is demonstrated for MEMS structures such as cantilever, diaphragms, accelerometer, microheater inspection. Further, both static and dynamic characterizations of these MEMS structures are illustrated.
Development of a simple user-friendly commercial digital holographic microscope
2008
We report the development of a simple commercial digital holographic microscope. The hologram is recorded using a CCD sensor and numerically reconstructed to provide quantitative analysis of the object. The laser source is coupled via fibre optics and the opto-mechanical setup is flexible and customizable for either the reflection or transmission mode. The user-friendly software allows live reconstruction, simultaneously providing both the amplitude and phase images. System performance is improved with phase unwrapping and interferometric comparison. Additional features include various image enhancements, cross-sectional and line profiling, measurement and data analysis tools for quantitative 3D imaging and surface topography measurement. The performance of the product is tested on different micro devices, glass and silicon surfaces.
Phase measurement via in-line digital holographic microscopy
2008
In-line digital holographic microscopy as a phase measurement tool for the inspection of micro-components is presented. Light diffracted by the micro-components interferes with the directly propagating beams to give the in-line digital hologram recorded by the CCD camera. The convolution method is used to calculate the diffractive propagation of the light in order to reconstruct the wavefront of the test specimen. A reference hologram without the test specimen is recorded for the phase reconstruction. Finally, the method is applied on a phase grating to test its refractive index of the coating material.
Extended depth-of-focus by digital holographic microscopy
Optics Letters, 2010
A recurrent problem in microscopy is the finite depth-of-focus linked to the NA of microscope objectives. Digital holographic microscopy (DHM) has the unique feature of being able to numerically change the focus from a single hologram without the need of moving the sample. Extended depth of focus of amplitude images has been demonstrated, but it has marginal interest for the metrological application of DHM that needs the topography. In this Letter, we demonstrate that DHM is able to provide not only extended depth-of-focus amplitude images but extended focused complex data from which the topography is computed. For this purpose, reflection and transmission measurements on micro-optics (microlens and retroreflector) performed by using standard reconstruction or the extended focused complex data are compared. These experiments demonstrate that DHM measures, from a single hologram acquisition, the accurate sample topography on a numerically increased depth-of-focus.