Digital holographic microscope with automatic focus tracking by detecting sample displacement in real time (original) (raw)
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.
Physical phase compensation in digital holographic microscopy
2009
In digital holographic microscopy (DHM), using microscope objective for sample imaging may introduce additional spherical phase curvature. It can be physically compensated by introducing a same phase curvature in the reference beam. A theoretical analysis of the wavefront interefence between the reference beam and object beam is provided to indicate the physical phase compensation. The spatial frequency spectra of the hologram are involved for the judgement of the physical phase compensation status. Different DHM setups are presented in order to fulfill the physical compensation of the introduced spherical phase. In the DHM setups based on the Michelson interferometric configuration, an adjustable lens is used to perform the quasi physical phase compensation during the hologram recording. In the common-path DHM setups, digital off-axis holograms are recorded by using a single cube beam splitter in a non-conventional configuration so as to both split and combine a diverging spherical wavefront emerging from a microscope objective. A simple plane numerical reference wavefront is used for the reconstruction and the correct quantitative phase map of the test object is obtained after phase unwrapping. Its simplicity of the presented setups make it easy to be well aligned and with lower cost.
Controlling Images Parameters in the Reconstruction Process of Digital Holograms
IEEE Journal of Selected Topics in Quantum Electronics, 2004
Digital holograms recorded with a charge-coupled device array are numerically reconstructed in amplitude and phase through calculation of the Fresnel-Kirchhoff integral. The flexibility offered by the reconstruction process in digital holography allows exploitation of new possibilities of application in microscopy. Through the reconstruction process we will show that it is possible to control image parameters as focus distance, image size, and image resolution. Those explored potentialities open further the novel prospective of application of digital holography in single-and multiwavelengths operation either for display or metrological applications. We demonstrate the concept of controlling parameters in image reconstruction of digital holograms in some real situations for inspecting silicon microelectronic-mechanical systems structures.
Automatic procedure for aberrations compensation in digital holographic microscopy
Optical Micro- and Nanometrology in Microsystems Technology, 2006
Digital Holographic Microscopy (DHM) is a powerful imaging technique allowing, from a single amplitude image acquisition (hologram), the reconstruction of the entire complex wave front (amplitude and phase), reflected by or transmitted through an object. Because holography is an interferometric technique, the reconstructed phase leads to a sub-wavelength axial accuracy (below λ/100). Nevertheless, this accuracy is difficult to obtain from a single hologram. Indeed, the reconstruction process consisting to process the hologram with a digital reference wave (similar to classical holographic reconstruction) seems to need a-priori knowledge about the physical values of the setup. Furthermore, the introduction of a microscope objective (MO), used to improve the lateral resolution, introduces a wave front curvature in the object wave front. Finally, the optics of the setup can introduce different aberrations that decrease the quality and the accuracy of the phase images. We propose here an automatic procedure allowing the adjustment of the physical values and the compensation for the phase aberrations. The method is based on the extraction of reconstructed phase values, along line profiles, located on or around the sample, in assumed to be flat area, and which serve as reference surfaces. The phase reconstruction parameters are then automatically adjusted by applying curve-fitting procedures on the extracted phase profiles. An example of a mirror and a USAF test target recorded with high order aberrations (introduced by a thick tilted plate placed in the setup) shows that our procedure reduces the phase standard deviation from 45 degrees to 5 degrees.
Applied Optics, 2014
The utilization of microscope objectives (MOs) in digital holographic microscopy (DHM) has associated effects that are not present in conventional optical microscopy. The remaining phase curvature, which can ruin the quantitative phase imaging, is the most evident and analyzed. As phase imaging is considered, this interest has made possible the development of different methods of overcoming its undesired consequences. Additionally to the effects in phase imaging, there exist a set of less obvious conditions that have to be accounted for as MOs are utilized in DHM to achieve diffraction-limit operation. These conditions have to be considered even in the case in which only amplitude or intensity imaging is of interest. In this paper, a thorough analysis of the physical parameters that control the appropriate utilization of MOs in DHM is presented. A regular DHM system is theoretically modeled on the basis of the imaging theory. The Fourier spectrum of the recorded hologram is analyzed to evaluate the performance of the DHM. A set of the criteria that consider the microscope features and the recording parameters to achieve DHM operation at the diffraction limit is derived. Numerical modeling and experimental results are shown to validate our findings.
Extended focus imaging in digital holographic microscopy: a review
Optical Engineering, 2014
The microscope is one of the most useful tools for exploring and measuring the microscopic world. However, it has some restrictions in its applications because the microscope's depth of field (DOF) is not sufficient for obtaining a single image with the necessary magnification in which the whole longitudinal object volume is in focus. Currently, the answer to this issue is the extended focused image. Techniques proposed over the years to overcome the limited DOF constraint of the holographic systems and to obtain a completely in-focus image are discussed. We divide them in two macro categories: the first one involves methods used to reconstruct three-dimensional generic objects (including techniques inherited from traditional microscopy, such as the sectioning and merging approach, or multiplane imaging), while the second area involves methods for objects recorded on a tilted plane with respect to hologram one (including not only the use of reconstruction techniques and rotation matrices, but also the introduction of a numerical cubic phase plate or hologram deformations). The aim is to compare these methods and to show how they work under the same conditions, proposing different applications for each. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.