Characterization of MEMS structures by microscopic digital holography

Abstract

We propose the use of digital holography (DH) as a metrological tool for inspection and characterization of MEMS structures. We show that DH can be efficiently employed to assess the fabrication process of micro structures as well as to test their behaviour in operative conditions. DH allows reconstructing both the amplitude and phase of microscopic objects and, compared to traditional microscopy, it provides quantitative phase determination. We demonstrate that DH allows determination of full field deformation maps that can be compared with analytical and/or numerical models of the deformed microstructure. Application of DH on structures with several different geometries and shapes, like cantilever beams, bridges and membranes is reported and result will be discussed. Dimensions of the inspected microstructures ranging from 1 to 50µm. Examples of application are presented were DH allows determination with high accuracy out of plane deformations due to the residual stress introduced by the fabrication process. An optical setup for recording digital holograms based on a Mach-Zehnder interferometer was adopted and a laser source which wavelength is λ=532nm was employed. The light reflected by the object under investigation was made to interfere with a plane wave front. Holograms were recorded by a CCD array with 1024 x 1280 square pixels with 6.7 µm size. A mirror mounted on a piezo-actuator was inserted along the reference arm of the interferometric in order to introduce controlled phase steps and to employ phase shifting technique. This technique allows suppressing both the zeroth-order and the conjugate wave-front in the numerical holographic reconstruction process. A method for compensating numerically curvature of the wave front and introduced by the microscopic objective lens is proposed and discussed.

Automatic digital filtering for the accuracy improving of a digital holographic measurement system

“…Furthermore, as an interferometric method, DH offers both a no-destructive and no-contact approach to very fragile objects combined with flexibility and a high sensitivity to geometric quantities such as thicknesses and displacements [5]. These features recommend it for the solution of many imaging and measurements problems, such as microelectro-optomechanical systems (MEMS/MEOMS) inspection and characterization [6][7][8][9][10][11][12][13]. These structures are generally realized using different material layers superimposed in various process steps [14].…”

Study of spatial lateral resolution in off-axis digital holographic microscopy

“…Digital holographic microscopy (DHM) is a well-established technique for MEMS evaluation [1][2][3], living cell screening [4][5][6][7] and particle tracking [8][9][10][11][12][13]. Based on the original Gabor's idea [14], DHM allows the retrieval of the complex wavefield scattered by samples from variety of fields [15][16][17][18].…”

Reconstruction Method of Wavefronts in Digital Micro-Holography

“…Therefore, digital holography has been paid a wide attention in recent years. The main applications includes observation of biological samples [1,2], living cells analysis [3][4][5][6], 3D pattern recognition [7,8], vibration analysis [9], measurement of MEMS and MOEMS [10] and so on.…”