High-precision large deflection measurements of thin films using time sequence speckle pattern interferometry

Abstract

The rapid development of microelectromechanical systems, thin-film technology and material research in mechanical behaviours has provided new impetus to handle high-precision and large-amplitude deformation measurements. Optical methods are very useful in making non-contact and full field measurements of the deformation of an object. However, there is a gap between the sensitivity and the measurement range in the existing coherent and incoherent optical methods. In this paper, we propose some approaches to employ time sequence speckle pattern interferometry to measure large deformations of pure Ni thin films subjected to bending with high sensitivity. We introduce a new large deformation measurement system, which includes a digital speckle interferometry and a micro loading system, and we propose two methods of phase demodulation in this paper. The new system is capable of automatically applying micro loading and continuously capturing the temporal speckle patterns imaged from the flexural surfaces of the thin film. Forty thousand frames of the time sequence speckle patterns have been recorded and the maximum deflection of about 1400 µm has been measured with sensitivities of λ/2 and λ/4 by the different proposed methods, respectively.

A sequence pulse counting method for shape measurement in dual-beam speckle interferometry

“…x y t  caused by a monotone displacement or deformation is resolved as [25]: where ( , , ) m x y t is the half-period number of the temporal speckle patterns (integer), and 1 ( , , ) m x y t  and 2 ( , , ) m x y t  , the fractional half-period numbers (positive decimals). Detailed expressions of 1 ( , , ) m x y t  and 2 ( , , ) m x y t  are given in ref.…”

“…Detailed expressions of 1 ( , , ) m x y t  and 2 ( , , ) m x y t  are given in ref. [25]. Thus, the in-plane displacement related to the position-or shape-change of the object can be expressed as:…”

“…(6) indicates that the greater the tilting angle is, the higher the resolution of the height measurement is. On the other hand, in SPCM the measurable lowest phase value is smaller than , i.e., the half period of the interferometric pattern ( , , ) 1 M x y t  [25]. Therefore, the resolution of the height measurement can be estimated as:…”

“…However, the measurable minimal height is limited by the requirement of a considerable number of speckle patterns in the Fourier-transform TSPI. In the present work, we will apply our previously developed sequence pulse counting method (SPCM) [25]-a special kind of TSPI methods-to directly extract the object shape with the help of the conventional dual-beam interferometric system.…”

Fractionized calibration of the sample stage used in an AFM-probe mechanical testing system

“…The displacement demodulation of TSPI sequences with the least-square fitting TSPI sequence analysis are suitable for a large range and a high accuracy displacement measurements, such as the methods of Fourier transform [13], scanning phase [14], time sequence phase [15], and sequence pulse counting [16]. In this section, the leastsquare fitting method [17] in TSPI is applied to the speckle pattern sequences to obtain the displacement of the cascading sample stage in the calibration.…”