various micromachined structures were designed for bulge test to determine the mechanical properties such as fatigue, creep, delamination, of thin films.
Measurement of Mechanical Properties of Thin Films Using Bulge Test Chung-Lin Wu, Weileun Fang, Ming-Chuen Yip Department of Power Mechanical Engineering National Tsing Hua University Hsinchu, Taiwan, R.O.C. ABSTRACT The mechanical properties of thin films are critical to precisely predict the mechanical behavior of MEMS devices. In this study, various micromachined structures were designed for bulge test to determine the mechanical properties such as fatigue, creep, delamination, of thin films. The micromachined test patterns on top of circular and square membranes were used as the test specimens. The circular membrane was fabricated using three different dry-etching processes, including RIE (reactive ion etching), deep RIE, and XeF2 chemical etching. Moreover, the square membrane was fabricated using the backside etching of the Si substrate. The dimensions of the membranes were precisely defined using the photolithography technique. The air pressure was employed to apply load (bulge test) on the membrane. The primary test setup was a vacuum system containing a vacuum chamber, a vacuum pump, two pair of needle, and a pneumatic valve. The deformation of the membrane was measured by the optical interferometer. A static pressure load was used for the thin film creep/delamination tests. Three different test patterns such as central pre-crack and cantilevers were used for the delamination tests. By varying the static pressure in chamber, the pre-crack between these two films would induce the delamination. In this test, the Young’s modulus and residual stresses were also determined from the relationship between pressure and deformation. In addition, a cycling pressure load was used for the thin film fatigue test. A comparison of the measured results with the nanoindentation test was also available in this study. Key words：bulge test, creep, delamination, XeF2, pre-crack INTRODUCTION Semiconductor materials such as polysilicon, silicon dioxide or metal thin films have been extensively adopted in MEMS (Micro-Electro-Mechanical System). MEMS application have been demonstrated and developed in the form of sensors or actuators. However, the mechanical properties of these thin films are critical to the reliability of these micro-devices and should be accurately obtained. For creating reliable MEMS devices, it is critical for designer to realize the mechanical property of the thin-film materials. By traditionally method, it is difficult to find out the parameter of mechanical properties because of the issue of small scale, alignment and gripper problems. Despite the availability of numerous methods to measure the mechanical properties of such materials, e.g., tensile test [1], cantilever beam bending test [2], nanoindentation [3] and frequency resonant method [4-5], the shape of the samples significantly affect the observed values. Therefore, mechanical tests of thin films require novel thinking that can incorporate the geometry and microstructure of the films. Based on the reasons mentioned above, a testing frames approach is established for a micro-sample that adopts the optics-interference method to measure the mechanical properties including the Young’s modulus and the adhesive strength of the thin film materials. Moreover, the non-contact method-bulge test can also be used to investigate the mechanical properties of creep and fatigue behavior in the future. The experimental technique of bulge test was first instituted by Beams et al [6]. They measured the mechanical properties of polycrystalline and single-crystal gold and silver films deposited on substrate and calculated stress and strain using the assumption of spherical cap. Besides, there are many techniques to measure the adhesion of thin film including blister test [7], pull test, shear-test and scratch test [8]. In this study, it was adopted to measure the adhesive strength of film/film by manufactured pre-crack. It was something different from the blister test. From the experiments, an appropriate formulation from mechanics of material can be derived to accurately estimate the Young’s modulus of thin film materials. Moreover, the result of Young’s modulus measured by bulge test was compared with the nanoindentation method. On the other hand, the adhesion of the two thin films can be extracted from the applied pressure. FUNDAMENTAL PRINCIPLE (i) Bulge test for circular membrane For the thin-walled sphere, the stress σ in a spherical pressure vessel of the thin film membrane can be expressed as the formula: PR σ = (1) 2t where R is the radius of spherical; P is the applied pressure; and t is the thickness of the thin film. In bulge test, the model derived from Beam [6] was based on the assumption of geometry as shown in Fig.1. The circular membrane have a radius a. When the pressure is applied to the specimens, the deformation of the membrane produced a height h.
h a
R θ
Fig.1 The geometry of spherical cap model From the geometry, the follow relation is obtained.
h a2 + 2 2h 2 for the small deformation, h
