Weixing Xu e-mail: weixing.xu@unsw.edu.au L. C. Zhang 1 e-mail: liangchi.zhang@unsw.edu.au School of Mechanical and Manufacturing Engineering, The University of New South Wales, New South Wales 2052, Australia Xuyue Wang Key Lab for Precision and Non-Traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, China e-mail: wbzzd@dlut.edu.cn Laser Bending of Silicon Sheet: Absorption Factor and Mechanisms Laser bending of silicon sheet is a process to form three-dimensional microstructural sili- con elements in an ambient environment. This study aims to investigate the process mech- anism with the aid of both experimental and numerical analyses. To this end, a thin-film thermocouple was prepared to capture the temperature field within the heating zone of the laser beam. A new method was then developed to precisely determine the absorption factor by coupling numerical simulation of the laser bending results with the experimen- tal results. It was found that each laser pulse causes a cycle of sharp temperature rise- drop in a silicon sheet. When the temperature in the heating zone is low, the sheet deforms elastically. When it is beyond the brittle–ductile transition temperature of sili- con, however, plastic deformation in the sheet takes place and bending occurs. The bend- ing angle becomes larger with increasing the number of laser beam scanning, once the temperature gradient in the scanning area is large enough. [DOI: 10.1115/1.4025579] Keywords: laser bending, silicon sheet, absorption factor, thin-film thermocouple, bending angle 1 Introduction Single-crystal silicon is a semiconductor material that has been widely used in various fields for its attractive electronic and me- chanical properties. However, to manufacture intricate silicon structures has been a significant challenge to both process design and production. For example, in micro-electromechanical sys- tems, the three-dimensional (3D) microstructural elements of sili- con are mainly prepared by anisotropic etching. This means that the structures are within the wafer plane [1]. To create an out of plane microstructure, plastic reshaping is often necessary. In an environment of room temperature, nevertheless, it is impossible to realize plastic reshaping because of the high brittleness of silicon. Fortunately, it has been found that when temperature is above 790 K, a deformation transition from brittle to ductile can take place in silicon. At a temperature beyond 920 K, the plasticity of silicon becomes easier to realize [2,3]. This property of silicon can therefore be utilized to form complex, out of plane micro- structures of a silicon component. For example, a silicon wafer or rod can be heated to its ductile temperature in a furnace, and the shaping can be subsequently conducted in a mould or be forced to bend [4,5]. By using this kind of forming process, however, the manufacture of the mould or the heating process is costly and time-consuming; yet the shapes producible are rather limited. Laser bending has been found to be a flexible forming tech- nique [2,6–8]. The shape change of a sheet is induced by heating- induced irreversible deformation. This is because heating a sheet within a small zone will create a nonuniform temperature distribu- tion through the sheet thickness, which, in turn, will give rise to a nonuniform stress–strain distribution through the thickness, as shown in Fig. 1. If the material’s yield stress is reached, plastic deformation in the sheet will take place. Hence, laser bending can facilitate the shaping of hard and brittle material, provided that the material has a workable brittle-to-ductile transition zone. Moreover, a laser bending can be of high shaping accuracy, which is particularly required in producing a micro/nanodevice, because a laser energy transfer can be precisely controlled. On the other hand, compared with a cold forming [9] or an integral hot forming [10], laser bending can be realized without a die. This is an important advantage because it can make the bending process very fast at a low cost. As an economic and efficient technique, laser bending has gained its significance over the past few years in several fields of industrial manufacturing [11]. To form a 3D microstructure at room temperature, G€ artner et al. [4,12,13] studied the ideal bend- ing parameters, and analyzed the bending mechanisms from the view of temperature gradient based on a simplified thermal model. Zhang and Xu [14,15] used different types of laser to bend the sheets of ceramics, glass, and silicon under either a continuous or a pulse mode. Wang and Wu [16–18] also carried out a series of work on laser bending of silicon sheets. After investigating the effects of laser parameters on bending angles and surface proper- ties of bent silicon elements, they obtained large bending angles on different thicknesses of silicon sheets without damage. A sim- plified finite-element (FE) model was then built up for theoretical analysis. However some of the assumptions, such as the ideal rec- tangular pulse, temperature independent material properties, and uniform laser absorption factor, caused a large deviation between the simulation and experimental results. The purpose of this study is to build up an accurate theoretical laser bending model through coupling thermal and mechanical analysis and explore the silicon sheet bending mechanism with the aid of both experimental and numerical analyses. 2 Modelling of Pulsed Laser Bending Figure 1 illustrates a schematic process of pulsed laser bending, where a silicon sheet is irradiated by a focused laser beam passing across the sheet surface with a certain scanning speed. The laser beam generates a steep temperature gradient in the thickness direction, causing the upper layers of the heated material to expand more than the lower layers, and consequently making the sheet to bend away from the laser beam. After the scanning, the originally heated zone cools down and the material shrinks. How- ever, because the material in the heated zone has been signifi- cantly softened, a reverse bending of the sheet takes place. In other words, the resultant bending is toward the laser-heated sheet surface. Clearly, the above process involves the coupled effect of many variables, such as the heat flux of the laser beam, 1 Corresponding author. Manuscript received April 1, 2013; final manuscript received September 26, 2013; published online November 5, 2013. Assoc. Editor: Yung Shin. Journal of Manufacturing Science and Engineering DECEMBER 2013, Vol. 135 / 061005-1 Copyright V C 2013 by ASME Downloaded From: http://manufacturingscience.asmedigitalcollection.asme.org/ on 11/06/2013 Terms of Use: http://asme.org/terms