A simple approach to determine five thermomechanical properties of thin films

Key words: thin film; thermal expansion coefficient; polyimide; elastic modulus; stress
1 Introduction
With the development of micro-electronics and micro-electro-mechanical systems (MEMS), new thin film materials and their related property studies are receiving more attentions since they are regarded as one kind of important functional materials in this field. For instance, considering their low thermal conductivity and high breakdown voltage, organic polymer films are often used as thermal and electrical insulating materials in MEMS[ 11. However, the higher CTE of organic polymer films compared with that of the insulated substrates (e.g. ceramics or metals) will cause thermal stress build-up and even results in device failure through peeling and crack between the insulating film and substrates. Therefore, to ensure the reliability of the device in the application, the CTE of thin films must be characterized before time. Based on the reported studying results, the mechanical properties (e.g. the residual stress[2], elastic constant [3] and Poisson’s ratio [4]) of thin films may not be the same as that of the bulk materials, and it is more reliable to directly characterize the mechanical properties from the thin film to be determined. About CTE measurement, some traditional techniques have been developed for bulk materials[5-91, but they are not suitable for thin films. Although some new techniques were especially designed for thin films, e.g. the
Abstract: A new technique was proposed to determine the coefficient of thermal expansion (CTE) of thin films at low temperature. Different pre-stress could be applied and the elastic modulus of materials at different temperatures was measured with CTE simultaneously to eliminate the influence of mechanical deformation caused by the pre-stress. By using this technique, the CTEs of polyimide/silica nanocomposite films with different silica doping levels were experimentally studied at temperature from 77 K to 287 K, and some characteristics related to this new technique were discussed.
Foundation item: Project (TJJOSOOS) supported by the National Natural Science Foundation of China and Beijing Jiaotong University Corresponding author: WANG Zheng-dao;Tel: +86-lO-S 1687257; Fax: +86-10-5 1682094; E-mail: zhdwang@
WANG Zheng-dao, et aVTrans. Nonferrous Met. SOC. China 16(2006)
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whether CTE may change for the case of stressed-body thermal expansion. ZHU et a1 [12] ever tried to give a study on this topic, but since they neglected the effect of the temperature on elastic modulus of materials, the experimental result is much higher than ever reported value. In this paper, the theoretical analysis about CTE experiment of pre-stressed body was given and a new CTE testing device suitable for all thin films was introduced. By this device, the CTE of polyimide/silica(PI/SiOz) nanocomposite films with different silica doping levels from low temperature (77 K) to room temperature (287 K) was researched.
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Coefficient of thermal expansion of stressed thin films
WANG Zheng-dao(€E%), JIANG Shao-qing(%*!@)
Institute of Mechanics, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China Received 10 April 2006; accepted 25 April 2006
techniques are not suitable and a tensile pre-stress must be applied. However, for CTE specimens with a certain pre-stress, the traditional testing techniques commonly neglect the influence of mechanical deformation and simplify Eqn.(4) as
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techniques of X-ray diffraction[ lo], micro-cantilever and micro-bridge[l 11, they are often limited for some special materials. For instance, the X-ray diffraction method is only appropriate for crystalline structure. Micro-cantilever and other related techniques are not suitable for such specimens, like a paper, that only one dimension is under the order of sub millimeter or microns. Therefore, sonle more universal CTE testing techniques suitable for thin film materials must be developed. About the CTE experiment, there also has another topic deserving to be paid attention. Basically, the present available techniques on CTE measurement can be divided into two types. One is that there is no load applied on specimens during the testing process and the specimen keeps free-state, e.g. strain gauge technique and micro-machined cantilever technique. The other is that a certain pre-load is applied on specimens during the testing process, e.g. a constant compressive force applied through the rigid expansion probe to the specimen for the technique of thermal mechanical analyzer (TMA), the mass of a quartz-rod applied through the rigid expansion probe to the specimen for the technique of quartz-tube dilatometer. For bulk materials, since the cross-sectional area is much larger than that of thin films, the load-induced pre-stress is very small and can be neglected. But for thin films, it may be different. Moreover, pre-stress state, which is caused by thermal and mechanical loads, the question asked here is that