Adhesion of Tungsten Fine Lines on SiO 2 by Micro-Wedge Scratching
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M. P. dE BOER, H. HUANG, J. C. NELSON, F. WANG and W. W. GERBERICH Dept. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Microscratch testing of thin films has mainly been done with a conical tip. In the wedge scratch technique introduced in this paper, thin film fine lines are scratched by a wedge-shaped diamond tip until a spallation occurs. A near plane strain situation arises, and finite element method (FEM) analysis is applied for more accurate evaluation of the work of adhesion. The material system used to demonstrate the micro-wedge scratch technique is rf-sputtered tungsten on thermally grown SiO 2. When scratching a long line, delamination occurs by two mechanisms. At loads above 8-9mN/lgm (normal load per unit line width), tensile stresses behind the indenter tip cause the tungsten to crack, and the thin film subsequently curls up. At loads above 1 lmN/tm, high shear stresses in front of the indenter cause delamination of the tungsten at the interface. This latter event is modeled by FEM using a bimaterial fracture mechanics approach to obtain a practical work of adhesion value. INTRODUCTION Good adhesion of fine lines in microelectronics is essential for high yield and reliability. In some cases, a planar thin film may adhere, but fine lines may delaminate after photolithography, and etching. This is because the side constraint is removed. Furthermore, in response to this, a shear stress at the fine line edge arises'. Therefore it is important to test the adhesion of fine lines after etching. However, adhesion of4thin films 2 3 is typically tested on unetched films by a peel test , abrasion test , or shear test . Relative
adhesion may be assessed by these methods, but no accurate adhesion values can be inferred. In microscratch testing, a conical tip is applied directly to the thin film 5 . This method appears to give good agreement with the four point bending results analyzed by bimaterial fracture mechanics 6 . However, stresses in the substrate are modeled approximately, and this technique is not applicable to a fine line. In wedge scratching, fine lines are probed directly, and evaluation by FEM using a bimaterial linear elastic fracture mechanics approach 7 can be done because a near plane strain situation arises. Therefore this technique is satisfying from both practical and theoretical perspectives. Sputtered tungsten on thermally grown SiO 2 was chosen as a model system because of its importance in the microelectronics industry as a "glue" layer for chemically vapor deposited (CVD) tungsten. Also, tungsten has a high shear strength, and hence transfers high shear stresses to the thin film-substrate interface. Finally, tungsten is elastically isotropic, which simplifies FEM analysis. EXPERIMENTAL Standard integrated circuit processing was utilized to fabricate the structures. Silicon wafers were thermally oxidized at 1100 'C in steam to grow a 1.5 gtm layer of Si0 2 . Tungsten was rf- sputter deposited to a thickness of 0.8jtm. The specime
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